Advances in reagent addition for droplet-based microfluidics: from interfacial engineering to high-throughput screening

Droplet-based microfluidics has transformed high-throughput screening by compartmentalizing biochemical reactions into nanoliter-to-picoliter soft microenvironments. However, precise on-demand reagent addition after droplet formation remains challenging due to interfacial tension barriers. This review comprehensively summarizes recent advances in reagent addition technologies from the perspectives of soft matter physics and interfacial engineering. Strategies are categorized into passive hydrodynamic merging and active injection approaches, highlighting the mechanisms used to disrupt surfactant-stabilized interfaces for reagent delivery. The broad impact of these spatiotemporally decoupled addition strategies is further discussed in applications including multistep synthesis of soft functional materials, drug screening, single-cell multi-omics, and directed enzyme evolution. Finally, current challenges limiting industrial translation are critically assessed. Continued advances in this field are expected to drive the development of intelligent droplet microreactor systems for next-generation biomedical and materials applications.