Actuation behavior of PNIPAM-based bilayer hydrogel regulated by polyvinyl alcohol polymer film

Responsive hydrogels based on Poly(N-isopropylacrylamide) (PNIPAM) are known to exhibit distinctive thermosensitive properties. However, isotropic PNIPAM hydrogels with weak mechanical properties and low deformation rates tend to exhibit only regular volume expansion/contraction, which limits them to promising applications such as intelligent actuators. In order to prepare programmable hydrogel actuators with satisfactory mechanical properties and fast deformation capability, a polyvinyl alcohol/PNIPAM (PVA/PNIPAM) bilayer hydrogel with anisotropic structure is proposed by combining preprepared PVA polymer film with outstanding mechanical properties as the passive layer and thermosensitive PNIPAM hydrogel as the active layer, forming a semi-interpenetrating network structure at the interface via the hydrogen-bond interaction between PNIPAM and PVA. The microstructures, mechanical properties and actuation behaviors of bilayer hydrogel were studied by scanning electron microscope, Fourier transform infrared spectrograph, mechanical testing machine and actuation test device. Results show that the introduction of PVA can improve the tensile stress of the bilayer hydrogel from 23.6 kPa to 62.6 kPa, and favor the hydrogel actuator excellent actuation preformation with a maximum bending amplitude of 500° and a maximum bending velocity of 13°/s within first 40 s. The bilayer hydrogel is further designed to work as a fluidic system valve that can recognize various temperature solutions and control solution flow rate. This design provides a simple and practical strategy to construct responsive hydrogels with anisotropic structure for further development in the field of intelligent actuators and flexible microfluidic systems.