Elsevier

Applied Surface Science

Volume 258, Issue 24, 1 October 2012, Pages 9840-9848
Applied Surface Science

The bioactivity mechanism of magnetron sputtered bioglass thin films

https://doi.org/10.1016/j.apsusc.2012.06.039Get rights and content

Abstract

Smooth and adherent bioactive coatings with ∼0.5 μm thickness were deposited onto Si substrates by the radiofrequency-magnetron sputtering method at 150 °C under 0.4 Pa of Ar atmosphere using a bioglass powder as target with a composition in the SiO2–CaO–MgO–P2O5–CaF2–B2O3–Na2O system. The bioactivity of the as-prepared bioglass samples was assessed by immersion in simulated body fluid for different periods of time up to 30 days. Grazing incidence X-ray diffraction, Fourier transform infra-red spectrometry and energy dispersive spectroscopy revealed that important structural and compositional changes took place upon immersing the samples in SBF. Whilst the excellent biomineralisation capability of the BG thin films was demonstrated by the in vitro induction of extensive and homogenous crystalline hydroxyapatite in-growths on their surfaces, a series of bioactivity process kinetics peculiarities (derogations from the classical model) were emphasised and thoroughly discussed.

Highlights

► RF-sputtering: an efficient solution for synthesizing bioactive glass thin films. ► Electrostatic interactions between charged surface and ions from stagnant solution. ► Heterogeneous-type nucleation of apatite from the SBF solution on the glass surface. ► Peculiarities of hydroxyapatite nucleation process and biomineralisation kinetics.

Introduction

Since the discovery of 45S5 bioglass by Hench et al., the bioactive glasses (BG) have proved their suitability to form a bond with the living bone tissue [1]. The enhanced bone layer grown at the implant site is due to favourable chemical interactions with the body fluid in the tissue rehabilitation process. Accordingly, considerable attention has been given to the use of implants with bioactive fixation [2]. Bioactive glasses and glass–ceramics have been extensively developed and investigated for non-load bearing applications as bone grafts or fillers owing to their ability to form bonds with the living bones and put into clinical use following many years of animal testing in a variety of experimental models [3], [4], [5]. The dissolution of the glass structure, leading to the formation of a silica-rich gel layer and subsequent deposition of an apatite-like layer on the glass surface, were found to be an essential step in the bioactivity evaluation both in vivo and in vitro studies [4], [6]. The formation of stable, mechanically strong interface with both bone and soft tissues is essential for clinical success [4], [7], [8].

In the last years bioglasses have been regarded as alternative candidate materials to commercial hydroxyapatite (HA) plasma spray coatings due to their osteoproductive properties [9], [10]. Various techniques, including enamelling [11], plasma spraying [12], sol–gel [13], electrophoretic deposition [14], and pulsed laser deposition [15] have been proposed to prepare high quality bioactive glass coatings. Unfortunately, many attempts to coat metallic implants with BGs have had limited success, due to the poor BG coatings’ adherence [11], [12]. An ideal implant-type coating requires excellent biocompatibility and mechanical stability. The bone forming ability of bioactive glasses depends on their chemical composition and textural properties [16], [17], [18]. Namely, it has been proved that the bioactive properties and the growth rate of the apatite-like layer could be increased by suitably adjusting the bioglass composition [19], [20].

Radio-frequency magnetron sputtering (RF-MS) deposition is nowadays one of the most popular techniques to grow thin films in the research field and in decorative and semiconductor industries [21], [22], [23]. Recently, RF-MS has emerged as a promising alternative for preparing adherent and highly bioactive bioglass films due to a number of advantages that include: (i) low pressure operation; (ii) low substrate temperature; (iii) high purity of films; (iv) excellent uniformity on large area substrates; (v) ease of process automation [24], [25], [26].

The aim of this work is to emphasise the potential of highly bioactive and adherent glass thin films deposited by the short time consuming RF-MS techniques as a viable alternative to the thick coatings produced by other deposition techniques. From the best of our knowledge this is the first article reporting on the in vitro biomineralisation process kinetics of such thin BG films under biomimetic conditions (ISO/FDIS 23317).

Section snippets

Powder preparation

A bioactive glass composition (SiO2 – 40.08, CaO – 29.1, Na2O – 4.59, P2O5 – 6.32, MgO – 8.96, CaF2 – 5.79, and B2O3 – 5.16) was recently developed and tailored to obtain a thermal expansion coefficient (CTE200–400 °C) of around 10.7 × 10−6/°C, close to those of Ti and its alloys [27]. Silicon oxide and calcium carbonate powders of technical grade purity greater than 99.5%, and reagent grade of H3BO3, 4MgCO3·Mg(OH)2·5H2O, Na2CO3, CaF2, and NH4H2PO4 were used for BG preparation. Homogeneous

XRD characterisation

Fig. 1 displays the XRD patterns of the target powder and of the as-deposited BG film recorded in symmetric geometry. The target powder is amorphous as suggested by the pronounced halo centred at 2θ  30°, which can be attributed to the amorphous silica. Within the experimental sensitivity limit, the as-deposited BG film seems to keep the amorphous structure, but the amorphous hump is shifted to larger angles in comparison to the target powder (maximum at 2θ  33°), indicating a slight modification

Discussion

Our goal was to propose an alternative solution for synthesizing biofunctional BG thin films. The traditional methods in use, enamelling and plasma spraying produce thick coatings (tens or even hundreds of μm), which are thus prone to cracking or delaminating. Our belief is that the use of thin films should be safer in operation than the use of micrometric coatings, as in the case of the latter, the adherence problems could determine micromovements of the implant in situ which could result in

Conclusions

Bioactive glass thin films were successfully obtained by the magnetron sputtering deposition technique. A quasi-stoichiometrical target-to-substrate atomic transfer has been noticed for the BG films due to physical processes of magnetron sputtering in non-equilibrium conditions and to the complexity of the target material structure and composition. The nucleation process of carbonated hydroxyapatite on the bioglass sputtered films’ surface might be influenced by the electrostatic interactions

Acknowledgements

G.E. Stan acknowledges with thanks the financial support of this work by the Romanian National Authority for Scientific Research through the UEFISCDI TE 49/05.10.2011 (TE-0164).

Thanks are due to CICECO for the support and to the Portuguese Foundation for Science and Technology for the fellowship grant of S. Pina (SFRH/BPD/64119/2009).

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