Abstrict A disposable syringe for administering liquid medicinal agents
to live animal or human subjects is disclosed. The syringe comprises
a hollow cylinder, filled with a medicinal agent, with which an
injection needle is attached. The cylinder surrounds a piston that
can be moved by means of a piston rod towards the injection needle.
The piston is produced from a thermoplastically processable rubber
mixture and is delimited in the direction of the medicinal agent
by a peripheral sealing lip that contacts the hollow cylinder and
that can be deflected in the radial direction.
Claims What is claimed is:
1. A disposable syringe for administering liquid medicinal agents;
comprising:
a hollow cylinder having proximal and distal ends, said hollow
cylinder being configured to receive a liquid;
an injection needle located at the proximal end of the hollow cylinder;
a piston made from a thermoplastically processable rubber mixture
and being delimited in the proximal direction by a radially deflectable
peripheral sealing lip, said piston being provided with a recess
radially inside the sealing lip; and a piston rod, said piston being
located at one end of the piston rod, said piston rod and said piston
being configured to be slidable within the hollow cylinder so that
the piston can be displaced by the piston rod within the hollow
cylinder towards the injection needle at the proximal end of the
cylinder;
wherein the peripheral sealing lip on the piston contacts the hollow
cylinder and is configured to be radially deformed so as to reduce
the recess when driven by the piston rod towards the proximal end
of the hollow cylinder.
2. A disposable syringe according to claim 1 wherein the piston
is fastened by means of internal threads to external threads on
the piston rod, said internal threads being axially shorter than
the external threads of the piston rod.
3. A disposable syringe according to claim 1 wherein the piston
is fastened by means of external threads to internal threads on
the piston rod, said internal threads being axially shorter than
the external threads of the piston.
4. A disposable syringe according to claim 1 wherein the rubber
mixture consists predominantly of a block polymer.
5. A disposable syringe according to claim 4 wherein the rubber
mixture contains a quantity of a polyolefin.
6. A disposable syringe according to claim 5 wherein polypropylene
is used as the polyolefin; and the quantity in which it is provided
is 1 to 30 wt %.
7. A disposable syringe according to claim 4 wherein the rubber
mixture contains a quantity of 0.1 to 2 wt % of a silicone rubber.
8. A disposable syringe according to claim 1 wherein the sealing
lip has at least one sealing edge that is delimited on the outside
by two conical surfaces that intersect one another.
9. A disposable syringe according to claim 1 wherein the recess
comprises a peripheral groove penetrating into the end surface of
the piston.
10. A disposable syringe according to claim 9 wherein the hollow
cylinder has an end wall, and wherein radially inside the groove
the piston has an end surface that has a shape conforming to said
end wall of the hollow cylinder.
11. A disposable syringe according to claim 1 wherein the end
surface of the piston delimits a projection that extends beyond
the sealing lip in the direction of the end wall.
12. A disposable syringe according to claim 11 wherein the piston
comprises an excess length at its proximal side and this excess
length is dimensioned so that the radial displacement of the sealing
lip that results when the piston is pressed against the end wall
substantially fills the groove.
13. A disposable syringe according to claim 10 wherein the end
surface is interrupted by at least one recess, running radially,
that extends from the outer circumference to the center of the piston.
14. A disposable syringe according to claim 8 wherein the recess
comprises a peripheral groove penetrating into the end surface of
the piston.
Description BACKGROUND OF THE INVENTION
The invention generally relates to a disposable syringe for administering
liquid medicinal agents to live animal or human subjects. Generally
speaking, the syringe comprises a hollow cylinder filled with a
medicinal agent that terminates with an injection needle. The cylinder
surrounds a piston that can be moved by means of a piston rod toward
the injection needle.
Disposable syringes of this kind are generally known. Typically,
the piston and the piston rod are configured of a single material
and are integrally and continuously linked with one another. The
piston rod and piston are made, for example, of polyethylene or
nylon; the hollow cylinder preferably is made of polyethylene. However,
the arrangement often provides an unsatisfactory sealing effect
between the piston and the hollow cylinder. Even when very tight
manufacturing tolerances are maintained, the pressure of the piston
against the inner wall of the hollow cylinder varies among peripherally
adjacent subregions of the piston. This increases the risk of leakage
due to relaxation phenomena within the piston. Moreover, the cylindrical
piston has on the side facing the medicinal agents a flat end wall
extending in the radial direction, which prevents the full discharge
of the contents of the hollow cylinder.
There remains a need to further develop a disposable syringe of
the aforesaid type so that the above-identified shortcomings are
avoided. There further remains a need for such a syringe having
improved operating characteristics. In particular, there remains
a need to improve the sealing between the piston and the hollow
cylinder so as to prevent leaks in this region and so that medicinal
agent can be discharged almost completely from the hollow cylinder.
SUMMARY OF THE INVENTION
In the present invention, the piston is produced from a thermoplastically
processable rubber mixture. It is delimited in the direction of
the medicinal agent by a peripheral sealing lip that contacts the
hollow cylinder and which can be deflected in the radial direction.
The piston is configured as a sealing element and is made of an
elastomeric rubber material, so that the sealing lip, which is resilient
in the radial direction, is in sealing engagement with the inner
surface of the hollow cylinder. The sealing lip contacts the hollow
cylinder on the inside along a circumferential line with a consistent
radial preload. Manufacturing-related tolerances of the hollow cylinder
and/or the piston can thereby easily be compensated for, thus resulting
in reliable sealing.
Still greater security against the unwanted loss of medicinal agent
via leakage can be obtained by using a plurality of sealing lips
arranged in functional succession in the direction of the piston
rod movement. On the other hand, the use of only one such peripheral
sealing lip has the advantage of minimizing any friction between
the piston and the hollow cylinder so as to provide a smooth, low-force
actuation of the piston rod. With such an arrangement, the medicinal
agent being administered can be metered easily and precisely.
According to one embodiment, the piston can be fastened by means
of internal threads to external threads of the piston rod, the internal
threads preferably being shorter in the movement direction of the
piston than the external threads. The piston rod is preferably configured
with a dished pressure plate on the side facing away from the medicinal
agent.
The shaft of the piston rod preferably has a cruciform cross section.
The cruciform cross section reliably eliminates the risk of bending
the shaft during actuation, especially when the piston rod is used
in hollow cylinders with small diameters and/or with viscous medicinal
agents.
Arranged adjacent to the shaft on the side facing the medicinal
agent is a dished support washer that is configured integrally and
continuously with the piston rod and delimits, with the inner surface
of the hollow cylinder, an annular gap. Low-friction relative mobility
of the piston rod within the hollow cylinder is assured because
the support washer does not lie in contact with the inside diameter
of the hollow cylinder along its entire outer periphery.
By fastening the piston to the piston rod by means of a threaded
joint, there is obtained enhanced relative immobility of the two
parts with respect to one another in the axial direction, which
is advantageous in helping to provide an exact metering of the substance
being administered. A prerequisite for ensuring this kind of mutually
immobile fastening of the two parts to one another is that the internal
threads and external threads be fastened to one another without
axial play, i.e., with a degree of axial preload. An axial preload
of this kind can be effected, for example, by providing that the
internal threads of the piston be shorter in the latter's movement
direction than the external threads of the piston rod. The substantially
cup-shaped piston is threaded onto the external threads of the piston
rod until the bottom surface of the piston is in contact with the
end surface of the piston rod (which constitutes the end of the
external threads) with an elastic preload. The elastomeric material
of the internal threads is thereby braced against the turns of the
external threads of the piston rod, thus eliminating the thread
play.
According to another embodiment, the internal threads can be longer
in the movement direction of the piston than the external threads.
This is advantageous in that when the cup-shaped piston is threaded
onto the external threads of the piston rod, the annular surface
of the piston first comes into contact with the support washer of
the piston rod. When the piston is then threaded further onto the
external threads of the piston rod, until the bottom contacts the
end surface, the support washer, designed as a buttress, produces
bulging of the elastomeric material of the piston on the side opposite
the medicinal agent, and a radial deflection movement, so that the
bulged material contacts and seals against the inner periphery of
the hollow cylinder. This embodiment not only eliminates thread
play, but also further improves protection against fluid losses.
According to another embodiment, the piston can be fastened by
means of external threads to internal threads of the piston rod,
the internal threads being shorter along the axial direction of
piston movement than the external threads of the piston. One advantage
of this arrangement is that radial deflection movements of the elastomeric
material of the piston do not occur upon fastening to the piston
rod in the direction of the hollow cylinder, thus reliably eliminating
high actuation forces and immobilization of the piston in the hollow
cylinder. The piston is threaded into the piston rod until the turns
of the external threads and the turns of the internal threads are
braced against one another.
The rubber mixture can consist predominantly of a block polymer,
and can contain a quantity of a polyolefin. Such a composition of
rubber mixture advantageously provides good resilience, while presenting
minimal relaxation phenomena in the course of use. Furthermore,
a rubber mixture of this kind is easily and economically manufactured.
It is also advantageous that a block polymer with a polyolefin quantity
is resistant to the most common medicinal agents. Polypropylene
can be used as the polyolefin, the quantity preferably being 1 to
30 wt %. The resilience of the piston can be adapted to the particular
conditions of the application as a function of the quantity of polypropylene
in the rubber mixture. For example, if a comparatively viscous medicinal
agent is to be administered, it has proven to be advantageous that
the polypropylene quantity be 20 to 30 wt %, which provides the
piston with the required rigidity.
To improve the sliding characteristics of the piston in the hollow
cylinder, the rubber mixture can contain a quantity of 0.1 to 2
wt % of a silicone rubber. The improved sliding characteristics
eliminate stick-slip effects. Operation of the disposable syringe
is simplified, and the medicinal agent can be administered in accurately
predetermined doses.
The rubber mixture can be composed predominantly of an at least
partially crosslinked rubber and a thermoplastic material, the rubber
mixture being, for example, composed of styrene-ethylene-butylene-styrene
(SEBS).
The sealing lip can have at least one sealing edge that is preferably
delimited on the outside by two conical surfaces that intersect
one another. In an embodiment of this kind, the sealing edge extends
along the piston substantially linearly and on the outer circumference,
and reliably seals off the inner wall of the hollow cylinder. According
to a further embodiment, the sealing edge is delimited on the outside
by a surface with a semicircular cross section. An embodiment of
this kind is particularly advantageous when the radial pressure
of the sealing lip against the hollow cylinder and/or the pressure
inside the hollow cylinder is relatively elevated by administration
of the medicinal agent. As a result, the sealing lip is better protected
against abrasive wear and therefore seals optimally even under such
conditions.
The piston can be provided with a recess radially inside the sealing
lip. The recess preferably consists of a peripheral groove penetrating
into the end surface of the piston. During movement of the piston
rod in the direction of the injection needle and injection of the
medicinal agent, excess pressure relative to the atmosphere builds
up inside the annular peripheral groove, additionally reinforcing
the pressure, conditioned by both design and material, of the sealing
lip against the inner wall of the hollow cylinder. The pressure
of the sealing lip against the hollow cylinder automatically adapts
as a function of the viscosity of the medicinal agent and the pressure
that thereby builds up inside the hollow cylinder during administration
of the medicinal agent.
The piston can have, radially inside the groove, an end surface
that has a shape conforming to the end wall of the hollow cylinder.
The result of this embodiment is that the dead space inside the
hollow cylinder when the piston rod is completely inserted is kept
to a minimum. When the disposable syringe is actuated, the end surface
of the elastomeric piston is moved almost to the outlet opening
of the hollow cylinder, and almost completely displaces the medicinal
agent present in the hollow cylinder.
The end surface preferably delimits a projection that extends beyond
the sealing lip in the direction of the end wall. As a result, liquid
components of the medicinal agent that are located in the narrowed
outlet region of the hollow cylinder are almost completely administered.
The excess length of the projection formed by the extended end
can, according to one embodiment, be dimensioned so that the radial
displacement of the sealing lip that results when the piston is
pressed against the end wall substantially fills the groove. The
liquid components of the medicinal agent that are located inside
the groove during movement of the piston rod toward the injection
needle are forced out of the groove toward the injection needle,
as the piston comes to a stop against the end wall of the hollow
cylinder, by the elastic deformation of the piston. This ensures
that despite the deformation of the piston, no liquid components
of the medicinal agent are forced past the sealing lip toward the
atmosphere.
The end surface can be interrupted by at least one recess, running
radially, that extends from the outer circumference to the center.
This assures that the medicinal agent is not enclosed in cavities
when the end of the piston comes to a stop against the end wall
of the hollow cylinder, but rather that a liquid-carrying connection
toward the injection needle always exists.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of a disposable syringe constructed according to
the principles of the invention will be explained below in greater
detail with reference to the drawings.
FIG. 1 is a longitudinal view in cross section of the disposable
syringe of the invention, filled with a medicinal agent.
FIG. 2 is a view similar to that of FIG. 1 but with the syringe
discharged.
FIG. 3 is a fragmentary sectional view of region X in FIG. 2 showing
a section of the disposable syringe just prior to the complete discharge
of the medicinal agent.
FIG. 4 is a fragmentary view similar to FIG. 3 in which the piston
rod has been displaced relatively farther toward the injection needle,
and the medicinal agent has been discharged as completely as possible.
FIGS. 5 and 6 show a further embodiment in which the piston is
fastened by external threads in internal threads of the piston rod.
DETAILED DESCRIPTION
FIG. 1 shows a first embodiment of a disposable syringe that is
filled with a liquid medicinal agent 1. The disposable syringe comprises
a hollow cylinder 2 one axial end of which is closed off in a liquid-tight
manner by a piston 4 placed on a piston rod 5 and the other axial
end of which is provided with an injection needle 3. Piston 4 consists
of a resilient, thermoplastically processable rubber material, preferably
TPE, and seals the medicinal agent inside the hollow cylinder with
respect to the environment. Piston 4 is cup-shaped, and has inside
a recess internal threads 7 with which it is mounted onto external
threads 8 of piston rod 5. Piston 4 is fastened onto the piston
rod under an axial preload, which compensates for thread play. In
the embodiment depicted here, the annular rim of piston 4 is in
sealing contact with support washer 22 of piston rod 5 under elastic
preload, the bulging of the elastomeric material causing a radial
expansion and additional sealing against the inner wall of hollow
cylinder 2.
FIG. 2 shows the disposable syringe of the invention in the discharged
state. End surface 14 (illustrated in FIG. 3) of piston 4 facing
the medicinal agent 1 lies in contact with the inside end wall 17
of the hollow cylinder 2. A projection 18 located at the proximal
end of the piston at least partially fills the outlet opening of
the hollow cylinder 2.
In FIG. 3 the disposable syringe is depicted in the region of its
outlet opening, just prior to the complete discharge of the medicinal
agent 1. Piston 4 is in sealing contact with the inner peripheral
wall of the hollow cylinder 2 with its sealing lip 6 under a radial
preload. In this embodiment sealing lip 6 has a sealing edge 10.
Piston 4 also contacts the inner wall of hollow cylinder 2 with
a sealing lug 23 radially outside a groove 15 and with end surface
16 radially inside groove 15. In this operating state, projection
18 is centered in the illustrated position by the outlet opening
at the proximal end of the hollow cylinder 2. Recesses 19 are provided
for delivery of medicinal agent 1 toward injection needle 3 as
they channel medicinal agent 1 toward injection needle 3. The recesses
extend inside projection 18 in the direction of flow.
FIG. 4 depicts with a level of detail similar to that of FIG. 3
the configuration of the piston 4 when piston rod 5 is moved farther
toward the outlet opening of hollow cylinder 2. Proceeding from
the depiction of FIG. 3 the subregion of the piston radially outside
groove 15 has been displaced radially inwardly as a result of its
contact with the inner wall of the hollow cylinder 2. As before,
sealing edge 10 of sealing lip 6 is in sealing contact with the
hollow cylinder 2. Sealing lug 23 provides guidance of sealing lip
6; because of the radial displacement, groove 15 is substantially
closed and the liquid components of medicinal agent 1 located therein
are forced through recesses 19 of projection 18 toward the injection
needle 3. In this state, When all liquid components of medicinal
agent 1 have been forced out of groove 15 piston 4 completely closes
off the outlet opening of hollow cylinder 2. Excess piston length
A (shown in the figure) must be dimensioned, in conjunction with
the geometric configuration of projection 19 in such a way that
end surface 14 of piston 4 does not completely close off the outlet
opening of hollow cylinder 2 until no further medicinal agent 1
is located radially outside of and in groove 15.
FIG. 5 shows a further embodiment of the disposable syringe according
to the invention, in which medicinal agent 1 has been largely discharged.
In contrast to the embodiment depicted in FIG. 1 piston 4 has on
the side facing away from medicinal agent 1 integrally formed external
threads 24. The external threads 24 are joined to internal threads
25 of piston rod 5. End surface 14 of the piston, facing medicinal
agent 1 is identical to the corresponding portion of the embodiment
of FIG. 3.
FIG. 6 depicts a section of the disposable syringe of FIG. 5 at
enlarged scale. This Figure clearly shows that the outer circumferential
surface of piston 4 does not bulge subsequent to assembly, and therefore
does not expand radially. The advantage here is that on the outer
circumference, piston 4 contacts the inner wall of the hollow cylinder
only with sealing edge 10 of sealing lip 6 thus reducing to a minimum
the actuation forces necessary to administer an injection. |