Abstract:
The present invention is directed to syringes that have needles that retract after an injection to provide safe syringes which reduce the possibility of a sharps injury through retraction of the needle after use into the barrel of the syringe. The syringe includes a barrel, a needle assembly positioned in the hollow body, a fluid retention system positioned in the hollow body, and a plunger assembly that moves in the hollow body. A fluid retention system retains in the syringe any fluid remaining in the needle after the plunger assembly approaches tha distal end of the hollow body. The fluid retention system includes a variable volume vacuum cavity in fluid communication with the needle passage, the vaccum cavity expanding prior to retraction of the needle assembly and providing a pressure drop in the needle, tending to draw into the hollow body any fluid remaining in the needle.

Description:
[0001]    Under 35 U.S.C. 119(e), applicant claims the benefits of and priority based on provisional patent application serial No. 60/260,096, filed Jan. 5, 2001 and entitled “Syringe That Inhibits Fluid Loss Injections”, and provisional patent application serial No. 60/264,554, filed Jan. 26, 2001 and entitled “Syringe That Inhibits Fluid Loss After Injection,” the disclosures of which are hereby incorporated by reference as if fully disclosed herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The field of the present invention relates generally to apparatus and methods for protection against an accidental sharps injury or stick from an unprotected needle and for inhibition of fluid loss from a syringe.  
           [0003]    For some time, the art has recognized the desirability of protecting personnel from accidental sharps injuries or needle sticks and against contact with fluid that might leak, drip, or be sprayed from a syringe after the syringe is used to deliver an injection. Oftentimes, after a syringe is used to inject fluid into a patient, some fluid remains in the syringe, particularly at the tip of the needle. This fluid may include the fluid injected into the patient from the syringe, and may also include bodily fluids from the patient, such as blood. Any fluids remaining in the syringe after use of the syringe may leave the syringe, such as by leaking, spraying or dripping from the syringe, and may contact persons or objects in the area. Syringes with retractable needles may be especially prone to this loss of fluid when the needle quickly retracts into the barrel of the syringe after injection.  
           [0004]    More recently, concerns have been expressed about the possibility of transmitting serious or potentially fatal infection as a result of such accidents. Most recently, legislation requiring the use of safe needle technology is pending in a number of States and before the Occupation Safety and Health Administration. Although the art has recognized the desirability of protecting against accidental sharps injuries or needle sticks, it is believed that available devices allow fluid spraying or other loss from a syringe after injection, or require the syringe user to carry out further or non-routine steps in addition to those required to use a conventional syringe to inhibit fluid loss. The further a device is from routine operation, the less accepted it is by the medical community.  
           [0005]    Various retractable syringes are disclosed in co-owned PCT Application No. PCT/US97/20646, International Publication No. WO 98/20923, the disclosure of which is hereby incorporated by reference as if fully set forth herein. While these devices operate successfully, it has been found that the devices may be improved by inhibiting fluid loss after injection.  
           [0006]    Another concern with prior art devices is the complicated and costly manufacturing processes. With the tremendous number of syringes and other needle devices used by the medical community, any substantial rise in cost of the products is undesirable and generally unacceptable.  
           [0007]    Accordingly, there is a need for syringes, both retractable and non-retractable, that inhibit or prevent fluid loss after the syringe is used and that can be used in a conventional manner and do not require elaborate manufacturing.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention is directed to syringes that have needles that retract after an injection. Various configurations are contemplated, presented as preferred embodiments. A principle object of the present invention is to provide safe syringes which reduce the possibility of a sharps injury through retraction of the needle after use into the barrel of the syringe. Other and further objects and advantages will appear hereinafter  
           [0009]    The syringe includes a barrel, a needle assembly positioned in the hollow body, a fluid retention system positioned in the hollow body, and a plunger assembly that moves in the hollow body. In some embodiments, the needle assembly is retractable.  
           [0010]    In a first separate aspect of the present invention, a fluid retention system retains in the syringe any fluid remaining in the needle after the plunger assembly approaches the distal end of the hollow body.  
           [0011]    In a second separate aspect of the present invention, the fluid retention system includes a variable volume vacuum cavity in fluid communication with the needle passage, the vacuum cavity expanding prior to retraction of the needle assembly and providing a pressure drop in the needle, tending to draw into the hollow body any fluid remaining in the needle.  
           [0012]    In a third separate aspect of the present invention, the fluid retention system includes a variable volume vacuum cavity that, during initial retraction of the mandrel, is substantially sealed against fluid communication other than to provide fluid communication with the needle.  
           [0013]    In a fourth separate aspect of the present invention, the fluid retention system includes a cylinder defined within the needle assembly and a piston connected with the mandrel and slidingly and sealingly engaging the cylinder during initial retraction of the mandrel.  
           [0014]    In a fifth separate aspect of the present invention, a method of inhibiting fluid loss from a syringe following injection includes the step of reducing pressure in the needle passage after the plunger assembly has been substantially completely depressed.  
           [0015]    In a sixth separate aspect of the present invention, it is contemplated that combinations of the foregoing separate aspects may be incorporated into a single embodiment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is an expanded view of a retractable syringe in accordance with the present invention.  
         [0017]    [0017]FIG. 2 is a cross sectional view of one embodiment of the syringe barrel.  
         [0018]    [0018]FIG. 3 is an exploded view of the closed end of the syringe barrel of FIG. 2.  
         [0019]    [0019]FIG. 4 is an exploded view of another embodiment of the closed end of the syringe barrel.  
         [0020]    [0020]FIG. 5 is a side elevation of one embodiment of the needle assembly.  
         [0021]    [0021]FIG. 6 is a side elevation of another embodiment of the needle assembly.  
         [0022]    [0022]FIG. 7 is an exploded view of the closed end of another embodiment of the syringe barrel and another embodiment of the needle assembly.  
         [0023]    [0023]FIG. 8 is an isometric view of one embodiment of a plunger assembly.  
         [0024]    [0024]FIG. 9 is a cross sectional view of the plunger frame of the plunger assembly of FIG. 8.  
         [0025]    [0025]FIG. 10 is a cross sectional view of another embodiment of a plunger assembly.  
         [0026]    [0026]FIG. 11 is an enlarged view of part of the plunger assembly of FIG. 10.  
         [0027]    [0027]FIG. 12 is one embodiment of a retraction assembly.  
         [0028]    [0028]FIG. 13 is one embodiment of a catch member.  
         [0029]    [0029]FIG. 14 is an exploded view of the sealing end of the plunger frame of FIG. 9.  
         [0030]    [0030]FIG. 15 is a cross sectional view of another embodiment of the sealing end of a plunger frame.  
         [0031]    [0031]FIG. 16 is a cross sectional view of the plunger assembly of FIG. 8.  
         [0032]    [0032]FIG. 17 is an exploded view of the first end of the plunger assembly of FIG. 16.  
         [0033]    [0033]FIG. 18 is a side elevation view of one embodiment of the mandrel of the present invention.  
         [0034]    [0034]FIG. 19 is a side elevation view of another embodiment of the mandrel and a cross sectional view of the mandrel seal of the present invention.  
         [0035]    [0035]FIG. 20 is a side elevation view of one embodiment of the catch member of the present invention.  
         [0036]    [0036]FIG. 21 illustrates an embodiment of the syringe in an assembled but unused condition.  
         [0037]    [0037]FIG. 22 illustrates the syringe of FIG. 21 upon initial substantial depression of the plunger assembly.  
         [0038]    [0038]FIG. 23 illustrates the syringe of FIG. 21 after loading of the syringe.  
         [0039]    [0039]FIG. 24 illustrates the forward portion of the syringe of FIG. 21 as it is inserted in a patient.  
         [0040]    [0040]FIG. 25 illustrates the forward portion of the syringe of FIG. 21 upon substantial injection depression of the plunger assembly.  
         [0041]    [0041]FIG. 26 illustrates the forward portion of the syringe of FIG. 21 after the mandrel tip has entered the needle assembly cavity.  
         [0042]    [0042]FIG. 27 illustrates the forward portion of the syringe of FIG. 21 upon complete depression of the plunger assembly.  
         [0043]    [0043]FIG. 28 illustrates the syringe of FIG. 21 after retraction of the needle.  
         [0044]    [0044]FIG. 29A is a side elevation of a first embodiment of a fluid retention system.  
         [0045]    [0045]FIGS. 29B and 29C are a cross-sectional view of the fluid retention system of FIG. 29A.  
         [0046]    [0046]FIG. 30 is a cross-sectional view of a second embodiment of a fluid retention system.  
         [0047]    [0047]FIG. 31 is a side elevation of a third embodiment of a fluid retention system.  
         [0048]    [0048]FIG. 31A is a top view of the fluid retention system of FIG. 31.  
         [0049]    [0049]FIGS. 32A and 32B are cross-sectional views of a fourth embodiment of a fluid retention system.  
         [0050]    [0050]FIG. 33A is a cross-sectional view of a sixth embodiment of a fluid retention system.  
         [0051]    [0051]FIG. 33B is a perspective view of the mandrel of the fluid retention system of FIG. 33A.  
         [0052]    [0052]FIG. 34A is a cross-sectional view of a seventh embodiment of a fluid retention system.  
         [0053]    [0053]FIG. 34B is a perspective view of the washer of the fluid retention system of FIG. 34A.  
         [0054]    [0054]FIG. 35A is a cross-sectional view of an eighth embodiment of a fluid retention system.  
         [0055]    [0055]FIG. 35B is a perspective view of the fluid retention system of FIG. 35A. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0056]    The preferred embodiments will be described with reference to drawing figures.  
         [0057]    A. Retractable Syringes  
         [0058]    Various retractable syringes are disclosed in co-owned PCT Application No. PCT/US97/20646, International Publication No. WO 98/20923, the disclosure of which is hereby incorporated by reference as if fully set forth herein.  
         [0059]    Referring to FIG. 1, an embodiment of a syringe assembly  8  is comprised generally of a cap member  10 , a syringe barrel  40 , a needle assembly  70 , and a plunger assembly  100 .  
         [0060]    The cap member  10  includes an open, mating end  12  and a closed cone section  14 . The mating end  12  is preferably configured to slidingly engage the syringe barrel  40 . Alternatively, the mating end  12  may be provided with threads (not shown) which may engage corresponding threads (not shown) on the syringe barrel  40 . Other cap and corresponding barrel configurations are known and may also be employed. The closed cone section  14  preferably includes a plurality of ribs  16  which assist gripping of the cap member  10 .  
         [0061]    Referring to FIGS. 2 and 3, one embodiment of the syringe barrel  40  is comprised of a hollow body portion  42  which has a closed end  44  and an open end  54 . An external stabilized grip member  56  extends from the body  42  adjacent to, but forward of the open end  54 . The grip member  56  may have various configurations, the preferred elliptical configuration being shown. An internal annular shoulder  60  is defined in the hollow body  42  at approximately the same position as the grip member  56 . The open end  54  defines an open cavity  58  rear of the internal annular shoulder  60 . An internal annular lip  62  may also be provided adjacent the open end  54 .  
         [0062]    The closed end  44  is defined by a truncated cone  46  which includes a truncating plane having an aperture  48 . Referring to FIG. 3, in one embodiment of the syringe barrel  40 , a retaining groove  50  is located on the interior of the syringe barrel  40  at a position adjacent to the closed end  44 . The retaining groove  50  retains the needle assembly  70  in position during use as will be described in more detail hereinafter. In one embodiment, the closed end  44  proximate the truncated cone  46  has a generally convex taper  47  and at least one internal ramp  52 , the functions of which will be described hereinafter.  
         [0063]    Referring to FIG. 5, in another embodiment of the syringe barrel  40 , retaining fingers  51  are attached to the interior of the syringe barrel  40  at a position adjacent to the closed end  44 . The retaining fingers  51  retain the needle assembly  70  in position prior to retraction of the needle as will be described in more detail hereinafter.  
         [0064]    [0064]FIG. 5 depicts one embodiment of the needle assembly  70 . In this embodiment, the needle assembly  70  is comprised of a needle  72  which is centrally positioned in the hollow projection  74 . The hollow projection  74  generally complements the interior of the truncated cone  46  of the syringe barrel  40 . Immediately adjacent to the projection  74  is a sealing ring  76 . The projection  74  and the sealing ring  76  preferably are formed as a unitary molding, but may be formed as separate components. The interior passage  78  of the needle assembly  70  is in fluid communication with the hollow needle  72  and the geometrically configured cavity  80  extending into the rear surface of the needle assembly  70 . The cavity  80  preferably has a cylinder portion  80   a  and a hemispheric portion  80   b  which complement the geometrically configured tip  176  of the plunger mandrel  170  (shown in FIG. 18). The needle assembly  70  is positioned within the syringe barrel  40  such that the needle  72  extends through the aperture  48  and the sealing ring  76  is positioned in and retained by the retaining groove  50 . Preferably, the sealing ring  76  sealingly engages the truncated cone  46  of the syringe barrel  40  and the hollow projection  74 .  
         [0065]    [0065]FIG. 6 depicts another embodiment of the needle assembly  70 . In this embodiment, the needle assembly  70  has a needle seal lip  75  that engages and retains a needle seal  77 . The needle seal lip  75  may comprise an annular collar or other surface that retains the needle seal  77 . When the needle assembly  70  is inserted into the truncated cone  46  of the syringe barrel  40 , the needle seal  77  is positioned between and sealingly engages the needle seal lip  75  and the closed end  44  of the syringe barrel  40 . Alternately, the needle seal  77  may sealingly engage the hollow projection  74  and the closed end  44  of the syringe barrel  40 . To facilitate sealing engagement of the needle seal  77  with the closed end  44  of the syringe barrel  40 , the closed end  44  may have a shelf  43  (shown in FIG. 4) that engages the needle seal  77 . The needle seal  77  is preferably annular, and may comprise an O-ring.  
         [0066]    Other embodiments of the syringe barrel  40  and needle assembly are shown in FIG. 7. In these embodiments, a needle seal  53  (shown in cross-section) is inserted through the aperture  48  at the closed end  44  of the syringe barrel  40 . The needle seal  53  is positioned between and sealingly engages the hollow projection  74  and the truncated cone  46 . In this embodiment, a smaller end  55  of the truncated cone  46  opens up to receive the needle seal  53  and then closes to retain the seal  53  in sealing engagement. To accomplish this opening and closing, the smaller end  55  of the truncated cone  46  is comprised of a plurality of retaining arms  57 . A first end  61  of each retaining arm  57  is attached to a larger end  59  of the truncated cone  46 . The first ends  61  are arranged about the circumference of the larger end  59 . Each of the retaining arms  57  has a second end  63  that may move between an open position and a closed position when the retaining arms  57  are bent. An area defined by the second ends  63  of the retaining arms  57  while in the closed position is smaller than an area defined by the second ends  63  of the retaining arms  57  while in the open position. The area defined by the arms  57  in the closed position is smaller than a cross-sectional area of the needle seal  53  so that the needle seal  53  will not pass out through the aperture. When the needle seal retainer is in the closed position, the needle seal  53  sealingly engages the truncated cone  46  and the hollow projection  74 , and when the needle seal retainer is in the open position, the needle seal  53  is preferably released from sealing engagement with the truncated cone  46  and the hollow projection  74 . Retaining fingers  51  are attached to the interior of the syringe barrel  40  at a position adjacent to the closed end  44 . The retaining fingers  51  help retain the needle assembly  70  in position prior to retraction of the needle as will be described in more detail hereinafter.  
         [0067]    In the embodiments described herein, the needle assembly  70  is retained in a position that is adjacent to the closed end  44  of the barrel  40  by a needle assembly retainer prior to retraction. In the embodiments shown in FIGS. 3 and 5, the needle assembly retainer comprises the sealing ring  76  that is positioned in and retained by the retaining groove  50 . In the embodiment shown in FIGS. 4 and 7, the needle assembly retainer comprises the retaining fingers  51 .  
         [0068]    The retaining fingers  51  are preferably disposed about the circumference of a larger end  59  of the truncated cone  46 . A first end  65  of each of the retaining fingers  51  is connected with the syringe barrel  40  at a location adjacent to the closed end  44 . The retaining fingers  51  preferably comprise L-shaped members with a retaining lip  67  near a second end  69 . The retaining fingers  51  are bendable between a closed position and an open position. While in the closed position, the second ends  69  of the fingers  51  define an area that is smaller than the area defined by the second ends  69  when they are in the open position. The smaller area defined by the fingers  51  in the closed position is smaller than a cross-sectional area of the needle assembly  70 , such that the needle assembly does not pass through the needle assembly retainer when the fingers  51  are in the closed position.  
         [0069]    Referring to FIGS. 1 and 8- 20 , the plunger assembly  100  includes a plunger frame  110 , a retraction assembly  160 , a thumb pad  104  and a sealing member  150 . The plunger frame  110  includes a first end  114  and a sealing end  130  with a pair of opposed connecting rods  126  extending therebetween. The opposed connecting rods  126  define opposed retraction assembly guide tracks  128 .  
         [0070]    As shown in FIG. 9, in one embodiment of the plunger frame, the first end  114  of the plunger frame  110  includes a terminating plate  116  extending between and bridging the opposed connecting rods  126 . An annular thumb pad retaining ring  117  extends about the terminating plate  116 . Additionally, a guide member  118  may extend outward from each connecting rod  126  proximate the terminating plate  116 . A retention assembly  120  extends inward from the terminating plate  116  between the opposed connecting rods  126 . This embodiment of the retention assembly  120  includes a pair of opposed L-shaped members  122 , each L-shaped member having a beveled catch  124  extending therefrom. Other retention assemblies which permit inward passage and then retention of a geometrically configured tip are within the scope of the invention.  
         [0071]    As shown in FIGS. 10 and 11, another embodiment of the plunger frame has a retention assembly that comprises a plurality of retention teeth  103  arranged along a surface of the connecting rods  126  of the plunger assembly, substantially parallel to a longitudinal axis of the elongated frame portion of the plunger assembly.  
         [0072]    Each component of the plunger frame  110  is preferably manufactured from polypropylene or glass filled polypropylene. Other materials, including various plastics, may also be used. As described in more detail hereinafter, the plunger frame  110 , in addition to components of the retraction assembly  160 , is preferably formed as a first shot of a multiple shot injection molding procedure.  
         [0073]    As shown in FIG. 14, in one embodiment of the plunger assembly, the sealing end  130  includes a sealing platform  132  extending between the connecting rods  126  and including an apertured cylinder  134  terminating in an apertured pressure cone  138 . The apertures are preferably concentric such that a continuous hollow integral shaft  140  passes through the sealing end  130  from the sealing platform  132  to the pressure cone  138 . The hollow shaft  140  is preferably tapered such that the diameter is greater within the sealing platform  132  than within the pressure cone  138 . Additionally, an internal annular ring  142  extends into the hollow shaft  140  proximate the pressure cone  138 . An external annular retaining ring  136  is positioned about the juncture of the cylinder  134  and pressure cone  138 . The function of the taper aid the internal and external rings  136  and  142  will be described in more detail hereinafter.  
         [0074]    As shown in FIG. 15, in another embodiment of the plunger assembly, the hollow shaft  140  has a first end  141  and a second end  143 , wherein the first end  141  defines a cross-sectional area that is smaller than a cross-sectional area defined by the second end  143 . The transition between the smaller and larger cross-sectional areas is depicted to be abrupt, but may alternately be gradual.  
         [0075]    Referring to FIG. 16, the plunger sealing member  150  is positioned about the cylinder  134  and the external retaining ring  136  and is maintained in position by the external ring  136 . The sealing member  150  includes annular seals  152  and  154  at each end with a narrower portion  156  positioned therebetween. When the plunger assembly  100  is positioned in the syringe barrel  40 , annular seal  152  sealingly engages the inside surface of the hollow body portion  42  with an area of open space about the narrower portion  156 . Annular seal  154  may also sealingly engage the hollow body  142 , but may also include a passage to prevent creating a vacuum in the narrower portion  156 . The sealing member  150  is preferably manufactured from an elastomer. A material found to be suitable is Kraton™ manufactured by Shell Oil. A preferred material is Kraton™ G2706 manufactured by Shell Oil. As will be described in greater detail hereinafter, the sealing member  150  is preferably overmolded directly in position, but may be manufactured separately and subsequently positioned about the cylinder  134  and retaining ring  136 .  
         [0076]    The first end  114  of plunger frame  110  can be utilized with the terminating plate  116  and no thumb pad  104 . However, it is preferable to provide a thumb pad  104  about the terminating plate  116  and retained by the retaining ring  117  as shown in FIG. 17. The thumb pad  104  is also preferably manufactured from an elastomer, preferably Kraton™. As with the sealing member  150 , it is preferable that the thumb pad  104  be overmolded directly in position, but it too may be manufactured separately and subsequently positioned and secured about the terminating plate  116 .  
         [0077]    The preferred retraction assembly  160  will be described with reference to FIGS. 8, 12,  13 ,  16 ,  18 ,  19  and  20 . The retraction assembly  160  includes a mandrel  170 , a catch member  190  and an elastic member  164 .  
         [0078]    Referring to FIG. 18, one embodiment of the mandrel  170  includes a generally cylindrical body  172  with a tapered portion  174  extending from one end and a shaft portion  180  extending from the other. The tapered portion  174  terminates in a geometrically configured tip  176 . A mandrel annular retaining ring  178  extends about the cylindrical body  172  proximate the juncture with the tapered portion  174 . The mandrel  170  is releasably secured to the plunger frame by a retention means, which includes the retaining ring  178 . The mandrel retaining ring  178  preferably is part of the mandrel  170  and does not separate from the mandrel  170 . The shaft portion  180  includes a plurality of barbs  182  or the like extending therefrom for retaining the elastic member  164 .  
         [0079]    In another embodiment of the mandrel  170 , as shown in FIG. 19, the cylindrical body  172  has a groove  173  and a mandrel seal  171  (a cross-section of which is depicted) is positioned about the groove  173  between the mandrel  170  and the hollow shaft  140 . The mandrel seal  171  releasably and sealingly engages the mandrel  170  and the hollow shaft  140 . The mandrel seal  171  is depicted as an O-ring, but may be of any configuration that releasably and sealingly engages the hollow shaft  140  and the mandrel  170 .  
         [0080]    The mandrel seal  171  initially is in sealing engagement with the mandrel  170  and the first end  141  of the hollow shaft  140  and inhibits the passage of fluid therebetween prior to retraction of the needle. The sealing engagement creates holding forces between the hollow shaft  140  and the mandrel seal  171 , as well as between the mandrel seal  171  and the mandrel  170 . The holding forces inhibit movement of the seal  171  relative to the mandrel  170  and relative to the hollow shaft  140 , and thus inhibit retraction of the needle. Accordingly, the retention means that releasably secures the mandrel  170  to the plunger frame includes the mandrel seal  171 .  
         [0081]    In the smaller-diameter area, the holding force between the mandrel seal  171  and the mandrel  170  is greater than the holding force between the mandrel seal  171  and the hollow shaft  140 , such that the mandrel seal  171  does not move relative to the mandrel  170 , but instead moves relative to the hollow shaft  140  when the mandrel  170  is moved relative to the hollow shaft  140 .  
         [0082]    The desired distribution of holding forces may provided by various means. Preferably, the mandrel  170  has an annular groove  173  about the circumference of the mandrel  170  and located at a seal position on the mandrel  170 . The annular groove  173  is configured so that the mandrel seal  171  abuts or fits into the groove  173  and is held in place on the mandrel  170  at the seal position so long as the mandrel seal  171  is located in the smaller-area portion of the hollow shaft  140 . Once the mandrel seal  171  moves from the smaller area to the larger area of the hollow shaft  140 , the mandrel seal  171  may either remain in the groove  173  or expand away from the groove  173 , as described in greater detail below.  
         [0083]    It is noted that configurations other than a groove may also be employed to retain the mandrel seal fixed with respect to the mandrel. For example, the mandrel seal could be positioned between two annular collars on the mandrel. Additionally, any other non-smooth or irregular surface may be employed to mechanically inhibit movement of the seal relative to the mandrel. Alternately, an adhesive could be used to attach the mandrel seal to the mandrel. The mandrel seal could also be formed as an integral part of the mandrel.  
         [0084]    Referring to FIG. 16, in one embodiment of the retraction assembly  160 , an annular stop  184  extends about the cylindrical body  172  of the mandrel  170  adjacent the end of the hollow shaft. The stop  184  is preferably elastomeric and therefore is preferably formed in conjunction with the elastic member  164 .  
         [0085]    Referring to FIGS. 8 and 20, one embodiment of the catch member  190  includes an elongated plate  192  which is sized such that each end of the elongated plate  192  extends into and travels within a respective retraction assembly guide track  128  of the plunger frame  110 . Extending from one side of the elongated plate  192  is a shaft  194  with barbs  196  or the like extending therefrom for retaining the elastic member  164 . In the embodiment shown in FIG. 20, a second shaft  198  extends from the opposite side of the elongated plate  192  and terminates in a geometrically configured catch tip  200 . The tip  200  is configured to mate with and be retained by the retention assembly  120  of the plunger frame  110 .  
         [0086]    As shown in FIGS. 12 and 13, another embodiment of the catch member  105  has a guide  107  configured to travel in the guide track  128  between the connecting rods of the plunger frame  110 . The catch member  105  has a catch tooth  109  situated on either side of the catch member  105 . Each catch tooth  109  engages with one of the retention teeth  103  (the retention teeth  103  are part of the retention assembly shown in FIGS. 10 and 11) at a time. After one of the retention teeth  103  receives one of the catch teeth  109 , the catch tooth  109  is inhibited from moving away from the first end  114  of the elongated frame portion. Accordingly, the catch member  105  is retained by the retention teeth  103  of the retention assembly and the elastic member  164  is thereby held in tension on one end by the sealing platform and on the other end by the catch member  105 .  
         [0087]    The mandrel  170  and the catch member  190   105  are preferably manufactured from the same material as the plunger frame  110 . As such, these components can also be formed during the first shot of the multiple shot injection molding procedure used to form the plunger frame  110 .  
         [0088]    The elastic member  164  extends between the mandrel  170  and the catch member  190   105 . The elastic member  164  is manufactured from a resilient material, which is preferably an elastomer, but which can be other materials, for example a stainless steel spring or the like. The elastic member  164  is preferably manufactured from Kraton™. In the preferred method of manufacture described below, the elastic member  164  is formed between the mandrel  170  and the catch member  190   105  with a second injection, overmolding shot. Do to the elastic member  164  being directly overmolded over the barbed shafts  180  and  194  and the innate bonding property of the preferred material, there is generally not a need for additional securing means, for example adhesive, to maintain the elastic member  164  secured to the mandrel  170  and catch member  190   105 . It is contemplated that the elastic member  164 , irrespective of the material from which it is manufactured, may also be manufactured separately and secured to the mandrel  170  and catch member  190   105 . Since the sealing member  150 , thumb pad  104  and stop  184  are all also preferably manufactured from the same material as the elastic member  164 , they are also preferably formed during the second injection, overmolding shot.  
         [0089]    Having described the components of the preferred retractable syringe  8 , its assembly and use will now be described with reference to FIGS.  21 - 28 . The needle assembly  70  is positioned in the syringe barrel  40  with the needle  72  extending through the aperture  48 .  
         [0090]    In the embodiment shown in FIGS. 3 and 5, the needle assembly  70  is inserted until the sealing ring  76  seats in and is retained by the syringe body retaining groove  50 . In the embodiment shown in FIGS. 4 and 6, the needle assembly  70  with the needle seal  77  is inserted into the truncated cone  46  of the syringe barrel  40  and is retained by the retaining fingers  51 . When the needle assembly  70  is being inserted into the truncated cone  46 , it contacts the retaining fingers  51 . Further insertion of the needle assembly  70  moves the retaining fingers  51  into the open position so that the needle assembly  70  may pass through the opening defined by the retaining fingers  51 .  
         [0091]    When the needle assembly  70  is inserted sufficiently into the truncated cone  46 , the retaining fingers  51  return to the closed position. The retaining fingers  51  are formed from an elastic material so that they return to the closed position if they are displaced from the closed position and no external forces are acting on the retaining fingers  51 . If a force is applied tending to push the needle assembly  70  out of the truncated cone  46  toward the open end of the barrel  40  when the retaining fingers  51  are in the closed position, the needle assembly  70  will contact at least one of the retaining lips  67 , and at least one of the retaining fingers  51  will resist that force. The orientation of the surface of the lips  67  in a substantially perpendicular relationship to the direction in which such a force would be applied decreases the component of the force that would act to spread the fingers  51  into the open position.  
         [0092]    In the embodiment shown in FIG. 7, the needle seal  53  is inserted into the truncated cone  46  through the aperture  48  rather than through the open end of the syringe barrel  40 . To insert the needle seal  53  into the truncated cone  46  through the aperture  48 , the retaining arms  57  are moved into the open position and held in the open position so that the needle seal  53  may pass through the opening defined by the arms  57 . Once the needle seal  53  is sufficiently inserted into the truncated cone  46 , the arms  57  are released and move toward the closed position. The arms  57  are formed of an elastic material so that the arms  57  return to the closed position if they are displaced from the closed position and no external forces are acting on the arms  57 . Once the needle seal  53  is inserted into the truncated cone  46  and the arms  57  return to the closed position, the needle seal  53  is retained in the truncated cone  46  by the arms  57 .  
         [0093]    The needle assembly  70  may be inserted through the aperture  48  or through the open end of the barrel  40 . If the needle assembly  70  is inserted through the open end of the barrel  40 , it is done in the same manner as described above in connection with the embodiments shown in FIGS. 4 and 6. Accordingly, the needle assembly  70  contacts the retaining fingers  51  and pushes them into the open position. Once the needle assembly  70  is sufficiently inserted, the retaining fingers  51  move into the closed position and retain the hollow projection  74  of the needle assembly  70 . If the needle assembly  70  is inserted through the aperture  48 , it may be inserted in the same manner as, and at the same time as, the seal  53 . If inserted through the aperture  48 , the retaining fingers  51  need not move to the open position to accept the needle assembly  70 .  
         [0094]    In the embodiments shown in FIGS.  3 - 6 , the cap member  10  may be mated with the closed end  44  of the syringe barrel  40  either before or after insertion of the needle assembly  70 . In the embodiments shown in FIG. 7, the cap member  10  may be mated with the closed end  44  of the syringe barrel  40  after insertion of the needle seal  53 .  
         [0095]    The plunger assembly  100  is assembled by assembling the plunger frame  100 , which already has the thumb pad  104  and sealing member  150  positioned thereon, and the retraction assembly  160 . As explained above, the elastic member  164  is preferably molded directly to the mandrel  170  and catch member  190   105 , to form the retraction assembly  160 . If not formed integrally, the elastic member  164  is secured to the mandrel and catch member barbed shafts  180  and  194 .  
         [0096]    With the retraction assembly  160  complete, the mandrel  170  is inserted through the hollow shaft  140  passing through the plunger frame sealing end  130 . In the embodiment shown in FIGS. 14 and 18, the mandrel  170  is inserted until the mandrel retaining ring  178  is secured by the sealing end internal annular ring  142 . The mandrel retaining ring  178  forms a fluid tight seal with the plunger sealing end  130  proximate the pressure cone  138 , thereby sealing the hollow shaft  140 . The resilient stop  184  abuts against the rear surface of the sealing platform  132 , thereby forming a fluid tight seal about that end of the hollow shaft  140 . As the plunger assembly is withdrawn from the hollow body  42 , a vacuum is created therein. The seal provided by the resilient stop  184  helps prevent air or other materials from be pulled past the mandrel  170  into the syringe body  42  by the internal vacuum force.  
         [0097]    In the embodiment shown in FIGS. 15 and 19, the mandrel  170  is inserted to place the mandrel seal  171  within the first end  141  of the hollow shaft  140 . The mandrel seal  171  may abut a pressure cone  138  or a lip (not shown) on the hollow shaft  140  that inhibits excessive insertion of the mandrel  170 .  
         [0098]    With the mandrel  170  in place, the plunger assembly  100  is ready to be inserted into the syringe barrel  40  through the open end  54 . The annular seals  152  and  154  sealingly engage the inside of the syringe barrel  40  as the plunger assembly  100  is inserted. The plunger assembly  100  is inserted approximately half-way into the syringe barrel  40  until the catch member  190   105  abuts the shoulder  60 , as shown in FIG. 21. The syringe  8  is ready for packaging and delivery. It should be noted that at this time the elastic member  164  is not tensioned. This helps increase the shelf life of the syringe  8  since the elastic member  164  is not under constant tension. If shelf life is not a concern, the catch member  190   105  can be secured to the retention assembly  120  prior to packaging, whereby the syringe  8  would have a preloaded elastic member.  
         [0099]    After removing the syringe assembly  8  from the packaging, the operator can hold the syringe in a typical one hand manner, i.e. with two fingers abutting the grip member  56  and the thumb on the thumb pad  104 . The operator presses on the thumb pad  104  to depress the plunger assembly  100  into the syringe barrel  40  with a substantially complete depression to expel air from the syringe hollow body  42 . This is similar to standard syringe operation. As the plunger assembly  100  is depressed, the catch member  190   105  is retained by the shoulder  60  such that the catch member  190   105  cannot travel forward. However, the plunger frame  110  continues its forward travel. Since the catch member  190   105  is retained but the plunger frame  110  and secured mandrel  170  continue forward, the elastic member  164  begins to stretch and tension.  
         [0100]    As travel continues forward, the catch element  200   109  of the catch member  190   105  is received by the retention assembly  120 . In the embodiment shown in FIG. 20, the catch element is a geometrically configured catch tip  200 . In the embodiment shown in FIGS. 12 and 13, the catch element is comprised of catch teeth  109  and the retention assembly is comprised of retention teeth  103 . As shown in FIG. 22 for the embodiment with a catch tip  200 , the catch element  200   109  is secured by the retention assembly  120  of the plunger frame  110 . The elastic member  164  is thereby secured in a loaded condition between the secured mandrel  170  and the secured catch member  190   105 . As the catch element  200   109  and retention assembly  120  mate, an audible “click” may occur to provide a signal of proper mating to the operator.  
         [0101]    Additionally, the syringe barrel annular lip  62  adjacent the first end  114  of the plunger frame will discourage complete depression of the plunger assembly  100  as the catch element  200   109  is received by the retention assembly  120 .  
         [0102]    With the elastic member  164  loaded, the syringe  8  can be loaded in a typical fashion by removing the cap  10 , inserting the needle  42  into a desired vial or the like, and withdrawing the plunger assembly  100  to draw up a desired dose as shown in FIG. 23. Since the elastic member  164  is tensioned between two components secured to the plunger frame  110 , withdrawal of the plunger assembly  100  will not trigger the elastic member  164 . Instead, the plunger assembly  100  will operate as a standard syringe plunger.  
         [0103]    Once any air has been purged from the syringe barrel  40  in a known manner, the device  8  is ready for injection of the needle  72  into the patient. As stated above, the elastic member  164  is tensioned between two fixed components, and therefore, is not acting to move the plunger assembly  100  in either direction. As such, the user does not have to maintain constant pressure on the plunger assembly, but is free to hold the syringe  8  in the traditional dart like fashion between their thumb and forefinger of one hand, and use the other hand to pinch the patient&#39;s skin at the point of insertion for subcutaneous injection, spread the skin for intramuscular injection, and stabilize the skin for IV injection. These methods of injection are the generally preferred methods in the medical field.  
         [0104]    As the needle  72  is inserted, a rearward force, indicated by the arrow A in FIG. 24, is applied against the needle assembly  70 . To resist this force, the needle assembly sealing ring  76  is secured within the retaining groove  50  in the embodiment shown in FIGS. 3 and 5. Additionally, since the syringe barrel tapered surface  47  about the truncated cone  46  is convex, the rearward force causes the syringe barrel surface to urge inward, as indicated by arrows B, thereby creating a tighter retention force about the needle assembly  70 . Once the user has inserted the needle  72  into the patient, the user injects the substance into the patient by depressing the thumb pad  104 . In the embodiments shown in FIGS. 4, 6, and  7 , the force applied against the needle assembly is resisted by the retaining fingers  51 .  
         [0105]    In an embodiment shown in FIG. 25, upon substantial depression of the plunger assembly  100 , the mandrel tip  176  begins to enter the needle assembly cavity  80 . At approximately the same time, the first annular seal  152  meets and is deflected by the ramps  52  adjacent the closed end  44  of the syringe barrel  40 , thereby breaking the fluid tight seal. Any fluid trapped between the plunger sealing member  150  and the needle assembly  70  is permitted to pass the deflected annular seal  152  into the open space around narrower portion  156 . The second annular seal  154  may remain in sealing engagement with the syringe barrel  40  to prevent any unwanted inward or outward flow past the sealing member  150 . However, the annular seal  154  may include a small passage to let trapped air about the narrower portion  156  escape.  
         [0106]    In this embodiment, the mandrel tip  176  passes through the needle assembly cavity cylindrical portion  80   a  into the geometrically configured cavity hemispherical portion  80   b  whereby the mandrel  170  is secured to the needle assembly  70  as shown in FIG. 26.  
         [0107]    At approximately the same time the mandrel  170  and needle assembly  70  attach, the retaining fingers  51  are moved into the open position by the plunger assembly  100  so that the needle assembly  70  may pass through the opening defined by the fingers  51 . A holding force exerted by the fingers  51  on the needle assembly  70  is lower when the fingers  51  are in the open position than when the fingers  51  are in the closed position. The tension in the elastic member is sufficient to overcome any remaining holding force exerted by the fingers  51  on the needle assembly  70 .  
         [0108]    Once the mandrel  170  is secured to the needle assembly  70  as shown in FIG. 26, the mandrel tip  176  has moved as far into the needle assembly  70  as possible, yet the plunger frame  110  has not completed its full stroke. As such, continued force on the thumb pad  104  will continue to move the plunger frame  110  forward. Since the mandrel  170  position is fixed and the plunger frame  110  is being forced forward, the mandrel is pushed backward.  
         [0109]    In the embodiment shown in FIGS. 14 and 18, the retaining ring  178  will be forced inward past the plunger sealing end retaining ring  136 , thereby releasing the mandrel  170  as shown in FIG. 18. That is, the mandrel retaining ring  178  moves behind the retaining ring  136  as shown in phantom.  
         [0110]    In the embodiments shown in FIGS. 15 and 19, as the needle user continues depressing the plunger, the plunger frame continues moving forward relative to the mandrel  170 , so that the mandrel  170  is moved from a first position in the smaller-area first end  141  of the hollow shaft  140  to a second position in the larger-area second end  143  of the hollow shaft  140 .  
         [0111]    As the mandrel  170  moves from the first position toward the second position, the holding force between the hollow shaft  140  and the mandrel seal  171  is overcome and the mandrel seal  171  slides against the hollow shaft  140 . The mandrel seal  171  moves with the mandrel  170  because the holding force between the mandrel seal  171  and the mandrel  170  is greater than the holding force between the mandrel seal  171  and the hollow shaft  140 .  
         [0112]    Once the mandrel seal  171  moves into the larger area of the second end  143  of the hollow shaft  140 , the mandrel seal  171  either expands away from the mandrel  170  or remains engaged with the mandrel  170 . The seal  171  would expand if it were compressed around the mandrel  170  by the hollow shaft  140  during the assembly of the syringe. If the seal  171  expands, the area defined by an opening in the seal  171  (the inner seal area) preferably becomes large enough as the seal  171  moves from the first position to the second position, such that the mandrel  170  may retract the needle through the inner seal area.  
         [0113]    In any case, the mandrel seal  171  no longer secures the mandrel  170  to the hollow shaft  140  after the mandrel seal  171  moves from the first position to the second position within the hollow shaft  140 .  
         [0114]    Since the mandrel  170  is under the load of the elastic member  164  but no longer secured to the hollow shaft  140 , the load of the elastic member  164  automatically retracts the mandrel  170  into the plunger frame  110  between the connecting rods  126 . Through the connection of mandrel tip  176  and the geometrically configured needle assembly cavity  80 , the needle assembly  70  is also retracted into the plunger frame  110 , as shown in FIG. 28.  
         [0115]    In one embodiment, the tapered pressure cone  138  on the sealing end of the plunger frame  110  contacts the convex, tapered portion  47  of cone  46  and causes it to spread slightly. This reduces the retaining force of cone  46  on the needle assembly  70  to assist retraction of the needle assembly  70 . Furthermore, since the tapered portion  47  is convex, the forward fluid and plunger force, as indicated by arrow C in FIG. 27, urge the tapered portion  47  outward, as indicated by arrows D, further easing the retaining force and thereby reducing the requisite retraction force. This flexing preferably occurs simultaneously or slightly after the mandrel  176  enters the hemispherical portion  80   a  of the needle assembly cavity  80 .  
         [0116]    Referring again to FIG. 28, as the plunger assembly  100  completes its stroke, the thumb pad  104  enters the open cavity  58  at the end of the syringe barrel  40 . In the preferred embodiment, the thumb pad  104  is manufactured from a resilient material which sealingly engages the syringe barrel  40  wall, thereby closing the open end  54  and preventing any inadvertent fluid flow out of the syringe barrel  40 . Additionally, since the thumb pad  104  enters and is recessed in the open cavity  58 , it makes it difficult for anyone to inadvertently or intentionally remove the plunger assembly  100  and expose the used needle  72 . The thumb pad  104  preferably has a semi-domed configuration which enhances its inaccessibility. Additionally, the thumb pad  104  is preferably inserted past the inner annular lip  62  and retained thereby, further enhancing inaccessibility.  
         [0117]    B. Fluid Retention Systems  
         [0118]    [0118]FIGS. 29A, 29B, and  29 C depict an embodiment of a fluid retention system that includes a cylinder  580  and a piston  576 . The piston  576  fits within and slidingly engages the cylinder  580 . The cylinder  580  may fluidly communicate with the needle passage  561  of the needle  572  through an interior passage  578  of the needle assembly  560 . In this embodiment, the piston  576  is integral with the mandrel  570  and the cylinder  580  is defined within the hollow projection  574  of the needle assembly  560  of a retractable syringe.  
         [0119]    After the syringe is charged with fluid to be injected into, a patient, the plunger assembly may be depressed to inject the patient. When the plunger assembly is depressed substantially completely, the piston  576  begins to enter the cylinder  580 . As the plunger is further depressed, a shoulder  553  of the piston  576  slides past a lip  555  of the cylinder  580  and enters the cylinder  580 , the abutment of the shoulder  553  and lip  555  securing the piston inside the cylinder. The cylinder preferably is defined at least partially by a resilient material such that the shoulder  553  may urge the lip  555  open as it moves into the cylinder  580 , and the lip  555  may return to a relaxed state after the shoulder  553  moves past the lip. As the plunger assembly is depressed still further, the piston  576  contacts a stop  583  that inhibits further forward movement of the piston  576  relative to the cylinder  580 .  
         [0120]    As the plunger is further depressed, the mandrel  570  becomes disengaged from the hollow shaft that initially secures the mandrel  570  in place as described in connection with FIGS. 15 and 19, and the elastic member pulls the mandrel  570  into the hollow body.  
         [0121]    As the mandrel retracts into the barrel, the needle assembly  560  initially tends not to retract with the mandrel because of inertia of the needle assembly or because of holding forces exerted on the needle assembly by the patient&#39;s skin or by parts of the needle assembly that contact the rest of the syringe, such as the sealing ring and retaining groove described in connection with FIGS. 3 and 5. The piston  576  initially moves relative to the cylinder  580 , increasing the volume of a vacuum cavity located between the piston  576  and the tip of the needle  572 . As the volume of the cavity increases, the pressure inside the cavity initially decreases and remains at a decreased level until air or other fluid entering the cavity equalizes the pressure in the cavity with ambient pressure.  
         [0122]    The pressure drop inside the vacuum cavity preferably tends to draw into the cavity any fluid remaining in the needle passage  561 , such as fluid remaining at the tip of the needle  572 , thereby inhibiting fluid loss from the needle  572  during retraction. Preferably, the vacuum cavity is substantially sealed other than at the needle tip and at least partially open at the needle tip so that air or other fluid may enter the vacuum cavity only (or at least most easily) through the needle tip and is drawn away from the needle tip into the vacuum cavity. To provide the preferred sealing, the piston  576  substantially sealingly engages the cylinder  580 . This sealing may be accomplished with a piston seal  581 , such as an annular ring, that sealingly engages the piston  576  and the cylinder  580 . The piston  576  may have a seal retaining groove  551  that positions and retains the piston seal  581 .  
         [0123]    After the piston  576  retracts and moves relative to cylinder  580  sufficiently, the shoulder  553  of the piston  576  abuts the lip  555  of the cylinder  580 , inhibiting further relative movement of the piston  576  to the cylinder  580 . As the piston  576  further retracts, it retracts the needle assembly  560  along with it, retracting the needle  572  into the hollow barrel of the syringe.  
         [0124]    [0124]FIG. 30 depicts another embodiment of a fluid retention system that includes a cylinder  680  and a piston  656 . In this embodiment, the piston  656  is secured in and slideable within the cylinder  680 . The piston  656  is secured from excessive axial movement within the cylinder  680  by a lip  655  and a stop  683  disposed opposite each other. The piston has a piston passage  687  that is in fluid communication with the needle passage  661  through an interior passage  678  of the needle assembly  660 .  
         [0125]    The mandrel  670  has a geometrically configured tip  656  and the piston passage  687  is configured to engage the geometrically configured tip  676  of the mandrel  670 . The hollow projection  674  defines a keyway  631  through which the geometrically configured tip  676  of the mandrel  670  may enter to engage the piston passage  687 . In this embodiment, the cylinder  680  is defined within the hollow projection  674  of the needle assembly  660  of a retractable syringe.  
         [0126]    After the syringe is charged with fluid to be injected into a patient, the plunger assembly may be depressed to inject the patient. When the plunger assembly is depressed substantially completely, the mandrel tip  676  enters the keyway  631  and pushes against the piston. As the plunger assembly is further depressed, the mandrel tip  676  engages the piston passage  687  and the mandrel tip  676  moves the piston  656  toward the needle  672  until the piston  656  contacts the top  683  of the cylinder  680 , inhibiting further forward movement of the piston  656  relative to the cylinder  680 .  
         [0127]    As the plunger is further depressed, the mandrel  670  becomes disengaged from the hollow shaft that initially secures the mandrel  670  in place as described in connection with FIGS. 15 and 19, and the elastic member pulls the mandrel  670  into the hollow body of the syringe.  
         [0128]    As the mandrel  670  retracts into the barrel, the needle assembly  660  initially tends not to retract with the mandrel  670  because of inertia of the needle assembly  660  or because of holding forces exerted on the needle assembly  660  by the patient&#39;s skin or by parts of the needle assembly that contact the rest of the syringe, such as the sealing ring and retaining groove described in connection with FIGS. 3 and 5. The piston  656  initially moves relative to the cylinder  5 = 680 , increasing the volume of a vacuum cavity located between the piston  656  and the tip of the needle  672 . As the volume of the cavity increases, the pressure inside the cavity initially decreases and remains at a decreased level until air or other fluid entering the cavity equalizes the pressure in the cavity with ambient pressure.  
         [0129]    The pressure drop inside the vacuum cavity preferably tends to draw into the cavity any fluid remaining in the needle passage  561 , such as fluid remaining at the tip of the needle  572 , thereby inhibiting fluid loss from the needle  572  during retraction. Preferably, the vacuum cavity is substantially sealed other than at the needle tip and at least partially open at the needle tip so that air or other fluid may enter the vacuum cavity only (or at least most easily) through the needle tip and is drawn away from the needle tip into the vacuum cavity. To provide the preferred sealing, the mandrel tip  676  preferably substantially sealingly engages the piston passage  687  and the piston  656  substantially sealingly engages the cylinder  680 . Sealing may be aided by one or more piston seals  681 , which may comprise an annular ring. The piston  656  may have a seal retaining groove (not shown) that positions and retains the piston seal  681 .  
         [0130]    After the piston  576  retracts and moves relative to cylinder  580  sufficiently, the 553 of the piston  656  abuts the lip  655  of the cylinder  680 , inhibiting further relative movement of the piston  656  to the cylinder  680 . As the piston  656  further retracts, it retracts the needle assembly  660  along with it, retracting the needle  672  into the hollow barrel of the syringe.  
         [0131]    [0131]FIGS. 31 and 31A depict another embodiment of a fluid retention system. This embodiment includes a diaphragm cavity  780  and a diaphragm  756  having a centrally-disposed slit  757  and a spacer  755 . The diaphragm  756  is disposed within and preferably sealingly engages a diaphragm cavity  780  defined by the hollow projection  774 . The slit cavity  780  may fluidly communicate with the needle passage  761  through an interior passage  778  of the needle assembly  760 .  
         [0132]    This embodiment is particularly well-suited for use in either retractable syringes or conventional, non-retractable syringes. In a retractable syringe, the diaphragm preferably is disposed in the hollow projection  774  or otherwise formed into the needle assembly  760 , but alternately may be disposed outside of the needle assembly  760  near the closed end of the hollow body and connected with the needle assembly  760 . Further, the diaphragm may include a geometric configuration securable with the mandrel  770 .  
         [0133]    The diaphragm selectively provides fluid communication through the slit. The diaphragm preferably comprises a resilient material that bulges under fluid pressure and relaxes when the pressure is decreased. Such material may be, for example, a polymer or a flexible membrane. When the diaphragm is relaxed and not bulging, fluid communication through the slit is inhibited, and when the diaphragm is bulging under fluid pressure that is not equal on both sides of the diaphragm, fluid communication through the slit is provided. The spacer  755  provides clearance within the hollow projection  774  to accommodate bulging of the diaphragm  756 .  
         [0134]    As the plunger assembly is withdrawn to charge the syringe or depressed to inject the charge into a patient, pressure in the hollow barrel of the syringe increases. The fluid pressure acts against the diaphragm  756 , urging the slit  767  and allowing fluid communication through the diaphragm  756 . When the plunger assembly is depressed substantially completely, a geometrically configured tip  776  of the mandrel  770  enters and engages the needle assembly  760  and further forward movement of the tip  776  relative to the needle assembly  760  is inhibited, allowing the fluid pressure on the diaphragm to drop.  
         [0135]    When fluid pressure on the diaphragm drops, the bulging in the flexible diaphragm subsides and fluid no longer flows through the slit. As the bulge subsides, the volume of a vacuum cavity located between the diaphragm and the needle tip increases. As the volume of the cavity increases, the pressure inside the cavity initially decreases and remains at a decreased level until air or other fluid entering the cavity equalizes the pressure in the cavity with ambient pressure. The diaphragm substantially sealingly engages the slit cavity and is substantially sealed other than at the needle tip. As explained in connection with other embodiments, the pressure drop inside the cavity preferably tends to draw into the vacuum cavity any fluid remaining in the needle passage, thereby inhibiting fluid loss from the needle during retraction of the needle.  
         [0136]    As the plunger is further depressed, the mandrel  770  becomes disengaged from the hollow shaft that initially secures the mandrel  770  in place as described in connection with FIGS. 15 and 19, and the elastic member pulls the mandrel  770  into the hollow body.  
         [0137]    [0137]FIGS. 32A and 32B depict another embodiment of a fluid retention system that includes a plunger seal  1217  secured with the sealing platform  1214  of the plunger assembly and sealingly engaging the plunger assembly and the hollow body  1225  of the syringe. The plunger seal  1217  and the hollow body  1225  together define an annular cavity  1219 . Ribs  1241  are positioned near the closed end of the hollow body  1225 . Preferably, there are three or four ribs positioned radially about the closed end of the hollow body  1225 .  
         [0138]    After the syringe is charged with fluid to be injected into a patient, the plunger assembly may be depressed to inject the patient. As the plunger assembly is depressed, the seal is moved toward the closed end of the hollow body. When the plunger is substantially completely depressed, the top  1231  of the seal contacts one or more ribs  1241  and is prevented from moving further forward. As the plunger is further depressed, it moves the bottom portion of the seal toward the top  1231  of the seal, compressing the seal. As the seal is compressed, the top surface of the seal no longer sealingly engages the hollow body, such that the annular cavity fluidly communicates with the needle passage  1261 . This fluid communication preferably is provided through spaces between the ribs. Alternately, this communication may be provided through a passage in the seal (not shown). The bottom portion of the plunger seal continues to sealingly engage the plunger assembly with the barrel even while the seal is being compressed.  
         [0139]    As the seal is compressed, the annular cavity expands into the seal, increasing the volume of a vacuum cavity located between the annular cavity and the needle tip, and the mandrel  1270  engages the hollow projection  1274  of the needle assembly.  
         [0140]    Preferably, the vacuum cavity is substantially sealed other than at the needle tip. As the annular cavity expands and the cavity volume increases, a pressure drop is created in the vacuum cavity, tending to draw into the cavity any fluid remaining at the tip of the needle. As the plunger is further depressed, the mandrel  1270  becomes disengaged from the hollow shaft that initially secures the mandrel  1270  in place as described in connection with FIGS. 15 and 19, and the elastic member pulls the mandrel  1270  into the hollow body  1225  of the syringe.  
         [0141]    The annular cavity may expand after the mandrel engages the needle assembly. In such a case, fluid communication between the annular cavity and the needle passage  1261  may be provided by a gap (not shown) between the mandrel and the interior surface of the hollow projection  1274 . Such a gap may be in the form of a groove in the interior surface of the hollow projection  1274  or a groove in the mandrel.  
         [0142]    As the annular cavity expands into the internal cavity of the plunger seal, air preferably is displaced from the internal cavity of the plunger seal through one or more air vents  1252  that provide fluid communication with the interior portion of the hollow body located proximally of the seal. If the air is not displaced from the cavity, then pressure in the internal cavity will increase as the annular cavity expands. This increasing pressure may inhibit the expansion of the annular cavity.  
         [0143]    [0143]FIGS. 33A and 33B depict another embodiment of a fluid retention system that includes a plunger seal  1317  secured with the sealing platform  1314  of the plunger assembly and sealingly engaging the plunger assembly and the hollow body of the syringe. The plunger seal has an internal collar that is positioned in an internal cavity  1337  of the plunger seal. The internal collar defines a central cavity  1319  together with the mandrel  1370 . The central cavity fluidly communicates with the needle passage. Fluid communication is preferably provided by one or more grooves  1361  in the mandrel, but alternately may be provided by a passage through the top portion  1331  of the plunger seal.  
         [0144]    After the syringe is charged with fluid to be injected into a patient, the plunger assembly may be depressed to inject the patient. As the plunger assembly is depressed, the seal is moved toward the closed end of the hollow body. When the plunger is substantially completely depressed, the top portion  1331  of the seal contacts the closed end of the hollow body of the syringe and is prevented from moving further forward. As the plunger is further depressed, it moves the bottom portion of the seal toward the top portion  1331  of the seal, compressing the seal and the internal collar. As the internal collar is compressed, the volume of the central cavity increases, increasing the volume of a vacuum cavity located between the internal collar and the needle tip, and the mandrel engages the needle assembly.  
         [0145]    Preferably, the vacuum cavity is substantially sealed other than at the needle tip. As the central cavity expands and the cavity volume increases, a pressure drop is created in the vacuum cavity, tending to draw into the cavity any fluid remaining at the tip of the needle. As the plunger is further depressed, the mandrel  1370  becomes disengaged from the hollow shaft that initially secures the mandrel  1370  in place as described in connection with FIGS. 15 and 19, and the elastic member pulls the mandrel  1370  into the hollow body of the syringe.  
         [0146]    The central cavity may expand after the mandrel engages the needle assembly. In such a case, fluid communication between the central cavity and the needle passage may be provided by a gap (not shown) between the mandrel and the interior surface of the hollow projection. Such a gap may be in the form of a groove in the interior surface of the hollow projection  1274  or a groove in the mandrel.  
         [0147]    As the internal collar expands into the internal cavity of the plunger seal, air preferably is displaced from the internal cavity of the plunger seal through one or more air vents  1352  that provide fluid communication with the interior portion of the hollow body located proximally of the seal. If the air is not displaced from the cavity, then pressure in the internal cavity will increase as the collar expands. This increasing pressure may inhibit the expansion of the collar.  
         [0148]    [0148]FIGS. 34A and 34B depict an embodiment of a fluid retention system that includes a washer cavity  1480  and a washer  1456  contained by and moveable within the washer cavity. The washer cavity  1480  is defined within the hollow projection  1474  of the needle assembly  1460 . The washer is invertable between and stable in each of a first configuration and a second configuration, both shown in FIG. 34A. The washer substantially sealingly engages the washer cylinder as the washer is inverted (i.e., flipped inside-out) between the first configuration and the second configuration. The washer  1456  has a washer passage  1487  providing fluid communication through the washer, and having a geometric configuration securable with the mandrel. The washer cavity  1480  fluidly communicates with the needle passage  1461  of the needle  1472 .  
         [0149]    The washer of this embodiment is contained in the washer cavity, but alternately may be connected with or integral with the mandrel and enter the washer cavity when the plunger assembly is depressed substantially completely.  
         [0150]    After the syringe is charged with fluid to be injected into a patient, the plunger assembly may be depressed to inject the patient. When the plunger assembly is depressed substantially completely, the geometrically configured tip  1476  of the mandrel enters and engages the washer passage, and moves the washer toward an upper surface  1483  of the washer retainer that inhibits further forward movement of the washer  1456  relative to the washer cavity  1480 .  
         [0151]    As the plunger is further depressed, the mandrel  1470  becomes disengaged from the hollow shaft that initially secures the mandrel  1470  in place as described in connection with FIGS. 15 and 19, and the elastic member pulls the mandrel  1470  into the hollow body.  
         [0152]    As the mandrel retracts into the barrel, the needle assembly  1460  initially tends not to retract with the mandrel because of inertia of the needle assembly or because of holding forces exerted on the needle assembly by the patient&#39;s skin or by parts of the needle assembly that contact the rest of the syringe, such as the sealing ring and retaining groove described in connection with FIGS. 3 and 5. As the mandrel retracts, the washer is pulled away from the needle and comes into contact with a lip  1455  of the washer cavity  1480 . The lip constrains the washer about its circumference, and the retracting mandrel influences the center portion of the washer to move relative to the circumferential portion. The mandrel applies sufficient force to cause the washer to invert. The washer being unstable until it reaches its second stable configuration, the washer while under the influence of the retracting mandrel tends to move toward the second configuration until it is reached. When the washer inverts, a volume of a vacuum cavity located between the washer and the tip of the needle increases, decreasing the pressure inside the cavity.  
         [0153]    The pressure drop inside the vacuum cavity preferably tends to draw into the cavity any fluid remaining in the needle passage  561 , such as fluid remaining at the tip of the needle  572 , thereby inhibiting fluid loss from the needle  572  during retraction. Preferably, the vacuum cavity is substantially sealed other than at the needle tip and at least partially open at the needle tip so that air or other fluid may enter the vacuum cavity only (or at least most easily) through the needle tip and is drawn away from the needle tip into the vacuum cavity. To provide the preferred sealing, the geometrically configured tip of the mandrel preferably substantially sealingly engages the washer passage, and the washer preferably sealingly engages the washer cavity. This sealing may be accomplished with a washer seal (not shown), such as an annular ring, that sealingly engages the washer  1476  and the washer cavity  1480 .  
         [0154]    As the mandrel further retracts, it retracts the needle assembly  1460  along with it, retracting the needle  1472  into the hollow barrel of the syringe.  
         [0155]    [0155]FIGS. 35A and 35B depict an embodiment of a fluid retention system that includes an anvil cylinder  1580  and an anvil  1573  slideably secureable with the anvil cylinder. The anvil cylinder  1580  is defined within the hollow projection  1574  of the needle assembly  1560  and in fluid communication with the needle passage. The anvil  1573  has an anvil base  1556  and an anvil head  1576  having a geometric configuration that is secureable with the anvil cavity  1580  of the needle assembly. The anvil base is invertable between and stable in each of a first configuration shown in FIG. 35A and a second configuration shown in FIG. 35B. The anvil head substantially sealingly engages the anvil cylinder as the anvil base is inverted (i.e., flipped inside-out) between the first configuration and the second configuration. An anvil base cavity is defined by the retraction member and includes a retaining groove disposed about a perimeter of the cavity. The anvil base preferably is seated in the retaining groove.  
         [0156]    After the syringe is charged with fluid to be injected into a patient, the plunger assembly may be depressed to inject the patient. When the plunger assembly is depressed substantially completely, the geometrically configured anvil head  1576  enters and slidingly engages the anvil cylinder, and moves forward toward the needle until it contacts a stop  1583  in the cylinder that inhibits further forward movement of the anvil head  1576  relative to the anvil cylinder  1580 , or until the anvil base contacts the hollow projection.  
         [0157]    As the plunger is further depressed, the anvil base inverts to its second configuration and the mandrel  1570  becomes disengaged from the hollow shaft that initially secures the mandrel  1570  in place as described in connection with FIGS. 15 and 19, and the elastic member pulls the mandrel  1570  into the hollow body.  
         [0158]    As the mandrel retracts and the anvil inverts, the anvil head is pulled away from the needle. The needle assembly  1560  initially tends not to retract with the mandrel because of inertia of the needle assembly or because of holding forces exerted on the needle assembly by the patient&#39;s skin or by parts of the needle assembly that contact the rest of the syringe, such as the sealing ring and retaining groove described in connection with FIGS. 3 and 5. The anvil moves relative to the anvil cylinder away from the needle. When the anvil base inverts and the anvil head slides relative to the anvil cylinder, a volume of a vacuum cavity located between the anvil head and the tip of the needle increases, decreasing the pressure inside the vacuum cavity.  
         [0159]    The pressure drop inside the vacuum cavity preferably tends to draw into the cavity any fluid remaining in the needle passage  1561 , such as fluid remaining at the tip of the needle  1572 , thereby inhibiting fluid loss from the needle  1572  during retraction. Preferably, the vacuum cavity is substantially sealed other than at the needle tip and at least partially open at the needle tip so that air or other fluid may enter the vacuum cavity only (or at least most easily) through the needle tip and is drawn away from the needle tip into the vacuum cavity. To provide the preferred sealing, the geometrically configured anvil head preferably substantially sealingly engages the anvil cylinder. As the mandrel further retracts, it retracts the needle assembly  1560  along with it, retracting the needle  1452  into the hollow barrel of the syringe.  
         [0160]    It should be appreciated that many of the embodiments described herein are adaptable for use in non-retractable syringes. Embodiments that rely on the retraction mechanism to move a component of the vacuum assembly, such as a piston or stretchable tube may be implemented in a non-retractable syringe by using a biasing element to substitute for the retraction mechanism. For example, a spring may be interposed between the cylinder and the piston of the embodiment described in FIG. 30? so that, after the piston is pressed toward the top of the cylinder during injection of a patient, the piston driver drives the piston back down toward the bottom of the cylinder, creating a pressure drop and tending to draw fluid away from the needle tip.