Living, Self‐Replicating Ferrofluids for Fluidic Transport

Abstract

Magnetic actuation offers a means to wirelessly control flow in ferrofluids for applications including microfluidic pumping and targeted drug delivery. Despite the promise of these concepts, practical use of synthetic ferrofluids as actuators of flow frequently requires high concentrations and is hindered by low ferrohydrodynamic coupling efficiency and inhomogeneous flow fields. Inspired by the magnetic properties and hydrodynamic forms displayed by magnetotactic bacteria (MTB), this work studies the use of these microbes as a living, self‐replicating ferrofluid for improved fluidic transport via magnetically coerced rotation. Using multicore iron oxide nanoparticles as a performance benchmark, MTB under rotating magnetic fields are shown to produce more homogeneous and efficient flow. Coupling is enhanced whether the comparison is made in terms of volume of magnetic material or total volume fraction. To clarify the mechanistic role of interactions with boundaries in transport, a computational model is developed and validated experimentally. Applying this model, two distinct and feasible magnetic control strategies are predicted: a rotating gradient field that generates directional flow despite boundaries that promote flow in opposing directions and a magnetostatic gating field that enables spatially selective actuation. The advantageous properties identified for MTB open a design space for these strategies to be realized.

Magnetic torque-driven living microrobots for enhanced tumor infiltration

“…To increase the available magnetic torque, which is a function of magnetic moment and magnetic field strength, the applied field magnitude was increased to 20 mT. At this field strength, the step out frequency of our MTB strain of interest was found to be 24 Hz (37), in agreement with the magnetometer data (Fig. 2F).…”

“…Gradient fields that select signal from image voxels in MPI have proven challenging to scale up to humans, but recent efforts have demonstrated MPI in deep regions in human brains with moderate resolution (52). Our recent work has shown that torque-based actuation, as well as the resulting inductive signals generated by the MTB, should be compatible with selection fields (37). One foreseeable challenge to simultaneous inductive monitoring and actuation of MTB with RMF is that frequencies of tens of hertz are used rather than the kilohertz signals generated in MPI, resulting in correspondingly weaker inductive signals.…”

“…TEM images and multisizer data were used to determine the approximate size of a single bacterium, which were estimated to be 0.45 and 1.8 μm for the short and long axis respectively (fig. S1) (37). Hydrodynamic interactions were modeled as linear and rotational viscous damping acting on the rigid body.…”

“…An inductive background signal at the same frequency and at 2 mT was collected both before and after every sample measurement. This approach assumes that the response of the bacteria at 2 mT and the investigated frequencies will be far smaller than their response at higher amplitudes, an assumption motivated by previous studies (37). Prior to conducting measurements at each condition, inductive background was minimized by minutely mechanically adjusting the angle between the sense and compensation coil to balance phase and using the potentiometer to minimize the output signal of the amplifier.…”

Facile Synthesis, Static, and Dynamic Magnetic Characteristics of Varying Size Double-Surfactant-Coated Mesoscopic Magnetic Nanoparticles Dispersed Stable Aqueous Magnetic Fluids

“…MFs display unique properties such as ultra-low friction, self-levitation/sealing, magnetic bearing formation, sticking to the magnet, enhanced thermal conductivity, variable density, and viscoelasticity [21,22]. The motion of the MFs can easily be controlled by an external magnetic field.…”