Semi-conducting properties of titanium dioxide surfaces on titanium implants

Biomaterials 30 (2009) 4471–4479

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Biomaterials journal homepage: www.elsevier.com/locate/biomaterials

Semi-conducting properties of titanium dioxide surfaces on titanium implants Ingela U. Petersson a, *, Johanna E.L. Lo¨berg a, b, Anette S. Fredriksson a, Elisabet K. Ahlberg b a b

¨lndal, Sweden Astra Tech AB, SE-431 21 Mo Department of Chemistry, University of Gothenburg, Sweden

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 April 2009 Accepted 19 May 2009 Available online 12 June 2009

The properties of the TiO2 layer on titanium implant surfaces are decisive for good contact with the surrounding tissue. The oxide properties can be deliberately changed by for example chemical etching, ion incorporation or anodisation. In the present study impedance spectroscopy was used to study the semi-conducting properties of the naturally formed oxide for different pre-treatment of the surface. A turned surface was used as a reference and both physical (blasting) and chemical (hydrofluoric acid etching) treatments were investigated. Blasting of a titanium sample introduces defects in the metal surface and the study clearly shows that also the oxide layer contains defects leading to a higher number of charge carriers (increased conductivity) compared with the oxide on the turned surface. The hydrofluoric acid etching of the blasted surface results in an oxide film with even higher conductivity. Indication of the defect oxide structure for fluoride treated samples was also seen when analysing the TiOþ/ Tiþ ratio from ToF-SIMS data. The lowest value of this ratio was obtained for the HF etched sample, indicating a less stoichiometric oxide compared to the other surfaces. This is a result of incorporation of fluoride ions in the oxide, as proven by adsorption studies on a TiO2 suspension. The results were treated in the context of surface complexation and two surface complexes were identified. Our results are discussed in relation to pull-out data on rabbit. The pull-out forces depend primarily on surface roughness but the contribution from the hydrofluoric acid etching might be explained by fluoride ion incorporation and the resulting increase in oxide conductivity. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Titanium implant ToF-SIMS Semi-conducting properties Electrochemistry Surface modifications Impedance spectroscopy

1. Introduction Titanium has long been used as an implant material in different medical applications, showing excellent performance in forming a close contact to the surrounding tissues [1,2], which is attributed the natural oxide film covering the metal [3,4]. Although a great deal of research has been conducted on the use of titanium as an implant material, the mechanism of the implant–tissue interaction is not entirely understood [3,5]. The functional activity of cells close to the implant surface is expected to be influenced by the properties of the implant surface. This cell-surface interaction is likely to exist via an adsorbed layer of water, ions and biomolecules covering the implant surface, rather than a direct interaction [4]. The build-up of this interface with time is a complex process which involves numerous factors. These include not only implant related properties such as material, shape, topography and surface chemistry but also biomechanical properties, surgical technique and patient variables such as bone quality and quantity [6–8].

* Corresponding author. Tel.: þ46 31 776 3261. E-mail address: [email protected] (I.U. Petersson). 0142-9612/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2009.05.042

When an implant is placed within the body, the first contact will be to blood. The implants ability to react with the blood components are thought to be of high importance for early bone formation. Blood platelets role in wound healing is particular important since their activation results in the release of growth factors and cytokines that are known to accelerate wound healing [9]. Blood platelet and their ability to coagulate are one of the most important features to form contact osteogenesis [9]. It is the blood clot that forms the network which osteogenic cells can migrate on towards the implant surface and lay down bone matrix directly on the implant. The secreted matrix is a non-collagenous organic matrix which provides nucleation sites for calcium phosphate mineralisation which induces the formation of collagen [10]. The creation of blood clots involves many of the proteins in blood. One of the proteins is fibrinogen which have been found to adsorb on titanium surfaces in vivo and in vitro [11,12]. Fibrinogen binds to the implant surface and to activated blood platelets and forms a weak aggregation of bonded activated blood platelets. Through protein reactions, which among others involves the protein thrombin, the fibrinogen is transformed into fibrin which is a non-soluble polymer and a stable blood clot is formed [13]. Biomaterials trigger the intrinsic coagulation pathway which is activated by the adsorption

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and binding of plasma proteins to the implant surface [14]. On titanium surfaces blood platelets have been shown to form within a few seconds [15]. In order to achieve a desired biologic response numerous surface modifications of the implant have been developed. These include blasting, wet chemical etching, porous-sintering, anodisation, plasma-spraying, hydroxyapatite coating incorporation of ions in the oxide and a combination of these [16]. There has been a substantial improvement of the physical structure of the surface i.e., increased surface roughness resulting in improved bone response [6,8,9,17–20]. By changing the properties of the oxide, different tissue responses can be induced. One simple oxide modification method is heat treatment which induces a much thicker oxide film, which in turn changes the tissue response. Heat treatment at temperatures <200  C has been shown to influence the adsorption of specific proteins and induces bone growth towards or on the implant surface [16]. A different way to obtain a thicker oxide has been presented by Pan et al. [21]. They performed several studies on titanium dental implants pre-treated with H2O2, where they found that this treatment induces a thicker, more porous titanium oxide with a slightly decrease in oxygen–titanium ratio (O/Ti). Higher amounts of calcium were found in cell studies with H2O2 pre-treated surfaces, indicating an earlier maturation of osteoblasts and mineralisation [21]. Another commonly used technique is anodisation. By applying high potentials and using concentrated electrolyte solutions a porous surface with large pore sizes can be obtained [22]. Improved biocompatibility of titanium implants can also be induced by changes in the chemical composition and oxide structure. The oxide can for example be modified by incorporation of additional ions [23] and a large amount of research has focused on incorporation of mineral ions such as calcium (Ca), phosphorus (P), and sulphur (S), since these are naturally occurring in the human bone [2,16,24]. Sul et al. concluded that both the presence of sulphur and phosphate, on electrochemically oxidized titanium surfaces, resulted in increased removal torque forces and higher bone-to-implant contact compared to a reference sample [25]. Increased removal torque forces for titanium dental implants were also observed with incorporated magnesium ions (Mg2þ), where bond failure of the magnesium modified implants mainly occurred in the bone, while regular titanium implant failure takes place at the bone/implant interface [26]. In a number of papers the effect of fluoride ions on titanium implant surfaces has been reported [6,27–30]. Fluoride ions have been reported to enhance the incorporation of newly formed collagen into the bone matrix and increase the rate of seeding of apatite crystals [30]. By incorporation of fluoride ions into titanium oxide several studies have shown greater pull out and removal torque forces as well as increased thrombogenic properties and increased osteoblast differentiation [6,27,28,30]. Using these different surface modifications substantial improvement in implant performance has been found and related to surface roughness and chemical composition of the oxide surface. A limited number of studies have also considered changes in the electrical properties of the titanium oxide as a consequence of the performed modifications [31–33]. It has for example been concluded that blood compatibility on heart valve materials is improved by the semi-conducting nature of non-stoichiometric titanium oxide coatings [33]. In this investigation a thicker amorphous titanium oxide film showed decreased reaction with the surrounding blood, which induced a more hemocompatible surface than both natural titanium oxide film and Low-Temperature Isotropic Pyrolytic Carbon (LTIC), the most common material for such applications [33]. A surface less reactive to blood components were also created by sputtering titanium oxide with tantalum (Ti(Ta5þ)O2) [31] . On this surface the interfacial surface tension between the blood and the implant

surface was lowered indicating that adsorbed proteins kept their original conformation and function and less platelet adhesion and fibrinogen adsorption was observed [31]. Decreased fibrinogen adsorption was explained by fibrinogens inability to transfer charge to the material [33]. Fibrinogen has an electronic structure similar to that of a semiconductor. When electrons are transferred from the occupied valence band of fibrinogen to the material this induces the decomposition into fibrin monomers [33]. For the artificial heart valves application, a passive oxide is desired, while in the application for dental implants, a reactive oxide should be one way of inducing stimulation of the coagulation pathway. By growing the oxide in different ways the number of defects inside the oxide can be changed, altering the conductivity of the normally passivating film [34], which possibly can be used for tuning the oxide properties for implant applications. In the present investigation semiconducting properties of physically as well as chemically modified titanium samples were investigated by electrochemical impedance spectroscopy. Four differently prepared samples were compared: A) turned reference sample, B) TiO2-blasted with small particles, C) TiO2-blasted with large particles and D) surface C treated in diluted hydrofluoric acid. Sample B and D represent the commercially available TiOblastÔ and OsseoSpeedÔ implant surfaces, respectively. The electrochemical experiments were complemented by ToF-SIMS analysis in order to obtain information about surface composition as a function of preparation method. The obtained data were also extended to include analysis of surface stoichiometry as well as degree of hydration and hydroxylation. Topographic information of the surfaces was obtained by scanning electron microscopy (SEM) and the same technique was also used for measuring the roughness of the samples. A study of the interaction between TiO2 and fluoride ions is also included in this work. 2. Experimental 2.1. Sample preparation Four differently prepared titanium oxide disks, (C.p titanium, grade IV, Ø ¼ 5 mm), were included in this study, A) a turned reference sample, and B–D) three blasted samples. Initially all samples were degreased and rinsed. After that procedure sample A was ready for analysis while the three other samples were continuously processed. Samples B–D was blasted with two different sizes of blasting powder (TiO2), a powder with smaller particles for sample B) and a powder with larger particles for sample C) and D). In addition, sample D was treated in diluted hydrofluoric acid. Sample B and D represent the commercially available TiOblastÔ and OsseoSpeedÔ implant surfaces, respectively. All samples were birradiated before analysis.

2.2. Electrochemical cell All the electrochemical experiments were performed in a conventional threeelectrode cell. The electrolyte solution was bubbled with purified nitrogen for at least 30 min prior to the experiment and this atmosphere was kept constant during the experiment. All potentials are referred to a double-junction electrode AgjAgCl, E ¼ 197 mV with respect to SHE. The inner compartment was filled with a solution saturated with KCl and AgCl, and the outer compartment contained 0.5 M Na2SO4. The working electrode consisted of titanium disks, Ø ¼ 5 mm, with only one side exposed to the electrolyte. A platinum gauze was used as auxiliary electrode and the potential was controlled by an EG&G Princeton Applied Research potentiostat/galvanostat model 273A. The impedance was measured using a Schlumberger frequency response analyser SI1255. All experiments were carried out at room temperature.

2.3. Scanning electron microscopy (SEM) The topography of the four differently prepared surfaces used in this study was characterised using a scanning electron microscope (ESEM XL30, FEI Company). Images were collected using an acceleration potential of 30 kV. The surface roughness of the four surfaces was measured using 3D-SEM in combination with the evaluation program MeXÒ (Alicona Imaging).

I.U. Petersson et al. / Biomaterials 30 (2009) 4471–4479

2.00E-04

I /A

1.60E-04

B

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spectroscopy (ToF-SIMS), using a ToF-SIMS IV instrument (Ion-Tof GmbH, Germany). For the analysis a primary ion beam of 25 keV Biþ was used and the analysis area was 500  500 mm2. Mass spectra were extracted from the outermost surface of the oxide layers.

1.20E-04

2.5. Electrochemical impedance spectroscopy (EIS)

8.00E-05

Impedance measurements were performed at the open circuit potential followed by measurements between 1 and 1 V in 25 mV intervals. In this potential range titanium is not oxidized as can be seen in the LSV shown in Fig. 1. A frequency sweep from 100 kHz to 5 mHz was used with an amplitude of the applied sine wave of 5 mV r.m.s. Capacitance data from these measurements were extracted for the Mott–Schottky analysis.

4.00E-05 0.00E+00 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 E /V Fig. 1. A linear sweep voltammogram between 0.5 V and 1.5 V for sample B in 0.5 M Na2SO4. Sweep rate 2 mV s1. 2.4. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) In order to obtain detailed information about the chemical composition of the included surfaces the samples were analysed with time-of-flight secondary ion mass

3. Results and discussion 3.1. SEM The topography of the four titanium oxide surfaces compared in this work was investigated by SEM and images are presented in Fig. 2. The turned sample has the smoothest surface, where the topography stems mainly from the manufacturing process, Fig. 2a.

Fig. 2. SEM images for (a) turned surface, (b) surface blasted with fine particles, (c) surface blasted with coarse particles, (d) surface (c) etched in HF and (e) an enlargement of figure (d).

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0.8

Table 1 Sa values, flat band potentials and donor densities for samples A–D. Sample

Sa

Efb/V

ND/10

Turned surface (A) Fine blasted (B) Coarse blasted (C) Surface (C) with HF (D)

0.21 0.79 1.31 1.38

0.64 0.65 0.62 0.87

8.37 22.4 27.9 70.5

20

0.7

3.2. ToF-SIMS Surface composition and structure of a biomaterial is always different from its bulk composition and structure surface-sensitive techniques are needed for proper characterisation. SIMS is one of the few surface-sensitive techniques were hydrogen can be detected and mass spectra are produces from the outer 10–20 Å of the material. This technique is therefore capable of providing detailed information about the chemical composition of a surface. However, quantitative analysis is difficult because the secondary ion yield is not directly proportional to the concentration and also the matrix itself effects the analysis. It is therefore common to plot ratios of different signals to obtain comparable information. In the present study, ToF-SIMS data were used to analyse the surface stoichiometry (TiOþ/Tiþ), degree of hydroxylation (TiOHþ/TiOþ) and hydration (TiHþ/Tiþ), as a function of preparation method. Detailed studies on the hydroxylation of TiO2 have previously been performed using SIMS by measuring the reactivity of the titanium oxide surface towards water [37–39]. SIMS was used in these studies in a static, low damaging mode which provides information on surface species by the study of relative intensities of cluster ions. The surface stoichiometry was changed by argon bombardments inducing preferential sputtering of oxygen and was measured using the TiOþ/Tiþ ratio. The reactivity towards water is largely dependent on surface stoichiometry with faster rates as the surface is less stoichiometric. The reaction taking place leads to a hydroxylated surface. The oxygen ion in the oxide was shown to take part in the formation of the hydroxide groups [36–38], which is in general agreement with the common view in surface complexation, see Section 3.4. The hydroxide coverage on the surface was measured by the TiOHþ/TiOþ ratio. The influence of traces of fluoride ions on the hydroxylation was also studied. The adsorption of fluoride was not intentional but a result of pollution in the vacuum system. The results showed that the hydroxylation rate increases in the presence of fluoride ions in the oxide [37]. In the present study the positive fragments were measured and following the suggestions in the literature different ratios were plotted to obtain information on the surface properties. In Fig. 3 the surface stoichiometry ratio (TiOþ/Tiþ) is plotted for the different

TiOH+/TiO+

TiO+/Ti+ C D

50TiH+/50Ti+

0.6 0.5 0.4

When blasting the samples the surface area increases significantly, Fig. 2b–d. In Fig. 2b a blasting powder with small particles has been used while in Fig. 2c and d the size of the used blasting particles are larger. In Fig. 2d and e the effect of chemical treatment is clearly observed, resulting in additional roughness on a smaller scale compared to the blasting procedure. It is clear from the SEM images that the topography of these four samples differs. The roughness of the four samples was determined using 3D-SEM in combination with the analysis software MexÒ (Alicona Imaging), Table 1. The most common way of describing 3D surface roughness on dental implants is the Sa parameter, which was also used in this study. In agreement with the SEM images the Sa values increase significantly for the blasted samples. The Sa value for surfaces C and D are the same within experimental error, thus the influence of chemical treatment cannot be revealed using 3D-SEM. A more thorough description of roughness parameters will be discussed in a forthcoming paper [35].

A

0.3 0.2

A

C

D A

C

D

0.1 0 Fig. 3. Surface stoichiometry (TiOþ/Tiþ), surface hydroxylation (TiOHþ/TiOþ) and surface hydration (TiHþ/Tiþ) for the three different samples A, C and D calculated from ToF-SIMS data.

samples together with the ratio showing the hydroxylation of the surface (TiOHþ/TiOþ). The TiOþ/Tiþ ratio varies from 0.4 to 0.7 with the lowest value obtained for the HF treated sample (D). These values are similar to the values found in the literature [36–38]. The low value for sample D indicates that this surface is less stoichiometric compared to the other surfaces. This is probably a result of incorporation of fluoride ions in the oxide. In aqueous solution, flouride ions can take part in an exchange reaction (Eq. (8)) where surface hydroxide ions are replaced. Also a surface complex without exchange of the hydroxide ion is formed, see Section 3.4. The TiOHþ/TiOþ ratio seems to be the same irrespective of surface treatment with an exception for the turned reference sample (A). Thus the hydroxylation of the surface is not influenced by the chemical treatment of sample D even though the surface stoichiometry is changed. This indicates that the amount of fluoride ions on the surface is low, in agreement with the surface complexation results [29] and XPS analysis [39]. Since fluorine is very electronegative the space charge layer at the TiO2 surface is expected to be influenced and indeed, the impedance measurement clearly show an increase in the oxide conductivity for the HF treated surface (D), see Section 3.3. In Fig. 3 also the ratio related to hydrated species is shown, TiHþ/ Tiþ. The m/z ¼ 51 signal was used for this analysis since no other species interfere using the 50Ti isotope. For all other isotopes the TiH signal would overlap with the Ti signal. The presence of TiHþ could be related to the presence of titanium hydrides at the oxide/ metal interface. However, the ratio is very low and since the pure TiO2-sample also display a TiHþ signal it is not likely that the origin is the hydride. TiH fragments may be produced during sputtering if hydrogen containing species are present at the surface.

3.3. Impedance spectroscopy and Mott–Schottky analysis A typical impedance diagram is shown in Fig. 4 in the form of a Bode plot. The measurements were made at open circuit potential in the frequency range 100 kHz to 5 mHz. At the highest frequencies the impedance is constant and related to the resistance in solution, Rsol. In the intermediate frequency range the log(Z) is linearly related to log(u) showing capacitive behaviour. The slope for an ideal capacitor is 1 but for real systems frequency dispersion is commonly observed. The non ideality can be simulated using a constant phase element, CPE [40]. At the lowest frequencies the impedance again approaches a resistive behaviour where the resistance is the sum of the solution resistance and the resistance of the thin oxide layer, Roxide. The resistance of the oxide is inversely related

4 3 2 1 0

-3

-2

-1

0

1 2 log (f/Hz)

3

4

5

6

3.0

to the conductivity. Since the resistance is high the conductivity of the oxide layer is low. Assuming that the oxide is insulating the interface can be described as a parallel plate capacitor, Eq. (1),

3r 30 A

(1)

d

where C is the capacitance, 3r the dielectric constant of TiO2, 30 the dielectric constant of vacuum, A is the surface area and d the thickness of the oxide layer. However, for a semiconductor, the space charge region of the oxide depends on potential and the total capacitance can be written, Eq. (2),

1 1 1 1 ¼ þ þ C tot C oxide C sc C dl

(2)

where Csc is the space charge capacitance and Cdl is the double layer capacitance. The double layer capacitance for the oxide water interface is in the order of 100 mF cm2 [41,42]. Since the capacitances are coupled inversely the smallest capacitance will dominate and therefore the contribution from the double layer capacitance can be ignored. The electric properties of the oxide film were determined using the Mott–Schottky relationship, Eq. (3):

1 C 2sc

 ¼

2 3r 30 eN D



E  E fb 

kT e



2.5

8 6 4 2 0

4475

A

A

B

C

D

2.0

B

1.5

C

1.0

D

0.5

Fig. 4. Impedance spectra for the Ti/TiO2/solution interface at the open circuit potential in the frequency range 100 kHz to 5 mHz. Impedance spectra is also shown in a restricted frequency region as a function of potential. The capacitance increases as the potential becomes less positive showing a n-type behaviour.

C oxide ¼

ND/1021

5

4.0 3.5

φ-

log (Z/Ω)

6

100 90 80 70 60 50 40 30 20 10 0

C-2/F-2cm2

7

x109

I.U. Petersson et al. / Biomaterials 30 (2009) 4471–4479

(3)

where E is the applied potential, Efb the flat band potential, ND the number of charge carriers, k the Boltzmann’s constant, T the temperature and e the charge of the electron. The impedance was measured as a function of potential from 1 V to 1 V with 25 mV steps. At each potential a delay time of 300 s was used to ensure stable conditions before the impedance spectra was taken. A restricted frequency range, 100–0.1 Hz, was used where the impedance was mainly capacitive, Fig. 4. The data were fitted to an equivalent circuit containing a constant phase element coupled in parallel with a resistor. In this procedure, the frequency dispersion is handled in the fitting procedure and the value obtained from the CPE element was used as the capacitance value. The Mott–Schootky plots for the four different surface studied in the present paper are given in Fig. 5. For all surfaces a linear relationship is observed and the number of charge carriers can be calculated from the slope while the flat band potential can be calculated from the intercept (Table 1). The turned surface has been taken as a reference for which the geometric area was used. To calculate the other surface areas, the Sa values were used and

0.0 -0.75 -0.5

-0.5

-0.25 -1E-15 0.25

0.5

0.75

1

E/V vs. Ag/AgCl

Fig. 5. Mott–Schottky plots for (A) turned surface, (B) surface blasted with fine particles, (C) surface blasted with coarse particles and (D) surface (C) etched in HF. The inset figure show the number of charge carriers for the different surfaces.

normalised by the Sa value for the turned surface. The number of charge carriers calculated according to this procedure is relatively high (Fig. 5 and Table 1). The lowest number of charge carriers was obtained for the turned surface. The blasting introduces defects in the oxide structure and the number of charge carriers is about three times larger than for the turned surface. A further increase in ND is observed for the HF treated samples, indicating an incorporation of fluoride ions in the oxide. The donor density depends on the nature of the oxide and the form of the electrode and vary from w1018 for bulk oxides to w1022 for highly defective oxides [7,43–49]. The flat band potential is in good agreement with literature values for the turned and the blasted surfaces. However, for the HF etched surface the flat band potential is considerably more negative. The flat band potential varies with the pH of solution according to Eq. (4):

E fb ¼ E 0fb 

RTlnð10Þ pH F

(4)

where E0fb is the flat band potential in a 1 M acid solution. The value of E0fb has recently been summarised and varies with the crystallographic structure of the oxide and the nature of the electrode material [43,45,46]. The high value of the charge carrier concentration determined in the present work raises the question of the validity of the Mott– Schottky relationship for thin films. The thickness of the native TiO2 layer has been reported to be in the range 2–5 nm [7,43,44]. In the present work the native oxide thickness was determined to be 5.4 nm using anodisation to different potentials and extrapolating to zero applied potential. For the Mott–Schottky relationship to be valid the oxide thickness must be larger than the thickness of the space charge layer. The thickness of the space charge layer depends on potential and can be calculated according to Eq. (5) [45]

dsc ¼

23r 30 ðE  Efb  kT eÞ eND

!1=2 (5)

By using the values for Efb and ND obtained experimentally the thickness of the space charge region can be calculated in the potential region used. Since the charge density is high the thickness of the space charge layer is small, <3.5 nm, and therefore the thickness of the oxide layer will not limit the capacitance value obtained. The fact that the Mott–Schottky plot is linear over a potential region of more than 1 V also shows that the Mott– Schottky relationship holds in the present case.

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Scheme 1. Schematic picture of hydroxylation of an oxide surface in water (A) and the amphoteric nature of the surface (B).

3.4. Surface complexation When an oxide gets in contact with water, surface hydroxide groups are formed. These surface groups can adsorb and/or desorbs protons which will affect the charge on the surface (Scheme 1). Most metal oxides are amphoteric and can, depending on the pH, become either positively charged by adsorption of protons or negatively charged by desorption of a proton. At low pH values the surface will thus be positively charged and at high pH negatively charged. The pH of zero charge, pHpzc, depends on the nature of the oxide and can be determined from potentiometric titrations or electrokinetic methods such as electroacoustics [50]. The surface hydroxyl groups are coordinated to one, two, or three underlying titanium atoms. The configurations of the different types of surface groups depend on the crystal structure of the oxide and not all of the groups exhibit acid base properties in the pH range accessible [51,52]. Over the last decades, surface complexation models have been developed to describe the chemical reactions that occur at oxide surfaces. These surface reactions are described in the same way as reactions in solution and it is possible to define surface equilibrium constants. However, the reactions at the surface are affected by the surface charge and therefore, a physical description of the liquid outside the charged surface is also necessary [53]. In the present work the surface properties of TiO2 and the interaction between TiO2 and fluoride ions, F, are discussed to get

0.15

σ0/cm2

0.1 0.05 0

1=2

hXOH2 4hXOH1=2 þ Hþ h

Ka ¼

i

h

gXOH hXOH1=2 gHþ Hþ h

gXOH2 hXOH1=2 2

4

5

6

7

8

pH

Fig. 6. Two different surface charge curves obtained from potentiometric titrations of TiO2 (P25) in 0.1 M NaNO3.

i

i

(6)

hXOH1/2 and hXOH1/2 are the protonated and deprotonated 2 surface groups, respectively. Ka is the surface equilibrium constant and g the activity coefficient. The concentration of protons near the surface will be different than in the bulk due to electrostatic effects and Ka will change with pH. It is possible to treat the electrostatic effects separately using electrical double layer theory and an intrinsic equilibrium constant, Kint is defined, which is independent of charge. The adsorption of fluoride on TiO2 (P25) is shown in Fig. 7 as % F adsorbed vs. pH. The fluoride adsorption starts around pH 8 and reaches full coverage for pH values less than 3. In general, specific sorption of anions on oxides often occurs via ligand exchange reactions in which hydroxyl groups at the surface are replaced by the sorbing anions, Eq. (7).

hTiOH þ F 4hTiF þ OH

-0.05 -0.1

a better understanding of the proposed beneficial effects of flouride ions on the osseointegration process. Surface charge curves obtained from potentiometric titration of TiO2 (P25) in 0.1 M NaNO3 are shown in Fig. 6. The pH interval is restricted to 4–8 where experimental data are reliable. The surface charge curves were calculated using pHpzc ¼ 6.9 as determined by electroacoustic measurements [29]. The 1-pK formalism was used to model the acid base properties [54], where the proton affinity constant can be directly determined experimentally by measuring pHpzc (Eq. (6))

pK

(7)

A pK value of 6.2 has been reported for such a complexation by Herrmann et al. [55] who also reported nearly equivalent amounts of strongly and weakly acidic groups on the surface and a pzc of 6.6 for TiO2 (P25). The fluoride ion adsorption was best modelled with the 1-pK Basic Stern Model (BSM) in the absence of electrolyte ion adsorption. Two surface reactions were defined to describe the sorption process, i) an ion exchange reaction which is accompanied by an increase in pH and ii) adsorption of fluoride ions on the

I.U. Petersson et al. / Biomaterials 30 (2009) 4471–4479

% adsorbed F-

100 80 60 40 20 0

2

3

4

5 pH

6

7

8

Fig. 7. The adsorption of fluoride ions presented as percent adsorbed F vs. pH.

surface without a change in pH [29]. From Fig. 7 it can be seen that the adsorption of fluoride starts around pH 8, where the overall surface charge is still negative. This surface complexation reaction does not include co-adsorption of Hþ and can be described by reaction Eq. (8):

hTiOH þ F 4hTiOHF

(8)

Below pHpzc the surface is protonated and positively charged and the reaction can be described by purely electrostatic interactions, Eq. (9).  hTiOHþ 2 þ F 4hTiOH2 F

(9)

The effect of fluoride ions on the properties of TiO2 has previously been studied in relation to photocatalytic reactions [57,58]. Two different properties were explored, doping by fluoride ions forming a bulk material where the fluoride ion replaces the oxygen ion (Ti2xFx) and surface complexation involving ligand exchange with surface hydroxyl groups [55], Eq. (7). Doping with fluoride ions was shown to improve photocatalytic reactivity and crystallinity. The presence of fluoride ions in the lattice also influences the electric properties by creating fewer anion vacancies [57]. On the other hand, the ligand exchange reaction mainly influences the surface properties, for example the value of pHpzc. In the presence of fluoride ions the pzc shifts to lower pH values and the positive charge on the surface at acidic pH is much reduced since the hTi–OHþ 2 groups are replace by hTi–F groups. It is clear from the experimental data on TiO2 (P25) that the exchange reaction mechanism can explain the fluoride adsorption on HF treated implants, which means that fluoride ions replace hydroxide ions at the implant surface. From the adsorption curve in Fig. 7 it can be observed that, at the low pH values, all added fluoride ions are adsorbed at the oxide surface provided there is enough oxide surfaces to adsorb on. However, at pH 7 approximately 85–90% of the adsorbed fluoride ions are released according to the same graph. As implants are rinsed in water following the hydrofluoric acid treatment with a rise in pH as a result, it can be expected that there will be a loss of fluoride from the surface. The fluoride content, remaining on the surface oxide show fluoride levels of approximately 1 at%. A fluoride signal of around 1 at% means approximately 5% of all active sites or that every 20th site is occupied by an exchanged fluoride ion in the oxide surface. 3.5. Relevance to dental implants Ellingsen et al. [58] have investigated the bone attachment to titanium implants with surfaces that had been modified with respect to surface roughness and chemistry. It was concluded that an increase in the size of the TiO2 blasting particles improved the bone-to-implant attachment after an 8 week healing period in

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rabbit. Further improvement was observed by hydrofluoric acid treatment. A shorter healing time was envisaged due to the improved bone attachment [58]. In the present study it was concluded that blasting of titanium samples introduces defects in the metal surface which means that even the oxide obtain more defects. The defects generate increase in conductivity in the material which can be one possible explanation for the increased pull-out forces seen in the in vivo study [58]. In the present investigation, it was also seen that fluoride ions are incorporated into the TiO2 structure lowering the surface stoichiometry and increasing the number of charge carriers in the oxide film. Indication of a defect oxide structure for the fluoride treated samples compared to turned and blasted was supported by the ToF-SIMS analysis. These findings interpreted together with earlier in vivo results and literature [31–33] indicates that alteration of the semi-conducting properties of the oxide could be a tool for inducing a wanted biological response. In the newly formed wound in the implant insertion process the pH value is low compared to the normal physiological pH and consequently the implant surface is positively charged [59]. The positive surface charge on the TiO2 surface decreases in the presence of fluoride ions since the hTi–OHþ 2 groups are replace by hTi–F groups. Thus the electrostatic repulsion is decreased and cations like Ca2þ and positively charged plasma proteins such as fibrinogen [60] can adsorb more readily at the surface, which is important for the use of titanium as implant material. The adsorption of calcium on rutile has been studied in a large temperature range [61] and it has been shown that adsorption starts at about one pH unit above the pHpzc. Extrapolating these findings to the conditions for implant insertion, shows that fluoride ions are beneficial for the adsorption of calcium ions on the surface. In addition, the surface group hTi–F act as an electron trapping site [56]. This may have beneficial effects since radicals formed on insertion of the titanium implant can be captured, facilitating the healing process. An enhanced adsorption of positively charged plasma proteins is beneficial for the adsorption of blood platelets [62] and formation of blood clots and consequently the contact osteogenesis is facilitated [9]. The results from the present investigation may also provide an explanation as to the fate of fluoride ions when the implants are introduced into bone. At the implant installation there is likely a pH decrease due to inflammation in the surrounding tissue compared to the normal value in the body. At low pH values, the fluoride ions are adsorbed at the oxide surface by a Columbic force i.e., an electrostatic force due to the positively charged surface complexes. Since the implants are rinsed in water during the manufacturing process, the excess of fluoride ions are probably already released from the surface. Depending on how the remaining fluoride ions are bound to the surface no further release of fluoride ions are likely to take place during the pH increase following the healing process.

4. Conclusions The results from this study show that the electric properties of the oxide film are changed both by physical surface treatment (blasting) and chemical treatment (HF etching). Blasting introduces defects in the oxide and the conductivity increases compared to a turned surface. In the chemical treatment, fluoride ions are incorporated in the surface of TiO2. This results in lower surface stoichiometry and a higher number of charge carriers compared with the same surface without HF etching. The beneficial effects of HF treated surface, reported on in the literature, can possibly be explained by the combined effect of

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