fig1 shows two longitudinal sections in two section planes of an auto - injector 1 with force control in an as shipped state . the auto - injector 1 comprises an elongate housing 2 , an essentially tubular shroud 3 arranged inside the housing 2 and slidable in longitudinal direction with respect to the housing 2 . a distal portion 3 . 1 of the shroud has an external diameter selected to fit into the housing 2 . the distal portion 3 . 1 extends essentially through the entire housing 2 to the distal end d . the biggest part of the distal portion 3 . 1 consists of two longitudinal extensions rather than a tube shape in order to allow other parts of the auto - injector 1 to engage in the housing 2 for preventing relative rotation . this could alternatively be achieved by a tubular distal portion 3 . 1 with longitudinal slots . a proximal portion 3 . 2 of the shroud 3 has a reduced diameter compared to the distal portion 3 . 1 for slidably accommodating a syringe carrier 4 . the syringe carrier 4 holds a syringe 5 and supports it at its proximal end in order to avoid stress to its finger flanges 5 . 1 . a hollow injection needle 6 is attached to the proximal end of the syringe 5 . a stopper 7 serves for sealing the distal end of the syringe 5 . a liquid medicament m stored in the syringe 5 may be displaced through the needle 6 by pushing the stopper 7 in proximal direction p by means of a plunger 8 . in the as shipped state the syringe 5 and syringe carrier 4 are locked in position thus providing a clearance between the plunger 8 and the stopper 7 . although the needle 6 protrudes beyond the proximal end p of the auto - injector 1 , needle stick injuries are avoided by a protective needle shield 9 attached to the needle 6 in the as shipped state . the syringe carrier 4 is biased in distal direction d with respect to the shroud 3 by means of a syringe spring 10 bearing against a shoulder 3 . 3 in the shroud 3 and against a rear flange 4 . 1 in the syringe carrier 4 . the shoulder 3 . 3 is defined between the distal portion 3 . 1 and the proximal portion 3 . 2 . the rear flange 4 . 1 is arranged at the distal end of the syringe carrier 4 . a drive spring 11 is arranged near the distal end d of the auto - injector 1 inside the shroud 3 . the drive spring 11 is preferably arranged as a compression spring . the distal end of the drive spring 11 is grounded in the housing 2 or in a grounding member 12 fixed to the housing 2 . the proximal end of the drive spring 11 bears against a drive collar 13 which is arranged inside the drive spring 11 , rotationally fixed by splines 2 . 4 in the housing 2 but translatable in longitudinal direction . inside the drive collar 13 a drive sleeve 14 is arranged which in turn is arranged around the plunger 8 . the drive sleeve 14 is rotationally free and axially fixed by distally bearing against the grounding member 12 and proximally bearing against a second bulkhead 2 . 2 . the plunger 8 is rotationally fixed and axially free . the drive collar 13 is engaged with the drive sleeve 14 by a first screw thread 15 . the drive sleeve 14 is engaged with the plunger 8 by a second screw thread 16 . hence , when the drive collar 13 is pushed in proximal direction p by the drive spring 11 , the drive sleeve 14 is caused to rotate which causes axial movement of the plunger 8 . the first screw thread 15 and the second screw thread 16 are like - handed . by varying the pitch of the two screw threads 15 , 16 a ratio of translation of the drive collar 13 and the plunger 8 is changed and hence the transmitted force is amplified or reduced . the screw threads 15 , 16 may have cam tracks and followers or ball bearings . a syringe viewing window 17 for inspecting the syringe contents is provided in the proximal portion 3 . 2 of the shroud 3 . a housing cap 18 and a trigger button 19 are arranged at the distal end d of the auto - injector 1 . in the shipped state in fig1 , the drive spring &# 39 ; s 11 preload on the drive collar 13 is statically resolved through distal snap arms 13 . 1 of the drive collar 13 which are engaged behind a shoulder 12 . 1 in the grounding member 12 . the syringe carrier 4 is prevented from moving in proximal direction p by the rear flange 4 . 1 bearing against a first bulkhead 2 . 1 provided in the housing 2 . withdrawal of the shroud 3 into the housing 2 is resisted by flexural elements 3 . 4 on the shroud 3 acting against the first bulkhead 2 . 1 of the housing 2 from the proximal side ( see fig1 c for details ). extension of the shroud 3 in proximal direction p is prevented by inward protrusions 3 . 5 contacting the drive collar 13 . the syringe carrier 4 is locked to the shroud 3 by at least two snaps 4 . 2 . the protective needle shield 9 is interlocked to the syringe carrier 4 . for this purpose the syringe carrier 4 has a pair of resilient snaps 4 . 2 extending proximally beyond the section of the syringe carrier 4 supporting the proximal end of the syringe 5 . in the state as shipped these snaps 4 . 2 are snapped into corresponding recesses provided in the proximal portion 3 . 2 of the shroud 3 thus preventing relative axial translation between the shroud 3 and the syringe carrier 4 . the snaps 4 . 2 are kept from flexing inwards and disengaging from the recesses by the protective needle shield 9 . the protective needle shield 9 protrudes beyond the proximal end of the shroud 3 through an orifice . in order to arm the auto - injector 1 the protruding part of the protective needle shield 9 is gripped by a user and pulled off the syringe 5 and needle 6 in proximal direction p ( see fig2 ). once the protective needle shield 9 has been removed the snaps 4 . 2 are no longer supported inwardly and disengage from the recesses 3 . 6 ( see fig3 ). preferably the snaps 4 . 2 are biased to relax inwards when not supported . in an alternative embodiment the snaps 4 . 2 and the shroud 3 may have angled mating surfaces for moving the snaps 4 . 2 inward thus disengaging them from the recesses 3 . 6 under the force from the syringe spring 10 when the snaps 4 . 2 are not supported inwardly . in this case the snaps 4 . 2 do not have to be biased inwardly . now delatched from the shroud 3 the syringe carrier 4 together with the syringe 5 and the needle 6 are translated in distal direction d due to the load of the syringe spring 10 ( see fig4 ). thus the needle 6 is hidden inside the shroud 3 and the user protected from accidental needle stick injuries . in the next operating step the user places the proximal end p of the auto - injector 1 against an injection site , e . g . a patient &# 39 ; s skin . when contacting the injection site the shroud 3 is depressed and translates by a small distance in distal direction d into the housing 2 ( see fig5 ) against the load of the flexural elements 3 . 4 . as the shroud 3 translates apertures in the shroud 3 move from a blocking position against trigger button resilient extensions 19 . 1 , thus unlocking the trigger button 19 . the trigger button 19 comprises a number of resilient extensions 19 . 1 facing the grounding member 12 . the grounding member 12 has a tapering surface 12 . 2 facing the trigger button 19 . the resilient extensions 19 . 1 are at least partially arranged inside the snap arms 13 . 1 of the drive collar 13 . when the trigger button 19 is pushed in proximal direction p the resilient extensions 19 . 1 contact the tapering surface 12 . 2 and are splayed apart . consequently , the resilient snap arms 13 . 1 are also splayed apart and disengaged from the shoulder 12 . 1 of the grounding member 12 . thus , the drive collar 13 is no longer axially restricted and will be released in the proximal direction p by the drive spring 11 . before the shroud 3 is depressed ( fig1 to 4 ) the distal portion 3 . 1 of the shroud 3 prevents the trigger button &# 39 ; s 19 resilient extensions 19 . 1 from splaying out . as the shroud 3 is depressed ( fig5 ) it travels in distal direction d to such an extend that the resilient extensions 19 . 1 meet respective distal recesses 3 . 7 thus allowing the resilient extensions 19 . 1 to be splayed apart ( see fig5 c for details ). at the same time the syringe spring 10 is partially compressed by depressing the shroud 3 . when the trigger button 19 is pressed and the drive collar 13 is delatched from the grounding member 12 the force of the drive spring 11 translates the drive collar 13 in proximal direction p ( see fig6 ). rotation of the drive collar 13 is prevented by splined engagement in the housing 2 . the drive sleeve 14 is forced to rotate by engagement to the drive collar 13 through the first screw thread 15 . the plunger 8 extends towards the stopper 7 by engagement to the drive sleeve 14 through the second screw thread 16 . the first screw thread 15 may comprise an external , right - handed screw thread in the drive sleeve 14 engaged with a ball located in a pocket on the internal surface of the drive collar 13 . the second screw thread 16 may comprise an external right - handed screw thread in the plunger 8 engaged with a ball located in a pocket on the internal surface of the drive sleeve 14 . alternatively , both screw threads 15 , 16 may be left - handed . rotation of the plunger 8 is prevented by splined engagement in the grounding member 12 . this may be achieved by corresponding non - circular cross sections , e . g . square cross sections . the gear ratio of the gear box comprising the drive collar 13 , the first screw thread 15 , the drive sleeve 14 and the second screw thread 16 is defined by the pitch angles of the two screw threads 15 , 16 . if the pitch angle of the drive sleeve 14 is greater than that of the plunger 8 , the gear ration will be greater than 1 , i . e . the gear box acts as a distance multiplier . conversely , when the plunger 8 pitch angle is greater than that of the drive sleeve 14 , the gear ratio will be less than 1 , i . e . the gear box acts as a force multiplier . in an alternative embodiment the drive sleeve 14 may have an internal screw thread engaged with the plunger 8 and the drive collar 13 may have an internal screw thread engaged with the drive sleeve 14 . as the plunger 8 travels forward it meets the stopper 7 and applies a force on it which is resolved through the syringe spring 10 . the counteracting force of the syringe spring 10 during compression has to be greater than a counteracting force of the stopper 7 due to friction between the stopper 7 and the inner wall of the syringe 5 and due to the hydrostatic resistance of the liquid medicament m to be displaced through the hollow needle 6 . as the syringe spring 10 is compressed the syringe carrier 4 travels in proximal direction p together with the syringe 5 and the needle 6 . hence , the needle 6 is inserted into the injection site . the injection depth is set by the rear flange 4 . 1 of the syringe carrier 4 contacting the first bulkhead 2 . 1 . when the rear flange 4 . 1 hits the first bulkhead 2 . 1 the force of the plunger 8 pushes the stopper 7 in proximal direction p thus displacing the liquid medicament m from the syringe 5 through the needle 6 and into the injection site . during injection of the medicament m , the pitch angles of the screw threads 15 , 16 may vary in order to adapt the mechanical advantage of the gearbox . fig7 a and 7 c show the auto - injector 1 during removal from the injection site in the course of an injection cycle . if this happens , the shroud 3 will extend to cover the needle 6 under load of the syringe spring 10 . in parallel , internal splines 3 . 8 on the distal part 3 . 1 of the shroud 3 engage in teeth 14 . 1 on the outer surface of the drive sleeve 14 . this prevents further rotation of the drive sleeve 14 and hence expansion of the plunger 8 and further emptying of the syringe 5 . fig7 b shows the drive sleeve 14 and the shroud 3 during injection with the shroud 3 persistently pressed against the injection site . hence , the splines 3 . 8 and teeth 14 . 1 do not engage and the drive sleeve 14 continues rotating . by contrast , fig7 c shows the drive sleeve 14 and the shroud 3 during removal of the auto - injector 1 from the injection site in the course of an injection cycle . the shroud 3 is translated in proximal direction p to an extent bringing the teeth 14 . 1 and the splines 3 . 8 into engagement . even though the forward extension of the shroud 3 is limited by the position of the drive collar 13 , the auto - injector 1 is configured to provide needle safety at all stages of the operational cycle : with a transmission ratio of 1 or less as illustrated , as the needle is inserted into the injection site , the drive collar 13 moves in sync with the syringe 5 . therefore , given that the needle 6 is initially fully covered when the shroud 3 is fully extended and the needle 6 moves in sync with the drive collar 13 , the needle can not protrude a greater distance from the shroud 3 than the distance permitted by the drive collar 13 and the inward protrusions 3 . 5 . if the transmission ratio were greater than 1 , the design may be modified to position the needle 6 further from the proximal end of the shroud 3 to ensure needle safety at all stages of the injection . in fig8 the stopper 7 has reached the end of the syringe 5 and the dose is fully delivered . the auto - injector 1 is sized so that this occurs prior to the drive collar 13 and / or plunger 8 reaching end - of - travel on their respective screw threads 15 , 16 , in particular before a flange 13 . 2 of the drive collar 13 contacts a flange 14 . 2 of the drive sleeve 14 . the end of dose may be indicated to the user by an elapsed time ( e . g . ten seconds ), visible inspection through the syringe viewing window 17 , or audible detection of movement of a ratchet engaged between any two parts with relative motion , for example the housing 2 and the drive sleeve 14 . when the dose has been fully delivered the user may remove the auto - injector 1 from the injection site thus extracting the needle 6 . as the auto - injector 1 is removed , the shroud 3 extends under bias of the syringe spring 10 ( see fig9 ). when the shroud 3 is at least almost fully extended , resilient latches 3 . 9 in the distal portion 3 . 1 of the shroud 3 snap into respective recesses 2 . 3 arranged in the housing 2 thus preventing the shroud 3 from being pushed in distal direction d again , so post injection needle safety is provided ( fig1 ). this applies for both cases , when the auto - injector 1 is removed from the injection site during injection ( fig7 a ) or after delivering the full dose ( fig8 ). the syringe carrier 4 has lateral apertures corresponding to the syringe viewing window 17 in order to allow visual inspection of the syringe 5 . in another embodiment the gearbox as shown in the preceding figures comprising the drive collar 13 , the first screw thread 15 , the drive sleeve 14 and the second screw thread 16 may be replaced by a rotary friction element shown in fig1 to 13 . the rotary friction element comprises a friction collar 20 . in this embodiment the drive spring 11 bears against the drive collar 13 which in turn pushes against the friction collar 20 in sync with the plunger 8 . at predetermined times the friction collar 20 is forced to rotate by its engagement in a cam track 12 . 3 provided in the grounding member 12 . for this purpose the friction collar 20 has a cam follower 20 . 1 . as in the gearbox ( fig1 ) the drive collar 13 is rotationally fixed and axially free . the friction collar 20 in contrast is axially free and rotationally constrained by friction and by the cam track 12 . 3 . the plunger 8 comprises a hollow distal portion 8 . 1 fitting onto a shaft of the grounding member 12 and a proximal portion 8 . 2 with a reduced diameter . the cross sections of both the hollow distal portion 8 . 1 and the shaft of the grounding member 12 may be designed to prevent rotation of the plunger 8 . for instance the plunger 8 may have a cam follower running in a straight section of the cam track 12 . 3 . rotation of the friction collar 20 is defined by the cam track 12 . 3 . the load on the friction collar 20 is coupled to the rotationally fixed , axially free plunger 8 which applies a force to the stopper 7 . the compressive force of the drive spring 11 acting on both mating surfaces of the friction collar 20 towards the drive collar 13 and towards the plunger 8 introduces a friction force opposing rotation . as the friction collar 20 axially translates , it is forced to rotate by any section of the cam track 12 . 3 that is not parallel with the longitudinal axis of the auto - injector 1 . the friction collar 20 will only rotate if the friction force between the mating surfaces of the drive collar 13 and the plunger 8 is overcome . the torque to overcome this friction force is generated by the contact force between the cam follower 20 . 1 and the cam track 12 . 3 , in which the cam follower 20 . 1 is engaged . the contact force is generated by the spring force of the drive spring 11 . a degree of coupling between the spring force and the contact force is defined by the respective angle of the cam track 12 . 3 . by an appropriate modification to the cam track angle , the amount of spring force required to overcome the friction force can be modified . for instance , increasing the cam angle requires more of the spring force to be reacted through the cam track 12 . 3 , thereby reducing the force applied to the stopper 7 . in an alternative embodiment an intermediary component may be provided for first coupling the plunger 8 to the syringe carrier 4 or the syringe 5 directly without acting on the stopper 7 until the needle 6 has reached its injection depth . the plunger 8 would then be decoupled from the syringe 5 or syringe carrier 4 by the intermediary component and instead be coupled to the stopper 7 in order to displace the medicament m from the syringe 5 . thus , wet injection is avoided , i . e . the medicament is not leaking out of the needle tip before the needle is inserted . the intermediary component may be a transfer sleeve or an additional feature at the syringe carrier 4 . the transfer sleeve and plunger 8 would be initially coupled and translate together . however , when the syringe carrier 4 nears the end of its travel during needle insertion , the transfer sleeve would decouple from the plunger . from this point forwards , the plunger 8 load would be transferred directly to the stopper 7 . this decoupling arrangement may be embodied in any suitable auto - injector arrangement . for example , the transfer sleeve could be clipped to the plunger by some clips . near the end of travel the clips could find some place to splay or be pushed away from the plunger 8 in order to decouple the plunger 8 from the transfer sleeve .
an auto - injector may be used to administer a dose of a liquid medicament . in some embodiments , a shroud is arranged inside a housing , the shroud slidable in longitudinal direction between at least a retracted position , in which a hollow injection needle is exposable and an advanced position , in which the needle is covered by the shroud . a protective needle shield is attachable to the needle in a manner to partially protrude beyond the proximal end of the shroud through the orifice with the needle in its advanced position . a syringe carrier is arranged inside the shroud and comprises at least one snap for locking it to the shroud in order to prevent relative axial motion . the snap is configured to automatically disengage upon removal of the protective needle shield to allow the syringe and needle to be retracted into the covered position under load of the syringe spring .