Med-Tech Innovation Materials


which is what we have done by looking at the impact of varying volume. Thus, the rational approach can result in different conclusions and gives much greater confidence in those conclusions.


Why isn’t everyone doing it? If the logic is clear, why isn’t everyone doing rational selection? Of course, the approach has some practical challenges: • You need data on the “universe” of materials • Data must be normalised so that sensible comparisons can be made


• Data cannot have gaps or materials excluded for no better reason than they have merely been missed


• You need information beyond the traditional “engineering properties” found in databases or handbooks such as price, biocompatibility and sterilisability


• It is complicated to efficiently apply constraints and to study the trade-off between objectives


• How do you pick the right performance index? The barrier to overcoming these challenges has been high and the motivation to do so has been low, thus traditional approaches to selection have sufficed. But these factors are changing. Rational selection software is now relatively mature. Moreover, medical device organisations face pressures that are driving them to look harder at materials selection or that create more demand to replace materials. These include the need to differentiate themselves in competitive global markets, regulatory requirements that demand greater auditability, disruption to materials supplies and price volatility.


Enabling rational selection


Granta Design is a Cambridge University spin-off founded by Mike Ashby in 1994. In the intervening period materials selection has become a standard element of many university engineering courses. Therefore, an emerging generation of engineers and designers is familiar with the principles. The tools for rational selection and supporting technology have developed substantially in two major areas: data and software tools.


The MaterialUniverse database, developed at Granta and Cambridge University, provides engineering, economic and environmental properties on more than 3,000 materials. It has been designed for selection. Data is comparable and there are no gaps, and unknown values are estimated using proven data models. In recent years, a medical version has been developed that adds properties such as toxicity, biocompatibility and sterilisability. The database covers the full range of commercially available materials types with the properties that device designers need. It includes estimated data and does not describe specific grades, which is the right level of detail for an initial screening that identifies the best classes of material. Designers can then use one of the more specialist databases such as CAMPUS Plastics database or their own in-house data to make final choices.


The second element is software. The company’s CES Selector has a simple user interface that makes


20 ¦ November/December 2011 www.med-techinnovation.com


it quick for users to specify constraints on materials choice. A graphical Performance Index Finder (Figure 2) lets designers pick a design scenario that is close to their application and they do not need to learn the mathematics. The software produces clear materials property charts that make it easy to visualise and trade-off materials options. On-line information resources and handbooks still play an important role. Several resources are available providing extensive information on cardiovascular and orthopaedic materials and devices, and on human biological materials (Figure 3), including those from ASM International. These resources help designers to specify


Figure 2: A graphical performance


index finder allows designers to select a design scenario that is close to their application


Figure 3: Today’s software allows designers to specify their selection problem and explore in greater depth the candidate materials generated during selection


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