When it comes to sweat, the first impression would be a stinky workout byproduct for most people. However, sweat is an interesting biological fluid for researchers who focus on health-assessing wearable microfluidic sensors, because it contains electrolytes, molecules, lactate, and proteins working as excellent non-invasive biomarkers of physiological health.
An Overview of Microfluidics
Microfluidics are small volumes of fluids, down to a quadrillionth of a liter. The behaviors of fluids are very different on the micrometric scale compared with that in daily life, which inspires new and innovative scientific experiments. This term could refer to both studies of the fluid behaviors through micro-channels and the technology of manufacturing microminiaturized devices, in which chambers and tunnels are designed for fluids flowing or confinement. The key concept of microfluidics is the lab-on-a-chip technology that integrates a simple micro-sized system or a whole laboratory into a chip.
About Microfluidic Chips
Microfluidic chips are molded or engraved microchannels, forming distinct compartments of inlets, channel mixers, and signal detectors. Some channels with different dimensions are hollowed out through the chips so that fluids can be injected into and evacuated from the microfluidic chips through these pathways.
Several materials are applicable in the manufacturing of microfluidic chip components, including organic materials like polydimethylsiloxane (PDMS), polystyrene (PS), polymethyl methacrylate (PMMA), and paper, and inorganic materials like silica and glass, as well as some composite materials. And these microfluidic components along with specific systems to manage these microchannels are assembled to make a functional chip. Microfluidic chips are widely used for laboratory-on-a-chip construction, detection of pathogens, electrophoresis, DNA analysis, microfluidic diagnostics, etc.
Microfluidic Sweat Sensors
The advancement in miniaturization and material science prompts a variety of electronic devices for healthier lives. Wearable microfluidic sweat sensors are such kinds of tools that monitor and reflect the health condition of a person in real-time by measuring and quantifying biomarkers.
Sweat analysis on microfluidic chips has been widely used because sweat is a local and noninvasive health indicator that carries diverse biomolecules, such as small electrolytes, metabolites, hormones, lactate, and larger proteins. Most of the microfluidic sweat sensors are layered devices containing microfluidic channels that can suck sweat into the sensor, which is a piece of textile embedded in the center of the device with a pH-sensitive dye, for further sweat analysis.
How Does It Work?
After sweat flows into the microfluidic chip through pathways, it then reacts with different preset chemicals in individual sensor chambers, by which the sensor will show a color change based on the pH, lactate, chloride, and glucose concentrations of the sweat.
And modern technology provides a wireless communication environment for microfluidic sweat sensors, meaning people can monitor their physiological health conditions with the colorimetric changes and a real-time readout of the results, if only appropriate applications are developed on the smartphone.
Recent Research on Microfluidic Sweat Sensors
A recent study of microfluidic sweat sensors to measure real-time lactate concentration was conducted by scientists from the Department of Pure and Applied Chemistry at the Tokyo University of Science, Japan.
Lactate is one of the most important biomarkers for both blood and sweat and has already been a common material for microfluidic wearable sensors. It's a product of the glucose breakdown in the absence of oxygen in tissues, so it can display the intensity of physical exercise and the oxygenation of muscles. Most lactate will be released to the bloodstream while some will be eliminated via perspiration. As a result, microfluidic wearable chemical sweat sensors could measure the lactate concentration and return the real-time monitoring result.
Research of the Tokyo University of Science mainly focuses on the sensing mechanism and the material of the microchannels. Previous microfluidic sweat sensors would immobilize lactate oxidase and a mediator on an electrode, and their chemical reactions result in measurable current to show the lactate concentration. However, most of them apply materials that irritate the skin. This recent research changes the way of immobilizing the enzyme and mediator on an electrode with a method called electron beam-induced graft polymerization, and the microfluidic chips of the novel microfluidic sweat sensor are made of PDMS, a soft and nonirritating material.
In a conclusion, promising wearable microfluidics enables the collection and analysis of sweat, allowing real-time and continuous monitoring of vital signs, including glucose, lactate, sodium ion/potassium ion, calcium ion, and pH.
