Biodegradable hydrogels based on stereocomplex formation between lactic acid oligomers grafted to dextran

Abstract

A novel hydrogel system in which crosslinking is established by stereocomplex formation between lactic acid oligomers of opposite chirality is proposed. To investigate the feasibility of this novel system, we first investigate whether there is an operation window where lactic acid oligomers in either the d- or l-form do not give a crystalline phase, whereas in a blend of the d- and l-form stereocomplex formation occurs. Therefore, d- and l-lactic acid oligomers with different degrees of polymerization (DP) were prepared and analyzed using DSC. It was shown that crystallinity was present in d- or l-oligomers with DP ≥11. On the other hand, in blends of d- and l-oligomers of lactic acid crystallinity (stereocomplexation) was already observed at a DP ≥7. In the next step, l- and d-lactic acid oligomers were coupled via their terminal hydroxyl group to dextran, yielding dex-( l)lactate and dex-( d)lactate, respectively. Upon dissolving each product in water separately and mixing the solutions, a hydrogel is formed at room temperature as demonstrated by rheological measurements. The storage modulus of the obtained hydrogel strongly decreased upon heating to 80°C, while it was restored upon cooling to 20°C demonstrating the thermo-reversibility and the physical nature of the cross-links. The storage modulus of the gels depends on the degree of polymerization of the lactate acid grafts and their degree of substitution on dextran. Interestingly, gel formation was favored when one lactic oligomer was coupled via its hydroxyl group whereas the oligomer of opposite chirality was coupled via its carboxylic acid group. This is ascribed to the parallel packing of the oligomers in stereocomplexes.