Screen-Printed Paper-Based Impedimetric Sensor Using Flexible Silver Ink for Sweat Conductivity Measurement

Abstract

Recently, electrochemical wearable devices have been attracted attention as a device for evaluating the health condition. Flexibility of a substrate and conductive wiring with low cost is key point to attach the sensor on the human body . Our group has reported several paper-based biosensor and biofuel cells [1,2]. A printable paper-based devise is suitable for disposable wearable sensors. In this study, we newly prepared a flexible silver ink as a conductive wiring, and a flexible electrochemical impedimetric sensor was fabricated by screen printing using Japanese paper with surgical tape as the substrate for measurement of sweat conductivity. Flexible silver ink was prepared by mixing a silver flakes (Fukuda Metal Foil Powder Co.) and a urethane resin (Dispercoll U42, Sumika Bayer Co. Ltd.) with a commercially available silver ink. Figure 1 shows the schematic illustration of the sensor. Silver wiring was prepared by printing of the flexible silver ink mentioned above on the surgical tape having two holes. The durability of the silver wiring was evaluated by bending test, in which the silver wiring was bended with a constant curvature at 50 times. Commercially available carbon ink was printed on the Japanese paper as a sensor part. It is noted that the carbon layer and silver wiring were connected through the holes on the surgical tape. A conductivity of a test solution (artificial sweat) in the paper was determined by electrochemical impedance measurement. The conductivity of the solution was compared with that estimated using a commercially available conductivity meter (Toa DKK made, CM-25G). The durability of the silver wiring using the silver flake and urethane resin was drastically improved, comparing with that using only commercially available silver ink. The increasing rate of the resistance of the silver wiring using the silver flake and urethane resin after the bending test was 260%. On the other hand, that using only the commercially available silver ink was 580%. Cracks on the silver wiring were found to be suppressed by using the urethane resin. The silver flakes have a role as a binder to connect between silver particles in the ink. The conductivity of the artificial sweat in the paper measured by the sensor was good agreement with that measured by the conductivity meter. Furthermore, in the present paper, we measured change in a human sweat conductivity during the exercise using our wearable sensor.