High Strength Magnetic/Temperature Dual-Response Hydrogels for Applications as Actuators

Anisotropically structured magnetic/temperature dual-response hydrogels have great application prospects as actuators because they can exhibit controlled, complex behaviors. However, one key issue hindering the application of such hydrogels is the imbalance of the mechanical and response properties. This study used a combination of flexible chain polymers such as poly( N -isopropylacrylamide) (PNIPAM), poly(vinyl alcohol) (PVA), and polyacrylamide (PAM) to build a multinetwork structure. The introduction of TEMPO-oxidized cellulose nanofibrils (TOCNF) as a nanofiber reinforcement agent led to a key improvement to ensure a high mechanical strength by creating additional hydrogen bonding. The cross-linking density was further increased through a salting out treatment to obtain a greater mechanical strength while improving the dissipation of energy applied by external sources. The obtained temperature responsive layer featured a high tensile strength (1.97 MPa) while the magnetically responsive layer showed a high magnetization (6.1 emu/g) with a good tensile strength (0.47 MPa). The main idea of this study was in combining two hydrogel layers with different polymer network structures, with magnetic nanoparticles being dispersed within one layer, whereas the other layer was designed as temperature-sensitive. The obtained bilayer hydrogel had suitable mechanical properties (the tensile strength reaching 0.81 MPa) coupled with strong dissipation of the applied external energy and could rapidly and reversibly undergo bending deformations upon a temperature change within a narrow range, 25–37 °C (bending angle up to 160° within 5 min). With high magnetization characteristics for the magnetically responsive layer, the bilayer hydrogel could easily be driven by an external magnetic field to transport a target object, which was “grabbed” due to the gel bending. It also showed good biocompatibility, thus enabling applications in the field of invasive medical actuators.