Needleless injector with shock absorbing means between ram and piston

Abstract

A needleless injector has a syringe body having an opening at one end, a piston housed within the syringe body for urging a liquid within the syringe body through the opening and a ram for driving the piston. A shock absorbing component is provided for reducing an initial transfer of force from the ram to the piston. The initial rate of pressure increase on the liquid is reduced, thereby controlling the rate of collapse of gas bubbles in the liquid.

Description

BACKGROUND TO THE INVENTION [0001] Needlefree injectors are used as an alternative to needle-type hypodermic injectors for injecting liquid drugs through the epidermis and into the underlying tissues. The usual form of construction for such a device is a syringe having a small discharge orifice which is placed in contact with the skin, and through which the drug is injected at a sufficiently high speed to penetrate the skin of the patient. The energy required to pressurise the drug may be derived from a compressed coil spring, compressed gas, explosive charge or some other form of stored energy. [0002] There are a number of different ways in which the energy may be converted from the stored form into pressure in the liquid. These may include rupturing a seal, so allowing gas to escape from a canister and causing pressure to build up behind a piston which pressurises the gas. Alternatively, a gas may cause a ram to accelerate across a gap, prior to impacting on the back of a piston. ...

AI Technical Summary

Benefits of technology

[0005] The inventors have recognised that it would therefore be beneficial to ensure that the collapse of any bubble that may be present in the capsule (either because of the filling process, or because of any gas that may come out of the drug solution due to changes in temperature, pressure etc.) is in a slow controlled manner. It has been found that, depending on the material and geometry of the capsule, bubbles greater than around 0.5-1 μl can increase the probability of the capsule breaking.

[0006] It has also been found that the key factor giving rise to these shock waves is the initial rate of pressure increase. The peak pressure in the fluid may be around 200-400 Bars, and yet by slowing the rate of pressure increase from atmospheric pressure to around 20 Bars still has a dramatic effect on reducing the shock wave generation, even if the remainder of the pressure increase occurs at the same rate as previously.

[0007] The invention provides a method of preventing a: collapsing bubble from causing a needle free injector capsule to break, and comprises the step of causing the bubble to collapse in a slower, controlled manner immediately prior to the normal injection cycle, without changing the peak value or the shape of the remainder of the pressure profile. The invention also provides an apparatus for this purpose.

[0013] shock absorbing means for reducing an initial transfer of force from the ram to the piston.

[0014] This apparatus provides reduction of the initial force applied to the ram, so that the initial rate of pressure increase on the liquid is reduced, thereby controlling the rate of collapse of gas bubbles in the liquid.

[0018] This provides a step in the internal profile, which provides a local pressure peak which results in a low initial pressure for ensuring bubble collapse. The internal opening of the cylinder can have three internal diameters, a third internal diameter at an end of the cylinder for cooperation with the piston, the third internal diameter being greater than the second internal diameter. The third internal diameter can-be equal to or greater than the diameter of the ram, so that this section of the component does not increase the frictional resistance to the ram, but provides length over which the initial pressure continues to act before impact of the ram with the piston.

Problems solved by technology

This bubble collapse, if it occurs quickly enough, can cause shock waves within the capsule, which can give rise to extremely high localised stresses.

These stresses can sometimes cause the capsule to fail during this initial pressure peak, which is likely to result in an incomplete injection.

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