Synthesis and Characterization of Thermally and Chemically Gelling Injectable Hydrogels for Tissue Engineering

Novel, injectable hydrogels were developed that solidify through a dual-gelation, physical and chemical, mechanism upon preparation and elevation of temperature to 37°C. A thermogelling, poly(N-isopropylacrylamide)-based macromer with pendant epoxy rings and a hydrolyticallydegradable polyamidoamine-based diamine crosslinker were synthesized, characterized, and combined to produce in situ forming hydrogel constructs. Network formation through the epoxyamine reaction was shown to be rapid and facile, and the progressive incorporation of the hydrophilic polyamidoamine crosslinker into the hydrogel was shown to mitigate the often problematic tendency of thermogelling materials to undergo significant post-formation gel syneresis. The results suggest that this novel class of injectable hydrogels may be attractive substrates for tissue engineering applications due to the synthetic versatility of the component materials and beneficial hydrogel gelation kinetics and stability

Thermoresponsive Injectable Hydrogels Cross-Linked by Native Chemical Ligation

Temperature-induced physical gelation was combined with native chemical ligation (NCL) as a chemical cross-linking mechanism to yield rapid network formation and mechanically strong hydrogels. To this end, a novel monomer N-(2-hydroxypropyl)methacrylamide-cysteine (HPMA-Cys) was synthesized that copolymerizes with N-isopropylacrylamide (NIPAAm) to yield thermoresponsive polymers decorated with cysteine functionalities. Triblock copolymers consisting of a poly(ethylene glycol) (PEG) middle block flanked by random blocks of NIPAAm and HPMA-Cys were successfully synthesized and characterized. Additionally, thioester cross-linkers were synthesized based on PEG and hyaluronic acid, respectively. Upon mixing the thermoresponsive polymer with PEG or hyaluronic acid cross-linker, cysteine and thioester functionalities react under physiological conditions to generate a native peptide bond. An immediate physical network was formed after elevation of the temperature to 37 °C due to the self-assembly of the pNIPAAm chains. This network was stabilized in time by covalent cross-linking due to NCL reaction between thioester and cysteine functionalities, resulting in hydrogels with up to 10 times higher storage moduli than without chemical cross-links. Finally, a collagen mimicking peptide sequence was successfully ligated to this hydrogel using the same reaction mechanism, showing the potential of this hydrogel for tissue engineering applications. © 2014 American Chemical Society