Biomedical Engineering Reference
In-Depth Information
approximately 4 MPa, whereas tendons and ligaments experience
peak stresses in the range 40-80 MPa. The hip joints are subjected
to an average load up to three times body weight (3,000 N) and peak
loads experienced during jumping can be as high as 10 times body
weight. These stresses are repetitive and fluctuating depending
on the nature of the activities, which can include standing, sitting,
jogging, stretching and climbing. Therefore, all types of potential
biomaterials and bioceramics must sustain attacks of a great variety
of aggressive conditions [298]. Regrettably, there is presently no
material fulfilling all these requirements.
On the other hand, any ceramics, when they fail, tend to do so in a
dramatic manner. Namely, a brittle nature of calcium orthophosphate
bioceramics is attributed to high-strength ionic bonds. Thus, it is not
possible for plastic deformation to happen prior to failure, as slip
cannot occur. Consequently, if a crack is initiated, its progress will
not be hindered by the deformation of material ahead of the crack, as
would be the case in a ductile material (e.g
, a metal). In ceramics, the
crack will continue to propagate, rapidly resulting in a catastrophic
failure [195].
For dense bioceramics, the strength is a function of the grain sizes.
It appears to be very sensitive to a slow crack growth [299]. Finer
grain size bioceramics have smaller flaws at the grain boundaries
and thus are stronger than one with larger grain sizes. In general,
the mechanical properties decrease significantly with increasing
content of an amorphous phase, microporosity and grain sizes, while
a high crystallinity, a low porosity and small grain sizes tend to give
a higher stiffness, a higher compressive and tensile strength and a
greater fracture toughness. Accordingly, from the mechanical point
of view, calcium orthophosphate bioceramics appear to be brittle
polycrystalline materials for which the mechanical properties are
governed by crystallinity, grain size, grain boundaries, porosity and
composition [203]. Thus, it possesses poor mechanical properties
(for instance, a low impact and fracture resistances) that do not allow
calcium orthophosphate bioceramics to be used in load-bearing
areas, such as artificial teeth or bones [49-55, 300]. For example,
fracture toughness [301] of HA bioceramics does not exceed the
value of ~1.2 MPa·m
.
1/2
1/2
). It
decreases almost linearly with a porosity increasing [233]. Generally,
fracture toughness increases with grain size decreasing. However, in
some materials, especially non-cubic ceramics, fracture toughness
[302] (human bone: 2-12 MPa·m
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