Fast capillary flow on μPADs with hollow channels packaged by a thermal contraction tube

Microfluidic paper-based analytical devices (μPADs), as an excellent platform for point-of-care diagnostics, are becoming more and more popular. Flow rate control of liquid samples on μPADs is highly intriguing for improving the detection performance of these devices. In this work, we develop a novel package method for μPADs using a thermal contraction tube, which can be easily used to create hollow channels on μPADs and generate fast capillary flow of liquid samples. Using this package method, we create hollow channels above a single layer of a cellulose paper strip and between two layers of cellulose paper strips. We investigate the influence of the height of the hollow channels on the capillary flow rate, and find that the average flow rate increases with the height of hollow channels with height up to 0.8 mm. For the μPADs with hollow channels above a single layer of a paper strip, the maximum increase can reach 865% while the maximum increase is 1328% for μPADs with hollow channels between two layers of paper strips when the height of the hollow channels is 0.8 mm. As a proof of concept, we use μPADs with hollow channels above a single layer of a paper strip for glucose detection in blood plasma, showing much higher efficiency compared to a naked paper strip. In addition, we can achieve a fluidic switch function on these μPADs with hollow channels by simply pressing and releasing the thermal contraction tubes, with the possibility to speed up or slow down. Furthermore, we use μPADs with 0.8 mm hollow channels between two layers of paper strips for fast measurement of viscosity of liquid samples. We successfully measure viscosity of liquid samples including water, mixture of glycerol and water, mixture of poly(ethylene glycol) (PEG) and water, mixture of sodium carboxymethylcellulose (SCMC) and water, as well as blood plasma and whole blood, with viscosities having a range of three orders of magnitude. With advantages including easy handling and low cost, we believe μPADs packaged by this method can find more applications in point-of-care diagnostics.