Controlled actuation of alternating magnetic field-sensitive tunable hydrogels

The feasibility of using tunable magnetic nano-particles embedded in cylindrical hydrogel materials for guided actuation via controlled modulation of oscillating magnetic field and frequency is investigated. Ferromagnetic nano-particles (Fe 3 O 4 ) encapsulated within a thermo-sensitive polymer network [-poly(N-isopropylacrylamide) (PNIPAM)] were polymerized inside 1.5 mm diameter capillary tubes. Inside alternating magnetic field (25–70 Oe, 150–280 kHz), the polymer monolith quickly bends along the longitudinal axis. The bending behaviour of the polymer monolith was influenced by the following factors: (a) mechanical strength of the monolith, (b) ac field-induced temperature regulation and (c) the surface evaporation. The equilibrium bending angle reached a maximum value of 74° at 30 Oe, 200 kHz, between 15% and 35% relative humidity conditions. In addition, we found that micro-scale monolith (300 µm diameter) exhibited significantly faster actuation response compared with the 1500 µm diameter hydrogel cylinder. Both de-swelling efficiency and volumetric transition temperature were not affected due to the nano-magnet incorporation. As ac magnetic field-induced controlled modulation can directly transform the absorbed energy into bending and shrinkage simultaneously for temperature sensitive polymers, i.e. the absorbed energy is converted into mechanical work, this novel approach may lead to a new category of magnetically responsive polymeric structures for potential applications in the field of smart gel-based devices, such as micro-sensors and actuators, and particularly in biomedical fields.