Injectable hydrogels based on carboxymethyl cellulose and dextran, reinforced with rigid rod-like cellulose nanocrystals (CNCs) and aldehyde-functionalized CNCs (CHO-CNCs), were prepared and characterized. The mechanical properties, internal morphology, and swelling of injectable hydrogels with unmodified and modified CNCs at various loadings were examined. In all cases, gelation occurred within seconds as the hydrogel components were extruded from a double-barrel syringe, and the CNCs were evenly distributed throughout the composite, as observed by scanning and transmission electron microscopy. When immersed in purified water or 10 mM PBS, all CNC-reinforced hydrogels maintained their original shape for more than 60 days. The maximum storage modulus was observed in hydrogels with 0.250 wt % of unmodified CNCs and 0.375 wt % of CHO-CNCs. CHO-CNCs acted as both a filler and a chemical cross-linker, making the CHO-CNC-reinforced hydrogels more elastic, more dimensionally stable, and capable of facilitating higher nanoparticle loadings compared to hydrogels with unmodified CNCs, without sacrificing mechanical strength. No significant cytotoxicity to NIH 3T3 fibroblast cells was observed for the hydrogels or their individual components. These properties make CNC-reinforced injectable hydrogels of potential interest for various biomedical applications such as drug delivery vehicles or tissue engineering matrices.

Injectable PEG-analogue hydrogels based on poly(oligoethylene glycol methacrylate) have been developed based on complementary hydrazide and aldehyde reactive linear polymer precursors. These hydrogels display the desired biological properties of PEG, form covalent networks in situ following injection, and are easily modulated for improved control over their functionality and physiochemical properties.

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The design criteria for injectable, in situ-gelling hydrogels are reviewed in conjunction with highlights on recent progress in the preparation of injectable PEG and PEG-analogue poly(oligoethylene glycol methacrylate) (POEGMA) hydrogels.

Mixing POEGMA precursor polymers with different LCSTs leads to linear changes in macroscopic gel properties (e.g. mechanics, swelling) but non-linear changes in properties dependent on gel microstructure (e.g. protein adsorption, cell adhesion).