Biomedical Engineering Reference
In-Depth Information
oxide layer on their surface, just a few nanometres thick. Since corrosion is a
surface phenomenon, this oxide layer acts as a passivating layer, preventing
the transport of metallic ions and electrons between the metal implant and
body fluids. This serves to prevent both aqueous corrosion and leaching of any
potentially irritating or toxic ions, such as Ni
2+
, Cr
3+
or Co
2+
. Al, Cr and ti
are so reactive in air that they are almost always included as components in
alloys to ensure an oxide layer will form. A nitric acid treatment on stainless
steel is another common way to create this layer.
Once in the body, the passivating oxide layer does not always completely
protect the metallic implant from corrosion. First, mechanical stresses may
cause the oxide film to wear off. While it can reform, biological factors may
affect its ability to do so. Biological macromolecules, such as proteins, cells
and bacteria can upset the equilibria of corrosion reactions by altering the
local ph, affecting electrode potential and affecting the amount of oxygen
available to reform and maintain the layer. Weak points in the oxide layer
can also cause what is known as pitting corrosion. this forms holes in the
oxide layer that cannot be reformed. Fretting corrosion is a cyclic process
in which the passive oxide layer is continuously removed and reformed,
gradually wearing away at the surface of the implant. Finally, stress corrosion
can occur in the presence of an applied load. it reacts by attacking the oxide
layer at its weakest and forming small cracks that grow in length and depth
over time, eventually leading to device failure.
5.4.2 Ceramic dissolution
Metals are made passive to aqueous corrosion in the body by first creating
an oxide layer through dry corrosion. this is the reason why ceramics are
not subject to corrosion. the Al
2
O
3
, ZrO
2
, tiO
2
, passivating layers that are
formed are ceramic in structure and nature, and the strongly directional
interatomic bonds in the structures mean that large amounts of energy are
required for their disruption. instead, ceramics may have a tendency to
dissolve in aqueous environments, depending on their chemical composition
and microstructure. In some cases this 'flaw' is turned into advantage by
creating ceramic implants with a controlled dissolution rate.
Bioceramics (discussed in more detail in a later chapter) include Al
2
O
3
,
ZrO
2
, calcium phosphates, calcium carbonates and bioglasses, ranging on the
scale of
in vivo
behaviour from bioinert (the body produces a fibrous capsule
around the implant) to bioactive (the body directly bonds with the material)
to bioresorbable (the body dissolves the material completely). Factors such as
chemical composition (i.e. Ca/P ratio for calcium phosphates), porosity and
even mechanical stress play a role in the rate and extent of degradation. the
more chemically similar a material is to bone, the more likely the body is to
recognise it and bond to it or interact with it in some way. indeed, complete
