*6.1. Conduit Composition*

Regardless of their composition, empty conduits longer than 1 cm in length, generally induce poor, if any, axon regeneration. However, conduit composition is important. Silicon conduits induce axon regeneration [149,150] but can have the complication of rigidity, which can result in significant chronic nerve compression and irritation at the implantation site [151]. In this case, they may have to be removed, which endangers any recovered neurological function. Therefore, it is advisable to use a fully degradable matrix that does not negatively affect axon regeneration [152].

Axon regeneration is extensive through conduits composed of fibrin [145,153], hydrogel tubes [154], alginate/chitosan polyelectrolyte [155–157], poly epsilon-caprolactone [158,159], polyglycolic acid [160], poly(lactic-co-glycolic acid) or silk-based [161–164]. Conduits composed of decellularized human umbilical artery [165] and muscles [166] are also effective in promoting axon regeneration across nerve gaps.

Veins are promising as conduits [167,168]. Clinically, empty vein conduits appear to have a 3-cm-long limit for their ability to induce axon regeneration [169]. A meta-analysis of published papers determined that for nerve gaps up to 4 cm in length, vein conduits did not induce any significant improvement in sensory recovery outcome compared to conduits of other materials [170]. However, the axon regeneration-promoting e fficacy of vein conduits is enhanced when they are filled with PRP [171–173], muscle [174], pre-degenerated muscle [175,176], muscle seeded with neural-transdi fferentiated human mesenchymal stem cells [177], and minced peripheral nerves [178]. One study in sheep showed that using the median epineural sheath as a conduit can restore median nerve function across 6-cm-long nerve gaps [179].
