The development of sensors for measuring biomarkers in eccrine sweat has seen significant advancements, with the creation of flexible, stretchable planar devices and tattoo-format sensors. These innovations allow for real-time, non-invasive monitoring of hydration, athletic performance, and stress through the measurement of salts, metabolic products, and other biomarkers. Current research focuses on integrating these sensors into one-dimensional (1D) weavable textile materials to create invisible sensor fibers that could form a grid within a textile, providing more comprehensive physiological data. Textile-based sensors, utilizing novel nanomaterials, offer a unique solution, providing strong, electrochemically stable, and tunable materials that function as both electrodes and weavable electrical interconnects. Several materials are candidates for flexible electrochemical textile electrodes, including CNT fibers/yarns, nanowire elastomeric fibers, carbon-coated threads, and reduced graphene oxide (rGO) fibers. rGO fibers, produced from inexpensive graphite flakes, are a particularly economically viable option due to their mechanical robustness, conductivity, and ease of functionalization. While rGO fibers have been used in various sensor applications, their use in wearable enzymatic sensors remains largely unexplored.

Existing literature highlights the progress in sweat-based biosensing, showcasing advancements in flexible and stretchable planar sensors and tattoo-like devices. These technologies enable real-time monitoring of various physiological parameters. The integration of such sensors into 1D weavable textiles is a growing area of research, aiming to create unobtrusive and large-area sensing capabilities. Different nanomaterials, including CNTs, nanowires, and rGO, have been explored for their suitability as textile-integrated electrodes. rGO, in particular, presents an attractive option due to its cost-effectiveness and tunability. However, its application in wearable enzymatic sensors remains limited, prompting the need for this research.

This study developed rGO yarns for wearable sweat sensing. The rGO fibers were fabricated using a dry-spinning method from liquid crystal GO, which was produced via an improved Hummers' method. The fibers exhibited strong mechanical properties and high conductivity. Platinum nanoparticles (Pt NPs) were deposited onto the rGO fibers using a one-pot polyol synthesis process to enhance their electrochemical properties. The lactate sensor was constructed using a multilayer approach, incorporating a permselective Nafion membrane, a biopolymer layer containing lactate oxidase (LOx), and a polyurethane (PU) diffusion-limiting layer. The pH sensor employed a spray-coated polymeric ISE membrane on a ferrocene-functionalized rGO fiber. A four-fiber braid, incorporating the lactate sensor (reference, counter, and working electrode fibers) and a pH sensor, was created and integrated into a wearable textile patch. The patch facilitated sweat wicking and channeled it to the sensor. A custom-designed wireless readout circuit with Bluetooth capabilities and a mobile app were used for data acquisition. Electrochemical characterization, including cyclic voltammetry (CV) and chronoamperometry (CA), was performed to assess the sensors' performance. On-body testing was conducted on human subjects during exercise to validate the system's accuracy and functionality against commercial pH meters and lactate assay kits. The rGO fibers' synthesis involved dry-spinning GO dope and reduction in hydriodic acid and ethanol. Pt NP deposition used a polyol synthesis process. The lactate sensor fabrication involved sequential coatings of Nafion, a LOx-containing biopolymer, and PU. The electrochemical yarn was characterized via CV, CA, and EIS. On-body testing involved comparing sensor readings with those from commercial pH meters and lactate assays.

The fabricated PtNP-rGO fibers demonstrated high sensitivity (67.8 ± 0.3 µA mM⁻¹ or 2.4 mA mM⁻¹ cm²) and a low limit of detection (LOD) of 0.2 µM for H₂O₂. The lactate sensor exhibited a linear range up to 30 mM lactate, a sensitivity of 3.8 µA mM⁻¹ cm⁻³, and an LOD of 0.7 mM. The sensor demonstrated negligible interference from common sweat components. The temperature and pH sensitivity of the lactate sensor were characterized, showing a linear correlation with temperature (4.0% °C⁻¹) and a significant pH dependence, highlighting the importance of simultaneous pH monitoring for accurate lactate measurements. On-body testing during exercise showed a high correlation between the yarn-based sensor readings and measurements from commercial pH meters and lactate assays. The custom-designed wireless readout system successfully transmitted data to the mobile app in real-time, displaying simultaneous pH and lactate levels. The system demonstrated minimal crosstalk between the pH and lactate sensing channels.

The results demonstrate the successful development of a multi-sensing electrochemical yarn for continuous sweat lactate monitoring. The high sensitivity, extended linear range, and minimal interference of the lactate sensor address the challenges associated with accurate lactate detection in complex biological fluids. The integration of pH sensing allows for real-time calibration and correction of lactate readings, improving accuracy. The wireless data transmission capability enhances the system's practical utility for real-world applications. The on-body validation provides strong evidence of the technology's viability for wearable health monitoring. The findings of this study contribute significantly to the field of wearable biosensors, advancing the development of accurate and convenient tools for continuous physiological monitoring.

This research successfully demonstrated a novel multi-sensing electrochemical yarn for continuous sweat lactate monitoring. The use of rGO fibers provides a cost-effective and scalable platform for fabricating textile-integrated biosensors. Future work could focus on integrating additional sensors for monitoring other sweat biomarkers, improving the patch design for faster hydration, and exploring different textile materials for optimal comfort and performance. Further miniaturization of the electronics and the development of more sophisticated data analysis algorithms could further enhance the system's capabilities.

The current study focused on lactate and pH sensing; the inclusion of other biomarkers may require additional sensor integration and data processing. The hydration time of the patch could be improved through material selection or patch design. The number of participants in the on-body testing was limited, and further studies with a larger and more diverse population are needed to confirm the system's generalizability. The study's focus was on a proof-of-concept, and further optimization may be required for large-scale manufacturing.