Patent Publication Number: US-2012046734-A1

Title: Medical devices and methods of making the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority under 35 U.S.C. §120 to U.S. application Ser. No. 11/855,019, filed on Sep. 13, 2007, which is a non-provisional of U.S. Provisional Application Ser. No. 60/845,046, filed on Sep. 15, 2006, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to medical devices, such as, for example, endoprostheses, and to related methods. 
     BACKGROUND 
     The body includes various passageways such as arteries, other blood vessels, and other body lumens. These passageways sometimes become occluded or weakened. For example, the passageways can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, a passageway can be reopened or reinforced, or even replaced, with a medical endoprosthesis. An endoprosthesis is typically a tubular member that is placed in a lumen in the body. Examples of endoprostheses include stents, stent-grafts, and covered stents. 
     An endoprosthesis can be delivered inside the body by a catheter that supports the endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. Upon reaching the site, the endoprosthesis is expanded, for example, so that it can contact the walls of the lumen. 
     The expansion mechanism may include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries a balloon-expandable endoprosthesis. The balloon can be inflated to deform and to fix the expanded endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter withdrawn. 
     In another delivery technique, the endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded (e.g., elastically or through a material phase transition). During introduction into the body, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force. 
     To support a passageway and keep the passageway open, endoprostheses are sometimes made of relatively strong materials, such as stainless steel or Nitinol (a nickel-titanium alloy), formed into struts or wires. 
     SUMMARY 
     In one aspect, the invention features an endoprosthesis including a generally tubular member having a lumen and including at least one component selected from struts, bands, and combinations thereof. The component includes a bioerodible material selected from bioerodible metals, bioerodible metal alloys, and combinations thereof. The component has a first region including pores and a second region including pores, and the average maximum dimension (e.g., diameter) of the pores in the second region is greater than the average maximum dimension (e.g., diameter) of the pores in the first region. 
     In another aspect, the invention features an endoprosthesis including a generally tubular member having a lumen and including at least one component selected from struts, bands, and combinations thereof. The component includes a bioerodible material selected from bioerodible metals, bioerodible metal alloys, and combinations thereof. The component has a first region and a second region having a higher pore density than the first region. 
     In an additional aspect, the invention features an endoprosthesis including a generally tubular member having a lumen. The generally tubular member includes at least one component selected from struts, bands, and combinations thereof. The component includes a bioerodible material selected from bioerodible metals, bioerodible metal alloys, and combinations thereof. The component has at least one pore, and the endoprosthesis includes a polymer that is disposed within the pore. 
     In a further aspect, the invention features an endoprosthesis including a generally tubular member having a first region including pores and a second region including pores. The first region defines an interior surface of the generally tubular member, and the second region defines an exterior surface of the generally tubular member. The average maximum dimension of the pores in the second region is greater than the average maximum dimension of the pores in the first region. The generally tubular member includes a bioerodible material selected from bioerodible metals, bioerodible metal alloys, and combinations thereof. 
     In another aspect, the invention features an endoprosthesis including a generally tubular member. The generally tubular member has a first region defining an interior surface of the generally tubular member and a second region defining an exterior surface of the generally tubular member. The second region has a higher pore density than the first region. The generally tubular member includes a bioerodible material selected from bioerodible metals, bioerodible metal alloys, and combinations thereof. 
     In an additional aspect, the invention features an endoprosthesis including a generally tubular member and a polymer. The generally tubular member has at least one pore, and the polymer is disposed within the pore. The generally tubular member includes a bioerodible material selected from bioerodible metals, bioerodible metal alloys, and combinations thereof. 
     In a further aspect, the invention features a method including delivering an endoprosthesis into a lumen of a subject. The endoprosthesis includes a generally tubular member including a therapeutic agent and a bioerodible material selected from bioerodible metals, bioerodible metal alloys, and combinations thereof. The generally tubular member erodes at an erosion rate and the therapeutic agent elutes into the lumen of the subject at an elution rate. The elution rate is slower than the erosion rate. 
     In another aspect, the invention features an endoprosthesis including a generally tubular member having a lumen. The generally tubular member includes at least one component selected from struts, bands, and combinations thereof. The component includes a reservoir surrounded by a matrix including a bioerodible material and having at least one pore. The bioerodible material is selected from bioerodible metals, bioerodible metal alloys, and combinations thereof. 
     Embodiments can include one or more of the following features. 
     The first and/or second region of the component and/or the generally tubular member can include at least one pore (e.g., multiple pores). The average maximum dimension of the pores in the second region can be different from (e.g., greater than) the average maximum dimension of the pores in the first region. In some embodiments, the average maximum dimension of the pores in the second region can be at least about 1.5 times greater (e.g., at least about two times greater, at least about five times greater, at least about 10 times greater) than the average maximum dimension of the pores in the first region. 
     The endoprosthesis can include a therapeutic agent. The reservoir can contain a therapeutic agent. The endoprosthesis can include a polymer (e.g., a bioerodible polymer). The polymer can be supported by the component and/or the generally tubular member. The polymer can be disposed within pores of the component and/or the generally tubular member. In some embodiments, the polymer can be disposed within at least one pore (e.g., multiple pores) in the first region and/or the second region of the component and/or the generally tubular member. In certain embodiments, the endoprosthesis can include a composite including a therapeutic agent and a polymer. 
     The generally tubular member can have an exterior surface and an interior surface that defines the lumen of the generally tubular member. In some embodiments, the first region of the component can define at least a portion of the interior surface of the generally tubular member. In certain embodiments, the second region of the component can define at least a portion of the exterior surface of the generally tubular member. 
     The pore density of the second region of the component and/or the generally tubular member can be different from (e.g., higher than) the pore density of the first region of the component and/or the generally tubular member. In some embodiments, the pore density of the second region can be at least about 1.5 times higher (e.g., at least about two times higher, at least about five times higher, at least about 10 times higher) than the pore density of the first region. 
     In certain embodiments, the first and/or second regions of the component and/or the generally tubular member may not include any pores. 
     Embodiments can include one or more of the following advantages. 
     In some embodiments, a medical device (e.g., an endoprosthesis) including a bioerodible material can be used to temporarily treat a subject without permanently remaining in the body of the subject. For example, the medical device can be used for a certain period of time (e.g., to support a lumen of a subject), and then can erode after that period of time is over. 
     In certain embodiments, a medical device (e.g., an endoprosthesis) including a bioerodible metal and/or a bioerodible metal alloy can be relatively strong and/or can have relatively high structural integrity, while also having the ability to erode after being used at a target site. 
     In some embodiments, a medical device (e.g., an endoprosthesis) including a bioerodible material and having regions with different pore densities and/or with pores having different average maximum dimensions can erode at different rates in the different regions. In certain embodiments, a medical device can be designed to erode at a faster rate in some regions than in other regions. For example, an endoprosthesis can be designed so that its end regions erode at a faster rate than its center region. The result can be that the endoprosthesis erodes as one piece, starting at its end regions and progressing toward its center region. 
     In some embodiments, a medical device (e.g., an endoprosthesis) that includes a bioerodible material can also include at least one other material that is either bioerodible or non-bioerodible. The other material can, for example, enhance the strength and/or structural integrity of the medical device. 
     In certain embodiments, a medical device (e.g., an endoprosthesis) can provide a controlled release of one or more therapeutic agents into the body of a subject. For example, in some embodiments in which a medical device includes a bioerodible material and a therapeutic agent, the erosion of the bioerodible material can result in the release of the therapeutic agent over a period of time. 
     In some embodiments, a medical device (e.g., an endoprosthesis) having regions with different pore densities and/or with pores having different average maximum dimensions can deliver therapeutic agents at different rates and/or in different amounts from the different regions. For example, a region of an endoprosthesis having a relatively high pore density and/or having pores with a relatively high average maximum dimension may deliver therapeutic agent at a faster rate, and/or may deliver a greater total volume of therapeutic agent, than another region of the endoprosthesis having a relatively low pore density and/or having pores with a relatively low average maximum dimension. In certain embodiments, one region of a medical device can be designed to deliver more therapeutic agent, and/or to deliver therapeutic agent at a faster rate, than another region of the medical device. For example, a region of an endoprosthesis that is located along an outer diameter of the endoprosthesis can be designed to deliver a greater volume of therapeutic agent, and/or to deliver therapeutic agent at a faster rate, than a region of the endoprosthesis that is located along an inner diameter of the endoprosthesis. 
     In certain embodiments, a medical device (e.g., an endoprosthesis) having regions with different pore densities and/or with pores having different average maximum dimensions can deliver different therapeutic agents from the different regions. As an example, in some embodiments, a region of an endoprosthesis having a relatively high pore density and including pores having a relatively high average maximum dimension can deliver a therapeutic agent at a relatively fast rate, while another region of the endoprosthesis having a relatively low pore density and including pores having a relatively low average maximum dimension can be used to deliver a different therapeutic agent at a relatively slow rate. 
     In some embodiments in which a medical device (e.g., an endoprosthesis) includes both a bioerodible material (e.g., a bioerodible metal) and a therapeutic agent, the erosion rate of the bioerodible material can be independent of the elution rate of the therapeutic agent. As an example, in certain embodiments, a medical device can be formed of a porous bioerodible metal, and can include a composite including a bioerodible polymer combined with a therapeutic agent that is disposed within the pores of the bioerodible metal. As the polymer erodes, it can release the therapeutic agent at a rate that is different from the erosion rate of the bioerodible metal. In certain embodiments, the bioerodible metal can erode before all of the therapeutic agent has been released from the polymer. The remaining polymer can continue to elute the therapeutic agent. The therapeutic agent can be selected, for example, to help alleviate the effects, if any, of the erosion of the bioerodible metal on the body of the subject. 
     In some embodiments, a medical device (e.g., an endoprosthesis) including one or more metals (e.g., bioerodible metals) can be relatively radiopaque. This radiopacity can give the medical device enhanced visibility under X-ray fluoroscopy. Thus, the position of the medical device within the body of a subject may be able to be determined relatively easily. 
     An erodible or bioerodible endoprosthesis, e.g., a stent, refers to a device, or a portion thereof, that exhibits substantial mass or density reduction or chemical transformation, after it is introduced into a patient, e.g., a human patient. Mass reduction can occur by, e.g., dissolution of the material that forms the device and/or fragmenting of the device. Chemical transformation can include oxidation/reduction, hydrolysis, substitution, and/or addition reactions, or other chemical reactions of the material from which the device, or a portion thereof, is made. The erosion can be the result of a chemical and/or biological interaction of the device with the body environment, e.g., the body itself or body fluids, into which it is implanted and/or erosion can be triggered by applying a triggering influence, such as a chemical reactant or energy to the device, e.g., to increase a reaction rate. For example, a device, or a portion thereof, can be formed from an active metal, e.g., Mg or Ca or an alloy thereof, and which can erode by reaction with water, producing the corresponding metal oxide and hydrogen gas (a redox reaction). For example, a device, or a portion thereof, can be formed from an erodible or bioerodible polymer, or an alloy or blend erodible or bioerodible polymers which can erode by hydrolysis with water. The erosion occurs to a desirable extent in a time frame that can provide a therapeutic benefit. For example, in embodiments, the device exhibits substantial mass reduction after a period of time which a function of the device, such as support of the lumen wall or drug delivery is no longer needed or desirable. In particular embodiments, the device exhibits a mass reduction of about 10 percent or more, e.g. about 50 percent or more, after a period of implantation of one day or more, e.g. about 60 days or more, about 180 days or more, about 600 days or more, or 1000 days or less. In embodiments, the device exhibits fragmentation by erosion processes. The fragmentation occurs as, e.g., some regions of the device erode more rapidly than other regions. The faster eroding regions become weakened by more quickly eroding through the body of the endoprosthesis and fragment from the slower eroding regions. The faster eroding and slower eroding regions may be random or predefined. For example, faster eroding regions may be predefined by treating the regions to enhance chemical reactivity of the regions. Alternatively, regions may be treated to reduce erosion rates, e.g., by using coatings. In embodiments, only portions of the device exhibits erodibilty. For example, an exterior layer or coating may be erodible, while an interior layer or body is non-erodible. In embodiments, the endoprosthesis is formed from an erodible material dispersed within a non-erodible material such that after erosion, the device has increased porosity by erosion of the erodible material. 
     Erosion rates can be measured with a test device suspended in a stream of Ringer&#39;s solution flowing at a rate of 0.2 m/second. During testing, all surfaces of the test device can be exposed to the stream. For the purposes of this disclosure, Ringer&#39;s solution is a solution of recently boiled distilled water containing 8.6 gram sodium chloride, 0.3 gram potassium chloride, and 0.33 gram calcium chloride per liter. 
     Other aspects, features, and advantages are in the description, drawings, and claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a perspective view of an embodiment of a stent in a compressed condition. 
         FIG. 1B  is a perspective view of the stent of  FIG. 1A , in an expanded condition. 
         FIG. 1C  is a cross-sectional view of the stent of  FIG. 1A , taken along line  1 C- 1 C. 
         FIG. 1D  is an enlarged view of region  1 D of the stent of  FIG. 1C . 
         FIG. 2A  is a cross-sectional view of an embodiment of a stent. 
         FIG. 2B  is an enlarged view of region  2 B of the stent of  FIG. 2A . 
         FIG. 3  is a cross-sectional view of an embodiment of a stent. 
         FIG. 4A  is a perspective view of an embodiment of a stent. 
         FIG. 4B  is a cross-sectional view of the stent of  FIG. 4A , taken along line  4 B- 4 B. 
         FIG. 5  is a cross-sectional view of an embodiment of a stent. 
         FIG. 6A  is a perspective view of an embodiment of a stent. 
         FIG. 6B  is a cross-sectional view of the stent of  FIG. 6A , taken along line  6 B- 6 B. 
         FIG. 7  is a cross-sectional view of an embodiment of a stent. 
         FIG. 8A  is a perspective view of an embodiment of a stent. 
         FIG. 8B  is an enlarged view of region  8 B of the stent of  FIG. 8A . 
         FIG. 8C  is a cross-sectional view of region  8 B of  FIG. 8B , taken along line  8 C- 8 C. 
         FIG. 9  is a cross-sectional view of an embodiment of a component of a stent. 
         FIG. 10A  is a perspective view of an embodiment of a stent. 
         FIG. 10B  is a cross-sectional view of the stent of  FIG. 10A , taken along line  10 B- 10 B. 
         FIG. 11A  is a perspective view of an embodiment of a stent. 
         FIG. 11B  is a cross-sectional view of the stent of  FIG. 11A , taken along line  11 B- 11 B. 
         FIG. 12A  is a perspective view of an embodiment of a stent. 
         FIG. 12B  is a cross-sectional view of the stent of  FIG. 12A , taken along line  12 B- 12 B. 
         FIG. 13A  is a perspective view of an embodiment of a stent. 
         FIG. 13B  is an enlarged view of region  13 B of the stent of  FIG. 13A . 
         FIG. 13C  is a cross-sectional view of region  13 B of  FIG. 13B , taken along line  13 C- 13 C. 
         FIG. 14A  is a perspective view of an embodiment of a stent. 
         FIG. 14B  is a cross-sectional view of the stent of  FIG. 14A , taken along line  14 B- 14 B. 
         FIG. 15A  is a perspective view of an embodiment of a stent. 
         FIG. 15B  is a cross-sectional view of the stent of  FIG. 15A , taken along line  15 B- 15 B. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  shows a stent  10  including a generally tubular member  12  capable of supporting a body lumen and having a longitudinal axis A-A and a lumen  13 . Generally tubular member  12  includes apertures  14  that are provided in a pattern to facilitate stent functions (e.g., radial expansion) and lateral flexibility.  FIG. 1A  shows stent  10  in a compressed condition, such that stent  10  has a relatively small diameter D c  suitable for delivery into a lumen of a subject. As shown in  FIG. 1B , once stent  10  has been delivered into a lumen of a subject, stent  10  is expanded to a larger diameter, D exp . This larger diameter can allow stent  10  to contact the walls of the lumen. In some embodiments, a stent such as stent  10  can be expanded by a mechanical expander (e.g., an inflatable balloon). 
       FIG. 1C  shows a cross-sectional view of stent  10 . As shown in  FIG. 1C , generally tubular member  12  has in interior surface  15  and an exterior surface  17 , and is formed of a metal matrix  16  including pores  18 . Pores  18  can form an open pore system (in which different pores  18  are interconnected) or a closed pore system (in which different pores  18  are not interconnected). In certain embodiments, some pores  18  can be interconnected, and other pores  18  may not be interconnected. While pores  18  are shown as having an irregular cross-sectional shape, in some embodiments, the pores in a metal matrix can have one or more other cross-sectional shapes. For example, a pore in a metal matrix can be circular, oval (e.g., elliptical), and/or polygonal (e.g., triangular, square) in cross-section. 
     Metal matrix  16  includes (e.g., is formed of) one or more bioerodible metals and/or bioerodible metal alloys. In some embodiments (e.g., some embodiments in which metal matrix  16  is formed entirely of bioerodible metals and/or bioerodible metal alloys), generally tubular member  12  is bioerodible. In certain embodiments, generally tubular member  12  can erode after stent  10  has been used at a target site. 
     As shown in  FIGS. 1C and 1D , different regions of generally tubular member  12  have different pore densities and/or include pores having different average maximum dimensions. As used herein, the pore density of a region is equal to the number of pores per square centimeter in that region. As an example,  FIG. 1D  shows a portion of generally tubular member  12  that has been divided by a line L 1  into a region R 1  and a region R 2 . Region R 1  has a lower pore density than region R 2 , and also has pores with a lower average maximum dimension than the pores in region R 2 . 
     The variation in pore density and in the average maximum dimension of pores in different regions of generally tubular member  12  can be designed, for example, to result in a particular pattern and/or rate of erosion by generally tubular member  12 . Typically, as the pore density and/or average maximum dimension of the pores in a region of generally tubular member  12  increases, the erosion rate of that region can also increase. Without wishing to be bound by theory, it is believed that as the pore density and/or average pore volume of a region of generally tubular member  12  increases, the surface area of bioerodible material in that region that is exposed to blood and/or other body fluids (e.g., at a target site) can also increase. As a result, region R 2  of generally tubular member  12 , with its relatively high pore density and with its pores having a relatively high average maximum dimension, may erode at a faster rate than region R 1  of generally tubular member  12 , with its relatively low pore density and with its pores having a relatively low average maximum dimension. 
     In some embodiments, a medical device (e.g., stent  10 ) or a component of a medical device (e.g., generally tubular member  12 ) that is formed of one or more bioerodible materials can substantially erode (can exhibit a mass reduction of about 95 percent or more) over a period of at least about five days (e.g., at least about seven days, at least about 14 days, at least about 21 days, at least about 28 days, at least about 30 days, at least about six weeks, at least about eight weeks, at least about 12 weeks, at least about 16 weeks, at least about 20 weeks, at least about six months, at least about 12 months). In some embodiments in which a medical device includes one or more radiopaque materials, the erosion of the medical device within the body of a subject can be monitored using X-ray fluoroscopy. In certain embodiments, the erosion of a medical device within the body of a subject can be monitored using intravascular ultrasound. 
     In some embodiments, region R 1  can have a pore density of at least about 100 pores per square centimeter (e.g., at least about 500 pores per square centimeter, at least about 1000 pores per square centimeter, at least about 10 4  pores per square centimeter, at least about 10 5  pores per square centimeter, at least about 10 6  pores per square centimeter, at least about 10 7  pores per square centimeter, at least about 10 8  pores per square centimeter) and/or at most about 10 9  pores per square centimeter (e.g., at most about 10 8  pores per square centimeter, at most about 10 7  pores per square centimeter, at most about 10 6  pores per square centimeter, at most about 10 5  pores per square centimeter, at most about 10 4  pores per square centimeter, at most about 1000 pores per square centimeter, at most about 500 pores per square centimeter). In certain embodiments, region R 2  can have a pore density of at least about 100 pores per square centimeter (e.g., at least about 500 pores per square centimeter, at least about 1000 pores per square centimeter, at least about 10 4  pores per square centimeter, at least about 10 5  pores per square centimeter, at least about 10 6  pores per square centimeter, at least about 10 7  pores per square centimeter, at least about 10 8  pores per square centimeter) and/or at most about 10 9  pores per square centimeter (e.g., at most about 10 8  pores per square centimeter, at most about 10 7  pores per square centimeter, at most about 10 6  pores per square centimeter, at most about 10 5  pores per square centimeter, at most about 10 4  pores per square centimeter, at most about 1000 pores per square centimeter, at most about 500 pores per square centimeter). In some embodiments, the pore density of region R 2  can be at least about 1.5 times greater (e.g., at least about two times greater, at least about five times greater, at least about 10 times greater, at least about 25 times greater, at least about 50 times greater, at least about 75 times greater), and/or at most about 100 times greater (e.g., at most about 75 times greater, at most about 50 times greater, at most about 25 times greater, at most about 10 times greater, at most about five times greater, at most about two times greater), than the pore density of region R 1 . While  FIG. 1D  shows both region R 1  and region R 2  as including pores  18 , in certain embodiments, a generally tubular member such as generally tubular member  12  can have one or more regions that do not include any pores. 
     In some embodiments, the average maximum dimension (e.g., diameter, length, width) of the pores in region R 1  can be at least 0.01 micron (e.g., at least 0.05 micron, at least about 0.1 micron, at least about 0.5 micron, at least about one micron, at least about five microns) and/or at most about 10 microns (e.g., at most about five microns, at most about one micron, at most about 0.5 micron, at most about 0.1 micron, at most 0.05 micron). In certain embodiments, the average maximum dimension (e.g., diameter, length, width) of the pores in region R 2  can be at least 0.01 micron (e.g., at least 0.05 micron, at least about 0.1 micron, at least about 0.5 micron, at least about one micron, at least about five microns) and/or at most about 10 microns (e.g., at most about five microns, at most about one micron, at most about 0.5 micron, at most about 0.1 micron, at most 0.05 micron). In some embodiments, the average maximum dimension of the pores in region R 2  can be at least about 1.5 times greater (e.g., at least about five times greater, at least about 10 times greater, at least about 25 times greater, at least about 50 times greater, at least about 75 times greater), and/or at most about 100 times greater (e.g., at most about 75 times greater, at most about 50 times greater, at most about 25 times greater, at most about 10 times greater, at most about five times greater), than the average maximum dimension of the pores in region R 1 . 
     The bioerodible materials that are included in a medical device can include one or more metals and/or one or more metal alloys. Examples of bioerodible metals include alkali metals, alkaline earth metals (e.g., magnesium), iron, zinc, and aluminum. As used herein, a metal alloy refers to a substance that is composed of two or more metals or of a metal and a nonmetal intimately united, for example, by being fused together and dissolving in each other when molten. Examples of bioerodible metal alloys include alkali metal alloys, alkaline earth metal alloys (e.g., magnesium alloys), iron alloys (e.g., alloys including iron and up to seven percent carbon), zinc alloys, and aluminum alloys. Metal matrix  16  of generally tubular member  12  can include one metal or metal alloy, or can include more than one (e.g., two, three, four, five) metal or metal alloy. In some embodiments, metal matrix  16  can include one or more metals and one or more metal alloys. Bioerodible materials are described, for example, in Weber, U.S. Patent Application Publication No. US 2005/0261760 A1, published on Nov. 24, 2005, and entitled “Medical Devices and Methods of Making the Same”; Colen et al., U.S. Patent Application Publication No. US 2005/0192657 A1, published on Sep. 1, 2005, and entitled “Medical Devices”; Weber, U.S. patent application Ser. No. 11/327,149, filed on Jan. 5, 2006, and entitled “Bioerodible Endoprostheses and Methods of Making the Same”; Bolz, U.S. Pat. No. 6,287,332; Heublein, U.S. Patent Application Publication No. US 2002/0004060 A1, published on Jan. 10, 2002, and entitled “Metallic Implant Which is Degradable In Vivo”; and Park,  Science and Technology of Advanced Materials,  2, 73-78 (2001). 
     In some embodiments, stent  10  can include one or more therapeutic agents. As an example, stent  10  can include one or more therapeutic agents that are disposed within pores  18  of generally tubular member  12 . During delivery and/or use in a body of a subject, stent  10  can elute the therapeutic agents. For example, as generally tubular member  12  erodes, the therapeutic agents within pores  18  can be released into the body. The erosion of generally tubular member  12  can result in a relatively consistent release of therapeutic agent, as pores  18  continue to become exposed. 
     The variation in pore density and in the average maximum dimension of the pores in different regions of generally tubular member  12  can be designed, for example, to result in a particular pattern and/or rate of therapeutic agent elution from generally tubular member  12 . Typically, a region of generally tubular member  12  having a relatively high pore density and/or including pores with a relatively high average maximum dimension can elute therapeutic agent at a faster rate than a region of generally tubular member  12  having a relatively low pore density and/or including pores with a relatively low average maximum dimension. For example, region R 2  of generally tubular member  12  may elute therapeutic agent at a faster rate, and/or may elute a higher total volume of therapeutic agent, than region R 1 . 
     Examples of therapeutic agents include non-genetic therapeutic agents, genetic therapeutic agents, vectors for delivery of genetic therapeutic agents, cells, and therapeutic agents identified as candidates for vascular treatment regimens, for example, as agents targeting restenosis. In some embodiments, one or more therapeutic agents that are used in a medical device such as a stent can be dried (e.g., lyophilized) prior to use, and can become reconstituted once the medical device has been delivered into the body of a subject. A dry therapeutic agent may be relatively unlikely to come out of a medical device (e.g., a stent) prematurely, such as when the medical device is in storage. 
     Therapeutic agents are described, for example, in Weber, U.S. Patent Application Publication No. US 2005/0261760 A1, published on Nov. 24, 2005, and entitled “Medical Devices and Methods of Making the Same”, and in Colen et al., U.S. Patent Application Publication No. US 2005/0192657 A1, published on Sep. 1, 2005, and entitled “Medical Devices”. 
     Generally tubular member  12  of stent  10  can be formed by any of a number of different methods. In some embodiments, generally tubular member  12  can be formed by molding a mixture of a bioerodible metal and a second bioerodible material into a generally tubular shape, and exposing the generally tubular shape to a solvent that solvates the second bioerodible material (without also solvating the bioerodible metal), and/or to a temperature that causes the second bioerodible material to melt (without also causing the bioerodible metal to melt). When the second bioerodible material is solvated and/or when it melts, it can result in the formation of pores in the metal, thereby producing metal matrix  16 . 
     While a stent including regions having different pore densities and having pores with different average maximum dimensions has been described, in some embodiments, a stent can alternatively or additionally include regions having the same pore density and/or having pores with the same average maximum dimension. For example,  FIG. 2A  shows a cross-sectional view of a stent  100  including a generally tubular member  112 . Generally tubular member  112  has an interior surface  113 , an exterior surface  114 , and a lumen  115 , and is formed out of a metal matrix  116  formed of one or more bioerodible metals and/or bioerodible metal alloys. Metal matrix  116  includes pores  118 . 
       FIG. 2B  shows a portion of generally tubular member  112  that has been divided by a line L 2  into regions R 3  and R 4 . As shown in  FIG. 2B , regions R 3  and R 4  have the same pore density, and also include pores  118  having the same average maximum dimension. 
     While stents including generally tubular members formed out of a metal matrix and/or including a therapeutic agent have been described, in some embodiments, a stent can include one or more other materials. The other materials can be used, for example, to enhance the strength and/or structural support of the stent. Examples of other materials that can be used in conjunction with a metal matrix in a stent include metals (e.g., gold, platinum, niobium, tantalum), metal alloys, and/or polymers (e.g., styrene-isobutylene styrene (SIBS), poly(n-butyl methacrylate) (PBMA)). Examples of metal alloys include cobalt-chromium alloys (e.g., L 605 ), Elgiloy® (a cobalt-chromium-nickel-molybdenum-iron alloy), and niobium-1 Zr alloy. In some embodiments, a stent can include a generally tubular member formed out of a porous magnesium matrix, and the pores in the magnesium matrix can be filled with iron compounded with a therapeutic agent. 
     In certain embodiments, a stent can include both a bioerodible metal matrix and one or more additional bioerodible materials that are different from the bioerodible materials in the bioerodible metal matrix. For example, in some embodiments, a stent can include both a bioerodible metal matrix and one or more non-metallic bioerodible materials (e.g., starches, sugars). In certain embodiments, a stent can include a bioerodible metal matrix and one or more additional bioerodible materials that erode at a different rate from the bioerodible metal matrix. The additional bioerodible materials can be added to the bioerodible metal matrix to, for example, tailor the erosion rate of the stent. For example, in some embodiments, a stent can include a generally tubular member that is formed of a porous bioerodible metal matrix, and a bioerodible polymer can be disposed within some or all of the pores of the bioerodible metal matrix. For example,  FIG. 3  shows a cross-sectional view of a stent  200  including a generally tubular member  202 . Generally tubular member  202  has an exterior surface  204 , an interior surface  206 , and a lumen  208 , and is formed of a metal matrix  210  that is formed of one or more bioerodible metals and/or bioerodible metal alloys. Metal matrix  210  includes pores  212  that are filled with a bioerodible polymer  214 . Examples of bioerodible polymers include polyiminocarbonates, polycarbonates, polyarylates, polylactides, and polyglycolic esters. A stent including a metal matrix and a bioerodible polymer disposed within the pores of the metal matrix can be made, for example, by forming a generally tubular member out of a metal matrix (e.g., as described above), immersing the generally tubular member in a solution of the polymer, and allowing the solution to dry, so that the solvent in the solution evaporates, and the polymer is left behind on the stent. 
     In some embodiments, a stent can include both a bioerodible metal matrix and one or more materials that carry a therapeutic agent. For example, a stent can include a generally tubular member that is formed of a porous bioerodible metal matrix, and a polymer containing a therapeutic agent can be disposed within the pores of the metal matrix. The polymer can be non-bioerodible, or can be bioerodible. In some embodiments in which the polymer is bioerodible, the polymer can erode at a different rate from the metal matrix. As an example, in some embodiments, the polymer can erode at a faster rate than the metal matrix, causing all of the therapeutic agent to be released into the body before the generally tubular member has completely eroded. As another example, in certain embodiments, the polymer can erode at a slower rate than the metal matrix. The result can be that after the matrix has completely eroded, at least some of the therapeutic-agent containing polymer can remain in the body (e.g., in the form of polymeric particles). In some embodiments in which the stent has been delivered into a lumen of a subject, the polymer can be at least partially embedded in a wall of the lumen. As the polymer continues to erode, it can release the therapeutic agent into the body. Thus, the body can continue to be treated with the therapeutic agent, even after the generally tubular member has eroded. The therapeutic agent can be selected, for example, to alleviate the effects, if any, of the erosion of the stent on the body. By including a material (such as a polymer) containing a therapeutic agent, the stent can have a therapeutic agent elution rate that is independent of the erosion rate of its generally tubular member. 
     In certain embodiments, a stent can include one or more coatings on one or more surfaces of the stent. For example,  FIGS. 4A and 4B  show a stent  300  including a generally tubular member  302  defining a lumen  304 . Generally tubular member  302  is formed of a metal matrix  306  that is formed of one or more bioerodible metals and/or bioerodible metal alloys, and that includes pores  308 . Stent  300  further includes a coating  310  disposed on the exterior surface  312  of generally tubular member  302 . Coating  310  can be used, for example, to regulate therapeutic agent release from generally tubular member  302 . For example, pores  308  can contain one or more therapeutic agents, and coating  310  (e.g., which can be bioerodible) can be used to control the release of the therapeutic agent(s) from pores  308  (e.g., by delaying the release of the therapeutic agent(s) until stent  300  has reached a target site). 
     In certain embodiments, a stent can include a coating that contains a therapeutic agent or that is formed of a therapeutic agent. For example, a stent can include a coating that is formed of a polymer and a therapeutic agent. The coating can be applied to a generally tubular member of the stent by, for example, dip-coating the generally tubular member in a solution including the polymer and the therapeutic agent. Methods that can be used to apply a coating to a generally tubular member of a stent are described, for example, in U.S. Provisional Patent Application Ser. No. 60/844,967, filed concurrently herewith and entitled “Medical Devices”. 
     While a stent with one coating has been shown, in some embodiments, a stent can include more than one (e.g., two, three, four, five) coating. For example,  FIG. 5  shows a cross-sectional view of a stent  350  having a lumen  352 . Stent  350  includes a generally tubular member  353  formed of metal matrix  354  that is formed of one or more bioerodible metals and/or bioerodible metal alloys, and that includes pores  355 . Stent  350  also includes a coating  356  on the exterior surface  358  of generally tubular member  353 , and a coating  360  on the interior surface  362  of generally tubular member  353 . Coatings  356  and  360  can include one or more of the same materials, or can be formed of different materials. 
     Examples of coating materials that can be used on a stent include metals (e.g., tantalum, gold, platinum), metal oxides (e.g., iridium oxide, titanium oxide, tin oxide), and/or polymers (e.g., SIBS, PBMA). Coatings can be applied to a stent using, for example, dip-coating and/or spraying processes. 
     While stents including generally tubular members formed of a porous metal matrix have been described, in certain embodiments, a stent can alternatively or additionally include a coating that is formed of a porous metal matrix. For example,  FIGS. 6A and 6B  show a stent  400  having a lumen  402 . Stent  400  includes a generally tubular member  404  that is not formed of a porous metal matrix. Generally tubular member  404  can be formed of, for example, one or more metals (e.g., gold, platinum, niobium, tantalum), metal alloys, and/or polymers (e.g., SIBS, PBMA). Examples of metal alloys include cobalt-chromium alloys (e.g., L 605 ), Elgiloy® (a cobalt-chromium-nickel-molybdenum-iron alloy), and niobium-1 Zr alloy. Stent  400  further includes a coating  406  that is disposed on the exterior surface  408  of generally tubular member  404 . Coating  406  is formed of a metal matrix  410  that is formed of one or more bioerodible metals and/or bioerodible metal alloys, and that includes pores  412 . Metal matrix  410  can be used, for example, as a reservoir for one or more therapeutic agents. For example, one or more therapeutic agents can be disposed within pores  412  of metal matrix  410 . During and/or after delivery of stent  400  to a target site in a body of a subject, metal matrix  410  can erode, thereby eluting therapeutic agent into the body of the subject. 
     A coating such as coating  406  can be formed using, for example, one or more sintering and/or vapor deposition processes. 
     While coating  406  is shown as having a relatively uniform pore density and as including pores having a relatively uniform average maximum dimension, in some embodiments, a porous coating on a stent can have a non-uniform pore density and/or can include pores having a non-uniform average maximum dimension. For example,  FIG. 7  shows a cross-sectional view of a stent  450  including a generally tubular member  452  that is not formed of a porous metal matrix. Generally tubular member  452  can be formed of, for example, one or more metals (e.g., gold, platinum, niobium, tantalum), metal alloys, and/or polymers (e.g., SIBS, PBMA). Examples of metal alloys include cobalt-chromium alloys (e.g., L 605 ), Elgiloy® (a cobalt-chromium-nickel-molybdenum-iron alloy), and niobium-1 Zr alloy. Stent  400  further includes a coating  456  that is disposed on the exterior surface  458  of generally tubular member  452 . Coating  456  is formed of a metal matrix  460  that is formed of one or more bioerodible metals and/or bioerodible metal alloys, and that includes pores  462 . Metal matrix  460  can be used, for example, as a reservoir for one or more therapeutic agents. As shown in  FIG. 7 , coating  456  has an interior surface  464  and an exterior surface  466 . The pore density of metal matrix  460  is higher, and the average maximum dimension of pores  462  in metal matrix  460  is greater, in the regions of generally tubular member  452  that are closer to exterior surface  466  than in the regions of generally tubular member  42  that are closer to interior surface  464 . 
     While stents having certain configurations have been described, in some embodiments, a stent including one or more bioerodible metals and/or bioerodible metal alloys can have a different configuration. For example,  FIG. 8A  shows a stent  520  that is in the form of a generally tubular member  521  formed of one or more bioerodible metals and/or bioerodible metal alloys. Generally tubular member  521  is defined by a plurality of bands  522  and a plurality of connectors  524  that extend between and connect adjacent bands. Generally tubular member  521  has a lumen  523 . 
       FIG. 8B  shows an enlarged view of a connector  524  of stent  520 , and  FIG. 8C  shows a cross-sectional view of the connector of  FIG. 8B . As shown in  FIG. 8C , connector  524  is formed of a metal matrix  530  including pores  534 . Metal matrix  530  is formed of one or more bioerodible metals and/or bioerodible metal alloys. A line L 3  divides connector  524  into regions R 5  and R 6 . As shown in  FIG. 8C , region R 5  has a higher pore density than region R 6 , and the pores in region R 5  have a higher average maximum dimension than the pores in region R 6 . 
     During delivery and/or use of stent  520 , bands  522  and/or connectors  524  can erode. The presence of pores  534  in connectors  524  can help to accelerate and/or control the erosion of connectors  524 . In some embodiments, the presence of pores  534  in connectors  524  can result in connectors  524  eroding at a faster rate than bands  522 . In certain embodiments, it may be desirable for connectors  524  to completely erode before bands  522 , allowing stent  520  to move and flex within a target site (e.g., within a lumen in a body of a subject). By the time connectors  524  have completely eroded, tissue may have grown over the remaining parts of stent  520  (e.g., bands  522 ), thereby helping to hold bands  522  (and, therefore, stent  520 ) in place. 
     While a stent including connectors having regions with different pore densities and with pores having different average maximum dimensions has been described, in some embodiments, a stent can include one or more components having regions with relatively uniform pore densities and/or with pores having relatively uniform average maximum dimensions. For example,  FIG. 9  shows a cross-sectional view of a connector  550  of a stent. As shown in  FIG. 9 , connector  550  is formed of a metal matrix  554  including pores  558 . Metal matrix  554  is formed of one or more bioerodible metals and/or bioerodible metal alloys. A line L 4  divides connector  550  into regions R 7  and R 8 . As shown in  FIG. 9 , regions R 7  and R 8  have the same pore density and the pores in regions R 7  and R 8  have the same average maximum dimension. 
     While stents including connectors including pores have been described, in some embodiments, a stent can alternatively or additionally include one or more other components (e.g., bands) having pores. 
     While certain embodiments have been described, other embodiments are possible. 
     As an example, in some embodiments, a stent including a generally tubular member formed of a bioerodible metal can be manufactured using powder metallurgy methods. For example, a stent can be formed by sintering and compacting bioerodible metal particles and/or metal alloy particles into the shape of a generally tubular member. A metal particle or metal alloy particle can have a dimension (e.g., a width, a length, a diameter) of, for example, at least about 0.1 micron (e.g., at least about 0.5 micron, at least about one micron, at least about five microns) and/or at most about 10 microns (e.g., at most about five microns, at most about one micron, at most about 0.5 micron). Sintering the metal particles and/or the metal alloy particles can include exposing the metal particles and/or the metal alloy particles to heat and pressure to cause some coalescence of the particles. A generally tubular member that is formed by a sintering process can be porous or non-porous, or can include both porous regions and non-porous regions. In some embodiments in which the generally tubular member includes pores, the sizes of the pores can be controlled by the length of the sintering and compacting period, and/or by the temperature and/or pressure of the sintering process. Typically, as the temperature and/or pressure of a sintering process increases, the pore density of the resulting generally tubular member, and the average maximum dimension of the pores in the generally tubular member, can decrease. In certain embodiments, a generally tubular member can be formed by sintering metal particles and/or metal alloy particles having different sizes. 
     In certain embodiments in which a generally tubular member includes different regions having different pore densities and/or having pores with different average maximum dimensions, the generally tubular member can be formed using a sintering process employing thermal gradients. The sintering process can include exposing certain regions of the generally tubular member, as it is being formed, to higher temperatures than other regions of the generally tubular member. The regions that are exposed to higher temperatures ultimately can have relatively low pore densities and/or pores with relatively small average maximum dimensions, while the regions that are exposed to lower temperatures can have relatively high pore densities and/or pores with relatively large average maximum dimensions. Without wishing to be bound by theory, it is believed that this variation in pore density and in the average maximum dimension of the pores can occur because as the temperature of the sintering process decreases, the extent by which the metal particles and/or the metal alloy particles come together can decrease as well. In some embodiments, a sintering process that is used to form a stent can include forming a generally tubular member around a mandrel that is selectively heated so that certain regions of the mandrel are hotter than other regions of the mandrel. The result can be that the generally tubular member has different regions having different average pore volumes and/or having pores with different average maximum dimensions. 
     In some embodiments, a stent that is formed by sintering metal particles and/or metal alloy particles can erode after being used at a target site in a body of a subject, and the erosion of the stent can result in the formation of metal particles and/or metal alloy particles having the same size as the particles that were originally sintered together to form the stent. Thus, the size of the particles formed from the erosion of a stent can be selected, for example, by sintering metal particles and/or metal alloy particles of the desired size to form the stent. 
     As another example, while stents with certain porosity patterns have been described, in some embodiments, a stent can have a different porosity pattern. For example,  FIGS. 10A and 10B  show a stent  570  including a generally tubular member  574  having an interior surface  578 , an exterior surface  582 , and a lumen  586 . Generally tubular member  574  is formed of a metal matrix  590  that is formed of one or more bioerodible metals and/or bioerodible metal alloys, and that includes pores  594 . As shown in  FIG. 10B , the pores in generally tubular member  574  that are relatively far from both interior surface  578  and exterior surface  582  are relatively large, while the pores that are relatively close to interior surface  578  or exterior surface  582  are relatively small. Stent  570  can be used, for example, to store a relatively large volume of therapeutic agent in the relatively large pores, and to provide a slow and/or controlled release of the therapeutic agent into the target site through the relatively small pores. 
     As an additional example, in some embodiments, a stent can include a porous generally tubular member that includes more than one therapeutic agent in its pores. For example,  FIGS. 11A and 11B  show a stent  600  including a generally tubular member  604  having an interior surface  605 , an exterior surface  606 , and a lumen  607 . Generally tubular member  604  is formed of a metal matrix  608  that is formed of one or more bioerodible metals and/or bioerodible metal alloys. Metal matrix  608  includes pores  610 . As shown in  FIG. 11B , pores  610  are aligned in an inner circle  614  close to interior surface  605 , and in an outer circle  618  close to exterior surface  606 . In some embodiments, the pores that form inner circle  614  can be filled with one type of therapeutic agent (e.g., an anticoagulant, such as heparin), while the pores that form outer circle  618  can be filled with a different type of therapeutic agent (e.g., an anti-proliferative, such as paclitaxel). 
     As a further example, in some embodiments, a stent can include a porous bioerodible metal matrix surrounding a therapeutic agent-containing layer. For example,  FIGS. 12A and 12B  show a stent  650  including a generally tubular member  652  formed of three layers  654 ,  656 , and  658 . Layer  654  is formed of a metal matrix  660  that is formed of one or more bioerodible metals and/or bioerodible metal alloys, and that includes pores  662 . Similarly, layer  658  is formed of a metal matrix  664  that is formed of one or more bioerodible metals and/or bioerodible metal alloys, and that includes pores  668 . Layer  656 , which is located between layer  654  and layer  658 , includes one or more therapeutic agents. For example, layer  656  can be formed entirely of one or more therapeutic agents, or can be formed of one or more materials (e.g., a bioerodible polymer) that are combined with one or more therapeutic agents. Layers  654  and  658  can regulate the release of the therapeutic agent(s) from layer  656  into a target site. 
     As another example, in certain embodiments, a stent can include one or more components (e.g., bands and/or connectors) including a hollow reservoir that can be filled with, for example, one or more therapeutic agents. For example,  FIG. 13A  shows a stent  720  that is in the form of a generally tubular member  721  formed of one or more bioerodible metals and/or bioerodible metal alloys. Generally tubular member  721  is defined by a plurality of bands  722  and a plurality of connectors  724  that extend between and connect adjacent bands. Generally tubular member  721  has a lumen  723 . 
       FIG. 13B  shows an enlarged view of a connector  724  of stent  720 , and  FIG. 13C  shows a cross-sectional view of the connector of  FIG. 13B . As shown in  FIG. 13C , connector  724  is formed of a metal matrix  730  surrounding a reservoir  732  and including pores  734 . Metal matrix  730  is formed of one or more bioerodible metals and/or bioerodible metal alloys. Reservoir  732  is filled with a therapeutic agent  750  that can, for example, elute through pores  734  during and/or after delivery of stent  720  to a target site. 
     As an additional example, in some embodiments, a stent can include a generally tubular member having different regions along its length that have different pore densities and/or that include pores having different average maximum dimensions. 
     For example,  FIGS. 14A and 14B  show a stent  800  including a generally tubular member  802  having a lumen  804 . Generally tubular member  802  is formed of a metal matrix  806  including pores  808 . Metal matrix  806  is formed of one or more bioerodible metals and/or bioerodible metal alloys. As shown in  FIG. 14B , different regions R 9 , R 10 , and R 11  of generally tubular member  802  along the length L 5  of generally tubular member  802  have different pore densities and include pores having different average maximum dimensions. More specifically, region R 9  has a higher pore density than region R 10 , and includes pores with a higher average maximum dimension than the pores in region R 10 . Region R 10 , in turn, has a higher pore density than region R 11 , and includes pores with a higher average maximum dimension than the pores in region R 11 . These differences in the pore densities and average maximum dimensions of the pores in regions R 9 , R 10 , and R 11  can, for example, result in region R 9  eroding at a faster rate than both regions R 10  and R 11 , and region R 10  eroding at a faster rate than region R 11 . 
       FIGS. 15A and 15B  show a stent including a generally tubular member having different regions along its length that include pores having different average maximum dimensions. As shown in  FIGS. 15A and 15B , a stent  850  includes a generally tubular member  852  having a lumen  854 . Generally tubular member  852  is formed of a metal matrix  856  including pores  858 . Metal matrix  856  is formed of one or more bioerodible metals and/or bioerodible metal alloys. As shown in  FIG. 15B , different regions R 12 , R 13 , and R 14  of generally tubular member  852  along the length L 6  of generally tubular member  852  include pores having different average maximum dimensions. More specifically, the pores in end regions R 12  and R 14  have higher average maximum dimensions than the pores in middle region R 13 . In some embodiments, one or more of the pores in generally tubular member  852  can contain one or more therapeutic agents that can treat thrombosis. The relatively large pores in end regions R 12  and R 14  can contain a higher volume of the therapeutic agent(s) than the relatively small pores in middle region R 13 . 
     As another example, in some embodiments, a stent including a metal matrix including pores can be a self-expanding stent. For example, in certain embodiments, a self-expanding stent can include a generally tubular member that is formed of Nitinol, and can further include a porous bioerodible metal supported by the generally tubular member (e.g., the porous bioerodible metal can be in the form of a coating on the generally tubular member). 
     As a further example, while stents have been described, in some embodiments, other medical devices can include pores, bioerodible metals, and/or bioerodible metal alloys. For example, other types of endoprostheses, such as grafts and/or stent-grafts, can include one or more of the features of the stents described above. Additional examples of medical devices that can have one or more of these features include bone screws. 
     As another example, in some embodiments, a medical device can include regions that are formed of a porous metal and/or a porous metal alloy (e.g., a bioerodible porous metal and/or a bioerodible porous metal alloy), and regions that are not formed of a porous metal or metal alloy. For example, a stent may include regions that are formed of a bioerodible porous metal, and regions that are formed of a metal that is neither bioerodible nor porous. 
     As an additional example, in certain embodiments, a medical device (e.g., a stent) including a coating formed of a porous metal and/or a porous metal alloy can be further coated with one or more other coatings. The other coatings can be formed of porous metals and/or porous metal alloys, or may not be formed of porous metals or porous metal alloys. 
     As a further example, in some embodiments, a coating can be applied to certain regions of a medical device, while not being applied to other regions of the medical device. 
     As another example, in certain embodiments, a medical device (e.g., a stent) can include one or more metal foams, such as one or more bioerodible metal foams. Medical devices including metal foams are described, for example, in U.S. Provisional Patent Application Ser. No. 60/844,967, which is incorporated by reference, filed Sep. 15, 2006 and entitled “Medical Devices”. 
     All publications, applications, references, and patents referred to in this application are herein incorporated by reference in their entirety. 
     Other embodiments are within the claims.