1). Development of arc-poration device

To disrupt the stratum corneum, we developed an arc-poration device (Figure 1). In this device, the penetration of biomolecules into the skin can be promoted through micropores created by an arc discharge that is ignited by high voltage-induced dielectric breakdown (Figure 1A, left panel). Since the breakdown voltage of air is known to be approximately 3 kV/mm and the applied voltage generally used in handheld electronic devices is very low, we used transformer in which the voltage applied to the primary coil was elevated to the winding ratio of the secondary coil via electromagnetic induction to generate high voltage from low applied voltage (Figure 1A, left panel). As a result, 3.3 Vdc of the applied voltage was transformed to 1.91 ± 0.4 kVpp of alternating current (AC) voltage (Figure 1A, right top panel). To prevent severe burning of the skin caused by thermal energy of continued arc discharge, we also generated pulsed arc discharge using 20 Hz of burst frequency, applying duty cycle in the range of 10%–90%, regulating the burning period (Figure 1A, right bottom panel). Although the output voltage was elevated to 1.91 ± 0.4 kVpp, it was not sufficient to ignite an arc discharge between the electrode and the skin surface. Since dielectric breakdown strength reportedly decreases as the frequency increases, we applied a frequency of 90 kHz as a carrier frequency into the burst frequency to ignite the arc discharge efficiently (Figure 1A, right bottom panel), and then the output carrier frequency was measured at 74.17 ± 0.36 kHz under the no load condition (Figure 1A, right top panel). Figure 1B describes the process of arc-poration once the device contacts the skin. Initially, one of electrodes is in contact with the skin surface as a ground electrode. Once the other electrode enters within the distance where dielectric breakdown can occur, an arc discharge is generated, and micropores are created on the skin surface by the thermal energy. When both electrodes contact the skin surface, the AC current drops due to high skin resistance and flows between the two electrodes through the skin (Figure 1A, right top panel). From the moment one electrode is detached from the skin, an arc discharge occurs and micropores are created on the skin until that electrode is outside the range where dielectric breakdown can occur. Through this mechanism, micropores can be easily created by sweeping, brushing, or tapping the skin with the arc-poration device

FIGURE 1

Development of arc-poration device. (A) Schematic diagram of transdermal delivery system using arc-poration (left panel). Changes in voltage and frequency at the indicated step are shown (right top panel). Data are expressed as mean ± SD (n = 5). Schematic diagram of carrier frequency in burst frequency used in this study are shown (right bottom panel). (B) Schematic diagram for a process of arc-poration on the skin surface.

 

2). Skin micropore generation using arc-poration device

To analyze the ability of arc-poration to create micropores, we applied this device to both porcine skin and reconstituted human skin (Figure 2). Since a porcine skin has many pores, stamp ink was applied before the arc-poration treatment to distinguish newly created micropores (Figure 2A, top panel). As expected, several newly created micropores with an average pores' diameter of 97.45 ± 19.33 μm were observed on the porcine skin treated with arc-poration (Figure 2A), which is similar to the diameter of the pores generated by microporation or microneedle. Similarly, our histological analysis showed that arc-poration created micropores on reconstituted human skin and the micropores were formed only on the stratum corneum (Figure 2B). Together, these data suggest that arc-poration can disrupt the skin barrier that prevents substance penetration, making a channel for transdermal delivery.

FIGURE 2

Arc-poration creates micropores and increases permeability on the skin. (A) Microscopic images of untreated and Arc-poration-treated porcine skin are shown. Arrows indicate the micropores (top panel). The micropores' diameter was calculated (bottom panel). Error bars represent standard deviation (n = 40). (B) Microscopic images of H&E-stained reconstituted human skin (top panel) and arc-poration-treated reconstituted human skin (bottom panel) are shown. Arrows indicate the micropores. (C) The skin permeability through arc-poration-treated (AP) or nonarc-poration-treated (No AP) reconstituted human skin was analyzed with caffeine using the Franz diffusion cell experiment. At each indicated time point, samples were taken, and the caffeine concentration was analyzed using HPLC. The experiment was duplicated.

3). Increased permeability of caffeine by arc-poration treatment

Given the stratum corneum's function as a skin barrier, it can be assumed that the stratum corneum-specific micropores created by the arc-poration facilitate the penetration of biomolecules into the skin. Therefore, we performed a Franz diffusion cell experiment with both arc-poration-treated and nonarc-poration-treated reconstituted human skin. Caffeine, which is widely used in dermatological applications, was used as a test substance to measure permeability. The concentration of caffeine penetrated through the stratum corneum to the dermal layer was measured using HPLC. At 2, 4, and 8 h after caffeine treatment, the average concentrations of permeated caffeine are 5.9, 4.8, and 4.5-fold higher in arc-poration-treated sample than those in the nontreated sample, respectively (Figure 2C). This result indicates the enhancement of transdermal delivery through stratum corneum-specific micropores.

4). Clinical evaluation of arc-poration treatment

Next, we investigated whether increased skin permeation through the stratum corneum-specific micropores enhanced the clinical effect of active ingredients. Two types of cosmetics containing antioxidants and anti-inflammatory, moisturizing, and soothing ingredients were applied after pretreatment or nontreatment of arc-poration (Figure 3). After 4 weeks of application of the study products, significant improvements in skin gloss, dermal hydration, skin flakiness, skin texture, skin tone, skin tone evenness, and skin pigmentation were observed in the application area (cosmetics only area) in comparison to that at baseline, confirming the skin improvement effects of cosmetics. These were further improved when arc-poration device was used together with cosmetics (AP area); these significant improvements in the AP area were statistically higher than those in the cosmetic area (p < 0.05; Figure 3). The evaluation of skin pore tightening after 4 weeks showed that the pore volume in the cosmetics only area decreased compared to that at baseline, but the difference was not statistically significant. In contrast, the AP area showed significant improvement in skin pore tightening (p = 0.002), which was statistically significantly different from that of the cosmetics only area (p < 0.05; Figure 3). These clinical results indicate that the skin improvement effect of cosmetics is enhanced by pretreatment of arc-poration.

FIGURE 3

Arc-poration enhanced the skin improvement effect of cosmetics. After 4 weeks of cosmetics usage in the absence or presence of arc-poration treatment, skin gloss improvement, dermal hydration, skin flakiness improvement, skin pores improvement, skin texture improvement, skin tone improvement, skin tone evenness improvement, and skin pigmentation improvement was measured compared to baselines.  Data are expressed as mean ± SD (n = 21).