Ultrasonic aqueous synthesis of corrosion resistant hydroxyapatite coating on magnesium alloys for the application of long-term implant
Introduction
As an ideal biodegradable and bioactivity metal device in the orthopedic clinical application, it should perfectly meet with the injured tissue reconstruction process in human environment by virtue of its excellent biodegradability, mechanical compatibility and biocompatibility [1]. Magnesium and its alloys have considerable potential as the promising temporary implant biomaterials, owing to elastic modulus similar to nature bone, biodegradability, favorable strength and low toxicity [2]. Unfortunately, too rapid degradation rate of magnesium alloys brings about consequent excessive loss in mechanical integrity and a huge hydrogen release, which impedes their clinical application [3]. From the standpoint of clinical application requirement of biodegradable materials, the ideal degradation cycle of biodegradable implants should take at least 3–6 months to provide early support and fixation for bone reconstruction, and the implants finally are gradually absorbed by degradation or eliminated through metabolism.
It is well known to us, the corrosion degradation property is closely associated with the surface structure and composition of magnesium alloys. In recent years, to overcome the demerits of sever corrosion, numerous attempts focused on developing specific coatings on the surface of magnesium alloys have been made, one of which is bioactive HA coating that exerts an enhanced role on improving the corrosion resistance and promoting adhesion, proliferation and differentiation of cells, and new bone formation due to its close chemical resemblance to natural bones. Li et al. [4] fabricated a HA/MgO bilayer coating on magnesium using microarc oxidation and followed hydrothermal treatment of 24 h, and revealed a better long-term corrosion resistance with 90 days immersion in physiological saline (PS). Song et al. [5] developed a F-doped hydroxyapatite (FHA) coating on Mg-Zn alloys via electrodeposition method. After soaking in modified simulated biological fluid for 1 month, the FHA coating could promote the nucleation of osteoconductive minerals at later stages. Obviously, the aforementioned HA coating was effective on the long-term protection for magnesium alloys during immersion, which was mainly account for less intrinsic defects, such as holes and cracks in the coatings. The lack of defects could avoid the penetration of the corrosive medium resulting in the corrosion of magnesium alloys to a great extent [6], [7]. In addition to this, good interface bonding strength between protective coating and substrate is another vital prerequisite to furnish long-term protection in vivo service period. In the meantime, the mineralized layer precipitated quickly on the protective HA coating surface also played a role in compensating the degradation of the original coating for later-stage protection [8]. However, some surface modification techniques are not available for the preparation of surface coatings on complex shape devices. The CaP coatings developed via the common surface modification methods, such as modified biomimetic method and electrochemical deposition exhibited better corrosion resistance according to the immersion test results. Whereas, the bonding strength of these coatings ranging from 3 MPa to 13 MPa was not enough to attain the requirement for clinical application [9], [10], [11]. Therefore, it is necessary to find a more appropriate approach to fabricate the coating with good quality, excellent adhesion property and long-term corrosion resistance.
To date, ultrasonic technology is widely used in HA powder synthesis [12], [13]. It was generally believed that HA nucleation in aqueous solutions was favored by the electrostatic bonding of hydroxyl groups with calcium ions and the ultrasonic could provide high driving force for HA crystals growth [12]. Additionally, many researchers reported that, in chemical aqueous solutions systems, the ultrasonic could remarkably accelerate the reaction by decreasing the chemical reaction activation free energy, decreasing the induction period of crystallization and the width of the metastable zone when used in crystallization process [14], [15]. Furthermore, it is worth noting that ultrasonic could be conductive to dispersion and extension of HA [12], which may be advantaged for an integral coating architecture and good interface bonding of coating to the substrate [16]. Therefore, it is hopeful to prepare HA coatings on magnesium alloys and obtain strong interface bonding strength by using ultrasonic method that prevents peeling off at following implantation period, and finally acquire optimal implantation effect.
Many researches on the synthesis of different powders by ultrasonic, including HA power, have been reported, however, there are very few studies on the preparation of HA coating on the surface of magnesium alloys by ultrasonic method. Aiming at simultaneously endowing the coating with better bonding strength and long-term protective efficacy on magnesium alloy, in this work, a ultrasonic assisted deposition was applied to synthesize HA coating on magnesium alloys, and the proper conditions under ultrasonic route were investigated. Moreover, the structure morphology, bonding strength, electrochemical property, degradation and mineralization of HA coating were explored, respectively.
Section snippets
Ultrasonic aqueous synthesis of HA coatings on magnesium alloy
Commercial AZ31 magnesium alloy discs (3Al-1Zn-0.2Mn-Fe < 0.005, all in wt%) (Henan Yuhang Material Co., Ltd., China) with a size of 10 × 10 × 4 mm were used as substrates, which were ground progressively to 2000# grit with SiC papers, then ultrasonically cleaned with ethanol and distilled water for 5 min, respectively.
The prepared magnesium alloy plates were immersed in 1.5 M NaOH solution at 80 °C for 1 h. Thereafter, they were rinsed thoroughly with distilled water and dried for use. It was
Composition and microstructure of HA coating
The phase components of coatings after ultrasonic time for 10 min, 30 min, 1 h and 1.5 h in aqueous solution containing Ca2+ and PO43− ions were shown in Fig. 2. It was depicted that diffraction peaks at 2θ of 25.87°, 32.27° and 35.69° for all the four samples were matched closely with the hexagonal hydroxyapatite standard (JCPDS No. 86-0740), demonstrating that HA coating could be synthesized successfully on AZ31 magnesium alloy via the ultrasonic cavitation only for 10 min. However, there
Conclusion
In this work, a dense HA coating was fabricated on magnesium alloy by rapid ultrasonic cavitation in aqueous synthesis containing Ca2+ and PO43−. When the ultrasonic time was 1 h, a compact HA coating with a suitable coating thickness was obtained, and the bonding strength of the HA coated magnesium alloy reached 18.1 ± 2.2 MPa. The electrochemical measurement showed that sample UM1h possessed a good corrosion resistance with the highest Ecorr (−1.57 V), the greatest Rt (11.00 kΩ.cm2) and the
Acknowledgements
The work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51872197, 51572186 and 81772363), the Scientific Research Project of Tianjin Education Commission (Grant No. 2018KJ264), and Shanghai Committee of Science and Technology, China [Grant No. 15411951000].
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