TESTING 57


skin diseases and conditions involve a disrupted barrier. In occupational health, monitoring the TEWL will show minuscule skin damage before it becomes manifest and needs to be treated. Its measurement is also a good indicator for assessing the irritational potential of substances getting in contact with the skin. Other medical indications where the TEWL might be interesting would be premature babies, wound healing, burns and scars, sleep and sports medicine and many more. On the International Space Station ISS,


during two missions (2006-2018), the Tewameter®


has been used to monitor the


impact of long-term stays in space on the skin condition of astronauts.4 The evaporation of the water through


the skin into the environment is influenced by many different parameters. In warm and dry environments, TEWL and evaporation will increase. Not only the natural diffusion of water molecules, but additionally an elevated expulsion of water from the sweat glands will contribute to this process. Additionally, there are many intrinsic influences like general health and especially skin health, hormonal status and nutrition impacting the TEWL. Also, emotional state and high stress


levels induced by physical and psychological stress will increase sweating and thereby influence the Transepidermal Water Loss. In this case, it will not be possible to properly determine the skin barrier function. Therefore, TEWL measurement needs to


be undertaken in a climatised environment at ideally 20-22°C and an ambient relative humidity of 40-60%. In order to avoid any excitement or stress, the acclimatisation to standardised conditions before the measurement should take place in a quiet location. Anything that gets in contact with the


skin, will also affect the water loss. This is especially interesting for products that are applied topically on the skin. Any care product should be tested for its effect on the skin barrier, but especially skin regeneration, protection or repair claims of cosmetic products and ingredients can be supported by the measurement of the


Figure 2: Local heat balance divided in heat diffusion and evaporation cooling.


Transepidermal Water Loss. Additionally, TEWL measurement can


prove that an applied product does not impair the skin barrier. This is an essential test for all kind of products but especially for skin cleansers and detergents. Not only applied products but also fabrics


and textiles getting in contact with the skin, can affect the skin barrier, for example by rubbing over the skin and causing friction irritation. Therefore, TEWL measurement is also quite useful to demonstrate the skin- friendliness of fabrics. But it can also show the properties of the material itself – does it let water vapour pass into the environment like fleece textiles do, or does it absorb water or even have an occlusive effect, so that the water is constantly trapped between skin and garment which might be potentially skin- damaging.


TEWL and young skin Assessment of the Transepidermal Water Loss is particularly important for products for babies and toddlers. Especially preterm neonates and newborns do not have a fully developed skin barrier. Special products and humid environments have been shown to promote the building of the skin barrier which can be monitored by measuring the TEWL.5


Additionally, in incubators heat


convection prevents elevated loss of heat and calories by the water lost through evaporation. The new local skin energy


balance parameter might be an interesting measure for further studies into this process. Also, diapers that are supposed to keep


the sensitive baby skin dry are in constant contact and can irritate the skin by occlusion and by not fully absorbing the expelled urine. TEWL measurement can assess their efficacy in keeping the skin dry, as well as the effects of lotions and ointments in protecting and recovering the skin barrier. Not only diapers but also the effect of wipes and bathing products used for babies can be tested.6 For the food industry, skin barrier


measurements can be interesting to show the effect of so-called “superfoods” and food supplements with a positive effect on skin health.7


Even specific additives for pet


food, for example to render the coat shinier, might have a positive effect on other skin properties. For longer measurements, it is possible


to stick a wearable lightweight adaptation of a TEWL probe onto the skin. This might be useful for small children that cannot keep still for any prolonged period. The newly available local skin energy


balance could be an exciting additional parameter for different research fields such as sports, nutrition & food supplements, textiles, micro-circulation, sleep medicine, veterinary research and many more. At some body sites, TEWL


measurements are difficult to perform since the measuring surface is rather small or uneven, and a full contact of the regular probe with the skin cannot be ensured. For such locations, for example on the scalp or lips, the measurement surface of the probe needs to be rather small. Special probe adaptations such as the Tewameter®


Nano Figure 3: Transepidermal Water Loss measurements on board of the International Space Station (ISS). September 2020


with a 2-mm diameter of the measurement cylinder cover this requirement. With these very small diameters, also the measurement of the so-called Transonychial Water Loss (TOWL) on the curved surface of finger- and toenails is possible.


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