A Programmable Nanodroplet Device with Direct Sample-to-Droplet Interface toward High-Throughput Screening

Droplet microfluidics offers a promising technology for high-throughput screening due to its potential for massive parallelization of nano- to picoliter-sized reactions. Fulfillment of this promise, however, has been impeded by ineffective "sample-to-droplet" interfaces with shortcomings ranging from significant waste of sample volume, limited scalability of the number of samples, to lengthy idle time due to sample injection and switch. In response, we have developed a programmable nanodroplet device with a "direct" sample-to-droplet interface that has low sample waste, high scalability, and minimal idle time. In our device, each sample (0.8 mu L) is directly spotted onto an open-to-atmosphere device inlet. The sample-to-droplet interface of our device uses programmable microfluidic valves to regulate vacuum-assisted infusion of the sample from the open inlet into the device, as well as pressure-driven generation of nanodroplets (as small as similar to 5 nL) from the sample with minimal waste. Our device ensures high scalability, as series of samples are simply processed by the sample-to-droplet interface in succession. Parallelization of a pair of our sampleto-droplet interfaces and effective cleaning between successive samples help minimize idle time and cross-contamination. Finally, we have integrated nanodroplet assembly and on-chip incubation capabilities in our device and have coupled our device with an in-house laser-induced fluorescence detector to achieve in-line fluorescence detection of our nanodroplets. For demonstration, we perform a cytochrome P450-based drug screening assay in 50-nL nanodroplets - a 2000-fold reduction from 100-mu L benchtop reactions - in our device and achieve excellent differentiation of no-drug controls from inhibitor controls (Z' = 0.84).