Mg-MOF-74/MgF2 Composite Coating for Improving the Properties of Magnesium Alloy Implants: Hydrophilicity and Corrosion Resistance

Surface modification on Mg alloys is highly promising for their application in the field of bone repair. In this study, a new metal–organic framework/MgF2 (Mg-MOF-74/MgF2) composite coating was prepared on the surface of AZ31B Mg alloy via pre-treatment of hydrofluoric acid and in situ hydrothermal synthesis methods. The surface topography of the composite coating is compact and homogeneous, and Mg-MOF-74 has good crystallinity. The corrosion resistance of this composite coating was investigated through Tafel polarization test and immersion test in simulated body fluid at 37 °C. It was found that Mg-MOF-74/MgF2 composite coating significantly slowed down the corrosion rate of Mg alloy. Additionally, Mg-MOF-74/MgF2 composite coating expresses super-hydrophilicity with the water contact angle of nearly 0°. In conclusion, on the basis of MgF2 anticorrosive coating, the introduction of Mg-MOF-74 further improves the biological property of Mg alloys. At last, we propose that the hydrophilicity of the composite coating is mainly owing to the large number of hydroxyl groups, the high specific surface area of Mg-MOF-74, and the rough coating produced by Mg-MOF-74 particles. Hence, Mg-MOF-74 has a great advantage in enhancing the hydrophilicity of Mg alloy surface.


Introduction
Magnesium (Mg) alloys have drawn interest as biomaterials to repair bone defects in recent years [1][2][3][4][5]. Compared to other biomaterials, such as titanium alloys and stainless steel, Mg alloys have a lot of advantages: (i) Mg alloys are much closer to natural bone in density and elastic modulus, eliminating the stress shielding effects [6][7][8]; (ii) Mg 2+ is essential to human beings [9]; (iii) because of degradability of Mg in the human body, implants made of Mg alloys will not require a secondary removal surgery [10]. However, their disadvantage as implants is their poor corrosion resistance, greatly hindering their widespread clinical application so far.
Surface modification of Mg alloys using a biocompatible coating has shown to be a promising method, which can lower the corrosion rate, and also improve the interaction between implant and tissue [11]. Thus far, a variety of materials-such as bioactive ceramics [12], bioinert ceramics [5], MgF 2 coating [13], anode oxide film [14], biodegradable polymers [4,15], etc.-have been used as a coating material. Among them, MgF 2 coating is a common one because of simple process, low cost, and good corrosion resistance [16][17][18][19]. Recently, considerable efforts have been made to prepare composite coating of MgF 2 and other materials for better property. Ren [20]. Feng et al. prepared PRC-HA/MgF 2 coating on Mg-Zn-Ca alloy by pulse reverse current electrodeposition based on MgF 2 coating, improving the corrosion resistance and the bioactivity [21]. However, a hydrophilic surface is believed to be important for bone repair biomaterials, it can improve the biocompatibility of implant in blood [22], and it is also beneficial for bone cells adhesion and proliferation to promote further bone growth [23,24]. There have been many studies on hydrophilicity surface of titanium alloys [25][26][27], but the study of hydrophilicity on magnesium alloys is relatively less [28]. So far, few studies have been published about the hydrophilicity coating of Mg alloys based on MgF 2 . Metal-organic frameworks (MOFs), comprising of metal ions and various organic ligands, have been used over past several years in many fields, such as gas separation [29], drug delivery systems [30][31][32], corrosion protection of metals [33,34], and so on. Due to the excellent characteristics of MOFs with high specific surface area and various functional groups, some MOFs are especially biocompatible, it is possible to introduce MOF as the hydrophilicity coating.
The purpose of this study is to improve both hydrophilicity and corrosion resistance of Mg alloys intended as implant. Here, a new Mg-MOF-74/MgF 2 composite coating was fabricated by pre-treatment of hydrofluoric acid and hydrothermal in situ growth methods. Mg-MOF-74 is a classic biocompatible MOF material constructed with Mg 2+ ion and 2,5-dihydroxyterephthalic [35]. It is selected here because: (i) Mg-MOF-74 contains the same metal Mg with MgF 2 and Mg alloy, avoiding the introduction of other disadvantageous metals and being helpful to the formation of compact composite coating as well; (ii) a large number of hydroxyl groups on the outer and pore surface of Mg-MOF-74 might be beneficial to improve hydrophilicity; (iii) it has big surface area, good biocompatibility, and stability. Hydrophilicity was investigated through water contact angle testing, and the mechanism was discussed. The study on corrosion resistance was performed using an electrochemical test and the H 2 gassing experiment in simulated body fluid (SBF) at physiological temperature, respectively.

Instrumentation
The phases of the samples were analyzed on a Bruker D8 Advance X-ray diffractometer (Cu-Kα radiation) with the scan speed and step of 10 • ·min −1 and 0.02 • , respectively. The surface, cross-sectional morphologies and elemental composition of the coating were observed by scanning electron microscopy (SEM, FEI INSPECT F50, Hillsboro, OR, USA) equipped with energy dispersive X-ray spectrometer (EDS, BRUKER QUANTAX 400, Karlsruhe, Germany). Electrochemical tests were carried out using an electrochemical workstation (RST5000, Zhengzhou Shiruisi Inc., Zhengzhou, China) with a three-electrode system. In immersion testing, a simple setup was made by ourselves to record the evolved hydrogen volumes [36]. The static water contact angles were measured by using a contact angle meter (JC2000D1, Shanghai Zhongchen Inc., Shanghai, China) to examine surface wettability of all samples.

Sample Preparation
Pretreatment of Mg Alloy: AZ31B Mg alloy was cut into 20 × 20 × 2 mm sheets as substrates. Then Mg alloy sheet was ground sequentially with 240 # , 600 # , and 1000 # silicon carbide paper. Next, the sheet was rinsed ultrasonically in ethanol for 15 minutes. At last, it was washed with deionized water and dried in air.
Preparation of MgF 2 coating: To obtain the MgF 2 coating, the treated substrate was immersed in 40% (v/v) hydrofluoric acid at room temperature for 24 h. The specimen was later rinsed in deionized water and dried in the air.
Fabrication of Mg-MOF-74/MgF 2 composite coating: Mg-MOF-74 coating was provided by hydrothermal synthesis method [37], and the preparation of reaction solution is as follows. Magnesium nitrate hyxahydrate (2.5 mmoL) and 2,5-dihydroxyterephthalic acid (0.8 mmoL) were dissolved in a solvent mixture of N,N-dimethylformamide (60 mL), ethanol (4 mL) and deionized water (4 mL). The solution was transferred to an autoclave. Then specimen above was dipped vertically into the solution in an oven for 24 h at 125 • C to yield Mg-MOF-74 crystals on the MgF 2 coating. The process of purification and activation is as follows: the specimen with Mg-MOF-74/MgF 2 composite coating was steeped in anhydrous methanol for 12 h, then this sample was treated under vacuum at 100 • C for 3 h to remove the solvent molecules from the pores of Mg-MOF-74. The process on fabrication of Mg-MOF-74/MgF 2 composite coating is illustrated schematically in Figure 1. Zhengzhou, China) with a three-electrode system. In immersion testing, a simple setup was made by ourselves to record the evolved hydrogen volumes [36]. The static water contact angles were measured by using a contact angle meter (JC2000D1, Shanghai Zhongchen Inc., Shanghai, China) to examine surface wettability of all samples.

Sample Preparation
Pretreatment of Mg Alloy: AZ31B Mg alloy was cut into 20 × 20 × 2 mm sheets as substrates. Then Mg alloy sheet was ground sequentially with 240 # , 600 # , and 1000 # silicon carbide paper. Next, the sheet was rinsed ultrasonically in ethanol for 15 minutes. At last, it was washed with deionized water and dried in air.
Preparation of MgF2 coating: To obtain the MgF2 coating, the treated substrate was immersed in 40% (v/v) hydrofluoric acid at room temperature for 24 h. The specimen was later rinsed in deionized water and dried in the air.
Fabrication of Mg-MOF-74/MgF2 composite coating: Mg-MOF-74 coating was provided by hydrothermal synthesis method [37], and the preparation of reaction solution is as follows. Magnesium nitrate hyxahydrate (2.5 mmoL) and 2,5-dihydroxyterephthalic acid (0.8 mmoL) were dissolved in a solvent mixture of N,N-dimethylformamide (60 mL), ethanol (4 mL) and deionized water (4 mL). The solution was transferred to an autoclave. Then specimen above was dipped vertically into the solution in an oven for 24 h at 125 °C to yield Mg-MOF-74 crystals on the MgF2 coating. The process of purification and activation is as follows: the specimen with Mg-MOF-74/MgF2 composite coating was steeped in anhydrous methanol for 12 h, then this sample was treated under vacuum at 100 °C for 3 h to remove the solvent molecules from the pores of Mg-MOF-74. The process on fabrication of Mg-MOF-74/MgF2 composite coating is illustrated schematically in Figure 1.

Corrosion Resistance Study
Tafel polarization tests: A three-electrode system was employed, where the samples (with 1 cm 2 exposed area), saturated calomel electrode (SCE), and a platinum wire served as the working electrode (WE), reference electrode (RE), and counter electrode (CE), respectively. The solution was SBF at 37 °C, which was made up according to previous studies [38], and the information about reagents used can be found in Section 2.1 above. Tafel polarization tests were performed at a scan rate of 0.5 Mv·s −1 , and the samples were immersed in the SBF solution for about 30 min to obtain an electrochemical steady state before the tests.
Immersion tests: The samples were immersed separately in SBF solution controlled with a water bath at 37 °C. The SBF volume to the exposed area of the sample was 100 mL·cm −2 . The volumes of hydrogen produced were recorded at different times to investigate the corrosion resistance of samples.

Corrosion Resistance Study
Tafel polarization tests: A three-electrode system was employed, where the samples (with 1 cm 2 exposed area), saturated calomel electrode (SCE), and a platinum wire served as the working electrode (WE), reference electrode (RE), and counter electrode (CE), respectively. The solution was SBF at 37 • C, which was made up according to previous studies [38], and the information about reagents used can be found in Section 2.1 above. Tafel polarization tests were performed at a scan rate of 0.5 Mv·s −1 , and the samples were immersed in the SBF solution for about 30 min to obtain an electrochemical steady state before the tests.
Immersion tests: The samples were immersed separately in SBF solution controlled with a water bath at 37 • C. The SBF volume to the exposed area of the sample was 100 mL·cm −2 . The volumes of hydrogen produced were recorded at different times to investigate the corrosion resistance of samples.

Hydrophilicity Study
In the process of water contact angle measurements, 1 µL of water was dropped on the surface of samples, and then the images of the droplet were captured for 5 s after dropping. Three spots were chosen at random on each sample surface to make the measurement and the results obtained were averaged.  Figure 2b, the scratch caused by the grinding is still obvious after hydrofluoric acid etching, because the MgF 2 layer is very thin. It also can be seen from the cross-sectional micrograph of Mg alloy substrates with MgF 2 coating (Figure 2d). Moreover, a dense intergrown Mg-MOF-74 crystal layer is observed on the MgF 2 coating (Figure 2c,e). It can be attributed to that Mg source in MgF 2 could provide activated sites to connect with organic ligands, while Mg source in reaction solution is employed for growth of Mg-MOF-74 coating as well, named 'dual-Mg-source' method. Notably, this method not only benefits the coating compactness, but also enhances the binding force between coating and substrate [39,40]. The EDS spectrum on MgF 2 middle layer (area circled in Figure 2e) shown in Figure 2f displays the existence of fluoride, suggesting that MgF 2 still existed after Mg-MOF-74 was coated onto it.

Hydrophilicity Study
In the process of water contact angle measurements, 1 µL of water was dropped on the surface of samples, and then the images of the droplet were captured for 5 s after dropping. Three spots were chosen at random on each sample surface to make the measurement and the results obtained were averaged.  Figure 2b, the scratch caused by the grinding is still obvious after hydrofluoric acid etching, because the MgF2 layer is very thin. It also can be seen from the cross-sectional micrograph of Mg alloy substrates with MgF2 coating (Figure 2d). Moreover, a dense intergrown Mg-MOF-74 crystal layer is observed on the MgF2 coating (Figure 2c,e). It can be attributed to that Mg source in MgF2 could provide activated sites to connect with organic ligands, while Mg source in reaction solution is employed for growth of Mg-MOF-74 coating as well, named 'dual-Mg-source' method. Notably, this method not only benefits the coating compactness, but also enhances the binding force between coating and substrate [39,40]. The EDS spectrum on MgF2 middle layer (area circled in Figure 2e) shown in Figure 2f displays the existence of fluoride, suggesting that MgF2 still existed after Mg-MOF-74 was coated onto it.

Corrosion Resistance Property
Electrochemical test: Tafel polarization test is a common electrochemical test used to investigate the corrosion resistance property of Mg alloy. The high corrosion potential (Ecorr) and low corrosion current density (icorr) indicate the good corrosion resistance [42]. Figure 4 shows the Tafel polarization curves of different samples in SBF solution at 37 °C, and the Ecorr and icorr obtained are summarized in Table 1

Corrosion Resistance Property
Electrochemical test: Tafel polarization test is a common electrochemical test used to investigate the corrosion resistance property of Mg alloy. The high corrosion potential (E corr ) and low corrosion current density (i corr ) indicate the good corrosion resistance [42]. Figure 4 shows the Tafel polarization curves of different samples in SBF solution at 37 • C, and the E corr and i corr obtained are summarized in Table 1

Corrosion Resistance Property
Electrochemical test: Tafel polarization test is a common electrochemical test used to investigate the corrosion resistance property of Mg alloy. The high corrosion potential (Ecorr) and low corrosion current density (icorr) indicate the good corrosion resistance [42]. Figure 4 shows the Tafel polarization curves of different samples in SBF solution at 37 °C, and the Ecorr and icorr obtained are summarized in Table 1     Immersion test: H 2 will be produced when Mg alloy is in corrosive medium, and the larger the hydrogen volume is, the more serious the corrosion [4]. Herein, immersion test was carried out to further evaluate the corrosion resistance of the composite coating. Figure 5  Immersion test: H2 will be produced when Mg alloy is in corrosive medium, and the larger the hydrogen volume is, the more serious the corrosion [4]. Herein, immersion test was carried out to further evaluate the corrosion resistance of the composite coating. Figure   The results of these two tests prove that Mg-MOF-74/MgF2 composite coating could obviously enhance the corrosion resistance of Mg alloy, although its anticorrosion effect is slightly worse than that of single MgF2 coating. It is potentially due to partly dissolution of MgF2 layer during the Mg-MOF-74 synthesis, leading to greater probability of pit corrosion.

Hydrophilicity
The good hydrophilicity surface of bone repair implants is beneficial for bone cells adhesion and proliferation, promoting further bone growth. The water contact angle is often used to evaluate the hydrophilicity of material surface. Moreover, the smaller the contact angle is, the better the hydrophilicity [43]. Figure 6 shows the photographs of water droplets on the surfaces of uncoated, MgF2-coated and Mg-MOF-74/MgF2-coated Mg alloy samples. The surface wettability of Mg alloy is obviously changed after being coated. The contact angles of the uncoated, MgF2, and Mg-MOF-74/MgF2 coated samples are around 87.4°, 52.3°, and 0°, respectively, indicating the outer Mg-MOF-74 coating is superhydrophilic. According to previous study [44], hydrophilicity is connected with chemical constitution, microstructure and roughness. The superhydrophilicity of Mg-MOF-74 can be attributed to the points as follows: (i) the massive hydroxyl groups exposed on the pore surface of Mg-MOF-74, which can form hydrogen bond with water; (ii) the high specific surface area of Mg-MOF-74 as a MOF material [45]. According to the Wenzel model [44], a high specific surface area leads to excellent hydrophilicity; (iii) the rough coating produced by Mg-MOF-74 particles could decrease the contact angle between the water drop and the solid surface. Therefore, Mg-MOF-74 has great advantage to increase the hydrophilicity of Mg alloy surface. The results of these two tests prove that Mg-MOF-74/MgF 2 composite coating could obviously enhance the corrosion resistance of Mg alloy, although its anticorrosion effect is slightly worse than that of single MgF 2 coating. It is potentially due to partly dissolution of MgF 2 layer during the Mg-MOF-74 synthesis, leading to greater probability of pit corrosion.

Hydrophilicity
The good hydrophilicity surface of bone repair implants is beneficial for bone cells adhesion and proliferation, promoting further bone growth. The water contact angle is often used to evaluate the hydrophilicity of material surface. Moreover, the smaller the contact angle is, the better the hydrophilicity [43]. Figure 6 shows the photographs of water droplets on the surfaces of uncoated, MgF 2 -coated and Mg-MOF-74/MgF 2 -coated Mg alloy samples. The surface wettability of Mg alloy is obviously changed after being coated. The contact angles of the uncoated, MgF 2 , and Mg-MOF-74/MgF 2 coated samples are around 87.4 • , 52.3 • , and 0 • , respectively, indicating the outer Mg-MOF-74 coating is superhydrophilic. According to previous study [44], hydrophilicity is connected with chemical constitution, microstructure and roughness. The superhydrophilicity of Mg-MOF-74 can be attributed to the points as follows: (i) the massive hydroxyl groups exposed on the pore surface of Mg-MOF-74, which can form hydrogen bond with water; (ii) the high specific surface area of Mg-MOF-74 as a MOF material [45]. According to the Wenzel model [44], a high specific surface area leads to excellent hydrophilicity; (iii) the rough coating produced by Mg-MOF-74 particles could decrease the contact angle between the water drop and the solid surface. Therefore, Mg-MOF-74 has great advantage to increase the hydrophilicity of Mg alloy surface.

Conclusions
In this work, the Mg-MOF-74/MgF2 composite coating is fabricated on the AZ31B Mg alloy surface by pre-treatment of hydrofluoric acid and hydrothermal in situ growth methods. This coating has greatly improved the anti-corrosion and hydrophilicity of Mg alloys. In this composite coating, the role of MgF2 is as an anticorrosion coating to lower degradation rate. Mg-MOF-74 mainly plays a part in improving the hydrophilicity, which is probably conducive to bone cell adhesion and proliferation on Mg alloys. Furthermore, the mechanism has been discussed and we propose that the massive hydroxyl groups exposed on the pore surface, the high specific surface area of Mg-MOF-74, and the rough coating produced by Mg-MOF-74 particles contribute mainly to improve hydrophilicity of the coating. Especially, this work would offer a thought for the application of MOFs to bone tissue engineering.

Conclusions
In this work, the Mg-MOF-74/MgF 2 composite coating is fabricated on the AZ31B Mg alloy surface by pre-treatment of hydrofluoric acid and hydrothermal in situ growth methods. This coating has greatly improved the anti-corrosion and hydrophilicity of Mg alloys. In this composite coating, the role of MgF 2 is as an anticorrosion coating to lower degradation rate. Mg-MOF-74 mainly plays a part in improving the hydrophilicity, which is probably conducive to bone cell adhesion and proliferation on Mg alloys. Furthermore, the mechanism has been discussed and we propose that the massive hydroxyl groups exposed on the pore surface, the high specific surface area of Mg-MOF-74, and the rough coating produced by Mg-MOF-74 particles contribute mainly to improve hydrophilicity of the coating. Especially, this work would offer a thought for the application of MOFs to bone tissue engineering.