*2.1.4 Excipients*

Non-active ingredients, commonly termed excipients, are extensively used in the formulation of topical moisturisers and typically make up the majority (≥90%) of topical product content [41, 55]. By their physicochemical nature, different classes of excipients are used to enhance the functionality of active ingredients in therapeutic products, as well as to aid with formulation challenges. Excipients are often used to: (1) improve solubility to allow incorporation of an active; (2) control the release, penetration and permeation of an active; (3) improve the overall aesthetics of the product to increase patient compliance; (4) improve active and product stability; (5) prevent microbial growth and contamination (e.g., preservatives) and (6) balance the pH of water-based moisturisers, so that they are compatible with the skin's naturally slightly acidic pH [41].

#### *2.1.4.1 Penetration enhancers*

Penetration enhancers are chemicals that readily disrupt the structure of the SC and are commonly used to facilitate active (drug) delivery. The cutaneous inflammation experienced by patients with psoriasis promotes hypersensitivity and also suppresses skin barrier function. Therefore, the effective delivery of anti-inflammatory actives such as corticosteroids, aided by appropriate penetration enhancers, can bring about a net improvement in the skin's barrier function [41]. Many penetration

### *Topical Moisturisers for the Management of Psoriasis Vulgaris DOI: http://dx.doi.org/10.5772/intechopen.101964*

enhancers, like propylene glycol, are also solvents, and so can be used alone or in combination with other penetration enhancers to help facilitate both the partitioning into and the passage through the SC. However, care must be taken when selecting and using chemical penetration enhancers since their excessive use can potentially lead to systemic absorption of the active [41, 56]. As such, a careful tradeoff must be made between delivering a therapeutic active dose and protecting the integrity of the skin barrier. Penetration enhancers composed of short chain fatty acids, such as propylene glycol, are thought to integrate into the hydrophilic regions of the packed SC lipids and increase the solubility of this domain for the permeant [41], yet at high concentrations (above 10%) they can irritate the skin [41, 57, 58]. In contrast, penetration enhancers composed of long chain fatty acids like oleic acid insert themselves between the hydrophobic lipid tails to increase the fluidity of the SC lipid bilayers [41].
