3D Printing with Light 9783110570588, 9783110569476

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Pu Xiao, Jing Zhang (Eds.) 3D Printing with Light

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3D Printing with Light Edited by Pu Xiao and Jing Zhang

Editors Dr. Pu Xiao Research School of Chemistry Australian National University Canberra, ACT 2601 Australia [email protected] Dr. Jing Zhang Department of Chemical Engineering Monash University Clayton, Victoria 3800 Australia [email protected]

ISBN 978-3-11-056947-6 e-ISBN (PDF) 978-3-11-057058-8 e-ISBN (EPUB) 978-3-11-056984-1 Library of Congress Control Number: 2021930972 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2021 Walter de Gruyter GmbH, Berlin/Boston Cover image: Scharfsinn86/iStock/Getty Images Plus Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Preface 3D printing, different from traditional subtractive manufacturing techniques, is an additive manufacturing technology in which objects can be produced through computer-aided design without the need of molds. Since its invention in the last century, this promising technology has developed rapidly and has found numerous applications in various fields, ranging from personalized consumer products and food industry to drug delivery and tissue engineering and so on. Particularly, this versatile technology has also been used to produce personal protective equipment (e.g., face shields), medical devices (e.g., ventilator valves) and isolation wards to fight against the ongoing coronavirus pandemic. Among various 3D printing approaches (e.g., selective laser sintering, fused deposition modeling, direct metal laser sintering, electron beam melting and stereolithography), the one based on photopolymerization, that is, polymerization reactions induced by light, is extremely attractive. In this approach, objects with well-defined structures can be created from the photopolymerization of liquid photocrosslinkable resin during light irradiation controlled by computer-aided design models. While the engineering, electronic and optical technologies of this 3D printing approach are almost mature, the design and development of high-performance printable materials/inks remains a key challenge. This book thus aims to provide the recent progress in the aspects of chemistry and materials of 3D printing with light. Specifically, Chapter 1 contains information on the recently developed highperformance photoinitiating systems applicable to the vat photopolymerization 3D printing technology. Chapter 2 introduces the newly developed two-photon photoinitiators and their applications in 3D printing and mircofabrication. Chapter 3 critically reviews the recent advancements in the use of dyes in light-induced 3D printing. In particular, functional dyes can be exploited to create stimuli-responsive 3D-shaped polymers. Chapter 4 discusses the role of resin composition for the stereolithographic 3D printing of microfluidic devices, with a specific focus on the methods to successfully print enclosed channels of comparable dimensions to existing microfluidic technologies. The performance of printed objects in microfluidic applications is also reviewed. Chapter 5 discusses novel 3D printable photopolymerizable biomacromolecules and their applications in 3D printing of biomaterials, which demonstrate significant potential for clinical diagnosis and therapeutics. Chapter 6 introduces the principle, development and applications of photocurable 3D printing technology on the market, and puts forward some key problems of the technique which are required to be solved. Chapter 7 discusses the dual wavelength photochemistry for the polymerization reactions and the application in photocuring 3D printing. Chapter 8 introduces a 3D nanoprinting technique based on two-photon photopolymerization, which has been widely used to fabricate various functional micro-/nanodevices. Chapter 9 is dedicated to the application of photocontrolled reversible addition fragmentation chain transfer polymerization (photoRAFT) in 3D printing, which can add https://doi.org/10.1515/9783110570588-202

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Preface

a new dimension to the current manufacturing technologies. Chapter 10 discusses the main challenges in 3D printing focusing on printing speed and applications in biomedical areas. Finally, we would like to thank all chapter authors and De Gruyter, especially those who have been significantly impacted by the coronavirus pandemic in different cities around the world, for their efforts and contributions to the book in this challenging and tough time. Nothing lasts forever, even the worst disasters must end some day. Until the day when nature and science shall bring the disasters to the end, all human wisdom is summed up in two words: wait and hope (Alexandre Dumas). Pu Xiao and Jing Zhang Melbourne, Australia February 2021

Contents Preface

V

List of contributing authors

IX

D. Zhu, J. Zhang, J. Lalevée, P. Xiao Chapter 1 Novel photoinitiating systems for 3D printing

1

Shixiong Chen, Ruchun Zhou, Ming Jin Chapter 2 New free radical and cationic photoinitiators for two-photon 3D printing Ignazio Roppolo, Annalisa Chiappone Chapter 3 Functional dyes in light-induced 3D printing

107

Lubna Shahzadi, Feng Li, Fernando Maya Alejandro, Michael C. Breadmore, Stuart C. Thickett Chapter 4 Resin design in stereolithography 3D printing for microfluidic applications 135 Xiao Chen, Mengfan Zhang, Tingting Wan, Penghui Fan, Kai Shi, Yingshan Zhou, Weilin Xu, Pu Xiao Chapter 5 3D printing of biomaterials 175 Xiaoqun Zhu, Guoqiang Lu, Jun Nie Chapter 6 Photopolymerization and its application in 3D printing of customized objects 203 Yang-Yang Xu, Zhaofu Ding, Haibin Zhu, Yijun Zhang, Pu Xiao, Jean Pierre Fouassier, Jacques Lalevée Chapter 7 Dual wavelength systems in 3D printing 231

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Xuewen Wang, Yunfan Yue, Nianyao Chai, Yibing Chen Chapter 8 Functional 3D nanoprinting via femtosecond laser nonlinear lithography Ali Bagheri, Jianyong Jin Chapter 9 3D printing mediated by photoRAFT polymerization process H. Lai, P. Xiao Chapter 10 Main challenges in 3D printing: printing speed and biomedical applications 317 Index

335

295

249

List of contributing authors Fernando Maya Alejandro Australian Centre for Research on Separation Science (ACROSS) University of Tasmania Hobart TAS 7001 Australia

Yibing Cheng Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory Foshan 528216 People’s Republic of China

Ali Bagheri School of Science and Technology The University of New England Armidale NSW 2351 Australia [email protected]

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing Wuhan University of Technology Wuhan 430070 People’s Republic of China

Michael C. Breadmore Australian Centre for Research on Separation Science (ACROSS) University of Tasmania Hobart TAS 7001 Australia

Annalisa Chiappone Department of Applied Science and Technology DISAT Politecnico di Torino Corso Duca degli Abruzzi, 24 Torino 10129 Italy

Nianyao Chai International School of Materials Science and Engineering Wuhan University of Technology Wuhan 430070 People’s Republic of China

Zhaofu Ding College of Chemistry and Materials Science Anhui Normal University South Jiuhua Rd. 189 Wuhu 241002 People’s Republic of China

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing Wuhan University of Technology Wuhan 430070 People’s Republic of China Shixiong Chen Department of Polymer Materials School of Materials Science and Engineering Tongji University 4800 Caoan Road Shanghai 201804 People’s Republic of China Xiao Chen College of Materials Science and Engineering Wuhan Textile University Wuhan 430073 People’s Republic of China

https://doi.org/10.1515/9783110570588-204

Penghui Fan College of Materials Science and Engineering Wuhan Textile University Wuhan 430073 People’s Republic of China Jean Pierre Fouassier Université de Haute-Alsace France Jianyong Jin School of Chemical Sciences The University of Auckland Auckland 1010 New Zealand

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List of contributing authors

Ming Jin Department of Polymer Materials School of Materials Science and Engineering Tongji University 4800 Caoan Road Shanghai 201804 People’s Republic of China [email protected]

Ignazio Roppolo Department of Applied Science and Technology DISAT Politecnico di Torino Corso Duca degli Abruzzi, 24 Torino 10129 Italy [email protected]

H. Lai Research School of Chemistry Australian National University Canberra, ACT 2601 Australia

Lubna Shahzadi Australian Centre for Research on Separation Science (ACROSS) University of Tasmania Hobart TAS 7001 Australia

J. Lalevée Université de Haute-Alsace CNRS, Institut de Science des Matériaux de Mulhouse (IS2M) 15, rue Jean Starcky Cedex 68057 Mulhouse France [email protected] Feng Li Australian Centre for Research on Separation Science (ACROSS) University of Tasmania Hobart TAS 7001, Australia Guoqiang Lu College of Materials Engineering and Science Beijing University of Chemical Technology Beijing 100029 People’s Republic of China Jun Nie College of Materials Engineering and Science Beijing University of Chemical Technology Beijing 100029 People’s Republic of China

Interdisciplinary Research Center in Biomedical Materials COMSATS University Islamabad Lahore Campus 54000 Pakistan Kai Shi College of Materials Science and Engineering Wuhan Textile University Wuhan 430073 People’s Republic of China Stuart C. Thickett School of Natural Sciences (Chemistry) University of Tasmania Hobart TAS 7001 Australia [email protected] Tingting Wan College of Materials Science and Engineering Wuhan Textile University Wuhan 430073 People’s Republic of China

List of contributing authors

Xuewen Wang International School of Materials Science and Engineering Wuhan University of Technology Wuhan 430070 People’s Republic of China [email protected] Foshan Xianhu Laboratory of the Advanced Energy Science and Technology Guangdong Laboratory Foshan 528216 People’s Republic of China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing Wuhan University of Technology Wuhan 430070 People’s Republic of China P. Xiao Research School of Chemistry Australian National University Canberra, ACT 2601 Australia [email protected]

XI

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing Wuhan University of Technology Wuhan 430070 People’s Republic of China J. Zhang Department of Chemical Engineering Monash University Clayton, Victoria 3800 Australia Mengfan Zhang College of Materials Science and Engineering Wuhan Textile University Wuhan 430073 People’s Republic of China Yijun Zhang Université de Haute-Alsace CNRS, Institut de Science des Matériaux de Mulhouse (IS2M) 15, rue Jean Starcky Cedex 68057 Mulhouse France

Weilin Xu State Key Laboratory of New Textile Materials and Advanced Processing Technologies Wuhan Textile University Wuhan 430073 People’s Republic of China

Ruchun Zhou Department of Polymer Materials School of Materials Science and Engineering Tongji University 4800 Caoan Road Shanghai 201804 People’s Republic of China

Yang-Yang Xu College of Chemistry and Materials Science Anhui Normal University South Jiuhua Rd. 189 Wuhu 241002 People’s Republic of China [email protected]

Yingshan Zhou College of Materials Science and Engineering Wuhan Textile University Wuhan 430073 People’s Republic of China

Yunfan Yue International School of Materials Science and Engineering Wuhan University of Technology Wuhan 430070 People’s Republic of China

State Key Laboratory of New Textile Materials and Advanced Processing Technologies Wuhan Textile University Wuhan 430073 People’s Republic of China [email protected]

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List of contributing authors

D. Zhu Research School of Chemistry Australian National University Canberra, ACT 2601 Australia Haibin Zhu College of Chemistry and Materials Science Anhui Normal University South Jiuhua Rd. 189 Wuhu 241002 People’s Republic of China

Xiaoqun Zhu College of Materials Engineering and Science Beijing University of Chemical Technology Beijing 100029 People’s Republic of China

D. Zhu, J. Zhang, J. Lalevée, P. Xiao

Chapter 1 Novel photoinitiating systems for 3D printing The photosensitive formula for one-photon and two-photon 3D printing consists of photoinitiating system (PIS) and photocurable monomers/oligomers [1, 2]. PIS refers to the components group that is irradiated directly by light and produces active initiating species (free radicals and/or cations) [1, 2]. In more detail, photoinitiator (PI) is excited by photons, then either homocleaved into free radicals or reacted with additives via oxidation and energy transfer to generate radicals and/ or cations for polymerization initiation [1, 3]. Therefore, the properties of PIs in terms of functional group conversion (FC), rate of polymerization (RP,max), color, transparency and photobleaching affect toughness of 3D printed objects and 3D printer parameters such as printing layer thickness and printing duration for each layer [2]. Commercial photoinitiators, 2,4,6-trimethylbenzoyldiphenyl phosphine oxide (TPO), phenylbis(2,4,6-trimethyl-benzoyl)phosphine oxide (BAPO), (1-hydroxycyclohexyl)(phenyl)methanone (Irgacure 184), 2-benzyl-2-(dimethylamino)-1-(4-morpholinophenyl)butan-1-one (Irgacure 369), etc., are efficient and commonly used in PISs for 3D printing [4]. However, 3D printing technology has been applied to various fields such as tissue engineering, oral drug production, electrically conductive constructs, etc. [5–23]. Commercial PISs have obvious drawbacks such as UV initiation, toxicity and low efficiency in further advanced use of 3D printing technology. To improve the efficiency of PISs, and to fit low intensity commercial 3D printer projector in order to meet the standards in some special fields (e.g., medicine, scaffolds, etc.), additional requirements (e.g., non-toxic, visible-light initiable and highly efficient) have to be considered. Tons of efficiently designed type I and type II photoinitiators for 3D printing have been reported (e.g., metal complexes, flavone derivatives and naphthalimide derivatives) [24–26]. In this chapter, an overview of the newly developed PISs of centimeter-scale and nanoscale 3D printing are discussed. Guidance on how to evaluate if a PI or a PIS is suitable for 3D printing is provided as well.

D. Zhu, P. Xiao, Research School of Chemistry, Australian National University, Canberra, Australia J. Zhang, Department of Chemical Engineering, Monash University, Clayton, Victoria, Australia J. Lalevée, Université de Haute-Alsace, CNRS, Institut de Science des Matériaux de Mulhouse (IS2M), Mulhouse, France https://doi.org/10.1515/9783110570588-001

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1.1 Novel photoinitiating systems in 3D printing 1.1.1 Naturally originated and derived photoinitiators Due to abundant brightly colored substances (i.e., dyes), nature is a large provider of potential PIs that are sensitive to visible light LEDs exposure. For instance, curcumin, a bright yellow turmeric extract, has been observed to be a blue-light-sensitive PI [27]; riboflavin, also called vitamin B, is another well-investigated natural PI under blue lights [28, 29]. Several naturally originated and derived compounds have been reported to be efficient PIs [30–36]. Flavone and coumarin derivatives are presented in this subsection to introduce their properties and abilities for 3D printing.

1.1.1.1 Flavone-derivative-based photoinitiating systems Flavone derivatives reported as antioxidants [37] have been discovered to be a new series of blue-light-sensitive PIs in recent years [24, 38]. A general mechanism of photoinitiation process was proposed [24, 38]. Specifically, flavones are excited to singlet or triplet state by photons (r1) and then reacted with NPG or Iod forming intermediate free radicals (r2–r4). An alternate route is two additives (i.e., NPG and Iod) interact and produce a charge transfer complex (CTC) (r5). This CTC can then be decomposed to phenyl radicals (Ar•) (r6). The Ar• is then initiated the following polymerization: Flavone ! 1,3 FlavoneðhνÞ 1,3

Flavone + NPG ! NPGð− HÞ • + Flavone − H• NPGð− HÞ • ! CO2 + NPGð− H, − CO2Þ •

1,3

Flavone + Ar2 I + ! Flavone• + + Ar• + Ar − I

(r1) (r2) (r3) (r4)

NPG + Iod ! ½NPG − IodCTC

(r5)

½NPG − IodCTC ! ! ! Ar• ðhνÞ

(r6)

Photoinitiators 3-Hydroxyflavone (3HF), 6-hydroxyflavone (6HF) and 7-hydroxyflavone (7HF) (Scheme 1.1) are monohydroxy-substituted flavone derivatives. Chrysin and myricetin (Scheme 1.1) are multi-hydroxyl-substituted flavone derivatives. Of all, myricetin is the reddest shifted compound in terms of light absorption among reported flavone derivatives (Table 1.1: λmax = ∼375 nm). All presented flavone derivatives have no maximum absorption peak in visible light range, but their light absorption profiles overlap with the emission profile of LED@405 nm, and the corresponding extinction coefficients

Chapter 1 Novel photoinitiating systems for 3D printing

O

O

OH O 3-Hydroxyflavone

HO

3

O

HO O 6-Hydroxyflavone

O 7-Hydroxyflavone

OH OH HO

O

HO

O

OH

OH OH O Myricetin

OH O Chrysin

O

O Flavone

Scheme 1.1: Chemical structures of flavone derivatives. Table 1.1: Light absorption properties of flavone derivatives in methanol: maximum absorption wavelength (λmax), molar extinction coefficients at λmax and at the maximum LEDs emission wavelengths (εLED) of irradiation sources. PI Flavone HF HF HF Chrysin Myricetin

λmax (nm)

εmax (M− cm−)

ε nm (M− cm−)

Ref.

-* ~ -* ~ ~ 

-* ~, -* ~, ~, ,

~ ~ ~ ~ ~ ,

[] [] [] [] [] []

*:no obvious maximum absorption peak.

are listed in Table 1.1. Monohydroxy- and dihydroxy-substituted flavones exhibit extinction coefficients at used LED@405 nm as ∼70 M−1 cm−1–∼450 M−1 cm−1, while the hexahydroxy substituted flavones, myricetin, has the extinction coefficient of 4,800 M−1 cm−1 at 405 nm. Non-hydroxy substituted flavone (i.e., flavone itself) can trigger free radical polymerization of methacrylates (bisphenol-A-glycidyl methacrylate (Bis-GMA) and trethylene glycol dimethacrylate (TEGDMA), 70 wt%/30 wt%) but cannot promote the polymerization propagation (double bond functional group conversion (FC) of methacrylates at 100-s irradiation is only 4%). 3HF exhibits minor extinction coefficient at 405 nm in methanol (ε405 nm = ∼250 M−1 cm−1). In addition of the amine, N-phenylglycine (NPG), 3HF-based PIS can efficiently initiate photopolymerization of the blend of Bis-GMA/TEGDMA (thickness = 1.4 mm) under the irradiation of LED@405 nm. The double bond conversion of these methacrylates is 71% at 100-s

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irradiation (Table 1.2). In the same conditions as 3HF-based PIS, 6HF- and 7HF-based PISs initiate the blend of Bis-GMA/TEGDMA quite less than 3HF-based two-component PIS (FCt = 100 s(6HF/NPG) = 43%; FCt = 100 s(7HF/NPG) = 31.3%), illustrating that the location of substituents affects a flavone’s photophysics and photoinitiation abilities (Tables 1.1 and 1.2). 20% and 0% methacrylates conversion were attained using PISs of chrysin/NPG and myricetin/NPG, respectively, although the myricetin absorbs the most photon at 405 nm irradiation (ε405 nm = 4,800 M−1 cm−1) (Table 1.1 and 1.2). This might be ascribed to the number of the hydroxyl group. Hydroxyl group is a common radical scavenger that can catch free radicals and prevent active radicals from initiating polymerization [39]. Therefore, the multi-hydroxy flavone, myricetin, can hardly initiate photopolymerization of methacrylates in the presence of NPG upon exposure to LED@405 nm (Table 1.2 and Figure 1.1(a)). Table 1.2: Functional group conversion (FC) of Bis-GMA/TEGDMA (70%/30%, w/w), when using diverse photoinitiating systems (0.5 wt% PI/1 wt% NPG, thickness = 1.4 mm) under air upon exposure to LED@405 nm (I0 = 110 mW cm−2) at 100 s [24, 38]. PIS FC (%)

Flavone/NPG

HF/NPG

HF/NPG

HF/NPG

Chrysin/NPG

Myricetin/NPG







.



n.p.a

a

n.p.: no polymerization

Additives Two monohydroxy flavones, 3HF and 6HF, were chosen as target PIs for 3D printing due to their relatively high efficiency in the presence of NPG in free radical photopolymerization of methacrylates studies, as shown in Table 1.2 and Figure 1.1(a). As stated, the discussed flavone derivatives enclosed in Scheme 1.1 are type II PIs. Different additives can affect the photoinitiation ability of flavone derivatives positively or negatively, to various levels (Table 1.3 and Figure 1.1(b)). The effect of additives on NPG, diphenyliodonium hexafluorophosphate (Iod), ethyl 4-(dimethylamino)benzoate (EDB) and 4-diphenylphosphinobenzoic acid (4-DPPBA) were investigated. As for 3HF, PISs in the presence of NPG alone and Iod/NPG presented efficiency in free radical polymerization of methacrylates (Table 1.3: FCt = 100 s(3HF/NPG) = 71%, FCt = 100 s(3HF/Iod/ NPG) = 79%, while FCt = 100 s(3HF/Iod/EDB) is only 17%). The Iod/NPG is the best additives combination among studied PISs, though NPG alone can improve 3HF photoinitiation ability excellently (Table 1.3). Therefore, 3HF/NPG (0.5%/1%, wt) in the blend of Bis-GMA/TEGDMA (70%/30%, wt) was then prepared as 3D printing resin. As shown in Table 1.3, 6HF-based PISs were studied in two monomer systems – a blend of Bis-GMA/TEGDMA (70%/30%, wt) and trimethylolpropane triacrylate (TMPTA) alone. In photopolymerization of the blend of Bis-GMA/TEGDMA, additive combinations Iod/NPG and Iod/EDB in the presence of 6HF gave the highest double

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Chapter 1 Novel photoinitiating systems for 3D printing

(a)

Conversion (%)

50 1

40 30

4

20

2

10 5 6 3 100

0 50

0

1. 6HF/NPG (0.5/1wt%). LED@405 nm. 2. Chrysin/NPG (0.5/1wt%). LED@405 nm. 3. Myricetin/NPG (0.5/1wt%). LED@405 nm. 4. 7HF/NPG (0.5/1/1wt%). LED@405 nm. 5. Flavone/NPG (0.5/1wt%). LED@405 nm. 6. NPG (1 wt%). LED@405 nm.

Time (s)

Conversion (%)

(b) 80 70 60 50 40 30 20 10 0

(1) (5) (4) (3) (2) (10) (6) (7) (8) (9) 0 10 20 30 40 50 60 70 80 90 100 Time (s)

1. 6HF/lod/NPG (0.5/1/1wt%). LED@405 nm. 2. 6HF/NPG (0.5/1wt%). LED@405 nm. 3. 6HF/lod/EDB (0.5/1/1wt%). LED@385 nm. 4. 6HF/lod/EDB (0.5/1/1wt%). LED@405 nm. 5. lod/NPG (0.5/1wt%). LED@405 nm. 6. 6HF/lod (0.5/1wt%). LED@385 nm. 7. 6HF/lod (0.5/1wt%). LED@405 nm. 8. 6HF/EDB (0.5/1wt%). LED@405 nm. 9. lod/EDB (1/1 wt%). LED@405 nm. 10. NPG (1 wt%). LED@405 nm.

Figure 1.1: Photopolymerization of Bis-GMA/TEGDMA (70%/30%, wt, thickness = 1.4 mm) (FC vs. time) under air in presence of (a) flavone derivatives/NPG and (b) 6HF/additives upon exposure to LED@405 nm and/or LED@385 nm [38]. Reproduced with permission from [38]. Copyright 2020 Wiley Periodicals, Inc. Table 1.3: Photopolymerization of Bis-GMA/TEGDMA (70%/30%, wt, thickness = 1.4 mm) at 100 s and TMPTA (thickness = 25 µm) under air at 200 s in the presence of flavone derivatives and diverse additive combinations upon exposure to LED@405 nm [24, 38]. PI

Additives

HF(DR ) HF(DRa) HF(TMPTA) a

a

NPG

Iod

% % .%

– .% c

EDB

Iod/NPG

Iod/EDB

Iod/-DPPBA

– % 

% .% %c

% .% .%c

– –

: DR: dental resin, Bis-GMA/TEGDMA (70%/30%, wt) : irradiation at 385 nm c : in laminate b

b

%b,c

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bond conversion of methacrylates (FCt = 100 s(6HF/Iod/NPG) = 78.4%, FCt = 100 s (6HF/Iod/EDB) = 56.6%) (Table 1.3). While using TMPTA as monomers, Iod/4DPPBA and Iod/NPG are two efficient additive combinations in the presence of 6HF (FCt = 100 s(6HF/Iod/4-DPPBA) = 43%, FCt = 100 s(6HF/Iod/NPG) = 40%) (Table 1.3). This is ascribed to the viscosity of the blend of Bis-GMA/TEGDMA. Viscous monomer prevents oxygen diffusion and reduces oxygen inhibition in free radical polymerization [40]. 3D printing After confirming the components of 3D printing resin, the 3HF/NPG (0.5%/1%, wt) and 6HF/Iod/4-DPPBA that demonstrated high initiation efficiency to methacrylates were used as PISs in 3D printing. Figure 1.2 presents 3D printed objects and their characterization by numerical optical microscopy. Figure 1.2(A, B) show the 3D printed logo “natural.” The thickness of the 3D printed object is 1.9 mm according to profilometry of numerical optical microscopy (Figure 1.2(C)). Figure 1.2(a, b) show the 3D printed letter “n” and a cube. The thickness of the 3D printed “n” is 30 μm, and the thickness of the 3D printed cube is 1.8 mm, according to profilometry of numerical optical microscopy (Figure 1.2(c, d)). Among presented flavone derivatives, 3HF/NPG, 3HF/Iod/NPG and 6HF/Iod/NPG and 6HF/Iod/4-DPPBA are promising PISs for 3D printing.

1.1.1.2 Coumarin-derivative-based photoinitiating systems Photophysical properties Various coumarin derivatives have been explored as UV-sensitive or blue-lightsensitive photoinitiators for both free radical and cationic polymerization [41, 42]. An oxime-ester coumarin has also been introduced to upconversion nanoparticles as a near-infrared photoinitiator for free radical polymerization [43]. The coumarin derivatives shown in Scheme 1.2 have turned out to be efficient in blue-light-initiating 3D printing technology [44, 45]. As presented in Table 1.4, the most listed coumarin derivatives exhibit light absorption maxima above 400 nm (except for λmax(KC-E) = 343 nm, λmax(KC-G) = 294 nm and λmax(KC-H) = 349 nm). Even though KC-E and KC-G presented light absorption maxima in UV range, their light absorption profile contained a tail above 400 nm [44]. The order of magnitude of the extinction coefficients at their absorption maxima (for all) and at 405 nm is 4 (for all but ε405 nm(KC-E) = 260 M−1 cm−1, ε405 nm(KC-G) = 160 M−1 cm−1 and ε405 nm(KC-H) = 3,260 M−1 cm−1) (Table 1.4). A few possible effects of functional group on absorption maxima in UV range can be discovered from Scheme 1.2 and Table 1.4: comparing KC-F with KC-G, the diethyl amine substitution in KC-F can red shift the light absorption profile; when comparing KC-C and KC-E, the methoxyl group instead of diethyl amine group offers the same results as KC-C in blue shifting; all three compounds with absorption maxima

Chapter 1 Novel photoinitiating systems for 3D printing

Figure 1.2: 3D printed object using 3HF/NPG (A-C) and 6HF/Iod/4-DPPBA (a-d) under irradiation of LED@405 nm [24, 38]. Adapted with permission from [24, 38]. Copyright 2018 American Chemical Society and Copyright 2020 Wiley Periodicals, Inc.

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in UV range are in the absence of diethyl amine substitution (KC-E, KC-G and KC-H) at coumarin moiety. A reasonable deduction is that the presence of diethyl amine substitute in KC-C and KC-F can induce red-shifted light absorption. Photoinitiation abilities Photoinitiation abilities of coumarin derivatives (Scheme 1.2) in the presence of Iod and/or NPG were illustrated via polymerization of TMPTA (thickness = 1.4 mm) under air with irradiation of LED@405 nm (Table 1.5). For two-component PISs (with Iod or NPG), Coum-A1 and KC-H are ineffective (no polymerization detected) for free radical polymerization of TMPTA under the stated conditions (Table 1.5 caption). This can be ascribed to photoinitiation abilities – the limited light penetration due to the thickness of sample and the oxygen inhibition effect due to the under air condition – which was validated by the fact that the same PISs (Coum-A1/Iod, Coum-A1/NPG and KC-H/NPG) can effectively initiate polymerization of TMPTA (thickness = 25 μm) in laminate [44]. As for PISs of coumarin derivatives in the presence of Iod (Table 1.5), KC-E/ Iod PIS-initiated polymerization of TMPTA ended at 100 s with FC = 37%; CoumA/ Iod PIS gave increase in FC of TMPTA to 45%; KC-G/Iod led to no polymerization initiation. The other two-component PISs (CoumB/Iod, KC-C/Iod, KC-D/Iod and KC-F/Iod) brought FC within the range of ∼58% to ∼70%. In addition to Iod, NPG was also investigated as an additive to coumarin derivatives. As presented in Table 1.5. Thiophene (derivatives)-substituted coumarins, CoumA or CoumB, in the presence of NPG showed no effect on polymerization of TMPTA (FC = 0%) (Table 1.5). Benzophenone (derivatives)-substituted coumarins, KCs, presented more efficient photoinitiation abilities with NPG, compared to Iod (Table 1.5). NPG, in place of Iod, increased FC of TMPTA by ∼10% for KC-C, KC-D and KC-F, while it increased FC incredibly for KC-E-based two-component PIS in TMPTA by 49%, and for KC-G-based two-component PIS in TMPTA, from no polymerization to FC = 78% (Table 1.5). When combining Iod and NPG with coumarin derivatives as three-component PISs, the presence of two additives enhanced photoinitiation abilities of coumarin derivatives or at least retained the effectiveness, when using corresponding twocomponent PISs (Table 1.5). All coumarin-derivative-based three-component PISs, except for Coum-A1- and KC-H-based PISs, can efficiently initiate polymerization of TMPTA. As for CoumB/Iod/NPG in Table 1.5, the FC(TMPTA) = 93%. Other effective coumarin-derivative-based three-component PISs can initiate photopolymerization of TMPTA with high double bond conversions from 79% to 86% (Table 1.5). 3D printable resins and 3D printing Besides TMPTA, the blend of Bis-GMA/TEGDA (70%/30%, wt) was also used for 3D printing with coumarin-derivative-based PISs. As shown in Table 1.5, Iod-added

Scheme 1.2: Chemical structures of coumarin derivatives.

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Table 1.4: Light absorption properties of coumarin derivatives: maximum absorption wavelength (λmax), molar extinction coefficients at λmax and at the maximum LED@405 nm emission wavelengths (ε405 nm) of irradiation sources. PI

λmax (nm)

εmax (M− cm−)

ε nm (M− cm−)

ref

         

, , , , , , , , , ,

, , , , , ,  ,  ,

[] [] [] [] [] [] [] [] [] []

CoumA CoumB Coum-A Coum-B KC-C KC-D KC-E KC-F KC-G KC-H

Table 1.5: Functional group conversion of TMPTA for 100-s irradiation and Bis-GMA/TEGDMA (70%/ 30%, wt) for 150-s irradiation under air upon exposure to LED@405 nm in the presence of coumarin-derivative-based PISs (PI: 0.2 wt%, Iod: 1 wt%, NPG: 1 wt%; thickness = 1.4 mm) [44, 45]. PI

Monomer + additives TMPTA Iod

CoumA CoumB Coum-A Coum-B KC-C KC-D KC-E KC-F KC-G KC-H

% % n.p.a – % % % % n.p.a n.p.a

Bis-GMA/TEGDMA

NPG

Iod/NPG

Iod

NPG

Iod/NPG

a

% % n.p.a % % % % % % n.p.a

% % – – – – – – – –

a

% % – – – – – – – –

n.p. n.p.a n.p.a – % % % % % n.p.a

n.p. n.p.a – – – – – – – –

a

n.p.: no polymerization

CoumA- and CoumB-based two-component PISs can effectively initiate polymerization of Bis-GMA/TEGDA (FC(CoumA/Iod)t = 100 s = 36%; FC(CoumB/Iod)t = 100 s = 57%). Although another additive, NPG, has no effect on CoumA- and CoumB-based twocomponent PISs initiating polymerization of Bis-GMA/TEGDA (70%/30%, wt), the addition of NPG can enhance the CoumA or CoumB/Iod PIS photoinitiation abilities (Table 1.5). Specifically, the double bond conversions of Bis-GMA/TEGDMA are 69% and 74% for CoumA/Iod and CoumB/Iod, respectively, in the presence of NPG, while they are 36% and 57% correspondingly in the absence of NPG (Table 1.5). With the

Chapter 1 Novel photoinitiating systems for 3D printing

11

results of polymerization of TMPTA and Bis-GMA/TEGDMA, further letter patterns are 3D printed using LED@405 nm under air. The letter patterns shown in Figure 1.3 used two- and three-component PISs (CoumA(B)/Iod, CoumA(B)/Iod/NPG, KC-C/NPG and KC-C(D)/Iod/NPG) to initiate 3D photocuring of TMPTA and Bis-GMA/TEGDMA; and

Figure 1.3: Characterization of 3D printed object by numerical optical microscopy using CoumA- and CoumB-based PISs (A-I) and KCs-based PISs (a-c) under irradiation of LED@405 nm: (A) CoumB/Iod/ NPG (0.012%/0.061%/0.061% w/w) in Bis-GMA/TEGDMA (thickness = 1,880 µm); (B) CoumA/Iod/ NPG (0.025%/0.125%/0.125% w/w) in Bis-GMA/TEGDMA (thickness = 2,200 µm); (C) CoumA/Iod (0.04%/0.2% w/w) in (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX)/TMPTA (thickness = 2,340 µm); (D) CoumA/Iod (0.05%/0.25% w/w) in TMPTA (thickness = 2,420 µm); (E) CoumB/Iod (0.015%/0.077% w/w) in BisGMA/TEGDMA (thickness = 2,460 µm); (F) CoumA/Iod (0.04%/0.2% w/w) in TMPTA (thickness = 2,840 µm); (G) CoumB/Iod/NPG (0.018%/0.091%/0.091% w/w) in TMPTA (thickness = 2,400 µm); (H) CoumA/Iod/NPG (0.02%/0.1%/0.1% w/w) in TMPTA (thickness = 3,200 µm); (I) CoumB/Iod (0.05%/0.025% w/w) in TMPTA (thickness = 2,620 µm); (a) KC-D/Iod/NPG (0.02%/0.1%/0.1% w/w) in TMPTA (thickness = 2,220 µm); (b) KC-C/Iod/NPG (0.016%/0.083%/0.083% w/w) in TMPTA (thickness = 2,220 µm); and (c) KC-C/NPG (0.025%/0.125% w/w) in TMPTA (thickness = 1,590 µm) [44, 45]. Adapted with permission from [44, 45]. Copyright 2020 Wiley Periodicals, Inc. and Copyright 2019 The Royal Society of Chemistry.

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the thickness of the 3D printed patterns range from 1,590 μm to 3,200 μm (Figure 1.3). All thick patterns were completely printed within 1 min [44, 45].

1.1.2 Carbazole-derivative-based photoinitiating systems Some commonly used coinitiators or their derivatives later turned out to be efficient PIS (e.g., NPG derivative: N-phenylglycine-o-carboxylic acid) [33]. A well-known coinitiator for cationic polymerization initiation is a carbazole derivative (i.e., N-vinylcarbazole (NVK)) [31, 36]. A series of carbazole derivatives have been reported as efficient one-photon PIs and two-photon PIs that initiate polymerization [46, 47]. Recently, some carbazole derivatives were reported as efficient PIs for 3D printing (Scheme 1.3) [48–51]. All the presented carbazole derivatives illustrate absorption maxima in UV range (Figure 1.4).

1.1.2.1 Carbazole-oxime-esters: for both one-photon and two-photon 3D printing Carbazole-oxime-esters (4a–4d) (Scheme 1.3) showed light absorption at 405 nm (Figure 1.4), and the oxime-ester moiety provides them with the capability of twophoton polymerization (TPP) initiation [52, 53]. The free radical of 4d trapped by PBN was generated by light excitation, indicating that investigated carbazole-oximeesters can initiate polymerization as a one-component PI [52]. These four compounds as one-component PIs can efficiently initiate polymerization of TMPTA/ethoxylated trimethylolpropane triacrylate (TMP3EOTA) (1:1, equality of functional groups) upon LED@405 nm (45 mW cm−2) in the absence of air (double bond FC: 40%–60%) [52]. Among these, 4b and 4d were comparable to 2,4,6trimethylbenzoyldiphenyl phosphine oxide (TPO) [52]. 4d was chosen as PI for one-photon digital light process (DLP) 3D printing (Figure 1.5). The resolution of 3D printed deer figurine (Figure 1.5(a)) is 50 μm [52]. The deer were printed in high fidelity, which is validated by the vertical view of deer antler in Figure 1.5(b). Figure 1.5(c, d) reveal the deviation between printed object and designed model. According to the color band in Figure 1.5(d), the green part indicates the deviation is within ± 0.1 mm. The more reddish and the more bluish mean that the absolute deviation is larger. Therefore, it was proved that 4d could produce a precise 3D printed object, limiting the deviation to 0.1 mm. As for two-photon 3D printing initiated by carbazole-oxime-esters, the σ2PA of 4a–4d at 800 nm was investigated via z-scan technique [52]. Due to presence of the electron donating group (EDG) and electron withdrawing group (EWG) in carbazole-oxime-ester structure, the two-photon cross section (σ2PA, 800 nm) of 4a–4d are higher than a reported efficient two-photon PI, and the laser transmittance of 4d in chloroform is the highest among the same concentration of 4a–4d [52, 54].

Scheme 1.3: Chemical structures of investigated carbazole derivatives.

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Figure 1.4: Light absorption maxima and extinction coefficients at maxima of carbazole derivatives (bubble size: extinction coefficient) [48–51].

1.1.2.2 Other carbazole derivatives: for one-photon 3D printing In Scheme 1.3, the other compounds (i.e., A1–A4, Cd1–Cd7 and C1–C4) are used in multicomponent PISs. Their light absorption maxima present within UV range of 320 nm–390 nm (Figure 1.4). However, they illustrate light absorption at 405 nm with other magnitude of molar extinction coefficients from 2 to 5 (except for Cd2 and Cd6 in Figure 1.4), which is the irradiation wavelength of the projector in the 3D printer that is used [49, 50, 52, 55]. As for free radical polymerization, the photocuring of acrylate (TMPTA) in the presence of Iod and/or EDB as well as methacrylates (Bis-GMA/TEGDMA) in the presence of Iod mostly occurred (Table 1.6). While using two-component PISs, the FC of TMPTA is from 0% to 58% (Table 1.6). The A3/Iod (FCt =100s = 58%), Cd7/Iod (FCt =100s = 57%) and C2/Iod (FCt =100s = 56%) are the most efficient twocomponent PISs in each group (Table 1.6). When EDB, instead of Iod, was added as the coinitiator in two-component PISs, the FC of TMPTA were less than or, at most,

Chapter 1 Novel photoinitiating systems for 3D printing

(a)

(b)

15

(c)

5 mm

(d)

0.200 0.180 0.160 0.140 0.120 0.100

–0.100 –0.120 –0.140 –0.160 –0.180 –0.200 mm Figure 1.5: (a) 3D printing model of deer; (b) the vertical view of the printed object; (c) 3D printing model of plant; and (d) 3D scanning picture (4d was used as PI, the wavelength of printer source is 405 nm, and the coin diameter is 25 mm) [48]. Adapted with permission from [48]. Copyright 2020 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim.

similar to the FC of TMPTA using Iod as the coinitiator (Table 1.6). The FC difference between PI/Iod and PI/EDB was from 0% (C3/Iod and C3/EDB) to 19% (A2/Iod and A2/EDB) (Table 1.6). While using three-component PISs in addition to EDB to Iod, the FCs of TMPTA were enhanced for all presented PISs (Table 1.6). While using Bis-GMA/TEGDMA (70%/30%, wt) instead of TMTPA, the double bond conversion of methacrylate improved by ∼10% for A2/Iod and Cd1/Iod (Table 1.6). This might be ascribed to less oxygen diffusion due to monomer viscosity [40].

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Table 1.6: Photopolymerization of EPOX, acrylates (TMPTA) and methacrylate (Bis-GMA/TEGDMA) upon exposure to LED@405 nm (110 mW cm−2) under air for 100 s, unless otherwise noted [48–51]. Monomer + additives

PI

A A A A Cd Cd Cd Cd Cd Cd Cd C C C C

EPOX

Bis-GMA/TEGDMA

Iod

Iod

Iod

NPG

Iod/NPG

% a % a % a % a % n.p. b % % % – % % a % a % a % a

n.p.b,c % c –

% d % d % d % d % d n.p. b,d % d % d % d % d % d % d % d % d % d

% d % d % d – – – – – – – – % d % d % d % d

% d % d % d – – – – – – – – – % d – % d

% c n.p. b,c – – – – % – % c – –

TMPTA

a

: no polymerization : t = 800 s c : thickness = 1.4 nm d : in laminate b

Although the free radical polymerization of TMPTA and Bis-GMA/TEDGMA showed good efficiency with investigated two- or three-component carbazole-derivatives-based PISs, the shrinkage effect of polymerization is due to the nature of methacrylates and acrylates [56, 57]. On the contrary, cationic polymerization of EPOX is a ring-opening polymerization that shows little shrinkage [58]. Hence, cationic polymerization of EPOX upon exposure to LED@405 nm under air was performed, and the efficiency order of carbazole derivatives was: C1 > C4 > C3 > C2 > A2∼A3 > A4∼A1 > Cd7∼Cd1 > Cd3 > Cd5 > Cd4 > Cd6∼Cd2 (Table 1.6), which aligns with light absorption properties (Figure 1.4) of ε405 nm(Cd2) = 0 M−1 cm−1 and ε405 nm (Cd6) = 0 M −1 cm−1. Therefore, two-component PISs in the presence of Iod (1 wt%) with A3, Cd7 or C2 (0.5 wt%) were then set as PISs for 3D printing [49, 50, 55]. EPOX was used in 3D printing due to its less shrinkage during polymerization, unlike the free radical polymerization of acrylates and methacrylates [58]. The PISs can apply to a thick layer (thickness = 0.5 mm), which can reduce overall printing time due to the smallest number of object layers.

Chapter 1 Novel photoinitiating systems for 3D printing

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Therefore the printing time was within 1 min, even for a 0.5-mm thick 3D printed cube [49, 50, 55].

1.1.3 Charge transfer complexes A number of charge transfer complexes (CTCs) have been found to be efficient in both visible-light-photoinitiated polymerization – due to their red-shifted light absorption and allowing extremely deep light penetration (i.e., 31 cm in [59]) – as also in thermally initiated polymerization [60–64]. The well-investigated CTC combination is an [amine-iodonium salt]CTC-([donor-acceptor]CTC) combination (e.g., NPG/ Iod) [62]. Chemical structures of discussed CTCs (donors and acceptors) in this chapter are enclosed in Scheme 1.4.

1.1.3.1 Diverse phenylamines and iodonium salts NPG was found as a group of [NPG-Iod]CTC that can photoinitiate 9-cm-thick monomer composite [33, 62]. The phenyl amines and tertiary amines were investigated in PISs for 3D printing (Am1-Am7 in Scheme 1.4) [59]. As presented in Figure 1.6(a), the obvious red shift was found for [Am1-Iod]CTC through calculation. Meanwhile, the experimental light absorption profiles (Figure 1.6b) also validated that the CTC

Scheme 1.4: Chemical structures of donors and acceptors in CTCs.

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Figure 1.6: (a) Optimized (at the UB3LYP/LANL2DZ level) Am1, Iod, and [Am1-Iod]CTC geometries and their respective UV-vis spectra (single point in DCM). (b) Experimental UV-vis [59]. Adapted with permission from [59]. Copyright 2017 American Chemical Society.

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complex of amine in the presence of Iod ([Am1-Iod]C T C CTC curve 3) exhibits light absorption above 400 nm, up to 550 nm from blue light to green light, while Am1 (curve 1) or Iod (curve 2) alone demonstrate light absorption only within UV range. This might be ascribed to the HOMO located at electron rich N-aromatic amines and LUMO located at iodonium salt, inducing lower HOMO-LUMO gap (i.e., ΔE = 3.13 eV) [33, 59]. Due to the drastic red-shift effect, it allowed application of UVabsorbing PIs to blue-light-triggered 3D printing. Amines and onium salts: effect on CTC As NPG/Iod has been investigated as an efficient CTC complex for photopolymerization initiation [62], Iod was used as model acceptor for amines in the CTC complex study. Among Am1-7, Am1/Iod and Am2/Iod were exceptional for FC of resin 1 (1,4butanediol dimethacrylate (1,4-BDMA)/hydroxypropyl methacrylate (HPMA)/urethane dimethacrylate oligomer (33%/33%/33%, wt)) and irradiation time (Table 1.8: FCt = 150 s (Am1/Iod) = 77% and FCt = 160 s(Am2/Iod) = 71%). This also aligned with their UV-vis spectra in the presence of Iod: [Am1-Iod]CTC and [Am2-Iod]CTC absorbed the most light at 405 nm in dichloromethane (DCM) (Ams: 29 mM; Iod: 13.8 mM) [59]. It was noted that the Iod2 in CTC complexes presented similar results of FCs as did using Iod (Table 1.8). The author opines that this might be due to the electron donating group (EDG) located on aromatic ring and increasing electron density on aromatic N atom; the electron withdrawing group (EWG), ester group on Am3 diluting the electron density on aromatic N resulted in less red-shift effect and less efficiency on FC and irradiation time (Scheme 1.4 and Table 1.8) [59]. Furthermore, this also aligned with the results of HOMO energy. Am1 and Am2 have the highest HOMO energy among Am1Am7 (HOMO energy (Am1) = HOMO energy (Am2) = –4.84 eV) (Table 1.7), which gave the lowest HOMO-LUMO gap when using the same acceptor, Iod. Table 1.7: Summary of donors (amines) HOMO energies in the presence of Iod and acceptors (onium salts) LUMO energies in the presence of Am1 (calculated at the UB3LYP/LANL2DZ Level) [59]. Amines HOMO energy (eV)

Am −.

Am −.

Am −.

Am −.

Am −.

Am −.

Am −.

Onium salts LUMO energy (eV)

Iod −.

Iod −.

Iod −.

Iod −.

PSI −.

– –

– –

As Am1 showed the best efficiency on FC and irradiation time, it was used as model donor in acceptor studies. Studied acceptors (i.e., iodonium salts) are presented in Scheme 1.4. As mentioned, a small HOMO-LUMO gap could induce red-shift effect for CTC complexes. For donors (i.e., amines), high HOMO energy is attractive, and for acceptors, low LUMO energy is preferred. As presented in Table 1.7, theoretically, Iod and Iod4 should be more efficient in CTC systems than Iod2 and Iod3 for

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their cationic form LUMO energies, which are −6.15 eV and −6.53 eV, respectively. However, the [Am1-Iod2]CTC molecular orbital property showed a localized LUMO, and [Am1-Iod]CTC LUMO was somewhat delocalized [59]. Although CTC complexes turned out to be super efficient in deep resin photocuring, thermal free radical polymerization and 3D printing [59, 62], migration issues due to PI molecular weight remain serious. Therefore, a macro-acceptor (Scheme 1.4), iodinated polystyrene (PSI), was synthesized and used in CTC complex studies [64]. [Am1-PSI]CTC red-shifted light absorption from range 100–300 nm to range 200–400 nm (λmax = 307 nm) [64]. Irradiation at 405 nm can result in radical generation via bond photoinduced dehalogenation [64]. Photopolymerization and 3D printing The photoinitiation abilities of the relevant donor/acceptor/phosphine (1%/2%/ 1.5%) as PISs were investigated. The photoinitiation abilities of Iod- and Iod2-based CTC complexes are pretty similar in the polymerization of resin 1 upon exposure to LED@405 nm (Table 1.8). As Am1 and Am2 are of similar structure and show similar photoinitiation abilities in CTC complexes of [amine-onium salt]CTC (Table 1.8), and as Am2 is less toxic than Am1 according to manufacturer’s data [59], [Am2-Iod2]CTC was used for the photopolymerization and 3D printing. 4-(Diphenylphosphino)styrene (4dpps) was a phosphine additive in addition to CTC complex ([Am2-Iod2]CTC) for photopolymerization of resin 1, which shortened the polymerization time from 100 s to 50 s [59]. With Am2/Iod2/4dpps, depth of photopolymerization was investigated with resin 1 and resin 2 (Bis-GMA/TEGDMA: 70%/30%, wt). Table 1.8: Photopolymerization double bond FC of resin 1 under air in the presence of amine and Iod or Iod2 upon exposure to LED@405 nm [59]. Donor

Am

Am

Am

Accepter FC (%) Irradiation time (s)

Am

Am

Am

 

 

 

~ ~

~ 

~ ~

Iod  

 

 

Acceptor FC (%) Irradiation time (s)

Am

  Iod

~ ~

~ ~

~ 

~ ~

According to Raman analysis, the sample depth at 9 cm was of FC(C =C) = ∼40% for resin 2 within 10 min and of FC(C =C) = ∼60% for resin 1 within 12 min [59]. Moreover, the deepest cured sample in the presence of Am2/Iod2/4dpps for resin 2 is 31 cm [59]. Such deep photocuring ability provides a broad avenue for application optimization (e.g., shortening 3D printing time). Using PSI as acceptor, Am1-based CTC complex,

Chapter 1 Novel photoinitiating systems for 3D printing

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in the absence of phosphine additive, can result in double bond conversion of TMPTA of 40% at irradiation time of 100 s [64]. With the same PISs (Am2/Iod2/3dpps: 1%/2%/1.5% and Am1/PSI: 0.5%/1%, wt), laser-written thick “CTC” and letter “P” were successfully printed in 200-μm and 96-μm thickness, respectively (laser diode at 405 nm) [59, 64]. The spatial resolution of “CTC” was pretty good as the pattern edge width was only 54 μm [59].

1.1.3.2 Phosphines instead of amines as donors As mentioned in amine-based donors in CTC complexes, a phosphine additive (i.e., 4dpps) was used in PISs [59]. Due to lone pairs, phosphine-atom and nitrogen-atom are promising electron-rich centers (e.g., NPG, Am1 and Am2) [59, 62]. Several phosphine-centered compounds were investigated as donors in CTC complexes (Scheme 1.5) [65]. [Phosphine-Iod2]CTC complexes can red-shift light absorption. Among three CTC complexes with Iod2 as acceptor, ΔE([2DPP1NA-Iod2]CTC) = 3.29 eV was the highest, which aligned with its light absorption profile and absorption maxima that its redshifted light absorption is the closest to UV range (i.e., the least extent of red-shift) [65]. Other [phosphine-Iod2]CTC complexes can red-shift light absorption from UV range to visible light range (blue or green) (e.g., λmax(Iod2) = 277 nm; λmax([DMAPDPIod2]CTC) = 320 nm, 393 nm and 564 nm) due to reduction of HOMO-LUMO gap (Table 1.9) [65]. Hence, 2DPPBA and DMAPDP were involved in studies of Bis-GMA/ TEGDMA (70%/30%, wt) photopolymerization. Table 1.9: Summary of HOMO and LUMO energies of phosphines in the absence and the presence of Iod2 (calculated at the UB3LYP/LANL2DZ Level) [65]. Amines

DPPINA

DPPBA

DMAPDP

HOMO energy (eV) LUMO energy (eV) ΔE (eV) Absorption maxima (nm)

−. −.  , 

−. −. . , 

−. −. . , , 

+Iod HOMO energy (eV) LUMO energy (eV) ΔE (eV) Absorption maxima (nm)

−. −. . , , 

−. −. . , , 

−. −. . , , 

1

cationic form

For the photopolymerization of 1.4-mm-thick Bis-GMA/TEGDMA (70%/30%, wt) under air, with irradiation at 405 nm, photoinitiation ability of [2DPP1NA-Iod2]CTC with low concentration (donor/acceptor: 0.5%/0.5%, wt) was better than [DMAPDP-Iod2]CTC on

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double bond FC (FCt = 100s ([2DPP1NA-Iod2]CTC) = 70%, FCt = 100s ([DMAPDP-Iod2]CTC) = 40%)[65]. Addition of concentration of [donor-acceptor]CTC for [2DPP1NA-Iod2]CTC prompted a few double bond FCs, while that for [DMAPDP-Iod2]CTC (2%/2%, wt) increased double bond FCs by ∼40%, similar to FC initiated by [2DPP1NA-Iod2]CTC (0.5%/0.5%, wt). Laser-writing experiments using [2DPP1NA-Iod2]CTC and [DMAPDP-Iod2]CTC with Bis-GMA/TEGDMA were conducted to illustrate the capability of investigated CTC complexes of 3D printing application. Pattern margin width along cured and uncured resin board was only 54 μm (similar to the one of the laser beam: ∼50 μm), which demonstrated that the spatial resolution control of 3D printing using [2DPP1NA-Iod2]CTC and [DMAPDP-Iod2]CTC were good [65].

Scheme 1.5: Chemical structures of phosphines as donors in CTC systems.

1.1.4 Metal-contained photoinitiators Metal-contained compound is a large group of visible light PI category [66–69]. For instance, three metal complexes, zinc tetraphenylporphyrin (ZnTPP), CuC-4 and Ru, presented in Scheme 1.6, were reported as efficient PIs for FRP and CP. Meanwhile, they were also applied to 3D printing [25, 67, 69].

1.1.4.1 ZnTPP ZnTPP is a tetraphenylporphyrin complexed with zinc (Scheme 1.6). As depicted in Table 1.10, ZnTPP can absorb light intensely at 420 nm (λ420 nm = ∼400,000 M−1 cm−1) and at 552 nm (λ552 nm = ∼200,000 M−1 cm−1) [25]. Light absorption properties of ZnTPP in combination of photopolymerization of Bis-GMA/TEGDMA and EPOX validated ZnTPP capability of production of active species when excited by blue and green lights [25]. For photopolymerization of Bis-GMA/TEGDMA (sample thickness = 1.4 mm), diverse visible light LEDs, including LED@405 nm, LED@455 nm, LED@477 nm and

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Scheme 1.6: Chemical structures of introduced metal complexes.

Table 1.10: UV-vis light absorption properties of discussed metal complexes [25].

λmax (nm) εmax (M− cm−) ε nm (M− cm−)

ZnTPP

Ru

CuC-A

CuC-

CuC-

CuC-

CuC-

 nm , –

 nm – –

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(a)

Layer 2 exposure Layer 3 exposure

Layer 0 exposure Layer 1 exposure

Layer 4 exposure Total exposure

6

Normalized dose, Ω(z,τl)

5

Layer index: 0

1

2

3

4 τl = 2.0 ha = 72.5 ζl = 0.69

4 3 2 1 0 0

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50

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150

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(b) 6 0

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6

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8

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Normalized dose, Ω(z,τl)

5 Single layer parameters ζl = 0.69 τl = Ωfront = 2.0 Ωback = 1.0

4 3 2 1 0

0

2

4

6 𝛾

8

10

12

Figure 4.15: Optical dose applied to an SLA resin under a variety of conditions. Panel (a): variation of normalized dose across a printed object consisting of 5 layers. A normalized dose of 1 represents a fully cured layer. (b) Modeled normalized dose across a 12-layer object with channel in layers 5 and 6. In this case, overcuring has occurred as the normalized dose is > 1 within layer 6. (c) Effect of varying print layer thickness for printing a 200 μm channel. Reproduced with permission from [89]. Copyright 2015, Royal Society of Chemistry.

and ~ 5.5 ha for resins with ha < 40 μm. An optimized PEGDA resin was developed incorporating 3 % w/w NPS as photoblocker (ha = 8 μm) enabled the successful printing of channels of height 2.3 ha (18 μm); the cross-sectional area of these channels was as small as 18 μm × 20 μm [34].

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(c)

6

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zl = 50.00 μm ζl = 1.50 τl = 4.47 zl = 25.00 μm ζl = 0.75 τl = 2.11 zl = 10.00 μm ζl = 0.30 τl = 1.35

Normalized dose, Ω(z,τl)

5 4 3

ha = 33.4 μm L = 200.0 μm L/ha = 5.99

2 1 0 0

100

200

z (μm)

300

400

500

Figure 4.15 (continued)

Figure 4.16: Optical microscopy images demonstrating the impact of print layer thickness on the quality of enclosed channels (200 μm high, 135 μm wide) within a PEGDA resin using Sudan I as photoblocker. Left to right: 50 μm, 25 μm and 10 μm print layers respectively. Reproduced with permission from [89]. Copyright 2015, Royal Society of Chemistry.

4.4.2 Materials considerations One of the main limitations for expanding the scope of 3D printing applications by stereolithography (SLA) in the field of microfluidics is the limited selection of materials available to date. While the amount of novel materials for fused deposition modeling is increasing at a faster pace, the limited amount of materials that can be photopolymerized is the main bottleneck in 3D printing by SLA. The development of novel photopolymerizable formulations will be critical for the advancement of 3D printing by SLA as a technique for the fabrication of microfluidic devices. The development of novel materials for 3D printing by SLA has been focused towards obtaining 3D printed parts with improved mechanical properties, chemical stability and functionality. Typical resin formulations for SLA 3D printing are based on acrylate monomers and oligomers, which result in polymers with a relatively limited stability at higher

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temperatures or when exposed to organic solvents. This will limit the application of microfluidics based on such materials for applications required high temperature, the use of organic solvents typically used in analytical sample preparation, or liquid chromatographic techniques. These disadvantages greatly limit the application of this additive manufacturing technique for the fabrication of devices for analytical separation science. This limitation is circumvented easily in FDM 3D printing by fabricating filaments using more resistant polymers such as polyether ether ketone (PEEK). In SLA 3D printing, a route to circumvent this limitation is the 3D printing of composites based on the dispersion of ceramic particles on an acrylate-based resin [113]. A critical parameter in the photopolymerization of composite resins is the stability of the photopolymerizable resin containing a high load of the ceramic particles, as the aggregation or precipitation of these particles will lead to a final 3D printed part with a non-homogeneous distribution of the ceramic particles. This was achieved by dispersing amorphous silica nanoparticles (40 nm, mean diameter) in the polymerization mixture. The polymerization mixture consists mostly on the monomer hydroxyethyl methacrylate (HEMA). The use of this hydrophilic monomer enabled the dispersion of a high load of silica nanoparticles without requiring further additives. Using SLA, 3D printed parts based on the previous composite resins were fabricated. The initial 3D printed part was thermally debound, removing the polymer matrix by thermal decomposition. The resulting part was sintered at 1,300 °C to obtain highquality fused silica glass with no remnants of porosity or cracks. An alternative to the addition of silica nanoparticles to the polymerization mixture is the use of photosensitive methylsilsesquioxane preceramic polymers. In this case, the resin was prepared from a commercially available methylsilsesquioxane resin (SILRES MK, Wacker‐Chemie GmbH, Nuenchritz, Germany) [114]. The 3D printed parts, based on this polymerization mixture containing SILRES, were converted to SiOC by pyrolysis at 1,000 °C under nitrogen atmosphere. In a different example, a UV-curable siloxane formulation was prepared from (mercaptopropyl) methylsiloxane and vinylmethoxysiloxane combined with a photoinitiator that is active at 405 nm [115]. After carbonization at 1,000 °C, the preceramic polymer is converted to an amorphous silicon oxycarbide containing sulphur. While glass is the preferred materials for microfluidics because of its chemical and thermal stability, fluorinated polymers such as polytetrafluoroethylene (PTFE) also have high chemical and thermal stability. PTFE is not a polymer suitable for stereolithography. However, PTFE nanoparticles can be dispersed in a photocurable resin and the resulting composite resin can be 3D printed by stereolithography [116]. In a similar procedure, as mentioned for the 3D printing of glass devices, the PTFE/polymer 3D printed part was sintered at 370–400 °C. A limitation here is that the resulting PTFE-based 3D printed devices would be non-transparent, making it difficult for certain types of applications such as UV-vis monitoring. Highly fluorinated perfluoropolyether (PFPE) methacrylates are interesting alternatives for transparent and chemically resistant microfluidic chips. A new

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coating material, named Fluoropor, has been developed by the photopolymerization of a PFPE‐methacrylate using cyclohexanol as non-solvent and a fluorinated alcohol as an emulsifying agent. Fluoropor has been shown to be an interesting candidate for 3D printing of chemically resistant parts since it can be printed using benchtop SLA 3D printers [117]. The developed formulation was used to 3D print microfluidic chips with a minimal cross-sectional area of 600 µm for monolithic embedded channels and 200 µm for open channels. In addition, the PFPE-methacrylate used in this application showed a transmittance of above 70% for light with wavelengths between 520–900 nm. It also showed a high chemical resistance against organic solvents. As a proof of concept of microfluidic application, a gradient generator was 3D printed (Figure 4.18). The channels of the gradient generator have a width and height of 800 µm (Figure 4.17(a, b)), or 600 µm (Figure 4.17(c, d)).

Figure 4.17: 3D printed microfluidic gradient generator based on PFPE-methacrylate. Scale bars: (a) and (c), 2 mm; (b) and (d), 500 µm. Reproduced with permission from [117]. Copyright 2018, MDPI.

The development of simple and efficient strategies to SLA 3D print devices with high resolution in glass, or fluorinated polymers will be critical for the development of robust devices for microfluidic applications.

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4.4.3 Examples and performance of printed objects SLA 3D printing is currently the best option for microfluidics fabrication owing to its high printing resolution compared to FDM and unlike inkjet printers such as PolyJet, it does not require supporting material, which normally takes days and weeks to remove from inside the microchannel. Channel dimensions and optical transparency of the printed device are two important factors affecting the performance of the SLA printed microfluidics. Shallan and co-workers directly 3D printed microfluidic devices with cross sectional area of 250 µm, with an SLA 3D printer for the first time and used these 3D printed devices for various applications including micromixing, gradient generation and isotachophoresis [118]. The minimum microchannel size fabricated with a commercially available SLA 3D printer and resins was 154 µm by Macdonald et al. [119]. Nordin’s group has done tremendous work in improving the SLA printing resolution for truly microfluidic devices fabrication through optimizing the resin formulations and the printer light source using a customized printing setup. Using their customized SLA printer and resin formulations, microfluidic channels with cross sections as small as 18 µm × 20 µm were successfully created as shown in Figure 4.18 [34]. This printer, with high printing resolution, has also been used for other applications such as for microchip electrophoresis of preterm birth markers detection with the limit of detection in the high pM to low nM range. This was the first report of the creation of microchip electrophoresis devices with < 50 µm cross-sectional dimensions by 3D printing [120]. Optical detection is the most common detection method for microfluidics, so the optical transparency property of the printed device can affect the detection significantly. Both Rapp’s and Folch’s groups have achieved SLA printing with highly transparent materials such as glass and PDMS, respectively. Glass is one of the materials used in the early stages of microfluidic research, given its unmatchable optical transmittance, high chemical and thermal resistance, and low non-specific adsorption that enable its wide application in optical detection and capillary chromatography. Kotz et.al developed an SLA-based approach for transparent glass 3D printing with photocurable silica nanocomposite at a resolution of 60 μm and surface roughness of 2 nm. The printing process is shown in Figure 4.19 [113]. More recently, Kotz and co-workers fabricated hollow 3D microchannels in glass indirectly by combining sacrificial template replication with a photopolymerization process [121]. PDMS is currently the most widely used material for microfluidics fabrication due to its high transparency, biocompatibility and gas permeability. However, PDMS molding is a manual procedure and requires tedious and time-consuming assembly steps. Femmer et al. developed an SLA approach for printing PDMS through the use of a PDMS copolymer functionalized with 7–9 mol % pendant methacrylate groups as the main resin component with ethyl (2,4,6-trimethylbenzoyl) phenyl phosphinate (TPO-L) as photoinitiator and Orasol Orange as photo-blocker [122]. This

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Figure 4.18: (a) Primary and additional edge exposure patterns for a single layer containing a flow channel. (b) Channel narrowing for 2% NPS resin for additional edge exposure. The build layer thickness is 8.3 μm and the designed flow channel height is 25 μm. (c) Same as (b) except for 3% NPS resin with 6 μm layers and a designed flow channel height of 18 μm. (d), (e) measured channel width and height, respectively, as a function of edge exposure time. Reproduced with permission from [34]. Copyright 2017, Royal Society of Chemistry.

resin system enabled the printing of a cross-flow gas-liquid PDMS membrane with the connectors integrated as part of the print. Bhattacharjee and co-workers used a similar approach for the 3D printing of PDMS microfluidics [123]. The resins consisted of a functionalized 3-methacryloxypropyl-PDMS copolymer as well as methacryloylterminated PDMS as monomers in addition to TPO-L and isopropylthioxanthone

Figure 4.19: (a) Ultraviolet-curable monomer mixed with amorphous silica nanopowder is structured in a stereolithography system. The resulting polymerized composite is turned into fused silica glass through thermal debinding and sintering (scale bar, 7 mm). (b, c), Examples of printed and sintered glass structures: Karlsruhe Institute of Technology logo (b; scale bar, 5 mm) and pretzel ((c); scale bar, 5 mm). (d), Demonstration of the high thermal resistance of printed fused silica glass (scale bar, 1 cm). The flame had a temperature of about 800 °C. Reproduced with permission from [113]. Copyright 2017, Nature Publishing Group.

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(ITX) as photoinitiators and photo-blockers, respectively. The resins were polymerized with a 385 nm light source to produce transparent microfluidic devices with cross sectional areas of 300 µm × 500 µm for use in mammalian cell culture. Another interesting application of SLA printing in microfluidics is the fabrication of integrated devices to increase the functionality of the devices [124]. Multimaterial 3D printing is the most ideal approach for integrating microfluidics fabrication, but it is difficult to easily achieve multimaterial printing with SLA printer. Currently, SLA printing integrated microfluidics is mainly through single material printing, which can be obtained by combining the 3D geometry and intrinsic material properties. Rogers and co-workers fabricated a microfluidic device with integrated pneumatic valves capable of operating for 800 actuations. The device was SLA printed with a customized resin and integrated with a 3D printed 100 μm membrane as part of the device. Due to the flexibility of the thin membrane, an external pressure caused the localized deflection and was used for sealing the inlet and outlet openings, effectively closing the valve. The valve was opened when the external pressure was released and the membrane returned to its original position [125]. Modular microfluidic devices were also fabricated with SLA printing – Bhargava and co-workers fabricated integrated microfluidic devices including a gradient generator, microdroplet generator and optical droplet sensing system by assembling discrete reconfigurable components printed with a SLA printer [126]. Yuen also presented a ‘Plug-n-plug’ modular microfluidic system with multiple microfluidic components, including a motherboard with fluidic interconnects, fittings, and fluidic inserts fabricated with SLA printing individually [127].

4.5 Conclusions and perspectives Over the past decade, SLA based techniques have shown to be an exceptionally exciting approach in the fabrication of microfluidic devices with tailored feature size, materials properties and functionality. Compared to the historical state of the art of PDMS molding to produce devices, the move towards printed objects via CAD design represents a significant step towards standardized printing with increased reproducibility. While the learning curve can be potentially steep for researchers entering the field, the wealth of printed designs that are freely available for download should encourage more non-specialists in the field. Compared to other 3D printing techniques, SLA is particularly appealing for microfluidic applications, given the ability to print features with small size at high resolution. One major drawback has been the relatively limited choice of materials for printing. However, this is an active area of research through the development of new resin chemistries amenable to SLA, specifically with a view towards improved mechanical properties and chemical resistance. An ongoing challenge is the fabrication

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of a fully integrated device with parts that require different materials properties as opposed to separate part fabrication across multiple steps. Multi-material 3D printing can potentially address this question. However, it is a technique that is still in its infancy and challenges exist with respect to the printer setup to successfully print different resins (or different resin components) in one print. We consider this an exciting area of research in the near future. Great advances have been made in the successful printing of devices with enclosed channels via SLA-based 3D printing. This has been made possible through the rational control of light penetration through the resin by specific additives such as photo-blockers and a systematic understanding of print parameters such as print layer thickness and print time. Such understanding has enabled SLA to move from being only able to print millifluidic devices to those truly in the microfluidic range. Other approaches to control or constrain the printed object thickness at the resin interface present opportunities to push the capabilities of SLA even further in the coming years.

References [1] [2] [3] [4] [5] [6]

[7]

[8] [9] [10] [11] [12]

Chapiro, M. Current achievements and future outlook for composites in 3D printing, Reinf Plast, 2016, 60, 372–375. Wang, X., Jiang, M., Zhou, Z., Gou, J., Hui, D. 3D printing of polymer matrix composites: A review and prospective, Composites Part B Eng, 2017, 110, 442–458. Manapat, J. Z., Chen, Q., Ye, P., Advincula, R. C. 3D printing of polymer nanocomposites via stereolithography, Macromol Mater Eng, 2017, 302, 1600553. Stansbury, J. W., Idacavage, M. J. 3D printing with polymers: Challenges among expanding options and opportunities, Dent Mater, 2016, 32, 54–64. Buchanan, C., Gardner, L. Metal 3D printing in construction: A review of methods, research, applications, opportunities and challenges, Eng Struct, 2019, 180, 332–348. Sanatgar, R. H., Campagne, C., Nierstrasz, V. Investigation of the adhesion properties of direct 3D printing of polymers and nanocomposites on textiles: Effect of FDM printing process parameters, Appl Surf Sci, 2017, 403, 551–563. Morris, V. B., Nimbalkar, S., Younesi, M., McClellan, P., Akkus, O. Mechanical properties, cytocompatibility and manufacturability of chitosan: PEGDAHybrid-Gel Scaffolds by stereolithography, Ann Biomed Eng, 2017, 45, 286–296. Lee, J.-Y., An, J., Chua, C. K. Fundamentals and applications of 3D printing for novel materials, Appl Mater Today, 2017, 7, 120–133. Peels, J. 3D Printing in the Military. 3d Print 2017. Crivello, J. V., Reichmanis, E. Photopolymer materials and processes for advanced technologies, Chem Mater, 2014, 26, 533–548. Global 3D printing products and services market size from 2020 to 2024. Jul 23, 2020 ed. United States of America: Statista, Inc, 2020. Campbell, T., Williams, C., Ivanova, O., Garrett, B. Could 3D printing change the world. Technologies, Potential, and Implications of Additive Manufacturing, Atlantic Council, Washington, DC. 2011.

Chapter 4 Resin design in stereolithography 3D printing for microfluidic applications

[13] [14] [15] [16]

[17] [18]

[19] [20]

[21]

[22] [23] [24] [25]

[26]

[27] [28]

[29]

[30]

[31]

169

Standard, A. ISO/ASTM 52900: 2015 Additive manufacturing-General principles-terminology. ASTM F2792-10e1. 2012. Grimm, T. User’s guide to rapid prototyping, Society of Manufacturing Engineers, 2004. the text. Truby, R. L., Lewis, J. A. Printing soft matter in three dimensions, Nature, 2016, 540, 371. Heller, C., Schwentenwein, M., Russmueller, G., Varga, F., Stampfl, J., Liska, R. Vinyl esters: Low cytotoxicity monomers for the fabrication of biocompatible 3D scaffolds by lithography based additive manufacturing, J Polym Sci A Polym Chem, 2009, 47, 6941–6954. Cho, Y. H., Lee, I. H., Cho, D.-W. Laser scanning path generation considering photopolymer solidification in micro-stereolithography, Microsyst Technol, 2005, 11, 158–167. de Leon, A. C., Chen, Q., Palaganas, N. B., Palaganas, J. O., Manapat, J., Advincula, R. C. High performance polymer nanocomposites for additive manufacturing applications, React Funct Polym, 2016, 103, 141–155. Schmidleithner, C., Kalaskar, D. M. Stereolithography. IntechOpen; 2018. Elomaa, L., Pan, -C.-C., Shanjani, Y., Malkovskiy, A., Seppälä, J. V., Yang, Y. Threedimensional fabrication of cell-laden biodegradable poly (ethylene glycol-co-depsipeptide) hydrogels by visible light stereolithography, J Mater Chem B, 2015, 3, 8348–8358. Palaganas, N. B., Mangadlao, J. D., de Leon, A. C. C., Palaganas, J. O., Pangilinan, K. D., Lee, Y. J., et al. 3D printing of photocurable cellulose nanocrystal composite for fabrication of complex architectures via stereolithography, ACS Appl Mater Interfaces, 2017, 9, 34314–34324. Zhu, W., Qu, X., Zhu, J., Ma, X., Patel, S., Liu, J., et al. Direct 3D bioprinting of prevascularized tissue constructs with complex microarchitecture, Biomaterials, 2017, 124, 106–115. Katal, G., Tyagi, N., Joshi, A. Digital light processing and its future applications, Int J Sci Eng Res Publ, 2013, 3, 1997–2004. Ligon, S. C., Liska, R., Stampfl, J., Gurr, M., Mülhaupt, R. Polymers for 3D printing and customized additive manufacturing, Chem Rev, 2017, 117, 10212–10290. Hatzenbichler, M., Geppert, M., Seemann, R., Stampfl, J. Additive manufacturing of photopolymers using the Texas Instruments DLP lightcrafter. Emerging Digital Micromirror Device Based Systems and Applications V: International Society for Optics and Photonics; 2013, 86180A. Baumgartner, S., Pfaffinger, M., Busetti, B., Stampfl, J. Comparison of dynamic mask-and vector-based ceramic stereolithography. Proceedings of the 41st International Conference on Advanced Ceramics and Composites Hoboken, Wiley: Wiley Online Library, 2018, 163–173. Tumbleston, J. R., Shirvanyants, D., Ermoshkin, N., Janusziewicz, R., Johnson, A. R., Kelly, D., et al. Continuous liquid interface production of 3D objects, Science, 2015, 347, 1349–1352. Janusziewicz, R., Tumbleston, J. R., Quintanilla, A. L., Mecham, S. J., DeSimone, J. M. Layerless fabrication with continuous liquid interface production, Proc Natl Acad Sci, 2016, 113, 11703–11708. Shusteff, M., Browar, A. E., Kelly, B. E., Henriksson, J., Weisgraber, T. H., Panas, R. M., et al. One-step volumetric additive manufacturing of complex polymer structures, Sci Adv, 2017, 3, eaao5496. Kelly, B. E., Bhattacharya, I., Heidari, H., Shusteff, M., Spadaccini, C. M., Taylor, H. K. Volumetric additive manufacturing via tomographic reconstruction, Science, 2019, 363, 1075–1079. Loterie, D., Delrot, P., Moser, C. High-resolution tomographic volumetric additive manufacturing, Nat Commun, 2020, 11, 1–6.

170

Lubna Shahzadi et al.

[32] Van Der Laan, H. L., Burns, M. A., Scott, T. F. Volumetric photopolymerization confinement through dual-wavelength photoinitiation and photoinhibition, ACS Macro Lett, 2019, 8, 899–904. [33] Li, F., Thickett, S. C., Maya, F., Doeven, E. H., Guijt, R. M., Breadmore, M. C. Rapid Additive Manufacturing of 3D Geometric Structures via Dual-Wavelength Polymerization, ACS Macro Lett, 2020, 9(10), 1409–1414. [34] Gong, H., Bickham, B. P., Woolley, A. T., Nordin, G. P. Custom 3D printer and resin for 18 μm × 20 μm microfluidic flow channels, Lab Chip, 2017, 17, 2899–2909. [35] Zarek, M., Mansour, N., Shapira, S., Cohn, D. 4D printing of shape memory-based personalized endoluminal medical devices, Macromol Rapid Commun, 2017, 38, 1600628. [36] Erkal, J. L., Selimovic, A., Gross, B. C., Lockwood, S. Y., Walton, E. L., McNamara, S., et al. 3D printed microfluidic devices with integrated versatile and reusable electrodes, Lab Chip, 2014, 14, 2023–2032. [37] Han, Y., Wang, F., Wang, H., Jiao, X., Chen, D. High-strength boehmite-acrylate composites for 3D printing: Reinforced filler-matrix interactions, Compos Sci Technol, 2018, 154, 104–109. [38] Ma, Y., Dong, J., Bhattacharjee, S., Wijeratne, S., Bruening, M. L., Baker, G. L. Increased protein sorption in poly (acrylic acid)-containing films through incorporation of comb-like polymers and film adsorption at low pH and high ionic strength, Langmuir, 2013, 29, 2946–2954. [39] Gao, W., Zhang, Y., Ramanujan, D., Ramani, K., Chen, Y., Williams, C. B., et al. The status, challenges, and future of additive manufacturing in engineering, Comput-Aided Des, 2015, 69, 65–89. [40] Xing, J.-F., Zheng, M.-L., Duan, X.-M. Two-photon polymerization microfabrication of hydrogels: An advanced 3D printing technology for tissue engineering and drug delivery, Chem Soc Rev, 2015, 44, 5031–5039. [41] Zhang, J., Xiao, P. 3D printing of photopolymers, Polym Chem, 2018, 9, 1530–1540. [42] Zheng, X., Smith, W., Jackson, J., Moran, B., Cui, H., Chen, D., et al. Multiscale metallic metamaterials, Nat Mater, 2016, 15, 1100. [43] Zarek, M., Layani, M., Cooperstein, I., Sachyani, E., Cohn, D., Magdassi, S. 3D printing of shape memory polymers for flexible electronic devices, Adv Mater, 2016, 28, 4449–4454. [44] Palaganas, J., de Leon, A. C., Mangadlao, J., Palaganas, N., Mael, A., Lee, Y. J., et al. Facile preparation of photocurable siloxane composite for 3D printing, Macromol Mater Eng, 2017, 302, 1600477. [45] Patel, D. K., Sakhaei, A. H., Layani, M., Zhang, B., Ge, Q., Magdassi, S. Highly stretchable and UV curable elastomers for digital light processing based 3D printing, Adv Mater, 2017, 29, 1606000. [46] Stassi, S., Fantino, E., Calmo, R., Chiappone, A., Gillono, M., Scaiola, D., et al. Polymeric 3D printed functional microcantilevers for biosensing applications, ACS Appl Mater Interfaces, 2017, 9, 19193–19201. [47] Moszner, N., Salz, U. New developments of polymeric dental composites, Prog Polym Sci, 2001, 26, 535–576. [48] Warner, J., Soman, P., Zhu, W., Tom, M., Chen, S. Design and 3D printing of hydrogel scaffolds with fractal geometries, ACS Biomater Sci Eng, 2016, 2, 1763–1770. [49] Al Mousawi, A., Garra, P., Sallenave, X., Dumur, F., Toufaily, J., Hamieh, T., et al. πconjugated dithienophosphole derivatives as high performance photoinitiators for 3D printing resins, Macromolecules, 2018, 51, 1811–1821.

Chapter 4 Resin design in stereolithography 3D printing for microfluidic applications

171

[50] Al Mousawi, A., Garra, P., Schmitt, M., Toufaily, J., Hamieh, T., Graff, B., et al. 3-hydroxyflavone and N-phenylglycine in high performance photoinitiating systems for 3D printing and photocomposites synthesis, Macromolecules, 2018, 51, 4633–4641. [51] Yue, J., Zhao, P., Gerasimov, J. Y., van de Lagemaat, M., Grotenhuis, A., Rustema‐Abbing, M., et al. 3D‐printable antimicrobial composite resins, Adv Funct Mater, 2015, 25, 6756–6767. [52] Credi, C., Fiorese, A., Tironi, M., Bernasconi, R., Magagnin, L., Levi, M., et al. 3D printing of cantilever-type microstructures by stereolithography of ferromagnetic photopolymers, ACS Appl Mater Interfaces, 2016, 8, 26332–26342. [53] Goodner, M. D., Bowman, C. N. Development of a comprehensive free radical photopolymerization model incorporating heat and mass transfer effects in thick films, Chem Eng Sci, 2002, 57, 887–900. [54] Elliott, J. E., Lovell, L., Bowman, C. Primary cyclization in the polymerization of bis-GMA and TEGDMA: A modeling approach to understanding the cure of dental resins, Dent Mater, 2001, 17, 221–229. [55] Chiappone, A., Fantino, E., Roppolo, I., Lorusso, M., Manfredi, D., Fino, P., et al. 3D printed PEG-based hybrid nanocomposites obtained by sol–gel technique, ACS Appl Mater Interfaces, 2016, 8, 5627–5633. [56] Yagci, Y., Jockusch, S., Turro, N. J. Photoinitiated polymerization: Advances, challenges, and opportunities, Macromolecules, 2010, 43, 6245–6260. [57] Mirle, S. K., Kumpfmiller, R. J. Photosensitive compositions useful in three-dimensional partbuilding and having improved photospeed. Google Patents; 1995. [58] Steinmann, B. Stereolithographic resins with high temperature and high impact resistance. Google Patents; 2006. [59] Bagheri, R., Marouf, B., Pearson, R. Rubber-toughened epoxies: A critical review, J Macromol Sci, Part C: Polym Rev, 2009, 49, 201–225. [60] Matsumura, S., Hlil, A. R., Lepiller, C., Gaudet, J., Guay, D., Shi, Z., et al. Ionomers for proton exchange membrane fuel cells with sulfonic acid groups on the end groups: Novel branched poly (ether− ketone) s, Macromolecules, 2008, 41, 281–284. [61] Crivello, J. V. Design of photoacid generating systems, J Photopolym Sci Technol, 2009, 22, 575–582. [62] Crivello, J. V. The discovery and development of onium salt cationic photoinitiators, J Polym Sci A Polym Chem, 1999, 37, 4241–4254. [63] Al Mousawi, A., Poriel, C., Dumur, F., Toufaily, J., Hamieh, T., Fouassier, J. P., et al. Zinc tetraphenylporphyrin as high performance visible light photoinitiator of cationic photosensitive resins for LED projector 3D printing applications, Macromolecules, 2017, 50, 746–753. [64] Xu, J. Photo-curable resin composition. Google Patents; 2015. [65] Steinmann, B., Schulthess, A. Liquid, radiation-curable composition, especially for stereolithography. Google Patents; 1999. [66] Lapin, S. C., Brautigam, R. J. Stereolithography using vinyl ether based polymers. Google Patents; 1996. [67] Xiao, P., Dumur, F., Frigoli, M., Tehfe, M.-A., Graff, B., Fouassier, J. P., et al. Naphthalimide based methacrylated photoinitiators in radical and cationic photopolymerization under visible light, Polym Chem, 2013, 4, 5440–5448. [68] Cai, Y., Jessop, J. L. Decreased oxygen inhibition in photopolymerized acrylate/epoxide hybrid polymer coatings as demonstrated by Raman spectroscopy, Polymer, 2006, 47, 6560–6566. [69] Ohkawa, K., Saito, S. Resin Composition for Optical Modeling. Patent NumberEP 0360869. 1990.

172

Lubna Shahzadi et al.

[70] Kade, M. J., Burke, D. J., Hawker, C. J. The power of thiol‐ene chemistry, J Polym Sci A Polym Chem, 2010, 48, 743–750. [71] Ligon-Auer, S. C., Schwentenwein, M., Gorsche, C., Stampfl, J., Liska, R. Toughening of photo-curable polymer networks: A review, Polym Chem, 2016, 7, 257–286. [72] Fairbanks, B. D., Sims, E. A., Anseth, K. S., Bowman, C. N. Reaction rates and mechanisms for radical, photoinitiated addition of thiols to alkynes, and implications for thiol− Yne photopolymerizations and click reactions, Macromolecules, 2010, 43, 4113–4119. [73] Husár, B., Ligon, S. C., Wutzel, H., Hoffmann, H., Liska, R. The formulator’s guide to antioxygen inhibition additives, Prog Org Coat, 2014, 77, 1789–1798. [74] McNair, O. D., Janisse, A. P., Krzeminski, D. E., Brent, D. E., Gould, T. E., Rawlins, J. W., et al. Impact properties of thiol–ene networks, ACS Appl Mater Interfaces, 2013, 5, 11004–11013. [75] Oesterreicher, A., Wiener, J., Roth, M., Moser, A., Gmeiner, R., Edler, M., et al. Tough and degradable photopolymers derived from alkyne monomers for 3D printing of biomedical materials, Polym Chem, 2016, 7, 5169–5180. [76] Bertlein, S., Brown, G., Lim, K. S., Jungst, T., Boeck, T., Blunk, T., et al. Thiol–ene clickable gelatin: A platform bioink for multiple 3D biofabrication technologies, Adv Mater, 2017, 29, 1703404. [77] Kumpfmueller, J., Stadlmann, K., Li, Z., Satzinger, V., Stampfl, J., Liska, R. Two-photoninduced thiol-ene polymerization as a fabrication tool for flexible optical waveguides, Des Monomers Polym, 2014, 17, 390–400. [78] Chen, L., Wu, Q., Wei, G., Liu, R., Li, Z. Highly stable thiol–ene systems: From their structure–property relationship to DLP 3D printing, J Mater Chem C, 2018, 6, 11561–11568. [79] Sycks, D. G., Wu, T., Park, H. S., Gall, K. Tough, stable spiroacetal thiol‐ene resin for 3D printing, J Appl Polym Sci, 2018, 135, 46259. [80] Mongkhontreerat, S., Öberg, K., Erixon, L., Löwenhielm, P., Hult, A., Malkoch, M. UV initiated thiol–ene chemistry: A facile and modular synthetic methodology for the construction of functional 3D networks with tunable properties, J Mater Chem A, 2013, 1, 13732–13737. [81] Senyurt, A. F., Hoyle, C. E., Wei, H., Piland, S. G., Gould, T. E. Thermal and mechanical properties of cross-linked photopolymers based on multifunctional thiol− urethane ene monomers, Macromolecules, 2007, 40, 3174–3182. [82] Hoyle, C. E., Lee, T. Y., Roper, T. Thiol–enes: Chemistry of the past with promise for the future, J Polym Sci A Polym Chem, 2004, 42, 5301–5338. [83] Belbakra, Z., Cherkaoui, Z. M., Allonas, X. Photocurable polythiol based (meth) acrylate resins stabilization: New powerful stabilizers and stabilization systems, Polym Degrad Stab, 2014, 110, 298–307. [84] Bail, R., Patel, A., Yang, H., Rogers, C., Rose, F., Segal, J., et al. The effect of a type I photoinitiator on cure kinetics and cell toxicity in projection-microstereolithography, Procedia CIRP, 2013, 5, 222–225. [85] Bagheri, A., Jin, J. Photopolymerization in 3D printing, ACS Appl Polym Mater, 2019, 1, 593–611. [86] Jasinski, F., Zetterlund, P. B., Braun, A. M., Chemtob, A. Photopolymerization in dispersed systems, Prog Polym Sci, 2018, 84, 47–88. [87] Hofstetter, C., Orman, S., Baudis, S., Stampfl, J. Combining cure depth and cure degree, a new way to fully characterize novel photopolymers, Addit Manuf, 2018, 24, 166–172. [88] Bail, R., Hong, J. Y., Chin, B. D. Effect of a red-shifted benzotriazole UV absorber on curing depth and kinetics in visible light initiated photopolymer resins for 3D printing, J Ind Eng Chem, 2016, 38, 141–145. [89] Gong, H., Beauchamp, M., Perry, S., Woolley, A. T., Nordin, G. P. Optical approach to resin formulation for 3D printed microfluidics, RSC Adv, 2015, 5, 106621–106632.

Chapter 4 Resin design in stereolithography 3D printing for microfluidic applications

173

[90] Badev, A., Abouliatim, Y., Chartier, T., Lecamp, L., Lebaudy, P., Chaput, C., et al. Photopolymerization kinetics of a polyether acrylate in the presence of ceramic fillers used in stereolithography, J Photochem Photobiol A Chem, 2011, 222, 117–122. [91] Messe, L., Chapelat, C. Curable composition. Google Patents; 2013. [92] Melisaris, A. P., Renyi, W., Pang, T. H. Liquid, radiation-curable composition, especially for producing flexible cured articles by stereolithography. Google Patents; 2002. [93] Odian, G. Principles of polymerization, 4th edn, Wiley Interscience: Hoboken, New Jersey, USA, 2004. [94] Terrones, G., Pearlstein, A. J. Effects of optical attenuation and consumption of a photobleaching initiator on local initiation rates in photopolymerizations, Macromolecules, 2001, 34, 3195–3204. [95] Lin, J.-T., Liu, H.-W., Chen, K.-T., Cheng, D.-C. Modeling the kinetics, curing depth, and efficacy of radical-mediated photopolymerization: The role of oxygen inhibition, viscosity, and dynamic light intensity, Front Chem, 2019, 7, 760. [96] Lin, J.-T., Wang, K.-C. Analytic formulas and numerical simulations for the dynamics of thick and non-uniform polymerization by a UV light, J Polym Res, 2016, 23, 53. [97] Miller, G. A., Gou, L., Narayanan, V., Scranton, A. B. Modeling of photobleaching for the photoinitiation of thick polymerization systems, J Polym Sci A Polym Chem, 2002, 40, 793–808. [98] Ivanov, V. V., Decker, C. Kinetic study of photoinitiated frontal polymerization, Polym Int, 2001, 50, 113–118. [99] O’Brien, A. K., Bowman, C. N. Modeling thermal and optical effects on photopolymerization systems, Macromolecules, 2003, 36, 7777–7782. [100] Goodner, M. D., Bowman, C. N. Modeling primary radical termination and its effects on autoacceleration in photopolymerization kinetics, Macromolecules, 1999, 32, 6552–6559. [101] Cramer, N. B., Davies, T., O’Brien, A. K., Bowman, C. N. Mechanism and modeling of a thiol−ene photopolymerization, Macromolecules, 2003, 36, 4631–4636. [102] Cramer, N. B., Reddy, S. K., O’Brien, A. K., Bowman, C. N. Thiol−ene photopolymerization mechanism and rate limiting step changes for various vinyl functional group chemistries, Macromolecules, 2003, 36, 7964–7969. [103] Flory, P. J. Molecular size distribution in three dimensional polymers. I. Gelation1, J Am Chem Soc, 1941, 63, 3083–3090. [104] Carothers, W. H. Polymerization, Chem Rev, 1931, 8, 353–426. [105] Carothers, W. H. Polymers and polyfunctionality, Trans Faraday Soc, 1936, 32, 39–49. [106] Cabral, J. T., Hudson, S. D., Harrison, C., Douglas, J. F. Frontal photopolymerization for microfluidic applications, Langmuir, 2004, 20, 10020–10029. [107] Lee, J. H., Prud’homme, R. K., Askay, I. A. Cure depth in photopolymerization: Experiments and theory, J Mater Res, 2001, 16, 3536–3544. [108] Boddapati, A., Rahane, S. B., Slopek, R. P., Breedveld, V., Henderson, C. L., Grover, M. A. Gel time prediction of multifunctional acrylates using a kinetics model, Polymer, 2011, 52, 866–873. [109] Boddapati, A. Modelling Cure Depth During Photopolymerization of Multifunctional Acrylates: Georgia Institute of Technology; 2010. [110] Slopek, R. P., McKinley, H. K., Henderson, C. L., Breedveld, V. In situ monitoring of mechanical properties during photopolymerization with particle tracking microrheology, Polymer, 2006, 47, 2263–2268. [111] Jacobs, P. F. Rapid Prototyping and Manufacturing: Fundamentals of Sterolithography: Society of Manufacturing Engineers; 1992.

174

Lubna Shahzadi et al.

[112] Bennett, J. Measuring UV curing parameters of commercial photopolymers used in additive manufacturing, Addit Manuf, 2017, 18, 203–212. [113] Kotz, F., Arnold, K., Bauer, W., Schild, D., Keller, N., Sachsenheimer, K., et al. Threedimensional printing of transparent fused silica glass, Nature, 2017, 544, 337–339. [114] Zanchetta, E., Cattaldo, M., Franchin, G., Schwentenwein, M., Homa, J., Brusatin, G., et al. Stereolithography of SiOC Ceramic Microcomponents, Adv Mater, 2016, 28, 370–376. [115] Hundley, J. M., Eckel, Z. C., Schueller, E., Cante, K., Biesboer, S. M., Yahata, B. D., et al. Geometric characterization of additively manufactured polymer derived ceramics, Addit Manuf, 2017, 18, 95–102. [116] Helmer, D., Voigt, A., Wagner, S., Keller, N., Sachsenheimer, K., Kotz, F., et al. Suspended liquid subtractive lithography: One-step generation of 3D channel geometries in viscous curable polymer matrices, Sci Rep, 2017, 7, 7387. [117] Kotz, F., Risch, P., Helmer, D., Rapp, B. E. Highly fluorinated methacrylates for optical 3D printing of microfluidic devices, Micromachines, 2018, 9, 115. [118] Shallan, A. I., Smejkal, P., Corban, M., Guijt, R. M., Breadmore, M. C. Cost-effective threedimensional printing of visibly transparent microchips within minutes, Anal Chem, 2014, 86, 3124–3130. [119] Macdonald, N. P., Cabot, J. M., Smejkal, P., Guijt, R. M., Paull, B., Breadmore, M. C. Comparing microfluidic performance of three-dimensional (3D) printing platforms, Anal Chem, 2017, 89, 3858–3866. [120] Beauchamp, M. J., Nielsen, A. V., Gong, H., Nordin, G. P., Woolley, A. T. 3d printed microfluidic devices for microchip electrophoresis of preterm birth biomarkers, Anal Chem, 2019, 91, 7418–7425. [121] Kotz, F., Risch, P., Arnold, K., Sevim, S., Puigmartí-Luis, J., Quick, A., et al. Fabrication of arbitrary three-dimensional suspended hollow microstructures in transparent fused silica glass, Nat Commun, 2019, 10, 1–7. [122] Femmer, T., Kuehne, A. J. C., Wessling, M. Print your own membrane: Direct rapid prototyping of polydimethylsiloxane, Lab Chip, 2014, 14, 2610–2613. [123] Bhattacharjee, N., Parra‐Cabrera, C., Kim, Y. T., Kuo, A. P., Folch, A. Desktop‐ stereolithography 3D‐printing of a poly(dimethylsiloxane)‐based material with sylgard‐184 properties, Adv Mater, 2018, 30(22), 1800001. [124] Li, F., Macdonald, N. P., Guijt, R. M., Breadmore, M. C. Increasing the functionalities of 3D printed microchemical devices by single material, multimaterial, and print-pause-print 3D printing, Lab Chip, 2019, 19, 35–49. [125] Rogers, C. I., Qaderi, K., Woolley, A. T., Nordin, G. P. 3D printed microfluidic devices with integrated valves, Biomicrofluidics, 2015, 9, 016501. [126] Bhargava, K. C., Thompson, B., Malmstadt, N. Discrete elements for 3D microfluidics, Proc Natl Acad Sci, 2014, 111, 15013–15018. [127] Yuen, P. K. SmartBuild–A truly plug-n-play modular microfluidic system, Lab Chip, 2008, 8, 1374–1378.

Xiao Chen, Mengfan Zhang, Tingting Wan, Penghui Fan, Kai Shi, Yingshan Zhou, Weilin Xu, Pu Xiao

Chapter 5 3D printing of biomaterials 5.1 Introduction Three-dimensional (3D) printing, as a rapid prototyping technology, can fabricate customized or complex objects via layer-by-layer printing through computer-aided design (CAD), without the need for molds [1, 2]. Compared to conventional subtractive manufacturing technologies, 3D printing is more flexible and efficient, wastes lesser materials and fabrication of complex elaborate structures can be carried out more easily [3]. Due to these merits, 3D printing has found wide application in various fields ranging from jewelry, robotics and electronic products to food, pharmaceutical and biomedical devices [4, 5]. In the last few decades, 3D printing technology has developed rapidly and many technologies have emerged. Of these technologies, photopolymerization-based 3D printing – for example, stereolithography (SLA), digital light processing (DLP) and continuous liquid interface production (CLIP) as well as two-photon polymerization (2PP) – are capable of producing complex 3D substructures with high precision, fast formation and spatial/temporal control [6, 7]. In photopolymerization photocuring 3D printing, printable materials mainly consist of (meth) acrylate- or epoxide-based monomers and oligomers and photoinitiators. Upon exposure to light source, photoinitiators can be converted into the reactive species, for example, radicals or cations, which then react with monomer and oligomer units to drive the chain growth by radical or cationic mechanism [8]. Therefore, a liquid system (monomers and oligomers) is rapidly transformed into solids with regular geometry. Considering their biomedical applications, photoactive biopolymer precursors are usually used instead of monomers/oligomers in photopolymerization photocuring 3D printing systems. The typical photopolymerization mechanisms are thus chain growth, chain step growth and cycloaddition [9].

Xiao Chen, Mengfan Zhang, Tingting Wan, Penghui Fan, Kai Shi, College of Materials Science and Engineering, Wuhan Textile University, Wuhan, People’s Republic of China Yingshan Zhou, College of Materials Science and Engineering, Wuhan Textile University, Wuhan, People’s Republic of China; State Key Laboratory of New Textile Materials and Advanced Processing Technologies, Wuhan Textile University, Wuhan, People’s Republic of China Weilin Xu, State Key Laboratory of New Textile Materials and Advanced Processing Technologies, Wuhan Textile University, Wuhan, People’s Republic of China Pu Xiao, Research School of Chemistry, Australian National University, Canberra, Australia https://doi.org/10.1515/9783110570588-005

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In this chapter, an overview of recent progress in the field of photocuring 3D printing and highlights of novel 3D printable photopolymerizable bio macromolecules are provided. Also, the emerging representative biomedical applications regarding the 3D printing of biomaterials, including tissue regeneration or repair, drug delivery and other applications are also discussed. In the end, this chapter also provides current and future perspective of 3D printing of biomaterials and their limitations.

5.2 Photopolymerizable systems for 3D printable biomaterials Using 3D printing techniques via photopolymerization, photopolymerizable systems can be converted into solid 3D printing objects. These 3D printing techniques via photopolymerization mainly include stereolithography (SLA), digital light processing (DLP) and continuous liquid interface production (CLIP) [10–12].

5.2.1 Photoinitiators In photopolymerization-based 3D printing systems, a suitable photoinitiator is a primary requisite to guarantee an adequate polymerization rate and to avoid toxic effects of the produced materials for cells and surrounding tissues [13]. In addition, important characteristics, for example, water solubility, stability and absorption spectrum, should be considered [14]. A number of available biocompatible photoinitiators are listed below [1, 15]. As shown in Table 5.1, these are mainly radical photoinitiators (e.g., UVlight-sensitive or visible-light-sensitive photoinitiators), which are widely used in biomedical applications, due to their biocompatibility. Although cationic photoinitiators are commonly used in photocuring systems, their potential toxicity from protonic acids produced in the process of photopolymerization limit their use as biomaterials [16]. Up to now, many commercially available cytocompatible UV-light-sensitive photoinitiators have been used in 3D printing applications [17–20]. Irgacure 819 was reported for the photopolymerization of PEGDA in a DLP process [17]. Considering the fact that a number of bio macromolecules are water-soluble photopolymerizable precursors, a water-soluble UV-sensitive photoinitiator is required. Water-soluble Irgacure 2959, VA-086 and Irgacure 651 have been exploited in biocompatible 3D printing systems; for example, the fabrication of hydrogel scaffolds for cell culture [18–20]. Another water-soluble photoinitiator, Lithium phenyl (2,4,6-trimethylbenzoyl) phosphinate (LAP), can absorb light at longer wavelength than UV to initiate the photopolymerization reaction in 3D printed systems [21]. Although 3D printing using UV photopolymerization is available, high-energy UV light exposure may demonstrate potential deleterious effects on the cells or

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Table 5.1: Examples of common photoinitiators used for 3D printing. Name

Abbreviation

Category

-hydroxy-ʹ-(-hydroxyethoxy)-methylpropiophenone

Irgacure 

UV-light-sensitive

,-dimethoxy--phenylacetophenone

Irgacure , DMPA

Phenyl bis(,,-trimethylbenzoyl) phosphine oxide

Irgacure , BAPO

Diphenyl(,,-trimethylbenzoyl) phosphine oxide

Darocure TPO

,ʹ-azobis[-methyl-n-(-hydroxyethyl) propionamide]

VA-

Lithium phenyl (,,-trimethylbenzoyl) phosphinate

LAP

Camphorquinone

CQ

Tris(,-bipyridyl) dichlororuthenium (II) hexahydrate

Ru

Eosin Y

–

Riboflavin

–

Visible-lightsensitive

surrounding tissues [22]. Compared to the UV light, visible light with longer wavelengths than UV is friendlier to living cells and is preferable for biomedical applications and in dentistry [23, 24]. Due to this benefit, several photopolymerizable systems containing visible-light-sensitive photoinitiators (e.g., camphorquinone, eosin Y and Ru) have been used in biocompatible 3D printing applications [13]. More details on the recently developed novel photoinitiating systems for one-photon and two-photon 3D printing are presented in Chapter 1 and Chapter 2.

5.2.2 Photopolymers As mentioned earlier, photopolymers precursors are always used to replace small molecular monomers/oligomers to fabricate biomaterials via photocuring 3D printing techniques. These photopolymers should be biocompatible, and have mechanical and biological features that are fit for human organs or tissues. And they should be able to be prepared using synthetic or natural polymers. Synthetic polymers possess good mechanical performance but poor biological properties, while natural polymers provide inherent biological properties and functions that resemble the ECM but present limited mechanical strength and potential immunogenicity [25]. However, most of synthetic or natural polymers have no photoreactive moieties in their molecular chains, and thus, are unable to undergo light-mediated reactions [26]. An available strategy is the introduction of photoreactive pendant groups (e.g., vinyl group, methyl acryloyl group, norbornene and tyramine) into the polymer backbone to

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obtain photo-cross-linkable polymer precursors [27–30]. A variety of synthetic and natural polymers, including poly(ethylene glycol), poly(vinyl alcohol), chitosan, alginate, hyaluronic acid, cellulose, collagen or gelatin and silk fibroin, have been exploited to prepare these photopolymers precursors.

5.2.2.1 Synthetic polymer-based photopolymerizable precursors (Meth)acrylated poly(ethylene glycol) Polyethylene glycol (PEG) is a nontoxic water-soluble polymer, which is approved by the food and drug administration (FDA). It is widely used in food, cosmetics and pharmaceuticals. (Meth)acrylated poly(ethylene glycol) (e.g., PEGDA, PEGDMA and PEGMA) can generally be synthesized by the reaction of PEG with (meth) acryloyl chloride (Figure 5.1). PEGMA can be used to yield biocompatible 3D structures with diverse architectures via a 3D printing/ vat photopolymerization [31]. PEGDA can also be copolymerized with other polymers to prepare tablets with various geometries, for delivery of poorly soluble drugs using photoinitiated 3D inkjet printing [32]. By using acrylamide-PEGDA mixture as photopolymerizable precursors, highly stretchable hydrogels can be prepared via DLP based 3D printing (Figure 5.2) [33]. More interestingly, these hydrogels are capable of forming strong interfacial bonding with commercial 3D printing elastomers, realizing 3D printing of hydrogel-elastomer hybrids, which significantly simplifies the process of fabricating conductive hydrogel-based flexible electronics.

Figure 5.1: Synthesis of PEGDA.

Methacrylated polycaprolactone Polycaprolactone (PCL) is semicrystalline aliphatic polyester synthesized by ringopening polymerization of caprolactone. PCL is biocompatible and biodegradable. It can be degraded by hydrolysis of its ester linkages in physiological conditions and has, therefore, attracted more attention for use as implantable biomaterials. Methacrylated polycaprolactone can be synthesized by the reaction between PCL and methacrylic anhydride (Figure 5.3) [34]. It was often used to fabricate multiscale porous scaffolds as bone repair materials via photopolymerization-based 3D printing.

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Load Unload

Stretch 150 μm

Unstretch

Figure 5.2: Highly stretchable hydrogels from PEGDA for UV curing based 3D printing. Reproduced with permission from [33]. Copyright 2018 Royal Society of Chemistry.

Figure 5.3: Synthesis of methacrylated polycaprolactone.

Methacrylated poly(vinyl alcohol) Poly(vinyl alcohol) (PVA) is a water-soluble synthetic polymer produced by the hydrolysis of polyvinyl acetate. It has been widely investigated as promising biomaterial for artificial cartilage, cardiovascular tissue and drug delivery systems because of its biocompatibility and tissue-like viscoelasticity. Methacrylated PVA can be easily synthesized by using methacrylic anhydride to modify -OH of PVA (Figure 5.4) [35]. It can be frequently cross-linked with gelatin methacryloyl to fabricate complex cellladen constructs by DLP process. To improve mechanical strength and cell adhesion of PVA-based materials, methacrloyl PVA with ultralow degrees of substitution was synthesized, and subsequent methacrloyl PVA/methacrylate-functionalized silica nanocomposite hydrogels were developed via the photopolymerization approach [36]. This kind of system is expected to be a photopolymerizable precursor in photopolymerization-based 3D printing.

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Figure 5.4: Synthesis of methacrylated PVA.

Poly (propylene fumarate) Poly (propylene fumarate) (PPF) is an unsaturated linear polyester synthesized from propylene glycol and fumaric acid, using an acidic catalyst (Figure 5.5). PPF has double bonds in the backbone, allowing its cross-link via free radical polymerization with other photoactive monomers, using a photoinitiator. PPF has tunable degradation, biocompatibility and osteoconductivity [37]. In particular, PPF-based materials have well-matched surface properties suitable for regeneration of bone, which makes them promising candidates for bone tissue engineering [38]. Based on those reasons, PPF has been used for the fabrication of tissue engineering scaffolds via SLA 3D printing technique [39]. O

O HO

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Figure 5.5: Synthesis of PPF by a one-step method.

5.2.2.2 Natural polymer based photopolymerizable precursors Protein-based photopolymerizable precursors Photoactive gelatin Gelatin, derived from collagen, is an FDA-approved biopolymer. Its backbone contains Arg-Gly-Asp (RGD) tripeptides that are able to bind cell surface integrins, making it suitable for promoting cell adhesion and proliferation [40]. Compared to its precursor collagen, gelatin is less immunogenic and exhibits high water solubility [41]. Gelatin has been widely used for 3D printing in various biomedical applications. However, its gelation kinetics is too slow to be efficient for the 3D printing process [42]. Therefore, photoactive gelatin has been extensively developed by methacrylation (Figure 5.6). And methacrylated gelatin (GelMA)-based tissue constructs with mesoscale pore networks have been fabricated via photopolymerization 3D bioprinting (Figure 5.7).

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O O

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Figure 5.6: Synthesis of photoactive gelatin.

However, methacrylated gelatin (GelMA) cross-linking is still slow and often inhibited by dissolved oxygen (i.e., the well-known oxygen inhibition effect for free radical polymerization) [43]. Although the oxygen inhibition can be overcome by improving UV light dosage, this approach is undesirable due to the potential damage to cells [44]. To solve these problems, the UV-initiated thiol-ene click reaction has emerged as a promising alternative. Compared to conventional chain-growth polymerization mechanisms, thiol-ene click reaction (step-growth polymerization mechanism) exhibits faster reaction kinetics, relatively homogenous networks and improved cell viability [43]. Thiol-norbornene gelatin-based 3D printing objects (Figure 5.8) via photopolymerization have been created through reaction between norbornene-modified gelation (GelNB, Figure 5.6) and thiol-containing (macro)molecules in the presence or absence of a photoinitiator [43]. And these photoclick scaffolds can be a benefit for cell viability, cell proliferation and cell difference, which make them an excellent candidate for use in tissue engineering applications and as a valuable alternative to replace the GelMA [45]. Methacrylated collagen Collagen is the most abundant mammalian protein and the main component of connective tissues. It shows excellent biological performance and can promote the adhesion and differentiation of cells as well as the formation of functional tissues, which make it useful in tissue engineering; for example, skin tissue and vascular tissue scaffolds [28]. Similar to a methacrylated gelatin, methacrylated collagen can also be synthesized by the reaction between collagen and methacrylic anhydride. Methacrylated collagen, together with photoinitiators – for example, I2959 or VA-086 – may be exposed to UV light to cross-link the collagen fibers and create a cell-laden 3D printing construct. Although these 3D printed constructs have structural rigidity, they are still mechanically weak and need further cross-linking [47].

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Figure 5.7: Bioprinting of complex constructs with mesoscale pore networks. Reproduced with permission from [46]. Copyright 2020 Springer Nature.

Methacrylated bovine serum albumin Bovine serum albumin (BSA) is a water-soluble globular protein with a net negatively charged surface. The globular shape and negatively charged surface of the protein confer an extremely high solubility in water (reaching up to ~ 40 % w/v) and low intrinsic viscosity (not self-assembling or aggregating), which perfectly meets the designing requirement of new oligomeric and polymeric resins for SLA. Based on that, methacrylated BSA was firstly used to afford a 3D printed construct via SLA process, and then the 3D printed construct was thermally cured to afford a denatured protein construct with improved mechanical properties [48].

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Figure 5.8: Z stability in printed constructs fabricated from A) GelNB + LAP, B) GelMA + LAP, C) and GelNB + I2959, D) and GelMA + I2959. Scale bars are 10 mm. Reproduced with permission from [43]. Copyright 2019 American Chemical Society.

Methacrylated silk fibroin Silk fibroin (SF), derived from Bombyx mori, has been used for various biomedical applications, due to its biocompatibility, biodegradability, promotion of cell adherence and proliferation and low inflammation [49]. Similar to gelatin, SF has amine groups in its molecular chain and can be modified with glycidyl methacrylate to obtain methacrylated SF (Figure 5.9). Methacrylated SF was used to fabricate 3D printed silk fibroin hydrogels by DLP process [50]. This type of hydrogel enabled cell viability, proliferation and differentiation to chondrogenesis, and can be expected to be used for cartilage regeneration. O O

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Figure 5.9: Schematic presentation for methacrylation of silk fibroin with glycidyl methacrylate.

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Soy protein isolate Soy protein isolate (SPI), a plant derived protein, is an attractive material for biomedical applications, due to its ease of isolation and processing, tailorable biodegradability and low immunogenicity. SPI contains 3.7 mol % tyrosine, which can be used for photo-cross-linked materials via dityrosine cross-linking [51]. Based on that, SPI was used to fabricate 3D-printable photo-cross-linked scaffolds or hybrid scaffolds for tissue engineering applications. Polysaccharide-based photopolymerizable precursors (Meth)acrylated chitosan Chitosan, a natural polysaccharide with a structure similar to glycosaminoglycan in extracellular matrix of nature tissue, is one of the most promising bio macromolecules for biomedical applications, due to its excellent biological properties such as biodegradability, biocompatibility and promotion of tissue regeneration [52]. (Meth) acrylated chitosan can be synthesized by the reaction between chitosan and various anhydrides or acrylates (Figure 5.10). Photo-cross-linkable N-maleyl chitosan methacrylate

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(MA-CS-GMA) was prepared and then was used to fabricate 3D grip-shaped MA-CSGMA-based scaffolds, via multi-photon polymerization [53]. This 3D hybrid scaffold could promote matrix mineralization that is suitable for bone tissue engineering. To mimic the dynamic movement of the tissues, hydroxybutyl methacrylated chitosan (HBC-MA) was synthesized and further used to fabricate hydrogel scaffolds with a shape memory property, via SLA process [54]. To increase polymerization rates and overcome the disadvantage of cell damage from UV exposure, methacrylated chitosan, was mixed with LAP to be processed into complex 3D hydrogel structures with high resolution, high-fidelity and good biocompatibility through DLP printing, using blue light [55]. Methacrylated alginate Alginate, a linear unbranched polysaccharide derived from seaweed that contains the repeating units of 1,4-linked β-D-mannuronic acid and α-L-guluronic acid, has been used in drug delivery, cell encapsulation and tissue regeneration, due to its biocompatibility and biodegradability [56]. Methacrylated alginate (MAA) can be synthesized by the reaction of alginate with methacrylic anhydride or mechacrylate (Figure 5.11). Just like GelMA, MAA-based materials lack versatility in mechanical properties and lasting 3D structures. Therefore, MAA is usually compounded with other polymers or nanoparticles to fabricate 3D printed scaffold via photopolymerization. For instance, MAA was used to prepare composite long-lasting scaffolds for 3D bioprinting of highly aligned muscle tissue [57]. In addition, MAA and graphene oxide were also used to fabricate 3D printed scaffolds for cartilage tissue engineering [58]. By sacrificing some component in printed hydrogel, complex channels within cell-laden hydrogels could be fabricated via 3D bioprinting (Figure 5.12) [59]. This approach can potentially provide a platform for fabricating vascularized tissues. Photoactive hyaluronic acid Hyaluronic acid (HA), a main component of the extracellular matrix found in various tissues of the body, is an attractive candidate for tissue scaffolds because of its biocompatibility, biodegradability and non-immunogenic property [60]. Photoactive hyaluronic acid can be synthesized by molecular modification of methacrylic anhydride, norbornene and tyramine. Methacrylated HA was used to prepare cellladen hydrogels with complex channels via 3D bioprinting [59]. To produce stableshaped 3D printed construct, norbornene-modified HA (Figure 5.13), together with thiol-containing molecules was used to obtain this kind of construct via photopolymerization-based 3D printing [61, 62]. In the same way, a tyramine-modified HA was synthesized, and it could undergo a dual cross-linking mediated by horseradish peroxidase and hydrogen peroxide followed by a visible light cross-linking to obtain structurally stable soft hydrogel [63].

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Figure 5.11: Synthetic procedures of methacrylated alginate.

Photoactive cellulose Cellulose is the most abundant biopolymer with wide applicability in pharmaceutical industry. A number of photo-cross-linkable cellulose resins have been synthesized from cellulose and commercial cellulose ethers; for example, carboxymethyl cellulose and methyl cellulose. Methacrylated cellulose (Figure 5.14) was prepared and used as a precursor to produce composite scaffold for 3D bioprinting of highly aligned muscle tissue [57]. Tyramine-functionalized carboxylmethyl cellulose (Figure 5.15) was also synthesized and used to fabricate 3D printed in situ tissue sealant with visible-lightcross-linking [30]. Also, tyramine-modified cellulose was used for other 3D bioprinting applications [64].

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Figure 5.12: Digital designs and corresponding 3D devices. All channels were injected with a red food coloring. Reproduced with permission from [59]. Copyright 2019 Elsevier Ltd.

Other methacrylated polysaccharides Other methacrylated polysaccharides, for example, pectin and pullulan, were used to fabricate 3D constructs via multiscale light-assisted 3D printing for cell scaffolds or dermal tissue engineering [65, 66]. For example, a methacrylated pectin was synthesized by the reaction pectin and methacrylic anhydride, and its bioink was designed to tethering of integrin-binding motifs and preparation of hydrogels via UV photopolymerization and ionic gelation (Figure 5.16). These printed constructs can provide a suitable microenvironment for cell behaviors.

5.2.2.3 Extracellular matrix-derived photopolymerizable precursors Native extracellular matrix (ECM) can provide a structure architecture that contains adhesion sites for cell surface receptors, facilitating cell attachment, growth and maturation [29]. The material derived from decellularized tissue-specific ECM can

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Figure 5.14: Synthesis of methacrylated cellulose.

Figure 5.15: Synthesis of tyramine functionalized carboxylmethyl cellulose.

provide these same functions as native ECM [67]. Based on this, a methacrylated kidney ECM-derived bioink was developed and used to form ECM-like hydrogel by 3D bioprinting via photopolymerization (Figure 5.17) [68]. And it was found that this hydrogel could accelerate renal tissue formation. Through the same strategy, methacrylated liver ECM-derived bioink was applied to fabricate liver microtissue by DLP bioprinting [69]. The results showed that hiHep cells could spread farther and show better hepatocyte-specific functions in this liver microtissue, indicating it would be a potential liver tissue engineering product that can help restore hepatic functions.

5.3 Applications in biomedical fields 5.3.1 3D printing of biomaterials for bone regeneration To simulate composition and geometric structure of natural bone, hydroxyapatite and tricalcium phosphate (HA/TCP), the major constituents of natural bone and teeth, together with photocurable resins were used to fabricate HA/TCP scaffolds with complex geometry including biomimetic features and hierarchical porosity via SLA process and sintering (Figure 5.18) [70]. This 3D printed HA/TCP scaffold with biomimetic hierarchical structure is biocompatible and has sufficient mechanical strength for surgery, as investigated by experiments on cell proliferation and surgery in a nude mouse in vivo

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Figure 5.16: Schematic illustration of the design and preparation of the biofunctional bioink for extrusion bioprinting. Reproduced with permission from [66]. Copyright 2018 Royal Society of Chemistry.

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Figure 5.17: Schematic illustration of a photo-cross-linkable kidney-specific ECM hydrogel. Reproduced with permission from [68]. Copyright 2019 John Wiley & Sons.

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Figure 5.18: Schematic diagram of 3D printed HA/TCP scaffold with bioinspired hierarchical porous structure. a. A nude mouse long bone image; b. Microscopy of printed green part of biomimetic HA/ TCP scaffold; c. Microscope and SEM images of biomimetic HA/TCP scaffolds after sintering; and d. Fabrication of HA/TCP scaffold. Reproduced with permission from [70]. Copyright 2020 Springer Nature.

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model of long bone. In the future, other biodegradable polymers or growth factors will be integrated into this scaffold to further accelerate bond healing.

5.3.2 3D printing of biomaterials for soft tissue repair 5.3.2.1 Skin regeneration 3D bioprinted cellularized constructs can instruct entrapped cells to secrete new ECM components, which are attractive for tissue repair, especially for dermal repair because of limited self-reparative ability of the dermis upon injury [71]. Pectin-based 3D constructs were prepared via 3D printing and ionic cross-linking (shown in Figure 5.16) [66]. These printed constructs can support the deposition of endogenous ECM, rich in collagen and fibronectin by entrapped dermal fibroblasts, and are required for the generation of biomimetic skin constructs.

5.3.2.2 Cartilage regeneration Cartilage injury is often caused by traumas, diseases and sport accident, and does not self-heal soon, due to avascularity and a poor supply of repair cells around the tissue [72]. 3D bioprinting offers an alternative strategy for dealing with cartilage damage by simultaneously integrating living cells, biomaterials and biological cues to provide a scaffold. GelMA based cell-laden cartilage tissue constructs with coreshell nanospheres were prepared via SLA process [73]. This construct showed high cell viability and proliferation rate, indicating its potential for cartilage repair. To enhance the printability and cell proliferation further, a photo-cross-linkable alginate/gelatin/chondroitin sulfate composite construct involving the graphene oxide nanofiller was fabricated via 3D printing [58]. The composite scaffold presented high cell proliferation and chondrogenic differentiation, favoring cartilage tissue regeneration. In order to improve mechanical properties of scaffolds to satisfy the stringent requirement for load-bearing as bioscaffolds, poly (N-acryloyl-2-glycine)/ GelMA (PACG-GelMA) biohybrid scaffold consisting of top layer of PACG-GelMA hydrogel-Mn2+ and bottom layer of hydrogel-bioactive glass was fabricated for the repair of osteochondral defects by a 3D printing technique (Figure 5.19) [74]. The biohybrid hydrogel scaffold can support cell attachment and spreading and enhance gene expression of chondrogenic-related/osteogenic-related differentiation of human bone marrow stem cells, facilitating concurrent regeneration of cartilage and subchondral bone in a rat model. To prolong the degradation time matching to cartilage regeneration, gelatin/hyaluronic acid composite construct was fabricated by integrating photocuring 3D printing and lyophilization techniques [75]. This scaffold combined with chondrocytes can regenerate mature cartilage with typical

Osteochondral regeneration

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Figure 5.19: Schematic illustration of 3D printing of the biohybrid gradient scaffolds for repair of osteochondral defect. Reproduced with permission from [74]. Copyright 2019 John Wiley & Sons.

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lacunae structure and cartilage-specific extracellular matrixes both in vitro and in the autologous goat model, making it a potential candidate for use in cartilage regeneration (Figure 5.20). A1

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Figure 5.20: Gross view and histological examinations of the regenerated cartilage in and autologous goat. After 8 weeks of autologous implantation in the goat, 2-week in vitro samples successfully regenerate relatively homogeneous mature cartilage with typical lacunae structures and cartilagespecific ECM deposition. Reproduced with permission from [75]. Copyright 2018 American Chemical Society.

5.3.2.3 Vascularized tissues 3D bioprinting can fabricate hydrogel constructs with embedded microchannels, which are useful for basic studies on vascularization and angiogenesis, and for the development of organ-on-a-chip biodevices for disease modeling. By sacrificing some component in printed hydrogel, complex channels within cell-laden hydrogels could be fabricated via 3D bioprinting [59]. This hydrogel construct can promote adherence and confluence of endothelial cells within microchannels (Figure 5.21), favoring the formation of vascularized tissues.

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(a)

(b)

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Figure 5.21: Fluorescent images of the hUVECs cultured within channels. (a) Maximum projection image from top. Cross-sectional (b) and top (c) view of the channels. Reproduced with permission from [59]. Copyright 2019 Elsevier Ltd.

5.3.2.4 Neuronal repair Neurodegenerative diseases always result in irreversible neuronal damage and death. To treat these diseases, an alternative strategy is to deliver cells to targeted location and to provide a suitable microenvironment to differentiate these cells into functional neuronal networks. GelMA-multiferroic nanoparticles composite helical hydrogel microswimmer was prepared via photocuring 3D printing [76]. While soft GelMA hydrogel chassis was degraded by enzymes secreted by cells, multiferroic nanoparticles displayed magnetoelectric features to transport the bioactive chassis with neuron-like cells loaded to targeted location. The approach has good potential in targeted cell therapies for traumatic injuries and diseases in the central nervous system.

5.3.2.5 Renal/liver tissue repair Decellularized tissue-specific ECM can provide the same functions as native ECM. Based on this, a methacrylated kidney ECM-derived bioink was developed and could be used to form ECM-like hydrogel by 3D bioprinting via photopolymerization [68]. It was found that this hydrogel could accelerate renal tissue formation. Through the same strategy, methacrylated liver ECM-derived bioink was used to fabricate liver microtissue by DLP bioprinting [69]. This construct could promote the spread of hiHep

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cells and have better hepatocyte-specific functions, making it a potential liver tissue engineering product to help restore hepatic functions.

5.3.3 Drug delivery systems 3D printing has been investigated as a new route to manufacture tables, mainly due to its precise control over tablet geometry and porosity, which can confer tunable control over drug dissolution. PEGDA-based tablets were prepared for the delivery of poorly soluble drugs (carvedilol) using photoinitiated 3D inkjet printing [32]. These tablets can release drug accurately by varying the tablet geometry. Specifically, the release behavior of carvedilol was fastest for the thin films, followed by the ring, mesh and cylindrical geometries.

5.4 Conclusions and perspectives As a model-free manufacturing technology, photopolymerization-based 3D printing can fabricate various complex and fine constructions in relatively high printing speed, if a suitable efficient photoinitiator is used. These printed objects have been studied in various fields such as tissue engineering, biomedical devices and drug delivery. However, despite its rapid progress, several challenges still exist and limit its further development. First, more biocompatible photocurable precursors (photosensitive resins) with low viscosity and high performance must be developed to match the fast and precise printing of 3D biomaterials. Second, the development of biocompatible photoinitiators with both longer wavelengths absorbance and high efficiency of initiation is of considerable importance to rapid and accurate formation of 3D printed biomaterials under mild condition. There is still room for improvement in this field even though significant progresses have been made recently, as discussed in Chapter 1 and Chapter 2. Third, for 3D printed biomaterials as tissue engineering scaffolds, the realization of sufficient vascularization in material-cell constructs to obtain a whole organ with the biological functionality and structure similar to a native organ, may be a great challenge. All in all, photopolymerization-based 3D printing opens a new paradigm in tissue engineering and biomedicine fields.

References [1] [2]

Bagheri, A., Jin, J. Photopolymerization in 3D printing, ACS Appl Polym Mater, 2019, 1, 593–611. Weems, A. C., Perez-Madrigal, M. M., Arno, M. C., Dove, A. P. 3D printing for the clinic: Examining contemporary polymeric biomaterials and their clinical utility, Bio macromolecules, 2020, 21, 1037–1059.

198

[3] [4] [5]

[6] [7] [8]

[9] [10] [11] [12]

[13]

[14] [15]

[16] [17]

[18]

[19]

[20] [21]

[22]

Xiao Chen et al.

Li, J., Wu, C., Chu, P. K., Gelinsky, M. 3D printing of hydrogels: Rational design strategies and emerging biomedical applications, Mater Sci Eng R-Rep, 2020, 140, 100543. You, S., Li, J., Zhu, W., Yu, C., Mei, D., Chen, S. Nanoscale 3D printing of hydrogels for cellular tissue engineering, J Mat Chem B, 2018, 6, 2187–2197. Xu, X., Robles-Martinez, P., Madla, C. M., Joubert, F., Goyanes, A., Basit, A. W., Gaisford, S. Stereolithography (SLA) 3D printing of an antihypertensive polyprintlet: Case study of an unexpected photopolymer-drug reaction, Addit Manuf, 2020, 33, 101071. Zhang, J., Xiao, P. 3D printing of photopolymers, Polym Chem, 2018, 9, 1530–1540. Quan, H., Zhang, T., Xu, H., Luo, S., Nie, J., Zhu, X. Photo-Curing 3D printing technique and its challenges, Bioact Mater, 2020, 5, 110–115. Fu, J., Yin, H., Yu, X., Xie, C., Jiang, H., Jin, Y., Sheng, F. Combination of 3D printing technologies and compressed tablets for preparation of riboflavin floating tablet-in device (TiD) systems, Int J Pharm, 2018, 549, 370–379. Pei, M., Mao, J., Xu, W., Zhou, Y., Xiao, P. Photocrosslinkable chitosan hydrogels and their biomedical applications, J Polym Sci Pol Chem, 2019, 57, 1862–1871. Rossi, S., Puglisi, A., Benaglia, M. Additive manufacturing technologies: 3D printing in organic synthesis, ChemCatChem, 2018, 10, 1512–1525. Layani, M., Wang, X., Magdassi, S. Novel Materials for 3D printing by photopolymerization, Adv Mater, 2018, 30, 1706344. Tumbleston, J. R., Shirvanyants, D., Ermoshkin, N., Janusziewicz, R., Johnson, A. R., Kelly, D., Chen, K., Pinschmidt, R., Rolland, J. P., Ermoshkin, A., Samulski, E. T., Desimone, J. M. Continuous liquid interface of 3D objects, Science, 2015, 347, 1349–1352. Guerra, A. J., Lara-Padilla, H., Becker, M. L., Rodriguez, C. A., Dean, D. Photopolymerizable resins for 3D-printing solid-cured tissue engineered implants, Curr Drug Targets, 2019, 20, 823–838. Shih, H., Lin, C. C. Visible-light-mediated thiol-ene hydrogelation using eosin-Y as the only photoinitiator, Macromol Rapid Commun, 2013, 34, 269–273. Lin, H., Zhang, D., Alexander, P. G., Yang, G., Tan, J., Cheng, A. W. M., Tuan, R. S. Application of visible light-based projection stereolithography for live cell-scaffold fabrication with designed architecture, Biomaterials, 2013, 34, 331–339. Nguyen, K. T., West, J. L. Photopolymerizable hydrogels for tissue engineering applications, Biomaterials, 2002, 23, 4307–4314. Chiappone, A., Fantino, E., Roppolo, I., Lorusso, M., Manfredi, D., Fino, P., Pirri, C. F., Calignano, F. 3D printed PEG-based hybrid nanocomposites obtained by sol-gel technique, ACS Appl Mater Interfaces, 2016, 8, 5627–5633. Chan, V., Zorlutuna, P., Jeong, J. H., Kong, H., Bashir, R. Three-dimensional photopatterning of hydrogels using stereolithography for long-term cell encapsulation, Lab Chip, 2010, 10, 2062–2070. Occhetta, P., Visone, R., Russo, L., Cipolla, L., Moretti, M., Rasponi, M. VA-086 Methacrylate gelatine photopolymerizable hydrogels: A parametric study for highly biocompatible 3D cell embedding, J Biomed Mater Res Part A, 2015, 103, 2109–2117. Warner, J., Soman, P., Zhu, W., Tom, M., Chen, S. Design and 3D printing of hydrogel scaffolds with fractal geometries, ACS Biomater Sci Eng, 2016, 2, 1763–1770. Palaganas, N. B., Mangadlao, J. D., De Leon, A. C. C., Palaganas, J. O., Pangilinan, K. D., Lee, Y. J., Advincula, R. C. 3D printing of photocurable cellulose nanocrystal composite for fabrication of complex architectures via stereolithography, ACS Appl Mater Interfaces, 2017, 9, 34314–34324. Hu, J., Hou, Y., Park, H., Choi, B., Hou, S., Chung, A., Lee, M. Visible light crosslinkable chitosan hydrogels for tissue engineering, Acta Biomater, 2012, 8, 1730–1738.

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[23] Zhang, J., Dumur, F., Xiao, P., Graff, B., Bardelang, D., Gigmes, D., Fouassier, J. P., Lalevee, J. Structure design of naphthalimide derivatives: Toward versatile photoinitiators for near-UV/ visible LEDs, 3D printing, and water-soluble photoinitiating systems, Macromolecules, 2015, 48, 2054–2063. [24] Bagheri, A., Arandiyan, H., Boyer, C., Lim, M. Lanthanide-doped upconversion nanoparticles: Emerging intelligent light-activated drug delivery systems, Adv Sci, 2016, 3, 1500437. [25] Annabi, N., Tamayol, A., Uquillas, J. A., Akbari, M., Bertassoni, L. E., Cha, C., Khademhosseini, A. 25th anniversary article: Rational design and applications of hydrogels in regenerative medicine, Adv Sci, 2014, 26, 85–124. [26] Fertier, L., Koleilat, H., Stemmelen, M., Giani, O., Joly-Duhamel, C., Lapinte, V., Robin, J. J. The use of renewable feedstock in UV-curable materials-A new age for polymers and green chemistry, Prog Polym Sci, 2013, 38, 932–962. [27] Xiang, H., Wang, X., Ou, Z., Lin, G., Yin, J., Liu, Z., Zhang, L., Liu, X. UV-curable, 3D printable and biocompatible silicone elastomers, Prog Org Coat, 2019, 137, 105372. [28] Gao, Q., Niu, X., Shao, L., Zhou, L., Lin, Z., Sun, A., Fu, J., Chen, Z., Hu, J., Liu, Y., He, Y. 3D printing of complex GelMA-based scaffolds with nanoclay, Biofabrication, 2019, 11, 035006. [29] Dobos, A., Hoorick, J. V., Steiger, W., Gruber, P., Markovic, M., Andriotis, O. G., Rohatschek, A., Dubruel, P., Thurner, P. J., Vlierberghe, S. V., Baudis, S., Ovsianikov, A. Thiol-gelatin-norbornene bioink for laser-based high-definition bioprinting, Adv Healthc Mater, 2019, 9, 1900752. [30] Al-Abboodi, A., Zhang, S., Al-Saady, M., Ong, J. W., Chan, P. P. Y., Fu, J. Printing in situ tissue sealant with visible-light-crosslinked porous hydrogel, Biomed Mater, 2019, 14, 045010. [31] Aduba, D. C. Jr, Margaretta, E. D., Marnot, A. E. C., Heifferon, K. V., Surbey, W. R., Chartrain, N. A., Whittington, A. R., Long, T. E., Williams, C. B. Vat photopolymerization 3D printing of acid-cleavable PEG-methacrylate networks for biomaterial applications, Mater Today Commun, 2019, 19, 204–211. [32] Clark, E. A., Alexander, M. R., Irvine, D. J., Roberts, C. J., Wallace, M. J., Yoo, J., Wildman, R. D. Making tablets for delivery of poorly soluble drugs using photoinitiated 3D inkjet printing, Int J Pharm, 2020, 578, 118805. [33] Zhang, B., Li, S., Hingorani, H., Serjouei, A., Larush, L., Pawar, A. A., Goh, W. H., Sakhaei, A. H., Hashimoto, M., Kowsari, K., Magdassi, S., Ge, Q. Highly stretchable hydrogels for UV curing based high-resolution multimaterial 3D printing, J Mat Chem B, 2018, 6, 3246–3253. [34] Dikici, B. A., Reilly, G. C., Claeyssens, F. Boosting the osteogenic and angiogenic performance of multiscale porous polycaprolactone scaffolds by in vitro generated extracellular matrix decoration, ACS Appl Mater Interfaces, 2020, 12, 12510–12524. [35] Lim, K. S., Levato, R., Costa, P. F., Castilho, M. D., Alcala-Orozco, C. R., Dorenmalen, K. M. A. V., Melchels, F. P. W., Gawlitta, D., Hooper, G. J., Malda, J., Woodfield, T. B. F. Bio-resin for high resolution lithography-based biofabrication of complex cell-laden constructs, Biofabrication, 2018, 10, 034101. [36] Zhang, C., Liang, K., Zhou, D., Yang, H., Liu, X., Yin, X., Xu, W., Zhou, Y., Xiao, P. Highperformance photopolymerized poly (vinyl alcohol)/silica nanocomposite hydrogels with enhanced cell adhesion, ACS Appl Mater Interfaces, 2018, 10, 27692–27700. [37] Gresser, J. D., Hsu, S. H., Nagaoka, H., Lyons, C. M., Nieratko, D. P., Wise, D. L., Barabino, G. A., Trantolo, D. J. Analysis of a vinyl pyrrolidone/poly (propylene fumarate) resorbable bone cement, J Biomed Mater Res, 1995, 29, 1241–1247. [38] Cai, Z., Wan, Y., Becher, M. L., Long, Y. Z., Dean, D. Poly (propylene fumarate)-based materials: Synthesis, functionalization, properties, device fabrication and biomedical applications, Biomaterials, 2019, 208, 45–71.

200

Xiao Chen et al.

[39] Guerra, A. J., Lammel-Lindemann, J., Katko, A., Kleinfehn, A., Rodriguez, C. A., Catalani, L. H., Becker, M. L., Ciurana, J., Dean, D. Optimization of photocrosslinkable resin components and 3D printing process parameters, Acta Biomater, 2019, 97, 154–161. [40] Tytgat, L., Damme, L. V., Arevalo, M. D. P. O., Declercq, H., Thienpont, H., Otteveare, H., Blondeel, P., Dubruel, P., Vlierberghe, S. V. Extrusion-based 3D printing of photocrossllinkable gelatin and k-carrageenan hydrogel blends for adipose tissue regeneration, Int J Biol Macromol, 2019, 140, 929–938. [41] Gasperini, L., Mano, J. F., Reis, R. L. Natural polymers for the microencapsulation of cells, J R Soc Interface, 2014, 11, 2014087. [42] Boza, A. M., Biegun, M. K. W., Campo, A. D., Lasa, B. V., Roman, J. S. Glycerylphytate as an ionic crosslinker for 3D printing of multi-layered scaffolds with improved shape fidelity and biological features, Biomater Sci, 2020, 8, 506–516. [43] Tigner, T. J., Rajput, S., Gaharwar, A. K., Alge, D. L. Comparison of photocrosslinkable gelatin derivatives and initiators for three-dimensional extrusion bioprinting, Biomacromolecules, 2020, 21, 454–463. [44] Svobodova, A. R., Galandakova, A., Sianska, J., Dolezal, D., Lichnovska, R., Ulrichova, J., Vostalova, J. DNA damage after acute exposure of mice skin to physiological doses of UVB and UVA light, Arch Dermatol Res, 2012, 304, 407–412. [45] Tytgat, L., Damme, L. V., Hoorick, J. V., Declercq, H., Thienpont, H., Ottevaere, H., Blondeel, P., Dubruel, P., Vlierberghe, S. V. Additive manufacturing of photo-crosslinked gelatin scaffolds for adipose tissue engineering, Acta Biomater, 2019, 94, 340–350. [46] Shao, L., Gao, Q., Xie, C., Fu, J., Xiang, M., Liu, Z., Xiang, L., He, Y. Sacrificial microgelladen bioink-enabled 3D bioprinting of mesoscale pore networks, Bio-Des Manuf, 2020, 3, 30–39. [47] Kajave, N. S., Schmitt, T., Nguyen, T. U., Kishore, V. Dual crosslinking strategy to generate mechanically visible cell-laden printable constructs using methacrylated collagen bioinks, Mat Sci Eng C, 2020, 107, 110290. [48] Smith, P. T., Narupai, B., Tsui, J. H., Millik, T. S. C., Shafranek, R. T., Kim, D. H., Nelson, A. Additive manufacturing of bovine serum albumin-based hydrogels and bioplastics, Biomacromolecules, 2020, 21, 484–492. [49] Park, H. J., Lee, O. J., Lee, M. C., Moon, B. M., Ju, H. W., Min Lee, J., Kim, J., Kim, D. W., Park, C. H. Fabrication of 3D porous silk scaffolds by particulate (salt/sucrose) leaching for bone tissue reconstruction, Int J Biol Macromol, 2015, 78, 215–223. [50] Hong, H., Seo, Y. B., Kim, D. Y., Lee, J. S., Lee, Y. J., Lee, H., Ajiteru, O., Sultan, M. T., Lee, O. J., Kim, S. H., Park, C. H. Digital light processing 3D printed silk fibroin hydrogel for cartilage tissue engineering, Biomaterials, 2020, 232, 119679. [51] Dorishetty, P., Balu, R., Sreekumar, A., Campo, L. D., Mata, J. P., Choudhury, N. R., Dutta, N. K. Robust and tunable hybrid hydrogels from photo-cross-linked soy protein isolate and regenerated silk fibroin, ACS Sustain Chem Eng, 2019, 7, 9257–9271. [52] Zhou, Y., Zhao, S., Zhang, C., Liang, K., Li, J., Yang, H., Gu, S., Bai, Z., Ye, D., Xu, W. Photopolymerized maleilated chitosan/thiol-terminated poly (vinyl alcohol) hydrogels as potential tissue engineering scaffolds, Carbohydr Polym, 2018, 184, 383–389. [53] Parkatzidis, K., Chatzinikolaidou, M., Kaliva, M., Bakopoulou, A., Farsari, M., Vamvakaki, M. Multiphoton 3D printing of biopolymer-based hydrogels, ACS Biomater Sci Eng, 2019, 5, 6161–6170. [54] Seo, J. W., Shin, S. R., Park, Y. J., Bae, H. Hydrogel production platform with dynamic movement using photo-crosslinkable /temperature reversible chitosan polymer and stereolithography 4D printing technology, Tissue Eng Regen Med, 2020, 17, 423–431.

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[55] Shen, Y., Tang, H., Huang, X., Hang, R., Zhang, X., Wang, Y., Yao, X. DLP printing photocurable chitosan to build bio-constructs for tissue engineering, Carbohydr Polym, 2020, 235, 115970. [56] Jeon, O., Bouhadir, K. H., Mansour, J. M., Alsberg, E. Photocrosslinked alginate hydrogels with tunable biodegradation rates and mechanical properties, Biomaterials, 2009, 30, 2724–2734. [57] Garcia-Lizarribar, A., Fernandez-Garibay, X., Velasco-Mallorqui, F., Castano, A. G., RamonAzcon, S. J. Composite biomaterials as long-lasting scaffolds for 3D bioprinting of highly aligned muscle tissue, Macromol Biosci, 2018, 18, 1800167. [58] Olate-Moya, F. A., Arens, L., Wilhelm, M., Mateos-Timoneda, M. A., Engel, E., Palza, H. Chondroinductive alginate-based hydrogels having graphene oxide for 3D printed scaffolds fabrication, ACS Appl Mater Interfaces, 2020, 12, 4343–4357. [59] Ji, S., Almeida, E., Guvendiren, M. 3D bioprinting of complex channels within cell-laden hydrogels, Acta Biomater, 2019, 95, 214–224. [60] Zhang, C., Dong, Q., Liang, K., Zhou, D., Yang, H., Liu, X., Xu, W., Zhou, Y., Xiao, P. Photopolymerizable thiol-acrylate maleiated hyaluronic acid/thiol-terminated poly (ethylene glycol) hydrogels as potential in-situ formable scaffolds, Int J Biol Macromol, 2018, 119, 270–277. [61] Highley, C. B., Song, K. H., Daly, A. C., Burdick, J. A. Jammed microgel inks for 3D printing applications, Adv Sci, 2019, 6, 1801076. [62] Galarraga, J. H., Kwon, M., Burdick, J. A. 3D bioprinting via an in situ crosslinking technique towards engineering cartilage tissue, Sci Rep, 2019, 9, 19987. [63] Petta, D., Armiento, A. R., Grijpma, D., Alini, M., Eglin, D., D’Este, M. 3D bioprinting of a hyaluronan bioink through enzymatic- and visible light-crosslinking, Biofabrication, 2018, 10, 044104. [64] Shin, J. Y., Yeo, Y. H., Jeong, J. E., Park, S. A., Park, W. H. Dual-crosslinked methylcellulose hydrogels for 3D bioprinting applications, Carbohydr Polym, 2020, 238, 116192. [65] Giustina, G. D., Gandin, A., Brigo, L., Panciera, T., Giulitti, S., Sgarbossa, P., D’Alessandro, D., Trombi, L., Danti, S., Brusatin, G. Polysaccharide hydrogels for multiscale 3D printing of pullulan scaffolds, Mater Des, 2019, 165, 107566. [66] Pereira, R. F., Sousa, A., Barrias, C. C., Bartolo, P. J., Granja, P. L. A single-component hydrogel bioink for bioprinting of bioengineered 3D constructs for dermal tissue engineering, Mater Horiz, 2018, 5, 1100–1111. [67] Annabi, N., Tamayol, A., Uquillas, J. A., Akbari, M., Bertassoni, L. E., Cha, C., Camci‐Unal, G., Dokmeci, M. R., Peppas, N. A., Khademhosseini, A. 25th anniversary article: Rational design and applications of hydrogels in regenerative medicine, Adv Mater, 2014, 26, 85–124. [68] Ali, M., Kumar, P. R. A., Yoo, J. J., Zahran, F., Atala, A., Lee, S. J. A photo-crosslinkable kidney ECM-derived bioink accelerates renal tissue formation, Adv Healthc Mater, 2019, 8, 1800992. [69] Mao, Q., Wang, Y., Li, Y., Juengpanich, S., Li, W., Chen, M., Yin, J., Fu, J., Cai, X. Fabrication of liver microtissue with liver decellularized extracellular matrix (dECM) bioink by digital light processing (DLP) bioprinting, Mat Sci Eng C, 2020, 109, 110625. [70] Li, X., Yuan, Y., Liu, L., Leung, Y. S., Chen, Y., Guo, Y., Chai, Y., Chen, Y. 3D printing of hydroxyapatite/ tricalcium phosphate scaffold with hierarchical porous structure for bone regeneration, Bio-Des Manuf, 2020, 3, 15–29. [71] Pereira, R. F., Sousa, A., Barrias, C. C., Bayat, A., Granja, P. L., Bartolo, P. J. Advances in bioprinted cell-laden hydrogels for skin tissue engineering, Biomanuf Rev, 2017, 2, 1. [72] Zhou, Y., Liang, K., Zhao, S., Zhang, C., Li, J., Yang, H., Liu, X., Yin, X., Chen, D., Xu, W., Xiao, P. Photopolymerized maleilated chitosan/methacrylated silk fibroin micro/nanocomposite hydrogels as potential scaffolds for cartilage tissue engineering, Int J Biol Macromol, 2018, 108, 383–390.

202

Xiao Chen et al.

[73] Zhu, W., Cui, H., Boualam, B., Masood, F., Flynn, E., Rao, R. D., Zhang, Z. Y., Zhang, L. G. 3D bioprinting mesenchymal stem cell-laden construct with core-shell nanospheres for cartilage tissue engineering, Nanotechnology, 2018, 29, 185101. [74] Gao, F., Xu, Z., Liang, Q., Li, H., Peng, L., Wu, M., Zhao, X., Cui, X., Ruan, C., Liu, W. Osteochondral regeneration with 3D-printed biodegradable high-strength supramolecular polymer reinforce-gelatin hydrogel scaffolds, Adv Sci, 2019, 6, 1900867. [75] Xia, H., Zhao, D., Zhu, H., Hua, Y., Xiao, K., Xu, Y., Liu, Y., Chen, W., Liu, Y., Zhang, W., Liu, W., Tang, S., Cao, Y., Wang, X., Chen, H. H., Zhou, G. Lyophilized scaffolds fabricated from 3Dprinted photocurable natural hydrogel for cartilage regeneration, ACS Appl Mater Interfaces, 2018, 10, 31704–31715. [76] Dong, M., Wang, X., Chen, X., Mushtaq, F., Deng, S., Zhu, C., Terzopoulou, T. H., Qin, X., Xiao, X., Luis, J. P., Choi, H., Pego, A. P., Shen, Q., Nelson, B. J., Pane, S. 3D-printed soft magnetoelectric microswimmers for delivery and differentiation of neuron-like cells, Adv Funct Mater, 2020, 30, 1910323.

Xiaoqun Zhu, Guoqiang Lu, Jun Nie

Chapter 6 Photopolymerization and its application in 3D printing of customized objects 6.1 Overview of 3D printing In contrast to traditional subtractive manufacturing, 3D printing is a kind of additive manufacturing with rapid prototyping. Subtractive manufacturing involves processes that reduce materials, like cutting and polishing during jade bracelet making. In contrast, objects are fabricated through a process of layer-by-layer depositing of materials for additive manufacturing. Any complex three-dimensional structure can be cut into numerous two-dimensional planes along one direction. This creation of two-dimensional planes supports 3D printing fundamentally. Digital software can help construct the model data of all these two-dimensional planes and transmit it to the printer. Then, a three-dimensional object is printed layer-by-layer under data command. 3D printing is an interesting crossover of machinery, computer technique and material science; none is dispensable. It is model-less, designable, customizable and intelligent, greatly reducing the production cycle cost while improving the performance of products. 3D printing truly strikes at the core of the traditional manufacturing industry. 3D printing has emerged more than 40 years [1] and many technologies, such as selective laser melting (SLM), selective laser sintering (SLS), fused deposition modelling (FDM) and stereolithography (SLA), were developed [2–5]. Materials, such as metal, inorganic powder, plastic, liquid photosensitive resin and even paper, can be used in 3D printing [6, 7]. Now, 3D printing technology is applied widely in aerospace, automobile, medical treatment, jewelry, movie making and other industries. It has penetrated into our daily life and brings about great changes to our life constantly.

6.2 Development of photocuring 3D printing Photocuring 3D printing is the earliest one among 3D printing technologies [8, 9]. The first 3D printer was fabricated based on the principle of photopolymerization. In the 1970s, four scientists from America and Japan came up with the concept of

Xiaoqun Zhu, Guoqiang Lu, Jun Nie, College of Materials Engineering and Science, Beijing University of Chemical Technology, Beijing, P. R. China https://doi.org/10.1515/9783110570588-006

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3D printing almost at the same time. However, only Chuck Hull turned the concept into a reality and founded the first 3D printing company – 3D Systems Corp Company in 1986 [10]. In the next year, the first 3D printer named SLA-1 was developed based on stereolithographic appearance (SLA). Due to these outstanding contributions, Chuck Hull was inducted into the American inventor Hall of Fame in 2014. Over the last 30 years, many new technologies evolved from photocuring 3D printing [11–13], such as digital light processing (DLP), [14, 15] liquid crystal display (LCD), [16, 17] Multi-jet printing (MJP), [18, 19] two-photo 3D printing (TPP), [20] continuous liquid interface production (CLIP) [21] and so on. The following sections will introduce some mainstream and cutting-edge photocuring 3D printing technologies.

6.3 Basic knowledge of photopolymerization Photocuring 3D printing, based on the principle of photopolymerization, has advantages and disadvantages similar to photopolymerization [6, 9, 22–26]. In this section, a basic knowledge of photopolymerization related to photocuring 3D printing will be introduced. Firstly, the raw materials used in photopolymerization are liquid photosensitive resins. These resins consist of monomers, oligomers and initiators [9, 27–30]. According to the different polymerization mechanisms, they can be divided into free radical and cationic photopolymerization. The materials used in free radical photopolymerization can be acrylates and vinyl ethers [28, 31, 32]. The materials used in cationic photopolymerization contain vinyl ethers and epoxies [33, 34]. Monomers, also called active diluents, have low viscosity and just like solvents can dilute oligomers and dissolve initiators. But different from solvents, their structures contain active functional groups such as the double-bonds of acrylate and vinyl ether, the epoxy groups of epoxy resins and oxetane, which can be used for the reaction of photopolymerization. The monomers may contain one, two or more reactive functional groups. Oligomers, usually having high viscosity, are a class of polymers with low molecular weight, which can determine the final properties of the printed objects [31, 35, 36]. The oligomers mainly include epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates, epoxy resins and others [34, 37]. Photoinitiators are chemicals that are sensitive to light. They produce active species when exposed to suitable light [38–42]. These active species include free radicals, cations and anions, which further initiate the polymerization of monomers and oligomers [43]. In general, the liquid photosensitive resins are very stable without irradiation. They can be preserved stably for half a year to one year or even longer. But, once

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these systems are exposed to suitable light, they will cure rapidly and form a crosslinked polymer network. For photopolymerization and photocuring 3D printing, the wavelength of the light source can deterimine the reaction mechanism and the intensity can affect the reaction speed and printing efficiency, which finally affect the performance of the printed objects. Photopolymerization has some advantages. Firstly, the reaction can be easily controlled by switching light on or off. What’s more, light can be controlled spatiotemporally, endowing possibility to print objects with complex structures. More importantly, the light can be controlled precisely by corresponding equipment, which will endow the printed objects with high definition. This is one of the reasons why photocuring 3D printing has the highest molding precision among all the 3D printing technologies. Secondly, photopolymerization involves a process that changes the system from liquid to solid. This means that the printed objects are easily separated from the raw materials, thus allowing a high utilization of raw materials. If equipped with a high-precision 3D printer, it can even achieve the smooth surface like injection moldings. But, such a liquid-solid phase transition also has disadvantages for photocuring 3D printing. For example, it is a common method to add supports for printing the suspended structures from liquid resins and so it is inevitable to remove these supports after printing, which is time-consuming and affect the precision of objects. They can even cause damage to the printed objects while removing these supports. Thirdly, the speed of photopolymerization is pretty fast, only taking a few seconds or even less than one second, that’s why the rapid speed of photocuring 3D printing. So far, photocuring 3D printing has the fastest printing speed among all types of 3D printing technologies. Every coin has two sides and photopolymerization is also no exception. The advantages of photopolymerization results in photocuring 3D printing and bring many advantages along with it. Meanwhile, there are also some shortcomings in photocuring 3D printing that are difficult to overcome. First of all, free radicals are very sensitive to oxygen and can be quenched by oxygen, which affect the surface curing especially. Usually, photocured objects present a state where the internal structures are solidified completely, but the surface of the objects remains an uncured liquid state or viscous state. However sometimes, oxygen inhibition is an advantage in 3D printing. For example, it can solve the problem of adhesion between layers during printing. Secondly, volume shrinkage is another drawback which is difficult to overcome in photopolymerization, which means the volume of the whole system will contract after polymerization. Before polymerization, the distance between the liquid molecules is the van der Waals distance, which is generally 3.54Å. Nevertheless, it will be converted to the length of the covalent bond (carbon-carbon single bonds) after polymerization, 1.54Å, which is nearly 2Å less. The higher the double bonds ratio,

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the higher is the conversion rate and greater is the volume shrinkage. Usually, the volume shrinkage of epoxy systems is relatively smaller than free radical systems, which is due to the ring tension. After polymerization, the ring structures open and become straight chains, which can partially replenish the loss of the van der Waals distance. This is the reason why the volume shrinkage of cationic polymerization is relatively small. What’s more, volume shrinkage will cause great internal stress in objects. Due to the rapid photopolymerization, the rate of volume shrinkage can’t match well with the rate of polymerization. Hence, the volume shrinkage will be not synchronized during polymerization, which induced the internal stress of the printed objects. To lessen the internal stress, polymer chains will move after polymerization, which would lead to deformation and instability of the printed objects, consequently the printed object is broken. This is the main reason why photocured objects easily warp while using. In addition, liquid photosensitive resins used for photocuring 3D printing always contain a lot of small molecules, which will result in a low crosslinking density after photopolymerization, obviously affecting the performances of printed objects. Therefore, printed objects usually have poor mechanical properties such as hardness, brittleness, insufficient toughness and so on. These drawbacks severely limit the application areas and are the main reason why it is difficult to improve the market share of photocuring 3D printing. At present, photocuring 3D printing technology is mainly used in fields that require only transitional and temporary features.

6.4 Advantages and disadvantages of photocuring 3D printing It is easy to summarize the advantages and disadvantages of photocuring 3D printing by combing the characteristics of photopolymerization with 3D printing. The advantages of photocuring 3D printing include: (1) Fast molding speed (2) High precision (3) Good surface quality (Smooth surface) (4) Customizable objects with complex and fine structure (5) High utilization of raw materials At the same time, photocuring 3D printing also has disadvantages: (1) Limited strength, stiffness, heat resistance and usually brittleness (2) Easy deformation of the printed objects (3) High cost of production and maintenance

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(4) Require post-curing (5) Require supports for some objects with suspended structures that can easily damage printed objects while removing the supports

6.5 Types of photocuring 3D printing To understand and master photocuring 3D printing technologies quickly, it can proceed from the following key points. The first is the wavelength of the light source which determines the polymerization mechanism and the kind of initiators be used. Because the initiators can’t be excited by a lamp with mismatched wavelength, the reaction can’t be initiated consequently. Secondly, it is necessary to know the intensity of the light source, as it affects the speed of photopolymerization and further influences the printing efficiency. The stronger the intensity of the light source, the faster is the speed of photopolymerization and higher is the conversion rate of polymerization. Specifically, cationic photopolymerization is sensitive to light intensity and cationic initiators are difficult to be initiated under low light intensity. The third is the exposure method, it can be divided into point and surface exposure or up and down exposure. By understanding the above information, researchers can simply judge the printing precision and speed of the machine. In this section, some mainstream photocuring 3D printing will be briefly introduced.

6.5.1 Stereo lithography apparatus (SLA) Stereo lithography apparatus (SLA) is the earliest and most mature 3D printing technology among all the available technologies. It is widely used in the industry [11, 12]. There is a lifting platform in a resin tank and lasers can move and scan above the material tank during printing (Figure 6.1). The process can be explained in more detail as follows. Firstly, the lifting platform rises to the height of a layer-thickness above the liquid surface. Next, the scraper wipes the liquid level and the laser beam scans the liquid level point-by-point to initiate polymerization. Then, the lifting platform descends a distance of one layer after one layer of resin is cured. Finally, a threedimensional object is printed by repeating the above processes. Usually, the wavelength of the lamp used in the SLA machine is 355 nm, which has a strong light intensity. Because it is equipped with a point light source, the pattern formation is controlled by the path of the laser beam. Theoretically, the beam can move over an infinite range, so the SLA can print objects of large size. But the indensity of laser beam will gradually decay with time, which may result in different extent of polymerization, consequently affect the stability of the printed objects. It is therefore necessary to repair or change the light source regularly.

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Laser beams

Scraper Cured parts Supporting platform

Liquid photosensitive resins

Lifting platform

Resin tank

Figure 6.1: The schematic diagram of SLA.

What’s more, the printing speed is relatively slow due to point exposure. More importantly, the printing precision depends on the size of the laser spots. Thus, the SLA has low precision when compared with other technologies. Generally, the polymerization mechanism of SLA can be free radical or cationic due to the short wavelength and the strong intensity of the light source. Therefore, it has a broad choice of initiators [44]. But, pure cationic polymerization is rarely selected due to the less number of choices of resins and the high price of cation initiators. Hence, hybrid photopolymerization (radical and cation polymerization) is generally adopted. Therefore, the volume shrinkage of printed objects is relatively low. To date, SLA is the unique choice for printing large-sized objects, thus having broad application in fields such as electronics, medical devices, automotives, aerospace and so on.

6.5.2 Digital light processing (DLP) The principle of DLP is shown in Figure 6.2. Here, a projector (digital microscope device), which consists of a light source and an image control system that are integrated in an exposure module, is used to project the cross-section images of objects into liquid photosensitive resin [15]. Commonly, it adopts a down-exposure and the projection system is placed under the resin tank. The bottom of the resin tank is transparent. There is a release membrane on the side that is in contact with the liquid resin, which helps the cured parts separate from the bottom of resin tank. The working process is as follows. First, the print platform descends to a layer-thickness from the bottom of the tank. Then, the projection system casts a light pattern for curing one-layer liquid resin. Next, the platform goes up to the original position.

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Lifting platform Resin tank Cured parts

Liquid photosensitive resins

Digital microscope device Figure 6.2: The schematic diagram of DLP.

Finally, the machine repeats the above processes after the liquid resins replenish the cured areas until the whole process is finished. The core part of DLP is its projection system, also called optical semiconductor, and digital microscope device or DLP chip. This method was invented by Dr. Larry Hornback in 1977 and commercialized by Texas instruments in 1996. Currently, no technology can compete with it, which result in a high price of the machine. Usually, the projection system works with a high precision and adopts surface exposure. That is to say, a whole two-dimensional plane can be solidified through one projection and only a few seconds are needed to print one layer. Hence, DLP has a high printing speed and precision. The light source of DLP is LED lamp, which is a stable light source and has a long service-life. But, these components of the DLP machine are not tolerant to ultraviolet light. Thus, DLP usually adopts light source with a wavelength of 405 nm. To maintain high printing precision, the size of the printed objects is small, generally between 100 mm × 60 mm and 190 mm × 120 mm. So, it has to sacrifice some precision to print a large-sized object by DLP. With the evolution of DLP, many advanced DLP technologies with large molding have appeared in the market. Due to the weak intensity of the light source, DLP can only adopt the mechanism of free radical photopolymerization and there is need to choose some longwavelength initiators such as Irgacure 819, TPO, TPO-L and so on. DLP can print objects with small size and high precision. Thus, it is more suitable in the fields such as jewelry casting, dentistry and so on [15].

6.5.3 Liquid crystal display (LCD) LCD, basically having the same printing process as the DLP, is a rapidly developing 3D printing technology in recent years. And the main difference with DLP is

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the light source and imaging system [17]. The principle of LCD printing technology is shown in Figure 6.3. Its light source is an array of LED beads that are integrated on a flat plate. For LCD 3D printing technology, the liquid crystal display is used as the imaging system. When an electric field is applied to the liquid crystal display, it changes its molecular arrangement and prevents light from passing through. Consequently, it forms alternate intervals of light and shade, thus realizing the pattern control of light.

Figure 6.3: The schematic diagram of LCD.

The machine structure is very simple. The LCD screen is a mass-product and pretty cheap, so the the LCD printer is not expensive. Moreover, the LCD screen of 8.9-inch, 2 K pixel is widely used at present. This allows a good control for printing precision. LCD has high molding precision and the printed objects also have a smooth surface. According to the characteristics of the machine, the size of the printed objects is slightly larger than those objects printed by DLP. But, the liquid crystal is not tolerant to ultraviolet rays. Thus, 405nm-LED lamp is adopted. Moreover, the liquid crystal screen is composed of multiple layers of film, which strongly block light transmission, so the transmittance is relatively low and usually less than 10%. Further, it has low polymerization efficiency due to the weak light intensity. Therefore, LCD printing technology can only choose the mechanism of free radical polymerization. The LCD screen has a limited service life and needs to be repaired or replaced regularly. What’s more, there exists a backlight, which allows a small amount of light to pass through unexposed areas. If the printer is operated for a long time, the unexposed areas are cured, so it is necessary to clean the resin tank regularly. Finally, As the light intensity between the lamp beads is nonuniform, the equipment requires to be treated for uniform light. At present, it is widely used in dentistry, manual toys, jewelry and so on.

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6.5.4 Multi-jet printing/PolyJet (MJP) MJP, was developed by 3D systems company in 1996. While it was called PolyJet by Object company from Israeli and later the technology was incorporated into Stratasys company in 2012. The principle of MJP is shown in Figure 6.4. The printed pattern is controlled by nozzle array and a light source is used for curing the liquid resin. Usually, the number of nozzles can reach hundreds or thousands, so the molding area is, theoretically, not limited. The principle of MJP is as follows. Photosensitive resins are sprayed on a working platform and cured by a UV lamp. At the same time, some fusible or soluble materials are used as supporting structures during printing. Printing is carried out by a set of special nozzles. After finishing these processes, the molding platform accurately descends a layer thickness and repeats the above processes until the whole part is printed. Importantly, MJP can provide the highest Z-axis resolution, about 16 microns. What’s more, since this technology has a separate light source system, the wavelength and intensity of the light can be selected, theoretically, according to resin formulas.

Figure 6.4: The schematic diagram of MJP.

As different nozzles can print different materials, MJP is the only technology that can print objects with multiple kinds of materials and multiple colors, among all of the currently available photocuring printing technologies. What’s more, objects with high precision can be also printed by MJP technology. More importantly, the supporting materials are soluble or fusible. This is equivalent to embedding the printed parts in supporting materials. Thus, some soft objects can be printed by MJP. Finally, the surface of these printed objects is pretty smooth as there are no sharp supporting-points to other printed parts. However, the threshold of the nozzle technology is very high, so the equipment is pretty expensive. MJP can be mainly used in fields requiring high precision such as mold casting, medical health field, color printing and so on.

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6.5.5 Two-photon 3D printing Two-photon initiators are the key factors for two-photon 3D printing technology [20, 45]. Common photoinitiators need to absorb only one photon to produce an active center and initiate polymerization. However, two-photon initiators need to absorb two photons to generate free radicals, so the reaction can take place only at the focus of two light rays. Thus, the printing precision can reach nano-level. But the molding speed is very slow, so it is generally used in the area of micro-nano devices and the machine is expensive.

6.5.6 Continuous liquid interface production (CLIP) Although many technologies based on photocuring 3D printing have emerged, no one reflects the advantages of rapid curing of photopolymerization The time for photocuring can be in milliseconds, but the activities of rising and falling of the platform take more than 10 s, which greatly weakens the advantages of photocuring. Until 2015, the continuous and fast printing technology was developed by Carbon company which broken through the technical bottleneck of traditional photocuring 3D. The technology was reported as a cover article in Science. The printing speed could reach up to 480 mm/h. This technique was called Continuous Liquid Interface Production technology (CLIP) [21]. Briefly, CLIP is an advanced technology based on DLP. The principle of CLIP is shown in Figure 6.5. The key technique is the oxygen-permeable membrane, which is helpful for continuous printing due to oxygen permeation that inhibits radical polymerization. The oxygen-permeable membrane lies at the bottom of resin tank. It permits oxygen to pass through so that these resins that are close to the bottom of resin tank can’t be cured, allowing stable liquid states to be maintained during printing. In this way, the liquid resin quickly fills into the groove. Therefore, the platform needn’t repeat the lifting and falling processes, ensuring continuous printing and greatly increasing the printing speed. There are two necessary conditions to achieve a high printing speed in CLIP. One is to print hollow structures. Such structures ensure that the curing area of each layer is small and that the surrounding liquid resins can replenish to these printed areas quickly. But, for solid parts, it takes some time for the liquid resins to flow from around to the middle, which greatly slows down the printing speed. The second condition is that the photosensitive resin should have low viscosity. The liquid resins requires to have good liquidity, ensuring that they can flow to the printed areas quickly. Therefore, the printing speed will be slow for printing solid objects or using liquid resins with high viscosity. Now, the most successful application of CLIP is printing of shoe soles or shoe molds. To obtain good mechanical properties such as good elasticity, stretchability,

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Lifting platform

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Figure 6.5: The schematic diagram of CLIP.

high wear resistance and so on, it is necessary to adopt a double-curing mode such as light-heat curing. The shoe soles or shoe molds are first printed by the photocuring 3D printing technology and then the printed objects are placed in an oven for secondary curing. The photocuring 3D printing technologies described so far are relatively mature and have been widely used in the industry. Recently, new photocuring 3D printing technologies have also emerged. In 2019, research teams at UC Berkeley and Kelly of Lawrence Livermore National Laboratory published a report on Volumetric 3D printing technology (specifically, Volumetric additive manufacturing via tomographic reconstruction) in Science, which was regarded as a technology that overturned the traditional photocuring 3D printing technology [46]. Volumetric 3D printing technology breaks the traditional 3D printing technology where objects are printed layer-by-layer. The working principle of volumetric 3D printing technology is just like the reverse computed tomography (CT) scan in which X-ray tubes rotate around patients to take pictures of the body’s internal organs. These projections are then used to reconstruct a 3D image by computer. Next, the researchers calculate the shape of the object from several different angles using a computer. Subsequently, the resulting 2D images are input into a slide projector and cast into a cylindrical container containing photosensitive liquid resins. The container then rotates at a certain angle with the rotation of projector. Any position that can receive light can be controlled independently and the liquid resin solidifies when the total amount of light exceeds a certain value. In general, the key point of volumetric 3D printing technology is CAL algorithm, which is relatively complex and has a high threshold due to the large computation, which requires the support of high-performance computers. Of course, there are also many problems during printing such as oxygen inhibition,

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light field interference, conversion and matching between 3D spaces and 2D projections and so on. The advantage of volumetric 3D printing technology is its high printing speed – it can print a small model in dozens of seconds – and high tolerance for the viscosity of photosensitive resins. As we all know, greater the molecular weight greater is the viscosity of the resin and better is the performance of the printed objects. Therefore, volumetric 3D printing technology can almost print any structure. But, this technology requires transparent resins and only can print objects with small-size. Moreover, the key algorithm is relatively complex. In 2019, Martin P. de Beer and Harry L. van der Laan, from Michigan University published an article titled “Rapid, continuous additive manufacturing by volumetric polymerization inhibition patterning” in Science Advance. The printing speed can reach two meters per hour [47]. Its operating principle is also to keep the bottom of resin tank in liquid state during printing, thereby ensuring that the printing platform can pull up continuously. But, the method, different from CLIP, achieved the bottom inhibition by designing the resin formulas. There are two kinds of photosensitizers in the formulas; one is the visible photoinitiator camphorquinone (CQ), whose maximum absorption wavelength can reach more than 500 nm (The 458 nm-wavelength LED lamp is adopted during printing). The other photosensitizer is the free radical inhibitor bis[2-(o-chlorophenyl)-4,5-diphenylimidazole] (o-CI-HABI), which can produce free radical inhibitors to capture free radicals and prevent them from initiating polymerization. Here, the absorption wavelength of inhibitors is less than 450nm, which can be initiated by the 365 nm-wavelength LED lamp. Two rays of different wavelengths irradiate the same position at the same time during printing. Rays that have a long wavelength exhibit stronger penetrability than rays that have short wavelength, so free radicals and inhibitors can generate simultaneously at the bottom of resin tank. Here, the free radicals are suppressed and thus can’t initiate polymerization. The resins at the bottom of the tank are maintained in liquid state during printing. Because the rays of short wavelength can’t penetrate the upper layer, they only produce free radicals to initiate polymerization. In this way, the resins at the bottom of the tank are always kept in liquid state, guaranteeing continuous photocuring 3D printing.

6.6 Materials of photocuring 3D printing Photosensitive resins are the main materials for photocuring 3D printing and directly determine the performances of printed objects [6, 9, 26–28, 37, 48, 49]. Therefore, it is necessary for photosensitive resins to meet these following requirements: (1) Good matching of the machine’s light source and light intensity – Different printers have different light sources; their wavelengths and light intensities differ. The

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wavelength determines the type of initiators and the photocuring mechanism, while the light intensity affects the usage of initiators or the rate of photocuring. Low viscosity – Whether the process is layer-by-layer pull-down or continuous pull-up, the resins should have good leveling and should be tiled at the bottom of the material tank or tiled on the surface of cured objects within a short time during printing (especially in the process of continuous pull-up). All these processes require resins with low viscosity and the requirements of viscosity vary greatly for different printing technologies. Generally, the main purpose of adding reactive diluent is to adjust and reduce the viscosity of the photosensitive resin formulas. Small volume shrinkage – Volume shrinkage, caused by the transformation of Van der Waals distance between the liquid molecules into covalent bond after photocuring, is an inherent and inevitable phenomenon in photopolymerization. What’s more, there are still some unreacted double-bonds that are enclosed in printed objects after finishing, which may undergo slow polymerization. And generally, there is a need for post-curing treatment of photocuring objects. These phenomena also result in volume shrinkage, to a certain extent. More importantly, it is difficult to ensure uniform light for three-dimensional objects at the same time. This results in a nonuniform conversion rate of double-bonds, further generating uneven stress relaxation. Generally, the wall thickness of the objects is uneven due to the asymmetry of the three-dimensional objects in space. As a result, the stress relaxation generated in each direction is inhomogeneous. These stresses are difficult to neutralize, finally causing some problems such as deformation, cracking, crimp and imprecise size for printed objects. A highly primary curing degree – The primary curing refers to the curing that occurs during printing. To some extent, the volume shrinking during printing can be reduced by supplementing with surrounding liquid resins. Therefore, post-curing is seldom required if the double-bonds conversion is high during primary curing, resulting in a shrinking of a small volume or a low degree of deformation. High strength under wet state – It refers to the strength of the printed objects at primary-molding, which will guarantee less deformation, expansion and interlaminar peeling during the post-curing process. The material strength of the primary-molding includes a variety of mechanical properties such as stress, strain, modulus, hardness and adhesion between layer and layer. Only the high strength of primary-molding can ensure that the printed parts will not deform and the layers will not separate from each other when they are pulled out or scraped during printing. Small swelling degree – When all or some of the printed parts are immersed in the liquid photosensitive resin during printing, the size of the printed objects will be inaccurate if swelling occurs. Specifically, if the polarity of the monomers matches well with the polymer or if the polymer has a low cross-linking density, the monomer with small molecules may enter the polymer networks

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when these cured parts are immersed in the liquid resins. Therefore, it is necessary to design an appropriate resin formula and cross-linking density of the polymer to ensure low swelling. (7) Stable storage, no reaction and no sedimentation at room temperature – The resins need to have stable performances without irradiation, which will be beneficial for transportation and storage. (8) Good photosensitivity and high curing speed – The resins are sensitive to UV light and cured quickly under low light intensity. (9) Low odor, low irritation and low toxicity – It is necessary to consider the odor and toxicity of monomers and oligomers when designing the formulas. Apart from above requirements, there are also several points required to be considered. The systems used for photocuring 3D printing can be divided into free radical, cationic and hybrid photopolymerization according to the mechanisms. Free radical photopolymerization is a reaction where free radicals induce the polymerization of double-bonds [25]. It has many advantages, such as a wide variety of monomers, resins and initiators for choosing, high technical maturity, rapid curing speed and so on, so have broad application in traditional UV industry. The commonlyused free radical monomers are acrylate, vinyl and vinyl ether, whose degree of functionality can be 1, 2, 3 or multi-functionality. These monomers usually have low viscosity and can be used to dilute resins and dissolve initiators. Resins, which can be used for free radical photopolymerization, include urethane acrylates, epoxy acrylates, polyester acrylates and polyether acrylates [29–32, 35, 50]. Generally speaking, urethane acrylates have good flexibility and wear resistance, but there are many intermolecular or intramolecular hydrogen bonds due to the presence of a large number of amine-ester bonds, hence they have high viscosity. Epoxy resins have high polymerization speed and high strength, but high brittleness and are easily prone to yellowing. Polyester acrylates have good curing quality and the resin properties can be adjusted over a wide range. Polyether acrylates have good flexibility and good anti-yellowing properties owing to the existence of a large number of ether bonds in their molecular chain, but the materials made printed by polyether acrylates are relatively soft and have poor mechanical strength and chemical resistance. Free radical photoinitiators can be divided into cracking-type and hydrogengrabbing type [38–44]. Cracking-type initiators are generally used in photocuring 3D printing owing to their high initiation speed. It should choose the photoinitiators of matched absorption wavelength with that of the light source of the printer. For example, we can choose initiators that have high efficiency and low cost such as Irgacure 1173, 184, for the light source with 355 nm-wavelength. While the initiators, such as Irgacure TPO and 819, can be selected when the machine is equipped with the light source of 385 nm, 395 nm or 405 nm-wavelengths. If there is a visible light in the printer, the photoinitiators can be Irgacure 784, camphor quinone (CQ), etc. But they

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are high cost and colored, especially Irgacure 784 which is red, thus limiting the application in light or colorless resins. In summary, free radical photosensitive resins have good photosensitivity, fast curing speed and low cost. They are the earliest materials used in photocuring 3D printing. However, there is a large volume shrinkage in the printed molds and parts, which easily deformed while using. Cationic photopolymerization is a reaction that proton-acid induces the polymerization of epoxy, oxetane and vinyl ether [25, 33, 34, 37]. The ring-opening polymerization has low volume shrinkage, no oxygen inhibition. But, the induction period of cationic photopolymerization is relatively long and these reactions are at low speed. Therefore, these systems have relatively few applications in traditional photopolymerization. More importantly, they offer a less choices of monomers, resins and initiators, so the cost is higher than those of free radical systems. At present, pure cationic photopolymerization system is seldom used in photocuring 3D printing. The monomers that can be used for cationic photopolymerization include epoxy, oxetane and vinyl ether, and the most commonly used monomers are epoxies. The cationic initiators include sulfonium salt, iodonium salt and sensitizing agent [39, 43]. However, the current technology of cationic photopolymerization with long wavelength is not mature, so the mechanism of cationic photopolymerization can’t be used in photocuring 3D printing except SLA. Therefore, it has great significance in photocuring 3D printing fields to research the cationic photopolymerization with long wavelength, low-intensity of light and fast reaction speed. By combining the advantages of free-radical and cationic photonitiators, hybrid photopolymerization will be a better choice for photocuring 3D printing. At present, hybrid photosensitive systems are used mainly in SLA (355 nm) [51]. The printed objects and parts have characteristics such as low volume shrinkage and high precision. Due to the numerous advantages of photopolymerization, it can be used in many fields. However, the traditional UV materials are developed mainly for surface coating materials, just for protection and decoration and not for bulk materials. Photocuring 3D printed objects are bulk materials, requiring materials with good physical and mechanical properties, heat resistance, biocompatibility, casting and degradable properties. In order to expand the application field and market share of photocuring 3D printing, it is very important to research new photocurable materials with properties similar to engineering plastics [11, 23, 26, 48, 52, 53]. The group of Zhou Feng synthesized a kind of polyimide acrylate resins by combining the structure of polyimide with the functional group of acrylates, which are used for photocuring 3D printing to print objects with high temperature resistance (as shown in Figure 6.6) [53]. The printed objects can be used for a long time at 300 °C by blending these resins with active diluents (as shown in Figure 6.7), whose glass transition temperature is 242 °C and the tensile properties of the printed materials at 300 °C are slightly lower than those at room temperature.

SiDA

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Figure 6.6: The process of synthesizing polyimide acrylate resin. Reproduced with permission from [53]. Copyright 2017, Royal Society of Chemistry.

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Figure 6.7: Tensile properties (left) and dynamic mechanical properties (right) of polyimide photosensitive resins at different temperatures. Reproduced with permission from [53]. Copyright 2017, Royal Society of Chemistry.

6.7 Application of photocuring 3D printing With the collaborative development of hardware, software and materials, photocuring 3D printing has developed rapidly in recent years and its application fields are also continuously expanding [54, 55]. At present, the application fields of photocuring 3D printing have these following characteristics: personalization, small batch, complex structures, high precision and so on. It is mainly used in dental medicine [56], jewelry, earphone hearing aid, glasses, shoes manufacturing, education and

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so on [57–60]. Some optical images of printed objects by photocuring 3D printing are shown in Figure 6.8.

Figure 6.8: Optical images of the printed objects by photocuring 3D printing.

6.7.1 Application of photocuring 3D printing in dentistry Dentistry is the most typical and successful application of photocuring 3D printing [5]. Dental orthodontics and dental implant are the most widely used projects as well as having high added value. The emergence of 3D printing has overturned the traditional dental medical technology, which make dental orthodontics and implants more precise, efficient and comfortable. Nowadays, people have more demands on the quality of life and pay more attention to their personal image. Having a nice smile and even teeth can provides people with more confidence. So, dental medicine has a huge market all over the world. According to the survey, the dental market in China is growing rapidly, with an annual growth rate of about 15% in the recent years. Therefore, as an emerging and advanced technology, 3D printing technology has a broad application in the field of dentistry.

6.7.1.1 Application of photocuring 3D printing in dental orthodontics Orthodontics correct the deformity in teeth by using the principle of biomechanics. Mechanical forces are applied to the teeth by means of attaching various orthodontic devices and used to align teeth.

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Metal braces correction, known as “steel teeth”, is one of the most commonly used methods to correct teeth deformity. Specifically, this method uses steel wires to fix the teeth firstly. The doctor then adjust the teeth to desired position based on their own experience and skill. We can clearly see metal braces and steel wires on the surface of teeth which gives others a feeling of “iron tooth and bronze tooth”. More importantly, the metal braces and steel wires are easy to cause friction with the mouth. This brings foreign body sensation and stimulates oral mucosa. Further, food residues are easy to be remained on the metal braces and steel wires, growing bacteria and causing oral inflammation. Invisible orthodontics, also called “Invisalign appliance”, are widely favored by people who want to have beautiful teeth. There are no steel wires and metal braces in the process of correction. More importantly, traditional technology of invisible orthodontics is time-consuming, costly and the precision is reduced after several times of turning-model. Digital invisible orthodontic use the mechanism of force application and absorption by biomechanics. First, suitable force is applied on the teeth. Then, the force is transmitted to the alveolar bone of the stressed side due to the hardness of the teeth (teeth do not undergo any significant change). Next, the alveolar bone will slowly absorb the force and realign the teeth. At the same time, the other side of alveolar bone will slowly reconstruct and regenerate owing to the drag force. Here, the most important process is the biological force applied to the teeth. It must be accurately calculated to balance the reconstruction tension with the absorption pressure of the alveolar bone. If the doctor can operate well, they will safely complete the objective of aligning the teeth and adjusting the occlusal relationship of the upper and lower jaws. By using digital invisible orthodontics, doctors can obtain the patient’s oral data using a oral scanner, which is used for constructing 3D models. After diagnosis and personalized design, the doctors can accurately calculate the biological force of each pair of braces by using digital software and can deduce the model evolution. The model data is then transmitted to printer. Finally, doctors turn these molds into transparent braces so that the patients can wear them in sequence. This method not only saves time and causes less damage to teeth, it also enables patients wearing the braces comfortably, looking beautiful and cleaning their mouth easily and maintaining oral health. According to the statistics of Henry Schein, there are 2.06 million cases of orthodontics in China and the market size was 24.72 billion yuan in 2017, with an annual growth rate of about 15%. It can be forecasted that there is a huge potential for dental orthodontics in the future and photocuring 3D printing will develop rapidly in the field of dental orthodontics.

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6.7.1.2 Application of photocuring 3D printing in dental prostheses The same as traditional invisible orthodontics, traditional dental prostheses also involve a series of complicated processes such as biting mold, turning mold, implanting and so on. Such a complex process will be precision-damaging and timeconsuming. Dental prostheses usually require a high precision and are of small size, which are in accordance with the technical characteristics of photocuring 3D printing. With the development of 3D printing technology, digital dental prostheses have been realized through a series of precise operations such as scanning the patient’s oral, converting them into model data and then transmitting to photocuring 3D printer. These processes replace the plaster model and ensures accuracy while repairing.

6.7.1.3 Application of photocuring 3D printing in dental surgery guide plate Dental guide plate planting technology has evolved recently. Simply speaking, guide plate is just like navigation while driving a car. The structures of the teeth are so complex that only gums can be seen from the surface. Doctors can’t see the location of the alveolar bones and neural tube during traditional dental implant. They may complete the entire process of dental plant by flap surgery, incision or other operations. Such a process can result in a large wound and need long time for patients to recover. With the help of digital technology, doctors can get the location of alveolar bone and neural tube through cone beam computer tomography (CBCT) and design the implant guide plate by software before the operation. In this way, doctors can ensure the success of the surgery and post-repair by controlling the position, direction and size of the implant. In short, digital guide plant can greatly improve the accuracy of surgery, especially for some complex implant surgery that may damage important anatomical structures.

6.7.2 Application of photocuring 3D printing in jewelry manufacturing For a long time, jewelry manufacturing relied on hand-sketched and handcarved wax molds, which is time-consuming and inefficient. 3D printing technology can greatly shorten the production time of jewelry prototypes by combining CAM (computer-aided manufacturing) and CAD (computer-aided design). In comparison to traditional hand-drawing, designers can create new and unique designs more easily using CAD, which can also help those people who are not good at painting to transform their ideas into reality, so everyone can easily make and modify a piece of jewelry. What’s more, it is very convenient to design

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models using CAD and designers needn’t worry about the problem of dimension accuracy because CAD programs will automatically adjust the model size. No matter how complex the model is, it only takes a few hours to finish the model design completely. In addition, these designs can be stored in a computer permanently and it is also easy to get a part of the model and copy it into other models. By using shadow and reflection to let the preview more realistic during designing, designers and customers can also clearly see the appearance of jewelry from the designs. The model designed by CAD can be converted into threedimensional data and the output quickly obtained from a 3D printer. Owing to the high precision of photocuring 3D printing, it can make more complex and fine structures than hand-carving. More importantly, many models can be printed together, which greatly saves time and improves efficiency and accuracy. Finally, the printed models can be turned into various kinds of metal jewelry by casting. The photosensitive resins used for printing jewelry are required to have excellent characteristics such as low volume shrinkage, high dimensional accuracy, small thermal expansion, no residue after burning and no corrosion to plaster molds. Due to its difference from the traditional wax models, the casting parameters and the process of photocuring 3D printing should be readjusted. It is therefore necessary for foundry and researchers to explore suitable casting processes. But the casting process is relatively complex and confidential among foundries, so the technique level is uneven, which is one of the main factors for 3D printing being difficult to popularize in the field of jewelry.

6.7.3 Application of photocuring 3D printing in earphones and hearing-aids Customized earphones and hearing-aids can match well with the owner’s auricle, which will insulate users from outside noise and achieve the optimal hearing. Specially, the auricle model can be obtained by scanner, then these 3D data of auricle model will be transmitted to printer, finally the earphones or hearing-aids will be printed by photocuring 3D printer. Because the earphones and hearing-aids are directly contacted with human skin for a long time, the materials of earphones must be biocompatible. At the same time, it is necessary to have high precision, transparence and good resistance to yellowing for the printed earphones. According to the statistical data of the world health organization, 15% of adults have hearing loss all over the world. The total market of hearing-aids was 4.7 billion dollars in 2015, and will reach 7 billion dollars by 2024, with an average annual growth rate of 4.5%.

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6.7.4 Application of photocuring 3D printing in customized glasses Nowadays, more and more people wear glasses to correct vision and spectacles manufacturing has developed into a large-scale industry. Commonly, spectacle frames are basically the same due to standardized and large-scaled manufacturing. However, there are no two identical faces in the world and the space between the pupils is exactly not the same. Therefore, there is an urgent need in the market for precise and customized glasses. The breakthrough in 3D printing has brought a solution for customized glasses. Optometrists can design the structure of glasses on demand so as to enhance the comfort of the owner and create a personalized style. Specially, the facial profile of a customer can be obtained using a scanner and the most comfortable position of the spectacle lens can be determined by analyzing the facial data. Then, the frame, whose color and other details can be adjusted easily, can be customized according to the unique facial features. What’s more, customers can observe the wearing effect by an online design system.

6.7.5 Application of photocuring 3D printing in shoe manufacturing Apart from dentistry, shoe manufacturing is also likely to be a new large-scale and high value-added area for photocuring 3D printing. In 2017, the market of China’s shoe industry reached 400 billion yuan. Each person has feet that are special in size and shape. The size may be the same but the shape may be totally different. Traditional shoe manufacturing often uses one mold for the same size, which can’t meet personalized needs. However, photocuring 3D printing coupled with software design can easily meet these demands. According to the characteristics of consumers, personalized shoes can be designed, which can not only effectively reduce the weight of the shoes but also provide excellent shock absorption so as to achieve the best wearing experience. In 2015, the development of CLIP laid the foundation for rapid 3D printing. Later, Adidas and Carbon printed the middle soles of shoes by CLIP. They produced 5,000 pairs of shoes in 2018 and the price was 333 dollars per pair. They produced 100,000 pairs in 2019 and they planed to produce 5 million pairs in 2020. More importantly, Carbon company layed out comprehensive patent to protect the technology and corresponding materials. If other companies want to make shoes by photocuring 3D printing, they must break through these pioneering effort and establish their own proprietary mechanism. Else, when there is a conflict of intellectual property, it will bring huge trouble to their company.

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Because the shoes need to bear the weight of the whole person, the performance requirements of the materials are very high –good elongation at break, tear strength, rebound resilience, wear-resistance and so on. In addition to these applications, photocuring 3D printing also has been applicated in garage kits, model design, culture products and so on.

6.8 Problems in photocuring 3D printing Although photocuring 3D printing came into being more than 30 years, it still commands a low market share compared to metal 3D printing and plastic 3D printing. The main reason is the low performance of the printed objects due to aspects such as high brittleness, insufficient toughness and so on. Thus, it is necessary to improve performances and there are two points that need to be considered. One is to design and synthesize new resins suitable for photocuring 3D printing, which can combine the structural unit of engineering plastics with photosensitive structures so as to improve the mechanical and thermal performance of printed objects [11, 26, 48, 61–63]. The second is software design which can indirectly improve properties by designing corresponding three-dimensional spatial structures [52]. At present, the existing problems include: (1) Lack of photosensitive resin with high performance (low viscosity, low volume shrinkage) (2) Large volume shrinkage of free radical photopolymerization and low reactivity of cationic photopolymerization (3) Incompatibility between high quantity of fillers and fast printing for composite systems (4) Low storage stability of double-component (photo-thermal) systems (5) Treatment of solid waste after using the models (6) Printing only small-sized objects using DLP and LCD (7) Long curing time and low conversion rate in LCD (8) Low printing precision in SLA (9) Problem between high-speed printing and poor surface quality or precision (10) Printers with high printing speed only for hollow structures (11) Requirement of supporting structures and post-processing is time-consuming and manpower-wasting (12) Difficulty in printing soft objects and using resins with high viscosity (13) Absence of high-level education combined with 3D printing (14) Absence of competitiveness and independent innovation (15) Require to increase the online market for 3D printing

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6.9 Conclusions and perspectives Although there are many problems, we still can believe that photocuring 3D printing has a promising prospect. First of all, photocuring 3D printers had the fastest growth rate in the global layout of 3D printers from 2018 to 2020. The amount of materials also grew rapidly in the same period – it increased by 25% compared to the previous year. Secondly, the development of additive manufacturing has been strongly supported by government and it has been recognized that additive manufacturing will bring the fourth industrial revolution. Thirdly, the huge consumer markets offer tremendous opportunities for 3D printing and there will be a real chance for photocuring 3D printing from the existing 300 billion market of polymer injection molding. Most importantly, the photocuring 3D printing technology and its materials are far away from the stage of booming rapid growth. However, by coupling with the highly developed internet, the photocuring 3D printing technology will develop rapidly.

References [1] [2] [3] [4] [5]

[6] [7] [8] [9] [10] [11] [12] [13] [14]

Kruth, J. P., Leu, M. C., Nakagawa, T. Progress in additive manufacturing and rapid prototyping, CIRP Annals, 1998, 47, 525–540. Bikas, H., Stavropoulos, P., Chryssolouris, G. Additive manufacturing methods and modelling approaches: A critical review, Int J Adv Manuf Technol, 2015, 83, 389–405. Gao, W., et al. The status, challenges, and future of additive manufacturing in engineering, Comput-Aided Des, 2015, 69, 65–89. Huang, S. H., et al. Additive manufacturing and its societal impact: A literature review, Int J Adv Manuf Technol 2012, 67, 1191–1203. Revilla-Leon, M., Ozcan, M. Additive manufacturing technologies used for processing polymers: Current status and potential application in prosthetic dentistry, J Prosthodont, 2019, 28, 146–158. Ligon, S. C., et al. Polymers for 3D printing and customized additive manufacturing, Chem Rev, 2017, 117, 10212–10290. Ngo, T. D., et al. Additive manufacturing (3D printing): A review of materials, methods, application and challenges, Composites, Part B, 2018, 143, 172–196. Quan, H., et al. Photo-curing 3D printing technique and its challenges, Bioact Mater, 2020, 5, 110–115. Zhang, J., Xiao, P. 3D printing of photopolymers, Polym Chem, 2018, 9, 1530–1540. Hull, C. W., Uvp, I. Apparatus for production of three-dimensional objects by stereolithography. 1986. Herzberger, J., et al. 3D Printing All-Aromatic Polyimides Using Stereolithographic 3D Printing of Polyamic Acid Salts, ACS Macro Lett, 2018, 7, 493–497. Khatri, B., et al. Development of a multi-material stereolithography 3D printing device, Micromachines, 2020, 11, 532. Melchels, F. P., Feijen, J., Grijpma, D. W. A review on stereolithography and its application in biomedical engineering, Biomaterials, 2010, 31, 6121–6130. Kim, S. H., et al. Precisely printable and biocompatible silk fibroin bioink for digital light processing 3D printing, Nat Commun, 2018, 9, 1620.

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[15] Shen, Y., et al. DLP printing photocurable chitosan to build bio-constructs for tissue engineering, Carbohydr Polym, 2020, 235, 115970. [16] Li, J., et al. Optically rewritable transparent liquid crystal displays enabled by light-driven chiral fluorescent molecular switches, Adv Mater, 2019, 31, e1807751. [17] Shan, J., et al. Design and synthesis of free-radical/cationic photosensitive resin applied for 3D printer with Liquid Crystal Display (LCD) irradiation, Polymers, 2020, 12, 1346. [18] Castiaux, A. D., et al. PolyJet 3D-printed enclosed microfluidic channels without photocurable supports, Anal Chem, 2019, 91, 6910–6917. [19] Cazón, A., Morer, P., Matey, L. PolyJet technology for product prototyping: Tensile strength and surface roughness properties, Proc Inst Mech Eng, Part B: J Eng Manuf, 2014, 228, 1664–1675. [20] Xing, J. F., Zheng, M. L., Duan, X. M. Two-photon polymerization microfabrication of hydrogels: An advanced 3D printing technology for tissue engineering and drug delivery, Chem Soc Rev, 2015, 44, 5031–5039. [21] Tumbleston, J. R., et al. Continuous liquid interface production of 3D objects, Science, 2015, 347, 1349–1352. [22] Bagheri, A., Jin, J. Photopolymerization in 3D printing, ACS Appl Polym Mater, 2019, 1, 593–611. [23] Ji, Z., et al. 3D printing of photocuring elastomers with excellent mechanical strength and resilience, Macromol Rapid Commun, 2019, 40, e1800873. [24] Tay, Y. W., et al. Processing and properties of construction materials for 3D printing, Mater Sci Forum, 2016, 861, 177–181. [25] Yagci, Y., Jockusch, S., Turro, N. J. Photoinitiated polymerization: Advances, challenges, and opportunities, Macromolecules, 2010, 43, 6245–6260. [26] Herzberger, J., et al. Polymer design for 3D printing elastomers: Recent advances in structure, properties, and printing, Prog Polym Sci, 2019, 97, 101144. [27] Ligon-Auer, S. C., et al. Toughening of photo-curable polymer networks: A review, Polym Chem, 2016, 7, 257–286. [28] Zhang, X., et al. Acrylate-based photosensitive resin for stereolithographic three-dimensional printing, J Appl Polym Sci, 2019, 136, 47487. [29] Degirmenci, M., Hepuzer, Y., Yagci, Y. One-step, one-pot photoinitiation of free radical and free radical promoted cationic polymerizations, J Appl Polym Sci, 2002, 85, 2389–2395. [30] Ding, R., et al. Sustainable near UV-curable acrylates based on natural phenolics for stereolithography 3D printing, Polym Chem, 2019, 10, 1067–1077. [31] Xu, D. M., Zhang, K. D., Zhu, X. L. A novel dendritic acrylate oligomer: Synthesis and UV curable properties, J Appl Polym Sci, 2004, 92, 1018–1022. [32] Iedema, P. D., et al. Photocuring of di-acrylate, Chem Eng Sci, 2018, 176, 491–502. [33] Li, C., et al. Photopolymerization kinetics and properties of a trifunctional epoxy acrylate, Des Monomers Polym, 2012, 16, 274–282. [34] Wan, J., et al. A sustainable, eugenol-derived epoxy resin with high biobased content, modulus, hardness and low flammability: Synthesis, curing kinetics and structure–property relationship, Chem Eng J, 2016, 284, 1080–1093. [35] Maurya, S. D., et al. A review on acrylate-terminated urethane oligomers and polymers: Synthesis and applications, Polym Plast Technol Eng, 2017, 57, 625–656. [36] Li, J., et al. Synthesis and properties of a low-viscosity UV-curable oligomer for threedimensional printing, Polymer Bulletin, 2015, 73, 571–585. [37] Atif, M., Bongiovanni, R., Yang, J. Cationically UV-cured epoxy composites, Polymer Reviews, 2015, 55, 90–106.

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[38] Fouassier, J. P., et al. Photoinitiators for free radical polymerization reactions, John Wiley & Sons, Inc, 2010, 351. [39] Mousawi, A. A., et al. 3-Hydroxyflavone and N-phenylglycine in high performance photoinitiating systems for 3D printing and photocomposites synthesis, Macromolecules, 2018, 51, 12, 4633–4641. [40] Kara, M., Dadashi-Silab, S., Yagci, Y. Phenacyl ethyl carbazolium as a long wavelength photoinitiator for free radical polymerization, Macromol Rapid Commun, 2015, 36, 2070–2075. [41] Li, J., et al. Synthesis of furan derivative as LED light photoinitiator: One-pot, low usage, photobleaching for light color 3D printing, Dyes Pigm, 2019, 165, 467–473. [42] Pietrzak, M., Wrzyszczyński, A. Novel sulfur-containing benzophenone derivative as radical photoinitiator for photopolymerization, J Appl Polym Sci, 2011, 122, 2604–2608. [43] Tang, L., Nie, J., Zhu, X. A high performance phenyl-free LED photoinitiator for cationic or hybrid photopolymerization and its application in LED cationic 3D printing, Polym Chem, 2020, 11, 2855–2863. [44] Zhou, R., et al. A two-photon active chevron-shaped type I photoinitiator designed for 3D stereolithography, Chem Commun (Camb), 2019, 55, 6233–6236. [45] Nguyen, L. H., Straub, M., Gu, M. Acrylate-based photopolymer for two-photon microfabrication and photonic applications, Adv Funct Mater, 2005, 15, 209–216. [46] Kelly, B. E., et al. Volumetric additive manufacturing via tomographic reconstruction, Science, 2019, 363, 1075-+. [47] de Beer, M. P., et al. Rapid, continuous additive manufacturing by volumetric polymerization inhibition patterning, Science Advances, 2019, 5, eaau8723. [48] Chen, L., et al. Highly stable thiol–ene systems: From their structure–property relationship to DLP 3D printing, J Mater Chem C, 2018, 6, 11561–11568. [49] Zhao, T., et al. Silicone–Epoxy‐based hybrid photopolymers for 3D printing, Macromol Chem Phys, 2018, 219, 1700530 [50] Xiao, M., Li, Z., Nie, J. Synthesis and photopolymerization of 2-(acryloyloxy)ethyl piperidine-1carboxylate and 2-(acryloyloxy)ethyl morpholone-4-carboxylate, J Appl Polym Sci, 2011, 119, 1978–1985. [51] Xiao, M., He, Y., Nie, J. Novel bisphenol a epoxide–acrylate hybrid oligomer and its photopolymerization, Des Monomers Polym, 2012, 11, 383–394. [52] Guerra, A. J., et al. Optimization of photocrosslinkable resin components and 3D printing process parameters, Acta Biomater, 2019, 97, 154–161. [53] Guo, Y., et al. Solvent-free and photocurable polyimide inks for 3D printing, J Mater Chem A, 2017, 5, 16307–16314. [54] Macdonald, E., et al. 3D printing for the rapid prototyping of structural electronics, IEEE Access, 2014, 2, 234–242. [55] MacDonald, E., Wicker, R. Multiprocess 3D printing for increasing component functionality, Science, 2016, 353, aaf2093. [56] Bhargav, A., et al. Applications of additive manufacturing in dentistry: A review, J Biomed Mater Res B Appl Biomater, 2018, 106, 2058–2064. [57] Chia, H. N., Wu, B. M. Recent advances in 3D printing of biomaterials, J Biol Eng, 2015, 9, 4. [58] Espalin, D., et al. 3D Printing multifunctionality: Structures with electronics, Int J. Adv Manuf Technol, 2014, 72, 963–978. [59] Leong, K. F., Cheah, C. M., Chua, C. K. Solid freeform fabrication of three-dimensional scaffolds for engineering replacement tissues and organs, Biomaterials, 2003, 24, 2363–2378. [60] Li, H., Tan, C., Li, L. Review of 3D printable hydrogels and constructs, Mater Des, 2018, 159, 20–38.

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[61] Moszner, N., Salz, U. New developments of polymeric dental composites, Prog Polym Sci, 2001, 26, 535–576. [62] Peutzfeldt, A. Resin composites in dentistry: The monomer systems, Eur J Oral Sci, 1997, 105, 97–116. [63] Ji, Z., et al. Facile photo and thermal two-stage curing for high-performance 3D printing of poly(Dimethylsiloxane), Macromol Rapid Commun, 2020, 41, e2000064.

Yang-Yang Xu, Zhaofu Ding, Haibin Zhu, Yijun Zhang, Pu Xiao, Jean Pierre Fouassier, Jacques Lalevée

Chapter 7 Dual wavelength systems in 3D printing 7.1 Introduction 3D printing, also known as additive manufacturing, is a promising technology in producing three dimensional objects through layer-by-layer printing [1]. Since the proposal of the concept of 3D printing in 1980s, the 3D printing technique has witnessed a rapid development and has expanded its application from traditional manufacturing field to medical, electronics, photonics, precision machinery and other high-tech fields [2]. Among the many 3D printing techniques, photocuring 3D printing possesses some outstanding advantages, including high printing speed, high precision, green chemistry and smooth surface for the printed objects. Photocuring 3D printing (also called stereolithography) is based on photopolymerization reaction of formulated photosensitive liquid resin upon light irradiation. Since the photocuring process only happens at the irradiated part of the resin, it is very easy and the printed model can be quickly separated from the liquid resin. Therefore, a 3D model can be fabricated rapidly with high precision and fast printing rate [3, 4]. As one of the earliest 3D printing techniques, photocuring 3D printing has been the subject of intensive study and deep exploration. According to the different principles of pattern formation and control systems, photocuring 3D printing has developed some unique techniques, including direct laser writing (DLW), stereolithography appearance (SLA), digital light processing (DLP), liquid crystal display (LCD), multi-jet printing (MJP), continuous liquid interface production (CLIP), holographic 3D printing technology, among others [5–7]. Photochemical reactions have been extensively exploited in the fields of organic chemistry and polymer chemistry. In general, compared to traditional thermal polymerization, light induced polymerization reaction or photopolymerization benefits from environmentally friendly conditions (usually at room temperature without release of volatile organic compounds), is energy saving and low cost, shows

Yang-Yang Xu, Zhaofu Ding, Haibin Zhu, College of Chemistry and Materials Science, Anhui Normal University, Wuhu, P. R. China Yijun Zhang, Jacques Lalevée, Université de Haute-Alsace, CNRS, Institut de Science des Matériaux de Mulhouse (IS2M), Mulhouse, France Pu Xiao, Research School of Chemistry, Australian National University, Canberra, Australia Jean Pierre Fouassier, Université de Haute-Alsace https://doi.org/10.1515/9783110570588-007

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facile operation, non-invasiveness, high reaction rate (overcoming large activation barriers in relatively short time) and monomeric conversion, and offers the possibility of orthogonal as well as excellent spatial and temporal control [8–10]. Tremendous effort has been devoted over the past few years to developing light-mediated polymerization reactions, such as free radical polymerization (FRP), cationic polymerization (CP), free radical promoted cationic polymerization (FRPCP) and ring opening polymerization (ROP) [11–13]. The usage of wavelength-selective reactions in a one-pot system can be explained as the preference of one reaction channel over another through utilizing a specific wavelength regime. The reactions are selectively inducible in response to the irradiation wavelengths; hence, multiple reactions may be regulated in one pot by simply altering the irradiation wavelengths [14, 15]. But in most published reports, only single wavelength irradiation is used to induce just one kind of photopolymerization [16]. In particular, for sequential polymerizations, either a single photo trigger (at a single wavelength) or a combination of thermal/photo trigger was utilized to control the polymer network formation [17]. Systems completely controlled by changing irradiation wavelengths have been seldom reported. Now, the most commonly and typically utilized light source for the photocuring 3D printing is the low-cost and safe violet LED@405 nm or laser [18, 19]. However, one of the main obstacles for photocuring 3D printing is that the existing photoinitiators cannot be fully adapted to the current irradiation wavelength of 405 nm. The development of orthogonal photoinitiators, which allow a separate radical/cationic polymerization dependent on the wavelengths, is supposed to be a key point in broadening the possibilities [20]. Wavelength-selective polymerization for photocuring would greatly expand the application in 3D printing fields, but is still in its infancy. In order to stress upon this issue, this chapter is divided into two parts. In the first part, dual wavelength photopolymerization reactions are introduced; and in the second part, the application of dual wavelength photochemistry in 3D printing during the last five years is further discussed. This chapter would bring on more attention to the investigation of dual wavelength systems in the field of photocuring 3D printing.

7.2 Dual wavelength photopolymerization reactions 7.2.1 λ-Orthogonality click pericyclic reaction for linear polymers using lights with different wavelengths The click chemistry, first introduced by Sharpless in 2001, possesses some unique features: fast reaction kinetics, quantitative yield, simple product isolation without byproducts, eco-friendly conditions, stereoselectivity, equimolarity, uphill photosensitization and orthogonal reactivity [21, 22]. The λ-orthogonality of click reactions is

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defined as the kinetic preference of one-click reaction channel by using a specific wavelength regime. The key to achieving orthogonality in photoinduced click reactions is to select appropriate moieties that are kinetically preferred by a specific wavelength. A one-pot reaction of equimolar quantities of maleimide with two poly (ethylene glycol) (PEG) chains carrying different maleimide-reactive endgroups, that is, a photoactive diene (photoenol) and a nitrile imine (tetrazole), is illustrated in Figure 7.1. Firstly, upon selective UV irradiation (310–350 nm), the maleimide compound reacts exclusively with the photoenol derivative 1. After complete conversion of the photoenol derivative 1, subsequent irradiation at another UV light (270–310 nm) activates the click reaction between maleimide and tetrazole derivative 2. Based on light-induced click reactions performed at specific wavelengths, the λ-orthogonal pericyclic photochemistry offers an effective avenue for synthesizing macromolecules by addressing individual parts of an oligomer [23].

Figure 7.1: λ-orthogonal pericyclic reactions in a one-pot system consisting of a dienophile (maleimide), a photoenol derivative 1, and a tetrazole derivative 2. Reproduced with permission from [23]. Copyright 2015 John Wiley and Sons.

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7.2.2 λ-orthogonal photopolymerization of star shaped polymers Compared to traditional linear polymers, the star-shaped polymers have a much higher degree of branching, which can lead to some unique physical and mechanical properties [24]. Many controlled polymerization techniques have been used for the synthesis of well-defined star polymers, such as atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, nitroxide mediated polymerization (NMP) directly based on multi-arm initiators, as well as in combination with modular ligation strategies [25]. The sequential λ-orthogonal photochemistry would develop a new method for the synthesis of star polymers. A trifunctional maleimide center and a bifunctional oligomer carrying two different photoactive termini, a benzaldehyde group and a tetrazole group, are shown in Figure 7.2. A lightinduced sequence cycloaddition between benzaldehyde/ tetrazole group with maleimide core is realized with UV irradiation of different wavelengths. For the first reaction path induced by low-energy UV (300–440 nm), Diels–Alder ligation of benzaldehyde to the maleimide core is carried out first, and then with a more energetic UV light irradiation (280–440 nm), subsequent tetrazole is activated, leading to the attachment of an ene terminated polymer to the star-shaped precursor. On the contrary, for the reverse reaction path, tetrazole is activated prior to the benzaldehyde, with a more energetic UV light. Then, upon the irradiation of low-energy UV, the benzaldehyde ligation is selectively attached to the maleimide center [26]. Interestingly, the sequential UV irradiation in one-pot system allows the synthesis of two different star polymers in an orthogonal fashion from identical starting materials. This new λ-orthogonal reaction sequence demonstrates the possibility of the photochemical synthesis of complex macromolecular architectures, based on different wavelengths.

7.2.3 Orthogonal polymerization of graft copolymers by different visible lights Although UV is usually used to induce photopolymerization reactions, the application of environmentally benign visible light for photopolymerization is much more interesting and promising, considering that intensive UV is harmful to both environment and human body [27, 28]. A dual-wavelength visible light-regulated orthogonal polymerization system capable of alternating two polymerizations for the synthesis of graft copolymers in one pot was designed. Under blue light (460 nm) irradiation, a merocyanine-based photoacid (PAH) could act as a photocatalyst for the ring opening polymerization (ROP) of δ-VL, a six-membered cyclic ester δ-valerolactone. Subsequently, after the light was switched from blue to red (635 nm), the photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer radical (PET-RAFT) polymerization of methyl acrylate (MA), catalyzed by zinc tetraphenylporphyrin (ZnTPP), was activated. Therefore, a graft copolymer could be

Figure 7.2: Schematic illustration for the orthogonal light induced synthesis of star shaped polymers depending on different wavelengths. Adapted with permission from [26]. Copyright 2016 Royal Society of Chemistry.

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synthesized by combining the photoacid catalyzed ROP and ZnTPP catalyzed PET-RAFT polymerization in one pot by alternating visible wavelengths, as shown in Figure 7.3. Besides graft copolymers, this alternating photo-ROP and PET-RAFT in one-pot polymerization process can also be repeated several times to produce block copolymers [29].

Figure 7.3: Schematic illustration of one-pot sequential photo-ROP and PET-RAFT polymerization for the synthesis of graft copolymer PMA-b-(PHEA-g-PVL) induced by alternative blue (460 nm) and red light (635 nm) irradiation. Reproduced with permission from [29]. Copyright 2016 Royal Society of Chemistry.

7.2.4 Interconversion of cationic and radical polymerizations for block copolymers Free radical polymerization and cationic polymerization make for a big share in the photopolymerization area. The ability to combine these two polymerization mechanisms in a one-pot setup and realize the facile switching of the monomer selectivity via external stimuli provides a wonderful opportunity to modulate polymer sequence and structure [30]. Light, as a clean energy source and one of the most powerful external stimuli, may play a crucial role in addressing this challenge [31]. It is a great strategy to determine monomer incorporation via the specific wavelength of light, through selective activation of either free radical or cationic polymerization processes. In a mixture system of equimolar isobutyl vinyl ether (IBVE) and methyl

Figure 7.4: (a) Mechanism for the polymerization switching between cationic and free radical polymerization by changing wavelength; (b) Block copolymers generated with monomer selectivity upon blue or green LED irradiation. Adapted with permission from [32]. Copyright 2017 American Chemical Society.

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acrylate (MA), the trithiocarbonate acts as the chain transfer agent (CTA) (1), while 2,4,6-tris(p-methoxyphenyl) pyrylium tetrafluoroborate (2) and Ir(ppy)3 (3) act as oxidizing photocatalyst and reducing photocatalyst, respectively. As shown in Figure 7.4, the green light (520 nm) irradiation exclusively induces the cationic photopolymerization, whereas the successive exposure from green light to blue light (450 nm) could produce a tapered di-block copolymer. Notably, a multiblock copolymer is created with only blue light irradiation, under the same reaction condition [32]. Additionally, adjusting the ratios between the two photocatalysts could further afford complementary chemical control over these two photopolymerization reactions to design elaborate polymeric structures.

7.2.5 Sequential photopolymerization of IPN by UV/visible light Interpenetrating polymer networks (IPNs) are composed of two (or more) distinct polymer networks that are held together by mutual entanglements within the same polymerization system [33]. For the simultaneous formation of IPN, the two polymer networks are formed at the same time. On the contrary, for the sequential formation of IPN, one stage network is constructed first; and then, at a later stage, swollen unreacted moieties are reacted to form a secondary cross-linking system [34]. A photobase generator (NPPOC-TMG) and a photoinitiator (Irgacure 2959) are selected as the appropriate pair for sequential thiol-Michael polymerization and free radical polymerization of acrylates upon visible/UV light irradiation, respectively. As shown in Figure 7.5, in a nonstoichiometric thiol-acrylate system with excess acrylate, the thiol-Michael polymerization is initiated by photobase catalyst upon exposure to

400–500 nm

PETMP

TCDDA

1st Stage: photobase induced loosely thiolMichael polymer network formation

365 nm

2nd Stage: radical initiator induced highly cross-linked polymer network formation

Figure 7.5: Mechanism of wavelength-selective thiol-Michael/free radical homopolymer network formation controlled with different wavelength induction lights (400–500 nm and 365 nm). Reproduced with permission from [35]. Copyright 2017 American Chemical Society.

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visible light (400–500 nm) in the first stage, resulting in a loosely cross-linked network. Then, for the second-stage reaction, the subsequent free radical homopolymerization of the remaining excess acrylate moieties is initiated by radical initiator upon UV exposure (365 nm), and finally, a highly cross-linked network material is formed with both high stiffness and high glass transition temperature [35]. In addition to the thiol-acrylate polymerization system discussed earlier, many other types of polymerizations could also be controllably induced by a similar initiation system to generate various IPN structures.

7.3 Dual wavelength systems for 3D printing 7.3.1 Spatial photopatterning of soft matter materials with two colors of light using a photomask The photocuring of soft matter materials has found applications spanning from traditional coatings to high-performance dental materials. Particular attention has been paid to network systems formed from light-activated (spatially resolved) functionalization step with two wavelengths independently, allowing access to disparate materials from one resist resin, simply by selecting a specific wavelength, which is highly attractive [36, 37]. Two photoactive groups can be dimerized upon specific wavelength: o-methyl benzaldehyde (o-MBA) upon UV and styrylpyrene (StyP) upon visible light irradiation, respectively. Interestingly, these two dimerization reactions are orthogonal: the dimerization of o-MBA is exclusively initiated at 330 nm, while the StyP is solely dimerized at 435 nm. In other words, each wavelength is highly selective towards a specific photoligation reaction within the resist, and the shorter wavelength cannot induce ligation of the longer wavelength selective species within the same resist mixture, and vice versa. Therefore, the selective dimerization of these two reactive species, that is, o-MBA or StyP attached to poly (methyl methacrylate) (PMMA) chains, can induce the exclusive network formation of one polymer component, as shown in Figure 7.6. This orthogonal photodimerization technology allows selective curing of one out of the two soft matter materials by simply selecting the irradiation wavelength [38]. For each wavelength, one half of the photomask is blocked, resulting in spatially resolved patterns in predetermined areas. This dual-color addressable resist system can find a vast application ranging from coatings to 3D additive manufacturing of multimaterial architectures by simply selecting the curing wavelength.

Figure 7.6: Laterally resolved patterning of the wavelength-orthogonal multimaterial resist with a photomask by two colors of light: UV (λ1 = 330 nm) for o-MBA and visible light (λ2 = 435 nm) for StyP, respectively. Adapted with permission from [38]. Copyright 2019 Wiley-Blackwell.

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7.3.2 3D printing of polydiacetylene (PDA) photocomposite materials: 2 wavelengths for 2 orthogonal chemistries Polydiacetylene (PDA), whose backbone is composed of alternating ene-yne conjugated structure, is well known for its outstanding chromatic transition (usually from blue to red) and fluorescence enhancement effects upon exposure to various environmental stimuli, including heat, pH, organic solvents, biomolecular affinity, and so on [39]. However, due to its innate weak mechanical strength, the spatially controllable photopolymerization and subsequent 3D printing of PDA has seldom been reported, in spite of the traditional photomask method [40]. Therefore, the preparation of PDA photocomposite materials based on polyacrylate is first reported by taking advantage of the dual wavelengths photopolymerization strategy and orthogonal chemistry. Firstly, the diacetylene (DA) microaggregates are homogeneously dispersed in acrylate resin. Then, a visible LED@405 nm is used for the free radical polymerization of polyacrylate. Finally, UV irradiation is used to induce the 1,4-topotactic polymerization of PDA, which can illustrate a successive blue-topurple-to-red colorimetric transition, in response to increment in the temperature gradient. Interestingly and more importantly, thanks to the acrylate matrix resin, 3D printing technique can be applied directly to this composite material, in a laser dioxide at 405 nm and UV combination, and successfully realizes the spatially controlled polymerization of PDA (Figure 7.7). Using this method, many macroscopically three dimensional patterns can be fabricated with high resolution, which also

Figure 7.7: Numerical optical microscopy observation of the 3D printing stereo pattern: (a) top surface morphology; (b) 3-D overall appearance in color pattern, respectively. (c) In-situ photographs of the generated stereo patterns: UV- induced topopolymerization of PDA and following colorimetric change from blue to red upon heated to 80 °C (from left to right). Reproduced with permission from [41]. Copyright 2020 American Chemical Society.

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show thermochromic behavior [41]. This novel kind of functional photocomposite material demonstrates a wide application prospect in various fields, such as laser writing, colorimetric sensing, information encryption, anti-counterfeiting, and so on.

7.3.3 Continuous and rapid additive manufacturing by volumetric polymerization inhibition patterning Traditional layer-wise additive manufacturing approaches often yield objects with ridged surfaces and at sluggish fabrication rates. Use of continuous stereolithographic printing can overcome the disadvantages of layer-wise operation and generate objects that have smooth surfaces and achievable high printing speeds [42, 43]. In a complex photoinitiating system, camphorquinone (CQ) and ethyl 4-(dimethylamino)benzoate (EDAB) act as a visible-light photoinitiator and co-initiator, while bis[2-(o-chlorophenyl)- 4,5-diphenylimidazole] (o-Cl-HABI) acts as a UV photoinhibitor, respectively. Therefore, the polymerization of acrylate resin can be selectively initiated with blue light (458 nm) irradiation and inhibited by UV light (365 nm), in the presence of the complementary photoinitiator and photoinhibitor species. A unique feature of this method can achieve volumetric patterning through the usage of a multicolor system (UV and blue light) with both polymerization initiation and polymerization inhibition species. The thickness of the polymerization inhibition volume can be controlled by varying the ratio of the intensities of the two illuminating light sources and the concentration of the UV absorber. When both UV and blue light are supplied to the resin, an inhibition volume with no polymerization is generated adjacent to the window. Above this region, polymerization occurs, allowing the continuous printing of objects. Notably, in conjunction with high photoinitiation rates, the large inhibition volume can facilitate continuous and rapid object printing [44]. The proposed concurrent photoinitiation and photoinhibition process overcomes numerous time-consuming steps currently used in micro fabrication, and allows single-step fabrication of cured materials with intricate surface topographies and rapid generation of personalized 3D products.

7.3.4 One-step, multimaterial 3D printing by solution mask liquid lithography Traditional objects printing systems use only a single starting material, while multimaterial printing (e.g., polymer–polymer composites) can be particularly attractive in designing advanced materials with unique properties. The development of effective mixture resins incorporating multiple photochemical reactions that allow spatial resolution and orthogonal cross-linking has been a major challenge to realize this goal [45, 46]. The solution mask liquid lithography (SMaLL) approach can be

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used for the fabrication of 3D objects with spatially resolved mechanical and chemical properties. This novel methodology takes advantage of coherent photobleaching fronts arising from the use of photochromic molecules (solution masks) to provide rapid build rates, large depths of cure without the need for moving parts, excellent feature resolution and 3D objects without layering defects. In a mixture resin system, camphorquinone (CQ) is examined as the photosensitizer, since it is capable of inducing both radical and cationic polymerizations. Through wavelength-selective activation, blue light (λmax ≈ 470 nm) or green light (λmax ≈ 530 nm), this SMaLL methodology is used to induce selective radical/cationic dual photocuring, producing 3D objects with a range of mechanical properties (≈4 orders of magnitude variation in moduli) from commercially viable monomer systems. The coupling of these photoswitches with resin mixtures containing orthogonal photo-cross-linking systems allows simultaneous and selective curing of multiple networks, providing access to 3D objects with chemically and mechanically distinct domains. The versatility of the SMaLL method can be demonstrated through the fabrication of a bioinspired butterfly template, which has soft and flexible joints to connect the stiff and structural “wings” and “body” parts, as shown in Figure 7.8. Interestingly, upon application of tension to the “wings,” local strain and flexing is observed selectively in the flexible, acrylate-based joint regions while the “body” and “wings” remain rigid due to the stiff epoxy domains [47].

Mixed resin 470 nm Exposure

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Soft, rubbery radical only

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(d) Local strain

2 mm

Figure 7.8: (Up) Schematic illustration of network structure formation: blue exposure (470 nm) leads to dual radical and cationic curing, while green exposure (530 nm) results in only radical cross-linking. (Down) Digital butterfly design: blue areas correlate to stiff sections and green areas correlate to soft joints. Adapted with permission from [47]. Copyright 2018 Wiley-Blackwell.

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7.3.5 Multimaterial actinic spatial control with UV/visible light Additive manufacturing could make production of multimaterial objects with varied mechanical properties possible. However, achieving multimaterial control along all three axes of printing [48, 49] still remains a huge challenge. A multiwavelength photopolymerization method utilizing multimaterial actinic spatial control (MASC) during additive manufacturing is developed to provide chemoselective wavelength control over material composition. Notably, the radical and cationic photopolymerization of acrylate and epoxide can be viewed as orthogonal upon different wavelengths with specific photoinitiators. Figure 7.9 shows a multicomponent photoresin including both acrylate- and epoxide-based monomers as well as corresponding radical and cationic photoinitiators. Specifically, under long wavelength (visible light) irradiation, the Irgacure initiator (Irgacure 819) can be selectively activated, leading to preferential curing of acrylate-based resin components with free radical polymerization

Figure 7.9: (a) MASC formulation for the synthesis of multimaterials with different wavelengths of light; (b) computer-aided design (CAD) model; and (c) printed tetragonal lattice: purple part corresponds to regions printed with UV light and white/grey part corresponds to visible light. Adapted with permission from [50]. Copyright 2019 Nature Publishing Group.

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mechanism; however, under short wavelength (UV) irradiation, a combination of free radical polymerization and cationic polymerization is observed. This wavelength-dependent photopolymerization enables the production of multimaterials containing both stiff epoxide networks and flexible, elastomeric acrylate parts. Also, the variation in the photoresin formulation with different wavelengths can drastically adjust the mechanical properties of printed samples. In a tetragonal lattice design, the x-axis is printed with UV light to produce stiff rows, the z-axis is prepared predominately with visible light to produce a viscoelastic part, while the y-axis is printed with either UV or visible light to provide a composite response. Compression along these three axes reveals different degrees of stiffness and flexibility [50]. Besides, the spatially controlled swelling behavior of the printed objects through the MASC method can create multimaterial actuators for the purpose of 4D printing, considering the various swelling ratios in deionized water or organic solvents for the multimaterials components fabricated with either UV or visible light.

7.4 Conclusions and perspectives 3D printing techniques are now quite well established. However, for the photopolymerization approaches, it mainly depends on the use of a single wavelength (@405 nm). We have shown here that the use of multiple wavelengths can be very attractive for the introduction of new functionalities in 3D printed materials. Although this approach remains in an infant stage, important developments are underway, leading to new materials.

References [1] [2] [3]

[4] [5]

[6]

Tay, Y. W., Panda, B., Paul, S. C., Tan, M. J., Qian, S., Leong, K. F., Chua, C. K. Processing and properties of construction materials for 3D printing, Mater Sci Forum, 2016, 861, 177–181. Quan, H., Zhang, T., Xu, H., Luo, S., Nie, J., Zhu, X. Photo-curing 3D printing technique and its challenges, Bioact Mater, 2020, 5, 110–115. Sanders, P., Young, A. J., Qin, Y., Fancey, K. S., Reithofer, M. R., Guillet-Nicolas, R., Kleitz, F., Pamme, N., Chin, J. M. Stereolithographic 3D printing of extrinsically self-healing composites, Sci Rep, 2019, 9, 388. Wang, X., Jiang, M., Zhou, Z., Gou, J., Hui, D. 3D printing of polymer matrix composites: A review and prospective, Compos Part B, 2017, 110, 442–458. Ji, Z., Zhang, X., Yan, C., Jia, X., Xia, Y., Wang, X., Zhou, F. 3D printing of photocuring elastomers with excellent mechanical strength and resilience, Macromol Rapid Commun, 2019, 40, 1800873. Liu, Y., Lin, Y., Jiao, T., Lu, G., Liu, J. Photocurable modification of inorganic fillers and their application in photopolymers for 3D printing, Polym Chem, 2019, 10, 6350–6359.

246

[7] [8] [9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21] [22]

Yang-Yang Xu et al.

Guo, Y., Ji, Z., Zhang, Y., Wang, X., Zhou, F. Solvent-free and photocurable polyimide inks for 3D printing, J Mater Chem A, 2017, 5, 16307–16314. Jasinski, F., Zetterlund, P. B., Braun, A. M., Chemtob, A. Photopolymerization in dispersed systems, Prog Polym Sci, 2018, 84, 47–88. Kasisomayajula, S., Jadhav, N., Gelling, V. J. Conductive polypyrrole and acrylate nanocomposite coatings: Mechanistic study on simultaneous photopolymerization, Prog Org Coat, 2016, 101, 440–454. Chemtob, A., Feillée, N., Ley, C., Ponche, A., Rigolet, S., Soraru, C., Ploux, L., Nouen, D. L. Oxidative photopolymerization of thiol-terminated polysulfide resins. Application in antibacterial coatings, Prog Org Coat, 2018, 121, 80–88. Riad, K. B., Arnold, A. A., Claverie, J. P., Hoa, S. V., Wood-Adams, P. M. Photopolymerization using metal oxide semiconducting nanoparticles for epoxy-based coatings and patterned films, ACS Appl Nano Mater, 2020, 3, 2875–2880. Wang, D., Szillat, F., Fouassier, J. P., Lalevée, J. Remarkable versatility of Silane/Iodonium salt as redox free radical, cationic, and photopolymerization initiators, Macromolecules, 2019, 52, 5638–5645. Hola, E., Pilch, M., Galek, M., Ortyl, J. New versatile bimolecular photoinitiating systems based on amino-m-terphenyl derivatives for cationic, free-radical and thiol-ene photopolymerization under low intensity U -A and visible light sources, Polym Chem, 2020, 11, 480–495. Eibel, A., Fast, D. E., Sattelkow, J., Zalibera, M., Wang, J., Huber, A., Müller, G., Neshchadin, D., Dietliker, K., Plank, H., Grützmacher, H., Gescheidt, G. Star-shaped polymers through simple wavelength -selective free radical photopolymerization, Angew Chem Int Ed, 2017, 56, 14306–14309. Kaupp, M., Hiltebrandt, K., Trouillet, V., Mueller, P., Quick, A. S., Wegener, M., BarnerKowollik, C. Wavelength selective polymer network formation of end-functional star polymers, Chem Commun, 2016, 52, 1975–1978. Zhang, J., Hill, N. S., Lalevée, J., Fouassier, J.-P., Zhao, J., Graff, B., Schmidt, T. W., Kable, S. H., Stenzel, M. H., Coote, M. L., Xiao, P. Multihydroxy-anthraquinone derivatives as free radical and cationic photoinitiators of various photopolymerization under green LED, Macromol Rapid Commun, 2018, 39, 1800172. Wang, D., Kaya, K., Garra, P., Fouassier, J.-P., Graff, B., Yagci, Y., Lalevée, J. Sulfonium salt based charge transfer complexes as dual thermal and photochemical polymerization initiators for composites and 3D printing, Polym Chem, 2019, 10, 4690–4698. Dumur, F., Gigmes, D., Fouassier, J.-P., Lalevée, J. Organic electronics: An El dorado in the quest of new photocatalysts for polymerization reactions, Acc Chem Res, 2016, 49, 1980–1989. Sun, K., Xu, Y., Dumur, F., Morlet-Savary, F., Chen, H., Dietlin, C., Graff, B., Lalevée, J., Xiao, P. In silico rational design by molecular modeling of new ketones as photoinitiators in threecomponent photoinitiating systems: Application in 3D printing, Polym Chem, 2020, 11, 2230–2242. Hurrle, S., Lauer, A., Gliemann, H., Mutlu, H., Wöll, C., Goldmann, A. S., Barner-Kowollik, C. Two-in-one: λ-orthogonal photochemistry on a radical photoinitiating system, Macromol Rapid Commun, 2017, 38, 1600598. Kolb, H. C., Finn, M. G., Sharpless, K. B. Click chemistry: Diverse chemical function from a few good reactions, Angew Chem Int Ed, 2001, 40, 2004–2021. Lutz, J.-F. 1,3-Dipolar cycloadditions of azides and alkynes: A universal ligation tool in polymer and materials science, Angew Chem Int Ed, 2007, 46, 1018–1025.

Chapter 7 Dual wavelength systems in 3D printing

247

[23] Hiltebrandt, K., Pauloehrl, T., Blinco, J. P., Linkert, K., Borner, H. G., Barner-Kowollik, C. λ-Orthogonal pericyclic macromolecular photoligation, Angew Chem Int Ed, 2015, 54, 2838–2843. [24] Chaffey-Millar, H., Stenzel, M. H., Davis, T. P., Coote, M. L., Barner-Kowollik, C. Design criteria for star polymer formation processes via living free radical polymerization, Macromolecules, 2006, 39, 6406–6419. [25] Wenn, B., Martens, A. C., Chuang, Y. M., Gruber, J., Junkers, T. Efficient multiblock star polymer synthesis from photo-induced copper-mediated polymerization with up to 21 arms, Polym Chem, 2016, 7, 2720–2727. [26] Hiltebrandt, K., Kaupp, M., Molle, E., Menzel, J. P., Blinco, J. P., Barner-Kowollik, C. Star polymer synthesis via λ-orthogonal photochemistry, Chem Commun, 2016, 52, 9426–9429. [27] Qiu, W., Li, M., Yang, Y., Li, Z., Dietliker, K. Cleavable coumarin-based oxime esters with terminal heterocyclic moieties: Photobleachable initiators for deep photocuring under visible LED light irradiation, Polym Chem, 2020, 11, 1356–1363. [28] Li, Y.-H., Chen, Y.-C. Triphenylamine-hexaarylbiimidazole derivatives as hydrogen-acceptor photoinitiators for free radical photopolymerization under UV and LED light, Polym Chem, 2020, 11, 1504–1513. [29] Fu, C., Xu, J., Boyer, C. Photoacid-mediated ring opening polymerization driven by visible light, Chem Commun, 2016, 52, 7126–7129. [30] Chen, M., Deng, S., Gu, Y., Lin, J., MacLeod, M. J., Johnson, J. A. Logic-controlled radical polymerization with heat and light: Multiple-stimuli switching of polymer chain growth via a recyclable, thermally responsive gel photoredox catalyst, J Am Chem Soc, 2017, 139, 2257–2266. [31] Xu, J., Shanmugam, S., Fu, C., Aguey-Zinsou, K.-F., Boyer, C. Selective photoactivation: From a single unit monomer insertion reaction to controlled polymer architectures, J Am Chem Soc, 2016, 138, 3094–3106. [32] Kottisch, V., Michaudel, Q., Fors, B. P. Photocontrolled interconversion of cationic and radical polymerizations, J Am Chem Soc, 2017, 139, 10665–10668. [33] Hola, E., Ortyl, J., Jankowska, M., Pilch, M., Galek, M., Morlet-Savary, F., Graff, B., Dietlinc, C., Lalevée, J. New bimolecular photoinitiating systems based on terphenyl derivatives as highly efficient photosensitizers for 3D printing application, Polym Chem, 2020, 11, 922–935. [34] Tehfe, M. A., Lalevee, J., Telitel, S., Contal, E., Dumur, F., Gigmes, D., Bertin, D., Nechab, M., Graff, B., Morlet-Savary, F., Fouassier, J. P. Polyaromatic structures as organo-photoinitiator catalysts for efficient visible light induced dual radical /cationic photopolymerization and interpenetrated polymer networks synthesis, Macromolecules, 2012, 45, 4454–4460. [35] Zhang, X., Xi, W., Huang, S., Long, K., Bowman, C. N. Wavelength-selective sequential polymer network formation controlled with a two-color responsive initiation system, Macromolecules, 2017, 50, 5652–5660. [36] Gräfe, D., Wickberg, A., Zieger, M. M., Wegener, M., Blasco, E., Barner-Kowollik, C. Adding chemically selective subtraction to multi-material 3D additive manufacturing, Nat Commun, 2018, 9, 2788. [37] Liang, Y., Kiick, K. L. Multifunctional lipid-coated polymer nanogels crosslinked by phototriggered Michael-type addition, Polym Chem, 2014, 5, 1728–1736. [38] Bialas, S., Michalek, L., Marschner, D. E., Krappitz, T., Wegener, M., Blinco, J., Blasco, E., Frisch, H., Barner-Kowollik, C. Access to disparate soft matter materials by curing with two colors of light, Adv Mater, 2019, 31, 1807288. [39] Weston, M., Tjandra, A. D., Chandrawati, R. Tuning chromatic response, sensitivity, and specificity of polydiacetylene-based sensors, Polym Chem, 2020, 11, 166–183.

248

Yang-Yang Xu et al.

[40] Lee, J., Seo, S., Kim, J. Rapid light-driven color transition of novel photoresponsive polydiacetylene molecules, ACS Appl Mater Interfaces, 2018, 10, 3164–3169. [41] Xu, -Y.-Y., Ding, Z.-F., Liu, F.-Y., Sun, K., Dietlin, C., Lalevée, J., Xiao, P. 3D printing of polydiacetylene photocomposite materials: Two wavelengths for two orthogonal chemistries, ACS Appl Mater Interfaces, 2020, 12, 1658–1664. [42] Tumbleston, J. R., Shirvanyants, D., Ermoshkin, N., Janusziewicz, R., Johnson, A. R., Kelly, D., Chen, K., Pinschmidt, R., Rolland, J. P., Ermoshkin, A., Samulski, E. T., DeSimone, J. M. Continuous liquid interface production of 3D objects, Science, 2015, 347, 1349–1352. [43] Zhu, W., Tringale, K. R., Woller, S. A., You, S., Johnson, S., Shen, H., Schimelman, J., Whitney, M., Steinauer, J., Xu, W., Yaksh, T. L., Nguyen, Q. T., Chen, S. Rapid continuous 3D printing of customizable peripheral nerve guidance conduits, Mater Today, 2018, 21, 951–959. [44] Beer, M. P., Laan, H. L., Cole, M. A., Whelan, R. J., Burns, M. A., Scott, T. F. Rapid, continuous additive manufacturing by volumetric polymerization inhibition patterning, Sci Adv, 2019, 5, eaau8723. [45] Studart, A. R. Additive manufacturing of biologically-inspired materials, Chem Soc Rev, 2016, 45, 359–376. [46] Compton, B. G., Lewis, J. A. 3D-printing of lightweight cellular composites, Adv Mater, 2014, 26, 5930–5935. [47] Dolinski, N. D., Page, Z. A., Callaway, E. B., Eisenreich, F., Garcia, R. V., Chavez, R., Bothman, D. P., Hecht, S., Zok, F. W., Hawker, C. J. Solution mask liquid lithography (SMaLL) for onestep, multimaterial 3D printing, Adv Mater, 2018, 30, 1800364. [48] Shusteff, M., Browar, A. E. M., Kelly, B. E., Henriksson, J., Weisgraber, T. H., Panas, R. M., Fang, N. X., Spadaccini, C. M. One-step volumetric additive manufacturing of complex polymer structures, Sci Adv, 2017, 3, eaao5496. [49] Nadgorny, M., Ameli, A. Functional polymers and nanocomposites for 3D printing of smart structures and devices, ACS Appl Mater Interfaces, 2018, 10, 17489–17507. [50] Schwartz, J. J., Boydston, A. J. Multimaterial actinic spatial control 3D and 4D printing, Nat Commun, 2019, 10, 791.

Xuewen Wang, Yunfan Yue, Nianyao Chai, Yibing Chen

Chapter 8 Functional 3D nanoprinting via femtosecond laser nonlinear lithography 8.1 Introduction Continuous development in the field of three-dimensional (3D) additive manufacturing (AM) techniques makes it possible to generate complex geometry 3D structure [1]. Driven by femtosecond direct laser writing (fs-DLW) via nonlinear lithography, the technology has reached a new level, where printing accurate and complex 3D structures in the micro/nanometer scale can be considered [2]. Up to now, it has been widely used in the fields of science and engineering, in fabrication of dielectric geometric phase optical elements, laser plasmonic coloration of metal films and laser direct printing meta-atom structures [3–6]. Currently, Two-photon lithography (TPL) via femtosecond laser is the efficient way for fabricating complex architectures, with the resolution beyond the diffraction limit [7, 8]. TPL induced by nonlinear absorption contains two critical steps: two-photon absorption and two-photon polymerization [9]. Photoresist is a key material involved in these two steps. Its solubility will change under exposure to electron beam, ion beam or femtosecond laser beam, etc., so as to achieve the targeted structures. According to the photochemical reaction mechanism, photoresist can be divided into two categories: positive and negative photoresist, whose solubility is increased or decreased after laser exposure. Different types of photoresists and their applications will be discussed in Section 4. As a simple way to fabricate function materials at micro/nanometer scale, TPL has attracted broad interest [10]. In order to fabricate more precise and controllable structures rationally, a variety of 3D printing technologies have developed, such as inkjet printing, selective laser sintering (SLS) and selective laser melting (SLM) [1].

Xuewen Wang, International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, P. R. China; Foshan Xianhu Laboratory of the Advanced Energy Science and Technology, Guangdong Laboratory, Foshan, P. R. China; State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, P. R. China Yunfan Yue, Nianyao Chai, International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, P. R. China; State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, P. R. China Yibing Cheng, Foshan Xianhu Laboratory of the Advanced Energy Science and Technology, Guangdong Laboratory, Foshan, P. R. China; State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, P. R. China https://doi.org/10.1515/9783110570588-008

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However, when compared to these technologies, TPL has more advantages. One is able to achieve controllable spatial resolution beyond diffraction limit [11, 12]. In terms of manufacturing accuracy, the process is no longer layer-to-layer, but bottom-up through moving the focal spot, which avoids precision deviations due to layer thickness. Another advantage is that the extensive materials can be used via this technique. As long as TPL technique uses photoinitiators and monomers, theoretically, a variety of materials can be photopolymerized for designed properties. In this chapter, we will focus on the functional 3D nanoprinting technique via TPL, starting from the theory to emerging applications. This systematic interpretation is helpful for readers to build a comprehensive understanding of the existing literature on this technology.

8.2 Classical theory 8.2.1 Multiphoton absorption process Multiphoton absorption is an optical phenomenon behind the TPL process. Simply, we will concentrate on two-photon absorption (TPA) which can be easily extrapolated to other number of photons. The concept of TPA, which was a two-order nonlinear optical absorption, was born in 1931, with the seminal paper of GoeppertMayer et al. [13]. It is defined as “an absorption event caused by the collective action of two or more photons, all of which must be present simultaneously to impart enough energy to drive a transition” [13]. But, limited by technology and equipment, this concept was not experimentally verified over 30 years, until the emergence of lasers, by Kaiser and Garret [14]. However, the lack of large TPA cross-section materials ( 1) objective lens immersed in oil. A more accurate expression is as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi λ n2oil − NA2 π*NA sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   λz 2 ωð z Þ = ω 0 1 + πω20

ω0 =

(8:6)

(8:7)

where noil is the refractive index of immersion oil. And, it can also be an identical quantity, as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   λ z2 2 ωð z Þ ≡ (8:8) zR + 2 nπ zR where, zR is the Rayleigh length, as follows: zR =

πω20 λ

(8:9)

When I reaches the threshold intensity Ith for initiating polymerization, that is, I(r,z) = Ith, combining eqs. (8.2), (8.6)–(8.9), the diameter D and the length L which are relative can be shown as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   I0 (8:10) D = ω0 ln Ith

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sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   ffi I0 −1 L = 2zR Ith

(8:11)

If the photopolymerization is dependent on the intensity as I ∝ IthN, and N is the order of nonlinear absorption, the D and L in nonlinear condition are as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   ffi I0 1 * (8:12) D = ω0 ln Ith N sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  1 I0 N L = 2zR −1 Ith

(8:13)

The intensity of threshold (C) is relative to the primary initiator particle density (ρ0) and threshold initiator density (ρth), as follows:   ρo (8:14) C = ln ρ0 − ρth As we know, the voxel radius is proportional to I(r) during linear absorption, and proportional to I2(r) during TPA process. Thus, after normalization, the relationship between I(r) and r is shown in Figure 8.4.

Intensity (a.u)

Single-photon absorption

Threshold Two-photon absorption

–750

–500

–250 0 250 Wavenumbers (nm)

500

750

Figure 8.4: The diagram of spot radius for single-photon (yellow) and two-photon (blue) absorption.

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The violet and blue (three types of color depths) curves represent the singlephoton absorption and two-photon absorption process, respectively. The different depths mean different nonlinear absorption curves under laser intensity changes. The red line indicates that when the light intensity changes, the diameter corresponding to the full width at half maximum (FWHM) is a fixed value, which is determined only by the focusing condition, N.A., and laser wavelength [32]. The length of the intersection of the threshold (indicated by the gray line) and the curve is the diameter of the voxel. It will change with variation in the laser intensity. In summary, we can change the radical density (adjusting different contents of photoinitiators) to change the threshold. And we are also able to adjust the laser intensity, scanning speed, laser power and exposure time to control the resolution, when fabricating [33]. After years of development, the resolution of TPL has improved well. Many groups have contributed a lot on improving the resolution, including changing the process parameters, adding radical quencher, mixing photoinitiators or other additives and using STED-like lithography. From the earlier discussion, we can confirm that the main process parameters are laser power, exposure time and the N.A. of the objective. They can easily and effectively affect the result of resolution in fabrication. Thus, more efforts on tuning these parameters were taken in order to improve the fabrication accuracy. In 2001, Kawata et al. [34] first used TPL to fabricate microdevices with feature size close to the diffraction limit. A 3D microstructure with spatial resolution of 120 nm was first reported. The bull sculpture (Figure 8.5(a)) produced by laser scanning in sub-micro scale led to the development of TPL over the next several decades. In order to control the exposure time, switching the shutter and adjusting the scanning speed are direct methods that are widely used in pinpoint scanning mode (PSM) and continuing scanning mode (CSM), respectively. Because of the lower mechanical response of the shutter, PSM needs more exposure time, which results in lower resolution. However, in CSM, the scanning speed can be easily changed to get a higher resolution. Tan’s group [33] explored the effect of laser scanning speed on line width through experiments (Figure 8.5(b)). They fabricated a fiber with feature size of sub-25 nm at a laser power of 20 mW and scanning speed up to 700 um/s. In The next year, Dong et al. [21] improved lateral spatial resolution to 20 nm by changing the exposure time. Besides, the AR, which is critical for fine 3D micro/nanodevices fabrication was also reduced to 1.38. Wang et al. [35] demonstrated a new strategy to fabricate suspended nanonetworks with feature size below 10 nm and nearly 7 nm in width, taking advantage of nonlinear lithography. In this work, they focused on exploiting initial stages of cross-linking in the photoresist (IP-Dip, Nanoscribe Gmbh) (Figure 8.5(c)). They investigated a method to increase resolution up to several nanometers by research for sub-threshold conditions. The supporting walls were fabricated first, which leads

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Figure 8.5: (a) Bull sculpture produced by raster scanning. Reproduced with permission from [34]. Copyright 2001 Springer Nature BV. (b) The relationship between the feature size of the suspended fibers and architectures. Reproduced with permission from [33]. Copyright 2007 American Institute of Physics. (c) Schematic and fabricated results from different cross-linking process. (i) – (ii) Random nucleation cross-linkers are distributed homogeneously. (iii) – (iv) Regular cross-linking: laser exposure well above the threshold results in cross-linkers distributed in high concentrations. (d) Schematic of 3D wall structure and Scanning electronic microscopy (SEM) images of the narrowest nanoweb with 7 nm feature size. Reproduced with permission from [35]. Copyright 2018 IOP Publishing Ltd.

the cross-linkers to distribute homogeneously; then, sub-threshold exposures were used to the produce nanolines (Figure 8.5(d)). The theoretical issue of TPP has discussed earlier. As for quenching process, it is expressed in eqs. (8.15)–(8.17) which is different from TPP process [36]. The photoinitiators (PI) absorb photons from TPA chromophores to the excited state (PI*) and decompose to produced radicals (R·) at the same time. Radical quenchers (Q) replace the monomers (M) to combine with radials and then produce quenched radicals. Lastly, the RQ· can be deactivated by release of heat or irradiation. Thus, radical quenchers

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can inhibit monomer polymerization to a certain extent, in order to enhance the resolution of photocured product. PI ! PI* ! R · + R ·

(8:15)

R · + Q ! RQ ·

(8:16)

RQ · ! RQ + heat or hν

(8:17)

Figure 8.6 shows the schematic diagram of radical quenching process [36]. The radical quencher is evenly dispersed inside the photoresist. As we mentioned earlier, the intensity of laser beam approximately has a Gaussian distribution, which determines the density of radicals. At the center of the focus area, higher light intensity results in higher radical density enough to diminish or avoid the quenching effect. But as the distance goes away from the focus, the density of radical decreases, so that the quencher can terminate the early polymerization reaction and chain growth.

Figure 8.6: Schematic of the mechanism of radical quenching. Reproduced with permission from [36]. Copyright 2003 Springer Nature BV.

In 2005, Takada et al. intentionally added radical quenchers into photoresist in which the concentrations of the additive initiator and quencher were 0% and 0.8%, respectively [37]. The next year, S. H. Park et al. similarly proposed a method using 2,6-di-tert-butyl-4-methylphenol (DBMP) as a radical quencher to increase the resolution up to 100 nm, and experimentally investigated the relationship between the quenching effect and the amount of radical quencher [36]. Besides, comparing the strength of the contents of radical quencher with the original resin, they pointed out that the quenching effect would reduce the mechanical strength of polymerized structures because of their short chain lengths. From the previous content, it can be known that the feature size is also related to the polymerization threshold, in addition to the exposure time and laser power.

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According to eq. (8.14), the photoinitiator is one of the keys to determine the threshold [16, 38]. In 1999, Perry and Marder firstly put forward a series of rules for choosing photoinitiators [16]: (i) Chromophore with a large two-photon absorption cross-section (δTPA), such as D-π-D structure. When a laser beam passes through a nonlinear medium, the photoresist will absorb photons to excite the system, which will cause the attenuation of the laser beam. δTPA refers to the probability that the incident photon is captured by the target substance. (ii) High initiation efficiency. In order to initiate photopolymerization with high efficiency, a photoinitiator that can polymerize monomers with a shorter exposure time can be carefully selected, resulting in a higher resolution. (iii) A mechanism that can activate chemical functions through chromophores, such as charge transfer. In short, highly sensitive and efficient photoinitiators lead to a lower threshold, in order to increase the resolution effectively. More details on the progress of the newly developed two-photon initiators are presented in Chapter 2. Xing et al. [38] prepared a photoresist mixing 9,10-bispentyloxy-2,7-bis[2-(4dimethylamino-phenyl)-vinyl] (BPDPA, a highly sensitive and efficient photoinitiator) (Figure 8.7(a)), and dipentaerythritol hexaacrylate (DEP, cross-linker, providing amounts of polymerizable double bounds (Figure 8.7(a)). The components successfully improve lateral spatial resolution up to 80 nm (Figure 8.7(b)), under a laser power of 0.8 mW and a linear scanning speed of 50 μm/s. In addition to selecting different photoinitiators, scholars have been trying to add other additives to the photoresist to obtain specific properties. Peng et al. [39] proved that the presence of Quantum dots (QDs) can increase the lateral resolution, and they produced polymer lines that reached 75 nm (Figure 8.7(c, d)). The mechanism is that the QDs absorbed photons during writing, leading to lesser photons being absorbed by the photoinitiators. Stimulated emission depletion microscopy (STED) proposed by Stefan W. Hell in 1994 has been continuously developed over recent years, providing a lot of help for the observation and analysis of microstructures in the fields of biomedicine and materials. Inspired by this, scholars applied STED to TPP-based nonlinear lithography, which significantly improved the resolution of fabricated architectures [30]. Photon-induced mechanism that locally prohibits the formation of insoluble crosslinked polymers is suitable. There are several types of STED-like lithography according to the depletion mechanisms, as shown in Figure 8.8 [40–42]: (i) STED lithography. PI molecules are initiated to S1 during TPA process, and then are returned back to the ground state (S0) through stimulated emission (SE) in STED lithography. At the same time, a part of the PIs proceeds to the triplet (T1) via intersystem crossing (ISC) and generates R · to cross-link polymers (Figure 8.8(a)); (ii) Resolution Augmentation through Photo-Induced Deactivation (RAPID). In this process, an intermediate state will result after TPA, but upon light excitation, the intermediate state loses its activity and cannot cross-link the polymers (Figure 8.8(b)); (iii) Two-color photoinitiation/inhibition (2PII) lithography. PI molecules are excited by one of the photons through

Figure 8.7: (a) Molecular structure of photoinitiator and cross-linker. (b) SEM images of photopolymeric lines with feature size of 80 nm. Reproduced with permission from [38]. Copyright 2007 American Institute of Physics. (c) SEM images of polymer limes adding QDs with features size of 75 nm (d) Photoluminescence images of QDs-containing 3D truss lattice showing emission at 460 nm.

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SE

T1

ISC

Q.

Scavenging

R.

S0

S0 *

1

T1

Cross-linked polymer

Cross-linked polymer

RM.

Termination

RM.

R.

(e) Excited-state absorption + Resist heating Sn Tn S1* ISC S

S0

Sn

S0

Sn

(c) 2PII lithography (photoinhibitor)

S0

S0*

S1

TPA

Initiator

Initiator

TPA

Intermediate state

Non-radiative decay or non-initiating product

S0

S0*

S1

S1 *

Sn ISC

T1

Tn

R.

RM.

(d) Excited-state absorption + Non-radiative decay

S0* S0

S1

Sn

(b) RAPID lithography

TPA

(a) STED lithography S1*

TPA

Cross-linked polymer

Cross-linked polymer

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Figure 8.8: Several transitions for the different depletion mechanisms in STED-like lithography. (a) STED lithography. (b) RAPID lithography. (c) 2PII lithography. Excited-state absorption and (d)nonradiative decay; (e) resist heating. Reproduced with permission from [40]. Copyright 2014 Wiley‐VCH Verlag GmbH and. KGaA Weinheim.

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one-photon absorption (OPA) to generate radicals(R ·), which can initiate the polymerization. But another photon plays an opposite role in generating noninitiating radicals (Q ·), which limits the degree of polymer cross-linking (Figure 8.8(c)). (iv) In excitedstate absorption, after excitation, the depletion light can be absorbed by several intermediate states. From such highly excited states, the polymerization reaction occurs after the nonradiative decay to the ground state (Figure 8.8(d)). (v) During resist heating, it can be heated by repeated absorption from the excited state and by nonradiative attenuation to the same state. As the temperature rises, the properties of several resists will change and inhibit excitation, initiation or polymerization (Figure 8.8(e)). Utilizing spatial phase-shaping, Linjie Li et al. fabricated nanolines with scalable resolution of the deactivation down to 40 nm feature sizes [12]. The laser emitted two light beams to play different roles. By using visible light for TPP-DLW (780 nm) and STED 352 nm as shown in Figure 8.9, Richard Wollhofen and his group used a mixed of tri- and tetra-acrylates and 7-Diethylamino-3-thenoylcoumarin as photostarter to produce a structure with lateral sizes of 55 nm [43]. They proved that the resolution could be controlled by the components of photoresist, the position of the focal spot, the scanning speed and the power of the STED and femtosecond laser.

Figure 8.9: Setup for STED lithography. Reproduced with permission from [43]. Copyright 2013 Optical Society of America.

Figure 8.10 summarizes the development of resolution through TPP in the past 20 years. Scholars constantly seek new ways to improve resolution. Although the resolution continues to increase over time, it is limited to several nanometers. On the one hand, there are certain bottlenecks in the development of resolution. On the other hand, when the resolution reached a certain level, scholars began to turn to functional applications and manufacturing speed gradually.

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Changing process parameters Adding radical quencher Mixing photoinitiators and additives Combining STED-like lithography

Resolution (or feature size) (nm)

120

100

80

60

40

20

0 2000

2005

2010 Year

2015

2020

Figure 8.10: Improvement of resolution through 2 PPM with different methods.

8.3.3 Making 3D micro/nanoprinting faster Ultrafast laser for TPL is a powerful device for fabricating 3D micro/nanostructures. However, the printing efficiency is always a critical problem in inhibiting the development of TPL in industrial manufacturing. In order to deal with this problem, many scholars have explored several methods and devices to increase the efficiency such as multiple-beam interferometry method, multifocus parallel processing and space-time focusing technique. Hence, it has realized cross-scale micro/nano processing and manufacturing from nanometer to centimeter size, making it useful in a variety of functional fields [44]. The relative speed between focal spot and sample contributes to printing efficiency directly. In order to realize the manufacturing at the 3D scale, the sample stage usually moves in the lateral axis (x-y), and the objective moves in the axial axis (z), which results in the focal spot moving in any direction. Besides, the galvoscanners can also be rotated to tilt the wavefront of the laser beam, which results in the focal spot moving accurately (Figure 8.11). Notably, aberrations and vignetting can easily lead to displacement and distortion of the focal intensity distribution. In the previous section, we detailed the relationships between the processing size and each parameter, the printing speed is in inverse proportion to the exposure time. Recently, many scholars have proposed several works to improve printing speed, while ensuring the resolution. In 2014, Hayaski et al. used a pair of spatial light modulators

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Resist Substrate Displaced focal spot

Objective

Tip

Tilt

Structuring beam Figure 8.11: Schematic of galvanometric mirror moving in printing. Reproduced with permission from [1]. Copyright 2015 John Wiley and Sons Books.

to perform arbitrary and variable beam shaping of femtosecond lasers and demonstrated vector wave femtosecond laser processing, based on wavefront and polarization holographic processing [45]. This method provides a guidance to fabricate periodic complex nanostructures rapidly and achieve high-speed imaging via femtosecond laser, but unfortunately, the researchers did not show the resolution and printing speed accurately in detail. In 2015, taking advantage of a spatial light modulator, Yang et al. loaded computer-generated holograms (CGHs) into it, so that the beam was pre-modulated into multiple foci to achieve multifocus spots parallel processing (Figure 8.12(a)) [46]. With multiple foci, the processing time could be reduced greatly, because of the decrease of numbers of repeated 3D scanning in fabricating such spiral photonic structures. It took only 6 m to fabricate one microlens, but 20 m in the conventional single laser spot fabrication. Similarly, Abid and his group successfully printed large-area complex structures with cascade resolution and 3D profile, by using multiple exposure of two-beam laser interference with angle variation and period modulation [47]. Through experimental testing, they proved that the structure processed could simulate a variety of supernatural phenomena, such as super hydrophobicity, iridescence, directionality of reflectivity and polarization at different colors. In 2019, Wang et al. achieved fabrication efficiency up to 0.18 mm3/s (8200 voxels/s), when the scanning speed and layer spacing were 40 mm/s and 50 μm, respectively, by combing a 2D scanning galvanometer with a 2D mobile platform [48]. This motion system could lead focal spot to move to any displacement accurately, to complete 3D additive manufacturing (Figure 8.12(b)). This design would achieve high printing speed and large printing area, which could further improve printing efficiency in the future. In fact, the serial point-by-point printing set up of nonlinear lithography is too slow for many applications. Parallelization technology does not have

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Figure 8.12: (a) Schematic experimental illustration of the multiple beams by the spatial light modulator, and SEM image of fabricated “L” structure. Reproduced with permission from [46]. Copyright 2015 Elsevier Science and Technology Journals. (b) Schematic experimental illustration of the rapid printing and SEM images of printed sculpture of Albert Einstein’s head and Confucius in fused silica. Reproduced with permission from [48]. Copyright 2019 MDPI.

sub-micro resolution and cannot fabricate complex architectures. In the same year, Saha et al. realized the hierarchical parallel manufacturing based on projection by spatially and temporally focusing an ultrafast laser, which effectively solved this problem [49]. This technology has advantages that are three orders of magnitude above the existing serial technology in terms of resolution, while ensuring high manufacturing rates [49].

8.4 Photoresist In addictive manufacturing, fs-DLW uses negative photoresist a lot, hoping to be able to manufacture finely controllable structures in designated areas. In general, there are

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two basic components of photoresist: (i) monomers or a mixture of monomers and oligomers, which are the basic ingredients of forming polymers. (ii) a photoinitiator, which will absorb photons produced by laser beam and generate the active radicals to initiate polymerization [50]. They mainly include organic materials, SU-8, photosensitive hybrid materials, hydrogels, natural polymers and proteins. Their improvement determines the development and application of TPP-based nonlinear lithography to a certain extent.

8.4.1 Organic polymers Over a century of development, use of organic polymers has expanded from hightech devices to people’s daily necessities. With external stimulants, monomeric units are induced to form large polymer networks. After absorbing photons, photopolymerization is represented as chain growth and synthesis of polymers. As one of the earliest photoresists, organic polymer comprises simple raw materials and can be obtained through a single step. Most of these use acrylates as polymerization monomers, and then are mixed with an appropriate amount of photoinitiators. In 1997, Maruo et al. mixed photoinitiators, urethane acrylate monomers/oligomers to generate a photoresist in order to fabricate a spiral microstructure [51]. This was the first time that scholars used acrylate monomers in DLW-based addictive manufacturing. Taking advantage of low cost and easy availability, this type of material has been widely applied into photopolymerization. In 2008, Tayalia et al. mixed SCR368 with SR400 in the ratio 48:49, adding 3% of photoinitiators as two-monomer composition resin for manufacturing (Figure 8.13(a)) [52]. It is a pioneering work that can provide precise and independent control of architectural parameters as a model 3D extracellular matrix to research on cell adhesion and migration. Later, Weib et al. found that three different types of methacrylate photopolymerizable monomers (methacrylated oligolactones, urethane dimethacrylate and poly(ethylene glycol diacrylate)) could be used as a highly efficient photoresist and relied on fs-DLW to fabricate them into geometric structures at different scales and sizes (Figure 8.13(b)) [53]. As technology continues to mature, more and more complex structures are manufactured with a high resolution (Figure 8.13(c, d)) [54]. SU-8 plays an important role in organic polymer photoresists. It is an epoxybased negative photoresist that has been widely used in additive manufacturing [55]. Figure 8.14 shows a single chemical structure of an SU-8 molecule [56]. It can be seen that the epoxy group is bound by aromatic hydrocarbon. Thus, as opposed to normal organic photopolymers, the principle of polymerization is a cationic ring-opening process, which leads to the cyclic compound monomer being converted into a linear polymer through the ring-opening addition [57]. Benefitting from this, SU-8 has cross-linked networks with a high degree after polymerization. As a result, it has excellent chemical resistance, high temperature resistance and high dimensional

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Figure 8.13: SEM images of (a) a scaffold; (b) 2PP-derived scaffolds. Reproduced with permission from [52, 53]. Copyright 2018 and 2011 Elsevier Science and Technology Journals. SEM images of (c) A logo sized 280 μm × 280 μm with feature size of (d) 200 nm. Reproduced with permission from [54]. Copyright 2017 Elsevier Science and Technology Journals. Scale bars are 25 μm for (a), 400 μm for (b), 25 μm for (c) and 4 μm for (d).

stability. Besides, it also has a high sensitivity to other photoresists, because of the high functionality of the epoxy group, so that it has been applied in various lithography technologies. Since two-photon polymerization of SU-8 came out in 1998, scholars have applied it into a lot of researches. In the past few years, many groups have found that it is suitable for fabrication of microfluidics constructs, micro electromechanical systems and bio-applications [58]. Taking advantage of the flexibility of nonlinear lithography, SU-8 was fabricated into a complex suspended microchannel resonator [59]. In another field, SU-8 was used to design burr-like spherical micro-biorobot to carry and deliver targeted cells [60].

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O

O

CH2

CH

O

CH2

CH

CH H 2C

H2C

H 2C

O

O

O

O

C

CH3 H3C

O

H2 C

C

CH3 H3C

O

H 2C

O

C

CH3 H3C

O

CH3

H 2C CH

CH2

C

O

H 2C CH

CH2

H2 C

O

H2 C CH

CH2

CH

H2C

H2 C

H 3C

O

CH2

269

O

CH CH2

O

CH2

Figure 8.14: Chemical structure of SU-8.

8.4.2 Hybrid photosensitive resin With the development of TPL technology, scholars no longer want to limit it to high resolution in fabrication, but would like to create a variety of functional materials in 3D. However, there is a limit to selecting proper materials for AM at the micro/ nanoscale, which is especially pronounced for optical, magnetic and piezoelectric properties. Thus, scholars have paid attention to the improvement of hybrid composites [61]. As a commercial silicon-based photoresist, ORMOCER is the most widely used and studied material due to its hardness, chemical and thermal stability. It contains highly cross-linked organic networks as well as inorganic components and has been used in photonic applications. Its performance surpasses those of inorganic or polymeric materials [62]. Besides, scholars used metal-containing acrylic-based materials as polymerization monomers to synthesize organic-inorganic photoresist, in order to obtain specific properties. In essence, it still uses photopolymerization of organic components to form the framework, but will perform the properties of ceramics after pyrolysis. It provides a new way to effectively solve the problem that ceramics are difficult to

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Figure 8.15: (a) The voxel height is relative to the average laser power and Zr content in the photoresist. (b) SEM images of fabricated photonic crystal structures and the linear relation in refractive index of the material with different molar ratio. Reproduced with permission from [64]. Copyright 2008 American Chemical Society. (c) SEM images of the changes of woodpile structures after pyrolysis. (d) EDS spectrums shows only TiO2 remain after pyrolysis. Reproduced with permission from [66]. Copyright 2020 American Chemical Society.

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fabricate in complex structures at the nanoscale [63]. As a typical example, Ovsianikov et al. combined Zirconium n-propoxide with methacrylic acid to synthesize the Zr-containing acrylic photoresist, taking full advantage of the sol-gel organicinorganic hybrid technology [64]. During fabrication, these materials were similar to organic polymers, and the resolution was not only controlled by the laser power but also affected by the components’ molar ratio (Figure 8.15(a)). As a result, the refractive index of the prepared structure to light after pyrolysis could be modified by varying the molar ratio (Figure 8.15(b)). Similarly, Greer et al. added acrylic acid to titanium(IV) ethoxide to obtain a titania photoresist [65, 66]. After pyrolysis shrinking, the architecture could be transformed from polymer to ceramic, in which the carbon component in the form of carbon dioxide was completely eliminated (Figure 8.15(c, d)). Benefitting from the controlled architecture in sub-micro scale of ceramics, the titania photoresist was manufactured into cubic and truss for solar water purification [65] and 3D dielectric photonic crystals [66], respectively. To some extent, we are able to fabricate any complex ceramics in different elements by varying the hybrid of metal-based ethoxide. Till now, scholars have proposed a series of functional oxides photoresist (Al2O3, HfO2, TiO2, ZrO2, Ta2O5 and Nb2O5), which could be applied in various fields by controlling structures and resolution after shrinkage [67]. The emergence of this type of organic-inorganic photoresist preparation technology has greatly expanded the application range of nonlinear lithography.

8.4.3 Hydrogels Hydrogels are a type of extremely hydrophilic gel with 3D cross-linked network structure. They swell rapidly in water and maintain a large volume of water, without dissolving in the swelling state. The cross-linked network leads to swell and hold a large volume of water, and the degree of cross-linking determines the amount of water absorption [68]. A higher degree of crosslinking results in lower water absorption. Due to the presence of hydrophilic groups (such as -NH2, -COOH, -OH, -CONH2, -CONH and -SO3H), hydrogel monomers should exhibit sufficient water solubility and biocompatibility that resembles the extra-cellular matrix from the view of biology[69]. For a long time in the past, natural and synthetic polymers have been used as raw materials for hydrogels, and then modified natural polymers have been derived. Naturally, most kinds of hydrogels can be broadly divided into three categories: (i) natural polymers and proteins, (ii) synthetic hydrogels and (iii) modified natural hydrogels [70]. They are widely applied in cell engineering, tissue engineering and drug delivery. Natural hydrogels occupy a decisive position in simulating native extracellular matrix due to their good biocompatibility, low toxicity and easy degradability [69]. There are several kinds of organic polymers that can be used as raw materials for

Figure 8.16: (a) SEM images of dynamic caging and culture for living cells. Reproduced with permission from [71]. Copyright 2018American Chemical Society. (b) Schematic for fabricating microrobots. Reproduced with permission from [72]. Copyright 2019 John Wiley and Sons Books. (c) Schematic of swelling process in the air and in the water. Reproduced with permission from [73]. Copyright 2018 Elsevier Science and Technology Journals.

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synthetic hydrogels, such as PEGDA (the most popular), GELMA and MMP2. It is possible to fine-tune specific mechanical properties during manufacturing, because the component ratio can be precisely adjusted. Modified hydrogels are chemically modified in the laboratory to improve or adjust specific properties of natural hydrogels. The hydrogels mentioned here, along with their material properties, have been widely investigated for various bio-applications [69]. Pennacchio et al. combined acrylamide-modified gelatin B with azo cross-linker and Irgacure 369 to synthesis a kind of modified photoresist, which could be fabricated into a “dynamic” caging as cell niches (Figure 8.16(a)) [71]. One of the advantages of TPL is that we can add various components into photoresist. For example, the magnetically actuated and degradable 3D helical microrobots were fabricated by adding PETA which contains magnetic Fe3O4 nano particles(NPs) and 5-FU (Figure 8.16(b, c)) into PEGDA [72]. Using the water-absorption properties of PEG-based hydrogels to advantage, Lv et al. created a humidity-response microstructure that was able to expand when humidity changed (Figure 8.16(d)) [73].

8.4.4 Summary of photoresist development Table 8.1 shows several types of photoresists and their properties. It can be clearly seen that the development of photoresist has gone through a long process. Organic polymers are the earliest photoresists used in TPL, and they are also the most complete system, at present, that have mature commercial products (such as SU-8, SCR500 etc). The emergence of hybrid photosensitive materials fully demonstrates the wide range of applications of this technology, which provide a new idea in fabricating ceramic in complex and sub-micrometers structures. As for hydrogels, since it mainly based on PEG and other biocompatible gelatins as raw materials, it has been getting a lot of attention to the biological field. The development of photoresists is aimed to continuously improve and enhance the properties that attract various applications and will be discussed in the next section.

8.5 Applications of functional 3D nanoprinting In the past decades, photoresists and TPL have made great development in enriching types of raw materials and achieving small feature sizes [11, 95]. While much progress has been made in expanding functional applications through the years, by printing precise and complex structures, one can confer the various structures with specific properties to adapt to peculiar and novel environment. The reason why this technology can be applied into many fields is also based on its unparalleled advantages.

Shell

Acrylate resign SU- SU-

SU-

Hydrogels (synthesis)

Tissue engineering PhCs; ceramics

Bulk

D scaffold D scaffold

PEG; FeO

Process optimization Magnetic sensors

PhCs; ceramics

Woodpile

Woodpile

Cell engineering PhCs

Microrobot Woodpile

PEtOx-DA

SU- Hybrid Titanium ethoxide;SCRphotosensitive  Zirconium propoxide (ZPO); methacrylic acid (MAA) Titanium ethoxide; acrylic acid(AA) Titanium ethoxide; AA

Woodpile D scaffold D scaffold

UDMA;DLMA Cell migration / Process optimization Resonator

Biointerfaces

D scaffold

SR-; SR-

Application Process optimization Cell engineering

SCR-

Organic polymer

Structure Spiral structure D scaffold

Raw materials

Category

Table 8.1: Various photoresists and their properties.

A magnetically actuated D hydrogel microstructure was fabricated with size up to  μm.

A fabricated cubic nanostructure was applied in solar water purification. The as-fabricated materials were dense and homogeneous after pyrolysis. Improved accuracy and speed in manufacturing.

The single step fabricated suspended microchannel resonator. High magnetic field drove ability and cell viability. It was a method for combining Ti+ with urethane acrylate resigns to obtain complex ceramics. It referred the relationship between refractive index of structures and ZPO content.

It is the earliest method proposed for D microfabrication with PP. It has precise and independent control of architectural parameters to determine cell adhesion and migration. Provided a new idea for cell to be cultured in vitro close to natural conditions. Controllable of pore size and topology was prepared. SU- was used in PP, first. Fabricated in various micro/nanostructures rapidly.

Properties

year

[] 

[] 

[] 

[] 

[] 

[]  [] 

[] 

[]  []  [] 

[] 

[] 

[] 

Reference

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Hydrogels (modified)

Hydrogels (natural)

Cell engineering Cell engineering

D scaffold D scaffold

BSA

BSA D scaffold

Process optimization Process optimization Tissue engineering

D scaffold

HA-modified

Process optimization Cell-engineering

D free-form

Microswimmer Drug delivery

Microswimmer Drug delivery

D scaffold Woodpiles

Bovine serum album (BSA) Hyaluronic acid (HA)

PEG Methacrylate gelatin (GELMA) Methacrylate gelatin (GELMA) Matrix metalloproteinase

PEGDA;PETA;FeO;-FU

PEGDA

PEGDA

Microcavity

PEGDA

Drug delivery

Tissue engineering Flower-shaped Tissue engineering Pyramid-like; Tissue Dome-like engineering Microrobot Drug delivery

Microrobots

PEGDA

Specified the relationship between parameters and feature size. The PP feasibility was proved with a low writing speed.

Provided simultaneous topographical and chemical cues to cells. Improved fabrication efficiency.

It achieved controllability of drug delivery release and hyperthermia therapy. Selectivity immobilized a particular cell by cross-linking. Promoted cell adhesion, migration and toxicity reduction. It showed nontoxicity and high values of forward velocity. It represented magnetic driving and controlled enzymatic degrade response to the markers. Cross-linked various proteins in fabrication.

Improved reproducibility and endurance for humidity response. It was suitable for PH sensing in soft biological tissues.

Superparamagnetic hydrogels composites were fabricated to microdevices whose framework was biodegradable. Responded to change in humidity.

(continued )

[] 

[] 

[] 

[] 

[] 

[] 

[] 

[]  [] 

[] 

[] 

[] 

[] 

[] 

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Category

Tissue engineering Process optimization Cell engineering

D scaffold

D scaffold

D scaffold

Tissue engineering

D scaffold

Methacrylamidemodified gelation (GELMOD); HA BSA-modified

Hyaluronic acid − tyramine Acrylamide-modified gelatin

Application

Structure

Raw materials

Table 8.1 (continued )

Different degrees of cross-linking induced different swelling shrinking. Photoactuable cell confining system

It showed superior cytocompatibility.

The scaffold had excellent stability in culture medium.

Properties

year

[] 

[] 

[] 

[] 

Reference

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At present, it is widely applied in the fields of mechanical metamaterials, MEMS, electronic, biomedicine, etc. This section mainly discusses their valuable functionalized devices in various fields.

8.5.1 Micromechanics Mechanical metamaterials refer to a kind of man-made materials with special mechanical properties (such as Young’s modulus enhancement, pressure twist response, negative Poisson ratio, etc.), whose characteristics come from their precise structural design [96]. TPL has recently opened the doors to build complex mechanical metamaterial architectures in the micro/nanoscale, which were previously inaccessible. It is a prominent tool for the preparation of metamaterials in the microscopic field [97]. The progress of metamaterials development, like most other fields, is from theory to practice. As a manufacturing technology, TPL was first combined with metamaterials only to show that such structures could be manufactured, and the performance analysis mostly relies on the finite element simulation rather than actual manufacturing with mechanical test. Kadic et al. designed pentamode mechanical metamaterials based on the diamond model [96]. By adjusting the structural parameters, they found that the ratio of bulk modulus to shear modulus can reach 1000 (Figure 8.17(a)), via the finite element simulation (named COMSOL Multiphysics). Unfortunately, They only fabricated such a structure through TPL, but did not carry out more in-depth analysis and application expansion. In the case of this system, Buckmann et al., from the same group, fabricated the same polymer microstructures and conducted elastomechanical measurements that were applied in elasto-mechanical unfeelability cloaks (Figure 8.17(b)) [98]. They demonstrated that it was difficult to find the rigid cell covering a cloak with additional pentamode-metamaterials. With the development of TPL and mechanical test at the nanoscale, scholars have combined simulation with fabrication to verify the performance of microstructure mechanical metamaterials. In 2017, Frenzel et al. designed a chiral structure that could perform twists when an axial stress was applied [99]. Furthermore, they fabricated polymer samples to prove for twists properly. Portela et al. also combined fabrication with analysis to explore the impact of node geometry on the effective stiffness of 3D micro metamaterials structure (Figure 8.17(c)) [100]. In their study, the authors selected several specific parameters to fabricate and analyze to verify the correction of simulation. Dejean et al. reported a 3D nano-plate lattice structure which exhibit optimal isotropic stiffness (Figure 8.17(d)) [101]. Latter, Crook et al. followed similar approach to prepare this structure and made a compression experiments of various parameters with different relative densities (Figure 8.17(e)) [102]. Deeply, they proved that Young’s modulus and strength are relevant to the structural relative density (Figure 8.17(f)).

Figure 8.17: (a) Finite element simulation of pentamode with different structural parameters. Reproduced with permission from [96]. Copyright 2012 American Institute of Physics. (b) A mode of unfeelability cloak. Reproduced with permission from [98]. Copyright 2014 Springer Nature. (c) Relative Young’s modulus testing with different nodes combing simulation and fabrication. Reproduced with permission from [100]. Copyright 2018 Elsevier Science and Technology Journals. (d) Measured Young’s moduli for different types of plate lattices. Reproduced with permission from [101]. Copyright 2018 John Wiley and Sons. (e) Young’s modulus of cubic + octet platenanolattices with different relative density. (f) Compression experiments of pyrolytic carbon cubic + octet plate-nanolattices. Reproduced with permission from [102]. Copyright 2020 Springer Nature.

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Actually, the current application of TPL in metamaterials is to combine the finite element simulation with nonlinear lithography. It is a trend to use simulation software to systematically analyze various structural parameters of metamaterials, and then select feature parameters for microstructure manufacturing to verify the correctness of the simulation results. Of course, as a powerful tool for precise 3D manufacturing at the micro/nanoscale, TPL has been investigated for such system in order to achieve industry applications in the future. It is important to find materials that can intrinsically generate movement under stimulation, when we want to scale down machinery to the level of micro-electromechanical system (MEMS). The critical points of using TPL to fabricate MEMS are: (i) controlling the microstructure at the sub-micro scale (ii) manipulating the motion of the material under the influence of the external environment [103]. Here, we have given full play to the technical advantages of TPL based on twophoton polymerization and use materials that respond to specific stimuli to form the structure for mechanical motion [104]. Tian et al. fabricated methacrylate-based polymer microwires that the polymer networks would expand or shrink when the interfacial solvent polarity changed, and the responsive motion was reliable under repetition, several times [104]. As mentioned earlier, Lv et al. made use of the hygroscopic swelling properties of hydrogel to produce a humidity-responsive actuator (Figure 8.16(c)) [73]. Similarly, the microstructures that Scalper and coworkers designed, relying on fs-DLW, could swell under the responsibility to PH [82]. Since these processes are diffusion-controlled, the response will become slow and delayed. A type of temperature and light respond soft microrobots were produced by Hippler et al.[105]. They demonstrated that not only could the bending motion be controlled by changing the temperature of the solution, but also a spatial controlled response by inducing a local temperature increase under laser radiation (Figure 8.18(a)). Besides, the properties of liquid crystals (LCs) have also been used in TPL. LCs can flow like a liquid, but their molecules are aligned and ordered like a road (anisotropy) [106]. When energized, it is turned on and the array becomes orderly, allowing light to pass; when it is not energized, the array is chaotic, preventing light from passing. The LCs block or allow light to penetrate like a gate. In 2013, Zeng et al. demonstrated that the molecular orientation of the polymer network could be maintained when LCs phase microstructure was prepared [107]. Following previous work, Descrovi et al. fabricated a light-responsive device using a photocurable azopolymeric compound [108]. When irradiated by a light source, the fabricated film would show expansion process, then, would relax to the initial state as the irradiation was switched off (Figure 8.17(b)). Barbot et al. reported that they could fabricate a micropiston with maximum dimension of 150 nm onto the top of 140-μm diameter capillaries, utilizing TPL technology [109]. When integrated with a microgripper, this actuator could grasp and release microsphere (Figure 8.18(c)). It is a great progress in catheter actuation at the sub-micro scale.

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Figure 8.18: (a) Bright-field optical micrographs of temperature-induced actuation and SEM images of light-induced actuation. Reproduced with permission from [105]. Copyright 2019 Springer Nature. (b) Unwrapped phase distributions of light transmitted under laser off/on showing membrane edges motion. Reproduced with permission from [108]. Copyright 2018 Royal Society of Chemistry. (c) SEM image of a micropiston integrated into a compliant gripper and demonstration of fetching a microsphere.

8.5.2 Biomedical applications Since TPL technology was originally generated for other fields, it requires interdisciplinary cooperation for biomedicine research. In practical applications, it is necessary to make use of TPL to fabricate precise microstructures, but it also requires special physical, chemical and biological environments to perform the intended functions [110].

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In the previous section, we have introduced several photoresists that can be used in the biomedical field. Here, we will focus on how these materials are applied into biomedicine in combination with TPL. In cell engineering, hydrogels are widely used in femtosecond nonlinear lithography because of their better biocompatibility and lower toxicity, compared to other photoresists, as shown in Table 8.1. In order to research on the relationship between cell shape and function, hydrogels have been fabricated into a variety of complex structures to fix cells or adsorb cells to induce migration. Brigol et al. used hydrogels to prepare a 3D woodpile structure with a controllable degree of cross-linking and found that changes in the degree of cross-linking directly impact cell migration and adsorption efficiency (Figure 8.19(a)) [84]. Another effective method is to connect the specific cells together through the cross-linking network in the manufacturing process. The hydrogel line connecting cells can control the flow and migration of cells [83]. This technology can be used not only to induce migration, but also for cell fixation and shape control. Pennacchio et al. designed a new “dynamic” caging culture system that could fix cells (Figure 8.16(a)) [71]. Moreover, they found that light irradiation would change the structure of the cage, so that the shape of the cell could be modified. This research paves a new way to shape cells in the engineered niches. Mobile micro/nanorobots are one of the future biomedical research trends [111]. They are effective in controlling the directional movement of microstructures that can deliver drugs to target cells. As we mentioned earlier in actuation applications, one of the keys to drug delivery is to be able to generate a controllable driving force, such as magnetic motive force. Other essential elements are biocompatibility and biodegradability [110]. In 2016, Peters et al. fabricated a robotic device using superparamagnetic polymer composite (Figure 8.19(c)) [80]. They added Fe3O4 into hydrogels to make the device magnetic, in order to control motivation by adjusting the external magnetic field (Figure 8.19(d)), and demonstrated that it could be naturally degraded into nontoxic substances in the physiological environment. Later, a nontoxic photo-crosslinkable hydrogel gelatinmethacryloyl (GelMA) was reported and has been fabricated into biodegradable soft helical microswimmers via TPL [85]. Compared to rigid microswimmers, it could keep high values of forward velocity when the step-out frequency increased. With the deepening of research, scholars have become more accurate and precise in drug delivery. In 2019, Park, with his coworkers, investigated that actively controlled drug delivery, release and hyperthermia therapy could be enabled into microrobots with a 3D helical structure, through adding 5-fluorourcil(5-FU) [72]. In the same year, Cevlan et al. demonstrated a 3D-printed biodegradable microswimmer, which could also release other functional cargos to label specific cells, in addition to delivery of drugs to the given microenvironment [86]. This type of biodegradable microrobot can be well applied in minimally invasive surgery to reduce the patient’s treatment and recovery time as much as possible. In

Figure 8.19: (a) SEM images of woodpile structures fabricated by TPL and representative images of cell adhesion. Autofluorescence (red), phalloidin (magenta) and hoechst (blue) was used to mark woodpile structure, cells and nuclei, respectively. Reproduced with permission from [84]. Copyright 2017 Elsevier Science and Technology Journals. (b) Confocal fluorescence and SEM image of grid scaffold which mitochondrial regions of cells was selectively dyed to deep red. Reproduced with permission from [117]. Copyright 2019 American Chemical Society. (c) SEM images of fabricated microrobots (microswimmers). (d) The process of swimmers approach motion. Reproduced with permission from [80]. Copyright 2016 John Wiley and Sons Books.

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recent years, benefitting from materials and structural design, biomicrorobots have been able to perform more and more interesting functions, which is a direct result of adding TPL to various materials. Tissue engineering is a multifaceted application that combines materials, cells and biological environments. It aims to produce a scaffold as biological substitute for damaged tissue. There are several rules that should be considered before fabrication: (i) suitable biocompatibility and biodegradability, enough mechanical stability to support the attached cells; (ii) interconnected networks so that cells can be adhered and allowed to proliferate and migrate deeply; (iii) nutrients and oxygen that can easily reach cells through the network, and cellular waste products can also be easily excreted [112, 113]. As an excellent technology allowing preciseness and a high degree of freedom, TPL can fabricate a macroscopic scaffold and apply it to neural tissue engineering to help nerve repair. Koroleba et al. used photopolymerizable polylactic acid (PLA) to produce scaffolds [114]. Through a wide biocompatibility study, the photocured PLA scaffold was demonstrated to support primary cell growth, which proved that it could be applied into neural tissue engineering appropriately. Deeply benefitting from cross-linked PEGDA and hyaluronic acid, a scaffold was fabricated for efficient colonization of neuro cells and 3D confocal imaging [115]. Recently, the passage of foreign substances through the blood-brain barrier (BBB) has attracted widespread attention in the field of biomedicine [116]. Using nonlinear lithography, a type of microtube inspired by the brain capillaries was fabricated into a BBB model, and it performed well in terms of hindering dextran diffusion through the barrier. It is expected to be used in treating and diagnosing a variety of brain diseases, including brain cancer. Zheng et al. introduced a novel water-soluble photoinitiator that could effectively improve cross-section absorption [117]. And then, a hydrogel scaffold was fabricated and proved to show the potential of cell culture and observation (Figure 8.19(b)).

8.5.3 Microelectronics The preparation of high-performance 3D printable conductive materials is of great significance to the development of MEMS. Among the many 3D printing technologies, TPL has the smallest spatial resolution and plays an important role to manufacture the smallest conductive devices [118]. The critical point of making a cross-linked network with conductive properties is that it can hold nanoparticles (NPs) and allow charge transfer. The most direct way to obtain conductive properties is to add conductive components to the photoresist to form a hybrid material. Among them, a mixed photoresist composed of carbon nanotubes (CNT) has been studied a lot, because of its reasonable conductivity [119]. Staudinger et al. mixed single-walled carbon nanotubes

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(SW-CNT) with commercial photoresist equably to fabricate a few microstructures that demonstrated the viability of fabricating conductive microdevices using femtosecond laser and nanocomposites [120]. Similarly, Xiong and his coworkers developed a method to fabricate 3D conductive structures through incorporating a large-amount of well-aligned multiwalled carbon nanotubes (MWNTs) into photoresists [119]. To demonstrate the potential of TPL fabrication in MEMS, they prepared a variety of conductive microelectronic devices, including arrays of capacitors (Figure 8.20(a)) and resistors (Figure 8.20(b)), which could be fabricated on the polyethylene terephthalate (PET) substrate (Figure 8.20(c)). The network generated by the dispersed nanotubes resulted in reasonable conductivity. Their works pave the way towards fabrication of precise micro/nano architectures in applications of MEMS/NEMS.

Figure 8.20: Optical micrographs of a capacitor (a) and a resistor (b) with different structural arrays. (c) A bent PET substrate used for MEMS. (d) SEM image of perpendicular connections between four gold pads. Reproduced with permission from [119, 123]. Copyright 2016 John Wiley and Sons.

Another effective method to make fabricated structures conductive consists of photoinduced reduction of metal ions. The use of ultrafast laser can induce photoreduction of metal ions in the presence of a photosensitizer, while printing 3D structures. It has

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been investigated in directly writing metal structures in solution or mixing in a polymer matrix to form conductive architectures. Taking advantage of commercial photoresist and photoinitiator containing HAuCl4, Shukla et al. and Nakamura et al. wrote conductive patterns characterized by high conductivity [121, 122]. They demonstrated that the HAuCl4-doped photosensitizer could induce the reduction and aggregation of gold ions by two-photon absorption when the laser was irradiated, while only cross-link a small amount of SU-8. Remarkably, Blasco et al. replaced SU-8 with PEG-based photoresist that Au could be photoreduced, while two-photon polymerizing the cross-linking network. They successfully fabricated conductive 3D wires that connected different gold pads and showed conductivity as high as bulk gold (2.2 × 106 S m−1, Figure 8.20(d)) [123]. Piezoelectric effect means that when certain dielectrics are deformed by external forces in a certain direction, polarization will occur inside them, and at the same time, positive and negative charges will appear on its two opposite surfaces. In histological applications, the appearance of piezoelectric effect can stimulate cell stimulation and differentiation [10, 124]. Marino et al. fabricated bioinspired 3D structures (Osteo-prints) with a nanocomposite resist, Ormocomp, containing 10% piezoelectric barium titanate NPs (BTNPs). Under mechanical simulation, these scaffolds could enhance the osteogenic differentiation of SaOS-2 bone-like cells. This ability to feed mechanical action back to electrical stimulation was expected to be used for tissue and cell control [124].

8.5.4 Microoptics There are many interesting applications in the field of optical and photonic devices including on-demand complex (micro) structures with high refractive index (n) and tailored bandgap. The properties of light beam carrying angular momentum have been investigated from the research of quantum optics to microscopy and micromanipulation. Femtosecond laser nonlinear lithography to modify the microstructure of optical components can achieve peculiar phenomena [10]. Beresna et al. produced a radical polarizer that could generate optical vortices on a small scale, using femtosecond laser corrosion [125]. They demonstrated that the orbital angular momentum could appear in the micro voids after nonlinear lithography. Because of angular momentum conservation, the spherical interface would cause the generation of optical vortices. This technology provided a new idea for the development of radial polarizers. Following this thought, Wei et al. prepared microscale spiral phase structures that could generate orbital-angular momentum light. Moreover, for real applications, TPL could fabricate spiral phase plates at different operating wavelengths to fit various work environments [126].

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As a high-resolution 3D manufacturing technology, TPL has become the main method for the fabrication of nanophotonic devices. Photonic crystal is an artificial periodic dielectric structure that can be precisely controlled by nonlinear lithography. Rybin et al. investigated several types of 3D photonic crystals and found that the complete photonic band gap appears in the inverted opal (Figure 8.21(a)) [127]. They demonstrated that changes of complete photonic band gap were relative to different woodpile structures, which indicated the optical tunable properties via this technology. Similarly, Greer’s group proposed a new method to create complex ceramics through nonlinear lithography and pyrolysis [66]. Taking advantage of high refractive index of Ti02 and using Ti-containing photoresist, they prepared a photonic crystal with all photonic band gaps. It is worth noting that they combined controllable ceramic shrinkage with nonlinear lithography to precisely control the microscopic morphology of the photonic crystal. As a controllable technology, Purtov et al. fabricated a series of nanopillar diffraction gratings by changing laser power (Figure 8.21(b)) [128]. They proved that the color would change towards a blueish hue gradually, when the pillar size increased.

Figure 8.21: (a) Woodpile crystals with specific filling factor (w/a = 0.28) and the photonic band structure. Reproduced with permission from [127]. Copyright 2015 MDPI. (b) Optical appearance and SEM images of nanopillar gratings under different laser power. Reproduced with permission from [128]. Copyright 2018 MDPI. (c) SEM image of a photonic follower. (d) Crossed polarized micrographs of photonic flower response to temperature and humidity. RH, relative humidity. Reproduced with permission from [130]. Copyright 2020 American Chemical Society.

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TPL based on nonlinear absorption is also suitable for nanofabrication of integrated photonics devices [129]. Besides, 4D structural color microactuators also can be printed by using a photonic photoresist (Figure 8.21(c)) [130]. It would change color and structure with variation in temperature or humidity, which showed dualresponse (Figure 8.21(d)). Figure 8.21(d) shows the dynamic response in the flower structure’s height with changes in humidity or temperature. It is supposed to belong to the actuation, but it can also act as a photonic device. It proved that variations in the reflection band were related to changes of the ordered cholesteric liquid crystals (CLC), which was triggered by expansion. This is a kind of four-dimensional photon microactuator with great potential.

8.6 Conclusions and perspectives As mentioned before, TPL is based on the mature femtosecond laser technology. The development of TPL is not only depend on the establishment of two-photon absorption and two-photon polymerization theoretical system, but also on the output of femtosecond laser, a key device. As for the photoresist, scholars are no longer limited to commercial materials, but exploring a variety of methods to enrich characteristics and apply them into more fields. Starting from the principle of nonlinear lithography, this chapter introduces methods for controlling resolution and efficiency in 3D manufacturing. Later, based on recent research, several photoresists currently used have been introduced. The composition of photoresist is not complex – it must contain photoinitiator and polymerized monomer, but the preparation of photoresist with excellent performance still needs continuous exploration. The ultimate goal of these studies is to be applicable in various fields. In the past few years, researches based on this 3D additive manufacturing technology have shown explosive growth. This proves that it has a wide range applications and infinite developing potential in science and industry. Among the various 3D manufacturing technologies, TPL has the highest precision, accurately fabricating complex structures with a resolution exceeding the diffraction limit and opening a new world in architecture and applications. However, while further expanding the scope of application of nonlinear lithography, some problems still remain. TPL has lower speed of fabrication compared to other 3D additive manufacturing technologies. Moreover, it takes a tedious process and a lot of time to get a complete and usable device. These shortcomings prevent this technology from being widely used in industrial fabrication. But, we believe that it will lead the industrial micro/nanofabrication to a higher level. Scholars are also constantly optimizing the process and increasing its manufacturing efficiency. Once the conditions are met, it will inevitably be integrated into various aspects of life.

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References Hohmann, J. K., Renner, M., Waller, E. H., von Freymann, G. Three-dimensional μ-printing: An enabling technology, Adv Opt Mater, 2015, 3, 1488–1507. [2] Tommaso, B. Three-dimensional microfabrication using two-photon polymerization: fundamentals, technology, and applications, Matthew Deans: USA, 2016. [3] Zhang, Y.-L., Chen, Q.-D., Xia, H., Sun, H.-B. Designable 3D nanofabrication by femtosecond laser direct writing, Nano Today, 2010, 5, 435–448. [4] Wang, X. W., Kuchmizhak, A. A., Brasselet, E., Juodkazis, S. Dielectric geometric phase optical elements fabricated by femtosecond direct laser writing in photoresists, Appl Phys Lett, 2017, 110, 4. [5] Wang, X. W., Kuchmizhak, A., Storozhenko, D., Makarov, S., Juodkazis, S. Single-step laser plasmonic coloration of metal films, ACS Appl Mater Interfaces, 2018, 10, 1422–1427. [6] Wang, X. W., Kuchmizhak, A. A., Li, X., Juodkazis, S., Vitrik, O. B., Kulchin, Y. N., Zhakhovsky, V. V., Danilov, P. A., Ionin, A. A., Kudryashov, S. I., Rudenko, A. A., Inogamov, N. A. Laserinduced translative hydrodynamic mass snapshots: Noninvasive characterization and predictive modeling via mapping at nanoscale, Phys Rev Appl, 2017, 8, 17. [7] Malinauskas, M., Farsari, M., Piskarskas, A., Juodkazis, S. Ultrafast laser nanostructuring of photopolymers: A decade of advances, Phys Rep, 2013, 533, 1–31. [8] Ovsianikov, A., Mironov, V., Stampf, J., Liska, R. Engineering 3D cell-culture matrices: Multiphoton processing technologies for biological and tissue engineering applications, Expert Rev Med Devices, 2012, 9, 613–633. [9] Xing, J. F., Zheng, M. L., Duan, X. M. Two-photon polymerization microfabrication of hydrogels: An advanced 3D printing technology for tissue engineering and drug delivery, Chem Soc Rev, 2015, 44, 5031–5039. [10] Carlotti, M., Mattoli, V. Functional materials for two-photon polymerization in microfabrication, Small, 2019, 15, e1902687. [11] Sakellari, I., Kabouraki, E., Gray, D., Purlys, V., Fotakis, C., Pikulin, A., Bityurin, N., Vamvakaki, M., Farsari, M. Diffusion-assisted high-resolution direct femtosecond laser writing, ACS Nano, 2012, 6, 2302–2311. [12] Li, L., Gattass, R. R., Gershgoren, E., Hwang, H., Fourkas, J. T. Achieving lambda/20 resolution by one-color initiation and deactivation of polymerization, Science, 2009, 324, 910–913. [13] Goppert-Mayer, M. Uber Elementarakte mit zwei quantensprungen, 1931, 273–294. [14] Kaiser, W. K., Garrett, C. G. B. 2-Photon excitation in Caf2 – Eu2+, Phys Rev Lett, 1961, 7, 229–231. [15] Marius, A. D. B., Jean-Luc, B., Jeffrey, E., Jia-Ying, F., Ahmed, A. H., Hess, S. E., Kogej, T., Levin, M. D., Marder, S. R., McCord-Maughon, D., Perry, J. W., Rockel, H., Rumi, M., Subramaniam, G., Watt, W. W., Xiang-Li, W., Chris, X. Design of organic molecules with large two-photon absorption cross Sections, Science, 1998, 281, 1653–1656. [16] Cumpston, B. H., Ananthavel, S. P., Barlow, S., Dyer, D. L., Ehrlich, J. E., Erskine, L. L., Heikal, A. A., Kuebler, S. M., Lee, I. Y. S., McCord-Maughon, D., Qin, J., Röckel, H., Rumi, M., Wu, X.-L., Marder, S. R., Perry, J. W. Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication, Nature, 1999, 398, 51–54. [17] Wood, D. Microstereolithography and other fabrication techniques for 3D MEMS [Book Review], Eng Sci Educ J, 2002, 11, 65–65. [18] Boyd, R. W. Nonlinear optics, Third edn, Academic Press: Burlington, 2008. [19] LaFratta, C. N., Fourkas, J. T., Baldacchini, T., Farrer, R. A. Multiphoton fabrication, Angew Chem Int Ed Engl, 2007, 46, 6238–6258. [1]

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289

[20] Maruo, S., Nakamura, O., Kawata, S. Three-dimensional microfabrication with two-photonabsorbed photopolymerization, Opt Lett, 1997, 22, 132–134. [21] Sun, H.-B., Kawata, S. NMR 3D Analysis Photopolymerization. 2006. [22] Fischer, J., Wegener, M. Three-dimensional direct laser writing inspired by stimulatedemission-depletion microscopy [Invited], Opt Mater Express, 2011, 1, 614–624. [23] He, G. S., Tan, L. S., Zheng, Q., Prasad, P. N. Multiphoton absorbing materials: Molecular designs, characterizations, and applications, Chem Rev, 2008, 108, 1245–1330. [24] Wu, S., Serbin, J., Gu, M. Two-photon polymerisation for three-dimensional micro-fabrication, J Photochem Photobiol A Chem, 2006, 181, 1–11. [25] Park, S. H., Yang, D. Y., Lee, K. S. Two-photon stereolithography for realizing ultraprecise three-dimensional nano/microdevices, Laser Photon Rev, 2009, 3, 1–11. [26] Xing, J., Liu, J., Zhang, T., Zhang, L., Zheng, M., Duan, X. A water soluble initiator prepared through host-guest chemical interaction for microfabrication of 3D hydrogels via two-photon polymerization, J Mater Chem B, 2014, 2, 4318–4323. [27] Dong, X.-Z., Zhao, Z.-S., Duan, X.-M. Improving spatial resolution and reducing aspect ratio in multiphoton polymerization nanofabrication, Appl Phys Lett, 2008, 92, 091113. [28] Bourdon, L., Maurin, J.-C., Gritsch, K., Brioude, A., Salles, V. Improvements in resolution of additive manufacturing: Advances in two-photon polymerization and direct-writing electrospinning techniques, ACS Biomater Sci Eng, 2018, 4, 3927–3938. [29] Born, M. Book principles of optics – electromagnetic theory of propagation, interference and diffraction of light, Astronomische Nachrichten, 1980, 301. [30] Zhou, X., Hou, Y., Lin, J. A review on the processing accuracy of two-photon polymerization, AIP Adv, 2015, 5, 030701. [31] Yang, Z.-J., Zhang, S.-M., Li, X.-L., Pang, Z.-G. Variable sinh-Gaussian solitons in nonlocal nonlinear Schrödinger equation, Appl Math Lett, 2018, 82, 64–70. [32] Juodkazis, S., Mizeikis, V., Seet, K. K., Miwa, M., Misawa, H. Two-photon lithography of nanorods in SU-8 photoresist, Nanotechnology, 2005, 16, 846–849. [33] Tan, D., Li, Y., Qi, F., Yang, H., Gong, Q., Dong, X., Duan, X. Reduction in feature size of two-photon polymerization using SCR500, Appl Phys Lett, 2007, 90, 71106. [34] Kawata, S., Sun, H.-B., Tanaka, T., Takada, K. Finer features for functional microdevices, Nature, 2001, 412, 697–698. [35] Wang, S., Yu, Y., Liu, H., Lim, K. T. P., Srinivasan, B. M., Zhang, Y. W., Yang, J. K. W. Sub-10nm suspended nano-web formation by direct laser writing, Nano Futures, 2018, 2, 025006. [36] Park, S. H., Lim, T. W., Yang, D.-Y., Kim, R. H., Lee, K.-S. Improvement of spatial resolution in nano-stereolithography using radical quencher, Macromol Res, 2006, 14, 559–564. [37] Takada, K., Sun, H.-B., Kawata, S. Improved spatial resolution and surface roughness in photopolymerization-based laser nanowriting, Appl Phys Lett, 2005, 86, 071122. [38] Xing, J.-F., Dong, X.-Z., Chen, W.-Q., Duan, X.-M., Takeyasu, N., Tanaka, T., Kawata, S. Improving spatial resolution of two-photon microfabrication by using photoinitiator with high initiating efficiency, Appl Phys Lett, 2007, 90, 131106. [39] Peng, Y., Jradi, S., Yang, X., Dupont, M., Hamie, F., Dinh, X. Q., Sun, X. W., Xu, T., Bachelot, R. 3D photoluminescent nanostructures containing quantum dots fabricated by two-photon polymerization: Influence of quantum dots on the spatial resolution of laser writing, Adv Mater Technol, 2019, 4, 1800522. [40] Fischer, J., Wegener, M. Three-dimensional optical laser lithography beyond the diffraction limit, Laser Photon Rev, 2013, 7, 22–44. [41] Yaoyu, C., Zongsong, G., Baohua, J., Evans, R. A., Min, G. High-photosensitive resin for super-resolution direct-laser-writing based on photoinhibited polymerization, Opt Express, 2011, 19, 19486.

290

Xuewen Wang et al.

[42] Fischer, J., Von, Freymann, G., Martin, W. The materials challenge in diffraction-unlimited direct-laser-writing optical lithography, Adv Mater, 2010, 22, 3578–3582. [43] Wollhofen, R., Katzmann, J., Hrelescu, C., Jacak, J., Klar, T. A. 120 nm resolution and 55 nm structure size in STED-lithography, Opt Express, 2013, 21, 10831–10840. [44] Bae, W. G., Kim, H. N., Kim, D., Park, S. H., Jeong, H. E., Suh, K. Y. 25th anniversary article: scalable multiscale patterned structures inspired by nature: The role of hierarchy, Adv Mater, 2014, 26, 675–700. [45] Hasegawa, S., Hayasaki, Y. Holographic vector wave femtosecond laser processing, Int J Optomechatronics, 2014, 8, 73–88. [46] Yang, L., El-Tamer, A., Hinze, U., Li, J., Hu, Y., Huang, W., Chu, J., Chichkov, B. N. Parallel direct laser writing of micro-optical and photonic structures using spatial light modulator, Opt Lasers Eng, 2015, 70, 26–32. [47] Abid, M. I., Wang, L., Chen, Q.-D., Wang, X.-W., Juodkazis, S., Sun, H.-B. Angle-multiplexed optical printing of biomimetic hierarchical 3D textures, Laser Photon Rev, 2017, 11, 1600187. [48] Wang, P., Chu, W., Li, W., Tan, Y., Liu, F., Wang, M., Qi, J., Lin, J., Zhang, F., Wang, Z., Cheng, Y. Three-dimensional laser printing of macro-scale glass objects at a micro-scale resolution, Micromachines (Basel), 2019, 10, 565. [49] Saha, S. K., Wang, D., Nguyen, V. H., Chang, Y., Oakdale, J. S., Chen, S. C. Scalable submicrometer additive manufacturing, Science, 2019, 366, 105–109. [50] Selimis, A., Mironov, V., Farsari, M. Direct laser writing: Principles and materials for scaffold 3D printing, Microelectron Eng, 2015, 132, 83–89. [51] Maruo, S., Nakamura, O., Kawata, S. Three-dimensional microfabrication with two-photonabsorbed photopolymerization, Ol/22/2/ol Pdf, 1997, 22, 132–0. [52] Tayalia, P., Mendonca, C. R., Baldacchini, T., Mooney, D. J., Mazur, E. 3D cell-migration studies using two-photon engineered polymer scaffolds, Adv Mater, 2008, 20, 4494–4498. [53] Weiß, T., Schade, R., Laube, T., Berg, A., Hildebrand, G., Wyrwa, R., Schnabelrauch, M., Liefeith, K. Two-photon polymerization of biocompatible photopolymers for microstructured 3D biointerfaces, Adv Eng Mater, 2011, 13, B264–B73. [54] Ummethala, G., Jaiswal, A., Chaudhary, R. P., Hawal, S., Saxena, S., Shukla, S. Localized polymerization using single photon photoinitiators in two-photon process for fabricating subwavelength structures, Polymer, 2017, 117, 364–369. [55] Juodkazis, S., Mizeikis, V., Misawa, H. Three-dimensional microfabrication of materials by femtosecond lasers for photonics applications, J Appl Phys, 2009, 106, 051101. [56] Sabel, T. Volume hologram formation in SU-8 photoresist, Polymers (Basel), 2017, 9, 198. [57] Zhang, J., Tan, K. L., Hong, G. D., Yang, L. J., Gong, H. Q. Polymerization optimization of SU-8 photoresist and its applications in microfluidic systems and MEMS, J Micromech Microeng, 2001, 11, 20–26. [58] Ovsianikov, A., Schlie, S., Ngezahayo, A., Haverich, A., Chichkov, B. N. Two-photon polymerization technique for microfabrication of CAD-designed 3D scaffolds from commercially available photosensitive materials, J Tissue Eng Regen Med, 2007, 1, 443–449. [59] Accoto, C., Qualtieri, A., Pisanello, F., Ricciardi, C., Pirri, C. F., Vittorio, M. D., Rizzi, F. Two-photon polymerization lithography and laser doppler vibrometry of a SU-8-based suspended microchannel resonator, J Microelectromech Syst, 2015, 24, 1038–1042. [60] Li, J. Y., Li, X. J., Luo, T., Wang, R., Liu, C. C., Chen, S. X., Li, D. F., Yue, J. B., Cheng, S. H., Sun, D. Development of a magnetic microrobot for carrying and delivering targeted cells, Sci Robot, 2018, 3, 11. [61] Farsari, M., Vamvakaki, M., Chichkov, B. N. Multiphoton polymerization of hybrid materials, J Opt, 2010, 12, 124001.

Chapter 8 Functional 3D nanoprinting via femtosecond laser nonlinear lithography

291

[62] Dinca, V., Kasotakis, E., Catherine, J., Mourka, A., Ranella, A., Ovsianikov, A., Chichkov, B. N., Farsari, M., Mitraki, A., Fotakis, C. Directed three-dimensional patterning of self-assembled peptide fibrils, Nano Lett, 2008, 8, 538–543. [63] Farsari, M., Chichkov, B. N. Two-photon fabrication, Nat Photonics, 2009, 3, 450–452. [64] Ovsianikov, A., Viertl, J., Chichkov, B., Oubaha, M., MacCraith, B., Sakellari, I., Giakoumaki, A., Gray, D., Vamvakaki, M., Farsari, M., Fotakis, C. Ultra-low shrinkage hybrid photosensitive material for two-photon polymerization microfabrication, ACS Nano, 2008, 2, 2257–2262. [65] Vyatskikh, A., Kudo, A., Delalande, S., Greer, J. R. Additive manufacturing of polymer-derived titania for one-step solar water purification, Mater Today Commun, 2018, 15, 288–293. [66] Vyatskikh, A., Ng, R. C., Edwards, B., Briggs, R. M., Greer, J. R. Additive manufacturing of high-refractive-index, nanoarchitected titanium dioxide for 3D dielectric photonic crystals, Nano Lett, 2020, 20, 3513–3520. [67] Dinachali, S. S., Dumond, J., Saifullah, M. S., Ansah-Antwi, K. K., Ganesan, R., Thian, E. S., He, C. Large area, facile oxide nanofabrication via step-and-flash imprint lithography of metal-organic hybrid resins, ACS Appl Mater Interfaces, 2013, 5, 13113–13123. [68] Drury, J. L., Mooney, D. J. Hydrogels for tissue engineering: Scaffold design variables and applications, Biomaterials, 2003, 24, 4337–4351. [69] Liao, C., Wuethrich, A., Trau, M. A material odyssey for 3D nano/microstructures: Two photon polymerization based nanolithography in bioapplications, Appl Mater Today, 2020, 19, 100635. [70] You, S., Li, J., Zhu, W., Yu, C., Mei, D., Chen, S. Nanoscale 3D printing of hydrogels for cellular tissue engineering, J Mater Chem B, 2018, 6, 2187–2197. [71] Pennacchio, F. A., Fedele, C., De Martino, S., Cavalli, S., Vecchione, R., Netti, P. A. Threedimensional microstructured azobenzene-containing gelatin as a photoactuable cell confining system, ACS Appl Mater Interfaces, 2018, 10, 91–97. [72] Park, J., Jin, C., Lee, S., Kim, J. Y., Choi, H. Magnetically actuated degradable microrobots for actively controlled drug release and hyperthermia therapy, Adv Healthc Mater, 2019, 8, e1900213. [73] Lv, C., Sun, X.-C., Xia, H., Yu, Y.-H., Wang, G., Cao, X.-W., Li, S.-X., Wang, Y.-S., Chen, Q.-D., Yu, Y.-D., Sun, H.-B. Humidity-responsive actuation of programmable hydrogel microstructures based on 3D printing, Sens Actuators B Chem, 2018, 259, 736–744. [74] Olsen, M. H., Hjorto, G. M., Hansen, M., Met, O., Svane, I. M., Larsen, N. B. In-chip fabrication of free-form 3D constructs for directed cell migration analysis, Lab Chip, 2013, 13, 4800–4809. [75] Witzgall, V., Yablonovitch, D. Single-shot two-photon exposure of commercial photoresist for the production of three-dimensional structures, Opt Lett, 1998, 23, 1745–1747. [76] Teh, W. H., Dürig, U., Salis, G., Harbers, R., Drechsler, U., Mahrt, R. F., Smith, C. G., Güntherodt, H. J. SU-8 for real three-dimensional subdiffraction-limit two-photon microfabrication, Appl Phys Lett, 2004, 84, 4095–4097. [77] Duan, X.-M., Sun, H.-B., Kaneko, K., Kawata, S. Two-photon polymerization of metal ions doped acrylate monomers and oligomers for three-dimensional structure fabrication, Thin Solid Films, 2004, 453–454, 518–521. [78] Wloka, T., Czich, S., Kleinsteuber, M., Moek, E., Weber, C., Gottschaldt, M., Liefeith, K., Schubert, U. S. Microfabrication of 3D-hydrogels via two-photon polymerization of poly(2-ethyl-2-oxazoline) diacrylates, Eur Polym J, 2020, 122, 109295. [79] Xiong, Z., Zheng, C., Jin, F., Wei, R., Zhao, Y., Gao, X., Xia, Y., Dong, X., Zheng, M., Duan, X. Magnetic-field-driven ultra-small 3D hydrogel microstructures: Preparation of gel photoresist and two-photon polymerization microfabrication, Sens Actuators B Chem, 2018, 274, 541–550.

292

Xuewen Wang et al.

[80] Peters, C., Hoop, M., Pane, S., Nelson, B. J., Hierold, C. Degradable magnetic composites for minimally invasive interventions: Device fabrication, targeted drug delivery, and cytotoxicity tests, Adv Mater, 2016, 28, 533–538. [81] Huang, Q.-L., Xu, H.-L., Li, M.-T., Hou, Z.-S., Lv, C., Zhan, X.-P., Li, H.-L., Xia, H., Wang, H.-Y., Sun, H.-B. Stretchable PEG-DA hydrogel-based whispering-gallery-mode microlaser with humidity responsiveness, J Lightwave Technol, 2018, 36, 819–824. [82] Scarpa, E., Lemma, E. D., Fiammengo, R., Cipolla, M. P., Pisanello, F., Rizzi, F., De Vittorio, M. Microfabrication of pH-responsive 3D hydrogel structures via two-photon polymerization of high-molecular-weight poly(ethylene glycol) diacrylates, Sens Actuators B Chem, 2019, 279, 418–426. [83] Hasselmann, N. F., Hackmann, M. J., Horn, W. Two-photon fabrication of hydrogel microstructures for excitation and immobilization of cells, Biomed Microdevices, 2017, 20, 8. [84] Brigo, L., Urciuolo, A., Giulitti, S., Della Giustina, G., Tromayer, M., Liska, R., Elvassore, N., Brusatin, G. 3D high-resolution two-photon crosslinked hydrogel structures for biological studies, Acta Biomater, 2017, 55, 373–384. [85] Wang, X., Qin, X.-H., Hu, C., Terzopoulou, A., Chen, X.-Z., Huang, T.-Y., Maniura-Weber, K., Pané, S., Nelson, B. J. 3D printed enzymatically biodegradable soft helical microswimmers, Adv Funct Mater, 2018, 28, 1804107. [86] Ceylan, H., Yasa, I. C., Yasa, O., Tabak, A. F., Giltinan, J., Sitti, M. 3D-printed biodegradable microswimmer for theranostic cargo delivery and release, ACS Nano, 2019, 13, 3353–3362. [87] Pitts, J. D., Howell, A. R., Taboada, R., Banerjee, I., Wang, J., Goodman, S. L., Campagnola, P. J. New photoactivators for multiphoton excited three-dimensional submicron cross-linking of proteins: Bovine serum albumin and type 1 collagen¶†, Photochem Photobiol, 2002, 76, 135–144. [88] Seidlits, S. K., Schmidt, C. E., Shear, J. B. High-resolution patterning of hydrogels in three dimensions using direct-write photofabrication for cell guidance, Adv Funct Mater, 2009, 19, 3543–3551. [89] Ritschdorff, E. T., Shear, J. B. Multiphoton lithography using a high-repetition rate microchip laser, Anal Chem, 2010, 82, 8733–8737. [90] Turunen, S., Kapyla, E., Terzaki, K., Viitanen, J., Fotakis, C., Kellomaki, M., Farsari, M. Picoand femtosecond laser-induced crosslinking of protein microstructures: Evaluation of processability and bioactivity, Biofabrication, 2011, 3, 045002. [91] Berg, A., Wyrwa, R., Weisser, J., Weiss, T., Schade, R., Hildebrand, G., Liefeith, K., Schneider, B., Ellinger, R., Schnabelrauch, M. Synthesis of photopolymerizable hydrophilic macromers and evaluation of their applicability as reactive resin components for the fabrication of threedimensionally structured hydrogel matrices by 2-photon-polymerization, Adv Eng Mater, 2011, 13, B274–B84. [92] Ovsianikov, A., Deiwick, A., Van Vlierberghe, S., Dubruel, P., Moller, L., Drager, G., Chichkov, B. Laser fabrication of three-dimensional CAD scaffolds from photosensitive gelatin for applications in tissue engineering, Biomacromolecules, 2011, 12, 851–858. [93] Qin, X.-H., Torgersen, J., Saf, R., Mühleder, S., Pucher, N., Ligon, S. C., Holnthoner, W., Redl, H., Ovsianikov, A., Stampfl, J., Liska, R. Three-dimensional microfabrication of protein hydrogels via two-photon-excited thiol-vinyl ester photopolymerization, J Polym Sci A Polym Chem, 2013, 51, 4799–4810. [94] Loebel, C., Broguiere, N., Alini, M., Zenobi-Wong, M., Eglin, D. Microfabrication of photo-cross-linked hyaluronan hydrogels by single- and two-photon tyramine oxidation, Biomacromolecules, 2015, 16, 2624–2630. [95] Yulianto, E., Chatterjee, S., Purlys, V., Mizeikis, V. Imaging of latent three-dimensional exposure patterns created by direct laser writing in photoresists, Appl Surf Sci, 2019, 479, 822–827.

Chapter 8 Functional 3D nanoprinting via femtosecond laser nonlinear lithography

293

[96] Kaschke, J., Wegener, M. Optical and infrared helical metamaterials, Nanophotonics, 2016, 5, 510–523. [97] Kadic, M., Bückmann, T., Stenger, N., Thiel, M., Wegener, M. On the practicability of pentamode mechanical metamaterials, Appl Phys Lett, 2012, 100, 191901. [98] Buckmann, T., Thiel, M., Kadic, M., Schittny, R., Wegener, M. An elasto-mechanical unfeelability cloak made of pentamode metamaterials, Nat Commun, 2014, 5, 4130. [99] Frenzel, T., Kadic, M., Wegener, M. Three-dimensional mechanical metamaterials with a twist, Science, 2017, 358, 1072–1074. [100] Portela, C. M., Greer, J. R., Kochmann, D. M. Impact of node geometry on the effective stiffness of non-slender three-dimensional truss lattice architectures, Extreme Mech Lett, 2018, 22, 138–148. [101] Tancogne-Dejean, T., Diamantopoulou, M., Gorji, M. B., Bonatti, C., Mohr, D. 3D platelattices: An emerging class of low-density metamaterial exhibiting optimal isotropic stiffness, Adv Mater, 2018, 30, e1803334. [102] Crook, C., Bauer, J., Guell Izard, A., Santos de Oliveira, C., Martins de Souza, E. S. J., Berger, J. B., Valdevit, L. Plate-nanolattices at the theoretical limit of stiffness and strength, Nat Commun, 2020, 11, 1579. [103] Oakdale, J. S., Smith, R. F., Forien, J.-B., Smith, W. L., Ali, S. J., Bayu Aji, L. B., Willey, T. M., Ye, J., van Buuren, A. W., Worthington, M. A., Prisbrey, S. T., Park, H.-S., Amendt, P. A., Baumann, T. F., Biener, J. Direct laser writing of low-density interdigitated foams for plasma drive shaping, Adv Funct Mater, 2017, 27, 1702425. [104] Tian, Y., Zhang, Y. L., Xia, H., Guo, L., Ku, J. F., He, Y., Zhang, R., Xu, B. Z., Chen, Q. D., Sun, H. B. Solvent response of polymers for micromachine manipulation, Phys Chem Phys, 2011, 13, 4835–4838. [105] Hippler, M., Blasco, E., Qu, J., Tanaka, M., Barner-Kowollik, C., Wegener, M., Bastmeyer, M. Controlling the shape of 3D microstructures by temperature and light, Nat Commun, 2019, 10, 232. [106] Li, M. H., Keller, P., Li, B., Wang, X., Brunet, M. Light-driven side-on nematic elastomer actuators, Adv Mater, 2003, 15, 569–572. [107] Zeng, H., Martella, D., Wasylczyk, P., Cerretti, G., Lavocat, J. C., Ho, C. H., Parmeggiani, C., Wiersma, D. S. High-resolution 3D direct laser writing for liquid-crystalline elastomer microstructures, Adv Mater, 2014, 26, 2319–2322. [108] Descrovi, E., Pirani, F., . P., Rajamanickam, V., Licheri, S., Liberale, C. Photo-responsive suspended micro-membranes, J Mater Chem C, 2018, 6, 10428–10434. [109] Barbot, A., Power, M., Seichepine, F., Yang, G. Z. Liquid seal for compact micropiston actuation at the capillary tip, Sci Adv, 2020, 6, 8. [110] Yi, S., Ding, F., Gong, L., Gu, X. Extracellular matrix scaffolds for tissue engineering and regenerative medicine, Curr Stem Cell Res Ther, 2017, 12, 233–246. [111] Nelson, B. J., Kaliakatsos, I. K., Abbott, J. J. Microrobots for minimally invasive medicine, Annu Rev Biomed Eng, 2010, 12, 55–85. [112] Billiet, T., Vandenhaute, M., Schelfhout, J., Van Vlierberghe, S., Dubruel, P. A review of trends and limitations in hydrogel-rapid prototyping for tissue engineering, Biomaterials, 2012, 33, 6020–6041. [113] Yeong, W. Y., Chua, C. K., Leong, K. F., Chandrasekaran, M. Rapid prototyping in tissue engineering: Challenges and potential, Trends Biotechnol, 2004, 22, 643–652. [114] Koroleva, A., Gill, A. A., Ortega, I., Haycock, J. W., Schlie, S., Gittard, S. D., Chichkov, B. N., Claeyssens, F. Two-photon polymerization-generated and micromolding-replicated 3D scaffolds for peripheral neural tissue engineering applications, Biofabrication, 2012, 4, 025005.

294

Xuewen Wang et al.

[115] Accardo, A., Blatché, M.-C., Courson, R., Loubinoux, I., Vieu, C., Malaquin, L. Two-photon lithography and microscopy of 3D hydrogel scaffolds for neuronal cell growth, Biomed Phys Eng Express, 2018, 4, 027009. [116] Marino, A., Tricinci, O., Battaglini, M., Filippeschi, C., Mattoli, V., Sinibaldi, E., Ciofani, G. A 3D real-scale, biomimetic, and biohybrid model of the blood-brain barrier fabricated through two-photon lithography, Small, 2018, 14, 1702959. [117] Zheng, Y. C., Zhao, Y. Y., Zheng, M. L., Chen, S. L., Liu, J., Jin, F., Dong, X. Z., Zhao, Z. S., Duan, X. M. Cucurbit[7]uril-Carbazole two-photon photoinitiators for the fabrication of biocompatible three-dimensional hydrogel scaffolds by laser direct writing in aqueous solutions, ACS Appl Mater Interfaces, 2019, 11, 1782–1789. [118] Lin, Y., Xu, J. Microstructures fabricated by two-photon polymerization and their remote manipulation techniques: Toward 3D printing of micromachines, Adv Opt Mater, 2018, 6, 1701359. [119] Xiong, W., Liu, Y., Jiang, L. J., Zhou, Y. S., Li, D. W., Jiang, L., Silvain, J. F., Lu, Y. F. Laserdirected assembly of aligned carbon nanotubes in three dimensions for multifunctional device fabrication, Adv Mater, 2016, 28, 2002–2009. [120] Staudinger, U., Zyla, G., Krause, B., Janke, A., Fischer, D., Esen, C., Voit, B., Ostendorf, A. Development of electrically conductive microstructures based on polymer/CNT nanocomposites via two-photon polymerization, Microelectron Eng, 2017, 179, 48–55. [121] Nakamura, R., Kinashi, K., Sakai, W., Tsutsumi, N. Fabrication of gold microstructures using negative photoresists doped with gold ions through two-photon excitation, Phys Chem Chem Phys, 2016, 18, 17024–17028. [122] Shukla, S., Vidal, X., Furlani, E. P., Swihart, M. T., Kim, K. T., Yoon, Y. K., Urbas, A., Prasad, P. N. Subwavelength direct laser patterning of conductive gold nanostructures by simultaneous photopolymerization and photoreduction, ACS Nano, 2011, 5, 1947–1957. [123] Blasco, E., Muller, J., Muller, P., Trouillet, V., Schon, M., Scherer, T., Barner-Kowollik, C., Wegener, M. Fabrication of conductive 3D gold-containing microstructures via direct laser writing, Adv Mater, 2016, 28, 3592–3595. [124] Marino, A., Barsotti, J., de Vito, G., Filippeschi, C., Mazzolai, B., Piazza, V., Labardi, M., Mattoli, V., Ciofani, G. Two-photon lithography of 3D nanocomposite piezoelectric scaffolds for cell stimulation, ACS Appl Mater Interfaces, 2015, 7, 25574–25579. [125] Beresna, M., Gecevičius, M., Bulgakova, N. M., Kazansky, P. G. Twisting light with micro-spheres produced by ultrashort light pulses, Opt Express, 2011, 19, 18989. [126] Wei, H., Amrithanath, A. K., Krishnaswamy, S. 3D printing of micro-optic spiral phase plates for the generation of optical vortex beams, IEEE Photonics Technol Lett, 2019, 31, 599–602. [127] Rybin, M., Shishkin, I., Samusev, K., Belov, P., Kivshar, Y., Kiyan, R., Chichkov, B., Limonov, M. Band structure of photonic crystals fabricated by two-photon polymerization, Crystals, 2015, 5, 61–73. [128] Purtov, J., Rogin, P., Verch, A., Johansen, V. E., Hensel, R. Nanopillar diffraction gratings by two-photon lithography, Nanomaterials (Basel), 2019, 9, 1495. [129] Abrashitova, K., Gulkin, D., Safronov, K., Kokareva, N., Antropov, I., Bessonov, V., Fedyanin, A. Bloch surface wave photonic device fabricated by femtosecond laser polymerisation technique, Appl Sci, 2018, 8, 63. [130] Del Pozo, M., Delaney, C., Bastiaansen, C. W. M., Diamond, D., Schenning, A., Florea, L. Direct laser writing of four-dimensional structural color microactuators using a photonic photoresist, ACS Nano, 2020, 14, 9832–9839.

Ali Bagheri, Jianyong Jin

Chapter 9 3D printing mediated by photoRAFT polymerization process 9.1 Introduction 3D printing technology (otherwise known as additive manufacturing) has changed the world of manufacturing as it offers a programmable pathway for the layer-bylayer fabrication of customized and on-demand 3D objects tailored to meet the demands of individuals and specific applications [1]. Among the different techniques, 3D printing via photopolymerization that includes stereolithography (SLA), digital light processing (DLP) and continuous liquid interface production (CLIP) is one of the most attractive methods due to the limitless innovations that can be provided by polymer chemistry [2, 3]. This technology has contributed to various fields such as microfluidics, biomedical devices, soft robotics, medical surgery, tissue engineering, dentistry and drug delivery [4–6]. The photopolymerization mechanism used in 3D printing is normally based on radical and cationic polymerizations. The radical polymerization (commonly referred to as “free radical polymerization”)1 finds favor due to its broad monomer and functionality scope. These mechanisms have proved effective in 3D printing of a wide range of materials with different functionalities and characteristics [3, 7–10]. However, the application of an alternative chemistry such as photocontrolled reversible addition fragmentation chain transfer polymerization (photoRAFT) to 3D printing can offer additional possibilities for advanced material manufacturing. Indeed, RAFT polymerization is one of the most straightforward, well-established and versatile reversible deactivation radical polymerization (RDRP) technique. RDRP techniques are typically used for producing polymer chains of controlled molecular weight, structure and architecture. One of the greatest possibilities provided by RDRP is the ability to produce polymer materials with dormant functionalities. In the context of polymer crosslinked networks, these dormant functionalities can be used to enable numerous potential modifications in a postsynthetic stage such as self-healing/welding [11, 12], grafting polymer side chains [13], expansion of network structures [12, 14], biofunctionalization [15] and spatial differentiation [16].

1 The International Union of Pure and Applied Chemistry recommends adjective “free” not to be used to describe radical polymerization. Ali Bagheri, School of Science and Technology, The University of New England, Armidale, Australia Jianyong Jin, School of Chemical Sciences, The University of Auckland, Auckland, New Zealand https://doi.org/10.1515/9783110570588-009

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Moreover, some of RDRP systems are oxygen-tolerant, a characteristic that is required for conducting fully open-to-air 3D printing processes [17–26]. In the context of 3D materials with transformability features, 4D printing (fourth dimension being time) concept has been developed which enables changes in shapes, sizes and properties of 3D printed materials in response to external stimuli such as temperature, water, pH, light, etc. [27–34]. This ability is normally realized by integration of stimuli responsive functionalities into the resin formulations which can become part of the printed materials [32]. Integration of light-responsive functional groups such as RAFT agents within the structure of 3D printed materials – which is the focus of this chapter – can be also considered as a subclass of 4D printing. However, the main difference between conventional 4D printing cases and introduction of RAFT agents into the resin formulation is that RAFT agents can participate in mediating the photoreaction process. This means that RAFT agents can have two roles: i) mediating the photopolymerization (printing process) and ii) serving as reactivatable specie to enable postprinting monomer addition onto the surface or within the structure of an already printed object via a subsequent RAFT process. Recently, the first proof of concept of photoRAFT-mediated 3D printing of materials capable of postprinting modification has been demonstrated [35–37]. In this chapter, we first provide an overview of the conventional photopolymerization-based 3D printing techniques. We then discuss the mechanisms behind (photo) RAFT polymerization alongside the possibilities that this technique has to offer to the crosslinked networks, particularly to the 3D printing technology. The recent key works on the application of photoRAFT in 3D printing have also been highlighted. The major challenges that restrict scaling and translation of photoRAFT facilitated 3D printing into industrial settings are also discussed.

9.2 Conventional photopolymerization techniques used in 3D printing Three main techniques are typically used in 3D printing via photopolymerization. They are stereolithography (SLA) [6, 38–41], digital light processing (DLP) [42–54] and continuous liquid interface production (CLIP) [55, 56]. These techniques are fundamentally based on crosslinking of liquid monomer/oligomers, which proceed via radical or cationic polymerization in the presence of photoinitiators. The most commonly used photocurable formulations contain (meth)acrylate monomers/oligomers that can be cured via radical polymerization. The mechanism can be broken into three stages: initiation, propagation and termination. In the initiation stage, photocleavable single molecules, known as type I photoinitiators (e.g. benzil ketals and phosphine oxide containing compounds), are typically exploited for radical generation under exposure to light. These photoinitiators have high molar

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extinction coefficients at short wavelengths (e.g. UV light) [7, 8]. Type II photoinitiators which are based on the interaction of an uncleavable sensitizer and a coinitiator can be also used for radical generation. We refer the readers to other reviews for more information about the photoinitiator structures and the mechanisms by which reactive radical species are formed [7, 8, 57]. The recent advancements on the novel one-photon and two-photon photoinitiators are also demonstrated in Chapter 1 and Chapter 2. In the propagation stage (photocrosslinking), (meth)acrylate monomer/oligomers containing multifunctional monomers such as poly(ethylene glycol)diacrylate (PEGDA) [44, 58, 59], urethane dimethacrylate [60, 61], triethylene glycol dimethacrylate [62–65], bisphenol A glycidyl methacrylate [62–65], trimethylolpropane triacrylate [55, 56, 60], and bisphenol A ethoxylate diacrylate [41] can be used. Inherently, in radical polymerization, the rate of initiation is relatively slower than propagation and the termination reactions are mobility-restricted. This results in a high kinetic chain length and, ultimately, the formation of networks with heterogeneity and thus, high brittleness [66, 67]. It has been demonstrated that the use of addition fragmentation chain transfer (AFCT) agents in the resin formulations reduces the kinetic chain length and therefore delays the gel point to higher monomer conversion. This results in 3D printing of more uniform networks with reduced shrinkage stress and improved toughness [68–70]. It is known that oxygen can inhibit the radical polymerization of (meth)acrylate systems by quenching the excited-state photoinitiator and/or forming a peroxide, upon interaction with propagating radical species [71, 72]. Tertiary amines or triphenylphosphine have been exploited as additives in resin formulations to minimize the oxygen inhibition in 3D systems. In addition to the (meth)acrylate systems, thiol-ene /yne chemistry has also been used in 3D resin formulations [67, 73–76]. The stepgrowth mechanism dictating the thiol-ene systems delays the gelation point to a higher monomer conversion and, therefore, offers lower shrinkage stress of the 3D printed materials as compared to the (meth)acrylate-based systems [77]. Thiol-ene/ yne systems also offer higher biocompatibility [76] that can be used in 3D fabrication of biocompatible and biodegradable hydrogel constructs [78, 79]. Thiol-yne chemistry provides higher glass transition and higher modulus as compared to the thiol-enederived materials. This characteristic is mainly attributed to the vinyl sulfide as the reaction intermediate capable of undergoing a second addition with excess thiol [76, 80, 81]. For instance, materials printed using the tricyclo [5.2.1.02,6] decane-4,8dimethanol dibut-3-yn-1-yl carbonate displayed high strength and lower cytotoxicity than their corresponding meth(acrylates) formulations [76]. Cationic polymerization is another type of chemistry used in 3D applications which is typically based on ring opening photopolymerization of epoxides [10, 65, 82]. In all the systems described here, UV or visible light-sensitive initiators are commonly used to generate reactive species to mediate radical or cationic photopolymerization for 3D applications [8, 57, 83, 84]. Although the conventional photopolymerization techniques have proved highly effective for 3D printing of a wide range of materials, application of an

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alternative chemistry such as photoRAFT to 3D printing can add a new dimension to the current technologies.

9.3 Fundamental aspects of RAFT polymerization and its mechanistic progression Before discussing the application of photoRAFT to polymer-crosslinked networks and 3D printing, we provide an overview of the mechanisms of RAFT polymerization. RAFT process is a family of RDRP reactions [12, 85–89] discovered in the 1990s that has become increasingly popular as it can enable controlled synthesis of welldefined polymers with high chemical fidelity [90]. RAFT polymerization has found its application in different fields due to its simplicity of implementation as well as the availability of a wide range of compatible reagents. The mechanism of RAFT process presented in Figure 9.1(a) shows the RAFT equilibria as an additional step to the main stages involved in a radical polymerization (e.g. initiation, propagation and termination) [90]. In RAFT polymerization, propagating radical (P•m) is produced by radical addition to the monomers (step i). This propagating radical is then added to thiocarbonylthio RAFT agents followed by a chain transfer reaction (fragmentation of the intermediate radical) to generate a polymeric thiocarbonylthio compound [PmSC(Z) = S] and a new initiating R• radical (step ii). This new radical is capable of forming a new propagating radical, P•n (step iii), which is capable of entering into an equilibrium between the active P•m and P•n species (polymeric transfer agents) and dormant thiocarbonylthio compounds via reversible addition fragmentation reactions (step ii and iv). In an effective RAFT process, the rate of propagation is lower than that of the addition/fragmentation equilibrium (reversible chain transfer), which maintains the majority of chains in a dormant form and, therefore, enables all the propagating chains to grow at equal probability; making it possible to produce linear polymers with narrow polydispersity [68]. More critically, RAFT functionality remains at the chain-end or within the polymer backbone (depending on the structure of the RAFT agent used) that can be further used for postpolymerization reactions (e.g. chain extension to form di or triblock copolymers) [68]. The initiation step in RAFT polymerization was traditionally based on the decomposition of thermal initiators [91]. Although thermally initiated systems are broadly studied, such thermally labile initiators are typically hazardous and in the polymerization process, the initiator fragment can be attached to a fraction of the polymer chain-ends, which compromises the chain-end fidelity of the polymer products. As RAFT process developed over the years, alternative routes to generate radicals and activate RAFT polymerization have been developed using ultrasound, enzymes, electrochemical, light, etc. [94]. In particular, photoactivation pathway enables high degrees of spatial and temporal control on polymerization at low temperatures and it

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Figure 9.1: (a) Traditional RAFT polymerization activated via radical addition [91]. Radical can be formed by different stimuli. Note that the termination process is not shown in this scheme, (b) A reversible termination polymerization [92]., (c) Photoiniferter polymerization [93], and (d) Photoinduced electron/energy-transfer reversible addition–fragmentation chain-transfer (PET-RAFT) polymerization [71]. P: polymer chain, Pn•: propagating radical species, Z: reactivity-modifying group, M: monomer and PC: photocatalyst.

also facilitates synthesis of polymer chains with high chain-end fidelity [85–87]. Photoactivation can be either through direct activation of RAFT agents (known as photoiniferter mechanism) or indirectly using chromophores such as photocatalysts – known as photoinduced electron or energy transfer RAFT (PET-RAFT). Before the first report of RAFT polymerization, Otsu and coworkers introduced the concept of photoinduced initiator-transfer agent terminators (photoiniferters) based on

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sulphur-containing compounds to synthesize polymers via a degenerative chain transfer (Figure 9.1(a)) and reversible termination mechanism (Figure 9.1(b)) [93]. Following the pioneering works of Otsu, various thiocarbonylthio RAFT agents (Figure 9.2) were explored as photoiniferters to control the radical polymerizations. In early systems, UV light was utilized to induce a thiocarbonyl π → π* transition, resulting in C–S bond cleavage, to produce an active carbon-centered radical to control the radical polymerization [95, 96]. The UV-mediated systems showed relatively poor control over the molecular distribution of polymer chains, especially at high monomer conversions, which was mainly attributed to irreversible photolytic degradation of the fragmented RAFT agents under UV exposure (Figure 9.1(c)) [97]. To suppress the degradation of RAFT agents, visible light has been recently employed to induce n → π* transitions, resulting in C–S bond cleavage and, thus, radical generation to activate radical polymerization via two simultaneous pathways: i) reversible termination and ii) radical activation of a RAFT-type degenerative chain transfer process following photolysis of the thiocarbonylthio compounds (Figure 9.1(c)) [93, 98–101]. It should be mentioned that not all the RAFT agents are capable of undergoing photolysis under visible light. Specific trithiocarbonates (e.g. possessing tertiary R groups) can be activated under visible light without the presence of external initiators. Experiments undertaken by Davis, Rizzardo and coworkers elucidated that the degenerative chain transfer (e.g. RAFT process) is the

Figure 9.2: Typical examples of thiocarbonylthio compounds used in photoRAFT polymerization systems. Some of these thiocarbonylthio compounds can be used as photoiniferters while some can be used as RAFT agents in PET-RAFT systems. CDTPA: 4-cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid, BSTP: 3-benzylsulfanyl-thiocarbonylthiosulfanyl propionic acid, BTPA: (2-(nbutyltrithiocarbonate)-propionic acid, DBTTC: dibenzyl trithiocarbonate, DDMAT: S-1-Dodecyl-S′(α, α′-dimethyl-α′′-acetic acid) trithiocarbonate, CPD-TTC: 2-cyano-2-propyl dodecyl trithiocarbonate, CMP: 2-(1-Carboxy-1-methylethylsulfanylthiocarbonylsulfanyl)-2-methylpropionic acid, CPADB: 4-cyanopentanoic acid dithiobenzoate, and CDB: 2-phenyl-2-propyl benzodithioate.

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main process that provides control over the polymerization, while the extent to which the mechanistic pathway of reversible termination occurs is unclear [92]. Photoactivation of thiocarbonylthio compounds can be also realized indirectly using external redox-active catalytic species. Excited photoredox catalysts can interact with the RAFT agents and control the activation-deactivation equilibrium of RAFT polymerization via electron/energy transfer cyclic processes. This process is known as photoinduced electron or energy transfer RAFT (PET-RAFT) (Figure 9.1(d)) [71, 102–105]. Compared to the thiocarbonylthio iniferters, photoredox catalysts have better photochemical properties with relatively long-lived excited states and suitable redox potentials. Therefore, PET-RAFT systems offer faster polymerization rates as compared to the photoiniferter-mediated polymerization [85, 106]. It should be pointed out that even in the presence of photoredox catalysts (especially when light sources of shorter wavelengths are used), thiocarbonylthio compounds can undergo photolysis and, therefore, multiple mechanisms of activation are likely to happen simultaneously [107]. Having said that, the dictating mechanism is highly dependent upon the irradiation wavelength and the reagents used. Thus far, a wide range of photocatalysts with strong absorption in the visible and NIR range have proved compatible with PET-RAFT polymerization, which opened up new possibilities in polymer synthesis [71, 108]. More importantly, various photoredox catalysts (Figure 9.3) such as Ir(ppy)3 [71], Ru(bpy)3Cl2 [109], eosin Y (EY) [103, 107], pheophorbide a (PheoA) [108], and zinc tetraphenylporphine (ZnTPP) [12, 87, 110] can provide oxygen-tolerant systems via different mechanisms that are well studied in the literature [22, 94, 109, 111]. While the potential application of photoRAFT technique is not covered in this chapter, it is worth mentioning that this technique has been exploited in a variety of contexts including but not limited to the synthesis of well-defined block copolymers and nano/micro size polymeric particles, surface modification of diverse materials, sequence-defined polymers, chemically and architecturally diverse polymers (e.g. self-assemblies, brushes, stars and multi-blocks), etc. [94, 112]. In the context of polymer crosslinked networks, RAFT polymerization has the ability to reversibly deactivate the propagating radial species, which enables improved control over polymer chain growth, and therefore making it possible to relatively tune the network uniformity [91]. However, using RAFT polymerization does not allow direct control over the distance between the crosslinking points in covalently crosslinked networks, or direct control of the pore sizes [113].

Figure 9.3: Typical examples of chromophores used as photocatalysts in PET-RAFT polymerization; PTH: 10phenylphenothiazine, Ru(bpy)32+: tris(2,2′-bipyridine) ruthenium(II), EY: eosin Y, ZnTPP: zinc tetraphenylporphine, PheoA: pheophorbide a, AIPc: aluminum phthalocyanine, and Ir(ppy)3: tris-phenylpyridine cyclometalated fac-Ir(ppy)3.

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9.4 Use of RAFT polymerization in the synthesis of polymeric crosslinked networks Before discussing the application of photoRAFT in 3D printing, we provide a general overview of the polymeric crosslinked networks that contain RAFT agents within their structures [1, 28, 89, 114]. As we discussed in section 9.3, dynamic equilibrium between the dormant and active species is the foundation of RAFT polymerization, which is facilitated by the RAFT agents [115, 116]. The reversible bond exchange between the trithiocarbonate (TTC) units can enable crosslinked polymer networks to undergo selfhealing via reshuffling of reversible covalent bonds (Figure 9.4(a)) [115]. For instance,

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Matyjaszewski and coworkers used UV light to induce homolysis of TTCs (present within crosslinked networks) and generate carbon radicals, enabling reactions with other TTCs via degenerative exchange process [11]. In the RAFT process, the thiocarbonylthio groups preserved in the polymer products can be further used for postpolymerization reactions (e.g. chain extension to form di or triblock copolymers). In polymer crosslinked networks that are formed using RAFT agents, postsynthetic modifications can be realized through reactivation of the preserved RAFT functionalities. Depending upon the nature of the RAFT agents and their molecular structure, the postsynthetic reactions differ. For instance, when the RAFT functionalities are present at the chain-ends, surface-initiated polymerization can be obtained. When the preserved functionalities are present in the core and connect the arms of the RAFT-derived polymer chains, monomer insertion within the structure of crosslinked networks can be realized [113]. In a study by Johnson and coworkers, UV light was used to activate TTCs embedded within a polymer network to facilitate monomer insertion into an already synthesized network, resulting in an increase in the average molecular weights between the crosslinking points [117]. The field of photopolymerization is moving towards the use of benign visible lights to avoid the limitations of using high-energy UV light. Some of the main advantages of using visible light are as follows: Low-energy visible light sources are eco-friendly with low thermal effects and minimal side reactions. They are also preferred for bio-related applications [9, 118]. To this end, various types of visible light sensitive compounds with the ability to initiate photopolymerization in RDRP systems [85, 86, 106, 119–121] and in 3D printing [1, 9, 10, 45, 65, 76, 122] have been investigated. It is worth mentioning that in the context of layer-by-layer 3D printing process, higher wavelength light sources can penetrate deeper and offer higher 3D printing speed. For example, Matyjaszewski and coworkers demonstrated that green and blue visible light can be used to activate the TTCs present within the crosslinked networks to enable different postsynthetic modification pathways such as network expansion or surface modification [14]. In a similar study, Bagheri, Jin and coworkers reported the synthesis of photoexpandable/transformable-polymer networks (PET-PNs) using photoiniferter polymerization [12]. In their approach, a symmetric DBTTC RAFT agent was activated by blue LED light to mediate inferter-based radical polymerization to fabricate PET-PNs. The preserved DBTTC groups within the structure of networks were reactivated (in a second photoiniferter process) to facilitate the addition of new monomers within or onto the surface of pristine networks [12]. Even though the addition of monomers into the PET-PNs was confirmed by an increase in the mass and swelling ratio of the modified networks, the uniformity of the photogrowth process was not controlled. This was explained to be partly due to the use of conventional crosslinkers and limited accessibility/reactivity of the TTCs within the network [12].

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Instead of photoiniferter-derived polymerization, photoredox catalyzed RAFT polymerization (commonly known as PET-RAFT polymerization) [71, 102–105] can be also used in the context of crosslinked networks [123]. An early example was the work by Johnson and coworkers in which a PTH photocatalyst was excited under blue LED light exposure to interact with the TTC units embedded within a crosslinked network and, therefore, mediate a RAFT polymerization for insertion of new monomers into an already synthesized network (Figure 9.4(b)) [89]. Inspired by these advancements, Bagheri, Jin and coworkers reported the synthesis of PET-PNs using ZnTPP as a photoredox catalyst in the presence of DBTTC RAFT agents and a diacrylate crosslinker. In the postsynthesis stage, the preserved TTC functionalities found within the networks were reactivated to enable addition of new monomers onto the surface and within the structure of an existing network [12]. One of the ultimate goals of the studies was to implement these systems in a practical 3D printing system to render postprinting modification a reality. This area of research which was historically unexplored (due to the slow polymerization rate and oxygen sensitivity of RDRP techniques) has been recently comprehended by the pioneering works of Bagheri and Jin in collaboration with Boyer’s group, which are elaborated in the following section.

9.5 Use of photo RAFT polymerization in 3D printing Polymer networks have traditionally been synthesized using solution or bulk polymerization, while the emergence of 3D printing technology enabled an alternative way for on-demand production of polymer networks with complex structures in streamlining the additive manufacturing processes [124, 125]. As elaborated in Section 9.2, the polymer chemistry used in photopolymerization-based 3D printing is typically based on radical and/or cationic polymerization [47, 126]. The use of an alternative chemistry, for example photoRAFT, can broaden the possible scope of 3D technology. Perhaps one of the opportunities is postprinting modification facilitated by (re)activation of RAFT functionalities integrated within the structure of 3D printed materials. The first example of photoiniferter facilitated 3D printing was reported by Bagheri and Jin in collaboration with Boyer [35]. A formulation containing TTC units (e.g. CDTPA or DBTTC) and a diacrylate monomer (e.g. PEGDA) was prepared in the complete absence of exogenous initiators/catalysts and solvent (Figure 9.5(a) i and ii). This formulation was used in a layer-by-layer photopolymerization process facilitated by a modified DLP 3D printer equipped with a photomask and LED lights (λ max = 405 nm, 1.8 mW/cm2). Upon exposure to light, TTCs acted as photoiniferters and mediated the photopolymerization of diacrylate monomers. In a postprinting stage, the preserved TTC species present in the 3D printed networks were reactivated to enable monomer addition (e.g. 1-pyrenemethyl methacrylate (PyMA) or n-butyl acrylate (BA)) within or on the surface of already printed materials [35]. As can be seen from Figure 9.6(a), the

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Figure 9.5: (a) Chemical structures of components in the formulation used for 3D printing: i) CDTPA and DBTTC, ii) PEGDA (Mn = 250 g/mol) and BA monomer, and iii) EY and TEA. For the photoiniferter-based 3D printing, a formulation containing i and ii was used, while for the PET-RAFT based 3D printing, EY and TEA (iii) were added to the formulation, (b) Proposed photopolymerization mechanism under visible light irradiation. This mechanism has been previously reported in both solution [86] and bulk [129] RDRP for preparing linear polymers [107], (c) UV-vis absorption spectrum of EY and the emission spectrum of the in house build blue LED light source of 3D printer, (d) UV-vis absorption spectrum of EY and the emission spectrum of the in house build green LED light source of 3D printer, (e) Schematic of 3D printing process using a modified bottom-up DLP printer equipped with a photomask and blue (λ max = 483 nm, 4.16 mW/cm2) or green (λ max = 532 nm, 0.48 mW/cm2) LED lights, at room temperature and fully open to air, (f) An optical image and a microscope image of a 3D object printed using PET formulation, and (g-h) Representative SEM images of the printed objects. Reproduced with permission from [36]. Copyright 2020 American Chemical Society.

polyPyMA (PPyMA)-modified network exhibited excimer emission of pyrene moieties under 365 nm UV light exposure. The optical image of the PPyMA-modified disc taken under UV light exposure, exhibited stronger excimer emission from the outer regions as compared to the central areas of the disc, suggesting a bigger block of PPyMA added to the areas where TTCs were possibly more accessible [12, 127]. It should

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Figure 9.6: (a) Optical images of an initially 3D printed network and its subsequent modified network after PyMA monomer addition (image taken under 365 nm UV light exposure). Reproduced with permission from [35]. Copyright 2020 Royal Society of Chemistry. (b) Water contact angle measurements of a 3D printed network with high surface wettability and its subsequent modified network with decreased surface wettability after BA monomer addition. Reproduced with permission from [37]. Copyright 2019 John Wiley and Sons.

be mentioned that postmanufacturing monomer insertion within the structure of highly crosslinked networks, especially those that are formed using symmetrical RAFT, are complex and require further in-depth exploration [113]. Moreover, factors such as crosslinking density, (re)activatability of the preserved TTC species, the distribution of the TTCs within the network, and the uniformity of light exposure/penetration can affect the postprinting modification reactions. The printing speed of the photoiniferter-based formulation was significantly slower than the systems based on conventional formulations that proceed via free radical polymerization using type I or type II photoinitiators [1, 126]. This was partly due to the slow activation rate of TTCs (via photolysis) as well as chain transfer reactions that result in slower propagation rates [91]. It is known that carbon-centered radicals can react with molecular oxygen to form peroxy radical and hydroperoxides that are not reactive for reinitiating polymerization [17–20, 71, 128]. To avoid oxygen inhibition, the 3D printing process was conducted in an inert atmosphere by the physical displacement of oxygen from the resin vat using a glovebox [35]. Oxygen sensitivity makes conducting a 3D printing process challenging at a large scale where displacement of oxygen is demanding without increased manufacturing costs. Therefore, translation of RAFT-based 3D printing into industrial setting requires the use of an oxygen-tolerant system [130]. Thus far, various chemical approaches have been reported to scrub oxygen from the polymerization reaction, for instance, polymerizing through oxygen, [20–22] enzyme-mediated deoxygenation [23, 24] and the consumption of molecular oxygen via a PET process initiated from photoredox catalysts [25, 26, 111, 131]. Organic dyes such as EY have also been used as a PET catalyst for preparing linear polymers in the presence of oxygen [86, 103,

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107, 129, 132]. To realize a fully open 3D printing process, Bagheri et al. complemented their initial photoiniferter-based formulation (which was proceeded via photolysis of RAFT agents) with the addition of a EY photocatalyst and a sacrificial triethylamine reducing agent (Figure 9.5(a) iii). The photopolymerization of a diacrylate monomer (e.g. PEGDA), proceeded, in principle, via a PET pathway, as shown in Figure 9.5(b) [36]. Using this formulation, the active molecular oxygen can be consumed to form inactive superoxide anions by electron transfer from an eosin-radical anion or anion RAFT species [17, 86, 133]. The layer-by-layer 3D printing process was carried out in a completely open-to-air environment using a modified DLP printer (with a photomask) equipped with blue (λmax = 483 nm, 4.16 mW/cm2) or green (λmax = 532 nm, 0.48 mW/cm2) LED lights (Figure 9.5(c–e)) [36]. The use of green LED light showed higher 3D printing build speed compared to blue LED-induced printing, which was due to the greater absorbance-emission overlap between EY and green LED emission as well as the higher penetration depth of the green light [36]. To demonstrate that the photopolymerization proceeded via a layer-by-layer process, [35] scanning electron microscopy (SEM) analysis was used to obtain images of the printed object. The analysis confirmed high accuracy in layer uniformity and thickness that are in agreement with the predefined values using slicing software (Figure 9.5(f–h)) [36]. As the major motivation behind using RAFT-based systems in 3D printing was to produce materials containing dormant reactivatable species, postprinting modification was demonstrated by the addition of BA monomers from the preserved and accessible TTC units to change the surface hydrophilicity of the printed object [36]. At the same time, Boyer, Corrigan and colleagues also exploited an oxygen-tolerant PET-RAFT system based on a water soluble Erythrosin B photocatalyst and a triethanolamine reducing agent, which facilitated 3D printing in aqueous solutions. The preserved TTC units were then reactivated in a postprinting stage to produce materials with different functionalities, such as materials with different surface wettability (Figure 9.6(b)) [37].

9.6 Conclusions and perspectives 3D printing via photopolymerization is typically conducted using radical, cationic and/or thiol-ene/yne polymerization. These mechanisms have enabled 3D printing of materials with a wide range of functionalities and applications. However, the use of an alternative chemistry based on RAFT polymerization can add a new dimension to the existing techniques. In particular, integration of RAFT functionalities within the structure of (3D) crosslinked networks can provide different post-manufacturing possibilities that is not achievable using conventional formulations. Having said that, although the application of RAFT in 3D printing technology represents a noteworthy progress, its development has been constrained due to the slow polymerization rate

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and the oxygen sensitivity of the RAFT systems. Further success in the application of photoRAFT to 3D printing requires the following essential challenges to be overcome: (i) photopolymerization must achieve fast kinetics at room temperature; (ii) the presence of a small amount of oxygen must not inhibit the reaction. Moreover, in the field of 3D bioprinting, the activation wavelength required to mediate the photopolymerization must not harm biosensitive materials or degrade the reactants – this means that developing printable formulations that are responsive to longer wavelengths is highly demanded. We envisage that photoRAFT will open up new opportunities in the field of 3D printing for fabrication of advanced functional materials.

References [1] [2] [3] [4]

[5] [6] [7] [8] [9]

[10]

[11]

[12] [13]

Bagheri, A., Jin, J. Photopolymerization in 3D printing, ACS Appl Polym Mater, 2019, 1, 593–611. Layani, M., Wang, X., Magdassi, S. Novel materials for 3D printing by photopolymerization, Adv Mater, 2018, 30, 1–7. Zhang, J., Xiao, P. 3D printing of photopolymers, Polym Chem, Royal Society of Chemistry, 2018, 9, 1530–1540. Wang, M. O., Vorwald, C. E., Dreher, M. L., Mott, E. J., Cheng, M. H., Cinar, A., et al. Evaluating 3D-printed biomaterials as scaffolds for vascularized bone tissue engineering, Adv Mater, 2014, 27, 138–144. Rusling, J. F. Developing microfluidic sensing devices using 3D printing, ACS Sensors, 2018, 3, 522–526. Zorlutuna, P., Jeong, J. H., Kong, H., Bashir, R. Stereolithography-based hydrogel microenvironments to examine cellular interactions, Adv Funct Mater, 2011, 21, 3642–3651. Ligon, S. C., Liska, R., Stampfl, J., Gurr, M., Mülhaupt, R. Polymers for 3D printing and customized additive manufacturing, Chem Rev, 2017, 117, 10212–10290. Jasinski, F., Zetterlund, P. B., Braun, A. M., Chemtob, A. Photopolymerization in dispersed systems, Prog Polym Sci, Elsevier Ltd, 2018, 84, 47–88. Zhang, J., Dumur, F., Xiao, P., Graff, B., Bardelang, D., Gigmes, D., et al. Structure design of naphthalimide derivatives: Toward versatile photoinitiators for Near-UV/Visible LEDs, 3D printing, and water-soluble photoinitiating systems, Macromolecules, 2015, 48, 2054–2063. Zhang, J., Dumur, F., Xiao, P., Graff, B., Gigmes, D., Pierre Fouassier, J., et al. Aminothiazonaphthalic anhydride derivatives as photoinitiators for violet/blue LED-Induced cationic and radical photopolymerizations and 3D-Printing resins, J Polym Sci Part A Polym Chem, 2016, 54, 1189–1196. Amamoto, Y., Kamada, J., Otsuka, H., Takahara, A., Matyjaszewski, K. Repeatable photoinduced self-healing of covalently cross-linked polymers through reshuffling of trithiocarbonate units, Angew Chemie – Int Ed, 2011, 50, 1660–1663. Bagheri, A., Bainbridge, C., Jin, J. Visible light-induced transformation of polymer networks, ACS Appl Polym Mater, 2019, 1, 1896–1904. Cuthbert, J., Zhang, T., Biswas, S., Olszewski, M., Shanmugam, S., Fu, T., et al. Structurally tailored and engineered macromolecular (STEM) gels as soft elastomers and hard/soft interfaces, Macromolecules, 2018, 51, 9184–9191.

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[15] [16]

[17] [18] [19]

[20] [21]

[22]

[23]

[24] [25]

[26]

[27] [28] [29] [30] [31] [32] [33]

Ali Bagheri, Jianyong Jin

Shanmugam, S., Cuthbert, J., Flum, J., Fantin, M., Boyer, C., Kowalewski, T., et al. Transformation of gels: Via catalyst-free selective RAFT photoactivation, Polym Chem, Royal Society of Chemistry, 2019, 10, 2477–2483. He, H., Averick, S., Mandal, P., Ding, H., Li, S., Gelb, J., et al. Multifunctional hydrogels with reversible 3D ordered macroporous structures, Adv Sci, 2015, 2, 1–6. Cuthbert, J., Beziau, A., Gottlieb, E., Fu, L., Yuan, R., Balazs, A. C., et al. Transformable materials: Structurally tailored and engineered macromolecular (STEM) gels by controlled radical polymerization, Macromolecules, American Chemical Society, 2018, 51, 3808–3817. Ligon, S. C., Husár, B., Wutzel, H., Holman, R., Liska, R. Strategies to reduce oxygen inhibition in photoinduced polymerization, Chem Rev, 2014, 114, 577–589. Wilson, O. R., Magenau, A. J. D. Oxygen tolerant and room temperature RAFT through alkylborane initiation, ACS Macro Lett, 2018, 7, 370–375. Enciso, A. E., Fu, L., Russell, A. J., Matyjaszewski, K. A breathing atom-transfer radical polymerization: Fully oxygen-tolerant polymerization inspired by aerobic respiration of cells, Angew Chemie – Int Ed, 2018, 57, 933–936. Gody, G., Barbey, R., Danial, M., Perrier, S. Ultrafast RAFT polymerization: Multiblock copolymers within minutes, Polym Chem, Royal Society of Chemistry, 2015, 6, 1502–1511. Reyhani, A., McKenzie, T. G., Ranji-Burachaloo, H., Fu, Q., Qiao, G. G. Fenton-RAFT polymerization: An “On-Demand” chain-growth method, Chem – A Eur J, 2017, 23, 7221–7226. Lamb, J. R., Qin, K. P., Johnson, J. A. Visible-light-mediated, additive-free, and open-to-air controlled radical polymerization of acrylates and acrylamides, Polym Chem, Royal Society of Chemistry, 2019, 10, 1585–1590. Chapman, R., Gormley, A. J., Stenzel, M. H., Stevens, M. M. Combinatorial low-volume synthesis of well-defined polymers by enzyme degassing, Angew Chemie – Int Ed, 2016, 55, 4500–4503. Oytun, F., Kahveci, M. U., Yagci, Y. Sugar overcomes oxygen inhibition in photoinitiated free radical polymerization, J Polym Sci Part A Polym Chem, 2013, 51, 1685–1689. Niu, J., Page, Z. A., Dolinski, N. D., Anastasaki, A., Hsueh, A. T., Soh, H. T., et al. Rapid visible light-mediated controlled aqueous polymerization with in situ monitoring, ACS Macro Lett, 2017, 6, 1109–1113. Ren, K., Perez-Mercader, J. Thermoresponsive gels directly obtained: Via visible lightmediated polymerization-induced self-assembly with oxygen tolerance, Polym Chem, Royal Society of Chemistry, 2017, 8, 3548–3552. Lee, J., Kim, H. C., Choi, J. W., Lee, I. H. A review on 3D printed smart devices for 4D printing, Int J Precis Eng Manuf – Green Technol, 2017, 4, 373–383. Sydney Gladman, A., Matsumoto, E. A., Nuzzo, R. G., Mahadevan, L., Lewis, J. A. Biomimetic 4D printing, Nat Mater, 2016, 15, 413–418. Momeni, F., Mehdi, M., Hassani, N. S., Liu, X., Ni, J. A review of 4D printing, Mater Des, Elsevier Ltd, 2017, 122, 42–79. Yang, H., Leow, W. R., Wang, T., Wang, J., Yu, J., He, K., et al. 3D printed photoresponsive devices based on shape memory composites, Adv Mater, 2017, 29, 1701627. Kotikian, A., Truby, R. L., Boley, J. W., White, T. J., Lewis, J. A. 3D printing of liquid crystal elastomeric actuators with spatially programed nematic order, Adv Mater, 2018, 30, 1–6. Dutta, S., Cohn, D. Temperature and pH responsive 3D printed scaffolds, J Mater Chem B, Royal Society of Chemistry, 2017, 5, 9514–9521. Roppolo, I., Chiappone, A., Angelini, A., Stassi, S., Frascella, F., Pirri, C. F., et al. 3D printable light-responsive polymers, Mater Horizons, Royal Society of Chemistry, 2017, 4, 396–401.

Chapter 9 3D printing mediated by photoRAFT polymerization process

311

[34] Zhao, Z., Kuang, X., Yuan, C., Qi, H. J., Fang, D. Hydrophilic/hydrophobic composite shapeshifting structures, ACS Appl Mater Interfaces, American Chemical Society, 2018, 10, 19932–19939. [35] Bagheri, A., Engel, K. E., Bainbridge, C. W. A., Xu, J., Boyer, C., Jin, J. 3D printing of polymeric materials based on photo-RAFT polymerization, Polym Chem, Royal Society of Chemistry, 2020, 11, 641–647. [36] Bagheri, A., Bainbridge, C. W. A., Engel, K. E., Qiao, G. G., Xu, J., Boyer, C., et al. Oxygen tolerant PET-RAFT Facilitated 3D printing of polymeric materials under visible LEDs, ACS Appl Polym Mater, 2020, 2, 782–790. [37] Zhang, Z., Corrigan, N., Bagheri, A., Jin, J., Boyer, C., Versatile, A. 3D and 4D printing system through photocontrolled RAFT polymerization, Angew Chemie – Int Ed, 2019, 58, 17954–17963. [38] Hull, C. W. Apparatus for production of three-dimensional objects by stereolithography. U.S. Patent 4575330. 1986. US4575330A. [39] Elomaa, L., Pan, C. C., Shanjani, Y., Malkovskiy, A., Seppälä, J. V., Yang, Y. Three-dimensional fabrication of cell-laden biodegradable poly(ethylene glycol-co-depsipeptide) hydrogels by visible light stereolithography, J Mater Chem B, 2015, 3, 8348–8358. [40] Wang, X., Jiang, M., Zhou, Z., Gou, J., Hui, D. 3D printing of polymer matrix composites: A review and prospective, Compos Part B Eng, Elsevier Ltd, 2017, 110, 442–458. [41] Credi, C., Fiorese, A., Tironi, M., Bernasconi, R., Magagnin, L., Levi, M., et al. 3D printing of cantilever-type microstructures by stereolithography of ferromagnetic photopolymers, ACS Appl Mater Interfaces, 2016, 8, 26332–26342. [42] Rossi, S., Puglisi, A., Benaglia, M. Additive manufacturing technologies: 3D printing in organic synthesis, ChemCatChem, 2018, 10, 1512–1525. [43] Wu, G. H., Hsu, S. H. Review: Polymeric-based 3D printing for tissue engineering, J Med Biol Eng, Springer Berlin Heidelberg, 2015, 35, 285–292. [44] Chiappone, A., Fantino, E., Roppolo, I., Lorusso, M., Manfredi, D., Fino, P., et al. 3D printed PEG-based hybrid nanocomposites obtained by Sol–Gel technique, ACS Appl Mater Interfaces, 2016, 8, 5627–5633. [45] Wang, J., Stanic, S., Altun, A. A., Schwentenwein, M., Dietliker, K., Jin, L., et al. A highly efficient waterborne photoinitiator for visible-light-induced three-dimensional printing of hydrogels, Chem Commun, 2018, 54, 920–923. [46] Dilla, R. A., Motta, C. M. M., Snyder, S. R., Wilson, J. A., Wesdemiotis, C., Becker, M. L. Synthesis and 3D printing of PEG-Poly(propylene fumarate) diblock and triblock copolymer hydrogels, ACS Macro Lett, 2018, 7, 1254–1260. [47] Zhu, W., Qu, X., Zhu, J., Ma, X., Patel, S., Liu, J., et al. Direct 3D bioprinting of prevascularized tissue constructs with complex microarchitecture, Biomaterials, Elsevier Ltd, 2017, 124, 106–115. [48] Soman, P., Chung, P. H., Zhang, A. P., Chen, S. Digital microfabrication of user-defined 3D microstructures in cell-laden hydrogels, Biotechnol Bioeng, 2013, 110, 3038–3047. [49] De Leon, A. C., Chen, Q., Palaganas, N. B., Palaganas, J. O., Manapat, J., Advincula, R. C. High performance polymer nanocomposites for additive manufacturing applications, React Funct Polym, Elsevier B.V, 2016, 103, 141–155. [50] Wang, F., Chong, Y., Wang, F. K., He, C. Photopolymer resins for luminescent threedimensional printing, J Appl Polym Sci, 2017, 134, 1–8. [51] Patel, D. K., Sakhaei, A. H., Layani, M., Zhang, B., Ge, Q., Magdassi, S. Highly stretchable and UV curable elastomers for digital light processing based 3D printing, Adv Mater, 2017, 29, 1–7.

312

Ali Bagheri, Jianyong Jin

[52] Zhang, B., Kowsari, K., Serjouei, A., Dunn, M. L., Ge, Q. Reprocessable thermosets for sustainable three-dimensional printing, Nat Commun, 2018, 9, 1831. [53] Mu, Q., Wang, L., Dunn, C. K., Kuang, X., Duan, F., Zhang, Z., et al. Digital light processing 3D printing of conductive complex structures, Addit Manuf, Elsevier B.V, 2017, 18, 74–83. [54] Cullen, A. T., Price, A. D. Digital light processing for the fabrication of 3D intrinsically conductive polymer structures, Synth Met, Elsevier, 2018, 235, 34–41. [55] Tumbleston, J. R., Shirvanyants, D., Ermoshkin, N., Janusziewicz, R., Johnson, A. R., Kelly, D., et al. Continuous liquid interface production of 3D objects, Science, 2015, 347, 1349–1352. [56] Janusziewicz, R., Tumbleston, J. R., Quintanilla, A. L., Mecham, S. J., DeSimone, J. M. Layerless fabrication with continuous liquid interface production, Proc Natl Acad Sci, 2016, 113, 11703–11708. [57] Fouassier, J. P., Allonas, X., Lalevée, J., Dietlin, C. Photoinitiators for Free Radical Polymerization Reactions, Photochemistry and Photophysics of Polymeric Materials, Allen, N. S. eds, John Wiley & Sons, Inc., Hoboken, New Jersey, 2010, 351–419. [58] Palaganas, N. B., Mangadlao, J. D., De Leon, A. C. C., Palaganas, J. O., Pangilinan, K. D., Lee, Y. J., et al. 3D printing of photocurable cellulose nanocrystal composite for fabrication of complex architectures via stereolithography, ACS Appl Mater Interfaces, 2017, 9, 34314–34324. [59] Warner, J., Soman, P., Zhu, W., Tom, M., Chen, S. Design and 3D printing of hydrogel scaffolds with fractal geometries, ACS Biomater Sci Eng, 2016, 2, 1763–1770. [60] Liska, R., Schuster, M., Inführ, R., Turecek, C., Fritscher, C., Seidl, B., et al. Photopolymers for rapid prototyping, J Coatings Technol Res, 2007, 4, 505–510. [61] Yue, J., Zhao, P., Gerasimov, J. Y., Van De Lagemaat, M., Grotenhuis, A., Rustema-Abbing, M., et al. 3D-printable antimicrobial composite resins, Adv Funct Mater, 2015, 25, 6756–6767. [62] Al Mousawi, A., Garra, P., Sallenave, X., Dumur, F., Toufaily, J., Hamieh, T., et al. πconjugated dithienophosphole derivatives as high performance photoinitiators for 3D printing resins, Macromolecules, 2018, 51, 1811–1821. [63] Al Mousawi, A., Kermagoret, A., Versace, D. L., Toufaily, J., Hamieh, T., Graff, B., et al. Copper photoredox catalysts for polymerization upon near UV or visible light: Structure/reactivity/ efficiency relationships and use in LED projector 3D printing resins, Polym Chem, 2017, 8, 568–580. [64] Al Mousawi, A., Dumur, F., Garra, P., Toufaily, J., Hamieh, T., Graff, B., et al. Carbazole scaffold based photoinitiator/photoredox catalysts: Toward new high performance photoinitiating systems and application in LED projector 3D printing resins, Macromolecules, 2017, 50, 2747–2758. [65] Al Mousawi, A., Garra, P., Schmitt, M., Toufaily, J., Hamieh, T., Graff, B., et al. 3Hydroxyflavone and N-Phenylglycine in high performance photoinitiating systems for 3D printing and photocomposites synthesis, Macromolecules, 2018, 51, 4633–4641. [66] Lovestead, T. M., O’Brien, A. K., Bowman, C. N. Models of multivinyl free radical photopolymerization kinetics, J Photochem Photobiol A Chem, 2003, 159, 135–143. [67] Ligon-Auer, S. C., Schwentenwein, M., Gorsche, C., Stampfl, J., Liska, R. Toughening of photo-curable polymer networks: A review, Polym Chem, 2016, 7, 257–286. [68] Moad, G., Rizzardo, E., Thang, S. H. Radical addition-fragmentation chemistry in polymer synthesis, Polymer, 2008, 49, 1079–1131. [69] Gorsche, C., Seidler, K., Knaack, P., Dorfinger, P., Koch, T., Stampfl, J., et al. Rapid formation of regulated methacrylate networks yielding tough materials for lithography-based 3D printing, Polym Chem, 2016, 7, 2009–2014.

Chapter 9 3D printing mediated by photoRAFT polymerization process

313

[70] Seidler, K., Griesser, M., Kury, M., Harikrishna, R., Dorfinger, P., Koch, T., et al. Vinyl sulfonate esters: Efficient chain transfer agents for the 3D printing of tough photopolymers without retardation, Angew Chemie – Int Ed, 2018, 57, 9165–9169. [71] Xu, J., Jung, K., Atme, A., Shanmugam, S., Boyer, C. A robust and versatile photoinduced living polymerization of conjugated and unconjugated monomers and its oxygen tolerance, J Am Chem Soc, 2014, 136, 5508–5519. [72] Yagci, Y., Jockusch, S., Turro, N. J. Photoinitiated polymerization: Advances, challenges, and opportunities, Macromolecules, 2010, 43, 6245–6260. [73] Matthew, J. K., Daniel, J. B., Craig, J. H. The power of thiol‐ene chemistry, J Polym Sci Part A Polym Chem, 2010, 48, 743–750. [74] Senyurt, A. F., Wei, H., Phillips, B., Cole, M., Nazarenko, S., Hoyle, C. E., et al. Physical and mechanical properties of photopolymerized thiol-ene/acrylates, Macromolecules, 2006, 39, 6315–6317. [75] Sycks, D. G., Wu, T., Park, H. S., Gall, K. Tough, stable spiroacetal thiol-ene resin for 3D printing, J Appl Polym Sci, 2018, 135, 1–12. [76] Oesterreicher, A., Wiener, J., Roth, M., Moser, A., Gmeiner, R., Edler, M., et al. Tough and degradable photopolymers derived from alkyne monomers for 3D printing of biomedical materials, Polym Chem, Royal Society of Chemistry, 2016, 7, 5169–5180. [77] McNair, O. D., Janisse, A. P., Krzeminski, D. E., Brent, D. E., Gould, T. E., Rawlins, J. W., et al. Impact properties of thiol-ene networks, ACS Appl Mater Interfaces, 2013, 5, 11004–11013. [78] Qin, X. H., Gruber, P., Markovic, M., Plochberger, B., Klotzsch, E., Stampfl, J., et al. Enzymatic synthesis of hyaluronic acid vinyl esters for two-photon microfabrication of biocompatible and biodegradable hydrogel constructs, Polym Chem, 2014, 5, 6523–6533. [79] Bertlein, S., Brown, G., Lim, K. S., Jungst, T., Boeck, T., Blunk, T., et al. Thiol–Ene clickable gelatin: A platform bioink for multiple 3D biofabrication technologies, Adv Mater, 2017, 29, 1–6. [80] Fairbanks, B. D., Sims, E. A., Anseth, K. S., Bowman, C. N. Reaction rates and mechanisms for radical, photoinitiated addition of thiols to alkynes, and implications for thiol-yne photopolymerizations and click reactions, Macromolecules, 2010, 43, 4113–4119. [81] Chan, J. W., Shin, J., Hoyle, C. E., Bowman, C. N., Lowe, A. B. Synthesis, thiol-yne “click” photopolymerization, and physical properties of networks derived from novel multifunctional alkynes, Macromolecules, 2010, 43, 4937–4942. [82] Al Mousawi, A., Lara, D. M., Noirbent, G., Dumur, F., Toufaily, J., Hamieh, T., et al. Carbazole derivatives with thermally activated delayed fluorescence property as photoinitiators/ photoredox catalysts for LED 3D printing technology, Macromolecules, 2017, 50, 4913–4926. [83] Lalevée, J., Tehfe, M. A., Dumur, F., Gigmes, D., Graff, B., Morlet-Savary, F., et al. Lightharvesting organic photoinitiators of polymerization, Macromol Rapid Commun, 2013, 34, 239–245. [84] Fouassier, J. P., Allonas, X., Burget, D. Photopolymerization reactions under visible lights: Principle, mechanisms and examples of applications, Prog Org Coatings, 2003, 47, 16–36. [85] Fors, B. P., Hawker, C. J. Control of a living radical polymerization of methacrylates by light, Angew Chemie – Int Ed, 2012, 51, 8850–8853. [86] Xu, J., Shanmugam, S., Duong, H. T., Boyer, C. Organo-photocatalysts for photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, Polym Chem, Royal Society of Chemistry, 2015, 6, 5615–5624. [87] Bagheri, A., Arandiyan, H., Adnan, N. N. M., Boyer, C., Lim, M. Controlled direct growth of polymer shell on upconversion nanoparticle surface via visible light regulated polymerization, Macromolecules, 2017, 50, 7137–7147. [88] Cuthbert, J., Balazs, A. C., Kowalewski, T., Matyjaszewski, K. STEM Gels by controlled radical polymerization, Trends Chem, Elsevier Inc, 2020, 2, 341–353.

314

Ali Bagheri, Jianyong Jin

[89] Chen, M., Gu, Y., Singh, A., Zhong, M., Jordan, A. M., Biswas, S., et al. Living additive manufacturing: Transformation of parent gels into diversely functionalized daughter gels made possible by visible light photoredox catalysis, ACS Cent Sci, 2017, 3, 124–134. [90] Hill, M. R., Carmean, R. N., Sumerlin, B. S. Expanding the scope of RAFT polymerization: Recent advances and new horizons, Macromolecules, 2015, 48, 5459–5469. [91] Moad, G., Rizzardo, E., Thang, S. H. Living radical polymerization by the RAFT process, Aust J Chem, 2002, 58, 379–410. [92] Quinn, J. F., Barner, L., Barner-Kowollik, C., Rizzardo, E., Davis, T. P. Reversible addition – Fragmentation chain transfer polymerization initiated with ultraviolet radiation, Macromolecules, 2002, 35, 7620–7627. [93] Otsu, T. Iniferter concept and living radical polymerization, J Polym Sci Part A Polym Chem, 2000, 38, 2121–2136. [94] Nothling, M. D., Fu, Q., Reyhani, A., Allison-Logan, S., Jung, K., Zhu, J., et al. Progress and perspectives beyond traditional RAFT polymerization, Adv Sci, 2020, 2001656, 1–12. [95] Gruendling, T., Kaupp, M., Blinco, J. P., Barner-Kowollik, C. Photoinduced conjugation of dithioester- and trithiocarbonate-functional raft polymers with alkenes, Macromolecules, 2011, 44, 166–174. [96] Chen, M., Johnson, J. A. Improving photo-controlled living radical polymerization from trithiocarbonates through the use of continuous-flow techniques, Chem Commun, 2015, 51, 6742–6745. [97] Lu, L., Yang, N., Cai, Y. Well-controlled reversible addition-fragmentation chain transfer radical polymerisation under ultraviolet radiation at ambient temperature, Chem Commun, 2005, 42, 5287–5288. [98] Bagheri, A., Sadrearhami, Z., Adnan, N. N. M., Boyer, C., Lim, M. Surface functionalization of upconversion nanoparticles using visible light-mediated polymerization, Polymer, Elsevier Ltd, 2018, 151, 6–14. [99] Amamoto, Y., Otsuka, H., Takahara, A., Matyjaszewski, K. Self-healing of covalently crosslinked polymers by reshuffling thiuram disulfide moieties in air under visible light, Adv Mater, 2012, 24, 3975–3980. [100] Pan, X., Jiang, G., Zhu, X., Zhu, J., Fan, C., Zhang, Z., et al. Photocatalyst-free and blue lightinduced RAFT polymerization of vinyl acetate at ambient temperature, Macromol Rapid Commun, 2015, 36, 2181–2185. [101] McKenzie, T. G., Fu, Q., Wong, E. H. H., Dunstan, D. E., Qiao, G. G. Visible light mediated controlled radical polymerization in the absence of exogenous radical sources or catalysts, Macromolecules, 2015, 48, 3864–3872. [102] Xu, J., Shanmugam, S., Boyer, C. Organic electron donor-acceptor photoredox catalysts: Enhanced catalytic efficiency toward controlled radical polymerization, ACS Macro Lett, 2015, 4, 926–932. [103] Niu, J., Lunn, D. J., Pusuluri, A., Yoo, J. I., O’Malley, M. A., Mitragotri, S., et al. Engineering live cell surfaces with functional polymers via cytocompatible controlled radical polymerization, Nat Chem, Nature Publishing Group, 2017, 9, 537–545. [104] Tucker, B. S., Coughlin, M. L., Figg, C. A., Sumerlin, B. S. Grafting-from proteins using metalfree PET-RAFT polymerizations under mild visible-light irradiation, ACS Macro Lett, 2017, 6, 452–457. [105] Yang, Y., Liu, X., Ye, G., Zhu, S., Wang, Z., Huo, X., et al. Metal-free photoinduced electron transfer-atom transfer radical polymerization integrated with bioinspired polydopamine chemistry as a green strategy for surface engineering of magnetic nanoparticles, ACS Appl Mater Interfaces, 2017, 9, 13637–13646.

Chapter 9 3D printing mediated by photoRAFT polymerization process

315

[106] Ohtsuki, A., Goto, A., Kaji, H. Visible-light-induced reversible complexation mediated living radical polymerization of methacrylates with organic catalysts, Macromolecules, 2013, 46, 96–102. [107] Figg, C. A., Hickman, J. D., Scheutz, G. M., Shanmugam, S., Carmean, R. N., Tucker, B. S., et al. Color-coding visible light polymerizations to elucidate the activation of trithiocarbonates using eosin y, Macromolecules, 2018, 51, 1370–1376. [108] Bagheri, A., Yeow, J., Arandiyan, H., Xu, J., Boyer, C., Lim, M. Polymerization of a photocleavable monomer using visible light, Macromol Rapid Commun, 2016, 37, 905–910. [109] Xu, J., Jung, K., Boyer, C. Oxygen tolerance study of photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization mediated by Ru(bpy)3Cl2, Macromolecules, 2014, 47, 4217–4229. [110] Shanmugam, S., Xu, J., Boyer, C. Exploiting Metalloporphyrins for Selective Living Radical Polymerization Tunable over Visible Wavelengths, J Am Chem Soc, 2015, 137, 9174–9185. [111] Corrigan, N., Rosli, D., Jones, J. W. J., Xu, J., Boyer, C. Oxygen tolerance in living radical polymerization: Investigation of mechanism and implementation in continuous flow polymerization, Macromolecules, 2016, 49, 6779–6789. [112] Li, S., Han, G., Zhang, W. Photoregulated reversible addition-fragmentation chain transfer (RAFT) polymerization, Polym Chem, Royal Society of Chemistry, 2020, 11, 1830–1844. [113] Moad, G. RAFT (Reversible addition-fragmentation chain transfer) crosslinking (co) polymerization of multi-olefinic monomers to form polymer networks, Polym Int, 2015, 64, 15–24. [114] Scott, T. F., Schneider, A. D., Cook, W. D., Bowman, C. N. Photoinduced plasticity in crosslinked polymers, Science, 2005, 308, 1615–1617. [115] Nicolaÿ, R., Kamada, J., Van Wassen, A., Matyjaszewski, K. Responsive gels based on a dynamic covalent trithiocarbonate cross-linker, Macromolecules, 2010, 43, 4355–4361. [116] Amamoto, Y., Otsuka, H., Takahara, A., Matyjaszewski, K. Changes in network structure of chemical gels controlled by solvent quality through photoinduced radical reshuffling reactions of trithiocarbonate units, ACS Macro Lett, 2012, 1, 478–481. [117] Zhou, H., Johnson, J. A. Photo-controlled growth of telechelic polymers and end-linked polymer gels, Angew Chemie – Int Ed, 2013, 52, 2235–2238. [118] Bagheri, A., Arandiyan, H., Boyer, C., Lim, M. Lanthanide-doped upconversion nanoparticles: Emerging intelligent light-activated drug delivery systems, Adv Sci, 2016, 3, 1500437. [119] Dadashi-Silab, S., Doran, S., Yagci, Y. Photoinduced electron transfer reactions for macromolecular syntheses, Chem Rev, 2016, 116, 10212–10275. [120] Ciftci, M., Batat, P., Demirel, A. L., Xu, G., Buchmeiser, M., Yagci, Y. Visible light-induced grafting from polyolefins, Macromolecules, 2013, 46, 6395–6401. [121] Tasdelen, M. A., Ciftci, M., Yagci, Y. Visible light-induced atom transfer radical polymerization, Macromol Chem Phys, 2012, 213, 1391–1396. [122] Al Mousawi, A., Dumur, F., Garra, P., Toufaily, J., Hamieh, T., Goubard, F., et al. Azahelicenes as visible light photoinitiators for cationic and radical polymerization: Preparation of photoluminescent polymers and use in high performance LED projector 3D printing resins, J Polym Sci Part A Polym Chem, 2017, 55, 1189–1199. [123] Lampley, M. W., Harth, E. Photocontrolled growth of cross-linked nanonetworks, ACS Macro Lett, American Chemical Society, 2018, 7, 745–750. [124] Wendel, B., Rietzel, D., Kühnlein, F., Feulner, R., Hülder, G., Schmachtenberg, E. Additive processing of polymers, Macromol Mater Eng, 2008, 293, 799–809. [125] Jungst, T., Smolan, W., Schacht, K., Scheibel, T., Groll, J. Strategies and molecular design criteria for 3D printable hydrogels, Chem Rev, 2016, 116, 1496–1539.

316

Ali Bagheri, Jianyong Jin

[126] Mondschein, R. J., Kanitkar, A., Williams, C. B., Verbridge, S. S., Long, T. E. Polymer structure-property requirements for stereolithographic 3D printing of soft tissue engineering scaffolds, Biomaterials, Elsevier Ltd, 2017, 140, 170–188. [127] Bagheri, A., Boyer, C., Lim, M. Synthesis of light-responsive pyrene-based polymer nanoparticles via polymerization-induced self-assembly, Macromol Rapid Commun, 2019, 40, 1800510. [128] McKenzie, T. G., Fu, Q., Uchiyama, M., Satoh, K., Xu, J., Boyer, C., et al. Beyond traditional RAFT: Alternative activation of thiocarbonylthio compounds for controlled polymerization, Adv Sci, 2016, 3, 1–9. [129] Lee, I. H., Discekici, E. H., Anastasaki, A., De Alaniz, J. R., Hawker, C. J. Controlled radical polymerization of vinyl ketones using visible light, Polym Chem, Royal Society of Chemistry, 2017, 8, 3351–3356. [130] Matyjaszewski, K., Coca, S., Gaynor, S. G., Wei, M., Woodworth, B. E. Controlled radical polymerization in the presence of oxygen, Macromolecules, 1998, 31, 5967–5969. [131] Xu, J., Shanmugam, S., Fu, C., Aguey-Zinsou, K. F., Boyer, C. Selective photoactivation: From a single unit monomer insertion reaction to controlled polymer architectures, J Am Chem Soc, 2016, 138, 3094–3106. [132] Nomeir, B., Fabre, O., Ferji, K. Effect of tertiary amines on the photoinduced electron transferreversible addition–fragmentation chain transfer (PET-RAFT) polymerization, Macromolecules, 2019, 52, 6898–6903. [133] Fu, Q., Xie, K., McKenzie, T. G., Qiao, G. G. Trithiocarbonates as intrinsic photoredox catalysts and RAFT agents for oxygen tolerant controlled radical polymerization, Polym Chem, Royal Society of Chemistry, 2017, 8, 1519–1526.

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Chapter 10 Main challenges in 3D printing: Printing speed and biomedical applications 10.1 Introduction 3D-printing, also known as additive manufacturing, is defined as a process to create 3D objects by the sequential layer-by-layer addition of materials with the assistance of a computer-aided design (CAD). Compared to conventional formative or subtractive manufacturing that is based on molds or machining, 3D printing is more energy- and material-efficient, capable of creating objects with complex geometries and is compatible with bio systems [1, 2]. From the time of its development in the early 1980s for the purpose of model making and rapid prototyping, this technology has been advancing rapidly to find wide applications in engineering, chemistry, biology, medicine and materials science [2–5]. Several 3D-printing approaches have been developed, which are defined by the patterning and solidification process [6]. These include methods based on jetting, powder bed fusion, extrusion and photopolymerization, involving applicable materials ranging from thermoplastics and polymeric resins to inorganic powders [7]. Comprehensive introductions about the categories of 3D printing technologies and their respective working methodology have been given in previous publications [2, 6, 8]. The current research efforts in 3D printing are mainly devoted to the innovations in 3D printing methods and the adaptation of the developed techniques in various sectors including customized manufacturing, architecture design and medical fields [9–12]. One major issue for 3D printing to be used in manufacturing on a large scale is related to the printing speed – the rate to convert the digital information stored as units of bits in CAD model into physical objects [13]. However, most of the established 3D printing methods that rely on layer-by-layer deposition are too sluggish to meet the demands for printing of large objects and high-throughput manufacturing. It can take hours to build an object with height of several centimeters by the current stereolithography (SLA) systems [14, 15]. It has been indicated that an order of magnitude increase in printing speed without a compromise on part accuracy (resolution or voxel size) is the premise for these methods in mass production [15]. However, this seems to be a difficult goal to achieve as an inherent trade-off exists between the resolution, build volume and speed with all 3D printing methods [6]. For instance, a reduction of the voxel size by a factor of ten requires a 1000-fold H. Lai, P. Xiao, Research School of Chemistry, The Australian National University, Canberra, Australia https://doi.org/10.1515/9783110570588-010

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increase in the printing speed [13]. To tackle this challenge, innovations in 3D printing have emerged with new materials and processes towards faster solidification and deposition. They are discussed in this chapter. On the other hand, 3D printing was born to limitlessly build structures with geometric complexities. This feature has been frequently exploited in the pharmaceutical field for on-demand dosage forms and personalized drug delivery. In addition, human body represents one of the most complex systems, organized with tissues and organs that comprise multiple cell types and the related supporting extracellular matrix (ECM). Even since the first U.S. patent was awarded in 2006 for 3D bioprinting [2], 3D printing of tissues and organs has become a focus of interest in 3D printing area and the field of regenerative medicine. These include the bioprinting of skin, vascular grafts, heart tissue and cartilaginous structures, representing the state-ofthe-art applications of 3D printing in tissue engineering and regenerative medicine. Herein, we focus on the recent innovations and challenges in which the 3D printing technology has advanced with a dramatic reduction in build-time. We also highlight the applications of 3D printing in biomedical areas (including pharmaceutics and bioprinting of tissues and organs) with detailed examples. In addition, we share our perspective on the future directions about this rapidly evolving area of research.

10.2 Innovation and challenges to advance the printing speed The traditional 3D printing was limited by printing speed with the bottleneck being the time-consuming step-wise point-by-point or layer-by-layer deposition processes [1, 15]. To unleash the practical potential of 3D printing, innovations utilizing new materials and printing methods to allow for rapid material deposition with high precision have accelerated the printing process significantly over the past few years [6].

10.2.1 New materials for fast printing It has been indicated that an instantaneous replenishment of the liquid resin after the curing of one layer in photocuring could lead to great advance in printing speed [16]. This generally requires the employment of monomers with low viscosity. The digital light projection (DLP) printing of thermoplastic polymers from a monofunctional monomer, 4-acryloylmorpholine (ACMO in Figure 10.1(a)), was demonstrated [16]. Specifically, this low-viscosity monomer showed fast polymerization kinetic to generate high molecular weight polymers with Tg above the print temperature (Figure 10.1(b)), which was adapted to a top-down DLP printing process. Natural air was exploited to create an inhibition layer on the top of the resin to favor the fast

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Figure 10.1: Rapid open‐air DLP Printing of thermoplastic ACMO resin. (a) The printing setup and formulations. (b) The curing kinetics of the ACMO resin and a commercial thermoset acrylate resin. (c) Printed parts. (d) Printed pillars with different radii (left to right, 150, 200, 400 and 800 µm) demonstrating the resolution. All the printed 150 µm pillars and half of the 200 µm pillars collapsed. (e) Micro Kelvin lattice with fine struts. Reprinted with permission from [16]. Copyright 2019 John Wiley and Sons.

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spreading of fresh resin onto the printed part, allowing the successful printing of various 3D objects with complex features (overhang, hollow and lattice showed in Figure 10.1(c)). This requires an upper limit of 3s on the irradiation time to maintain the inhibition layer. A print speed of 730 mm h−1 could be achieved with good structural fidelity, with resolution in the range of 200 and 400 µm (Figure 10.1(d, e)). With its unique thermoplastic and water-soluble feature, the printed objects were sought after for uses as sacrificial molds. The printed resin could be removed by melting or dissolution to fabricate 3D functional devices such as microfluidic devices [16]. Besides, 3D objects can also be developed from 2D structures. These are rapidly printed with digital projectors capable of dynamic spatial control of the light exposure time [1]. The different light exposure results in a variable crosslinking density within the created 2D network, leading to stresses that can be released upon triggering and the development of the network into 3D designed objects. This concept is also widely used in 4D printing. The single brief digital light exposure overcomes the layer-by-layer deposition of material and enables quick printing of 3D objects with stimuli response [1]. More interestingly, the development of highly efficient photoinitiators capable of inducing fast photopolymerization reactions is a promising and emerging strategy to enhance the 3D printing speed especially under the longer wavelength light irradiation. The relevant details are presented in Chapter 1.

10.2.2 New approaches for fast printing Apparently, there is a leap in the printing speed by the replacement of the rastering laser in SLA with DLP. Printers that use a bladder to replenish the resin after each printing cycle and subsequently reposition consume a large percentage of time collectively to complete the printing of an object. This process is found in DLP, selective laser sintering (SLS) and binder jetting (BJ) technology. The printing in additive manufacturing can be greatly accelerated and a remarkable reduction of printing time can be achieved if the inherent flaws of layer-wise addition process can be mitigated or overcome [17]. The continuous liquid interface production (CLIP) method takes advantage of the oxygen-inhibition effect in photopolymerization [15]. In this approach, a “dead zone” is created between the cured frontier and oxygen-permeable window, ensuring a continuous interface of the liquid resin below the advancing part (Figure 10.2 (a)), thereby eliminating the resin replenishment process. The continuous elevation of the build-support and the projection of the UV images through the window enable a continuous printing process. The printed objects are drawn out from the resin bath at rates of hundreds of millimeters per hour, in contrast to a few millimeters per hour in the layer-wise approaches. Moreover, CLIP also shows promise in the fabrication of large overhangs that benefit from the isotropic mechanical

Figure 10.2: Schematic illustration of continuous 3D printing using (a) oxygen inhibition technique (b) concurrent photopolymerization and photoinhibition.

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properties, which are independent of the slice layer thickness. It was pointed out that the print speed for CLIP could be further increased by optimizing the resin cure rates and viscosity [15]. Following the CLIP work, a photoinhibition process is utilized instead of oxygen to create the “dead zone”, which is called the “inhibition volume” (Figure 10.2(b)) [14]. The bis[2-(ochlorophenyl)-4,5-diphenylimidazole] (o-Cl-HABI; Figure 10.2(b)) is adopted in the system as a photoinhibitor to trap the propagating species generated from camphorquinone (CQ)/ethyl-4-dimethylamino benzoate (EDAB) two-component photoinitiating system. Photoinitiation and photoinhibition are achieved by the blue and UV light, respectively, according to their absorbance spectra. The oxygenpermeable window in CLIP is thus replaced by a glass-transparent window. The thickness of the “inhibition volume” above the glass window is manipulated by changing the relative intensities of the two illuminating light sources and the concentrations of o-Cl-HABI. The thickness can, therefore, be tuned to hundreds of micrometers, which is much higher than the tens of micrometers in the CLIP approach. This greatly facilitates the reflow of resin underneath the printing frontier, allowing the printing of viscous resin and large cross-sectional areas objects. The printing speed is dependent on the absorption heights and the intensity of the two irradiations. Objects can be printed with a speed of up to 2 m/hour by optimizing these parameters. By flowing a layer of fluorinated oil beneath the photocuring resin, the continuous high-area rapid printing (HARP) process was developed [18]. In this approach, a slip boundary is created beneath the emerging part as a result of a shear stress caused by the oil motion (Figure 10.3). It was found that a mobile interface and

Figure 10.3: Schematic illustration of high-area rapid printing.

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active cooling are essential for heat dissipation, otherwise the accumulating heat would exceed the flashpoint of the resin, leading to part-cracking or result in reduced effectiveness at the slip boundary. By cooling the flowing oil, large parts can be printed with volumetric throughputs of 100 liters/hour. Moreover, without depending on oxygen inhibition to create a dead zone, HARP printing was also compatible with oxygen sensitive resin. Additionally, the HARP-printed material exhibited isotropic mechanical properties with high structure fidelity. These features together make the approach promising for rapid printing of large-objects. The transition from the step-wise process to a continuous process significantly accelerates printing, but the CLIP and HARP printing methods still remain in the category of layer-by-layer printing. Inspired by the image reconstruction of computed tomography (CT), the computed axial lithography (CAL) 3D printing technique, which photopolymerizes all points within the 3D object at once, was developed [19, 20]. It is based on the time-sequenced projection of light pattern to a rotating viscous resin [21]. Photocuring occurs at the superposed position from multiple angles, which accumulates sufficient exposures to overcome oxygen inhibition. Objects with lateral sizes up to ~ 55 mm can be printed rapidly in the range of 30 to 120 s and all parts materialize as a whole simultaneously in a support-free manner. The layerless printing allows objects to be printed with exceptionally smooth surface finishes or without support. Printing on preexisted parts can also be achieved, opening avenues for multi-materials fabrication, soft materials creation and lenses with smooth curved surfaces [19]. It was proved that a feedback camera could be integrated to CLA printing, perpendicular to the direction of illumination, to record the photocuring information, which could be used to optimize subsequent prints and improve the geometric fidelity of the object solidification [21]. Even though extensive efforts have been devoted to enhancing 3D printing speed, it still remains a great challenge especially in industrial applications.

10.3 Advancement in the biomedical applications of 3D printing 10.3.1 3D-printed drug delivery devices 3D printing for drug delivery is rapidly expanding with the aim of reducing the side effects and ensuring personalized medicine such as on-demand dosage forms that meet the specific pharmacogenomic, anatomical and physiological demands of patients [4, 22]. Different drug releasing profiles are needed corresponding to the different clinical circumstances such as constant, pulsatile, increasing and decreasing release. A general method for customizing the drug carriers for various release profiles has been challenging with the conventional materials-filling approach.

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3D printing is employed to fabricate drug tablets with any kind of desired drug release profiles [23]. Specifically, a polymeric template with an embossed feature of the desired shape and an impermeable poly(L-lactic acid) (PLA) container is at first 3D printed with a commercially available extrusion 3D printer. A mold is then cured complementary to the template. A surface-eroding polymer containing the drug is photocured in the mold and is inserted into the PLA container, which is followed by infilling of a polymer solution without the drug into the void spaces and photocured. The drug is allowed to release from the top of the tablet that is capped with an additional layer of surface-eroding polymer. The release is 1dimentional and follows the shape of the drug-containing polymer. The versatility of this method for 3D printed tablets was proved with constant, pulsed, decreasing and increasing profiles and arbitrary profiles (Figure 10.4). The entire duration of the release can be tuned by adjusting the crosslinking density within the surface-eroding polymer. Interestingly, multiple drugs with different release profiles can be encapsulated in one tablet, demonstrating the possibility of simultaneous administration of multiple drugs [23]. A 3D printed personalized wearable mouthguard with sustained drug release capability was demonstrated [24]. In this approach, the dentition impressions obtained from individualized intraoral scans were used to build CAD models tailored to patient-specific anatomic features. Clobetasol propionate (CBS), which is an effective oral anti-inflammation drug with high thermal stability, was chosen as the model drug and blended with PLA and poly(vinyl alcohol) (PVA) before 3D printing into the mouthguard via fused deposition modeling (FDM) printing. The release rate of CBS from the mouthguard can be tuned by varying the feed ration of PLA and PVA, with higher PVA content facilitating a faster release. The drug loading regions were also manipulated to allow spatial placement of drugs. CBS was replaced with the food-grade flavor vanillic acid (VA) and the mouthguards were printed with pharmaceutical-grade filaments for human release study. A sustained release of VA from the mouthguard over the duration of wearing was found, which can boost treatment efficacy and reduce unwanted side effects.

Figure 10.4: The expected and the experimentally determined release profiles of dye from the tablets. Reprinted with permission from [23]. Copyright 2015 John Wiley and Sons.

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10.3.2 Innovations of 3D printing for engineered tissues and organs Due to its potential to fabricate personalized complex constructs, 3D printing of engineered tissues and organs has been treated as one of the promising approaches to address the rising demand for transplantation in opposition to the limited availability of donors [5, 25–27]. Compared to nonbiological 3D-printed constructs, 3D bioprinting faces more complexities: 1) They need to be made of printable and biologically functional materials (bioink) that are compatible with cells and can be degraded into nonharmful byproducts while being capable of maintaining the structural and mechanical properties corresponding to the native organs; 2) There is a need to establish methods to expand and harvest sufficient cells for printing; 3) The printing calls for deposition of multiple cell types within patterns that mimic the complex microarchitecture; 4) The printing should create vasculature networks mimicking arteries, veins and capillaries within the printed constructs to allow the perfusion of nutrition and oxygen. With the development of engineering in 3D printers, biomaterials science and cell biology, the printing of simple tissues or simplified models of organs has succeeded as exemplified below [5, 25, 28]. As mentioned, the properties of bioinks are of great importance to affect the mechanical performance and the viability of cells in the printed constructs. Thus, effective bioinks for printing of 3D tissues are highly sought after. It was reported that methacrylated silk fibroin (Sil-MA) could be used to print highly complex organ structures via DLP 3D bioprinting [29]. Sil was chemically modified in the amino groups by glycidyl methacrylate (GMA) and the modification degree was tuned by varying the amount of GMA. The mechanical properties of the hydrogels prepared via photopolymerization were thus manipulated by the modification degree, with a higher methacrylation resulting in an increased tensile strength. The successful DLP printing of methacrylated silk fibroin with a 30% methacrylation degree was proved with various objects including the Eiffel Tower apart from the brain and ear that could return to their original shapes without deformation after compression. Tubular organs and tissues with complex structures were also successfully printed with good mechanical integrity (Figure 10.5). The hydrogel was found to be compatible with cells, regardless of the concentration and methacrylation degree. Moreover, the hydrogel exhibited excellent degradability, with 50% degradation after 4 weeks’ cultivation in 30% Sil-MA hydrogel [29]. The capability of an integrated tissue–organ printer (ITPO) was demonstrated to fabricate human-scale constructs using cell-laden hydrogels and biodegradable polymers [26]. Specifically, four-cartridge modules were employed as material dispensers. Among the four modules, two of them delivered poly(ε-caprolactone) (PCL) polymer and a sacrificial Pluronic F-127 hydrogel, respectively, to impart mechanical strength. The other two modules delivered cell-laden hydrogels. Two patterns were designed by varying the placement of PCL for different resultant mechanical strengths, depending

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Figure 10.5: Printed products by DLP using Sil-MA showed complex structure reflecting their CAD images, including veins, arteries, folds and holes. Adapted with permission from [29]. Copyright 2018 Springer Nature.

on the aimed tissues or organs. The Type I pattern was designed with PCL frames woven throughout the constructs to engineer mandible and calvarial bone and ear cartilage. In contrast, PCL only formed the exterior in Type II pattern with a softer structure to mimic the skeletal muscle. The cell-laden hydrogels were organized to create microchannels to allow maximal diffusion of nutrients and oxygen, which maintained cell viability and proliferation in the constructs. The cells embedded in the constructs can thereby differentiate and mature into vascularized functional tissues. This was demonstrated by the observation of calcium deposition in the mandible bone and in the newly formed vascularized bone tissue throughout the calvarial bone, both of which were printed with osteogenic stem cell-laden hydrogel. In addition, the ear cartilage exhibited much higher resilience after in vivo implantation. The implanted muscle constructs printed with Type II pattern also showed

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response to electrical stimulation. Based on these results, it is envisioned that ITPO may produce tissues for human applications, benefitting from the transport of nutrients and oxygen within the vascular constructs. By considering the immune risk of biomaterials and heterogeneous tissues, autologous materials were utilized to engineer personalized cardiac patches and heart via 3D printing [30]. The strategy began from the extraction of a biopsy of omental tissue from a patient (Figure 10.6). The extra-cellular matrix was processed into a personalized bioink, while cells were separated and reprogrammed to form induced pluripotent stem cells (iPSCs). The iPSCs were further developed into cardoimyocytes (CMs) and endothelial cells (ECs) to construct the parenchymal cardiac tissue and blood vessels. A patch model was obtained from the CT of a patient heart and with assistance of a mathematical model. A thick patch (~2 mm) was 3D printed with two print heads. One contained CMs in personalized hydrogel and the other contained EMs mixed with gelatin. Gelatin was sacrificed after printing and a vascularized cardia patch with physical robustness was obtained, which enabled perfusion of liquid. The in vivo contractility potential was also verified. To print a heart with significantly higher weight, a supporting medium composed of alginate microparticles in xanthan gum-supplemented growth medium was introduced. The printed heart with major blood vessels was shown to possess basic anatomical architecture

Figure 10.6: Schematic process of 3D printing of personalized thick and perfusable cardiac patches and hearts. Reprinted with permission from [30]. Copyright 2019 John Wiley and Sons.

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and mechanical strength. Challenges remain in whole organ printing, including imaging of the finer vessels and reproducing them during printing. Reproducing a more specific heart anatomical structure was realized by the 3Dbioprint collagen using freeform reversible embedding of suspended hydrogels (FRESH) [31]. The challenge to support soft materials during printing to maintain high resolution and fidelity was overcome by a rapid pH change-driven-collagen self-assembly within a buffered support material (from ~ 3.5 to 7.4). The support material was composed of gelatin microparticle slurry and underwent reversible melting at 37 °C to release the printed scaffolds. The slurry, called FRESH v2.0, was modified from the previous report with improved resolution in extruded collage filament [32]. With this support, collagen can be directly printed to form the walls of functional vasculature, promoting cell infiltration and microvascularization. A model of the left ventricle of a human heart was printed using collagen and cellladen as dual bio-inks with an ellipsoidal shape and a sandwich structure, where the inner and outer shells were made of collagen-encapsulated human embryonic stem cell–derived cardiomyocytes (hESC-CMs). Directional propagating waves during the spontaneous contractions of hESC-CMs could be captured. A tri-leaflet heart valve with a diameter of 28 mm was built and its mechanical function to cyclically open and close the valve leaflets was proved in a pulsatile flow test. A vascular network from the left coronary arteries was reproduced with FRESH approach. Ultimately, half of a neonatal scale human heart was printed with well-defined microstructures, including trabeculae, atrial and ventricular chambers. The stereolithographic apparatus for tissue engineering (SLATE) was used to fabricate multivascular networks from 3D printed hydrogels composed of water and poly(ethylene glycol) diacrylate in the presence of photoabsorbing additives [33, 34]. A water-soluble food additive, tartrazine, was added to the formulation as a photo-blocker to minimize the light that penetrated the printing layer during polymerization, thereby increasing the z resolution and ensuring the creation of a hollow perfusable vasculature. With this approach, monolithic hydrogels with an integrated static mixer or an integrated 3D bicuspid valve within the vasculature were printed. Rapid mixing dependent on the fin numbers of mixer and rapid response to pulsatile anterograde and retrograde flows by the valve, respectively, were both demonstrated. Complex entangled networks capable of independently flowing fluids were constructed based on 3D mathematical algorithms. One entangled helical topology was printed with a serpentine channel and perfused with oxygen gas. As a result, the inflowed deoxygenated RBCs changed color from dark red to bright red, suggesting intervascular interstitial transport. Besides, a biomimetic alveolar model with a shared airway atrium and an ensheathing vasculature were developed and printed into hydrogels. The airway was cyclically ventilated by oxygen to oxygenate the RBC streams perfusing in the surrounding vasculature. The architecture also supported the proliferation and differentiation of human stem cells [33].

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The application of a microscale continuous projection printing (μCPP) method was demonstrated to fabricate biomimetic hydrogel scaffolds that could accommodate neural progenitor cells (NPCs) for axon regeneration from spinal cord injury in the central nervous system [35]. Polyethylene glycol–gelatin methacrylate was used as the photo-bioink to engineer the rat spinal cord scaffold, which mimicked the geometries with an H-shaped solid area representing the ‘gray matter’ area and microchannels of 200 μm diameter to host NPCs and provide alignment with the host axonal tracts, above and below the injury. Host and stem cell-derived axons were found to linearly traverse the scaffold channels and form neural relays via electrophysiological transmission after implantation of the NPCs-loaded scaffolds at the transection site. The method opens up an avenue for the fabrication of personalized scaffolds that ‘fit’ the precise anatomy of an individual injury to enhance the central nervous system regeneration. Moreover, the state-of-the-art development of novel 3D printable photocurable biomacromolecules, especially in the aspect of the design of relevant chemical structures, is given in Chapter 5.

10.4 Conclusions and perspectives The demands for innovations in 3D printing are accompanied by the wide applications of 3D printing in various areas, emphasizing a more rapid fabrication with higher resolutions in larger volumes. Once a new printing technique is developed with better performance in speed, resolution or size, it is rapidly adopted by the research community to advance the respective areas. For instance, the CLIP has been widely used in 3D printing of tissue scaffolds [35] and medical devices [36, 37], soon after its release in 2015. The potential of CAL 3D printing to rapidly create entire objects at once was exploited to fabricate, within seconds, entire cell-laden constructs with arbitrary size and architecture [19, 38] Despite the remarkable achievements, there are still several aspects of the printing technology that need to be enhanced to completely meet the requirements of printing constructs with complexities that are relevant to tissues and organs. The current 3D printing approaches remain generally limited to single-material fabrication. Progress that allows the local fabrication with customized 3D spatial integration of dissimilar materials (such as metals, ceramics and polymers) in a single build sequence can unleash the end-use functionality of the fabricated structures. Simultaneous printing of several materials can be achieved with several syringes in direct ink writing [39]. The jetting of resin droplets composed of different materials at the same location, followed by polymerization, enables the formation of hybrid solid voxel in inkjet-based 3D printing [39]. The ability of CAL techniques to

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photopolymerize resins around a preexisting structure is also highly promising to integrate other materials for multimaterial fabrication [19]. The bioprinting of tissues and organs is also facing significant hurdles and challenges before real applications. The fabrication must be accelerated to a level that clinically-relevant-sized-tissues or organs can be manufactured with high cell viability in the constructs [5]. Methods to produce large number of functional cells from stem cells need to be developed for large organs printing. The integration of multiple types of functional, progenitor and supporting cell types in printed tissues remains elusive. Moreover, within tissues or organs of clinically relevant size, vascularization is a critical factor for oxygen and nutrient diffusion to allow cell survival in clinically relevant-sized 3D printed tissues. A complete multiscale vascular network, spanning arteries and veins down to the smallest capillaries, is still beyond the current fabrication approaches. Significant progresses in cell biology and the combination of different bioprinting modalities and bioinks are required to solve these problems to allow fabrication of clinically relevant-sized tissues or organs that can be used for transplantation.

References [1]

Huang, L., Jiang, R., Wu, J., Song, J., Bai, H., Li, B., Zhao, Q., Xie, T. Ultrafast digital printing toward 4D shape changing materials, Adv Mater, 2017, 29(7), 1605390. [2] Ligon, S. C., Liska, R., Stampfl, J., Gurr, M., Mülhaupt, R. Polymers for 3D printing and customized additive manufacturing, Chem Rev, 2017, 117(15), 10212–10290. [3] Hartings, M. R., Ahmed, Z. Chemistry from 3D printed objects, Nat Rev Chem, 2019, 3(5), 305–314. [4] Capel, A. J., Rimington, R. P., Lewis, M. P., Christie, S. D. R. 3D printing for chemical, pharmaceutical and biological applications, Nat Rev Chem, 2018, 2(12), 422–436. [5] Murphy, S. V., Atala, A. 3D bioprinting of tissues and organs, Nat Biotechnol, 2014, 32(8), 773–785. [6] Truby, R. L., Lewis, J. A. Printing soft matter in three dimensions, Nature, 2016, 540(7633), 371–378. [7] Ahn, D., Stevens, L. M., Zhou, K., Page, Z. A. Rapid high-resolution visible light 3D printing, ACS Cent Sci, 2020, 6(9), 1555–1563. [8] Ambrosi, A., Pumera, M. 3D-printing technologies for electrochemical applications, Chem Soc Rev, 2016, 45(10), 2740–2755. [9] Au, A. K., Huynh, W., Horowitz, L. F., Folch, A. 3D-Printed Microfluidics, Angew Chem Int Ed, 2016, 55(12), 3862–3881. [10] Zhou, L.-Y., Fu, J., He, Y. A Review of 3D printing technologies for soft polymer materials, Adv Funct Mater, 2020, 30(28), 2000187. [11] Powell, S. K., Cruz, R. L. J., Ross, M. T., Woodruff, M. A. Past, present, and future of softtissue prosthetics: Advanced polymers and advanced manufacturing, Adv Mater, 2020, 32(42), 2001122. [12] Wallin, T. J., Pikul, J., Shepherd, R. F. 3D printing of soft robotic systems, Nat Rev Mater, 2018, 3(6), 84–100.

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[13] Hahn, V., Kiefer, P., Frenzel, T., Qu, J., Blasco, E., Barner-Kowollik, C., Wegener, M. Rapid assembly of small materials building blocks (Voxels) into large functional 3D metamaterials, Adv Funct Mater, 2020, 30(26), 1907795. [14] de Beer, M. P., van der Laan, H. L., Cole, M. A., Whelan, R. J., Burns, M. A., Scott, T. F. Rapid, continuous additive manufacturing by volumetric polymerization inhibition patterning, Sci Adv, 2019, 5(1), eaau8723. [15] Tumbleston, J. R., Shirvanyants, D., Ermoshkin, N., Janusziewicz, R., Johnson, A. R., Kelly, D., Chen, K., Pinschmidt, R., Rolland, J. P., Ermoshkin, A., Samulski, E. T., DeSimone, J. M. Continuous liquid interface production of 3D objects, Science, 2015, 347(6228), 1349–1352. [16] Deng, S., Wu, J., Dickey, M. D., Zhao, Q., Xie, T. Rapid open-air digital light 3D printing of thermoplastic polymer, Adv Mater, 2019, 31(39), 1903970. [17] Janusziewicz, R., Tumbleston, J. R., Quintanilla, A. L., Mecham, S. J., DeSimone, J. M. Layerless fabrication with continuous liquid interface production, PNAS, 2016, 113(42), 11703–11708. [18] Walker, D. A., Hedrick, J. L., Mirkin, C. A. Rapid, large-volume, thermally controlled 3D printing using a mobile liquid interface, Science, 2019, 366(6463), 360–364. [19] Kelly, B. E., Bhattacharya, I., Heidari, H., Shusteff, M., Spadaccini, C. M., Taylor, H. K. Volumetric additive manufacturing via tomographic reconstruction, Science, 2019, 363(6431), 1075–1079. [20] Hart, A. J., Rao, A. How to print a 3D object all at once, Science, 2019, 363(6431), 1042–1043. [21] Loterie, D., Delrot, P., Moser, C. High-resolution tomographic volumetric additive manufacturing, Nat Commun, 2020, 11(1), 852. [22] Norman, J., Madurawe, R. D., Moore, C. M. V., Khan, M. A., Khairuzzaman, A. A new chapter in pharmaceutical manufacturing: 3D-printed drug products, Adv Drug Deliv Rev, 2017, 108, 39–50. [23] Sun, Y., Soh, S. Printing tablets with fully customizable release profiles for personalized medicine, Adv Mater, 2015, 27(47), 7847–7853. [24] Liang, K., Carmone, S., Brambilla, D., Leroux, J.-C. 3D printing of a wearable personalized oral delivery device: A first-in-human study, Sci Adv, 2018, 4(5), eaat2544. [25] Murphy, S. V., De Coppi, P., Atala, A. Opportunities and challenges of translational 3D bioprinting, Nat Biomed Eng, 2020, 4, 370–380. [26] Kang, H.-W., Lee, S. J., Ko, I. K., Kengla, C., Yoo, J. J., Atala, A. A 3D bioprinting system to produce human-scale tissue constructs with structural integrity, Nat Biotechnol, 2016, 34(3), 312–319. [27] Moroni, L., Burdick, J. A., Highley, C., Lee, S. J., Morimoto, Y., Takeuchi, S., Yoo, J. J. Biofabrication strategies for 3D in vitro models and regenerative medicine, Nat Rev Mater, 2018, 3(5), 21–37. [28] Giannopoulos, A. A., Mitsouras, D., Yoo, S.-J., Liu, P. P., Chatzizisis, Y. S., Rybicki, F. J. Applications of 3D printing in cardiovascular diseases, Nat Rev Cardiol, 2016, 13(12), 701–718. [29] Kim, S. H., Yeon, Y. K., Lee, J. M., Chao, J. R., Lee, Y. J., Seo, Y. B., Sultan, M. T., Lee, O. J., Lee, J. S., Yoon, S.-I., Hong, I.-S., Khang, G., Lee, S. J., Yoo, J. J., Park, C. H. Precisely printable and biocompatible silk fibroin bioink for digital light processing 3D printing, Nat Commun, 2018, 9(1), 1620. [30] Noor, N., Shapira, A., Edri, R., Gal, I., Wertheim, L., Dvir, T. 3D printing of personalized thick and perfusable cardiac patches and hearts, Adv Sci, 2019, 6(11), 1900344.

Chapter 10 Main challenges in 3D printing

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[31] Lee, A., Hudson, A. R., Shiwarski, D. J., Tashman, J. W., Hinton, T. J., Yerneni, S., Bliley, J. M., Campbell, P. G., Feinberg, A. W. 3D bioprinting of collagen to rebuild components of the human heart, Science, 2019, 365(6452), 482–487. [32] Hinton, T. J., Jallerat, Q., Palchesko, R. N., Park, J. H., Grodzicki, M. S., Shue, H.-J., Ramadan, M. H., Hudson, A. R., Feinberg, A. W. Three-dimensional printing of complex biological structures by freeform reversible embedding of suspended hydrogels, Sci Adv, 2015, 1(9), e1500758. [33] Grigoryan, B., Paulsen, S. J., Corbett, D. C., Sazer, D. W., Fortin, C. L., Zaita, A. J., Greenfield, P. T., Calafat, N. J., Gounley, J. P., Ta, A. H., Johansson, F., Randles, A., Rosenkrantz, J. E., Louis-Rosenberg, J. D., Galie, P. A., Stevens, K. R., Miller, J. S. Multivascular networks and functional intravascular topologies within biocompatible hydrogels, science, 2019, 364(6439), 458–464. [34] Dasgupta, Q., Black, L. D. A FRESH SLATE for 3D bioprinting, Science, 2019, 365(6452), 446–447. [35] Koffler, J., Zhu, W., Qu, X., Platoshyn, O., Dulin, J. N., Brock, J., Graham, L., Lu, P., Sakamoto, J., Marsala, M., Chen, S., Tuszynski, M. H. Biomimetic 3D-printed scaffolds for spinal cord injury repair, Nat Med, 2019, 25(2), 263–269. [36] Johnson, A. R., Tumbleston, J. R., Bloomquist, C. J., Moga, K. A., Ermoshkin, A., et al. Singlestep fabrication of computationally designed microneedles by continuous liquid interface production, PLoS ONE, 2016, 11(9), e0162518. [37] Lim, S. H., Kathuria, H., Tan, J. J. Y., Kang, L. 3D printed drug delivery and testing systems – a passing fad or the future?, Adv Drug Deliv Rev, 2018, 132, 139–168. [38] Bernal, P. N., Delrot, P., Loterie, D., Li, Y., Malda, J., Moser, C., Levato, R. Volumetric bioprinting of complex living-tissue constructs within seconds, Adv Mater, 2019, 31(42), 1904209. [39] Layani, M., Wang, X., Magdassi, S. Novel materials for 3D printing by photopolymerization, Adv Mater, 2018, 30(41), 1706344.

Index (mercaptopropyl)methylsiloxane 162 (meth)acrylated chitosan 184 (meth)acrylated poly(ethylene glycol) 178 “dynamic” caging 281 2-nitrophenyl phenyl sulfide 152 3D helical mircorobots 273 3D printing 6, 11, 17, 20, 24, 29, 33, 35, 37, 231–232, 239, 241–242, 245, 295–298, 303–308 3D printing and microfabrication 83 405 nm 232, 241, 245 4-acryloylmorpholine (ACMO) 318 4D printing 296, 320 800-nm infrared 40 absorber. See photo-blocker accuracy 254 acrylate 144–149, 153–154, 161 additive manufacturing 203, 213–214, 226, 249 AMF file 136 analytical devices 143 anthracene 152 anthraquinone – 15-DAAQ 32 application 206, 208, 212, 216–217, 219–221 applications 189 aspect ratio 254 ASTM 135 attenuation coefficient 154–156 average photon flux intensity 255 barium titanate NPs 285 beam shaping 265 Beer-Lambert law 154 benzil ketals 151 benzoin 151 benzophenone 151 benzotriazole 152 benzyl-N,N’-dimethylamine 153 beyond diffraction limit 250 binder jetting (BJ) 320 bio-applications 273 Biological 83 biomaterials 145 biomedical 281

https://doi.org/10.1515/9783110570588-011

biomedicine 281 bisphenol A 147 blood-brain barrier 283 blue light 6, 23 bone regeneration 189 bottom-up 250 butyl nitrite 141 butylated hydroxytoluene 153 CAD. See computer-aided design CAL 330 camphorquinone 141, 151 capacitors 284 carbazole 12 – other carbazole 14 – oxime-esters 12 carbon nanotubes 283 cardiomyocytes 329 cardoimyocytes (CMs) 328 cartilage regeneration 193 cationic 232, 236–237, 243–244 cationic resins 146 cell culture 283 cell engineering 281 cell fixation 281 cell migration 281 cell stimulation 285 ceramic particles 162 ceramics 269, 286 chain inhibition 252 chain reaction 252 chain transfer 252 charge transfer 283 chemical stability 161–162, 164 chiral 277 cholesteric liquid crystals 287 CLIP. See continuous light interface production clobetasol propionate (CBS) 324 color microactuators 287 Composite materials 84 computed axial lithography (CAL) 323 computed axial lithography 141 computed tomography (CT), 323 computed tomography 141 computer-aided design (CAD) 317 computer-aided design 136, 143–144

336

Index

conductive 283 continuing scanning 254 continuing scanning mode 257 continuous light interface production 140 continuous liquid interface production 212 continuous liquid interface production (CLIP) 320 coumarin 52 CP 25–26, 31 critical conversion 155–156 critical energy 156 CT 328 CTC 17, 30 – amine 17 – amine-iodonium 17 – phenylamine 17 – phosphine 21 cure depth 33 curing time 155 dead zone 140, 320, 323 degenerative exchange 303–304 degraded – biodegradable 281 dental resin 5, 23, 31 diameter 255 differentiation – cell differentiation 285 digital light processing 138, 208 digital light projection (DLP) 318 digital mirror device 139 diluents 152 DLP 319–320, 326–327 dormant 295, 298, 303, 308 drug delivery 281 drug delivery systems 197 dual-response 287 dye – dyes 107–108, 110–111, 113–115, 118–128 D-π-A-π-D 52 elasto-mechanical unfeelability cloaks 277 elastomers 145 electrodeless reduction 283 electron donating group – EDG 12, 19, 26 electron transfer 252 electron withdrawing group – EWG 12, 19, 26 endothelial cells (ECs) 328

energy conversion 252 epoxide 147–148 epoxy group 267 epoxy resins 146 ethyl 4-(dimethylamino) benzoate 141 extracellular matrix 187 feature size 254 femtosecond direct laser writing (fs-DLW) – fs-DLW 249 fillers 153 flavone – 3HF 2, 4 – 6HF 2, 4 – 7HF 2 – chrysin 2 – myricetin 2, 4 fluorene 54 fluorinated oil 322 Fluoropor 163 fractional conversion 154 free radical 150 free radical polymerization 252 freeform reversible embedding of suspended hydrogels (FRESH) 329 frontal polymerization 155 FRP 26, 30 fs-DLW 249 full width at half maximum 257 functional oxides photoresist 271 functionalities 295–296, 304–305, 308 fused deposition modeling (FDM) 135, 161, 324 galvanometer 137–138 gel point 148, 150, 154 gelation 153 glass transition temperature 146, 148 glycidyl methacrylate (GMA) 326 green light 22, 26 HARP 323 HAuCl4 285 hierarchical parallel manufacturing 266 high reaction rate 252 high refractive index 285 high-area rapid printing (HARP) 322 HOMO 19 HOMO-LUMO gap 19 Hull, Charles 136, 144

Index

humidity-responsive 279 hybrid composites 269 hydrogel 36 hydrogels 271 hydrogen atom transfer 151 hydroxyacetophenone 151 hydroxyethyl methacrylate 162 improve printing speed 264 improvement of resolution 257 induction time 155–156 inhibition 137, 146, 148, 154 inhibition volume 322 inhibitors 152 initial stages 257 integrated tissue–organ printer (ITPO) 326 interfacial solvent polarity 279 iodonium salts 147 iquid crystal display 209 isopropylthioxanthone 165 isotropic stiffness 277 Jacobs curve 156 laminated object manufacturing 135 laser stereolithography. See SLA layer by layer 305 layer print time 158 layer thickness 155–156, 158 layer-by-layer 295, 304, 308 layer-by-layer printing 143 layerless printing 140 length 255 light dose 158 light intensity 154 light penetration 8, 33 light-responsive 279 liquid chromatography 162 liquid crystal display 210 liquid crystals 279 liquid photosensitive resins 204, 206 Lithography 91 LUMO 19 mechanical metamaterials 277 mechanical properties 146, 149–150, 158, 161 mechanisms 295–296, 301, 308 MEMS 279, 283

metal 22 – Cu(I) complex 25 – ruthenium complex 24 – ZnTPP 22 methacrylate 144–146, 153 – perfluoropolyether 162 methacrylated alginate 185 methacrylated bovine serum albumin 182 methacrylated collagen 181 methacrylated poly(vinyl alcohol) 179 methacrylated polycaprolactone 178 methacrylated silk fibroin (Sil-MA) 326 methoxy hydroquinone 153 methyl ethyl hydroquinone 144 methylsilsesquioxane 162 Michael addition 148 microfluidics 135, 143, 157–158, 162–164 – channel dimensions 143, 164 – electrophoresis 164 – gradient generator 163–164 – integrated device 167 – isotachophoresis 164 – micromixing 164 – modular devices 167 – optical detection 144, 164 – PDMS 164–165 microscale continuous projection printing (μCPP) 330 microscope objective – to tightly focus laser beam 253 microswimmers 281 migration – cell migration 281 mircogripper 279 modifications 295, 304 modified BAPO – CNC-BAPO 38 – PEG-BAPO 38 modified hydrogels 273 molar absorption coefficient 154 monomer functionality 155 monomer radicals 252 monomers 204 motion system 253 multi-focus parallel processing 264 multihyroxy 4 multi-jet printing 211 multi-material printing 136, 143, 167 multi-photon absorption 250

337

338

Index

multiple foci 265 multiple-beam interferometry 264 multiwalled carbon nanotubes 284 nano particles 273 nanopillar diffraction gratings 286 natural 2 – coumarin 6 – flavone 2 natural hydrogels 271 neural 283 neuronal repair 196 node geometry 277 nonlinear condition 256 non-radiation state 250 NPG 4, 10 nucleation site 283 observation – cell observation 283 oligomers 204 optical 285 Optical materials 89 optical model 158 optical tunable 286 optical vortices 285 organic polymers 267 organic-inorganic photoresist 269 ORMOCER 269 orthogonal 232, 234–235, 239–242, 244 others – acridone 30 – anthraquinone 32 – dithienophosphole 31 – modified BAPO 35 – naphthalimide 26 – safranine O 33 – semiconductor-metal hybrid nanoparticle 38 – triazine 34 overcuring 152, 158 oxygen 296–297, 305, 307, 309 oxygen diffusion 6, 15 oxygen inhibition 24 oxygen permeable window 322 oxygen tolerant 301, 307–308 PDMS 144 PEGDA 145, 159 penetration depth 151–153, 156

pentaerythritoltetrakis (3-mercaptopropionate) 148 personalized bioink 328 personalized medicine 323 PET-RAFT 299–302, 305–306, 308 PH – PH-resposive 279 phenylamine – NPG 17 photoabsorber 110, 118–119 – photoabsorbers 110, 113, 118, 120 photoactivation 298–299, 301 photoactive cellulose 186 photoactive gelatin 180 photoactive hyaluronic acid 185 photo-blocker 137, 144, 151, 153–154, 157–158, 167 photocleavage 60 photocuring 3D printing 203–204 photocuring 231–232, 239, 243 photoinhibition 322 photoiniferters 299–301, 305 photoinitiation 322 photoinitiator 137, 144, 149, 153–154, 157, 162, 167 – cationic 147 – Type I 148, 150–151 – Type II 150–151 photoinitiators 176, 204, 251 photon flux intensity 254 photonic 285 photonic band gap 286 photonic crystal 286 photonic crystals 286 photopolymerization 157, 203–207, 209, 215–217, 231–232, 234, 236, 238, 241, 244–245, 295–297, 304–306, 308 – kinetics 153, 156 – rate 153–154 photopolymers 177 photoRAFT 295–296, 298, 300–301, 303, 305, 309 photoredox catalysts 301, 305, 307 photoresist 267 piezoelectric 285 pinpoint scanning mode 257 pinpointing scanning 254 PLA 324 poly (propylene fumarate) 180 poly(L-lactic acid) (PLA) 324

Index

poly(vinyl alcohol) (PVA) 324 poly(ε-caprolactone) (PCL) 326 polydimethylsiloxane. See PDMS polyethylene glycol diacrylate. See PEGDA polymer crosslinked networks 295, 298, 301, 304 polymeric crosslinked networks 303 polymerization depth 157 polytetrafluoroethylene 162 postpolymerization 298, 304 postprinting 296, 305, 308 postprinting modification 296, 305, 307–308 precursors 178 preserved 296, 304–305, 308 printing efficiency 264 process parameters 257 PTFE. See polytetrafluoroethylene pyrene 152 pyrogallol 153 pyrolysis 269 quenching effect 259 quenching effect. 259 quenching process 258–259 radical 232, 234, 236–238, 241, 243–244 radical polarizer 285 radical scavenger 4 RAFT agents 296, 298–300, 303–305, 308 RAFT polymerization 298–299, 301–302, 304–306, 308 Rayleigh length 255 reactivatable species 296, 308 relative speed 264 renal/liver tissue repair 196 resins 136, 144 – composites 162 – dual-cure 148 – optical model 158 – thiol-ene 148–149, 154 – thiol-yne 148–149 resistors 284 ring-opening addition 267 rules for choosing photoinitiators 260 scaffold 283 scanning galvanometer 265 Schiff bases 52 SCR368 267

selective laser sintering 135 selective laser sintering (SLS) 320 separation science 162 sequential absorption 250 shape cells 281 shape-memory polymers 145 shrinkage 147–148 silica nanoparticles 162 Sil-MA 327 simultaneous absorption 250 single walled carbon nanotubes 283 single-photon absorption 250 skin regeneration 193 SLA 135–136, 143, 157, 161, 320 – advantages and disadvantages 143 – bottom-up configuration 137, 139 – glass 164 – materials 161 – PDMS 164 – top-down configuration 137 sodium persulfate 24 software 254 sol-gel 271 SOLIDWORKS 136 solvent stability 143 soy protein isolate 184 space-time focusing technique 264 spatial light modulator 265 spatial light modulators 264 spinal cord 330 spiral phase 285 SR400 267 stabilizer 137 stereo lithography Apparatus 207 stereolithographic apparatus for tissue engineering (SLATE) 329 stereolithography (SLA) 317 stereolithography. See SLA stimulated emission depletion microscopy – STED 260 STL file 136 SU-8 267, 285 subdiffraction-limit resolution 252 sub-threshold 257 Sudan Orange 152 sulfonium salts 68 supernatural phenomena 265 superparamagnetic 281 surface roughness 139

339

340

Index

surface-eroding polymer 324 synthetic hydrogels 273 tailored bandgap 285 temperature and light respond 279 the finite element simulation 277 thermal stability 162 thiocarbonylthio 298, 300–301, 304 thioxanthone 151 tissue engineering 283 titania photoresist 271 TPA cross-section 250 TPL 249–250 triazine 33 triethylene glycol dimethacrylate 145 trimethylolpropane triacrylate 146, 156 trimethylolpropanetris (3-mercaptopropionate) 148 tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate 148 trithiocarbonate 300, 303 TTC 300, 303–305, 308 TTCs 303–306 tubular organs and tissues 326 two-order nonlinear optical absorption 250 two-photon 3D printing 212 two-photon 12, 38 – HAPI 40 – oxime-esters 40 – z-scan 12 two-photon absorption (TPA) 250 Two-photon absorption 49, 249 Two-photon cationic photoinitiators 68 Two-photon free-radical photoinitiator 51

two-photon lithography (TPL) – TPL 249 two-photon polymerization 49 – TPP 249 Two-photon sensitizers 70 ultrafast laser 253 ultrafast shutter 254 under air 5, 23, 26 urethane acrylate 267 urethane diacrylate 144 urethane dimethacrylate 145 UV 6, 12, 26, 233–234, 238–242, 244–245 vanillic acid (VA) 324 vascularized tissues 195 vinylmethoxysiloxane 162 virtual state 250 viscosity 146, 150, 152 visible light 30, 234, 238–240, 242, 244, 297, 300, 304, 306 vitrification 150 volumetric photopolymerization 141 water soluble 71 water-soluble PI 28 wavelength 231–241, 243–245 Young’s modulus 146, 148 Zr-containing acrylic photoresist 271 z-resolution 137, 152 α-cleavage 150