Thermo/Shear-Responsive Injectable Hydrogels from an Alginate/PNIPAM-Based Graft Copolymer: Effect of Divalent Cations Ca2+

This work is focused on the design and development of biocompatible self-assembling hydrogels, which behave as soft gels at room temperature and strong gels at the physiological temperature, suitable for potential bio-applications. A graft copolymer of sodium-alginate, bearing eight side chains of poly(N-isopropylacrylamide), enriched with the hydrophobic comonomer N-tertiary-butyl-acrylamide (NtBAM), (NaALG-g-P(NIPAM-co-NtBAM)) were used as gelator. In total, 5 wt% aqueous polymer solutions in the presence of Ca2+ cations were prepared and evaluated as thermo-responsive hydrogels. Rheological experiments revealed a twostep reversible gelation either upon heating or cooling. The divalent cations operated as a cross-linking agent through ionic interactions, inducing the formation of a network at low temperatures. Upon heating, an additional crosslinking develops through thermo-induced hydrophobic association of the thermo-responsive P(NIPAM-co-NtBAM) side chains above a critical temperature. The combination of thermo- and shear-responsiveness provides shelf-assembling systems as potential candidates for injectable strategies. For instance, the system under investigation could be used for cell transplantation, which requires a weak gel to protect the cells during injection and a gel strengthening after the injection at a physiological temperature to immobilize the created scaffold in the targeting position of the host tissue.