Efficient mixing of microliter droplets as micro-bioreactors using paramagnetic microparticles manipulated by external magnetic field.

Magnetic manipulation of paramagnetic particles had great potential for efficient bioprocessing. In this study, we stirred microliter-volume water droplets formed on a superhydrophobic surface as micro-bioreactors by using paramagnetic magnetite microparticles manipulated by an external magnetic field. We showed that magnetite microparticles in the droplets spontaneously formed rod-like aggregates, which were like commercial stir bars, in an external magnetic field and spun with rotating of magnetic field. Increasing the rotating rate of the magnetic field and increasing the concentration of the microparticles caused the microparticles to fixate at the air/water interface of the droplets and their rotation at the interface with rotating of magnetic field. The active mixing enhanced the enzyme reaction and microorganism proliferation in the droplets. These results demonstrated that manipulating the magnetite microparticles by an external magnetic field efficiently mixed the small droplets as micro-bioreactors.