Abstract:
A method and apparatus for infusing medication into a patient&#39;s body using a medication chamber referenced to ambient pressure. The apparatus includes a medication chamber enclosed by a peripheral wall which includes a movable portion configured to transfer exterior ambient pressure into the chamber. Means are provided for exerting a negative bias force acting on the movable portion in a direction opposed to the ambient pressure force. Thus, the resultant pressure in the chamber will be negative with respect to ambient pressure, reducing the risk that the chamber can be overpressurized and produce an unintended medication discharge.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of PCT/US2004/020117 which claims priority based on U.S. Provisional Application 60/483,015 filed on 25 Jun. 2003. This application claims priority based on both aforementioned applications. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to medication infusion devices which include a chamber for storing fluid medication and means for extracting medication from the chamber for delivery to a patient&#39;s body site. 
       BACKGROUND OF THE INVENTION 
       [0003]    Various types of implantable and/or external medication infusion devices are described in the literature. For example only, see U.S. Pat. Nos. 4,772,263 and 6,283,943 and the references cited therein which relate primarily to implantable devices. Many such devices employ a medication chamber together with a propellant reservoir which functions to isolate the chamber from changes in ambient pressure attributable, for example, to changes in altitude. More particularly, a typical propellant reservoir contains a biphasic propellant balanced between gas and liquid phases to maintain a constant pressure regardless of changes in reservoir volume. The pressure in the medication chamber is typically referenced (either positive or negative) to the constant reservoir pressure. Positive referenced devices have the advantage that the propellant can be selected to provide a constant driving pressure under defined operating conditions (e.g., constant flow applications) acting in a direction to force medication out of the chamber. Alternatively, negative referenced devices have inherent safety advantages; e.g., when refilling the chamber with a hypodermic needle, medication can be drawn into the chamber without the application of manual pressure to the needle. This assures that the needle will not discharge medication unless it has been properly placed in a device fill port and reduces the possibility of chamber overpressurization. Also, during normal operation, since chamber pressure is lower than ambient pressure, the pressure differential acts in a direction to draw fluid from the outlet catheter toward the chamber thus tending to reduce the risk of medication leakage into the patient&#39;s body. 
         [0004]    Although the use of a propellant reservoir has the advantage of isolating the medication chamber from changes in ambient pressure, it nevertheless adds to device size, complexity, and cost. Accordingly, it has been recognized that, in some situations, it may be preferable to reference the medication chamber directly to ambient pressure. For example, U.S. Pat. No. 4,772,263 describes an infusion pump which includes a spring for producing a positive force on the drug chamber to force the solution therefrom. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is directed to a method and apparatus for infusing medication into a patient&#39;s body using a medication chamber referenced to ambient pressure (so as to avoid the need for, and attendant complexity of, a propellant reservoir) while achieving the safety and reliability of negative referenced propellant reservoir designs. Embodiments of the invention can be configured for use either exterior to a patient&#39;s body or implanted within a patient&#39;s body 
         [0006]    An apparatus in accordance with the invention includes a medication chamber enclosed by a peripheral (or boundary) member which includes a movable portion configured to transfer exterior ambient pressure into the chamber. Means are provided in accordance with the invention for exerting a negative bias force acting on the movable portion in a direction opposed to the force produced by the ambient pressure. Thus, the resultant pressure in the chamber will always be negative with respect to ambient pressure, reducing the risk: that the chamber can be overpressurized and produce an unintended medication discharge. 
         [0007]    The peripheral member defining the chamber can be variously formed in accordance with the invention. For example, the peripheral member (or wall) can be comprised of one or more rigid and/or flexible wall portions which cooperate to fully enclose the chamber. At least one wall portion is movable and has an exterior surface exposed to ambient pressure. 
         [0008]    In one preferred embodiment, the peripheral member is defined by a rigid wall portion and a flexible wall portion, e.g., a resilient membrane, secured around its edge to the rigid wall portion to enclose the chamber therebetween. The exterior surface of the flexible wall portion is exposed to ambient pressure and a negative bias force is applied to the flexible wall portion acting in opposition to the ambient pressure. The negative bias force can be provided by various types of force generators, e.g., a magnet, the inherent resiliency of a properly configured resilient membrane, or by a spring member (e.g., leaf, coil, bellows, elastomeric material, etc). In any event, the bias force acts to create a pressure in the chamber which is negative with reference to ambient. 
         [0009]    In accordance with the invention, medication is extracted from the negatively biased chamber by a selectively actuatable outlet pump. 
         [0010]    In one alternative preferred embodiment, the chamber peripheral wall member can be comprised of first and second rigid wall portions connected by a flexible wall portion, e.g., a flexible shroud or bellows, which permits the rigid wall portions to move toward and away from one another to vary the chamber volume therebetween. 
         [0011]    In a still further preferred embodiment, the chamber peripheral wall can be formed by the interior wall surface of a hollow cylinder and by a piston mounted for reciprocal linear movement in the cylindrical volume. 
         [0012]    Regardless of the particular implementation of the chamber peripheral wall, embodiments of the invention are characterized by a movable wall portion which is exposed to ambient pressure and a bias force acting in opposition to the ambient pressure to produce a resultant chamber pressure which is negative with respect to the ambient pressure. The chamber peripheral wall, including the moveable wall portion, preferably has a geometry which optimizes volumetric efficiency, i.e., maximizes the useable volume and minimizes dead space volume or ullage. The bias force can be produced by a variety of force members including, for example, discrete springs of various types, elastomeric material, magnets, etc. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0013]      FIG. 1  is a schematic plan view of a preferred medication infusion device in accordance with the invention; 
           [0014]      FIG. 2  is a schematic sectional view through the device of  FIG. 1  showing the movable portion (e.g., resilient membrane) of the chamber peripheral (or boundary) wall in a fully extended (i.e., chamber full) position; 
           [0015]      FIG. 3  is a schematic representation of the movable chamber wall portion depicting the application of ambient and bias forces to the wall portion in accordance with the invention; 
           [0016]      FIGS. 4A and 4B  schematically depict an alternative embodiment of the invention using a spring to provide the bias and respectively showing the movable wall portion in its compressed and extended positions; 
           [0017]      FIG. 5  is a schematic illustration of a further alternative embodiment of the invention using a bellows or exterior spring to provide the bias force; 
           [0018]      FIG. 6  is a schematic illustration of a still further alternative embodiment using magnetic repulsion to provide the bias force; and 
           [0019]      FIG. 7  is a schematic illustration of a still further embodiment using a hollow cylinder and a movable piston to define the chamber. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Attention is now directed to  FIGS. 1 and 2  which illustrate a preferred embodiment of a medication device  20  in accordance with the present invention for controllably delivering medication to a patients body site. Although the particular device  20  illustrated in  FIGS. 1 and 2  is intended for implanting in a patient&#39;s body, it should be understood that the invention also finds utility in applications where the device is carried externally. 
         [0021]    As depicted in  FIGS. 1 and 2 , the device  20  is comprised of a housing  24  including a base plate  26  and a cover  28  supported on the base plate  26 . The base plate  26  and cover  28  define one or more compartments therebetween, e.g., compartments  30 ,  34 ,  38  for housing various components such as battery  32 , an electronics module  36 , and an active medication pump  40 . 
         [0022]      FIG. 2  depicts a flexible and resilient membrane  44  secured along its edge  46  to the underside of the base plate  26 . As will be explained in greater detail hereinafter, the membrane  44  is configured to naturally expand to the fully extended position shown in  FIG. 2  to maximize the space, i.e., volume, of the closed medication chamber  50  formed between the membrane  44  and base plate surface  47 . When the content of chamber  50  is evacuated, the ambient pressure acting against the membrane  44  will collapse it against base plate surface  47 . 
         [0023]    An inlet valve  54  is supported by the cover  28  and base plate  26  and affords communication to the interior of chamber  50 . The inlet valve  54  can be conventionally constructed comprising a self healing septum  56  through which a hypodermic needle can be inserted to discharge medication into the chamber  50 . As will be discussed hereinafter, inasmuch as the medication chamber  50 , in accordance with the present invention, is maintained at a negative pressure relative to ambient pressure, the hypodermic needle, when properly inserted through septum  56 , is able to discharge medication into the chamber  50  without the application of manual pressure to the hypodermic needle. 
         [0024]    The active pump  40  has an inlet  60  which communicates with the chamber  50  for extracting medication therefrom. The pump  40  is coupled to a catheter outlet connector  62  through which medication is pumped for distribution to a body site. 
         [0025]    In accordance with the present invention, the membrane  44  comprises a movable portion of a peripheral member or wall which defines and encloses the medication chamber  50 . The exterior surface  68  of the membrane  44  is configured to be exposed to ambient pressure, i.e., that is the internal body pressure when the implantable device  20  is in situ. Typically, this ambient pressure will be very close to atmospheric pressure, which of course is dependent upon altitude, temperature, etc. The ambient pressure acts in a direction tending to compress the membrane  44  against the base plate  26 . More particularly, when the chamber  50  is filled with medication, the membrane  44  will expand to its natural fully extended position shown in  FIG. 2 . However, when the medication is evacuated by action of pump  40 , then the ambient pressure acts to collapse the membrane  44  toward the base plate surface  47 . 
         [0026]    In accordance with the present invention, a spring bias force is applied to the chamber movable wall portion, i.e., membrane  44  in  FIG. 2 , which acts in a direction to oppose the ambient pressure force so as to create a residual pressure in the chamber which is negative with reference to ambient. 
         [0027]    More particularly, with reference to  FIG. 3 , note that the ambient pressure P A  acting on the exterior surface  68  of movable wall portion  44  produces a force F A  tending to move the wall portion  44  toward the base plate  26 , i.e., to collapse the chamber  50 . In accordance with the present invention, a bias force F B  is created which acts in opposition to the force F A . As shown, the chamber pressure P C  will be negative with respect to the ambient pressure P A  attributable to the negative bias force F B . 
         [0028]    The force F B  can be provided in a variety of different ways. For example, the membrane  44  of  FIG. 2  can comprise a part formed of metal or plastic material (e.g., nitinol, titanium, stainless steel, super alloys, composite material) configured so that in its natural or quiescent state it resiliently expands to the extended position represented in  FIGS. 2 and 3 . Thus, as the ambient pressure bears against, the movable wall portion of membrane  44  tending to move it toward its compressed position, it will develop a restoration force F B  acting to oppose the compression. As an alternative to configuring wall portion  44  to inherently exhibit the desired resilient characteristic, a separate force generator, e.g., a spring, a magnet, a frictional member, etc. can be incorporated into the device structure. 
         [0029]    Attention is now directed to  FIG. 4A  which depicts an alternative embodiment  100 . The embodiment  100  includes a base plate  102  and cover  104  which can be considered identical to the corresponding components  26  and  28  discussed in connection with  FIG. 2 . The plate  102  defines a substantially rigid portion of a peripheral wall extending around and enclosing a medication chamber  106 . The chamber peripheral wall, in accordance with the present invention, also includes a movable portion which in embodiment  100  comprises a flexible boot or shroud  108 . The boot  108  carries a rigid wall portion  110  which is spaced from and oriented substantially parallel to plate  102 . Thus, the chamber  106  in embodiment  100  is defined by the inner surfaces of wall portions  102  and  110  and flexible wall portion or boot  108 . 
         [0030]    In the embodiment  100 , a force generator, or member, comprises a coil spring  120  mounted between the inner surfaces of wall portions  102  and  110 . The spring member  120  is shown as a coil spring which is configured so that in its natural or quiescent state, e.g., in a vacuum, it is extended to the position shown in  FIG. 4B . Ambient pressure acting on the outer surface of movable portion  110  acts in the direction to compress spring member  120  with the spring member thus providing a restoration or bias force acting in opposition to the force of the ambient pressure. Thus, the pressure within the chamber  106  will be maintained below the ambient pressure as a consequence of the force produced by spring  120  as was discussed in connection with  FIG. 3 . Typically, this spring force is selected to produce a chamber pressure which is negative with respect to ambient pressure by a differential within the range 0.1 to 5.0 psig. 
         [0031]      FIG. 5  depicts a further alternative embodiment  200  in which the inner surfaces of a base plate  202 , a movable rigid portion  204 , and a flexible shroud or bellows  206  define and enclose a medication chamber  208 . A coil spring  210  is depicted as being formed around the exterior of the shroud  206 . The shroud  206  and coil spring  210  be formed separately or alternatively can be formed as an integral bellows member. 
         [0032]    The embodiment of  FIG. 5  operates identically to the embodiment of  FIGS. 4A and 4B  in that the spring  210  produces a bias force opposing the force of the ambient pressure bearing on wall portion  204 . As medication is drawn from the chamber  208  by action of the active pump, the ambient pressure will displace wall portion  204  toward support plate  202  acting against the bias force provided by spring  210 . 
         [0033]      FIG. 6  illustrates an embodiment  300  which is similar in construction to the embodiment  200  in  FIG. 5 . However, in lieu of using a spring member to provide the negative bias force, the embodiment  300  uses magnetic repulsion to develop the negative bias force. More particularly, note in  FIG. 6  that adjacent magnets  302  and  304  are similarly poled. Also note that adjacent magnets  306  and  308  are similarly poled. Thus, as the force produced by ambient pressure on the exterior surface of wall portion  310  acts to displace wall portion  310  toward base plate  312 , the repulsion force produced by the magnets will increase in opposition to the ambient force. Of course, as has been discussed in connection with the earlier embodiments, this negative bias force will produce a chamber pressure which is negative with respect to the ambient pressure. 
         [0034]      FIG. 7  depicts a still further embodiment  400 . In the embodiment of  FIG. 7 , a hollow cylinder  402  is provided defining an interior wall surface  404 . A piston  408  is mounted for reciprocal linear motion within the cylindrical volume defined by the interior wall surface  404 . The piston  408  interior surface  409 , together with wall surface  404 , defines a medication chamber  410 . An inlet valve  412  opens to the medication chamber and an outlet  414  couples the chamber  410  to an actuatable pump  416 . A force generator, e.g., spring member  418 , is shown mounted in the chamber  410  bearing against piston interior surface  409 . The piston outer surface  420  is exposed to ambient pressure. 
         [0035]      FIG. 7  depicts spring  418  in its expanded state with the chamber  410  filled with medication supplied via inlet valve  412 . As medication is extracted from the chamber  410  by action of the pump  416 , ambient pressure acting on the piston outer surface  420  will act to move the piston  408  along the interior wall surface  404  to compress spring  418  and diminish the volume of chamber  410 . This action will be opposed by the restoration force of spring member  418  thus producing a pressure in chamber  410  which is negative with respect to the ambient pressure applied to the piston surface  420 . 
         [0036]    From the foregoing, it should now be appreciated that multiple exemplary embodiments have been described herein characterized by a chamber peripheral wall portion which is exposed to ambient pressure together with means for producing a bias force acting in opposition to the ambient pressure to produce a pressure within the chamber which is negative with respect to the ambient pressure. Although only a limited number of embodiments have been specifically described, it should be recognized by those skilled in the art that the invention can be implemented by a variety of alternative, essentially equivalent, structures conforming to the spirit of the invention and within the intended scope of the appended claims.