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Rotating magnetic fields (RMFs), when used to actuate biomedical microrobots for targeted delivery to tumors, have been shown to enable them to overcome physiological barriers and promote their accumulation and penetration into tissue. Nevertheless, directly applying a RMF to a deeply situated target site also leads to off-target actuation in surrounding healthy tissue. Here, we investigate an open-loop control strategy for delivering torque density to diffuse distributions of microrobots at focal points by combining RMFs with magnetostatic gating fields. Taking magnetotactic bacteria (MTB) as a model biohybrid microrobotic system for torque-based actuation, we first use simulation to elucidate off-target torque suppression and find that resolution is set by the relative magnitude of the magnetostatic field and RMF. We study focal torque delivery in vitro, observing off-target suppression of translational velocity of MTB, convection-driven accumulation of companion nanoparticles, and tumor spheroid colonization. We then design, construct, and validate a mouse-scale torque-focusing apparatus incorporating a permanent magnet array, three-phase RMF coils, and offset coils to maneuver the focal point. Our control scheme enables the advantages of torque-based actuation to be combined with spatial targeting, and could be broadly applied to other microrobotic designs for improved drug delivery.

Combining rotating magnetic fields with gating fields enables focused delivery of torque density to dispersed microrobots.