The team led by Suraj Shinde at Jeonbuk National University systematically reviewed the latest advancements in wearable sweat-sensing patches (WSPs) for personalized healthcare monitoring, offering a pathway to integrate WSPs into flexible human-machine interfaces, personalized healthcare solutions, and closed-loop systems.
With advancements in technology, there is growing interest in real-time monitoring of vital signs. Wearable sweat sensors can adhere to organ surfaces, analyzing bodily fluids (such as sweat, saliva, and tears) to accurately detect subtle changes in biochemical components, electrolytes, metabolites, and exogenous substances. Therefore, secretion monitoring is crucial for maintaining physiological homeostasis. Compared to traditional blood and urine analyses, sweat analysis has gained attention due to its unique advantages of high accessibility, non-invasiveness, and ease of use. Wearable sweat sensing patches (WSPs) enable continuous, real-time monitoring of biomarkers and track dynamic changes in health indicators.
The fundamental principles and sensing methods of WSP
WSP can collect, process, and quantitatively analyze sweat. It is typically constructed with multiple layers, including a bonding layer for secure skin attachment, a sampling layer for efficient sweat transport, and a sensing layer for targeted analyte detection. Each layer is designed for specific functions, such as sweat collection, which is often achieved using microfluidic, porous adhesive patches, or microporous patch channels. These channels utilize capillary action or hydrophilic pathways to facilitate smooth flow and minimize sample loss, thereby guiding sweat from the skin to the sensor.
Despite the crucial role of the adhesive layer in securely and stably attaching WSP to the skin, sensors capable of maintaining long-term reusability and durability in complex biological fluid systems are required due to the continuous renewal of the stratum corneum, skin oils, and prolonged exposure to external environments (e.g., water, heat, chemicals, and sunlight). Wang et al. developed a tough silk fibroin-polyacrylamide (SF-PAAm) hydrogel patch with robust, tunable, and durable adhesive properties. Compared to the original SF hydrogel, the SF-PAAm DN hydrogel retains high water content, enabling it to form a firm and tight bond with the DN hydrogel structure featuring high porosity, thereby ensuring close adherence to the skin surface and reducing the likelihood of detachment during movement. Meanwhile, Sharifi et al. developed a smart wearable optical sensor (SWOS) that combines sweat sensor patches with an IoT-enabled readout module for continuous, real-time monitoring of sweat volume and pH levels.
Due to the accessibility of sweat and its wide range of analytes, WSP employs various sensing modes, utilizing electrochemical, optical, and colorimetric methods to detect and quantify biomarkers in sweat. Among these, chromatography can separate individual components from complex mixtures for analysis, while mass spectrometry quantifies biomarkers by examining the mass-to-charge ratio, supporting both targeted and untargeted detection. Electrochemical methods such as amperometry and potentiometry measure analyte concentrations by monitoring electrochemical changes at the electrode surface, whereas colorimetry relies on specific chemical reactions to produce color or fluorescence changes for semi-quantitative analysis. Ultimately, these detection data can be wirelessly transmitted to personal devices, enabling portable and continuous monitoring for remote healthcare.
Innovative Sweat Collection Technology in WSP
Since the concentration of analytes in sweat is lower than that in blood, effective sweat sampling is crucial for the continuous and accurate monitoring of biomarkers in sweat by WSP. Iontophoresis is a process that uses a mild electric current to transport ions or charged molecules through the skin, whereas reverse iontophoresis operates by extracting ions or charged molecules from the body through the skin using low current, rather than introducing substances into the body. Paz et al. introduced a flexible epidermal adhesive patch that integrates a reverse iontophoresis (RI) system with an amperometric lactate biosensor on the anode electrode, working in conjunction with a porous hydrogel reservoir to achieve sweat lactate extraction and electrochemical measurement. This patch effectively reduces the time required for RI, lowers the risk of skin damage, and promotes rapid sweat collection and continuous lactate level monitoring. These innovative designs have the potential to detect other biomolecules in sweat, thereby expanding its applications in broader health monitoring.
The Impact of WSP Attachment Position
The composition of sweat, secretion rate, and skin characteristics may vary depending on the placement of sweat patches. Therefore, selecting the appropriate patch location is crucial for obtaining meaningful and informative data for monitoring. In their study, Hooton et al. collected sweat over a period (e.g., 24 hours) using non-occlusive sweat patches attached to the skin and combined it with a method known as "differential chemical isotope labeling (CIL) LC-MS" to analyze the metabolic profiles of sweat samples obtained from the epidermis of the left forearm, lower back, and neck in 20 healthy participants. Additionally, Baker et al. conducted an experiment involving 11 amateur athletes (7 males and 4 females, with an average body weight of 71.5 ± 8.4 kg) who participated in two randomized cycling trials. The results indicated a significant increase in WB sweat [Na], [Cl+−], and [K] from LOW to MOD intensity. Consequently, effective and convenient sites for sweat sampling are the fingertips, palms, and dorsum of the hands, as these areas are rich in eccrine glands and allow for rapid absorption of minimal natural sweat secretion.
WSP's healthcare applications
WSP has shown great potential in the healthcare field as a biomarker associated with metabolic health, hydration, and chronic diseases. For example, they can track glucose levels in sweat, providing a non-invasive alternative to diabetes management. Similarly, WSP can measure electrolyte levels, which is crucial for diagnosing dehydration and electrolyte imbalances, especially in patients with kidney or endocrine disorders. These sensors have great potential in disease diagnosis, management, and personalized healthcare due to their sensitivity, selectivity, and seamless integration with digital health technology.
Wearable sweat sensing patches (WSPs) are at the forefront of wearable technology, providing a non-invasive and continuous way to monitor various biomarkers in real-time. With its adaptability and accessibility, sweat sensing patches have the potential to significantly improve personalized healthcare, support early disease detection, enhance athletic performance, and achieve environmental monitoring for public health. However, to achieve widespread adoption, challenges such as standardization, reliability, and data interpretation must be addressed. The continuous advancement of WSP technology requires collaboration between scientists, engineers, and healthcare providers to overcome these barriers and unleash the full potential of this innovative technology.
Source: Sensor Expert Network