Abstract:
An injection device ( 1 ) for injecting apportioned doses of liquid drug. The injection device ( 1 ) includes a dose setting structure, an injection structure, a removable cap ( 15 ) and a cap receiving part ( 9 ) adapted to abut or engage with the cap ( 15 ) when the cap ( 15 ) is mounted on the injection device ( 1 ). The dose setting structure is operatively coupled to the cap receiving part ( 9 ) in such a manner that mounting and/or dismounting of the cap ( 15 ) on/from the injection device ( 1 ) causes the dose setting structure to set a dose. Thereby a correct dose of drug is automatically set during a cap on/cap off cycle. Since such a cycle is normally performed between subsequent injections, the number of steps required to be performed by the user is reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/842,337, filed Jul. 23, 2010 (Notice of Allowance received), which is a continuation of Ser. No. 12/864,279, filed Jul. 23, 2010 (U.S. Pat. No. 8,348,905, issued Jan. 8, 2013), which is a 35 U.S.C. §371 national stage application of International Patent Application PCT/EP2009/050797 (published as WO 2009092807), filed Jan. 23, 2009, which claimed priority of European Patent Application 08150533.1, filed Jan. 23, 2008; this application further claims priority under 35 U.S.C. §119 of U.S. Provisional Application 61/027,099, filed Feb. 8, 2008, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an injection device for injecting apportioned doses of liquid drug, such as for injecting doses of a single fixed amount of drug, or for injecting doses of a limited number of different amounts of drug. More particularly, the present invention relates to an injection device in which the number of operations to be performed by a user is minimised. The injection device is particularly suitable for self-injection by the user of a liquid drug, e.g. insulin for treating diabetes. 
       BACKGROUND OF THE INVENTION 
       [0003]    Within some therapy areas the tendency of a patient to adhere to the prescribed therapy is dependent on the simplicity of the specific treatment regimen. For example, many people with type 2 diabetes are diagnosed with the disease at a relatively high age where they are less prone to accept a treatment that intervenes too much with their normal way of living. Most of these people do not like constantly being reminded of their disease and, as a consequence, they do not want to be entangled in complex treatment patterns or waste time on learning to operate cumbersome delivery systems. 
         [0004]    Basically, people with diabetes need to keep track of, and minimise, their glucose excursions. Insulin is a well-known glucose lowering agent which has to be administered parenterally to be effective in the body. The presently most common way of administering insulin is by subcutaneous injections. Such injections have previously been performed using a vial and a syringe, but in recent years so-called injection devices, or injection pens, have gained more and more attention in the marketplace. This is for one thing due to the fact that for many people these injection devices are easier to handle, particularly as they do not require the user to carry out a separate filling procedure before each injection. 
         [0005]    In some prior art injection devices which are suitable for self-injection, the user has to set a desired dose using a dose setting mechanism of the injection device and subsequently inject the previously set dose using an injection mechanism of the injection device. In this case the dose is variable, i.e. the user must set a dose which is suitable in the specific situation each time a dose is to be injected. 
         [0006]    Other prior art injection devices are adapted to inject a fixed dose each time it is operated. In this case the user has to prepare the injection device, thereby setting the fixed dose, using a dose setting or loading mechanism, and subsequently inject the dose using an injection mechanism. 
         [0007]    U.S. Pat. No. 4,973,318 discloses a disposable syringe comprising a protective cap which is removably mounted over a first housing element of the syringe. The cap is configured to abut a second housing element while mounted in place on the first housing element. The protective cap is engaged with the first housing element such that rotation of the cap with respect to the second housing element causes rotation of the first housing element with respect to the second housing element. This relative rotation causes a variable dose to be set, i.e. the protective cap is used when setting a dose. However, it is necessary for the user to perform the step of setting a dose as well as the step of injecting the set dose. 
         [0008]    U.S. Pat. No. 5,674,204 discloses a medication delivery pen having a medication cartridge, a pen body assembly and a cap. The pen body assembly includes a dose setting mechanism and a dose delivery mechanism that are selectively disconnected and connected by attaching and removing, respectively, the cap of the medication delivery pen. When the cap is attached to the medication delivery pen the user can easily dial in and correct the dialled in dosage and when the cap is removed the medication delivery pen is ready to dispense the dialled in dose. Thus, attaching/removing the cap to/from the medication delivery pen causes a clutch mechanism to be operated to switch the medication delivery pen between a dose setting mode and an injection mode. Also in this device it is necessary for the user to perform the step of setting a dose as well as the step of injecting the set dose. 
         [0009]    An example from another medical device area, U.S. Pat. No. 7,302,948 discloses a nasal applicator in which a drug container is able to slide back and forth in response to the cocking and actuation of a spring. The drug container is slided backwards when a cap is attached to the nasal applicator. An abrupt stop during the forward movement of the drug container causes a piston to move and eject a dose of the drug through a dispensing nozzle. 
         [0010]    U.S. Pat. No. 6,056,728 discloses an injection device which offers automatic needle insertion. It includes an intermediate chamber between the drug reservoir and the injection outlet for receiving a volume of the drug during preparation of the device for injection. The device has a rather bulky construction which makes it less attractive to carry around in e.g. a handbag. 
         [0011]    It is desirable to provide an injection device which is simple to handle and which is intuitive and easy for the patient to learn how to use. In particular, it is desirable to provide an injection device which is capable of administering a number of doses of liquid drug, while at the same time requiring a minimum number of operations to be performed by the user. It is also desirable to provide an injection device which clearly indicates to the user when it is ready for injection and when the remaining volume of drug in the reservoir is insufficient to provide a full dose, and which then automatically renders further activation of the injection mechanism impossible. It is further desirable to provide an injection device which has a non-bulky design, so the user is not tempted to leave it at home instead of carrying it along during the day. 
         [0012]    Some prior art injection devices offer so-called automatic delivery. These injection devices use energy from an internal energy source, typically a spring, to advance the piston in the reservoir. Automatic injection devices intend to reduce the force required by the user to eject the drug out of the reservoir. An example of such an injection device is found in U.S. Pat. No. 5,104,380. 
         [0013]    In an automatic, spring-powered injection device where an engagement member is retracted axially along a toothed piston rod when the device is readied for injection, it must be ensured that the spring is cocked and secured against release at the same time as the engagement member moves into engagement with a dedicated tooth on the piston rod. If the engagement member has moved into engagement with a tooth on the piston rod but the spring has not been secured against release, the device will deliver an unintended dose. On the other hand, if the spring has been cocked and secured against release without the engagement member having moved into engagement with a tooth on the piston rod, no dose will be delivered when the injection mechanism is activated. 
         [0014]    It is therefore desirable to provide an automatic injection device with which the user is ensured that a dose is either set correctly, and secured against delivery until the user activates the injection mechanism, or not set at all. 
         [0015]    In U.S. Pat. No. 6,193,698 a spring is used to bias a dosing button and a drive arrangement towards a proximal position in an injection apparatus. During an injection the dosing button and the drive arrangement are pushed towards a distal position. To prevent uncontrolled injections, a locking member prevents return movement of the dosing button and the drive arrangement against the biasing force of the spring. In order to release the dosing button the user must manually press a trigger button which is accessible only after manual placement of two sleeves in a “zero” position relative to each other. 
         [0016]    It is desirable to provide an injection device which locks the dosing button in a distal position following an injection, and which automatically releases the dosing button and moves it axially back into a proximal position when the injection device is readied for an injection so the user can see that the device is handled properly. 
         [0017]    EP 1 304 129 discloses an injection device which includes a mechanism for automatically locking out the dose dial from an inadvertent injection after the dial has been retracted to set a dose. The lockout mechanism comprises an interference fit between flexible fingers formed in the dial and a groove in the device housing. These fingers must be able to withstand large compressive forces in order to prevent the dial from being depressed in case of misuse or accidental handling of the device. 
         [0018]    US 20070135767 discloses another example of an injection device which includes a mechanism for preventing inadvertent depression of an injection button. 
         [0019]    It is desirable to provide an injection device which the user does not risk inadvertently activating to eject a dose of drug while the protective cap is still on and which at the same time does not require a mechanical lock that is able to resist large forces. 
         [0020]    It is further desirable to provide an injection device which is both safe and effective to use and safe to carry around. 
         [0021]    Generally, when manufacturing injection devices which comprise a piston rod adapted to move a piston in a reservoir to thereby expel drug out of the reservoir it is essential that the piston rod is in engagement with the piston during the entire course of injection. If this is not the case the user may risk injecting a smaller amount of drug than intended. However, it is for several reasons preferred that the drug is not pressurised in the reservoir when the user takes the injection device into use for the first time. Injection devices are therefore often manufactured in such a way that a small clearance is deliberately provided between the piston rod and the piston to allow some play of the piston rod during transportation. In case of variable dose injection devices, when taking the device into use for the first time, the user sets a small dose and ejects it into the air. This action primes the injection device such that when a next dose is set the user is sure to inject the correct amount of drug, since the piston rod and the piston are now connected. In some fixed dose injection devices the piston rod travels a substantial distance each time a dose is injected. If the user in this case initially sets a dose and fires it into the air to prime the device, a substantial amount of drug may be wasted to the surroundings. This is particularly unwanted if the drug is expensive. 
         [0022]    It is therefore desirable to provide a fixed dose injection device with which a user can perform an initial priming without wasting an approximately full dose of drug. 
       SUMMARY OF THE INVENTION 
       [0023]    It is an object of the invention to provide an injection device in which the number of operating steps required to be performed by the user is reduced as compared to similar prior art injection devices. 
         [0024]    It is a further object of the invention to provide an injection device which is intuitive and thereby easy to learn how to use. 
         [0025]    It is an even further object of the invention to provide an injection device in which the dose setting procedure is simplified as compared to similar prior art injection devices. 
         [0026]    It is an even further object of the invention to provide an injection device which clearly signals to the user when it is ready for an injection. 
         [0027]    It is an even further object of the invention to provide an injection device which clearly signals to the user when the remaining amount of medicament in the reservoir is insufficient to provide another full dose and which automatically renders further use of the device impossible. 
         [0028]    It is an even further object of the invention to provide an injection device in which the injection means is automatically disabled when the protective cap is mounted on the device and automatically enabled when the protective cap is dismounted from the device, thereby ensuring that the user do not risk involuntarily ejecting a dose of medicament into the cap when for example carrying the device in a hand bag. 
         [0029]    It is an even further object of the invention to provide an injection device which automatically sets a correct dose, thereby eliminating the risk of a user setting an incorrect dose. 
         [0030]    It is an even further object of the invention to provide an injection device which is capable of injecting a predetermined dose and which has an initial priming feature allowing the user to perform a first shot with the injection device that results in an ejection of a smaller volume of drug than the predetermined dose. 
         [0031]    In the following disclosure of the present invention, aspects and embodiments will be described which address one or more of the above objects or which address objects apparent from the disclosure as well as from the description of exemplary embodiments. 
         [0032]    Thus, according to a first aspect of the invention a mechanical injection device for injecting apportioned doses of liquid drug is provided, the injection device comprising dose setting means being operable to set a dose, injection means being operable to inject a previously set dose, a removable cap, and a cap receiving part adapted to abut or engage with the cap when the cap is mounted on the injection device, wherein the dose setting means is operatively coupled to the cap receiving part in such a manner that mounting and/or dismounting of the cap on/from the injection device causes the dose setting means to set a dose. 
         [0033]    The injection device may for example be of the kind which is able to repeatedly set and deliver a predetermined dose. 
         [0034]    In the present context the term ‘mechanical injection device’ should be interpreted to mean an injection device which is mechanically operated as opposed to motor driven injection devices. 
         [0035]    In the present context the term ‘liquid drug’ should be interpreted to mean a drug in a liquid state, such as, e.g., a solution or a suspension. 
         [0036]    In the present context the term ‘predetermined dose’ should be interpreted in such a manner that when the dose setting means is operated a specific fixed dose is set, i.e. it is not possible to set an arbitrary dose. However, the predetermined dose may be variable in the sense that it may be possible to initially set the injection device to a selected dose, and the dose setting means will then set this selected dose each time the dose setting means is operated. It should also be noted that the term ‘predetermined dose’ does not rule out that the injection device has a priming function. 
         [0037]    The injection device is preferably capable of injecting multiple doses of liquid drug. 
         [0038]    The dose setting means is the part of the injection device which is operated when a dose is being set. Similarly, the injection means is the part of the injection device which, when operated, is causing a set dose to be injected. The injection means often comprises a movable piston rod being adapted to cooperate with a piston arranged in a cartridge containing the liquid drug in such a manner that operation of the injection means causes the piston rod to move in such a manner that the piston is moved inside the cartridge in a direction which causes liquid drug to be expelled from the cartridge via a needle arranged to penetrate a septum of the cartridge. 
         [0039]    The injection device comprises a removable cap which may be adapted to cover a needle holding part of the injection device when the injection device is not in use. Thereby the removable cap is capable of protecting a needle mounted on the needle holding part, preventing needle sticks and preventing accidental spilling of liquid drug. The cap can be removed when it is desired to inject a dose, thereby uncovering the needle holding part. 
         [0040]    The cap receiving part is a part of the injection device which is adapted to receive and hold the removable cap when it is mounted on the injection device. It may comprise means for retaining the cap, such as a bayonet joint, a threaded portion, a snap lock, etc. The cap receiving part may be adapted to receive the cap when the cap is mounted on the injection device to cover the distal part of the injection device. Alternatively, the cap receiving part may be adapted to receive the cap when the cap is mounted on the proximal part of the injection device. 
         [0041]    The dose setting means is operatively coupled to the cap receiving part, i.e. performing specific operations of the cap receiving part affects the dose setting means. More particularly, the dose setting means and the cap receiving part are coupled in such a manner that mounting and/or dismounting of the cap on/from the injection device causes the dose setting means to set a dose. The dose setting means and the cap receiving part may be mechanically coupled, either directly or via one or more intermediate parts, or they may be coupled in any other suitable way as long as specific operations of the cap receiving part affects the dose setting means in such a manner that the dose is set. Thus, the dose may be set when the cap is mounted or when the cap is dismounted. Alternatively, the dose may be partly set when the cap is mounted, the remaining part of the dose being set when the cap is subsequently dismounted. In any event, performing a cycle of operations comprising mounting and dismounting the cap results in the dose being set by the dose setting means, preferably automatically. 
         [0042]    The removable cap is normally positioned at the cap receiving part, preferably covering a needle holding part or a jet orifice, when the injection device is not in use, and the cap is removed when it is desired to inject a dose of drug by means of the injection device. After the dose has been injected the cap is once again mounted at the cap receiving part. Thus, each time a dose is injected the cap has been mounted and dismounted since the previous dose was injected. Since the dose setting means and the cap receiving part are coupled as described above, such a mounting/dismounting cycle of the cap automatically results in a dose being set. Therefore, when the user has removed the cap in order to inject a dose, the dose is already set, and it is not necessary for the user to perform additional operating steps in order to set the dose. Thereby the number of steps to be performed by the user is reduced. Furthermore, since the dose is automatically set, the risk of introducing errors during dose setting is reduced. 
         [0043]    Thus, in one particular embodiment of the invention an injection device for administering apportioned doses of a liquid drug is provided, the injection device comprising dose setting means operable to set a dose, injection means operable to inject the set dose, a removable cap, a cap receiving part adapted to abut or engage with the cap when the cap is mounted on the injection device, wherein the dose setting means is operatively coupled to the cap receiving part in such a manner that mounting and/or dismounting of the cap on/from the injection device causes the dose setting means to set a single dose. 
         [0044]    In the present context, the term ‘to set a single dose’ should be interpreted as outlined in the above, i.e. mounting the cap on, or dismounting the cap from, the injection device causes the dose setting means to set one dose which is deliverable upon operation of the injection means. By such an arrangement it is therefore not possible to inject two consecutive doses without performing a cycle of mounting and dismounting the cap on/from the injection device. This constitutes a safety feature of the device since if it was possible to inject a multiple number of doses, such as the same dose a multiple number of times, without performing the cap mounting/dismounting cycle, the user would have to keep count of how many times the injection means had been operated. This could lead to confusion and uncertainty regarding the actual dose delivered. 
         [0045]    A substantially linear movement of the cap may cause the dose setting means to set the dose, i.e. the movement of the cap may involve a substantially linear translation which causes the dose setting means to set the dose. According to this embodiment, the cap is mounted and/or dismounted in a substantially linear movement. In this case the cap is preferably retained in the mounted position by means of a snap lock, a bayonet joint or the like. According to this embodiment, the cap may move an element in a substantially axial direction when it is mounted or dismounted. The movement of the element may cause the dose to be set, e.g. by storing energy in a spring member and/or by moving an injection button in an axial direction. 
         [0046]    Alternatively, or additionally, a rotational movement of the cap may cause the dose setting means to set the dose, i.e. the movement of the cap may involve a rotational movement which causes the dose setting means to set the dose. According to this embodiment, the cap is preferably mounted and/or dismounted in a movement which is at least partly rotational, e.g. a purely rotational movement or a spiralling movement. The cap may, in this case, be retained at the cap receiving part by means of a threaded connection, a bayonet joint or the like. The rotational part of the movement may alone be responsible for setting of the dose, e.g. by causing an element of the injection device to rotate along. For instance, in the case that the cap is retained by means of a bayonet joint, the rotating part of the mounting or dismounting operation may cause an element to rotate along. It may be envisaged that the cap is mounted in a substantially linear movement, pressing the cap past a threaded portion, and that the cap must be rotated along the threaded portion in order to dismount the cap. In this case the rotating part of the dismounting movement may advantageously cause the dose to be set. This has the advantage that the dose is not set until immediately prior to the intended injection of the dose, and it can thereby be avoided that a loaded injection device must be carried in a pocket or a handbag. Thereby the risk of accidentally ejecting the set dose prematurely in the cap is minimised. However, this could alternatively be obtained by means of a suitable locking mechanism preventing injection of the set dose until the cap has been removed. 
         [0047]    As an alternative to the purely rotational movement, a combination of a linear and a rotational movement, i.e. a spiralling movement may cause the dose setting means to set the dose. 
         [0048]    In a particular embodiment of the invention, mounting the cap on the injection device causes an element to move axially with respect to the piston rod to thereby move an engagement member along the piston rod to a more proximal position. Each time a dose is set by mounting the cap on the injection device the engagement member is thus moved further along the piston rod towards the proximal end thereof. 
         [0049]    In an exemplary embodiment of the invention an injection device for administering apportioned doses of a liquid drug is provided, the injection device comprising a proximal part and an opposite distal part, a cartridge adapted to hold the liquid drug and comprising a movable piston, dose setting means operable to set a dose, injection means operable to inject the set dose and comprising a piston rod adapted to sequentially advance the piston in the cartridge, each sequential advancement corresponding to the set dose, a removable cap adapted to cover the distal part of the injection device, and a cap receiving part adapted to abut or engage with the cap when the cap is mounted on the injection device, wherein the dose setting means is operatively coupled to the cap receiving part in such a manner that mounting and/or dismounting of the cap on/from the distal part of the injection device causes the dose setting means to set a dose. Since the distal part of the injection device is the part from which the drug is ejected out of the reservoir, the removable cap is adapted to cover and protect the drug outlet. 
         [0050]    The injection device may further comprise energy means connected to the dose setting means and the injection means in such a manner that energy is stored in the energy means during setting of a dose, and in such a manner that previously stored energy is released from the energy means during injection of a dose, thereby causing the dose to be injected. 
         [0051]    The energy means may be a spring member which may be adapted to be loaded along its centre axis, e.g. by compressing the spring or elongating the spring. The spring member may be a compressible spring or a torsion spring. In the case that the spring member is a compressible spring, the injection device may advantageously be operated in the following manner. When the cap is either mounted on or dismounted from the cap receiving part a spring compressing element is moved, preferably in an axial direction, thereby compressing the spring. The spring compressing element is locked in this position, thereby retaining the spring member in the compressed state. When the injection needle has been inserted at a desired injection site, the injection button is pressed. This causes the spring compressing element to be moved out of the locked position, and the energy stored in the spring is thereby released in such a manner that it causes a piston rod to move while pressing a piston of a cartridge forward, thereby causing a dose of drug to be injected from the cartridge, via the injection needle. 
         [0052]    According to a second aspect of the invention an injection device is provided comprising a housing, dose setting means operable to set a dose, injection means operable to inject the set dose and comprising an at least partly toothed rod, a drive member adapted to undergo relative motion with respect to the toothed rod when the dose setting means is operated and to transmit a driving force to the toothed rod when the injection means is operated, the drive member comprising an engagement element adapted to engage with the toothed rod, guiding means adapted to guide the movement of the drive member and/or the toothed rod, and energy means operatively coupled to the dose setting means and the injection means and adapted to store and release energy for translational and rotational motion. 
         [0053]    The drive member may be coupled with the energy means in such a manner that movement of the drive member causes the energy means to store and/or release energy and, conversely, in such a manner that release of energy from the energy means causes the drive member to move. The energy means may comprise a compression spring being rotationally pre-stressed to bias the drive member in a specific rotational direction. 
         [0054]    When the dose setting means is operated to set a dose the drive member will undergo a relative motion with respect to the toothed rod whereby the engagement element will be moved out of engagement with a tooth on the toothed rod and moved along the toothed rod to pass a more proximally positioned tooth. This relative motion is guided by the guiding means. The guiding means may form part of the housing or may be a separate element coupled to the housing. When the injection means is subsequently operated to inject the set dose the engagement member will engage the tooth it just passed and the drive member will move distally in the housing while slaving the toothed rod. Also this motion is guided by the guiding means. 
         [0055]    Hence, in the present context the term ‘the dose setting means is operated to set a dose’ should be interpreted to mean that the dose setting means is operated to a degree where a dose is actually set. Just operating the dose setting means does not necessarily lead to a dose being set, as will be clear from the below. 
         [0056]    Further, in the present context the term ‘tooth’ should be interpreted to mean any lateral structural irregularity on the rod, such as e.g. a protrusion or an indentation, capable of receiving an engagement element and allowing for a mutual axial displacement of the rod and the engagement element. 
         [0057]    In an exemplary embodiment of the invention the guiding means comprises a structure which enables the drive member and the toothed rod to perform a purely translational relative motion during one part of the relative motion and to perform a combined translational and rotational relative motion during another part of the relative motion. In this particular embodiment the guiding means is provided with a longitudinal first guiding surface which is substantially parallel with the toothed rod and which enables the purely translational relative motion between the drive member and the toothed rod. The guiding means is further provided with a sloping second guiding surface which meets the first guiding surface at a transition point and which enables the combined translational and rotational relative motion between the drive member and the toothed rod. The second guiding surface and the first guiding surface are preferably mutually angled between 180° and 270°, more preferably between 225° and 270°, and most preferably between 240° and 270°. In any case the angle between the first guiding surface and the second guiding surface and the traversable dimension of the second guiding surface constitute two parameters which should be fitted so that when the drive member traverses the second guiding surface during dose setting the drive member and the toothed rod perform a combined translational and rotational relative motion during which the engagement element passes a tooth on the toothed rod. 
         [0058]    The energy means may comprise a compression spring being rotationally pre-stressed to constantly bias the drive member in a specific rotational direction. The spring may further be axially pre-stressed to constantly bias the drive member in the distal direction of the injection device. This means that when the drive member is traversing the first guiding surface of the guide means it is exposed to the axial force of the spring seeking to displace it distally in the injection device. During dose setting the drive member may thus traverse the first guiding surface against the force of the spring, while during injection the drive member may traverse the first guiding surface under the force of the spring. Further, when the drive member is traversing the first guiding surface it may be exposed to the rotational force of the spring. However, the drive member is prevented from being rotated in accordance with a biasing rotational force of the spring when traversing the first guiding surface. This is due to the first guiding surface being arranged longitudinally and substantially in parallel with the toothed rod. 
         [0059]    The transition point denotes the position where the first guiding surface and the second guiding surface meet, i.e. the point where the drive member is transitioned from traversing the first guiding surface to traversing the second guiding surface, and vice versa. When the drive member is traversing the second guiding surface it may be exposed to both the axial and the rotational biasing forces of the spring. Since the second guiding surface is sloped these biasing spring forces enable a combined translational and rotational movement of the drive member with respect to the toothed rod. During dose setting the drive member may traverse the second guiding surface under the rotational force of the spring, but against the axial force of the spring. During operation of the injection means the drive member may traverse the second guiding surface against the biasing rotational force of the spring. 
         [0060]    The spring and the sloping angle of the second guiding surface are preferably dimensioned so that the biasing rotational force of the spring is able to move the drive member along the second guiding surface against the biasing axial force of the spring. 
         [0061]    The guiding means is preferably provided with an abutting surface adapted to stop the combined translational and rotational movement of the drive member when the drive member is in a position where the engagement element has just passed a tooth on the toothed rod. In this position the spring is both cocked and secured against release since the biasing rotational force of the spring is able to overcome the biasing axial force of the spring and thereby retain the drive member in a stabile stationary state. 
         [0062]    The guiding means is further preferably provided with an abutting surface adapted to stop the translational movement of the drive member during injection, thereby indicating an end-of-dose position, i.e. a position of the drive member corresponding to the complete delivery of the set dose. The actual dose delivered may be determined by the distance between two consecutive teeth on the toothed rod. This distance is greater than the axial distance travelled by the drive member along the first guiding surface of the guiding means, but smaller than the total axial distance travelled by the drive member following activation of the injection means, i.e. smaller than the combined axial dimension of the first and the second guiding surfaces. The actual dose delivered may alternatively be determined by the total axial distance, which the drive member travels following activation of the injection means. 
         [0063]    The above described arrangement is particularly advantageous since when the user operates the dose setting means to set a dose the last part of the dose setting may be performed automatically by the injection device. This is due to the fact that during the first part of the dose setting the user manually operates the dose setting means to move the drive member proximally along the first guiding surface against the biasing translational force of the spring. If the spring is rotationally pre-stressed it constantly exerts a force on the drive member which may bias the drive member against the first guiding surface. Hence, in this case, as long as the drive member is guided by the first guiding surface it is prevented from rotating and it therefore performs a purely translational movement during which the engagement element is slided along the toothed rod. When the user has forced the drive member so far proximally that it reaches the transition point at the connection between the first guiding surface and the second guiding surface the biasing rotational force of the spring may begin rotating the drive member and force it to travel along the second guiding surface until it reaches the stop at the abutting surface. Since the second guiding surface is sloped the drive member will thereby be performing a combined rotational and translational movement with respect to the toothed rod. The configuration of the first and second guiding surfaces may be such that the axial displacement which the drive member undergoes from the transition point to the stop at the abutting surface moves the engagement element from a position just below, or distally of, a given tooth on the toothed rod to a position just above, or proximally of, the tooth. This ensures that when the injection means is operated the engagement element will be able to move into engagement with this tooth and slave the toothed rod axially towards the distal end of the injection device. During movement of the drive member along the second guiding surface, the spring may release rotational energy while it stores translational energy. In that case, when the drive member reaches the stop at the abutting surface the spring is cocked as well as secured against release until the next activation of the injection means. 
         [0064]    As long as the dose setting means is manipulated in such a way that the drive member travels along the first guiding surface no dose is set, and if the user ends the manipulation of the dose setting means before the drive member has reached the transition point the biasing translational force of the spring may just return the drive member to the starting point, i.e. the end-of-dose position. However, when the dose setting means is manipulated to an extent where the drive member reaches the transition point the spring may take control of the remaining part of the dose setting and ensure that the dose is actually being set, i.e. that the engagement element actually passes the intended tooth on the toothed rod, and that the drive member is retained in a stabile stationary state from which it can not be moved unless the injection means is manipulated. In that case, the last part of the dose setting procedure is carried out automatically by the injection device and the user does not have any options of intervention. 
         [0065]    When the user operates the injection means to inject the set dose the drive member may be initially forced along the second guiding surface against the biasing rotational force of the spring. At some point during this movement the engagement element will move into engagement with a tooth on the toothed rod. When the drive member reaches the transition point the biasing translational force of the spring may move the drive member and the toothed rod axially in the distal direction until the drive member meets the abutting surface. 
         [0066]    The toothed rod may be operatively coupled with a drug containing reservoir in the injection device in such a manner that the axial distance travelled by the toothed rod correlates with the actual dose delivered from the reservoir. The drug containing reservoir may be a rigid reservoir, such as a cartridge, comprising an axially moveable piston and the toothed rod may be operatively coupled to the reservoir via the piston. Alternatively, the drug containing reservoir may be a flexible reservoir which is adapted to undergo a controlled deformation when the toothed rod is moved axially in the injection device. In any case, the axial movement of the toothed rod may cause a volume reduction of the drug containing reservoir corresponding to the delivered dose. 
         [0067]    The dose setting means may be operated by pulling the dose button in a proximal direction away from the housing. Alternatively, the dose setting means may be operated as described in the following. The injection device may further comprise a removable cap and a cap receiving part adapted to abut or engage with the cap when the cap is mounted on the injection device. The dose setting means may be operatively coupled with the cap receiving part in such a manner that mounting the cap on the injection device causes the drive member to move axially along the toothed rod while being guided in this movement by the energy means and the geometry of the guiding means, as described above. In this particular embodiment, mounting the cap on the injection device causes the injection device to automatically set a dose. The guiding means may be configured so that each time the cap is mounted on the injection device the drive member travels the same distance proximally and each time the injection means is activated the drive member travels the same distance distally, in which case the injection device is a fixed dose delivery device. However, the guiding means and/or the toothed rod may alternatively, or additionally, be configured so that it is possible to pre-calibrate the zero dose position before a dose setting, thereby in practice providing a variable dose delivery device capable of delivering a limited number of different doses of drug. This could for example be implemented by providing means for regulating the axial dimension of the first guiding surface. 
         [0068]    The energy means may comprise a compression spring being rotationally pre-stressed as described above. However, other suitable energy means may be used such as for example two or more springs each being able to provide a part of the energy needed for translational and rotational motion, e.g. a compression spring capable of providing energy for translational motion and a torsion spring capable of providing energy for rotational motion, an axially compressible torsion rod or an arrangement comprising a tension spring. 
         [0069]    According to a third aspect of the invention an injection device for administering predetermined doses of liquid drug is provided comprising dose setting means operable to set a dose, injection means operable to inject the set dose and comprising an at least partly toothed rod, and a drive member adapted to undergo relative motion with respect to the toothed rod during dose setting and to transmit a driving force to the toothed rod during injection, wherein the injection device has a priming feature which allows the user to prime the injection device without ejecting an approximately full predetermined dose. 
         [0070]    The priming feature may be implemented by providing guiding means as described in connection with the second aspect of the invention, the guiding means further comprising a second longitudinal guiding surface. This second longitudinal guiding surface may be identical to the abutting surface adapted to stop the movement of the drive member along the sloping ramp surface during dose setting. Alternatively, it may be another longitudinal surface being physically connected to the sloping ramp surface. In any case, the second longitudinal guiding surface is preferably connected to a support shelf in such a way that before the user takes the injection device into use for the first time, such as when the injection device is delivered from the manufacturer, the drive member rests on the support shelf and when the user performs the very first operation of the injection means, the drive member is automatically caused to travel the second longitudinal guiding surface to take in a position on the sloping ramp surface. In case the injection device further comprises energy means being operatively coupled to the dose setting means and the injection means and being adapted to store and release energy for translational and rotational motion when the user performs this first operation of the injection means the energy means may be activated to execute the initial movement of the drive member. 
         [0071]    The longitudinal dimension of the second longitudinal guiding surface is smaller than the longitudinal dimension of the first guiding surface which guides the drive member between the transition point and the end of dose position. This means that the initial axial movement of the drive member is smaller than the axial movement it undergoes during regular injection. In other words, since the drive member slaves the toothed rod during injection the toothed rod is displaced axially a smaller distance upon the initial activation of the injection means than upon a subsequent activation of the injection means leading to the delivery of a set dose. Thereby it is possible to perform an initial priming of the injection device without wasting an amount of drug approximately equal to a predetermined dose. 
         [0072]    The injection device may be provided with a tamper band which the user can pull off to start the priming shot. This tamper band may e.g. be placed at the distal end of the housing or just distally of the injection button. Alternatively to the user pressing the injection button to perform the initial priming, the priming may be activated by turning the injection button clockwise or anti-clockwise a certain number of degrees to remove the slider from the initial shelf position. 
         [0073]    According to a fourth aspect of the invention a mechanical injection device for injecting apportioned doses of liquid drug is provided, the injection device comprising dose setting means being operable to set a dose, injection means being operable to inject a previously set dose, an injection button being operatively coupled to the dose setting means and the injection means and being axially moveable between a first position in which a dose is set and the injection device is ready for injection and a second position in which the injection means has been activated to inject the set dose, and retaining means for retaining the injection button in the second position upon activation of the injection means to inject the set dose. When the user removes his/her finger from the injection button after an injection the injection button will thus stay in the second position, thereby signalling to the user that the injection device is not yet ready for another injection. 
         [0074]    In the present context the term ‘a second position in which the injection means has been activated to inject the set dose’ should be interpreted to mean a position where the injection means has been activated to a degree allowing for the complete set dose to be injected. 
         [0075]    The injection device comprises a proximal part and an opposite, distal part, and it is preferably of an elongated shape, defining a general axis which in the abstract bridges the proximal part and the distal part. In the present context, an ‘axially moveable’ or ‘axially displaceable’ element should thus be interpreted as an element which is moveable or displaceable along the general axis of the injection device. 
         [0076]    The retaining means may be operatively coupled to the dose setting means in such a manner that when the dose setting means is operated to set a dose the retaining means is automatically disabled. This will enable the injection button to move from the second position back to the first position. In a particular embodiment, when the dose setting means is operated to set a dose the injection button is automatically moved from the second position to the first position, whereby the injection device signals to the user that it is ready for injection. 
         [0077]    The second position may be a position in which the injection button is fully depressed in or against the housing and in which only the top part or the push surface of the injection button can be seen and/or touched by the user. The first position may, conversely, be a position in which the injection button clearly protrudes from the housing. Preferably, the axial distance which the injection button travels between the first position and the second position is sufficiently large to provide a clear indication of whether the injection device is ready for injection or not. 
         [0078]    When the dose setting means is operated to set a dose, the injection button may be moved from the second position to the first position by a force transmitting member abutting or engaging with the injection button in such a manner that a translational, rotational or spiralling movement of the force transmitting member causes the movement of the injection button. The injection button may be moved substantially linearly, i.e. non-rotationally, between the first position and the second position. Alternatively, or additionally, the movement may involve a rotation of the injection button. 
         [0079]    The retaining means holding the injection button in the second position when the injection means has been activated to inject the set dose may comprise a simple friction fit between the injection button and the housing or the force transmitting member, e.g. between the exterior of the injection button and the interior of the housing. Alternatively, or additionally, the retaining means may comprise a snap fit between the injection button and the housing or the force transmitting member. The injection button may comprise a catch member adapted to engage with a locking geometry on the housing, e.g. on the interior of the housing. Conversely, the housing may be provided with a catch member adapted to engage with a locking geometry on the injection button. During injection when the injection button reaches the second position the catch member moves into engagement with the locking geometry and retains the injection button from reverse movement. During operation of the dose setting means the engagement may be released by another force transmitting member acting to move the catch member away from the locking geometry. 
         [0080]    Energy means may act on the injection button to bias the injection button towards the first position. In this case, disabling the retaining means may cause the energy means to automatically release energy for moving the injection button to the first position. In an exemplary embodiment, the energy means comprises a spring being compressed during injection by the movement of the injection button from the first position to the second position. Disabling the retaining means when operating the dose setting means to set a dose causes the spring to push the injection button back to the first position, thereby indicating that a dose has been properly set and that the injection device is ready for injection. 
         [0081]    In a particular embodiment an injection device for administering apportioned doses of a liquid drug is provided comprising a reservoir adapted to hold the liquid drug and comprising a moveable piston, dose setting means being operable to set a dose, injection means being operable to inject a previously set dose and comprising a piston rod adapted to sequentially advance the piston in the reservoir, each sequential advancement corresponding to the set dose, an injection button being operatively coupled to the dose setting means and the injection means and being axially moveable between a first position in which a dose is set and the injection device is ready for injection and a second position in which the injection means has been activated to inject the set dose, retaining means for retaining the injection button in the second position upon activation of the injection means to inject the set dose, a removable cap, and a cap receiving part adapted to abut or engage with the cap when the cap is mounted on the injection device, wherein the dose setting means is operatively coupled to the cap receiving part in such a manner that mounting the cap on the injection device causes the dose setting means to substantially simultaneously set a dose, disable the retaining means, and move the injection button from the second position to the first position. 
         [0082]    In a further embodiment an injection device is provided comprising a variable volume reservoir, dose setting means operable to set a dose, injection means operable to inject the set dose and comprising a piston rod adapted to cause a volume reduction of the reservoir, a drive member adapted to perform relative translational and rotational motion with respect to the piston rod during dose setting and to transmit a driving force to the piston rod during injection, a removable cap, a cap receiving part adapted to abut or engage with the cap when the cap is mounted on the injection device, an injection button operatively coupled to the dose setting means and the injection means and axially moveable between a first position in which a dose is set and the injection device is ready for injection and a second position in which the injection means has been activated to inject the set dose, and energy means operatively coupled to the dose setting means and the injection means and adapted to store and release energy for translational and rotational motion, wherein the piston rod comprises a structural element adapted to engage with the drive member to prevent the drive member from rotating during operation of the dose setting means when the remaining amount of drug in the reservoir is insufficient to provide another full dose, thereby providing an end-of-contents indication. 
         [0083]    By such an arrangement, when the piston has been advanced to a point where an insignificant or insufficient amount of drug is left in the reservoir the drive member is still able to move axially along the piston rod during dose setting, but it is prevented from rotating with respect to the piston rod. In case mounting the cap on the injection device affects the dose setting means by causing the drive member move relative to the piston rod, it is thus still possible to mount the cap on the injection device. However, an axial displacement alone will not cause a dose to be set, and the device is therefore rendered impossible to use for further injections. 
         [0084]    As described above, the injection button may be automatically moved from the second position to the first position when the dose setting means is operated to set a dose. However, when the dose setting means is operated without a dose actually being set, the injection button will not move to the first position, and the above arrangement will therefore signal to the user mounting the cap on the cap receiving part after injection of the last dose that no further doses remain in the injection device. 
         [0085]    The structural element provided on the piston rod may be a bead, a hammerhead construction or another configuration suited for engagement with the drive member in order to prevent the drive member from rotating relative to the piston rod. In case the piston rod is a toothed rod, the structural element may advantageously be provided at the proximal end of the piston rod, e.g. to rotationally lock the drive member after passage and activation of the most proximally positioned tooth on the piston rod. 
         [0086]    According to a fifth aspect of the invention an injection device is provided which comprises a locking mechanism preventing injection of a set dose. Such a locking mechanism is preferably used for preventing that a set dose is accidentally expelled before it is intended to inject the dose, e.g. before a needle or a jet nozzle has been appropriately positioned at a suitable and desired injection site. This is particularly useful in the case that mounting of the cap causes the dose setting means to set a dose, since in this case some time will elapse between setting the dose and injecting it, and it may very well be necessary to carry the injection device along during this time interval, e.g. in a pocket or a handbag. 
         [0087]    The locking mechanism may be automatically activated when the cap is mounted on the injection device. According to this embodiment, mounting the cap may advantageously result in setting the dose as well as activating the locking mechanism. Alternatively, the locking mechanism may be manually and/or separately operable, or it may be automatically activated by other suitable actions than mounting the cap. 
         [0088]    Thus, according to an exemplary embodiment an injection device for administering apportioned doses of a liquid drug is provided comprising a variable volume reservoir adapted to hold the liquid drug, dose setting means operable to set a dose, injection means operable to inject the set dose, a removable cap, a cap receiving part adapted to abut or engage with the cap when the cap is mounted on the injection device, wherein the injection means is operatively coupled to the cap receiving part in such a manner that mounting the cap on the injection device disables the injection means, thereby preventing an ejection of drug from the reservoir. This type of arrangement is advantageous since the user is prevented from risking inadvertently activating the injection means when e.g. carrying the injection device in a pocket or a handbag. 
         [0089]    According to one embodiment, the locking mechanism must be separately switched to an unlocking state prior to injection of a set dose. This may be performed manually and/or separately. Alternatively, the locking mechanism may be automatically switched to the unlocking state when the cap is dismounted. In the case that mounting the cap activates the locking mechanism and dismounting the cap switches the locking mechanism to the unlocking state, the cap may be regarded as forming part of the locking mechanism. This embodiment is very safe, since the locking mechanism is automatically activated and deactivated, and therefore the user does not have to consider this or remember to activate/deactivate the locking mechanism. 
         [0090]    The reservoir may be a rigid cartridge comprising an axially moveable piston or it may be a flexible reservoir capable of undergoing controlled volume reduction. The injection means may comprise an axially moveable piston rod being adapted to act on the reservoir, either directly or via a coupling device, to reduce the volume of the reservoir, causing drug to be expelled therefrom. Mounting the cap on the cap receiving part may affect the injection means directly or indirectly by affecting an associated element. In any case, the cap, preferably the edge of the cap, affects the injection means mechanically in such a way that the injection means is incapable of being activated even if the user applies a very large force to the injection device. 
         [0091]    The injection device may comprise a drive member adapted to slave the piston rod during forward axial movement in the injection device, i.e. during movement towards the distal end of the device. In that case when mounted on the cap receiving part the cap may physically block the drive member from axial forward movement, e.g. through abutting surfaces. The cap may, however, at the same time allow rotational movement of the drive member. 
         [0092]    In a particular embodiment the injection device further comprises guiding means adapted to guide the movement of the drive member and/or the piston rod, and an injection button operatively coupled to the dose setting means and the injection means and axially moveable between a first position corresponding to a position in which the dose is set and a second position corresponding to a position in which the injection means has been activated to inject the set dose, wherein the injection button is able to perform substantially unimpeded movement from the first position to the second position and back to the first position when the cap is mounted on the injection device. In other words, the injection button can be manipulated, e.g. depressed, while the cap is mounted on the device. Such an arrangement allows for an injection device which is secured against premature activation of the injection mechanism, without incorporating a mechanical lock capable of withstanding large forces being applied to the injection button, e.g. as a result of the user playing with, mishandling or dropping the device. 
         [0093]    If the guiding means comprises a first longitudinal guiding surface and a second sloping guiding surface, as described in connection with the second aspect of the invention, this may be implemented by arranging the drive member such that a part of the drive member abuts with the cap edge when the cap is mounted on the injection device. The cap thereby blocks the drive member from moving axially and the piston rod is thus also prevented from moving axially, in which case no dose can be ejected from the reservoir. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0094]    The invention will now be described in further detail with reference to the accompanying drawings in which 
           [0095]      FIG. 1  is a cross sectional view of an injection device according to a first embodiment of the invention in an unloaded state, 
           [0096]      FIG. 2  is a cross sectional view of the injection device of  FIG. 1  in a loaded state, 
           [0097]      FIG. 3  is a perspective view of an injection device according to a second embodiment of the invention, 
           [0098]      FIG. 4  is a perspective view of the injection device of  FIG. 3  with some parts removed, 
           [0099]      FIG. 5  is a detail of the injection device of  FIGS. 3 and 4 , 
           [0100]      FIG. 6  is a cross sectional view of an injection device according to a third embodiment of the invention. 
           [0101]      FIG. 7  is a cross sectional view of an injection device according to a fourth embodiment of the invention, 
           [0102]      FIG. 8  is a cross sectional perspective view of the housing of the injection device, showing a guiding means in detail, 
           [0103]      FIG. 9  is a perspective view of the housing of the injection device, showing the position of the guiding means in the housing, 
           [0104]      FIG. 10   a  and  FIG. 10   b  show the front side, respectively the back side of a piston rod of the injection device, 
           [0105]      FIG. 11  is a perspective view of a drive member of the injection device, 
           [0106]      FIG. 12  is a two-dimensional representation of the guiding means and the drive member, showing the drive member in two different positions with respect to the guiding means, 
           [0107]      FIG. 13  is a two-dimensional representation of guiding means further comprising a support shelf to enable initial priming, 
           [0108]      FIG. 14  is a perspective view of a push button of the injection device, 
           [0109]      FIG. 15  is a perspective view of a coupling element of the injection device, 
           [0110]      FIG. 16  is a perspective view of a spring retaining means of the injection device, 
           [0111]      FIG. 17  is a perspective view showing an assembly of the drive member, a spring, and spring retaining means, 
           [0112]      FIG. 18  is a perspective view illustrating the functional connection between the push button and the drive member, 
           [0113]      FIG. 19  is a perspective view of the injection device with the housing removed, showing an interaction between the drive member and the protective cap, 
           [0114]      FIG. 20  is a perspective view of an end-of-contents mechanism in the injection device, 
           [0115]      FIG. 21  is a cross sectional view of an injection device according to a fifth embodiment of the invention, 
           [0116]      FIG. 22  is a perspective view of a guiding means seen from the side, 
           [0117]      FIG. 23  is a perspective view of a guiding means seen from the distal end, 
           [0118]      FIG. 24  is a perspective view of a drive member, 
           [0119]      FIG. 25  is a perspective view of a push button, 
           [0120]      FIG. 26  is a perspective view illustrating the functional connection between the push button and the drive member, 
           [0121]      FIG. 27  is a perspective view showing the engagement between the drive member and a piston rod, 
           [0122]      FIG. 28  is a perspective view showing an assembly of the piston rod, the drive member, the guiding means, and a spring, in a situation where the drive member rests on the dose shelves of the guiding means corresponding to a loaded condition of the injection device, 
           [0123]      FIG. 29  is a perspective view of an end-of-contents mechanism in the injection device, 
           [0124]      FIG. 30  is a perspective view of the injection device with the housing removed, showing an interaction between the drive member and the protective cap, 
           [0125]      FIG. 31  is a cross sectional view of an injection device according to a sixth embodiment of the invention, and 
           [0126]      FIGS. 32   a - c  show a push button release mechanism of the injection device in detail. 
       
    
    
       [0127]    In the figures like structures are mainly identified by like reference numerals. 
       DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0128]      FIG. 1  is a cross sectional view of an injection device  1  according to a first embodiment of the invention. In  FIG. 1  the injection device  1  is shown in an unloaded state, i.e. a dose has not yet been set. 
         [0129]    The injection device  1  comprises a housing  2 , a cartridge holding part  3  having a cartridge  4  arranged therein, and an injection button  5 . At a distal end of the cartridge holding part  3  an injection needle  6  is attached. A piston rod  7  is arranged in abutment with a piston  8  arranged in an interior part of the cartridge  3  in such a manner that moving the piston rod  7  in a distal direction will cause the piston  8  to move in a distal direction, thereby causing liquid drug from the cartridge  4  to be expelled via the injection needle  6 . 
         [0130]    When a user has completed an injection a cap (not shown in  FIG. 1 ) is mounted on the injection device  1  at cap receiving part  9  in such a manner that the injection needle  6  is covered. When the cap is mounted at the cap receiving part  9  it pushes against slider  10 , thereby moving it in a proximal direction. This causes spring  11  to be compressed, thereby storing energy in the spring  11 , and moves snap arms  12  in a proximal direction to a position beyond protrusions  13  arranged on the housing  2 . The protrusions  13  ensure that the snap arms  12  are retained in this position. 
         [0131]    The slider  10  is connected to the piston rod  7  via teeth (not shown) formed on the piston rod  7  and a teeth engaging part  14  formed on the slider  10 . The teeth and the teeth engaging part  14  are arranged in such a manner that the teeth engaging part  14  is allowed to pass over the teeth when the slider  10  is moved in a proximal direction relative to the piston rod  7 , but the piston rod  7  must move along with the slider  10  when the slider  10  is moved in a reverse direction. Thus, moving the slider  10  in a proximal direction as described above causes the slider  10  to move relative to the piston rod  7 , the moved distance corresponding to a predetermined dose, since the piston rod  7 , and thereby the piston  8 , will be moved along the same distance when the slider  10  is subsequently moved in a reverse direction. 
         [0132]    Furthermore, the movement of the slider  10  in a proximal direction as described above causes the injection button  5  to be moved in a proximal direction, i.e. causing the injection button  5  to protrude from the housing  2 , thereby indicating to a user that the injection device  1  has been loaded, i.e. a dose has been set. 
         [0133]      FIG. 2  is a cross sectional view of the injection device  1  of  FIG. 1  in a loaded state. In  FIG. 2  a cap  15  has been mounted on the injection device  1  at the cap receiving part  9 . It is clear that the injection button  5  has been moved in a proximal direction as compared to the position shown in  FIG. 1 . It is also clear that the snap arms  12  have been moved in a proximal direction beyond the protrusions  13 , and that the protrusions  13  retain the snap arms  12  in this position. 
         [0134]    When it is desired to inject the set dose, the user removes the cap  15 , thereby uncovering the injection needle  6 , and inserts the injection needle  6  at a suitable injection site. The injection button  5  is then pushed in a distal direction, i.e. towards the housing  2  and the position shown in  FIG. 1 . This causes pushing surfaces  16  to push snap arms  12  towards the centre of the injection device  1 , thereby releasing them from the protrusions  13 . Accordingly, the slider  10  is allowed to move in a distal direction, and the energy stored in the spring  11  during setting of the dose will cause this movement to take place. Due to the engagement between the teeth of the piston rod  7  and the teeth engaging part  14  of the slider  10 , the piston rod  7  is moved along. Thereby the piston  8  is also moved in a distal direction, thereby causing the predetermined dose to be expelled from the cartridge  4  via the injection needle  6 . 
         [0135]    When the injection has been completed, the cap  15  is once again mounted on the injection device  1  at the cap receiving part  9 , thereby causing a new dose to be set as described above. It should be noted that since the slider  10  is moved the same distance each time the cap  15  is mounted on the injection device  1 , the set dose is a predetermined, fixed dose. 
         [0136]      FIG. 3  is a perspective view of an injection device  100  according to a second embodiment of the invention. A housing  102  and an injection button  105  are visible, and a cap  115  is mounted on the injection device  100 . Since the injection button  105  is positioned relatively close to the housing  102 , it can be seen that the injection device  100  is not loaded, i.e. a dose has not been set. 
         [0137]      FIG. 4  is a perspective view of the injection device  100  of  FIG. 3 . For the sake of clarity, some of the parts, notably the cap and the housing, have been removed. This allows the cartridge holding part  103  and the injection button  105  to be seen. The injection device  100  of  FIGS. 3 and 4  is preferably operated in the following manner. When it is desired to inject a dose, the cap  115  is removed from the injection device  100  by rotating the cap  115  relative to the housing  102 , thereby uncovering an injection needle (not shown). The cap  115  engages the cartridge holding part  103  via teeth  117  arranged at the cap receiving part  109  in such a manner that when the cap  115  is rotated, the cartridge holding part  103  is rotated along. Rotating the cartridge holding part  103  in this manner causes track portion  118 , which is actually a part of the cartridge holding part  103 , to rotate. An inclined portion of the track of the track portion  118  engages a protrusion (not shown) formed on an inner part of the housing, and thereby the rotation of the track portion  118  causes the track portion  118  to be moved axially in a proximal direction relative to the housing. 
         [0138]    Furthermore, rotating the cartridge holding part  103  causes the piston rod  107  to rotate. The injection button  105  is connected to a thread formed on the piston rod  107 , and therefore rotation of the piston rod  107  results in a prolongation of the piston rod  107 /injection button  105  assembly. As the piston rod  107  is not allowed to move in a distal direction, this prolongation causes the injection button  105  to move in a proximal direction, i.e. out of the housing. Thereby the injection device  100  is loaded. 
         [0139]    Finally, the axial movement of the track portion  118  causes the cap  115  to be pushed away from the injection device  100 . 
         [0140]    After the cap  115  has been removed and the injection device  100  has been loaded as described above, the injection needle is inserted at a suitable injection site. The injection button  105  is then pushed in a distal direction, i.e. towards the housing  102 . Due to axial locking between the injection button  105  and the piston rod  107  this movement results in axial movement of the piston rod  107 , and thereby drug injection. 
         [0141]    When the injection has been completed, the cap  115  is once again mounted on the injection device  100 . This is done by pushing the cap  115  onto the injection device  100  in a substantially axial movement. Simultaneously, the injection button  105  must be pushed in a distal direction in order to properly fit the cap  115  onto the injection device  100 . 
         [0142]      FIG. 5  is a detail of  FIG. 4 , in which the cap receiving part  109 , the teeth  117  and the track portion  118  are seen more clearly. 
         [0143]      FIG. 6  is a cross sectional view of an injection device  200  according to a third embodiment of the invention. The injection device  200  is in a loaded state. The injection device  200  operates in a manner similar to the injection device  1  shown in  FIGS. 1 and 2 . However, in this case energy is not stored in a spring, and the user has to manually press the injection button  205  home in order to expel a set dose. 
         [0144]    When an injection has been completed, the cap  215  is mounted on the injection device  200  at the cap receiving part  209 . The cap  215  pushes against slider  210 , thereby moving it in a proximal direction, the slider  210  thereby pushing the injection button  205  in a proximal direction, i.e. away from the housing  202  to the position shown in  FIG. 6 . 
         [0145]    The slider  210  and the piston rod  207  are engaged via teeth  219  formed on the piston rod  207  and teeth engaging parts  214  formed on the slider  210 . The teeth  219  and the teeth engaging parts  214  are arranged in such a manner that the teeth engaging parts  214  are allowed to pass over the teeth  219  when the slider  210  is moved in a proximal direction relative to the piston rod  207 , and the piston rod  207  must be moved along when the slider  210  is moved in a reverse direction. Accordingly, moving the slider  210  in a proximal direction as described above, results in the slider  210  moving relative to the piston rod  207 . The distance moved corresponds to a predetermined dose as described above. 
         [0146]    When it is desired to inject the set dose, the user removes the cap  215 , thereby uncovering the injection needle  206 , and the injection needle  206  is inserted at a suitable injection site. The injection button  205  is then pushed in a distal direction, i.e. towards the housing  202 . This causes the slider  210  to move in a distal direction, and due to the engagement between the teeth  219  and the teeth engaging parts  214 , the piston rod  207  is moved along. Thereby the piston  208  is also moved in a distal direction, and the set dose of drug is expelled via the injection needle  206 . 
         [0147]      FIG. 7  is a cross sectional view of an injection device  300  according to a fourth embodiment of the invention. The injection device  300  generally comprises a housing  302  and a cartridge holding part  303  for supporting a cartridge  304  which contains the liquid drug. The liquid drug is positioned between a piston  308 , which is capable of moving axially in the cartridge  304 , a tubular cartridge wall  340 , and a self-sealing septum  342  covering a drug outlet  341  through which the liquid drug is intended to flow when the piston is advanced in the cartridge  304  and when an injection needle (not shown) is attached to the drug outlet  341  via a needle hub interface  343 . A cap  315  is mounted on a cap receiving part  309  in the housing  302 , whereby it protects the cartridge  304  and covers the drug outlet  341 . An injection button  305  being capable of reciprocating axial motion with respect to the housing  302  is shown in a position where it protrudes from the distal end of the housing  302 . This indicates that the injection device  300  is in a loaded state, i.e. that a dose has been set and that the injection device  300  is ready to perform an injection (upon removal of the cap  315 ). This will be explained in greater detail below. 
         [0148]    A piston rod  307  is attached to the piston  308  via a piston rod foot  347  and operatively coupled to the injection button  305  such that when the cap  315  is off, an injection needle has been attached to the needle hub interface  343 , and the injection button  305  is pressed against the housing  302  the piston rod  307  will advance axially through the housing  302  a certain distance, thereby displacing the piston  308  in the cartridge  304  an equivalent distance to inject a desired amount of drug through the outlet  341 . 
         [0149]    The movement of the piston rod  307  is realised through a coupling ring  330  being in engagement with a helical track  351  in the injection button  305 , and a driver  310  which is in engagement with the coupling ring  330  and which is adapted to engage with, and transmit a driving force to, the piston rod  307 . The driver  310  is powered by a spring  311  which is a torsionally pre-tensioned compression spring capable of storing and releasing energy for both translational and rotational motion. One end of the spring  311  is retained in a spring base  360  and the other end of the spring  311  is in engagement with the driver  310  in such a way that the spring  311  and the driver  310  are able to interchange both forces and torques. The driver  310  is thus capable of performing both translational and rotational motion relative to the housing  302 . The spring  311  may for example be torsionally pre-tensioned during assemblage of the injection device  300 , e.g. by mutually twisting its two end parts a half or a full turn. When the cap  315  is dismounted from the injection device  300 , the movement of the driver  310  is guided by a guide member  320 . 
         [0150]    The housing  302  has two radially opposite apertures  361 , each adapted to receive a hook  362  provided on the spring base  360  to thereby retain the spring base  360  from translational as well as rotational movement relative to the housing  302 . The housing  302  further has a window  399  useable for inspection of the current position of the driver  310  in the injection device  300 . 
         [0151]      FIG. 8  is a cross sectional perspective view of the housing  302 , which shows the guide member  320  in more detail. For the sake of clarity the proximal end  344  of the cartridge holding part  303  has been removed from the figure. The guide member  320  comprises a dose shelf  323  adapted to support and guide the driver  310  during the second part of the dose setting and the first part of the injection. A longitudinal guide surface  324  leads from the dose shelf  323  to an end of dose stop  325 . The dose shelf  323  is a helical ramp segment which extends circumferentially from a connection with the longitudinal guide surface  324  to a longitudinal stop surface  322 . A click finger  326  is provided on the guide member, the click finger  326  having a tip  327  for engagement with the piston rod  307 . 
         [0152]      FIG. 9  is a perspective view of the housing  302  as seen from the distal end. It shows the position of the guide member  320  within the housing  302 . Again, for the sake of clarity the proximal end  344  of the cartridge holding part  303  has been removed from the figure. The guide member  320  is a generally tubular structure positioned concentrically with the housing  302  and connected to the housing  302  via a number of spacers  386 . This connection to the housing  302  provides a tubular clearance  389  between the outer wall  329  of the guide member  320  and the inside of the housing  302 . Some of this tubular clearance is, however, taken up by the tubular proximal end  344  of the cartridge holding part  303 . The cap receiving part  309  comprises the remaining part of the tubular clearance  389  and a distal housing edge  385 . A central bore  380  is provided allowing passage of the piston rod  307  through the guide member  320 . The central bore  380  is adapted to guide axial movements of the piston rod  307 . 
         [0153]      FIG. 10   a  shows a first side of the piston rod  307 . A number of teeth  319  are distributed along the piston rod  307 , the distance between two consecutive teeth  319  being constant throughout the entire distribution. The teeth  319  are adapted for engagement with the driver  310  during dose injection where the driver  310  engages a tooth  319  and slaves the piston rod  307  in a forward motion. At its proximal end the piston rod  307  is provided with a stop face  393  adapted to restrict the movement of the driver  310  in an end-of-content situation. 
         [0154]      FIG. 10   b  shows a second side of the piston rod  307 . On this side a number of smaller teeth  396  are distributed, the distance between two consecutive teeth  396  equalling the distance between two consecutive teeth  319  on the first side of the piston rod  307 . Between two consecutive teeth  396  a number of even smaller teeth  395  are distributed, the distance between two consecutive teeth  395  being constant throughout the distribution. The teeth  395  and  396  are intended to be overridden by tip  327  of click finger  326  during advancement of the piston rod  307  through the central bore  380 . At its proximal end the piston rod  307  is provided with a longitudinal track  394  adapted to restrict the movement of the driver  310  in an end-of-content situation, preferably co-acting with stop face  393 . 
         [0155]      FIG. 11  is a perspective view of the drive member  310 , generally comprising a tubular body  370  having two radially opposite longitudinal grooves  371  extending from its proximal end, and a pair of shoulders  377  connecting the tubular body  370  with a distal part which comprises various engagement elements. From the shoulders  377  project two legs  372  which are adapted to move in the tubular clearance  389 . Each leg  372  has a foot section, the bottom of which constitutes a contact sole  374 . The distal part of the driver  310  further comprises two slider elements  373  adapted to travel the guide surfaces of the guide member  320 . One of the slider elements  373  is provided with a catch element  375 . A tooth engaging element  376  is placed circumferentially between the two slider elements  373  and are rigidly connected to them such that the tooth engaging element  376  undergoes the same translational and/or rotational movement as the slider elements  373 , and vice versa. 
         [0156]    During dose setting the tooth engaging element  376  is able to perform axial relative motion with respect to the piston rod  307 , whereas during injection the tooth engaging element  376  is adapted to move into contact with a tooth  319  on the piston rod  307  and move the piston rod  307  axially a distance through the housing  302 . The shoulders  377  act as a bearing face for the spring  311  and are thereby the physical interface for the exchange of axial forces between the spring  311  and the driver  310 . Adjacent one of the shoulders  377  is an abutting surface  378  adapted to abut with the distal end of the spring  311  for the exchange of torques between the spring  311  and the driver  310 . 
         [0157]      FIG. 12  is a two-dimensional representation of the guide member  320  and the driver  310 , showing one of the slider elements  373  in two different positions on the guide member  320 . It is understood that the guide member  320  comprises two sets of guiding surfaces which the two slider elements  373  travel simultaneously. However, as this movement of the slider elements  373  along the respective guiding surfaces is identical only one of them is presented. The dose shelf  323  and the longitudinal guide surface  324  are mutually connected at an angle  8 . The connection point between the dose shelf  323  and the longitudinal guide surface  324  can be termed a ramp edge  328  and it constitutes a transition point between sliding motion of the slider element  373  along the longitudinal guide surface  324  and sliding motion of the slider element  373  along the sloping dose shelf  323 . The spiralling motion of the slider element  373  along the dose shelf  323  is limited by the stop surface  322  and the axial motion of the slider element  373  along the longitudinal guide surface  324  is limited by the end of dose stop  325 . The axial length of the longitudinal guide surface  324  is H, i.e. when the slider element  373  is positioned exactly at the ramp edge  328  it is lifted a distance H from the end of dose stop  325 . Due to the slope of the dose shelf  323  in addition to a rotational movement the slider element  373  also performs an axial movement, D, when it travels the dose shelf  323  from the ramp edge  328  to the stop surface  322 . When the slider element  373  is positioned at the stop surface  322  it is thus lifted an axial distance equalling H+D from the end of dose stop  325 . The axial dimension H+D is notably larger than the distance between two consecutive teeth  319  on the piston rod  307 , which is again larger than, or equal to, the axial dimension H. 
         [0158]    Due to the torsionally pre-tensioned spring  311 , the slider element  373  is biased against the stop surface  322  when it is positioned on the dose shelf  323  and against the longitudinal guide surface  324  when it is positioned at the end of dose stop  325  (it is in fact biased against the longitudinal guide surface  324  at any position below the ramp edge  328 ). The spring  311  is also axially pre-tensioned biasing the slider element  373  towards the end of dose stop  325 . The characteristics of the spring  311  and the slope of the dose shelf  323  are, however, dimensioned such that when the slider element  373  is positioned above the ramp edge  328  the torque provided by the spring  311  is able to overcome the axial force of the spring  311  and the slider element  373  will be forced towards the stop surface  322 . 
         [0159]      FIG. 13  is a two-dimensional representation of the guide member  320  and the driver  310  in a variant where the guide member  320  further comprises a support shelf  321  for supporting the slider element  373  prior to the first use of the injection device  300 . Due to the biasing torque of the spring  311  the slider element  373  is secured on the support shelf  321  until the device is taken into use. The slider element  373  is capable of sliding along the support shelf  321  and the stop surface  322  to take up a position on the dose shelf  323  in a manner similar to its movement from the dose shelf  323  to the end of dose stop  325 . The slider element  373  is, however, not able to move from the dose shelf  323  back to the support shelf  321 , i.e. once the slider element  373  has been transferred away from the support shelf  321 , it can only move between the dose shelf  323  and the end of dose stop  325 . The axial length of the stop surface  322  is P, i.e. the slider element  373  travels the axial distance P when moved from the support shelf  321  to the dose shelf  323 . Since P is, relatively, much smaller than H, and there may further be a small axial clearance between the piston rod  307  and the piston  308  when the injection device  300  is supplied by the manufacturer, when the injection device  300  is used for the first time the piston rod  307  will perform a much smaller axial movement than during subsequent uses, thereby enabling an initial priming that does not waste an approximately full dose of the liquid drug. 
         [0160]      FIG. 14  is a perspective view of the injection button  305  comprising a push face  352  for interfacing with an operator of the injection device  300 . The injection button  305  further comprises two flanges  353 , each provided with a helical track  351  and two flange sides  354 . 
         [0161]      FIG. 15  is a perspective view of the coupling ring  330  adapted to couple the injection button  305  and the driver  310 . The coupling ring  330  has a proximal face  331  and a distal face  332  and two radially opposite tongues  333  adapted to engage with the grooves  371  in the tubular body  370  of the driver  310  to rotationally lock the coupling ring  330  to the driver  310 . The tongues  333  are further adapted to engage with the spring base  360  to translationally lock the coupling ring  330  to the spring base  360 . The coupling ring  330  and the driver  310  are able to perform relative translatory motion limited by the length of the grooves  371 . Two protrusions  334  are provided to engage with, and travel, the helical tracks  351  in the flanges  353  to thereby transform rotational motion of the coupling ring  330  to translational motion of the injection button  305 , and vice versa. 
         [0162]      FIG. 16  is a perspective view of the spring base  360  which is adapted to hold one end of the spring  311  in a permanent position with respect to the housing  302 . The spring base  360  has two radially opposite arms  364  each comprising a hook  362  for engagement with the respective apertures  361  in the housing  302 , and two contact faces  365  adapted to abut with the flange sides  354 , thereby preventing the injection button  305  from rotating relative to the spring base  360 . Due to the engagement between the hooks  362  and the apertures  361  the spring base  360  is completely locked to the housing  302 , i.e. the spring base  360  is prevented from performing rotational as well as translatory motion relative to the housing  302 . A spring retaining groove  366  is provided for retaining the proximal end of the spring  311 . The spring base  360  further comprises a proximal face  363  adapted to abut with the distal face  332  of the coupling ring  330 , and two circumferential grooves  367  adapted to slidably engage with the tongues  333  and to retain the tongues  333  with respect to axial movement. The coupling ring  330  is thereby translationally locked to the spring base  360 , but capable of rotating relative thereto, the rotational motion being limited by the circumferential dimension of the grooves  367 . 
         [0163]      FIG. 17  is a perspective view showing an assembly of the driver  310 , the spring  311 , and the spring base  360 . The proximal spring end  397  is retained in the spring base  360  and the distal spring end  398  is in connection with the driver  310 . As the spring base  360  is locked to the housing  302  and thereby unable to move the torsionally pre-tensioned spring  311  will bias the driver  310  anti-clockwise, as seen from the spring base  360 . 
         [0164]      FIG. 18  is a perspective view illustrating the functional connection between the injection button  305  and the driver  310 . A push on the push face  352  will force the injection button  305  downwards towards the spring base  360 . As the injection button  305  is locked against rotation relative to the spring base  360  this downwards movement is purely translational. During the translational movement of the injection button  305  the protrusions  334  travel the helical tracks  351 . This engagement converts the movement of the injection button  305  to a rotational movement of the coupling ring  330 , and since the coupling ring  330  is rotationally locked to the driver  310 , also the driver  310  will rotate. The helical tracks  351  are arranged such that when the injection button  305  is pushed towards the spring base  360  the coupling ring  330 , and thereby the driver  310 , will rotate clockwise, as seen from the spring base  360 , i.e. against the rotational bias of the spring  311 . 
         [0165]      FIG. 19  is a perspective view of the injection device  300  with the housing  302  removed, showing an interaction between the driver  310  and the cap  315  when the cap  315  is mounted on the injection device  300  to cover and protect the distal part of the injection device  300 . For the sake of clarity the proximal end  344  of the cartridge holding part  303  has been removed from the figure. When the cap  315  is completely received in the cap receiving part  309  an annular abutting surface  381  on the cap  315  abuts the distal housing edge  385 , and a cap edge  382  abuts the contact soles  374  of the driver  310 . This corresponds to a situation where the injection device  300  is loaded, i.e. a dose has been set. As long as the injection device  300  still contains enough liquid drug to provide a full dose the injection button  305  will in such a situation protrude from a proximal housing opening  384 . In  FIG. 19 , however, the injection button  305  is depressed against the spring base  360 . This illustrates a situation where a user has tried to activate the injection mechanism to eject the set dose from the cartridge  304  when the cap  315  is mounted on the injection device  300 . In such a case depressing the injection button  305  will still cause a rotation of the driver  310  against the biasing torque of the spring  311 , but since the cap edge  382  blocks against translational movement of the driver  310  via the interface with the contact soles  374  the contact soles  374  will just slide along the cap edge  382  and no ejection will take place. When the user removes the push force from the injection button  305  the biasing torque of the spring  311  will force the driver  310  to rotate in the opposite direction until the slider elements  373  meet the respective stop surfaces  322  (not visible). During this movement the contact soles  374  will slide along the cap edge  382  back to their original positions on the cap edge  382 , and the injection button  305  will be forced to project out of the proximal housing opening  384  due to the threaded engagement with the coupling ring  330 . Apertures  383  in the cap  315  are adapted to receive respective beads (not shown) on the cartridge holding part  303  to ensure that the cap  315  is able to withstand a certain push force from the contact soles  374  due to the translational bias of the spring  311  when the driver  310  is rotated to a position corresponding to the slider elements&#39;  373  passage of the ramp edges  328  (not visible). 
         [0166]      FIG. 20  is a perspective view of an end-of-content mechanism in the injection device  300 . In  FIG. 20  the tooth engaging element  376  has passed the most proximal positioned tooth  390  on the piston rod  307  and slaved the piston rod  307  to cause an injection of the last remaining full dose in the cartridge  304 , and the driver  310  has responded to a remounting of the cap  315  by moving proximally with respect to the piston rod  307 . As long as more doses remain in the cartridge  304  repositioning the cap  315  in the cap receiving part  309  will lead to both a translatory and a rotational movement of the driver  310 , as will be explained in greater detail below. However, when the last full dose has been injected repositioning the cap  315  in the cap receiving part  309  will only lead to a translatory movement of the driver  310  due to the construction of the proximal end of the piston rod  307  and the distal part of the respective slider elements  373 . The stop face  393  at the proximal end of the piston rod  307  is adapted to abut with the catch element  375  to prevent anti-clockwise rotation of the driver  310  with respect to the piston rod  307 . Further, the longitudinal track  394  (not visible) is adapted to receive a protrusion  379  to thereby prevent the driver  310  from rotating with respect to the piston rod  307 . The protrusion  379  is received in the longitudinal track  394  at the same time as the catch element  375  engages with the stop face  393 , and the stop face  393  and the longitudinal track  394  thus reinforce each others individual restriction of the freedom of movement of the driver  310 . 
       Operation of the Injection Device Represented by FIGS. 7-20 
       [0167]    In the following a situation of use of the injection device according to the fourth embodiment of the invention, as depicted in  FIGS. 7-20 , will be described. The injection device  300  shown in  FIG. 7  is in a non-use state having the cap  315  mounted thereon. When the user needs to perform an injection he/she removes the cap  315  from the injection device  300  and attaches an injection needle to the needle hub interface  343 . The injection device  300  is already loaded and ready to inject the set dose so the user simply selects the injection site, positions the injection needle in the skin and pushes the injection button  305  which protrudes from the housing opening  384  at the proximal end of the housing  302 . Pushing the injection button  305  in the distal direction towards the housing  302  causes a substantially pure translatory displacement of the injection button  305  relative to the housing  302  due to the contact faces  365  preventing rotation of the injection button  305  via the engagement with the flange sides  354 . This translatory displacement of the injection button  305  causes the protrusions  334  to travel the helical tracks  351 . Since the coupling ring  330  is axially locked with respect to the housing  302 , due to the engagement between the tongues  333  and the circumferential grooves  367  in the spring base  360  being completely locked with respect to the housing  302 , when the protrusions  334  travel the helical tracks  351  the translatory movement of the injection button  305  is transformed into a rotational movement of the coupling ring  330  relative to the housing  302 . The engagement between the tongues  333  and the longitudinal grooves  371  in the tubular body  370  forces the driver  310  to rotate along with the coupling ring  330 . 
         [0168]    The rotation of the driver  310  caused by the depression of the injection button  305  is performed against the biasing torque of the spring  311 . In the situation where the user has not yet depressed the injection button  305  the slider elements  373  rests on the respective dose shelves  323  being biased against the stop surfaces  322  by the spring torque. Depressing the injection button  305 , and thereby causing a rotation of the driver  310 , will lead to the slider elements  373  being slid down the dose shelves  323  towards the ramp edges  328 . When the injection button  305  is substantially fully depressed in the housing  302  the slider elements  373  reach the transition point at the ramp edges  328  at which point the spring  311  will release its stored translational energy and force the slider elements  373 , via the abutment with the shoulders  377  on the driver  310 , down along the longitudinal guide surfaces  324  towards the end of dose stop  325 . During the movement of the slider elements  373  along the longitudinal guide surfaces  324  the tooth engaging element  376 , being in engagement with a tooth  319  of the piston rod  307 , will move along and thereby force the piston rod  307  to perform a corresponding axial forward movement through the central bore  380 . Since the piston rod  307  is connected to the piston  308  such a forward movement of the piston rod  307  will cause a corresponding advancement of the piston  308  in the cartridge  304 , leading to the ejection of the set dose from the cartridge  304 . Hence, following a substantially full depression of the injection button  305 , the spring  311  will automatically eject the liquid drug from the injection device  300  whether or not the user keeps a pressure on the push face  352 . While the piston rod  307  moves axially to expel the set dose the tip  327  of the click finger  326  rides over the teeth  395  of the piston rod  307  being distributed between two consecutive larger teeth  396 , thereby providing an audible feedback mechanism indicating to the user through audible clicks that the dosage is progressing. Just as the piston rod  307  stops its forward movement when the slider elements  373  are at the end of dose stop  325  the tip  327  rides over a larger tooth  396  providing a distinguishable audible indication, such as a larger click sound, of the fact that the movement of the piston  308  has stopped and that the dosage in principle is completed. When the slider elements  373  are at the end of dose stop  325  the driver  310  will be positioned such in the housing  302  that it completely covers the window  399 , thereby also providing a visual end of dose indication. The user may then wait a few seconds before taking the injection needle out of the skin. However, when this happens, all the user has to do to prepare the injection device  300  for the next injection is to reposition the cap  315  in the cap receiving part  309 , i.e. to put the cap  315  back on the injection device  300 . 
         [0169]    Repositioning the cap  315  in the cap receiving part  309  after an injection will cause a next dose to be set, as explained in the following. Due to the threaded engagement between the injection button  305  and the coupling ring  330  the injection button  305  will stay depressed in the housing  302  when the user releases the pressure from the push face  352  following the activation of the injection mechanism. This indicates to the user that the injection mechanism has been activated and that a dose is either being expelled or has been expelled from the cartridge  304 . When the cap  315  is repositioned in the cap receiving part  309  after an injection the cap edge  382  will abut the contact soles  374  on the legs  372  of the driver  310 . As the cap edge  382  during this repositioning of the cap  315  is moved gradually further towards the proximal end of the injection device  300  (when the annular abutting surface  381  moves towards abutment with the distal housing edge  385 ) it will push the driver  310  in the proximal direction accordingly. The driver  310  is thereby displaced axially against the translational biasing force of the spring  311  as the slider elements  373  move up along the longitudinal guide surfaces  324  towards the ramp edges  328 . When the driver  310  is moved so far proximally by the cap edge  382  that the slider elements  373  reach the ramp edges  328  the torsional tension of the spring  311 , having actually been increased during the injection procedure, will move the slider elements  373  up along the dose shelves  323  to the stop surfaces  322 , thereby rotating the driver  310  relative to both the piston rod  307  and the housing  302 . Since the dose shelves  323  are sloped this movement of the slider elements  373  will also cause the driver  310  to perform a small additional axial movement. The movement of the driver  310  causes the tooth engaging element  376  to perform an identical combined translatory and rotational movement, whereby it is lifted out of engagement with one tooth  391  on the piston rod  307  to pass the next more proximally positioned tooth  392 . Since the axial displacement of the driver  310  during movement of the slider elements  373  from the end of dose stop  325  to the stop surfaces  322 , H+D, is larger than the distance between two consecutive teeth  319  on the piston rod  307  the tooth engaging element  376  actually passes the next tooth  392  and leaves a small space between them, as shown in  FIG. 18 . As long as the cap  315  is positioned in the cap receiving part  309  in such a way that the cap edge  382  has not yet moved the driver  310  proximally to a point where the slider elements  373  have passed the ramp edges  328 , the tooth engaging element  376  has not passed the next tooth  392 , and if the cap  315  in such a situation is removed from the injection device  300  the translational bias of the spring  311  will move the slider elements  373  back to the end of dose stop  325 , whereby the driver  310  will return to the position it takes following an injection and the tooth engaging element  376  will move back into engagement with the tooth  391 . In other words the injection mechanism is not activated. However, if the cap  315  is positioned in the cap receiving part  309  such that the cap edge  382  moves the driver  310  proximally to an extent where the slider elements  373  pass the transition point at the ramp edges  328  the spring  311  will release its stored rotational energy and move the slider elements  373  along the dose shelves  323  to a position at the stop surfaces  322 . This rotational movement is performed against the translational bias of the spring  311  which means that the slope of the dose shelves  323  must be within certain limits to enable the angular displacement of the driver  310 . In this case, the slope of the dose shelves is approximately 10°, i.e. the angle  8  is approximately 260°. 
         [0170]    When the driver  310  is rotated due to the slider elements  373  travelling the dose shelves  323  the tooth engaging element  376  is moved from a position just below the next tooth  392  (corresponding to the position of the slider elements  373  just below the ramp edges  328 ) to a position above the tooth  392  in a combined translatory and rotational motion. This combined translatory and rotational motion is caused by the spring  311  releasing its stored rotational energy when the slider elements  373  pass the ramp edges  328 , i.e. when this transition point is reached the user is no longer in control of the dose setting and the dose will be set no matter if the user dismounts the cap  315  from the injection device  300  or tries other maneuvers to stop it. Furthermore, the torsional pre-tensioning of the spring  311  assures a stabile position of the slider elements  373  on the dose shelves  323  at the stop surfaces  322 , whereby the injection device  300  is also secured from being fired until the user activates the injection mechanism to inject the set dose. 
         [0171]    Since the driver  310  and the coupling ring  330  are rotationally locked the rotation of the driver  310  in connection with the dose being set causes the coupling ring  330  to rotate along, whereby the protrusions  334  will travel the helical tracks  351  in the injection button  305  and cause the injection button  305  to translate out of the proximal housing opening  384 . As the driver  310  only rotates when the slider elements  373  have passed the ramp edges  328  and a dose is actually being set, the injection button  305  will only protrude from the housing  302  when a dose is set. This gives a clear signal to the user that either no dose is set or a dose is set and the injection device is ready for injection. In other words, when the cap  315  is mounted properly on the injection device  300  a dose is automatically set by the injection device  300  and the injection button  305  is automatically moved out of the housing  302  to indicate that the device is ready for use. 
         [0172]    When the cap  315  is mounted on the injection device  300  it is not possible to activate the injection mechanism to eject a dose out of the reservoir  304 . This will be explained in the following. As mentioned above, when positioning the cap  315  in the cap receiving part  309  the cap edge  382  will abut the contact soles  374  and move the driver  310  proximally in the housing  302 . This action will move the driver  310  away from the window  399 , and when the slider elements  373  are secured on the dose shelves  323  and a dose is set, the user can not see the driver  310  through the window  399 . As long as the cap  315  is mounted on the injection device  300  the cap edge  382  abuts the contact soles  374 . If the user tries to activate the injection mechanism by pressing the injection button  305  towards the housing  302 , he/she will be unsuccessful because the cap edge  382  prevents any advancement of the piston rod  307  through the housing  302 . The injection button  305  is free to move towards the housing  302 . As the injection button  305  moves towards a depressed state the coupling ring  330  will rotate and this will cause the driver  310  to rotate against the rotational bias of the spring  311 , as explained above in connection with an injection procedure. However, instead of moving down the sloping dose shelves  323  the slider elements  373  will just rotate while maintaining the same axial position. This is due to the contact soles  374  sliding along the cap edge  382  and not being able to perform an axial movement. When the injection button  305  is fully depressed in the housing  302  the driver  310  has been subjected to an angular displacement corresponding to a displacement of the slider elements  373  from the position on the dose shelves  323  at the stop surfaces  322  to a position past the ramp edges  328 , at a height H+D above the end of dose stop  325 . If the user releases the pressure from the injection button  305  the torsionally pre-tensioned spring  311  will immediately force the driver  310  to perform a reverse rotation. This is possible since the slider elements  373  are positioned above the ramp edges  328 , and when the driver  310  rotates so does the coupling ring  330 . The reverse rotation of the coupling ring  330  then causes the injection button  305  to travel out of the housing opening  384  and back to its most proximal position indicating that a dose is set and the injection device  300  is ready for injection. In other words, the user is able to press the injection button  305  against the housing  302  when the cap  315  is mounted on the injection device  300  without this leading to any drug being expelled from the reservoir  304 . And when the user releases the pressure from the injection button  305 , the injection device  300  will automatically push the injection button  305  back out of the housing  302  due to the spring  311  releasing stored energy for rotational reverse motion of the driver  310 . When the slider elements  373  are at the position above the end of dose stop  325 , i.e. on the other side of the ramp edges  328 , the translational bias of the spring  311  will try to force the driver  310  axially in the distal direction against the contact force from the cap edge  382 . The spring  311  is however not capable of moving the cap  315  out of the cap receiving part  309  due to the engagement between the cap  315  and the cap receiving part  309 . Furthermore, beads (not shown) on the cartridge holding part  303  engage with the apertures  383  in the cap  315  to reinforce the connection. An injection device is thereby provided which is incapable of ejecting drug out of the reservoir when capped, while at the same time allowing the injection button to move freely in and out of the housing. 
         [0173]    When the injection device  300  has been used for injection a number of times and the last full dose has just been ejected out of the cartridge  304  the tooth engaging element  376  is in engagement with the most proximal tooth  390  on the piston rod  307 . If the user puts the cap  315  back on the injection device  300  the cap edge  382  will, as before explained, move the driver  310  proximally in the housing  302  whereby the tooth engaging element  376  will be lifted out of engagement with the tooth  390 . However, as the slider elements  373  approach the transition point at the ramp edges  328 , the catch element  375  engages with the stop face  393  and the protrusion  379  slides into the longitudinal track  394 . The driver  310  is thereby prevented from rotating with respect to the piston rod  307 . Since the piston rod  307  is rotationally locked in the central bore  380  it is not able to rotate with respect to the housing  302 . The driver  310  is therefore in this particular situation not able to rotate relative to the housing  302 . As the driver  310  and the coupling ring  330  are rotationally locked the coupling ring  330  will also not rotate and the injection button  305  is thus not moved out of the housing opening  384 . This is a clear signal to the user that the last dose has been injected and the injection device  300  is empty. 
         [0174]    It is clear from the above description that the transition point at the ramp edges  328  constitutes a boundary between manual and automatic actions in the respect that during dose setting all that happens before the slider elements  373  reach the ramp edges  328  is in the hands of the user, whereas when the slider elements  373  pass the ramp edges  328  the injection device  300  will take over and automatically set the dose and secure the injection mechanism, while during injection all that happens as long as the slider elements  373  are positioned on the dose shelves  328  is in the hands of the user, whereas when the slider elements  373  pass the ramp edges  328  the injection device  300  will perform an automatic injection which can not be aborted. 
         [0175]      FIGS. 21-30  show an injection device  400  according to a fifth embodiment of the invention. The injection device  400  is operationally identical to the injection device  300  and it generally includes the same features as that device. However, there are certain structural differences between the two which will be clear from the below description. 
         [0176]      FIG. 21  is a cross sectional view of the injection device  400  which comprises a housing  402 , a cartridge  404  containing a liquid drug, a cartridge holding part  403 , a cap receiving part  409  and a cap  415 . The liquid drug is positioned between a piston  408 , which is capable of moving axially in the cartridge  404 , a tubular cartridge wall  440 , and a self-sealing septum  442  covering a drug outlet  441 . An injection needle  406  is attached to the injection device  400  via a needle hub interface  443 . An axially moveable piston rod  407  is coupled to the piston  408  via a piston rod foot  447 . The piston rod  407  is adapted to be moved axially by a driver  410 . A guide member  420  guides the movement of the driver  410  and the piston rod  407 . The injection device  400  is powered by a rotationally pre-stressed spring  411  which is locked to the housing  402  at its proximal end, via a spring base  460 , and which is coupled to the driver  410  at its distal end. The spring base  460  further holds the distal end of a button spring  450  adapted to bias an injection button  405  towards a position in which it protrudes from the proximal end of the housing  402 . 
         [0177]      FIGS. 22 and 23  show the guide member  420  in more detail. The guide member  420  comprises a dose shelf  423  adapted to support and guide the driver  410  during the second part of the dose setting procedure and the first part of the injection procedure. A longitudinal guide surface  424  leads from the dose shelf  423  to an end of dose stop  425 . The dose shelf  423  is a helical ramp segment which extends circumferentially from a connection with the longitudinal guide surface  424  to a longitudinal stop surface  422 . It is to be understood, that radially opposite this set of guide surfaces is a similar set of guide surfaces. This set is, however, not visible on the figures. A click finger  426  is provided on the guide member  420  for engagement with the piston rod  407 . A tubular clearance  489  is provided between an outer wall  429  of an inner tubular structure of the guide member  420  and the wall of the guide member  420 . Two radially opposite guide elements  436  are adapted to engage the through-going piston rod  407  (not shown) and guide the axial movements of the piston rod  407  while preventing the piston rod  407  from rotating relative to the housing  402 . A distal edge  485  of the guide member  420  is adapted to abut the cap  415  when the cap  415  is engaged in the cap receiving part  409 . 
         [0178]      FIG. 24  shows the driver  410  which comprises a tubular body  470 , a couple of pushing surfaces  469 , two slider elements  473  adapted to travel the guide surfaces of the guide member  420 , two contact soles  474 , and a tooth engaging element  476  adapted to engage with teeth on the piston rod  407  in order to slave the piston rod  407  in a forward motion towards the distal end of the injection device  400 . A catch element  475  is further provided for engagement with the proximal end of the piston rod  407  after the last full dose has been delivered from the cartridge  404 . 
         [0179]      FIG. 25  shows the injection button  405  comprising a push face  452  for interfacing with an operator of the injection device  400 . The injection button  405  further has two sets of flanges  453 ,  455  each provided with pushing surfaces  416  for sliding engagement with the pushing surfaces  469  on the driver  410 . The flanges  455  are each provided with a hook  456  adapted to engage with respective catch members (not shown) in the housing  402  for holding the injection button  405  depressed in the housing  402  against the bias of the button spring  450  when the injection mechanism has been activated to inject a set dose. The flanges  453  are each provided with a longitudinal slit  457  adapted to engage with respective protrusions (not shown) in the housing  402  thereby rotationally locking the injection button  405  with respect to the housing  402 . The protrusions (not shown) in the housing  402  are capable of axially travelling the slits  457 , whereby the injection button  405  is able to move axially relative to the housing  402  a distance determined by the axial dimension of the longitudinal slits  457 . 
         [0180]      FIG. 26  illustrates the functional connection between the injection button  405  and the driver  410 . The shown assembly has been separated from the rest of the injection device for the sake of clarity. When the user depresses the injection button  405  by pushing on the push face  452  the pushing surfaces  416  move into engagement with the pushing surfaces  469  on the driver  410 . The purely translational movement of the injection button  405  causes the pushing surfaces  469  to slide along the pushing surfaces  416  whereby the driver  410  is rotated clockwise with respect to the injection button  405  (and the housing  402 ). In an injection situation this will lead to the tooth engaging element  476  moving into engagement with a tooth on the piston rod  407 , and when the slider elements  473  pass the transition point at the ramp edges  428  the spring  411  will force the driver  410 , and thereby the tooth engaging element  476  and the piston rod  407 , to advance axially in the distal direction to inject the set dose. The interface between the injection button  405  and the driver  410  works both ways, i.e. if the driver  410  is rotated anti-clockwise, e.g. during a dose setting, the pushing surfaces  469  will slide along the pushing surfaces  416  on the flanges  453 ,  455 , whereby the injection button  405  will be released from its retained position and displaced axially out of the housing  402  by the button spring  450 . 
         [0181]      FIG. 27  depicts the situation where the tooth engaging element  476  has been moved from one tooth  491  to a more proximally positioned tooth  492  during a dose setting. The shown assembly has been separated from the rest of the injection device for the sake of clarity. This has artificially uncovered the proximal spring end  497  which is actually retained in the spring base  460 . 
         [0182]      FIG. 28  depicts the situation where the slider elements  473  are positioned on the dose shelves  423  at the stop surfaces  422  (not visible) and the injection device is ready for injection. Again the shown assembly has been separated from the rest of the injection device for the sake of clarity. A push on the injection button  405  (not shown) will cause the driver  410  to rotate clockwise (as seen from the injection button) against the rotational bias of the spring  411 . At the passage of the ramp edges  428  the slider elements  473  will be forced along the longitudinal guide surfaces  424  towards the end of dose stop  425 . 
         [0183]      FIG. 29  illustrates an end of content situation. When the last full dose has been delivered from the injection device  400  and the user repositions the cap  415  in the cap receiving part  409  to thereby move the driver  410  proximally in a manner similar to what has previously been described in relation to the injection device  300  the tooth engaging element  476  is moved out of engagement with the most proximal positioned tooth  490  and up along the piston rod  407 . This movement is performed synchronously with the movement of the slider elements  473  up along the longitudinal guide surfaces  424 . However, as the slider elements  473  approach the transition point at the ramp edges  428 , the catch element  475  engages with the stop face  493  and the driver  410  is thereby prevented from rotating with respect to the piston rod  407 . Since the piston rod  407  is rotationally locked to the housing  402  the driver  310  is in this particular situation not able to rotate relative to the housing  402 . In any previous case, when the user has repositioned the cap  415  in the cap receiving part  409  following an injection and the driver  410  has been moved proximally such that the slider elements  473  have passed the ramp edges  428  and a dose thereby has been set, the last part of the dose setting has been performed automatically by the spring  411  releasing its stored energy for rotational motion of the driver  410 . This rotation of the driver  410  has caused a simultaneous translation of the injection button  405  due to the interface between the pushing surfaces  469 ,  416  deflecting the hooks  456  out of engagement with the catch members (not shown) in the housing  402  thereby releasing the button spring  450  and consequently forcing the injection button  405  out of the housing  402 , signalling to the user that a dose has been set and that the device is ready for the next injection. 
         [0184]    As the driver  410  is incapable of rotation when the cap  415  is mounted on the injection device  400  after injection of the last full dose, the button spring  450  will not be released and the injection button  405  will therefore not be moved out of the housing  402 . This is a signal to the user that the injection device  400  has been emptied. 
         [0185]      FIG. 30  is a perspective view of the injection device  400  with the housing  402  removed, showing an interaction between the driver  410  and the protective cap  415 . The figure illustrates a situation where the user tries to eject a dose from the cartridge  404  (not visible) while the cap  415  is mounted on the injection device  400 . The basic movement pattern is similar to that described in relation to  FIG. 19 , the only real difference being the action of the button spring  450  which realises the proximal movement of the injection button  405 . A cap edge  482  abuts the contact soles  474  and prevents the driver  410  from undergoing axial displacement towards the distal end of the injection device  400 . Depressing the injection button  405  causes the contact soles  474  to slide along the cap edge  482 , whereby the tooth engaging element  476  is prevented from moving into contact with a tooth  419  on the piston rod  407 . The rotational bias of the spring  411  will pose a return torque on the driver  410  which will again release a locking of the button spring  450  due to the interaction between the pushing surfaces  469  and the flanges  453 ,  455 . When the cap  415  is properly positioned in the cap receiving part  409  an annular abutting surface  481  abuts the distal edge  485  of the guide member  420 . A couple of beads  488  on the cap  415  are adapted to engage with a bead receiving track on the inside wall of the guide member  420  for leading the cap  415  properly onto the injection device  400 . 
         [0186]      FIG. 31  is a cross sectional view of an injection device  500  according to a sixth embodiment of the invention, in a loaded state, i.e. where a dose is set. The injection device  500  comprises a housing  502 , a cartridge  504  with a piston  508 , a cap  515 , a toothed piston rod  507 , a driver  510  comprising a tooth engaging element  576  adapted to engage a tooth  519  on the piston rod  507  and slave the piston rod  507  towards the needle end of the injection device  500 . The driver  510  further comprises radially deflectable snap arms  512  adapted to engage with respective protrusions  587  on the housing  502 . The snap arms  512  are elastically biased towards the inside wall of the housing  502 . A main spring  511  is provided for powering the driver  510  during injection, and a secondary spring  550  is provided for biasing an injection button  505  towards a position in which it protrudes from the proximal end of the housing  502 . The injection button  505  has longitudinally extending arms  558  ending in respective hooks  556  adapted to engage with radially protruding catch elements  546  to hold the injection button  505  in a depressed position in the housing  502  against the bias of the secondary spring  550 . The injection device  500  further has an injection needle  506  attached. 
         [0187]      FIGS. 32   a - c  show an injection button release mechanism of the injection device  500  in detail. In  FIG. 32   a  the injection button  505  is depressed in the housing  502  illustrating a situation where the user has performed an injection. The injection button  505  is held in this position against the biasing force of the secondary spring  550  due to the engagement between the hooks  556  and the catch elements  546 . 
         [0188]    In  FIG. 32   b  the driver  510  has been displaced proximally in the housing  502  due to the user mounting the cap  515  on the injection device  500 . This has caused inclined push faces  548  of the snap arms  512  to slide along corresponding inclined faces  549  of the protrusions  587  and move into contact with corresponding inclined push faces  559  of the hooks  556 . Further proximal movement of the driver  510 , and thereby the snap arms  512 , will cause the inclined push faces  548  of the snap arms  512  to slide along the inclined push faces  559  of the hooks  556 , whereby the hooks  556  will be moved out of engagement with the catch elements  546 . When the hooks  556  are moved completely out of engagement with the catch elements  546  three things happen simultaneously. The secondary spring  550  releases its stored energy and forces the injection button  505  proximally out of the housing  502 , and the snap arms  512  deflect back towards the inside wall of the housing  502  to move into engagement with the proximal faces of the protrusions  587 , thereby cocking the main spring  511 . This is shown in  FIG. 32   c . Furthermore, the tooth engaging element  576  passes a more proximally positioned tooth  519  on the piston rod  507 , whereby a dose is set. 
         [0189]    When the user pushes the injection button  505  to inject a set dose the arms  558  are moved distally in the housing  502  while being deflected radially outwards by a sliding engagement with the catch elements  546 . The inclined push faces  559  of the hooks  556  are hereby brought into engagement with the inclined push faces  548  of the snap arms  512 . As the injection button  505  is being fully depressed against the housing  502  the arms  558  will force the inclined push faces  548  of the snap arms  512  to slide along the inclined push faces  559  of the hooks  556  to a point where the snap arms  512  are moved out of engagement with the protrusions  587 . This will cause the main spring  511  to release its stored energy and move the driver  510  distally in the housing  502  to expel the dose of drug through the injection needle  506 . Simultaneously, the elastic recovery of the arms  558  will cause the hooks  556  to move into engagement with the catch elements  546 , thereby cocking the secondary spring  550  and retaining the injection button  505  within the housing  502 .