INJECTION DEVICES

Abstract: In a medical injection device a capsule (1) containing the fluid to be injected has a resilient piston (5) acted upon by a plunger (8') which moves a set distance when pressed or released, to inject a predetermined dose. To reduce errors arising from the resilience, instead of a wholly flat end face the plunger (8') has a centrally recessed (13) working end, leaving an annular face (14) in a radial plate to act on the plunger (5).

Description: INJECTION DEVICES This invention relates to injection devices. It is particularly concerned with those where a measured dose is dispensed from a replaceable capsule. There are several injection devices which can be set sc tnat a plunger, when pressed or released, can only move a pre-determmed distance. This can reasonably be expected tc shift a piston a capsule on which it acts by the same distance, and therefore cause a known, accurate dose to be dispensed. However, the piston is generally of stiffly, resilient material, such as rubber, to make a good seal. Also, the plunger head usually has radial clearance with respect to the inside of the capsule for easy, unobstructed action Therefore, it does not engage the whole area of the rear face of the piston. When tne plunger head presses against the piston it sinks into it to a small degree, and causes a meniscus of rubber to be formed oy the outer peripnery cf the rear face of the piston, swelling back around the plunger head. The effect of this is that the plunger tends to travel a greater distance than the piston, whicn does not start to move until fractionally after engagement by the plunger . After an injection, the piston may slowly recover sc that the meniscus disappears and its rear face becomes flat again, but the needle will by then have been witndrawn. St a subsequent injection will have the same error. It is the aim of this invention to reduce this error According to the present invention there is provided a plunger for an injection device m which a cylindrical capsule with an open rear end has a stiffly resilient piston to be acted upon by the head of the plunger, wherein the head has a central recess within an annular face a radial plane which bears on the piston. Preferably, the recess is dome shaped, although it could be conical, cylindrical, or nearly cylindrical. The included angle of the mouth of the recess will generally be the range 20° to 85°, making the transition between the annular face and the peripheral portion of the recess an ODtuse angle. The outside diameter of the head will be the same as known piston heads, giving radial clearance between the head and the interior of the capsule. Currently, this outside diameter is either 6 mm or 9.1 mm (for small and large capsules) and the radial width of the annular face is then preferably m the range 0.4 to 1.2 mm. Often, the piston has small projections m a regular array at least on its rear face, and with conventional flat headed plungers, these are simply squashed into the mam body of the piston. They may contribute to the inaccuracy referred to above. But with the present plunger, the annular face can surround these projections, leaving them freely to enter the recess. For a better understanding of the invention, one embodiment will now be described, by way of example, with reference to the accompanying drawing, m which: Figure 1 is a side elevation of a capsule for use m an injection device, Figure 2 is an axial section of the rear end of a capsule engaged by the head of a known plunger, Figure 3 is an axial section of the rear end of a capsule engaged by a plunger according to the present invention, and Figures 4 (a) and (b) and 5 (a) and (b) show preferred non-limitmg dimensions for a plunger according to the present invention. The capsule has a cylindrical glass body 1 throughout most of its length, open at one end (the rear end) and narrowing to a neck 2 at the other end where it is capped off by a collar 3 and a membrane 4. When fitted m an injection device, the membrane is pierced by the rear end of the needle. The contents are contained by a rubber piston 5, initially at the rear end, formed with annular ribs 6 and grooves 7 to give a tight sliding fit withm the capsule 1. The piston 5 is acted upon by a plunger 8 having a cylindrical head 9 with a radial clearance gap 10 m relation to the interior of the capsule. In a conventional injection device, as shown m Figure 2, the forward, working face 11 of the head 9 is flat and m a radial plane. When pressure is applied by the plunger 8 to the piston 5, the immediate result is that the periphery of the rear face of the piston is squeezed into the clearance gap 10 m the manner of a meniscus 12. For reasons explained, this leads to inaccurate dosage. Referring now to Figure 3, the plunger 8' has a head 9', again with a clearance gap 10' radially between it and the inside of the capsule. But instead of an entirely flat end face 11 there is a central dome shaped recess 13 sur- rounded by an annular face 14 a radial plane. When the plunger is thrust forwards, it is this face 14 alone which engages the piston 5. However, instead of a meniscus 12 forming, the central portion of the rear face of the piston bulges nto the recess 13, as shown. Experiments have shown that the resultant travel of the piston 5 is much closer to the travel of tne plunger 8, and therefore a more accurate dose is dispensed. Referring to Figures 4 and 5, ranges of preferred included angles and annulus width dimensions are shown, for a curved concave recess and a conical recess respectively. Thus the included angle of the recess at its mouth preferably lies m the range of from 20° to 85°, and the annulus width dimension preferably lies m the range of from 0.4 to 1.2 mm. This applies whether the plunger head is small (6 mm diameter) or large (9.1 mm diameter) , and would apply to plungers of that order of magnitude.

Claims: CLAIMS 1. A plunger for an injection device m which a cylindrical capsule with an open rear end has a stiffly resilient piston to be acted upon by the head of the plunger, wherein the head has a central recess within an annular face m a radial plane which bears on the piston. 2. A plunger as claimed m Claim 1, wherein the recess is dome shaped. 3. A plunger as claimed m Claim 1, wherein the recess is conical. 4. A plunger as claimed m Claim 1, wherein the recess is substantially cylindrical. 5. A plunger as claimed any preceding claim, wherein the included angle of the mouth of the recess is the range 20° to 85°. 6. A plunger as claimed m any preceding claim, wherein the radial width of the annular face is m the range 0.4 to 1.2 mm. 7. A plunger as claimed any preceding claim, m combination with a capsule and piston accessible through the rear end of the capsule, wherein the piston has small projections at least on its rear face, and the annular face of the plunger surrounds these projections, leaving them freely to enter the recess. 8. An injection device having a plunger as claimed m any preceding claim.

Inventors: MARTEAU DENIS, YEAGER HAROLD, MARTEAU, DENIS, YEAGER, HAROLD
Applicants: LILLY CO ELI, MARTEAU DENIS, OWEN MUMFORD LTD, YEAGER HAROLD, ELI LILLY AND COMPANY, MARTEAU, DENIS, OWEN MUMFORD LIMITED, YEAGER, HAROLD
Application year: 1997
Main classification: A61M5
IPC codes: A61M5