Electrospun Nanofibers Research: Recent Developments Edited by A. K. Haghi

CONTENTS

PREFACE
Nanotechnology is revolutionizing the world of materials. The research and development of nanofibers has gained much prominence in recent years due to the heightened awareness of its potential applications in the medical, engineering and defense fields. Among the most successful methods for producing nanofibers is the electrospinning process. Electrospinning introduces a new level of versatility and broader range of materials into the micro/nanofiber range. An old technology, electrospinning has been rediscovered, refined, and expanded into non-textile applications.

This new book offers an overview of structure–property relationships, synthesis and purification, and potential applications of electrospun nanofibers. The collection of topics aims to reflect the diversity of recent advances in electrospun nanofibers with a broad perspective which may be useful for scientists as well as for graduate students and engineers. The book presents leading-edge research from around the world in this dynamic field. Diverse topics on electrospun Nanofibers published in this book are the original works of some world wide well-known scientists.

Chapter 1 – Electrospinning is one of three techniques available nowadays for the processing of fibers mimicking the extracellular environment at the nanoscale, the so-called nanofibers. This technique allows the fabrication of a controllable continuous nanofiber scaffold made of natural polymers, of synthetic polymers or of inorganic substances. Moreover, through secondary processing, the nanofiber surface can be functionalized to display specific biochemical characteristics.

This chapter will discuss/summarize in detail the currently available electrospinning techniques, recent trends on nanofiber processing and characterization and their current biomedical applications, with particular emphasis on the most recent tissue engineering applications for regenerative medicine.

Chapter 2 – Polymer non-woven mats are often considered as potential three-dimensional (3D) supports (scaffolds) for tissue engineering applications, where cells and bioactive molecules are combined with a proper scaffold to repair and regenerate damaged biological tissues. Electrospinning is a promising technology for the fabrication of nanofibrous nonwoven mats that resemble the morphological nano-features of the extracellular matrix (ECM).

For this reason, electrospun meshes are widely used as ECM-mimicking scaffolds to enhance cell-material interactions and tissue regeneration. In order to properly use electrospun scaffolds it is important to take into account on one hand the well-known problem of electrospinning process reproducibility and, on the other hand, all practical aspects related with scaffold handling and scaffold preparation for cell culture experiments. As a matter of fact, in some cases the above mentioned issues can dramatically change fibre morphology, that is known to affect viability, attachment and migration of cells seeded on the scaffold. In this chapter the reproducibility of the electrospinning process will be discussed and practical hints, concerning for example wetting procedure, scaffold sterilization, mat shrinkage, scaffold handling, etc., will be provided to tissue engineers using electrospun scaffolds in cell culturing experiments. It will be also pointed out that a proper understanding of polymeric solid state properties is required in order to improve standard operating procedures to manufacture electrospun scaffolds for regenerative medicine use.

Chapter 3 – Fiber diameter is an important structural characteristic for electrospinning process, due to its direct influence on the properties of the produced webs. In this chapter, an image analysis based method called Direct Tracking for measuring electrospun fiber diameter has been developed. Another image analysis method, Distance Transform, was also adapted to that end. In order to evaluate the accuracy of the methods, samples with known characteristics were generated using a simulation scheme known as μ-randomness. Some electrospun webs of PVA were used to verify the capability of the method for the real webs. Due to the necessity of binary input images, micrographs of the real webs obtained from scanning electron microscopy were first segmented using local thresholding. The results obtained from the methods were compared to simulation for simulated images and manual method for the real webs. For instance, in the case of the simulated image with the mean of 15.24 and standard deviation of 5.77 pixels, mean and standard deviation obtained from distance transform were 17.14 and 7.60 pixels and from direct tracking 16.25 and 6.13 pixels respectively. For an electrospun web with the mean of 246.3 nm and standard deviation of 26.0 nm, distance transform and direct tracking resulted in mean of 301.9 and 286.7 nm and standard deviation of 91.6 and 55.1 nm respectively. Results obtained by direct tracking significantly excelled distance transform, indicating that the method could be used for measuring electrospun fiber diameter.

Chapter 4 – Electrospinning of emeraldine base Polyaniline/Polyacrylonitrile (PANI/PAN) blends with different composition ratios were performed using N-Methyl-2- pyrrolidon (NMP) as solvent. The blends were electrospun at various electrospinning temperature and electric fields. Morphology and fibers diameters were investigated by scanning electronic microscopy (SEM). The average diameter of nanofibers and their distributions were determined from 100 measurements of the random fibers with image analyzer software (manual microstructure distance measurement). Electrical conductivity of the prepared mats was characterized using standard four point probe method. The nanofibers with diameter ranging from 60 to 600 nm were obtained. The PANI/PAN blends containing up to the PANI content of 30% could be electrospun into the continuous fibrous structure, although pure PANI solution was not able to be electrospun into the fibrous structure. Average of fiber diameter was decreased with increasing in PANI content and electrospinning temperature. The electrospun PANI/PAN fibers at 50 °C and 75 °C showed smaller diameters with much better uniformity than those electrospun at 25 °C. The electrical conductivity of the mats was increased with the increase of PANI content in the blend with percolation threshold of 0.5%.

Chapter 5 – An emerging technology of manufacturing of thin natural fibers is based on the principle of electrospinning process. In conventional fiber spinning, the mechanical force is applied to the end of a jet. Whereas in the electrospinnig process the electric body force act on element of charged fluid. Electrospinning has emerged as a specialized processing technique for the formation of sub-micron fibers (typically between 100 nm and 1 μ m in diameter), with high specific surface areas. Due to their high specific surface area, high porosity, and small pore size, the unique fibers have been suggested for wide range of applications. Electrospinning of natural fibers offers unique capabilities for producing novel natural nanofibers and fabrics with controllable pore structure. Current research effort has focused in understanding the electrospinning of natural fibers in which the influence of different governing parameters are discussed.