fig1 shows a drug delivery device 1 in the form of an injection pen . the device has a distal end ( upper end in fig1 ) and a proximal end ( lower end in fig1 ). the component parts of the drug delivery device 1 are shown in fig2 in more detail . the drug delivery device 1 comprises an outer housing part 10 , an inner body 20 , a piston rod 30 , a driver 40 , a nut 50 , a display member 60 , a button 70 , a cartridge 80 and a cap 90 , i . e . in total nine separate component parts . as shown in fig2 , a needle arrangement comprising a needle hub 2 and a needle cover 3 may be provided as additional components , which can be exchanged as explained above . the outer housing part 10 is a generally tubular element having a distal part , which forms a cartridge holder 11 for receiving cartridge 80 , and a proximal part , which forms an outer body 12 . in a preferred embodiment , the outer housing part 10 is transparent , with the outer body 12 being provided with an opaque layer 13 . in fig2 , the opaque layer 13 covers most of the outer body 12 with the exception of a transparent window 14 . apertures 15 may be provided in the cartridge holder 11 . further , at its distal end the cartridge holder 11 has a thread 16 or the like for attaching the needle hub 2 . the inner body 20 is a generally tubular element having different diameter regions . as can be seen in fig1 a to 16 c , the inner body 20 is received in the outer body 12 and permanently fixed therein to prevent any relative movement of the inner body 20 with respect to the outer body 12 . an external thread 21 is provided on the outer surface of the inner body 20 . further , splines 22 are provided on the inner surface of the inner body 20 which are shown in fig8 and 11 . as can be taken from fig7 , the inner body 20 has near its distal end an inner thread 23 . the piston rod 30 is an elongate element having two external threads 31 , 32 with opposite hand which overlap each other . one of these threads 31 engages the inner thread 23 of the inner body 20 . a disk - like bearing 33 is provided at the distal end of the piston rod 30 . as shown in fig2 , the bearing 33 may be attached to the piston rod 30 as a one - piece component via a predetermined breaking point . this allows that the bearing 33 is separated from the piston rod 30 such that the bearing 33 remains seated on the distal end of the piston rod 30 to allow relative rotation between the bearing 33 and the piston rod 30 . the driver 40 is a generally tubular element having different diameter regions . a distal region of the driver 40 has an external thread 41 . an inner surface of the driver 40 has an inner thread 42 ( fig1 a and 12 b ) engaging one of the external threads 32 of the piston rod 30 . the driver 40 surrounds the piston rod 30 and is at least partly located within inner body 20 . the driver has at least one proximal opening 43 which will be explained in more detail below . further , a resilient finger 44 ( fig6 a and 6 b ) is provided on the driver 40 by a u - shaped cut in the skirt of the driver 40 . the finger 44 is allowed to flex in the axial direction and engages button 70 . in addition , a flexibly hinged protrusion 45 ( fig8 and 9 a ) is provided on the driver 40 by a similar cut out in the skirt of the driver 40 . the protrusion 45 is allowed to flex radially inwardly and is provided with lateral flaps 46 . protrusion 45 engages splines 22 of the inner body 20 . the nut 50 is provided between the inner body 20 and the driver 40 . external ribs 51 of the nut 50 engage splines 22 of the nut 50 . an internal thread 52 of the nut engages the external thread 41 of the driver 40 . as an alternative , splines and ribs could be provided on the interface between the nut 50 and the driver 40 and threads could be provided on the interface between the nut 50 and the inner body 20 . as a further alternative , the nut 50 may be designed as e . g . a half nut . further , in the embodiment of fig1 , four rotational hard stops 53 are provided on nut 50 for interaction with corresponding stops 47 on the driver 40 at the proximal end of thread 41 . the display member 60 is a generally tubular element with an internal thread 61 engaging the external thread 21 of the inner body 20 . thus , the display member 60 is interposed between the inner body 20 and the outer body 12 . a series of numbers is provided , e . g . printed , on the outer surface of the display member 60 . the numbers are arranged on a helical line such that only one number or only a few numbers are visible in through window 14 of the outer body 12 . as will be explained in more detail below , the display member 60 is attached to the driver 40 preventing relative axial movement but allowing relative rotation . fig4 a and 4 b show in more detail a zero unit rotational hard stop formed by a stop wall 62 in thread 61 of the display member 60 and a corresponding stop face 24 on the inner body 20 . fig5 a and 5 b show in more detail a maximum dose ( e . g . a 80 units ) rotational hard stop formed by a finger 63 at the distal end of the display member 60 and a protrusion 25 in thread 21 of the inner body 20 . thus , a user is prevented from dialing below zero units and above e . g . 80 units . the button 70 has a proximal end with an , e . g . serrated , flange or outer skirt 71 allowing a user to easily grip and dial button 70 . a sleeve - like part 72 of the button 70 with a reduced diameter extends in the distal direction and is inserted into the driver 40 such that a limited relative axial movement is allowed but relative rotation is prevented . this is achieved by a rib 73 on the sleeve - like part 72 which is guided in a proximal opening 43 of the driver 40 . a recess 73 which generally has the outline of the protrusion 45 and its lateral flaps 46 is provided in the sleeve - like part 72 of button 70 . a clutch is provided between the display member 60 and the button 70 by corresponding teeth 64 and 74 ( fig1 a and 13 b ). if teeth 74 of the button 70 engage teeth 64 of the display member 60 , these components are rotationally locked . the resilient finger 44 of the driver 40 biases the button 70 in the proximal direction of the device 1 , i . e . in a direction engaging the clutch teeth . the clutch can be released allowing relative rotation by shifting the button 70 axially with respect to the display member 60 against the bias of finger 44 . further , a dispense clicker is provided by flexible arms 65 on the display member 60 and a toothed profile 75 on the inner side of flange 71 of button 70 . this clicker is shown in fig1 a to 17 c . the cartridge 80 includes a pre - filled , necked - down cartridge reservoir 81 , which may be typically made of glass . a rubber type bung 82 or stopper is located at the proximal end of the cartridge reservoir 81 , and a pierceable rubber seal ( not shown ) is located at the other , distal , end . a crimped annular metal band 83 is used to hold the rubber seal in place . the cartridge 80 is provided within the cartridge holder 11 with bearing 33 of piston rod 30 abutting bund 82 . fig1 shows the cap 90 attached to the distal end of the device 1 , thus covering the cartridge holder 11 . the cap 90 may be releasable snapped onto the outer housing 10 and can be taken off for use of the device 1 . in the following , the function of the disposable drug delivery device 1 and its components will be explained in more detail . to use the device , a user has to select a dose . in the start ( at rest ) condition as shown in fig3 a and 6 a the display member 60 indicates the number of doses dialed to the user . the number of dialed units can be viewed through the dose window 14 in the outer body 12 . due to the threaded engagement between the display member 60 and the inner body 20 rotation of the button 70 in a clockwise fashion causes the display member 60 to wind out of the device and incrementally count the number of units to be delivered . fig3 b shows an intermediate stage of dialing ( 47 of 80 units ). during dose setting button 70 , driver 40 and display member 60 are rotationally locked together via clutch teeth 64 , 74 . further , button 70 , driver 40 and display member 60 are axially coupled . thus , these three components wind out of the outer housing 10 during dose setting . clockwise rotation of the button 70 causes the driver 40 to rotate and in doing so it advances along the piston rod 30 which remains fixed throughout dialing . the protrusion 45 and splines 22 form a clicker arrangement that provides tactile and audible feedback to the user when dialing doses . this clicker arrangement has the further functions of defining discrete positions for the display member 60 when dialing and of providing a method of locking the rotation of the driver 40 and hence button 70 when dosing . during dialing ( dose setting ) the button 70 is in an axial position relative to the driver 40 such that the pocket or recess 73 is located radially inwards of the protrusion . thus , the protrusion 45 is allowed to flex radially inwards to overcome splines 22 thereby providing a tactile and audible feedback to the user . fig8 shows the flexible protrusion arm 45 located between splines 22 which are e . g . 15 ° apart . at the maximum settable dose of 80 units , the stop features 63 , 25 shown in fig5 a and 5 b engage to prevent further dialing . this position of the device is shown in fig3 c and 6 b . the last dose nut 50 provides the function of counting the number of dispensed units . the nut 50 locks the device 1 at the end of life and as such no more drug can be dialed or dispensed by the user . the last dose nut 50 and the driver 40 are connected via a threaded interface 41 , 52 as explained above . further , the last dose nut 50 is assembled into splines 22 as shown in fig1 such that the nut 50 and the inner body 20 are rotationally locked together ( at all times ). rotation of the driver 40 during dialing causes the nut 50 to advance along the driver 40 thread 41 . the nut 50 is free to slide axially within the inner body 20 at all times which allows advancement of the nut . the change in pitch shown in fig1 a and 12 b towards the final doses axially accelerates the advancement of the nut 50 towards the end of life lockout condition . at the end of life condition , the stop features 53 of the last dose nut 50 contact the corresponding features 47 on the driver 40 . the splined contact with inner body 20 reacts any torque transmitted by these stop features 47 . with the desired dose dialed , the device 1 is ready for dose dispensing . this basically requires pushing button 70 which will result in a disengagement of the clutch teeth 64 , 74 . as mentioned above , when dialing a dose the button 70 is ‘ biased out ’ and the clutch features 64 , 74 which rotationally lock the driver 40 , button 70 and display member 60 together are engaged as shown in fig1 a . upon pressing the button 70 the clutch features 64 , 74 disengage as shown in fig1 b and relative rotation between the display member 60 and the button 70 is possible . in all conditions the driver 40 and the button 70 are rotationally locked together by engagement rib 73 and opening 43 . thus , with the clutch 64 , 74 disengaged ( button 70 pushed in ) button 70 and driver 40 are rotationally locked together with the button 70 , the driver 40 and the display member 60 still being axially coupled . at the same time the relative axial movement of the button 70 with respect to the driver 40 results in the pocket or recess 73 being shifted relative to the protrusion 45 . thus , the protrusion 45 is prevented from flexing inwards because flaps 46 rest on a non - recessed area of button 70 . a comparison of fig9 c and 9 d shows this activation of the lockout feature preventing the flexible protrusion arm 45 from overcoming splines 22 if the button 70 is pressed . in this condition , the driver 40 and the button 70 are rotationally constrained to the inner body 20 thus preventing any rotation relative to the outer housing 10 if the splines 22 are axially aligned with the device as shown in fig1 c . the above mentioned alternative embodiment with twisted splines 22 is shown in fig1 . with the desired dose dialed the button 70 can be depressed and the piston rod 30 driven forward to dispense drug from the cartridge . the interaction of mating threads between the piston rod 30 , driver 40 and inner body 20 delivers a mechanical advantage of 2 : 1 . the sequence of dispensing is depicted in fig1 a to 16 c with fig1 a showing the device 1 with 80 units dialed prior to pushing button 70 , fig1 b shows the device 1 with 80 units dialed and button 70 pushed and fig1 c shows the device 1 with 80 units dispensed . during dose dispensing a dispense clicker is active which involves button 70 and display member 60 . the dispense clicker provides primarily audible feedback to the user that drug is being dispensed . the interaction between the flexible arms 65 on the display member 60 and the toothed profile 75 on the button flange 71 provide this dispense click . relative rotation is only allowed in one direction . this occurs when the components are decoupled during dispense and a click is produced for every unit .
the invention relates to a disposable drug delivery device for selecting and dispensing a number of user variable doses of a medicament . the device comprises a housing , a cartridge holder for retaining a cartridge containing the medicament , a piston rod displaceable relative to the cartridge holder , a driver coupled to the piston rod , a display member for indicating a set dose and being coupled to the housing and to the driver , and a button coupled to the display member and to the driver .