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What Is Next for Electrospinning?

2020; Elsevier BV; Volume: 2; Issue: 2 Linguagem: Inglês

10.1016/j.matt.2020.01.004

2590-2393

Mike Tebyetekerwa, Seeram Ramakrishna,

Advanced Sensor and Energy Harvesting Materials

Electrospinning is a powerful and scalable synthesis technique capable of producing spun polymer fibers across a range of diameters from nano to micro. It has been extensively studied and successfully implemented for over 100 years. But what does the future hold for electrospinning methods and its products? Electrospinning is a powerful and scalable synthesis technique capable of producing spun polymer fibers across a range of diameters from nano to micro. It has been extensively studied and successfully implemented for over 100 years. But what does the future hold for electrospinning methods and its products? Since the first reports of “electrical spinning” by C.V. Boys1Boys C.V. On the Production, Properties, and some suggested Uses of the Finest Threads.Proc. Phys. Soc. Lond. 1887; 9: 8Crossref Scopus (53) Google Scholar in 1887, the subsequent patents of Cooley and Morton2Cooley J.F. Apparatus for electrically dispersing fluids. Google Patents, 1902. W.J. Morton, Google Patents, 1902Google Scholar in 1902, and the growing importance of nanotechnology in the 2000s, the electrospinning technique has received enormous attention both from science and engineering communities. Electrospinning (ES) is sometimes referred to as electrostatic spinning or electro-hydrodynamic spinning, and these terms are used interchangeably. Simply put, in ES, an electric force is used to draw charged threads of polymer melts into fibers. It is closely related to electrospraying except that in electrospraying, particles are collected whereas in electrospinning continuous or non-continuous uniform and non-uniform fibers are obtained. Move over wool and cotton, the era of plastic textiles has arrived! Using this technique, a wide range of fascinating fibrous materials ranging from nano to micro realms have been produced, investigated, and explored for use in various fields, thus contributing to the emergence and development of nanotechnologies. Electrospun materials have found real-life uses in biomedicine, energy, environmental applications, space exploration, food, water, and agriculture, to mention but a few. Indeed, many commercial products have long been available for sale on the market, including Smart Mask (from NASK, nask.hk), cell culture dishes (Sigma Aldrich), FERENA filtering units (KOKEN LTD, http://www.koken-ltd.co.jp/), ReBOSSIS synthetic bone (Ortho ReBirth, orthorebirth.com), Pest Control (fiber trap, fibertrap.com), and many others. Over time, beyond simple variation and/or substitution of constituent polymers, the basic ES technique has been profoundly revolutionized by various research groups and companies. The primary focuses of redesign were the collectors and nozzles, which can be tweaked to change the morphology, alignment, fiber diameter, porosity, and the overall output of the resultant fibrous materials. As an essential ingredient of the setup, polymers are widely involved in all processes of fiber formation during ES. Many spinnable polymers with the right viscosity in solution are now available, and they can be virginly spun in desired fibers or can be blended with the non-spinnable materials such as metals, metal oxides, or carbon particles to obtain composite fibers. What is next for science to achieve with regards to ES? 1. The pre-solid. ES is one of the most highly studied methods in material science, and this implies that much is already known about the technique. It is worth noting that one of the defining mechanisms of ES—the actual process of fiber formation in the fast acceleration zone—is not fully understood. Fiber formation during ES is widely described to occur within two major flight zones, i.e., a slow acceleration zone near the solution side and a fast acceleration zone closer to the collector side (see Figure 1A). Most importantly, the works of the Reznik and Hohman groups theoretically described the mechanisms, Taylor cone formation, and effect of the electric field on the bending instability of the electrically forced jet before fiber formation.3Hohman M.M. Shin M. Rutledge G. Brenner M.P. Electrospinning and electrically forced jets. II. Applications.Phys. Fluids. 2001; 13: 2221Crossref Scopus (660) Google Scholar,4Reznik S.N. Yarin A.L. Theron A. Zussman E. Transient and steady shapes of droplets attached to a surface in a strong electric field.J. Fluid Mech. 2004; 516: 349-377Crossref Scopus (184) Google Scholar Also, Dzenis’s group studied solvent evaporation from electrospun polymer jets and presented results critical to predicting the drying and the resultant nanofiber inhomogeneity.5Wu X.-F. Salkovskiy Y. Dzenis Y.A. Modeling of solvent evaporation from polymer jets in electrospinning.Appl. Phys. Lett. 2011; 98: 223108Crossref Scopus (71) Google Scholar Reneker’s group also reported essential findings on the dynamics of hollow nanofiber formation.6Guenthner A.J. Khombhongse S. Liu W. Dayal P. Reneker D.H. Kyu T. Dynamics of hollow nanofiber formation during solidification subjected to solvent evaporation.Macromol. Theory Simul. 2006; 15: 87-93Crossref Scopus (70) Google Scholar However, these studies were limited to Zone 1, because Zone 2 is a fast acceleration zone and very hard to study with existing (traditional) techniques. The actual dynamics of how and what happens to the polymer are just predictions with no sound scientific understanding to date. Once we clearly understand what happens in this high-voltage transition fiber forming zone, it means we can control the most critical fiber properties like size, orientation, texture, and shape. Also, factors like the voltage, collector distance, and solution viscosity can be predetermined before industrial or experimental setup with ease. This can guide us to obtaining fibers of any required properties with optimum parameters in a novel way. 2. At solid state. During the ES process, polymeric nanofibers are subjected to elongation as they travel from the source dope to the collector through the high-electric-field zone. This scenario tends to give resultant fibers a stretched non-equilibrium state. Depending on the polymer species, the concentration, and the spinning parameters, the nanofibers are expected to undergo either elastic or plastic deformation (or even a combination of both). To understand and quantify these scenarios is a significant challenge both theoretically and experimentally but is critical for controlling the behavior of the resultant nanofibers and/or later using them suitably in various engineering devices and applications. Only Zussman’s group has attempted to estimate the degree of nanofiber stretching caused by ES.7Vasilyev G. Burman M. Arinstein A. Zussman E. Estimating the Degree of Polymer Stretching during Electrospinning: An Experimental Imitation Method.Macromol. Mater. Eng. 2017; 302: 1600554Crossref Scopus (11) Google Scholar In their work, they experimentally determined the stretching of a polyurethane segmented block copolymer with reference to stretched cast polyurethane films. According to the authors, this was possible because of the similarity in the behavior of polyurethane nanofibers and stretched cast films, which both show massive contraction upon heating. However, it is worth noting that this strategy is limited to similar kinds of polymers. Here, we suggest introducing material species into an ES dope whose properties can be dictated by their strain or stress or electronic states behaviors. For example, 2D monolayer transitional metal dichalcogenides (TMDs) are known to have strain-dependent electronic structures and optical properties. This can be tracked within the nanofibers to predict the extent of strain, stress, and many other properties across various spinning parameters as shown in Figure 1B. The results can later be compared to known or simulated data. ES-induced strains and electrical/electronic states in final fibers, if ably controlled, can be intentionally induced to give new materials with various properties and applications. This is indeed the next big thing in this field. In the recent past, ES has also been utilized as a platform to obtain materials beyond polymers/plastics, including novel carbon materials (through polymer carbonization), inorganic particle-decorated functional fibers, and multimetallic alloys with excellent functionalities (Figure 2A).8Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (666) Google Scholar,9Radacsi N. Campos F.D. Chisholm C.R.I. Giapis K.P. Spontaneous formation of nanoparticles on electrospun nanofibers.Nat. Commun. 2018; 9: 4740Crossref PubMed Scopus (55) Google Scholar Just recently, Yao et al.8Yao Y. Huang Z. Xie P. Lacey S.D. Jacob R.J. Xie H. Chen F. Nie A. Pu T. Rehwoldt M. et al.Carbothermal shock synthesis of high-entropy-alloy nanoparticles.Science. 2018; 359: 1489-1494Crossref PubMed Scopus (666) Google Scholar employed electrospun nanofibers as the platform to alloy up to eight disparate elements into single-phase solid-solution nanoparticles (Figure 2B). When used as ammonia oxidation catalysts, their novel-obtained materials showed extraordinary performance of ∼100% conversion and also showed >99% nitrogen oxide selectivity, even for extended usage. Can ES be pushed further into the metallic world and attain pure metal spinning to obtain high-performance nanometals, i.e., ES to obtain nano- to microfiber metals, ceramics, and alloys with different properties? Currently, there exists modern processing techniques that are capable of producing polymer-free ceramic and small diameter glass fibers with an average size of ∼0.2 μm (Figure 2C)10Quintero F. Dieste Ó. Penide J. Lusquiños F. Riveiro A. Pou J. Nonconventional Production of Glass Nanofibers by Laser Spinning.J. Am. Ceram. Soc. 2014; 97: 3116-3121Crossref Scopus (5) Google Scholar and alloyed fibers close to 10 μm in diameter (Figure 2D). These materials have unique properties hardly found in the same materials at millimeter scale and on 3D sheets—for example, flexibility, the high surface-to-volume ratio, electron mobility, and many others. There is a high possibility of pushing new, novel properties in materials with very low dimensions if this is attempted. A perspective schematic has been given for the possible ways to utilize ES together with the existing technology of laser spinning to obtain new materials of very low dimensions and better orientation. (Figure 2E) Engineered electrospun nanofibers can be adopted for a wide range of applications, from energy and environment to medicine and health to food and agriculture. Given the pressing demands of the circular economy or zero waste vision, new material design is paramount. ES, being one core technique in the nanotechnology realm, needs to be able to give such materials that can be used in a wide range of applications and be high-performance, recyclable, or reusable if possible. Most of these named applications require finer fibers but with functional properties. Purely organic or polymeric fibers might not be able to fit in such a challenging category. However, a better material design through optimization techniques can at least guarantee most of these properties in individual nanofibers (Figure 3), e.g., multimetallic-polymeric nanofibers. Electrospun materials are also needed in the emerging new world because of their many properties, such as guaranteed lightweight, flexibility, self-healing, and high surface area on a scaled-up production. The issue of reproducibility, if tackled by understanding the actual behavior of fiber formation as suggested in the previous sections, can contribute to material homogeneity. Currently, many electrospun nanofibers’ applications are focused on as-spun membranes. There is a dynamic shift toward post-treating the nanofibers for new applications, but this is at the infant stage and needs systematic approaches. An example is carbonization of nanofibers to form highly conductive carbon nanofibers. These have intensively found use in energy and electronic applications because of their conductive nature. Another strategy is carbothermal shock synthesis, in which electrospun nanofibers are used as a platform to form multimetallic materials with unique and exciting properties for catalysis, sensors, energy, and more. Post-ES has also enabled incorporation of enzymes and other biological species into the nanofibers, which find applications in biomedical and health-related applications. But beyond these properties brought about by post-ES, there is a hidden lot of features in as-electrospun nanofibers, owing to the fact that the spinning parameters themselves change the resultant nanofibers’ properties, the induced electrical and electronic states in solidified fibers, the on-collection medium functionalization/polymerizations which can happen in the solid/solid interfaces or solid/liquid or solid/air interfaces between the solidified fibers and collector medium (air or water or solid). With the right materials, optimized spinning parameters, and carefully selected spinning medium, it’s possible to obtain all these properties in new electrospun materials with new applications. This is the emerging future of ES. Despite the productive research output and general interest in ES fabrication and its resultant materials, there is a lot left to be done. The next wave of this is posed to hinge mostly on three factors: (1) better understanding of the nanofiber formation and formed nanofibers, (2) obtaining of a new class of nanofibers (metallic and hybrids), and (3) engineering the ES technique to get novel materials for a wide range of applications (through post-treatment and the like). The ongoing development of ES will likely continue for the next 100 years. M.T. acknowledges the research support of the Australian Government Research Training Program (RTP) Scholarship.