(483g) Microswimmer Combing: A Dynamic and Ultra-Simple Approach for Rapid Isolation of Active Microswimmers on an Open Surface

High-throughput isolation of bioparticles like cells and micro-organisms, is an essential technical need for a wide range of applications, such as image-based phenotypical screening and single-cell transcriptomic profiling. Various microfluidic methods have been developed to isolate static bioparticles including cells and embryos. Among these methods, open-surface microfluidic techniques, such as open-surface droplet arrays and under oil open-channel microfluidics, possess a unique advantage as they do not require sophisticated off-chip controllers and tubing connections. They hence greatly lower the operational barrier for general biologists and pharmaceutical researchers. Most open-surface techniques rely on engineered surface wettability or surface morphology to partition liquid buffers into nanoliter or microliter droplets for sample isolation. While these open-surface methods are successful in manipulating nonmoving bioparticles, adapting them for screening multicellular and active microswimmers is not as straightforward. The challenge is that these microswimmers usually have irregular body shapes and are highly mobile, which make them difficult to handle and isolate by existing sample loading mechanism based on droplet partitioning.

To address this challenge, we develop an ultra-simple approach termed “microswimmer combing” that can in parallel isolate highly active small animals from a suspension and individually address them on an open surface. Our approach exploits a novel dynamic sample loading mechanism by engineering local contact line dynamics on a 2D microgel array. The unique interfacial dynamics dominates the active locomotion of microswimmers, and hence achieves open-surface sample isolation. We demonstrate the efficacy of this approach using one of the most studied and hardest to isolate microswimmer, Caenorhabditis elegans, as a model. The open-surface substrate for housing individual C. elegans consists of an array of separated hydrophilic PEG-based microgel pads surrounded by a continuous, less-hydrophilic plastic surface made of structured Kapton tape. To capture individual microswimmers on the microgel array, we move a thin film of active worm suspension across the microgel array by a glass slide on top of the substrate, which we call “combing”. When the receding edge of the liquid film slides through, the imbalance of local capillary pressures between the microgel pad and the surrounding plastic surface due to their different wettabilities drives a rapid collapsing of opposite meniscus between two adjacent microgel pads. Such a fast contact line dynamic can hence pin the individual animal on a microgel pad autonomously. This approach can be performed simply by hand without any external equipment. More than 100 worms can be isolate on a single chip with 80% single isolation rate, within 30 seconds.

We provide a simple scaling theory to guide the design of the device for adapting this approach to different active bioparticles. The mechanistic principle underpins the robust sample loading which does not rely on user expertise; the isolation performance is largely guaranteed a priori by design. We demonstrate the effectiveness and simplicity of this approach by characterizing the consistent sample isolation performance with different device designs, loading conditions, and users. The open-surface array of isolated microswimmers can be directly used for multiple screening applications. Here we demonstrate the utility of this strategy with high-resolution image-based screening of synaptic development in larval C. elegans. Other screening functions, such as selective sample recovery, can also be enabled by the open-accessibility of individual samples isolated on our device. Although we demonstrate this method using nematode C. elegans as an example, we envision that our device, with proper scaling, can be easily modified to study other active microswimmer model systems, such as parasitic nematodes, D. rerio and Ciona larvae, and motile bacteria.