Bioactive glass thin films deposited by magnetron sputtering technique: The role of working pressure
Introduction
Bioactive glasses (BG) represent a new family of biomaterials which proved to possess very good osteointegrative properties as bulk materials. The bio-functionalization of metallic implants with BG coatings has been regarded as an excellent option for obtaining a rapid bond with the living tissue and to help bone reparation and regeneration by offering support to bone growth [1], [2], [3], [4]. Therefore, considerable attention has been given to study the bioactive fixation of such implants in the past decade [1], [2], [3], [4]. Various methods of deposition were used with promising biological results [5], [6], [7], [8], [9]. Unfortunately the adherence values of BG films at titanium substrates reported in the last years are too low for a safe use as implant-type coatings in biomedical applications [8], [10], [11].
Radio frequency-magnetron sputtering (RF-MS) is a very powerful technique which is used in a wide range of applications due to its excellent control over thickness and uniformity, excellent adherence of the films and its versatility in automatization [12]. BG films prepared by magnetron sputtering (MS) deposition technique were not studied in depth up to now [11], [13]. Recently we have optimized our deposition system for BG films and obtained both superior bioactive and adherence properties [14], [15].
In the present study we have used as starting material a new bioactive glass composition (wt.%: SiO2 – 40.08, CaO – 29.1, MgO – 8.96, P2O5 – 6.32, CaF2 – 5.79, B2O3 – 5.16, and Na2O – 4.59), incorporating modifiers such as magnesium and boron, with the aim of improving specific characteristics such as biomineralization capability and thermal expansion coefficient.
The main goal of this work was to evaluate the influence of RF-MS deposition pressure on the structure, composition and biomineralization capability of the BG films after 30 days of immersion in simulated body fluid (SBF), expressed by the formation of a carbonated hydroxyapatite (CHA) layer and its morphological features. The biomineralization capability is an essential property in osseous tissue healing process, enhancing the chemical interactions with the body fluid which lead to the implant bond with the surrounding tissue.
The argon pressure is related to the probability of back-scattering of the sputtered particles, thereby influencing the deposition rate. The atomistic theories of nucleation suggest the important role of individual atoms and clusters during the earliest stages of film formation. At a higher pressure the sputtered atoms undergo more collisions; hence the atoms impinge the film with lower kinetic energy. In the nucleation stages, the first nuclei incorporate the impinging sputtered atoms and clusters and grow in size forming islands whose density rapidly saturates. Thereby, one can expect that the bonding configuration, the structure and morphology of bioglass films can be tailored by varying the argon sputtering pressure. The changes occurred at the surface of the bioglass films during the biomineralization testes were monitored by FTIR, XRD and SEM-EDS techniques.
Section snippets
Powder preparation
Powders of technical grade (purity >99.5%) of silicon oxide and calcium carbonate, and of reagent grade of H3BO3, 4MgCO3·Mg(OH)2·5H2O, Na2CO3, CaF2, and NH4H2PO4 were used for BG preparation. Homogeneous mixtures of batches (∼100 g), obtained by ball milling, were preheated at 1000 °C for 1 h for decarbonization and then melted in a Pt crucible at 1400 °C for 1 h, in air. Frit glass was obtained by quenching of the melt into cold water. To obtain a fine powder with a mean particle size of about 11–14
Influence of working pressure on film thickness
The film thicknesses values as determined by optical transmittance measurements are reported in Table 1. One can notice a monotonous film thickness decrease with increasing sputtering argon pressure. A possible explanation for the decrease of deposition rate with increasing the argon pressure is the shortening of the mean free path of the sputtered particles. With increasing the sputtering pressure, the sputtered particles could suffer more collisions while traveling towards the substrate
Discussion
There is a direct correlation between the glass thin films’ composition and their structure that reflects directly in their biomineralization capability. FTIR analysis performed before immersion in SBF for the BG films revealed a dependence between argon deposition pressure and the short-range order of the glass structure. One can see a decrease in intensity of bioglass films spectra with deposition pressure that might weaken the structural bonds sustained by the bridging oxygen atoms, due to
Conclusions
The results presented and discussed along this work demonstrated the possibility of tailoring the structure, morphology and biomineralization capacity of bioglass films by changing the sputtering pressure. The argon pressure in the magnetron sputtering deposition chamber affected the composition and short-range structure of bioglass films deposited on silicon substrates, and the in vitro CHA formation capability of the bioglass layers. Pronounced bioactivity was showed for all the films upon
Acknowledgements
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) and to Romanian Ministry of Education and Research for the scientific project and financial support (CNMP PN II 71-110/2007). The financial support of “BD” PhD research scholarship offered by CNCSIS is gratefully acknowledged by G.E. Stan.
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