Biomedical Engineering Reference
In-Depth Information
factors. So, for example, the rate of growth of a crack may be strongly
influenced by the pH of the corroding solution, but this pH will itself change
in the rather stagnant region inside the crack. Thus even measuring the local
chemical conditions can be a challenge. Normally designers approach this
problem by avoiding it, choosing to use material/environment combinations
which are not susceptible to SCC. However, there have been some notable
exceptions, for example Fig. 12.7 shows a laparoscopic forceps which failed
in use: the particular stainless steel alloy was found to be very susceptible
to SCC in the presence of chloride ions.
For more detailed discussion of the above issues the reader is referred to
some excellent textbooks which compare the various types of mechanical
failure and give many examples of their occurrence in engineering components
(Wulpi, 1985; ASM, 2002).
12.2.5 Other chemical effects and
physiological reactions
It goes without saying that we avoid using materials inside the body if they
are known to react strongly with the physiological environment, causing loss
of material by corrosion or releasing substances which are toxic. But we
should acknowledge the fact that there is probably no material in existence
which is completely inert when placed in the human body. Titanium, for
example, although it functions very well as an implant material, certainly
reacts, as evidenced by the staining of surrounding tissues. Fatigue in most
materials is often accelerated by the presence of a chemical environment,
12.7
A laparoscopic forceps which failed during use by stress-
corrosion cracking.
