Biomedical Engineering Reference
In-Depth Information
fact that the distances of material transport during the sintering
becomes shorter for ultrafine powders with a high specific surface
area, resulting in a densification at a low temperature. Therefore,
due to low grain growth rates, a low-temperature sintering appears
to be effective to produce fine-grained apatite bioceramics [148].
Furthermore, the mechanical properties (namely, hardness and
toughness) of HA bioceramics appeared to increase as the grain size
decreased from sub-micrometers to nanometers [149].
More to the point, nano-sized HA is also expected to have a
better bioactivity than coarser crystals [150-152]. Namely, Kim et
al
found that osteoblasts (bone-forming cells) attached to the nano-
sized HA/gelatin biocomposites to a significantly higher degree
than to micrometer size analog did [153]. An increased osteoblast
and decreased fibroblast (fibrous tissue-forming cells) adhesion on
nanophase ceramics [134, 154-158], as well as on nanocrystalline
HA coatings on titanium, if compared to traditionally used plasma-
sprayed HA coatings, was also discovered by other researchers
[159-161]. Scientists also observed enhanced osteoclast (bone-
resorbing cells) functions to show healthy remodeling of bone at
the simulated implant surface [151]. Besides, the proliferation
and osteogenic differentiation of periodontal ligament cells were
found to be promoted when a nanophase HA was used, if compared
to dense HA bioceramics [162]. Thus, the underlying material
property, responsible for this enhanced osteoblast function, is the
surface roughness of the nanostructured surface [18]. Interestingly,
but an increased osteoblast adhesion was discovered on nano-sized
calcium orthophosphate powders with higher Ca/P ratios [163],
which points out to some advantages of apatites over other calcium
orthophosphates. Furthermore, a histological analysis revealed
a superior biocompatibility and osteointegration of bone graft
substitutes when nano-sized HA was employed in biocomposites
[164, 165]. However, data are available that nano-sized HA could
inhibit growth of osteoblasts in a dose-dependent manner [166].
Obviously, the volume fraction of grain boundaries in
nanodimensional calcium orthophosphates is increased significantly
leading to improved osteoblast adhesion, proliferation, and
mineralization. Therefore, a composition of these biomaterials
at the nano-scale emulates the bone's hierarchic organization, to
initiate the growth of an apatite layer and to allow for the cellular
.
Search WWH ::
Custom Search