Tailored Thermoresponsive Polyurethane Hydrogels: Structure–Property Relationships for Injectable Biomedical Applications

Thermoresponsive hydrogels that undergo reversible sol-gel transitions near physiological temperatures are highly attractive for biomedical applications, such as injectable drug delivery and embolization therapies. In this study, a library of polyurethane-based hydrogels was synthesized via step-growth polymerization using polyethylene glycol (PEG) of varying molecular weights, different diisocyanates, and a series of functional diols derived from diethanolamine with increasing hydrophobicity. The resulting polymers exhibited sol-gel transition behaviors without the need for external crosslinkers, relying solely on non-covalent interactions. The thermal responsiveness was systematically investigated using UV-Vis turbidimetry, and the cloud point temperature (T_CP) was found to be tunable within a range of 26-49 °C by modulating the monomer composition. Statistical modeling identified PEG molecular weight and diol structure as the primary determinants of T_CP, while diisocyanate type and diol-to-PEG ratio had negligible effects. Only diethanolamine (DEA)-based polymers formed stable hydrogels above a critical gelation temperature (LCGT), attributed to enhanced intermolecular interactions via free amine groups. In vitro degradation assays confirmed good hydrolytic stability under physiological conditions over four weeks, with degradation profiles strongly influenced by the PEG chain length and hydrophobic content. These findings establish a structure-property framework for the rational design of injectable, thermoresponsive polyurethane hydrogels with tailored sol-gel behavior for biomedical applications.