ir easy aerosolization, controlled degradation properties, and ability to resist phagocytic clearance mechanisms.
             However, the ability to optimize new formulations for pulmonary drug delivery has been limited by the inadequate models used to mimic the conditions the particles encounter in the various regions of the lung. Conventional "complete immersion" methods used to characterize microparticle water uptake rates, polymer degradation kinetics, and drug diffusion rates may not be relevant for large light particles designed for inhalation due to the extremely thin aqueous layers in the lungs and nasal passages. This problem is especially acute in the alveolar region of the lung, where the average thickness of fluid bathing epithelial cells is only approximately 0.07 mm, or two orders of magnitude smaller than our typical aerosol particle. The aqueous layers covering the lung walls in the trachea have an average thickness of 5-30
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