Abstract:
A spring-powered infusion pump includes a syringe barrel  20  having two opposing openings  30, 55  forming two chambers  80, 90  between the first opening  30  and the plunger  40,  and the second opening  55  and the plunger  40.  The dispenser opening  30  has a one-way valve  35  to selectively release fluid retained within the first chamber  80.  The second opening  55  is capped, and a spring  60  is compressed between the plunger  40  and syringe cap  50  within the second chamber  90.  The spring  60  applies a force to the plunger  40  in the direction of the first opening  30.  However, the one-way valve  35  retains the fluid within the first chamber  80,  despite the force applied to the plunger  40,  until a tubing set  70  equipped with an infuser connector  75  is attached to the dispenser opening  30  of the syringe barrel  20.  The infuser connector  75  is insertable through the one-way valve  35  and thus provides a passageway for the fluid retained within the first chamber  80  of the syringe barrel  20.  Once attached, the force from the spring  60  causes the plunger  40  to move toward the dispenser opening  30,  thereby dispensing the fluid from the first chamber  80  through the infuser connector  75  and one-way valve  35  into the tubing set  75.

Description:
RELATED APPLICATIONS  
       [0001]    The present application is based in part on the Applicant&#39;s International Patent Application PCT/US99/30890, entitled “Spring-Powered Infusion Pump,” filed on Dec. 27, 1999, which is based on U.S. Provisional Patent Application Ser. No. 60/114,206, filed on Dec. 29, 1998. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to the field of spring-powered infusion pumps. More specifically, the present invention discloses a single-dose spring-powered infusion pump.  
           [0004]    2. Statement of the Problem  
           [0005]    Syringe-type infusers typically require the user to manually dispense the fluid contents (e.g., by pressing the syringe plunger). Thus, it is difficult to deliver a steady flow over time, especially when there is a large amount of fluid to be dispensed over a period of time. Spring-powered infusers, on the other hand, deliver fluid at a continuous rate, but require elaborate mechanisms, caps or clips to retain the fluid within the syringe because the plunger is under pressure.  
           [0006]    Conventional spring-loaded infusers are initially loaded under pressure. Therefore, a secondary problem is created when the filling apparatus is disconnected from the infuser. If tubing is not immediately attached or the connection is not otherwise capped or clipped, the pressurized liquid will be lost, thus making the amount delivered inaccurate.  
           [0007]    An additional problem associated with conventional spring-loaded infusers is limited delivery accuracy. The manufacturing process for springs introduces a significant amount of natural variation from spring to spring. This natural variation is a primary factor in limiting the accuracy with which a spring-loaded infuser can deliver liquids.  
           [0008]    Syringe type infusers used in the past include the following:  
                                                       Inventor   Patent No.   Issue Date                           Calhoun   1,123,990   Jan. 5, 1915           Bessesen   1,476,946   Dec. 11, 1923           Kollsman   2,605,765   Aug. 5, 1952           Jinotti   3,565,292   Feb. 23, 1971           Magoon, et al.   4,312,347   Jan. 26, 1982           Genese   4,381,006   Apr. 26, 1983           Vaillancourt   4,813,937   Mar. 21, 1989           Chang   4,991,742   Feb. 12, 1991           LeFevre   4,997,420   Mar. 5, 1991           Reese   5,078,679   Jan. 7, 1992           Vaillancourt   5,100,389   Mar. 31, 1992           Zdeb   5,135,500   Aug. 4, 1992           Ishikawa   5,178,609   Jan. 12, 1993           Elson   5,346,476   Sep. 13, 1994           Kriesel   5,569,236   Oct. 29, 1996           McPhee   5,599,315   Feb. 4, 1997           Ragsdale et al.   5,607,395   Mar. 4, 1997                      
 
           [0009]    Chang teaches an automatic drip bottle set. A cover and basin connect to hold a spring in the basin. The spring provides pressure urging the basin downward and applying uniform pressure on an expansion drip bottle.  
           [0010]    LeFevre discloses a portable drug delivery device for delivering a drug in liquid form at a constant and self-regulated rate. A syringe having a spring-loaded piston in a cylinder forces the liquid out through a tubing having a restrictor in the length of tubing to impede flow and achieve a desired flow rate.  
           [0011]    Zdeb teaches a self-driven pump device for delivering fluid at a relatively constant, controlled rate. A vacuum power means collapses under atmospheric pressure and drives a plunger to deliver fluid from the fluid storage means. The fluid storage means is filled by attaching a male luer opening to a female luer opening associated with the fluid storage means. As the male luer is pushed into the duck-bill valve, the tapered end portion opens to allow fluid to pass through the valve and into the fluid storage means. Fluid can then be delivered from the fluid storage means as the plunger moves under atmospheric pressure. A generally similar vacuum-powered infusion pump has been marketed by McKinley Medical, LLLP, of Wheat Ridge, Colo., as the “Outbound” disposable syringe infuser.  
           [0012]    Calhoun discloses a type of syringe for dosing or inoculating animals. The syringe automatically discharges the contents when the user forces a small plunger inward, permitting a spring to draw a piston into the syringe barrel, thus forcing the contents out in a controlled manner.  
           [0013]    Bessesen discloses a fluid-pressure device. When the aperture is closed and the barrel filled with fluid, pressure is created in the barrel by turning the handle to release the spring. When the barrel is emptied, the piston is retracted by turning the handle the opposite direction.  
           [0014]    Kolisman teaches an automatic syringe. After removing a release cap from the end of the piston guide rod, a spring expands moving the piston toward a partition plug against the resistance of a viscous liquid in chamber  33 . The liquid flows slowly through a capillary passage  36  into a chamber  32 , which moves a piston  15  displacing fluid from the chamber  14  through an injection needle over a predetermined time.  
           [0015]    Jinotti discloses an apparatus for holding a blood bag and causing the blood to be fed out of the bag. A piston is retracted by turning a handle to the desired position and then released so that the piston is under pressure created by the spring, which in turn forces blood out of the bag gradually and constantly.  
           [0016]    Magoon et al. teach a positive-pressure drug releasing device. A chamber is filled with a liquid drug and placed under continuous positive pressure by a spring and plunger device. Fluid diffuses at a predetermined rate through a membrane opposite the plunger.  
           [0017]    Genese discloses a continuous low flow rate fluid dispenser. Two spiral coiled springs move the driver member toward the abutment member, forcing the plunger stopper toward the nozzle portion expelling fluid from the syringe barrel at a slow and steady rate.  
           [0018]    Vaillancourt (&#39;937) teaches an ambulatory disposable infusion delivery system. Inflow of a fluid causes an elastomeric member attached to a piston to stretch, which pushes the fluid out of the bore when the tubing line is opened. The housing is provided with a discharge fluid conduit and a restrictor controlling the rate of flow.  
           [0019]    Reese discloses a method of administering anesthesia directly to the surgical site. The plunger of a spring-loaded syringe creates pressure thus causing the medication to flow through a cannula and catheter into the wound. Flow of the medication is regulated by the micro-bore cannula to ensure delivery at very small rates.  
           [0020]    Vaillancourt (&#39;389) teaches an ambulatory infusion pump with a preloaded spring having a fixed spring constant. The preloaded spring is released by a tab and biases the piston of the pump. The biasing force of the spring and the stroke of the piston are coordinated to maintain pressure on the fluid and dispense the fluid at a slow rate.  
           [0021]    Ishikawa discloses a medical liquid injector for continuous transfusion. A syringe is fitted with a piston and a cap having an elastic pressing device for continuously pressing the piston to force the liquid from the syringe. Flow is controlled using a flow rate control tube having a given inner diameter.  
           [0022]    Elson discloses a fluid delivery system having a bladder enclosed in a cap and drive mechanism. A piston driven by a constant force spring delivers the fluid at a predetermined rate based on the spring design.  
           [0023]    Kriesel discloses a fluid container assembly having a plunger that is powered by a stored energy source, such as a compressible cellular mass or an elastomeric membrane, to dispense fluid.  
           [0024]    McPhee discloses a syringe actuation device that uses a spring-biased piston.  
           [0025]    Ragsdale et al. disclose a spring-powered injection device.  
           [0026]    3. Solution to the Problem  
           [0027]    None of the prior art references discussed above show a spring-powered infusion pump having a one-way valve (e.g., a duck-bill valve) for retaining the fluid within the dispenser while under pressure from a spring. The spring compressed between the syringe plunger and the syringe cap provides pressure to the plunger so that the fluid contained within the syringe barrel can be released automatically at a continuous rate of flow and the user does not have to manually dispense the fluid contents. A one-way valve retains the fluid within the syringe barrel against the pressure on the plunger from the spring. By inserting an infuser connector through the one-way valve, the fluid retained within the dispenser can be selectively released.  
         SUMMARY OF THE INVENTION  
         [0028]    The present invention is a spring-powered infusion pump. The spring-powered infusion pump has a syringe barrel with two opposing openings and a plunger disposed between the openings within the syringe barrel. Thus, two chambers are formed between the first opening and the plunger, and the second opening and the plunger. The first opening is fitted with a one-way valve to selectively release fluid retained within the first chamber. The second opening is capped, and a spring is biased between the plunger and syringe cap within the second chamber. The spring applies a force to the plunger in the direction of the first opening. However, the one-way valve retains the fluid within the first chamber, despite the force applied to the plunger, until a tubing set equipped with an infuser connector is attached to the first opening of the syringe barrel. The infuser connector is insertable through the one-way valve and thus provides a passageway for the fluid retained within the first chamber of the syringe barrel. Once attached, the force exerted by the spring causes the plunger to move toward the first opening, thereby dispensing the fluid from the first chamber through the infuser connector and one-way valve into the tubing set.  
           [0029]    A primary object of the present invention is to provide a single predetermined dose of fluid at a continuous rate of flow. Inconsistencies in the flow rate associated with manual operation of the syringe plunger are largely eliminated by the spring-powered syringe of the present invention.  
           [0030]    Another object of the present invention is to provide a sterile, disposable device for dispensing predetermined amounts of fluid. The syringe barrel of the present invention can be prefilled in a sterile environment so that the fluid is not contaminated prior to being dispensed.  
           [0031]    Yet another object of the present invention is to retain the fluid within the syringe barrel prior to being dispensed without the need for caps, clips or other stoppers. When caps or other stoppers are removed, the fluid immediately is released from the syringe barrel, and if tubing is not immediately attached, this fluid is lost and the amount delivered is thus inaccurate. In addition, caps and clips can easily be lost. Fluid is retained within the syringe barrel of the present invention by the one-way valve and released only when an infuser connector fitted within the tubing, is connected to the dispenser. Therefore, when the one-way valve is opened, fluid flows immediately into the tubing and none is lost.  
           [0032]    Another object of the present invention is to provide an adjustment mechanism to improve delivery accuracy. A shim or other height adjustment mechanism is used to adjust the spring when manufacturing the infusion pump. The natural variation from spring to spring is reduced or eliminated, resulting in a more uniform driving force in the infusion pump from unit to unit, leading to improved accuracy in the rate of fluid delivery.  
           [0033]    These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    The present invention can be more readily understood in conjunction with the accompanying drawings, in which:  
         [0035]    [0035]FIG. 1 is an exploded perspective view of the spring-powered infusion pump.  
         [0036]    [0036]FIG. 2 is a cross-sectional view of the assembled infusion pump.  
         [0037]    [0037]FIG. 3 is a detail cross-sectional view showing an infuser connector penetrating the one-way valve.  
         [0038]    [0038]FIG. 4 is a detail cross-sectional view corresponding to FIG. 3 after the infuser connector has been completely attached the infusion pump.  
         [0039]    FIGS.  5 ( a ) and  5 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating a series of shims  81  to adjust the force exerted by the spring  60  on the plunger  40 .  
         [0040]    FIGS.  6 ( a ) and  6 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating a helical shim  82  that can be trimmed to a desired thickness to adjust the spring force.  
         [0041]    FIGS.  7 ( a ) and  7 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating coaxial beveled segments  83  to adjust the spring force.  
         [0042]    FIGS.  8 ( a ) and  8 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating a jackscrew mechanism  84  to adjust the spring force.  
         [0043]    FIGS.  9 ( a ) and  9 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating a threaded post  85  to adjust the spring force.  
         [0044]    [0044]FIG. 10 is bottom perspective view of another embodiment of the plunger  40  having a longer skirt and a plurality of raised circumferential ridges  42 .  
         [0045]    [0045]FIG. 11 is a top perspective view of the plunger  40  corresponding to FIG. 10.  
         [0046]    [0046]FIG. 12 is an exploded perspective view of an embodiment of the infusion pump in which a cap ring  225  on the interior periphery of the cap  50  interlocks with a barrel flange  220  extending about the exterior periphery of the syringe barrel  20  to attach the cap  50  to the syringe barrel  20 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0047]    Turning to FIG. 1, the spring-powered infusion pump has a syringe barrel  20  with two opposing openings  30  and  55 . The bottom portion of the syringe barrel  20  is tapered to form a dispenser opening  30 . Opposite the dispenser opening  30 , the syringe barrel  20  forms a top opening  55  that is substantially the same diameter as the syringe barrel  20 . A plunger  40  can thus be inserted into, and slidable within, the syringe barrel  20  through the top opening  55 . The plunger  40  is surrounded by a circumferential compressible seal  45  (e.g., an o-ring) to form a seal between the plunger  40  and the inside surface of the syringe barrel  20 .  
         [0048]    As shown in FIG. 2, a fluid chamber  80  is formed within the lower portion of the syringe barrel  20  between the dispenser opening  30  and the plunger  40 . Similarly, as shown in FIG. 1, a spring chamber  90  is formed in the upper portion of the syringe barrel  20  between the top opening  55 , which is covered by a cap  50 , and the plunger  40 . Before the cap  50  is secured over the top opening, a spring  60  is compressed within the spring chamber  90  to exert a force against the plunger  40  in the direction of arrow  65  shown in FIG. 2.  
         [0049]    Although in the preferred embodiment the syringe barrel  20  is cylindrical, it is to be expressly understood that the syringe barrel  20 , and hence the plunger  40  and compressible seal  45  can be any suitable shape. It is only important that a sealed fluid chamber and a separate spring chamber be formed adjacent one another. In the preferred embodiment, the syringe barrel  20 , plunger  40 , and cap  50  are made from a hard plastic, such as polypropylene or polycarbonate, so that these parts can be disposed or recycled after use. However, other materials such as metal or glass can also be used for the various components. In addition, the terms “dispenser opening” and “top opening” are intended only to differentiate the two openings, and not to limit the present invention to its orientation.  
         [0050]    [0050]FIG. 2 shows the present infusion pump after it has been assembled. As depicted in FIG. 1, a series of barrel tabs  200  are formed on the outside of the syringe barrel  20 . The cap  50  includes a lip  210  that fits over the top opening  55  of the syringe barrel  20 . A series of cap tabs  215  are formed within the lip  210  of the cap  50 , which can be snapped over the barrel tabs  200  to securely hold the cap  50  over the top opening  55  of the syringe barrel  20 .  
         [0051]    In the preferred embodiment, the cap  50  is separate from the barrel  20  to simplify manufacturing and assembly of the present invention. The cap  50  readily fits over the top opening of the barrel  20  and is locked in place. Thus, it is difficult to remove the cap  50  once it is assembled to prevent the cap  50  from popping off due to pressure from the spring  60 .  
         [0052]    In the preferred embodiment of the present invention, there are more cap tabs  215  than barrel tabs  200  so that the cap  50  and barrel  20  do not have to be perfectly aligned, and so that the cap  50  does not come off under pressure from the spring  60  in the event that cap  50  is rotated. However, it is to be expressly understood that the number and placement of cap tabs  215  and barrel tabs  200  are not important to the present invention so long as the cap  50  can be securely fitted to top opening  55 . Likewise, the cap  50  can be secured over the top opening  55  of the syringe barrel  20  in any suitable manner, including but not limited to: permanently bonding the plastic cap to the syringe barrel  20 , mechanical latches, or any other suitable design for retaining the cap  50  on the syringe barrel  20  under pressure from the spring  60 .  
         [0053]    [0053]FIGS. 3 and 4 show the details of the one-way valve  35 . The preferred embodiment of the one-way valve is a duck-bill, as depicted in FIGS. 3 and 4. However, it is to be expressly understood that any appropriate type of one-way valve may be used, including but not limited to a ball valve, flapper valve, umbrella valve, disc valve, or any other suitable design for allowing flow in one direction while preventing flow in the opposite direction except when pierced or otherwise opened with a mating component. The one-way valve  35  is securely fitted within the dispenser opening  30 . The duck-bill embodiment of the one-way valve  35  typically is formed by two “duck-bills”  310  made of a soft, sealable plastic or rubber having an opening between the duck-bills  310 . In its normal position shown, the one-way valve  35  is in a closed position (e.g., the duck-bills  310  are collapsed against one another by fluid pressure) so that the fluid is retained within the fluid chamber  80 . The dispenser opening  30  includes threads  300  so that the threaded connector  305  of the tubing  70  can be secured to the dispenser opening  30 . As the threaded end of tubing  70  is threaded onto the dispenser opening  30 , a tubular portion  315  of the infuser connector  75  is inserted between the duck-bills  310 , thus spreading duck-bills  310  and forming a conduit from the fluid chamber  80  through the dispenser opening  30  (via the one-way valve  35 ) and through the infuser connector  75  (via the needle-like portion  315 ) and into the tubing  70 . Thus, fluid retained within fluid chamber  80  is allowed to flow, under the force of the spring  60 , into the tubing  70  to be dispensed (e.g., as an IV into a patient).  
         [0054]    Note that in the duck-bill embodiment of the one-way valve  35 , the duck-bills  310  and infuser connector  75  are designed so that upon insertion of the infuser connector  75 , the duck-bills  310  form a seal around the tubular portion  315  of the infuser connector. This seal prevents fluid from leaking through the one-way valve  35  and around the outside of the infuser connector  75 .  
         [0055]    In the preferred embodiment, the dispenser opening  30  and the tubing connector  305  are threaded. However, any suitable means for securely attaching the tubing  70  to the dispenser opening  30  can be used without departing from the scope of the present invention. For instance, the dispenser opening  30  may be ribbed to receive the tubing  70 , or any other suitable design may be used so long as the tubing  70  is held securely to the dispenser opening  30  when the infuser connector  75  opens the one-way valve  35 .  
         [0056]    In manufacturing springs, there is some degree of variation from spring to spring. In other words, if a group of springs are compressed to the same height, the force exerted by each spring will vary throughout the group. In the embodiment of the infusion pump shown in FIGS.  1 - 4 , the spring will be compressed to the same height (i.e., the distance in the infuser between the cap  50  and the plunger  40 ). The pressure created in the infusion pump is proportional to the force generated by the spring  60 . Since springs vary in force from spring to spring, the pressure created in each infusion pump will vary from unit to unit. Flow rate is proportional to the infuser pressure, so the variation in spring force ultimately leads to variations in flow rate, which is highly undesirable.  
         [0057]    To give some indication of the importance of this variation in spring force, infusion pumps are typically required to deliver fluid at a flow rate that is within 15% of an ideal, nominal flow rate. The spring used in current spring-powered infusion pumps varies by about 7%, or almost half of the permissible variation for the entire assembly. If the variation in spring force can be reduced to 2%, for example, we could then have a device with 10% accuracy, instead of 15%. Alternatively, other sources of variation could be more loosely controlled to reduce the cost of the infusion pump.  
         [0058]    The embodiments shown in FIGS.  5 ( a ) through  9 ( b ) employ shims or other height adjustment mechanisms to adjust the spring compression within each infusion pump to reduce the variation in spring force from unit to unit, thereby effectively reducing variation in flow rate. The further a spring is compressed, the greater the force generated by the spring. If one spring is slightly weaker than another, the same force can be generated by both springs by compressing the weaker spring slightly more.  
         [0059]    To illustrate this concept, assume there is a two inch space between the cap  50  and plunger  40 . Without any adjustments, each spring will be compressed to two inches once it is assembled into an infuser. Assume the spring rate is 16 lbs per inch. Also assume the average force when compressed to two inches is 40 lbs, but the springs have a 10% variation, so the actual force will vary from 36 to 44 lbs. Using a height adjustment mechanism, all of the springs can be adjusted to 44 lbs by compressing the weaker springs to heights less than two inches. For the weakest springs (i.e., those generating 36 lbs), we need to add another 8 lbs of force. Adding a 0.5 inch shim (so the spring is compressed to 1.5 inches) will increase the spring force to 44 lbs. For an average spring, adding a 0.25 inch shim (so the spring is compressed to 1.75 inches) will add 4 lbs of force to bring those up to 44 lbs, too.  
         [0060]    If desired, this approach can be applied to all of the springs in a selected group. However, this approach could be applied to only a portion of the springs, or only those springs falling outside of a predetermined tolerance. For example, adjusting half of the springs would cut variation in half. Adjusting that half of the springs having the worst variations would cut variation by more than half.  
         [0061]    FIGS.  5 ( a ) and  5 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating a stacked series of shims  81  of equal or varying thickness to adjust the force exerted by the spring  60  on the plunger  40 . For example, the shims  81  could be shaped as disks or washers.  
         [0062]    FIGS.  6 ( a ) and  6 ( b ) are cross-sectional views of an embodiment of the infusion pump using a helical shim  82  that can be trimmed to a desired height to adjust the spring force. The manufacturer would simply cut off the proper number of coils to achieve the desired height for the helical shim  82 .  
         [0063]    FIGS.  7 ( a ) and  7 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating coaxial beveled segments  83  to adjust the spring force. This embodiment is similar to the device used to damp a swinging door. Two beveled cylindrical segments are stacked atop one another. Rotating the cylindrical segments with respect to one another increases or decreases the overall height of the assembly.  
         [0064]    FIGS.  8 ( a ) and  8 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating a jackscrew mechanism  84  to adjust the spring force. The upper end of the spring  60  abuts a plate threaded on a screw attached to the cap  50  of the infusion pump. Turning the screw or the plate moves the plate up or down to adjust spring compression.  
         [0065]    FIGS.  9 ( a ) and  9 ( b ) are cross-sectional views of an embodiment of the infusion pump incorporating a threaded post  85  to adjust the spring force. The spring  60  threads down over the post  85 . The effective length of the spring  60  is adjusted by how far it is screwed onto the post  85 .  
         [0066]    [0066]FIGS. 10 and 11 are bottom and top perspective views, respectively, of an embodiment of the plunger  40  having an elongated side wall and a plurality of raised circumferential ridges  42  extending about the periphery the plunger  40 . These ridges  42  are separated by recessed areas  41 . FIG. 12 is a cross-sectional view of an infusion pump using the plunger  40  from FIGS. 10 and 11. This embodiment of the plunger  40  is less likely to become jammed in the syringe barrel  20 , reduces potential friction, and provides a more continuous flow rate.  
         [0067]    The embodiment of the plunger  40  shown in FIG. 11 includes a plurality of contoured ribs  44  on the interior surface of the plunger  40 . The coils of the spring  60  tend to catch on any exposed edges of the plunger  40 , including the upper edge of the plunger  40 . This results in uneven friction as each spring coil catches and then slips free. The end results are dips and spikes in the flow rate from the infuser, with flow slowing as a coil catches, and then a spike in the flow rate when the coil slips free. The interior ribs  44  bear on the cylindrical surface of the spring  60 . The upper ends of the ribs taper away from the spring  60 , thereby eliminating any corners or edges that the spring  60  could rub on.  
         [0068]    [0068]FIG. 13 is an exploded perspective view of an embodiment of the infusion pump in which a cap ring  225  on the interior periphery of the cap  50  interlocks with a barrel flange  220  extending about the exterior periphery of the syringe barrel  20  to attach the cap  50  to the syringe barrel  20 . This configuration helps to ensure that the cap  50  is permanently secured to the syringe barrel  20  and minimizes the risk that the infuser might be accidentally disassembled or tampered with.  
         [0069]    Design considerations will determine the rate at which fluid is dispensed from the syringe barrel  20 . For instance, the diameter of the dispenser opening  30 , the size of the opening created by the one-way valve  35  and the infuser connector  75 , the diameter of the tubing  70 , and the spring constant of the spring  60  can be selected to achieve the desired flow characteristics. Flow restrictor elements attached to the tubing  70  can also serve to regulate the flow. Likewise, the amount of pressure exerted by the spring  60  will determine the characteristics of the one-way valve  35  required to retain the fluid within the fluid chamber  80 . Markings on the syringe barrel can be used to indicate the amount of fluid retained in the fluid chamber  80 .  
         [0070]    The fluid chamber  80  of the present invention is preferably filled by the manufacturer or the health care provider. The device is assembled with the plunger  40  disposed within the syringe barrel  20  and the spring  60  compressed between the plunger  40  and the cap  50 . The health care provider then connects a filling apparatus to the device. Pressurized fluid is fed into the fluid chamber  80  with sufficient force to overcome the force of the spring  60 . The one-way valve  35  opens with the applied force of the fluid and allows the fluid to flow into the fluid chamber  80 . Once the fluid chamber  80  is filled to a predetermined level, the filling apparatus is removed and the one-way valve  35  returns to its sealed position to retain the fluid within the fluid chamber  80 . The fluid is retained by the one-way valve  35  until the spring-powered infusion pump is ready for use, at which time, tubing  70  having an infuser connector  75  is connected to the dispenser opening  30 , and the fluid is allowed to flow from the fluid chamber  80  as described above.  
         [0071]    The flow rate delivered by the infusion pump gradually slows as it empties due to relaxation of the spring. This is clinically desirable in some applications, such as pain management, in which the patient&#39;s need for medication tapers off with time. There may be other applications in which changing the flow rate over time would be desirable. By changing the spring geometry or using multiple springs, compound delivery profiles can be obtained, with step-changes in flow rate or differing flow rates for different portions of the delivery.  
         [0072]    It is to be expressly understood that the spring-powered infusion pump of the present invention can be filled by the manufacturer and disposed of after use, or the present invention may be refilled for repeated use. Alternatively, the infusion pump can be shipped empty and filled by the healthcare provider before use. The preferred embodiment offers the advantage of accurate filling and sterility.  
         [0073]    Alternatively, the user may wish to fill the infuser with fluid and then freeze it. This is often not feasible with other types of infusers because the fluid expands when frozen and damages the device (e.g., cracks the housing, breaks seals, stretches seals open). The present infuser has additional capacity allowing overfill. If the infuser has not been purposely overfilled, it can be frozen and the plunger  40  and spring  60  will simply move further back to accommodate the increased volume due to expansion upon freezing.  
         [0074]    The above disclosure sets forth a number of embodiments of the present invention. Other arrangements or embodiments, not precisely set forth, could be practiced under the teachings of the present invention and as set forth in the following claims.