Biomedical Engineering Reference
In-Depth Information
It is important to recognize, that the major biomedical
properties (such as bioactivity, bioresorbability, osteoconductivity
and osteoinductivity) of the multiphasic and/or polyphasic
compositions might be adjusted by changing the ratio among the
calcium orthophosphate phases. When compared to both α- and
β-TCP, HA is a more stable phase under the physiological conditions,
as it has a lower solubility (Table 1.1) and, thus, slower resorption
kinetics. Therefore, due to a higher biodegradability of the α- or
β-TCP component, the reactivity of BCP increases with the TCP/HA
ratio increasing. Thus,
bioresorbability of BCP can be adjusted
through the phase composition. Similar conclusions are also valid
for both the biphasic TCP (in which α-TCP is a more soluble phase)
and the triphasic (HA, α-TCP and β-TCP) formulation.
A phase transition from α-TCP into β-TCP in three types of BCPs
(HA + TCP) was investigated and the experimental results indicated
that a sintering temperature for the complete phase transition
from α-TCP into β-TCP increased with HA content increasing in BCP
[511].
in vivo
1.3.15 Ion-substituted Calcium Orthophosphates
Finally, one should very briefly mention on existence of
carbonateapatite [512-518], chlorapatite [519-521], as well as on
a great number of various ion-substituted calcium orthophosphates
[82, 522, 523]. Usually, they are of a non-stoichiometric nature and
there are too many of them to be mentioned in one review. Currently
this is a hot investigation topic; therefore, the readers are referred
to other topics and reviews [27-29, 35, 37, 47, 107, 277, 432]. In
addition, there is a very good review, in which the structures of more
than 75 chemically different apatites have been discussed [114].
To conclude this topic, it is interesting to note, that chemical
elements not found in natural bones can be intentionally incorporated
into calcium orthophosphate biomaterials to get special properties.
For example, addition of Ag
+
2+
2+
[524-526], Zn
[526, 527] and Cu
[526-528] has been used for imparting antimicrobial effect, while
radioactive isotopes of
re [530]
have been incorporated into HA bioceramics and injected into knee
joints to treat rheumatoid joint synovitis [529, 530, 532]. more to
the point, apatites were found to incorporate individual molecules,
such as water, oxygen and carbon dioxide [82].
90
Y [529],
153
Sm [530-532] and
186
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