Silicon as a microfluidic material for imaging and incubation of droplets

Droplet microfluidics has become a powerful tool in life sciences, underlying digital assays, single-cell sequencing or directed evolution, and it is making foray in physical sciences as well. Imaging and incubation of droplets are crucial, yet they are encumbered by the poor optical, thermal and mechanical properties of PDMS - the de facto material for microfluidics. Here we show that silicon is an ideal material for droplet chambers. Si chambers pack droplets in a crystalline and immobile monolayer, are immune to evaporation or sagging, boost the number of collected photons, and tightly control the temperature field sensed by droplets. We use the mechanical and optical benefits of Si chambers to image ∼1 million of droplets from a multiplexed digital assay - with an acquisition rate similar to the best in-line methods. Lastly, we demonstrate their applicability with a demanding assay that maps the thermal dependence of Michaelis-Menten constants with an array of ∼150,000. The design of the Si chambers is streamlined to avoid complicated fabrication and improve reproducibility, which makes Silicon a complementary material to PDMS in the toolbox of droplet microfluidics. Significance Statement As the technological engine behind single-cell sequencing and digital assays, droplets microfluidics has revolutionized life science and molecular diagnosis, and is making foray into physical sciences as well. Observing droplets in a controlled manner is becoming crucial, but PDMS - the de facto material of microfluidics – hampers imaging and incubation. Here we revisit silicon as a microfluidic material and show that its superior mechanical, optical and thermal performances improve the throughput and operation of droplets assay. ### Competing Interest Statement The authors have declared no competing interest.