Applications for electrospun fiber bundle

Advancement in electrospinning has given rise to various nanofibrous structures beyond simple nonwoven membrane. Electrospun fiber bundle and continuous yarn have use for many applications and twisted form generally gives better mechanical stability and consistency compared to untwisted yarns. However, it will take a significant increase in the production rate of electrospun twisted nanofibrous rope to compete with conventional twisted yarns in the textile industry. Nevertheless, such form of nanofibrous structure may be useful in other applications which demand certain functionality beyond mechanical stability. Lotus et al (2009) twisted electrospun ZnO yarn and NiO yarn together to form free standing p-n junctions. Beyond textile industry, there are many other potential applications for electrospun fiber yarn of finite length both in its twisted and untwisted form.

Electrospun fibers are regularly made from conductive materials such that they may be used in applications where its conductivity is a desirable functional property. Zheng et al (2015) constructed polyvinylidene fluoride (PVDF)/carbon nanotube (CNT) nanofibers twisted ropes using electrospinning.

Figure 1. Electrospinning setup for constructing twisted ropes [Zheng et al 2015. This work is licensed under a Creative Commons Attribution 4.0 International.]

It is interesting to note that the conductivity of twisted ropes was higher than aligned nanofibers as shown in figure 2. The improved conductivity was attributed to greater physical contact between the nanofibers [Zheng et al 2015] which may allow electrons to take the fastest path. Good conductivity may also contribute to the sensitivity of the twisted ropes to strain. A strain sensor was constructed of twisted rope fixed on a flexible insulating substrate and the ends connected to electrodes. When the strain sensor was bent, a corresponding reduction in the current was observed. After the applied force was released, the current recovered within a second. The stability of the sensor was also good after 20 cycles of testing [Zheng et al 2015]. Lin et al (2014) also showed using their electrospun twisted poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)-polyvinyl pyrrolidone (PEDOT:PSS-PVP) nanofiber microropes doped with ionic liquid (IL) to response to increasing tensile strain with decreasing conductivity in a linear manner for strain up to 35%. This tensile strain sensor was also found to exhibit repeatable strain cycle response.

Figure 2 (a) I-V curves of composite fiber bundles and twisted ropes. (b) Electric resistance [Zheng et al 2015. This work is licensed under a Creative Commons Attribution 4.0 International.]

Electrospun twisted yarn has also been tested for its wicking properties for use as a flexible absorbing material and conduit to transport small quantities of fluids. Such applications typically require relatively short strand of yarns and may be useful in flexible micro- and nanofluidic systems. Tsai and Kornev (2013) showed that the twisted electrospun yarn made of polyacrylonitrile/cellulose acetate showed permeability of 1.3x10^-13 while polyacrylonitrile yarn showed permeability of 8.8 x10^-14 which is lower than the larger Dacron fibers with diameters of about 20 µm and permeability of 1.9x10^-11. In general, as the fiber diameter decrease, so is its permeability due to smaller interfiber distance and space for liquid transportation.

Nanofibrous yarn for insertion into nerve conduits to function as contact guidance.

Natural tissue extracellular matrices (ECM) are known to be made out of collagen nanofibers. This makes electrospun nanofibers attractive for reconstruction and restoration of injured tissues and for other medical applications. Long strands of electrospun nanofibers yarn has already been tested for use as sutures [Liu 2008]. Drugs have also been loaded to electrospun nanofibers yarn for added functionality to the suture [Liu et al 2010]. Tendons and ligaments are also ideal candidates for investigating potential use of electrospun yarns as their replacement grafts. Study by Bosworth et al (2014) has already shown that human mesenchymal stem cells (hMSC) seeded on electrospun yarn demonstrated differentiation towards tendon lineage when cultured under mechanical loading. Other researchers have shown that short strand nanofibrous yarn can be used as intra-luminal contact guidance for peripheral nerve repair [Li et al 2015; Koh 2009].

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Content [Biomedical (Stent)]