Abstract:
A plunger is provided for a syringe that includes a pressure relief mechanism that exhausts any fluid trapped in the chamber between sealing flanges carried by the plunger when the plunger is advanced to dispense fluid from the syringe. The contact pressure between the barrel-contacting surface of the rearmost sealing flange nearest the access opening of the syringe barrel is weakened relative to the contact pressure provided by the barrel-contacting surface of the sealing flange that isolates the fluid within the barrel. The contact pressure of the rearmost sealing flange may be lessened by either reducing its radial diameter relative to the radial diameter of the other sealing flange, removing an underlying portion of the plunger, or configuring the rearmost sealing flange as deflectable sealing lip.

Description:
FIELD OF THE INVENTION 
     This invention relates to syringes, and more particularly, to a plunger having an improved sealing ability for use with a syringe. 
     BACKGROUND OF THE INVENTION 
     Syringes are devices routinely employed for providing a quantity of a medical liquid to a patient, typically into the patient&#39;s arteriovenous system, and for dispensing liquids in other non-medical applications. Syringes are traditionally configured with a tubular barrel for holding the liquid, a plunger within the barrel, an access opening for the plunger at a proximal end of the barrel, and a smaller discharge outlet at a distal end of the barrel. The plunger has a proximal portion adapted to receive a dispensing force, a central section with sealing flanges, and a distal head that contacts the liquid within the barrel. During a dispensing procedure, a dispensing force moves the plunger in a distal direction along the longitudinal length of the barrel, if the magnitude of the dispensing force is sufficient, to urge the liquid contained in the barrel through the discharge outlet. The sufficiency of the dispensing force depends, among other factors, upon the viscosity of the liquid held by the barrel. 
     Plungers usually carry two annular sealing flanges of substantially identical dimension and in a spaced relationship that extend about the circumference of an outer surface of the plunger. Each flange has a barrel-contacting sealing surface configured to compressively engage the interior of the barrel. An annular chamber is defined in the space bounded by the adjacent sealing flanges, the cylindrical side wall of the plunger and the interior cylindrical surface of the barrel. During a dispensing procedure, the distal sealing flange exerts a contact pressure against the interior wall of the barrel sufficient to prevent unwanted leakage of the liquid past the plunger in a proximal direction. The proximal sealing flange is present to ensure proper alignment of the plunger during a dispensing procedure and to prevent the passage of air during when the syringe is aspirated to, for example, fill the interior of the barrel forward of the plunger with liquid. 
     As the dispensing force advances the plunger, the liquid in the barrel exerts a significant hydrostatic pressure against a distal surface of the plunger. The plunger is formed of a resilient material that responds to the hydrostatic pressure by compressing along its longitudinal axis. The axial compression is accommodated by an outward distention of the plunger body. The sealing flanges likewise move radially outward so that the contact pressure is enhanced between the barrel-contacting sealing surface of the distal sealing flange and the interior of the barrel. The enhanced contact pressure offsets the increased hydrostatic pressure exerted by the liquid against the distal sealing flange and preserves the integrity of the fluid-tight barrel-contacting engagement. However, the outward distension of the side walls also reduces the volume of the annular chamber. 
     The annular chamber is normally occupied by a compressible fluid, such as air, that can accommodate a change in volume without supplying a significant opposing force against the contact pressure. In certain situations, however, a quantity of liquid may pass the distal sealing flange and become trapped in the annular chamber nearest to the distal sealing flange prior to either initiating or completing the dispensing procedure. For example, liquid may become trapped in the annular chamber when the syringe is filled or if the plunger cants during the dispensing procedure. Liquids are relatively incompressible fluids that occupy roughly a constant volume independent of an applied external pressure. 
     If a significant quantity of liquid is trapped in the annular chamber, the contact pressure exerted against the interior of the barrel by the sealing surfaces of the sealing flanges will be reduced. In particular, the distal sealing flange may actually exert a lesser contact pressure against the interior of the barrel than the proximal sealing flange. During the dispensing operation, the contact pressure between the distal sealing flange and the interior of the barrel cannot increase commensurate with the axial compression due to the presence of the incompressible trapped liquid. As a result, the sealing ability of the distal sealing flange may be compromised. 
     If the distal sealing flange loses contact with the interior of barrel, the highly pressurized liquid within the barrel will rush through the newly-created gap. The proximal sealing flange is not configured to resist the flow of highly pressurized liquid and readily yields to the leaking liquid, which exits past the plunger in a proximal direction toward the access opening. When such “blow-by” events occur, significant quantities of liquid can stream past the plunger. Among the undesirable side effects of blow-by, the patient will receive less liquid per unit distance of plunger movement than the prescribed dosage. If the syringe is recycled to perform multiple dispensing operations, the likelihood of a blow-by event increases with each subsequent dispensing operation. 
     Liquid may be dispensed in an precise fashion over a lengthy duration with the assistance of a power injector. Power injectors utilize a motor-driven plunger drive adapted to engage and continuously advance the plunger for incrementally dispensing the contents of the syringe over an extended time according to predetermined injection parameters such as flow rate, volume, duration, and time. Power injectors are commonly used for carrying out extended infusions of a liquid, such as an imaging contrast agent, into the vascular system of a patient, because of the greater reliability and consistency in infusion rates and dosage when compared to a manual injector. The occurrence of blow-by in a syringe being actuated by a power injector is more deleterious that blow-by associated with a manual injection because the injection pressure can approach 1200 pounds per square inch (psi). However, blow-by has been noted for injection pressures as low as 50 psi. In addition to the uncertainties in patient dosage discussed above, the power injector itself can be damaged by the introduction of liquid into the mechanism or by the need to overdrive the motor to compensate for a reduced delivery rate. Further, the injector and the portion of the procedure room near the injector may be soiled by the escaped liquid. Still further, the sterility of the equipment may be compromised. 
     Thus, an improved plunger is needed for a syringe having a configuration of sealing flanges that will prevent blow-by of the liquid held within the syringe barrel during a dispensing procedure and that can do so without significantly increasing the sliding frictional force between the sealing flanges and the interior of the barrel. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these and other problems by defining a plunger with dual sealing flanges for use with a syringe, where the plunger is configured to prevent a blow-by event. Further, the present invention provides a plunger configured with a pressure relief mechanism that preferentially ejects any liquid trapped between the sealing flanges of the plunger so as to prevent blow-by. The present invention provides sealing flanges with optimized dimensions or modified structure that eliminates blow-by without significantly altering the frictional force between the sealing flanges and the interior of the syringe barrel. Further, the present invention provides a plunger that is not sensitive to trapped liquid so that the more complex factors that contribute to the trapping of the liquid do not have to be addressed and solved. 
     According to the present invention, one embodiment of the plunger has a piston with a chamber susceptive of trapping a portion of the liquid when the plunger is in motion. When the plunger is moved by a dispensing force in a proximal-to-distal direction to dispense liquid from the syringe, the piston is configured to maintain a fluid-tight contact pressure with the interior surface of the syringe barrel and to preferentially exhaust any trapped liquid in a proximal direction. 
     In another embodiment, the piston has first and second spaced circumferential sealing flanges that project radially outwardly from a cylindrical outer surface. The second or proximal sealing flange is spaced from the first sealing flange to define a chamber, which may have an annular configuration. The first or distal sealing flange is substantially circumferentially continuous about the cylindrical outer surface of the plunger and has a barrel-contacting sealing surface that exerts a first contact pressure on the interior surface of the syringe barrel sufficient to normally maintain a fluid-tight seal therewith. The second sealing flange likewise has a barrel-contacting sealing surface configured to exhaust liquid trapped in the chamber in a proximal direction. The piston has a head, that is usually conical and integral with the cylindrical outer surface, that contacts the liquid held by the barrel and, according to one embodiment, that tapers to an included angle of about 120°. 
     According to one embodiment of the present invention, the second sealing flange may be substantially circumferentially continuous and exert a second contact pressure on the inside surface of the barrel that is lesser than the first contact pressure exerted by the first sealing flange. As a result, any liquid trapped in the chamber preferentially overcomes the second contact pressure and is exhausted in a proximal direction without disturbing the fluid-tight seal provided by the first sealing flange. The differential in contact pressure may be established by mismatching the radial distance of the sealing flanges, measured relative to the longitudinal axis of the barrel, so that the first sealing flange has a larger radial distance than the second sealing flange. Increasing the radial distance of the first sealing flange by 10 mils has been found to optimize the pressure relief without significantly altering the sliding frictional forces between the sealing flanges and the interior of the barrel. 
     In another embodiment, the second sealing flange is a circumferential sealing lip that projects in a radially outward and proximal direction from the cylindrical outer surface of the plunger. The sealing lip has a barrel-contacting sealing surface that contacts the inside flange of the syringe barrel. The sealing surface exerts a lesser contact pressure with the interior of the barrel than the first sealing flange and, as a result, the sealing lip can deflect proximally to exhaust any pressurized fluid trapped in the chamber between the sealing flanges without compromising the sealing ability of the first sealing flange. 
     In another embodiment, the plunger may comprise an outer sheath that surrounds the exterior of an inner member. The sheath includes first and second spaced circumferential sealing flanges that project radially outwardly from an exterior surface of a cylindrical sidewall for compressively engaging the interior of the syringe barrel. 
     In other embodiments, the contact pressure exerted by the second sealing flange may be reduced by removing a portion of the exterior of the piston or by removing a portion of the interior of the sheath that, in each case, is radially inwardly of the second sealing flange. The reduction in the contact pressure exerted by the second sealing flange against the interior of the barrel relative to the contact pressure of the first sealing flange provides a path of lesser resistance for pressurized trapped fluid to exhaust in a proximal direction. 
     The present invention shall become more apparent from the accompanying drawings and description thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
     FIG. 1 is a side view of a plunger for a syringe, which is shown in longitudinal cross-section, according to the present invention; 
     FIG. 2 is an enlarged view of encircled area “ 2 ” of FIG. 1; 
     FIG. 3 is a section of the plunger shown in FIG. 2; 
     FIG. 4 is a view, similar to FIG. 2, during a filling operation; 
     FIG. 5 is a view, similar to FIG. 4, following a filling operation; 
     FIG. 6 is a view, similar to FIG. 5, during a dispensing procedure; 
     FIG. 7 is a section view of an alternative embodiment of a plunger for a syringe according to the present invention; 
     FIG. 8 is a section view of another alternative embodiment of a plunger for a syringe according to the present invention; 
     FIG. 9 is a section view of yet another alternative embodiment of a plunger for a syringe according to the present invention; and 
     FIG. 10 is a section view of an alternative embodiment of the plunger shown in FIG.  8 . 
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to plunger having dual sealing flanges for use with an otherwise conventional syringe, wherein the sealing flanges are configured to prevent pressurized liquid trapped between the sealing flanges from compromising the sealing ability of the plunger during a dispensing procedure. In particular, the plunger has a pressure relief mechanism, which is operable to preferentially eject any trapped liquid when a dispensing force is applied to the plunger. More particularly, one sealing flange is configured to have a reduced contact pressure with the interior of the syringe barrel. Pressurized trapped liquid is preferentially exhausted by this sealing flange without affecting the sealing ability of the plunger. As a result, the phenomenon known as blow-by is eliminated during a dispensing procedure. As used hereinafter, blow-by is defined as allowing a significant amount of liquid to leak past the sealing flanges of the plunger during a dispensing procedure. 
     Referring to FIGS. 1-3, a syringe  20  includes an elongated, tubular barrel  22  extending along a longitudinal axis between a distal end  21  and a proximal end  23 . A discharge outlet  26  is attached to a frustoconical portion  28  of distal end  21  and terminates with a discharge tip  30 . Tip  30  is adapted to attach the interior of barrel  22  in fluid communication with a cannula or a Luer connector interfaced with a length of tubing and a catheter (not shown). Inserted through an access opening  31  in proximal end  23  of syringe  20  is a plunger  32 , which comprises a piston-like structure positioned for sliding movement within the interior of barrel  22  and along a longitudinal axis thereof. 
     The portion of the interior of barrel  22  between tip  30  and plunger  32  defines a volume for containing a quantity of a liquid  24 . This portion of the interior of barrel  22  may be either prefilled with liquid  24  or filled with liquid  24  by placing discharge outlet  26  in fluid communication with a liquid reservoir and moving plunger  32  in a proximal direction to siphon liquid  24  through discharge outlet  26 . Alternatively, syringe  20  may be filled by pouring liquid  24  directly into the rear access opening  31  of barrel  22  and inserting the plunger  32 . Syringe  20  may hold, for example, a maximum volume of 125 ml, 200 ml, etc. depending on the amount of liquid  24  to be delivered to the patient. It will be understood that the dimensions and capacity of syringe  20  may be varied without departing from the spirit and scope of the present invention. 
     Liquid  24  may comprise a substance suitable for injection into a patient, such as a pharmaceutical liquid or an imaging contrast agent. If liquid  24  is an imaging contrast agent, liquid  24  should be provided in barrel  22  with a quantity sufficient to facilitate an imaging operation. 
     The proximal side of the plunger  32  includes an attachment tab  38  adapted to couple plunger  32  with a gripper carried by a plunger ram (not shown). When a dispensing force of a sufficient magnitude is applied to the attachment tab  38  in a direction indicated by arrow  40 , plunger  32  urges liquid  24  through outlet  26 . In particular, syringe  20  may be configured for use with a power injector and the plunger ram may be attached to a power injector (not shown), for supplying the dispensing force to move plunger  32  within barrel  22 . 
     As best shown in FIG. 3, one embodiment of the plunger  32  includes an outer sheath  34  having a cylindrical sidewall  41  and a conical head  43  integrally interconnected with the proximal end of sidewall  41 . A substantial portion of conical head  43  contacts the liquid  24  contained within barrel  22 . Conical head  43  has a predetermined included angle that, in the embodiment  10  shown in FIG. 1, tapers to an included angle, φ, of about 120°. However, it is understood that other included angles may be selected without departing from the spirit and scope of the present invention. It is further understood that head  43  may have a geometrical shape other than that of a conical without departing from the spirit and scope of the present invention. 
     Sheath  34  is configured be received and held in a snug, resilient fit by an inner member  36 . To that end, sheath  34  includes an annular ridge  41   a  extending about the interior of sidewall  41  and projecting in a radially inward direction. Annular ridge  41   a  engages an annular notch  36   a  provided on the exterior of inner member  36 . Sheath  34  is formed of a low durometer elastomer, such as a natural rubber or an isoprene, and inner member  36  is formed of a higher durometer polymer, such as a delrin, a polycarbonate, or preferably a polypropylene. 
     A first sealing flange  44  and a second sealing flange  46  are circumferentially positioned about an outer surface  39  of sidewall  41 . As best shown in FIGS. 2 and 3, sealing flanges  44 ,  46  are positioned in a spaced relationship and each projects radially outwardly from outer surface  39 . Each sealing flange  44 ,  46  has a barrel-contacting sealing surface  45 ,  47 , respectively, that compressively engages an interior surface  37  of barrel  22 . The first sealing flange  44  is substantially circumferentially continuous and sealing surface  45  exerts a sufficient first contact pressure to normally provide a fluid-tight compressive engagement with the interior of barrel  22 . First sealing flange  44  has a generally semi-circular cross-sectional profile with a predetermined radius of curvature RC 1  when in an uncompressed state. The second sealing flange  46  also has a substantially circumferentially continuous sealing surface  47  that exerts a second contact pressure on the interior of barrel  22 . Second sealing flange  46  has a generally circular cross-sectional profile with a predetermined radius of curvature RC 2 , when in an uncompressed state. The compressive engagement with the interior surface  37  deforms and distorts sealing surfaces  45 ,  47  such that, when plunger  32  is inserted into barrel  22 , each of the sealing surfaces  45 ,  47  and the interior surface  37  is substantially coplanar. When plunger  32  is moved by a dispensing force in a proximal-to-distal direction, the frictional force between each of the sealing surfaces  45 ,  47  and the interior surface  37  of barrel  22  is determined by an appropriate coefficient of dynamic friction characteristic of the combination of respective materials and the radially outward force produced by the respective first or second contact pressure. 
     An annular chamber  48  is defined between the first and second sealing flanges  44 ,  46 . Chamber  48  is an continuous, annular open volume disposed about the circumference of outer surface  39 . It will be understood that chamber  48  may comprise multiple subchambers partitioned by sections of the outer surface  39  of cylindrical wall  41  without departing from the spirit and scope of the present invention. Annular chamber  48  is susceptible to trapping a portion of the liquid  24  that would otherwise reside in barrel  22 , when plunger  32  is in motion. 
     In one embodiment of the present invention, the first and second flanges  44 ,  46  are configured to exhaust any trapped liquid  50  (FIG. 5) trapped by the annular chamber  48  in a distal-to-proximal direction toward access opening  31 . Trapped liquid  50  is exhausted from chamber  48  without substantially disturbing the fluid-tight sealing ability of the first sealing flange  44 . In particular, the sealing surface  47  of second sealing flange  46  exerts a contact pressure against the interior surface  37  of barrel  22  that is smaller than the contact pressure exerted by the sealing surface  45  of first sealing flange  44  against the interior surface  37  of barrel  22  when a dispensing force is applied to move plunger  32  in a proximal-to-distal direction for dispensing liquid  24  through the discharge outlet  26 . 
     The relative contact pressures of the first and second sealing flanges  44 ,  46  may be controlled by adjusting their respective radial distances, measured with respect to the longitudinal axis of barrel  22 . In particular the radial distance, measured relative to the longitudinal axis of barrel  22 , of the first sealing flange  44  is adjusted to be greater than the radial distance of the second sealing flange  46 . The difference in radial distance can be accomplished by mismatching the radius of curvature RC 1  and RC 2  of the sealing flanges  44  and  46 , respectively. However, it is understood that the mismatch could be otherwise accomplished, such as by inserting a radial spacer between the curved portion of sealing flange  44  and outer surface  39 , without departing from the spirit and scope of the present invention. 
     According to the present invention, the ability to exhaust trapped liquid  50  in a proximal direction toward access opening  31  is optimized by adjusting the radial distance of the second sealing flange  46  to be about 10 mils (0.010″) less than the radial distance of the first sealing flange  44 , when sealing flanges  44 ,  46  are in an uncompressed state. This 10 mil difference in radial distance RD (see FIG. 3) has been found to eliminate blow-by without significantly altering the dynamic frictional force between the sealing surfaces  45 ,  47  and the interior surface  37  of barrel  22 . 
     By way of example, and not by way of limitation, a plunger  32  adapted for a syringe  20  having a capacity of 200 ml may have a first sealing flange  44  with a radial distance of about 0.951″ and a second sealing flange  46  with a radial distance of about 0.9″, wherein the radii of curvature RC 1 , RC 2  of the respective sealing flanges  44 ,  46  are 20 mils and 30 mils, respectively. In this specific embodiment, sheath  34  has a side wall  45  with a radial distance of about 0.9135″ and a wall thickness of about 80 mils to about 100 mils, preferably about 90 mils. 
     The operation of plunger  32  according to the present invention is diagrammatically illustrated in FIGS. 4-6. Referring to FIG. 4, the plunger  32  is moved in a proximal direction, indicated by arrow  52 , to fill the barrel  22  with liquid  24  by aspiration during a filling operation. Under certain circumstances, the vacuum generated by the aspiration induces the barrel-contacting sealing surface  45  of first sealing flange  44  to lose contact with the interior surface  37  of barrel  22 . A quantity of liquid  24  can pass between the first sealing flange  44  and interior surface  37  of barrel  22  and become trapped in annular chamber  48 , as shown in FIG.  5 . Alternatively, trapped fluid  50  can originate from canting of the plunger  32  during a dispensing procedure if either the attachment tab  38  is slightly off-center or the frictional force between the sealing flanges  44 ,  46  and the interior surface  37  of barrel  22  is circumferentially nonuniform. 
     As shown in FIG. 6, if a quantity of trapped liquid  50  otherwise sufficient to produce blow-by during a dispensing procedure becomes trapped in annular chamber  48 , the reduced contact pressure between the sealing surface  47  of the second sealing flange  46  and the interior surface  37  of the barrel  22  permits all or a portion of the trapped liquid  50  to pass as in a proximal direction when a dispensing force is applied to plunger  32 . As plunger  32  is advanced by the dispensing force in a distal direction, indicated by arrow  54 , the liquid  24  in barrel  22  exerts a hydrostatic pressure on the conical head  43  of sheath  34 . The hydrostatic pressure axially compresses the sheath  34  so that cylindrical sidewall  41  distends radially outwardly toward the interior surface  37  of barrel  22 . As a result, the volume of annular chamber  48  is reduced and any trapped fluid  50  is pressurized. 
     The axial compression increases the contact pressure exerted on the interior surface  37  of barrel  22  by sealing surface  45  of the first sealing flange  44 , which is urged radially outwardly in proportion to the hydrostatic pressure. The axial compression also increases the contact pressure exerted on the interior surface  37  of barrel  22  by sealing surface  47  of the second sealing flange  46 , which is urged radially outwardly in proportion to the hydrostatic pressure. 
     Due to the difference in radial distance RD, sealing surface  47  of the second sealing flange  46  exerts a weakened contact pressure against the interior surface  37  of barrel  22  relative to the contact pressure exerted by the first sealing flange  44 . As a result, sealing surface  47  may preferentially lose contact with the interior of barrel  22  so that all or a portion of the pressurized trapped liquid  50  can be exhausted in a proximal direction as exhausted fluid  56 . However, the contact pressure exerted by sealing surface  45  of the first sealing flange  44  against the interior surface  37  of barrel  22  is not significantly effected by the trapped fluid  50 . Sealing surface  45  remains compressively engaged against the interior surface  37  of barrel  22  with a contact pressure sufficient to resist leakage of fluid  24 . 
     According to the present invention, an alternative embodiment of a plunger  59  having a sheath  60  covering an inner member  36  is presented in FIG. 7 in which like parts have like numerals as in FIGS. 1-3. Sheath  60  may carry a circumferential lip  58  having a spaced relationship with respect to a generally semi-circular sealing flange  62 . Sealing flange  62  has a barrel-contacting sealing surface  62   a  that compressively engages the interior surface  37  of barrel  22  with a contact pressure sufficient to establish a fluid-tight engagement during a dispensing procedure. Circumferential lip  58  projects in a radially outwardly and proximal direction from an outer surface  64  of a cylindrical sidewall  66 . A contact portion  58   a  of lip  58  contacts the interior surface  37  of the barrel  22 , with a contact pressure. During a dispensing operation with a sufficient amount of a trapped liquid  68  confined in an annular chamber  70  between lip  58  and sealing flange  62 , the proximally-directed force exerted by trapped liquid  68  deflects lip  58  in a distal-to-proximal direction indicated by arrow  72  on FIG. 7, so that contact portion  58   a  loses contact with the interior surface  37  of barrel  22  to create a gap therebetween. All or a portion of the trapped liquid  68  flows through the gap and past the deflected lip  58  in a proximal direction toward the access opening  31  (FIG.  1 ). 
     In accordance with the present invention, a plunger  79  having a sheath  80  covering an inner member  81  is presented in FIG. 8 in which like parts have like numerals as in FIGS. 1-3. An outer surface  83  of sheath  80  carries a first sealing flange  82  and a second sealing flange  84 , which have barrel-contacting sealing surfaces  82   a ,  84   a , respectively. An annular chamber  90  is defined in the space bounded by the sealing flanges  82 ,  84 , the interior surface  37  of barrel  22 , and the outer surface  83  of sheath  80 . The contact pressure exerted by the sealing surface  84   a  of the second sealing flange  84  against the interior surface  37  of barrel  22  is reduced, relative to the contact pressure exerted by sealing surface  82   a , by removing material from the exterior of inner member  81 . Specifically, material is removed at a location beneath the second sealing flange  84  to provide an annular recess  86  of a semi-circular cross-sectional profile positioned radially inward from the second sealing flange  84 . It is understood that the shape or size of recess  86  may be varied without departing from the spirit and scope of the present invention. For example, recess  86  may have a rectangular cross-sectional profile, rather than the semi-circular cross-sectional profile illustrated in FIG.  8 . 
     During a dispensing procedure with a sufficient quantity of a pressurized trapped fluid  88  trapped in annular chamber  90 , second sealing flange  84  will preferentially yield under the force applied by trapped fluid  88 . As a result, the trapped fluid  88  will exhaust in a proximal direction, as indicated by arrow  92 , through the gap created between the sealing surface  84   a  of the second sealing flange  84  and the interior surface  37  of barrel  22 . Sealing surface  86   a  will remain compressively engaged against the interior surface  37  of barrel  22  with a contact pressure sufficient to resist the hydrostatic pressure exerted in a proximal direction by liquid  24  held by barrel  22 . 
     In accordance with the present invention, a plunger  99  having a sheath  100  covering an inner member  101  is presented in FIG. 9 in which like parts have like numerals as in FIGS. 1-3. An outer surface  102  of sheath  100  carries a first sealing flange  104  and a second sealing flange  106  having respective barrel-contacting sealing surfaces  104   a ,  106   a . An annular chamber  108  is defined in the space bounded by the sealing flanges  104 ,  106 , the interior surface  37  of barrel  22 , and the outer surface  102  of sheath  100 . Material is removed from the interior of sheath  100  radially inward from the second sealing flange  106  to provide an annular recess  110 . Recess  110  functions to reduce the contact pressure that the sealing surface  106   a  of second sealing flange  106  exerts against the interior surface  37  of barrel  22 . It is understood that the shape or size of recess  110  may be varied without departing from the spirit and scope of the present invention. For example, recess  110  may have a rectangular cross-sectional profile, rather than the semi-circular cross-sectional profile illustrated in FIG.  9 . 
     During a dispensing procedure, second sealing flange  106  will preferentially yield under the force applied by pressurized trapped fluid  112 , if trapped fluid  112  is present in a sufficient quantity in annular chamber  108 . As a result, the trapped fluid  112  will exhaust in a proximal direction, as indicated by arrow  114  on FIG. 9, through the gap created between the sealing surface  106   a  of second sealing flange  106  and the interior surface  37  of barrel  22 . Barrel-contacting sealing surface  104   a  of first sealing flange  104  will remain compressively engaged against the interior surface  37  of barrel  22  with a contact pressure sufficient to resist the hydrostatic pressure exerted proximally by liquid  24  within barrel  22 . 
     In accordance with the present invention, a plunger  119  having a sheath  120  covering an inner member  121  is presented in FIG. 10 in which like parts have like numerals as in FIGS. 1-3. An outer surface  122  of sheath  120  carries a first sealing flange  124  and a second sealing flange  126  in a spaced relationship and having respective barrel-contacting sealing surfaces  124   a ,  126   a . An annular chamber  128  is defined in the space bounded longitudinally by the sealing flanges  124 ,  126 , and bounded radially between the interior surface  37  of barrel  22  and the outer surface  122  of sheath  120 . An annular ridge  127  on the interior of sheath  120  engages an annular notch  129  provided on the exterior of inner member  121  for securing sheath  120  to inner member  121 . 
     An annular recess  130  is positioned radially inward of second sealing flange  126  and provides a circumferential cavity that separates annular notch  129  and annular ridge  127 . Recess  130  is created by removing material from either sheath  120  to increase the inner radial distance of annular ridge  127  or, in the alternative, by removing material from inner member  121  to reduce the outer radial distance of the annular notch  129 . It is understood that the shape or size of recess  130  may be varied without departing from the spirit and scope of the present invention. 
     During a dispensing procedure, recess  130  functions to reduce the contact pressure exerted by the barrel-contacting sealing surface  126   a  of second sealing flange  126  against the interior surface  37  of barrel  22 . Recess  130  permits annular ridge  127  to move radially inward in response to the axial compression of sheath  120 . As a result, sealing surface  126   a  of second sealing flange  126  will preferentially yield under the force applied by pressurized trapped fluid  132 , if a sufficient quantity of trapped fluid  132  is present in annular chamber  128 . Trapped fluid  132  exhausts in a proximal direction, as indicated by arrow  134  on FIG. 10, through the gap created between sealing surface  126   a  and the interior surface  37  of barrel  22 . Barrel-contacting sealing surface  124   a  of first sealing flange  124  will remain compressively engaged against the interior surface  37  of barrel  22  with a contact pressure sufficient to resist the hydrostatic pressure exerted proximally by liquid  24  within barrel  22 . 
     While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, the present invention is not limited to dispensing pharmaceutical liquids and may be used in conjunction with a syringe for dispensing non-pharmaceutical liquids, such as adhesives. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of applicant&#39;s general inventive concept.