CBE Doctoral Dissertation Defense: “DROPS-ON-STILTS MICROROBOTS FOR TRANSPORT AND DELIVERY OF HYDROPHOBIC CARGO” (Oluwafemi Ligan)

Abstract:

Magnetically actuated microrobots composed of dynamic assemblies of iron oxide nanoparticles (IONPs) offer versatile opportunities for biomedical applications. While such assemblies can reconfigure and perform diverse tasks under applied magnetic fields, the effective transport of hydrophobic cargo remains a significant challenge. To address this limitation, we introduce “drops-on-stilts” microrobots formed by the co-assembly of IONPs with oil droplets, enabling magnetic field-guided locomotion and delivery of hydrophobic payloads, including therapeutic agents. Using a 3D Helmholtz coil to generate uniform rotating magnetic fields, we induce the assembly of IONPs and droplets into stilt-like architectures that lift and mobilize droplets across surfaces. The formation of these structures depends critically on particle wetting behavior, interfacial packing, and initial suspension conditions. At low IONP volume fractions, magnetic dipole interactions drive IONP accumulation at droplet poles, facilitating rotational motion. At higher concentrations, elongated IONP chains in suspension merge with interfacially -trapped particles to create stilts, enabling droplet walking with velocities that scale linearly with stilt length. High droplet number densities further promote chaining and merging into “log-like” collectives capable of rolling or walking. We demonstrate that drops-on-stilts and their collectives can navigate obstacles and transport both IONPs and oil as cargo. Monte Carlo simulations, modeling IONPs as hard spheres with magnetic dipolar interactions and adhesion to droplet interfaces, reproduce the experimentally observed chain formation and collective dynamics, validating the proposed mechanism. Modulation of magnetic field conditions and pH tunes nanoparticle adhesion to the oil interface, yielding distinct structural and dynamical behaviors. Finally, we apply this platform to the delivery of hydrophobic therapeutics against Candida albicans biofilms, highlighting its potential for biomedical treatment. Overall, this work establishes a tunable microrobotic strategy with enhanced adaptability to complex environments, offering a promising route for magnetically controlled transport and delivery of hydrophobic agents.

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