Soft, Skin-Integrated Multifunctional Microfluidic Systems for Accurate Colorimetric Analysis of Sweat Biomarkers and Temperature

Abstract

Real-time measurements of the total loss of sweat, the rate of sweating, the temperature of sweat, and the concentrations of electrolytes and metabolites in sweat can provide important insights into human physiology. Conventional methods use manual collection processes (e.g., absorbent pads) to determine sweat loss and lab-based instrumentation to analyze its chemical composition. Although such schemes can yield accurate data, they cannot be used outside of laboratories or clinics. Recently reported wearable electrochemical devices for sweat sensing bypass these limitations, but they typically involve on-board electronics, electrodes, and/or batteries for measurement, signal processing, and wireless transmission, without direct means for measuring sweat loss or capturing and storing small volumes of sweat. Alternative approaches exploit soft, skin-integrated microfluidic systems for collection and colorimetric chemical techniques for analysis. Here, we present the most advanced platforms of this type, in which optimized chemistries, microfluidic designs, and device layouts enable accurate assessments not only of total loss of sweat and sweat rate but also of quantitatively accurate values of the pH and temperature of sweat, and of the concentrations of chloride, glucose, and lactate across physiologically relevant ranges. Color calibration markings integrated into a graphics overlayer allow precise readout by digital image analysis, applicable in various lighting conditions. Field studies conducted on healthy volunteers demonstrate the full capabilities in measuring sweat loss/rate and analyzing multiple sweat biomarkers and temperature, with performance that quantitatively matches that of conventional lab-based measurement systems.

Advanced Wearable Microfluidic Sensors for Healthcare Monitoring

“…CBVs utilize capillary effects for liquid manipulation, which is determined by the surface tension of the liquid and the geometry and surface chemistry of the µ‐channels. CBVs can be molded with the same manufacturing material as that of the microfluidic networks and can be easily adjusted in terms of their geometry . Furthermore, utilizing a series of CBVs, a microfluidic system can manipulate liquids in a preprogrammed manner and realize time‐dependent sampling, which has great potential in capturing chronographic physiological information …”

“…b) Schematic illustration of microfluidic sweat sensor for colorimetric detection of sweat temperature and other biomarkers (left), and optical image of temperature sensor's color and RGB level at different temperatures (right). Reproduced with permission . Copyright 2019, American Chemical Society.…”

Application of Microfluidics in Wearable Devices

“…C) Schematic illustration of a unit cell in device and sketch of CBVs. Reproduced with permission . Copyright 2019, American Chemical Society.…”

“…For example, Rogers et al designed capillary bursting valves (CBVs) in their skin‐mounted microfluidic devices to realize precise sampling capability. Each CBV is a microfluidic channel with a diverging section, and has its characteristic bursting pressure (BP) by setting specific diverging angle, width, and height values of the channel (Figure C) . The liquid samples will first flow through the valve with lower BP and then reach other valves.…”

Biofunctionalization of Glass‐ and Paper‐Based Microfluidic Devices: A Review

“…Microfluidic devices and lab‐on‐a‐chip technology have been distinguished as a promising platform for ultrasensitive bioanalytical devices, point‐of‐care (POC) diagnostics and health monitoring systems, investigation of cell biology and tissue engineering, mixing and separation of biospecies, drug discovery, biohazard detection, as well as automating synthetic biology to create micro‐organism and DNA foundries . Incorporating microfluidics in different bioanalytical systems such as those based on fluorescence, colorimetric, electrochemical, and chemiluminescence readout, as well as mass spectrometry has significantly enhanced the limit of detection (LOD) of these assays . In cell biology, microfluidics enables the integration of multiple cell lines to mimic “organs‐on‐chips” with the desired physiological conditions and hemodynamic forces.…”