- Email: [email protected]
Ion beam processing of surgical materials
Nuclear Instruments
616
ION BEAM PROCESSING
OF SURGICAL
MATERIALS
and Methods
in Physics
Research B37/38 (1989) 676-681 North-Holland. Amsterdam
*
James M. WILLIAMS Solid State Division, Oak Ridge National Laboratory,
Raymond Department
A. BUCHANAN of Materials
and In-Seop
Science and Engineering,
Oak Ridge, TN 37831.6057,
USA.
LEE The Unwersity of Tennessee, Knoxuille,
TN 37996-2200,
USA
Ion beam processing has now achieved a secure place in surface treatment of biomaterials. This development is largely a result of the success of the process for wear prevention of orthopedic Ti-alloy in rubbing contact with ultrahigh molecular-weight polyethylene. Basic contributions of the authors in this area, together with other pertinent literature will be reviewed. Research in ion beam processing of biomaterials is turning to other areas. Among these, bioelectronics is considered to be a promising area for further effort. Pertinent experiments on effects of implantation of iridium into titanium and Ti-6Al-4V alloy on corrosion and charge injection properties are presented.
1. Introduction and overview
1.1. Wear inhibition for Ti-alloy against ultrahigh molecular polyethylene
Ion implantation doping of semiconductors had become well established commercially by the early to mid 1970s. Soon the power and versatility of the ion implantation technique for modification of other surface-related properties of materials was being explored. From anecdotes we know that pioneers quickly recognized the eminent suitability of the technique for treatment of biomaterials. The challenge to materials of the in vivo environment is extreme, but few materials have been developed specifically for surgical use. Instead the bioengineer has usually had to select his materials from those developed for more general applications in the economy. A surface modification technique with the capabilities of ion beam processing for enhancement, customization, or optimization of near-surface properties of surgical materials could clearly be of great advantage. Ion beam processing lends itself to economical research, development, and fine tuning of processes. Improved products that emerge can be rapidly commercialized because for this technique there are not necessarily important differences between laboratory and commercial operations as regards size of operations, skill levels required, protocols, and quality assurance. In many respects, ion beam processing and the biomedical materials industry appear to be ideally matched.
* Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract DE-ACOS-
84OR21400 with Martin Marietta Energy Systems, Inc. 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
The authors’ contribution has been in the area of ion implantation for wear inhibition of the articulating surfaces of total hips and knees [l-6]. Concurrent and related publications are ref. [7-131. Most of these publications deal with wear of the Ti-6Al-4V alloy against ultrahigh molecular-weight polyethylene (UHMWPE). Wear did not come to the front as an important issue in orthopedics until about 1980 when the widespread use of Ti-6Al-4V alloy, instead of the, then standard, cast Co-Cr-Mo alloys, began to be contemplated. Principal advantages of the alloy were thought to be its very high biocompatibility and its low modulus of elasticity [14]. It was believed that these advantages could be used in designs which would ameliorate loosening of the surgical implant from the bone. Wear was always an issue, however, because it was known that Ti had suspect wear properties. It was argued, however, that wear might be satisfactory against a soft wear partner such as UHMWPE if the interface were free of abrasive particles and well lubricated. Clarke et al. [15] summarized the wear controversy as it stood in 1981, based on laboratory experience. It may be significant that the worst experience was with a journal bearing type of test arrangement, similar to that used by the present authors [2], although it was claimed that contamination with particles of acrylic bone cement was necessary to trigger the type of severe wear described below. For our tests the geometry consisted of the loaded cylindrical UHMWPE brake pads (concave cylinder) against the outside cylindrical surface of the alloy jour-
nal. The assembly could be immersed in electrolytes chosen to simulate body chemistry during the mechanical tests. Saline solution (0.9%) and saline with bovine serum were used. The apparatus provided for electrochemical polarization and measurement of wear-corrosion currents during the tests. Average interfacial stresses ranging from 1.8 to 6.9 MPa were used. A few summary points are as follows: _ For all stresses, both media, and all polarizations (open circuit and about 550 mV in the anodic direction from open circuit) wear was generally severe for unimplanted alloy samples. Troughs of nearly 100 urn in depth were noted. Average wear often amounted to removal of several microns of material in a few hours of testing at an interfacial velocity of 5 cm/s. _ Wear of UHMWPE, although quite significant, was not as severe as for the alloy. This was apparently because oxide wear debris became embedded in the plastic and continued to “machine” the alloy during subsequent rotations. _ Most material was removed by this mechanical process rather than by corrosion at open circuit. _ For anodic polarizations large wear-corrosion currents could be measured as the alloy attempted to repassivate itself. - Wear rates as reflected by these anodic currents were independent of nominal interfacial stress from 1.8 to 6.9 MPa. (Since increasing stresses increased elastic conformance of the pads to the journal, we do not know the true stresses.) _ For all media, all polarizations and all stresses, severe wear was eliminated by ion implantation of about 1.7 x lOI /cm’ of nitrogen ions distributed on a depth of about 120 nm. Wear-corrosion currents were reduced by factors of up to 10000. Alloy samples and plastic pads remained essentially as-new if the alloy samples were ion implanted. We do not know whether corrosion inhibition or hardening [9,12] is the more important reason for the striking improvement produced by ion implantation. The open circuit corrosion potential for the alloy in saline is made somewhat more noble (about 150 mV) by nitrogen implantation, and thus some corrosion inhibition is to be expected. The wear process is thought to be autocatalytic because the passivating oxide for Ti is only weakly adherent to the metal surface. Once a particle of oxide is removed and becomes embedded in the UHMWPE, an associated wear band is formed for which the sample is deeply grooved, and a great deal of new wear debris is generated. This process could be suppressed by the amount or nature of native corrosion product initially on the surface. Alternatively, ion implantation hardening might arguably make the surface harder than the abrasive corrosion product [16]. Ion implantation of orthopedic prosthetics is now rather mature as a commercial process. Approximately
40000 hips and knees will be ion implanted in 1988 in the US [17], and quite likely, thousands more in the world. Apparently the ion implantation treatment will become the industry standard for devices that have articulating surfaces of Ti-alloy against UHMWPE. 1.2. Future possibilities for ion implantation ais
in biomateri-
Orthopedic prosthetics is by far the largest consumer of surgically implantable biomate~als, at least in terms of volume of material, if not numbers of devices [IS]. About 1 M intraocular lenses are implanted each year in the US as against 200000 “total” hips and knees. However, orthopedics in general includes “ partial” joints, joints other than hips and knees, fixation devices, pins, screws and the like, many for both traumatic and chronic conditions. Most of these work quite satisfactorily. However patients, physicians, and government funding and regulatory agencies are seeking ever higher standards of reliability and performance. In part for this reason there might be significant opportunity for further contributions of ion beam technologies in areas such as notch/corrosion fatigue and crevice corrosion. A recent symposium brought together investigators from throughout the world who were interested in ion beam processing of biomaterials. Two presentations [19,20] were concerned with sputter deposition of hydroxylapatite to orthopedic alloy substrates for improved accommodation of bone to the structural device. (Hydroxylapatite consists of the same mineral content as bone.) This potentially fruitful area of endeavor represents part of a broader class of problems involving joining of dissimilar materials. Joining of dissimilar materials is a large part of bioengineering, and important problems exist in several areas, including in particular, the field of dental reconstruction. In addition to joining of dissimilar artificial materials, the effects of ion implantation on interaction between tissue and artificial materials (biocompatibility) is also an active area of investigation [21,22]. In the opinion of the authors the technology of biosensors, electrodes and neural prostheses represents perhaps the greatest area of opportunity for ion beam processing left in the field of biomate~als. A large variety of transducers and controllers are needed 123,241, and it is anticipated that use will be widespread as performance, quality, and price are improved. The field has many points in common with that of microelectronics, already familiar to the practitioner of ion implantation. In addition such problems as corrosion. joining, and insulation are often special for these devices because of the severity of the in vivo environment and the need for biocompatibility. The basic research described in the next section was undertaken with an eye to such applications. VI. APPLICATIONS
678
J. M. Williams ei al. / Ion beam ~rQcess~~~of surgical materi&
2. Biocorrosion studies in k-implanted Ti and Ti-alloy For many purposes in construction of neural electrodes Ir, with its native oxide formed by the thermal activation process appears to be almost ideal [24]. Iridium is the most corrosion resistant metal known. The metal is considered to be biocompatible. but since the surface oxide provides the interface with the environment, it can be presumed that the biocompatibility is due to the oxide properties. Most importantly, the charge injection capability of the “activated” (thermally oxidized) Ir is the best of any presently known surface [25,26]. This charge injection capability is, in effect, the maximum area of a voltammetric cycle (see below), for which the limiting polarization potentials are chosen so as not to produce significant corrosion of the electrode or significant decomposition of water. Fundamental aspects of the charge injection reaction are discussed in ref. [27]. The currently prevailing view is that the charge injection results from the reversible transition of IrO, to Ir,O, within the hydrated oxide layer. Apparently, many details of the electrochemistry are not fully understood, however. The density of charge injection is important in allowing design of miniature devices. The disadvantages af Ir are the cost of the material, together with the challenge of fabricating devices. The fabrication problems arise in part because the corrosion resistance, which is otherwise desirable, makes etching and lithography of miniature devices difficult. Moreover the material is very hard and brittle, and therefore difficult to form mechanically. Because of these considerations, Robblee and co-workers [26] have used a technique for deposition of Ir onto Ti substrates from IrCl, solutions. The films were then activated by heating. Charge injection properties of such films were excellent. The present research is intended to explore the possibilities of ion implantation as another way of economic management of the fabrication problem. Ion implantation is not necessarily highly efficient in terms of fraction of feed material that can be converted to beam, but is very efficient in utilization of a beam, once formed.
These percentage designations will be used from now on in identifying samples. After ion implantation, standard potentiometric electrochemical techniques, as stated in the results, were used in determination of corrosion properties. Data are referred to the standard hydrogen electrode (SHE). First the samples were exposed to a solution of IN deaerated H,SO,, and properties were measured. This treatment also amounted to a pretreatment for bringing the implanted Ir to the surface, as will be seen. Properties were then measured for the samples in a solution of 0.9% NaCl at 25 o C. Analytical techniques employed were were RBS and ESCA, as stated in the results. 2.2. Results Corrosion properties of the ion-implanted samples were first studied for the IN H,SO, solution. The collection of samples carried throughout the corrosion studies included control samples of both the CP-Ti and the Ti-alloy, both in prepassivated and ordinary mechanically polished surface states, and pure metallic Ir. The effects of the ion-implanted Ir in reducing corrosion currents under anodic polarization and in increasing the open circuit corrosion potential were immediately apparent, even though most of the implanted Ir was still below the surface. After tests at anodic potential, open circuit corrosion potentials were measured over a period of 280 h. After dissolution of anodically-formed oxides corrosion potentials were near that of Ir ( + 100 mV vs SHE) for implanted samples of both doses and both compositions. Corrosion potentials of u~mplant~ CPTi and Ti-alloy were about -450 and - 500 mV, respectively, as is well known. There is no oxide passivation of these materials in this solution at equilibrium. With increasing exposure time to the acid solution the corrosion potentials of the implanted samples approached still closer to that of Ir.
-
~--I-
700
----we -‘-.
.,
.-
600 -
2.1. Procedures Coupons of commercially pure (CP) Ti and surgical Ti-6Al-4V alloy were ion implanted with Ir at fluences of either 7.4 x 1Or5 or 1.48 x 1016 /cm2. The energy was 244 keV, and the calculated average projected range was 50 nm for that energy. Rutherford backscattering (RBS) measurements of the as-implanted profiles are presented as part of the results (see below). The lower dose was designed to produce a peak concentration of about 2.5 at.% Ir and the higher was intended to produce about 5 at.%. These results were confirmed by RBS.
500 q400 s-
PURE ,r 2.5 a,. % lr IN Tb6Al-W 5.0 at. % Ir IN TiCGAl-.@/ PREPASSIVATED T&Al-4V AS-POLISHED TibAb4V
\* ____ ---‘*
____________---_-----___----___
300 2oo _
-.\. .._
.._..-._-..
. . I..---
100 J
,,“o: 0
10
20
30
40
50
TIME (d)
Fig. 1. Free corrosion potential versus time for the collection of samples in the 0.9% saline.
J.M.
DEPTH
Williams et al. / Ion beam processing
ofsurgical maierials
,microns,
679
DEPTH (microns)
Fig. 2. (a) Rutherford backscattering yield (2.0-l&V He ions) versus depth of Ir for ion-implanted Ir (5 at.%) in Ti-6Al-4V alloy, as-implanted and after a corrosion treatment. (b) Rutherford backscattering yield (2.0 MeV ions) versus depth of Ir for ion-implanted Ir (5 at.%) in CP-Ti, as-implanted and after a corrosion treatment.
Following the HaSO, treatment the samples were in the NaCl solution, except that new control samples were used instead of the ones that had been in the H2S0,. Fig. 1 shows open circuit corrosion potentials versus time for the Ti-alloy samples in the saline solution. Prepassivated and as-polished samples approached the state of equilibrium passivation for saline. This means a reduction in potential for the prepassivated and an increase for the polished. Except for this latter behavior the data of fig. 1 are similar in form to the data for H2SO4. Corrosion potentials for the implanted samples are near that of pure Ir, very stable, and if anything, still increasing after 45 d. The analyses to be given below will show that this more noble potential is related to segregation of the ion-implanted Ir to the surface during the H,SO, treatment. After these long exposures in the saline the samples were analysed by RBS and ESCA. Fig. 2 shows depth profiles for Ir in the alloy and in CP-Ti for the as-implanted condition and then after the treatment described above, Clearly the Ir is much more highly segregated to the surface for the alloy than for the CP-Ti. In fact, several analyses support the idea that, for the alloy, Ir enrichment at the surface is near IOO%, so that the alloy is completely covered with a few atomic layers of Ir. In form, the RBS response of fig. 2 is best simulated for constituent layer models in which the first layer is 100% Ir, and layers of increasing depth have progressively less fraction of Ir. Naturally such a calculation of RBS response to be expected from a model of constituent layers, in comparison with actual response, does not provide a unique determination of the constitution. Depending on simulation model, at least SO%, and perhaps more, of the ion-impl~ted Ir has been retained on the surface. The data of fig. 3 contribute further. These data show a clear offset in the energy placed
I
I
0.
Fig. 3. Rutherford backscattering yield from Ti versus depth, where the depth calibration assumes stopping-power values for pure
Ir.
(V “8. SHE)
SCAN RATE: ELECTROLYTE:
0.8
to0 mV/s 0.9% N’dCl
I
CURRENT
DENSITY
(mAkm*t
Fig. 4. Cyclic voltammetry curves (potential vs current density) for the sample materials indicated. The implanted alloy sample was also acid treated. See text. VI. APPLICATIONS
680
J.M. Williams et al. / ion beam processing of surgical materials
(depth) of the Ti edge in the RBS histogram due to the Ir coverage. This offset, represented by only about two channels of spectrometer resolution, was nevertheless unmistakable. It is consistent with a depth of 2 or 3 nm of Ir, or perhaps nearly all of the implanted Ir in oxidized form. No such offset was found for the CP-Ti, in which cases the Ir was distributed as indicated in fig. 2b. To save space, ESCA results will not be presented graphically. The results were also consistent with the idea that the surface of the alloy has practically solid coverage with Ir after the implantation and corrosion treatment. Titanium (and aluminum) peaks were almost completely suppressed; the 450-eV peak was just barely in evidence for Ti. Iridium peaks were very prominent, but were shifted in energy to values higher than either those for metallic Ir or IrO,. The reason for this result is not known, but presumably the Ir is in some oxide alloy state involving mostly IrO, with some TiO,. The potential practical implications of the research described above are represented in fig. 4, which shows cyclic voltammetric curves for Ir and for the alloy in the unimplanted state and after the implantation and corrosion treatment which brought the Ir to the surface. Such data are obtained by monitoring the current while scanning the voltage at the rate indicated. For a fixed scan rate the area of the curve is proportional to the total charge transported during a cycle. Values are 0.28, 1.64, and 2.50 mC/cm*, respectively, for the virgin alloy, Ir, and the alloy sample with the surface enrichment of Ir. Thus, the implanted alloy sample is better than Ir at this stage of development. For no case, virgin, as-implanted, or implanted and treated, did CP-Ti have significant charge injection compared to the values given above. The best was for 5% implanted and treated, for which the value was 0.18 mC/cm2. 2.3. Discussion Although the samples were not analysed by RBS between the H,SO, treatment and the NaCl treatment, it seems clear that, since the acid is so much more corrosive than the saline, it was the acid treatment that brought the Ir to the surface for the alloy. The fact that segregation to the surface for the CP-Ti was not as marked as for the alloy probably means that the pure Ti is much more corrosion resistant than the alloy in the acid. Either a stronger solution or more time would be required to produce a high degree of segregation for Ir in the Ti. Tests to confirm this hypothesis are under way. It is interesting that the charge injection capability of the alloy with the segregated Ir exceeds that of Ir itself. On the whole this finding would seem to be a favorable indication for practicality. At present we cannot say whether the difference in performance between the samples is due to some rather superficial difference, beyond
the control of our experiment (such as differences in surface texturing between the two samples), or whether the effect is fundamental, and related to the differences in the method of formation of the two surfaces. The most important point is that the ion implanted sample is at least not worse than the solid Ir. This result is not yet necessarily practical, however, in that the magnitude of the charge is clearly less than that achieved (at the same scan rate) by Robblee et al. [25] by their process of deposition and thermal activation. (This latter result is only presented graphically, and thus we are not able to quote a precise value). Part of the reason for the difference is that we have chosen more conservative potential limits, so as to be sure of not corroding the electrode or decomposing water. A basis for direct comparison will not exist, however, until we have made efforts to “activate” the oxides by thermal treatments or other means. At present such activation that exists has been produced by the anodic polarization treatment that was described. It is known [28] that anodic oxides are not as effective in charge transport as those produced thermally. The behavior of ion-implanted Ir in Ti and the Ti-6Al-4V alloy is apparently similar in most respects to that of ion-implanted Rh [29].
3. Conclusion Ion implantation for wear inhibition in orthopedics has moved rapidly since completion of the seminal basic research to the the high degree of commercial maturity and sizable market that exists today. This success has heightened the interest of ion beam technologists in possible applications in biomaterials. Simultaneously the bioengineer has become more aware of the promise of ion beam materials processing. Government funding and regulatory agencies seem more receptive to the idea of a rapidly changing biomaterials technology than in the past. The future for ion beam processing of biomaterials seems secure. Research has turned from orthopedics to emphasis in other areas of biomaterials. The authors believe that the field of bioelectronics presents good opportunities for ion beam applications. Thanks are due to D.B. Poker who wrote the software for simulation and other analyses by RBS.
References [l] R.A. Buchanan, E.D. Rigney, Biomed. Mater. [2] R.A. Buchanan, p. 355.
Jr. and J.M. Williams, J. Res. 21 (1987) 367. E.D. Rigney, Jr. and J.M. Williams, ibid.,
J.&f. WiNiams et al. / Ion beam processing of surgical materials [3] J.M. Williams, R.A. Buchanan and E.D. Rigney, Jr., in: Ion Plating and Implantation, ed. Robert F. Ho&man (American Society of Metals, Metals Park, OH, 1986) p. 141. [4] J.M. Williams, Nucl. Instr. and Meth. BlO/ll (1985) 539 (Proc. 8 Int. Conf. on the Application of Accelerators in Research and Industry). [5] J.M. Williams and R.A. Buchanan, Mater. Sci. Eng. 69 (1985) 237 (Proc. Int. Conf. on Surface Modification of Metals by Ion Beams, 1984). [6] J.M. Williams, G.M. Beardsley, R.A. Buchanan and R.K. Bacon, in: Ion Implantation and Ion Beam Processing of Materials, eds. G.K. Hubler, O.W. Holland, C.R. Clayton and C.W. White, Mater. Res. Sot. Symp. Proc. vol. 27 (Elsevier, New York, 1984) p. 735. [7] Piran Sioshansi, Richard W. Oliver and Frank D. Matthews, J. Vat. Sci. Technol. A3 (1985) 2670. [8] Zhang Jianqiang, Zhang Xiaozhong, Guo Zintang and Li Hengde. in: Biomedical Materials, eds. J.M. Williams, M.F. Nichols and W. Zingg, Mater. Res. Sot. Symp. Proc. vol. 55 (Materials Research Society, Pittsburg, 1986) p. 229. [9] Piran Sioshansi and Richard W. Oliver, ibid., p. 237. [lo] Frank D. Matthews, Keith W. Greer and Douglas L. Armstrong, ibid., p. 243. [ll] R. Hutchings and W.C. Oliver, Wear 92 (1983) 43. [12] J.B. Pethica, R. Hutchings and W.C. Oliver, Nucl. Instr. and Meth. 209/210 (1983) 995 (Proc. 3rd Int. Conf. on Ion Beam Modification of Materials). [13] R. Martinella, S. Giovanardi, G. Chevallard and M. Villani, Mater. Sci. Eng. 69 (1985) 247 (Proc. Int. Conf. on Surface Modification of Metals). [14] B.P. Bannon and E.E. Mild, in: Titanium Alloys in Surgical Implants, eds. Hugh A. Luckey and Fred Kubli, Jr. (American Society for Testing and Materials, Baltimore, 1983) p. 7. [15] I.C. Clarke, H.A. McKellop, P. McGuire, R. Okuda and A. Sarmiento, ibid., p. 136.
681
[16] G. Dearnaley, Nucl. Instr. and Meth. B7/8 (1985) 517 (Proc. 4th Int. Conf. on Ion Beam Modification of Materials). [17] Piran Sioshansi, these Proceedings (7th Int. Conf. on Ion Implantation Technology, Kyoto, Japan 1988) Nucl. Instr. and Meth. B37/38 (1989) 667. [18] L.L. Hench and June Wilson, in: Biomedical Materials, eds. J.M. Williams, M.F. Nichols and W. Zingg, Mater. Res. Sot. Symp. Proc. vol. 55 (Materials Research Society, Pittsburg, 1986) p. 65. [19] J.R. Stephenson, H. Solnick-Legg and K.O. Legg, in: Biomedical Materials and Devices, eds. Jacob S. Hanker and Beverly L. Giammara, Mater. Res. Sot. Symp. Proc. Vol. 110 (Materials Research Society, Pittsburg) to be published. [20] Barry L. Barthell and Thomas Archuleta, ibid. [21] Yoshiaki Suzuki, Masahiro Kusakabe, Masaya Iwaki, Kiyoko Kusakabe, Hiromichi Akiba and Masaaki Suzuki, ibid. [22] K.S. Grabowski, F.A. Young, and J.C. Keller, ibid. [23] William F. Regnault and Grace Lee Picciolo, in: Biomedical Materials, eds. J.M. Williams, M.F. Nichols and W.F. Zingg, Mater. Res. Sot. Symp. Proc. vol. 55 (Materials Research Society, Pittsburg, 1986) p. 273. [24] Piet Bergveld, Sensors and Actuators 10 (1986) 165. [25] F. Terry Hambrecht, ref. [23], p. 265. [26] Lois S. Robblee, Michael J. Mangaudis, Ellen D. Lasinsky, Angela G. Kimball and S. Barry Brummer, ref. [23], p. 303. [27] T. Katsube, I. Lauks and J.N. Zemel, Sensors and Actuators 2 (1982) 399. [28] Matijan Vukovic, J. Appl. Electrochem. 17 (1987) 737. [29] L-S. Lee, R.A. Buchanan and J.M. Williams, in: Biomedical Materials and Devices, eds. Jacob S. Hanker and Beverly L. Giammara, Mater. Res. Sot. Symp. Proc. vol. 110 (Materials Research Society, Pittsburg) to be published.
VI. APPLICATIONS
616
ION BEAM PROCESSING
OF SURGICAL
MATERIALS
and Methods
in Physics
Research B37/38 (1989) 676-681 North-Holland. Amsterdam
*
James M. WILLIAMS Solid State Division, Oak Ridge National Laboratory,
Raymond Department
A. BUCHANAN of Materials
and In-Seop
Science and Engineering,
Oak Ridge, TN 37831.6057,
USA.
LEE The Unwersity of Tennessee, Knoxuille,
TN 37996-2200,
USA
Ion beam processing has now achieved a secure place in surface treatment of biomaterials. This development is largely a result of the success of the process for wear prevention of orthopedic Ti-alloy in rubbing contact with ultrahigh molecular-weight polyethylene. Basic contributions of the authors in this area, together with other pertinent literature will be reviewed. Research in ion beam processing of biomaterials is turning to other areas. Among these, bioelectronics is considered to be a promising area for further effort. Pertinent experiments on effects of implantation of iridium into titanium and Ti-6Al-4V alloy on corrosion and charge injection properties are presented.
1. Introduction and overview
1.1. Wear inhibition for Ti-alloy against ultrahigh molecular polyethylene
Ion implantation doping of semiconductors had become well established commercially by the early to mid 1970s. Soon the power and versatility of the ion implantation technique for modification of other surface-related properties of materials was being explored. From anecdotes we know that pioneers quickly recognized the eminent suitability of the technique for treatment of biomaterials. The challenge to materials of the in vivo environment is extreme, but few materials have been developed specifically for surgical use. Instead the bioengineer has usually had to select his materials from those developed for more general applications in the economy. A surface modification technique with the capabilities of ion beam processing for enhancement, customization, or optimization of near-surface properties of surgical materials could clearly be of great advantage. Ion beam processing lends itself to economical research, development, and fine tuning of processes. Improved products that emerge can be rapidly commercialized because for this technique there are not necessarily important differences between laboratory and commercial operations as regards size of operations, skill levels required, protocols, and quality assurance. In many respects, ion beam processing and the biomedical materials industry appear to be ideally matched.
* Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract DE-ACOS-
84OR21400 with Martin Marietta Energy Systems, Inc. 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
The authors’ contribution has been in the area of ion implantation for wear inhibition of the articulating surfaces of total hips and knees [l-6]. Concurrent and related publications are ref. [7-131. Most of these publications deal with wear of the Ti-6Al-4V alloy against ultrahigh molecular-weight polyethylene (UHMWPE). Wear did not come to the front as an important issue in orthopedics until about 1980 when the widespread use of Ti-6Al-4V alloy, instead of the, then standard, cast Co-Cr-Mo alloys, began to be contemplated. Principal advantages of the alloy were thought to be its very high biocompatibility and its low modulus of elasticity [14]. It was believed that these advantages could be used in designs which would ameliorate loosening of the surgical implant from the bone. Wear was always an issue, however, because it was known that Ti had suspect wear properties. It was argued, however, that wear might be satisfactory against a soft wear partner such as UHMWPE if the interface were free of abrasive particles and well lubricated. Clarke et al. [15] summarized the wear controversy as it stood in 1981, based on laboratory experience. It may be significant that the worst experience was with a journal bearing type of test arrangement, similar to that used by the present authors [2], although it was claimed that contamination with particles of acrylic bone cement was necessary to trigger the type of severe wear described below. For our tests the geometry consisted of the loaded cylindrical UHMWPE brake pads (concave cylinder) against the outside cylindrical surface of the alloy jour-
nal. The assembly could be immersed in electrolytes chosen to simulate body chemistry during the mechanical tests. Saline solution (0.9%) and saline with bovine serum were used. The apparatus provided for electrochemical polarization and measurement of wear-corrosion currents during the tests. Average interfacial stresses ranging from 1.8 to 6.9 MPa were used. A few summary points are as follows: _ For all stresses, both media, and all polarizations (open circuit and about 550 mV in the anodic direction from open circuit) wear was generally severe for unimplanted alloy samples. Troughs of nearly 100 urn in depth were noted. Average wear often amounted to removal of several microns of material in a few hours of testing at an interfacial velocity of 5 cm/s. _ Wear of UHMWPE, although quite significant, was not as severe as for the alloy. This was apparently because oxide wear debris became embedded in the plastic and continued to “machine” the alloy during subsequent rotations. _ Most material was removed by this mechanical process rather than by corrosion at open circuit. _ For anodic polarizations large wear-corrosion currents could be measured as the alloy attempted to repassivate itself. - Wear rates as reflected by these anodic currents were independent of nominal interfacial stress from 1.8 to 6.9 MPa. (Since increasing stresses increased elastic conformance of the pads to the journal, we do not know the true stresses.) _ For all media, all polarizations and all stresses, severe wear was eliminated by ion implantation of about 1.7 x lOI /cm’ of nitrogen ions distributed on a depth of about 120 nm. Wear-corrosion currents were reduced by factors of up to 10000. Alloy samples and plastic pads remained essentially as-new if the alloy samples were ion implanted. We do not know whether corrosion inhibition or hardening [9,12] is the more important reason for the striking improvement produced by ion implantation. The open circuit corrosion potential for the alloy in saline is made somewhat more noble (about 150 mV) by nitrogen implantation, and thus some corrosion inhibition is to be expected. The wear process is thought to be autocatalytic because the passivating oxide for Ti is only weakly adherent to the metal surface. Once a particle of oxide is removed and becomes embedded in the UHMWPE, an associated wear band is formed for which the sample is deeply grooved, and a great deal of new wear debris is generated. This process could be suppressed by the amount or nature of native corrosion product initially on the surface. Alternatively, ion implantation hardening might arguably make the surface harder than the abrasive corrosion product [16]. Ion implantation of orthopedic prosthetics is now rather mature as a commercial process. Approximately
40000 hips and knees will be ion implanted in 1988 in the US [17], and quite likely, thousands more in the world. Apparently the ion implantation treatment will become the industry standard for devices that have articulating surfaces of Ti-alloy against UHMWPE. 1.2. Future possibilities for ion implantation ais
in biomateri-
Orthopedic prosthetics is by far the largest consumer of surgically implantable biomate~als, at least in terms of volume of material, if not numbers of devices [IS]. About 1 M intraocular lenses are implanted each year in the US as against 200000 “total” hips and knees. However, orthopedics in general includes “ partial” joints, joints other than hips and knees, fixation devices, pins, screws and the like, many for both traumatic and chronic conditions. Most of these work quite satisfactorily. However patients, physicians, and government funding and regulatory agencies are seeking ever higher standards of reliability and performance. In part for this reason there might be significant opportunity for further contributions of ion beam technologies in areas such as notch/corrosion fatigue and crevice corrosion. A recent symposium brought together investigators from throughout the world who were interested in ion beam processing of biomaterials. Two presentations [19,20] were concerned with sputter deposition of hydroxylapatite to orthopedic alloy substrates for improved accommodation of bone to the structural device. (Hydroxylapatite consists of the same mineral content as bone.) This potentially fruitful area of endeavor represents part of a broader class of problems involving joining of dissimilar materials. Joining of dissimilar materials is a large part of bioengineering, and important problems exist in several areas, including in particular, the field of dental reconstruction. In addition to joining of dissimilar artificial materials, the effects of ion implantation on interaction between tissue and artificial materials (biocompatibility) is also an active area of investigation [21,22]. In the opinion of the authors the technology of biosensors, electrodes and neural prostheses represents perhaps the greatest area of opportunity for ion beam processing left in the field of biomate~als. A large variety of transducers and controllers are needed 123,241, and it is anticipated that use will be widespread as performance, quality, and price are improved. The field has many points in common with that of microelectronics, already familiar to the practitioner of ion implantation. In addition such problems as corrosion. joining, and insulation are often special for these devices because of the severity of the in vivo environment and the need for biocompatibility. The basic research described in the next section was undertaken with an eye to such applications. VI. APPLICATIONS
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2. Biocorrosion studies in k-implanted Ti and Ti-alloy For many purposes in construction of neural electrodes Ir, with its native oxide formed by the thermal activation process appears to be almost ideal [24]. Iridium is the most corrosion resistant metal known. The metal is considered to be biocompatible. but since the surface oxide provides the interface with the environment, it can be presumed that the biocompatibility is due to the oxide properties. Most importantly, the charge injection capability of the “activated” (thermally oxidized) Ir is the best of any presently known surface [25,26]. This charge injection capability is, in effect, the maximum area of a voltammetric cycle (see below), for which the limiting polarization potentials are chosen so as not to produce significant corrosion of the electrode or significant decomposition of water. Fundamental aspects of the charge injection reaction are discussed in ref. [27]. The currently prevailing view is that the charge injection results from the reversible transition of IrO, to Ir,O, within the hydrated oxide layer. Apparently, many details of the electrochemistry are not fully understood, however. The density of charge injection is important in allowing design of miniature devices. The disadvantages af Ir are the cost of the material, together with the challenge of fabricating devices. The fabrication problems arise in part because the corrosion resistance, which is otherwise desirable, makes etching and lithography of miniature devices difficult. Moreover the material is very hard and brittle, and therefore difficult to form mechanically. Because of these considerations, Robblee and co-workers [26] have used a technique for deposition of Ir onto Ti substrates from IrCl, solutions. The films were then activated by heating. Charge injection properties of such films were excellent. The present research is intended to explore the possibilities of ion implantation as another way of economic management of the fabrication problem. Ion implantation is not necessarily highly efficient in terms of fraction of feed material that can be converted to beam, but is very efficient in utilization of a beam, once formed.
These percentage designations will be used from now on in identifying samples. After ion implantation, standard potentiometric electrochemical techniques, as stated in the results, were used in determination of corrosion properties. Data are referred to the standard hydrogen electrode (SHE). First the samples were exposed to a solution of IN deaerated H,SO,, and properties were measured. This treatment also amounted to a pretreatment for bringing the implanted Ir to the surface, as will be seen. Properties were then measured for the samples in a solution of 0.9% NaCl at 25 o C. Analytical techniques employed were were RBS and ESCA, as stated in the results. 2.2. Results Corrosion properties of the ion-implanted samples were first studied for the IN H,SO, solution. The collection of samples carried throughout the corrosion studies included control samples of both the CP-Ti and the Ti-alloy, both in prepassivated and ordinary mechanically polished surface states, and pure metallic Ir. The effects of the ion-implanted Ir in reducing corrosion currents under anodic polarization and in increasing the open circuit corrosion potential were immediately apparent, even though most of the implanted Ir was still below the surface. After tests at anodic potential, open circuit corrosion potentials were measured over a period of 280 h. After dissolution of anodically-formed oxides corrosion potentials were near that of Ir ( + 100 mV vs SHE) for implanted samples of both doses and both compositions. Corrosion potentials of u~mplant~ CPTi and Ti-alloy were about -450 and - 500 mV, respectively, as is well known. There is no oxide passivation of these materials in this solution at equilibrium. With increasing exposure time to the acid solution the corrosion potentials of the implanted samples approached still closer to that of Ir.
-
~--I-
700
----we -‘-.
.,
.-
600 -
2.1. Procedures Coupons of commercially pure (CP) Ti and surgical Ti-6Al-4V alloy were ion implanted with Ir at fluences of either 7.4 x 1Or5 or 1.48 x 1016 /cm2. The energy was 244 keV, and the calculated average projected range was 50 nm for that energy. Rutherford backscattering (RBS) measurements of the as-implanted profiles are presented as part of the results (see below). The lower dose was designed to produce a peak concentration of about 2.5 at.% Ir and the higher was intended to produce about 5 at.%. These results were confirmed by RBS.
500 q400 s-
PURE ,r 2.5 a,. % lr IN Tb6Al-W 5.0 at. % Ir IN TiCGAl-.@/ PREPASSIVATED T&Al-4V AS-POLISHED TibAb4V
\* ____ ---‘*
____________---_-----___----___
300 2oo _
-.\. .._
.._..-._-..
. . I..---
100 J
,,“o: 0
10
20
30
40
50
TIME (d)
Fig. 1. Free corrosion potential versus time for the collection of samples in the 0.9% saline.
J.M.
DEPTH
Williams et al. / Ion beam processing
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,microns,
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DEPTH (microns)
Fig. 2. (a) Rutherford backscattering yield (2.0-l&V He ions) versus depth of Ir for ion-implanted Ir (5 at.%) in Ti-6Al-4V alloy, as-implanted and after a corrosion treatment. (b) Rutherford backscattering yield (2.0 MeV ions) versus depth of Ir for ion-implanted Ir (5 at.%) in CP-Ti, as-implanted and after a corrosion treatment.
Following the HaSO, treatment the samples were in the NaCl solution, except that new control samples were used instead of the ones that had been in the H2S0,. Fig. 1 shows open circuit corrosion potentials versus time for the Ti-alloy samples in the saline solution. Prepassivated and as-polished samples approached the state of equilibrium passivation for saline. This means a reduction in potential for the prepassivated and an increase for the polished. Except for this latter behavior the data of fig. 1 are similar in form to the data for H2SO4. Corrosion potentials for the implanted samples are near that of pure Ir, very stable, and if anything, still increasing after 45 d. The analyses to be given below will show that this more noble potential is related to segregation of the ion-implanted Ir to the surface during the H,SO, treatment. After these long exposures in the saline the samples were analysed by RBS and ESCA. Fig. 2 shows depth profiles for Ir in the alloy and in CP-Ti for the as-implanted condition and then after the treatment described above, Clearly the Ir is much more highly segregated to the surface for the alloy than for the CP-Ti. In fact, several analyses support the idea that, for the alloy, Ir enrichment at the surface is near IOO%, so that the alloy is completely covered with a few atomic layers of Ir. In form, the RBS response of fig. 2 is best simulated for constituent layer models in which the first layer is 100% Ir, and layers of increasing depth have progressively less fraction of Ir. Naturally such a calculation of RBS response to be expected from a model of constituent layers, in comparison with actual response, does not provide a unique determination of the constitution. Depending on simulation model, at least SO%, and perhaps more, of the ion-impl~ted Ir has been retained on the surface. The data of fig. 3 contribute further. These data show a clear offset in the energy placed
I
I
0.
Fig. 3. Rutherford backscattering yield from Ti versus depth, where the depth calibration assumes stopping-power values for pure
Ir.
(V “8. SHE)
SCAN RATE: ELECTROLYTE:
0.8
to0 mV/s 0.9% N’dCl
I
CURRENT
DENSITY
(mAkm*t
Fig. 4. Cyclic voltammetry curves (potential vs current density) for the sample materials indicated. The implanted alloy sample was also acid treated. See text. VI. APPLICATIONS
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J.M. Williams et al. / ion beam processing of surgical materials
(depth) of the Ti edge in the RBS histogram due to the Ir coverage. This offset, represented by only about two channels of spectrometer resolution, was nevertheless unmistakable. It is consistent with a depth of 2 or 3 nm of Ir, or perhaps nearly all of the implanted Ir in oxidized form. No such offset was found for the CP-Ti, in which cases the Ir was distributed as indicated in fig. 2b. To save space, ESCA results will not be presented graphically. The results were also consistent with the idea that the surface of the alloy has practically solid coverage with Ir after the implantation and corrosion treatment. Titanium (and aluminum) peaks were almost completely suppressed; the 450-eV peak was just barely in evidence for Ti. Iridium peaks were very prominent, but were shifted in energy to values higher than either those for metallic Ir or IrO,. The reason for this result is not known, but presumably the Ir is in some oxide alloy state involving mostly IrO, with some TiO,. The potential practical implications of the research described above are represented in fig. 4, which shows cyclic voltammetric curves for Ir and for the alloy in the unimplanted state and after the implantation and corrosion treatment which brought the Ir to the surface. Such data are obtained by monitoring the current while scanning the voltage at the rate indicated. For a fixed scan rate the area of the curve is proportional to the total charge transported during a cycle. Values are 0.28, 1.64, and 2.50 mC/cm*, respectively, for the virgin alloy, Ir, and the alloy sample with the surface enrichment of Ir. Thus, the implanted alloy sample is better than Ir at this stage of development. For no case, virgin, as-implanted, or implanted and treated, did CP-Ti have significant charge injection compared to the values given above. The best was for 5% implanted and treated, for which the value was 0.18 mC/cm2. 2.3. Discussion Although the samples were not analysed by RBS between the H,SO, treatment and the NaCl treatment, it seems clear that, since the acid is so much more corrosive than the saline, it was the acid treatment that brought the Ir to the surface for the alloy. The fact that segregation to the surface for the CP-Ti was not as marked as for the alloy probably means that the pure Ti is much more corrosion resistant than the alloy in the acid. Either a stronger solution or more time would be required to produce a high degree of segregation for Ir in the Ti. Tests to confirm this hypothesis are under way. It is interesting that the charge injection capability of the alloy with the segregated Ir exceeds that of Ir itself. On the whole this finding would seem to be a favorable indication for practicality. At present we cannot say whether the difference in performance between the samples is due to some rather superficial difference, beyond
the control of our experiment (such as differences in surface texturing between the two samples), or whether the effect is fundamental, and related to the differences in the method of formation of the two surfaces. The most important point is that the ion implanted sample is at least not worse than the solid Ir. This result is not yet necessarily practical, however, in that the magnitude of the charge is clearly less than that achieved (at the same scan rate) by Robblee et al. [25] by their process of deposition and thermal activation. (This latter result is only presented graphically, and thus we are not able to quote a precise value). Part of the reason for the difference is that we have chosen more conservative potential limits, so as to be sure of not corroding the electrode or decomposing water. A basis for direct comparison will not exist, however, until we have made efforts to “activate” the oxides by thermal treatments or other means. At present such activation that exists has been produced by the anodic polarization treatment that was described. It is known [28] that anodic oxides are not as effective in charge transport as those produced thermally. The behavior of ion-implanted Ir in Ti and the Ti-6Al-4V alloy is apparently similar in most respects to that of ion-implanted Rh [29].
3. Conclusion Ion implantation for wear inhibition in orthopedics has moved rapidly since completion of the seminal basic research to the the high degree of commercial maturity and sizable market that exists today. This success has heightened the interest of ion beam technologists in possible applications in biomaterials. Simultaneously the bioengineer has become more aware of the promise of ion beam materials processing. Government funding and regulatory agencies seem more receptive to the idea of a rapidly changing biomaterials technology than in the past. The future for ion beam processing of biomaterials seems secure. Research has turned from orthopedics to emphasis in other areas of biomaterials. The authors believe that the field of bioelectronics presents good opportunities for ion beam applications. Thanks are due to D.B. Poker who wrote the software for simulation and other analyses by RBS.
References [l] R.A. Buchanan, E.D. Rigney, Biomed. Mater. [2] R.A. Buchanan, p. 355.
Jr. and J.M. Williams, J. Res. 21 (1987) 367. E.D. Rigney, Jr. and J.M. Williams, ibid.,
J.&f. WiNiams et al. / Ion beam processing of surgical materials [3] J.M. Williams, R.A. Buchanan and E.D. Rigney, Jr., in: Ion Plating and Implantation, ed. Robert F. Ho&man (American Society of Metals, Metals Park, OH, 1986) p. 141. [4] J.M. Williams, Nucl. Instr. and Meth. BlO/ll (1985) 539 (Proc. 8 Int. Conf. on the Application of Accelerators in Research and Industry). [5] J.M. Williams and R.A. Buchanan, Mater. Sci. Eng. 69 (1985) 237 (Proc. Int. Conf. on Surface Modification of Metals by Ion Beams, 1984). [6] J.M. Williams, G.M. Beardsley, R.A. Buchanan and R.K. Bacon, in: Ion Implantation and Ion Beam Processing of Materials, eds. G.K. Hubler, O.W. Holland, C.R. Clayton and C.W. White, Mater. Res. Sot. Symp. Proc. vol. 27 (Elsevier, New York, 1984) p. 735. [7] Piran Sioshansi, Richard W. Oliver and Frank D. Matthews, J. Vat. Sci. Technol. A3 (1985) 2670. [8] Zhang Jianqiang, Zhang Xiaozhong, Guo Zintang and Li Hengde. in: Biomedical Materials, eds. J.M. Williams, M.F. Nichols and W. Zingg, Mater. Res. Sot. Symp. Proc. vol. 55 (Materials Research Society, Pittsburg, 1986) p. 229. [9] Piran Sioshansi and Richard W. Oliver, ibid., p. 237. [lo] Frank D. Matthews, Keith W. Greer and Douglas L. Armstrong, ibid., p. 243. [ll] R. Hutchings and W.C. Oliver, Wear 92 (1983) 43. [12] J.B. Pethica, R. Hutchings and W.C. Oliver, Nucl. Instr. and Meth. 209/210 (1983) 995 (Proc. 3rd Int. Conf. on Ion Beam Modification of Materials). [13] R. Martinella, S. Giovanardi, G. Chevallard and M. Villani, Mater. Sci. Eng. 69 (1985) 247 (Proc. Int. Conf. on Surface Modification of Metals). [14] B.P. Bannon and E.E. Mild, in: Titanium Alloys in Surgical Implants, eds. Hugh A. Luckey and Fred Kubli, Jr. (American Society for Testing and Materials, Baltimore, 1983) p. 7. [15] I.C. Clarke, H.A. McKellop, P. McGuire, R. Okuda and A. Sarmiento, ibid., p. 136.
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[16] G. Dearnaley, Nucl. Instr. and Meth. B7/8 (1985) 517 (Proc. 4th Int. Conf. on Ion Beam Modification of Materials). [17] Piran Sioshansi, these Proceedings (7th Int. Conf. on Ion Implantation Technology, Kyoto, Japan 1988) Nucl. Instr. and Meth. B37/38 (1989) 667. [18] L.L. Hench and June Wilson, in: Biomedical Materials, eds. J.M. Williams, M.F. Nichols and W. Zingg, Mater. Res. Sot. Symp. Proc. vol. 55 (Materials Research Society, Pittsburg, 1986) p. 65. [19] J.R. Stephenson, H. Solnick-Legg and K.O. Legg, in: Biomedical Materials and Devices, eds. Jacob S. Hanker and Beverly L. Giammara, Mater. Res. Sot. Symp. Proc. Vol. 110 (Materials Research Society, Pittsburg) to be published. [20] Barry L. Barthell and Thomas Archuleta, ibid. [21] Yoshiaki Suzuki, Masahiro Kusakabe, Masaya Iwaki, Kiyoko Kusakabe, Hiromichi Akiba and Masaaki Suzuki, ibid. [22] K.S. Grabowski, F.A. Young, and J.C. Keller, ibid. [23] William F. Regnault and Grace Lee Picciolo, in: Biomedical Materials, eds. J.M. Williams, M.F. Nichols and W.F. Zingg, Mater. Res. Sot. Symp. Proc. vol. 55 (Materials Research Society, Pittsburg, 1986) p. 273. [24] Piet Bergveld, Sensors and Actuators 10 (1986) 165. [25] F. Terry Hambrecht, ref. [23], p. 265. [26] Lois S. Robblee, Michael J. Mangaudis, Ellen D. Lasinsky, Angela G. Kimball and S. Barry Brummer, ref. [23], p. 303. [27] T. Katsube, I. Lauks and J.N. Zemel, Sensors and Actuators 2 (1982) 399. [28] Matijan Vukovic, J. Appl. Electrochem. 17 (1987) 737. [29] L-S. Lee, R.A. Buchanan and J.M. Williams, in: Biomedical Materials and Devices, eds. Jacob S. Hanker and Beverly L. Giammara, Mater. Res. Sot. Symp. Proc. vol. 110 (Materials Research Society, Pittsburg) to be published.
VI. APPLICATIONS