## 5 Substances, Standards, Methods--New Trends in Psoriatic Skin Care
### Barrier-Restoring Substances  
Another equally important group of preparations used in the care of psoriatic skin are fatty substances that supplement the deficiencies of naturally occurring lipids in human skin. Ingredients that support the restoration of the lipid barrier include waves, cholesterol, ceramides and essential fatty acids (EFAs) [13; 69]. Due to impaired ceramide synthesis in patients with psoriasis, their use improves skin barrier function by stimulating lipid synthesis [61]. On the other hand, EFAs, which are found in large amounts in, for example, hemp oil and oil or evening primrose seeds, facilitate the absorption of fat-soluble vitamins, restore the water-lipid balance and support the regenerative processes of the epidermis [13; 69]. The authors [72] report that in a study of 25 patients with plaque psoriasis, a significant reduction in psoriasis plaque thickness was observed when fish oil (containing 15.8% eicosapentaenoic acid and 10.1% docosahexaenoicenoic acid) was applied topically to the skin for 4 weeks. The moisturizers used in psoriatic skin care described elsewhere in this review are emollients, the regular use of which improves the comfort of psoriasis patients. Non-medicated moisturizers are available as cosmetic preparations and are available in drugstores in the form of both creams, ointments, liquids and gels [73]. In addition, there are also cosmetic preparations containing emollients and humectants in their formulation. Humectants are substances whose ability to moisturize the skin is due to the binding and retention of water in the epidermis [13; 61; 69]. Examples of humectants that are used in cosmetics aimed at psoriatic skin include glycerin, urea and hyaluronic acid [69]. The ability of humectant molecules to penetrate deep into the epidermal layers depends on their size. Smaller molecules, such as urea or glycerol, can penetrate the deeper layers of the epidermis, while larger molecules, such as hyaluronic acid, remain in the stratum corneum [69].  
Urea belongs to the group of natural skin moisturizing factors (NMF). In low concentrations (5-10%) it has moisturizing and exfoliating properties, and in concentrations >10% it has a keratolytic effect and increases the penetration of other topical preparations [61]. The use of urea allows an increase in the penetration of corticosteroids, which significantly reduces the amount of corticosteroid delivered into the skin [2]. According to the authors [25], topical application of highly concentrated urea increases lipid biosynthesis. In addition, studies [2] conducted in this area confirmed that urea (10%) regulates the expression of genes involved in keratinocyte differentiation, increases the synthesis of antimicrobial peptides and lipids responsible for the normal skin barrier, thus confirming that proper skin care of psoriatic skin is important even during periods of remission. In vivo and in vitro studies [25; 73] have shown that urea reduces epidermal hyperproliferation. Thinning of the epidermis by 20%, reduction in the number of cells in its layers and prolongation of the maturation time of cells after division were observed. According to the authors [73], urea's mechanism of action involves breaking hydrogen bonds, which disrupts the quaternary structure of keratin, thereby denaturing keratin. Urea does not disrupt the skin's hydrolipid barrier. The authors emphasize that the use of urea preparations enhances barrier function by, among other things, increasing lipid synthesis within the skin [73; 74].