Abstract:
The invention relates to a front end for an auto-injector, the front end comprising a syringe with an injection needle and a stopper, wherein the syringe is slidably arranged within a syringe retainer, wherein an outer sleeve for preventing access to the needle is removably arrangeable over the syringe retainer, wherein the syringe retainer comprises a screw thread for connecting it to a correspondent screw thread of a mounting of a re-usable auto-injector back end comprising a drive means for advancing the syringe and needle in a proximal direction for needle insertion and for advancing the stopper within the syringe for injection, wherein a resilient locking barb is arranged to rotationally lock the syringe retainer to the mounting in an unscrewing direction when the front end is screwed to the re-usable back end and when the outer sleeve is removed, wherein the outer sleeve is arranged to unlock the resilient locking barb when arranged over the syringe retainer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Phase Application pursuant to 35 U.S.C. §371 of International Application No. PCT/EP2011/073516 filed Dec. 21, 2011, which claims priority to European Patent Application No. 10196080.5 filed Dec. 21, 2010. The entire disclosure contents of these applications are herewith incorporated by reference into the present application. 
     FIELD OF INVENTION 
     The invention relates to a front end for an auto-injector according to the preamble of claim  1 . 
     BACKGROUND 
     Administering an injection is a process which presents a number of risks and challenges for users and healthcare professionals, both mental and physical. 
     Injection devices (i.e. devices capable of delivering medicaments from a medication container) typically fall into two categories—manual devices and auto-injectors. 
     In a manual device—the user must provide the mechanical energy to drive the fluid through the needle. This is typically done by some form of button/plunger that has to be continuously pressed by the user during the injection. There are numerous disadvantages to the user from this approach. If the user stops pressing the button/plunger then the injection will also stop. This means that the user can deliver an under dose if the device is not used properly (i.e. the plunger is not fully pressed to its end position). Injection forces may be too high for the user, in particular if the patient is elderly or has dexterity problems. 
     The extension of the button/plunger may be too great. Thus it can be inconvenient for the user to reach a fully extended button. The combination of injection force and button extension can cause trembling/shaking of the hand which in turn increases discomfort as the inserted needle moves. 
     Auto-injector devices aim to make self-administration of injected therapies easier for patients. Current therapies delivered by means of self-administered injections include drugs for diabetes (both insulin and newer GLP-1 class drugs), migraine, hormone therapies, anticoagulants etc. 
     Auto-injectors are devices which completely or partially replace activities involved in parenteral drug delivery from standard syringes. These activities may include removal of a protective syringe cap, insertion of a needle into a patient&#39;s skin, injection of the medicament, removal of the needle, shielding of the needle and preventing reuse of the device. This overcomes many of the disadvantages of manual devices. Injection forces/button extension, hand-shaking and the likelihood of delivering an incomplete dose are reduced. Triggering may be performed by numerous means, for example a trigger button or the action of the needle reaching its injection depth. In some devices the energy to deliver the fluid is provided by a spring. 
     US 2002/0095120 A1 discloses an automatic injection device which automatically injects a pre-measured quantity of fluid medicine when a tension spring is released. The tension spring moves an ampoule and the injection needle from a storage position to a deployed position when it is released. The content of the ampoule is thereafter expelled by the tension spring forcing a piston forward inside the ampoule. After the fluid medicine has been injected, torsion stored in the tension spring is released and the injection needle is automatically retracted back to its original storage position. 
     Most conventional art auto-injectors are disposable, i.e. they are used and completely disposed of after use. For reasons of sustainability it may be desirable to re-use at least a part of the auto-injector. An auto-injector may therefore consist of a re-usable back end engine containing drive means and a front end containing the syringe with the needle which must not be re-used. 
     SUMMARY 
     It is an object of the present invention to provide an improved front end for an auto-injector. 
     The object is achieved by a front end according to claim  1 . 
     Preferred embodiments of the invention are given in the dependent claims. 
     In the context of this specification the term proximal refers to the direction pointing towards the patient during an injection while the term distal refers to the opposite direction pointing away from the patient. 
     A front end for an auto-injector according to the invention comprises a syringe with an injection needle and a stopper. The syringe is slidably arranged within a syringe retainer. An outer sleeve or cap for preventing pre and post injection access to the needle is removably arrangeable over the syringe and syringe retainer. The syringe retainer comprises a screw thread for connecting it to a correspondent screw thread of a mounting of a re-usable auto-injector back end, the back end comprising a drive means for advancing the syringe and needle in a proximal direction for needle insertion and for advancing the stopper within the syringe for injection. A resilient locking barb is arranged to rotationally lock the syringe retainer to the mounting in an unscrewing direction of rotation when the front end is screwed to the re-usable back end and when the outer sleeve is removed. The outer sleeve is arranged to unlock the resilient locking barb when arranged over the syringe retainer. This avoids removal of the front end from the back end without the outer sleeve or cap being reapplied to the front end thus reducing the risk of post injection needle stick injuries. 
     The invention provides a means to ensure fitment of the correct syringe to the back end. This may help to ensure that the intended drug is used with the re-usable auto-injector. 
     The locking barb may be arranged on the syringe retainer proximally from the screw thread. The locking barb is arranged to engage an internal groove arranged in the mounting for locking the syringe retainer to the mounting. The locking barb is arranged to partially remain outside of the mounting when the syringe retainer is fully screwed to the mounting so as to allow the outer sleeve to engage it and to inwardly deflect the locking barb to unlock it so that the syringe retainer can be unscrewed from the mounting. 
     The locking barb may exhibit a chamfer for inwardly deflecting the locking barb by an internal surface of the mounting when rotated in a screwing direction. This allows for screwing the front end to the mounting even if the outer sleeve or cap is already removed. 
     If a protective needle sheath on the needle is intended to be pulled off the needle on removal of the outer sleeve the syringe has to be prevented from translating out of the distal position. A syringe lock may be arranged for preventing translation of the syringe from a distal position in the proximal direction when the outer sleeve is arranged over the syringe retainer. This ensures that the syringe remains in the distal position. 
     The syringe may be arranged in a tubular syringe carrier arranged to support the syringe at its proximal end. This is preferred over holding the syringe at a finger flange since a neck at the proximal end of the syringe is less prone to break when subjected to impact load. The syringe carrier is slidably arranged in the syringe retainer. 
     The syringe lock may comprise a hub extending through a lateral aperture in the syringe retainer in a manner to engage the finger flange of the syringe or a second shoulder on a distal end of the syringe carrier. The hub is outwardly biased, e.g. by a spring or resilient arms on the hub so as to disengage the finger flange or second shoulder in a manner to allow translation in the proximal direction. The outer sleeve is arranged to outwardly support the hub when attached to the syringe retainer so as to prevent outward deflection and disengagement of the syringe lock. 
     A syringe spring may be arranged to bias the syringe in the distal direction. This allows for a defined position of the syringe prior to needle insertion. The syringe may be advanced against the load of the syringe spring by a plunger of the re-usable back end pushing on the stopper for needle insertion. After injection, the plunger may be retracted by the back-end allowing the syringe spring to retract the syringe and needle for providing post-injection needle safety. 
     The syringe may be arranged within an elongate housing telescoped within the syringe retainer. The housing may be biased in the proximal direction and arranged to be translated in the distal direction when applied against an injection site thus serving as a transfer sleeve indicating skin contact. A distal end of the housing is arranged to protrude towards the re-usable back end to indicate its longitudinal position to the re-usable back end. The re-usable back end may have a mechanical means or an electrical or optical sensor engaged to the distal end of the housing. An injection would preferably only be triggered when the housing has been translated indicating skin contact thus enforcing a sequence of operation and increasing pre injection needle safety. 
     A cover retainer may be arranged in the outer sleeve extending distally and inwardly at an angle, wherein the cover retainer is inwardly biased so as to engage an outward surface of the housing in an initial state and during removal of the outer sleeve. The cover retainer is arranged to deflect inwards when having travelled beyond a proximal end of the housing such that the cover retainer cannot re-enter the space between the housing and outer sleeve when the outer sleeve is reapplied to the housing. Instead the cover retainer hits the proximal end of the housing and is deformed on re-application resulting in a shape opposing removal of the outer sleeve after reapplication. This reduces the risk of a user tampering with the front end post injection and after removal from the re-usable back end. 
     At least one transfer spring may be arranged as a resilient part integrally moulded with the housing for biasing the housing in the proximal direction against the syringe retainer. 
     The protective needle sheath may be removably arrangeable over the needle. The protective needle sheath may be attached to the outer sleeve so as to be removed from the needle on removal of the outer sleeve from the syringe retainer. 
     For this purpose the outer sleeve may be attached to the protective needle sheath by a push fit or bump or snap fit or glued or ultrasonic welded. 
     In another embodiment a star washer may be arranged over an external diameter of the protective needle sheath and outwardly engaged to an internal diameter of the outer sleeve so as to couple the protective needle sheath to the outer sleeve for joint axial translation. 
     In yet another embodiment the protective needle sheath may have an external profiled structure in the shape of a finger grip, wherein the outer sleeve has at least one profiled arm extending in the distal direction from the proximal end face of the outer sleeve. The profiled arm is engaged to the profiled structure of the protective needle sheath. The housing is arranged to outwardly support the profiled arm when the outer sleeve is attached to the syringe retainer such that the profiled arm is prevented from deflecting outwards. 
     In yet another embodiment the outer sleeve may have at least one distal snap engaged distally behind a distal edge or shoulder of the protective needle sheath. 
     The syringe may be arranged in the front end in a manner to prevent its removal after injection. Such an arrangement may be referred to as a packaged syringe which has to be entirely disposed of after use. This embodiment ensures that contact with the user, in an area immediately surrounding the injection site, is limited to the single-use packaged syringe. This may help to prevent cross contamination between consecutive injections and may eliminate the requirement for cleaning re-usable elements. Re-use of an emptied syringe is prevented. This reduces the chance of the user being subjected to the health risks associated. 
     In another embodiment the syringe may be replaceably arranged in the front end. 
     The re-usable back end may comprise a conventional spring, a gas spring, an electric motor or another drive means for advancing the syringe and the stopper. 
     The front end 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 protein, antibodies and complex carbohydrates. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1  is an isometric view of a first embodiment of a front end, 
         FIG. 2  is an isometric longitudinal section of the first embodiment, 
         FIG. 3  is a longitudinal section of a second embodiment of the front end, 
         FIG. 4  is a longitudinal section of the second embodiment upon removal of a protective needle sheath, 
         FIG. 5  is a longitudinal section of the second embodiment with a syringe retracted after removal of the protective needle sheath, 
         FIG. 6  is a longitudinal section of the second embodiment in a different section plane in the situation as in  FIG. 5 , 
         FIG. 7  is an isometric longitudinal section of a third embodiment of the front end prior to use, 
         FIG. 8  is an isometric longitudinal section of the third embodiment in a different section plane in the situation as in  FIG. 7 , 
         FIG. 9  is an isometric longitudinal section of the third embodiment post use in the section plane as in  FIG. 7 , 
         FIG. 10  is an isometric longitudinal section of the third embodiment in the section plane as in  FIG. 8  in the situation as in  FIG. 9 , 
         FIG. 11  is an isometric longitudinal section of a fourth embodiment prior to use, 
         FIG. 12  is an isometric longitudinal section of the fourth embodiment prior to use in a different section plane, 
         FIG. 13  is an isometric longitudinal section of the fourth embodiment post use in the section plane as in  FIG. 11 , 
         FIG. 14  is an isometric longitudinal section of the fourth embodiment post use in the section plane as in  FIG. 12 , 
         FIG. 15  is a longitudinal section of a fifth embodiment of a front end, 
         FIG. 16  is a longitudinal section of the fifth embodiment in a different section plane, 
         FIG. 17  is a schematic view of a cover retainer with a locking barb of the fifth embodiment, 
         FIG. 18  is another schematic view of the cover retainer with the locking barb, 
         FIG. 19  is yet another schematic view of the cover retainer with the locking barb, 
         FIG. 20  is an isometric detail view of the fifth embodiment mounted to a re-usable back end, 
         FIG. 21  is an isometric detail view of a distal end of the fifth embodiment, 
         FIG. 22  is an isometric detail view of a proximal end of the re-usable back end, 
         FIG. 23  is a longitudinal section of the protective needle sheath engaged to the cover, 
         FIG. 24  is a longitudinal section of another embodiment of the engagement between the protective needle sheath and the cover, 
         FIG. 25  is a longitudinal section of a third embodiment of the engagement between the protective needle sheath and the cover, 
         FIG. 26  is a longitudinal section of a fourth embodiment of the engagement between the protective needle sheath and the cover, 
         FIG. 27  is a longitudinal section of a fifth embodiment of the engagement between the protective needle sheath and the cover, and 
         FIG. 28  is a longitudinal section of a sixth embodiment of the engagement between the protective needle sheath and the cover. 
     
    
    
     Corresponding parts are marked with the same reference symbols in all figures. 
     DETAILED DESCRIPTION 
       FIG. 1  is an isometric view of a first embodiment of a front end  1  intended to be attached to a re-usable engine back end of an auto-injector (not illustrated).  FIG. 2  is a respective longitudinal section of the front end  1 . 
     The front end  1  comprises a syringe  2  with an injection needle  3  attached to a proximal end of the syringe  2 . At a distal end the syringe  2  comprises a finger flange  4 . A stopper  5  is slidably arranged in the syringe  2  for distally sealing a cavity in the syringe  2  arranged to contain a liquid medicament M. The stopper  5  may be translated in a proximal direction P within the syringe in order to displace the medicament M through the needle  3 . A protective needle sheath  6  is removably arranged over the needle  3  for protecting the needle  3  from mechanical damage and for preventing finger access to the needle  3  thus reducing the risk of needle stick injuries. 
     An outer sleeve  7  is removably arranged over at least part of the length of the syringe  2  thereby entirely covering the protective needle sheath  6 . A substantial length of the outer sleeve  7  extends proximally beyond the protective needle sheath  6 . 
     The outer sleeve  7  or cap may be applied to the syringe  2  prior to injection, in particular post filling. The cap may likewise be assembled prior to filling. As the outer sleeve  7  is pushed onto the syringe  2 , inwardly biased resilient first barbs  8  in the outer sleeve  7  deflect outwards. First ribs  9  within the outer sleeve  7  arranged to abut against a proximal end of the protective needle sheath  6  determine a maximum depth by which the outer sleeve  7  can be applied. Once the outer sleeve  7  has reached the maximum depth, the first barbs  8  engage on a first shoulder  10  of the protective needle sheath  6  in a manner to prevent removal of the outer sleeve  7  from the protective needle sheath  6 . 
     In order to prepare for an injection the front end  1  is loaded into the re-usable back end (not illustrated). The outer sleeve  7  is removed by being pulled in the proximal direction P thereby also removing the protective needle sheath  6 . 
     Optionally the outer sleeve  7  and the protective needle sheath  6  may be combined as one component. 
     The front end  1  according to the first embodiment may facilitate loading into the re-usable back end without prior removal of the protective needle sheath  6 . This may reduce the chance of the user incurring an injury when loading the syringe  2 . 
     If the protective needle sheath  6  is removed once loaded in the re-usable back end, the outer sleeve  7  being considerably longer than the protective needle sheath  6  may be used to improve access to the protective needle sheath  6 . For this purpose the outer sleeve  7  may protrude through a proximal opening in the re-usable back end. This may enable the re-usable back end to be configured with the syringe  2  recessed within the back end. This should ensure that the user cannot access the needle  3  and thus improve needle safety. 
     The front end  1  may be assembled into the re-usable backend by loading it from the distal direction D. 
       FIG. 3  is a longitudinal section of a second embodiment of the front end  1  prior to use. 
     The front end  1  comprises a syringe  2  with an injection needle  3  attached to a proximal end of the syringe  2 . At a distal end the syringe  2  comprises a finger flange  4 . A stopper  5  is slidably arranged in the syringe  2  for distally sealing a cavity in the syringe  2  arranged to contain a liquid medicament M. The stopper  5  may be translated in a proximal direction P within the syringe in order to displace the medicament M through the needle  3 . A protective needle sheath  6  is removably arranged over the needle  3  for protecting the needle  3  from mechanical damage and for preventing finger access to the needle  3  thus reducing the risk of needle stick injuries. 
     The syringe  2  is arranged within a syringe carrier  11 , wherein the finger flange  4  proximally abuts against a second shoulder  12  on the distal end of the syringe carrier  11  so as to couple the syringe  2  to the syringe carrier  11  for joint translation in the proximal direction P. The syringe carrier  11  is slidably arranged within an elongate housing  13 . A syringe spring  14  in the shape of a compression spring is arranged between the second shoulder  12  and a third shoulder  15  in the housing  13  so as to bias the syringe carrier  11  in the distal direction with respect to the housing  13 . 
     The syringe  2  can be assembled into the housing  13  post filling and application of the protective needle sheath  6  over the needle  3 . 
     The front end  1  is shipped with the syringe spring  14  compressed, i.e. with the syringe carrier  11  and syringe  2  in a maximum proximal position. The force from the syringe spring  14  is resolved through resilient latch arms  16  on the proximal end of the syringe carrier  11  which are latched on to the housing  13 . The latch arms  16  are outwardly deflected by the protective needle sheath  6  in a manner to engage a respective outward protrusion  17  on the latch arm  16  to the housing  13  in a manner to prevent translation of the syringe carrier  11  in the distal direction D. The syringe  2  may be prevented from moving in the distal direction D in this stage by friction between the syringe  2  and syringe carrier  11 . 
       FIG. 4  is a longitudinal section of the second embodiment of the front end  1  upon removal of the protective needle sheath  6 . 
     The user removes the protective needle sheath  6  from the syringe  2  by pulling it off in the proximal direction P. Thus the latch arms  16  may deflect inwards and close in around the needle tip thereby disengaging the outward protrusions  17  from the housing  13  thus allowing translation of the syringe carrier  11  in the distal direction D. The outward protrusions  17  may have a ramp on their distal face, to pull the latch arms  16  inwards under the action of the syringe spring  14 . The compressed syringe spring  14  causes the syringe carrier  11  to recede in the distal direction D entirely retracting the needle  3  within the housing  13  and protecting the user from accidental needle stick (see  FIG. 5 ). The syringe  2  is locked within the housing  13  by resilient first snaps  18  within the housing  13  latching to the second shoulder  12  on the syringe carrier  11  respectively. The first snaps  18  exhibit a proximal ramp allowing the syringe carrier  11  to deflect the first snaps  18  outwards as it is translated in the distal direction D. A distal side of the first snaps  18  is essentially perpendicular to a longitudinal axis of the front end  1  preventing the syringe carrier  11  from re-translating in the proximal direction P as soon as the first snaps  18  flex inwards behind the syringe carrier  11 . The protective needle sheath  6  may be discarded after removal. 
       FIG. 6  is a longitudinal section of the second embodiment in a different section plane approximately 90° rotated relative to the section planes in  FIGS. 3 to 5  in the situation as in  FIG. 5 . A pair of resilient second snaps  19  is arranged in the housing  13  in a manner to prevent the syringe  2  from being translated out of the distal end of the housing  13 . The second snaps  19  are distally ramped to allow insertion of the syringe  2  into the housing  13  during assembly. 
     In order to prepare for an injection the front end  1  is connected to the re-usable back end (not illustrated). The re-usable back end is arranged to deflect the first snaps  18  outwards in a manner to de-latch them and allow translation of the syringe carrier  11  and syringe  2  in the proximal direction P. A plunger in the re-usable back end may then push on the stopper  5  and translate the syringe  2  and needle  3  in the proximal direction P into the position as in  FIG. 4 . In this position the syringe  2  cannot translate further. Hence the stopper  5  is translated in the proximal direction P within the syringe  2  injecting the dose of medicament into the injection site, e.g. a patient&#39;s skin. At the end of injection when the stopper  5  has bottomed out in the syringe  2  the plunger may be retracted thus allowing the syringe spring  14  to retract the syringe  2  and needle  3  in the distal direction D into a needle safe position as in  FIGS. 5 and 6 . 
     The front end  1  may be combined with the re-useable back end pre or post 
     removal of the protective needle sheath  6 . 
     The front end  1  according to the second embodiment ensures the access to the needle  3  is prevented once the protective needle sheath  6  is removed. This would reduce the chance of the user incurring an injury. The housing  13  serving as a needle shroud is locked in position post injection and after removal of the front end from the back end such that needle access can only be gained through abuse of the assembly. 
       FIG. 7  is an isometric longitudinal section of a third embodiment of the front end  1  prior to use.  FIG. 8  is an isometric longitudinal section of the front end in a different section plane approximately 90° rotated relative to the section plane in  FIG. 7  prior to use. 
     The front end  1  comprises a syringe  2  with an injection needle  3  attached to a proximal end of the syringe  2 . At a distal end the syringe  2  comprises a finger flange  4 . A stopper  5  is slidably arranged in the syringe  2  for distally sealing a cavity in the syringe  2  arranged to contain a liquid medicament M. The stopper  5  may be translated in a proximal direction P within the syringe  2  in order to displace the medicament M through the needle  3 . A protective needle sheath  6  is removably arranged over the needle  3  for protecting the needle  3  from mechanical damage and for preventing finger access to the needle  3  thus reducing the risk of needle stick injuries. 
     The syringe  2  is arranged within an elongate housing  13 . 
     The syringe  2  is assembled into the housing  13  through the distal end post both filling and application of the protective needle sheath  6 . The syringe  2  may likewise be assembled into the housing  13  pre filling. During assembly the syringe  2  is translated in the proximal direction P until the finger flange  4  abuts against a second rib  20  in the housing  13 . The syringe  2  is held in this position by friction between resilient compliant arms  21  on the proximal end of the housing  13  outwardly deflected by the protective needle sheath  6 . The housing  13  may be locked to the syringe  2  in a proximal position by resilient first snaps  18  within the housing  13  latching to the syringe finger flange  4 . The first snaps  18  exhibit a proximal ramp allowing the finger flange  4  to deflect the first snaps  18  outwards as the housing  13  is translated in the proximal direction P relative to the syringe  2 . A distal side of the first snaps  18  is essentially perpendicular to a longitudinal axis of the front end  1  preventing the housing  13  from re-translating in the distal direction D as soon as the first snaps  18  flex inwards behind the finger flange  4 . A pair of resilient second snaps  19  is arranged in the housing  13  in a manner to prevent the syringe  2  from being translated out of the distal end of the housing  13 . The second snaps  19  are distally ramped to allow insertion of the syringe  2  into the housing  13  during assembly. 
     In order to prepare for an injection the front end  1  is connected to the re-usable back end (not illustrated). 
     The user then removes the protective needle sheath  6  from the front end  1 . The compliant arms  21  on the housing  13  are thus allowed to relax and flex inwards for closing in around the tip of the needle  3 . The syringe  2  is now free to move with respect to the housing  13 . 
     During operation of the re-usable back end, the housing  13  can be moved as required. 
     The re-usable back end may be arranged to move the housing  13  in the proximal direction P relative to the syringe  2  at the end of injection. For this purpose means may be provided by which the re-usable back end may hold the syringe  2  to move it distally relative to the housing  13 , e.g. a syringe carrier. A syringe return biasing the syringe  2  in the distal direction D relative to the housing  13  spring may likewise be provided. The housing  13  is locked in the proximal position by the first snaps  18  preventing access to the needle  3  (see  FIGS. 9 and 10 ). The front end  1  may then be removed form the re-usable back end. 
       FIG. 11  is an isometric longitudinal section of a fourth embodiment of the front end  1  prior to use.  FIG. 12  is an isometric longitudinal section of the fourth embodiment prior to use in a different section plane approximately 45° rotated relative to the section plane in  FIG. 11 . 
     The front end  1  comprises a syringe  2  with an injection needle  3  attached to a proximal end of the syringe  2 . At a distal end the syringe  2  comprises a finger flange  4 . A stopper  5  is slidably arranged in the syringe  2  for distally sealing a cavity in the syringe  2  arranged to contain a liquid medicament M. The stopper  5  may be translated in a proximal direction P within the syringe  2  in order to displace the medicament M through the needle  3 . A protective needle sheath  6  is removably arranged over the needle  3  for protecting the needle  3  from mechanical damage and for preventing finger access to the needle  3  thus reducing the risk of needle stick injuries. 
     The syringe  2  is arranged within an elongate housing  13 . A sliding shroud  22  is telescoped in a proximal part of the housing  13 . Prior to use the sliding shroud  22  is in a distal position. The protective needle sheath  6  is telescoped in the sliding shroud  22  and engaged to it by a friction fit. 
     The syringe  2  is assembled into the housing  13  through the distal end post both filling and application of the protective needle sheath  6 . The syringe  2  may likewise be assembled into the housing  13  pre filling. During assembly the syringe  2  is translated in the proximal direction P until the finger flange  4  abuts against a second rib  20  in the housing  13 . A pair of resilient second snaps  19  is arranged in the housing  13  for engaging the finger flange  4  in a manner to prevent the syringe  2  from being translated out of the distal end of the housing  13 . The second snaps  19  are distally ramped to allow insertion of the syringe  2  into the housing  13  during assembly. 
     In order to prepare for an injection the front end  1  is loaded into the re-usable back end (not illustrated). The user removes the protective needle sheath  6  from the front end  1  by pulling it in the proximal direction P. As the user pulls the protective needle sheath  6  off the needle  3 , the friction fit between the protective needle sheath  6  and the sliding shroud  22  causes the sliding shroud  22  to also be drawn in the proximal direction P and begin to obscure the needle  3 . The re-usable back may be arranged to restrict the motion of the sliding shroud  22  by engaging a fourth shoulder  23  on the sliding shroud  22  such that the user can fully remove the protective needle sheath  6  from the needle  3  by overcoming the friction force between the protective needle sheath  6  and the sliding shroud  22 . The sliding shroud  22  is now free to move with respect to the syringe  2 . 
     During operation of the re-usable back end, the sliding shroud  22  can be moved as required by the reusable back-end. 
     At the end of the injection, the re-usable back end drives the sliding shroud  22  fully out in the proximal direction P in a manner to cover the needle  3  such that resilient second barbs  24  on the sliding shroud  22  engage into recesses  25  within the housing  13 . This prevents access to the needle  3  once the front end  1  is removed from the reusable back end. 
       FIG. 13  is an isometric longitudinal section of the fourth embodiment of the front end  1  post use after removal from the re-usable back end in the section plane as in  FIG. 11 .  FIG. 14  is an isometric longitudinal section of the fourth embodiment of the front end  1  post use in the section plane as in  FIG. 12 . 
       FIG. 15  is a longitudinal section of a fifth embodiment of a front end  1 .  FIG. 16  is a longitudinal section of the fifth embodiment in a different section plane. 
     The front end  1  comprises a syringe  2  with an injection needle  3  attached to a proximal end of the syringe  2 . At a distal end the syringe  2  comprises a finger flange  4 . A stopper  5  is slidably arranged in the syringe  2  for distally sealing a cavity in the syringe  2  arranged to contain a liquid medicament M. The stopper  5  may be translated in a proximal direction P within the syringe  2  in order to displace the medicament M through the needle  3 . A protective needle sheath  6  is removably arranged over the needle  3  for protecting the needle  3  from mechanical damage and for preventing finger access to the needle  3  thus reducing the risk of needle stick injuries. 
     The syringe  2  is arranged within a syringe carrier  11  arranged to support the syringe  2  at its proximal end so as to couple the syringe  2  to the syringe carrier  11  for joint translation. The syringe carrier  11  is slidably arranged within an elongate housing  13 . A syringe spring  14  in the shape of a compression spring is arranged between a second shoulder  12  on the distal end of the syringe carrier  11  and a third shoulder  15  in the housing  13  so as to bias the syringe carrier  11  in the distal direction D with respect to the housing  13 . 
     The housing  13  is telescoped within a syringe retainer  26  at the distal end of the front end  1  arranged to be attached to a re-usable back end. 
     An outer sleeve  7  or cap is removably arranged over a substantial length of the housing  13  and the syringe retainer  26 . The outer sleeve  7  is engaged to the protective needle sheath  6  for joint axial translation. 
     The syringe  2  is assembled into the housing  13  and syringe retainer  26  through the distal end post both filling and application of the protective needle sheath  6 . The syringe  2  may likewise be assembled into the housing  13  pre filling. A pair of resilient second snaps  19  is arranged in the syringe retainer  26  for engaging the finger flange  4  in a manner to prevent the syringe  2  from being translated out of the distal end of the syringe retainer  26 . The second snaps  19  are distally ramped to allow insertion of the syringe  2  into the syringe retainer  26  during assembly. 
     In order to prepare for an injection the user attaches the front end  1  to the re-usable back end using a screw thread  27  on the syringe retainer  26 . The method of attachment is illustrated with a screw thread  27  by way of example however a snap, bayonet, or push fit could be used instead. 
     The user removes the outer sleeve  7  from the syringe retainer  26 . The protective needle sheath  6  is attached to the outer sleeve  7  and therefore the protective needle sheath  6  is removed with the outer sleeve  7  or cap. 
     The protective needle sheath  6  may be attached to the outer sleeve  7  by one of the options illustrated in the  FIGS. 23 to 28 . 
       FIG. 23  illustrates the outer sleeve  7  having an inward boss  28  attached through an opening in the proximal end of the protective needle sheath  6  by a push fit or bump or glued or ultrasonic welded inside the opening. 
       FIG. 24  illustrates the outer sleeve  7  being push fitted over an external diameter of the protective needle sheath  6  or glued or ultrasonic welded to the external diameter. 
       FIG. 25  illustrates the outer sleeve  7  and the protective needle sheath  6  being connected by an additional snap fit part  29  engaged in openings in both the protective needle sheath  6  and the outer sleeve  7 . 
       FIG. 26  illustrates the outer sleeve  7  and the protective needle sheath  6  being connected by a star washer  30  arranged over the external diameter of the protective needle sheath  6  and outwardly engaged to an internal diameter of the outer sleeve  7 . 
     In  FIG. 27  the protective needle sheath  6  has an external structure in the shape of a finger grip. The outer sleeve  7  or cap has at least two profiled arms  31  extending in the distal direction D from a proximal face of the outer sleeve  7 . The profiles of the profiled arms  31  are engaged to the finger grip of the protective needle sheath  6 . The profiled arms  31  are outwardly supported by the housing  13  telescoped inside the outer sleeve  7  such that the profiled arms  31  cannot flex outwards and disengage the protective needle sheath  6 . As the outer sleeve  7  is being pulled off in the proximal direction P the protective needle sheath  6  is thus firmly held by the profiled arms  31 . When the outer sleeve  7  is pulled to such an extent that the profiled arms  31  are no longer supported by the housing  13  the friction between the protective needle sheath  6  and a hub on the syringe  2  mounting the needle  3  has already been overcome. 
       FIG. 28  illustrates the outer sleeve  7  having at least two distal snaps  32  engaged distally behind a distal edge of the protective needle sheath  6 . The distal snaps  32  could likewise be engaged behind a shoulder on the protective needle sheath  6 . Distal ramps on the distal snaps  32  are arranged to facilitate assembly of the outer sleeve  7  over the protective needle sheath  6 . 
     As the outer sleeve  7  is removed, movement of the syringe  2  and compression of the syringe spring  14  is prevented by a syringe lock  33  to ensure the protective needle shield  6  is dislodged and ensure that the syringe  2  does not move in the proximal direction P. 
     The syringe lock  33  comprises a hub  34  extending through a lateral aperture in the syringe retainer  26  in a manner to engage the second shoulder  12  on the syringe carrier  11  to prevent to translation the syringe carrier  11  in the proximal direction P from the initial distal position shown in  FIGS. 15 and 16 . Two resilient arms  35  on the syringe lock  33  are arranged against an outer surface of the syringe retainer  26  in a manner to bias the hub  34  outwardly so as to disengage it from the second shoulder  12 . However, in the initial state, the syringe lock  33  is outwardly supported by the outer sleeve  7  so as to prevent outward deflection of the hub  34  and hence disengagement of the syringe lock  33 . 
     The syringe lock  33  locks the syringe  2  to the syringe retainer  26  when the outer sleeve  7  or cap is in place, thereby preventing syringe  2  being translated in the proximal direction under the load generated by removal of the needle shield  6 . The syringe lock  33  is deactivated upon partial removal of the outer sleeve  7 , at which position the load generated by removal of the needle shield has diminished, to enable the syringe  2  to be moved by a plunger of the re-usable back end (not illustrated). 
       FIG. 20  shows an isometric detail view of the front end  1  according to the fifth embodiment mounted to a re-usable back end  36 .  FIG. 21  shows the distal end of the front end  1  and  FIG. 22  a proximal end of the re-usable back end  36  with an internally threaded mounting  39  for being connected to the front end  1 . When the front end  1  is screwed into the mounting  39  as in  FIG. 20  the outer sleeve  7  may be removed. A resilient locking barb  37  arranged proximally from the screw thread  27  on the syringe retainer  26  engages into an internal groove  38  within the internally threaded mounting  39  of the re-usable back end  36  and prevents rotation of the front end  1  thus preventing removal of the front end  1  from the re-usable back end  36  (see  FIG. 20 ). A number of circumferential bumps  43  on the syringe retainer  26  are arranged proximally from the locking barb  37  to oppose removal of the outer sleeve  7  which may have a respective notch or shoulder for engaging the bumps  43  (not illustrated). Spaces on the perimeter of the syringe retainer  26  between the bumps  43  are arranged to receive drive dogs in the outer sleeve  7  in a manner to couple the outer sleeve  7  to the syringe retainer  26  for joint rotation when the outer sleeve  7  is applied to the syringe retainer  26 . 
     The illustrated screw thread  27  is right-handed, hence the locking barb  37  and the internal groove  38  are arranged to prevent left-hand rotation. Screwing the front end  1  to the re-usable back end  36  is thought to be more intuitive with a right hand rotation. However, the required sense of rotation could be reversed by a left handed screw thread  27  and the locking barb  37  and internal groove  38  arranged to prevent right-hand rotation. 
     As the locking barb  37  partially remains proximally outside the mounting  39  it is inwardly deflected by the outer sleeve  7  prior to removal of the outer sleeve  7  and it can be re-deflected by the outer sleeve  7  being put back in place after injection so as to disengage the locking barb  37  from the internal groove  38  and allow removal of the front end  1  from the re-usable back end  36 . 
     The front end  1  may be combined with the re-useable back end  36  pre or post 
     removal of the protective needle sheath  6 . As the front end  1  is rotated in a 
     clockwise direction for attaching it to the re-usable back end  36  a chamfer  40  on the locking barb  37  ensures the locking barb  37  deflects inwardly upon contact with the re-usable back-end  36 . Thus the function of the outer sleeve  7  in depressing the locking barb  37  can be dispensed with, i.e. the front end  1  may be assembled to the re-usable back end  36  with the outer sleeve  7  already removed. 
     When the user applies the proximal end of the auto-injector composed of the front end  1  and the re-usable back end to an injection site, e.g. a patient&#39;s skin, the housing  13  serving as a transfer sleeve is translated depressing an integrally moulded transfer spring  41  within the housing  13 . The movement of the housing  13  can be detected in the re-usable back end  36  to ensure the auto-injector is only actuated when in contact with the injection site. The syringe spring  14  in the front end  1  permits movement of the syringe  2  under the driving force of the plunger of the re-usable back end  36 . An injection depth is limited to the range of movement permitted by the syringe carrier  11  within the syringe retainer  26 . 
     The load of the syringe spring  14  is resolved through a neck at the proximal end of the syringe  2  as opposed to the finger flange  4 . This reduces the chance of damage to the syringe  2  during the injection process and also gives better injection depth control. 
     Once the injection has been completed, the user must reapply the outer sleeve  7  to the syringe retainer  26  in order to unscrew the front end  1  from the re-usable back end  36 . As the outer sleeve  7  is reapplied, a cover retainer  42  in the shape of a metal clip attached to an inside of the outer sleeve  7  is deformed by contact to the housing  13  such that, once reapplied, the outer sleeve  7  cannot be removed.  FIG. 17  illustrates the cover retainer  42  outwardly deflected by the housing  13  during removal of the outer sleeve  7 , i.e. the outer sleeve  7  is moved in the proximal direction P relative to the housing  13 .  FIG. 18  shows the cover retainer  42  having travelled beyond the proximal end of the housing  13  thus deflecting in an inward direction I.  FIG. 19  shows the outer sleeve  7  being reapplied over the housing  13 . The cover retainer  42  hits the proximal end of the housing  13  and is bent on further distal motion of the outer sleeve  7  in a manner opposing removal of the outer sleeve  7  after reapplication. 
     The front end  1  according to the fifth embodiment ensures the access to the needle  3  is prevented once the protective needle sheath  6  is removed. This would reduce the chance of the user incurring an injury. 
     The options illustrated in the  FIGS. 23 to 28  may likewise be combined with the first embodiment illustrated in  FIGS. 1 and 2 . 
     Once loaded into or attached to the re-usable back end  36 , the syringe spring  14  of the front end  1  according to the second or fifth embodiment may be used to generate the force required to withdraw the needle  3  from the patient so this function does not have to be implemented in the re-usable back end  36  reducing the part count of the re-usable back end  36 . 
     In the second, third and fifth embodiment of the front end the protective needle sheath may be removed before the front end  1  is combined with the re-usable back end  36 . This may help to reduce the complexity and physical size of the assembled auto-injector. 
     The front end  1  according to the third, fourth and fifth embodiment prevents re-use of an emptied syringe  2  by enforcing the user to replace the protective needle sheath  6  post injection, and locking it in place. This reduces the health risk to the user of cross-contamination whether un-intended or through abuse of the front end  1 . 
     The front end  1  according to either embodiment is preferably arranged as a packaged syringe  1  in the sense of a disposable device which is intended to be used once and to be disposed of after use thus reducing the risk of cross contamination between consecutive injections and eliminating the requirement for cleaning re-usable elements. 
     The front end  1  could also be arranged as a re-usable device, wherein the syringe  5  would be replaceably arranged in the front end  1  in such a manner that all components except the syringe  5  could be re-used. 
     The front end  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 protein, antibodies and complex carbohydrates.