Large-scale fully printed “Lego Bricks” type wearable sweat sensor for physical activity monitoring

Traditional methods for monitoring biomarkers in blood and saliva for thermal-related diseases are invasive, costly, and time-consuming. Sweat, however, offers a non-invasive alternative, containing valuable biochemical information such as sodium (Na⁺) and potassium (K⁺) concentrations, which are crucial indicators of hydration status and potential thermal-related diseases. Abnormal levels can lead to various health issues. Wearable sweat ion electrochemical biosensors offer a promising solution for continuous, in situ monitoring. These sensors generally consist of a microfluidic layer, an ion-selective electrode (ISE), and a data transmission system. While existing research focuses on electrolyte level monitoring, limited studies explore changes under external interference. This research aims to develop a low-cost, high-performance, large-scale wearable sweat biosensor to monitor these electrolyte changes under diverse conditions, providing valuable insights into exercise and health management. The ISE is critical, determining the accuracy of the detection signal. It comprises a conductive substrate, an ion-electron transducer layer, and an ion-selective membrane. Maintaining stability and repeatability is crucial for practical applications. Existing fabrication methods often suffer from limitations like high cost, complex procedures, and material constraints. Screen printing, a high-throughput and cost-effective technique, offers a solution for producing flexible electrodes. Paper-based microfluidic devices, known for their low-cost and large-scale preparation capabilities, further enhance the sensor’s potential. This study proposes a “Lego Bricks” type wearable sensor using screen-printed electrodes and wax-printed paper microfluidics, offering a customizable and scalable solution for monitoring Na⁺ and K⁺ in sweat.

Existing literature highlights the growing interest in wearable sweat sensors for non-invasive, continuous health monitoring. Researchers have made significant advancements in designing novel ISEs with improved stability and repeatability. However, much research focuses solely on monitoring electrolyte levels, neglecting the influence of external factors. The development of low-cost, high-throughput fabrication methods is essential for large-scale applications. Current methods, such as laser etching and 3D printing, often present limitations. Screen printing and paper-based microfluidics emerge as promising alternatives for cost-effective, high-throughput production. Multifunctional wearable sweat sensors, capable of simultaneously detecting multiple biomarkers, have been developed, but customization remains a challenge. This study addresses these limitations by introducing a modular ‘Lego Bricks’ design.

The researchers utilized a ‘Screen+Wax’ printing technique to fabricate the “Lego Bricks” type wearable sweat sensor. This involved screen printing flexible electrode arrays on polyethylene terephthalate (PET) substrates and wax printing paper-based microfluidic layers. Gold nanoparticles (AuNPs) were electrodeposited onto the carbon electrodes, enhancing ion-electron transfer efficiency. Na⁺/K⁺ ion-selective membranes were then drop-coated onto the AuNP layer. Three different paper-based microfluidic layers (3DM-P, 3DM-W, and adjusted 3DM-W) were designed and fabricated using wax printing. The characterization involved cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), open-circuit potential tests (OCPT), scanning electron microscopy (SEM), transmission electron microscopy (TEM), contact angle measurements, and energy dispersive spectroscopy (EDS). The mechanical deformation and stability of the sensors were also investigated by bending tests. For in vitro characterization of sodium and potassium ion sensors, OCPT response, hysteresis tests, reproducibility tests, and selectivity tests were performed. The sensors were tested by monitoring sweat Na⁺ and K⁺ from different body regions during exercise and assessing the impact of different interventions (electrolyte water vs. pure water supplementation) during prolonged exercise. Data from the wearable sensors were compared with standard methods, such as inductively coupled plasma optical emission spectrometry (ICP-OES), to validate accuracy. The study involved three healthy volunteers.

The “Screen+Wax” printing technique successfully created a cost-effective and scalable method for producing the wearable sweat sensors. The electrodeposition of AuNPs significantly improved the ion-electron transfer rate and the electrochemical surface area (ECSA) of the electrodes. The optimized number of electrodeposition cycles for AuNPs was determined to be 12. The in vitro tests demonstrated the high sensitivity, selectivity, reproducibility, and stability of the Na⁺ and K⁺ sensors. The sensors showed excellent performance even under mechanical deformation. In vivo testing during exercise showed that sweat Na⁺ concentrations initially increased and then stabilized, while K⁺ concentrations initially decreased and then stabilized. Back sweat generally showed slightly higher Na⁺ and K⁺ concentrations compared to forearm sweat. Supplementation with electrolyte water effectively reduced Na⁺ concentration (dehydration indicator) during prolonged exercise compared to pure water supplementation, highlighting the importance of electrolyte replacement during physical activity.

The study successfully demonstrated the feasibility and advantages of the fully printed, modular “Lego Bricks” type wearable sweat sensor for large-scale monitoring of sweat electrolytes. The modular design enables easy customization and mass production at low cost, addressing limitations of existing methods. The high sensitivity, selectivity, and stability of the sensor ensure accurate and reliable measurements during exercise. The findings underscore the importance of considering electrolyte balance during prolonged physical activity, suggesting the benefits of electrolyte supplementation for maintaining hydration and preventing dehydration-related health issues. The correlation between sweat electrolyte levels and exercise intensity and type provides valuable information for personalized health management. Future research could incorporate additional biomarkers and further explore the relationship between sweat composition and physiological responses under different environmental and exercise conditions.

This research presents a novel, fully printed, ‘Lego Bricks’ type wearable sweat sensor for large-scale monitoring of sweat Na⁺ and K⁺. The modular design, coupled with the cost-effective fabrication method, enables customization and scalability. The sensor’s high performance and accuracy, validated through in vitro and in vivo testing, establish its potential for personalized health monitoring and management. Future work should focus on integrating additional biomarkers and expanding the sensor’s applications in various settings.

The study was conducted with a small sample size of three participants. Further studies with a larger, more diverse population are needed to enhance the generalizability of the findings. The long-term stability of the sensor in real-world conditions needs more comprehensive investigation. The study focused primarily on Na⁺ and K⁺; expanding to include other sweat biomarkers could provide a more comprehensive health assessment. While the modular design is advantageous, the assembly process could be further optimized for ease of use and reduced potential errors.