The range of drugs that can be delivered transdermally to therapeutic levels is restricted due to the effective barrier provided by skin, in particular the stratum corneum, as described in the earlier chapters. The magnitude of permeant flux was given by Equation 2.5 in Chapter 2 as: Flux = Diffusion   coefficient × partition   coefficient × concentration   in   the   donor   phase Membrane   thickness https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429059872/f8a8b035-c40f-4052-9bf6-919fb8589d2b/content/TNF-CH003_eqn_0001.tif"/>