Biomedical Engineering Reference
In-Depth Information
For calcium orthophosphate-based biocomposites able to sustain
a high-temperature sintering (valid for the formulations consisting of
inorganic components only), an inter-diffusion of chemical elements
might take place among the phases. Such effect was detected by
energy-dispersive X-ray spectroscopy in HA/TiO
biocomposite
particles with partial formation of calcium titanates; this process
was found to be favorable to enhancing the cohesive strength of
particles in the composite coating [997]. A similar high-temperature
interaction between HA and zirconia [911, 940], as well as between
HA and Ti [661, 1030, 1032-1034] was also detected. Namely, lower
Ti content composites sintered at 1200 ºC showed main crystalline
phases as CaTiO
2
, CaO and Ti
P
, while an increase in Ti content to
3
x
y
50 vol.% revealed Ti
Ti2O and residual α-Ti as additional phases. Thus,
the chemical reactions between HA and Ti were expressed by the
following unbalanced illustrative equation [1032]:
2
Ti + Ca
(PO
)
(OH)
Æ
CaTiO
+ CaO + Ti
P
+ (Ti
O)
10
4
6
2
3
x
y
2
O (6.1)
Besides, partial decomposition of HA and formation of different
calcium aluminates were detected in HA/Al
+ (Ca
P
O
) + H
4
2
9
2
biocomposites after
sintering at 1200-1300 °C. This has been attributed to the diffusion
of Ca
O
2
3
2+
2+
from HA into the alumina matrix and the depletion of Ca
from HA leads to the decomposition of HA into β-TCP [968, 974-976].
Presumably, all these processes influence the mechanical strength of
the biocomposites.
6.6
Bioactivity and Biodegradation of Calcium
Orthophosphate-Based Biocomposites
The continuous degradation of an implant causes a gradual load
transfer to the healing tissue, preventing stress-shielding atrophy and
stimulates the healing and remodeling of bones. Some requirements
must be fulfilled by the ideal prosthetic biodegradable materials,
such as biocompatibility, adequate initial strength and stiffness,
retention of mechanical properties throughout sufficient time to
assure its biofunctionality and non-toxicity of the degradation
by-products [178]. Generally speaking, bioactivity (i.e
, ability of
bonding to bones) of biologically relevant calcium orthophosphates
reinforced by other materials is usually lower than that of pure
calcium orthophosphates [30, 31, 1160].
.
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