Skin Impedance Measurements with Nanomesh Electrodes for Monitoring Skin Hydration

Abstract

The importance of continuous monitoring of skin hydration in daily life, to aid in the diagnosis of skin diseases, is rising. Electrodes that can be worn directly on the skin are attracting attention as an effective means. However, they should not inhibit natural water evaporation from the skin and should not cause inflammation or irritation even if they are attached to the body for long periods of time. In this study, nanomesh electrodes that have previously been reported to exhibit high biocompatibility are also found to exhibit high water vapor permeability, resulting in properties that prevent skin dampness. Furthermore, the skin impedance measured using nanomesh electrodes is found to correlate with the hydration level of skin measured using existing medical equipment. This study provides a new approach to measure skin hydration in conditions close to bare skin. Skin is the outermost organ of the body and acts as a barrier separating the inside of the body from the outside world. Skin is essential to prevent the entry of pathogens and contaminants from the outside environment into the body, as well as to prevent excessive evaporation of water from the body. The stratum corneum, the outermost layer of the skin, serves as a barrier that inhibits water evaporation from the skin called trans-epidermal water loss (TEWL). [1] When the stratum corneum is damaged, the water in skin evaporates more easily to the outside world, making the skin prone to dryness. [2] Therefore, it is important to measure the skin hydration levels to assess the barrier function of the stratum corneum. The evaluation of the stratum corneum's barrier function has attracted significant attention from the cosmetic and medical fields as it is useful for understanding the characteristics of an individual's skin and diagnosing skin diseases such as atopic dermatitis, [3] ichthyosis vulgaris, [4] and psoriasis. [5] Skin hydration levels are commonly measured indirectly by measuring the skin's electrical properties, such as the skin impedance and skin capacitance. This is because the higher the hydration level of the skin, the higher the conductivity and dielectric constant. [6,7] In the medical community, the electrical properties of the skin are measured using probes with rigid electrodes

Time Evolution of the Skin–Electrode Interface Impedance under Different Skin Treatments

“…It is also worth noting that the trends seen in the overall impedance as it changes over time are expected, given the various treatment types. The milder treatments—NT, SA, and µNA—showed decreases in their 10 Hz impedance from the t 0 to t f timepoints ( Figure 2 d–f), which is consistent with the hypothesis that natural skin hydration decreases the interfacial impedance due to the presence of ions in human sweat [ 12 ]. Additionally, there was a slight increase in the impedance of the skin under the AT treatment over 24 h. While the change was minimal over time, it may still be explained from a biological standpoint.…”

“…The motion artifact susceptibility of skin-based electrodes may also increase with a higher impedance, further degrading signatures such as the QRS complex in ECG recordings [ 10 , 11 ]. A high impedance electrode may also make it difficult to monitor skin hydration levels over time, which can hinder both sweat-based measurements, and investigations of certain skin diseases [ 12 ]. Finally, in long-term monitoring and ambulatory applications, impedance stability is highly desirable, as changes in the electrode response as a result of increased impedance may be detrimental to diagnostic accuracy [ 13 , 14 ].…”

“…While additional factors such as the electrode surface area and the amount of pressure applied to the recording contact can also produce slight variations in impedance and recording quality [ 20 , 23 ], skin properties, rather than electrode properties, have been postulated to have the greatest impact on the interface impedance and its stability over time [ 4 , 24 ]. Indeed, a recent report found that the interface impedance is inversely correlated with skin hydration levels, which can decrease by up to 50% over the course of 2 h, especially if non-breathable materials and electrode geometries are used [ 12 ]. Thus, an understanding of the multifaceted nature of the skin–electrode interface and its dynamics has serious implications on the design and engineering of low-noise and long-term stable wearable biotechnologies.…”

“…The composition of the outermost skin layer—the stratified epidermis and its uppermost layer, the stratum corneum—is perhaps the largest factor contributing to the variability of the interfacial impedance. The epidermis is a highly complex system with chemical, mechanical, and electrical properties that depend on sex, age, diet, sweat and activity levels, as well as environmental conditions such as temperature and humidity, among others [ 4 , 12 , 17 , 24 ]. To moderate some of these sources of variability, skin treatments are often applied before placing the electrode, with the most common intervention being abrasion to remove dead skin, oils, dirt, and other debris from the uppermost layers [ 17 , 25 , 26 ].…”

“…To moderate some of these sources of variability, skin treatments are often applied before placing the electrode, with the most common intervention being abrasion to remove dead skin, oils, dirt, and other debris from the uppermost layers [ 17 , 25 , 26 ]. However, abrasion can irritate the skin and cause discomfort to the subject, and the initial effects of abrasive skin treatments do not persist over extended periods of time, since the impedance will naturally decrease once the skin rehydrates with sweat [ 7 , 12 , 22 , 25 , 27 , 28 ].…”