Ageing processes of biomedical polymers in the body - Durability and Reliability of Medical Polymers - page 161

previous literature on the effect of natural ageing on polymers in vivo and
in vitro and in section 8.4, the concept of using accelerated ageing to predict
the behaviour of polymers in vivo is discussed. Finally, section 8.5 presents
conclusions and a summary.

8.2 Principles of chemical and biochemical
degradation and calcification
Biodegradation of polymers in the human body can be described as the
chemical breakdown of a material by the physiological environment of
the body, implying that it is host-induced (Coury et al. , 2004). Before a
polymer is classified as being suitable to be implanted in the body, it is
subjected to rigorous and extensive laboratory testing and goes through a
careful selection process (Coury et al. , 2004). Hence, in most instances, it
is generally found that the polymer performs satisfactorily when it is first
implanted (Coury et al. , 2004). However, problems may arise over time, as
it is virtually impossible to predict precisely the behaviour of the materials
after a period of extended use in the body (Coury et al. , 2004). Prediction
techniques employed in the laboratory, such as accelerated ageing (Coury
et al. , 2004; Hukins et al. , 2008), cannot identify all the factors that may
cause the material to deteriorate unacceptably during their intended period
of use in vivo . Consequently, many previously unknown factors are only
identified from retrieval studies after an extended period of use in vivo
(Coury et al. , 2004).

8.2.1 Polymer degradation
Biomedical-grade polymers degrade because the body constituents attack
the materials, either directly or indirectly through other parts attached to
the polymer in the medical device (Coury et al. , 2004). For example, some
biological processes which should only become active to attack foreign
organisms invading the human body attack and break down polymers implanted
in the body (Coury et al. , 2004). After implantation, polymers absorb soluble
components (proteins, lipids, water and ions) and adsorb proteinaceous
components and as a result, cells attach to the surface of the materials and
initiate chemical processes that may result in a change in their mechanical
properties (Coury et al. , 2004). Furthermore, joints in the human body are
loaded cyclically, i.e. subjected to cyclic stresses, and various joint implant
testing standards are recommended (BS-ISO-14242-1:2002; BS-ISO-18192-
1:2008). Joints also experience other forms of deformation such as abrasion,
flexion, extension and bending, in an aqueous ionic environment which can
be electrochemically active and cause the polymer to soften (Coury et al. ,
2004). Biological processes that are activated, as a result, can allow cells to

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