BACKGROUND OF THE INVENTION 
     This invention relates to autoinjection devices and, in particular but not exclusively, to such devices intended for multiple use. 
     DESCRIPTION OF THE RELATED ART 
     Although single use autoinjection devices are common there are many instances where the autoinjection device is designed for reuse by the user or clinician and this is becoming more frequent due to environmental awareness. Where an autoinjection device is to be reused, it is important that the loading, priming, firing and unloading of the syringe is achieved simply and consistently by a wide range of potential users and also that there are safeguards against inadvertent operation. 
     SUMMARY OF THE INVENTION 
     According to one aspect of this invention there is provided an autoinjection device comprising: 
     a housing including a main body portion and a front body portion moveable longitudinally between a closed position and an open position allowing access to enable in use a syringe to be loaded into said housing, the syringe having a body, a plunger and a needle at a forward end of the body; 
     a drive member and a drive bias means disposed in said housing, the drive member being moveable against said drive bias means to a primed position and operable in use when released from said primed position to urge the syringe forwardly to an injection position, and to expel a dose therefrom; and 
     trigger means for releasably retaining said drive member in its primed position, 
     the device being operable to move said drive member to its primed position as the main body and the front housing portion are moved to their closed position. 
     In this way preferred arrangements of the device may be opened to insert a syringe and then closed to prime the drive mechanism. 
     To provide a safety feature, at least one of said front housing portion and said drive member is preferably moveable between an inactive configuration, in which closing movement of the front housing portion does not cause engagement with said drive member, and an active configuration, in which closing movement of said front housing portion applies directly or indirectly a rearward force to said drive member to move it to said primed position. 
     Thus the front housing portion may include a drive face adapted to be moved to cooperate with a drive face on the drive member. 
     An externally operable actuating member may be disposed on the main body portion and actuable to urge the drive faces into lateral engagement against a bias. 
     Conveniently, said front housing portion is operable to apply directly or indirectly a force to release said trigger means on movement of the front housing portion rearwardly from said closed position. 
     Again, for safety, at least one of said front housing portion and said trigger means may be changeable between an inactive configuration in which rearward movement of said front housing to engage said trigger means is prevented, and an active configuration, wherein, on rearward movement from said closed position, said front housing portion releases said trigger means. 
     An externally operable actuating member may be disposed on the main body portion and operable to switch the front housing portion and the trigger means to their active configuration. 
     A dual function actuating member may be provided for activating the priming stroke and for freeing the front housing portion for rearward motion to release the trigger means. 
     This safety feature may be provided by providing releasable safety latch means for preventing rearward movement of said front housing portion to release said trigger means until after said safety latch has been released. 
     Still further, to ensure that the needle is shrouded after an injection a lock out latch means may be provided for latching the forward housing portion against retracting means when it returns to a forward position on completion of an injection. 
     Both latching functions may be performed by a common releasable latching means. Indeed, in a particularly preferred arrangement a common actuating member and latching means may be provided. 
     To facilitate the injection, the housing may have an associated injection site contacting element having two lobes spaced to either side of the longitudinal axis of the needle and adapted in use, when the contact element is pressed against the user&#39;s flesh, to compress the flesh at spaced locations to either side of the injection axis and thereby to cause a bulge at the injection site. 
     Preferably said injection site contact element has only two lobes, and the contact element has a profile comprising a central concave region with two convex regions to either side thereof to define said lobes. In one arrangement said lobes are adapted to move towards each other as pressure is applied to said device, thereby to exert a pinching action to enhance said bulging effect. 
     In another aspect, this invention provides an autoinjection device comprising: 
     a housing having a main body portion, containing a drive mechanism, and 
     a front body portion moveable longitudinally between a closed position and an open position allowing access to enable a syringe to be loaded therein for an autoinjection cycle, wherein closing of said housing is operable to energise the drive mechanism for the autoinjection cycle, and rearward movement of said front body portion beyond the closed position releases said drive mechanism. 
     The device advantageously includes an externally operable actuating member, operarable to affect at least one of the following functions: 
     to engage and/or disengage a load path between the forward body portion and a prime mover in the drive mechanism, 
     to engage and/or disengage a load path between the forward body portion and a trigger for the drive mechanism, 
     to prevent and/or allow rearward movement of the forward body portion prior to release of said trigger, 
     to prevent and/or allow rearward movement of the forward drive portion from a shrouding position after completion of said injection operation. 
     In yet a further aspect, this invention provides an injection device comprising: 
     a housing including a main body portion and a front body portion relatively moveable longitudinally; 
     the housing being openable to provide access to allow a syringe to be loaded into the housing in use, the syringe having a body, a plunger and a needle at its forward end; 
     a drive member disposed within the housing and moveable against a drive spring bias to a primed position and operable in use when released from said primed position to urge the syringe forwardly within the housing to an injection position in which the syringe needle projects from the front end of the housing and to expel a dose therefrom; 
     a trigger for latching said drive member in its primed position; 
     said front body portion further being moveable against a housing spring bias rearwardly from a closed position to a fire position in which it unlatches said firing latch to free said drive member for forward movement under the influence of said drive spring bias; 
     wherein as said housing is moved away from the injection site following an injection, said housing spring bias urges said front body portion forwardly back towards its closed position to shroud said needle; 
     a releasable safety latch preventing rearward movement of said front body portion from its closed position until release of the safety latch, and 
     a latch for latching said forward body portion in its extended, shrouding, position. 
     Preferably said front body portion is movable longitudinally relative to said main body portion from said closed position to an open position providing access for said syringe. The front housing portion may include a drive face adapted to be moved to cooperate with the drive member to move the drive member to its primed position as the housing is moved from its open position to its closed position. An actuating member may be disposed on the main body portion for urging the drive face on the front body portion laterally into engagement with the drive member. 
     The said actuating member may also be operable to unlatch said safety latch whereby, at different phases of operation of the device, said actuating member is operable 
     to urge said drive face on the forward drive portion into engagement with the drive member 
     to release said forward body portion to allow it to move from said closed position to its firing position, and 
     to lock out the forward drive portion on completion of an injection. 
     Preferably said front body portion includes a unitary element slideably mounted for telescopic movement relating to said main body portion. Said unitary front body portion may include a shroud region for said needle, a latch surface for cooperating with said releasable safety latch, and a drive face resiliently moveable into engagement with said drive member. The drive member is preferably longitudinally slideably mounted within said housing, with said drive spring bias comprising at least one spring. The drive spring bias preferably comprises two co-acting springs, which advantageously each comprise a constant force spring. 
     Conveniently, said housing includes at its forward end thereof an interchangeable nose element, the contact nose element being interchangeable to adjust the penetration depth of the injection. 
     In another aspect, this invention provides an injection device comprising: 
     a housing; 
     said housing being adapted in use to receive a syringe having needle at its forward end; 
     a drive member moveable against a spring bias to a primed position and releasable in use to urge said syringe to a forward position and to expel a dose; 
     a trigger for latching said drive member in its primed position; 
     an actuating member having a shroud portion at a forward region thereof and moveable between a forward position and a rearward position; 
     actuating member bias means biasing said actuating member towards its forward position; 
     said actuating member being adapted in use when moved to its rearward position to release said firing latch to free said drive member for movement; 
     the actuating member thereafter being movable forward under the influence of said actuating member bias means to return to its forward position with said shrouding region in use shrouding the needle when the needle is at its forward position, and 
     releasable latch means for locking said shroud member against movement from its forward position. 
     In yet another aspect this invention provides an injection device comprising a needle through which a dose is delivered and, adjacent said needle, a skin contact surface, said skin contacting surface comprising two lobes spaced to either side of the longitudinal axis of the needle and, adapted in use, when the contact element is pressed against the skin, to compress the flesh space locations to cause a bulge at the injection site. 
     In still a further aspect, this invention provides an autoinjection device comprising a housing for receiving a syringe, and having a drive mechanism for releasable to urge the syringe forwardly to an injection position and to expel a dose therefrom, said drive mechanism comprising a drive member mounted for sliding movement within said housing and acted upon by two constant force spring arrangements. 
     Whilst the invention has been described above, it extends to any inventive combination or sub-combination of the features set out above, or in the following description or claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The invention may be performed in various ways, and embodiment thereof will now be described by way of example only, reference being made to the accompanying drawings in which: 
         FIG. 1  is a top perspective view of the injection device when closed; 
         FIG. 2  is a top perspective view of the injection device when open for insertion of a syringe; 
         FIG. 3  is a bottom perspective view when closed; 
         FIG. 4  is an exploded view of the injection device; 
         FIGS. 5( a ) and ( b )  are a view of the injection device when closed in a primed condition with the top cover removed, and a detail showing the latch mechanism respectively; 
         FIG. 6  are respective views of the injection device at the start of a priming action with the multi-function button depressed and urging the rear part of the drawer into engagement with the drive member, a detail on said view, and a detailed section view respectively; 
         FIG. 7  is a part-assembled view of the injection device; 
         FIG. 8  is a view from the underside of the injection device with the lower body removed showing interaction of the button and the drawer; 
         FIG. 9  is a schematic view showing the effect of the wide footprint of the nose of the injection device when applied to a user&#39;s skin; 
         FIGS. 10( a ) and ( b )  are schematic section views of a modified form of nose designed actively to exert a pinching action on the injection site, and 
         FIGS. 11( a ) to ( f )  illustrate successive stages during operation of the device of  FIGS. 1 to 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to  FIGS. 1 to 4  the reusable auto injector of the present invention is designed to have a slim compact rectangular form closed of length not much greater than that of the syringe and needle cap. 
     The auto injector indicated generally at  10  comprises a main body portion  12  made up of a lower casing part  14  and an upper casing part  16  to define a body of generally rectangular form open at one end to receive slideably a drawer or front body portion  18 . On the forward end of the drawer  18  is a nose piece  20 . The nose piece may be interchangeable so as to provide a number of options for the injection depth. Typically injection depth may be 8, 10 and 12 mm penetration depth. On the underside of the housing, as seen in  FIGS. 3 and 4 , there is a multi-function button  22  having an integral forwardly extending arm  24  and being mounted for rocking movement in the lower casing part  14 . 
     Turning more specifically to  FIG. 4 , the drawer  18  comprises a main transverse web  26  from which extends forwardly a cylindrical tubular portion  28 . From the front end of the tubular portion  28  extends first laterally then rearwardly two spring arms  30  with finger pads  32  that align with apertures  34  in the nose piece  20 . At their rearward ends, the spring arms  30  terminate in barbed portions  33  which latch behind abutment surfaces inside an inner lip  36  of the main body portion  12  releasably to latch the drawer in its closed position. Extending rearwardly from the main web  26  are two side webs  38  between which there is a floor  40  extending well to the rear of the webs  38 . The drawer in this embodiment is formed integrally from a single plastic moulding. In the floor, just to the rear of the side webs  38  is a latch aperture  42  that cooperates with the arm  24  of the multi-function button  22  in a manner to be described below. 
     At the rear end the floor  40  has a cut out to accommodate a pivot post  46  upstanding from the lower casing  14 . Upstanding from the spaced edges of the floor  40  to either side of the cut out portion  44  are upwardly projecting ribs  48 . Slideably received in the inside of the tubular portion  28  of the drawer  18  is a compression spring  50  and the forward end of a part cylindrical syringe carriage  51  cut away to allow a syringe to be introduced as to be described below. The drawer  18  is urged forwardly by twin drawer springs  52  which are received inside pockets on the lower body casing and act against the main web  26  of the drawer to push it forwardly so that the barbs  33  are normally in engagement with the lip  36  on the housing. 
     Slideably mounted in the lower casing part  14  is a drive member  54 , the forward extent of movement of the drive member  54  being limited by horns  56  on the drive member abutting internal ribs  58  on the lower casing  14 . The drive member is urged forwardly relative to the lower casing by a pair of constant spring arrangements each comprising a drum  58  rotatably mounted on the drive member around which is wrapped a constant force spring  60  whose apertured free end is anchored to the lower casing  14  by a peg  62 . By using twin constant force spring arrangements the loading on the drive member is symmetric and also the size of the springs required can be reduced thereby giving a flatter, more compact arrangement. The lower casing  14  may be provided with ‘end of dose’ apertures through which part of the drive member  54  is visible only when it is in its fully forward position with the horns  56  adjacent the abutments  58 . The drive member  54  has a latching aperture  64  which cooperates with a T-shaped latch  66  which is mounted for pivoting movement on the pivot post  48 . At the base of the T of the latch  66  is a barbed latch surface which engages the latch aperture  64  releasably to retain the drive member in a primed position against the bias afforded by the constant force springs  60 . One of the cross limbs of the latch  66  is anchored in the lower casing  14  and acts as a resilient bias  70  urging the barb  68  to its latching position. The other cross piece of the latch comprises an abutment surface  72  that is engageable by one of the ribs  48  on the rear end of the drawer when suitably deflected upwards by the multi-function button  22 . 
     Referring now to  FIGS. 11( a ) to ( f )  operation of the device will now be described. From a closed position as shown in  FIGS. 1 and 3 , the device is moved to an open position by squeezing the finger pads  32  thus pulling the barbs  33  out of engagement with the lip  36  and pulling the drawer forwardly to allow access to the syringe carriage  51  ( FIG. 11( a ) ). A syringe may then be inserted into the carriage  51  and dropped into place ( FIG. 11( b ) ). In doing this the needle cap is trapped by the keyhole shaped opening  21  in the nose piece  20 , thus holding the syringe forwardly in the drawer against the bias of the carriage spring  50  ( FIG. 11( c ) ). When the needle cap is removed, the syringe springs back to become captive inside the device thereby shrouding the needle. 
     In order to prime the device, the multi-function button  22  is pressed which in turn presses the floor  40  of the drawer away from the opposed surface of the lower casing  14  so that the ribs  48  are aligned with a transverse surface of the drive member  54 . At the same time the device is pushed down on a flat surface ( FIG. 11( d ) ) to push the drawer  18  back into the main body and at the same time pushing the drive member  54  rearwardly until it is latched in the primed position by means of the latch  66 . As the drawer returns to its closed position, the barbs  33  move past the inward lip  36  and prevent forward movement of the drawer. At the same time the arm  24  on the multi-function button  22  latches in latch aperture  42  in the floor  40  of the drawer  18 , to prevent rearward movement. It will be noted that the device cannot be primed without the compound action of both pressing the multi-function button  22  and pushing the nose against a firm surface to move the drive member  54  rearwardly to latch it. 
     In order to fire the device it is offered up to the injection site ( FIG. 11( e ) ) and the multi-function button  22  depressed to exert two functions; firstly, it lifts the arm  24  out of the latching recess  42  so allowing the drawer to move back into the housing towards the firing position and secondly, it again lifts the floor  40  of the drawer away from the internal surface of the lower casing so that the right hand rear rib  48  as viewed in  FIG. 4  is aligned with the abutment surface on the trigger and so as the drawer reaches its rearmost position, it pivots the latch  66  thus releasing the drive member  54 . The drive member moves under the influence of the constant force springs firstly to drive the syringe forwardly and thereafter to expel a dose as the plunger moves relative to the syringe. It is to be noted that, unless the button  22  is pressed to lift the floor of the drawer  40  upwardly, the rib  48  will simply pass underneath the abutment  72  and thus not trip the latch. This embodiment therefore requires that, for an injection to occur, there must be sustained pressure applied to the multi-function button  22  not only to release the drawer for rearward movement but also to ensure that the rib  48  engages the release abutment. 
     When the user releases pressure from the main body portion on completion of an injection, the drawer  18  moves back forwardly to its initial position under the influence of the drawer springs  52  and in so doing, shrouds the needle of the syringe. As it nears the rest position, the finger  24  again latches into the latch aperture  42  on the drawer thus effectively locking the drawer against retraction movement and ensuring that the needle remains shrouded. 
     Finally, to unload the device, the finger pads  32  are pressed and the drawer moved forwardly to allow access to the syringe which is removed. 
     Referring to  FIGS. 9 and 10 , the traditional method of administering a subcutaneous injection with a syringe (without an auto injector) is to pinch the flesh skin at the injection site and to insert the needle at the centre of the pinched fold of skin. This creates depth in the subcutaneous layer ensuring the needle does not enter the muscle layer below, it also relieves pressure under the skin thereby making it easier to inject the drug. With some known types of auto injector, the surface area of the device contacting the skin is typically relatively small and immediately adjacent the injection axis. Such devices can therefore have the effect of actually thinning the subcutaneous layer due to the pressure and increasing pressure under the skin, especially if the footprint is small. However in the present embodiments it will be noted that the nose  20  has a rounded ‘w’ profile where it contacts the injection site, defining two lobes  72  to either side of the injection axis. Applying pressure to two spaced locations to either side of the injection axis provides an effect similar to the pinching action commonly used when injecting into the subcutaneous layer. 
     It will be appreciated that this contact surface may be used on a wide variety of different devices beyond the embodiments described herein. 
       FIGS. 10 ( a ) and ( b )  describe a variation of this technique where the lobes of the injection contact surface instead of being static are defined on the ends of flexible arms which are configured such that as pressure is applied to the arms, the contact surfaces tend to move together thus enhancing the pinch effect. Thus in these Figures, the forward end of the housing is provided with two inwardly facing L-shaped members with rounded contact surfaces designed to roll inwards as pressure is applied.
An autoinjection device includes a main body in the front end of which is slideably mounted a drawer or front body portion. The drawer can be opened to remove or insert a syringe. Closing the drawer while pressing a multifunction button primes the drive mechanism by latching it against trigger latch. Pressing the button gain while urging the front end causes the drawer to release the trigger latch. After the injection the drawer shrouds the used needle and can be released only by pressing the button.