Although a major analysis of dry skin treatments are outside of the scope of this review it is worth mentioning just briefly the biology that needs to be corrected in cosmetic dry skin conditions and some key examples of suitable treatments.
Traditionally, humectants, occlusives, and emollients have been, and will continue to be, the mainstay of cosmetic treatments (92):
Arguably, the most widely used and effective humectant used in cosmetic treatments for xerotic skin is glycerol, due to its excellent safety profile, cost, and simply outstanding water-retaining (humectant) and hygroscopic properties. There is now much evidence, however, that glycerol is not only a “mere” humectant, but also (i) is a lipid fluidizer (93), modulating the temperature-dependent rheology of SC lipid, thus preventing a loss of fluidity of their lamellar structure at low relative humidities and (ii) has corneodesmolytic activity, facilitating the proteolytic digestion of superficial corneodesmosomes in dry skin (94). Humectants are also an essential requirement for most of the additional approaches. In O/W creams occlusives and bilayers-forming lipids (described below) also require glycerol to alleviate dry skin. Moreover, humectants are required for the transglutaminase-mediated CE maturation that is required for a healthy SC (95). In this respect, combinations of humectants including glycerol have been shown to be more effective than just using glycerol alone. Glycerol has also been shown to enhance the barrier function of the SC (96).
Like glycerol, urea is a natural component of the SC NMF and has been used as a humectant in creams since 1943 (97). Ten-percent urea has been shown to be more efficacious than salicylic acid and petroleum jelly. Urea-containing moisturizers have been reported to improve barrier function and reduce TEWL, increase skin capacitance, and reduce irritation reactions (98-101).
As a principal component of NMF, considerable interest has been paid to the ability of PCA and its derivatives to moisturise the SC. Creams and lotions containing the sodium salt of PCA are widely reported to help hydrate the SC and improve dry flaky skin conditions (102-106).
Petroleum jelly acts primarly as an occlusive agent having been shown to reduce TEWL by over 98%, whereas other oils only manage a 20-30% reduction. Yet this agent does not simply act as an occlusive film over the surface of the skin; it has been shown to diffuse into the SC intercellular domains which may add to its efficacy. On penetrating the epidermis it was also shown to accelerate lipid biosynthesis, thereby aiding barrier repair (107).
Recent years, however, have seen a dramatic increase in the development and inclusion of novel technologies that complement these mainstays of moisturization.