Suspensions are not just vehicles but products consisting of particles, generally actives or functional excipients, that are dispersed in a liquid or semisolid medium that functions as a vehicle. Nevertheless, a suspension is also a type of formulation that may be used for application on the skin and to deliver substances to a target. In this way, a suspension can be regarded as a vehicle entity affecting the application site. Examples are sun-protection products or pearlescent nail lacquers containing pigments.

In suspension, sedimentation of unsoluble particles may happen because of difference in density. In order to guarantee a homogeneous product when applied, the particles must be redispersible by shaking before use. Alternatively, sedimentation must be hindered or at least reduced during storage. This is achieved by reduction of particle size and/or by increasing the viscosity of the vehicle, ideally creating a thixotropic system. The vehicle effect of the suspension on the skin is primarily caused by the liquid or semisolid phase of the vehicle comparable to solutions and emulsions.

Source: Handbook of Cosmetic Science and Technology - André O. Barel, Marc Paye, Howard I. Maibach

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