Hydroxyapatite - Hydroxyapatite Coatings An Overview

Chemical Formula

Ca10(PO4)6(OH)2

Topics Covered

Background

Advantages of Hydroxyapatite

Disadvantages of Hydroxyapatite

Hydroxyapatite Coatings

Coating Techniques

Plasma Sprayed Hydroxyapatite Coatings

Clinical Behaviour

Background

Advantages of Hydroxyapatite

The beneficial biocompatible properties of hydroxyapatite are well documented. It is rapidly integrated into the human body, while at the same time the body is none the wiser as to the invasion by a foreign body, albeit a friendly invasion. Perhaps its most interesting property is that hydroxyapatite will bond to bone forming indistinguishable unions.

Disadvantages of Hydroxyapatite

However, poor mechanical properties (in particular fatigue properties) mean that hydroxyapatite cannot be used in bulk form for load bearing applications such as orthopaedics.

Hydroxyapatite Coatings

Coatings of hydroxyapatite have good potential as they can exploit the biocompatible and bone bonding properties of the ceramic, while utilising the mechanical properties of substrates such as Ti6Al4V and other biocompatible alloys. While the metallic materials have the required mechanical properties, they benefit from the hydroxyapatite which provides an osteophilic surface for bone to bond to, anchoring the implant and transferring load to the skeleton, helping to combat bone atrophy.

Coating Techniques

Some techniques that have been tried are summarised in the following table.

Technique

Thickness

Advantages

Disadvantages

Dip Coating

0.05-0.5mm

Inexpensive

Coatings applied quickly

Can coat complex substrates

Requires high sintering temperatures

Thermal expansion  mismatch

Sputter Coating

0.02-1μm

Uniform coating thickness on flat substrates

Line of sight technique

Expensive

Time consuming

Cannot coat complex substrates

Produces amorphous coatings

Pulsed Laser Deposition

0.05- 5μm

As for sputter coating

As for sputter coating

Hot Pressing and Hot Isostatic Pressing

0.2-2.0mm

Produces dense coatings

HP cannot coat complex substrates

High temperature required

Thermal expansion mismatch

Elastic property differences

Expensive

Removal/Interaction  of encapsulation material

Electrophoretic Deposition

0.1-2.0mm

Uniform coating thickness

Rapid deposition rates

Can coat complex substrates

Difficult to produce crack-free coatings

Requires high sintering temperatures

Thermal Spraying

30-200μm

High deposition rates

 

Line of sight technique

High temperatures induce decomposition

Rapid cooling produces amorphous coatings

Sol-Gel

<1μm

Can coat complex shapes

Low processing temperatures

Relatively cheap as coatings are very thin

Some processes require controlled atmosphere processing

Expensive raw materials

Information in the table above has been sourced from various scientific publications. Some of the coating techniques have been purely experimental.

Plasma Sprayed Hydroxyapatite Coatings

Of the techniques outlined above, only thermal spraying, in particular, plasma spraying is the only commercially accepted method for producing hydroxyapatite coatings.

While plasma spraying is a well understood process, control of variables is quite complicated. Small changes to processing variables can vastly affect the properties of the final coating.

In the case of hydroxyapatite coatings, this type of problem can be exacerbated due to its thermal instability. Variations in such things as powder feedstock may produce soluble coatings. The reason for this is the high processing temperatures encountered by the hydroxyapatite induce some decomposition to soluble calcium phosphate compounds.

Clinical Behaviour

Plasma sprayed hydroxyapatite coated implants have become widely used over the last twenty years, with several companies manufacturing devices for orthopaedic and dental applications.

In general, the implants have performed well. In fact, when removed from patients (sometimes post humously) and separated from the surrounding bone, it has been the the bone/coating interface that has failed. This indicates that the bond between the bone and the coating has formed a stronger bond than that which exists between the metal substarte and hydroxyapatite coating.

 

Primary author: AZoM.com

 

Date Added: May 9, 2002 | Updated: Jul 12, 2013
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