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

Abstract

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. Applying droplet microfluidics to cells trapped within water-in-oil droplets offers a powerful way to study single cells in action. Wilhelm Huck and colleagues at Radboud University in the Netherlands have reviewed the application of this technique and research toward the study of individual cells. Unlike most cell analysis methods, it allows specific chemicals to be associated with the individual cell they come from or interact with. Researchers are currently addressing many technical challenges, including performing washing steps inside droplets and developing better ways to monitor and manipulate the cells at high-throughput rates. Current work is focused on reducing the complexity and enhancing the sensitivity of the technique. The authors also review applications that could soon become routine, including drug and antibody studies, tracking single-cell gene expression, studies of cell evolution and analysis of cellular interactions when droplets fuse. This review highlights the recent advances in droplet-based microfluidics for studying the properties of single cells, with a specific interest in quantitative studies into the heterogeneity of large populations of cells