Published on 2024/01/08                                                                                                            Research powered by Mightex’s Polygon1000     

Bingzhi Wu and Sambeeta Das, Upstream mobility and swarming of light activated micromotors. Materials Advances, (2024). 

Introduction

Micromotors are small particles that can autonomously propel themselves in a particular direction. Biological micromotors pose potential use for “Lab-on-a-chip”, cell sorting, and drug delivery applications across many biomedical fields. Most research to date has focused on the development and proof of principle of individual micromotors (references). However, individual micromotors can only carry a limited payload, and are restricted in the force they can exert. As such, the application of such technology is thwarted. 

Recent advances have focused on using a micromotor “swarming” approach where many micromotors can be controlled together, overcoming many of the limitations of single micromotor approaches. In particular, swarming micromotors are able to exert higher force, carry greater payload, and travel against the flow of the liquid in which they are held. These features are  important for transport of molecules in blood flow and through various media in microfluidic devices. As such, their promise for biomedical applications is broader and more tangible. 

Micromotor swarms are generated by the application of various forces, including magnetic fields, chemical effects, electrical fields, thermal gradients, and acoustic waves (ref). However, all of these approaches require relatively high concentrations of chemicals, such as hydrogen peroxide, to propel their movement. These chemical concentrations unfortunately cause toxicity in living tissues, thus limiting the application of such methods for biological applications (Dong et al., 2016; 33). As such, a new approach with greater bio-compatibility is required to shift forward this technology for use in the biomedical field. With this goal in mind, Wu and colleagues have developed a light-driven technique to instigate micromotor swarming. 

Wu and colleagues first used an electron-beam evaporation technique to coat titanium dioxide (Ti02) micromotors in a nickel-iron alloy, allowing the micromotors to become magnetically responsive. This process was then followed by adding layers of platinum and silver in order to increase the light-inducible movement properties of the micromotors. The micromotors were then placed into 35um x 16um microfluidic devices, generated via a standard photolithography process (ref). The Mightex Polygon was then used with a 365nm UV LED (500mW/cm2) to provide patterned illumination, allowing the micromotors to become mobile. A magnetic field was then applied to create uniform movement across the width of the microfluidic channel.

A schematic of the experimental set up can be seen below (figure 1)

 Figure 1:Adapted from Wu et al., (2023); (A) A microfluidic device is positioned on the stage of an inverted microscope. A microfluidic pump fills the channels of the device with micromotors in hydrogen peroxide solution. Their activity can then be observed through the objective of the microscope. (B) The Mightex Polygon DMD Pattern Illumınator is then used to project UV patterned stimulation to the field of view, resulting in controlled movement of the micromotors within the microfluidic device.

Methods

Application of UV light by the Polygon is able to generate movement in the micromotors by initializing a self-electrophoretic mechanism, such that a flow of charge is generated on the surface of the micromotor relative to the surrounding medium. This in turn results in an anisotropic electric charge that produces motion. When the patterned light from the Polygon is not present, the micromotors move passively with the fluid flow. When only a portion of the field of view and channel containing the micromotors is illuminated by the Polygon, particles in that region begin to move against the fluid flow, reaching dynamic equilibrium (active propulsion upstream balanced with passive flow downstream) at the edge of the patterned illumination. 

Over time, the passive flow of micromotors into the illuminated region allows a growing cluster of micromotors to collect at the dynamic equilibrium point, creating a micromotor swarm. The result of this process is seen in Figure 2 and Video 1 below.

 Figure 2: Adapted from Wu et al., (2023) (A) Schematic of the swarm creation process, using patterned illumination from the Polygon DMD to cluster micromotors at upstream edge in dynamic equilibrium. (B) Image of micromotor swarm at edge of patterned illumination region.