One drop at a time: toward droplet microfluidics as a versatile tool for single-cell analysis

Miniaturization has been the key driver for many remarkable technological developments in recent decades. Miniaturization has now also extended into biology, thereby setting the stage for high-throughput single-cell analysis. This advancement is important because, despite detailed molecular information on individual cell subtypes, virtually no information is available on the functional capacities of individual cells. Typical in vivo animal models, as well as in vitro laboratory test tube experiments, only yield a global outcome of interactions of often millions of cells rather than providing insight into the functional contribution of individual cells. Reaction volumes of biological experiments have generally been reduced from milliliters to microliters. Tools and methods that study single-cell behavior have become increasingly important, but often do not allow for high-throughput manipulation. Recent advances in (droplet-based) microfluidics enable systematic high-throughput analyses of individual cells in a highly controlled manner. The implementation of microfluidic technologies in single-cell analysis is one of the most promising approaches that not only offers new information and high-throughput screening but also enables the creation of innovative conditions that are impractical or impossible by conventional methods. In this review, we provide a comprehensive overview of recent developments in droplet-based microfluidics for single-cell studies.