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Challenges And Opportunities In Dermal/transdermal Delivery




The pH of the skin’s surface has been reported to be in the range of 5.4 to 5.9 and is important in the maintenance of skin barrier function and defense against infection and disease [88]. The skin also has an excellent buffering capacity against large changes in pH. Albeit, external factors such as washing and applying solutions, drugs and cosmetics to the surface of the skin can raise its surface pH and can likewise increase or induce skin irritation. For example, alkaline solutions of pH 9 and above applied to the skin have been reported to cause skin irritation. In the same study, aqueous solutions of pH 5 and 7 did not cause irritation when applied to the skin [89]. In another study, Ananthapadmanabhan et al. showed that a solution at pH 10 when applied to the skin, compared with a pH of 4 or 6.5, increased the transition temperature of stratum corneum lipids [90]. Observed adverse effects were swelling of the stratum corneum and disruption of the skin barrier function, as indicated by an increase in transepidermal water loss. Therefore, to avoid skin irritation it is very important to buffer formulations applied to the skin as close to the skin’s surface pH as possible. Some universal pharmaceutical solvents are irritating to the skin and therefore cannot be used in topical preparations. Prior to in vivo investigation, these solvents should be replaced by alternative solvents with acceptable irritation and safety profiles. Topical solvents deemed to be safe for use include isopropyl alcohol, propylene glycol, isopropyl myristate and polyethylene glycols to a certain percentage (up to 60% used in marketed products) [91].


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