Fabrication of magneto-responsive microgears based on magnetic nanoparticle embedded PDMS
We present a new fabrication method for nonspherical magnetically responsive microparticles. It is based on photo- and soft-lithography and is suitable for production of prism shaped magnetic microparticles. Approximately 105 particles per run can be produced. The key element of the fabrication is the soft polydimethylsiloxane (PDMS) mold with hollows obtained by replica molding from a hard (SU-8 photoresist) master. The master is microfabricated by photolithography. The PDMS mold is filled with commercially available magnetic PDMS followed by the addition of superparamagnetic nanoclusters which enhance the magnetic susceptibility of the particles. After the cross-linking process particles are extracted from the mold and dispersed in the water. A magneto-responsive behavior of so produced microparticles is demonstrated in an experiment with magnetic microgears subjected to a rotating magnetic field of different strengths and frequencies. At low frequencies a microgear follows the rotation of the field whereas above the critical frequency the microgear rotation frequency decreases with increasing field frequency. The observed dependence is well explained with a model assuming that the magnetic torque on a microgear results from an anisotropic effective susceptibility as well as finite relaxation time of the magnetization. We also demonstrate that a magnetic microgear can transmit rotation to one or several non-magnetic microgears.