Patent Publication Number: US-2007118176-A1

Title: Radiopaque bioabsorbable occluder

Description:
REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 60/729,549, filed Oct. 24, 2005, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to an occlusion device for the closure of physical anomalies, such as an atrial septal defect, a patent foramen ovale, and other septal and vascular defects. The invention also relates to making such a device or other medical implant radiopaque.  
     BACKGROUND OF THE INVENTION  
      A patent foramen ovale (PFO), illustrated in  FIG. 1 , is a persistent, one-way, usually flap-like opening in the wall between the right atrium  11  and left atrium  13  of the heart  10 . Because left atrial (LA) pressure is normally higher than right atrial (RA) pressure, the flap usually stays closed. Under certain conditions, however, right atrial pressure can exceed left atrial pressure, creating the possibility that blood could pass from the right atrium  11  to the left atrium  13  and blood clots could enter the systemic circulation. It is desirable that this circumstance be eliminated.  
      The foramen ovale serves a desired purpose when a fetus is gestating in utero. Because blood is oxygenated through the umbilical chord, and not through the developing lungs, the circulatory system of the fetal heart allows the blood to flow through the foramen ovale as a physiologic conduit for right-to-left shunting. After birth, with the establishment of pulmonary circulation, the increased left atrial blood flow and pressure results in functional closure of the foramen ovale. This functional closure is subsequently followed by anatomical closure of the two over-lapping layers of tissue: septum primum  14  and septum secundum  16 . However, a PFO has been shown to persist in a number of adults.  
      The presence of a PFO is generally considered to have no therapeutic consequence in otherwise healthy adults. Paradoxical embolism via a PFO is considered in the diagnosis for patients who have suffered a stroke or transient ischemic attack (TIA) in the presence of a PFO and without another identified cause of ischemic stroke. While there is currently no definitive proof of a cause-effect relationship, many studies have confirmed a strong association between the presence of a PFO and the risk for paradoxical embolism or stroke. In addition, there is significant evidence that patients with a PFO who have had a cerebral vascular event are at increased risk for future, recurrent cerebrovascular events.  
      Accordingly, patients at such an increased risk are considered for prophylactic medical therapy to reduce the risk of a recurrent embolic event. These patients are commonly treated with oral anticoagulants, which potentially have adverse side effects, such as hemorrhaging, hematoma, and interactions with a variety of other drugs. The use of these drugs can alter a person&#39;s recovery and necessitate adjustments in a person&#39;s daily living pattern.  
      In certain cases, such as when anticoagulation is contraindicated, surgery may be necessary or desirable to close a PFO. The surgery would typically include suturing a PFO closed by attaching septum secundum to septum primum. This sutured attachment can be accomplished using either an interrupted or a continuous stitch and is a common way a surgeon shuts a PFO under direct visualization.  
      Umbrella devices and a variety of other similar mechanical closure devices, developed initially for percutaneous closure of atrial septal defects (ASDs), have been used in some instances to close PFOs. These devices potentially allow patients to avoid the side effects often associated with anticoagulation therapies and the risks of invasive surgery. However, umbrella devices and the like that are designed for ASDs are not optimally suited for use as PFO closure devices.  
      Currently available septal closure devices present drawbacks, including technically complex implantation procedures. Additionally, there are not insignificant complications due to thrombus, fractures of the components, conduction system disturbances, perforations of heart tissue, and residual leaks. Many devices have high septal profile and include large masses of foreign material, which may lead to unfavorable body adaptation of a device. Given that ASD devices are designed to occlude holes, many lack anatomic conformability to the flap-like anatomy of PFOs. Thus, when inserting an ASD device to close a PFO, the narrow opening and the thin flap may form impediments to proper deployment. Even if an occlusive seal is formed, the device may be deployed in the heart on an angle, leaving some components insecurely seated against the septum and, thereby, risking thrombus formation due to hemodynamic disturbances. Finally, some septal closure devices are complex to manufacture, which may result in inconsistent product performance.  
      A septal defect closure device can promote tissue growth and healing of the defect. A permanent implant may not be necessary. Bioabsorbable materials can be useful material for implantable devices such as a septal closure device because they degrade over time into non-toxic materials and are absorbed into bodily tissue. The body, therefore, may accept the implant without long-term medication to suppress an immunal or inflammatory response. Bodily tissue may even grow “through” the bioabsorbable material. In addition, because these materials degrade over a known period of time, determined in part by the characteristics of the material, eventually the device will be entirely absorbed by the body. Because the device is absorbed in the body, removal procedures via catheter or invasive surgery are unnecessary.  
      When devices are implanted percutaneously (e.g., via a catheter) it is important to be able to observe the location and position of the devices by some technique such as fluoroscopy or X-ray. Bioabsorbable materials are typically radiotranslucent and cannot be viewed easily using fluoroscopy or X-ray. This characteristic makes the implantation of a device made of a bioabsorbable material challenging because the position of the device cannot be determined with precision. Some techniques are known for making a bioabsorbable implant partially radiopaque (i.e., can be seen under fluoroscopy or X-ray) so that the position of the device can be viewed during implantation by fluoroscopy and X-ray.  
      One technique for making a device viewable under fluoroscopy involves attaching a small radiopaque marker “band” at a predetermined location on the device. When marker bands are applied to a device, the location and orientation of the device can be inferred based on the location and orientation of the visible radiopaque marker bands. Accordingly, the device can be delivered under the guidance of fluoroscopy and X-ray equipment to monitor the position of the device relative to the desired implantation site in a patient&#39;s body, and to ensure their proper orientation, position, and/or deployment. A radiopaque marker band is usually made of metal because many metals have radiopaque properties.  
      Devices with radiopaque marker “band(s)” can also suffer from limited visibility. Specifically, the radiographic visibility of a device incorporating a marker band(s) is limited to specified regions of the marker band itself. As noted above, when a marker band is placed on a device, the location of the device at the delivery site is only known by inference relative to the marker band, not by actually viewing the device. Under these conditions, the placement of a device in the body requires alignment of the device based on limited viewing of radiopaque areas.  
      The use of (non-bioabsorbable) metal radiopaque marker band(s) with bioabsorbable devices may have other potential problems. After the bioabsorbable portion of the device is absorbed, the marker band remains behind and is usually embedded in the surrounding tissues leaving a foreign mass in the body. In addition, the body might recognize a metallic mass as a foreign material, and respond with an immune reaction or long-term inflammation. Such responses can adversely impact the usefulness of a medical implant.  
      Instead of, or in addition to, a marker band, a radiopaque agent, such as barium sulfate, can be mixed with a non-radiopaque material, to form a radiopaque blend. This agent usually provides a non-radiopaque material with some degree of radiopacity to allow the device made of such radiopaque blend to be visible under fluoroscopy. Generally, the more radiopaque agent that is added in the blend, the greater radiopacity that can be achieved.  
      Adding a radiopaque agent to bioabsorbable material presents design considerations and challenges. One such consideration is whether the radiopaque agent can be safely processed, e.g. degraded, absorbed, and/or excreted, by the body as opposed to generating an inflammatory reaction, toxicin, or being collected in an organ such as the liver.  
      Additionally, even if a radiopaque agent can be safely processed by the body, the addition of the radiopaque agent to the non-radiopaque material changes the mechanical and/or thermal properties of the non-radiopaque material. The viscosity, maximum stress, modulus, elongation, and glass transition temperature of the non-radiopaque material may all be significantly altered by the presence of the radiopaque agent.  
      Therefore, there is a need for an improved radiopaque septal defect closure device that can be viewed through fluoroscopy and X-ray and can also be safely processed by the body.  
      The present invention is designed to address these and other deficiencies of prior art septal closure devices.  
     SUMMARY OF THE INVENTION  
      In one aspect, the present invention provides an occluder for a biological defect to be introduced into the body through the vasculature. In one aspect of the invention, the occluder includes a structural member formed of a radiopaque bioabsorbable material which is made of a blend of a bioabsorbable material and a radiopaque material. In certain embodiments, the radiopaque bioabsorbable material has a thickness between 500 and 750 microns. In certain embodiments, the radiopaque material has a linear attenuation coefficient greater than about 9 cm −1 , and the radiopaque bioabsorbable material contains between about 20 to about 35 percent by weight of the radiopaque material, and preferably between 20 to 35 percent by weight of the radiopaque material.  
      In certain embodiments, the bioabsorbable material is selected from a group consisting of polyglycolic acid, polylactic acid, poly caprolactone, poly (hyrodxybutyrate), poly(hydroxyvalerate), poly(sebacic acid-headecanoic acid anhydride), polyorthoester, polydioxanone, polygluconate, poly(amino acid), poly(alpha hydroxyl acid), and co-polymers thereof. In certain embodiments, the radiopaque material is tungsten in the form of a powder. In certain embodiments, the tungsten powder has a particle size in the range between about 0.5 to about 2.0 microns.  
      In another aspect of the invention, a structural member of an occluder for a biological defect is made of a radiopaque, bioabsorbable material. The material is preferably a blend of a bioabsorbable polymer having a molecular weight of 300,000 or greater and a radiopaque agent. According to some embodiments, the radiopaque agent preferably has a linear attenuation coefficient greater than about 9 cm −1 . According to some embodiments, the radiopaque material has a mass attenuation coefficient greater than about 1.2 cm 2 /gm  
      According to at least some embodiments, the device is formed from a tube. According to some embodiments, the occluder has a proximal side and a distal side that cooperate to close the defect, and at least one of the proximal side or the distal side includes petals that are formed by the structural member made of a bioabsorbable radiopaque material. According to some embodiments, the occluder further includes tissue scaffolding attached to the occluder.  
      According to some embodiments, the bioabsorbable material is a bioabsorbable polymer. According to some embodiments, the bioabsorbable polymer is selected from a group consisting of polyglycolic acid, polylactic acid, poly caprolactone, poly (hyrodxybutyrate), poly(hydroxyvalerate), poly(sebacic acid-headecanoic acid anhydride), polyorthoester, polydioxanone, polygluconate, poly(amino acid), poly(alpha hydroxyl acid), and co-polymers thereof.  
      According to some embodiments, the radiopaque material is tungsten in the form of a powder. According to some embodiments, the tungsten powder has a particle size in the range between about 0.5 and about 2.0 microns and preferably is between 0.5 and 2.0 microns. According to some embodiments, the weight percent of tungsten in the radiopaque bioabsorbable material is in the range between about 20 and 35 weight percent.  
      According to some embodiments, the radiopaque, bioabsorbable material has a thickness between 500 and 750 microns.  
      According to some embodiments, the occluder is made from a tube with slits that form petals when the tube changes from a delivery configuration to a deployed configuration. According to some embodiments, the occluder has a proximal side and a distal side that cooperate to close the defect and the proximal side includes proximal petals and the distal side includes distal petals. According to some embodiments, the occluder further comprises tissue scaffolding attached to at least one of the distal petals or the proximal petals. In certain embodiments, the occluder is a patent foramen ovale (PFO) occluder. In some embodiments, the tube consists essentially of the radiopaque bioabsorbable material.  
      According to some embodiments, the bioabsorbable material is a bioabsorbable polymer. According to some embodiments, the bioabsorbable polymer is selected from a group consisting of polyglycolic acid, polylactic acid, poly caprolactone, poly (hyrodxybutyrate), poly(hydroxyvalerate), poly(sebacic acid-headecanoic acid anhydride), polyorthoester, polydioxanone, polygluconate, poly(amino acid), poly(alpha hydroxyl acid), and co-polymers thereof.  
      According to some embodiments, the radiopaque material is tungsten in the form of a powder. According to some embodiments, the tungsten powder has a particle size in the range between about 0.5 and about 2.0 microns and preferably is between 0.5 and 2.0 microns. According to some embodiments, the weight percent of tungsten in the radiopaque bioabsorbable material is in the range between about 20 and 35 weight percent.  
      According to some embodiments, the radiopaque, bioabsorbable material has a thickness between 500 and 750 microns.  
      In another aspect of the invention, includes implanting a radiopaque, bioabsorbable occluder is implanted by insertion into the vasculature of a body. One aspect of the invention includes a method of implanting an occluder for a biological defect, including the steps of providing an occluder, having a structural member consisting essentially of a radiopaque bioabsorbable material. The radiopaque bioabsorbable material comprises a blend of a bioabsorbable material and a radiopaque material. The bioabsorbable material has a molecular weight of at least about 300,000, and the radiopaque material having a linear attenuation coefficient greater than about 9 cm −1 . The occluder is inserted into a subject using a catheter and the position and orientation of the device are viewed radiographically during implantation.  
      In another aspect of the invention, a method of making a radiopaque, bioabsorbable medical implant is provided. A method of making a radiopaque, bioabsorbable medical implant having a structural member formed of a blended radiopaque bioabsorbable material, made of a bioabsorbable polymer and a radiopaque agent, includes the following steps. A biocompatible radiopaque agent for blending with a bioabsorbable polymer is selected. A concentration of the radiopaque agent in the radiopaque bioabsorbable material to attain a desired level of radiopacity is determined. For a physical property of the radiopaque bioabsorbable material that will vary as the material is bioabsorbed after implantation, a desired initial criteria is identified and a bioabsorbable polymer is selected according to the desired initial criteria. The selected radiopaque agent and the selected bioabsorbable polymer are blended according to the determined concentration to form the blended material and the structural member is formed using the blended material. According to certain embodiments, the desired initial criteria is determined based on an expected rate of bioabsorption and an expected life of the implant. According to certain embodiments, the physical property is molecular weight. According to certain embodiments, the radiopaque agent is tungsten. According to certain embodiments, the concentration of the radiopaque agent is between about 20 and 35 weight percent. According to certain embodiments, the bioabsorbable polymer is selected from a group consisting of polyglycolic acid, polylactic acid, poly caprolactone, poly (hyrodxybutyrate), poly(hydroxyvalerate), poly(sebacic acid-headecanoic acid anhydride), polyorthoester, polydioxanone, polygluconate, poly(amino acid), poly(alpha hydroxyl acid), and co-polymers thereof.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of a human heart including various septal defects;  
       FIGS. 2A-2D  are isometric views of an embodiment of an occluder according to the present invention;  
       FIGS. 2E-2H  are isometric views of an embodiment of an occluder according to the present invention;  
       FIGS. 2I-2K  are isometric views of occluders according to various embodiments of the invention;  
       FIGS. 2L and 2M  are side and top views, respectively, of an alternate embodiment of an occluder according to the present invention;  
       FIGS. 3A-3C  are front elevational, side, and cross-sectional views, respectively, of the occluder of  FIGS. 2A-2D ;  
       FIGS. 4A-4B  are front elevational and side views, respectively, of another embodiment of an occluder according to the present invention;  
       FIGS. 5A-5B  are front and side views, respectively, of still another embodiment of an occluder according to the present invention;  
       FIGS. 6A-6E  are isometric views of one embodiment of a catch system according to the present invention;  
       FIGS. 7A-7C  are side views of another embodiment of a locking mechanism according to the present invention;  
       FIGS. 8A-8C  are isometric views of yet another embodiment of an occluder according to the present invention;  
       FIGS. 9A-9H  are side views of one method for delivering an occluder according to the present invention to a septal defect; and  
       FIGS. 10A-10D  are side views of one method for retrieving an occluder according to the present invention from a septal defect;  
       FIG. 11  is a side view of an embodiment of the occluder of the present invention;  
       FIG. 12  is an isometric view of an embodiment of the occluder of the present invention; and  
       FIG. 13  is a side view of the occluder of  FIGS. 21-2K  deployed in vivo. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention provides a device for occluding an aperture within body tissue. This device relates particularly to, but is not limited to, a septal occluder made from a polymer tube. In particular and as described in detail below, the occluder of the present invention may be used for closing an ASD or PFO in the atrial septum of a heart. Although the embodiments of the invention are described with reference to an ASD or PFO, one skilled in the art will recognize that the device and methods of the present invention may be used to treat other anatomical conditions. As such, the invention should not be considered limited in applicability to any particular anatomical condition.  
       FIG. 1  illustrates a human heart  10 , having a right atrium  11  and a left atrium  13  and including various anatomical anomalies  18   a  and  18   b . The atrial septum  12  includes septum primum  14  and septum secundum  16 . The anatomy of the septum  12  varies widely within the population. In some people, septum primum  14  extends to and overlaps with septum secundum  16 . The septum primum  14  may be quite thin. When a PFO is present, blood could travel through the passage  18   a  between septum primum  14  and septum secundum  16  (referred to as “the PFO tunnel”). Additionally or alternatively, the presence of an ASD could permit blood to travel through an aperture in the septum, such as that schematically illustrated by aperture  18   b.    
      The term “bioabsorbable,” as used in this application, is also understood to mean “bioresorbable.” 
      In this application, “distal” refers to the direction away from a catheter insertion location and “proximal” refers to the direction nearer the insertion location.  
      Referring to occluder  20 , distal side  30  and proximal side  40  are connected by central tube  22 . As illustrated, e.g., in  FIGS. 9 and 10  the central tube  22  is an uncut central part of the tube used to form occluder  20 . As described below, the entire tube is indicated by reference numeral  25 . As shown in  FIGS. 9 and 10 , the occluder  20  may be inserted into the septum  12  to prevent the flow of blood through the aperture  18   a , e.g., the occluder may extend through the PFO tunnel such that the distal side  30  is located in the left atrium  13  and the proximal side  40  is located in the right atrium  11 . Additionally or alternatively, the occluder  20  may be inserted into the septum  12  so as to prevent the flow of blood through the aperture  18   b , e.g., the occluder may extend through the ASD such that the distal side  30  is located in the left atrium  13  and the proximal side  40  is located in the right atrium  11 . As used in this application, unless otherwise indicated, the term “aperture  18 ” refers to any anatomical anomaly that may be treated by use of occluder  20 , such as PFO  18   a  or ASD  18   b.    
      The occluder  20  is constructed of one or more metal or polymer tube(s), referred to collectively as “tube”  25 . Tube  25  includes slits  31  and  41  (or  231  and  241 ), which are formed using an etching or cutting process that produces a particular cutting pattern on tube  25 . For example, as shown in  FIG. 2K , slits  31  (or  231 ) are cut along the axial length of the upper half of tube  25  using a cutting tool, e.g., a razor blade. According to some embodiments of the present invention and as shown in  FIG. 2K , slits  31  (or  231 ) are cut without removing any significant amount of material from tube  25 , i.e., the formation of slits  31  (or  231 ) does not significantly reduce the overall volume of tube  25 . According to other embodiments of the present invention, slits  31  (or  231 ) are formed by cutting material out of tube  25  such that the volume of tube  25  is reduced. Both ends of each of slits  31  are rounded so as to relieve stresses at the axial ends of the slits  31 . This prevents slits  31  from lengthening due to cyclic stresses present in a beating heart and the resultant material fatigue. In those embodiments where slits  31  are cut without removing any significant amount of material from tube  25 , rounded ends or holes  33  may be produced by burning holes at both ends of each of slits  31 . In those embodiments where slits  31  are formed by cutting material out of tube  25 , rounded ends  33  may be formed during the cutting process. The size of rounded ends  33  may vary depending upon the dimensions of tube  25  and the amount of stress release required by the deformation.  
       FIG. 2D and 2H  illustrate exemplary occluder  20  formed from a tube  25 , according to some embodiments of the present invention. Configuration of the occluder  20  is determined by the cutting pattern on tube  25 . For example, and as shown in FIGS.  2 A,  2 B- 2 D, and  3 A- 3 C, petal-shaped loops  32 ,  42  ( FIGS. 2A-2D  and  FIG. 3A ) are produced by cutting slits  31  in the distal side  30  of tube  25 , and cutting slits  41  in the proximal side  40  of tube  25  according to the cutting pattern shown in  FIG. 2A . As shown in  FIG. 2B , the distal side  30  of tube  25  is cut in half from a center portion  22  to a distal distance to form half sections  91   a  and  91   b . The half sections  91   a  and  91   b  are further cut to a proximal distance from the distal end  39  into quarter sections  92   a ,  93   a ,  92   b , and  93   b . The cuts are discontinued and quarter sections  92   a  and  92   b  form half section  94   a  at end  39 , and quarter sections  93   a  and  93   b  form half section  94   b  at end  39 . Upon application of force F d  to end  39 , struts bow and twist outward to form petal-shaped loops  32  in distal side  30 , as shown in  FIGS. 2C-2D . The movement of the struts during deployment is such that the struts rotate in an orthogonal plane relative to the axis of the device. Central tube  22  may be constrained during the application of force F d , or any combination of forces sufficient to reduce the axial length of the tube  25  may be applied. One end of each of petal-shaped loops  32  originates from central tube  22 , while the other end originates from end  39  ( FIGS. 2B-2C  and  FIG. 3A ). Petal-shaped loops  42  may be formed in proximal side  40  of tube  25 , as shown in  FIGS. 2B-2D , using the same cutting pattern described above.  
      Given that the surface of occluder  20  will contact septum  12  once it is deployed in vivo, slits  31  and  41  are cut so as to prevent the formation of sharp, potentially damaging edges along their length. For example, a heated cutting tool may be used to cut slits  31  and  41  such that the material of tube  25  melts slightly when placed in contact with the cutting tool. Such melting rounds the edges of the sections. Lasers may also be used to cut slits  31  and  41 . According to this process, the edges of loops  32  and  42  formed by the cutting of slits  31  and  41  are blunted (due to melting) to prevent tissue damage in vivo. One skilled in the art will recognize that same considerations and techniques also apply to slits  231  and  241 .  
      The tube(s)  25  forming occluder  20  includes a biocompatible metal or polymer. In at least some embodiments, the occluder  20  is formed of a bioabsorbable polymer, or a shape memory polymer. In other embodiments, the occluder  20  is formed of a biocompatible metal, such as a shape memory alloy (e.g., nitinol). The thermal shape memory and/or superelastic properties of shape memory polymers and alloys permit the occluder  20  to resume and maintain its intended shape in vivo despite being distorted during the delivery process. In addition, shape memory polymers and metals can be advantageous so that the structure of the device assists in compressing the PFO tunnel closed. Alternatively, or additionally, the occluder  20  may be formed of a bioabsorbable metal, such as iron, magnesium, or combinations of these and similar materials. Exemplary bioabsorbable polymers include polyhydroxyalkanoate compositions, for example poly-4-hydroxybutyrate (P4HB) compositions, disclosed in U.S. Pat. No. 6,610,764, entitled Polyhydroxyalkanoate Compositions Having Controlled Degradation Rate and U.S. Pat. No. 6,548,569, entitled Medical Devices and Applications of Polyhydroxyalkanoate Polymers, both of which are incorporated herein by reference in their entirety.  
      In certain embodiments, the occluder  20  is partially or completely radiopaque, and, in particular, is partially or completely formed of radiopaque bioabsorbable materials.  
      Preferred materials for making the occluder  20  are radiopaque under standard X-ray and fluoroscopy equipment and bioprocessable, for example through absorption, degradation, excretion, or otherwise processed (generally referred to as bioabsorbed) safely by the body over a predetermined period of time. In addition, in preferred embodiments, the implantation and subsequent absorption of the material will not cause safety concerns, including inflammation, toxicity, tissue accumulation and rejection. As the material is absorbed, the device “disappears” so as not to leave any implant behind, and the radiopaque agent does not create an embolic risk but degrades and is excreted from the body.  
      The radiopaque bioabsorbable materials can be used as a main component, e.g. a structural member, or a frame, of a medical implant such as occluder frame  20 , allowing all or nearly all of the implant to be monitored with radiography during implantation.  
      Further, by selecting an appropriate bioabsorbable material, the device can be manufactured to be bioabsorbed over a desired period of time, and/or with proper in-growth of native tissue. For example, in the case of an occlusive device designed to seal a hole or a defect in tissue such as occluder  20 , it might be desirable that the bioabsorption process is not completed until the hole completely heals through regeneration of native tissue. For example, native tissue growth may begin in  6  months after implantation and complete healing may take two years. The radiopaque characteristics of the materials of preferred embodiments will allow the progress kinetics of the bioabsorption of the device, as well as any changes in the device location and orientation, to be easily monitored.  
      According to preferred embodiments, a radiopaque, bioabsorbable occluder  20  is formed from a bioabsorbable material selected as a base material, e.g., a bioabsorbable polymer, preferably blended with a radiopaque agent to form a radiopaque bioabsorbable material. In a preferred embodiment, the radiopaque agent is biocompatible and is capable of being broken down in the body and flushed out so that the radiopaque agent does not accumulate in major organs, such as the liver.  
      Additionally, the radiopaque agent at a suitable concentration to obtain the desired physical properties of the blended material should provide sufficient radiopacity so that during fluoroscopic examination the material can be reliably viewed. The choice of a radiopaque agent  
      on many factors including the biocompatibility of the agent, the bioabsorbability of the agent and the impact of the radiopaque agent on the structural integrity of a device constructed with the radiopaque bioabsorbable material. One important consideration is the ability of the material to “attenuate” the mono energetic photons emitted by the fluoroscopic and X-ray device, this quality is described as mass attenuation coefficient, (μ/ρ), where μ is linear attenuation coefficient, and ρ is the density of the material. The mass attenuation coefficient of a material is directly related to the visibility of the material under fluoroscopy and X-ray, i.e., its radiopacity.  
      The relationship can be expressed thus:
 
 I/I   0 =exp[−(μ/ρ)χ]  (1)
 
 Where a narrow beam of mono energetic photons with an incident intensity of I 0 , penetrates a layer of material with mass thickness, χ, and density, ρ, and emerges with an intensity, I. The equation can be rewritten as:
 
μ/ρ=χ −1 ln( I   0   /I )  (2)
 
 mass thickness, χ, is defined at the mass per unit area and is obtained by multiplying the thickness t by the density, ρ, i.e. χ=ρ·t. The equation can be further rewritten as
 
−(μ/ρ)χ=−(μ/ρ)(ρ· t )=−(μ· t )  (3)
 
 I/I   0 =exp[−(μ/ρ)χ]=exp[−(μ· t )]  (4)
 
      Accordingly, assuming the same thickness t, the degree of radiopacity is largely determined by the material&#39;s linear attenuation coefficient, μ, since a higher linear attenuation coefficient, μ, will result in a lower emerging intensity, I, which indicates an higher degree of radiopacity. Furthermore, to obtain the same degree of radiopacity, i.e. the same emerging intensity, I, a lower thickness, t, of the material with a higher linear attenuation coefficient, μ, will meet the requirement. Considering the linear attenuation coefficient, μ, in the table below, a preferred material for blending with a bioabsorbable material will have a linear attenuation coefficient larger than 35 cm −1 . Of course, in other embodiments of the invention, materials with smaller linear attenuation coefficient, μ, could be used with corresponding decreases in their radiopacity.  
      The table below provides several elements that can be used as the radiopaque agent. In one embodiment of the invention biocompatible materials with mass attenuation coefficient, (μ/ρ), of 1.20 cm 2 /g or greater are suitable for various embodiments of the invention. In another embodiment, materials with greater density, ρ, and greater mass attenuation coefficient, (μ/ρ), are preferred.  
                                   TABLE 1                                   Mass                           Attenuation           Chemi-       Coefficient       Linear       Atomic   cal       @60 KeV X-   Density   Attenuation       Number   Symbol   Element   ray energy   (g/cm 3 )   Coefficient                                                        26   Fe   Iron   1.21 cm 2 /g   7.87    9.523 cm −1         28   Ni   Nickel   1.51 cm 2 /g   8.9   13.439 cm −1         30   Zn   Zinc   1.76 cm 2 /g   7.13   12.549 cm −1         34   Se   Selenium   2.34 cm 2 /g   4.8   11.232 cm −1         42   Mo   Molyb-   4.27 cm 2 /g   10.2   43.554 cm −1                 denum       53   I   Iodine   7.58 cm 2 /g   4.94   37.445 cm −1         56   Ba   Barium   8.51 cm 2 /g   3.6   30.636 cm −1         74   W   Tungsten   3.71 cm 2 /g   19.3   71.603 cm −1         83   Bi   Bismuth   5.233 cm 2 /g    9.8   51.283 cm −1                    
 
      A preferred radiopaque agent has a mass attenuation coefficient, (μ/ρ), greater than 1.2 cm 2 /g, more preferably above 3.0 cm 2 /g. A preferred radiopaque agent has a linear attenuation coefficient, μ, greater than 9 cm −1 , more preferably above 30 cm −1 . For a material with greater radiopacity, only a small amount would provide a sufficient degree of radiopacity for the resulting material, and therefore, properties of the original material would not be significantly adversely affected by the presence of the radiopaque agent. Specifically, when blending such radiopaque agent with a bioabsorbable material, the mechanical and/or thermal properties of the bioabsorbable material are maintained within an appropriate range.  
      A particularly preferred radiopaque agent for some embodiments of the invention is tungsten. Tungsten has a linear attenuation coefficient, μ, of 71.603 cm −1 , a mass attenuation coefficient, (μ/ρ), of 3.71 cm 2 /g, and is denser than some other materials that could be considered for use as radiopaque agents, such as barium sulfate. Therefore a much smaller amount of tungsten than barium sulfate would provide an equivalent level of radiopacity in the resulting blend. Because a relatively small amount of tungsten is needed, the mechanical and/or thermal properties of the blend relative to the base polymer should have less degradation than otherwise. For example, in one or more embodiments, only 20-35 weight percent, or 7.6-9.6 volume percent, tungsten in the blend provides a useful level of radiopacity. Additionally, because the resulting blend is highly radiopaque, implantable devices can be fabricated with relatively thin features, while still being easily visualized with standard radiographic equipment.  
      In addition to being highly radiopaque, tungsten is also biocompatible. As the bioabsorbable material degrades and its byproducts of that process are absorbed or excreted by the body. A similar process occurs for the tungsten. Tungsten has been commonly used in embolization coils. It is known that the coils degrade and disappear over time in the body. As tungsten degrades, it can be eliminated readily from the body, primarily in the urine. In general, the presence of elevated tungsten levels in the blood does not appear to have a detrimental affect on human health.  
      The degradation of the radiopaque agent, including tungsten could be controlled by selecting the amount of tungsten and the particle size used. The particle size of radiopaque agents identified above, including tungsten, preferably ranges from 0.5 microns to 400 microns. The smaller the particle size, the less property degradation of the original bioabsorbable material. The volume of the radiopaque agents, identified above, including tungsten that can be added to the blend could range from 1% to 80%.  
      The bioabsorbable material used can be one or more of a variety of bioabsorbable materials known in the art. For example, the bioabsorbable material can be a polymer of glycolide (commonly referred to as polyglycolic acid), a polymer of lactide (commonly referred to as polylactic acid), polycaprolactone, poly(hydroxybutyrate), poly(hydroxyvalerate), poly(sebacic acid-hexadecanoic acid anhydride), polyorthoesters, polydioxanone, polygluconate, poly(amino acids), poly(alpha hydroxy acids), co-polymers of the above (for example poly(galactide-co-lactide) which is commonly referred to as poly glycoic and lactic acid or PGLA), and other bioabsorbable materials, including collagen-based materials. Exemplary bioabsorbable polymers include polyhydroxyalkanoate compositions, for example poly-4-hydroxybutyrate (P4HB) compositions, disclosed in U.S. Pat. No. 6,610,764, entitled Polyhydroxyalkanoate Compositions Having Controlled Degradation Rate and U.S. Pat. No. 6,548,569, entitled Medical Devices and Applications of Polyhydroxyalkanoate Polymers, both of which are incorporated by reference in their entirety. Poly (hydroxybutyrate), in particular, is a preferred material for use as a base material.  
      The bioabsorption of the radiopaque, bioabsorbable material changes the structure and physical properties and chemical composition of the device over time. It may be medically necessary for the device to function with a certain level of performance for a certain amount of time. In one embodiment, the degradation rate of the radiopaque bioabsorbable material described herein can be controlled by properly selecting the bioabsorbable material, the amount of radiopaque agent blended and the particle size of the radiopaque agent blended. In another embodiment, devices made of the radiopaque bioabsorbable material described in the present invention could have a controlled degradation rate, and desired mechanical and/or thermal properties that meet the requirements of the particular application over time, for example the proper in-growth of native tissue. Selection of a suitable bioabsorbable material, factoring into account the bioabsorption of the blended material, is discussed further.  
      In one aspect of the invention, the bioabsorbable base material is selected such that the blend will have certain characteristics suitable for a particular application factoring in both the amount of radiopaque agent and also the bioabsorbable and degradable nature of the final material, and useful life of the device. Desired criteria for physical properties (e.g., strength) and bioabsorption characteristics can be used to extrapolate desired properties of the starting material. The desired properties can be used to select a starting material.  
      To achieve a desired degree of radiopacity, about 20-35 weight percent of tungsten is sufficient in certain embodiments of the invention. When combined, for example, with poly (hydroxybutyrate), the resulting material is sufficiently strong to form a reliable occluder device for implantation. As the bioabsorption process takes place, however, the composition of the occluder  20  changes over time. Different processing rates for the base material (e.g., poly (hydroxybutyrate)) and the radiopaque agent cause the concentration of the radiopaque agent and the strength of the frame  20  to change over time. Using a predicted degradation rate, a desired life span and minimum characteristic of the device  20 , it is possible to calculate a desired starting criteria. In preferred embodiments, the base material used to form occluder frame  20 , has a molecular weight greater than 300,000 to give the frame sufficient strength over the useful life of the device until the septal defect has healed.  
      The described radiopaque bioabsorbable material could be made by mixing the selected radiopaque agent with the selected bioabsorbable material together. This could be done by grinding the selected radiopaque agent into fine powder and physically blending the fine radiopaque agent powder with melted or non melted bioabsorbable material to form a composite. This could also be done by melting the selected radiopaque agent and mixing the melt with melted or non melted bioabsorbable material to form a composite. The process of making the composite could take place in an extruder or other machines. The composite could then be used to make implantable devices. The methods of making implantable devices with the composite include, but are not limited to, injection molding, extrusion, thermoforming, casting, and rotational molding.  
      In one embodiment, the radiopaque bioabsorbable material for occluder  20  can be manufactured by blending fine tungsten particles and the bioabsorbable material together in an extruder. In a preferred embodiment, the tungsten particles can be, for example, between 0.5 and 2.0 microns in diameter. In another preferred embodiment, the weight percent of tungsten in the resulting material is between about 20% and about 35%, preferably between 20% and 35%. In another preferred embodiment, the material is extruded to a thickness of, for example, between 500 and 750 microns to form the occluder. As the bioabsorbable material degrades and its byproducts are absorbed and excreted by the body, tungsten particles are exposed, processed and excreted readily from the body, primarily in the urine.  
      The cross-sectional shape of tube  25  may be circular or polygonal, for example square, or hexagonal. The slits  31  and  41  (or  231  and  241 ) may be disposed on the face of the polygon (i.e., the flat part) or on the intersection of the faces.  
      The tube  25  can be extruded or constructed of a sheet of material and rolled into a tube. The sheet of material could be a single ply sheet or multiple ply. The slits that form the struts could be cut or stamped into the tube prior to rolling the tube to connect the ends to form an enclosed cross section. Various geometrical cross sections are possible including circular, square, hexagonal and octagonal and the joint could be at the vertex or along the flat of a wall if the cross section is of a particular geometry. Various attachment techniques could be used to join the ends of the sheet to form a tube, including welding, heat adhesives, non-heat adhesives and other joining techniques suitable for in-vivo application. One advantage of using a blended radiopaque bioabsorbable material for forming one or more structural members or even the whole occluder frame is that a radiopaque occluder  20  can readily be formed using this processing the radiopaque, bioabsorbable material to form the tube.  
      The surface of tube  25  may be textured or smooth. An occluder  20  having a rough surface produces an inflammatory response upon contact with septum  12  in vivo, thereby promoting faster tissue ingrowth, healing, and closure of aperture  18   a  (shown in  FIG. 1 ). Such a rough surface may be produced, for example, by shaving tube  25  to produce whiskers along its surface. For example, central tube  22  may include such whiskers. Additionally or alternatively, the surface of tube  25  may be porous to facilitate cell ingrowth.  
      The distal side  30  of the occluder  20  (also called the “anchor portion”) is shown in  FIGS. 2C and 2D . The distal side  30  includes four loops  32   a ,  32   b ,  32   c , and  32   d  (collectively referred to as loops  32 ). As previously described, each of loops  32   a - 32   d  are formed by corresponding cut sections  92   b ,  93   b ,  92   a ,  93   a , produced by cutting slits  31 . The application of force F d  to end  39  of tube  25  brings the axial ends of slits  31  together such that struts bow and twist outwardly to form loops  32  of distal side  30  ( FIGS. 2B-2C ). Central tube  22  may be constrained during the application of force F d . One skilled in the art will recognize that any combination of forces sufficient to reduce the axial length of the tube  25  would be sufficient to deploy the distal side  30  of occluder  20 .  
      As illustrated, the loops  32  are evenly distributed about central tube  22  and end  39 . Thus, when the distal side  30  includes four loops  32  (as shown in  FIGS. 2C and 2D ), the four slits  31  are spaced 90 degrees radially apart. Similarly, when the distal side  30  includes six loops  32 , the six slits  31  are spaced 60 degrees radially apart. The angle between radially equally-spaced spaced is determined by the formula (360/n d ), where nd is the total number of loops  32 .  
      Although the distal side  30  of the occluder  20  shown in  FIG. 3A  includes four loops  32 , occluders according to the present invention may include any number of loops  32  necessary for a given application. In particular embodiments, the distal side  30  of occluder  20  includes six loops  32  ( FIG. 4A ). Occluders having between four and ten loops  32  may be formed without requiring significant adjustments in the processes described in this application. However, occluders having less than four or more than ten loops  32  may be complicated to manufacture and difficult deliver through the vasculature.  
      Regardless of the number of loops included in distal side  30  and depending upon the material used to form occluder  20 , the outer perimeter of loops  32  may vary. In at least some embodiments, the outer perimeter of loops  32  is rounded to provide an occluder  20  having a smooth, circular perimeter. As the number of loops  32  in the distal side  30  of occluder  20  increases, it becomes desirable to round the outer perimeters of the loops  32  so as to prevent the infliction of trauma on the surrounding septum  12 .  
      The proximal side  40  of the occluder  20 , shown in side view in  FIG. 2D , also includes four loops,  42   a ,  42   b ,  42   c , and  42   d  (collectively referred to as loops  42 ). As previously described, each of loops  42   a - 42   d  are formed by corresponding cut sections, produced by cutting slits  41 . The application of force F p  to tip  44  of tube  25  brings the axial ends of slits  41  together such that struts bow and twist outwardly to form loops  42  of proximal side  40  ( FIGS. 2C-2D ). Central tube  22  may be constrained during the application of force F p . One skilled in the art will recognize that any combination of forces sufficient to reduce the axial length of the tube  25  would be sufficient to deploy the proximal side  40  of occluder  20 . As described above for distal loops  32 , the loops  42  are evenly distributed about central tube  22  and tip  44 . Similarly, the angle between radially equally-spaced slits  41  in the proximal side  40  is determined by the formula (360/n d ), where nd is the total number of loops  42 .  
      Although the proximal side  40  of the occluder  20  shown in  FIG. 2D  includes four loops  42 , one skilled in the art will recognize that the proximal side  40  of an occluder according to the present invention may include any number of loops  42  required and suitable for a given application. In particular embodiments, the proximal side  40  of occluder  20  includes six loops  42  ( FIG. 4A ). Further, although as illustrated, distal side  30  and proximal side  40  both include four loops, there is no requirement that distal side  30  and proximal side  40  of occluder  20  include the same number of loops. In fact, in particular applications, it may be advantageous to use an occluder  20  in which the distal side  30  contains fewer loops than the proximal side  40 , or vice versa.  
      It will be apparent to one skilled in the art that loops  32  and loops  42  (or loops  232  and  242 ) do not have to be the same size. In one embodiment, loops  32  (or  232 ) are larger in size than loops  42  (or  242 ). In another embodiment, loops  32  (or  232 ) are smaller in size than loops  42  (or  242 ). Size of loops  32  and  42  (or  232  and  242 ) is determined by the lengths of slits  31  and  41  (or  231  and  241 ), respectively. Therefore, absolute and relative lengths of slits  31  and  41  (or  232  and  241 ) can be varied to achieve desired absolute and relative sizes of loops  32  and  42  (or  232  and  242 ).  
      In at least some embodiments, illustrated in  FIGS. 4A , loops  42  of the proximal side  40  are radially offset from loops  32  of the distal side  30  to provide a better distribution of forces around the aperture  18   a . This can be achieved by making cuts to create slits  31  and  41  such that they are radially offset relative to each other. The maximum degree of offset will depend on the number of slits. In general, if slits are equally spaced, the maximum possible offset will be one half of the angle between the loops. For example, if distal side  30  (or proximal side  40 ) contains 4 slits (and therefore 4 loops), loops will be 90 degrees apart (see the formula described above), thereby allowing for maximum degree of offset of one half of 90 degrees (which is 45 degrees) between loops  32  and loops  42 . In a preferred form, when distal side  30  (or proximal side  40 ) contains 4 slits (and therefore 4 loops), loops  42  and loops  32  are offset by 45 degrees. In an alternative embodiment, the degree of offset between loops  32  and  42  ranges from about 30 to about 45 degrees.  
       FIGS. 2E-2H  illustrate another embodiment of the invention, where the occluder  20  is formed from a tube with loops  232  and  242 , produced from the cutting pattern shown in  FIG. 2E . In one embodiment, the proximal side  40  and the distal side  30  of occluder  20  each include eight loops or petals. As shown in  FIG. 2E , the distal portion  30  of the tube  25  includes 8 slits  231  that form  8  extended segments of the tube that form the distal loops or petals  232 . As apparent from the figures, the slits extend the entire distance of the distal portion  30  of the tube  25 , i.e. between central tube  22  and distal end  39 , so that the loops of identical cross-sections are formed. Upon application of force F d  to distal end  39 , extended segments defined by slits  231  bow and twist outward to form distal petals  232  in distal side  30  of the occluder  20 . The movement of the segments during deployment is such that the segments rotate in an orthogonal plane relative to the axis of the device. Central tube  22  may be constrained during the application of force F d , or any combination of forces sufficient to reduce the axial length of the tube may be applied. One end of each of distal petals  232  originates from central tube  22 , while the other end originates from distal end  39 . Proximal petals  242  may be formed in proximal portion  40 , as shown in  FIGS. 2E-2H , making slits  241  between central tube  22  and proximal tip  44 , using the same cutting pattern described above and applying force F p  or combination of forces sufficient to reduce the axial length of the tube by allowing slits  241  to bow and twist outward to form proximal petals  242  in proximal portion  40  of the occluder  20 . One end of each of proximal petals  242  originates from central tube  22 , while the other end originates from proximal tip  44 .  
      One embodiment of the distal side  30  of the occluder  20  (also called the “anchor portion”) is shown in  FIG. 2G and 2H . The distal side  30  includes eight loops  232   a ,  232   b ,  232   c ,  232   d ,  232   e ,  323   f ,  232   g , and  232   h  (collectively referred to as loops  232 ). As previously described, each of loops  232   a - 232   h  is produced by cutting slits  231 . The application of force Fd to end  39  of tube  25  brings the axial ends of slits  231  together such that struts bow and/or twist outwardly to form loops  232  of distal side  30  ( FIGS. 2F-2G ). Central tube  22  may be constrained during the application of force F d . One skilled in the art will recognize that any combination of forces sufficient to reduce the axial length of the tube  25  would be sufficient to deploy the distal side  30  of occluder  20 .  
      As illustrated, the loops  232  are evenly distributed about central tube  22  and end  39 . Thus, when proximal side  30  includes eight loops  232  (as shown in  FIGS. 2G and 2H ), the eight slits  231  are spaced 45 degrees radially apart. The angle between radially equally-spaced slits  231  in distal side  30  is determined by the formula (360/n d ) where n d  is the total number of loops  232 .  
      The proximal side  40  of the occluder  20 , shown in side view in  FIG. 2H , also includes eight loops,  242   a ,  242   b ,  242   c ,  242   d ,  242   e ,  242   f ,  242   g , and  242   h  (collectively referred to as loops  242 ). As previously described, each of loops  242   a - 242   h  is produced by cutting slits  241 . The application of force F p  to tip  44  of tube  25  brings the axial ends of slits  241  together such that struts bow and twist outwardly to form loops  242  of proximal side  40  ( FIGS. 2G-2H ). Central tube  22  may be constrained during the application of force F p . One skilled in the art will recognize that any combination of forces sufficient to reduce the axial length of the tube  25  would be sufficient to deploy the proximal side  40  of occluder  20 . As described above for distal side  30 , the loops  242  are evenly distributed about central tube  22  and tip  44 . Similarly, the angle between radially equally-spaced slits  241  in proximal side  40  is determined by the formula (360/n d ) where n d  is the total number of loops  242 .  
      Although the distal side  30  and the proximal side  40  of the occluder  20 , shown in  FIG. 2H , each include eight loops  232  and  242 , respectively, one skilled in the art will recognize that the distal side  30  and proximal side  40  of an occluder  20  according to the present invention may include any number of loops  232  and  242 , respectively, required and/suitable for a given application. Further, although as illustrated, distal side  30  and proximal side  40  both include eight loops, there is no requirement that distal side  30  and proximal side  40  include the same number of loops. In fact, in particular applications, it may be advantageous to use an occluder  20  in which distal side  30  contains fewer loops than proximal side  40 , or vice versa.  
      It will be apparent to one skilled in the art that loops  232  and loops  242  do not have to be the same size. In one embodiment, loops  232  are larger in size than loops  242 . In another embodiment, loops  232  are smaller in size than loops  242 . Size of loops  232  and  242  is determined by the lengths of slits  231  and  241 , respectively. Therefore, absolute and relative lengths of slits  231  and  241  can be varied to achieve desired absolute and relative sizes of loops  232  and  242 .  
      While loops  232  and  242 , shown in  FIGS. 2F-2H  are illustrated as aligned, this does not have to be the case. In one embodiment, loops  232  and  242  are radially offset from each other. This can be achieved by making cuts to create slits  231  and  241  such that they are radially offset relative to each other. The maximum degree of offset will depend on the number of slits. In general, if slits are equally spaced, the maximum possible offset will be one half of the angle between the loops. For example, if distal side  30  (or proximal side  40 ) contains 8 slits (and therefore 8 loops), the loops will be 45 degrees apart (see the formula described above), thereby allowing for maximum degree of offset of one half of 45 degrees, which is 22.5 degrees between loops  232  and loops  242 . It is understood, that offset can be in either rotational direction (i.e., clockwise and counterclockwise). Therefore, in this example with 8 slits, an offset of 30 degrees is equivalent to an offset of 7.5 degrees in the opposite direction.  
      The cutting pattern illustrated in  FIG. 2E  can be varied, as shown in  FIGS. 2I-2K . According to one embodiment of the invention, the number of slits  231  and  241  cut in the tube  25  can be changed according to the desired number of loops  232  and  242  in the occluder  20  when deployed. The cross-sectional dimensions of loops  232  and  242  are determined by the thickness of tube  25  and the distance between adjacent slits  231  and  241 . The length of slits  231  and  241  determines the length of loops  232  and  242  and the radial dimensions of the deployed occluder  20 . In this manner, the dimensions of loops  232  and  242  can be controlled during production of occluder  20 . For example, as more material is removed from tube  25  during the cutting process used to form slits  231  and  241 , the thickness of loops  232  and  242  decreases. Moreover, any or all of slits  231  and  241  can be cut such that thickness of loops  232  and  242  varies along their length. In some embodiments, it may be desirable to have wider loops  232  and  242  at the location where the loops join tube  25  to create a sturdier device. Alternatively, it may be desirable to have a wider portion elsewhere along the loops  232  and  242  such that occluder  20  is predisposed to bend into a certain shape and arrangement. For example, the portion of loops  232  and  242  nearer central tube  22  may be thinner than the portion of loops  232  and  242  nearer end  39  and tip  44 , respectively, to facilitate bending of the loops  232  and  242 .  
      Slits  231  and  241 , as shown in  FIG. 2J , are cut axially along the length of tube  25 . However, as one of skill in the art will recognize, slits  231  and/or  241  may also be cut along other dimensions of tube  25 . For example, as shown in  FIG. 2I , slits  231  and  241  may be cut at an angle such that they are helically disposed on tube  25 . Angled slits  231  and  241  produce angled loops  232  and  242  during deployment. Further, slits  231  and  241  need not be straight; for example, slits  231  and  241  may be cut as zigzags, S-shaped slits, or C-shaped slits. One skilled in the art will be capable of selecting the angle for the slits  231  and/or  241  and the loop  232  and  242  shape(s) appropriate for a given clinical application. For example, when occluder  20  is formed from a polymer tube  25 , straight loops  232  and  242  may be preferable because they will impart maximum stiffness to occluder  20 . If the tube  25  is formed of a stiffer material, the angled slits  231  and/or  241  may provide a more desired stiffness to the occluder  20 .  
      In one embodiment, the occluder  20  has loops according to  FIGS. 2A-2D  on one side and loops according to  FIGS. 2E-2H  on the other side. For example, occluder  20  may comprise loops  42  on the proximal side  40  and loops  232  on the distal side  30 , or it may comprise loops  242  on the proximal side  40  and loops  32  on the distal side  30 .  
      In one embodiment, for example as shown in  FIG. 2H , each loop  242  and  232  has some amount of twist, i.e., when the loop is formed, the proximal side of the loop is radially offset with respect to the distal side of the loop. Loops  242  and/or  232 , however, need not have any twist.  
       FIG. 2M , for example, illustrates an embodiment of the occluder with slits cut as illustrated in  FIG. 2L . In this embodiment, neither loops  32  nor loops  42  are twisted. It will be apparent to one skilled in the art that any combination of twisted and untwisted loops may be used. Furthermore, an occluder can have any combination of loops with different bends and twists if desired.  
      In one embodiment, loops  32  (or  232 ) of distal side  30  are bent to form concave loops, while loops  42  (or  242 ) of proximal side  40  are flat ( FIG. 11 ). In this embodiment, the outermost portions of loops  42  (or  242 ) of proximal side  40  oppose the outermost portions of the loops  32  (or  232 ) of the proximal side  30 , as described in more detail below, thereby creating a desirable opposing force that secures the occluder  20  at its desired location in vivo. So configured, the opposing compressive forces exerted by sides  30  and  40  on the septum  12  following deployment of occluder  20  in vivo is advantageous in certain circumstances, such as closing certain kinds of PFOs. In another embodiment, loops  42  (or  242  of the proximal side  40  are bent, while loops  32  (or  232 ) of the distal side  30  are flat. In yet another embodiment, loops  42  (or  242 ) of the proximal side  40  and loops  32  (or  232 ) of the distal side  30  are bent.  
      Whatever the number and shapes of loops  32  and  42  (or  232  and  242 ), the loops  32  and  42  (or  232  and  242 ) may be of varied sizes to facilitate delivery of occluder  20 , e.g. to improve collapsibility of the occluder  20  or to enhance its securement at the delivery site. For example, loops  32  and  42  (or  232  and  242 ) that are sized to better conform with anatomical landmarks enhance securement of the occluder  20  to the septum  12  in vivo. As indicated above, the cross-sectional dimensions of loops  32  and  42  (or  232  and  242 ) are determined by the thickness of tube  25  and the distance between adjacent slits  31  and  41  (or  231  and  241 ). The length of slits  31  and  41  (or  231  and  241 ) determines the size of loops  32  and  42  (or  232  and  242 ) and the radial extent of the deployed occluder  20 . In at least some embodiments, each of distal side  30  and proximal side  40  has a diameter in the range of about 10 mm to about 45 mm, with the particular diameter determined by the size of the particular defect being treated. In particular embodiments, the diameter of distal side  30  will be different than that of proximal side  40  so as to better conform to the anatomy of the patient&#39;s heart.  
      According to one embodiment of the invention, the loops of the occluder are formed by struts as illustrated in  FIG. 2B . Sections  91   a ,  91   b ,  92   a ,  92   b ,  93   a ,  93   b ,  94   a , and  94   b  are of equal distance, being about ⅓ the length of distal side  30  (i.e., the distance between central tube  22  and end  39 ) of the tube  25 . According to another embodiment of the invention, other lengths of sections can be used to produce advantageous results. In general, the longer the length of the hemispherical struts, such as half sections  91   a ,  91   b ,  94   a , and  94   b , the stiffer the occluder will be. The longer the length of the quarter (as shown) struts, such as half sections  92   a ,  92   b ,  93   a , and  93   b , the less stiff the occluder will be. In general, the hemispherical cut (one of the two) may be 20-40% of the overall length of the distal side (or proximal side) the tube. Specifically, the hemispherical cuts could be 40% of the overall length of the distal side (or proximal side) and then the quarter cut could be 20% of the overall length of the distal side (or proximal side) of the tube  25 . Also, the lengths of the hemispherical cuts need not be the same. It may be advantageous to shorten one or the other side of the hemispherical cut based on a desired stiffness characteristic for a particular application of the occluder. In an alternative structure, the hemispherical cuts can be extended in a range up to 100% of the length of the distal side (or the proximal side) of the occluder, while still enabling the bow and twist of the struts.  
      As indicated previously and shown in  FIGS. 2A-2H , distal side  30  and proximal side  40  of occluder  20  are connected by central tube  22 . The central tube  22  is formed by the portion of tube  25  between the distal side  30  of tube  25 , which contains slits  31 , (or  231 ) and the proximal side  40  of tube  25 , which contains slits  41  (or  241 ). Given that the central portion of tube  25  remains uncut during the cutting process, the central portion of the tube maintains its profile upon the application of forces F d  and F p  and does not bow and twist outward as the proximal and distal sides are adapted to do.  
      According to one embodiment, central tube  22  is straight, as illustrated in  FIGS. 2D and 2H , where the central tube  22  is perpendicular to loops  32  and  42  (or  232  and  242 ). According to another embodiment of the invention, central tube  22  is positioned at an angle θ relative to the proximal side  40  of the occluder  20 , as shown, for example, in  FIGS. 5B and 11 . The shape of central tube  22  included in a given occluder is, at least in part, determined by the nature of the aperture  18 . An occluder having a straight central tube  22  is particularly suited to treat an anatomical anomaly including a perpendicular aperture, such as an ASD and certain PFOs. Often, however, anatomical anomalies, such as certain PFOs, have non-perpendicular apertures and are sometimes quite significantly non-perpendicular. An occluder having an angled central tube  22  is well-suited for treatment of such defects, such that the angle of the anatomical aperture  18  is more closely matched by the pre-formed angle θ of the occluder  20 . Also, the length of central tube  22  can be varied depending on the anatomy of the defect being closed. Accordingly, the distal side  30  and proximal side  40  of occluder  20  are more likely to be seated against and minimize distortion to the septum  12  surrounding the aperture  18 , as shown in  FIG. 13 . A well-seated occluder  20  is less likely to permit blood leakage between the right  11  and left  13  atria, and the patient into which the occluder  20  has been placed is, therefore, less likely to suffer embolisms and other adverse events.  
      Advantageously, angled central tube  22  also facilitates delivery of occluder  20  because it is angled toward the end of the delivery sheath. In at least some embodiments, the angle θ is about 0-45 degrees. To form the angle θ, proximal side  40  of the occluder  20  bends depending upon, among other factors, the material used to form occluder  20 . Accordingly, depending upon design considerations, tip  44  and end  39  may be aligned with central tube  22  or perpendicular to proximal side  40  or some variation in between. One skilled in the art will be capable of determining whether a straight or angled central tube  22  is best suited for treatment of a given anatomical aperture  18  and the appropriate angle θ, typically in the range between about 30 and about 90 degrees. Sometimes, angles of about 0 degrees to about 30 degrees can be used in an oblique passageway such as a very long tunnel PFO. One skilled in the art will recognize that the concept of an angled central tube may be applied to septal occluders other than those disclosed herein.  
      When central tube  22  is positioned at angle θ, distal side  30  and proximal side  40  of occluder  20  may be configured such that they are either directly opposing or, as shown in  FIGS. 5B, 11  and  12 , offset by distance A. One skilled in the art will, of course, recognize that the shape and arrangement of either or both of distal side  30  and proximal side  40  may be adjusted such that the compressive forces they apply are as directly opposing as possible. However, in some clinical applications, an occluder  20  having an offset of distance A may be particularly desirable. For example, as shown in  FIGS. 5B , and  11 - 12 , if the septum  12  surrounding aperture  18  includes a disproportionately thick portion (e.g. septum secundum  16  as compared to septum primum  14 ), the offset A may be used to seat occluder  20  more securely upon septum  12 . Moreover, the offset A allows each of sides  30  and  40  to be centered around each side of an asymmetric aperture  18 .  
      When a central tube  22  at angle θ is included in occluder  20 , a marker is required to properly orient the occluder  20  in its intended in vivo delivery location. For example, a platinum wire may be wrapped around one of loops  32  or  42  (or one of loops  232  or  242 ) so as to permit visualization of the orientation of the occluder  20  using fluoroscopy. Alternatively, other types of markers may be used, e.g. coatings, clips, etc. As one skilled in the art would appreciate, the radiopaque marker could be blended in with the extrudate and thus provide visibility under fluoroscopy. As will be readily understood by one skilled in the art, the orientation of a non-symmetrical occluder  20  during delivery is of great importance. Of course, when a non-symmetrical occluder  20  is used, the periphery of the occluder  20  may be configured such that the clamping force applied by the proximal side  40  is directly opposed to that applied by the distal side  30 .  
      Upon deployment in vivo (a process described in detail below), an occluder  20  according to the present invention applies a compressive force to the septum  12 . Distal side  30  is seated against the septum  12  in the left atrium  13 , central tube  22  extends through the aperture  18 , and proximal side  40  is seated against the septum  12  in the right atrium  11 . At least some portion of each of loops  32  and  42  (or  232  and  242 ) contacts septum  12 . In particular embodiments, a substantial length of each of loops  32  and  42  (or  232  and  242 ) contacts septum  12 . As illustrated in the representative Figures, the proximal side  40  and distal side  30  of occluder  20  overlap significantly, such that the septum  12  is “sandwiched” between them once the occluder  20  is deployed. According to at least some embodiments and depending upon the material used to form occluder  20 , the loops  32  and  42  (or  232  and  242 ) provide both a radially-extending compressive force and a circumferential compressive force to septum  12 . In these embodiments, the compressive forces are more evenly and more widely distributed across the surface of the septum  12  surrounding the aperture  18  and, therefore, provide the occluder  20  with superior dislodgement resistance as compared to prior art devices. As used in this application, “dislodgement resistance” refers to the ability of an occluder  20  to resist the tendency of the force applied by the unequal pressures between the right  11  and left  13  atria (i.e. the “dislodging force”) to separate the occluder  20  from the septum  12 . Generally, a high dislodgement resistance is desirable.  
      Loops  32  and  42  (or  232  and  242 ) are also configured to minimize the trauma they inflict on the septum  12  surrounding aperture  18 . Specifically, as indicated previously, the outer perimeter of loops  32  and  42  (or  232  and  242 ) may be rounded.  
      According to one embodiment of the invention, for example, as illustrated in  FIGS. 2B-2D , the circumferential portions of loops  32  and  42  are thinner than the orthogonally-extending portions of loops  32  and  42 ; therefore, the center of the occluder  20  is stronger than its perimeter. Accordingly, outer perimeter of loops  32  and  42  of occluder  20  has a low compression resistance. As used in this application, “compression resistance” refers to the ability of an occluder  20  to resist the lateral compressive force applied by the heart as it contracts during a heartbeat. Generally, an occluder that resists compressive force, i.e. has high compression resistance, is undesirable because its rigid shape and arrangement may cause trauma to the septum  12 , the right atrium  11 , and/or the left atrium  13 .  
      According to at least some embodiments of the present invention, occluder  20  further includes a catch system, generally indicated at  131 , that secures the occluder  20  in its deployed state. The catch system  131 , in general, maintains the shape and arrangement of loops  32  and  42  (or  232  and  242 ) of occluder  20 , once the occluder  20  has been deployed. Catch system 131  reduces and maintains the axial length of the occluder  20  so that occluder  20  maintains its deployed state, is secured in the aperture  18 , and consistently applies a compressive force to septum  12  that is sufficient to close aperture  18 . Catch system  131  is particularly advantageous when the occluder  20  is formed of a polymeric material, as previously described, because the polymeric occluder  20  may be deformed during delivery such that it may not fully recover its intended shape once deployed. By reducing and maintaining the axial length of occluder  20  once it has been deployed in vivo, catch system  131  compensates for any undesirable structural changes suffered by occluder  20  during delivery. In some embodiments, catch system  131  includes a ceramic material or a material selected from the group consisting of metals, shape memory materials, alloys, polymers, bioabsorbable polymers, and combinations thereof. In particular embodiments, the catch system may include nitinol or a shape memory polymer. Further, the catch system may include a material selected from the group consisting Teflon-based materials, polyurethanes, metals, polyvinyl alcohol (PVA), extracellular matrix (ECM) or other bioengineered materials, synthetic bioabsorbable polymeric scaffolds, collagen, and combinations thereof.  
      Catch system  131  may take a variety of forms, non-limiting examples of which are provided in  FIGS. 6A-6E . For example, as shown in  FIG. 6A , catch system  131  includes two catch elements, e.g., balls,  133  and  135 , connected by wire  134 . The catch system and catch element are preferably made of the same material as the occluder, although based on design selection, they could be made of the same or different material. In certain circumstances, it may be necessary to make them of different material. As illustrated in  FIG. 6A , delivery string  137  is attached to ball  133  and is then extended through end  39 , distal portion  30  of tube  25 , central tube  22 , proximal portion  40  of tube  25 , and tip  44 , such that ball  133  is located between central tube  22  and end  39  and ball  135  is located on the distal side of central tube  22 . The function of catch system  131  is shown in  FIGS. 6B-6E . Ball  133  is designed such that, upon the application of sufficient pulling force F 1 , to delivery string  137 , it passes through central tube  22  ( FIG. 6B ) and tip  44  ( FIG. 6C ). Ball  133  cannot reenter tip  44  or central tube  22  without the application of a sufficient, additional force. In this manner, ball  133  may be used to bring together the distal side  30  and the proximal side  40 , thereby reducing and maintaining the axial length of occluder  20 . Obviously, during the application of pulling force F 1 , the tip  44  of occluder  20  must be held against an object, such as a delivery sheath. Ball  135  is designed such that, upon application of sufficient pulling force F 2  to delivery string  137 , it passes through end  39  ( FIG. 6D ) and central tube  22  ( FIG. 6E ). The pulling force F 2  required to move ball  135  through end  39  and central tube  22  is greater than the pulling force F 1 , required to move ball  133  through central tube  22  and tip  44 . However, ball  135  cannot pass through tip  44 . Thus, the application of sufficient pulling force F 2  to ball  135  releases distal side  30  and proximal side  40 , as described in more detail below. It should be noted that while catch elements  133  and  135  are illustrated as spherical elements in  FIGS. 6A-6E , catch elements  133  and  135  may take any suitable shape. For example, catch elements  133  and  135  may be conical. The narrow portions of conical catch elements  133  and  135  point toward tip  44  of proximal side  40 . One possible mode of recovery or retrieval for this device is simply reversing the implantation procedure. Of course, other modes of recovery or retrieval are possible, some of which are described in this specification.  
      A different system for securing the device in the deployed state is shown in  FIGS. 7A-7C . A locking mechanism  191  includes a hollow cylinder  141  having at least two half-arrows  143  and  145  located at its proximal end ( FIG. 7A ). Cylinder  141  enters tip  44  under application of pulling force F. to delivery string  137 . As cylinder  141  enters tip  44 , half-arrows  143  and  145  are forced together such that the diameter of the proximal end of cylinder  141  is reduced ( FIG. 7B ). Under continued application of pulling force F 1 , half-arrows  143  and  145  pass through tip  44  and expand to their original shape and arrangement ( FIG. 7C ). Given that half-arrows  143  and  145  extend beyond the diameter of tip  44 , the axial length of an occluder  20  including the locking mechanism  191  shown in  FIGS. 7A-7C  is maintained in its reduced state. If the implant needs to be removed or repositioned, the locking mechanism  191  shown in  FIGS. 7A-7C  may be released by moving half-arrows  143  and  145  together such that the diameter of the proximal end of cylinder  141  is smaller than that of tip  44  and cylinder  141  passes through tip  44 . Cylinder  141  may then be withdrawn from tip  44 .  
      One skilled in the art will recognize that catch system  131  may assume numerous configurations while retaining its capability to reduce and maintain the axial length of occluder  20  such that occluder  20  maintains its deployed state. For example, catch system  131  may include a threaded screw, a tie-wrap, or a combination of catch systems  131 . Furthermore, catch system  131  may include multiple members that may provide a stepped deployment process. For example, catch system  131  as depicted in  FIGS. 6A-6E  may include three balls. In this configuration, one ball is used to secure the distal end  30  of occluder  20  and another ball is used to secure the proximal end  40  of occluder  20 , and the third ball is secured to the distal end. Any suitable catch system  131  may be incorporated into any of the embodiments of occluder  20  described herein. One skilled in the art will be capable of selecting the catch system  131  suitable for use in a given clinical application.  
      Occluder  20  may be modified in various ways. According to some embodiments of the present invention, distal side  30  and/or proximal  40  side of occluder  20  may include a tissue scaffold. The tissue scaffold ensures more complete coverage of aperture  18  and promotes encapsulation and endothelialization of septum  12 , thereby further encouraging anatomical closure of the septum  12 . The tissue scaffold may be formed of any flexible, biocompatible material capable of promoting tissue growth, including but not limited to polyester fabrics, Teflon-based materials, ePTFE, polyurethanes, metallic materials, polyvinyl alcohol (PVA), extracellular matrix (ECM) or other bioengineered materials, synthetic bioabsorbable polymeric scaffolds, other natural materials (e.g. collagen), or combinations of the foregoing materials. For example, the tissue scaffold may be formed of a thin metallic film or foil, e.g. a nitinol film or foil, as described in United States Patent Publ. No. 2003/0059640 (the entirety of which is incorporated herein by reference). In those embodiments, where occluder  20  includes a tissue scaffold, the scaffold may be located on the outside the face of distal side  30  and proximal side  40  of the occluder, with an alternative of including scaffold also inside the face of distal side  30  and proximal side  40  of the occluder. Also, the tissue scaffold could be disposed against the tissue that is sought to be occluded, such as the septum  12  so that the proximity of the tissue scaffold and septum  12  promotes endothelialization. Loops  32  and  42 , (or  232  and  242 ), can be laser welded, ultrasonically welded, thermally welded, glued, or stitched to the tissue scaffold to securely fasten the scaffold to occluder  20 . One skilled in the art will be able to determine those clinical applications in which the use of tissue scaffolds and/or stitches is appropriate.  
      Occluder  20  may be further modified so that it lacks end  39  and tip  44 , as shown in  FIGS. 8A-8C , and, therefore, has a reduced septal profile. Such an occluder may be formed in several ways. For example, according to one embodiment, slits  31  and  41  are extended through end  39  and tip  44 , respectively, of tube  25  during the cutting process. This cutting pattern produces struts  32  that deform during deployment to produce incomplete loops  32 . One side of the device, facing the viewer as shown in  FIG. 8A , is formed by slits  31  that extend along the tube  25  to varying lengths. The tube  25  is cut in half to form half sections  154   a  and  154   b . The half sections  154   a  and  154   b  are further cut to a proximal distance from the end  39  into quarter sections  155   a ,  156   a ,  155   b , and  156   b . The ends of the quarter sections  155   a  and  155   b  are joined at “free” ends  153  to close the loop  32 . Similarly, the free ends of quarter sections  156   a  and  156   b  may be joined by appropriate cutting, see  FIG. 8   b . The ends may be joined using any suitable connectors, e.g.,  151 , e.g., welds. One of skill in the art will recognize that the free ends  153  of loops  32  connected using other means, including but not limited to seams and bonds obtained by heat or vibration.  
      In the above embodiment, the slits in the quarter sections are run completely through the end of the tube  39 . In an alternative embodiment, the end  39  may remain uncut, thereby eliminating the need for a weld to join the quarter sections together.  
      The embodiment illustrated in  FIGS. 8A-8C  depicts an occluder  20  in which both sides are formed according to the above-described design. Alternatively, an occluder  20  according to the present invention may include a hybrid structure, wherein one side is designed according to the embodiment shown in  FIGS. 8A-8C  and the other side is designed according to other types of structures disclosed in this application.  
      Occluder  20  may be prepared for delivery to an aperture  18  in any one of several ways. Slits  31  and  41  (or  231  and  241 ) may be cut such that tube  25  bends into its intended configuration following deployment in vivo. Specifically, slits  31  and  41  (or  231  and  241 ) may be cut to a thickness that facilitates the bending and formation of loops  32  and  42  (or  232  and  242 ). Upon the application of forces F d  and F p , tube  25  bends into its intended deployed configuration. Alternatively and/or additionally, tube  25  formed of a shape memory material may be preformed into its intended configuration ex vivo so that it will recover its preformed shape once deployed in vivo. According to at least some embodiments, these preforming techniques produce reliable deployment and bending of occluder  20  in vivo. An intermediate approach may also be used: tube  25  may be only slightly preformed ex vivo such that it is predisposed to bend into its intended deployed configuration in vivo upon application of forces F d  and F p.    
      An occluder  20  as described herein may be delivered to an anatomical aperture  18  using any suitable delivery technique. For example, distal side  30  and proximal side  40  of occluder  20  may be deployed in separate steps, or both distal side  30  and proximal side  40  of occluder  20  may be deployed in the same step. One delivery method will be described in detail herein.  
      When a patient has an implanted device made of a radiopaque bioabsorbable material, the position and orientation of the device can be monitored during the implantation procedure. At a later time, the device can be again viewed radiographically, and changes in its density or size resulting from bioabsorption, as well as in its position and/or orientation, can be assessed. This feature can be useful both for monitoring the health and recovery progress of patients, as well as for developing and improving the form of and materials used in future devices based on the observed results.  
      As shown in  FIGS. 9A-9H , a delivery sheath  161  containing pusher sleeve  169  (shown in  FIG. 9H ) is used to deliver occluder  20  including the catch system  131  illustrated in  FIGS. 6A-6E . Sheath  161  contains occluder  20  in its elongated, delivery form ( FIG. 9A ). As shown in  FIG. 9B , delivery sheath  161  is first inserted into the right atrium  11  of the patient&#39;s heart. Sheath  161  is next inserted through aperture  18  located in the septum  12  (which, in this example, is a PFO tunnel) and into the left atrium  13  ( FIG. 9C ). Distal side  30  of occluder  20  is then exposed into the left atrium  13 , as shown in  FIG. 9D . Following deployment of distal side  30 , pulling force F 1  is applied to delivery string  137  while pusher sleeve  169  is holding the occluder  20  in place such that ball  133  passes through the central tube  22 , thereby securing distal side  30  into its deployed state ( FIG. 9E ). Sheath  161  is further withdrawn through the aperture  18  and into the right atrium  11 , such that central tube  22  is deployed through the aperture  18  ( FIG. 9F ). Proximal side  40  of occluder  20  is then exposed into the right atrium  11  ( FIG. 9G ), and pulling force F 1  is again applied to delivery string  137  while pusher sleeve  169  is holding the occluder  20  in place such that ball  133  passes through tip  44 , thereby securing the proximal side  40  into its deployed state ( FIG. 9H ). When properly deployed, occluder  20  rests within the aperture  18 , and the distal side  30  and proximal side  40  exert a compressive force against septum primum  14  and septum secundum  16  in the left  13  and right  11  atria, respectively, to close the aperture  18 , i.e. the PFO. When occluder  20  is properly deployed, delivery string  137  is detached from catch system  131 , including balls  133  and  135  and a connecting member, and sheath  161  is then withdrawn from the heart. In the event occluder  20  is not properly deployed after performing the procedure described above, the occluder  20  may be recovered by reversing the steps of the above described delivery sequence.  
      In an alternative recovery technique, the occluder  20  may be recovered and repositioned by catch system  131  as shown in  FIG. 10A-10D . Pusher sleeve  169  in sheath  161  is positioned against tip  44  of the occluder  20  in the right atrium  11  ( FIG. 10A ). Pulling force F 2  is applied to delivery string  137 , such that ball  135  passes through end  39  and into central tube  22 , thereby releasing distal side  30  from its deployed state ( FIG. 10B ). Force F 2  is again applied to delivery string  137  so that ball  135  subsequently passes through central tube  22 , thereby releasing proximal side  40  from its deployed state ( FIG. 10C ). Delivery string  137  is then pulled further such that occluder  20 , now in its elongated state, is retracted into sheath  161  ( FIG. 10D ). Following recovery of occluder  20 , sheath  161  may be withdrawn from the heart and another occluder inserted in the desired delivery location as described above and shown in  FIGS. 9A-9H .  
      One skilled in the art will recognize that the occluders described herein may be used with anti-thrombogenic compounds, including but not limited to heparin and peptides, to reduce thrombogenicity of the occluder and/or to enhance the healing response of the septum  12  following deployment of the occluder in vivo. Similarly, the occluders described herein may be used to deliver other drugs or pharmaceutical agents (e.g. growth factors, peptides). The anti-thrombogenic compounds, drugs, and/or pharmaceutical agents may be included in the occluders of the present invention in several ways, including by incorporation into the tissue scaffold, as previously described, or as a coating, e.g. a polymeric coating, on the tube(s)  25  forming the distal side  30  and proximal side  40  of the occluder  20 . Furthermore, the occluders described herein may include cells that have been seeded within the tissue scaffold or coated upon the tube(s)  25  forming the distal side  30  and proximal side  40  of the occluder  20 .  
      One skilled in the art will further recognize that occluders according to this invention could be used to occlude other vascular and non-vascular openings. For example, the device could be inserted into a left atrial appendage or other tunnels or tubular openings within the body.  
      The radiopaque bioabsorbable material described in present invention could be used to make devices for repairing, replacing, remodeling or closing intracardiac septal and atrial appendage defects, for sealing of a percutaneous puncture in a blood vessel or organ; stents, sutures and varies and orthopedic applications.  
      Having described certain embodiments, it should be apparent that modifications can be made without departing from the scope of the invention as defined by the appended claims. For example, certain materials have been stated, although other suitable materials could be used. In another example, a radiopaque bioabsorbable material made with one radiopaque agent blending with one bioabsorbable material have been described, although a mixture of radiopaque agents could be blended with one or a mixture of bioabsorbable material to make a radiopaque bioabsorbable material.  
      Having described preferred embodiments of the invention, it should be apparent that various modifications may be made without departing from the spirit and scope of the invention, which is defined in the claims below.