Capillary Microfluidics for Bioanalysis: Design Principles and Application Perspectives

Capillary flow-driven microfluidic devices enable autonomous fluid transport for bioanalytical applications by using surface tension and wettability in microscale channels and porous structures, eliminating the need for external pumps. This review develops a coherent physical framework for capillary devices, based on the Young-Laplace equation, Jurin's law, the Lucas-Washburn equation, and Darcy's law, that relates interfacial curvature, capillary pressure, viscous dissipation, and permeability to fluid behavior in both hollow channels and porous media. From this framework, we derive practical design principles for priming, timing, valving, and sample metering in paper-based, laminate, and hybrid devices and demonstrate how these principles enable autonomous bioanalytical functions, including plasma separation, nucleic acid extraction, analyte preconcentration, immunoassays, and nucleic acid tests in both lithographically fabricated and paper-based platforms. The review places strong emphasis on mechanistic design rules that link capillary physics to assay performance, reproducibility, and integration level, reduce user intervention, shorten assay time, and approach laboratory-grade analytical performance while preserving low cost and portability. Finally, we discuss emerging wearable and body-interfaced devices that integrate capillary-driven sampling of sweat, interstitial fluid, and other biofluids with colorimetric, electrochemical, and optical readouts for continuous or time-resolved measurements. Together, these developments show how capillary actuation, grounded in classical transport laws, is enabling increasingly sophisticated pump-free microfluidic systems that are well suited to decentralized diagnostics, environmental monitoring, and point-of-need bioanalysis.