There are a number of attributes of topical drug systems that may be classified as cosmetic that make patients more or less willing to use their medications (compliant). These include the ease of application, the feel of the preparation once it is on the skin, and the appearance of the applied film. Ideally, the application should be undetectable to the eye and neither tacky nor greasy. Certain items, such as ointments and pastes, are of course intrinsically greasy, and suspensions of all types tend to leave an opaque, easily detectable film. Thus, the extent to which the cosmetic features can be idealized is dependent on the nature and purpose of the dosage form.
The ease of application and method of application of a formulation depend on the physicochemical attributes of the system involved. Solutions and other highly fluid systems may be swabbed on, sprayed on, or rolled on. A cotton pledget or other applicator is often necessary to obtain an even application. Soft semisolid systems, on the other hand, may be spread evenly and massaged into the skin with the fingers, a procedure technically referred to as inunction. The spreadability is a rheological quality related to the nature and degree of internal structure of the formulation. Formulations such as pastes that are very stiff tend to be hard to apply; their application over broken or irritated skin can be disagreeable. The stiffness of a preparation can be upregulated or downregulated by manipulating the amounts of structure-building components of a vehicle and in some instances by adjusting the phase/ volume ratio of semisolid emulsions. Thus, for ointments, increased spreadability can be obtained by decreasing the ratio of the waxy components (waxes and petrolatum) to fluid vehicle components (mineral oil, fixed oils). Greasiness of such preparations goes in the opposite direction. For o/w creams, decreasing the ratio of the internal phase to the external phase tends to make the systems more fluid. Substitution of more liquid oils for some of the high-melting waxy components of creams achieves the same end.
Tackiness and greasiness are determined by physicochemical properties of the vehicle constituents that comprise the formed film on the skin. A sticky film is extremely uncomfortable and, generally, considerable effort is directed to minimizing this inelegant feature. Where creams are concerned, waxy ingredients such as stearic acid and cetyl alcohol produce noticeably nontacky films. Stearic acid is the principal internal phase component of vanishing creams, systems that are virtually undetectable visually or by touch after inunction. On the other hand, propylene glycol, which may be added to creams and gels to solubilize a drug, tends to make systems as these tacky. The synthetic and natural gums used as thickening and suspending agents in gels and lotions tend to increase their tackiness and, therefore, these materials are used as sparingly as function allows.
Creams tend to be invisible on the skin. The same is true for ointments, although the oiliness of ointments causes them to glisten to an extent. Whatever opacity creams and ointments have is due primarily to the presence of insoluble solids. These often imbue applications with a powdery or even crusty appearance. Dispersed solids are usually functional, as in calamine lotion, zinc sulfide lotion, zinc oxide paste, and so on, and are an implacable feature of these preparations. However, at times insoluble solids are added as tints to match the color of the skin and to impart opacity. Since individual skins vary widely in hue (pigmentation) and texture, tinting to a single color and texture is generally unsuccessful.
Evaluation of the cosmetic elegance of topical preparations can be accomplished scientifically, but it is questionable whether physical experiments on system rheology and the like offer appreciable advantage over the subjective evaluations of the pharmacist, the formulator, or other experienced people. Persons who use cosmetics are particularly adept and helpful as evaluators.
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