Numerical simulation of Franz diffusion experiment: Application to drug loaded soft contact lenses

Drug loaded soft contact lenses (SCLs) have been considered as an alternative to drug administration via eye-drops to improve bioavailability and patient comfort. Understanding the drug delivery characteristics is key to the design of therapeutic drug delivery systems. In this study, Franz diffusion cell experiments were carried out in order to analyse such characteristics of hydrogel used in SCLs. Two types of experiments were carried out. In one case, the donor compartment was loaded with an initial concentration of diclofenac while in the other case the lens material was pre-loaded with diclofenac. Experiments were carried out with varied initial concentration and temperature. In order to simulate the experimental data and estimate relevant physical parameters of the SCL material, we introduce a general mathematical multi-layer model that takes into account diffusion within each layer as well as the effects of both partition and barriers between layers. The model includes a novel treatment of the discontinuity in drug concentration between layers in an accurate manner. We succeeded in simulating all these experiments. However, the donor-loaded experiments proved more consistent than the pre-loaded case as the same set of estimated parameters could consistently explain varied starting conditions in the simulation of donor-loaded experiments.

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