PLASMA ELECTROLYTIC OXIDATION COATINGS
FOR IMPLANTS SURGERY
S.V. Gnedenkov, S.L. Sinebryukhov, O.A. Khrisanfova, A.V. Puz’, M.V. Nistratova
Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, Vladivostok, Russia
Abstract. The possibilities of plasma electrolytic oxidation (PEO) for development of the composite coatings containing hydroxyapatite and calcium phosphates on the titanium and nitinol (NiTi) surface were demonstrated. Such layers include pores in the surface part of the coating which can be used as carriers for medicine (antibiotics, hydroxyapatite or other phosphate-containing substances providing the best compatibility of implant with bone tissues). Use of superdispersed polytetrafluoroethylene (SPTFE) in the coatings composition enables one to increase stability of substrate materials in the corrosion active media. Peo with subsequent SPTFE treatment makes it possible to obtain a bioactive or bioinert implant surface. In this case the polymer partly seals the pores where medicine previously was inserted. It decreases the medicine diffusion from the surface layer and, therefore, increases the duration of the therapeutic effect. anticorrosion protective coatings decreasing the nickel ions diffusion from nitinol substrate prevent nickel accumulation in human tissues and its harmful after-effect. Moreover, the way of hydroxyapatite formation in the coatings composition directly to over PEO was found. In this case the ratio of Ca/P equals to 1.4, i.e. it is close for human bone tissue ratio Ca/P (1.67).
Introduction
Titanium alloys have found the wide application in restorative surgery as basic biomaterials for manufacturing of implant prostheses. Amongst the various materials currently employed, the commercial pure titanium and alloy Ti–6Al–4V have found extensive biomedical applications due to their good mechanical properties. These properties combine good mechanical characteristics, high corrosion resistance and good compatibility with biological materials. The passivity is due to the very stable and tenaciously adherent oxide films spontaneously formed over the surface [1–3]. Besides, a bioinert material NiTi or nitinol (40–50 % at. Ti and 50–60 % at. Ni) having a unique memory shape effect has been recently introduced in implantation surgery [4, 5]. In particular, it is used to make holders for treating spinal traumas and dystrophic illnesses, tack-implants used for junction of breast bone during cardiologic surgeries etc. However, the diffusion and accumulation of nickel ions in the organism soft tissues might have adverse effects, for example, of carcinogenic nature.
Bioactivity of titanium surfaces is not high enough to induce direct growth of the bone tissue, and good bone fixation takes several months. Modifications of metal surfaces are often employed as a means of controlling tissue–titanium interactions and shortening the time for bone fixation [6]. Hydroxyapatite (HA) is a major component of bone [7]. The hydroxyapatite-coated metallic implants show high tensile strength and ductility of the metal, and bioactivity of hydroxyapatite. Enhanced biocompatibility and bioactivity of titanium-base materials may be achieved by coating them with ceramic and composite materials with using the plasma electrolytic oxidation (PEO) method. It was shown as a result of experiments, that this method enables one to obtain protective layers on the surface of a material with far better efficiency than by any other processing methods.
The PEO method is found to induce the emerging plasma micro discharges on the electrode surface during the anodic or AC current polarization of the processed material under the high voltage. As a result of local high energy effect, the layers including as elements of the substrate (oxidized material) as well those of electrolyte are formed on the surface of materials [8, 9]. The properties of such layers differ from those of conventional anodic films. Subsequent treatment of the previously created PEO structure (filling pores by bioactive and/or bioinert composites) allows building composite coatings that could be prospective in terms of practical application in the implant surgery.
At present, there is insufficient information in literature concerning the development of protective coatings on nitinol (TiNi) regarding the use of the PEO method. That is why it appears to be advisable to search the electrolyte compositions and oxidation conditions to build composite structures with protective properties on the nitinol surface to study their phase composition, surface morphology, anticorrosion and mechanical properties.
Hydroxyapatite coatings on titanium
Commercially pure titanium VT1-0 (Ti – 99.4 %) plates (70 mm × 15 mm × 1 mm) were used as the substrates for PEO. As a pretreatment procedure, all samples were ground using #400–#1000 SiC sandpaper gradually and then washed with acetone and distilled water. The electrolyte was prepared by dissolving 30 g/l disodium hydrogen phosphate dodecahydrous (Na2HPO4∙12H2O) and 30 g/l calcium citrate (Ca3(C6H5O7)2∙4H2O) in bidistilled water.
PEO treatment were implemented with using the automatic control system (ACS) consisting of the power source, control and measurement unit, computer and software. The standard three-phase thyristor rectifier of type TEP-100.460H-2-2UHL4 was used as a power source. The ACS ensured the real time conditions of the technological process parameters and detected the appearance of failures in the system functioning. PEO process was carried out in two steps for 10 minute. First one is the anodic polarization in the unipolar potentiostatic mode at 310 V for 400 seconds. The second step is the combined mode, which is the combination of the anodic potentiodynamic (dU/dt = 1.25 V/s) and cathodic galvanostatic (jc = 1 A/cm2) polarization modes for 200 seconds. The duration ratio of the anodic and cathodic periods of polarization was τa/τc = 4.
Figure 1 shows the micrographs of the surface (Fig. 1 a) and the SEM cross-sectional view (Fig. 1 b) of titanium substrate treated with PEO in the Ca- and P-containing solution. It could be seen that a porous network structure was formed on the surface of titanium. The pores were well separated and homogeneously distribute over the surface and bulk of the coating, and the pores in sizes varied from 1 to 20 mm. The thickness of the coating was approximately 120 mm. It was established that the thickness of the coatings increases not proportionally during the PEO process (Fig. 2). The coating thickness grows linearly up to 110 mm during a first stage of the process for 300 seconds and then this increases by 10 mm only. According to the calculation with the software of ImageJ 1.38x the porosity of the coating was about 10 %.
a) b)
Fig. 1. SEM surface morphologies (a) and cross-section view (b) of Ti sample with coating.
Fig. 2. Dependence of the coatings thickness on the treatment time of the titanium by PEO.
According to the X-ray data (Fig. 3) in this experiments the Ca- and P-containing coatings including HA were obtained. The coating formed in the Ca- and P-containing solution contained Ca and P with Ti and O, as shown in Fig. 4. It was implied that the elemental component in an electrolytic solution was introduced into the coating by PEO. Relative concentrations of elements on the surface and in the interface of titanium specimens treated with PEO and the ratios of concentrations of calcium to those of phosphorus are not equal. The ratio of Ca to P on the surface was 1.4 while that in the interface was 0.1. The content of Ca was gradually raised and concentration of Ti was gradually cut down while concentrations of O and P were relatively unchanged from the titanium substrate to the surface. Thus we have in surface region the layer which contains the Ca- and P-compounds without the titanium oxides. The thickness of this layer on different samples varied from 10 mm to 30 mm. Formation of an apatite layer on the surface of metal implant provides the living body a favorable condition for this material to bond to the living bone. According to the detailed analyses of the surface apatite layer (Fig. 1), it was revealed that the surface layer consisting of nano-size apatites similar to the bone mineral in its structure and composition. As a result, the surrounding bone can come into the direct contact with the surface HA-layer on the implant. When this occurs, a chemical adhesion is between the surface of apatite and bone mineral in order to reduce the interface energy between them. It can be inferred from these that a new biomaterial is able to form bone-like apatite on its surface in the living bone through the porous apatite layer. The calcium phosphate is a precursor forming apatite and HA. The porous surface of implants is beneficial to bone tissue growth and enhances the anchorage of implants to the bone. At the same time, a defined porous structure may be valuable as a depot for bioactive constituents such as growth factors or bone morphogenetic proteins. Therefore, the porous and HA-containing coating on the titanium formed by PEO method and presented in this study is expected to be significant for medical applications.
Fig. 3. Diffractogram of titanium sample surface processed by the plasma electrolytic oxidation method (PEO).
Fig. 4. Depth profiles of the elements in the PEO coating on the titanium.
Composite protective coatings on the nickel-titanium alloy
Nickel-titanium alloy (NiTi) is the one of the most popular materials for various biomedical applications. The almost equiatomic nickel-titanium alloy is unique in that it possesses interesting properties such as shape memory effect and superelasticity. The nickel-titanium alloy is successfully applied in manufacturing of special devices for the medicine due to its mechanical properties. However, implantation of nickel containing materials in human’s body requires some caution. The metallic implants inevitably undergo in some degree of corrosion in body fluids. This processes lead to the releasing of the nickel from implants into the human’s body. It is well known, that the nickel is capable to cause a toxic and allergic responses when its concentration exceeds a certain limit.
A selection of electrolytes providing the possibility of obtaining the titanium oxides, aluminum oxides and phosphates or spinels in the composition of anticorrosion layers was used in our experiments. The above electrolytes included those containing aluminates, phosphates, carbonates and vanadates. In accordance with the X-ray analysis data, the aluminum phosphate AlPO4 and nickel-aluminium double oxide NiAl2O4 were presented in the coating obtained by PEO method (Fig. 5). During the analysis of diffractograms of surface layers of some samples, the presence of an oxygen-containing compound of nickel and titanium Ni3Ti3O was detected (its concentration was negligibly small on the diffractogramm shown in Fig. 5). At the same time, the titanium oxides were not found in the surface layers composition.
Fig. 5. Diffractogram of nitinol sample surface processed by the plasma electrolytic oxidation method (PEO).
The ESM pictures of the coating surface (a) and the optical picture (´1000) of the sample cross-section with the surface PEO-coating are shown in Fig. 6.
a) b)
Fig. 6. ESM (a) and cross-section photo (b) of PEO coating formed on the nitinol surface.
The obtained PEO-coatings were studied by electrochemical impedance spectroscopy. This method enables one to investigate the processes occurring at the electrode/electrolyte interface with taking into account specific features of the surface structure. The impedance spectra presented in a Bode plot (the dependence of impedance magnitude |Z| and phase angle theta versus the frequency) for nitinol samples after different kinds of treatment are shown in Fig. 7.
The impedance spectra of the samples with and without SPTFE coating are virtually identical (see Fig. 7, curves 1 and 2). The processing of nitinol surface by SPTFE powder has small effect on the state of the electrode/electrolyte interface and increases insignificantly the impedance. The low effect of such processing must be related to weak adhesion of the polymer to the metal substrate and insufficient homogeneity of the formed protective layer. The sample processing by the PEO method results in increase of the nitinol stability in the corrosion media. The addition of dimethylglioxyme into the electrolyte somewhat increases the coatings protective properties (see Fig. 7, curve 5) although this difference is not very significant, as it seen from the impedance spectra.
Fig. 7. Bode plots for the investigated nitinol samples: 1 – without coating; 2 – coated by SPTFE (heating at 100°С, 1 h) without the preliminary treatment; 3 – with PEO-coatng formed in unipolar mode (electrolyte: Na3PO4·12H2O – 10 g/l, NaAlO2 – 20 g/l, Na2CO3 – 10 g/l); 4 – with PEO-coatng formed in bipolar mode (electrolyte: Na3PO4·12H2O – 10 g/l, NaAlO2 – 20 g/l, Na2CO3 – 10 g/l); 5 – with PEO-coating formed in unipolar mode (electrolyte: Na3PO4·12H2O – 10 g/l, NaAlO2 – 20 g/l, Na2CO3 – 10 g/l, dimethylglioxyme – 1 g/l); 6 – as the sample № 3 and treatment by SPTFE; 7 – as the sample № 4 and treatment by SPTFE. The equivalent circuit simulating the experimental data is presented in the insert.
The only appreciable increasing of the impedance was detected on the polarization curves (see Fig 7, curve 5). One can suggest that a chelate compound – nickel dimethylglioxymate – deposited in pores of the oxide layer stimulates the increasing of the oxide layer protective properties. However, its concentration in the film is not high (less than 10 %), because the lines attributed to this phase were not detected in the diffractogram. Being a thermally unstable compound, nickel dimethylglioxymate is partly decomposed under the effect of PEO that is known to involve attainment of high temperatures in short-lived plasma channels and due to thermolysis in the coating area adjacent to the plasma channel. That is why low content of the nickel dimethylglioxymate in the coating pores does not allow attaining the efficient corrosion protection under significant field shifts.
Two time constants can be seen on the diagram of the phase angle dependence versus the frequency for the samples with PEO coating (Fig. 7) as compared to the samples without coating and containing SPTFE only. The above data indicate to the two-layer coating structure: the upper layer is porous while the lower one is pore-free. It is in a good agreement with the earlier suggested model of the PEO-coating [10]. As a rule, the porous layer has crater-like cavities with the diameters up to few micrometers. So the pore sizes are larger than the size of SPTFE powder (nearly 1 µm) applied for surface treatment. The development of the porous structure can be an additional advantage of the PEO method, since the developed surface promotes the best overgrowing of implant by bone tissue and allows filling pores with bioinert or bioactive composites.