MARKET PLACE
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Pressurised metered dose inhalers (pMDIs) are the preferred drug delivery device for the treatment of asthma and chronic obstructive pulmonary disease. They are low cost, convenient, reliable and offer accurate dosing. However, in most cases they carry a major patient safety risk: they do not display the number of doses remaining. Patients are expected to record the number of actuations, but studies show that most rely on guesstimates and, because the canisters are over-filled during manufacture, they often continue using their inhalers beyond the labelled number of doses anyway.
Companies are under increasing pressure to improve patient safety and compliance for pMDIs by designing-in dose indicators or counters. The United States Food and Drug Administration (FDA)1 and European regulatory bodies have already issued recommendations on the ideal feature set and in the near future these are likely to become mandatory. Devices should count down to display the number of doses remaining while also allowing patients to continue using them beyond the labelled number of doses. Most patients carry only one inhaler and in a severe, potentially life-threatening attack they may have to rely on using a device that is theoretically “empty,” but still able to deliver multiple sub-therapeutic doses.
A brief survey of patents reveals that many companies are actively developing electronic and mechanical counters for a range of delivery products. Surprisingly these typically contain 8 to 12 moulded components, often with a spring. The individual parts within a medical device each have manufacturing tolerances associated with them, which result in considerable variability when they are assembled together (tolerance build up). Devices designed along these lines not only carry significant development risk, but also will be costly to manufacture and assemble.
Potential solutions
The project was to find a way to design-out high-tolerance components. As a result, three low part count dose indicators based on existing, well-documented technologies have been designed. Each indicator is comprised of no more than five parts and offers low manufacturing and assembly costs. They have also been designed to use simple plastic moulding and normal manufacturing tolerances, thus are expected to be robust and
reliable.
Dose indicator 1 comprises four components and the plastic body of the actuator and is based on a simple, but powerful “wobble wheel” mechanism. The display wheel is typically marked in 20 unit divisions to allow patients to see the number of doses remaining to the nearest ten.
Dose indicator 2 is a five-part device that uses common tooling to produce the two largest components, that is, the actuator body and canister housing, and the parts that can also be used for the numeric dose counter (as described below). This makes it suitable for manufacturers who want to implement indicating and counting technologies within their product lines. This device uses eccentric gearing principles whereby a mismatch between the number of teeth is deliberately used to create a high-ratio gearing mechanism.
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Figure 1: The five-part numeric dose counter is based on a modified Sully escapement mechanism. |
Dose indicator 3 is a numeric dose counter that uses five parts and is based on a modified Sully escapement mechanism (Figure 1) that has been employed for centuries in clocks to convert reciprocating movements into a rotary motion. In this case, the depression and release of the canister during actuation counts out each dose on a wheel. One wheel is used to record units from 0–9 and a second escapement mechanism is used to index the wheel every tenth dose and to count in tens. The viewing window is obscured once the “final” dose has been delivered in accordance with the pack labelling, although the inhaler can continue to be actuated in an emergency situation.
There is always a compromise to be made between the size of the numerals and the overall space that is available. The aim is to have numerals or characters that are between 2.5- and 3-mm high. The products designed here have numeral heights of 2.7 mm. With regard to the potential issue of debris, the mechanisms do not become disengaged and therefore should be resistant to dust ingress.
All three solutions have low part counts, eliminate the need for high-tolerance moulded components and have been designed and animated in Pro-Engineer to show that all parts are moving exactly as they should. The next stage is to develop manufactured prototypes to fully validate the designs. The approach could also be incorporated into other devices for nasal drug delivery, pill dispensing or inhaled therapeutics such as insulin.
For more information, contact Nick Campling, Managing Director, 42 Technology, St Ives PE27 4LG, UK, tel. +44 1480 302 700, e-mail: nick.campling@42technology.com, www.42technology.com.
