Skin surface water loss

FIGURE 22.7 Measurement of skin surface water loss (Evaporimeter EP-1, Servomed, Stockholm) during application and after removal of two hydrophilic pastes for 30 min on previously hydrated skin of n = 6 healthy volunteers. Means =t HSD for the sake of clarity. Closed squares control, hydrated skin triangles HP2 open circles HP3 statistically significant differences with the control group. M indicates homogenous subsets. 

In vivo, hydrophilic pastes showed different interactions with the skin. Some pastes clearly hydrated the skin, others could indeed remove water from a preliminary hydrated horny layer. Elements contributing to these properties may be the presence of humectants such as glycerol, contributing to a long-lasting presence of water on the skin in the first case, or the acceleration of skin surface water loss, contributing to an accelerated removal of water from a hydrated horny layer in the second case. However, this represents, in our opinion, at most one of the elements contributing to the measured events and may simply be due to a different water content of the pastes. 

The interaction of harsh surfactants on SC proteins results in an increase in skin surface water loss (SSWL). This is evident in the results shown in Figure 31.5. Water loss, measured using an evaporimeter immediately after a wash, show that harsher soap induces a higher rate of evaporation than milder syndet. The implications of this high rate of evaporation are examined further. 

NIR and Raman spectroscopy Properties affected by hydration state Skin surface water loss Elasticity... 

Instrument used (measured response) Dia-Stron erythema meter , Minolta Chromameter , Cortex Dermaspectrometer (all measure a redness index of erythema) laser Doppler Velocimeter (blood flow) Servo-Med evaporimeter (transepidermal or skin surface water loss)... 

Emulsion components enter the stratum corneum and other epidermal layers at different rates. Most of the water evaporates, and a residue of emulsifiers, Hpids, and other nonvolatile constituents remains on the skin. Some of these materials and other product ingredients may permeate the skin others remain on the surface. If the blend of nonvolatiles materially reduces the evaporative loss of water from the skin, known as the transepidermal water loss (TEWL), the film is identified as occlusive. AppHcation of a layer of petrolatum to normal skin can reduce the TEWL, which is normally about 4—8 g/(m h), by as much as 50 to 75% for several hours. The evaporated water is to a large extent trapped under the occlusive layer hydrating or moisturizing the dead cells of the stratum corneum. The flexibiHty of isolated stratum corneum is dependent on the presence of water dry stratum corneum is britde and difficult to stretch or bend. Thus, any increase in the water content of skin is beHeved to improve the skin quaHty. 

Desert rodents lead the most water-independent life of all vertebrates. Kangaroo rats can so reduce their evaporation that they are able to maintain water balance on only metabolic water. Other species survive on only meiabolic water plus free water in air-dry seeds. Respiratory water loss is reduced by cool nasal mucosal surfaces, which condense water from warm air coming from the lungs, before it can be expired. Skin impermeability involves a physical vapor barrier in the epidermis, pins unknown physiological factors. 

Water homeostasis is a strict requirement for normal physiological function. The most important task of the human skin is thus to create a watertight enclosure of the body to prevent water loss. It is the intercellular lipid matrix of the outermost keratinized horny layer of the skin (possibly together with recently reported claudin-based tight-junctions Furuse et al., 2002) that represents the skin barrier proper as once this lipid matrix (composed foremostly of saturated long chain ceramides ( 50% wt/wt) and cholesterol (—30% wt/wt) (Wertz and Norlen, 2002)) has been removed, substances diffuse freely into or out of the body system (Blank, 1952 Breathnach et al., 1973 Elias and Friend, 1975). At the same time the intercellular lipid matrix ensures that the stratum corneum remains hydrated and thus the skin surface appears healthy and smooth. 

Kamiya, T., Tsuchiya, S., Hara, K., Okamoto, K., Hattori, A., and Taguchi, N. Study of dry skin in chronic dialysis of skin surface hydration, transepidermal water loss and skin surface structure. Jpn J. 

Dry, scaly skin is characterized by a decrease in the water retention capacity of the stratum corneum (SC),1 with water content diminished to less than 10%. Barrier function of the SC is usually declined, and transepidermal water loss (TEWL) is increased because of an abnormality on barrier homeostasis.2 People feel tightness of their skin, and the skin surface becomes rough, scaly, and sensitive. Hyperkeratosis, abnormal scaling, and epidermal hyperplasia are usually observed in the dry skin.2 Keratinization also shows abnormal features.2 These phenomena are commonly observed in atopic dermatitis and psoriasis.3 Dermatitis induced by environmental factors such as exposure to chemicals, low humidity, and UV radiation also shows these features. Thus, many researchers have been investigating the cause and treatment of dry skin, and there is currently great interest in adequate model systems for dry skin studies. In this chapter, I will describe several model systems of dry skin for clinical research of dermatitis associated with skin surface dryness and also mention recent studies to improve the dry skin. 

Previously13 investigators usually used back or forearm skin for the experiment. It was easier to induce scaly skin on back skin than on forearm skin. In the case of back skin, we stripped SC nine times with adhesive cellophane tape. At that time, the transepidermal water loss (TEWL) value was over 10 mg/cm2/h and most of the SC was removed. In the case of forearm, to induce dry, scaly skin, stripping for 30 to 50 times was needed. One week after treatment, TEWL was higher than the normal level, skin surface conductance decreased, and SC cell area also decreased (Table 10.1). The skin surface became scaly and flaky. Figure 10.1 shows skin surface pictures of the forearm skin with and without barrier disruption. Abnormal scaling is observed on the surface of skin, which was treated with tape stripping. These phenomena are commonly observed in natural dry skin, such as atopic dermatitis and psoriasis. 

As described previously, one can induce dry, scaly skin, which shows features very similar to dermatitis such as atopic dermatitis and psoriasis. Use of this experimentally induced dry skin should enable the discovery of a new clinical methodology to cure or care for skin problems. Recently, several excellent in vitro skin models have been reported. Although they are also very useful models for the study of cutaneous metabolism, their function and microstructure are still different from those of intact skin. On the other hand, the mechanisms underlying abnormal desquamation, that is, scaling in the dry skin such as atopic dermatitis, are not completely known. Sato et al. reported55 the inhibition of protease in the SC induced scale without affecting epidermal mitosis. This result seems to be no direct relationship between skin surface appearance and epidermal proliferation. However, decline of SC barrier function induced epidermal hyperplasia, as described earlier.30 The loss of water content from SC also induced epidermal DNA synthesis.30 Further mechanistic studies on each of the dry skin features are required. 

Wilhelm, K.P., Cua, A.B., and Maibach, H.I., Skin aging effect on transepidermal water loss, stratum corneum hydration, skin surface pH, and casual sebum content, Arch. Dermatol., 127,1806-1809,1991. 

Treatment of xerosis on the plantar surface of the feet for two weeks gave more pronounced improvement in skin roughness, fissures, and dryness by a 40% urea cream (Carmol 40) than from a 12% ammonium lactate lotion (Lac-Hydrin).40 No change in transepidermal water loss (TEWL) was noted from urea-treatment. Both therapies showed sustained benefit during the next two weeks. Furthermore, a cream containing 10% urea and 4% lactic acid provided faster and better improvement with significantly less xerosis regression in patients with diabetes.41... 

Powers and Fox demonstrated that lanolin is semi-occlusive and can reduce transepidermal water loss (TEWL).36 The application of lanolin and lanolin oil (5.0-6.25 mg/cm2, equivalent to a film thickness of 54-68 /tm) to the inner surface of the forearm reduced the TEWL by 32 and 22%, respectively.36 This is in comparison with petrolatum which reduced the TEWL by 48%. Spruits reported a 20 to 30% reduction in TEWL using a 50-/xm lanolin film.37 The clinical improvement in xerotic skin is not simply due to a transient reduction in TEWL, because a completely impermeable plastic film applied twice a day had no beneficial effects. 

Visible skin dryness has been found to correlate positively with surface hydration, but not necessarily with an increase in transepidermal water loss (TEWL).30 This suggests that significant barrier breakdown is not a requirement for skin dryness. A continued increase in dryness to values above a certain level may, however, lead to scaling, cracking and chapping, barrier breakdown, and, eventually, to irritation. 

Leveque, J., Hydration in psoriasis and eczema the dry surface-high evaporative water loss paradox, in Eisner, P., Berardesca, E., and Maibach, H. (Eds.), Bioengineering of the Skin Water and the Stratum Comeum, CRC Press, Boca Raton, pp. 243-249, 1994. 

The stratum comeum is usefully thought of as a brick wall , with the fully differentiated comeocytes comprising the bricks , embedded in the mortar created by the intercellular lipids. A layer of lipid covalently bound to the comified envelope of the comeocyte contributes to this exquisite organization. The intercellular lipids of the stratum comeum include no phosphohpids, comprising an approximately equimolar mixture of ceramides, cholesterol and free fatty acids. These non-polar and somewhat rigid components of the stratum comeum s cement play a critical role in barrier function. On average, there are about 20 cell layers in the stratum comeum, each of which is about 0.5 fim in thickness. Yet, the architecture of the membrane is such that this very thin structure limits, under normal conditions, the passive loss of water across the entire skin surface to only about 250 mL per day, a volume easily replaced in order to maintain homeostasis. 

Treffel, P., Panisset, F., Faivre, B. and Agache, P. (1994). Hydration, transepidermal water loss, pH and skin surface parameters Correlations and variations between dominant and non-dominant forearms. Br. J. Dermatol. 130 325. 

Heat loss by evaporation is maximum when the skin is completely wetted. Also, clothing offers resistance to evaporation, and the rate of evaporation in clothed bodies depends on the moisture permeability of the clothes. Tlie maximum evaporation rale for an average man is about 1 L/h (0.3 g/s), which represents an upper limit of 730 W for the evaporative cooling rate. A person cM lose as much as 2 kg of water per hour during a workout on a hot day, biit any excess sweat slides off the skin surface without evaporating (Fig. 13-44). 

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