The deposition, characterisation and biocompatibility of hydroxyapatite and silicon doped hydroxyapatite thin film coatings for orthopaedic applications

Coe, Samuel C. (2008) The deposition, characterisation and biocompatibility of hydroxyapatite and silicon doped hydroxyapatite thin film coatings for orthopaedic applications. PhD thesis, University of Nottingham.

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Silicon doped hydroxyapatite (SiHA) could be used as a thin film coating on load bearing bone implants to provide a bioactive layer enabling bone to form a direct bond with the implant/bone interface thus increasing implant lifetime by lowering the chances of aseptic loosening. This study has been undertaken to investigate silicon additions to RF magnetron sputtered hydroxyapatite (HA) thin films. Detailed characterisation was carried out on SiHA thin films to establish the structural, chemical, mechanical and compositional properties. Silicon content was altered by adjusting the power density applied to silicon targets in a co-deposition process resulting in SiHA films containing 0.0, 1.8, 4.2 and 13.4 wt.% silicon. All as-deposited thin films were found to be amorphous. After annealing at 600˚C in flowing argon for 2 h, it was found that films exhibited a single phase HA structure. The addition of silicon inhibited HA crystallite growth and acted to lower the stability of HA films in aqueous solutions. The 13.4 wt.% SiHA thin film did not recrystallise until a heat treatment at 800˚C.

From the work presented here, it is proposed that, in post-plasma-deposited heat treated films, silicon substitutes as silicate species into the HA lattice. Asdeposited silicon containing thin films were found to be amorphous and have a polymeric silicate configuration, suggesting that, silicate groups may be randomly distributed throughout the amorphous film. After post-deposition annealing silicon containing films were in a monomeric state suggesting silicate groups had substituted for phosphate tetrahedra in the HA lattice. Furthermore, an HA-like phase was found to be present. Contrary to these findings, FTIR analysis did not manifest any silicate-based bands. This may, however, be due to the fact that technique used only samples a very small amount of material and, due to the low doping quantities of silicon in the HA films. Furthermore, Ca/P ratios consistently differed from the stoichiometric value of HA (1.67). This combined evidence raises the question of whether the post-deposition heat-treated films have a true HA-like structure. More work is required in order to truly understand the structures present in heat-treated SiHA thin films. HA thin film composition is commonly measured in terms of the Ca/P ratio. Energy dispersive X-ray analysis (EDX) and XPS were evaluated in terms of accuracy in conjunction with Rutherford backscattering spectroscopy (RBS) to measure the Ca/P ratio of HA thin films to establish the most appropriate technique for accurate compositional analysis. This was was found to the RBS, achieving an accuracy of within 2 %, with EDX averaging 8 % and XPS ranging from 25 - 42%. It was concluded that XPS gave such large differences in values because the top few atomic layers of thin films was of a different composition than the bulk of the coating.

A Human osteoblast cell (HOB) model was used to establish the in vitro cellular response of SiHA thin films. Initially, HA and SiHA thin films annealed at 600˚C were compared. Cells attached and proliferated well on HA surfaces compared to SiHA surfaces, however, improved cell growth was seen with increasing silicon content. Dissolution studies showed that SiHA thin films were highly unstable in cell culture media and it is thought that the films dissolved, and where cell adhesion and growth did occur it was because cells adhered to the titanium substrates beneath the films. This was then compared with HA and SiHA thin films annealed at 700˚C. No significant difference was found between the two surfaces in terms of cell growth or protein expression indicating that silicon content and crystallinity play an important role in the cellular response of SiHA thin film.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Grant, D.
Walker, G.S.
Scotchford, C.A.
Subjects: R Medicine > RD Surgery
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 10984
Depositing User: EP, Services
Date Deposited: 13 Nov 2009 11:22
Last Modified: 15 Oct 2017 05:02

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