Biomimetic apatite formation on chemically treated titanium

Lenka Joná
ová^{, , a, b}, Frank A. Müller^a, Ale
Helebrant^b, Jakub Strnad^c and Peter Greil<sup>a

a</sup> Department of Materials Science (III), University of Erlangen-Nuremberg, Martensstrasse 5, Erlangen 91058, Germany
^b Department of Glass and Ceramics, Institute of Chemical Technology, Technická 5, Prague 166 28, Czech Republic
^c Lasak Co Ltd., Papírenská 25, Prague 160 00, Czech Republic

Received 10 January 2003;  accepted 11 August 2003. ; Available online 14 October 2003.
Biomaterials
Volume 25, Issues 7-8 , March-April 2004, Pages 1187-1194

1. Abstract

Titanium treated in NaOH can form hydroxycarbonated apatite (HCA) after exposition in simulated body fluid (SBF). Generally, titanium is covered with a passive oxide layer. In NaOH this passive film dissolves and an amorphous layer containing alkali ions is formed on the surface. When exposed to SBF, the alkali ions are released from the amorphous layer and hydronium ions enter into the surface layer, resulting in the formation of Ti-OH groups in the surface. The released Na⁺ ions increase the degree of supersaturation of the soaking solution with respect to apatite by increasing pH, and Ti-OH groups induce apatite nucleation on the titanium surface. The acid etching of titanium in HCl under inert atmosphere was examined as a pretreatment to obtain a uniform initial titanium surface before alkali treatment. Acid etching in HCl leads to the formation of a micro-roughened surface, which remains after alkali treatment in NaOH. It was shown by SEM, gravimetric and solution analysis that the apatite nucleation was uniform and the thickness of precipitated HCA layer increased continuously with time. The treatment of titanium by acid etching in HCl and subsequently in NaOH is a suitable method for providing the metal implant with bone-bonding ability.

Author Keywords: Titanium; Bioactive; Chemical treatment; SBF

2. Article Outline

1. Introduction

2. Materials and methods

2.1. Sample preparation

2.2. Soaking of samples in SBF

2.3. Analysis of sample surface and SBF

3. Results and discussion

4. Conclusions

Acknowledgements

References

3. 1. Introduction

Titanium and its alloys (e.g. Ti6Al4V) are now being used as a common material for bone implants under biomechanical loading conditions. Bioinert titanium based materials are generally encapsulated after implantation into the living body by fibrous tissue that isolates them from the surrounding bone [1 and 2]. It was shown that a limited kind of ceramics [3, 4 and 5], called bioactive, could bond to living bone without formation of fibrous tissue. These materials create a bone-like apatite layer on their surface after implantation. Thus the apatite formation is believed to be the main requirement for bone-bonding ability of materials in our understanding of the bioactive behavior. The formation of a bone-like apatite surface layer can be reproduced in an acellular simulated body fluid (SBF) with ion concentration nearly equal to those of inorganic part of human blood plasma [6].

In order to combine the mechanical properties of metals with bone-bonding ability of bioactive ceramics, various methods of coating the titanium surface have been tested [7 and 8]. Hydroxyapatite (HA) plasma spraying is one of the most widely investigated methods for coating titanium. By this method HA powder is heated at extremely high temperatures and deposited with very high velocity on the metal surface. The coating quality, composition and crystallinity of plasma sprayed HA are difficult to control [9 and 10].

Recently, it has been described that bioactive titanium surface can be prepared by a simple chemical treatment in NaOH [11 and 12]. The structural changes of the titanium surface during chemical treatment and subsequent apatite formation in SBF have been already described. Leaching of titanium in NaOH results in the formation of a hydrated titanium oxide gel layer containing alkali ions on its surface. This gel layer is dehydrated and densified to form an amorphous alkali titanate layer by heat treatment below 600°C. When this pre-treated titanium is exposed to SBF, the alkali ions are released from the amorphous alkali titanate layer and hydronium ions enter into the surface layer, resulting in the formation of a titanium oxide hydrogel layer. The released Na⁺ ions increase the degree of supersaturation of the soaking solution with respect to apatite by increasing pH, and the titanium oxide hydrogel induces apatite nucleation on the titanium surface [13, 14, 15 and 16].

Lately, it was shown that the reproducibility of the sodium titanate hydrogel layer formation after alkali treatment and subsequent precipitation of HA is poor due to differences in the titanium surface structure that depends on the titanium processing preceding the alkali treatment [17]. The thickness of the passive oxide layer on Ti depends on temperature. Maximum oxidation occurs at 770-870°C [18]. Furthermore, the composition of the passive film is not uniform. It was shown to be composed of an amorphous TiO₂ outer layer with 10-20 nm thickness and an intermediate TiO_x layer with 10-40 nm thickness, in contact with the TiO₂ layer and the metallic substrate [19].

The purpose of this study was to examine a chemical treatment of the titanium surface that would inhibit the negative effect of titanium processing on the amorphous layer formation and subsequently HA precipitation.

4. 2. Materials and methods

2.1. Sample preparation

Commercially pure titanium discs (TIMET GERMANY GmbH, Duesseldorf, Germany) with a diameter of 10 mm and a thickness of 1 mm were washed in ethanol in an ultrasonic cleaner and dried at 100°C. The sample marked as A was soaked in 10 
NaOH aqueous solution at 60°C for 24 h, washed with distilled water and dried at 100°C. Sample B was etched in HCl under inert atmosphere of CO₂ for 2 h and then soaked in 10 
NaOH aqueous solution at 60°C for 24 h, washed with distilled water and dried at 100°C.

2.2. Soaking of samples in SBF

The effect of titanium pre-treatment in HCl on HA formation was examined in modified SBF which simulates the inorganic part of human blood plasma (Table 1). The solution was prepared by dissolving reagent-grade NaCl, KCl, NaHCO₃, MgSO₄·12H₂O, CaCl₂ and KH₂PO₄ into distilled water and buffered at pH=7.3 with tris-hydroxymethyl aminomethane (TRIS) and HCl at 37°C. NaN₃ was added into the solution to inhibit the growth of bacteria [6].

Table 1. Composition of SBF and the inorganic part of human blood plasma (mmol/l)

The samples were exposed to the solutions under static conditions in a biological thermostat at 37°C for 2, 5, 10, 14 and 20 days, respectively. For each time period four samples A and B were exposed (i.e. total number of samples marked as A was 20). The ratio of sample geometrical surface area to soaking solution volume S/V was 0.005 cm⁻¹. After exposure the samples were washed in distilled water.

2.3. Analysis of sample surface and SBF

Changes in the Ti surface after chemical treatment and soaking in SBF were determined by SEM-EDX (Cambridge Instruments, USA) on carbon-sputtered samples and XRD (Siemens DIFFRAC 500, Germany). The surface roughness was measured by laser scanning microscopy (UBM Messtechnik GmbH, Microfocus COMPACT, Germany).

Sample-solution interactions were quantified on the basis of gravimetric and solution analysis. The results in Fig. 3, Fig. 4 and Fig. 10 are average values from four samples in every time period. To evaluate the ability and rate of HCA formation on the sample surface, changes in the concentration of phosphates and calcium in the solution were determined using spectrophotometric measurement and atomic absorption spectroscopy (AAS), respectively. The measurement error for the phosphate and calcium concentration is ±3.9 and ±5.1 mg/l, respectively. A consumption of these components may be directly correlated with the formation of phosphorus and calcium enriched surface layer.

(5K)

Fig. 3. Gravimetric analysis of precipitated layers after soaking of samples A and B in SBF for different time.

(6K)

Fig. 4. Time dependence of changes in (PO₄)³⁻ and Ca²⁺ concentration after soaking of sample A in SBF.

(6K)

Fig. 10. Time dependence of changes in (PO₄)³⁻ and Ca²⁺ concentration after soaking of sample B in SBF.

5. 3. Results and discussion

Fig. 1 shows the titanium surface after treatment in 10 
NaOH at 60°C for 24 h. After 2 days in SBF the surface remained unchanged compared to unexposed sample (Fig. 1 and Fig. 2). The ability of alkali treated titanium to induce HCA formation was evaluated on the basis of gravimetric analysis (Fig. 3). It may be assumed that an increase in sample weight indicates the formation of a surface layer. A notable increase in the weight of sample A by 1 and 1.5 mg was measured after 10 and 14 days in SBF, respectively. The results correspond well with the decrease of (PO₄)³⁻ concentration in the solution (Fig. 4). It can be supposed that the weight of the precipitated layer should increase with time. However, the weight of precipitate after 20 days was 0.6 mg less than after 14 days.

(27K)

Fig. 1. SEM micrographs of titanium surface after treatment in 10 
NaOH at 60°C for 24 h.

(102K)

Fig. 2. SEM micrographs of titanium surface after treatment in 10 
NaOH at 60°C for 24 h (sample A) and exposure to SBF for (a) 2 days, (b) 10 days, (c)14 days, and (d) 20 days.

The results evaluated by gravimetric and solution analyses were in agreement with those obtained by SEM (Fig. 2). Isolated spheroid particles that are typical for apatite crystallized from SBF deposited on the sample surface after 10 days ( Fig. 2b). The number of particles and their maximum diameter about 10 
m were comparable with those crystallized after 20 days. This corresponds well with the increase in sample weight after 10 and 20 days by 1 and 0.9 mg, respectively (Fig. 3). After 14 days in SBF an inhomogeneous HCA layer was observed on sample A using SEM ( Fig. 2c). Two areas were analyzed on this sample by X-ray diffraction analysis. In the first area (referred as "1" in Fig. 2c) HCA was confirmed ( Fig. 5b). On the contrary only titanium was detected in the second area (referred "2" in Fig. 2c), ( Fig. 5c). The results described above indicate that the reproducibility of apatite crystallization on alkali treated titanium is low which is in disagreement with the experimental findings discussed in literature [13 and 16]. The different ability of alkali treated titanium to induce apatite nucleation could be caused by differences in the titanium surface that depend on titanium processing. The surface of titanium is covered with a passive oxide layer, whose thickness depends on temperature with maximum oxidation about 770-870°C [18]. Furthermore, the composition of the passive film is not uniform. It was shown to be composed of an amorphous TiO₂ outer layer and an intermediate TiO_x layer [19]. However, from the results discussed above, the leaching of titanium in NaOH seems to be not sufficient to completely dissolve the passive oxide layer. Upon exposure to NaOH this passive layer partially dissolves to form HTiO₃⁻. Simultaneously, titanium is hydrated to form HTiO₃⁻·nH₂O. These negatively charged groups react with positively charged alkali ions to produce an alkali titanate hydrogel layer on the titanium surface [14 and 16].

(7K)

Fig. 5. XRD of sample A (a) without exposure to SBF and (b) and (c) after 14 days in SBF in two different areas.

Acid etching of titanium in HCl under inert atmosphere was examined as an alternative pretreatment to obtain a uniform initial titanium surface before alkali treatment. Fig. 6 shows the titanium surface after acid etching in HCl with an average surface roughness R_z=6 
m measured by laser scanning microscopy. After subsequent alkali treatment in NaOH the initial surface topography is maintained (Fig. 7). No changes were observed in the surface of sample B after 2 days in SBF using SEM (Fig. 8a) compared to unexposed one (Fig. 7). EDX analysis revealed calcium, magnesium, sodium and phosphorus to be present in the surface ( Fig. 9). Sodium probably remains after alkali treatment. The detected Ca/P molar ratio was 8.6. Since no stable calcium phosphate compounds with Ca/P molar ratios higher than 2 are known it can be supposed that these components are present as ions in the sample surface. This result shows that in the first step the cations like Ca²⁺ and Mg²⁺ are incorporated in the titanium surface. Furthermore, no decrease in calcium or phosphate concentration in SBF and no increase of sample weight were measured by solution and gravimetric analysis, respectively (Fig. 10 and Fig. 3).

(21K)

Fig. 6. SEM micrographs of titanium surface after acid etching in HCl under inert atmosphere.

(26K)

Fig. 7. SEM micrographs of titanium surface after acid etching in HCl under inert atmosphere and subsequent alkali treatment in NaOH at 60°C for 24 h (sample B).

(70K)

Fig. 8. SEM micrographs of titanium surface after acid etching in HCl and treatment in 10 
NaOH at 60°C for 24 h (sample B) and exposure to SBF for (a) 2 days, (b,c) 5 days, (d) 10 days, (e) 14 days, (f) 20 days.

(3K)

Fig. 9. EDX analysis of sample B soaked for 2 days in SBF.

After 5 days in SBF isolated spheroids with a diameter of 5 
m were deposited in the surface of sample B (Fig. 8b and c). The composition of the particles analyzed by EDX showed the presence of calcium, magnesium and phosphorus ( Fig. 11). The molar Ca/P ratio was 1.3, which corresponds to that one of octacalcium phosphate (OCP: Ca₈H₂(PO₄)₆·2H₂O). OCP is believed to be a precursor in the crystallization of bone-like apatite [20 and 21]. On the contrary, when EDX point analysis was conducted in the area without spheroids, a Ca/P molar ratio of 2.7 was detected ( Fig. 12). After 10, 14 and 20 days in SBF a compact and homogeneous surface layer was formed (Fig. 8d-f). X-ray diffraction analysis confirmed that the composition of the layer corresponds to HCA ( Fig. 13c and d) with a Ca/P molar ratio of 1.6.

(3K)

Fig. 11. EDX analysis of spheroids crystals in the surface of sample B soaked for 5 days in SBF.

(3K)

Fig. 12. EDX analysis of sample B soaked for 5 days in SBF.

(9K)

Fig. 13. XRD of: (a) titaniuim after acid etching in HCl and of sample B; (b) without exposure to SBF; and after exposition in SBF for (c) 14 days and (d) 20 days.

The Ca²⁺ and (PO₄)³⁻ concentration in SBF after soaking of sample B are given in Fig. 10 and compared with SBF containing no sample (referred as "blank" in Fig. 10). After 2 and 5 days the changes of calcium and phosphate concentration were negligible. A notable decrease of (PO₄)³⁻ was measured after 10 days in SBF which indicates the deposition of a surface layer enriched with phosphorus. Furthermore, the weight of sample B after 10 days increased by 2 mg that corresponds to a HCA layer thickness of 20 
m (Fig. 3). The weight of sample B after exposure in SBF increased continuously with time.

Based on the results described above the possible structural changes of titanium surface during chemical treatment and subsequent apatite formation in SBF are as follows (Fig. 14): Metal titanium is passivated by an oxide film that forms spontaneously. Its composition is reported to be of a TiO₂ outer layer and a TiO_x intermediate layer [19] ( Fig. 14a). During acid etching in HCl the passive oxide film degradates according to

TiO2+4HCl→TiCl4+2H2O.

(1)

(24K)

Fig. 14. Possible structural changes of the titanium surface: (a) during acid etching; (b) alkali treatment (c); and subsequent apatite formation in SBF (d-f).

Simultaneously titanium reacts with HCl to form TiCl₃ and H₂:

2Ti+6HCl→2TiCl3+3H2.

(2)

TiH₂ that was detected using X-ray diffraction analysis (Fig. 13) can be formed by

Ti+H2→TiH2.

(3)

On the TiH₂ intermediate layer a new oxide layer can form in contact with air moisture (Fig. 14b). It can be supposed that the titanium oxide layer is thinner than the initial one because it was not exposed to high temperature. Upon exposure of acid etched titanium in NaOH the passive oxide layer dissolves to form amorphous titania layer containing Na⁺ ions [16] ( Fig. 14c):

TiO2+NaOH→HTiO3^−+Na^+.

(4)

Immediately after immersion in SBF, Na⁺ ions from the amorphous layer will be exchanged by H₃O⁺ ions from the surrounding fluid resulting in Ti-OH layer formation [14 and 16] ( Fig. 14d). Simultaneously, with increasing pH the apatite nucleation is accelerated by increasing the supersaturation of the solution with respect to apatite. Calcium ions are incorporated in the hydrated Ti-OH layer. The positively charged Ca²⁺ may act as nucleation sites for HCA by attaching to negatively charged (PO₄)³⁻ and (CO₃)²⁻ to form Ca-P enriched surface layer (Fig. 14e) which crystallizes to bone-like apatite (HCA) ( Fig. 14f). The apatite formation on chemically treated titanium seems to be similar to that on bioactive glasses. The exchange of Na⁺ from glass by H⁺ or H₃O⁺ from the fluid leads to the formation of a Si-OH layer that was described to induce apatite nucleation via incorporation of Ca²⁺. The consumption of H₃O⁺ causes an increase of pH and apatite will be precipitated [22 and 23].

6. 4. Conclusions

Titanium treated in NaOH can form hydroxycarbonated apatite (HCA) after exposition in SBF. The HCA layer, however, was inhomogeneous and non-uniform even after 20 days in SBF. Acid etching of titanium in HCl under inert atmosphere leads to the formation of a uniform micro-roughened surface that provides improved condition for in situ HCA formation. After alkali treatment in NaOH the apatite nucleation was homogenous and the thickness of precipitated HCA layer increased continuously with time. The treatment of titanium by a two step HCl and subsequent NaOH treatment seems to be a suitable method for providing the titanium surface with bone-bonding ability. The process of apatite formation on chemically treated titanium is supposed to be similar to that on bioactive glasses.

7. Acknowledgements

Financial support by the Grant Agency of the Czech Republic under the project no. GACR 106/97/0957, Deutsche Forschunggemeinschaft, Fonds der chemische Industrie and DAAD is gratefully acknowledged.

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Corresponding author. Department of Materials Science (III), University of Erlangen-Nuremberg, Martensstrasse 5, , Erlangen 91058, , Germany. Tel.: [phone]; fax: [phone]

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