fig1 a and 1 b show an injection device 110 , having an injection device housing 112 . the injection device 110 has a removable cap 190 . with the cap 190 removed , as shown in fig2 the end of the housing 112 can be seen to have an exit aperture 128 , through which the end of a sleeve 119 can emerge . the injection device 110 also has a trigger 180 . as shown in fig2 a and 2 b , the housing 112 contains a hypodermic syringe 114 of conventional type , including a syringe body 116 defining a reservoir and terminating at one end in a hypodermic needle ( not shown ) and at the other in a flange 120 . the hypodermic needle is covered by a needle shield 118 . the needle shield 118 is fixed inside the cap 190 . the syringe body 116 is of substantially constant diameter along the length of the reservoir , and is of significantly smaller diameter close to the end of the syringe which terminates in the hypodermic needle . a drive element 134 ( syringe piston ) acts through the bung of the syringe to discharge the contents of the syringe 114 through the needle 118 . this drive element 134 constrains a drug ( contained in the syringe ) to be administered within the reservoir defined by syringe body 116 . whilst the syringe illustrated is of hypodermic type , this need not necessarily be so . transcutaneous or ballistic dermal and subcutaneous syringes may also be used with the injection device of the present invention . the housing 112 comprises a case nose 113 which is integrally formed with a sleeve 160 . the sleeve 160 surrounds a syringe carrier 150 which is moveable within the sleeve 160 along its longitudinal axis . as illustrated , the syringe 114 is housed within the syringe carrier 150 . the syringe carrier 150 has a first end 151 and a reduced diameter section 151 a . the section 151 a of the syringe carrier supports the end of the syringe 114 nearest to the hypodermic needle . the syringe carrier 150 comprises a bearing surface 153 on which an end of a return spring 126 is located . the return spring 126 , via the syringe carrier 150 biases the syringe 114 from an extended position in which the needle 118 extends from the aperture 128 in the housing 112 to a retracted position in which the needle 118 is contained within the housing 112 . if the syringe were to fail or break , the syringe carrier 150 , which substantially surrounds the syringe 114 along its length , would contain the broken pieces of syringe and reduce the likelihood of them from escaping from the injection device . the housing 112 also includes a trigger 180 , and a drive which here takes the form of a compression drive spring 130 . drive from the drive spring 130 is transmitted via a multi - component drive ( 118 a ) to the drive element 134 of the syringe 114 to advance the syringe from its retracted position to its extended position and discharge its contents through the needle 118 . the drive accomplishes this task by acting directly on the syringe 114 and the drug in the syringe . static friction between the drive element 134 and the syringe body 116 initially ensures that both the syringe 114 and bung advance together , until the return spring 126 bottoms out when the bearing surface 153 on the syringe carrier 150 comes up against an opposing bearing surface 161 on the sleeve 160 . the trigger 180 is provided on the housing 112 remote from the exit aperture 128 . the trigger , when operated , serves to decouple a drive sleeve 131 on which the drive spring 130 acts from the housing 112 , allowing it to move relative to the housing 112 under the influence of the drive spring 130 . the operation of the device is then as follows . the cap 190 can be removed by a user with a twist and pull action or simply by pulling the cap . the exact action required depends on the type of syringe 114 being used . in one embodiment , the syringe 114 will comprise a rigid needle shield 118 containing a rubber boot ( not shown ) in which the needle is contained . in this embodiment , the needle shield 118 simply needs to be removed by pulling the cap 190 along the longitudinal axis of the device 110 . in an alternative embodiment , the syringe 114 comprises a plastic needle shield 118 which is held to the syringe 114 by a frangible connection . in order to break the frangible connection , the cap 190 must be first twisted and then pulled along the longitudinal axis of the device 110 . a guiding element 191 on the end cap 113 serves to guide the removal of the cap 190 in the way that is required to remove the needle shield 118 . since the needle shield 118 is held inside the cap 190 , removal of the cap 190 , causes the needle shield to be removed , thereby exposing the needle of the syringe 114 within the injection device . at this time , the needle is still enclosed by the housing 112 . initially , the syringe carrier 150 and syringe 114 , are prevented from movement by a resilient latch member 162 . by moving the sleeve 119 in a direction into the housing 112 , the latch member 162 moves outwards disengaging from the syringe carrier 150 . once the latch member 162 has disengaged from the syringe carrier 150 , the syringe 114 and syringe carrier 150 are free to move . the trigger 180 can then be depressed by a user and the drive spring 130 is released . the drive spring 130 moves the drive sleeve 131 , the piston 134 and , by virtue of static friction and hydrostatic forces acting through the drug to be administered , moves the syringe body 114 against the action of the return spring 126 . the syringe body 114 moves the syringe carrier 150 , which compresses the return spring 126 . the hypodermic needle 118 emerges from the exit aperture 128 of the housing 112 . this continues until the return spring 126 bottoms out or the syringe body 116 meets some other obstruction ( not shown ) that retards its motion . because the static friction between the second drive element 134 and the syringe body 116 and the hydrostatic forces acting through the drug 124 to be administered are not sufficient to resist the full drive force developed by the drive spring 130 , at this point the second drive element 134 begins to move within the syringe body 116 and the drug begins to be discharged . the cap 190 of the injection device 110 of the present invention is depicted in fig3 . the cap 190 includes an annular protrusion 191 which extends into the exit aperture 128 when it is attached to the injection device 110 . the annular protrusion 191 includes grip means 191 a which grip the boot 118 of the syringe 114 so that the boot is removed when the cap 190 is removed from the injection device 114 . at an end of the annular protrusion 191 , where it opposes the exit aperture 128 , there is an edge of the annular protrusion 191 which provides a first planar interface 192 for interfacing with the first end 151 of the syringe carrier 150 , on which resides a second planar interface 151 b . the annular protrusion 191 and syringe carrier 150 are dimensioned so that the first and second planar interfaces 192 , 151 b are in juxtaposition with each other when the cap 190 is in place on the injection device 110 . thus , when the cap 190 is in place , movement of the syringe carrier 150 in a direction f out of the injection device 110 is prevented , for example , when the injection device 110 experiences an external impact force , when it hits a hard surface . since forward movement is inhibited , damage to an integrity seal 196 and / or needle 197 of the syringe is prevented . the syringe carrier 150 is shown with an intermediate section 151 a of reduced diameter which acts to prevent forward movement of the syringe 114 in the syringe carrier 150 by gripping the syringe 114 between the discharge nozzle and the syringe body 116 . fig4 shows the first end of the syringe carrier 150 , on which the second planar interface 151 b is located . the syringe carrier 150 is in the form of a split annular sheath , with a split 193 in each diametrically opposing side of the sheath at the first end 151 of the syringe carrier 150 . each split 193 provides a restraining interface 194 . when the sleeve 119 is in its extended ( unactuated ) position , the resilient latch members 161 are in juxtaposition with the restraining interfaces 194 , thereby preventing forward movement of the syringe carrier 150 . when the sleeve 119 is pushed into the injection device 110 , the latch members splay away from the syringe carrier 150 , permitting the syringe carrier 150 to travel forward on actuation of the trigger 180 . when the cap 190 is in place on the injection device 110 , juxtaposition of the interfaces 192 and 151 b prevents loading of ( and hence damage to ) the latch members 161 during high loading of the impact of the injection device 110 with , for example , external forces . fig5 shows the cap 190 of the injection device as depicted in fig3 without the boot 118 of the syringe 114 in place . the grip means 191 a is seen to comprise rearward protrusions 198 a which engage the boot 118 such that movement of the boot 118 in a direction out of the cap ( i . e . opposite to direction f ) is prevented . however , the grip means 191 a is formed of resilient metallic material so that insertion of the boot 118 into the cap 190 is permitted , following which the protrusions engage the rubber material of the boot 118 to prevent its removal out of the cap 190 . it will of course be understood that the present invention has been described above purely by way of example and modifications of detail can be made within the scope of the invention .
an injection device 110 is described having a housing 112 that receives a syringe 114 having a needle 118 , wherein the syringe is supported in a syringe carrier 150 . the injection device 110 has a removable cap 190 . the syringe 114 and syringe carrier 150 are biased by a return spring 126 from an extended position in which the needle 118 extends from the housing 112 through an exit aperture 128 to a retracted position in which it does not . the syringe carrier 150 abuts a surface inside the removable cap 190 which prevents forward movement of the syringe carrier 150 when the cap is in place . the injection device is less prone to failure than prior art devices and is safer should failure occur .