Preventing auto injector activation during cap removal

Abstract

An auto injector comprises a housing (2), a rotatable driver (10) and an axially slidable (5) syringe assembly (4) with ramps (28). When the user presses a button (12) to actuate the device, the driver (10) rotates under the force of a drive spring (8) and acts on the ramps (28) to advance the syringe assembly (4). The syringe assembly (4) and the driver (10) comprise mutually facing stop surfaces (99,101), which engage when the driver (10) is in an initial position to prevent the syringe assembly (4) advancing before the device is actuated, in (10) particular when an end cap (7) is removed. The stop surfaces (99,101) may be inclined at an angle greater than or equal to the angle of the ramps (28) to avoid interfering with the advancement of the syringe assembly (4) when the device is actuated.

Description

[0001] TITLE

[0002] Preventing auto injector activation during cap removal

[0003] DESCRIPTION

[0004] Technical field

[0005] The invention relates to auto injectors, which, when applied to the skin of a subject and triggered in a pre-defined manner, automatically deliver a known quantity of a drug through a hollow needle into the skin, then retract the needle into the device to avoid the risk of it causing injury or contamination. The automated sequence of steps performed by such auto injectors might or might not include an initial step of advancing the needle from the device to penetrate the skin.

[0006] Technical background

[0007] Various mechanisms are known for controlling the sequence of steps to be performed by an auto injector. One example is disclosed in published international patent application WO 2019 / 192662 Al, describing an auto injector that is driven by a prestrained torsion spring, which rotates a driver. During a first functional sequence, a threaded connection between the rotating driver and a syringe assembly drives the syringe assembly axially to expose the needle from the device. During a second functional sequence, rotation of the driver drives a screw to rotate within a threaded connection to the syringe assembly, which advances the screw and expels a drug from the device. At the same time, the driver rotates along a further threaded connection with the housing to determine the number of turns made by the screw during drug delivery until, in a third functional sequence, the further threaded connection runs out. This frees the syringe assembly to retract under the force of a compression spring and to withdraw the needle into the device. The auto injector is actuated by the movement of a sleeve, which slides axially relative to a main housing when a user holds the sleeve and presses the auto injector against the skin of a subject.

[0008] Auto injectors are typically used for self-administration of drugs and sometimes for administering emergency medication. For these reasons, it is particularly important that they should be capable of reliably delivering an accurate dose, even when operated

[0009] P51752WO by inexperienced users or medically untrained persons. They should preferably not permit any drug to be released before the needle has been exposed from the device or after it has been withdrawn from the skin. For safety, they should be resistant to accidental actuation before the intended time of use and also resistant to any form of actuation or exposure of the needle after the auto injector has already been used.

[0010] Summary of the invention

[0011] The invention provides an auto injector comprising: a housing, which defines an axis; a syringe assembly which is fixed against rotation relative to the housing, the syringe assembly comprising a syringe, a hollow needle and a ramp; a driver which is capable of rotation about the axis relative to the housing but is restrained against movement in a distal direction beyond an axial limit; and a drive spring configured to urge the driver to rotate, whereby the driver acts on the ramp to drive the syringe assembly to move in a distal direction and advance the needle relative to the housing; wherein the syringe assembly further comprises a stop surface, which faces generally in the distal direction, and the driver comprises a corresponding stop surface, which faces generally in the opposite direction, such that, when the driver is in an initial position before actuation of the auto injector, the respective stop surfaces engage one another to prevent the syringe assembly being moved in the distal direction and, when the driver rotates away from its initial position, the stop surfaces disengage one another to permit the syringe assembly to move in the distal direction.

[0012] The mutual engagement of the stop surfaces on the driver and the syringe assembly secures the syringe assembly against unwanted movement in the distal direction before the device has been actuated. In particular, such movement is prevented when a distal force is applied to the syringe assembly by removal of an end cap that protects the needle before use.

[0013] The housing preferably comprises a shoulder that faces away from the distal direction and prevents the driver moving beyond the axial limit. The axial force exerted on the driver by the mutual engagement of the respective stop surfaces needs to be balanced by a reaction force and such a shoulder is an easy way to provide it in a robust manner.

[0014] P51752WO The auto injector may further comprise a button at a proximal end of the autoinjector, wherein: the button comprises at least one arm that extends in the distal direction; the button is capable of moving in the distal direction from an unactuated position to an actuated position; when the button is in the unactuated position, the arm blocks rotation of the driver away from its initial position; and when the button moves to the actuated position, the arm releases the driver to rotate away from the initial position under the force of the drive spring and drive the movement of the syringe assembly. The button arm prevents actuation of the device only by blocking rotation of the driver until the button is pressed. The button does not directly hold the driver or syringe assembly against axial movement, therefore there exists no mechanism by which a distal force applied to the syringe assembly - in particular, when the end cap is removed - may be transmitted to the button and result in the device being actuated prematurely.

[0015] The stop surfaces of the driver and the syringe assembly are preferably inclined to the axis such that when a force is exerted on the syringe assembly in the distal direction, the mutually engaging stop surfaces act as a cam to push the driver with greater force against the arm of the button in the direction of rotation.

[0016] The angle of inclination of the stop surface of the syringe assembly is preferably greater than or equal to the angle of inclination of the ramp. This ensures that, when the device is actuated and the driver rotates simultaneously with the advancement of the syringe assembly, the stop surfaces will disengage in a manner that avoids interfering with that advancement.

[0017] The stop surface of the syringe assembly may be formed in a recess of the syringe assembly; and the stop surface of the driver may be formed on a corresponding projection of the driver.

[0018] The auto injector may further comprise a needle sleeve, which is releasably secured to the syringe; and a cap, which is engaged with the needle sleeve such that withdrawing the cap in the distal direction removes the needle sleeve from the syringe and uncovers

[0019] P51752WO the needle. The needle and cap are necessary to protect the needle during transport and storage of the auto injector. Removal of the cap and the needle sleeve prior to use are the most probable cause of axial forces being applied to the syringe assembly, which, but for the present invention, would risk advancing the syringe assembly or actuating the auto injector prematurely.

[0020] The drive spring is preferably a torsion spring. This is the simplest and therefore the most efficient and reliable way of driving the rotational movement of the driver. However, the rotational movement could in principle be driven by a linear spring acting through a gear.

[0021] In this specification, the term “distal” refers to the end of the auto injector that is directed towards the skin of the subject during use and the term “proximal” refers to the opposite direction.

[0022] The person operating the auto injector is referred to as the “user” and the person (or possibly animal) into whom the drug is injected is referred to as the “subject”. Often auto injector will be used for self-administration of the drug, in which case the user and the subject are the same person.

[0023] The term “drug” refers to any liquid composition, which it is desired to deliver into the skin of a subject. It is typically but not necessarily a pharmaceutical for treatment or management of a medical condition. Features of the auto injector such as the strength of the torsion spring and the length and pitch of the respective threaded connections can be varied. This may accommodate different volumes and viscosities of drug that are to be delivered and allow an appropriate duration of the injection to be determined.

[0024] The drawings

[0025] Figure 1 is a longitudinal cross section of an auto injector according to the invention, seen in a state prior to use.

[0026] Figure 2 is a side view of the syringe assembly of the auto injector of Figure 1.

[0027] Figure 3 is a partial longitudinal cross section of the auto injector of Figure 1, showing removal of the end cap.

[0028] P51752WO Figure 4 is a side view of a sub-assembly of the auto injector of Figure 1.

[0029] Figure 5 is a proximal end view of the sub-assembly of Figure 4.

[0030] Figure 6 is a perspective view of the proximal end of the auto injector of Figure 1.

[0031] Figure 7 is a sectional perspective view of the proximal end of the auto injector of Figure 1, after unlocking but prior to activation.

[0032] Figure 8 is a sectional perspective view similar to Figure 7, immediately after activation.

[0033] Figure 9 shows, in section, a detail of the button of the auto injector of Figure 1.

[0034] Figure 10 is a sectional perspective view of the proximal end of the auto injector of Figure 1, prior to unlocking.

[0035] Figure 11 is a sectional perspective view of the proximal end of the auto injector of Figure 1, after unlocking.

[0036] Figure 12 is a longitudinal cross section of the auto injector of Figure 1, in a state during drug delivery.

[0037] Figure 13 is a longitudinal cross section of the auto injector of Figure 1, in a state at the end of drug delivery.

[0038] Figure 14 is a longitudinal cross section of the auto injector of Figure 1, in a final state following retraction of the needle.

[0039] Figure 15 is a partial elevation of the plunger of the auto injector of Figure 1.

[0040] Figure 16 is a partial perspective view of the plunger of the auto injector of Figure 1.

[0041] Figure 17 is a sectional view of the auto injector of Figure 1, on a plane parallel to the axis, showing a latching mechanism at the end of drug delivery.

[0042] Figure 18 is a sectional view similar to Figure 17, showing the latching mechanism after retraction of the needle.

[0043] Figure 19 is a perspective view, in section on a plane parallel to the axis of the auto injector of Figure 1, of an engagement between the connecting tube and the driver.

[0044] Figure 20 is a partial, longitudinal cross section of the auto injector of Figure 1, showing an engagement between the driver and the housing.

[0045] Detailed description of preferred embodiments

[0046] The auto injector device of Figure 1 is enclosed in a housing 2, which defines a longitudinal axis 3. Within the housing 2, a syringe assembly 4 is mounted for axial

[0047] P51752WO movement to expose or retract a hollow needle 6 from a distal end of the device. In the initial configuration of the device shown in Figure 1, the distal end is protected by an end cap 7. Also mounted within the housing 2 is a pre-strained torsion spring 8, of which one end is attached to the syringe assembly 4 to be angularly fixed relative to the housing 2 and the other end is attached to a driver 10. The driver 10 is capable of rotation relative to the housing 2 but in the initial configuration it is restrained from rotating. To activate the auto injector, a button 12 at the proximal end may be pressed to release the driver 10 to rotate under the torque of the torsion spring 8. Through means that will be explained below, the rotation of the driver 10 controls, in sequence, a first movement of the syringe assembly 4 in the distal direction to expose the needle 6, the advance of a plunger 14 relative to the syringe assembly 4 to deliver a drug from the device through the needle 6, and a further movement of the syringe assembly 4 in the proximal direction to retract the needle 6 into the device.

[0048] Figure 2 shows the syringe assembly 4 separately from the device. An upper syringe clamp 16 and a lower syringe clamp 18 are snap-fitted together to secure between them the flange 20 at the proximal end of a syringe 22. The syringe 22 has an internal sliding piston 24 and the hollow needle 6 is attached to the distal end of the syringe in a conventional syringe arrangement. The syringe 22 is pre-filled with a drug (not illustrated) so that, by moving the piston 24 in the distal direction, the drug may be expelled from the syringe 22 and delivered to the subject through the needle 6. A connecting tube 26 is secured to the upper syringe clamp 16 and extends from it in the proximal direction, terminating in a pair of helical ramps 28. The upper and lower syringe clamps 16,18, the syringe 22, the needle 6 and the connecting tube 26 are preferably formed as separate parts but, once the device has been assembled, there is no relative movement between them and they function as a single part, termed the syringe assembly 4, during subsequent use of the device. The mounting of the syringe assembly 4 in the housing 2 is such that it can only slide in the axial direction but cannot rotate.

[0049] An end casing 30 is snap-fitted into the distal end of the housing 2 to surround and support the syringe 22. The end casing 30 also provides a distal face 32 of the device,

[0050] P51752WO which can rest against the skin of the subject when the auto injector is being used to deliver the drug. The distal face 32 has a central opening 33 and also provides a set of circumferentially spaced apertures 34, which can receive a corresponding set of resilient tabs 36 of the end cap 7 to secure the end cap 7 releasably to the device. The tabs 36 may be hooked so that the end cap 7 can be removed from the device by a twist- and-pull action. Additionally or alternatively, recesses (not illustrated) in the tabs 36 may releasably engage bump features (not illustrated) in the apertures 34 of the end casing 30 to resist removal of the end cap 7 until the user pulls with sufficient force.

[0051] As seen in Figure 1, a resilient needle sleeve (RNS) 38 engages the syringe and surrounds the needle 6 to protect and shield it from the environment during assembly and transport prior to use. The end cap 7 further comprises a plurality of snap arms 40, which extend in the proximal direction and end in hooks 42. When the end cap 7 is fitted to the device during assembly, the snap arms 40 flex outwards and over the needle sleeve 38 until the hooks 42 snap behind its proximal end. Figure 3 shows that when the end cap 7 is removed again, immediately prior to use of the device, the hooks 42 pull the needle sleeve 38 away from the syringe 22 and withdraw it through the central opening 33 of the end casing 30 to uncover the needle 6. The needle sleeve 38 is thereafter retained by the hooks 42 within the end cap 7.

[0052] Figures 4 and 5 show a sub-assembly comprising the driver 10 and the plunger 14. The plunger 14 is rotationally coupled to the driver 10 so that rotation of the driver 10 forces the plunger 14 to rotate but allows it to slide axially. In this embodiment of the invention, a pair of arms 44 formed at the proximal end of the plunger 14 slide in a pair of axial channels 46 formed in the driver. Distally of the arms 44, the plunger 14 comprises a shaft 48 that lies on the axis 3 of the assembled device. A mid-portion of the shaft 48 is provided with an external thread 50, which is capable of engagement with an internal thread 51 of the upper syringe clamp 16 in a first threaded connection. The aforementioned torsion spring 8 is seated at a distal end in the upper syringe clamp 16, which is rotationally locked to the housing. The torsion spring 8 is seated at a proximal end in the driver 10 so that, when the driver 10 is permitted to rotate under the torque exerted by the spring 8, it causes the plunger 14 to rotate within the first

[0053] P51752WO threaded connection 50,51 and to advance in the distal direction relative to the syringe assembly 4. A distal end of the plunger 14 extends into the syringe 22 and carries a bearing 52, which bears against the piston 24 to drive it forwards and expel a measured quantity of the drug from the syringe 22. The quantity is proportional to the number of turns the plunger 14 makes within the first threaded connection 50,51.

[0054] There exist possible alternative embodiments of the invention (not illustrated) in which, instead of the plunger 14 having a sliding engagement with the driver 10 and a threaded engagement with the upper syringe clamp 16, the plunger has a threaded engagement with the driver and a sliding engagement with the upper syringe clamp. Thereby, the plunger is rotationally fixed relative to the housing and, as the driver rotates around it, the first threaded engagement drives the plunger in the distal direction.

[0055] An external thread 54 of the driver 10 selectively engages a short internal thread 55 of the connecting tube 26 to form a second threaded connection. During delivery of the drug, the second threaded connection 54,55 couples the driver 10 to the syringe assembly 4 and is handed such that rotation of the driver 10 by the torsion spring 8 causes the syringe assembly 4 to advance gradually in the distal direction relative to the driver 10. The pitch of the second threaded connection 54,55 is smaller than the pitch of the first threaded connection 50,51.

[0056] A further coupling between the driver 10 and the syringe assembly 4 will now be described. The proximal end of the driver 10 comprises a radially extending flange 56, in which are formed a pair of arcuate slots 57. Each slot terminates in an angled internal surface. In the initial position of the auto injector illustrated in Figure 1, the pair of helical ramps 28 at the proximal end of the connecting tube 26 are located in the respective arcuate slots 57. When the driver 10 is first released for rotation under the torque of the torsion spring 8, the angled internal surfaces of the arcuate slots 57 bear against the helical ramps 28 and urge the connecting tube 26 - and hence the entire syringe assembly 4 - to move in the distal direction. After rotation of the driver 10 through approximately one half turn, the syringe assembly has moved through a predetermined distance that is sufficient for the ramps 28 to move clear of the arcuate

[0057] P51752WO slots 57 and disengage from the driver 10. The predetermined distance is also sufficient to advance the needle 6 beyond the distal face 32 of the auto injector and, if that distal face is applied to the skin of a subject, to insert the needle 6 into the skin.

[0058] While the driver 10 is engaged with the steeply pitched helical ramps 28 on the connecting tube 26, it cannot simultaneously be engaged with the short internal thread 55 of the connecting tube 26, which has a much shallower pitch. Accordingly, the external thread 54 may be omitted from a proximal end of the driver 10 over the predetermined distance. Alternatively, as illustrated in Figure 4, the external thread 54 may be provided with a lead-in portion 58 of comparable pitch to the helical ramps 28, which will not interfere with the rapid advancement of the connecting tube 26 but will guide the internal thread 55 into subsequent engagement with the main portion of the external thread 54 so that drug delivery can reliably follow.

[0059] A compression spring 60 is seated between the end casing 30 and the lower syringe clamp 18. (In other embodiments, the spring could be seated on the upper syringe clamp 16.) The compression spring 60 therefore resists the advancement of the syringe assembly 4 but the axial force on the syringe assembly 4 resulting from the torque exerted by the torsion spring 8 on the helical ramps 28 is sufficient to overcome the resistance of the compression spring 60. Energy is stored in the compression spring 60 as it becomes shorter.

[0060] Figures 6 and 7 show a locking dial 62 attached to the proximal end of the housing 2 so as to be axially fixed but capable of relative rotation. The locking function of the dial 62 will be explained below. A thread element 64 is fixed inside the locking dial 62 on the axis 3 of the auto injector and provides an external thread 65 that is stationary in the axial direction relative to the housing 2. As seen in Figures 12 to 14, the driver 10 further comprises an internal partial thread 66 that extends between the plunger arms 44 to engage the external thread 65 in a third threaded connection. The third threaded connection 65,66 couples the driver 10 to the housing 2 and is handed such that rotation of the driver 10 by the torsion spring 8 causes the driver 10 to retreat gradually in the proximal direction relative to the housing 2. The pitch of the third threaded

[0061] P51752WO connection 65,66 is equal to the pitch of the second threaded connection 54,55 whereby, as rotation of the driver 10 causes it to retreat relative to the housing 2, it also causes the syringe assembly 4 to advance relative to the driver 10 at the same rate. The net effect is that the syringe assembly 4 remains stationary relative to the housing 2 during delivery of the drug from the auto injector. When the driver initially exerts a force on the helical ramps 28 to advance the syringe assembly 4 in the distal direction, the reaction force on the driver 10 moves it in the proximal direction to come into engagement with the start of the third threaded connection 65,66 (if it is not already engaged as a result of the action of the compression spring 60).

[0062] Figure 7 shows the button 12, which is mounted inside the locking dial 62 and closes the proximal end of the auto injector. For ease of manufacture, in this embodiment of the invention the button 12 comprises a button cap 68, which is snap-fitted into a button sleeve 70. The button cap and sleeve 68,70 do not move relative to one another during use of the auto injector and in other embodiments they could be formed as a single part. The button sleeve 70 comprises a pair of arms 72 that extend in the distal direction and are received in corresponding axial channels 74 formed in the proximal end of the housing 2 to limit the button 12 to move only axially relative to the housing 2, without any rotation. The distal ends of the button arms 72 are radially enlarged to form circumferentially facing stop surfaces 76. When the button 12 is in its extended, unactuated position, as shown in Figure 1, the stop surfaces 76 of the button arms 72 are axially aligned with corresponding circumferential stop surfaces 78 formed on the flange 56 at the proximal end of the driver 10. The torque exerted by the torsion spring 8 on the driver 10 urges the respective stop surfaces 76,78 into engagement and blocks rotation of the driver 10.

[0063] Figure 8 shows that, when a user presses the button 12 to advance it in the distal direction to its actuated position, the stop surfaces 76 of the button arms 72 move clear of the stop surfaces 78 of the driver 10. This frees the driver 10 to rotate and activates the sequence of steps performed by the auto injector. Figure 9 shows how, when the button 12 is fully depressed, locking hooks 80 on the button cap 68 snap fit behind corresponding features on the locking dial 62 to prevent the button 12 returning to its

[0064] P51752WO unactuated position. This makes it clear to the user that the auto injector has already been actuated once and cannot be used again.

[0065] The locking dial 62 is provided to prevent the user from actuating the auto injector prematurely by accidentally pressing the button 12. As previously described, the locking dial 62 is attached to the housing 2 to as to be axially fixed but capable of a limited degree of rotation about the axis 3. The limits of rotation are defined by inward projections 82 of the locking dial 62, which butt in each rotational direction against the arms 72 of the button 12 that extend axially through the locking dial 62. The auto injector is supplied with the locking dial 62 in the locked position shown in Figure 10. The distal faces of arcuate tabs 84 on the button 12 engage the inward projections 82 of the locking dial 62 to block movement of the button 12 in the distal direction. By rotating the locking dial 62 to the unlocked position shown in Figure 11, the user can move its inward projections 82 out of alignment with the arcuate tabs 84, which frees the button 12 to be advanced through a sufficient distance to release the driver for rotation. Means such as a bump and a ramp (not illustrated) may be provided on the housing 2 and the locking dial 62 respectively to provide audible and / or tactile feedback to the user when they move the locking dial 62 between its locked and unlocked positions. Indicia may be provided on opposite sides of the junction between the housing 2 and the locking dial 62 to provide a visual indication to the user of whether the locking dial 62 is in its locked or unlocked position.

[0066] When the button 12 has been advanced to its actuated position, any attempt to return the locking dial 62 to its locked position will be blocked by the inward projections 82 butting against circumferential surfaces of the arcuate tabs 84. As previously described in relation to Figure 9, upon actuation locking hooks 80 on the button cap 68 prevent the button 12 returning to its unactuated position, which thereby also prevents the locking dial returning to its unlocked position.

[0067] The operation of the auto injector will now be described. From the initial state shown in Figure 1, the user removes the end cap 7 and applies the distal face 32 of the auto injector against the skin of the subject. They then rotate the locking dial 62 from the

[0068] P51752WO locked to the unlocked position. (This may be done before or after applying the device to the skin.) This frees the button 12 to be depressed from its unactuated to its actuated position. As previously described, the movement of the button arms 72 in the distal direction frees the driver 10 to rotate under the influence of the torsion spring 8.

[0069] Initially, the rotating driver 10 acts on the ramps 28 of the connecting tube 26 to drive the syringe assembly 4 in the distal direction. This rapidly advances the needle 6 from the distal end of the device to penetrate the skin of the subject and it also stores energy in the compression spring 60. The driver 10 also moves a short distance in the proximal direction to engage the third threaded connection 65,66 between it and the locking dial 62. When the syringe assembly 4 has moved through the predetermined distance, the driver 10 disengages from the ramps 28 and engages with the connecting tube 26 to form the second threaded connection 54,55. This state of the device is shown in Figure 12.

[0070] Continued rotation of the driver 10 now causes it to follow the second and third threaded connections 54,55; 65,66 and to retreat slowly in the proximal direction, while the syringe assembly 4 remains stationary relative to the housing 2. The rotation of the driver 10 also drives co-rotation of the plunger 14, which thereby advances into the syringe 22 along the first threaded connection 50,51 and displaces a predetermined dose of the drug through the needle 6. The third threaded connection 65,66 restrains movement of the driver 10 in the proximal direction, except by following that threaded connection. Eventually, after the plunger 14 has advanced sufficiently to deliver the desired dose, the driver 10 reaches the proximal end of the third threaded connection 65,66, as shown in Figure 13. This frees the compression spring 60 to push the syringe assembly 4 and the driver 10 back in the proximal direction, thereby withdrawing the needle 6 from the skin of the subject to reach the final state shown in Figure 14. The syringe assembly 4 and the driver 10 remain axially coupled via the second threaded connection 54,55.

[0071] It is important that the dose of drug delivered into the skin by the auto injector should be precisely controlled. The piston 24 must therefore be advanced in the syringe 22

[0072] P51752WO through the correct distance corresponding to the desired dose, and no drug should be expelled from the needle 6 before it is inserted into the skin or after it has been removed from the skin. This can be managed by careful design of the start and end of the first threaded connection 50,51.

[0073] The driver 10 rotates through approximately one half turn as it acts on the ramps 68 of the connecting tube 26 to drive insertion of the needle 6. The plunger 14 is rotationally locked to the driver 10 so it rotates by the same amount. However, the first threaded connection 50,51 between the plunger 14 and the upper syringe clamp 16 can be designed so that its threads do not engage during this initial rotation of the plunger 14 or, at least, the first part of it. For example, Figure 15 shows the plunger thread 50 in the form of a channel. The outline of the corresponding thread element 51 of the upper syringe clamp 16 is shown schematically in a first position (solid lines) and a second position (dashed lines). There is a cut-out 85 close to the distal end of the thread 50, in which a circumferential segment of both the proximal and distal walls of the channel is omitted. This allows the plunger to rotate through part of a turn while the thread element 51 moves from the first position to the second position relative to the cutout 85, without forcing the plunger 14 to advance. This allows time for the needle 6 to be inserted into the skin before the drug starts to be delivered through the needle 6. Alternatively, in the initial state of the device a sufficient gap can be left between the plunger bearing 52 and the piston 24 (as seen in Figure 1), such that during this phase the plunger 14 moves forwards but does not contact the piston 24 to advance it within the syringe 22.

[0074] At the end of the drug delivery step, it is important that the needle 6 should not be withdrawn from the skin before drug delivery is complete. Therefore, the first threaded connection 50,51, which controls drug delivery, is designed to run out (i.e.to reach the end of the helical thread and disengage) shortly before the third threaded connection 65,66, which determines when the syringe assembly 4 is retracted, runs out. To accommodate the continued rotation of the plunger 14 for a short period after it has already been advanced through the desired distance, an axial gap 86 extends circumferentially about the plunger shaft 48 at the proximal end of its external

[0075] P51752WO thread 50. This permits the plunger 14 to rotate without engaging the internal thread 51 of the upper syringe clamp 16 and without advancing further. Beside ensuring correct timing of the sequence of steps, this allows a short period for any inertial or non-steadystate effects to be resolved. For example, if the resilient piston 24 has been compressed while being driven forwards by the plunger 14, it can re-expand to its original size, which may displace further drug through the needle 6.

[0076] Figure 16 shows a cut-out 88 at the proximal end of the plunger thread 50. Specifically, the plunger thread 50 is in the form of a channel and, in the region of the cut-out 88, a circumferential segment is omitted, which forms a generally triangular portion between the proximal wall of the channel and the end of the thread 50. (The term “cut-out” in this specification should not be taken to imply that its formation necessarily involves any cutting process.) This omission has no effect during drug delivery, when it is the distal wall of the channel that is active, but it means that when the syringe assembly 4 is retracted, the plunger thread 50 does not have a rigid connection to the upper syringe clamp 16 which could cause further drug to be ejected after the needle 6 has been withdrawn from the skin. Some situations where such unwanted ejection of drug might otherwise occur will now be described.

[0077] • To simplify construction and ease the assembly process, the flange 20 of the syringe 22 is not held rigidly between the upper and lower syringe clamps 16,18 but has some axial freedom to move through a very short distance. When the retraction of the needle comes to an end, the driver 10 and syringe clamps 16,18 suddenly stop moving in the proximal direction but the inertia of the syringe 22 can cause it to continue moving through that short distance. If the plunger 14 were rigidly axially fixed to the upper syringe clamp 16, this would result in a small relative movement between the plunger 14 and the syringe 22, which could expel a few drops of the drug. Instead, the cut-out 88 permits some proximal movement of the plunger 14 relative to the upper syringe clamp 16, whereby the plunger follows the movement of the syringe 22 and does not cause displacement of the piston 24.

[0078] P51752WO • Similarly, there may be some axial play in the coupling between the upper and lower syringe clamps 16,18. During drug delivery, the plunger 14 acting through the first threaded connection 50,51 pulls the upper syringe clamp 16 in the proximal direction, while friction and viscous forces in the syringe 22 pull the lower syringe clamp 18 in the distal direction so the coupling is in tension. When the device switches to the needle retraction phase, the situation is reversed. There is no longer any significant axial force on the upper syringe clamp 16 but the compression spring 60 pushes the lower syringe clamp 18 in the proximal direction so the coupling moves into compression. This may cause the lower syringe clamp 18 and with it the syringe 22 to move proximally relative to the upper syringe clamp 16. If the plunger 14 were rigidly axially fixed to the upper syringe clamp 16, this would result in a small relative movement between the plunger 14 and the syringe 22, which could expel a few drops of the drug. Instead, the cut-out 88 permits proximal movement of the plunger 14 relative to the upper syringe clamp 16, whereby the plunger follows the movement of the syringe 22 and does not cause displacement of the piston 24.

[0079] • Finally, it has already been mentioned that the rubber piston 24 might expand axially when the pressure on it is released at the end of drug delivery and the start of needle retraction. If the plunger 14 were rigidly axially fixed to the upper syringe clamp 16, the plunger bearing 52 would prevent any expansion of the piston in the proximal direction so it could only expand distally, which might expel a few drops of the drug. Instead, the cut-out 88 permits some proximal movement of the plunger 14 relative to the upper syringe clamp 16, whereby the piston 24 can expand proximally and push the plunger back, without expelling any further drug from the syringe 22.

[0080] It has been explained that, once the button 12 has been fully depressed, it becomes locked in its actuated position and cannot be used to actuate the device again. The positions of the torsion spring 8 and the respective threaded connections 50,51; 54,55; 65,66 also preclude the device being re-actuated by the same mechanism. However, it

[0081] P51752WO would in principle be possible for the needle 6 to be extended again, perhaps by shaking the device or by subjecting it to an impact sufficient to overcome the force of the compression spring 60. This could cause physical injury or cross-contamination. To avoid this and increase the safety of the device, the auto injector preferably provides latching means for securing the driver 10 and syringe assembly 4 against distal movement after they have been retracted.

[0082] Figure 17 shows that the locking dial 62 includes a guide element 90, which extends generally in the proximal direction. At a proximal end of the guide element 90, it transitions into a ledge 92, faces generally in the proximal direction. As previously explained, after the auto injector has been actuated the locking dial 62 is blocked against rotation, therefore both the guide element 90 and the ledge 92 are stationary with respect to the housing 2. The flange 56 of the driver 10 comprises a latching element 94, which has both circumferentially and distally facing surfaces.

[0083] When the third threaded connection 65,66 between the driver 10 and the thread element 64 of the locking dial 62 runs out, the driver 10 is pushed in the proximal direction by the compression spring 60. It also continues to rotate under the torque of the torsion spring 8, which brings the circumferential face of the latching element 94 into contact with the stationary guide element 90. The proximal movement of the driver 10 continues and the latching element 94 slides along the guide element 90 until it reaches the proximal end. There the torque of the torsion spring 8 moves the latching element 94 behind the ledge 92 so that if the device is manipulated in a way that might cause the driver 10 to move in the distal direction, the latching element 94 will contact the ledge 92 and block such movement.

[0084] Further axial movement of the driver 10 may be blocked by the inward projections 82 of the locking dial 62. Further rotation of the driver 10 may be blocked by a generally axially aligned stop surface 96 of the locking dial 62, which is angularly offset from the guide element 90 and against which the latching element 94 of the driver 10 comes to rest, as shown in Figure 18. The residual torque of the torsion spring 8 pushing the latching element 94 against the stop surface 96 should be sufficient to hold the latching

[0085] P51752WO element 94 on the ledge 92 but the ledge 92 could be inclined to the axis 3 or could be provided with a lip (not illustrated) to hold the latching element 94 more securely.

[0086] In the unactuated state of the device, the driver 10 is prevented from rotating by the abutment between the circumferential stop surfaces 78 of the driver 10 and the stop surfaces 76 of the button arms 72. The action of removing the end cap 7 to pull the resilient needle sleeve 38 away from the needle 6 exerts a distal force on the syringe 22 and hence on the whole syringe assembly 4, which the compression spring 60 is not sufficiently strong to resist. Figure 19 illustrates one way of preventing this unwanted distal movement of the syringe before the device has been actuated. The connecting tube 26 of the syringe assembly 4 further comprises one or more recesses 98, which each include a generally distally facing, inclined stop surface 99. The flange 56 of the driver 10 comprises corresponding projections 100, which each include a generally proximally facing, inclined stop surface 101. When the driver 10 is in its initial position and the button 12 is in its unactuated position, if a distal force is exerted on the syringe assembly 4, it will be prevented from moving in the distal direction by engagement between the respective stop surfaces 99,101. The driver 10 is in turn prevented from moving in the distal direction by its flange 56 abutting against an internal shoulder 102 of the housing, as shown in Figure 20.

[0087] The angle of inclination of the respective stop surfaces 99,101 is greater than or equal to the angle of inclination of the ramps 28. This means that, when the device is actuated and the driver 10 rotates away from its initial position to drive axial movement of the syringe assembly via the ramps 28, the stop surfaces 99,101 will disengage and avoid interfering with that movement. Prior to actuation of the device, the inclined stop surfaces 99,101 will act as a cam to resolve any axial force exerted on the syringe assembly 4 partly in the circumferential direction and thereby push the circumferential stop surfaces 78 of the driver 10 with greater force against the stop surfaces 76 of the button arms 72.

[0088] It should be apparent that many of the aspects of the disclosed auto injector are capable of being adopted independently in alternative embodiments. For example:

[0089] P51752WO • The means for unlocking and triggering actuation of the auto injector are independent of the means for controlling the sequence of steps performed by it, namely, advancing the needle, delivering the drug and retracting the needle.

[0090] • The ledge that prevents the syringe assembly advancing again after it has been retracted is independent of the means for controlling the initial advancement of the needle (if any) or for delivering the drug. It has potential application to any auto injector comprising an element that is driven rotationally and is also axially coupled to a retractable syringe assembly.

[0091] • The means for controlling the relative timing of the end of drug delivery and retraction of the needle is independent of the means for controlling advancement of the needle (if any).

[0092] • The means for preventing premature actuation when the end cap is removed is independent of the means for controlling the initial advancement of the needle (if any) or for delivering the drug. It has potential application to any auto injector comprising an element that is driven to rotate but is restrained from rotating until freed by the distal movement of another element.

[0093] P51752WO

Claims

CLAIMS1. An auto injector comprising: a housing, which defines an axis; a syringe assembly which is fixed against rotation relative to the housing, the syringe assembly comprising a syringe, a hollow needle and a ramp; a driver which is capable of rotation about the axis relative to the housing but is restrained against movement in a distal direction beyond an axial limit; and a drive spring configured to urge the driver to rotate, whereby the driver acts on the ramp to drive the syringe assembly to move in a distal direction and advance the needle relative to the housing; wherein the syringe assembly further comprises a stop surface, which faces generally in the distal direction, and the driver comprises a corresponding stop surface, which faces generally in the opposite direction, such that, when the driver is in an initial position before actuation of the auto injector, the respective stop surfaces engage one another to prevent the syringe assembly being moved in the distal direction and, when the driver rotates away from its initial position, the stop surfaces disengage one another to permit the syringe assembly to move in the distal direction.

2. An auto inj ector according to claim 1 , wherein the housing comprises a shoulder that faces away from the distal direction and prevents the driver moving beyond the axial limit.

3. An auto injector according to claim 1 or claim 2, further comprising a button at a proximal end of the auto injector, wherein: the button comprises at least one arm that extends in the distal direction; the button is capable of moving in the distal direction from an unactuated position to an actuated position; when the button is in the unactuated position, the arm blocks rotation of the driver away from its initial position; andP51752WOwhen the button moves to the actuated position, the arm releases the driver to rotate away from the initial position under the force of the drive spring and drive the movement of the syringe assembly.

4. An auto injector according to claim 3, wherein the stop surfaces of the driver and the syringe assembly are inclined to the axis such that when a force is exerted on the syringe assembly in the distal direction, the mutually engaging stop surfaces act as a cam to push the driver with greater force against the arm of the button in the direction of rotation.

5. An auto injector according to claim 4, wherein the angle of inclination of the stop surface of the syringe assembly is greater than or equal to the angle of inclination of the ramp.

6. An auto injector according to any preceding claim, wherein the stop surface of the syringe assembly is formed in a recess of the syringe assembly; and wherein the stop surface of the driver is formed on a corresponding projection of the driver.

7. An auto injector according to any preceding claim, further comprising a needle sleeve, which is releasably secured to the syringe; and a cap, which is engaged with the needle sleeve such that withdrawing the cap in the distal direction removes the needle sleeve from the syringe and uncovers the needle.

8. An autoinjector according to any preceding claim, wherein the drive spring is a torsion spring.P51752WO

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