Biomedical Engineering Reference
In-Depth Information
Chapter 8
Metal Implants Bound to Disappear
Since the dawn of the
Bronze Age
some ten thousand years ago our fellow ances-
tors started the extremely tedious pilgrimage to more efficient production processes
of metals and to master composition, heat and mechanical treatments to get an
appropriate ratio between mechanical properties and density or, within mechanical
properties, between say fatigue strength and Young's modulus, fitting unconsciously
the modified form of the term
biological performance
, all this of course without the
slightest suspicion of what was going on at the atomic scale.
1
Gold was the almost
exclusive metal to be found as native metal in nature. Silver or copper was excep-
tionally found in native form (silver was more expensive than gold in ancient Egypt).
The straightforward explanation for this behavior is the high positive redox potential
(see Table
C.1
), where gold occupies the very top position of the
E
0
ranking. Gold,
silver and copper figure in the same column of Mendelejev's wonderful periodic
system (Table
A.1
). All three have an f.c.c. crystal structure and share a number
of chemical and mechanical properties, e.g., workability intrinsically linked to its
f.c.c. structure. Silver and copper are relatively easily reduced to metal and this is
obviously the reason why the
Iron Age
follows the
Bronze Age
and not vice versa:
iron is less easily reduced than copper. Native iron is only found in meteorites.
Cast iron (
3%C) is brittle. During the nineteenth century the carbon content
was gradually lowered resulting in alloys combining good strength, ductility and
machinability. This development is the culmination point of the
Iron Age
, embodied
in the icons of this age: Iron Bridge in Shropshire (England, 1781) and the
Eiffel
tower
in Paris (1887-1889). The iron alloys with a carbon content
>
% are housed
in the somewhat vague generic family named
steel
. But steel corrodes and the Eiffel
tower needs 50-60 tonnes of paint every seven years to protect it from rust. This
makes clear why the progressing steel industry was accompanied by an emerging
new technology and science:
corrosion science
.
Covering unhealthy teeth by gold was good fashion for those who could pay for.
Insertion of a gold foil in joints was practiced with good long-term success near
the end of the nineteenth century. They were bound to stay in the body. Copper
or bronze was used only for surgical instruments but not for implants. But things
<2
1
see Helsen and Breme, p. 25 [35
].
