TLDR Different factors like pH, gel type, and chemical enhancers affect how well hydrocortisone gets into hair follicles, and less hydrated skin doesn't work well with the test method.
In the 2008 study using the in vitro human skin sandwich system, researchers investigated how formulation variables such as pH, viscosity, and the presence of chemical enhancers (carvone, menthone, oleic acid, and sodium lauryl sulphate) affected hydrocortisone permeation into hair follicles. They also examined the effect of varying hydration levels of the skin tissues on the method's efficacy. The study found that increasing the pH from 3 to 8.8 slightly decreased hydrocortisone flux into follicles. The use of HPMC gels instead of aqueous solutions significantly reduced intrafollicular penetration, and pretreatment with chemical enhancers decreased follicular flux, likely due to increased permeability of the stratum corneum. Additionally, the skin sandwich method could not be effectively modified to work with less hydrated skin tissues.
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