I am about to prepare a formulation of transdermal patch. So I was going through various journals. Many of these a term called "flatness" to evaluate the prepared patches.
But I am not clear about the concept of it. What is flatness in regards to transdermal patches?
A transdermal patch should possess a smooth surface and should not constrict with time. This can be demonstrated with flatness study. For flatness determination, one strip is cut from the centre and two from each side of patches. The length of each strip is measured and variation in length is measured by determining percent constriction. Zero percent constriction is equivalent to 100 percent flatness.
A transdermal patch should possess a smooth surface and should not constrict with time. This can be demonstrated with flatness study. For flatness determination, one strip can be divided in such a way that, one from the center and two from each side of patches. The length of each strip is measured and variation in length is measured by determining percent constriction. 0% constriction is equivalent to 100% flatness.
% Constriction =L1 – L2/L1X 100
L2 = Final length of each strip
L1 = Initial length of each strip
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