A Wearable Graphene Sweat Sensor for Real-time Stress Monitoring

Introduction:: Wearable devices show great promise to noninvasively and continuously monitor stress biomarkers, such as cortisol in sweat. Here, we report a wearable graphene sweat sensor to enable simultaneous real-time quantification of sweat volume, sweat rate, pH, and sodium and cortisol concentrations. Molecularly imprinted polymers (MIPs)-based cortisol sensor was designed to detect and quantify cortisol in sweat at physiological concentrations. Compared to existing approaches, the cortisol sensors utilize label free impedance method without using antibodies, enzymes or other redox active materials.

Materials and Methods:: A CO₂ laser cutter was used to fabricate the laser induced graphene (LIG) electrodes. MIP and non-imprinted polymer were then electropolymerized on the LIG electrodes. A Na⁺ selective membrane, was also prepared on the LIG electrode. A microfluidic chamber was constructed over this electrode to enable the flow of sweat over the electrode. This laser cutter was also used to engrave LIG electrodes, which were modified with carbachol hydrogels to produce sweat through iontophoresis. A paper based microfluidic design allows for the real-time quantification of sweat rate. The calibration of the sensor was done with varying concentrations of cortisol, generally present in human sweat. The sweat pH and sodium concentrations were quantified by the pH and ionic sensors. As a proof of concept, the sweat rate estimations were done after iontophoresis and cycling.






Results, Conclusions, and Discussions:: Figure 1(a) shows the key functional layers in the wearable sweat sensor. Electrochemical impedance spectroscopy was employed to measure the impedance of the MIP electrodes after exposing the sensor to varying concentrations of cortisol (0.1 pM to 1 µM). Figure 1(b) and (c) shows the increasing impedance as a function of increasing cortisol concentrations. The NIP electrode shows much lower impedance change at these concentrations. The sodium selective membrane electrode showed linear response to varying sodium concentrations (160 to 10 mM), with a sensitivity around 50 mV/dec (Figure 1(d)). Figure 1(e) shows the impedimetric response of the cortisol sensor towards changing cortisol concentrations continuously. Figure 1(f) shows the comparison of sweat rate after iontophoresis and cycling with the wicking rate of chromatography paper.

Conclusions: In this work, we reported the development of laser induced graphene-based sensor for the sensitive and accurate detection of cortisol as low of 0.1 pM. The sweat rate, pH, and sodium concentrations can also be quantified in real time. We demonstrated the continuous monitoring of cortisol at physiologically relevant concentrations. The cortisol sensor can be adapted for the detection and quantification of other clinically relevant biomarkers by changing the template used during the MIP formation.




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