Synthesis, test, calibration and modeling of a temperature-actuated hydrogel bilayer

Abstract

Smart structures such as temperature-sensitive actuators made of soft hydrogels can be useful for designing biomedical devices. In this study, free swelling and force generation behavior of a bilayer actuator based on thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogel and an elastomer under various loading profiles has been investigated. The thermo-sensitive hydrogel was synthesized by redox-initiated radical polymerization. We then prepared a bilayer actuator made of temperature-sensitive PNIPAM hydrogel, and examined its behavior under different loading conditions such as free swelling and force generation to study different aspects of the response of the hydrogel, including homogeneous free swelling and confined swelling using two approaches for (monolayer) hydrogel. Experiments on free swelling and force generation swelling of the bilayer actuator were also performed. After implementing an appropriate hydrogel constitutive model within a finite element framework, material parameters (of both the hydrogel and an elastomer) required in fabrication of the bilayer were calibrated. Using these calibrated material parameters, the same test conditions were simulated, and the numerical results were compared to those of experiments; a good agreement was observed between the experimental tests and numerical outcomes. The results show that the material model parameters are reliable, and the finite element method can be used for designing more complicated structures based on thermo-sensitive hydrogels.