**2.1 Fundamental ingredients**

Topical moisturisers usually contain, at a minimum, one or a combination of the key moisturising ingredients, namely emollients (e.g., dimethicone) (Section 2.1.1), humectants (e.g., glycerin) (Section 2.1.2) and occludents (e.g., petrolatum/ petroleum jelly) (Section 2.1.3), as well as numerous excipients (e.g., penetration enhancers, preservatives, pH adjusters) to stabilise the formulation (Section 2.1.4). Additional ingredients often include selected specialty ingredients (Section 2.2) and actives (Section 2.3). Ingredient selection, and moisturiser composition and formulation are crucial considerations when choosing an appropriate moisturiser. Specifically for psoriasis, these considerations can determine whether the product will repair and strengthen or further deteriorate the psoriatic skin barrier [6, 35, 45].

### *2.1.1 Emollients*

Emollients are used to improve the appearance and texture of skin by filling in the crevices between corneocytes. This contributes to increased softness, smoothness and suppleness of the skin and improves its overall appearance [42, 46, 47]. The most common types of emollients are silicones such as dimethicone, which is a hypoallergenic and non-comedogenic polymer and is used extensively in topical moisturisers. It exerts a protective effect on the skin by locking in moisture and decreasing TEWL [48]. Dimethicone's low surface energy and highly flexible silicone polymer backbone allows for effective spreading on the skin and a pleasant skin feel. The physical and aesthetic properties of silicones can be controlled by varying the chain length and molecular weight of the polymer. As chain length increases, the viscosity of silicones also increases, and vice-versa. Low viscosity means that the silicone is able to spread quickly and easily while providing a light, silky skin feel, whereas higher viscosities enable silicones to form more persistent hydrophobic (water-repelling) films with good water barrier properties [49].

### *2.1.2 Humectants*

Humectants are hygroscopic (water-attracting) substances that are able to increase the water content of the skin by enhancing water absorption from the underlying skin layers, namely the deeper epidermis and dermis. Humectants penetrate the SC readily and act like biological sponges that promote water retention in the skin [47].

In addition, humectants are also able to hydrate the SC by absorbing water from the external environment. As a consequence, the SC tends to have greater water content in areas in which humectants are localised [42, 46, 47].

Glycerin is the most widely-studied and used humectant. It is also an endogenous component of the human skin. Glycerin is transported from the dermis through the keratinocytes by a transmembrane water/glycerol transport protein, Aquaporin 3 (AQP3) [50–52], and its hygroscopic properties enable it to increase the water holding capacity of an impaired SC. Glycerin functions in a way similar to the skin's own natural moisturising factor (NMF), which is an essential skin process responsible for appropriate SC hydration, barrier homeostasis, desquamation and plasticity. When used topically, glycerin protects the skin from irritant-associated skin conditions and accelerates recovery of irritated skin, while also improving overall skin hydration. Topical glycerin also helps barrier recovery through corneocyte desquamation regulation and is able to restore skin hydration at low usage levels (from as little as 2% v/v up to 10% v/v) [47, 52].
