Irritation barrier function measurement

To test the irritancy potential of substances, two tests which can reliably distinguish between skin corrosives and noncorrosives are endorsed by the European Centre for the Validation of Alternative Methods (ECVAM). The testing procedures are based on the transcutaneous electrical resistance (TER) measurements of rat skin and on a human skin model. Both test systems [141-145] will be briefly outlined below. Nevertheless, these tests are not suited for the group of mild irritants which do not induce an acute effect on the barrier function. For those substances, new markers need to be evaluated. First results are available for heat shock protein 27 where higher levels were observed in skin models after exposure to mildly irritating chemicals [146, 147]. 

The principle of the TER test is that the test material is applied for up to 24 h to the epidermal surfaces of skin discs taken from the pelts of humanely killed young rats. Corrosive materials are identified by their abUity to produce a loss of normal stratum comeum integrity and barrier function, which is measured as a reduction in the inherent TER below a threshold level (5 kil). Generally, materials that are noncorrosive in animals, but are irritating or nonirritating, do not reduce the TER below the threshold level. A dye-binding step can be incorporated into the test procedure for... 

Porcine corneal models have been used to develop in vitro/ex vivo models able to detect recovery of ocular injury. In preliminary studies porcine corneas cultured for at least 120 h showed regeneration of the damaged stratified epithelium by treatment with 3 % SLS and Ethanol [84], The model was further optimized and developed towards an ocular irritancy assay based on porcine corneas with reversibility as an endpoint, called the Porcine Corneal Ocular Reversibility Assay (PorCORA) [85]. The assay uses an air-interface porcine corneal culture system, and is maintained in culture for 21 days, similar to the in vivo observation period, to determine reversibility of corneal injury as measured by sodium fluorescein and to detect potential compromised epithelial barrier function. 

Although visual scoring of hand irritation indices are simple and generally are effective in providing important information concerning irritation potential, transepidermal water loss rate and skin moisture content (corneometer) measurements are better (more accurate and precise) for such evaluations. These measure the barrier function of the stratum corneum, a main indicator of skin irritation [16]. 

In addition to their regeneration effects, emollients have also been shown not only to treat but also to prevent ICD. A recently performed study showed on experimentally irritated skin both a significant preventive effect and a therapeutic effect of a moisturizer (Ramsing and Agner 1997). The product tested prevented irritant skin reaction due to SLS and it accelerated regeneration of skin barrier function of SLS-irritated skin of the hands judged by measurements of TEWL and electrical capacitance. 

To build on this work showing the concurrent effects of various irritants on skin, other exogenous factors were studied as well. Thermal stimuli can affect the skin s barrier function. Researchers found that when human skin was pretreated with SLS for 48 h under patching and then exposed to a thermal stimulus on the test sites, skin blood flow measures increased more quickly and sharply in those pretreated to higher concentrations of SLS. This indicated that stimulated blood flow measures may be a good model to evaluate nonchemical inducted irritation, such as by climatic factors [59]. 

The development in recent years of noninvasive instrumental techniques has considerably increased the level of discrimination between products or surfactants. Significant differences in terms of interaction of products with skin surface are now detected much earlier than clinical signs of irritation. Different test protocols have been described in the literature [86] and have been developed in order to induce no or minimal clinical irritation and compare the effect of siufactants by means of instrumental measurements. Transepidermal water loss (evaporimetry), skin capacitance/conductance (skin surface electrical measurements), and vascular status (laser Doppler flowmetry) measurements seem among the most sensitive bioengineering methods for such a purpose. They assess the effect of the surfactants on alteration of the skin barrier function, skin surface hydration, and microvessels blood flow, respectively. 

The transepidermal water loss (TEWL) measurement consists of a measurement of the total amount of water vapor crossing through the stratum comeum. When the barrier function of the skin is altered by a surfactant or another irritant, the amount of water vapor passing through the stratum comeum and evaporating is increased and determined as an... 

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