Electrospun Materials and Their Allied Applications

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Phillip Carmical (pcarmical@scrivenerpublishing.com)

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Publishers at ScrivenerMartin Scrivener (martin@scrivenerpublishing.com)

Phillip Carmical (pcarmical@scrivenerpublishing.com) Electrospun Materials and Their Allied Applications

Edited byInamuddin, Rajender Boddula,

Mohd Imran Ahamed and Abdullah M. Asiri

This edition first published 2020 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2020 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.

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Library of Congress Cataloging-in-Publication Data

ISBN 978-1-119-65486-5

Cover image: Pixabay.comCover design by Russell Richardson

Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines

Printed in the USA

10 9 8 7 6 5 4 3 2 1

v

Contents

Preface xv1 Electrospinning Fabrication Strategies: From Conventional

to Advanced Approaches 1J.R. Dias, Alexandra I. F. Alves, Carolina A. Marzia-Ferreira and Nuno M. Alves1.1 Introduction 21.2 Conventional Fabrication Approaches 3

1.2.1 Randomly Oriented Fiber Meshes 31.2.2 Aligned Fiber Meshes 81.2.3 Fibers With Core/Shell Structure 14

1.3 Advanced Fabrication Approaches 191.3.1 Melt Electrospinning 191.3.2 Near Field Electrospinning 221.3.3 Electroblowing 231.3.4 Hybrid Structures 251.3.5 Cell Electrospinning 301.3.6 In Situ Electrospinning 33

1.4 Conclusions and Future Perspectives 36 Acknowledgments 37 References 37

2 History, Basics, and Parameters of Electrospinning Technique 53Aysel Kantürk Figen2.1 Definitions 532.2 Milestone of Electrospinning Technique 542.3 Setup and Configuration of Electrospinning Technique 562.4 Parameters 59

2.4.1 Polymer Solutions 592.4.2 Spin Parameters 622.4.3 Environmental Parameters 63

vi Contents

2.5 Concluding Remarks 64 References 65

3 Physical Characterization of Electrospun Fibers 71Anushka Purabgola and Balasubramanian Kandasubramanian3.1 Introduction 723.2 Characterization Techniques 76

3.2.1 Scanning Electron Microscopy (SEM) 763.2.2 Field Emission Scanning Electron

Microscopy (FESEM) 773.2.3 Transmission Electron Microscopy (TEM) 793.2.4 High-Resolution TEM (HRTEM) 803.2.5 Atomic Force Microscopy (AFM) 813.2.6 X-Ray Diffraction (XRD) 833.2.7 Nanoindentation 843.2.8 Differential Scanning Calorimetry (DSC) 853.2.9 Thermalgravimetric Analysis (TGA) 85

3.3 Physical Characterization of Electrospun Fibers 873.3.1 Electrospun Polymer Nanofibers 87

3.3.1.1 Polyacrylonitrile (PAN) Nanofiber 873.3.1.2 Polyvinylidene Fluoride (PVDF)

Fibrous Nanofibers 913.3.1.3 Polydodecylthiophene (PDT)

Core–Polyethylene Oxide (PEO) Shell Polymer Nanofiber 92

3.3.1.4 Polymethylmethacrylate (PMMA) Nanofiber 92

3.3.2 Electrospun Metal (Oxide) Nanofiber 943.3.2.1 Polyvinyl Alcohol (PVA)/Nickel Acetate 953.3.2.2 Polyvinyl Pyrrolidone (PVP)/TiO2 Nanofibers 963.3.2.3 Polyethylene Oxide/Polyvinylpyrrolidone–

Iron Oxide Nanofiber 963.3.3 Electrospun Nanocomposite Nanofibers 97

3.3.3.1 TiO2/SiO2/C (TSC) Nanofibers 983.3.3.2 Polyvinylidene Fluoride (PVDF)/ZnO

Nanocomposite Nanofiber 1003.3.3.3 Polyvinyl Alcohol (PVA)/Cellulose

Nanocrystals Composite Nanofibers 1013.3.4 Electrospun Carbon Nanofibers (CNFs) 104

3.3.4.1 Polyacrylonitrile (PAN)/N-Doped CNFs 1043.3.4.2 Lignan-Derived CNFs/PAN 104

Contents vii

3.3.4.3 Poly(L-Laticide-Co- -Caprolactone) (PLCL)/MWCNTs Nanofibers 105

3.4 Conclusion 108 References 109

4 Application of Electrospun Materials in Catalysis 113Bilge Coşkuner Filiz4.1 Introduction 1134.2 Type of Catalysts 115

4.2.1 Catalyst Supports 1154.2.2 Template for Catalytic Nanotubes 1164.2.3 Metal Oxide Catalysts 117

4.3 Catalytic Applications 1174.3.1 Energy Field 118

4.3.1.1 Oxidation Reactions 1184.3.1.2 Reduction Reactions 1194.3.1.3 Hydrogen Generation Reactions 120

4.3.2 Environment Field 1214.3.2.1 Oxidation Reactions 1214.3.2.2 Reduction Reactions 1224.3.2.3 Degradation Reactions 122

4.4 Conclusion 124 References 125

5 Application of Electrospun Materials in Packaging Industry 131Samson Rwahwire, Catherine Namuga and Nibikora Ildephonse5.1 Packaging Industry 1315.2 Electrospinning 1325.3 Nanofibers 1355.4 Biopolymers 135

5.4.1 Nanoencapsulation 1355.4.2 Methods of Encapsulation Application

in Food Packaging 1395.4.3 Drying 1405.4.4 Nano-Enabled Packaging Solutions 1405.4.5 Food Packaging 1415.4.6 Active Food Packaging 142

5.5 Future Perspectives 144 References 145

viii Contents

6 Application of Electrospun Materials in Water Treatment 151Shivani Rastogi and Balasubramanian Kandasubramanian6.1 Introduction 1526.2 Heavy Metal Ion Removal From Wastewater 154

6.2.1 Cellulose/Camphor Soot Nanofibers 1576.2.2 Spider-Web Textured Electrospun

Graphene Composite Fibers 1586.2.3 Resorcinol–Formaldehyde Nanofibers 1616.2.4 Ion-Imprinted Chitosan/1-Butyl-3-

Methylimidazolium Tetrafluoroborate Fibers 1626.2.5 Molecular Imprinted Camphor Soot

Functionalized PAN Nanofibers 1646.2.6 Iron Functionalized Chitosan Electrospun

NFs (ICS-ENF) 1666.2.7 Cellulose/Organically Modified

Montmorillonite 1666.3 Dye Removal From Wastewater 167

6.3.1 Zein Nanofibers 1676.3.2 β-Cyclodextrin Based Nanofibers 1696.3.3 3-Mercapto Propionic Acid Coated Fe3O4 NP

Immobilized Amidoximated Polyacrylonitrile 1716.3.4 Functionalized Polyacrylonitrile Membrane 171

6.4 Oil–Water Separation 1726.4.1 Wettable Cotton-Based Janus Bio Fabric

(PLA/Functionalized Organoclay) 1726.4.2 Camphor Soot Immobilized Fluoroelastomer

Membrane 1746.4.3 Polycaprolactone/Beeswax Membrane 174

6.5 Microbe Elimination From Wastewater 1766.5.1 β-Cyclodextrin/Cellulose Acetate Embedded

Ag and Ag/Fe Nanoparticles 1766.5.2 Silver Coated Polyacrylonitrile (PAN) Membrane 177

6.6 Antibiotic Removal From Wastewater 1786.7 Conclusion 180 References 180

7 Application of Electrospun Materials in Oil–Water Separations 185T.C. Mokhena, M.J. John, M.J. Mochane and P.C. Tsipa7.1 Introduction 1857.2 Oil Spill Clean-Up 187

Contents ix

7.2.1 Hydrophobic–Oleophilic Polymer Nanofiber 1877.2.2 Blends 1917.2.3 Composites 194

7.3 Separation Membranes 1957.4 Thin-Film Composite (TFC) Membranes 2027.5 Three Dimensional (3D) Nanofibrous Membranes 2037.6 Smart Membranes 2047.7 Conclusions and Future Trends 208 Acknowledgments 209 References 209

8 Application of Electrospun Materials in Industrial Applications 215Anisa Andleeb and Muhammad Yar8.1 Introduction 2168.2 Technology Transfer From Research Laboratories

to Industries 2188.3 Industrial Applications of Electrospun Materials 220

8.3.1 Biomedical Materials 2218.3.2 Defense and Security 2278.3.3 Textile Industry 2278.3.4 Catalyst 2288.3.5 Energy Harvest 2298.3.6 Filtration 2308.3.7 Sensor Applications 2328.3.8 Food 234

8.4 Current and Future Developments 236 References 237

9 Antimicrobial Electrospun Materials 243Samson Afewerki, Guillermo U. Ruiz-Esparza and Anderson O. Lobo9.1 Introduction 244

9.1.1 Electrospinning Technology 2449.1.2 Antimicrobial Materials 2469.1.3 Antimicrobial Electrospun Materials 2469.1.4 Conclusions and Future Directions 254

Acknowledgments 255 References 255

x Contents

10 Application of Electrospun Materials in Gene Delivery 265GSN Koteswara Rao, Mallesh Kurakula and Khushwant S. Yadav 10.1 Introduction 26610.2 Gene Therapy 26610.3 Cellular Uptake of Nonviral Gene Delivery 26810.4 Vectors 269

10.4.1 Viral Vectors 26910.4.2 Nonviral Vectors 27010.4.3 Delivery of Genes through Vectors 271

10.5 Nanofibers/Scaffolds 27310.6 Electrospinning 275

10.6.1 Steps Involved in the Electrospinning Process 27610.6.2 Types of Electrospinning 279

10.7 Characterization 28110.8 Applications of Electrospun Materials 282

10.8.1 Electrospun Materials in Gene Delivery 28210.8.1.1 Tissue Engineering 28210.8.1.2 Regenerative Medicine 28410.8.1.3 Vascular Grafts 28410.8.1.4 Bone Regeneration 28510.8.1.5 Diabetic Ulcer Treatment 28610.8.1.6 Cancer Treatment 28710.8.1.7 Blood Vessel Regeneration 28710.8.1.8 Wound Management 28810.8.1.9 Carrier for Genetic Material

Loaded Nanoparticles 28810.8.1.10 Myocardial Infarction Treatment 28810.8.1.11 Stem Cell-Based Therapy 28910.8.1.12 Gene Silencing 28910.8.1.13 Controlled Release of Gene 29010.8.1.14 DNA Delivery 290

10.8.2 Electrospun Materials in Drug Delivery 29110.8.2.1 Antibiotics and Various

Antibacterial Agents 29210.8.2.2 Anticancer Drugs 29210.8.2.3 Cancer Diagnosis 29210.8.2.4 Wound Management 29310.8.2.5 Tissue Engineering 29310.8.2.6 Bone Tissue Engineering 293

Contents xi

10.8.2.7 Dental Growth 29410.8.2.8 Therapeutic Delivery Systems 294

10.8.3 Electrospun Materials in Miscellaneous Applications 294

10.9 Future Scope and Challenges 29610.10 Conclusion 296 References 297

11 Application of Electrospun Materials in Bioinspired Systems 307Anca Filimon, Adina Maria Dobos, Oana Dumbrava and Adriana Popa11.1 Introduction 30811.2 Composite Materials Based on Cellulosic Nanofibers 309

11.2.1 Processing of Cellulose-Based Materials 31011.2.2 Structure–Property–Biological Activity

Relationship 31011.2.2.1 Biosensors Based on Cellulosic Fibers 31011.2.2.2 Delivery Systems and Controlled

Release of Drugs 31211.2.2.3 Wound Dressing 31611.2.2.4 Tissue Engineering 317

11.3 Chitosan Nanofibrous Scaffolds 32211.3.1 Overview on Obtained Chitosan From

Bio-Waste Source 32211.3.2 Specific Applications of Chitosan Nanofibers

in Bio Inspired Systems 32511.3.2.1 Wound Dressing 32511.3.2.2 Drug Delivery 32911.3.2.3 Tissue Engineering 33011.3.2.4 Antibacterial Activity 336

11.4 Conclusions 339 References 339

12 Smart Electrospun Materials 351Gaurav Sharma, Shivani Rastogi and Balasubramanian Kandasubramanian12.1 Introduction 35212.2 Smart Electrospun Materials in Biomedical Applications 354

12.2.1 Tissue Engineering 35412.2.2 Controlled Drug Delivery 35512.2.3 Wound Healing 356

xii Contents

12.3 Smart Electrospun Materials for Environmental Remediation 35712.3.1 Water Pollution Control 35712.3.2 Air Pollution Control 35912.3.3 Noise Pollution Control 360

12.4 Smart Electrospun Materials in Electronics 36112.4.1 Solar Cell 36112.4.2 Energy Harvesters 36212.4.3 Shape-Memory Polymers 36312.4.4 Batteries and Supercapacitors 36412.4.5 Sensors, Transistors, and Diodes 366

12.5 Smart Electrospun Materials in Textiles 36812.5.1 Biomedical Parameter Regulation 36812.5.2 Protection from Environment Threat 36912.5.3 Energy Harvesters in Textiles 37012.5.4 Smart Textile Project 370

12.6 Smart Electrospun Materials in Food Packaging 37112.7 Conclusion 372 References 373

13 Advances in Electrospinning Technique in the Manufacturing Process of Nanofibrous Materials 379Karine Cappuccio de Castro, Josiel Martins Costa and Lucia Helena Innocentini Mei13.1 Introduction 38013.2 Process 38013.3 Important Parameters 382

13.3.1 Effects of the Applied Tension 38213.3.2 Effects of Solution Eject Rate 38213.3.3 Effects of Needle-to-Collector Distance

and Needle Diameter 38413.3.4 Effects of Solution Concentration and Viscosity 38413.3.5 Effects of Solution Conductivity 38513.3.6 Solvent Effects 38513.3.7 Effects of Surface Tension 38513.3.8 Humidity and Temperature Effects 386

13.4 Recent Advances in the Technique 38613.4.1 Electrospinning Coaxial 38613.4.2 Electrospinning Triaxial 38713.4.3 Multiple Needle Electrospinning 38713.4.4 Electroblowing 387

Contents xiii

13.4.5 Magnetic Electrospinning 38813.4.6 Centrifugal Electrospinning 38813.4.7 Needleless Electrospinning 388

13.5 Coaxial Electrospinning as an Excellent Process for Hollow Fiber and Drug Delivery Device Production 389

13.6 Applications 39013.7 Conclusions and Future Perspectives 393 References 393

14 Application of Electrospun Materials in Filtration and Sorbents 401T.S. Motsoeneg, T.E. Mokoena, T.C. Mokhena and M.J. Mochane14.1 Introduction 40214.2 Morphology of Sorbents With Concomitant

Sorption Capacity 40314.3 Mechanistic Overview in Purification During Filtration 40614.4 Conclusion and Future Prospects 410 References 411

15 Application of Electrospun Materials in Batteries 415Subhash B. Kondawar and Monali V. Bhute 15.1 Introduction 41615.2 Electrospun Nanofibers as Anodes 418

15.2.1 Carbon Nanofibers as Anode 41815.2.2 Metal Oxide Nanofibers as Anode 419

15.3 Electrospun Nanofibers as Cathode 42315.3.1 Lithium Metal Oxide Nanofibers as Cathode 42315.3.2 Transition Metal Oxides Nanofibers as Cathode 424

15.4 Electrospun Nanofibers as Separator 42515.4.1 Polymer Nanofibers as Separator 42615.4.2 Polymer–Inorganic Nanofiber Separators 430

15.5 Conclusions and Outlook 432 References 433

16 State-of-the-Art and Future Electrospun Technology 441Prasansha Rastogi and Balasubramanian Kandasubramanian16.1 Introduction 44216.2 Some General Smart Applications

of Electrospun Membranes 44516.3 Stimuli Responsive or Shape Memory

Electrospun Membranes 454

xiv Contents

16.4 Conclusion 473 Acknowledgment 474 References 474

17 Antimicrobial Electrospun Materials 483Rushikesh S. Ambekar and Balasubramanian Kandasubramanian17.1 Introduction 48417.2 Drug-Loaded Polymer Nanofibers 48517.3 Drug-Loaded Biodegradable Polymer Nanofibers 48517.4 Drug-Loaded Non-Biodegradable Polymer Nanofibers 50117.5 Conclusion and Future Scope 507 References 508

Index 515

xv

Preface

The electrospinning technique uses an electrically charged jet of polymer solution or melt of both natural and synthetic polymers to produce fibers of submicron to nanometer size. Fibers with various morphologies and structures can be easily prepared by electrospinning by altering the pro-cessing parameters. Electrospinning is a voltage-driven process by which a wide range of materials, including polymers, biomaterials, inorganic sol–gels, colloidal particles, additives like fillers, plasticizers, etc., can be spun into nanofibers. Electrospinning can be traced back to the 17th century, about 400 years ago, when William Gilbert observed the deformation of a liquid droplet into conical form when a piece of statically charged amber was placed closer to the liquid. Later, in the early 18th century, John Zeleny worked on the mathematical model of the effect of electric field on the liquid meniscus. In 1934, Formhals filed his first patent for drawing artifi-cial threads. In the 1960s, Taylor expanded the work of William Gilbert by using conducting fluid and showed the conical shape of the droplet in the presence of the electric field, hence named a Taylor cone. After the 1960s, researchers started studying the morphology, structure, operation param-eters, etc., of electrospun nanofibers, which are still being expanded upon for their applications as smart materials. Despite the fact of the technology having already been developed, the surge in the utilization of electrospin-ning for the production of fibrous materials by both academia and industry intensified during the last decade. A variety of nanofibers can be prepared by electrospinning technology for a wide range of applications in tissue engineering, drug delivery, biotechnology, wound healing, environmental protection, energy harvesting and storage, electronics and defense, and security purposes. The materials possess higher mechanical performance, large surface area-to-volume ratio, and functional properties.

The aim of this edition of Electrospun Materials and Their Allied Applications is to explore the history, fundamentals, manufacturing processes, optimization parameters, and applications of electrospun

xvi Preface

materials. This book includes various types of electrospun materials such as antimicrobial, smart, bioinspired systems, and so on. The electrospun materials have applications in areas such as energy storage, catalysis, biomedical, separation, adsorption, and water treatment technologies. The book emphasizes the enhanced sustainable properties of electro-spun materials, with the challenges and prospectives being discussed in detail. The chapters are written by top-class researchers and experts from throughout the world. This book is envisioned for faculty members and students of engineering, materials science, engineers, and materials designers who need to consider the morphological design of materials for versatile applications. Based on thematic topics, this edition contains the following 17 chapters:

Chapter 1 discusses the current and advanced electrospinning fabrica-tion strategies. The technological limitations of conventional strategies and their reduced ability to achieve 3D structures are also discussed. Advanced strategies, such as melt electrospinning, near-field electro-spinning, electroblowing, hybrid structures, cell electrospinning, and in  situ electrospinning, are highlighted with respect to the way they may contribute to circumvent the limitations of conventional strategies.

Chapter 2 discusses the development of electrospinning techniques and provides information about the theory of electrospinning. The setup and configurations of electrospinning are discussed in detail for the fabrication of nanofibers. The effect of processing conditions on geometry, morphol-ogy, and functionality of nanofibers are also presented.

Chapter 3 briefly provides information about certain physical characteri-zation techniques that are relevant with respect to electrospinning and the changes observed in the physical properties of the material.

Chapter 4 focuses on the applications of electrospun materials in areas such as catalysis. Several reactions such as oxidation, reduction, and deg-radation in the field of energy and environmental applications are men-tioned, in which the presence of heterogeneous catalyst is prepared by electrospinning technique. This chapter investigates recent approaches for these specific applications of catalysis.

Chapter 5 discusses developments in the packaging industry, specifi-cally food packaging; the science of electrospinning and parameters that

Preface xvii

influence the process are presented, and, after that, electrospun materials in the food packaging industry and their application thereof.

Chapter 6 summarizes the advanced applications of distinct electrospun materials in the growing water treatment sector. Covered in this chapter are various organic/biomaterials as well as inorganic/synthetic materials with improved properties due to electrospinning procedure. It describes the power of electrospun materials for resolving the problem of hazardous water contaminants like heavy metal ions, dyes, microbial growth, and pharma-ceutical waste (antibiotics), along with problems related to oil spills.

Chapter 7 summarizes the design, manufacturing, and recent develop-ments of electrospun nanofibers with tailorable surface wettability for oily wastewater purification. The chapter also discusses various electrospun nanofibrous materials having different mechanisms for oil-in-water sepa-ration and their challenges and prospects.

Chapter 8 describes various industrial applications of electrospun mate-rials, including the transfer of electrospun materials from research lab-oratories to industries for commercialization. The main focus is on the applications of electrospun materials in different industrial fields such as biomedical, filtration, textiles, sensors, protective clothing, energy harvest-ing, and storage devices.

Chapter 9 highlights electrospinning technology for the fabrication of electrospun materials and their advantages and wide range of potential applications. Due to the fast-growing problem of infections and the preva-lence of antibiotic-resistance microbes, the focus is on electrospun materi-als with antimicrobial property.

Chapter 10 discusses the various applications of electrospun materials in gene delivery. It emphasizes the delivery of genes, DNA, RNA, peptides, antibodies, growth factors, and many drugs by electrospun materials, including nanofibers. The major applications that are elaborated on include their role in tissue engineering, bone regeneration, wound healing, stem cell treatment, blood vessel growth, dentistry, gene expression/ silencing, and controlled release of biomolecules/drugs.

Chapter 11 presents information on the natural polymers and how they can be processed by electrospinning to obtain properties required by target

xviii Preface

applications. The role of methods in the development of electrospun mate-rials is studied in correlation with the way in which they can be adapted for bioinspired applications.

Chapter 12 discusses the various applications of electrospun materials in various sectors like air, water, and noise pollution control. Some of the important applications of electrospun materials in areas such as solar cells, energy harvesters, batteries, supercapacitors, and sensor diodes are extensively discussed along with their use in textiles and at industrial levels. The main focus is on the application areas of these materials for a wider explanation of the numerous studies reported in the literature and inventories.

Chapter 13 details the main concepts involved in the electrospinning technique, discusses the parameters that influence the morphology of nanofibers, and presents the main advances related to the process and the applications that have been highlighted in recent years.

Chapter 14 outlines the synthesis of sorptive mats by electrospinning methods for use in the filtration processes to eradicate contaminants pre-dominantly in wastewater and terrains for the alleviation of environmental pollution. Recent developments in the manufacturing of new electrospun mats for use as sorbents in the purification processes and the in-depth mechanistic binding between sorptive mats and unwanted impurities during filtration are covered.

Chapter 15 elaborates on the recent development of the electrospun nano-fiber-based materials in terms of synthesis and application for lithium-ion battery components such as anodes, cathodes, and separators. A short overview of the challenges and prospects of electrospun nanofibers for lithium-ion battery components is also presented.

Chapter 16 summarizes the employment of a robust electrospinning tech-nique in membrane fabrication for varied applications. Performance of the same is diversified by utilizing stimuli-responsive/shape memory materi-als, which react to triggers from the external environment with a widened scope in biomedical treatment, fuel cells, filtration, etc.

Preface xix

Chapter 17 discusses the classification of antibacterial nanofibers based on biodegradability, which includes drugs such as synthetic drugs, natural drugs, or nanoparticle-embedded biodegradable and non-biodegradable nanofibers with applications. The ideal design of antibacterial nanofibers based on comparative study of recently developed antibacterial nanofibers is also reported in the chapter.

EditorsInamuddin

Rajender Boddula Mohd Imran Ahamed

Abdullah M. AsiriDecember 2019

1

Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Electrospun Materials and Their Allied Applications, (1–52) © 2020 Scrivener Publishing LLC

1

Electrospinning Fabrication Strategies: From Conventional

to Advanced ApproachesJ.R. Dias*, Alexandra I. F. Alves, Carolina A. Marzia-Ferreira

and Nuno M. Alves

Centre for Rapid and Sustainable Product Development (CDRsp), Polytechnic Institute of Leiria, Leiria, Portugal

AbstractElectrospinning is a widely used technique in several fields to produce micro- nanofibers due to its versatility, low cost and easy use. Moreover, electrospun meshes present some advantages like high surface area, small pore size, high poros-ity, and interconnectivity. Present, also, the possibility to control the nanofiber composition and orientation to achieve desired properties and/or functionalities. These outstanding properties make the electrospun nanofibers good candidates for many applications such as filtration, tissue engineering, wound dressings, energy conversion and storage, catalysts and enzyme carriers, protective clothing, sensors, drug delivery, electronic and semi-conductive materials.

This chapter presents a comprehensive review of current and advanced elec-trospinning fabrication strategies. Recent advances have been mainly focused on the materials used rather than on sophisticated fabrication strategies to generate complex structures. The technological limitations of conventional strategies, such as random, aligned, and core–shell technologies, and their reduced capacity to achieve 3D structures will be discussed. Advanced strategies, such as melt electro-spinning, near field electrospinning, electroblowing, hybrid structures, cell elec-trospinning and in situ electrospinning will be highlighted in the way they may contribute to circumvent the limitations of conventional strategies, through the combination of different technologies and approaches. The main research chal-lenges and future trends of fabrication electrospinning strategies will be discussed.

*Corresponding author: juliana.dias@ipleiria.pt

2 Electrospun Materials and Their Allied Applications

Keywords: Conventional/advanced strategies, hybrid fibers, hierarchical structures

1.1 Introduction

Although the electrospinning technique is under growing development in several fields its principles emerged around the 1600s. However, since the 1980s, several research groups demonstrated that it is possible to pro-duce electrospun fibers with organic polymers increasing, since then, the number of publications exponentially [1, 2]. Electrospinning is a technique allowing to create submicron to nanometer scale fibers from polymer solu-tions or melts and was developed from a basis of electrospraying, widely used for more than 100 years [3, 4]. It is also known as electrostatic spin-ning, with some common characteristics to electrospraying and the tradi-tional fiber drawing process [5].

The conventional setup for an electrospinning system consists of three major components: a high voltage power supply, a spinneret, and a col-lector that can be used with horizontal or vertical arrangement [1, 3, 6]. The syringe contains a polymeric solution or a melt polymer, pumped at a constant and controllable rate. The polymer jet is initiated when the voltage is turned on and the opposing electrostatic forces overcome the surface tension of the polymer solution. Just before the jet formation, the polymer droplet under the influence of the electric field assumes the cone shape with convex sides and a rounded tip, known as the Taylor cone [5, 7, 8]. During the jet’s travel, the solvent gradually evaporates and charged polymer fibers are randomly deposited or oriented in the collector [8]. The electrospinning process can be influenced by several parameters, such as solution parameters (viscosity, concentration, type of solvent), processing parameters (flow rate, distance between needle and collector, voltage supply, type of collector), and ambient parameters (temperature and humidity). The technique is also highly versatile since, in addition to the conventional fiber configuration, it is possible to obtain a variety of other configurations, namely core/shell (co-axial) or emul-sion configurations and, according to the fiber orientation, it is possible to produce aligned or randomly oriented fibers depending the type of the collector used. More recently emerged several advanced fabrication strategies that allow making structures more complex and multifunc-tion. The present chapter intends to give an overview of the fabrication

From Conventional to Advanced Strategies 3

strategies used in electrospinning technique from conventional to the recent advanced strategies.

1.2 Conventional Fabrication Approaches

1.2.1 Randomly Oriented Fiber Meshes

Conventional electrospinning setup configuration consists of fibers ran-domly deposited over the grounded collector, which is usually a metal plate [1, 9, 10]. The random deposition is a consequence of the jet instability resulting from the electric field applied to overcome the polymeric solution surface tension [7, 11]. There are several studies comparing random and aligned deposition strategies in terms of nanofibers morphology, hydro-philicity, mechanical properties, and cell adhesion and proliferation [12, 13].

In terms of biological response, numerous studies demonstrated that aligned fibers usually exert a more relevant influence on cellular behavior including cell morphology, cellular density and gene expression. In terms of mechanical properties, the elongation at break presents better results when fibers are randomly oriented [12, 14].

The electrospun fibers without defined orientation are often produced when using the electrospinning technique, due to the simplicity of the pro-cess and the collector type associated with this process, which is usually a planar and static collector as a standard [15]. The collecting method of elec-trospun nanofibers is one of the parameters that influence their orientation, and by consequence, their shape, size, and mechanical properties [15, 16]. The collector must be a conductive metal plate, to reduce the loads and avoid repulsive forces between the fibers themselves. Aluminum foil, copper plates, paper and water bath are some of collector types used by some authors to produce non-woven meshes with smooth surface and dense structure [15].

The electrospun meshes have certain characteristics, resulting from their unstructured deposition, such as high specific surface area to volume ratio, with high porosity (>90%), wettability, and appreciable mechanical prop-erties [17–19]. Characteristics that make its use advantageous in several fields, such as biomedical applications, environmental protection, energy store cells, catalysts, defense clothes, among others [17–20].

Electrospinning serves as a popular technique for fabricating porous scaffolds with diverse properties for culturing cells to be used in engineer tissues [16]. In wound healing, electrospun fibers accurately mimic the in vivo environment of cells, such as fibroblasts and keratinocytes, allowing

4 Electrospun Materials and Their Allied Applications

their adhesion, proliferation and growth. Thus, by reproducing the extra-cellular matrix (ECM), since it also has a disorganized structure formed by nanofibers of collagen and elastin, allows a rapid and efficient tissue replacement [21].

Sobhanian and colleagues [22], developed electrospun nanofibrous of poly(vinyl alcohol) (PVA)/gelatin and alginate grafted with collagen (extracted from rat tail) as a potential skin substitute. The results demon-strated that the structures grafted with collagen potentiate their function-ality, hydrophilicity and cells adhesion and proliferation. Some mechanical properties have been improved with the addition of collagen, such as elon-gation at break, resulting in a successful technique and reasonable cost to repair damaged skin when compared to solutions that already exist in the market as autograft, allograft, or xenograft [16, 22].

In the wound healing process, the first procedures of the damaged tissue are crucial to ensure the organism hemostasis [23, 24]. The application of a wound dressing is an essential procedure to prevent infections and promote exudate absorption. The electrospinning technique allows creating more functional wound dressings than conventional ones since it is quite versa-tile in the materials and parameters that can be used [17–19]. Electrospun wound meshes can fit the ideal requirements such as gas permeation, wound protection, and prevent wound dehydration. For this reason, it is necessary to have a high porosity, which is only possible with nanofibers with random orientation. The great advantage of nanofibers is the possibility of incorpo-rating drugs and other substances that potentiate their functionality [25].

To study the potential of electrospun meshes in wound healing, Li and his colleagues [26], produced nanofibers based on hydrophilic poly(vinylpyrro-lidone) (PVP) and hydrophobic ethyl cellulose (EC). Fibers were, also, col-lected in aluminum foil and directly on gauze. Ciprofloxacin (CIF), a model antibiotic, was loaded into fibers to avoid bacterial infection [26]. The results obtained showed a faster CIF release, when compared with their hydrophobic analogs. While EC nanofibers in 3 days had a release to zero-order. Cell via-bility assays with human dermal fibroblasts (HDF) cells have close to 100% viability for all fibers types [26]. Fibers with EC formulations cell growth is assured, with cell adhesion and proliferation. The antibacterial tests with S. aureus, a gram-positive, and E. coli, a gram-negative showed that both polymers have antibacterial activity, although PVP fibers had greater activ-ity. There were also no differences in the fibers when deposited in different collectors, allowing the application directly in gauzes for a smart fabric [26].

In terms of water and air purification, electrospun membranes serve as an alternative to non-membrane based purification methods, which often are not easily recycled or reused [20].

From Conventional to Advanced Strategies 5

In addition to the features already mentioned, nanofibers have good mechanical and thermal properties, that give them more resistance, when compared with other fibers like glass fibers, melt-blown and spunblow-ing fibers, and others materials for the same application [16, 20, 27]. The changeability of the technique allows several polymers to be used for this purpose, even if they are synthetic or organic. The most commonly used are poly(acrylonitrile) (PAN), chitosan (CS), cellulose, PVA, and polysty-rene (PS) [16, 20–22, 24].

Bortolassi et al. [27] utilized PAN electrospun nanofibers containing different percentages of silver (Ag) to be used as air filters by removing nanoparticles from the air, and, evaluating their antibacterial activity against E. coli [27]. The results demonstrate that when 50 wt% silver nitrate AgNO3 (50AgF) was added to the PAN nanofibers, although, had the low-est filtration efficiency (>98%) comparing with other Ag concentrations, being the best candidate to be applied in air filter because had the best high quality factor with low-pressure drop as well the highest antibacterial activity [20]. To remove micropollutants, such pharmaceuticals, personal care products, radioactive or biologically harmful metals, pesticides or endocrine disrupters, from waters, Fan et al. [28] produced electrospun nanofibers with β-cyclodextrin (β-CD), CS, and PVA. They conclude that the randomly oriented nanofibers can rapidly remove organic pollutants and heavy metals by adsorption, like lead (Pb2+), mercury (Hg2+), cad-mium (Cd2+), nickel (Ni2+), cooper (Cu2+), and dichromate Cr O2 7

2−( ). These heavy metals are naturally present in the environment, but due to anthro-pogenic activity, their concentrations may exceed the desired limits, caus-ing problems for the organism, the human being even [28].

In addition, random nanofibers can also be used in the desalination process. The range of electrospun meshes characteristics makes them indispensable in the membrane distillation since its reduced and sturdy structure increases hydrophobicity, which is necessary for this process [28–30]. An efficient separation membrane must present high porosity and hydrophobic character to not allow the passage of liquid water [28–30].

According to Woo et al. [29] study, polyvinylidene fluoride-co-hexaflu-oropropylene (PH) was loaded with different concentrations of graphene, between 0–10 wt%, to be used as membrane distillation via air gap (AGMD). The results presented show that graphene, at a concentration of 5 wt%, potentiates the structure of the fibers, increasing its roughness and thus improving its absorption/desorption capacity [29]. When exposed to salts, the superhydrophobicity of the membrane avoids the penetration of water, and due to its porous structure and high volume/ratio only water vapor passes through it (Figure 1.1).

6 Electrospun Materials and Their Allied Applications

Although the technique still has some challenges to overcome, such as optimization of parameters, mechanical properties and scale-up produc-tion, the easy production of randomly oriented fibers through electro-spinning, as well as the variety of materials that can be used to make the membranes more robust, makes the technique very attractive to purify air and water, compared to the traditional ones [30].

Electrospun nanofibers have been receiving more attention over the last few years in the chemistry sector since they are a more ecological and economical option than traditional ones that often-including hazardous chemicals. These characteristics are due to the possibility of using a vast range of natural and semi-natural polymers that are eco-friendly, and due to the possibility of reuse these electrospun nanofibers without losing their functionalities [16].

G/PH electrospunnano�ber membrane

GrapheneNano�ber

Grapheneprotrusions

E�ects of graphene incorporation

− Provides surface roughness and hydrophobicity− Improves anti-wetting property− Provides diffusion path for water vapour (i.e., rapid adsortion/desorption capacity)− Improves thermal stability & mechanical properties of the composite membrane

Graphene/PH membraneproperties

Superhydrophobic

Fast transport alonggraphene surface

Direct permeation throughmembrane

Activated diffusion viaadsorption/desorptionon graphene surface

Highly porous surface

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Figure 1.1 Scheme of the effect of graphene nanofibers on AGMD process. Reproduced with permission from Ref. [29].

From Conventional to Advanced Strategies 7

In addition, the catalysis reactions are those that occur more frequently in the chemical processes, it is estimated that 90% of the processes use heterogeneous catalysts, especially in a more industrial component. Other nanostructures beyond nanofibers can play this role such as nanotubes, nanoparticles, and nanowires [31]. In fact, they can be used together, i.e., Xu et al. [32] used electrospun nanofibers with random orientation, to be introduced into a halloysite nanotube (HNts) with the function of absorb-ing dyes and catalyst support. The fibers are made of PAN and polyimide (PI), providing mechanical elasticity and stability, while PVA was utilized as a binding agent. The results demonstrated that using electrospun nano-fibers as a skeleton of HNTs sponges allowing to remove 90% of dye after five cycles of adsorption/desorption and can be reused up to five times [32].

Recent studies have demonstrated the use of carbon nanofibers (CNFs), and other similar ones, such as graphene, in energy cells such as high-rate batteries [33]. CNFs have good mechanical and conductive properties and can work as the anode to be applied at lithium-ion batteries (LIBs) [33]. In 2019, Bhute & Kondawar [34], produced by electrospinning poly(vinylidenefluoride) (PVDF)/cellulose acetate/silver-titanium dioxide (AgTiO2) nanofibers to loaded in lithium batteries (Figure 1.2). The ionic mobility and polymer segmental motion were increased with the aid of Ag–TiO2 in the membrane [34].

These are just a few examples of recent works that prove the diversity and applicability of nanofibers in this field, although its applications may still extend to other examples. Namely photochemical energy, by dye-sensitized solar cells, where electrospun nanofibers increased the surface area of pho-toelectrode and the overall performance [16]. Supercapacitors development is another example, those, are used as an energy store because possesses high power densities and lifetimes compared with LIBs [35]. The performance of supercapacitors with electrospun nanofibers composed by PAN and 40 wt% of manganese acetylacetonate (MnACAC) as precursors improved their performance from 90 Fg−1 specific capacitance to 200 Fg−1 [35].

Electrospun fibers demonstrated great potential as reaction catalysts, but especially their role in generating and storing energy in appreciable quan-tities, in view of the available commercial devices. At the same time, using electrospun nanofibers, become an environmental and economic solution, by reducing the need to consume energy from fossil fuels to store energy [16].

In the section were highlighted the topics that address the main applica-tions of randomly oriented fiber meshes. However, they can also be applied in the textile industry, protective materials, sensors, agriculture, and food packing [16–18, 20]. Despite this technique has unique applications, some

8 Electrospun Materials and Their Allied Applications

associated production challenges remain, such as method reproducibility, and large-scale production [16, 19, 20].

1.2.2 Aligned Fiber Meshes

Depending on the field of application, highly aligned micro or nanofibers are often required in order to attend to specific needs, either in the bio-medical field [36–38], energy and electronics [39–42], or reinforcement in composite materials [43–47].

Electrospinning comprises two regimes of jet movement upon jet emis-sion from the Taylor Cone: a minor short-distance segment in a straight line (stable) followed by a dominant whipping motion (unstable) [48, 49]. Regarding unstable jet-based, electrospun fibers spatial orientation can be achieved through modification of the collector (rotating, parallel, water bath) or by manipulation of external forces (magnetic field, electric field, post-drawing, centrifugal force, or gas force) [50]. On the other hand, the use of stable jet region is highly desirable to align fibers and can be achieved

Electrospinning

PVdF/CA-AgTiO2 NanofibersPolymer electrolyte

PVdF/CA-AgTiO2 Nanofibers membrane

AgTiO2

Li+

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Cellulose acetate

SolventDMF: Acetone

PVdF

Figure 1.2 Diagram of production of electrospun nanofibers with (PVDF)/cellulose acetate/AgTiO2. Reproduced with permission from Ref. [34].