Abstract:
An implantable drug delivery system utilizing a non-magnetic valve/accumulator metering assembly is disclosed. The solenoids of the prior art, which respond to magnetic fields, are replaced by Shape Memory Alloy (SMA) wires and associated control electronics. By exploiting the inherent characteristics of SMA wires, which can expand and contract based on their temperature, the movements required to actuate the metering assembly can be achieved. This configuration retains the benefits associated with the prior art, while eliminating the major drawback.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Implantable valve accumulator pump systems for the delivery of mediation or other fluids to a patient are well known and described in U.S. Pat. Nos. 4,838,887 and 5,049,141, the disclosures of which are hereby incorporated by reference. 
         [0002]    U.S. patent Nos. &#39;887 and &#39;141 disclose an implantable valve accumulator pump system for the delivery of infusate, such as medication. The implantable pump portion is comprised of four essential assemblies, as shown in  FIG. 1 . The first major assembly is a rechargeable, constant pressure drug reservoir  10  in series with a bacteria/air filter  24 . In one embodiment, the reservoir  10  comprises a sealed housing  14  containing a bellows  16 . The bellows  16  separates the housing  14  into two parts. Chamber  18  is used to hold the drug or other medicinal fluid. Second zone  20  is normally filled with a two-phase fluid, such as Freon®, that has a significant vapor pressure at body temperature. Thus, as the fluid within the second zone  20  vaporizes, it compresses the bellows  16 , thereby pressurizing the drug in the chamber  18 . The drug can be refilled via septum  12 . 
         [0003]    The two-phase fluid helps maintain the chamber  18  under a constant pressure. When the chamber is refilled, the two-phase fluid is pressurized thereby condensing a portion of the vapor and converting it to liquid. As the chamber  18  is emptied, this liquid vaporizes, thus maintaining the pressure on the bellows  16 . 
         [0004]    Since the infusate in chamber  18  is under positive pressure, it is urged out of the chamber, through a bacterial filter  24  and toward the metering assembly. 
         [0005]    The second major assembly is an electronically controlled metering assembly comprising two normally closed solenoid valves  26 ,  28 , which are positioned on the inlet and outlet sides of a fixed volume accumulator  30 . The valves are controlled electronically via a module  32 , which can be programmed utilizing an external programmer  34 . The metering assembly is designed such that the inlet valve  26  and the outlet valve  28  are never simultaneously open. 
         [0006]    The third major assembly is an outlet catheter  36  for medication infusion in a localized area. The delivery of fluid occurs at an infusion site that is below the accumulator pressure, thereby forcing discharge through the catheter  36 . 
         [0007]    The fourth assembly of this system is the external programmer  34  used to communicate and program the desired medication regimen. This programmer is preferably a handheld unit with a touch screen. It provides a data transfer link to the implanted electronics  32  and is able to exchange information with the electronics  32 , including but not limited to battery status, diagnostic information, calibration information, etc. 
         [0008]    Returning to the metering assembly,  FIG. 2  illustrates the normal sequence used to fill and dispense infusate. The valves in the medication metering assembly alternately open and close to admit infusate from the reservoir  18  into the accumulator  30  and to dispense a precise volume spike to an outlet catheter  36 . During the first step, both valves are closed and the accumulator is empty. In this step, no fluid is moved. During the second step, the inlet valve  26  opens while the outlet valve  28  remains closed. Since the incoming fluid is at a higher pressure than the accumulator  30 , fluid fills the accumulator. The accumulator preferably has a fixed volume such that exact amounts of fluid can be dispensed. Once the accumulator  30  is filled, no fluid movement occurs. During the third step, the inlet valve  26  closes, thereby separating the reservoir from the accumulator. At this step, the accumulator  30  is filled. Finally, during the fourth step, the outlet valve  28  opens. Since the accumulator  30  is at a higher pressure than the outlet canella, the fluid exits the accumulator through outlet valve  28 . 
         [0009]      FIG. 3  illustrates the components used in the metering assembly of the prior art. Valves  26  and  28  are implemented as miniature solenoid valves. The valves are preferably disposed in a side-by-side arrangement having two solenoid assemblies  74 , each receiving power via a corresponding electrical lead  76 . The valves are operably powered to drive a working plunger  78  biased by means of spring  80 . The working plunger and return spring assembly are isolated from the solenoids  74  by means of an isolation diaphragm  82 . As is customary, the solenoid is actuated by a magnetic field that drives the working plunger  78 . Once charged, the solenoid overcomes the force of the bias spring  80 , and pulls the plunger  78  off the valve seat  84 , allowing fluid flow. 
         [0010]    The flow path of the infusate or medicinal fluid is illustrated by the arrows in  FIG. 3 . As described above, with valve  26  in the open position, fluid communication is established between the accumulator  30  and the inlet conduit  54 . The infusate is thereby delivered upward through the valve seat  84  (shown closed in  FIG. 3 ), into the accumulator flow passage  86 . The area between the valve seats comprises the accumulator storage space. When valve  26  is closed, the accumulator  30  is isolated from the reservoir  18 . 
         [0011]    When valve  28  is opened, fluid communication is established between the accumulator and the outlet conduit  55 . The infusate is thereby delivered downward from the accumulator storage space, through the valve seat  84  (shown closed in  FIG. 3 ), and into the outlet conduit  55 . Furthermore, the system is preferably designed such that valves  26  and  28  cannot be opened at the same time in order to prevent the metering function of the accumulator  30  from being bypassed. 
         [0012]    This system is highly effective in most situations. However, when a patient with such a device enters an MRI (magnetic resonant imaging) environment, the presence of large magnetic fields above a certain threshold may affect the operation of the pump&#39;s valves such that the metering function of the accumulator is bypassed. Consequently, patients implanted with these devices must be instructed to have the pump reservoir  18  emptied before undergoing an MRI procedure. This prohibition and warning is commonplace for patients implanted with such medical devices. 
         [0013]    It would be beneficial for such an implantable drug delivery system to remain operational even in the presence of strong magnetic fields, such as those encountered during MRI procedures. 
       SUMMARY OF THE INVENTION 
       [0014]    The problems of the prior art have been overcome by the present invention through the use of a non-magnetic valve/accumulator metering assembly. The solenoids of the prior art, which are susceptible to magnetic fields, are replaced by Shape Memory Alloy (SMA) wires and associated control electronics. By exploiting the inherent characteristics of SMA wires, which can expand and contract based on their temperature, the movements required to actuate the metering assembly can be achieved. This configuration retains the benefits associated with the prior art, while eliminating the major drawback. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic diagram of the implantable drug delivery system of the prior art; 
           [0016]      FIG. 2  is a representation of the sequence of steps performed by the metering assembly; 
           [0017]      FIG. 3  is a schematic representation of the metering assembly of the prior art; 
           [0018]      FIG. 4  is a first embodiment of the present invention; 
           [0019]      FIG. 5  is a second embodiment or the present invention; and 
           [0020]      FIG. 6  is a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    As described above, solenoids are used to cause the movement required to open and close the valve assemblies. However, in a magnetized environment, such as an MRI chamber, these solenoids cannot be controlled. Thus, it is possible to bypass the accumulator. To overcome this limitation, the movement, formerly provided by the solenoid, is now provided via a shape metal alloy (SMA) wire. 
         [0022]    Shape memory alloys are metals that display property changes as their temperature changes. Most useful is the fact that, at elevated temperatures, these alloys transform to a memorized shape. 
         [0023]    Several metals exhibit these properties, including a nickel-titanium alloy, comprising roughly equal parts of each. Other alloys, such as CuAlNi, can also be used. The nickel-titanium alloy is particularly useful because of its electrical characteristics. In wire form, this alloy can be heated simply by passing an electrical current through it. Thus, no external or addition heat generators are required. 
         [0024]    As stated above, the SMA wire is actuated preferably by passing current through it, so as to heat the wire. Since an electrical current is passing through the wire, it is preferable to separate the wire from the fluid path in the assembly. A number of different techniques can be utilized to achieve this result. 
         [0025]    In a first embodiment, shown in  FIG. 4 , each solenoid is replaced with a single SMA wire. The valve system consists of two valves  110 ,  120 , both in communication with an accumulator  130 . The normal or default state of both valves is closed, and each is independently actuated using a respective SMA wire  112 ,  122 . As done in the prior art, the inlet valve  116  is actuated to allow the passage of fluid through the inlet  140  and into the accumulator  130 . The accumulator is preferably pressurized with a fluid, such as Argon (Ar) gas. Once the accumulator  130  reaches a maximum volume, such as about 2 μL, the inlet valve  116  can be closed. After the inlet valve  116  has been closed for a period of time, the outlet valve  126  can be safely actuated to allow the accumulator  130  to drain through the outlet valve  126  and through the outlet  150 . The device follows the sequence of steps described in  FIG. 2 . 
         [0026]    Each valve is actuated using a respective SMA wire  112 ,  122 . These wires are preferably located outside of the fluid path, since electrical current is passed through them. Thus, the valve is separated into at least two chambers, a fluid chamber through which infusate can flow, and a second chamber in which the SMA wire is attached. Within the fluid chamber is an actuated member, having two settings; a first setting wherein fluid communication occurs between the fluid chamber and the accumulator and a second setting wherein the fluid chamber is isolated from the accumulator. 
         [0027]    In one embodiment, one end of a wire is attached to a fixed point  114 ,  124 , while the opposite end is connected to the actuated member, such as plunger assembly  116 ,  126 . The plunger assemblies  116 ,  126  are preferably attached to flexible fluid impermeable barriers  117 ,  127  that separate the fluid chamber from the wire chamber of the valve. These flexible fluid impermeable barriers, which can be made of titanium or any other suitable material, deflect as the SMA wire contracts allowing the valve to open. In some embodiments, the SMA wire contracts about 3% of its total length when an electrical current is ran through the wire. This reduction in length pulls the actuated member, such as plunger  116 , 126  to its first setting, away from the valve seat, thereby allowing the valve to open. 
         [0028]    The SMA wire is connected to a power supply through a lead near the fixed point  114 ,  124 . The return path for the current is provided via an insulted lead located near the plunger (not shown). When power is removed from the wire  112 ,  122 , the wire relaxes back to its longer length. A biasing element, such as return spring  118 ,  128  preferably located inside the fluid path, aids in returning the actuated member, such as plunger  116 ,  126 , to its second setting where it presses against the valve seat. In some embodiments, the plunger is titanium with a molded silicone seat. In addition, in some embodiments, the forces resulting from the deflection of flexible fluid barrier  117 ,  127  also help return the assembly to the second setting, which is its default position. 
         [0029]    As stated above,  FIG. 4  depicts an assembly utilizing a fluid barrier to separate the fluid chamber from the wire chamber. This design represents one example of a possible configuration utilizing a flexible fluid barrier. Furthermore,  FIG. 4  depicts the use of a plunger assembly as the actuated member used to enable and disable fluid communication between the fluid chamber and the accumulator. Those skilled in the art will appreciate that other designs are also possible and within the scope of the invention. 
         [0030]    For example,  FIG. 5  depicts an assembly utilizing a bellows configuration as another approach to separate the fluid chamber from the wire chamber. The bellows also serves as the actuated member. The bellows is preferably made of titanium with plungers  116 ,  126  welded to the bellows. The bellows can withstand repeated cycling without any material changes in performance. In this configuration, all similar components are numbered using like reference designators and operate as described above. In this figure, the bellows  210 ,  220  act as a flexible fluid barrier and replace the springs  118 , 128  shown in  FIG. 4 . The bellows  210 ,  220  is maintained in a slight compressed state when the valve is closed, so as to keep the valve seat engaged. When the SMA wire  212 ,  222  is energized, it begins to contract. This force further compresses the bellows  210 ,  220  and causes the valve to open, thereby allowing fluid flow. When power to the wire is removed, the SMA wire relaxes to its original length. In this relaxed state, the spring force from bellows  210 ,  220  returns the seat to its normally closed position. 
         [0031]    This figure shows the SMA wire  212 ,  222  in a looped configuration. This configuration allows twice the pull forces in a similarly sized physical space. In this configuration, two leads are provided to each valve, one attached to each end of the SMA wire. Either the straight or looped wire design may be used in any of the embodiments. For example, the looped configuration shown in  FIG. 5  can be applied to the assembly shown in  FIG. 4 . Similarly, the straight wire configuration of  FIG. 4  can be used in the bellows configuration of  FIG. 5 . 
         [0032]      FIG. 6  shows a third embodiment of the assembly of the present invention. In this embodiment, the actuated member is a plunger  310 ,  320 , and is surrounded by an elastic material  330 ,  332 , such as an elastomer. Silicone or a soft plastic material, such as urethane can also be used. The plunger and the surrounding material is sized and shaped such that under normal conditions, the elastic material is under slight compression so as to force the valve seat closed. The plunger and elastic material also serve as a flexible fluid barrier. A SMA wire  312 , 322 , in either a straight or looped configuration, is attached to the bottom of the plunger  310 ,  320 . 
         [0033]    When the SMA wire  312 ,  322  is energized and contracts, the plunger is pulled away from the valve seat. This causes the elastic material to deflect and allows the valve seat to open, thereby allowing fluid flow. When the current through the SMA wire  312 ,  322  is removed, the wire relaxes to its original length. The spring force from the deflected, or compressed, elastic material pushes toward the valve, and urges the valve seat to its closed position. 
         [0034]    In one embodiment, the plunger assembly is a over molded unit where the titanium plungers  310 ,  320  are over molded with a polymer to form a single unit. In an alternative embodiment, the plunger  310  and the surrounding elastic material  330  are molded as a single unit. 
         [0035]    The valve electronic system can be controlled using a closed or open loop system. In an open loop system, a known amount of electrical current is passed through the valve (i.e. the SMA wire) in order to actuate the device. In a closed loop system, there is electrical/mechanical feedback that allows the valve to be opened to the same distance and only applies the amount of current required to actuate the wire to this distance. This feedback can be created in a variety of ways, including but not limited to electronic sensors, or mechanical proximity switches. A closed loop system prevents the wire from over stress and helps to maintain the power at a lower level than in an open loop system, since the current passed through the wire is more tightly regulated. The closed loop system should therefore provide lower power consumption and long cycle life of the SMA wire. However, in certain implementations, the simplicity of an open loop system may be preferable. 
         [0036]    Each of these embodiments illustrates the basic requirements associated with the present invention. Because of the current flowing through the SMA wires, the fluid path must be isolated from the wire. While three embodiments are described above, the invention is not so limited. Any mechanism that successfully isolates the fluid chamber from the SMA wire chamber can be used. A second requirement is that there be a biasing element to help stretch the SMA wire back from its energized length to its relaxed length. Again, the present disclosure describes springs, bellows and elastic materials as three embodiments of implementing this biasing element. However, other biasing elements are known in the art and within the scope of the present invention. Finally, an actuated member is required to open and close fluid communications between the accumulator and the fluid chamber