Wearable Patch-Type Transepidermal Potential Measurement Device with Porous Microneedle
Authors: Takizawa Ryohei , Abe Yuina , Raut Bibek , Konno Hajime , Kimura Natsumi , Yoshida Shotaro , Abe Hiroya , Nishizawa Matsuhiko 
Published: December 2020

The skin is a structure that covers the surface of the body and separates the inside and outside of the body. The stratum corneum on the surface of the skin has a barrier function that prevents the entry of foreign substances and excessive moisture evaporation. It has been reported that impaired barrier function can lead to dryness and diseases of the skin. Therefore, it is important to evaluate the barrier function. Currently, the measurement of transepidermal water loss (TEWL) is used as the main method. The barrier function is evaluated based on the fact that water evaporation increases when barrier function is reduced. This method, however, requires strict temperature and humidity control, and can be used only in designated locations as a result. As another indicator of barrier function, it has been known that the transepidermal potential (TEP), a potential difference in the thickness direction in the epidermis, can be used [1], but there was no practical method to measure it.
In this study, we developed a wearable patch-type TEP measurement device that can evaluate barrier function with minimal invasiveness. To measure the TEP, it is necessary to connect electrodes to the inside and outside the epidermis. In order to connect electrodes to the inside the epidermis, we developed a tip-conducting porous microneedle array by insulating except for the tip of a porous microneedle array that is entirely conductive [2]. In order to make the device thinner and more flexible, an Ag/AgCl reference electrode was developed, in which the electrode and liquid junction were screen-printed on a thin film. We then combined a microneedle array and a flexible electrode into a patch-type device. In addition, we are now developing a small voltmeter that can be connected to a patch-type device to make a wearable device for long-term TEP monitoring.
First, a tip-conducting porous microneedle array was developed to connect minimally invasively to the interior of the epidermis. The microneedle array was prepared by mixing the monomer solution with the porogen solution, pouring it into a mold and solidifying it with UV light. The microneedle array was placed on a block of Polydimethylsiloxane (PDMS), pressed down with a 10 g weight, and the needle tips were buried in PDMS block while the vapor deposition of the parylene C coating. The porogen was then dissolved in an organic solvent. The distribution of parylene C at the tip of the needle was evaluated by using energy dispersive X-ray spectroscopy (EDX). In addition, the electrical resistance of the microneedle array was measured to evaluate the insulation. Next, a flexible Ag/AgCl reference electrode was developed. Plastic wrapping film was used as a substrate, Ag/AgCl paste was printed by screen printing, and polyvinylpyrrolidone (PVP) gel containing KCl was dripped over it and dried. A waterproof adhesive tape with a 0.1 mm diameter hole was then applied, and a liquid mixture of polyurethane and cellulose acetate dissolved in an organic solvent and KCl powder was applied and dried. The prepared electrode was then immersed in Ringer's solution and the electrode potential was measured for 24 hours to evaluate the deviation of the electrode potential. Then, a patch-type device was fabricated by combining a tip-conducting porous microneedle array with a flexible Ag/AgCl reference electrode. A microneedle array was attached to the electrode using a waterproof adhesive tape. The device was used to measure the TEP of porcine skin samples, and the measured values were evaluated in comparison with the results obtained by the conventional method, which is not practical due to its high invasiveness.
The fabricated microneedle array was successfully insulated except for the tip of the needle (Fig. 1A). The deviation of the electrode potential of the fabricated flexible electrodes was less than 5 mV after 24 hours of measurement. The fabricated device was thin and flexible (Fig. 1B). The measured TEP values of the fabricated device were almost consistent with those of the conventional method (Fig. 1C). Furthermore, we are now developing a small wristwatch-type voltmeter. In conclusion, this device could be a wearable device that can monitor TEP minimally invasively and for a long time.
References
[1] E. Kawai, J. Nakanishi, N. Kumazawa, K. Ozawa, and M. Denda, “Skin surface electric potential as an indicator of skin condition: A new, non-invasive method to evaluate epidermal condition,” Exp. Dermatol., 17, 688–692 (2008).
[2] K. Nagamine, J. Kubota, H. Kai, Y. Ono, and M. Nishizawa, “An array of porous microneedles for transdermal monitoring of intercellular swelling,” Biomed. Microdevices, 19, 1–6 (2017)

Figure 1