Abstract:
An apparatus for delivering a fluid includes a housing, an inlet in the housing for receiving the fluid, and an outlet in the housing for discharging the fluid. A piston channel is provided within the housing through which the fluid flows from the inlet to the outlet. An actuator is positioned within the housing and is moveable between a retracted position and a forward position, the actuator defining a piston chamber for storing fluid received through the inlet when the actuator is in the retracted position and for driving the fluid stored in the piston chamber toward the outlet when the actuator transitions from the retracted position to the forward position. The actuator includes an armature and a piston coupled to the armature and moveable within the piston channel. The piston is provided with a groove in an outer surface for conducting fluid from the inlet to the outlet.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to infusion devices and, more particularly, to an actuator for use in an infusion device drive mechanism, the actuator being configured to facilitate periodic cleaning of the infusion device and to generally improve fluid flow from the infusion pump&#39;s inlet reservoir to the pump&#39;s outlet chamber. 
   BACKGROUND OF THE INVENTION 
   Infusion devices may be used to deliver an infusion media (e.g. a medication such as insulin) to a patient. Such devices may be designed to be implanted into a patient&#39;s body to deliver predetermined dosages of the infusion media to a particular location within the patient&#39;s body; e.g. in the venous system, the spinal column, or within the peritoneal cavity. 
   A known infusion device of the type described above includes a drive mechanism that includes a reciprocating pumping element made of a ferrous material. The reciprocating pumping element includes an actuator including a piston portion that is coupled to an armature portion. The piston portion is configured to reciprocate within a piston channel when a solenoid coil is alternately energized and de-energized. That is, when the solenoid is energized, magnetic flux causes the actuator to move very quickly (i.e. in the order of 2-3 milliseconds) until it reaches a stop member. This corresponds to the pump&#39;s forward stroke and results in the delivery of a predetermined dosage of infusion media from an outlet chamber to the patient. When the solenoid is de-energized, the lack of magnetic flux allows the actuator to return to its original position under the force of a spring. This, in turn, causes the pressure in the piston chamber to fall. The reduced pressure in the piston chamber causes infusion media to flow from a reservoir through an annulus between the actuator piston and the piston cylinder wall to refill the piston chamber thus equalizing the pressure between the reservoir and the piston chamber and preparing the pump for its next pumping or delivery stroke. This is referred to as the refill stroke. The annulus between the actuator piston and the piston cylinder is very small (i.e. in the order of 150 to 250 microinches radially) resulting in an outlet chamber refill process that takes between about 1 to 2 seconds. In contrast, the pump&#39;s forward (delivery) stroke may be approximately 500 times faster than the refill process. 
   Over time, protein drugs such as insulin denature resulting in the deposition of protein on the surfaces of fluid paths; for example, on the surfaces that form the annulus between the actuator piston and the pistol cylinder. Such deposits may cause valves to leak, impede the motion of moving parts, and/or otherwise degrade device performance. Typically, such deposits are removed periodically (e.g. once per year) by rinsing the implanted pump with a solvent (for example, sodium hydroxide (NaOH)) causing the deposits to dissolve. 
   The rinsing procedure is typically performed as follows. The infusion device&#39;s reservoir is first filled with a desired buffer or rinsing solution. Since the device is implanted near the patient&#39;s skin, the reservoir may be filled utilizing a first syringe. A second syringe engages the device&#39;s outlet to produce a negative pressure differential and therefore help pull the fluid through the pump. The pump itself is operated during this procedure to assist fluid flow through the pump. In the case of insulin, it is an established goal that the rinsing procedure should result in the transport of at least 1 cc of rinsing fluid from the inlet reservoir to the pump&#39;s outlet in approximately ten minutes. Rinse cycles less than ten minutes in duration may result in failure to dissolve all deposits, and rinse cycles greater than ten minutes may result in undue discomfort to the patient. The rinse procedure may include a multi-stage operation that involves emptying and refilling the pump&#39;s reservoir several times with different fluids, and different drug therapies may require the use of different rinsing agents. It is to be understood that other protein drugs may require different rinse times and/or volumes. 
   As previously stated, the space or annulus between the surface of the actuator piston and the piston cylinder wall is approximately 150-200 micro-inches radially, a fairly tight fit, and it takes approximately 1 to 2 seconds to refill the piston chamber via this annulus. Deposits of the type described above that form on the annulus walls will restrict fluid flow thus increasing the time it takes to refill the piston chamber, which, in turn, lowers the stroking frequency and causes the corrective rinse procedure to be protracted; e.g. it could take 30 minutes or more instead of the desired 10 minutes. The deposit build-up could be so extreme so as to cause the pump to jam. In this case, it could take more than 30 minutes to pass ¼-½ cc of rinsing fluid. This may not be sufficient to render the pump operational. 
   BRIEF SUMMARY OF THE INVENTION 
   According to an aspect of the invention, there is provided an apparatus for delivering a fluid. The apparatus includes a housing, an inlet in the housing for receiving the fluid, an outlet in the housing for discharging the fluid, a piston channel within the housing through which the fluid flows from the inlet to the outlet, and an actuator positioned within the housing and moveable between a retracted position and a forward position. The actuator in conjunction with the piston channel defines a piston chamber for storing fluid received through the inlet when the actuator is in the retracted position. The actuator drives the fluid stored in the piston chamber toward the outlet when the actuator transitions from the retracted (or refill) position to the forward (or delivery) position. The actuator includes an armature and a piston coupled to the armature and moveable within the piston channel. The piston has a groove in an outer surface for conducting fluid from the inlet to the outlet. 
   According to a further aspect of the invention, there is provided an actuator for delivering fluid through a piston channel from an inlet to an outlet. The actuator includes an armature configured to move between a forward position and a retracted position, and a piston that is coupled to the armature and moveable within the piston channel. The piston has a groove in an outer surface for conducting fluid through the groove. 
   According to a still further aspect of the invention, there is provided an actuator mechanism including an armature portion and a piston portion coupled to the armature portion and having a groove in an outer surface thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will hereinafter be described in conjunction with the following drawings wherein like reference numerals denote like elements throughout, and 
       FIG. 1  is an isometric view of an implantable infusion device in accordance with the prior art; 
       FIG. 2  is an isometric view of a drive mechanism for the implantable infusion device shown in  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of a drive mechanism in accordance with a first embodiment of the present invention; 
       FIG. 4  is an exploded view of an embodiment of the drive mechanism shown in  FIG. 3 ; 
       FIG. 5  is an isometric view of an embodiment of an actuator including an armature and a grooved piston for use in the drive mechanism shown in  FIGS. 3 and 4 ; 
       FIGS. 6 ,  7 , and  8  are simplified cross-sectional views of the drive mechanism shown in  FIG. 3  in quiescent, forward, and retracted states, respectively; 
       FIGS. 9 ,  10 , and  11  are cross-sectional views of three piston grooves in accordance with an embodiment of the present invention; 
       FIG. 12  is a graph illustrating the relationship between pressure differential and volume pull-through for grooved and ungrooved actuator pistons; 
       FIG. 13  is a graph illustrating the relationship between stroke volume and pulse period for grooved and ungrooved actuator pistons; 
       FIG. 14  is an isometric view of a portion of an actuator piston having first and second oppositely directed helical grooves; 
       FIG. 15  is an isometric view of a portion of an actuator piston having a helical groove with very few turns; and 
       FIG. 16  is an isometric view of a portion of an actuator piston having a plurality of longitudinal straight grooves. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description is of the best presently contemplated mode of implementing the invention. This description is not to be taken in a limiting sense, but is merely for the purpose of illustrating the general principles of embodiments of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. The scope of the invention is best defined by the appended claims. 
   As discussed above, embodiments of the present invention relate to an infusion device and to a drive mechanism including an actuator that improves fluid flow from the device&#39;s inlet reservoir to the device&#39;s outlet and facilitates the periodic cleaning of the device. 
     FIG. 1  shows an implantable infusion device  10  in accordance with the teachings of the prior art. The illustrated device  10  is configured to be surgically implanted into a patient, for example, in the abdominal region, between the skin and the abdominal wall. A catheter connected to the pump may deliver infusion medium to the patient, for example, by feeding infusion medium to a particular location in the venous system, within the spinal column, or in the peritoneal cavity of the patient. As described below, embodiments of the device  10  are configured in accordance with one or more aspects of the invention for enhancing prolonged usage and cleaning after implantation. However, further embodiments of the invention may be implemented as external infusion devices, which connect to patients through suitable catheter devices or the like. Yet further embodiments of the invention may be used in other contexts; e.g. for delivery of a medium into other suitable environments. Therefore, for purposes of simplifying the present disclosure, the term “patient” is used herein to refer to any environment in which an implantable device is implanted or to which an external device is connected, whether or not the implant or connection is carried out for medical purposes. Also, the term “infusion medium” is used herein to refer to any suitable medium delivered by the drive device. 
   The device  10  includes a generally disc-shaped housing  14 . While a generally circular disc-shaped embodiment is illustrated in  FIG. 1 , it will be understood that further embodiments of the invention may employ housing of other shapes, including, but not limited to, oval, oblong, rectangular, or other curved or polygonal shapes. In implantable devices, the housing  14  is made of a biocompatible material and most often has a relatively small diameter and thickness to reduce patient trauma during implant surgery and after implantation. 
   The housing  14  includes a reservoir  16  for holding a volume of infusion medium, such as, but not limited to, a liquid medication to be administered to the patient. Housing  14  also contains a drive mechanism  18  (e.g. a pump), a power source  13 , and control electronics  20  described below. Pump  18  is configured to receive infusion media from reservoir  16  via a pump inlet  22 . Inlet structure  22  provides a closeable and sealable fluid flow path to the reservoir in the reservoir portion of the housing. The inlet structure includes a port for receiving a needle through which fluid may be transferred to the infusion device; for example, to fill or re-fill the reservoir of the device with the infusion media or a rinsing fluid as will be more fully discussed below. In particular embodiments, the inlet structure is configured to re-seal after a fill or re-fill operation, and to allow multiple re-fill and re-seal operations. One example of an inlet structure is described in U.S. Pat. No. 6,652,510, titled “Infusion Device and Reservoir for Same,” which is incorporated herein by reference. However, further embodiments may employ other suitable inlet structures, including, but not limited to, those described in U.S. Pat. Nos. 5,514,103 and 5,176,644, each to Srisathapat et al.; U.S. Pat. No. 5,167,633 to Mann et al.; U.S. Pat. No. 4,697,622 to Swift; and U.S. Pat. No. 4,573,994 to Fischell et al. Representative examples of reservoir housing portions and reservoirs which may be employed in embodiments of the invention are described in the above referred to U.S. Pat. No. 6,652,510, and further embodiments may employ other suitable reservoir configurations, including, but not limited to, those described in the above referred to U.S. Pat. Nos. 5,514,103; 5,176,644; 5,167,633; 4,697,622; and 4,573,994. 
   Returning now to  FIGS. 1 and 2 , pump  18  has an outlet  24  through which the infusion medium may be expelled. When the device  10  is implanted in a patient or connected externally to a patient, a catheter  12  may be connected to the outlet  24  to deliver expelled infusion medium into the patient&#39;s blood stream or to a selected location in the patient&#39;s body. 
   The infusion device  10  includes a drive mechanism  18  such as a pump, and an electronic control system  20  located in the housing portion  14 . The drive mechanism  18  is connected between the reservoir and the outlet of the infusion device. The electronic control system  20  includes a power source  13 , such as a battery, and control electronics for controlling the drive mechanism  18  to deliver infusion medium from the reservoir to the patient in a prescribed manner. The drive mechanism may be controlled to deliver infusion medium in any suitable manner; for example, according to a programmed dispensing rate or schedule or according to an actuation signal from a sensor, timer or other suitable source. 
   In particular embodiments, both the drive mechanism  18  and the reservoir  16  are hermetically sealed. In such embodiments, the housing  14  containing drive mechanism  18  and control electronics  20  may be made from titanium or titanium alloy or other biocompatible metals, while the reservoir portion  16  of the housing may be made from such metals or a biocompatible and infusion medium compatible plastic as long as the material is such as to permit the required hermeticity. 
   The drive mechanism  18  includes mechanical and electromagnetic components that inhabit a volume of space within the housing  14  in which the components reside and operate. In that regard, the drive mechanism can contribute to the thickness requirements of the housing  14 , and thus to the overall thickness of the device  10 . The ability to reduce or minimize the device thickness without compromising the drive capabilities can provide significant advantages with respect to patient comfort, appearance and flexibility in selecting implant locations of the body. In particular embodiments, the drive mechanism  18  is configured to have a relatively small thickness thus allowing the device  10  to have a relative small thickness. Also in particular embodiments, the device  10  is configured such that, once implanted, it functions for a relatively long period of time to administer infusion medium to the patient to periodically be replenished from the outside of patient&#39;s body, and to be periodically rinsed to remove unwanted protein build-up on the fluid path surfaces that may degrade the performance of the infusion device. 
     FIG. 2  illustrates a drive mechanism  18  in accordance with the prior art. The drive mechanism  18  has a partially cylindrical, disc-shaped configuration having an inlet  30  and an outlet  24 . The inlet  30  may be coupled in fluid communication with reservoir  16  of device  10  ( FIG. 1 ) through a suitable conduit (not shown) within the device  10 . Similarly, the outlet  24  may be coupled in fluid communication with outlet  12  of the device  10  in  FIG. 1 , through a suitable conduit (not shown) within the device  10 . 
     FIG. 3  is a cross-sectional view of a drive mechanism  18  in a retracted position or state in accordance with an embodiment of the present invention. As described in more detail below, the drive mechanism  18  employs electromagnetic and mechanical forces to change (or move) between retracted and forward states to cause infusion medium to be drawn in through the inlet  30  and forced out of the outlet  24 , respectively. The assembly of components shown in  FIG. 3  is also shown in an exploded view in  FIG. 4 . 
   Referring to  FIGS. 3 and 4 , the drive mechanism  18  includes a housing member  32  that is open on one side to a hollow, annular interior section  34 . The housing  32  has a central hub portion  36  with a central piston channel  38 . The bottom side of the housing member  32  (with reference to the orientation shown in  FIG. 3 ) includes an opening to the hollow interior section  34  through which coil wires may pass, as described below. The bottom side of the housing member also includes a configuration of recesses and cavities for providing an outlet chamber and an outlet passage. The housing member  32  is most often made of generally rigid, biocompatible and infusion medium compatible material having no or low magnetic permeability such as, but not limited to, titanium, stainless steel, bio-compatible plastic, ceramic, glass or the like. 
   As shown in  FIGS. 3 and 4 , a coil cup  40  is located within the annular interior section  34  of the housing  32 . The coil cup  40  has a generally cylinder shape, open on one side to a hollow, annular interior. The coil cup  40  includes a bore  42  located in a central hub portion  44  and extending axially relative to the annular interior. The hub portion  44  of the cup member defines an inner annular wall  46  having an end surface  48  (or inner pole surface) having a width W 1 . The cup member  40  has an outer wall  50  having an end surface  52  (or outer pole surface) having a width W 2 . The outer wall  50  is connected to the inner wall  46  of hub portion  44  by a backiron portion  51  of the cup member  40 . As described in further detail below, at the open end of cup member  40 , the end surfaces  48  and  52  of the inner and outer walls  46  and  50 , respectively, define pole surfaces that cooperate with pole surfaces on an armature to provide a path for electromagnetic flux during a forward stroke of the drive mechanism. In particular embodiments, the width W 1  of inner pole surface  48  is greater than the width W 2  of the outer pole surface  52  to provide certain electromagnetic characteristics as described below. 
   When assembled, the coil cup  40  is located in the hollow interior of the housing member  32 , with the central portion  36  of the housing  32  extending through channel  42  of the coil cup  40  as shown in  FIG. 3 . A coil  54  is located within the hollow, annular interior of the coil cup  40  and is disposed around the axis of the annular interior of the coil cup  40 . The coil cup  40  is provided with an opening  56  through which coil leads extend, as shown in  FIGS. 3 and 4 . The coil cup  40  is most often made of generally rigid material having a relatively high magnetic permeability such as, but not limited to, low carbon steel, iron, nickel, ferritic stainless steel, ferrite, other ferrous materials, or the like. The coil  54  includes a conductive wire wound in a coil configuration. The coil wire may include any suitable conductive material such as, but not limited to, silver, copper, gold or the like, with each turn electrically insulated from adjacent turns and the housing. In one particular embodiment, the coil wire has a square or rectangular cross-section to achieve minimal space between windings and a greater number of coil turns thus improving electrical efficiency. 
   The drive mechanism  18  also includes an actuator member  58 , which has an armature portion  60  and a piston portion  62 . The actuator member is most often made of a generally rigid, biocompatible and infusion medium compatible material having a relatively high magnetic permeability such as, but not limited to, ferrous materials, ferritic stainless steel with high corrosion resistance, or the like. In the embodiment of  FIGS. 3 ,  4 , and  5 , the actuator (with an armature portion  60  and a piston portion  62 ) is formed as a single, unitary structure. In other embodiments as described below, the piston portion may be a separate structure with respect to the armature portion. 
   A perspective view of the example actuator member  58  is shown in  FIG. 5 , where the armature portion  60  of the actuator member has a round, disc shape, and may be provided with at least one opening, and most often a plurality of openings as shown in the drawing. The openings in the illustrated example include a plurality of substantially circular openings  66 . The sections  68  of the armature  60  between openings  66  generally define radial struts coupling an annular outer section (or outer pole)  70  to an inner section (or inner pole)  72  of the armature. In particular embodiments, the width W 1  of the inner pole surface is greater than the width W 2  of the outer pole surface corresponding to the difference between the width of the pole surface  48  on the inner wall  46  of the cup member and the width of the pole surface  52  on the outer wall  50  of the cup member. 
   As described in more detail below, the armature  60  cooperates with the inner and outer walls of the coil cup  40  to provide a flux path for electromagnetic flux. The spacing between the pole surfaces on the armature  60  and the pole surfaces on the coil cup walls define gaps in the flux path. In particular embodiments, the spacing between the surface of outer pole  70  of the armature  60  and the surface of outer pole  52  of the outer wall  50  of the coil cup  40  (or a barrier  74  described below) is greater than the spacing between the surface of inner pole  72  of the armature and the pole surface  48  of the inner wall  46  of the coil cup (or the barrier  74 ) when the actuator is in the retracted position shown in  FIG. 3 . 
   The radial struts  68  in the armature provide radial paths for electromagnetic flux between the outer and inner pole sections  70  and  72  of the armature. The openings  66  provide a passage for infusion medium to pass as the actuator  58  is moved between retracted and forward stroke positions to reduce resistance to the actuator motion that the infusion medium may otherwise produce. The configuration of openings is most often designed to provide a sufficient conductor for electromagnetic flux and yet minimize or reduce viscous resistance to actuator motion. With reference to  FIG. 3 , the actuator member  58  is arranged with the piston portion  62  extending through the axial channel  38  of the housing  32  and with the armature portion  60  positioned adjacent to the open side of the coil cup  40 . An actuator spring  78  is positioned to force the armature portion  60  of the actuator  58  in the direction away from the open side of the coil cup  40  to provide a gap between the armature  60  and the open side of the coil cup  40 . A biocompatible and infusion medium compatible barrier  74  is located over the open side of the coil cup  40  between the armature  60  and the coil cup  40  to help seal the annular interior of the coil cup  40  and coil  54 . In other embodiments in which infusion medium may contact the coil, the barrier  74  may be omitted. 
   The actuator spring  78  in the illustrated embodiment includes a coil spring disposed around the piston portion  62  of the actuator  58  adjacent the armature portion  60 . One end of the coil spring abuts the armature portion  60  of the actuator, while the opposite end of the coil spring abuts a shoulder  81  in the piston channel  38  of the housing  32 . In this manner, the actuator spring  78  imparts a spring force between the housing and the actuator  58  to urge the actuator toward its retracted position shown in  FIG. 3 . 
   In the illustrated embodiment, by using a coil spring  78  located around and coaxial with the piston portion  62  and disposed partially within the piston channel  38 , the actuator spring may have minimal or no contribution to the overall thickness dimension of the drive mechanism. However, in other embodiments, actuator springs may have other suitable forms and may be located in other positions suitable for urging the actuator toward its retracted position shown in  FIG. 3 . The actuator spring  78  is most often made of a biocompatible and infusion medium compatible material that exhibits a suitable spring force such as, but not limited to, titanium, stainless steel, MP35N cobalt steel or the like. 
   The drive mechanism  18  further includes a cover member  80  which attaches to the housing member  32  over the open side of the housing member and the barrier  74 . The cover member  80  is most often made of a generally rigid, biocompatible and infusion medium compatible material having a relatively low magnetic permeability (being relatively magnetically opaque) such as, but not limited to, titanium, stainless steel, biocompatible plastic, ceramic, glass or the like. 
   The cover member  80  defines an interior volume  82  between the barrier  74  and the inner surface of the cover member. The armature portion  60  of the actuator member  58  resides within the interior volume  82  when the cover is attached to the housing below, the armature  60  is moveable in the axial direction within the volume  82  between a retracted position shown in  FIG. 3  and a forward stroke position. This movement is created by the action of electromagnetic force generated when a current is passed through the coil  54  and the mechanical return action of the actuator spring  78 . 
   An adjusting plunger  84  is located within the cover  80  for contacting the armature  60  when the armature is in the fully retracted position shown in  FIG. 3  to set the retracted position of the armature. In particular embodiments, a seal (e.g. a silicon rubber sealing ring) may be disposed between the plunger  84  and the cover member  80 . In further embodiments, a flexible diaphragm  85  (such as, but not limited to, a thin titanium sheet or foil) may be coupled to the inside surface of the cover  80  and sealed around the opening through which the plunger  84  extends. The diaphragm will flex to allow the plunger to define an adjustable retracted position and, yet, provide sealing functions for inhibiting leakage at the interface between the plunger  84  and the cover  80 . In other embodiments, after a proper armature position is set, the plunger is fixed in place with respect to the cover member, for example, by adhering the plunger to the cover member with one or more welds, adhesives or other securing methods. 
   The cover member  80  includes the inlet  30  of the drive mechanism, which has an inlet opening  86  in fluid flow communication with the interior volume  82  as described below. The inlet opening  86  connects in fluid flow communication with the reservoir of the infusion device  10  ( FIG. 1 ) to receive infusion medium from the reservoir. Connection of the inlet opening  86  and the reservoir may be through suitable conduit (not shown), such as tubing made of suitable infusion medium compatible material, including, but not limited to, titanium, stainless steel, biocompatible plastic, ceramic, glass or the like. 
   The inlet opening  86  provides a flow path to an inlet chamber  88  formed in the cover member  80  adjacent the inlet opening. A filter or screen member, such as a porous or screen material  90 , may be disposed within the inlet chamber  88 . The filter or screen member  90  is provided in a flow path between the inlet opening  86  and an inlet port  92  to the volume  82 . A one-way inlet valve (not shown) may also be provided in the flow path between the inlet opening  86  and the inlet port  92  or within the inlet port  92  to allow medium to flow into but not out of the interior volume  82  through the inlet. The cover member  82  may be provided with an inlet cover  94  that, when removed, allows access to the inlet chamber  88  to, for example, install, replace or service a filter  90  or inlet valve, or to service or clean the inlet  86 . 
   As shown in  FIG. 3 , the piston portion  62  of the actuator  58  extends through the axial channel  38  in the housing  32  toward an outlet chamber  98  at the end of the axial channel  38 . The channel  38  has an inside diameter which is larger than the outside diameter of the piston portion  62 . As a result, an annular volume is defined between the piston portion  62  and the wall of the axial channel  38  along the length of the axial channel  38 . Infusion medium may flow through the annular volume  82  within the cover  80  to a piston chamber  100  located between the free end of the piston portion  62  and a valve member  102  of a valve assembly  96 . In particular embodiments, the radial spacing between the piston portion  62  and the wall of the channel  38  is selected to provide a suitable flow toward the piston chamber  100  to refill the piston chamber  100  (during a return stroke of the piston portion), but small enough to sufficiently inhibit back flow of medium from the piston chamber  100  (during a forward stroke of the piston portion). 
   The actual radial spacing between the piston portion  62  and the wall of the channel  38  to achieve such results depends, in part, on the overall dimensions of those components, the pressure differentials created in the mechanism, and the viscosity of the infusion medium. In particular embodiments, the radial spacing is selected such that the volume of medium for refilling is between about 1 and 4 orders of magnitude (and, most often, about 2 orders of magnitude) greater than the volume of medium that back-flows through the space. Alternatively, or in addition, the radial spacing may be defined by the ratio of the diameter D P  of the piston portion  62  to the diameter D C  of the channel  38 , where the ratio D P /D C  is most often within a range of about 0.990 to about 0.995. As a representative example, a total spacing of about 400 to 600 micro-inches and, most often, an average radial gap of about 250 micro-inches annularly around the piston portion  62  may be employed. 
   The valve assembly  96  in the embodiment of  FIG. 3  includes the valve member  102  and a valve spring  106 . The valve member  102  is located within the outlet chamber  98  and, as shown in  FIG. 3 , is positioned to close the opening between the axial channel  38  and the outlet chamber  98  when the actuator  58  is in the retracted position. During the forward stroke, the valve member  102  is positioned to open a flow passage between the axial channel  38  and the outlet chamber  98 . The valve spring  106  is located within the outlet chamber  98  to support the valve member  102 . The spring  106  imparts a spring force on the valve member  102  in the direction toward piston  62  urging the valve member  102  toward a closed position to block the opening between the axial channel  38  and the outlet chamber  98 . 
   The valve member  102  is most often made of generally rigid, biocompatible and infusion medium compatible material, such as, but not limited to, titanium, stainless steel, biocompatible plastic, ceramic, glass, gold, platinum or the like. A layer of silicon rubber or other suitable material may be attached to the rigid valve member material on the surface facing the channel  38  to help seal the opening to channel  38  when the valve member is in the closed position shown in  FIG. 3 . 
   The valve spring  106  is most often made of biocompatible and infusion medium compatible material that exhibits a suitable spring force such as, but not limited to, titanium, stainless steel, MP35N cobalt steel or the like. In the illustrated embodiment, the valve spring  106  is a coil spring. In other embodiments, other suitable valve spring configurations may be employed, including, but not limited to, helical, flat, radial, spiral, barrel, hourglass, constant or variable pitch springs or the like. 
   The embodiment shown in  FIG. 3  utilizes a valve cover  110  sealed to the housing  32  to enclose the outlet chamber  98 . The valve cover  110  is most often made of a generally rigid, biocompatible and infusion medium compatible material, such as, but not limited to, titanium, stainless steel, biocompatible plastic, ceramic, glass, gold, platinum or the like. 
   The coil  54  may be inserted into the annular interior of the coil cup  40  with the coil leads extended through a coil lead opening  56  in the coil cup. The coil may be impregnated or partially impregnated with a fill material of epoxy or the like for adhering the coil to the coil cup and for sealing or partially sealing the coil. The fill material may also be used to adhere the barrier plate to the coil members to avoid warping or bulging of the barrier plate after assembly. 
   The coil cup  40  and the coil  54  may be inserted into the interior of the housing  32  with the coil leads (which may be wire leads or flexible conductive tabs) extending through a coil lead opening  56  in the housing  32 . In particular embodiments, the coil cup and housing are configured to provide a tight friction fit that does not require additional means to adhere the two components together. In other embodiments, the coil cup  40  and housing  32  may be coupled together by a suitable adhesive material or other adhering methods, including, but not limited to, welding, brazing or the like. 
   The barrier  74  may be placed over the coil, coil cup and housing sub-assembly. The barrier  74  may be adhered to the housing by one or more adhering points or continuously secured along the circumference of the barrier  74  with any suitable adhesive material or other adhering methods including, but not limited to, welding, brazing, soldering, or the like. Alternatively, or in addition, the barrier  74  may be held in place by a shoulder portion of the cover  80 , as shown in  FIG. 3 . In addition, as noted above, the barrier  74  may be adhered to the coil  54  by fill material in the coil. In particular embodiments, the barrier  74  is held in a generally flat position relative to the coil cup and coil. To enhance this flat relationship, the coil cup and housing may be assembled together and then machined to planarize the barrier contact surfaces prior to inserting the coil in the coil cup and prior to adding fill material to the coil. 
   After the barrier  74  is placed over the coil, coil cup and housing, the actuator  58  may be added to the sub-assembly. First, however, the actuator spring  78  is placed around the piston portion  62  adjacent the armature portion  60  of the actuator. Then the free end of the piston portion  62  is passed through the axial channel  38  of the housing  32  with the armature end of the actuator arranged adjacent the barrier  74 . 
   The cover member  80  may then be disposed over the armature end of the actuator and secured to the housing  32 . In particular embodiments, the cover member  80  is adhered to the housing by one or more adhering points or continuously along the circumference of the cover member  80  with one or more welds or any other suitable adhering methods, including, but not limited to, adhesive materials, brazing or the like. The inlet filter  90  and the inlet cover  94  may be pre-assembled with the cover member  80  prior to adding the cover member to the sub-assembly. Alternatively, the filter  90  and the inlet cover  94  may be added to the cover member  80  after the cover member  80  is assembled onto the housing  32 . In particular embodiments, the filter  90  is disposed within the inlet chamber  88  and then the inlet cover  94  is adhered to the cover member  80  by one or more adhering points or continuously along the circumference of the inlet cover with one or more welds or any other suitable adhering methods, including, but not limited to, adhesive materials, brazing or the like. 
   The valve side of the drive mechanism may be assembled before or after the above-described components are assembled. On the valve side of the drive mechanism, the valve member  102  is disposed within the outlet chamber cavity  98  of the housing  32  adjacent the opening to the axial channel  38 . The valve spring  106  is then disposed within the outlet chamber cavity  98  adjacent the valve member  102 . The valve cover  110  may then be placed over the outlet chamber cavity  98 . In particular embodiments, the valve cover  110  is adhered to the housing  32  by one or more adhering points or continuously along the circumference of the valve cover with one or more welds or any other suitable adhering methods, including, but not limited to, adhesive materials, brazing or the like. 
   The volume of piston chamber  100 , the compression of the actuator spring  78 , and the position of the actuator  58  in the retracted position shown in  FIG. 3  may be adjusted by adjusting the position of the adjusting plunger  84 . In one particular embodiment, the adjusting plunger includes a threaded cylindrical member that engages corresponding threads in a plunger aperture in the cover member  80  to allow adjustment in a screw-threading manner. The diaphragm  85  under the plunger  84  contacts the armature portion  60  of the actuator inside of the cover member  80 . The other end of the plunger  84  may be provided with a tool-engagement depression for allowing engagement by a tool, such as a screw-driver, Allen wrench or the like, from outside of the cover member  80 . By engaging and rotating the plunger  84  with a suitable tool, the depth that the plunger extends into the cover member  80  may be adjusted to adjust the retracted position of the armature portion  60  relative to the barrier  74  (to adjust the gaps between the pole sections  70  and  72  of the armature and pole sections formed by the coil cup  40  when the actuator is in the retracted position of  FIG. 3 ). In one particular embodiment, adjustments of the plunger  84  are made during manufacture. In that embodiment, the adjusted position is determined and set by welding or otherwise adhering the plunger  84  in the adjusted position during the manufacture. In other embodiments, the plunger  84  is not set and welded during manufacture to allow adjustment of plunger  84  after manufacture. 
     FIGS. 6 ,  7  and  8  are simplified cross-sectional views of the drive mechanism  18  shown in  FIG. 3  and will be useful in explaining the operation of drive mechanism  18 . In the interest of clarity, only major functional features and components are illustrated, and these are identified by reference numerals corresponding to reference numerals used in  FIG. 3  to denote like features and components. 
     FIG. 6  illustrates drive mechanism  18  in its quiescent state. That is, valve member  102  is fully extended under the force of spring  106 , piston chamber  100  and inlet chamber  88  are substantially filled with infusion media (or rinsing media as the case may be), and coil  54  is de-activated (not energized or inadequately energized) in a manner to overcome the force of spring  78 . Drive mechanism  18  employs electromagnetic and mechanical forces to move between retracted ( FIG. 8 ) and forward ( FIG. 7 ) positions to cause infusion medium to be drawn into and driven out of the mechanism in a controlled manner. In the retracted position, the spring  78  urges the actuator  58  toward its retracted position shown in  FIG. 8 . When the coil  54  is energized to overcome the spring force of spring  78 , the actuator  58  moves to its forward stroke position shown in  FIG. 7 . The movement of the actuator between retracted and forward positions creates pressure differentials within the internal chambers and volumes of the drive mechanism  18  to draw medium into the inlet  86  and drive medium out the outlet  24 . 
   More specifically, when the coil  54  is de-activated, the actuator  58  is held in its retracted position ( FIGS. 6 and 8 ) under the force of the spring  78 . When coil is de-activated immediately following a forward stroke, the spring  78  moves the actuator  58  to the retracted position of  FIG. 8  from the forward position shown in  FIG. 7 . The openings  66  ( FIG. 5 ) in the armature portion  60  of the actuator  58  provide passages for medium to pass and, thus, reduce viscous drag on the actuator. As a result, the actuator  58  may move to its retracted position ( FIG. 8 ) relatively quickly. 
   As the actuator  58  retracts, the piston portion  62  of the actuator is retracted relative to the valve member  102  such that a piston chamber  100  volume is formed between the end of the piston portion  62  and the valve member  102 . The formation of the piston chamber  100  volume creates a negative pressure which draws infusion medium (or rinsing fluid) from the volume  82  of the cover member  80  through the annular space between the piston portion  62  and the wall of the channel  38  and into the piston chamber  100  as is indicated by arrows  120 . While not shown, one or more channels could be provided through the piston portion  62  to provide one or more additional flow paths to the piston chamber  100  if desired. 
   In the retracted position, a gap is formed between each of the annular pole surfaces  48  and  52  defined by the inner and outer walls  46  and  50  of the coil cup  40  and respective annular surfaces of the inner and outer pole sections  72  and  70  of the actuator&#39;s armature portion  60 . With particular reference to  FIG. 3 , a first gap is formed between the annular pole surface  48  of the inner cup member wall  46  and the annular surface of the inner pole section  72 . A second gap is formed between the annular surface  52  of the outer cup member wall  50  and the annular surface of the outer pole section  70 . 
   When the coil  54  is energized in a manner to overcome spring force  78 , the actuator  58  is forced in the direction to close the gaps and moves to its forward position ( FIG. 7 ) under the influence of electromagnetic flux generated by the energized coil. In particular, the coil may be energized by passing an electrical current through the coil conductor to create electromagnetic flux. The electromagnetic flux defines a flux path through the coil cup walls across the gaps and through the armature portion of the actuator. The electromagnetic flux provides an attraction force between the annular surfaces  48  and  52  of the coil cup  40  and the annular surfaces of the armature&#39;s pole sections  70  and  72  to overcome the spring force of spring  78  and draw the armature  60  toward the coil cup. 
   As the armature portion  60  of the actuator is drawn toward the coil cup  40 , the piston portion  62  of the actuator is moved axially through the channel  38  in the direction toward the outlet chamber  98 . With the coil energized, the piston portion  62  continues to move under the action of the armature until a mechanical stop is reached, for example, mechanical contact of the actuator  58  with the barrier  74 , a portion of the housing  32  or cover member  80 . In other embodiments, the motion may continue until the return force of the spring and fluid pressure overcomes the electromagnetic force provided by energizing the coil. 
   The movement of the piston portion  62  towards the stopping point reduces the volume of the piston chamber  100  and increases the pressure within the piston chamber until the pressure is sufficient to overcome the force of the valve spring  106 . As the valve spring force is overcome by the pressure within the piston chamber, the valve member  102  is moved toward an open position, away from the opening between the piston chamber  100  outlet chamber  98 . When the valve member  102  is in the open position, medium is discharged through the outlet chamber  98  and outlet  24  as is indicated by arrow  128  in  FIG. 7 . 
   When the coil is deactivated and the piston portion  62  is moved back to its retracted position, the pressure in the piston chamber  100  reduces and the valve member  102  is reseated under the action of the valve spring  106 . This prevents fluid from flowing back into the drive mechanism through the outlet. In addition, a negative pressure is created in the piston chamber  100  to draw medium into the chamber for the next forward stroke, as described above. 
   In this manner, energization of the coil  54  to move the actuator  58  to its forward position ( FIG. 7 ) causes a measured volume of medium to be discharged from the outlet. As described above, when the coil  54  is de-energized, the actuator  58  is returned to the retracted position ( FIG. 8 ) under the force of spring  106  and an additional volume of medium is drawn into the piston chamber  100  for the next discharging operation. Accordingly, the coil  54  may be energized and de-energized by a controlled electronic pulse signal where each pulse may actuate the drive mechanism  100  to discharge a measured volume of medium. In particular embodiments, the coil  54  may be electrically coupled to an electronic control circuit (not shown) to receive an electronic pulse signal from the control circuit; for example, in response to a sensor signal, timer signal or other control signal input to the control circuit. 
   In particular embodiments, when the piston motion is stopped at the end of the forward stroke, the valve-facing end of the piston portion  62  is in close proximity to the valve member  102 , for example, spaced from the valve member  102  by a distance that is no more than two to three percent (2-3%) of the piston diameter. In further embodiments, the valve facing end of the piston portion  62  is in contact with the valve member  102  at the end of the forward stroke. In this manner, gas that may be present in the infusion medium is less likely to accumulate within the piston chamber  100 . More specifically, in some operational contexts, infusion medium may contain gas in the form of small bubbles that may migrate into the piston chamber  100  during filling of the piston chamber. As gas is significantly more compressible than liquid, too much gas within the piston chamber may adversely affect the ability of the drive mechanism to self prime. 
   In yet another embodiment, the piston portion  62  may contact the valve member  102  at the end of the forward stroke and push the valve member  102  open. In this embodiment, it is less likely that gas will be trapped between the piston portion  62  and the valve member  102  and more likely that the chamber will be purged of gas. 
   As already described, protein drugs such as insulin denature resulting in the deposition of denatured protein on the surfaces of the fluid delivery path. Over time, such deposits may (1) occlude the delivery path to the therapy site; (2) reduce clearances between moving parts and thus slow operation and perhaps ultimately cause jamming; (3) compromise the condition of valve mating surfaces causing the valve not to seat properly; and (4) create areas of precipitant coagulation that may grow and collect debris thus further impacting fluid flow and device operation. 
   These deposits may be periodically removed (e.g. once per year) by rinsing the implanted pump with a solvent (e.g. sodium hydroxide) to dissolve the deposits. The infusion device&#39;s reservoir is first filled with a desired buffer or rinsing solution. Since the device is implanted near the patient&#39;s skin, the reservoir may be filled utilizing a first syringe. A second syringe engages the device&#39;s outlet to produce a negative pressure differential and therefore help pull the fluid through the pump. The pump itself may be operated during this procedure to assist fluid flow through the pump. It is an established goal that the rinsing procedure should result in the transport of at least 1 cc of rinsing fluid from the inlet reservoir to the pump&#39;s outlet in approximately ten minutes. Rinse cycles less than ten minutes in duration may result in failure to dissolve all deposits, and rinse cycles greater than ten minutes may result in undue discomfort to the patient. The rinse procedure may include a multi-stage operation that involves emptying and refilling the pump&#39;s reservoir several times with different fluids, and different drugs may require the use of different rinsing agents. However, other time periods may be used depending on the agent used, the frequency between rinsings, the amount of deposits and/or the like. 
   As previously stated, the space or annulus between the actuator piston and the piston cylinder is approximately 150-200 micro-inches radially, a fairly tight fit, and it takes approximately 1 to 2 seconds to refill the piston chamber via the annulus. Deposits on the annulus walls, however, will restrict fluid flow thus increasing the time to refill the piston chamber, which, in turn, lowers the stroking frequency and causes the corrective rinse procedure to be protracted; e.g. it could take 30 minutes or so instead of the desired 10 minutes. The deposit build-up could be so severe so as to cause the pump to jam. In this case, it could take more than 30 minutes to pass ¼-½ cc of rinsing fluid and thus may not be sufficient to render the pump operational. 
   To overcome these problems and provide a more effective flow path for the rinsing agent, a groove is provided in the outer surface of the actuator piston. For example, actuator piston  62  is provided with a helical groove  64  and is shown in  FIGS. 3 and 5 . Groove  64  may have, for example, a hemispherical cross-section as shown in  FIG. 9  of sufficient cross-sectional area to ensure that a flow path will always exist regardless of the amount of deposits in the bore from the inlet end  130  of actuator piston  62  to the outlet end  132  of actuator piston  62 . In addition, the helical groove  64  is most often configured to deliver rinsing agent in close proximity to any protein deposit in the annulus between the actuator piston  62  and the surface of the central piston channel  38 . In this manner, rinsing agent can be effectively applied to deposits even when the actuator is jammed. 
   In particular, groove  64  is configured to conduct rinsing agent to within approximately 0.015 inch of any deposit in the annulus. To this end, it has been found that for devices of this nature, a groove having a depth that is approximately 1.5-6% of the diameter of the piston, a width that is approximately 3-30% of the diameter of the piston, a pitch that is approximately 8-70% of the diameter of the piston, and/or a cross-sectional area that is approximately 0.2-0.6% of the area of the piston face is helpful. More specifically, a groove having a width of substantially 0.012 inch, a depth of substantially 0.0035 inch, and a pitch of about 0.025-0.035 inch works quite well. In this case, the groove  64  will have approximately seven turns. More specifically, the groove may have 1-2 turns in the area occupied by piston spring  78  and 5-6 turns in the remainder of the piston  62 . It will be appreciated that a tight spiral path (i.e. many turns) makes it more certain that the rinsing agent will reach deposits in the annulus; however, too many turns could result in back leakage during the pumping stroke due to the corresponding reduction in the piston&#39;s regions of higher diameter which are responsible for the piston&#39;s tight fit within the piston channel. It is to be noted, however, that since the forward stroke is very fast (e.g. 1.5 milliseconds) and the refill time is much longer (e.g. 100-150 times longer), the back leakage is dramatically smaller than the forward flow. Furthermore, a helical groove of the type shown in  FIG. 5  causes fluid flow to transition from laminar to turbulent thereby restricting fluid flow. Thus, the groove generally provides a flow path when the actuator is moving relatively slowly (retracting) and provides a sealing action when the actuator is moving fast (pumping). 
   While the groove in the outer surface of the actuator piston is shown in  FIG. 9  as having a hemispherical cross-section, it is known that sharp edges in the flow path (such as edges  134  in  FIG. 9 ) may cause further denaturing of the protein therapy drug. Therefore, in a particular embodiment shown in  FIG. 10 , groove  64  has a cross-sectional shape including rounded edges  136 . It should be clear, however, that grooves having a variety of cross-sectional shapes may be employed depending on the particular application and circumstances, and all such grooves are considered to be within the scope of the present invention. For example, groove  64  shown in  FIG. 11  includes rounded edges for the reasons described above, but has a cross-section that is generally more conical. 
   Groove  64  enhances the operation of drive mechanism  18  in several ways. First, it can assure that a flow path will exist between the pump&#39;s inlet  86  and outlet  24  even if there are heavy protein deposits on the surfaces of the flow path. This permits rinsing agent to pass through the mechanism even if the mechanism is jammed. Second, it can significantly shorten the refill period by 75 percent or more compared to that of a smooth, ungrooved actuator of similar dimensions thus increasing the amount of rinsing agent that may be pumped by the actuator. Under normal operation, the increased frequency of operation permits infusion rates to be increased thus permitting therapy drugs to be delivered to the patient more expeditiously. 
   The graph shown in  FIG. 12  illustrates the relationship between the inlet-to-outlet pressure differential (horizontal axis) and the volume of fluid pulled through a pump in ten minutes (vertical axis) for a drive mechanism having a smooth actuator (curve  140 ) and an actuator including a seven-turn helical groove having a width of approximately 0.01 inch and a depth of 0.004 inch in the surface thereof (curve  142 ). As can be seen, at pressure differentials greater than −8 psig, the volume pull-through in the grooved actuator increases dramatically above that of the smooth actuator. In fact, at a differential pressure of −13 psig, the pull-through of the grooved actuator is over two times that of the ungrooved actuator. If the pump is operated while the differential pressure is applied, the volume passed through the pump will exceed 1 cc in 10 minutes. 
     FIG. 13  illustrates the relationship between stroke refill volume (vertical axis) and pump pulse period (horizontal axis) for a standard ungrooved actuator (curve  144 ), an actuator having a 0.0025 inch deep helical groove in its surface (curve  146 ), and an actuator having a 0.004 inch deep helical groove therein (curve  148 ). As can be seen, the stroke refill volume for the ungrooved actuator peaks at about 1.0 second, and the actuator with the 0.0025 inch deep helical groove therein peaks at about 0.6 second. The actuator having the 0.004 inch deep helical groove therein has a stroke refill volume that peaks in about 0.2 second, approximately five times faster than the ungrooved actuator piston. Thus, an infusion pump equipped with the grooved actuator piston characterized by curve  148  in  FIG. 13  can be operated at approximately five times the speed of a pump having an ungrooved actuator piston. 
   Thus far, the inventive drive mechanism/actuator has been described in accordance with particular embodiments; i.e. one in which the actuator piston has a helical groove in the surface thereof. It should be appreciated, however, that different configurations and/or numbers of grooves may be utilized. For example,  FIG. 14  illustrates an actuator piston  62  that includes a double helical groove formed by a right-spiral groove  150  and a left-spiral groove  152 . The use of two or more grooves such as is shown in  FIG. 14  may permit the grooves to be shallower and still provide the desired results.  FIG. 15  illustrates a helical groove  154  including a lesser number of turns, perhaps less than one turn, and  FIG. 16  illustrates an actuator piston  62  having one or more straight grooves  156  in its surface. While reducing the number turns or utilizing straight grooves may result in increased back leakage during the forward stroke of the piston, the forward stroke (pumping) of the piston will still be substantially faster than the rearward stroke (refill) and the back leakage will still be substantially less that the forward flow. Finally, one or more such grooves may be provided in the cylinder wall to facilitate fluid flow as described above in connection with the groove or grooves in the armature piston. 
   Thus, there has been provided an infusion pump that dispenses predetermined dosages of a protein drug (e.g. insulin) and is configured to facilitate the passage of rinsing fluid to remove undesirable protein building on the fluid path surfaces. The infusion pump includes a piston pumping mechanism that includes an actuator configured to dissolve protein build-up on the surfaces of the piston and piston walls. In addition, the drive mechanism is configured to reduce the time it takes to refill the outlet chamber of the infusion pump to an acceptable time despite the build-up of protein deposits on the walls of the pump&#39;s fluid path. 
   While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing exemplary embodiments of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.