Biomedical Engineering Reference
In-Depth Information
multifunctional surfaces. Some are indebted with an impressive set of properties:
low surface energy (hydrophobe, low friction coefficient), high hardness (wear resis-
tance) and high corrosion resistance. However, the literature data are still too scarce
to compose a coherent table on engineering properties or on potential biomedical
properties.
Aluminum alloys were intensively studied since many decades to improve their
mechanical properties (lighter metals for aircrafts). As already discussed mechanical
properties of metals can be improved by precipitating small grains during cooling
or grain refining, both pinning down deformations of the lattice. During cooling
aluminum
copper copper-rich precipitates are formed reinforcing the aluminum
(duralumins). In 1982, Danny Schechtman and coworkers discovered in these alloys
Al-Mn crystals with five-fold symmetry, till then unseen in crystallography. One
can imagine the delight of the materials scientist when admiring this crystallo-
graphic anomaly:
it turned out to be the most sophisticated achievement of Nature
in the world of complex intermetallics
. Meanwhile, they can be produced at indus-
trial scale by conventional metallurgical techniques. The set of strange properties
are recognized by industry and some are turned into commercial products. Compre-
hensive texts on this subject are the topics of Janot
Quasicrystals, A Primer
[230]
and the Dubois's recent book
Useful Quasicrystals
[231].
Penrose, one of the most prominent mathematicians of former century, forced
fivefold symmetry into an aperiodic plane filling tiling. It also appeared to be
possible to construct a three-dimensional Penrose aperiodic network maintaining
five-fold symmetry. Herewith, a fascinating research field was born and a crystal-
lographer's frustration about five-fold symmetry, dodecahedra and icosahedra has
been lifted. For the interested reader, the topics of Janot and Dubois (under some
reserve for his experiments on wear) are recommended literature.
Listed below are the eye-catching properties of these peculiar materials with
respect to future biomedical applications. Quasicrystals:
C
300 .cm)
1
at 300
ı
K. This phenomenon
is more reminiscent to metallic glasses or amorphous semiconductors than to
the constituting metals; the reason for this phenomenon is theoretically not yet
cleared out.
Exhibit low electrical conductivity (
Exhibit low heat conductivity substantially lower than the metallic constituent
elements.
With good lattice perfection and diamagnetic (in contrast to Al and Al-based
crystalline intermetallics that are diamagnetic).
Have low corrosion currents.
Are hard and brittle and do not show any work hardening like conventional
metals.
Have low friction coefficients.
Are poorly wetted by polar liquids.
Is it only a dream thinking that the combination of low wettability with high
hardness and a low friction coefficient might one day permit to produce more ideal
joint prostheses or some other device we are not aware of yet?
