Dosage control torque tests for metered inhalers, auto-injectors and insulin pens

Requirement

It goes without saying that the correct administering and proper dose of medicine is fundamental to ensuring the best chance of patient recovery and wellbeing. Medical devices are becoming increasingly more sophisticated as patients are able to self-administer their own medication, and all remain subject to stringent quality controls.

Auto-injectors, which allow patients to quickly and safely have liquid medication administered by a needle, are no longer reserved only for emergency situations like adrenaline for anaphylactic shock or atropine for wounded soldiers. In the case of diabetic treatment, injector pens (both disposable and re-useable) have increasingly become the preferred device of many patients for taking insulin over the traditional vial/syringe method. One of the primary reasons for this is a greater confidence by patients in the dose accuracy when using pen injectors.

The use of inhaled medication is central to managing patients with asthma and chronic obstructive pulmonary disease (COPD). There is a wide selection of inhaler devices available from pressurized metered-dosed inhalers (pMDIs), dry powder inhalers (DPIs) to soft mist inhalers (SMIs). All feature dose control which is normally achieved by the action of a rotating or sliding mechanism within the drug delivery device.

Various studies have shown that patient satisfaction with their device is often linked to their ability to interact ergonomically with the device. Physical constraints such as reduced strength and manual dexterity, loss of touch and sensitivity in the hands all have a significant impact on the ability to administer appropriate doses of medication—especially amongst elderly patients.

Mecmesin torque testing equipment is used by manufacturers of needle-based injector pens and inhaler devices around the world to check the ergonomic dialing torque and verify the mechanical performance of their medical devices.

Solution

In the case of pen injectors, the effectiveness of the device and its dosage control often comes from the frictional resistance afforded by the movement of the helically-threaded dose sleeve. By testing the torque required to rotate the dose sleeve through a specified number of degrees in both directions, it is possible to optimise the device’s design by careful selection of a range of plastic materials, additives and lubricants. The same test also helps characterise its performance when controlling batches of production units to meet stringent quality-control requirements.

For inhalers, dose counters are increasingly common and impose specific requirements on the valve as they rely on displacement or force to indirectly count remaining doses. It is particularly important that the design and mechanical functionality is completely reliable otherwise the dose may not be correctly counted. This means overcounting (a ‘count-although-not-fired’ event) and undercounting (a ‘fire-although-not-counted’ event), the latter being the more serious failure mode since it misleads a patient to believe a dose is present when it may not be. Torque and force testing is critical in maintaining objective quality-control of such devices.

Although positive engagement and a level of feedback are required for the desired feel, the efforts involved in using the controls on these devices may be quite small. Correct specification of torque (or load) cell to ensure accuracy and the ability to measure small, yet significant differences in torque (or force) is essential. Additionally, correct gripping of the device body and the moving component optimises repeatability. Mecmesin manufactures custom-fit fixtures to the exact geometry of manufacturer’s design.

Test equipment

• Helixa precision torque test system with capacity matched torque cell
• Special fixtures to hold the device body and grip the dosing control mechanism