High-throughput magnetic particle washing in nanoliter droplets using serial injection and splitting

Droplet microfluidics has emerged as a promising technique to perform high-throughput, massively-parallel chemical and molecular biological reactions. Droplet microfluidic operations such as droplet generation, sorting, and fluid addition are well established; however, fluid exchange (i.e. washing) at high-throughput is challenging to implement. Here we present a microfluidic device architecture that utilizes wash buffer injection preceding a splitting junction in proximity to a magnetic field to transfer paramagnetic microparticles across a concentration gradient within a single droplet. The device can operate at high throughput (50?Hz) while preserving input droplet volume at the collection outlet as verified using high speed imaging. Using a two-stage device, combined microparticle retention rates (up to 97.5%) and high wash efficiency (92.9%) is demonstrated using dye absorbance and fluorescence. This method can be performed in a serial array to obtain an arbitrary degree of wash efficiency and integrated into lab-on-a-chip systems for use in multi-step microfluidic bioassays or single-cell genomic applications requiring high-fidelity washing steps within droplets.