Soft mist inhalers are a relatively new concept in the inhalation space. In principle, they use mechanical energy in place of an HFA propellant to aerosolise the drug. The mist created lasts slightly longer than that of a pressurised metered dose inhaler (pMDI), reducing the reliance on users coordinating actuation and inhalation.
It was first available as a single use inhaler, but later reusable cartridges were introduced making the inhaler body reusable with up to six cartridges. With the current push for more sustainable inhalers, which is a key part of the circular model presented by the EPRS. See image to the right. So how sustainable is this ‘reusable’ inhaler?
Presented with a single pack, you receive a reusable inhaler body and a 60-puff (30 dose) cartridge.
On first inspection the body feels sturdy, visible bits of metal such as the nozzle also give of the impression of high quality.
Without consulting the IFU, when exploring the device you notice a few things; a small button on the exterior releases the clear cover from the body, inside the device is a visible spring and a long, thin capillary tube. There is also a cartridge counter graphic visible.
When you flip up the green cap, a mouthpiece is revealed with a metal nozzle and a different grey plastic button is revealed, which you cannot press at this point.
By inspecting the inhaler, you notice that the clear lid rotates through 180 degrees before clicking and locking. The grey button then pops out. If you press this button, you feel a ‘firing’ happen inside the device.
Taking the clear cover off, you can rotate the inner part of the device, but it locks at around 120 degrees rotation. When you replace the clear cover, the mechanism is released, allowing you to rotate through 180 degrees.
Looking at the cartridge, there is a number on the bottom and a small diagram showing how the cartridge is inserted into the inhaler body. To aid with this discovery, there is only one way that the cartridge can be inserted into the inhaler body. You can pierce the cannister either just by adding the cartridge or doing 2 in 1 and placing the clear cover over the top and pressing down. Either way, you discover that the clear cover must be over the cartridge to rotate through 180 degrees.
Unlike a standard pMDI such as Evohaler, the device has 2 primary actions – rotation of the back of the device and pressing the grey button when you are ready to inhale. This separation allows the user to inhale when they are ready, and timing becomes less of an issue.
Like many other inhalers, the use of Respimat can be distilled to three key areas:
PRIMING:
INHALING:
CLOSING
To allow Respimat to achieve these user steps, we have looked at what happen mechanically inside the device.
In the priming phase, the primary action is to rotate the clear cover, this clear cover is connected to the central body via some clicks, and the geometry allows it to be keyed to the rotating housing. The rotation of the device does two main things, it compresses the drive spring and moves the cannister around 10mm backwards inside the case, due to the internal profile of the main body. This works like a syringe; the drug fluid is fed into the dosing chamber through the capillary tube and non-return valve. The dose counter uses a small post on the back of the clear cover and ratchets one position forward, in turn decreasing the dose counter by 1. There is a small locking mechanism covered on the rotating housing which triggers at 180 degrees by passing over some fixed geometry in the main body. As the cannister is pushed back, the firing button and its connector works like a ‘chock’ and slides into place after the 180-degree rotation, at this point it holds the firing spring back. The inhaling phase is much simpler, When the firing button is pressed, this releases the spring, firing the cannister and capillary shaft- forward (using the metal shaft to displace the drug) forcing the dose through the Respimat’s Uniblock nozzle, creating a fine mist that lasts for a few seconds. In pressing the button, the internal part holding the cannister is pushed through another 180 degrees into its default state.
The process of creating a soft-mist with no propellant involves a highly engineered mechanism. This makes for a lot of different materials, increased quantity of material, and total number of parts in use compared to common pMDI’s. Using the PrescQIPP October 2021 Database we were able to compare Respimat’s embodied CO2 per puff against other inhalers providing similar treatment.
Excluding Handihaler, for similar treatment classes the Respimat is one of the best performing inhalers for CO2 per puff. A key contributor to this was its reusability. By averaging using data for the refills over each inhaler lifespan, the base indicative carbon footprint was reduced from 12.95 (for 1x cannister and the body). Reuse of inhaler bodies, especially within the DPI class can really bring down the CO2e for inhalers, as over time the CO2e tends to the value for the refill. The propellant used in pMDI’s is the biggest net contributor to the environmental footprint of the inhalers.
The benefit of an SMI over a pMDI is obvious, but in many cases, the SMI serves different therapeutic use categories where DPI’s are more common. Respimat can get the drug to much more of the lung compared to DPI’s and has a reduced carbon footprint compare to disposable DPI’s such as Ellipta. Currently the SMI is limited to LAMA and LABA classes, however, with the benefits over pMDI’s, expect them to become more prevalent in industry for short acting therapeutic uses where pMDI’s are still dominant.
Here at HD we have a wealth of experience in inhalation devices, whether that be knowledge gained from design, or from actual use. Having insight from real world users, such as our Design Director Mark, allows us to develop designs that are dealing with problems and frustrations faced by users day-to-day.
Design Development Engineer
Rob graduated from the Dyson School of Design Engineering at Imperial College London in 2021 with a Master’s in Design Engineering. Prior to joining HD, he worked in the automotive sector using state of the art 3D scanning and 3D printing techniques. He has a keen interest in human centred, interaction and experience design and has expertise in additive manufacturing, CAD, IoT, UI and mechatronics.
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