COILED STRIP SPRING AND ITS APPLICATION IN AN INJECTION DEVICE

Abstract: A coil spring of strip material is of non-uniform width. There may be a progressive taper from one end to the other, or there may be steps between portions (1, 2) of uniform width, each portion having a constant force characteristic. Other configurations are possible. For application to an injection device in which a capsule is spring-driven for its needle to project, and in which a piston in the capsule is then spring-driven to eject a dose, the spring has a wide portion (1) wound innermost which urges the capsule forwards, and a narrow portion (2) which then takes over to eject the dose.

Description: COILED STRIP SPRING AND ITS APPLICATION IN AN INJECTION DEVICE This invention relates to springs. It has been developed primarily for syringes or injection devices of the kind in which a capsule is released and driven forwards by a spring to project a needle (thrusting it into the flesh when the nose is held against the skin) , and then further spring action pushes the plunger of the capsule forwards to eject the dose through the needle. This is favoured for self- injection devices, as the action is automatic once some form of catch is released, and the dosage can be accurately preset . However, although the invention will mainly be described in that context, it will be understood that it may well have other applications. In an injection device of the kind described, it has long been a desired objective to apply a constant spring force throughout both phases. However, the usual spring used has been a coil spring, which is initially compressed and expands when released. The extension required is substantial and it is difficult if not impossible to produce one that will exert as much force at the end of its expansion as at the beginning. It declines steadily as the spring lengthens. A remedy has been sought by having two coil springs in tandem, but that is complicated, more expensive, and not a complete solution. Another drive arrangement has been proposed in EP-A- 0666084, with a constant force spring in the form of a flat strip of spring steel coiled about an axis transverse to the length of the injection device. However, although that works well enough, it does not actually produce the ideal action. The requirements of thrusting the needle into the flesh and of ejecting the dose are very different. Normally, the needle penetration should be positive and rapid, so that the user has no time to flinch and partially withdraw. Then the dose should be squeezed out slowly, with a firm but gentle pressure on the plunger. In other words, it is desirable to have one spring rate for the first phase and another spring rate for the second phase . According to the present invention there is provided a coil spring of strip material characterised in that the strip is of non-uniform width. The strip may be progressively wider from one end to the other, and this progression may be by a continuous taper or by steps, with portions of substantially uniform width. Each portion should then have a substantially constant force characteristic. For most applications, it is expected that the coil will be wound widest end innermost. For the injection device of the kind described the strip may be in two portions one wider than the other but each portion having its own constant force characteristic. With the wider portion forming turns inside those of the thinner portion, when the spring is substantially unwound, with a 'tail' from the innermost turn or two, and then released with the contracting tail restrained into a straight line, it rewinds with a first constant force until the thinner portion starts to wind, when a second, smaller constant force prevails. This can be harnessed in a manner similar to EP-A-0666084 but impelling the capsule forwards with greater force than that applied subsequently to the plunger of the capsule. As indicated above, this may be just one of many non- uniform rate springs. For example, there could be more than two different widths, and the order in which they occur can differ. They do not necessarily have to increase (or decrease) at every step: the strip could be wide, narrow and then wider again. Gradual change could be achieved by tapering the width, and this of course need not be over the entire length of the strip. An example of the invention is shown in the accompany- ing drawing, in which the single figure is a perspective view of a coil spring, semi-unwound. The spring is of flat spring-steel strip. Its inner wound portion 1 is of greater width than what is normally its outer wound portion 2, here illustrated as a straight tail tangential from the turns of the portion 2. Each portion will have its own constant force characteristic, that of portion 1 obviously being greater than that of portion 2. In an injection device, similar to that of EP-A-666084 for example, the coil is substantially unwound, with some of the portion 1 and all of the portion 2 straight and leading forwardly, and the innermost turn or two of the portion 1 bearing on the rear of a drive member which will urge a capsule forwards. When the spring is released, the coil winds up, the turns acting on the drive member to propel that forwards. At the point at which the capsule is arrested with a needle at its forward end fully projected, the portion 1 is fully wound and the portion 2 takes over. The drive is now concentrated on a piston within the capsule and the forward movement of this piston, which ejects the dose through the needle, is generated by the lesser force of the portion 2.

Claims: CLAIMS 1. A coil spring of strip material characterised in that the strip is of non-uniform width. 2. A coil spring as claimed in Claim 1, characterised in that the strip is progressively wider from one end to the other. 3. A coil spring as claimed in Claim 2, characterised in that the progression is by a continuous taper. 4. A coil spring as claimed in Claim 2, characterised in that the progression is in steps, with portions (1, 2) of substantially uniform width. 5. A coil spring as claimed in Claim 4, wherein each portion (1, 2) has a substantially constant force characteristic. 6. A coil spring as claimed in any one of claims 2 to 5, wherein the coil is wound widest end (1) innermost. 7. An injection device in which a capsule is spring driven to project a needle and in which a piston within the capsule is subsequently spring driven to eject a dose, the spring drive for both operations being provided by a single coil spring of strip material, characterised in that the spring is as claimed in Claims 4 and 6, with two portions, the wider portion (1) acting first for the needle projection and the narrower portion (2) for the dose ejection.

Inventors: CROSSMAN DAVID DANVERS, CROSSMAN, DAVID, DANVERS
Applicants: CROSSMAN DAVID DANVERS, OWEN MUMFORD LTD, CROSSMAN, DAVID, DANVERS, OWEN MUMFORD LIMITED
Application year: 1998
Main classification: F16F1
IPC codes: A61M5, F16F1