Microswimmers chemotaxis behavior in a stop flow gradient generation platform

This short review about microswimmers chemotaxis is based on the article published in Angewandte Chemie International Edition written by Zuyao Xiao, Audrey Nsamela, Benjamin Garlan and Juliane Simmchen.

One of the authors, Audrey Nsamela, is a PhD candidate in the Active Matter project which has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 812780.

Abstract

The ability of artificial microswimmers to respond to external stimuli and the mechanistical details of their origins belong to the most disputed challenges in interdisciplinary science. Therein, the creation of chemical gradients is technically challenging, because they quickly level out due to diffusion. Inspired by pivotal stopped flow experiments in chemical kinetics, we show that microfluidics gradient generation combined with a pressure feedback loop for precisely controlling the stop of the flows, can enable us to study mechanistical details of chemotaxis of artificial Janus micromotors, based on a catalytic reaction. We find that these copper Janus particles display a chemotactic motion along the concentration gradient in both, positive and negative direction and we demonstrate the mechanical reaction of the particles to unbalanced drag forces, explaining this behaviour.

Motivation – Understanding living systems at the microscale

Taking a closer look at natural survival skills of most living creatures on earth, from bacteria to humans, one of the common traits is migration to search for food. However, if it is now fairly easy to understand the mechanisms that drives your neighbor Charlie to go to the supermarket and buy supplies for a lovely dinner, it is a whole other story when it comes to study self-propelling microorganisms’ ways of finding nutrients. At this scale, and especially in non-equilibrium physical phenomena, many things differ from the macro world; mass and heat transfer are enhanced, viscous forces are dominant over inertial forces, that is by definition a low Reynolds number regime, thermal fluctuations become non negligible. Understanding the behavior of such microsystems requires a multidisciplinary approach, with biology, physics and chemistry being closely interlinked. In this study, the focus is on the physical response of artificial microswimmers to a fuel concentration gradient. The microswimmers are actually silica microparticles half coated with a thin copper layer [1]. When in contact with hydrogen peroxide, the copper side degrades the fuel to form oxygen and water and this chemical reaction initiates the motion of the particles.

Figure 1 – Illustration of the catalytic reaction driving the self-propelling microparticles and SEM picture of a Cu@SiO2 particle.

How does microfluidics come into play for microswimmers study?

Microfluidics has mainly been used to fabricate artificial or biohybrid microswimmers but recently, its potential for micro-environment design has been explored as well [2]. In this paper, the aim was to perform a chemotaxis assay in a well-defined hydrogen peroxide gradient without any external flows. Performing a net zero flow in microfluidics is not a trivial task, especially when such light microparticles sense the slightest flow (well below the detection limit of common flow sensors). For this purpose, a platform with pressure driven flow controller has been developed to combine gradient generation with stop flow actuation. A set of microfluidic valves enables sample injection into the microfluidic chip and a retroactive pressure loop to ensure no pressure difference across the microfluidic setup during stop flow. A short sequence was created on the ESI for automated gradient creation and stop flow. Videos of the particles were recorded in the first 30 seconds of stop flow.

Figure 2 – Schematic of the microfluidic setup and snapshot of the ESI sequence for automated gradient generation and stop flow actuation.

Key findings on microswimmers chemotaxis

A stable flow-based gradient was achieved in the microfluidic chip as the active particles were flowing in the water stream. Once the flow was stopped, the particles were able to move freely in the microfluidic chip. We found that:

• There was a clear trend for the particles to swim downwards or upwards the fuel gradient, as compared to the control experiment (no gradient).
• This tendency is depending on the fuel initial concentration, with barely any change when the concentration is below 0.1% of H₂O₂.
• This phenomenon is explained by a change in drag force experienced by particles with orientation askew to the gradient, pushing them to reorient themselves towards the more concentrated region.
• By symmetry, particles with original orientation downward the gradient will not be affected by this mechanical orientational change.

Next steps include testing this platform with other types of microswimmers and fuels, to gain more insight into the physics behind chemotaxis [3].

Figure 3 – Illustration of the chemotaxis assay with artificial microswimmers at the different stages: (i) flow-based gradient generation and particles injection, (ii) stop flow, (iii) observation of particles swimming direction in fading gradient.

References