Spreading of Semisolid Formulations An Update

pastes, cream emulsions, gels, and rigid foams. Their common property is the ability to cling to the application surface for a reasonable period of time before they are washed off or worn off. They usually serve as vehicles for topically applied drugs, as emollients, or as protective or occlusive dressings, or they may be applied to the skin and membranes such as the rectal, buccal, nasal, and vaginal mucosa, urethral membrane, external ear lining, or the cornea (1). These preparations are widely used as a means of altering the hydration state of the substrate (i.e., the skin or the mucous membrane) and for delivering the drugs (topical or systemic) by means of the topical–mucosal route. One of the serious problems associated with the formulation and manufacture of topical–mucosal preparations is the establishment of reliable techniques for their characterization, mainly because of the complexity of their physical structure (2). Consumer preference for such products depends on various properties of the preparation, collectively known as the textural profile, which includes appearance, odor, extrudability (when applicable), initial sensations upon contact with the skin, spreading properties, tackiness, and residual greasiness after application (3). Ultimate acceptability and clinical efficacy of such preparations require them to possess optimal mechanical properties (ease of removal from the container, spreadability on the substrate), rheological properties (viscosity, elasticity, thixotropy, flowability), and other desired properties such as bioadhesion, desired drug release, and absorption (4). The efficacy of topical therapy depends on the patient spreading the formulation in an even layer to deliver a standard dose. The optimum consistency of such a formulation helps ensure that a suitable dose is applied or delivered to the target site. This is particularly important with formulations of potent drugs. A reduced dose would not deliver the desired effect, and an excessive dose may lead to undesirable side effects. The delivery of the correct dose of the drug depends highly on the spreadability of the formulation. Spreadability, in principle, is related to the contact angle of the drop of a liquid or a semisolid preparation on a standardized substrate and is a measure of lubricity, which is directly related to the coefficient of friction (5). Spreadability is subjectively assessed at shear rates varying from 102 to 105 s 1. The rate of shear during spreading, s 1, is cal-