Dendrimer crosslinked collagen as a corneal tissue engineering scaffold: Mechanical properties and corneal epithelial cell interactions

Generation 2 polypropyleneimine octaamine dendrimers were used to generate highly crosslinked collagen with mechanical properties that would make it appropriate for use as a corneal tissue-engineering scaffold. Crosslinking of a highly concentrated collagen solution (2-4%) was effected using the water-soluble carbodiimide 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide hydrochloride (EDC). The multifunctional dendrimers were introduced as novel multifunctional crosslinkers after the activation of the carboxylic acid groups of glutamic and aspartic acid residues in collagen. Glutaraldehyde, a common collagen crosslinker, was used as comparison, as was EDC, itself an alternative crosslinker, which forms "zero-length or self-crosslinking". The mechanical properties resultant gels were determined. Young's modulus of the dendrimer crosslinked gels was significantly higher than that observed with the other crosslinkers, increasing to 5 MPa compared with 0.1 MPa for the EDC crosslinked gels. Transmission electron microscopy (TEM) analysis of the gels demonstrated the presence of fibrils in the thermally gelled collagen controls; no fibrils were observed in the dendrimer crosslinked gels. As a result, the optical transparency of the dendrimer crosslinked collagen was significantly better than that of the collagen thermal gels. The EDC and glutaraldehyde crosslinked gels were generally less transparent than those crosslinked with the dendrimers. Glucose permeation results demonstrated that the dendrimer crosslinked collagen had higher glucose permeability than natural human cornea. Dendrimer crosslinked collagen gels supported human corneal epithelial cell growth and adhesion, with no cell toxicity. In comparison, some potentially cytotoxic effects were observed with glutaraldehyde crosslinked collagen. Overall, the dendrimer crosslinked collagen gels showed promising properties that suggest that these might be suitable scaffolds for corneal tissue engineering and potentially other tissue engineering applications.