Understanding partial saturation in paper microfluidics enables alternative device architectures†
Paper-microfluidic devices are becoming more common due to their ability to automate diagnostic assays at low cost. Increasingly, paper-microfluidic devices automate more complex, multi-step assays, with the aim of bringing these assays out of the laboratory to the point-of-care. However, some common assay procedures have resisted easy automation by paper-microfluidic devices. Sample preparation procedures in particular, due to the complexity of real biological samples, still largely involve the manual execution of multiple user steps before addition to a paper device, often with the aid of specialized equipment. In the laboratory, most sample types are exposed to (bio)chemical treatment to release target species, which often requires additional purification steps prior to detection. Some samples, such as urine, also require concentration of target species; others, such as nasal swabs or blood, also require removal or dilution of non-target species that interfere with downstream assay steps. Additionally, the samples used in many laboratory assays require volumes (milliliter scale) too large to process efficiently with paper devices. To broaden the effectiveness and availability of point-of-care testing, we have developed and characterized a technique that automates either the concentration or dilution of large-volume samples in paper-microfluidic devices. Here we demonstrate its simplicity and broad utility in two ways: (1) an automated parallel dilution system for an immunoassay detection antibody and (2) an automated DNA extraction and concentration system for mL-sized samples. This and similar techniques rely on an improved understanding of how fluids wet porous materials and flow through multi-material networks; they confer important new capabilities to paper-microfluidic devices, the most important of which is the integration of complex sample preparation with downstream biomolecule detection.