Synthesis and characterization of physically cross-linked GrapheneOxide(GO)/collagen hydrogels for tissue engineering

Collagen is the most abundant protein of the ECM and, for this reason, collagen hydrogels provide an in-vivo like three-dimensional environment suitable for creating tissue equivalents and studying cell-matrix interactions. Nevertheless, collagen hydrogels exhibit limited mechanical properties if compared to native collagen tissues. Chemical cross-linking is a common method to synthesize hydrogels but cross-linking agents are usually cytotoxic against cells. For this reason, physically cross-linked hydrogels are considered an attractive alternative for cell culture applications. Graphene oxide (GO) has been recently attracting attention due to its unique properties such as large surface area, flexibility, high mechanical strength, high dispersibility, hydrophilicity, biocompatibility and ability to promote cell proliferation. In this study, physically cross-linked GO-collagen nanocomposite hydrogels at increasing concentrations of GO up to 5% w/w% were fabricated by self-assembly of collagen at 37°C, without the use of chemical agents. The effect of GO on the secondary structure of collagen was investigated through circular dichroism (CD) spectroscopy. Spectra confirmed that the helical structure of collagen was preserved even in presence of GO. Overall, a concentration of GO equal to 0.5% turned out to be the most effective amount of oxide in terms of cross-linking density, mechanical properties and cell culture suitability of the resulting GO-collagen nanocomposite hydrogels. GO was successfully sterilized and found to be cytotoxic for concentrations above 2%.