Dipolar Rings of Microscopic Ellipsoids: Magnetic Manipulation and Cell Entrapment

Abstract

We study the formation and dynamics of dipolar rings composed by microscopic ferromagnetic ellipsoids, which self-assemble in water by switching the direction of the applied field. We show how to manipulate these fragile structures and control their shape via application of external static and oscillating magnetic fields. We introduce a theoretical framework which describes the ring deformation under an applied field, allowing to understand the underlying physical mechanism. Our microscopic rings are finally used to capture, entrap and later release a biological cell via magnetic command, i.e. performing a simple operation which can be implemented in other microfluidic devices which make use of ferromagnetic particles.

Emergent propagation modes of ferromagnetic swimmers in constrained geometries

“…[9][10][11][12][13][14][15][16][17][18][19] Magnetic actuation enables propulsion to be controlled wirelessly without affecting biological viability, an essential requirement of many biomedical applications, with the added benefit that the magnetic field also determines the direction of motion. Simple magnetic attraction of particles can be effective, but requires the magnetic field to have both a strong magnitude and gradient, so this solution is only viable when the particles are close to the magnetic poles (e.g.…”

“…[9][10][11][12][13] Alternatively, swimming has been demonstrated using interacting superparamagnetic particles, either to produce flagellum-like motion when the particles are physically connected 22 or to produce swarm-like collective motion of individual particles. [14][15][16][17] Our group proposed a third strategy based on two ferromagnetic particles with differing size and magnetic anisotropies, connected by an elastic link. 18,19 Experimental demonstration of a prototype magneto-elastic swimmer can be found in the supplementary material of reference 19.…”

“…Indeed, many swarm-type swimmers cannot propagate in bulk fluids and require the interface to generate propulsion. [15][16][17] While necessary for establishing proof of principle for swimming, this approach has neglected to consider the effects of encountering a geometrical barrier on motion of otherwise free swimmers. However, biomedical and microfluidic applications will require swimmers initially moving in bulk fluids to interact with surface barriers, such as the cell membrane, blood vessel wall or the sides of a microfluidic chamber.…”

Colloidal Microworms Propelling via a Cooperative Hydrodynamic Conveyor Belt

“…Propulsion arises due to the nonlinear cooperative rectification of flows generated by the spinning particles close to the bounding wall. The hydrodynamic flow generated by our magnetic prototype can be used as an efficient mechanism to transport biological or colloidal cargos entrapped and translated by the conveyor belt [53]. On the other hand, trapping the leading rotor of the worm by optical tweezers could convert the magnetic propeller into a fluid pump which can be readily employed in microand nanofluidc systems.…”

Flexible mechanism of magnetic microbeads chains in an oscillating field

“…[6][7][8][9][10][11] Magnetic actuation enables motion of the magnetic micro devises to be controlled wirelessly without affecting biological viability but with the extra benefit that the direction of motion can be determined by the field. 12,13 One of the challenges in proposing the micro devices to move in the low-Reynolds-number regime should be that a microswimmer must deform without structural instability in a way that is not invariant under time-reversal.…”