*6.4. Conduits Containing Cells*

The e fficacy of fibrin conduits is enhanced when they are filled with autologous undi fferentiated adipose-derived stem cells [145] or contain mesenchymal stem cells [195], and when hydrogel conduits contain mesenchymal stem cells [196]. The e fficacy of chitosan conduits is enhanced when they contain combinations of fibronectin and laminin with mesenchymal stem cells (MSCs) or Schwann cells [190], chitosan/PLGA sca ffolds are combined with mesenchymal stem cells [197], and when three-dimensional alginate/chitosan conduits are filled with muscle fibers [198]. Adding PRP to the inside of silicon tubes bridging nerve gaps increases the extent of axon regeneration compared to that induced by empty silicon conduits [149,198,199]. Axon regeneration through conduits can be enhanced by adding olfactory ensheathing cells [200] or dissociated Schwann cells, which release axon regeneration-promoting neurotrophic factors, such as NGF, BDNF, NT-3, CNTF, GDNF, and cell adhesion molecules (CAMs) [175] and by building up a basement membrane [69].

Clinically, the number of axons and the distance they regenerate, are significantly increased by adding minced pieces of peripheral nerve to empty conduits [184]. Clinically, bridging a 5-cm-long radial nerve gap with two sensory nerve grafts within a pure fibrin-filled collagen tube induces excellent sensory and motor recovery across a 5-cm-long nerve gap [201].

#### *6.5. Conduit Composition and Electrical Stimulation*

Neurite outgrowth from neuron-like cells in vitro is enhanced when they are grown on nerve guidance channels composed of an electrically conductive polymer (oxidized polypyrrole) [202]. Electrically conductive biodegradable polymer composite materials enhance the rate of neurite outgrowth from cultured PC12 cells [101]. Similarly, electrical stimulation of stem cells seeded onto electrospun conducting polymer nanofibers induces neurite extension [203]. Thus, electrical stimulation of conduits composed of such material may enhance axon regeneration in vivo.
