General Application of electrospun 3D Structures

Early development of electrospun three-dimensional (3D) structures are geared towards tissue scaffold where certain implants require volume and not just plain 2D mesh. With the development of process and techniques to electrospin 3D structure, potential applications for electrospun fibers significantly increases.

In most bone regeneration and soft tissue fillers applications, having a scaffold with substantial volume in terms of thickness is necessary for clinical use. Further, studies using other substrates have provided evidence that cell behavior on two-dimensional substrate and three-dimensional scaffold differs with the later phenotypic cell expression resembling in vivo behavior [Baker et al 2012]. A three-dimensional space provides a larger spatial environment for cell proliferation and several studies have shown greater cell proliferation on three-dimensional electrospun scaffold compared to two-dimensional scaffold [Blakeney et al 2011, Cai et al 2013, Lee et al 2011]. INS-1 (832/13) cells showed significantly greater proliferation on three-dimensional electrospun scaffold compared to two-dimensional scaffold [Blakeney et al 2011]. Fibroblast cultured on a three-dimensional fluffy scaffold was also shown to adopt a spherical and three-dimensional morphology with much higher level of proliferation (nearly 5 times) than on a two-dimensional membrane after 7 days of culture [Cai et al 2013].

Figure 1. Cell culture in 3D nanofibrous scaffold (left) SEM image of osteoblast (right) DAPI staining of cell nucleus.

The effect of dimensionality on cell response goes beyond proliferation. An in vitro study by Luong et al (2012) on electrospun two-dimensional and three-dimensional poly-L-lactide/collagen nanofibrous scaffold found significant difference in mesenchymal stem cell (MSC) response. Contrary to other studies, cell proliferation on the two-dimensional scaffold is significantly greater than three-dimensional (cultured in osteogenic medium). However, bone mineral deposits on three-dimensional scaffold by MSC were significantly greater and denser than the deposits on two-dimensional scaffold after 14 days [Luong 2012]. Stem cells undergoing differentiation are known to exhibit slower proliferation and thus the lower proliferation of MSC on 3D scaffold. The electrospun 3D scaffold used by Luong et al (2012) was comprised of bundles of yarns with micrometer scale diameter made of aligned nanofibers instead of single strand nanofibers and this may also yield different cell behavior.

The effectiveness of a passive fiber-based sound absorbance material involves several parameters such as porosity, tortuosity, fiber diameter, density, airflow resistance, thickness etc. and the absorbance coefficient of a material is dependent on the amount of energy absorbed by it. Electrospun 3D scaffold in particular those made out of individual nanofiber strands has the potential to be a good sound absorbance material as its high surface area and tortuous path through the thickness of the mesh exposes more material for sound absorbance while retarding airflow. However, stacking layers of electrospun nonwoven membrane to build up sufficient thickness may also be used as sound absorbent material. Petrone et al (2016) tested the performance of multiple stacked layers of electrospun PVP fibers membrane (average diameter of 2.8 µm) with minimum of 6 layers as sound absorbers in aircraft. As the layers of PVP membranes increases, the peak absorption shifted towards lower frequency (about 300 Hz) when the weight was 18 g. With 6 layers at a weight of 7g, the peak absorption is at about 700 Hz. The sound absorption coefficient was consistently above 0.8 at its peak across all samples. In contrast, glass wool fibres at 12 g showed better sound absorbance at higher frequency. Sound absorption coefficient of 12 g electrospun fiber multi-layer membranes performs better than aerogel (12 g) and polyester (10 g) across most frequency range from 300 Hz to 1600 Hz. For glass wool fibers (12 g), sound absorption starts to surpass electrospun PVP layers at frequency above 650 Hz. Given the performance of stacked electrospun membrane in sound absorbance, it would be interesting to determine the sound absorbent property of cotton-like bulk structure made of electrospun nanofibers.

Electrospun foam was also found to be useful in water-in-oil emulsion separation. In electrospun membrane, surface water contact angle was found to increase if the material used is hydrophobic. Si et al (2014) showed that electrospun PAN/BA-a and SiO₂ nanofibrous foam demonstrated a high water contact angle and superoleophilicity. The foam readily allows oil to permeate through while water droplets were retained above. Similarly, Duan et al (2015) also showed that their electrospun methylacrylate copolymers foam was able to selectively absorb oil from water-in-oil mixture.

With the combination of macropores and micropores within the foam structure, it may also be used as a depth filter. Deuber et al (2016) used a combination of short nanofibers suspension and controlled freeze drying to construct a sponge comprising of macropores from the freeze drying and micropores from the distance between nanofibers. Their preliminary study showed that the filtration efficiency increased from 91% to 99.96% when the macropores were reduced from 122.6 µm to 15.2 µm. However, the most penetrating particle size (MPPS) remained from 134 to 202 nm across the sponges with different macropore sizes.

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