Biomedical Engineering Reference
In-Depth Information
applications are the most demanding of bioceramics. However, their
inherent brittleness derived from their low fracture toughness has prevented
their use in some applications. Moreover, the presence of flaws or defects in
the material can lead to catastrophic failure during mechanical loading.
Therefore, new kinds of materials have been studied for increasing the
performance of ceramic matrix materials. For ceramics used in biomedical
applications, which are extensively called bioceramics, this problem still
remains. Nanophased ceramics are being investigated as a way of solving
some of the structural and bio-related problems. For example, nanometric
features in the surface of a prosthesis seem to reduce the risk of rejection and
enhance the proliferation of osteoblasts (bone-forming cells). Nanophased
or nanostructured ceramics can be obtained either by nanocrystalline
materials or with nanocomposites.
Nanocrystalline materials are solids with a nanometric microstructure,
consisting of polycrystals with one or several nanometric phases (Gleiter,
2000). Nanocomposites are materials with at least one of the solid phases in
the nanometric range. Both nanomaterials are structurally characterized by
a large volume fraction of grain boundaries, which can significantly alter
their physical, mechanical and chemical properties.
16.2.1 Nanocrystalline ceramics
In the case of nanocrystalline ceramics, as the grain size is reduced, the grain
volume at grain boundaries is increased (Meyers et al., 2006). Thus, due to
the high density of interfaces, an important fraction of atoms will be at the
interface. This fact allows nanocrystalline materials to offer unusual and
improved properties when compared to microscale materials.
There are studies (Webster et al., 1999) that provide evidence that
nanophase ceramics could promote osseointegration, which is critical for the
clinical success of orthopaedic/dental implants. Webster et al. (2000)
synthesized dense nanophase alumina (Al
2
O
3
) materials and showed a
significant increase in protein absorption and osteoblast adhesion on the
nano-sized ceramic materials compared to traditional micron-sized ceramic
materials. Other studies (Du et al., 1999) have suggested that better
osteoconductivity would be achieved if synthetic HAp could more resemble
bone minerals in composition, size and morphology.
The use of nanocrystalline materials can thus offer advantages for use in
biomedical applications, such as:
increased resistance/hardness
.
improved toughness
.
lower elastic modulus and lower ductility
.
reduced risk of rejection
.
