An experimental design for the control and assembly of magnetic microwheels

Abstract: Superparamagnetic colloidal particles can be reversibly assembled into wheel-like structures called microwheels ($\mu$wheels) which roll on surfaces due to friction and can be driven at user-controlled speeds and directions using rotating magnetic fields. Here, we describe the hardware and software to create and control the magnetic fields that assemble and direct wheel motion and the optics to visualize them. Motivated by portability, adaptability and low-cost, an extruded aluminum heat dissipating frame incorporating open optics and audio speaker coils outfitted with high magnetic permeability cores was constructed. Open-source software was developed to define the magnitude, frequency, and orientation of the magnetic field, allowing for real time joystick control of $\mu$wheels through two-dimensional (2D) and three-dimensional (3D) fluidic environments. With this combination of hardware and software, $\mu$wheels translate at speeds up to 50 $\mu$m/s through sample sizes up to 5 cm x 5 cm x 5 cm using 0.75-2.5 mT magnetic fields with rotation frequencies of 5-40 Hz. Heat dissipation by aluminum coil clamps maintained sample temperatures within 3 C of ambient temperature, a range conducive for biological applications. With this design, $\mu$wheels can be manipulated and imaged in 2D and 3D networks at length scales of micrometers to centimeters