Advances in microfluidics-based single-cell secretion analysis

Secretion is one of the most fundamental and essential cellular functions, playing a critical role in regulating a wide range of biological processes, including cell-to-cell communication, proliferation, migration, differentiation, and apoptosis. Importantly, individual cells exhibit significant heterogeneity in their secretory behavior, reflecting diverse functional states and responses to environmental cues. Conventional population-based analyses average the signals from large cell ensembles, thereby masking this cellular heterogeneity and impeding our understanding of the diversity and dynamics of cellular functions. To address this limitation, single-cell secretion profiling has emerged as a powerful approach to dissect functional diversity, uncover regulatory mechanisms in biological systems, and enable the development of precision diagnostics and therapeutics. However, single-cell secretion analysis poses substantial technical challenges, such as isolating and tracking individual cells, maintaining cellular viability and function during analysis, achieving sufficient sensitivity and multiplexity in protein detection, and scaling the process for high-throughput applications. Recent advances in microfluidic technologies have provided innovative solutions to many of these challenges by enabling precise fluid handling, integration of complex operations, and miniaturization of reaction volumes. Three major classes of microfluidic platforms have been widely adopted for single-cell secretion analysis: integrated microfluidics, droplet microfluidics, and microwell arrays, each offering distinct advantages and trade-offs. Integrated microfluidics employs networks of microchannels, valves, and chambers to perform highly controlled, programmable operations such as single-cell trapping, reagent exchange, and multiplex detection. This platform provides high precision, environmental control, and reaction integration, making it suitable for dynamic studies and complex workflows. However, it often requires elaborate chip fabrication and external pressure or pneumatic systems, limiting its accessibility and scalability. Droplet microfluidics achieves ultra-high throughput by encapsulating single cells into nanoliter or picoliter droplets, effectively creating thousands of isolated microreactors per second. This approach allows rapid screening and enables integration with barcoded beads for multiplex protein detection and multi-omics profiling, such as combining secretome with transcriptome or surface proteome analysis. Despite its scalability, droplet microfluidics faces challenges including low single-cell occupancy rates, limited reagent exchange within closed droplets, and relatively lower detection multiplexity compared to other formats. Microwell arrays offer a structurally simple yet highly addressable platform, where single cells are randomly loaded into microwells via gravity without the need for complex fluidic control. This simplicity facilitates integration with antibody barcode chips, enabling highly multiplexed protein detection (up to 40+ analytes). Furthermore, spatial addressability allows the correlation of secretion data with off-chip assays such as transcriptomics or imaging. Nonetheless, microwell systems typically suffer from lower throughput and reduced environmental control compared to the other two platforms. This review provides a comprehensive overview of these microfluidic technologies, comparing their working principles, technical advantages, limitations, and application scenarios. We further highlight their use in immunological studies, such as T-cell functional profiling, antibody discovery, and real-time monitoring of immune responses. Additionally, we discuss emerging trends and future directions in this field, including (1) enhancing information richness, by shifting from static to dynamic, time-resolved secretion monitoring and integrating multi-omics such as transcriptomics, epigenomics, metabolomics, and spatial transcriptomics; (2) improving data utilization, through AI-powered data processing and integration to extract meaningful insights from high-dimensional datasets; and (3) developing high-dimensional functional sorting technologies, enabling the identification and engineering of multifunctional cells for applications in immunotherapy, aging research, and personalized medicine. With continued advancements in microfluidic design, materials, automation, and interdisciplinary collaboration, we believe microfluidics-based single-cell secretion analysis is poised to become a cornerstone technology for understanding complex biological systems and driving next-generation biomedical innovations.