Electrophoretic deposition of hydroxyapatite-iron oxide-chitosan composite coatings on Ti–13Nb–13Zr alloy for biomedical applications

Highlights

• HA–Fe₃O₄–CS composite coatings were fabricated on Ti–13Nb–13Zr alloy by EPD.

• The surface of the composite coatings was free from cracks.

• Composite coating of HA@1 wt% Fe₃O₄ revealed the finest corrosion resistance.

• The generation of apatite mineralization of composite coating enhances with incorporation of Fe₃O₄.

Abstract

In this study iron oxide (Fe₃O₄) incorporated hydroxyapatite (HA) and chitosan (CS) composite coatings were developed to enhance the corrosion resistance and surface properties of Ti–13Nb–13Zr alloy. Pure HA and composites with different percentage (1, 3 and 5%, wt%) of Fe₃O₄ have been fabricated by using the electrophoretic deposition method (EPD). The morphology of the composite coating indicated porous nature with average 10 ± 0.2 µm coating thickness, were examined by scanning electron microscopy (SEM). The wettability behavior and surface roughness values of the composite coatings were measured and discussed. The Electrochemical test was performed to evaluate the corrosion behavior in Ringer's solution. The HA-1wt% Fe₃O₄ composite coating exhibited the preeminent corrosion resistance with lowest I_corr and the highest E_corr value as a contrast to HA-3wt% Fe₃O₄ and HA-5wt% Fe₃O₄ coatings. Additionally, in vitro bioactivity test were conducted in Ringer's solution for 7 days. It was found that the composite coating enhanced the apatite formation ability, which was further endorsed by SEM-EDX analysis. In conclusion, the composite HA-CS bioactive coating incorporated with Fe₃O₄ is proposed as a promising candidate for biomedical applications.

Introduction

During the last decades, there is an increasing demand for replacement of damaged bones or hard tissues because of various diseases like osteoarthritis (inflammation in the joints of bone) [1], osteoporosis (weakening of the bones) [2], dentistry [3] and war-related injuries [4]. For this purpose, the biomaterials are widely used to regenerate the skeletal tissues in the body. Presently, the biomaterials used for dental and orthopedic applications are Titanium (Ti) based alloys, Stainless steel, Magnesium based alloys and Cobalt chromium alloys [5]. Among these biomaterials, Ti-based alloys proved to be the relevant material for their better corrosion resistance, attractive biocompatibility, moderate elastic modulus and favorable strength to weight ratio [6]. Moreover, in various fields such as osteosynthesis, oral implantology and joint prosthetics Ti alloys have exhibited their potential as an implant material [7]. In biomedical engineering, generally two phases (α + β) such as Ti–6Al–4V and Ti–6Al–7Nb alloys have been broadly employed as metallic implants but their long term use causes a stress shielding problem due to their biochemical incompatibility issues [8]. These two particular alloys contain vanadium and aluminum compounds that release toxic ions and produce adverse health effects [9]. Vanadium causes cytotoxicity whereas neurological disorder (Alzheimer's disease) is caused by aluminum [10]. Therefore, current studies have been focused on β alloys (Zr, Nb, Ta and Mo as alloy additives) which possesses better properties such as good ductility, similar mechanical strength, enhanced corrosion resistance and lower elastic modulus as compared to Ti–6Al–4V alloy [11,12]. The Nb and Zr into Ti are more biocompatible than V and Al and govern to the formation of β alloy structure [13].

In spite of all these features, Ti-based alloys are incapable to induce bone formation on their surface as they are classified as bioinert material. Moreover, implant failure occurs as there is insufficient growth of bone tissues [14]. Therefore, to improve the osseointegration phenomena various surface modifications have been done on Ti-based alloys. Among these surface modifications, HA has proved to be a potential candidate for bone tissue applications. HA boosts osteoconductivity and cell responses as it possesses similar compositional resemblance and crystal structure to that of human bone [15]. Moreover, HA coating helps to reduce the Ti-body fluid contact because of their good interfacial bond to Ti alloy substrate [16]. Various methods such as the sol-gel method [17], electrochemical method [18], plasma spray method [19] and electrophoretic deposition [20] were engaged to achieve the HA coating. Among these methods, EPD has shown more interest as it could be operated under atmospheric pressure and relatively low processing temperature [21]. As a contrast to other coating methods, EPD has many advantages such as low cost, simplicity, control the coating thickness, high production rates and ability to coat a substrate with intricate shapes [22], [23], [24].

Reportedly, implant failure occurs due to the high degradation rate of pure HA coating, which reduces the long term survivability of the implants in the body environment [25]. Huang et al. [26] demonstrated that the reinforced- HA coating has better potential to enhance the life of the implant than pure HA coating. To improve the properties of HA coating several reinforcements had been studied in combination with ceramic and polymer phases such as titanium oxide [27], silica oxide [28], iron oxide [29], zinc oxide [30], polycaprolactide [31], chitosan [32] and polylactide [33]. However, there is a growing interest for the Fe₃O₄ particles among the research community for their excellent characteristics such as tissue repair, small size, good mass transfer efficiency, the high surface area, non-toxicity, biocompatibility, easy preparation, cell imaging, drug delivery and purification of cell populations [34], [35], [36], [37]. In addition, Fe₃O₄ particles play a vital role in various in vitro and in vivo applications [38].

In order to refine the mechanical and biological behavior of inorganic composite materials, a great deal of interest is focused on organic polymer coatings [39]. Among the organic materials, chitosan acts as a capping agent due to their remarkable properties such as wound healing activity, biocompatibility, biodegradability, antibacterial and environmental friendliness [40], [41], [42]. CS is a naturally occurring polysaccharide which strongly interacts with negatively charged bodies [43]. Moreover, the utilization of CS vanish the disadvantage of high-temperature and prevent the substrate from micro-cracking and shrinkage [44]. In the earlier study, it has been suggested that the corrosion resistance and biological performance was improved by CS composite coatings on Ti substrate [45]. Yadav et al. [46] prepared the composites by utilizing the combination of metal oxide particles and CS. It was observed that the biocompatibility and surface properties were improved by incorporation of Fe₃O₄ particles. Due to the presence of amine groups, CS provides a hydrophilic environment which is more compatible for the biomolecules [47]. Therefore, reinforcement of Fe₃O₄ as a secondary phase in HA and CS composite coating is a favorable approach to enhance the surface properties as well as corrosion resistance.

In the present investigation, the Ti–13Nb–13Zr alloy was electrophoretically coated with pure HA, pure Fe3O4 and HA–Fe₃O₄–CS coatings by altering the concentration of Fe₃O₄ at three different levels i.e. 1, 3 and 5 wt%. As per literature available, the findings of corrosion resistance and analysis of surface properties of EPD coated Fe₃O₄ particles with HA and chitosan-based coatings on the Ti–13Nb–13Zr alloy are yet to be established. In addition, the bioactivity behavior, surface roughness and wettability of the samples were observed and discussed.