Elevating Healthcare—Flexible Electrochemical Sensing of Cytokines in Sweat for Wearables

In recent years, wearable devices have become essential tools in our daily lives, supporting a variety of activities ranging from human-machine interfaces to health monitoring. Especially the application of wearable sensors in healthcare allows for continuous, minimally invasive monitoring of human health indicators in accessible biofluids like sweat. Among the various biomarkers measurable in sweat, cytokines are particularly compelling for health monitoring as they are essential signaling proteins that play a key role in understanding immune activity and diagnosing diseases like sepsis, diabetes, and cancers. Given the strong correlation between sweat and blood levels, the continuous and quantitative detection of sweat cytokines via wearable devices offers valuable insights into both physiological and pathological conditions, thereby enhancing human healthcare.<br/>While various biosensing techniques for cytokine detection exist, our work focuses on a flexible screen-printed carbon-based three-electrode (SPCE) electrochemical platform, due to its accuracy, sensitivity, miniaturization, and continuous monitoring capabilities, enabling low-cost, accessible, and highly performing wearable monitoring. Due to the low physiological levels of cytokines in sweat (pg/mL range), achieving sensitive detection requires meticulous geometric optimization of the transducer. Here, we detail the fabrication and geometric optimization of the SPCE platform (i.e., active area and electrode distance), through experimental and simulated electrochemical validation via electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The geometrical optimization findings indicate that an increase of 300% in the active area leads to a doubled peak current during CV. Additionally, for integration into wearables, a compact configuration appears optimal. Decreasing the distance between WE and CE by half results in a 60% increase in current density in the electrolyte, thus contributing to increasing peak currents and reduced diffusive effects observed during experimental CV. Moreover, the electrochemical reaction occurs solely on the WE, leading to a higher current density in the electrolyte and thereby enhancing sensitivity.<br/>Following geometrical optimization to boost sensitivity, the WE is modified with electrodeposited gold nanoparticles (AuNPs) to improve electron transfer and increase the surface area, while biofunctionalization with thiolated aptamers ensures selective detection of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α), two critical cytokines. Results show that detection occurs in artificial sweat under constant flow conditions, utilizing a microfluidic system designed for wearable applications.<br/>The sensor successfully detects IL-6 and TNF-α within their physiological sweat ranges of 5 to 15 pg/mL and 9 to 362 pg/mL, respectively, with a limit of detection of 0.2 pg/mL for both analytes, using EIS. Importantly, the sensor exhibits a negligible response to non-specific analytes, ensuring high selectivity.<br/>In comparison to other works, which employ more complex and costly materials, our work emphasizes simplicity and cost-effectiveness, as we use simple AuNP-modified SPCEs. Moreover, we apply EIS to detect cytokines in artificial sweat without additional redox reporters, a novel approach that significantly advances biosensor technology, making it ideal for integration into wearable devices designed for sweat-sensing applications, however necessitates a wearable electronic readout suitable for on-body use. Achieving this integration necessitates advancements in low-power electronics and energy harvesting technologies, enabling continuous energy-autonomous sensing without frequent recharging. This integration enhances user convenience and enables on-body applications, thereby paving the way for self-sustaining, high-performance wearable health monitoring devices that have the potential to elevate human healthcare.