Biomedical Engineering Reference
In-Depth Information
[266]. However, Raman spectroscopic data indicate that ACP in
matrix vesicles is not in a pure chemical state but instead calcium
and orthophosphate ions are combined in a single-phase complex
with lipid and protein moieties found within the vesicles [267]. Also
consistent with formation of an amorphous precursor phase are
infrared and Raman spectroscopic findings that the first crystals in
matrix vesicles are OCP and not CDHA [267, 268]. These crystals,
in turn, penetrate the enclosing membrane and initiate a chain of
crystallization events that appears to trigger the mineralization
of the collagenous matrix. Thus ACP, even when present in small
amounts, may be an important initiating factor in the calcification of
skeletal tissues [20].
Besides the questionable cases of calcified tissues of mammals,
ACPs are found in mammalian milk [7, 8, 73]. Clearly, the presence
of the most easily biodegradable calcium orthophosphates in the
form of ACPs in milk is necessary to construct skeletons of young
organisms.
2.5
Biomedical Application of ACPs
Currently, biomaterials and bioceramics of calcium orthophosphates
are available in various physical forms: powders, particles, granules,
dense blocks, porous scaffolds, injectable formulations, self-setting
cements and concretes, implant coatings, as well as composite
components of different origin (natural, biological or synthetic) often
with the specific shapes, such as implants, prostheses or prosthetic
devices (see Refs. [32, 33, 269] and Chapter 4). In principle, all these
physical forms should apply to ACPs; however, not all of them have
been realized yet. Namely, it is easy to prepare ACPs in a powder
form by a wet-precipitation technique (see section 2.3.2); however,
manufacturing of other physical forms of ACPs is not so simple.
Furthermore, to be used in surgery, all implantable 3D constructions
must possess the necessary mechanical properties, which is difficult
to achieve in the case of ACPs. For example, both dense blocks
and 3D porous scaffolds made of calcium orthophosphates reach
their mechanical properties only after sintering at temperatures,
exceeding ~ 1000°C, which is impossible in the case of ACPs (see
section 2.3.6). Furthermore, an ACP powder might be easily added
as a component to self-setting calcium orthophosphate formulations
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