Biomedical Engineering Reference
In-Depth Information
of HA at temperatures exceeding ~900°C (e.g., during plasma-spray
of HA) only in the absence of water vapor [28, 29, 484, 485]. It also
might be crystallized in glass-ceramics [486]. Computer modeling
techniques have been employed to qualitatively and quantitatively
investigate the dehydration of HA to OA [487]. OA has the hexagonal
space group symmetry
—
(174) of cesanite type [114], while the
space group symmetry for partially dehydrated HA was found to
change from hexagonal P63/m to triclinic
P
—
when more than
.
35% of the structurally bound water had been removed [485]. OA
has no stability field in aqueous conditions [488]; it is very reactive
and transforms to HA in contact with water vapor [484]. Due to the
aforementioned problems with OA preparation, no information on
biomedical applications of pure OA is available. Plasma-sprayed
coatings of HA, in which OA might be present as an admixture phase,
seems to be the only application.
P
ca
1.3.13
TTCP (or TetCP)
Tetracalcium phosphate or tetracalcium orthophosphate monoxide
(Ca
O; the chemically correct name is tetracalcium oxide
bis(phosphate); the mineral hilgenstockite [489]) is the most basic
calcium orthophosphate. However, its solubility in water is higher
than that of HA (Table 1.1). TTCP cannot be precipitated from
aqueous solutions. It can be prepared only by a solid-state reaction at
temperatures above 1300°C, e.g., by heating homogenized equimolar
quantities of DCPA and CaCO
(PO
)
4
4
2
in dry air, or in a flow of dry nitrogen
[28, 277, 490, 491]. These reactions should be carried out in a dry
atmosphere, in vacuum or with rapid cooling (to prevent uptake
of water and formation of HA). Easily DCPA might be replaced by
ammonium orthophosphates [492, 493], while calcium carbonate
might be replaced by calcium acetate [493]. Furthermore, TTCP often
appears as an unwanted by-product in plasma-sprayed HA coatings,
where it is formed as a result of the thermal decomposition of HA to
a mixture of high-temperature phases of α-TCP, TTCP and CaO [494].
TTCP is metastable: in both wet environment and aqueous solutions
it slowly hydrolyses to HA and calcium hydroxide [28, 277, 495].
Consequently, TTCP is never found in biological calcifications. In
medicine, TTCP is widely used for preparation of various self-setting
calcium orthophosphate cements [120, 132, 148, 164, 170, 171,
263, 494, 496]; however, to the best of my knowledge, there is no
3
Search WWH ::
Custom Search