fig1 depicts an endoprosthesis in the form of a stent 20 . stent 20 could have the form of a tubular member defined by a plurality of bands 22 and a plurality of connectors 24 that extend between and connect adjacent bands . during use , bands 22 could be expanded from an initial , small diameter to a larger diameter to contact stent 20 against a wall of a vessel , thereby maintaining the patency of the vessel . connectors 24 could provide stent 20 with flexibility and conformability that allow the stent to adapt to the contours of the vessel . stent 20 could 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 , stent 20 could have a diameter of between , for example , 1 mm to 46 mm . in certain embodiments , a coronary stent could have an expanded diameter of from 2 mm to 6 mm . in some embodiments , a peripheral stent could have an expanded diameter of from 5 mm to 24 mm . in certain embodiments , a gastrointestinal and / or urology stent could have an expanded diameter of from 6 mm to about 30 mm . in some embodiments , a neurology stent could 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 could have a diameter from about 20 mm to about 46 mm . stent 20 could be balloon - expandable , self - expandable , or a combination of both ( see , e . g ., u . s . pat . no . 5 , 366 , 504 ). stent 20 could be delivered to a desired location within a body lumen and expanded using a catheter delivery system . catheter delivery 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 . stents and stent delivery systems are also exemplified by the radius ® or symbiot ® systems , available from boston scientific scimed , maple grove , minn . referring to fig2 , all or a portion of stent 20 could include a base portion 26 provided with a porous cladding 28 . in fig2 , porous cladding 28 is shown on the outer surface of stent 20 . however , it would also be possible to include porous cladding 28 on the inner surface of stent 20 . porous cladding 28 could include an outer “ capping ” layer 30 and an underlying porous layer 32 . capping layer 30 could have a porosity that is lower than the porosity of the underlying porous layer 32 , and preferably could be substantially non - porous . porous cladding 28 could be formed by applying a metal hydride - containing powder to the base portion 26 of stent 20 . examples of suitable materials for base portion 26 could include titanium alloys and composites ( e . g ., nickel - titanium alloys , commonly referred to as “ nitinol ” alloys ), niobium alloys and composites , cobalt - chromium alloys , stainless steel , and the like . examples of suitable metal hydride - containing powders could include transition metal hydrides such as titanium hydride , zirconium hydride , and the like . to promote adhesion between the porous cladding and the base portion , the powder could also include a metal powder in which the metal is the same as one of the metals in the base portion . for example , if the base portion includes a nickel - titanium alloy , the metal hydride - containing powder could contain a mixture of titanium hydride powder and titanium metal powder . the metal hydride - containing powder could be applied to base portion 26 of stent 20 in the form of a thin coating by coating techniques such as spraying , roll coating , stencil printing , dipping , and the like . adhesion promoters could be added to the powder to promote initial adhesion between the powder and base portion 26 of stent 20 . following the coating step , the powder coating could be exposed to a heat source to melt the powder and liberate hydrogen gas . as the gas escapes , it creates pores , thereby resulting in the formation of porous cladding 28 on base portion 26 . suitable heat sources are capable of delivering energy quickly such that the powder is melted , but not the underlying base portion 26 of the stent . particularly useful heat sources could include lasers . the lasers could be continuous wave lasers or pulsed lasers . specific examples could include yag lasers and co 2 lasers . other examples of potentially useful heat sources include radiofrequency heat sources capable of producing eddy currents . the laser exposure could take place in an inert atmosphere . alternatively , it could take place in the presence of a gas such as oxygen or nitrogen . using an oxygen - containing atmosphere could create metal oxide porous claddings , while a nitrogen - containing atmosphere could create metal nitride porous claddings . the resulting porous cladding 28 could include an outer capping layer 30 and an underlying porous layer 32 . capping layer 30 could have a porosity that is lower than the porosity of the underlying porous layer 32 , and preferably could be substantially non - porous . the particular characteristics of the layers 30 and 32 would depend on both the composition of the metal hydride - containing powder and the heating conditions . for example , increasing the amount of metal hydride in the powder could result in smaller and / or fewer pores , and could favor the formation of closed pores . capping layer 30 could be treated with a separate laser to ablate all or selected portions of the layer to create pores in the capping layer . the resulting stent could thus feature a porous cladding having an underlying porous layer and an overlying surface layer with pores that are smaller than the pores of the underlying porous layer . this structure could be useful , e . g ., as a drug eluting stent . the underlying porous layer could act as a reservoir for the drug , while the smaller pores of the overlying capping layer could control the elution rate of the drug . examples of suitable drugs that could be incorporated into the pores are well - known include therapeutic agents such as paclitaxel . the drugs could be incorporated into the pores using conventional techniques that include exposing the stent to a vacuum , followed by spraying the drug onto the stent to force the drug into the pores , or by exposing the stent to a solution of the drug to imbibe the drug into the pores . supercritical co 2 could also be used to incorporate a drug into the pores . suitable lasers for ablating crust layer 30 could include lasers such as femtosecond lasers and excimer lasers that deliver very rapid bursts of energy . the size , number , and condition of the holes ( e . g ., open - cell vs . closed - cell ) could be selected by selecting appropriate laser exposure conditions . it also would be possible to remove all or a selected portion of capping layer 30 completely using the laser . this would expose the underlying porous layer 32 , thereby creating a surface that could promote endothelialization ( i . e ., a “ pro - healing ” surface ). in another embodiment , all or a selected portion of capping layer 30 could be removed to expose the underlying porous layer 32 . drug could then be incorporated into the pores of porous layer 32 . the resulting structure could then be provided with an overlying porous membrane ( e . g ., a polymeric or inorganic membrane ) to form a drug eluting stent . alternatively , a second porous cladding , having a second porous layer and a second capping layer , could be created on top of porous layer 32 following the above - described process . the second porous cladding could then be treated ( e . g ., by ablating all or a portion of the second capping layer to create pores or to remove the second capping layer ) to create a drug eluting stent . in these embodiments , the pore size of the structure could be fine - tuned to control the drug eluting properties of the stent . in yet another embodiment for creating a drug eluting stent , the drug could be deposited directly on the endoprosthesis surface , followed by creation of the cladding layer 28 and subsequent ablation of the capping layer 30 . porous cladding 28 could be created on the stent itself , after the stent has been formed from precursor tubing . alternatively , porous cladding 28 could be created on the precursor tubing , which could be processed subsequently to form the stent . a stent made of stainless steel 316lvm is provided . a powder mixture is coated onto the surface of the stent by spraying , roll coating , stencil printing , dipping , or the like to form a thin coating on the surface of the stent . the powder mixture contains an adhesive or fluid additive , and is in the form of a paste to promote adhesion to the stent surface . the powder mixture also contains particles of titanium hydride and ss316 stainless steel in portions chosen to achieve a predetermine porosity in the final coating . the paste is left to dry on the surface of the stent . a laser is then used to melt the paste in a nitrogen atmosphere . the molten material cools and forms a porous cladding on the surface of the stent as hydrogen outgases . the nitrogen in the surrounding atmosphere combines with some of the titanium in the molten powder to form titanium nitride ( tin ). the final porous cladding contains an outer skin overlying a porous network . another laser is then used to bore multiple holes having diameters less about 1 micron in the outer skin to enable access to the porous network . a solution of drug is then introduced into the porous network via the holes to form a drug eluting stent . a tube made of stainless steel 316lvm is provided . a powder mixture is coated onto the surface of the tube by spraying , roll coating , stencil printing , dipping , or the like to form a thin coating on the surface of the tube . the powder mixture contains an adhesive or fluid additive , and is in the form of a paste to promote adhesion to the tube surface . the powder mixture also contains particles of titanium hydride and ss316 stainless steel in portions chosen to achieve a predetermine porosity in the final coating . the paste is left to dry on the surface of the tube . a laser is then used to melt the paste . the molten material cools and forms a porous cladding on the surface of the tube as hydrogen outgases . the tube is later cut into a stent using the same or different laser to expose sides of the porous coating , thereby creating a stent with an abluminal porous coating that can be used for abluminal drug delivery . a stent made of nitinol ( niti ) is provided . a powder mixture is coated onto the surface of the stent by spraying , roll coating , stencil printing , dipping , or the like to form a thin coating on the surface of the stent . the powder mixture contains an adhesive or fluid additive , and is in the form of a paste to promote adhesion to the stent surface . the powder mixture also contains particles of titanium hydride and niti in portions chosen to achieve a predetermine porosity in the final coating . the paste is left to dry on the surface of the stent . a laser is then used to melt the paste in a nitrogen atmosphere . the molten material cools and forms a porous cladding on the surface of the stent as hydrogen outgases . the nitrogen in the surrounding atmosphere combines with some of the titanium in the molten powder to form titanium nitride ( tin ). the final porous cladding contains an outer skin overlying a porous network . another laser is then used to bore multiple holes having diameters less about 1 micron in the outer skin to enable access to the porous network . a solution of drug is then introduced into the porous network via the holes to form a drug eluting stent . the inside surface of the stent is ablated using a laser to expose the underlying porous network and form a surface that promotes endothelial cell growth . a stent made of nitinol ( niti ) is provided . a powder mixture is coated onto the surface of the stent by spraying , roll coating , stencil printing , dipping , or the like to form a thin coating on the surface of the stent . the powder mixture contains an adhesive or fluid additive , and is in the form of a paste to promote adhesion to the stent surface . the powder mixture also contains particles of titanium hydride and niti in portions chosen to achieve a predetermine porosity in the final coating . the paste is left to dry on the surface of the stent . a laser is then used to melt the paste in an oxygen atmosphere . the molten material cools and forms a porous cladding on the surface of the stent as hydrogen outgases . the oxygen in the surrounding atmosphere combines with some of the titanium in the molten powder to form titanium oxide . the final porous cladding contains an outer skin overlying a porous network . another laser is then used to bore multiple holes having diameters less about 1 micron in the outer skin to enable access to the porous network . a solution of drug is then introduced into the porous network via the holes to form a drug eluting stent . the inside surface of the stent is ablated using a laser to expose the underlying porous network and form a surface that promotes endothelial cell growth . a stent made of a titanium alloy is provided for use as an mri compatible stent . a powder mixture is coated onto the surface of the stent by spraying , roll coating , stencil printing , dipping , or the like to form a thin coating on the surface of the stent . the powder mixture contains an adhesive or fluid additive , and is in the form of a paste to promote adhesion to the stent surface . the powder mixture also contains particles of titanium hydride and titanium in portions chosen to achieve a predetermine porosity in the final coating . the paste is left to dry on the surface of the stent . a laser is then used to melt the paste in a nitrogen atmosphere . the molten material cools and forms a porous cladding on the surface of the stent as hydrogen outgases . the nitrogen in the surrounding atmosphere combines with some of the titanium in the molten powder to form titanium nitride ( tin ). the final porous cladding contains an outer skin overlying a porous network . another laser is then used to bore multiple holes having diameters less about 1 micron in the outer skin to enable access to the porous network . a solution of drug is then introduced into the porous network via the holes to form a drug eluting stent . a stent made of a titanium alloy is provided for use as an mri compatible stent . a powder mixture is coated onto the surface of the stent by spraying , roll coating , stencil printing , dipping , or the like to form a thin coating on the surface of the stent . the powder mixture contains an adhesive or fluid additive , and is in the form of a paste to promote adhesion to the stent surface . the powder mixture also contains particles of zirconium hydride and titanium in portions chosen to achieve a predetermine porosity in the final coating . the paste is left to dry on the surface of the stent . a laser is then used to melt the paste in a nitrogen atmosphere . the molten material cools and forms a porous cladding on the surface of the stent as hydrogen outgases . the nitrogen in the surrounding atmosphere combines with some of the zirconium and titanium in the molten powder to form titanium nitride ( tin ) and zirconium nitride ( zrn ). the final porous cladding contains an outer skin overlying a porous network . a number of embodiments have been described . nevertheless , it will be understood that various modifications may be made without departing from the spirit and scope of the invention . accordingly , other embodiments are within the scope of the following claims .
a process for making an endoprosthesis comprising : applying a powder that includes a metal hydride to a surface of a metal endoprosthesis , or precursor tubing thereof ; and exposing the powder to a heat source to melt the powder and liberate hydrogen gas , thereby forming a porous coating on the surface of the endoprosthesis , or precursor tubing thereof .