fig1 shows an auto - injector 1 . the auto - injector 1 comprises a reusable engine 1 . 1 and a disposable packaged syringe 1 . 2 . the disposable packaged syringe 1 . 2 comprises a cap 2 , a syringe 3 with a hollow injection needle 4 and a stopper 5 for sealing the syringe 3 and displacing a liquid medicament m . the syringe 3 is arranged inside a support sleeve 6 which in turn is arranged inside a tubular transfer sleeve 7 which may be translated within a tubular syringe retainer 8 . the syringe retainer 8 is arranged to restrict longitudinal translation of the syringe 3 . the reusable engine 1 . 1 comprises a case 9 with an open proximal end . a gas spring 10 is arranged in the case 9 , the gas spring 10 comprising a cylinder 10 . 1 with a closed distal end and a piston 10 . 2 arranged to reciprocate in the cylinder 10 . 1 and seal against the cylinder walls . a high pressure cavity filled with a compressed gas g is defined between the distal end of the cylinder 10 . 1 and the piston 10 . 2 . the gas g may be nitrogen at a pressure of approximately 10 bar when the gas spring 10 is in a loaded or compressed state . the piston 10 . 2 is arranged at a distal end of a plunger 10 . 3 . the cylinder 10 . 1 may be translated in longitudinal direction in the case 9 . a latch 14 is provided for preventing this longitudinal motion by locking the cylinder 10 . 1 to the case 9 . a flexible diaphragm ( not illustrated ) may be arranged in the cylinder 10 . 1 under the latch 14 for sealing the cylinder 10 . 1 . a cylinder spring 15 , arranged between an internal shoulder 9 . 1 in the case 9 and an external shoulder 10 . 4 at the cylinder biases the cylinder 10 . 1 in distal direction d with respect to the case 9 . the cylinder spring 15 is compressed in an initial state of the auto - injector 1 . the plunger 10 . 3 extends in proximal direction p in a manner to allow it to push against the stopper 5 of the syringe 3 when the packaged syringe 1 . 2 is attached to the reusable engine 1 . 1 . a trigger button 11 is arranged in a lateral aperture in the case 9 . inside the case 9 a button latch 12 is slidably arranged . it may be slid between a locking position obstructing the trigger button 11 in a manner to prevent it from being pressed and an unlocked position allowing the trigger button 11 to be pressed inwardly . the trigger button 11 has a dog 11 . 1 arranged inwardly . the dog 11 . 1 has as an aperture for the plunger 10 . 3 , the aperture comprising a narrow portion 11 . 1 . 1 and a wide portion 11 . 1 . 2 . the narrow portion 11 . 1 . 1 has a diameter smaller than a diameter of a main section 10 . 3 . 1 of the plunger 10 . 3 but wider than the diameter of a locking section 10 . 3 . 2 of the plunger 10 . 3 . when the gas spring 10 is in its compressed state , the locking section 10 . 3 . 2 is right outside the proximal end of the cylinder 10 . 1 . as long as the trigger button 11 is not pressed the locking section 10 . 3 . 2 is held in the narrow portion 11 . 1 . 1 of the aperture . the main section 10 . 3 . 1 is thicker than the narrow portion 11 . 1 . 1 so the gas spring 10 is locked . when the trigger button 11 is being depressed the wide portion 11 . 1 . 2 becomes aligned with the plunger 10 . 3 , thus allowing the main section 10 . 3 . 1 to pass through the aperture and the gas spring 10 to expand . beyond the locking section 10 . 3 . 2 in proximal direction p , the plunger 10 . 3 comprises a shoulder 10 . 3 . 3 followed by a pin 10 . 3 . 4 with the shoulder 10 . 3 . 3 having a greater diameter than the pin 10 . 3 . 4 . a plunger front part 10 . 3 . 5 is telescoped with the pin 10 . 3 . 4 . a plunger spring 10 . 6 is arranged on the pin 10 . 3 . 4 in a manner to bias the plunger front part 10 . 3 . 5 against the shoulder 10 . 3 . 3 and the rest of the plunger 10 . 3 . a valve 10 . 5 is arranged in or near a proximal face of the cylinder 10 . 1 . a sequence of operation of the auto - injector 1 is as follows : the user assembles the packaged syringe 1 . 2 to a proximal end of the reusable engine 1 . 1 . the connection may be a screw connection as illustrated . alternative embodiments may involve bayonet connections etc . the assembled auto - injector 1 is shown in fig1 a , 1 b and 3 . then the user removes the cap 2 thereby also removing a protective needle sheath 16 arranged on the needle 4 . this situation is illustrated in fig2 . the user places the proximal end p of the auto - injector 1 , i . e . the transfer sleeve 7 against an injection site , e . g . a patient &# 39 ; s skin . as the auto - injector 1 is pushed against the injection site , the transfer sleeve 7 translates by a small distance in distal direction d against the bias of at least one resilient member 7 . 1 and displaces the button latch 12 along the longitudinal axis of the auto - injector 1 from the locking position into the unlocked position . this removes the obstruction preventing depression of the trigger button 11 . the button latch 12 may be biased in the locking position by a bias spring ( not illustrated ). another bias spring may be provided for biasing the trigger button 11 outwardly ( not illustrated ). the user depresses the trigger button 11 to commence an injection cycle . as the trigger button 11 is depressed , the dog 11 . 1 preventing movement of the plunger 10 . 3 is disengaged . the piston 10 . 2 and plunger 10 . 3 start to move in proximal direction p due to the force of the compressed gas g in the cavity . as the plunger 10 . 3 moves through the cylinder 10 . 1 , gas , e . g . air on a proximal , low - pressure side of the piston 10 . 2 is displaced and vented into the atmosphere . the valve 10 . 5 is arranged to restrict the venting thereby introducing a speed dependent force , hence a damping opposing the plunger 10 . 3 motion . the valve 10 . 5 is defined to ensure a damping rate greater than a damping force generated due to friction between the stopper 5 and the inner wall of the syringe 3 and due to hydraulic resistance of the liquid medicament m forced through the small fluid channel in the hollow needle 4 . the force on the piston 10 . 2 from the compressed gas g is always distributed between this damping force and any load applied to the proximal end of the plunger 10 . 3 . upon contact with the stopper 5 , the plunger 10 . 3 applies a force to the stopper 5 . a syringe spring 13 arranged in the packaged syringe 1 . 2 , the syringe spring 13 arranged to bias the syringe 3 in distal direction with respect to the transfer sleeve 7 . the force from the plunger 10 . 3 is transferred to the syringe spring 13 since a friction force generated by the friction between the stopper 5 and the inner wall of the syringe 3 and by the hydraulic resistance of the liquid medicament m forced through the small fluid channel in the hollow needle 4 is greater than the force of the syringe spring 13 and the force required to insert the needle 4 into the injection site . thus , the syringe spring 13 is compressed and the needle 4 is inserted into the injection site . an injection depth is controlled by the design of the packaged syringe 1 . 2 , i . e . by the distance , the syringe 3 and support sleeve 6 can travel forwardly before the syringe spring 13 is fully compressed . in alternative embodiments the injection depth may be controlled by a depth stop within the reusable engine , or within the packaged syringe . the force applied to the stopper 5 by the plunger 10 . 3 increases until the friction opposing motion of the stopper 5 within the syringe 3 is overcome . as this force increases , the plunger spring 10 . 6 will become partially compressed . once the friction force is exceeded , the stopper 5 starts to move and the medicament m is displaced from the syringe 3 through the needle 4 into the injection site . when the stopper 5 reaches the end of travel , i . e . the syringe 3 is fully emptied , the gas spring 10 continues to move the plunger 10 . 3 . this additional motion is accommodated by the plunger spring 10 . 6 . the plunger spring 10 . 6 is defined to generate a force greater than the hydraulic force of emptying the syringe yet weaker than the force of the gas spring 10 . at full extension of the gas spring 10 , the piston 10 . 2 triggers the latch 14 that disengages the cylinder 10 . 1 from the case 9 . if a flexible diaphragm is provided in the cylinder 10 . 1 under the latch , the piston 10 . 2 would operate the latch 14 through the diaphragm to decouple the gas spring 10 from the case 9 . the cylinder 10 . 1 then translates in distal direction d under the force of the cylinder spring 15 . since the gas spring 10 has reached its full extension , the plunger 10 . 3 is withdrawn together with the cylinder 10 . 1 . this removes the force from the stopper 5 allowing the syringe spring 13 to retract the syringe 3 and the needle 4 , whereby the needle 4 is extracted from the injection site and gets hidden inside the transfer sleeve 7 . the user may now remove the packaged syringe 1 . 2 from the reusable engine 1 . 1 . the reusable engine 1 . 1 may be reset using a tool or a base station ( not illustrated ). when resetting , the cylinder 10 . 1 has to be translated in proximal direction p and reengaged to the case 9 by the latch 14 . the latch 14 may be biased to return to its position locking the cylinder 10 . 1 to the case 9 . the gas spring 10 has to be recharged by pushing the plunger 10 . 3 and the piston 10 . 2 in distal direction d . the cylinder valve 10 . 5 is a one way valve . hence , it exerts a minimal force when intaking air into the low pressure side of the cylinder 10 . 1 when resetting the reusable engine 1 . 1 . the reusable engine 1 . 1 may be combined with other types of packaged syringes 1 . 2 . an arrangement for avoiding wet injection may be optionally provided , i . e . the liquid medicament is not leaking out of the hollow needle 4 before or during needle insertion . this may be achieved by a component which couples the plunger 10 . 3 to the syringe 3 during needle insertion so the stopper 5 is not pushed . when the injection depth has been reached the plunger 10 . 3 is decoupled from the syringe 3 and coupled to the stopper 5 to inject the medicament m . the auto - injector 1 may preferably be used for subcutaneous or intra - muscular injection , particularly for delivering one of an analgetic , an anticoagulant , insulin , an insulin derivate , heparin , lovenox , a vaccine , a growth hormone , a peptide hormone , a proteine , antibodies and complex carbohydrates .
an engine for an - auto injector for administering a dose of a liquid medicament includes a case arranged to be attached to a package syringe containing a syringe . the case contains a drive capable of , upon activation , pushing the needle from a covered position into an exposed position , and operating the syringe to supply the dose of medicament . the case contains a trigger arranged to lock the drive in a loaded state prior to manual operation and capable of , upon manual operation , releasing the drive for injections . the drive is arranged as a gas spring including a cylinder and a piston with a plunger for transmitting the spring force . a latch is arranged for preventing translating between the cylinder and the case . the latch is disengageable by the released gas spring being at least nearly fully extended thus allowing the extended gas spring to be translated in a distal direction .