Biological Factors

Beyond considering the solvent perse, formulators must also consider the biology and state of the skin, and whether transient or sustained delivery is desired. Generally, semisolid formulations are selected for increased residence on the skin, and liquid formulations for a rapid short-term input of permeant into the skin. In both the clinical and cosmetic domains, skin type can affect the choice of formulation base in that usually for normal to oily skin types, gels are preferred but for normal to dry skin types lotions are usually selected and for dry skin, creams are often the preferred base. As well as skin type, the skin site to be treated can affect vehicle selection. For example, for hairy areas lotions, gels or sprays are usually preferable as these spread better whereas for intertriginous areas, creams or lotions are usually employed.

However, it is mainly clinical rationale as to which formulation type (and hence vehicle) is selected for topical therapy. Depending upon the lesion type of the condition to be treated then:

• For a wet, vesicular or weeping lesion, a "wet", usually aqueous based, formulation is generally preferred (cream, lotion, gel).

• For a dry, thickened scaly lesion, a "dry", usually fatty, formulation is preferred (ointments, pastes)

Beyond these simple considerations, clinicians tend not to apply liniments, which may be alcoholic or oily solutions, to broken skin. Evaporation from aqueous solutions (lotions) cools and soothes the skin which can be valuable when treating inflamed skin sites. Alcohol can increase the soothing effect, but should only be used on intact skin. Paints and tinctures generally contain volatile solvents (e.g. acetone or ether) designed to deposit a layer of the active agent onto the skin. Clearly solvent selection is important in clinical efficacy but also impacts upon patient compliance since patients tend to favour creams rather than gels or ointments.

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