Quantitative analysis for sweat-absorbing times of paper-based microfluidic chips

Structures of paper-based microfluidic chips affect the sweat-absorbing time when they are used for sweat analysis. For the first time, we use COMSOL to establish two types of paper-based chip sweat-absorbing models that can quantitatively analyze this phenomenon. The standard model contains 1089 sweat glands, and the simplified model simplifies it according to the idea of finite element division, including 81 sweat glands. Sweat flows in from the bottom of the paper-based chip and out from the electrode contact surface (the upper surface of the central cylinder of the paper-based chip). Both models contain six paper-based chip structures, use Richards’ equation as the governing equation, set the outflow velocity to 0, and set the sweating rate of a sweat gland at 0.6 μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mu$$\end{document}L/min. In the standard model, it takes only 46 s for the paper-based structure with the fastest sweat-absorbing speed to completely saturate the electrode contact surface with sweat (meaning the sweat-absorbing time is 46 s), which is 13.06%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\%$$\end{document} shorter than that of the slowest structure. In the simplified model, the top 3 structures of sweat-absorbing speed are consistent with the standard model. The simulation results show that the sweat-absorbing time is positively correlated with the H value of the bottom surface of the paper-based structure (defined as the area of the bottom surface /the area of sweat glands covered by the bottom surface), which can be proved by analytical and experimental methods. The analytical method proves that this conclusion can be generalized to other sweating rate conditions.