Med-Tech Innovation Materials


Table I continued... Guidance on materials factors for designers and failure investigation of medical devices Polymer materials history


Batch-to-batch and other variations leading to problems/ failures


Materials evaluation for finished medical devices


Medical devices made of polymers need to be fully characterised with respect to the tests and factors in the right-hand columns:


Tests/methods for materials characterisation


Materials composition


Important parameters/factors to know


Using spectroscopic and/or chromatographic analysis and fingerprints to identify all


composition specified to a medical product


Traces of metallic and organic contaminants


Electrical properties (if applicable)


Materials changes caused by post- product treatment


All implants and most medical devices need to be sterilised before use, this can be conducted by manufacturers as finished products or prior to use


Autoclaving (steam or dry heat) Ethylene oxide


Radiation (gamma or electron-beam radiation)


Dynamic SIMS, ToFSIMS, ICP, GCMS etc.


This is a different subject and will be discussed in a separate article


Case-to-case variation in sterilisation caused product failure. In general, heating, chemicals (water, ethylene oxide) and high energy radiation can all cause a polymer to change its properties to a certain degree. It is a matter of how serious it is; normally too little to cause a performance problem, although some do have a major effect


Material changes during storage and transportation


Ageing is one of the problems that causes a polymer to degrade


Case-to-case variation


Case-to case-variation: Polymer degrades with time; transportation and storage in a harbour under sunshine or rain all have an impact on polymer degradation


Material changes at use


People get used to “cleaning” a device before use – that is one potential problem causing a short term or long term effect on product performance


Polymers: The fundamentals Although polymers are the most widely used material in the medical device industry and, indeed, have been used for many years, they can often be the root cause of why medical devices fail. This is because of the complexity of polymeric materials; nano and/or microstructures of the polymer can change at any stage during manufacture or use. Table 1 lists many of the changes that a polymer may go through. To have a better understanding of polymers, let’s examine the basic polymer chain structure. Figure 1 is a schematic drawing of a polymer molecule of a polyethylene (in this case a medical hip implant grade of a molecular weight of between 4 to 6 million g/mole). If stretched, the polymer chain has a diameter of approximately 0.5 nm and an average length of 6600 nm to 9900 nm (6.6–9.9 µm) for an average molecular weight of between 4 and 6 million g/mole. What would this polymer chain, with such a high length to diameter ratio and the freedom to rotate at an angle of 109.5 degrees for each of tens of thousands of C – C units look like? Surely, it must be a randomised coiled sphere. From here, it is not difficult to imagine what billions and billions of these polymer chains, some short and some long, would look like if they were gathered together. Many of them would be entangled together due to the high ratio of length to diameter. The overall configuration of long chain polymers remains more


32 ¦ November/December 2011 Case-to-case variation


Case-to-case variation: Types of detergents and solvents used will affect performance and even cause failure depending on the types of polymer and cleaning agents used


or less the same under most applied circumstances, that is, randomised coiled spheres, within which crystals are embedded even when the polymer forms crystals. Further to these randomised configurations, it is essential to understand the fact that all applicable properties are borne from two physical properties: intra-polymer chains (some soft and others stiff) and inter-polymer chains (from weak van der Waals forces to relatively strong). These two physical factors are the fundamental reason why polymers are extremely different from materials such as metals and ceramics. Understanding the two factors is critical to understanding the mechanical properties of polymers. For example, because of their relatively weak van der Waals forces compared with those in metallic and ceramic bonding, long chain polymers are easy to deform in most circumstances.


Polymer material changes We will now examine where and how changes in polymers can affect the design of medical devices. Variations in supplied materials. “Raw” materials


refers to the pellets that are made from polymerisation or the post-compounding process. Batch-to-batch variations are common for polymers. Variations can of course exist when sourcing material from different suppliers but, even when material comes from one


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