TORSION SPRING FIXATION IN AUTOMATIC DRUG DELIVERY DEVICE

Abstract

A drug injection device (1) comprising a dose expelling mechanism for expelling drug from a reservoir through a reservoir outlet, the dose expelling mechanism comprising: a first component (40), a second component (10, 10, 15, 10, 15), and a torsion spring element (20) for inducing relative rotation between the first component (40) and the second component (10, 10, 15, 10, 15) to execute a dose expelling operation, the torsion spring element (20) extending along a longitudinal axis and comprising: a first spring end portion (22) being rotationally restrained with respect to the first component (40), and a second spring end portion (23) exhibiting an end portion inner diameter in a relaxed state of the torsion spring element (20), wherein the second spring end portion (23) is fitted over a spring receiving portion (11) of the second component (10, 10, 15, 10, 15), the spring receiving portion (11) having a transversal dimension which is larger than the end portion inner diameter, and the fitting of the second spring end portion (23) over the spring receiving portion (11) being configured to provide a sticking friction engagement between the two when the torsion spring element (20) is strained to exert a maximum in-use torque.

Claims

1. A drug injection device comprising a dose expelling mechanism for expelling drug from a reservoir through a reservoir outlet, the dose expelling mechanism comprising: a first component, a second component, and a torsion spring element for inducing relative rotation between the first component and the second component to execute a dose expelling operation, the torsion spring element extending along a longitudinal axis and comprising: a first spring end portion being rotationally restrained with respect to the first component, and a second spring end portion exhibiting an end portion inner diameter in a relaxed state of the torsion spring element, wherein the second spring end portion is fitted over a spring receiving portion of the second component, the spring receiving portion having a transversal dimension which is larger than the end portion inner diameter, and the fitting of the second spring end portion over the spring receiving portion being configured to provide a sticking friction engagement between the two when the torsion spring element is strained to exert a maximum in-use torque.

2. A drug injection device according to claim 1, further comprising a housing accommodating at least a portion of the dose expelling mechanism, wherein one of the first component and the second component is arranged stationarily relative to the housing, and the other of the first component and the second component is capable of rotation relative to the housing about the longitudinal axis.

3. A drug injection device according to claim 2, further comprising a dose setting mechanism for setting of a dose of drug to be expelled by the dose expelling mechanism, the dose setting mechanism comprising a dose setting button being rotatable about the longitudinal axis between a zero dose set position and a maximum dose set position by a user providing a torque to overcome a ratchet coupling between the dose setting button and the housing, wherein the second component is axially movable between a first position in which the spring receiving portion is rotationally interlocked with the dose setting button and a second position in which the spring receiving portion is rotationally decoupled from the dose setting button.

4. A drug injection device according to claim 3, further comprising a dose release button for activating the dose expelling mechanism to expel a set dose of drug, wherein the second component is adapted to, during movement from the first position to the second position, rotationally interlock with a rotatable piston rod guide being rotationally interlocked with the piston rod, and wherein the second component is operatively coupled with the dose release button and configured to move from the first position to the second position in response to the dose release button moving relative to the housing from a proximal position to a distal position.

5. A drug injection device according to claim 4, wherein the second component is further configured to move from the second position to the first position in response to the dose release button moving relative to the housing from the distal position to the proximal position.

6. A drug injection device according to claim 5, wherein the dose release button is biased towards the proximal position.

7. A drug injection device according to claim 1, wherein the second component comprises a first part and a second part arranged end to end and friction fitted in an overlap zone, wherein at least a portion of the overlap zone is surrounded by the spring receiving portion.

8. A drug injection device according to claim 1, wherein the first spring end portion exhibits a second end portion inner diameter in the relaxed state of the torsion spring element, and wherein the first spring end portion is fitted over a spring receiving portion of the first component, the spring receiving portion of the first component having a transversal dimension which is larger than the second end portion inner diameter, and the fitting of the first spring end portion over the spring receiving portion of the first component being configured to provide a sticking friction engagement between the two when the torsion spring element is strained to exert the maximum in-use torque.

9. A drug injection device according to claim 1, wherein the torsion spring element is a helical spring, and wherein the second spring end portion comprises at least one winding and at most five windings.

10. A drug injection device according to claim 9, wherein the first spring end portion comprises at least one winding and at most five windings.

11. A drug injection device according to claim 9, wherein the helical spring has a constant inner diameter when in the relaxed state.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In the following the invention will be further described with references to the drawings, wherein

[0044] FIG. 1 shows a torsion spring as used in state of the art automatic injection devices,

[0045] FIG. 2 shows a drive member and a torsion spring member of a drive assembly for use in a drug delivery device according to an embodiment of the invention,

[0046] FIG. 3 is a longitudinal perspective section view of the drive assembly members of FIG. 2,

[0047] FIG. 4 is a perspective cut-out of a drive member in an alternative configuration, and

[0048] FIG. 5 is a longitudinal section view of a proximal portion of a drug delivery device according to another embodiment of the invention.

[0049] In the figures like structures are mainly identified by like reference numerals.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0050] When in the following relative expressions, such as upper and lower, are used, these refer to the appended figures and not necessarily to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as their relative dimensions are intended to serve illustrative purposes only.

[0051] FIG. 1 shows a torsion spring 120 as conventionally used in the art of torsion spring based automatic injection devices. The torsion spring 120 comprises a helically coiled body 121 which extends longitudinally between a proximal end portion 122 and a distal end portion 123. The proximal end portion 122 terminates in a proximal (open) hook 124 while the distal end portion 123 terminates in a distal (open) hook 125. In conventional applications the proximal hook 124 is attached to a proximal receiving section which is stationary with respect to the injection device housing, and the distal hook 125 is attached to a distal receiving section arranged on a rotatable drive member capable of causing activation of a piston rod by rotation about its own axis of extension. The torsion spring 120 thus serves as a rotational energy storage component and is arranged to exert a torque to rotate the drive member during release of the stored energy.

[0052] In relation to the manufacturing of such a torsion spring the bending of a spring end is a cost increasing procedure. Furthermore, given the intended use of the torsion spring in a high precision drug injection device the position of the hooked ends relative to one another is of great importance, since the manufacturer optimally wants each hooked end to engage the dedicated receiving section in the injection device in a fully relaxed state of the torsion spring, such that a predictable torque level is produced in the torsion spring during the subsequent pre-straining. Such pre-straining is performed in order to ensure that a certain torque level is always available in the torsion spring so that even the smallest dose can be expelled. In other words it is important for this type of torsion spring that the hooked ends are arranged such that they align with their respective receiving sections during assembly of the injection device. This is a difficult and even further cost increasing task.

[0053] According to the present invention, this task can be eliminated by using an arrangement as indicated in FIGS. 2, 3 and 5.

[0054] FIGS. 2 and 3 show, in a perspective view, respectively a longitudinal perspective section view, a longitudinally extending torsion spring 20 arranged about a two-component drive member 10, 15 consisting of an actuation member 10 and a transmission tube 15 arranged in longitudinal extension of one another and operatively coupled by a snap fit connection.

[0055] The function of the drive member 10, 15 is similar to that of an alternative single-component drive tube 10 described in detail below in the context of an injection device. Thus, the two-component embodiment simply serves to illustrate the principle solution to the above highlighted challenges.

[0056] The torsion spring 20 comprises a helical coil body 21, a proximal end portion 22 terminating in a hook 24, and a distal end portion 23, which is wrapped around a spring receiving portion 11 of the actuation member 10. It is noted that in a relaxed state, i.e. before attachment to the actuation member 10, the torsion spring 20 has a constant spring inner diameter, d. However, since the spring receiving portion 11 has an outer diameter, D, which is greater than the spring inner diameter, d, the distal end portion 23 is actually radially expanded over the spring receiving portion 11, and as a result a sticking friction engagement between the two is provided due to the compressional force exerted by the distal end portion 23.

[0057] The number of windings and the radial expansion of the distal end portion 23 about the spring receiving portion 11 required to hold a specific torque exerted by the torsion spring 20 can be determined from the belt friction equation. The spring/drive member connection should be dimensioned to hold a specific torque which corresponds to the maximum in-use torque of the torsion spring 20, i.e. the torque exerted by the torsion spring 20 when strained to deliver the maximum dose offered by the injection device. Hence, the following expression must be satisfied:

[00002]$\mathrm{EID}.Math.\frac{R_2-R_1}{2.Math.R_1.Math.R_2^2}.Math.\left(e^{2.Math.N.Math..Math.}-1\right)T_{\mathrm{ma}.Math..Math.x}$

[0058] where E is the elastic modulus of the spring, I is the area moment of inertia of the spring, D is the diameter of the clutch arbor (the spring receiving portion), R.sub.1 is the radius of the neutral axis of the spring end portion when in a free, unexpanded state, R.sub.2 is the radius of the neutral axis of the spring end portion when wrapped around the spring receiving portion, N is the number of spring coils being wrapped around the spring receiving portion, is the coefficient of friction, and T.sub.max is the maximum in-use torque of the spring.

[0059] The fact that the distal end portion 23 is wrapped around the spring receiving portion 11 in this manner and thereby is rotationally fixed with respect to the actuation member 10 eliminates the need for a hook at this end of the torsion spring 20. The cost of producing the torsion spring 20 is thereby reduced. Furthermore, the assembly process is simplified because only one hooked spring end needs to be aligned with a dedicated receiving section. The distal end portion 23 will be in sticking friction engagement with the spring receiving portion 11 regardless of the angular orientation of the distal end portion 23.

[0060] Notably, the wrapped distal end portion 23 additionally tightens the snap fit connection between the actuation member 10 and the transmission tube 15 in that the compressional force exerted on the spring receiving portion 11 tends to press the spring receiving portion 11 radially against a coupling portion 16 of the transmission tube 15. The tightened snap fit connection increases the rotational stiffness of the two-component drive member 10, 15 and improves its reliability as a torque resisting structure.

[0061] FIG. 4 shows a portion of an alternative two-component drive member 10, 15 consisting of an actuation member 10 and a transmission tube 15 arranged in longitudinal extension of one another and operatively coupled by a snap fit connection, similar to the snap fit connection of the drive member 10, 15 shown in FIGS. 2 and 3.

[0062] The actuation member 10 has a proximal interface 12 which is adapted for interaction with the torsion spring 20. The proximal interface 12 has a circular-cylindrical basic shape of diameter, d, but is provided with a plurality of circumferentially distributed, longitudinally extending ridges 11, whereby the transversal dimension of the proximal interface 12 is increased so that a radial expansion of the distal end portion 23 is needed for it to fit over the proximal interface 12. An arrangement is thus provided whereby the distal end portion 23 may be rotationally fixed to a spring receiving section in a sticking friction engagement, similarly to what is described above, without requiring the basic shape diameter of the spring receiving section be greater than the spring inner diameter, d.

[0063] FIG. 5 shows an exemplary use of the torsion spring 20 in a pen-type drug injection device 1. Only a proximal portion of the injection device 1 is shown. The undisclosed distal portion may be realised as such are conventionally in the art of pen-type injection devices.

[0064] The injection device 1 has a generally circular-cylindrical housing 2 which accommodates a portion of a drug containing cartridge 30, being sealed by, respectively, a slidable piston 31 and a needle-penetrable rubber septum (not shown), as well as a drive mechanism for advancing the piston 31 through the cartridge 30. A rotatable, 2K moulded, dose dial 3 is arranged at the proximal end of the housing 2 for allowing a user to select a dose to be delivered from the cartridge 30.

[0065] A piston rod 9 extends longitudinally within the housing 2 and is in threaded engagement with a nut member 7 thereof. The piston rod 9 has a distal end face which during use of the injection device 1 abuts a piston washer 8 serving as a force distributor for the piston 31. The piston rod 9 further has a longitudinally extending groove (not visible) providing for rotational interlocking engagement with a piston rod guide 70, a key 71 of the latter being slidably received in the groove.

[0066] The piston rod guide 70 is axially fixed in, but capable of unidirectional rotation relative to, the housing 2. On an interior surface the piston rod guide 70 is provided with teeth 72 adapted for disengageable engagement with mating teeth 19 on an exterior portion of a drive tube 10.

[0067] The drive tube 10 is a unitary structure which extends axially in the housing 2 and is arranged about a portion of the piston rod 9. A distal portion of the drive tube 10 has longitudinal slits (not visible) to allow an otherwise exteriorly arranged scale drum 60 to extend radially through the drive tube 10 and threadedly engage with the piston rod 9 via an interior nut member 61. In addition to coupling the scale drum 60 directly to the piston rod 9 this provides a rotational interlocking connection between the drive tube 10 and the scale drum 60.

[0068] A proximal portion 15 of the drive tube 10 comprises a circumferential toothing 18 adapted for disengageable engagement with an interior toothed collar 4 of the dose dial 3, and a radially inwardly extending catch portion 17 firmly gripping a harpoon member 6 of a proximal injection button 5, thereby providing a translational interlocking connection between the drive tube 10 and the injection button 5. The injection button 5 is axially movable between an inactive position (shown in FIG. 5), in which the injection device 1 is in a dose setting state, and an active position, in which the injection device 1 is in a dose delivery state. A compression spring 50 arranged to act between an interior surface of the injection button 5 and an upper transversal surface of the toothed collar 4 biases the injection button 5 towards the inactive position. The proximal most position of the injection button 5 relative to the housing 2 is defined by a flange 14 provided on the proximal portion 15 of the drive tube 10, which flange 14 serves to limit proximal movement of the drive tube 10 relative to a spring base 40 being axially and rotationally fixed with respect to the housing 2.

[0069] The drive mechanism is powered by the torsion spring 20. The hook 24 of the proximal end portion 22 is rotationally anchored to the spring base 40, in a manner conventionally known in the art, and the spring base 40 thus provides a stationary reference point for the torsion spring 20. The distal end portion 23 is wrapped around a spring receiving portion 11 of the drive tube 10 so as to establish a rotational interlocking connection between the distal end portion 23 and the drive tube 10. The spring receiving portion 11 has an outer diameter, D, which is larger than the inner diameter, d, of the torsion spring 20, which means that the distal end portion 23 is elastically expanded over the spring receiving portion 11 and thus exerts a compressional force thereonto. In the shown embodiment three windings of the torsion spring 20 overlap the spring receiving portion 11 and thus ensure a sticking friction engagement with the drive tube 10.

[0070] The spring base 40 has a proximally facing serrated surface 44 capable of slipping engagement with a distally facing serrated interior surface 84 of the dose dial 3, thereby providing a ratchet mechanism for preventing relaxation of the torsion spring 20 during dose setting.

[0071] In the following a situation of use of the injection device 1 will be described.

[0072] The injection device 1 is prefilled in the sense that it carries the cartridge 30 when delivered from the manufacturer, and the basic user steps required to administer a dose of the contained drug (e.g. insulin, glp-1 or a mixture thereof) are simple and relatively fast to execute: 1) attach an injection needle assembly (not shown) to the distal end of the cartridge 30, 2) dial a desired dose, 3) insert the injection needle at a suitable injection site, and 4) initiate an expelling of the set dose by releasing the torsion spring 20. Steps 1) and 3) may be performed in accordance with common ways of attaching an injection needle assembly to a pen-type injection device and subsequently inserting the injection needle into the skin, and since these steps are irrelevant to the description of the present invention they will not be discussed any further in this text.

[0073] So, in order to set a dose to be administered the user may hold the housing 2 in one hand and use the other hand to rotate the dose dial 3 relative to the housing 2, about a longitudinal axis of the injection device 1. The injection button 5 is in its inactive position, i.e. its proximal most position relative to the housing 2, due to the biasing force from the compression spring 50. Due to the engagement between the harpoon member 6 and the catch portion 17 the drive tube 10 is accordingly situated in a proximal most position. In the proximal most position of the drive tube 10 the circumferential toothing 18 is radially aligned with the interior toothed collar 4, thereby rotationally interlocking the drive tube 10 and the dose dial 3. A rotation of the dose dial 3 about the longitudinal axis, in either direction, thus results in a corresponding rotation of the drive tube 10.

[0074] The rotation of the drive tube 10 occasions both a helical travel of the scale drum 60 within the housing 2 and a twisting of the torsion spring 20. Due to the rotational interlocking connection between the drive tube 10 and the scale drum 60 the scale drum 60 is forced to rotate in accordance with the rotation of the drive tube 10, and as the piston rod 9 is held stationary by the piston rod guide 70, the threaded engagement between the piston rod 9 and the nut member 61 causes the scale drum 60 to move helically about the piston rod 9. The scale drum 60 carries a plurality of dose related indicia (not shown), and a window (not visible) in the housing 2 allows the user to see a subset of these dose related indicia pass by as the scale drum 60 moves and to determine the size of the set dose from the current position of the scale drum 60. Furthermore, as the spring receiving portion 11 of the drive tube 10 is rotated relative to the spring base 40 the wrapped distal spring end portion 23 is angularly displaced relative to the proximal spring end portion 22, an amount corresponding to the angular displacement of the spring receiving portion 11 due to the sticking friction engagement between the two, whereby rotational energy is stored in the torsion spring 20. The ratchet connection between the proximally facing serrated surface 44 of the spring base 40 and the distally facing serrated interior surface 84 of the dose dial 3 allows the user to set the desired dose by rotating the dose dial 3 relative to the housing 2 in discrete steps, and to reduce a set dose by rotating the dose dial 3 in the reverse direction.

[0075] In order to expel a set dose from the cartridge 30 the injection button 5 is depressed, whereby the compression spring 50 is compressed and the drive tube 10 is moved distally in the housing 2. The distal movement of the drive tube 10 causes the circumferential toothing 18 to slide out of engagement with the interior toothed collar 4 and the teeth 19 to slide into engagement with the mating teeth 72 on the interior surface of the piston rod guide 70. Notably, the resulting rotational interlocking of the drive tube 10 with the piston rod guide 70 takes effect before complete disengagement of the circumferential toothing 18 from the interior toothed collar 4.

[0076] At some point during the depression of the injection button 5 the circumferential toothing 18 disengages completely from the interior toothed collar 4, whereby the energy stored in the torsion spring 20 is released and the distal end portion 23 is returned to its pre-dose setting position. The rotation of the distal end portion 23 causes a corresponding rotation of the drive tube 10 due to the sticking friction engagement between the wrapped spring windings and the spring receiving portion 11.

[0077] Due to the established rotational interlocking engagement between the drive tube 10 and the piston rod guide 70 the piston rod guide 70 is forced to rotate, whereby the key 71, being engaged in the longitudinally extending groove of the piston rod 9, causes a corresponding rotation of the piston rod 9, which is then helically advanced through the nut member 7 to displace the piston washer 8 and the piston 31 distally in the cartridge 30.

[0078] Simultaneously, the scale drum 60 rotates along with the drive tube 10, and is thereby helically displaced in the distal direction along with the piston rod 9, until a portion of the scale drum 60 meets a rotational stop surface (not visible) in the housing 2. At this stop surface the scale drum 60 is in an end-of-dose position in the housing 2.

[0079] As the rotation of the scale drum 60 stops, so does the rotation of the drive tube 10 and thereby also the rotation of the piston rod guide 70 and the piston rod 9 is resultantly halted in the nut member 7. When the user discontinues her force on the injection button 5 the compression spring 50 expands and returns the injection button 5 to the inactive position. The drive tube 10 is thereby moved proximally in the housing 2 until the flange 14 meets a stop surface (not visible) on the spring base 40. In this position of the drive tube 10 the circumferential toothing 18 has re-engaged with the interior toothed collar 4 and the teeth 19 have disengaged from the teeth 72, rotationally decoupling the drive tube 10 and the piston rod guide 70. The injection device 1 is now ready for the setting of a new dose.

[0080] It is noted that even though the injection device 1 exemplifies the invention by the distal end portion 23 of the torsion spring 20 being wrapped around the spring receiving portion 11, an alternative embodiment could be envisaged in which the proximal end portion 22 was wrapped around e.g. an axially protruding structure in the spring base 40 and the distal end portion 23 had a hook for attachment to the drive tube 10. In a further alternative embodiment both the proximal end portion 22 and the distal end portion 23 could be wrapped around suitable geometries on, respectively, the spring base 40 and the drive tube 10.