Ageing processes of biomedical polymers in the body - Durability and Reliability of Medical Polymers - page 163

that can cause degradation of the material (Coury et al. , 2004). The rate of
degradation as a result of hydrolysis depends on intrinsic properties, such as
the functional groups and the morphological and molecular characteristics
of the polymer (Coury et al. , 2004). Examples of factors that reduce the
likelihood of hydrolysis include crosslinking, crystallinity and thermal
annealing (Coury et al. , 2004).

8.2.3 Oxidative degradation
Oxidative degradation (Coury et al. , 2004; Griesser, 1991) in polymers
occurs as a result of a chemical reaction that takes place when the material
is exposed to oxygen, for example hydrogen peroxide can degrade polymers
oxidatively (Griesser, 1991). Such a degradation process can be initiated by
the physiological environment of the body or by the external environment
(Coury et al. , 2004). Stress cracking is an example of oxidative degradation
(Coury et al. , 2004; Griesser, 1991). This form of degradation attacks the
surface of the polymer and causes chemical changes that occur in vivo or in
vitro oxidising conditions (Coury et al. , 2004). For example, stress cracking in
polyether urethans elastomers has been reported and common characteristics
found in these elastomers include presence of oxidative aliphatic ether groups
(Coury et al. , 2004; Griesser, 1991).

8.2.4 Enzymatic degradation
Oxidative and hydrolytic enzymes have been reported to attack polymers
(Griesser, 1991). Examples of such enzymes include papain, urease, leucine
aminopeptidase, esterase and trypsin, and trypsin derivatives, ficin and
bromelain (Griesser, 1991). It is likely that different enzymes degrade the
polymer through different mechanisms (Griesser, 1991). The major result
of enzymatic degradation is that it alters the mechanical properties of the
polymer and it is possible that it could release potentially harmful degradation
products into the body fluid and tissue (Griesser, 1991).

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8.2.5 Calcification
Deposits of calcium-containing compounds, such as calcium phosphate, can
form on biomaterials in the human body (Coury et al. , 2004). This process is
known as calcification and, although it is normal and desirable for deposits
of calcium to form on teeth and bones, in the human body and on some
biomaterials [e.g. osteoconductive materials used in dental and orthopaedic
applications (Begley et al. , 1995)], it is not desirable for other medical devices
such as silicone breast implants and heart valves to calcify because this can
cause the devices to fail (Coury et al. , 2004). Incidences of failure because of