To keep up with the demand of high standards and fast time to market in medical device development, it is a necessity to ensure development methods are optimised. Harnessing inspection processes during the development phase can ensure a smooth transition between design and production. However, if inspection processes are not utilised correctly, there will likely be implications effecting aspects such as: the overall project cost, timelines and product quality. This article details 3 misconceptions related to inspection processes during medical device development.
1: The more inspection, the better the components will be
Current CAD and technical drawing software allows addition of inspection notes and dimensions at the click of a mouse; not only is it easy, but it may also seem logical to designers that including a more detailed component specification will lead to more thoroughly checked and accurate parts; leading to the question “why not just get it measured?” but taking this approach can lead to unwanted drawbacks:
Increased inspection time – In general, the more dimensions/inspection points specified on the technical drawings, the longer the parts will take to inspect. Given some components can go through many samples before finalisation, removing unwanted inspections should reduce development time.
Specification changes – Due to the complexity of production manufacturing processes and plastic materials, it may be that initial component samples are not completely accurate in all areas. If component specifications include irrelevant inspection points, these irrelevant areas of the component could end up out-of-specification. To stop manufactures wasting time fixing insignificant features in further samples, component specifications can be loosened, but this may increase the development time and cost.
Prioritisation of the specification – If the specification is over-extensive for each component, the manufacturers may not know which areas to prioritise. Even if critical dimensions are specified, the quantity can blur the importance, which means that aspects of the specification may get overlooked. Not only that, but manufacturers may not take an overpopulated specification as seriously; if the specification is simple, the manufacturing experts can narrow their focus.
Ideally, from design procedures such as risk analysis and understanding the device requirements, the features which require more control can be identified, and anything irrelevant can be omitted, or deemed acceptable through testing.
2: Widening tolerances and component specifications will reduce project costs
Contrary to the previous misconception, designers will sometimes focus on loosening tolerances and reducing testing to try and speed up development and save money; and when it comes to things like injection moulding, a simpler specification can reduce the cost of the initial tool. However, even though the tool cost may be reduced, the project costs may not follow the same trend as part quality may be compromised. Certain things should be considered before widening part specifications:
Design constraints - Considering the design inputs (performance and user requirements etc.) will set constraints for the design such as size and weight, and when all these constraints are laid out, defining the individual component specifications should be easier. If defined correctly, there will be less chance that theses specifications will need revisiting later.
Tolerance stack-up - If tolerances are loosened without consideration of the device’s intended function, the risk of device failures may increase. The device components should have adequately tight tolerances, such that they satisfy tolerance stack-up calculations. Tolerance stack-up calculations should help determine which tolerances can be loosened, and statistical approaches can be taken to make the tolerance stack-ups more realistic.
Manufacturing capabilities – Liaising with the manufacturer could bring to light where the tolerances can influence cost, perhaps it is simple for the manufacturer to achieve ±0.1mm tolerances across the board without influencing cost, meaning there would be less reason to loosen the dimensions past ±0.1mm.
Material capabilities – For moulding processes, the material itself can also limit tolerance capabilities. Raw material suppliers are generally aware of the limitations of their materials, and they may have alternate grades if required.
Ultimately, initially defining the component parameters well initially, should mean less component and specification modifications, this can directly improve overall product quality, reduce project cost and ensure the project is delivered on time.
3: Inspection is sufficient on its own, there is no need to test
It is likely that inspection methods are not sufficient on their own, and it may benefit development to accompany inspection with testing or other development tools. There is a wealth of potential testing methods for many scenarios, and it should be evaluated whether there is a reason to test a device or individual components during early development. If inspection is the sole method of sample analysis during development, certain elements could be missed:
Unseen imperfections – Components may have imperceptible imperfections which influence their properties, such as: mould imperfections (contamination, weld lines, internal flash etc.), pre-conditioning influences, moisture absorption etc., inspection alone may not always identify these occurrences.
Complex interactions – Inspection can be a vital tool in reviewing components individually, and engineering calculation combined with material research can help designers understand the interactions between parts. However, testing real part interactions is another development avenue which could highlight any potential issues with interactions.
Complex properties – Plastics and their properties do not always easily translate to theory, take for example a device with plastic components with a shelf-life of 3 years, there may be occurrences such as creep, stress relaxation, degradation etc., these issues should be picked up by aging studies but may not be picked up by theoretical approaches.
Inexperienced medical device designers can rely too much on theory and sometimes miss the reality of the actual device samples, inspection can provide information where testing does not, and vice versa, so using both inspection and testing methods rather than relying on inspection alone helps give the device the best chance of moving quickly through the development phase.
Inspection can be an extremely useful tool to ensure successful development of medical devices when used correctly; and can be an expensive, time consuming, burden when used incorrectly. Ensuring component specifications are defined realistically and with justification, and at the same time understanding the limitations to inspection, should translate into a more successful project/s.
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Tom Milnes is a Design Engineer at IDC. Since graduating from the University of Sheffield with a Master's Degree in Mechanical Engineering, he has gained solid experience developing many products and devices across a wide range of industries and sectors including drug delivery.