Abnormal Liquid Chasing Effect in Paper Capillary Enables Versatile Gradient Generation on Microfluidic Paper Analytical Devices.

Microfluidic paper analytical devices (μPADs) are smart and accessible substituents to traditional counterparts in point-of-care tests (POCT), which exploited delicate control over passive fluid in microscale for rich functions. In this work, we are extending such control by introducing novel ways to generate 1D and 2D gradients on μPADs. It is achieved by using paper-capillary-based serial sampling. The paper capillary is composed of a concaved paper channel sealed with tape, with test pads properly distributed aside. In such a structure, the liquid can not only quickly and automatically flow along the capillary but also be continuously consumed by the peripheral test pads. Thus, when we do serial sampling, an abnormal liquid chasing effect can be observed, where the later liquid sample chases and surpasses the earlier parts in the paper capillary, leading to reversely ordered sample distribution compared with that in a typical glass capillary. This specialty allows for fast, ordered, and tunable sequential sampling and enables efficient generation of 1D and 2D concentration gradient with one, two, and even three components on μPADs. Besides, we verified the applicability of this technique for arrayed assays, including 1D serial dilution-based metal ion colorimetry and a 2D bacterial antibiotic susceptibility test for synergic effect evaluations, which paves the way for high-throughput sample analysis and information-rich condition screening on μPADs.