referring initially to fig1 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 fig3 ( 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 fig1 ( 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 . fig3 ( 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 fig4 ( 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 ( fig5 ( 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 ( fig5 ( 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 fig5 ( 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 fig6 , 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 fig7 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 .
an injection device includes an elongate housing , a syringe disposed in the housing , and an internal piston to express a dose from a needle at its front end . the injection device further includes a shroud element movable forwardly relative to the syringe to shroud the needle after use and a latch 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 .