Methods of integrating electrospun fibers with other parts

Electrospinning may be used to produce free-standing nanofibrous products or as part of a hybrid material. Due to the slow volume buildup of nanofibers, commercial application of electrospinning and its nanofibers are generally geared towards hybrid material where electrospun nanofibers are combined with other materials or used as a coating. This strategy has been used successfully in air filter media where electrospun nanofibers form a thin layer over a supporting nonwoven substrate. This combines the enhanced filtration performance from the nanofibers while the substrate takes on a supporting function. Other applications that may potentially benefit from this arrangement includes sensor, water filtration media, facemask, composite and implants.

The most straight forward method of integrating electrospun fibers on the substrate material is by direct fiber deposition. The manufacturing process typically involves passing the fabric under electrospinning jets such that a layer of nanofibers are coated on the surface. For filtration, a supporting substrate is typically required for holding a thin layer of electrospun nanofibers. It is also common to directly electrospin fibers on the supporting substrate since it is usually made of soft fabric. Adomaviciute et al (2011) studied the effect of different support materials (polypropylene, polyethylene, polyethylene terephthalate, aluminum, acetate fiber, paper) and grounding electrodes (drum, wire or support material 40 mm from wire electrode) located behind the supporting material on the collection of polyvinyl alcohol (PVA) using free surface electrospinning (Nanospider from Elmarco). It was found that the type of grounding electrodes have a greater influence on the number of fibers collected than the support material with the drum electrode having the most fiber collected. This is probably due to greater potential difference generated between the solution surface and the wider surface area of drum grounding electrode. Direct electrospinning of fibers is also suitable as coating for uneven surfaces. In medical applications, electrospinning has been tested for stent coating and bone implant coating. Direct electrospinning will be able to apply a continuous coating of nanofibers on the surface with relatively good uniformity. Another advantage of this method is that it eliminates the need for a transfer substrate which increases contamination risks. While application of fibers through direct electrospinning has several advantages, there are some cases where it is necessary or beneficial to use a transfer substrate.

Direct coating may also have the advantage of enhancing performance compared to breaking the fibers into short strands prior to coating. Conventional method of coating sensor substrate with inorganic fibers involved using short strand fibers. However, there are several advantages if intact fibers are used instead. In gas sensor application, an intact fiber network allows better gas diffusion through the pores between the fibers. With an intact fiber network, the connections to the two electrodes are better as the long fibers are better able to bridge the gap. In contrast, electron transport may be inefficient in a fractured network as there are numerous joints, points of poor contact and dead end paths. Ning et al (2021) constructed a H₂S sensor by directly coating a ceramic tube with CuO-doped SnO₂ precursors using electrospinning and sintering to form CuO-doped SnO₂ fibers. By direct coating of the sensor part using electrospinning, it avoided the need for pulverizing the fibers for coating. Comparing the sensors constructed by in situ electrospun fibers on the ceramic tube carrier and coated with pulverized electrospun fibers, the former showed significantly better performance with faster response and recovery.

Using a transfer substrate to collect electrospun fibers for later application on the product have some distinct advantages. The collector for the electrospun fibers may be selected to maximize fiber production and quality. This may improve the overall production and quality of the final product. Xu et al (2016) found that roll-to-roll transfer of electrospun nanofiber film onto an air filter substrate demonstrated better filtration performance than air filter membrane using direct electrospun fiber deposition. In other cases, the electrospun fibers may require further processing before it is applied to the next substrate. This may apply to inorganic fibers where the precursor fibers need to undergo thermal processing for conversion into its inorganic form and the required substrate is unable to withstand high temperature. In this case, a transfer substrate suitable for conversion is used and subsequently transferred to the final product after the necessary post-electrospinning treatments. A transfer substrate is also useful for off-site fabrication of nanofibers prior to integrating with other parts. This may be found in applications such as composite interleaves where a layer of electrospun fibers membrane is transferred onto a composite pre-preg as the composite layers are built up. Such usage can be found commercially as Xantu.Layer from RevolutionFibres.

The transfer substrate need not be a holding material where the fibers are kept in storage while waiting to be transferred. The transfer substrate is just an intermediate collector where fibers are deposited and immediately transferred onto the next material in a single process flow. An example of this scenario will be a large rotating metal drum collector for fiber collection on one side and the deposited fibers are immediately transferred onto a filter substrate at the other side. The filter substrate may be pressed onto the rotating metal drum collector so that the fibers are pressed against the filter substrate for better adhesion.

Electrospun fibers may be post-processed to form short strand fibers which may be mixed with other material to form a paste or as coating. Conventional methods of mechanically shortening of nanofibers such as grinding [Fujihara et al 2007], milling [Krysova et al 2013] and cutting [Jiang et al 2013] have been employed for this purpose. Generally, the nanofiber needs to be sufficiently brittle to fracture under mechanical loading instead of undergoing stretching. Grinding and milling has been successfully used in making inorganic nanorods from electrospun fibers [Fujihara et al 2007, Krysova et al 2013]. Grinding and milling is generally fast but control of fiber length is limited. Organic nanofibers are generally too ductile for conventional mechanical cutting. However, Jiang et al (2013) was able to cut polyimide (PI) nanofibers by immersing the mat in liquid nitrogen. Although cutting offers a more controlled way of managing the fiber length, this process is slower than grinding and milling. Given the limitation of shortening organic nanofiber by conventional mechanical means, other nonconventional mechanical methods have been tested.

Short strand fibers have been used in various applications by mixing with suitable carrier matrices. Lee et al (2009) ground electrospun Fe nanofibers into short fibers for mixing with an epoxy resin. The epoxy resin with short Fe nanofibers may be applied as a coating on other surfaces for EMI shielding purposes. Li et al (2014) constructed a biosensor by mixing short strands of electrospinning derived carbon nanofibers with Nafion and laccase before loading onto glass carbon electrodes. The performance of the sensor compares well to other reported laccase-based biosensors and may be attributed to the presence of electrospun-derived carbon nanofibers which enhanced the conductivity of the composite and hence a faster electron transfer.

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