Influence of Surface Modification on Corrosion Behavior of the Implant Grade Titanium Alloy Ti-6Al-4V, in Simulated Body Fluid: An In Vitro Study

INTRODUCTION

Metallic biomaterials, such as, 316L stainless steel, cobalt-chromium-molybdenum, pure titanium, and titanium-based alloys, are commonly used as orthopedic and dental implants. Biomaterials are commonly defined as non-viable materials intended to interact with biological systems to evaluate, treat, augment, or replace any tissue, organ, or function of the body.¹ Biocompatibility is an essential requirement of a biomaterial. A biocompatible material performs an appropriate host response (i.e., minimum disruption of normal body function) in a specific application.² It implies that the material should not cause any toxic or inflammatory reaction when placed in the human body. The factors that determine the biocompatibility are, the host reactions induced by the material and the degradation/corrosion of the material in the body/oral environment.

Commercially pure titanium (CP-Ti) and its alloys are mostly used as medical implants.³ They are corrosion resistant due to the formation of a protective oxide layer on its surface, thus preventing corrosion. Consequently, the release of ionic or by-product residue into the periprosthetic tissue is minimal and these biomaterials may be classified as biologically inert or electrochemically passive in the whole range of clinically relevant potential-pH combinations.⁴ Ti-alloys with their excellent chemical and mechanical properties and biocompatibility are a suitable choice for implant applications, thus reducing the utilization of rare metals. These alloys have become the structural materials for replacing hard human tissue. Among the other mechanical properties, the low Young’s modulus of these alloys avoids the occurrence of stress shielding after implantation.⁵

Although the CP-Ti and Ti-6Al-4V alloys exhibit excellent resistance against general and pitting corrosion, the low wear resistance and the possibility of vanadium release from Ti-6Al-4V may induce aseptic loosening during long-term implantation.^6,7 Even though Ti-6Al-4V releases undesirable vanadium, the formation of a uniform, adherent TiO₂ film, makes it a choice for bioapplications. Ti-6Al-4V is modified by replacing V with Nb, Zr, or Ta to make it more biocompatible and corrosion-resistant. Another approach adopted was surface modifications, such as, laser (laser surface melting and laser surface alloying), electrochemical oxidation, and thermal oxidation techniques to enhance the corrosion resistance and biocompatibility.⁸ Also, modification of the surface of the dental implants by changing the roughness and porosity of the surface is known to enhance the process of osseointegration,^9–16 which is considered to be crucial for ideal prosthetic fixation.

On implantation, body temperature remains 37°C, but pH drops from 7.4 to 4.00 due to the formation of hematoma around the implants.¹⁷ Intraorally, exposed parts of the implant come in contact with highly acidic food and fluorides of dental products that are corrosive. Hence, there is a need to assess the biocompatibility of an implant material as such and with surface modifications, thus enabling us to understand the degradation/corrosion of the material in the body/oral environment on long-standing.

Corrosion behavior of dental alloys is assessed in simulated body fluid (SBF) using different techniques, such as, potentiostatic, anodic polarization, and impedance spectroscopy for characterization of corrosion.^18–22 Various solutions like artificial saliva, Ringer’s solution, and SBF have been used^23–26 as corrosive media to simulate the conditions encountered inside the body.

The main objectives of the present study are to improve the surface area of implant grade Ti-6Al-4V alloy by modifying its surface through hydroxyapatite coating, surface texturing, and sintering with TiO₂ using LASER, a combination of chemical and thermal treatments, and surface oxidation treatments; evaluate their corrosion resistance which governs the biocompatibility and durability of the modified surfaces by subjecting them to corrosion tests namely, electrochemical impedance and cyclic polarization tests (CPTs) in SBF. The null hypothesis being that the different surface modifications of implant grade Ti-6Al-4V alloy surfaces do not influence its corrosion behavior.

The present investigation is concerned with the study of the influence of surface modifications of the implant grade titanium alloy, Ti-6Al-4V, on its in vitro biocompatibility through electrochemical impedance spectroscopy (EIS) and CPTs in SBF at 37°C.

MATERIALS AND METHODS

RESULTS

DISCUSSION

As stated by Eliaz,³⁴ the corrosion resistance of an implant material affects its functionality and durability; it is a primary factor governing biocompatibility. Implantation may lead to hypersensitivity and cancer due to the exposure of fluctuating temperature and body fluids, despite the implant showing good clinical performance. Intraorally, partial dental frameworks and dental implants are exposed to saliva and varying acidic and alkaline conditions. Hence, understanding the corrosion behavior of dental implants in specific environments is very essential.

In vitro evaluation of localized corrosion, such as, pitting corrosion, crevice corrosion, and stress corrosion cracking of the metallic biomaterials can be evaluated using cyclic polarization techniques.³⁵ In the present study, Electrochemical impedance spectroscopy test and CPTs were carried for all the groups using GAMRY Potentiostat following the instructions by ASTM standards.²⁸ Polarization resistance (R_p) is known to be an important parameter to characterize corrosion resistance of materials derived from the EIS test; higher the value of R_p betters the corrosion resistance. In the present study, corrosion resistance was markedly improved in TG concerning UMS (control group). On the contrary, corrosion resistance of the other surface-modified groups HA, LS, LT, HT850, and HT1050, was found to be reduced as compared to that of the UMS. Corrosion resistance was lowest in the HT1050 conditions.

Silverman stated that pitting potential (E_pit), repassivation or protection potential (E_rp), the potential of anodic to cathodic transition, hysteresis, and active-passive transition (anodic nose) are the parameters used to interpret the cyclic polarization curves.³³ The more difference between the E_pit and E_rp and the larger area of a positive loop (hysteresis) demonstrating the probability of low pitting corrosion resistance.²⁸

The relative position of pitting potential and repassivation potential or protective potential concerning the corrosion potential are the most important parameters for evaluating the pitting corrosion behavior.³⁶ In the polarization curve, the scanning direction changes at pitting potential as a forward curve, scanning continues until the reverse curve crosses the forward polarization curve and this intersection point is named protection potential. Pitting potential is the minimum potential at which the material tends to the pitting corrosion. Above the pitting potential, new pits will initiate and develop.³⁷ Repassivation potential is the potential at which the growth rate of pits is stopped.

The occurrence of hysteresis in the curve is when the forward curve is not overlaid with the reverse scanning curve. The difference between forward and reverse current density at the same potential demonstrates the size of hysteresis. The amount of hysteresis or in the other words, the difference between E_pit-E_rp indicates the amount of localized corrosion.³⁸ The difference between the E_pit and E_rp and also the area of the hysteresis loop indicate the probability of localized corrosion.

The representative cyclic polarization curves of all the groups obtained in SBF in the present study are shown in Figure 2. It may be seen that there is a wide variation in the protection potential and the area of the hysteresis loops. The absence of a hysteresis loop during the potential scan (the forward curve coincides with the reverse curve) indicates that there was no occurrence of the localized corrosion^39,40 which is seen in the case of TG. Positive loops concerning UMS, HA, LS, LT, HT850, and HT1050 show a decrease in passivity due to localized corrosion (pitting and crevice corrosion).

However, it is important to understand the present in vitro study results have to be applied cautiously as it differs from in vivo tests. In vitro and in vivo corrosion data were reviewed by Kuhn et al.⁴¹ and concluded that the use of electrochemical techniques conducted in simple electrolytes affords a valid means of ranking or screening biomaterials.

Revie and Uhlig⁴² summarized the characteristics of surface oxide film on various metallic biomaterials. Commercially pure Ti surface oxides constitute Ti0, Ti2+, and Ti3+ and Ti4+. Ti-6Al-4V alloy surface consisted of TiO₂, Al₂O₃, hydroxyl groups, and bound water. The alloying element V was not detected. These findings matched the surface oxides of UMS group of our study.

It may be seen that while there was the only presence of TiO₂ in the TG sample, there was TiO₂ as well as Al₂O₃ in the LS condition. X-ray diffraction analysis found the formation of newer oxides [LASER textured (LS)—V₂O; oxidized state (HT850)—V₆O₁₃ and AlTi₃; oxidized state (HT1050)—V₂O₅, VO₂, and oxidized state (HT1050)—V₂O₅, VO₂] in their respective modifications as compared with unmodified surface alloy of the present study. These newer oxides may be detrimental to the biocompatibility of the Ti-6Al-4V alloy which needs to be correlated with the results of corrosion tests.

Titanium and its alloys are the most widely used as medical implants as they are light, biologically, and chemically inert and have a low electrical and thermal conductivity in comparison to other metals. The modulus of elasticity of titanium and its alloys is closer to that of bone than the moduli of stainless steels and cobalt-based alloys, thus reducing the occurrence of stress shielding after implantation. Minimal image interferences are seen in magnetic resonance imaging (MRI) or computed tomography (CT) scans due to their weak paramagnetism. Their coefficient of thermal expansion is similar to that of bone, thus minimizing the distortion of MRI images. The formation of a thick oxide layer on the surface of titanium and its alloys is due to their high affinity to oxygen, providing corrosion resistance and histological osseointegration. This oxide layer is thermodynamically stable and found to repassivate.³

Ti-6Al-4V used in the present study is the most common alloy used in medicine.²⁷ It combines good workability, heat treatment ability, and weldability, as well as high corrosion resistance, high strength, and biocompatibility.

In the present study, on optical microscopic evaluation of the Ti-6Al-4V alloy showed a dual microstructure (Fig. 1), which consisted of mainly fine-grained hcp (α) phase distributed in bcc (β) phase indicating higher strength and corrosion resistance. The presence of elements, such as, Al, O, N, Ga, and C was found to stabilize the phase, whereas metals, such as, V, Nb, Ta, and Mo stabilized the β phase. The microstructure and mechanical properties of Ti-6Al-4V are greatly dependent on the thermomechanical processing treatments. If the material is cooled too slowly, the β phase becomes more noticeable and lowers the strength and corrosion resistance of the alloy.³

Aksakal et al.⁴³ stated that leaching of the metallic ions in vivo due to corrosion leads to various biological effects. The biomaterial Ti-6Al-4V alloy showed leaching of aluminum (Al) causing epileptic effects and Alzheimer’s disease; vanadium (V) was toxic in the elementary state. Asri et al.⁴⁴ stated that surface modifications may be considered as a “best solution” to enhance corrosion resistance; besides achieving superior biocompatibility and promoting osseointegration of biomaterials. Several surface modification techniques include deposition of the coating, development of passivating oxide layer and ion beam surface modification, and also surface texturing methods, such as, plasma spraying, chemical etching, blasting, electropolishing, and laser treatment. A systematic review by Wennerberg and Albrektsson stated that the titanium surface topography influenced bone response at the micrometer level.⁴⁵

Queiroz et al.⁴⁶ assessed the surfaces of commercially pure titanium implants (cp Ti) with modified surfaces by a laser beam (LS); with and without hydroxyapatite (HA) deposition; without (HAB) and with (HABT) thermal treatment utilizing histomorphometric and descriptive histological analyzes in their in vivo study. It provided evidence that LS, HAB, and HABT-modified surfaces improved bone-to-implant contact and increased bone formation around osseointegrated implants compared to conventional machined implants favoring the osseointegration process. But, the R_p values of LASER sintering (LS) were found to be second-lowest in our study consisting of Ti-6Al-4V alloy which suggests lower biocompatibility; LASER texturing (LT) was closer to UMS (control) and HA.

Tian et al.⁴⁷ stated that lasers have the intrinsic properties of high coherence and directionality, as its beam can focus onto the metallic surface to perform a broad range of treatments, such as, remelting, alloying, and cladding, which are used to improve the wear and corrosion resistance of titanium alloys. del Pino et al.⁴⁸ performed surface oxidation of titanium by irradiation in the air with an Nd:YAG (λ = 1.064 μm) laser operating in pulsed mode and reported that their compositional studies performed by XRD showed that the coatings were mainly composed of Ti₂O and TiO which are similar to our study. Excimer laser surface treatment of Ti-6Al-4V alloy was done by Yue et al.⁴⁹ to improve the pitting corrosion resistance of the alloy. The electrochemical behaviors of the untreated and the laser-treated specimens were evaluated by electrochemical polarization tests. Excimer laser surface treatment significantly increased the pitting potential of the Ti alloy, especially when the material was treated in argon gas.

In the present study, an attempt was made to evaluate and compare chemical, thermal, and lasers surface modifications on the same experimental setup and to understand the corrosion behavior of surface-modified Ti-6Al-4V alloy along with the unmodified surface. It is obvious that while in the HA sample there is hydroxyapatite, a biomaterial, and there are TiO₂ and Al₂O₃ in LASER sintered (LS) and combined chemical and thermal treated (TG)—TiO₂ surface-treated conditions. The TiO₂ and Al₂O₃ oxides are known to be bio-inert⁵⁰ materials. TG, HA, and LS oxides are suggestive of good osseointegration but when correlated with the results of corrosion tests, LS shows less R_p indicating less corrosion resistance.

Hydrogen peroxide (H₂O₂) is found to be an oxidizing molecule, which is secreted by cells and is associated with wound healing and is thus present in significant quantities in fresh implant environments. Also, the highest corrosion resistance of the TG sample [surface treatment consisted of soaking of the sample in a 6% (by mass) hydrogen peroxide solution at 60°C for 4 hours and subsequent heating at 400°C for 1 hour, followed by furnace cooling] may be attributed to the formation of a thin, compact, and adherent film of titania gel (TiO₂) on its surface which match the results of studies done by Bearinger et al.^51,52 They studied the morphological changes of TiO₂ oxide films on CP-Ti and Ti-6Al-4V alloy; upon exposure to PBS and hydrogen peroxide-modified PBS solutions, all the samples were found covered with protective titanium oxide domes developed in that area.

Corrosion tests, a tool widely used to assess the self-passivating ability and resistance against pitting in all groups, showed enhanced polarization resistance (R_p) and low susceptibility to localized corrosion (retracing of forward and backward scan without the formation of hysteresis loop) in TG sample suggests its improved biocompatibility. Thus, the null hypothesis stating that the different surface modifications of implant grade Ti-6Al-4V alloy have no influence on its corrosion behavior is being rejected.

The study could not mimic the in vivo environment due to the high concentration of dissolved in O₂ in isotonic solutions as compared to venous blood which may lead to passivation. The circular flat disks samples used in the present in vitro study do not match the implant geometry which has threads for mechanical engagement in bone; size, shape, and surface wettability of the alloy are found to influence biocompatibility.

There is a need to evaluate the corrosion behavior of new Ti alloys without vanadium, such as, Ti–15Mo–3Nb, Ti–Mo–Nb–Al, Ti–Mo–Nb–Al–Cr–Zr, Ti–13Zr–13Nb, Ti–15Zr–4Nb, Ti–6Al–7Nb, Ti–Zr–Nb–Ta–Pd, Ti–Mo–Nb–Al, Ti–Mo–Nb–Al–Cr–Zr, Ti–Zr–Nb, and Ti–Zr–Nb–Ta–Pd. To assess the influence of fluoride ions, present in dental gels on the surface-modified layer of titanium alloy Ti-6Al-4V and the stability of the corrosion products formed.

CONCLUSION

• Combined chemical and thermal treated (TG) condition has highest corrosion resistance in comparison to unmodified titanium surfaces (UMS), whereas hydroxyapatite-coated, LASER-modified surfaces, oxidized states (850 and 1,050°C) are lower to UMS. Oxidized state at higher temperatures (1,050°C) was the lowest corrosion-resistant surface.
• Surface modifications have enhanced the resistance against localized corrosion in TG followed by HA, LS, LT, and HT850 than unmodified titanium surfaces (UMS) by increasing the surface passivity except in oxidized state at higher temperatures (1,050°C) within the limitations of the study.
• Combined chemical and thermal treatment of titanium alloy showed greater biocompatibility than the unmodified surface.

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