SEM, Scanning Auger and XPS characterization of chemically pretreated Ti surfaces intended for biomedical applications

Materials Chemistry and Physics 104 (2007) 93–97

SEM, Scanning Auger and XPS characterization of chemically pretreated Ti surfaces intended for biomedical applications M. Pisarek a,b,∗ , M. Lewandowska a , A. Roguska a,b , K.J. Kurzydłowski a , M. Janik-Czachor b a

Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland b Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Received 2 August 2006; received in revised form 5 February 2007; accepted 24 February 2007

Abstract Titanium is known as a biocompatible metal characterized by biological and corrosion immunity and good mechanical properties, including a high fracture toughness. In a variety of environments, this metal undergoes “natural” oxidation which determine its resistance to corrosion. It can also be exposed to chemical treatments in acidic or alkaline solutions which “enforces” chemical and morphological changes of Ti surface. Those methods, if well controlled, may increase the effective Ti surface area, making it more biocompatible. However, the morphological and chemical factors responsible for their interactions with biological cells are still not well known. The aim of this work was to compare surface chemical and morphological changes introduced by commonly used aqueous NaOH pretreatment with those occurring in a new “piranha” acidic solution. Particular attention was paid to possible changes which may be decisive for the biocompatibility of the Ti-elements subjected to these surface modifications. Surface analytical techniques such as Auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS) combined with Ar+ ion sputtering allowed us to investigate in detail the chemical composition of Ti oxide layers. SEM examinations provided morphological characterization of the surface of Ti samples. The results revealed large difference in morphology of Ti surfaces pretreated with different procedures whereas only minor difference in the chemistry of the surfaces were detected. © 2007 Elsevier B.V. All rights reserved. Keywords: Biomaterials; Surface chemical treatment; Auger electron spectroscopy (AES); X-ray photoelectron spectroscopy (XPS), SEM

1. Introduction Metallic biomaterials are a subject of extensive investigations in many scientific centers [1–4], worldwide. However, the morphological and chemical factors responsible for their interactions with human cells are still not well known. It has been only revealed that the contact of cells with materials of different morphology and chemistry results in a modification of their shape and bioactivity [5–9]. Titanium is known as a biocompatible metal [10,11] characterized by biological and corrosion immunity, as well as a high fracture toughness. Therefore, it is used in applications for load-bearing bone substitutes such as hip joints and dental roots. For metallic implants, high mechanical stability and cor-

∗ Corresponding author at: Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland. Tel.: +48 22 343 3325; fax: +48 22 343 3333/632 5276. E-mail address: [email protected] (M. Pisarek).

0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.088

rosion resistance are required to ensure negligible ion release from the implant into the human body. Ti provides an excellent corrosion resistance, due to passive film which is stable in different media within a large pH range. Some authors are of the opinion that a thin oxide film (a few nm) formed easily and spontaneously on Ti surfaces reduces a tendency of infection in an area of contact between the metal and the living cells [12]. Electrochemical measurements (polarization and impedance in simulated body solutions like Ringer’s or Hank’s solution) confirm a spontaneous passivation of Ti-based alloys [13,14]. Moreover, Ti possess quite stable passivity even in Cl− containing solutions being not susceptible to pitting corrosion at the redox potentials encountered in body fluids (stable pitting of Ti was detected only at potentials well above these redox potentials [15]). A stable fixation of Ti to the surrounding bone have been long considered as a problem in clinical applications. The titanium treated chemically was found to bond and to integrate with living bone by forming a bonelike apatite layer (via incorporation of Ca, P, S into the thin oxide layer) on its surface in the

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body [16,17]. High biocompatibility of Ti is achieved by simply soaking the metal into aqueous NaOH solution followed by an appropriate heat treatment [18,19]. This pretreatment brings about some chemical [20,21] and morphological [22,23] changes to the surface of Ti [24] and its alloys [25], which apparently are crucial for achieving biocompatibility. However, other chemical treatments resulting in distinct chemical and morphological changes of the Ti surface may also successfully introduce appropriate modifications of the Ti surface thus making it even more biocompatible [11,26,27]. Recent results suggest that such modifications may occur also in acidic aqueous solutions. The aim of this work was to compare chemical and morphological changes introduced to Ti surface by a well known aqueous NaOH pretreatment with those occurring in a new “piranha” [28–30] acidic solution. A pure titanium mesh (Tiomesh) which has been commonly used in clinical applications was used for the investigations. High resolution surface analytical methods like Auger electron spectroscopy (AES), Xray photoelectron spectroscopy (XPS) and SEM were applied to investigate the chemistry of the oxides obtained in the different treatments. Attention was paid to possible changes which may be decisive for the biocompatibility achieved. 2. Experimental 2.1. Materials For the investigations, a pure titanium was used in two different forms: (1) a commercially available titanium mesh (Tiomesh) commonly used in clinical applications with the mesh diameter of 1 mm, 0.2 mm thickness and (2) a pure titanium (Grade 2) rod Ø6 mm cut into cylinders, 2 mm thickness. The latter was used only for XPS investigations. From the 30 mm × 30 mm mesh, the samples in the form of 6 mm discs were cut. All the samples were mechanically polished, and rinsed in distilled water.

2.2. Pretreatments The samples were modified by using two different solutions: • soaking in 5.0 M aqueous NaOH at 60 ◦ C for 24 h; the samples were then gently washed in distilled water, at 40 ◦ C for 48 h; • etching in “piranha” solution (98% H2 SO4 + 30% H2 O2 mixture, the volume ratio 1:1). This type of pretreatment was carried out at room temperature (RT) for 4 h or in boiling solution (BS) for 10 min. The samples, after soaking in aqueous NaOH and washing in water, were additionally heat treated with a rate of 5 ◦ C h−1 up to 600 ◦ C, annealed at this temperature for 1 h, and then cooled with furnace. The “piranha” treated samples were not heat treated.

2.3. Surface analytical investigations Subtle changes in modified surface layer of titanium samples were examined using the Auger electron microanalysis and photoelectron spectroscopy. Auger and XPS studies provide “surface” information about the few uppermost nanometers of the samples [33–35]. Therefore, Auger microprobe analyzer, Microlab 350 (Thermo Electron) was applied in order to monitor surface morphology and local chemical composition, utilizing the AES and XPS functions of the Microlab with a lateral resolution of about 20 nm for AES and several mm for XPS. The chemical state of surface species was identified using the high-energy resolution of the Auger spectrometer (the energy resolution of the spherical sector analyzer is continuously variable between 0.6 and 0.06%) and of

XPS spectrometer (the maximum energy resolution is 0.83 eV). The appropriate standards for AES and XPS reference spectra were also used. XPS spectra were excited using Al K␣ (hν = 1486.6 eV) radiation as a source. Survey spectra and high resolution spectra were recorded using 150 and 50 eV pass energy. A linear or Shirley background subtraction was made to obtain XPS signal intensity. The peaks were fitted using an asymmetric Gaussian/Lorentzian mixed function. The measured binding energies were corrected referring to energy of C1s at 285 eV. An advantage based data system software (Version 3.44) was used for data acquisition and processing.

3. Results 3.1. Surface morphology observation Fig. 1 presents typical morphology of Ti before (Fig. 1a) and after the pretreatments (Fig. 1b–d). The SEM images suggest the following. Immersion in 5 M NaOH and subsequent heating up to 600 ◦ C result in formation of a developed morphology similar to “honeycomb” (Fig. 1b). The surface layer seems also quite porous. Pores of Ø0.1–0.2 ␮m are well visible in the “honeycomb” islands, surrounded by elongated depressions and cracks, several ␮m long and 0.4 ␮m wide. After pretreatment in “piranha” solution at room temperature or in boiling “piranha” solution (Fig. 1c and d), the morphology is quite different and less developed than that produced in the course of NaOH pretreatment. At an elevated temperature, a high population of shallow depressions Ø0.5–1 ␮m is formed. 3.2. Surface analytical investigations (AES) Microlab 350 made it possible to investigate meshes made of Ti. Primary field emission electron beam (some 12 nm in diameter) was able to excite the thin Ti metal surface between two meshes. Results of these investigations for not pretreated mesh and for the meshes pretreated in both solutions are presented in Figs. 1 and 2. A set of AES survey spectra placed in the lower part of Fig. 1 shows that signals from Ti and O are well distinguishable. This implies that Ti is oxidized on all the surfaces investigated. Notably, for a mesh pretreated in NaOH the relative peak ratio of O KLL signal to Ti LMM signal is much higher than those for all other samples (compare Table 1). This would suggest that the NaOH pretreatment provides more oxidized species at the very surface than the “piranha” pretreatment. Contamination with C and a beneficial incorporation of Ca is also noted for the NaOH pretreatment, which apparently results from the preparation procedure. Table 1 Auger peak height ratio of Ti LMM and O KLL signals estimated from the differentiated Auger spectra Samples’ pretreatment

Ti LM3/Ti LM2

O KL1/Ti LM2

As-received “Piranha”, RT “Piranha”, BS Aqueous NaOH solution

1.6 1.4 1.4 1.9

1.3 1.1 1.0 3.4

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Fig. 1. SEM images of typical morphologies of a commercially available Ti mesh subjected to different chemical modifications: (a) as-received, (b) after pretreatment in NaOH, (c) after “piranha” pretreatment at room temperature, (d) after “piranha” pretreatment in boiling solution. Below: typical Auger survey spectra taken at Ti mesh surface chemically modified in different solutions, respectively (a) as-received, (b) after pretreatment in NaOH, (c) after “piranha” pretreatment at room temperature and (d) after “piranha” pretreatment in boiling solution.

Fig. 2 presents a set of the high resolution Ti LMM spectra showing Ti LMM signals for the samples under investigation, as well as Ti metallic and TiO2 oxide reference spectra. Careful inspection of all those spectra confirms that there are Ti-oxides on the surface of all meshes investigated before and after the NaOH or “piranha” pretreatment. Comparison of these spectra does not reveal any very distinct difference in their shape suggesting that similar Ti-oxides are formed at the surface as a result of each of the pretreatments.

3.3. Surface analytical investigations (XPS) In order to get a better insight into the chemical state of titanium in the Ti-oxygen surface compounds, additional XPS measurements were performed on aqueous NaOH and “piranha” treated pure Ti rods. Unfortunately, it was not possible to carry out these measurements on meshes, since a large Ti area was required to get a sufficient XPS signal. Contamination with carbon of the samples, in partic-

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M. Pisarek et al. / Materials Chemistry and Physics 104 (2007) 93–97 Table 2 Ti2p3/2 and O1s binding energies as measured from the corrected XPS spectra and the surface Ti O compounds evaluated from a deconvolution procedure

Fig. 2. High resolution Auger spectra (energy analyzer resolution 0.1 eV within the energy range 350–440 eV) taken at Ti mesh surface chemically modified in different solutions: as-received, after “piranha” pretreatment at room temperature (RT), after “piranha” pretreatment in boiling solution (BS), after pretreatment in NaOH. For a comparison Ti LMM reference spectra for metallic Ti and titanium oxide TiOx are given, as indicated in the figure.

ular for those pretreated with “piranha”, made the analysis difficult. Fig. 3 demonstrates the corresponding Ti2p spectra before Ar+ ion sputtering. These results confirm that TiO2 is the main component of the chemically modified Ti surface. Deconvolution of the Ti2p signals suggested, some lower Ti-oxides are also present for pretreated samples, see Table 2. NaOH treated samples produced an oxide film which was rather quite stable against Ar+ ion bombardment. The spectra did not change much after sputtering. Deconvolution of the peaks after sputtering suggested appearance of a small amount of TiO species together with TiO2 . However, this could be a result of a preferential sputtering and other well known probing effects occurring in the course of the surface analytical measurement, as already reported elsewhere [31,32]. One has to mention, that

Samples’ pretreatment

Binding energy (eV)

Compound

O1s

Ti2p3/2

Main component

Minor component

As-received

530.3

458.9

TiO2

–

“Piranha”, RT

529.9

458.3 456.2 454.6

TiO2

458.5

TiO2

458.8

TiO2

“Piranha”, BS

530

Aqueous NaOH solution

530.1

457.8

Ti2 O3 TiO – Ti2 O3

the appreciable stability of the above oxide films against the Ar+ ion sputtering may be a result of the heat treatment applied. This may produce rutile from anatase which is stable at room temperature. The high temperature modification of TiO2 (rutile) presents a shorter Ti O interatomic distance, thus a stronger Ti O bond. “Piranha”-treated Ti shows XPS spectra, where the binding energies of Ti2p3/2 and Ti2p1/2 of the two spectra are lower than those of NaOH-treated specimen. This may reflect a surface oxide (probably anatase) being more prone to the modifications introduced by Ar+ ion sputtering, than that produced by the NaOH pretreatment. Table 2 presents binding energies of Ti2p3/2 and O1s electrons for all the samples investigated. The treated Ti surface exhibited in all cases a clear O1s signal at 529.9–530.3 eV ascribed to the Ti O bond, due to presence of titanium oxide on its surface. Analysis of the XPS data suggest an absence of Ti OH bonds at the surface. While presence of Ti OH bonds is considered to be crucial for biocompatibility of Ti (they are able to incorporate immediately Ca2+ from the human fluid to form a calcium titanate which then incorporates PO4 −3 and converts into apatite [11]) particular attention was paid to its presence on the modified surface. Apparently, the Ti OH containing species, notably Ti(OH)4 which is not a stable compound [36], may only be formed in situ, in a simulated body fluid, or in vivo, in the presence of blood plasma. For “piranha”-treated samples, e.g. the higher binding energy shoulder was present in O1s signal. In particular, it was distinct for “piranha” pretreatment at room temperature. This may suggest the presence of some OH groups [37] on the surface or some oxygen containing carbon compounds, which however, are not bound to Ti, as a corresponding feature in the Ti2p signal was not detected. This point requires, however, further investigations. 4. Discussion

Fig. 3. Ti2p XPS spectra taken at Ti rod surface chemically modified in different solutions: as-received, after “piranha” pretreatment at room temperature (RT), after “piranha” pretreatment in boiling solution (BS), after pretreatment in NaOH.

The results have shown that chemical surface modifications in both alkaline and acidic solutions result in significant changes in topography/morphology and only minor difference in chemical composition. Both these factors strongly influence the cells’ behavior.

M. Pisarek et al. / Materials Chemistry and Physics 104 (2007) 93–97

Surface composition is considered to influence the properties of adherent cells [7]. Preliminary qualitative observations of the MG 63 osteoblast-like cells in the direct contact with the surface of the investigated materials show that both the not pretreated titanium surface and the surfaces modified by the methods used in this work are all well tolerated by the living cells [38]. Within the experimental scatter, all the surfaces provided good substrate for proliferation and growth of the cells [38]. Quantitative measurements (cell number, viability and differentiation) will be a subject of further research. However, one has to take into account that for a successful bone fixation, the morphology of the implant and thus the specific surface area of a contact implant-bone may be of a great importance. Moreover, the recent studies [6] show that osteoblast-like cells tend to attach more readily to surface with a rougher microtopography. The difference in surface topography (surface roughness) after various chemical modifications may be caused by different chemical reactions leading to oxide formation. In the case of “piranha” solution, the oxidizing factor is H2 O2 , whereas for alkaline solution-H2 O (corrosion under water decomposition, see ref. [39]). The kinetics of these reactions are various depending on temperature and environment. Thus, the different topography of surface may result from various oxide thickness. 5. Conclusions (1) SEM investigations reveal large difference in the morphology of the surface of mesh made of Ti after chemical modifications applied (“piranha” pretreatment at room temperature, “piranha” pretreatment in boiling solution, NaOH pretreatment). (2) AES investigations have shown that all the chemical modifications of Ti mesh surfaces resulted in formation of a top TiO2 . High resolution Auger spectra exhibited subtle difference, depending upon pretreatment, which may reflect minor difference in chemistry or structure of the oxide layers formed. (3) XPS confirmed the AES results. Moreover, stability of TiO2 during sputtering was found for samples modified with aqueous NaOH solution or in “piranha” boiling solution. (4) Above results from one side, and those of preliminary biological tests—from the other suggest that, in this case, investigated chemistry of a modified Ti-mesh surface may play a more important role in determining early cell response than the type of topography. Acknowledgments Surface characterizations were performed using a Microlab 350 located at the Physical Chemistry of Materials Center of the Institute of Physical Chemistry, PAS, and of the Faculty of Materials Science and Engineering, WUT. This work was financially supported by the Polish Ministry of Science and Higher Education (Grant No. 3 T08A 01829), the Institute of Physical Chemistry, PAS, and by the Foundation for Polish Science through a generous fellowship for M. Pisarek.

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