Abstract:
Implantable medical devices, including stents, grafts, covered stents, catheters, patches or the like having regions of the device which are functionalized employing microelectromechanical systems that are capable of acting as electromechanical sensors or biosensors in response to either an endogenous event, such as tissue growth, biochemical binding events, pressure changes, or respond to an externally applied stimulus, such as RF energy, to cause a change in the state of the device, such as to induce an oscillation signal which may be interrogated and interpreted external the body or may generate an induced electrical or electromagnetic potential in the device to activate micromotors to effect a geometric change in the device.

Description:
CROSS-REFERENCE TO RELATED INVENTIONS  
       [0001]     This application is a continuation application of co-pending U.S. patent application Ser. No. 10/945,203 filed Sep. 20, 2004, which claims benefit of U.S. Provisional Patent Application Ser. No. 60/503,988 filed Sep. 18, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to the field of medical devices suitable for in vivo use, such as implantable devices, indwelling devices, catheters and delivery systems. More particularly, the present invention relates to implantable medical devices, such as endoluminal stents, that are capable of acting as sensors and/or actuators in vivo.  
         [0003]     With the advent of microelectromechanical system (MEMs) technology, manufacture of very small scale devices has become feasible. The principal application of MEMs technology has, heretofore, been in the electromechanical arts, in particular fluidics and fluid sensors. The present invention, however, adapts MEMs technology to the field of medical devices and, in particular, to the field of implantable medical devices that are designed to sense in vivo conditions, alter the geometry of the device, and/or deliver metered doses of bioactive substances in vivo.  
         [0004]     The field of implantable MEMs based medical devices has extended to diagnostic Microsystems, including miniature mass spectrometers, molecular-recognition biosensors, and microfluidic processors, surgical Microsystems, such as microsensors and micromotors, and therapeutic Microsystems, such as implantable and transdermal drug delivery Microsystems. Such types of microdevices are described in Polla, D. L., et al., “Microdevices in Medicine,”  Ann. Rev. Biomed. Eng.  2000, 02:551-576, which is hereby incorporated by reference. Further description of implantable medical sensors is found in U.S. Pat. No. 6,201,980, which is hereby incorporated by reference. Further description of a microactuator for controlled drug delivery may be found at Low, L. M., et al., “Microactuators toward microvalves for responsive controlled drug delivery,”  Sensors and Actuators,  B 678 (2000) 149-160, which is also hereby incorporated by reference.  
         [0005]     Micropumps, high resolution microaccelerometers, and electrostatic linear motors are examples of micro-scale electromechanical machines that rely upon low-voltage and low power consumption requirements. See, e.g., Yun, K. S., et al., “A Surface-Tension Driven Micropump for Low-voltage and Low-Power Operations,”  J. Microelectromechanical Sys.,  11:5, October 2002, 454-461, Yeh, R., et al., “Single Mask, Large Force, and Large Displacement Electrostatic Linear Inchworm Motors,”  J. Microelectromechanical Sys.,  11:4, p August 2002, 330-336, and Loh, N. C., et al., “Sub-10 cm 3  Interferometric Accelerometer with Nano-g Resolution,”  J. Microelectromechanical Sys.,  11:3, June. 2002, 182-187, each of which is hereby incorporated by reference.  
         [0006]     Conducting polymers have been used as sensors for the development of electronic tongues by fabricating nanostructured films for use as individual sensing units. The films operate by impedance spectroscopy for signal transduction in the frequency range of 1-1 MHz to detect trace amounts of tastants and inorganic contaminants in liquid systems. Riul, Jr., A., et al., “An Artificial Taste Sensor Based On Conducting Polymers,”  Biosensors and Bioelectronics,  00 (2003) 1-5, which is hereby incorporated by reference. In a related vein, hydrogels and conducting polymers have been combined as an electroactive hydrogel composite that traps enzymes within the composite matrix for biosensor construction and chemically stimulated controlled release. Glucose, cholesterol and glactose amperometric biosensors have been made using this composite material that display extended linear response ranges between  10   −5  to 10 −2  M with response times of less than sixty seconds. pH sensors were made by cross-linking the hydrogel component with dimethylaminoethyl methacrylate monomer. See, Brahim, S., et al., “Bio-smart Hydrogels: Co-joined Molecular Recognition and Signal Transduction in Biosensor Fabrication and Drug Delivery,”  Biosensors and Bioelectronics,  17 (2003) 973-981, which is hereby incorporated by reference.  
         [0007]     Single crystalline MgO nanotubes filed with Gallium have been used as wide-temperature range nanothermometers. See, e.g., Li, Y.B., et al. “Ga-filled Single-Crystalline MgO Nanotube: Wide-temperature Range Nanothermometer,”  App. Phys. Let.,  83:5, August 2003, 999-1001, which is hereby incorporated by reference.  
         [0008]     It has been recognized that ion-channel switches may be used in biosensors and the current flux generated by ion&#39;s passing through the ion channel may serve as a basis for sensing a given condition. For example, an ion-channel switch has been made of a lipid membrane containing gramicidin ion channels linked to antibodies and tethered to a gold electrode. This tethered membrane creates an ionic reservoir between the gold electrode and the membrane which is electrically accessed through connection to the gold electrode. In the presence of an applied potential, ions flow between the reservoir and the external solution when the channels are conductive. When the ion current is switched off, mobile channels diffusing within the outer half of the membrane become crosslinked to the antibodies and immobilized. See, Cornell, B. A., et al., “A Biosensor that uses Ion-channel Switches,” Letters to Nature, 1997.  
         [0009]     Finally, it is now known that electrical fields effect endothelial cell migration. See, Li, X., et al., “Effects of Direct Current Electric Fields on Cell Migration and Actin Filament Distribution in Bovine Vascular Endothelial Cells,”  J. Vasc. Res.,  2002;39:391-404, which is hereby incorporated by reference. Controlling endothelial cell migration is a significant step toward designing implantable devices that exhibit greater healing responses. Thus, by designing implantable devices that employ controlled electrical fields, endothelial cells will be more susceptible to binding to the device and propagating along the device surfaces to promote rapid and complete healing and minimize smooth muscle cell proliferation or thrombogenic effects.  
         [0010]     In order to design implantable devices having controlled electrical fields, advantageous use may be made of interdigitated electrodes to create a galvanotactic medical device. Interdigitated electrodes have been employed in dielectrophoresis to separate live and heat-treated Listeria innocua cells on microfabricated devices employing interdigitated electrodes by utilizing the difference in dielectric properties between the alive and dead cells, Li, H., et al., at http://www.nnf.cornell.edu/2002cnfra/2002cnfra54.pdf and Li, H., “Dielectrophoretic Separation and Manipulation of Live and Heat-Treated Cells of Listeria on Microfabricated Devices with Interdigitated Electrodes,”  J. Sensors and Actuators,  April 2002 which are hereby incorporated by reference. Interdigitated microsensor electrodes, also called interdigitated arrays are microfabricated from patterns of noble metals deposited on an insulating substrate chip. These devices are designed for simultaneous interrogation of electrical, electrochemical or optical properties of polymeric films and coatings in microelectrochemistry and electrical/electrochemical impedance spectroscopy. See, e.g., Guiseppi-Elie, A., “Measuring Electrical Materials Properties Using Microfabricated Interdigitated microsensor Electrodes (IMEs) and Independently Addressable Microband Electrodes (IAMEs),” An ABTECH Application Note, http://www.abtechsci.com/pdfs/resist0501.pdf, May, 2001, which is hereby incorporated by reference.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention provides several embodiments of stent-based or graft-based sensors and actuators. In accordance with a first embodiment of the invention there is provided a galvanotactic stent in which the stent material is at least partially fabricated by multi-layer physical vapor deposition. A first substrate layer is deposited, then a conductive layer is deposited and interdigitated electrodes formed in the conductive layer, with adjacent electrodes being separated by dielectric material, a final top insulating layer is deposited and a plurality of openings formed and patterned to match the position of the interdigitated electrodes in the intermediate conductive layer. Upon application of an electrical current to the device, the interdigitated electrodes become charged and a focused current emanates from the openings in the top insulating layer and is patterned in correspondence with the pattern of openings in the top layer.  
         [0012]     Suitable power sources may include an externally applied RF source that induces resonator circuitry in the stent to charge a solid state capacitor formed in the stent, thus providing an integrated power supply within the stent to maintain a charge source for the interdigitated electrodes. Alternative power sources include, without limitation, externally applied electromagnetic fields, ultrasound, UV or photoemissive energy, or thermal energy.  
         [0013]     Since it is known that endothelial cells migrate under the influence of an applied field, the presence of an electrical field integral with the stent is expected to enhance endothelialization of the stent surfaces and promote the formation of healthy neointimal tissue, while lowering the incidence of restenosis associated with stent implantation.  
         [0014]     It is also contemplated in accordance with the present invention that an endoluminal stent may include a conductive polymer with a biological element that forms an embedded circuit in the stent that responds to changes in a physiological condition in the body and produces a change in conductivity in known relationship with the change in the physiological condition. Examples of this type of mechanism of action are conductive polymers such as polypyroles and polypyrolidones used as artificial tongues in the food industry.  
         [0015]     Another aspect of the invention is that conductive polymers on a stent may be used to bind oxidases and generate peroxides to yield free electrons and provide a source of electrical current for the sensor device on the stent. Alternatively, voltage generated by ion channel activity from receptor binding mediated events may be employed to generate a voltage for a stent-based sensor.  
         [0016]     For each of the types of inventive microsensor devices contemplated by the present invention, it is necessary to have an external means for interrogating the microsensor device to determine its state. Transcutaneously applied RF energy is preferably employed to interrogate the inventive microsensor devices, or cause the inventive microsensor devices to actuate for either drug delivery or micromachine actuation. There is an exponential relationship between frequency and data density that may be transmitted over a given frequency is known in the art. Similarly, there is an inverse relationship between frequency and range. See, e.g., Leeper, D. G., Scientific American, May, 2002, which is hereby incorporated by reference. In the ultrawideband frequency, large gigabyte level data densities may be obtained, but over relatively short distances of a few meters. It is contemplated that in the present invention, at terahertz frequencies it is expected that higher data densities may be obtained even while sacrificing range. Since a range of only a few centimeters is required to transcutaneously interrogate an implanted medical device, very high frequencies in the terahertz range may be employed with the concomitant effect of yielding terabyte data densities that are expected to yield sufficient data streams to construct real-time 3D images representative of the condition of the implanted microsensor medical device. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0017]      FIG. 1  is a sequential diagram with panels A-G illustrating fabrication of a cantilever structure in a MEMs device.  
         [0018]      FIG. 2A  is a circuit diagram depicting an interrogator circuit for generating a first electronic signal.  
         [0019]      FIG. 2B  is a circuit diagram depicting a passive resonator circuit for sensing the first electronic signal and activating a variable capacitor.  
         [0020]      FIG. 3  is a graph illustrating the relationship between spatial capacity of different bandwidths of conventional wireless communication modalities.  
         [0021]      FIG. 4  is a perspective view of a nanothermometer construct useful with the inventive medical device of the present invention.  
         [0022]      FIG. 5  is a diagrammatic view depicting the fundamental elements of the inventive functional medical device and their interactions.  
         [0023]      FIG. 6  is a diagrammatic view of an accelerometer construct useful with the inventive medical device of the present invention.  
         [0024]      FIG. 7  is a fragmentary perspective view of an embodiment of the inventive medical device of the present invention depicting a galvanotactic construct for stimulating endothelial cell attachment and proliferation along the surface of the inventive medical device.  
         [0025]      FIG. 8  is a diagrammatic elevational view of a cantilever structure of the MEMs functionalized medical device with dampening due to tissue growth around the cantilever structure.  
         [0026]      FIG. 9  is a diagrammatic elevational view of an embodiment of the cantilever structure of the MEMs functionalized medical device of the present invention for physiochemical affinity binding of biochemical species.  
         [0027]      FIG. 10  is a table illustrating the relationship of specificity of MEMs device functionality and the type of functionality applicable based upon a range of specificity.  
         [0028]      FIG. 11  is a diagrammatic perspective view of an embodiment of the present invention employing impedance spectroscopy as the functional approach.  
         [0029]      FIG. 12  is a diagrammatic view of an embodiment of the present invention employing amperometric measurement by peroxide generation in a composite bioactive hydrogel membrane.  
         [0030]      FIGS. 13A and 13B  are schematic representation of an embodiment of the present invention employing an antibody/ion-channel switch as a synthetic biosensor.  
         [0031]      FIG. 14  is a perspective, partial cut-away view of an embodiment of the present invention employing electrocorrosion to release a bioactive agent.  
         [0032]      FIG. 15A  is a photomicrograph depicting a MEMs device forming an artificial muscle valve, in an open position, to regulate release of a bioactive agent, in accordance with an alternative embodiment of the present invention.  
         [0033]      FIG. 15B  is a photomicrograph depicting a MEMs device forming an artificial muscle valve, in a closed position, to regulate release of a bioactive agent, in accordance with an alternative embodiment of the present invention.  
         [0034]      FIG. 16A  is a diagrammatic cross-sectional view of a continuous electro-wetting micropump in accordance with an alternative embodiment of the MEMs functional medical device of the present invention, depicting the micropump in a loading state.  
         [0035]      FIG. 16B  is a diagrammatic cross-sectional view of a continuous electro-wetting micropump in accordance with an alternative embodiment of the MEMs functional medical device of the present invention, depicting the micropump in a loading state.  
         [0036]      FIGS. 17A-17D  are sequential fragmentary diagrammatic views depicting an actuating stent having micromotors and actuating rails for expanding the stent and the process of actuating the micromotors and moving the actuating rails.  
         [0037]      FIG. 18A  is an exploded diagrammatic view of the inventive actuating stent.  
         [0038]      FIG. 18B  is a diagrammatic view of the inventive actuating stent in its fully diametrically expanded state. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]     The accompanying  FIGS. 1-18  depict different aspects of the present invention, including physical vapor deposition and formation of microcantilevers for use a microsensors or as drug delivery fluidic pumps, an exemplary variably resonant circuit with communicating interrogator circuits outside the body and a passive resonator sensor circuit implanted in the body, a graph depicting the relationship between RF frequency, special capacity and range, a nanothermometer, basic elements of an interactive implantable stent, a lateral accelerometer fabricated using MEMs technology, a diagrammatic galvanotactic device for generating a field gradient for imparting endothelial cell migration, a microcantilever based sensor for detection of thrombus formation and tissue thickness, a microcantilever based sensor for detection of molecular species based upon binding events, a table of different contemplated approaches to electronic biosensing, an diagrammatic device made of composite films of conductive polymers and lipids for impedance spectroscopy, a system for amperometric measurement by peroxide generation, an antibody/ion-channel switch as a biosensor, a drug release valve actuated by electro corrosion, a drug release valve by an artificial muscle, a micropump, sequential figures illustrating movement of a linear micromotor employing interdigitated elements and a corresponding track for relative movement of the interdigitated elements along the track ( FIGS. 17A-17D ), and an actuating stent having linear micromotors and tracts forming actuating ring units for purpose of radially expanding the stent in response to an applied current to the linear micromotor.  
         [0040]     In accordance with each preferred embodiment of the invention, it is contemplated that an implantable medical device, such as a stent, stent-graft, covered stent, graft, or other similar device is fabricated in such a manner as to accommodate a MEMs device either as a discrete component coupled to the medical device, but preferably, the medical device is fabricated using physical vapor techniques as described in co-pending application Ser. No. 10/211,489 filed Aug. 2, 2002, published as U.S. Published Application U.S. 2003/0059640 on Mar. 27, 2003, which is hereby incorporated by reference in its entirety.  
         [0041]     It is contemplated that during fabrication of the inventive medical device, the MEMs sensors and/or actuators of the present invention will be formed during fabrication of the device and be an integral component of the device. For purposes of illustration only, reference will be made to an endoluminal stent having a plurality of structural members, with the MEMs sensors and/or actuators being described as being associated with at least one of the plurality of structural members. Those of ordinary skill in the art, however, will understand that a wide variety of medical devices are contemplated by the present invention and may serve as the carrier substrate for the MEMs sensors and/or actuators of the present invention, including, without limitation, patches, grafts, catheters, balloons, filters, coils, covered stents, or the like.  
         [0042]     A first embodiment  10  of the present invention is illustrated with reference to  FIG. 1 . The structural member of the stent  12  is formed with at least one recess  13 . The at least one recess is filled with a sacrificial material  14 , and a microcantilever layer  16  is formed on the structural member  12  and covering the sacrificial material  14 . Optionally, a piezoelectric material  18  may be provided covering the microcantilever layer  16 . The piezoelectric material  18  and the microcantilever layer  16  are partially removed to form a length of the microcantilever layer  16  and piezoelectric material  18  which corresponds to the desired length of microcantilever  16 , with a portion of the sacrificial material  14  now being exposed. A contact  20  and space between an adjacent cantilever  22  may be provided. Removal of the sacrificial material  14  from beneath the cantilever  16  reopens recess  13  and leaves a space  24  between a terminus of the cantilever and a terminus of the recess in which the cantilever may freely oscillate.  
         [0043]     As illustrated in  FIGS. 8-9 , alternative embodiments  90  and  100  of the microcantilever concept may be used to sense thrombus  98  or vascular tissue binding to the respective medical device  92 ,  102 , in embodiment  90 , or physiochemical binding of sub-cellular components such as antibodies  105 , DNA, antisense DNA or the like, to the cantilever member  94 ,  104 , which attenuates the ability of the associated microcantilever  94 ,  104  to oscillate upon application of an external signal, which, therefore, returns an altered signal indicative of the condition of the device. Similarly, the microcantilevers may be used as a gate to open or close an aperture for purposes of drug delivery and may be stimulated to open or close in response either to an applied external RF signal or a binding event of an endogenous or exogenous substance.  
         [0044]     A sample resonant circuit having an ex vivo interrogator circuit  30  and an in vivo passive resonator circuit  32  is illustrated in  FIGS. 2A and 2B . The interrogator circuit  30  consists generally of a closed loop circuit having an oscillator  34 , a resistor  36  and a power supply  38 , while the resonator circuit  32  consists generally of a loop circuit having a power supply  40  and a variable capacitor  42 .  
         [0045]     As illustrated in  FIG. 3 , a significant relationship exists between communication frequency and spatial capacity of the bandwidth. Similarly, an inverse relationship exists between range of signal and spatial capacity. Thus, for the inventive medical application, it is desirable to employ ultrawideband frequencies which have a spatial capacity in excel of 1,000 kbps/m 2  and a range of 10 meters which is more than sufficient for medical applications.  
         [0046]      FIG. 4  depicts a nanothermometer which may be incorporated into or associated with the structural elements of a medical device. The nanothermometer consists of an array of gallium filled single crystal MgO tubules  52 .  
         [0047]      FIG. 5  depicts an overview of an actuating system  60  for actuating relative movement of component parts in a medical device, such as a stent. The general components of the system are a power generator  68  coupled to logic control circuitry  66 , which is, in turn coupled to a communications module  64 , which communicates with actuator assemblies  62  associated with the component parts of the medical device. Thus, relative movement of the component parts relative to one another is controlled by the logic circuit  66 , and overseen by the communications module  64 . Interconnections  67 ,  65 ,  63  may be electrical, RF, electromagnetic, magnetic or such other functional interconnections as is known in the art. It is regarded as within the skill of the artisan to design and program the specific implements of the logic circuit  66  and the communications module  64  without the exercise of undue experimentation.  
         [0048]      FIG. 6  depicts an accelerometer  70  in accordance with one embodiment of the present invention. The accelerometer  70  may be incorporated in a medical device for the purpose of monitoring patient vital signs such as pressure, pulse or flow. Accelerometer  70  consists generally of a stationary element  74  which may be part of or affixed to a structural element in the medical device and a moveable element  72 . The moveable element  72  is suspended within the stationary element  74  by spring elements  73  which permits relative movement of the moveable element  72 . A first projection  75  from the moveable element  72  interlaces with second projections  76  on the stationary element  74  such that movement of the first projection  75  within the second projections  76  induces a current within the stationary element  74 . The relative strength of the induced current is then correlated to the pressure on the moveable element  72  and indicative of the stimulus being sensed.  
         [0049]     An embodiment of the inventive medical device employing a galvanotactic field gradient  80  is depicted in  FIG. 7 . In accordance with this embodiment of the invention, there is provided a galvanotactic stent in which the stent material is at least partially fabricated by multi-layer physical vapor deposition. A first substrate layer is deposited, then a conductive layer is deposited and interdigitated electrodes formed in the conductive layer, with adjacent electrodes being separated by dielectric material, a final top insulating layer is deposited and a plurality of openings formed and patterned to match the position of the interdigitated electrodes in the intermediate conductive layer. Upon application of an electrical current to the device, the interdigitated electrodes become charged and a focused current emanates from the openings in the top insulating layer and is patterned in correspondence with the pattern of openings in the top layer.  
         [0050]     A suitable power sources may be an externally applied RF source that induces resonator circuitry in the stent to charge a solid state capacitor formed in the stent, thus providing an integrated power supply within the stent to maintain a charge source for the interdigitated electrodes.  
         [0051]     Since it is known that endothelial cells migrate under the influence of an applied field, the presence of an electrical field integral with the stent is expected to enhance endothelialization of the stent surfaces and promote the formation of healthy neointimal tissue, while lowering the incidence of restenosis associated with stent implantation.  
         [0052]     Thus, by forming a plurality of openings  84  in an arrayed pattern in a structural element  82  of a medical device, and providing an array of electrodes  86  electrically connected  88  to one another in adjacent proximity to the plurality of openings  84 , then applying a voltage  81  to the electrodes  86 , an electrical field gradient  83  is created along the pathway of endothelial cell migration  85  across the structural surface  82  of the medical device. Thus, the applied electrical field gradient  83  may be employed in conjunction with a medical device, such as a stent, to preferentially enhance endothelial cell binding and migration to provide surface coverage and healing of the device.  
         [0053]      FIG. 10  differentiates different approaches to biosensing based upon their sensitivity, with impedance spectroscopy having a low sensitivity and receptor/ion-channel completing having a high sensitivity. Thus, an endoluminal stent may include a conductive polymer with a biological element, such as a lipid, that forms an embedded circuit in the stent that responds to changes in a physiological condition in the body and produces a change in conductivity in known relationship with the change in the physiological condition. Examples of this type of mechanism of action are conductive polymers such as polypyroles and polypyrolidones used as artificial tongues in the food industry.  FIG. 11  depicts a representative type of spectroscopic device  110 . An example of a device  110  suitable for impedance spectroscopy using composite films of conductive polymers and lipids in a membrane is illustrated in  FIG. 11 . A conductive polymer membrane  112  encloses a first electrode  114  and a second electrode  116  which are interlaced relative to one another. Binding of an external component to the polymer membrane  112  causes an impedance change in the voltage between the first and second electrodes  112 ,  116  which is detectable by impedance spectroscopy.  
         [0054]     Another aspect of the invention is that conductive polymers on a stent may be a device  120  for amperometric measurement by oxidase binding that generates peroxides to yield free electrons and provides detectable signal from the sensor device as depicted in  FIG. 12 . Thus, polyHEMA (polyhydroxyethyl methacrylate)  126  or polyethylene glycol may be used as the hydrogel matrix with polypyrole in combination with enzyme as a counteranion, may be employed. The conductive polymers may be coated onto stents or other medical devices to sense chemical moiety binding, such as blood glucose, and produce an amperometric measurement indicative of the chemical moiety binding.  
         [0055]     Alternatively, voltage generated by ion channel activity from receptor binding mediated events may be employed to generate a voltage signal for a stent-based sensor  130 , as illustrated in  FIGS. 13A and 13B . Immobilized ion channels (GT), synthetic archaebacterial membrane spanning lipids and half-membrane-spanning tethered lipids are attached to a conductive surface via polar linkers and sulphur bonds. Polar spacer molecules are directly attached to the conductive surface using the same chemistry. Mobile half-membrane-spanning lipids and mobile ion channels (Ga) complete the membrane. The mobile ion channels are biotinylated and coupled to biotinylated antibody fragments Fab9  132 ,  134  using streptavidin (SA) intermediates. Some of the membrane spanning lipids possess biotin-tethered Fab9  132 ,  134 . In the absence of analyte (A), the mobile ion channels diffuse within the outer monolayer of the tethered membrane, intermittently forming conducting dimers (GD). The addition of the targeted analyte crosslinks the Fabs on the lipids and Ga and forms complexes that tether the Ga distant from their immobilized inner-layer partners. This prevents the formation of channel dimers and lowers the electrical conductivity of the membrane.  
         [0056]      FIG. 14  illustrates a type of drug delivery mechanism that functions by electrocorrosion  140  and may be incorporated into an implantable medical device, such as a stent. The device consists generally of a conductive metal foil  142  forming a cap  144  over a reservoir  148  containing a bioactive agent. The entire assembly is formed onto a surface of a medical device which may act as a sealing layer  146  to retain the bioactive agent. Upon application of an electrical field to the conductive metal foil  142 , the cap  144  will corrode at a known rate and emit a flow  145  of the bioactive agent after the cap has corroded. Thus, controlled drug elution may be accomplished employing the electrocorrosion mediated drug delivery device  140  of the present invention.  
         [0057]      FIGS. 15A and 15B  depict a first closed state and a first open state, respectively, of a microvalve  150 . The microvalve  150  consists generally of a scaffold  152  which may be coated with polyHEMA, an actuatable microvalve  154  which functions as a variable opening to control passage of fluids, such as bioactive agents through the microvalve  154 . The microvalve  154  may be fabricated from shape memory materials or conductive polymers, such as polyanaline. The microvalves  150  are preferably provided in an array and associated with drug-eluting reservoirs formed in structural elements of a medical device.  
         [0058]      FIGS. 16A and 16B  depict an electrowetting micropump  160  in which a nanofabricated or microfabricated fluid flow pathway is formed between structures. A first reservoir  161  terminates with a first gate valve  166  which permits or restricts fluid flow between the first reservoir  161  and a second reservoir  173 . An electrolytic pump  185  drives a first diaphragm  165  which is communication with the second reservoir  173 , to close the first gate valve  166 , and pulls a second diaphragm  169 , which opens a second gate valve  168  to drive fluid from the second reservoir  173  to a third reservoir  173 . The electrolytic pump  185  is driven by electrowetting of a first membrane  162  on the first gate valve  16  side of the pump. By switching to electrowetting of a second membrane  163 , as depicted in  FIG. 16B , fluid within the third reservoir  173  is emitted from an exit opening  170  by actuation of the second diaphragm  169 .  
         [0059]     For each of the types of inventive microsensor devices contemplated by the present invention, it is necessary to have an external means for interrogating the microsensor device to determine its state. Transcutaneously applied RF energy is preferably employed to interrogate the inventive microsensor devices, or cause the inventive microsensor devices to actuate for either drug delivery or micromachine actuation. There is an exponential relationship between frequency and data density that may be transmitted over a given frequency is known in the art. Similarly, there is an inverse relationship between frequency and range. See, e.g.,  FIG. 3  and Leeper, D. G., Scientific American, May, 2002, which is hereby incorporated by reference. In the ultrawideband frequency, large gigabyte level data densities may be obtained, but over relatively short distances of a few meters. It is contemplated that in the present invention, at terahertz frequencies it is expected that higher data densities may be obtained even while sacrificing range. Since a range of only a few centimeters is required to transcutaneously interrogate an implanted medical device, very high frequencies in the terahertz range may be employed with the concomitant effect of yielding terabyte data densities that are expected to yield sufficient data streams to construct real-time 3D images representative of the condition of the implanted microsensor medical device.  
         [0060]     As noted above, stent based actuators may have the basic elements of a power generator, logic circuit, communications module and an actuator assembly as depicted in  FIG. 5 . A particular embodiment of a stent actuator capable of enlarging the diameter of the stent is illustrated in  FIGS. 17A-17D  and  18 A and  18 B, in which a linear micromotor having at least two cooperating elements  194 ,  196  is employed in conjunction with a drive track  192  associated with at least one arcuate section of a stent  202 ,  204  or  206  to axially drive the arcuate stent sections  202 ,  204 ,  206 , thereby diametrically expanding the stent as depicted in  FIG. 18B . A stent may be fabricated of a plurality of micromotors and a plurality of curved tracks, it being understood that each curved track is generally linear in nanoscale. Each of the micromotors  194 ,  196  are configured as “inchworm” type devices, in which there are first  191  and second  193  interlacing comb members, each of the first  191  and second  193  comb members are electromechanically coupled to a contact  195  which drives relative movement of one of the first  191  or second  193  comb members relative to one another. A first comb member  191  has a plurality of projections  199  extending therefrom which engage a mating plurality of projections  192  on the drive track  192  associated with the stent sections  202 ,  204  or  206 . In operation, an electrical signal transmitted through the contact  195  drives a first comb member  191  toward the contact, thereby displacing the comb member  191  toward the drive track  192  and toward the second comb member  193 , whereupon the projections  199  on the first comb member mate with the projections  197  on the drive track. Another electrical signal applied to the contact  195 , then drives causes the second comb member  193  to displace, thereby moving the first comb member  191  and the drive track  192  axially relative to the drive track  192 . An adjacent micromotor  196  undergoes sequentially identical steps in step-wise fashion to axially move the drive track  192  in an “inchworm” fashion.  
         [0061]     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 device design, device selection, design of the MEMs device integral with the implantable medical device, and the functionality of the MEMs device may be made without departing from the scope of the invention.