Keywords: Eccrine sweat gland; Skin capacitance mapping; Cycloergometer
At rest and cool conditions, water evaporation from sweat pores at the skin surface is added to the regular Transepidermal Water Loss (TEWL) to create the so-called insensible perspiration corresponding to the global Skin Surface Water Loss (SSWL). This latter parameter is not perceptible at the clinical inspection alone. The literature remains occasionally quite confusing regarding the clear-cut distinction between TEWL and SSWL.
Over the past decade, an innovative progress afforded the Skin Capacitance Mapping (SCM) method representing a specific type of nonoptical skin surface imaging [4-7]. This method relied on fine-tuned electrometric measurements of the skin surface properties. Such electrometric assessments of the upper layers of the epidermis represented a convenient noninvasive way for assessing the SC hydration. Such properties are closely related to a series of specific SC structures and functions [6-9].
The virtue of the SCM method relies on the specific information avoiding some misleading concepts about the reality of SC hydration. The method has probably greater impact for assessing a wide range of physiological and pathological changes altering functional skin characteristics than for providing diagnostic clues in dermatology. Anyway, some SCM aspects are seemingly typical for some disorders [7,10,11].
In practice, the real-time SCM nonoptical images are acquired and displayed on a computer screen. They provide simultaneously SC capacitance measurements every 50 μm over the explored area. Capacitance values of the superficial SC appear as pixels in a range of 256 grey levels. The darker pixels correspond to high capacitance (hydrated) spots, while the clear ones represent the lower capacitance (dry) values.
The SCM method is noninvasive, ethical and applicable without any restriction in humans. It provides objective and quantitative information that is not provided by any other method. The SCM procedure has been validated by several research groups. SCM combines accuracy, precision, sensitivity and reproducibility. With regard to SC moisturization and detection of sweat gland activity, SCM has a low limit of detection allowing detecting minute changes in capacitance. In addition, the range of detectable values is large at any of the measuring points.
TEWL measurements are commonly used for testing the SC barrier function in absence of sweating activity, but this condition is probably not fulfilled in all circumstances. A number of variables affect TEWL, including person-linked factors as well as environmental and instrumental variables. In particular, any physical, thermal and emotional stress ainfluence TEWL measurements. Therefore, a premeasurement 15 to 30 min rest time out of physical activity in a temperature-controlled room at 20°C - 22°C is respected in most studies. The same considerations apply to electrometric measurements including SCM. In these different technical approaches, it is often hardly possible to distinguish the genuine TEWL from the so-called "imperceptible perspiration" by sweat glands . The contribution of this latter physiological parameter in the instrumentally-measured TEWL values has never been thoroughly assessed and is commonly neglected in the interpretation of TEWL data.
Sweat glands excrete sweat only intermittently, in cyclic alternations of periodic discharges and pauses. The pulsatility reaches about 0.3 to 12 sprouts per minute . Such cyclic activity depends on individuals, circumstances, and body areas. It results from spasmodic contractions of periluminal myoepithelial cells once distended by the flow of sweat. Apparently, the different glandular groups are active alternately . Even in case of profuse sweating, about 50% of the pores do not give out liquid at a given time.
Sweating and thermoregulation are impaired with age. The number of eccrine glands diminishes and the output of eccrine sweat is reduced. Men over 60-70 years old display lower sweat rates and higher core temperatures in response to exercise than younger men and boys , and the temperature threshold to induce sweating is 0.5°C higher in aged men. The elevated temperature threshold for sweating and reduced sweat response was more pronounced in aged women. However, a small study comparing 8 women aged 50-62 with 8 young women aged 20-30 found that in a hot-dry environment, the older women's whole body and local sweat rates were significantly lower than those of younger women, but in a warm-humid environment, there was no age-related difference .
At the onset of sweating remaining clinically imperceptible, only black dots appeared, marking the active sweat gland openings . This finding questions the interpretation to be given to blind TEWL determinations which are indeed influenced by imperceptible sweating. Progressively, the SCM black dots become larger and larger till merging to form continuous black puddles.
This study was extended by the inclusion of the 36 additional healthy Caucasian volunteers of both genders. Their ages ranged 21-56 years. At inclusion, two sites were delimited by self-adhesive rings placed on the volar aspect of each forearm, either in distal or proximal locations. In the test procedure, they remained at rest for at least 20 minutes before the first SCM assessment was performed. Then, they started a mild training program using an exercise cycloergometer for 10 min in a room at controlled temperature (21 ± 1°C) and relative humidity (RH: 52 ± 3 %). Other SCM captures were performed at the completion of exercise as well as 1 min and 5 min later. Skin temperature was measured using a Skin Thermometer® ST500 (C + K electronic, Cologne, Germany), and TEWL/SSWL was measured using a Tewameter® TM300 (C + K electronic).
Data of the present study were combined to those of the previous one . They were expressed as means and standard deviations or as medians and ranges according to the type of data distribution. Statistical comparisons were performed using the two-sided paired t-test, the Mann-Whitney test or the Kruskal- Wallis test, as appropriate. The linear regression analysis model was applied to evaluate the relationships between paired variables. A p value < 0.05 was considered significant.
None of the volunteers developed visible sweat running at the skin surface during the test procedure. At rest, TEWL, skin temperature, and the mean SCM showed no significant differences between both forearms, and between genders.
Compared with the resting condition (Figure 1), significant changes were found after mild physical exercise (Table 1). The SSWL values were markedly increased for at least 5 min following exercise (Table 1). Of note, a large range of SSWL assessments (6.0 – 78.9 g/h/m2) were found 1 min after exercise. Compared to the rest condition, skin temperature was initially significantly decreased at completion of exercise, but increased significantly 5 min later (Table 1). The SCM values were significantly increased at completion of exercise and 1 min later, but returned to normal after 5 min.
SCM after exercise showed a combination of two major changes affecting the sweat gland activity and the SC hydration.
10-min physical exercise
Skin temperature (°C)
1 min later
5 min later
6.6 ± 1.1
60.3 ± 9.8***
27.4 ± 15.6***
9.8 ± 1.7**
30.1 ± 0.2
29.1 ± 0.5***
29.8 ± 1.3
30.4 ± 0.1**
27.4 ± 7.3
41.6 ± 8.4**
36.4 ± 6.3**
29.8 ± 6.6
SSWL, Skin Surface Water Loss
SCM, Skin Capacitance Mapping
Significant linear correlations (p < 0.05) were found between TEWL and the mean SCM 1 min after exercise (r = 0.73) and 5 min after exercise (r = 0.59). A negative correlation (p < 0.05) was found between skin temperature and the mean SCM 1 min after exercise (r = -0.37).
In our panel of volunteers, we did not encountered intraindividual variations of SCM patterns of the SC as reported on the chest of aged individuals in a previous study .
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