There are three main approaches to filling blisters with dry powder for inhalation: vacuum drum, membrane dosing and partial compaction and each has its place. But they behave very differently when confronted with real inhalation powder formulations, designed with the patient first, the device second and production equipment third (if you’re lucky).
Vacuum drum filling is well established and can operate at high speed. Powder is drawn into cavities in a rotating drum before being transferred into the blister. It can deliver excellent results with the right formulation, but performance is influenced by powder properties and the interaction between vacuum, cavity filling and release. Change the powder, and the process window can change with it.
Membrane filling uses vacuum to draw powder through a membrane into a defined dosing volume. It offers accurate, low-dose filling and is an established approach for DPI applications. But the principle itself introduces a potential production challenge: progressive membrane blinding. As the membrane becomes increasingly blocked, filling performance can change, potentially meaning cleaning, intervention and downtime are required to maintain the process.
Partial compaction takes a different approach. Our Rotary Turret system uses a dosing principle closer to that of a tablet press. Powder is given time and controlled agitation to fill the dosing pocket under gravity, drawing on well-established tablet press technology. A controlled quantity is then partially compacted to create a coherent dose before being positively ejected into the blister. The partial compaction and ejection are controlled by precision cams and physical movement of the pins, rather than relying on powder passing continuously through a fine membrane.
Of course, it isn't a magic solution. The formulation still needs to fill the pockets repeatably under gravity, be positively extracted from the pocket, and, most importantly, deliver effectively through the patient's device. But there are some significant advantages, for instance, there is no membrane to progressively blind. The process is inherently continuous and less dependent on the time available for powder to flow through small apertures. That can make it particularly interesting for cohesive or otherwise difficult-to-handle formulations. And because production performance matters just as much as initial dosing accuracy, perhaps the more important question when selecting DPI filling technology isn't simply What accuracy can it achieve? It's: What accuracy can it maintain after four hours, eight hours and thousands of doses?
A filling technology shouldn't just perform during an acceptance test. It needs to keep performing when production starts.