Table A2.

Studies for the measurement of skin hydration using electrical-based techniques.

Author(s)	Year	Aim	Major Findings	Measurement Technique
Logger, J. [38]	2019	Authors designed a study to investigate the anatomical site variation of water in the stratum corneum layer	Significant differences found in water content with large interindividual variations, the largest being in the cheek and smallest in the mid-calf region. The resulting values obtained from the Epsilon device were lower than conventional Corneometers, although they followed a similar trend.	Electrical—Capacitance
Flament, F. [39]	2021	Authors investigated skin moisture assessment using hydration sensor patches coupled with smartphones via near field communication	Values recorded by the SHSP were found to be highly correlated with those provided by the Corneometer. Observed significant differences in skin hydration of both sites at all times, as compared to values obtained before the application of the gel.	Electrical—Capacitance
Batisse, D. [40]	2006	Authors developed a device called the SkinChip, which works by using the capacitance method to obtain components from the grey-level histogram of images of the skin	A linear correlation that is shown to be highly significant (R = 0.88, p < 0.00) between the Corneometer recordings and the grey levels measured by the SkinChip device.	Electrical—Capacitance
Nishimura, N. [41]	2019	Authors investigated the effect of fine water particles on the moisture and viscoelasticity of facial skin	Skin conductance of the stratum corneum was higher with smaller water particles. At a 120 min interval after spraying the water particles, the conductance was significantly increased in comparison with its baseline under all conditions.	Electrical—Conductance
André, T. [42]	2008	Authors developed a device known as the Moisture Evaluator that is used to directly measure skin hydration during object manipulation	Coefficients of determination were large and correlation was highly significant between devices. Reduced sensitivity of the Moisture Evaluator for moisture values below 40 (dry skin) on the Corneometer scale. Moisture Evaluator had improved sensitivity at high hydration levels than the Corneometer.	Electrical—Conductance
Davies, L. [43]	2016	Authors produced a three-dimensional cell model using CAD software, formed of 10 thin layers of hexagonal cells acting as the stratum corneum and four thick layers of epidermal cells positioned below	At low frequencies, the conductivity had no effect on the overall impedance. However, when the frequency exceeded 100 kHz, the impedance was shown to decrease with lower conductivity.	Electrical—Bioimpedance
Leonov, V. [44]	2019	Authors conducted a study on cyclists to assess body hydration using the bioimpedance method	Correlation between hydration and the measured resistance. Sensitivity was shown up to around a 700 mL change in hydration across multiple tests.	Electrical—Bioimpedance
Matsukawa, R. [45]	2020	Authors measured skin impedance using nanomesh electrodes to monitor skin hydration levels	Recorded hydration levels showed a decrease as the skin impedance increased, with a negative correlation coefficient of R = −0.86.	Electrical—Bioimpedance
Sunny, A. I. [46]	2019	Authors developed a low-cost bioimpedance sensor to measure skin hydration and tested it with salt water mixtures, a gelatine-based phantom and human participants	The average impedance changes acquired over a frequency range of 30–50 kHz followed an exponential dependence with salt concentration. The gelatine phantom measurements displayed an increase in impedance as water content decreased over time.	Electrical—Bioimpedance
Ameri, S. K. [47]	2017	Authors designed a skin sensor similar to that of a tattoo that had a high stretchability while avoiding a loss of conductivity	Recorded results from this developed sensor were highly consistent with those obtained using silver–silver chloride gel electrodes.	Electrical—Bioimpedance
Yao, S. [36]	2017	Authors developed a skin hydration sensor that consisted of conformal silver nanowire electrodes	Impedance measurements on artificial skin confirmed that the sensor gave stable readings despite changes in external surroundings. Impedance measurements exhibited an increase as water content decreased, with an exponential relationship when compared to the MoistureMeterD device. In vivo experiments presented a decrease in skin impedance after the application of lotion and so an increase in skin hydration.	Electrical—Bioimpedance
Clarys, P. [48]	2012	Authors investigated a comparison between the capacitance and impedance method for hydration measurements of the stratum corneum	High correlations between the amount of water in the filter and the capacitance and conductance readings, with r being 0.89 and 0.99, respectively. In vivo experiments showed highly significant correlations between devices, with an r of 0.98 to the analogue Corneometer and 0.97 to the digital version.	Electrical
Westermann, T. V. A. [49]	2020	Authors developed a device called SkinUp which uses the impedance method to measure skin moisture and oil levels	The Corneometer had a higher standard error value of 2.26–2.99, whilst the SkinUp device was 1.01–1.53. Corneometer showed higher sensibility after the application of water, showing a variation of 93.8% in capacitance. However, the SkinUp device expressed higher sensibility after the application of a cream emulsion.	Electrical
Krishnan, S. [69]	2017	Authors developed a device that introduced multimodal sensors using both thermal transport and electrical properties to allow for measurements from sensitive areas of the skin	Temperature distributions of the porcine skin present that the level of skin hydration has a direct effect on its thermal behaviours. Results conveyed a strong agreement between the developed multi-modal device and state-of-the-art techniques.	Multi-Modal—Thermal and Electrical
Cho et al. [70]	2019	Authors used a multi-modal approach where they developed a chronic wound monitoring system that could interface multiple signals, being voltage, resistance and capacitance measurements, using an integrated circuit	Results concluded that the developed multi-modal sensor system is able to deliver an accurate and reliable diagnosis of chronic wounds at a lower cost and consumption of energy.	Multi-Modal—Electrical