Abstract
Glial scar is a significant barrier to neural implant function. Micromotion between the implant and tissue is suspected to be a key driver of glial scar formation around neural implants. This study explores the ability of soft hydrogel coatings to modulate glial scar formation by reducing local strain. PEG hydrogels with controllable thickness and elastic moduli were formed on the surface of neural probes. These coatings significantly reduced the local strain resulting from micromotion around the implants. Coated implants were found to significantly reduce scarring in vivo, compared to hard implants of identical diameter. Increasing implant diameter was found to significantly increase scarring for glass implants, as well as increase local BBB permeability, increase macrophage activation, and decrease the local neural density. These results highlight the tradeoff in mechanical benefit with the size effects from increasing the overall diameter following the addition of a hydrogel coating. This study emphasizes the importance of both mechanical and geometric factors of neural implants on chronic timescales.
Highlights
The acute response is primarily dominated by microglia and occurs over the first few weeks following implantation
These results suggest that neural implant designs could incorporate materials with lower mechanical stiffness and/or coatings that promote adhesion with neural tissue to reduce the extent of glial scar formation
polyethylene glycol dimethacrylate (PEG-DMA) hydrogel coatings were formed on 150 μm borosilicate capillaries via a UV polymerization process (Fig. 1a)
Summary
The acute response is primarily dominated by microglia and occurs over the first few weeks following implantation. Glial scar forms with the function of protecting the rest of the central nervous system from the foreign body within the tissue as well as from reactive species released following injury to the brain[5,15,16] Despite these acute benefits, glial scar formation is a key contributor to neural implant failure. Subbaroyan et al conducted simulations that suggested that a probe composed of a hypothetical soft material with modulus of 6 MPa results in a strain two orders of magnitude less than that of a silicon probe (E = 200 GPa)[27] These results suggest that neural implant designs could incorporate materials with lower mechanical stiffness and/or coatings that promote adhesion with neural tissue to reduce the extent of glial scar formation
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