Thermal-Drawn Fiber-Based Multiplexed Electrochemical Sensor for Sweat Sensing Applications

The sweat, representing the most easily accessible body fluids, contains important biomarkers, including electrolytes and metabolites that are directly associated with our health status. Recently, a significant amount of research effort has been devoted to developing sweat sensors for wearable applications. However, photolithography or printing techniques are often the mainstream of the fabrication process, resulting in sweat sensors based on 2D planar structures with limited surface area for body contact. The strict definition for wearable sensors is the ones that we wear daily, which are clothes. In this study, we developed a flexible fiber with all-in-one multiplexed sensing capabilities, which has a footprint of 750 µm × 370 µm to 1100 µm × 550 µm and can be weaved into textile for on-body sweat sensing. Such fibers were fabricated by the scalable and versatile thermal drawing process that has been conventionally used in the telecommunication industries to produce optical fibers. Here it has been adopted to fabricate multiplexed electrochemical sensing fibers. The fiber is composed of two polymer electrodes based on carbon black loaded polyethylene (CPE) and stainless-steel wires right beneath them, an internal microfluidic channel between the electrodes, which then were further encapsulated within the polycarbonate (PC) cladding. To realize the longitudinal functions, we used laser machining techniques to expose the CPE within the PC cladding at the designated positions along the length of the fiber. The exposed electrodes were further modified with sensing materials, such as gold nanoparticles (AuNP)s as well as ion-sensitive membranes (ISM) for either direct monitoring of uric acid oxidation or sensing Na+ concentration. In addition, the pseudo reference electrode (p-RE) based on Ag/AgCl was also incorporated into the fiber by integrating Ag/AgCl ink, NaCl-containing hydrogel buffer zone and membrane passive layers, to realize all-in-one electrochemical sensing. We carefully characterized the electrochemical functions of each component. First, the electrode modified with AuNPs exhibits a high sensitivity of 0.82 ± 0.08 nA/µM towards UA in the physiological ranges of 0 to 500 µM. Secondly, Na+ ion-sensitive electrodes (ISE) achieved via the ISM showed sensitivity to Na+ of up to 55.2 ± 0.8 mV/decade, close to the theoretical Nernstian value of 59.2 mV/decade. In addition, p-RE had excellent stability, which had no response in NaCl solution with varying concentrations (- 4.0 ± 0.5 mV/decade). Lastly, we integrated all the functions within a single fiber as an all-in-one sensor and characterized its performance in the artificial sweat. It exhibited UA sensitivity of 0.62 ± 0.1 nA/µM and Na+ sensitivity of 43.1 ± 3.5 mV/decade, the performance of which can meet requirements for on-body applications. Additionally, we also showed the proof-of-concept of the fluid delivery capability of its internal microfluidic channels, for washing and resetting the sensor surfaces. In summary, the thermally drawn fibers and textiles-based multiplexed electrochemical sensing system shows great promise in its future deployment as wearable clothes for monitoring important physiological signals in sweat and beyond.