The failure of synthetic polymeric medical devices - Durability and Reliability of Medical Polymers - page 179

9
The failure of synthetic polymeric
medical devices

P. R. Lewis, The Open University, UK

Abstract: several case studies are used to show how failed medical devices
are analysed for their failure modes and linked to witness evidence to show
how accidents occurred. The analytical tools needed to investigate failures
of medical products is reviewed briefly, with microscopy, spectroscopy and
thermal analysis among the most important tools available for polymers.
Other useful methods include mechanical testing, stress analysis, and gel
permeation chromatography for molecular weight. Microscopic techniques
include visual inspection, optical microscopy and scanning electron
microscopy or environmental scanning electron microscopy. Fractures are
often the starting point in any study, analysis of the fracture surface and
surrounding features frequently allowing the failure mode to be identified
uniquely. However, any single result must be corroborated with independent
evidence, especially in cases which result in litigation. improvements in
medical product design must involve forensic study of failures, which
implies preservation and conservation of the evidence, and an objective
approach to independent analysis.

Key words: catheters, polymers, brittle cracking, failure, sutures, breast
implants.

Note: This chapter is a revised and updated version of Chapters 1
'introduction', 2 'examination and analysis of failed components' and 3
'Polymer medical devices' by P. R. Lewis, originally published in Forensic
polymer engineering: why polymer products fail in service, P. R. Lewis and
C. Gagg, woodhead Publishing Limited, 2010, isBN: 978-1-84569-185-1.

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9.1 Introduction
if one were to examine areas of great advance in the use of new materials,
medical devices would surely be among the first to be noticed. One reason
why synthetic polymers are now so widely used is their similarity to the
proteins from which our bodies are built. They have similar mechanical
properties, and so are flexible in response to body stresses. Some polymers
are inert and unreactive to body fluids, and all can be designed into products
of some complexity with great ease. The body environment is highly reactive
because it is in a continual state of producing energy for body functions (such
as muscle movement), with many complex chemical pathways both in the
fluids (such as blood) and tissues (such as muscle and bone). Enzymes, or

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