Skin Hydration Measurement

Stratum corneum hydration is essential for proper function and appearance of the skin. 

The moisture content can be measured in vitro by means of gravimetry or electron microscopy, or by magnetic resonance techniques in vivo. The resolution of the latter technique is, however, currently not sufficiently high to enable isolated measurements on the stratum corneum. Compared with these techniques, assessment of stratum corneum hydration by means of electrical measurements (susceptance) represents an important reduction in instrumental cost and complexity. 

A prerequisite for using electrical measurements in this way is a detailed knowledge of how the different parts of the skin influence electrical impedance. Furthermore, the current and potential distribution in the skin will also be determined by the electrode geometry, which must be taken into account. As explained in Section 4.2.6, the complex conductance of the stratum corneum and the viable skin converge as the measuring frequency is increased. Measurements at high frequencies will hence normally be largely influenced by 

Because the sweat ducts largely contribute to the DC conductance of the skin, the proper choice of electrical parameter for stratum comeum hydration assessment is consequently low-frequency AC conductance (where DC conductance has been removed), or susceptance. 

There are a number of instmments for skin hydration assessment on the market. Most of them measure at rather high frequencies, which means that they measure deep into the viable skin. Some instmments use closely spaced interdigitated microelectrodes. This somewhat reduces the contribution from viable skin layers, but the chance of only measuring in redundant moisture on the skin surface is obvious for such systems. Rationales for using a low-frequency electrical susceptance method for skin hydration assessment and description of a method for absolute calibration of the measurements can be found elsewhere (Martinsen et al., 1998b, 2008 Martinsen and Grimnes, 2001). 

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A 5% urea cream (Canoderm , ACO Hud AB, Sweden) increased skin hydration (measured as capacitance)34 and showed similar efficacy as a 4% urea cream also containing 4% sodium chloride as active ingredient (Fenuril , ACO Hud AB, Sweden) in a double-blind, randomized, and parallel study on 48 atopic patients.35 The clinical and instrumental assessment showed improvements in both groups during the treatment period.35 In another study on atopic dry skin, the 4% urea-formulation... 

Gravimetric and other methods have been used for in vitro calibration of skin hydration measurements (Martinsen et al. 2008). 

Martinsen 0G, Grimnes S, Nilsen JK, Tronstad C, Jang W, Kim H, Shin K. 2008. Gravimetric method for in vitro cahbration of skin hydration measurements. IEEE Trans Biomed Eng 55(2), 728-732. 

Despite the well-known clinical coupling of dry skin and itch, studies to objectively compare the degree of skin hydration or measurements of the transepidermal water losses with severity of pruritus have provided conflicting results on this close association. This review will discuss the existing data, breakdown the pathophysiology of xerotic itch, and describe the role of moisturizers in alleviating both entities. 

Different types of evidence exist for the clinical efficacy of 10% urea in the treatment of psoriasis (Table 19.1). Early clinical data from a clinical study on various types of hyperkeratosis showed no superior effects on from 10% urea cream compared to ordinary aqueous cream BP in the treatment of psoriasis.10 However, five psoriatic patients with chronic therapy-resistant lesions obtained soft and pliable skin after treatment with 10% urea, but no effect on erythema was observed.17 Psoriatic lesions on the extremities (at least 5 cm in diameter in size) also showed clinical improvement after two weeks of treatment with an ointment containing 10% urea (Basodexan S ointment) in a placebo-controlled study on ten patients.26 Higher values of skin capacitance (suggested to reflect skin hydration) were also noted on urea-treated areas. Increased hygroscopicity and water content were also obtained after treatment with 10% urea ointment in patients with psoriasis vulgaris.27 Moreover, urea treatment reduced epidermal proliferation, measured as an altered expression of involucrin and cytokeratins.26 Treatment of psoriasis vulgaris with 10% urea-formulations support clinical efficacy at evidence-level lb (cf. Figure 19.1). 

Interactions of the pastes with the skin in vivo all in vivo measurements were conducted in a climatized room under standardized temperature and humidity conditions (22°C, 45 5% rh). Six healthy volunteers participated in the study. In the first part, after measurement of skin hydration with the NOVA DPM 9003,11 the pastes were randomly applied at a rate of 10 mg/cm2 on different areas (2x2 cm including one untreated control area) of the ventral forearms for 5, 30, and 120 min. Thereafter, the pastes were removed with a soft paper tissue and skin hydration was measured at 1, 2, 3, 4, 5, and 15 min. The second part of this study was conducted on the same volunteers following exactly the same procedures, but the skin was preliminary hydrated by an occlusive application of a moisturizer (an O/W lotion containing 5% urea and 10% glycerol) for 1 h. This was intended to mimic a clinical situation where the pastes are applied on wet skin states with the explicit goal of drying the skin. 

FIGURE 22.6 DPM measurements of skin hydration after application of pastes (10 mg/cm2) during 5 (left), 30 (middle), or 120 min (right) on previously hydrated skin of n = 6 healthy volunteers. The measurements were done 1, 2, 3, 4, 5, and 15 min after removal of the pastes. For the sake of clarity, only means are indicated. Squares control, hydrated skin closed circles HP1 triangles HP2 open circles HP3 statistically significant differences with the control group. M indicates homogenous subsets. 

We conclude that pastes cannot be pooled in a single group and be generally characterized as drying and exudate binding. Lipophilic pastes did not bind any water at all and were highly occlusive. Thus, they are likely to hydrate the skin through an impairment of the transepidermal water loss. They should be preferably used for skin protection. Hydrophilic pastes, on the other hand, hydrated the skin or maintained an elevated hydration state if they contained humectants. Only an hydrophilic paste without any additional component was able to reduce a hydrated state and led to measurably decreased skin hydration values. 

Barlow, T., Measuring skin hydration. Cosmet. Toiletries 114 47-53, 1999. 

Studying the friction of skin supplements other mechanical tests. Friction studies can be conducted with noninvasive methods and give a measure of the skin s health — skin hydration, for example Naylor1 showed that moistened skin has an elevated friction response and El-Shimi2 demonstrated that drier skin has a lowered friction response. Friction provides a quantitative measurement to assess skin. 

To obtain reliable values, it is necessary to have a standardization of procedures (EEMCO).21 In this chapter, therefore, attention is focused on the standardization of measurements by determining the variables associated with the method of measuring skin hydration, assessing the extent to which they influence the measured value, and elaborating the techniques by which they may be resolved. 

Rogiers, V., Derde, M.P., Verleye, G., and Roseeuw, D. Standardized conditions needed for skin surface hydration measurements. Cosmet. Toiletries 1990 105 73-82. 

Loden, M., Hagforsen, E., and Lindberg, M. The presence of body hair influences the measurement of skin hydration with the Comeometer. Acfa Derm. Venereol. 1995 75 449 150. 

Determination of basal biophysiological parameters may identify subjects with sensitive skin. Earlier studies have shown that increased skin susceptibility has been correlated with an increased basal TEWL, 1 41 skin surface pH,42 and fair skin complexion (measured by chromametric L values),43 whereas no relationship was shown for basal skin thickness, skin blood flow, sebum excretion, and skin hydration.29 However, a recent study by Seidenari et al. utilizing multiple bioengineering techniques showed significant correlations only for capacitance and colorimetric a values.44... 

Individuals with sensitive skin often have associated dry skin. In a recent study of subjects with sensitive hands, no difference in skin hydration was seen macroscopically between normal subjects and sensitive hand subjects (who had self-perceived dry skin). However, measurement with the corneometer confirmed reduced skin surface moisture in the group with sensitive hands, and D-squame analysis showed greater loss of cohesiveness between corneocytes harvested from the... 

Specialized testing equipment and conventions have been developed to aid in the formulation and testing of such products. For example, the corneometer is a capacitance-based instrument used to measure the degree of skin hydration based on the principle that the dielectric constant for a skin surface is related to its water content [13]. The measurements are relative and are typically recorded in arbitrary hydration units called relative corneometer units ircu), where the rcu scale ranges from 0 to 120. 

The impedance of the skin is dominated by the SC at low frequencies. It has generally been stated that skin impedance is determined mainly by the SC at frequencies below 10 kHz and by the viable skin at higher frequencies (Ackmann and Seitz, 1984). This will of course be dependent on factors such as skin hydration, electrode size and geometry etc., but it may nevertheless serve as a rough guideline. A finite element simulation on a concentric two-electrode system used by Yamamoto et al. (1986) showed that the SC accounted for approximately 50% of the measured skin impedance at 10 kHz but only approximately 10% at 100 kHz (Martinsen et al., 1999). 

Martinsen, 0.G., Grimnes, S., 1998b. On using single frequency electrical measurements for skin hydration assessment. Innov. Technol. Biol. Med. 19 (5), 395—399. 

The instmment is very convenient to carry and easy to use. The contact surface of the measuring probe is relatively small (7X7 mm). It is mounted on a spring in order to deliver a constant pressure (3.5 N) on skin during measurements and ensure reproducible measurements. Measurement is very quick and capacitance values, expressed in arbitrary units, are obtained in 1 s a longer measurement period would cause skin occlusion and locally increase the water content at that site. This is also why, when several measurements on the same site have to be taken, it is advisable to wait a few seconds between successive measurements to allow the skin to recover each time from the previous application of the probe. The Comeometer has been reported to measure skin hydration to a depth of 0.1 mm [111], which represents the stratum comeum and the upper part of the epidermis. 

The Nova DPM 9003 measures hydration even more superficially than the Comeometer—at the level of the stratum comeum only—and is used mainly for measurements of improved skin hydration. Different probe sizes are available depending on the specific testing needs. The instrument is easy to carry (small size) and easy to use (self-calibration, pressure control by means of a spring into the probe, automatic switch when applied to the skin, etc.) it can be used with a battery. A characteristic of the dermal phase meter (DPM) is the ability to record skin surface hydration in a continuous mode, which makes this instrument very suitable for specific studies investigating the kinetics of water desorption from skin [115]. 

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