Abstract:
A syringe may include various forms of a releasable stop adapted to selectively limit the travel of the syringe plunger within the syringe barrel such that a first amount of therapeutic substance may be discharged from the syringe while a residual supply of the therapeutic substance remains in the syringe. Movement of the plunger is permitted to displace the residual supply of fluid from the barrel. Syringes according to the invention find particular application as diluent syringes in a wet/dry drug delivery syringe system having two cooperating syringe barrels in which the residual supply of diluent in the diluent syringe may be used to flush additional and otherwise wasted drug from a drug syringe barrel.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to syringes and syringe systems for packaging and delivering therapeutic substances or medicaments to a living body. The invention also relates to syringe systems for mixing a controlled amount of a first therapeutic substance or constituent, such as a liquid diluent with a second therapeutic substance or constituent, such as a dry or powdered drug, for subsequently dispensing the constituent mixture and for subsequently flushing leftover amounts of the second constituent from the syringe system with a residual supply of the first constituent.  
       BACKGROUND OF THE INVENTION AND TECHNICAL PROBLEMS POSED BY THE PRIOR ART  
       [0002]     It is known in the prior art to provide syringe systems that incorporate two pre-filled syringe barrels which cooperate to provide for the mixing of two constituents previously maintained in isolation from one another. Such systems are useful in pharmaceutical applications where pre-mixed solutions or suspensions of a drug are not stable enough to withstand prolonged periods of storage. Frequently, one of the two constituents is provided as a dry lyophilized, powdered drug and the other constituent is provided as a diluent. A first syringe barrel contains the dry drug within an expandable mixing chamber defined by a barrel wall, a dispensing end and a reciprocable stopper housed in the first syringe barrel. A second syringe barrel contains a supply of diluent for reconstituting the drug. Using a movable plunger reciprocably disposed in the second syringe barrel, diluent is displaced from the second syringe barrel through a discharge passage, through the stopper in the first syringe barrel and into the expandable mixing chamber where the dry drug is reconstituted. Once a predetermined supply of diluent has been discharged from the second syringe barrel, continued movement of the second syringe within the first syringe barrel results in dispensing of the mixed drug to a patient. These so-called “wet/dry” drug delivery systems are disclosed in U.S. Pat. No. 5,779,668, the subject matter and entire writing of which is incorporated herein by reference.  
         [0003]     In many cases, the active drug in a two-constituent delivery system is substantially more expensive than the diluent. Thus, it is economically desirable to maximize the amount of active drug dispensed from the syringe system to the patient. In prior art systems, however, when the diluent supply has been exhausted from the diluent syringe, there often remains a residual amount of drug in the system. In particular, drug may remain in small amounts in the mixing chamber and in the syringe dispensing passage. As such, the amount of drug packaged in the pre-filled system is typically in excess of the actual amount needed to fulfill patient dosage requirements. This results in increased cost.  
         [0004]     It would therefore be desirable to provide an improved syringe system that permits an increased amount of drug to be dispensed from the mixing chamber in comparison to prior art syringes and systems. In particular, it would be advantageous to provide an improved syringe which provides for the dispensing of a first predetermined amount of diluent or first constituent for mixing with a supply of drug or second constituent while providing for the retention of a residual supply of diluent or first constituent for flushing remaining amounts of drug from the mixing chamber.  
         [0005]     It would also be beneficial to provide an improved syringe system which provides the aforementioned desirable features and which is economical to manufacture and use. Additionally, it would be desirable if such an improved syringe and syringe system could be easily and safely operated by an operator administering the therapeutic substance.  
       SUMMARY OF THE INVENTION  
       [0006]     The benefits and advantages described above are realized by the present invention which provides a syringe that permits a user to discharge a first predetermined amount of diluent, while reserving a residual supply of diluent for later use. A syringe according to the invention includes a barrel including a discharge end defining a discharge passage, a plunger disposed within the barrel, the plunger and discharge end defining a chamber within the barrel, the plunger being adapted to move within the barrel and thereby displace fluid from the barrel through a discharge end, and a releasable stop or limiting means that is operatively associated with the syringe plunger and adapted to move between a deployed position, in which the plunger movement is limited such that a first supply of fluid is displaced from the barrel while a residual supply of fluid remains in the barrel, and a retracted position, in which further movement of the plunger is permitted to displace the residual supply of fluid from the barrel.  
         [0007]     Syringes according to the invention find particular application as diluent syringes in a wet/dry drug delivery syringe system having two cooperating syringe barrels. Accordingly, a prefilled syringe system according to the invention comprises a first barrel having an open end and an opposite mixture delivery end defining a mixture delivery passage; a stopper slidably disposed in the first barrel, the stopper and mixture delivery end defining a first chamber for containing the first constituent; a second barrel disposed within the first barrel, the second barrel having an open end and an opposite discharge end defining a discharge passage for conveying the second constituent into the first chamber of the first barrel; a plunger disposed within the second barrel, the plunger and discharge end defining a second chamber within the barrel, the plunger being adapted to move within the barrel and thereby displace the second constituent from the second barrel through the discharge end and into the first chamber of the first barrel for mixing with the first constituent; and a releasable stop or limiting means that is operatively associated with the plunger and adapted to move between a deployed position, in which the plunger movement is limited such that a first supply of the second constituent is displaced from the second barrel while a residual supply of fluid remains in the second barrel, and a retracted position, in which further movement of the plunger is permitted to displace the residual supply of the second constituent from the second barrel.  
         [0008]     The releasable stop or limiting means according to the present invention may take various forms. In one preferred embodiment, the releasable stop is provided as a thumb lever integrally molded with the syringe plunger and having an engaging end for engaging a portion of the syringe barrel when the thumb lever is in a deployed position. The thumb lever pivots relative to the plunger shaft about a pivot end opposite the engaging end and is biased in an outward radial direction from the longitudinal axis of the plunger shaft. When a lateral force is applied to a gripping surface of the thumb lever, the thumb lever pivots within a channel formed in the plunger shaft radially inward towards the plunger shaft axis to a retracted position. Preferably, the thumb lever includes a a bi-stable, over-center detent which maintains the thumb lever in in the retracted position once pivoted beyond a center point. The plunger shaft is then permitted to continue to travel or move within the syringe barrel. The thumb lever may be actuated with the same hand used to actuate the plunger within the syringe barrel and therefore provides single-hand operation.  
         [0009]     According to another preferred embodiment, the releasable stop is provided as a compressible cylindrical element situated around the plunger shaft beneath the plunger push button. In a deployed position, the cylindrical element assumes an external diameter that is larger than the internal diameter of the syringe barrel such that an engaging end of the cylindrical element prevents movement of the plunger. The cylindrical element may be compressed by application of a suitable lateral force to a retracted position in which the external diameter of the cylindrical element becomes smaller than the internal diameter of the syringe barrel, thereby permitting the cylindrical element to move with the plunger shaft within the syringe barrel and thereby permitting further displacement of the plunger shaft.  
         [0010]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a pair of flaps extending from and pivotable with respect to a central portion of the plunger shaft. In a deployed position, the flaps extend outward beyond the internal diameter of the syringe barrel such that the flaps are adapted to engage the syringe barrel flange and prevent movement of the plunger. In a retracted position, the flaps are pivoted about the central portion of the plunger shaft to a position in which the flaps are folded about the central portion of the plunger shaft, thereby permitting the plunger shaft to undergo continued displacement within the syringe barrel.  
         [0011]     According to still another preferred embodiment, the releasable stop or limiting means is provided as a lever pivotably attached to the plunger shaft at a generally central location and having an actuating handle that extends upward beyond the plunger push button. A slot is formed in the plunger push button to accommodate radially inward movement of the actuating handle. On an end of the lever opposite the actuating handle is an engaging end of the lever. When the actuating lever resides in the slot in the plunger push button, the lever is in a deployed position in which the engaging end is positioned to engage the syringe barrel flange. When the lever actuating handle is moved radially outward from the slot in the push button, the lever assumes a retracted position in which the engaging end is disposed adjacent the plunger shaft to permit further displacement of the plunger shaft within the syringe barrel.  
         [0012]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a lever extending from the plunger shaft and pivoting about a pivot joint disposed at an end of the lever opposite an actuating handle. The actuating handle, when the lever is in a deployed position, is disposed in a position remote from the plunger push button and a recess or catch formed on the lever engages a complimentarily-shaped annular projecting lip formed on the barrel flange. The actuating handle may be moved to a retracted position by application of a radially inward force, causing the handle to move into a slot formed in the push button and causing the recess or catch to become disengaged from the annular projecting lip, thus permitting continued displacement of the plunger within the syringe barrel.  
         [0013]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a pair of projections extending from the plunger push button. The plunger shaft and push button are permitted to rotate about a longitudinal axis of the plunger shaft relative to the syringe barrel. In a deployed position, the projections are rotated to a position where they engage the syringe barrel flange and therefore prevent movement of the plunger shaft. In a retracted position, the projections are rotated to a position where they clear the syringe barrel flange and therefore permit movement of the plunger within the syringe barrel.  
         [0014]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a breakaway projection extending from the syringe barrel flange or from the plunger shaft and adapted to engage the push button of the syringe plunger in a deployed position. The breakaway projection is moved to its retracted position by separating the breakaway projection from the syringe barrel, preferably at a frangible or weakened portion of the breakaway projection. With the breakaway projection removed, further downward travel of the plunger is permitted within the barrel flange.  
         [0015]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a pair of interfering rings, one formed on the plunger shaft and one formed on an interior surface of the syringe barrel. Upon application of a suitable force to the syringe plunger push button, one or both of the rings deforms from a deployed position to a retracted position such that the plunger shaft ring is permitted to pass the syringe barrel ring and further movement of the plunger within the syringe barrel is permitted.  
         [0016]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a leaf spring formed on the plunger shaft and having a tab or projection for engaging an internal annular ledge formed on the syringe barrel interior. The ledge extends inward to engage the tab or projection as the plunger shaft is displaced within the syringe barrel and provides suitable tactile indication to a user that the plunger displacement has reached a first limit. Upon application of a suitable increased force to the plunger push button by the user, the annular ledge causes the leaf spring to moves from a deployed position to a retracted position in which further movement of the plunger into the syringe barrel is permitted.  
         [0017]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a removable element disposed in a slot formed in the plunger shaft. The removable element is of a generally U-shaped construction and includes a recess formed therein which cooperates with the slot on the plunger shaft. When the removable element is in a deployed position in which it is disposed in the slot on the plunger shaft, movement of the plunger shaft is limited by interference between the removable element and the barrel flange. When the removable element is moved to a retracted position in which it is separated from the plunger shaft, the plunger shaft is permitted to move within the syringe barrel.  
         [0018]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a breakaway cylindrical element disposed around the plunger shaft. The cylindrical element has an outer diameter that is greater than the inner diameter of the syringe barrel such that an engaging end of the cylindrical element engages the syringe barrel flange and an opposite end of the cylindrical element engages the plunger push button. In this deployed position, movement of the plunger relative to the syringe barrel is limited. The cylindrical element is moved to its retracted position by fracturing the cylindrical element, preferably into two halves along one or more weakened or frangible fines or areas. Fracture of the cylindrical element permits removal and further movement of the plunger within the syringe barrel.  
         [0019]     According to yet another preferred embodiment, the releasable stop or limiting means is provided as a pair of pivotable finger tabs extending outward from a central portion of the plunger shaft. When the finger tabs are pivoted to a deployed position stop edges on the finger tabs are adapted to engage the syringe barrel. When the finger tabs are pivoted to a retracted position, the stop edges are disposed in a position where they do not engage the syringe barrel and further movement of the plunger shaft within the syringe barrel is permitted.  
         [0020]     According to still another preferred embodiment, the releasable stop or limiting means is provided as an externally threaded ring on the plunger shaft which cooperates with an internally threaded insert on the syringe barrel. Upon downward movement of the plunger shaft within the syringe barrel, the externally threaded ring engages the internally threaded insert and prevents further movement of the plunger shaft. To continue movement of the plunger shaft within the syringe barrel, the plunger shaft must be rotated about a longitudinal axis relative to the syringe barrel, thus causing the externally threaded ring to become threaded into and through the internally threaded insert and permitting further movement of the plunger shaft within the syringe barrel.  
         [0021]     Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention, from the claims, and from the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     In the accompanying drawings that form part of the specification, and in which like numerals are employed to designate like parts throughout the same,  
         [0023]      FIG. 1A  is a partial cross-sectional view of a first embodiment of an exemplary syringe according to the present invention showing the syringe prior to discharge of a diluent or first constituent, with the releasable stop being in the deployed position;  
         [0024]      FIG. 1B  is a partial cross-sectional view of the embodiment of  FIG. 1A  showing the displacement of the syringe plunger being limited by the releasable stop;  
         [0025]      FIG. 1C  is a partial cross-sectional view of the embodiment of  FIG. 1A  showing the releasable stop in the retracted position, permitting further displacement of the diluent or first constituent from the syringe;  
         [0026]      FIG. 1D  is a cross-sectional view in reference to line  1 D- 1 D of  FIG. 1B ;  
         [0027]      FIGS. 2A-2E  are partial cross-sectional views of a syringe system including an embodiment of a syringe similar to that shown in  FIG. 1A  in an exemplary combination with a second syringe according to another preferred embodiment of the invention and illustrating the operation of the syringe system to mix and deliver a mixture of diluent and drug and to dispense a residual supply of diluent;  
         [0028]      FIGS. 3A and 3B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0029]      FIG. 3C  is a cross-sectional view in reference to line  3 C- 3 C in  FIG. 3B ;  
         [0030]      FIGS. 4A and 4B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in a deployed and retracted positions, respectively;  
         [0031]      FIG. 4C  is a cross-sectional view in reference to line  4 C- 4 C in  FIG. 4B ;  
         [0032]      FIG. 5A  is a perspective view of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in a deployed position;  
         [0033]      FIG. 5B  is a partial section showing the releasable stop in a retracted position;  
         [0034]      FIG. 5C  is a cross-sectional view of a plunger shaft in accordance with the embodiment of  FIG. 5A ;  
         [0035]      FIG. 5D  is a top view in reference to line  5 D- 5 D in  FIG. 5B ;  
         [0036]      FIGS. 6A and 6B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0037]      FIG. 6C  is a cross-sectional view in reference to line  6 C- 6 C in  FIG. 6B ;  
         [0038]      FIG. 7A  is a perspective view and  FIG. 7B  is a top view of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0039]      FIGS. 8A and 8B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0040]      FIGS. 9A and 9B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0041]      FIGS. 10A and 10B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0042]      FIGS. 11A and 11B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0043]      FIGS. 12A and 12B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0044]      FIGS. 12C and 12D  are cross-sectional views in reference to line  12 C- 12 C in  FIG. 12A  and line  12 D- 12 D in  FIG. 12B , respectively;  
         [0045]      FIGS. 13A and 13B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0046]      FIGS. 13C and 13D  are cross-sectional views in reference to line  13 C- 13 C in  FIG. 13A  and line  13 D- 13 D in  FIG. 13B , respectively;  
         [0047]      FIG. 14A  is a perspective view and  FIG. 14B  is a partial sectional view of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively;  
         [0048]      FIGS. 15A and 15B  are partial cross-sectional views of an exemplary syringe according to another preferred embodiment of the invention showing a releasable stop in deployed and retracted positions, respectively.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0049]     While this invention is susceptible of embodiment in many different forms, this specification and the accompanying drawings disclose only some specific forms as examples of the invention. The invention is not intended to be limited to the embodiments so described. The scope of the invention is pointed out in the appended claims.  
         [0050]     Figures illustrating the apparatus show some mechanical elements that are known and that will be recognized by one skilled in the art. The detailed descriptions of such elements are not necessary to an understanding of the invention, and accordingly, are herein presented only to the degree necessary to facilitate an understanding of the novel features of the present invention.  
         [0051]     An exemplary syringe according to a first embodiment of the present invention is illustrated in  FIGS. 1A through 1D  and is designated generally by the reference numeral  5 . The syringe  5  includes a generally cylindrical syringe barrel  10  having a discharge end  12  defining a delivery passage  14  therein. Opposite the discharge end  12  is an open end  16  of the syringe barrel  10  and a barrel flange  18  which may typically have a circular or rectangular shape extending from the periphery of the open end  12 . A removable closure  21  is provided on the discharge end  12  for sealing the syringe  5  prior to discharge of the diluent  23  contained therein. The barrel  10  is preferably fabricated by injection molding a synthetic polymer, such as a thermoplastic material. Alternatively, the barrel  10  may be made of other suitable materials including glass.  
         [0052]     Reciprocally disposed in the open end  16  of the syringe barrel  10  is a plunger  20  for permitting displacement of a piston  22  affixed to an end thereof, typically using a threaded connector. Extending from the piston  22  is a plunger shaft  24  having a push button  26  at an end opposite piston  22 . The push button  26  permits a user to apply a displacing force to the plunger  20  and thereby displace diluent  23  from the diluent chamber  25  defined by the syringe barrel  10 , discharge end  12  and piston  22 .  
         [0053]     In accordance with the invention, a releasable stop or limiting means  30  is operatively associated with the plunger shaft  24  to selectively limit the displacement of the plunger shaft  24  relative to the syringe barrel  10 . Releasable stop  30  comprises a thumb lever  32 , which is preferably integrally molded with the plunger shaft  24  and movably disposed within a cavity  33  formed in the plunger shaft  24 . The thumb lever  32  includes a pivot end  34  about which the thumb lever  32  pivots relative to the plunger shaft  24  between a deployed position, illustrated in  FIGS. 1A, 1B  and  1 D, and a retracted position, illustrated in  FIG. 1C . The thumb lever  32  also includes an engaging end  36  oppsite the pivot end  34  for ultimately engaging the barrel flange  18 . The thumb lever includes a wider gripping portion  37  and a generally narrow guide portion  39  extending within the cavity  33 . The thumb lever  32  also includes a bi-stable, over-center detent member  40  molded therewith for limiting outward movement of the thumb lever  32  relative to the plunger shaft  24  and for providing a biasing force for maintaining the thumb lever  32  in its deployed position and for retaining the thumb lever  32  in the retracted position. The plunger shaft  24 , push button  26  and thumb lever  32  are preferably molded as an integral homogenous one-piece construction from polypropylene or acetal. Over-center detent  40  includes a linkage  41  which undergoes a slight compression, and pivots as thumb lever  32  moves from its deployed position to its retracted position. The linkage  41  is constructed to provide a biasing force to the thumb lever  32  in an outwardly direction (toward the deployed position) and provide a biasing force in an opposite inwardly direction (toward the retraced position) once the linkage  32 , pivots beyond a center point (as a result of pressure upon the thumb lever  32  from the user) relative to the diluent syringe  10 . In this embodiment, the thumb lever  32  is maintained in the retracted position without any aid from the user.  
         [0054]     Referring now to  FIG. 1C , after a user depresses the thumb lever  32  to the retracted position as shown, the engaging end  36  is moved to an interior position within the cavity  33  formed in the plunger shaft  24 . Thus, the plunger  20  is permitted further movement within the syringe barrel  10  and a residual supply  41  of diluent can be displaced from the syringe barrel  10 .  
         [0055]      FIGS. 2A-2E  illustrate an exemplary syringe similar to that described relative to  FIGS. 1A-1D  in combination with a drug syringe, generally referenced by the numeral  60 , and having a drug syringe barrel  61  containing a supply of dry drug  65 . Drug syringe barrel  61  has an open end  63  opposite a dispensing end This general combination, exclusive of the features of the present invention, is described in U.S. Pat. No. 5,876,372, the subject matter and entire writing of which is hereby incorporated by reference. Referring to  FIGS. 2A and 2B , during a mixing stage, a closure, typically a luer cap, is first removed from the diluent syringe  10  thereby permitting egress of the diluent through the discharge passage  14 . A back cap (not shown) is also removed from the secondary syringe  60  to permit insertion of the diluent syringe  10  in such a fashion that the discharge end  12  extends through a passage in a stopper element  62  provided on the secondary syringe  60 . This ensures a secure fluid connection between the diluent syringe  10  and the mixing chamber  64  defined between the stopper  62  and a delivery end  64  of the drug syringe  60 .  
         [0056]     Referring to  FIG. 2B , the next step in the operation of the device provides for mixing of the diluent  23  and dry drug  66 . During this step the plunger  20  of the diluent syringe  10  is depressed thereby displacing a first amount of diluent from the chamber  25  of the diluent syringe  10  and into the mixing chamber  66  of the secondary syringe  60  where the diluent  23  can be mixed with the dry drug. As diluent  23  is discharged into the mixing chamber  66 , the stopper  62  of the drug syringe  60  is displaced (upwards in  FIG. 2B ) relative to the drug syringe barrel  61 . According to the invention, the releasable stop  30  provided on the diluent syringe  10  provides for discharge of a first amount of diluent from the chamber  25 —an amount necessary for proper reconstitution of the dry drug  65 —while a residual supply  41  of diluent remains in the diluent chamber  25 .  
         [0057]     Referring now to  FIG. 2C , the mixture of the first amount of diluent  23  and the dry drug  65  is dispensed from the secondary syringe as, the user applies a continued downward force on the push button  26 , thereby displacing the diluent syringe  10  and stopper  62  of the drug syringe  60  within the drug syringe barrel  61  to displace the now-mixed drug from the mixing chamber  66  and out through the dispensing passage  67  formed in the dispensing end of the secondary syringe. As will be appreciated by those of ordinary skill in the art, a residual amount of mixed drug remains in the mixing chamber  66  and within the dispensing passage  67  formed in the dispensing end  64  of the drug syringe  60 .  
         [0058]     Referring to  FIGS. 2D and 2E , the residual supply of mixed drug may be flushed out using the residual supply  41  of diluent that is retained in the diluent syringe  10 . To do this, an operator will depress the thumb lever  30  thereby permitting the plunger to undergo a further displacement within the first barrel and permitting the residual supply of diluent  41  to flow into the mixing chamber  66  to flush any residual drug out from the mixing chamber for delivery to the patient. As will be understood by those of ordinary skill in the arts the additional diluent does not create any adverse conditions within the patient and the amount of drug provided in the syringe can be closely controlled to permit a precise amount of drug to be delivered to the patient assuming that the entire amount of drug will be dispensed from the mixing chamber. This is contrast to the prior art where a residual supply of drug would remain in the syringe and therefore the actual amount of drug provided in the syringe during packaging would have to be above the amount required for patient dosage.  
         [0059]     The remaining figures illustrate various other exemplary ways of providing the releasable stop or limiting means on a syringe in accordance with the invention.  FIGS. 3A through 3C  illustrate an exemplary syringe according to another preferred embodiment of the invention. Here a releasable stop is provided in the form of a compressible cylindrical element  120  which includes an outer surface  122  and an inner surface  124 . The compressible cylindrical element  120  includes a slot forming two opposing slot faces  130  which are tapered to provide an apex  132  at a central portion thereof. The compressible element  120  is disposed around the shaft  24  of the plunger  20  and is compressible to a smaller diameter such that an insertion end  134  of the compressible element  120  may be inserted into the inner diameter of the syringe barrel  10 . Thus a user may apply lateral force to the sides of the cylindrical element  120  to force the slot faces together and compress an end of the cylindrical element  120 , thereby permitting the insertion end  134  to be inserted into the syringe barrel  10  with continued downward force on the plunger button resulting in continued compression of the cylindrical element until it eventually assumes a retracted position and resides entirely within the syringe barrel  10  as shown in  FIG. 3B . As will be understood by those of ordinary skill in the art, the compressible cylindrical element provides for a releasable stop which is adapted to move between a deployed position as shown in  FIG. 3 -A and a retracted position shown in  FIG. 3 -B. Thus the compressible element interacts with the plunger  24  to provide for displacement of a first amount of diluent or other constituent from the syringe barrel while retaining a residual supply of diluent therein.  
         [0060]     Referring now to  FIGS. 4A-4C , there is illustrated another exemplary syringe according to yet another embodiment of the present invention. In this embodiment, the releasable stop  30  or limiting means is provided in the form of a pair of pivotable flaps  140  depending from a central portion  142  of the plunger shaft  24 . The flaps  140  may be molded with a live joint which may comprise a molded plastic area of slight dimension enabling the elements to pivot with respect to the central portion  142  of the plunger shaft. As shown in  FIG. 4A , the flaps  140  are in a deployed position and extend outward from the central portion of the plunger shaft where an engaging end  143  of each of the flaps  140  may engage the barrel flange  18 . Thus in the deployed position, the flaps  140  limit the displacement of the plunger shaft  24  within the syringe barrel. Referring now to  FIGS. 4B and 4C , the releasable stop  30  is shown in a retracted position in which the flaps  140  are pivoted to an inward position in such a fashion that the engaging ends  143  of the flaps  140  no longer engage the barrel flange  18  and the plunger  24  is therefore permitted to be further displaced within the syringe barrel  10 .  
         [0061]     Referring now to  FIG. 5A , there is illustrated yet another exemplary syringe in accordance with yet another preferred embodiment of the present invention. In this embodiment, the releasable stop  30  or limiting means is provided as a pivotable lever  150  that is integrally molded with the plunger shaft  24 . The lever is provided with a central pivot  151  which may include a living hinge molded of a slight dimension to thereby permit a slight deformation and therefore pivoting of the lever  150 . The lever also includes an engaging end  152  for engaging a corresponding portion of the syringe barrel flange  18 . As shown in  FIG. 5C , the lever extends from a plunger shaft  24  of generally X-shaped cross section. At an end opposite the engaging end  152 , the pivotable lever  150  is provided with an actuating handle  153  which extends upward beyond the plunger push button  26 .  FIG. 5A  illustrates the lever  150  in its deployed position in which the stopping surface or engaging surface of the lever  150  is positioned to engage the syringe barrel flange  18 . In accordance with a feature of this embodiment, as shown in  FIG. 5D , the push button  26  is provided with a recess  154  for receiving the actuator handle  153  and thereby permitting the actuator handle  153  to undergo a radial inward displacement beyond the peripheral extent of the push button  26 . Also in accordance with this aspect of the invention, the push button  26  is provided with a pair of retaining protuberances  155  on the recess to retain the actuator handle  153  therein unless overcome by suitable force applied by the user. Thus to move the lever  150  from its deployed position to its retracted position, the user must overcome the resistive forces provided by the protuberances  155  and thereby deform the protuberances or the actuating handle  153  to such a degree that lateral movement of the actuating lever  153  in an outward radial direction relative to the push button  26  is permitted. Such movement therefore results in the lever  150  assuming a retracted position, as shown in  FIG. 5B  in which the engaging surface is no longer positioned to engage the syringe barrel flange  18  and wherein the plunger shaft is permitted to undergo further displacement downward into the syringe barrel  10 . Those of ordinary skill will recognize that the contour of the lever will be provided in such a fashion to provide minimal interference with the syringe barrel  10  as the plunger  20  is displaced downward into the syringe barrel  10 .  
         [0062]     Referring now to  FIGS. 6A-6C , there is illustrated another exemplary syringe in accordance with yet another preferred embodiment of the invention. In this embodiment, the releasable stop  30  or limiting means is provided as a flexible lever  160  extending from and formed integrally with the plunger shaft  24 . Preferably, the lever  160  is formed with a radially outward bias such that a force is required to displace the lever in a radially inward direction.  FIG. 6A  illustrates the lever  160  in a deployed position in which a central portion  161  of the lever is provided with a notch or recess  162  which is adapted to engage complementarily-shaped annular lip  163  formed on the syringe barrel  10 . As shown in  FIG. 6A , the push button  26  of the plunger is provided with a recess  164  similar to the recess  154  described relative to the embodiment shown with respect to FIGS.  5 -A through  5 -D. Recess  164  permits a further radially inward displacement of an actuator handle  165  of the lever  160  in order to move the lever  160  from the deployed position shown in  FIG. 6A  to the retracted position shown in  FIG. 6B .  FIG. 6 -C is a cross section taken along lines  6 C- 6 C of  FIG. 6A  illustrating that the lever is formed as an integral part of the plunger shaft which has a generally X-shaped cross section. To move the actuator lever  160  from the deployed position to the retracted position, the user applies a radially inward force to the actuator handle  165  thereby displacing the actuator lever  160  to a radially inward position and thereby disengaging the notch or recess  162  of the lever  160  from the annular lip  163  formed on the syringe barrel. It will be noted that a slightly upward force may have to be applied to the plunger shaft  24  in order to remove the recess  162  from the annular lip  163 . When the actuator is moved to a radially inward position, the plunger is permitted to undergo further displacement downward and the lever being moved to its retracted position may be further displaced within the syringe barrel.  
         [0063]      FIGS. 7A and 7B  illustrate another exemplary syringe according to yet another preferred embodiment of the present invention. A releasable stop  200  is provided as a pair of projections  210  extending downward from the push button  26  of the plunger  20  to engage the barrel flange  18 . The solid lines in  FIGS. 7A and 7B  illustrate the releasable stop  200  in a deployed position in which push button projections  210  are positioned to engage the barrel flange  18  and thereby prevent further downward movement of the plunger. In accordance with this aspect of the invention, projections  210  are moved to a retracted position, shown in phantom in  FIGS. 7A and 7B , by rotation of the plunger  20  and therefore the plunger button  26  about a longitudinal axis (A) relative to the syringe barrel  10 . After approximately 90-degrees of rotation in either a clockwise or counterclockwise direction, push button projections  210  are moved to a position in which their respective engaging ends  212  avoid engagement with the barrel flange  18 . Thus, the plunger is permitted to undergo further downward displacement upon application of a suitable downward force by a user to the plunger pushbutton. It will be recognized by those of ordinary skill in the art that the barrel flange  18  is preferably formed as an element that is not radially symmetrical, thus permitting selective engagement of the projections  210 . However, the barrel flange could be provided as a generally radially symmetrical element, but having holes formed therein for receiving the projections  210  and thereby permitting further downward displacement of the plunger when the projections are rotated to the retracted position.  
         [0064]      FIGS. 8A and 8B  illustrate another exemplary syringe according to yet another preferred embodiment of the present invention. In this embodiment, a releasable stop  220  is provided as a bendable or breakable flange projection  222  extending from the transverse flange of the syringe barrel. Referring to  FIG. 8A , in a deployed position, the flange projection  222  extends in a direction generally parallel to the displacement direction of the plunger shaft and is disposed at a radial location relative to the plunger shaft axis such that an engaging end  224  of the projection  222  is positioned to engage the underside of the plunger push button. Flange projection  222  may be formed of the same material as and integrally molded with the syringe barrel. Preferably, flange projection  222  is molded with a weakened or frangible portion  226  enabling relatively quick and easy removal by a user. Flange projection  222  is thus movable to a retracted position shown in  FIG. 8B  in which flange projection  222  is removed or separated from the transverse flange of the syringe, thereby permitting continued movement of the plunger within the syringe barrel and corresponding displacement of a residual supply of diluent or other constituent from the syringe barrel.  
         [0065]      FIGS. 9A and 9B  illustrate another exemplary syringe according to yet another preferred embodiment of the present invention. In this embodiment, a releasable stop  230  is provided in the form of two cooperating rings, one formed on the plunger shaft and one formed on the interior of the syringe barrel. A plunger shaft ring  232  may be formed integrally with the plunger shaft and is provided with a radial extent, measured outward from the plunger shaft axis, that is less than the general interior diameter of the syringe barrel. Likewise, the syringe barrel is formed with an interior barrel ring  234  dimensioned to interfere to a controlled degree with the plunger shaft ring  232 . Preferably, the dimensions, including the radial extent of the respective rings are selected to require a substantial force before deformation and therefore release of the plunger occurs to prevent inappropriate deformation of the rings. One or both of the plunger shaft ring  232  and/or the interior barrel ring  234  undergoes a deformation and thus moves to a retracted position when a suitable force is applied to the plunger push button to permit additional displacement of the plunger within the syringe barrel and corresponding displacement of the residual supply of diluent or other constituent from the syringe barrel.  
         [0066]      FIGS. 10A and 10B  illustrate yet another exemplary syringe according to yet another preferred embodiment of the present invention. In this embodiment, a releasable stop  240  is provided as a breakaway projection  242  extending from the plunger shaft. The deployed position of breakaway projection  242  is illustrated in  FIG. 10A , where the projection extends radially outward from the plunger shaft and is adapted to engage the transverse flange of the syringe barrel and thereby prevent further displacement of the plunger within the syringe barrel. The retracted position of the breakaway projection  242  is illustrated in  FIG. 10B . Here, the breakaway projection  242  is separated from the plunger shaft, which is permitted to undergo additional displacement within the syringe barrel. The breakaway projection  242  is preferably provided with a weakened or frangible portion  244  securing it to the plunger shaft in such a manner that upon application of a suitable force, the breakaway projection  242  may be separated from the plunger shaft. As will be appreciated by those of ordinary skill in the art, separation of the breakaway projection from the plunger shaft preferably occurs by application of a suitable force on the plunger push button and a fracturing or separating force being applied to the breakaway projection  242  by the lateral flange of the syringe barrel.  
         [0067]      FIGS. 11A and 11B  illustrate yet another exemplary syringe according to yet another preferred embodiment of the present invention. In this embodiment, a releasable stop  250  is provided as a leaf spring  252  formed integrally with the plunger shaft and cooperating with an annular ledge  255  formed on the interior of the syringe barrel. Leaf spring  252  preferably has a generally curved shape and molded with the plunger shaft to be biased radially outward. A recess  256  is also formed in the plunger shaft to accommodate the leaf spring and thereby permit radially inward displacement of the leaf spring  252  relative to the plunger shaft. Leaf spring  252  includes a generally round or arcuate projection  254  adapted to engage the annular ledge  254 . Annular ledge  255  is dimensioned to provide a relatively large displacement of leaf spring  252  as leaf spring  252  moves from a deployed position, shown in  FIG. 11A  to a retracted position, during engagement with annular ledge  255 .  
         [0068]      FIGS. 12A-12D  illustrate yet, another exemplary syringe according to yet another preferred embodiment of the present invention. According to this embodiment, a releasable stop  260  is provided as a removable element  262  cooperating with a slot  264  formed in the plunger shaft. Preferably, removable element  262  is formed as a generally U-shaped element having a recess  266  formed therein. Recess  266  cooperates with the slot  264  formed in the plunger shaft to retain the removable element  262  in a deployed position shown in  FIGS. 12A and 12C , where the removable element  262  is positioned to engage the lateral flange of the syringe barrel.  FIGS. 12B and 12D  illustrate the removable element  262  in a retracted position, where the removable element  262  is removed from the slot  264  formed in the plunger shaft. It will be recognized by those of ordinary skill that removal of the removable element  262  may occur by simple manual operation by a user of the syringe. With the removable element  262  removed to its retracted position, further displacement of the plunger within the syringe barrel and the corresponding displacement of diluent or constituent may occur.  
         [0069]      FIGS. 13A-13D  illustrate yet another exemplary syringe according to yet another preferred embodiment of the present invention. In this embodiment, a releasable stop  270  or limiting means is provided as a breakaway cylindrical element  272  disposed around the plunger shaft  24  in a deployed position shown in  FIGS. 13A and 13C . The breakaway cylindrical element  27 , has an outer diameter (D) that is greater than the inner diameter (d) of the syringe barrel  10  such that an engaging end  273  of the breakaway cylindrical element  272  engages the syringe barrel flange  18  and an opposite end  274  of the cylindrical element engages the plunger push button  26 . In this deployed position, movement of the plunger relative to the syringe barrel is limited. The breakaway cylindrical element  270  is moved to its retracted position, shown in  FIGS. 13B and 13D  by fracturing the cylindrical element, preferably into plural portions  272 A and  272 B along one or more weakened or frangible lines or areas  275 . Fracture of the cylindrical element permits removal and further movement of the plunger  20  within the syringe barrel  16 .  
         [0070]      FIGS. 14A and 14B  illustrate yet another exemplary syringe according to yet another preferred embodiment of the invention. In this embodiment, a releasable stop  280  is provided as a pair of pivotable finger tabs  281  extending outward from a central portion  282  of the plunger shaft  24 . When the finger tabs  281  are pivoted to a deployed position, shown in solid lines, stop edges  283  on the finger tabs are adapted to engage the syringe barrel. When the finger tabs are pivoted to a retracted position, shown in phantom, the stop edges  283  are disposed in a position where they do not engage the syringe barrel flange  18  and further movement of the plunger shaft  24  within the syringe barrel  10  is permitted.  
         [0071]      FIGS. 15A and 15B  illustrate yet another preferred embodiment of the invention. In this embodiment, the releasable stop  290  or limiting means is provided as an externally threaded ring  291  on the plunger shaft  24  which cooperates with an internally threaded insert  292  on the syringe barrel  10 . Upon downward movement of the plunger shaft  24  within the syringe barrel  10 , the externally threaded ring  291  engages the internally threaded insert  292  and prevents further movement of the plunger shaft  24 . To continue movement of the plunger shaft  24  within the syringe barrel  10 , the plunger shaft  10  must be rotated about a longitudinal axis (A) relative to the syringe barrel, thus causing the externally threaded ring  291  to become threaded into and through the internally threaded insert  292  and permitting further movement of the plunger shaft within the syringe barrel.  
         [0072]     Attached are 11 sheets of schematic figure drawings to be incorporated in the application. Such sheets are labeled Exhibit A.  
         [0073]     It will be readily apparent from the foregoing detailed description of the invention and from the illustrations thereof that numerous variations and modifications may be effected without departing from the true spirit and scope of the novel concepts or principles of this invention, the scope of which is defined in the appended claims.