FIELD OF THE INVENTION

The present invention relates to needle assemblies for hypodermic drug administration.

BACKGROUND OF THE INVENTION

Parenteral drug administration carried out using a traditional vial and syringe system is in creasingly being substituted by administration using a pen injection device. Pen injection devices are particularly convenient in that they allow the user to perform a dosed injection from a prefilled drug reservoir without first having to manually transfer the particular dose from one reservoir (the vial) to another (the syringe).

Predominantly, two types of pen injection devices are available, durable injection devices being capable of delivering one or more doses of drug from a prefilled drug cartridge which can be loaded into the device before use and replaced after exhaustion, and disposable in jection devices being capable of delivering one or more doses of drug from a prefilled and non-exchangeable drug cartridge. Each of these types of pen injection devices are, or may in principle be, realised in various sub-types, such as e.g. single shot devices adapted to deliver only one dose of a predetermined, or selected, size from a drug cartridge, multi-shot devices capable of delivering a plurality of doses from a drug cartridge, manual devices, where the user provides the force needed for injection, automatic devices having a built-in energy source releasable to occasion the injection, fixed dose devices adapted to deliver doses of identical size, variable dose devices offering delivery of a plurality of doses of drug, each settable by the user from a range of possible dose sizes, etc.

As the labels suggest a durable injection device is intended for use over a considerable pe riod of time during which multiple drug cartridges are exhausted and replaced, whereas a disposable injection device is intended for use until its dedicated drug cartridge is exhausted, after which the entire injection device is discarded.

Pen injection devices are conventionally used with matching pen needle assemblies which provide access to a subcutaneous compartment and serve as a means for administration of the drug thereto. However, many people dislike the thought of having an injection needle inserted through the skin. An undisclosed number of people even suffer from needle-phobia, and these people often benefit from using needle units with shielded needles, where the injection needle remains out of sight during handling of the needle unit, including insertion of the injection needle into the skin.

Typically, this type of needle unit comprises an axially movable sheath which can be slid between a first position in which it covers the injection needle and a second position in which the injection needle is exposed and ready for injection. In some cases, the sheath is spring loaded such that it is automatically slid back to the first position when the injection needle is retracted from the skin. An example of this is disclosed in US 2003/0078546 (Jensen).

Users of such injection systems are recommended to discard the needle assembly, or nee dle unit, after a single injection to minimise the risk of contamination. Hence, in the course of its lifetime a multi-use injection device is by default used with multiple needle units.

Needle units are typically wrapped and sealed individually to ensure sterility prior to use. In connection with a dose administration action the user must therefore unwrap the needle unit, mount it on the injection device, perform the injection, dismount it from the injection device, re-wrap or otherwise encapsulate it to prevent needle stick injuries, and finally dispose of it, preferably in a dedicated sharps container.

The readying and subsequent removal of the needle unit is both the most complicated and the most time-consuming part of the injection procedure. Especially for young and elderly users the handling of the small items and foils can present a challenge and make the task of injection a bit cumbersome. As a result, some users reuse the needle unit several times. In fact, some users only change the needle unit when the injection device is empty or if the needle for example exhibits clogging or hooking. This reduces the number of times these users have to carry out needle handling activities significantly.

However, it also entails increased risks of both infections and needle stick injuries, the for mer due to needle contamination and the latter due to the users typically disposing of the original needle unit packaging in connection with the fitting of the needle and the first use and therefore do not have this available as receptacle for when they eventually change the needle unit after several times of reuse.

WO 2014/064100 (Novo Nordisk A/S) discloses a multi-use needle unit for a pen injection device where a portion of the front needle is housed between injections in a cleaning medi um, such as a solid plug containing anti-bacterial particles. Said portion of the front needle is thus cleaned by, and stored in, the cleaning medium following each injection action, thereby reducing the risk of microbial contamination and preventing accidental needle stick injuries.

Hence, this needle unit concept allows for safe multiple reuse of the injection needle. How ever, using a solid plug as cleaning medium requires a tight enclosure of the portion of the needle to be cleaned. Consequently, the contact interface between the solid plug and the injection needle exhibits significant friction when the needle shield moves. A drawback of this friction is the increased resistance to the axial displacement of the needle shield, result ing in the need for a more powerful spring element for the automatic return of the needle shield after use, adding to the cost and weight of the whole device.

A concomitant issue with shielded injection needles is the fact that it is impossible for the user to visually ascertain that the needle is properly inserted in the skin. With such needle units there is a risk that the skin portion around the injection site, during displacement of the needle shield, deflects and forms a cavity in which the needle tip can rest without penetrat ing the epidermis. Thus, if the user initiates an injection before the needle shield is fully pushed back this may lead to a so-called wet-shot, where the dose of drug is undesirably expelled onto the surface of the skin instead of into the body.

Normally, the user will determine that the needle shield is fully pushed back tactilely by sens ing the difference in resistance to the applied motion of the injection device towards the skin experienced as the resistance transitions from being due to the compression of a weak spring to being due to compression of the skin and tissue. If a more powerful spring is em ployed to accommodate the need for overcoming the friction in returning a solid plug along the injection needle surface this tactile feedback will become less pronounced, leading to an increased risk of the user not being able to correctly determine the point of full proximal nee dle shield displacement.

SUMMARY OF THE INVENTION

It is an object of the invention to eliminate or reduce at least one drawback of the prior art, or to provide a useful alternative to prior art solutions.

In particular, it is an object of the invention to provide a needle unit for multiple use with an injection device and having a plug for cleaning a shielded injection needle between use, which needle unit is functionable with a conventional, weak needle shield returning spring. It is a further object of the invention to provide such a needle unit which is relatively small, and which therefore does not add significantly to the bulk of the injection device.

It is a further object of the invention to provide such a needle unit which is simple and easy to operate, and which offers a high degree of safety against accidental needle stick injuries.

In the disclosure of the present invention, aspects and embodiments will be described which will address one or more of the above objects and/or which will address objects apparent from the following text.

In one aspect the invention provides an injection needle unit as defined in claim 1.

Accordingly, a needle unit is provided comprising a needle hub in which an injection needle is fixed, a plug member tightly surrounding a portion of the injection needle and capable of sliding along an exterior surface of the injection needle, and a needle shield surrounding, at least portions of, the plug member and the injection needle.

The injection needle extends along a reference axis and comprises a distal needle end por tion adapted for insertion into skin. The plug member is rotationally locked with respect to the needle hub but capable of axial displacement relative thereto between an extended plug position in which the plug member fits tightly around the distal needle end portion and a re tracted plug position in which the distal needle end portion extends distally beyond the plug member. The plug member comprises a penetrable self-sealing front section which a distal tip of the injection needle is able to transpierce during displacement of the plug member from the extended plug position to the retracted plug position and which re-seals itself to provide a fluid tight enclosure for the distal needle end portion upon return displacement of the plug member to the extended plug position.

The needle shield comprises a shield body extending between a proximal shield end and a distal shield end. The needle shield is axially displaceable relative to the needle hub be tween a forward shield position in which the distal shield end extends distally beyond, and thereby covers, the distal needle end portion, and a rearward shield position in which the distal shield end is positioned proximally of the distal needle end portion, and the distal nee dle end portion thereby is exposed to the surroundings.

The needle unit distinguishes from the aforementioned prior art in that it further comprises a plug displacement mechanism operable to displace the plug member axially relative to the needle hub when the needle shield is in the forward shield position. The plug displacement mechanism thus allows for displacement of the plug member between the extended plug position and the retracted plug position independently of the needle shield, i.e. without an accompanying displacement of the needle shield. Thereby, the plug member can be dis placed manually by the user from the extended plug position to the retracted plug position as well as from the retracted plug position to the extended plug position while the injection nee dle remains covered by the needle shield. Hence, the force required to return the needle shield to the forward shield position following a dose expelling action does not need to over come an additional friction between the injection needle and the plug member, and, result- antly, a conventional weak spring element can be employed in the needle unit.

Since a weak spring element can be employed the tactile feedback to the user is not com promised by a harder to compress spring, and so the present needle unit with a plug mem ber for cleaning the injection needle provides an equally good indication to the user of when the needle shield is fully pushed back during insertion of the injection needle into the skin as a needle unit without such a plug member.

In exemplary embodiments of the invention the plug member comprises a radially protruding geometry, such as e.g. a pair of oppositely projecting arms, and the plug displacement mechanism comprises a guide structure arranged concentrically about the plug member and the needle shield and comprising a plug guiding track for sliding engagement with the radial ly protruding geometry. The plug guiding track comprises a helical track segment configured to prompt axial displacement of the radially protruding geometry in response to a rotation of the guide structure.

This provides a particularly compact and easily operable solution for the displacement of the plug member in that it merely adds another layer to the cylindrical configuration of the needle unit with shield. Since the plug member is rotationally fixed with respect to the needle hub a simple rotation of the guide structure about the reference axis relative to the needle hub will cause the radially protruding geometry to travel the helical track segment and thereby dis place the plug member axially, i.e. proximally or distally depending on the direction of rota tion.

The plug guiding track may further comprise a circumferential track segment leading into the helical track segment, and the radially protruding geometry may be adapted to reside in the circumferential track segment when the plug member is in the extended plug position. In that case the plug member will be prevented from accidental axial displacement, e.g. due to the plug member being subjected to axial impact from the surroundings, as could be the case if the needle unit is carried about in a bag. Such accidental axial displacement is undesirable as it exposes the distal needle end portion to the surrounding environment prematurely, and thereby increases the risk of needle stick injuries and/or surface contamination of the injec tion needle.

The circumferential track segment may lead into the helical track segment via a junction, and the junction may comprise a track narrowing geometry, e.g. a bulb member, such that an additional torque is needed during rotation of the guide structure to allow the radially protrud ing geometry to exit the circumferential track segment and enter the helical track segment. The track narrowing geometry thus constitutes an additional safety feature against prema ture exposure of the distal needle end portion.

The shield body may comprise a radial stud, and the guide structure may further comprise an axially extending shield guiding track for sliding engagement with the radial stud. The guide structure is thereby also used to define the potential motion of the needle shield, elim inating the need for an additional component for that purpose.

The guide structure may be a two-part component comprising an outer track sleeve having an interior wall portion in which is formed a plurality of distal guide surfaces and an inner track sleeve in which is formed a plurality of proximal guide surfaces. The inner track sleeve is then adapted for mating connection with the interior wall portion such that the plurality of distal guide surfaces and the plurality of proximal guide surfaces together form the plug guid ing track and the shield guiding track.

A two-part solution for the guide structure is advantageous from a manufacturing perspective as it provides for easy arrangement of the radially protruding geometry and the radial stud in the respective guide tracks during assembly of the plug member and the needle shield with the guide structure. The plug member and the needle shield are simply slid into position within the outer track sleeve from the proximal end thereof, after which the inner track sleeve is slid into engagement with the interior wall portion.

The outer track sleeve may be mounted on the needle hub so as to prevent relative axial motion but allow relative rotation between the two. In particular embodiments of the inven tion the needle hub comprises a distal circumferential groove and the outer track sleeve comprises a proximal catch portion adapted to snap into the circumferential groove. The needle hub may comprise a pair of opposite axially extending flanges defining a pair of axial hub tracks and the plug member may be rotationally locked but axially displaceable with respect to the needle hub by the radially protruding geometry being slidably accommo dated in the axial hub tracks. Thereby, a very compact and structurally simple needle unit can be provided.

The shield body may rest against the radially protruding geometry when the plug member is in the extended plug position and the needle shield is in the forward shield position. The ra dially protruding geometry will thereby prevent the shield body from passing the plug mem ber, and the needle shield is resultantly prevented from moving away from the forward shield position as long as the plug member is in the extended plug position and the radially protrud ing geometry resides in the in the circumferential track segment, further increasing the pro tection against accidental needle stick injuries.

The plug member may be a biostatic component further comprising a micro- bacteria I growth inhibitor. In particular, the plug member may be made of a thermoplastic elastomer contain ing immobilised Zinc (Zi ⁺⁺ ) or immobilised silver (Ag ⁺⁺ ), as these ions are known to inhibit micro-bacterial growth. Since the plug member fits tightly around a portion of the needle any micro-bacterial contaminants thereon will resultantly be neutralised. The needle is thereby kept in a biostatic environment between injections and the needle assembly is accordingly suitable for multiple use.

The plug member may further comprise a distal plug surface which may be flush with or po sitioned distally of the distal shield end when the plug member is in the extended plug posi tion and the needle shield is in the forward shield position. Thereby, any contaminants de posited on the distal plug surface after the plug member has slid along and scraped the sur face of the injection needle can be easily removed, e.g. using an alcohol swap, before the next use of the needle unit. If the distal plug surface is positioned proximally of the distal shield end there is a risk that some contaminants may be caught in small crevices between the shield body and the plug member and thereby remain on the needle unit even though the distal plug surface is swept with an alcohol swab or the like.

In another aspect the invention provides a system comprising an injection needle unit as described above and a drug injection device. The drug injection device comprises a car tridge holder for holding a drug cartridge containing a variable volume of drug, and a drug expelling mechanism operable to expel a dose of drug from the drug cartridge. The cartridge holder comprises distal attachment means to which the needle hub is attached.

The distal attachment means may e.g. comprise a thread or a bayonet track for receiving a proximal connecting sleeve of the needle unit, enabling easy attachment and detachment of the needle unit. Alternatively, the distal attachment means may be adapted to receive a por tion of the needle unit in a non-releasable fitting, whereby the needle unit is discarded along with the cartridge holder. Hence, in case the drug injection device is of the disposable type the needle unit is adapted to be used therewith until the drug cartridge has been emptied, or substantially emptied.

For the avoidance of any doubt, in the present context the term “injection device” designates an apparatus suitable for injecting fluid media into the body, e.g. with the aid of an attacha ble needle device, and the term “drug” designates a medium which is used in the treatment, prevention or diagnosis of a condition, i.e. including a medium having a therapeutic or meta bolic effect in the body. Further, the terms "distal" and "proximal" denote positions at, or di rections along, a drug delivery device, a drug reservoir, or a needle unit, where "distal" refers to the drug outlet end and "proximal" refers to the end opposite the drug outlet end.

In the present specification, reference to a certain aspect or a certain embodiment (e.g. "an aspect", "a first aspect", "one embodiment", "an exemplary embodiment", or the like) signi fies that a particular feature, structure, or characteristic described in connection with the re spective aspect or embodiment is included in, or inherent of, at least that one aspect or em bodiment of the invention, but not necessarily in/of all aspects or embodiments of the inven tion. It is emphasized, however, that any combination of the various features, structures and/or characteristics described in relation to the invention is encompassed by the invention unless expressly stated herein or clearly contradicted by context.

The use of any and all examples, or exemplary language (e.g., such as, etc.), in the text is intended to merely illuminate the invention and does not pose a limitation on the scope of the same, unless otherwise claimed. Further, no language or wording in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 illustrates a needle assembly according to the prior art,

Fig. 2 is an exploded view of a needle assembly according to an exemplary embodiment of the present invention,

Figs. 3-8 are different views of various components of the needle assembly of Fig. 2,

Fig. 9 is a sectioned perspective view of the needle assembly in a pre-use state,

Fig. 10 is a perspective view of an injection system comprising an injection device with the needle assembly pre-mounted thereon, and

Fig. 11 shows the needle assembly in different states before and during use.

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

DESCRIPTION OF EXEMPLARY EMBODIMENTS

When/If relative expressions, such as "upper" and "lower", "left" and "right", "horizontal" and "vertical", "clockwise" and "counter-clockwise", etc., are used in the following, 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.

Fig. 1 shows a needle assembly T for multiple use according to the prior art. The needle assembly T comprises a needle hub 4’ carrying an injection needle 15’, and a displaceable needle shield 12’. The needle hub 4’ is slidable along a longitudinal axis within a needle hub guide member 14’ which is mounted over a drug cartridge 3T forming part of a drug injection device (not shown). Fig. 1a depicts the needle assembly T in a pre-use (or between-use) state where a rear needle end portion 15r’ is spaced apart from a penetrable self-sealing septum 32’ which seals the drug cartridge 3T and a front needle end portion 15f resides in a solid plug 10’ with antibacterial particles, which is fixed to the needle shield 12’. The needle shield 12’ is in a forward position, biased by a shield spring 5’, and the needle hub 4’ is in a top position in the needle hub guide member 14’, biased by a hub spring 7’. In the pre-use state the needle shield 12’ thus covers the front needle end portion 15f and there is no fluid connection to the interior of the drug cartridge 31’.

Fig. 1b depicts the needle assembly 1’ in an intermediate state during an injection proce dure, where a user has placed a distal end surface 10s’ of the solid plug 10’ against a skin site (not shown) and applied a force F thereto in order to initiate a needle insertion step. The needle shield 12’ is thereby pushed away from the forward position, while the shield spring 5’ is compressed, causing the front needle end portion 15f to transpierce the solid plug 10’ and enter the injection site. By application of an additional force the needle hub 4’ will be displaced downwards in the needle hub guide member 14’, compressing the hub spring 7’, until the rear needle end portion 15r’ has penetrated the self-sealing septum 32’ and estab lished fluid connection to the interior of the drug cartridge 3T, after which an injection can be carried out.

Upon completion of the injection the applied force is removed whereby the hub spring 7’ automatically returns the needle hub 4’ to the top position, retracting the rear needle end portion 15r’ from the drug cartridge 3T, and the shield spring 5’ automatically returns the needle shield 12’ to the forward position, withdrawing the front needle end portion 15f from the skin and re-positioning it in the solid plug 10’.

For the solid plug 10’ to provide satisfactory sealing and cleaning effects it needs to enclose the front needle end portion 15f tightly. This means that when the user applies the above described force F to push back the needle shield 12’ and insert the front needle end portion 15f this force F must overcome not only the bias force from the shield spring 5’ but also the friction in the interface between the injection needle 15’ and the solid plug 10’. Similarly, when the shield spring 5’ returns the needle shield 12’ following a completed injection it must be able to provide a force which overcomes this friction, i.e. a greater force than if the solid plug 10’ were not present.

All in all, the force required to push the needle shield 12’ back is markedly greater than the force required to push back a needle shield of a similar construction without the solid plug. Resultantly, a larger and more powerful shield spring must be included in the needle assem bly 1, and it becomes more difficult for the user to determine the point where the resistance to the needle shield depression transitions from being due to the compression of the shield spring 5’ and movement of the solid plug 10’ to being due to a compression of the skin and tissue, potentially negatively affecting the decision on when the needle shield 12’ is fully pushed back, and thereby on whether the injection needle 15’ is inserted correctly and an injection can be performed without risking a wet-shot.

Fig. 2 is an exploded view of a needle assembly 1 according to an exemplary embodiment of the invention. The needle assembly 1 comprises a needle hub 4 in which an injection needle 15 is fixedly mounted, an outer track sleeve 2, an inner track sleeve 3, a needle shield 12, a shield spring 5 in the form of a small compression spring, and a plug 10. The injection nee dle 15 is a straight tubing extending along a reference axis, and the plug 10 is biostatic in that it is made of a thermoplastic elastomer containing immobilised Zinc (Zi ⁺⁺ ) for neutralis ing micro-bacterial contaminants. The outer track sleeve 2 and the inner track sleeve 3 are both axially and rotationally interlocked, functioning as a single unit.

Fig. 3 is a longitudinal section view of the needle shield 12 which has a hollow, generally cylindrical shield body 12.1 with a distal shield end 12.2. A pair of opposite radial protrusions 12.5 are provided on an exterior wall of the shield body 12.1. Furthermore, an interior portion of the shield body 12.1 is formed with a wall thickening to provide an interior chamber 12.6 for sliding reception of the plug 10 as well as a shield spring socket 12.7 for support of a distal end portion of the shield spring 5.

Fig. 4 is a longitudinal section view of the plug 10 which has a generally cylindrical plug body 10.1 with a distal end surface 10.6, a proximal receiving chamber 10.2 capable of accom modating a front portion of the needle hub 4 and a cylindrical channel 10.3 of a diameter slightly smaller than the transversal dimension of the injection needle 15 to ensure a fluid tight slidable reception and accommodation thereof. The cylindrical channel 10.3 is sealed distally by a self-sealing front section 10.4 which the injection needle 15 is able to penetrate. At its proximal end portion, the plug 10 is provided with a pair of transversally extending plug arms 10.5.

Figs. 5a and 5b are, respectively, a longitudinal section view and a perspective view of the needle hub 4 having a central needle carrier 4.1 to which the injection needle 15 is glued. The injection needle 15 comprises a front needle end portion 15f for insertion into skin and a rear needle end portion 15r for insertion into a drug reservoir. The front needle end portion 15f comprises a front needle tip 15t which is ground to enable easy and painless insertion of the front needle end portion 15f into the skin. The needle hub 4 comprises a transversal sur- face from which an annular sleeve 4.2 extends proximally and a pair of opposite plug guides

4.3 extend distally.

The annular sleeve 4.2 houses the rear needle end portion 15r and defines a receiving space 4.9 for reception of a cartridge holder (not shown) in such a manner that the rear nee dle end portion 15r fluidly connects with an interior of a drug cartridge (not shown) held in the cartridge holder. The annular sleeve 4.2 comprises a plurality of interior protuberances 4.7 for securing a firm attachment of the needle hub 4 to the cartridge holder. The interior protuberances 4.7 are engageable with a bayonet track interface of the cartridge holder.

The plug guides 4.3 are arranged to form axial guiding slots 4.5 for the plug arms 10.5, pre venting rotation of the plug 10 relative to the needle hub 4, while allowing relative axial mo tion between the two. The periphery of the transversal surface forms a circumferential flange

4.4 which together with a neighbouring annular recess 4.6 function as attachment means for axial fixation of the outer track sleeve 2 to the needle hub 4.

Fig. 6 is a perspective view of the outer track sleeve 2 which has a generally cylindrical sleeve wall 2.1 of larger dimension than the shield body 12.1. In Fig. 6 an interior portion 2.2 of the sleeve wall 2.1 is indicated with dotted lines to provide a three-dimensional visualisa tion of inner geometries which include a pair of diametrically opposite start seats 2.3, a pair of diametrically opposite distal helical guide surfaces 2.6, a pair of diametrically opposite male coupling portions 2.4, and a pair of diametrically opposite distal shield track portions 2.5. Each of the distal shield track portions 2.5 is configured for sliding engagement with one of the protrusions 12.5 on the shield body 12.1. An interior rim 2.8 (cf. Fig. 9) is provided at a proximal end portion of the sleeve wall 2.1 for snap engagement with the circumferential flange 4.4.

Fig. 7 is a perspective view of the inner track sleeve 3 which has an inner track sleeve body 3.1 with a pair of female coupling portions 3.4 for engagement with the male coupling por tions 2.4, ensuring a rotationally interlocked relationship between the outer track sleeve 2 and the inner track sleeve 3.

The inner track sleeve body 3.1 is further provided with respective start seat tops 3.3, re spective proximal helical guide surfaces 3.6, respective bulbs 3.7 separating the two, and respective proximal shield track portions 3.5, which in cooperation with the respective start seats 2.3, the respective distal helical guide surfaces 2.6, and the respective distal shield track portions 2.5 provide two types of track configurations defining, respectively, the extent of motion of the plug arms 10.5 and the protrusions 12.5, and thereby of the plug 10 and the needle shield 12.

Fig. 8 is a perspective view of a track sleeve assembly 90 as formed by the combination of the outer track sleeve 2 with the inner track sleeve 3. The outer track sleeve 2 is for the sake of clarity indicated as a see-through component to illustrate how the above described re spective seats and guide surfaces of the inner track sleeve 3 and the outer track sleeve 2 form the different track configurations for the plug arms 10.5 and the protrusions 12.5 to fol low. Only one of each of the two types of track configurations are visualised, and in the fol lowing the details of the structure and use of the track sleeve assembly 90 will be explained based on these respective representatives, it being implicit that diametrically opposite track configurations with similar characteristics exist.

Thus, the track sleeve assembly 90 comprises a needle shield track 91, defined by the distal shield track portion 2.5 and the proximal shield track portion 3.5, for guiding an axial move ment of the needle shield 12 relative to the needle hub 4 via the radial protrusion 12.5, a circumferential track section 93 for accommodating the plug arm 10.5 in an advanced posi tion of the plug 10, and a helical track section for guiding an axial movement of the plug 10 relative to the needle hub 4 via the plug arm 10.5.

Fig. 9 shows the needle assembly in a pre-use state, sectioned by a 90° cut. In the pre-use state, which also corresponds to a between-use state, the plug 10 is in the advanced posi tion, housing the front needle end portion 15f in the cylindrical channel 10.3, and the needle shield 12 is in a forward position, covering both the plug 10 and the front needle end portion 15f. It is seen that the shield body 12.1 rests against the plug arm 10.5, which prevents prox imal movement of the needle shield 12 relative to the plug 10.

Fig. 10 is a perspective view of the needle assembly 1 mounted on an exemplary suitable injection device 100. The injection device 100 comprises a housing 50 accommodating a dose expelling mechanism (not visible) activatable by means of an injection button 65, and a cartridge holder 20 in axial extension thereof, the cartridge holder 20 carrying a prefilled drug cartridge 30 of the type having a generally cylindrical body with a distal seal in the form of a penetrable septum (not visible) and a proximal seal in the form of an axially displaceable piston (not visible). The injection device 100 further comprises a dose setting mechanism including a dose dial 69 and a scale drum 59 by which the user can set a desired dose to be injected in conventional manner. The invention will now be described in more detail with reference to Fig. 11 which illustrates the movement patterns of the plug 10 and the needle shield 12 during use of the injection device.

Figs. 11 a-11 g shows the needle assembly 1 in different states during an injection procedure. In each figure the outer track sleeve 2 is depicted as a see-through component to allow presentation of the internal track configurations.

In Fig. 11a the needle assembly 1 is in the pre-use state where the front needle end portion 15f is covered by both the plug 10 and the needle shield 12, corresponding to the state shown in Fig. 9. In this state the plug arm 10.5 resides in the circumferential track section 93 and the plug 10 is thereby prevented from axial displacement. The radial protrusion 12.5 is positioned at a distal end portion of the needle shield track 91, but since the needle shield 12 is prevented from proximal motion relative to the plug 10 (cf. Fig. 9) the radial protrusion 12.5 is unable to leave its position at this point. Consequently, there is no risk of accidental needle stick injuries, and the mere mounting of the needle assembly 1 on the injection de vice 100 does not present any safety issues.

When an injection procedure is initiated the user firstly rotates the outer track sleeve 2 coun ter-clockwise (seen from a distal perspective) relative to the needle hub 4, the cartridge holder 30, and the housing 50, as illustrated in Figs. 11 b-11 f. The plug arm 10.5, which is rotationally fixed with respect to the needle hub 4 via its engagement with the guiding slot 4.5, thereby travels the circumferential track section 93 until it reaches the bulb 3.7. This is illustrated in Fig. 11b.

An additional torque to the outer track sleeve 2 is needed to move the plug arm 10.5 past the bulb 3.7 and into the helical track section 94, as the plug arm 10.5 must deform elastical ly to pass through the narrowed pathway provided by the bulb 3.7. This is illustrated in Fig. 11c.

Once the plug arm 10.5 has passed the bulb 3.7 continued counter-clockwise rotation of the outer track sleeve 2 causes the plug arm 10.5 to travel the helical track section 94 (Figs. 11 d and 11 e), which includes an axial displacement component. Resultantly, the plug 10 experi ences an axial retraction from the advanced position, leading to the front needle tip 15t pen etrating the self-sealing front section 10.4 and eventually to the front needle end portion 15f being exposed within the needle shield 12 when the plug arm 10.5 reaches a closed end of the helical track section 94, as shown in Fig. 11 f. The plug 10 is thereby moved to a retracted position independently of the needle shield 12 which remains in the forward position, still covering the front needle end portion 15f and pre venting accidental encounters with the front needle tip 15t. However, since the plug arm 10.5 has now been moved proximally the needle shield 12 is no longer prevented from axial dis placement, so when the user places the distal shield end 12.2 on the skin at a desired site and presses the injection device against the skin the radial protrusion 12.5 is free to travel the needle shield track 91 as the needle shield 12 moves from the forward position to a rearward position against the bias force from the shield spring 5. The front needle end por tion 15f consequently penetrates the skin and enters a subcutaneous compartment to which a dose of the drug may then be administered.

Once the dose administration is completed and the injection device is removed from the skin the shield spring 5 returns the needle shield to the forward position, whereby the front nee dle end portion 15f is automatically re-covered. The user now rotates the outer track sleeve 2 clockwise relative to the needle hub 4, the cartridge holder 30, and the housing 50. This leads the plug arm 10.5 backwards in the helical track section 94 towards the bulb 3.7. When the plug arm 10.5 reaches the bulb 3.7, corresponding to the position shown in Fig. 11 d an additional torque to the outer track sleeve 2 is required to move it past the bulb 3.7 and back into the circumferential track section 93. However, once the plug arm 10.5 reaches the start position (Fig. 11a) the plug 10 has been returned to the advanced position where the front needle end portion 15f resides in the cylindrical channel 10.3 covered by the self sealing front section 10.4.

The biostatic property of the plug 10 provides a microbe hostile environment for the front needle end portion 15f between use of the injection needle 15 which ensures that the portion of the injection needle going into the skin of the user is kept sufficiently clean to allow for safe multiple reuse. If, for example, the injection device 100 is of the disposable type the needle assembly 1 may be used with the injection device 100 until the drug cartridge 30 is emptied of injectable content, thereby reducing the needle handling activities significantly compared to conventional needle assembly solutions.

Furthermore, due to the independent axial displacement of the plug 10 by the manual rota tion of the outer track sleeve 2 the force required to automatically return the needle shield 12 to the forward position is relatively small, and a weak, low-cost shield spring 5 can thereby be incorporated in the needle assembly 1. The tactile feedback to the user when pressing the injection device against the skin during insertion of the front needle end portion 15f is consequently sufficiently large to distinguish when the needle shield is fully pushed back and thereby to establish when the injection can be performed.

Hence, the needle assembly 1 , which can be user mountable onto, or pre-mounted by the manufacturer on, the injection device 100, is easily and simply operated during each injec- tion procedure by rotation of the outer track sleeve 2 in a specific direction to displace the plug 10 proximally away from an initial needle covering position, depression against the skin to displace the needle shield 12 proximally and insert the front needle end portion 15f, and rotation of the outer track sleeve 2 oppositely to the specific direction to return the plug 10 distally to the initial needle covering position.