Progress in the study of micro-arc oxidation film layers on biomedical metal surfaces

2.1 Antimicrobial

Magnesium alloy slowly releases magnesium ions (Mg²⁺) in the physiological environment, which has a certain antibacterial activity and can inhibit the proliferation of bacteria. Moreover, after micro-arc oxidation treatment, a dense magnesium oxide film is formed on the surface of the magnesium alloy, and the chemical composition and structural changes of the oxide film will affect the adhesion and growth of bacteria.

Generally, constructing an antimicrobial film layer on the implant surface is an effective measure to deal with bacterial infections, and micro-arc oxidized implants with different functional elements show excellent antimicrobial properties. The release of antibacterial ions in MAO coating alloy can effectively inhibit the proliferation of bacteria (Liang et al. 2021). Using the MAO method, the functional elements in the electrolyte are introduced into the oxidized layer on the implant surface through the discharge so that the membrane layer possesses antibacterial activity, which is not a characteristic of traditional modification technology. The growth of Escherichia coli and Staphylococcus aureus was inhibited by the inhibitory effect of the Cu coating when copper (Cu), an osteogenic element with good bacteriostatic properties, was added to the MAO electrolyte (H. Chen et al. 2019; J. Chen et al. 2019). Song et al. (2019) prepared pure magnesium poly hydrogen biofilms using ultrasonic MAO (UMAO) and phytate-plating-copper conversion method, and the Cu coating significantly inhibited the growth of E. coli and S. aureus and the pure magnesium UMAO-phytate-Cu3min planted membranes had good antibacterial activity. MAO films containing silver (Ag) elements also demonstrated good antibacterial properties, adding Ag-containing electrolytes during the MAO process and gradually releasing Ag⁺ during degradation (Chen et al. 2021). Sukuroglu (2018) introduced an Ag-coated MAO coating to enhance the antibacterial properties of AZ91 alloy. As shown in Figure 1(a–d), the surface morphology of Ag-free and Ag-containing MAO coatings shows that the coating surface grown in the electrolyte with silver is more uniform. As shown in Figure 1(b–e), the cross-sectional morphology of MAO coating without Ag and MAO coating with Ag indicates almost no difference in the thickness of the two coating samples. As shown in Figure 1(c–f), the inhibition zone diameters of Ag-free and Ag-containing MAO coatings are more significant, and the Ag-coated MAO coatings have good antibacterial properties.

Figure 1:

SEM images after MAO and samples in different electrolytes: (a) surface image, (b) cross-sectional image, (c) diameter of the suppression zone for samples coated in silver-deficient electrolyte; (d) surface image, (e) cross-sectional image, (f) diameter of the suppression zone for samples coated in silver-containing electrolyte. Images taken from Sukuroglu (2018) used with permission from © Ebru Emine Sukuroglu 2018.

In addition to the elemental infiltration method to improve the antimicrobial properties of micro-arc oxidized magnesium alloys, the preparation of composite coatings with antimicrobial properties can also significantly improve the antimicrobial properties of implants (Xue et al. 2021). It is worth noting that the parameter settings of the MAO power supply affect the growth rate of the film layer and the effect of the film-forming reaction, and thus, the antimicrobial properties of the final film layer. Miao and Jiang et al. (2009) performed MAO on medical magnesium alloys. They obtained calcium–phosphorus–silver composite coatings with excellent antibacterial properties, which helped to promote bone repair more effectively. Li et al. (2022) prepared an MAO coating with photo-thermal antibacterial properties on Mg–Zn–Ca alloy. The antibacterial mechanism is shown in Figure 2(a). As MAO coatings usually have good insulating properties and high hardness, different chemical compositions and phase structures may develop inside and outside the coating. This results in potential differences between the coating and the substrate material or between the inside and outside of the coating, galvanic corrosion occurs, and the coating surface releases OH⁻, which constantly reacts with H⁺ outside bacteria, consuming a lot of H⁺, leading to the destruction of bacterial function and eventually death. Secondly, the MAO coating was subjected to 808 nm near-infrared laser irradiation, a photothermal treatment that results in localized high temperatures on the coating surface, which can cause bacterial damage. The light absorption capacity and photothermal performance at 808 nm are the best. After 5 min of laser irradiation at 808 nm, the MAO coating of sample P3 shows the same antibacterial performance as bare magnesium alloy. It has good antibacterial activity against E. coli and S. aureus, as shown in Figure 2(b and c).

Figure 2:

Antibacterial mechanism and test of MAO coating: (a) antimicrobial mechanism of MAO coating under photothermal action; (b) antimicrobial performance of samples against Staphylococcus aureus; (c) antimicrobial performance of samples against Escherichia coli. Images taken from Li et al. (2022) used with permission from © MDPI 2022.

2.2 Corrosion resistance

Magnesium alloy is a material that can be completely degraded in organisms and has potential biomedical application prospects. However, its active chemical properties and poor corrosion resistance limit its broader application in the biomedical field. Therefore, the surface of magnesium alloy is treated by micro-arc oxidation technology to improve its corrosion resistance (Zhu et al. 2019). MAO coating was prepared on magnesium alloy and a porous Mg sample was coated with phosphate/calcium electrolyte by micro-arc oxidation technology (Rúa et al. 2019), and dense and thick micro-arc oxidation coating was prepared at constant current density (Zhao et al. 2010), which can significantly improve the corrosion resistance of magnesium alloy, and Mg²⁺ released by implant degradation can also promote fracture healing (Wu et al. 2017). Wang et al. (2015) studied the Ca–P self-sealing MAO coating. The self-sealing MAO coating containing Ca–P can effectively control the degradation of pure Mg in vitro, and the coating is non-toxic. It can effectively promote the formation of new bones. Ca–P self-sealing MAO coating may be a potential candidate for biodegradable magnesium-based implants in bone fixation.

Wang et al. (2018) studied the degradation behavior of ZK60 alloy with and without MAO coating in simulated body fluid (SBF). The surface and cross-sectional morphology of MAO coating are shown in Figure 3(a). As shown in Figure 3(b), the FTIR spectrum of MAO coating samples shows that there is a P–O tensile vibration mode at 890 cm⁻¹, which also confirms the existence of Ca₃Mg₃(PO₄)₄ soaked in SBF for a long time. The intensity of the P–O peak decreases with soaking time, indicating that Ca₃Mg₃(PO₄)₄ gradually degrades because there is almost no P–O peak after soaking for 30 days, which indicates that MAO film has good biodegradability. Figure 3(d–g) shows the degradation model of MAO-coated samples after soaking in SBF. Only magnesium oxide in the coating reacts with the solution, as shown in Figure 3(d). As the immersion test continues, SBF solution penetrates the loose layer. It diffuses into and expands the micropores of the loose layer so that SBF penetrates the dense layer and even the matrix alloy, as shown in Figure 3(e). Compared with the uncoated sample, the pH value of SBF of the coated sample is always kept at a low level, and the coated sample shows a higher degradation rate.

Figure 3:

Morphology, test and mechanism model of MAO coating: (a) surface morphology, (b) cross-sectional morphology of MAO coatings; (c) Fourier transform infrared (FTIR) spectra of corrosion products on MAO layers; (d–g) degradation modeling of MAO coated samples after immersion in SBF. Images taken from Wang et al. (2018) used with permission from © MDPI 2018.

Surface treatment is essential to enhance the corrosion resistance of magnesium alloys, and the MAO technique is regarded as a highly promising surface treatment that is expected to reduce the corrosion rate of magnesium alloys. However, intense spark discharges may lead to the appearance of micropores and microcracks during the formation of the MAO film. These porous structures provide a pathway for corrosive solutions to infiltrate or penetrate the interface between the MAO coating and its substrate, leading to corrosion, which in turn accelerates the corrosion rate of the substrate (Chu et al. 2013; Li et al. 2014). Therefore, improved sealing measures were taken to improve the MAO coatings. Studies have shown that composite coatings prepared on micro-arc oxidized magnesium alloys using deposition (Bai et al. 2017), sol-gel technology (Tang et al. 2018), powder spraying (Tang et al. 2020b), geraniol dip coating (Peng et al. 2019) and sliding friction treatment (SFT)-MAO bonding (Cao et al. 2018) improved the electrochemical behavior of the substrate and greatly enhanced the corrosion resistance and osteogenic capacity of the implanted material (Rojaee et al. 2014), Shang et al. (2020) successfully prepared an MAO/graphene oxide (MAO/GO) composite coating on the surface of magnesium alloys by using MAO and electrodeposition technology. The coating closes the micropores of MAO film, the surface elements are evenly distributed, the content of carbon (C) elements is high, and the composite coating is smoother than the MAO film. The corrosion resistance of the MAO/GO composite coating is significantly better than that of the single MAO film. Yang et al. (2018) successfully prepared a double-layer composite coating system on the surface of AZ31 magnesium alloy by using MAO + electrostatic powder spraying (EPS) technology. This MAO + EPS composite coating has excellent binding force, so the corrosion resistance of AZ31 magnesium alloy has been effectively improved. L. Zhang et al. (2021) successfully prepared aminated hydroxyethyl cellulose (AHEC) coating on the MAO AZ31 magnesium alloy surface by sol-gel method. Polarization potential test analysis showed that the AHEC-sealed coating significantly improved the corrosion resistance of AZ31 magnesium alloy and reduced its degradation rate in simulated body fluid compared to a single micro-arc oxidized coating.

Butt et al. (2020) composite samples of several magnesium alloys combined with a biodegradable polymer, polylactic acid (PLA), were prepared by injection molding and treated magnesium alloy with four different frequencies of MAO. The schematic diagram of the preparation of magnesium composite rods treated with PLA cladding MAO is shown in Figure 4(a). As shown in Figure 4(b), the macro photos of the magnesium rod, PLA-coated Mg rod, and pure PLA rod treated by Mao are shown. As shown in Figure 4(c), the surface morphology of the coating at different frequencies is observed by scanning electron microscope. It can be seen that higher frequencies seem to merge tiny pores to form larger pores, which can be classified as small craters on the surface. Figure 4(d) shows that MgO ceramic porous coating was formed after AO treatment. Figure 4(e) shows the mass changes of four composites and pure PLA after soaking in Hank’s solution for 3, 7, and 14 days. The quality of pure polylactic acid (PLA) decreased in the degradation process, but the change was not significant, and the weight loss rate was only 0.14 % after soaking for 14 days. Figure 4(f) is a schematic diagram of the degradation process. During the destruction process, the interfacial adhesion is remarkable, and the magnesium alloy does not appear to have severe corrosion. Surface-treated composite samples based on magnesium alloys show a lower degradation rate, and magnesium-reinforced PLA composite rods are promising candidates for orthopedic implants.

Figure 4:

Morphology, test and mechanism analysis of MAO coating: (a) demonstration of the preparation process; (b) photographs of Mg samples as well as PLA-coated MAO-treated Mg rods; (c) surface morphology of the MAO coatings produced at different pulse frequencies; (d) XRD patterns of the surfaces of the pure and MAO-treated Mg rods; (e) loss of Mg versus immersion time in Hank’s solution; and (f) schematic diagrams of degradation mechanism in four different stages. Images taken from Butt et al. (2020) used with permission from © American Chemical Society 2020.

Changing electrolytes and optimizing process parameters in the process of film preparation are also important methods to improve the corrosion resistance of magnesium alloys. MAO coatings are formed on pure magnesium by adding hydroxyapatite (HA) particles to the Ca–P-based electrolyte, and the HA particles improve the corrosion resistance by increasing the densification of the coating (Zheng et al. 2021). Ceramic MgO coatings can also be applied on AZ31 magnesium alloys by MAO process under pulsed bipolar constant–current conditions in silicon and phosphorus-containing electrolytes, and the optimum corrosion protection process parameters were obtained by the Taguchi method (Jian et al. 2019). Applying electrochemical principles and combining MAO technology and ultrasonic technology is promising (B. Qu et al. 2014; L.J. Qu et al. 2014). Mu et al. (2020) prepared coatings on pure magnesium substrates using ultrasonic MAO and self-assembly techniques. The effect of safranin on the properties of ultrasonic MAO/poly (lactic and glycolic acid) copolymer/safranin (UMAO/PLGA/BR) coatings was investigated. The UMAO/PLGA/BR coatings showed the lowest current density (3.14 × 10⁻⁸ A/cm²) and improved corrosion resistance when the safranin content was 3.0 g/L.

2.3 Biocompatibility

MAO is an effective surface treatment technique, and the porous structure formed on the coated surface by spark discharge helps to enhance osteoblast attachment and growth, thus improving the biocompatibility of magnesium alloy surfaces (B. Qu et al. 2014; L.J. Qu et al. 2014). Zhang et al. (2013) used MAO-coated ZK60 alloy sheets and medical-grade calcium sulfate micro pellets as the test and control materials, respectively, and compared with the calcium sulfate treatment, the magnesium treatment improved defect repair. The magnesium coating with added silicon did not significantly produce cytotoxicity on the surface of magnesium alloys by MAO treatment in silicate electrolyte solution with constant–current power supply mode, and the MAO coating with low crystalline concentration and porous surface improved the bioactivity of ceramic film (Yu et al. 2016; Jiao et al. 2021). Chen and Hao (2014) successfully prepared a bioactive ceramic membrane on the magnesium alloy surface using MAO technology in the solution of the zirconium salt and silicate systems. As shown in Figure 5(a–d), the surface of the ceramic layer grew with worm-like hydroxyapatite morphology after being soaked in a silicate system solution and zirconium salt system solution for 21 days. As shown in Figure 5(b, c, e, and f), the content of Ca and P elements on the surface of the ceramic membrane increases gradually with the extension of soaking time, which can effectively improve the bone bioactivity of the membrane.

Figure 5:

Morphology and EDS analysis of coatings in two solutions: (a) surface morphology, (b) EDS analysis, and (c) surface Ca and P elemental content of ceramic membranes prepared from zirconium salt system solution after immersion; (d) surface morphology, (e) EDS analysis, and (f) surface Ca and P elemental content of ceramic membranes prepared from silicate system solution after immersion. Images taken from Chen and Hao (2014) used with permission from © CNKI 1998–2025.

3.1 Antimicrobial

In the process of micro-arc oxidation, the antibacterial properties of materials can be enhanced by doping antibacterial metal ions such as silver and copper. In addition, the composition and structure of micro-arc oxidation film and metal ions released in the physiological environment can inhibit the growth and reproduction of bacteria.

The antimicrobial properties of titanium implant surfaces were enhanced by MAO treatment. To further enhance its antimicrobial properties, embedding bioactive elements into the MAO coating was used titanium implant surfaces, thereby mitigating the risk of implant-related infections (Teker Aydogan et al. 2018; Zhang et al. 2017). Silver has strong antibacterial properties and is not easy to produce drug resistance. Therefore, the preparation of micro-arc oxidation film which can release silver ions stably can effectively inhibit the growth and adhesion of bacteria (Lee et al. 2016; Yu et al. 2013). Numerous studies have shown that zinc (Zn) exhibits enhanced antimicrobial effects over time, and samples fabricated by MAO containing both Ag and Zn showed more robust antimicrobial properties (Tsutsumi et al. 2021). Strontium (Sr), on the other hand, is the most valuable element for stimulating bone formation in titanium coatings. Simultaneous doping of Sr and Ag into TiO₂ coatings at optimal concentrations will result in titanium materials with suitable osteogenic and antibacterial activities (Y. Zhang et al. 2021). Zhang et al. (2016) successfully prepared TiO₂ coatings with the microporous structure on a titanium surface by MAO technique and successfully doped them with Zn²⁺ and Ag nanoparticles. The Ag and Zn content on the coating surface increased with the increase of MAO time. The co-doping of Zn²⁺ and Ag nanoparticles in this coating significantly reduced the attachment of S. aureus and decreased the number of planktonic bacteria in the culture medium. Surface biomodification can improve the antimicrobial properties or biocompatibility of materials. Cu ions have been used to develop antimicrobial coatings, and Ti–Cu alloys have been shown to have excellent antimicrobial properties in vitro and in vivo (Hu et al. 2020). Xinkun Shen et al. (2020) doped silicon (Si) and/or copper (Cu) ions into the coating treated by high-energy shot peening (HESP)/MAO, and the Si-doped sample (MAO-Si) significantly improved the cell viability, alkaline phosphatase (ALP) activity, mineralization and the expression of osteoblasts in MC3T3-E1 cells. Figure 6(a) shows the live/dead staining of MC3T3-E1 cells co-cultured with Streptococcus mutans for 24 h. Only a few cells died on Si/Cu-MAO substrate, which indicates that it can effectively ensure the survival of MC3T3-E1 cells in the presence of S. mutans. Figure 6(b and c) further shows that MC3T3-E1 cells co-cultured on Si/Cu-MAO substrate have significant ALP activity and osteocalcin (ALP and OCN) expression. The introduction of Cu showed good bactericidal performance against S. aureus and S. mutans, and the inhibition rates of both bacteria exceeded 95 %. Studies have shown that elements Ag and Cu have good antimicrobial ability but also have certain cytotoxicity. Element Zn has weaker cytotoxicity elements Ag and Cu but also has weaker antimicrobial ability (Du et al. 2018). Therefore, there is an urgent need to develop new antimicrobial coatings that are efficient, healthy, environmentally friendly, and low-cost to overcome the drawbacks of conventional coatings. W-containing TiO₂ coatings exhibit excellent antimicrobial properties, which are highly correlated with the formation of intracellular reactive oxygen species (ROS) (Wang et al. 2021). Zhou et al. (2019) prepared antimicrobial W-containing MAO coatings by doping Na₂WO₄ in the electrolyte. The antimicrobial mechanism of W-containing coatings is related to the extracellular and intracellular reactive oxygen species. The bacterial deaths were caused by the synergistic effect of extracellular and intracellular ROS, and W-containing coatings are effective against the planktonic and adherent E. coli and S. aureus, both planktonic and adherent showed significant bactericidal properties.

Figure 6:

Morphology and antibacterial test of MAO coating: (a) SEM (red arrows: spreading MC3T3-E1 cells) and live/dead staining (red color: dead MC3T3-E1cells; green color: living MC3T3-E1cells) images of co-cultured MC3T3-E1 cells and Streptococcus mutans on MAO, Si-MAO, and Si/Cu-MAO substrates at 24 h; ALP activity (b, c) osteogenic gene expression of co-cultured MC3T3-E1 cells on MAO, Si-MAO, and Si/Cu-MAO substrates at 7 day. Error bars represent mean ± SEM for n = 6, **p < 0.01. Images taken from Shen et al. (2020) used with permission from © Shen et al. 2020.

Adding inorganic antibacterial metal elements in micro-arc oxidation can effectively reduce the probability of implant infection in the human body. Besides this method, micro-arc oxidation composite coating technology can effectively ensure the biocompatibility of metals, and it is not toxic, which is expected to reduce the risk of bacterial adhesion and peri-implant inflammation (Huang et al. 2019). Cao et al. (2019) provided a new dual coating of micro-arc oxidation and absorbable polylactic acid copolymer on Ti–6Al–4V implant. The micro-arc oxidation coating with antibiotics improved the adhesive strength coating of poly-l-lactide-co-ε-caprolactone (PLC), which was enough to reduce the risk of infection after implantation and effectively improve biocompatibility. He et al. (2018) adopted the surface microporous structure and antibacterial peptide coating. After micro-arc oxidation treatment, the multi-layer coating composed of poly-dopamine, cationic antibacterial peptide LL-37, and phospholipid (POPC) was coated on MAO substrate. The combination of poly-dopamine-LL-37-POPC can reduce the sudden release of LL-37 in the initial stage and has good antibacterial performance and biological activity.

3.2 Corrosion resistance

The MAO technique has received more attention due to its ease and effectiveness in preparing oxide ceramic coatings with porous structures on the surface of Ti, Al, Mg, and their alloys. The MAO surface treatment technique can substantially improve the corrosion resistance of titanium alloys (Xu et al. 2008). Micro-arc oxidation coating can also be prepared on Ti alloy, and the prepared ceramic coating is porous and can be firmly adhered to the alloy substrate, which is beneficial to improve the biocompatibility of the implant (,b). Alemayehu et al. (2023) used micro-arc oxidation (MAO) or plasma electrolytic oxidation (PEO) technology and mineral solution containing Ca and P to modify the surface of commercially pure titanium. The results show that mechanical grain refinement methods and surface modification can improve the corrosion resistance of commercially pure titanium. Hu et al. (2021) successfully formed two kinds of micro-arc oxidation coatings with different thicknesses on the commercial pure titanium (TA2) surface by adjusting the oxidation time. As shown in Figure 7(b and c), the coating thickness of MAO-1 and MAO-2 is 10 μm and 25 μm, respectively, and Figure 7(d) shows MAO-2 coating with a thin surface layer removed. Figure 7(a) shows a schematic diagram of galvanic corrosion measurement. Figure 7(e) shows the Nyquist plots measured after 36 h of immersion in 3.5 % NaCl solution, which shows that although the MAO treatment significantly improves the corrosion resistance of TA2, increasing the thickness of the coating does not further improve its corrosion resistance. Figure 7(f) shows the Nyquist plots of EIS over time in a 3.5 % NaCl solution, indicating that MAO coating alone cannot inhibit the galvanic coupling corrosion of titanium and other metals.

Figure 7:

Morphology and corrosion resistance test of MAO coating: (a) schematic diagram of galvanic coupling corrosion measurements between the micro-arc oxidized electrode and the steel electrodes; (b–d) cross-sectional SEM images of the coatings prepared by micro-arc oxidization at different times; (e) EIS Nyquist plots of MAO-1 and MAO-2 measured after a 36 h immersion in 3.5 % NaCl solution; (f) EIS over time Nyquist plots of S/MAO-2 coupled with S/TA2 coupled together in Nyquist plots of EIS over time. Images taken from Hu et al. (2021) used with permission from © MDPI 2021.

Electrolyte plays a crucial role in the MAO of titanium alloys, such as silicate system and aluminate system, and its composition has an essential influence on the properties of the final oxide layer (Xu et al. 2009; Yu et al. 2015). ,b) prepared ceramic coating on NiTi alloy by micro-arc oxidation in sodium aluminate (NaAlO₂) solution at constant current density. As shown in Figure 8(a and b), high temperature in a micro-arc discharge channel will form a porous surface. The porous surface coated with nickel and titanium benefits cell attachment, reproduction, and implant anchoring on bone. As shown in Figure 8(c), the potential dynamic polarization curve shows that the corrosion resistance of NiTi alloy coated with Al₂O₃ is significantly improved. As shown in Figure 8(d), the amount of nickel released by the coated nickel–titanium sample is much lower than that of the uncoated nickel–titanium sample, and the corrosion resistance of Al₂O₃ coating is better than that of uncoated NiTi coating.

Figure 8:

SEM micrographs of the surface of ceramic coatings formed by microarray oxidation at (a) 20 min and (b) 40 min; (c) potential kinetic polarization curves of uncoated NiTi and coated NiTi; (d) nickel release from Hank’s solution of coated and uncoated NiTi samples. Images taken from, b) used with permission from © The Ceramic Society of Japan 2010.

However, compared with the composite film layer with excellent performance, the application width of single-layer MAO films must be increased for practical applications. Therefore, MAO technology has been combined with other surface modification technologies to cope with situations arising in practical applications (Yu et al. 2009). Xu et al. (2019) prepared a composite coating using MAO technology combined with the sol-gel method. Comparing the corrosion resistance of the three surface treatment technologies, standard titanium alloy, MAO ceramic film, and composite coating, the composite film layer showed excellent performance compared to the other specimens. Jiang et al. (2015) used MAO technology and subsequent superhydrophobic treatment to prepare a superhydrophobic TiO₂ coating on a biomedical Ti–6Al–4V alloy. A single anatase TiO₂ coating was formed on the surface of the Ti–6Al–4V alloy with a rough and porous micron-sized structure, which improved the corrosion resistance of the superhydrophobic sample by an order of magnitude compared to the uncoated Ti–6Al–4V alloy.

3.3 Biocompatibility

The MAO ceramic layer exhibits a dense inner layer and porous outer layer, in which the porous morphology facilitates the attachment of osteoblasts while promoting the growth of bone tissue, thus improving the bonding of bone tissue to the implant (Wang et al. 2016). Xu et al. (2012) investigated the effects of MAO surface modification on the phase transition behavior, shape memory properties, in vitro haemocompatibility, and cytocompatibility of biomedical NiTi alloys. The results showed that MAO surface modification could effectively improve the haemocompatibility of coated NiTi alloys by decreasing the hemolysis rate, prolonging the dynamic coagulation time, and reducing the number of platelet adhesions.

Hydroxyapatite is a typical bioactive material, and its chemical formula is Ca₁₀(PO₄)₆(OH)₂, which is the main inorganic component of human bones and teeth. However, these materials are unsuitable for load-bearing applications due to their poor mechanical properties. Bioactive coatings on titanium alloys offer a way to overcome the disadvantages of ceramic and metallic biomaterials. Therefore, titanium alloys with biocompatible coatings are suitable materials for load-bearing conditions (Terleeva et al. 2010; Wang and Di 2011). Tang et al. (2020) prepared TiO₂–BaTiO₃ coatings on medical titanium alloys by MAO and hydrothermal reaction. The coatings have bioelectrical activity similar to that of natural bone after the polarization of the electric field, and the negative charge attracts Ca²⁺ to aggregate on the coating surface, thus obtaining a high deposition of apatite, increasing the Ca/P ratio and promoting the crystallinity of apatite.

Micro-arc oxidation coating was prepared on titanium alloy, and then the sample was placed in an alkaline solution for electrochemical reduction. This double-coating process can effectively improve biocompatibility and promote the application of the alloy in dentistry and plastic surgery (Li et al. 2016). In addition, the MAO technique can generate ceramic films in situ on non-ferrous surfaces, which have a high bonding strength to the substrate. By adjusting the formulation of the electrolyte, the effect of increasing the bioactivity of the film layer can also be achieved. Zhang et al. (2021) treated Ti–6Al–4V alloy samples with different concentrations of EDTA-MgNa₂(Na₂MgY) and potassium hydroxide (KOH) in heteropoly acid (H₁₂Phy) electrolyte by micro-arc oxidation. As shown in Figure 9(a), the surface morphology of the film prepared in the mixed solution of 15 g/L H12Phy and 10 g/L LNA₂MGY; Figure 9(b and c) shows the surface morphology of the film prepared by adding 8 g/L KOH and 11 g/L KOH respectively based on the mixed solution. It can be seen that with the increase of KOH concentration, the micro-arc oxidation film becomes rough and the distribution of micropores is uneven. As shown in Figure 9(f), the fluorescence image of bone cells in the mixed solution of 10 g/L Na₂MgY and 5 g/L KOH has wider cell distribution and more vital filiform foot extension ability compared with the untreated control group and 10 g/L Na₂MgY and 8 g/L KOH system in Figure 9(d and e), which can promote the initial attachment of cells. MAO coating with appropriate magnesium content improves the cell compatibility of Ti–6Al–4V alloy and has excellent potential in orthopedic applications.

Figure 9:

Morphology, test and mechanism analysis of MAO coating: (a,b,c) SEM images of the MAO coatings produced in solutions with different Na2MgY and KOH concentrations; (d,e,f) fluorescence images of MC3T3-E1 preosteoblasts after 4 h of incubation. Images taken from R. Zhang et al. (2021) used with permission from © MDPI 2021.