Magnesium and its alloys have gained increasing interest in the field of temporary implant materials . Biodegradable cardiovascular magnesium stents were successfully tested for clinical applications [2, 3]. Due to its mechanical properties, which are similar to those of bone, magnesium is also of high interest for orthopedic research . Also, metallic magnesium and its degradation product magnesium hydroxide show osteoconductive properties [5, 6].
As magnesium is highly susceptible to degradation in aqueous environments, efforts have been made both to decelerate as well as to exploit this process. For the application as a part of large, load-bearing prosthesis, an uncontrolled, i.e. too rapid, degradation is obviously a major disadvantage. On the other hand, the osteoconductive properties mentioned can support the incorporation of an implant made of or coated with magnesium or magnesium compounds [7, 8]. Thus, the degradation of metallic magnesium is an obstacle as well as an opportunity in the field of degradable biomaterials. A thorough understanding of the kinetics and mechanisms of this process is therefore crucial in order to better evaluate the degradation behavior and in this way increase the predictive power of in vitro studies, the results of which have so far often differed considerably from those of in vivo studies [4, 9]. In vitro degradation setups provide many opportunities to follow the course of decomposition online, such as pH and conductivity monitoring [10, 11], but the identity of the degradation products formed from magnesium and the supplements of the media cannot be derived from these parameters alone. Electron-dispersive X-ray spectroscopy (EDS) analysis can be performed discontinuously on the degradation products to gain information on their composition, but without providing speciation between different products with the same elemental formula. Moreover, most instruments only give semi-quantitative data, because they are not calibrated to the respective elements. X-ray diffraction (XRD) measurements provide more detailed information at least about the crystalline degradation products.
Magnesium hydroxide is the dominant crystalline degradation product of magnesium and its alloys upon exposure to aqueous media, as has been shown before by various studies . As the solubility of the hardly soluble Mg(OH)2 can be increased, for example, by the presence of chloride ions [13, 14] and by buffering of pH, magnesium hydroxide is not the only product which can be expected. Alternative products such as phosphates and carbonates, respectively, were found in various studies. In an in vivo approach, a magnesium containing calcium phosphate (MgxCay(PO4)z) was found after the degradation on the surface of a Mg-Mn-Zn alloy , while the degradation of a magnesium rare-earth alloy led to the in vitro formation of complex products with the general formula (Mg,Ca)x(PO4)y(CO3)z(OH)I . The formation of magnesium carbonates (MgCO3) was reported by various authors, when pure magnesium was exposed to different aqueous media [16, 17]. However, many studies did not distinguish between different modifications of the carbonates and phosphates, respectively.
With regard to the issues mentioned, it is of central interest to estimate the degradation rates and products of magnesium exposed to aqueous media under conditions similar and relevant to in vivo conditions. For this purpose, we have studied the degradation of pure magnesium under cell culture conditions in cell-free media. As the respective media Hank’s Buffered Salt Solution (HBSS), simulated body fluid (SBF) and Dulbecco’s Modified Eagle Medium (DMEM) were applied. The cell culture conditions included a CO2 (5%) and O2 (21%) gassing, as well as a temperature of 37 °C and 95% relative humidity over the entire experiment. In an additional set of experiments, fetal bovine serum was added to all the above-mentioned media to include the influence of bioorganic components. Degradation rates were determined by measuring the weight loss of the samples. In order to identify the degradation products, the specimens were investigated by scanning electron microscopy (SEM) and XRD. The latter is suitable for the identification of crystalline phases that may have formed and have been deposited during the degradation process either directly or by follow-up reactions.
Materials and methods
A magnesium ingot (purity 99.95 wt.%) was prepared by permanent mold casting. The material was molten under a protective atmosphere (Ar+2% SF6) at a temperature of 750 °C. The melt was stirred for 30 min with 200 rpm prior to casting the material into preheated molds (550 °C) made out of mild steel. To assure cleanliness of the cast ingots, a filter (Foseco SIVEX FC, Foseco GmbH, Borken, Germany) was used. Cylindrical specimens with a diameter of 10 mm and a height of 1.5 mm were cut from the cast blocks via electrical discharge machining. Apart from sterilization the samples were used without further treatment.
Three different media were used for the degradation tests: (I) HBSS without Ca2+ and Mg2+ ions (Life Technologies, Darmstadt, Germany) as a low-buffered solution; (II) DMEM Glutamax-I (Life Technologies, Darmstadt, Germany) as a more complex, highly buffered solution, and (III) SBF (SBF-JL2, according to ). For the preparation of the SBF two solutions were prepared. All chemicals were obtained from Merck (Darmstadt, Germany). (I) Solution A: 6.129 g NaCl, 5.89 g NaHCO3 and 0.498 g Na2HPO4·2H2O were dissolved in 1 L of double-distiled water. After dissolution, 0.934 mL of 1 M HCl was added. (II) Solution B: 6.129 g NaCl and 0.54 g CaCl2 were dissolved in 1 L double-distiled water and 0.934 mL of 1 M HCl was added. Both solutions were kept separately in the fridge and mixed (1:1) directly before the experiment. The pH value of the resulting solution was 7.4. A comparison of the compositions is given in Table 1. In a second set of experiments, 10% or 20% FBS (FBS, PAA Laboratories, Linz, Austria) were added to these media.
Determination of degradation rate, pH and osmolality
The degradation rate was determined by weight loss. Each sample was weighed (Scaltec SBA52, Scaltec Instruments GmbH, Göttingen, Germany) and sterilized (20 min in ultrasonic bath with 70% ethanol) prior to the immersion (n=6 per solution). After 3 days of incubation under cell culture conditions the samples were taken from the different media and dried at 37 °C under vacuum overnight. Degradation residues from the samples were removed by immersion in chromic acid (180 g/L in distiled water, VWR International, Darmstadt, Germany). The immersion was carried out two times for 10 min, with the samples being turned over in between. Afterwards the samples were rinsed in distiled water and 100% ethanol. After drying, the weight of the samples was measured again and the degradation rate (DR) was calculated according to equation 1 .
where A is the surface area in cm2, t is the time in hours, ρ is the density of pure Mg (1.74 g/cm3) and ΔW is the observed mass loss in g.
After the immersion of the specimens the different media were removed from the wells and used for analysis of the pH value and osmolality. Solutions without samples were incubated in parallel as controls. The pH value was measured by a standard laboratory pH meter (Sentron SI600, Sentron Europe BV, Roden, The Netherlands) with an ISFET electrode. The osmolality of the supernatants was analyzed by a cryoscopic osmometer (Osmomat Auto, Gonotec GmbH, Berlin, Germany). Fifty microliters of the supernatant were used for the measurement. The device is a freezing point osmometer which determines the osmolality by measuring the freezing point. This decreases by 1.86 °C when 1 mole of a nonionic solute is added to 1 kg of solvent .
Identification of degradation products
Pure magnesium samples were sonificated for 20 min in dry isopropanol, dried and gamma-sterilized at the in core irradiation (ICI) facility of the Geesthacht Neutron Facility with a total dosage of 29 kGy. All specimens were immersed under sterile conditions in 1 mL (solid/liquid ratio of 0.2 g/mL) of the respective media in agarose-coated 12-well plates (Greiner Bio-One, Frickenhausen, Germany). Then the samples were incubated under cell culture conditions (37 °C, 5% CO2, 21% O2 and 95% relative humidity) in an incubator (Heraeus BBD 6620, Fisher Scientific, Schwerte, Germany). The immersion time was 1, 2, 3, 4 and 7 days, respectively. Neither a pH adjustment nor a liquid stream was applied. After the immersion all samples were rinsed with deionized water for 1 min, dried under vacuum and stored at room temperature. Immersion experiments were performed in duplicate.
The samples were visualized using a scanning electron microscope (SEM; Auriga, Zeiss, Oberkochen, Germany) to visualize the degradation layers. Images were taken at an accelerating voltage of 20 keV with the secondary electron detector (SE2). In order to identify the crystalline products formed and deposited, XRD measurements were performed on a Stoe Theta/Theta diffractometer (Stoe, Darmstadt, Germany) in reflection mode using Cu Kα radiation (λ=1.54060 Å). The 2Θ range was scanned applying a step size of 0.01° (3.0 s per step). The diffractometer was operated at 40 kV and 30 mA. No further sample preparations were made for the XRD measurements. All samples were placed flatly in a sample holder and adjusted to the X-ray beam.
Thermodynamical solubility calculations:
Additionally to the formation of brucite and magnesite in HBSS some other degradation products can form. Besides Mg2+ and carbonates, SBF and cell culture media may contain various ions, including Na+, K+, Ca2+, Cl–, phosphate and sulfate ions. In standard incubation conditions [temperature T=37 °C, partial CO2 pressure p(CO2)=5.0%, pH close to 7.5], the dominating carbonate and phosphate soluble species are HCO3– and HPO42–, respectively. In SBF and culture media containing Ca2+ and phosphate ions, the following precipitation reactions should also be considered:
Precipitation of magnesium phosphate (Mg3(PO4)2·8H2O, bobierrite):
Precipitation of calcium hydroxide (Ca(OH)2, portlandite):
Precipitation of calcium carbonate (CaCO3, calcite):
Precipitation of calcium phosphate (Ca5OH(PO4)3, hydroxyapatite):
Some other reactions taking place in aqueous solutions deserve to be mentioned.
Dissociation of water (pK° (37 °C)=13.61):
Dissociation of hydrogen carbonate ions (pK° (37 °C)=10.24):
Dissociation of hydrogen phosphate ions (pK° (37 °C)=12.32):
Table 2 presents the equilibrium dissolution constants at 37 °C for the precipitates considered in the present study (pK=–log10K). Tables 1 and 3 summarizes the compositions and main physico-chemical parameters of the incubating solutions considered in the present study. The saturation index S°(X) is given by:
where Π(X) is the ionic product of the ions released upon dissolution of precipitate X, and K(X) the equilibrium constant of the dissolution reaction; for instance, in the case of brucite:
A positive S°(X) means that the solution is supersaturated in X, so that the precipitation of X is thermodynamically possible. However, the actual formation of a precipitate X depends on the kinetics of nucleation and growth of X crystals: if the kinetic process is inhibited for some reason (e.g. large free energy of nucleation), the solution can remain in a metastable state without precipitate formation.
Data are displayed as mean±standard deviation (SD). Statistical analysis was performed by using the SigmaStat Software (Systat GmbH, Erkrath, Germany; version 11.0). Comparison of degradation rates was analyzed by applying the one-way analysis of variance (ANOVA) with Holm-Sidak post-hoc test. The significance level was p<0.05.
Editing of figures
All figures were finished with PowerPoint (Microsoft Office 2007 and 2010). Raw figure material was created and edited, respectively, using: SigmaPlot (Version 11, Systat Software Inc.) for Figures 5 and 6; Origin (OriginLab) for Figures 7–9; PowerPoint (Microsoft Office 2007 and 2010) for Figure 10.
All samples showed obvious signs of degradation on the surface. Exemplary photographs of samples are given in Figures 1–3. Figure 1 shows the surfaces of samples immersed in HBSS medium and in HBSS supplemented with 10% FBS, Figure 2 shows corresponding photographs for samples from DMEM and the corresponding 10% FBS-supplemented medium, and Figure 3 for the samples from SBF-based media. In general, the amount of white crust and flakes fallen off from the samples, respectively, increased with immersion time; however, the exact time points of the chipping off were not determined. According to the appearances, degradation occurred fastest in HBSS. When FBS was supplemented to HBSS and DMEM, degradation seemed to be reduced. In contrast, degradation did apparently not decrease when FBS was supplemented to SBF.
In general, according to the SEM images, two types of crystals with different morphologies predominate within the layer of degradation products (Figure 4). Firstly, intergrown needles, and secondly, crystals with a flake-like shape. However, SEM images from samples which were immersed in different media showed differences between the surface morphologies. Samples immersed in HBSS exhibited mostly flake-like crystals, whereas on DMEM-immersed samples mainly interconnected needles were visible. In all cases, the appearance of the degradation layer changed with the addition of proteins.
Degradation rate, pH and osmolality
The degradation rate was highest in HBSS and lowest in SBF. According to the statistical analysis, the HBSS treatment led to significantly higher degradation rates (compared to all other treatments, Figure 5). The actual degradation rates are compared in Figure 6, Top. Using HBSS the addition of proteins exhibited a significant decrease of the degradation rate due to the addition of proteins. This effect was weaker for DMEM, where only the addition of 20% FBS led to a significant decrease. Presumably, due to the low amount of samples, other differences between the degradation rates in DMEM and SBF were not statistically significant. The addition of proteins generally resulted in a decrease of the degradation rate, except for SBF, where the higher amount of protein increased the degradation rate again. The measurement of osmolality reproduces this behavior in the case of DMEM and SBF, but shows a different trend for HBSS (Figure 6 middle). The increase in pH (Figure 6 bottom) appears to show a loose correlation to the degradation rate, when the pH increases as well as the degradation rates in HBSS-based media are both higher. DMEM- and SBF-based media have a higher buffering capacity than HBSS, correspondingly a weaker pH increase is observed; degradation rates are generally smaller as those in HBSS-based media, as well.
Phase identification via XRD
X-ray diffraction (XRD) investigations allow the qualitative identification (speciation) of different crystalline degradation products. Beforehand, it has to be pointed out that a quantitative evaluation of the species by XRD is not the aim in the present study. Even if this is in principle possible using elaborated standardization and calibration experiments, it does not make sense regarding the way the samples were produced. Crumbles of different sizes fell off from most samples at undetermined time points. Possible texture effects, which are likely to occur with surface-crystallized or deposited material, would further complicate a quantitative evaluation. Also, reflections that can be qualitatively assigned to brucite (Mg(OH)2) or to the substrate magnesium could in some cases hardly be distinguished from background noise.
Exemplary XRD patterns of samples immersed in HBSS or in HBSS supplemented with 10% FBS are shown in Figure 7A; Figure 7B and C show diffractograms for samples immersed in corresponding DMEM- and SBF-based media, respectively.
Reflections associated with metallic magnesium (2Θ=32.2°, 34.4° and 36.6°) were present in all diffractograms, allowing the conclusion that the penetration depth of the X-ray beam was sufficient to examine the crusts in their entire thickness. However, the relative intensities were remarkably low in the DMEM series. For clarity, in the figures the reflections ascribed to magnesium are marked only in the patterns obtained from samples with an immersion time of 1 day. Reflections ascribed to degradation products are marked, if appropriate, in all respective patterns. Brucite, Mg(OH)2, a degradation product observed in many degradation processes of magnesium and magnesium alloys, was in our study only found as one of the products on samples from HBSS-based media (corresponding reflections at 2Θ=18.5° and 38.0°). Nesquehonite, a magnesium hydroxycarbonate hydroxide hydrate of formula Mg(HCO3)(OH)·2H2O (with characteristic reflections at 2Θ=13.6°, 23.0°, 29.5° and 34.2°), was the dominant degradation product in our study. It was observed in all diffractograms from samples from HBSS-based media, and in all diffractograms of specimens that had been exposed to SBF- and DMEM-based media for longer than 1 day. Calcium phosphates were found on a sample immersed in SBF supplemented with 10% FBS for 7 days (Figure 7C). The reflections can be assigned to Ca2P4O12·4H2O and Ca2P2O7·2H2O. However, some reflections, e.g. at 2Θ=10.2° and 11.6°, remained unassigned.
Influence of the medium and proteins on the degradation rates and osmolality
HBSS has been evaluated to be the simplest medium, followed by SBF, while DMEM also includes organic substances . Degradation rates decreased in the order of HBSS – DMEM – SBF. As proteins are known to retard magnesium degradation under certain conditions , a reduction of the decomposition rate was expected for DMEM. The expectation was confirmed for samples immersed in HBSS and DMEM, but not for samples immersed in SBF. As stated above, osmolality examinations reproduced this behavior for DMEM and SBF, but not for HBSS. Presumably due to the high degradation rates in HBSS, which were the highest in this study, and due to the high salt content of the solutions, a simple correlation could not be found.
Two magnesium compounds were found to be the dominant degradation products in this study: Mg(OH)2 and nesquehonite (Mg(HCO3)(OH)·2H2O). The latter was found on samples from all media, while Mg(OH)2 was found exclusively for specimens which had been exposed to HBSS. Measurements revealed the highest pH when HBSS was used as the medium, hence, the presence of Mg(OH)2 in the crusts is plausible. Unfortunately, as described above, the XRD investigations in this study were not suited to be used for quantification. The detection of nesquehonite is in line with the observation of needle-like crystals in SEM images, as this is a typical morphology for this magnesium compound.
In various previous studies, Mg(OH)2 has been described as the only crystalline degradation product. However, these studies did not include CO2 gassing as part of their setup . Therefore, we ascribe the formation of nesquehonite to the introduction of CO2 in our experiments, as carbon dioxide will serve as an additional source of hydrogen carbonate and carbonate ions. In this context, it is of interest that the formation of magnesium carbonates is of relevance for CO2 storage using mineral carbonation . The family of magnesium carbonates consists of a variety of compounds, and in other studies with slightly changed parameters in the immersion experiment setups, other members of this family might be formed. Nesquehonite, can be directly synthesized by the addition of a NaCO3 solution to a MgCl2 solution at room temperature . At slightly elevated temperatures, the NaCO3-MgCl2 system leads to the formation of hydromagnesite (Mg5(CO3)4(OH)2·4H2O) . Furthermore, the transformation of nesquehonite to hydromagnesite was reported in the context of dypingite (Mg5(CO3)4(OH)2·5H2O) [24, 25], which was also found as a byproduct in the synthesis of magnesite (MgCO3) . Hydromagnesite and magnesite, respectively, were occasionally found to be associated with dolomite (South Australia) . It should be noted that some formulae only differ in one equivalent of crystal water, a difference that in some cases semi-quantitative methods are probably not able to detect reliably, and the formulae of dypingite and giorgiosite are the same (Mg5(CO3)4(OH)2·5H2O).
The formation of calcium phosphates on samples from the SBF immersion can be traced back to the composition of this medium, which corresponds to a supersaturated calcium phosphate system. Reflections of crystalline magnesium phosphates were not detected in this study. However, by the characterization methods applied, it cannot be excluded that certain amounts of Ca2+ were replaced with Mg2+. The formation of the calcium phosphates could also be responsible for the small degradation rate of Mg in SBF, as calcium phosphates are a well-known means to stabilize magnesium against too fast degradation .
Probability of degradation products determined by thermodynamical solubility calculations
This solution (HBSS), besides NaCl, also contains KCl, NaHCO3, KH2PO4 and Na2HPO4: the latter two components are included as pH buffering additives. The presence of phosphate ions makes possible the precipitation of bobierrite. During the increase in Mg concentration resulting from the corrosion process, the pH increases from 6.69 up to 8.63, where the solution is saturated in brucite; at this point, the solution is supersaturated in nesquehonite (S°(MC-n)=0.50) and in bobierrite (S°(TMPO)=1.27) (Figure 8). At saturation in nesquehonite, a lower pH of 8.28 is reached, the solution being then undersaturated in brucite (S°(MH)=–0.34), but still supersaturated in bobierrite (S°(TMPO)=1.11) (Figure 9). Although magnesite is less soluble than nesquehonite (Table 2), it is not observed experimentally. This suggests that, in the present reaction conditions, the nucleation and growth of nesquehonite is favored over that of magnesite. Likewise, the absence of experimentally observed bobierrite suggests that its spontaneous nucleation is inhibited in the present experimental conditions.
The solution, besides NaCl, also contains CaCl2, NaHCO3 and Na2HPO4; pH buffering is obtained by the presence of carbonates under p(CO2)=5%. The simultaneous presence of Ca2+ and HPO42– in the solution makes possible the precipitation of calcite, hydroxyapatite and bobierrite, which show positive initial values of S°(X) (Table 3). However, long lasting experience has shown that SBF solutions are metastable and can be kept for several weeks without any noticeable precipitation, probably due to the absence of spontaneous nucleation in the bulk of the solution. Only artificial nucleation, for instance in the presence of “bioactive” surfaces can trigger the formation of calcite or hydroxyapatite . The absence of bobierrite found in the present study also suggests that the nucleation of this precipitate is inhibited just like that of hydroxyapatite. During the increase in Mg concentration resulting from the corrosion process, the pH increases from 7.61 up to 8.64, where the solution is saturated in brucite; at this point, the solution is supersaturated in nesquehonite (S°(MC-n)=0.50), in calcite (S°(CC)=1.17), in hydroxyapatite (S°(HA)=0.99) and in bobierrite (S°(TMPO)=1.38) (Figure 8). At saturation in nesquehonite, a lower pH of 8.31 is reached, the solution being then undersaturated in brucite (S°(MH)=–0.34), but still supersaturated in calcite (S°(CC)=1.01), hydroxyapatite (S°(HA)=1.18) and bobierrite (S°(TMPO)=1.21) (Figure 9). Also in this case the formation of nesquehonite seems to be favored over magnesite. The observation, that calcium phosphates were only observed after 7 days’ immersion and in presence of 10% FBS shows that the presence of proteins may be one of the important factors under these experimental conditions.
Medium has an inorganic composition close to that of SBF (Table 1) but also contains many organics (e.g. glucose, amino-acids). The present thermodynamic calculations only simulate the behavior of DMEM solutions free of any organic additives. As for SBF the precipitation of calcite, hydroxyapatite and bobierrite is thermodynamically possible, since they show positive initial values of S°(X) (Table 3). As for SBF solutions, DMEM media seem to be metastable and can be stored for several weeks without any noticeable precipitation. During the increase in Mg concentration resulting from the corrosion process, the pH increases from 7.70 up to 8.64, where the solution is saturated in brucite; at this point, the solution is supersaturated in nesquehonite (S°(MC-n)=0.50), in calcite (S°(CC)=1.16), hydroxyapatite (S°(HA)=0.92) and bobierrite (S°(TMPO)=1.29) (Figure 8). At saturation in nesquehonite, a lower pH of 8.31 is reached, the solution being then undersaturated in brucite (S°(MH)=–0.34), but still supersaturated in calcite (S°(CC)=1.01), hydroxyapatite (S°(HA)=1.11) and bobierrite (S°(TMPO)=1.13) (Figure 9). Again, the absence of any noticeable precipitate besides nesquehonite and brucite suggests that magnesite, bobierrite, calcite and hydroxyapatite cannot nucleate in DMEM, in the experimental conditions of the present study.
A possible reaction scheme for the in vitro degradation of magnesium in different media, which is in line with earlier findings , is shown in Figure 10. It contains the often observed formation of magnesium hydroxide, but also takes into account the increased solubility of Mg(OH)2 in the presence of chlorides . When additional hydroxycarbonate or carbonate ions are present, as in our case due to the CO2 gassing, magnesium hydroxyl carbonate (hydrates) can be formed. It is noteworthy that we find Mg(OH)2 only in the case of degradation in HBSS which has the lowest salt carbonate concentration of the tested media. Possibly, in DMEM and SBF with higher carbonate concentrations, formed Mg(OH)2 is transformed (either directly or via dissolution in the presence of Cl–) to magnesium carbonate species. When the degradation solution contains high amounts of phosphate, as SBF, calcium phosphates may be formed which in turn may decelerate the degradation reaction. Also the presence of proteins, in our investigation modeled by the addition of FBS, will affect the degradation pathways. In addition to surface effects like the adsorption of proteins on magnesium or on degradation layers, proteins could also act directly, as some proteins are known to bind CO2 .
This study has shown that the conditions under which the degradation of pure magnesium is investigated strongly influence the degradation rate and the degradation products. We have chosen conditions which are close to cell-culture studies, including CO2 gassing and, in part of the experiments, addition of a widely used serum-supplement containing proteins. Degradation occurred fastest in HBSS, while the supplement with proteins led to a decrease of degradation in case of HBSS and DMEM, but not in case of SBF. As expected, brucite Mg(OH)2 could be found as one of the degradation products when HBSS was used. More surprisingly, nesquehonite (Mg(HCO3)(OH)·2H2O), was found to be the dominant, crystalline degradation product, formed within short terms in all media. Since it is likely that the additional carbonate source CO2 promoted the formation of this magnesium hydrogen carbonate, we propose that CO2 pressure ought to be taken into account in further in vitro studies. Also, the CO2 pressure of the intended in vivo environment should be considered.
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About the article
Published Online: 2016-04-29
Published in Print: 2016-09-01
Author’s statementConflict of interest: Authors state no conflict of interest.
Materials and methodsInformed consent: Informed consent has been obtained from all individuals included in this study.
Ethical approval: The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.