“Nothing is softer or more flexible than water, yet nothing can resist it.” - Lao Tzu
Paradoxically it is as strong as it is soft. This is the case with the human body, soft and strong and composed of more than 60% water (adult) with the skin itself comprising 65-70% water. Maintaining a healthy hydration of the body is vital for helping to maintain a hydrated skin. We often hear a common question — what is the difference between hydration and moisturisation? Hydration is the capacity of the skin to hold water. Moisturisation on the other hand tends to include a multiple of other factors as well, such as preventing transepidermal water loss and emolliency. Water is vital for proper functioning of the skin and homeostasis of the stratum corneum (SC), and loss of water from the skin is a carefully regulated process fully dependant on the structure and behaviour of the SC. The skins water holding capacity and its retention within the lower SC is dependent on two crucial structural factors: (a) The presence of natural moisturisation factor (also termed NMF), within the corneocytes. These are a mix of low molecular weight hygroscopic molecules that reside within the corneocyte structure and enable them to remain hydrated. NMF is about 50:50 amino acids: salts including lactic acid and urea. The amino acids are primarily derived from filaggrin, a precursor protein in keratinocytes which is processed into amino acids within the corneocytes. If external humidity is low, NMF is produced lower down in the SC than if the humidity is high. (b) The Stratum corneum lipid process forming the lipid matrix of the SC whereby intercellular lipids orderly arranged form a barrier to transepidermal water loss (TEWL). These specialised lipids occupy the intercellular space of the SC, and are produced within the keratinocytes and discharged at the base of the SC as the keratinocytes transform into corneocytes. Fatty acids, ceramides and cholesterol (the stratum corneum barrier lipids) spontaneously arrange in multiple bilayers (lamellar structure). An orderly release of corneocytes at the skins surface is required for soft smooth skin, and it is the water content of the SC which is necessary for normal corneocyte and SC maturation. Desquamation or detachment and shedding of corneocytes from the skin surface occurs via enzymatic degradation of the protein links (corneodesmosomes) keeping corneocytes attached to one another. These enzymes do not function if the SC surface is dry — leading to dry skin, an accumulation of corneocytes occurs at the skins surface. An accumulation of corneocytes becomes visible and rough to the touch. Moreover, increased TEWL impairs enzymatic functions required for normal desquamation thus resulting further in the visible appearance of dry, flaky skin. Also of importance is the role of the water-transporting protein aquaporin-3 in the viable epidermis (1). Furthermore knowledge of the presence of tight junction structures at the junction between the stratum granulosum and the SC bring new insights into the mechanisms of skin water distribution and it importance in skin barrier function. The mechanisms of skin hydration are complex, and key to any soft supple and hydrated skin is the presence of adequate water. This water is vital especially in its bound form in order to ensure that the NMF process works effectively. Glycerol, a well-known cosmetic ingredient (as glycerin and butylene glycol), exists in the SC as a natural humectant. It is also well known that adding too much of this particular humectant to a formulation, will result in water being drawn out of the skin. If an impaired barrier is not rectified with the additional presence of appropriate lipids, then TEWL will be exacerbated, despite any impressive Corneometer readings! The ‘moisturiser’ hyaluronic acid (hyaluronan), which has been regarded mainly as dermal component, is found in the epidermis and is important for maintaining normal SC structure and epidermal barrier function. Epidermal hyaluronic acid is rapidly lost during ageing, and so its replenishment is very important. In recent years, the growing interest in microcrystalline hyaluronic acid formed into “needles” or micro-cones has resulted in the development of a variety of skin patches to replace lost epidermal HA, as well as to provide delivery of other beneficial skin actives such as peptides. The nature of the importance of bound water versus free or unbound water was demonstrated as far back as the 1990s where it was shown that changes in structural proteins and associated hydration during the photo-ageing process was responsible for altered skin morphological and mechanical properties. Raman studies showed that proteins in photo-aged skin are more compact and only able to interact with water in a very limited way, thus leaving them ‘dry’, as compared to young skin and chronologically aged skin, where water was mostly present in the bound form. Thus impairment in protein hydration adds to the understanding of ultrastructural, mechanical, and biochemical changes in structural proteins in photo-aged skin. It has never been easier or more complex to measure the intricacies of skin hydration, from barrier functioning through to dermal ageing. Be it water capacitance, TEWL, or more detailed Raman analysis of water at different layers of the skin, through to the presence of bound or unbound water and its significance in ageing skin hydration, a plethora of bio-instrumental methods are available to the skin researcher. Skin hydration and moisturisation claims are still the number one in the eyes of the consumer. However, I will leave on a cautionary note, if the understanding, knowledge and products designed for improved skin hydration, do not meet consumer expectations, then in the words of Anton Chekov “ When one longs for a drink, it seems as though one could drink a whole ocean—that is faith; but when one begins to drink, one can only drink altogether two glasses—that is science”.
