Measuring DNA Microswimmer Locomotion in Complex Flow Environments

TL;DR

This work tackles the challenge of quantifying synthetic microswimmer locomotion within complex flow environments by introducing fiducial microspheres that track fluid motion and enable subtraction of flow- and gradient-induced motion from active swimming. The authors fabricate DNA-linked, ferromagnetic/non-magnetic colloidal swimmers and actuate them with orthogonal Helmholtz coils across constant, rotating, and oscillating magnetic fields, while tracking both fiducials and swimmers with high precision. A key innovation is the estimation of local flow via non-magnetic fiducials and the use of an exclusion zone to mitigate swimmer-induced flow perturbations, allowing the calculation of the field-driven translation $\\Delta_{swim} - \\\Delta_{mag}$. Results show that oscillating fields drive net field-driven locomotion in multiple swimmers, confirming theoretical predictions for flexibly linked microswimmers in low Reynolds number regimes, and establishing a method to ground truth swimmer motion in complex fluids for future design and control efforts.

Abstract

Microswimmers are sub-millimeter swimming microrobots that show potential as a platform for controllable locomotion in applications including targeted cargo delivery and minimally invasive surgery. To be viable for these target applications, microswimmers will eventually need to be able to navigate in environments with dynamic fluid flows and forces. Experimental studies with microswimmers towards this goal are currently rare because of the difficulty isolating intentional microswimmer motion from environment-induced motion. In this work, we present a method for measuring microswimmer locomotion within a complex flow environment using fiducial microspheres. By tracking the particle motion of ferromagnetic and non-magnetic polystyrene fiducial microspheres, we capture the effect of fluid flow and field gradients on microswimmer trajectories. We then determine the field-driven translation of these microswimmers relative to fluid flow and demonstrate the effectiveness of this method by illustrating the motion of multiple microswimmers through different flows.

Figures (8)

• Figure 1: Microscope image of non-magnetic fiducial microspheres, a magnetic fiducial microsphere, and a microswimmer. The inset shows the microswimmer structure (a non-magnetic polystyrene microsphere connected to a ferromagnetic polystyrene microsphere by a network of DNA nanotubes). The fiducial microspheres allow us to track fluid flow during experiments. Scale bar is 50µm.
• Figure 2: a) Chamber and fluid contents used in microswimmer swimming studies. b) Possible sources of fluid flow and microswimmer drift include evaporation (orange arrows), fluid mixing between solution layers (black arrows), and magnetic field gradients (red arrow).
• Figure 3: Diagram showing microswimmer motion due to a) constant, b) rotating, and c) oscillating magnetic fields. Only the oscillating field generates the non-reciprocal body deformations needed to break the "Scallop Theorem" and achieve net translation.
• Figure 4: a) Orthogonal pairs of Helmholtz coils were used to generate uniform, controlled magnetic field inputs. b) Each coil (green: Bx, blue By) generated a component vector of the net magnetic field (red).
• Figure 5: a) The magnetic field orientation generated by the coils. b) Rotations in the magnetic field lead to rotations in the microswimmer. c) The median and standard deviation of 4 non-magnetic fiducial displacements over time. d) The difference between the magnetic fiducial translation and the non-magnetic fiducial translation over the same time period. e) The difference between the microswimmer translation and the magnetic fiducial translation.