Biomedical Engineering Reference
In-Depth Information
evidences for ACP in skeletal tissues of mammals have been difficult
to establish directly, much of our progress in clarifying the possible
roles of ACP in biogenic calcification has come from both synthetic
and
in vitro
studies [20].
2.3.2
Preparation
2.3.2.1
Wet-chemistry
Already in early 1970s, researchers established that the final and
stable product of a reaction between calcium and orthophosphate
salts in neutral or basic aqueous solutions was crystalline
stoichiometric HA. However, the stoichiometric well crystalline HA
might be prepared at elevated temperatures only; thus, in the vast
majority cases, in aqueous solutions CDHA is formed instead (Chapter
1). Furthermore, during CDHA precipitation, over a broad range of
the solution conditions, an ACP precursor phase is often formed
[113-123], in some cases, via a short intermediate stage of OCP
formation [124, 125]. Data are available that CDHA crystallization
from ACP simply involves a LRO increase in the structure [126]. One
should stress, that already in the mid of 1970s ACP was found to
be not the mandatory precursor to CDHA. Namely, in dilute aqueous
solutions CDHA was found to precipitate without going through an
ACP precursor [126]. Afterward, a model was developed to illustrate
factors influencing the nature of non-stoichiometric amorphous
precursor phases precipitating in highly supersaturated solutions
[127].
The basic approach to synthesize ACP still consists of a spontaneous
precipitation by mixing concentrated aqueous solutions of calcium
and orthophosphate ions, first developed in 1953 by Watson and
Robinson [98]. Another commonly used method is to prepare an
acidic (pH within 1-5) sub-saturated aqueous solution of a calcium
orthophosphate salt (e.g., DCPD, MCPM) and afterward to induce
precipitation by a rapid addition of a strong base (e.g., NaOH, KOH,
NH
OH) to reach the desired solution pH [128]. Vigorous mixing is
highly desirable. By means of both approaches, various types of ACPs
have been prepared from solutions encompassing a wide range of
pH (from ~6.5 to ~13), Ca/P ionic ratios (from ~0.1 to ~10), calcium
4
Search WWH ::
Custom Search