Characterization of the network structure of carbodiimide cross-linked gelatin gels

The network structure of native and carbodiimide cross-linked gelatin A and B gels was studied based on their rheological behavior. Gelatin A and B contain different numbers of carboxylic acid groups caused by different preparation conditions and had previously shown different characteristics in controlled release applications. It was evaluated to which extent chemical cross-linking densified the network structure of physical gelatin gels. After normalization of the equilibrium shear modulus (G(e)) with respect to swelling (Q), it was observed that the normalized G(e) values largely depend on the way gelatin is prepared from collagen. At an equal number of chemical junctions, chemically cross-linked gelatin B gels had a lower elasticity modulus than chemically cross-linked gelatin A gels. This seemed contradictory as gelatin B contains more carboxylic acid groups, available for cross-linking, but is related to a higher probability for intramolecular cross-linking, as was validated quantitatively by chemical and rheological analysis of the number of cross-links. Assuming an ideal network, the average molecular weight of the elastic network chains (M-c) was calculated for physical and chemical gelatin A and B networks, and on the basis of M-c the mesh sizes of the gels were estimated. The calculated mesh sizes were experimentally confirmed by lysozyme and albumin diffusion. Chemical cross-linking increased the resistance of the gels toward thermal degradation, resulting in a more gradual disintegration of physical cross-links upon heating. Moreover, chemical cross-linking prevented recombination of these cross-links upon cooling