Accurate monitoring of sweat loss is crucial, particularly for individuals wearing Personal Protective Equipment (PPE) in harsh conditions, such as firefighters who are at high risk of dehydration. Existing sweat sensors often face challenges in measuring sweat loss under clothing due to bulky instrumentation or transduction methods requiring direct line of sight. This research aims to develop a low-cost, comfortable, and accurate sweat sensor wearable under PPE. The study focuses on resonant sensors—small circuits whose resonance frequency changes with the permittivity of the surrounding medium—as a potential solution for miniaturization and wireless interrogation. Current methods for sweat sampling and interrogation have limitations. Microfluidics, wicking materials, and natural ventilation are common sampling methods; natural ventilation is most accurate but bulky, while microfluidics and wicking materials are suitable for wearables. Existing interrogation methods, like potentiometry, capacitance, and impedance, often require direct line-of-sight or complex circuitry. This research proposes a resonant sensor approach, which has seen success in other biomedical applications and offers a simple, compact solution.

The literature review covers existing sweat analytical devices, categorized by sweat sampling methods (microfluidics, wicking materials, natural ventilation) and interrogation methods (potentiometry, capacitance, impedance). While many studies explore sweat sensors, few address the challenge of measuring perspiration under clothing, particularly thick PPE. The authors highlight the advantages of resonant sensors for size reduction and wireless integration in wearable applications, citing their prior use in intracardiac monitoring and other biomedical applications.

The study details the fabrication of a resonant sweat sensor sticker comprising a laser-ablated microfluidic channel and an Archimedian spiral resonator chemically etched from a copper-coated polyimide substrate. The microfluidic channels are fabricated in PDMS using a laser cutter and sealed via plasma bonding. A mathematical model is developed to predict channel dimensions based on laser power and cutting speed. The sensor is interrogated wirelessly using a handheld vector network analyzer (VNA) and a custom reader with a double-loop antenna. The sensor's response to varying NaCl concentrations (as a sweat proxy) is characterized, revealing a relationship between transmission loss (TL) and conductivity. The effect of sweat volume on the resonant frequency is also investigated, exhibiting a quadratic relationship with the filling distance. A small human study (four participants) is conducted to assess the effect of tissue dielectric heterogeneity and sensor-reader orientation on the sensor's variability. The study includes detailed explanations of sensor fabrication, channel fabrication through laser ablation, conductive NaCl solution preparation, channel filling setup, mechanical stability testing of the sensor, and the human study methodology, including participant selection, data acquisition, and analysis. The custom Matlab code used for data analysis is also mentioned.

The sensor successfully measures both sweat conductivity and sweat rate. The transmission loss (TL) magnitude correlates with conductivity, while the resonant frequency shift (Δf0) correlates with sweat volume. A mathematical model is developed to predict channel dimensions based on laser cutting parameters. A quadratic model accurately predicts the change in TL (ΔTL) as a function of conductivity. The effect of PPE thickness on the conductivity response is shown to be correctable via a linear offset. However, the effect of PPE thickness on sweat rate requires a calibration for different materials and thicknesses. The human study reveals a significant influence of sensor-reader orientation on the sensor's response, particularly for resonant frequency. The study quantified variability in TL and f0 responses due to inter- and intra-subject variations and sensor-reader repositioning. The signal-to-noise ratio for conductivity is high, but the one for sweat rate is affected by orientation.

The study demonstrates a feasible prototype for wireless, under-garment sweat monitoring using resonant sensors. The orthogonal response for conductivity (TL) and sweat rate (Δf0) is a key advantage. The use of a low-cost VNA makes the system potentially inexpensive. While the sensor shows promise, limitations related to sensor-reader orientation and the effect of PPE thickness on sweat rate need addressing. Strategies for improving orientation consistency (mechanical guides, sensor arrays) and calibration methods for different PPE types are discussed.

This study presents a prototype wireless sweat sensor capable of simultaneously measuring sweat conductivity and rate through clothing. While promising, challenges remain in optimizing sensor-reader orientation and accounting for PPE thickness variability. Future work should focus on improving sensor design, integrating improved orientation control mechanisms, and conducting larger human studies to validate the sensor's performance and reliability.

The study's limitations include the small sample size (four participants) in the human study, the use of NaCl solution as a sweat proxy, and the observed sensitivity to sensor-reader orientation. The effect of various PPE types on the sensor's response requires further investigation, as does the long-term stability and reliability of the sensor under real-world conditions.