2340042 Hypodermic Syringe This invention relates to hypodermic syringes
of the piston and cylinder type in which a needle is disposed at one end of the cylinder and through which a liquid substance can be forced to flow under pressure fi7om the piston or through suction caused by withdrawing the piston from the needle end of the cylinder.
A known problem with hypodermic syringes is that they can easily provide cross contamination of diseases if they are re-used or if the use of one hypodermic syringe is shared between a number of parties. This problem is exacerbated in particular in the case of drug abusers and this is one of the reasons why such syringes are often made freely available to the public so as to discourage the habit of re-use or shared use of a single syringe.
It is an object of the present invention to provide a hypodermic syringe in which the aforementioned disadvantages are obviated and in particular to provide a syringe which can ordinaril y only be used once so that the choice of re- using the needle is not available to drug abusers.
According to the invention, there is provided a hypodermic syringe comprising a cylinder having an open end and a closed end with a hollow needle at the closed end for dispensing liquid therefrom, a piston for forcing liquid out of the cylinder via the needle under lateral pressure from a plunger, the piston being deformable against the walls of the cylinder to prevent or inhibit leakage of liquid passed the piston in the direction of the open end of the cylinder, characterised in that the cylinder is constricted in the region of the closed end such that pressure on the piston from the plunger to force the piston into the constricted part of the cylinder to fully evacuate liquid therefrom causes the piston to become trapped such that withdrawal of the plunger causes it to disconnect from the piston so that the syringe cannot be re-used.
Conveniently, the constriction of the closed end of the cylinder is itself cylindrical and of smaller diameter but large enough to permit the piston to be forced therethrough under pressure from the plunger and small enough to ensure that, in doing so, the piston is trapped or gripped sufficiently to ensure that if the plunger is withdrawn it disconnects from the piston, leaving the piston at the closed end of the cylinder.
The piston is conveniently made of an elastomeric material and has voids or ridges in the outer circumference thereof to permit the elastomeric material to be easily deformed when entering the constricted part of the cylinder, which constricted part may have bevelled sufface corresponding to beveUed surfaces at the forward end of the piston to allow the latter to easily enter the constricted part under pressure from the plunger.
The plunger may be connected to the piston either permanently, with a weakened section which can break off when the plunger is withdrawn after the piston has been trapped within the constricted part of the cylinder. More typically, it may be of conventional design and comprise a plastics rod having a flanged formation at the forward end co-operable,%ith a corresponding formation on the rear of the piston so that they can be releasably connected together such that the piston can be used to suck liquid into the cylinder via the needle in a conventional manner without becoming disconnected from the plunger.
It will be understood that with the arrangement of the invention a perceived problem may be the presence of a void at the constricted part of the cylinder when the hypodermic syringe is not in use such that when it is used to suck out liquid from, for example, an ampoule, a relatively large volume of air is also present which makes it difficult to gauge how much liquid has been sucked into the cylinder before the syringe is inverted in a conventional manner and the air expelled by pressing the plunger and hence the piston towards the closed end of the cylinder. However, in accordance with another aspect of the invention the outside of the cylinder, which may suitably be of a clear plastics material, is calibrated in two ways, one to indicate how much liquid is being sucked into the cylinder via the needle, taking into account the volume of air, and another to indicate how much liquid is in the cylinder after it has been inverted and the air expelled therefrom, such that the syringe can still accurately dispense a required quantity of liquid.
The invention therefore provides a hypodermic syringe that can be used once only and, in particular, for drug abusers to be able to receive the full charge of a drug they must push the plunger all of the way but, in so doing, the syringe then becomes non re-useable because the piston is then trapped within the constricted portion of the cylinder.
The invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Figure I is a side view of a hypodermic syringe according to the invention and:
Figure 2 is a longitudinal cross section of the syringe shown in Figure but showing the plunger partially withdrawn.
Refer-ring to the drawings of the hypodermic syringe, the invention comprises a hollow plastics cylinder I having a closed end 2 of conical shape and from which extends a hollow needle 3. Into the open end 4 of the cylinder I extends an elongated cylindrical plunger 5 having a flanged portion 6 whereby it can be actuated by thumb pressure exerted against a pair of reinforced finger grips 7,8 on diametrically opposed sides of the cylinder 1.
As shown in Figure 2, the end of the plunger 5 remote from the flange 6 includes an inwardly facing flanged portion 9 into which is disposed a correspondingly shaped flanged portion 10 of a cylindrical elastomeric piston 11 slidably received within the cyfinder 1. The piston I I has a pair of concentric voids 12,13 to allow surrounding elastomeric material of the piston 11 to be compressed in a manner to be described.
Towards the closed end 2 of the cylinder I is a constricted region 14 of smaller diameter than the major portion 15 of the cylinder I but sufficient to allow the piston 11 to be slidingly received therein under pressure from the plunger 5. To assist in this the forward face 16 of the piston is of conical shape corresponding to a bevelled concentric region 17 at the beginning of the constricted region 16 with the conical inner surface 18 of the closed end 2 corresponding in shape to the forward face 16 of the piston -5 11, although of smaller diameter.
In operation the hypodermic syringe according to the invention is used by pushing the piston I I towards the closed end 2 of the cylinder I until it abuts the bevelled portion 17 at the opening to the constricted region 14. The needle 3 is then connected to a source of liquid such as an ampoule containing a drug, and a quantity of the drug is sucked through the needle 3 into the cylinder 1. To assist in gauging the amount of liquid being received the outer surface of the cylinder I may have a volumetric scale which takes into account the void of air within the constricted region at the commencement of the process. When a sufficient quantity of the drug has been sucked into the cylinder 1 it can be inverted such that the needle 3 is pointing upwards and the liquid rests against the piston I I with a quantity of air therebetween corresponding to the volume of the constricted region 14. At this point the plunger 5 can then be moved inwardly relative to the cylinder I such that the piston 11 drives the air out through the needle 3, at which point a second volumetric scale may be used to determine the exact amount of drug to be dispensed.
After completion of these preparations, the drug can be dispensed in the usual manner but in order to dispense the final volume contained in the constricted region 14 the plunger 5 must be pressed harder at that point to force the piston 11 into that region, thereby allowing the elastomeric material around the voids 12,13 to flatten out and tightly grip the inner walls of the cylinder I in the constricted region 14. When the drug has been completely dispensed the piston I I is thereby firmly trapped within the constricted region 14 to the extent that upon withdrawal of the plunger 5 the flanged regions 9, 10 on, respectively, the forward end of the plunger 5 and rear end of the piston I I become detached leaving the piston I I within the cylinder I adjacent the closed end 2.
It will be apparent that in this position the piston I I can no longer be easily withdrawn such that, effectively, the hypodermic syringe can only be used once and a fresh syringe must be obtained to administer further injections.
The invention therefore provides a relatively simple but efficient way of ensuring the hypodermic syringes are only used once, such that where they are available free for drug abusers, the tendency will be for them to be thrown away or otherwise disposed of after only being used once since the difficulties in extracting the piston I I from the cylinder I would outweigh the ease by which additional supplies of fresh syringes could be obtained.
Although the invention is primarily designed to ensure that drug abusers do not re-use syringes, it will be understood that the hypodermic syringe of the invention may also be used in conventional medicine or for any other use. Similarly, it will be understood that although the embodiment described above shows the piston being trapped within a constricted region of the cylinder to ensure that the plunger becomes detached therefrom, other methods by which the piston may become trapped at the closed end of the cylinder may be used including, without limitation, formations on the inside of the cylinder towards the closed end which permit the piston to pass towards the closed end but not to return and such formations may, in particular, be bevelled portions which allow the piston to pass one way but not return the other.