2018 年 58 巻 8 号 p. 1519-1523
Titanium and stainless steel have become attractive materials especially in medical applications due to their improvement of implant performance and longevity. The corrosion behaviour for stainless steel 316L (316L SS) and Ti-6Al-4V under simulated biological conditions with the presence of proteins were investigated using electrochemical methods. The electrochemical tests were performed with the addition of bovine serum albumin (BSA) with concentrations of 2%, 4%, 6%, 8% and 10% at 37°C. The electrochemical methods used in this study were open circuit potential (OCP), potentiodynamics polarization and electrochemical impedance spectroscopy (EIS). The results showed that stable corrosion resistance of the two alloys can be obtained under protein-containing solutions. The protein adsorption to the metal surface at high impedance results indicated that at all protein concentrations showed great performance and the most stable film formed on the 316L SS and Ti-6Al-4V are in the solution that contain 8% of BSA.
The metallic bio-implant materials such as stainless steel (SS), cobalt chromium, titanium-based alloys, composites and polymer are widely used in biomedical applications.1) These implants have a very different chemical nature and possess different biological environments when they interact with the tissues and bones in the body.2) Some major concerns should be taken into consideration when the implant interact with tissues around the implanted area such as corrosion, biodegradation of the implant surface and the changes or instability of the biological implant itself.3) This is because the interaction between the surface of the metal with human body fluids can have further influence on the mode and rate of metal ion release. They should not cause any negative effects like allergy, inflammation and toxicity either immediately after surgery or under post conditions. If this happens, it will take longer time to recover.4) Table 1 shows a survey of the failed implants.
| Reported cause | Number of implants | Incidence (%) |
|---|---|---|
| Fracture | 21 | 42 |
| Corrosion | 12 | 24 |
| Adverse tissue reaction | 7 | 14 |
| Cracks without fracture | 4 | 8 |
| Wear | 3 | 6 |
| Bending without fracture | 3 | 6 |
316L SS is the type of stainless steel that is commonly used for orthopedic, orthodontic, and cardiovascular implants.6) They have good biocompatibility and corrosion resistance, as well as excellent mechanical properties such as strong formability, high yield strength and high modulus of elasticity.7) The corrosion resistance of 316L SS depends on its chromium content. This is because chromium rapidly forms a Cr2O3 layer when they are under the influence of oxygen from air or water.8)
On the other hand, titanium alloys are commonly used as metal implant in human body because of its high mechanical resistance, low modulus of elasticity, excellent corrosion resistance, high biocompatibility and non-toxicity.3) Due to its light weight, as well as the similar characteristics with human bone gives many advantages when they are attached to the bone cells, which is the main reason why titanium is biocompatible without toxicity and able to adapt to the tissues in the human body.9)
In the human body, biomolecules such as protein ranges between 33 and 52 g/L of albumin in healthy humans. The component of albumin consist of 584 amino acids. The total protein content of plasma in albumin is about 50–60%,10) which is the most abundant plasma protein as well as the best known binder of biological fluids. These characteristic of albumin is the reason why it is often chosen to be added in simulated body fluid solutions to study the corrosion behaviour of bio-implant materials.11) Many recent studies have discovered that the protein, cell and tissue reactions play an important role when in contact with material implants.12) According to Wataha et al.,13) the protein-containing solutions could accelerate the release of elements from alloys. He found that more elements are released from alloys in solutions that contain BSA. Kanathila et al.14) also reported a maximum release of elements from Ni–Cr and Co–Cr alloys in BSA. No toxic effects occurred from the elements released. However, the presence of protein has been reported to either decrease or increase the corrosion rate, or have no effect in which created conflict that has to be further investigated.15)
Therefore, the aim of this work was to study the effect of different concentrations of bovine serum albumin (BSA) on the corrosion of Ti-6Al-4V and 316L SS. This work will improve the understanding of the metal-tissue interactions at the implant surface in real body environments.
A titanium aluminum vanadium alloy (ESPI Metals, Ashland) or Ti-6Al-4V and 316L SS (AK Steel Corporation) was used as the working electrode in the form of rods, with a diameter of 10 mm and 6 mm, respectively. The composition of the alloys is shown in Table 2 according to ASTM F-136-84 and ASTM A 240. For the electrochemical tests, the samples preparation was performed by sanding with 320 up to 600 grit finish, cleaned and rinsed with distilled water, dried with compressed air and left in open air for 5 minutes and then transferred quickly to the electrolytic cell. The working electrodes of the electrochemical measurements were glued to the wire using a conductive glue leaving a surface area for exposure to the solution of 4.7 cm2 and 2.5 cm2 for Ti-6Al-4V and 316L SS, respectively. A three-electrode cell setup was used, which consisted of a graphite rod as the counter electrode, and a saturated calomel reference electrode (SCE).
| Metal | Elements | Composition (%) |
|---|---|---|
| Ti-6Al-4V | Ti | 88.0 |
| Al | 5.5 | |
| V | 3.5 | |
| Fe | < 1 | |
| SS 316L | C | 0.03 |
| Mn | 2.0 | |
| P | 0.05 | |
| S | 0.03 | |
| Si | 0.75 | |
| Cr | 16.0 | |
| Ni | 10.0 | |
| Mo | 2.0 | |
| N | 0.10 | |
| Fe | Bal |
All electrochemical tests were performed using polished samples and fresh solution for every experiment. All potentials in the potentiodynamic polarization tests were measured versus SCE. The scan was commenced from −0.5 V below Eoc up to +0.5 V with a scan rate of 5 mV s−1. EIS was performed on five different albumin concentrations (2, 4, 6, 8 and 10 g/L) of each sample after 24 hours of immersion. EIS was conducted by imposing a 10 mV sinusoidal oscillation over a frequency range of 300000 Hz to 0.01 Hz. The impedance behavior of the specimen can be expressed either in Bode plots of impedance modulus (|Z|) as a function of frequency or in Nyquist plots of Z”(ω) as a function of Z’(ω), where ω = 2πf. The composition of the phosphate buffer solution (PBS) was (in g/L) 8 NaCl, 0.224 KCl, 1.42 Na2HPO4 and 0.272 KH2PO4.
Electrochemical test were conducted at a temperature of 37°C and maintained at pH 7.4. All electrochemical measurements were carried out with a Gamry model G300 potentiostat/galvanostat, which was controlled by a software provided by Gamry Instruments Company. The EIS tests were performed when the OCPs became relatively stable after immersion for 1 h. Table 3 shows the chemical composition of phosphate buffer solution (PBS) used as an electrolyte for the electrochemical cell.
| Solutions | Compounds | Concentration (g/L) |
|---|---|---|
| PBS | NaCl | 8.19 |
| KCl | 0.2235 | |
| Na2HPO4 | 1.42 | |
| KH2PO4 | 0.272 |
In order to evaluate the effect of protein concentration on the corrosion resistance of 316L SS and Ti-6Al-4V, polarization curves were obtained in the PBS solution, with different concentration of BSA (0%, 2%, 4% 6%, 8% and 10%) and the results are presented in Figs. 1 and 2, respectively.
Potentiodynamic polarization curves for 316L SS alloy in PBS solution with different concentrations of BSA.
Potentiodynamic polarization curves for Ti-6Al-4V in PBS solution with different concentrations of BSA.
Figure 1 shows the potentiodynamic polarization for the highest corrosion potential (Ecorr) happened in the solution with 8% of BSA. Even though for solutions that contained 0% and 10% of BSA showed the lowest corrosion potential at the beginning, the pattern of the graph looked similar with 2%, 4% and 6% of BSA at −0.6 mV to −0.2 mV. This shows that at concentration of 8% BSA, the proteins are most effective to decrease the corrosion rate of 316L SS by acting as a protective barrier from the initial stages of corrosion. This is probably due to an increase in thickness and density of the Cr2O3 passive film that was reported by Hashemi et al.16)
However, in Fig. 2, Ti-6Al-4V showed a different trend where at 0%, 2% and 8% of BSA, the corrosion potential starts to increase slowly as compared to 4%, 6% and 10% of BSA. The graph is clearly shown that the formation of TiO2 layers are most stable in solutions that contain 4%, 6% and 10% of protein concentrations. Previous studies stated that with the presence of albumin, the cathodic reaction of Ti alloys decreases, showing that albumin acts as a cathodic inhibitor.15)
3.1.2. EIS MeasurementsElectrochemical impedance spectroscopy (EIS) measurements were performed at open circuit potential conditions after the materials attained a stable open circuit potential in protein-containing solution. Impedance spectra for all investigated samples were recorded and presented in Figs. 3 and 4. The impedance behavior of the specimen is expressed in Bode plots of impedance modulus (|Z|) as a function of frequency. The impedance of constant phase elements (CPE) is defined by the Eq. (1):
| (1) |
Bode plots diagrams for 316L SS surfaces, after OCP, in PBS solution with 2%, 4%, 6%, 8% and 10% of BSA.
Bode plots diagrams for Ti-6Al-4V surfaces, after OCP, in PBS solution with 2%, 4%, 6%, 8% and 10% of BSA.
Equivalent circuit model used in the fitting of the impedance data at different conditions, Rs = solution resistance, Rct = charge-transfer resistance, Cdl = double layer capacitance, Rp = passive film resistance and Cp = passive film capacitan.
By analyzing Figs. 3 and 4, it can be seen that the impedance spectra for 2%, 4%, 6%, and 10% of BSA for both graphs show almost similar behaviour as the frequencies increases from −2 Hz to closely 6 Hz. This indicates a decrease in the protective ability of the Cr2O3 and TiO2 oxide film for 316L SS and Ti-6Al-4V. However, the Bode plot diagram showed a slightly different pattern at 8% BSA for both alloys. According to previous study, Pieretti et al.5) found that the addition of 10% of albumin in the solution acted as a protective role to corrosion of metallic implants. This suggests that the addition of 8% BSA creates a layer that avoids corrosion from happening due to passive film breakdown. For 316L SS, the presence of Cr2O3 film on the surface may have also played a role as protective layer.17)
The values of the equivalent circuit parameters for the 316L SS and Ti-6Al-4V in various BSA concentration in phosphate buffer solution are reported in Tables 4 and 5, respectively. Rp represent the passive film resistance in EIS spectra which can be used to evaluate the corrosion resistance. 316L SS and Ti-6Al-4V have negative film resistance at 8% albumin concentration. This demonstrates that the OCP stabilizes at a potential in the active–passive transition region again. However the data obtained at 8% of BSA for both metals were excluded, as Wang et al.18) has reported that it is meaningless to assess the corrosion resistance with the negative resistance.
| BSA concentration | Rs/Ω | Cdl/µF cm−2 | Rp/Ω cm2 | Cp/µF cm−2 |
|---|---|---|---|---|
| 2% BSA | 33.6 | −3.7 | 482.8 | −3.4 |
| 4% BSA | 29.8 | −3.7 | 351.7 | −3.5 |
| 6% BSA | 30.7 | −3.7 | 479.4 | −3.5 |
| 8% BSA | 3.1 | −3.5 | −3.4 | 3.2 |
| 10% BSA | 37.2 | −5.4 | 573.2 | −3.4 |
| BSA concentration | Rs/Ω | Cdl/µF cm−2 | Rp/Ω cm2 | Cp/µF cm−2 |
|---|---|---|---|---|
| 2% BSA | 29.2 | −4.3 | 3.0 | −3.4 |
| 4% BSA | 29.6 | −3.2 | 3.0 | −5.4 |
| 6% BSA | 32.7 | −3.6 | 971.2 | −3.5 |
| 8% BSA | 3.0 | −3.3 | −8.2 | −0.5 |
| 10% BSA | 33.0 | −3.2 | 4.9 | 406.4 |
Samples were also observed by optical microscope in order to analyze the changes in their surface morphology. The corrosion morphologies of 316L SS and Ti-6Al-4V after 24 h immersion and the electrochemical test in PBS solutions containing albumin are shown in Figs. 6 and 7, respectively.
Optical microscope images for 316L SS alloy obtained after 24 hours of immersion in (a) 2%, (b) 4%, (c) 6%, (d) 8% and (e) 10% of protein-containing solution.
Optical microscope images for Ti-6Al-4V obtained after 24 hours of immersion in (a) 2%, (b) 4%, (c) 6%, (d) 8% and (e) 10% of protein-containing solution.
For 316L SS alloy (Fig. 6), when the albumin concentration of 2% is added, it can be seen that the changes of appearance on the metal surface which showed obvious corrosion occurs compared to the absence of BSA in Fig. 6(a). In contrast, there is no obvious corrosion that occurs at 8% BSA (Fig. 6(d)). This means that 316L SS has mostly high passivity at protein concentration of 8%, which is supported by the results from polarization curve at Fig. 1 and Bode plots diagram in Fig. 3.
For Ti-6Al-4V (Fig. 7), most of the metal surfaces for every concentration also appeared rougher and produced white corrosion products except for 8% BSA. The brown corrosion products obviously are more than the white corrosion products. This white colour product may be related to the presence of the proteins adsorbed on the metal surfaces. The interaction between the metal surface and proteins may be different in each concentration due to different distribution of alloying elements (Ti, Al, V, Fe, Cr and Ni) in the microscopic scale and the microstructure of alloys, which are the fundamental reasons for the metal corrosion morphology and corrosion resistance.19)
The electrochemical techniques used in this investigation led to the following conclusions. The corrosion behaviour were shown for 316L SS and Ti-6Al-4V in PBS with addition of BSA conclude that there is slightly no significant difference at each of BSA concentration. The impedance results indicated that the film formed on the 316L SS and Ti-6Al-4V consisted of an inner barrier layer associated with high impedance and was responsible for the corrosion of the sample. The results revealed that 316L SS and Ti-6Al-4V both have stable corrosion resistance with the presence of albumin. These two alloys can perform well with the presence of protein-containing solutions thus they are compatible to use as implant materials in human body environment.
The authors gratefully acknowledge the help of the Research Management Centre (RMC) in providing the Fundamental Research Grant Scheme (Project Number: 600-RMI/FRGS 5/3 (80/2015)) and 600-RMI/RAGS 5/3 (37/2015) research grants and Universiti Teknologi MARA for their support.
