Biomedical Engineering Reference
In-Depth Information
log IP
supersaturated
-100
stability domain
HAP
-110
HAP
-120
undersaturated
4
5
6
7
8
9
pH
Fig. 10.14
Stability domain of HAP. Modified from [341]
G
log
.IP =K
sp
/:
(10.4)
The reader will notice that this proportionality function is similar to the Nernst equa-
tion, where
acts as the driving force in electrochemical reactions. The active
concentration of PO
4
in solution is function of pH. Consequently, the chemical
equilibrium between precipitate and solution will be pH dependent and we will
be able to construct the plot of log
IP
versus pH shown in Fig.
10.14
, a plot sus-
piciously similar to Fig. 3.4,the
E
-pH or Pourbaix diagram for chromium. The
practical limitation on both plots are absolutely similar.
Considering the fairly large differences in
E
K
sol
, selective precipitation of the dif-
ferent phosphates should be a piece of cake. Unfortunately, it is not. Precipitation
is a consecutive process of nucleation, kinetics of crystal growth, accompanied or
disturbed by absorption and, at high supersaturation, polynucleation of other phases
on the growing crystals. At pH
7 and a ratio Ca/P equal to 1, the expected inter-
mediates between ACP (or WH) and HAP is DCPD but experiment shows them to
be OCP and DOHA. It is crystal clear that simulating the formation of a given
phosphate is not simple business. Moreover, it is not impossible that more reli-
able solubility products will be obtained by a new and interesting determination
technique promoted by Darvell and collaborators [342-344].
In order to be of any practical use, one should be able to master a number of
specific material characteristics. Three applications are shortly discussed: cements,
porous structures as drug carriers and scaffolding.
Bone cements
. Four obvious characteristics for cements are setting time, tem-
perature increase during setting, mechanical performance and biocompatibility. The
interest in calcium phosphate cements (CPCs) started in the early 1980s [345]. In the
subsequent years, setting time of CPCs could be tailored to practical clinical limits,
D
