Mechanically Reinforced Smart Hydrogels with pH-Responsive Colorimetric Behavior for Potential Wound Care Applications

Hydrogels are widely utilized in biomedical applications due to their water-rich nature, biocompatibility, and tunable properties. Our earlier reported phosphine oxide hydrogel (Gel-A) exhibited promising cationic pH sensitivity but lacked sufficient mechanical strength for long-term or load-bearing use, prompting structural enhancement. To address these issues, this work presents two cationic hydrogels, Gel I and Gel II, designed to enhance mechanical performance over Gel-A. Gel-A was synthesized via one-step Michael addition of piperazine (PP) and trivinylphosphine oxide (TVPO) at 80 °C as a control. Gel I and Gel II incorporated poly(ethylene glycol) diglycidyl ether (PEGDE) and poly(ethylene glycol) diacrylate (PEGDA), respectively, as third monomers to reduce the cross-linking density and increase the toughness. The resulting hydrogels showed improved mechanical strength, with Gel I supporting over 16 times more compressive stress than Gel-A. All hydrogels exhibited cationic pH-responsive behavior; however, Gel II dissolved after 6 h at a pH of 10, most likely due to hydrolysis of the acrylate ester bond. The incorporation of PEGDE modified the swelling behavior of Gel I. Unlike the highly cationic Gel-A, which swells significantly in acidic media, Gel I exhibits reduced swelling at low pH due to a lower charge density. However, under alkaline conditions, Gel I shows enhanced swelling (200% increase) compared to Gel-A, driven by electrostatic repulsion between deprotonated oxygen groups (O–). SEM revealed that Gel-A had a porous structure with large pores, while Gel I showed a layered morphology with thick walls. To enhance usability, we developed a full-IPN nanofibrous hydrogel by cross-linking poly(vinyl alcohol) (PVA) with citric acid (CA) and Gel I via electrospinning and postheat treatment. Cytotoxicity assays confirmed its nontoxic, nonsensitizing nature. Its potential as a colorimetric wound dressing was demonstrated using covalently immobilized 2-hydroxyethylsulfonyl-based dyes to avoid leaching, enabling visible, pH indication for real-time monitoring of wound infections.