Abstract:
An implantable and extractable sensor is used for monitoring blood flow and vessel characteristics within a patient. The sensor includes a structurally supportive shuttle that has an angularly offset shelf. A transducer is mounted to this shelf and offset at the same angle so as to utilizes the Doppler effect. Silicone is injection molded around the assembly to provide a housing having a plurality of cutouts that expose portions of release wires running through the housing. The sensor is attached to the vessel by suturing around the exposed portions of the release wires. When the wires are retracted, the sensor can be extracted from the patient without having to reopen the surgical wound. The shuttle provides a consistent location to mount a transducer and also provides the structural support for the silicone housing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a removable implanted device that includes a sensor for measuring blood flow and other vessel characteristics. In particular the present invention relates to an encasement and a support structure to house such an implantable device. 
     2. Description of the Related Art 
     After certain medical conditions have occurred or certain medical procedures have been performed, it is often desirable to monitor blood flow through various veins or arteries, especially those in close proximity to the heart. For example, monitoring the flow of blood out of a patient&#39;s aorta immediately following heart surgery can provide doctors with very valuable information. 
     Various devices have been designed to obtain blood flow information by making appropriate measurements. One device includes a transducer mounted within a housing. When attached to a blood vessel, the transducer utilizes the Doppler effect to monitor blood flow and the A- or M-mode to monitor the diameter of the vessel. The housing has a plurality of cutouts, with one or more release wires extending through the housing as illustrated in U.S. Pat. No. 5,205,292. The wires are releasably coupled to a front wall of the device and are exposed outside of the housing through the cutouts. These wires allow for the releaseable attachment of the device to a patient&#39;s aorta. 
     A tube is coupled to the proximal end of the housing and extends out of the corpus of the patient. The necessary electrical/data connections made to the device are contained within the tube and provide the appropriate signals to monitoring equipment. The above mentioned release wires also extend through the tube to a point where they are accessible by the surgeon. 
     In use, the surgeon makes an incision into the patient. The housing of the device is positioned on the appropriate blood vessel, and sutures are used to connect the vessel to the housing via the exposed portions of the release wire. In other words, the exposed portions of the release wire provide anchoring points for the device. With the tube extending out of the corpus, the implantation wound is then sutured. The data lines are connected and patient information is transmitted to the monitoring equipment. 
     When specific blood flow data is no longer needed, the surgeon locates the portions of the release wires which extend out of the tube and retracts them. This causes a separation of the release wires from the front wall of the housing. As the wires are pulled backwards, they slide through the cutouts, thus releasing the sutures. Once the device is completely free, the surgeon can pull it out of the corpus by retracting the tube. Because the device is so small, it can exit through the opening left for the tube, and hence the surgeon need not create or reopen a wound in order to remove it. This represents a tremendous advantage in patient care, in that the corpus does not have to be needlessly and repeatedly opened to obtain blood flow measurements. 
     The housing and the transducer must be extremely small in order to be effective. As stated above, the device must unobtrusively sutured to various blood vessels and then removed through a surgical wound. It is critical that the wound remain as small as possible and that it not be unnecessarily disturbed. Obviously, making many of these components as small as possible, while also remaining operable, is often a complicated task. All materials and part configurations need to be optimized so as to meet all of the aforementioned goals. Also, manufacturing challenges need to be accounted for to insure an operating product can be fabricated without incurring exorbitant costs. 
     In order to be operable and effective, the transducer must be positioned at a specified angle with respect to the major axis of the vessel. Typically, this angle is accomplished by configuring the housing such that a predetermined angle is maintained between the transducer and the bottom of the housing. This is a difficult manufacturing step because the housing is made from compliant silicone. When the transducer is placed into the housing it is difficult to align properly. The most obvious method of fabricating an appropriate housing is to first mold a housing and then have the transducer attached thereto. In the context of this device, this is not an acceptable process as too many variables can be introduced. Specifically, a layer of adhesive is used to attach the transducer. This adhesive introduces a layer of material below the transducer. It is virtually impossible to insure that the layer of adhesive has parallel surfaces, and is free of air bubbles. The presence of either air bubbles or unparallel surfaces can effect the operation of the device. 
     If the transducer is being attached to a surface of the housing, it is then possible for the surface to be deformed. A deformation in the surface can also adversely effect the operation of the device. 
     Even if properly aligned, it is difficult to maintain this arrangement as the silicone housing sets. As such, the data obtained may potentially be skewed because of an unnoticed misalignment. 
     Additionally, the appropriate placement and retention of the release wires creates another manufacturing challenge. As can be appreciated, the release wires must be sufficiently retained within the housing so that suturing will hold the housing in place. Concurrently, the release wires must be sufficiently releaseable so they can be easily withdrawn at an appropriate point in time. 
     Again, one obvious way of manufacturing a housing which includes these release wires is to take the above discussed preformed housing, drill appropriate passage ways through the housing, and insert the release wires therethrough. However, these drilled holes are typically sized larger than the release wire itself, thus eliminating the natural ability of the housing to retain the release wires. In this configuration, the release wires are thus prone to slide out of the housing when being sutured. If this occurs, the device becomes virtually unusable as the measurements cannot be relied upon. 
     Another way of manufacturing a housing which includes these release wires is to take the above discussed preformed housing and pierce the release wires through the housing appropriately. This is prone to manufacturing inconsistencies which result in stress or strain on the compliant housing, unequal gripping force by the housing on the wires, ripping of the compliant housing, or improper placement of the wires. 
     As such, there exits a need to provide an easily removable implant containing a transducer securely and accurately fixed at a predetermined angle with respect to the surface of the implant contacting the wall of the vessel. There also exists a need to provide an implantable device which will maintain and position release wires in a consistent and predictable manner to accommodate appropriate retention of the device while also accommodating desired releaseability. 
     SUMMARY OF THE INVENTION 
     The present invention includes a structurally supportive shuttle member having a planar shelf which is offset at a predetermined angle from the main axis of the shuttle. A transducer is then mounted on the shelf, and as such, maintains the same predetermined angle. A cavity exists immediately behind the shelf and opposite the transducer. Acoustical backing material is placed in the cavity to enhance transducer operation. The various electrical components and connections required to operate the transducer are then placed within the space remaining in this cavity, beside the acoustical backing material. Connection to the actual transducer is made via conductive flex tapes which pass through appropriate openings in the shuttle. 
     Initially, the shuttle assembly will have three struts extending outwardly from its main body segment. The struts are used to provide support alignment and anchoring points for the shuttle during the assembly process. Initially, the transducer is mounted to the shelf and the various electrical connections are made. Next, acoustic backing material is placed in the cavity behind the transducer. The assembly is then insulated with parylene and plated with a gold shield. Release wires are then placed parallel to the major axis of the shuttle and are run through a coil stop that is located adjacent to one end of the shuttle. With all of these components properly in place, the shuttle is ready for encasement. 
     A housing is injection molded around the shuttle and its attached components. While this molding is used to produce the housing, it is more of an encasement than classical “housing.” The housing is formed so as to provide cutouts that provide the necessary suture points. That is, the portions of the release wire exposed through the cutouts accommodate sutures when attached to a patient. Portions of the struts extend out through the housing, and continue to support the assembly until the silicone solidifies. Once the silicone solidifies the extended portions of the struts and one end of the release wires are severed. Lastly, final assembly operations are completed such as placing all associated wiring into a protective tubing. At this point, a properly configured implantable device has been fabricated and is ready to be utilized. 
     It is an object of the present invention to provide a functioning transducer mounted within a structurally sound housing, wherein an angle of the transducer with respect to a lower wall of the housing is fixed. 
     It is another object of the present invention to provide a shuttle that can be used during the manufacture of an implanted device which utilizes an ultrasound transducer to measure certain characteristics of the body&#39;s function. 
     It is yet still another object of the present invention to configure the shuttle so that it provides structural support to the housing, once completed. 
     It is still another object of the present invention to provide an easily manufacturable ultrasound transducer device which includes a shuttle that can support and properly align the components of an implanted device to enhance operation. This includes proper alignment of the transducer itself and appropriate positioning of connections and other acoustic materials. The use of this shuttle also provides for protection of the transducer during manufacturing. 
     It is yet another object of the present invention which provides for the appropriate retention of release wires to insure they stay in place during suturing, while also easily release when desired. 
     It is still yet a further object of the present invention to provide an implantable device wherein the electrical and mechanical connections are made without impeding the acoustical performance of the transducer. 
     It is another object of the present invention to provide a shuttle assembly for an implantable device wherein the shuttle includes a cavity for containing acoustical backing material to further enhance the operation of a transducer. 
     It is still another object of the present invention to electrically shield the components at an implantable device so that patient safety is assured and so that the components function properly. 
     It is a further object of the present invention to encase the electrically shielded components of an implantable device in a silicone housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top, planar view of a completed housing for an implanted device with the release wires extending through cutouts in the housing and trailing from the proximal end of the housing. 
     FIG. 2 is side planar view of a completed housing for an implanted device. 
     FIG. 3 is a top view of a completed housing attached to a vessel, by having the exposed portions of the release wires sutured to the wall of the vessel. 
     FIG. 4 is a top planar view of a shuttle assembly, prior to injection molding, with the release wires and coil stop properly positioned. 
     FIG. 5 is a front, planar view of a coil stop. 
     FIG. 6 is a bottom, planar view of the shuttle assembly. 
     FIG. 7 is a side, planar view of the shuttle assembly. 
     FIG. 8 is a lower perspective view of the shuttle assembly in relation to the silicone housing which will be later be formed around the shuttle assembly. 
     FIG. 9 is an upper perspective view of a shuttle assembly in relation to the silicone housing which will be later be formed around the shuttle assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, a substantially complete implantable sensor head is illustrated and generally referred to as  10 . Sensor head  10  has a contoured housing  12  that is formed from an injection molding process. Preferably, silicone is used to form housing  12 , as silicone is structurally reliable and medically safe for implantation procedures. Furthermore, the implant utilizes an acoustic transducer and silicone has excellent acoustic properties. Housing  12  has a number of arcuate cutouts  13  spaced along its outer perimeter. A pair of release wires  14  are shown to extend through housing  12 , and are partially exposed as they pass through each of the cutouts  13 . The release wires  14  are releasably secured within housing  12  by the frictional engagement of the silicone. Release wires  14  exit the proximal end  17  of housing  12  and extend to a release mechanism (not shown). Similarly, electrical wires  16  exit proximal end  17  and are coupleable to appropriate monitoring equipment (again, not shown). 
     As will later be explained in greater detail, sensor head  10  is a sensor or probe used to measure fluid flow. Housing  12  encases an ultrasound transducer that sends and receives signals. The transducer is preferably ultrasonic and is mounted onto a shuttle assembly, which is also encased within housing  12 . 
     FIG. 3 illustrates sensor head  10  as it is attached to an artery  18 . Sutures  20  are wrapped about the portions of release wires  14  that are exposed at cutouts  13  and are attached to the wall of artery  18 . Tubing  22  is coupled to the proximal end  17  of housing  12  and encases the extending portions of release wires  14  and electrical wires  16 . After implantation, the tubing extends out of the corpus of the patient. While in use, the electrical wires  16  are coupled to various types of monitoring equipment. The transducer operates to obtain various measurements, including blood flow and arterial dimensions. When monitoring is no longer needed, the surgeon pulls release wires  14 , dislodging them from their encasement in the walls of housing  12  and retracts them out through tubing  22 . Release wires  14  can be fully retracted but must at least be retracted beyond the cutouts  13 . Once release wires  14  are retracted beyond cutouts  13 , the sutures  20  no longer bind housing  12  to artery  18 . As such, the surgeon can then extract sensor head  10  from the patient by retracting tubing  22 . The sutures  20  will harmlessly remain in the patient or dissolve over time. 
     As previously mentioned, the sensor head  10  utilizes an ultrasonic transducer to perform appropriate monitoring. Appropriate alignment and handling of this sensor is critical to insure proper operation. As can be appreciated, this makes fabrication very complicated. Further, issues of appropriate electrical connections and shielding all further complicate the fabrication process. Consequently, a internal structure for sensor head  10  must be carefully designed to allow for easy fabrication. 
     Referring to FIGS. 4-7, a shuttle  24  forms the foundation for the assembly of components into the sensor head  10 . Shuttle  24  is a solid and rigid mechanical structure having a medially disposed main body  26 . Shuttle  24  can be made from any suitable material such as polycarbonate. A distal tee  28  and a proximal tee  30  extend from main body  26 . Right strut  32 , left strut  34 , and tail strut  36  also extend away from the main body  26 . 
     Main body  26  includes a cavity  38  with a slanted shelf  54  disposed forming bottom of the cavity  38 . A slot  40  is provided which connects the shelf  54  to the remainder of cavity  54 . Slanted shelf  54  is specifically configured at a predetermined angle with respect to the main body  26  of shuttle  24 . The molding process used to fabricate shuttle  24  provides for tight control of this angle, thus insuring a repeatable configuration for shelf  54 . 
     As seen in FIG. 8, a transducer  56  is coupled to the shelf  54  and assumes the same angle with respect to main body  26 . In order to utilize the Doppler effect to obtain measurements, the transducer  56  must be angled with respect to the artery  18  wall. The use of shelf  54  as a mounting structure provides a convenient and consistent way to precisely control this angle in sensor head  10 . 
     Flexible contacts (not shown) are flushly coupled to each major face of the transducer  56  providing the appropriate connections. These flexible contacts can then be connected with additional circuitry to provide efficient transducer operation. By connecting in this way, the attachment of the contacts will not acoustically interfere with the transducer. 
     Acoustic backing material  48  is placed within cavity  38 , leaving enough room within cavity  38  for any electrical connections or components (including the flex contacts) necessary to operate transducer  56 . The acoustic backing material  48  is provided to increase the sensitivity of the transducer  56  and to accommodate broadband operation (i.e., having a short ring-down time). It is advantageous to utilize the acoustic backing material because it has an impedance close to that of air, but also provides a mechanical damping effect. It is a soft material, making it easier to work with, yet still having a very high attenuation. 
     Electrical wires  16 , coupled to transducer  56  through the flexible contacts extend towards and beyond proximal tee  30 . Various notches and grooves can be cut into shuttle  24  to direct and retain the various electrical wires. The shuttle  24 , transducer  56 , and all of the electrical contacts and components are coated with a layer of parylene which acts as an electrical insulator. Alternatively, any other acoustically transparent conformal coating could be used. The shuttle  24  is also plated with gold (or any other appropriate metal) to shield the transducer, minimizing its susceptibility to noise and electromagnetic emissions. A shield wire is attached (shown as part of electrical wire  16 ) to maintain appropriate contact with the shield. 
     Right strut  32  and left strut  34  are provided with right notch  42  and left notch  44 , respectively. To encase transducer  56  and all other components, shuttle  24  is positioned in an injection molding device (not shown) such that is supported by the three struts  32 ,  34 ,  36 , leaving the remainder of the shuttle  24  and its attached components entirely unencumbered. Release wires  14  are coated with parylene and then placed parallel to a main axis of shuttle  24 . The release wires  14  are sufficiently rigid to facilitate this arrangement. The release wires  14  pass through right and left notches  42 , 44  so that they are in close proximity to main body  26 . Care must be taken so that the release wires  14  do not contact the gold plated shuttle  24 , as this represents a potential risk to the patient&#39;s safety. A yoke shaped coil stop  46  is positioned near proximal tee  30  and the release wires  14  extend through wire release holes  52 , thus supporting coil stop  46  in the position illustrated. 
     Once the arrangement of FIG. 4 is achieved and the components are in place, silicone is injected into a mold surrounding these elements, thus forming contoured housing  12 . The contoured housing provides a structured encasement for shuttle  24  and all related components. Most significantly, this provides an encasement for transducer  56  which is conducive to transmitting sound to/from the patient&#39;s tissue. Further, because the shuttle can be easily aligned during this molding operation, the alignment of transducer  56  with respect to the bottom surface of housing  12  can be carefully controlled. As previously mentioned, sensor head  10  is sutured to an artery in order to perform its intended monitoring. In this arrangement, the bottom surface of sensor head  10  is placed in direct contact with the artery surface. Due to the controlled alignment between transducer  56  and the bottom surface of sensor head  10 , a controlled angle is also provided when sensor  10  is attached to the patient&#39;s artery. 
     The cutouts  13  are located so that release wires  14  are partially exposed, and a gap is provided between the exposed portion and the remainder of housing  12 . The shape of the cutouts  13  is significant. In previous removable implantable devices, the cutouts have been relatively long, large radiused, arcuate configurations. As such, relative little force was required to cause the release wires to bend or arch away from the housing. When the sutures were placed around the release wires, they could be pulled away from the housing. In order to prevent the wires from being pulled out of the housing by the tension generated by the sutures, a relatively firm clamping mechanism had to be placed into the housing that rigidly and securely held the release wires in place. Hence, upon removal, a large amount of force had to be applied to release the wire, which necessitated the use of coil stop  46 . In the present invention, the cutouts  13  are relatively short and provide a nearly perpendicular plane to the entrance and exit of the release wires  14 . Hence, much more force would have to be applied to cause the release wires  14  to arch away from the housing. This is because only a short segment of the wire is exposed and it passes through walls that are perpendicularly offset from it. As such, the suturing process will not flex the wires  14 . Therefore, less force is required to retract the wires  14 . Since less force is required, the silicone encasement of the wires  14  is sufficient to hold them in place. Furthermore, the restriction forces are minimal, hence it is very unlikely that the coils (not shown) would be forced into housing  12  (and blocking cutout  13 ). Therefore, coil stop  46  is now optional. 
     The coil stop  46  (when used) is put in place so that when the release wires  14  are retracted, the coils (not shown) do not enter the silicone housing  12  and obstruct the cut outs  13 . Were this to happen, the surgeon would not be able to retract the implant and would have to reenter the corpus. Each release wire  14  is held in place due to the frictional engagement of the silicone that surrounds it. In addition, each wire  14  has a pair of metal coils surrounding the wire  14  outside of but proximate to the housing  12 . The surgeon manipulates the wires  14  and coils so that the coils move towards the housing  12 , causing the wires to move away from (out of) housing  12 . The parylene coated release wires  14  are fairly easy to extract from the silicone. As such, the coil stop  46  is optional and is only necessary if it is expected that the separation of the wires  14  from the housing  12  will be made more difficult. 
     FIGS. 8 and 9 illustrate a comparison between the shuttle  24  (and its attached components) prior to injection molding and the formed silicone portion of housing  12 . As shown, various elements will extend beyond the housing  12 , once the silicone is molded. For example, release wires  14  will extend in both a proximal and distal direction beyond housing  12 . Furthermore, right, left, and tail struts  32 ,  34 ,  36  will extend beyond housing  12 . Once the silicone has hardened, the extended portions of struts  32 ,  34 ,  36  are trimmed away flush with housing  12 . The release wires  14  may simply be cut to terminate in the silicone housing  12 , with the silicone providing a sufficient clamping action on the wires  14  to hold them in place, until they are forcibly pulled from the silicone. Another alternative is to use a single release wire  14  that it is looped through the distal end  15  of the housing  12 , with each of the ends of wire  14  then extending outside of the patient, where they are secured. Instead of releasing from the silicone housing or a release mechanism, the looped wire is simply disconnected outside of the patient, and then one end is pulled, until the sutures  20  are free. The tab  50  of coil stop  46  is also trimmed away. 
     Since silicone is generally pliable, the remaining portion of shuttle  24  provides most of the structural support for the sensor head  10 . The distal and proximal tees  28 ,  30  are embedded within the silicone and form a “hammerhead” structure that increases the overall structural integrity of the device. The angled portion of main body  26  provides depth to the shuttle  26 , further increasing its rigidity. Finally, the untrimmed portions of right and left struts  32  and  40  that remain within housing  12  form a “medial tee” and offer additional rigidity. The shuttle  24  provides an optimal balance between providing sufficient rigidity, while ensuring that the shuttle assembly is fully encased. 
     As shown, the bottom surface of housing  12 , which contacts the wall of the artery is slightly convex. When applied to the convex wall of an artery, the housing  12  causes the artery to become partially concave to receive the housing  12 . As such, this contact will force any air trapped between the housing  12  and the artery outward, thus assuring flush contact. Also, this will eliminate air bubbles, which can detrimentally effect the operation of the device. 
     To complete the implant, a sufficient length of the release wires  14  and electrical wires  16  are inserted into a similar length of tubing  22 . An end of the tubing  22  is then coupled to the proximal end of housing  12 , forming a completed sensor head  10  as shown in FIGS. 1-3. 
     Those skilled in the art will further appreciate that the present invention may be embodied in other specific forms without departing from the spirit or central attributes thereof. In that the foregoing description of the present invention discloses only exemplary embodiments thereof, it is to be understood that other variations are contemplated as being within the scope of the present invention. Accordingly, the present invention is not limited in the particular embodiments which have been described in detail therein. Rather, reference should be made to the appended claims as indicative of the scope and content of the present invention.