Mass transfer studies through biological membranes. A chemical engineering approach
Résumé
This contribution describes the classical way that the process of drug diffusion through skin is studied experimentally. One use the Franz cell in association with a linear model based on Fick diffusion model, linear equilibrium relations and the assumption that the skin can be assumed to be a dense membrane. It appears that the way the Franz cell is used is related to the existence of an analytical solution of the model that is available in classical textbooks [1]. Due to numerical methods, improvements in the description of the process of drug transfer can be proposed. Firstly, the highly complex multiphase/multilayer structure of the skin can be taken into account. Such models have been proposed in the literature devoted to mass transfer through skin [2]. They are more or less complex depending on the properties of the studied layer. In the Stratum Corneum, the pure diffusion model is always assumed due to its avascular nature. However, some papers propose to take into account its structural properties by 2D or 3D "biphasic brick-and-mortar" models with simple partition coefficients between the two phases but diffusion coefficients variable in space [3]. Some improvements were further made adding porosity and tortuosity in the model. In the viable epidermis (VE), some authors have added a convection term to model the transport into blood and lymphatic capillary partition. But the transport has also been modelled using a dispersion term, which includes diffusion and convection at the same time. As far as we know, all these models remain generally based on a linear description of thermodynamic equilibrium and diffusion in order to get, if possible, analytical solutions [4]. Compartment models are also commonly used within this framework [2]. Secondly, modern diffusion theory (Irreversible thermodynamics approach, Maxwell-Stefan approach [5,6]) as well as network model approach could be used provided that more sophisticated thermodynamic models of the skin and knowledge on skin structure to be available.