Drug delivery diffusion mechanism

Figure 1. Mechanisms of controlled release drug delivery. Diffusion-controlled (A), dissolution-controlled (B), osmoticSly controlled (C), ion-exchange controlled (D), and degradation-controlled (polymeric prodnig, E) systems.

Thus, the side-by-side device, when properly calibrated, is a versatile and useful method to determine diffusion coefficients, to evaluate mass transport mechanisms, and to evaluate up-to-date drug delivery systems. 

An ideal in vitro model for the characterization of aerosol formulations would incorporate cell types from various regions of the lung (tracheal, bronchial, and alveolar) and would facilitate simulation of deposition mechanisms by impaction, sedimentation, and diffusion of a high-metered singlebolus inhalation. In the future, such systems may reduce the need for animal studies and may offer to correlate in a predictive way the results from such in vitro tests to clinical bioavailability data after pulmonary drug delivery in vivo. 

The permeability barrier in the respiratory region of the nose includes the mucus layer and the airway cells. Mucus potentially affects drug delivery by acting as a barrier to diffusion. In vitro studies have indicated that the presence of mucus has the potential to retard the transport of many compounds, although mechanisms are complex and the impact of mucus is difficult to predict. 

The majority of controlled drug delivery systems now being marketed or under development are based on diffusion of the drug through a semipermeable membrane to achieve the requisite release rate. Diffusion control is particularly important to transdermal delivery, where biodegradation and dissolution are not viable mechanisms of controlling the release rate. Provided the process is Fickian, the rate of diffusion through the semipermeable polymer is determined by... 

Siepmann, J., Kranz, H., Bodmeier, R., and Peppas, N., HPMC-matrices for controlled drug delivery A new model combining diffusion, swelling, and dissolution mechanisms and predicting the release kinetics, Pharmaceutical Research, Vol. 16, No. 11, 1999, pp. 1748-1756. 

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