Calcium Phosphate Coatings
Porous hydroxyapatite has been accepted that due to its unfavourable mechanical properties it cannot be used under load bearing purposes. For this reason hydroxyapatite has been used as thin film coatings on metallic alloys. Of the metallic alloys investigated titanium based alloys have shown to be the material of preference for thin film coatings. Titanium alloys possesses good mechanical strength and fatigue resistance under load bearing conditions. They are lightweight, with high strength to weight ratios.
Calcium Phosphate Coating Deposition Processes
Of the coating techniques utilized, thermal spraying tends to be the most commonly used and analysed. This technique has been faced with challenges of producing a controllable resorption response in clinical situations. Besides the set backs, thermally sprayed coatings are continually being improved by using different compositions and post heat treatments which converts amorphous phases to crystalline calcium phosphates. Other techniques are being investigated. Techniques that are capable of producing thin coatings include pulsed-laser deposition and sputtering which, like thermal spraying involves high – temperature processing. Other techniques such as electrodeposition, and sol-gel utilise lower temperatures and avoid the challenge associated with the structural instability of hydroxyapatite at elevated temperatures.
The Advantages of the Sol-Gel Process
The advantages of sol-gel technique are numerous; it results in a stoichiometric, homogeneous and pure coating due to mixing on the molecular scale; reduced firing temperatures due to small particles sizes with high surface areas; it has the ability to produce uniform fine-grained structures (Figure 1); the use of different chemical routes (alkoxide or aqueous based); and their ease of application to complex shapes with a range of coating techniques those being dip, spin, and spray coating. The lower processing temperature has another advantage; it avoids the phase transition (~1156 K) observed in titanium based alloys used for biomedical devices.
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Figure 1. (a) SEM and (b) AFM image of a sol-gel (alkoxide) derived hydroxyapatite coating. |
Calcium Phosphate Biomaterials – Solubility of Calcium Phosphates
Most calcium phosphates are classified as resorbable biomaterials. This means that under physiological conditions they will dissolve. The benefit of calcium phosphate biomaterials is that the dissolution products can be readily assimilated by the human body.
Calcium Phosphates as Bone Defect Fillers
Due the resorbable nature of calcium phosphate, with the general exception of hydroxyapatite, they have been proposed as potential bone defect fillers. In this application, they would fill the void and gradually dissolve away, being replaced by bone. However, the uncontrollable resorption rate has hindered their uptake in clinical applications.
Calcium Phosphate Compounds
Listed in the table below are some calcium phosphate compounds of biomaterials interest.
Table 1. Some calcium phosphate compounds of biomaterials interest.
|
Chemical Name |
Abbr |
Chemical Formula |
Phase |
Ca/P |
|
Amorphous calcium phosphate |
ACP |
- |
- |
- |
|
Dicalcium Phosphate |
DCP |
CaHPO4 |
Monetite |
1.00 |
|
Tricalcium Phosphate |
α-TCP |
Ca3(PO4)2 |
|
1.50 |
|
Tricalcium Phosphate |
β-TCP |
Ca3(PO4)2 |
Whitlockite |
1.50 |
|
Pentacalcium Hydroxyl Apatite |
HAp |
Ca10(PO4)6(OH)2 |
Hydroxyapatite |
1.67 |
|
Tetracalcium Phosphate Monoxide |
TTCP |
Ca4O(PO4)2 |
Hilgenstockite |
2.00 |
Amorphous calcium phosphate is a phase that is often formed during high temperature processing, such as is the case with plasma spraying of hydroxyapatite. It and other phases, may be associated with hydroxyapatite after high temperature processing and the subsequent decomposition when dealing with hydroxyapatite
Solubility of Calcium Phosphate Compounds
While the forming method and exact stoichiometry will have an effect on solubility, the generally accepted order of solubility is:
ACP > DCP > TTCP > α-TCP > β-TCP >> HAp
The relative insolubility of hydroxyapatite compared to the other calcium phosphate phases is not surprising as it is the only stable calcium phosphate compound at pH’s above 4.2. Below this, dicalcium phosphate dihydrate (CaHPO4.2H2O) is the stable compound. It is not uncommon for unstable calcium phosphates to dissolve and repreciptate as the stable compound at a given pH.
Under normal physiological conditions of pH 7.2, hydroxyapatite is the stable calcium phosphate compound. This may drop to as low as pH 5.5 in the region of tissue damage, although this would eventually return to pH 7.2 over a period of time. Even under these conditions hydroxyapatite is still the stable phase.
A complete set of references can be found by referring to the original paper.
