Degradation Behavior Of Magnesium Alloy Research Articles

Effect of medium renewal mode on the degradation behavior of Mg alloys for biomedical applications during the long-term in vitro test

To build a suitable corrosion environment for testing the degradation behavior of magnesium alloy in vitro, the medium and test environment are constantly optimized. However, the applicability of medium renewal mode is still vague. In this study, based on three representative test protocols, the influence of the specific ratio of media volume to surface area and the disposable medium real-time renewal mode (flow-through) on Mg corrosion were investigated. It is found that in flow-through medium renewal mode, the composition of the medium is maintained quasi-constant which is essential for corrosion tests. The selection suggestion on medium renewal mode was also proposed. This work is beneficial for ultimately establishing the representative in vitro testing protocols for Mg bioabsorbable materials.

Experimental insights toward understanding how the morphology of Mg2Si particles affects degradation behavior of the biodegradable as-cast Mg–Si alloys

Considering the remarkable function of silicon in the development and growth of connective tissue and bone, building the immune system, and tissue healing, together with its positive effects on improving mechanical and degradation behavior of magnesium alloys, Mg–Si alloys can be a promising alloying system for making biodegradable implants. However, the morphology, size, and distribution of Mg2Si particles, as the only intermetallic phase in Mg–Si alloys, can affect the corrosion behavior of these alloys significantly. Therefore, in this investigation, the effects of Y addition (0.5 wt%) and heat treatment (HT, at 420 °C for 24 h) on degradation behavior of two hypo-eutectic Mg–1Si and hyper-eutectic Mg–2Si alloys are studied by hydrogen evolution, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) tests, all done in phosphate-buffered saline (PBS) solution. Also, the appearance of the corroded surfaces of the studied alloys after electrochemical and immersion tests was evaluated by scanning electron microscopy equipped with electron disperse spectroscopy. It was found that Y addition to the as-cast Mg–1Si alloy increased the corrosion resistance, corresponding to a decrease in the corrosion current density (Icorr) from 5.60 to 3.24 μA/cm2. On the contrary, the addition of Y to the as-cast Mg–2Si alloy did not significantly change the corrosion resistance. In addition, it was found that after applying HT on the as-cast Mg–1Si and Mg–2Si alloys, corrosion resistance was improved, which was mainly attributed to the modified eutectic Mg2Si particles.

In Vivo Degradation Behavior of Magnesium Alloy for Bone Implants with Improving Biological Activity, Mechanical Properties, and Corrosion Resistance.

This study aimed to establish a surface modification technology for ZK60 magnesium alloy implants that can degrade uniformly over time and promote bone healing. It proposes a special micro-arc oxidation (MAO) treatment on ZK60 alloy that enables the composite electrolytes to create a coating with better corrosion resistance and solve the problems of uneven and excessive degradation. A magnesium alloy bone screw made in this way was able to promote the bone healing reaction after implantation in rabbits. Additionally, it was found that the MAO-treated samples could be sustained in simulated body-fluid solution, exhibiting excellent corrosion resistance and electrochemical stability. The Ca ions deposited in the MAO coating were not cytotoxic and were beneficial in enhancing bone healing after implantation.

Effects of aging treatment and pre-deformation on stress corrosion cracking of magnesium alloy

As a medical implant material, the mechanical degradation behavior of magnesium alloy in service environment is of great significance. The Mg–4Zn–0.1Sr alloy is an ideal biomedical alloy, while solute Zn atoms are easy to cause the stress corrosion cracking (SCC) of the alloy. In this paper, the distribution of Zn atoms in Mg–4Zn–0.1Sr alloy is changed by aging treatment, so as to reduce the SCC of the alloy promoted by the Portevin-Le Chatelier effect. In comparison with the as-solutionized alloy, the tensile strength and the elongation of the alloy aged at 160 °C for 400 h in Hanks' solution are increased by 42% and 107%, respectively. In order to improve the aging kinetics, twins and dislocations are introduced by pre-compression. Twins and dislocations promote the diffusion of Zn atoms during aging treatment and make the distribution of precipitates more dispersed. The dislocations can be recovered by aging treatment to avoid promoting anodic dissolution and hydrogen embrittlement. Twin boundaries cannot easily migrate due to the nailing of Zn atoms, the latter of which improves the mechanical properties and leads to grain refinement, and have a good effect on reducing the anodic dissolution rate of the alloy. In comparison with the as-solutionized alloy, the tensile strength and the elongation of the alloy with 3% pre-compression and aging at 160 °C for 200 h are increased by 47% and 77%, respectively. The mechanism of mechanical property improvement of the alloy in Hanks' solution by the thermomechanical treatment is revealed.

Assessing the long-term in vivo degradation behavior of magnesium alloys - a high resolution synchrotron radiation micro computed tomography study

Biodegradable magnesium (Mg) implants are emerging as a potential game changer in implant technology in situations where the implant temporarily supports the bone thereby avoiding secondary surgery for implant removal. However, the consequences of the alteration in the degradation rate to bone healing and the localization of degradation and alloying products in the long term remain unknown. In this study, we present the long-term osseointegration of three different biodegradable Mg alloys, Mg-10Gd, Mg-4Y-3RE and Mg-2Ag, which were implanted into rabbit femur for 6 and 9 months. In addition, we have investigated the effect of blood pre-incubation on the in vivo performance of the aforementioned alloys. Using high-resolution synchrotron radiation based micro computed tomography, the bone implant contact (BIC), bone volume fraction (BV/TV) and implant morphology were studied. The elemental traces have been characterized using micro X-ray fluorescence. Qualitative histological evaluation of the surrounding bone was also performed. Matured bone formed around all three implant types and Ca as well as P which represent parts of the degradation layer were in intimate contact with the bone. Blood pre-incubation prior to implantation significantly improved BIC in Mg-2Ag screws at 9 months. Despite different implant degradation morphologies pointing toward different degradation dynamics, Mg-10Gd, Mg-4Y-3RE and Mg-2Ag induced a similar long-term bone response based on our quantified parameters. Importantly, RE elements Gd and Y used in the alloys remained at the implantation site implying that they might be released later on or might persist in the implantation site forever. As the bone formation was not disturbed by their presence, it might be concluded that Gd and Y are non-deleterious. Consequently, we have shown that short and mid-term in vivo evaluations do not fully represent indicators for long-term osseointegration of Mg-based implants.

Predicting the degradation behavior of magnesium alloys with a diffusion-based theoretical model and in vitro corrosion testing

Magnesium alloys have shown great potential for their use in the medical device field, due to the promising biodegradability. However, it remains a challenge to characterize the degradation behavior of the Mg alloys in a quantitative manner. As such, controlling the degradation rate of the Mg alloys as per our needs is still hard, which greatly limits the practical application of the Mg alloys as a degradable biomaterial. This paper discussed a numerical model developed based on the diffusion theory, which can capture the experimental degradation behavior of the Mg alloys precisely. The numerical model is then implemented into a finite element scheme, where the model is calibrated with the data from our previous studies on the corrosion of the as-cast Mg-1Ca and the as-rolled Mg-3Ge binary alloys. The degradation behavior of a pin implant is predicted using the calibrated model to demonstrate the model’s capability. A standard flow is provided in a practical framework for obtaining the degradation behavior of any biomedical Mg alloys. This methodology was further verified via the comparison with enormous available experimental results. Lastly, the material parameters defined in this model were provided as a new kind of material property.

In vitro and in vivo studies of biodegradable fine grained AZ31 magnesium alloy produced by equal channel angular pressing

The objective of the present work is to investigate the role of different grain sizes produced by equal channel angular pressing (ECAP) on the degradation behavior of magnesium alloy using in vitro and in vivo studies. Commercially available AZ31 magnesium alloy was selected and processed by ECAP at 300°C for up to four passes using route Bc. Grain refinement from a starting size of 46μm to a grain size distribution of 1–5μm was successfully achieved after the 4th pass. Wettability of ECAPed samples assessed by contact angle measurements was found to increase due to the fine grain structure. In vitro degradation and bioactivity of the samples studied by immersing in super saturated simulated body fluid (SBF 5×) showed rapid mineralization within 24h due to the increased wettability in fine grained AZ31 Mg alloy. Corrosion behavior of the samples assessed by weight loss and electrochemical tests conducted in SBF 5× clearly showed the prominent role of enhanced mineral deposition on ECAPed AZ31 Mg in controlling the abnormal degradation. Cytotoxicity studies by MTT colorimetric assay showed that all the samples are viable. Additionally, cell adhesion was excellent for ECAPed samples particularly for the 3rd and 4th pass samples. In vivo experiments conducted using New Zealand White rabbits clearly showed lower degradation rate for ECAPed sample compared with annealed AZ31 Mg alloy and all the samples showed biocompatibility and no health abnormalities were noticed in the animals after 60days of in vivo studies. These results suggest that the grain size plays an important role in degradation management of magnesium alloys and ECAP technique can be adopted to achieve fine grain structures for developing degradable magnesium alloys for biomedical applications.

Cytotoxicity studies of AZ31D alloy and the effects of carbon dioxide on its biodegradation behavior in vitro

Magnesium alloys have been advocated as potential artificial bone materials due to their biocompatibility and biodegradability. The understanding of their corrosive mechanism in physiological environments is therefore essential for making application-orientated designs. Thus, this in vitro study was designed to assess the effects of CO2 on corrosive behavior of AZ31D to mimic in vivo special ingredient. Electrochemical technologies accompanied with Scanning electron microscope, Fourier transform infrared, X-ray diffraction, Energy dispersive spectroscopy and hydrogen evolution measurement were employed to analyze corrosive rates and mechanisms of AZ31D. Moreover, the biocompatibility of AZ31D was assessed with a direct cell attachment assay and an indirect cytotoxicity test in different diluted extracts. The ion concentrations in extracts were measured using inductively coupled plasma mass spectrometry to offer explanations on the differences of cell viability in the indirect test. The results of the direct cytotoxicity assay showed that the corrosive rate of AZ31D was too rapid to allow for cell adhesion. Extracts diluted less than 20 times would cause adverse effects on cell proliferation, likely due to excessive ions and gas release. Moreover, the presence of CO2 did not cause significant differences on corrosive behavior of AZ31D according to the results of electrochemical testing and hydrogen evolution measurement. This might be caused by the simultaneous process of precipitation and dissolution of MgCO3 due to the penetration role of CO2. This analysis of corrosive atmospheres on the degradation behavior of magnesium alloys would contribute to the design of more scientific in vitro testing systems in the future.

In Vivo and In Vitro Degradation Behavior of Magnesium Alloys as Biomaterials

The corrosion behavior of pure Mg, AZ31, and AZ91D were evaluated in various in vitro and in vivo environments to investigate the potential application of these metals as biodegradable implant materials. DC polarization tests and immersion tests were performed in different simulated body solutions, such as distilled (DI) water, simulated body fluid (SBF) and phosphate buffered solution (PBS). Mg/Mg alloys were also implanted in different places in a mouse for in vivo weight loss and biocompatibility investigations. The in vivo subcutis bio-corrosion rate was lower than the corrosion rate for all of the in vitro simulated corrosive environments. The Mg/Mg alloys were biocompatible based on histology results for the liver, heart, kidney, skin and lung of the mouse during the two months implantation. Optical microscopy and scanning electron microscopy were carried out to investigate the morphology and topography of Mg/Mg alloys after immersion testing and implantation to understand the corrosion mechanisms.

In vitro degradation behavior of M1A magnesium alloy in protein-containing simulated body fluid

Magnesium alloys possess unique advantages to be used as biodegradable implants for clinical applications. In this study, in vitro cells responses and degradation behaviors of magnesium alloy M1A in simulated body fluid (SBF) and albumin-containing SBF (A-SBF) were systematically investigated. Cell responses, in terms of Cell morphology and cell proliferation, imply that M1A possesses good viability for MG63 cells. The corrosion behaviors of M1A are strongly affected by the addition of albumin through the combined effects of adsorption and chelation. Electrochemical testing indicates that such an absorbed albumin layer makes M1A to be more noble with a smaller corrosion current. Corrosion rate monitored by hydrogen evolution rate suggests that the quickly adsorbed albumin serves as an effective protective layer, resulting in a much slower hydrogen release rate at initial stage. With increasing immersion time, a higher corrosion rate is observed since the chelation effect exerts more significant acceleration effects on the removal of the passivation layer. The corrosion mode evaluated by surface morphology of the samples changes from a nonuniform-anisotropic mode for M1A in SBF to a uniform-isotropic mode for M1A in A-SBF.

Modification of degradation behavior of magnesium alloy by IBAD coating of calcium phosphate

In order to modify the degradation behavior of magnesium alloy, hydroxyapatite (Ca10(PO4)6(OH)2) coating is formed on magnesium alloy by ion-beam assisted deposition (IBAD) and heat treatment. The morphology, composition and phase structure of the coated samples were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy, respectively. The width of atomic mixed interface between the coating and substrate formed by IBAD was approximate 3 μm, which was measured by Auger electron spectroscopy. The hardness and elastic modulus of coating were measured with a depth-sensing nanoindenter system. The results of accelerated test (AT) of degradation indicate that the coatings can decrease degradation rate of original Mg alloy significantly.