Although collagen is commonly used as components of tissue-engineered nerve-guidance channels, little is known about the effect of the mechanical properties of commonly used gel concentrations on the extension of neurites. This study focused on neurite extension of dissociated chick dorsal root ganglia in vitro over a range of collagen concentrations (0.4-2.0 mg/ml). Neurite length increased in all gels between day 1 and day 4, except at the highest collagen concentration, where a 9% decrease was noted at day 4. Although maximum neurite extension was seen in lower concentration gels (0.6-0.8 mg/ml), mechanical stiffness of each gel significantly increased with increasing concentration, from 2.2 Pa at 0.4 mg/ml to 17.0 Pa at 2.0 mg/ml. A previous model of mechanical stiffness versus neurite outgrowth did not fit this data well, likely because of interactions between the growth cone and the collagen fibers. Overall, these results provided insight regarding factors that influence neurite elongation and may be utilized to further optimize tissue-engineered scaffolds.

The goal of this work was to investigate methods to retain bioactivity of nerve growth factor (NGF) after encapsulation in poly(lactic-co-glycolic acid) (PLGA) discs for controlled release. Poly(ethylene glycol) (PEG) was chosen as a porogen not only to control the release rate of NGF but also because it has been used to help maintain bioactivity of molecules in organic solvents. NGF and PEG were encapsulated in PLGA via standard dissolution-evaporation techniques with methylene chloride as the solvent. Morita et al. (Pharm Res 2000; 17:1367-1373) indicated that colyophilization of PEG and bioactive factors before exposure to organic solvents increased the retention of bioactivity. Therefore, various PEG:NGF mass ratios were colyophilized before encapsulation within PLGA to examine retained activity of NGF. When PEG was not colyophilized before encapsulation, NGF activity was lost during the fabrication process. In contrast, colyophilization of PEG and NGF supported retention of NGF activity during the entire fabrication process. The amount of PEG encapsulated was the dominating factor in the rate of NGF release regardless of the fabrication method. These results demonstrate the usefulness of PEG in both acting as a porogen to modulate release and aiding in the retention of activity of NGF. This process may be extended to other methods to enhance activity of growth factors after exposure to organic solvents.