Alternative Methods for Fabricating Aligned Electrospun Fibers

Various setups have been proposed for production of aligned electrospun fibers and most of these are based on mechanical rotation, electrical field manipulation on their combination. However, there are other concepts and methods of collecting aligned fibers that have been shown to be effective in obtaining highly aligned fibers. One such concept is to eliminate or significantly reduce the speed of the electrospinning jet/fiber prior to collection using a rotating drum.

Two very different approaches have been developed to reduce the speed of the electrospinning jet/fiber. The first approach uses opposing charge electrospinning jet to encourage mid-air collision of the jets from opposing ends. This significantly reduces the speed of the jets to almost a stand-still which is then brought to a rotating collector placed at a distance normal to the jet path. Cui et al (2010) were able to produce highly aligned polyvinylpyrrolidone (PVP) fibers using this technique. The same team has also constructed highly aligned inorganic bicomponent fibers using precursors of SnO₂ and TiO₂ with PVP [Xu et al 2012]. While this technique has shown to be capable of fabricating highly aligned fibers, its consistency and ease of producing large area of aligned fibers needs to be demonstrated. This technique has also been used to produce continuous fibrous yarn with larger fiber count per bundle. Given that fibers with smaller diameter are more susceptible to breakage, consistent drawing of fiber bundle comprising of just two fibers may be a challenge. If yarns comprising of highly aligned nanofibers can be stacked together in parallel, this offers an opportunity to construct a large area of aligned nanofibers.

Deposition of nanofibers on a water surface has the benefit of bringing the nanofiber to a complete stop before collection on a rotating drum. This is similar to static liquid deposition technique for collecting aligned nanofiber yarn. Studies have shown that this technique is able to construct yarn with highly aligned fibers. Shang et al (2010) was able to use this technique to construct a mesh comprising of parallel and cross-aligned fibrous poly (lactide-co-glycolide) (PLGA). Bajaj et al (2012) used this technique to successfully construct highly aligned blended Poly (amide-coimide) (PAI)/Poly (trimellitic anhydride chloride-co-4, 4'-methylene dianiline) (PTACM) electrospun fibers with a drum rotation speed of 1000 - 1500 rpm from the edge of the coagulation bath. The rotating of the drum should be sufficiently high such that the fibers do not form nanofiber bundles on the water surface prior to collection onthe drum.

In a concept whose arrangement is at first sight similar to parallel electrodes collector, a single, finite strand of fiber is ejected from the spinneret towards a target collector before it rests with one end at the collector and the other end resting on a base plate below it [Rafique et al 2007]. Continuous "shooting" of single strands of fiber and repetition of the process led to a mesh of aligned fibers. A significant difference between this and parallel electrodes collector is that continuous spinning jet is used for parallel collector system while this concept is based on repeatedly "shooting" a single strand of fiber. In this setup, aligned fibers were not collected when the spinning is allowed to proceed continuously [Rafique et al 2007]. Production of the finite fiber length is through having a solution ejection rate due to the high voltage that is faster than the feed rate. Thus the electrospinning jet will periodically break off due to insufficient solution at the spinneret. Fiber length of more than 25 cm and distributed over a lateral range of 63 cm has been constructed using this technique [Rafique et al 2007]. It is not immediately clear why continuous spinning of fiber will result in non-aligned fibers given the similarity of the setup with parallel electrodes collector.

A charged electrospinning jet will be attracted to a grounded electrode and for parallel electrodes collector, fiber alignment across a space is due to part of the continuous jet being attracted to the first electrode and another part to the other electrode. In a continuous jet, movement by other segment of the jet may disrupt parts of the same jet that is settling on the two electrodes and this may cause misalignment of the fibers. However, for individual strand of fibers within a length range, such interference is reduced as each fiber is allowed to settle across the space without a long trailing section throwing the settling fiber off course.

Stable jet electrospinning has been used for realizing precision fiber deposition. The same concept can be used in getting highly aligned fibers. To collect highly aligned poly(L-lactide) (PLLA) fibers, Zhou et al (2013) optimizes the conditions for getting a stable electrospinning jet. Their study showed that higher concentration, molecular weight and solution volatility contributed to the generation of a long stable electrospinning jet which was up to 25 cm length. This enables them to collect highly aligned fibers. Similarly, Yuan et al (2012) used ultrahigh molecular weight poly(ethylene oxide) (UHMWPEO) (Mw > 5000kDa) for blending with other polymers to form long and stable electrospinning jet. The presence of UHMWPEO raises the solution viscoelasticity and this contributes to the jet stability. Doping poly(L-lactide), polycaprolactone and chitosan solution with UHMWPEO has been shown to generate a stable electrospinning jet across a tip to collector distance of 15 cm. The resultant fiber diameters were about 1.5 µm or more. With the stable electrospinning jet, they were able to construct highly ordered structures. Unlike near-field electrospinning, given the substantial tip to collector distance, they were able to use this method to construct patterned three-dimensional fibrous scaffold [Yuan et al 2015].

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