Influence of Tris in simulated body fluid on degradation behavior of pure magnesium

In current paper, influence of tris-hydroxymethyl-aminomethane (tris) in simulated body fluid (SBF) on degradation behavior of pure magnesium is investigated using electrochemical tests as well as degradation measurement. Our results shows that tris mainly affects earlier degradation behavior of pure magnesium alloy. Tris and HCl used in preparation of SBF will form Tris–HCl which only lowers corrosion potential of magnesium slightly but accelerates degradation rates of pure magnesium by teens times. Consumption of OH^? generated during magnesium dissolution by Tris–HCl progressively promotes transformation from Mg to Mg²⁺, which is the main reason for quite high degradation rate of pure magnesium in SBF. Pure magnesium is also more sensitive to pitting corrosion due to inclusion of Tris–HCl in SBF. This study deepens the understanding on degradation mechanism of biomedical magnesium alloys.     

表面改性后AZ31B镁合金在不同模拟体液中的降解性能研究

采用化学沉积法在AZ31B镁合金表面制备了钙磷陶瓷涂层,通过浸泡试验和电化学试验研究了其在3种不同模拟体液(生理盐水、PBS、Hank′s)中的降解性能。结果表明,沉积处理改变了AZ31B镁合金在模拟体液中的降解性能,明显抑制了其降解速度;浸泡后溶液的pH值变化结果和电化学实验结果均表明,在3种不同模拟体液中,表面处理后AZ31B镁合金显示出不同的降解速度,顺序依次为:生理盐水>PBS溶液>Hank′s溶液。     

可降解血管支架材料降解行为研究进展

目前可降解血管支架材料包括聚合物、镁合金、铁合金及锌合金,它们的降解特性直接影响其作为血管支架植入后的支撑能力、局部反应和血管修复的预后。聚合物降解时间较易调整、生物相容性较好,但力学性能不足;镁合金的降解存在降解速率快、释氢反应和微环境pH值变化较大的问题;铁合金降解速率太慢;锌合金的降解速率适中,是近年可降解血管内植入材料研究热点。除了材料自身的特性,可降解材料的血管内降解行为还受到环境的离子浓度、酶、pH值和温度等多种因素的影响。综述了目前不同血管内可降解支架材料在模拟体液及动物体内生物降解行为的研究结果,以期为血管内可降解材料研究和产品开发提供参考。     

New insights into the effect of Tris-HCl and Tris on corrosion of magnesium alloy in presence of bicarbonate,sulfate,hydrogen phosphate and dihydrogen phosphate ions

In vitro degradation is an important approach to screening appropriate biomedical magnesium(Mg) alloys at low cost. However, corrosion products deposited on Mg alloys exert a critical impact on corrosion resistance. There are no acceptable criteria on the evaluation on degradation rate of Mg alloys. Understanding the degradation behavior of Mg alloys in presence of Tris buffer is necessary. An investigation was made to compare the influence of Tris-HCl and Tris on the corrosion behavior of Mg alloy AZ31 in the presence of various anions of simulated body fluids via hydrogen evolution, p H value and electrochemical tests.The results demonstrated that the Tris-HCl buffer resulted in general corrosion due to the inhibition of the formation of corrosion products and thus increased the corrosion rate of the AZ31 alloy. Whereas Tris gave rise to pitting corrosion or general corrosion due to the fact that the hydrolysis of the amino-group of Tris led to an increase in solution p H value, and promoted the formation of corrosion products and thus a significant reduction in corrosion rate. In addition, the corrosion mechanisms in the presence of Tris-HCl and Tris were proposed. Tris-HCl as a buffer prevented the formation of precipitates of HCO_3~-, SO_4~(2-),HPO_4~(2-) and H_2PO_4~- ions during the corrosion of the AZ31 alloy due to its lower buffering p H value(x.x).Thus, both the hydrogen evolution rate and corrosion current density of the alloy were approximately one order of magnitude higher in presence of Tris-HCl than Tris and Tris-free saline solutions.     

Corrosion behaviour of some vapour deposited magnesium alloys

Abstract

Binary magnesium alloys containing chromium, manganese, or titanium were made using a physical vapour deposition technique. The corrosion resistance of the alloys was assessed in aqueous chloride solutions using total immersion tests in quiescent 600 mmol L^?1 NaCI solutions. Alloying with manganese or titanium was found to lower the corrosion rate of magnesium over most of the compositional ranges of interest, whereas addition of chromium had a detrimental effect on the corrosion resistance of magnesium. The lowest corrosion rate was recorded for a Mg–Ti alloy where the value obtained was about 80 times lower than that found for vapour deposited pure magnesium. Open circuit corrosion potential measurements conducted in 600 mmol L^?1 NaCl solution showed that additions of chromium, titanium, and manganese also produced deposits which were significantly more noble than pure magnesium, suggesting that these alloys would be less susceptible to galvanic corrosion.

MST/3064     

In vitro corrosion of magnesium alloy AZ31 --- a synergetic influence of glucose and Tris

Biodegradable Mg alloys have generated great interest for biomedical applications. Accurate predictions of in vivo degradation of Mg alloys through cost-effective in vitro evaluations require the latter to be conducted in an environment close to that of physiological scenarios. However, the roles of glucose and buffering agents in regulating the in vitro degradation performance of Mg alloys has not been elucidated. Herein, degradation behavior of AZ31 alloy is investigated by hydrogen evolution measurements, pH monitoring and electrochemical tests. Results indicate that glucose plays a content-dependent role in degradation of AZ31 alloy in buffer-free saline solution. The presence of a low concentration of glucose, i.e. 1.0 g/L, decreases the corrosion rate of Mg alloy AZ31, whereas the presence of 2.0 and 3.0 g/L glucose accelerates the corrosion rate during long term immersion in saline solution. In terms of Tris-buffered saline solution, the addition of glucose increases pH value and promotes pitting corrosion or general corrosion of AZ31 alloy. This study provides a novel perspective to understand the bio-corrosion of Mg alloys in buffering agents and glucose containing solutions.     

<Emphasis Type="Italic">In vitro</Emphasis> corrosion of magnesium alloy AZ31 — a synergetic influence of glucose and Tris

Biodegradable Mg alloys have generated great interest for biomedical applications. Accurate predictions of in vivo degradation of Mg alloys through cost-effective in vivo evaluations require the latter to be conducted in an environment close to that of physiological scenarios. However, the roles of glucose and buffering agents in regulating the in vivo degradation performance of Mg alloys has not been elucidated. Herein, degradation behavior of AZ31 alloy is investigated by hydrogen evolution measurements, pH monitoring and electrochemical tests. Results indicate that glucose plays a content-dependent role in degradation of AZ31 alloy in buffer-free saline solution. The presence of a low concentration of glucose, i.e. 1.0 g/L, decreases the corrosion rate of Mg alloy AZ31, whereas the presence of 2.0 and 3.0 g/L glucose accelerates the corrosion rate during long term immersion in saline solution. In terms of Tris-buffered saline solution, the addition of glucose increases pH value and promotes pitting corrosion or general corrosion of AZ31 alloy. This study provides a novel perspective to understand the bio-corrosion of Mg alloys in buffering agents and glucose containing solutions.     

Biocompatibility and corrosion behavior of the shape memory NiTi alloy in the physiological environments simulated with body fluids for medical applications

Due to unique properties of NiTi shape memory alloys such as high corrosion resistance, biocompatibility, super elasticity and shape memory behavior, NiTi shape memory alloys are suitable materials for medical applications. Although TiO₂ passive layer in these alloys can prevent releasing of nickel to the environment, high nickel content and stability of passive layer in these alloys are very debatable subjects. In this study a NiTi shape memory alloy with nominal composition of 50.7 atom% Ni was investigated by corrosion tests. Electrochemical tests were performed in two physiological environments of Ringer solution and NaCl 0.9% solution. Results indicate that the breakdown potential of the NiTi alloy in NaCl 0.9% solution is higher than that in Ringer solution. The results of Scanning Electron Microscope (SEM) reveal that low pitting corrosion occurred in Ringer solution compared with NaCl solution at potentiostatic tests. The pH value of the solutions increases after the electrochemical tests. The existence of hydride products in the X-ray diffraction analysis confirms the decrease of the concentration of hydrogen ion in solutions. Topographical evaluations show that corrosion products are nearly same in all samples. The biocompatibility tests were performed by reaction of mouse fibroblast cells (L929). The growth and development of cells for different times were measured by numbering the cells or statistics investigations. The figures of cells for different times showed natural growth of cells. The different of the cell numbers between the test specimen and control specimen was negligible; therefore it may be concluded that the NiTi shape memory alloy is not toxic in the physiological environments simulated with body fluids.     

Rapid degradation of biomedical magnesium induced by zinc ion implantation

The degradation rate is important to biodegradable magnesium materials. In this study, Zn is implanted using a cathodic arc source into pure magnesium at an accelerating voltage of 35 kV. The nominal ion implant fluence is 2.5 × 10¹⁷ ions cm⁻². After Zn implantation, the degradation rate in simulated body fluids is increased significantly. It is postulated that because Zn exists in the metallic state in the implanted layer, the galvanic effect between the Zn rich surface region and magnesium matrix induces the observed accelerated degradation.     

Bio-corrosion of a magnesium alloy with different processing histories

High rates of degradation in corrosive media represent the Achilles heel of Mg alloys, which hinders their applications in various areas, particularly in prosthetics. We present an investigation of the degradation behaviour of magnesium alloy AZ31 in Hank's solution that simulates bodily fluids. The degradation rate is shown to be significantly reduced by grain refinement produced by mechanical processing. In particular, hot rolling does lead to a desirable retardation of degradation, while subsequent equal channel angular pressing does not result in any further reduction of degradation rate.     

In vitro degradation and cell attachment of a PLGA coated biodegradable Mg–6Zn based alloy

Currently available engineering magnesium alloys have several critical concerns if they are about to be used as biomaterials, particularly the concern about the toxicity of the common alloying elements such as aluminum and rare earth (RE). There is an increasing demand to develop new magnesium alloys that do not contain any toxic elements. It is also desirable, yet challenging, to develop such a material that has a controllable degradation rate in the human fluid environment. This paper presents mechanical properties, degradation, and in vitro cell attachment of a newly developed Mg–6Zn magnesium alloy. The alloy demonstrated comparable mechanical properties with typical engineering magnesium alloys. However, the bare alloy did not show an acceptable corrosion (degradation) rate. Application of a polymeric PLGA or poly(lactide-co-glycolide) coating significantly decreased the degradation rate. The results obtained from cell attachment experiments indicated that the mouse osteoblast-like MC3T3 cells could develop enhanced confluence on and interactions with the coated samples.     

Degradable silver-based alloys

Noble metals solved in iron implants are effective cathodes, which can suit to accelerate the corrosion rate of the base material. In terms of its antibacterial behavior as well as lower costs in comparison with gold or platinum, silver seems to be an attractive candidate to adapt the corrosion rate of implants to the medical requirements. However, the degradation of silver in human bodies is a time-consuming process, and is controversially discussed due to the unknown long-term effect of silver on the human organism. Alloying silver with chemical elements less resistant to corrosion in aqueous mediums, particularly, in simulated body fluid, can improve the degradability of silver. Therefore, the current study addresses the design of adapted silver alloys exhibiting improved degradability in comparison with pure silver. Pure silver and binary silver alloys containing silicon, magnesium and calcium are studied in terms of their microstructure, open-circuit potential and degradation rate.     

Ion release from magnesium materials in physiological solutions under different oxygen tensions

Although magnesium as degradable biomaterial already showed clinical proof of concepts, the design of new alloys requires predictive in vitro methods, which are still lacking. Incubation under cell culture conditions to obtain “physiological” corrosion may be a solution. The aim of this study was to analyse the influence of different solutions, addition of proteins and of oxygen availability on the corrosion of different magnesium materials (pure Mg, WE43, and E11) with different surface finishing. Oxygen content in solution, pH, osmolality and ion release were determined. Corrosion led to a reduction of oxygen in solution. The influence of oxygen on pH was enhanced by proteins, while osmolality was not influenced. Magnesium ion release was solution-dependent and enhanced in the initial phase by proteins with delayed release of alloying elements. The main corrosion product formed was magnesium carbonate. Therefore, cell culture conditions are proposed as first step toward physiological corrosion.     

Blood triggered corrosion of magnesium alloys

Intravascular stents manufactured out of bioabsorbable magnesium (Mg) or Mg-alloys are considered as auspicious candidates for the next stent generation. However, before clinical application numerous physical and biological tests, especially to predict the clinically highly important degradation kinetics in vivo, have to be performed. In a Chandler-Loop model, the initial degradation of eight different magnesium alloys during 6 h in contact with human whole blood was investigated. The magnesium release varied between 0.91 ± 0.33 mg/cm² (MgAl9Zn1) and 2.57 ± 0.38 mg/cm² (MgZn1). No correlation could be found with Mg release data obtained after immersion in simulated body fluid (SBF). This pilot study showed that Mg corrosion is highly influenced by the biological test environment (SBF or blood, etc.) and that a modified Chandler-Loop model with human whole blood may be superior to predict corrosion of Mg alloys under clinical conditions than the SBF models presently used.     

In vitro degradation of four magnesium–zinc–strontium alloys and their cytocompatibility with human embryonic stem cells

Magnesium alloys have attracted great interest for medical applications due to their unique biodegradable capability and desirable mechanical properties. When designed for medical applications, these alloys must have suitable degradation properties, i.e., their degradation rate should not exceed the rate at which the degradation products can be excreted from the body. Cellular responses and tissue integration around the Mg-based implants are critical for clinical success. Four magnesium–zinc–strontium (ZSr41) alloys were developed in this study. The degradation properties of the ZSr41 alloys and their cytocompatibility were studied using an in vitro human embryonic stem cell (hESC) model due to the greater sensitivity of hESCs to known toxicants which allows to potentially detect toxicological effects of new biomaterials at an early stage. Four distinct ZSr41 alloys with 4 wt% zinc and a series of strontium compositions (0.15, 0.5, 1, and 1.5 wt% Sr) were produced through metallurgical processing. Their degradation was characterized by measuring total mass loss of samples and pH change in the cell culture media. The concentration of Mg ions released from ZSr41 alloy into the cell culture media was analyzed using inductively coupled plasma atomic emission spectroscopy. Surface microstructure and composition before and after culturing with hESCs were characterized using field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. Pure Mg was used as a control during cell culture studies. Results indicated that the Mg–Zn–Sr alloy with 0.15 wt% Sr provided slower degradation and improved cytocompatibility as compared with pure Mg control.     

In vitro Degradation of Pure Mg for Esophageal Stent in Artificial Saliva

Magnesium and its alloys as biodegradable implant materials can be potentially used in cardiovascular and orthopedic devices.However,few studies have focused on its application in esophageal stents.In this paper,time-lapse degradation characteristics of pure Mg were analyzed by scanning electron microscopy,energy dispersive spectroscopy,X-ray diffraction,Fourier transform infrared spectroscopy,hydrogen evolution,pH and electrochemical measurements after immersion in arti?cial saliva for different times.Results revealed that a dense degradation product?lm formed on samples,which mainly consisted of two kinds of layers:one was calcium phosphate compounds with different structures;the other was thin magnesium hydrate layer close to the substrate.Less pH increase and low degradation rate were observed in the?rst 5 days of immersion,which can be ascribed to the formation of a thicker and denser layer on the sample surface with increasing immersion time.And then there was an increase in degradation rate and pH values;the deposition layer remained almost intact after immersion for 6 and 8 days.After 10 days of immersion,the degradation rate and pH value remained stable,and the calcium phosphate layer was delaminated and the inner magnesium hydrate layer was exposed.This study indicated that pure Mg exhibited desirable degradation resistance in arti?cial saliva,which provided magnesiumbased materials with the potential to be used as esophageal stents.     

A Comparison of Corrosion Behavior in Saline Environment: Rare Earth Metals (Y, Nd, Gd, Dy) for Alloying of Biodegradable Magnesium Alloys

Rare earth(RE) metals are widely used as the alloying elements in biodegradable magnesium alloys as medical implants.However,corrosion behavior of pure RE metals not only in physiological media but also in chlorinated saline environment is not well understood.In the present work,the RE metals Y,Nd,Gd and Dy are selected to investigate their corrosion behavior in 0.1 mol/L NaCl solution with immersion and electrochemistry techniques.As indicated,corrosion of the currently investigated RE metals is promoted in the order of Dy,Y,Gd and Nd.In terms of electrochemical response,such a sequence correlates with the increased impedance and the decreased corrosion rate(CR).These RE metals manifest weak ability for passivation in the native surface.Then,reaction with aqueous solution easily happens through the anodic dissolution and cathodic hydrogen evolution.The corrosion products,RE(OH)3,adhered on the surface of RE metals,do not have an appreciable power to resist the reaction proceeding with corrosive chloride ions.In contrast to pure Mg,the RE metals,including Y,Nd,Gd and Dy,exhibit significantly fragile corrosion resistance in saline media.Therefore,with the current findings,it is impossible to reveal a well-defined correlation of corrosion resistance between RE-containing Mg alloy and RE metal itself.     

Degradation behavior of novel Fe/ß-TCP composites produced by powder injection molding for cortical bone replacement

When it comes to bone replacement in load-bearing areas, there are currently no adequate biodegradable implants available. Several non-degradable metallic materials fulfill the requirements of biocompatibility and mechanical strength. However, besides magnesium, only iron is a degradable metallic material. The aim of this long-term degradation study was to investigate the effects of iron beta-tricalcium phosphate interpenetrating phase composite on degradation rate and strength in comparison to pure iron. Cylindrical samples with 0–50 vol% beta-tricalcium phosphate (ß-TCP) were prepared by powder injection molding. In addition to dense samples, porous iron samples with a porosity of 60.3 % were produced with polyoxymethylene as a placeholder. Dense and porous samples were immersed in 0.9 % sodium chloride solution (NaCl) or in phosphate buffered saline solution (PBS) for 56 days. Following immersion, the degradation rate, compressive yield strength, and ion release were determined. A maximum degradation rate of 196 µm/year was observed after 56 days for iron with 40 vol% ß-TCP. This was found to be 28 % higher than for pure iron. After immersion, the compressive yield strength of pure iron decreased by 44 % (NaCl) and 48 % (PBS). In comparison, iron with 40 % ß-TCP samples lost <1 % (NaCl) and 9 % (PBS) of strength following immersion. It was demonstrated that the solubility of calcium phosphate enhanced the corrosion processes and led to an increase in degradation, thus showing that the addition of ß-TCP to pure iron can be a promising route for a novel degradable bone substitute material, particularly for load-bearing areas due to the increased strength.     

Bio-corrosion characterization of Mg–Zn–X (X = Ca, Mn, Si) alloys for biomedical applications

The successful applications of magnesium-based alloys as biodegradable orthopedic implants are mainly inhibited due to their high degradation rates in physiological environment. This study examines the bio-corrosion behaviour of Mg–2Zn–0.2X (X = Ca, Mn, Si) alloys in Ringer’s physiological solution that simulates bodily fluids, and compares it with that of AZ91 magnesium alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy results showed a better corrosion behaviour of AZ91 alloy with respect to Mg–2Zn–0.2Ca and Mg–2Zn–0.2Si alloys. On the contrary, enhanced corrosion resistance was observed for Mg–2Zn–0.2Mn alloy compared to the AZ91 one: Mg–2Zn–0.2Mn alloy exhibited a four-fold increase in the polarization resistance than AZ91 alloy after 168 h exposure to the Ringer’s physiological solution. The improved corrosion behaviour of the Mg–2Zn–0.2Mn alloy with respect to the AZ91 one can be ascribed to enhanced protective properties of the Mg(OH)₂ surface layer. The present study suggests the Mg–2Zn–0.2Mn alloy as a promising candidate for its applications in degradable orthopedic implants, and is worthwhile to further investigate the in vivo corrosion behaviour as well as assessed the mechanical properties of this alloy.     

Stress corrosion cracking of rare-earth containing magnesium alloys ZE41, QE22 and Elektron 21 (EV31A) compared with AZ80

Stress corrosion cracking (SCC) of the high-performance rare-earth containing magnesium alloys ZE41, QE22 and Elektron 21 (EV31A) was studied using slow strain rate test (SSRT) method in air, distilled water and 0.5 wt.% NaCl solution. For comparison, the well-known AZ80 alloy was also studied. All alloys were susceptible to SCC in 0.5 wt.% NaCl solution and distilled water to some extent. AZ80 had similar SCC susceptibility in distilled water and 0.5 wt.% NaCl solution. ZE41, QE22 and EV31A had higher susceptibility to SCC in 0.5 wt.% NaCl solution than in distilled water. EV31A had the highest resistance to SCC compared to AZ80, ZE41 and QE22 in both distilled water and 0.5 wt.% NaCl solution. The fractography was consistent with (i) largely transgranular SCC (TGSCC) in distilled water for AZ80, ZE41 and QE22 and also for AZ80 in 0.5 wt.% NaCl solution, and (ii) a significant component of intergranular SCC (IGSCC) in 0.5 wt.% NaCl solution for QE22, ZE41 and EV31A. The TGSCC fracture path in AZ80, ZE41 and QE22 is consistent with a mechanism involving hydrogen. In each case, the IGSCC appeared to be associated with the second-phase particles along grain boundaries. For IGSCC of EV31A and QE22, the fractography was consistent with micro-galvanic acceleration of the corrosion of -magnesium by the second-phase particles, whereas it appeared that the second-phase particles had corroded itself in the case of ZE41 in 0.5 wt.% NaCl solution. The study suggests that rare-earth elements in magnesium alloys can improve SCC resistance significantly as observed in the case of EV31A. However, the SCC resistance also depends on the other critical alloying elements such as zinc (in ZE41) and silver (in QE22) and the microstructure.