Ageing processes of biomedical polymers in the body - Durability and Reliability of Medical Polymers - page 162

secrete oxidising agents and enzymes that digest the implant material (Coury
et al. , 2004; Griesser, 1991).
Another factor to consider is that, before being implanted in the body,
polymers undergo a treatment process that can degrade the properties of
the material (Coury et al. , 2004). For example, it has been shown that
gamma irradiation sterilisation of ultra high molecular weight polyethylene
(UHMWPE) in hip implants can generate free radicals in the materials that
react with oxygen in the body and result in undesirable oxidation products
(Coury et al. , 2004). This, in turn, over the short and long term, causes
oxidation and scission of the polymer chains, leading to embrittlement and
reduced strength of the material (Coury et al. , 2004).
Table 8.1, which has been adapted from Coury et al. (2004), lists the
chemical and physical processes that can lead to the degradation of polymers
in the body.

8.2.2 Hydrolytic degradation
Hydrolysis is a chemical reaction which involves the splitting of molecules
because of a reaction with water (Coury et al. , 2004). The likelihood of
hydrolysis of a polymer occurring in the human body depends on several
factors, including the physiological environment, the chemical structure,
the length of time since implantation, the morphology and the dimensions
(Coury et al. , 2004). For example, urethans, esters, amides, anyhydrides
and carbonates are polymers that are highly hydrolysable in the body, i.e.
they consist of carbonyls bonded to heterochain elements (O, N, S) (Coury
et al. , 2004). The process of hydrolysis of polymers in the body creates
more hydrophilic species, thus causing the polymer to swell. Swelling of
polymers allows degrading species to enter the bulk of the material and thus
both swelling and the uptake of water increases the number of reaction sites

￿ ￿ ￿ ￿ ￿ ￿

Table 8.1 The chemical and physical processes that can lead to the
degradation of polymers in the body

Chemical Physical

Thermolysis e.g. radial scission
Oxidation
Solvolysis e.g. hydrolysis
Photolysis e.g. visible, ultraviolet
Radiolysis e.g. x-rays, gamma rays
Fracture-induced radical reactions

Swelling
Softening
Sorption
Impact fracture
Fatigue fracture
Stress cracking
Dissolution
Extraction
Crystallisation

Source: adapted from Coury et al ., 2004. Reproduced with permission
from Elsevier.