Apatite Formation on Cobalt and Titanium Alloys by a Biomimetic Process

J. C. Escobedo Bocardo, M. A. López Heredia, D. A. Cortés Hernández, A. Medina Ramírez and J. M. Almanza Robles

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Posted: November 2005
AZojomo (ISSN 1833-122X) Volume 1 November 2005

Topics Covered

Abstract

Keywords

Introduction

Materials and Methods

Results and Discussion

Chemical Analysis

Mechanical Properties

SEM and EDX

X-Ray Diffraction

Conclusions

Acknowledgements

References

Contact Details

Abstract

Ti and Co alloys were subjected to a biomimetic process.  The Ti and Co alloy samples were immersed in 10M and 5M NaOH solutions, respectively.  The samples were heat treated and finally, immersed in either a simulated body fluid (SBF) with a lower ionic concentration to that of human blood plasma (0.85SBF) or a simulated body fluid with a higher ionic concentration (1.3SBF) for a period of 21 days.  A more modified surface, due to the chemical and heat treatment, was observed on the Ti alloys than on the Co alloys.  An apatite layer was observed after the immersion of the samples in 0.85SBF for both, Ti and Co alloys.  After the immersion in 0.85SBF the observed ceramic layer on the Ti alloy samples was more homogeneous and thicker than that observed on the Co alloy samples.  However, the ceramic layer on both samples was not continuous along the surface.  A thick apatite layer was formed on samples immersed in 1.3SBF and this was attributed to the higher ion concentration, mainly of calcium and phosphorus.  The Ca/P ratios measured in the apatite layers formed in 1.3SBF were closer to that of bone.

Keywords

Apatite, Bioactive, Biomimetic, Simulated Body Fluid

Introduction

A biomimetic process modifies a biocompatible material to give it bioactive characteristics [1, 2, 3].  In the case of the implants used for orthopedics or dental applications, such as hip and maxillae facial prostheses, this process allows the deposition of apatite layer on the surface of the implant [1, 2, 4].  Such methods are used to improve the bioactivity of materials such as titanium alloys, tantalum, alumina, and biodegradable polymer composites [5, 6].  This method has shown the following advantages in comparison with the traditional methods [2]: 1) it is a low temperature process that can be applied to any temperature sensitive substrate, 2) it forms apatite crystals, similar to those of bone, showing good bioactivity and good reabsorption characteristics, 3) it can be deposited even in porous substrates or implants of complex geometries, 4) it can incorporate bone growth features.  This biomimetic process, in the case of metallic materials, generally consists of a chemical treatment in an alkaline solution, followed by a heat treatment and ending with an immersion in a simulated body fluid (SBF).  The immersion in SBF can be considered as a first-stage procedure for the bioactivity assessment of a biocompatible material [6, 7].  Some researchers [7-10] have studied and elucidated the apatite formation mechanism on pure Ti and its alloys, Ta and alumina, finding that once the apatite nuclei are formed, crystals spontaneously start to grow by consuming calcium and phosphorous ions from the surrounding solution.  This crystal growth is controlled by the ion transfer through the interface between the substrate and the fluid [11].  For cobalt alloys (Vitallium®) under certain conditions, by using a chemical treatment of NaOH 10M at 60°C for 24 hours, followed by a heat treatment at 600°C, no significatives changes on the samples were reported [6].  After the chemical treatment, deposits of an unknown phase were found and there was no formation of an apatite layer after the immersion in SBF [6].  On the other hand by using a chemical treatment of NaOH 5M at 60°C for 24 hours, followed by a heat treatment at 600°C and immersing the sample in SBF [12, 13] the spontaneous formation of a bone-like apatite layer on this alloy has been reported.  However, the apatite formation mechanism for the Co alloys has not fully been understood.  This work presents the comparative results of the effect of the SBF concentration on the apatite formation between Ti and Co alloys.

Materials and Methods

To obtain a Co alloy (ASTM F75), containing approximately 0.018 %wt. C, the investment casting technique was used via two raw materials, a wrought alloy and a powder metallurgy alloy (Carpenter Technology Co).  The Ti alloy used was the Ti6Al4V ELI in annealed condition as supplied (Carpenter Technology Co).  The cast bars of the cobalt alloy obtained were heat treated at 1224°C for 75 minutes.  Tension test were performed for this material (Instron, model 4206) at a crosshead speed of 3 mm/min according to the ASTM E8 standard.  The mechanical properties and chemical analysis for the Ti alloy were provided by the supplier.  The chemical analysis of the cast cobalt alloy was performed by emission spectrophotometry (Lab S, Spectro Analytical Instruments) and direct combustion with infrared detection for carbon (LECO model CS 244-748-000, Leco Corporation).

To apply the biomimetic process, samples of the alloys with dimensions of 12.7 mm in diameter and 2 mm in height were obtained.  These samples were ground with silicon carbide papers ranging from 80 to 1200 grit.  Finally, the samples were washed with deionised water and ethanol, dried by compressed air and stored in a desiccator before testing.

For the chemical treatment the samples were immersed in 5M and 10M NaOH aqueous solutions for Co alloy and for Ti alloy samples, respectively.  The samples immersed were kept at a constant temperature of 60°C for 24 hours.  After that period, the samples were rinsed with deionised water and dried for 24 hours at 37°C.  A couple of samples were kept to further observation.  Then, the samples were heat treated at 600°C for 1 hour and cooled down inside the furnace.  After the chemical and heat treatments, one couple of samples was taken for further observation.  Following the heat treatment, the samples were immersed in an 0.85SBF or in a 1.3SBF solutions.  The 0.85SBF and the 1.3SBF were prepared according to the method proposed by Oyane [14].  The ion concentration of the 0.85SBF, SBF, 1.3SBF and that of the human blood plasma are shown in Table 1.

Table 1. Ionic concentration.

The samples were immersed in SBF solutions (0.85SBF or 1.3SBF) at 37°C for 21 days.  During the immersion period, these solutions were renewed each 7 days.  At the end of the immersion period the samples were washed gently with deionised water.  The samples surfaces were characterized using Scanning Electron Microscopy (SEM; microscope Philips, model XL 30 ESEM), Energy Dispersive X-Ray Analysis (EDX; software Genesis, EDAX) and X-Ray Diffraction (XRD; Diffractometer Xpert Philips, model PW3040).

Results and Discussion

Chemical Analysis

The chemical analysis of the samples and the requirements of the ASTM F75 and F136 standards are shown in Table 2.  Both alloys are within the parameters of their respective standards.

Table 2. Chemical analysis results [wt %].

Mechanical Properties

Table 3 shows the results of the mechanical properties and the requirements of the ASTM F75 and F136 standards.  The alloy obtained by the investment casting process satisfied the standard.

Table 3. Mechanical properties.

SEM and EDX

Before the microscopy characterization the samples were visually analyzed after the chemical and heat treatment.  After the chemical treatment a change in color was observed on the surface of the Ti alloy, while the Co alloy samples remained unchanged.  After the heat treatment the Ti alloy samples presented an homogeneous layer on its surface and a change in the surface color.  The Co alloy samples presented only a visible change in color.

Figures 1 and 2 show the SEM images and their respective EDX spectrum for the Ti and Co alloys samples after chemical treatment.

Figure 1. Chemically-treated Ti alloy sample.  (a) SEM image and (b) EDX spectrum.

Figure 2. Chemically-treated Co alloy sample.  (a) SEM image and (b) EDX spectrum.

The chemical treatment modifies more markedly the surface of the Ti alloy than the Co alloys, as reported in the literature [7].  The SEM images show that on the Co alloy the grinding marks are more visible after the chemical treatment, not been the same for the Ti alloys where they have almost disappeared.  The chemical treatment forms an alkali titanate hydrogel, which contains some Na or K [1, 4, 9, 10] on the surface of the Ti alloys.  For the Co alloys a similar behavior in a certain degree could be expected.  However, from the EDX spectrum, as perceived by the surface modification, the Ti alloy sample seems to be more affected by this treatment and the corresponding spectrum presents a more defined and higher Na peak than that in the spectrum for the Co alloy sample.

Figures 3 and 4 show the SEM images and their respective EDX spectrum for a Ti alloy and a Co alloy samples with a chemical and heat treatments.

Figure 3. Ti alloy sample with chemical and heat treatments.  (a) SEM image and (b) EDX spectrum.

Figure 4. Co alloy sample with chemical and heat treatments.  (a) SEM image and (b) EDX spectrum.

After the chemical and heat treatments for the Ti alloys the formation of an amorphous alkali titanate in which the Na or K ions are stabilized is reported [1, 4, 9, 10].  Supposing that on the Co alloy a compound containing Na is formed due to the chemical treatment, which may be stabilized during the heat treatment, another surface modification could be expected.  From the SEM images these modifications on both alloys are appreciated.  Nevertheless, once again for the Ti alloy the effect produced by the two consecutive treatments is more evident than that for the Co alloys, in which the grinding marks are still visible and the change in color is thought to be due to an oxidation mechanism.  The EDX spectrum shows that for the Ti alloy the presence of Na is more visible than on the Co alloy, but both exhibit a certain modification due to the previous treatments.

Figures 5 and 6 show the SEM images and their respective EDX spectrum for a Ti alloy and a Co alloy samples chemically treated, heat treated and immersed in 0.85SBF for 21 days.

After the respective treatments and the immersion in 0.85SBF the formation of a ceramic layer, on both samples was observed.  However, the layer formed on the Ti alloy seems to be more homogeneous and thicker than that formed on the Co alloy, in which now the grinding marks have almost disappeared. Using the respective alloy peaks on the EDX spectrum, it can be inferred that on both cases the layer is extremely thin.  The Ca/P ratios on these samples are shown in Table 4.

Table 4. Ca/P ratios for the samples immersed in SBF.

The Ca/P ratio range for the apatite is 1.2 – 1.66, for the hydroxyapatite it is 1.67.  On the Ti alloy a Ca-deficient apatite seems to be formed, while a compound with a Ca-excess was formed on the Co alloy.

Figures 7 and 8 show the SEM images and their respective EDX spectrum for a Ti alloy and a Co alloy samples chemically treated, heat treated and immersed in 1.3SBF for 21 days.

An apatite layer was observed on both samples.  These layers were thicker than those of the samples immersed in 0.85SBF (Figures 5 and 6).  Thus, the ionic concentration of the simulated body fluids has a considerable effect on the thickness and composition of the layers formed on the metallic substrates.  The Ca/P ratios for the samples immersed in 1.3SBF are shown in Table 5.

Table 5. Ca/P ratios for the samples immersed in 1.5SBF.

The apatite formed on both alloys was Ca deficient.  The layer formed on the Ti alloy showed a closer Ca/P ratio to that of hydroxyapatite.  Furthermore, this layer seemed to be thicker and better adhered than that on the Co alloy.  The thickness of the layer can be inferred form the EDX spectrum by observing the relative intensity of the peaks for Ca, P, and those of the alloy.  The adherence can be inferred from the size and number of cracks observed on the ceramic layers [3,15].  During the treatments of the surface, the surface properties are modified, improving the attachment of the layer formed [3,15].  Once again, the surface modification seems to be more effective on the Ti alloy sample than on the Co alloy sample.

X-Ray Diffraction

The X-Ray diffraction patterns are given from Figure 9 to Figure 12 for all the samples after the immersion in 0.85SBF or 1.3SBF.

For the Ti alloy sample immersed in 0.85SBF (Figure 9) the XRD pattern shows only the corresponding peaks of the Ti alloy. For the Co alloy sample (Figure 10) the result indicated the presence a complex CoCr oxide on the surface.  These results may indicate that the layer obtained by the immersion in 0.85SBF is extremely thin, undetectable by XRD.  For the Ti alloy (Figure 11) and Co alloy samples (Figure 12), immersed in 1.3SBF, the XRD patterns indicate the presence of hydroxyapatite.  A thicker layer was formed when samples were immersed in a 1.3SBF solution.

It is possible to observe that the biomimetic method used, consisting of chemical and heat treatments, led to a more modified surface on the Ti alloy than on the Co alloy.  After 21 days of immersion in 1.3SBF, the apatite layer formed on the surface of the Ti alloy was thicker than that formed on the Co alloy.  However, the Ca/P ratios and morphological characteristics of the layers formed on both alloys are similar.  Results obtained when the plasma spray method is used indicate that the apatite layers obtained on the surface of Ti and Co alloys have similar mechanical and histological characteristics and also the same thickness [16].

Regarding the mechanism of formation of the apatite layer on the Co alloy by using the biomimetic method, it is only possible to present two observations: 1) The presence of Na on the surface of the Co alloy sample before the chemical treatment with a NaOH solution is detected, this may indicate the formation of a compound containing Na, and 2) According to thermodynamics, the formation of compounds containing Na, such as chromates, from both chromium or chromite is feasible.  However, taking only into account the presence of Na and the thermodynamics it is not possible to elucidate the mechanism of formation of apatite on the Co alloy samples.

Conclusions

As stated in the literature, the formation of an apatite layer on the Ti and Co alloys is possible by using a biomimetic process.  The surface modification, due to chemical and heat treatment, was more effective on the Ti alloy samples than that on the Co alloy samples.  This surface modification has an effect on the features of the formed layer.  To obtain an apatite layer with better characteristics, another way of modifying the surface of the Co alloys must be explored.  A Thicker layer of apatite was observed on the alloys immersed in 1.3SBF in comparison with the layer formed on the alloys immersed in 0.85SBF.  As the ion concentration on the SBF was increased, the growth rate of the layer was also increased.

Acknowledgements

The authors would like to thank the Mexican National Council of Science and Technology (CONACyT) for their financial support to this work.

References

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Contact Details

J. C. Escobedo Bocardo

Centro de Investigación y de Estudios Avanzados – Instituto Politécnico Nacional (IPN), Unidad Saltillo.

Carretera Saltillo-Monterrey Km. 13.5, A.P. 663, C.P. 25000

Saltillo, Coahuila

México.

E-mail: [email protected]

M. A. López Heredia

Centro de Investigación y de Estudios Avanzados – Instituto Politécnico Nacional (IPN), Unidad Saltillo.

Carretera Saltillo-Monterrey Km. 13.5, A.P. 663, C.P. 25000

Saltillo, Coahuila

México.

D. A. Cortés Hernández

Centro de Investigación y de Estudios Avanzados – Instituto Politécnico Nacional (IPN), Unidad Saltillo.

Carretera Saltillo-Monterrey Km. 13.5, A.P. 663, C.P. 25000

Saltillo, Coahuila

México.

A. Medina Ramírez

Centro de Investigación y de Estudios Avanzados – Instituto Politécnico Nacional (IPN), Unidad Saltillo.

Carretera Saltillo-Monterrey Km. 13.5, A.P. 663, C.P. 25000

Saltillo, Coahuila

México.

J. M. Almanza Robles

Centro de Investigación y de Estudios Avanzados – Instituto Politécnico Nacional (IPN), Unidad Saltillo.

Carretera Saltillo-Monterrey Km. 13.5, A.P. 663, C.P. 25000

Saltillo, Coahuila

México.

This paper was also published in print form in “Advances in Technology of Materials and Materials Processing”, 7[2] (2005) 141-148.