Introduction: Non-invasive measurement of the stratum corneum hydration (SCH) with capacitance-based instrumentation is established in dermatological and cosmetic studies. We wanted to test the reliability of non-invasive self-measurements for SCH performed under real-life conditions by volunteers with a Bluetooth-based (wireless) probe Corneometer® (CM 825i) transmitting the data via a smartphone application to a central server. Probes and smartphones communicated using Bluetooth Low Energy. Data from the smartphone were securely transferred to a remote server in a different country with TLS encryption using HTTPS protocols. CM 825i values were correlated with the established CM 825 under laboratory conditions. The primary endpoint was the correlation of the two probes. Secondary endpoints were the coefficient of variation (CV) and delta values (before and after treatment). Methods: Eighteen healthy volunteers (f: 8; m: 10) participated in the prospective observational study. The real-world home use of the wireless CM 825i was performed before and after treatments with base cream DAC for 7 days. Results: Both instruments showed a significant and relevant correlation (p < 0.0001; Spearman coefficient of r = 0.8647). CM 825i and CM 825 differentiate significantly between normal and high SCH. Both devices showed comparable robustness in repeated measurements with a CV between 5.6% and 9.2%. Conclusion: We could show a significant correlation between both devices and a comparable differentiation between low and high SCH and comparable CVs. The real-life use demonstrated adequate acquiring and transmitting of in vivo data to a smartphone and subsequently transmitting to the secure server with low numbers of missed transmissions (<0.2%) and missed measurements (<5%).

Optimal skin hydration is a prerequisite for competent epidermal barrier functions [1, 2]. The water content of the stratum corneum (SC) affects barrier permeability, its mechanical properties, and the process of normal corneocyte desquamation [3, 4]. Failure of the SC to retain water induces dryness and impairs epidermal barrier function [2, 5, 6]. Different in vivo methods for the assessment of SCH have been described, including microwave, thermal, and spectroscopic methods as well as nuclear magnetic resonance, infrared, and Raman spectroscopy [7‒9]. However, the most commonly applied methods are based on measuring the electrical conductance, capacitance, or impedance as an indirect indicator for SC water content. Low-frequency skin impedance measurement reflects rather the water content in the living tissues of the skin [10], whereas high-frequency conductance detects more selectively the hydration of SC [11]. The electrical methods give an integrated value of the SCH, rather than the actual water distribution of the superficial epidermal layers.

Half of the patients do not take their medications as prescribed, representing a major barrier to medication adherence [12]. In dermatology, adherence to topical therapy may even pose additional challenges related to the specifics of medications and their route of application [13, 14]. Estimation of the dynamics in SC hydration is used in efficacy claim studies on topically applied potentially hydrating (moisturizing) agents [15, 16]. In skin physiology studies, measurements are usually performed in a laboratory under standardized and controlled conditions. Introducing a home-based remote device for SCH measurement could potentially increase patient adherence to topical treatment in study settings [17, 18].

The objective of this study is to compare the new remote wireless CM 825i with the classic CM 825. Our primary hypothesis was that the new CM 825i has a significant and relevant correlation with the classic CM 825 with normal and high SCH (after standard moisturizer application) and pooled data. Our secondary hypotheses under laboratory conditions were that (1) CM 825i is non-inferior in differentiating between low and high SCH states to the classic CM 825 and (2) the CM 825i is not inferior to the CM 825 in terms of robustness of repeated measurements. The tertiary hypotheses under real-world home-use conditions included the following: (1) the new device is capable of acquiring and transmitting in vivo data at home to a smart phone via Bluetooth and subsequently encrypted to a secure server in a different country; (2) the device can register and transmit data from measurements performed by volunteers under real life conditions; (3) it is capable to differentiate between two states of hydration, namely, before and after application of a standard moisturizer; (4) not more than 5% missing home measurement time points and not more than 1% of transmission failures.

Measured Variables

Both Corneometer® probes are capacitance-based measurement devices. The Corneometer® measurements are performed directly on the skin (online suppl. Fig. S1; for all online suppl. material, see https://doi.org/10.1159/000539411). By gently pressing Corneometer®’s sensor at the tip of the probe on the measurement location, the measurement process was started. The measurement process lasts for 1 s. The electrical field depth of the emitted electrical capacitance field allows the assessment of water content in the SC compartment from 0 down to approximately 20 µm [19, 20]. The Corneometer® CM 825i works with the same principle as the established CM 825. The principle of the capacitance-based Corneometer® probes is depicted in online supplementary Figure S1.

The data were transferred via Bluetooth to a smartphone. From the smartphone, data were transmitted in an encoded form to the study server of the sponsor, which was located in a different country (the study was performed in Bulgaria, and the server was located in Germany). The published guidelines regarding the non-invasive assessment of SCH were respected [20, 21].

Concept of Transmission Software

CliniScale is an eHealth solution in cosmetic or clinical trials. It is, on the one hand, a dynamic study design with integrated tools for medical specialists and, on the other hand, a mobile application for volunteers, which can be connected via an AES-secured Bluetooth connection to a mobile sensor. Excerpts from the CliniScale software are presented in online supplementary Figure S2. In the present study, we used the Corneometer® CM 825i for measurements at home and to capture real-world data on the use of a cosmetic product. The connection of the mobile sensor to the CliniScale mobile application with subsequent data transmission to the remote online server allows for real-time data evaluation. Once a volunteer completes an entry or measurement, the CliniScale mobile application sends the data via a TLS-secured data connection to the CliniScale server for further evaluation. In addition to a classic time-shifted evaluation of developed studies, the CliniScale system also enables the possibility to either make adjustments to the study design during the study execution in real time or to establish direct contact with the probands, e.g., to gain more detailed insights into the behavior of the probands or to achieve higher compliance. The CliniScale system was designed using user-centered design [22] principles to achieve optimal usability of the system.

The wireless Corneometer® CM 825i comprises a module with the original Corneometer® CM 825 measurement system and a wireless data transmission module using the Bluetooth protocol. CliniScale mobile application with Bluetooth and location permissions are required for the volunteers in order to perform measurements. The mobile application automatically connects to the probe. In case of an error, the user can manually connect to the device. Once a connection is established, the measurement module is activated by placing the probe measurement electrodes on the designated skin location and gently pushing it to the skin. The force needed is approximately 1 N. After one second, the measurement result is taken and transmitted to the mobile device. A string of the probes’ serial numbers, measurement results, or error codes is transmitted to the application. The application checks the string and saves the data. On each measurement, a timestamp is added to the measurement data. Once the required number of measurements are taken, the results are securely transmitted to a remote server using TLS encryption via the HTTPS protocol where the results can be analyzed by authenticated study admins.

Study Design

This exploratory and comparative, observational study was conducted on 18 healthy volunteers. Due to the exploratory character of the study, no formal power calculation was performed. We estimated the sample size based on published comparative studies [23]. The present study was conducted respecting the published guidelines for skin hydration [20]. Room temperature and relative humidity were recorded in the measurement room.

Ten repeated baseline (BL) measurements were performed on the volar forearm with the new Corneometer® CM 825i and the established CM 825 at BL. Subsequently, topical treatment with base cream DAC and measurements with both instruments were performed 5 min after the cream application. The base cream DAC (Bombastus, Freital, Germany) (INCI list: glycerol monostearate 60, cetylalcohol, mid-chain triglycerides, white Vaseline, macrogol-20[1]-glycerolmonostearat, 1,2-propandiol, purified water) was a standard cream with known hydrating properties [24]. During the 7 days, the volunteers performed the measurements before and after base cream application twice per day on the according areas with the CM 825i device. At the end of the 7 days’ home-treatment and home-measurement period, the volunteers came back to the laboratory. Again, before- and after-treatment measurements were performed with both instruments.

All volunteers met all of the following inclusion criteria to participate in this study: male or female patient aged 18–65 years; oral and written informed consent; volunteers able and willing to comply with study procedures as per protocol; no topical treatment (medication) on the volar forearm for 14 days before the patient’s first visit and the duration of the study. Female patients who are pregnant or lactating; institutionalized; persons aged <18 years or >65 years; known contact allergy to one or more components of base cream DAC; the intake of at least one medication of immunosuppressive treatment or systemic glucocorticosteroids as well as any topical treatment (cosmetic or medication) on the locations to be tested were regarded as exclusion criteria. The study was performed according to the Declaration of Helsinki with a positive approval by the local Ethics Committee of the EuroDerma Clinic – Sofia, Bulgaria (approval number 2022-1). All participants signed a written informed consent prior to the inclusion and participation in the study.

Statistical Analysis

The statistical analysis was performed with GraphPad Prism 6. The normality was tested with the D’Agostino and Pearson omnibus normality test. Since not all groups showed a normal distribution, the subsequent tests were adjusted to a non-parametric approach. More than two groups were compared with an ANOVA test (Friedman test) and an alpha-adjusted pairwise comparison (Dunn’s multiple comparison test). The correlation was tested with the Spearman test. The sample size was estimated based on previous studies of this group with a similar design.

Study Population

The 18 healthy volunteers (8 females, 10 males) with a mean age of 29.3 years (22–42), mean weight of 76.4 (SD 28.1), mean height of 172.9 (SD 10.1), mean BMI of 25.3 (SD 8.7) participated in the study. No drop-outs and no adverse events due to the application of base cream DAC or the measurement probes were recorded. All included subjects completed the study as per protocol. The mean room temperature during the measurements in the laboratory was 22.0°C (SD 1.39) and the relative humidity was 49.9% (SD 8.1).

Measurement under Laboratory Conditions

The first part of the study was performed with both instruments under standardized laboratory conditions with both instruments. The emphasis was on the comparison of CM 825i and CM 825 along with the robustness of the assessed values in repeated measurements. Figure 1 depicts the mean ± SD of 10 repeated measurements from all 18 healthy volunteers with BL measurements (Fig. 1a) and measurements after 7 days of treatment (Fig. 1b) with both instruments before and after cream application. It can be seen that both Corneometer® instruments showed increased values after the cream application at the level of the 18 individual volunteers.

Fig. 1.

Individual measurements in the laboratory of 18 volunteers. Measurements in the laboratory of the 18 individual volunteers: each point depicts the mean ± SD of 10 repeated measurements. Circles represent the CM 825 (○before and ●after cream application) and triangles represent CM 825i (△before and ▲after cream application). a BL measurements. b Measurements after 7 days of treatment. Note: all measurements recorded an increase in Corneometer® values after the application of the cream at the level of each individual.

Fig. 1.

Individual measurements in the laboratory of 18 volunteers. Measurements in the laboratory of the 18 individual volunteers: each point depicts the mean ± SD of 10 repeated measurements. Circles represent the CM 825 (○before and ●after cream application) and triangles represent CM 825i (△before and ▲after cream application). a BL measurements. b Measurements after 7 days of treatment. Note: all measurements recorded an increase in Corneometer® values after the application of the cream at the level of each individual.

Close modal

Depicting difference after treatment: combined data of the 18 volunteers are depicted in Figure 2. We compared the values before and after treatment at BL (Fig. 2a) and after 7 days of treatment (Fig. 2b). Both instruments depicted an increase of values for the treatment at BL and day 7 (p < 0.001 for all comparisons). When comparing the delta values at BL, Corneometer® CM 825 depicted a significantly higher delta compared to CM 825i (p < 0.01); after 7 days of treatment, the delta values showed no significant difference (Fig. 2c).

Fig. 2.

Collective data of the laboratory measurements. Collective data points of measurements in the different groups. Each point depicts the mean ± SD of the 18 volunteers. CM 825 (○before and ●after cream application) and CM 825i (△before and ▲after cream application) recorded a significant increase in measured values after cream application. a BL measurements: in the pairwise comparison, CM 825 (p < 0.001) and CM 825i (p < 0.0001) showed a significant increase in values after the application of the cream. b Measurements after 7 days of treatment: again, both CM 825 (p < 0.0001) and CM 825i (p < 0.0001) depicted a significant increase in values after cream application. c Comparison of the delta values (AU) before and after cream application for both instruments at BL under laboratory conditions showed significantly higher differences (p < 0.01) for CM 825i compared to CM 825. After 7 days of treatment, the delta values did not differ significantly.

Fig. 2.

Collective data of the laboratory measurements. Collective data points of measurements in the different groups. Each point depicts the mean ± SD of the 18 volunteers. CM 825 (○before and ●after cream application) and CM 825i (△before and ▲after cream application) recorded a significant increase in measured values after cream application. a BL measurements: in the pairwise comparison, CM 825 (p < 0.001) and CM 825i (p < 0.0001) showed a significant increase in values after the application of the cream. b Measurements after 7 days of treatment: again, both CM 825 (p < 0.0001) and CM 825i (p < 0.0001) depicted a significant increase in values after cream application. c Comparison of the delta values (AU) before and after cream application for both instruments at BL under laboratory conditions showed significantly higher differences (p < 0.01) for CM 825i compared to CM 825. After 7 days of treatment, the delta values did not differ significantly.

Close modal

The next series tested the robustness of both devices (Fig. 3): CV under laboratory conditions was calculated from 10 repeated measurements as the ratio of standard deviation divided by the mean. At BL (Fig. 3a) and after 7 days of treatment (Fig. 3b), ANOVA showed a significant difference (BL: p = 0.047; D7 p = 0.0341); but post-hoc pairwise comparisons showed no significant difference at both time points. The mean CV was low with a percentage between 6.5% and 9.2% at BL and between 5.6% and 8.9% after 7 days of treatment.

Fig. 3.

Robustness of repeated measurements: coefficient of variance (CV) under laboratory conditions. The coefficient of variation was calculated from 10 repeated measurements (in the laboratory) with both instruments as the ratio of standard deviation divided by the mean. a BL measurements: ANOVA (Friedman test) (p = 0.047), post hoc pairwise comparisons (Dunn’s multiple comparisons test) showed no significant difference. The mean CV was low with a percentage between 6.5% and 9.2%. b Measurement after 7 days of treatment: ANOVA (Friedman test) (p = 0.0341), post hoc pairwise comparisons (Dunn’s multiple comparisons test) depicted no significant difference. The mean CV was low with a percentage between 5.6% and 8.9%.

Fig. 3.

Robustness of repeated measurements: coefficient of variance (CV) under laboratory conditions. The coefficient of variation was calculated from 10 repeated measurements (in the laboratory) with both instruments as the ratio of standard deviation divided by the mean. a BL measurements: ANOVA (Friedman test) (p = 0.047), post hoc pairwise comparisons (Dunn’s multiple comparisons test) showed no significant difference. The mean CV was low with a percentage between 6.5% and 9.2%. b Measurement after 7 days of treatment: ANOVA (Friedman test) (p = 0.0341), post hoc pairwise comparisons (Dunn’s multiple comparisons test) depicted no significant difference. The mean CV was low with a percentage between 5.6% and 8.9%.

Close modal

Correlations were calculated for the laboratory measurements between CM 825i and CM 825 Fig. 4) before and after treatment at BL and after 7 days. Significant correlations between CM 825i and CM 825 were shown at BL before (p = 0.004; Spearman coefficient of r = 0.7197) (Fig. 4a) and after treatment (p = 0.0022; Spearman coefficient of r = 0.6367) (Fig. 4b). These findings were partially confirmed after 7 days of treatment with significant correlations between CM 825i and CM 825 before (p = 0.001; Spearman coefficient of r = 0.7544) (Fig. 4c) but not after treatment (p = 0.0986; Spearman coefficient of r = 0.3189) (Fig. 4d). By cumulating the data of all four groups, a significant correlation (p < 0.0001; Spearman coefficient of r = 0.8647) between CM 825i and CM 825 in the repeated laboratory measurement was calculated.

Fig. 4.

Correlation between CM 825i and CM 825 from laboratory measurements. The 10 repeated measurements with CM 825i and CM 825 in the laboratory were correlated against each other before and after treatment at BL and after 7 days. a Significant correlation (p = 0.004) between CM 825i and CM 825 at BL before cream application with a Spearman coefficient of r = 0.7197. b Significant correlation (p = 0.0022) between CM 825i and CM 825 at BL after cream application with a Spearman coefficient of r = 0.6367. c Significant correlation (p = 0.0001) between CM 825i and CM 825 after 7 days of treatment before cream application with a Spearman coefficient of r = 0.7544. d After 7 days of treatment and after cream application, no significant correlation (p = 0.0986) between CM 825i and CM 825 could be depicted with a Spearman coefficient of r = 0.3189.

Fig. 4.

Correlation between CM 825i and CM 825 from laboratory measurements. The 10 repeated measurements with CM 825i and CM 825 in the laboratory were correlated against each other before and after treatment at BL and after 7 days. a Significant correlation (p = 0.004) between CM 825i and CM 825 at BL before cream application with a Spearman coefficient of r = 0.7197. b Significant correlation (p = 0.0022) between CM 825i and CM 825 at BL after cream application with a Spearman coefficient of r = 0.6367. c Significant correlation (p = 0.0001) between CM 825i and CM 825 after 7 days of treatment before cream application with a Spearman coefficient of r = 0.7544. d After 7 days of treatment and after cream application, no significant correlation (p = 0.0986) between CM 825i and CM 825 could be depicted with a Spearman coefficient of r = 0.3189.

Close modal

Home measurements with the CM 825i under real-life conditions are depicted in Figure 5. Fig. 5a shows the values of the 18 individual volunteers before and after the cream application with increased values for all volunteers after treatments. Cumulative data over the 7 days of treatment before and after the treatment showed significant increases in the pairwise comparison for all time points (p values between p < 0.001 and p < 0.0001) (Fig. 5b).

Fig. 5.

Home application with the CM 825i. a Values of the 18 individual volunteers before (△) and after (▲) cream application. b Cumulative data over the 7 days of treatment before (gray bar) and after (black bar) cream application. Bars represent mean ± SD. ANOVA (Kruskal-Wallis test) showed significant differences (p < 0.0001), which was confirmed with significant differences in the pairwise comparison (Dunn’s multiple comparisons test) for all time points (p values between p < 0.001 and p < 0.0001).

Fig. 5.

Home application with the CM 825i. a Values of the 18 individual volunteers before (△) and after (▲) cream application. b Cumulative data over the 7 days of treatment before (gray bar) and after (black bar) cream application. Bars represent mean ± SD. ANOVA (Kruskal-Wallis test) showed significant differences (p < 0.0001), which was confirmed with significant differences in the pairwise comparison (Dunn’s multiple comparisons test) for all time points (p values between p < 0.001 and p < 0.0001).

Close modal

Missing values were quantified as a measure of reliability in home measurements and data transfer. A total of 234 home events (18 volunteers × 13 time points) were performed. We recorded 10 events that were missed by the volunteers (4.27%). A total of 18 volunteers × 13 time points × 6 measurements at home plus 18 volunteers × 20 measurements in the laboratory makes 2,124 measurements. In these 2,124 measurements, only three missing values due to communication problems (mobile application ≤ device) and one due to device error (value reading = 0) occurred, which totals 0.14% of missing data transmission.

In the present study, we were able to confirm that the SCH measurement data are consistent with previously published data with the classical Corneometer® CM 825 and other measurement devices [25]. The requested ambient conditions for non-invasive studies were respected according to the published guidelines with 22.0°C (SD 1.39) and relative humidity of 49.9% (SD 8.1) [20, 21]. The present study confirms our primary hypothesis that the new CM 825i has a significant and relevant correlation with the classic CM 825 at normal and high SCH states (after standard moisturizer cream application) (p < 0.0001; Spearman coefficient of r = 0.8647 for pooled data; Fig. 5). Our secondary hypothesis under laboratory conditions that the CM 825i is non-inferior in differentiating between low and high SCH states to the classic CM 825 could be confirmed. At BL, the new CM 825i showed an even higher difference than the classical CM 825 (Fig. 2c). The additional secondary hypothesis that the CM 825i is not inferior to the CM 825 in terms of robustness in repeated measurements was confirmed. The CV was comparable in all groups for both instruments at BL and after 7 days of treatment (Fig. 3).

The tertiary hypotheses were tested under real-world home-use conditions to determine whether the new CM 825i is capable of acquiring and transmitting in vivo data at home to a smartphone via Bluetooth and subsequently allowing an encrypted transmission to a secure server in a different country. Only 0.14% (pre-set threshold was 1%) of acquired data failed to be recorded or to be transferred to the server. The volunteers themselves missed performing 4.27% (the pre-set threshold was 5%) of the measurements at home. Additionally, we could show that the new CM 825i could differentiate between two states of hydration, namely, before and after applying a standard moisturizer cream over the 7 days of treatment (Fig. 5).

In standardized laboratory conditions, the CM 825i showed comparable properties to the established CM 825. Furthermore, we were able to evidence the reliability of the CM 825i under real-life conditions with home measurements. The limitation of the present study is that only 18 volunteers were included over a relatively short study period.

Patient adherence is an essential prerequisite for therapeutic success. In dermatology, many diseases require frequent and prolonged topical medication application. In alopecia, patient non-adherence to topical treatment is the major reason for therapy failure [26]. Poor patient-physician relationship during the management course could further impede treatment adherence [27]. Even distant but regular contact with the study crew increases the efficacy of topical psoriasis treatments [28]. Digital health programs including medication reminders can increase patient adherence to therapy [29]. Mobile phone applications increase patient involvement, e.g., in oncology [30]. Therefore, it is possible that the use of mobile technologies and distant data transmission can increase volunteer adherence to topical treatments, e.g., self-measurements in skin physiology and cosmetic studies.

To date, the adherence to topical therapy in drug as well as in cosmetic studies has not been systematically analyzed. This new device allows new approaches in clinical research: (i) controlling and supporting a higher adherence rate in studies with topical applications, (ii) performing long-term studies, e.g., for cosmetic claims related to SCH with a minimum of visits to the study sites.

A critical point using the new CM 825i is the adequate training of the volunteers on the proper handling of the device to ensure a maximum of performed measurements. The missing measurements in our study were mainly related to the lack of applications by the volunteers themselves. An additional feature of the software reminding the volunteers shortly before the scheduled measurement/treatment might further increase adherence.

In conclusion, the present study could show a significant and relevant correlation between the new CM 825i and the classic CM 825 with a comparable differentiating capacity between low and high SCH. Both instruments had a comparable robustness in terms of CV in repeated measurements. The real-world home use showed acquiring and transmitting in vivo data at home to a smartphone via Bluetooth and subsequently transmission to a secure server in a different country with a low number of missed transmissions (<0.2%) and missed measurements (<5%). Future studies with larger populations during longer periods will further establish the CM 825i as a new tool to measure SCH in clinical studies.

SGS proderm, Schenefeld, Germany, provided input in the development of the APP.

The study was performed according to the Declaration of Helsinki with a positive approval by the local Ethics Committee of the EuroDerma Clinic – Sofia, Bulgaria (approval number 2022-1). All participants signed a written informed consent prior to the inclusion and participation in the study.

J.W.F. and R.D. have received consulting fees from Courage + Khazaka in the past. G.W. and J.G. are employed by Courage + Khazaka.

This study was sponsored by Courage + Khazaka electronic GmbH.

Joachim W. Fluhr: conceptualization, methodology, data curation, writing – original draft preparation, and writing – reviewing and editing. Agnès Voisard and Nicolas J. Lehmann: writing – original draft preparation, writing – reviewing and editing, and software. Dessyslava G. Nikolaeva: methodology, data curation, software, validation, and writing – reviewing and editing. Leonie Herzog: conceptualization, writing – original draft preparation, and writing – reviewing and editing. Georg Wiora: conceptualization and writing – reviewing and editing. Jeremias Gayer: software and writing – reviewing and editing. Razvigor Darlenski: writing – original draft preparation, writing – reviewing and editing, visualization, investigation, and supervision.

All data generated or analyzed during this study are included in this article.

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