Patent Publication Number: US-2021169667-A1

Title: In vivo adjustment mechanism and associated systems and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Provisional Application No. 62/683,295, filed Jun. 11, 2018, which is incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD 
     The present disclosure relates generally to implantable medical devices, and more specifically to mechanisms for adjusting the diameter of implantable medical devices and associated methods thereof. 
     BACKGROUND 
     Implantable medical devices such as stents, stent-grafts, valves, and other intraluminal devices are used in a variety of medical procedures including to maintain, open, or adjust various body passageways or body lumens to maintain, prevent, and/or adjust fluid flow therethrough. Such devices may be implanted in various locations within the body of a patient including in the vascular system, coronary system, urinary tract, and bile ducts, among others. 
     In some instances, the size of the medical device required may change over time. For example, devices implanted in children may need to be removed and replaced with larger diameter devices as the child grows. In other scenarios, it may be beneficial to implant a larger diameter device and incrementally decrease the diameter, for example, to seal off a defect or slow fluid flow to a certain, afflicted area such as an aneurysm. It may also be beneficial to increase or decrease the size of a body lumen to adjust the rate of fluid flow therethrough such as during dialysis or instances of heart or kidney failure. 
     The diameters of implantable medical devices are often not easily adjustable or customizable, and many devices do not permit intravenous or percutaneous diametric adjustments. Current practices often require replacement of the device with a new, differently sized device altogether, which may require further operation and/or invasive procedures, causing added risk, stress and discomfort to the patient. 
     SUMMARY 
     Various examples relate to mechanisms for adjusting the diameter of a variety of implantable medical devices and methods thereof. In particular, various examples relate to diametric adjustment mechanisms having a track defining a series of diametric setpoints, a rider engaged with the track and movable between the series of diametric setpoints, and a biasing element biasing the rider in a certain direction along the track. 
     According to one example (“Example 1”), a diametric adjustment mechanism for an implantable medical device includes a track. The track defines a series of diametric setpoints including a first diametric setpoint and a second diametric setpoint. The adjustment mechanism also includes a rider engaged with the track. The rider is selectively movable along the track from the first diametric setpoint to the second diametric setpoint and/or from the second diametric setpoint to the first diametric setpoint. The adjustment mechanism also includes a biasing element biasing the rider toward the first diametric setpoint when the rider is at the second diametric setpoint. 
     According to another example (“Example 2”) further to Example 1, the track defines a stepped path. The first diametric setpoint is at a first step location of the stepped path and the second diametric setpoint is at a second step location of the stepped path. 
     According to another example (“Example 3”) further to any of Examples 1 to 2, the track defines a first adjustment path between the first diametric setpoint and the second diametric setpoint and a return path from the second diametric setpoint and the first diametric setpoint. 
     According to another example (“Example 4”) further to any of Examples 1 to 3, the track defines an intermediate diametric setpoint between the first diametric setpoint and the second diametric setpoint. The rider is engaged with the track such that the rider is selectively movable along the track from the first diametric setpoint to the intermediate diametric setpoint prior to moving to the second diametric setpoint, and from the intermediate diametric setpoint to the second diametric setpoint. 
     According to another example (“Example 5”) further to any of Examples 1 to 4, the biasing element is a collar having elastic properties. 
     According to another example (“Example 6”) further to any of Examples 1 to 5, the track defines a continuous loop. 
     According to another example (“Example 7”) further to any of Examples 1 to 6, the track includes at least one of a groove, a channel, a notch, an indentation, and a rail. 
     According to another example (“Example 8”) further to any of Examples 1 to 7, the biasing element is configured to maintain the rider at the first diametric setpoint until a biasing force of the biasing element is exceeded by an external force to move the rider to the second diametric setpoint. The biasing element maintains the rider at the second diametric setpoint until the biasing force of the biasing element is exceeded by an external force. 
     According to another example (“Example 9”) further to any of Examples 1 to 8, the rider is a projection and the track is a depression slidably receiving the projection. 
     According to another example (“Example 10”), a medical device includes a tubular implant and the adjustment mechanism of any of Examples 1 to 9. The adjustment mechanism is coupled to the tubular implant. The biasing element of the adjustment mechanism includes a collar formed of a resilient material, the collar being coupled to the tubular implant. 
     According to another example (“Example 11”) further to Example 10, the biasing element overlaps itself to form the collar. 
     According to another example (“Example 12”) further to Example 11, the biasing element includes a first portion and a second portion. The first portion overlaps the second portion to engage the rider with the track. 
     According to another example (“Example 13”), a method of adjusting the diameter of the medical device of any of Example 1 to 12 includes imparting a first diametric force on the tubular element to move the rider from the first diametric setpoint to the second diametric setpoint. The method also includes imparting a second diametric force on the tubular element to move the rider from the second diametric setpoint to the first diametric setpoint. 
     According to another example (“Example 14”) further to Example 13, the diametric force is an expanding force imparted on an interior of the tubular implant with a balloon catheter. 
     According to another example (“Example 15”) further to any of Examples 13 to 14, moving the rider from the first diametric setpoint to the second diametric setpoint adjusts the diameter of the medical device from a first diameter to a second diameter. Moving the rider from the second diametric setpoint to the first diametric setpoint adjusts the diameter of the medical device from the second diameter to the first diameter. 
     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments, and together with the description serve to explain the principles of the disclosure. 
         FIG. 1  is a side view of a diametric adjustment mechanism for an implantable medical device, according to some embodiments; 
         FIG. 2  is a top view of a diametric adjustment mechanism for an implantable medical device, according to some embodiments; 
         FIG. 3A  is a side view of a diametric adjustment mechanism coupled to an implantable medical device at a first diametric setpoint, according to some embodiments; 
         FIG. 3B  is a side view of a diametric adjustment mechanism coupled to an implantable medical device at an intermediate diametric setpoint, according to some embodiments; 
         FIG. 3C  is a side view of a diametric adjustment mechanism coupled to an implantable medical device at a second diametric setpoint, according to some embodiments; 
         FIG. 4A  is a side view of a diametric adjustment mechanism coupled to an implantable medical device at a first diametric setpoint, according to some embodiments; 
         FIG. 4B  is a side view of a diametric adjustment mechanism coupled to an implantable medical device at an intermediate diametric setpoint, according to some embodiments; and 
         FIG. 4C  is a side view of a diametric adjustment mechanism coupled to an implantable medical device at a second diametric setpoint, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Various aspects of the present disclosure relate to adjustment mechanisms for adjusting diameters of implantable medical devices. Examples of implantable medical devices can include stents, stent-grafts, valves, and devices for occlusion and/or anastomosis, among others. In certain examples, the implantable medical devices may be configured to adjust (e.g., increase and/or decrease) the size of a particular artificial or natural body lumen, passageway, and/or conduit to promote, restrict, or otherwise adjust fluid flow therethrough. For reference, the term “lumen” should be read broadly to include any of a variety of passages, such as those associated with the vasculature, biliary tract, urinary tract, lymph system, reproductive system, gastrointestinal system, or others. 
     In certain instances, it may be beneficial to adjust the diameter of implantable medical devices after implantation inside the body of a patient. For example, in certain applications where the size of the body lumen increases or decreases over time, it may be beneficial to increase and/or decrease the diameter of the device to fit the changing size of the body lumen. In other instances, it may be beneficial to gradually reduce or restrict the flow of blood to a certain area, such as slowing blood flow to an aneurysm, adjusting urine flow during and/or after dialysis, and restricting and/or decreasing blood flow during heart or kidney failure. 
     In the above examples, it may also be beneficial to be able to adjust the implantable medical devices without additional, invasive procedures. Procedures such as these can impart added stress and discomfort on the patient. Therefore, a device that reduces potential, additional burden on the patient and/or medical provider would be desirable. 
       FIG. 1  shows a diametric adjustment mechanism for an implantable medical device, according to some embodiments. The diametric adjustment mechanism  100  includes a track  102  defining a series of diametric setpoints  104 , a rider  106  engaged with the track  102  and selectively movable along the track  102  between the series of diametric setpoints  104 , and a biasing element  108  to promote movement of the rider  106  in a certain direction along the track  102 . In some embodiments, the diametric adjustment mechanism  100 , also referred to herein simply as the adjustment mechanism  100 , is coupled to an implantable medical device  200  ( FIG. 3 ). The series of diametric setpoints  104  includes at least two setpoints, for example, a first diametric setpoint  110  ( FIG. 3A ) and a second diametric setpoint  112  ( FIG. 3C ), but may include more setpoints as desired. As the rider  106  moves along the track  102  between the series of diametric setpoints  104 , the diameter D of the adjustment mechanism  100  is either increased or decreased depending on the direction in which the rider  106  moves along the track  102 . For example, the rider  106  could move in a clockwise direction around the track  102  or in a counter-clockwise direction around the track  102  depending on the configuration of the track  102 . As the diameter D of the adjustment mechanism  100  is increased or decreased, the diameter d ( FIG. 3A ) of the implantable medical device  200  also increases or decreases. 
     In some embodiments, the series of diametric setpoints  104  correspond to a series of stop points spaced along the track  102  configured to keep the rider  106  at a certain location along the track  102  until a biasing force imparted by the biasing element  108  on the rider  106  is overcome. In other words, each of the stop points keep the adjustment mechanism  100  at a respective, desired diameter D until the biasing force is overcome and the rider  106  moves to the subsequent stop point. The biasing force can be overcome by application of a diametric force (e.g., a radial force applied in a radially outward direction from the longitudinal axis A of the adjustment mechanism  100 ), a magnetic force (e.g., applied externally through the skin of a patient), or any other applied force that exceeds the biasing force and to cause the rider  106  to move along the series of diametric setpoints  104 . In some examples, the diametric force is an expansion force imparted on an interior of the implantable medical device  200  using a balloon catheter, although other methods of imparting an expansion force upon the adjustment mechanism  100  are also contemplated. 
     In various embodiments, the series of diametric setpoints  104  can be any of a series of notches, steps, grooves, bends, curves, crooks, or any other configuration capable of keeping the rider  106  at a certain location along the track  102 . In some examples, the series of diametric setpoints  104  may include portions that are flat, upwardly angled, or otherwise inflected as compared to the rest of the track  102  so that the rider  106  may sit, rest, or lodge at the respective one of the series of setpoints  104  until the biasing force is overcome, as shown in  FIG. 1 . 
     In some embodiments, the series of diametric setpoints  104  includes a first diametric setpoint  110  and a second diametric setpoint  112 . The first diametric setpoint  110  corresponds to a first diameter D 1  of the adjustment mechanism  100  and the second diametric setpoint  112  corresponds to a second diameter D 2  of the adjustment mechanism  100 . Thus, moving the rider  106  between the first diametric setpoint  110  and the second diametric setpoint  112  causes the diametric adjustment mechanism  100  to increase and/or decrease from the first diameter D 1  to the second diameter D 2  and, in turn, causes the implantable medical device  200  to also increase or decrease from a first device diameter d 1  to a second device diameter d 2 . 
     The track  102  can include additional setpoints, as desired, for adjusting the diameter D of the adjustment mechanism  100 . For example, the track  102  can include an intermediate diametric setpoint  124  located between the first diametric setpoint  110  and the second diametric setpoint  112 . Similar to the first and second diametric setpoints  110 ,  112 , the intermediate diametric setpoint  124  corresponds to an intermediate diameter D 1  of the diametric adjustment mechanism  100 , the intermediate diameter D 1  being between the first diameter D 1  and the second diameter D 2 . Additional diametric setpoints may allow for incremental adjustment of the adjustment mechanism  100  and/or the implantable medical device  200  between any number of diameters as desired. For example, in certain instances, a larger number of smaller, incremental diametric adjustments may be necessary or beneficial where, in other instances, fewer, larger adjustments may be desired. 
     In some embodiments, the track  102  defines a stepped path  114 , as shown in  FIG. 1 , with each of the series of diametric setpoints  104  spaced along the stepped path  114 . In some embodiments, the first diametric setpoint  110  is located at a first step location  116  along the stepped path  114  and the second diametric setpoint  112  is located at a second step location  118  along the stepped path  114 . As discussed above, moving the rider  106  between the first diametric setpoint  110  (e.g., the first step location  116 ) and the second diametric setpoint  112  (e.g., the second step location  118 ) causes the adjustment mechanism  100  to increase and/or decrease from the first diameter D 1  to the second diameter D 2 . In some embodiments, the first step location  116  may be near a first end  120  of the track  102  and the second step location  118  may be near a second end  122  of the track  102 . However, the first and second step locations  116 ,  118  can be located anywhere along the track  102  as desired. 
     In some embodiments, the track  102  defines a first adjustment path P 1 . The first adjustment path P 1  may be, for example, between the first diametric setpoint  116  and the second diametric setpoint  118 . For example, the rider  106  can move along the first adjustment path P 1  to adjust the adjustment mechanism  100  between the first diameter D 1  and the second diameter D 2 . In some embodiments, the first adjustment path P 1  may also be between the first diametric setpoint  116  and the intermediate diametric setpoint  124 . For example, the rider  106  can move along the first adjustment path P 1  from the first diametric setpoint  116  to the intermediate diametric setpoint  124  prior to moving to the second diametric setpoint  118 . 
     In some embodiments, the track  102  also defines a second adjustment path P 2 . For example, after moving from the first diametric setpoint  116  to the intermediate diametric setpoint  124 , the rider  106  may then move along the second adjustment path P 2  from the intermediate diametric setpoint  124  to the second diametric setpoint  118 . In various embodiments, the track  102  may define a third adjustment path, a fourth adjustment path, or any number of adjustment paths between each setpoint of the series of diametric setpoints  104  as desired. 
     Although the adjustment mechanism  100  is described above and shown in  FIG. 1  to decrease in diameter as the rider  106  moves along the first adjustment path P 1  and the second adjustment path P 2 , the mechanism  100  can also be configured to increase in diameter as the rider  106  moves along the first and second adjustment paths P 1 , P 2 . For example, as the rider  106  moves along the first adjustment path P 1  from the first diametric setpoint  116  to the intermediate diametric setpoint  124 , the diameter D of the adjustment mechanism  100  may increase (i.e., from a smaller diameter to a larger diameter), and may further increase as the rider  106  moves along the second adjustment path P 2  from the intermediate setpoint  124  to the second diametric setpoint  118 . 
     In some embodiments, the track also defines a return path  126  between the second diametric setpoint  112  and the first diametric setpoint  110 . The return path  126  allows for diametric adjustment of the adjustment mechanism  100  from the second diameter D 2  to the first diameter D 1 . In some embodiments, the return path  126  may be located adjacent and substantially parallel to the stepped path  114 . In some embodiments, the return path  126  may be substantially straight such that the rider  106  can move continuously and uninterrupted from the second diametric setpoint  112  to the first diametric setpoint  110 . In some embodiments, the return path  126  allows for return of the rider  106  to its original location (e.g., the first step location  116 ) so that in use the rider  106  remains continually engaged with the track  102 . 
     In some embodiments, the track  102  may define a continuous loop, as shown in  FIG. 1 . This allows the rider  106  to move along the track  102  (e.g., between the series of diametric setpoints  104 ) without disengaging from the track  102 . For example, the rider  106  can move along the stepped path  114  from the first diametric setpoint  110  to the second diametric setpoint  112  and then move along the return path  126  from the second diametric setpoint  112  to the first diametric setpoint  110  without disengaging from the track  102 . 
     In various examples, the rider  106  and the track  102  are complementary features that are configured to remain slidably coupled during diametric adjustment. In some embodiments, the rider  106  may be a projection, groove, or other feature capable of slidably engaging with the track  102 . The track  102  may define a depression or relief feature capable of receiving the rider  106 , or a raised rail or other feature on which the rider  106  traverses. For example, the track  102  can include at least one of a groove, a channel, a notch, an indentation, a rail, or any other feature capable of receiving or otherwise engaging with the rider  106 . 
     In some embodiments, the diametric adjustment mechanism  100  includes a biasing element  108 , as shown in  FIG. 1 . As discussed above, the biasing element  108  promotes movement of the rider in a certain, desired direction along the track  102 . For example, the biasing element  108  may bias or promote movement of the rider  106  toward the second diametric setpoint  112  when the rider  106  is at the first diametric setpoint  110 , or toward the first diametric setpoint  110  when the rider  106  is at the second diametric setpoint  112 . In some embodiments, the biasing element  108  is configured to maintain the rider  106  at the first diametric setpoint  110  until the biasing force of the biasing element  108  is exceeded by the diametric force (i.e., from the catheter balloon, the magnet, or a bodily function such as, for example, a heartbeat) to move the rider  106  to the second diametric setpoint  112 , at which movement of the rider  106  is halted until the biasing force of the biasing element  108  is again exceeded by the diametric force. 
       FIG. 2  shows the diametric adjustment mechanism  100  coupled to the biasing element  108 , according to some embodiments. In some embodiments, the biasing element  108  is formed of a resilient material capable of imparting a bias on the rider  106 , as discussed above. In other words, the biasing element  108  promotes movement of the rider  106  in a certain direction along the track  102 . For example, where the capability to incrementally adjust an increasing diameter is desired, then the biasing element  108  may be configured to bias the adjustment mechanism  100  toward the smaller diametric setpoint. In turn, wherein the capability to incrementally adjust from a larger diameter to a smaller diameter is desired, then the biasing element  108  may be configured to bias the adjustment mechanism  100  toward the smaller diametric setpoint (e.g., biasing the rider  106  in a direction along the track  102  from the first diametric setpoint  110  toward the second diametric setpoint  112 ). In some embodiments, the biasing element  108  is formed of a substantially flat sheet of material capable of overlapping itself to form a generally tubular or cylindrical shape, as is shown in  FIG. 1 . In some embodiments, the biasing element  108  may include a length L extending from a first portion  128  to a second portion  130 , and a width W, the width W being a dimension perpendicular to the length L. The biasing element  108  also includes an outer surface  132 , an inner surface  134  ( FIG. 1 ), and a longitudinal axis A. In some embodiments, the track  102  is coupled to the outer surface  132 , near the second portion  130 , of the biasing element  108  and the rider  106  is coupled to the inner surface  134 , near the first portion  128 , of the biasing element  108  such that, when the biasing element  108  is folded over itself and overlaps (e.g., the first portion  128  overlaps the second portion  130 ), the rider  106  engages the track  102  so that the rider  106  is slidably coupled to the track  102 . 
       FIGS. 3A to 3C  show the adjustment mechanism  100  coupled to an implantable medical device  200  in the form of a tubular implant. As shown, the adjustment mechanism  100  can be coupled to the implantable medical device  200  such that, when the adjustment mechanism  100  changes from the first diameter D 1  to the second diameter D 2 , the implantable medical device  200  also changes from the first device diameter d 1  to the second device diameter d 2 . As discussed above, examples of implantable medical devices  200  in the form of tubular implants may include stents and stent-grafts, among other tubular, cylindrically-shaped devices (e.g., heart valves, vascular filters, anastomosis devices, occluders, and others). 
       FIG. 3A  shows the diametric adjustment mechanism  100  coupled to the implantable medical device  200  in an expanded configuration or, in other words, at the first diameter D 1 . As shown, the rider  106  is located at the first diametric setpoint  110  (e.g., at the first step location  116 ). The first diametric setpoint  110  corresponds to the first diameter D 1  of the adjustment mechanism  100  and the first device diameter d 1  of the implantable medical device  200 . 
     As shown, the biasing element  108  can be a cylindrical member  136 , also described as a collar  136 , configured to surround an outer surface or a portion of the exterior surface of the implantable medical device  200 . In some embodiments, the collar  136  has elastic properties that impart the bias on the adjustment mechanism  100 . As discussed above, when the biasing force is overcome by the diametric force, the rider  106  moves, for example, from the first diametric setpoint  110  to the second diametric setpoint  112 , adjusting the diameters of the adjustment mechanism  100  and implantable medical device  200  as described above. 
       FIG. 3B  shows the adjustment mechanism  100  at the intermediate diameter D 1 . The rider  106  is located at the intermediate diametric setpoint  124 . As discussed above, the intermediate diametric setpoint  124  corresponds to an intermediate diameter D 1  of the diametric adjustment mechanism  100  and an intermediate device diameter d 1  of the implantable medical device  200 . As shown, movement of the rider  106  along the track  102  or, in some examples, along the stepped path  114 , facilitates adjustment of the implantable medical device  200  from the first device diameter d 1  to the intermediate device diameter d 1 . 
       FIG. 3C  shows the adjustment mechanism  100  in a compressed configuration or, in other words, at the second diameter D 2 . As shown, the rider  106  is located at the second diametric setpoint  112  (e.g., at the second step location  118 ). The second diametric setpoint  112  corresponds to the second diameter D 2  of the adjustment mechanism  100  and the second device diameter d 2  of the implantable medical device  200 . 
     In some embodiments, a method of adjusting the diameter d of the implantable medical device  200  includes imparting a first diametric force on the diametric adjustment mechanism  100 . The first diametric force moves the rider  106  from the first diametric setpoint  110  to the second diametric setpoint  112  along the stepped path  114  according to a first configuration as can be seen in  FIGS. 3A to 3C  (configured to transition from a larger diameter to a smaller diameter under the first diametric force, which is a compressive force) or according to a second configuration as can be seen in  FIGS. 4A to 4C  (configured to transition from a smaller diameter to a larger diameter under the first diametric force F D1 , which is an expansion force). 
     According to  FIGS. 3A to 3C , in some embodiments, the first diametric force may be a constrictive or compressive force configured to alter the diameter d of the implantable medical device  200 . In some examples, the diametric force is imparted by a manual force applied through the skin (e.g., by hand) or a force applied using one or more transcatheter devices (e.g., a balloon catheter or other device capable of diametric adjustment). The diametric force may also be applied (whether internally or externally) as a magnetic force that interacts with the rider  106  and “forces” or moves the rider  106  along the track  102  between any setpoints of the series of diametric setpoints  104  as desired. The method also includes imparting a second diametric force on the diametric adjustment mechanism  100 . In some examples, the second diametric force releases the rider  106  from the second diametric setpoint  112  and allows movement of the rider  106  from the second diametric setpoint  112  to the first diametric setpoint  110  along the return path  126  (e.g., as a result of the biasing force). 
     As shown in  FIGS. 4A to 4C , in some embodiments, the first diametric force is an expanding force. The expanding force may be imparted on an interior of the implantable medical device  200  in a variety of ways such as with a balloon catheter, as discussed above. 
       FIGS. 4A to 4C  show the adjustment mechanism  100  and implantable medical device  200  as an incremental expansion force is applied. As shown in  FIG. 4A , the rider  106  is positioned at the first diametric setpoint  110  and the implantable medical device  200  is at the second (e.g., reduced) device diameter d 2 .  FIG. 4B  shows the adjustment mechanism  100  after a first diametric force has been applied. As shown, the rider  106  is positioned at the intermediate diametric setpoint  124  and the implantable medical device  200  is at the intermediate device diameter d 1 .  FIG. 4C  shows the adjustment mechanism  100  after a second diametric force has been applied. The rider  106  is positioned at the second diametric setpoint  112  and the implantable medical device  200  is at the first (e.g., expanded) device diameter d 1 . 
     The first and second diametric forces and are described above as both compressive and expansion forces, the first and second diametric force and can be any of a variety of forces capable of overcoming the biasing force and moving the rider  106  along the track  102  from the first diametric setpoint  110  to the second diametric setpoint  112  and vice versa. In some examples, the diametric force is imparted by a manual force applied through the skin (e.g., by hand) or a force applied using one or more transcatheter devices (e.g., a balloon catheter or other device capable of diametric adjustment). The diametric force may also be applied (whether internally or externally) as a magnetic force that interacts with the rider  106  and “forces” or moves the rider  106  along the track  102  from the first diametric setpoint  110  to the second diametric setpoint  112 , from the second diametric setpoint  112  to the first diametric setpoint  110 , and/or between any setpoints of the series of diametric setpoints  104  as desired. For example, the rider  106  optionally includes a ferromagnetic material upon which an internal or external magnet may act. It should be understood that any other types of diametric forces FD may be used, as desired, to impart an applied force on the rider  106  and overcome the biasing force to move the rider  106  along the track  102 . 
     The adjustment mechanism  100  is optionally employed in a variety of applications. For example, the adjustment mechanism  100  is optionally employed to control flow through an intrahepatic portosystemic shunt device (e.g., in association with devices such as W.L. Gore &amp; Associates Inc.&#39;s product sold under the trade name “GORE® VIATORR® TIPS Endoprosthesis.” In other examples, the adjustment mechanism  100  is employed in an arteriovenous access application (e.g., to control flow through a fistula or graft, for example). In still further examples, the adjustment mechanism  100  is employed to control flow through a prosthetic valve (e.g., heart valve). In still further examples, the adjustment mechanism is employed to control flow in an aorta of a patient to control flow into the renal arteries of the patient (e.g., by controlling a diameter of a portion of an aortic stent graft). Although a few examples are provided, it should be understood that any of a variety of applications are contemplated. Methods of using the adjustment mechanism include a one-time adjustment, multiple adjustments, and adjustments of any frequency or periodicity (e.g., an adjustment per minute, hour, day, week, year, or per every heart beat). 
     In some examples, the adjustment mechanism  100  may be configured to adjust the diameter of a medical device with each of the patient&#39;s heartbeats. For example, the adjustment mechanism  100  may have many small diametric setpoints that each require a small biasing force to adjust. Therefore, with each heartbeat, the mechanism  100  may incrementally increase in diameter until reaching its full diameter, at which point the mechanism  100  may reset to its minimum diameter and repeat the cycle. The mechanism  100  may also be configured to incrementally decrease in diameter until reaching its minimum diameter, at which point the mechanism  100  may reset to its maximum diameter and repeat. Such continuous increasing or decreasing of the adjustment mechanism  100  may prevent the patient&#39;s body from adjusting to a new pressure, flow, or other property created by the presence of the adjustment mechanism  100 . 
     Persons skilled in the art will readily appreciate that various aspects of the present disclosure can be realized by any number of methods and apparatus configured to perform the intended functions. It should also be noted that the accompanying drawing figures referred to herein are not necessarily drawn to scale, but may be exaggerated to illustrate various aspects of the present disclosure, and in that regard, the drawing figures should not be construed as limiting.