Water is life – all life on earth requires water to survive. We are about 70% water, and it is a vital chemical to the correct functioning of the skin. Water helps keep the outermost layer of the skin (the Stratum Corneum or SC) flexible. Without water the SC becomes hard and brittle and prone to cracking and splitting. It is also necessary for the biochemical processes going on within the skin which help regulate its growth, and eventually how the outermost cells are lost.
We have to be careful with water though – just as too little of it is a problem, too much also causes problems. Too much water, or hyperhydration, results in swelling of the SC, and it becomes much weaker. Too much water also washes out Natural Moisturising Factors (NMF) from the skin which reduces its ability to hold on to water when it dries out again.
A number of methods exist to look at hydration within the SC. Most rely on changes in the electrical properties of the skin as a consequence of its degree of hydration. Dry skin has both a enhanced ability to storage an electric charge applied to it, and an increased electrical resistance to charge passing across it. Techniques such as the Corneometer, Skicon, and Dermal Phase Meter, look at these electrical properties and use them to deduce hydration status of the skin. The water of the SC can also be directly measured using devices such as Confocal Raman Spectroscopy, or Infra Red spectroscopy, however these are significantly more expensive and complex than the electrical measures, and as such are currently not as widely used (although they can most definitely be used to provide a more accurate assessment of water content).
As with most skin methods, no two devices are ever precisely the same in how they measure the water, and as such it is good practice to use the same device throughout a study, and to always do change from the original condition, and also to measure untreated sites of skin, to determine whether there has been any day to day change in the skin itself.
Skin hydration measures are used for measuring the efficacy of moisturisers, and also for testing the drying effects of cleansers. With moisturiser testing an area on the skin is marked out, and the baseline hydration state measured. A standard amount of moisturiser is applied to the skin in the marked out area (typically 2μl cm-2), rubbed in and allowed to dry. As most moisturisers contain water, if the area where the product has been applied is measured too quickly, the water in the product itself is detected, and gives a false reading of the skins hydration. It is recommended that atleast 30 minutes is left between applying the product and the first reading to give the water from the product time to evaporate. The site can then be measured over time to see how the skins hydration level changes after product application. These types of tests are used to develop “hydrates your skin for 24 hours”, or “more hydrating than leading competitor” types of claims. When looking at cleansers and how they can dry the skin, typically longer studies are done (up to 2 weeks) with regular product usage, and the hydration state of the skin measured at the beginning and end of the study.
There are watchouts when measuring skin hydration. As well as the issues with moisturisers as discussed above, the devices are sensitive to how they are placed in contact with the skin, so the operator needs to be well trained, and familiar with the device being used. As they rely on contact with the skin, the presence of hair will cause them to give low values for hydration. As with all skin measures, the panellist need to be acclimatised in the room where the measurements are done so that they become used to the temperature.
I use a Corneometer (one of the electrical devices) for the majority of my work on hydration, as I have found this to work well for moisturiser testing. However I have worked extensively on Confocal Raman Spectroscopy of skin which can provide direct measurement of water levels within the skin (see my publication list and the section on Confocal Raman Spectroscopy for more details).