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Enhanced corrosion resistance and biocompatibilty of PMMA-coated ZK60 magnesium alloy Weihong Jin a , Qi Hao a,b , Xiang Peng a , Paul K. Chu a,n a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China b Department of Physics and Jiangsu Key Laboratory for Advanced Metallic Materials, Southeast University, Nanjing 211189, China article info Article history: Received 14 January 2016 Received in revised form 3 March 2016 Accepted 12 March 2016 Available online 13 March 2016 Keywords: Magnesium alloy Thin lm Polymer Corrosion Biocompatibility abstract Poly(methyl methacrylate) (PMMA) lms are deposited on the ZK60 magnesium alloy to improve the corrosion resistance and biocompatibility. The lm thickness, surface composition, corrosion behavior, and biocompatibility are evaluated. Polarization, electrochemical impedance spectroscopy, and immer- sion tests show that the PMMA lm with a thickness of about 470 nm provides good protection against attack in simulated body uids. At the same time, good cell adhesion and cell viability are observed from the PMMA-coated ZK60 magnesium alloy in vitro. & 2016 Elsevier B.V. All rights reserved. 1. Introduction Magnesium-based alloys are used in the aerospace and auto- motive industry due to good properties such as light weight and good strength-to-weight ratio and have recently aroused a lot of interest in the biomedical elds as temporary orthopedic implants and cardiovascular stents [1,2]. Owing to natural degradation in the physiological medium, a second removal surgery can be avoided after tissue healing [3]. Meanwhile, the stress-shielding effect can be mitigated since the elastic modulus of magnesium- based alloys matches that of human bone better than conventional bio-metals such as stainless steel and titanium [4]. However, de- gradation of magnesium alloys in the aggressive physiological environment is too rapid resulting in a local alkaline environment, excessive evolution of hydrogen bubbles [5], and even mechanical failure of the implant before the tissues heal completely [6]. These problems must be solved in order to widen the usage of magne- sium alloys in biomedical engineering. Surface treatment is a practical way to enhance the surface properties of magnesium alloys [7,8]. Polymer coatings have been extensively used for corrosion protection of metals due to their superior performance in an aggressive environment [911]. In particular, poly(methyl methacrylate) (PMMA) has good bio- compatibility with human tissues and been implemented on bone cements in orthopedic and dental applications as well as contact and intraocular lens [12,13]. However, there have been few reports on the application of PMMA to improve the properties of mag- nesium alloys so far. In this work, PMMA is spin-coated on ZK60 magnesium alloy and the corrosion resistance and biocompat- ibility are studied. 2. Experimental details The ZK60 (Mg 5.19 wt% Zn 0.53 wt% Zr) specimens with dimensions of Ф12 mm 4 mm were ground successively to 1200 grit SiC abrasive papers, ultrasonically rinsed with ethanol, and dried by nitrogen. A 4% poly(methyl methacrylate) (PMMA) with a molecular weight of M.W.950,000 solution in anisole was used to prepare the PMMA lm on the ZK60 substrate by spin coating at 600 rpm for 15 s followed by 3000 rpm for 1 min. Attenuated total reectance Fourier transformed infrared spectroscopy (ATR-FTIR) was utilized to determine the functional groups and X-ray photoelectron spectroscopy (XPS) was employed to determine the elemental chemical states of the PMMA lm. The binding energies were calibrated according to the Au 4f line at 84.0 eV. The cross-section of the PMMA-coated silicon was ex- amined by eld-emission scanning electron microscopy (FE-SEM) to measure the PMMA lm thickness. The image of the PMMA coating on ZK60 after the adhesion test according to ASTM D3359- 09B was obtained by optical microscopy. The electrochemical Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2016.03.071 0167-577X/& 2016 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: [email protected] (P.K. Chu). Materials Letters 173 (2016) 178181
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Page 1: Enhanced corrosion resistance and biocompatibilty of PMMA ...Poly(methyl methacrylate) (PMMA) films are deposited on the ZK60 magnesium alloy to improve the corrosion resistance and

Materials Letters 173 (2016) 178–181

Contents lists available at ScienceDirect

Materials Letters

http://d0167-57

n CorrE-m

journal homepage: www.elsevier.com/locate/matlet

Enhanced corrosion resistance and biocompatibilty of PMMA-coatedZK60 magnesium alloy

Weihong Jin a, Qi Hao a,b, Xiang Peng a, Paul K. Chu a,n

a Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, Chinab Department of Physics and Jiangsu Key Laboratory for Advanced Metallic Materials, Southeast University, Nanjing 211189, China

a r t i c l e i n f o

Article history:Received 14 January 2016Received in revised form3 March 2016Accepted 12 March 2016Available online 13 March 2016

Keywords:Magnesium alloyThin filmPolymerCorrosionBiocompatibility

x.doi.org/10.1016/j.matlet.2016.03.0717X/& 2016 Elsevier B.V. All rights reserved.

esponding author.ail address: [email protected] (P.K. Chu).

a b s t r a c t

Poly(methyl methacrylate) (PMMA) films are deposited on the ZK60 magnesium alloy to improve thecorrosion resistance and biocompatibility. The film thickness, surface composition, corrosion behavior,and biocompatibility are evaluated. Polarization, electrochemical impedance spectroscopy, and immer-sion tests show that the PMMA film with a thickness of about 470 nm provides good protection againstattack in simulated body fluids. At the same time, good cell adhesion and cell viability are observed fromthe PMMA-coated ZK60 magnesium alloy in vitro.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

Magnesium-based alloys are used in the aerospace and auto-motive industry due to good properties such as light weight andgood strength-to-weight ratio and have recently aroused a lot ofinterest in the biomedical fields as temporary orthopedic implantsand cardiovascular stents [1,2]. Owing to natural degradation inthe physiological medium, a second removal surgery can beavoided after tissue healing [3]. Meanwhile, the stress-shieldingeffect can be mitigated since the elastic modulus of magnesium-based alloys matches that of human bone better than conventionalbio-metals such as stainless steel and titanium [4]. However, de-gradation of magnesium alloys in the aggressive physiologicalenvironment is too rapid resulting in a local alkaline environment,excessive evolution of hydrogen bubbles [5], and even mechanicalfailure of the implant before the tissues heal completely [6]. Theseproblems must be solved in order to widen the usage of magne-sium alloys in biomedical engineering.

Surface treatment is a practical way to enhance the surfaceproperties of magnesium alloys [7,8]. Polymer coatings have beenextensively used for corrosion protection of metals due to theirsuperior performance in an aggressive environment [9–11]. Inparticular, poly(methyl methacrylate) (PMMA) has good bio-compatibility with human tissues and been implemented on bone

cements in orthopedic and dental applications as well as contactand intraocular lens [12,13]. However, there have been few reportson the application of PMMA to improve the properties of mag-nesium alloys so far. In this work, PMMA is spin-coated on ZK60magnesium alloy and the corrosion resistance and biocompat-ibility are studied.

2. Experimental details

The ZK60 (Mg – 5.19 wt% Zn – 0.53 wt% Zr) specimens withdimensions of Ф12 mm�4 mm were ground successively to 1200grit SiC abrasive papers, ultrasonically rinsed with ethanol, anddried by nitrogen. A 4% poly(methyl methacrylate) (PMMA) with amolecular weight of M.W.950,000 solution in anisole was used toprepare the PMMA film on the ZK60 substrate by spin coating at600 rpm for 15 s followed by 3000 rpm for 1 min.

Attenuated total reflectance Fourier transformed infraredspectroscopy (ATR-FTIR) was utilized to determine the functionalgroups and X-ray photoelectron spectroscopy (XPS) was employedto determine the elemental chemical states of the PMMA film. Thebinding energies were calibrated according to the Au 4f line at84.0 eV. The cross-section of the PMMA-coated silicon was ex-amined by field-emission scanning electron microscopy (FE-SEM)to measure the PMMA film thickness. The image of the PMMAcoating on ZK60 after the adhesion test according to ASTM D3359-09B was obtained by optical microscopy. The electrochemical

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Fig. 1. (a) FE-SEM image of the cross-section of the PMMA-coated silicon and optical image of the PMMA coating on ZK60 demonstrating good adhesion; (b) ATR-FTIRspectra of the PMMA-coated ZK60; High-resolution XPS spectra of (c) C 1s and (d) O 1s acquired from the PMMA-coated ZK60.

W. Jin et al. / Materials Letters 173 (2016) 178–181 179

experiments were conducted in simulated body fluids (SBF) at37 °C on an electrochemical workstation according to the methoddescribed in our previous paper [14] by exposing 1.131 cm2 of thesample surface. The surface morphology of the PMMA-coatedZK60 sample after immersion in SBF for 2 h was monitored byscanning electron microscopy (SEM). In the biological studies,1.0�105 MC3T3-E1 pre-osteoblasts per well were seeded onto theuntreated and PMMA-coated ZK60 samples on a 12-well cultureplate. The cell attachment after incubation for 7 h incubation andcell viability after 28 h were assessed following the proceduresdescribed previously [14].

3. Results and discussion

Fig. 1(a) depicts the FE-SEM image of the cross-section of thePMMA film revealing a PMMA film thickness of about 470 nm andthe PMMA film is uniform and compact with few defects, althoughpartial curling is observed from the surface. The inset image inFig. 1(a) of the PMMA film on ZK60 after the adhesion test alsoshows that the PMMA film exhibits good adhesion with the sub-strate. Fig. 1(b) shows the characteristic C¼O vibration of thecarbonyl group in PMMA at 1728.2 cm�1 [15,16] and the twodoublet bands at (1271.0, 1242.1 cm�1) and (1192, 1149.5 cm�1)are attributed to C–O stretching [17]. The C 1s spectrum can be

deconvoluted into three peaks at 284.5 eV, 285.5 eV, and 288.0 eVcorresponding to C-Hx, C–O, and C¼O, respectively [18,19], asshown in Fig. 1(c). The two peaks of O 1 s at 531.6 eV and 533.2 eVin Fig. 1(d) are assigned to C–O and C¼O [20]. The results provideevidence of successful preparation of the PMMA coating.

The polarization curves are shown in Fig. 2(a) and the corrosionpotential (Ecorr) and corrosion current density (Icorr) listed inTable 1 are determined from the cathodic polarization curve byTafel extrapolation. The PMMA-coated ZK60 sample exhibits muchlower Icorr of 0.355 mA-cm�2 in SBF compared to the untreatedZK60 sample (757.4 mA-cm�2), indicating three orders of magni-tude reduction of Icorr for ZK60 in SBF after PMMA deposition.Degradation of ZK60 is notably retarded by the PMMA film be-cause the small corrosion current density corresponds to the lowcorrosion rate. The Nyquist plots and corresponding electricalequivalent circuit with three time constants used to fit the ex-perimental data are shown in Fig. 2(b). In the equivalent circuit, Rscorresponds to the solution resistance between the referenceelectrode and sample, CPE1 is the capacitance of the film or cor-rosion products, R1 is the corresponding resistance, CPE2 re-presents the double layer capacitance at the electrolyte/substrateinterface, and R2 represents the relevant charge transfer resistance.The fitted EIS data in Table 1 show that the PMMA-coated sampleshows 5256 and 6817 folds increase in R1 and R2, respectively.The results indicate that corrosion of ZK60 is significantly

Page 3: Enhanced corrosion resistance and biocompatibilty of PMMA ...Poly(methyl methacrylate) (PMMA) films are deposited on the ZK60 magnesium alloy to improve the corrosion resistance and

Fig. 2. (a) Polarization curves and (b) Nyquist plots in SBF with the equivalent circuits; SEM images of the surface morphology of (c) ZK60 and (d) PMMA-coated ZK60 afterimmersion in SBF for 2 h.

Table 1Calculated Ecorr and Icorr, and fitted EIS results.

Ecorr (V vs. SCE) Icorr (mA/cm2) CPE1 (Ω�2 cm�2 S�n) n1 R1 (Ω cm2) CPE2 (Ω�2 cm�2 S�n) n2 R2 (Ω cm2)

ZK60 �1.671 757.4 2.35�10�3 0.447 35.58 1.45�10�5 1 38.14PMMA-coated ZK60 �1.561 0.355 4.70�10�7 0.850 1.87�105 1.44�10�8 0.897 2.60�105

W. Jin et al. / Materials Letters 173 (2016) 178–181180

mitigated by the PMMA film in SBF. As shown in Fig. 2(c) and (d),after immersion in SBF for 2 h, the untreated ZK60 shows a se-verely corroded surface with some cracks, but no corrosion pits orcracks are observed from the PMMA-coated sample, indicatingthat the PMMA film provides good protection against attack bycorrosive electrolytes.

Fig. 3(a)–(c) show that the number of MC3T3-E1 cells attachedon the PMMA-treated ZK60 sample is significantly larger than thaton the untreated substrate after incubation for 7 h. Fig. 3(d) further shows that the viability of MC3T3-E1 cells incubated onthe PMMA-coated ZK60 magnesium alloy for 28 h is much higherthan that on the untreated ZK60 sample. The biological assess-ment indicates that the PMMA coating improves not only thecorrosion resistance, but also cell attachment and proliferation onthe ZK60 magnesium alloy. The enhanced biocompatibility of themagnesium alloys is generally related to the improved corrosionresistance. Cells are sensitive to fluctuations in the surroundingsand likely to grow better in an environment with less adversestimulation. Owing to the improved corrosion resistance, the

PMMA-coated ZK60 sample is able to suppress hydrogen evolutionand ion release thus maintaining a normal pH environment tobenefit adhesion, spreading, and proliferation. Moreover, thechange in the surface characteristics such as composition, surfaceenergy, and surface roughness rendered by the biocompatiblePMMA coating may influence the interactions between cells andimplants. Therefore, formation of an anti-corrosion and bio-compatible PMMA surface layer provides a more favorable andstable environment for cell attachment and growth.

4. Conclusion

A PMMA film is deposited on ZK60 magnesium alloy to retardsurface corrosion and enhance the biocompatibility. After PMMAdeposition, the ZK60 sample exhibits much lower corrosion cur-rent density, higher charge transfer resistance, and higher masstransportation resistance in SBF. Nearly no surface corrosion isobserved from the PMMA-treated ZK60 sample after immersion in

Page 4: Enhanced corrosion resistance and biocompatibilty of PMMA ...Poly(methyl methacrylate) (PMMA) films are deposited on the ZK60 magnesium alloy to improve the corrosion resistance and

Fig. 3. Nuclei in blue stained with DAPI of MC3T3-E1 pre-osteoblasts on (a) ZK60 and (b) PMMA-coated ZK60 after incubation for 7 h and (c) corresponding cell numbermeasured by counting the cell nuclei. ***Po0.001 compared to ZK60. The quantitative data represent mean7SD (n¼10). (d) Cell viability of MC3T3-E1 pre-osteoblastscultured for 28 h. ***Po0.001 compared to ZK60. The quantitative data represent mean7SD (n¼5).

W. Jin et al. / Materials Letters 173 (2016) 178–181 181

SBF for 2 h. Moreover, the ZK60 magnesium alloy after PMMAdeposition also shows good cell attachment and viability. Our re-sults show that PMMA deposition improves both the corrosionresistance and biocompatibility of ZK60 magnesium alloy.

Acknowledgements

This work is financially supported by Hong Kong ResearchGrants Council (RGC) General Research Funds (GRF) Nos. CityU112212 and 11301215 as well as City University of Hong KongStrategic Research Grant (SRG) No. 7004188.

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