Micro-arc oxidation coating on porous magnesium foam and its potential biomedical applications

Accepted Manuscript Micro-arc oxidation coating on porous magnesium foam and its potential biomedical applications

J.M. Rúa, A.A. Zuleta, J. Ramírez, P. Fernández-Morales PII: DOI: Reference:

S0257-8972(18)31431-2 https://doi.org/10.1016/j.surfcoat.2018.12.106 SCT 24177

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

4 September 2018 23 December 2018 25 December 2018

Please cite this article as: J.M. Rúa, A.A. Zuleta, J. Ramírez, P. Fernández-Morales , Micro-arc oxidation coating on porous magnesium foam and its potential biomedical applications. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.12.106

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Micro-arc oxidation coating on porous magnesium foam and its potential biomedical applications J. M Rúa 1,*, A.A. Zuleta2, J. Ramírez3, P. Fernández-Morales 4

1

70-01, Medellín, Colombia

Grupo de Investigación de Estudios en Diseño - GED, Facultad de Diseño Industrial,

RI

2

PT

Facultad de Ingeniería Aeronáutica, Universidad Pontificia Bolivariana, Circular 1ª. Nº

3

SC

Universidad Pontificia Bolivariana, Circular 1ª. Nº 70-01, Medellín, Colombia Facultad de Minas, Universidad Nacional de Colombia, Cra. 80 # 65-223, Medellín,

NU

Colombia 4

MA

Facultad de Ingeniería Industrial, Universidad Pontificia Bolivariana, Circular 1ª. Nº

D

70-01, Medellín, Colombia

PT E

Abstract

In this study, we fabricated a biodegradable porous metal using the AZ31 Mg alloy

CE

substrate with anodic coating for application in bioimplant scaffolds. The cellular Mg alloy was obtained by employing replication processes that use NaCl as a space holder

AC

under a controlled atmosphere. Further, vacuum pressure was applied for achieving the infiltration. The Mg porous samples were coated by a micro-arc oxidation (MAO) technique using a phosphate/calcium electrolyte. Further, the samples were examined by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), compression tests and surface area measurements. The MAO technique allowed

*

corresponding author Tel.: +57 4 448 83 88 Ext.12177 E-mail: [email protected]

ACCEPTED MANUSCRIPT us to obtain a homogeneous coating layer of ~16.0 µm thickness with the surface across the foam structure comprising rounded pores with diameters of less than 3.0 µm. Ca, P, Mg and O are the main elements of the anodic coating. Furthermore, both the coated and uncoated porous samples exhibit similar values of approximately 1.5 GPa and 5.0 MPa for the Young’s modulus and compressive strength, respectively.

PT

Keywords: Magnesium foam, Micro-arc oxidation, Inorganic coating, Infiltration

RI

process, Cancellous bone, Biodegradable Mg.

SC

1. Introduction

NU

Several studies have been conducted to investigate whether magnesium (Mg) and its alloys can be considered to be suitable candidate materials for developing medical

MA

implants because of their properties such as good biocompatibility, non-toxicity and high specific strength [1–5]. Therefore, porous Mg structures, such as foams, have also been

D

investigated as scaffolds because the biomimetic structure of the open porous foam

PT E

ensures the chemical stability of implants while a patient is healing. Despite the aforementioned advantages, Mg exhibits low ductility and poor creep as well

CE

as corrosion resistance, which has limited its application as a biomaterial [6]. Further, a previous study implies that the presence of Mg in human body results in a favourable

AC

environment for the growth of bone cells [7]. Additionally, this effect can be improved when the surface area of the material is increased [8], which is similar to that observed in case of metal foams. This indicates that the porous morphology of the Mg alloy foams allows bone regeneration because of the presence of an appropriate osteoconductive environment. Furthermore, the mechanical behaviour of the AZ31 Mg foams is highly similar with that of the cancellous bone tissue having a Young’s modulus and compressive strengths of 2.0 GPa and 0.2–80 MPa, respectively, which effectively

ACCEPTED MANUSCRIPT reduces the stress shielding induced between the implants and surrounding bone tissue [9–11]. Mg and its alloys are known to exhibit a high chemical/electrochemical activity that could lead to corrosion failures in different surroundings, including biological environments [12]. Further, hydrogen gas bubbles are generated owing to the anodic reaction that occurs

PT

during the Mg corrosion process. This hydrogen generation increases proportionally with

RI

the Mg dissolution rate [13] and inhibits cell attachment, causing inflammation of the

SC

surrounding tissue [1]. In case of Mg foams, this phenomenon is considerably pronounced because of the large surface area [14]. Therefore, developing new processes for

NU

conducting surface treatments of Mg substrates is necessary to improve their surface properties, thereby enabling the usage of Mg and its alloys in biological environments.

MA

This constitutes an interesting subject of research, which focuses on obtaining

D

biodegradable materials with good biocompatibility and adjustable corrosion rates.

PT E

The micro-arc oxidation (MAO) technique, which is also known as plasma electrolytic oxidation (PEO), improves the corrosion behaviour and wear resistance of metal substrates, such as titanium, aluminium and magnesium alloys, through the formation of

CE

a strong, thick, relatively porous and adherent ceramic film exhibiting minimal changes

AC

in the substrate properties. The MAO technique exhibits surface modification with promising results for preventing corrosion along with the potential biomedical application of Mg and Mg alloys in bulk [15]. Technically, the MAO technique uses conventional anodised Mg. The applied voltage and current are considered to be important for initiating the MAO technique and for creating a circuit that allows the oxidation of a metal surface employed as the anode [16]. The MAO technique parameters, such as current density, voltage, cathode metal,

ACCEPTED MANUSCRIPT conductivity and pH electrolyte, as well as some substrate factors, such as microstructure and chemical composition, strongly influence the coating properties. The phosphate and silicate electrolytes are commonly used because of their optimal conductivity, and each of these electrolytes exhibit respective characteristic responses [17]. Specifically, the phosphate electrolyte accelerates the discharge process and allows rapid coating growth,

PT

which is considered to be an advantage for the application of this technique to samples

RI

with a large exposed area [16–19].

SC

In particular, the MAO coatings are being developed on Mg alloys to enhance their biocompatibility with coatings similar to bone tissue. In this case, phosphate coatings are

NU

considered to be good candidates for improving the bioactivity and biodegradation properties of the substrate [20]. Because the calcium phosphate compounds are important

MA

for functionalising the surface metal, researchers have employed some salt types as

D

alternatives for phosphate electrolytes [21–23].

PT E

No Mg porous materials that have been successfully modified with MAO have been reported to date. Therefore, to establish MAO coating on AZ31 alloys, studies, such as the one conducted by Li et al. [24], were evaluated. Li et al. used a sodium phosphate

CE

solution to obtain a biocompatible surface, where the final reaction formed products that

AC

exhibited both hydroxyapatite and calcium phosphate phases. Similarly, Chen et al. [25] coated AZ31 with a combination of silicate and phosphate using NaOH to adjust the pH level. Furthermore, other solutions have been applied to protect the Mg substrates, among which silicates have also been applied [26–28]. Despite the fact that the AZ31 Mg alloy is already coated by means of the MAO process using a phosphate electrolyte, we failed to find any reports on the application of MAO coatings on Mg foams during our literature review. Thus, we assume that this study is the first to describe the Mg foam that has been treated with MAO. The objective of this study

ACCEPTED MANUSCRIPT is to investigate the formation of MAO coating on AZ31 Mg foams using an electrolyte comprising calcium and phosphorus. Further, the mechanical properties, chemical composition and morphology of the coatings were assessed. The presented coatings are

PT

potentially suited to protect the Mg foams in biomedical applications.

RI

2. Experimental details

SC

2.1.1 Magnesium foam fabrication

The AZ31 Mg foams (wt. %: 3 Al, 1 Zn, bal. Mg) were produced using the infiltration

NU

process under the conditions that were reported in a previous study [29]. Further, the NaCl particles were used as space holders with a size ranging between 500 and 600 µm. The

MA

argon gas that was combined with the salt fluxes was used as the control atmosphere for performing the Mg melting process, and the infiltration was performed under vacuum

D

conditions. The infiltrated material comprising AZ31/NaCl was cut and mechanised into

PT E

cylinders having a diameter of ~8.0 mm. The samples height to mechanical test was of 16 mm and 4.0 mm to morphological characterisation. To reveal the porous structure, the

CE

specimens were cleaned using deionised water in an ultrasonic bath for 15 minutes. Further, the samples underwent pickling with 80% nitric acid and were neutralised with

AC

40% sodium hydroxide, rinsed in deionised water and dried in an air stream. This process was terminated when the value of electroconductivity became ~97.0 µS/cm. 2.1.2 MAO coating process The MAO process was conducted in potentiostatic mode for 5 min using the Matsusada power supply (1000 W). The electrolyte was prepared by dissolving NaOH, Ca(OH)2, Ca(HPO4), and metallic calcium in 8 L of deionised water with a final alkalinity and electroconductivity of pH 12.5 and 96.7 µS/cm [30], respectively. The bath was stirred

ACCEPTED MANUSCRIPT during the process using a magnetic stirrer at 600 rev/min. Further, a two-electrode electrochemical cell was employed using a stainless steel vessel as the cathode. Subsequently, the specimens were immediately rinsed in ethanol and dried in a cool air stream. Each experiment was performed thrice to verify the reproducibility.

PT

2.1.3 Metal foam characterisation The average roughness for uncoated and coated foam, (Ra) and (Rq), respectively, was

RI

assessed by atomic force microscopy (AFM) using the Infinity Asylum (Oxford

SC

Instruments) Research MFP-3D microscope. The analysis area was 20 × 20 μm using a

NU

silicon tip with a diameter of 9.0 nm for conducting topographic measurements. To measure the surface area of coated specimens, degassing of the specimen was

MA

conducted overnight at a temperature of 110 °C and a vacuum pressure of 1 × 10−6 mbar; the adsorption/desorption isotherms of a pressure sorption were obtained using a

D

Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. Further, the N2 gas was

PT E

used for obtaining adsorption isotherms, and the pore size distribution was analysed using

CE

the density functional method (DFT).

AC

2.1.4 Coating characterisation The morphological and chemical composition examination of the coatings was conducted by scanning electron microscopy (SEM) using the JEOL 6490LV instrument, equipped with energy dispersive X-ray (EDX) spectroscopy (OXFORD INCAPentaFET-x3). Further, cross-sections of the coatings were prepared by polishing using SiC paper (#120, #320, #400, #600 and #1200 grit), followed by polishing with diamond to a roughness of 1 μm, and were further rinsed with ethanol and dried in a cool air stream.

ACCEPTED MANUSCRIPT The coating composition was investigated by X-ray diffraction (XRD) using an X'Pert APD X-ray diffractometer with Co Kα radiation. The data were recorded over the 2θ range of 10°–80°, with a step size of 0.02°. In addition, a Fourier transform infrared spectroscopy (FTIR) was employed to identify the phosphate compounds in the coating

PT

using a Shimadzu (FTIR) Model Tracer-100 in the spectral range from 400 to 4000 cm−1.

RI

2.1.5 Mechanical characterisation

SC

Finally, the mechanical compression response was determined under a uniaxial

NU

compression load using the Instron 3366 Universal testing machine (Instron Corp., USA) with a crosshead speed of 0.50 mms−1. The compression tests were conducted on the

MA

cylindrical specimens having a height of 16.0 mm at room temperature in accordance

PT E

D

with the ISO 13314 standard (compression test for porous and cellular metals) [31].

3. Results and discussion

CE

3.1 MAO coating

The current and voltage–time response during the MAO process for the AZ31 Mg foam

AC

is presented in Figure 1.

RI

PT

ACCEPTED MANUSCRIPT

SC

Figure 1. Current and voltage–time plots during the potentiostatic MAO process of the AZ31 magnesium

NU

alloy foam at 500 V.

MA

A curve with three main segments is observed in agreement with previous reports [16,32]: (i) a current increase from 0 to 2 A is observed due to the formation of a barrier layer

D

[33], which is followed by the initial state of the constant current that lasts for up to 50 s,

PT E

corresponding to an initial galvanostatic period; (ii) subsequently, the voltage becomes constant, and a decrease in current to 1 A is observed using the fluctuations that are caused

CE

by localised sparks on the surface of the AZ31 foam; (iii) finally, the current is stabilised until the end of the process at ~0.65 A. Further, it is important to mention that the applied

AC

current during the MAO process was experimentally fixed because the Mg foam suffered severe corrosion at low current values and because the formed coating was considered to be unsatisfactory owing to the presence of cracks and corrosion products.

3.2 Morphology, surface area and density of AZ31 foams Figure 2

(a) and (b) depict the SEM images of the uncoated and coated AZ31 Mg foam

morphologies. The uncoated foam exhibits a uniform pore distribution with

ACCEPTED MANUSCRIPT interconnected pore sizes of between 100 and 500 μm. Further, the pore size and morphology replicates the shape of the salt particles that are used as space holders, which is an observation that is characteristic of the infiltration process [29, 34–36]. The porosity percentage of the AZ31 Mg foam can be determined using the following expression: 𝑃𝑟 (%) = (1 − 𝜌𝑅 ) × 100,

PT

(1)

where Pr (%) denotes the porosity percentage and ρR denotes the relative density of the 𝜌

RI

foam, equivalent to the ratio between the foam and bulk Mg densities ( 𝐹 ) [37–39]. The 𝜌𝐵

SC

Pr (%) was 35.679% ± 0.011% and 53.381% ± 0.002% for uncoated and coated foams,

NU

respectively. These results are similar to those reported for the Mg foams obtained by the same infiltration process [14,40]. This result is important with respect to the biomedical

MA

field because the minimal porosity values for applications in cancellous, cortical or trabecular bones lie between 35% and 80% [8]; further, angiogenesis occurs in sites or

D

cavities with mean diameters of between 100 and 500 μm [41–43].

PT E

After the MAO coating process is completed, the AZ31 Mg foam relative density was observed to be ~28% lower than that of the uncoated foam (1.093 ± 0.003 for uncoated

CE

foam and 0.724 ± 0.019 for coated foam). The porosity percentage measured using the physisorption technique was 50.6%, which was 38% higher than that of the uncoated

AC

foam. This could be explained by the fact that the MAO coating process involves the surface dissolution of the Mg substrate, which is replaced by a porous coating layer [44,45]. The surface morphology of the coated AZ31 Mg foam is depicted in Figure 2 (a). This coating exhibits a homogeneous layer with a volcano-like morphology presenting circular pores and some coalescent pores, which are randomly distributed across the foam surface (Figure 2 (b)). The pore formation is typical of MAO processing on light metals. Further,

ACCEPTED MANUSCRIPT pores are formed by the discharges and gas evolution across the metal surface of the sample [16,46–48]. Such pores have sizes in the micro-/nano-scale, with diameters ranging from 3.0 to 8.5 µm in case of micropores. Further, the porous surface at the nanoscale level measured using the physisorption technique shows a bimodal pore distribution, with one peak around 8.3 nm and another peak between 18.5 and 50 nm. Some studies

PT

have implied that the nanopores that are present in the coating may decrease the corrosion

RI

resistance. Thus, the coating with nanopores may establish degradation control [49–52].

SC

This decrease in corrosion velocity is considered to be favourable while controlling the degradability and bioabsorbability of the Mg scaffolds. From this viewpoint, the MAO

NU

process ensures the porous morphology surface of the AZ31 foam that may enhance the bone cell adhesion, while the Mg scaffold is being metabolised by the body [17,53].

MA

Figure 2 (c) demonstrates the circularity of the pores on the coating, where a homogeneous distribution of large pores can be observed. Regardless, some micropores

D

can be observed, confirming the pore distribution on the coating.

PT E

A cross-section of the coated specimen is depicted in Figure 2 (d). The cross-section exhibits a relatively uniform thickness of 16.3 µm, with pores that tend to be distributed

AC

CE

throughout the coating thickness.

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Figure 2. SEM images of (a) the uncoated magnesium foam, (b–c) surface of the MAO-coated AZ31 Mg

PT E

D

foam, (d) cross-section of the MAO-coated AZ31 Mg foam.

CE

The surface topography three-dimensional images that are obtained by AFM for the uncoated and coated AZ31 magnesium foams are observed in Figure 3 (a) and (b),

AC

respectively. The uncoated foam exhibits a softer surface when compared with that exhibited by the coated foam, which can be attributed to the little holes that are present on the coated surface foam. The coating layer includes volcano-type pores with valleys and peaks between approximately 2.0 and 1.7 μm, increasing the roughness from Ra = 380.09 ± 97.73 nm (Rq = 477.063 ± 120.75 nm) for uncoated foam to Ra = 658.55 ± 107.483 nm (Rq = 831.876 ± 111.86 nm) for coated foam, which is an increase of approximately 57%.

RI

PT

ACCEPTED MANUSCRIPT

PT E

D

MA

NU

SC

(a)

(b)

CE

Figure 3. AFM of the AZ31 magnesium foam surface topography for (a) uncoated foam and (b) coated foam.

AC

Reports have observed that cell adhesion is improved when the roughness becomes greater than 200 nm [8,54]. The surface area, measured using DFT analysis, yields values of 0.0260 and 6.845 m2/g for uncoated and coated foams, respectively. Both the results imply the presence of a Mg foam with a sufficiently large exposed area and a minimum roughness (200 nm) for cell anchoring. However, the increase in the surface area of the Mg foam after the MAO process could be attributed to the formation of a porous magnesium oxide layer. This is relevant because of the following two reasons: (i) while

ACCEPTED MANUSCRIPT maintaining porosity, large surface areas lead to better anchoring and colonisation sites for the cells [4,20,55,56] and (ii) the porosity of the oxide layer allows to control the corrosion rate of Mg, thereby avoiding its total degradation [40,57]. The aforementioned aspects suggest that the coated AZ31 foams become a potential biodegradable or bioabsorbable implant [34,58–60]. The SEM and AFM analysis present the AZ31 foams

PT

as materials with an interconnected porous and rough surface that exhibits appropriate

RI

topographic conditions for promoting cell growth and for the possible proliferation of

SC

bone cells [41,61].

Figure 4 (a) depicts the XRD patterns of the coated and uncoated AZ31 foams. The

NU

difference between both the samples can be observed in the signals corresponding to magnesium oxide (MgO) and calcium phosphate (CaP) that appear on the pattern of the

MA

coated sample [18,62,63]. The low intensity of CaP peaks can be attributed to the low amount of this compound or the presence of an amorphous phase [64]. Because rapid

D

cooling of ceramic particles may occur during the MAO process, the presence of

PT E

amorphous CaP on the coating is a possibility [64]. Previous studies have reported that the CaP compound hydroxyapatite-type signals are present in a range of 2θ = 25°–30°

CE

[21,47]. Further, CaP coating may enhance the biocompatibility of an implant-body

AC

because it presents a chemical composition similar to that of the bone, thereby avoiding the inclusion of elements that are located outside the body. In addition, these coatings prevent direct contact between Mg and the body fluids. This may result in a low corrosion level of Mg, thereby facilitating cellular proliferation [21,65,66]. To detect the presence of the nanocrystalline phase, Infrared spectroscopy (IR) spectroscopy was conducted as a complementary technique for the chemical characterisation of the coating. The IR spectra in Figure 4 (b) exhibit bands at 628, 1041

ACCEPTED MANUSCRIPT and 1502 cm−1, which are associated with PO−3 4 , and confirm the presence of the CaP

MA

NU

SC

RI

PT

compound, as previously exhibited by energy dispersive spectroscopy (EDS).

AC

CE

PT E

D

(a)

(b) Figure 4. (a) XRD patterns for uncoated and coated foams and (b) IR for coated foam

ACCEPTED MANUSCRIPT Ca3O8P2 and MgO were detected in small amounts in the coating because MgO is a hard compound with less dissolution, while Ca3O8P2 is used to develop osteoconductive biomaterials [67,68]. Figure 5 illustrates the elemental distribution, denoting the homogeneous distribution of Ca and P. The surface mapping exhibits that the predominant elements of the coating are

PT

not preferentially segregated. Further, the chemical composition and elemental

RI

distribution of the coating were obtained using EDS analysis that presented the following

SC

results in wt. %: Mg: 31.92; O: 36.68; Al: 2.01; P: 19.52; and Ca: 9.87. This supports the results that have been obtained from the XRD and IR spectra. The XRD analysis did not

NU

exhibit any high intensity peaks associated with calcium phosphates; however, the EDS and mapping demonstrated that the compounds that are abundant in phosphorus and

MA

calcium are not segregated at preferential sites, possibly because they are present in small

AC

CE

PT E

D

quantities.

Figure 5. Elemental mapping of the MAO coating on the AZ31 foam.

3.3 Mechanical characterisation

ACCEPTED MANUSCRIPT The nominal stress–strain curves of the uncoated and coated samples are presented in Figure 6. Both the uncoated and coated Mg foams exhibited similar mechanical behaviours with similar values of Young’s modulus and compression strength (Table 1), which indicated that surface modification does not affect the mechanical properties of the AZ31 Mg foam [69]. The AZ31 magnesium foam presents an elastic region and exhibits

PT

considerable strain with respect to other porous metals such as titanium foams.

RI

Specifically, the Mg foam presents a considerable strain for a stress of approximately 5.0

SC

MPa, which indicates an elastic region with a low stiffness that is similar to the values

AC

CE

PT E

D

MA

NU

obtained by the cancellous bone [70–72] .

Figure 6. Compression stress vs strain curve for the uncoated and coated foams. Insert denotes the elastic zone.

According to Ashby et al. [37,73], Young’s modulus is inversely proportional to the foam porosity, which indicates the possibility that the mechanical properties can be modulated with respect to the particular bone application requirements. In this sense, when compared to bulk implants, metal foams present a considerably good alternative for osteointegration

ACCEPTED MANUSCRIPT to reduce the stress shielding effect [37,40,74,75]. The aforementioned data indicate that the obtained Mg foam could be a potential alternative for application as a bone scaffold because the pore size and porosity promotes osseointegration while offering good

PT

biomechanical behaviour [34,60].

1.484 ± 0.340

Coated AZ31 foam

1.452 ± 0.008

Magnesium foams

1.2–4.46

Titanium foams

0.40–50

Natural bone Cancellous bone

[8,76]

23–102

[34,43,60]

0.1–20.0

2.0–180

[69,77]

0.01–2.00

0.2–80

[70–72]

67

[78]

D

MA

5–46

3–23

CE

4. Conclusions

This work

5.88 ± 0.211

PT E

Cortical bone

References

5.347 ± 0.351

NU

Uncoated AZ31 foam

Compression strength (MPa)

SC

Young’s modulus (GPa)

Specimen

RI

Table 1. Comparative mechanical properties of the coated and uncoated foams with respect to other studies.

The electrical parameters of the MAO process were adapted for samples with

AC

considerable amount of exposed areas, such as magnesium foams, to yield coating with pore sizes of less than 3.0 μm and a layer thickness of ~16 μm as well as to obtain a chemical composition that is close to that of the bone tissue. These conditions contribute to obtain better anchoring of the cells. The AZ31 Mg foam density was ~28% lower than that of the uncoated foam, while the porosity of the coated foam was 38% higher than that of the uncoated foam. In addition, there was an increase in roughness, which can be considered to be an advantage because

ACCEPTED MANUSCRIPT it may enhance the cellular activity. Hence, the MAO coating can be considered to be an alternative for application on magnesium foams as a potential material for the medical implants industry. The chemical composition of the coating obtained using calcium phosphate compounds indicates that the coating exhibits a potential to be used as a biomaterial because these

PT

compounds contain the main elements that constitute the bone system, thereby alleviating

RI

the allergic reactions and other traumas that are associated with the presence of a foreign

SC

material inside the body.

Further, the mechanical compression tests did not show significant differences between

NU

the coated foam and the uncoated foam. Additionally, the Young’s modulus is observed

MA

to be within the range of values that is required for bone tissue applications.

PT E

D

Acknowledgements

The authors are grateful to “Centro de Investigación para el Desarrollo y la Innovación (CIDI)” from the Universidad Pontificia Bolivariana through UPB-Innova

CE

Rad: 635B-06/16-18 and “Departamento Administrativo de Ciencia, Tecnología e Innovación (COLCIENCIAS) of the Colombian Government (Contract N°392-2016).

ysis.

AC

The authors also thank Prof. Zulamita Zapata for collaboration in the DFT tests and anal-

5. References [1]

E. Lukyanova, N. Anisimova, N. Martynenko, M. Kiselevsky, S. Dobatkin, Y. Estrin, Features of in vitro and in vivo behaviour of magnesium alloy WE43, Materials Letters. 215 (2018) 308–311. doi:10.1016/j.matlet.2017.12.125.

ACCEPTED MANUSCRIPT [2]

F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Degradable biomaterials based on magnesium corrosion, Current Opinion in Solid State and Materials Science. 12 (2008) 63–72. doi:10.1016/j.cossms.2009.04.001.

[3]

R. Zeng, W. Dietzel, F. Witte, N. Hort, C. Blawert, Progress and challenge for

PT

magnesium alloys as biomaterials, Advanced Engineering Materials. 10 (2008)

S. Agarwal, J. Curtin, B. Duffy, S. Jaiswal, Biodegradable magnesium alloys for

SC

[4]

RI

3–14. doi:10.1002/adem.200800035.

orthopaedic applications: A review on corrosion, biocompatibility and surface

[5]

MA

doi:10.1016/j.msec.2016.06.020.

NU

modifications, Materials Science and Engineering C. 68 (2016) 948–963.

U. Thormann, V. Alt, L. Heimann, C. Gasquere, C. Heiss, G. Szalay, J. Franke,

D

R. Schnettler, K.S. Lips, The biocompatibility of degradable magnesium

PT E

interference screws: An experimental study with sheep, BioMed Research International. 2015 (2015). doi:10.1155/2015/943603. N. Li, Y. Zheng, Novel Magnesium Alloys Developed for Biomedical

CE

[6]

Application: A Review, Journal of Materials Science and Technology. 29 (2013)

[7]

AC

489–502. doi:10.1016/j.jmst.2013.02.005. Y. Gu, C.F. Chen, S. Bandopadhyay, C. Ning, Y. Zhang, Y. Guo, Corrosion mechanism and model of pulsed DC microarc oxidation treated AZ31 alloy in simulated body fluid, Applied Surface Science. 258 (2012) 6116–6126. doi:10.1016/j.apsusc.2012.03.016. [8]

M. Cheng, T. Wahafu, G. Jiang, W. Liu, Y. Qiao, X. Peng, T. Cheng, X. Zhang, G. He, X. Liu, A novel open-porous magnesium scaffold with controllable

ACCEPTED MANUSCRIPT microstructures and properties for bone regeneration, Scientific Reports. 6 (2016) 24134. doi:10.1038/srep24134. [9]

J. Yang, F. Cui, I.S. Lee, Surface modifications of magnesium alloys for biomedical applications, Annals of Biomedical Engineering. 39 (2011) 1857–

PT

1871. doi:10.1007/s10439-011-0300-y. [10] S. V. Dorozhkin, Calcium orthophosphate coatings on magnesium and its

RI

biodegradable alloys, Acta Biomaterialia. 10 (2014) 2919–2934.

SC

doi:10.1016/j.actbio.2014.02.026.

NU

[11] S. Shadanbaz, G.J. Dias, Calcium phosphate coatings on magnesium alloys for biomedical applications: A review, Acta Biomaterialia. 8 (2012) 20–30.

MA

doi:10.1016/j.actbio.2011.10.016.

[12] H. Duan, C. Yan, F. Wang, Effect of electrolyte additives on performance of

D

plasma electrolytic oxidation films formed on magnesium alloy AZ91D,

PT E

Electrochimica Acta. 52 (2007) 3785–3793. doi:10.1016/J.ELECTACTA.2006.10.066.

CE

[13] M. Curioni, The behaviour of magnesium during free corrosion and

AC

potentiodynamic polarization investigated by real-time hydrogen measurement and optical imaging, Electrochimica Acta. 120 (2014) 284–292. doi:10.1016/j.electacta.2013.12.109. [14] E. Aghion, Y. Perez, Effects of porosity on corrosion resistance of Mg alloy foam produced by powder metallurgy technology, Materials Characterization. 96 (2014) 78–83. doi:10.1016/j.matchar.2014.07.012. [15] L. Xia, J. Han, J.P. Domblesky, Z. Yang, W. Li, Investigation of the Scanning

ACCEPTED MANUSCRIPT Microarc Oxidation Process, Advances in Materials Science and Engineering. 2017 (2017). [16] P.H. Sobrinho, Y. Savguira, Q. Ni, S.J. Thorpe, Statistical analysis of the voltage-time response produced during PEO coating of AZ31B magnesium alloy, Surface and Coatings Technology. 315 (2017) 530–545.

PT

doi:10.1016/j.surfcoat.2017.02.029.

RI

[17] A. Ghanbari, A.B. Khiabani, A. Zamanian, B. Yarmand, The competitive

SC

mechanism of plasma electrolyte oxidation for the formation of magnesium oxide

doi:10.1016/j.matpr.2018.04.178.

NU

bioceramic coatings, Materials Today: Proceedings. 5 (2018) 15677–15685.

MA

[18] W. Yang, D. Xu, X. Yao, J. Wang, J. Chen, Stable preparation and characterization of yellow micro arc oxidation coating on magnesium alloy,

D

Journal of Alloys and Compounds. 745 (2018) 609–616.

PT E

doi:10.1016/j.jallcom.2018.02.192. [19] S. Aliasghari, P. Skeldon, G.E. Thompson, Plasma electrolytic oxidation of

CE

titanium in a phosphate/silicate electrolyte and tribological performance of the coatings, Applied Surface Science. 316 (2014) 463–476.

AC

doi:https://doi.org/10.1016/j.apsusc.2014.08.037. [20] J. Lu, M. Descamps, J. Dejou, G. Koubi, P. Hardouin, J. Lemaitre, J.P. Proust, The biodegradation mechanism of calcium phosphate biomaterials in bone, Journal of Biomedical Materials Research. 63 (2002) 408–412. doi:10.1002/jbm.10259. [21] H. Tang, Y. Gao, Preparation and characterization of hydroxyapatite containing coating on AZ31 magnesium alloy by micro-arc oxidation, Journal of Alloys and

ACCEPTED MANUSCRIPT Compounds. 688 (2016) 699–708. doi:10.1016/j.jallcom.2016.07.079. [22] S. Durdu, S. Bayramoglu, A. Demirtaş, M. Usta, A.H. Üçşk, Characterization of AZ31 Mg Alloy coated by plasma electrolytic oxidation, Vacuum. 88 (2013) 130–133. doi:10.1016/j.vacuum.2012.01.009.

PT

[23] H. Nasiri Vatan, R. Ebrahimi-Kahrizsangi, M. Kasiri-Asgarani, Structural, tribological and electrochemical behavior of SiC nanocomposite oxide coatings

RI

fabricated by plasma electrolytic oxidation (PEO) on AZ31 magnesium alloy,

SC

Journal of Alloys and Compounds. 683 (2016) 241–255.

NU

doi:10.1016/j.jallcom.2016.05.096.

[24] Y. Li, F. Lu, H. Li, W. Zhu, H. Pan, G. Tan, Y. Lao, C. Ning, G. Ni, Corrosion

MA

mechanism of micro-arc oxidation treated biocompatible AZ31 magnesium alloy in simulated body fluid, Progress in Natural Science: Materials International. 24

D

(2014) 516–522. doi:10.1016/j.pnsc.2014.08.007.

PT E

[25] H. Chen, G. Lv, G. Zhang, H. Pang, X. Wang, H. Lee, S. Yang, Corrosion performance of plasma electrolytic oxidized AZ31 magnesium alloy in silicate

CE

solutions with different additives, Surface and Coatings Technology. 205 (2010)

AC

S32–S35. doi:10.1016/j.surfcoat.2010.03.032. [26] J.H. Gao, S.K. Guan, J. Chen, L.G. Wang, S.J. Zhu, J.H. Hu, Z.W. Ren, Fabrication and characterization of rod-like nano-hydroxyapatite on MAO coating supported on Mg-Zn-Ca alloy, Applied Surface Science. 257 (2011) 2231–2237. doi:10.1016/j.apsusc.2010.09.080. [27] X. Guo, K. Du, Y. Wang, Y. Shao, F. Wang, A new nanoparticle penetrant used for plasma electrolytic oxidation film coated on AZ31 Mg alloy in service environment, Surface and Coatings Technology. 206 (2012) 4833–4839.

ACCEPTED MANUSCRIPT doi:10.1016/j.surfcoat.2012.05.063. [28] Y. Jang, B. Collins, J. Sankar, Y. Yun, Effect of biologically relevant ions on the corrosion products formed on alloy AZ31B: An improved understanding of magnesium corrosion, Acta Biomaterialia. 9 (2013) 8761–8770.

PT

doi:10.1016/j.actbio.2013.03.026. [29] J.F. Ramírez, M. Cardona, J.A. Isaza, P. Fernández-, Pores distribution analysis

RI

for metal foams obtained by Casting- Dissolution Process., (2014) 1–13.

SC

[30] Y. Gao, A. Yerokhin, A. Matthews, Effect of current mode on PEO treatment of

NU

magnesium in Ca- and P-containing electrolyte and resulting coatings, Applied Surface Science. 316 (2014) 558–567. doi:10.1016/j.apsusc.2014.08.035.

MA

[31] D.I. Standard, Mechanical testing of metals — Ductility testing — Compression, International Organization. (2010) 9–10.

PT E

D

https://www.iso.org/standard/53669.html. [32] G. Barati Darband, M. Aliofkhazraei, P. Hamghalam, N. Valizade, Plasma electrolytic oxidation of magnesium and its alloys: Mechanism, properties and

CE

applications, Journal of Magnesium and Alloys. (2017).

AC

doi:10.1016/j.jma.2017.02.004. [33] D. Quintero, M.A. Gómez, J.G. Castaño, E. Tsuji, Y. Aoki, F. Echeverría, H. Habazaki, Anodic films obtained on Ti6Al4V in aluminate solutions by spark anodizing: Effect of OH− and WO4−2 additions on the tribological properties, Surface and Coatings Technology. 310 (2017) 180–189. doi:https://doi.org/10.1016/j.surfcoat.2016.12.063. [34] S. Muñoz, S.M. Castillo, Y. Torres, Different models for simulation of

ACCEPTED MANUSCRIPT mechanical behaviour of porous materials, Journal of the Mechanical Behavior of Biomedical Materials. 80 (2018) 88–96. doi:10.1016/j.jmbbm.2018.01.026. [35] A. Kucharczyk, K. Naplocha, J.W. Kaczmar, H. Dieringa, K.U. Kainer, Current Status and Recent Developments in Porous Magnesium Fabrication, Advanced

PT

Engineering Materials. 20 (2018) 1–16. doi:10.1002/adem.201700562. [36] C. Choong, J.T. Triffitt, Z.F. Cui, Polycaprolactone Scaffolds for Bone Tissue

RI

Engineering, Food and Bioproducts Processing. 82 (2004) 117–125.

SC

doi:10.1205/0960308041614864.

NU

[37] G.L. Hao, F.S. Han, W.D. Li, Processing and mechanical properties of magnesium foams, Journal of Porous Materials. 16 (2009) 251–256.

MA

doi:10.1007/s10934-008-9194-y.

[38] X.Z. Yue, K. Kitazono, X.J. Yue, B.Y. Hur, Effect of fluidity on the

D

manufacturing of open cell magnesium alloy foams, Journal of Magnesium and

PT E

Alloys. 4 (2016) 1–7. doi:10.1016/j.jma.2015.11.007. [39] I.A. Figueroa, M.A. Suarez, M. Velasco-Castro, H. Pfeiffer, B. Alcántar-

CE

Vázquez, G. González, I. Alfonso, G.A. Lara-Rodríguez, Development of pure

AC

Mg open-cell foams as structured CO2 captor, Thermochimica Acta. 621 (2015) 74–80. doi:10.1016/j.tca.2015.10.011. [40] H. Zhuang, Y. Han, A. Feng, Preparation, mechanical properties and in vitro biodegradation of porous magnesium scaffolds, Materials Science and Engineering C. 28 (2008) 1462–1466. doi:10.1016/j.msec.2008.04.001. [41] Y. Li, J. Zhou, P. Pavanram, M.A. Leeflang, L.I. Fockaert, B. Pouran, N. Tümer, K.U. Schröder, J.M.C. Mol, H. Weinans, H. Jahr, A.A. Zadpoor, Additively

ACCEPTED MANUSCRIPT manufactured biodegradable porous magnesium, Acta Biomaterialia. 67 (2017) 378–392. doi:10.1016/j.actbio.2017.12.008. [42] S. Dutta, K. Bavya Devi, M. Roy, Processing and degradation behavior of porous magnesium scaffold for biomedical applications, Advanced Powder Technology.

PT

28 (2017) 3204–3212. doi:10.1016/j.apt.2017.09.024. [43] M. Menhal Shbeh, A. Yerokhin, R. Goodall, Microporous Titanium through

RI

Metal Injection Moulding of Coarse Powder and Surface Modification by Plasma

SC

Oxidation, Applied Sciences. 7 (2017) 105. doi:10.3390/app7010105.

NU

[44] A.H. Yusop, A.A. Bakir, N.A. Shaharom, M.R. Abdul Kadir, H. Hermawan, Porous biodegradable metals for hard tissue scaffolds: A review, International

MA

Journal of Biomaterials. 2012 (2012). doi:10.1155/2012/641430. [45] Y. Qian, Y. Li, S. Jungwirth, N. Seely, Y. Fang, X. Shi, The Application of Anti-

D

Corrosion Coating for Preserving the Value of Equipment Asset in Chloride-

PT E

Laden Environments: A Review, Int. J. Electrochem. Sci. 10 (2015) 10756– 10780. www.electrochemsci.org.

CE

[46] F. Zhu, J. Wang, S. Li, J. Zhang, Preparation and characterization of anodic films

AC

on AZ31B Mg alloy formed in the silicate electrolytes with ethylene glycol oligomers as additives, Applied Surface Science. 258 (2012) 8985–8990. doi:10.1016/j.apsusc.2012.05.135. [47] C. Liu, J. Liang, J. Zhou, Q. Li, L. Wang, Characterization of AZ31 magnesium alloy by duplex process combining laser surface melting and plasma electrolytic oxidation, Applied Surface Science. 382 (2016) 47–55. doi:10.1016/j.apsusc.2016.04.047.

ACCEPTED MANUSCRIPT [48] O.A. Galvis, D. Quintero, J.G. Castaño, H. Liu, G.E. Thompson, P. Skeldon, F. Echeverría, Formation of grooved and porous coatings on titanium by plasma electrolytic oxidation in H2SO4/H3PO4 electrolytes and effects of coating morphology on adhesive bonding, Surface and Coatings Technology. 269 (2015) 238–249. doi:https://doi.org/10.1016/j.surfcoat.2015.02.036.

PT

[49] H.H. Huang, C.P. Wu, Y.S. Sun, T.H. Lee, Improvements in the corrosion

RI

resistance and biocompatibility of biomedical Ti-6Al-7Nb alloy using an

SC

electrochemical anodization treatment, Thin Solid Films. 528 (2013) 157–162. doi:10.1016/j.tsf.2012.08.063.

NU

[50] A. Zeng, E. Liu, I.F. Annergren, S.N. Tan, S. Zhang, P. Hing, J. Gao, EIS

MA

capacitance diagnosis of nanoporosity effect on the corrosion protection of DLC films, Diamond and Related Materials. 11 (2002) 160–168. doi:10.1016/S0925-

D

9635(01)00568-4.

PT E

[51] S. V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, R. Serra, S.K. Poznyak, M.G.S. Ferreira, Nanoporous titania interlayer as reservoir of corrosion inhibitors

CE

for coatings with self-healing ability, Progress in Organic Coatings. 58 (2007) 127–135. doi:10.1016/j.porgcoat.2006.08.029.

AC

[52] C. Jeong, J. Lee, K. Sheppard, C.H. Choi, Air-Impregnated Nanoporous Anodic Aluminum Oxide Layers for Enhancing the Corrosion Resistance of Aluminum, Langmuir. 31 (2015) 11040–11050. doi:10.1021/acs.langmuir.5b02392. [53] P. Li, Biomimetic nano-apatite coating capable of promoting bone ingrowth., Journal of Biomedical Materials Research. Part A. 66 (2003) 79–85. doi:10.1002/jbm.a.10519. [54] F.S.L. Bobbert, A.A. Zadpoor, Effects of bone substitute architecture and surface

ACCEPTED MANUSCRIPT properties on cell response, angiogenesis, and structure of new bone, J. Mater. Chem. B. 5 (2017) 6175–6192. doi:10.1039/C7TB00741H. [55] E. Birmingham, G.L. Niebur, P.E. Mchugh, G. Shaw, F.P. Barry, L.M. McNamara, Osteogenic differentiation of mesenchymal stem cells is regulated by osteocyte and osteoblast cells in a simplified bone niche, European Cells and

PT

Materials. 23 (2012) 13–27. doi:vol023a02 [pii].

RI

[56] Anawati, H. Asoh, S. Ono, Enhanced uniformity of apatite coating on a PEO film

SC

formed on AZ31 Mg alloy by an alkali pretreatment, Surface and Coatings

NU

Technology. 272 (2015) 182–189. doi:10.1016/j.surfcoat.2015.04.007. [57] G.E.J. Poinern, R.K. Brundavanam, X. Thi Le, P.K. Nicholls, M.A. Cake, D.

MA

Fawcett, The synthesis, characterisation and in vivo study of a bioceramic for potential tissue regeneration applications, Scientific Reports. 4 (2014) 6235.

D

doi:10.1038/srep06235.

PT E

[58] Z. Wei, P. Tian, X. Liu, B. Zhou, In vitro degradation, hemolysis, and cytocompatibility of PEO/PLLA composite coating on biodegradable AZ31

CE

alloy, Journal of Biomedical Materials Research - Part B Applied Biomaterials.

AC

103 (2015) 342–354. doi:10.1002/jbm.b.33208. [59] R. Montoya, C. Iglesias, M.L. Escudero, M.C. García-Alonso, Modeling in vivo corrosion of AZ31 as temporary biodegradable implants. Experimental validation in rats, Materials Science and Engineering C. 41 (2014) 127–133. doi:10.1016/j.msec.2014.04.033. [60] I. García, E. Gracia-Escosa, M. Bayod, A. Conde, M.A. Arenas, J. Damborenea, A. Romero, G. Rodríguez, Sustainable production of titanium foams for biomedical applications by Concentrated Solar Energy sintering, Materials

ACCEPTED MANUSCRIPT Letters. 185 (2016) 420–423. doi:10.1016/j.matlet.2016.09.037. [61] Y.J. Liu, Z.Y. Yang, L.L. Tan, H. Li, Y.Z. Zhang, An animal experimental study of porous magnesium scaffold degradation and osteogenesis, Brazilian Journal of Medical and Biological Research. 47 (2014) 715–720. doi:10.1590/1414-

PT

431X20144009. [62] Q.H. Bao, L.Q. Zhao, H.M. Jing, Q. Xu, Microstructure of

RI

Hydroxyapatite/Collagen Coating on AZ31 Magnesium Alloy by a Solution

SC

Treatment, Journal of Biomimetics, Biomaterials and Biomedical Engineering.

NU

30 (2017) 38–44. doi:10.4028/www.scientific.net/JBBBE.30.38. [63] C. Wen, S. Guan, L. Peng, C. Ren, X. Wang, Z. Hu, Characterization and

MA

degradation behavior of AZ31 alloy surface modified by bone-like hydroxyapatite for implant applications, Applied Surface Science. 255 (2009)

D

6433–6438. doi:10.1016/j.apsusc.2008.09.078.

PT E

[64] Z. Yao, L. Li, Z. Jiang, Adjustment of the ratio of Ca/P in the ceramic coating on Mg alloy by plasma electrolytic oxidation, Applied Surface Science. 255 (2009)

CE

6724–6728. doi:10.1016/j.apsusc.2009.02.082.

AC

[65] W.H. Ma, Y.J. Liu, W. Wang, Y.Z. Zhang, Improved biological performance of magnesium by micro-arc oxidation, Brazilian Journal of Medical and Biological Research. 48 (2015) 214–225. doi:10.1590/1414-431X20144171. [66] J.W. Guo, S.Y. Sun, Y.M. Wang, Y. Zhou, D.Q. Wei, D.C. Jia, Hydrothermal biomimetic modification of microarc oxidized magnesium alloy for enhanced corrosion resistance and deposition behaviors in SBF, Surface and Coatings Technology. 269 (2015) 183–190. doi:10.1016/j.surfcoat.2015.02.010.

ACCEPTED MANUSCRIPT [67] G. Hulsart-Billström, Q. Hu, K. Bergman, K.B. Jonsson, J. Åberg, R. Tang, S. Larsson, J. Hilborn, Calcium phosphates compounds in conjunction with hydrogel as carrier for BMP-2: A study on ectopic bone formation in rats, Acta Biomaterialia. 7 (2011) 3042–3049. doi:10.1016/j.actbio.2011.04.021. [68] D.R. Di, Z.Z. He, Z.Q. Sun, J. Liu, A new nano-cryosurgical modality for tumor

PT

treatment using biodegradable MgO nanoparticles, Nanomedicine:

RI

Nanotechnology, Biology, and Medicine. 8 (2012) 1233–1241.

SC

doi:10.1016/j.nano.2012.02.010.

[69] X. Xia, W. Zhao, X. Feng, H. Feng, X. Zhang, Effect of homogenizing heat

NU

treatment on the compressive properties of closed-cell Mg alloy foams, Materials

MA

and Design. 49 (2013) 19–24. doi:10.1016/j.matdes.2013.01.064. [70] M. Yazdimamaghani, M. Razavi, D. Vashaee, K. Moharamzadeh, A.R.

D

Boccaccini, L. Tayebi, Porous magnesium-based scaffolds for tissue engineering,

PT E

Materials Science and Engineering C. 71 (2017) 1253–1266. doi:10.1016/j.msec.2016.11.027.

CE

[71] M. Prot, D. Saletti, S. Pattofatto, V. Bousson, S. Laporte, Links between mechanical behavior of cancellous bone and its microstructural properties under

AC

dynamic loading, Journal of Biomechanics. 48 (2015) 498–503. doi:10.1016/j.jbiomech.2014.12.002. [72] M. Prot, D. Saletti, S. Pattofatto, V. Bousson, S. Laporte, A.P. Md Saad, A. Syahrom, Study of dynamic degradation behaviour of porous magnesium under physiological environment of human cancellous bone, Journal of Biomechanics. 131 (2015) 45–56. doi:10.1016/j.jbiomech.2014.12.002. [73] L.J. Gibson, Cellular Solids, MRS Bulletin. 28 (2003) 270–274.

ACCEPTED MANUSCRIPT doi:10.1557/mrs2003.79. [74] T.L. Nguyen, Synthesis of Topologically-Ordered, (2011) 3. [75] N.T. Kirkland, I. Kolbeinsson, T. Woodfield, G.J. Dias, M.P. Staiger, Synthesis and properties of topologically ordered porous magnesium, Materials Science

PT

and Engineering B: Solid-State Materials for Advanced Technology. 176 (2011)

RI

1666–1672. doi:10.1016/j.mseb.2011.04.006.

[76] L.C. Chan, X.Z. Lu, C.P. Lai, L.W.P. Chow, Compressive Behaviours of Raw

SC

and Cased Porous Magnesium-alloys determined by X-ray CT, Procedia

NU

Engineering. 207 (2017) 1427–1432. doi:10.1016/j.proeng.2017.10.908. [77] L. White, Y. Koo, S. Neralla, J. Sankar, Y. Yun, Enhanced mechanical properties

MA

and increased corrosion resistance of a biodegradable magnesium alloy by plasma electrolytic oxidation (PEO), Materials Science and Engineering B:

D

Solid-State Materials for Advanced Technology. 208 (2016) 29–46.

PT E

doi:10.1016/j.mseb.2016.02.005. [78] H.M. Wong, K.W.K. Yeung, K.O. Lam, V. Tam, P.K. Chu, K.D.K. Luk, K.M.C.

CE

Cheung, A biodegradable polymer-based coating to control the performance of

AC

magnesium alloy orthopaedic implants, Biomaterials. 31 (2010) 2084–2096. doi:10.1016/j.biomaterials.2009.11.111.

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Graphical Abstract

ACCEPTED MANUSCRIPT

Highlights

PT

RI SC NU MA D PT E



CE



MAO coatings on AZ31 magnesium foam obtained with satisfactory results. Magnesium foam was coated with friendly alkali electrolyte getting a homogeneus coating. Magnesium foam with surface and mechanical behavior are like cancellous bone. The porosity and chemical composition of MAO coating do not show harmless for biological applications.

AC

 