Unless the injector is delivered as a one shot disposable device which is filled with a medi- cine dose, an injector has to be filled through its outlet immediately before use., e. g. as de- scribed in WO 96/19252. This is an inconvenient process.

Consequently it is an object of the invention to provide a jet injector which is designed to be used for several injections and which do not request a filling procedure in advance of each injection.

This is obtained by a jet injector of the kind described in the opening paragraph of this appli- cation which injector is characterised in that the cylinder is a changeable cylinder ampoule containing medicine for a number of injections.

The cylinder ampoule may be of the kind having a first and a second end, wherein a dose of medicine is pressed out through an injection member mounted at the first end when a piston closing the second end is pressed into the ampoule.

Further dose setting means may be provided by which the distance which the piston is moved into the ampoule by the driven piston rod during an injection can be set in advance of each release of the piston rod.

According to the invention the nozzle may be integral with the ampoule or the first end of the ampoule may be closed by a rubber membrane and the injection member may comprise a nozzle mounted in a hub with a needle which penetrates the rubber membrane to make the nozzle communicate with the medicine in the ampoule. The last mentioned embodiment en- ables a changing of the nozzle between the injections and further the access to the ampoule is closed when the nozzle is dismounted.

In an embodiment of the invention the injector may comprise a distal part accommodating the ampoule and a proximal part containing the drive mechanism and the piston rod which can be advanced a set distance by said drive mechanism when released the distal and the proximal parts being provided with threads so that one of these parts can successively be screwed into the other to set a dose to be delivered by the device.

In the following the invention is described in further details with references to the drawing, wherein Figure 1 schematically shows a sectional view of an injector according to the invention.

Figure 2 shows in enlarged scale the ampoule end and a mounted injection member.

The injector shown in figure 1 comprises a distal part 1 accommodating an ampoule 2 which is at an outlet end closed by a rubber membrane 3 and at its other end closed by a piston 4 which can be forced into the ampoule to press out some of the content of the ampoule through a nozzle 5 at the end of a hollow needle 6 which pierces the rubber membrane 3 to make the nozzle communicate with the interior of the ampoule 2 when the nozzle by a hub 7 is mounted on a receiving part at the outlet end of the ampoule. Figure 2 shows in an en- larged scale the outlet end of the ampoule with the mounted hub 7.

Further the injector comprises a proximal part forming a power unit comprising a housing 8 in which a piston rod 9 is mounted to be moved axially between a proximal and a distal posi- tion. In figure 1 the piston rod 9 is in its proximal position in which it is held against the force of a spring 10 which positioned between an inward flange 11 in the housing 8 and a ring shaped flange 12 on the piston rod 9 forces the said piston rod in a distal direction. The piston rod is held in its proximal position by a number of hooks 13 provided on flexible arms 14 at the proximal end of the piston rod. Said arms 14 projects through an opening left in the centre of the inward flange 11 so that the hooks grips behind this flange. The hooks present collectively a conical surface on which the edge of an inner bore in a release button 15 can act when the release button is pressed to press hook 13 and the arms inward towards the axis of the piston rod 9 until the hooks loose their grip over the flange 11 and can be drawn through the opening at the centre of said flange 11 when the piston rod 9 by the spring is

forced to its distal position which is defined by the ring shaped flange 12 on the piston rod abutting a second inward flange 16 in the housing 8.

From its distal position the piston can be returned to its proximal cocked position by a cock- ing device comprising a not shown pin which mounted through a hole 17 which runs through the piston rod 9 perpendicularly to the longitudinal axis of this piston rod and which through slots in the housing 8 projects from the outer side of this housing. The ends of this pin can be gripped and manually drawn in a proximal direction to return the piston rod 9 to its proxi- mal position. During this movement of the piston rod 9, the hooks 13 will be moved through the opening at the centre of the inward flange 11 an will due to the flexibility of the arms snap outward and grip behind the flange 11 to lock the piston rod in its proximal position.

The distal part 1 of the injector containing the ampoule 2 is at its proximal end provided with an outer thread which engage an inner thread in the tubular distal end of the housing 8 of the proximal part so that the distal part can be screwed into the proximal part of the injector.

This screw connection allows the parts to be screwed apart to allow a changing of the am- poule and to be screwed successively into each other to allow a setting of a dose to be in- jected.

When a dose is going to be injected, the procedure is the following starting with the piston rod in its distal position: A hub 7 with a nozzle 5 is mounted on the receiving member at the outlet end of the ampoule 2, the distal part is screwed into the proximal part until a visual inspection shows that medicine is pressed out through the nozzle, the power unit is cocked by drawing the piston in a proximal direction until the hooks 13 grips behind the flange 11 (the end of the piston rod 9 now has a distance to the piston 4 corresponding to the stroke of the piston rod), a dose is set by screwing the distal part into the proximal part a distance corre- sponding to the movement of the piston which is necessary to press out the wanted

dose (the distance between the end of the piston rod 9 and the piston 4 now corre- sponds to the stroke of the piston rod minus the distance which the piston must be moved to inject the wanted dose), and the nozzle is held against the skin where the injection is wanted and the release but- ton 15 is pressed (the piston rod is now moved by the spring in the distal direction a distance corresponding to its stroke. During the last part of this movement the piston is moved into the ampoule a distance corresponding to the set dose).

A counter may be installed to count units in accordance with the extend of the screwing to set a dose. Further a detent may be provided so that the distal part can only be unscrewed from the proximal part when the detent is released either to allow a partly unscrewing to an- nul a wrongly set dose or a totally unscrewing to allow changing of the ampoule.

The cocking of the injector may be performed by an build in motor which is powered either from a build in battery or through an external connection to a power source. Also gas pres- sure from a build in or an external gas cartridge may be used.

In the embodiment described the piston rod is driven by a cocked spring but the piston rod may also be driven by compressed gas, or by an electric motor, or by a carefully adapted charge of an explosive.