With many autoinjectors a training device is required to safely teach patients and users how to use the device without the risk of accidental injection. These are surprisingly complicated devices as they must accurately mimic the functioning of the active device whilst also being resettable so they can be used repeatedly. We have worked on device specific training devices in the past for a few customers but due to the confidential nature of projects we haven’t been able to showcase what we have achieved. Therefore, we decided to design a capability demonstrator which could be use to show our service offering.
We designed a training device which mimics the functioning of typical autoinjectors. Whereby the blue safety cap is removed, and the needle end of the device is pushed against skin. At a pre-determined force, the device activates, and a needle followed by drug is pushed into the patients’ muscle, in the case of the trainer a safe short plastic peg emulates the needle along with an audible click to inform the user that it has activated. Once the device has been removed from the skin a shroud protrudes forwards over the needle to protect against needle stick injury.
Normally at this stage an active device would be disposed of or kept to show a paramedic that it has been used, however there are no more steps required. With a training device it needs to be reset so that it can be reused. Our device is designed so that when the safety cap is refitted, and the needle shroud is pushed back in, the mechanism is pushed back as well so the device resets.
We started the project with brainstorming sessions to produce a wide range of suitable concepts for the internal mechanism which the device could be based on. Using tools such as ideal judgement matrix’s we refined the concepts down to the most viable options. Some of the criteria for selection included, number of components, number of user steps, method of resetting, and confidence in reliability of the mechanism. We weighted these criteria against each other using Pugh matrix as are more important than others, as it’s a training device we don’t want to confuse the user on the operation of the active device so the method of resetting has to be such that they don’t try resetting an active device in an emergency or only remember how to reset the device and not how to inject someone safely.
For the design of the housing, we wanted something that looked very different to commonly available devices so that it couldn’t be confused. As well as brainstorming concepts for the mechanism we came up with a range of ideas for the industrial design of the injector. We settled on a concept which balanced aesthetics, functionality, and manufacturability. The design of the shell flares outwards towards the needle end of the device to naturally encourage the user to hold it closer to the safety cap end.
The device uses 2 springs to provide the force for each movement (needle firing and movement of the needle shroud). During the needle shroud movement one of the springs compresses the other spring to relatch some of the clips, to simplify the part count we decided we wanted to use the same spring in both positions therefore we had to find a spring which had the correct geometry as well as a suitable K (Spring rate) value to provide the correct forces at it’s intended extension.
To calculate the force requirements of the springs we used a combination of FEA and 3D printed components to physically test parts.
As well as knowing what the force requirements were, we wanted to minimise these as much as possible. Excessive forces from the springs would cause problems such as creep of components over time due to prolonged loading and excessive impact forces when the device fires reducing the life of the device. It may also require clips to be thicker to hold the forces which in turn would require more force to bend, this leads to a cycle of chasing the clips size and spring force.
We used FEA to refine the clips to the point where they required minimal reclipping force and still functioned reliably.
Labelling was developed to inform people how to use the device. The instructions are typical of an autoinjector trainers which features details such as: dummy expiry date, information that the devices do not contain active medicines or needles, arrows to help inform users of which end the needles would be. The labels were designed so that it is clear which side are the activation/firing instructions, and which side is the resetting instructions. An important part of the training process for these types of devices is teaching the user to check the expiry date prior to using the autoinjector and to hold the device against the skin for 10 seconds so that the full drug does can be delivered.
As this project wasn’t for a customer the branding on the label has been created in line with the Haughton Design corporate guidelines.
Training devices don’t require the same rigorous packaging design that active devices do. Commonly at a training venue they will be taken out of there shipping box and placed in tubs where they can easily be accessed. They still require transportation packaging so that they don’t get damaged, we designed a plastic free box which is made from recyclable materials to minimise waste.
With many of our projects we will progress from 3D prints straight to soft tooling this is because small complex mechanisms often don’t function properly unless they are accurately produced using high quality resins such as Acetal. 3D printing can be useful for doing fit checks or checking the logic of a device works however it typically won’t be as strong as moulded components due to the method of laying down layers and it’s doesn’t have the same accuracy as injection moulding with parts varying batch to batch or even part to part based on where they are on the printer bed.
A halfway house between 3D printing and injection moulding can be vacuum casting which is a process where Polyurethane is moulded in soft Silicone moulds. Due to the material and method of manufacture, the moulds are much lower cost (less than 10% of a prototype injection mould tool). It’s not all low cost however the process of vacuum casting is labour intensive which makes the unit price roughly 10x more expensive the injection moulding. The tools also have a limited life of 15-30 components. For these reasons vacuum casting is good for low volume prototype manufacture, if the strength properties of the material can be replicated, with Polyurethane substitutes. Amorphous polymers such as ABS and Polycarbonates can usually be replicated in Polyurethane, however engineering grades such as Acetal and Polyamide cannot.
Cost wise we reviewed the cost of manufacturing this training device prototype via injection moulding and vacuum casting and found that the cross over point between the two processes was around 200 assemblies.
Following production of the first off prototypes we preformed functional testing which included; checking the device couldn’t be fired with the safety cap on, activation force, resetting force, as well as some preliminary life testing.
For the cycle testing we fired a sample of devices 300 times and measured the activation and resetting force on every 10th activation. We didn’t see any drop off in performance over the 300 cycles.
Due to the Polyurethane material used on these prototypes we were unable to perform accelerated lift testing using elevated temperatures. If Polyurethane is heated up it creeps and deforms at a rate which isn’t proportional to its chemical aging therefore it doesn’t accurately simulate how it would naturally age. On previous projects we used to Arrhenius model to calculate the acceleration rate of different temperatures in an incubation chamber. Typically, we would use an acceleration temperature of +25 degrees from ambient which ages the plastics at roughly 1 year for every 100 days. Each 100 days we would take the device out of the chamber and allow them to cool to room temperature over a day before perform force and functionality testing on them before returning to the chamber. We can then monitor whether there is an quantifiable change in performance over the period.
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