Abstract
Titanium surface was chemically modified with sodium hydroxide, sodium silicate and calcium hydroxide with the purpose of improving metal prosthesis to bone adhesion. Results indicate that Ti modified plates immersed in Hanks solution exhibit increased calcium adsorption in comparison to untreated titanium. Calcium silicotitanate, beta-tricalcium phosphate and apatites were detected on the coating.
Titanium; Calcium phosphates; Titanite; Calcium silicotitanate; Coatings
SURFACES, INTERFACES, AND THIN FILMS
Triple alkaline treatment of titanium surfaces for calcium phosphates growth
Luis Carlos Moreno; Jazmín García-Piñeros; Edgar Delgado-Mejía
Grupo de Aplicación de Materiales a la Odontología (GRAMO), Departamento de Química, Universidad Nacional de Colombia, Kra 30 #45-03, Fax +57-1-3165220, Bogotá D.C., Colombia
ABSTRACT
Titanium surface was chemically modified with sodium hydroxide, sodium silicate and calcium hydroxide with the purpose of improving metal prosthesis to bone adhesion. Results indicate that Ti modified plates immersed in Hanks solution exhibit increased calcium adsorption in comparison to untreated titanium. Calcium silicotitanate, b-tricalcium phosphate and apatites were detected on the coating.
Keywords: Titanium; Calcium phosphates; Titanite; Calcium silicotitanate; Coatings
I. INTRODUCTION
Titanium and its alloys are widely used as biomaterials mainly because of its low density and the outstanding mechanical properties, nevertheless its biointegration falls short from that of calcium phosphates ceramics [1-2]. Obtention of a composite material showing simultaneously the advantages of both, metal and ceramics, has long been sought and has been reached via superficial coatings [3-4].
Perhaps the most known titanium coating process is plasma spray but it has several drawbacks including the use of temperatures as high as 20.000K that negatively affect the phases so deposited converting hydroxypapatite (HA) into tricalcium phosphates and others phases that are more soluble and therefore more easily resorbed [5-6]. Electrodeposition allows for coating preselected phases but it lacks suitable adherence [7].
In this article two methods for the chemical modification of titanium surfaces were conducted for obtaining coatings with chemical and physical properties with closer resemblance of calcium phosphates. This similarity is important for nucleating the above mentioned phosphates on the metal matrix for increased biocompatibility useful in dental and bone implant applications.
The titanium hydrothermal modification with NaOH was undertaken because it creates microrugosity [8] which in the second step, namely sodium silicate treatment, can work as lodging or anchor. The final chemical step purpose is to perform the ionic exchange of calcium for sodium creating a low solubility calcium and silicon containing layer that might induce calcium apatite growth [9-10].The titanium surface hiydrothermal modification with calcium hydroxide has been shown to increase the in Vitro deposition of various calcium phosphates and it was used as a comparison [11-13].
II. MATERIALS AND METHODS
Chemically pure titanium (99.7%) 15x15x0.89 mm plates were cut from foils, sanded with number 600 SiC grit paper in one direction , washed and thoroughly cleaned in acetone with ultrasound and then in acetone in a vapour phase cleaning apparatus for four hours.
For the Ca(OH)2 method used as comparison, freshly cleaned titanium plates were immersed in 0.02M Ca(OH)2 aqueous solution at 121 C and 101.300 Pa. (local pressure is 76.642 Pa.) for two hours [11-13]. For the proposed triple alkaline treatment (TAT) also freshly cleaned plates were submerged in 5.0 M NaOH aqueous solution for 24 hours at 95.5C at local pressure and rinsed in distilled water followed by immersion in sodium silicate aqueous solution (Na2O 8%, SiO2 27%, density 1.37 kg/L) for 15 minutes, rinsed and finally were treated with 0.02M Ca(OH)2 solution for two hours.
The plates so treated were rinsed three times with distilled water and placed in Hanks solution containing the main inorganic electrolytes in concentrations equivalent to those in human blood serum (Table 1) [14] at 37C and pH 7.4 for thirty days with Hanks solution renewal every two days. Plates were then heated at 300C for one hour and blown clean of loose particles with oil free air compressed at 138.000 Pa. through a 1 mm diameter bore nozzle.
Coated plates were characterized by X ray fluorescence (XRF) in a Philips Analytical PW240 apparatus with 2q ranging from 12º to 147º. Phases composition was studied in a low angle X ray diffractometer (LAXRD) D8 Advanced
Bruker AXS with parallel beam geometry and copper anode (Ka, l= 1,5405), using incident angle of 2º. Surface morphology and elemental chemical analysis were done by SEM with a Philips XL30 ESEM attached with an EDX system.
III. RESULTS AND DISCUSSION
Non modified plates surface morphology shows deep parallel sanding scratches that are absent or shallower in treated titanium (Fig.1).
Surface produced by the Ca(OH)2 (Fig. 2), displays crystals of around 10 µm size in which EDAX detected Ca, C, O and Ti indicating the possibility of calcium carbonates, titanates or more complex compounds some of which have been found on titanium under similar conditions [11,12,13].
TAT surfaces (Fig. 3) show aggregates and microcracks, soaking in sodium hydroxide produces the latter solution, result that agrees with those reported by some researchers [8]. These microcracks did not show on Ca(OH)2 or sodium silicate plates (Figs 2, 4 respectively).
XRF results appearing on Table 2 support that Ca(OH)2 plates and those obtained with the TAT have a larger calcium adsorption than that of unmodified metal, indicating that both processes are effective.
Calcium and phosphorus surface contents after nucleation of TAT plates are larger than the contents of reference treatment plates, observation also supported by micrographs for chemically modified and nucleated plates (Figs. 5 and 6).
Nucleated TAT plates contain less calcium than non-nucleated TAT plates indicating that some calcium dissolution occurs during the month immersion in Hanks solution and that remaining calcium, as well as other elements, are probably irreversibly bond to the surface layer.
EDAX elemental analysis in the circle in Fig. 6 taken on metal surface, not on crystals, indicated Ti, Ca, P, Si, Mg and O suggesting that besides crystals there is a complex layer or a layer mixture of simpler compounds (Fig. 7).
Ca/P ratios calculated from table 2 differ from values expected for biomedical applications calcium phosphate compounds such as hydroxyapatite (stoichiometric HA Ca/P 1.67) and beta tricalcium phosphate (stoichiometric b-TCP Ca/P 1,5). This can be explained in terms of partial substitution of magnesium, sodium, potassium or vacancies for calcium in the case of cations and 
or 
for the anions. These ions originate from the Hanks solution at the given pH except for carbonate that comes also from atmospheric CO2 that was not excluded from reactions or solutions because natural bone and teeth hard tissues contain carbonates since they are cell synthesized in carbonate containing biological fluids. Other possible causes stem from the formation of salts with other anions during the process namely silicates, titanates and silicotitanates as detected by XRF, EDX and LAXRD.
Also Ca/P weight ratios are an average value for the XRF analytical volume that probably are non uniformly distributed but follow a gradient profile originated in the different treatments, nucleation and heating.
LAXRD showed b-TCP and calcium apatites on modified and nucleated plates. TTA plates showed calcium silicotitanate or so called titanite (Fig. 7), results that agree with those of EDX and XRF. Coating on Ca(OH)2 plates contained calcium perovskite (Fig. 8).
IV. CONCLUSIONS
Titanium plates modified with TAT develop an increased capacity for calcium and phosphate adsorption and nucleation compared to unmodified titanium and also to Ca(OH)2 modified titanium. b-TCP, calcium apatites and titanite are produced on triple alkaline treated titanium surface.
Acknowledgements
The authors wish to thank División de Investigaciones Sede Bogotá (DIB) Universidad Nacional de Colombia for the financial support and Ingeominas sede Bogotá for their invaluable cooperation with analytical equipment. Thanks also to the members of the biomaterial research group GRAMO for their efforts and cooperation.
Received on 8 December, 2005
- [1] W. Raembonck, P. Ducheyne, and P. Meester. Calcium Phosohate Ceramics, in Metal and ceramic biomaterial, P. Ducheyne (ed), CRC Press, Inc., Florida, 2000.
- [2] F. Silver, D. Christiansen. Biomaterials Science and Biocompatibility, Springer Verlag, New York, 1999.
- [3] T. Kasuga, T. Mizuno, M. Watabane, M. Nogami, and M. Niinomi. Biomaterials, 22, 577 (2001).
- [4] P. Molitor, T. Young. Int. J. Adhesion & Adhesives, 22, 101 (2001).
- [5] A. Ravaglioli, A. Krajewski, Bioceramics, Chapman & May, Great Britain, 1992.
- [6] R. B. Heimann, R. Wirth, Biomaterials, 27, 823 (2006).
- [7] A. Guzmán, C. Randall, P. W. Brown, and E. Delgado, Biomecánica, 9, 29 (2001).
- [8] K. Hamada, M. Kon, T. Hanawa, K. Yokoyama, Y. Miyamoto, and K. Asaoka, Biomaterials, 23, 2265 (2002).
- [9] P. N. Aza de, F. Guitian, and S. Aza de, Scripta Metallurgica et Materialia, 31, 1001 (1994).
- [10] N. Sahai, M. Anseau, Biomaterials, 26, 5763 (2005).
- [11] T. Hanawa, M. Kon, H. Ukai, K. Murakami, Y. Miyamoto, and K. Asaoka, J. Biomed. Mater. Res., 34, 273 (1997).
- [12] K. Hamada, M. Kon, T. Hanawa, Y. Yokoyama, Y. Miyamoto, and K. Asaoka, Biomaterials, 23, 2265 (2002).
- [13] B. Feng, J. Y. Chen, S. K. Qi, L. He, L. Z. Zhao, and X. D. Zhang, Biomaterials, 23, 173 (2002).
- [14] J. Bronzino (ed), The biomedical engineering handbook, CRC Press and IEEE Press, U.S.A., 1995.
Publication Dates
-
Publication in this collection
29 Nov 2006 -
Date of issue
Sept 2006
History
-
Received
08 Dec 2005
