Minimally-Invasive Transepidermal Potential Measurement System with Microneedle Salt Bridge
Authors: Abe Yuina , Nagamine Kuniaki , Nakabayashi Mayu , Kai Hiroyuki , Yamauchi Takeshi , Yamasaki Kenshi , Nishizawa Matsuhiko 
Published: February 2020

1. Background
The epidermis, the surface layer of the skin, works as a barrier to protect inner organ against external stimulation. Malfunction of the skin barrier can be developed into skin diseases such as dry skin, dermatitis, etc. Thus, the evaluation of integrity of the skin barrier functions is required in medicine and cosmetics. The transepidermal potential (TEP), potential difference across the epidermis generated with ion localization, has been suggested to be related to the skin barrier functions1–3. In the TEP measurement, a pair of electrodes are required to be vertically aligned, where an inner electrode is set under the epidermis, while a surface electrode is on the skin. Conventionally, the inner electrode is placed on a wound, surgically exposed dermis1,3. As a non-invasive alternative to the wound, the inner electrode can be placed on a sublingual area for evaluation of the TEP variation2. For the measurement of absolute TEP, to juxtapose the inner and the surface electrodes is desirable for suppression of iR drop generated in dermis and other tissues.
In this study, we developed a minimally invasive system for absolute TEP measurement with the inner electrode made of a fine microneedle (0.18 mm in diameter). The microneedle filled with agar works as a salt bridge, and the TEPs of a piece of porcine skin were measured. Furthermore, the inner and surface electrodes were integrated into a compact probe for the measurements of local skin barrier disruption based on TEP value.
2. Materials and methods
A painless syringe needle with a plastic cartridge was purchased from Terumo (Nanopass 34G). Total length of the needle was 4 mm, the tip diameter was 0.18 mm, and the length of the lumen at the tip was ca. 0.7 mm. The needle surface was hydrophilized by an ozone oxidation treatment. 2 wt% of agarose in Ringer’s solution was injected to the preheated needle and its cartridge. Gelation of the agarose solution was induced at room temperature. The connecting hole was created on the side of the cartridge by a biopsy punch, and the conventional tubular salt bridge filled with Ringer’s solution in agar was connected through the hole. A silicone elastomer ring was equipped as a spacer to control the penetration depth of the needle within the subepidermal region. All the salt bridges were stored in Ringer’s solution until each measurement was conducted.
3. Results and discussion
Figure 1(a) is a schematic diagram of the entire TEP measurement systems using two kinds of salt bridge electrodes. One electrode was a conventional tubular salt bridge (inner diameter: 3mm), and another was a needle-based salt bridge as shown in Figure 1(b). Both contained Ringer’s solution in agar, which has similar electrolyte composition to the tissue fluid. The tubular electrode is put on the target surface of skin, and its potential was measured against that of the needle electrode inserted to subepidermal regions. Figure 1(c) shows TEP measured for a piece of porcine skin using this system with changing the insertion point of the inner needle electrode (20 mm, 10 mm, and 2 mm from the target area). Stable potential ca. -15 mV for this sample was measured regardless of the position of needle electrode.
This result motivated us to develop a compact device by integrating the tubule and needle electrodes for the local mapping of TEP. As shown in Figure 2(a), two thin Ag/AgCl electrodes were connected to the Ringer’s solution in agar filled inside and outside of the needle. The distance between the needle and the neighbor salt bridge was ca. 2 mm. The localized TEP measurement was demonstrated for the porcine skin partially disrupted by tape-stripping and defatting as indicated in the photographs of Figure 2(b). The potential measured by the sensor probe showed smaller potential for the area B (disrupted) than that of its adjacent areas A and C (undisrupted).
As future works, the probe for a large animal with similar skin to human will be developed for more quantitative local TEP measurements.
(Adapted from “Minimally-Invasive Transepidermal Potentiometry with Microneedle Salt Bridge”, Y. Abe et al.(submitted))
1. Barker, A. T. et al., Am. J. Physiol.
242,R358–R366 (1982).
2. Kawai, E. et al., Exp. Dermatol.
17,688–692 (2008).
3. Dubé, J. et al., Tissue Eng. Part A
16, 3055–3063 (2010).

Figure 1