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Med-Tech Innovation Materials


weight, all other possible variables are listed in Table I, including composition, crystal structures, viscoelastic property, spherulite crystal phase, nano/micro structure and processibility.


Materials changes during manufacturing. A


Figure 1. Schematic chain length and diameter of one polyethylene molecule (average molecular weight equates to 4 to 6 millions g/mole)


Figure 2 A failed catheter during manufacturing


Figure 3. Gamma radiation induced ductile-to-brittle transition of polymer


product can fail during any part of the manufacturing stage because processing and all that it entails, such as changes in temperature and pressure, can change the nano/microstructure of the polymer. In the manufacturing environment, modifying processes, equipment, manufacturing material and even personnel will inevitably have an impact on the quality of products produced, which quite often leads to product failure investigations. Figure 2 is an example of a catheter that has failed during a manufacturing stage. This is a typical example of how the combined effect of materials variation and processing conditions can lead to failures. Here, the catheter tube positioning, over-heating (processing factor) and the subsequent change in the material’s thermal properties (materials factor) have all led to the failure of the product. There are several ways of evaluating changes in materials that are introduced during manufacturing: surface analysis, bulk mechanical properties, nano and microstructural analysis, thermal analysis (because the polymer must be heated to be processed). Table I lists a few of the techniques commonly used to characterise materials.


Materials changes caused by post product


source, the material can still vary from batch-to-batch. This is because most polymers used for medical device applications are made by free radical or condensation polymerisation and, as such, have a wide range of molecular weight distributions. Let’s use molecular weight as an example to explain some changes and what these changes may mean. For ease of understanding we will assume that our polymeric material is a mixture of only two molecules as follows:


Case 1: One molecular weight is 2k g/mole and the other 1000k g/mole


Case 2: One molecular weight is 491k g/mole and the other 511k g/mole.


Both cases have the same average molecular weight (actually number average) of 501k g/mole. However, they have different properties in many ways, including polymer chain configuration, nano/micro phase distribution of amorphous and crystalline structure (if they are semi- crystalline polymers), viscoelastic property, rheology (processing flow behaviour), mechanical properties (performance of a product) and so on. For a given processing stage, changing molecular weight and molecular weight distribution parameters can be highly problematic, depending on the degree of the variation. In addition to batch-to-batch variations of molecular


34 ¦ November/December 2011


treatment. All implants and most medical devices need to be sterilised before use, either at the end of manufacture or prior to use. The most common methods of sterilisation are autoclaving (steam or dry heat) at temperatures well above 100°C, using ethylene oxide at lower temperatures below 60°C or the use of radiation (gamma or electron-beam radiation) at room temperature. However, damage to polymeric medical devices can also be introduced at this stage. For example, Figure 3 is a schematic illustration that shows that gamma-ray radiation alone can change the property of a polymer from very ductile (a good mechanical property) to completely brittle (useless). This is because gamma ray is a radiation of high energy that can induce a chemical reaction in solid state, hence affecting the performance of the polymer.1-3


Generally speaking, heating and chemicals


(water, ethylene oxide) and high energy radiation can all cause a polymer to change its nano/microstructure and hence its properties to a certain degree, depending, of course, on temperature and/or length of time of sterilisation. Often the material changes have little effect; sometimes they have a significant effect. For example, medical grade polyethylene acetabular


liners are normally shown to have good mechanical properties after gamma treatment (inducing cross-linking and sterilisation), that is, they are still ductile polymers. This is because the radiation doses employed are normally well below those critical radiation doses that would cause the polymer to become brittle and lose its mechanical properties (Figure 3). However, this does not mean that the gamma rays cause no structural damage to the polymer. The fact is that the damage is too small to


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