Abstract:
An injection device includes an elongate housing ( 10 ), a syringe ( 12 ) disposed in the housing, and an internal piston ( 20 ) to express a dose from a needle ( 16 ) at its front end. The injection device further includes a shroud element ( 24 ) movable forwardly relative to the syringe to shroud the needle after use and a latch ( 74 ) movable between a latched state in which it restricts rearward movement of the shroud element, and a release state. The latch is moved to its latch state by a magnetic force ( 56 ).

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to injection devices and in particular, but not exclusively, to injection devices of the type where a syringe is disposed in an elongate housing and a shroud element moves forwardly relative to the syringe after use to shroud the needle and thereby minimise the risk of accidental needle stick injuries. 
     It is important for the latching mechanism to be highly reliable and to have latch characteristics that do not alter significantly during long term storage, for example due to changes in spring characteristics, plastic creep etc, especially if the injection device is reusable. The prior art contains many examples of devices where the shroud element is latched out by snapping past a flexible plastic finger or the like. We have devised an alternative to such devices where a magnetic force is used to enable the latch. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, in one aspect, this invention provides an injection device comprising:
         an elongate housing;   a syringe disposed in said housing and having an internal piston to express a dose from a needle at its front end;   a shroud element movable forwardly relative to the syringe to shroud the needle after use;   a latch movable between a latched state in which it restricts rearward movement of the shroud element, and a release state;   wherein said latch is moved to its latch state by a magnetic force.       

     The manner in which the magnetic force is provided may vary widely from application to application. In one arrangement, where the syringe has a longitudinally movable plunger for driving the piston, the latch may be moved by a magnetic force acting between the latch (or a part associated therewith) and said plunger. In this arrangement, the arrival of the plunger at or near the end of its stroke can be used magnetically to drive the latch into its latched state by attraction or repulsion. In such arrangements, one of the plunger and the latch may be provided with a magnet or be magnetic, with the other having a ferro-magnetic portion. Alternatively, the plunger and the latch may each be provided with magnets either in poles together or poles apart orientation. 
     In one particular arrangement, the latch may be formed of a ferro-magnetic metal. Although the latch could be an element that shuttles back and forth between the latched and release states, in one particular arrangement, a rear end portion of the latch is anchored within the housing of the injection device and extends forwardly from said anchorage to provide a latch surface at a forward region thereof. The shroud element may take many forms, but may conveniently comprise a cylindrical portion telescopically movable relative to the housing. 
     Preferably at least a component of said magnetic force acting between the latch and said plunger is in a direction to assist forward urging of the piston towards the end of the stroke of the plunger. 
     The invention also extends more generally to injection devices in which certain actions during the injection phase are enabled, initiated, or enhanced utilising magnetic force 
     A potential problem encountered in the design of injection devices is that the force needed to expel a dose at a uniform rate from a syringe can increase towards the end of the stroke due to a reduced siliconisation down the length of the internal bore of the syringe. In many spring-driven systems, the spring obeys Hooke&#39;s law so that the force generated decreases as it extends, thereby providing a lower force during the stroke, where an increasing force may actually be desirable. This issue can be partly addressed by the use of constant force springs but these are expensive and still do not provide compensation for the increasing force required. We have therefore developed an injection device in which the force applied to the piston towards the end of the stroke is enhanced by the addition of a magnetic force. 
     Accordingly, in another aspect, this invention provides an injection arrangement for injecting a dose, said arrangement comprising a syringe having an internal piston for expressing a dose from the forward end; drive means for urging the piston forwardly in the syringe to express a dose, and further including means for applying a magnetic force directly or indirectly to said piston to assist forward movement at least towards the end of the forward stroke of the piston. 
     In many injection devices the movement of a drive plunger is initially applied to the body of a syringe to move the syringe forward to extend the needle to penetrate an injection site, with the plunger restrained against forward movement relative to the syringe. Once the syringe has reached a predetermined forward position, the plunger is released to move relative to the syringe to urge the piston to express a dose. Various systems exist to effect this sequencing, with some being quite complex requiring intricate delatching mechanisms and consequently having a high component count with attendant high tooling and assembly costs. We have previously described in WO2005/002653 a device with a low component count which uses an “O” ring as a friction coupling. We have now developed a further design which does not employ friction and which still retains a low component count. Furthermore, once the coupling has yielded, there is little or no residual friction inherent in the coupling.
         In this aspect, the invention provides a n injection device comprising:   a housing;   a syringe having a generally hollow cylindrical body with a needle at its forward end and a bore slideably receiving an internal piston for expressing a dose through said needle;   the syringe being mounted within said body for movement between a rearward position and a forward limit position;   a drive plunger releasable to move forwardly to move the syringe forwardly and then to express a dose;   a magnetic coupling acting between the plunger and said syringe body for transmitting forward motion of the plunger to said syringe body but yielding as said syringe arrives at or near said forward limit position to allow a forward end of said plunger to urge said piston forwardly within the syringe bore to express said dose.       

     In this manner, once the magnetic coupling between plunger and the syringe body has yielded there is little or no friction or drag interaction between the plunger and said syringe body and so substantially all of the first of the plunger force is applied to the piston. 
     The magnetic coupling may take a variety of forms but typically may comprise a thrust member for engaging and urging said syringe forwardly, the thrust member being magnetically coupled to said plunger. Thus one of the thrust member and the plunger may include a magnetised portion and the other thereof may include a ferro-magnetic portion, or each of said thrust member and said plunger may include a magnetised portion. 
     Whilst the invention has been described above, it extends to any inventive combination set out above, or in the following description or drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be performed in various ways, and two embodiments thereof will now be described by way of example only, reference being made to the accompanying drawings, in which: 
         FIG. 1  is an exploded view of the first embodiment of an autoinjector in accordance with this invention; 
         FIG. 2  is a side section view through the autoinjector of  FIG. 1  assembled and prior to use; 
         FIGS. 3( a ) and ( b )  are respective side and top section views of the autoinjector with the cap in place, and removed; 
         FIGS. 4( a ) and ( b )  are respective side and top section views of the autoinjector with the firing button partially depressed immediately prior to the release of the plunger; 
         FIGS. 5( a ), ( b ) and ( c )  are respective side section views showing the autoinjector with the syringe in its forwardmost position, shortly after breakout of the piston, and at the injection complete stage respectively; 
         FIG. 6  is a view of the device after use, with the shroud extended and locked out by the magnetically enabled latch; 
         FIG. 7  is a side view through a second embodiment of an autoinjector, and 
         FIG. 8  is a shaded side view. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIGS. 1 to 3 , the autoinjector comprises an outer housing  10  of cylindrical form in the bore of which is disposed a syringe  12  of known form with a barrel  14 , a needle  16  extending from the forward end, and a flange  18  at its rear end. A medicament is contained within the syringe and can be expressed through the needle by a piston  20  inside the barrel. The syringe is supported and surrounded by moulded plastics shroud/carrier  23  assembly comprising a forward hollow cylindrical portion  24  integrally formed with diametrically opposed springs  26  to either side, and a collar  28  adapted to engage the forward face of the syringe flange. Extending rearwardly from the collar are two diametrically opposed clearance fingers  30  with barbed teeth  32  that engage the intermediate member, as to be described below. In the pre-use position as shown in  FIG. 3( a ) , the barbed fingers are prevented from outward splaying movement by the base of respective diametrically opposed grooves  34  on the inner surface of the rear part of the housing. In the pre-use position, the shroud portion  24  is telescopically received within the forward end of the housing and co-terminous therewith. 
     In the rear of the housing is provided a drive mechanism which comprises a first outer spring  36  which acts between the front face of a transverse inner wall  38  at the rear of the housing and a forward flange  40  of a top hat-shaped intermediate member  42 . An inner, second, spring  44  is received within a cylindrical part of the intermediate member  42  and acts between an inner face of the rear end wall thereof and a circumferential rib  46  on the forward part of a plunger  48 . At the rear end, the plunger has a resilient hooked arm  50  (see  FIGS. 1, 3 ( b )) which latches around the edge of an aperture in the transverse inner wall  38  of the housing. Projecting rearwardly from the rear end of the housing is a captive axially slideable trigger button  52  movable against a rearward bias from the position shown in e.g.  FIG. 3( b ) , where a release finger  54  is spaced rearwardly of the hooked arm  50 , to a forward portion where the finger  54  cams the hooked arm to release its retention by the wall  38 , thereby allowing the springs  36 ,  44  to drive the plunger  48  forwardly. The plunger is shaped and sized so that it can pass into and down the internal bore of the syringe barrel  14 , to urge the piston  20  to express a dose. In the forward end of the plunger is a cylindrical recess in which is located a small powerful magnet  56 . 
     The trigger button  52  is biased rearwardly by means of two integral forwardly extending sprung arms  58  with cam surfaces  60  which ride over respective abutments  62  inside the rear of the housing. However, initially, forward movement of the trigger button is prevented by means of two rearwardly extending locking arms  64  which extend back from the rear end of a cap  66 . The cap covers the whole of the forward end of the housing and has a re-entrant cylindrical portion  68  with claw features  70 . The claw features  70  slip over the rear end of a needle shield  72  which is secured to the front end of the needle during manufacture. Thus the cap  66  fulfils the functions of acting as a safety catch for the trigger button  52 , serving as a shield remover. Anchored inside the forward end of the housing is a latch  74  formed of pressed steel or other ferro-magnetic material to provide two latch arms  76  which extend forwardly from an anchorage normally to sit in an annular space between the shroud  24  and an inner part of the housing wall. 
     In operation, the user pulls the cap  66  off forwardly which removes the needle shield  22  from the syringe and arms the device by rendering the trigger button  52  operational. The user then offers the injection device up to the injection site and presses the trigger button  52 . This releases the hooked arm  50  of the plunger  48  as shown more particularly in  FIG. 4( b ) . Once the plunger is released, the first spring  36  expands to extend the syringe  20  so that the needle penetrates the flesh. During this period ( FIG. 5( a ) ), the second spring  44  remains substantially fully compressed, with the plunger  48  bearing against the piston  20  within the syringe but not moving it relative to the syringe. During the initial phase of penetration, the clearance fingers  32  on the syringe supporting collar  28  are constrained against outward splaying movement by the grooves  34  and so a gap is preserved between the syringe flange  18  and the flange  40  of the intermediate member  42 , as long as the fingers are still in engagement with the constraining grooves. 
     The fingers  32  exit the constraining grooves  34  at about the same time as forward movement of the syringe is arrested by the compression spring portions  26  bottoming out, the shroud  24  being held against movement by contact with the skin surface. When the syringe is arrested, the first spring  36  continues to expand to drive the flange  40  of the intermediate member into engagement with the syringe flange  18  thereby contributing to the force required to initiate movement of the piston down the syringe ( FIG. 5( b ) ). From this position the second spring  44  expands to drive the piston down the barrel of the syringe to express a dose. At the end of its travel, it will be noted that the magnet  56  in the plunger is spaced between the latching arms  76  of the latch  74 , as shown in  FIG. 5( c ) . When the dose has been expressed, the user pulls the device away from the flesh and so the shroud portion  24  is free to expand under the influence of the compression spring portions  26 . The shroud portion is driven by the springs  26  forwardly beyond the front tips of the latching arms  76 . Once this happens, the latching arms are free to move inwardly to the latching positions shown in  FIG. 6 , under the influence of the magnet  20 . The shroud portion  24  is therefore locked out and the device thereby rendered safe. 
     In other embodiments, not shown, one or more further magnets or ferro-magnet material may be disposed in the front end of the housing around or in front of the syringe in order to provide a magnetic boost effect as the plunger nears the end of its stroke. It will also be appreciated that the exact position of the magnet(s) and interacting components can be varied. 
     Referring now to  FIGS. 7 and 8  the second embodiment has many of the same components as the first embodiment and which act in a similar fashion. These components are given the same reference numbers and will not therefore be described in detail again. This second embodiment incorporates a magnetic coupling embodiment between the plunger  48  and the syringe designed so that during an initial penetration phase of movement the plunger is coupled magnetically to a thrust collar  80  of ferro-magnetic material that is positioned in contact with the rear face of the syringe flange  18 . Thus the plunger  48  and the syringe  18  are initially held against relative movement and so move as one during this phase, until the syringe is arrested by reaching its forwardmost position, with the needle inserted into the injection site. Upon arrest of the syringe, the spring force acting on the plunger overcomes the magnetic coupling force and the coupling yields so that the plunger is released to move forward relative to the syringe to move the plunger into contact with and to urge the piston forwardly to expel a dose. As previously, as the plunger moves alongside the ferro-magnetic latching arms  76  and they are attracted inwardly. This is enhanced in the embodiment by provision of two magnets  82  on the forward ends of the latching arms. These magnets are aligned so as to be attached towards and to exert a pull on the magnet  56  in the plunger to provide a magnetically influenced forward boost to the plunger toward the forwardmost end of its stroke. Upon completion of the injection and removal of the device from the site, the shroud  24  extends forwardly as the spring portions  26  re-expand, and as a rearward lip on the shroud passes the magnets, the latching arms move inwards to block retraction movement of the shroud and thereby lock it out. The magnets  82  may be housed slideably in through-holes in the housing wall, allowing them also to move inwards with the latching arms as the rearward lip on the shroud passes the magnets, and so provide a visual and tactile confirmation of locking out of the shroud  24 .