In medical device design, a number of key factors need to be considered when deciding whether or not a material, and its specific grade, is appropriate for use on a component:
Frequently the first characteristic to be considered when choosing a material as these determine its suitability for the functions that the component will have to carry out. Properties such as stiffness, toughness and yield strength quickly narrow down the options, while additional factors such as environmental conditions of storage and use (hot, cold, humid and so on), and loading conditions (compression/tension, continuous/cycled, single or repeat use) will help the engineer home in on potential candidates
Hugely important when assessing possible materials, particularly if friction plays a crucial role in successful device operation. If friction co-efficient data is unavailable for the material combinations under consideration (which is often the case), then further analyses and testing is frequently required to check performance of the specific grade combinations chosen. Surface properties can also significantly influence scope for processes such as bonding, labelling and printing.
Broader physical properties such as density, transparency and electrical conductivity are critical to many applications, including some not immediately evident (for example, a build-up of static electricity on a component in the drug path can significantly affect the performance of a dry powder inhaler). Physical properties can also impact on manufacturing options such as injection moulding processing or, for assembly, ultrasonic or laser welding.
These can influence resistance to degradation though contact with lubricants, solvents, moisture or electro-magnetic radiation (such as UV light), and can also raise concerns about susceptibility to the influence of sterilisation techniques such as gamma or ETO. The chemical make-up of a material also determines whether certain moulding techniques such as co-moulding can be used to achieve, say, a multi-textured exterior finish (for ‘soft-touch’ grips) or different features within a single part (a clear window within an opaque body or casing, or a compliant sealing element for example).
Relating more to manufacturing considerations than performance in use, properties such as melt temperature and viscosity have a major impact on injection moulding capability, and hence can rule out some materials for components with sections that would be difficult to fill. They can also influence a material’s tendency to ‘sink’ or ‘flash’.
Shrinkage rates can also affect component stability, especially if rates are different parallel and perpendicular to the flow, and hence they potentially limit scope for the designer with respect to part geometry. Component design influences material selection and vice versa; the decision process is not ‘one-way’.
Biocompatibility & stability:
Key requirements for many medical devices, especially drug delivery devices which must be made of materials suitable for contact with both the drug (that they do not influence drug characteristics, and are not influenced by them) and the user (such as skin contact or implantation – long or short term). For materials not already approved, extensive test programmes could be required to check extractables and leachables, toxicity or irritation, depending on the application and risk. For considerations such as long term implantation or drug primary packaging, where permeability to substances including moisture or oxygen can also be key properties, these issues can be the dominant factor driving material/material grade selection.
An important element of product design and often used to help create the visual language necessary to encourage correct use, and to make a device more ‘appealing’. But colour selection for medical devices can present a number of different hurdles. Only a limited palette is approved as medical grade, for example, constraining choice for device designs unless budgets and timescales can accommodate a lengthy approvals process. Colour – or colour change – can also impact on moulding tolerances, highly relevant when components feature tightly controlled critical dimensions.
Materials contribute significantly towards the physical feel of a device, giving the user important tactile and visual clues. An expensive device has to look and feel solid and substantial, whereas a disposable device may need to be considered ‘OK’ to be thrown away guilt-free. Any requirements for audible feedback, such as device ‘clicks’ to indicate correct use throughout the lifetime of the product, will also have a bearing on material selection.
Almost always a factor in the decision making process, and for devices in highly competitive markets this can be the dominant driver. Materials selection can become a process of ‘moving up the price list’ from materials and grades which are ‘affordable’ but which aren’t quite up to the job to ones which challenge target product costs but which can deliver technical performance. This is not an ideal approach; selection should be based, wherever possible, on technical criteria, but devices need to be commercially viable as well as technically sound, and compromise is often required, where risk profiles permit.
In an ideal world, all of these different factors would be considered in an organised, structured fashion as part of the design development process, resulting in a clear set of component materials specifications for the device. In reality, however, material selection rarely occurs as a steady progression through a strictly defined decision tree. It is invariably influenced by other factors – borne out of the reality of the development process – which makes a theoretically well-defined process less straightforward than might be expected.
To begin with, material selection demands discussion between a number of different parties, all of whom bring different and influential perspectives to the negotiations. These are likely to include:
Continually seeking the best combination of industrial design and mechanical engineering, driven by the ‘vision’ for the device and by user needs and aspirations, branding and visual language, and the need for reliable and robustly performing devices produced from capable manufacturing and assembly processes. All at the target cost. Within the development team itself there may be different and sometimes conflicting views on the best way to achieve good solutions, and there can be many potential compromises with no clear favourite. It is imperative that the device development team understands the different priorities and perspectives at work and is also not too influenced by personal material preferences. There can be a strong tendency to ‘trust’ a material used successfully in the past, even if the new application is slightly different.
Key partners in materials selection, able to provide detailed information and recommendations on the materials and grades they supply. They are, of course, primarily focused on their own product range and hence not necessarily best placed to give broad, independent advice. Building good relationships with a range of materials suppliers, including some independent organisations, is one way to help ensure that balanced views and recommendations are obtained. It is also often the case that detailed information required for some engineering analysis such as FEA, Mouldflow and fatigue or creep calculations may not be available from the suppliers for the grade under consideration. If there is neither the time, budget or resources to generate this data – and engineers should not be surprised to be told that the amount of material for an application is too low to warrant such investment – a designer may be pushed to make assumptions, which can be risky, or chose a grade for which good data is available, which may result in compromise.
The people who, at the end of the day, will have to manufacture the device, in the quantities required, in a commercially viable way – which means low reject rates, straightforward inspection, low tool maintenance and so on. They may be limited to – or prefer to stick with – existing technologies, processes and supply chains and hence may not be open to new material options. They might also have preferences for certain materials with, for example, wide processing windows or well understood storage and handling procedures.
Registered device manufacturers:
Responsible for placing the device on the market, and focussed on the need to ensure that it will be safe, commercially viable, will meet regulatory requirements and can be manufactured reliably in the volumes and geographical locations required. Risk reduction is a key driver, so they may prefer well established ‘industry standard’ materials compared to newer or less frequently used polymers and grades, especially those where the supply chain is less well established.
Unexpected events and late design changes
As well as the influence of individuals, ‘events’ can also complicate the matter of material selection. Health scares – justified or unfounded – can very quickly shift industry or regulatory opinion and render some materials suddenly unacceptable, or at least highly restricted. Examples include the risks centred on formaldehyde release from acetal polymers which became a serious issue in the 1990s, or recent concerns over potentially carcinogenic Bisphenol A residues in polycarbonate containers. For this or other reasons, some suppliers may suddenly decide to withdraw approval for the use of their material in medical applications, especially if higher production volumes are coming from sectors other than healthcare, and the added control and documentation overheads required by medical device supply may simply not be worthwhile.
With environmental concerns rising up priority lists for many companies, the desire to use recyclable materials – or at least materials offering minimal environmental impact on disposal – is increasing, which introduces a new selection criteria for consideration.
And finally, back to the bottom line. Changes in raw materials costs, such as oil prices, can significantly affect price – and availability – of a desired polymer or grade, adding further uncertainty to the economic aspects of material selection.
Once a preferred material is selected and designs detailed, it will be hoped that no further changes are required. However, updates to device requirements late in the day, or changes in the understanding of a design envelope – for example when issues are discovered during performance testing – are just two instances that may prompt a material change late in a development programme, which can be extremely time consuming and expensive to implement.
Some design changes result from issues with ’bulk’ properties of the material, such as the creep performance of a plastic spring element or the stiffness of a retention clip, but these issues should have been identified and resolved earlier in development. Performance issues late in the day are frequently associated with surface properties, such as: excessive friction leading to poor mechanism function; poor stability with a drug due to surface interactions; reduction in impact strength due to micro-cracks, discovered during drop testing perhaps; degradation in properties due to surface contact with other substances; labelling/bonding/printing issues due to low surface energy. German physicist Wolfgang Pauli (1900-1958) summed it up nicely when he said: “God made the bulk; surfaces were invented by the devil”.
All of which may seem like an endless set of hurdles to overcome, but it’s not that bad. To begin with, surfaces can be modified… With thorough forward planning, rigorous empirical and analytical development work, good relationships between all stakeholders involved, and detailed consideration given to materials selection from early on in the design process, it should be possible to identify the ideal materials for a medical device. With almost 4,500 materials and grades of plastic and elastomers on the Campus database for a start, there are quite a few to work with.