Introduction to Electrospun peripheral nerve regeneration scaffold

Fluorescent microscopy micrographs of RT4-D6P2T Schwann cell lines cultured for 1 hour on electrospun matrices of nanofiber [Tsai et al 2014]

Material and structural versatility has made electrospinning an attractive process for manufacturing peripheral nerve regeneration scaffold. The fibrous porous scaffold produced by electrospinning has been compared with porous scaffold produced by lyophilization for Schwann cells (SCs) growth. Using chitosan as the model polymer in a comparative study, Wu et al (2021) showed that both structures were able to support growth of Schwann cells (SCs) . The pores on the electrospun membrane were much smaller than the scaffold produced by lyophilization. SCs on the electrospun membrane were spread out across the surface of the membrane as the small pore size prevented the SCs from penetrating into it. On a lyophilized scaffold, the SCs form clusters within each pore. Proliferation and adhesion was found to be higher on electrospun scaffold which may be due to greater similarity of fibrous electrospun scaffold to natural extracellular matrix (ECM) and better cell-cell communication. Greater amount of brain-derived neurotrophic factor was also secreted by SCs cultured on electrospun scaffold compared to lyophilized scaffold. Therefore, the fibrous electrospun scaffold may be more suitable for neural regeneration. Beyond the wide range of materials that can be electrospun, drugs, growth factors and other additives may also be incorporated into the electrospun fibers. In terms of physical form, the fibers can be constructed with diameters from nanometer to micrometer scale. At a higher level, the fibers may collect as nonwoven mesh, aligned mesh, yarn or tubular scaffold.

A wide range of materials may be used for electrospinning depending on the required property. Man-made biodegradable polymers such as poly(lactic acid) has been electrospun with bioactive molecules such as peptides [Schaub et al 2015] collagen [Koh 2009] and laminin [Leach et al 2013] added for better biocompatibility. Various growth factors have also been blended into electrospun fibers to improve its performance [Dinis et al 2014]. Han et al (2022) prepared electrospun poly(3(S)-methyl-morpholine-2,5-dione-co-lactone) (PDPLA) fibers blended with deferoxamine (DFO) in the form of a nerve guidance guidance conduit. DFO is an iron chelator in clinical practice to relieve iron overload. Nerve injuries and subsequent Wallerian degeneration causes iron overload at the distal end and this causes stress to cells leading to their death. PDPLA only fibers may increase iron overloading with overexpression of HO-1. However the presence of DFO in PDPLA was able to bring the expression of HO-1 to normal. PDPLA loaded with DFO was also found to effectively reduce endoplasmic reticulum (ER) and mitochondrial stress. In vivo tests of the PDPLA/DFO conduit in a rat sciatic nerve injury model showed recovery of nerve function that is close to that of autograft after 4 weeks. To imitate electrical signaling in natural nerves,conductive [Ghasemi-Mobarakeh et al 2009] and piezoelectric [Biazar et al 2013] materials have also been electrospun and the resultant scaffold has been shown to be suitable for nerve regeneration.

Electrospun membrane has been shown to support the growth of a variety of nerve cells such as Schwann cells and neuronal stem cells. To investigate the benefit of co-culture system for neural regeneration, Fan et al (2017) co-cultured neural stem cells (NSC) and activated Schwann cells (ASC) on electrospun polycaprolactone (PCL) fibers with average diameter of 8 µm. With only NSC on electrospun PCL scaffold, most of them differentiated into astrocytes, some into neurons and a few into oligodendrocytes after 7 days. With co-culture system, ASCs were found to express myelin basic protein (MBP) while NSCs differentiated into neurons. Neurons were found to be attached to ASCs expressing MBP which supported axon growth. The co-culture system also produced more extracellular matrix.

Several studies have shown that cells cultured on aligned electrospun fibrous substrate, exhibit contact guidance. This is particularly useful in neural engineering where the suitability of the scaffold is determined in part from the length neurite extension. In highly aligned nanofiber substrate, neurite outgrowth can be as much as 20% longer than that on random fibers [Corey et al 2007]. Schwann cells were also shown to elongate along the direction of fiber orientation [Corey et al 2007, Gnavi et al 2015]. When compared to film matrix of the same material composition, aligned nanofibers comprising of carbon nanotube/polycaprolactone/gelatin showed greater NRG1 and P0 protein expression levels in Schwann cell [Tsai et al 2014].

The versatility of electrospinning has also allowed researchers to experiment with different electrospun nerve scaffold assemblies and check its ability to promote axon growth. Simple empty tube can be easily electrospun to bridge transected nerve gap. Different forms of electrospun guidance channel have also been constructed and shown to facilitate axon growth. These include sheets [Clements et al 2009], yarns [Koh et al 2010] and smaller diameter tubes [Dinis et al 2014].