Human Skin Hydration

Human skin is the largest organ in the human body. It is fundamentally important to health as the semi-permeable barrier - the first line of defence - between the body and the external world. However, it remains relatively inaccessible to conventional magnetic resonance imaging, firstly because it is thin and therefore requires high spatial resolution, and secondly because it is characterized by relatively short T2 relaxation times, particularly in the outermost stratum comeum. Conventional studies have not usually achieved a resolution better than 70-150 pm, with an echo time of the order of a millisecond or so. As a planar sample, skin has proved amenable to GARField study where it has been possible to use both a shorter echo time and achieve a better spatial resolution, albeit in one direction only. Such studies have attracted the interest of the pharmaceutical and cosmetic industries that are interested in skin hydration and the transport of creams and lotions across the skin. 

Preliminary studies, which have repeated many of the in vitro experiments in vivo, reported that the differentiation between stratum corneum and viable epidermis is at least as good, if not better in vivo and that many of the other experiments are similarly reproduced [16]. 

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Bond, J.R., and B.W. Barry. 1986. Limitations of hairless mouse skin as a model for in vitro permeation studies through human skin Hydration damage. J Invest Dermatol 90 486. 

Van Neste, D. Comparative study of normal and rough human skin hydration in vivo evaluation with four different instruments. J. Dermatol. Sci. 1991 2 119. 

Salter, D.C. Monitoring human skin hydration in vivo using electrical impedance — a model of skin as a solid ionic conductor. Proceedings IX International Conference on Electrical Bio-Impedance and European Community Concerted Action on Impedance Tomography, Heidelberg, September 26-30, 1995, pp. 17-20. 

D Southwell, BW Barry. Pentration enhancement in human skin. Effect of 2-pyrroli-done, dimethylformamide and increased hydration on finite dose permeation of aspirin and caffeine. Int J Pharm 22 291-298, 1984. 

Squalene is a polyunsaturated hydrocarbon with a formula of C30H50 Squalene can be found in certain fish oils, especially shark liver oil, in high amounts and some vegetable oils in relatively smaller amounts. Human sebum also contains 13% squalene as one of its major constituents. Squalane is a saturated derivative of squalene and also found in these sources. Interest in squalene has been raised after its characterization in shark liver oil which is used as a traditional medicine for decades. Several studies exhibited results that prove certain bioactivities for squalene and squalane. Up to date, anticancer, antioxidant, drug carrier, detoxifier, skin hydrating, and emollient activities of these substances have been... 

According to results, squalene-including mixtures were able to increase the barrier to maintain hydration in a comparable manner to vernix caseosa. Five percent sodium lauryl sulfate-treated rat and human skin showed increased transepidermal water loss and riboflavin penetration. However, squalene treatment reverts the effects of sodium lauryl sulfate. 

When investigating the effects of water on transdermal permeation, animal skin may yield results markedly different to human data. For example, hairless mouse skin is unsuitable for modeling human stratum corneum regarding hydration effects the murine skin, when hydrated for 24 h, became grossly more permeable than human skin membranes [8]. Thus water effects on skin permeability obtained using animal models need cautious assessment. 

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. 

Hendriks, F.M., Brokken, D., Oomens, C.W., and Baaijens, F.P. Influence of hydration and experimental length scale on the mechanical response of human skin in vivo, using optical coherence tomography. Skin Res. Technol. 2004 10 231—41. 

Jemec, G.B., Jemec, B., Jemec, B.I., and Serup, J. The effect of superficial hydration on the mechanical properties of human skin in vivo implicalions for plastic surgery. Plast. Reconstr. Surg. 1990 85 100-3. 

Cua, A., Wilheim, K.P., and Maibach, H.I., Frictional properties of human skin relation to age, sex and anatomical region, stratum comeum hydration and transepidermal water loss, Br. J. Dermatol., 1990 123 473-479. 

A number of investigators have utilized the technique of reflectance spectroscopy to characterize human skin in vivo [57-60]. In particular, the technique has been extensively used for ascertaining the extent of skin hydration, both in pathological skin conditions and in the evaluation of moisturizing treatments (see later). 

SAFETY PROFILE Poison by skin contact and intraperitoneal routes. Moderately toxic by ingestion. A severe human skin and eye irritant. Acts as a primary irritant as well as a sensitizer of skin. An allergen. Mutation data reported. Combustible when exposed to heat or flame. A moderate explosion hazard when exposed to flame, sparks, heated to 150°, or when shocked in a sealed container. Explosive reaction with ammonia at 170°C/40 bar. To fight fire, use CO2, dry chemical. Reacts violently with hydrazine sulfate or hydrazine hydrate. See also NITRO COMPOUNDS of AROMATIC HYDROCARBONS. 

A natural concentration gradient across human skin exists the inner dermal layer is essentially fully hydrated whereas the outer stratum corneum water content depends on external factors such as relative humidity and temperature typically, stratum corneum water content is around 20-30% of the tissue dry weight. Of this, approximately 25-35% is associated with some structural component of the tissue (i.e. bound ) with the remainder free (Walkley, 1972). Increasing the water content of the barrier stratum corneum is well known to... 

Due to protein-water binding, human skin has the capacity to absorb five to six times its weight in water. The extent of skin hydration is directly related to the penetration rate of toxic materials. Controversy exists about whether hair follicles contribute to increased penetration, but some studies do show increased absorption through hairy skin. Inflammation in the skin leads to vasodilatation and increased blood flow and subsequently increased penetration of toxic material. Aging contributes to decreased lipid barrier protection and decreased intercellular cohesion and increased penetration of toxic material. 

In Equation 10.11, C (t) is the concentration of the compound in the SC lipid phase at time t, 5 is its solubility in these lipids (taken to be the product of water solubility and octanol-water partition coefficient K a)> id b is a positive exponent with a value of about 2.7 (Kasting and Saiyasombati, 2001). The value of b was estimated based on an analysis of the Flynn skin permeability database (Johnson et al., 1997), which represents steady-state permeabilities obtained with hydrated human skin in vitro. It is possible that a somewhat higher value of b may apply for volatile disposition on air-dried skin if it is more size selective than hydrated skin. Such a refinement has not been attempted here. 

Ceramides are the main constituents of the lipid lamellas present in the intocellular domains of the stratum comeum from human skin. They have a main role in the structure and the barrier flmction of the skin, vdiich is fimdamental in the sensitive skin. Recent studies have demonstrated that the liposome structures finmed widi internal wool lipids with a relevant amount of ceramides provide bodi a reinforc ent of the barrier function of the skin and an increase of the cutaneous hydration (1,2). 

Wanitphakdeedecha R, Eimpimth S, Manuskiatti W (2009) The effects of tetrahydrocurcmnin in curmin cream on the hydration, elasticity, and color of human skin. J Am Acad Dermatol 60(3) AB83... 

Hydration Injury to Human Skin A View from the Horny Layer... 

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