Topical treatments are commonly administered to treat skin conditions or inflammation and irritation. Active components such as proteins and biotmaterials need to be delivered effectively to the surface of the skin or tissue and often to the deeper dermal layers. They may also be applied to tissues other than skin such as eye drops for eye infections or nasal sprays for respiratory conditions. The delivery systems may come in the form of creams, lotions, gels, mousses or foams, ointments and solutions. How these are formulated, stored and applied to the skin is very much down to their rheological behavior.
The physical stability to ensure the product does not phase separate or undergo syneresis can be tested on a rheometer. Formulating these treatments to possess the correct viscosity for the type of ailment is important. This will affect how the product sits on the skin and therefore how quickly it is absorbed. Whether we want a thicker consistency (viscosity) ointment to provide more of a emollient barrier or low viscosity drops to be compatible with the tissue such as eyes, ears or nose.
The graph below shows comparative viscosity results for a topical lotion, gel and foam. The foam has a much more delicate structure and yield StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress than the gel and lotion. However, this may vary depending on the type of foam and condition it is designed for. This is also true for any topical treatment; the rheological properties may significantly differ for each formulation and is important they are appropriate for the intended application. This is why measuring and understanding the rheology of new formulations is so crucial.
To learn more about the rheology of emulsions and lotions you can sign up to watch the webinar here.