Evaluation of Wearable Potassium Ion Sensor Fabricated By Heat Transfer Printing
Authors: Kimura Rio , Hoshi Yoshinao , Watanabe Hikari , Shitanda Isao , Itagaki Masayuki 
Published: December 2020

Printable wearable sensors for monitoring biomarkers in sweat of people exercising has attracted attention. For example, wearable ion sensors in sweat based on tattoo paper has been reported1).
A wearable textile-based sensor has advantages such as low cost, flexibility, and light weight. For example, a stretchable self-powered biosensor has been reported previously 2). On the other hand, when a sensor is formed by printing directly on a cloth, the problem is that the resistance value increases, and the accuracy of the sensor may decrease due to the irregularity of the electrode surface.
To solve the problem, we recently reported a wearable potentiometric sodium ion sensor by transfer printing technique3). In case of transfer printing, a sensor is firstly formed on a smooth substrate. Then the sensor is transferred by heating with pressure on a textile with high accuracy irregularities. In this study, we newly produced and evaluated potassium ion sensors by transfer printing technique.
Schematic diagram of electrodes and the potassium ion sensor is shown in Figure 1 (a). Electrodes of the sensor were formed on a PET film by laminating overcoat, resist ink, carbon ink, Ag/AgCl ink, and adhesive using screen printing. The potassium ion selective membrane was prepared on the carbon electrode surface by drop-casting 8 μL of the solution which valinomycin, PVC, DOS, and KTCIPB were dissolved in THF and drying overnight. The Ag/AgCl reference electrode was prepared by drop-casting 8 μL of the solution which PVB and NaCl were dissolved in methanol and drying overnight. The sensor on the PET film was formed on the cloth using a heat transfer machine, and the potential difference measurement between the potassium ion selective electrode and the reference electrode was measured using a potassium chloride solution of 10-7 ~ 10-1 M.
Figure 1 (b) shows the relationship between EMF of the sensor formed on the cloth by thermal transfer and potassium ion concentration. The sensor shows a linear region proportional to the potassium ion concentration in the range of 0.1 ~ 100 mM. Since the concentration of potassium ions in general human sweat is 4 ~ 24 mM4), the calculated detection lower limit suggests that this sensor may be used to quantify potassium ions in human sweat. Furthermore, selectivity coefficients of sensors for sodium ions and ammonium ions abundant in human sweat were calculated by a single solution method. The selectivity coefficient of sodium ions is , and the selectivity of ammonium ions is . These results suggest selectively measure potassium ion levels in human sweat.
References
1) Guinovart et al., Anal. Chem., 138, 7031-7038 (2013).
2) Jeerapan et al., J. Mater. Chem. A, 4, 18342 (2016).
3) Watanabe et al., Chemical Sensors, 33 (Supplement A), 67-69 (2017).
4) Sonner et al., Biomicrofluidics, 9, 031301-10 (2015).
Acknowledgments
This work was partially supported by JST-ASTEP Grant Number JPMJTS1513, JSPS Grant Number 17H02162 and Private University Research Branding Project (2017-2019) from Ministry of Education, Culture, Sports, Science and Technology, and Tokyo University of Science Grant for President's Research Promotion．

Figure 1