Recent advances in hydrothermal modification of calcium phosphorus coating on magnesium alloy

Lei Ling, Shu Ci,?, Qinqin Li, Jiyue Sun, Xiogng Bo, Guohu Xu

a Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin, China

b Department of Orthopedic Surgery, Spine Center, Changzheng Hospital, Naval Medical University, Shanghai, China

Abstract Magnesium (Mg) and its alloys have emerged as a favored candidate for bio-regenerative medical implants due to their superior biocompatibility, biodegradability and the elastic modulus close to that of human bone.Unfortunately, the rapid and uncontrollable degradation rate of Mg alloys in chloride-rich body microenvironments limits their clinical orthopedic applications.Recently, Calcium Phosphate (Ca-P)biomaterials, especially Hydroxyapatite (HA), have been broadly applied in the surface functional modification of metal-based biomaterials attributed to their excellent bioactivity and biocompatibility.Hydrothermal modification of Ca-P coatings on Mg alloys has been extensively exploited by researchers for its significant superiorities in controlling coating structure and improving interfacial bonding strength for better osseointegration and corrosion resistance.This work focuses on the up-to-the-minute advances in Ca-P coatings on the surface of Mg and its alloys via hydrothermal methods, including the strategies and mechanisms of hydrothermal modification.Herein, we are inclined to share some feasible and attractive hydrothermal surface modification strategies.From the perspectives of hydrothermal manufacturing technique innovation and coating structure optimization, we evaluate how to foster the corrosion resistance, coating bonding strength, osseointegration and antibacterial properties of Mg alloys with Ca-P coatings synthesized by hydrothermal method.The challenges and future perspectives on the follow-up exploration of Mg alloys for orthopedic applications are also elaborately proposed.

Keywords: Calcium phosphorus; Magnesium alloy; Coatings; Hydrothermal methods; Corrosion resistance; Biological properties.

1.Introduction

Magnesium (Mg) and its alloys are employed as loadbearing cardiovascular or orthopedic implants for the treatment of coronary heart disease, bone fractures and osteoporosis due to their excellent mechanical properties and biodegradability [1,2].Compared to other metallic materials (e.g., titanium, stainless steel, nickel, cobalt and their alloys), the Young’s modulus, fracture toughness and density of magnesium are similar to those of human bone, which mitigates the possible stress shielding effect when implanted and avoids bone remodeling and gradual bone loss in response to external stimuli [3-5].Mg-based implants are engineered to maintain mechanical integrity in the body for 12-18 weeks, to be completely degraded or replaced by normal bone tissue after assisting bone healing, avoiding the need for a second graft removal surgery, alleviating the psychological and economic burden on the patient and the potential for implant failure, and thus facilitating the bone healing process [6-9].Yu et al.[10]confirmed Mg-Zn-Ca-Y alloy hemostatic clips in SD rats could work for 8 months before they were completely degraded.Besides, Guan et al.[11]tested Mg?Nd?Zn?Zr alloys deposited with Ca-P coating (brushite, CaHPO4?2H2O)and obtained an expected complete degradation time of 10 to 12 months.Collectively, the above cases articulate the remarkable advantages of Mg alloys when used for regenerative orthopedic applications with a controlled biodegradation rate.

In fact, as an essential element for bone health, Mg is essential for normal metabolic processes within the body.Approximately 60% of our body’s Mg is stored in the bone matrix, and Mg deficiency can lead to osteoporosis.Wang et al.[12]reported that Mg2+ions released from Mg-based implants can create an alkaline microenvironment to promote proliferation and differentiation of osteoblasts through the MAPK/ERK signaling pathway, leading to the precipitation of calcium phosphate and the generation of Mg(OH)2corrosion products, which facilitate the formation of biological apatite.Liu et al.[13]found that Mg significantly increased the expression and secretion of PDGF-BB in MC3T3-E1 osteoblasts, thus promoting osteogenesis and human umbilical vein endothelial cells (HUVEC) vascularization.Zhang et al.[14]revealed that Mg could promote neuronal calcitonin gene related polypeptide-α(CGRP)-mediated osteogenesis and differentiation, and developed an innovative Mgcontaining intramedullary nail to foster femoral fracture repair in ovariectomy-induced osteoporotic rats.All things considered, above merits of Mg, combining with other biologically beneficial coatings, exhibit a great potential in strengthening the union of implant fractures.

Nevertheless, Mg is very chemically active with a low corrosion potential and is prone to react with chloride ions in body fluids, thus necessitating the need to overcome the excessive degradation rates in practical applications [15-17].Too fast corrosion rate of Mg can generate hydrogen gas cavities and excess hydroxide ions in the gap between the implant and surrounding tissues,affecting implant-to-tissue integration and pH balance in the organism, giving rise to inflammatory and cytotoxic reactions between the implant and host bone[18,19].Meanwhile, rapid degradation will render the mechanical properties of the implanted material ineffective, resulting in the loss of mechanical support.To properly control the degradation of Mg, researchers usually perform high purification for metals, or alloy with other metallic elements,or prepare dense polymers and bio-ceramics composite coatings on the surface of Mg alloys [20-22].Specifically, Ca-P coatings possess excellent biological activity, in which hydroxyapatite is a major inorganic component of human bones and teeth and make a striking difference on the metabolic processes of bone tissue[23-25].Hao et al.[26]have demonstrated that HA can promote osteogenic differentiation of human adipose-derived mesenehymal stem cells (hADSCs).Of note, the Ca-P coating on Mg alloys avoids the direct contact between Mg substrate and body fluids in the first instance,and serves a purpose to induce mineralization on the implant surface and enhance osteogenic differentiation of the scaffolds in body fluids on the other hand.

In recent decades, a plethora of coating modification methods have been proposed on Mg alloy including plasma spraying[30],sol-gel method[31],electrochemical deposition[32],micro-arc oxidation [33]and chemical conversion method[34].But multiple inevitable concerns could be encountered during the manufacture.For instance, even if plasma spraying is the only coating method to achieve commercial application, it may form various undesirable crystal phases and residual thermal stresses on the surface of implants.In addition, for electro-deposition, hydrogen bubbles will be generated on the surface of magnesium, which seriously affects the coating density and interfacial bonding strength.Among them, the hydrothermal method is facile and allows the precise control of the coating composition, microstructure and surface morphology.of the to prepare coatings on nonlinear and complex shaped surfaces [35,36].Additionally, calcium phosphate coating prepared by hydrothermal method has high interfacial bonding strength and density,which is beneficial to significantly improve the corrosion resistance of metal-based biomaterials [37].

Cutting to the chase, this paper comprehensively reviews the critical criteria and the state-of-the-art strategies for hydrothermal preparation of Ca-P coatings on Mg alloys from the perspective of preparation technology, coating composition and structure respectively as showed in Fig.1.Toward this end, we cherish the hope to furnish some effective methodologies for research related to ameliorating the corrosion resistance, osteogenic properties and antibacterial properties of Mg alloys.

2.Strategies for the hydrothermal Ca-P coating modification on Mg surface

2.1.Template-mediated method

The surface of bare Mg alloy lacks the adsorption sites related to nucleation of Ca-P crystals.In default of chelating agent in the hydrothermal coating process, the corrosion of Mg alloy at a high temperature will yield a myriad of Mg2+ions, resulting in the formation of Mg(OH)2and competitive adsorption with Ca2+ions.Accordingly, it is difficult to obtain a high-pure Ca-P coating on Mg alloy or the bond strength between the coating and the substrate is very low, which will affect the mechanical stability of the implant in the organism.In order to improve the phase purity,density and bonding strength of Ca-P coatings, many conjugates,organic compounds and polymers,such as Ethylene Diamine Tetraacetic Acid (EDTA), Monoethanolamine (MEA)and Polydopamine(PDA), are used as inducers or templates to accelerate the synthesis of calcium phosphate based on molecular recognition mechanisms [38-44].EDTA is an organic compound that adsorbs divalent metal cations and combines with calcium ions in the reaction to promote the nucleation and growth of HA nanocrystals.By analogy, MEA is an aminol, which enhances the solubility of the reactants in the hydrothermal reaction, promotes the formation of HA crystals, and can adjust the pH of the solution during the process [45,46].L-cysteine has been found to have a relatively good ability to inhibit metal corrosion by virtue of the fact that it contains -NH2and -COOH and -SH.It can coordinate with Mg via the nitrogen atom of -NH2, the oxygen atom of-COOH, and the sulfur atom of -SH to form a dense protective coating.Fan et al.[47]prepared an L-cysteine-bioinspired Ca-P coating on Mg alloy AZ31 hydrothermal method.The corrosion resistance of the Mg alloy was improved by theaddition of L-cysteine and the Ca-PL-Cys-coating formation mechanism is revealed in Fig.2(Ⅰ).

HA coating on the surface of Mg alloys by hydrothermal method using PDA as an intermediate layer, which will act like a glue.Zhou et al.[48]released a study that documented owing to the synergistic effect of the internal polydopamine coating and the external HA coating, the corrosion resistance and the stability and cytocompatibility under body fluid conditions of the PDA template-mediated coating modified Mg alloy were considerably promoted, as revealed in Fig.2(Ⅰ).The catechol functional group in PDA is highly susceptible to metal ion adsorption and facilitates the formation of HA coating by attracting Ca2+ions and further attracting PO43?to form HA nuclei, which enables cross-linking with HA.Similarly, the coordination of the polyhydroxyl unit of glucose with Ca2+ions in aqueous solution can be utilized to promote the nucleation of Ca-P crystals on the bare Mg surface.Li et al.[49]described the role of glucose as a "catalyst"that can be converted to gluconic acid with a carboxyl group under hydrothermal conditions (aldehyde-to-carboxyl transition), thereby leading to a negatively charged Mg surface which will attract Ca2+ions, thus promoting the formation of a corrosion-resistant Ca-P coating on the Mg alloy and reducing the anodic dynamics of the Mg substrate in Hank’s solution to a large extent.Fig.2(Ⅰ)is a schematic representation of the glucose-induced composite coating of Ca-P and Mg(OH)2induced by hydrothermal treatment of pure Mg.So, it can be seen that in future studies related to template-mediated hydrothermal preparation of HA coatings,researchers choose inducers from the perspective of biocompatible substances(e.g.,peptides, amino acids, proteins, glucose, and dopamine) that can chemically bond with HA crystals [50].The composition, structure and properties of chelating agents used in the preparation of Ca-P coating on magnesium alloy surface by hydrothermal method in recent years are listed in Table 1.

Table 1Chelating agents, corresponding active groups and coating properties in recent studies.

2.2.Two-step hydrothermal method

2.2.1.Microwave-assisted hydrothermal method

Microwaves refer to electromagnetic waves with frequencies between 300 MHz and 300 GHz.When microwaves are delivered to chemical synthesis reactions, they can cause thepolar molecules(such as water,alcohols and carboxylic acids)to vibrate in an ordered, high-frequency fashion.The collision, friction and squeezing of molecules convert microwave energy into heat energy,speeding up the reaction rate[55-58].Compared with the conventional hydrothermal method, the microwave-assisted hydrothermal method features fast heating speed, sharp response and homogenized heating system,which enables the synthesis of crystals with uniform particle size and topography in a short period of time and avoids the agglomeration phenomenon [59-61.Therefore, it is often used for the synthesis of HA powders.To the best of our knowledge, microwave hydrothermal method for the synthesis of Ca-P coating on Mg alloy surface was first adopted and published by our groups.Jiang et al.[62]prepared magnesium phytate/apatite composite coating on the surface of Mg alloy using microwave hydrothermal method, utilizing phytic acid (PA) with better biodegradability and biocompatibility in the human physiological environment, covalently fixed on the surface of Mg alloy through chelating reaction with hydroxyl and phosphate anions, interacting with Ca2+to improve the bonding strength of apatite coating and reduce its degradation rate.Yu et al.[63]fabricated a special double-layer structure uniformly crack-free strontium-doped HA coating on Mg alloy using microwave-assisted deposition to enhance the corrosion resistance and bioactivity of the Mg alloy.Immersion tests in simulated body fluid (SBF) demonstrated that Sr/HA coatings can provide protection to Mg alloys during the initial period, while inducing the precipitation of new apatite to form a dense layer on the coating surface and accelerating the precipitation of apatite, thereby strengthening the bioactivity of the Mg alloy substrate.Shen et al.[64]developed a needle-like fluorine-doped HA coating with micron/nano morphology using a microwave aqueous solution method on a Mg alloy substrate.The resultant dense FHA coating maintained its original integrity after 24 days immersion in simulated body fluid, and the pH value consistently fluctuated between 7.0 and 7.4 (Fig.3).

Fig.2.(Ⅰ) Schematic diagram of (a-c) Ca-P coating and (d-f) Ca-PL-Cys-coating formation mechanism [47], with permission from Elsevier;(Ⅱ)Formation mechanism of hydroxyapatite coating induced by polydopamine [48], with permission from Elsevier;(Ⅲ)Schematic representation of the glucose-induced composite coating of Ca-P and Mg(OH)2 by hydrothermal treatment of pure Mg, with permission from Elsevier [49].

The conventional hydrothermal method for the preparation of whisker-like HA coatings usually requires 1 h to 14 d Whereas the microwave hydrothermal method takes only 10 min, which greatly simplifies the experimental process and shortens the time needed.Unlike conventional electric heating, microwave heating can boost the chemical driving force of HA precipitation,thus promoting the growth of HA crystals along the c-axis [65,66].Simultaneously, microwave radiation reduces the activation free energy (ΔG) of the HA precipitation reaction, increases the diffusion rate of F-ions in the deposited coating solution, encourages F?ions to enter the HA lattice rapidly.Namely, the superposition of microwave radiation on both thermodynamics (increasing the chemical driving force for HA precipitation) and kinetics (promoting the fast entry of F?into the HA lattice) led to the synthesis of whisker-like fluorine-doped HA bilayer coatings with diameters ranging from 35 nm to 80 nm in only 10 min.Our study shows that the most prominent virtues of the microwave method are the fast synthesis and a small grain size, but the resulting bond strength between coating and substrate is usually less than 10 MPa, which does not meet the requirementfor clinical application.To tackle this obstacle, the preparation of composite coatings by chelator induced method can be considered in later studies to increase the bonding strength and wear resistance of HA coating.Compared with the conventional heating process, microwave heating can effectively reduce the thermal stress and temperature gradient of the reactants, allowing the reactants to be heated quickly and uniformly with efficient energy conversion.

Fig.3.(a) Weight loss of magnesium alloy with FHA coating.(b) pH variations of SBF containing magnesium alloy with FHA coating.(c) Microscopic appearance of magnesium alloy with FHA coating after immersion in SBF for up to 24 days [64], with permission from Elsevier.

2.2.2.Micro-arc oxidation/hydrothermal (M-H) methods

The Micro-Arc Oxidation process (MAO), also known as plasma electrolytic oxidation (PEO), is performed by placing Mg or Mg alloy as an anode in an aqueous electrolyte solution, which first generates an anodic oxide film on the surface when energized.Then the anodic potential is raised from tens of volts used for ordinary anodic oxidation to hundreds of volts, thereby rendering the previously generated oxide film punctured,causing a series of complex reactions such as plasma chemistry, electrochemistry, thermochemistry, diffusion reaction, high temperature phase transition, etc., which eventually generates a MgO ceramic coating with a two-layer structure (a porous layer and a dense layer) on the surface of Mg alloy, the porous layer increases the contact area with osteoblasts, which is conducive to the close integration with the bone interface, and the internal dense layer enhances the corrosion resistance of the substrate [67,68].

The MgO ceramic layer created by the micro-arc oxidation treatment grows in situ from the Mg substrate, so it is tightly bonded to the underlying substrate, thus significantly improving the corrosion and wear resistance of Mg alloy.Chang et al.[69]prepared a coating containing calcium phosphate dihydrate (DCPD) andβ-Ca3(PO4)2on the surface of Mg alloys using a micro-arc oxide (MAO) hydrothermal treatment method.The MAO layer is mainly composed of MgO and MgAl2O4, and the Ca and P in the oxide layer exist in amorphous phase.Hydrothermal treatment not only transforms the amorphous Ca and P into DCPD andβ-Ca3(PO4)2, but also promotes the biological corrosion resistance of Mg alloy, especially the pitting corrosion resistance.Yao et al.[70]investigated the effect of pH value of hydrothermal conditions on the properties of Mg alloy composite coatings prepared by a two-step process of micro-arc oxidation and hydrothermal treatment.The porous MAO layer on the surface of the Mg alloy was comprised of MgO and amorphous phosphate.A hydrothermal layer with a thickness of about 4 μm was deposited on the surface of the MAO coating.At pH 5.6,the coating was flake-like with the predominant crystals being brushite (DCPD, CaHPO3and HA).At pH 9.4, the coating is rod-like and the predominant crystalline form is HA.Owing to the excellent sealing effect and the increased thickness of the composite coating, its corrosion resistance is improved by one order of magnitude over the MAO coating and by two orders of magnitude over the Mg substrate.The composite coating of pH 5.6 has a slightly greater thickness and better effective sealing than that of the coating pH 9.4,so that the corrosion resistance of the former is superior to that of the latter.Hydrothermal treatment impaired the apatite induction capacity of the samples in SBF solution at 37 °C, and the composite coating had a better ability to induce apatite formation compared to the single MAO coating.For example, the rod-like HA prepared on MAO coating by hydrothermal method at pH 9.4 had a better apatite induction capacity than the flaky DCPD formed at pH 5.6.Bai et al.[71]prepared a MgO layer on the surface of Mg alloy using a micro-arc oxidation technique and then applied a composite coating consisting of MgOH, HA and MgO to the MgO layer by hydrothermal deposition,followed by implantation of the prepared MgO composite coating material into the rabbit femoral pulp cavity to evaluate the in vivo corrosion behavior of the implant.The Mg(OH)2in the composite coating had a low corrosion resistance.The presence of Mg(OH)2in the MgO pore may influence the in situ formation of HA as a confinement effect and Mg(OH)2can be generated in the Cl?contained environment, but MgCl2offers no corrosion protection.Therefore, decreasing the Mg(OH)2content and increasing the HA content will be the focus of future research on the microarc oxidation/hydrothermal method combination.The minuscule-arc oxidation/hydrothermal method is an effective way to synthesize Ca-P nanocrystals with small size,high purity and uniform particle size distribution in a short period of time.

2.2.3.Other "two-step hydrothermal method"

The idea of the two-step hydrothermal method is to first prepare the calcium phosphate precursor on the Mg alloy substrate, and then hydrothermally convert it into a HA coating with higher crystallinity and purity, besides the aforementioned micro-arc oxidation, the first step also includes plasma spraying,electro-deposition,and so on.To date,plasma spraying is the only commercially available technology for surface modification of metallic materials, which has the advantage of high efficiency and easy control of coating thickness.Nevertheless, plasma spraying requires high temperatures, which can lead to the decomposition of the calcium phosphate phase and the formation of amorphous phases,and the coatings tend to be of low purity and low crystallinity.Low crystallinity accelerates the dissolution of the HA coating, causing it to be out of sync with the life of the implant.Plasma sprayed HA coatings have a micro-scale roughness on the surface, whereby the osteo-conductivity is generally poor.The mechanism of the two-step plasma spray/hydrothermal preparation of calcium phosphate coatings is that plasma spraying produces amorphous phases and decomposition products (tricalcium phosphate and calcium oxide), which can serve as precursors and nucleation sites.The hydrothermal treatment is a dissolution-recrystallization process, and subsequently deposited coating on substrate.At a hydrothermal temperature of 180 °C, the tricalcium phosphate and amorphous phase dissolve into calcium and phosphate ions, which form Posner clusters in solution.Posner clusters are thought to be the growth units of HA crystals,which exhibit a positive charge on their surface and attract hydroxide ions to produce HA crystals.It is worth mentioning that the plasma spray/hydrothermal two-step process for the preparation of calcium phosphate coatings on Mg alloys is in principle feasible, but it has only been studied on titanium alloys and few researchers have tried and investigated the corrosion resistance and biological properties of coated Mg alloys by the plasma spray/hydrothermal two-step process [72].

Besides surface modification, alloying of Mg with biocompatible metal elements (e.g., Ca, Zn, Mn, Al), for microstructural optimization through drastic plastic deformation and amorphization, is an effective approach to upgrade the corrosion resistance of Mg alloys.Despite the high production cost of alloying method, complex manufacturing process,small number of alloying elements to choose from, and limited efficacy in controlling the degradation rate of Mg, Mg alloy with controlled degradation rate can be manufactured by combining the two processes of alloying and surface modification.Li et al.[73]pre-deformed Mg alloys via high-pressure torsion (HPT) process and then hydrothermally prepared HA coatings with different Mg(OH)2contents.Grain size plays a crucial role in the structural parameters of the polycrystalline metallic materials.HPT treatment refined the grain size,introduced a large number of twins, and improved the microhardness and Young’s modulus of the Mg alloy.At the same time,the fine and uniformly distributed second phase and a large number of fine crystals, twins, and grain boundaries in the substrate provide more nucleation sites for HA crystals.HPT technology and HA coating surface modification simultaneously improve the corrosion resistance of Mg alloys, which yields Mg alloys a favorable outlook within the orthopedic implant clinical application.

3.Regulation of the composition and structure of Ca-P coatings via hydrothermal method

3.1.Ca-P coating with various hydrothermal conditions

As a liquid phase technology, the hydrothermal method refers to creating a closed reaction environment with high temperature and pressure by heating the reaction system to grow specific crystals in aqueous solution, which is extensively applied to the powder synthesis and coating preparation such as piezoelectric, ferroelectric, ceramic and oxide film.The hydrothermal method has the advantages of convenient operation, simple synthesis process, controllable particle size [74,75].

The rate of crystals formation and the morphology of crystals in different hydrothermal systems vary dramatically.Zhu et al.[76]reported that pH value, reaction temperature, hydrothermal time and Ca2+ion concentration had a considerable role in the microstructure and growth process of HA coatings.As the hydrothermal reaction time, reaction temperature and calcium ion concentration increased, the HA crystals grew more fully and the length to diameter ratio tended to increase.Hao et al.[77]described the hydrothermal reaction for the preparation of HA crystals using polyacrylamide (PA) as pH regulator, where the reactant concentration (Ca2+ion and PA regulator) significantly affected the morphology of HA.At relatively low solution supersaturation, long and homogeneous fiber-like HA was obtained,while flower-like aggregated HA crystals appeared at highsupersaturation.Ham et al.[78]showed that the structure and morphology of calcium phosphate crystals are closely related to the pH value of the hydrothermal precursor solution.When pH＜6, rectangular flake-like dicalcium phosphate anhydrous (DCPA) microcrystals are formed, while at pH＞8,HA nanorods are primarily formed.In the hydrothermal reaction for the formation of HA crystals, crystalline deposition occurs by chemical polymerization in a solution containing calcium ions and phosphate, while water is involved as a chemical reagent in the reaction [79].In the hydrothermal system, the following changes occur in the chemical properties of water: (I) Under high-temperature and high-pressure hydrothermal conditions,according to the Arrhenius equation,the reaction rate constant is exponential with increasing temperature, and the ionization constant of water increases with increasing reaction temperature and pressure.With higher pressure and temperature, the water decomposes more hydroxide ions.(II) The viscosity and surface tension of water decrease with rising temperature.In a hydrothermal system,the mobility of molecules and ions in solution is greatly increased, thereby allowing crystals to grow faster than under other conditions.Calcium phosphate possess excellent bioactivities,and coatings composed of different calcium phosphate phases can affect the corrosion and biological behavior of the coatings.The different calcium phosphate phases obtained in the study under different hydrothermal conditions are shown in Table 2.

Table 2The different calcium phosphate phases obtained in the study under different hydrothermal conditions.

The microstructure of the coating not only affects the corrosion resistance, but also significantly determines the biological properties of the Mg alloy.Zhang et al.[82]used a hydrothermal method to prepare the F ion-doped HA coating and the morphology of the coating was changed from the flake-like (pure HA coating) to the nano-needles(FHA coating).The HA coating consists of two layers, the inner layer contains vertically growing crystals and is densely arranged in a sequence, while the outer layer is thick and loose.The fluoride treatment significantly reduced the corrosion tendency and rate of the AZ31 Mg alloy.The high resistance and low capacitance of the fluoride conversion film was more effective in retarding the degradation of the Mg alloy than the HA coating.The dense, fine-grained inner layer of the HA coating facilitated corrosion protection and delayed dissolution of the fluoride conversion film, while the firm outer layer facilitated osteoclast adhesion and growth within the implant due to the appropriate pore size.Sachiko et al.[85]obtained different crystalline phases and coating topography of calcium phosphate at different pH conditions (Fig.4).The OCP and HAp crystalline phases were composed of plate-like crystals of about 0.1 μm and rod-like crystals of about 0.1 μm diameter, respectively.Osteoblast MG63 formed focal con-tacts at the tips of the plate-like OCP and rod-like HAp crystals with a height of 2-5 μm, but the spacing between OCP tips was 0.8-1.1 μm, greater than the spacing between HAp tips of 0.2-0.3 μm.He concluded that cell proliferation depends on the microscopic morphology of the coating that controls the focal contact spacing and that HAp coatings are more suitable for improving the cytocompatibility and osteogenic ability of Mg alloys.However, OCP is more soluble in water and readily converts to HA under alkaline conditions.

Fig.4.(a, b) Surface and (c, d) cross sectional SEM images of (a, c) OCP- and (b, d) HAp-AZ31.For cross-sectional observation, OCP and HAp coating layers were scraped off the surface of the substrate [85], with permission from Taylor & Francis.

3.2.Superhydrophobic coating

Mg alloy is hydrophilic, the contact angle is less than 90°,therefore,corrosive solutions(such as acid,alkali and chloride solution) are easily spread on the surface of Mg and corrode it.The super-hydrophobic coating can prevent the direct contact between the coated Mg alloy and the corrosive solution,as elaborated in the Fig.5a[86,87]by Wang et al.Featuring a host of unique functions such as self-cleaning, anti-bacterial adhesion and anti-freeze properties, super hydrophobic surfaces have been gainfully exploited in recent years for oilwater separation, bio-catalysis and biosensors, and drug/gene delivery.The specific morphology and roughness of super hydrophobic surfaces, as well as surface convection and localization effects, facilitate the alleviation of undesired tissueimplant interactions and alteration of gene delivery pathways,offering new perspectives to effectively address certain limitations in biomedical technology.The air layer existing on the surface of the super-hydrophobic coating enables a diminution of the contact area of the erosive liquid with Mg alloy and significantly lengthens the penetration path of the erosive liquid, thereby effectively improving the corrosion resistance of Mg alloy, as exhibited in Fig.5b, 5c [88-91].

It is more profitable to fabricate super-hydrophobic composite coatings on Mg alloys through modifying the initial rough surface via changing the surface chemical properties or combining hydrophobic materials, which does not entail the hydrophobic nature of the original substrate.Therefore, the currently reported micro-nano-structured calcium-phosphorus coatings prepared on Mg alloys can be used as specific rough substrates, and their distinctive osseointegrative properties can facilitate the biocompatibility of Mg alloys.Through the adjustment of the substrate roughness and the complementary surface hydrophobic modification, it is expected to impart functional properties such as corrosion resistance, antibacterial and osteogenesis to Mg alloys.Inspired by the"petal effect", Peng et al.[92]successfully constructed superhydrophobic coatings on pure Mg by a simple hydrothermal treatment in a solution containing sodium oleate.The micro-nano coating structure and the low surface energy of the oleate group conferred the super-hydrophobicity.Sealed air pockets and the attraction of van der Waals forces at the solid-liquid interface rendered the water droplets on the superhydrophobic coating unable to roll off even when the sample was inverted.The low cytotoxicity and enhanced corro-sion resistance of the coating are expected to have a positive effect on the bone remodeling.Zhang et al.[93]deposited Mg(OH)2coatings on AZ31 Mg alloys by hydrothermal method and then prepared micron-sized spherical polypropylene (PP) films on the Mg(OH)2coatings step by step to obtain corrosion-resistant and super-hydrophobic coatings.Kang et al.[94]sedimented super-hydrophobic hydroxyl groups on Mg alloy substrates from solutions containing stearic acid and calcium phosphate compounds apatite/calcium stearate coating.The maximum contact angle of the prepared coating is 152.8° and the sliding angle is less than 2°.The prepared coating causes hemolysis in less than 0.1% and has excellent hemato-compatibility.Stearic acid possess a very low surface energy and considerably reduces the degradation rate of Mg alloys [95].However, in the above studies the effect of super-hydrophobicity on corrosion resistance was often only recorded, ignoring the possible changes and effects of superhydrophobic coatings in the in vivo environment.Modifying

Mg alloys with stearic acid could achieve super-hydrophobic surfaces, and such organic acid groups could bind to calcium or other ions in body fluids, and could regulate the pH of body fluids and promote the nucleation and growth of apatite nanocrystals.Especially, the change of surface structure or the formation of deposits on the super-hydrophobic coating can make the hydrophobicity weakened, which is expected to realize the conversion of the surface from hydrophobic to hydrophilic.The effect of stearic acid on osteoblasts is still unclear, and it will be an excellent strategy for bone tissue engineering if the excellent biological activity of Ca-P coating can be combined to engineer the Mg alloy surface in the follow-up study of biological properties.

Fig.5.Increased corrosion resistance and prevention of bacterial adhesion using superhydrophobic surfaces: (a) Schematic illustration of the protective mechanisms of superhydrophobic coatings against corrosion of Mg substrate, plotted by Wang et al.[87], with permission from American Chemical Society;(b) The air layer existing on the surface of the superhydrophobic coating, (c) The electrochemical impedance spectroscopy of LAS-Mg superhydrophobic coating obtained by Xun et al.[90], with permission from Elsevier; (d) The statistical results of E.coli adhesion on different samples and the illustrations of bacterial adhesion on (e) sample HA, (f) sample S100, and sample S100 representing a superhydrophobic coating [91], with permission from Springer.

3.3.Composite Ca-P coating with other biocompatible materials

The candidates chosen for hydrothermal Ca-P composite coatings involve metal oxides, medical polymers or template chelating agents such as tin dioxide, amino acids and graphene and its derivatives etc.These multifunctional materials can be used as reinforcing media or chelating agents to increase the physiological stability and mechanical properties of the Ca-P coatings.Zhou et al.[96]used a simple one-step hydrothermal method to fabricate multilayer nano-HA/ZnO coatings with a thickness of 16 μm on the surface of Mg alloys.After hydrothermal treatment, the nano-HA whiskers with high crystallinity were grown on the Mg alloy surface.HA/ZnO-coated Mg alloys have enhanced corrosion resistance in SBF compared to pure and uncoated Mg alloys.Due to the presence of ZnO in the coating, the in vitro antimicrobial rate of the Mg alloy is close to 100%,thereby providing nano HA/ZnO coated Mg alloys with good biocompatibility and excellent corrosion resistance.Cui et al.[97]developed SnO2-doped dicalcium phosphate coating on AZ31 alloy by hydrothermal deposition.The coating has a spherical morphology with a long laminar crystal structure and a thickness of about 40 μm.The corrosion current density and hydrogen release rate of the coatings obtained in the presence of SnO2were diminished and the corrosion resistance of the SnO2-doped coatings was imitated compared to the coatings without SnO2and the bare AZ31 substrates.Peng et al.[98]formulated a bilayer coating on AZ31 alloy with HA as the inner layer and graphene oxide (GO)as the outer layer.The graphene oxide could fully inhibit the formation and growth of corrosion cracks in the HA layer, thus conferring excellent corrosion resistance.In addition, in vitro cell culture results revealed that MC3T3-E1 cells showed excellent proliferation rate on the HA/GO coating.

3.4.Ion-substituted ha coating

Ion substitution of HA coatings has been categorized as a promising alternative to conventional Ca-P coatings, providing HAp with better osseoinduction, antibacterial activity,accelerated biomechanical fixation,and provisioning solutions in certain pathological scenarios such as infection and osteoporosis [99,100].The special hexagonal structure of HA crystals offers the possibility of multi-elemental ion substitution(Fig.6a).Elemental substitution is widely explored to modulate and tailor the growth and surface morphology of HA coatings.Ca2+ions in HA are readily replaced by other divalent metal cations of similar atomic radii such as Zn and Sr ions[101,102],and phosphate ions can be replaced by silicate,carbonate, and sulfate ions [103], leading to the formation of calcium-deficient HA(CDHA)with Ca/P ratios＜1.67 instead of stoichiometric HA.

Zhou et al.[104]used a one-step hydrothermal method to dope zinc into the HA coating on the surface of ZK60 Mg alloy to obtain a corrosion-resistant implant with osteogenic differentiation and antibacterial activity.The addition of Zn led to lattice distortion of the a/b crystal face, forming a growth trend along the c-axis and accelerating the generation of nanorod-like Zn/HA.ALP activity test showed that appropriate zinc concentration was conducive to the osteogenic differentiation of bone marrow mesenchymal stem cells.In addition to controlling the growth of HA crystals, the addition of trace elements also facilitates bone formation and integration.Wang et al.[105]successfully prepared Sr-doped HA coatings with good corrosion resistance and biocompatibility on the surface of ZK60 Mg alloy by hydrothermal method, revealing the potential of Sr-doped HA-coated Mg alloys for orthopedic applications.The zinc-doped coating reduces bacterial adhesion and inhibits bacterial proliferation by generating reactive oxygen species.Compared with pure HA coatings, most of the osteogenic differential markers, including Runx2, Osterix, OCN and Col-I, on F-doped HA micronano-structured coatings prepared by microwave hydrothermal method by Shen et al.[64]showed an increasing trend, indicating that F can enhance the osteogenic differentiation performance of HA coatings.In addition, the FHA coatings are highly resistant to solubilization due to the reduced crystalline spacing of fluorine doping, and the micron/nanometer morphology of FHA coatings provides large specific surface area and high roughness to support secondary nucleation, growth,and anchoring of HA mineralized layers (Fig.6).

4.Corrosion and biological properties of Ca-P coated Mg alloy via hydrothermal method

4.1.Corrosion resistance

Corrosion resistance, strong bonding strength between coating with substrate, osseointegration, and antimicrobial properties are among the properties that should be present in bone replacement candidates [106,107].Hydrothermal preparation of Ca-P coating on Mg alloy to improve its corrosion resistance can be explained by the fact that the dense coating physically isolates the Mg substrate from the corrosive fluid.In parallel, the Ca-P coating induces biomimetic mineralization in the body fluid, which precipitates biological apatite onthe surface of the Mg substrate, thus further preventing corrosion from occurring [108].In order to gain above characters,the follows need to be met: (Ⅰ) Ca-P coating should be dense,complete without defects; (Ⅱ) The bond strength between the coating and magnesium alloy substrate should be high enough to avoid the coating cracking and peeling off in the flowing body fluid; (Ⅲ) the Ca-P coatings possess a certain crystallinity, which can induce biomimetic mineralization in vivo and prevent the internal dissolution of the coating [109-111].Maintaining long-term mechanical integrity and appropriate degradation rates in the physiological environment is a key requirement for magnesium and Mg alloy implants [112-115].The dense coating structure and large coating thickness will effectively prevent the corrosion fluid from penetrating into the Mg alloy, thus providing effective initial protection for the Mg alloy substrate.To all intents and purposes, immersion tests of the coated Mg alloys in SBF are performed to evaluate the long-term protective properties of Ca-P coatings with different composition.The relevant corrosion resistance data in the previous reports are presented in Table 3.

Table 3Corrosion data from our previous work on immersion experiments with calcium-phosphorus-coated modified magnesium alloys.

Fig.6.The mechanism and an example of ion-substituted HA coating: (a) The special hexagonal structure of HA crystals offers the possibility of multielemental ion substitution [99], with permission from The Royal Society of Chemistry.The unit cell of hydroxyapatite projected along the a axis (left) and along the c axis (right) showing Ca1, Ca2, tetrahedral phosphates and hydroxyl sites; (b) TEM image of a nanoneedle on surface layer and the inset shows lattice fringe, (c) EDS elemental mapping of the nanoneedle, (d) XRD patterns of the FHA Coating, (e) Optical density values on FHA coating, HA coating and the blank sample.Relative mRNA levels of main osteogenic differentiation markers on FHA coating and HA coating, obtained by Shen et al.[64], with permission from Elsevier.

The bond strength of the coating is determined by the cohesion strength of the coating and the strength of the coating/Mg alloy interface.The cohesive strength of the coating increases with the increasing density of the coating; the interfacial bond strength of the coating/Mg alloy depends on the density of the coating and the residual stress at the interface.The dense coating structure increases the interface contact area of the coating/Mg alloy, strengthens the mechanical interlocking and chemical bonding, and then improves the interface bonding strength.Low bond strength increases the risk of magnesium-based grafts falling off during use, accelerates local corrosion, and affects the mechanical integrity of the stent [122-124].Lin et al.[125]prepared dense magnesium substitutedβ-tricalcium phosphatemagnesium hydroxide (β-TCMP/Mg(OH)2) composite coating with a bond strength of 20.88 ± 1.60 MPa on the surface of AZ31 alloy using a one-step hydrothermal method at 120 °C.Before hydrothermal synthesis, the bare Mg alloy is treated with alkali, and the formation of magnesium hydroxide on the surface can be used as a protective layer in addition to induce the nucleation of calcium phosphate crystals.Asif et al.[126]successfully hydrothermally synthesized high-strength CaHPO4coatings with high crystallinity,cytocompatibility, and biosorption on the surface of Mg alloy at 100 °C.The adhesion strength of the coatings was measured by lap shear test, and it was demonstrated that cohesion damage occurred under 21.89 MPa stress.Li et al.[127]prepared CDHA/MgF2bilayer coatings on magnesium surfaces using a combination of fluoride treatment and hydrothermal treatment.The coating has a nanoscale roughness and a bond strength of up to 27.64 ± 1.7 MPa with the Mg substrate.The use of MgF2as an intermediate layer between the calcium phosphate coating and the magnesium substrate alleviates the mismatch in hardness and thermal expansion coefficient between the HA coating and magnesium, reduces the residual stress between the HA coating and magnesium, and thus enhances the bond strength.Ji et al.[27]used polyacrylic acid to hydrothermally induce the deposition of HA coatings on the surface of Mg alloys, and the adhesion strength of HAp coatings expressed as critical loads measured by a nanolayer scratch tester indicated that the bond strength of PAA/GS-Hap composite coatings was 3928 mN.polyacrylic acid (PAA) is a carboxyl-containing anionic polyelectrolyte,and its free carboxylic acid groups have a strong affinity with the calcium ion, so the interfacial bonding strength is high.

When Mg alloys are applied to bone fixation devices (e.g.,intramedullary nails), they deform to fit the shape of the affected area, and are sometimes scratched by surgical tools or bones during surgery.This localized coating damage provides a pathway for the permeating of the corrosive solutions,which can lead to severe localized corrosion or even premature coating failure.Investigating self-healing anticorrosion coatings is an important strategy for achieving long-term corrosion resistance of Mg alloys.Self-healing coatings are smart coatings that actively or passively repair coating defects and restore coating performance by incorporating self-healing agents or corrosion inhibitors into the coating [128].Self-healing coating corrosion inhibitors for biomedical applications must have excellent biocompatibility, such as polysiloxane [129], phytic acid [34], 3-aminopropyltrimethoxysilane [130]and sodium polyacrylate[131].Sachiko[84]reported a calcium phosphate self-healing coating composed of Hydroxyapatite (HAp) and octacalcium phosphate(OCP)coatings,which exploit changes in pH around coating defects for self-healing.He confirmed the self-healing properties of the coating defects by immersing the scratched HAp and OCP-coated Mg alloys in cell culture medium.Specifically, in the scratch, the pH of the corrosion solution increased,releasing phosphate ions,and the HAp and OCP coatings healed themselves by forming a quasi-corrosion protective layer containing Mg and calcium phosphate in the coating defect.

4.2.Osseointegration properties

Creating a bioactive interface between the implant and the natural bone or the process of osteogenesis and remodeling after the implant is implanted into the host is called "osseoin-tegration".The calcium phosphate coating solves the problem of poor bone conductivity on the surface of bare Mg alloy.The formation of new bone at the interface of magnesium implant leads to the close interlocking between the implant and surrounding bone tissue, which leads to good osseointegration.With the increase of osseointegration, the mechanical stability of implant is improved and the probability of loosening is reduced.Therefore, in order to achieve good osseointegration, the surface chemical composition and the construction of nanostructured the implant interface should be considered [132-134.In fact, the bone formation of osteoblasts and the bone resorption of osteoclasts are regulated by multiple systems and local regulators such as inorganic ions and growth factors [135].

As already stated, specific inorganic ions are capable of strongly influencing the biological responsiveness of HA not only by modifying the physicochemical properties (e.g., crystal structure, crystallinity, surface charge, solubility etc.), but also because of their recognized and physiological role in bone metabolism [136-138.Here, we mainly discuss the effect of growth factors on the osteogenesis of magnesium alloy grafts.It has to be taken into account that growth factors are protein-type biomaterials that should not be exposed to high temperatures during the process,otherwise they will lose their bioactivity.Accordingly, the hydrothermal method is not suitable for the one-step preparation of growth factor/Ca-P composite coating and researchers often soak it in the second step to obtain the functional coating of growth factor [139].Bone morphogenetic protein-2 (BMP-2), a protein known to promote osteoblast differentiation, can promote bone formation around implants and effectively repair bone defects.Kim et al.[140]prepared calcium phosphate ceramics and oxide layers (MgO and Mg(OH)2) on Mg alloy by MAO/hydrothermal treatment, and then placed the specimens in a constant temperature 37 °C of 50 ng/mL of BMP-2 protein solution for 24 h.The resulting self-assembled specimens were found to promote bone formation through the sustained release of BMP-2.In contrast to bare Mg alloy, cell culture in vitro and animal experiments in vivo revealed that the presence of BMP-2 increased the expression of cranial osteoblast marker genes and the formation of bone-like nodules, and that the combination of BMP-2 and HA rapidly promoted early bone formation.As evident in the Histological images in Fig.7, no inflammatory cell infiltration or bone resorption was observed in the ALB100-implants during the implantation period.Four weeks after implantation, the bone marrow cells and bone structure had healed and increased in density.

4.3.Antibacterial properties

A suitable local alkaline environment effectively inhibits the growth of gram-positive and negative bacteria by inhibiting ATP synthesis and inducing oxidative stress [141].Mg corrosion causes an increase in the pH value of the surrounding micro-environment that may inhibit bacterial growth.However, the buffering system in the organism that maintains stability of physiological processes allows the buffering of the highly alkaline environment generated by Mg corrosion, thus limiting the antimicrobial activity of magnesium implant [142].Hydrothermal calcium phosphate coated Mg alloys is not only beneficial to the adhesion and growth of osteoblasts, but also facilitate the proliferation of bacteria, leading to severe biofilm-associated infections [143,144].Bacteria increase the rate of corrosion, so it is important to prevent inflammatory responses while preserving the surface.Consequently, exterminating pathogenic bacteria prior

to biofilm formation is a critical step in preventing bacterial colonization [145,146].

Fig.8.Enhancement of antimicrobial performance of SnO2 on hydrothermal Ca-P coatings:(a) Illustration of the preparing processes of the Ca-P-Sn coating on the Mg-Li-Ca alloy; SEM micrographs (b), chemical compositions (c) and XRD patterns (d) of the Ca-P-Sn coating on the Mg-Li-Ca alloy; (e) Numbers of E.coli colonies on Ca-P-Sn coating plotted by Cui et al.[154], with permission from Elsevier.

In order to prevent such infections, many studies have attempted to add antimicrobial active ions or antimicrobial drugs to magnesium-based implant materials.Antimicrobial drug (e.g., gentamicin sulfate) hydrothermally induced deposited HA coatings, conferring the ability of magnesiumbased implants to prevent bacterial infections [27,147].Additionally, researchers often choose antimicrobial ions such as silver, zinc, cesium, strontium, copper and manganese ions doped with HA to prepare hydrothermal coatings modified Mg alloy [148-150].The ability of zinc-doped nanowhisker hydroxyapatite coatings to reduce bacterial adhesion and inhibit bacterial proliferation by generating reactive oxygen species was investigated by Zhou et al..They have successfully prepared a bioactive and corrosion-resistant zinc-doped HA coating on ZK60 Mg alloy by a one-step hydrothermal method.Owning to the addition of zinc, the coating projects significant antibacterial activity.And the formation of good bioactive whisker nanostructures was directly related to the appropriate concentration of zinc.Under their experimental conditions, the optimum concentration of zinc was 5%.Compared to metal ion antimicrobial agents, metallic oxide (e.g.,ZnO, TiO2, SnO2, etc.) exhibits lower cytotoxicity and can enhance bone induction [151,152].Sun et al.[153]prepared a monolayer of zinc oxide (ZnO) coating on Mg alloy using microwave hydrothermal synthesis, the coated magnesium alloy was irradiated with ultraviolet light (UV) for different times and then immersed in SBF.The newly formed apatite was found to be dense and could activate the ZnO coating to obtain a more effective antimicrobial bioactivity.Cui et al.[154]found that the addition of SnO2to the Ca-P coating and the release of OH?from the Mg substrate (extract solution pH = 8.93) could improve the antimicrobial properties,as exhibited in Fig.8.Nevertheless, it should be noted that the high alkalinity during corrosion of the Mg alloy in this study is not representative of the in vivo situation, in which the pH is strictly limited to the range of 7.2?7.4.Still,the preliminary results highlight some antimicrobial effects of SnO2nanoparticles in Ca-P coatings.

5.Challenges and future perspectives

Biodegradable Mg alloys, as hard tissue repair materials,need to provide vital mechanical support before new bone generated.However, the failure of the mechanical properties due to its rapid degradation and high hydrogen release will lead to implantation failure.In order to acquire a better corrosion resistance and biocompatible coating modifeid Mg alloy,it is prevalent to endow the coating with low porosity, strong cohesive strength, high crystallinity, and high chemical and phase stability.However, it remains a challenge for conventional hydrothermal method to achieve certain specifci clinical criteria, the researchers have adopted different modifeid hydrothermal strategies so as to achieve multifunctional Ca-P coatings.

Corrosion resistance helps to ensure the stability of the graft.For the long-term protection of the coating on the Mg alloy, the density, thickness, solubility and mineralization capacity of the coating must be considered in a comprehensive manner.The dense coating structure and large coating thickness will effectively prevent the corrosion fluid from penetrating into the Mg alloy, thus providing effective initial protection for the Mg alloy substrate.While the solubility of the coating which is closely related to its chemical composition and microscopic morphology will directly affect the degradation rate.Ca-P bioceramics have excellent biocompatibility,but are fragile and therefore not suitable for use as loadbearing orthopedic graft materials.It is a versatile strategy to select bioactive metal materials and coat them with Ca-P coating, but the bond strength between the coating and the Mg substrate must be taken into account for clinical applications.

With regard to surface osteogenesis of magnesium alloy grafts, the controlled release of growth factors is becoming an increasingly urgent issue because the recovery cycle of grafts usually requires 3-6 months, and the layer-by-layer release method proposed only worked for 4 weeks.In future studies, it will remain a challenge to develop coatings with controlled growth factor release rates to facilitate continuous osteo-conduction of magnesium-based implants.

The bacterial adhesion and subsequent biofilm formation at the site of implantation will lead to the infection of orthopedic implants.The construction of superhydrophobic coating on Mg alloy is one of the novel and effective solutions.Superhydrophobic surface can trap air layer between Mg surface and liquid through the combination of layered structure and low surface energy, which can effectively reduce the adhesion of bacteria on materials, inhibit the formation of biofilm and improve the corrosion resistance of Mg substrate.

Significantly, as an implant material, cell adhesion, growth and differentiation have different requirements on the wettability of the Mg surface.Cell behavior requires the surface to be hydrophilic.Superhydrophobic surface is not conducive to cell adhesion, proliferation, differentiation and osteogenesis.In order to solve this contradiction, the superhydrophobic surface must be changed from superhydrophobic to hydrophilic during implantation to achieve the synergistic effect of antibacterial and osteogenesis.Therefore, the structural design of Ca-P coating on superhydrophobic magnesium alloy with changeable wettability will be a new research hotspot in the future.

6.Conclusion

Mg-based grafts are considered as the promising alternative substitutes for bone tissue engineering attributed to their intrinsic biodegradability and superior mechanical properties.However, the degradation rate of magnesium is too fast to control, which limits its wide application.To address the aforementioned concerns, hydrothermal method has been widely used as a modification technique for Mg alloy coatings because of the high interfacial bonding strength, high corrosion resistance and high bioactivity of the prepared coat-ings.This paper summarizes the recent progress in hydrothermal preparation of calcium phosphate coatings on magnesium and its alloys to improve their corrosion resistance, interfacial bonding strength, osteointegration and antibacterial ability.In order to improve the interfacial bonding strength of hydrothermal calcium phosphate coatings and magnesium substrates,the template-induced hydrothermal method has been widely used in recent years.The core of this improved hydrothermal method is the selection of biocompatible materials with functional groups that can coordinate with calcium ions, such as amino acids, polydopamine and phytic acid.In addition,when microwave treatment is performed during the hydrothermal preparation, it significantly refines the grains of synthetic HA crystals and reduces their clustering effect.Meanwhile,calcium phosphate coatings with different morphologies and phase compositions can be synthesized under different hydrothermal conditions.The preparation of superhydrophobic coatings can effectively enhance the corrosion resistance of Mg alloys, but the effect of superhydrophobicity on biological properties has to be further investigated.Ion-substitution and composite coatings can effectively improve the antimicrobial and osteoconductive properties of calcium phosphate coatings.Our reviewing paper will hopefully provide some ideas for subsequent research on Mg alloys applied to bone tissue engineering scaffolds.

Declaration of Competing Interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No.51872197, 81772363 and 81972076), and Shanghai Committee of Science and Technology, China (Grant No.15411951000).