Biomedical Engineering Reference
In-Depth Information
4.4.3
Possible Transparency
Singlecrystalsofallcalciumorthophosphatesareopticallytransparent
for the visible light. As bioceramics of calcium orthophosphates have
a polycrystalline nature with a random orientation of big amounts
of small crystals, it is opaque and of white color, unless colored
dopants have been added. However, in some cases, a transparency
is convenient to provide some essential advantages (e.g
, to
enable direct viewing of living cells, their attachment, spreading,
proliferation, and osteogenic differentiation cascade in a transmitted
light). Thus, transparent calcium orthophosphate bioceramics (Fig.
4.5) [361] have been prepared and investigated [204, 206, 289,
292, 361-370]. They can exhibit an optical transmittance of ~66 %
at a wavelength of 645 nm [370]. The preparation techniques, for
example, include a hot isostatic pressing [204, 206], an ambient-
pressure sintering [362], gel casting coupled with a low-temperature
sintering [365, 368], a pulse electric current sintering [366], as well
as a spark plasma sintering [289, 292]. Fully dense, transparent
calcium orthophosphate bioceramics was obtained at temperatures
above ~800°C. Depending on the preparation technique, the
transparent bioceramics has a uniform grain sizes ranging from ~81
nm [370] to ~250 μm [365] and always is pore-free. Furthermore,
a translucent calcium orthophosphate bioceramics is also known
[371-373]. However, due to a lack of both porosity and the necessity
to have see-through implants inside the body, the transparent and/
or translucent forms of calcium orthophosphate bioceramics will
hardly be ever used in medicine with the specific eye implants as the
only reasonable exception.
.
Figure 4.5
Transparent HA bioceramics prepared by spark plasma
sintering at 900°C from nano-sized HA single crystals.
Reprinted from Ref. [361] with permission.
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