Characterization of surface roughness in titanium dental implants measured with scanning tunnelling microscopy at atmospheric pressure

Characterizationof surface roughness in titaniumdental implantsmeasured with scanningtunnellingmicroscopy at atmosphericpressure AM. Barb, N.Garcia, R.Miranda, L.Vhzquez Universided Aot6noma de Madrid, Department0

de Fisica Fundamental, C-III. Cantoblanco, 28049

Madrid, Spain

C.Aparicio," J.Oliv6" Clinica de Pn5tesis Osteointegradas de Barcelona. Rda. General Mitre 90, Barcelona 080 17, Spain

J.Lausmaa Deparrment of Physics, Chalmers University of Technology, S-41296 Biotechnology, Box 33053 S-400 33, Sweden (Received 30 July 1985; revised 19 December 1985)

Gothenburg, Sweden

and The Institute

for Applied

Characterization of the surface topography of implant materials is important for understanding tissue response. We have measured, for the first time, the topography of titanium surfaces used in osseointegrated dental implants. Scanning tunnellling microscopy (STM) which provides 3D real space images was used. In addition to clinical samples, electropolished and anodically oxidized surfaces were also measured. Clinical samples are rather inhomogeneous in character showing grooves and steps with a maximum depth of 0.11 urn. Micropores with an average diameter of about 30 nm are also present. Electropolished samples are rather homogeneous and very smooth, showing steps of 1 to 5 nm in height. The measurements were performed under atmospheric conditions at a resolution in the subnanometer range. Keywords: Dental materials, scanning tunnelling microscopy (STM), titanium implants, osseointegration, microtopography

Under certain circumstances, implants manufactured from non-alloyed titanium and inserted into human bone will establish and maintain a direct contact with the bone tissue: this is called osseointegration’. One factor which has been claimed to be of crucial importance for the success or failure of a particular implant is the outermost surface layer of the implant materia12-5. For the titanium implants here, the surface has been shown to consist of a thin (2-5 nm) surface oxide6. The role and properties of this surface oxide, e.g. chemical composition and microstructure (crystal structure), for the biocompatibility of titanium have already been discussed in Refs 2-4. In this paper we are concerned with another property of the implant surface which may influence the biological response, namely the surface topography. Depending on the scale length involved, the topography of the surface may influence different aspects of the implant-tissue intet-face2-4. *Guests at the Institute for Applied Biotechnology for the purpose of this investigation. @ 1986

On the macroscopic level (> 10 pm), roughness or porosity will influence the mechanical properties of the interface and the way stresses are distributed and transmitted5. Porosity of similar dimensions may also provide mechanical interlocking of the implant by porous ingrowth of tissue. On a smaller scale surface topography in the range 10 nm to 10 ym may influence the interface biology, since it is of the same order of size as cells and large biomolecules. It should however be noted that the surface topography of these dimensions will not have any influence on the primary chemical interactions between the implant surface and the host tissue3. In contrast, topographic variations of the order of 10 nm and less may become important in this context. Microtopography on this scale length consists of material defects such as grain boundaries, steps and vacancies. These features are all known to be active sites for adsorption, and thus may influence the way biomolecules can bond to the implant surface2. Consequently it is of interest to be able to characterize the topography of implant surfaces, on these different

Butterworth Et Co (Publishers) Ltd. 0142-9612/86/060463-04$03.00 Biomaterials

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dimension scales. With scanning electron microscopy (SEM), features down to about 10 nm can be observed, but up until now there has been no appropriate method for studying the important nanometer region. Subnanometer resolution can be obtained by transmission electron microscopy (TEM), but such analysis requires sophisticated sample preparation and cannot be performed on real surfaces. Recently, scanning tunnelling microscopy (STM) was proposed as a new surface roughness standard instrumentg. The unique feature of STM is that it provides 3D real space images with atomic resolution both vertically and laterally. In addition, operation of STM at air ambient pressure makes this technique an interesting tool for technological and biological applications. The objective of this work has been to use STM to studythe microtopography of different titanium implant materials.

la

zpo;

EXPERIMENTAL

400A

Scanning tunnelling microscopy (STM)

X

The principle of STM has been described in Refs 1 O-l 2. Briefly, STM is based on the tunnelling current which flows when two electrodes are placed very short distances apart. In STM the tunnel barrier is a vacuum. The microscope works by scanning a sharp tungsten tip over the surface to be investigated, close enough for a measurable tunnel current to flow, i.e. at a distance of some 10 A. The scanning is done by means of the three mutually perpendicular piezos. In the constant current mode, the tip-surface distance remains roughly constant during the scan due to the extreme sensitivity of the tunnel current with respect to the tip-surface separation. Thus STM behaves like atracer point instrument with two important advantages. Firstly since there is no mechanical contact between tip and surface and since the electron energy lies in the meV to eV range the method is non-destructive and secondly due to the exponential dependence of the tunnelling current on the tip-sample distance, atomic resolution is obtained. We have used a STM microscope similar to the one recently developed at the IBM Zurich Laboratory by Binnig, Rohrer, Gerber and Weibel’0-‘2, but operated at air atmospheric pressure. The crucial point is that stability of the vacuum gap in the sub-A range is needed, and that requires excellent vibration damping.

Sample preparation Four different types of surfaces were studied, namely those of clinical dental implants, electropolished titanium, electropolished and anodically oxidized titanium, and electropolished titanium alloy. All samples were made from commercially pure titanium, except the titanium alloywhich wasTiGA14V. The clinical samples had been prepared according to the procedures developed by B&remark and co-workers’. The implants were machined to the desired shape (threaded screws), ultrasonically cleaned and sterilized by autoclaving. Electropolished samples were prepared in an electrolyte consisting of a mixture of 540 ml methanol, 350 ml n-butanol and 60 ml perchloric acid at 22.5 V and -30°C for 5 min.The anodic oxidation was performed in 1 M H2S04 at 80 V, producing an oxide thickness of about 1600 A. The electrochemically treated samples were ultrasonically cleaned in successive baths of methanol and n-butanol. For a more detailed description of the electrochemical treatment see Reference 6.

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figure 7

Topograpy of the clinicalsample of titanium: (a) and(b) show tw(0 different sites of the sample with an enalysed region of x = 1 lyn and y = 0.35 w at the same magnification; (c) shows the same region as in (b) but with a magnification 5 times higher.

#icmto~ogr~~y

RESULTS AND DISCUSSION Several topographic profiles, taken at different macroscopic sites on each sample were recorded. We started by scanning a region of about 1 urn length (which is the maximum range covered by the piezoelectric ceramics driving the tip) in order to obtain a general overview of the surface. As a general trend, all samples showed a structure in the form of grooves and/or steps of different width and height. In some cases a pore-like structure was also observed. The clinical sample was found to be of a rather inhomogeneous character. Topographic variations on the different dimension scales were found within each macroscopic analysis site, as well as between different sites. However, two different types of areas could be distinguished, as shown in Figure 7 (a and b). The area shown in Figure 1 a is characterized by a very rough surface which is rich in steps and grooves, with a maximum depth, R,,,of about 0.11 urn. Figure 7 &shows a smoother type of area on the clinical sample. At higher magnification, this type of area was found to contain a large number of, what appear to be, micropores with an average diameter of about 30 nm, as shown in Figure 1 c. It should be noted here that the area shown in Figure 1 c probably would appear to be perfectly smooth in the SEM. In contrast to the clinical surface, the electropolished one seemed to be rather hom~en~us and very smooth. Figure 2 shows a picture taken from the el~tro~lish~ titanium sample at high magnification. The only feature observed on this sample was steps with heights of 1 to 5 nm. The electropolished titanium alloy sample was found to be considerably rougher than the pure titanium. In Figure 3, a picture taken at high magnification from the anodicallyoxidized sample is shown. The surface of this sample is considerably smoother than the clinical surface. The channel shown in the picture is of unknown origin. It is worth remarking that this sample has a surface oxide thickness of about 1600 A, Assuming that, in this case, the tunnelling current still goes through the vacuum gap between tip and surface oxide, this means that the oxide layer is sufficiently conductive to provide for the tunnelling current. A voltage drop in the oxide is probably responsible for the high tip potential required for this experiment (2.9 V, tip positive)13. A similar effect has previously been observed on the Si(lll) surface’4. A summary of the results for all the analysed samples is shown in Table 7, which gives the values for R,,.

of titanium implents: AM. Bar& et al.

during machining, hence the variations in surface topography between the different sites in this sample. Electropolishing, on the other hand, works in such a way that the rate of removal (i.e. dissolution) of material is much higher at protruding parts of the surface than at the receding parts. For homogeneous samples, this produces an extremely smooth surface. For alloys, which often consist of different phases, a smooth surface is seldom obtained, as shown for the titanium alloy sample. In anodic oxidation, an oxide layer is being built up on the surface. Repending on the parameters during the oxidation process, the properties of the anodic oxide may vary considerably. For example, by using different electrolytes, porous oxide layers can be produced. Although the results presented here are of a preliminary character it is worthwhile to discuss some implications of this study. Surface topography on the scale of atomic dimensions and upwards is likely to be one of many factors which may influence the biocompatibility of a particular implant material. Thus it would be of great interest to be able

Figure 2 STM microgrephs of an electropolished titanium sample. Fores have been eliminated by this procedure leaving steps as the only residual structure. Magnification is the same as for Figure 1 c.

The observed differences between the samples are probably due to the different preparation techniques. The inhomogeneous character of the clinical surface is not surprising considering that it has been prepared by machining. One can except quite large statistical variations in the amount of material which is removed from the surface Table I

Values of R,,

Sample

Clinical

No. of analysed sites 5 Values of Rma, 0.11 in pm

for the different ana/ysed samples Elactropolished pure Ti

El~tro~lished Eiectro~lished anodic oxidized alloyed Ti pure Ti

2

5

2

0.005

0.1

0.02

*4n, is defined as the vertical distance between the highest and lowest point of the topographic profile.

Figure 3 Topography of a flat titanium sample electropolished and enodically oxidized (oxide thickness II 1600 A). Magnification is twice that for Figure 2

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to prepare and characterize implant surfaces with different topographies. STM seems to be a useful method in this context, since it reveals information which is generally not accessible using conventional methods such as SEM or the stylus. However, due to its extremely high resolution STM should preferably be used in combination with SEM if a more complete picture of the surface topography, especially on a coarser scale, is desired. Furthermore, before any unambiguous identification of the observed structures in these samples can be made, more background knowledge from SEM and TEM studies is needed. A comparison between the clinical samples and the electrochemically treated ones showed that it was possible to prepare titanium implant surfaces with considerably different surface topographies. At present, work is in progress to test the biological performances of these different titanium surfaces.

CONCLUSIONS

ing this work. We also thank J. Presa and J. de Miguel for help with the experiments. The financial support of IBM and the Ramon Areces Foundation is gratefully acknowledged.

REFERENCES 1

2

3 4

STM has been shown to reveal information about the surface topography of titanium implants, which is not accessible using conventional methods such as SEM or stylus measurements. The clinical surface exhibits features in the form of grooves, steps and pores, which are distributed quite inhomogeneously with regard to number, width and depth. The electropolished titanium sample shows a much more homogeneous and smooth surface, with steps of up to 5 nm in height as the only observed feature. This means that electropolishing can be used for varying the surface topography of titanium implants. Further studies are necessary before any unambiguous identification of the observed structures can be made.

10 11 12

ACKNOWLEDGEMENTS

13

It is a pleasure to thank Prof. Bengt Kasemo and Dr Lars Magnus Bjursten for giving us valuable suggestions concern-

14

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Brdnemark, P.-l., Hansson, 8.0.. Adeil, Ft., Breine, U., Lindstrom, J., Hallen, 0. and Ohman. A., Osseointegrated Implants in the Treatment of the Edentulous jaw. Experience fmm a lo-yeerperiod, Almquist 8 Wiksell International, Stockholm. Also published in &end. J. flast. Reconstr. Surg. 1977, Suppl. 16, 1OO-104 Kasemo, 6. and Lausmaa, J., Material selection surface characteristics and chemical processes at implant surfaces in Tissue integrated Prostheses; Osseointegration in Clinical Dentistry, (Eds P.-l., Br&nemark, G.A. Zarb and T. Albrektsson), Quintessence Publishing Co., Inc., Chicago, 1985, Ch. 4, pp 99-l 16 Kasemo, 8.. Biccompatibility of titanium implants: Surface science aspects, J. Pmsth. Dent. 1983,49, 832-837 Albrektsson,T., Branemark, P.I., Hansson, H.A., Krasemo, 8.. Larssson, K. and Lundstrom, II., The interface zone of inorganic implants in viva: Titanium implants in bone, Ann. Biomed. Eng. 1983, 11, l-27 Ratner, B.D.,Ann. Biomed. Eng. 1983, 11, 313-336 Lausmaa, J., Uvdal, P. and Ordell, E., Gothenburg Institute of Physics Report, No. GIPR-245, 1983 Lausmaa, J., Gothenburg Institute of Physics Report, No GIPR-250, 1984 Skelak, R., Biomechanicel considerations in osseointagrated prostheses, J. Pmsth, Dent. 1983,49, 843 Garcia, N.. Bar& A.M., Miranda, R., Rohrer, H., Gerber, Ch.. Garcia Cantu, R. and Pena, J.L., Surface roughness standards obtained with the Scanning Tunneling Microscope operated at atmospheric air pressure, Metrologia 1985, 21, 566 Binnig, G., Rohrer, H., Gerber, Ch. and Weibel, E., Tunneling through a controllable vacuum gap, App. Phys. Lett. 1982,40, 178-l 80 Binnig, G. and Rohrer, H., Scanning Tunneling Microscopy, /-/e/v. Phys. Acta, 1982, 56, 726 Binnig, G. and Rohrer, H., Scanning Tunneling Microscopy, Surface Sci. 1983,126,236 Binnig.G., Rohrer, H., Gerber, Ch. and Weibel, E., 7 x 7 reconstruction on Si(l1 1) resolved in real space, Phys. Rev. Lett. 1983, 50, 120-l 23 Flares, F. end Garcia, N., Voltage drop in the experiments of Scanning Tunneling Microscopy for Si Phys. Rev. 1984, 930, 2289