Syringe plunger tip

A sealing cap is provided for attachment to the end of a syringe plunger. The sealing cap has an annular sidewall with an annular first sealing wing projecting distally therefrom, an annular second sealing wing positioned proximal of the first sealing wing and projecting proximally, and an annular sealing ridge encircling and radially projecting out from between the first sealing wing and the second sealing wing. The sealing cap further includes an interior cavity having an annular side surface aligned with the annular sidewall. A side groove is recessed within the side surface so as to be aligned slightly proximal of the annular sealing ridge. The distal end of a syringe plunger has a head mounted thereon that is substantially complementary to the configuration of the internal cavity of the sealing cap. More specifically, a tapered ridge is formed on the periphery of the head to complementarily be received within the side groove. As a positive pressure is created against the sealing cap, the first sealing wing flexes against the interior surface of the barrel and the annular sealing ridge urges against the interior surface of the barrel, thereby increasing the effective seal. As a negative pressure is formed within the barrel, the second sealing wing flexes against the interior surface of the barrel to increase the effective seal.

BACKGROUND OF THE INVENTION 
1. The Field of the Invention 
The present invention relates to syringes and, more specifically, medical 
syringes having a syringe plunger with an improved tip for sealing against 
the interior surface of a syringe barrel. 
2. The Relevant Technology 
A variety of different syringes are used in the medical field. A typical 
syringe comprises a tubular barrel having an access opening formed at one 
end, and a smaller discharge opening formed at the opposing end. The lead 
end of an elongated plunger is received within the access opening of the 
barrel so as to be slidable within the barrel. Attached to the lead end of 
the plunger is a flexible sealing member that snugly seals against the 
interior surface of the barrel. A needle or threaded member is usually 
attached to the discharge opening on the barrel. The needle can be used to 
penetrate a surface while the threaded member can be used to attach the 
syringe to another medical device, such as a catheter. 
During use, the discharge end of the syringe is initially placed in contact 
with a fluid. For example, the needle on the syringe can be inserted into 
a liquid medication. As the plunger is retracted within the barrel, a 
process known as aspiration, a negative pressure is formed within the end 
of barrel so as to cause the fluid to be drawn into the barrel. The 
syringe can then be moved to a second location where advancing the plunger 
within the barrel causes the fluid to be pushed or expressed out the 
discharge end of the barrel. 
Syringes come in a variety of different sizes and configurations each 
having unique properties for their intended use. For example, at times it 
is necessary to use a syringe that is capable of withstanding high 
pressures within the barrel. Such high pressures may be encountered when 
it is necessary to introduce a large amount of fluid into a body of an 
individual in a short period of time. High syringe pressures may also be 
encountered when it is necessary to insert a large amount of fluid into 
relatively small structure, such as the small, narrow tube of a catheter. 
To accommodate the above situations, it is necessary that the seal between 
the sealing member and the barrel be sufficiently strong to prevent 
leaking. Failure of the seal would prevent at least a portion of the fluid 
from being discharge and, as such, could be critical in an emergency 
situation. 
To accommodate the formation of high pressures, conventional syringes use 
enlarged sealing members which are compressed within the barrel such that 
a large lateral force is continually urged against the interior surface of 
the barrel. As such, the syringe is able to withstand high pressures 
within the barrel without failure of the seal. The problem associated with 
such syringes, however, is that the increased force pushing on the 
interior surface of the barrel increases the friction between the sealing 
member and the barrel. As a result, it is more difficult to advance the 
plunger within the barrel. Furthermore, increasing the friction between 
the sealing member and the barrel decreases the medical practitioners 
ability to feel the pressure on the fluid within the syringe barrel. 
Alternative types of syringes are made exclusively to enable a medical 
practitioner to feel any pressure variance on the fluid in the barrel. For 
example, administration of an epidural anesthesia requires a needle to be 
located within an epidural space located around the spinal cord. To help 
determine the exact location of the epidural space, medical practitioners 
use syringes which have very low friction between the sealing member and 
the barrel. As a result, a medical practitioner is able to determine the 
location of the needle tip by sensing through the plunger the pressure 
variations within the various body spaces. 
One type of syringe that is designed for such use is a glass syringe. Glass 
syringes, however, are extremely expensive as they are handmade and can 
shatter if dropped. Other types of syringes which accommodate the ability 
to feel the fluid pressure within the barrel have been made by using 
sealing members which only slightly engage the interior surface of the 
barrel, resulting in a very low friction force. One drawback, however, is 
that such syringes are unable to withstand the injection of fluids at high 
pressures without failure of the seal. 
Other syringes are designed to carefully dispense micro amounts of 
medication. Such syringes, however, often encounter the problem of 
"stiction." Stiction refers to the phenomenon that a syringe will jump or 
skip during small advancement of the plunger. As a result, more medication 
is deliver then is desired. Stiction occurs because a larger amount of 
energy is needed to overcome the static force between the barrel and 
sealing member than is needed to slide the plunger the desired distance. 
To design a syringe with low stiction, it is again necessary to minimize 
the friction force between the sealing member and the barrel. As is 
evidenced from the foregoing examples, however, syringes found in the 
prior art that are designed to have low friction typically do so at the 
expense of reducing the ability of the syringe to seal under high 
pressure. Conversely, syringes found in the prior art that are designed 
for high pressure applications typically to do at the expense of reducing 
the syringes' ability to have a "sensitive feel" and low stiction. 
As a result of what appears to be mutually exclusive properties for 
syringes, a medical facility is required to purchase and store a vast 
array of different types of syringes having different properties. This 
large number of syringes increases overhead costs and takes up valuable 
storage space. Furthermore, the large number of syringes complicates 
medical producers since the medical practitioners must ensure that they 
have the proper syringe for the proper procedure. 
OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide improved 
syringes. 
It is another object of the present invention to provide improved syringes 
that can withstand high pressures within the barrel without failure of the 
seal between the barrel and the plunger. 
It is yet another object of the present invention to provide improved 
syringes that have low friction between the plunger and barrel under low 
pressures. 
Another object of the present invention is to provide improved syringes 
that minimize stiction at low pressures. 
It is still another object of the present invention to provide improved 
syringes that are able to both withstand high pressures without failure 
and have low friction between the plunger and barrel at low pressures. 
Still another object of the present invention is to provide improved 
syringes in which the effectiveness of the seal between the plunger and 
the barrel increases as the pressure within the barrel increases. 
Another object of the present invention is to provide improved syringes in 
which a single syringe is able to accomplish the task of two or more 
conventional syringes. 
Furthermore, it is another object of the present invention to provide such 
syringes at a relatively low cost. 
Finally, it is another object of the present invention to provide syringes 
that produce effective seals between the plunger and barrel during both 
injection and aspiration. 
To achieve the foregoing objectives and in accordance with the invention as 
embodied and broadly disclosed herein, an improved syringe is provided. 
The syringe includes a barrel having an interior surface defining a lumen 
longitudinally extending therethrough. The syringe further includes an 
elongated plunger having a distal end slidably received within the lumen 
of the barrel. The plunger comprises a conical head positioned at the 
distal end of the plunger. The conical head radially slopes outward to an 
annular outer edge. 
Mounted on the head of the plunger is a flexible sealing cap. The flexible 
sealing cap is preferably made of silicone and can be coated with a 
lubricant. In the preferred embodiment, the flexible sealing cap comprises 
an annular sidewall having an exterior surface extending between a 
proximal end and a distal end. A conical crown is mounted on the annular 
sidewall so as to cover the distal end of the sealing cap. 
An annular first sealing wing encircles and radially projects outward in a 
distal direction from the exterior surface of the sidewall. The first 
sealing wing preferably projects out at an angle of about 30 degrees 
relative to the longitudinal axis of the sealing cap. The first sealing 
wing contacts the interior surface of the barrel when the distal end of 
the plunger having the flexible sealing cap received thereon is positioned 
within the barrel. 
The flexible sealing cap further includes an annular second sealing wing 
encircling and radially projecting outward in a proximal direction from 
the exterior surface of the sidewall. As with the first sealing wing, the 
second sealing wing also projects at an angle of approximately 30 degrees 
relative to the longitudinal axis of the sealing cap. The second sealing 
wing is positioned proximal of the first sealing wing and contacts the 
interior surface of the barrel when the sealing cap attached to the head 
of the plunger is received within the barrel. 
Radially projecting out from the exterior surface of the sidewall between 
the first sealing wing and the second sealing wing is an annular sealing 
ridge. The peak of the sealing ridge may or may not contact the interior 
surface of the barrel during ambient conditions. 
The flexible sealing cap further includes an interior surface defining a 
cavity within the sealing cap. The cavity is configured to complementarily 
receive the head of the plunger. As such, the cavity includes an annular 
side groove configured to complementarily receive the outer edge of the 
head of the plunger. The side groove is positioned relative to the annular 
sealing ridge so that the annular sealing ridge radially urges out to 
increasingly engage the interior surface of the barrel as the plunger is 
advanced within the barrel to increase the pressure within the barrel. 
Finally, an annular end face is positioned at the proximal end of the 
sealing cap and defines an opening to the cavity through which the head of 
the plunger can be received within the cavity of the sealing cap. 
One of the unique properties of the present inventive syringe is its 
ability to vary the amount of friction and the effective sealing force 
between the sealing cap and the barrel. For example, when the syringe is 
used under relatively low barrel pressures, the tips of the first and 
second sealing wings only slightly engage the interior surface of the 
barrel with a minimal amount of force. As such, only minimal friction 
force occurs. In contrast, however, as the pressure within the barrel 
increases, the pressure within the barrel acts on the first sealing wing, 
urging it to flex radially outwardly, so as to apply a greater amount of 
force against the interior surface of the barrel. The amount of sealing 
force applied by the sealing wing against the interior surface of the 
syringe barrel is proportional to the amount of pressure generated within 
the syringe barrel. Furthermore, the sealing ridge increasingly engages 
against the interior surface of the barrel. As a result, the effective 
sealing force between the sealing cap and the interior surface of the 
barrel increases as the pressure within the barrel increases. 
These and other objects, features, and advantages of the present invention 
will become more fully apparent from the following description and 
appended claims, or may be learned by the practice of the invention as set 
forth hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Depicted in FIG. 1 is a syringe 10 comprising a plunger 12 slidably 
received within a barrel 14. Plunger 12 is shown as having an exterior 
surface 16 extending between a proximal end 18 and a distal end 20. Barrel 
14 is shown as comprising an interior surface 22 that defines a lumen 24 
longitudinally extending through barrel 14. Barrel 14 is further shown as 
comprising an access end 26 through which plunger 12 is received within 
lumen 24, and an opposing discharge end 28 through which fluid is received 
within and discharged from lumen 24. In the embodiment shown, a luer lock 
connector 30 is rotatably attached to discharge end 28 for selective 
attachment to alternative medical devices. In alternative embodiments, 
various types of needles or adapters can be attached to discharge end 28 
in fluid communication with lumen 24. 
Positioned at distal end 20 of plunger 12 is a plunger tip 32 having a 
sealing cap 34 attached thereto. As better shown in FIG. 2, plunger tip 32 
comprises an exterior surface 36 extending between a proximal end 38 and a 
distal end 40. Plunger tip 32 is further shown as comprising a 
substantially cylindrical portion 39 positioned at proximal end 38, a 
conical head 52 positioned at distal end 40, and an annular tapered 
portion 41 that extends therebetween. 
Encircling and radially extending out from exterior surface 36 of 
cylindrical portion 39 is a first stop ridge 42. As also shown in FIG. 2, 
plunger 12 is shown as having an interior surface 44 defining a chamber 
46. Proximal end 18 of plunger 12 is also shown as terminating at an 
annular end face 48 which defines an opening 50 to chamber 46. The inner 
diameter of interior surface 44 of plunger 12 is comparable to the outer 
diameter of proximal end 38 of plunger tip 32. As such, plunger tip 32 is 
mounted to plunger 12 by slidably receiving proximal end 38 of plunger tip 
32 within opening 50 of chamber 46 until end face 48 abuts first stop 
ridge 42. An adhesive or other bonding means can be positioned between 
interior surface 44 and exterior surface 36 so as to provide a tight, 
sealed bond between plunger tip 32 and plunger 12. 
In alternative embodiments, plunger tip 32 can be integrally molded on 
plunger 12. As such, plunger 12 need not be hollow but can be solid. In 
like manner, although plunger tip 32 is shown as having an internal 
chamber 51 to decrease material cost, plunger tip 32 can also be formed 
without chamber 51. 
Conical head 52 is shown as comprising an annular distal end face 54 that 
radially slopes out in a proximal direction to an outside shoulder 56. 
Shoulder 56 has a proximal portion 58 and a distal portion 60. Encircling 
and radially extending out from between portions 58 and 60 is an annular 
tapered ridge 62. Conical head 52 further includes an annular proximal end 
face 64 that extends between proximal portion 58 of shoulder 56 and 
tapered portion 41. As will be discussed later in greater detail, an 
annular second stop ridge 66 encircles and radially projects out from 
exterior surface 36 between first stop ridge 42 and head 52. 
Sealing cap 34 is shown as comprising an annular sidewall 68 extending 
between a proximal end 70 having an annular end face 71 and an opposing 
distal end 72. A conical crown 74 is integrally attached to sidewall 68 so 
as to cover distal end 72. An annular first sealing wing 76 encircles and 
radially projects outward in a distal direction from distal end 72. First 
sealing wing 76 tapers to an annular lip 77 at an angle .beta. in a range 
between about 20.degree. to about 40.degree. with about 25.degree. to 
about 35.degree. being more preferred. First sealing wing 76 also projects 
at an inside angle .alpha. that is less than 90.degree. relative to the 
longitudinal axis "A" of sealing cap 34. First sealing wing 76 typically 
projects at an angle in a range between about 20.degree. to about 
40.degree., and more preferably in a range from between about 25.degree. 
to about 35.degree. relative to longitudinal axis "A" of sealing cap 34. 
An annular second sealing wing 78 encircles and radially projects out from 
sidewall 68 proximal of first sealing wing 76. Second sealing wing 78 
tapers to an annular lip 79 at an angle comparable to angle .beta.. Second 
sealing wing 78 projects in a proximal direction at an inside angle less 
than 90.degree. relative to the longitudinal axis "A" of sealing cap 34. 
The angle at which second sealing wing 78 projects is comparable to the 
angle .alpha. at which first sealing wing 76 projects. 
Positioned between first sealing wing 76 and second sealing wing 78 is a 
tapered, annular sealing ridge 80 which encircles and radially projects 
out from sidewall 68. Sealing ridge 80 tapers to an annular lip 81 that 
has an outer diameter that is preferably, although not necessarily, 
smaller than the outer diameter of the annular lips 77 and 79 when sealing 
wings 76 and 78 are in an unflexed position. 
Sealing cap 34 is further shown as comprising an interior surface 82 
defining a cavity 84. Cavity 84 is configured to complementarily receive 
head 52 of plunger tip 32. More specifically, interior surface 82 
comprises an annular side face 86 which is adjacent to and substantially 
parallel to sidewall 68. Side face 86 includes a proximal shoulder 88 and 
a distal shoulder 90. Formed between shoulders 88 and 90 is a tapered side 
groove 92 that is recessed within side face 86. Side groove 92 tapers to 
an annular apex 93. In the preferred embodiment, apex 93 is positioned 
slightly proximal of annular lip 81 of sealing ridge 80 as designated by 
the distance "d". In alternative embodiments, apex 93 can be aligned with 
annular lip 81 or even distal of annular lip 81. 
Formed distal of side face 86 is a conical inside surface 94 that is formed 
to complementarily abut distal end face 54 of head 52. Radially projecting 
inward from proximal shoulder 88 is an annular face 96. A frustoconical 
inside wall 98 extends between annular face 96 and annular end face 71, 
thereby defining an opening 100 to cavity 84. Inside wall 98 and annular 
face 96 define a retaining lip 105. 
Sealing cap 34 is preferably made of a resiliently flexible material such 
as silicone or rubber. Alternatively, other medical grade materials having 
comparable properties can be used. As a result of the resilient and 
flexible nature of sealing cap 34, opening 100 can be radially expanded to 
allow head 52 to be received within cavity 84, as shown in FIG. 3. As seen 
in FIG. 3 but as primarily noted by the reference characters in FIG. 2, 
cavity 84 is configured so that as distal end face 54 of head 52 abuts 
against conical face 94, tapered ridge 62 is received within side groove 
92. Furthermore, proximal portion 58 and distal portion 60 of outside 
shoulder 56 are aligned against proximal shoulder 88 and distal shoulder 
90, respectively, of sealing cap 34. 
Once head 52 is positioned within cavity 84, inside wall 98 resiliently 
returns to its original relaxed or at-rest shape, such that annular face 
96 is aligned against proximal end face 64 of head 52 and annular end face 
71 of sealing cap 34 is biased against second stop ridge 66. As a result, 
sealing cap 34 is securely held on plunger tip 32. Sealing cap 34 is 
preferably sized so as to be unstressed once received on head 52. A slight 
gap may even exist between sealing cap 34 and head 52. 
Sealing cap 34 can be manufactured from a variety of conventional 
manufacturing processes. By way of example and not be limitation, sealing 
cap 34 can be manufactured by transfer molding or liquid injection 
molding. Furthermore, sealing cap 34 can be coated with a medical grade 
lubricant such as a silicone oil. In an alternative embodiment, sealing 
cap 34 can be coated with a fluorosilicon fluid. The fluorosilicon fluid 
is marketed as an improved lubricant since it does not soften the silicone 
material. 
FIG. 3 illustrates sealing cap 34 mounted on plunger tip 32 and the 
combination thereof being received within lumen 24 of barrel 14 at ambient 
conditions. As used in the specification and the appended claims, the term 
"ambient conditions" is defined as meaning that no external force is being 
applied to the proximal end of plunger 12 and, therefore, no internal 
pressure is being generated within lumen 24 of barrel 14. The inner 
diameter D.sub.1 of barrel 14 (see FIG. 3) is slightly smaller than the 
outer diameter D.sub.2 of first sealing wing 76 and second sealing wing 78 
(see FIG. 2) when sealing wings 76 and 78 are in an unflexed condition. 
Accordingly, as sealing cap 34 is positioned within lumen 24 of barrel 14, 
sealing wings 76 and 78 radially flex inward creating a continuous and 
positive bias between sealing wings 76 and 78 and interior surface 22 of 
barrel 14. As a result of the positive bias, a seal is formed between 
sealing wings 76 and 78 and interior surface 22 of barrel 14 at ambient 
conditions. 
Although annular lip 81 of sealing ridge 80 is shown in FIG. 3 as slightly 
touching interior surface 22 of barrel 14, this is not necessary. In 
alternative embodiments, sealing ridge 80 may be separated from interior 
surface 22 when no pressure is being exerted on sealing cap 34. 
As a result of the minimal surface contact and the low bias force between 
sealing wings 76 and 78 and interior surface 22, there is relatively low 
friction between sealing cap 34 and barrel 14. As a result, syringe 10 
minimizes stiction at low pressures within barrel 14 and is highly 
effective in delivering micro amounts of medication. Furthermore, syringe 
10 is highly sensitive to variations of fluid pressures within lumen 22 at 
low pressure ranges. 
Depicted in FIG. 4 is the syringe assembly of FIG. 3 wherein the sealing 
cap 34 is under an increased pressure. That is, with the application of an 
external force to the proximal end of plunger 12, plunger 12 has been 
advanced so as to compress a fluid within discharge end 28 of barrel 14, 
thereby generating a positive internal pressure within the discharge end 
28 of barrel 14. In ram, the fluid produces a corresponding compressive 
force against sealing cap 34. As the pressure gradually increases against 
sealing cap 34, several structural modifications of sealing cap 34 
gradually transpire. Initially, as the pressure increases, first sealing 
wing 76 flexes radially outward with an increased force so as to produce 
an increase in the effective seal between sealing wing 76 and interior 
surface 22 of barrel 14. 
Furthermore, since sealing cap 34 is made of a flexible material, sealing 
cap 34 in part reacts as a fluid. Accordingly, as the pressure against 
sealing cap 34 increases, the material of sealing cap 34 attempts to flow 
toward the low pressure zone which is located proximal of sealing cap 34. 
As the material of sealing cap 34 begins to flow around tapered ridge 62, 
the material is pushed radially outward causing sealing ridge 80 to 
complementarily radially project outward so as to bias against interior 
surface 22 of barrel 14. Furthermore, by positioning tapered ridge 62 
proximal of sealing ridge 80, tapered ridge 62 is better able to direct 
more of the material of sealing cap 34 against sealing ridge 80. As a 
result, sealing ridge 80 creates a second seal that incrementally 
increases in effectiveness as the pressure within barrel 14 increases. 
Although the effective increase in sealing also results in an increase of 
frictional forces, the resulting detriment of having an increase in 
friction is decreased as the pressure within barrel 14 increases. That is, 
the need for dispensing micro amounts of medication or sensing the 
variance in pressure of a fluid within barrel 14 is decreased as the 
pressure within barrel 14 increases. 
As the pressure within barrel 14 begins to decrease, the resilient nature 
of sealing cap 34 causes sealing cap 34 to return to its original 
configuration, as shown in FIG. 3. As a result of the above-described 
configuration, syringe 10 has low friction at low pressure and increased 
sealing effectiveness as the pressure increases. The variable properties 
of inventive syringe 10 thus enables it to be used in a variety of 
different applications. 
Second sealing wing 78 is of greatest effectiveness when syringe 10 is used 
in aspiration. That is, as syringe plunger 12 is retracted within barrel 
14, a negative pressure can be created distal of sealing cap 34. As used 
in the specification and the appended claims, the term "negative pressure" 
is defined as meaning a pressure less than the ambient pressure. In the 
embodiment as shown in FIG. 3, as the negative pressure is created, second 
sealing wing 78 radially flexes outward so as to increase the bias against 
interior surface 22. As such, the greater the negative pressure, the 
greater the seal produced between second sealing wing 78 and interior 
surface 22. Further, the adjacent positioning of tapered ridge 62 acts as 
a support for second sealing wing 78, thereby preventing second sealing 
wing 78 from folding back over on itself. 
Using the above teachings in the formation of syringe 10, syringe 10 can be 
constructed so as to provide an effective seal between sealing cap 34 and 
interior surface 22 of barrel 14 that will not leak when exposed to 
pressures greater than about 200 pounds per square inch, and preferably 
greater than about 400 pounds per square inch, and more preferably greater 
than about 600 pounds per square inch. Likewise, an effective seal between 
sealing cap 34 and interior surface 22 can be made that will prevent 
leaking against significant negative pressures. 
Using the above teachings, it is also possible to design syringes that can 
work over a broad spectrum of desired pressures or that can be 
specifically designed to operate within a desired range of pressures. For 
example, where it is desirable that the inventive syringe only be operated 
under high pressures, it may be desirable to have sealing wings 76 and 78 
project at a steeper angle so that a greater initial biasing force is 
produced between sealing wings 76 and 78 and interior surface 22. 
Alternatively, sealing ridge 80 could be increased in size so as to engage 
interior surface 22 at lower pressures, or even under ambient conditions, 
and with greater force. Based on the teachings disclosed herein, those 
skilled in the art will be able to adjust the size and configuration of 
the various elements of the present invention to obtain desired properties 
for both different sizes and different kinds of syringes. 
The present invention also provides primary sealing means formed on sealing 
cap 34 for producing a continuous sealing engagement between sealing cap 
34 and interior surface 22 of barrel 14. By way of example and not by 
limitation, the primary sealing means includes first sealing wing 76, as 
discussed above. First sealing wing 76 is disclosed as having an outer 
diameter that is smaller than the inner diameter of lumen 24. As such, 
first sealing wing 76 biases against interior surface 22 so as to produce 
a continuous sealing engagement therewith. In an alternative embodiment, 
the primary sealing means can include conventional structures used in 
sealing a conventional plunger against the interior surface of a syringe 
barrel. For example, the primary sealing means can comprise an annular 
gasket which encircles and radially extends out from sealing cap 34 to 
engage interior surface 22 of barrel 14. Such an annular gasket need not 
radially urge out under increased pressure but can be used to maintain a 
constant seal. 
Furthermore, the present invention provides variable sealing means for 
producing a sealing engagement between a portion of sidewall 68 and 
interior surface 22 of barrel 14 that increases in sealing effectiveness 
as plunger 12 advances within barrel 14 so as to increase the pressure 
within barrel 14. By way of example and not by limitation, the variable 
sealing means includes sealing ridge 80 that radially projects out from 
sidewall 68 and tapered ridge 62 that radially projects out from shoulder 
56. As discussed above, ridge 62 is received within side groove 92 which 
in mm is positioned adjacent to sealing ridge 80. Accordingly, as plunger 
12 advances within barrel 14 so as to increase the pressure within barrel 
14, sealing ridge 80 is urged radially outward to producing a sealing 
engagement with interior surface 22 of barrel 14. 
The present invention also comprises engagement means, responsive to 
pressure generated within the syringe barrel, for increasing the force 
with which the annular sealing ridge engages the interior surface of the 
syringe barrel in proportion to the pressure generated within the syringe 
barrel. In the preferred embodiment illustrated in FIGS. 1-4, the engaging 
means comprises the conical head 52 in combination with the complementary 
surfaces of cavity 84. As set forth above, when a positive pressure is 
generated within barrel 14, the sloping surfaces of conical head 52 
cooperate with the complementary sloping surfaces within cavity 84 to 
cause sealing cap 34 to compress or flex, thereby increasing the force 
with which sealing wing 76 and sealing ridge 80 engage the interior 
surface 22 of barrel 14. Similarly, when a negative pressure or partial 
vacuum is generated within barrel 14, the sloping surfaces of conical head 
52 cooperate with the complementary sloping surfaces within cavity 84 to 
cause sealing cap 34 to compress or flex, thereby increasing the force 
with which sealing wing 78 and sealing ridge 80 engage the interior 
surface 22 of barrel 14. The force with which the respective sealing 
members engage the interior surface 22 of barrel 14 is proportional to, 
and automatically increases in response to, any increase in pressure 
(either positive or negative) generated within barrel 14. 
An alternative embodiment of the variable sealing means is disclosed in 
FIG. 5A. As shown therein, a sealing cap 99 is provided having first 
sealing wing 76 and second sealing wing 78 comparable to sealing cap 34. 
In contrast, however, sealing cap 99 does not include annular sealing 
ridge 80 projecting between sealing wings 76 and 78. Rather, a recessed 
portion 102 is formed between sealing wings 78 and 80. Furthermore, a 
conical head 101 is shown as having a distal end face 103 that uniformly 
slopes to an annular outside edge 104. Interior surface 82 of sealing cap 
99 is designed to complementarily receive head 101. Outside edge 104 is 
positioned sufficiently close to recessed portion 102 that as pressure is 
applied to sealing cap 99, outside edge 104 causes the flow of the 
material of sealing cap 99 to push a portion of recessed portion 102 
radially outward, as shown in FIG. 5B, so as to bias and seal against 
interior surface 22 of barrel 14. 
It is evident that the ability of recessed portion 102 to effectively seal 
against interior surface 22 of barrel 14 is dependent on the distance 
between recessed portion 102 and interior surface 22 and the distance 
between outside edge 104 and recessed portion 102. Such distances, 
however, are not fixed but must be determined based on the size of the 
syringe and the amount of pressure that is desired to be withstood. For 
example, as the size of sealing cap 99 increases, the amount of material 
capable of flowing also increases. As such, outside edge 104 can be 
positioned farther away from interior surface 22 of barrel 14. 
FIGS. 6A and 6B disclose another alternative embodiment of the inventive 
aspect of the present invention. As disclosed therein, a sealing cap 106 
can be formed with first sealing wing 76, annular sealing ridge 80, but 
excluding second sealing wing 78. Furthermore, a conical head 108 is 
disclosed comprising a conical distal end face 110 that uniformly and 
radially slopes out to an annular outside shoulder 112. Shoulder 112 is 
shown as being substantially parallel with the longitudinal axis of 
plunger tip 32. Such an embodiment functions under positive pressure in 
substantially the same way as discussed above with regard to FIG. 3. That 
is, as shown in FIG. 6B, as the pressure increases against sealing cap 
106, first sealing wing 76 flexes to bias against interior surface 22 and 
sealing ridge 80 radially projects out to sealing engage interior surface 
22. Furthermore, even though second sealing wing 78 is removed, sealing 
cap 34 still functions as a seal under low negative pressure during 
aspiration. 
Finally, FIG. 7A and 7B disclose yet another alternative embodiment of an 
inventive aspect of the present invention. As disclosed therein, a sealing 
cap 114 is formed comparable to sealing cap 106 except that first sealing 
wing 76 has been removed. As such, sealing cap 114 comprises annular 
sidewall 68 on which annular sealing ridge 80 solitarily encircles and 
radially extends therefrom. In this embodiment, however, it is desired 
that sealing ridge 80 have an outer diameter sufficiently large to 
initially engage interior surface 22 of barrel 14 under ambient 
conditions. This is because annular sealing ridge 80 is the sole structure 
for producing a seal between sealing cap 114 and barrel 14. As depicted in 
FIG. 7B, as the pressure increases within barrel 14 the material of 
sealing cap 114 begins to flow so that outside edge 112 of head 108 
radially pushes sealing ridge 80 against interior surface 22, thereby 
effectively increasing the seal. 
The present invention may be embodied in other specific forms without 
departing from its spirit or essential characteristics. The described 
embodiments are to be considered in all respects only as illustrated and 
not restrictive. The scope of the invention is, therefore, indicated by 
the appended claims rather than by the foregoing description. All changes 
which come within the meaning and range of equivalency of the claims are 
to be embraced within their scope.