Li Zhang, The Chinese University of Hong Kong
Magnetic Colloidal Microswarm for Targeted Delivery
Written by Hortense Le Ferrand
Flying birds form swarms to escape predators, fire-ants form swarms to create physical bridges: large groups of insects or animals display functions that cannot be realized by single individuals. Applying this strategy in microrobots for minimal invasive treatment could present the advantages of increasing the local dose of a drug and enhancing its imaging or tracking within the body.
Microrobotic swarms have been realized by utilizing bacteria and attaching cargo nanoparticles or by synthesizing artificial microstructures whose motion is remotely controlled by light, electric fields, or ultrasounds. As a different approach, the research team of Li Zhang uses iron oxide nanoparticles of 100 nm to 500 nm diameter, and control their behavior via magnetic fields through 3-axis Helmholtz coils.
Magnetic nanoparticles are convenient as they are non-toxic, have well described and controlled synthesis, and can be easily functionalized. When the suspended nanoparticles are placed under a magnetic field, they will assemble into chains. Under a horizontally rotating field, vortex-like swarms of nanoparticles form. Tuning the frequency of rotation of the field and its strength, several regimes can be observed and are reversible. For example, under a constant magnetic field strength, multiple small swarms can be formed at low frequencies around 8 Hz, that will merge into one large swarm when the frequency is raised to 40 Hz and that then split into two compact swarms at 70 Hz. Using tilted rotating fields, translation of the swarms can also be controlled to allow its locomotion into microfluidic channels. Oscillating anisotropic field can also be used to elongate or contract the swarm into specific directions. Finally, this phenomenon occurs in water, but also in biofluids such as blood, which demonstrates the potential of microrobotic swarms as a means for controllable and adaptable localized therapy.