Influence of the Processes Parameters on the Properties of The ...

Chapter 1.
Polylactide Based Bio-Materials
solution [Rodrigues and Lebugle, 1998; Lebugle et al., 1986]. The presence <strong>of</strong> ethanol in <strong>the</strong> precipitation
medium influences <strong>the</strong> composition <strong>of</strong> <strong>the</strong> amorphous phase and especially its HPO 4 2- content and Ca/P ratio.
The main difficulties in <strong>the</strong> preparation <strong>of</strong> ATCP are related to its instability and reactivity.
Generally, ATCP cannot be obtained at neutral or slightly acidic pH. However, it can be stabilised by
various mineral ions or organic molecules which can be added to <strong>the</strong> precipitating and/or washing solutions.
For example, ATCP can be prepared under more acidic conditions (around pH 6) and in <strong>the</strong> presence <strong>of</strong>
magnesium or citrate ions, known as crystal growth inhibitors <strong>of</strong> apatite phase [Holt et al., 1989]. Owing to
its reactivity and ra<strong>the</strong>r high water content, precipitated ATCP is generally freeze-dried and stored at ~ 18°C
to prevent any fur<strong>the</strong>r evolution. Dry, heated ATCP can, however, be stored at room temperature in dry
atmosphere. In contrast with syn<strong>the</strong>sis methods in solution, ATCP can also be obtained via a dry, hightemperature
route through rapid quenching <strong>of</strong> melted calcium phosphate. In <strong>the</strong> absence <strong>of</strong> ions o<strong>the</strong>r than
Ca 2+ and PO 3- 4 , <strong>the</strong> amorphous phase which is forming is analogous to anhydrous precipitated ATCP [Ranz,
1996]. However, in practice, o<strong>the</strong>r anions such as O 2- are also observed in <strong>the</strong> high-temperature amorphous
phase, increasing its Ca/P atomic ratio (1.5). Observed by transmission electron microscopy (TEM),
ATCP morphology is mostly spherical, although <strong>the</strong>se particles generally tend to agglomerate into larger,
irregularly shaped, branched clusters [Chow and Eanes, 2001]. The size <strong>of</strong> <strong>the</strong> spheroid particles varies in a
large range (20 to 200 nm). Amjad [1997] reported that spheroidal particles <strong>of</strong> 25 nm in size were <strong>the</strong>
dominant and most stable morphology <strong>of</strong> ATCP.
Eanes [1970] also investigated ATCP through <strong>the</strong>rmo-chemical experiments and concluded that
such amorphous tricalcium phosphate corresponds to a hydrated tri-calcium phosphate phase, suggesting a
Ca 3 (PO 4 ) 2 ,n(H 2 O) chemical formula. The presence <strong>of</strong> water is an intrinsic feature <strong>of</strong> ATCP. Even freezedried,
amorphous calcium phosphate still contains around 15–20 wt. % <strong>of</strong> water. Sedlak and Beebe [1974]
concludes from temperature-programmed dehydration <strong>of</strong> ATCP that two types <strong>of</strong> bound water exist in this
compound: loosely held water, and tightly bound water held inside <strong>the</strong> amorphous particles. ATCP can also
easily incorporate ‘foreign’ ions through ionic substitutions. Interestingly, ATCP can trap carbonate ions
from <strong>the</strong> preparation solutions [Greenfield and Eanes, 1972], and <strong>the</strong> carbonate content tends to increase
with <strong>the</strong> solution pH. O<strong>the</strong>r substitutions have also been reported, including Mg 2+ and PO 4
3-
pyrophosphate
ions. More generally, ATCP can trap several mineral ions exhibiting biological activity (Sr 2+ , Zn 2+ , Mg 2+ ,
Mn + Cu + , etc.). The incorporation <strong>of</strong> silver was recently shown to bring antimicrobial performances to
ATCP [Aimanova et al., 2005].
3.2.2.2 Addition <strong>of</strong> Strontium
Strontium has been subject <strong>of</strong> study in recent years due to its relation to <strong>the</strong> prevention and
treatment <strong>of</strong> osteoporosis. It is a very widespread in nature with a chemical structure quite similar to that <strong>of</strong>
calcium. Indeed, strontium is an element belongs to group IIA <strong>of</strong> <strong>the</strong> periodic table <strong>of</strong> elements and <strong>the</strong>refore
it presents chemical properties very similar to calcium.
In <strong>the</strong> human body strontium is accumulated mainly in <strong>the</strong> bone, so 99% <strong>of</strong> its content is in this
tissue. The extraordinary similarity to <strong>the</strong> calcium to that strontium is metabolic routes are <strong>the</strong> same as those
<strong>of</strong> calcium, that is to say, absorption takes place in <strong>the</strong> intestine, <strong>the</strong> accumulation in bone and excretion
occur through urine [Cohen-Solal, 2002].
The major route <strong>of</strong> exposure is by ingestion <strong>of</strong> food or water. Compared to <strong>the</strong> amount <strong>of</strong>
strontium ingested only 25−30% is absorbed by <strong>the</strong> intestine. Distribution strontium bone is proportional to
plasma levels (between 0.11 − 0.31 mmol / l), duration <strong>of</strong> exposure and gender. The passage <strong>of</strong> strontium in
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