FIELD OF THE INVENTION

The present invention relates to reservoirs for medical use and particularly to reservoirs hav ing more than one chamber.

BACKGROUND OF THE INVENTION

Within some medical treatment areas a combination therapy involving co-administration of at least two drugs is advantageous because of synergistic or additive effects. For example, within diabetes care, in the management of type 2 diabetes mellitus, concomitant use of cer tain insulin and glp-1 products has been shown to reduce HbAi _c levels in subjects, thereby improving glycaemic control.

Many drugs must be administered parenterally to be effective in the body and some of these, e.g. insulin and glp-1 , may require one or more doses to be delivered subcutaneously on a daily basis. Subcutaneous drug delivery is often associated with discomfort as many people dislike the thought of having an injection needle inserted through the skin. An undis closed number of people even suffer from needle-phobia, and these people have a particu larly strong desire to escape multiple daily injection therapy.

One attractive scenario, therefore, is to reduce the number of required skin penetrations by administering the drugs at the same time, or substantially the same time, through a single injection needle. In some cases, this is achievable by co-formulation of the active ingredi ents, where the co-formulated product is administered using a conventional injection device. In other cases, e.g. if the active ingredients are unsuitable for co-formulation, the individual substances are stored in separate chambers of a dual chamber, or multi-chamber, reservoir device from which they can be expressed, simultaneously or sequentially, through a single injection needle by use of dedicated expressing means.

US 4,394,863 (Survival Technology, Inc.) discloses an example of a dual chamber reservoir device in the form of an automatic injector with a cartridge having a fixedly mounted hypo dermic needle. In a pre-use state of the device the cartridge holds a forward liquid medica ment in a front chamber and a rearward liquid medicament in a rear chamber. The two liq uids are separated by an intermediate piston, and the rear chamber is sealed proximally by a rearward piston. During use, in response to a release of a stressed spring, a plunger is urged forward, pushing the rearward piston and pressurising the rearward liquid medicament which transmits the movement of the rearward piston to the intermediate piston. Eventually, as the spring continues to provide a forward bias to the plunger, this leads to an expelling of the forward liquid medicament through the hypodermic needle, followed by an expelling of the rearward liquid medicament, via a distally arranged bypass section.

WO 2010/139793 (Novo Nordisk A/S) discloses an example of a dual chamber reservoir device in the form of a manually operated mixing device with a piston coupling arrangement allowing for an aspiration procedure to ensure proper insertion of an associated IV infusion needle. In a pre-use state of the device a dry drug, or a liquid, is held in a front chamber, and a liquid is held in a rear chamber. The two substances are separated by a front piston, and the rear chamber is sealed proximally by a rear piston through which a piston rod extends. During use the piston rod is manually advanced, slaving the rear piston and pressurising the rear chamber liquid which transmits the movement of the rear piston to the front piston. As the user continues to press the piston rod forward the front piston enters a bypass section and becomes immobilised because the pressure now forces the rear chamber liquid into the bypass, past the front piston and into the front chamber. In the front chamber the two sub stances mix as the rear chamber collapses. When the rear piston eventually reaches the front piston and the substances are thoroughly mixed the user can expel the mixed sub stance by continued advancement of the piston rod.

A common drawback of such devices is the fact that during storage, over time, the piston material tends to adhere to the reservoir material, which means that a significant static fric tion must be overcome in order to initiate a drug mixing and/or expelling. Due to the incom pressibility of the liquid in the rear chamber the two pistons will move in unison until the front piston reaches the bypass section. Resultantly, the force required to overcome this static friction is actually the sum of the forces required to break loose the individual pistons.

In case of a manually driven piston rod the sudden shift from static to kinetic friction as the pistons break loose is likely to cause a jerking forward motion of the pistons as the user tries to compensate for the sudden acceleration by significantly decreasing the force input. Apart from being an unpleasant user experience it may in fact lead to an overly fast transfer of the rear chamber liquid to the front chamber. If the front chamber carries a dry powder to be reconstituted the transfer process may even lead to undesired foaming. In connection with spring driven injection devices like the automatic injector of US 4,394,863 the spring needs to be relatively powerful to ensure availability of a sufficient break-loose force. A downside of this is that once the friction becomes kinetic the power available for the actual drug expelling is very high and may lead to an unpleasantly high speed of delivery. Adding to that, a powerful spring requires stronger interfacing injection device parts to avoid creep or breakage during a potential medium- or long-term storage period in pre-loaded state, increasing both the cost and the weight of the injection device.

SUMMARY OF THE INVENTION

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

In particular, it is an object of the invention to provide a drug reservoir for use in an injection device for delivery of more than one substance, where serially arranged pistons in the drug reservoir may be moved by application of a reduced force.

It is a further object of the invention to provide a drug delivery device for delivery of a plurali ty of substances through a single needle interface on the basis of a relatively small force input.

It is also an object of the invention to provide a cost-effective automatic injection device for delivery of a plurality of initially separated substances.

It is an even further object of the invention to provide a method for filling a drug reservoir having more than one chamber.

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

In one aspect the invention provides a drug reservoir according to claim 1.

Hence, a drug reservoir is provided which comprises a reservoir body extending along a reference axis between a proximal end and an outlet end, a front piston arranged, in a pre use position, within the reservoir body between the proximal end and the outlet end, a rear piston arranged within the reservoir body between the front piston and the proximal end, a distal chamber defined by the outlet end, a first portion of the reservoir body, and the front piston, a proximal chamber defined by the front piston, a second portion of the reservoir body, and the rear piston, and bypass means allowing fluid flow past the front piston in a particular advanced position of the front piston, i.e. distally of the pre-use position. The distal chamber holds first contents, e.g. comprising a distal liquid volume or a dry powder, and the proximal chamber holds second contents comprising a proximal liquid volume and a proxi mal gas volume. The proximal gas volume is a volume of non-liquid-bound gas, i.e. free gas which is not embedded on a molecular level in the liquid, lying within a volume range having a preset minimum value.

The non-liquid-bound gas is present as a gas volume between a surface portion of the prox imal liquid volume and a surface portion of the proximal chamber, or as a gas bubble in the proximal liquid volume. Being non-liquid-bound the proximal gas volume provides resilience to the proximal chamber, in the sense that the second contents becomes compressible, as opposed to if the second contents consisted of liquid only. This has the effect that the two pistons will be broken loose sequentially instead of simultaneously. When a force of suffi cient magnitude is applied to the rear piston the rear piston will be able to break loose from the inner wall of the reservoir body, whereby the force resisting movement of the rear piston will shift from a static friction force to a, lower, kinetic friction force, before the applied force is transferred to the front piston via the second contents. The break loose force needed to mobilise the two pistons is thus lower than if the second contents are incompressible and two static friction forces must be overcome at the same time.

As a consequence, when used in a drug delivery device the drug reservoir provides for a lower activation force during a drug expelling operation. For automatic injection devices, for example, this means that a less powerful spring may be employed to drive the drug expelling mechanism. A less powerful spring will in a pre-loaded state strain the interfacing device components less, reducing the risk of creep or breakage and/or allowing for use of less de formation resistant materials and/or or configurations, thereby reducing the cost of the drug delivery device.

The predetermined, or preset, minimum value of the volume range within which the proximal gas volume lies reflects the amount of gas needed to obtain the above described effect. This minimum value may depend on the transversal dimension of the reservoir body and the de sign of the rear piston. For drug reservoirs of the types and sizes commonly used in injection or infusion therapy such as that realised by subcutaneous self-administration of drugs, in- eluding reservoirs specified in ISO 11040-4 (2015): Prefilled syringes - part 4: Glass barrels for injectables and ready-to-use prefillable syringes, e.g. employing rubber pistons with an outer configuration as specified in ISO 11040-5 (2012): Prefilled syringes - part 5: Plunger stoppers for injectables, the inventors have established that a minimum gas volume of 15mI (e.g. for a standard 1 ml long PFS having an inner diameter of 6,35 mm) is required to pro vide sufficient resilience in the proximal chamber, allowing for a sequential release of the two pistons.

The volume range may further have a predetermined, or preset, maximum value, in which case the volume range is a predetermined closed volume range. The maximum value may reflect the maximum amount of gas guaranteed to not cause stability issues with the proxi mal liquid volume and/or certain practical considerations of the manufacturer, e.g. regarding the physical dimensions of the end product.

A maximum gas volume satisfying requirements to the physical size of the drug reservoir may, for example, be 200mI, 100mI, 75mI, or 50mI.

An optimum volume range may be established or approximated in view of the aforemen tioned requirements as well as the effect produced by a specific proximal gas volume. The inventors have determined that in some cases a predetermined volume range of [15mI; 200mI] is preferable, while in other cases e.g. a predetermined volume range of [20mI; 50mI] is preferable.

In some embodiments of the invention the first contents comprise a distal liquid volume and a distal (non-liquid-bound) gas volume, where the distal gas volume is smaller than the prox imal gas volume.

The distal liquid volume may contain or comprise a first drug substance, and the proximal liquid volume may contain or comprise a second drug substance.

In some embodiments of the invention the proximal gas volume comprises air. This is partic ularly attractive in relation to the manufacturing of the drug reservoir, as the reservoir body may be filled in a conventional cleanroom environment.

In other embodiments of the invention the proximal gas volume comprises an inert gas to reduce the risk of undesired chemical reactions with the proximal liquid volume. In those embodiments the reservoir body may be filled in a cleanroom environment of the inert gas. The drug reservoir may for example be a cartridge type reservoir, where a penetrable sep tum closes the outlet end, or a syringe type reservoir. In case of the latter the drug reservoir may further comprise a staked hollow needle, i.e. a hollow needle fixedly arranged at the outlet end and fluidly connected with the distal chamber. The hollow needle may be sealed off by a removable plug.

The bypass means may e.g. comprise a bypass channel as conventionally known from dual chamber medicament containers such as the one disclosed in US 4,394,863.

In another aspect of the invention a drug delivery device is provided comprising a drug res ervoir as described in the above. The drug delivery device may further comprise a dose ex pelling structure for pressurising the proximal chamber. The dose expelling structure may comprise an actuatable piston rod adapted to transfer an expelling force to the rear piston.

The piston rod may be attached, or attachable, to the rear piston and adapted for direct ma nipulation by a user of the drug delivery device. Alternatively, the drug reservoir may be coupled with, e.g. embedded in, a housing of the drug delivery device and the piston rod may be operable via other components in the housing.

The dose expelling structure may be powered by a spring member operatively coupled with the piston rod and adapted to store energy releasable to urge the piston rod towards the outlet end. This will provide an automatic drug delivery device capable of executing a drug expelling with a minimum of user effort.

The drug delivery device may further comprise a retention structure which when enabled retains the spring member in a tensioned state, and a sleeve member extending axially along a portion of the housing and comprising a release structure, where the sleeve member is configured for proximal displacement relative to the housing and the retention structure from a first position in which the retention structure is enabled to a second position in which the retention structure is disabled by the release structure and stored energy consequently is released from the spring member.

In the first position the sleeve member may extend a distance beyond the outlet end such that a hollow needle arranged at the outlet end is covered by a distal end portion thereof. Thereby, the sleeve member may function as a combined needle shield and trigger for the tensioned spring. Hence, by simply placing the sleeve on the skin and pressing the housing towards the skin the user will initiate an automatic injection because the release structure, during movement of the sleeve member to the second position, will disable the retention structure, thereby causing a release of stored energy from the spring, which energy is used to urge the piston rod distally relative to the housing, applying a force to the rear piston that causes a cascade of events including a compression of the second contents and a breaking loose of the rear piston from the inner wall of the reservoir body, a further pressurisation of the second con tents and a breaking loose of the front piston from the inner wall of the reservoir body, an axial displacement of the proximal chamber until the front piston reaches the particular ad vanced position, during which a portion of the first contents may have been expelled through the outlet end, a collapse of the proximal chamber as the second contents are forced past the front piston via the bypass means, and finally an emptying, or substantial emptying, of the distal chamber.

Notably, the size and power of the spring required to accomplish this may be smaller than with prior art drug reservoirs due to the initial presence of the proximal gas volume, as de scribed above.

In a further aspect of the invention a method of filling a drug reservoir comprising a generally cylindrical main body with a bypass section, a closed outlet end, and an open end is provid ed. The method comprises (i) arranging the drug reservoir at least substantially vertically with the open end facing upward, (ii) introducing a first liquid volume into the drug reservoir through the open end such that a first interior portion of the generally cylindrical main body, including the bypass section, is covered by liquid in a vertical position of the drug reservoir where the open end faces upward, (iii) in a first sub-atmospheric pressure environment in serting a first piston into the generally cylindrical main body to a first piston position at least substantially adjoining the free surface of the first liquid volume, thereby establishing a front chamber holding the first liquid volume, (iv) introducing a second liquid volume into the drug reservoir through the open end, and (v) in a second sub-atmospheric pressure environment of a surrounding gas inserting a second piston into the generally cylindrical main body to a second piston position, thereby establishing a rear chamber, where the second piston posi tion is determined such that the rear chamber holds the second liquid volume and a rear chamber gas volume lying within a volume range having a predetermined minimum value. The first sub-atmospheric pressure environment ensures that a negative pressure is estab lished in the front chamber which will pull the first piston towards the free surface of the first liquid volume, minimising a present front chamber gas volume.

The second sub-atmospheric pressure environment may be established in a surrounding gas selected by the manufacturer in accordance with the constitution of the second liquid volume, e.g. air or an inert gas. The latter may be chosen to minimise the risk of undesired chemical reactions with the second liquid volume.

The second sub-atmospheric pressure environment ensures that a negative pressure is es tablished in the rear chamber, which negative pressure may be controlled in order to place the second piston at the desired position within the generally cylindrical main body that, in view of the position of the first piston and the introduced second liquid volume, provides for a presence of gas in the predetermined minimum amount.

In some exemplary embodiments of the invention the predetermined minimum value is 15mI. In other exemplary embodiments of the invention the predetermined minimum value is 20mI.

In step (v) the second piston position may be determined such that the rear chamber gas volume lies within a predetermined closed volume range, i.e. such that the volume range further has a predetermined maximum value.

In some exemplary embodiments of the invention the predetermined maximum value is 200mI. In other exemplary embodiments of the invention the predetermined maximum value is 50mI.

The first piston and/or the second piston may be arranged in the desired piston position us ing an insertion tube having an external diameter which is smaller than an internal diameter of the generally cylindrical main body such that the insertion tube may be introduced through the open end and into the generally cylindrical main body without establishing physical con tact thereto. The piston in question is then pre-arranged slidably within the insertion tube, and the insertion tube is inserted into the generally cylindrical main body to a position proxi- mally of the desired piston position, whereupon the piston is slid out through a distal inser tion tube opening and brought into sealing contact with an interior wall portion of the general ly cylindrical main body. Many drug reservoirs used in injection or infusion therapy have a siliconized interior surface to improve the friction interface to a piston. By using an insertion tube the piston may be inserted without sliding along the interior surface of the drug reservoir and resultantly scrap ing off the silicone. Scraped off silicone may over time interact with the liquid drug substance and cause precipitation, in which case the product is rendered useless.

For the avoidance of any doubt, in the present context the terms "distal" and "proximal" de note positions at, or directions along, a drug delivery device, or a needle unit, where "distal" refers to the drug outlet end and "proximal" refers to the end opposite the drug outlet end.

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

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

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a longitudinal section view of a drug reservoir according to an exemplary embodi ment of the invention in a pre-use state,

Figs. 2a-2c are graphs showing an initial force application to the rear reservoir piston in cas es without a proximal gas volume, respectively with two different proximal gas volumes,

Fig. 3 is a principle sketch of the process for filling the drug reservoir with two liquid volumes, Fig. 4 is a longitudinal section view of an exemplary drug delivery device employing the drug reservoir of Fig. 1 ,

Fig. 5 is a longitudinal section view of the drug delivery device in a ready to use state,

Figs. 6-10 are longitudinal section views of the drug delivery device in different in-use states, and

Fig. 11 is a longitudinal section view of the drug delivery device in a post use, emptied state. In the figures like structures are mainly identified by like reference numerals.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

When/If relative expressions, such as "upper" and "lower", "left" and "right", "horizontal" and "vertical", "clockwise" and "counter-clockwise", etc., are used in the following, these refer to the appended figures and not necessarily to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as their relative dimensions are intended to serve illustrative purposes only.

Fig. 1 is a longitudinal section view of a drug reservoir 1 according to an exemplary embod iment of the invention. The drug reservoir 1 is depicted in a pre-use state, i.e. in a state as supplied by the manufacturer (albeit without a rigid needle protector).

The drug reservoir 1 has a generally cylindrical reservoir body 2 with a bypass channel 3 and a narrowed distal end portion 4. An injection needle 5 is fixed to the distal end portion 4 and establishes fluid communication to a reservoir outlet 6. A front piston 8 is arranged in the reservoir body 2 between the reservoir outlet 6 and an open proximal end 7, and a front chamber 10 is thereby defined by the reservoir outlet 6, a front portion of the reservoir body 2 comprising the bypass channel 3, and the front piston 8. A rear piston 9 is arranged in the reservoir body 2 between the front piston 8 and the open proximal end 7, and a rear cham ber 11 is thereby defined by the front piston 8, a middle portion of the reservoir body 2, and the rear piston 9. The rear piston 9 has a cavity 13 adapted to receive an end portion of a piston rod (not shown).

The front chamber 10 holds a first liquid substance 18, and the rear chamber 11 holds a second liquid substance 19 as well as a proximal gas volume 12, sketched in the form of a gas bubble in the liquid drug 19. The proximal gas volume 12 is deliberately introduced in the rear chamber 11 in order to reduce the force required to perform an expelling of the res ervoir contents through the injection needle 5, as will be described in further detail below. In the present example the proximal gas volume is 15mI.

If a piston rod is inserted into the cavity 13 and a distally directed force is applied to the rear piston 9 the rear piston 9 will stay in its initial position until the applied force exceeds a cer tain threshold required to overcome the static friction in the contact interface between the sealing exterior surface of the rear piston 9 and the inner wall of the reservoir body 2.

Figs. 2a-2c indicate the initial force required to set the rear piston 9 into motion in three dif ferent cases, where the graph in Fig. 2a is the force profile for a drug reservoir without a proximal gas volume, the graph in Fig. 2b is the force profile for a drug reservoir with a prox imal gas volume of 15mI, and the graph in Fig. 2c is the force profile for a drug reservoir with a proximal gas volume of 20mI.

In a dual chamber drug reservoir without a proximal gas volume in the rear chamber the liquid acts as a rigid connection between the front piston and the rear piston. The single force peak, F ₀ , in Fig. 2a reflects the fact that, in such a device, in order to set the rear piston into motion the front piston needs to be set into motion also, due to the incompressibility of the liquid. According to the present experiments a break loose force of approximately 15N is required to overcome the static friction in the system comprising both pistons.

In contrast thereto, as the graph in Fig. 2b shows, when a predetermined proximal gas vol ume of 15mI is present in the rear chamber 11 the gas will add some flexibility to the system which will result in the rear piston 9 breaking loose before the front piston 8. A smaller force, F _r, 15, just short of 7N is required in this case to set the rear piston 9 into motion, as the prox imal gas volume is compressed. The force rises subsequently, as the gas becomes fully compressed and the liquid/gas system consequently acts as a rigid connection between the two pistons, until the front piston 8 breaks loose at a next force peak, F _f,i s, around 11 N. A sudden drop in the force level following the breaking loose of the front piston 8 reflects the transition from static friction to kinetic friction between the pistons and the inner reservoir wall. As the liquid in the front chamber 10 is pressurised and forced out through the small lumen of the injection needle 5 at a continued motion of the front piston 8, the force again increases some, due to the flow resistance in the injection needle 5, but does not approach the level of Fo. In Fig. 2c the difference is even more pronounced. Depicting results of experiments with a proximal gas volume of 20mI in the rear chamber 11 , the graph reveals a comparable force, F _r, 2o, for breaking loose the rear piston 9 but a significantly smaller force, F _f, 2o, in the area of 9N, for subsequently breaking loose the front piston 8. All in all, as the graphs indicate, when a proximal gas volume of at least 15mI is present in the rear chamber 11 the required maxi mum force for initiating and carrying through a drug expelling action is reduced because the flexibility provided by said gas volume enables the rear piston 9 to break loose from the res ervoir wall separately from the front piston 8.

Fig. 3 is a principle sketch of the process for filling the drug reservoir 1 with two liquid vol umes. From left to right the process steps include holding the drug reservoir 1 in an upright position with the injection needle 5 sealed up by a needle plug 41 and introducing a prede termined volume of the first liquid substance 18 into the reservoir body 2 through the proxi mal end 7.

Having filled a distal portion of the reservoir body 2 to a level where the bypass channel 3 is covered the drug reservoir 1 is placed in a first sub-atmospheric pressure environment 100. The front piston 8 is arranged in a radially compressed state in an insertion tube 80 having an inner diameter which is smaller than the inner diameter of the reservoir body 2, and the insertion tube 80 is introduced into the reservoir body 2 through the proximal end 7, notably without touching the inner wall of the reservoir body 2. The front piston 8 is then pushed through the insertion tube 80 and expands into contact with the inner wall of reservoir body 2 just above the free surface of the first liquid substance 18, thereby establishing the front chamber 10, and the drug reservoir 1 is subsequently re-exposed to normalised pressure conditions. The negative pressure in the front chamber 10 due to the front piston 8 being inserted in the first sub-atmospheric pressure environment 100 will cause the front piston 8 to move towards the first liquid substance 18, closing any gap to the free surface thereof.

A predetermined volume of the second liquid substance 19 is introduced into the reservoir body 2 though the proximal end 7 and fills a space above the front piston 8. The drug reser voir 1 is then placed in a second sub-atmospheric pressure environment 200 of a surround ing gas, and the insertion tube 80, now carrying the rear piston 9 in a radially compressed state, is introduced into the reservoir body 2 in a manner similar to the above described. This time the pressure is controlled such that when the rear piston 9 is deposited in the reservoir body 2, thereby establishing the rear chamber 11 , and the drug reservoir 1 is subsequently re-exposed to normalised pressure conditions a volume of the surrounding gas remains in the rear chamber 11 as a free gas volume lying within a volume range having a predeter mined minimum value.

Fig. 4 is a longitudinal section view of the drug reservoir 1 forming part of an exemplary, dedicated auto-injector 20. The auto-injector 20 comprises a tubular housing 21 closed prox- imally by a transversal end wall 22 and accommodating a drug expelling mechanism includ ing a piston rod 30 having a head portion 31 inserted into the cavity 13 and a shoulder por tion 32 adapted to apply a distally directed force to the rear piston 9.

A couple of snap arms 24 extend distally from the transversal end wall 22 into the interior of the housing 21 , ending in respective claws 26 with "v"-shaped interfacing portions 27 config ured for engagement with corresponding depressions 33 in the piston rod 30. Each snap arm 24 has a proximal carving 25 which provides flexibility and allows for radial deflection of the claw 26.

A pre-tensioned compression spring 65 is arranged within the piston rod 30 and supported proximally by a central pin 23 which extends distally from the transversal end wall 22. The spring 65 is adapted to act between a distal end portion of the piston rod 30 and the trans versal end wall 22.

The drug reservoir 1 is held within the housing 21 and is closed distally by a rigid needle protector 40 carrying the needle plug 41. An elongated sleeve 50 is arranged concentrically with, and between, the drug reservoir 1 and the housing 21. The sleeve 50 is axially dis placeable relative to the housing 21 , biased in the proximal direction by a sleeve spring 75, and comprises a radially enlarged proximal end portion 51 with a narrow adjoining section 53. In the shown pre-use state of the auto-injector 20 the sleeve 50 is in its maximum ex tended position relative to the housing 21 , and the proximal end portion 51 is axially aligned with the claws 26, physically preventing the interfacing portions 27 from leaving the depres sions 33. The auto-injector 20 is thus safely cocked, as the spring 65 is maintained in its pre- tensioned state because the piston rod 30 is unable to undergo axial motion relative to the housing 21.

Fig. 5 is a longitudinal section view of the auto-injector 20 in a ready-to-use state, after re moval of the rigid needle protector 40 and the needle plug 41. The sleeve 50 is still in its maximum extended position relative to the housing 21 , where a distal sleeve end portion 52 covers the injection needle 5 and thus protects the user from accidental needle stick injuries. The distal sleeve end portion 52 has a sleeve rim 54 adapted to abut, and be pressed against, the user's skin at the desired injection site during drug expelling.

Figs. 6-11 illustrate in a step-wise manner the dose expelling sequence of the auto-injector 20. Firstly, the user places the sleeve rim 54 in contact with a desired skin location (not shown) and presses the housing 21 against the skin. This causes the distal sleeve end por tion 52 to compress the sleeve spring 75, as the housing 21 and the sleeve 50 undergo rela tive axial motion from the mutual position shown in fig. 5 to that shown in Fig. 6. In essence the sleeve 50 is displaced proximally relative to the housing 21 and this causes the respec tive enlarged proximal end portions 51 to slide proximally along the claws 26. At some point, when the distal sleeve end portion 52 is pressed back sufficiently far that the tip of the injec tion needle 5 is exposed and has penetrated the skin surface, the sleeve 50 reaches a posi tion relative to the snap arms 24 in which the enlarged proximal end portions 51 are no longer axially aligned with the claws 26. Instead, the claws 26 are axially aligned with the narrow adjoining section 53 and thereby no longer prevented from radial displacement.

The pre-tensioned spring 65 constantly provides a distally directed bias to the piston rod 30, so when the claws 26 are no longer radially fixated the axial force from the spring 65 and the respective configurations of the interfacing portions 27 and the depressions 33 will cause the snap arms 24 to deflect radially about the proximal carvings 25, leading to a disengagement of the claws 26 from the piston rod 30 and a resultant release of the spring 65. This is indi cated in Fig. 7.

The initial result of the release of the spring 65 is also seen in Fig. 7. The presence of the proximal gas volume 12 enables a small compression of the rear chamber 11 , so as the force from the expanding spring 65 pushes the piston rod 30 forward the rear piston 9 breaks loose from the inner wall of the reservoir body 2 while the front piston 8 remains sta tionary, the spring 65 at this point thus having to overcome only the static friction between the rear piston 9 and the reservoir body 2 (and not also the static friction between the front piston 8 and the reservoir body 2). In Fig. 7 the compression of the rear chamber 11 is illus trated by a reduced size of the proximal gas volume 12.

When the proximal gas volume 12 is fully compressed the contents of the rear chamber 11 will transfer the force from the spring 65 to the front piston 8 which will then break loose and move distally in the reservoir body 2 along with the rear piston 9, the second liquid sub stance 19 and the compressed proximal gas volume. The rear chamber 11 as such is thus displaced within the reservoir body 2, while a volume of the first liquid substance 18 is forced out through the injection needle 5, until the front piston 8 reaches the bypass channel 3, as shown in Fig. 8, at which point the second liquid substance 19 is forced into the bypass channel 3 and past the front piston 8 as the spring 65 keeps expanding.

The rear chamber 11 eventually collapses as the rear piston 9 approaches the front piston 8 and the second liquid substance 19 is transferred to the front chamber 10 where it mixes with the remains of the first liquid substance 18. Fig. 9 depicts the state of the auto-injector 20 immediately after the collapse of the rear chamber 11.

The mixed first liquid substance 18 and second liquid substance 19 is now expelled from the front chamber 10 through the injection needle 5 as the rear piston 9, under the influence of the piston rod 30 and the spring 65, pushes the front piston 8 further distally in the reservoir body 2. In Fig. 10 the front piston 8 covers the distal end of the bypass channel 3 and thus seals off the front chamber 10 in the proximal direction.

The drug expelling continues until the front piston 8 reaches a constriction of the reservoir body 2 at the reservoir outlet 6, after which the injection needle 5 is pulled out of the skin by the user moving the housing 21 away from the injection site. As the pressure between the skin surface and the sleeve rim 74 is relieved the sleeve spring 75 expands and urges the sleeve 50 distally relative to the housing 21 until the distal sleeve end portion 52 again co vers the injection needle 5. The auto-injector 20 is now in a post-use state, as shown in Fig. 11 , and may be discarded safely with no risk of accidental needle stick injuries.