Design and Control of a Surface-Dimple-Optimized Helical Microdrill for Motions in High-Viscosity Fluids

Magnetically controlled microrobots have attracted wide attention in noninvasive therapy. However, it is challenging to design a microrobot with low swimming drag force whose swimming step-out frequency can be dynamically adjusted. Here, we design a 75- μ m helical drill-like microrobot with the biodegradable materials gelatin methacryloyl and hyaluronic acid methacryloyl. In high-viscosity fluids, the microrobot is optimized with surface dimples to reduce the drag force in swimming and dynamically regulate forward direction and rotational frequency in external magnetic fields. Inspired by the phenomenon that rough surface can reduce the fluidic flow resistance at microscale, the microdrill is modified with a theoretical dimple-to-surface area ratio of 19.6% to effectively reduce the motion resistance. Considering tasks in high-viscosity situations, we implement a control strategy to dynamically switch rotating direction and frequency with visual feedback from the local environment, which actuates the microdrill to move flexibly and penetrate obstacles effectively. An experiment demonstrates the step-out frequency of a dimpled microdrill is 15.5 Hz and its maximum translational velocity is 52.5 μ m/s in water, which improvements are 55 and 51.6% compared with a nondimpled microdrill with the same architecture. Furthermore, the dimpled microdrills move with high flexibility in fluids with viscosities from 1 ${\rm{mPa}} \cdot {\rm{s}}$ (water) to 47.6 ${\rm{mPa}} \cdot {\rm{s}}$ , which makes it possible to apply them to in vivo hyperlipemia therapies in the future.