Abstract:
An automatic injection device for delivering a dose from a medicine containing syringe includes a housing for containing the syringe, a force applicator for applying a force to eject medicine from the syringe, a trigger coupled to the force applicator for releasing the force applicator to cause an injection, a boot covering a needle attached to the syringe to protect and maintain sterility of the needle, and a mechanical interlock. The mechanical interlock prevents actuation of the trigger prior to removal of the boot. When the boot is removed, the mechanical interlock allows for actuation of the trigger.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an automatic injection device for delivering a dose of medicine to a user from a medicine containing syringe. 
       BACKGROUND 
       [0002]    Automatic injection devices are routinely used in the medical field to deliver a measured dose of medicine to a user. Due to their user friendly design, they can be safely used by patients for self-administration, although in some circumstances they may be used by trained personnel. 
         [0003]    A typical automatic injection device comprises several parts which may include; a syringe containing medicine, a needle fixed to the end of the syringe, a firing mechanism including a spring (or possibly other drive means such as an electric motor or gas drive means), and a trigger. The spring may be preloaded, or may be set using a dose setting mechanism such as a dial. The firing mechanism is activated by the trigger and forces the medicine through the needle and into the user. A mechanical lock may be provided to prevent the trigger from being accidentally pressed. This could be, for example, simply a catch that must be moved out of the way in order to access the trigger. 
         [0004]    Single use, disposable automatic injection devices are delivered to end users in an assembled state, with a medicine syringe contained within the device housing and a needle fixed to the end of the syringe. In order to ensure sterility of the needle, the projecting end of the needle is contained within an rubber or elastomer “boot”. Typically, the boot forms an interference fit around the narrowed end portion of the syringe body. The tip of the needle may penetrate the end of the boot. In the case of re-useable automatic injection devices, an end user must typically open the housing and press a new single-use syringe into position. The single-use syringe will have a needle and boot already in place. 
         [0005]    The injection device may also comprise a boot remover to allow the end user to easily and safely remove the boot and thereby expose the needle. Typically, the boot remover is fitted around or inside the proximal end of the device prior to insertion of the syringe into the housing. When the syringe is pressed into the housing, the boot protecting the needle is captured by the boot remover, i.e. snaps into place within the boot remover. A needle shield may be further provided around the needle, such that the needle remains protected even after the boot has been removed. This is relevant to so-called “auto-injectors” which, in addition to driving the medicine through the needle, perform an initial step of inserting the needle through the skin using the force provided by the injection spring (or possible a secondary spring). 
         [0006]    When a single use automatic injection device is to be used, a user should first remove the boot remover and boot to expose the needle. NB. the needle remains surrounded by the needle shield at least in the case of an auto-injector. The user will then release the mechanical lock, such that the trigger can be pressed. The user can then place the auto-injector against the surface of the skin and press the trigger to push the needle through the skin and force the medicine through the needle. In the case of an auto-injector, a carriage and carriage-return spring may cause the needle to be returned to a position within the needle shield. 
         [0007]    A problem with single use automatic injection devices occurs when a user forgets to first remove the boot, and, instead, operates the trigger with the boot still in place. This is particularly likely in the case of an auto-injector, where the needle and boot are not readily visible. If the boot is not removed before firing, no drug is delivered to the user. Furthermore, since the medicine will now be under pressure, there is a risk that the user may inadvertently empty the syringe contents into the air if, when realising their error, they subsequently remove the boot. 
         [0008]    A user may not have an abundance of medicine and so waste may be a serious issue. Waste may also be undesirable due to cost implications: some medicines can be extremely expensive. Therefore, there exists a need to provide an automatic injection device that overcomes the problem of a device being fired prior to removal of a boot. 
       SUMMARY 
       [0009]    It is an object of the present invention to provide an automatic injection device that cannot be fired prior to removal of a boot. 
         [0010]    According to an aspect of the present invention there is provided an automatic injection device for delivering a dose from a medicine containing syringe. The automatic injection device comprises a housing for containing the syringe, a force applicator for applying a force to eject medicine from the syringe, a trigger coupled to the force applicator for releasing the force applicator to cause an injection, a boot covering a needle attached to the syringe to protect and maintain sterility of the needle, and a mechanical interlock. The mechanical interlock prevents actuation of the trigger prior to removal of the boot. When the boot is removed, the mechanical interlock allows for actuation of the trigger or commencement of an actuation sequence. 
         [0011]    The present invention overcomes problems associated with current automatic injection devices, where a user can accidentally fire the automatic injection device with the boot still in place. This can result in wasted medicine, which may be expensive to replace. The present invention overcomes this by providing a mechanical interlock, such that an automatic injection device cannot be fired prior to removal of the boot. 
         [0012]    As an option the automatic injection device comprises a boot remover for removing the boot. The boot remover may be formed integrally with the boot. Alternatively the boot and boot remover are formed as separate discrete components, and configured such that the boot is locked into the boot remover upon insertion of the syringe into the housing. 
         [0013]    In a first embodiment of the present invention, the mechanical interlock comprises a boot remover, wherein the boot remover is configured such that removal of the boot remover from the housing both removes the boot from the needle and facilitates access to the trigger. As an option the mechanical interlock comprises a cover attached to a distal end of the housing. The cover is locked in place when the boot remover is attached to the housing, and is removable from the housing to expose the trigger only after removal of the boot remover and boot. As another option the cover is coupled to the distal end of the housing by one or more flexible latches. When the boot remover has been removed, the latches may be disengaged and the cover removed. The boot remover may extend to cover the flexible latches when the boot remover is attached to the housing. 
         [0014]    In a second embodiment of the present invention, the housing comprises a first part for containing the syringe and a second part for attachment to the first part by a user. The mechanical interlock is configured to remove the boot upon coupling together of the first and second parts. As an option the trigger is provided on the second part. The first and second parts may be coupled together by relative axial motion of the parts, for example, by engaging complimentary screw threads formed on the first and second parts. The mechanical interlock may comprise a rod coupled to the first part of the housing and slideable relative thereto in an axial direction. The rod has a distal end that engages with the second part in order to axially displace the rod upon coupling together of the first and second parts. The rod has a proximal end that is coupled to the boot in order to remove the boot. As an option, the rod is coupled to the boot by way of the boot remover. As an option, the first part of the housing defines a channel within which the rod is slideably mounted. As an option the boot remover may comprise a peg that protrudes into the channel for engaging with the rod upon coupling together of the first and second parts in order to push the boot remover off of the housing. Alternatively, the housing comprises a spring coupled to the rod in order to return the proximal end of the rod into the channel upon disconnection of the first and second parts. 
         [0015]    In a third embodiment of the present invention, the mechanical interlock comprises a rod coupled to the boot remover and a trigger lock engaging with the trigger and with said rod. The mechanical interlock is configured such that removal of the boot results in rotation of the rod thereby releasing the trigger lock. The rod may have a helical track extending axially therealong and a peg is provided on the boot remover to engage with said track such that the rotation of the rod is caused by the axial motion of the peg as the boot remover is removed. As an option the boot remover may comprise a key and the housing may comprise an axially extending track. The key and axially extending track are arranged such that the key engages with the axially extending track to prevent rotation of the boot remover prior to removal of the boot remover. 
         [0016]    In a fourth embodiment of the present invention, the mechanical interlock comprises a rod coupled to the boot remover and a trigger lock engaging with the trigger and with said rod. The mechanical interlock is configured such that removal of the boot results in rotation of the rod thereby releasing the trigger lock. A torsion spring is coupled between the rod and the hosing to provide a rotational bias to the rod. A latch is provided on the boot remover to prevent rotation of the rod until the boot remover has been removed. As an option the rod comprises a pin for engaging with the latch. As an option the boot remover may comprise a key and the housing may comprise an axially extending track. The key and axially extending track are arranged such that the key engages with the axially extending track to prevent rotation of the boot remover prior to removal of the boot remover. 
         [0017]    In a fifth embodiment of the present invention, the housing comprises a first part for containing the syringe and a second part for attachment to the first part by the user. The mechanical interlock is provided by the boot remover such that, when attached to the first and second parts, the boot remover holds the first and second parts in a non-useable configuration and removal of the boot remover allows the first and second parts to be brought together into a useable configuration. As an option, there is provided a hinge that rotatably couples the first and second parts together. The mechanical interlock may further comprise a trigger lock for preventing actuation of the trigger when the device is in the non-useable configuration whilst allowing actuation of the trigger when the device is in the useable configuration. The trigger lock may comprise an elongate plate mounted to the first part by a pivot axle. There may be a biasing mechanism acting on the plate such that, when in the non-usable configuration, the plate prevents actuation of the trigger. When the device is brought together into a useable configuration, the plate is rotated about said axle to free the trigger. 
         [0018]    Further aspects of the present invention are set out in the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  shows a cross-section through an auto-injector according to a first embodiment; 
           [0020]      FIG. 2  shows a cross-section through the auto-injector of  FIG. 1  with a boot remover partially removed; 
           [0021]      FIG. 3  shows a cross-section through the auto-injector of  FIG. 1  with a trigger cover partially removed; 
           [0022]      FIG. 4  shows a perspective view of an auto-injector according to a second embodiment; 
           [0023]      FIG. 5  shows a perspective view of the auto-injector of  FIG. 4  with a boot remover partially removed; 
           [0024]      FIG. 6  shows a close up perspective view of a proximal end of the auto-injector of  FIG. 4 ; 
           [0025]      FIG. 7  shows a further close up perspective view of a proximal end of the auto-injector of  FIG. 4  with boot remover removed; 
           [0026]      FIG. 8  shows a cross-section through an auto-injector according to a third embodiment; 
           [0027]      FIG. 9  shows a perspective view of a boot remover according to an option of the third embodiment; 
           [0028]      FIG. 10  shows a side view of the boot remover of the third embodiment engaging with a rod; 
           [0029]      FIG. 11  shows a close up perspective view of the distal end of the auto-injector of  FIG. 8 ; 
           [0030]      FIG. 12  shows a further close up perspective view of the distal end of the auto-injector of  FIG. 8 ; 
           [0031]      FIG. 13  shows a further close up perspective view of the distal end of the auto-injector of  FIG. 8 ; 
           [0032]      FIG. 14  shows a cross-section through an auto-injector according to a fourth embodiment; 
           [0033]      FIG. 15  shows a perspective view of a boot remover engaging with a rod according an example of the fourth embodiment; 
           [0034]      FIG. 16  shows up perspective view of the distal end of the auto-injector of  FIG. 14 ; 
           [0035]      FIG. 17  shows a side view of the distal end of the auto-injector of  FIG. 14 ; 
           [0036]      FIG. 18  shows a cross-section through an auto-injector according to a fifth embodiment; 
           [0037]      FIG. 19  shows a cross-section through the auto-injector of  FIG. 18 , with a boot remover removed; 
           [0038]      FIG. 20  shows a cross-section through the auto-injector of  FIG. 18 , and a side view of said auto-injector; 
           [0039]      FIG. 21  shows a perspective view of a latch according to an option of the fifth embodiment; 
           [0040]      FIG. 22  shows a cross-section view of a trigger lock according to an option of the fifth embodiment; 
           [0041]      FIG. 23  shows a cross-section view of a trigger lock according to an option of the fifth embodiment in a different configuration; 
           [0042]      FIG. 24  shows a perspective view of a trigger lock according to an option of the fifth embodiment; 
           [0043]      FIG. 25  shows a close up perspective view of the auto-injector of  FIG. 18 ; 
           [0044]      FIG. 26  shows a perspective view of the auto-injector of  FIG. 18 ; 
           [0045]      FIG. 27  shows a cross-section view of an auto-injector according to a sixth embodiment; 
           [0046]      FIG. 28  shows a cross-section view of the auto-injector of  FIG. 27  with a boot remover removed; 
           [0047]      FIG. 29  shows a cross-section view of the auto-injector of  FIG. 27  when the trigger is pressed and the device is not pressed against the skin; 
           [0048]      FIG. 30  shows a cross-section view of the auto-injector of  FIG. 27  being activated; 
           [0049]      FIG. 31  shows a cross-section view of an auto-injector according to a seventh embodiment; 
           [0050]      FIG. 32  shows a cross-section view of the auto-injector of  FIG. 31  with a boot remover removed; 
           [0051]      FIG. 33  shows a cross-section view of the auto-injector of  FIG. 31  when pressed against the skin; 
           [0052]      FIG. 34  shows a cross-section view of the auto-injector of  FIG. 31  being activated; 
           [0053]      FIG. 35  shows a cross-section view of an auto-injector according to an eighth embodiment; 
           [0054]      FIG. 36  shows a cross-section view of the auto-injector of  FIG. 35  following removal of a boot remover; 
           [0055]      FIG. 37  shows a cross-section view of the auto-injector of  FIG. 35  when pressed against the skin; 
           [0056]      FIG. 38  shows a cross-section view of the auto-injector of  FIG. 35  with the trigger depressed; 
           [0057]      FIG. 39  shows alternative views of a locking mechanism; 
           [0058]      FIG. 40  shows a cross-section view of an auto-injector according to a ninth embodiment; 
           [0059]      FIG. 41  shows a cross-section view of the auto-injector of  FIG. 40  following removal of a boot remover; 
           [0060]      FIG. 42  shows a cross-section view of the auto-injector of  FIG. 40  following removal of a boot remover; 
           [0061]      FIG. 43  shows a cross-section view of the auto-injector of  FIG. 40  when pressed against the skin; 
           [0062]      FIG. 44  shows a cross-section view of the auto-injector of  FIG. 40  being activated; 
           [0063]      FIG. 45  shows a cross-section view of the auto-injector of  FIG. 40  after activation; 
           [0064]      FIG. 46  shows a cross-section view of an auto-injector according to a tenth embodiment; 
           [0065]      FIG. 47  shows a cross-section view of the auto-injector of  FIG. 46  with the boot remover removed; 
           [0066]      FIG. 48  shows a cross-section view of the auto-injector of  FIG. 46  when pressed against the skin; 
           [0067]      FIG. 49  shows a further cross-section view of the auto-injector of  FIG. 46  when pressed against the skin; 
           [0068]      FIG. 50  shows a further cross-section view of the auto-injector of  FIG. 46 ; 
           [0069]      FIG. 51  shows a cross-section view of a locking mechanism according to an eleventh embodiment; 
           [0070]      FIG. 52  shows a cross-section view of the locking mechanism after disengagement; 
           [0071]      FIG. 53  shows a cross-section view of the locking mechanism with the trigger depressed; 
           [0072]      FIG. 54  shows a perspective view of a release element according to a twelve embodiment of the present invention; 
           [0073]      FIG. 55  shows a perspective view of a trigger according to a twelfth embodiment of the present invention; 
           [0074]      FIG. 56  shows a cross section of the twelfth embodiment; 
           [0075]      FIG. 57  shows a cross section of the twelfth embodiment; 
           [0076]      FIG. 58  shows a cross section of the twelfth embodiment; 
           [0077]      FIG. 59  shows a boot remover of the twelfth embodiment; 
           [0078]      FIG. 60  shows a top cross sectional view and side view of an auto-injector according to thirteenth embodiment of the present invention; 
           [0079]      FIG. 61  shows a top cross sectional view and side view of the auto-injector of  FIG. 60  with the boot remover remvoed; 
           [0080]      FIG. 62  shows a top cross sectional view and side view of the auto-injector of  FIG. 60  when pressed against the skin; 
           [0081]      FIG. 63  shows a top cross sectional view and side view of the auto-injector of  FIG. 60  when activated; 
           [0082]      FIG. 64  shows a cross sectional view of an auto-injector according to fourteenth embodiment of the present invention; 
           [0083]      FIG. 65  shows a cross sectional view of the auto-injector of  FIG. 64  with the boot remover removed and the button depressed; 
           [0084]      FIG. 66  shows a cross sectional view of the auto-injector of  FIG. 64  when pressed against the skin; 
           [0085]      FIG. 67  shows a cross sectional view of the auto-injector of  FIG. 64  when the button is depressed and the device is pressed against the skin; and 
           [0086]      FIG. 68  shows a cross sectional view of the auto-injector of  FIG. 64  once activated. 
       
    
    
     DETAILED DESCRIPTION 
       [0087]    Embodiments to be described aim to provide an automatic injection device that cannot be fired until a boot protecting the syringe needle has been removed. The aim is to prevent the problem of wasted medicine and user frustration that may otherwise occur. Embodiments are described in the context of an auto-injector, that is an automatic injection device that has a spring or springs that not only drives the injection of medicine, but also pushes the needle into the patient&#39;s skin. Such a device is referred to as an auto-injector. However, the skilled person will appreciate that the approach may also be applied to automatic injection devices that only drive medicine delivery and do not push the needle into the skin. 
         [0088]    With reference to  FIGS. 1 to 3 , there will now be described a first embodiment, referred to here as the “enclosed button auto-injector”. To assist with an understanding of this and further embodiments described below it is helpful to define a “proximal” end of the auto-injector as being the end that is closest to the patient&#39;s skin when in use, and a “distal” end as being the end furthest from the patient&#39;s skin. 
         [0089]      FIG. 1  shows a cross-sectional view of an enclosed button auto-injector  101  comprising a needle  102 , syringe  103 , boot remover  104 , boot  111 , trigger  105 , trigger cover  106 , and housing  107 . The auto-injector has a proximal end  112  and a distal end  113 . The housing  107  houses the needle  102  for piercing a user&#39;s skin, and the syringe  103  for containing medicine. Activation of the trigger  105  actuates a firing mechanism  108 . The firing mechanism  108  drives the needle into the skin, and forces the medicine through the needle and into the user. Although not described in detail, the device also includes a carriage  114  and carriage return spring  115  within which the syringe  103  is mounted. 
         [0090]    To prevent the user from accidentally activating the trigger  105 , the trigger cover  106  is removably attached to the housing  107  such that it covers the trigger  105 . This provides a physical barrier that prevents the user from accidentally activating the trigger  105 . Any suitable mechanical interlock for preventing activation of the trigger  105 , such as a trigger lock, may be used instead of the trigger cover  106 . When the user wishes to use the enclosed button auto-injector  101 , he or she must first remove the trigger cover  106  in order to access the trigger  105 . 
         [0091]    The trigger cover  106  may be secured to the housing  107  by any suitable connection type. For example, in  FIGS. 1 and 2  the trigger cover  106  has ridges  109  for slotting into shoulders  110  formed in the housing  107 . 
         [0092]    The boot  111  is arranged to prevent contamination of the needle  102 . The boot remover  104  is connected to the boot, and facilitates removal of the boot. The boot remover  104  extends over the outer surface of the housing  107  and over the ridges  109  of the trigger cover  106 . By doing so, the boot remover  104  prevents any lateral displacement of ridges  109 , and therefore prevents the ridges  109  from being moved out of the shoulders  110 , preventing removal of the trigger cover  106 . The boot remover  104  may provide support to the ridges  109 , holding them in place within the shoulders  110 . 
         [0093]      FIG. 2  shows the boot remover  104  partially removed from the housing  107 , no longer preventing the ridges  109  from lateral movement. The ridges  109  are pre-stressed and splay outwardly upon removal of the boot remover  104  to disengage from the shoulders  110 . In an alternative configuration, the ridges may by displaced outwardly by a separate biasing mechanism, e.g. a spring.  FIG. 3  shows the boot remover  104  totally removed from the device  101 , and the trigger cover  106  partially removed. As a result, the trigger  105  is now exposed. 
         [0094]    This arrangement forces a user to perform the step of removing the boot  111  using the boot remover  104  before pressing the trigger  105 . By doing so, accidentally activating the enclosed button auto-injector  101  while the boot  111  is still in place is not possible. 
         [0095]    This example is but one of many ways in which the boot remover  104  can prevent removal of the trigger cover  106 . For example, the boot remover  104  may act as an interlock to a button, where the button may be used to facilitate removal of the trigger cover  106 . 
         [0096]    With reference to  FIGS. 4 to 7 , there will now be described a second embodiment, referred to here as the “embedded rod auto-injector”. 
         [0097]      FIG. 4  illustrates an embedded pin or rod auto-injector  201 , comprising a firing mechanism housing  202 , a syringe housing  203  containing a syringe, needle and boot (not shown), and a boot remover  204 . Initially, the firing mechanism housing  202  is separate from the syringe housing  203 , and therefore actuation of the firing mechanism within the firing mechanism housing  202  will not actuate the injection. On assembly of the embedded rod auto-injector  201 , a lip  205  on the firing mechanism housing  202  displaces a rod  207  residing in a channel  206  in the syringe housing  203 . Assembly may be achieved by screwing the firing mechanism housing  202  into the syringe housing  203 . 
         [0098]      FIG. 5  shows the firing mechanism housing  202  fully engaged with the syringe housing  203 . The lip  205  has displaced the rod  207 , which has in turn displaced the boot remover  204 , removing it from the syringe housing  203 . As a result, the boot will be removed before the auto-injector  201  can be actuated.  FIG. 6  shows a close up view of the rod  207  displacing the boot remover  204 . 
         [0099]    The boot remover  204  may have a peg  208  that protrudes into the channel  206  for engaging with the rod, shown in  FIG. 7 . This arrangement ensures that the boot remover  204  can be displaced and ejected without subsequent protrusion of the rod  207 , which may otherwise lead to an obstruction when administering an injection.  FIG. 7  also shows a track  209  running along the inner surface of the syringe housing  203 , for receiving a ridge (not shown) formed on the boot remover  204 , ensuring proper alignment of the boot remover  204 . 
         [0100]    The embedded rod auto-injector  201  may comprise a spring located within the syringe housing  203  that acts to push the rod  207  backwards (toward firing mechanism housing  202 ) in order to ensure that the rod  207  returns from the protruding position upon disassembly. This is relevant in particular to a re-useable device. 
         [0101]    With reference to  FIGS. 8 to 13 , there will now be described a third embodiment, referred to here as the “helical linkage auto-injector”. 
         [0102]      FIG. 8  shows a cross-section of a helical linkage auto-injector  301 , comprising a boot remover  302 , boot  312 , housing  303 , trigger  304  and trigger lock  305 . 
         [0103]    The boot remover  302  has a radially projecting peg  306  and a key  307 . The housing  303  has an axial track (not shown) for receiving the key  307 , and is arranged to prevent rotation of the boot remover  302  while the key  307  is engaged with the axial track. Any number of ways can be used to prevent rotation of the boot remover  302  while it is attached to the helical linkage auto-injector  301 . The use of a key  307  and axial track is just one of many possible alternatives. 
         [0104]    The housing  303  contains a rod  308  with a helical track  310  (not shown in  FIG. 8 ) running around its circumference. The helical track  310  is arranged to receive the peg  306 . Note that when the peg  306  is engaged with the helical track  310 , the connection between the peg  306  and helical track  310  may be sufficient to prevent rotational movement of the boot remover  302 . The key  307  and axial track may then not be required. At one end of the rod  308  there is a trigger lock  305  for preventing actuation of the trigger  304 . The trigger lock  305  features is shaped such that, in one orientation of the rod  308  and trigger lock  305 , the trigger  304  cannot be activated, but, when the rod and trigger lock are rotated 180 degrees, the trigger  304  can be activated. The shape may be a stepped shape for example.  FIG. 9  shows a perspective view of the boot remover  302 , showing the peg  306  and a key  307 , whilst  FIG. 10  shows the peg  306  engaging with the helical track  310  on the rod  308 . 
         [0105]      FIG. 11  shows a close up view of the trigger  304  and trigger lock  305 . The trigger  304  has a lip  311  that abuts the trigger lock  305 , and prevents downward motion of the trigger  304 . When the boot  312  is removed using the boot remover  302 , the axial motion of the peg  306  causes the rod  308  to rotate due to the interaction between the peg  306  and helical track  310 . The trigger lock  305 , being connected to the rod  308 , also rotates. 
         [0106]      FIG. 12  shows a close up view of the trigger  304  and trigger lock  305  after the boot remover  302  has been removed. The trigger lock  305  no longer blocks the path of the lip  311 , allowing the trigger  304  to be freely pushed downwards, activating the auto-injector.  FIG. 13  shows the trigger  304  following actuation. 
         [0107]    The helical linkage auto-injector  301  cannot be fired while the boot remover  302  is still in place. As the boot remover  302 , along with the boot  312 , is removed, the trigger lock  305  is disengaged. A user can then press the trigger  304  and activate the auto-injector  301 . 
         [0108]    With reference to  FIGS. 14 to 17 , there will now be described a fourth embodiment, referred to here as the “spring loaded lock auto-injector”. 
         [0109]      FIG. 14  shows a cross sectional view of a spring loaded lock auto-injector  401 , comprising a boot remover  402 , boot  412 , housing  403 , trigger  404  and trigger lock  405 . The boot remover  402  has a latch  406  and one or more keys  407  (not shown). The housing has one or more a linear, axially extending tracks (not shown) for engaging with the keys  405 . This arrangement restricts rotation of the boot remover  402  prior to removal. Any number of ways can be used to prevent rotation of the boot remover  402  while attached to the spring loaded lock auto-injector  401 . 
         [0110]    The housing  403  contains a rod  408  with a pin  409  for engaging with the latch  406 . The housing  403  also contains a torsion spring  410  that connects to the rod  408 , providing a torque to the rod  408  when the rod is rotationally displaced from a given orientation. At one end of the rod  408  there is a trigger lock  405  for preventing actuation of the trigger  404 . The trigger lock  405  is shaped such that, in one orientation, the trigger  404  cannot be activated, but, when the trigger lock  405  is rotated by 180 degrees, the trigger  404  can be activated. This may be facilitated by a stepped feature formed in the trigger lock  405 . In the auto-injector&#39;s unarmed state, the rod is rotationally displaced such that a torque is applied to the rod  408  by the torsion spring  410 , the rod  408  being held in place by the pin  409  being engaged with the latch  406 . A perspective view of the latch and pin is shown in  FIG. 15 . 
         [0111]      FIG. 16  shows close up view of the trigger  404  and trigger lock  405  in an unarmed position. The trigger  404  has a lip  411  that abuts the trigger lock  405 , preventing downward motion of the trigger  404 . 
         [0112]    When the boot remover  402  is removed, the pin  409  disengages with the latch  406 , allowing the rod  408  and trigger lock  405  to rotate due to the torque applied by the torque spring  410 . When the rod  408  and trigger lock  405  reach their final position, the trigger lock  405  no longer prevents the trigger  404  from being pressed. 
         [0113]      FIG. 17  shows a close up of the trigger  404  and trigger lock  405  in an armed position. It will be apparent that rotation of the rod  408  has caused the trigger lock  405  to be rotated such that it no longer blocks the path of the lip  411 , allowing the trigger  404  to be freely pushed downwards, activating the auto-injector  401 . 
         [0114]    The trigger lock in the helical linkage auto-injector and the spring loaded lock auto-injector have been described as having a trigger lock ( 305 ;  405 ) that is arranged to abut the trigger, preventing axial motion of the trigger ( 304 ;  404 ). It is noted that other trigger prevention mechanisms may be used instead. For example, the trigger lock may be a cover that prevents access to the trigger, wherein rotation of the rod causes the cover to move to into a position such that it does not prevent access to the trigger. 
         [0115]    With reference to  FIGS. 18 to 26 , there will now be described a fifth embodiment, referred to here as the “hinged auto-injector”.  FIG. 18  shows a cross sectional view of such a hinged auto-injector  501 , comprising a boot remover  502 , boot  514 , syringe housing  503 , trigger  504 , firing mechanism housing  505 , hinge  506  and needle  507 . When folded, the hinged auto-injector  501  is in an unarmed position, with the boot remover  502  covering both the trigger  504  and the needle  507 . In order to use the hinged auto-injector  501 , the boot remover  502  must first be removed. The hinged auto-injector  501  can then be unfolded into a firing position. 
         [0116]      FIG. 19  shows the hinged auto-injector  501  with the boot remover  502  and boot  513  removed, but still in the folded configuration.  FIG. 20  shows the hinged auto-injector  501  unfolded, as both a cross-section and in plan. A latch  508  may be used to lock the hinged auto-injector  501  in the unfolded position. 
         [0117]      FIG. 21  shows a close up view of the latch  508 . Note that the latch  508  may reside on either the firing mechanism  505  or the housing  503 . Other suitable mechanisms for locking the hinged auto-injector  501  in position will be readily apparent. 
         [0118]    The hinged auto-injector  501  may further comprise a mechanism that prevents actuation of the trigger  504  before the hinged auto-injector  501  has been fully unfolded. An example of such a mechanical interlock comprises a trigger lock comprising an elongate plate  509 , shown in  FIG. 22 . The elongate plate  509  features a boss  510  for preventing downward motion of the trigger  504 :  FIG. 22  illustrates the “locked” position. The elongate plate  509 , upon unfolding of the auto-injector, is caused to pivot about a central pivot axis from a position in which downward motion of the trigger  504  is prevented into one in which downward motion of the trigger is possible.  FIG. 23  shows the elongate plate  509  in an unlocked position after unfolding of the device. 
         [0119]    The elongate plate  509  is shown in more detail in  FIG. 24 . A front end of the trigger lock features an angled face  511  that acts as a spring and holds the elongate plate  509  in a locked position. The elongate plate  509  is mounted on a central pivot axle  512 . As shown in  FIG. 25 , interaction with a second latch  513  on the housing  503  causes the angled face  511  to be pressed into the firing mechanism housing  505 . This action causes the elongate plate  509  to pivot about the pivot axle  506 , causing the boss  510  to move clear of the path of the trigger  504 .  FIG. 26  shows the hinged auto-injector  501  in a locked position, with the elongate plate  509  disengaged, and the latch  508  engaged. 
         [0120]    With reference to  FIGS. 27 to 30 , there will now be described a sixth embodiment, referred to here as the “floating auto-injector”.  FIG. 27  shows a cross sectional view of such a floating auto-injector  601 , comprising a boot remover  602 , housing  603 , and trigger  604  attached to the housing  603 . The housing  603  houses a syringe, needle  609 , and firing mechanism, which are not shown  FIG. 27 . The floating auto-injector  601  also comprises an outer casing  605 , within which the housing  603  sits. The housing  603  is axially movable with respect to the outer casing  605 . 
         [0121]    The outer casing  605  has a first opening located at a proximal end and a second opening located at a distal end. The first opening is of sufficient size to allow the proximal end of the housing to pass through. The second opening is of sufficient size to allow the trigger to pass through. A spring  606  acts between the housing  603  and the outer casing  605  to bias the housing  603  in a distal direction. 
         [0122]    The boot remover  602  is arranged to remove the boot (not shown) and further arranged such that when it is attached to the housing  603 , the boot remover  602  holds the housing  603  forward in a first proximal position. While the housing  602  is held forward in the first position, the trigger  604  is held within the outer casing  605 , and access to the trigger  604  is restricted. The floating auto-injector  601  may also comprise an elastic membrane  607  which covers the distal opening, preventing access to the inside of outer casing  605 . 
         [0123]      FIG. 28  shows the floating auto-injector  601  once the boot remover  602  has been removed. With the boot remover  602  no longer holding the housing  603  in place, the spring  606  pushes the housing  603  distally to a second position, such that the trigger  604  extends through distal opening, stretching the elastic membrane  607 , and making the trigger  604  accessible to a user. 
         [0124]    When the floating auto-injector  601  is not pressed against the user&#39;s skin, pressing the trigger  604  pushes the housing into the outer casing  605  such that the proximal end of the housing  604  exits the proximal end of the outer casing  605 . The housing  604  is moved towards its first position, until the trigger  604  is no longer accessible. The force required to move the trigger  604  relative to the housing  603  is greater than the force required to move the housing  603  relative to the outer casing  605 . Therefore, pushing the trigger  604  moves the housing  603  through the outer casing  605 , and does not activate the floating auto-injector  601 . This is shown in  FIG. 29 . 
         [0125]      FIG. 30  shows the floating auto-injector  601  when the outer casing  605  is pressed against the skin  608 . By applying pressure to the trigger  604  in a proximal direction, the proximal end of the housing  603  is pressed against the skin  608 . With both the housing and outer casing  605  pressed against the skin, the two cannot move relative to one another, and so the force applied to the trigger  604  axially moves the trigger  604  relative to the housing  603 , activating the floating auto-injector  601 . 
         [0126]      FIGS. 31 to 34  show a seventh embodiment, the seventh embodiment being an alternative arrangement of the floating auto-injector embodiment. The alternative floating auto-injector  701  is structurally similar to the floating auto-injector  601 , but whereas the floating auto-injector  601  has a spring  601  that urges the housing  603  distally with respect to the outer casing  605 , the alternative floating auto-injector  701  has a spring  706  which urges the housing  703  proximally with respect to the outer casing  705 . When urged forward by the spring  706 , access to the trigger  704  is restricted, as shown in  FIGS. 31 and 32 . In order to access the trigger  704 , a proximal ending of the housing  703  must be pressed against a user&#39;s skin in order for the housing to move rearward relative to the outer casing  705 , as in sixth embodiment. When the boot remover  702  is attached, the boot remover  702  covers a proximal end of the outer casing  705 , such that the housing  703  cannot be pressed against the user&#39;s skin. Therefore, in order to activate the device  701 , the user must first remove the boot remover  702 , exposing the proximal end of the housing  703 , and then press the proximal end of the housing  703  against the skin, such that the trigger  704  becomes accessible. The device can then be activated by pressing the trigger. 
         [0127]    With reference to  FIGS. 35 to 39 , there will now be described a eighth embodiment, referred to here as the “toothed wheel auto-injector”.  FIG. 35  shows a cross sectional view of such a toothed wheel auto-injector  801 , comprising a boot remover  802 , housing  803 , trigger  804  and skin sensor  805 . The housing is arranged to house a syringe carrying needle and force applicator (not shown). When the boot remover  802  is attached to the device  801 , the boot remover  802  abuts the trigger  804 , preventing the trigger  804  from being displaced relative to the housing  803 .  FIG. 36  shows the toothed wheel auto-injector  801  following removal of the boot remover  802 . 
         [0128]    The toothed wheel auto-injector  801  further comprises a rotatable shaft  806  which is coupled to a plunger  807 . The plunger  807  is coupled to the trigger  804  such that by pressing the trigger  804 , the plunger is pushed into a bung of a syringe. The plunger  807  may be driven via a drive spring (not shown), or any other means. 
         [0129]    The rotatable shaft  806  is connected to the plunger  807  by a flexible member  808 , such as string. The rotatable shaft  806  has two toothed portions  809 ,  810  which extend circumferentially around the outer surface of the rotatable shaft  806 . The toothed portions  809 ,  810  are arranged to engage with locking levers  811 ,  812 , which prevent rotation of the shaft  806 . When the shaft  806  is rotationally fixed, the plunger  807  is restricted from axially movement due to the connection via the flexible member  809 . Alternatively, the flexible member  808  may be attached directly to the trigger  804 , preventing axial movement of the trigger  804  when the locking levers  811 ,  812  are engaged. 
         [0130]    The skin sensor  805  is arrange to extend beyond a proximal end of the housing  803 , and further arranged such that when the skin sensor  805  is pressed against the skin, the skin sensor  805  is pushed into the housing  803 . When pushed into the housing  803 , the skin sensor  805  acts on the locking levers  811 ,  812  via legs  813 ,  814 , disengaging the locking levers  811 ,  812  from the toothed portions  809 ,  810  of the shaft  806 , allowing rotation of the shaft  806 . 
         [0131]    The locking levers  811 ,  812  are pivotally attached to the housing  803  via pivot points  814 ,  815 . The pivot points  815 ,  816  are located between a part of the levers where the legs  813 ,  814  of the skin sensor  805  act, and a part of the levers that engage with the toothed portions  809 ,  810 . Therefore, when pressed against the skin, the skin sensor  805  causes the locking levers  811 ,  812  to pivot, disengaging them from the toothed portions  809 ,  810 , and allowing the device  801  to be actuated.  FIG. 37  shows the skin sensor having been pushed into the housing, such that the locking levers  811 ,  812  have pivoted about the pivot points  815 ,  816 , disengaging from the shaft  806 .  FIG. 38  shows the trigger  804  being pushed into the housing.  FIG. 39  shows alternative views of the plunger  807 , and shaft  806 . 
         [0132]    With reference to  FIGS. 40 to 45 , there will now be described a ninth embodiment, referred to here as the “toothed element auto-injector”.  FIG. 40  shows a cross sectional view of such a toothed element auto-injector  901 , comprising a boot remover  902 , housing  903 , trigger  904  and outer casing  905 . 
         [0133]    The boot remover  902  abuts the trigger  904 , prior to removal of the boot remover  902 . This prevents the trigger  904  from being displaced relative to the outer casing  905  and/or the outer casing  905  to activate the device  901 .  FIG. 41  shows the boot remover  902  removed. 
         [0134]    The housing  903  is located within the outer casing  905  and a part of the housing  903  protrudes from a proximal end of the outer casing  905 , such that it can be pressed against the skin. The outer casing  905  comprises a drive spring  906  and plunger  907 , for acting on a bung or plunger of a syringe  908  contained in the housing  903 . Pressing the trigger  904  activates the drive spring  906 , which drives the plunger  907 . 
         [0135]    The device  901  further comprises two locking elements  909 ,  910  axially fixed within the outer casing  905 . The locking elements  909 ,  910  have interlocking teeth which engage with interlocking teeth on the plunger  907 , preventing axial movement of the plunger  907 . When the locking elements  909 ,  910  are engaged, the device  901  cannot be activated ( FIG. 42 ). 
         [0136]    The housing  903  is axially moveable relative to the outer casing  905 , and when pressed against the user&#39;s skin, the housing  903  is pushed into the outer casing  905  ( FIG. 43 ). 
         [0137]    The outer casing  905  comprises biasing springs  911  which bias the locking elements  909 ,  910  against the plunger  907  such that the teeth interlock. While teeth have been described, it will be understood by the skilled person that any interlocking feature may be used. 
         [0138]    The housing  903  has two angled surfaces  912 ,  913  which interact with two angled surfaces  914 ,  915  on the locking elements  909 ,  910 , when the housing  903  is pressed into the outer casing  905 . These angled surfaces cause the locking elements to be radial displaced, disengaging them from the plunger  907  ( FIG. 43-45 ). 
         [0139]    Therefore, the boot remover must first be removed and then the device  901  pressed against the skin before pressing the trigger  904  activates the device  901 , driving a needle  916  into a user and dispensing medicine contained in the syringe  908 . 
         [0140]    With reference to  FIGS. 46 to 50 , there will now be described a tenth embodiment.  FIG. 46  shows a cross sectional view of an auto-injector  1001  according to the tenth embodiment. The auto-injector  10001  comprises a boot remover  1002  for removing a boot  1002   a , housing  1003 , trigger  1004  and skin sensor  1005 . The housing  1003  houses a syringe carrying medicine, a needle, and drive mechanism (not shown) for driving the needle into a user&#39;s skin, and dispensing the medicine. 
         [0141]    The skin sensor  1005  extends proximally from the housing  1002 , and is arranged to be pressed against the user&#39;s skin. The boot remover  1002  covers the skin sensor  1005  such that the skin sensor  1005  cannot be pressed against the skin. With the boot remover  1002  removed ( FIG. 47 ), the skin sensor  1005  can be pressed against the skin. The skin sensor  1005  has an elongated leg  1006  which abuts a corresponding elongated leg  1007  on the trigger  1004 . The abutment prevents the trigger  1004  from being axially displaced. 
         [0142]    The skin sensor  1005  further comprises a helical keyway  1008  which engages with a boss  1009  on an inner surface of the housing  1003 . When the skin sensor  1005  is pressed against the skin, the skin sensor rotates into the housing due to the boss  1009  following the helical keyway  1008  ( FIGS. 48 and 49 ). This allows the elongated leg  1006  of the skin sensor  1005  to disengage from the elongated leg  1007  of the trigger  1004 , thereby allowing displacement of the trigger. 
         [0143]    With reference to  FIGS. 51 to 53 , there will now be described an eleventh embodiment.  FIG. 51  shows a cross sectional view of a distal end of an auto-injector  1101  according to the eleventh embodiment. The auto-injector  11001  comprises a boot remover (not shown) for removing a boot, housing  1102 , trigger  1103  and skin sensor  1104 . As with the eighth embodiment, the skin sensor  1104  is arranged to be pressed against the skin, such that it enters the housing  1102 . The boot remover is arranged such that, prior to removal, it covers a proximal end of the skin sensor  1104 , preventing it from being pressed against the skin and into the housing  1102 . The trigger  1103  is prevented from axial movement in the proximal direction due to an abutment between the trigger  1103  and two flexible legs  1105 ,  1106  fixed on the inside of the housing  1102 . 
         [0144]    When the boot remover is removed, the skin sensor  1104  is pressed against the skin, such that it moves axially into the housing  1102 . The a distal end of the skin sensor  1104  pushes against the flexible legs  1105 ,  1106 , causing them to deform ( FIG. 52 ). Once deformed, the flexible legs  1105 ,  1106  no longer abut the trigger  1103 , allowing activation of the device  1101  ( FIG. 53 ). 
         [0145]    With reference to  FIGS. 54 to 59 , there will now be described a twelve embodiment.  FIG. 55  shows a trigger  1201  for activating an automatic injection device (not shown). The trigger  1201  has three latches  1202  which extend from the trigger  1201 , and are arranged to project through slots  1203  of a housing  1204  of the auto-injector ( FIG. 56 ). This engagement prevents actuation of the trigger  1201 . 
         [0146]    The auto injector has three release elements  1205  (a release element is shown in  FIG. 54  from a side view and front view). The release elements  1205  have a de-latching portion  1206 , which feature a raised, angled surface  1207  for engaging with the latch  1202  of the trigger  1201 . The release elements extend beyond a proximal end of the housing  1204  and are arranged such that, when the auto-injector is pressed against the skin, the release elements  1205  are pushed into the housing  1204  and the de-latching portions  1206  engage with, and slide past, the latches  1205  so as to press the latches back through the slots  1203  and allow the trigger  1201  to be actuated ( FIGS. 57 and 58 ). 
         [0147]    The release elements  1205  are fixed to the housing  1204 , and have integral spring elements  1207  between the attachment to the housing and the de-latching portion  1206 . The spring elements  1207  are arranged to compress when the release elements  1205  are pressed against the skin. Any number of release elements may be used. The release elements  1205  are distributed evenly around a circumference having its centre along an axial direction of the housing  1204 . This can be seen from  FIG. 55 , where the three latches  1202 , which engage with the release elements  1205 , are distributed evenly. 
         [0148]      FIG. 59  shows a boot remover  1208  which removes the boot (not shown), and covers the release elements  1205 . The boot remover  1208  must be removed before the release elements  1205  can be pressed against the skin. 
         [0149]    With reference to  FIGS. 60 to 63 , there will now be described a thirteenth embodiment.  FIGS. 60 to 63  show a top cross sectional view of an auto-injector of the thirteenth embodiment, and a partial cross sectional side view. The auto-injector  1301  of the thirteenth embodiment comprises a housing  1302  which houses a syringe and needle, and drive mechanism (not shown) for activating the device  1301 ; a trigger  1303  coupled to the housing  1302 , and arranged to activate the drive mechanism; a boot remover  1304  for removing the boot and covering the proximal end of the device  1301 ; a skin sensor  1305  protruding proximally beyond the housing  1302  and an out casing  1306 ; and an elliptical flexible collar  1307  axially fixed to the housing  1302 , and located between the trigger  1303  and the housing  1302 . 
         [0150]    The elliptical flexible collar  1307  is arranged to sit around a part of the trigger  1303 , and prevent the trigger  1303  from being fully depressed when in a first configuration, preventing activation of the device  1301 . In the first configuration the collar  1307  has a minor axis  1308  which is shorter than the diameter of an upper part of the trigger  1309 . This prevents the trigger  1303  from being fully depressed into the housing  1302  ( FIG. 60  shows the device  1301  with the boot remover  1304  attached, and  61  shows the device with the boot remover removed). Note that while the trigger is described as having a diameter, and the collar is describe as being elliptical, both may be any suitable shape such that deformation of the collar allows the trigger to pass through. 
         [0151]    The skin sensor  1305  has an angled surface  1310  at its distal end. When the skin sensor  1305  is pressed against the skin ( FIG. 62 ), it is moved into the outer casing  1306 , and the angled surface  1310  pushes against the vertices defining the major axis, so as to deform the elliptical flexible collar  1307 . The deformation causes the minor axis to increase in length, until the trigger  1303  is no longer blocked by the collar  1307 . In this second configuration, the device can then be activated ( FIG. 63 ). The boot remover  1304  covers the skin sensor  1305  prior to removal. Once removed, the skin sensor  1305  may be pressed against the skin. 
         [0152]    With reference to  FIGS. 64 to 68 , there will now be described a fourteenth embodiment.  FIGS. 64 to 68  show a cross section of an auto-injector of the thirteenth embodiment. The auto-injector  1401  comprises a housing  1402  housing a syringe, drive mechanism (not shown), and needle  1411 ; an outer casing  1403  within which the housing  1402  is located; a boot remover  1404  for removing a boot and covering the proximal end of the device  1401 ; and a trigger  1405 . 
         [0153]    The housing  1402  is contained within the outer casing  1403  and arranged to be axially moveable within the outer casing  1403 . The drive mechanism within the housing  1402  is prevented from being released and activating the device  1401  by being connected to flexible member  1406  which is clamped in two places. 
         [0154]    The auto-injector  1401  further comprises a spring  1407  which acts between the outer casing  1403  and the housing  1402  and biases the housing  1402  towards a proximal end of the outer casing  1403  such that a part of the housing  1402  protrudes from an opening of the proximal end of the outer casing  1403 . When held in this forward position by the spring  1407 , a part of the flexible element  1406  is clamped in a first clamped position  1408  between an outer surface of the housing  1402  and an inner surface of the outer casing  1403 . When the housing  1402  is pressed against the skin, the housing  1402  moves into the outer casing  1403 , which releases the clamp  1408  ( FIG. 66 ). 
         [0155]    The boot remover  1404  prevents the housing  1402  from being pressed against the skin, prior to removal. 
         [0156]    The flexible element  1406  is further clamped in a second position by a butterfly valve  1409  coupled to the housing  1402 . The trigger  1405  has two release pegs  1410 , which engage with the butterfly valve  1409  when the trigger  1405  is displaced. This engagement opens the valve  1409 , unclamping the flexible element  1406  from the second clamp ( FIG. 67 ). The drive mechanism is no longer prevented from being released by the flexible member, and so the device  1401  is activated, driving the needle  1411  and medicine into the user&#39;s skin. 
         [0157]    The device  1401  is a further arranged, such that when the housing  1402  is held in the forward position by the spring  1407 , the butterfly valve  1409  is held in a position which is inaccessible to the release pegs  1410  of the trigger  1404 . This is shown in  FIG. 65 , where the trigger  1405  is pressed into the housing, but does not reach the valves  1410 . While not shown, the trigger  1405  has limited axial movement due to a structural stop, such as a ledge within the outer casing  1403 . 
         [0158]    It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. Furthermore, while several separate embodiments have been described, the skilled person will recognise that some of these embodiments may be combined.