Poly(N-isopropylacrylamide) (PNIPAM) hydrogel nanospheres response to global temperature stimuli across the low critical solution temperature is studied as the water content in the polymer network is modified. The refractive index of gel nanoparticles was measured as a function of temperature using spectroscopic ellipsometry. The volume of the nanospheres reduces by 40% resulting in a modification of light scattering properties of the medium. A change in temperature from 33 to 34 °C results in a volume contraction of nanospheres which is accompanied by 8–10% enhancement in the refractive index of the gel network.

Biocompatible magnetic nanoparticles hold great therapeutic potential, but conventional particles can be toxic. Here, we report the synthesis and alternating magnetic field dependent actuation of a remotely controllable, multifunctional nano-scale system and its marked biocompatibility with mammalian cells. Monodisperse, magnetic nanospheres based on thermo-sensitive polymer network poly(ethylene glycol) ethyl ether methacrylate-co-poly(ethylene glycol) methyl ether methacrylate were synthesized using free radical polymerization. Synthesized nanospheres have oscillating magnetic field induced thermo-reversible behavior; exhibiting desirable characteristics comparable to the widely used poly-N-isopropylacrylamide-based systems in shrinkage plus a broader volumetric transition range. Remote heating and model drug release were characterized for different field strengths. Nanospheres containing nanoparticles up to an iron concentration of 6 mM were readily taken up by neuron-like PC12 pheochromocytoma cells and had reduced toxicity compared to other surface modified magnetic nanocarriers. Furthermore, nanosphere exposure did not inhibit the extension of cellular processes (neurite outgrowth) even at high iron concentrations (6 mM), indicating minimal negative effects in cellular systems. Excellent intracellular uptake and enhanced biocompatibility coupled with the lack of deleterious effects on neurite outgrowth and prior Food and Drug Administration (FDA) approval of PEG-based carriers suggest increased therapeutic potential of this system for manipulating axon regeneration following nervous system injury.

The feasibility of using tunable magnetic nanoparticles embedded in cylindrical hydrogel materials as a flow regulator via thermo-mechanical gating is studied within microfluidic channels. Ferromagnetic nanoparticles (Fe3O4) encapsulated within a thermo-sensitive polymer network (-poly(N-isopropylacrylamide) (PNIPAM)) was polymerized inside 300 µm diameter micro-capillary tubes. An oscillating magnetic field range 20–125 Oe, (100–1000 kHz) was used to induce heat and control the valving action. Valving action was effectively regulated by modulating the magnetically responsive PNIPAM networks (MPNIPAM) and thereby physically regulating the harmonics (swelling and shrinking) of the polymer monolith inside the microchannel. Magnetic properties in terms of saturation magnetization, remanence and coercivity of the designed system have been extracted for data accuracy. The optimum concentration of NIPAM monomer in the polymer matrix and the embedded nanoparticles yield ∼80% volume shrinkage inside the microchannel, which is close to the undoped PNIPAM system, without compromising the oscillating field induced heating. Very importantly, the oscillating field-actuated de-swelling response time is ∼3 s, which is significantly faster than the thermal actuation, and in addition the microvalve exhibits a faster response time compared with the macrovalve (MPNIPAM monolith inside 1500 µm diameter channel). The enhanced shrinkage rate and the actuation efficiency might be ideal for many biomedical applications, including synergistic application of heat and sustained releasing capability of chemotherapeutic agents.

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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 (Fe3O4) 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.