Microfluidic Static Droplet Arrays with Tunable Concentration Gradients

Recent advances in drop-based microfluidics are creating new opportunities for conducting high throughput analysis with significantly reduced sample and reagent volumes. In particular, microfluidic production of arrays of nanoliter-scale drops at prescribed coordinates could emerge as a powerful and inexpensive alternative to conventional multi-well-plate-based screening. However, the challenge is to develop effective strategies to vary the reagent concentration from drop-to-drop in the static droplet arrays (SDA). Here we describe a one-step passive strategy to create such concentration gradients in immobilized droplet arrays. The technique exploits controlled coalescence, and mixing of materials between moving plugs and stationary drops in a microfluidic parking network (MPN). We show that the concentration of one or more reagents can increase or decrease from drop-to-drop enabling a wide range of composition variation. This unique approach also generates concentration gradients of soluble reagents in drops containing insoluble materials such as particles and cells at well-defined time points.