Dimensionally stable thermal-stiffening composite hydrogels enabled by thermo-responsive SiO2 hybrid nanoparticles

Mechanically adaptive hydrogels with switchable mechanical properties have garnered significant attention due to their potential applications in various emerging fields. However, inevitable swelling and/or shrinkage associated with mechanical property switching lead to dimensional instability and structural failure of these hydrogels. This study synthesized dimensionally stable thermal-stiffening composite hydrogels by incorporating poly( N -isopropyl acrylamide) (PNIPAM)-grafted SiO 2 hybrid nanoparticles into a hydrogel matrix. By strategically localizing the thermo-responsive unit of PNIPAM at the interface between the SiO 2 nanoparticles and hydrogel matrix, the undesired swelling and shrinkage behavior are successfully eliminated, which are typically observed in conventional thermo-responsive mechanically adaptive hydrogels. The resulting composite hydrogels exhibit significantly enhanced dimensional stability compared to conventional counterparts and demonstrate superior response speed. Upon heating to 40 °C, the PNI-SiO 2 /P(AM- co -NIPAM)-2.0 hydrogel displays a tensile modulus of 22.4 kPa, strength of 60.5 kPa, and toughness of 127.1 kJ·m −3 , which is approximately twice that of its original state (12.2 kPa, 32.5 kPa, and 70.7 kJ·m −3 ). Moreover, both heating and cooling responses are completed within approximately 10 seconds while maintaining mass and volume changes below 3 %, with water content remaining at approximately 95 %. This work presents a design strategy for developing dimensionally stable mechanically adaptive composite hydrogels, which hold great promise for applications in tissue engineering, biomedical fields, and soft robotics.