In vivo biocompatibility of carbodiimide-crosslinked collagen matrices: effects of crosslink density, heparin immobilization, and bFGF loading

Collagen matrices, crosslinked using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (E) and N-hydroxysuccinimide (N), were previously developed as a substrate for endothelial cell seeding of small-diameter vascular grafts. In the present study, the biocompatibility of various EN-crosslinked collagen matrices was evaluated following subcutaneous implantation in rats for periods up to 10 weeks. The effects of the crosslink density, referred to as the number of free primary amino groups per 1,000 amino acid residues (EN10, EN14, EN18, or EN22), the amount of heparin immobilized to EN14, and the effect of preloading heparinized EN14 with basic fibroblast growth factor (bFGF) on the induced tissue reaction were studied. EN-crosslinked collagen was biocompatible at both early and late time intervals, and matrices with high crosslink densities (i.e., EN14, EN10) especially demonstrated a significantly decreased antigenic response when compared to noncrosslinked collagen. Furthermore, increased crosslinking resulted in a decreased degradation rate. Immobilization of heparin onto EN14 resulted in a similar to EN14 (thus without heparin) or somewhat reduced tissue reaction, but fibrin formation and vascularization were increased with increasing quantities of immobilized heparin. Matrices preloaded with bFGF also demonstrated good biocompatibility, especially in combination with higher amounts of immobilized heparin. The latter matrices [EN14 with high heparin and bFGF, thus EN14-H (0.4)F and EN14-H(1.0)F] demonstrated significantly increased vascularization for periods up to 3 weeks. Neither heparin immobilization nor bFGF preloading induced an increased antigenic response. It is concluded that the results of this study justify further evaluation of bFGF preloaded, heparin immobilized EN14 collagen, as a matrix for endothelial cell seeding in experimental animals.