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substances to kill possible invaders and trigger immune responses to fend off dangerous microbes. If anything overcomes the outmost barrier functions, different internal immune functions will fight the intruder.9 With a damaged skin barrier, the skin is
often itchy, its surface appears dry, flaky and cracked, it shows redness and irritation and is prone to breakouts of specific skin problems and infection. Skin barrier quality can be easily assessed
by measuring the so-called transepidermal water loss (TEWL). Water from the inside of the body will diffuse through the skin into the environment. If the skin barrier is strong and intact, it will only allow a small amount of water to pass through, on most body areas less than 15 gram per m2
per hour (g/m2 /h). If the
barrier is weakened and damaged, the TEWL rate that is measured will be much higher. The measurement itself takes approximately 30 seconds and is very precise. It enables you to detect any barrier impairment before it becomes obvious to the eye. Different devices on the market measure the TEWL either by a closed chamber or with an open measurement chimney. The latest EEMCO guidance for the in vivo measurement of water in the skin10 gives clear preference to the open chamber measurement since it is very reliable and does not influence in any way the skin within the measurement area. When the skin barrier is damaged, the
water-holding capacity of the upper skin layer, the stratum corneum, will also be diminished. The skin surface will dry out and might even show flakiness. There are different ways to assess this process.
Figure 2: The Tewameter® features an open chamber measurement method to assess the TEWL.
Measuring the hydration of the skin surface is the easiest way to show immediately if there is a decrease of the water content. The measurement by capacitance only takes one second. The Corneometer® is globally known as the most-used device to assess skin hydration. The measurement covers an area of 7 mm x 7 mm, the sensor averages the readings for this area and this way gives stable and reproducible results for the skin surface. Other methods use capacitance imaging, showing the distribution of water at contact surfaces. Healthy skin will constantly shed thin
layers of corneocytes from the skin surface to make room for new skin cells migrating from the basis of the epidermis to the outside. This shedding process is called desquamation, and
in healthy skin it should be imperceptible to the eye. It is triggered by enzymatic activities that are induced in different skin layers, often depending on special pH values.11
When
barrier and skin functions are disrupted, these minuscule dry flakes will not be regularly shed but will form accretions of thicker corneocyte formations that can often already be recognised by the eye. Desquamation can be easily measured. One very simple technique is collecting the flakes with special sticky tapes and analysing the removed corneocytes by optical means such as a camera or a microscope. Skin with a damaged barrier is also easily
irritated and tends to have inflammatory reactions. Skin irritation will show in increased microcirculation visible as an elevated redness. Skin redness can be easily quantified using probes to measure the overall skin colour or only certain colour spectra like the redness caused by enlarged blood vessels. Also, increased skin temperature as a result of irritation and inflammation can be easily measured, usually by contactless infra-red sensor technique. The skin surface has an acidic pH-value that is also reflected in the term “acidic mantle”. The gradient from more or less neutral pH values in the “living” epidermis to the acidic pH-value on the outer part of the stratum corneum is important for triggering enzymatic processes. Washing the skin with alkaline products such as soaps and foams, will increase the pH value on the skin. Even though the skin will recover from this attack on its acidic mantle, frequent use of such products can cumulate and change the pH-value for a longer period. In the new pH regime, specific processes can be impaired and again the skin barrier can be affected. And not only physiological processes will be influenced by a changed pH level, but also organisms dwelling within the microbiome that rely on specific pH environments may be impacted by increased pH values, causing a dysbalance of the microbiome.12
This imbalance will Figure 3: Moisture mapping shows the distribution of water at contact surfaces within the sensor area. PERSONAL CARE June 2021
contribute to the weakening of the skin barrier. The skin-pH-value and the influence of skin products on this value can be monitored by
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