Characterization of pharmaceutical powder inhalers: estimation of energy input for powder dispersion and effect of capsule device configuration

The present study was aimed to examine (i) the overall energy input (Edispersion) into two capsule-type pharmaceutical powder inhalers, the Dinkihaler® and the Rotahaler®, and (ii) the effect due to the presence of capsule, on powder dispersion using mannitol as a model compound. Powder dispersion was assessed by a liquid impinger with the fine particle fraction (FPF) defined as the wt. % of particles 5 μm in the aerosol. The overall energy input is given by Edispersion=ΔPVaftdispersion, where ΔP is the total pressure drop across the inhaler, Vaf the volume air flow through the inhaler, and tdispersion the time taken for powder to empty out of the capsule. The overall energy input, Edispersion, was found to correlate with the FPFs generated by either inhaler. Compared with the Rotahaler® at the same air flows, the Dinkihaler® had larger Edispersion values, due to higher ΔP and longer tdispersion. The larger Edispersion was consistent with the higher FPFs. For both inhalers, the presence of a capsule improved the FPFs, presumably via collision between the powder and capsule during dispersion. For the Dinkihaler® the capsule-end apertures also affected the FPF, powder emptying and impaction loss. The optimal aperture size was found between 1.00 and 2.38 mm to deliver a high FPF (50–60 wt. %) with minimal impaction loss and capsule and device retention at a moderate air flow (0.0015 m3/s).