A modular, plasmin-sensitive, clickable poly(ethylene glycol)-heparin-laminin microsphere system for establishing growth factor gradients in nerve guidance conduits

Abstract

Peripheral nerve regeneration is a complex problem that, despite many advancements and innovations, still has sub-optimal outcomes. Compared to biologically derived acelluar nerve grafts and autografts, completely synthetic nerve guidance conduits (NGC), which allow for precise engineering of their properties, are promising but still far from optimal. We have developed an almost entirely synthetic NGC that allows control of soluble growth factor delivery kinetics, cell-initiated degradability and cell attachment. We have focused on the spatial patterning of glial-cell derived human neurotrophic factor (GDNF), which promotes motor axon extension. The base scaffolds consisted of heparin-containing poly(ethylene glycol) (PEG) microspheres. The modular microsphere format greatly simplifies the formation of concentration gradients of reversibly bound GDNF. To facilitate axon extension, we engineered the microspheres with tunable plasmin degradability. ‘Click’ cross-linking chemistries were also added to allow scaffold formation without risk of covalently coupling the growth factor to the scaffold. Cell adhesion was promoted by covalently bound laminin. GDNF that was released from these microspheres was confirmed to retain its activity. Graded scaffolds were formed inside silicone conduits using 3D-printed holders. The fully formed NGC’s contained plasmin-degradable PEG/heparin scaffolds that developed linear gradients in reversibly bound GDNF. The NGC’s were implanted into rats with severed sciatic nerves to confirm in vivo degradability and lack of a major foreign body response. The NGC’s also promoted robust axonal regeneration into the conduit.

Biomimetic neural scaffolds: a crucial step towards optimal peripheral nerve regeneration

“…Growth factors integrated into the scaffold material can prolong localized drug availability over time. 133–136 Incorporation of various growth factors (such as NGF, 133,134 CNTF, 136 GDNF, 135 and VEGF 134 ) into the luminal wall has been studied extensively. Overall, integrating growth factors into the conduit walls may be a superior strategy compared to luminal fillings as the lumen can remain open for axonal growth.…”

3D melatonin nerve scaffold reduces oxidative stress and inflammation and increases autophagy in peripheral nerve regeneration

“…Linear ordered collagen conduit could effectively promote sciatic nerve transection repair, and the effect was further enhanced using laminin in combination with ciliary neurotrophic factor (CNTF) . Silicon scaffolds releasing CNTF‐modified poly(ethylene glycol) (PEG) microspheres, fabricated via a 3D printer, markedly improved motor axon restoration . Apart from growth factors, other agents are also promising in peripheral nerve regeneration.…”

Clickable Microgel Scaffolds as Platforms for 3D Cell Encapsulation

“…In particular, functionalizing microgels with specific differentiation factors or using microgels as drug depots could allow for a new class of cell instructive materials. Microgel scaffolds have already been synthesized with deliverable growth factors, [24,26] and the fabrication of scaffolds with multiple chemical cues could allow for the creation of complex culture environments.…”