Abstract:
An injection device includes a housing, a plunger, a drive gear and a release trigger. The plunger includes a leadscrew and a cap attached thereto where, in use, the cap engages a bung of a cartridge containing medicament. The drive gear within the housing engages with the leadscrew such that rotation of the drive gear around the leadscrew drives the plunger axially through the housing towards the bung. The trigger includes a locking ring having generally axially extending teeth for engaging with corresponding teeth on the drive gear. The trigger is generally axially movable between an engaged position where the teeth are engaged and prevent rotation of the drive gear, and a disengaged position where the teeth are disengaged and enable rotation of the drive gear. Movement of the trigger between the disengaged and engaged position causes counter-rotation of the drive gear and retracts the plunger away from the bung.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an injection device, and to leak prevention in such invention devices. 
       BACKGROUND 
       [0002]    WO2007063342 describes a generally pen-like syringe as shown in  FIG. 1  and which is suitable for use with a cartridge containing a medicament. The cartridge is held within a cartridge housing and is adapted to receive a needle (not shown) which pierces a rubber septum sealing a proximal end (with respect to a user&#39;s skin) of the cartridge. A cap  1  is provided for covering the cartridge housing  2  and cartridge, and any attached needle. The syringe incorporates a dose metering device provided by a dose knob  3  which is rotatable with respect to the main housing  4  to a position related to the dose of medication to be injected. Rotation of the cap stores energy in a torsion spring  5 , which is prevented from unwinding by cooperating splines. The user disengages the splines by means of a sliding trigger  6  coupled to the outside of the main housing  4 , resulting in the unwinding of the spring. This in turn causes rotation of a drive gear  7  which is coupled to a plunger  8  via a quick pitch screw thread  9 . During firing, the plunger is prevented from rotating by splines in a rewind knob  10  which is rotationally fixed relative to the main housing  4  when the cartridge housing  2  is attached to the main housing  4 . The splines interact with recesses along the axial direction of the plunger, such that rotation of the gear results in axial movement of the plunger  8  through the main housing. As illustrated schematically in  FIG. 2 , a cap  11  mounted on the end of the plunger  8  acts on a “bung”  12  of the cartridge  13 , driving medicament  14  from the cartridge, through an attached needle, as the plunger moves in the direction of arrow A. 
         [0003]    When a dose has been administered, the cap  11  remains in contact with the bung  12  after driving it forwards. This can lead to leakage of the drug, since the bung is compressed by the action of the cap during the driving stroke, and will tend to expand to its original shape once the stroke is complete. This expansion can force liquid from the cartridge. In addition, the movement of the dose knob  3  when the dose is being set can cause vibrations, and these vibrations may be transmitted down the plunger  8  to the cap  11  and thus the bung  12 , again forcing liquid from the capsule. 
       SUMMARY 
       [0004]    It would therefore be desirable to provide an injection device adapted to prevent undesired leakage. 
         [0005]    In accordance with one aspect of the present invention there is provided an injection device comprising a main housing, a plunger, a drive gear and a release trigger. The plunger comprises a leadscrew and a cap attached to the leadscrew such that, in use, the cap engages a bung of a cartridge containing medicament. The drive gear is located within the housing and engages with said leadscrew such that rotation of the drive gear drives the plunger axially through the housing towards the bung. The release trigger comprises a locking ring having generally axially extending teeth for engaging with corresponding teeth on the drive gear, the release trigger being generally axially movable between an engaged position in which the teeth are engaged so as to prevent rotation of the drive gear relative to the housing, and a disengaged position in which the teeth are disengaged so as to enable rotation of the drive gear. Movement of the release trigger between the disengaged position and the engaged position causes counter-rotation of the drive gear so as to retract the plunger away from the bung. 
         [0006]    The release trigger may be located in a slot in the main housing, the slot including an axially extending section for guiding the trigger to the disengaged position, and a dogleg section corresponding to the engaged position of the trigger, the dogleg section extending in a direction including a circumferential component so that, as the release trigger moves through the dogleg section, it causes rotation of the drive gear. 
         [0007]    The teeth of the locking ring may be bevelled at an end facing the drive gear such that, as the trigger moves from the disengaged position to the engaged position, they provide a rotation force on the teeth of the drive gear so as to cause the drive gear to counter-rotate. The teeth of the drive gear may have a corresponding bevel at an end facing the locking ring so as to react against the bevelled faces of the locking ring teeth. The counter-rotation may be one notch. 
         [0008]    The teeth of the locking ring and teeth of the drive gear may have a screw pitch so that axial movement of the trigger between the disengaged position and the engaged position causes counter-rotation of the drive gear. 
         [0009]    The drive gear may be configured to engage with a screw thread on the leadscrew on the plunger such that rotation of the drive gear around the leadscrew drives the plunger axially through the housing when the leadscrew is locked against rotation with respect to the housing. 
         [0010]    In an alternative embodiment, the device may further comprise a static gear configured to engage with a screw thread on the leadscrew. In this embodiment the plunger is rotationally fixed relative to the drive gear such that rotation of the drive gear causes rotation of the leadscrew resulting in axial movement of the plunger relative to the static gear when the static gear is locked against rotation with respect to the housing. In this embodiment the cap may be rotatable relative to the leadscrew. 
         [0011]    The invention also provides a method of operating the injection device described above. The method includes locating the release trigger in the engaged position so as to prevent rotation of the drive gear, storing rotational potential energy between the housing and the drive gear, moving the release trigger to the disengaged position, rotating the drive gear by release of the potential energy so as to drive the plunger axially towards the bung, returning the release trigger to the engaged position, and counter-rotating the drive gear so as to retract the plunger away from the bung 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  shows a horizontal cross-section through a prior art injector pen and a detail of the same horizontal cross-section; 
           [0014]      FIG. 2  is a perspective view of a plunger of the device of  FIG. 1 , interacting with a medicament containing cartridge; 
           [0015]      FIG. 3A  is a perspective exploded view of an injector pen; 
           [0016]      FIG. 3B  is a detailed view of a portion of a drive gear used in the injector pen of  FIG. 3A ; 
           [0017]      FIG. 3C  is a detailed view of a sliding trigger used in the injector pen of  FIG. 3A ; 
           [0018]      FIG. 3D  is a detailed view of the drive gear and sliding trigger of  FIGS. 3B and 3C  in combination; 
           [0019]      FIG. 4A  is a perspective view of a locking ring; 
           [0020]      FIG. 4B  is a perspective view of the locking ring of  FIG. 4A  engaged with a drive gear; 
           [0021]      FIG. 5A  is a perspective view of an alternative injector pen; 
           [0022]      FIG. 5B  is a top view of the housing of the injector pen of  FIG. 5A ; and 
           [0023]      FIG. 6  is a schematic view of parts of an alternative injector pen. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    An injection device  300  similar to that shown in  FIG. 1  is illustrated in exploded form in  FIG. 3A . Features common to the device of  FIG. 1  and the improved device are identified with like reference numerals. A dose knob  3  is provided at a proximal end of the device and which is rotatable with respect to the main housing  4 . Rotation of the knob causes rotation of a drive sleeve  316  which stores energy in a torsion spring  5  (not shown in  FIG. 3 ). The drive sleeve  316  includes an annular ring  317  projecting at its distal end having at least one sprung tooth  318  protruding therefrom. The sprung tooth  318  cooperates with ratchet teeth (not shown) formed on an internal surface of a drive gear  307 , and allows rotation of the drive sleeve  316  relative to the drive gear  307  in one direction only in a series of “clicks”. The dose can thus be wound up in that direction, and the drive sleeve is prevented from unwinding by the ratchet teeth. The drive gear itself is prevented from unwinding by external teeth  319  which cooperate with internal teeth  320  of a locking ring  321  of the sliding trigger  6 . The drive gear  307  and sliding trigger  306  are shown in more detail in  FIGS. 3B, 3C and 3D . 
         [0025]    The sliding trigger is coupled to the main housing  4  via a generally axial slot  322  such that a button  323  is located outside the housing  4  and the locking ring  321  is located inside the housing. The trigger  6  can therefore be moved axially but not circumferentially. Axial movement of the sliding trigger  6  disengages the teeth  320  of the locking ring  321  from the teeth  319  of the drive gear  307 , thus allowing the drive gear  307  and drive shaft  316  to be rotated by unwinding of the torsion spring  5 . The drive gear  307  is coupled to a plunger  8  via a quick pitch screw thread. During firing, the plunger  8  is prevented from rotating by splines  10  interacting with recesses  324  along the axial direction of the plunger, such that rotation of the drive gear  7  results in axial movement of the plunger through the main housing  4 . 
         [0026]    Rotation continues until an externally protruding latch  325  behind the annular ring  318  on the drive sleeve contacts a corresponding stop in a collar  326  fixed relative to the main housing  4 . The sliding trigger  306  is then retracted again so that the internal teeth  320  of the locking ring  321  re-engage with the teeth  319  of the drive gear  307 . An additional spring (not shown) may be provided to ensure that the trigger is retracted after use. 
         [0027]    In order to prevent leakage, the cap  11  should be withdrawn a short distance from the bung  12  once the dose has been dispensed. This can be achieved by counter-rotating the drive gear  7  after the plunger  8  has been driven the required distance, to bring the plunger back slightly. 
         [0028]    This is achieved in the arrangement of  FIG. 3A  by the fact that the inner teeth  320  of the locking ring  321  do not extend purely axially, but have a screw pitch and therefore act as a thread. The outer teeth  319  on the drive gear  307  have a corresponding pitch so that the drive gear  307  is rotated slightly by axial movement of the sliding trigger  306 . The direction of the thread is such that the forward movement of the trigger  306  rotates the drive gear  307  in the same direction as that caused by unwinding of the torsion spring  5 . 
         [0029]    When the user wishes to administer a dose, once the dose knob has been rotated to tension the torsion spring  5 , the sliding trigger  306  is moved forwards (against the force of the additional spring, if present). The forward motion of the sliding trigger causes rotation of the drive gear  307 , which intern causes the plunger  8  to be extended slightly. This short extension of the plunger  8  brings the cap  11  into contact with the bung  12 . Further forward movement of the slider disengages the teeth  321  of the sliding trigger from those of the drive gear  307  once the cap is in contact with the bung, at which point the gear is rotated by the torsion spring  5  so as to extend the plunger  8  and dispense the dose, in the same manner as the device of  FIGS. 1 and 2 . 
         [0030]    Once the latch  325  has contacted the stop on the collar  326 , the sliding trigger  306  is returned (by the additional spring, if present) along the axial slot  322  until the teeth (threads)  320  of the locking ring  321  engage those of the drive gear  307 . Once the threads are engaged, further axial retraction of the sliding trigger  307  results in counter-rotation of the drive gear  307 , which in turn retracts the plunger a short distance, thus lifting the cap  11  away from the bung  12 . When the next dose is administered the first part of the slider movement again causes forward rotation of the drive gear  307  so as to extend the plunger  8  so that the cap  11  contacts the bung  12 . 
         [0031]    An alternative approach which achieves a similar effect is illustrated in  FIG. 4B , which shows an alternative locking ring  421  and drive gear  407 , which may be used in place of the locking ring  321  and drive gear  307  shown in  FIG. 3A . The locking ring in isolation is shown in  FIG. 4A . In this approach, the teeth  419 ,  420 , of the drive gear  407  and locking ring  421  extend axially, but each tooth  420  of the locking ring  421  has a bevelled rear face  426 . A corresponding front face of each tooth  419  of the drive gear is also bevelled. This means that, when the sliding trigger  406  is pushed forwards to initiate dispensing of the dose, no rotation is applied to the drive gear  407 , and the plunger does not move axially until the teeth  419 ,  420  disengage. However, when the sliding trigger is retracted, the bevelled faces  426  of the teeth on the locking ring  421  contact the bevelled faces of the teeth  419  of the drive gear  407  and cause it to counter-rotate as the teeth re-engage, thus retracting the plunger  8  so that the cap  11  disengages from the bung  12 . The counter-rotation may be one notch (i.e. one tooth of the drive gear). 
         [0032]    A further alternative approach is illustrated in  FIGS. 5A and 5B .  FIG. 5A  shows an exploded view of a device  500  similar to the device  300  shown in  FIG. 3 . The device differs from that of  FIG. 3  in that the slot  522  in the main housing  504 , along which the sliding trigger passes, is not purely axial but includes a dogleg section. The housing  504  is shown in more detail  FIG. 5B , where it can be seen that the generally axial slot includes a front axial section  527  extending axially, a dogleg section  528  including a circumferential component, and a rear axial section  529 . A further difference from the device of  FIG. 3  is that the teeth  519 ,  520 , on the drive gear  7  and locking ring  521  respectively, extend generally axially in the same way as on the device of  FIG. 1 . 
         [0033]    When the device is at rest the sliding trigger  6  is held by a spring (not shown) at the rear end of the rear axial section of the slot such the teeth  520  of the locking ring engage the teeth  519  of the drive gear. When the user wishes to administer a dose, the sliding trigger  506  is moved forwards against the pressure of the spring. While the teeth are still engaged with the drive gear the sliding trigger moves along the dogleg section  528 , and this motion includes a circumferential component. This movement results in a small rotation of the drive gear  507 , resulting in a short extension of the plunger  8 . This short extension of the plunger  8  brings the cap  11  into contact with the bung  12 . Further forward movement of the sliding trigger  506  is along the front axial section  527  and this disengages the teeth  520  from those of the drive gear once the cap is in contact with the bung, at which point the gear is rotated by the torsion spring  5  so as to extend the plunger as before. 
         [0034]    Once the drug dispensation is complete, the sliding trigger  506  is returned by the spring along the front axial section  527  of the slot  522  until the teeth  520  of the locking ring  521  engage those  519  of the drive gear  507 . The sliding trigger  506  then continues to move along the dogleg section  528 , so that the movement includes a circumferential component which counter-rotates the drive gear  507  and retracts the plunger a short distance, thus lifting the cap  11  away from the bung  12 . When the next dose is administered the first part of the sliding trigger movement again includes a circumferential component so as to rotate the drive gear and extend the plunger  8  so that the cap  11  contacts the bung  12 . 
         [0035]    In each of these approaches it will be appreciated that, during dispensation, the first movement is a small rotation of the drive gear to bring the cap of the plunger into contact with the bung. The next movement is the release of the drive gear from the rotational constraint placed on it by the slider, at which stage it is rotated by the torsion spring (in the same direction as the initial small movement). Then as the slider is retracted there is a small counter-rotation of the drive gear which retracts the cap of the plunger away from contact with the bung. 
         [0036]    As noted above, in the prior art arrangement of  FIGS. 1 and 2 , when a dose is being administered, rotation of the drive gear is stopped when a latch on the drive shaft encounters a stop fixed relative to the main housing. In the prior art configuration, when the slider is retracted the latch remains in contact with the stop, and this assists in holding the tension of the spring. 
         [0037]    When the drive gear is counter-rotated slightly as the slider is retracted, this lifts the latch away from the stop. The torsion spring tension is therefore now held by the interaction between the ratchet teeth of the drive gear and the drive shaft. This means that, as the knob  3  is rotated to set the dose, there is no play in the initial movement. This reduces the manufacturing tolerances required in the manufacture of the drive shaft and drive gear. 
         [0038]    It will be appreciated that the three approaches described above may be combined: for example the pitched teeth shown in  FIGS. 3A to 3D  may have bevelled ends, and may be used in conjunction with a doglegged slot. 
         [0039]    It will also be appreciated that variations from the embodiments described above may fall within the scope of the invention. 
         [0040]    For example, in the devices described above, the drive gear includes an internal thread which engages with the screw thread on the leadscrew of the plunger so as to drive the plunger axially when it is locked against rotation relative to the housing. However, in another arrangement (illustrated in  FIG. 6 ) the plunger  8  may be locked rotationally relative to the drive gear  607 , for example by the use of splines  630  which interact the recesses  324  in the plunger, so that rotation of the drive gear causes rotation of the plunger. A static gear, for example the rewind knob  610 , may include an internal thread  631  so that rotation of the plunger causes axial movement relative to the rewind knob. The rewind knob  610  is lockable relative to the housing  4  when the cartridge housing  2  is attached to the main housing  4 . This may be achieved by use of a locking bar (not shown). If this arrangement is used the cap  11  needs to be rotatable relative to the leadscrew  9  so that it does not rotate when in contact with the bung  12 .