Biomedical Engineering Reference
In-Depth Information
Fig. 2.10
Metallographic section through a Ti6Al4V wire: the light-grey zone is the ˛-, the darker
the ˇ-phase. In the
upper right corner
, a surface layer of bioactive glass is visible
treatment, hot working and annealing. In Table
2.6
, typical values for a selection
of alloys are collected. To simplify the table, mid-range values of published data
are given only. The range can be quite broad and depends on slight changes in
composition and thermal history.
2.3
Skeletal Tissue
Thus far we were watching the fate of 'heavy duty' substitutes for bone. Right time
to turn our attention to bone itself, not for the pleasure of learning a petty fact more.
Understanding how complex a material is, that an implant has to substitute, is vital
to design better-performing implants.
Mechanical properties of polycrystalline alloys, and glassy inorganic and organic
materials are preponderately isotropic, unless anisotropy was intentionally induced.
Composites consist of two or more phases with the intention to generate properties
the individual phases do not and cannot have. A carbon fibers filled epoxy resin
is substantially different from the pure epoxy with isotropic mechanical properties.
When in the composite the fibers are nicely aligned, the bending strength perpen-
dicular to the fiber orientation will be an order or orders of magnitude higher than
that of the epoxy matrix. On the contrary, loading parallel to the alignment of the
fibers the strength might even be lower than the resin itself: an extreme example of
anisotropy.
Bone is a composite but complex and of 'intelligent design': complex because
it has much more than two phases, 'intelligent' because it is produced
in situ
in
the body with the properties the body imposes at that particular spot, the right stuff
at the right place! Skeletal tissues, bone and cartilage, form a class of structural
