Characterization of poly(ethylene glycol) gels with added collagen for neural tissue engineering

Over the past decade, it has been increasingly recognized that both chemical and mechanical properties of scaffolds influence neural cell behavior, ranging from growth to differentiation to migration. However, mechanical properties are difficult to control for in the design of scaffolds for nerve regeneration, as properties change over time for most biologically derived scaffolds. The focus of this project was to examine how the mechanical properties of a nondegradable scaffold, poly(ethylene glycol) (PEG) gels, influenced nerve cell behavior. Low concentration PEG gels, of 3, 4, or 5% PEG, with added collagen to alter chemical properties were examined for both their mechanical properties and their ability to support nerve expression and extension. Stiffness (G*) significantly increased with increased PEG concentration. The addition of chemically conjugated collagen significantly decreased the stiffness compared to plain gels. This phenomenon was confirmed to be an effect of the conjugate, and not the protein itself, as G* of gels containing conjugate, but no protein, was not significantly different than G* of gels with conjugated protein. PC12 cell neurite expression increased with decreasing PEG and increasing collagen concentration. At its best, the expression approached the value on collagen-coated tissue culture plastic, which is a substantial improvement over previous studies on PEG. Neurite extension of dorsal root ganglia was also improved on these same gels over gels with either higher PEG concentration or lower collagen amount. Overall, these results suggest that exploration of lower stiffness materials is necessary to improve neurite growth and extension in three-dimensional synthetic scaffolds.