Oscillating magnetic field-actuated microvalves for micro- and nanofluidics

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.