Patent Application
20080033530
According to one exemplary embodiment, the marker alloy is distinguished by (i) its low melting point (approximately 450° C. to 800° C. for the specified alloy compositions) and special suitability for typical thermal processing methods, such as soldering or laser welding, (ii) a homogeneous microstructure without intermetallic phases, which simplifies processability, and (iii) (at least partial) biocorrodibility. Both a homogeneous structure (mixed crystal) and the occurrence of intermetallic phases may be controlled by suitable selection of the production parameters. The production parameters essentially include, but are not limited to, the composition of the melt, the temperature of the marker melt and of the substrate, the surrounding atmosphere (inert, e.g., vacuum or argon gas; reactive, e.g., nitrogen) and pressure, and the cooling rate and further following heat treatment measures, which are, in turn, essentially characterized by the temperature and heating and cooling rates and the surrounding atmosphere.
Preferably, x equals 25 to 40 atomic percent and y equals 60 to 75 atomic percent, and especially preferably x equals 28 to 35 atomic percent and y equals 65 to 72 atomic percent. Particularly preferably, the marker alloy corresponds to the composition Mg31.5Yb68.5. It has been shown that marker alloys of the cited compositions have a sufficiently high mean mass absorption coefficient for the medical technology x-ray energy range of 80 keV to 100 keV and a melting temperature which is below the melting point of the biocorrodible magnesium alloys used up to this point for the main body of the implant. Furthermore, marker alloys of the cited composition are also stable for a sufficiently long time in aqueous or physiological solution for the intended purposes.
The addition of the component M is optional and is particularly used for lowering the melting point of the outlet. Preferably z equals 3 to 8 atomic percent.
The alloy composition Mg31.5Yb68.5 is a eutectic mixture, whose melting point is approximately 496° C., while, for example, the biocorrodible magnesium alloy WE43 has a melting point of approximately 590° C. A required material thickness of 51 μm for an attenuation of the intensity to the factor 0.86 may be calculated from the density of this marker alloy (5.9 g/cm3) and its mean mass absorption coefficient in the energy range from 80 to 100 keV (5.98 cm2/g). This value is significantly less than the wall thickness of typical magnesium stents. The cited factor corresponds to an attenuation coefficient as is observed in gold-coated steel stents, a thickness of the steel being 70 μm and a thickness of the gold coating being 14 μm. In other words, a material thickness of the marker alloy was calculated which is necessary to obtain the same intensity attenuation as in the steel/gold composite and the ascertained value of 51 μm illustrates that this marker alloy is suitable for the filigree structures of stents.
A special feature of the marker alloy is that the electronegativity of ytterbium is less than that of magnesium, so that an acceleration of the corrosion of the main body in the contact area to the marker material by the formation of local elements is prevented.
The biocorrodible metallic material is preferably, but not exclusively, a biocorrodible alloy selected from the group of elements consisting of magnesium, iron, and tungsten, in particular, a biocorrodible magnesium alloy, such as WE43. The cited elements are provided in the alloy as the main component, i.e., the mass proportion is greatest in comparison to the other elements present in the alloy. The mass proportion of the cited elements in the biocorrodible alloys is preferably more than 50 weight-percent, in particular, more than 70 weight-percent.
A biocorrodible magnesium alloy of the composition rare earth metals 5.2-9.9 weight-percent, yttrium 3.7-5.5 weight-percent, and the remainder less than 1 weight-percent, magnesium making up the proportion of the alloy to 100 weight-percent, is especially suitable as the implant material. This magnesium alloy has already confirmed its special suitability experimentally and in initial clinical trials, i.e., the magnesium alloy displays a high biocompatibility, favorable processing properties, good mechanical characteristics, and corrosion behavior adequate for the intended uses. For purposes of the present disclosure, the collective term “rare earth metals” includes scandium (21), yttrium (39), lanthanum (57) and the 14 elements following lanthanum (57), namely cerium (58), praseodymium (59), neodymium (60), promethium (61), samarium (62), europium (63), gadolinium (64), terbium (65), dysprosium (66), holmium (67), erbium (68), thulium (69), ytterbium (70), and lutetium (71).
The biocorrodible alloys of the elements magnesium, iron, or tungsten are to be selected in composition in such a way that the elements are biocorrodible. For purposes of the present disclosure, alloys are referred to as biocorrodible when degradation occurs in a physiological environment, which finally results in the entire implant or the part of the implant made of the material losing its mechanical integrity. Artificial plasma, as has been previously described according to EN ISO 10993-15:2000 for biocorrosion assays (composition NaCl 6.8 g/l, CaCl2 0.2 g/l, KCl 0.4 g/l, MgSO4 0.1 g/l, NaHCO3 2.2 g/l, Na2HPO4 0.126 g/l, NaH2PO4 0.026 g/l), is used as a testing medium for testing the corrosion behavior of an alloy under consideration. For this purpose, a sample of the alloy to be assayed is stored in a closed sample container with a defined quantity of the testing medium at 37° C. At time intervals, tailored to the corrosion behavior to be expected, of a few hours up to multiple months, the sample is removed and examined for corrosion traces by techniques known to those skilled in the art. The artificial plasma according to EN ISO 10993-15:2000 corresponds to a medium similar to blood and represents a possibility for reproducibly simulating a physiological environment.
The x-ray marker is preferably provided in solid embodiment as a solid material. Alternatively, the x-ray marker may also be embedded as a powder in an inorganic carrier matrix.
The implant is preferably a stent, in particular, made of a magnesium or iron alloy (e.g., the magnesium alloy WE43). There is a significant need for marker materials, which result from the special requirements for the design and material of the stent.
In an exemplary embodiment, the implant is produced from the marker material.
An alloy was produced by joint melting the alloy components in a graphite or boron nitride crucible, concretely by joint melting of 31.5 atomic percent magnesium and 68.5 atomic present ytterbium. Because both magnesium and ytterbium have a very high tendency to oxidize and low vaporization enthalpies, the melting process was performed under protective gas and with slight overpressure.
A stent made of the magnesium alloy WE43 (containing 93 weight-percent magnesium, 4 weight-percent yttrium (W) and 3 weight-percent rare earth metals besides yttrium (E)) was immersed on both sides at the ends up to a depth of approximately 1 mm and for 1-2 seconds in a melt made of Mg31.5Yb68.5 and was subsequently cooled. The cooled layer made of the marker material was approximately 50 μm thick.
All patents, patent applications and publications referred to herein are incorporated by reference in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 038 237.4 | Aug 2006 | DE | national |
