Biomedical Engineering Reference
In-Depth Information
conditions. Obtaining full dense nanostructures is the key to enhance the
mechanical and biological properties of HAp-based bioceramic materials.
The mechanical properties and microstructures of the HAp bioceramics
strictly depend on the characteristics of the original HAp powder, includ-
ing crystallinity, agglomeration, stoichiometry, and substitutions, and on the
processing conditions. Nanostructure processing can improve the sinter-
ability of bioceramics and enhances the mechanical reliability by reducing
flow sizes. The high volume fraction of grain boundaries in nanocrystalline
bioceramic compacts also provides for increased ductility and superplastic-
ity for low-temperature net-shape forming. Recent studies have reported that
the nanocrystalline HAp materials produced were thermally stable up to
1300°C, whereas conventional commercial HAp powders could not be sin-
tered to >70% theoretical density via pressureless sintering and underwent
decomposition to α-tricalcium phosphate by 900°C. Fully dense HApP com-
pacts can be achieved at temperatures as low as 900°C, while retaining the
ultrafine microstructure (Ahn et al. 2001).
Nanostructured bioceramics are usually processed by compacting
nanopowders at high pressures, and sintering at different times and temper-
atures and in various atmospheres. Pressure-assisted methods, such as hot
pressing, hot isostatic pressing, and sinter forging, are also applied to obtain
nanostructured ceramic materials (Groza 1999; Raynaud et al. 2002; Veljović et
al. 2009). Hot pressing makes it possible to enhance densification kinetics and
limit grain growth. This technique is usually used to process HAp ceram-
ics with a controlled microstructure. After the sintering and hot pressing,
the calcium deficient apatite turns into a mixture of HAp and β-tricalcium
phosphate (β-TCP). A high amount of β-TCP is highly detrimental to the sin-
tering and mechanical properties of HAp bioceramics (Raynaud, Champion,
Bernache-Assollant, et al. 2002). In order to avoid degradation of the mechani-
cal properties, the absence of the β-TCP in HAp bioceramics is advisable.
5.2.4 Trace Element Incorporating into Calcium Phosphate Bioceramics
One of the major drawbacks of stoichiometric CaPs is their inferior osteo-
genic capacity and poor mechanical strength compared with the living bone
tissue, and this has been attributed to the subtle but significant chemical dif-
ference found in the structure. Natural bone apatite is nonstoichiometric and
contains relatively higher levels of magnesium, sodium, and carbon (in the
form of carbonate groups, CO
3
2-
), and lower levels of trace minerals such as
Mg, Sr, Zn, and F. These trace ions at critical levels are considered to play piv-
otal roles in the process of biomineralization as well as other diverse effects
on nanocrystal size, dissolubility, and bioactivity of synthetic bioceramics.
In recent years, a large number of studies have been reported on the synthe-
sis of biologically essential trace mineral-doped CaPs (Boanini et al. 2010).
Bodhak et al. (2011) reported that the combined addition of MgO and SrO in
commercially procured phase pure HAp ceramics was found to be the most
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