with particular reference to fig1 and 2 , the drug - eluting device 10 of the present invention consists generally of a body element 12 , which for purposes of illustration only , is depicted in a generally tubular conformation having a first wall surface 14 and a second wall surface 16 , a first end surface 13 and an opposing second end surface 15 . a plurality of openings 20 pass through either or both of the first wall surface 14 and the second wall surface 16 and communicate between at least one chamber 21 , shown in phantom , which resides entirely within the z - axis thickness of the drug - eluting device 10 and is defined between the first wall surface 14 and the second wall surface 16 with only at least one of the plurality of openings 20 communicating between the internal chamber 21 and either the first 14 or second 16 wall surface of the drug - eluting device 10 . a plurality of cover members 18 are provided in or in association with either or both of the first wall surface 14 and the second wall surface 16 , and are positioned such that at least one of the plurality of openings 20 are covered by one of the plurality of cover members 18 . the plurality of openings 29 and the associated plurality of cover members 18 may , optionally , be arrayed in a pattern of groupings 23 a - 23 e of openings 20 and cover members 18 . each of the plurality of cover members 18 have generally binary functionality in that they have a first or closed position where the associated at least one opening 20 is covered and occluded by the cover member 18 , and a second or open position where the associated at least one opening 20 is uncovered by the cover member 18 . transition between the first position and the second position preferably occurs by either shape memory or superelastic phase transitions in the material used to fabricate the plurality of openings 20 . the binary transition of the plurality of cover members 18 may be synchronous or asynchronous . that is , that all of the plurality of cover members 18 may transition between the first to the second position under common conditions and , therefore , act synchronously ; alternatively , either individual cover members 18 or groups of cover members , but not all cover members 18 , may transition under common conditions , while other cover members 18 do not undergo a binary transition , therefore , acting asynchronously . while the drug - eluting device 10 of the present invention is illustrated and will be described with reference to a generally tubular embodiment , those of ordinary skill in the art will understand and appreciate that alternate geometric conformations are contemplated and feasible , including , without limitation , spherical , ovoid , planar , curvilinear or cylindrical conformations . in accordance with a preferred embodiment of the present invention , the plurality of cover members 18 comprise cantilever - like members fabricated of shape memory or superelastic metal or pseudometal material . the cantilever - like cover members 18 may be formed as integral components in the first wall surface 14 , the second wall surface 16 , or both , may be formed as a layer upon the first wall surface 14 , the second wall surface 16 or both , or , alternatively , may be discrete members which may be coupled to the first wall surface 14 , the second wall surface 16 , or both . further , the cantilever - like cover members may be provided in regular or irregular pattern arrays . the cantilever 15 can cover any or all openings 14 in any desired pattern . additionally , some of the plurality of openings 20 may have no associated cantilever - like cover member 18 , or all of the plurality of openings 20 may have associated cantilever - like cover members 18 . the plurality of openings 20 have dimensions sufficient to permit the bioactive agent to elute by diffusion , osmotic pressure or under the influence of a positive pressure applied by cellular in - growth into the at least one interior chamber 21 . fig3 and 4 illustrate the binary functioning of the plurality of cantilever - like cover members 18 . in the embodiments illustrated in fig3 and 4 , the plurality of cantilever - like cover members 18 are formed integrally in wall surface 14 , and each of the plurality of cantilever - like cover members 18 subtend an associated opening 20 which underlies the cover member 18 . those skilled in the art will appreciate that openings 20 and cover members 18 could also be formed in and associated with opposing wall surface 16 . the interior chamber 21 is defined entirely within the z - axis thickness of the device 10 and intermediate the first wall surface 14 and the second wall surface 16 . an elutable bioactive agent 24 is retained with the interior chamber 21 . fig3 illustrates the plurality of cantilever - like cover members 18 in their first or closed position where each of the plurality of cantilever - like cover members 18 are in co - planar relationship with one another along wall surface 14 . fig4 illustrates the plurality of cantilever cover members 18 in their second or open position where each of the plurality of cantilever - like cover members are deformed to uncover each associated opening 20 , and permit elution of the bioactive agent 24 from the interior chamber 21 and through the openings 20 . as noted above , while fig3 and 4 depict synchronous function of the plurality of cantilever - like cover members 18 , the plurality of cantilever - like cover members 18 may function asynchronously . fig5 is a transverse cross - sectional view taken along line 5 - 5 of fig1 and illustrates the drug - eluting implantable device 30 in an embodiment consisting of a generally tubular member 32 , which may be a cylindrical stent , or may be an individual strut of a stent . the tubular member 32 has at least one of a plurality of internal chambers 34 formed entirely between a first wall surface 14 and a second wall surface 16 of the tubular member 32 which act as a reservoir for a bioactive agent 36 . a central lumen 31 provides a fluid flow channel for bodily fluids to traverse the device 30 . alternatively , where the tubular member 32 may be an individual strut of a stent , the central lumen 32 may serve as the internal chambers 34 for retaining the bioactive agent 36 to be eluted from the device 30 , in which case , the plurality of internal chambers 24 may , optionally , be eliminated . the plurality of openings 38 communicate between the at least one of a plurality of internal chambers 34 , the central lumen 31 and external the device 30 . the plurality of cantilever - like cover members 42 are formed in an outer circumferential layer 40 which forms the first wall surface 14 of device 30 . fig6 is a transverse cross - section view of an alternate embodiment of the invention depicted in fig5 . the alternative embodiment of the drug - eluting device 45 depicted in fig6 is substantially similar to that in fig5 , with the exception that the plurality of openings 47 communicate between the plurality of interior chambers 43 and central lumen 31 of the device 45 . additionally , the plurality of cantilever - like members 48 are formed in a lumenal layer of material 46 and cover the plurality of openings 47 to control elution of the bioactive agent 36 from the interior chambers 43 . thus , in fig5 , when at least some of the plurality of cantilever - like cover members 42 transition from their first , closed position to their second , open position , the bioactive agent 36 is eluted abluminally from the device 30 , while in fig6 , the bioactive agent 36 is eluted luminally from the device 45 . the position of each of the plurality of openings 20 may vary dependent upon the particular indication or application for which the drug - eluting implantable device 10 is intended . the plurality of openings 20 may open to either a luminal wall surface 16 of the device 10 , or to an abluminal wall surface 14 of the device 10 , or both the luminal wall surface 16 and the abluminal wall surface 14 of the device 10 . as an alternative to having a uniform distribution of openings 20 about the circumferential and longitudinal axes of the device 10 , there may be provided a higher density of openings 20 toward a proximal or distal end of the device 10 . alternatively , a higher density of openings 20 may be provided along an intermediate region o the device 10 . it will be understood that where there is provided a higher density of openings 20 , a larger dosage of the bioactive agent 36 may be released at any one time due to the higher density of openings 20 . in addition to the foregoing positioning of the plurality of openings 21 , the plurality of internal chambers 34 , 41 , may be either continuous or discontinuous within the z - axis thickness of the device 10 and may be present in different circumferential or longitudinal regions of the device 10 . where discontinuous internal chambers 34 , 41 are provided , plural bioactive agents may be loaded into the device 10 for either synchronous or asynchronous elution . by employing asynchronous functioning plurality of cantilever - like cover members 18 , differential drug delivery may be accomplished based upon occurrence of different physiological conditions . the body element 12 is preferably fabricated of a biocompatible metal such as titanium , vanadium , aluminum , nickel , tantalum , zirconium , chromium , silver , gold , silicon , magnesium , niobium , scandium , platinum , cobalt , palladium , manganese , molybdenum and alloys thereof , such as zirconium - titanium - tantalum alloys , nickel - titanium alloy , chromium - cobalt alloy or stainless steel . the plurality of cantilever - like cover members 18 are preferably fabricated of a shape memory or superelastic material , such as nickel - titanium or chromium cobalt alloy . each of the plurality of cantilevers 18 may be fabricated of a material capable of undergoing elastic , plastic , shape memory and / or a superelastic deformation . materials such as stainless steel , titanium , nickel , tantalum , gold , vanadium , nickel - titanium , or alloys thereof may be employed to fabricate the plurality of cantilever members . different electrical , thermal or mechanical properties may be imparted to the cantilevers 18 by altering the alloy ratios of the material . it is preferable to vacuum deposit both the body element 12 and cantilevers 18 to permit tight control over the material composition , electrical , mechanical and thermal properties of the material , as well as provide for tight control over the tissue and fluid contacting surfaces and the bulk material of the device . for example with nickel - titanium alloys , the titanium content of the target , in a nickel - titanium binary target , may be changed a known amount to precisely alter the transition temperature of a cantilever members 18 . in accordance with one embodiment of the present invention either or both of the body member 12 and the plurality of cantilevers 18 are fabricated of thin metallic films . as used herein , the term “ thin metallic film ” or “ metal thin film ” are used synonymously to refer to biocompatible materials made of metallic or pseudometallic materials . the inventive thin metallic films may be fabricated by conventional wrought metal processing techniques , or may be made by nanofabrication techniques such as physical vapor deposition or chemical vapor deposition . such thin metallic films as are used with the present invention may be comprised of single or plural layer films fabricated of biocompatible metals or biocompatible pseudometals having thicknesses greater than 0 μm and less than about 125 μm . each of the plurality of cantilevers 18 preferably have binary functionality to provide a first “ closed ” position indicative of an austenite phase of the cantilevers 18 and a second “ open ” position indicative of a martensite phase of the cantilevers 18 . the closed position is configured such that it is in a lowered position that is substantially co - planar with the surface . on the other hand , the open position is configured such that it is in the raised position or projecting outwardly relative to the surface . it will be understood , therefore , that as the implanted temperature sensor encounters different in vivo temperatures , different sets of cantilever members will be exposed to their transition temperature and change from the “ closed ” position to the “ open ” position . once in the open position , the cantilevers do not impede elution of bioactive agents through the openings from the internal cavities . the plurality of cantilever - like cover members 18 function as sensors in that they may be fabricated to sense and respond to changes in a physiological state , such as pressure , temperature , cell or protein binding , the presence or absence of a given biochemical marker , or the like . alternatively , the plurality of cantilever - like cover members 18 may be fabricated to respond only to a specific externally applied stimulus . in this manner , an exogenous stimulus , such as a magnetic field , rf energy , ultrasound , heat or the like may be applied to actuate at least some of the plurality of cantilever - like cover members 18 and permit elution o the bioactive agent . as illustrated in fig1 , ordered arrays , generally denoted as element 23 , of cantilever like cover members 18 may form sensor groups , such that , for example , a cantilever - like cover members 18 forming a first array 23 a may be fabricated to have a martensitic stress / strain transition coefficient a , while cantilever - like cover members 18 forming second array 23 b are fabricated to have a transition coefficient σ + 1 , cantilever - like cover members 18 forming a third array 23 c are fabricated to have a transition coefficient of σ + 2 , etc . such that different cover members 18 or groups of cover members 18 change their position based upon a given quantum of stress or strain applied to the cantilever - like cover members 18 in vivo . alternatively rather than having merely binary functionality , each of the plurality of cover members 18 may have a response curve which is dependent upon the modulus of the material and the moment of inertia of each cantilever member . this response curve allows for varying degrees of impedance of the openings as the cover members 18 gradually shift from a closed to opening position , thereby , resulting in varying elution profiles through the openings . each of the cover members 18 may be configured to have a variation in z - axis thickness along an x - y axis of the cover members 18 . by configuring the cover members 18 with variable z - axis thicknesses , different cover members 18 or different groupings of cover members 18 will exhibit different stress - strain responses due to the different material modulus and different moment of inertia attendant to the altered geometry of the cover members 18 . with this alternate construct of the cover members 18 , for a given quantum of stress - strain applied to the cover members 18 , the cover members 18 will deflect and shift a returned resonance frequency applied from an external energy source . the degree of deflection will then correlate to the stress and strain forces acting upon the cover members 18 . it will be understood , of course , that this alternate construct of the cover members 18 still provides binary “ closed ” and “ open ” functionality with the “ closed ” and “ open ” positions merely being indicative of the outlying positions of the cover members 18 . it will be understood , therefore , that as the implanted sensor encounters different stress and strain associated with , for example , changes in physiological blood pressure , fluid shear stress , endothelialization , arterioschlerotic plaque development , different sets of cantilever members will be exposed to their transition conditions and change from the “ closed ” position to the “ open ” position . each of the above - described preferred embodiments of the present invention may be fabricated by a number of methods . in accordance with present invention , it is contemplated that either forming wrought metal parts , such as capillary tubing , into the implantable device or forming the implantable devices by vacuum deposition techniques are the preferred method of making the implantable structural elements of the present invention . where an implantable device is to be fabricated of a plurality of individual tubular elements , pre - existing microtubular members having an outer diameter , for example , between 60 and 400 μm and a wall thickness of between 10 and 350 μm , may be employed to fabricate extremely small dimensioned devices suitable for intracranial or coronary artery applications . the microtubular members may be formed into a cylindrical endoluminal device , such as by braiding or bending and joining microtubular members together by spot welding . where ends of the microtubular members are formed to be self - cannulating , the self - cannulating ends may be exposed on the abluminal surface of an endoluminal device at any point along the longitudinal axis thereof . the plurality of openings passing through the wall of each of the individual tubular elements may be formed by microdrilling the openings through the wall and into the internal cavity or lumen of the individual tubular members . the plurality of openings may be laser cut , etched or formed by edm methods , and may be formed either pre - or post - formation of the tubular elements into the three - dimensional conformation of the implantable device . where an implantable device is to be formed from non - preexisting structural elements , vacuum deposition techniques may be employed to form the implantable structural body , such as sputtering , reactive ion etching , chemical vapor deposition , plasma vapor deposition , or the like , as are known in the microelectronics fabrication arts and are more fully described in commonly assigned u . s . pat . no . 6 , 379 , 383 , issued apr . 30 , 2002 and commonly assigned u . s . patent application ser . no . 10 / 211 , 489 , published as u . s . published patent application no . 20030059640 published mar . 27 , 2003 , both of which are hereby incorporated by reference as teaching methods of fabrication of implantable materials using physical vapor deposition processes . the internal chambers , the plurality of openings and the cover members may each be formed during deposition . in order to form these elements by vacuum deposition , the vacuum deposition process may be modified requisite patterns of sacrificial material to form the regions of the internal chambers and openings , over a base layer of structural material , then depositing a second layer of structural material over the sacrificial material and the base layer . the sacrificial material may then be removed , such as by etching , to leave the internal cavities and plurality of openings formed within the deposited bulk material . the plurality of cover members may be formed by depositing a layer of cover material , then defining the cover members in the layer of cover material , such as by laser etching to define the cantilever - like cover members in the cover material . an exemplary method 60 for making the inventive drug - eluting medical device of the present invention is illustrated , sequentially , in fig9 a through 9g . as depicted in fig9 a , a substrate 62 is provided ; a first layer of biocompatible material 64 is deposited onto the substrate 62 , followed by a sacrificial material layer 66 . in fig9 b , the following step entails patterning the sacrificial material layer 66 to leave patterned sections 68 of the sacrificial layer 66 . the pattern sections 68 will , as described hereinafter , form the internal chambers 21 of the inventive device . a second layer of biocompatible material 70 is then deposited onto the patterned sections 68 and the first layer of biocompatible material 64 as illustrated in fig9 c . as illustrated in fig9 d , a plurality of openings 72 are formed in the second layer of biocompatible material 70 and communicate with the patterned sections 68 of the sacrificial material layer 66 . then , as illustrated in fig9 e , the sacrificial material remaining in the patterned sections 68 is removed through the openings 72 to leave interior chambers 72 bounded entirely by the second layer of biocompatible material 70 and the first layer of biocompatible material 64 . a third layer of biocompatible material 76 is then provided to cover the second layer of biocompatible material 70 and the plurality of openings 72 therein , as illustrated in fig9 f . this third layer of biocompatible material 76 may have been preformed with a plurality of cantilever members 78 having separation gaps 80 between adjacent cantilever members 78 and adhesion regions 82 formed between the gap 80 and the opening 72 which the cantilever member 78 covers , as illustrated in fig9 g . those skilled in the art will appreciate that the third layer of biocompatible material may be provided as a discrete layer of material or may consist of a plurality of individual cantilever members 78 each coupled to the second layer of biocompatible material 70 at adhesion regions 82 . the third biocompatible material 76 may be deposited directly onto the second layer of biocompatible material 70 , then the plurality of individual cantilever members 78 formed , such as by laser cutting or selective etching . however , it will be important to interpose a sacrificial interlayer mask which covers the second biocompatible layer 70 and the plurality of openings 72 , but exposes only the adhesion regions 82 so that , after removal of the sacrificial interlayer mask , the plurality of cantilever members 78 are free to deflect from and open the plurality of openings 72 . regardless of which fabrication method is employed , the bioactive agent must be loaded into the internal cavities of the implantable device . loading of the bioactive agent may be accomplished by flowing a liquid or semi - liquid state of the bioactive agent through the plurality of openings and into the internal cavities , either throughout the entire device or in regions of the implantable device . flow loading may be facilitated by applying positive pressure , temperature change or both , such as is used in hot isostatic pressing ( hip ). in hip the pressurizing medium is typically a gas , and the process is carried out at elevated temperatures for specific time periods . while hip is typically utilized to densify materials , to heal casting defects and voids , or to bond similar or dissimilar materials it may be used to drive a fluid or semi - fluid from external the implantable device into the internal cavities of the implantable device . alternative , diffusion - mediated loading , osmotic loading or vacuum loading may be employed to load the bioactive agent into the internal cavities . while the present invention has been described with reference to its preferred embodiments , those of ordinary skill in the art will understand and appreciate that variations in structural materials , bioactive agents , fabrication methods , device configuration or device indication and use may be made without departing from the invention , which is limited in scope only by the claims appended hereto .
the implantable structural element for in vivo controlled delivery of bioactive active agents to a situs in a body . the implantable structural element may be configured as an implantable prosthesis , such as an endoluminal stent , cardiac valve , osteal implant or the like , which serves a dual function of being prosthetic and a carrier for a bioactive agent . control over elution of the bioactive agents occurs through a plurality of cantilever - like cover members which prevent drug elution until an endogenous or exogenous stimulus causes the cover members to open and permit drug elution .