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CaO-P2O5 Glass-Hydroxyapatite Thin Films Obtained by Laser Ablation: Characterisation and In Vitro Bioactivity Evaluation

M. P. Ferraz1,2, F. J. Monteiro1,3, D. Gião1,3,4, B. Leon4, P. Gonzalez4, S. Liste4, J. Serra4, J. Arias4 and M. Perez-Amor4

1INEB, Instituto de Engenharia Biomédica, Laboratório de Biomateriais, Rua Campo Alegre 823, 4150-180 Porto, Portugal, mpferraz@ufp.pt

2Universidade Fernando Pessoa, Faculdade de Ciências da Saúde, Rua Carlos da Maia 296, 4200-150 Porto, Portugal

3Universidade do Porto, Faculdade de Engenharia, Departamento de Engenharia Metalúrgica e Materiais, Rua Roberto Frias, 4200-465, Porto,Portugal

4Universidad de Vigo, Departamento de Física Aplicada, Lagoas, Marcosende, 9, 36280 Vigo, Spain

Keywords: CaO-P2O5 glass, hydroxylapatite thin films, laser ablation, in vitro bioactivity.

Abstract. Hydroxyapatite (HA) coatings have been applied to improve adhesion of non-cemented implants to host bone. Plasma spraying is the most common technique leading to thick calcium phosphate films (>120µm). Pulse laser deposition (PLD), is a possible alternative method to obtain thin (<10 µm), well adherent hydroxyapatite (HA) films. Similarly to synthetic HA, biological apatites contain Ca2+, PO4

3-

and OH-, but also several trace ions, like Na+, Mg2+, K+ and F-, which may be introduced by CaO-P2O5 glasses. In this study, calcium phosphate coatings based on HA and glass modified HA were applied by PLD onto Ti-6Al-4V, using deposition times of 3 hours.

SBF immersion up to 1 week was used to test the films bioactivity. PLD thin films before and after SBF immersion were observed by SEM/EDS and analysed by XPS and XRD. PLD thin films presented columnar cross section structures, independently of the coatings’ chemical composition. After SBF immersion, apatite films formed on PLD coatings, both of HA and HA+1.5% glass, did not present the usual morphology of immersion films, but appeared to replicate the previous films. The main difference between HA and modified HA coatings could be seen in the XPS analyses at short immersion periods. Natural apatite was calcium deficient with a Ca/P of ±1.2-1.3. The results seem to indicate that modified HA coatings with lower Ca/P ratio induced earlier formation of natural apatite.

Introduction

Calcium phosphate ceramics have been often applied as coatings for implants, in most cases using thermal plasma spraying processes, leading to thick calcium phosphate films (>120µm). Recently various attempts have been done to produce thin (<5 µm) hydroxyapatite (HA) coatings. Pulse laser deposition (PLD), is based on the ablation of a target by a pulsed laser beam, producing a plasma plume and ablating products which are transferred to the substrate, forming a film, being a possible method to obtain thin film coatings. Previous works indicated that HA films obtained by PLD were thin, continuous, adherent to the substrate and presented adequate biocompatibility and bioactivity [1,2].

Similarly to synthetic HA, biological apatites contain Ca2+, PO43- and OH-, but the inorganic part of

bone also contain several trace ions, particularly Na+, Mg2+, K+ and F-, which may be introduced through CaO-P2O5 glasses. In an attempt to increase bioactivity and simultaneously maintain the bonding strength levels of HA, modified HA coatings were prepared based on PLD technique, applied to targets obtained by HA/CaO-P2O5 glass homogeneous mixtures. Due to the presence of the glass in the target the modified HA layer was expected to induce a rapid initial response, as previously observed in coatings obtained by plasma spraying [3].

Key Engineering Materials Vols. 254-256 (2004) pp. 347-350online at http://www.scientific.net© (2004) Trans Tech Publications, Switzerland

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In this study, calcium phosphate coatings based on HA and glass modified HA (HA+1.5G) have been applied by PLD on Ti-6Al-4V substrates, aiming at evaluating their possible use as biomaterials and compare their bioactivity.

Materials and Methods

A CaO-P2O5 glass containing 35, 35, 20 and 10 mol% of P2O5, CaO, Na2O and K2O respectively was prepared as previously described. The modified HA preparation method has been described elsewhere [4]. Glass addition of 1.5 wt% to HA (Pure, fully crystalline Ha, Ca/P= 1.67, by Plasma Biotal Ltd, UK) was used.

Ti-6Al-4V was used as substrate for these experiments. PLD coating technique applied to calcium phosphates was described elsewhere [2]. Coatings were produced at 460°C in a reactive atmosphere of water vapour, at constant pressure of 0.45 mbar, with a focussed ArF laser beam (193nm), operating at 10Hz and 200mJ/pulse. Targets were made of mixtures of HA and 1.5 wt% CaO-P2O5 glass, and coatings were produced using several deposition times (0.5, 1, 2 and 3 hours).

SBF immersion for several periods up to 1 week was used to test bioactivity of films obtained with 3 hour of deposition, as they presented the desired thickness. PLD thin films before and after immersion in SBF were observed by SEM/EDS and analysed by XPS and XRD. For the S.E.M analysis of cross sections, samples were bent up to 10º max, in a specific jig, to disrupt the coated layers and expose, under similar conditions, the cross-sections.

Results and Discussion

Under the above conditions, PLD treatment formed thin films on Ti-6Al-4V with columnar cross section structures, independently of the coating chemical composition, as found in previous works on films obtained with HA targets [2].

Coating thickness depends on the deposition time and not on the chemical composition of the target, being thicker with longer deposition times (Fig 1).

Fig. 1. Coating thickness versus deposition time In both cases the only crystalline phase observed is HA, no amorphous phase could be detected, but the coating deposited from the composite target shows preferential growth in the (002) and (112) directions. From these results, one can conclude that within the processes taking place during pulsed laser deposition, namely ablation, transport through a plasma plume and surface reactions on the substrate, the 1.5% of calcium phosphate glass added to the HA plays a modifying role in the deposited material.

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The surface morphologies are also similar for both types of targets, and none of them shows the droplet morphology, very often found in laser ablation studies of other materials. In spite of these common features and the very slight variation in composition of the targets (1.5%), the PLD coatings produced from the HA and the composite target have respectively Ca/P ratios of 1.65 + 0.04 and 1.52 + 0.03. These differences resulting from the ablation process are also clear from the XRD patterns, depicted in Fig. 2.

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Coatings are calcium deficient hydroxylapatite, in which the Na and K ions supplied by the glass are substituting Ca in the HA structure, and are responsible for the observed preferential orientation . After SBF immersion, chemical changes clearly indicate the transformations taking place at the surface (Fig 3). However the apatite film formed on PLD coatings (both HA and HA/1.5% glass (HA/1.5BG)) does

not present the usual morphology of immersion films, but instead it replicates the previous films (Fig 4a/4c and Fig 5a/5c). A possible explanation for this fact might be a substitution process taking place instead of the usual precipitation normally occurring on HA surfaces when immersed in SBF. The main difference between HA and HA/1.5BG coatings could be seen for short immersion periods both on the XPS analyses (Fig 3), SEM (Fig4b and 5b) and XRD.

As it can be seen on Fig 3, HA at 3 days immersion has nearly the same Ca/P ratio as the HA+1.5BG at 1 day of SBF immersion. The corresponding SEM images (I and II respectively) show a very similar morphology, indicating that HA+1.5BG induces a faster process than HA. Fig. 3- Ca/P relation versus immersion time in SBF obtained by XPS. (I) SEM insert of HA+1.5BG 1 day

immersed in SBF, (II) SEM insert of HA 3 day immersed in SBF.

Natural apatite is calcium deficient with a Ca/P of ±1.2-1.3. The results seem to indicate that HA+1.5BG coating with lower Ca/P ratio induced earlier formation of natural apatite. This is confirmed by SEM, once that at 1 day of SBF immersion, the HA+1.5BG samples present a morphology similar to that observed after 7 days immersion (Fig5a/5b). On the contrary, for the same immersion period, HA is still undergoing the surface modification process deriving from the apatite film being formed and apparently substituting the previous PLD film, although also replicating the previous coating layer with the formation of the new apatite film (Fig 4a/4b). Different tilting angles were used in these observations, eventually inducing misleading thickness evaluations.

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Fig 4 – SEM of HA coating (a), after 1day (b) and 7 days (c) of SBF immersion. Tilting angle was not kept constant.

Fig. 5 – SEM of HA+1.5BG coating (a), after 1day (b) and 7 days (c) of SBF immersion. Tilting angle was not kept constant.

Conclusions

PLD thin films presented columnar cross section structures, independently of the coatings’ chemical compositions. The apatite film formed on both HA and modified HA PLD coatings did not present the typical morphology of immersion films, but instead it replicated the previously existing PLD films. The main difference between HA and HA/1.5BG coatings could be seen for short immersion periods, where HA+1.5BG coating with lower Ca/P ratio induced earlier formation of natural apatite, and better defined crystallinity.

Acknowledgements The authors wish to thank F.C.T. project POCTI/CTM/35478/2000, and CRUP “Acções Integradas Portugal/ Espanha E-17/20” for their financial support, and Socrates/ Erasmus programme, for Ms Gião’s grant provided during her training period at University of Vigo.

References [1] J.L.Arias et al :J Mater. Sci. Mater.Med.Vol. 8 (1997), p. 873-876. [2] C. Peraire et al : 13th European Conference on Biomaterials, Göteborg, Sweden, 4-7 Sept. 1997, p. 75. [3] M. P. Ferraz, F. J. Monteiro, J. D. Santos: J Biomed Mater Res, Vol. 45 (1999), p.373. [4] M. P. Ferraz, M. H. Fernandes, A. Trigo-Cabral, J. D. Santos, F. J. Monteiro: J. Mat Sci: Mat. Med,

Vol.10(9) (1999), p.567.

a b c

c b a

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