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International Journal of Bioprinting 3D bioprinting of nerve guidance conduits

Lundborg et al. filled eight longitudinal polyamide fibers nerve conduits. In vitro studies show that their high surface
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into a silicone NGC for the repair of a 15 mm sciatic nerve area and porosity enhance cell penetration along the
defect in rats for the first time. Results showed that nerve nerve-directed channels. The small-sized gaps between the
regeneration was significantly improved compared with electrostatically spun fibers limited the migration of most
that of hollow silicone nerve conduits due to the presence of the cells to the channel walls and minimized the influx
of intraluminal fillers. However, a densely packed lumen of inflammatory cells from the surrounding tissues during
matrix was found to hinder axonal regeneration and nerve implantation. The multi-channel NGC, therefore, provided
cell migration. Several studies have demonstrated that the an area of low growth resistance, which is more suitable for
addition of fillers to hollow conduits can compensate for guiding axonal growth.
defects and promote nerve repair, but the type, structure,
and density of fillers still need further research. 2.3. Porous nerve conduit
Porous structures facilitate the infiltration of cells, the
2.2. Multi-channel nerve conduit diffusion of nutrients and molecular signals, and the
Studies have generally shown that multi-channel nerve drainage of metabolic waste products. These features
conduits are more conducive to promoting nerve repair promote early adhesion, spreading, proliferation, and
than hollow nerve conduits. First, the longitudinally aligned differentiation of SCs, as well as the formation of Bungner
lumens in multi-channel conduits can act as microtubules bands and vascularization, while reducing fibrous scar
with a large surface area required for synthesizing formation. In addition, studies have shown that the pore
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basement membranes, which has a significant positive structure can alter the degradation rate of nerve conduits by
effect on the attachment, proliferation, and migration of affecting the accumulation of localized monomers and pH.
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SCs. Moreover, the multiple longitudinal lumens within Particularly, Odelius et al. investigated the effect of pore
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the nerve conduits reduce axon dispersion and promote size on the degradability of polylactic acid (PLA) conduits
the longitudinal extension of axons. This longitudinal and the release of degradation monomer products. Their
arrangement also reduces the rate of axonal misrouting study showed that larger pore structures can accelerate
when connecting to distal nerve stumps. However, studies degradation through autocatalysis, whereas smaller
also demonstrated that additional internal structures can pore sizes lead to a slower degradation rate. Wu et al.
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affect NGC properties, such as permeability, mechanics, investigated the in vitro chemical degradation properties
and degradation. In addition, they are also structurally of 3D poly(lactic-co-glycolic acid) (PLGA) porous
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complex and difficult to fabricate. conduits, revealing that conduits with a higher porosity or
Over the past few decades, researchers have explored a smaller pore size degraded more slowly than the opposite
various approaches and fabrication methods to develop conditions, thus outlasting those with a lower porosity or
microchannels within nerve conduits, which can facilitate a larger pore size. The effects are attributed to both wall
nerve repair. In 2004, Moore et al. investigated injection thickness and surface area size because the scaffolds with
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lower porosities or larger pores possess thicker pore walls
molding and rapid solvent evaporation to fabricate nerve and smaller surface area, which depress the diffusion of
conduits with intraluminal channels. The in vivo study on acidic degradation products, resulting in stronger acid-
transected adult rat spinal cords further confirmed the 51
presence of regenerating axons 1 month after surgery. Lee catalyzed hydrolysis. Song et al. also investigated the
et al. also performed an in vivo study using arginine– degradation characteristics of porous polyester materials
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glycine–aspartate (RGD)-functionalized multi-channel using a mathematical modeling strategy. The increase in
nerve conduits in a 10-mm sciatic nerve transection porosity slows down the autocatalytic reaction and the
degradation process by enhancing the diffusion coefficient
rat model. The results showed that the axon density in of oligomers. The effect of porosity is gradually weakened
the conduit was significantly higher than that in the as further increases in porosity lead to a reduced rate of
polycaprolactone (PCL) group after 8 weeks, and the cross-
sectional area of gastrocnemius muscle fibers was restored improvement in diffusion. Numerous experimental results
to 80% of that of the healthy control, suggesting that the have also confirmed the importance of porous structures
conduit is close to the autograft in terms of structure, on the physicochemical and biological properties of
function, and muscle reinnervation. Alternatively, Wang nerve regeneration and repair, leading to their increased
widespread application in the design of nerve conduits.
et al. used electrostatic spinning to prepare a multi-
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channel NGC with directional arrangement of fibers. In Nonetheless, porous nerve conduits possess certain
vivo experiments demonstrated that the conduit had a good drawbacks, including the presence of large pore sizes
guiding effect on nerve repair. Additionally, Jeffries and (normally >200 μm) that can result in fibroblast
Wang used electrospinning to fabricate multi-channel deposition, thereby hindering axonal growth. Ideal
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Volume 11 Issue 4 (2025) 38 doi: 10.36922/IJB025140120