Electrospun Materials and Their Allied Applications E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
The aim of this book is to explore the history, fundamentals, manufacturing processes, optimization parameters, and applications of electrospun materials. The book includes various types of electrospun materials such as antimicrobial, smart, bioinspired systems. It focuses on the many application areas for electrospun materials such as energy storage and harvesting, catalysis, biomedical including gene delivery and tissue engineering, separation, adsorption and water treatment technologies, packaging. The book emphasizes the enhanced sustainable properties of electrospun materials, with the challenges and future developments being discussed in detail. The chapters are written by top-class researchers and experts from throughout the world.
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Veröffentlichungsjahr: 2020
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Table of Contents
Cover
Preface
1 Electrospinning Fabrication Strategies: From Conventional to Advanced Approaches
1.1 Introduction
1.2 Conventional Fabrication Approaches
1.3 Advanced Fabrication Approaches
1.4 Conclusions and Future Perspectives
Acknowledgments
References
2 History, Basics, and Parameters of Electrospinning Technique
2.1 Definitions
2.2 Milestone of Electrospinning Technique
2.3 Setup and Configuration of Electrospinning Technique
2.4 Parameters
2.5 Concluding Remarks
References
3 Physical Characterization of Electrospun Fibers
3.1 Introduction
3.2 Characterization Techniques
3.3 Physical Characterization of Electrospun Fibers
3.4 Conclusion
References
4 Application of Electrospun Materials in Catalysis
4.1 Introduction
4.2 Type of Catalysts
4.3 Catalytic Applications
4.4 Conclusion
References
5 Application of Electrospun Materials in Packaging Industry
5.1 Packaging Industry
5.2 Electrospinning
5.3 Nanofibers
5.4 Biopolymers
5.5 Future Perspectives
References
6 Application of Electrospun Materials in Water Treatment
6.1 Introduction
6.2 Heavy Metal Ion Removal From Wastewater
6.3 Dye Removal FromWastewater
6.4 Oil–Water Separation
6.5 Microbe Elimination From Wastewater
6.6 Antibiotic Removal From Wastewater
6.7 Conclusion
References
7 Application of Electrospun Materials in Oil–Water Separations
7.1 Introduction Oily wastewater resulting from oil discharged into lakes, sea, rivers, and
7.2 Oil Spill Clean-Up
7.3 Separation Membranes
7.4 Thin-Film Composite (TFC) Membranes
7.5 Three Dimensional (3D) Nanofibrous Membranes
7.6 Smart Membranes
7.7 Conclusions and Future Trends
Acknowledgments
References
8 Application of Electrospun Materials in Industrial Applications
8.1 Introduction
8.2 Technology Transfer From Research Laboratories to Industries
8.3 Industrial Applications of Electrospun Materials
8.4 Current and Future Developments
References
9 Antimicrobial Electrospun Materials
9.1 Introduction
Acknowledgments
References
10 Application of Electrospun Materials in Gene Delivery
10.1 Introduction
10.2 Gene Therapy
10.3 Cellular Uptake of Nonviral Gene Delivery
10.4 Vectors
10.5 Nanofibers/Scaffolds
10.6 Electrospinning
10.7 Characterization
10.8 Applications of Electrospun Materials
10.9 Future Scope and Challenges
10.10 Conclusion
References
11 Application of Electrospun Materials in Bioinspired Systems
11.1 Introduction
11.2 Composite Materials Based on Cellulosic Nanofibers
11.3 Chitosan Nanofibrous Scaffolds
11.4 Conclusions
References
12 Smart Electrospun Materials
12.1 Introduction
12.2 Smart Electrospun Materials in Biomedical Applications
12.3 Smart Electrospun Materials for Environmental Remediation
12.4 Smart Electrospun Materials in Electronics
12.5 Smart Electrospun Materials in Textiles
12.6 Smart Electrospun Materials in Food Packaging
12.7 Conclusion
References
13 Advances in Electrospinning Technique in the Manufacturing Process of Nanofibrous Materials
13.1 Introduction
13.2 Process
13.3 Important Parameters
13.4 Recent Advances in the Technique
13.5 Coaxial Electrospinning as an Excellent Process for Hollow Fiber and Drug Delivery Device Production
13.6 Applications
13.7 Conclusions and Future Perspectives
References
14 Application of Electrospun Materials in Filtration and Sorbents
14.1 Introduction
14.2 Morphology of Sorbents With Concomitant Sorption Capacity
14.3 Mechanistic Overview in Purification During Filtration
14.4 Conclusion and Future Prospects
References
15 Application of Electrospun Materials in Batteries
15.1 Introduction
15.2 Electrospun Nanofibers as Anodes
15.3 Electrospun Nanofibers as Cathode
15.4 Electrospun Nanofibers as Separator
15.5 Conclusions and Outlook
References
16 State-of-the-Art and Future Electrospun Technology
16.1 Introduction
16.2 Some General Smart Applications of Electrospun Membranes
16.3 Stimuli Responsive or Shape Memory Electrospun Membranes
16.4 Conclusion
Acknowledgment
References
17 Antimicrobial Electrospun Materials
17.1 Introduction
17.2 Drug-Loaded Polymer Nanofibers
17.3 Drug-Loaded Biodegradable Polymer Nanofibers
17.4 Drug-Loaded Non-Biodegradable Polymer Nanofibers
17.5 Conclusion and Future Scope
References
Index
End User License Agreement
List of Tables
Chapter 3
Table 3.1 Basic properties and applications of different materials.
Table 3.2 Parameters and their effect on fiber morphology.
Table 3.3 Summarized physical characterization techniques involved.
Table 3.4 Crystallinity, porosity, tensile strength, Young’s modulus, and e...
Table 3.5 Summary for different core–shell polymer fibers.
Table 3.6 Mean diameter of PVDF/ZnO nanofibers at different weight composit...
Table 3.7 Change in properties of PVDF/ZnO nanofibrous membrane on increasi...
Table 3.8 Physical characterization analysis of N-CNFs at three different c...
Table 3.9 Average diameter and tensile properties for different concentrati...
Chapter 6
Table 6.1 TDI value of heavy metal ions by WHO.
Table 6.2 Electrospun fibers for heavy metal ion adsorption.
Table 6.3 Desorption studies of heavy metal ion adsorbing electrospun mater...
Table 6.4 Electrospun fibers for dye adsorption.
Chapter 7
Table 7.1 Selected studies based on the utilization of electrospun nanofibr...
Table 7.2 Selected studies on electrospun nanofibrous filtration membranes.
Chapter 11
Table 11.1 Biosensors based on cellulose fibers, activation techniques of c...
Table 11.2 Celluloses used in obtaining of the controlled drug delivery sys...
Chapter 12
Table 12.1 Types of electrospinning techniques [6].
Table 12.2 Various electrospun fibers as gas sensors [53].
Chapter 13
Table 13.1 Electrospinning parameters and their usual effects on the morpho...
Table 13.2 Application of different electrospinning designers.
Chapter 14
Table 14.1 Different sorptive mats prepared by electrospinning their sorpti...
Chapter 16
Table 16.1 Tabulation of stimuli-responsive and shape memory data of electr...
Chapter 17
Table 17.1 Drug loaded biodegradable polymer nanofibers.
Table 17.2 Drug loaded non-biodegradable polymer nanofibers.
List of Illustrations
Chapter 1
Figure 1.1 Scheme of the effect of graphene nanofibers on AGMD process. Re...
Figure 1.2 Diagram of production of electrospun nanofibers with (PVDF)/cel...
Figure 1.3 (a) Frame electrode used for obtaining parallel fibers and scan...
Figure 1.4 (a) Schematic of the steps involved in the fabrication of RGO/C...
Figure 1.5 Core/shell fibers. (a) Transmission electron microscopy (TEM) i...
Figure 1.6 TEM of coaxially spun fibers with 53% isoprene: (a) An as-spun ...
Figure 1.7 (a) TEM images of microstructures rGO/SnO2 composites. (b) TEM ...
Figure 1.8 (a) Schematic setup of direct
in vitro
melt electrospinning. (b...
Figure 1.9 Near field electrospinning apparatus. Reproduced with permissio...
Figure 1.10 Increasing the concentration of CaCl
2
in HEK 293T cells decrea...
Figure 1.11 Schematic illustration of the electroblowing apparatus. Reprod...
Figure 1.12 Hybrid structures produced by combination of: (a) electrospinn...
Figure 1.13 Fabrication and morphologies of the CPAN NF/MNP hybrid nanofib...
Figure 1.14 (a) Chondrogenic behavior and hypertrophy evaluation of hybrid...
Figure 1.15 (a) The first approach of CE. Reproduced with permission from ...
Figure 1.16 Aqueous solution-electrospinning methodology. I: (a) Schematic...
Figure 1.17
In situ
electrospinning concept. (a) Portable electrospinning ...
Figure 1.18 SpinCare device and a case study of its use on wound treatment...
Chapter 2
Figure 2.1 Milestone of electrospinning technique.
Figure 2.2 Setup of electrospinning technique.
Chapter 3
Figure 3.1 Schematic diagram of electrospinning process.
Figure 3.2 Parameters that affects electrospinning.
Figure 3.3 Working principle of SEM.
Figure 3.4 SEM image of PLA/AgNO
3
electrospun nanofiber. Reproduced with p...
Figure 3.5 FESEM images of (a) PVA/PANI (1 μm), (b) Chitosan/BMIM-BF4 (300...
Figure 3.6 FESEM image of Au coated electrospun PAN nanofiber (Scale bar: ...
Figure 3.7 TEM image of PVA/PANI/Au nanoparticles. Reproduced with permiss...
Figure 3.8 HRTEM images of (a) PVA/PANI (scale bar: 1 μm) and (b) PVA-RF (...
Figure 3.9 Working principle of AFM.
Figure 3.10 (a) 2-D and (b) 3-D AFM images of PVA–Au nanocomposites. Repro...
Figure 3.11 Working principle of XRD.
Figure 3.12 Schematic diagram representing nanoindentation.
Figure 3.13 DSC curve representing PAN and PAN/SWNT (1 wt%) electrospun na...
Figure 3.14 TGA curve representing for PAN and PAN/Lavender oil nanofibers...
Figure 3.15 SEM images of PVDF fibrous membrane treated at various tempera...
Figure 3.16 Co-electrospinning spinneret setup.
Figure 3.17 (a) SEM and (b) FESEM images of PMMA nanofibers. Reproduced wi...
Figure 3.18 (a) SEM (scale bar: 5μm) and (b) FESEM (scale bar: 200 nm) ima...
Figure 3.19 SEM image of PVP/TiO
2
nanofibers after calcination (scale bar:...
Figure 3.20 SEM image of the as-spun nanofibers: (a) PEO nanofibers and (b...
Figure 3.21 STEM image of PEO containing magnetic iron-oxide nanoparticles...
Figure 3.22 (a–c) SEM images of TSC NFM at different magnifications (5 μm,...
Figure 3.23 (a) STEM and (b–e) EDX elemental images of C, O, Si, and Ti on...
Figure 3.24 FESEM images of (a) pure PVDF and (b) (19.90%) PVDF/(0.1%) ZnO...
Figure 3.25 TEM image of PVA/CNC (2 wt%) nanofibers. Reproduced with permi...
Figure 3.26 (a) FESEM and (b) HRTEM images of N-CNFs carbonized at 900°C. ...
Figure 3.27 FESEM images of as-spun nanofibers, stabilized nanofibers and ...
Chapter 5
Figure 5.1 Schematic flow chart of the production of fibrous web during el...
Figure 5.2 Electrospinning setup.
Figure 5.3 Schematic representation of the Taylor cone.
Figure 5.4 The Nanospider
™
commercial electrospinning unit.
Figure 5.5 Effect of CLZNs on lesion development on apples inoculated with...
Figure 5.6 Appearance changes of strawberries stored at 21°C. (a) Control;...
Figure 5.7 Effect of Carvacrol incorporated electrospun zein PLA films on ...
Chapter 6
Figure 6.1 Electrospinning setup for fiber drawing.
Figure 6.2 Camphor soot particle synthesis.
Figure 6.3 FESEM images of (a) electrospun cellulose fibers; (b) cellulose...
Figure 6.4 SEM images of (a) electrospun CFs; (b) cellulose embedded with ...
Figure 6.5 FESEM images of RF/PVA nanofibers as dense framework. Uniformly...
Figure 6.6 Mechanism and principle of ion imprinting.
Figure 6.7 FESEM images. (a) Chitosan NFs. (b) Ion-imprinted chitosan/RTIL...
Figure 6.8 Adsorption mechanism of thorium ions on ion imprinted chitosan/...
Figure 6.9 FESEM images of ion-imprinted camphor soot functionalized PAN n...
Figure 6.10 Possible mechanisms of RB5 adsorption by zein fibers.
Figure 6.11 Separation principle of MB–MO mixture on β-CD-based electrospu...
Figure 6.12 Oil/water separation by superhydrophobic/oleophilic Janus fabr...
Figure 6.13 Ag and Ag/Fe nanoparticle reduction on β-CD/CA fibers using UV...
Chapter 7
Figure 7.1 (a) Photographs of the centrifugal spinning (CS) technique and ...
Figure 7.2 Wetting ability of PVDF and fluorinated copolymer 15 wt% (FCP15...
Figure 7.3 (a) Schematic presentation of continuous oil/water separation e...
Figure 7.4 (a–c) Schematic presentation of gravity-driven petrol/water sep...
Figure 7.5 Three-tier TFC membrane. Reprinted with permission from Ref. [4...
Figure 7.6 (a) Schematic showing the synthetic steps for designing, proces...
Figure 7.7 Photographs of pH-controllable oil/water separation device: (a)...
Figure 7.8 Antifouling and self-cleaning behavior of the prepared T–P nano...
Chapter 8
Figure 8.1 Diagrammatic illustration exhibiting the practical applications...
Figure 8.2 (a) A photograph of a scaffold (Tegaderm, 3M Medical) prepared ...
Figure 8.3 The picture shows rabbit aortoiliac bypass procedure using a co...
Figure 8.4 Schematic representation of the concept of the anti-oxidant pac...
Chapter 9
Figure 9.1 Strategies for engineering antimicrobial properties onto electr...
Figure 9.2 (a) The basic setup for electrospinning technology and (b) the ...
Figure 9.3 The illustration for the application of electrospun nanofibers....
Figure 9.4 Electrospun nanofibers composed of (a) polycaprolactone (PCL), ...
Figure 9.5 (a) The structure of the daylight active molecules. (b) The mor...
Figure 9.6 (a) The structures and dimensions of the various cyclodextrin (...
Figure 9.7 (a) The chemical process for the modification generating GO–Ag ...
Chapter 10
Figure 10.1 Classification of gene delivery methods [16].
Figure 10.2 Advantages and disadvantages of viral and nonviral vectors for...
Figure 10.3 Experimental setup of the electrospinning process [39].
Figure 10.4 SEM images representing the structure of electrospun nanofiber...
Figure 10.5 Schematic representation of (a) coaxial electrospinning, (b) e...
Figure 10.6 Different methods of electrospinning [23, 39, 64].
Figure 10.7 SEM image of electrospun nanofibers: (a) aligned fibers, (b) r...
Figure 10.8 An overview of electrospun nanofibers.
Chapter 11
Figure 11.1 Development of the bioinspired systems by processing of natura...
Figure 11.2 SEM images of CA electrospun fiber containing beads obtained i...
Figure 11.3 Schematic representation of the desired characteristics for wo...
Figure 11.4 (a) Cellulose fiber dressings. (b) For better visualization, (...
Figure 11.5 Electrospun cellulose acetate fiber subjected to computer-assi...
Figure 11.6 (a) Human ear scaffolds based on cellulose; (b) 3D scaffolds p...
Figure 11.7 Schematic illustration of processes performed for extraction o...
Figure 11.8 Schematic presentation of the chitosan nanofibers obtained fro...
Figure 11.9 The evolution of wound healing at 1, 4, 7, and 10 days after t...
Figure 11.10 Morphological characterization of the human umbilical vein en...
Figure 11.11 (A) SEM images of randomly (a) and aligned (b) oriented chito...
Figure 11.12 Bilayered construct consisting of a PCL multiscale (micro/nan...
Figure 11.13 The inhibitory effect of chitosan/PEO/1wt% cefazolin nanofibe...
Chapter 12
Figure 12.1 Electrospinning technique.
Figure 12.2 Potential applications of smart electrospun materials in disti...
Figure 12.3 Process of tissue engineering.
Figure 12.4 Mechanisms for stimulation of electrospun nanofibers for drug ...
Figure 12.5 Mechanism of wound healing by electrospun nanofibrous mat.
Figure 12.6 Water filtration by electrospun nanofibrous membranes.
Figure 12.7 Nanofibrous membranes for air filtration [29].
Figure 12.8 Nanofibrous membranes for sound absorption.
Figure 12.9 Electrospun piezoelectric energy harvesting mechanism.
Figure 12.10 Li-ion battery for mobile phone scheming by electrospun smart...
Figure 12.11 Fabrication of electrospun CuO nanofiber FETs.
Figure 12.12 Fabric shape change in smart shape memory textiles for enhanc...
Figure 12.13 Smart electrospun T-shirts used in textile industry for monit...
Chapter 13
Figure 13.1 Schematic diagram of the polymeric jet path from the needle to...
Figure 13.2 Experimental arrangement models: (a) horizontal electrospinnin...
Figure 13.3 Cross-section of coaxial (a) drug-loaded core–shell fiber; (b)...
Chapter 14
Figure 14.1 Sorption capacity of the produced PVDF-based sorbents for the ...
Figure 14.2 Schematic diagram of the whole preparation process for ZIF-8/P...
Figure 14.3 The adsorption mechanisms of Cr(VI), Cu(II), and Co(II) on the...
Chapter 15
Figure 15.1 Diagram of a lithium-ion battery.
Figure 15.2 (a) Photographic image of the EPF membrane and MoS
2
@EPF electr...
Figure 15.3 (a) Schematic diagram of the formation mechanism of C-CVO/300,...
Figure 15.4 (a) SEM of the materials PVP/NH4VO3/H24Mo7N6O24 composite nano...
Figure 15.5 SEM image of electrospun (a) PVdF nanofibrous membrane, (b) PV...
Figure 15.6 SEM images and histograms (inset) of (a) pure PVdF, (b) TiO
2
/P...
Chapter 16
Figure 16.1 Representation of bilayer design of bead fibers and bead fiber...
Figure 16.2 Illustration of PAN–gelatin interface behavior, i.e., swelling...
Figure 16.3 Declining magnitude of emitted strand cross-sectional dimensio...
Figure 16.4 Combination of two fluorophores renders different color fluoro...
Figure 16.5 (a) Double-sided electospinning of modified cellulose acetate ...
Figure 16.6 Complex production of sandwich electrolyte membrane with elect...
Figure 16.7 Depiction of water droplet nucleation (within 5 s) and growth ...
Figure 16.8 Depicting the proportional swelling effect (Q) relation with p...
Figure 16.9 Emphasizing the inverse relation of swelling in membranes with...
Figure 16.10 Portray reversibility of membrane hydro/olephobicity and hydr...
Figure 16.11 Depiction of SEM (left) showing temperature-guided relaxation...
Figure 16.12 Depicting four temporary halts in programming electrospun mem...
Figure 16.13 Illustration of two extreme environments, i.e., cool liquid N
Figure 16.14 Illustration of 3D printed engendered non-uniform transformat...
Chapter 17
Figure 17.1 Classification of bacteria on the basis of gram stain and shap...
Figure 17.2 Classification of polymer nanofibers based on degradability.
Guide
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Scrivener Publishing
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Beverly, MA 01915-6106
Publishers at Scrivener
Martin Scrivener ([email protected])
Phillip Carmical ([email protected])
Electrospun Materials and Their Allied Applications
Edited by
Inamuddin, 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
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 processing 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 artificial 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 parameters, 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 electrospinning 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 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 electrospun 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 fabrication 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 electrospinning, 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, morphology, and functionality of nanofibers are also presented.
Chapter 3 briefly provides information about certain physical characterization 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 degradation in the field of energy and environmental applications are mentioned, 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, specifically food packaging; the science of electrospinning and parameters that 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 pharmaceutical waste (antibiotics), along with problems related to oil spills.
Chapter 7 summarizes the design, manufacturing, and recent developments 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 separation and their challenges and prospects.
Chapter 8 describes various industrial applications of electrospun materials, including the transfer of electrospun materials from research laboratories 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 harvesting, 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 prevalence of antibiotic-resistance microbes, the focus is on electrospun materials 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 applications. The role of methods in the development of electrospun materials 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 predominantly 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 technique in membrane fabrication for varied applications. Performance of the same is diversified by utilizing stimuli-responsive/shape memory materials, which react to triggers from the external environment with a widened scope in biomedical treatment, fuel cells, filtration, etc.
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.
Editors
Inamuddin
Rajender Boddula
Mohd Imran Ahamed
Abdullah M. Asiri
December 2019
1Electrospinning Fabrication Strategies: From Conventional to Advanced Approaches
J.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 porosity, 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 electrospinning 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 electrospinning, near field electrospinning, electroblowing, hybrid structures, cell electrospinning 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 challenges and future trends of fabrication electrospinning strategies will be discussed.
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 produce 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 solutions 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 spinning, with some common characteristics to electrospraying and the traditional 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 collector 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 emulsion 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 multifunction. The present chapter intends to give an overview of the fabrication 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 randomly 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, hydrophilicity, 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 process and the collector type associated with this process, which is usually a planar and static collector as a standard [15]. The collecting method of electrospun 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 properties [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 their adhesion, proliferation and growth. Thus, by reproducing the extracellular 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 demonstrated that the structures grafted with collagen potentiate their functionality, hydrophilicity and cells adhesion and proliferation. Some mechanical properties have been improved with the addition of collagen, such as elongation 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 versatile 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 incorporating 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(vinylpyrrolidone) (PVP) and hydrophobic ethyl cellulose (EC). Fibers were, also, collected 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 viability 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 activity. 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].
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 spunblowing 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 polystyrene (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 lowest 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+), cadmium (Cd2+), nickel (Ni2+), cooper (Cu2+), and dichromate (Cr2O72− ). These heavy metals are naturally present in the environment, but due to anthropogenic activity, their concentrations may exceed the desired limits, causing 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-hexafluoropropylene (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).
Figure 1.1 Scheme of the effect of graphene nanofibers on AGMD process. Reproduced with permission from Ref. [29].
Although the technique still has some challenges to overcome, such as optimization of parameters, mechanical properties and scale-up production, the easy production of randomly oriented fibers through electrospinning, 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].
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 absorbing 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 nanofibers 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 quantities, 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 applications 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 associated production challenges remain, such as method reproducibility, and large-scale production [16, 19, 20].
Figure 1.2 Diagram of production of electrospun nanofibers with (PVDF)/cellulose acetate/AgTiO2. Reproduced with permission from Ref. [34].
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 biomedical field [36–38], energy and electronics [39–42], or reinforcement in composite materials [43–47].
Electrospinning comprises two regimes of jet movement upon jet emission 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 with short or long range of stable jet electrospinning namely through near field electrospinning or melt electrospinning approaches [7, 50–52]. In both cases, the collector is moving in X and Y directions to induce filament orientation, and the process is characterized by short distances between the tip of the needle and the collector. To achieve a stable jet region for controllable deposition the average distance of near electrospinning lies between 500 µm and 3 mm and for melt electrospinning between 3 and 5 cm [53–55]. Due to the complexity of these two methodologies both will be discussed in detail in the section of advanced strategies.
A widely used approach to align fibers is the customization of the collecting setup with either parallel, rotating, or water bath collectors (Figure 1.3) [38, 56, 57].
Figure 1.3 (a) Frame electrode used for obtaining parallel fibers and scanning electron microscopy (SEM) image of parallel PLA fibers obtained. Reproduced with permission from Ref. [127]. (b) Schematic illustration of an electrospinning setup with two conducting silicon stripes separated by a void gap and SEM of the PVP nanofibers collected on top and across the gap. Reproduced with permission from Ref. [209]. (c) Drum collector and SEM of electrospun nylon nanofibers collected on the copper wires. Reproduced with permission from Ref. [66]. (d) Schematic setup of continuous production of twisted nanofiber yarn by liquid vortex and SEM image of the structure obtained. Reproduced with permission from Ref. [81].
Due to the fact of the electrospun solution is ejected at different velocities (from several hundreds of mm/s to 2 m/s) [58–62], the most common way to control the deposition pattern is through the use of a rotating collector moving at an equally high-speed rate [38, 57, 62]. Based on this, the rotation speed of the collector is known to affect the microstructure (orientation of polymeric crystals) and spatial distribution of the collected fibers [38, 63, 64]. However, thicker aligned electrospun meshes commonly present lower alignment degree and collection rate due to the repulsive effect of residual charges and the insulating effect of the deposited fiber [65, 66]. Moreover, rotating speed is inversely proportional to the diameter of electrospun fibers, i.e., increasing the mandrel speed results in a fiber diameter decrease between 15% and 40% (depending on the speed rate) [38, 57]. Nevertheless, an increment in rotation velocity increases fiber deformation, which can lead to fracture, resulting in decreased crystalline orientation within the fibers and reduction in fiber quantity and quality [62, 63, 67]. area [38]. The electrospinning jet is highly charged and sensitive to the surrounding electric field, thus aligning in certain field configurations. This can be accomplished using a rectangular collector or a parallel collector. The parallel collector consists of two grounded conducting parallel plates that are set up as the collecting target, creating an electric field that causes nanofibers to deposit and align perpendicularly to the plates across the air gap that stand between them. The void gap distance was proved to affect the degree of fiber orientation considerably as Liu and Dzenis [65] demonstrated that alignment improved substantially with the gap size increase. The electrical properties of a polymer solution affect the deposition of aligned nanofibers across conducting parallel plates [38, 57]. If the conductivity of a given solution is too low, the electrostatic forces will not be sufficient to stretch the fibers across the gap, on the other hand, if it is too high, then random whipping instability will be predominant in the jet’s motion, making it all too hard for an oriented deposition [71–73].
The application of rotating collector for the alignment of electrospun materials has been widely applied in in vitro and in vivo studies evidencing the positive effect of an aligned nanotopography in certain cells functions such as morphogenesis and differentiation, gene and protein expression, combined drug delivery and guided orientation [68, 69]. In optoelectronics, an ordered deposition of conducting/semiconducting fibers can be beneficial as a means to access and control anisotropic electronic properties [39]. Shim et al. [70] produced an arranged inorganic nanowire architecture consisting of planarly aligned TiO2 nanowire arrays comprising both uniaxially aligned and multiple layers of cross-aligned nanowire arrays with a conjugated polymer through electrospinning using a rotating collector. The nanowire architectures displayed an improvement of over 70% in power conversion than non-woven TiO2 nanowire, due to enhanced transport rate and charge collection [70].
In the electronic textiles area, Fu et al. [40] developed a portable, scalable and eco-friendly strain sensor composed of aligned cellulose acetate (CA) nanofibers with belt-like morphology and a reduced graphene oxide (rGO) layer. These researchers used a rotating collector for the alignment of the nanofibers that further functioned as a flexible substrate for graphene oxide (GO) sheets (Figure 1.4). The well-aligned CA nanofibers induced an enhanced capillary force responsible for drawing the GO solution downward, facilitating interaction between CA and GO (later rGO) and decreasing interface resistance [40].
Instead of resorting to mechanical stretching forces to align nanofibers with a rotating collector, the parallel collector technique allows the alignment of fibers through manipulation of the electric field in the collecting
Figure 1.4 (a) Schematic of the steps involved in the fabrication of RGO/CA fibrous membrane based on electrospinning and hot pressing. (b) Sheet resistance of the RGO/CA mats against hot-press time at 130°C. Inset shows that the RGO/CA mat can light up LEDs with two 1.5-V batteries in a circuit, after hot pressing for 700 s. (c) Resistance of the strain sensor shows regular change in monitoring (c) the bending and (d) the pushing, respectively. Reproduced with permission from Ref. [40].
In the nanogenerators’ field, Ma et al. [74] produced high-output piezoelectric energy-harvesting nanofibers through electrospinning of a polyvinylidene fluoride (PVDF)/zinc oxide (ZnO) nanowires solution using two grounded electrodes placed 5 cm apart. PVDF and ZnO are both piezoelectric materials, capable of converting mechanical forces into electrical energy. PVDF is a uncommon ferroelectric polymer that exhibits great promise in energy harvesting, data storage and sensing applications [75, 76]. Additionally, this polymer is biocompatible and low-cost, and imparts some flexibility to composites with ZnO, a known direct band-gap semiconductor and photoconductor, naturally brittle and hard to manipulate [74]. This study demonstrated that the maximum output voltage generated by a single hybrid PVDN/ZnO nanofiber was, reportedly, over 300% of the voltage generated by a single nanofiber of pure PVDF. Moreover, the material demonstrated higher values of electrical conductivity when exposed to ultraviolet light, serving not only has a piezoelectric material but also as a photoconductive material [74].
Another approach to assemble aligned fibers or continuous uniaxial fibrous yarns is by employing a water bath collector in the electrospinning process. Smit et al. [77] reported a method for producing continuous uniaxial fiber bundle yarns by electrospinning a polymer solution onto a water reservoir. The nonwoven web of nanofibers formed in the water surface was then drawn, with the help of a glass rod, across the surface of the water and then scooped off into the air to a rotating collector [77]. The collection liquid can be either water or another non solvent with low surface tension for example, acetone, methanol, boric acid, or ethanol have also been used [71–73, 78, 79]. A liquid system may present the potential to address the current limitations of electrospun fibers, namely their manipulation without fracturing them, obtaining a considerably high degree of alignment with a significant production rate [80]. Nonetheless, this method reveals a great difficulty for the production of nanofibers and nano yarns of water-soluble materials, giving preference to water-insoluble or coagulated materials or to the establishment of a non-solvent solution for the coagulation bath [81]. This is a dangerous and complex operation (with an intricate apparatus) due to the proximity between electricity and water [82].
Teo et al. [82] used a vortex created by a dynamic liquid system for the manipulation of electrospun nanofibers into a continuous yarn. However, their approach was unable to produce twisted nanofibers and presented some weaknesses as to maintaining the feed rate at a minimum to avoid fiber breakage would mean the increase in diameters to more than 1 µm, with large deviations [82].
Collection of electrospun nanofibers with a water bath collector is an approach that allows the production of scaffolds with large pores due to increased fiber dispersion and decreased fiber bonding [83]. This method for 3D scaffold production can be advantageous in bone and cartilage tissue engineering, due to large porosity and increased fiber thickness [84, 85]. Yang et al. [85] fabricated loose, cotton-like PLGA/PCL scaffolds using an ethanol bath that exhibited a porosity of 99.0 ± 0.2%, compared to the 79.4 ± 2.8% of porosity for conventional scaffolds. After 4 weeks of chondrogenic differentiation, cell infiltration of rat bone marrow cells (RBMCs) and plentiful cartilage matrix deposition were observed throughout the scaffolds. The cell-seeded scaffold was subcutaneously implanted into nude rats, exhibiting new bone formation through the remodeling template after 8 weeks, although without total ossification of the construct [85].
Through electrospinning, it is possible to tune many properties of the nanofibers and assemble them into aligned arrays with anisotropic mechanical properties, improved strength, Young’s modulus and toughness in the alignment direction [36, 86]. Additionally, the electrospinning technique also allows post-functionalization of nanofiber surface with bioactive molecules or ECM proteins [87]. This type of topography changes improves interactions with cells, translating in the modulation of cellular behaviors such as selective endocytosis, adhesion, migration, orientation, proliferation, matrix deposition and integrin expression [88–90]. Many tissues, such as the nervous [91, 92], cardiac [93, 94], tendon [69, 95], and vascular [96, 97] have unique anisotropic structures and architectures, exhibiting different conductivity, tensile and optical properties in different directions, whose regeneration benefits greatly with aligned nanofibers. In comparison to randomly aligned fibrous scaffolds, aligned electrospun nanofibers provide a more adequate microenvironment (in vitro and in vivo) for spatial orientation of cells and tissue regeneration [68, 98–100]. However, the overall development of oriented or aligned nanofibers with adequate topographical features remains a challenge due to the polymer jet’s trajectory which translates into a very complex 3D whipping rather than a straight line, caused by bending instability [101, 102].
In the electronics field, conductivity is a decisive property for the production of nanofibrous devices [103]. Laforgue and Robitaille [104] have demonstrated that aligned nanofibers possess higher conductivity than random ones and this property increases with the decrease of fiber diameter, which is in accordance with more recent studies [105]. This is due to the fact that aligned structures present thinner fibers, less porosity and higher surface area that translates into higher conductivity, while thicker fibers increase void space in the mesh and reduce the contact of conductive segments [103].
In contrast, conventional fiber manufacturing methods, such as dry spinning and wet spinning, allow the facile controlled fabrication of patterned and aligned fibers. These processes rely on the mechanical extrusion of fibers rather than the use of electrostatic forces, which results in fibers with bigger diameters than those obtained by electrospinning (tens of micrometers versus tens of nanometers to several microns), thus easy to align.
1.2.3 Fibers With Core/Shell Structure
The core/shell approach emerged among the most promising setups in the field of electrospinning since it is based on the combination of, at least, two different materials or substances. Using this approach, the same filament may have distinct inner and outer layers, allowing different compositions such as a material surrounded by another material or by a matrix loaded with dispersed particles [106, 107]. This design was developed to incorporate substances (e.g., drugs, enzymes, growth factors, or other biomolecules) inside the nanofibers. It presents two main advantages [108]: i) substances can be incorporated in the inner layer being protected from environmental factors, such as the organic solvents usually used in the electrospinning technique; ii) and the incorporated substance can be released from the inner layer and past the outer shell layer in a more controlled and sustained way [106
