Monitoring epidermal perspiration is crucial for managing chronic diseases and preventing life-threatening events. Sweat rate and composition provide valuable insights into hydration levels and potential health issues like hypoglycemic shock or heart attacks. Current sweat-sensing platforms often suffer from limited operating time due to sweat mixing and high energy consumption for continuous wireless data transmission. This limitation arises from the finite volume of sweat sensors and the energy cost of constantly transmitting data, much of which is redundant. To overcome these limitations, the researchers developed a novel epifluidic electronic patch that mimics the spike-encoding mechanism of sensory neurons. This event-driven approach transmits data only when significant changes occur, drastically reducing energy consumption while still capturing key information about sweat dynamics. The development of such a system has the potential to revolutionize long-term, non-invasive health monitoring by providing continuous, accurate data in an energy-efficient way, making it more practical for everyday use.

Existing sweat-sensing technologies face challenges in long-duration monitoring due to the limited volume of sweat sensors and the mixing of old and new sweat. Various approaches have been explored, including electrochemical measurements and microfluidic systems. While increasing sensor volume can extend operation time, it increases hydrodynamic resistance and size, compromising comfort. Recent advancements have employed thermal flowmeters for longer-term sweat rate measurement but lack the ability to simultaneously measure ionic conductivity. Another major limitation is the high energy cost of continuous wireless data transmission for long-term monitoring, generating large amounts of redundant data. Neuron-inspired spike encoding has shown promise in energy-efficient signal processing but hasn't been successfully applied to direct perspiration monitoring. This study aimed to address both limitations—limited operation time and high energy consumption—by developing a spiking sweat clearance system to enable event-driven perspiration monitoring.

The researchers designed a novel sweat VIA (vertical interconnect access) sensor with a truncated cone-shaped vertical sweat channel. This channel has nanomesh electrodes on its inner wall and a super-hydrophilic CNT-PDMS sponge at the top for rapid sweat absorption and clearance. The filling and abrupt emptying of the channel generates electrical spike patterns. The spike frequency is proportional to the sweat rate, and the amplitude is proportional to the ionic conductivity. The nanomesh electrodes, fabricated using a hydrogel-templated molding transfer method, consist of silver nanowires (AgNWs) and carbon nanotubes (CNTs), further electroplated with gold (Au) for biocompatibility. The CNT-PDMS sponge facilitates rapid sweat absorption and evaporation due to its high surface area and porous structure. The sensor’s admittance is measured using an AC voltage, with the frequency chosen to ensure the resistive component of the sweat dominates the impedance. The sweat VIA sensor is integrated into a wireless epifluidic patch that transmits data via Bluetooth Low Energy (BLE). A custom algorithm transmits data only during spiking events, minimizing energy consumption. The patch's performance was evaluated in human participants during exercise by comparing data to manual sweat collection using absorbent cotton pads. The sweat rate and conductivity were determined using the spike frequency and amplitude, respectively. Long-term stability and reproducibility were assessed through 24-hour tests.

The sweat VIA sensor successfully generated spike patterns in admittance, with the spike frequency linearly correlating with sweat flow rate up to 3 µL/min, and then increasing at a slower rate. The peak admittance of each spike was directly proportional to the ionic conductivity of the sweat. The addition of non-conductive components commonly found in sweat (urea, serine, glycine) did not affect the spike frequency or peak admittance, demonstrating the sensor's selectivity for ionic conductivity. The epifluidic wireless patch successfully transmitted data in a spiking-event-driven manner, requiring only 0.63% of the energy needed for continuous data transmission. In a human study using stationary cycling, the patch effectively monitored sweat rate and conductivity at different chest locations, with the center area showing earlier sweat secretion than the outer area. The results strongly correlated with manual sweat collection, but the event-driven system provided significantly more data points. The sensor system demonstrated long-term operation (up to 24 hours) and monitored a large cumulative sweat volume of 720µL in one test and over 30µL in the on-body experiment, showcasing its potential for prolonged monitoring. The overall energy consumption of the spiking-event driven transmission was significantly lower compared to the continuous data transmission with 0.57% (C) and 0.27% (R) of wireless data transmission required.

The findings demonstrate the successful development of an energy-efficient, long-term sweat monitoring system. The event-driven approach addresses the limitations of existing technologies by reducing energy consumption and providing sufficient information for accurate sweat analysis. The correlation between the sensor data and manual sweat collection validates the accuracy of the system. The ability to monitor sweat rate and ionic conductivity simultaneously using a single, small device is a significant advancement. The observed regional differences in sweat secretion highlight the potential of the patch for mapping sweat distribution on the skin, which could be useful for various applications, including disease diagnostics and personalized fitness monitoring. The long-term stability and reproducibility of the sensor further enhance its practicality for real-world applications.

This study successfully demonstrates a novel epifluidic wireless patch for event-driven perspiration monitoring. The spiking sweat clearance mechanism enables long-term, energy-efficient operation, significantly reducing power consumption compared to continuous monitoring. The patch accurately monitors sweat rate and ionic conductivity, providing detailed information about perspiration dynamics. Future research could explore integrating the sweat VIA sensor with other on-skin sensors and advanced computing technologies for even more comprehensive health monitoring.

While the study demonstrates the effectiveness of the system, potential limitations exist. The human study involved a limited number of participants and a specific type of exercise. Further research is needed to validate the system's performance across different populations, activities, and environmental conditions. The calibration of the sensor may vary across individuals due to differences in skin properties, and standardized calibration procedures are needed for wider adoption. The influence of sweat composition other than ionic conductivity on the sensor response warrants further investigation.