Quasi-static and dynamic nanoindentation for mechano-chemical analysis on biodegradable polymeric nanocomposites for regenerative medicine

Four different biomedical patches were bioprinted using nanocomposite hydrogels of sodium alginate/gelatin, sodium alginate/gelatin/indocyanine green freely dispersed, sodium alginate/gelatin/empty liposomes and sodium alginate/gelatin/indocyanine green loaded liposomes. Quasi-static and dynamic nanoindentations of the patch surfaces were performed to examine the effect of the single component on the mechanical response. The combination of results suggests that the mechanical structure of the gels is strongly influenced by crosslinking and the liposomes incorporating dye. Indeed, the non-crosslinked samples exhibit lower $G^{\prime }$ and $G^{\prime \prime }$ values, with more visco-elastic behavior. The absence of a marked increase in $G^{\prime }$ after crosslinking, both in samples with free dye and those with dye encapsulated in liposomes, indicates that strong interactions between dye, liposomes, and ionic calcium exist. Indeed, phosphatidylcholine and indocyanine green react electrostatically with calcium cations, sequestering crosslinkers. Crosslinking with $C{a}^{+2}$ improves matrix stiffness, but its effectiveness depends on the composition of the system. All results were validated with comprehensive statistical analysis among the different patches and their crosslinked counterparts.