Applications of Conductive Polymers 9781774695579, 9781774690765

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Applications of Conductive Polymers

APPLICATIONS OF CONDUCTIVE POLYMERS

Edited by: Saeed Farrokhpay

ARCLER

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www.arclerpress.com

Applications of Conductive Polymers Saeed Farrokhpay

Arcler Press 224 Shoreacres Road Burlington, ON L7L 2H2 Canada www.arclerpress.com Email: [email protected]

e-book Edition 2023 ISBN: 978-1-77469-557-9 (e-book)

This book contains information obtained from highly regarded resources. Reprinted material sources are indicated and copyright remains with the original owners. Copyright for images and other graphics remains with the original owners as indicated. A Wide variety of references are listed. Reasonable efforts have been made to publish reliable data. Authors or Editors or Publishers are not responsible for the accuracy of the information in the published chapters or consequences of their use. The publisher assumes no responsibility for any damage or grievance to the persons or property arising out of the use of any materials, instructions, methods or thoughts in the book. The authors or editors and the publisher have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission has not been obtained. If any copyright holder has not been acknowledged, please write to us so we may rectify.

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© 2023 Arcler Press ISBN: 978-1-77469-076-5 (Hardcover)

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ABOUT THE EDITOR

Dr Saeed Farrokhpay is a Chemical Engineer with several years of experience in mineral & material processing. He obtained his PhD from University of South Australia in 2005. He is currently a Technical Consultant in Australia. He has worked for more than 20 years at mineral and chemical industries, universities and research centers around the world. Dr Farrokhpay has published more than 90 papers in high ranked journals and conference proceedings. He has also edited several technical and scientific books, and served as an editorial board member of several international scientific journals.

TABLE OF CONTENTS



List of Figures.................................................................................................xi



List of Tables................................................................................................. xix



List of Abbreviations..................................................................................... xxi

Preface.................................................................................................... ....xxv Chapter 1

Fundamentals of Conducting Polymers...................................................... 1 1.1. Introduction......................................................................................... 2 1.2. History................................................................................................. 3 1.3. The Innovation of Conducting Polymers (CPS)...................................... 4 1.4. Types................................................................................................... 7 1.5. Synthesis.............................................................................................. 8 1.6. Molecular Basis of the Electrical Conductivity..................................... 9 1.7. Structural Characteristics and the Concept of Doping........................ 10 1.8. Charge Carriers and the Mechanism of Conducting........................... 14 References................................................................................................ 22

Chapter 2

Applications of Conducting Polymers in Drug Delivery........................... 31 2.1. Introduction....................................................................................... 32 2.2. Intrinsically Conducting Polymers (CPS)............................................. 34 2.3. Drug Loading..................................................................................... 37 2.4. Drug Release..................................................................................... 39 2.5. The Architecture of ICPS for DDS....................................................... 42 2.6. Summary and Outlook....................................................................... 59 References................................................................................................ 61

Chapter 3

Applications of Conductive Polymers in Textile Industry......................... 71 3.1. Introduction....................................................................................... 72 3.2. Conductive Polymers and Mechanism of Conductivity....................... 75 3.3. Production of Electrically Conductive Textiles.................................... 83

3.4. Coating Textile Techniques................................................................. 91 3.5. Embroidery Techniques...................................................................... 96 3.6. Electrically Conductive Textiles and Smart Textiles Applications......... 97 3.7. Future Prospects.............................................................................. 102 References.............................................................................................. 104 Chapter 4

Use of Conducting Polymers in Flexible Supercapacitors....................... 113 4.1. Introduction..................................................................................... 114 4.2. Flexible Supercapacitors from Conducting Polymer (CP).................. 117 4.3. Flexible Supercapacitors from Conducting Polymer (CP)-Based Films........................................................................... 126 4.4. Flexible Supercapacitors from Conducting Polymer (CP)-Based Fibers.......................................................................... 131 4.5. Summary and Future Scenarios........................................................ 135 References.............................................................................................. 137

Chapter 5

Conductive Polymer-Based Organic Solar Cells..................................... 145 5.1. Introduction..................................................................................... 146 5.2. The Present Situation........................................................................ 153 5.3. Properties of Organic Solar Cells..................................................... 154 5.4. Solar Cell Architectures.................................................................... 156 5.5. Operational Principles of OSCS....................................................... 160 5.6. Characterization of Organic Solar Cells........................................... 165 References.............................................................................................. 169

Chapter 6

Conductive Polymer-Based Membranes................................................. 179 6.1. Introduction..................................................................................... 180 6.2. Membranes Centered on Pani (Polyaniline) and Their Uses.............. 180 6.3. Polypyrrole-Centered Membranes and their Uses............................. 187 6.4. Conductive Polymers Centered Membranes Utilized For Fuel Cells. 192 6.5. Summary and Outlook..................................................................... 194 References.............................................................................................. 196

Chapter 7

Applications of Conducting Polymers in Tissue Engineering.................. 203 7.1. Introduction..................................................................................... 204 7.2. Pure Conducting Polymer (CP) Films for Tissue Engineering............. 205 7.3. Conducting Composite Films or Blends for Tissue Engineering......... 206 viii

7.4. The Conduction of Copolymer Films for Tissue Engineering............. 208 7.5. Bone Tissue Engineering.................................................................. 210 7.6. Cardiac Tissue Engineering.............................................................. 213 7.7. Skin Tissue Engineering.................................................................... 215 7.8. Nerve Tissue Engineering................................................................. 216 References.............................................................................................. 219 Chapter 8

Nanostructured Conductive Polymers for Energy Storage Applications........................................................................................... 223 8.1. Introduction..................................................................................... 224 8.2. Nanostructured Conductive Polymers as Active Electrodes For Electrochemical Capacitors (ECS)........................... 226 8.3. Nanostructured Conductive Polymers as Active Electrodes For Lithium-Ion Batteries............................................................... 231 8.4. Nanostructured Conductive Polymers as Functional Materials For Li-Ion Batteries......................................................... 233 References.............................................................................................. 237

Index...................................................................................................... 243

ix

LIST OF FIGURES

Figure 1.1. Structures of the nanostructured compounds utilized as conductive polymers Figure 1.2. The conductive materials and their comparative conductivities Figure 1.3. Molecular structure of PA(polyacetylene) Figure 1.4. Picture of the three awardees of Nobel Prize in 2000 in Chemistry Alan G. Prof. MacDiarmid (left), Prof. Hideki Shirakawa (middle), and Prof. Alan J. Heeger (right) Figure 1.5. A basic diagram of the conjugated support of the conductive polymer Figure 1.6. Molecular structure of the usual conducting polymers (a) trans-PA; (b) PTH (polythiophenes); (c) PPP (poly(p-phenylene)); (d) PPy (polypyrrole); (e) PPV (poly(pphenylenevinylene)); (f) PTV (poly(2, 5-thienylenevinylene)) Figure 1.7. The conductivity of the conducting polymers can normally cover the entire insulator to a semiconductor to the metal region by varying the degree of doping Figure 1.8. Representation structure of (a) the positive polaron; (b) the positive bipolaron; and (c) 2 positive bipolarons in the polythiophenes Figure 2.1. The reversible redox activity of PEDOT (poly(3,4-ethylene dioxythiophene) through de-doping (reduction processes) and doping (oxidation) Figure 2.2. (a) The scale of material’s conductivity; (b) chemical structure of the mostly employed ICPs (PPy, PAni, PEDOT, and PPy) Figure 2.3. Diagram representing the procedure of drug loading and release in ICPs: (a) anionic drug encapsulation throughout the polymerization procedure (one step) and discharge upon reduction of the matrix; (b) loading of anionic drugs in three-steps and discharge upon reduction; and (c) loading of cationic drug and discharge upon oxidation Figure 2.4. SEM images of PPy films doped with Dex: (a) as prepared; and (b) later the application of 50 CVs Figure 2.5. Scheme of diverse nanostructures Figure 2.6. (a) Optical diagram of the neural probe with four electrodes covered with PEDOT/Dex. (b) Scheme representing the probe inserting utilizing an optical fiber as a guide. (c) Scheme showing the location of the electrodes in the skull, it is probable to visualize either the active and passive probes. (d) Photograph of the animal’s head where the connector amongst the active probe and the recording/stimulation device is located

Figure 2.7. (a) Chemical structure of the different reagents. (b) Chemical structure of the drugs utilized in this work: first (fluorescein) and second (daunorubicin). (c) Picture showing the solid-gel transition of the injectable conductive hydrogel. (d) SEM micrograph of PPy NPs laden with fluorescein. (e) SEM micrograph of PPy NPs inserted on the hydrogel Figure 2.8. (a) Schematic diagram of DOX/PPy (DOX-attached PPy nanowires). EDC/ NHS (1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide/N-hydroxysuccinimide) was utilized to chemically conjugate DOX to the biotin dopants of the PPy nanowires. DOX/PPy nanowire ranges had a double function for cancer therapy. By application of electrical stimuli could release the chemotherapeutic agent and, through localized NIR irradiation, work as photothermal agents. (b) SEM micrographs of the fabricated PPy nanowire arrays. Electropolymerization was executed by applying a continuous potential of 1 V for 6 (left) and 18 min (right) to get PPy nanowires with a length of 5 (left), 15 (middle), and 25 μm (right), respectively Figure 2.9. Image of the procedure steps to produce the PPy scaffold and the loading of the drug (a). SEM micrographs of (b) PMMA colloidal crystal template on the stainless substrate; (c) the PPy film got through electropolymerization and following PMMA template elimination; (d) and (e) cross-sectional images of the PPy structures after the electropolymerization of the dense PPy layer Figure 2.10. (a) Scheme of reversible alterations in pore size (and the drug release rate) among reduction and oxidation states. (b) Representation of flux vs. time for a 110 nm pore diameter membrane at the reduction state (magenta closed circles) and the oxidation (blue open circles). The data point was gathered every 10 s. (c) In situ AFM height images corresponding to the reduction states (right)and the oxidation (left) Figure 2.11. Snaps of chitosan-g-PAni comprising solution (a) before crosslinking and (b) after crosslinking with OD. (c) Graph comparing the amoxicillin discharge in PBS (phosphate-buffered saline) solution under diverse electric potentials, which were repeated every 10 min Figure 2.12. (a) Scheme outlining the filling and carrying mechanisms of CNT platforms. SEM micrographs of (b, c) PPy/CNT and (d, e) PPy/CNTb matrixes. CNT: exterior diameter 110–170 nm; CNTb: exterior diameter 20–30 nm (ii) Figure 2.13. Corrosion-driven drug release. Dex discharge from PEDOT/GO/Dex films placed onto a magnesium surface in which the drug delivery is driven by the substrate corrosion. Magnesium samples had rather short or long exposed areas however, a similar amount of coverage through the PEDOT/GO/Dex coating Figure 2.14. (a) Cross-section of the cellulose PPy composite film. (b) High magnification picture of (a). (c) Energy-dispersive X-ray spectrum of the cellulosePPy composite film. (d) Digital photo of the drug delivery method with the coating of magnesium layer on a single side of the cellulose-PPy composite film Figure 2.15. Diagrams of the microelectronic cardiac patch concept

xii

Figure 3.1. Conductivity range of polymers matched to other materials conductivity Figure 3.2. Simplified diagram of a conjugated backbone: A chain comprising alternating single and double bonds Figure 3.3. Three, two, and one-dimensional carbon materials: (a) diamond; (b) graphite; (c) polyacetylene chain Figure 3.4. Soliton interference through the polyacetylene chain Figure 3.5. Diagram representation of the techniques of polymer modification Figure 3.6. Reaction structure for preparation of: (a) polyaniline founded graft copolymer; (b) pyrrole-styrene graft copolymers Figure 3.7. Chitosan-graft-polyaniline is created through the oxidative-radical graft copolymerization Figure 3.8. ICPs fibers: (a) melt rolled thermoplastic trans-1,4-polyisoprene and doping with iodine; (b) SEM image of PEDOT: PSS-PEG fiber turned into isopropanol Figure 3.9. Low vacuum SEM pictures: (a, b) binary blend fibers (PP/PANI complex); (c, d) ternary blend fibers (PP/PA6/PANI complex), organized at a draw ratio of 2 Figure 3.10. Textile sensor addition into fabric: (a) PEDOT: PSS yarn coated through the roll-to-roll coating; (b) 2D weaving fabric with incorporated textile sensors Figure 3.11. Sensing knitted fabric: (a) technical face with conductive yarns; (b) contact points among conductive loops Figure 3.12. SEM pictures of the alloy-coated fabrics got at different K4Fe(CN)6 concentration in the bath: (a) 0 ppm; (b) 1 ppm; (c) 2 ppm; (d) 4 ppm Figure 3.13. Optical microscopy pictures of CB coated cotton fabrics with diverse amount of CB loading: (a) 0.5 mg/ml; (b) 1 mg/ml; (c) 2 mg/ml; (d) 5 mg/ml Figure 3.14. SEM pictures of GF fibers later PPy coating: (a, b, c) concentration of doping agent from 0.05 M to 0.1 M; (d) two coatings of PPy at little concentration of tosylate Figure 3.15. Printable conductive textile structures: (a) conductive gel trial on fabric with connecting leads; (b) humidity sensors Figure 3.16. Sport and fitness, health applications: (a) textile wristband with a PEDOT: PSS electrode; (b) edema stocking device; (c) sensoria smart socks; (d) Adidas textile electrode for sports top; (e) fitness shirt having a biometric function; (f) life tech jacket Figure 3.17. Textile electrodes for health monitoring: (a) textile headband; (b) knitted directly into a shirt (1) and the electrical path to the electrode (2) Figure 3.18. Automotive applications: (a) thermoplastic cross stiffener growth (ENSAIT, Gemtex laboratory); (b) conductive inks published on flat plastic and shaped into 3-dimensional car constituents (T-Ink) Figure 3.19. Other applications: (a) superamphiphobic fabric; (b) spandex founded electrochromic textile device; (c) textile electrodes to gather the electrical reply of a Venus Pytrap; (d) artificial horizon/radar barrier xiii

Figure 4.1. Structure of the flexible supercapacitor Figure 4.2. Chemical arrangement of: (a) PANI; :(b) polypyrrole; (c) polythiophene; and (d) PEDOT Figure 4.3. Features and applications of the structural hydrogels Figure 4.4. (a) Graphic representations of the three-dimensional hierarchical microstructure of gelated polyaniline hydrogel where the phytic acid behaves as a crosslinker and the dopant. Three levels of the hierarchical permeability from angstrom, nm (nanometer) to the pores of micron size have been outlined by red arrows. (b) An image of the polyaniline hydrogel inside the glass vial. (c) Graphic representation of the creation of three-dimensional hierarchical nanostructured polyaniline hydrogel and the photograph of PANI hydrogel inside the glass vial Figure 4.5. (a) Synthesis of polyaniline bearing the boronic acid groups. (b) Polyaniline/ poly(vinyl alcohol) hydrogel at 3 length-scales. left: an image of polyaniline/poly(vinyl alcohol) hydrogel; middle: the SEM image of polyaniline/poly(vinyl alcohol) hydrogel; right: the schematic molecular structure of polyaniline/poly(vinyl alcohol) hydrogel displaying the crosslink between PVA and PANI. (c) SEM image of polyaniline/ poly(vinyl alcohol) hydrogel displaying the porous structure. (d) The photographs of reactions with diverse amalgamations of reagents. Vial 2 possesses all of the 4 reactants and offers polyaniline/poly(vinyl alcohol) hydrogel. Vial 1 doesn’t possess any APS; vial 3 doesn’t possess any PVA; vial 4 doesn’t possess any ABA; vial 5 doesn’t possess any aniline Figure 4.6. (a) Synthesis standard of the structurally stretchable and flexible along with the electrically conductive networks of a hybrid hydrogel. (b) Molecular structure of the Zn-tpy supramolecule. (c) The G-Zn-tpy displays the reversible sol-gel phase shift at 50°C above which the G-Zn-tpy becomes the homogeneous solution. (d) Graphic illustration of anticipated mechanisms of the self-healing conduct for the consequential supramolecular gels. The vigorous intermolecular coordination and interaction at the position of crack assists heal the material of gel Figure 4.7. The method of bottom-up infilling. Left-right: sol is cast on the porous electrode situated on the gas-permeable substrate and concealed with the impermeable film; gel shapes from the bottom-up till the complete spongy electrode is infilled with the gel; the free-standing electrode filled with gel is acquired after elimination of the substrate Figure 4.8. Graphic representation of the making of smart, flexible, and stretchable supercapacitor Figure 4.9. Graphic representation of the creation of flexible polyaniline/rGO/multiwall carbon nanotube ternary hybrid film Figure 4.10. Making of HCFs and creation of the hollow structures. (a) Schematic representation; (b, c) cross-sectional SEM-based images of HCF at high and low magnifications, correspondingly; (d, e) SEM-based images of HCF by the side view at high and low magnifications, correspondingly; (h, i) SEM-based images of HPF by the side view at high and low magnifications, correspondingly xiv

Figure 5.1. Illustration of bonding-antibonding interactions between the HOMO/ LUMO levels of an organic semiconductor Figure 5.2. Chemical organization of organic solar cell donor and acceptor materials Figure 5.3. Illustration of an organic solar cell (construction of an organic photovoltaic device. Glass is the substrate, the negative electrode is (indium tin oxide) ITO, aluminum, is a general transparent electrode. The illustration presents a BHJ (bulk heterojunction) active layer where the blending of an acceptor and donor creates phaseseparated domains inside the active layer. The structure of the BHJ is essential to the execution of a solar device Figure 5.4. A fullerene derivative and several solutions processible conjugated polymers used in organic solar cells. Chemical arrangement and abbreviations of some conjugated organic molecules. From left: poly(para-phenylene-vinylene) (PPV); poly(acetylene) (PA), a relieved PPV (MDMO-PPV), poly(3-hexyl thiophene) (P3HT); and a C60 derived In every compound a pattern of irregular single and double bonds could be identified Figure 5.5. Transparent and flexible solar cells Figure 5.6. The structure of a single-layer and a multilayer organic solar cell Figure 5.7. Organization of a bilayer solar cell Figure 5.8. Organization of a bulk heterojunction solar cell Figure 5.9. Structure of a tandem cell Figure 5.10. AM1.5G reference solar spectrum Figure 5.11. Band placement of donor and acceptor materials for a heterojunction Figure 5.12. Image of the nature of charge separation in a solar cell Figure 5.13. Operation of a solar cell at various biases: (i) huge reverse bias; (ii) little reverse bias; (iii) positive bias, 0 resultant internal fields, and complying to open circuit condition; (iv) positive bias, carrier injection Figure 6.1. Synthesis path and structure of PANI (polyaniline) Figure 6.2. Structural forms of PANI. (a) leucoemeraldine; (b) pernigraniline; and (c) emeraldine base Figure 6.3. Polymerization of polypyrrole via chemical oxidation Figure 6.4. Polymerization of polypyrrole via chemical oxidation in the existence of the dopant Figure 6.5. Dimer sequences inside the polypyrrole structure. (a) α,α,’; (b) α,β’; and (c) β,β’ Figure 7.1. Conductive biomaterials and conducting polymers and their tissue engineering use Figure 7.2. Picture of the PPY/PDLLA nerve conduit (II, III) and PPY/PDLLA film (I) Figure 7.3. (a) The synthesis scheme of PCL3000-AT (the molecular weight of PCL is xv

3000, aniline trimer: AT) electroactive copolymers. (b) Shape memory characteristics of the degradable conductive copolymers. (1) The spiral shape of PCL3000-AT5 film after fixation; (2) the redeemed shape of the spiral film; (3) the circle shape of PCL3000-AT5 film after fixation; (4) the redeemed shape of the circle film. (c) Myogenic differentiation of C2C12 myoblasts. Cultured on PCL80000 and PCL3000-AT5 (the AT matter in the copolymer is 5 wt.%), tubulin (green), and nuclei (blue) immunofluorescence staining of C2C12 cells at day 7. Scale bar: 200 μm Figure 7.4. PEDOT: PSS scaffolds scanning electron microscopy (SEM) pictures Figure 7.5. (a) conductive nanofibrous scaffolds’ schematic fabrication; (b) SEM pictures of PANI nanoparticles, nanofibrous conductive scaffolds, the magnified pictures of scaffolds displaying the nanofiber, and PLA/PANI10: 10%wt PANI in the composite scaffolds; (c) Alizarin red staining of BMSCs on diverse substrates for two weeks Figure 7.6. SEM pictures, characteristic fluorescence pictures, and scheme representing a new conception to make an injectable conductive hydrogel Figure 7.7. (a) Pictures of the polymerized PEDOT/chitosan/gelatin (PEDOT/Cs/gel) scaffolds with dissimilar molar ratios of ammonium persulfate (APS) to EDOT. Gene expression and Neurite growth protein (a1d2) of PC12 cells (d, e) in the scaffolds after 5 days of culture. 3D and 2D confocal fluorescence micrographs of immunostained cells on the PEDOT/Cs/Gel scaffold (b, d) and chitosan/gelatin (Cs/Gel) scaffold (a, c) Figure 8.1. (a) The chemical structures of conventional conductive polymers; (b) the procedure of the doping/dedoping process of PPy; (c) diagrammatic demonstration of the method of polymerization of PPy Figure 8.2. Diagram of a high-performance EC electrode with the given required properties: (i) large electrode surface and interface; (ii) high electrical conductivity; (iii) high ion accessibility; (iv) good electrochemical compatibility; and (v) excellent processability and scalability. The four basic resistances in the electrode are illustrated in the enlarged picture Figure 8.3. (a, b) SEM image and electrochemical properties of the nanosheet of a PPy thin film [29]. (c, d) TEM image of the dehydrated 3D nanostructured PANI hydrogel, and its related capacitive performance (current density versus specific capacitance). The cycling performance of the 3D PANI CPHs. 6 is shown in the inset. (e, f) SEM image and cycling performance of dehydrated polypyrrole hydrogel as supercapacitor electrodes Figure 8.4. Image of a flexible G-PNF film Figure 8.5. (a) TEM picture of arranged HCLO4-doped PANI nanotubes; (b) cycling properties of half cells constructed by HCLO4-doped PANI nanotubes on a current density of 20 mA/g; (c) SEM model of the PPy-ferrocene polymer accumulated on a stainless-steel mesh; the chemical structure of the pyrrole/[(ferrocene) amidopropyl] pyrrole copolymer is shown in the inset; (d) charge-discharge curves of PPy and PPyferrocene polymer cathodes; (e) The TEM representation; (f) and cycling performance of vacuum-assisted layer-by-layer PANI-carbon nanotube electrodes xvi

Figure 8.6. (a) The classical approach utilizing a polymer and a conductive additive being a mechanical binder may lead to broken electric contacts; (b) a nanostructured conductive polymer, which performs numerous functions, as a binder and a conductor, could keep up both the mechanical and electrical integrity of the electrode while cycling Figure 8.7. (a) Diagrammatic illustration of the D3PIE process of dynamic increase with LiFePO4 at the dichloromethane/water interface; (b) The TEM representation of the sulfur-PPy nanotube composites accompanying 30 wt.% sulfur; (c) Cycling performance of the pristine sulfur and sulfur-PPy nanotube composites; (d) A diagrammatic view of 3D porous Si nanoparticle-conductive polymer hydrogel composite electrodes; (e) Electrochemical performance of the in situ polymerized Si-PANI composite electrodes at a charge-discharge current of 1.0 A/g, illustrating a balanced capacity of B1500 mA h/g subsequent to 1000 cycles (shown with red line)

xvii

LIST OF TABLES

Table 1.1. Organic conductive polymers presented concerning their composition Table 1.2. Charge, spin, and chemical term of soliton, bipolaron, and polaron in the conducting polymers Table 2.1. Summary of DDSs founded on ICP-containing hydrogels* Table 3.1. Properties variations of conductive polymers in reply to redox transition Table 3.2. Conductivity and other properties of usual conductive polymers Table 3.3. Benefits and drawbacks of intrinsic and extrinsic conductive fibers/yarns Table 3.4. Yarns are covered with intrinsically conductive polymers Table 3.5. Benefits and drawbacks of electrically conductive textiles Table 4.1. Some conducting polymers-centered composite films and the electrochemical performances Table 4.2. The specific capacitance, flexibility, and electrical of usual conducting polymer-centered supercapacitors

LIST OF ABBREVIATIONS

3D

three-dimensional

AIBN

azobisisobutyronitrile

Al2O3

aluminum oxide

ALP

alkaline phosphatase

APS

ammonium persulfate

ATMP

amino trimethylene phosphonic acid

AZO

aluminum zinc oxide

B boron BC

bacterial cellulose

BHJ

bulk heterojunction

BMSCs

bone marrow stromal cells

C carbon C6H5NH2 aniline CA

chronoamperometry

CAB

cellulose acetate butyrate

CA-PCL-DA

poly(citric acid-co-polycaprolactone-co-dopamine)

CB

carbon black

CNTs

carbon nanotubes

CP

chronopotenciometry

CPCs

conductive polymer composites

CPs

conducting polymers

CSA

camphor sulfonic acid

CV

cyclic voltammetry

CX43

connexin-43

D3PIE

dynamic three-stage interline electropolymerization

DBS

dodecylbenzene sulfonate

DMSO

dimethyl sulfoxide

DOX

doxorubicin

ECPs

extrinsically conductive polymers

ECs

electrochemical capacitors

EDOT

ethylene dioxythiophene

EES

electrochemical energy storage

EG

ethylene glycol

EKG/ECG electrocardiogram EMI

electromagnetic interference

EQE

external quantum efficiency

ESD

electrostatic discharge

ESMP

electroactive shape memory polymer

ESR

electron spin response

ESR

equivalent series resistance

FAS

fluorinated alkyl silane

FD-POSS

fluorinated decyl polyhedral oligomeric silsesquioxane

FET

field-effect transistor

FF

fill factor

FIT

fluctuation induced tunneling

GO

graphene oxide

HCL

hydrochloric acid

HOMO

highest occupied molecular orbital

ICP

intrinsically conductive polymers

IPCE

incident photon to electron conversion efficiency

ITO

indium tin oxide

LBL

layer-by-layer

LED

light-emitting diodes

LUMO

lowest unoccupied molecular orbital

MIM

metal-insulator-metal

MWCNT

multi-walled carbon nanotube

N nitrogen NGF

nerve growth factor

NMP

N-methyl pyrrolidone

NPs

nanoparticles

OCN

osteocalcin xxii

OD

oxidized dextran

P phosphorus PA

polyacetylene

PAAM

polyacrylamide

PANI

polyaniline

PC

polycarbonate

PCE

power conversion efficiency

PCL

polycaprolactone

PDLLA

poly(D, L-lactic acid)

PDMS

polydimethylsiloxane

PEDOT

poly(3,4-ethylene dioxythiophene)

PEEK

polyetheretherketone

PEG

polyethylene glycol

PET

polyethylene terephthalate

pHEMA

poly(2-hydroxyethyl methacrylate)

PLA

poly(lactic acid)

PLGA

poly(lactic-co-glycolic acid)

PMMA

poly(methyl methacrylate)

PNMPy

poly(N-methyl pyrrole)

PPV

poly(phenylene vinylene)

PPy

polypyrrole

Psf

polysulfone

PSS

poly(styrene sulfonate)

PTH

polythiophene

PU

polyurethane

PUU

polyurethane-urea

PV

photovoltaic

PVA

poly(vinyl alcohol)

PVDF

polyvinylidene fluoride

PVS

poly(vinyl sulfonic acid)

rGO

reduced graphene oxide

S sulfur SA

salicylic acid

SCLC

space charge limited current xxiii

SMP

shape memory polymeric

SSA

sulfosalicylic acid

SWCNT

single-walled carbon nanotube

TCR

temperature coefficient of resistivity

THF

tetrahydrofuran

TMV

tobacco mosaic virus

TO

tin oxide

V voltages VRH

variable range hopping

VSC

voltage shorted compaction

xxiv

PREFACE

The past 20 years have witnessed immense exploration of conductive polymers as a substitute to metallic conductors and semiconductors. Moreover, conductive polymers also impart electroactive characteristics to typically passive electronic devices (e.g., tissue scaffolds). Although, conductive polymers have witnessed momentous developments to date but there are also some challenges that have repressed the extensive use of conductive polymers in various medical and electronic applications. There are two major factors that influence the performance of conductive polymers in a biological environment, i.e., mechanical tuneability and stability and persistence of FBR (foreign body response). Conventional conductive polymers are friable and stiff, having the moduli in the range of 50–100 MPa. The moduli of conductive polymers are far less than the metallic components, but their performance is better than many soft biological tissues and rubbers. Moreover, polymer constituents and manufacturing parameters can be varied to achieve tunable mechanical and electrical properties of conductive polymers. This book contains a broad range of the areas on the study of conducting polymers (CPs) and their applications. The book is divided into eight chapters. Each chapter of the book provides a comprehensive overview of the particular aspect of conductive polymers. Chapter 1 contains the introductory topics related to conducting polymers. The chapter briefly discusses about history, classification, synthesis routes and electrical properties of conductive polymers. Chapter 2 offers detailed information about applications of conductive polymers in drug delivery systems. The chapter focusses on a brief introduction of drug loading, drug release, and architectures of the conductive polymers. Textile industry has witnessed an immense change during the past few decades. The role of conductive polymers in revolutionizing the textile industry is phenomenal. Chapter 3 focuses on the applications of conductive polymers in different areas of the textile industry. The advent of micro and nano-technologies has greatly influenced the electronic industry. However, the synthesis of flexible electronic devices from conductive polymers has greatly enhanced the efficiency and quality of electronic devices. Chapter 4 discusses the applications of conductive polymers in the development of supercapacitors.

Chapter 5 focuses on the applications of conductive polymers in the development of organic solar cells. Different architectures of organic solar cells and their properties are discussed in the chapter. Chapter 6 summarizes the applications of conductive polymers in membrane development. The chapter contains a detailed discussion about the development of polyaniline (PANI) based and polypyrrole (PPy)-centered membranes. Chapter 7 discusses the medical applications of conductive polymers in detail. A comprehensive analysis of the applications of conductive polymers in tissue engineering is presented in the chapter. Finally, Chapter 8 offers a detailed description of different conductive polymer materials and their use in energy storage systems. This book can be used as a textbook for students from the field of materials science, polymer engineering, and electronics. Moreover, this book is equally beneficial for researchers, scientists, industrialists, and professional engineers.

CHAPTER

1

FUNDAMENTALS OF CONDUCTING POLYMERS

CONTENTS 1.1. Introduction......................................................................................... 2 1.2. History................................................................................................. 3 1.3. The Innovation of Conducting Polymers (CPS)...................................... 4 1.4. Types................................................................................................... 7 1.5. Synthesis.............................................................................................. 8 1.6. Molecular Basis of the Electrical Conductivity..................................... 9 1.7. Structural Characteristics and the Concept of Doping........................ 10 1.8. Charge Carriers and the Mechanism of Conducting........................... 14 References................................................................................................ 22

2

Applications of Conductive Polymers

1.1. INTRODUCTION Materials can normally be divided into four kinds concerning their electrical properties: (a) insulator, (b) semiconductor, (c) conductor, and (d) superconductor. Generally, a material having conductivity less than 107 Scm– 1 is considered an insulator. The material having conductivity greater than 103 Scm–1 is known as metal while the conductivity of the semiconductor lies in the range of 104–10–6 Scm–1 depending upon the degree of doping. Organic polymers are usually described by S bonds and V (sigma). The sigma bonds are immobile and fixed because of making the covalent bonds amongst the carbon atoms. In contrast, S-electrons in the conjugated polymers are comparatively localized, contrasting the sigma electrons. The plastics are usually organic polymers having saturated macromolecules and are normally utilized as outstanding electrical insulators. Since the discovery of conductive PA (polyacetylene) doped with I (iodine), a new area of conducting polymers (CPs), also known as synthetic metals, has been developed and got the Nobel Prize in 2000 in Chemistry (Figure 1.1) (Walatka et al., 1973; Qi et al., 2012).

Figure 1.1: Structures of the nanostructured compounds utilized as conductive polymers. Source: unauth.

https://pubs.rsc.org/en/content/articlelanding/2016/nr/c5nr08803h/

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These days, CPs hold an important and special position as functionalized materials in the area of material sciences. Here in this chapter, the doping concept, discovery, structural characteristics, conducting mechanism, and charge transport for these CPs will be discussed briefly (Figure 1.2).

Figure 1.2: The conductive materials and their comparative conductivities. Source: https://matmatch.com/blog/electrically-conductive-polymers/.

1.2. HISTORY Polyaniline (PANI) was described first by Henry Letheby in the midnineteenth century, who examined the chemical and electrochemical oxidation products of aniline (C6H5NH2) in the acidic media. He observed that the reduced form was colorless; however the oxidized forms possessed deep blue color (Skotheim, 1997; Kumar et al., 2003). The first very conductive organic compounds were generally the charge transfer complexes (Furukawa, 1996). In the 1950s, scholars stated that the polycyclic aromatic compounds created semi-conducting charge transfer intricate salts with halogens (Mikulski et al., 1975). In 1954, examiners at Bell Labs and somewhere else described organic charge-transfer complexes having resistivities as low as nearly 8 ohms-cm (Stenger-Smith, 1998; Kolla et al., 2005). In the initial 1970s, researchers established that salts of tetrathiafulvalene exhibit nearly metallic conductivity, whereas superconductivity was established in 1980. Extensive research on the

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Applications of Conductive Polymers

charge transfer salts is carried out today. Whereas these compounds weren’t polymers technically, this specified that the organic compounds can also carry current. Whereas organic conductors were formerly occasionally conversed, the field was mainly energized by the expectation of superconductivity after the innovation of BCS theory (Chiang and MacDiarmid, 1986; MacDiarmid and Epstein, 1989). In 1963 Australians D.E. Weiss, B.A. Bolto and coworkers described derivatives of polypyrroles (PPy) having resistivities of 1 ohm-cm cite several reports of analogous high-conductivity oxidized PAs (Bolto et al., 1963; Jagur‐Grodzinski, 2002). With the noteworthy omission of the charge transfer complexes, organic molecules were formerly well-thoughtout insulators or inadequately conducting semiconductors. Consequently, DeSurville, and the coworkers stated high conductivity in the PANI (Su et al., 1980). Similarly, Logan, and Diaz (1980) described PANI films that can act as electrodes. Whereas frequently functioning in the quantum realm of generally less than a hundred nanometers, molecular electronic processes can cooperatively manifest on the macro scale. Instances include negative resistance, quantum tunneling, polarons, and phonon-assisted hopping. In 1977, Alan MacDiarmid, Hideki Shirakawa, and Alan J. Heeger (1977) reported analogous high conductivity in the oxidized I-doped PA. They were given the Nobel Prize in 2000 in Chemistry “for innovation and development of the conductive polymers.” PA didn’t find practical applications by itself, but drew the consideration of scientists and stimulated the quick development of the field. In the late 1980s, OLEDs (organic light-emitting diodes) have appeared as a significant application of the CPs.

1.3. THE INNOVATION OF CONDUCTING POLYMERS (CPS) In the 1960s to 1970s, an innovation, polymer becoming conductive electrically, was almost coming-out. The discovery implied that the polymer has to replicate the metal, which gives the meaning that the electrons in polymers must have free motion and not be stuck to the atoms. Technically, a reduction or oxidation process is frequently accompanied by withdrawing or adding electrons, proposing that the electron can be eliminated from the material via oxidation or presented into the material via reduction. The above idea suggests that the polymer might be conductive electrically by withdrawing electron via oxidation or through adding electron via reduction,

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which procedure was lately defined by the item of doping. The innovation was comprehended by 3 awardees of the Nobel Prize in Chemistry in 2000, were Alan G. MacDiarmid, Alan J. Heeger, and Hideki Shirakawa (Qi et al., 2012). In 1977, they by coincidence found that insulating S-conjugated PA could become a conductor having a conductivity of around 103 Scm–1 by doping of iodine (Shirakawa et al., 1977). The unanticipated innovation not only broke the traditional idea, which the organic polymers were considered only as of the insulators but established the new area of CPs, also known as Synthetic Metals. According to the report of the Royal Swedish Academy of Sciences, there was a fascinating story regarding the discovery of CPs (Walatka et al., 1973). Based on the idea above of polymer replicating the metal, scientists believed that PA could be considered as an outstanding nominee of polymers to be replicating the metal since it has alternating single and double bonds, known as conjugated double bonds. From Figure 1.3, it can be seen, PA is a flat molecule having an angle of 120° amongst the bonds and exists in 2 diverse forms, the isomers trans-PA and cis-PA (Shirakawa et al., 1977).

Figure 1.3: Molecular structure of PA(polyacetylene). Source: https://pubs.rsc.org/en/content/articlelanding/1977/c3/c39770000578.

Thus, the synthesis of PA received great attention at that particular time. At the start of the 1970s, Hedeki Shirakawa was learning the acetylene polymerization into plastics by utilizing catalyst made by Ziegler-Natta, who was given the Nobel Prize of Chemistry in 1963 for the technique of polymerizing propylene or ethylene into plastics. Generally, only the black powder form could be synthesized with the help of the conventional polymerization technique. A visiting researcher in the Shirakawa’s group struggled to synthesize PA in a typical way. However, the beautifully glossy silver-colored film, instead of the black powder made by the conventional technique, was acquired. The unanticipated outcomes promised Shirakawa to validate the conditions of polymerization again and again, and Shirakawa ultimately discovered that the concentration of catalyst used was improved by 103 times. Shirakawa was motivated by the unintentional discovery and

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further discovered the molecular structure of consequential PA was affected by the reaction temperature, for example, the silvery glossy film was transPA while the copper-colored film was nearly pure cis-PA. Somewhere else, physicist Alan J. Heeger and chemist Alan G. MacDiarmid were understanding the first metal-alike inorganic polymer ((SN)x) sulfur nitride, which is the main instance of the covalent polymer without having metal atoms (Mikulski et al., 1975). Prof. MacDiarmid in 1975 visited the Tokyo Institute of Technology, Japan, and gave the talk on sulfur nitride. After the lecture, MacDiarmid come across Shirakawa at the coffee break and exhibited a specimen of the golden sulfur nitride to Shirakawa. Subsequently, Shirakawa also displayed MacDiarmid a specimen of silvery (CH)x. The beautiful glossy silvery film held the eyes of Prof. MacDiarmid and he instantly invited Shirakawa to the University of Pennsylvania in order to further study PA. Since Heeger and MacDiarmid had discovered formerly that the conductivity of sulfur nitride x could be enhanced by times after adding Br (bromine) to the golden sulfur nitride material, which is known as a doping item in the inorganic semiconductor. Thus, they concluded to add some Br to the silvery films of (CH)x to observe what happens. A miracle occurred in November 1976. On that specific day, Shirakawa was working with Dr. C.K. Chiang, the postdoctoral colleague under Professor Heeger, in order to measure the electrical conductivity of PA by the 4-probe method. To their surprise, the conductivity of PA was 10 million times higher as compared to before adding Br. On this day, the first time the “doping” effect was observed in the CPs. In mid-1977, MacDiarmid, Shirakawa, and Heeger co-published their innovation in the article titled “Synthesis of electrically conducting organic polymers: Halogen derivatives of PA (CH)n” in The Journal of Chemical Society, Chemical Communications (QI et al., 2012). After the discovery of conductive PA, important researches dealing with the synthesis of novel materials, solubility, and processability, structural characterization, structure-properties association, and conducting mechanism of the CPs along with their uses in the technology have been broadly studied and substantial progress has been accomplished. After 23 years, the Royal Swedish Academy of Sciences has concluded to give the Nobel Prize for Chemistry in 2000 jointly to “Alan G. MacDiarmid, Alan J. Heeger, and Hideki Shirakawa” for innovation and development of the conductive polymers. The picture of the three scientists is displayed as a Figure 1.4. At present, the area of CPs has been established well and CPs

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hold a vital position as functional materials in the area of material sciences. To date, several articles, books, and reviews dealing with the CPs have been published. In the list of books, “Handbook of Conducting Polymers” by Ed. T. A. Skotheim published in 1986 and re-published in the year 1998 (Skotheim, 1997) is an excellent and ultimately reference book for students and scientists studying CPs. Here in this chapter, thus, only basic concepts and knowledge, like doping, conducting mechanism, characteristic of the molecular structure and transport and electrical properties of the CPs, are discussed briefly.

Figure 1.4: Picture of the three awardees of Nobel Prize in 2000 in Chemistry Alan G. Prof. MacDiarmid (left), Prof. Hideki Shirakawa (middle), and Prof. Alan J. Heeger (right). Source: https://link.springer.com/chapter/10.1007/978-3-540-69323-9_1.

1.4. TYPES Linear-backbone polymer blacks (PPy, PA, PANI, and polyindole) and copolymers are the major types of conductive polymers. PPV (Poly(pphenylene vinylene)) and its soluble derivatives have appeared as the perfect electroluminescent semi-CPs. Nowadays, poly(3-alkyl thiophenes) are the typical materials for transistors and solar cells (Mikulski et al., 1975). Table 1.1 presents some of the organic conductive polymers concerning their composition.

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Applications of Conductive Polymers

Table 1.1: Organic Conductive Polymers Presented Concerning Their Composition The Major Chain Contains • • •

Aromatic cycles Double bonds Aromatic cycles and the double bonds

Heteroatoms Existent No Heteroatom • • • • • • •

polyphenylenes Poly(fluorene)s Polyazulenes Polypyrenes Polynaphthalenes PAC (poly(acetylene)s) PPV (Poly(p-phenylene vinylene))

Nitrogen (N)Containing Nitrogen is in the aromatic cycle: • PPY (poly(pyrrole)s) • Polyindoles • Polycarbazoles • Polyazepines Nitrogen is outside the aromatic cycle: • PANI (polyanilines)

Sulfur (S)-Containing Sulfur is in the aromatic cycle: • PT (poly(thiophene)s) • PEDOT (poly(3,4ethylene dioxythiophene)) Sulfur is outside the aromatic cycle: • PPS (poly(p-phenylene sulfide))

1.5. SYNTHESIS The conductive polymers are made by numerous methods. Most of the conductive polymers are made by oxidative coupling of the monocyclic precursors. Such type of reactions involve dehydrogenation: n H–[X]–H → H–[X]n – H + 2(n – 1) H+ + 2(n – 1) e–

The low solubility of a majority of the polymers presents problems. Some researchers normally add solubilizing functional groups to all or some monomers to upsurge solubility. Other research address this via the creation of nanostructures and the surfactant-stabilized CPs dispersions in the water. These comprise PEDOT: PSS and PANI nanofibers. In several circumstances, the molecular weight of the conductive polymers is quite lower as compared to the conventional polymers like polyethylene. Though, in some of the cases, molecular weight must not be high in order to accomplish the anticipated properties (Wessling and Nalwa, 2000). There are 2 major methods utilized to synthesize the conductive polymers, electro (co)polymerization, and chemical synthesis. Chemical synthesis means linking the C-C bond of the monomers by employing the simple monomers under several conditions, like heating, light exposure, pressing, and catalyst. The benefit is high yield. Though, there are several impurities possible in the end products. Electro (co)polymerization means injecting three electrodes (counter electrode, a working electrode, and

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reference electrode) into a solution including monomers and reactors. By applying voltages(V) to electrodes, a redox reaction to make polymer is encouraged. Electro (co)polymerization can normally be divided into the potentiostatic and cyclic voltammetry (CV) technique by applying constant voltage and cyclic voltage (Gaikwad, 2019). The benefit of electro (co) polymerization is the purity of products, but the technique can synthesize only a few products at one time.

1.6. MOLECULAR BASIS OF THE ELECTRICAL CONDUCTIVITY The conductivity of such kinds of polymers is an outcome of various processes. For instance, in traditional polymers like polyethylenes, valence electrons are stuck in the sp3 hybridized covalent bonds. Sigma-bonding electrons have quite low mobility and don’t add to the material’s electrical conductivity (Peters, 1986; Otero and Cortes, 2001). Though, in the conjugated materials, the condition is entirely different. The CPs generally have supports for connecting sp2 hybridized C (carbon) centers. One valence electron on every center resides in the pz orbital, which is generally orthogonal to other three sigma-bonds. All of the pz orbitals syndicate with one another to the molecule broad delocalized set of the orbitals. The electrons in the delocalized orbitals possess high mobility when a material is doped by oxidation, which eliminates some of the delocalized electrons. Therefore, the interconnected p-orbitals create the 1-D electronic band and the electrons inside this band become movable when it is to some extent emptied (Akamatu et al., 1954). Band structures of the conductive polymers can be easily calculated with the tight-binding model. Theoretically, these identical materials can be “doped” by reduction, which generally adds electrons to an else empty band. In reality, most of the organic conductors are “doped” oxidatively to form p-type materials. Redox doping of the organic conductors is similar to the “doping” of silicon semiconductors, where the small fraction of silicon atoms are substituted by electron-abundant, e.g., P (phosphorus), or the electron-poor, e.g., B (boron), atoms to form n-and p-type semiconductors, correspondingly (Baughman and Shacklette, 1989; György, 2008). Even though usually doping conductive polymers involves reducing or oxidizing the material, the conductive organic polymers linked with the protic solvent might also be self-doped (Hush, 2003).

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Applications of Conductive Polymers

Undoped interconnected polymers states are insulators or semiconductors. In such type of compounds, the gap of energy can be greater than 2 eV, which is quite high for the thermally activated conduction. Thus, undoped interconnected polymers, like polythiophenes, PAs only have a low electrical conductivity of nearly 10–10–10–8 Scm–1. Even at the very low doping level (< 1%), electrical conductivity upsurges various orders of magnitude to the values of nearly 0.1 Scm–1. Consequent doping of CPs will outcome in the saturation of conductivity at the values nearly 0.1 to 10 kScm–1 for diverse polymers. The highest values stated till now are for the conductivity of the stretch-oriented PA with established values of around 80 kScm–1 (Inzelt, 2012). Even though the pi-electrons in PA are delocalized alongside the chain, pristine PA isn’t a metal. PA has alternating double and single bonds which possess lengths of 1.36 and 1.44 Å, correspondingly (Conwell and Mizes, 1990; Paasch, 1992). On doping, the alteration of a bond is reduced in the increase of conductivity. Non-doping upsurges in conductivity can be achieved in a FET (field effect transistor) (OFET or organic FET) and through irradiation. Some of the materials also display negative differential resistance and the voltage-controlled switching similar to that observed in the inorganic amorphous semiconductors. In spite of rigorous research, the association between morphology, conductivity, and chain structure is still poorly comprehended. Usually, it is supposed that conductivity must be higher in order to have better alignment of chains and a higher degree of crystallinity, though this couldn’t be established for PANI and was freshly established for PEDOT, which are very largely amorphous (Menon et al., 1993).

1.7. STRUCTURAL CHARACTERISTICS AND THE CONCEPT OF DOPING Since innovation of conductive PA by doping of iodine, other S-conjugated polymers, like PPy, PANI, PTH (polythiophenes), PPP (poly(p-phenylene)), PPV (poly(p-phenylenevinylene)), and PTV (poly(2,5-thienyl-enevinylene)) have been stated as the CPs, the molecular structure of which is exhibited in Figure 1.6. Normally the ground states of interconnected polymers are divided into non-degenerate and degenerate. An example of degenerate polymers is the trans-PA, which has to interchange C=C and CüC bonds as displayed in Figure 1.5. The total curve of the energy of the trans-PA has two equivalent minima, where the interchanging C=C and CüC bonds are reversed. Conversely, the non-degenerate polymer possesses no 2

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identical structures in its ground state. Most of the conjugated polymers, like PANI and PPy, belong to the non-degenerate group. Band gaps of the conjugated polymers are projected to be usually in the range of 1–3 eV from the electronic absorption spectra. The observations are constant with their semiconductor or insulator electrical properties. From the molecular structure as displayed in Figure 1.6, furthermore, the polymer support in CPs comprises the S-conjugated chain. The overlap of a wave is known as conjugation since it leads to the sequence of alternating single and double bonds, consequential in the unpaired electrons delocalized alongside the polymeric chain (Ferraris and Walatka, 1973).

Figure 1.5: A basic diagram of the conjugated support of the conductive polymer. Source: https://www.sciencedirect.com/science/article/pii/ S1742706114000671.

Figure 1.6: Molecular structure of the usual conducting polymers (a) trans-PA; (b) PTH (polythiophenes); (c) PPP (poly(p-phenylene)); (d) PPy (polypyrrole); (e) PPV (poly(p-phenylenevinylene)); (f) PTV (poly(2, 5-thienylenevinylene)). Source: http://www.nanotech-now.com/encyclopedia-nanoscience-nanotechnology.htm.

Applications of Conductive Polymers

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As mentioned above, PA is the easiest model system for interconnected polymers and is the first specimen for the polymer being CPs, demonstrating S-conjugated chain of polymer is the basic necessity for the polymer in order to become the CP. Delocalization of the S-bonded electrons over a polymeric backbone, existing with uncommon low ionization potentials and high electron affinities triggers exceptional electrical properties of the CPs (Bolto et al., 1963). In contrast, S-conjugated CPs chain triggers poor and insoluble mechanical properties of the CPs, restricting their usage in technology. Thus continue the effort to enhance solubility and to improve the mechanic strength of the CPs is required (Ohnishi et al., 1991; Sato et al., 1991). As described above, the alteration of the S-interconnected polymer from the insulator to the metal is executed by the process of doping. Though, the doping item utilized in CPs varies considerably from customary inorganic semiconductors. Dissimilarities in the doping item between CPs and inorganic semiconductors are exhibited as follows (Karasz et al., 1985; Shen and Wan, 1998): •

The intrinsic “doping” item in the CPs is a reduction (n-type doping) or oxidation (p-type doping) process, instead of atom replacement in the inorganic semiconductors. Using PA as a specimen, for example, the reaction of n-and p-doping is given as: Oxidation with a halogen (p-doping): [CH]n + 3x/2I2 →[CH]nx+ + xI3 (1) Reduction with an alkali metal (n-doping): [CH]n + xNa →[CH]nx– + xNa (2) •

p-doping (removing an electron from the polymeric chain) or the n-doping (addition of electron into a polymeric chain) in the CPs can usually be assimilated and subsequently accompanied with the integration of counterion, like anion for n-doping or cation for p-doping, into the polymer chain to fulfill the electrical nature. In the circumstance of oxidation, taking PA as a specimen again, the molecule of iodine draws an electron from the chain of PA and becomes I3. The molecule of PA, positively charged, is called radical cation (Ginzburg, 1970). Centered on the description above, thus, CPs not only comprise of the S-conjugated chain but also contain counter-ions triggered by doping. This varies from the conventional inorganic semiconductors, in which the counterions are lacking. The exceptional chain structure of the CPs outcomes in the electrical properties being disturbed by the dopant nature and structure of the polymeric chain. The process

Fundamentals of Conducting Polymers

•

13

of doping can be finished through electrochemical or chemical methods. Excluding electrochemical or chemical doping, other methods of doping, like photo-doping and charge-injection doping, are also feasible (De Surville et al., 1968). For example, solar cells are based on photo-doping while LEDs (light-emitting diodes) outcomes from charge-injection doping, correspondingly. Besides, proton doping found in PANI is a rare and effective doping method in the CPs (Shirakawa et al., 1977). The method of proton doping doesn’t involve an alteration in the number of electrons linked with the chain of the polymer that is dissimilar from redox doping where the incomplete addition or removal of the electrons to or from the S-system of the polymer support occurred (Burroughes et al., 1990; Menon, 2000). The insulating S-interconnected polymers can be transformed into CPs by electrochemical or chemical doping and then can be subsequently recombacked to the insulating state by the process of de-doping. This recommends that not just de-doping can occur in the CPs, but the de-doping/reversible doping process, which is quite different from the inorganic semiconductor where the process of de-doping cannot occur. As an outcome, the conductivity of CPs at room temperature covers the entire insulator to a semiconductor to the metal region by varying doping degree as exhibited in Figure 1.7 (Basescu et al., 1987; Naarmann and Theophilou, 1987). In contrast, those procedures are impossible to happen in the inorganic semiconductors!

Figure 1.7: The conductivity of the conducting polymers can normally cover the entire insulator to a semiconductor to the metal region by varying the degree of doping. Source: http://www.nanotech-now.com/encyclopedia-nanoscience-nanotechnology.htm.

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The degree of doping in the inorganic semiconductor is quite low (around a tenth of a thousand), while the degree of doping in the CPs can usually be accomplished as high as 50%. Thus electron density in the CP is quite higher as compared to the inorganic semiconductor; though, the movement of charge carriers is quite lower as compared to the inorganic semiconductor because of poor crystalline or defects (Bi et al., 1985; Long et al., 2011). The CPs are generally composed of O, H, N, and C elements, and the chain structure can normally be altered by adding replaced groups alongside the chain or as a side chain that outcome in CPs preserving flexibility and light-weight of the conventional polymers. Centered on the descriptions above, the CPs are intrinsic instead of conducting plasters made by the physical mixture of an insulating polymer with conducting filler. The dissimilarities of conducting plastics from the CPs also display as follows: conductivity of the conducting plasters increases rapidly at the percolation verge, at this threshold the conductive phase which is dispersed in a nonconductive matrix generally becomes continuous, whereas conductivity of CPs upsurges with an upsurge of the degree of doping. Second is that the conductivity of conducting plastics is quite lower as compared to doped CPs, for example, their conductivity of conducting plastics above the percolation threshold is 0.1 0.5 Scm–1 at 10 wt.% 40 wt.% fractions of conductive filler. Additionally, the position of the percolation threshold is disturbed by the shape of the filler and the particle size (Friend et al., 1999).

1.8. CHARGE CARRIERS AND THE MECHANISM OF CONDUCTING As is quite famous, conductivity (V), as measured by the four-probe technique, is a vital property for the assessment of the CPs. Generally, V is given as neP, where; e is the charge of an electron; n and P are the density and mobility of the charge carriers, correspondingly. The concept of doping in CPs entirely differs from the inorganic semiconductors, as described above, leading to an important difference in the electrical properties between inorganic semiconductors and CPs, which are described as follows: •

Inorganic semiconductor process only a few charge carriers, but the charge carriers possess high mobility because of the purity and high crystalline degree given by these materials. In contrast, CPs possess a high number of these carriers because of the large

Fundamentals of Conducting Polymers

•

15

doping degree (>50%), but the low mobility accredited to the structural defects. The free-electron in metal is considered as the charge carrier; and the temperature dependency of conductivity for the metal upsurges with reducing temperature. In contrast, a hole or electron is assigned as the charge carrier in the inorganic semiconductor, and electrical properties of the semiconductors are usually dominated by the minion charge carrier produced by p- or n-type doping. The transport of charge in the semiconductor is defined by the band model, in this model the electrical properties are controlled by the width of the energy gap, which is described as the difference in energy amongst the conducting band and valence band, as given by Eg. The transport of charge in the semiconductor can be thus given by the following equation:

(3) where; σ0 is the constant; ΔE is the activation energy; к is the Northman constant; T is the temperature. For the CP, polarons, bipolarons, and solitons are anticipated to interpret improvement of conductivity of the S-interconnected polymers from insulator to the metal regime through the process of doping (Kesik et al., 2014). Generally, a soliton is aided as a charge carrier for the degenerated CP (e.g., PA) while bipolaron or polaron is utilized as a charge carrier in the nondegenerated CP (e.g., PANI, and PPy). The model anticipated that soliton can travel along the PA backbone carrying charge but spinless, and if the electron is added to action or is taken away from an anion, the neutral radical soliton is established again (Heeger, 1997). In the mechanism comprising solitons, the electron conduction includes only completely occupied bands in a ground state and triggers the formation of the half-occupied electronic level within the gap. Hypothetical models also validate that the two radical ions react exothermically in order to yield a dianion or dication. The polaron is more stable thermodynamically as compared to two polarons because of electronic repulsion displayed by two charges restricted in a similar site and trigger strong lattice distortions (Brazovskii and Kirova, 1981). In the meantime, polaron is spin while bipolaron is spinless. As an outcome, bipolaron, and polaron can be differentiated through ESR (electron spin response). Schematic positive bipolaron and polaron as 2 positive polarons

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Applications of Conductive Polymers

in the PTH are as displayed in Figure 1.8. The charge, spin, and chemical terms for soliton, bipolaron, and polaron are given in Table 1.2 (Balogun et al., 2015). Therefore charge carrier in the CPs is diverse from free-electron in the metal or hole/electron in the inorganic semiconductor. It must be pointed out that the element of polaron, bipolaron, and soliton is only utilized to understand the electronic motion alongside the section of the polymeric chain (Singh et al., 2017). As mentioned above, the polymeric chain of doped CPs constitutes of S-interconnected length and the counter-ions, depending on doping fashion. Understandably, the conductivity of CPs is disturbed by the parameters as given in Table 1.2 (Cao and Heeger, 1992).

Figure 1.8: Representation structure of (a) the positive polaron; (b) the positive bipolaron; and (c) 2 positive bipolarons in the polythiophenes. Source: https://pubs.acs.org/doi/10.1021/j100068a025. Table 1.2: Charge, Spin, and Chemical Term of Soliton, Bipolaron, and Polaron in the Conducting Polymers Carrier Nature

Chemical Term

Spin

Charge

Neutral soliton

Neutral radical

1/2

0

Negative soliton

Anion

0

E

Positive soliton

Cation

0

+e

Negative polaron (electron polaron)

Radical anion

1/2

E

Positive polaron (hole polaron)

Radical cation

1/2

+e

Negative bipoloron

Dianion

0

2e

Positive bipolaron

Dication

0

+2e

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Chain structure comprises S-conjugated length and structure, crystalline, and replaced grounds, and confined fashion to a polymeric chain. Regarding the structure of the polymeric chain, for example, the maximum value of conductivity in I-doped PA was around 103 Scm–1. In contrast, the maximum conductivity for doped PTH and PPy were below 200 Scm–1. Regarding S-interconnected length, it is discovered that the high number of the conjugated length for the high conductive polymer is pointless since the conductivity of the oligomers is analogous to its long-interconnected polymers (Chiang et al., 1977; Friend et al., 1998). Regarding crystalline, generally, electrical conductivity at room temperature is directly proportional to a crystalline degree due to nearer intermolecular distance in the crystalline phase (MacDiarmid and Epstein, 1993; Roncali et al., 2005). Thus, CPs with the branched-chain possess low conductivity at room temperature is anticipated because of less crystalline: •

•

Dopant structure and the degree of doping are keys to comprehending an insulating S-interconnected polymer to become the CP. The molecular structure of dopants affects the electrical properties as well as solubility in water or organic solvent. For instance, CSA (camphor sulfonic acid) doped PANI not just has high conductivity, but is also soluble in the m-cresol (Cao et al., 1992). Moreover, the diameter and morphology of the CP nanostructures made by either soft- and hard-template techniques are strongly disturbed by dopant degree and dopant nature. Regarding the degree of doping, generally, the roomtemperature conductivity of CPs, as determined by the fourprobe technique, is the function of doping degree, displaying the conductivity upsurges with an upsurge of the degree of doping undergoing from insulator to the metal via a semiconductor (LinLiu et al., 1988; Furukawa, 1996). The conditions of polymerization comprising a concentration of dopant, oxidant, and monomer, the molar ratio of the dopant and oxidant to a monomer, and the polymerization time and temperature are other vital parameters affecting the conductivity since these are added to chain conformation, crystalline, and morphology of the ultimate product. The parameters mentioned above must be kept in mind while studying CPs although their nanostructures!

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Applications of Conductive Polymers

Technically, temperature dependency of conductivity, as determined by the four-probe technique, can be utilized to describe a feature of the charge transport for the material. Temperature dependency of the conductivity can generally be expressed by the logarithmic derivative, . The metal possesses a positive temperature coefficient and the finite dc conductivity when the T→0 is witnessed. In contrast, for semiconductors or insulators is the negative temperature coefficient. The symbol thus can be utilized to differentiate between semiconductor and metal or insulator. The metalliclike conductivity of the CPs at ambient temperature has been perceived (Kaiser, 1989). Furthermore, the metallic properties of doped CPs have been disclosed by the optical properties, magnetic susceptibility, and thermo-electrical power (Fincher et al., 1978). Likewise, heavily doped PTH exhibits metallic properties, like Pauli spin susceptibility and the linear temperature dependency of thermoelectric power has been perceived (Park et al., 1979). Centered on the theory of one-electron band, Furukawa (1996) recommended that the interaction amongst polarons in a polaron lattice triggers the creation of the half-filled band accountable for metallic properties, since the electronic wave-function of every polaron in a polaron lattice is overlaid, demonstrating the electronic states aren’t localized (Weinberger et al., 1979; Mizoguchi et al., 1998). Though the metallic temperature dependency of conductivity isn’t perceived rather than the thermally activated conduction feature of the semiconductor, particularly, the negative temperature coefficient was perceived. Furthermore, finite dc conductivity as the T approaches zero wasn’t observed (Gould et al., 1981; Kaneto et al., 1985). This is accredited to inter-contact confrontation in the inter-granular, inter-crystallinity, or inter-febrile regions of the CPs. An analogous issue has been faced in the measurement of temperature dependency of conductivity of the polycrystalline powder compactions, as determined by the fourprobe technique. Coleman (1978) suggested the VSC (voltage shorted compaction) method could efficiently short circuit the contact resistance of inter-crystallinity, exhibiting true temperature dependency of conductivity. The VSC method is analogous to the four-probe technique, for example, 4 metallic wires with the same distance are utilized as the probes. Though, the sample between 2 voltage terminals is usually shorted by the thin deposit of silver paste (Meixiang et al., 1983). The author demonstrated the validity of the VSC technique by comparison of temperature dependency of conductivity of the Qn(TCNQ)2 polycrystalline powder determined by VSC technique with that of the single crystal determined by four-probe technique (Wan, 2008). Though, the specific conductivity, as determined

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by the VSC method, is meaningless, as the measured conductivity includes the resistance of silver paste. Moreover, the author also recommended the simple physical model to infer why the VSC technique can qualitatively be utilized to conclude the intrinsic properties of temperature dependency of the conductivity of test materials (Wang et al., 2020). The model was supposed that the resistance measured is subjugated by the resistance of 3 layers: resistance of silver paste (signified by layer A), the resistance of particles of the CPs, or the organic poly-crystals submerged in the layer of silver (signified by layer B) and the resistance of tested materials (signified by layer C). The inter-particle interaction resistance in a layer B is entirely shorted by the silver paste. Furthermore, the resistance of layer B can be well-thought-out as the resistance of test material and the silver paste in series. The resistance between the voltage terminals of VSC device, thus, comprises of resistance of the A, B, and C layers in parallel (Cao et al., 1985; Tsukamoto, 1992). The considerable progress in emerging novel CPs and in the improvement of conductivity was evidently for the last 30 years. In 1987, for instance, Murase et al. (1987) stated that conductivity of the doped PA was very high of 104 Scm–1 analogous to those of the traditional metals, displaying the start of a novel generation of the CPs. The conductivity of the doped PA continually increased after that to 105 Scm–1 stated by Hagiwara et al. (1990). Additionally, conductivity of doped (Poly(p-phenylene vinylene)) on order of around 104 Scm–1 was also stated (Park et al., 1988). Furthermore, the PF6-doped PPy made by electrochemical polymerization at the lower temperature exhibited the high conductivity (nearly 103 Scm–1) and for the 1st time, a positive TCR (temperature coefficient of resistivity) was witnessed at the temperature below 20 K (Matveeva, 1996). In the meantime, the conductivity of (CSA) doped PANI as high as 300–400 Scm–1 and the substantial positive TCR in a temperature range of 160–300 K was also witnessed (Rahman and Maiti, 1990). Even though a great development has been made in the improvement of the room-temperature conductivity of CPs, as mentioned above, it is essential to decrease the macroscopic and microscopic disorder and thus discover the intrinsic metallic characteristics of CPs. To date, several models have been suggested to infer charge transport of the CPs. For example, a model of well-thought-out charge transport by the inter-chain hopping has normally been recommended by Matveeva (1996). The model recommended that single chain or the intra-molecular transport, across the inter-particle contact and inter-chain transport contribute to the

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Applications of Conductive Polymers

complete conductivity of CPs, as determined by the four-probe technique. This model also specified that the conductivity and mobility of the CPs are subjugated by both microscopic and macroscopic levels (Rahman and Maiti, 1990). Even though room-temperature conductivity of CPs exhibits the metal-like behavior, the transport of charge across inter-particles and inter-chain in the CPs, as signified by temperature dependency of the conductivity, only shows the semiconductor behavior and follows the VRH (variable range hopping) model recommended by Davis and Mott (1979), which is given as: (4) where; T0 and σ0 are the constants and the value of n= 1,2, and 3 for 3-D, 2-D, and 1-D conduction, correspondingly. The value of n is well-defined as the dimensionality of conduction, which can normally be attained from the graph of log σ against the T1/(n1). The parameter of Mott (T0) is directly proportional to the density of the state at the Fermi level and is also inversely proportional to the length of the location (Chen et al., 2015). It is discovered that the n (dimensionality) is disturbed by the nanostructure’s diameter, for example, 3D-electronic conduction in PPy or PANI nanotubes with the 400 nm is frequently observed while 1D-conduction is happening for the small diameter tubules because of the large ratio of ordered material (Knotek et al., 1973; Sheng and Klafter, 1983). Sheng and Klasfter’s FIT (fluctuation induced tunneling) model has been widely utilized to infer temperature dependency of conductivity of the CPs (Voit and Büttner, 1988). Though some debates on the FIT model were ascended and alteration on the FIT model was also suggested (Kaiser and Graham, 1990). Baughman and Shacklette (1989) suggested the random resistor network model with the broad dissemination of activation energies in order to infer temperature dependency of conductivity for the CPs. An analogous correction amongst the length of conjugation, conductivity, and the exponential temperature dependency of σ (T) was stated by Shklovskii and Spivak (1991). Epstein et al. (1987) offered the metallic islands model, which is the composite model comprising of very high conductivity crystalline areas enclosed by insulating amorphous areas, to infer the transport properties of PANIs protonated by the common acids. Further, some other models, like the random dimer model with the set of conducting delocalized states (Phillips and Wu, 1991) and bipolaron/polaron model comprising multi-phonon and correlated hopping process (Bishop et al., 1981) have

Fundamentals of Conducting Polymers

21

been offered for enlightening the exponential temperature dependency of conductivity perceived in the CPs (Chiang et al., 1978; Stafström et al., 1987). Based on the discussions above, in summary, the CPs not just possess the metal and semiconductor properties, but also preserve many benefits of the conventional polymers, like lightweight, processability, and flexible chain. The exclusive properties are occasioned from delocalized S-interconnected structure and rare doping idea and reversible process of doping/dedoping, and the conducting mechanism entirely differ from either metals or inorganic semiconductors (Br, 1982; Kaufman et al., 1984).

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CHAPTER

2

APPLICATIONS OF CONDUCTING POLYMERS IN DRUG DELIVERY

CONTENTS 2.1. Introduction....................................................................................... 32 2.2. Intrinsically Conducting Polymers (CPS)............................................. 34 2.3. Drug Loading..................................................................................... 37 2.4. Drug Release..................................................................................... 39 2.5. The Architecture of ICPS for DDS....................................................... 42 2.6. Summary and Outlook....................................................................... 59 References................................................................................................ 61

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2.1. INTRODUCTION This work gives an outline of the up-to-date research associated with ICPs (intrinsically conducting polymers (CPs)) and their function as new DDSs (drug delivery systems). Drugs directed to patients don’t continually reach the targeted organ, which might affect other tissues providing undesired side effects. To overwhelmed these issues, DDSs are under development. Currently, it is probable to target the administration and, most significantly, to attain a controlled drug dosage upon exterior stimuli. Mainly, the attention of this work emphasizes the drug release upon electrical stimuli engaging ICPs. These are well recognized organic polymers with exceptional electrical properties similar to metals; however also holding some advantageous characteristics usually associated with polymers, like the easiness of processing and mechanical stability. Relying on the redox state, ICPs could easiness of processing or incorporate cationic or anionic molecules on-demand. Moreover, the releasing rate might be well-tuned by the kind of electrical stimulation applied. Another exciting feature is that ICPs can sense redox molecules like serotonin, ascorbic acid, or dopamine, among others. Thus, future predictions go towards the structure of materials where the releasing rate might be self-adjusted in reply to fluctuations in the surrounding environment. This recompilation of projects and ideas gives a critical outline of ICPs synthesis growth related to their usage as DDSs. Certainly, ICPs are a very favorable branch of DDSs where the dose could be well-tuned through the exertion of an exterior stimulus, hence enhancing the repercussions of the drug and decreasing its side effects (Torchilin, 2005; Chertok et al., 2008). The understanding of animal and human diseases rises year by year, which consequences in a greater capacity to plan and synthesize new drugs for curing them. Though, the effectiveness of such drugs becomes less important if they are not delivered professionally with the suitable dosage for the disease stage, if they not focused on the correct target, or/and if they are left to interrelate with unwanted targets for an extended time, leading to severe side effects (Puiggalí-Jou et al., 2019). Swift developments are also underway in the biomaterials field, which had a great effect on patient care. Though the release and effectiveness of a wide variety of drugs, enzymes, antibodies, and vaccines had been enhanced for decades, the necessity of DDSs (drug delivery systems) with controlled, localized, and effective drug dosage remains (Vickers, 2017). The principal complications of conventional drug administration techniques are the poor

Applications of Conducting Polymers in Drug Delivery

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solubility and quick degradation of the drugs, the probable normal tissue damage, the low biodistribution, the unfavorable pharmacokinetics, and the absence of selectivity. DDSs could be utilized to improve the drug’s solubility through incorporating amphiphilic components (for example, specially designed polymers or lipids). Moreover, DDSs give a carrier for the drug, making problematic the accidental extravasation and enhancing the protection from initial stage degradation. Further, they could slow down the renal clearance, decrease side effects, and enhance drug concentrations in the diseased tissue through the enhanced permeability and ligand-mediated targeting and retention effect (Tiwari et al., 2012). The initial steps aimed at the manufacturing of DDSs were done in 1950, when drugs were integrated into solid polymers to attain constant drug release for agricultural purposes. In the next decades, those methods were extended to biomedicine and, later then, this field has not stopped expanding (Langer and Peppas, 1981). The initial patent for precise drug release, which comprised of the utilization of coatings on edible tablets, was placed by Wurster (1953). In 1968 Zaffaroni established ALZA, the first company devoted to the commercialization of DDSs, which initiated releasing low molecular weight drugs, such as pilocarpine, utilizing poly(hydroxyethylmethacrylate) and ethylene-vinyl acetate copolymer. Later on, numerous other polymers were utilized to retain both high and low molecular weight molecules, which were discharged in a slow fashion manner when opened to aqueous conditions (Langer and Folkman, 1976). In the 1960s, Horne and Bangham studied lipids as units for the bilayered structures, encouraging the research of liposomes as probable drug carriers (Bangham and Horne, 1964). Currently, some systems founded on lipids are commercially utilized for cancer therapy, for instance, Doxil (liposomal doxorubicin), DaunoXome (liposomal Daunorubicin), and Ara-C liposomal (liposomal Cytarabine). Future on, DDSs stretched towards numerous other inorganic materials (for example, iron oxide, gold, and silicon) and organic materials (Quan et al., 2011; Farooq et al., 2018). Amongst the latter, polymers outstand from the other since they could be modeled and managed into an extensive range of forms, like as foams, dendrimers, micelles membranes, fibers, hydrogels, and nanoparticles (NPs). This adaptable feature is joined with their ease of handling, consequences in materials that could be accommodated for the treatment of a large number of medical conditions. Certain of these materials are utilized clinically for an extensive range of therapies. For instance, Lupron Depot, which comprises PLGA (poly(lactic-

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co-glycolic) acid) microspheres that summarize the hormone leuprolide, is utilized to treat advanced prostate cancer (Dreaden et al., 2012; Xia et al., 2014). Biodegradable polymers, for instance, PLGA and PLA (poly(lactic acid), are appropriate materials for constant long-term discharge in vivo. Though, there is no control when the drug is required to be delivered as a reply to an alteration or on-demand. Newborn systems are those termed smart biomaterials or stimuli-responsive (Anglin et al., 2008). When there is an environmental alteration, certain materials are designed to apply a response (for example, enzymes, pH, pressure, temperature, or level of glucose) or, otherwise, the responses could be remotely trigged through exterior stimuli (for example, ultrasounds, near-infrared light, electric currents or magnetic fields). The earlier materials respond to localized alterations on the ambient of pathological irregularities and endorse the drug release, while the latter ones are inspired on demand for pulsatile drug delivery. Generally, it had been claimed that the global advanced market for DDSs is projected to grow from roughly $178.8 billion in 2015 to approximately $227.3 billion by 2020 (Fenton et al., 2018; Senapati et al., 2018).

2.2. INTRINSICALLY CONDUCTING POLYMERS (CPS) Amongst all the DDSs triggerable through exterior stimuli, herein we focus on the latest advances in ICPs (intrinsically CPs), also known as conductive polymers or semi-CPs. In common, ICPs are organic materials with characteristics like those encountered in metals (for example good optical, and electrical properties) and with the exceptional properties of conventional polymers (for example, lightness of weight, easiness in synthesis, and flexibility in processing). In the context of DDSs, ICPs are deliberated as electrochemically dynamic and conducting biomaterials that permit the delivery of electrical and/or electrochemical stimuli (Shirakawa, 2001). The recent era of ICPs was initiated in the late 70s when Heeger, MacDiarmid, and Shirakawa revealed that the conductivity of doped polyacetylene (PA) could enhance considerably upon the addition of iodine counter ions (Shirakawa et al., 1977). The key structural characteristic of ICPs is their conjugated π-system that permits the electron flow by this delocalized path. Hence, the conductivity of ICPs depends on their characteristic electronic structure, which comprises of the alternating single (σ) and double bonds (π) along the polymer chain. The doping procedure

Applications of Conducting Polymers in Drug Delivery

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facilitates the creation of radical dications/dianions (bipolarons) or cations/ anions (polarons) in the backbone, however, counter-ions from the solution pass into the polymeric material to counterbalance the charge (Torras et al., 2012). This procedure is reversible upon de-doping, as revealed in Figure 2.1 for PEDOT (poly(3,4-ethylene dioxythiophene), a typical biocompatible ICP. When electrons are withdrawn from the valence band through the termed p-doping procedure, positively charged holes are made in the electronic structure, however negative charges are produced when electrons are inserted into the conduction band through the n-doping procedure. In both circumstances, there is the need to counterbalance the electronic charge with oppositely charged ions (De Giglio et al., 1999; Hardy et al., 2015).

Figure 2.1: The reversible redox activity of PEDOT (poly(3,4-ethylene dioxythiophene) through de-doping (reduction processes) and doping (oxidation). Source: https://www.sciencedirect.com/science/article/abs/pii/ S016836591930450X.

The electrical conductivity of ICPs rises (Figure 2.2(a)) with the extension of the doping procedure (i.e., doping level). Currently, there are around 25 reported ICPs, which could be synthesized by electrochemical polymerization or oxidative chemical. However, PPy (polypyrrole), PANI (polyaniline), and PEDOT, which are shown in Figure 2.2(b), are the most employed because of their biocompatibility, stability, and good electrochemical and electrical properties. However, in some situations, these materials present certain drawbacks, like rigidness, insolubility, brittleness, poor processability, and lack of biodegradability. To overwhelmed these

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limitations, ICPs could be joined with biodegradable and/or extra flexible polymers, giving place to electroactive blends and copolymers (Huang et al., 2010).

Figure 2.2: (a) The scale of material’s conductivity; (b) chemical structure of the mostly employed ICPs (PPy, PAni, PEDOT, and PPy). Source: https://pubs.rsc.org/en/content/articlelanding/2015/cc/c5cc07405c.

The biocompatibility of ICPs had been studied in a huge variety of cell lines, comprising fibroblasts, endothelial, bone, keratinocytes, myoblast, neural, glial, and mesenchymal stem cells. After the valuation that ICPs were biocompatible with biological structures, their employment in the biomedical field enhanced remarkably (Fielding et al., 2015). Nevertheless, the elimination of unreacted monomers, residual solvent, or extra dopant ions is critical to get non-toxic or, at least, very minute toxic ICPs. Furthermore, it had been claimed that when the materials show nanofeatures could alter the toxicity values, producing adverse biological effects because of the superficial area enhancement. Therefore, it is vital a continuous toxicity assessment of ICP based materials to confirm the safety of the formed devices when utilized for biomedical applications (Ateh et al., 2006a, b; Tandon et al., 2018).

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2.3. DRUG LOADING It is well recognized that DDSs founded on ICPs take profit from their ability to reversibly oxidize and decrease to promote the uptake and exclusion of charged molecules from the polymer backbone. The methodology employed for the loading procedure depends on the physical and chemical characteristics of the drug (for example, molecular charge, weight, chemical composition). An extensive range of medicinal compounds had been discovered, comprising anti-inflammatory, anti-cancer, antibiotics, growth factors, peptides, and proteins. In a very latest review, Tandon et al. (2018) categorized the drug loading as per three different mechanisms: one-step loading of anionic drugs, three-step loading of cationic drugs, and loading of anionic drugs (Oh et al., 2013; Krukiewicz and Zak, 2014). Usually, small anionic drugs could be integrated as dopants by a one-step immobilization procedure during the monomer oxidation. Though caution is needed since relying on the drug characteristics, this simple strategy could strongly interfere with the closing properties of the ICP (for example. diminishing its conductivity, and enhancing its roughness and brittleness), leading in certain cases to low loading effectiveness. A successful alternate for anionic drug incorporation comprises of the following three-step methodology (Wadhwa et al., 2006): • • •

The ICP is synthesized through an anionic primary dopant; A reduction is applied potential to eject this primary dopant however the polymer chain is neutralized; and The preferred biological molecule is integrated as a secondary dopant when the film is oxidized once more. This technique represents a ground-breaking alternate to the earlier described onestep approach since the selected drug doesn’t affect the monomer polymerization. The limitation exists in the possible decrease of the incorporated drug since when the molecule is loaded throughout the polymerization is more possibly to be entrapped at the interior parts of the material. The three methodologies are showed in Figure 2.3 (Alizadeh and Shamaeli, 2014; Novikova et al., 2014).

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Figure 2.3: Diagram representing the procedure of drug loading and release in ICPs: (a) anionic drug encapsulation throughout the polymerization procedure (one step) and discharge upon reduction of the matrix; (b) loading of anionic drugs in three-steps and discharge upon reduction; and (c) loading of cationic drug and discharge upon oxidation. Source: https://www.sciencedirect.com/science/article/abs/pii/ S016836591930450X.

Cationic drugs could be loaded utilizing initially a primary anionic dopant and, afterward, decreasing the film to insert electrons within the polymer backbone. Though the loading of cationic drugs is less common than anionic ones, some instances, comprising neurotrophin growth factor-3, dopamine, chlorpromazine, and N-methyl phenothiazine, had been reported (Puiggalí‐Jou et al., 2017). Seemingly, the entrapment of neutral drugs is more complicated, even though the use of simple physical principles, for instance, the hydrophobichydrophilic effect amongst the anionic dopant and the drug, had been reported. Another approach comprises the creation of strong hydrogen bonds amongst the oxidized polymer chains and the drug. The strength of these intermolecular interactions reduces significantly upon the de-doping of the ICP, assisting the drug release. A more sophisticated path is created on the generation of covalent bonds amongst the ICP and the drug, even

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though in that situation, the release would depend fundamentally on the bond hydrolysis rather than on the exterior triggerable stimuli (Wang et al., 2009; Puiggalí‐Jou et al., 2016).

2.4. DRUG RELEASE Electrostatic forces play a significant role in the discharge of drugs from ICP matrices. Furthermore, the contraction and expansion movements of ICPs, which are because of their electro-chemo-mechanical reply, contribute to the mechanical exclusion of the drug. Since these mechanisms happen simultaneously, it is not probable to distinct them to determine the predominant release driving force (Thompson et al., 2006; Bax et al., 2012). Drugs loaded as secondary or primary dopants on the ICP matrix could be delivered through cyclic voltammetry (CV) (for example, the potential of the operational electrode is ramped overtime at the chosen scan speed and, when it reaches the required potential, it is ramped on the contrary direction to return to the early potential), chronopotentiometry (for example. a fixed current is fixed over a determined period) and chronoamperometry (CA) (for example a fixed potential is fixed over a determined period), hereafter denoted CV, CP, and CA, respectively (George et al., 2005; Li et al., 2006).

2.4.1. Cyclic Voltammetry (CV) Cyclic voltammetry (CV) favors the interchange of counterions amongst the ICP matrix and the medium through oxidation and reduction procedures, the kinetics of such exchange procedures depends on the scan rate. Moreover, there is no fixed potential and, thus, the release could be conducted without a strong optimization. Nonetheless, when numerous oxidation-reduction cycles are applied, the mechanical uniformity of the films might fail because of the electro-chemo-mechanical response of ICPs (Wang et al., 2004; Jang et al., 2017). This signifies a restriction for devices assumed to have a long lifetime. Further, the equipment needed to conduct CVs is more complicated than for fixed potentials. For instance, films of PPy doped with Dex (dexamethasone) were verified as micromachines for recording neural activity through modulating the provocative implant-host tissue reaction (Bastian et al., 2008; Kaur and Dhawan, 2012). After around 30 CV cycles from –0.8 to +1.4 V at 100 mV/s, the film experienced certain physical changes because of the induced actuation movement (Figure 2.4).

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Figure 2.4: SEM images of PPy films doped with Dex: (a) as prepared; and (b) later the application of 50 CVs. Source: https://www.sciencedirect.com/science/article/abs/pii/ S0168365905005687.

In an additional study, PPy was doped with antibiotics (P/S, penicillin/ streptomycin,) or an anti-inflammatory drug (Dex) and placed onto titanium. The displayed data showed that 80% of the loaded drug was discharged after only 5 CVs at 100 mV/s. Excitingly, the reduction and oxidation drug peaks were identified in the voltammograms, therefore, permitting a correlation with the quantity of the drug that was kept in the films. Otherwise, the smart release of an NT-3 (neurotrophic) factor from PPy was observed by CV, pulsed current, and pulsed potential (Jiang et al., 2013; Wang et al., 2016). It was noticed that, independently of the film width (3.6 or 26 μm), CV is more efficient than the other two electrochemical methods. Though, delamination of the polymer from the electrode surface-initiated after 12 min of CV. Besides, Jiang et al. resolved that fluctuations in the velocity of scan could

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regulate the discharge of ATP loaded into a PPy matrix. More precisely, around 57, 89, and 95% of loaded ATP were released after applying diverse velocities; 50, 100, and 200 mV/s, correspondingly, during 10 h. Nevertheless, it must be revealed that other molecules, for instance, monomers, oligomers, and/or salts, could also be discharged during electrochemical stimuli (Hepel and Mahdavi, 1997; Krukiewicz et al., 2015). The methods to quantify the released drugs, for instance, UV absorption spectroscopy, electrochemical quartz crystal microbalance, enzyme-linked immunosorbent assays, and radiometric measurements, might lead to misunderstanding of the results. Asplund and Boehler recommended the usage of high-performance liquid chromatography, which permits the distinction of the signal coming from the monomer or the drug with a detection limit under 5 ng/mL, although this is not the most straightforward approach. In vivo, drug release tests were also done utilizing CV. More precisely, Dex was free from PEDOT utilizing CV scans amongst –0.3 V and 0.45 V at 100 mV/s, both undesired electrochemical reactions and EDOT leakage being evaded under such electrochemical situations (Miller and Zhou, 1987; Zhou et al., 1989). Although ICPs could be employed for in vivo implantable applications, their usage is sometimes limited by their null or low biodegradability. Hence, some strategies to get biodegradable ICPs had been proposed. One approach is the creation of copolymers made of one conducting block associated with biodegradable. For example, Hardy et al. (2014) attained electro responsive oligoaniline related via ester bonds to PCL (polycaprolactone) and PEG (polyethylene glycol). Moreover, the researchers reported their capability to release in a precise manner an anti-inflammatory drug when there is an electrochemical stimulus. Specifically, CVs were conducted amongst 0.7 V and –0.5 V at 50 mV/s. More lately, a similar group prepared supramolecular polymers founded on peptides (oligoalanines) joint with oligoanilines (Hardy et al., 2015; Boehler et al., 2017). The resulting material was doped with Dex phosphate and CSA (camphor sulfonic acid), the concluding being released by CV (amongst 0.7 and –0.5 V at a scan rate of 50 mV/s during 62 s).

2.4.2. Chronoamperometry (CA) and Chronopotentiometry (CP) Rather than sweeping the voltage on diverse potential directions, it is probable to keep the same current or voltage during the chosen time (Pyo and Reynolds, 1996; Xiao et al., 2012).

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The usually employed methodologies are Dex phosphate and CA (steady voltage). When longer periods of times or higher voltages are performed, larger is the drug release. Though, the utilization of extreme power or time is not always the finest option. It is well recognized that, if the potential is very high, the drug could undergo irrevocable damages, dropping the effectiveness. Furthermore, if the voltage is greater than 1.229 V hydrolysis could occur. Hence, it is significant to check the reduction and oxidation potentials of the biological molecules earlier to any usage of CP and CA techniques. The usage of ICPs permits the employment of currents and voltages since these polymers are functional to small electrical variations (Murdan, 2003). The application of negative and positive voltages would depend on the drug charge. Negative voltages would be applied to discharge anionic drugs, however the opposite for cationic drugs. Yet, there are certain exceptions. For instance, George et al. (2006) applied 3 V throughout 150 s to discharge an anionic complex comprising the NGF (nerve growth factor). Other interesting studies demanded that it is probable to employ very little voltages (e.g., –0.05 V) for drug delivery when FeCl3 functions as the oxidizing agent (Uppalapati et al., 2016; Samanta et al., 2018). Though CA is the most well-established approach for drug release, there is yet a clear necessity to exam whether the delivered drugs are still bioactive. Therefore, restrictions in the usage of prolonged times or high voltages currently limit the general applicability of this approach.

2.5. THE ARCHITECTURE OF ICPS FOR DDS ICPs could be prepared through chemical (utilizing an oxidant agent) or electrochemical (applying an oxidizing potential by electrodes) synthesis. Repeatedly, electrochemical approaches are far more employed since providing better control over the charge rate and deposition, which are critical parameters to control the final electrical and electrochemical properties of the material (Syritski et al., 2003; Evans et al., 2009). For their procedure, simple ICP films don’t provide the most effective drug loading capacity, however nanostructures and micro usually offer a larger drug loading capacity because of their higher surface area ratio. Therefore, relying on the final application, three methods, which vary in the template, could be employed throughout ICPs synthesis to direct the desired structure: hard templates, template free, or soft templates (Ghosh et al., 2016).

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The usage of already prevailing nanostructures or micro as hard templates to direct the polymer development is a very simple and manageable approach, although it is restricted through the later elimination of the template. Sometimes, this could lead to film ruptures, loss of consistency, or structural collapses. Also, the dimension of the template limits the amount of ICP that could be produced and might limit the scale-up potentials (Xia et al., 2010). In the situation of chemical synthesis; the mold is required to be added to the reaction solution, which also comprises the monomer, dopant, and oxidizing agent. Instead, when electrochemical synthesis is active, the template is required to be conductive or be positioned close to the electrodes (Richardson et al., 2007; Boehler and Asplund, 2015). The most usual hard templates comprise nanoporous membranes formed of track-etch PC (polycarbonate) or Al2O3 (porous alumina membranes). In a revolutionary work, Martin et al. (1993) made nanofibers of ICPs inside the pores of PC membranes. Excitingly, nanowires as minor as 3 nm in diameter were attained following this strategy. Moreover, this approach could be utilized to form diverse nanostructured morphologies. For instance, Feng et al. (2013) made hollow PEDOT nanotubes through-loading PLGA with the 3, EDOT (4-ethylene dioxythiophene) monomer to form aligned nanofibers utilizing a rotating glass mandrel throughout electrospinning. EDOT was polymerized throughout electrospinning through exposure to an oxidative agent (FeCl3), however, the PLGA and the residual monomer were detached from the resulting nanofibers by absorption in chloroform. In contrast, Zhang et al. (2005) got spheres of PAni and hollow octahedrons utilizing Cu2O as a template in the existence of (NH4)2S2O8 and H3PO4 as an oxidant and a dopant, respectively (Martin et al., 1993; Han and Foulger, 2005). On the other hand, soft templates are founded of self-assembled molecules, which interrelate by non-covalent forces (for example, van der Waals π-π stacking and hydrogen bonds, among others). This method is simple, cheap, and powerful. Molecules utilized as soft-templates comprise oligomers, structure-directing molecules, and surfactants. Also, gas bubbles (for example, H2 or O2 from H2O electrolysis) steadied with surfactants had been utilized as soft templates, which are recognized as soap bubbles (Mucic et al., 1998; Qu and Shi, 2003). Surfactants could self-assemble into micelles, defining the polymer nanostructure. For instance, coreshell nanostructured PPy were produced using different oxidants through microemulsion polymerization. Also, a combination of cationic surfactants (for example, cetyltrimethylammonium bromide, dodecyl trimethyl ammonium bromide, and cetyltrimethylammonium ammonium bromide)

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was utilized to selectively fabricate PEDOT nanocapsules (Fang et al., 2018). The template-free approach is founded on the ability to regulate the nanostructures throughout the polymerization reaction without the utilization of any template. This could be done electrochemically by utilizing high currents, which are reported to create NPs rather than films on the electrodes. Liang et al. attained PAni NPs when current densities of 0.08 mA/cm2 were utilized. The chemical fusion without any template could be done on the interface between liquid and air or when the monomers and the oxidant are dissolved in two immiscible liquids. Nuraje et al. (2008) reported the creation of nanoneedles from the oxidation of pyrrole and aniline at the organic/aqueous interface. Thus, the development and structural design of the ICP DDSs is related to system stability and drug loading capacity. Nanostructures and micro-offer important advantages, like sensitivity and enhanced loading capacity towards the electrical stimuli. Despite these significant properties, a straightforward contrast among the diverse reported ICP structures, which comprise nanowires, fibers, hydrogels, NPs, films, and other 3D organizations that integrate the advantages of nanomaterials (Figure 2.5), had not been stated experimental yet.

Figure 2.5: Scheme of diverse nanostructures. Source: https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.201605529.

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2.5.1. Polymer Films Electrochemically grown thin films are the most common and easy format of ICPs. Electropolymerization of ICPs is a well-recognized methodology that could control the thickness of films whereas integrating biomedical active compounds as doping agents. Therefore, electrochemically synthesized polymer films had been largely employed to discharge drugs on demand. Earlier research has shown that drugs with very diverse chemical properties, such as Dex, SSA (sulfosalicylic acid), chlorpromazine, methotrexate, and ATP, risperidone, NGF, or heparin, could be incorporated into ICP films and unconfined in a controlled fashion. There are outstanding instances reported in literature founded on films prepared with PEDOT, PPy, PNMPy (poly(N-methyl pyrrole), oligoaniline-PCL, oligoaniline, oligoaniline-PCL, oligoaniline-alanine, and oligoaniline-PEG (Cai et al., 2017). For instance, Krukiewicz et al. (2017) established that PEDOT films integrating botulin, a biologically active molecule against a diversity of tumors, show high cytotoxicity against MCF-7 and KB cell lines. The cellkilling enhanced considerably when the botulin discharge was facilitated through applying a difference in potential. Thus, these matrices embrace the great potential for local chemotherapy applications. In a very latest study, neural activity was recorded through inserting electrodes in the hippocampus of rats, local inflammation being effectively avoided through a weekly drug release triggered by CV. Figure 2.6 drafts the embedding of such flexible electrodes. Anti-inflammatory Dex was kept in the PEDOT film and released in an organized manner. The evaluation took 12 weeks and the electrodes discharging the drug had closer neurons than the electrodes working as controls.

Figure 2.6: (a) Optical diagram of the neural probe with four electrodes covered with PEDOT/Dex. (b) Scheme representing the probe inserting utilizing

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an optical fiber as a guide. (c) Scheme showing the location of the electrodes in the skull, it is probable to visualize either the active and passive probes. (d) Photograph of the animal’s head where the connector amongst the active probe and the recording/stimulation device is located. Source: https://pubmed.ncbi.nlm.nih.gov/28343004/.

2.5.2. Polymer Nanoparticles (NPs) The development of NPs formed of ICPs for electro responsive drug delivery needs chemical synthesis and the usage of surfactants to increase their colloidal stability. Matched to electrochemically synthesized films, NPs permit easier scalability (for example, there is no restriction due to the electrochemical cell size or substrate dimensions), electrochemical cell size, and greater processability (for example, superficial area is larger). It had been reported drug loadings of 51 wt.%, which are much higher than those generally achieved with ICP films. The main issue of ICP NPs is associated with the set-up needed to stimulate the release: is it essential to fix the NPs on the electrode surface? Or is it superior to had them dispersed on the solution? To reply to these questions, it is essential to take into account that the reduction and oxidation of ICP NPs in the electrolyte solution are restricted by their diffusion from the bulk to the anode surface. Ge et al. (2012) made a gel of PLGA-PEGPLGA comprising 1 wt.% of PPy NPs (Figure 2.7), which was affixed to the electrode surface. Fluorescein was unconfined in vivo through applying an electric field of –1.5 V/cm for 40 s through needles. In another study, electrodes were covered with PPy NPs and drop-casted with 0.05 wt.% chitosan in 0.1 M HCl to avoid the detachment of such NPs throughout the electrostimulation. It had also been stated the electrostimulation of the NPs kept in dialysis bags located in the electrochemical cell (for example the drug could diffuse across the dialysis bag however NPs can’t) and the discharge of curcumin from PEDOT NPs affixed onto a glassy carbon surface (Bidan et al., 1995; Carli et al., 2018).

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Figure 2.7: (a) Chemical structure of the different reagents. (b) Chemical structure of the drugs utilized in this work: first (fluorescein) and second (daunorubicin). (c) Picture showing the solid-gel transition of the injectable conductive hydrogel. (d) SEM micrograph of PPy NPs laden with fluorescein. (e) SEM micrograph of PPy NPs inserted on the hydrogel. Source: https://pubmed.ncbi.nlm.nih.gov/22111891/.

2.5.3. Polymer Nanowires, Fibers, and Nanotubes In this section, all diverse types of microstructures and tubular nano made of ICPs had been grouped. Nanowires, which comprise of intertwined and elongated nanotubes creating a mesh, had been extensively investigated utilizing PPy. To prepare them diverse approaches had been followed, like those based on the usage of functional molecules, seeding progress, and interfacial polymerization. Though the performance of these nanostructures in energy storage and biosensing had been significantly explored, there is very little information regarding their utilization as DDSs.

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Ru et al. (2011) utilized ATP as both model delivery drug and morphology-directing agent. Astonishingly, data demonstrated that there is an enormous difference among the 53% release for conventional PPy morphologies to 90% for PPy nanowires later 45 h of electrical stimulation. It was visualized through a CV that PPy nanowires are far more electroactive than conventional PPy layouts, however, electrochemical impedance spectroscopy examines corroborated that the material resistance was far lower. On the other hand, Lee et al. (2015) made arrays of nanowires through electrochemical deposition of a blend of biotin and pyrrole monomers as a dopant in anodic alumina oxide membranes with a pore size of 0.2 μm as a sacrificial pattern (Figure 2.8). The number of DOX (doxorubicin) molecules conjugated with the biotin dopant (for example, the encapsulation efficiency) enhanced with the nanowire length, which extends from 5 to 25 μm relying on the polymerization time. Stimulated drug delivery examines utilizing the potential of +0.5 V for 1 min induced few releases than a negative potential of –1 V. In fact, the amount of drug discharged utilizing positive potentials was like that carried from non-stimulated controls, in which natural diffusion was the merely driven force.

Figure 2.8: (a) Schematic diagram of DOX/PPy (DOX-attached PPy nanowires). EDC/NHS (1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide/N-hydroxysuccinimide) was utilized to chemically conjugate DOX to the biotin dopants of the PPy nanowires. DOX/PPy nanowire ranges had a double function for cancer therapy. By application of electrical stimuli could release the

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chemotherapeutic agent and through localized NIR irradiation work as photothermal agents. (b) SEM micrographs of the fabricated PPy nanowire arrays. Electropolymerization was executed by applying a continuous potential of 1 V for 6 (left) and 18 min (right) to get PPy nanowires with a length of 5 (left), 15 (middle), and 25 μm (right), respectively. Source: https://pubmed.ncbi.nlm.nih.gov/25815804/.

Drugs could also be encapsulated or absorbed into PPy nanowires without keeping precise interactions with the doping agent of the ICP. More precisely, Jiang et al. utilized the nanogaps and micro produced amongst the nanowires as reservoirs for drug storage. These observations proposed that the volume and charge of the drug to be load are not conclusive for the loading capacity in this situation. Later on, to avoid drug leakage and to attain a controlled release, the mesh was enclosed with a layer of PPy, making a “sandwich.” In an important paper, Abidian et al. (2006) stated the synthesis of conducting nanotubes utilizing nanofibers of biodegradable PLGA or PLA as hard templates. First, PLGA or PLA with Dex thawed in chloroform were electrospun on the surface of a probe followed through electrochemical deposition of ICPs round the electrospun nanofibers. Drug delivery was attained by regulating the degradation of the PLGA/PLA or by keenly acting on the ICP with an electrical field. Chen et al. (2017) loaded diclofenac into BC (bacterial cellulose) microfibers, which were later covered by a PEDOT shell. During electrical stimulation, PEDOT shrank and generate movement that applied pressure to the BC microfiber, endorsing the drug release. Likewise, the actuation of ICPs was utilized to release the drug from electrospun PLA fibers loaded with curcumin and PEDOT NPs. Esrafilzade et al. (2013) prepared PEDOT: PSS (poly(styrene sulfonate) fibers through wet-spinning, which were used as a template to electropolymerized an exterior shell layer of PPy. Ciprofloxacin hydrochloride, which was nominated as a model drug, was utilized as the dopant agent throughout the PPy synthesis. Drug release saw upon decrease of the PPy layer. The preparation of fibers, nanotubes, and polymer nanowires provides an enhancement in surface/volume ratio, assisting a higher drug encapsulation effectiveness. Moreover, to develop such structures numerous times, it is necessary to associate ICPs with other polymers like PLGA or PLA, conferring to the material improved properties like mechanical stability and more flexibility.

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2.5.4. Polymer Nanoporous Films and Sponges Nanoporous sponges and films were designed to enhance drug retention and get regulated and prolonged drug release profiles. The preparation of porous surfaces is a good approach to enhance the superficial area of the desired layout. Though drugs could be charged into the bulk throughout the ICPs polymerization, some additional drug, not necessarily working as dopant agent, could be loaded inside the nanopores. These pores could be open or sealed through a thin layer of ICP on top (Sirivisoot et al., 2011). The latter method provides ICP founded DDSs with higher loading capacity as it is not expected to be restricted by the size and the charge of the drug. Hard templates, for instance, PSS, and PMMA (poly(methyl methacrylate) beads, which could be later dissolved with the suitable solvent, could be used to achieve porous morphologies (Esrafilzadeh et al., 2013). Figure 2.9 displays the procedure for the creation of sponge-like structures. Initially, PMMA beads were dispersed as in a colloidal crystal onto a stainless-steel substrate. Then, the electropolymerization of PPy was made around the PMMA hard template. Later, PMMA particles were dissolved utilizing a 1:3 v/v toluene: acetone mixture. Afterward, a drug solution was added dropwise on the sponge-like PPy films and air-dried at 20 °C overnight. Lastly, a tinny layer of PPy was electropolymerized above to capture the drug. The extreme drug release was witnessed when films were decreased at –0.6 V, which was attributed to the improvement of the drug diffusion when the spacing among the nanopores raised.

Figure 2.9: Image of the procedure steps to produce the PPy scaffold and the loading of the drug (a). SEM micrographs of (b) PMMA colloidal crystal tem-

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plate on the stainless substrate; (c) the PPy film got through electropolymerization and following PMMA template elimination; (d) and (e) cross-sectional images of the PPy structures after the electropolymerization of the dense PPy layer. Source: https://www.sciencedirect.com/science/article/abs/pii/ S0378517313000197#:~:text=The%20high%20surface%20area%20 PPy,of%20the%20porous%20polypyrrole%20films.

Researchers utilized ICPs as actuators to control the opening and closing of artificial pores (Figure 2.10) (Lee et al., 2015).

Figure 2.10: (a) Scheme of reversible alterations in pore size (and the drug release rate) among reduction and oxidation states. (b) Representation of flux vs time for a 110 nm pore diameter membrane at the reduction state (magenta closed circles) and the oxidation (blue open circles). The data point was gath-

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ered every 10 s. (c) In situ AFM height images corresponding to the reduction states (right)and the oxidation (left). Source: https://pubs.acs.org/doi/10.1021/nl104329y.

They were capable to promote ion diffusion through a porous membrane when the pores were exposed, stopping the diffusion when pores were shut. Particularly, the pore size reduced at the reduction state while it enhanced at the oxidation state. PPy doped with DBS (dodecylbenzene sulfonate), hereafter PPy/DBS, was electropolymerized on a porous aluminum oxide (Al2O3) membrane covered with a thin gold layer. PPy/DBS was selected as electrically responsive material since its volume experiences very huge changes (up to 35%) with the electrochemical state. The reason that produces such alterations is a complex interplay of numerous factors. In brief, the polymer matrix enlarges when solvated ions come into it, while the matrix shrinks when solvated ions discharge from it. These compositional fluctuations, which correspond to the doping procedure, alter the chemical nature and length of carbon‑carbon bonds and the angles amongst adjacent monomer units, disturbing polymer-solvent interactions. Remarkably, the pulsatile discharge of the drug was attained, the on-demand response taking less than a few seconds.

2.5.5. Polymer Hydrogels Hydrogels absorb and store a massive amount of water molecules because of their hydrophilic nature and provide a flexible 3D polymeric network with rubbery nature. These properties make them look like many biological tissues, being thus appropriate as implantable DDSs. Also, they could be utilized for stretchable and flexible bioelectronics. Besides, they could be modeled in a diversity of shapes and with diverse mechanical strength, relying on the implantation zone. Hydrogels formed of conventional naturally occurring synthetic polymers (e.g., polyacrylamide (PAAM), and PEG) or biopolymers (e.g., calcium alginate or agarose) need the usage of relatively high power (for example, from 2 to 25 V) for their operation. Though, much lower voltages (i.e., from 0.1 to 3 V) could be utilized when ICPs are added to the polymer matrix. Polymerization of ICPs on hydrogels could enhance the amount of entrapped drug and the polymer surface area. A representative DDS founded on ICP comprising hydrogel was made by crosslinking chitosan-g-PAni copolymer with OD (oxidized dextran) however, the loading with amoxicillin is done. This system provides a

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repeatable “on-off” pulse release, which enhances the voltage. Figure 2.11(a) and (b) associates the grafted copolymer before and after crosslinking, which Figure 2.11(c) displays the release behavior attained utilizing 0, 1, and 3 V potentials, continually applied every 10 min. It was witnessed that the cumulative discharge percentage of amoxicillin was around 34% after 80 min at 0 V, while this value raised to 69% and 82% at 1 V and 3 V, respectively.

Figure 2.11: Snaps of chitosan-g-PAni comprising solution (a) before crosslinking and (b) after crosslinking with OD. (c) Graph comparing the amoxicillin discharge in PBS (phosphate-buffered saline) solution under diverse electric potentials, which were repeated every 10 min. Source: https://pubmed.ncbi.nlm.nih.gov/29555459/.

Doroudian and Pourjavadi prepared a hydrogel founded on hydrolyzed collagen improved with PCL (Abidian et al., 2006). Conductive fibers were integrated through in situ polymerization of aniline for the controlled discharge of hydrocortisone. In vitro delivery tests displayed that the drug release profile of this material could be tailored through regulating the conductive stimulus. A different method was proposed by assembling an ICP film with a biohydrogel by a gelatin intermediate layer. In this situation, the biohydrogel was working as a dopamine container, however, the ICP film examined the drug release. Furthermore, the dopamine release was controlled by electrochemical stimulation, putting on potential ramps in cyclic stage to such ICP-biohydrogel assembly.

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More instances of DDSs founded on ICP-containing hydrogels comprise PPy electropolymerized into poly(acrylic acid) hydrogel and SA (salicylic acid) doped PPV (poly(phenylene vinylene)/PAAM hydrogels. Regarding the latter, it was clear that in lack of electric field the SA diffusion was late in the first 3 h because of the electrostatic attraction among the PPV and the anionic drug, whereas instant release was perceived upon the application of an electric stimulus.

2.5.6. Polymer Composites An alternative method to expand the superficial area concerning that of the film format is the accumulation of additional materials into the polymer matrix. Currently, CNTs (carbon nanotubes) represent a hot subject in many research fields linked with Materials Science and Engineering. Though these nanostructured compounds are regularly used to enhance mechanical and electrical properties of the primary materials, CNTs had also worked as drug nano-reservoirs for precise release since they could increase significantly the drug retention efficiency. Wang et al. (2016) demonstrated that Dex-loaded CNTs discharge this drug in its active state later the application of electrical stimuli. Particularly, the drug was entrapped in the interior of the nanotube, which was sealed at the ends with PPy layers formed by electropolymerization. CNTs not only permit the loading of a huge quantity of drugs however also provided a constant drug release profile (Table 2.1) (Del Valle et al., 2007). Table 2.1: Summary of DDSs Founded on ICP-Containing Hydrogels* Polymer

Drug

Dopant

Synthesis

Release

Anionic PAni

Amoxicillin (0.004 M) and ibuprofen (0.007 M)

HCl (0.1 M)

Chitosan-g-PAni was thawed in acetic acid aqueous solution and mixed with OD dissolved in PBS (pH = 7.4) at 37 °C for 1 h. Ibuprofen and amoxicillin was added to the mixture earlier gelation

The constant potential at 0, 1, and 3 V for 3 min. Repetitive every 30 m or 60 m

Neutral PAni

Hydrocortisone (100 mg)

HCl (1 M)

Hydrogels were distended in an HCl solution comprising aniline. After 1 day, the hydrogels were washed and positioned in an ammonium persulfate solution. Lastly, the dried hydrogel was dipped in a PBS solution with hydrocortisone

The constant potential at 3 V for 3 m

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Cationic PPy

Safranin (0.01 M)

NaNO3 (1 M)

CA with NaNO3 at +0.6 V till reaching 10.5°C. The subsequent hydrogel was dipped in a safranin solution for 5 h

Constant potential at 0.4 and –0.4 V utilizing pH = 3.8 or 6.4

PPV

SA (0.0125 M)

Drug

A PPV solution with H2O2 and SA was moved for 24 h. SA-doped PPV particles were sieved and vacuum dried for 24 h. Then, a free radical polymerization of the acrylamide with SA doped PPV was led

Constant potential at 0, 0.01, 0.03, 0.05, 0.07, 0.09, and 0.1 V utilizing. pH = 5.5 for 48 h

*The most significant characteristics (for example, preparation method, drug, polymer, dopant, and discharge mechanism) are revealed. The entries are categorized by the drug charge at neutral pH. Figure 2.12 displays a scheme of the synthetic procedure and SEM micrographs of 2 kinds of CNTs: CNTa (inner diameter: 3–8 nm; outer diameter: 100–170 nm) and CNTb (inner diameter: 5–10 nm; outer diameter: 20–30 nm). Drug release was much greater from CNTs wrapped with PPy than from PPy only. Regarding the size of the CNTs, CNTb gives the larger release, meaning that the loading was much greater. This fact was because they had an extensive inner diameter and a thin outer diameter compared to CNTa (i.e., the drug loading capability per unit weight was greater for the earlier than for the latter). Other methods founded on the addition of CNTs had been employed for electrically driven drug discharge and also for a dual application like the mixture of controlled release and the finding of the neural network activity. Related material is GO (graphene oxide), which comprises a hexagonal ring-based carbon 2D network (for example, a sheet) that had mutually sp2 and sp3-hybridized carbon atoms and having a diversity of reactive oxygen functional group (i.e., carboxyl, carbonyl, hydroxyl, and epoxide). The preparation of composites comprising GO is a burgeoning field as this material could enhance not only the chemical stability however also the electrical and mechanical properties. Besides, the use of GO for DDSs had been extensively explored as its oxygen comprising functional groups permit the attachment of fluorophores or site-directed moieties, while the occurrence of localized πelectrons at the surface sheet allows the creation of intermolecular ππ interactions enabling the entrapment of aromatic drugs. Researchers produced PPy matrixes through electropolymerization utilizing the anionic drug (Dex) and GO as dopants agents for the oxidation procedure (Yi and Abidian, 2016). Note that; GO nanosheets comprise lots of carboxylic acid groups on their boundaries, giving a negatively charged

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material. As it was predicted, the nanocomposite discharge Dex linearly in reply to electrical stimuli, and the quantity could be modified by altering the stimulation magnitude. Moreover, the release was kept over 400 stimulations however the drug was not able to diffuse inertly from the non-stimulated composite. A different view is given from the point that direct energy sources are not essential for electrostimulated discharge. From this point of view, selfpowered devices hold a huge potential for evading the dependence on power supplies. Besides, this could enable their integration into the biomedical market. For instance, scientists enclosed the surface of magnesium, which is striking for medical implantation because of its full in vivo degradation, with electropolymerized PEDOT utilizing GO as dopant agent and Dex carrier (Jang et al., 2004). Magnesium substrates corrosion consequence in a local discharge of OH–, Mg2+ and H+, rather, when the surface is covered with ICPs, the corrosion rate reduces down. In this work, writers played with electro-coating diverse substrate areas, producing the corrosion of the material at diverse rates (Bajpai et al., 2006).

Figure 2.12: (a) Scheme outlining the filling and carrying mechanisms of CNT platforms. SEM micrographs of (b, c) PPy/CNT and (d, e) PPy/CNTb matrixes. CNT: exterior diameter 110–170 nm; CNTb: exterior diameter 20–30 nm. Source: https://www.sciencedirect.com/science/article/abs/pii/ S0142961211005394.

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As it is shown in Figure 2.13, Dex release increases with the larger uncovered area of magnesium because of the increase in existing generation from corrosion, while there is practically no discharge when PEDOT/ GO/Dex is placed on the inert gold surfaces. Generally, PEDOT/GO/ Dex films, which provide Dex where corrosion is happening minimizing its disadvantages, could be turned into novel auto-powered drug delivery devices in which the corrosion is the motivating force for the energy source. as well as the controlled drug delivery.

Figure 2.13: Corrosion driven drug release. Dex discharge from PEDOT/GO/ Dex films placed onto a magnesium surface in which the drug delivery is driven by the substrate corrosion. Magnesium samples had rather short or long exposed areas however a similar amount of coverage through the PEDOT/GO/ Dex coating. Source: https://www.sciencedirect.com/science/article/abs/pii/ S1549963417300849.

The same effect was perceived when PPy-doped with ATP was added on the outer and inner portion of a cellulose matrix, and a tinny layer of Mg+ was placed on one side (Figure 2.14). Therefore, the ATP-doped PPy film, Mg+ layer, and the electrolyte solution made a galvanic cell. A galvanic couple of the PPy film (cathode) to the magnesium layer (anode) was the motivating force for drug release. In summary, magnesium was oxidized and transferred into the solution as an ion, however, the PPy was condensed facilitating the ATP release.

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Figure 2.14: (a) Cross-section of the cellulose PPy composite film. (b) High magnification picture of (a). (c) Energy-dispersive X-ray spectrum of the cellulose-PPy composite film. (d) Digital photo of the drug delivery method with the coating of magnesium layer on a single side of the cellulose-PPy composite film. Source: https://www.sciencedirect.com/science/article/pii/ S1388248110003206.

2.5.7. Hybrid 3D-Structures Hybrid materials, which were engineered to attain multitasking capabilities and superior properties, had also been lately employed as DDSs. Scientists made a hybrid cardiac matrix to note the cell electrical signals and deliver drugs when required (Feng et al., 2013). These were compromised of an epoxy mesh, termed SU-8, and a tinny layer of gold that was also enclosed with a rough nanoscale layer of titanium nitride. PPy was positioned on specified places and, lastly, electrospinning was utilized to deposit PCLgelatin fibers onto the electronics, enabling cardiac cell attachment (Figure 2.15). The gains of the fiber addition were high cell addition, avoidance of polymer delamination from the electrode surface, and drug dispersion from the ICP. The last goal of this material was to gather data from its surrounding environment and, therefore, determine when to induce electrical stimulation to discharge drugs (Wu and Bein, 1994).

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Figure 2.15: Diagrams of the microelectronic cardiac patch concept. Source: https://pubmed.ncbi.nlm.nih.gov/26974408/.

2.6. SUMMARY AND OUTLOOK The exponential progress of articles focused on ICPs for biomedical devices seems encouraging for their implantation in biomedicine, from electrode coverings for deep brain stimulation to scaffolds for tissue restoration. Regardless of being biocompatible when it comes to bioabsorbability, there are huge confines because of their inherent incapability to degrade naturally. Though, more in-depth studies are required to be done, like as, in vivo experiments to assess degradation kinetics and their nanotoxicology position earlier to embarking more challenging phases, like clinical tests in humans. The application of ICPs for utilization in humans is yet in its early stage (Krukiewicz et al., 2015). It is of utmost importance to design and develop the nanostructured material and micro material that would fit the finest with the chosen application. It is essential to balance the complexity, costs, production time, and functionality of the last device production. ICPs film preparation

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is the stress-free and most straightforward approach; however, usually, these ICPs films present brittleness and poor mechanical stability. Thus, a good approach to overwhelmed these drawbacks is to join ICPs with other polymers deliberating flexibility to the material. Moreover, increasing the surface volume ratio or the productions of voids to be full with the drug (physical entrapment) would considerably improve drug retention. Presently, ICPs for electrochemical and/or electrical controlled drug release had reached a high level of superiority for on-demand releasing profiles and on/off the trigger. Till now, the self-powered devices founded on magnesium substrates, which would decrease the utilization of complicated and expensive electronic devices able to perceive real-time alterations in the environment for persuading the release, are among the most exceptional DDSs. Future devices would gain from features that would spread their mechanical stability attaining stretchable electronics, which would permit a proper integration in the human body. Further, systems with extra sensing capabilities like neuronal recording, mechanical stress, pH, and heat could be utilized for, at a similar time, deciding the peak amount of drug to be released. Future technology, which would flourish from the utilization of ICPs as nano-reservoirs, is predicted to notify physicians of patients’ health situations followed by the probability of remote drug triggering. Moreover, it is even probable to imagine the capability to integrate a response loop into the system that would self-regulate and where physician help might not be needed. For instance, self-regulated insulin delivery remains one of the most severe technologies to be further developed. Rather than multiple regular injections of insulin, millions of diabetes patients could maintain their glucose level for months with one injection of self-regulated insulin DDS. Thus, we expect a rise in the sensitivity, selectivity, and adaptability of drug release profiles relying on the patient and permitting a better understanding of body electrophysiology. From the material’s viewpoint, DDSs required further development in the following directions: (1) more biodegradability and flexibility to increase the inserting of foreign material into the human body (2) improved integration of the diverse components into one single device; and (3) study the understanding of closed-looped DDSs through the incorporation of multiple advanced electronic devices, like as a portable battery. or wireless transductor (Schönenberger et al., 1971).

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73. Tiwari, G., Tiwari, R., Sriwastawa, B., Bhati, L., Pandey, S., Pandey, P., & Bannerjee, S. K., (2012). Drug delivery systems: An updated review. International Journal of Pharmaceutical Investigation, 2(1), 2. 74. Torchilin, V. P., (2005). Recent advances with liposomes as pharmaceutical carriers. Nature Reviews Drug Discovery, 4(2), 145– 160. 75. Torras, J., Casanovas, J., & Alemán, C., (2012). Reviewing extrapolation procedures of the electronic properties on the π-conjugated polymer limit. The Journal of Physical Chemistry A, 116(28), 7571–7583. 76. Uppalapati, D., Boyd, B. J., Garg, S., Travas-Sejdic, J., & Svirskis, D., (2016). Conducting polymers with defined micro-or nanostructures for drug delivery. Biomaterials, 111, 149–162. 77. Vickers, N. J., (2017). Animal communication: When I’m calling you, will you answer too? Current Biology, 27(14), R713–R715. 78. Wadhwa, R., Lagenaur, C. F., & Cui, X. T., (2006). Electrochemically controlled release of dexamethasone from conducting polymer polypyrrole coated electrode. Journal of Controlled Release, 110(3), 531–541. 79. Wang, X., Gu, X., Yuan, C., Chen, S., Zhang, P., Zhang, T., & Chen, G., (2004). Evaluation of biocompatibility of polypyrrole in vitro and in vivo. Journal of Biomedical Materials Research Part A: An Official Journal of The Society for Biomaterials (Vol. 68, No. 3, pp. 411–422). The Japanese Society for biomaterials, and the Australian Society for biomaterials and the Korean society for biomaterials. 80. Wang, X., Li, Y., Xiong, J., & Hodgson, P. D., (2009). Porous TiNbZr alloy scaffolds for biomedical applications. Acta Biomaterialia, 5(9), 3616–3624. 81. Wang, Y., Chen, B., Zhang, Y., Fu, L., Zhu, Y., Zhang, L., & Wu, Y., (2016). ZIF-8@ MWCNT-derived carbon composite as electrode of high performance for supercapacitor. Electrochimica Acta, 213, 260– 269. 82. Wu, C. G., & Bein, T., (1994). Conducting polyaniline filaments in a mesoporous channel host. Science, 264(5166), 1757–1759. 83. Wurster, D. E., (1953). U.S. Patent No. 2,648,609 (pp. 1–33). Washington, DC: U.S. Patent and Trademark Office. 84. Xia, B., Zhang, W., Shi, J., & Xiao, S. J., (2014). A novel strategy to fabricate doxorubicin/bovine serum albumin/porous silicon

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nanocomposites with pH-triggered drug delivery for cancer therapy in vitro. Journal of Materials Chemistry B, 2(32), 5280–5286. Xia, L., Wei, Z., & Wan, M., (2010). Conducting polymer nanostructures and their application in biosensors. Journal of Colloid and Interface Science, 341(1), 1–11. Xiao, Y., Ye, X., He, L., & Che, J., (2012). New carbon nanotubeconducting polymer composite electrodes for drug delivery applications. Polymer International, 61(2), 190–196. Yi, N., & Abidian, M. R., (2016). Conducting polymers and their biomedical applications. In: Biosynthetic Polymers for Medical Applications (Vol. 1, pp. 243–276). Woodhead Publishing. Zhang, Z., Sui, J., Zhang, L., Wan, M., Wei, Y., & Yu, L., (2005). Synthesis of polyaniline with a hollow, octahedral morphology by using a cuprous oxide template. Advanced Materials, 17(23), 2854– 2857. Zhou, Q. X., Miller, L. L., & Valentine, J. R., (1989). Electrochemically controlled binding and release of protonated dimethyl dopamine and other cations from poly(N-methyl-pyrrole)/polyanion composite redox polymers. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 261(1), 147–164.

CHAPTER

3

APPLICATIONS OF CONDUCTIVE POLYMERS IN TEXTILE INDUSTRY

CONTENTS 3.1. Introduction....................................................................................... 72 3.2. Conductive Polymers and Mechanism of Conductivity....................... 75 3.3. Production of Electrically Conductive Textiles.................................... 83 3.4. Coating Textile Techniques................................................................. 91 3.5. Embroidery Techniques...................................................................... 96 3.6. Electrically Conductive Textiles and Smart Textiles Applications......... 97 3.7. Future Prospects.............................................................................. 102 References.............................................................................................. 104

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3.1. INTRODUCTION Smart textiles are fabrics capable of sensing exterior conditions or stimuli, reacting, and adjust behavior to them in an intellectual way, and gives a challenge in numerous fields today like automotive, sport, aerospace, and health. Electrically conductive textiles comprise yarns, fabrics, conductive fibers, and finishing products made from them. Regularly they are essential to functioning smart textiles, and their quality controls launderability, reusability durability, and fibrous performances of smart textiles. A significant part of smart textiles progress has conductive polymers, which are defined as organic polymers capable of conducting electricity (Zhang and Tao, 2001; Grancarić et al., 2018). They combine certain mechanical features of plastics with electrical properties, usually for metals. The most attractive in a polymers group are polypyrrole (PPy), poly(3,4-ethylene dioxythiophene) (PEDOT), and polyaniline (PANI), as one of the PTH (polythiophene) derivatives. Commercially accessible smart textile products where conductive polymers had a critical role in their progress are protective clothing, touch screen displays, flexible fabric keyboards, medical textiles, and sensors for numerous areas. This chapter attentive to conductive polymer’s description, the process of their conductivity, and numerous approaches to generate electrically conductive textiles for smart textiles requirements. Marketable products of conductive polymers founded smart textiles are also presented as the objective of an amount of lab-scale items (Tao, 2005; Tang and Stylios, 2006; Schwarz, 2011). Smart textiles are fabrics capable of sensing exterior conditions or stimuli, reacting, and adapt behavior to them in an intellectual way. The stimuli could be chemical, electrical, mechanical, optical, thermal, magnetic, etc., (Hufenbach et al., 2011; Nauman et al., 2011). There is no contract regarding the definition of “smart.” Amongst other, better-recognized terms for similar purposes are intelligent, interactive, connected, responsive, and adaptive. Smart textiles are fabrics that interrelate with the user or the surrounding environment. They could provide the information required in the existing situations, assist to master daily life more efficiently and they give a challenge in numerous fields of application like as automotive, sport, aerospace, and health. Some of their initial usages were in medical and military applications. Progressive functionalized materials, like breathing, ultra-strong fabrics, or fire-resistant are not deliberated as smart, no matter exactly how high technological they could be. Smart textiles had benefits like continuous monitoring and non-invasive (Jocic, 2008; Cadenius and Gartvall, 2015).

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As per the applications developed throughout the last decades, smart textiles could be classified as (Kiekens et al., 2010; Frydrysiak et al., 2013): •

Passive Smart Textiles: The initial generation of smart textiles incorporates sensors that could detect (sense) environmental fluctuations or stimuli (conductive materials, optical fibers, thermocouple, etc.). • Active Smart Textiles: The next generation of smart textiles that could detect and react to stimuli from the user or the environment. The textiles comprise sensors and actuators that give the ability to detect and actuate or move a portion of their environment (shapememory materials, hydrogels, chromatic materials, phase change materials, and membranes). • Very Smart Textiles: The third generation of smart textiles that could detect, react, and adapt to exterior conditions or stimuli (Thermoregulating clothing, space suits, health monitoring apparel). Furthermore, it is significant to differentiate “Smart” and “Intelligent” clothing (Odhiambo et al., 2013). “Smart” clothing (furnished with functionalized materials) gives a set of data that users must infer themselves. Smart clothing incorporates sensors into the garment, and the sensors could track a user’s steps (for example, sleep rhythm, stress levels, and calories burned). Generally, smart clothing is incapable of automatically correct concerns for the end-user (Guo and Berglin, 2009). “Intelligent” clothing are garments that could infer data automatically and modify themselves to accommodate the wearers’ precise needs. They are equipped with actuators, sensors, that are textile-based or flexible and with the decision unit able to make decisions about the outputs of the sensors and to regulate actuators (Knittel and Schollmeyer, 2009; Stoppa and Chiolerio, 2014). The term “electrically conductive textiles” is utilized for an extensive range of textile fiber founded products with broadly differing precise electrical conductivity. Electrically conductive textiles comprise yarns, fabrics, conductive fibers, and finishing products made from them. Regularly they are essential to functioning smart textiles. Their quality decides the durability, reusability, launderability, and fibrous performances of smart textiles (Kim et al., 2004; Ankhili et al., 2018).

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A significant portion of smart textiles progress has conductive polymers, which are defined as organic polymers capable of conducting electricity, showing a semiconductive or a conductive behavior. Currently, there are over 25 recognized conductive polymers. They create an interesting class of materials that join certain mechanical features of plastics with the electrical properties, usually for metals. These polymers had become a widespread choice of conductive materials due to their lightweight and cost-effective, show comparatively high adjustable electrical conductivity (appreciations to doping process), biocompatibility, flexibility, could be tailored to have an actuating and sensing function, and are easy to form (Trifigny et al., 2013; Roh and Kim, 2016).). The greatest benefit of conductive polymers is their processability, chiefly by dispersion. They could be applied in energy conversion and energy storage devices, supercapacitors, fuel cells, as adsorbents, conducting inks, heterogeneous catalysts, in antistatic packaging, ESD (electrostatic discharge) control, metallic corrosion protection, smart membrane technology, EMI (electromagnetic interference) shielding applications, etc. Conductive polymers are appropriate for the fabrication of light smart materials without the integration of any metals (Wallace et al., 2007; Mattana, 2011). Commercially accessible smart textile products where conductive polymers had a vital role in their development are protective clothing, medical textiles, flexible fabric keyboards, touch screen displays, and sensors for numerous areas of application (Balint et al., 2014; Benhamou and Hamouni, 2014). The future predictions for the conductive polymers applications in the textile area comprise enhancing usage in health, fitness, and sports (clinical applications, sportswear, health monitoring), fashion (functional clothing), security (uniforms for firefighters), and non-clothing applications (home textile, automotive, etc.). This chapter has three major parts. The initial one attentive to conductive polymers description and mechanism of conductivity. Part two explains various methods to generate electrically conductive textiles for smart textiles requirements. The last one attentive mostly to commercial products of conductive polymers founded smart textiles as the objective of an amount of lab-scale items (Tamburri et al., 2009; Ding et al., 2010).

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3.2. CONDUCTIVE POLYMERS AND MECHANISM OF CONDUCTIVITY Conductivity borderlines among semiconductors, electrical insulators, and conductive materials are fluent and not exactly defined. The conductivity of conductive polymers is in the extent of semiconductors’ conductivity. The cause for the strong usage of these polymers is their great prospective for new applications. The conductivity of polymers matched to those of other materials is shown in Figure 3.1. Conductive polymers unite the positive properties of conventional polymers and metals. They conduct electrical charges and display great electrical properties. There are two subgroups of conductive polymers (Nardes et al., 2007; Ala and Fan, 2009): •

•

Intrinsically or Inherently Conductive Polymers (ICPs): Also identified as synthetic metals and conjugated polymers, show interesting optical and electrical properties earlier found only in inorganic systems. Different kinds of ICPs could be prepared with an extensive range of conductivities from 10–10 to 10+5 Scm– 1 . The most attractive in these polymers groups are polypyrrole (PPy), PANI, and poly(3,4-ethylene dioxythiophene) (PEDOT) as one of the PTH (polythiophene) derivatives. They display environmental stability and high electrical conductivity, they are produced easily, however, have poor mechanical properties (Bajgar et al., 2016; Gharghabi et al., 2018). Extrinsically Conductive Polymers (ECPs): Or conductive polymer composites (CPCs) are obtained through blending (melt mixing) an insulating thermosetting plastic or thermoplastic, polymer matrix, with conductive fillers. The three most significant conductive fillers are carbon (CB (carbon black) and carbon nanotubes (CNTs)), metal powders, and their compounds (ITO: indium tin oxide) and AZO (aluminum zinc oxide)), and ICPs (PANI, PPy). ECPs had special properties like as good thermal and electrical conductivity, good mechanical properties, and corrosion resistance. They are utilized as semi-conductive and conductive polymer fibers, ESD materials, electronics, corrosionresistant coatings, solar collectors. Their conductivity values are much lesser than the conductivity of ICPs, generally in the range among 105 and 103 Scm-1 relying on the applications (Heeger et al., 2000; Rasmussen, 2018).

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Figure 3.1: Conductivity range of polymers matched to other materials conductivity. Source: https://pubs.rsc.org/en/content/articlehtml/2015/ra/c5ra01851j.

There are two situations for polymers to become conductive. The first situation is that conductive polymers comprise alternating single and double bonds, named conjugated double bonds. Such bonds comprise a localized “sigma” (s) bond which creates strong chemical bonds. Also, every double bond comprises a less strongly localized “pi” (p) bond which is feebler (Figure 3.2). Though, bond conjugation is not sufficient to form the polymer material conductive. Therefore, the second situation is that polymer structure had to be concerned-by eliminating electrons from (oxidation), or injecting them into it (reduction). The procedures are termed p-doping and n-doping. They could distress its surface and bulk structural properties (porosity, volume, color) (Kaur et al., 2015).

Figure 3.2: Simplified diagram of a conjugated backbone: A chain comprising alternating single and double bonds. Source: https://www.sciencedirect.com/science/article/pii/ S1742706114000671.

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Doping reaction in organic conjugated polymers is a charge transference reaction and might be more appropriately classified as a chemical redox reaction. Typical alterations in properties of conductive polymers, gratitude to doping are presented in Table 3.1 (Gerard et al., 2002). Factors that impact polymer conductivity are the density of charge carriers, their mobility, and direction, the existence of doping temperature and materials (Bashir, 2013). Table 3.1: Properties Variations of Conductive Polymers in Reply to Redox Transition Oxidized

Reduced

Less transparent

More transparent

More capacitance Expanded More conductive Less hydrophobic Higher modulus

Less capacitance Contracted Less conductive More hydrophobic Lower modulus

Conductive polymers are inappropriate reduced or oxidized state conductors because of their stretched, unique, p-conjugation. Overlapped p*-orbitals form a conductive band and p-orbitals from the valence band. Due to electrochemical or chemical oxidation of conductive polymers, the electrons are eliminated from the valence band, which leads to the existence of charge on the conductive polymer which is delocalized in numerous monomer units in the polymer and makes easing of the polymer geometry in the steadiest form. Dopants could be integrated into the polymer throughout synthesis or could be retrofitted. They can be cations or anions, e.g., Naþ, ClO4, or greater polymer particles like PVS (poly(vinyl sulfonic acid), PSS (poly(styrene sulfonic acid), polyelectrolytes. The degree of doping signifies the ratio of monomers and counterion in the polymer (Bhadra et al., 2009). In 1977, it was revealed that the conductivity of polyacetylene (PA) could be increased through doping with iodine. The 2000 Nobel Prize in chemistry was awarded to MacDiarmid, Shirakawa, and Heeger for the discovery of conductive polymers (Cochrane et al., 2007). Some excitations happen in polyenes, which are associated with solitons and solitary waves. It is exciting to compare three simple carbon compounds, graphite, diamond, and PA. They might be regarded as three, two, and onedimensional forms of carbon materials (Figure 3.3).

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Figure 3.3: Three, two, and one-dimensional carbon materials: (a) diamond; (b) graphite; (c) polyacetylene chain. Source: https://www.sciencedirect.com/science/article/pii/ S1369702101801784/pdf?md5=c633cf26ea7b11d4527b85d78ae7ee46&p id=1-s2.0-S1369702101801784-main.pdf&_valck=1.

Graphite and diamond are modifications of pure carbon, however, in PA, one hydrogen atom is bound to every carbon atom. Diamond comprises only s bonds, and it is an insulator. High symmetry provides isotropic properties. PA and graphite had mobile p electrons, and for this cause, they are highly anisotropic conductors (Trifigny et al., 2013). PA as the simplest probable conjugated polymer got through polymerization of acetylene shows principles of conduction mechanisms in polymers (Figure 3.4). As an essential result of the asymmetry of the PA ground sate, 2 corresponding polyene chains, L, and R, are interconverted by the intervention of a soliton. The soliton as a neutral defect or mobile charge or a “kink” in the PA chain transmits down the chain and decreases the barrier for interconversion. The charge carrier in n-doped PA is a significantly stabilized polyenyl anion of around 29 to 31 CH units in length, which the maximum amplitude is at the mid of the defect. Soliton transferring from one end of the sample to another is described by the bipolaron hopping mechanism (Perumalraj et al., 2009). The uncertainty of PA in the air (covalent bonds are made among carbon and oxygen atoms, and these bonds reduce the conductivity of PA) strengthened research towards the study of additional conductive polymers (Table 3.2).

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Figure 3.4: Soliton interference through the polyacetylene chain. Source: https://www.ch.ic.ac.uk/local/organic/tutorial/ steinke/4yrPolyConduct2003.pdf. Table 3.2: Conductivity and Other Properties of Usual Conductive Polymers Polymer

Conductivity (Scm–1)

Type of Doping

0.4–400

n, p

• Environmentally stable; • Diverse structural forms; • Low cost; • Hard to process; • Limited solubility; • Non-biodegradable.

30–200

n, p

• Hard to process; • Low cost; • Diverse structural forms; • Limited solubility; • Environmentally stable; • Non-biodegradable.

7.5 103

p

• Easy of preparation and surface modification; • Insoluble; • High conductivity; • Brittle, rigid.

Properties

Conductivity in PPy is generally because of p-type conduction, the motion of cations or anions, and the inter-chain hopping of electrons inside the material. PPy could possess a conductivity of up to 7.5103 Scm-1. The key factor that restricts the conductivity of PPy is the “disorder” in the PPy backbone. More of these defects could form as a consequence of redox switching or exposure to water and oxygen, causing the slow decline of conductivity (Agrawal et al., 2013). PPy is one of the most significant conductive polymers that is being used in smart textiles because of its high conductivity, good adhesion, good environmental stability, and non-toxicity. This polymer had been studied in biosensors, microsurgical tools, microelectronics applications, neural tissue engineering, etc. (Paradiso and Pacelli, 2011). PANI had got much interest globally because of its chemical and thermal

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stability. Its production cost is less, and it could be easily doped with organic and inorganic acids to make the conductive form. The thermal stability of PANI is greater than other ICPs. Emeraldine base PANI has less conductivity in the range of 1010 Scm-1, however, its salt created through altering the base’s oxidative state is conductive with 30 Scm-1. In its base condition, the polymer chains are coiled, however, in its salt form, the extra positive charges in the polymer resist each other, prolonging the chains. In extended coil, form electrons are at ease to delocalize, therefore resulting in an enhanced conductivity. PANI has numerous sensing applications like EMI shielding, valuable metals recovery, and ammonia sensors. PANI is explored also for controlled drug delivery, neural probes, and tissue engineering (Li et al., 2005; Harel et al., 2013). The solubility issue of PEDOT was evaded with PSS, a water-dispersible polyelectrolyte. PSS is utilized like a charge balancing counterion throughout oxidative polymerization of the 3,4-EDOT (ethylene dioxythiophene) monomer; this produced polymer complex PEDOT: PSS (poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate). The polyion complex could be simply dispersed in water as colloidal gel particles with diameters of some tens of nanometers, and it had high stability (Feller and Grohens, 2004). The properties of this complex could be adjusted with the addition of wetting agents, organic co-solvents, surfactants to the aqueous coating blend to enhance the compatibility of the polymer on mechanical properties and hydrophobic substrates. Secondary dopants are utilized in small amounts (sorbitol, DMSO (dimethyl sulfoxide), glycerol), EG (ethylene glycol), NMP (N-methyl pyrrolidone), etc. The PEDOT: PSS polymer complex is utilized as a hole transport layer in organic LED (light-emitting diodes) or as electrode material in organic thin Elm transistors. This complex could coat hard surfaces of microelectronics also as fabrics and Ebres and other stretchable substrates (Hooshmand et al., 2011). In general, there are numerous techniques for polymer modification, through blending, grafting, and curing (Figure 3.5). Blending is the physical blend of two (or more) polymers to attain the requisite properties. Grafting is a technique where monomers are covalently bonded onto the polymer chain, while in curing, the polymerization of an oligomer blend forms a coating that follows the substrate through physical forces (Soroudi and Skrifvars, 2010). Substantial work had been done on methods of graft co-polymerization of diverse monomers on polymeric backbones. These techniques comprise photochemical, radiation, chemical, enzymatic grafting, and plasma-induced techniques (Dai et al., 2004).

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To get a material appropriate for applications in numerous technologies, the conductive polymers had to overcome certain confines (poor mechanical properties and issues in processing, uncertainty under the relevant ambient situations). Grafting enhances the processability of conductive polymers and gives the possibility to attain the particular use properties needed for a target application (Giamarchi and Schulz, 1988). Self-doped conductive polymers made by grafting methods afford a water-soluble PPy. The grafting of Py (pyrrole) ontop-amino diphenylamine moieties of water-soluble 2-acrylamide-2-methyl-1-propane sulfonic acid-N-(4 aniline phenyl) methacrylate co-polymers creates the desirable property. PANI was also prepared soluble through polymerizing aniline in an aqueous solution of poly(amino styrene) to make a graft copolymer (Figure 3.6(a)) that is soluble in certain solvents. Grafting also can decrease the rigidity of the polymer chain, as a consequence, solvation could occur through the solvents, imparting solubility (Bakhshi and Bhalla, 2004). A styrene-based composite had been prepared through copolymerizing styrene with 4-chloromethyl styrene, utilizing (azobisisobutyronitrile) (AIBN) as initiator and refusing the blend overnight with an additional solution of potassium pyrrolate in THF (tetrahydrofuran) medium to implant the Py moiety on the copolymer matrix. The reaction structure is shown in Figure 3.6(b). The conductivity of the composite is 10 Scm–1.

Figure 3.5: Diagram representation of the techniques of polymer modification. Source: https://www.infona.pl/resource/bwmeta1.element.elsevier-f50ee022f2ca-3f31-9100-fff3be9f3909.

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Figure 3.6: Reaction structure for preparation of: (a) polyaniline founded graft copolymer; (b) pyrrole-styrene graft copolymers. Source: https://www.infona.pl/resource/bwmeta1.element.elsevier-f50ee022f2ca-3f31-9100-fff3be9f3909.

The other reason for implanting is to alter the surface of the conductive polymer. The hydrophilicity of the PANI Elm surface could be improved through grafting hydrophilic monomers, for example, acrylic acid, acrylamide, the Na salt of 4-styrene sulfonic acid. This could be done by implanting co-polymerization of the monomers through treating the emeraldine Elms with a mixture of oxygen and ozone or with Ar plasma. Grafted PPy could function as an ion-sensor also, for example. it could sense the existence of ions in the solution (Jerkovic et al., 2017). An inherently anisotropic graft copolymer was made through the grafting of PANI onto chitosan utilizing persulfate as an originator in acidic conditions (Figure 3.7). The grafted biomaterial showed electrical conductivity with pH changing behavior like PANI. PANI-grafted biopolymer gives good processability with enhanced solubility, controlled electrical properties, and mechanical strength. Therefore, it could be used for biosensor and chemical applications (Li et al., 2009; Cristia et al., 2011).

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Figure 3.7: Chitosan-graft-polyaniline is created through the oxidative-radical graft copolymerization. Source: https://www.researchgate.net/publication/244756754_Synthesis_and_ characterization_of_electrical_conducting_chitosan-graft-polyaniline.

3.3. PRODUCTION OF ELECTRICALLY CONDUCTIVE TEXTILES Electrically conductive textiles had been deliberated in diverse applications because of their desirable properties in terms of EMI protection, radio frequency interference protection, thermal extension matching ESD, etc. Various methods were developed to made electrically conductive textiles comprising conductive yarns/fibers production, conductive yarns insertion after/during fabric producing, embroidery techniques, coating textile methods, etc. (Wallace et al., 2008; Baćani et al., 2009).

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3.3.1. Conductive Fibers/Yarns Production Conductive fibers/yarns were primarily utilized in technical areas like electronic manufacturing and medical applications. They had diverse functions, like EMI shielding, electronic applications, infrared absorption, and antistatic applications (Novak et al., 2010). Following is the manufacturing procedures, the conductive fibers/yarns are segregated into: • •

Intrinsic conductive fibers/yarns (naturally conductive); Extrinsic conductive fibers/yarns (treated conductive) (Table 3.3).

Table 3.3: Benefits and Drawbacks of Intrinsic and Extrinsic Conductive Fibers/Yarns Process Manufacturing Fibers/Yarns

Benefits

Drawbacks

Intrinsic

Carbon

High conductivity • Dark color; (104–106 Scm-1) • Hard integration into the structure.

ICPs

Lightweight

Metallic

High conductivity • High weight, expensive, (106 Scm-1) brittle; • Low comfortability, and flexibility; • Sensitive to sweating/washing, ununiform heating in clothing.

• Brittleness; • Poor mechanical strength, difficult processing.

Stiffness, strength, • Harmful to health. fatigue resistance Extrinsic

Conductive coated

High conductivity • High manufacturing costs, (106 Scm-1 stiffness Good mechanical properties.

Conductive filled

• Brittleness, high weight.

High conductivity • Effect on processing and fiber; Formability

• Properties; • High manufacturing costs.

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3.3.2. Intrinsic Conductive Fibers/Yarns Intrinsic conductive fibers/yarns are produced through materials containing high electrical conductivity (exclusively conductive substrate). Metallic fibers are made from electrically conductive materials like nickel, titanium, ferrous alloys, aluminum, copper, stainless steel through shaving procedure or a bundle-drawing or shaved off the border of thin metal sheeting. These fibers are highly conductive, however heavier and brittle than most textile fibers. The Sprint Metal Company (2015) differentiates fine metal wires (30 mm to 1.4 mm) and metallic fibers (2 to 40 mm) according to their diameter. Metallic threads/yarns made up of metallic fibers are very thin and might be woven or knitted into a textile utilized to form interconnections among components (Castano and Flatau, 2014). Carbon fibers/yarns are stringy carbon materials with a carbon content of more than 90%. They are transmuted from organic matter by 1000 to 1500°C heat treatment. These fibers/yarns had attractive properties (stability, electrical conductivity, low density, strength, heat resistance, low-to negative coefficient of thermal expansion). They are utilized as ESD materials, absorption materials, and strengthening in composites. Variations in structure and composition, associated with the conditions of their production and to scums present in the structure, are the reason for alterations in their electrical resistivity, resultant in properties extending from those of conductors to semiconductors (Skrifvars et al., 2008; Molina et al., 2009). Fibers/yarns made entirely of ICPs are generally produced through electrospinning or wet spinning, melt spinning. The ICPs are generally nonthermoplastic materials that decay at a temperature lesser than their melting point. As a consequence, melt spinning is not a very good method for ICPs fibers/yarns spinning. In the case of electrospinning, low concentration and instability problems are the main confines of this procedure. Han et al. (2016) prepared ICPs fibers through melt spinning low-cost thermoplastic trans-1,4-polyisoprene and doping with iodine, which could be as 0.01 mm, and electrical resistivity could be as low as 102 m (Figure 3.8(a)). Drawing could enhance the conductivity of the fibers and the orientation of trans-1,4polyisoprene crystals in the fibers. Such fibers could be utilized in textile and other fields (Zhou et al., 2001; Xue et al., 2007). Jalili et al. (2011) reported a new one-step fiber wet spinning route for the making of PEDOT: PSS-PEG (poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate)-polyethylene glycol (PEG)) fibers with improved

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electrical conductivity from 9 to 264 Scm–1 and redox cycling properties about EG (ethylene glycol) post-treated PEDOT: PSS fibers (Figure 3.8(b)). The quality of fibers was influenced by the choice of coagulation bath and the spinning formulation. The one-step PEDOT: PSS-PEG fiber method signified an advantage over the post-EG-treated PEDOT: PSS fibers. Spinning parameters, Relying on molecular ordering, charge delocalization, and treatment situations employed was obvious to the resultant electrochemical, mechanical, and electrical properties of the fibers (Qu and Skorobogatiy, 2015).

Figure 3.8: ICPs fibers: (a) melt rolled thermoplastic trans-1,4-polyisoprene and doping with iodine; (b) SEM image of PEDOT: PSS-PEG fiber turned into isopropanol. Source: https://pubs.acs.org/doi/abs/10.1021/acs.langmuir.6b01333.

3.3.3. Extrinsic Conductive Fibers/Yarns Extrinsic conductive fibers/yarns are produced through the combination of non-conductive and conductive materials, and they display high electrical properties. Special treatment includes the blending, mixing, or coating process (Soroudi and Skrifvars, 2012). Conductive-filled fibers/yarns are produced by adding conductive fillers (metallic nanowires, metallic powder, CNTs, ICPs CB) into non-conductive polymers (polyethylene or polystyrene, polypropylene). Wet spinning and melt spinning are the common procedures to develop this type of fibers/ yarns. The wet spinning procedure guarantees fibers/yarns with enhanced mechanical and electrical properties over those made through the melt spinning (Kaushik et al., 2015). To produce CB-polymer fibers/yarns, a particular amount of CB particles, 20–100 mm, is directly added to thermoplastic. or melts polymers. Then, the

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polymer melts could be melt turned into conductive polymer fibers/yarns. The concentration of CB in the fiber is greater than 10 wt.%, however in the yarns is in the extent from 10 to 40 wt.%. The mechanical properties of developed yarns are tainted with increasing CB concentrations, which confines yarn conductivity. The main benefit of CB-filled polymer yarns is the commercial availability of CB, low cost, and the ease of the synthesis procedure of the yarns. Because of the black color of CB, produced yarns made had an unattractive appearance for numerous applications (Lee et al., 2013). CNTs feature an exclusive one-dimensional structure and amazing physical properties (lightweight, good thermal and electrical conductivities, high aspect ratio). Because of these features, CNTs are deliberated as a superior conductive additive to CB for the fabrication of yarns/fibers. The mechanical properties of CNTs occupied fibers/yarns are better as compared to the actual fibers/yarns. These fibers/yarns could be spun through wet spinning or melt spinning methods (Jerkovic et al., 2015). Soroudi and Skrifvars (2012) done studies on the conductive poly blend elements made through melt spinning procedure (Figure 3.9). The ternary mixture of PP/PA6/PANI complex displayed a matrix/core-shell disseminated phase morphology with extensively varying droplet size. The ternary PP/PA6/PANI complex blends and the binary PP/PANI complex displayed that their conductivity relies on the fiber draw ratio. The ternary blend fibers showed a smoother surface and more even fibers (Atalay and Kennon, 2014).

Figure 3.9: Low vacuum SEM pictures: (a, b) binary blend fibers (PP/PANI complex); (c, d) ternary blend fibers (PP/PA6/PANI complex), organized at a draw ratio of 2. Source: https://onlinelibrary.wiley.com/doi/abs/10.1002/pen.23074.

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Conductive-coated yarns/fibers are produced through coating insulating yarns/fibers with conductive ones (CB, CNTs, metals, or ICPs). The properties of these fibers/yarns rely on the kind of conductive materials and the manufacturing procedure (Avloni and Henn, 2007). Various kinds of metals, like aluminum, copper, gold, and silver could be deposited on polymer yarns prepared of PA6, PP, etc. Diverse coating methods could be used comprising metallic-paint brushing, physical vapor deposition, polymer-metal lamination, and electroless plating. The drawback of the metal-coated yarns is that the metal layer might be peeled off because of washing or other kinds of mechanical abrasion. Lee et al. (2013) revealed the electroless plating of aluminum on the cotton threads, and the confrontation of the conductive cotton threads was measured to be 0.2/cm (National Fire Protection Association, 2000). Coating of CB or CNTs on polymer yarns is generally carried out through a simple dipping-and-drying method. Nauman et al. (2011) demonstrated coating nylon, cotton, and polyethylene yarns with a CB-Evoprene layer (a copolymer of styrene-butadiene-styrene units). The trial volume resistivity of the filaments or yarns was 0.2–1 kcm. They also utilized these conductive yarns to make a smart fabric that could be utilized as a piezoresistive strain sensor (Gan et al., 2008). Two main techniques are generally utilized to coat ICP layers onto textile yarns (Table 3.4): the chemical solution/vapor polymerization and dipping-and-drying method. The impact of ICP coating on the mechanical properties of the yarns relies on yarn type and the ICP, Oxidative agent and concentration of ICP, coating consistency, and thickness (Gültekin and İsmail, 2015).

3.3.4. Conductive Yarn Insertion Into the Fabric Conductive yarns are generally incorporated in textile structures through weaving, knitting, or braiding technology. Though, their incorporation into structures is a complicated process (Abbasi and Militky, 2013). GF/PP (E-glass/polypropylene) commingled yarn made by P-D FibreGlass Group, Germany, was utilized for PEDOT: PSS-coated yarn manufacturing as a strain sensor (Figure 3.10). The process was developed in the GEMTEX laboratory at the Ecole Nationale Supe´ rieure des Arts et Industries Textiles, Roubaix, France (Calvert et al., 2008).

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Table 3.4: Yarns are Covered with Intrinsically Conductive Polymers Textile Yarn

ICP

Coating Technique

Linear Resistivity (Conductivity)

Nylon-6, polyurethane

PPy

Vapor polymerization

–

Polyethylene terephthalate

PANI

Dipping and drying

100/cm

Wool

PPy

Wet polymerization

4.8 k/cm

Silk

PEDOT: PSS

Dipping and drying

2 k/mm

Wool, cotton, nylon, polyester

PANI

Wet polymerization

23 k/cm/filament

Silk

PEDOT: PSS

Dipping and drying

8.5 S/cm

Wool, cotton, nylon

PPy

Vapor polymerization

0.37–3 k/mm

Viscose

PEDOT: PSS

Vapor polymerization

–

Figure 3.10: Textile sensor addition into fabric: (a) PEDOT: PSS yarn coated through the roll-to-roll coating; (b) 2D weaving fabric with incorporated textile sensors. Source: https://www.researchgate.net/publication/307156302_E-GLASSPOLYPROPYLENE_SENSOR_YARNS_DEVELOPED_BY_ROLL_TO_ROLL_ COATING_PROCEDURE.

Aluminum roll to roll mechanism was produced for textile sensors production required for the structural health monitoring of the textile reinforced compounds. Four PEDOT: PSS textile sensors (2 cm distance) were incorporated in the warp and weft direction, distinctly, throughout the weaving of 2D structures, 4-end satin (weft density, 6 ends/cm and warp density, 4 ends/cm), at the ARM computer-controlled hand weaving loom. Three layers of 2D fabrics with the central layer incorporated textile

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sensors were combined at the heating press through 5 min (temperature of 185°C and pressure of 20–30 bar and). Textile sensors worked later on its production and incorporation in 2D structures (electrical resistance values 100 k), however its validation afterward consolidation of 2D fabrics had to be enhanced (Soroudi et al., 2011). Elastomeric yarns and silver-plated nylon were utilized for detecting knitted fabrics manufacturing through flat-bed knitting technology. Developed sensors are appropriate for the measurement of physiological signals and human body articulations. Electro-mechanical trials were performed on the specimens. The conductive yarn was placed only on the technical face of the knitted fabric (Figure 3.11) (Takamatsu et al., 2015). The number of contact areas and the contact pressure between the conductive loops shows their high values earlier to the extension of the knitted sensor.

Figure 3.11: Sensing knitted fabric: (a) technical face with conductive yarns; (b) contact points among conductive loops. Source: https://www.mdpi.com/1424–8220/14/3/4712.

However during the force loading phase. Enhancing the number of conductive courses forms a greater tendency to buckle, and this has a measurable impact on the initiating point of the linear working range (Sinha et al., 2017).

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3.4. COATING TEXTILE TECHNIQUES The probability to incorporate electronics in textiles is a developing research objective, as many commercially significant applications could be seen. Smart textiles and electronics have applications in medical, sports, leisure, military as well as the industrial textile zones. Many of the existing demonstrators and prototypes are cumbersome and awkward in usage and wear and impractical for regular use. To make more user-friendly electronic textiles and wearable smart novel textile fabrics and fibers must be formed and taken into saleable production. Optical and metallic fibers, conductive fabrics and yarns, conductive coatings, and inks are currently under development. These must though be tailored to fulfill the demands fixed by the textile manufacturing and design. Conductivity is a key requirement in electronic and smart textiles, and there are numerous options for attaining this (Tyler, 2016). Metal fibers in the shape of thin metal filaments could be utilized, however, these are heavier, brittle, and tougher to process than conventional textile fibers. Covering textile fibers with metallic salts is another choice, however, these have limited constancy during laundering. The development of ICP (intrinsically conductive polymers) had opened up new prospects for conductive textile materials. These polymers are conjugated polymers whose electrical conductivity is intensely increased through doping (Dogan et al., 2019). In the doping, a little quantity of chemical agent is added, and the electronic structure is altered. The doping procedure is reversible and contains a redox procedure. Conductive polymers are given both as liquid solutions or dispersions or solid compounds. The liquid versions could easily be applied to a textile substrate through coating techniques. There are numerous reports of this concept in the literature. Polyester fabrics had been coated with PPy (polypyrrole) for attaining heat generation textiles. The fabric could produce heat when a voltage was applied to the fabric (Avloni and Henn, 2007). In-situ polymerization of the conductive polymer on the textile surface had also been stated. PEDOT (Poly-3,4-ethylene dioxythiophene) and PPy had been deposited through electrochemical and chemical oxidation on a polyester textile. These textiles displayed also reduce in conductivity upon stretching, therefore enabling the textiles to be utilized as strain sensors. Same attempts had also been stated by others (Khan, 2016).

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Through applying conductive coverings on textiles, a new and technically interesting textile material should be attained. Textiles are regularly coated and printed to get diverse surface appearances and properties. There had though not been done many inquiries regarding the possibilities to apply conductive coverings on textile through similar methods as utilized in the textile industry. Though the concept looks straightforward, numerous technical obstacles must be overcome (Paul et al., 2014). Most techniques for imparting conductivity of textiles and other elastic substrates depend on the coating of CB, conductive polymers, metals, and paints on their surfaces. Coating textile methods comprise electroless (autocatalytic or chemical), vapor deposition, printing, electrodeposition (electroplating process), sputtering of thin Elms, spraying, plating, knifeover-roll coating. The thick coating could be applied manually or through masking techniques, soft lithography, embossing, dip-coating, or imprint. Other kinds of coating applied to fabrics are pH-sensitive, humidity sensitive, light-sensitive, electrochromic, etc. The parameters that impact the uniformity of coating comprise the viscosity and uniformity of the covering material, porosity, flexural rigidity, tension, and coating factor of the substrate (Carvalho et al., 2014). The final factor gives a measure of the fabric openness, degree of moisture resistance, air permeability, and adhesion. With a surface pre-treatment, however, fewer surface energy materials could be made conductive on the surface with a better coating adhesion (fluoropolymers, silicones, and polyolefins). Metal-coated textile materials are lightweight and breathable associated with metal fiber fabrics. Their electrical conductivity could be tailored by choosing the right metals and controlling the coating thickness. They provide functions like shielding against EMI, antistatic properties, and radiofrequency interference. These fabrics are utilized for camouflage, military, and antimicrobial applications. Gan et al. (2008) covered PET (polyethylene terephthalate) fabrics (64 g/cm2, taffeta fabric, 5138 counts/cm2) with CU-Ni-P alloy through using the electroless plating technique. Conductive fabrics with larger EMI shielding efficiency could be prepared at an optimum situation. A Cu-Ni-P alloy put the weight of 40 g/m2 made a shielding efficiency of more than 85 dB over the 100 MHz to 20 GHz frequency range (Figure 3.12) (Risicato et al., 2015; Legrand et al., 2016).

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Figure 3.12: SEM pictures of the alloy-coated fabrics got at different K4Fe(CN)6 concentration in the bath: (a) 0 ppm; (b) 1 ppm; (c) 2 ppm; (d) 4 ppm. Source: https://www.sciencedirect.com/science/article/abs/pii/ S092583880700093X.

Gültekin and İsmail (2015) examined the properties of CB coatedfabrics (Figure 3.13). The dipping-drying procedure was repeated numerous times to enhance the CB loading in simple weaved cotton (30 warp/cm, 22 weft/cm). The basic weight of the fabric is 113 g/m2. Agglomeration of CB nanoparticles (NPs) was witnessed on the fabric surface. The electrical resistivity reduced from 1.4Eþ09cm (neat fabric) to 5.8Eþ07 cm at 5 mg/ml CB concentration. At greater CB loads, the resistivity reduces, signifying that CB NPs work as electrically conductive bridges (Invernale et al., 2010). Features of conductive polymer-coated textile materials comprise deposition of conductive polymers on to numerous textile forms; coherent and uniform coating, development of lightweight, surface resistivity in a range among 10 and a billion, fabric widths range from 0.1 mm to a few mm, and, durable, and flexible materials. These fabrics had found use in applications like non-radar-reflective, radar-absorbing materials, antennas, camouflage netting, etc. Conductive polymers could be applied also to

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silicone, polyurethane, and polyimide foams. These foams are finding utilized in EMI suppression, static dissipation, against high strength radar absorption, etc., (Lim et al., 2010). Abbasi and Militky (2013) covered PPy on E-glass fabric (3/1 twill having 5430 warp and filling per inch) through vapor deposition of pyrrole monomer in the existence of tetraethylammonium p-toluene sulfonate termed as TsO-(tosylate) as doping agent with FeCl3 as an oxidizing agent. The conductive fabric gives 98.67% to 99.23% loss in power at the frequency range of 800–2400 MHz. These fabrics could be utilized to shield the household appliances, buildings, numerous electronic gadgets, cellular phones that operate up to 2.4 GHz frequency, etc.

Figure 3.13: Optical microscopy pictures of CB coated cotton fabrics with diverse amount of CB loading: (a) 0.5 mg/ml; (b) 1 mg/ml; (c) 2 mg/ml; (d) 5 mg/ ml. Source: https://www.researchgate.net/publication/281730940_Investigation_ of_Thermal_and_Electrical_Conductivity_Properties_of_Carbon_Black_ Coated_Cotton_Fabrics.

Besides, numerous printing processes are used for electrically conductive textile fabric creation: gravure printing, inkjet printing, screen printing, and roll to roll method. One of the issues with the printing of conductive polymers is the firmness of the paste. Their thickness differs relying on the process applied. Diverse conductivity values could be obtained which

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are appropriate for applications like as touch screens, notebook PCs, and keyboards, electrocardiogram (EKG/ECG) electrodes, etc., (Figure 3.14) (Shawl et al., 2007). Calvert et al. (2008) utilized ink-jet printing and electroless plating method to deposit detecting lines of PEDOT: PSS polymer complex on weave fabrics and to print silver lines to attach the strain sensors formed to the monitoring equipment (Figure 3.15(a)). They also examined the strained reply of printable conductive compounds. In this research, the fabric kind was crucial to get a better sensor response. Fabrics displayed a higher gauge factor for the lines printed through the fabric axes. Humidity sensors can be printed directly on the textile utilizing ink-jet printing technology. Gold or silver NPs were utilized for electrodes (Figure 3.15(b)). A humidity sensitive layer was made by combining PEDOT: PSS, pHEMA (poly(2-hydroxyethyl methacrylate), CAB (cellulose acetate butyrate), and Nafionsolution (sulfonated tetrafluoroethylene). Sensors were fabricated mostly on Kapton polyimide sheets and then woven into a textile (Taji et al., 2013).

Figure 3.14: SEM pictures of GF fibers later PPy coating: (a, b, c) concentration of doping agent from 0.05 M to 0.1 M; (d) two coatings of PPy at little concentration of tosylate. Source: https://www.researchgate.net/publication/309417770_EMI_Shielding_Effectiveness_of_Polypyrrole_Coated_Glass_Fabric.

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Figure 3.15: Printable conductive textile structures: (a) conductive gel trial on fabric with connecting leads; (b) humidity sensors. Source: https://www.tandfonline.com/doi/abs/10.1080/15421400801904690?j ournalCode=gmcl20.

3.5. EMBROIDERY TECHNIQUES Another probability to attain a conductive fabric is to fasten a conductive structure (copper, thin stainless steel, or other metal wires) to a ground structure through utilizing the embroidery techniques. Generally, such techniques are mostly integrated into textile industries to produce interconnections among sensors and output electronic systems. These techniques permit precisely identifying the circuit layout and stitching pattern in a computer-assisted design environment, under machine regulation and integration of yarns with diverse electrical properties (Nagaraju et al., 2016). According to explanation of various methods to produce electrically conductive textiles, their benefits and drawbacks are summarized in Table 3.5 (Marozas et al., 2011).

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Table 3.5: Benefits and Drawbacks of Electrically Conductive Textiles Process Manufac- Advantages turing

Disadvantages

Embroidery techniques

• Identifying the circuit layout and stitching pattern; • Applied on numerous textile types.

• Large amount of wires is required to be used.

Conductive fibers/ yarns

• Processes parameters adjusting.

• Differences in washability conductivity.

Coating textile techniques

• Deposition onto numerous textile forms, flexible, less expensive, uniform coatings, good adhesion, durable, compared to conductive fibers

• Thickness is too high for the particular application.

Conductive yarn injected into the fabric

• Complicated integration onto fabric; • Conductivity establishment relying on yarns types.

• High manufacturing costs.

3.6. ELECTRICALLY CONDUCTIVE TEXTILES AND SMART TEXTILES APPLICATIONS A huge number of lab-scale electrically conductive textiles for the requirements of the smart textile could be found in literature, however, yet not enough amount of them is available in the market. The emphasis of interest of this chapter is to provide commercially accessible conductive polymer-based smart textile products for sport, fitness, health, and automotive applications. Moreover, the great prospect in a group of smart textile materials displays electrochromic textiles, superamphiphobic electrically conductive textiles, and EMI shielding textiles for selected applications (Maity and Chatterjee, 2018).

3.6.1. Health, Sport, and Fitness Applications Takamatsu et al. (2015) made textile-based wearable devices encouraged by the Japanese kimono dyeing method (Figure 3.16(a)) to note high-quality EKG in the clinic and ambulatory situations, and to decide heart rate. PDMS (polydimethylsiloxane) stencil was utilized due to its hydrophobic nature to restrict the dispersion of the aqueous PEDOT: PSS dispersion on to interlock knit polyester fabric. PEDOT: PSS electrodes fictitious in this way

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and covered with an ionic liquid gel displayed a low impedance contact with the skin (Park et al., 2008). New checking Edema Stocking device (Figure 3.16(b)) made by Edema ApS allows measurements of volume fluctuations in the lower limbs. The system gives supplementary knowledge regarding fluid retention and the effectiveness of treatment for drainage through objective scientific measurement (Ko et al., 2018). Sensoria company produced smart socks (Figure 3.16(c)) filled with textile sensors that could detect foot pressure. Conductive fibers in the sock transmit data to an anklet which utilizes Bluetooth to connect with a mobile app. Production of predictable and reliable textile sensors capable of supporting machine wash cycles is vital in the development of efficient smart fabrics (Bashir et al., 2011). Adidas company incorporated conductive yarns into flexible garments through knitting technology. Textile electrodes, untraceable, and friendly for skin (Figure 3.16(d)), could pick up signals from the heart and other muscles. The data collected by these electrodes are transferred through the garment to a small gadget shattered into a sports top.

Figure 3.16: Sport and fitness, health applications: (a) textile wristband with a PEDOT: PSS electrode; (b) edema stocking device; (c) sensoria smart socks; (d) Adidas textile electrode for sports top; (e) fitness shirt having a biometric function; (f) life tech jacket. Source: https://www.nature.com/articles/srep15003.

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DuPont Microcircuit Materials launched the initial products of stretchable conductive inks published on to the textile (Figure 3.16(e)). The inks exhibit strong performance after 100 washing cycles when they were published on the PU film. Seymourpowell company produced a new smart 3-in-1 jacket termed as the Life Tech jacket (Figure 3.16(f)) for Korean outside sportswear brand KolonSport. This jacket keeps users active in tough conditions. Paul et al. (2014) produced textile electrodes appropriate for human biopotential monitoring (Figure 3.17(a)). They utilized screen printing to make encapsulated conductive tracks. Diverse pastes were carried for the electrode networks: a screen printable silver conductor, a screen printable polyurethane, and a stencil printed conductive rubber. Woven textile termed “Escalade” was utilized as a printed substrate (woven in a 31 twill, the thickness of 410 mm, the density of 295 g/m2). Conductive encapsulation was stencil published at electrode sites keeping an electrical connection to the skin surface. Carvalho et al. (2014) explained a shirt for use in risk environments, health, and sport monitoring. Thus, basic weft structures were produced (simple pique, locknut, jersey). The electrode areas (Figure 3.17(b)) were woolen with a textured multifilament polyamide yarn with a tinny silver covering (less than 10 nm). The ECG connections were moved to a single line, positioned on the back of the shirt, and from there to a distinct place to affix to the precise acquisition circuit.

3.6.2. Automotive Applications One-shot manufacturing of complicated shaped parts might be considered as one of the finest solutions for the quick production of complex pieces dedicated to the heavy and automotive industries. Replacement of metallic cross stiffeners through their composite parts created of GF/PP commingled yarns was stated by Risicato et al. (2015) (Figure 3.18(a)). The PEDOT: PSS sensors founded on glass yarns were injected into braided fabrics coupons (earlier thermal consolidation) and gives the capability to observe the structural health of these complex parts in real-time to select the appropriate composite architecture for the last automotive applications.

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Figure 3.17: Textile electrodes for health monitoring: (a) textile headband; (b) knitted directly into a shirt (1) and the electrical path to the electrode (2). Source: https://www.sciencedirect.com/science/article/abs/pii/ S0924424713005803.

Figure 3.18: Automotive applications: (a) thermoplastic cross stiffener growth (ENSAIT, Gemtex laboratory); (b) conductive inks published on flat plastic and shaped into 3-dimensional car constituents (T-Ink). Source: https://link.springer.com/article/10.1007/s10443-014-9400-9.

The T-Ink company makes conductive inks (Figure 3.18(b)) so robust that sensors, switches, and circuits, could be printed on flat plastic and then shaped into 3-dimensional components that regulate overhead lights and sunroofs in cars. Parts formed with this technology could replace thicker assemblies decreasing the weight and dimensions of wired constituents.

3.6.3. Other Applications Wang et al. (2013) made durable superamphiphobic (superoleophobic and superhydrophobic) electrically conductive fabric through one-step vapor phase polymerization of EDOT with the existence of FD-POSS (fluorinated

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decyl polyhedral oligomeric silsesquioxane) and FAS (fluorinated alkyl silane) (Figure 3.19(a)). Commercial polyester (plain weave, 168 g/m2) was utilized as the substrate. The addition of FAS and FD-POSS to the PEDOT exhibited little impact on the surface resistance. It could impart the PEDOT covering with self-healing capability and durable liquid repellency to auto repair from chemical harm. FD-POSS was found to play a significant role in improving the abrasion and washing durability and self-healing function of the covering. Chromic materials are very exciting in terms of their incorporation into textiles. Metallic materials might harm the textile structure or disturb the handling and comfort properties of it. The substitution of inorganic materials with organic materials could be a solution to these issues. Invernale et al. (2010) made electrochromic flexible textile electrodes through PEDOT: PSS impregnated spandex (Lycra) spray-covered with electrochromic polymer (gel electrolyte used) (Figure 3.19(b)). Bajgar et al. (2016) showed the cotton fabric (plain weave, bleached, specific mass 120 g/m2, sett: 28.0 n/cm (weft), yarn count 7.4/2 tex (warp), 14.5 tex (weft), and 51.2 n/cm (warp)) covered with conducting polymers (CPs), PPy or PANI, in situ throughout the oxidation of relevant monomers in an aqueous medium (Figure 3.19(c)).

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Figure 3.19: Other applications: (a) superamphiphobic fabric; (b) spandex founded electrochromic textile device; (c) textile electrodes to gather the electrical reply of a Venus Pytrap; (d) artificial horizon/radar barrier. Source: https://pubs.acs.org/doi/10.1021/am900767p.

Improved fabrics were covered again with PANI or PPy to examine synergetic effect among both polymers concerning conductivity and its steadiness during frequent dry cleaning. The resulting fabrics were utilized as electrodes to gather the electrical reply of a Venus Pytrap plant. This cost-effective procedure enables attaining textiles with exclusive properties appropriate for novel end-use applications and attractive niche markets products growth. Conductive polymers are deliberated also for EMI shielding applications, for example, PPy-coated woven polyester twill fabric (Figure 3.19(d)), EeonTexTM T-PI-365 (223 g/m2), is utilized to make artificial horizon, radar obstacles for military aerospace purposes.

3.7. FUTURE PROSPECTS Based on numerous advantages of smart textiles, the development in healthcare and medical applications is clear. New end-user applications are required to be launched to the market to fulfill customer’s wants with a substantial focus on these areas. Sport and fitness applications should be the regular basis of consumer products. Because of the fast rhythm of

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life in many areas, smart textiles should be focused on other applications, particularly to smart home textile applications growth. Various methods are utilized in this chapter for the growth of electrically conductive textiles as a portion of the smart textiles area. Only a scarce commercial smart textile products founded on conductive polymers usage are existing today though a huge number of lab-scale structures could be found. It is essential to look for new methods of their development, however, the washability concern of these textiles is required to be taken into greater deliberation for diverse commercialized applications. Thus, the industry plan and good research strategy had the most significant role following the market requirements and novel technologies.

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CHAPTER

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USE OF CONDUCTING POLYMERS IN FLEXIBLE SUPERCAPACITORS

CONTENTS 4.1. Introduction..................................................................................... 114 4.2. Flexible Supercapacitors from Conducting Polymer (CP).................. 117 4.3. Flexible Supercapacitors from Conducting Polymer (CP)-Based Films........................................................................... 126 4.4. Flexible Supercapacitors from Conducting Polymer (CP)-Based Fibers.......................................................................... 131 4.5. Summary and Future Scenarios........................................................ 135 References.............................................................................................. 137

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4.1. INTRODUCTION Due to their good processability, tunable conductivity, environmental friendliness, and exclusive doping-dedoping characteristics, CP (conducting polymer)-centered flexible supercapacitors have been investigated widely. The inclusion of CPs into the stretchy substrates has been confirmed to be an effective way for making CP-centered flexible supercapacitors. An emphasized review on current developments in research and development of CP-based flexible supercapacitors has been provided in this chapter. The electrochemical performances and synthesis of CP-based hydrogels, comprising the hybrid hydrogels, pristine CP hydrogels, and all of them in just one supercapacitor, are emphasized. The current advancement in the growth of flexible supercapacitors centered on ternary and binary hybrid films comprising of CP/carbon nanotubes (CNTs), CP/graphene/carbon, CP/ graphene nanotubes is discoursed. Moreover, a comprehensive summary of the utilization of CP-based fibers and textiles for the flexible supercapacitors is signified, along with the latest challenges and future viewpoints for CPcentered supercapacitors (Bonaccorso et al., 2015; Yu et al., 2015). With the diminution of fossil fuels, global warming, and climate change, there is a growing demand for renewable and clean energy storage and conversions. In this framework, rigorous research has been dedicated to emerging environmental friendly technologies of high-power energy storage, comprising supercapacitors (Kyeremateng et al., 2017). The supercapacitors, also known as electrochemical capacitors (ECs) or ultracapacitors, are significant for energy recycle and storage. In contrast, the recent quick development of wearable and flexible electronics needs flexible supercapacitors, which normally are foldable, or/and bendable, stretchable, as the source of power (Gibney, 2015; Mosa et al., 2017). The flexible supercapacitors are well-thought-out as the ideal source of power for wearable/portable electronic devices and light-weight electronic vehicles. Analogous to conventional supercapacitors, the flexible supercapacitors are light-weight and flexible energy storage devices having a long-life cycle and a high volumetric power density (Fan et al., 2016). Similarly, the flexible supercapacitors can be categorized into two types: EDLCs (electrical double-layer capacitors) and pseudocapacitors, dependent on their distinguished mechanisms of charge storage. The particular capacitance of EDLCs, linked with charge stored through electrostatic charge gathering at the electrolyte/electrode interface, depends on the size of pore distribution and the effective surface area of electrode materials

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whereas pseudocapacitors have comparatively high specific capacitances because of the reversible and fast redox reactions near or at the surface of the electrode (Son et al., 2014). The carbon nanomaterials, like CNTs and graphene, are outstanding electrode materials for EDLCs while transition metal hydroxides/oxides and CPs are perfect electrode materials for the pseudocapacitors (Conway and Pell, 2003; Shin et al., 2016). Amongst them, CPs have been well-thoughtout as the most auspicious materials for the flexible supercapacitors because of their high conductivity, high intrinsic flexibility, and ease of synthesis (Figure 4.1).

Figure 4.1: Structure of the flexible supercapacitor. Source: https://pubs.rsc.org/en/content/articlelanding/2014/ta/c4ta00567h.

Polymers have been utilized as the electrically insulating materials until the discovery of PA (polyacetylene), which can be doped in order to be conductive, made by Shirakawa et al. (1977). Several CPs with alternating double and single bonds can be synthesized now with uncommon electrical properties via π-electron delocalization as well as their polymer backbones. In current years, CPs, including PANI (polyaniline), PPy (polypyrrole), PTh (polythiophene), and PEDOT (poly(3,4-ethylene dioxythiophene)) with their structures displayed in Figure 4.2, have attracted a great deal of curiosity (MacDiarmid et al., 2001; Dai, 2004). Of specific concern is that the CPs reverse doping-dedoping performance has been utilized for several smart applications in advanced electronic devices, supercapacitors, sensors,

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and batteries. The conductivity of CPs can be adjusted over 13 orders of magnitude by tuning the level of doping (Zhou et al., 2016; Malik et al., 2017). CPs can be synthesized by oxidative polymerization utilizing common oxidants or simply by electrochemical polymerization into diverse forms, including hydrogels, powders, sponges, and films. A large multiplicity of morphologies, like nanofibers, nanotubes, nanowires, nanorods, nanoarrays, and nanorings, have been obtained by utilizing different synthetic or processing methods, including the soft or hard template seeding, assisted, interfacial, and electrochemical polymerization, and the post-synthesis electrospinning (Zhang et al., 2004; Feng et al., 2013). The accessibility of several CPs in different morphologies and forms offers substantial room for making CP-centered flexible supercapacitors as new renewable devices of energy storage with great promise. Because of the volume variation of CPs during the process of doping-dedoping, however, CP-based supercapacitors normally possess poor cycle stability. Thus, it is a major challenge to design and make flexible CP-based composites with enhanced mechanical properties and porous microstructures for high-performance CP-centered flexible supercapacitors (Zhang et al., 2005; Han et al., 2010).

Figure 4.2: Chemical arrangement of: (a) PANI;(b) polypyrrole; (c) polythiophene; and (d) PEDOT. Source: https://onlinelibrary.wiley.com/doi/10.1002/macp.201800355.

Here in this chapter, an emphasized review on current advances in research and development of CP-based supercapacitors. At first synthesis and

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the electrochemical performance of CP-centered hydrogels, including the hybrid hydrogels, pristine CP hydrogels, and all of them in one supercapacitor are highlighted. This is trailed by discussions on current developments in flexible supercapacitors centered on ternary and binary hybrid films of CPs/ graphene, CPs/graphene/CNTs, and CPs/CNTs. Consequently, the utilization of CP-based fibers and textiles for flexible supercapacitors are defined, along with the association of built microstructure with the electrochemical behavior of the CP-based flexible electrodes. Lastly, the latest challenges and future viewpoints on CP-centered supercapacitors are also defined (Mao et al., 2010; Li et al., 2011).

4.2. FLEXIBLE SUPERCAPACITORS FROM CONDUCTING POLYMER (CP) 4.2.1. Hydrogels These are three-dimensional (3D) crosslinked networks of the hydrophilic polymers, which comprise a substantial amount of water and generally can provide high flexibility. Hydrogels can be made from several hydrophilic monomers, comprising acrylamide, vinyl acetate, and acrylic acid, by step or chain-growth polymerization, trailed by crosslinking with the functional crosslinker in order to form the 3D porous network (Figure 4.3) (Yu et al., 2009; Sun et al., 2012).

Figure 4.3: Features and applications of the structural hydrogels. Source: https://www.sciencedirect.com/science/article/pii/ S0032386116305390.

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Polymeric hydrogels, like PVA (poly(vinyl alcohol)) and PAAM (polyacrylamide), have been utilized as solid-state electrolytes for the flexible supercapacitors because of their ionic conductivity and superior flexibility (Ahmed, 2015; Tang et al., 2015a, b). Currently, rigorous research struggles have been dedicated to the design and making of CPbased hydrogels as the flexible supercapacitor electrodes. By integration of CPs into the flexible hydrogels, several advantages, together with the intrinsic 3D porous structure, fast ion/transfer of charge, and redox activity can be conferred into hydrogels as the flexible supercapacitor electrodes. Furthermore to the CP-hybrid hydrogels, pristine CPs hydrogels have also been examined as the materials of an electrode for supercapacitors over the last few years. A multiplicity of strategies, like crosslinking with the crosslinkers and self-crosslinking, and the oxidant-templating, have been evolved to prepare flexible CP-based hydrogels with 3D crosslinked conductive structures (Zhao et al., 2013; Li et al., 2014). The electrical conductivity, mechanical properties, and the supercapacitive performance of CP-hybrid hydrogels and pristine CPs hydrogels in the supercapacitors have been studied systematically and are appraised in the subsequent sections.

4.2.2. Pristine Conducting Polymer (CP) Hydrogel Supercapacitors Ghosh and Inganäs (1999) described the first highly enhanced CP hydrogel electrodes for the supercapacitors, in which the crosslinked PEDOTPSS (polystyrene sulfonate) colloidal particles created the nanoscale uninterrupted conductive network with the high surface area suitable for storage of charge. This new hydrogel electrode created new chances for supercapacitors, particularly flexible supercapacitors. Since then, much advancement has been achieved in preparing to conduct polymer hydrogels, including PEDOT, PPy, and PANI hydrogels, for the supercapacitors (Xu et al., 2013; Guo et al., 2015). As displayed in Figure 4.4(a) and (b), the phytic acid comprising several hydroxyl groups was utilized as a cross-linker and the dopant for creation of the PANI hydrogel, which might be spray-coated or inkjet printer to display the conductivity of around 0.11 S cm–1. Having the hierarchical 3D porous structure, electrical conductivity, and high surface area, the subsequent PANI hydrogel supercapacitor electrodes offered the high specific capacitance of nearly 480 F/g with good rate capability and cycling stability (Wang et al., 2014). The symmetric malleable solid-state supercapacitor was made

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from the PANI hydrogel assisted by the carbon cloth of fiber as electrodes and the PVA/Sulfuric acid hydrogel as a solid electrolyte. In an analogous, but autonomous study, Shi et al. (2014) developed the hierarchically 3D nanostructured elastic polypyrrole hydrogel through interfacial polymerization of Py at the isopropanol water/alcohol interface utilizing phytic acid as dopant and crosslinker, and APS (ammonium persulfate) as oxidant.

Figure 4.4: (a) Graphic representations of the three-dimensional hierarchical microstructure of gelated polyaniline hydrogel where the phytic acid behaves as a crosslinker and the dopant. Three levels of the hierarchical permeability from angstrom, nm (nanometer) to the pores of micron size have been outlined by red arrows. (b) An image of the polyaniline hydrogel inside the glass vial. (c) Graphic representation of the creation of three-dimensional hierarchical nanostructured polyaniline hydrogel and the photograph of PANI hydrogel inside the glass vial (Pan et al., 2012).

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The polypyrrole hydrogel, therefore, prepared exhibited a conductivity of around 0.5 S/cm in a dry state with the high mechanical strength and an elasticity attractive for the flexible supercapacitors. By altering the molar ratio of Py to phytic acid, the size of the pore in the 3D spongy hierarchical nanostructure can generally be tuned. The symmetric flexible polypyrrole hydrogel supercapacitors displayed a high areal capacitance of nearly 6.4 F/cm at mass loading of around 20 mg/cm with good cycling stability. Likewise, one more polyhydroxy acid (i.e., ATMP (amino trimethylene phosphonic acid), Figure 4.4(c)) has been utilized as a dopant and gelator for the creation of PANI hydrogels with good electrochemical performance and high electronic conductivity (Dou et al., 2016; Huang et al., 2019). As the institution of the non-conducting crosslinkers might decrease the PANI hydrogel’s electrical conductivity, self-crosslinked PANI hydrogels were also made by the sol-gel method utilizing aniline hydrochloric salt as a monomer and the APS as an oxidant (Li et al., 2016). The 3D porous PANI hydrogel, therefore, produced with the coral-alike morphology of the ramous nanofibers is advantageous for electrochemical performance improvement. As an outcome, the resultant PANI hydrogel electrode displayed the maximum specific capacitance of around 750 F/g at the current density of nearly 1 A/g. Currently, Zhou et al. (2016) stated the hydrogel of pure PANI nanofibers utilizing V2O5.nH2O as an oxidizing agent and the hard template through in-situ polymerization. An oxidant template-aided quick creation of ultrathin PANI nanofibers encouraged the 3D assembly of as-synthesized PANI hydrogel with a 3D crosslinked structure and high conductivity. The resultant PANI hydrogel electrode displayed the high specific capacitance of around 626 and 636 F/g at a current density of nearly 2.5 and 2.0 A/g, correspondingly (Li et al., 2016, 2017). The retention of capacitance reached 83.3% after 10 thousand cycles, representing the ideal cycle groups were utilized to copolymerize with the aniline and crosslink with PVA by the condensation reaction of the -OH (Figure 4.5). The optimum molar ratio of the ABA to aniline for making the PVA/PANI hybrid hydrogel is around 0.07:1 without any hydrogel creation at ABA: aniline greater than 0.03:1. The consequential hydrogel exhibited the porous network structure comprising of interrelated nanoparticles (NPs) with a conductivity of nearly 0.1 S/cm, stretchability, high compress stress, and high tensile strength, comparable to the other hydrogels. The flexible supercapacitors centered on PANI/PVA composite hydrogel displayed the electrochemical capacitance of around 306 mF/cm and 153 F/g, along with an energy density of around 13.6 Wh/ kg. Moreover, 100% and 90% capacitance retentions were attained after

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thousand charge-discharge cycles and thousand mechanical folding cycles, correspondingly. These exceptional properties are linked to conductive hydrogel crosslinked amongst the soft and rigid polymer chains with the mesoporous structure. Therefore, the supermolecular approach presents a favorable route for making flexible, robust, and conductive electrode materials (Zang et al., 2017).

Figure 4.5: (a) Synthesis of polyaniline bearing the boronic acid groups. (b) Polyaniline/poly(vinyl alcohol) hydrogel at 3 length-scales. left: an image of polyaniline/poly(vinyl alcohol) hydrogel; middle: the SEM image of polyaniline/poly(vinyl alcohol) hydrogel; right: the schematic molecular structure of polyaniline/poly(vinyl alcohol) hydrogel displaying the crosslink between PVA and PANI. (c) SEM image of polyaniline/poly(vinyl alcohol) hydrogel displaying the porous structure. (d) The photographs of reactions with diverse amalgamations of reagents. Vial 2 possesses all of the 4 reactants and offers polyaniline/poly(vinyl alcohol) hydrogel. Vial 1 doesn’t possess any APS; vial 3 doesn’t possess any PVA; vial 4 doesn’t possess any ABA; vial 5 doesn’t possess any aniline (Li et al., 2016).

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Later, the post-synthesis process of the freeze-thaw cycle was utilized to further enhance the mechanical properties of PANI/PVA hybrid hydrogel (Lu et al., 2017).

4.2.3. Conducting Polymer (CP) Hybrid Hydrogel Supercapacitors To enhance the flexibility and mechanical properties of the pristine CPcentered conductive hydrogels, several hybrids CPs hydrogels have been established as electrode materials for the flexible supercapacitors. PVA is the most frequently utilized flexible substrate for CPs hybrid hydrogels whereas PAAM, sodium alginate, G-Zn-tPy, and graphene oxide (GO)/ graphene have also been utilized for the same goal. The hybrid hydrogels can usually be highly stretchable, compressible, foldable, and self-healable, favorable for applications in wearable/portable electronics (Smirnov et al., 2016). Because of the introduction of a non-conducting substrate, a big challenge still exists to design and fabricate hybrid hydrogels with a high surface area and high conductivity. However, the supramolecular approach has been established to crosslink PVA and PANI via dynamic binding, in which the functional monomer bearing both boronic and amino acid hydrogel displayed an improved tensile strength and an elongation break from 5.3– 16.3 M pascals and from 250% to 407%, correspondingly (Li et al., 2016). The consequential flexible supercapacitors also demonstrated good cycle stability and high energy density (Ates et al., 2018; Zou et al., 2018). Furthermore to crosslinking the firm CP chains with the soft polymer chains in order to form the conductive interrelated structure or the homogenous distribution of CP phase in a soft substrate, other methods have also been established for governed polymerization of CPs chains within the 3D crosslink networks comprising of the soft polymer substrate, like PAAM and PVA. Particularly, Zhang et al. established Polypyrrole/ Polyvinyl alcohol hybrid hydrogels through vapor phase polymerization of polypyrrole in the 3D crosslinked network of polyvinyl alcohol. The interpenetrating network created by the PVA network and PPy chains endowed the consequential hybrid hydrogel with good electrochemical performance and high mechanical strength in the flexible supercapacitors. Moreover, an analogous interconnected conductive structure centered on the 3D porous network of PAAM was stated by Lu et al. (2017). Moreover, α-cyclodextrin comprising hydrophilic exterior and the hydrophobic cavity has also been utilized to enhance the compatibility of hydrophilic PPy and hydrophobic PAAM (Figure 4.6(a)), leading to enhanced mechanical

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properties. Additionally, the perfect integration amongst the PANI guest and α-cyclodextrin-PAAM host was accomplished, in which the PANI chains were well-protected to confirm the satisfied conductivity and remarkable stability. Due to the high conductivity, stretchability, and flexibility, the PANI/PAAM hybrid hydrogels, when utilized as the flexible supercapacitor electrodes, exhibited a good rate performance, high specific capacitance, and outstanding cycling stability. The flexible supercapacitor having a high capacitance of around 364 F/g at 0.2 A/g, 50% capacity retention at the high current density of 16 A/g, and the high cycling stability has also been made-up by utilizing p-phenylenediamine not only as of the compatibilizer for PAAM and PANI hybrid but also the effective regulator for PANI morphology (Shi et al., 2015; Huang et al., 2017). To enhance the mechanical and electrical properties, graphene/PANI hybrid hydrogels were made by the hydrothermal treatment of the soluble PANI dispersion. The flexible solidstate supercapacitor centered on the hybrid hydrogel electrode exhibited a high specific capacitance of nearly 484 mF/cm2 and an energy density of 42.96 mW/cm2 (Figure 4.7) (Okumura and Ito, 2001).

Figure 4.6: (a) Synthesis standard of the structurally stretchable and flexible along with the electrically conductive networks of a hybrid hydrogel (Shi et

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al., 2015). (b) Molecular structure of the Zn-tpy supramolecule. (c) The G-Zntpy displays the reversible sol-gel phase shift at 50°C above which the G-Zntpy becomes the homogeneous solution. (d) Graphic illustration of anticipated mechanisms of the self-healing conduct for the consequential supramolecular gels. The vigorous intermolecular coordination and interaction at the position of crack assists heal the material of gel (Shi et al., 2015).

Figure 4.7: The method of bottom-up infilling. Left-right: sol is cast on the porous electrode situated on the gas-permeable substrate and concealed with the impermeable film; gel shapes from the bottom-up till the complete spongy electrode is infilled with the gel; the free-standing electrode filled with gel is acquired after elimination of the substrate (Li et al., 2018).

One other binary network gel made-up of polypyrrole and an acetonitrilecentered supermolecular gel (G-Zn-tPy) with the cubic architecture was made (Figure 4.6(b)–(d)). Precisely, polypyrrole, and the phytic acid hydrogel stated above were freeze-dried first into an aerogel. Afterward, the polypyrrole aerogel was swelled in a G-Zn-tPy solution above 50°C. On cooling the gel which is swollen, the binary network gel of supermolecule G-Zn-tPy/PPy is made, in which a G-Zn-tPy is homogeneously dispersed in the polypyrrole matrix during a sol-gel shift of G-Zn-Py. The consequential hybrid hydrogel film exhibited the 4-probe electrical conductivity of nearly 12 S/m with enhanced mechanical strength, self-healing capability, and elasticity (Haraguchi et al., 2012).

4.2.4. All in One Hydrogel Supercapacitors As observed from the discussion above, the electrochemical performance of the flexible supercapacitors is strongly dependent on the materials of an electrode, device configurations, and electrolytes. For wearable electronics, it is desirable to develop all-in-one-lithe high-performance supercapacitors that can easily incorporate the electrolyte and flexible

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electrodes altogether. In this framework, Wang et al. (2018) developed an incorporated supercapacitor device with 2 PANI layers embedded in the 3D spongy PVA-sulfuric acid hydrogel film as a solid-state electrolyte. The subsurface and surface of PVA-sulfuric acid hydrogel film covered with an insitu polymerized PANI of around 40-µm thick assisted as the electrolytes and electrode for the flexible supercapacitors, which displayed the large areal capacitance of nearly 488 mF/cm2, excellent flexibility, and cycling stability (Gong et al., 2003; Malkoch et al., 2006). For the self-healing supercapacitors, self-healing matrices are normally synthesized from the supermolecular networks with several hydrogen bonds. Unluckily, most of the stated self-healing supercapacitors need auxiliary healable layers, which led to a complex assembly for supercapacitors and the limited energy density. Currently, a self-healable all-in-one flexible supercapacitor has been made through in situ chemical removal of PANI and SWCNT (single-walled CNT) hybrid onto the 2-sided faces of a healable PVA-sulfuric acid hydrogel. Various hydrogen bonds embedded in a crosslinked network of the PVA permitted for the self-healing by dynamic creation of the hydrogen bonds after getting damaged. The self-healable PVA-sulfuric acid hydrogel also displayed a high ionic conductivity of around 136.4 mS/cm and suitable mechanical properties. The as-fabricated all-in-one supercapacitor displayed the areal capacitance of nearly 15.8 mF/ cm2 at the current density of around 0.044 mA/cm2. After the 5th self-sealing cycles, nearly 80% areal capacitance retention was attained, implying the better self-healable performance for flexible all-in-one supercapacitor (Sasaki and Koga, 2002). The new bottom-up infilling approach has also been established to infill the solid-state electrolyte into a PEDOT/PSS-CNT spongy electrode for assembling all in one approach of bottom-up infilling. As shown schematically in Figure 4.7, the permeable electrode was kept on the PVDF (polyvinylidene fluoride) Millipore membrane, and the water-impermeable PET (polyethylene terephthalate) film was then positioned on the top of PVA/H3PO4 solution on the top of PEDOT/PSS-CNT spongy electrode, interpreting water in the solution of PVA/H3PO4 evaporate downward via PVDF membrane. Electrolyte gel of Polyvinyl alcohol/H3PO4, therefore, began to form through the method of bottom-up and ultimately entered into the permeable electrode. The consequential free-standing electrolyte gel infilled PEDOT/PSS-MWCNT (multi-walled CNT) electrodes were assembled into the all-in-one flexible supercapacitor via gluing and pressing the as-obtained flexible electrodes with an electrolyte gel together. The

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lithe supercapacitors, therefore, prepared displayed the excessive large areal capacitance of around 2662 mF/cm2. This methodology of bottom-up infilling can be made practical to flexible hydrogels and different porous electrodes for constructing several new high-performance all-in-one flexible supercapacitors (Huang et al., 2007; Simon and Gogotsi, 2010).

4.3. FLEXIBLE SUPERCAPACITORS FROM CONDUCTING POLYMER (CP)-BASED FILMS Even though self-standing CPs films, like PANI, PEDOT, and PPy, can be made-up by electrochemical polymerization, spin coating, and interfacial polymerization, they normally suffer from brittleness because of the firmness of interconnected structures in CPs. To construct film-kind flexible CP supercapacitors, numerous CP films have been amalgamated with CNTs, RuO2, graphene, layered double hydroxides, aramid nanofiber, and black phosphorus, correspondingly, for preparing flexible hybrids film electrodes. Amongst them, composites centered on carbon nanomaterials and CPs are favorable for the production of flexible film-kind supercapacitors. As nanostructured carbons, comprising graphene and CNTs, are conductive, chemically stable, and highly flexible, they can be utilized also as current collectors in the flexible CP-based supercapacitors (Wang et al., 2018).

4.3.1. Conducting Polymer (CP)/Carbon Nanotube Hybrid Film Supercapacitors Thin films centered on CNTs have been utilized as perfect flexible electronics for the supercapacitors, they experience low energy density because of the low specific surface area. However, the energy density of the carbon nanotube films can be enhanced by the presence of CNTs, and PANI has been investigated most frequently because of its high pseudocapacitance and variable oxidation state intrinsically linked with its exclusive dopingdedoping features. The maximum level of doping and hypothetical specific capacitance of the p-type PANI can reach up to 0.5 and 2000 F/g1, correspondingly (Wang et al., 2016). To date, several methods have been established for the making of flexible PANI/Carbon nanotube hybrid films. Of specific curiosity is that CNTs Bucky papers and the freestanding carbon nanotube networks were utilized as a model for the chemical polymerization of PANI, from which all of the solid-state paper-like supercapacitors having high specific capacitance have

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successfully been assembled. Nevertheless, the attained conductivity (less than 150 Scm-1) and energy density (less than 2.2 kW/kg1) for the freestanding PANI/Carbon nanotube films must be further enhanced. In this concern, Fan et al. (2014) established the “skeleton/skin” approach utilizing PANI as skin and SWCNT film as a skeleton to prepare PANI/carbon nanotube hybrid films. Predominantly, free chemical vapor deposition was engaged to electrochemically deposit PANI onto the carbon nanotube bundles within a conjugated and uninterrupted network of the SWCNT film in order to form the CNT skeleton/PANI skin structure, which confirmed effective electron transport over the large area. The conductivity of around 1138 S/cm1 was attained, which is about thirty times higher as compared to those reported for the conventional PANI/SWCNT hybrid films (Wang et al., 2014). For CP/carbon nanotube hybrid films with diverse carbon nanotube architectures, the aligned carbon nanotube arrays have been confirmed to assist ion transport (Peng et al., 2014). For example, Peng and the coworkers (2014) made PANI/Carbon nanotube composite films through electrodepositing PANI into nanovoids within an associated MWCNT film/ array, along with the PANI-coating on the surface of constituent aligned CNTs. The synthesized PANI/multi-wall carbon nanotube hybrid film can normally be twisted more than a hundred times. The Synergetic interactions amongst the MWCNT and PANI gave the PANI/MWCNT hybrid film having good cycling performance and high specific capacitance. By utilizing the pre-stretched PDMS (polydimethylsiloxane) film substrate made by spin coating, the authors have also established the smart stretchable supercapacitor having high stability. As displayed in Figure 4.8, the prestretched PDMS film was covered by the thin layer of associated carbon nanotube film, trailed by electropolymerization of the aniline. Afterward, the pre-stretched film was unconfined and covered with electrolyte in order to form the crumpled structure, imparting electrochemical stability and a high stretchability. The PAN/PDMS/MWCNT stretchable film, therefore, prepared displayed the high specific capacitance of nearly 308.4 F/g1 along with 100% and 95.8% retentions after stretching for two hundred cycles and bending for thousand cycles, correspondingly. This work provided an opportunity for the design and growth of highly stretchable and flexible supercapacitors (Liu et al., 2016; Guo et al., 2018).

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Figure 4.8: Graphic representation of the making of smart, flexible, and stretchable supercapacitor (Chen et al., 2009).

4.3.2. Graphene/CP Hybrid Film Supercapacitors Graphene, the two-dimensional atom-thick sheet of carbon having high electrical conductivity, excellent chemical stability, and mechanical flexibility, has been well-thought-out as amongst the most favorable candidates for flexible supercapacitors. Though, it is yet perplexing to accomplish the high hypothetical specific capacitance for supercapacitors centered on the pure graphene sheets as the individual graphene sheets combined to lose the surface area all through the process of fabrication. Thus, CNTs have been deposited amongst the sheets of graphene as spacers to form amalgamated films. Through LBL (layer-by-layer) electrostatic assembly of the poly(ethyleneimine)-modified rGO (reduced graphene oxide) sheets and the acid-oxidized CNTs, Dai (2004) made the first carbon nanotube/rGO hybrid film with CNTs intercalated between contiguous rGO sheets. The consequential hybrid films preserved the large surface area and excellent mechanical properties linked with the sheets of graphene and exhibited the nearly rectangular CV (cyclic voltammogram) curve even at a very high rate of a scan of one V/s–1 with the specific capacitance of around 120 F/g1. Just resembling the conductive carbon nanotube spacers, the institution of CPs into the layers of graphene would be an effective way to avert the aggregation of sheets of graphene to enhance the electrochemical performance. Therefore, intensive research struggles have been dedicated to developing flexible CP/graphene hybrid films as the supercapacitor electrodes. Examples comprise flexible supercapacitors centered on the flexible rGO/PANI hybrid films along with a vacuum filtrated bendable PANI/rGO and polypyrrole/rGO hybrid films (Li et al., 2018).

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Rather than the chemical reduction of graphene oxide with the reducing agents, like NaBH4 and hydrazine, that might cause damaging effects on the combined structures, the electrochemical reduction of graphene oxide all through the electrochemical polymerization of CPs was discovered to be an efficient method to high performance rGO/CP hybrid films. By utilizing this simple strategy of in-situ electrochemical reductions, Latonen, and Lindfors (2014) electropolymerized aniline in the GO solution to make PANI/rGO flexible hybrid film as the supercapacitor electrodes. The as-made hybrid film offered an improvement in conductivity by 30% and the capacitance by 15% up to 77 mF/cm2 by the institution of rGO. Even after 10 thousand cycles of charging-discharging, the high capacitance of around 15 mF/cm2 was maintained for the 2-electrode symmetric supercapacitor centered on the flexible PANI/rGO hybrid electrodes (Han and Hong, 2001). With the help of precipitating PEDOT-PSS from the aqueous dispersion via partial elimination of PSS with dilute H2SO4, Li, and his co-workers (2014) made the free-standing thick film of PEDOT-PSS by casting the paste of PEDOT-PSS onto the filter paper, trailed by submerging into acetone to eliminate the filter paper. The lithe film of PEDOT-PSS, therefore, attained, after being submerged into the concentrated sulfuric acid for protonic doping, displayed the high conductivity with the specific capacitance of around 120 F/g1 at 0.4 A/g1. Also, the volumetric energy densities of around 6.80 mW/cm3 and 3.15 mW/cm3 and the high cycle stability were accomplished for the flexible supercapacitor device centered on the film of PEDOTPSS. Moreover, rGO was introduced by the simple method of bar-coating to prepare the stretchy PEDOT: PSS/rGO hybrid film, which displayed higher cycle stability and flexibility than the film of pristine PEDOT-PSS film. Under the whole bending and rolling up conditions, 100% specific capacitance retention was attained. A 95% capacitance retention was attained after 10 thousand charging-discharging cycles and 85% after subsequent 10 thousand cycles at the current density of around 2 A/g1, implying the highrate capacity and cycling stability (Song et al., 2013).

4.3.3. Conducting Polymer (CP)/Graphene/Carbon Nanotube Ternary Hybrid Film Supercapacitors Further institution of one-dimensional CNTs into CPs/graphene composites is a good option in order to further enhance the electrical and mechanical properties of the composite electrodes. Mainly, the utilization of carboxylated CNTs can also trigger in-situ doping of composites all through

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the polymerization of CPs. Thus, the inclusion of CNTs into hybrid films conducting films/graphene has currently been stated by various groups (Dong et al., 2010). For example, Zhou and Han (2016) made PPy/rGO/ carbon nanotube and PEDOT/rGO/carbon nanotube ternary films through the simplistic electrochemical co-deposition of constituent components from the assorted solution of monomer, GO, and carboxylated CNTs onto the fluorine-doped tin oxide (TO) conducting glasses. The consequential ternary film displayed an enhanced electrochemical performance as compared to its binary counterpart. In the distinct study, a PEDOT/rGO/carbon nanotube ternary film was made by 1-step electrochemical deposition in the mixed solution containing GO, CNT-COOH, and EDOT utilizing the commercial carbon nanotube film as a working electrode. The consequential ternary hybrid film with a conjugated carbon nanotube nano-network in order to promote the transfer of charge and the ion diffusion is lightweight, highly flexible, and thin with high volumetric and areal specific capacitance, as well as an outstanding rate capability and the long cycle life (Figure 4.9) (Zhang et al., 2014).

Figure 4.9: Graphic representation of the creation of flexible polyaniline/rGO/ multi-wall carbon nanotube ternary hybrid film Fan et al. (2014).

Rather than using the method of electrochemical co-deposition to prepare to conduct polymer/graphene/carbon nanotube ternary films, Fan et al. (2014) executed solution polymerization of the nanostructured PANI in the existence of the graphene/carbon nanotube film made by vacuum filtration of the GO-MWCNT solution. The consequential PANI/rGO/ Multi-wall carbon nanotube ternary flexible electrode displayed the specific capacitance of around 498 F/g1 at the current density of nearly 0.5 A/g1 with outstanding cycle stability and 98.5% capacitance retention after three thousand cycles (Wang et al., 2021).

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Current advances confirmed the making of CP hybrid films combined with the transition metal oxides, black phosphorus, layered double hydroxides, celluloses, and aramid fibers by several methods, including self-assembly, aerosol-jet spraying, electrochemical deposition, ionic liquid-backed supramolecular assembling, and vacuum filtration and posttreatment. Table 4.1 provides some of the CP-centered hybrid films and the electrochemical performances (Zhu et al., 2018).

4.4. FLEXIBLE SUPERCAPACITORS FROM CONDUCTING POLYMER (CP)-BASED FIBERS In comparison with the other flexible two-dimensional supercapacitors like papers and films, textile-centered supercapacitors can provide outstanding stability and flexibility. Furthermore to those flexible two-dimensional supercapacitors mentioned above, several textile-centered supercapacitors have been developed. Particularly, Feng et al. (2013) made ultra-porous lithe PEDOT nanofiber non-knitted mats by the 2-step method involving vaporphase polymerization and electrospinning, to exhibit the high conductivity of nearly 60 ± 10 S/cm1, high stability, and good electrochemical performance. To develop the flexible textile-centered supercapacitors with enhanced mechanical flexibility and electrochemical performance, several textiles, including cotton fabrics, carbon cloth, cellulose, ceramic fabric, and nylon lycra, have been engaged as the flexible substrates for placing CPs (Zeng et al., 2015; Yu et al., 2016). Table 4.1: Some Conducting Polymers-Centered Composite Films and the Electrochemical Performances Electrode Materials

Energy Density and Specific Capacitance Power Density

Stability

PEDOT/RuO2

0.053 µWh cm–2 at 147 µW cm–2

190 F cm–3

93% after 6000 cycles

PEDOT/LDH

46.1 Wh kg–1 at 11.9 kW kg–1

960 F g–1 at 2 A g–1

93% after 1000 cycles

PEDOT/MnO2

–

391 F cm–3 at 3.75 A cm–3

92% after 5500 cycles

PEDOT/aramid

4.54 Wh kg–1 at 306.4 W kg–1

25.3 F cm–3 at 0.1 A cm–3

88.3% after 10000 cycles

PPy/Black P

30.8 Wh kg–1 at 700 W kg–1

452.8 F g–1 at 0.5 A g–1

nearly no change after 10000 cycles

PEDOT/Cellulose

13.2 Wh kg–1 at 0.126 kW kg–1

50.4 F cm–3 at 0.05 A cm–3

90% after 1000 cycles

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In situ growing is quite a simple, but efficient, a method for polymerizing CPs on flexible substrates. The outstanding conductivity of cloth of carbon made it a perfect flexible substrate for in-situ electrochemical polymerization of pyrrole or aniline, whereas the functionalization of carbon cloth is significant for combining with CPs. In this regard, researchers have established the mild electrochemical approach for functionalization of the carbon cloth with RCOOH, ROH, and O = S = O in the 2 Mol sulfuric acid (Shirakawa et al., 1977). Together with covalent functionalization, the methods of non-covalent functionalization, including the hydrogen bonding and π-π interactions, have also been established for polymerizing ordered CPs onto the cloth of carbon. Explicitly, well-ordered polypyrrole nanowire arrays were grown directly on the untreated cloth of carbon by π-π interactions of carbon cloth with p-toluene sulfonic acid and PPy. In this specific circumstance, p-toluene sulfonic acid was utilized as the soft-template for the growth of the PPy nanoarrays, which exhibited the specific capacitance of nearly 699 F/g1 at 1 A/g1. The assembled polypyrrole/carbon cloth supercapacitors displayed an outstanding rate capability with good cycling performance (de Souza et al., 2014). The in-situ chemical polymerization has been broadly utilized to fabricate polypyrrole hybrid fibers, comprising PPy/cotton, PPy/nylon lycra, PPy/ceramic fabrics, and PPy/cellulose, for flexible supercapacitors. Due to the plentiful electronegative groups bearing in the structure of the cellulose, cotton, nylon ceramic fabrics, and lycra, pyrrole monomers might be polymerized inside the fabric networks via multiple hydrogen bonds. Compared with flexible planar supercapacitors, the fiber formed capacitors have currently gained widespread attention because of their tiny volume, wearability, and high flexibility. The fiber-molded supercapacitors with one-dimensional structures can be designed directly as textile electronics. In the last decade, a lot of struggle has been dedicated to the growth of flexible supercapacitors centered on carbon nanomaterials. Apart from the traditional electrochemically or chemically synthesized CP-carbon nanomaterial-centered hybrid fibers for the flexible supercapacitors, CPcarbon composite fibers made by wet-spinning, dip-coating, or solution drying have also been utilized to offer high pseudocapacitance. For example, Wang et al. (2020) stated the creation of polypyrrole/graphene fibers having diameters in the range of 15 to 100 µm by direct wet-spinning (Zhou et al., 2016).

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Given that the hollow fiber can offer surplus specific capacitance through the extra inner surface, Peng and the co-workers (2014) made supercapacitors based on the hollow PEDOT-PSS/rGO composite fibers through the facile method. As displayed in Figure 4.10, the mixture solution of GO, PEDOT: PSS, and vitamin C was vacuum-packed in the sealed glass tube. With the reduction of GO, the two ends of the glass tube were opened to further decrease and dry the misty fiber, triggering the creation of the hollow PEDOT: PSS/rGO composite fiber with a wrinkled and rough surface (Figure 4.10(b)), offering additional benefits for enhancing the capacitance and specific surface area. The film centered on as-synthesized hollow hybrid fiber displayed the tensile strength of nearly 631 MPa, which might be tied into the knot without structural destruction, confirming the high flexibility and mechanical strength. Supercapacitors centered on the hollow fibers displayed the ultra-high areal capacitance of nearly 304.5 mF/ cm2 at the current density of around 0.08 mA/cm2, which is many times that of the carbon nanotube/PANI and carbon nanotube/Co3O4 fiber equivalents (Yang et al., 2015). Similar to CPs, the transition oxides of metals can also offer pseudocapacitances. Amongst the normally utilized transition metal oxides, MnO2 (manganese dioxide) is well-thought-out as amongst the most favorable electrode materials because of the high theoretical specific capacitance, low cost, and environment-friendliness. For the applications of a flexible supercapacitor, manganese dioxide NPs with diverse crystalline structures were grown on the carbon fibers in order to form, for instance, manganese dioxide/carbon black (CB)/CNTs hybrid films via electrochemical deposition. Afterward, PEDOT: PSS was covered on the manganese dioxide/carbon black/carbon nanotube hybrid by dip coating or electrochemical deposition. The multi-layered hybrid fibers, therefore, prepared exhibited a high capacitance, good cycling performance, and high flexibility (Wu et al., 2010). Moreover, the core-shell structure of manganese dioxide@ PAA (poly(acrylic acid))/polypyrrole fiber has been developed for making fiber-centered supercapacitors combining the capacitance of manganese dioxide NPs with the shield of polypyrrole shell, leading to the specific capacitance of nearly 580 F/g at 5 A/g and a stable cycle stability. In one more study, nanosheets of MnO2 were grown on the PEDOT: PSScovered carbon nanotube fiber to make the manganese dioxide/PEDOT: PSS/carbon nanotube ternary fiber via electrochemical deposition at the potential of around 0.92 V. For the electrode of ternary hybrid fiber, the layer of PEDOT: PSS in middle can’t only offer pseudocapacitance, together with

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the manganese dioxide NPs, but also acts as the binder to associate the outer manganese dioxide nanosheets with carbon nanotube fiber. Therefore, the manganese dioxide/PEDOT: PSS/carbon nanotube ternary fiber displayed the volumetric capacitance of almost 478.6 and 267.3 F/cm3 at the current density of nearly 0.05 and 10 A/cm3, correspondingly, along with the good cycling performance having the retention of capacitance up to 91% after 10 thousand cycles. Table 4.2 gives the specific capacitance, flexibility, and electrical conductivity for typical CP-centered hydrogels, fibers, and films. For fabrication of the device, hydrogels are usually coated on carbon papers, Pt foils, or carbon clothes as the collector of current on the electrodes, which are accumulated into a device by sandwiching the gel electrolyte, like PVAH3PO4 and PVA-H2SO4, between the two electrodes (Xue et al., 2020).

Figure 4.10: Making of HCFs and creation of the hollow structures. (a) Schematic representation; (b, c) cross-sectional SEM-based images of HCF at high and low magnifications, correspondingly; (d, e) SEM-based images of HCF by the side view at high and low magnifications, correspondingly; (h, i) SEMbased images of HPF by the side view at high and low magnifications, correspondingly (Qu et al., 2016).

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Table 4.2: The Specific Capacitance, Flexibility, and Electrical of Usual Conducting Polymer-Centered Supercapacitors Electrode Material

Specific Capacitance

Flexibility

Electrical Conductivity (S cm–1)

PANI hydrogel

480 F g–1 at 5 mV S–1

Bendable

–

PANI hydrogel

480 F g–1 at 0.5 A g–1

Can be inkjet printed

0.23

PPy hydrogel

380 F g–1 at 0.2 A g–1

Bendable

0.5

PANI-αCD hydrogel

322 F g–1 at 2 A g–1

Stretchable and foldable

0.39

PANI hydrogel

422 F g–1 at 0.2 A g–1

3D printing

0.35

PANI/PVA all in one hydrogel

15.8 mF cm–2 at 0.044 mA cm–2

Stretchable and healable

0.13

PANI/MWNT film

233 F g–1 at 1 A g–1

Bendable

424

PANI/rGO film

438.8 F g at 0.5 A g

Bendable

1138

PANI/CNT on PDMS film

308.4 F g at 1 A g

Stretchable and bendable

—

PEDOT:PSS/graphene film

448 mF cm–2 at 10 mV s–1

Foldable

92.5

PEDOT-PSS/rGO fibers

304.5 mF cm–2 at 0.08 mA cm–2

Bendable

47

PPy/graphene fiber

107.2 mF cm–2 at 0.24 mA cm–2

Bendable

1.44

PPy/PAA/MnO2 fiber

692 F g–1 at 0.5 A g–1

Bendable

1.19 × 10–5

–1 –1

–1

–1

4.5. SUMMARY AND FUTURE SCENARIOS In this chapter, the current progress in CP-centered flexible supercapacitors with the electrodes varying from one-dimensional fibers, through twodimensional films and the textiles, to 3D hydrogels has been discussed. As can be noticed, CPs can’t only provide interesting doping-dedoping features and captivating tunable conductivity and morphology, but also display structural variations during the process of charging-discharging. Numerous strategies, including interfacial polymerization, electrodeposition, crosslinking/ self-crosslinking, vacuum filtration, electrospinning, and supramolecular assembling, have been established to construct the high-performance CPcentered flexible supercapacitors with better structural stability. Several progressive flexible supercapacitors in fiber, film, and all in one form have

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been established from many outstanding CP-centered electrode materials, without, and with the existence of extra pseudocapacitance reagents. The vigorous supramolecular collaboration in the network of polymer even permitted the making of self-healing supercapacitors with high performance (Lindfors and Latonen, 2014). Despite the substantial progress in the growth of flexible CP-centered supercapacitors, their energy densities and power must be further improved for practical applications. Reasonably designed 3D CP-centered hydrogels having hierarchical porous structures of the well-dispersed CPs must be developed to confirm the high conductivity and specific area. Similarly, the well-governed deposition of CPs on flexible two-D films or one-D fibers with quantified nanostructures also have a significant role in amending the interface of flexible electrode materials, and henceforth the performance of supercapacitor. The tuning of structures of doping, the design of “all in one” flexible supercapacitors, or the presence of redox-active kinds can further upsurge the performance of CP-based flexible supercapacitors. Even though it is still perplexing to accurately govern the structure-property association for several electrode materials, constant research and expansion in this exciting area must open up chances for interpreting the technologies of flexible supercapacitors into the reality of the market, which will considerably enhance the quality of life in the future (Hong et al., 2017).

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CHAPTER

5

CONDUCTIVE POLYMER-BASED ORGANIC SOLAR CELLS

CONTENTS 5.1. Introduction..................................................................................... 146 5.2. The Present Situation........................................................................ 153 5.3. Properties of Organic Solar Cells..................................................... 154 5.4. Solar Cell Architectures.................................................................... 156 5.5. Operational Principles of OSCS....................................................... 160 5.6. Characterization of Organic Solar Cells........................................... 165 References.............................................................................................. 169

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5.1. INTRODUCTION Polymer solar cells possessed multiple intrinsic benefits, such as low manufacturing costs, flexibility, lightweight, and reduced material. Recently, due to the potential of achieving greater performance than certain cells, conductive polymer tandem solar cells had received considerable attention. Photovoltaics are concerned with the conversion of sunlight into electrical energy. After their preliminary realization of a silicon solar cell by Chapin, Pearson, and Fuller, in the Bell labs in 1954, contemporary photovoltaic solar cells established on inorganic semiconductors had experienced a lot of advancements (Chapin et al., 1954; Goetzberger et al., 2003). Presently, with power transformation efficiency nearing 15 to 20% for mono-crystalline devices, silicon is, however, still the basic technology on the international market of photovoltaic solar cells. Even though the solar energy industry is highly subsidized through numerous years, the expenses of silicon solar cell-based power plants or panels are not cost-effective with other traditional combustion techniques yet-apart from multiple niche products. Organic materials that could be prepared under low-demanding conditions can be used to decrease the manufacturing expenses of solar cells. In the last 10 years, a great expansion has been seen in the research field for organic photovoltaics, which was formed over three decades ago (Hoppe et al., 2004; Spanggaard and Krebs, 2004). The amount of solar energy illuminating Earth’s landmass every year is nearly three thousand times the total amount of annual human energy utilization. When competing with energy from fossil fuels, however, photovoltaic devices must convert sunlight into electricity with a certain measure of effectiveness. For conductive polymer-based organic photovoltaic cells, which are less expensive to create as compared silicon incorporated solar cells, scientists had believed for a long time that the reason behind the high efficiency lies in the clarity of the polymer/ organic cell’s two domains-donor and acceptor (Rowell et al., 2006; Zhao et al., 2017). The device design, production procedure, and the character of the materials can be used to differentiate organic solar cells. The two major production techniques could be identified as wet processing or thermal evaporation. Device architectures consist of bilayer heterojunction, bulk heterojunction (BHJ), as well as a single layer with the diffuse bilayer heterojunction that acts as an intermediary among the bilayer and the BHJ (Kawano et al., 2006; Cheng and Zhan, 2016). Where a single layer consists of only one active material, the other architectures are based on two types of materials: electron acceptors (A) and electron donors (D), respectively. The

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variance of these architectures rests in the charge formation process: singlelayer devices mostly require a Scotty barrier at one contact, which allows splitting photo excitations in the barrier field. To separate the electron from the hole the DA solar cells employed the photo persuaded electron transfer (Sariciftci et al., 1992; Kawano et al., 2006). The photoinduced electron transfer takes place from the excited state of the donor (LUMO: lowest unoccupied molecular orbital) to the LUMO of the acceptor, which therefore had to be an acceptable electron acceptor with a stronger electron affinity (Shrotriya et al., 2006; Roncali, 2009). The electron and the hole perform to stretch the opposite electrodes, the anode, and the cathode, accordingly after the charge separation. Hence, a direct current could be transferred to an exterior circuit. As global-warming has been indicated to increase, it is becoming apparent that techniques that form electricity without the production of carbon dioxide and other greenhouse gases need to be found. Fortunately, renewable energy sources exist, which neither finish out nor do they have any considerable harmful effects on the environment (Brabec et al., 2001; Li et al., 2011). Obtaining energy originally from the sunlight consuming photovoltaic (PV) technology is being greatly recognized as a significant component of future global energy formation (Wöhrle and Meissner, 1991; Clarke and Durrant, 2010).

5.1.1. Organic Solar Cells Monetarily feasible long-term technology could be developed through the use of organic materials for large-scale power generation established on environmentally harmless materials with unlimited availability. Inorganic semiconductors such as Si can be substituted by organic semiconductors, which are comparatively less costly than them. The organic semiconductors may possess extremely high optical absorption coefficients, which can open up an opportunity for the formation of very thin solar cells (Koster et al., 2012). Additional appealing characteristics of organic PVs is their potential for developing thin flexible devices that could be manufactured using low temperature, high throughput techniques that utilize well-established printing processes in a roll-to-roll method (Shaheen et al., 2001; Koster et al., 2005). A reduction in the structure cost for organic PVs, leading to a decreased energetic payback time can be experienced through the prospective utilization of flexible plastic substrates in a commonly scalable high-speed printing process (Gustafsson et al., 1992; Brabec, 2004). The electronic framework of all the organic semiconductors is based on conjugated πelectrons. A

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modification among the single and double carbon-carbon bonds results in a conjugated organic structure. While double bonds consist of σ-bond and a π-bond, single bonds are recognized as σ-bonds and are associated with localized electrons (Schilinsky et al., 2004; Waldauf, 2004). Being greatly mobile as compared to σ-electrons, π-electrons are capable of jumping from site to site between carbon atoms appreciations to the common overlap of π orbital’s along with the conjugation path, which creates the wave functions to delocalize above the conjugated backbone. The π-bands can either be occupied with electrons (known as the HOMO: highest occupied molecular orbital ) or vacant (called the LUMO-Lowest Unoccupied Molecular Orbit), the band-gap of these materials range from 1 to 4 eV. This π-electron system consisted of all the significant electronic characteristics of organic materials: transport, charge production, light emission, and absorption (Shaheen et al., 2001; Hoppe et al., 2003). Organic semiconductors have been researched extensively since 1954 the high conductivity present in the perylene-iodine complex was discovered (Goetzberger et al., 2003). When Tang et al. introduced the first OLED in the 1970s, possible applications of organic semiconductors started (Chapin et al., 1954). OLEDs had established an exceptional industry of ultra-thin and flexible displays due to the unique features of organic semiconductors such as simple fabrication method flexibility and thinness. Large-area displays, for instance, televisions, and commercial applications like small OLED displays on mobile appliances, have accepted OLEDs, which are getting additional attention (Riedel et al., 2004; Mihailetchi, 2005). In addition to OLEDs, another great application of organic semiconductors is OSCs (organic solar cells). As compared to OLEDs, OSCs employ organic semiconductors to engross light and convert it into electrical energy (Brabec et al., 2002a, b). The easy and economical manufacturing process of OSCs grants the tremendous potential for large-area applications, with inorganic solar cells technology edging towards cost blockages for huge area applications. Moreover, OSCs can lead to novel applications such as portable solar panels, because of their unique qualities of flexibility and lightweight. The characteristics of OSCs are extremely intriguing for comprehending the organic devices. This chapter explains their fundamental principles (Schilinsky et al., 2002; Dyakonov, 2004).

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5.1.2. Overview of Organic Semiconductors Consisting of semiconductor properties, organic semiconductors are carbonbased materials. Atoms that are a part of an organic semiconductor molecule are linked by conjugated p-bonds, whereas, molecules are linked through weak van der Waal’s force, in contrast to the tremendous covalent structure exhibited by inorganic semiconductors. The bonding structure gives organic semiconductors their particular weight, flexibility, and low sublimation point which allow simple processing (Akamatu et al., 1954; Tang et al., 1987). The band composition of organic semiconductors could be treated like inorganic semiconductors from the macroscopic point of view. While the conduction band is mostly free of electrons, the valence band mostly consists of electrons. The HOMO and the LUMO, inorganic semiconductors, happen to be analogs to the valence band and conduction band, respectively. The hybridization between anti-bonding and bonding of the conjugated p-electrons is denoted by the LUMO and HOMO of organic semiconductors (Bredas et al., 2002; Kymissis, 2009). Organic semiconductors are produced of organic molecules which are developed through a p-conjugated system. Since carbon atoms are sp2 hybridized and the sp2 bonds have shown to create 3 strong r-bonds with adjacent atoms, through the development of feebler p-bonds, the remaining p-orbitals of the C atoms form a delocalized cloud of electrons. A quasi-one-dimensional organization for the conjugated organic semiconductors is made by this bond configuration. As per the electron wave function overlap of neighboring atoms, the p-bond arrangement could have extensive bonding configurations (White et al., 2006; Kaltenbrunner et al., 2012). For example, Figure 5.1, depicts two diverse phases of the p-bonds, with the anti-bonding and bonding phases relative to diverse energy levels (Koster, 2006). Diverse energy levels of an organic semiconductor can be an outcome of the LUMO and HOMO of organic semiconductors’ elevated energy bands that connect to diverse hybridization states of the p-bonds. The molecule itself is excited into a higher energy status when an electron acts as a stimulator from the HOMO to the LUMO of an organic semiconductor, in comparison with the real excitation of a free electron, in inorganic semiconductors, from the valence band to the conduction band (Gregg, 2003).

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Figure 5.1: Illustration of bonding-antibonding interactions between the HOMO/LUMO levels of an organic semiconductor. Source: https://www.springer.com/gp/book/9781447148227.

The carrier transport process is different in organic semiconductors than that of inorganic semiconductors. The carrier transport inside the inorganic semiconductor takes place when they thermally stimulate the ‘hopping’ of carriers which overcomes the energy barriers inside the disarrayed conjugated conductive polymer framework (Hu et al., 2002; Sundar et al., 2004). This process is very dissimilar for charge transport in the inorganic semiconductors, which could be explained through the motion of free carriers in the conduction band or valence. In contrast to their inorganic complements, the hopping transport process gives organic semiconductors fairly low mobility. While silicon has mobility as far as *4.5 9 10–2 m2 V–1 s–1, small-molecule organic semiconductors gain up to *1.5 9 10–3 m2 V–1 s–1 hole mobility (McCulloch et al., 2006; Kietzke, 2007). On the other hand, silicon had greater electron mobility of 0.1 m2 V–1s–1, meanwhile, electron mobility for certain small molecule materials reaches *1 9 10–5 m2 V–1 s–1. The commonly studied P3HT, in solar cell applications: PCBM blend had a hole and electron mobilities of the arrangement of *10–7–10–8 m2 V–1 s–1 in the various blend film (Gundlach, 2005; Anthopoulos, 2006). The decrease in mobility, when linked to inorganic semiconductors, is the major pitfall for organic semiconductors, and resulting, different devices were considered to overwhelm this incapacity.

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5.1.3. Structure of Organic Solar Cell Parameters such as the choice of electrodes used, light intensity, thickness of the active layer, the solid-state morphology of the film, the composition of the components, and the temperature reckon the magnitude of FF, JSC, fullerene BHJs, and VOC, for the organic solar cells established on conductive polymer (Brütting, 2006; Mayer et al., 2007). Definite knowledge of the device photocurrent and operation, production, Jph, and restrictions in these devices is required for their optimization and maximization. The association between the material parameters and experimental Jph (i.e., band-gap, relative dielectric constant or molecular energy levels, charge-carrier mobility) need to be comprehended and managed to allow the additional design of novel materials that could enhance the effectiveness of this type of solar cells (Coakley and McGehee, 2004; Min, 2010). By employing numerical models and concepts that are famous for inorganic solar cells, such as the p-n junction model, the first attempt to apprehend the physics besides the organic BHJ solar cells was concluded. An extended replacement circuit had been introduced to improve the contract of the traditional p-n model through the experimental Jph of an organic BHJ cell (Andersson, 2008; Lee et al., 2009). Comprising of an indefinite physical meaning for an organic cell that model is an alternative of the photoactive layer through an ideal diode and a serial and a parallel resistance. BHJ cells consist of a particular combination of two (un-doped) intrinsic semiconductors which happen to be nanoscopically varied and that form a randomly oriented interface. However, it is different from conventional p-n junction cells with spatially detached p- and n-type parts of doped semiconductors. Moreover, the classical p-n junction model is unsuitable to explain the Jph of these solar cells due to the distinct charge generation, recombination, and transport procedures in BHJs (Peumans et al., 2000). The employment of the MIM (metal-insulator-metal) concept provides an alternative, in which a homogenous mixture of two unipolar semiconductors (acceptor/donor) is recognized as a single semiconductor with features derivative from the two materials. This implies that the photoactive layer is manifested as a single ‘virtual’ semiconductor considering that its conduction band is granted through the LUMO of the acceptor and the HOMO of the donor-type material defines its valence band. The variation between the work functions of the metal electrodes affects the probable difference accessible in the MIM device, which pushes the photo

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formed charge carriers to the collection electrodes, in PV operation mode (Figures 5.2–5.4) (Tang, 1986; Jean-Michel, 2002). Figure 5.2 exhibits the multiple donor and acceptor materials that have been reported, yet not a single one of them gets over 3% efficiency except for PCPDTBT/PCBM or P3HT/PCBM.

Figure 5.2: Chemical organization of organic solar cell donor and acceptor materials. Source: http://pubs.sciepub.com/rse/2/3/2/figure/1.

Figure 5.3: Illustration of an organic solar cell (construction of an organic photovoltaic device. Glass is the substrate, the negative electrode is (indium tin oxide) ITO, aluminum, is a general transparent electrode. The illustration presents

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a BHJ (bulk heterojunction) active layer where the blending of an acceptor and donor creates phase-separated domains inside the active layer. The structure of the BHJ is essential to the execution of a solar device. Source: http://www-ssrl.slac.stanford.edu/content/science/ highlight/2011-01-31/effects-thermal-annealing-morphologypolymer%E2%80%93fullereneblends-organic#sthash.iE7FUkF8.dpuf.

Figure 5.4: A fullerene derivative and several solutions processible conjugated polymers used in organic solar cells. Chemical arrangement and abbreviations of some conjugated organic molecules. From left: poly(para-phenylene-vinylene) (PPV); poly(acetylene) (PA), a relieved PPV (MDMO-PPV), poly(3-hexyl thiophene) (P3HT); and a C60 derived In every compound a pattern of irregular single and double bonds could be identified. Source: https://www.springer.com/gp/book/9781447148227.

5.2. THE PRESENT SITUATION Organic Solar cells had some disadvantages based on their short life span and low efficiency (only 5% efficiency in contrast to the 15% of silicon cells). Nevertheless, the multiple advantages that they offer justify their present global investment and research in developing novel structures and combinations, as well as novel conductive polymeric materials, to achieve low-cost, large-scale manufacturing, and increase efficiency within the next

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years. The aim of the next period is the commercially attainable organic solar cell creation (Figure 5.5).

Figure 5.5: Transparent and flexible solar cells. Source: https://onlinelibrary.wiley.com/doi/10.1002/aenm.201701791.

Greater than 3.5% power transformation effectiveness has been exhibited by donor-acceptor established organic solar cells recently. The development of new low bandgap materials along with improving the nanoscale morphology is anticipated to steer the power transformation efficiencies nearing 10% (Koster, 2005; Günes et al., 2007).

5.3. PROPERTIES OF ORGANIC SOLAR CELLS 5.3.1. Organic Solar or Photovoltaic Materials The conversion of solar to electrical energy is carried out constantly by polymers, small molecules, and dendrimers, which are a form of organic materials (carbon-compound based) used by plastic or organic solar cells. These semiconductive organic molecules are capable of absorbing light and induce the movement of electrical charges between the conduction band of the absorber to the conduction band of the acceptor molecule. There are several kinds of OPVs (organic photovoltaic cells), consisting of singlelayer and multilayered structured cells. Both types are currently being used in small area applications and research, and both have certain benefits and drawbacks (Figure 5.6).

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Figure 5.6: The structure of a single-layer and a multilayer organic solar cell.

5.3.2. Benefits of Flexible Organic Compared to Rigid Conventional Solar Cells A sequence of organic cell potential benefits has been brought to light by the latest progress in molecular engineering that may ultimately outweigh the advantages of silicon-based solar cells. Although traditional solar cells dominate the prevailing market currently, the case may be very different in the coming future.

5.3.3. Manufacturing Process and Expenses Based on the molecular nature of the used materials, organic solar cells could be simply manufactured in contrast to silicon produced cells. Molecules could be employed with thin-film substrates that are one thousand times thinner as compared to silicon cells (order of a rare hundred nanometers) in addition to being easy to work with. The production expenses could be decreased extensively based on that alone. Organic materials display flexibility in their production methods as they are very compatible with a wide range of substrates. These techniques consist of solution processes (paints or inks), roll-to-roll technology, high quantity printing methods, and many more, that allow organic solar cells to envelop huge thin film surfaces easily and cost-efficiently. All of the mentioned procedures could reduce cost by a factor of 10 or 20 and had low energy and temperature demands as compared to traditional semiconductive cells.

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5.3.4. Tailoring Molecular Characteristics A major advantage of organic materials used in solar cell manufacturing is the potential to modify the molecule characteristics to fit the application. Through altering, for instance, the length and functional group of conductive polymers, molecular engineering could modify the capability to form charges, molecular mass, and bandgap. Moreover, new unique formulations could be established with a mixture of organic and inorganic molecules, making it possible to print the organic solar cells in any suitable pattern or color.

5.3.5. Desirable Qualities The alteration of molecular characteristics and the flexibility of production procedures explained previously permit organic polymer solar cells to extant a sequence of desirable qualities. These solar molecules are less likely to experience failure or get harmed as they are extremely elastic and reliable than their heavy and rigid counterparts. They have the potential to exist in several movable forms (for instance, rolled form) and their flexibility makes transport, installation, and storage much easier.

5.3.6. Impact on the Environment Conventional inorganic cells require a lot more amount of energy for their production in contrast to a solar cell. Hence, energy transformation efficiency does not have to be as high as the traditional cell’s effectiveness. Extensive utilization of organic solar cells is the cause of the increased use of solar power globally and making renewable energy sources obtainable to the average consumer (Koster et al., 2005).

5.4. SOLAR CELL ARCHITECTURES 5.4.1. Bilayer Solar Cell The development of the heterojunction by Tang et al. (1986) was originally presented in the form of a bilayer solar cell. The common framework of a bilayer solar cell as illustrated in Figure 5.7, involves an anode, cathode fabricated, a hole collection layer, an active layer consisting of donor and acceptor, electron collection layer serially. The hole collection layer and electron collection layer are used to improve the work function of the electrodes to form an ohmic contact. Excitons detach in a well-defined,

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individual interface arising between the acceptor and donor. Possessing this arrangement, the bilayer solar cell is the most uncomplicated structure described by the basic operating principle of the solar cell. The small exciton diffusion length of organic materials limiting the thickness of the donor and acceptor layers is a major drawback of the bilayer solar cells. The excitons created far away from the heterojunction may recombine sooner, before they reach the heterojunction if the thickness of the acceptor or donor layer thicker than average. Additionally, the acceptor and donor layers are confined to tens of nanometers which results in weak absorption. Therefore, interference impact had to be included all through the arrangement of bilayer solar cells to make sure that excitons are formed near the heterojunction. These common trade-off factors take to low EQE and cause issues in the design of bilayer OSCs.

Figure 5.7: Organization of a bilayer solar cell. Source: https://www.researchgate.net/figure/Typical-device-structure-of-bilayer-solar-cell-The-charge-dissociation-only-occurs-at_fig11_270550059.

5.4.2. Bulk Heterojunction (BHJ) Solar Cells The inception of the BHJ in the mid-1990s was probably one of the most important inventions in the field of OSCs (Yu et al., 1995). Figure 5.8

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represents the structure of BHJ. Although thermal co-deposition methods could be employed to manufacture a BHJ, the junction is commonly produced by combining acceptor and donor materials in a solution, then forming the active layer by spin coating a mixed solution on a substrate. The obtained film is an interpenetrating nanoscale network of acceptor and donor materials. The phase separation present in the film is normally 10–20 nm, which is within the exciton diffusion length of several organic semiconductors. Hence, close unity internal quantum efficiency had been achieved for BHJ solar cell, which means that nearly all photogenerated excitons are separated. Carriers are then moved by percolated pathways within the active layer in the direction of certain contacts for collection (Chen et al., 2009; Park et al., 2009). When matched to bilayer solar cells, a thicker active layer could be formed in these cells because of the little nanoscale phase separation in BHJs. Even though the spin-coating process is intrinsically less regulated as compared to the vapor deposition process, which is frequently used in bilayer solar cells, the performance of BHJ solar cells is affected by multiple parameter modifications. The efficiency of solar cells is greatly dependent on the morphology of the BHJ and several techniques such as changing polymer functional groups, solvent annealing, and thermal annealing had been researched to improve the performance of OSCs (Savenije et al., 2005; Li et al., 2007).

Figure 5.8: Organization of a bulk heterojunction solar cell. Source: https://www.researchgate.net/figure/Bulk-heterojunction-organic-solar-cells_fig1_340090481.

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Among various materials, P3HT: PCBM BHJ is the most repeated used and well-optimized active layer employed in OSCs. Through the use of these mixed materials, Schilinsky et al. (2002) introduced a short-circuit current of 8.7 mA/cm2, which was the maximum current at that particular time. With the effectiveness of P3HT: PCBM solar cells, increasing efficiency percentage up to 3.5 in just a year, P3HT: PCBM was subjected to intense research (Padingeret al., 2003; Vanlaeke et al., 2006; Chen et al., 2009).

5.4.3. Tandem Solar Cells The manufacturing of solar cells in tandem was proposed to overcome the limitation of feeble absorption strength as well as the absorption extent of the active layer of OSCs. Piling the solar cells in a series (a 2-terminal arrangement) would generate a large Voc and active layers with various absorption regions in the tandem structure could allow the cell to absorb light over a wide wavelength range (HoppeSariciftci, 2006; Kim et al., 2006). For instance, it was shown in a ZnPc: C60 and P3HT: PCBM tandem solar cell which exhibited improved Voc and a wide absorption range. Tandem cells also efficiently employed a thin layer of Ag or Au as the intermediate layer manufactured through thermal evaporation (Li et al., 2005; Yao et al., 2008). It has been brought to light that tandem cells are capable of being complete solution-processed, through a conductive polymer: titanium oxide/PEDOT: PSS and small molecule active layers as the interlayer between subcells. Similarly, other interlayers such as ZnO/PEDOT: PSS were also validated successfully to create effective tandem solar cells (Gilot, 2007). Besides linking the subcells in series, it had been reported that linking the cells in parallel (a 3-terminal structure) could also lead to enhanced effectiveness (Figure 5.9).

Figure 5.9: Structure of a tandem cell. Source: https://pubs.rsc.org/en/content/articlelanding/2009/ee/b817952b.

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The short circuit current is seemingly the aggregate of the current outputs from the two subcells when the cells are linked in parallel. Sista et al. unveiled a large short-circuit current of 15.1 mA/cm2 and power conversion efficiency (PCE) of 4.8% employing a 3-terminal structure. In addition to that, it was presented that the common-anode and commoncathode configurations are feasible by using TiO2: Cs/Al/Au and PEDOT: PSS/Au/V2O5 as the intermediate layer, respectively (Dennler, 2006; Sista et al., 2010).

5.5. OPERATIONAL PRINCIPLES OF OSCS 5.5.1. Exciton Generation An electron present in the organic semiconductor is thrilled from the HOMO to the LUMO after the absorption of a photon. The process of thrilling an electron from the valence band to the conduction band of the inorganic semiconductors is identical to it. However, due to the reduced hole wave functions and dielectric constant along with localized electrons in organic semiconductors, strong Coulombic attraction arises between the electronhole pair. Containing binding energy ranging between 0.1 and 1.4 eV, the resultant bound electron-hole pair is known as an exciton, as compared to an increasing lower binding energy of a rare meV possessed by inorganic semiconductors. Thus, in an inorganic semiconductor, it is a relatively greater possibility to create free charge carriers later to the absorption of photons. For example, by absorbing thermal energy the electron-hole pairs detach; nevertheless, strongly bound excitons are formed in organic semiconductors (Mihailetchi et al., 2005). The organic materials have an absorption coefficient that is mostly high at *105 cm–1. The active layer absorbs an appropriate amount of light and shows considerable solar cell qualities if the active layer is a few hundred nanometers are dense enough (Lenes, 2006). Several light trapping methods such as lens concentrators, folded cells, and gratings have been suggested to increase absorption even more, especially in the wavelength range where the necessary absorption of the material is weak (Shrotriya, 2006). Commonly the little absorption range and the huge bandgap, which result in low absorption effectiveness of photons in the long-wavelength area are a major concern for organic materials. It has been reported that *77% of solar light is absorbed with a LUMO-HOMO modification of 1.1 eV. On the other hand, classic material for OSCs, P3HT, possessed a bandgap of *1.9 eV, but, most organic materials retained band

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gaps of *2 eV. The AM1.5G reference solar spectrum has been presented in Figure 5.10. Due to the presence of many materials that don’t absorb in the region (700 nm), it is imperative that a massive portion of energy could be absorbed in the long-wavelength regions (700 nm) by employing low bandgap materials (Keis et al., 2002).

` Figure 5.10: AM1.5G reference solar spectrum. Source: spectra.

https://www.pveducation.org/pvcdrom/appendices/standard-solar-

5.5.2. Exciton Diffusion and Dissociation When an exciton is created, the question revolves around how to differentiate the bound electron-hole pair to develop free charges which finally result in the formation of electricity. Tang et al. came forward with an innovative solution according to which using two distinct organic materials with appropriately aligned band levels could lead to effective solar cells. The heterojunction is what a junction between the two materials is called. The heterojunction had become the key principle of OSC design, even after that discovery. Two organic materials with band alignment exhibited in Figure 5.11 are situated near to each other to achieve exciton dissociation. The difference between the LUMO of material B and the HOMO of material A had to be smaller than the potential difference between the bound electronhole pair, for instance, the bandgap of any material A or B subtracting the

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exciton binding energy (Kuwabara et al., 2008). When an exciton is created in, for example, material A, it moves towards the direction of heterojunction. Namely, the energy of the exciton is greater than the potential difference between LUMOB and HOMOA. The transfer of an electron from the exciton to LUMOB is an intensively beneficial process.

Figure 5.11: Band placement of donor and acceptor materials for a heterojunction. Source: https://www.mdpi.com/2079-9292/6/4/75.

While a hole stays in HOMOA, an electron is transferred from the exciton to HOMOB. Due to this charge transfer process, materials A and B labeled a donor and acceptor accordingly. The competing process of luminescence, which consists of the radiative recombination of excitons, takes place at a timescale of *1 ns. On the contrary, the charge transfer process operates at a smaller timescale of *45 fs, allowing efficient exciton dissociation at the heterojunction. Following the separation, an electron-hole pair builds a charge pair called a geminate pair, which are charges however Coulombically bound and seem to detach through an internal field (Waldauf, 2006).

5.5.3. Carrier Transport After the exciton separation, the geminate pairs formed had to move to electrodes for collection in their lifetimes. Drift currents and diffusion are the key driving forces for the transference of holes to the anode and electrons to the cathode. Apart from the potential gradient inside the solar

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cell, the drift current responds to the movement of the carrier. The selection of the electrode in a solar cell mainly establishes the potential gradient. Mostly, a high work function anode and decreased work function cathode are used, and this distinction inside the solar cell makes a built-in electric field that manages the open-circuit voltage (Voc) of the cell. A fluctuation in the drift current and an enhancement in the internal electric field are experienced when an exterior bias is applied (Hayakawa, 2007). The carriers drift together with a successive internal electric field of the solar cell to the specific electrodes for collection. An alternate procedure of carrier transport is the diffusion current, which can be explained as the diffusion of carriers with the carrier absorption gradient inside a solar cell. The concentration of holes and electrons is normally higher around the heterojunction, as the geminate pairs are formed around the solar cell heterojunction. Hence, the carriers diffuse simultaneously with the concentration gradient left from the heterojunction, taking to the diffusion current. The diffusion current usually the one to influence when the applied bias changes the internal electric field to nearly zero, but, drift current direct when the internal electric field is great. The movement in the active layer is the main limitation of carrier transport. The active layer had to be kept relatively thin to allow carriers to get to the electrodes in their lifetimes since electron and hole mobilities in organic materials are commonly low. The mobility variance is also an important element in describing the properties of charge transport because variance greater than a factor of 10 would bring about SCLC (space charge limited current). SCLC increases when one type of carrier, for instance, electrons (electron mostly had greater mobility in OSC materials) are conveyed more adequately to the cathode. As the rate of holes reaching the anode is not as high as the electrons getting to the cathode, electrons may assemble in the active layer near the cathode interface. This may create the space charge effect, which modifies the active layer’s charge transport properties and forms an upper limit for the current efficiency of a solar cell. Electron mobilities and the balanced hole is required to achieve suitable carrier transport in the active layer of a solar cell (Brabec et al., 2001; Peng, 2011).

5.5.4. Charge Extraction at Electrodes The charge carriers are extracted from the active layer to the electrodes after they are transferred to the active electrode/layer interface. The expected

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barrier at the active layer/electrode interfaces had to be reduced to acquire high effectiveness in charge extraction (Xie, 2011). Hence, the work function of the anode is precisely expected to complement the donor HOMO, but, the work function of the cathode is expected to complement the acceptor LUMO. When this occurs, the contacts are called ohmic contacts, and Voc corresponds perfectly with the variation between the donor HOMO and acceptor LUMO. Meanwhile, an ohmic contact cannot be produced if the work functions of anode and cathode materials are not close to the donor HOMO or acceptor LUMO, successively. In this case, the MIM model directs the carrier extraction behavior (Parker, 1994; Han et al., 2009). Utilizing various types of materials as electrodes is a method employed to progress the work function similar at the electrodes. As the work function of ITO (indium tin oxide) is *4.7 eV works well with HOMO of the P3HT, it is normally used as an anode contact (Scharber et al., 2006). High work function metals, such as Au (5.1 eV) could also be employed as the anode contact. Mostly, low work function metals, on the cathode side, for instance, Al (4.2 eV) are adopted to match the LUMO of PCBM. Inter-layers could also be added between the active layer and the electrodes to well align the active layer HOMO or LUMO and the electrode work function to alter the material to achieve work function matching. In particular, a very small layer of LiF is normally evaporated on the active layer before constructing the cathode to maintain an ohmic contact (Brabec, 2002; Mihailetchi, 2003). A boost in the electron collection can be witnessed by using solution-processed materials such as TiO2 and ZnO. To form an ohmic contact, transition metal oxides, for instance, likeWO3 or MoO3 are used as an interlayer on the anode side (Kyaw, 2008; Schmidt, 2009). It was signified that even Al, which is one of the little work function metals could be evaporated on the upper to make an anode in an inverted arrangement when transition metal oxides are employed as the interlayer. In addition to changing the work function of the electrodes, improving the interface area or roughness of the electrodes may also support a large area for more efficient charge collection (Tao, 2008; Jiang et al., 2010).

5.5.5. Summary of the Operation The following 4 phases are used to outline the entire operation of solar cells: (1) Photon absorption taking to exciton production; (2) Exciton diffusion to an acceptor/donor heterojunction; (3) To form geminate pairs, exciton separation takes place at a heterojunction; (4) Carrier extraction including

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carrier transport at the electrodes. Figure 5.12 presents the mentioned four steps.

Figure 5.12: Image of the nature of charge separation in a solar cell. Source: https://www.springer.com/gp/book/9781447148227.

The potency of the four steps is signified through gA, gdiff, gdiss, and gC accordingly. The EQE (external quantum efficiency) can be described as: (1) The EQE represents the percentage of photons that are eventually altered to charge carriers assembled at the electrodes. Commonly, major elements restrict the EQE. (1) Recombination of excitons due to multiple reasons such as quenching at metal electrodes and limited phase separation in an active layer results in a reduced EQE and depletion of photogenerated excitons. (2) Incomplete absorption of the solar spectrum, either due to a thin active layer or because of a narrow absorption band, bringing about a decline in gA. To lower these reductions, solar cell architectures had been proposed, and they would be explained in further sections.

5.6. CHARACTERIZATION OF ORGANIC SOLAR CELLS 5.6.1. J-V Properties OSCs are mainly considered lower than 1000 W/m2 light of AM 1.5 solar spectrum. Figure 5.13 depicts the act of a solar cell at various bases (Shrotriya et al., 2006; Cravino et al., 2007).

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At (i) reverse bias, the functional bias assists the built-in electric field enhancing consequences in a large photocurrent, exciton dissociation, and enhancing charge transport. Drift current is directing because of the presence of a strong electric field. When (ii) the applied bias is approaching zero, mostly the built-in field exists in the device, hence the built-in field energies the carriers to the relative electrodes for assembling. The positive bias rivals the built-in field when the administered bias is accelerated in a positive direction. Thus the resultant field in the device diminishes, the measure of current decreases, and drift current turns slighter (Janssen, 2007; Berger et al., 2013). Finally, the field reaches a point where (iii) the built-in field and the applied field are equal. During this stage, diffusion current directs the current, while the electric field is negligible inside the device. (iv) The applied field is higher than the built-in field whereas the device’s potential gradient is overturned when the exterior bias is developed even more. As the barrier is currently triangular, carrier injection occurs through the tunneling system and positive current results (Peumans, 2003; Vakhshouri et al., 2013). The solar cell gives out power when the current and applied bias are opposite in direction. The maximum power output stage is exhibited on the stage when the magnitude of the product of J and V is intense. Some parameters that are normally used to assess solar cell performance have been described in Figure 5.13.

Figure 5.13: Operation of a solar cell at various biases: (i) huge reverse bias; (ii) little reverse bias; (iii) positive bias, 0 resultant internal fields, and complying to open circuit condition; (iv) positive bias, carrier injection. Source: https://www.springer.com/gp/book/9781447148227.

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Short circuit current (Jsc), is defined as the current at which the externally applied voltage is 0. Jsc indicates the number of charge carriers that are created and eventually assembled at the electrodes at a short circuit state. Improved electrical/optical parameters that increase Jsc include high carrier mobility, high absorption coefficient, small bandgap, and smaller phase separation (Fabregat-Santiago et al., 2011). Open circuit voltage (Voc). It can be defined as the voltage at which the current density output is zero. Voc was highly dependent on the work function variations of metal contacts. Voc is dependent on the HOMO-LUMO difference between the acceptor and the donor if an ohmic contact is formed at the electrodes (Kim et al., 2007). FF (fill factor) explains the shape of the J-V curve and is stated as:

(2) Jmpp and Vmpp in the equation represent current density and the voltage at the stage of maximum output power respectively. FF is the ratio of the maximum power output point and the maximum achievable power output, for instance, Jsc times Voc. FF portrays the dependence of current output on the internal field of the device and is quantified by the shunt resistance and series resistance. Namely, low carrier mobility would result in the recombination of carriers before they arrive at a heterojunction. In such circumstances, increasing the external bias would move the carriers, which would otherwise reassemble at reduced field strength, to the heterojunction for dissociation, leading to a boost in current output. As confirmed by a decreased FF, this results in a strong reliance on current the applied bias (Vandewal et al., 2008). PCE; the PCE exhibits the effectiveness of the solar cell and can be quantified as shown below:

(3) The input power density is signified by Pin.

5.6.2. Incident Photon to Electron Conversion Efficiency (IPCE) As J-V characteristics do not show optical factors thoroughly, they are insufficient for completely characterizing a solar cell. The magnitude of IPCE

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(incident photon to electron conversion efficiency) is important for further analysis of the effectiveness of the solar cell at all wavelengths. IPCE suggests the percentage of incident photons that are converted into carriers that are collected in the end at the electrodes under short circuit circumstances and is equivalent to EQE. Hence, the short circuit current is proportional to the assembling of an IPCE spectrum. The shape of the IPCE curve is extremely dependent on the absorption curve of the active layer. Particularly, IPCE is an appropriate sign when implementing procedures to elevate absorption in certain wavelength positions (for instance, plasmonic structures). It could be signified as development in IPCE at the relative wavelengths. In addition to electrical effects, optical effects such as improved charge collection are displayed as shifts of an entire IPCE curve as electrical properties are mostly wavelength independent.

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CHAPTER

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CONDUCTIVE POLYMER-BASED MEMBRANES

CONTENTS 6.1. Introduction..................................................................................... 180 6.2. Membranes Centered on Pani (Polyaniline) and Their Uses.............. 180 6.3. Polypyrrole-Centered Membranes and their Uses............................. 187 6.4. Conductive Polymers Centered Membranes Utilized For Fuel Cells. 192 6.5. Summary and Outlook..................................................................... 194 References.............................................................................................. 196

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6.1. INTRODUCTION Membranes centered on conductive polymers signify a novel class of developed materials that can normally be utilized for separation or the processes of interphase transfer of some chemical classes centered on the electrical properties. The membranes can be attained through the formerly described procedures for the creation of the classic membranes, their fastidiousness being associated with polymers’ doping methods to enhance the conductive properties (Nunes et al., 2002; Liu et al., 2021). The membranes of this kind are attained primarily from the diverse conductive polymers, but bearing in mind the number of references from literature, the most utilized polymers are PANI (polyaniline) and PPy (polypyrrole). Apart from these types of polymers, during the previous era, the utilization of functionalized PEEK (polyetheretherketone) as a conductive polymer and of the substantial number of some other polymers in the fuel cells was researched thoroughly. Centered on these details, three major classes of the conductive polymer‐centered membranes are offered. All of the polymers mentioned above have chemical-physical properties that don’t permit the creation of membranes utilizing only the particular polymer but only in amalgamation with the other polymers. Such type of membranes are normally lacking conductive properties but possess outstanding mechanical strengths. Thus, most of the conductive polymer‐centered membranes are composite membranes (Qiao et al., 2005; Spurgeon et al., 2011).

6.2. MEMBRANES CENTERED ON PANI (POLYANILINE) AND THEIR USES Polyaniline (PANI) is the macromolecular compound attained via oxidative polymerization of the aniline, as exhibited in Figure 6.1 (Schauer et al., 2011).

Figure 6.1: Synthesis path and structure of PANI (polyaniline). Source: https://www.intechopen.com/books/conducting-polymers/conductivepolymer-based-membranes.

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In the structure of PANI, there exist structural units created from benzene rings associated via aminic groups (‐NH‐) and the structural units created from the benzene ring associated with the quinone diimine. The proportion of 2 structural units inside the polymeric chain is changing depending on its oxidation degree. There are 3 specific structures of PANI, what distinguish among them is the function of 2 structural units ratio (Figure 6.2).

Figure 6.2: Structural forms of PANI. (a) leucoemeraldine; (b) pernigraniline; and (c) emeraldine base. Source: https://www.researchgate.net/figure/Oxidation-states-of-polyaniline-aleucoemeraldine-base-fully-reduced-state-b_fig1_314952001.

Therefore, in the limit circumstance of PANI comprising only structural units created from benzene rings associated via aminic groups (x = 1), a compound is named as PANI-leucoemeraldine base (Figure 6.2(a)) and in the limit circumstance in which inside the structure are just structural units created from 1 benzene ring associated with the quinone diamine (x = 0), a compound is named as PANI pernigraniline base (Figure 6.2(b)). The third specific circumstance is that in which inside the structure of a macromolecular compound, the ratio of the 2 structural units are just equal (x = 0.5), a compound being named as PANI-emeraldine base (Figure 6.2(c)). Through a process of the forms with bases or acids (doping), the reversible polymer moves from one form to the other and acquires electroconductive properties (Carothers, 1940; Pezzin et al., 2008).

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PANI in the form of the base has properties that don’t permit attaining simple membranes (the low solubility in most of the solvents commonly utilized for the preparation of membranes, thermal instability at temperatures above 160°C, low plasticity, etc.). For this particular reason, PANI‐centered membranes are acquired through blends with the other polymer appropriate for the preparation of membranes. The composite membranes centered on PANI with the conductive properties are attained. There are several inert polymers utilized for PANI‐centered composite preparation of membranes, the most utilized being cellulose and the derivatives, polystyrene, polypropylene, and polysulfone (Psf) (Staudinger, 1936; Storer, 2017). PANI‐centered composite membranes are utilized in most of the domains stated for conductive polymers, primarily for choosy separation procedures of some chemical classes from the choosy separation of gases, complex liquid solutions, development of the biosensors, electronic, and electric devices (photovoltaic cells, LED), anticorrosive films and production of the antistatic textile materials. The current scientific researches on the creation and particular applications of PANI‐centered composite membranes are signified in the subsequent paragraphs (Carothers, 1929, 1931). Fiber-kind cellulose was utilized in nanocomposites production via oxidative in the situ polymerization of aniline inside the microstructure of the fibers. The polymerization was done in the oxidative conditions utilizing ammonium peroxydisulfate in the hydrochloric acid (HCL) aqueous solutions in which the cellulose fibers saturated with aniline are adjourned. From the fibers disjointed after the procedure, which comprise PANI in the microporous structure, the polymeric films were attained through the process of phase inversion. Likewise, a composite material utilizing nanofibrils bacterial cellulose (BC) as a backup material for PANI was attained (Hu et al., 2011). Research works displayed the growth of PANI content inside the composite material simultaneously with the upsurge of electric conductivity, with extended reaction time from 30 min to 90 min. Extending the reaction time more than ninety min outcomes in the reduction of electric conductivity because of aggregation of PANI particles and formation of discontinuities inside the structure of nanocomposite. At the optimal time, the nanomaterial with the best conductivity of around cca. 5.0 S/m was attained. Utilizing this composite material, the flexible film that synergetically combines PANI conductive properties with the mechanical strength given by the BC is attained (Seeber et al., 2014; Batrinescu et al., 2016). The membranes attained are made practical in the area of electrochemical sensors, flexible displays, and flexible electrodes. Compared with the procedures in which

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first the nanocomposite material is attained and then is utilized for conductive membrane creation following the classic technique, research works were carried out to attain PANI‐centered composite conductive membranes after the next sequence: at first, the semi-permeable cellulose membrane is acquired and then on the surface of this membrane, the thin layer of PANI is applied (Formoso et al., 2017). Accumulation of a thin layer at the interface of the membrane is realized through in situ polymerizations of the aniline with an oxidative combination comprising ammonium peroxydisulfate in HCL aqueous solutions. The aniline conversion of 80% was attained after a reaction time of 24 hours. After the process, the residual aniline was discovered on the active site of the PANI membranes, and inferior reaction products attained from the ammonium peroxydisulfate were discovered on both sides of the membrane. The cellulose esters are demonstrating another type of polymeric materials utilized as an aid in the creation of PANI‐centered composite membranes. The investigations on PANI accumulation on the surface of certain microporous cellulose ester membranes were executed with the help of 2 divergent techniques: accumulation of the layer of PANI on the surface of the membrane via in situ aniline polymerization in the liquid phase or the polymerization of aniline in the vapor phase (Qaiser et al., 2012). The aniline’s in situ polymerization in the liquid phase was executed through dipping of 1 membrane from the mixture of cellulose esters in the aniline solution (PANI monomer) and the FeCl3 as an antioxidant. In the other experimental alternate, aniline polymerization in the liquid phase is prepared by immersion of the pre‐formed cellulose esters membrane in the solution that comprises aniline and hydrochloric acid followed by the addition of the oxidative agent like ammonium peroxydisulfate solution (Shehzad et al., 2015). The aniline polymerization in the vapor phase is performed by soaking cellulose ester‐centered membrane in the solution of HCl and aniline and preserving it in the closed tank with the oxidizing agent‐ saturated vapor atmosphere heated at around 65 to 70°C. The Cellulose acetate—PANI composite membranes having electric conductivities varying from 10–3–11 S/m and correspondingly 98 Sm–1 were attained. Psf is another often-utilized polymer in the creation of polymeric membranes utilized in membrane processes centered on the gradient pressure driving forces (ultrafiltration, microfiltration, reverse osmosis). The diversity of applications is because of polymers’ physical-chemical features (very good plasticity, chemical inertness, outstanding solubility

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in the usual solvents utilized inside phase inversion process, excellent mechanical strength, etc.). Psf is utilized as the substrate for the creation of Psf-PANI composite membranes intended for developed separation of compounds with the polar groups from several mixtures, exploiting the conductive properties of PANI from their structure (Janata and Josowicz, 2003; Balint et al., 2014). Inside the Psf-PANI composite membranes, PANI is existent in the entire microporous structure of the membrane, not only on the surface. The method of preparation is also a particular one and is distinguishing from those described already. Therefore, research works designed to prepare Psf-PANI composite membranes via simultaneous creation of Psf‐base membrane and the aniline polymerization in the conditions of oxidation inside membranes under creation pores (Cuciureanu et al., 2010). The process comprises of solubilization of Psf polymer inside the particular solvent (dimethylformamide or N-methyl‐pyrrolidone) and aniline (PANI monomer), skinning of the polymeric solution on the surface of the plane, and absorption of the polymeric film inside the oxidative coagulation solution. As the process of phase inversion advances, a Psf membrane is created and inside its pores, PANI occasioned from aniline polymerization in oxidative conditions. Six kinds of composite membranes were attained, utilizing three polymeric solutions with 14, 12, and 10% Psf and two kinds of coagulants (distilled water with 1.9% aniline and distilled water). In all of the polymeric solutions, Psf was dissolved in a mixture of aniline and N‐methyl pyrrolidone. Attained membranes were categorized from the viewpoint of flow and the electroconductive properties via flow determination for the solutions with flexible pH (1, 3, 5, 7, 9, and 11), and choosy separation properties were stressed through the determination of the degree of retention for standard proteins. BSA separation experiments demonstrated that membranes attained through coagulation from aniline solution and water provide higher flows and the degree of retention associated with the membranes attained by coagulation with just distilled water. For instance, membranes attained from 10% solution coagulated with aniline and water present the flow of nearly 151.2 Lm–2h at pH = 4.9 and around 196.3 Lm–2h at pH = 7.4 matched with membranes attained from the similar solution but coagulated in the distilled water that displayed a flow of almost 140.1 Lm–2h at pH = 4.9 and nearly 189.4 Lm–2h at pH = 7.4. The degrees of retention for membranes coagulated in aniline and water varied between 92.16 and 81.84% compared

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with 81.4–75.36% concluded for membranes coagulated in the distilled water (Li et al., 2009; Ma et al., 2010). Via this procedure, composite membranes with conductive properties are attained, having separation features better than those attained utilizing analogous polymeric conditions that comprise only Psf. Executed research stressed the dependency of aniline/Psf ratio from the polymeric solution and hydrodynamic and structural characteristics of Psf-PANI composite membranes (Selampinar et al., 1995; Gerard et al., 2002). To reduce errors at the laboratory level, primarily the physical skinning of a polymeric solution and discrepancy during the process of phase inversion of a coagulation solution composition, Psf-PANI‐centered composite membrane creation was studied in the steady‐state installation (Batrinescu et al., 2014). This induces the alteration of composite membrane creation technology such that in 1 tank Psf membrane forming occurs. Psf membrane has the pre‐formed pores a specific quantity of aniline and finalization of the structure is made in the reaction tank packed with an oxidative combination in which aniline’s polymerization from pores takes place. Utilizing the 10% Psf solution dissolved in the mixture of aniline and N‐methyl pyrrolidone, membranes in the continuous system in resulting working conditions were made: the thickness of a polymeric film is 0.2 mm, the temperature of an oxidative solution is 25°C, speed of the carrier in tanks is 1 m/min and the reaction time is 2 hours. The characterization through distilled water flow for 9 samples from the similar membrane led to the maximum comparative deviation of values of the flow of 2.45%, demonstrating that through this method, Psf-PANI composite membranes with reproducible conductive and hydrodynamic properties in the complete surface and from 1 batch to the other are attained (Arshak et al., 2009; Yoon, 2013). Other polymers researched as backup materials to develop the composite membranes centered on PANI, with the conductive polymers, are polypropylene and polystyrene. Therefore, from blends comprising polystyrene and PANI in several ratios, dissolved in the N‐methyl‐2pyrrolidone, stretchy polymeric films were attained through the process of phase inversion, phase changing occurring via precipitation in the vapor phase (da Silva Guimarães et al., 2012). The fastidiousness of this approach comprises in the point that PANI was attained in the separate process of oxidative polymerization: the reaction media created from alcoholic and aniline solution of nearly 0.1 M H2SO4 in the volumetric ratio of 25/1 is

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cooled to around –5°C; the solution comprising ammonium peroxydisulfate as the originator is added slowly in the 2‐h period inside the reaction mixture; attained PANI polymer is cleaned and then washed with acetone and the solution of almost 0.1 M NH4OH and assorted for 24 hours; after that time the polymer is cleaned again, washed with the distilled water and then dried at 60°C for 24 hours. After the drying process, polystyrene, and PANI are dissolved in the N‐methyl‐2‐pyrrolidone, a solution being coated on the assistance that is generally heated in the oven at 60°C for 24 hours. Lastly, the composite membrane attained is eliminated from the backup and subjected to the process of doping by immersing in the 5 M hydrochloric solution for nine minutes and afterward dried (Moussa and Rehim, 2015). Attained polystyrene‐PANI composite membranes give conductive properties dependent on the polystyrene/PANI ratio inside the solution. The polystyrene is also utilized in the functionalized sulfonated form for the creation of the PANI‐centered composite materials. The polymers are attained instantaneously in a similar reaction environment comprising of aniline (PANI monomer), 4‐styrene sulfonic acid Na salt hydrate, HCl, and ammonium peroxydisulfate (Yang et al., 1996). The working process is the following: inside the 3.47 mM aqueous solution of the 4‐styrene sulfonic acid Na salt hydrate, heated at around 80°C, the ammonium peroxydisulfate (oxidant) was added in drops in the volumetric ration of around 1/7 to the styrene solution, under mingling for 1 hour; after that, the aqueous solution of an aniline chlorhydrate 0.58 M is added and 15 min later another quantity of the 4‐styrene sulfonic acid Na salt hydrate was added in drops; the media of reaction is preserved under mixing at 80°C for 3 hours, and then afterward the temperature falls to the surrounding temperature in the 24 hours (Muskovich and Bettinger, 2012). Attained composite material comprises in its structure PANI and the sulfonated polystyrene macromolecular associated with chemical bonds via diamino protonated PANI groups and sulfonic groups of the sulfonated polystyrene. Because of these bonds, the conductive capability of composite material is quite lower as compared with the composite materials presented above, being practically the semiconductor. The composite material is utilized for polymeric membrane creation via classic processes, anticorrosive material, semiconductors, or dense films for the antistatic packages (Elhalawany et al., 2014).

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6.3. POLYPYRROLE-CENTERED MEMBRANES AND THEIR USES PPy (polypyrrole) is the polymer acquired by Py (pyrrole) oxidation in the shape of the black powder. The polymer possesses poor mechanical strength and therefore low processibility. The conductive properties of the polymer in the natural state are quite low, and it is quickly oxidized in exposure with air, varying its properties. Polypyrrole conductivity is generally given by the presence within the structure of π interconnected electron systems with p electrons accessible at the N atom from a pyrrole ring. This particular property is considerably improved by polypyrrole doping with anions like chloride, perchlorate, sulfate, dodecyl sulfate, and other organic compounds. Doped polypyrrole is the polymer categorized by good thermal and chemical stability and better conductivity matched with some other conductive polymers (Persano et al., 2015). The drawbacks associated with polypyrrole mechanical strength, elasticity, and plasticity are enhanced both via the process of doping and by inclusion inside the inorganic and polymeric composite materials structure. Polypyrrole ‐centered composite materials are often utilized as membranes inside the procedures in which the driving force is not the pressure gradient but the electric potential gradient and concentration gradient (Peron et al., 2011). Polypyrrole polymerization in Polypyrrol ‐centered composite materials is carried out through two techniques: electrochemical polymerization and polymerization via chemical oxidation (Wang et al., 2001; Ivan et al., 2012). Hydrogen peroxide, ammonium peroxydisulfate, and several compounds centered on transitional metals salts (Cu2+; Fe2+; Mn2+; Cr6+; etc.), are often utilized as oxidative agents for polymerization via chemical oxidation. The procedure is demonstrated in Figure 6.3.

Figure 6.3: Polymerization of polypyrrole via chemical oxidation. Source: https://www.researchgate.net/figure/Chemical-polymerization-mechanism-of-PPy_fig2_301788462.

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Polypyrrole polymeric chain can comprise Py associated with three kinds of dimer sequences, demonstrated in Figure 6.5. Addition inside the chemical oxidation media of reaction of surfactants like sodium alkyl sulfonate and sodium dodecylbenzene sulfonate (DBS) and alkyl naphthalene sulfonate outcomes in an upsurge of the electric conductivity of composite material and pyrrole polymerization efficiency. The electrochemical polymerization is carried out centered on the application of the electric current amongst 2 electrodes immersed in the pyrrole solution that also comprises the dopant, in line with Figure 6.4.

Figure 6.4: Polymerization of polypyrrole via chemical oxidation in the existence of the dopant. Source: https://www.researchgate.net/figure/Chemical-oxidative-polymerization-of-a-PPy-and-b-PT_fig1_335322965.

where; C– is the counterion.

Figure 6.5: Dimer sequences inside the polypyrrole structure. (a) α,α,’; (b) α,β’; and (c) β,β’. Source: https://www.intechopen.com/books/conducting-polymers/conductivepolymer-based-membranes.

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The pyrrole chemical oxidative polymerization was made practical to acquire membranes having high permeability for the separation of gas. Membranes were attained through the method of accumulation in thin layers via interfacial polymerization (Son et al., 2005). The self-supporting polymeric film, 200 to 300 nm thick, was attained by pyrrole polymerization on the inert glass support via mingling of the oxidative agent aqueous solutions comprising ferric chloride (0.5 M) and ferrous chloride (0.4 M) with the solution of pyrrole dissolved in the organic solvent (n‐hexane). Membranes were made by pouring the polydimethylsiloxane’s (PDMS) solution dissolved in the n‐hexane on the surface of PPy, after oxidant surplus elimination and the washing of polymeric film with methanol. The interaction amongst components was preserved for 24 hours, and after then the composite membrane was forged in the air at 80°C for 15 minutes. Finally, the composite membrane is removed from the glass surface by simple washing with water. Attained composite membranes signify high selectivity for split-up of nitrogen and oxygen from several mixtures, separation proportions O2/N2 of nearly 17.2 being stated. The permeability for oxygen was nearly 40.2 barrier. Utilizing the same technique of interfacial polymerization via chemical oxidation of pyrrole or its derivatives, PPy‐ centered conductive composite membranes with uses in the area of separation of gas and pervaporation were attained on the microporous membrane supports surface (Martin et al., 1993). Polymerization was carried out at the chamber temperature for 4 hours utilizing an aqueous 0.5 M pyrrole solution and the ferric chloride solutions (3, 2, 1, or 0.5 M) as an oxidative agent. The attained membranes were then washed with the deionized water and kept in the 1 M solution of hydrochloric acid, and the operation was reiterated daily for one week. At last, membranes were kept in the deionized water before characterization and utilization. The pyrrole chemical oxidative polymerization was also utilized for the creation of PPy‐centered composite membranes with pre‐formed membranes from the sulfonated poly(styrene‐co-divinylbenzene) as a backbone, with biotechnological uses and uses in the wastewater treatment via electro‐ dialysis (Scherer et al., 2001). Analogous properties associated with the ion exchange and likelihood to apply in an electro‐dialysis also signify PPy‐centered membranes attained through pyrrole chemical oxidative polymerization inside the microporous structure of an inert polymeric supports or the inorganic supports (Muscalu et al., 2009; Baicea et al., 2010). In the case of inert polymeric supports, a

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composite membrane is created in 1 single-stage via phase inversion methods and the chemical oxidation reactions. The solution of PPy and Psf dissolved in the methanol/N, N‐dimethylformamide solvent system is covered on the plane surface and immersed in the solution of ferric chloride oxidant. Simultaneously, with the creation of the microporous Psf support, chemical oxidative polymerization of pyrrole inside the preformed pores occurs. In the case of inorganic supports, the process comprises of polymerization of the adsorbed pyrrole into silica via chemical oxidation, consequential in PPy organic‐inorganic composite membranes. Polypyrrole attained via chemical oxidative polymerization is utilized as a base material for medical devices because of its conductive properties (Huang et al., 2014). Pyrrol polymerization occurs in governed conditions, an exceptional consideration being given to polypyrrole doping and modification to be biocompatible. On the structure of polypyrrole are engrafted cells or biomolecules through adsorption of the covalent bonding. Some other studies stressed the utilization of polypyrrole integrated into poly(ε‐caprolactone) and the gelatin nanofibers for the cardiac tissues (Kai et al., 2011). The experiments executed proved that upsurge of polypyrrole concentration up to 30% inside the composite material occasioned a decrease of the diameter of the typical fibers from 239 ± 37–191 ± 45 nm, simultaneously, with an upsurge of around six times of the tensile modulus. Other uses of PPy‐centered composite membranes made through pyrrole chemical oxidation are in the anticorrosive and antistatic materials field. The polyethylene utilized as modified or natural polymer establishes an outstanding polymeric material for the creation of the polypyrrole composite membranes because of its properties required for the preparation of membranes (Elyashevich et al., 2002). Therefore, inside the polymeric films pores attained through melt extrusion with consequent annealing, thermal fixation, and uniaxial extension, polypyrrole was accumulated through pyrrole chemical oxidative polymerization. Utilizing another method, the polyethylene polymeric film is altered through engraftment inside the structure of one more polymer comprehended via acrylic acid irradiation with the γ rays. Attained material is very hydrophilic, which backs to the better pyrrole retention via surface adsorption which is further polymerized via chemical oxidation utilizing ammonium peroxydisulfate or iron chloride (III). Carried out researches were emphasized an upsurge of the electric conductivity of composite material via an increase of pyrrole concentration inside the media of reaction from 0.3–0.9 M but attained outcomes validated that this was unimportant. Introducing a novel pyrrole polymerization phase

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occasioned in an upsurge of the electric conductivity from 166–543 S/m (Ilicheva et al., 2012). Polypyrrole obtained through electrochemical technique has the same use as those of polypyrrole attained via chemical oxidation. Therefore, a Polypyrrole membrane made on the stainless-steel net is utilized for the separation of cyclohexane and ethanol mixtures via pervaporation (Zhou et al., 1995). The electrochemical polymerization occurs in the 4‐cell installation having porous glass walls and three electrodes. One electrode is created from the stainless-steel net attained through a weaving of the stainless-steel fiber with 18 or 25 µm diameter. Inside the cell that comprises this electrode, the solution of 0.1 M pyrrole, acetonitrile as a solvent, and the doping agent are administered. The pervaporation membranes made were tested in the particular installation and are demonstrated to be choosy for ethanol, the infusion being dependent on pyrrole polymerization degree and the type of dopant (Porter, 1989; Mulder and Mulder, 1996). Utilizing a similar technique, electrochemically polymerized polypyrrole accumulated onto the platinum sputter-covered polyvinylidene filters was acquired (Misoska et al., 2001). The process was comprehended through introduction inside the cell comprising filters of the pyrrole aqueous solution comprising as doping agents HQS (8-hydroxyquinoline‐5‐sulfonic acid) or BCS (2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline sulfonic acid). Polypyrrole/BCS‐kind conductive membranes are porous to the series of ions like Co2+, Zn2+, Ni2+, K+, Ca2+, Mg2+, Mn2+, Cu2+, and Fe3+. Polypyrrole/ HQS conductive membranes aren’t permeable to all ions mentioned above, significant outcomes being attained only for K+, Cu2+, and Co2+. Further, ion fluxes are higher for Polypyrrol/BCS compared with Polypyrrole/ HQS membranes. The Conductive membranes having permeability for K+, Na+, Mg2+, and Ca2+ ions were attained in analogous conditions with those designated above with the dissimilarity that Polypyrrol was accumulated onto the platinum sputter‐covered polyvinylidene fluoride (PVDF) filters and DBS/polystyrene sulfonate (1%) or dodecyl benzenesulfonate/polyvinyl phosphate systems were utilized for doping (Davey et al., 2001). One more domain that was studied carefully in the last era and applications polypyrrole polymers attained in the shape of membrane films through electrochemical polymerization is that of electroanalysis. Through an accumulation of the polypyrrole membrane on the Al2O3 surface of just one electrode, the amperometric sensors with various uses in the area of

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analytical chemistry are attained (Michalska et al., 2003). Polypyrrole membrane is made via electrochemical polymerization of the pyrrole on the surface of electrode utilizing the aqueous solution 0.2 M pyrrole and 0.1 M KCl or 0.5 M K4[Fe(CN)6] and 0.1 M Py. Polypyrrole-centered membranes use inside the fuel cells domain must be also mentioned. The applications of conductive polymers in the area of fuel cells are given in the next section.

6.4. CONDUCTIVE POLYMERS CENTERED MEMBRANES UTILIZED FOR FUEL CELLS Fuel cells are the devices that produce electricity centered on the free energy of the chemical reaction. The typical fuel cell comprises of the porous anode fed having gas fuel that after the oxidation led to the release of electrons; the porous cathode served with an oxidant, which produces protons and the electrolyte situated amongst the two electrodes; and two bipolar plates and the electrical connectors that are connecting electrodes through the exterior circuit. Chemical reactions for the typical combustion cell are given in Eqns. (1)–(3). 2H2 + 4HO– = 4H O2 + 4e– → Anode reaction

O2 + 2H O2 + 4e = 4HO → Cathode reaction –

–

2H2 + =O2 → 4H O2 + H+E → Overall reaction

(1) (2) (3)

where; H is the heat; and E is the electrical energy. These devices signify large spectra of uses because of the point that global efficiencies attained for electricity are much higher as compared to the classical systems. Simultaneously, the effects tempted on the environment are comparatively less harmful as compared to those produced by the other electricity‐yielding systems like fossil fuels burning (Kesting, 1990). Fuel cells can generally be classified centered on two major standards: electrolyte nature and the operation temperature. Centered on the later criterion, the types of fuel cell are: low temperature—AFCs (alkaline fuel cells —T < 100°C), PEMFCs (polymer electrolyte fuel cells—T = 60 to 120°C), DMFCs (direct methanol fuel cells—T = 60–120°C), PAFCs (phosphonic acid fuel cells—T = 160 to 220°C) and the high temperature— MCFCs (molten carbonate fuel cells—T = 600 to 800°C), SOFCs (solid oxide fuel cells—T = 800 to 1000°C).

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The conductive polymer membranes are studied carefully because of the advantages provided by the point that they function as solid electrolytes as well as the selective separation barriers for species implied in the generation of electricity inside the fuel cells (Bose et al., 2011). The first applications were centered on making and inclusion inside the structure of fuel cells of the protons exchange membranes—Nafion, attained from PTFE and persulfonic acid by Dupont Company 30 years ago. At present researches is emphasized on the creation of conductive polymers membranes with enhanced mechanical and electric properties. The recent researches in the creation of the conductive membranes having protons exchange properties, appropriate in PEMFCs, and the conductive membranes appropriate for AFCs are appraised in the subsequent paragraphs (Ray et al., 1985). The study devoted to this domain is focused on the various composite membranes centered on conductive polymers utilized for the fabrication of the high-temperature exchange of proton membrane fuel cells. The organic composite membranes centered on polymers with electric properties like sulfonated poly(ether ether ketone), sulfonated poly(p‐phenylene), sulfonated poly(arylene ether sulfone), sulfonated Psf, sulfonated poly(sulfide ketone), sulfonated poly(aryl ether nitrile), and inorganic‐ organic composite membranes like polyalkoxysilane/phosphotungstic acid, fluorinated polymer/SiO2, Nafion/TiO2, Nafion/SiO2, and Nafion/PTFE/ zirconium phosphate are reviewed (Altena and Smolders, 1982; Wijmans et al., 1983). Amongst the most considered polymers for the organic composite membranes with uses in the fuel cells is PEEK (poly(ether-ether ketone)), which is utilized in a base or altered form. Therefore, from SPEEK (sulfonated poly(ether ether ketone)), asymmetric microporous membranes can normally be attained through the phase inversion technique and the immersion‐precipitation method. The polymer of SPEEK was attained by dissolving (poly(ether-ether ketone)) in concentrated sulfuric acid added in the proportion of 5 wt.%, at the room temperature, the media of reaction being maintained by mingling for 24 hours. The attained membrane is varied via in situ polymerizations of pyrrole doped with cerium sulfate and iron chloride (Baicea et al., 2011). The composite membranes are acquired, utilizing SPEEK as the base material, via inclusion within the structure of heteropolycompounds centered on tungsten, wolfram, or molybdenum (Zaidi et al., 200).

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Some other studies on the applications of SPEEk for fuel cells displayed that it can be made functional with quaternary imidazolium hydroxide and amine hydroxide or by the PANI inclusion inside membrane structure (Yan et al., 2013). Psf and the poly(1,4‐phenylene ether-ether sulfone) are some other polymeric materials that normally can be utilized to attain composite membranes having conductive properties for making fuel cells. Utilizing Psf, the asymmetric membranes can be attained through the classical phase inversion process and afterward functionalized via integrating acrylamide‐centered ionomers having the groups of proton‐conducting sulfonic. The incorporation procedure of the novel polymer is centered on photopolymerization. Fuel cells exchange of proton composite membranes, that can be utilized at a temperature above 100°C, were attained from N‐(3‐aminopropyl) imidazole, poly(2,6‐dimethyl‐1,4‐phenylene oxide), and the metal-organic frameworks. The composite membranes with uses at very high temperatures, for the fuel cells, are also attained from bi‐functionalized copolymer made via radical copolymerization, with SiO2 within the structure.

One more interesting point within the literature is signified by the creation of membranes for the alkaline fuel cells. The studies carried out in this domain categorize conductive membranes for the creation of AFCs in homogeneous and heterogeneous membranes. Amongst the most current researches associated with membranes for the alkaline fuel cells is concentrating on the creation of the high ionic conductivity membranes from the crosslinked poly(arylene ether sulfones). In addition to the traditional techniques for preparation of the polymeric membranes having conductive polymers, a novel technique was currently developed—plasma methods both for plasma modification and for plasma polymerization of the membrane surfaces (Nagarale et al., 2006).

6.5. SUMMARY AND OUTLOOK The conductive polymers themselves aren’t making membranes that can generally be utilized in several processes because of low mechanical strength, absence of plasticity, and elasticity. Due to this reason, the conductive polymer‐centered membranes are primarily composite (Hu et al., 2011).

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The methods of preparation of the composite membranes centered on the conductive polymers are quite similar to those utilized for simple membranes for the preparation of a support polymer that confers elasticity and mechanical strength. There are 3 ways for the creation of composite membranes given within the literature (Nechifor and Popescu, 1990; Yoon, 2013): •

A conductive polymer is made simultaneously with backbone membrane; in this situation, the composite membrane comprises conductive polymer in its structure. • A conductive polymer is made after support membrane creation and its sopping in the monomer solution trailed by polymerization via chemical oxidation; now in this situation, the composite membrane comprises conductive polymer in the microporous structure. • A conductive polymer is made only after the creation of support membrane via accumulation on the surface of a conductive polymeric film; here in this situation, the composite membrane comprises in its structure two diverse layers—sandwich type. The polymer’s conductive properties that comprise interconnected electron systems are low as compared to metals, being at the semiconductors level. To attain polymers having better conductive properties, doping technique is utilized, through institution within the polymeric chain of groups of atoms or atoms that creates defects within the macromolecule structure as an outcome having more quick jumping of electrons amongst the polar centers (Mattoso et al., 2009; Muralidhara, 2010). The conductive polymer‐centered composite membranes are utilized in the membrane processes that normally utilize electric potential gradient and concentration gradient as driving forces. There also exist cases in which the membranes are utilized in processes that utilize pressure gradient. Various researches are engrossed on conductive polymer‐centered composite membranes’ utilization for fuel cells (Garganciuc et al., 2008a, b).

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CHAPTER

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APPLICATIONS OF CONDUCTING POLYMERS IN TISSUE ENGINEERING

CONTENTS 7.1. Introduction..................................................................................... 204 7.2. Pure Conducting Polymer (CP) Films for Tissue Engineering............. 205 7.3. Conducting Composite Films or Blends for Tissue Engineering......... 206 7.4. The Conduction of Copolymer Films for Tissue Engineering............. 208 7.5. Bone Tissue Engineering.................................................................. 210 7.6. Cardiac Tissue Engineering.............................................................. 213 7.7. Skin Tissue Engineering.................................................................... 215 7.8. Nerve Tissue Engineering................................................................. 216 References.............................................................................................. 219

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7.1. INTRODUCTION Ensuing in about 8,000,000 surgical processes and 40–90 million hospital days/year for treatment, organ breakdown and tissue depletion account for approximately half of the medical rate in the United States (Karp and Langer, 2007; Smith et al., 2008). The development of functional biological alternatives that reinstate, preserve, or refine tissue activity by merging a scaffold, cells, and biological molecules, is the goal of tissue engineering or regenerative medicine, which is an interdisciplinary and multidisciplinary domain (Vacanti and Vacanti, 2014; Makris et al., 2015). Numerous functions containing tunable biodegradation occurrence and non-hazardous degradation items, biocompatibility with host tissues, and appropriate porosity for the transference of nutrients and wastes, mechanical power and sterilization, is united by scaffolds (Schumann et al., 2007; Guo and Ma, 2014). Biocompatibility and biodegradability are one of the most extensively utilized scaffolding biomaterials as polymers own immense processing flexibility. The leading biomaterials as scaffolds for tissue engineering are organic polymers such as gelatin, alginate, chitosan, collagen, and so on and artificial polymers containing poly(lactic-co-glycolic acid) (PLGA), polylactide (PLA), poly(glycerol sebacate), polycaprolactone (PCL), and polyurethane (PU). At the time of tissue restoration, biomaterials play an essential part. They regulate the cellular actions, such as cell differentiation and proliferation, in addition to neo-tissue genesis, also should enhance the exchanges amongst the biomaterials and the seeding cells and assist as matrices for cellular adhesion. The development of bioactive biomaterials that can improve cell proliferation and lead distinction of cells is nonetheless, still a challenge (Zustiak and Leach, 2010; Van Vlierberghe et al., 2011). Owing to the high electrical conductivity and sturdiness in recent times, extensive inquiry in biosensor and bone tissue engineering programs has been done of conducting biomaterials centered on carbon nanowires, CNTs, graphene, and metallic particle (e.g., gold nanoparticle). Their potent and prevalent use was constrained by downsides, including problems of inexact long-standing in vivo toxicity, the heterogeneous division of the conducting particles in composite structure, and non-biodegradability. Features comprising of ease of synthesis and flexibility in processing is displayed by conducting polymers (CPs), which as a new range of biological materials also showed electrical and optical features similar to metals and inorganic semiconductors (Shoichet, 2010; Inkinen et al., 2011). As compared to traditional electronic inorganic and metal components, the soft essence of organic conductive polymers offers improved structural and mechanical

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tunability with organs and cells. Due to the simple synthesis and simple alteration, their capability to electronically regulate a variety of chemical and physical features by (i) the usage of an extensive range of molecules that can be secured or used as dopants; (ii) surface functionalization methods and biocompatibility CPs such as polyaniline (PANI), polythiophene, and polypyrrole (PPY) and their derivatives and compounds are appealing biomaterials (Guo et al., 2008, 2010; Wu et al., 2016).

7.2. PURE CONDUCTING POLYMER (CP) FILMS FOR TISSUE ENGINEERING The proliferation, in vitro adhesion, and distinction of a great assortment of cell types is aided by CPs (e.g., PANI, PPY, and polythiophene) signifying that they are cytocompatible (Sukmana, 2012). Moreover, animal replicas have established CP’s good biocompatibility. CP caused only a slight tissue response or exhibited no noteworthy long-term effect in vivo as proposed by numerous results (Guo et al., 2011). CPs can be electrochemically or chemically synthesized. Utilized for cell structure, electrochemically manufactured CPs are typically in the structure of films on the electrode. For instance, the electric deposition indium-tin-oxide glass slide of PPY membrane at the nanoscale. The uniform division of nanoscaled PPY particles with a regular diameter of 62 nm is done. Joint mechanical or electrical stimulus hugely endorse the proliferation and variation of MC3T3-E1 cells as compared to single stimulation as denoted by the results in terms of cell proliferation and gene expression of collagen-I after pre-osteoblasts MC3T3-E1 cells are cultured on PPY membrane under both mechanical and electrical stimulation. This specifies that the nano-PPY membrane may provide a way to stimulate bone tissue repair. A layer-by-layer (LBL) selfdoped sulfonated PANI copolymers on a polyethylene terephthalate (PET) film is made by the utilization of in situ polymerization. The cell growth and cell attachment amplified with increasing degree of sulfonation as directed by the cultures of bone marrow stromal cells (BMSCs) and pre-osteoblast cells (MC3T3-E1). The von Kossa staining of both the cells exposes that as compared to the mineralization of the cells without electrical stimulation, that the respective controls under electrical stimulation have intensely risen (Figure 7.1) (Sukmana, 2012).

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Figure 7.1: Conductive biomaterials and conducting polymers and their tissue engineering use.

7.3. CONDUCTING COMPOSITE FILMS OR BLENDS FOR TISSUE ENGINEERING It is very hard to fabricate pure CP film from CPs, and they are very fragile. Hence, for tissue engineering, combining CPS with other degradable polymers is extensively used as conductive biomaterials. CPs such as PANI and PPY are merged with natural and synthetic polymers which include chitosan, PLA, silk fibroin, PCL, and PLGA. For instance, initially immersing PCL film into pyrrole DI water and polystyrene sulfonic acid mixture develops a conductive PPY/PCL film (Zhang et al., 2001). The interpenetrating web of PPY coated PCL conductive film is the outcome of ferric chloride being put into the mixture as an oxidant. Akin to the inherent cardiac tissue, the film exhibits a resistivity of 1.0 ± 0.4 kΩ cm. Dip coating procedures and emulsion polymerization is employed to fabricate nerve conduits and conductive PPY/poly(D, L-lactic acid) (PDLLA) composite films. Figure 7.2 (I, II, and III) displays scanning electron pictures and photographic micrographs of the PPY/PDLLA films and conduits (Hopley et al., 2014).

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Figure 7.2: Picture of the PPY/PDLLA nerve conduit (II, III) and PPY/PDLLA film (I). Source: https://pubmed.ncbi.nlm.nih.gov/24138830/.

Note: Fluorescent pictures of PC12 cells categorized for actin (red) and nuclei (blue). (a), (c), (e), and (g) are PDLLA, 5% PPY/PDLLA, 10% PPY/ PDLLA, and 15% PPY/PDLLA in the absence of electrical stimulations. (b), (d), (f), and (h) are PDLLA, 5% PPY/PDLLA, 10% PPY/PDLLA, and 15% PPY/PDLLA in the presence of electrical stimulations of 100 mV for 2 h. Scale bar: 200 mm. (j) Percentage of neurite-bearing PC12 cells on conductive composite films with various PPY components (n= 4, *p < 0.05). (k) Median neurite length on PPY/PDLLA composite films with fluctuating PPY composition (Ates et al., 2018).

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Exceptional biocompatibility, physical features, biological features, and tunable mechanical features were displayed by polyurethane (PU) because of the segmented-block structure. Derived from PPY and polyurethane, an electrically conducting composite is established. The attained composite is squeezed into film and dried in a vacuum chamber after being processed by in situ polymerizations of pyrrole in the PU emulsion mixture. The supreme elongation length reduces while the stiffness rises by elevating the mass ratio of PPY to PU (Goenka et al., 2014). In the modulation of cell conduct, morphology, and surface property play a significant role. The making of electrically conductive silk film-based nerve tissue scaffolds with micrometer-scale grooves takes place. Drying is done to impart β-sheets development of silk after the silk solution is fit on polydimethylsiloxane (PDMS) substrates with micrometer-scale grooves on the surfaces. Afterward, to produce an interpenetrating network of PPY and polystyrene sulfonate in the silk matrix, the attained surfaces are absorbed in an aqueous solution of pyrrole, iron chloride, and polystyrene sulfonate. About 124 ± 23 kΩ square–1 is the resistance of the fashioned conductive silk sheet. The natural topography in the extracellular matrix is akin to the micrometer-scale grooves on the film surface (Fan et al., 2014).

7.4. THE CONDUCTION OF COPOLYMER FILMS FOR TISSUE ENGINEERING CPs’ non-degradability restricts the in vivo applications, although they are broadly employed as biomaterials in tissue engineering. The added benefit of being erodible (biodegradable) and the subsequent materials having comparable electroactivity as CPs is because of the use of aniline/ pyrrole-based copolymers operationalized with hydrolyzable groups, even though CPs are not characteristically biodegradable (Abarrategi et al., 2008). Conductive copolymers are synthesized as degradable conductive biomaterials by the employment of aniline/thiophene oligomers like aniline pentamer, aniline tetramer, and aniline trimer with precise structure, renal clearance feature, and good electroactivity. For instance, the sol-gel reaction of ricinoleic methyl ester, methoxy silane end-functionalized urethane prepolymers made of castor oil and methoxy silane functional aniline tetramer moieties develops electroactive polyurethane/siloxane/aniline tetramer composite film. Antioxidant characteristics and electroactivity is

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provided to the material by aniline tetramer, whereas the enrichment of the film’s mechanical attributes is done through the introduction of inorganic siloxane domain into the polyurethane matrices. For wound dressing use, water vapor transmission rate, suitable tensile strength at both dry and hydrated conditions, and appropriate water absorption are presented by the attained films (Mori et al., 2020). Derived from other degradable polymers and aniline oligomers, a sequence of conductive and degradable films were developed by the group for tissue engineering applications. Based on aniline trimer and polylactide, poly(ethylene glycol), the synthesis of a sequence of stretchable electroactive polyurethane-urea (PUU) elastomers is done. A modulus close to soft tissues and a strain at break higher than 1600% was exhibited by the films. The endowment of novel superelastic characteristics to the films was done by the vital physical interactions provided by the sphere-like hard domains self-assembled from aniline trimer sections. The high electric conductivity of 0.1 S/cm could be attained by the films after combining conductive fillers like nanosized carbon black (CB) and PANI nanofibers with the copolymers (Nair et al., 2017). A new prospect to less intrusive surgery in inserting these biomaterials into the human body has been provided by conductive biomaterials created from shape memory polymer (ESMP). As exhibited in Figure 7.3(a), by the use of hexamethylene diisocyanate as a crosslinker to crosslink electroactive amino capped aniline trimer segment and PCL segment, a sequence of flexible degradable electroactive shape memory polymer (ESMP) films that have tunable switching temperature was synthesized. Tunable recovery temperature around body temperature, good shape memory characteristics, and high elasticity was displayed by the biodegradable ESMP film (Figure 7.3(b)). Moreover, the synthesis of dual bioactive ESMP has also been done. For skeletal muscle restoration, a sequence of dopamine-incorporated polyurethane-based ESMPs have been prepared by researchers. By melting PCL, dopamine, and polymerization of citric acid, the poly(citric acid-copolycaprolactone-co-dopamine) (CA-PCL-DA) pre-polymer is synthesized. By the use of HDI as a crosslinker, the HDI-aniline hexamer is added into the CA-PCL-DA pre-polymer. Body temperature triggered shape memory feature, good electroactivity, and elongation was exhibited by the attained elastomers (Shevach et al., 2013).

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Figure 7.3: (a) The synthesis scheme of PCL3000-AT (the molecular weight of PCL is 3000, aniline trimer: AT) electroactive copolymers. (b) Shape memory characteristics of the degradable conductive copolymers. (1) The spiral shape of PCL3000-AT5 film after fixation; (2) the redeemed shape of the spiral film; (3) the circle shape of PCL3000-AT5 film after fixation; (4) the redeemed shape of the circle film. (c) Myogenic differentiation of C2C12 myoblasts. Cultured on PCL80000 and PCL3000-AT5 (the AT matter in the copolymer is 5 wt.%), tubulin (green), and nuclei (blue) immunofluorescence staining of C2C12 cells at day 7. Scale bar: 200 μm. Source: https://pubmed.ncbi.nlm.nih.gov/27640917/.

7.5. BONE TISSUE ENGINEERING The proliferation and bond of C2C12 cells, MC3T3-E1 cells, mesenchymal stem cells, and osteoblast-like SaOS-2 cells can be heightened utilizing conducting biomaterials. As exhibited in Figure 7.4(a)–(c), matrix deposition and MC3T3-E1 cell infiltration inside the void space was allowed by icetemplated porous and conductive PEDOT: PSS scaffold with median pore diameter above 50 μm. Moreover, as exhibited in Figure 7.4(c), the scaffold could also encourage osteocalcin (OCN) deposition of MC3T3-E1 cells, boost the gene expression levels of Runx2, COL1, and ALP, and enhance extracellular matrix mineralization (Bredas and Street, 1985). The differentiation into OCN positively stained osteoblasts (Figure 7.4(d) and (e)) of osteogenic precursor cells (MC3T3-E1) can be stimulated by

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the conductive PEDOT: PSS scaffold as proved by the results. According to the current study, the primary mechanisms of PEDOT: PSS scaffold’s gene expression of osteogenic markers and growing matrix mineralization is unclear. The pore size fluctuated from 110 μm to 160 μm by altering the PEDOT: PSS content, and around 60% porosity was displayed by the 3D conductive composite scaffolds based on bioactive glass nanoparticles (NPs), PEDOT: PSS and gelatin. Human mesenchymal stem cell adhesion was sustained by the scaffolds, and probably due to the increased electrical signaling between cells and enhanced microstructure of the scaffolds, there was an upsurge in the cell viability with the rise of the concentration of the conductive polymer in the scaffold (MacDiarmid et al., 1987).

Figure 7.4: PEDOT: PSS scaffolds scanning electron microscopy (SEM) pictures. Source: https://pubmed.ncbi.nlm.nih.gov/19830261/.

Note: The extremely interconnected, permeable structure is attained through ice template and following sublimation. (c) SEM pictures of MC3T3-E1 cells refined on PEDOT: PSS scaffolds. Models are harvested on day 1, 7, and 28 post-seeding. Matrix deposition and cell number are amplified over time, with a solidly formed tissue-construct, after 28 days in differentiation media. (d) False colored SEM picture of secondary and backscattered electrons (i, ii). Organic material is portrayed in green, calcified particles in red. (e) Confocal microscopy pictures of OsteoImage stained MC3T3-E1 cells on day 28 (i, ii). Nuclei are counterstained with DAPI (blue), while mineralized bone nodules looked green.

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Derived from aniline trimer and six-armed branched polylactide, electroactive shape memory polymeric (ESMP) films were synthesized. In comparison to aniline trimer-free SMP films, it was noticed that they could notably boost the proliferation of C2C12 cells. Likewise, derived from relative gene expression, ALP enzyme activity, and immunofluorescence staining, the osteogenic differentiation of C2C12 myoblast cells was enhanced by electroactive SMP films which displayed their immense potential for bone regeneration (Garg et al., 2015). Good cytocompatibility for bone marrow-derived mesenchymal stem cells (BMSCs) was laid out by 3D conductive PLA/PANI composite nanofibrous scaffolds. The expression levels of runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), mineralization of BMSCs (Figure 7.5), along with OCN were amplified by the osteogenic differentiation of BMSCs which were improved by the conductive nanofibrous scaffolds.

Figure 7.5: (a) conductive nanofibrous scaffolds’ schematic fabrication; (b) SEM pictures of PANI nanoparticles, nanofibrous conductive scaffolds, the magnified pictures of scaffolds displaying the nanofiber, and PLA/PANI10: 10%wt PANI in the composite scaffolds; (c) Alizarin red staining of BMSCs on diverse substrates for two weeks.

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Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6211800/pdf/nihms-993388.pdf.

Note: Scale bar = 200 μm. In comparison to PLA at day 14, a much greater area of positive alizarin red aggregates is existing in the BMSCs of conductive composite scaffolds (Chen et al., 2018). This specifies that for bone tissue engineering application, the osteogenic differentiation of BMSCs could be endorsed by reasonable content of PANI in the conductive scaffolds. An innovative biomaterial approach for the induction of osteogenic differentiation from BMSCs was supplied by these results (Ouyang et al., 2018).

7.6. CARDIAC TISSUE ENGINEERING The dissemination of electrical signals in a harmonized fashion through the cardiac cells causes the recognized behavior of excitation-contraction coupling of the heart. When likened to PCL film, the calcium transient extent of the cardiomyocyte monolayers could be reduced, and the velocity of calcium wave propagation could be amended by the PPY/PCL film. In comparison to pure PCL, peripheral localization of the gap junction protein connexin-43 (CX43) is presented more by cardiomyocytes on PPY/PCL film, and they also supported cardiomyocyte adhesion. Yet, no variance between the two materials was shown by the gene expression level of CX43 (Checkol et al., 2018). By electrospinning with a similar diameter of the nanofibers, PLA/PANI conductive nanofibrous sheets were readied (Figure 7.6(a)). In terms of fusion index and maturation index, they could endorse the differentiation of H9c2 cardio-myoblasts and exhibited high cell viability. Signifying PLA/PANI conductive nanofibrous sheets’ ability in cardiac tissue engineering application, PLA/PANI nanofibrous sheets maturation with extra secreted proteins which includes CX43 and αactinin, the unprompted beating of primary cardiomyocytes and also enhanced the cell-cell interaction. The enhancement of cell adhesion and attracting negatively charged adhesive proteins could be done by the doped conductive PLGA/PANI nanofiber meshes (Liu et al., 2010). The development of cell clusters can be done by cardiomyocytes on the nanofiber meshes which could link with each other. Along the main axis of the fibrous mesh, the cluster displayed aligned and elongated morphology. Also exhibited in the isolated clusters is the expression of the gap-junction protein CX43 and synchronous beating was displayed by the cells in the cluster. A native heart

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copying electrical stimulation could be used to synchronize the beating rate (Ravichandran et al., 2010).

Figure 7.6: SEM pictures, characteristic fluorescence pictures, and scheme representing a new conception to make an injectable conductive hydrogel. Source: https://pubmed.ncbi.nlm.nih.gov/28663141/.

All over the world, myocardial infarction is still a therapeutic challenge. MI typically leads to the development of a fibrous scar and mass death of cardiomyocytes. With the purpose of a cell delivery system for managing MI with good biocompatibility, a self-healing conductive chitosan-graft-aniline

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tetramer and dibenzaldehyde terminated poly(ethylene glycol) hydrogel was designed. In comparison to before injection, no clear difference was shown by the viability of C2C12 cells encapsulated in the hydrogels after injection of the hydrogels (Balint et al., 2014). Good in vivo cell retention and a tunable release profile was exhibited by the cell encapsulated hydrogel. The demonstration of in vivo biocompatibility and degradation ability of the hydrogel suggests its ability as a cell delivery vehicle for MI. Derived from thiolated hyaluronic acid and multi-armed conductive crosslinker tetraaniline-polyethylene glycol (PEG) diacrylate, an injectable conductive hydrogel was made by the group of Liu (Figure 7.6(c)). A conductivity equal to native myocardium was exhibited by the attained conductive hydrogels. They are filled with stem cells and plasmid DNA encoding eNOs nanocomplexes for treating MI. Accompanied with myocardium-related mRNA and upregulation of proangiogenic growth features, an amplified expression of eNOs in myocardial tissue has been detected after the injection of the hydrogel-based holistic system into the infarcted myocardium of SD rats (Guimard et al., 2007; Hardy et al., 2013).

7.7. SKIN TISSUE ENGINEERING The human body is defended by the skin from microbial attack and damage. Thermal burn wounds constitute about 265,000 deaths every year. The establishment of numerous biomaterials with good antibacterial activity has been done. With the addition of keratinocytes and fibroblasts, conductive materials have been confirmed to encourage cellular activities. Anti-bacterial characteristics were exhibited by conductive polymers such as PANI (Harris and Wallace, 2018). For wound healing material, they are considered promising biomaterials because of these advantages. For instance, by the use of a double full-thickness skin wounds model on the dorsum of SD rats, the wound healing demonstration of conductive nanofiber composites grounded on poly(vinyl alcohol) (PVA), chitosan oligosaccharide, and poly(aniline-co-aminobenzenesulfonic acid) are conducted. Through the test, it is established that the control group exhibited a more provocative response than the conductive nanofiber composite dressing. Signifying the auspicious application of conductive nanofiber composite for wound healing, in comparison to the control group after 15 days of treatment, the conductive dressing showed heightened granulation and collagen and almost perfect healing (Thompson et al., 2010). Exhibiting potential for wound healing application, the enhancement in both fibroblast and osteoblast growth is

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credited to the PANI/chitosan (1:3) nanofiber mat’s good conductivity and hydrophilicity which presented a synergistic effect in the improvement.

7.8. NERVE TISSUE ENGINEERING Electrically excitable cells in the nervous system which convey signals at a rapid pace are called neurons. For the improvement of the regeneration and growth of nerve tissue, the development of CPs which includes PPY and PANI as conductive scaffolds have been done by researchers. For the existence of neurite outgrowth from PC12 cells on the conductive nanofibers, the conductive PCL/PPY conductive nanofibers with proper cytocompatibility sustained PC12 cell differentiation (Mawad et al., 2012; Kaur et al., 2015). Moreover, as compared to cells on PCL sans PPY coating, the PC12 cells extend notably greater areas. The potential application of the conductive nanofibers scaffolds was proved by these results for nerve tissue engineering (Schmidt et al., 1997). As displayed in Figure 7.7(a), notably enhanced neuron-like rat PC12 cell proliferation and adhesion and good biocompatibility was shown by conductive PEDOT/chitosan/gelatin scaffolds. Further proof that it may be an auspicious conductive scaffold for neural tissue engineering is that the conductive scaffold could upregulate GAP43 and SYP protein and gene expression level, and sustain cells in more neurite growth and vigorous proliferation (Zhao et al., 2017). Augmentation of the length of neurites and development of neuronal cells was displayed by electroactive tobacco mosaic virus (TMV)/PANI/PSS nanofibers. Cells refined on the TMV-derived non-conductive nanofibers are lesser in number than the number of cells with neurites. Causing a bipolar cellular morphology, guiding the outgrowth course of neurites, and enhancing the percentage of cells with neurites could be done by the aligned electroactive TMV/PANI/PSS nanofibers. According to results, neurites outgrowth and the promotion of neural cell differentiation could be done by the electroactivity and topographical cues from TMV/PANI/PSS nanofibers (Qazi et al., 2014).

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Figure 7.7: (a) Pictures of the polymerized PEDOT/chitosan/gelatin (PEDOT/ Cs/gel) scaffolds with dissimilar molar ratios of ammonium persulfate (APS) to EDOT. Gene expression and Neurite growth protein (a1d2) of PC12 cells (d, e) in the scaffolds after 5 days of culture. 3D and 2D confocal fluorescence micrographs of immunostained cells on the PEDOT/Cs/Gel scaffold (b, d) and chitosan/gelatin (Cs/Gel) scaffold (a, c). Source: https://pubs.rsc.org/en/content/articlelanding/2017/tb/c7tb00608j. Note: SYP and GAP43 are displayed in green, and the nuclei are stained in blue (DAPI). (c1 and d1) scale bar = 200 mm, (a, b, c2, and d2) scale bar = 500 mm. qPCR examination of the gene expression stages of GAP43; (d) and SYP (e) on the Cs/Gel and PEDOT/Cs/Gel scaffold. *P