Printed Wearable Sensor Patch for Multi-Analytes Detection in the Sweat Range
Authors: Paul Brince , Demuru Silvia , Haque Rubaiyet Iftekharul , Van der wal Peter D. , Lafaye Céline , Saubade Mathieu , Briand Danick 
Published: May 2020

Wearable sensors have garnered significant attention in recent years owing to their great promise in personalized health care and fitness monitoring [1]. Among these, recent advancements in wearable technology have enabled the development of sweat analysing sensors for in situ real-time and continuous monitoring of ions, alcohol and metabolites. Comparison and correlation between sweat and blood samples provided a further demonstration of the medical and physiological relevance of sweat [2]. Continuous monitoring of ions present in sweat (such as Na+, K+, Cl- and H+), can provide multiple information on our health status, such as for dehydration. One of the main requirement for facilitating real-time analysis is the development of a soft and flexible platform integrated with microfluidics that can be easily interfaced with the epidermis. This allows direct extraction of sweat and continuous sampling [3]. In this context, printing technologies emerge as a promising method, over the traditional multi-step photolithography process, for rapid, high throughput and cost-effective fabrication of configurable flexible sensing platforms [4].
Here we present a microfluidics-integrated potentiometric-based chemical sensing patch for multiplexed sweat ions analysis. Potentiometric sensor arrays on flexible foil are fabricated by the screen printing process, and assembled into a polymeric microfluidic platform through vertical stacking of thin adhesive layers. The microfluidic structure, which comprises of sweat sampling inlet, guiding channels and collecting reservoirs, are devised and fabricated by a programmed laser-cutter. The developed sweat patch facilitates simultaneous real-time detection of ions present in sweat, such as sodium (Na+), potassium (K+), chloride (Cl-), and pH. Our patches pave the way for simple and low-cost fabrication of wearable sweat sensors for point-of-care medical applications and personalized fitness monitoring.
Figure 1(a) shows the optical image of the devised patch for sweat analysis. Flexible and transparent polymeric sheet was used as a substrate for creating microfluidic features and fabricating electrodes array for ion sensing. The microfluidic design, which includes sweat intake inlet, channel, reservoir and outlet, was fashioned by a software-based programmed laser cutter through 2D patterning of the polymeric film. Ion sensing electrodes array was fabricated by screen printing, which included printing of (i) silver layer for current collector, (ii) carbon layer as working electrode for Na+, K+, and pH sensing, and (iii) silver/silver chloride (Ag/AgCl) layer as both reference electrode and Cl- selective electrode. A polyurethane (PU) layer was screen printed as an insulator to encapsulate the electrodes in aqueous environments. Sodium and potassium ion selective electrodes (ISEs) were obtained by drop-casting Na+ and K+ ion-selective membranes (ISMs) onto their respective electrodes. The Ag/AgCl reference electrode was modified with a PVB membrane to get a stable reference. Polyaniline (PANI) was electrodeposited in order to obtain an ISE for pH sensing. The final 3D architecture of the sweat patch was obtained by stacking the sensing electrodes array layer and the microfluidic layer using laser patterned double-sided adhesive layers. A biocompatible medical-grade adhesive was used to mount the patch on the skin for real-time analysis.
To evaluate the performance of microfluidic patch for sweat secretion and collection, the patch (without electrodes array) was attached to subjects forearm and used to collect sweat. Figure 1(b) shows the optical image of the real-time, in situ collection of sweat using our patch during indoor activities. This continuous real-time test on human subjects validates the capability of our fabricated patch for sampling and collecting sweat through capillary effect. The sensing characteristics of the fabricated ISEs were tested. Each ion-selective sensor response was characterized individually with their respective analyte solution. Figure 1(c) and (d) show the open circuit potential (OCP) responses of the potassium and sodium ISEs, measured in 2-32 mM potassium chloride solutions and 10-160 mM sodium chloride solutions, respectively. Both ISEs showed a fast response and exhibited a linear relationship between OCP and analyte concentration in sweat range with a Nernstian behaviour. Figure 1(e) and (f) show the representative OCP responses of the pH and chloride ISEs measured in McIlwaine’s buffer (pH 4-8) and 5-160 mM sodium chloride solutions, respectively. The pH sensor showed a stable output voltage with nearly Nernstian behaviour. However, the chloride sensor exhibited low sensitivity, and is being further improved by incorporating Cl- selective membrane to the Ag/AgCl electrode. Interference studies have shown that the fabricated ISEs are selective in physiologically relevant concentrations, as shown for K+ ISE in Figure 1(g). The performance of the fabricated patch demonstrates the capability for real-time multiplexed monitoring of sweat analytes for health care applications with as next text its sensing validation on people.
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Figure 1