Autonomous Motion, Control Strategies & Sensing Applications Using Magnetic Micro-Robots

Abstract

In recent years, there has been growing interest in manoeuvring nanoscale objects in fluidic environments, as it holds significant potential in targeted therapies, diagnostics, and probing local rheological properties. Nanomotors are artificial structures, spanning from a few hundred nanometers to several microns. These can be powered by external energy sources such as chemical, optical stimuli, acoustic, electric, and magnetic fields. Among these various actuation methods, magnetic fields are especially promising due to their non-invasive nature and biocompatibility. Helical magnetic nanorobots (NR) (Nano helices) have emerged as one of the effective magnetic swimmers. When subjected to rotating magnetic fields, these helices exhibit corkscrew-like motions, enabling propulsion. This system has been extensively studied in recent years. In this work, I have focused on helical nanorobots and explored several key aspects, including novel fabrication methods, strategies for selective and collective control, their application in therapeutics, and their use in probing local environments.

Typically, these nanorobots are made with ferromagnetic material. When in dense suspensions, due to remnant magnetization, irreversible magnetic agglomeration arises, which severely hinders their motion. To address this, we developed superparamagnetic iron oxide nanoparticles (SPIONs) functionalized on helical nanorobots (SPION-NRs). Unlike their ferromagnetic counterparts, SPION-NRs exhibit no remnant magnetization and minimal coercivity, thereby maintaining stability and preventing agglomeration in dense suspensions. Furthermore, we observed that at higher magnetic fields, SPION-NRs outperform ferromagnetic nanorobots, as they avoid performance deterioration caused by remagnetization phenomena commonly observed in ferromagnetic materials. Additionally, we demonstrate their therapeutic potential, where SPION-NRs exposed to magnetic hyperthermia effectively induce localized heating, which leads to cell death while targeting cancer cells.

Selective control of individual nanobots at sub-micrometer scales remains a significant challenge, as homogeneous magnetic fields typically fail to distinguish between identical nanostructures. Addressing this, we leverage the intrinsic bistability of nanorobots actuated under homogeneous rotating magnetic fields, characterized by stochastic switching between precessional and tumbling states due to thermal fluctuations. Employing this strategy periodically, coupled with real-time orientation estimation algorithms, we achieve precise and selective control of identical nanobots within a swarm, enabling individual manipulation and navigation at nanoscale precision.

Expanding this study to collective behaviors, we investigated swarms of these nanobots, observing significant hydrodynamic interactions resulting in lateral motions in addition to forward propulsion. Our density-dependent analysis shows how proximity and interaction influence swarm dynamics. We also demonstrated that by using oscillating magnetic fields, we can make these driven swarms into active swarms.

Lastly, we explored the potential of magnetic nanorobots as precise rheological probes in complex fluids. While magnetic nanobots have traditionally been employed to measure viscosity in Newtonian fluids, their application in characterizing viscoelastic media is limited. Using magnetic rods subjected to in-plane rotating magnetic fields, we revealed a clear correlation between rod dynamics and the viscoelastic properties of the surrounding medium. Experimental results were validated through comprehensive numerical simulations, demonstrating the efficacy of this approach. Furthermore, we extended our technique to intracellular environments, successfully measuring viscoelastic properties within living cells, showcasing the versatility and biomedical potential of magnetic nanorobots as microrheological probes.