Injectable pH- and Temperature-Responsive Hydrogels for Scaffold Applications in Tissue Engineering

Injectable hydrogels offer promising alternatives for scaffold-based tissue engineering due to their minimally invasive delivery and in situ forming capability. In this study, we reported the first development of an injectable hydrogel scaffold combining carboxymethyl cellulose (CMC), poly(ethylene glycol) (PEG), and poly(ε-caprolactone) (PCL) into a single system. This novel approach integrated the biocompatibility of CMC, tunable responsiveness of PEG, and mechanical robustness/degradability of PCL, which had not been previously reported. A pH- and temperature-responsive carboxymethyl cellulose (CMC) grafted with a methoxy poly(ethylene glycol)-block-poly(ε-caprolactone) [CMC-g-(mPEG-b-PCL)] system was synthesized. The diblock copolymers were first prepared by ring-opening polymerization of ε-caprolactone using a poly(ethylene glycol) methyl ether (mPEG) in combination with a stannous octoate initiator, followed by grafting onto the pH-responsive CMC backbone using simple 1-ethyl-3-(3-(dimethylamino)propyl carbodiimide)/N-hydroxysuccinimide (EDC/NHS) coupling chemistry in N,N-dimethylformamide (DMF). Structural characterization by ¹H NMR and FTIR spectroscopy confirmed the presence of characteristic functional groups from both CMC and mPEG-b-PCL. Aqueous CMC-g-(mPEG-b-PCL) hydrogels were subsequently formulated, with 32 wt % CMC-g-(mPEG₁₇-b-PCL₁₂) showing the most favorable sol-gel phase-transition behavior based on the test tube inversion. Rheological analysis demonstrated that the hydrogel remained injectable in the sol state and formed a stable gel under physiological conditions, with the range of storage moduli comparable to that of early stage cartilage tissue. In addition, the hydrogel exhibited an interconnected porous structure, as observed by scanning electron microscopy. Cytocompatibility was validated through MTT and live/dead staining assays using L929 fibroblasts and MG63 osteoblast-like cells. The results showed that the cell morphology was preserved, and the cell viability was stable throughout 5 days of incubation. These findings support the cytocompatibility of the synthesized CMC-g-(mPEG-b-PCL) graft copolymer and suggest its potential for further investigation as an injectable hydrogel scaffold for bone and cartilage tissue engineering applications.