Light And Heat-Responsive Self-Healing Smart Gel System

Smart gels that respond to light often require complex synthesis, limiting their practical use. Naved Malek, Sardar Vallabhbhai National Institute of Technology, Surat, India, and collaeagues have developed a gel system that combines photoresponsiveness, temperature sensitivity, and self-healing ability using common, easily available components. Its simplicity and multifunctionality make it a strong candidate for applications in soft robotics, sensors, and controlled release systems.

The gel system, called CCG (CPC/[Ch][Ol]-based gel), was formed by mixing cetylpyridinium chloride (CPC); a surfactant [Ch][Ol]; a surface-active ionic liquid (SAIL); and diphenyliodonium nitrate (DPIN), a photoacid generator (PAG). At optimized concentrations (10% CPC, 5% [Ch][Ol], 30 mM DPIN), the components self-assembled into a gel through hydrogen bonding, electrostatic, and hydrophobic interactions.When exposed to UV light (270 nm), DPIN released protons, causing the pH to drop from 7.0 to 4.5. This protonation turned [Ch][Ol] into oleic acid, breaking apart the gel network and causing the gel to permanently liquefy (an irreversible sol transition).

Small-angle neutron scattering (SANS) confirmed structural changes from a mixed network to ellipsoidal micelles. Fourier-transform infrared spectroscopy (FTIR) showed molecular shifts consistent with protonation. Temperature sweeps revealed reversible gel-to-sol transitions, i.e., the gel liquefied at 50 °C and reformed at 25 °C. Rheological tests showed consistent viscosity changes across multiple heating-cooling cycles and a modulus crossover at 46 °C.

Self-healing was demonstrated by joining colored gel halves, which reconnected within minutes. Thixotropic tests showed recovery of mechanical strength after repeated strain cycles, with a healing efficiency of 94.9%. The CCG system thus offers a simple route to design smart materials that respond to light and temperature and repair themselves after damage. While the UV-triggered change is irreversible, this feature suits one-time applications such as drug release or temporary structural changes. Future work could explore reversible photoresponsive systems or biocompatible variants for medical use.