Abstract:
An injection device for delivering a dose of medicament from a syringe includes: a housing including a forward body and rearward body and wherein the forward and rearward body are axially moveably interconnected; a drive mechanism arranged, in use, to be released from an energized position to deliver a dose from the syringe; a latch arranged to hold the drive mechanism in the energized position; and a trigger for releasing the latch; wherein the injection device further includes an interlock arrangement moveable between a locked position in which release of the drive mechanism is prevented and an unlocked position in which the drive mechanism may be released, the interlock arrangement including: an internal blocking formation associated with the rearward body and positioned between the latch and trigger, and wherein in use, forward movement of the rearward body relative to the forward body moves the blocking formation into the unlocked position.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to injection devices for delivering a dose of medicament from a syringe. In particular, but not exclusively the invention relates to an “autoinjector” device. 
       BACKGROUND OF THE INVENTION 
       [0002]    Injection devices are used for the convenient administration of medicaments. For example, injection devices (which may typically be in the form of a pen injector) may be used for providing a single metered dose of a medicament, such as Epinephrine, in an emergency or for providing regular metered doses of a medicament, such as Insulin. Such devices may be either single use “disposable” devices in which the device is typically provided with a syringe already installed, and which is not user-replaceable, or “reusable” devices which allow the user to replace the syringe when the medicament has been used. 
         [0003]    It may be noted that whilst the term “syringe” is used herein for clarity and consistency, however the skilled person will appreciate that this term is intended to be broadly interpreted. In some arrangements the syringe may for example be a cartridge (which, for example, may be arranged to receive a disposable needle) or other medicament container. In some arrangements the syringe/cartridge/medicament container may formed integrally with the (or part of the) injection device. 
         [0004]    Injection devices may be provided in the form of an “autoinjector” device in which, in addition to automating the delivery of the medicament, the device is also arranged to automate the insertion of a needle into the skin prior to the delivery of the medicament. 
         [0005]    Injection devices generally comprise a delivery mechanism which is arranged to automatically deliver a dose from the syringe, and optionally (in the case of an autoinjector) to first displace the syringe within the housing to cause needle penetration. The delivery mechanism is typically released from an energised (or primed) position and may, for example, include one or more drive springs. The delivery mechanism may act upon a plunger which includes or acts against a piston (also referred to as a “bung”) which is slidably provided within the syringe. In the case of an autoinjector the initial stiction between the piston and syringe may resist forward movement of the piston relative to the syringe such that initially the drive mechanism moves the syringe into the needle insertion position (whereupon further movement of the syringe is blocked and the drive mechanism will continue to move forward thus moving the piston). 
         [0006]    A trigger, for example in the form of a button on the end of the device, is generally provided to allow the user to activate the delivery sequence. It is known to provide a safety arrangement in the form of a mechanical lock (which is referred to herein as an “interlock”) arranged to prevent actuation of the delivery mechanism by the user unless the lock is in the unlocked position. For example, such safety arrangements may require the user to perform a readying action prior to releasing the trigger or may be arranged to require the injection device to be in contact with an injection site before the trigger is able to activate the delivery mechanism. Examples of such arrangements are, for example, shown in: European Patent EP2429613, U.S. Pat. No. 5,137,516 and International Patent Application WO2012/110577. 
         [0007]    The provision of safety mechanisms is desirable to avoid inadvertent operation of injection devices (particularly, in auto-injectors where inadvertent operation could result in injury due to the projection of the needle during the insertion stage of the activation sequence). However, such arrangements may result in increased complexity of the device. Increased complexity may for example result in less intuitive use of the device for the end user. Further increased complexity may increase manufacturing costs, by increasing part count and/or increasing assembly complexity, which may be particularly disadvantageous for disposable injection devices. Thus, it is desirable to provide an alternate injection device which may at least partially address some of these disadvantages. 
       SUMMARY OF THE INVENTION 
       [0008]    Accordingly, in one aspect, this invention provides an injection device for delivering a dose of medicament from a syringe, the injection device comprising:
       a housing comprising a forward body and rearward body and wherein the forward and rearward body are axially moveably interconnected;   a drive mechanism arranged, in use, to be released from an energised position to deliver a dose from said syringe;   a latch arranged to hold the drive mechanism in the energised position; and   a trigger for releasing the latch; wherein   the injection device further comprises an interlock arrangement moveable between a locked position in which release of the drive mechanism is prevented and an unlocked position in which the drive mechanism may be released, the interlock arrangement comprising:   an internal blocking formation associated with the rearward body and positioned between the latch and trigger, and wherein   in use, forward movement of the rearward body relative to the forward body moves the blocking formation into the unlocked position.       
 
         [0016]    Typically, the front and rear bodies of the housing are slidably interconnected. The front and rear bodies jointly define the housing which encloses the injection device. The front and rear bodies may be arranged to move relative to one another by a limited amount in the axial direction of the injector device. 
         [0017]    The drive mechanism may further be arranged to displace the syringe within the housing to cause needle penetration in use. As such the injection device may further comprise a syringe carriage axially movable within the housing. For example the carriage may be moveable between a rearward (retracted) position and a forward (injection) position. The injection device may be an autoinjector. 
         [0018]    The trigger and latch may remain axially fixed relative to one another when the interlock is moved to the unlocked position. As such, the spacing between the latch and trigger may remain constant during the interlock operation. Movement of the rearward body relative to the forward body may move the blocking formation relative to both the latch and trigger. The latch and trigger may both be fixed relative to the forward body during forward movement of the rearward body (to move the interlock to the unlocked position. Fixing both the latch and trigger relative to the forward body (in the initial interlock operation) will ensure that the latch and trigger remain relatively fixed. 
         [0019]    In the locked position, the blocking formation may be arranged to block forward movement of the trigger. The blocking formation may, for example comprise a rearwardly facing surface, for example a shoulder. The rearwardly facing surface of the blocking member may be arranged for cooperating with a forwardly facing surface of the trigger when in the locked position. The forward facing surface may at least partially comprise a shoulder. The forward facing surface may at least partially comprise an end wall. A shoulder on the trigger may be formed on an inner surface of the trigger. 
         [0020]    The surfaces of the blocking member and forward facing surface may have matching profiles to positive engage or align in the locked position. The cooperating surfaces may be substantially radially extending with respect to the injection device. The cooperating surfaces may be sloped or angled. 
         [0021]    The latch may be arranged to expand in a radially outward direction to release the drive mechanism. At least a portion of the trigger may be disposed radially outside of the latch member. The trigger may prevent radial expansion of the latch in the pre-firing position. A surface of the trigger may abut an opposing surface to prevent radial movement of the latch member. When the trigger is in the release position the opposing surfaces of the trigger and latch may move out of axial alignment to enable the latch to expand. 
         [0022]    In the locked position, the rearward body may also block release of the latch. For example, the rearward body may prevent expansion of the latch. The inner surface of the rearward body may be profiled such that the relative forward movement of the rearward body creates clearance between the body and the portion of the latch to allow radial expansion of the latch. For example the inner diameter of the rearward body could be outwardly tapered towards the rear of the body. Alternatively or additionally, the inner surface of the rearward body may comprise at least one inwardly extending abutment arranged to be moved axially into and out of blocking alignment with a portion of the latch. 
         [0023]    The trigger may be disposed at a rearward end of the housing. The trigger may be arranged for forward movement relative to the housing to release the latch. The trigger may extend forwardly within the interior of the rearward body and the blocking formation may be at least partially within the interior of the trigger. Thus, the blocking formation may be at least partially captive within the trigger. 
         [0024]    The latch may be at least partially enclosed by the trigger. For example, the latch and trigger may be substantially concentric. The trigger and latch may further be substantially concentric with the rearward body. 
         [0025]    The trigger may extend forward to engage a feature of, or associated with, the forward body. The trigger may extend over a feature of, or associated with the rearward body to engage the feature of, or associated with, the forward body. The engagement between the trigger and forward body may be arranged to retain the forward and rearward bodies in slidable engagement. 
         [0026]    This arrangement may be particularly advantageous in providing an injector device which can be easily assembled. 
         [0027]    According to a further aspect the present invention comprises an injection device for delivering a dose of medicament from a syringe, the injection device comprising:
       a housing comprising a forward body and rearward body and wherein the forward and rearward body are axially movably interconnected;   a drive mechanism arranged, in use, to be released from an energised position to deliver a dose from said syringe;   a trigger disposed at a rearward end of the housing for releasing the drive mechanism; wherein   trigger arranged to extend forwardly within the interior of the rearward body and over at least one feature of, or associated, with the rearward body and engages a feature of, or associated with, the forward body to retain the forward and rearward bodies in slidable engagement.       
 
         [0032]    The axially movable interconnection between the forward and rearward body may be provided to enable relative movement during an activation sequence. 
         [0033]    An axially extending substantially annular space may be defined between the trigger and feature of, or associated with, the forward body. The at least one feature of, or associated, with the rearward body over which the trigger extends may be captively received within said substantially annular space. Thus, the feature of, or associated with, the rearward body may be captive within the space but provided with a degree of axial freedom to allow relative axial movement between the forward body and rearward body in use. 
         [0034]    The at least one feature of, or associated, with the rearward body may comprises a blocking formation of an interlock arrangement. 
         [0035]    The trigger may comprise a cup-shaped button. The button may have a rearward end which substantially closes the rear end of the housing. The button may have side portions which extend forwardly within the interior of the rearward body. 
         [0036]    The trigger may be arranged to have a snap fit engagement with the feature of, or associated with the forward body. The feature of, or associated with the forward body may be the latch (which may be connected to the forward body). 
         [0037]    The trigger may include a resilient catch for engaging the feature of, or associated with the forward body (for example the latch). The resilient catch may be arranged to disengage, in use, to allow forward movement of the trigger. For example the catch or corresponding surface may have a ramped profile to allow disengagement in only the forward direction. The resilient catch may comprise at least one sprung finger extending forwardly from the trigger. 
         [0038]    The resilient catch may be arranged to re-engage a feature of, or associated with the forward body (for example the latch) upon reaching an activation position. The resilient catch may re-engage at a second axial position relative to the forward body. Thus, the trigger may remain in an activation position after use to prevent re-activation and/or to provide a visual or tactile indication that the injector device has been activated. 
         [0039]    The trigger may be arranged to be retained in two axial positions. A first, rearward, position may correspond to the unactivated position. A second, forward, position may correspond to the activated position. The trigger may be prevented from rearward movement in both axial positions. As such, prior to use the trigger may only be moved forward of the first axial position (towards the second axial position) and after reaching the second axial position the trigger may be prevented from returning towards the first axial position. 
         [0040]    The rearward body may be arranged to prevent disengagement of the resilient catch until the rearward body is moved forward relative to the forward body. Such an arrangement provides a forced activation sequence whereby the injection device must be placed against the injection site before the trigger may be activated (in contrast to arrangements in which the two movements can be performed in either order or as a single motion). For example, the inner surface of the rearward body may be profiled such that the relative forward movement of the rearward body creates clearance between the resilient catch and the rearward body to allow disengagement of the resilient catch. For example the inner surface may include a tapered profile and/or may include an abutment surface which moves out of axial alignment with movement of the rearward body. 
         [0041]    The trigger may engage a feature of the latch. Thus, the feature of, or associated with, the forward body may comprise the latch. The latch may engage the forward body. For example the latch and forward body may resiliently engage with a snap-fit arrangement. This arrangement may further simplify assembly of the device as the injector may be assembled into forward and rearward sub-assemblies (each within the respective body portion) before the syringe is positioned within the device and the sub-assemblies brought together such that the latch and forward body positively engage. 
         [0042]    Thus, according to a further embodiment there is provided an injection device for delivering a dose of medicament from a syringe, the injection device comprising:
       a housing comprising a forward body and rearward body and wherein the forward and rearward body are axially moveably interconnected;   a drive mechanism arranged, in use, to be released from an energised position to deliver a dose from said syringe;   a latch arranged to hold the drive mechanism in the energised position; and   a trigger for releasing the latch; wherein   the latch is arranged to provide an interconnecting member between the forward body and rearward body.       
 
         [0048]    In an initial/pre-use condition the drive mechanism may be substantially contained within the rearward body. In an initial/pre-use condition the syringe may be substantially contained within the forward body. The forward body may contain a syringe carrier. The forward body may comprise or contain a moveable needle shroud. 
         [0049]    The latch may be arranged to engage the forward body. The trigger may be arranged to engage the latch. A feature of, or associated with, the rearward body may be restrained between the latch and trigger. 
         [0050]    The drive mechanism may comprise a plunger arranged for expelling medicament from the syringe. The plunger may be driven forwardly in use by the drive mechanism. The plunger may typically be arranged to engage a bung (or piston) within the syringe in use. The latch may engage a rearward portion of the plunger to hold the drive mechanism in the energised position. 
         [0051]    The blocking formation may be co-moulded with the rearward body. The blocking formation may be generally radially spaced from the inner surface of the rearward body. It will be appreciated that such a radially spaced arrangement may by supported by a plurality of supports which extend inwardly from the inner wall of the rearward body. The blocking formation may comprise an internal wall section within the bore of the rearward body. The blocking formation may comprise a radial flange. The blocking formation may be substantially annular. The blocking formation may be at least partially annular. The blocking formation may be generally concentric with the rearward body. Whilst the blocking formation may be formed as a substantially continuous circumferential feature, it will also be appreciated that the blocking formation could comprise a plurality of discreet sections or segments (for example circumferentially spaced around the inner wall of the rearward body). 
         [0052]    At least a portion of the internal wall extends generally axially relative to the rearward body. Such an axial portion may, for example, define a groove for retaining a spring. Alternatively, or additionally, an axial portion may be arranged to help align the rearward body and latch member. 
         [0053]    The rearward body may be biased rearwardly relative to the forward body. As such, the interlock arrangement may be normally engaged. The rearward body may need to be urged forward against said bias to move the interlock to the unlocked position. 
         [0054]    For example, the rearward and forward bodies are biased by a spring acting between the latch and blocking formation. The spring may be captive between the latch and blocking formation. The latch may comprise a rearwardly facing surface for engaging a first end of the spring. The blocking formation may comprise a forwardly facing surface for engaging a second end of the spring of the trigger. The spring may be a coil spring. Alternatively a moulded spring formation such as a spring arm may be provided acting between the body portions. 
         [0055]    The rearward body may be arranged to be held by the user during use. Thus, the rearward body may move forwardly relative to the forward body when the injection device is pressed against an injection site. 
         [0056]    Forward movement of the rearward body, in use, may be arranged to expose a portion of the trigger. Thus, the relative movement of the housing portions may provide a visual indication that the trigger is ready. 
         [0057]    The force required to enable the trigger to activate the drive mechanism may be selected such that the drive mechanism may only be released when the forward end of the injection device (for example the front end of the forward housing) is in contact with an injection site. Without the forward end of the housing in contact with an injection site no (or minimal) reaction force is available for forward pressure on the trigger to act against. Thus, it is not possible to transmit sufficient force to enable the trigger to release the latch and/or to disengage the trigger from its initial resilient engagement (for example with a feature of, or associated with, the forward body). 
         [0058]    The injection device may further comprise a cap. The cap may be arranged to close the forward end of the injector housing. The cap may be arranged to be removed prior to use by the user. Prior to removal the cap may obstructs forward movement of the rearward housing relative to the forward housing. 
         [0059]    According to a further aspect of the invention there is provided a method of assembling an injection device, comprising:
       providing a syringe containing medicament, a housing comprising a forward body and a rearward body, a drive mechanism and a trigger;   inserting the syringe, drive mechanism and forward housing into a forward end of the rearward housing; and   inserting the trigger into a rear end of the rearward housing.       
 
         [0063]    The method may comprise providing a syringe containing medicament, a housing comprising a forward body and a rearward body (which may be axially movably connected when the injection device is assembled), a drive mechanism and a trigger; inserting the syringe, drive mechanism and forward housing into a forward end of the rearward housing; and inserting the trigger into a rear end of the rearward housing. 
         [0064]    The method may further comprise forming a forward sub-assembly comprising at least the forward housing. The method may also comprise forming a rearward sub-assembly comprising at least the rearward assembly prior to assembling the injection device. For example, the forward sub-assembly may comprises components which are forward of the syringe and the rearward sub-assembly comprises components which are rearward of the syringe. The syringe may be placed between the forward and rearward sub-assemblies during assembly of the injection device. 
         [0065]    The rearward sub-assembly may include a latch (and the trigger may, for example, resiliently engage the latch). The forward sub-assembly may resiliently engage the latch. 
         [0066]    The forward body and rearward body may be axially moveable to provide an interlock arrangement. The method may further comprise temporarily preventing operation of the interlock during assembly. 
         [0067]    According to a further aspect of the present invention provides an injection device for delivering a dose of medicament from a syringe, the injection device comprising: 
         [0068]    a housing comprising a forward body and rearward body and wherein the forward and rearward body are axially moveably interconnected; 
         [0069]    a drive mechanism arranged, in use, to be released from an energised position to move a plunger to deliver a dose from said syringe; 
         [0070]    a latch arranged to hold the drive mechanism in the energised position; and 
         [0071]    a trigger for releasing the latch; wherein 
         [0072]    the injection device further comprises an injection complete indication arrangement comprising:
       a ribbon spring having a forward end coupled to the plunger and arranged to move with the plunger during activation,   the rear end of the ribbon spring being initially retained on a feature at the rear of the injection device, rearwardly of the latch, such that forward movement of the first end acts to unravel or unwind the ribbon spring; and wherein   the length of the ribbon spring is such that at a pre-determined point in the injection sequence the rear end of the ribbon passes beyond the latch member releasing the ribbon spring to return to a coiled or wrapped configuration to provide an audible and/or tactile indication.       
 
         [0076]    For example, upon recoiling the ribbon spring may strike an inner surface of the latch, drive spring, or other percussive surface within the injection device. The applicants have found that one convenient percussive surface may be part of an intermediate drive member provided as part of the drive mechanism. 
         [0077]    In such an arrangement it is important that any unintended audible and/or tactile emissions from the ribbon spring are reduced or prevented. Accordingly, the latch may be provided with a textured rearward face which may provide a cushioning and/or gripping effect between the latch and the ribbon spring. For example the rearward face may be overmoulded with a different material. Alternatively the latch may be formed by a co-injection moulding (also known as twin shot moulding) process which allows different materials to be moulded for a single component. 
         [0078]    The ribbon spring may be initially coiled around a surface or member associated with the trigger. Depending upon the particular configuration of the injection device (for example depending upon the space available) the ribbon spring could alternative be coiled around a surface or member associated with the latch. The surface may have a profile which is arranged to substantially match the shape of the ribbon spring coil. 
         [0079]    The trigger may include an internal member arranged to interact with the ribbon spring. The internal member may extend substantially transversely across the trigger. The member may have a chamfered side wall to provide an angled running surface for the ribbon spring. 
         [0080]    Whilst the invention has been described above, it extends to any inventive combination of the features set out above, or in the following description or drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0081]    The invention may be performed in various ways, and an embodiment thereof will now be described by way of example only, reference being made to the accompanying drawings, in which: 
           [0082]      FIG. 1  is an exploded view of an autoinjector according to a first embodiment showing the syringe, forward sub-assembly and rearward subassembly; 
           [0083]      FIG. 2  is an exploded view of the forward sub-assembly of  FIG. 1 ; 
           [0084]      FIG. 3  is an exploded view of the rearward sub-assembly of  FIG. 1 ; 
           [0085]      FIG. 4  shows two cross-sectional views of the first embodiment in a pre-use configuration; 
           [0086]      FIG. 5  shows two cross-sectional views of the first embodiment in a ready configuration (in which the injector has been pressed against an injection site); 
           [0087]      FIG. 6  shows two cross-sectional views of the first embodiment in an injection complete configuration (prior to removal of the device from the injection site); 
           [0088]      FIG. 7  shows two cross-sectional views of the first embodiment in a post-delivery configuration (after removal of the device from the injection site); 
           [0089]      FIG. 8 , shows partial cross-sectional views of alternate embodiments; 
           [0090]      FIG. 9 , shows an exploded partial cross-sectional view of an indicator arrangement for use in an embodiment; 
           [0091]      FIG. 10 , shows a partial cross-section showing the operational sequence of the indicator arrangement of  FIG. 9 ; 
           [0092]      FIG. 11 , shows a partial cross-section of a trigger button for use in an embodiment; and 
           [0093]      FIG. 12 , shows a view of an alternate trigger button for use in an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0094]    Front as used herein will be understood to refer to the end of the injector assembly (or components thereof) which, in use, are closest to the delivery needle delivery end of the injector (i.e. the end which is pointed at the skin). Rear as used herein will be understood to refer to the end of the pen injector assembly (or components thereof) which, in use, are furthest from the needle delivery end of the injector (i.e. the end which is pointed away from the skin). Forward and rearward will, likewise, be understood to refer to the directions orientated towards the front and rear of the injector assembly. 
         [0095]    Axial, radial and circumferential are used herein to conveniently refer to the general directions relative to the longitudinal direction of the injection device (or components thereof). The skilled person will, however, appreciated that these terms are not intended to be narrowly interpreted (and for example, the injection device may have a non-circular and/or irregular form). Typically, regardless of the chosen injector device external profile the cartridge or syringe will have a conventional generally cylindrical form and, as such, the longitudinal axis of the injection device will substantially coincide with (or be parallel to) the axial direction of the syringe. 
         [0096]    As seen in  FIG. 1 , an autoinjector  1  in accordance with an embodiment of the invention comprises a housing defined by a forward body  20  and a rearward body  30 . A syringe  5  of medicament is provided within the housing. The housing  20 ,  30  has a generally elongate tubular shape with a generally oval cross-sectional profile. 
         [0097]    The syringe is a conventional syringe having a bung or stopper  7  within its body (see  FIG. 4 ) and a needle at its forward end which may initially be protected (so as to remain sterile) by a removable needle shield  6 . The illustrated autoinjector  1  is generally intended to be a single use device (although the skilled person will appreciate that the invention is not limited to such devices) and, therefore, the exploded view of  FIG. 1  may typically represent a final assembly stage in which the syringe is placed into the housing (for example in a manufacturing facility). The end user would typically be provided with the autoinjector  1  preassembled around the syringe  5  (as for example, shown in  FIG. 4 ). As will be explained in further detail below, the autoinjector may conveniently be arranged during manufacture into a forward sub-assembly  2 , comprising the forward body  20  and those components which are initially forward of the syringe  5 , and a rearward sub-assembly  3 , comprising those components which are initially rearward of the syringe  5 . 
         [0098]    An exploded view of each of the forward and rearward sub-assemblies are shown in  FIGS. 2 and 3 . 
         [0099]    The forward sub-assembly  2  comprises the forward body  20  which is adapted to receive a cap  10  which closes the forward end of the autoinjector  1 . The cap  10  may include an internal formation, comprising rearwardly extending members  11 , arranged to engage the removable needle shield  6  of the syringe  5  such that removal of the cap  10  from the forward housing  20  during use also removes the removable needle shield  6  from the syringe  5 . Within the sub-assembly  2  there is a syringe carrier  55  for movably mounting the syringe within the forward body  20  to enable automatic needle penetration. It may be noted that prior to the removal of the cap  10 , the rearwardly extending members  11  of the cap  10  underlie spring fingers  56  of the syringe carrier  55 . This arrangement, thus prevents inward movement of the spin fingers  56  prior to removal of the cap  10  and, therefore, blocks unlatching of the syringe carrier  55  and prevents movement relative to the forward body  20 . 
         [0100]    A needle shroud  50  is also provided and arranged to shroud the needle after use (when the syringe  5  and syringe carrier  55  are in a forward position) to prevent needle stick injuries. The shroud  50  is activated by a pair of side-by-side shroud springs  51   a ,  51   b  carried on a spring guide  52 . The present application is not limited to any arrangement of the syringe carrier  55  and/or needle shroud  50 . As such the operation of the shroud  50  and carrier  55  is not described in detail. However, it may be noted that the arrangement substantially corresponds to the arrangement of the applicants&#39; earlier International Patent Application PCT/GB2011/052557. 
         [0101]    The rearward sub assembly  3  includes a trigger button  70  which is inserted into the rearward housing  30  from the rearward end so as to substantially close the rearward end of the injector housing. The trigger button  70  has a cup-like profile with side walls which are arranged to fit within (and be substantially concentric with) the rearward housing  30  and an end wall which closes the rear end of the housing. The trigger button  70  may comprise a single moulded part. However, for ease of manufacture and or assembly, in the illustrated embodiment the trigger button  70  includes a main body  72   a  and a rear cover  72   b . The main body  72   a  includes a pair of forwardly extending resilient arms  73   a  and  73   b  which are arranged to provide an engagement between the trigger button  70  and the injector  1 , as will be described in greater detail below. 
         [0102]    The rearward sub-assembly  3  also includes the drive mechanism  40 , which is inserted into the rearward housing  30  from the forward end. The drive mechanism  40  includes a plunger  42  which is arranged to engage the bung of the syringe in use. The plunger  42  is driven forwards in use by a pair of concentric drive springs  44   a  and  44   b  (although it will be appreciated that in other embodiments a single spring may be used) with an intermediate drive member  43  provided therebetween. A latch  60  is arranged concentrically around the drive springs  44 , intermediate member  43  and plunger  42 . The latch  60  is arranged to hold the plunger  42  against the bias of the springs  44  until the latch is released via the trigger button  70  (when both drive springs and the intermediate drive member are all released together). The latch  60  comprises a rear body portion  60   b  having a split cylinder profile and defining a latch aperture at its rear end and a forward connecting body portion  60   b . The forward body portion  60   a  has an external profile substantially corresponding to the interior profile of the rearward end of the forward housing  20 . The basic functional operation of the drive mechanism  40  is substantially as described, for example, in the applicants&#39; earlier International Patent Applications PCT/GB2011/051950 and PCT/GB2014/052276. 
         [0103]    The rearward assembly also includes an indicator arrangement  90  which will be described below with reference to  FIGS. 10 and 11 . 
         [0104]    An interlock spring  61  (the function of which is described below) is provided within the rearward sub-assembly  3  and is captive between the latch  60  and rearward housing  30 . 
         [0105]    The assembly of the autoinjector according to embodiments of the invention may be particularly convenient since only the trigger button  70  need be inserted from the rearward end of the rearward housing  30 . Further, with the components of the drive mechanism  40  inserted from the forward end of the rearward housing  30  the engagement of the latch  60  and trigger button  70  may enable the rearward sub assembly  3  to be initially assembled before being offered up to the forward sub assembly  2 . The components of the forward sub-assembly  2  may generally be arranged within the forward body housing  20  and the sub-assemblies snapped together with the syringe in place. All of the components of the forward sub-assembly  2  are therefore also connected to the forward end of the rearward sub-assembly  3 . In order to ensure that no unintended activation of the latch member  60  or interlock arrangement occurs during connection of the sub-assemblies  2 ,  3 , at least one aperture  35  may be provided in the rear housing  30  through which operation of the mechanism may be temporarily prevented. 
         [0106]    Further details of the autoinjector will now be described with reference to  FIGS. 4A and 4B  which show a cross section view of the injector in a fully-assembled, pre-use configuration. The two cross-sections are viewed from planes extending through the longitudinal axis of the autoinjector and substantially perpendicular to one another. As the housing of the autoinjector  1  has a substantially oval or elliptical profile it will be noted that  FIG. 4A  corresponds to a plane which is aligned with the major axis and  FIG. 4B  corresponds to a plane which is aligned with the minor axis. 
         [0107]    In  FIG. 4 , the forward and rearward sub-assemblies  2 ,  3  have been connected such that the housing fully encloses the syringe  5 . This is the configuration in which the autoinjector may typically be provided to the end-user. The cap  10  is in position and encloses the front end of the forward body  20  of the housing and the trigger button closes the rear end of the rearward body  30  of the housing. 
         [0108]    The connection between the sub-assemblies  2 ,  3  is provided via the latch  60  which it acts as an intermediate connecting member. The interconnection is a resilient snap fit connection which provides for ease of manufacture particularly for a single-use device (which the user will not be required to disassemble). Thus, it may be noted that the forward portion  60   a  of the latch  60  has been inserted into the rearward end of the forward housing  20 . The forward housing  20  and latch  60  are provided with cooperating engagement features  25 ,  65  to provide an automatic connection when brought together. In the illustrated embodiment the engagement features comprise a cut-out  25  in the forward housing and an outward projection  65  in the forward portion  60   a  of the latch. It may be noted that in the illustrated embodiment the rearward housing  30  extends over the cut out  25 , thereby covering the opening in the housing and preventing the user from disengaging the projection  65  to force separation of the housing body portions  20  and  30 . 
         [0109]    Within the rear sub-assembly  3 , the latch  60  is axially slidably connected to the rearward housing  30  via the trigger button  70 . The rearward housing  30  includes a radial flange  32  which extends from the interior bore of the rearward housing  30 . It will be noted that the radial flange is spaced radially apart from the inner surface of the rearward housing wall along part of its circumference as shown in  FIG. 4A  (whereas in the cross section of  4 B the flange includes connecting portions extending from the rearward housing). The resilient arms  73  of the trigger button  70  pass between the flange  32  and the inner side wall of the rearward housing  30  to engage the latch  60  (in other words the arms of the trigger button  70  pass over the flange  32  before engaging the latch  60 ). The forward ends of the resilient arms  73  are provided with inwardly projecting tabs which cooperate with corresponding outwardly projecting tabs  63  provided on a radially outer portion of the latch  60 . It will be noted that both the tabs  63  and  74  have an angled profile such that (as will be explained below) the resilient arms  73  may disengage and be deflected outwardly to allow forward movement of the trigger button  70  relative to the latch  60  but prevent disengagement in the reverse direction. 
         [0110]    Thus, it will be noted that an axially extending (and at least partially annular) space S is defined between interior of the forwardly extending arms  73  of the trigger button  70  and the exterior of the rearward portion  60   b  of the latch member  60 . The radially spaced portion of the flange  32  is captive within the space S. Also captive within the space S is the interlock spring  61 . The interlock spring  61  may, thus, act between a forward face of the flange  32  and a rearward face of the latch  60 . Each of the latch  60  and flange  32  may define a seat for receiving an end of the interlock spring (the seat may for example comprise an at least partial annular groove). As such, the interlock spring is arranged to bias the flange  32  rearwardly relative to the latch  60  and, as a result, biases the rearward housing  30  rearwardly relative to the forward housing  20 . 
         [0111]    In the initial position of  FIG. 4 , the flange  32  acts as a blocking formation preventing forward movement of the trigger button  70 . It will be noted that both a forward facing inner shoulder  75  and a forward facing end wall  76  of the button form forward surfaces which engage a corresponding rearward facing surface of the flange  32 . Thus, in the initial position the trigger button  70  is not free to move relative to the housing of the autoinjector and as such the flange  32  and rearward housing  30  provide an interlock arrangement. 
         [0112]    The firing sequence of the autoinjector according to the first embodiment will now be described with reference to  FIGS. 5 to 7  which correspond to the cross sections of  FIG. 4  in various stages of operation. 
         [0113]    After removal of the cap  10  (which also removes the removable needle shield  6 ) the user places the forward end of the autoinjector  1  against the injection site. The removal of the cap  10  enables the forward  20  and rearward  30  housing bodies to slide relative to one another (as previously the rearward end of the cap abutted the forward end of the rearward housing). After removal of the cap the device is ready to be fired but the interlock provided by the flange  32  will initially be held in place by the interlock spring  61 . Further it may be noted that as the user will grasp the rearward housing  30  and the trigger button  70  is blocked by the flange  32  of the housing any force applied to the button  70  prior to positioning the autoinjector against an injection site will only be reacted through the rear housing. In order for the trigger  70  to either be released from its engaged position on the latch  60  and release the latch  60  a reaction force must be transmitted through the injection site to the forward body  20  (or component associated with the forward body). 
         [0114]    Once the user presses the device against the skin the rearward body housing  30  is urged forwardly relative to the forward body housing  20 , by a distance X, as shown in  FIG. 5 . The reaction force through the forward body  20  allows the rearward body  30  to slides forward carrying with it the flange  32  (which therefore moves relative to the latch  60 ) to compresses the interlock spring  61 . This forward movement means that the blocking formation provided by the flange  32  is moved forward relative to the trigger button  70 . Thus the flange is spaced forwardly apart from the both the shoulder  75  and forward edge  76  of the trigger button  70 , by the same axial distance X. As the trigger button  70  is coupled to the latch  60 , it remains in a fixed axial position relative to the forward housing  20 . Thus, the trigger button  70  is partially revealed at the rear of the rearward housing  30  due to the relative forward movement of the rearward housing. It may be noted that in this position the outer surface of the rear portion  60   b  of the latch  60  remains radially adjacent to at least a portion  77  of the inner wall of the trigger button. In particular a plurality of radially outwardly projecting bosses  67  of the latch  60  are in abutment with the inner wall section  77  of the trigger button  70 . Thus, the rear portion  60   b  of the latch  60  is unable to deflect radially outwardly to release the plunger  42  to move under the bias of the drive spring  44 . 
         [0115]    With the interlocking arrangement moved to an unlocked position the user may now depress the trigger  70  in a forward direction. However, it is worth noting that in this particular embodiment the user&#39;s sequence of operation is not forced. In other words the user may either press the device against the injection site and then depress the trigger button or may alternatively hold the trigger button whilst pressing the device against the injection site (it is only a requirement that both activation steps are performed). 
         [0116]    As shown in  FIG. 6 , the trigger button  70  is displaced forwardly, by a distance Y, relative to the latch  60 . The force applied to the trigger button is reacted through the forward housing  20  and the latch  60  to allow the resilient arms  73  to deflect outwardly disengaging the tab  74  from the corresponding tab  63  of the latch and allowing the trigger to move forward. As the trigger moves forward the bosses  67  of the latch  60  move axially clear of the walls  77  on the interior of the trigger button  70 . Thus the split cylinder profile of the rearward portion  60   b  of the latch  60  may deflect radially outwardly to release then end of the plunger  42  to move forward under the force of the drive springs  44   a  and  44   b . Due to the initial stiction between the bung  7  and syringe  5  and/or the substantial incompressibility of the medicament initial movement of the plunger acts to move the syringe  5  and syringe carrier  55  forwardly to cause the needle of the syringe to penetrate the skin. Once the syringe reaches its forwardmost position the syringe carrier  55  reaches a stop position and continued movement of the plunger serves to dispense the medicament. 
         [0117]    It may be noted that the forward movement of the trigger button  70  causes the tabs  74  of the resilient arms  73  to ride over an overhang  64  formed forward of the tabs  63  on the latch  70 . Thus, the resilient arms  73  may be arranged to cause the tabs  74  to snap into engagement with the latch  60  in a second, axially forward position after activation. This both serves to retain the engagement between the rearward housing  30  and forward housing  20  (via the latch  60  and trigger  70  trapping the flange  32 ) and to ensure that the trigger button  70  remains in a forward position to indicate that the device has been used and cannot be re-fired. In this arrangement the forward body  20  and rearward body  30  of the housing may no longer be slidably engaged (rather they are axially fixed as the interlock spring  61  is fully compressed). 
         [0118]    During the activation movement of the syringe carrier  55 , the needle shroud  50  is released for forward movement relative to the forward housing  20  under the biasing of the shroud springs  51   a ,  51   b . Thus, as shown in  FIG. 7 , upon removal of the autoinjector from the injection site the needle shroud  50  is urged forward to automatically shroud the needle of the syringe  5  (which remains in the forward position). The shroud  50  is provided with latch features which lock the shroud in its fully forward position relative to the housing. 
         [0119]      FIG. 8A  illustrates the latch  60  and interlock arrangement according to an alternative embodiment. This embodiment works in the same manner as the first embodiment but includes additional optional features. In this embodiment the rearward body  30 ′ is provided with a profiled inner wall arranged to block radial deflection of the resilient legs of the trigger  70 ′ until the rearward body  30 ′ has moved forward to unlock the interlock. The inner wall includes radially inwardly extending abutments  38 ′ which are aligned with the tabs  74 ′ of the trigger  70 ′ so positively block disengagement between the trigger  70 ′ and the latch  60 ′ until the rearward body  30 ′ has moved forward against the interlock spring  61 ′. In contrast to the first embodiment, this embodiment ensures that the activation sequence must require the device  1  be pressed against the injection site before the trigger  70 ′ is depressed (in order to ensure that the blocking abutments  38 ′ clear the trigger  70 ′). 
         [0120]      FIGS. 8B and 8C  illustrates an alternate arrangement which operates in a similar manner to that of  FIG. 8A . The injector device of this embodiment includes a latch  160  and a trigger button  170  substantially as described above. The forward arms  173  of the trigger button  170  extend forwardly to engage the tabs  164 . In the pre-use position shown in the figure an external outwardly radially extending boss  176  abuts the inner surface of the rear body housing  130  and prevents outward deflection of the leg  174 . Thus, the button  170  cannot be urged forwardly to release the latch  160 . 
         [0121]    The housing  130  is provided with an aperture  138  rearwardly of the boss  176 . It will be appreciated that when the rear housing  130  moves forward as a result of the device being pressed against an injection site the aperture  138  moves into alignment with the boss  176  on the arm  173 . After the device has been presented to the injection site and the rear body  130  urged forwardly, the arm  173  is free to move radially outwardly and allows the button  170  to be depressed. Accordingly, the device of  FIG. 8B  may only be activated by sequentially pressing the device against the injection site prior to depressing th injection button  170 . 
         [0122]    The aperture  138  could alternatively be a recess but an aperture  138  may advantageously provide a visual indication of activation of the device by virtue of the bosses  176  being visible through the aperture  138  when they move into alignment. As such, the user is provided with an indication that the device is ready to be fired by means of the button  170 . 
         [0123]      FIG. 9  shows an exploded cross sectional view of an audible indication arrangement for use in embodiments. The indicator is typically arranged to provide an audible (and optionally tactile) indication which may be in the form of a “click”. The indication may be arranged to be triggered as the plunger  42  reaches a fully forward position to provide the user with an indication that a full dose of medicament has been delivered and, therefore, that the autoinjector may be removed from the injection site. The indicator comprises a flexible member in the form of a ribbon spring  90  (which may be a constant force spring). A forward end  92  of the ribbon is attached to the rearward end of the plunger  42  and the rearward portion  94  of the ribbon is in the form of a coil. 
         [0124]    As best seen in  FIG. 10A , which shows the unfired configuration, indicator  90  and end of the plunger  42  are initially positioned rearwardly of the latch member  60 . As the trigger is used to activate the drive mechanism ( FIG. 10B ) the plunger  42  is released from the latch  60  and moves forwardly. It may be noted that during activation the transverse member  79  which extends across the rear of the trigger button  70  may engage the end of the plunger  42  to help urge it forwards through the aperture of the latch. In other embodiments this function may be provided by a boss projecting from the rearward inner surface of the trigger button (see for example  FIG. 8 ). The provision of the transverse member  79  enables additional space to be provided between the rearward surface of the latch  60  and the forward surface of the end of the trigger  70  to accommodate the coil  94 . 
         [0125]    After release of the drive mechanism the plunger  42  moves forward within the autoinjector relative to the latch  60  and draws with it the forward end  92  of the indicator. The coil  94  at the rear of the indicator does not fit through the aperture of the latch  60  (and additionally may be wrapped around the transverse member  79 ) so is not carried forwardly but unravels. At, or typically slight prior to, the plunger  42  reaching its forwardmost position the indicator  90  will reach its fully unravelled arrangement. Thus, the rearwardmost end of the indicator will pass through the aperture of the latch  60 . Once the rearward end of the indicator is forward of the rearward portion of the latch  60 , the indicator will recoil due to its resilience. Thus, as shown in  FIG. 10C  the indicator will spring forwards towards the end of the plunger  42  and will strike a convenient surface of the autoinjector (which may for example be an inner surface of the latch, the inside of the drive spring or an intermediate drive member) to provide an audible click. In the illustrated embodiment the surface against which the coil  94  strikes is the intermediate drive member  43  (for example the rearward facing end surface of the intermediate member  43 ). 
         [0126]    In order to increase the effectiveness of audible indications it is desirable to eliminate or minimise any intermediate noises other than the final release/striking of the indicator  90 . One means of reducing such noises is to provide a suitable surface to the rear of the latch member over which the indicator  90  will travel. For example, an overmoulding or comoulded portion  69  may be provided on the rear surface. The overmoulded or comoulded portion  69  may use a material which is specifically chosen for acoustic reasons (whilst the material of the latch itself may be primarily defined by its function). For example a material for the portion  69  may have different shore hardness to the material of the main body of the latch  60 . For example, an appropriate shore hardness may be selected to cushion any unwanted contacts and prevent unwanted “clicks”. 
         [0127]    Further, as shown in  FIG. 11  the transverse member may be provided with at least one chamfered side wall  79   a ,  79   b  to provide a smooth running surface for the indicator  90 . The chamfer may be single sided  79   a  or double sided  79   b . Alternatively, as shown in  FIG. 12  the transverse member  79 ′ may have a profile which is intended to substantially match the curvature of the coil of the indicator  90 . Thus, the rear section may  94  of the indicator may be coiled around the member  79 ′ and have a smooth running surface across the member. 
         [0128]    Although the invention has been described above with reference to a preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.