Advances in Electrospun Functional Nanofibers

Abstract

It has been a great pleasure to serve as the guest editor of this Special Issue entitled “Functional Nanofibers”, which covers all major aspects of organic and inorganic nanofibers, with a particular focus on synthesis by electrospinning. Electrospinning has been recognized as one of the most efficient techniques for producing nonwoven fiber webs on the order of several hundreds of nanometers by electrically charging a suspended droplet of polymer solution with or without inorganic precursors or melts. Various types of materials with high degrees of porosity, large surface areas, superior mechanical properties, and modified surface functionalities, can be electrospun into nanofiber structures. These materials include polymeric nanofibers as well as metallic and metal-oxide nanofibers, which are prepared by subsequent heat treatment in a reducing or oxidizing atmosphere of metal salt precursor/polymer composite fibers. There are several key parameters, such as electric field strength, solution feeding rate, polymer solubility/viscosity, surface tension, dielectric constant, and the needle diameter, which have great influence on the phase, microstructure and morphology of the resulting electrospun fibers. Therefore, careful control of the processing parameters is critical in the exploration of a variety of nanofiber materials with unique microstructures, such as hollow tubes as well as polycrystalline, porous, and core–shell structures. The simplicity of electrospinning combined with the possibility of large-scale production through the use of multiple nozzles (>10 000) or cylindrical drums makes this process very attractive and therefore opens up new commercial markets for diverse applications. Thus far, electrospun functional nanofibers have served as highly optimized and versatile material platforms for a broad range of applications, such as for the filtration of liquids and gases, chemical sensors, nanofiber reinforced composites, active electrode materials for electrochemical cells, and photo-electrodes. In the biomedical field, they find application in enzyme immobilization, wound dressing, and tissue engineering based drug release. This dedicated Special Issue represents a collection of advances focused on the synthesis, characterization, and utilization of electrospun functional nanofibers. Persano and co-workers give a comprehensive review of the industrial up-scaling of electrospinning and potential applications of polymer nanofibers.1 They describe the perspectives for scaling up, technological weaknesses and strengths, and expected challenges to be faced by businesses in the electrospinning field. Cho and co-workers provide a detailed review of organic nanofiber-based electronics and photonics.2 Applications of organic nanofibers in organic photovoltaics, field-effect transistors (FETs), lasers, waveguides, and organic light-emitting diodes are highlighted in their contribution. Camposeo and co-workers give an in-depth review of the processes and properties of electrospun light-emitting nanofibers, which are especially optimized for applications in nanophotonics and optoelectronics.3 Three contributions deal with chemical, pressure, and wearable sensors. Choi and co-workers investigate the facile synthesis of p-type quaternary SrTixFe1-xO3 nanofibers and their excellent oxygen-sensing properties, which could be potentially used in automotive exhaust systems due to their ability to monitor oxygen concentration change at relatively low temperature.4 Ren and co-workers illustrate the promising development of flexible pressure sensors using P(VDF-TrFE) electroactive polymer nanofiber webs.5 Scalia and co-workers report the synthesis of liquid-crystal-functionalized, core–sheath fibers via coaxial electrospinning and their novel optical properties and strong response functions, which are useful in wearable sensors.6 Electrospun nonwoven membranes, which comprise a major portion of the scaffolds being studied for tissue engineering as well as various filters, have received significant attention with regard to their practical applications. Troung and co-workers demonstrate for the first time the use of electrospun gas diffusion nanofiber membranes with a hydrophobic nature for the accurate determination of dissolved CO2 concentration in water.7 Zhang and co-workers report the facile fabrication of aligned fiber scaffolds for nerve tissue engineering.8 Shin and co-workers illustrate how an electrospun fibrous mesh can be combined with a cell-adhesive peptide via a bio-inspired immobilization process,9 and Haynie and co-workers describe a model for the stability of electrospun fiber mats made of polypeptides in aqueous media.10 Son and co-workers introduce a functional silver–polydopamine coating of electrospun polymer nanofibers through the oxidation of dopamine and the related reductive potentials for metal ions.11 Hyun and co-workers describe a new method for preparing mesoporous polymer–nanoparticle composites containing a large amount of magnetic nanoparticles.12 Although the three review articles and nine full papers in this Special Issue cover very important aspects related to electrospun nanofibers, other interesting scientific challenges related to functional nanofibers and their practical use in various fields remain to be explored. I am grateful to all of the contributing authors and coauthors, and to the reviewers for their invaluable contributions. I deeply thank Dr. Stefan Spiegel, Editor of Macromolecular Materials and Engineering, for providing this opportunity to assemble key articles for this Special Issue and to the journal staff for their valuable assistance. I hope that this Special Issue will be beneficial to the readers who are working within or who plan to join the rapidly developing field of electrospinning. Il-Doo Kim received his PhD degree (2002) from Korea Institute of Science and Technology (KAIST). From 2003 to 2005, he was a postdoctoral fellow with Prof. Harry L. Tuller at MIT. He then returned to KAIST as a senior research scientist. In Feb. 2011, he became a faculty member in the Department of Materials Science and Engineering. Dr. Kim's current research emphasizes controlled processing and characterization of functional nanofibers via electrospinning for practical applications in exhaled breath sensors and energy storage devices.