Dynamic degradation behavior of MgZn alloy in circulating m-SBF

Dynamic degradation behavior of MgZn alloy in circulating m-SBF

Materials Letters 64 (2010) 1996–1999 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i ...

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Materials Letters 64 (2010) 1996–1999

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Dynamic degradation behavior of MgZn alloy in circulating m-SBF Ying Chen, Shaoxiang Zhang, Jianan Li, Yang Song, Changli Zhao, Xiaonong Zhang ⁎ State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

a r t i c l e

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Article history: Received 24 March 2010 Accepted 4 June 2010 Available online 10 June 2010 Keywords: MgZn Dynamic Degradation

a b s t r a c t A test platform was designed to mimic the conditions that an implanted coronary stent encounters in coronary arteries, and the degradation behavior of a biodegradable MgZn alloy in the circulating modified simulated body fluid (m-SBF) was investigated on the platform. Results indicated that the presence of circulating solution over the surface of the specimen increased its corrosion rate compared to that in static immersion test, and the diffusion of Mg2+ and other ions dominated the corrosion mechanism of the alloy. A corrosion layer with randomly scattering particles conglomerated to clusters on its surface was observed after 168 h of measurement. While the amount of Mg decreased from the matrix to the corrosion layer, the concentration ratio of Zn did not have a significant change. The concentrations of calcium and phosphor seemed to be gradually increased with the extension of distance from MgZn matrix to the degradation layer. © 2010 Elsevier B.V. All rights reserved.

1. Introduction As an essential element in the human body as well as the biodegradability and biocompatibility, magnesium (Mg) and its alloys are considered to be potential candidates for applications as materials of biodegradable coronary stents [1–3]. Mg alloy stents implanted into the stenosis human cardiovascular are expected to temporarily support pathological vascular walls and disappear after the healing of tissues. Consequently, they are thought to be superior to the permanent stents which may cause many questions, e.g. mismatch between vessel and stent, chronic inflammatory reaction and physical irritation. However, the rapid degradation rate of biodegradable Mgbased stents in vivo leads to an unexpected early loss of the mechanical integrity and results in restenosis [1,3]. Therefore, understanding the dynamic degradation behavior of Mg alloys in vitro and in vivo is an initial step to the future improvement in cardiovascular applications. Considering that the shear stress is also an important factor demonstrated to influence the corrosion behavior of Mg alloys [4], the static immersion tests used by researchers may be not so precise or even misleading in predicting the degradation characteristics of Mg stents corroded by both ions contained in blood and the shear stress applied by the blood. Witte et al. also suggested that the current ASTM standard in vitro immersion tests might not be used to predict corrosion rates of magnesium alloys in vivo [5]. Although dynamic tests have been used by researchers in tracing biomaterials' degradation in vitro, their test benches are either complicated or not so clear in details [6,7]. In this article, a newly simple and convenient test platform was designed and the degradation behavior of

⁎ Corresponding author: Tel./fax: + 86 21 3420 2759. E-mail address: [email protected] (X. Zhang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.06.011

a solid solution treated MgZn alloy in circulating m-SBF was studied on the platform.

2. Methods 2.1. Test platform Fig. 1 shows a test platform designed to simulate the environment encountered by stents in coronary arteries. Antifatigue silicone tubes with an inner diameter of 6.4 mm (in agreement with the inner diameter of the specimen) were used as channels of the circulating solution. The specimen with geometry of a circular tube was tightly locked by the pressure generated by enlarged silicone tubes on the outer wall of the specimen. The pressure is also able to prevent the circulating solution from penetrating from the interstice between the silicone tube and the outer wall of the specimen.

2.2. Specimen preparation The specimen with a shape of a circular tube of Φ1 9.4 mm × 15 mm was drilled and then machined from a solid solution treated MgZn alloy. Its exact fabrication, composition and microstructure have been reported [8]. Circular-tube configuration of the specimen is not only similar to the shape of coronary stents, but also ensures uniform shear stress applied by the fluid on the inner wall of the specimen. The inner diameter of the specimen (Φ2 = 6.4 mm) was the same as that of the silicone tubes. The identical inner diameter of specimen and the tubes was beneficial to the steady transition of the flow from the tube to the lumen of the specimen, ensuring that the flow profile in the lumen of the specimen was the same as that of the tube. The specimen was then

Y. Chen et al. / Materials Letters 64 (2010) 1996–1999

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Fig. 1. Diagram of test platform: (a) overview; (b) configuration of the specimen's location.

abraded and electropolished in the mixture of acid and ethanol with a direct current prior to testing. 2.3. Preparation of pseudo blood plasma The modified SBF (m-SBF) was chosen as the pseudo-physiological solution due to its stability and similar ion compositions to the human blood plasma. Table 1 lists the ion compositions of m-SBF and blood plasma. HEPES was also added as buffers. The m-SBF solution was prepared based on the steps described by Ayako Oyane et al. [9]. The volume of m-SBF was set to be 400 ml to reach a compromising ratio of the specimen's surface area to the solution volume. 2.4. Flow rate of m-SBF According to the research by Doriot et al. [10], the shear stress applied on the inner surface of the specimen was controlled to be 0.68 Pa by modifying the flow rate of solution according to the following formula:   3 τ = 32ηQ = πD where τ is shear stress (0.68 Pa), η is liquid viscosity (1 mPa s), Q is the flux of the solution and D (6.4 mm) is the diameter of the lumen of the specimen. The Reynolds number of the flow in the tubes was also checked to be inferior to 2100 in order to guarantee a laminar flow similar to that in human coronary arteries [9]. 2.5. Dynamic test and analysis The specimen was submitted to a shear stress of 0.68 Pa. Silicone tubes and m-SBF container were maintained at 37 ± 0.5 °C by a thermostatic bath. The weight of the specimen (without eliminating the corrosion products) and the pH values of the solution were measured after 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 24 h, 48 h 72 h, 96 h, 120 h, 144 h and 168 h, respectively. One of the terminals of the

Table 1 Ion concentrations in blood plasma and m-SBF [9].

specimen was marked with an oil pen on the outer wall in order to maintain the specimen a uniform orientation along the flow direction during the whole experiment. The weight loss (Δm) of the specimen was calculated according to the following equation: Δm =

  m0 −mðtÞ = s

where m0 is the initial weight, m(t) is the weight of the specimen after every time interval in circulating m-SBF and s is the area of the inner wall of the specimen. After 168 h of test, the specimen was removed from the test platform and dried at room temperature. The corrosion layer of the specimen was investigated by a JSM-7401F field-emission scanning electron microscope (FESEM) with an energy-dispersive spectroscope (EDS). 3. Results and discussion Fig. 2a displays the weight loss of the specimen over a range of one week in the circulating m-SBF. The rate of the weight loss began to decline after 48 h of corrosion and then maintained as a constant in general, which was mainly due to existence of deposited Mg(OH)2 and phosphates that hinder the matrix from being further corroded to some extent. Compared to the static immersion tests by Song et al. [11], the alloy degraded more rapidly in dynamic environment, indicating that the circulating solution was able to accelerate the degradation of MgZn alloy. While the pH value of the solution rose steadily in initial 48 h (Fig. 2b), it then grew with a quite slow rate. The ultimate pH value was approximately 8.60. The fact the pH value stabilized around 8 can be assumed to two aspects. On the one hand, since the deposited corrosion products slowed down the corrosion rate of the specimen, there would be less newly generated OH−. On the other hand, due to the supersaturation of the solution a lot of newly generated OH− was consumed and precipitated. As the two processes above interplayed, the degradation rate of the specimen was retarded. Fig. 2c shows that weight losses gained from different time intervals during the test were generally proportional to the square root of time (t1/2) with the following equation: y = 0:81919 + 0:68088x

Ion

Concentration/mM Blood plasma

m-SBF

Na+ K+ Mg2+ Ca2+ Cl− HCO− 3 HPO2− 4 SO2− 4

142.0 5.0 1.5 2.5 103.0 27.0 1.0 0.5

142.0 5.0 1.5 2.5 103.0 10.0 1.0 0.5

where y refers to the weight loss and x refers to t1/2. This linear relationship implied that diffusion of Mg2+ and other ions dominated the corrosion mechanism of the MgZn alloy during the test. SEM images and elemental analysis of the corrosion layer after 168 h of test are given in Fig. 3. A degradation layer with a total thickness of approximate 250 μm was seen in Fig. 3a. Formation of the hydrogen gaps (shown in Fig. 3a) among the corrosion layer was probably due to the accumulation of the evolved H2. A cracked surface

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Y. Chen et al. / Materials Letters 64 (2010) 1996–1999

Fig. 2. Behavior of the specimen in circulating m-SBF: (a) the change of weight loss; (b) the change of pH value of m-SBF; (c) weight loss plotted as a function of t1/2.

with randomly scattering particles which conglomerated to clusters on it was also observed in Fig. 3b. Fig. 3e showed the elemental compositions within the red square in Fig. 3b. The atomic ratio of P/O

was approximately 1/4 (calculated from Fig. 3e), indicating that those deposited particles were probably phosphates. Much attention should be paid to those depositions because they may disturb the flow of

Fig. 3. Morphologies and compositions of the corrosion layer: (a) cross section; (b) surface; (c) distribution of Mg and Zn; (d) distribution of Ca and P; (e) elemental composition within the red square in (b).

Y. Chen et al. / Materials Letters 64 (2010) 1996–1999

blood. Additionally, since the clusters are beneficial to the increase of the surface energy, they may be a potential threat leading to aggressive platelet adhesion when Mg-based stents are implanted into coronary arteries. Results of elemental line scans (Fig. 3c) showed that there was a sharp decrease of the concentration of magnesium from MgZn substrate to the corrosion layer, which demonstrated that much Mg had dissolved into the solution. On the other hand, the concentration of Zn did not have a significant change along the line scan, demonstrating that most of Zn still stayed in the corrosion layer. This proved that addition of Zn improved the corrosion resistance of the Mg in m-SBF. However, it should be noticed that the gathering of Zn in the surface corrosion layer may have an impact on the cytotoxicity and the hemolytic property of the alloy. Concentrations of calcium and phosphorus seemed to gradually increase along the line scan routine, which generally conformed to the in vivo analysis about the concentrations of calcium and phosphorus near the interface between the alloy and the corrosion layer by Gruhl et al. [12]. This distribution might be due to the diffusion mechanism of Ca and P from the solution to the degradation layer. 4. Conclusion This study designs a convenient platform to simulate the physiological conditions faced by stents implanted into coronary arteries, and the corrosion behavior of a solid solution MgZn alloy was investigated by using this test platform. The bare MgZn alloy showed a rapid degradation rate. Diffusion of Mg2+ and other ions dominated the corrosion mechanism of the alloy and a corrosion layer of approximately 250 μm was seen on the cross

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section. Zn tended to stay among the degradation layer and the concentration of calcium and phosphorus gradually increased with the extension of the distance from the base alloy to the corrosion layer. Acknowledgements The authors are grateful for the support from the Natural Science Foundation of China (no. 30772182 and no. 30901422), the Shanghai Jiao Tong University Interdisciplinary Research Grant (no. YG2009MS53) and the “863” High-Tech Plan of China (no. 2009AA03Z424). References [1] Heublein B, Rohde R, Kaese V, Niemeyer M, Hartung W, Haverich A. Heart 2003;9: 651–6. [2] Mani G, Feldman MD, Patel D, Agrawal CM. Coronary stents: a materials perspective. Biomaterials 2007;28:1689–710. [3] Erbel R, Di Mario C, Bartunek J, Bonnier J, de Bruyne B, Eberli FR, et al. Lancet 2007;369:1869–75. [4] Winzer N, Atrens A, Song GL, Edward Ghali E, Wolfgang, Dietzel, et al. Adv Eng Mater 2005;7(8):659–93. [5] Witte F, Fischer J, Nellesen J, Crostack H-A, Kaese V, Pisch A, et al. Biomaterials 2006;27(7):1013–8. [6] Levesque J, Hermawan H, Dube D. Mantovani D. Acta Biomater 2008;4(2):284–95. [7] Zhu SF, Huang N, Xu L, Zhang Y, Liu HQ, Sun H, et al. Mater Sci Eng 2009;29: 1589–93. [8] Zhang SX, Zhang XN, Zhao CL, Li JN, Song Y, Xie CY, et al. Acta Biomater 2010;6(2): 626–40. [9] Ayako O, Hyun-Min K, Takuo F, Tadashi K, Toshiki M, Takashi N. J Biomed Mater Res A 2003;65A:188–95. [10] Doriot P-A, Dorsaz P-A, Dorsaz L, De Benedetti E, Chatelain P, Delafontaine P. Coron Artery Dis 2000;11:495–502. [11] Song Y, Zhang SX, Li JN, Zhao CL, Zhang XN. Acta Biomater 2010;6(5):1736–42. [12] Gruhl S, Witte F, Vogt J, Vogt C. J Anal At Spectrom 2009;24:181–8.