Static Measurements

Short-Term Tests/Single Application

The tests are conducted on the forearm of healthy subjects and allow a randomized side-to-side comparison of test products with a placebo or vehicle, a known active product, and untreated control skin. Four to six products may be simultaneously tested. The products are applied at the rate of 2 mg/cm2. Two different experimental designs may be used.

1. The test products are left in place for 1 hour (or another suitable duration, e.g., 3 h [14]). Measurements are conducted at different times thereafter. Removal of excess or nonpenetrated product is preferable before measuring, especially if the preparation contains a high proportion of lipids. Most moisturizers show a rapid increase of measured hydration values (Fig. 1).

2. The test products may be applied on similar areas at the same rate but under occlusion with a standard occluding patch overnight for 16 hours. The next morning, measurements are conducted in the same way as in part 1 beginning 1 hour after removal of the occlusion patch (Fig. 2). This last procedure better picks up the activity of a humectant contained in the test preparation, whereas

Figure 1 Example of hydration changes after 1 h application of two different o/w moisturizers containing both 2% urea as humectant. Hydration evaluation: NOVA DPM 9003; Means + or - half SD: ▲ ▼; Moisturizer 1: ■; Moisturizer 2: □; Control (untreated skin): O. Start values (Time 0) measured before application of the products.

Figure 1 Example of hydration changes after 1 h application of two different o/w moisturizers containing both 2% urea as humectant. Hydration evaluation: NOVA DPM 9003; Means + or - half SD: ▲ ▼; Moisturizer 1: ■; Moisturizer 2: □; Control (untreated skin): O. Start values (Time 0) measured before application of the products.

Figure 2 Example of hydration changes after 16 h occlusive application of two different o/w moisturizers containing both 2% urea as humectant (same products as in Figure 1). Start values (Time 0) measured before the occlusive application of the products. (For further details, see legend of Figure 1.)

Figure 2 Example of hydration changes after 16 h occlusive application of two different o/w moisturizers containing both 2% urea as humectant (same products as in Figure 1). Start values (Time 0) measured before the occlusive application of the products. (For further details, see legend of Figure 1.)

the vehicle effect is strongly attenuated by the uniform conditions encountered under the occlusion patch.

Long-Term Tests/Multiple Applications

The design of these tests and selection of subjects is similar to the regression method previously described but with a modified and shortened regression protocol [15]. The treatment period extends over 1 week only, and the regression phase takes place over the following week. Bioengineering measurements are conducted 12 to 16 hours after the treatment or moisturizer application, and for the last time on the Monday following the regression week. Inclusion of these noninvasive measurements allowed rapid and reliable product-performance evaluation.

Dynamic Measurements

These tests, in addition to the classic evaluation of skin hydration, provide information on some dynamic properties of the SC [16-18]. These properties are likely to be modified by the humectants (e.g., glycerol, urea, and alpha-hydroxy acids) incorporated in the moisturizers used for treatment. Generally speaking, dynamic function tests are characterized by the assessment of the skin response to a given external stimulus that can be of physical (e.g., water, occlusion, stretch, heat) or chemical (e.g., drugs, irritants) nature. These dynamic tests may be used either during short-term or long-term product testing, and will usually be performed before and at different time points after treatment.

The Sorption-Desorption Test (SDT)

This test gives information about the water-binding capacity of the uppermost layers of the SC [16,18]. It is best conducted using measurement devices that are able to measure hydration on a wet surface and that give instantaneous readings on contact with the skin. This first value represents the hydration state of the SC. Then 50 |l of distilled water are pipetted onto the skin, left in place for exactly 10 seconds, wiped with a soft paper towel, and then hydration is immediately measured. This value represents the hygroscopicity of the superficial SC. Further measurements are taken at 0.5, 1, 1.5, and 2 minutes. The area under the curve from 0.5 minutes onwards represents the water-holding capacity of the superficial SC (Fig. 3).

The Moisture-Accumulation Test (MAT)

This test gives information about the quantity of moisture the SC may accumulate during a given time [17,18]. This test is conducted with a device able to measure continuously after bringing the probe in contact with the skin surface. The probe then remains on the skin for 3 minutes, thereby creating occlusive conditions. The MAT measures the accumulation of water under the probe every 0.5 minutes. Water accumulation is evaluated by calculating the area under the time curve until 3 minutes (Fig. 4).

The Plastic Occlusion Stress Test (POST)

The POST may also be considered a dynamic test and gives information about SC hydration, integrity of the barrier function, and SC water-holding capacity [19,20]. It consists of occluding the skin with a plastic chamber (e.g., Hilltop chamber or a similar occlusive device) for 24 hours. Then the occlusion is removed and the evaporation of the accumulated water is measured each minute for 30 minutes as TEWL. This technique has been

Figure 3 Time course of hydration changes during a sorption-desorption test (SDT) performed 60 min after a single 1 h short-term application of moisturizer 1. (For further details, see legend of Figure 1.)
Figure 4 Time course of hydration changes during a moisture accumulation test (MAT) performed 60 min after a single 1 h short-term application of moisturizer 1. (For further details, see legend of Figure 1.)

thoroughly described in recent guidelines [21,22]. The measurement is called skin surface water loss (SSWL) and not TEWL, because it does not represent the true TEWL but the sum of the TEWL and the evaporation of water trapped within and over the SC under the occlusive equipment, at least at the beginning of the measurement period. During these first minutes of evaporation, the SSWL is proportional to SC hydration. At the end of the dehydration time, SSWL is greatly reduced and mainly TEWL is measured. Therefore, changes induced in the last part of the curve reflects the barrier function of SC.

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