Enabling Torque-Based Magnetic Control of Living Bacteria for Targeted Drug Delivery Through Advanced Membrane-Bound Functionalization

Biohybrid microrobots combining living microorganisms such as bacteria with nanomaterials are poised to contribute to future cancer treatments by enabling tailored therapeutic functions and targeted non-invasive controllability. Magnetic materials are of particular interest to provide means for wireless magnetic control. Torque-based magnetic actuation using rotating magnetic fields (RMFs) is an advantageous strategy for remote control, as it can be implemented at clinically relevant scales, does not rely on positional feedback, and can be spatially focused.<br/><br/>However, such an actuation strategy requires robust, high-throughput functionalization capable of attaching several hundred of magnetic nanoparticles (MNPs) to the bacteria while leveraging magnetic anisotropy. Additionally, to ensure controllability, these strategies must remain stable and robust when exposed to physiological conditions.<br/>Previous biohybrid designs that have considered the introduction of magnetic material to the surface of clinically relevant bacterial strains have lacked adequate magnetic nanoparticle coverage to enable torque-based actuation.<br/><br/>Here, we present an efficient magnetic functionalization strategy for <i>Escherichia coli</i> that enables the application of sufficient torque to override natural locomotion at frequencies of several Hz. By studying the role of lipopolysaccharide in hindering the attachment of iron oxide nanoparticles and identifying a strategy employing calcium ions to mitigate this challenge, we were able to attach on the order of 10³ MNPs per bacterium. Moreover, by leveraging magnetic anisotropy, approximately 60% of the bacterial population became responsive to a 20 mT 2 Hz RMF. The resulting torques were on the same order of magnitude as magnetotactic bacteria, and exhibited a higher fraction of magnetic responsiveness. These biohybrid microrobots efficiently infiltrated the core region of 3D tumor spheroids, exhibiting up to a 5-fold enhancement of colonization under RMF exposure, highlighting their potential for enhancing tumor targeting and treatment efficacy.