fig1 a and 1b show two longitudinal sections in different section planes of an auto - injector 1 , the different section planes approximately 90 ° rotated to each other . the auto - injector 1 comprises an elongate housing 2 . a syringe 3 , e . g . a hypak syringe , with a hollow needle 4 is arranged in a proximal part of the auto - injector 1 . when the auto - injector 1 or the syringe 3 is assembled a protective needle shield may be attached to the needle ( not illustrated ). a stopper 6 is arranged for sealing the syringe 3 distally and for displacing a liquid medicament m through the hollow needle 4 . the syringe 3 is held in a tubular syringe carrier 7 and supported at its proximal end therein . a single drive spring 8 in the shape of a compression spring is arranged in a distal part of the auto - injector 1 . a plunger 9 is arranged for forwarding the spring force of the drive spring 8 . inside the housing 2 a retraction sleeve 10 is slidably arranged . before the injection is triggered the retraction sleeve 10 is in a maximum proximal position and prevented from moving in distal direction d by means of stops 11 caught behind latches 12 in the housing 2 . a distal end of the drive spring 8 bears against an end face 13 of the retraction sleeve 10 . due to the stops 11 and latches 12 the force of the drive spring 8 is thus reacted into the housing 2 . the proximal end of the drive spring 8 bears against a decoupling member 14 arranged around the plunger 9 . distally from the end face 13 the retraction sleeve has two or more resilient arms 15 for holding a stud 16 and keeping it from being moved in proximal direction p . the stud 16 is arranged at the distal end of the plunger 9 . the stud 16 and the resilient arms 15 have corresponding ramp features for pushing the resilient arms 15 apart in order to allow the stud 16 and the plunger 9 to move in proximal direction p . the decoupling member 14 comprises a thrust face 17 for bearing against a proximal end of the drive spring 8 . proximally from the thrust face 17 two or more resilient decoupling arms 18 are provided at the decoupling member 14 , the decoupling arms 18 having inner ramped surfaces bearing against a first shoulder 19 in the plunger 9 in proximal direction p . the resilient decoupling arms 18 are supported by an inner wall of the retraction sleeve 10 in this situation so they cannot flex outward and slip past the first shoulder 19 . a trigger button 20 is arranged at the distal end d of the auto - injector 1 . the trigger button 20 may be pushed in proximal direction p in order to start an injection . as long as the trigger button 20 is not pushed the resilient arms 15 are caught between two or more retainers 21 arranged at the trigger button 20 so the resilient arms 15 cannot flex outward and the stud 16 although proximally biased by the drive spring 8 cannot slip through . the syringe carrier 7 is engaged for joint axial movement with a syringe holder 22 which is slidably arranged in the retraction sleeve 10 . the syringe holder 22 is provided with two or more resilient syringe holder arms 23 arranged distally . the syringe holder arms 23 have a respective inclined surface for bearing against a second shoulder 24 in the plunger 9 arranged proximally from the first shoulder 19 . in the initial position shown in fig1 a and 1b , the syringe holder arms 23 are supported by an inner surface of the housing 2 so they cannot flex outward and the second shoulder 24 cannot slip through . in order to support the syringe holder arms 23 at the housing 2 a respective number of apertures are provided in the retraction sleeve 10 . two resilient wedges 10 . 1 are arranged at a proximal end of the retraction sleeve 10 . the housing 2 has two recesses 2 . 2 arranged to accommodate the resilient wedges 10 . 1 when the retraction sleeve 10 is in its proximal position . a skin interlock sleeve 25 is telescoped in the proximal end p of the housing 2 . the syringe carrier 7 in turn is telescoped in the interlock sleeve 25 . the interlock sleeve 25 is biased in proximal direction p by an interlock spring 26 . two resilient second latches 27 are arranged in the housing 2 near the proximal end p . in the state as delivered the second latches 27 are relaxed and extend inwardly through respective apertures 25 . 1 in the interlock sleeve 25 in a manner to prevent the syringe carrier 7 from translating in proximal direction p by the syringe carrier 7 abutting against respective distal faces 27 . 1 of the second latches 27 . the syringe carrier 7 , the syringe 3 and the needle 4 can therefore not be forwarded when pushed by the plunger 9 . in order to start an injection the auto - injector 1 has to be pressed against the injection site , e . g . a patient &# 39 ; s skin . as a result the interlock sleeve 25 translates in distal direction d into the housing 2 ( see fig2 a and 2b ). a proximal edge of the aperture 25 . 1 pushes against a proximal ramp 27 . 2 of the second latch 27 thereby flexing the second latch 27 outwards so the syringe carrier 7 comes clear of the distal faces 27 . 1 and may now translate in proximal direction p . when translated into the housing 2 as in fig2 a and 2b a distal end of the interlock sleeve 25 supports the resilient wedges 10 . 1 from inside so they cannot be flexed inwards thus preventing the retraction sleeve 10 from translating in distal direction d . the trigger button 20 can now be pushed to release the drive spring 8 in order to insert the needle 4 into the injection site and to inject the medicament m , as shown in fig2 c and 2d . if the auto - injector 1 is removed from the injection site without operating the trigger button 20 the interlock sleeve 25 will translate back into its proximal position under load of the interlock spring 26 . the second latches 27 will flex inwards and block the syringe carrier 7 so the auto - injector 1 is in its as delivered state again . the sequence of operation can be reversed in this embodiment , i . e . the trigger button 20 may be pushed before pressing the auto - injector 1 against the injection site . when the trigger button 20 is pushed the retainers 21 are pushed in proximal direction p so the resilient arms 15 are allowed to flex outward . under load of the drive spring 8 the inclined surfaces of the stud 16 force the resilient arms 15 apart until the stud 16 can slip through . the second shoulder 24 pushes the syringe holder 22 , syringe carrier 7 and syringe 3 forward while no load is exerted onto the stopper 6 . the hollow needle 4 appears from the proximal end p and is inserted into an injection site , e . g . a patient &# 39 ; s skin ( see fig2 c and 2d ). the forward movement continues until the syringe holder 22 bottoms out at a first abutment 32 in the housing 2 . the travel from the initial position up to this point defines an injection depth , i . e . needle insertion depth . when the syringe holder 22 has nearly bottomed out the resilient syringe holder arms 23 have reached a widened portion 2 . 1 of the housing 2 where they are no longer supported by the inner wall of the housing 2 . however , since the force required to insert the needle 4 is relatively low the second shoulder 24 will continue to drive forward the syringe holder 22 until proximal travel is halted at the first abutment 32 . at this point the syringe holder arms 23 are flexed out by the continued force of the second shoulder 24 and allow it to slip through . now the plunger 9 no longer pushes against the syringe holder 22 but against the stopper 6 for expelling the medicament m from the syringe 3 and injecting it into or through the patient &# 39 ; s skin . when the stopper 6 has nearly bottomed out in the syringe 3 the decoupling member 14 has reached a position where it pushes against the latches 12 in a manner to decouple the retraction sleeve 10 from the housing 2 . thus the drive spring 8 is no longer grounded with its distal end in the housing 2 by the latches 12 . instead , as soon as the decoupling member 14 has bottomed out at a second abutment 33 the proximal end of the drive spring 8 gets grounded in the housing 2 while its distal end is pulling the retraction sleeve 10 in distal direction d . just before the decoupling member 14 decouples the retraction sleeve 10 from the housing 2 the decoupling arms 18 reach an aperture 34 in the retraction sleeve 10 so they are no longer kept from being flexed outward . the decoupling arms 18 are thus pushed outward by the first shoulder 19 pushing against its ramped surfaces so the first shoulder 19 can slip through in distal direction d as soon as the decoupling member 14 has hit the second abutment 33 . although the latches 12 are disengaged now , the retraction sleeve 10 may not yet slide in distal direction d because of the resilient wedges 10 . 1 being held in the recess 2 . 2 between the housing 2 and the interlock sleeve 25 as long as the interlock sleeve 25 is in its distal position by the auto - injector 1 being kept pushed against the injection site . if the auto - injector 1 is taken away from the injection site the interlock sleeve 25 will return to its proximal position ( as in fig1 a and 1b ) under load of the interlock spring 26 so the resilient wedges 10 . 1 are no longer supported from inside . since the drive spring 8 tries to pull the retraction sleeve 10 in distal direction d , distal ramps of the resilient wedges 10 . 1 move along proximal ramps of the recesses 2 . 2 thereby flexing the resilient wedges inwards as the retraction sleeve 10 starts translating in distal direction . the syringe holder 22 is taken along in distal direction d by the retraction sleeve 10 , e . g . by a front face 35 . thus the syringe 3 and needle 4 are retracted into a safe position inside the housing 2 , e . g . into the initial position . the plunger 9 , no longer bearing against the decoupling arms 18 is pulled back too . in the embodiment illustrated in fig1 a , 1b , 2a , and 2b , the latches 12 and the stops 11 at the retraction sleeve 10 are not absolutely required . retraction can be triggered by removal of the auto - injector 1 from the injection site alone . however , the latches 12 and the stops 11 facilitate assembly of the auto - injector 1 and give an initial position to the retraction sleeve 10 . furthermore , the stops 11 as well as lugs on the decoupling member 14 for disengaging the latches 12 and the stops 11 run in a slot 2 . 6 in the housing 2 thus preventing the retraction sleeve 10 and the decoupling member 14 from rotating . fig3 a and 3b show two longitudinal sections in different section planes of another embodiment of an auto - injector 1 , the different section planes approximately 90 ° rotated to each other . the auto - injector 1 comprises an elongate housing 2 . a syringe 3 , e . g . a hypak syringe , with a hollow needle 4 is arranged in a proximal part of the auto - injector 1 . when the auto - injector 1 or the syringe 3 is assembled a protective needle shield may be attached to the needle ( not illustrated ). a stopper 6 is arranged for sealing the syringe 3 distally and for displacing a liquid medicament m through the hollow needle 4 . the syringe 3 is held in a tubular syringe carrier 7 and supported at its proximal end therein . a single drive spring 8 in the shape of a compression spring is arranged in a distal part of the auto - injector 1 . a plunger 9 is arranged for forwarding the spring force of the drive spring 8 . inside the housing 2 a retraction sleeve 10 is slidably arranged . before the injection is triggered the retraction sleeve 10 is in a maximum proximal position abutting against a stop 2 . 4 in the housing 2 . a distal end of the drive spring 8 bears against an end face 13 of the retraction sleeve 10 . the proximal end of the drive spring 8 bears against a decoupling member 14 arranged around the plunger 9 . the decoupling member 14 comprises a thrust face 17 for bearing against a proximal end of the drive spring 8 . proximally from the thrust face 17 two or more resilient decoupling arms 18 are provided at the decoupling member 14 , the decoupling arms 18 having inner ramped surfaces arranged for bearing against a first shoulder 19 in the plunger 9 in proximal direction p . in the as delivered state in fig3 a and 3b the first shoulder 19 is situated a small distance proximally from the ramped surfaces of the decoupling arms 18 . the resilient decoupling arms 18 may be supported by an inner wall of the retraction sleeve 10 for preventing them from being flexed outward and slip past the first shoulder 19 . apertures 34 are provided in the retraction sleeve 10 for allowing the decoupling arms 18 to flex outwards . a trigger button 20 is arranged at the distal end d of the auto - injector 1 . the trigger button 20 is arranged to be pushed in proximal direction p in order to start an injection . the syringe carrier 7 is engaged for joint axial movement with a syringe holder 22 which is slidably arranged in the retraction sleeve 10 . the syringe holder 22 is provided with two or more resilient syringe holder arms 23 arranged distally . the syringe holder arms 23 have a respective inclined surface for bearing against a second shoulder 24 in the plunger 9 arranged proximally from the first shoulder 19 . in the initial position shown in fig3 a and 3b , the syringe holder arms 23 are supported by an inner surface of the housing 2 so they cannot flex outward and the second shoulder 24 cannot slip through . in order to support the syringe holder arms 23 at the housing 2 a respective number of apertures are provided in the retraction sleeve 10 . two resilient wedges 10 . 1 are arranged at a proximal end of the retraction sleeve 10 . the housing 2 has two recesses 2 . 2 arranged to accommodate the resilient wedges 10 . 1 when the retraction sleeve 10 is in its proximal position . a skin interlock sleeve 45 is telescoped in the proximal end p of the housing 2 . the syringe carrier 7 in turn is telescoped in the interlock sleeve 45 . the interlock sleeve 45 is biased in proximal direction p by an interlock spring 26 . in the as delivered state as shown in fig3 a and 3b , the interlock sleeve 45 is coupled with the syringe carrier 7 by resilient clips 45 . 1 provided in the interlock sleeve 45 . the clips 45 . 1 are engaged in a respective recess in the syringe carrier 7 and outwardly supported by the housing 2 so they cannot flex outward . hence , in the state as delivered , the interlock sleeve 45 , the syringe carrier 7 with the syringe 3 and the needle 4 , the syringe holder 22 and the plunger 9 are coupled for joined axial translation . the recess in the syringe carrier 7 and the clip 45 . 1 have ramps arranged to flex the clip 45 . 1 outwards when the syringe carrier 7 and the interlock sleeve 45 are pushed against each other in longitudinal direction . the syringe carrier 7 , the syringe 3 and the needle 4 can therefore not be forwarded when pushed by the plunger 9 . the trigger button 20 is secured on the housing 2 by means of clips 20 . 1 so it cannot translate in distal direction d further than shown in fig4 . a central shaft 20 . 2 extends from the trigger button 20 in proximal direction towards the plunger 9 . another set of resilient clips 10 . 2 are arranged on the retraction sleeve 10 proximally from the end face 13 inside the drive spring 8 arranged for abutting against a shoulder 20 . 5 in the central shaft 20 . 2 in a manner to prevent the trigger button 20 from being actuated in the as delivered state ( see fig9 ). the retraction sleeve 10 can not translate in proximal direction p due to its proximal end abutting against the stop 2 . 4 . the resilient clips 10 . 2 may be disengaged from the trigger button 20 by respective bars 9 . 1 extending distally from the distal end of the plunger 9 upon translation of the plunger 9 in distal direction d . a set of resilient catches 10 . 3 ( shown in fig5 b ) are arranged on the retraction sleeve 10 proximally from the end face 13 inside the drive spring 8 for engaging respective catches 14 . 1 extending distally from the decoupling member 14 . the catches 14 . 1 and 10 . 3 are engaged with each other in a manner to prevent the decoupling member 14 from translating in proximal direction p as long as the trigger button 20 is not pushed . this occurs by respective dogs 20 . 3 on the proximal end of the trigger button 20 inwardly supporting the catches 10 . 3 so they cannot flex inwards . the catches 14 . 1 are outwardly supported by the drive spring 8 so they cannot flex outwards . when the trigger button 20 is actuated the dogs 20 . 3 are translated in proximal direction p so the catches 10 . 3 may flex inwards into spaces 20 . 4 as ramped surfaces of the catches 10 . 3 and 14 . 1 slide along each other under load of the drive spring 8 . in order to start an injection the auto - injector 1 has to be pressed against the injection site , e . g . a patient &# 39 ; s skin . as a result the interlock sleeve 45 translates in distal direction d into the housing 2 by a small distance taking with it the syringe carrier 7 , the syringe 3 with the needle 4 , the syringe holder 22 and the plunger 9 until the first shoulder 19 meets the ramped surfaces of the decoupling arms 18 . at the same time the clips 45 . 1 enter a space 2 . 3 in the housing 2 where they are no longer supported outwardly so they may now flex outwards for decoupling the interlock sleeve 45 from the syringe carrier 7 . when translated into the housing 2 a distal end of the interlock sleeve 45 supports the resilient wedges 10 . 1 from inside so they cannot be flexed inwards thus preventing the retraction sleeve 10 from translating in distal direction d . as the plunger 9 translates in distal direction d the bars 9 . 1 disengage the clips 10 . 2 so the trigger button 20 becomes unlocked and may now be actuated . if the auto - injector 1 is removed from the injection site without operating the trigger button 20 the interlock sleeve 45 will translate back into its proximal position under load of the interlock spring 26 . as the trigger button 20 is pushed dogs 20 . 3 no longer support catches 10 . 3 and so the catches 10 . 3 are flexed inwards into the spaces 20 . 4 as the ramped surfaces of the catches 10 . 3 and 14 . 1 slide along each other under load of the drive spring 8 . the drive spring 8 now advances the decoupling member 14 and thus the plunger 9 in proximal direction . the second shoulder 24 pushes the syringe holder 22 , syringe carrier 7 and syringe 3 forward while no load is exerted onto the stopper 6 . the hollow needle 4 appears from the proximal end p and is inserted into the injection site , e . g . a patient &# 39 ; s skin . the forward movement continues until the syringe holder 22 bottoms out at a first abutment 32 in the housing 2 . the travel from the initial position up to this point defines an injection depth , i . e . needle insertion depth . when the syringe holder 22 has nearly bottomed out the resilient syringe holder arms 23 have reached a widened portion 2 . 1 of the housing 2 where they are no longer supported by the inner wall of the housing 2 . however , since the force required to insert the needle 4 is relatively low the second shoulder 24 will continue to drive forward the syringe holder 22 until proximal travel is halted at the first abutment 32 . at this point the syringe holder arms 23 are flexed out by the continued force of the second shoulder 24 and allow it to slip through . now the plunger 9 no longer pushes against the syringe holder 22 but against the stopper 6 for expelling the medicament m from the syringe 3 and injecting it into or through the patient &# 39 ; s skin . when the stopper 6 has bottomed out in the syringe 3 , the retraction sleeve 10 may not yet slide in distal direction d because of the resilient wedges 10 . 1 being held in the recess 2 . 2 between the housing 2 and the interlock sleeve 45 as long as the interlock sleeve 45 is in its distal position by the auto - injector 1 being kept pushed against the injection site . if the auto - injector 1 is taken away from the injection site the interlock sleeve 45 will return to its proximal position ( as in fig3 a and 3b ) under load of the interlock spring 26 so the resilient wedges 10 . 1 are no longer supported from inside . since the drive spring 8 tries to pull the retraction sleeve 10 in distal direction d , distal ramps of the resilient wedges 10 . 1 move along proximal ramps of the recesses 2 . 2 thereby flexing the resilient wedges 10 . 1 inwards as the retraction sleeve 10 starts translating in distal direction d . the syringe holder 22 is taken along in distal direction d by the retraction sleeve 10 , e . g . by a front face 35 . thus the syringe 3 and needle 4 are retracted into a safe position inside the housing 2 , e . g . into the initial position . the plunger 9 , no longer bearing against the decoupling arms 18 is pulled back too . the auto - injector 1 of fig3 a and 3b is arranged to retract the syringe 3 and needle 4 at any time during the injection if pulled away from the skin . in fig1 a , 1b , 2a , and 2b , the retraction sleeve held by the interlock sleeve 25 if released before the end of injection will not retract the syringe 3 if the decoupling member 14 does not release the plunger 9 at the same time . the plunger 9 is only released at its end of travel when the decoupling member 14 meets the aperture 34 . in order to retract the syringe 3 before the end of injection the aperture 34 needs to be available at any time during plunger travel . the aperture 34 therefore extends right to the position of the decoupling arms 18 in the as delivered state . however , the aperture is angularly misaligned with respect to the decoupling arms 18 by a small angle so the decoupling member 14 and the plunger 9 cannot decouple . when the auto - injector 1 is removed from the injection site , the interlock sleeve 45 translates in proximal direction under load of the interlock spring 26 so the resilient wedges 10 . 1 are no longer supported from inside . since the drive spring 8 tries to pull the retraction sleeve 10 in distal direction d , distal ramps of the resilient wedges 10 . 1 move along proximal ramps of the recesses 2 . 2 thereby flexing the resilient wedges inwards as the retraction sleeve 10 starts translating in distal direction . the retraction sleeve 10 is arranged to rotate in this situation by a small angle thus aligning the aperture 34 and the decoupling arms 18 , so the decoupling member 18 decouples from the plunger 9 and the syringe 3 and the needle 4 are retracted into a safe position inside the housing 2 . decoupling part 14 and hence decoupling arms 18 are prevented from rotating in housing 2 by one or more lugs on decoupling member 14 which engage in longitudinal slots 2 . 6 in housing 2 , as shown in fig6 a and 6 b . in order to make the retraction sleeve 10 rotate a cam track 2 . 5 is arranged in the housing 2 and a cam follower 10 . 4 in the retraction sleeve 10 ( see fig6 a and 6 b ). the cam track 2 . 5 is essentially parallel to the longitudinal axis of the auto - injector 1 with a short angled section at its proximal end . as long as the cam follower 10 . 4 runs in the parallel section of the cam track , the retraction sleeve 10 is prevented from rotating . fig7 shows the auto - injector of fig3 a and 3b with the actuated trigger button 20 . fig8 is a cross section in the section plane viii - viii illustrating the angular misalignment before the retraction is triggered . in an alternative embodiment the auto - injector 1 of fig3 a and 3b may have the aperture 34 in the retraction sleeve 10 as in fig1 a , 1b , 2a , and 2b . in this case the needle retraction would occur only at the end of dose . likewise , the auto - injector 1 of fig1 a , 1b , 2a , and 2b could be combined with the aperture 34 , the cam track 2 . 5 and the cam follower 10 . 4 of fig3 to 8 in order to provide immediate needle retraction at any point during the injection cycle . the cam track may likewise be arranged in the retraction sleeve 10 and the cam follower in the housing 2 . the housing 2 may have at least one viewing window for inspecting the syringe 3 . the auto - injector 1 may preferably be used for subcutaneous or intra - muscular injection , particularly for delivering one of an analgetic , an anticoagulant , insulin , an insulin derivate , heparin , lovenox , a vaccine , a growth hormone , a peptide hormone , a proteine , antibodies and complex carbohydrates . the aforementioned arrangement for coupling the plunger 9 to either , the syringe 3 or the stopper 6 , may be applied in any auto - injector having a plunger for forwarding a force of a drive means to a syringe with a stopper . the primary advantage of this arrangement ensures the load from the drive means is not transferred directly to the stopper until the needle is inserted in the patient , thus avoiding a wet injection . the arrangement comprises the syringe holder 22 and associated syringe holder arms 23 , a shoulder ( e . g . the second shoulder 24 ) on the plunger 9 , the support of the holder arms 23 by an inner surface in order to prevent them from flexing out in a first position and , a widened portion 2 . 1 for allowing them to flex radially and to disconnect from the plunger when in a more proximal position . the spring means or other drive means , the ability to retract the syringe or to forward a needle shroud after injection and other features described herein are not required for the prevention of a wet injection .
an auto - injector comprising a housing arranged to contain a syringe . the syringe is slidably arranged with respect to the housing , a spring capable of pushing a needle from a covered position into an advanced position , operating the syringe to supply a dose of medicament , and retracting the syringe into a covered position and an activator . the spring is arranged to be grounded in the housing for advancing the needle and for injecting the dose of medicament . the drive spring grounded in the housing switched to its proximal end for retracting the syringe . an interlock sleeve telescoped within the housing . the activator comprises a trigger , wherein the trigger button is locked , thereby preventing actuation when the interlock sleeve is in its proximal position in an as delivered state and wherein translation of the interlock sleeve unlocks the trigger button so as to allow actuation .