Abstract:
An auto-injector comprising an elongate housing containing a syringe. The housing having a an orifice intended to be applied against an injection site. The syringe is slidably arranged with respect to the housing. A spring pushing the needle from a covered position into an advanced position, operating the syringe and covering the needle. An activator arranged to lock the spring prior to manual operation and capable of, upon manual operation, releasing the spring means for injection. A first gear arrangement and a second gear arrangement arranged for converting torque from a first end and a second end of the torsion spring into a translative force. The first end groundable in the housing while the second end acts on a plunger through the second gear arrangement for advancing the needle and supplying the dose.

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/EP2011/073508 filed Dec. 21, 2011, which claims priority to European Patent Application No. 10196073.0 filed Dec. 21, 2010 and U.S. Provisional Patent Application No. 61/432,241 filed Jan. 13, 2011. The entire disclosure contents of these applications are herewith incorporated by reference into the present application. 
    
    
     FIELD OF INVENTION 
     The invention relates to an auto-injector for administering a dose of a liquid medicament according to the preamble of claim  1  and to a method for operating the auto-injector according to claim  13 . 
     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 underdose 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. 
     SUMMARY 
     It is an object of the present invention to provide an improved auto-injector and an improved method for operating it. 
     The object is achieved by an auto-injector according to claim  1  and by a method according to claim  14 . 
     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. 
     An auto-injector for administering a dose of a liquid medicament according to the invention comprises:
         an elongate housing arranged to contain a syringe with a hollow needle and a stopper for sealing the syringe and displacing the medicament, the elongate housing having a distal end and a proximal end with an orifice intended to be applied against an injection site, wherein the syringe is slidably arranged with respect to the housing,   spring means capable of, upon activation, pushing the needle from a covered position inside the housing into an advanced position through the orifice and past the proximal end, operating the syringe to supply the dose of medicament, and covering the needle on removal of the auto-injector from the injection site,   activating means arranged to lock the spring means in a pressurized state prior to manual operation and capable of, upon manual operation, releasing the spring means for injection.       

     According to the invention the spring means is a single torsion spring. 
     In one embodiment a first gear arrangement and a second gear arrangement are arranged for respectively converting torque from a first end and a second end of the torsion spring into a translative force. The first end is arranged to be groundable in the housing while the second end is configured to act on a plunger through the second gear arrangement for advancing the needle and supplying the dose. The activating means is arranged to block or release the second gear arrangement. When the second gear arrangement is blocked by the activating means the torque from the second end of the torsion spring is also statically resolved in the housing. The first end is releasable from the ground in the housing for causing a translation through the first gear arrangement resulting in the needle getting covered. 
     This translation may be a needle retraction or preferably the advancement of a needle shroud over the needle. 
     The single torsion spring is used for both, inserting the needle and fully emptying the syringe. A major advantage of the torsion spring with gear arrangements is that force is exerted on the stopper and syringe in a smooth manner, whereas a conventional compression spring exhibits a rather abrupt force deployment which may spoil a glass syringe or other parts of the auto-injector. 
     The needle shroud may be arranged in the housing surrounding the syringe and translatable in longitudinal direction, wherein the needle shroud is coupled to the first gear arrangement in a manner to be translated in proximal direction over the advanced needle on release of the first end from the ground in the housing. 
     The torque required to advance the needle shroud may be configured to be less than the torque required to advance the plunger and the stopper. This allows for triggering the shroud advancement at any point during injection without having to ground the second end of the torsion spring in the housing while injection is immediately stopped. 
     The first gear arrangement and the second gear arrangement may respectively comprise a first gear member coupled to the respective end of the torsion spring, wherein the first gear member is engaged through a screw thread to a respective second gear member arranged to translate on rotation of the first gear member. 
     The needle shroud may be arranged to be in an initial position protruding from the proximal end of the housing in an initial state interlocked to the activating means for preventing manual operation. The needle shroud may be arranged to be translated in distal direction into the housing into a distal position against the load of a shroud spring when pushed against the injection site. The needle shroud is rotationally fixed to the housing and to the second gear member of the first gear arrangement. In the distal position the needle shroud is arranged to rotationally fix the first gear member to the second gear member of the first gear arrangement and to allow operation of the activating means. The needle shroud is thus used as a skin interlock means requiring a sequence of operation in order to increase needle safety. Furthermore, the needle shroud serves for blocking the first gear arrangement as long as it is maintained pressed against the injection site. When removed from the skin, the needle shroud returns into the initial position under load of the shroud spring thereby releasing the first gear arrangement for advancing the needle shroud further. 
     The activating means may be arranged to be in a splined engagement with the first gear member of the first gear arrangement in the initial state so as to rotationally fix it to the housing, wherein the activating means is arranged to remove this splined engagement on manual operation. This ensures that the first end of the torsion spring is grounded in the housing prior to injection. The activating means cannot be operated before the needle shroud has been depressed. On depression of the needle shroud the already grounded first end of the torsion spring becomes further grounded in the housing through the first gear arrangement. Operating the activating means removes only one of these grounds. This ensures that the needle shroud is only advanced over the needle when the needle shroud is allowed to return to the initial position on removal from the injection site after an injection has been triggered. 
     A clip arrangement may be provided comprising at least one resilient chassis clip attached to the housing. The chassis clip may be engageable proximally behind a shoulder in the plunger in a manner to prevent translation of the plunger in proximal direction. The shoulder may be arranged to flex the chassis clip outwards due to ramped engagement under force in proximal direction applied to the plunger from the second end of the torsion spring through the second gear arrangement. The activation means may comprise an end trigger button arranged at the distal end translatable between a distal position and a proximal position. At least one trigger beam may be arranged on the trigger button in a manner to outwardly support the chassis clip to prevent it from being outwardly deflected when the end trigger button is in the distal position. The trigger beam is arranged to be repositioned on translation of the end trigger button into the proximal position, i.e. depression in a manner to allow outward deflection of the chassis clip thus releasing the plunger for needle insertion and injection. The end trigger button may protrude from the distal end in a manner to be accessible for operation. 
     At least one flexible first beam element may be arranged on the housing, the flexible first beam element arranged to obstruct the path of the end trigger button so as to prevent its depression. A second beam element is arranged on the needle shroud in a manner to deflect the flexible first beam element out of the path of the end trigger button on depression of the needle shroud. This embodiment requires the user to first depress the needle shroud before the end trigger button can be translated. 
     In another embodiment a rib may be arranged in the housing in a manner to obstruct the path of a resilient part of the end trigger button so as to prevent depression of the end trigger button. A lateral trigger button may be laterally arranged on the housing arranged to inwardly deflect the resilient part of the end trigger button in a manner to bypass the rib thus allowing depression of the end trigger button. The needle shroud may be arranged to inwardly support the resilient part of the end trigger button when in the initial position so as to prevent deflection. The inward support of the resilient part of the end trigger button is arranged to be removed on translation of the needle shroud into the distal position. The end trigger button is biased in proximal direction against the housing by a trigger spring. In this embodiment the end trigger button may be hidden inside the distal end so only the lateral trigger button is operated by the user. 
     In yet another embodiment a wrap over sleeve trigger may be arranged over the distal end. The sleeve trigger is translatable in longitudinal direction between a distal position and a proximal position and has at least one locking feature engageable with a respective mating part on the needle shroud in the initial position so as to prevent depression of the sleeve trigger from the distal position into the proximal position. The mating part is arranged to be inwardly withdrawn by a cam feature on translation of the needle shroud into the distal position so as to allow the sleeve trigger to be depressed. The end trigger button exhibits at least one latch feature arranged to abut against a respective stop in the housing so as to prevent depression of the end trigger button. At least one latch actuation boss on the sleeve trigger is arranged to inwardly deflect the latch feature disengaging it from the stop. The end trigger button is biased in proximal direction against the housing by a trigger spring. A wrap over sleeve trigger may ease operation for users with reduced dexterity. 
     The trigger spring and the shroud spring are specified to balance each other&#39;s load. I.e. the relative strength of the shroud spring and the trigger spring are arranged such that when the auto-injector is pressed against the injection site the shroud will always move first thereby giving a two step feel to the operation. 
     The screw thread of the first gear arrangement may end with a pitch of zero on its proximal end allowing remaining torque in the torsion spring to be released when the second gear member reaches the zero pitch. 
     According to another aspect of the invention a method for operating the auto-injector comprises the steps of:
         grounding the first end of the torsion spring in the housing and blocking the second gear arrangement in an initial state prior to manual operation of the activating means,   releasing the second gear arrangement on manual operation of the activating means so as to translate the plunger in proximal direction for advancing the needle and supplying a dose of medicament from the syringe under torque from the second end of the torsion spring,   releasing the first end from the ground in the housing for causing a translation through the first gear arrangement under torque from the first end resulting in the needle getting covered.       

     The first gear arrangement may translate the needle shroud in proximal direction over the advanced needle on release of the first end from the ground in the housing. 
     The needle shroud may be held in an initial position protruding from the proximal end of the housing in the initial state, wherein the needle shroud is interlocked to the activating means for preventing manual operation in the initial state, wherein the needle shroud is translated in distal direction into the housing into a distal position against the load of a shroud spring when pushed against the injection site. The needle shroud is rotationally fixed to the housing and to the second gear member of the first gear arrangement. In the distal position the needle shroud rotationally fixes a first gear member to the second gear member of the first gear arrangement and releases the interlock so as to allow operation of the activating means. 
     The housing may have at least one viewing window for inspecting the syringe. 
     The auto-injector 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 partial section of an auto-injector, 
         FIG. 2  shows longitudinal sections of the auto-injector in an initial state, 
         FIG. 3  is a longitudinal section of the auto-injector after removal of a protective needle shroud, 
         FIG. 4  is a detail view of a distal end of the auto-injector with a trigger button in the situation as in  FIG. 3 , 
         FIG. 5  is a detail view of the trigger button with an interlock feature to a needle shroud in the situation as in  FIG. 3 , 
         FIG. 6  is a longitudinal section of the auto-injector after depression of the needle shroud, 
         FIG. 7  is an isometric detail view of a shroud follower locked in rotation to a shroud lead screw by depression of the needle shroud, 
         FIG. 8  is an isometric detail view of the interlock feature of  FIG. 5  disengaged by depression of the needle shroud, 
         FIG. 9  is a longitudinal section of the detail view of  FIG. 8 , 
         FIG. 10  is a longitudinal section of the auto-injector after depression of the trigger button, 
         FIG. 11  is an isometric detail view of the trigger button prior to depression of the trigger button, 
         FIG. 12  is an isometric detail view of the trigger button after depression of the trigger button, 
         FIG. 13  is another isometric detail view of the trigger button after depression of the trigger button, 
         FIG. 14  is another isometric detail view of the trigger button prior to depression of the trigger button, 
         FIG. 15  is yet another isometric detail view of the trigger button after depression of the trigger button, 
         FIG. 16  is the auto-injector with a syringe advanced for needle insertion, 
         FIG. 17  is the auto-injector with the emptied syringe, 
         FIG. 18  is the auto-injector on removal from the injection site after the syringe has been emptied, 
         FIG. 19  is a detail view in the situation as in  FIG. 18  with the shroud lead screw released for rotation, 
         FIG. 20  is the auto-injector with the needle shroud fully advanced for post injection needle safety, 
         FIG. 21  is a detail view of the shroud lead screw in the situation as in  FIG. 20 , 
         FIG. 22  is a detail view of the needle shroud locked in the position as in  FIG. 20 , 
         FIG. 23  is the auto-injector on removal from the injection site before the syringe has been emptied, 
         FIG. 24  is the auto-injector with the needle shroud fully advanced for post injection needle safety after delivery of a partial dose, 
         FIG. 25  is an alternative embodiment of the auto-injector with a lateral trigger button, 
         FIG. 26  is a detail view of an interlock mechanism between the lateral trigger button, the needle shroud and the end trigger button with the needle shroud depressed, 
         FIG. 27  is a detail view of the interlock mechanism with the lateral trigger button depressed and the end trigger button released, 
         FIG. 28  is a detail view of the interlock mechanism with the end trigger button translated in proximal direction for releasing the plunger, 
         FIG. 29  is an alternative embodiment of the auto-injector with a wrap over sleeve trigger, 
         FIG. 30  is a detail view of an interlock mechanism between the sleeve trigger, the needle shroud and the end trigger button during an attempt to translate the sleeve trigger in proximal direction without prior depression of the needle shroud, 
         FIG. 31  is a detail view of locking features of the interlock mechanism, and 
         FIG. 32  is a sectional detail view of the locking features of the interlock mechanism. 
     
    
    
     Corresponding parts are marked with the same reference symbols in all figures. 
     DETAILED DESCRIPTION 
       FIG. 1  shows an isometric partial section of an auto-injector  1  with an elongate housing  2  and a needle shroud  3  for protecting a needle (not shown). A trigger button  4  (e.g., an activating member) arranged at a distal end D of the auto-injector  1  may be depressed in a proximal direction P in order to trigger an automatic injection. The trigger button  4  is interlocked with the needle shroud  3  so it cannot be pressed until the needle shroud  3  is pushed into the housing  2  by placing it on an injection site, e.g. a patient&#39;s skin and applying pressure. The needle shroud  3  has longitudinal splines  5  engaged in corresponding grooves in the housing  2  for preventing relative rotation of the needle shroud  3  with respect to the housing  2   
       FIGS. 2 a  and 2 b    are longitudinal sections of the auto-injector  1  in different section planes approximately 90° offset from each other. The auto-injector  1  is in an initial state prior to use. A syringe  7  is partially surrounded and supported at a front end by a syringe carrier  8 . Attached at the front end of the syringe  7  is a hollow injection needle  19  for piercing a patient&#39;s skin and delivering a liquid medicament stored inside the syringe  7  (syringe and needle are not shown in  FIG. 2   b  for clarity). Near the distal end of the syringe  7  a stopper  10  is arranged for sealing and containing the medicament. The stopper  10  may be advanced by a plunger  11  (e.g., a second gear member of a second gear arrangement) in order to expel the medicament M from the syringe  7 . The syringe carrier  8  is slidably arranged inside the needle shroud  3 . The needle shroud  3  is biased towards a proximal end P by a shroud spring  12  in the shape of an integral compliant polymer beam element integrally moulded with the needle shroud  3  and acting against a first rib  2 . 1  in the housing  2 . The shroud spring  12  could likewise be of a different type, e.g. a compression spring. 
     The plunger  11  exhibits an external plunger lead screw  11 . 1  and is rotationally fixed to the housing  2  by a torque reaction rod  21  arranged in an axial bore of the plunger  11 . The axial bore and the torque reaction rod  21  both have a non-circular profile in order to keep the plunger  11  from rotating, e.g. a square profile or a profile with at least one spline or flat. The torque reaction rod  21  is attached to the housing  2  at the distal end D of the auto-injector  1  through a framework in a distal end cap  22  in such a manner that the torque reaction rod  21  is prevented from rotating relative to the housing  2 . 
     The plunger  11  is arranged inside a tubular plunger follower  15  (e.g., a first gear member of the second gear arrangement), which is engaged to the plunger lead screw  11 . 1  by at least one ball bearing (not illustrated). The plunger follower  15  is arranged inside a torsion spring  14  which in turn is arranged inside a tubular shroud lead screw  13  (e.g., a first gear member of a first gear arrangement) with an external shroud lead screw thread  13 . 1 . A tubular shroud follower  9  (e.g., a second gear member of the first gear arrangement) is arranged around the shroud lead screw  13  and inside a distal part of the needle shroud  3 . The shroud follower  9  is engaged to the shroud lead screw  13  by at least one ball bearing (not illustrated). 
     The extension of the needle shroud  3  from the proximal end P is limited by engagement of a pin  9 . 2  on the shroud follower  9  in a slot hole  3 . 3  in the needle shroud  3 . This engagement also fixes the shroud follower  9  rotationally to the needle shroud  3 . The shroud follower  9  is axially fixed to the tubular shroud lead screw  13  by the ball bearing. 
     Axial translation of the shroud lead screw  13  in the proximal direction P is prevented by an external lip  13 . 4  on the distal end of the shroud lead screw  13  abutting against a step  2 . 8  in the housing  2 . Axial translation of the shroud lead screw  13  in the distal direction D is prevented by an internal flange  13 . 5  distally bearing against the proximal end of the plunger follower  15 . The plunger follower  15  is axially fixed to the housing  2  at its distal end. In the initial state, rotation of the shroud lead screw  13  is prevented by splined engagement with the trigger button  4  (see  FIG. 4 ). For this purpose the trigger button  4  has an inward boss  4 . 3  engaged between circumferential outward bosses  13 . 3  on the shroud lead screw  13 . 
     The torque from the proximal end  14 . 1  of the torsion spring  14  is resolved into the shroud lead screw  13 . The torque from the distal end  14 . 2  is resolved into the plunger follower  15 . The torque from the plunger follower  15  is coupled through the ball bearing into an axial force in the plunger  11 . In the initial state, axial loads within the plunger  11  are resolved through into the housing  2  by means of a chassis clip arrangement at the distal end D. The chassis clip arrangement comprises two resilient chassis clips  16  fixed to the housing  2 . The chassis clips  16  are engaged proximally behind a shoulder  11 . 2  in the plunger  11  in a manner to prevent translation of the plunger  11  in proximal direction P. Due to ramped engagement the shoulder  11 . 2  is trying to flex the chassis clips  16  outwards which is prevented by two trigger beams  4 . 1  on the trigger button  4  respectively arranged between an outward pin on the chassis clip  16  and a rigid support beam  17 . 
     The trigger button  4  is locked in a distal position in the initial state to prevent unintended activation of the auto-injector  1 . Flexible first beam elements  2 . 2  integrally moulded with the housing  2  obstruct the motion of the trigger button  4  if attempts are made to depress it (see  FIGS. 5, 8 ). The first beam elements  2 . 2  are deflected out of the path of the trigger button  4  by a second beam element  3 . 1  on the needle shroud  3  on axial movement of the needle shroud  3  within the housing  2  (see  FIGS. 8, 9 ) in distal direction D. This is achieved when the proximal end P is pressed against the injection site. 
     In the as delivered initial state a protective needle shield  18  is arranged on the injection needle  19 . 
     A sequence of operation is as follows: 
     The user removes the protective needle shield  18  from the needle  19 . This can be achieved by a device cap engaged with the needle (not illustrated). The needle  19  is a safe distance back within the needle shroud  3  to protect the user from accidental needle stick injuries (see  FIG. 3 ). 
     The user places and pushes the proximal end P of the auto-injector  1  on the injection site, e.g. a patient&#39;s skin. The needle shroud  3  moves in distal direction D into the housing  2  by a small distance (see  FIG. 6 ). Due to the slot hole  3 . 3  this translation does not translate the shroud follower  9 . The translating second beam element  3 . 1  flexes the first beam elements  2 . 2  outwards thus clearing the path for the trigger button  4  and releasing the aforementioned interlock (see  FIGS. 8, 9 ). The moulded shroud spring  12  opposes this motion but is specified such that its spring rate and preload are low enough for this to feel natural. A resilient third beam element  3 . 2  on the needle shroud  3  is deflected inwards by contact with a ramped second rib  2 . 3  on the housing  2  on translation of the needle shroud  3 . The third beam element  3 . 2  deflects a fourth beam element  9 . 1  on the shroud follower  9  into a spline  13 . 2  on the shroud lead screw  13  (see  FIGS. 4, 7 ). 
     As the shroud follower  9  is rotationally fixed to the housing  2  through its engagement with the needle shroud  3 , the fourth beam element  9 . 1  provides further grounding of the shroud lead screw  13  to the housing  2 . As detailed above, the shroud lead screw  13  is already grounded to the housing  2  through a splined engagement with the trigger button  4 . 
     When ready to do so, the user depresses the trigger button  4 , translating it in proximal direction P (see  FIGS. 10, 12, 13, 15 ). The trigger beams  4 . 1  on the trigger button  4  are translated with the trigger button  4  in a manner to allow the outward pin  16 . 1  to enter an aperture  4 . 2  in the trigger beam  4 . 1  by the chassis clip  16  being flexed outwards due to ramped engagement with the shoulder  11 . 2  under the force pulling the plunger  11  in proximal direction P. 
     The plunger  11  moves in proximal direction P towards the stopper  10  driven by rotation of the plunger follower  15 . As stated, the plunger  11  is prevented from rotating by the torque reaction rod  21  down its centre. This could be achieved with one or more splines, flats or by using a square shaft as shown in this embodiment. One or more ball bearings provide a low friction contact between the plunger follower  15  and the plunger lead screw  11 . 1 . Depression of the trigger button  4  also removes the splined engagement of the trigger button  4  from the shroud lead screw  13  by translating the inward boss  4 . 3  out of engagement with the circumferential outward bosses  13 . 3  (see  FIG. 15 ). Now, the shroud lead screw  13  is grounded to the housing  2  through the shroud follower  9  only. This means at any point from now, a shroud extension mechanism described below would be triggered if the auto-injector  1  is removed from the injection site, thereby ensuring the needle  19  is covered. The trigger button  4  is locked in a fully depressed position by snaps  4 . 4  acting against the housing  2  (see  FIGS. 13 and 15 ). 
     The plunger  11  drives the syringe  7  forward by pushing on the stopper  10  thus inserting the needle  19  into the injection site. The subcutaneous injection depth is set by a rear flange  8 . 1  on the syringe carrier  8  contacting a stop  2 . 4  in the housing  2  (see  FIG. 16 ). 
     When the injection depth has been reached the stopper  10  is driven forwards in proximal direction P within the syringe  7 , injecting the dose of medicament M. The stopper  10  continues to move until it reaches the end of the syringe  7  thereby fully emptying the syringe  7 . This would require the user to hold the auto-injector  1  in place for a sufficient period of time. In this embodiment, the user is asked to keep pressure on the injection site for a short period of time (e.g. approximately 10 seconds), which can be communicated to the user through user instructions. Other options would be observance of moving components within the auto-injector  1 , or an audible ratchet detecting movement of the plunger  11 . 
     After confirmation that the full dose has been delivered, the user withdraws the auto-injector  1  from the injection site. With withdrawal, the needle  19  is extracted from the skin, and the needle shroud  3  extends under the force of the shroud spring  12  (see  FIG. 18 ). The third beam element  3 . 2  is translated back in proximal direction P thus no longer deflecting the fourth beam element  9 . 1  inwards. Hence the fourth beam element  9 . 1  flexes outwards again and disengages the shroud follower  9  from the shroud lead screw  13 . Therefore torque within the shroud lead screw  13  is no longer resolved through to the housing  2  (see  FIG. 19 ). With the grounding to the housing  2  removed, the torque is now resolved through the ball bearing interface to an axial force on the shroud follower  9 . Further movement of the plunger  11  in the proximal direction P is not possible in this situation, so when the shroud follower  9  is released it moves in the proximal direction P pushing the needle shroud  3  further out of the proximal end P of the housing  2  thus completely covering the needle  19  (see  FIG. 20 ). The shroud lead screw thread  13 . 1  ends with a pitch of zero on the proximal end allowing any remaining torque in the torsion spring  14  to be released (see  FIG. 21 ). This makes the auto-injector  1  tamper proof post injection, e.g. if the user tries to dismantle the auto-injector  1 . 
     The needle shroud  3  is locked in this extended position by snaps  2 . 5  within the case  2  (see  FIG. 22 ). 
       FIG. 23  illustrates removal from skin mid injection with the syringe  7  only partially emptied. As the shroud follower  9  disengages from the shroud lead screw  13 , the torque within the shroud lead screw  13  is no longer resolved through to the housing  2  but through the ball bearing interface to an axial force on the shroud follower  9 . Although the torque from the distal end  14 . 2  of the torsion spring  14  is still resolved through the plunger  11 , the torque required to extend the needle shroud  3  is less than the torque required to forward the plunger  11 , hence the distal end  14 . 2  becomes ground. 
       FIG. 24  shows the auto-injector  1  with the needle shroud  3  fully advanced for post injection needle safety after delivery of the partial dose. The zero end pitch of the shroud lead screw thread  13 . 1  allows the torsion spring  14  to be released preventing further drug delivery. 
       FIG. 25  shows an alternative embodiment of the auto-injector  1  with a lateral trigger button  20  (e.g., an activating member). The lateral trigger button  20  is an integral part of the housing  2 . It may be moulded orthogonal to the housing  2  and then folded into place. However, the auto-injector  1  also comprises the end trigger button  4  of the embodiment illustrated in  FIGS. 1 to 24 , but hidden inside the distal end D closed by the distal cap  22  attached to the housing  2 . A trigger spring  23  in the shape of a small compression spring applies a load between the distal cap  22  and the trigger button  4 . 
     The sequence of operation is as described above for the embodiment in  FIGS. 1 to 24  except in the following steps: 
     The initial position and function of all components is identical with the exception of the button interlock. The lateral trigger button  20  is locked in an extended position in the initial state to prevent unintended activation of the auto-injector  1 . A boss  20 . 1  on the distal end of the lateral trigger button  20  extends through an aperture in the housing  2 . In the initial state the boss  20 . 1  abuts against one of the inward bosses  4 . 3  on the end trigger button  4  which is inwardly supported by the shroud lead screw  13  and by a distal extension  3 . 4  on the needle shroud  3  arranged between the inward boss  4 . 3  and the shroud lead screw  13 . Any force applied to the lateral trigger button  20  is therefore statically resolved preventing its depression. The inward boss  4 . 3  on the end trigger button  4  abuts against a third rib  2 . 6  in the housing  2  in proximal direction P thus preventing release of the end trigger button  4 . When the auto-injector  1  is pressed against the skin, the needle shroud  3  translates within the housing  2  and a window  3 . 5  in the distal extension  3 . 4  becomes aligned with the boss  20 . 1  (see  FIG. 26 ) allowing the lateral trigger button  20  to be depressed (see  FIG. 27 ) thereby flexing the inward boss  4 . 3  inwards into the window  3 . 5  in such a manner that the inward boss  4 . 3  comes clear of the third rib  2 . 6  releasing the end trigger button  4  which is then translated under the action of the trigger spring  23  (see  FIG. 28 ). This releases the chassis clip  16  resolving the axial load on the plunger  11  as in the embodiment in  FIGS. 1 to 24 . 
       FIG. 29  shows another alternative embodiment of the auto-injector  1  with a wrap over sleeve trigger  24  (e.g., an activating member) arranged over the distal end D and extending over roughly half the length of the housing  2 . However, the auto-injector  1  also comprises the end trigger button  4  of the embodiment illustrated in  FIGS. 1 to 24 , but hidden inside the distal end D. A trigger spring  23  applies a load between the sleeve trigger  24  and the end trigger button  4 . The load from the trigger spring  23  is balanced by load from the shroud spring  12 . The end trigger button  4  exhibits latch features  4 . 5  initially abutted against a stop  2 . 7  in the housing  2  (see  FIGS. 29 to 32 ). 
     The sequence of operation is as described above for the embodiment in  FIGS. 1 to 24  except in the following steps: 
     The initial position and function of all components is identical with the exception of the button interlock. Movement of the trigger sleeve  24  is prevented to avoid unintended activation of the auto-injector  1 . If the trigger sleeve  24  is moved, a locking feature  24 . 1  engages with a mating part  3 . 6  on the needle shroud  3  guarding against the user grabbing the housing  2  and attempting to operate the trigger sleeve  24  (see  FIG. 30 ). When the auto-injector  1  is pressed against the skin, the needle shroud  3  translates within the housing  2  and the mating part  3 . 6  on the shroud  3  is inwardly withdrawn as the resilient third beam element  3 . 2  runs down a cam on the second rib  2 . 3  in the housing  2  allowing it to be depressed. 
     When ready to do so, the user translates the sleeve trigger  24  in proximal direction P. As the trigger sleeve  24  translates, a latch actuation boss  24 . 2  on the sleeve trigger  24  deflects the latch feature  4 . 5  inwards disengaging it from the stop  2 . 7  in the housing  2 . The end trigger button  4  is then translated under the action of the trigger spring  23 . This releases the chassis clip  16  resolving the axial load on the plunger  11 . 
     This allows for a forced activation sequence. The intended activation of the auto-injector  1  involves the needle shroud  3  (skin interlock) being depressed prior to the trigger sleeve  24  being translated. Both parts (needle shroud  3  and trigger sleeve  24 ) are sprung relative to the housing  2  in this embodiment. By careful selection of the spring properties of the shroud spring  12  and the trigger spring  23 , the correct sequence can be achieved. If the shroud spring  12  is less stiff than the trigger spring  23 , it will compress first when a load is applied between the needle shroud  3  and the trigger sleeve  24 . In the case of an end trigger button  4  only as in  FIGS. 1 to 24 , this is not required as the user holds the housing  2  and is able to move the two parts independently. 
     The embodiment of  FIGS. 29 to 32  furthermore allows for a non-return activation sequence, i.e. the embodiment allows the needle shroud  3  to be depressed and the trigger sleeve  24  to be moved up to the point of release of the end trigger button  4  and then released without changing the load paths within the auto-injector  1 . I.e. the auto-injector  1  can be placed on the skin and then removed and remain in a safe state. Once the trigger sleeve  24  moves beyond the point of release of the internal end trigger button  4 , the auto-injector  1  is activated under the action of the trigger spring  23  and cannot be stopped by the user. This prevents the user partially activating the auto-injector  1  and leaving it in a partially activated state, which would result in the auto-injector firing immediately on the next attempt without requiring the sequenced operation. 
     Furthermore the embodiment demonstrates the ability to convert the auto-injector  1  from an end trigger ( FIGS. 1 to 24 ) to a sleeve trigger ( FIGS. 29 to 32 ) by adding an additional part, thus achieving a platform design. 
     The auto-injector  1  may preferably be used 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.