Abstract:
An injection device houses a syringe and contains a drive spring, to urge the syringe forwards to project its needle and then eject the dose, and a return spring, to push the syringe back into the housing and retract the needle. A trigger is operated in one way to release the drive spring and in another way to release the return spring, these operations being mutually exclusive and there being parts for ensuring they are performed in the correct order.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to injection devices. 
     It concerns devices which are fitted with a syringe having a capsule with a needle projecting from the forward end and a plunger from the rear end. A trigger and spring mechanism, when operated, shoots the syringe forwardly to project the needle, and then continues to act on the plunger to eject the dose. There is also a return spring arrangement to retract the needle after use, thereby making it safe. 
     DESCRIPTION OF THE RELATED ART 
     In WO 99/03529 we have described such a device in which the trigger is arranges to provide an obstruction to the syringe, after its needle has been projected to prevent the syringe being retracted until the trigger is released or operated in another manner. Then the return spring can act. 
     This works well enough, but there is a potential problem with the device if it is used with a syringe with a low volume of prefill. The total movement of the plunger can then be insufficient to allow the differential between the drive and return springs to permit the syringe to retract satisfactorily. 
     SUMMARY OF THE INVENTION 
     It is the aim of this invention to ensure a more certain retraction. 
     According to the present invention there is provided an injection device for containing and operating a syringe having a capsule with a needle projecting from the forward end and a plunger from the rear end, the device having a barrel enclosing the syringe and first and second springs initially held by a trigger assembly carried by the barrel in compressed energised states respectively at the rear and towards the forward ends of the barrel, wherein a first operation of the trigger assembly releases the first spring but keeps the second spring compressed so that the first spring urges the syringe forwardly by acting on the plunger and thence through the dose within the syringe, and then, when the syringe reaches a needle projecting position, presses the plunger forwards relative to the capsule to eject the dose, and wherein a second, different operation of the trigger assembly releases the second spring which exerts itself to retract the syringe and its needle. 
     Preferably means will be provided for preventing the second operation of the trigger assembly before the first operation. 
     The first, drive spring is conveniently housed in a cylindrical slider and acts between the rear end of the barrel and the forward end of the slider. 
     Preferably the second, return spring is housed in a cylindrical carrier surrounding the capsule and acts between the rear end of the carrier and an abutment forward of the carrier provided by the barrel. This can locate in the barrel and provide a guide for the syringe as well as containment for the return spring. 
     The trigger assembly is conveniently a single rocker aligned longitudinally of the barrel, its rear end co-operating with the slider, to release that when its forward end is pressed in as said first operation, and its forward end co-operating with the carrier, to release that when its rear end is pressed in as said second operation. The co-operation of the rear end of the rocker with the slider can be the agency preventing that rear end being pressed in until after the slider has moved the syringe to its needle projecting position. 
     Since the return spring is out of commission until the injection has been carried out, a third, light spring may be provided to keep the syringe retracted against a stop until the device is fired. This will present no significant resistance to the main drive spring when that is released. 
     This containment of the return spring until it is actually needed means that it can be powerful enough to ensure retraction whatever the prefill and the amount the main drive spring hogan expanded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention one embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a part axial section of an injection device, the section being on line I—I of FIG.  2 . 
     FIG. 2 is a plan view of the device in a folded out condition before assembly is complete, and 
     FIG. 3 is a perspective view of the device in the condition of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The injection device has a barrel integrally moulded in two halves  1  and  2  connected by a thin web  3  which serves as a hinge to allow the halves to be closed together when the interior components have been assembled. 
     The device is for housing and operating a syringe  4  comprising a capsule  5 , a needle  6  projecting from its leading end, a piston  7  confining the dose, and a piston rod  8  projecting from the rear end of the capsule. The device projects the needle  6  by spring action on the piston rod  8 , which acts through the dose in the syringe, and then ejects the dose by continuance of that spring action. Finally it retracts the syringe by means of a further spring. 
     The main drive spring  9  is housed in a cylindrical slider  10  at the rear end of the barrel, the slider having a tongue  11  that is initially engaged by the rear end of a trigger  12 . This is integrally formed with the barrel and is of the rocker type. In its normal, initial position it keeps the spring  9  fully compressed between the closed rear end of the barrel and an inturned flange  13  at the leading end of the slider  10  The syringe is initially positioned so that the rear end of its piston rod  8  is immediately in front of the flange  13  of the slider  10  when that syringe has a full dose. For lesser prefills, there will be a gap between the rod  8  and slider  10 . 
     A return spring  14  is pre-compressed and held in its own carrier  15 . This is cylindrical and double walled to provide an annular space for the spring  14 , open at the forward end and closed at the rear end. It locates snugly within the barrel with the forward end of the spring  14  against a shoulder  16  provided by the barrel and the rear end of the carrier  15  initially engaged by the forward end of the trigger  12 , this holding the spring  14  compressed. The carrier  15  also provides a guide for the syringe  4 , around which it forms a collar. 
     At its rear end, the carrier  15  has an interior annular rebate  17  to locate the forward end of a third spring  18  which coils with coarse pitch around the syringe  4  back to an abutment  19  which fits the outwardly beaded rear end of the syringe. The bead itself is not prominent enough, and the abutment  19 , which slots over it laterally, effectively enlarges it. The abutment  19  also co-operates with lugs  20  on the inside of the barrel halves  1  and  2  which limit rearward movement Sliders  21  integral with the abutment  19  are guided longitudinally, of the barrel in grooves  22  completed when the halves  1  and  2  are closed together. The forward ends  23  of these grooves serve as stops. 
     As explained below, the trigger  12  is operated twice, in two different ways, and to ensure that this is done in the correct sequence the rear end of the trigger has a small lug  24  projecting back over the forward end of the tongue  11 . This prevents the rear end of the trigger  12  being pressed in when the device is in its initial, primed position as shown in FIG.  1 . 
     For use of the device, starting from that position, the forward end of the trigger  12  is pressed, lifting the rear end clear of the tongue  11 . The drive spring  9  is therefore released and shoots the slider  10  forwards, and with it the piston rod  8  and piston  7 . The dose, incapable of rapid escape, acts as a solid in known manner, and so the syringe  4  is carried forwards projecting the needle  6  from the barrel. The weak spring  18 , which serves only to keep the syringe retracted before firing, offers negligible resistance and is easily compressed. The trigger  12  keeps its engagement with the return spring carrier  15  at this point and the spring  14  therefore offers no resistance. 
     The syringe is arrested by the sliders  21  coming up against the stops  23 . The spring  9  continues to expand and, through the slider  10  and piston rod  8 , ejects the dose through the needle  6 . 
     The trigger  12  is then released. The slider  10 , having travelled beyond the rear end of the trigger, does not provide any obstruction to the resumption of its initial position. The trigger can then be pressed in at the rear end, bringing its forward end outwards and clear of the carrier  15 . The return spring  14  is therefore released and, through the abutment  19 , which has been closed up almost to the carrier  15 , it urges the syringe back into the barrel, retracting the needle Since this spring  14  is kept compressed until this stage and does not have to be energised by the drive spring, it can easily be powerful enough to overcome the extended drive spring and push the syringe back. 
     The syringe is thus spent and the device safe. 
     The trigger  12  in its rocker form is preferred for simplicity. However, its two functions could be performed by two separate triggers, although to ensure that they are operated in the correct sequence the one that releases the return spring would have to be incapacitated until after the device has been fired, requiring a more complicated interlock system than the lug  24  and the tongue  11 .