The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "Biomaterials are used in numerous medical applications today, such as fixation devices, replacements and surgical equipment. Implants are typical examples of a biomaterial application and there are several different implant materials used today. Many of these are however designed to stay in the body permanently even though they only serve their function temporarily. Even if the materials are biocompatible there are several complications associated with long term presence of implants, including allergy and sensitization. Many of these implants are only left in the body to eliminate risks concerning the removal process. Removing an implant usually involves surgery which increases both cost and patient morbidity. These negative consequences would be eliminated by using a biodegradable material. A completely biodegradable implant would dissolve and be absorbed by the body after the healing process is completed. Commonly used metallic implant materials include stainless steels, titanium alloys and cobalt-chromium alloys. These materials have great mechanical properties and are often used in load bearing applications. The mechanical properties of some common alloys can be seen in Table 1. However, many metallic corrosion products are harmful to the body and none of the implant metals used are biodegradable. Ceramic materials are known for their high strength and are generally biocompatible. Synthetic hydroxyapatite and other calcium phosphates as well as bioactive glass are commonly used materials for bone augmentation and bone replacement. They resemble the bone structure which gives good chemical bonding to bone and is therefore defined as bioactive. Alumina and zirconia are commonly used inert biomaterials. Ceramic coatings are frequently used on metallic implants to increase the biocompatibility and to induce bone ingrowth. The biggest disadvantage of ceramics is high brittleness, as can be seen in Table 1. There are numerous polymeric biomaterials used today, such as polyethylene (PE), polyvinylchloride (PVC), poly(methyl methacrylate) (PMMA) etcetera. However, all polymers have the disadvantage of low strength which eliminates their possibility to be used in load bearing applications, such as for example bone fixation devices.
"TABLE-US-00001 TABLE 1 Mechanical properties of magnesium, human bone and some commonly used biomaterials. Elastic Tensile modulus Density Yield strength strength (GPa) (g/cm.sup.3) (MPa) (MPa) Magnesium 45.sup.1 1.74.sup.1 70.sup.2 176.sup.2 Human cortical 5-23.sup.3 1.8-2.0.sup.3 106-224.sup.4 51-172.sup.4 bone (compressive) Stainless steel 190.sup.5 8.0.sup.3 300-1200.sup.5 480-620.sup.3 TI6Al4V 114.sup.1 4.43.sup.1 896.sup.1 1000.sup.1 Alumina 380.sup.4 3.95.sup.6 2260-2600.sup.6 270.sup.4 Bioactive glass 35.sup.3 -- 40-200.sup.2,3 Synthetic 73-117.sup.7 3.1.sup.7 600.sup.7 0.7.sup.7 hydroxyapatite (compressive) Biodegradable 12.8.sup.8 1.5.sup.9 -- 339-394.sup.9 PGA Biodegradable 1.2-3.sup.4 -- -- 28-48.sup.4 L-PLA The ranges of values are depending on testing conditions or anatomical location. References are compiled from different sources; .sup.1(
As a supplement to the background information on this patent application, NewsRx correspondents also obtained the inventors' summary information for this patent application: "There is no material used today that has the strength of a metal or ceramic material as well as biodegradable properties. Magnesium is potentially an excellent implant material due to its attractive mechanical properties and non-toxicity. It has a high corrosion rate, especially in chloride containing solutions, which means that it will degrade in the human body. If the corrosion rate can be controlled the material is a great candidate for use as a biodegradable implant.
"Magnesium alloys currently under investigation by researchers in the field for biomedical applications were originally designed for automotive and aerospace components with little consideration for their biocompatibility. As a result, most of the alloys currently being investigated contain toxic alloying elements. The inventors have sought to make a degradable implant material selecting the alloying elements for purposes of obtaining optimal mechanical functionality while maintaining biocompatibility. Calcium is an essential element for the human body and is non-toxic. Strontium is present in human bones and has been shown to promote osteoblast function and increase bone formation when added to hydroxyapatite, as compared to pure hydroxapatite. This creates the opportunity to develop metals that can completely dissolve within the body and that release dissolution products that are 100% biocompatible and enhance the biological processes in bone. In addition to their biological response, calcium and strontium are known to strengthen magnesium alloys while increasing their corrosion resistance. Controlling these elements and the corresponding microstructures that develop upon processing, our magnesium-based alloy can be designed with controllable degradation rates and mechanical properties. Hence, the inventors have shown that the magnesium-based alloy system containing calcium and strontium will produce promising results.
"Based on the research of inventors, it has been realized that magnesium alloys can be used in biomedical implant materials which will be advantageous over other materials as they can dissolve completely in the human body, while exhibiting the other desirous attributes of metal materials. The development of the alloy embodiments, has now enabled the development of medical devices that do not need additional surgeries for their removal. This greatly reduces the cost of treatment and patient morbidity. A magnesium-based alloy containing calcium and strontium is an improvement over other magnesium alloy systems being investigated as both calcium and strontium are elements present in bones and are biocompatible whereas the alloying elements being used in other studies are toxic. Thus, using magnesium alloy containing calcium and strontium greatly reduces the risk of potential toxicity by the degradation products being released from the medical device.
BRIEF DESCRIPTION OF THE DRAWINGS
"The advantages of this invention will be apparent upon consideration of the following detailed disclosure of the invention, especially when taken in conjunction with the accompanying drawings wherein:
"FIG. 1. Optical micrographs of a) Mg-0.5Ca-0.5Sr alloy, b) Mg-1.0Ca-0.5Sr alloy, c) Mg-1.0Ca-1.0Sr alloy, d) Mg-1.0Ca-2.0Sr alloy, and e) Mg-7.0Ca-3.5Sr alloy samples. The alloys show the characteristic dendritic structure associated with as-cast alloys. The darker regions are the Ca and Sr rich dendrites whereas the light regions are .alpha.-Mg regions.
"FIG. 2. SEM images of a-b) Mg-0.5Ca-0.5Sr alloy, c-d) Mg-1.0Ca-0.5Sr alloy, and e-f) Mg-1.0Ca-1.0Sr alloy. The (b), (d) and (f) pictures show the magnified images of the area in the squares and identify the phases present. Phase A is Mg17Sr2, Phase B is the Mg2Ca present in the eutectic and Phase C is the .alpha.-Mg phase.
"FIG. 3. XRD patterns of a) Mg-0.5Ca-0.5Sr alloy, b) Mg-1.0Ca-0.5Sr alloy, and c) Mg-1.0Ca-1.0Sr alloy samples. All three alloys display the same phases: .alpha.-Mg, Mg2Ca and Mg17Sr2.
"FIG. 4. Hydrogen evolution volumes of alloys immersed in Hank's solution. High purity Mg (99.95%) is shown for comparison.
"FIG. 5. SEM image and XRD pattern of the corroded surface of Mg-1.0Ca-0.5Sr alloy. The large striations on the surface of the samples are due to polishing effects during sample preparation. The microcracks, striations and corrosion products are labelled accordingly. It is apparent in this figure there is a significant number of microcracks forming on the sample surface.
"FIG. 6. SEM image and XRD pattern of the corroded surface of Mg-1.0Ca-1.0Sr alloy. The amount of corrosion products on the surface is significantly greater than that of Mg-1Ca-0.5Sr. The corrosion products, microcracks and holes in the surface layer are labeled. It can be seen that the holes run deep through the surface layer and can assist in the flow of media to unprotected surface beneath the corrosion layer. The XRD shows the presence of Mg(OH)2 and (Mg,Ca)3(PO4)2 phosphate on the surface of the corroded sample.
"FIG. 7. Toxicity on MC3T3-E1 cells expressed as a percentage of dead cells for different alloys after culturing in 10% alloy extraction media on day 3, 10% alloy extraction media on day 5, 50% alloy extraction media on day 3, and 50% alloy extraction media on day 5.
"FIG. 8. Optical Morphologies of MC3T3-E1 cells cultured in 50% concentration of a-b) Mg-0.5Ca-0.5Ca c-d) Mg-1.0Ca-0.5Sr and e-f) Mg-1.0Ca-1.0Sr alloy extracts respectively, after 3 and 5 days of culturing.
"FIG. 9. Alloy extract ion concentrations of a) Mg, b) Ca and c) Sr. The columns show the average value of five measurements on each sample with error bars showing .+-.1 standard deviation.
"FIG. 10: Optical micrographs of solution treated alloys (a) Mg-0.5Sr (b) Mg-1.0Sr © Mg-1.5Sr
"FIG. 11: Vickers microhardness of the binary Mg--Sr alloys
"FIG. 12: Hydrogen evolution plot"
For additional information on this patent application, see: Manuel,
Keywords for this news article include: Alloys, Magnesium, Light Metals,
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