Autoinjectors are medical devices designed for efficient subcutaneous and intramuscular drug delivery. Most autoinjectors are single-use, disposable, spring-loaded syringes. Differences in device and feature mechanical design can affect the autoinjector functionality and performance. To activate the device and deliver the dose, a powerful spring is used to drive the needle insertion and eject the drug from the cartridge. These springs are stored in a loaded state of typically 50N’s (5kg) force ready for the device to be used. It is essential that the spring force is held reliably until the device is intentionally activated as accidental firing will result in the device not working when needed, unintended injection, or injury whilst the needle is exposed. Jack Dunkley CEng, shares some hints and tips for optimising clip designs for auto-injector spring loads:
As with all engineering, clip design is about balancing requirements. For example, a clip which is holding the drive spring in an autoinjector needs to:
• Reliably hold a force for long periods of time.
• Not unclip when dropped.
• Reliably unclip when desired.
• Clip together during assembly.
• Fit within the device.
From this list, it is evident that some requirements conflict with others. A clip which can hold forces for long periods of time will likely be stiff therefore require more force to unclip. Therefore, the best solution(s) for one requirement might be the worst for another meaning that balance between multiple requirements is needed.
When optimising for strain in a bending clip, there are several geometric properties which can be varied to get a desired value. If subjected to a continuous stress over a long period of time, plastic components will creep until failure occurs. Therefore, it is essential to design the clips so that a continuous stress high enough to cause creep does not occur.
By plotting the variables over the expected tolerance range, the amount of strain in the clip is most sensitive to how much the clip is being bent. In the above equation, the length is squared so over larger ranges it will be more sensitive but, over the smaller scale the curve appears almost linear. It should also be noted that by understanding the effect of each variable, they can be balanced against each other to achieve the desired output. For example, if a clip requires a certain amount of engagement, the length and thickness can be tuned to give the required strain values.
As with many components, there are other variables that engineers have control over such as materials and manufacturing process. There are also factors that engineers have little to no control over such as contamination, age and environment. For a robust design, each of these foreseeable variables should be analysed. In many cases, FEA can be used to analyse a design by adjusting variables and examining the effect on the outputs.
Other variables, such as changes in material properties over time or contamination, may require physical samples to be tested. For example, accelerated life testing where a component or product is heated to above its normal operating condition can be used to reduce the testing time required to gather sufficient data.
We hope that some of these hints and tips surrounding optimising clip designs for autoinjector spring loads have helped. Please don’t hesitate to get in touch if you have any questions or would like to discuss a new product or medical device development project.
Engineering Director
Jacks is responsible for the engineering aspects of projects to ensure technical risk is minimised. With expertise in simulation, drug delivery devices, and electronics, Jack guides your project team on all related deliverables.
Jack is a Chartered Engineer with a Master’s Degree in Mechanical Engineering. He has over 10 years’ experience working in both the medical sector and on complex machinery.
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