Multifunctional inorganic-organic phase-change hydrogel with excellent light-to-thermal energy conversion and strain sensing capabilities for medical thermotherapy and monitoring

Flexible light/electrically responsive inorganic-organic phase-change materials (PCMs) have promising applications in thermal management and wearable health monitoring. Herein, flexible hydrogel PCMs with excellent light-to-thermal energy conversion efficiency and strain sensing capability were synthesized by loading sodium acetate trihydrate (SAT)/expanded graphite (EG)/borax/carboxymethylcellulose sodium (CMC) (termed SEBC) composites into a polyacrylamide (PAM)/chitosan (CS)/graphene (GNPs) (termed PCG) bi-networked hydrogel via a facile one-step strategy. The resulting composites showed excellent self-adhesion properties, tensile cycling stability, thermal stability, high melting enthalpy (119.7 J/g), and low water loss rate (0.41 %, at 70 °C). Benefiting from the light-absorbing ability and electrical conductivity of EG and GNPs, the hydrogel composite exhibited a high light-to-thermal energy conversion efficiency of 92.2 % and meanwhile can absorb excessive thermal energy to regulate the thermotherapy temperature on human joints under infrared light. Besides, the hydrogel composite can act as a strain sensor with an excellent gauge factor value of 2.73 by changing conductive network (EG and GNP) resistance to monitor the recovery of patient's joint activities. This multifunctional inorganic-organic phase-change hydrogels present good potential for personalized healthcare.