Hydrogel photopolymerization within microfluidic droplets for single cell encapsulation

Molecular investigation of specific gene expression patterns at the single-cell level reveals the heterogeneity of bulk cell populations, facilitating the understanding of interactive regulatory pathways that are hidden. However, this understanding has been constrained for decades by the length scale differences between traditional macroscopic tools and the size of a single cell. Additionally, these tools are inadequate in efficiently manipulating millions of individual cells within a reasonably short time period. Therefore, issues associated with the above limitations represent a challenge for single-cell encapsulation and subsequent analysis. This review highlights recent developments in droplet-microfluidic techniques that have enabled the high throughput fabrication of cell-laden picoliter droplets, which can be subsequently photopolymerized into microgels for further analytical assessment or bottom-up tissue engineering practices. Novel microfluidic droplet techniques developed for single-cell screening, analysis, and scaffold assembly for tissue engineering are introduced. Meanwhile, the constraints and challenges encountered in microfluidic droplet miniaturization processing are discussed in the context of enabling single-cell manipulation. In the end, an emerging alternative material, which holds great promise for single-cell encapsulation in miniaturized hydrogels, is introduced as a cell carrier with biocompatible polymerization kinetics and cytocompatibility.