Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 126

In vivo tests also demonstrated that HAp coatings produced by electrophoretic deposi-
tion on the surface of cortical screws have excellent biocompatibility and osteoconduc-
tivity [109]. Three groups of samples were prepared, these being uncoated, HAp-coated,
and coated with HAp + a xonotlite (6CaO·6SiO 2 ·H 2 TiO2, interlayer. As with Al 2 O 3 and TiO 2 ,
xonotlite has a coefficient of thermal expansion between that of HAp and Ti. The samples
were heated at 950°C for 1 h in argon. They then were implanted into sheep femurs for 2
months. After sacrifice, it was found that the bond at the bone/screw interface was much
stronger in the testing groups than that in the uncoated control. Extensive bone ingrowth
was observed for both test groups. The torques required to extract the test groups were
much greater than that for the control. Consequently, the screws with the electrophoreti-
cally deposited HAp coating had significantly improved bone-to-implant integration.
Electrophoretically deposited HAp coatings also were used in combination with bone
morphogenetic protein-2 (ng/rhBMP-2) to guide new bone growth in a murine calvarial
model [174]. Following retrieval after 3 weeks, it was found that the untreated implant,
which was a Ti dental screw, moved when touched. In comparison, the HAp-coated implant
demonstrated much better stability, even though no apparent bone formation occurred in
either case after such a short implantation period. When ng/rhBMP-2 was added, substan-
tially more supracalvarial bone formation was observed in both the implants with and
without the HAp coating. It appeared that the HAp coating assisted in retention of the
ng/rhBMP-2, resulting in sustained release of the growth factor. Three-dimensional com-
puter tomography (micro-CT) images of the implants are shown in Figure 3.17.
Summary
The present work has reviewed two main areas, citing 174 references:
• Principles of electrophoretic deposition
• Reports of electrophoretic deposition of hydroxyapatite and other biomaterials
The importance of the underpinning of the concepts of the DLVO theory, and in particular
the double layer, has been emphasized. Likewise, the assessment of the double layer in
terms of the zeta potential has been highlighted.
Complementing the preceding has been coverage of the importance of producing stable
suspensions and the means of doing so in both aqueous and organic media. These means
include electrostatic, steric, and electrosteric stabilization. Practical issues associated with
the achievement of suspension stability and electrophoretic deposition that have been dis-
cussed briefly include:
Suspension parameters
• Particle size
• Electrical conductivity of suspension
• Dielectric constant of medium
• Solids loading
• Viscosity of medium