The effect of rotation speed and flow rate on evacuation of particles from a spinning dry powder inhaler capsule

This study investigated a capsule’s powder evacuation behavior when rotating about its minor axis in a cross flow and considering the effects of rotation speed and flow rate on powder emission. The experimental platform, an optically accessible capsule chamber, was designed to uncover the independent effects of these variables by enabling high-speed imaging of the powder evacuation. The capsules were rotated at three speeds (1500, 2500, and 3650 RPM) and two constant flow rates, 30 SLPM and 60 SLPM (inlet velocity: 16.67 m/s and 33.33 m/s, respectively). Two powders were selected: a lactose carrier, Respitose (SV010, D50 = 104 µm) and Mannitol (D50 = 7 µm), the latter representing pure active pharmaceutical ingredient formulations that form agglomerates. In addition to imaging, the capsule was weighed before and after each device actuation to quantify powder emission. Increasing the flow rate was found to have the largest impact on the mass emitted from the capsule at all rotation speeds. The emitted mass for all cases was highly variable and influenced by the cohesiveness of the powder and subsequent blockage of the capsule aperture. The potential for blockage was more pronounced for mannitol at the high rotation speeds. Emitted dose over time was modeled using a natural logarithm function to describe the rate of emptying and demonstrate the advantage of increased flow rate and favorability of low/moderate rotation speeds. The study of powder size distribution during evacuation found no significant difference between flow conditions for mannitol, as dispersion was dominated by shearing at the capsule aperture.