On the Correlation between the Microscopic Structure and Properties of Phosphate-Cross-Linked Chitosan Gels

Ionic chitosan gels fabricated using multivalent anions, tripolyphosphate (TPP) or pyrophosphate (PPi), respectively, have been investigated as potential biomaterials to be used in tissue engineering. Starting from the hypothesis that the polymer mesh texture at the microscale affects the final performance of the resulting materials, an innovative image analysis approach is presented in the first part of the article, which is aimed at deriving quantitative information from transmission electron microscopy images. The image analysis of the (more extended) central area of the gel networks revealed differences between both the cross-linking densities and pore size distributions of the two systems, the TPP gels showing a higher connectivity. Chitosan-TPP gels showed a limited degradation in simulated physiological media up to 6 weeks, reasonably ascribed to the texture of the (more extended) central area of the gels, whereas PPi counterparts degraded almost immediately. The release profiles and the calculation of diffusion coefficients for bovine serum albumin and cytochrome c, herein used as model payloads, indicated a different release behavior depending on the polymer network homogeneity/inhomogeneity and molecular weight of loaded molecules. This finding was ascribed to the marked inhomogeneity of the PPi gels (at variance with the TPP ones), which had been demonstrated in our previous work. Finally, thorough in vitro studies demonstrated good biocompatibility of both chitosan gels, and because of this feature, they can be used as suitable scaffolds for cellular colonization and metabolic activity.