Reinforcement of collagen with covalently-functionalized single-walled carbon nanotube crosslinkers

Poly(ethyleneimine) (PEI) functionalized single-walled carbon nanotubes (SWNTs) and free PEI were investigated as collagen crosslinking agents to determine their ability to improve the Young's modulus of a collagen hydrogel. The crosslinked collagen matrices were prepared by blending Type I bovine collagen with either PEI-SWNT or free PEI and crosslinked utilizing carbodiimide chemistry. The resulting SWNT enriched material was a crosslinked collagen hydrogel with sufficient mechanical strength to be manipulated and transferred without damaging the matrix. Raman spectroscopy confirmed the presence of SWNTs within the PEI-SWNT/collagen hydrogels. In addition, it confirmed that crosslinking did not alter the SWNT delocalized network. Dynamic scanning calorimetry confirmed a change in the denaturation temperature for hydrogels prepared by PEI-SWNT or PEI crosslinkers. Water uptake analysis suggested very loosely crosslinked matrices were produced regardless of the crosslinking agent used. However, evaluating the Young's modulus, it was found that collagen hydrogels produced with PEI-SWNTs as the crosslinking agent had a modulus 4.5 to 9 times that of collagen hydrogels produced with PEI of the same amine concentration or in the absence of any crosslinking agent. In addition, PEI-SWNT crosslinked collagen hydrogels exhibited superior Young's modulus to control samples in which only the SWNTs or unlinked mixtures of SWNTs and PEI were used as crosslinkers. This comparative study confirms that PEI-SWNT crosslinked collagen gels exhibited Young's moduli significantly higher than collagen gels cross-linked with just nanotubes, PEI, or blends of nanotubes and PEI.