Patent Description:
Injection devices are used for the convenient administration of medicaments to patients. For example, injection devices, which may be auto-injectors, may be used for providing a single metered dose of a medicament. Such devices may be either single use "disposable" devices in which the device is typically provided with a syringe already installed, and which is not user-replaceable, or "reusable" devices which allow the user to replace the syringe when the medicament has been used.

It is noted that whilst the term "syringe" is used herein for clarity and consistency, this term is not intended to be limiting. In some arrangements the syringe may for example be a cartridge (which, for example, may be arranged to receive a disposable needle) or other medicament container. In some arrangements the syringe/cartridge/medicament container may be formed integrally with the (or part of the) injection device.

Injection devices may be provided in the form of an auto-injector device, in which delivery of the medicament is automated and the device may also be arranged to automate the insertion of a needle into the skin prior to the delivery of the medicament. However, it is noted that the term auto-injector may encompass injection devices that automatically insert the needle and devices which require the user to manually insert the needle.

Injection devices generally comprise a firing mechanism that is arranged to deliver a fluid from the syringe automatically under the force of a drive system, such as a drive spring. Optionally, injection devices may also comprise an insertion mechanism to displace the syringe within a housing of the injection device to cause needle penetration. The delivery arrangement generally acts via a plunger which includes a plunger and may also include or engage a piston (also referred to as a "bung") which is slidably provided within the syringe.

Injection devices may be designed and manufactured to accommodate different syringes. This provides a device that may be adapted to carry and operate syringes with different features and/or characteristics, such as different fill volumes. It is desirable to improve the safety and operability of such devices. <CIT> discloses an injection device having variable dosing.

Different syringes may be filled with different volumes of fluid, such as medicament. A difference in fill volume of a syringe results in a different axial position of the bung prior to use of the syringe as it is the bung that sets the useable volume within the barrel of the syringe. The inventors have realised that where a gap exists between an end of the plunger and the bung, e.g. because the fill volume is relatively low, the plunger moves in free space for a distance before engaging the bung. This movement in free space can cause the plunger to accelerate to velocities that are too high, such that when the plunger contacts the bung, damage may be caused to the syringe and discomfort may be caused to the recipient of an injection.

Methods and apparatus disclosed herein may be arranged to mitigate or solve one or more problems associated with the art, including those mentioned above and/or elsewhere herein.

According to the invention in an aspect, there is provided an assembly for a firing mechanism for use in an injection device and comprising: a rear cap having an elongate member extending axially forwards when installed within the injection device; a plunger releasably connected, directly or indirectly, to the elongate member and, upon release of the connection to the elongate member, axially displaceable in a forward direction; and a plunger driver to drive the plunger axially forwards upon release of the connection to the elongate member, wherein the plunger and the rear cap define an axial length of the assembly, which in turn determines a start position of a forward end of the plunger before release of the connection to the elongate member, and wherein, during construction of the assembly, the plunger is releasably connectable, directly or indirectly, to the elongate member at one of a plurality of positions for controlling the axial length of the assembly.

By controlling the length of the axial length of the assembly, the acceleration of the plunger under the force of the plunger driver before contacting the bung may be controlled. Further, it should be noted that the plurality of positions of the releasable connection may be discrete positions or may be a continuously varying.

Optionally, one or both of the plunger and the elongate member comprises a plurality of connection features for connection between the plunger and the elongate member during construction of the assembly.

Optionally, the plurality of connection features comprises a plurality of recesses or apertures on the elongate member, and wherein the plunger comprises a lug configured to be received in one of the plurality of recesses or apertures.

Optionally, the elongate member comprises an axially extending channel and wherein the recesses extend from a sidewall of the channel.

Optionally, the lug of the plunger is configured to travel within the channel and to enter one of the recesses on relative rotation between the plunger and the elongate member.

Optionally, the assembly further comprises a clutch that is rotationally coupled to the plunger, such that rotation of the clutch causes rotation of the plunger, and the clutch is configured to rotate upon activation of the injection device to rotate the plunger and move the lug from one of the plurality of recesses into the channel, to release the connection between the plunger and the elongate member.

Optionally, the plurality of connection features comprises a plurality of recesses or apertures on one of the plunger and the elongate member, and wherein the other of the plunger and the elongate member comprises a prong radially biased and having a resting position configured to engage a projection of the prong with one of the plurality of recesses or apertures.

Optionally, the plunger comprises the plurality of connection features and the rear cap comprises the prong.

Optionally, the rear cap comprises the plurality of connection features and the plunger comprises the prong.

Optionally, the assembly further comprises a clutch positioned around the plunger or the rear cap such that an inner wall of the clutch is presented to the prong, wherein the clutch has a fist inner diameter and a second inner diameter greater than the first inner diameter, and wherein the clutch is moveable between a first position in which the first diameter is presented to the prong and prevents radially outward displacement thereof, and a second position in which the second diameter is presented to the prong and provides a space between the prong and the inner wall of the clutch, allowing radially outward displacement of the prong.

Optionally, the rear cap comprises a keyed aperture, and the plunger comprises a plurality of keyed sections having a cross section corresponding to the keyed aperture and at least one lockout section positioned between the keyed sections and configured to permit rotation of the plunger within the keyed aperture.

Optionally, the assembly further comprises a plunger carrier releasably connected to the elongate member, the plunger being connected to the plunger carrier, such that the plunger is indirectly releasably connected to the elongate member, wherein, during construction of the assembly, the plunger is connectable to the plunger carrier at one of a plurality of positions for controlling the axial length of the assembly.

Optionally, one or both of the plunger and the plunger carrier comprises a plurality of connection features for connection between the plunger and the plunger carrier during construction of the assembly.

Optionally, the plunger is connectable to the plunger carrier by a thread on one of the plunger and the plunger carrier and a thread engagement feature on the other of the plunger and the plunger carrier comprises.

Optionally, the plunger carrier comprises a plurality of abutment surfaces at a plurality of axial positions on the plunger carrier, and the plunger comprises a corresponding configured to abut one of the abutment surfaces.

Optionally, the plunger is telescopically received within the elongate member.

According to the invention in an aspect, there is provided an injection device comprising the assembly according to any preceding claim.

According to the invention in an aspect, there is provided a method of constructing an assembly for a firing mechanism for an injection device, the method comprising: releasably connecting, directly or indirectly, a plunger to an elongate member of a rear cap, the plunger being axially displaceable in a forward direction upon release of the connection to the elongate member, wherein the plunger and the rear cap define an axial length of the assembly, which in turn determines a start position of a forward end of the plunger before release of the connection to the elongate member, and wherein the releasable connection is made at any of a plurality of positions to control the axial length of the assembly.

Exemplary embodiments will be described herein with reference to the accompanying drawings, in which:.

Generally, disclosed herein are methods and apparatus for controlling a gap between a bung of a syringe and a forward end of a plunger prior to activation of an injection device. For the remainder of this document, the term "auto-injector" will be used in place of "injection device" in order to aid description of some specific embodiments. However, this should not be seen as limiting.

In the following embodiments, the terms "forward" and "front" refer to the patient facing end of the injection device or component thereof. In other words, the front end of the injection device is the end proximal to the injection site during use. Likewise, the term "rear" refers to the non-patient end of the injection device assembly or component thereof. In other words, the term "rear" means distant or remote from the injection site during use.

Many features of the exemplary arrangements disclosed herein are described as being "coupled" to other features. This term encompasses any coupling that results in the coupled features moving together in any direction, whether that be on a <NUM>:<NUM> basis or on some geared basis. The term "coupled" also encompasses any one of a connection between features, an abutment of one feature against another and an engagement of one feature with another, and such coupling may be direct or may be indirect, i.e. with a third feature therebetween.

<FIG> shows an exploded view of an auto-injector <NUM>. The auto-injector <NUM> comprises a firing assembly <NUM>. The firing assembly comprises a rear cap <NUM> and a plunger <NUM>. The rear cap <NUM> comprises a head <NUM> and an elongate member <NUM>. The rear cap <NUM> and the plunger <NUM> are connected to each other such that before firing, relative axial movement between them is resisted or prevented. The connection between the rear cap <NUM> and the plunger <NUM> is releasable such that after activation of the auto-injector <NUM>, relative axial movement between them is permitted. The nature of the releasable connection is discussed in more detail below. As used herein, the term "axial" encompasses a direction parallel to or aligned with a longitudinal axis of the injection device.

The firing assembly <NUM> also comprises a biasing member <NUM> for driving the plunger <NUM> axially forwards and into a barrel of a syringe (shown in <FIG>) retained within the auto-injector <NUM>. In one example the biasing member <NUM> is a drive spring (e.g. a compression spring) and will be referred to as such throughout, although this should not be construed as limiting and the skilled person will appreciate that other means may be used to drive the plunger forwards.

In the example of <FIG>, the plunger <NUM> is telescopically received within the elongate member <NUM> of the rear cap <NUM>. The drive spring <NUM> is positioned between the rear cap <NUM> and the plunger <NUM> such that they are biased in opposite axial directions relative to each other. This is best shown in <FIG>, which is a section through an auto-injector <NUM> in an assembled state before activation and with a syringe <NUM> retained therein. The plunger <NUM> is received within the elongate member <NUM>. The plunger (<NUM>) is a hollow tube with an open end at the rear and the drive spring <NUM> is received within the plunger <NUM>. A first end of the drive spring <NUM> abuts a forward end of the plunger <NUM> and a second end of the drive spring <NUM> is fixed with respect to the rear cap at least during delivery of a medicament from the syringe. In some exemplary arrangements, the second end of the drive spring <NUM> may be coupled to (i.e. directly or indirectly abuts or is connected to) a reaction surface on the rear cap <NUM> or a further member directly or indirectly axially coupled to the rear cap <NUM>. In the example of <FIG>, the drive spring <NUM> is coupled to an end of dose indicator <NUM>, which in turn is coupled to the rear cap <NUM>. Expansion of the drive spring <NUM> drives the plunger <NUM> forwards into the barrel of the syringe <NUM> because, in the example of <FIG> and <FIG>, the position of the rear cap <NUM> is fixed.

In <FIG>, a forward end of the plunger (<NUM>) is shown abutting a bung <NUM>. This will not always be the case, as discussed above.

The auto-injector <NUM> also comprises a clutch <NUM>, which is positioned around the elongate member <NUM>. Before activation of the auto-injector <NUM>, the clutch <NUM> is rotationally coupled to the plunger <NUM>. Rotation of the clutch <NUM> therefore causes rotation of the plunger <NUM>. As explained below, on activation of the auto-injector <NUM>, the clutch <NUM> rotates, thereby rotating the plunger <NUM> relative to the rear cap <NUM> to release the connection therebetween. Operation of the clutch <NUM> is explained in more detail below.

The auto-injector <NUM> also comprises a main body <NUM>, which houses the firing mechanism <NUM>, the syringe <NUM> and other features necessary for operation of the auto-injector <NUM>. The main body <NUM> may comprise a plurality of separate parts. The main body <NUM> comprises a syringe locator, which in exemplary arrangements comprises one or more features for receiving and optionally retaining a syringe in position within the main body <NUM>.

The auto-injector also comprises a lockout spring <NUM> and lockout shroud <NUM>, wherein the lockout spring <NUM> is configured on release thereof to displace the lockout shroud <NUM> axially forwards to cover a needle of the syringe. A cap <NUM> also forms part of the auto-injector and covers a needle or forward end of the auto-injector prior to use.

<FIG> show perspective views of an assembly <NUM> for a firing mechanism. The assembly <NUM> comprises the rear cap <NUM> and the plunger <NUM>. The plunger <NUM> is telescopically received within the elongate member <NUM>. The assembly <NUM> also comprises a plunger driver to drive the plunger <NUM> axially forwards, which in the exemplary arrangements disclosed herein comprises a compression spring although the skilled person will understand that other arrangement are possible. The exemplary assembly <NUM> also comprises an end of dose indicator <NUM>, but this feature is not essential and its operation is beyond the scope of this description.

The exemplary plunger <NUM> comprises a cylindrical tube that is open at a rear end and closed at a forward end. The forward end of the plunger <NUM> comprises a shoulder <NUM> and a projection <NUM> configured to engage a bung <NUM> in a syringe barrel. The plunger <NUM> also comprises a lug <NUM> configured to engage with any of a plurality of recesses 308a, 308b in the elongate member <NUM>. In the exemplary arrangements of <FIG> the lug <NUM> extends radially from on outer surface of the plunger <NUM>.

The elongate member <NUM> comprises an axial channel <NUM>. The plurality of recesses 308a, 308b are formed in a sidewall of the channel <NUM>. That is, the plurality of recesses 308a, 308b extend circumferentially (or transverse to the axial channel) around the outer wall of the elongate member <NUM>. It is noted that while only two recesses 308a, 308b are shown in <FIG>, more recesses (e.g. three - as shown in <FIG> - or four) may be provided in the elongate member <NUM>. The recess 308b comprises an angled rear surface and a front surface that is perpendicular to an axial direction (or longitudinal axis) of the auto-injector <NUM>. The recesses 308a, 308b are configured to receive the lug <NUM> of the plunger <NUM>. <FIG> shows the lug <NUM> received in a rearward recess 308a and <FIG> shows the lug <NUM> received in a forward recess 308b.

The channel <NUM> and the recesses 308a, 308b are configured such that rotation of the plunger <NUM> relative to the elongate member <NUM> in a first direction moves the lug <NUM> into the recesses 308a, 308b and rotation in a second, opposite direction moves the lug <NUM> out of the recesses 308a, 308b.

The plunger <NUM> and the rear cap <NUM>, in particular the elongate member <NUM>, define an axial length of the assembly <NUM>. The axial length of the assembly <NUM> determines a start position of the forward end of the plunger <NUM> before release of the connection of the plunger <NUM> the elongate member <NUM>. This can be seen in <FIG>, which show the plunger <NUM> releasably connected to the elongate member <NUM> at different points, thereby controlling the overall axial length of the assembly <NUM>.

During assembly, the plunger <NUM> is connected to the elongate member <NUM> at any of a plurality of positions on the elongate member <NUM> and/or the plunger <NUM> to alter a combined axial length of the plunger <NUM> and the elongate member <NUM>. The connection may be made directly or indirectly through a plunger carrier, as explained below. In the example of <FIG>, a direct connection is made. The plunger <NUM> may be received within the elongate member <NUM> such that the lug <NUM> is in the channel <NUM>. The plunger <NUM> may then be displaced relative to the elongate member <NUM> until the lug <NUM> is aligned with one of the recesses 308a, 308b. The plunger <NUM> may then be rotated such that the lug is received within the one of the recesses 308a, 308b with which it was aligned. The force provided by the drive spring <NUM> retains the lug <NUM> within the recess 308a, 308b against the forward surface thereof. The combined axial length of the assembly (i.e. of the rear cap <NUM> and the plunger <NUM>) is increased in <FIG> when compared to <FIG>, in which the lug <NUM> is received within the rearward recess 308a.

<FIG> show alternative rear caps <NUM> having three recesses 408a, 408b, 408c. It will be appreciated that other numbers of recesses may be used. In addition, the recesses 408a, 408b, 408c may be positioned at any point on the elongate member <NUM>. In the exemplary arrangement of <FIG>, the recesses 408b and 408c have been displaced forwards with respect to those in <FIG>.

Other configurations of plunger and rear cap are possible. For example, <FIG> shows a rear cap <NUM> having at least one elastic prong <NUM> positioned on the elongate body <NUM>. In exemplary arrangements, the rear cap <NUM> comprises two prongs 502a, 502b, which are elastic. The elastic prongs 502a, 502b comprise projections 506a, 506b extending radially inwards. The projections 506a, 506b are configured to engage corresponding features 508a-508e of the plunger <NUM>. In the example shown in <FIG>, the features 508a-508e of the plunger <NUM> comprise recesses or apertures into which the projections 506a, 506b may be received. The elastic prongs 502a, 502b have a resting position in which the projections are received in the recesses of the plunger <NUM> and are biased radially inwards such that a force is required to displace them radially outwards. The projections 506a, 506b are thereby retained in the recesses of the plunger <NUM>.

In <FIG>, the recesses are formed by varying the diameter of the plunger <NUM>. In other arrangements, an alternative plunger <NUM> may comprise recesses formed by discrete apertures. The skilled person will be able to think of a number of different options for providing the features on the plunger corresponding to and for receiving the projections 506a, 506b.

<FIG> shows a schematic of a further arrangement of an assembly <NUM> for a firing mechanism. The assembly <NUM> comprises a rear cap <NUM> and a plunger <NUM>. The rear cap <NUM> comprises a keyed aperture <NUM>. At least one section of the plunger <NUM> has a cross section corresponding to and for passing through the keyed aperture <NUM>. In the example shown in <FIG>, five keyed sections 608a-608e of the plunger <NUM> have cross sections corresponding to the keyed aperture <NUM>, but other numbers of such sections are possible. In the example shown in <FIG>, the keyed aperture <NUM> has a length greater than a width thereof and may form a substantially rectangular or oblong shape. The width may substantially correspond to the width of the plunger <NUM> and the length may be greater than the width of the plunger <NUM>. The plunger <NUM> also comprises lockout sections 610a-610d between the keyed sections 608a-608e configured to be rotatable within the keyed aperture <NUM>. Rotation of the plunger <NUM> whilst one of the lockout sections 610a-610d is in the keyed aperture <NUM> allows the keying of the keyed aperture <NUM> to be out of alignment with the corresponding keying features 608a-608e of the plunger <NUM>, which prevents further passage of the plunger <NUM> through the keyed aperture <NUM>. In order to vary the connection between the plunger <NUM> and the rear cap <NUM>, the plunger <NUM> may be rotated such that the keyed sections 608a-608e are aligned with the keyed aperture <NUM>. The plunger <NUM> is passed through the keyed aperture <NUM> in either axial direction until the desired combined length of the assembly <NUM> is achieved. Then the plunger <NUM> is rotated whilst one of the lockout sections 610a-610d is within the keyed aperture <NUM> such that the keyed features 608a-608e and the keyed aperture are misaligned, thereby preventing further axial movement of the plunger rod <NUM> relative to the rear cap <NUM>. <FIG> shows the assembly <NUM> with the plunger <NUM> rotated by <NUM> degrees relative to the plunger <NUM> in <FIG>.

<FIG> show a further exemplary arrangement of an assembly <NUM> for a firing mechanism of an auto-injector. <FIG> show partial sections through the assembly <NUM>. In the exemplary arrangement, the rear cap <NUM> comprises a plurality of recesses 704a-704e. The plunger <NUM> comprises at least one elastic prong <NUM> having a radially inward facing projection <NUM>. In exemplary arrangements the plunger <NUM> may comprise a plurality of elastic prongs <NUM>.

The projection <NUM> is configured to be received within the recesses 704a-704e. The elastic prong <NUM> is biased radially inwards and has a resting state in which the projection <NUM> is received within a recess 704a, 704b. A force is required to displace the elastic prong <NUM> radially outwards to remove the projection <NUM> from a recess 704a, 704b. The projection <NUM> may have angled forward and/or rearward faces such that axial displacement of the plunger <NUM> with respect to the rear cap <NUM> provides the force required to remove the projection <NUM> from the recess 704a, 704b. That is, an axial force applied to the plunger <NUM> is translated to a radial force by interaction of the angled forward or rearward surface of the projection <NUM> with a sidewall of the recess 704a, 704b.

In the example of <FIG>, the rear cap <NUM> is telescopically received within the plunger <NUM>. This arrangement may apply to other exemplary assemblies described herein.

The assembly <NUM> may further comprise a clutch <NUM> that is positioned around the plunger <NUM> and the rear cap <NUM>. The clutch <NUM> has a varying internal diameter. In the example of <FIG>, the clutch has at least two internal diameters, a first internal diameter at which an internal wall of the clutch prevents the elastic prong <NUM> from being displaced radially outwards (shown in <FIG>), and a second internal diameter that provides a space <NUM> between the elastic prong <NUM> and the internal wall of the clutch <NUM> (shown in <FIG>). The space is sufficient to allow radially outward displacement of the elastic prong <NUM> such that the projection <NUM> can be removed from the recess 704a-704e and placed into another.

It is noted that the difference in internal diameter of the clutch <NUM> that provides the space <NUM> need not extend around the entire internal circumference of the clutch <NUM>. The second internal diameter may be obtained via a slot or other recess in the internal wall of the clutch <NUM>.

During assembly, the cutch <NUM> is moved (in this case rotated) to provide the space <NUM> radially outwards from the elastic prong <NUM>. The plunger <NUM> may then be axially displaced with respect to the rear cap <NUM> by first providing a force to displace the elastic prong <NUM> radially outwards and then axially moving the plunger <NUM> with respect to the rear cap <NUM>. When the projection <NUM> is aligned with a recess 704a-704e that provides the desired combined length of the rear cap <NUM> and the plunger <NUM>, the projection <NUM> will enter the recess 704a-704e under the bias of the elastic prong <NUM>. The clutch is then moved such that the first internal diameter is presented to the elastic prong <NUM> restricting or preventing outward radial movement thereof.

In another arrangement, the clutch may have the first internal diameter at one axial position and the second internal diameter at a second axial position such that axial movement, as opposed to rotation, presents a different internal diameter to the elastic prong. Further, in arrangements where the plunger is received within the elongate member such as those shown in <FIG>, the clutch may present the internal diameters to the elongate member and the elastic prong thereon.

Any of the assemblies for a firing mechanism of an auto-injector described above may be used in conjunction with the auto-injector <NUM> of <FIG> and <FIG>.

During assembly of the auto-injector, the assembly comprising the plunger and the rear cap is set using any of the methods and apparatus described herein. The plunger is connected to the elongate member of the rear cap. The connection is releasable in that upon activation of the auto-injector, the connection is released to allow relative axial movement of the plunger and the rear cap. Also, the connection may be made at any of a plurality of positions on the elongate member or the plunger. That is, one or both of the elongate member and the plunger may be configured to have a plurality of locations at which the connection may be made.

Accordingly, the combined axial length of the plunger and the rear cap is set to the desired length based on a fill volume (or bung position) of a syringe that the auto-injector is intended for use with. In so doing, the gap between the bung of the syringe, which sits at a position in the barrel that is dependent on the fill volume, is controlled. That is, if the combined axial length of the plunger and the rear cap is extended then the auto-injector may be used for syringes having a smaller fill volume, or otherwise having a bung that is initially positioned further forwards within the barrel, e.g. if the barrel is of a greater diameter but the fill volume remains the same. The start position of a forward end of the plunger is adjusted during assembly.

Once assembled, the user has no control over the combined length of the rear cap and the plunger.

Operation of the auto-injector <NUM> is described below using the reference numerals of the exemplary arrangement shown in <FIG> and <FIG>. As discussed above, other arrangements of the firing mechanism assembly may be used and the appropriate reference numerals may therefore be substituted into the following description.

In use, a user removes the cap <NUM> of the auto-injector <NUM>, which in turn removes a rigid needle shield covering the needle. Removal of the cap exposes the lockout shroud <NUM>, which protrudes from a forward end of the body <NUM>.

The user places a forward end of the lockout shroud <NUM> against an injection site and pushes the auto-injector <NUM> downwards onto the injection site. This action pushes the lockout shroud <NUM> rearwards within the auto-injector <NUM>. The lockout shroud interacts with the clutch <NUM> to rotate it. This may be done by forcing a surface (or pip) of the lockout shroud <NUM> against an angled surface on the clutch <NUM>, which translates the rearward motion of the lockout shroud <NUM> into rotational motion of the clutch <NUM>.

In some arrangements, an insertion spring may be activated by the action of pushing the auto-injector <NUM> onto the injection site and the insertion spring may drive the syringe forwards within the device to insert the needle into the injection site. In other arrangements, the force applied by the drive spring <NUM> acting against the bung may be used to insert the needle. In yet further arrangements, the syringe may be fixed in relation to the injection device <NUM> and the force applied by the user when pushing the auto-injector <NUM> onto the injection site may insert the needle into the injection site.

As the clutch <NUM> is rotationally coupled to the plunger <NUM>, rotation of the clutch <NUM> causes rotation of the plunger <NUM>. In some arrangements, the clutch <NUM> may have an internal track located on an internal wall thereof and that receives a lug of the plunger <NUM>. The lug may be the same as the lug <NUM> described with reference to <FIG>. Rotation of the plunger <NUM> with respect to the rear cap <NUM> releases the connection between the rear cap <NUM> and the plunger <NUM>, allowing the plunger <NUM> to be driven forwards under force of the drive spring <NUM>. In the examples of <FIG>, this is provided by rotating the lug <NUM> of the plunger rod <NUM> out of whichever recess 308a, 308b, 408a-408c the lug <NUM> was positioned in and into the axial channel <NUM>. The lug <NUM> is thereby allowed to travel forwards within the channel <NUM>.

The drive spring <NUM> then acts against the plunger <NUM> and the rear cap <NUM>. Because the rear cap <NUM> is fixed within the auto-injector <NUM>, the force delivered by the drive spring <NUM> acts to drive the plunger <NUM> into the barrel of the syringe. Because the gap between the forward end of the plunger <NUM> and the bung <NUM> has been controlled during assembly, the plunger <NUM> does not accelerate above a safe velocity that would risk damage to the syringe <NUM> or harm or discomfort to the subject of the injection.

After delivery of the contents of the syringe <NUM>, the lockout shroud is deployed under force of the lockout spring <NUM> in any of a number of ways that will be apparent to the skilled person.

<FIG> and <FIG> show arrangements in which a direct releasable connection is made between the plunger and the elongate member. This arrangement is less complex but results in an activation point (in some cases above, the lug <NUM>) being in different axial positions. Therefore, the activation mechanism needs to adapt to suit. <FIG> and <FIG> show arrangements in which the releasable connection is made indirectly to the elongate member via a plunger carrier. The plunger carrier is typically fixed in relation to the elongate member until activation of the injection device. The connection of the plunger to the plunger rod carrier may be made at a plurality of locations positions for controlling the axial length of the assembly.

<FIG> show an exemplary injection device <NUM>. The injection device <NUM> comprises an assembly for use as a firing mechanism. The assembly comprises a rear cap <NUM> comprising an elongate member <NUM>, a plunger <NUM>, a plunger driver <NUM> and a plunger carrier <NUM>.

The plunger carrier <NUM> is releasably connected to the elongate member <NUM>, wherein the connection is released on activation of the injection device <NUM>. The releasable connection may be achieved in any way described herein, or by any other means that will be known to a skilled person.

The plunger <NUM> is connected to the plunger carrier <NUM> at one of a plurality of positions to control the overall axial length of the assembly. This connection is made during construction of the assembly and/or the injection device <NUM>. In the exemplary arrangement of <FIG>, the plunger carrier <NUM> comprises a plurality of recesses <NUM> into which a lug (not visible in <FIG>) on the plunger <NUM> may be located, each recess having a different axial position. <FIG> shows the connection between the plunger <NUM> and the plunger carrier <NUM> made at a first position, and <FIG> shows the connection between the plunger <NUM> and the plunger carrier <NUM> made at a second position axially further forwards than the first position. As can be seen, the forward end of the plunger is also further forward in <FIG>.

<FIG> show a further exemplary injection device <NUM>. The injection device <NUM> comprises an assembly for use as a firing mechanism. The assembly comprises a rear cap <NUM> comprising an elongate member <NUM>, a plunger <NUM>, a plunger driver <NUM> and a plunger carrier <NUM>. As with the injection device <NUM> in <FIG>, the plunger carrier <NUM> is releasably connected to the elongate member <NUM>, wherein the connection is released on activation of the injection device <NUM>.

The plunger <NUM> comprises a thread at least partially along its length. The plunger carrier <NUM> comprises a thread engagement feature that engages the thread of the plunger <NUM>. The thread engagement feature can be a further thread corresponding to the thread on the plunger <NUM>, but need not be a full thread and may comprise one or more projections arranged to engage the thread. In the example shown in <FIG>, the plunger carrier <NUM> comprises a threaded aperture <NUM> in a forward end. Rotation of the plunger <NUM> relative to the plunger carrier <NUM> controls the extension of the plunger <NUM> from the threaded aperture <NUM>. This controls the axial length of the assembly and the start point of the forward end of the plunger <NUM>. It is noted that for the purposes of describing the invention, the phrase "connected to the elongate member at one of a plurality of positions for controlling the axial length of the assembly" encompasses a continuous connection arrangements, such as the continuaous nature of the controllability of the connection of the plunger <NUM> to the plunger carrier <NUM> (i.e. the threaded arrangement).

<FIG> show a further exemplary injection device <NUM>. The injection device <NUM> comprises an assembly for use as a firing mechanism. The assembly comprises a rear cap <NUM> comprising an elongate member <NUM>, a plunger <NUM>, a plunger driver <NUM> and a plunger carrier <NUM>. As with the injection devices <NUM> and <NUM>, the plunger carrier <NUM> is releasably connected to the elongate member <NUM>, wherein the connection is released on activation of the injection device <NUM>. In addition the plunger carrier <NUM> comprises two parts 1010a and 1010b, as explained below.

A first part 1010a of the plunger carrier <NUM> comprises a plurality of abutment surfaces <NUM> at different axial positions on the plunger carrier <NUM> and that, in the example shown are in a stepped arrangement, although they may be at the forward end of discrete channels.

In one arrangement not shown in <FIG>, the plunger <NUM> may include a corresponding abutment surface <NUM> that is configured to abut the abutment surfaces of the first part 1010a of the plunger carrier <NUM>. In such arrangements, the second part 1010b of the plunger carrier <NUM> may not be required. The plunger <NUM> may be rotated to a correct alignment with the plunger carrier <NUM> and then the abutment surfaces engaged. By selecting the rotational alignment, the extension of the plunger <NUM> and therefore the axial length of the assembly is controlled.

In the arrangement shown in <FIG>, the plunger carrier <NUM> also comprises a second part 1010b. The second part 1010b comprises a first abutment surface <NUM> for abutment with the abutment surfaces of the first part 1010a of the plunger carrier <NUM>. The second part 1010b may be rotationally aligned with the first part 1010a of the plunger carrier <NUM> such that the first abutment surface of the second part 1010b meets the correct abutment surface of the first part 1010a. The second part 1010b also comprises a plurality of second abutment surfaces <NUM> that are arranged to abut the abutment surface <NUM> of the plunger <NUM>. Accordingly, the plunger <NUM> may be rotationally aligned with the second part 1010b such that the abutment surface <NUM> of the plunger <NUM> is aligned with the correct second abutment surface <NUM> of the second part 1010b.

During construction of the assembly, the second part 1010b is inserted into the first part 1010a, with the correct rotational alignment. Then the plunger <NUM> is inserted into the second part 1010b, again with the correct rotational alignment. When the injection device <NUM> is activated, the first part 1010a, second part 1010b and the plunger <NUM> move forwards together.

The use of a plurality of abutment surfaces on the first part 1010a and the second part 1010b allow for greater control over the axial length of the assembly and therefore of the starting point of the forward end of the plunger <NUM>. <FIG> show different configurations in which the abutment surface <NUM> of the plunger <NUM> abuts different ones of the second abutment surfaces <NUM> of the second part 1010b, and the first abutment surface <NUM> of the second part 1010b abuts different abutment surfaces <NUM> of the first part 1010a.

The injection devices <NUM>, <NUM>, <NUM> of <FIG> operate in a similar fashion to that of the injection device <NUM>, operation of which is described above. The principal difference is that the connection between the plunger carrier and the elongate member is released on activation of the device and the plunger carrier and plunger are driven forwards together by the plunger driver.

Claim 1:
An assembly for a firing mechanism for use in an injection device and comprising:
a rear cap (<NUM>, <NUM>, <NUM>) having an elongate member (<NUM>, <NUM>, <NUM>, <NUM>) extending axially forwards when installed within the injection device;
a plunger carrier (<NUM>, <NUM>, <NUM>) releasably connected to the elongate member (<NUM>, <NUM>, <NUM>, <NUM>) and fixed in position relative to the elongate member (<NUM>, <NUM>, <NUM>, <NUM>) before release from the elongate member (<NUM>, <NUM>, <NUM>, <NUM>);
a plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) releasably connected to the elongate member (<NUM>, <NUM>, <NUM>, <NUM>) via the plunger carrier (<NUM>, <NUM>, <NUM>) and, upon release of the connection between the plunger carrier (<NUM>, <NUM>, <NUM>) and the elongate member (<NUM>, <NUM>, <NUM>, <NUM>), axially displaceable in a forward direction; and
a plunger driver (<NUM>, <NUM>) to drive the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) axially forwards upon release of the connection between the plunger carrier (<NUM>, <NUM>, <NUM>) and the elongate member (<NUM>, <NUM>, <NUM>, <NUM>), the plunger driver (<NUM>, <NUM>) positioned between the rear cap (<NUM>, <NUM>, <NUM>) and the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and received within the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) such that a first end of the plunger driver (<NUM>, <NUM>) abuts a forward end of the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>),
wherein the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and the rear cap (<NUM>, <NUM>, <NUM>) define an axial length of the assembly, which in turn determines a start position of a forward end of the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) before release of the connection to the elongate member (<NUM>, <NUM>, <NUM>, <NUM>),
wherein one or both of the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and the plunger carrier (<NUM>, <NUM>, <NUM>) comprises a plurality of connection features for connection between the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) and the plunger carrier (<NUM>, <NUM>, <NUM>) during construction of the assembly,
and wherein, during construction of the assembly, the plunger (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is releasably connectable to the plunger carrier (<NUM>, <NUM>, <NUM>) at one of a plurality of positions defined by the plurality of connection features for controlling the axial length of the assembly.