Abstract:
An autoinjector is provided with a magnetically operated “injection complete” indicator. A moveable magnet ( 44 ) associated with the drive piston ( 38 ) is kept at one end of a recess by a soft iron keeper ( 46 ) until the drive piston reaches its formal position where the moveable magnet ( 44 ) is attracted to impact a magnet ( 48 ) on the housing to generate an audible click. Also described is a spring finger ( 52 ) that slips behind the rear edge of the syringe boot ( 50 ) temporarily to hold the end of the boot well clear of the housing to that it can be gripped for removal.

Description:
This invention relates to injection devices, and in particular, but not exclusively, to reusable auto-injector devices comprising a housing into which a disposable syringe may be inserted to effect the injection and then removed and replaced as required for the next injection. 
     BACKGROUND OF THE INVENTION 
     It is a requirement that auto-injectors signal to the user when the injection is complete. Most have a visible indication, but sometimes the injection site is out of sight or would require some straining to see, for example in the buttocks or upper arm. It is therefore desirable to have an audible or tactile ‘injection complete’ signal. The term ‘injection complete’ is used to refer to a condition in which a satisfactory delivery of the drug has been achieved. 
     SUMMARY OF THE INVENTION 
     In one aspect this invention provides an injection device comprising a housing, a syringe disposed within said housing, and a magnetically-operated indicator for generating an injection complete indication. In the described embodiment, the use of a magnetically-operated device means that at least some of the energy required for creating the indication is derived from magnetic attraction (or repulsion) meaning that little or no energy is diverted from the main injection spring. 
     In another aspect, this invention provides an injection device including:
         a syringe disposed within a housing;   a drive element moveable from a first position to a second position to expel a dose from said syringe;   a magnetically operated indicator for indicating injection complete;   said indicator including two co-acting elements associated one with said drive element and one with said housing respectively, and arranged such that, as said drive element arrives at or near said second position, the relative position of said co-acting elements is changed by the influence of magnetic force acting therebetween.       

     As above, the injection complete indication is generated magnetically. The indication is generated by the change in position of the two co-acting elements. 
     Numerous different magnetic configurations are possible. In one arrangement, one of the co-acting elements comprises a captive element and the other comprises a relatively moveable element. Thus said captive element may be associated with the housing and the relatively moveable element may be associated with the drive element. Although not essential, it is preferred for both said co-acting elements to be magnetised. 
     In a preferred arrangement, the drive element has a ferro-magnetic keeper element associated with it adapted temporarily to restrain said moveable magnetic element until it is moved under the influence of magnetic force due to proximity of said captive magnetic element. In this way a light restraining force is applied to the moveable magnetic element until it is close enough to the captive element to be pulled away from the keeper by the magnetic force. 
     The injection complete indication may take many forms, for example it may comprise one or more of an audible indication, a visual indication or a tactile indication. Where the indicator provides an audible indication, this may be produced as a result of impact between the moveable magnetic element and the captive magnetic element (or a surface associated therewith). In this arrangement the drive element may have associated therewith a recess for receiving the relatively moveable magnetic element and allowing generally transverse movement of the moveable element due to magnetic interaction with the fixed magnetic element. The recess may extend transversely and have at one end thereof the keeper element, with the other end of the recess facing the fixed co-acting element when the drive element is in its second position. The recess may be provided in a drum which forms part of the drive assembly and which is acted upon by a spring. 
     Whilst in many embodiments the drive element will move linearly to expel a dose from the syringe, the invention extends to arrangements in which the movement of the drive member is non-linear, for example rotary. 
     In many auto-injector devices, a syringe with a sheath covering its needle is loaded into the device and the sheath has to be removed prior to operation. It is a feature of some existing auto-injector devices that the sheath covering the needle is difficult to remove once the syringe has been inserted into the device because only a very short portion of the sheath protrudes. Due to this problem, the user may decide to remove the sheath prior to inserting the syringe into the auto-injector and this increases the risk of accidental needle-stick injury. For a given length of sheath, if the device is designed so that a greater proportion of the sheath is accessible this would mean that the exposed needle tip would protrude beyond the housing and again this would lead to potential needle-stick injury. 
     Accordingly, in another aspect of this invention there is provided an injection device comprising: 
     a housing having a forward end; 
     a syringe having a needle at its forward end and disposed within said housing for longitudinal movement; 
     a removable needle sheath covering the needle prior to use; 
     the syringe being moveable forwards against a spring bias prior to injection to cause said needle sheath to move to an extended position in which a forward portion of said sheath projects from said housing to allow it to be gripped and removed from the needle, and 
     a latch for releasably latching at least one of said syringe and sheath so that said sheath is held in its extended position. 
     In this arrangement, the syringe may be moved forwardly against a spring bias so that the latch holds the syringe and/or sheath so that sufficient portion of the sheath is exposed to allow it to be gripped and removed. 
     It is preferred for the sheath to have a rearwardly facing surface and said latch to latch behind said rearwardly facing surface. This means that, on removal of the sheath, the syringe is urged rearwardly by its spring bias to move the needle back from the forward end of the housing. Thus for example, the rearwardly facing surface on the sheath may comprise the generally annular rim at the rear end of the sheath. 
     The latch may conveniently comprise a forwardly extending resiliently deflectable finger having a latch surface at a forward end region and its rearward end region being secured to said housing. 
     Whilst the invention has been described above, it extends to any inventive combination of the features set out above or in the following description or claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be performed in various ways and an embodiment thereof will now be described by way of example only, reference being made to the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view through an embodiment of the auto-injector in accordance with this invention, with the drive piston in the cocked position; 
         FIG. 2  is a side view of the auto-injector device; 
         FIG. 3  is a cross-sectional view through the auto-injector of  FIGS. 1 and 2  with the syringe extended and the dose expelled and generating an injection complete indication; 
         FIG. 4  is a cross-sectional view showing the front end of the auto-injector unscrewed and reversed to cock the drive piston; 
         FIG. 5  is a general perspective view showing the spring drum and constant force spring used in the embodiment of  FIGS. 1 to 4 ; 
         FIG. 6  is a longitudinal view with the syringe loaded and the needle sheath projecting a short distance from the front end of the housing; 
         FIG. 7  illustrates the difficulty of securely gripping the needle sheath for removal; 
         FIG. 8  is a longitudinal cross-sectional view through a second embodiment of auto-injector adapted to facilitate removal of the needle sheath; 
         FIG. 9  is a longitudinal view of the second embodiment of auto-injector; 
         FIG. 10  is a part-sectioned, detailed view on the front end of the housing showing the spring finger incorporated in the embodiment of  FIGS. 8  and  9 ; 
         FIG. 11  is a perspective view of the spring finger; 
         FIGS. 12 and 13  are successive views showing insertion of a syringe with sheath into the front end of the housing with the spring finger latching behind the sheath; 
         FIG. 14  is an enlarged detailed view on the front end of  FIG. 13 , and 
         FIGS. 15 and 16  show successive steps in removal of the needle sheath and subsequent rearward movement of the needle and syringe. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIGS. 1 to 5 , there is shown an auto-injector device comprising a front housing part  10  and a rear housing part  12 , having respective screw threaded portions  14 ,  16  to allow the parts to be screwed together as seen in  FIGS. 1 to 3 . The front and rear housing parts may be seen clearly in  FIG. 4 . 
     The front housing part is provided at its forward end with a captive depth adjustment cap  18  having an aperture  20  at its forward end through which the syringe needle may project. The forward housing part  10  is provided with an internal collar  22  which slideably receives a syringe carrier  24  which is biased rearwardly by a spring  26 . The syringe carrier  24  has an internal bore designed to receive a syringe  28  including a syringe plunger  30  and a needle  32 . The bore in the syringe carrier  24  may be designed to be an interference fit at its forward end with a part of the syringe so as lightly to grip the syringe. The syringe carrier  24  is provided at its forward end with a rib (not shown) that limits rearward movement of the syringe carrier under the influence of the spring  26 . 
     The rearward housing part  12  contains a constant force drive spring  34  wrapped around a drum  36 . The outer end of the constant force spring is secured to an anchorage (not shown) on the inner part of the rearward housing part  12 . The drum  36  is mounted for rotation in a drive piston  38  and the constant force spring is designed to be energised as the drive piston  38  is moved to its rearward position as shown in  FIG. 1 . The drive piston  38  may be latched in its cocked position and released by any suitable mechanism, but this is done in the present embodiment by means of a trigger  40  provided on the rearward housing part  12 . 
     The drum  36  defines a recess  42  in which is confined a spherical magnet  44  having north and south poles. In the base of the recess  42  is provide a soft iron keeper element  46  which, in the absence of other magnetic influences, retains the spherical magnet against the base of the recess  42 . 
     The rearward housing part  12  has provided in a wall thereof a captive, fixed, magnet  48  positioned at the forward end of the operational stroke of the drive piston  38 . The captive magnet  48  is positioned such that magnetic attraction between the captive magnet and the moveable magnet  44  overcomes the attraction between the moveable magnet  44  and the keeper  46  when the drive piston has moved to a position in which the injection is complete. The resultant ‘click’ as the moveable magnet impacts the fixed magnet therefore gives an audible indication that the injection is complete. The impact will also generate a tactile signal that can be felt by the user. 
     In other embodiments, not shown, different types of audible, tactile or visual indicators may be provided. For example, co-acting magnets may be provided which cause an element to project or retract into the housing so that the user can feel for an indication of injection complete. 
     An important feature of the magnetically-operated indicator is that it does not divert energy from the main constant force spring that expels the dose. In fact energy is required during priming of the device to separate the magnets but this does not significantly diminish the energy available for expelling the dose. 
     An injection cycle will now be described. The forward and rearward housing parts are unscrewed and the drive piston  38  pushed back to the cocked position by reversing the forward housing part and pushing the piston back using the nose of the cap  18  until it is latched by the trigger. As the drum  36  moves rearwardly, the spherical magnet  44  is pulled away from the captive magnet  48  and is pulled back towards the keeper  46  by magnetic attraction. 
     A syringe is then loaded into the syringe carrier  24  in the forward housing part  10  and the housing parts screwed together. The cap is removed during or after this operation by suitable means (such as is described below). The auto-injector is then in the condition shown in  FIG. 1  ready to be fired. On firing, the drum  36  and drive portion  38  are released for forward movement under the influence of the drive spring  26  so that the drive piston  38  moves the syringe plunger  30  forward to expel the dose. Towards the end of the stroke, when the required amount of the drug has been expelled, the drum  36  is close enough to the captive magnet for the captive magnet to attract the spherical magnet with a force greater than that exerted by the keeper so that there is a loud click indicating injection complete. 
     Referring now to  FIGS. 6 to 16 , there is now described an arrangement for facilitating removal of the needle sheath. As seen in  FIGS. 6 and 7 , when assembled with the syringe in place prior to injection, a typical needle sheath  50  extends only a short way forward of the cap  18 . This is not enough to enable someone to grip the needle sheath sufficiently tightly to remove it. There is a risk therefore that the user circumvents this problem by removing the sheath  50  before inserting the syringe into the syringe carrier, with the attendant risk of needle-stick injury. 
     Accordingly, as seen in  FIGS. 8 to 16 , a spring finger  52  is provided that is anchored to the forward housing part or a component affixed thereto and extends forwardly and inwardly. The spring finger  52  is designed so that its forward end can latch behind the rearward annular rib of the needle sheath  50  when the syringe has been inserted into the syringe carrier  24  and the syringe carrier pushed forwardly against the bias of the spring  26  as seen in  FIG. 13 . The spring finger retains the syringe and sheath in this forward position leaving a much greater portion of the sheath available to be gripped and removed. As soon as the sheath is removed by easing it off, and thus overcoming the friction fit between the needle sheath and the needle hub of the syringe, the syringe shoots back rearwardly under the influence of the spring  26  so that the needle is safely within the cap  18 . The device is then ready for use as previously described.