referring to fig1 a and 1b , an endoprosthesis 2 includes a plurality of generally circumferential struts 6 and connecting struts 8 . the circumferential struts 6 can be directly interconnected to one another and / or they can be connected by connecting struts 8 . the endoprosthesis can be delivered into a body lumen , such as a vasculature , in a reduced diameter configuration and then expanded into contact with the lumen wall to , e . g ., maintain patency at the site of an occlusion . referring as well to fig2 a and 2b , a perspective cross - sectional view through a strut 6 , 8 , the strut is formed of a body 4 made of a bioerodible material , e . g ., a metal such as magnesium . the strut also includes an erosion modifying material in layers 16 on the surface of the body 4 . the erosion modifying material modifies the rate at which portions of the body 4 are eroded when the stent is placed in the lumen . in embodiments , the erosion modifying material is a non - erodible material or an erodible material that erodes at a different rate than the erodible material forming the body 4 so as to form a barrier that reduces or prevents exposure of the body 4 to physiological body fluids . for example , the erosion modifying material can be a ceramic such as an oxide of the erodible material forming the body 4 ( e . g ., magnesium oxide ). the erosion modifying material can include a therapeutic drug . the pattern of the erosion modifying material on the surface of the body , and the geometry and dimensions of the body are selected so that a desirable erosion geometry forms as the body erodes . in particular , the geometry of the eroding body can be selected to maintain the mechanical strength of the strut , even after substantial erosion , and to reduce premature fragmentation or fragmentation into large pieces . continuing to refer to fig2 a and 2b , the strut 6 , 8 extends in a direction along a z - axis generally perpendicular to a plane on an x - y axis . the cross - section or strut dimensions in the x - y plane are generally smaller than along the z - axis . in the illustrated embodiment , the strut has an initial rectangular geometry with an abluminal surface 10 ( e . g ., an exterior surface 10 ), an adluminal surface 12 ( e . g ., an interior surface 12 ) and two side surfaces 14 . the strut has an initial thickness t between the surfaces 10 and 12 and an initial width w before the stent is implanted . the erosion modifying material is provided in corresponding coextensive layers 16 on the abluminal and adluminal surfaces of the body , which inhibit erosion from those surfaces . in some embodiments , each layer 16 can include a plurality of layers , which can include the same or different materials . within each layer , the composition can include the same or different materials at different portions of the layer . fig3 a and 3b illustrate the strut after erosion upon implantation in a body lumen . erosion of the body 4 occurs primarily at the side surfaces , yielding a i - beam geometry where the top and bottom of the i correspond to the abluminal and adluminal surfaces , respectively . the i - beam geometry provides mechanical strength to the strut even though a substantial amount of the body 4 , e . g ., 50 % or more of the area in the x - y plane has been eroded . moreover , at least one dimension of the strut , the vertical line of the i corresponding to the strut thickness is maintained . the geometry of the i - beam can result as a function of surface diffusion and / or mass transport processes of the eroding endoprosthesis . referring as well to fig3 c , after further erosion , e . g ., 75 percent or more of the strut has eroded , the i - beam geometry is still maintained . referring as well to fig3 d , after 90 percent or more of the strut has eroded , the i - beam geometry is broken , and the erosion modifying layer is itself eroded . the erosion modifying layer reduces the likelihood that loose fragments of the body 4 will break off ; the slow erosion of the strut also provides time for endothelialization prior to complete erosion . erosion to a desirable geometry can be controlled by selecting the initial dimensions and geometry of the bioerodible body , and the pattern and nature of the erosion control material . the bioerodible body preferably has a geometry such that the ratio of the characteristic dimensions in the x - y direction is between about 2 : 1 to 1 : 2 , e . g ., about 1 : 1 . for example , for a strut with a rectangular cross section , the ratio of the thickness to the width is about 2 : 1 to about 1 : 2 . for a strut with a circular cross section , the ratio of its radii or diameters in the cross section is 1 : 1 . as discussed above , the erosion modifying layer can include a biodegradable and / or non - biodegradable second material with a lower erosion rate than a first material of a bioerodible body of an endoprosthesis . examples are ceramics , metals or polymers , which provide a barrier that reduces the exposure of the erodible body to fluids by requiring diffusion of body fluids through the erosion modifying layer or preventing exposure of the erodible body to body fluids . in some embodiments , the erosion modifying layer can include a plurality of layers , which can include the same or different materials . within each layer , the composition can include the same or different materials at different portions of the layer . in some embodiments , the erosion rate of the erosion modifying layer is from about 10 % ( e . g ., from about 25 %, from about 50 %, from about 150 %, from about 200 percent , from about 400 percent , from about 600 percent , from about 8000 %) less than the erosion rate of a bioerodible body to about 1000 % ( e . g ., to about 800 %, to about 600 %, to about 400 %, to about 200 %, to about 150 %) less than the erosion rate of a bioerodible body . in some embodiments , the erosion rate of erosion modifying layer can range from about 0 . 001 % ( e . g ., from about 0 . 01 %, from about 0 . 1 %, from about 0 . 5 %) to about 1 % ( e . g ., to about 0 . 5 %, to about 0 . 1 %, to about 0 . 01 %) of the initial mass of that portion per day . the erosion rate of a bioerodible body can range from about 0 . 2 % ( e . g ., from about 0 . 5 %, from about 1 %, from about 2 %) to about 5 % ( e . g ., to about 2 %, to about 1 %, to about 0 . 5 %) of the initial mass of that portion per day . in some embodiments in which the erosion modifying layer includes a non - biodegradable second material , the second material is radiopaque and can provide long term identification of the endoprosthesis location ( e . g ., by x - ray , mri ) within a body . in some embodiments , the erosion modifying layer includes stainless steel , which can promote endothelialization of the endoprosthesis and / or reduce thrombus risk . the thickness of the erosion modifying material can be selected to control the rate of exposure of the erodible material to body fluid . the thickness can be uniform , variable in a gradient manner , variable in a stepwise manner , and / or variable in a random manner along a length or a width of an endoprosthesis . the thickness of the layers can range from about 1 nm ( e . g ., from about 5 nm , from about 25 nm , from about 100 nm , from about 500 nm , from about 800 nm , from about 1 μm , from about 2 μm , from about 3 μm , from about 4 μm , from about 5 μm , from about 6 μm , from about 7 μm , from about 8 μm , from about 9 μm ) to about 10 μm ( e . g ., to about 9 μm , to about 8 μm , to about 7 μm , to about 6 μm , to about 5 μm , to about 4 μm , to about 3 μm , to about 2 μm , to 1 μm , to about 800 nm , to about 500 nm ). the thickness of an erosion modifying layer can also be expressed as a fraction of a thickness of a bioerodible body . for example , the thickness of the erosion modifying layer can be at most about 50 % ( e . g ., at most about 40 %, at most about 30 %, at most about 20 %, at most about 10 %, at most about 5 %, at most about 2 %, at most about 1 %) the thickness of the bioerodible body and / or at least about 0 . 001 %, ( e . g ., at least about 0 . 004 %, at least about 0 . 01 %, at least about 0 . 1 %, at least about 1 %, at least about 2 %, at least about 5 %, at least about 10 %, at least about 25 %) the thickness of the bioerodible body . a strut can erode in a variety of erosion patterns and / or geometries . for example , referring to fig4 a , in some embodiments a strut 6 ′, 8 ′ has an initial rectangular geometry with an erodible body 4 ′, an abluminal surface 10 ′, an adluminal surface 12 ′ and two side surfaces 14 ′. the strut has an initial thickness t ′ between the surfaces 10 ′ and 12 ′ and an initial width w ′ before the stent is implanted . erosion modifying layers 16 ′ are provided in corresponding coextensive layers on the side surfaces of the body , which inhibit erosion from those surfaces . fig4 b illustrates the strut after partial erosion , upon implantation in a body lumen . erosion of body 4 ′ occurs primarily at the abluminal and adluminal surfaces , resulting in a geometry as shown in fig4 b . thickness t ′ decreases from the side surfaces toward the center of the strut . however , the width w ′ is maintained even though a substantial amount of the body 4 ′, e . g ., 50 % or more of the area in the x - y plane , has been eroded . the erosion geometry can provide mechanical strength to the eroded strut . in some embodiments , erosion modifying layers are provided intermittently along the perimeter of a strut . as an example , as shown in fig5 a , layers 22 are provided at the four edges of strut 20 . as shown in fig5 b , upon implantation , erosion of body 24 occurs starting at the uncoated side surfaces , resulting in a x - shaped geometry at the x - y plane . as an example , as shown in fig6 a , erosion modifying layers 32 are provided at opposing surfaces 35 , 36 including the four edges , and intermittently at the remaining surfaces 38 , 39 of strut 30 . upon implantation , as shown in fig6 b , partial erosion of body 34 occurs at the exposed surfaces , resulting in a strut having grooves along the abluminal and adluminal surfaces . referring to fig7 a , erosion modifying layers 42 are provided intermittently along the surfaces of strut 40 , which result in erosion of body 44 starting at the exposed surfaces upon implantation in a body lumen . referring to fig7 b , after erosion , the strut has a series of grooves forming an interdigitated pattern . the erosion geometry of struts 20 , 30 , 40 in fig5 b , 6 b , 7 b can provide mechanical strength to the eroded strut . in some embodiments , as shown in fig8 a , a strut 50 has an initial circular geometry at the x - y plane with initial radius and / or diameter . an erosion modifying layer 52 can intermittently coat the strut surface along the perimeter of the circular strut . upon implantation , erosion starting at the uncoated surface 54 of strut 50 can result in a grooved geometry that maintains the initial radii in the lobe regions , for example , as shown in fig8 b . referring to fig9 a and 9b , in some embodiments , a strut 60 is coated with an erosion modifying layer 62 . erosion modifying layer 62 can curve along the z - axis , for example , in a sinusoidal pattern . upon implantation , erosion of an erodible body 64 starting at the uncoated surfaces 66 can result in a curved geometry along the z axis . further erosion can result in a i - beam geometry or grooved geometry . in some embodiments , a strut can erode to generate a bioerodible body with , for example , an overall grooved geometry at the x - y plane , an overall rectangular geometry at the x - y plane , or combinations thereof . in some embodiments , an erosion modifying layer can coat two or more adjoining surfaces of a strut , or two or more non - adjoining surfaces of a strut . an endoprosthesis having a strut with one or more surfaces covered by an erosion modifying layer can , for example , have a lower erosion rate , maintain structural integrity for a longer duration , limit the degrees of freedom available for erosion , and reduce the risk associated with penetrating localized erosion and attendant fragmentation . referring to fig1 a and 11a , a strut 70 , 80 coated with erosion modifying layers 72 , 82 on three surfaces can erode to generate a bioerodible body 74 , 84 having , for example , an overall grooved or concave geometry at the x - y plane ( e . g ., fig1 b ), or an overall rectangular cross - section ( fig1 b and 11c ), or combinations thereof . the erosion modifying layer can have a uniform thickness along the length of the endoprosthesis , or the erosion modifying layer can have a variable thickness distribution , which can tailor the rate and directionality of endoprosthesis erosion . in certain embodiments , an erosion modifying layer can have variable thickness throughout the length of the endoprosthesis . for example , as shown in fig1 a , 12 b , 12 c , 12 d , and 12 e , an erosion modifying layer 96 can be thicker on a first end 92 of an endoprosthesis 90 and decrease gradually in thickness toward a second , opposite end 94 of endoprosthesis 90 , thus allowing the second end of the endoprosthesis to erode before the first end . layers 96 can be the same or different . as another example , as shown in fig1 a , 13 b , 13 c , 13 d , and 13 e , an erosion modifying layer 108 can be thicker at a middle portion 106 of an endoprosthesis 100 than at the ends 102 and 104 of the endoprosthesis , thus allowing the ends of the endoprosthesis to erode before the middle of the endoprosthesis . layers 108 can be the same or different . in some embodiments , the thicknesses of an erosion modifying layer at different surfaces on the strut can be the same or different . for example , to compensate for any difference in erosion rates between an interior surface and an exterior surface and to allow a cross - section of an endoprosthesis to erode relatively uniformly at the bioerodible body , an erosion modifying layer located at the interior may be thicker than a layer located at the exterior along the cross section of the endoprosthesis . in some embodiments , the thickness of an erosion modifying layer can change along a width of the strut . as shown in fig1 , a strut 110 with two opposite side surfaces 112 and 114 coated with an erosion modifying layer 116 can have increasing thicknesses of the erosion modifying layer from an abluminal surface 118 to an interior surface 119 . layers 116 can be the same or different . in some embodiments , each of layers 96 , 108 , and 116 can include a plurality of layers , which can include the same or different materials . in some embodiments , within each layer , the composition can include the same or different materials at different portions of the layer . an endoprosthesis can have struts having a rectangular cross - section , a square cross - section , a circular cross - section , an ovaloid cross - section , an elliptical cross - section , a polygonal cross - section ( e . g ., a hexagonal , an octagonal cross - section ), or an irregularly shaped cross - section . the endoprosthesis can have an erosion modifying layer covering a portion of a total surface area of the endoprosthesis . in some embodiments , an erosion modifying layer covers at most 99 percent ( e . g ., at most about 90 percent , at most about 80 percent , at most about 70 percent , at most about 60 percent , at most about 50 percent , at most about 40 percent , at most about 30 percent , at most about 20 percent ) and / or at least about 10 percent ( e . g ., at least about 20 percent , at least about 30 percent , at least about 40 percent , at least about 50 percent , at least about 60 percent , at least about 70 percent , at least about 80 percent ) of a total surface area of an endoprosthesis . in some embodiments , the erosion modifying layer can cover the entire surface of the endoprosthesis . the erosion modifying layer can include a plurality of layers , the composition of the layers can be the same or different . within each layer , the composition can include the same or different materials at different portions of the layer . depending on the materials in the erosion modifying layer ( s ), the erosion process can be tailored to follow a desired sequence . for example , one or more erosion modifying layers located at select portions of the endoprosthesis ( e . g ., the side surfaces of a strut ) can include a more erodible material ( s ) that erodes prior to the remaining layers ( e . g ., located at the abluminal and adluminal surfaces of a strut ), which can include a less erodible material ( s ). the erosion sequence can expose the endoprosthesis to body fluids at different locations and / or at different times during the lifetime of the endoprosthesis , which can produce a desired erosion geometry ( e . g ., an i - beam geometry ). in some embodiments , the erosion modifying layer and / or the bioerodible body have pores and / or patterns to adjust the erosion rate and / or erosion location of an endoprosthesis . as an example , an erosion modifying layer with open or closed pores extending throughout the layer can erode at a faster rate than a solid layer and / or allow the diffusion of body fluids through the erosion modifying layer , which can in turn allow the bioerodible body to erode at a faster rate . pores can range in volume from about 500 nm 3 ( e . g ., from about 0 . 00005 μm 3 , from about 0 . 0005 μm 3 , from about 0 . 005 μm 3 , from about 0 . 05 μm 3 , from about 0 . 5 μm 3 , from about 1 μm 3 , from about 5 μm 3 , from about 35 μm 3 , or from about 50 μm 3 ) to about 550 μm 3 ( e . g ., to about 450 μm 3 , to about 300 μm 3 , to about 200 μm 3 , to about 100 μm 3 , to about 75 μm 3 , to about 40 μm 3 , to about 10 μm 3 , to about 5 μm 3 , to about 1 μm 3 , to about 0 . 5 μm 3 , to about 0 . 05 μm 3 , to about 0 . 005 μm 3 , or to about 0 . 00005 μm 3 ). as another example , a bioerodible endoprosthesis coated with a patterned erosion modifying layer can preferentially erode at certain exposed locations and can have controlled erosion geometries . a pattern includes a repeating sequence of one or more shapes or motifs , for example , grids , squares , circles , and / or lines . in some embodiments , an endoprosthesis having a patterned erosion modifying layer has enhanced endothelialization and reduced thrombus in a body lumen . an erosion modifying layer located on the abluminal , adluminal , or the side surface of the strut can have the same chemical composition or different compositions . for example , an adluminal surface ( e . g ., fig2 , surface 12 ) can contact bodily fluid more than an abluminal surface ( e . g ., fig2 , surface 10 ), which can contact a wall of a body passageway , and as a result , the interior surface can erode more quickly than the exterior surface . to compensate for the difference in erosion and to allow a given cross - section of an endoprosthesis to erode relatively uniformly , the interior surface can have a layer having a chemical composition that erodes more slowly than the chemical composition of a layer at the exterior surface . in some embodiments , the erosion rate of an endoprosthesis is tailored by changing the percentage of cold working of a metal or an alloy . without being bound by theory , it is believed that cold working increases the susceptibility to erosion of a material by inducing dislocations and other defects in the structure , which tend to be anodic and corrode . for example , a bioerodible body can be cold - worked at a higher percentage than an erosion modifying layer so that the bioerodible body can erode before an erosion modifying layer . referring to fig1 , a method 200 of making an endoprosthesis as described herein is shown . method 200 includes forming a bioerodible tube ( step 202 ), forming a pre - endoprosthesis from the bioerodible tube ( step 204 ), and applying one or more erosion modifying layers to the pre - endoprosthesis ( step 206 ) to form an endoprosthesis . in some embodiments , one or more erosion modifying layers are applied to the bioerodible tube , and the tube with the applied erosion modifying layer ( s ) is subsequently formed into an endoprosthesis . the bioerodible tube can be formed ( step 202 ) by manufacturing a tubular member including ( e . g ., is formed of ) one or more bioerodible materials and capable of supporting a bodily lumen . for example , a mass of bioerodible material can be machined into a rod that is subsequently drilled to form the tubular member . as another example , a sheet of bioerodible material can be rolled to form a tubular member with overlapping portions , or opposing end portions of the rolled sheet can be joined ( e . g ., welded ) together to form a tubular member . a bioerodible material can also be extruded to form a tubular member . in certain embodiments , a bioerodible tube can be made by thermal spraying , powder metallurgy , thixomolding , die casting , gravity casting , and / or forging . the bioerodible or erodible material can be a substantially pure metallic element , an alloy , or a composite . examples of metallic elements include iron , magnesium , zinc , and alloys thereof . examples of alloys include iron alloys having , by weight , 88 - 99 . 8 % iron , 0 . 1 - 7 % chromium , 0 - 3 . 5 % nickel , and less than 5 % of other elements ( e . g ., magnesium and / or zinc ); or 90 - 96 % iron , 3 - 6 % chromium and 0 - 3 % nickel plus 0 - 5 % other metals . other examples of alloys include magnesium alloys , such as , by weight , 50 - 98 % magnesium , 0 - 40 % lithium , 0 - 5 % iron and less than 5 % other metals or rare earths ; or 79 - 97 % magnesium , 2 - 5 % aluminum , 0 - 12 % lithium and 1 - 4 % rare earths ( such as cerium , lanthanum , neodymium and / or praseodymium ); or 85 - 91 % magnesium , 6 - 12 % lithium , 2 % aluminum and 1 % rare earths ; or 86 - 97 % magnesium , 0 - 8 % lithium , 2 %- 4 % aluminum and 1 - 2 % rare earths ; or 8 . 5 - 9 . 5 % aluminum , 0 . 15 %- 0 . 4 % manganese , 0 . 45 - 0 . 9 % zinc and the remainder magnesium ; or 4 . 5 - 5 . 3 % aluminum , 0 . 28 %- 0 . 5 % manganese and the remainder magnesium ; or 55 - 65 % magnesium , 30 - 40 % lithium and 0 - 5 % other metals and / or rare earths . magnesium alloys are also available under the names az91d , am50a , and ae42 . other erodible materials are described in bolz , u . s . pat . no . 6 , 287 , 332 ( e . g ., zinc - titanium alloy and sodium - magnesium alloys ); heublein , u . s . patent application 2002000406 ; and park , science and technology of advanced materials , 2 , 73 - 78 ( 2001 ), all of which are hereby incorporated by reference herein in their entirety . in particular , park describes mg — x — ca alloys , e . g ., mg — al — si — ca , mg — zn — ca alloys . other suitable alloys include strontium . as an example , strontium can be a component in a magnesium alloy . the bioerodible tube can include more than one bioerodible material , such as different bioerodible materials physically mixed together , multiple layers of different bioerodible materials , and / or multiple sections of different bioerodible materials along a direction ( e . g ., length ) of the tube . an example of a composite is as a mixture of a magnesium alloy in a bioerodible polymer , in which two or more distinct substances ( e . g ., metals , ceramics , glasses , and / or polymers ) are intimately combined to form a complex material . as shown in fig1 , after the bioerodible tube is formed , the tube is formed into a pre - endoprosthesis ( step 204 ). in some embodiments , selected portions of the tube can be removed to form circular and connecting struts ( e . g ., 6 , 8 ) by laser cutting , as described in u . s . pat . no . 5 , 780 , 807 , hereby incorporated by reference in its entirety . other methods of removing portions of the tube can be used , such as mechanical machining ( e . g ., micro - machining , grit blasting or honing ), electrical discharge machining ( edm ), and photoetching ( e . g ., acid photoetching ). the pre - endoprosthesis can be etched and / or electropolished to provide a selected finish . in certain embodiments , such as jelly - roll type endoprostheses , step 204 is omitted . next , the erosion modifying layer ( s ) is applied to the pre - endoprosthesis ( step 206 ) to form an endoprosthesis . prior to applying the erosion modifying layer , selected surfaces ( e . g ., interior surface ) or portions ( e . g ., portion between the end portions of the endoprosthesis ) of the pre - endoprosthesis can be masked so that the erosion modifying layer will not be applied to the masked surfaces or portions . in some embodiments , prior to applying the erosion modifying layer , pores can be formed on the pre - endoprosthesis ( e . g ., by micro - arc surface modification , sol - gel templating processes , near net shape alloy processing technology such as powder injection molding , adding foaming structures into a melt or liquid metal , melting a powder compact containing a gas evolving element or a space holder material , incorporating a removable scaffold ( e . g ., polyurethane ) in a metal powder / slurry prior to sintering , sintering hollow spheres , sintering fibers , combustion synthesis , powder metallurgy , bonded fiber arrays , wire mesh constructions , vapor deposition , three - dimensional printing , and / or electrical discharge compaction ). in some embodiments , pores can be formed by incorporating embedded microparticles and / or compounds ( e . g ., a salt ) within the antioxidant layer ( e . g ., a polymerizable monomer , a polymer , a metal alloy ), forming the antioxidant layer , and removing ( e . g ., dissolving , leaching , burning ) the microparticles and / or compounds to form pores at locations where the microparticles and / or compounds were embedded . removable ( e . g ., dissolvable ) microparticles can be purchased , for example , from microparticles gmbh . in some embodiments , pores are formed by using a gas as a porogen , bonding fibers , and / or phase separation in materials such as polymers , metals , or metal alloys . suitable erosion modifying layer materials can include a polymer including covalently bound c , n , o , and halogen , a ceramic material , an oxide , a carbide , a halide , a metal , a metallic alloy , and / or a metal - containing polymer . for example , suitable polymers include bioerodible polymers as polylactic acid ( pla ), polylactic glycolic acid ( plga ), polyanhydrides ( e . g ., poly ( ester anhydride ) s , fatty acid - based polyanhydride , amino acid - based polyanhydride ), polyesters , polyester - polyanhydride blends , polycarbonate - polyanhydride blends , and / or combinations thereof . suitable ceramic materials include , for example , iridium oxide . suitable oxides include magnesium oxide , titanium oxide , and / or aluminum oxide . suitable nitrides include magnesium nitride , titanium nitride , titanium oxynitride , iron nitride , and / or silicon nitride . suitable carbides include iron carbide and silicon nitride . suitable halides include magnesium fluoride . suitable metals and / or a metallic alloys include stainless steel , titanium , niobium , a radiopaque metal such as gold , platinum , iridium , and alloys thereof ; an alloy such as bioerodible magnesium alloys and iron alloys as previously described having adjusted compositions so that erosion occurs at a different rate than the bioerodible body . suitable inert or dissolvable polymers including metals ( e . g ., fe , au , pt ) or metal compounds such as organometallic complexes . depending on the erosion modifying layer material , one or more material can be dissolved in a solvent and applied to the pre - endoprosthesis , and / or two or more different materials can be blended together in the form of , for example , a composite such as a metal matrix composite ( e . g ., in a manner that one material is embedded or encapsulated in a remaining material ) and applied to the pre - endoprosthesis . in some embodiments , for example , erosion modifying coatings are generated by physical or plasma vapor deposition , thermal metal spraying , dip coating , electrostatic spraying , conventional air atomization spraying , ion implantation ( e . g ., by plasma immersion ion implantation , by laser - driven ion implantation ), electrochemical deposition , oxidation ( e . g ., anodizations ), chemical grafting , interlayer transitional coatings to bond multiple layers , and / or metallurgical augmentation ( e . g ., peening , localized metallurgical treatments ). in some embodiments , pores are generated in an erosion modifying layer , e . g ., by powder injection molding sol - gel templating processes , near net shape alloy processing technology such as powder injection molding , micro - arc surface modification , sol - gel templating processes , adding foaming structures into a melt or liquid metal , melting a powder compact containing a gas evolving element or a space holder material , incorporating a removable scaffold ( e . g ., polyurethane ) in a metal powder / slurry prior to sintering , sintering hollow spheres , sintering fibers , combustion synthesis , powder metallurgy , bonded fiber arrays , wire mesh constructions , vapor deposition , three - dimensional printing , and / or electrical discharge compaction ). in some embodiments , pores can be formed by incorporating embedded microparticles and / or compounds ( e . g ., a salt ) within the antioxidant layer ( e . g ., a polymerizable monomer , a polymer , a metal alloy ), forming the antioxidant layer , and removing ( e . g ., dissolving , leaching , burning ) the microparticles and / or compounds to form pores at locations where the microparticles and / or compounds were embedded . removable ( e . g ., dissolvable ) microparticles can be purchased , for example , from microparticles gmbh . in some embodiments , pores are formed by using a gas as a porogen , bonding fibers , and / or phase separation in materials such as polymers , metals , or metal alloys . in certain embodiments , patterns are generated in an erosion modifying layer , e . g ., by laser ablation , lithography , ink - jet printing , and / or screen printing . in some embodiments , a medicament is incorporated into an erosion modifying coating on an endoprosthesis . for example , a medicament can be adsorbed onto an erosion modifying coating on an endoprosthesis . a medicament can be encapsulated in a bioerodible material and embedded in an erosion modifying coating on an endoprosthesis . as another example , a medicament can be dissolved in a polymer solution and coated onto an endoprosthesis . incorporation of a medicament is described in u . s . ser . no . 10 / 958 , 435 filed oct . 5 , 2004 , hereby incorporated by reference . in some embodiments , an endoprosthesis can have greater than one type of erosion modifying coating located at the same or different locations on the endoprosthesis . referring to fig1 , as an example , an endoprosthesis can have a polymer coating 210 superimposed upon a stainless steel coating 212 on a strut 214 . as another example , an endoprosthesis can have a ceramic coating on an exterior surface , and a polymer coating on an interior surface of a strut . in certain embodiments , an erosion modifying layer can be applied to a pre - endoprosthesis in one layer , or in multiple layers ( e . g ., at least two layers , at least three layers , at least four layers , at least five layers ) in order , for example , to provide greater control over the thickness of an erosion modifying layer . within an erosion modifying layer , the thickness and composition of a second material can be the same or different to provide desired erosion rates and erosion sequence . for example , the intermediate portion of an endoprosthesis can have a smaller thickness of a non - bioerodible second material than the end portions of the endoprosthesis , which can contain a greater thickness of a bioerodible second material . the erosion modifying layers can be applied the same way or in different ways . for example , a first , innermost erosion modifying layer can be plasma - deposited on the pre - endoprosthesis , and a second , outer erosion modifying layer can include a polymer that is dip - coated onto the first layer . in some embodiments , an erosion modifying coating partially coats one or more portions of an endoprosthesis . referring to fig1 , as an example , an endoprosthesis 220 can have a band ( s ) 222 of the same or different coatings along the length of the endoprosthesis . as shown in fig1 , as an example , an endoprosthesis 230 can have a strip ( s ) of the same or different coatings along the circumference of the endoprosthesis . bands and strips can be coated onto the endoprosthesis by selectively masking certain areas of the endoprosthesis . bands and strips of erosion modifying coating can have pore / patterns , and / or have different thicknesses as discussed above . referring now to fig1 , an endoprosthesis 300 having an increasing number of different erosion modifying layers along its length can be produced from a metallic pre - endoprosthesis 240 by masking selective portions of the endoprosthesis . for example , during production , all portions of the pre - endoprosthesis can be coated with a first erosion modifying layer to generate a pre - endoprosthesis 250 . next , a portion 252 of the pre - endoprosthesis is masked ( e . g ., with a protective polymeric coating such as a styrene - isoprene - butadiene - styrene ( sibs ) polymer ), which protects the masked portion from further erosion modifying layer coating , and the remaining section is coated with a second erosion modifying layer to make a pre - endoprosthesis 270 . finally , a second portion 272 of the pre - endoprosthesis is masked , and the remaining portion is further coated with a third erosion modifying layer to make pre - endoprosthesis 290 . the protective coatings can be removed , e . g ., by rinsing in a solvent such as toluene to complete the production of endoprosthesis 300 . an endoprosthesis having tapered thicknesses can be produced by masking the interior and / or outer portions with a movable sleeve and longitudinally moving the sleeve and / or the endoprosthesis relative to each other during implantation . in some embodiments , the erosion modifying layer ( s ) can be applied to the bioerodible tube prior to forming the bioerodible tube into an endoprosthesis ( if necessary ). as a result , the endoprosthesis can have its exterior and interior surfaces coated with the erosion modifying layer ( s ), and the side surfaces of the endoprosthesis can be free of the erosion modifying layer ( s ). prior to applying the erosion modifying layer ( s ), the interior surface or the exterior surface of the bioerodible tube can be masked to apply the erosion modifying layer ( s ) to only selected portion ( s ) of the tube . as another example , while the endoprosthesis can have both exterior and interior surfaces coated with a desired erosion modifying layer material , in other embodiments , one or more segments of an endoprosthesis have only the exterior surfaces or the interior surfaces coated with an erosion modifying layer having a second material . exterior surfaces of a pre - endoprosthesis can be coated with a desired second material , e . g ., by placing a mandrel , a pin or a sleeve that is sized to mate with the selected inner surface ( s ) of the pre - endoprosthesis so that during coating , the second material is effectively blocked from entering interior surface of the pre - endoprosthesis . such an endoprosthesis , after implantation , may have a cross - section that has only two materials : an exterior surface that is coated with the second material , and an interior surface that has not been coated . interior surfaces of a pre - endoprosthesis can be coated with a desired erosion modifying layer material , e . g ., by placing a polymeric coating on selected outer surface ( s ) of the pre - endoprosthesis so that during coating the second material can coat only the interior surface ( s ) and is prevented from coating the exterior surfaces . alternatively , exterior surfaces can be protected by placing the pre - endoprosthesis in a tight - fitting tube , e . g ., a heat shrink tube , to cover the exterior surfaces . in some embodiments , photo - lithography and / or stereo - lithography can be used to mask surfaces of a pre - endoprosthesis to prevent coating of an erosion modifying layer material . in use , the endoprostheses can be used , e . g ., delivered and expanded , using a catheter delivery system , such as a balloon catheter system . catheter systems are described in , for example , wang u . s . pat . no . 5 , 195 , 969 , hamlin u . s . pat . no . 5 , 270 , 086 , and raeder - devens , u . s . pat . no . 6 , 726 , 712 . endoprosthesis and endoprosthesis delivery are also exemplified by the radius ® or symbiot ® systems , available from boston scientific scimed , maple grove , minn . the endoprostheses described herein can be of a desired shape and size ( e . g ., coronary stents , aortic stents , peripheral vascular stents , gastrointestinal stents , urology stents , and neurology stents ). depending on the application , the stent can have a diameter of between , for example , 1 mm to 46 mm . in certain embodiments , a coronary stent can have an expanded diameter of from about 2 mm to about 6 mm . in some embodiments , a peripheral stent can have an expanded diameter of from about 5 mm to about 24 mm . in certain embodiments , a gastrointestinal and / or urology stent can have an expanded diameter of from about 6 mm to about 30 mm . in some embodiments , a neurology stent can have an expanded diameter of from about 1 mm to about 12 mm . an abdominal aortic aneurysm ( aaa ) stent and a thoracic aortic aneurysm ( taa ) stent can have a diameter from about 20 mm to about 46 mm . while a number of embodiments have been described , the invention is not so limited . in some embodiments , the erosion rate of a bioerodible material is increased by forming , for example , a galvanic couple that is exposed to body fluids or an electrolyte solution . for example , the erosion rate of a bioerodible material ( e . g ., a magnesium alloy ) can be increased by addition of one or more other materials such as iron , nickel , copper , and cobalt , and / or low level impurities such as gold , platinum , and iridium . referring to fig2 a , 20 b , and 20 c , an endoprosthesis 310 can have a strut 312 , which can have a bioerodible body 322 having an inner portion 318 , a center portion 320 , an exterior portion 316 , and two erosion modifying layers 314 . depending on the composition and thicknesses of the portions , the endoprosthesis can be configured to erode sequentially from an interior portion to an exterior portion , from an exterior surface to an interior surface , from a center portion to the exterior and interior portions , or from the exterior and interior portions to the center portion . this construction can allow the endoprosthesis to support the body vessel initially using the strength of multiple layers , and to reduce in thickness over time ( e . g ., after cells have endothelialized the endoprosthesis ). the reduction in thickness can enhance the flexibility the endoprosthesis to better match the natural state of the body vessel . as another example , an endoprosthesis can have multiple alloy compositions along the length of a bioerodible body . for example , an alloy composition having a greater rate of erosion can be located at a first end of the bioerodible body , while an alloy composition having a smaller rate of erosion can be located at a second end of the bioerodible body , such that the first end erodes at a faster rate than the second end . the erosion directionality can allow for increased maintenance of patency for certain locations ( e . g ., weakened locations ) in a body vessel . the endoprostheses described herein can be a part of a stent , a covered stent or a stent - graft . for example , an endoprosthesis can include and / or be attached to a biocompatible , non - porous or semi - porous polymer matrix made of polytetrafluoroethylene ( ptfe ), expanded ptfe , polyethylene , urethane , or polypropylene . the endoprostheses described herein can include non - metallic structural portions , e . g ., polymeric portions . the polymeric portions can be erodible . the polymeric portions can be formed from a polymeric alloy . polymeric stents have been described in u . s . patent application ser . no . 10 / 683 , 314 , filed oct . 10 , 2003 ; and u . s . patent application ser . no . 10 / 958 , 435 , filed oct . 5 , 2004 , the entire contents of each is hereby incorporated by reference herein . the endoprostheses can include a releasable therapeutic agent , drug , or a pharmaceutically active compound , such as described in u . s . pat . no . 5 , 674 , 242 , u . s . ser . no . 09 / 895 , 415 , filed jul . 2 , 2001 , u . s . ser . no . 11 / 111 , 509 , filed apr . 21 , 2005 , and u . s . ser . no . 10 / 232 , 265 , filed aug . 30 , 2002 . the therapeutic agents , drugs , or pharmaceutically active compounds can include , for example , anti - thrombogenic agents , antioxidants , anti - inflammatory agents , anesthetic agents , anti - coagulants , and antibiotics . the therapeutic agent , drug , or a pharmaceutically active compound can be dispersed in a polymeric coating carried by the endoprosthesis . the polymeric coating can include more than a single layer . for example , the coating can include two layers , three layers or more layers , e . g ., five layers . the therapeutic agent can be a genetic therapeutic agent , a non - genetic therapeutic agent , or cells . therapeutic agents can be used singularly , or in combination . therapeutic agents can be , for example , nonionic , or they may be anionic and / or cationic in nature . an example of a therapeutic agent is one that inhibits restenosis , such as paclitaxel . the therapeutic agent can also be used , e . g ., to treat and / or inhibit pain , encrustation of the endoprosthesis or sclerosing or necrosing of a treated lumen . any of the above coatings and / or polymeric portions can be dyed or rendered radio - opaque . the endoprostheses described herein can be configured for non - vascular lumens . for example , it can be configured for use in the esophagus or the prostate . other lumens include biliary lumens , hepatic lumens , pancreatic lumens , uretheral lumens and ureteral lumens . other configurations of endoprosthesis are also possible . referring to fig2 , an endoprosthesis 330 can have a tubular body with slots removed from the tubular body , an erosion modifying layer ( s ) can be coated onto an exterior surface 332 , an interior surface 334 , or any of the side surfaces 336 of the endoprosthesis . referring to fig2 , an endoprosthesis 340 can have a braided or woven tubular body made of intertwining filaments 338 . the endoprosthesis can be coated with an erosion modifying layer ( s ) on the exterior or the interior of the tubular body . in some embodiments , a braided endoprosthesis can include erosion modifying layer - coated and uncoated filaments . all references , such as patent applications , publications , and patents , referred to herein are incorporated by reference in their entirety .
a bioerodible endoprosthesis erodes to a desirable geometry that can provide , e . g ., improved mechanical properties or degradation characteristics .