Med-Tech Innovation Materials


be seen. Clinical studies show, however, that polyethylene implantations retrieved 10 months after surgery exhibited surface cracking.4,5


Implants like these will have much


shorter shelf life than expected; in addition, biological problems such as worn debris in the body can occur. Materials changes during storage and transportation relevant to shelf life. This is another factor that is often overlooked at the research and development stage. Some medical devices are extremely sensitive to high temperature and/or seasonal environmental changes such as humidity, yet all devices have an optimum shelf life. To determine shelf life, accelerated ageing tests can be designed to make reasonable and reliable shelf-life predictions; this is normally acceptable to the US Food and Drug Administration and other regulatory bodies. Much more difficult to assess is the impact that transportation may have on the device. Although some companies test the effects of transportation in real-life conditions, the best way to get realistic results is to use an environmental testing house/laboratory where all environmental conditions such as rain, snow, and hot and cool cycles can be simulated. Another area worth mentioning here is fluorescent lighting. Some polymers are sensitive to it, which is obviously a problem for those medical devices that are stored and subjected to fluorescent lighting for a length of time.


Materials changes in use. This appears to be the


This was probably due to an unfortunate design mistake because at that time polycarbonate was, and continues to be, widely used for medical devices. The solvent-induced crazing in polycarbonate was well discussed in 1977 by Bucknall.7


Environment plus stress


is a major cause of crazing, then cracking, and finally device failure during use.


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area where the fewest changes would be likely to occur; as such, materials changes during use have not been well addressed in the past. Some polymers, among them amorphous polymers, are solvent sensitive; if these are also to be put under stress, then product failure is likely to occur. A good example of this is polycarbonate. Solvents (such as detergents or other similar organic materials used or in the environment) can gradually migrate into polymer devices through defects (no material is defect-free) and induce crazing in the polymer, which eventually leads to cracking and failure of the product. Indeed, medical devices made of this polymer such as connectors and joints, have suffered failure during use in the 1990s.6


Future development Table I provides a general guide to evaluate the changing characteristics of polymer properties with the aim of making it easier for medical device designers to create innovative devices whilst avoiding failures. However, there are far too many factors and circumstances that can affect the nano/microstructure of polymeric materials to list them here. Variations in the types of polymers and nano/microstructures will all have an impact on the performance of medical devices. It is important for designers to understand these and take them into account as potential risk factors. The complexity and changing nature of the structure and properties of polymers at all stages, from raw materials to finished products and to end use has led to many serious medical accidents in the past. To avoid failures and to make safe and effective medical devices for the future, it is important to fully understand materials control and to incorporate it into medical device design.


References


1. X C. Zhang et al., “The Ductile–Brittle Transition of Irradiated Isotactic Polypropylene Studies Using Small Angle X-Ray Scattering and Tensile Deformation,” Polymer, 41, 3797–3807, (2000).


2. X.C. Zhang and R.E. Cameron, “The Morphology of Gamma- Ray Irradiated Isotatic Polypropylene,” J. Appl. Polym. Sci., 74, 2234–2242 (1999).


3. X.C. Zhang et al., “The Relationships Between Morphology, Irradiation and the Ductile–Brittle Transition of Isotactic Polypropylene,” Polymer Intl, 48, 11, 1173–1178 (1999).


4. K.D. Moore et al., “Early Failure of a Cross-Linked Polyethylene Acetabular Liner: A Case Report,” J. Bone and Joint Surgery, 90, 11, 2499–2504 (2008).


5 .L. Bradford et al., “Wear and Surface Cracking in Early Retrieved Highly Cross-linked Polyethylene Acetabular Liners,” J. Bone Joint Surg. Am., 86 1271–1282 (2004).


6. P.R. Lewis, “Environmental Stress Cracking of Polycarbonate Catheter Connectors,” Engineering Failure Analysis, 16, 6, 1816–1824 (2009).


7.C.B Bucknall, Toughened Plastics, Elsevier Science and Technology (1977).


Dr Xiang Zhang is Principal Consultant, Medical Materials at Ceram, Queens Road, Penkull, Stoke-on-Trent ST4 7LQ, UK, tel. +44 (0)1782 764 428, e-mail: enquiries@ceram.com, www.ceram.com/medical


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