throughout the description and claims of this specification , the words “ comprise ” and “ contain ” and variations of the words , for example “ comprising ” and “ comprises ”, means “ including but not limited to ”, and is not intended to ( and does not ) exclude other components , integers or steps . throughout the description and claims of this specification , the singular encompasses the plural unless the context otherwise requires . in particular , where the indefinite article is used , the specification is to be understood as contemplating plurality as well as singularity , unless the context requires otherwise . throughout the following description , reference to a “ forward ” direction means the direction which is towards the patient when the injection device is in use . the “ forward ” end of the injection device is the end nearest the patient &# 39 ; s skin when the device is in use . similarly , reference to a “ rearward ” direction means the direction which is away from the patient and the “ rearward ” end of the device is the end furthest from the patient &# 39 ; s skin when the injection device is in use . the “ plunger ” does not necessarily ( but may ) include any elastomeric stopper or the like which seals the chamber containing liquid medicament . features , integers , characteristics or groups described in conjunction with a particular aspect , embodiment or example of the invention are to be understood to be applicable to any other aspect , embodiment or example described herein unless incompatible therewith . referring to fig7 , a first embodiment of the invention is illustrated . in this embodiment , there is a biasing means in the form of a compression spring 200 between the rear of the outer housing 30 and the flange 8 f of the plunger 8 . the illustrated spring 200 is integrally formed with the plunger flange 8 f , but alternatively , a separate spring could be provided . the spring 200 has a flange 201 at the rear thereof to abut the rear of the outer housing . the spring 200 is a compression spring so that the plunger flange 8 f is biased axially forwards so that the syringe as a whole is also biased axially forwards ( the biasing force from the spring 200 being transmitted to the rest of the syringe including the barrel via the incompressible liquid medicament therein ). this means that the flange 90 of the syringe barrel is always initially located forward of the front tags 7 b of the inner housing . the forward axial force provided by the spring 200 is relatively weak and needs only be sufficient to bias the syringe forwards . the force is not sufficient to cause medicament to be ejected from the front of the device , nor is it sufficient to affect the operation of the main energy source ( usually a much more powerful spring ) which actuates the device when delivery of an injection is required . using this modified plunger , the performance of the device is improved , as one always knows that the syringe flange &# 39 ; s initial axial position is biased forward of the rear tags 7 b , avoiding the risk of malfunction . using the modified plunger , the axial position of the plunger is such that it is possible to for the tags 7 a , 7 b to be stored in their radially - outward position , which is preferable given the above - described tendency for the tags to acquire “ memory ” of their stored position . in order to avoid the risk of one set of the tags catching in the recesses intended for the other set of tags , the front tags 7 b are radially offset from the rear tags 7 a , for example by 45 °, as illustrated in fig8 . an alternative embodiment of the invention is shown in fig9 - 11 . in this embodiment , the biasing means are provided as part of a modified inner housing . referring to fig9 , the inner housing 7 has rear tags 7 a and front tags 7 b having the same function as described above . in addition , and as shown best in fig1 , the front end of the inner housing is provided with one or more biasing means in the form of radially flexible legs 250 . the legs are preferably integrally formed with the inner housing 7 . each flexible leg 250 may have an enlarged head 251 . normally , before the autoinjector is fired , the legs 250 are flexed radially inwards so that they are in the axial path of the finger flange 90 of the syringe barrel . fig1 and 11 show the finger flange 90 of the syringe barrel and the barrel seat 91 at the rear of the device &# 39 ; s syringe holder . the legs 250 may be moulded so that this is their default position . the legs 250 need not abut the finger flange 90 ( although they may do so ) but they must at least partially block its rearward axial path . in this way , undesirable rearward axial movement of the finger flange 90 is prevented and the syringe barrel is biased to a position forward of the front tags 7 b . again , the performance of the device is improved , as one always knows that the syringe flange &# 39 ; s initial axial position is biased forward of the rear tags 7 b , avoiding the risk of malfunction . the legs 250 of the inner housing 7 are relatively weak and need only to be strong enough to resist the weight of the syringe to prevent undesirable axial movement . they do not impede the firing of the autoinjector to deliver medicament . when the autoinjector is actuated , the inner housing 7 is urged forward by the full force of the main energy source . the legs 250 cannot resist this force and so they are forced radially - outwardly , out of the path of the finger flange 90 . therefore it is possible for the front tags 7 b to engage the finger flange 90 in the normal way to move the syringe axially forward enough to move the needle out of the housing into the injection site . an alternative embodiment of the invention is shown schematically in fig1 - 17 . in this embodiment , the rear of the plunger 8 is attached to a chamber 300 filled with a shear thickening fluid . a shear thickening fluid is one whose viscosity increases with the rate of shear and is sometimes referred to as a “ dilatant ” material . the chamber 300 is fixed with respect to the plunger 8 . the chamber 300 contains an axially - moveable piston 301 which does not fill the cross - section of the chamber , so that the shear thickening fluid is able to pass freely around the piston . the piston 301 is attached to and fixed with respect to a rear flange 302 . the rear tags 7 a of the inner housing engage behind the rear flange 302 , in the same way as if the rear flange were the flange of the plunger ( 8 f ) in the fig7 embodiment . a compression spring 303 is disposed between the rear flange 302 and the rear surface of the chamber 300 , as shown in fig1 , so that the rear flange 302 is always biased against the rear tags 7 a of the inner housing . the outer housing of the device is labelled as item 30 and the barrel of the syringe is labelled as item 9 in fig1 . the chamber 300 acts as a linear dashpot and , in combination with the spring 303 may be referred to as biasing means . fig1 shows what happens when the plunger 8 moves during storage and / or transportation of the device . for example , the pressure inside the syringe may change if the device is transported by airfreight when any gas bubble inside the syringe may change in volume , causing the plunger 8 to move axially . the axial movement is relatively slow and is not forceful enough to change the viscosity of the shear thickening fluid . consequently , as the plunger 8 moves rearwardly ( upwards as illustrated in fig1 ) so does the chamber 300 . however , the piston 301 does not move rearwardly ( as it is attached to the rear flange 302 ) and so the shear thickening fluid moves around it as the chamber moves rearwardly . the distance between the rear flange 302 and the rear surface of the chamber 300 reduces as the spring 303 is compressed . if the plunger moves forwards , the opposite happens i . e . the distance between the rear flange 302 and the rear surface of the chamber 300 increases . the spring 303 always keeps the rear flange 302 biased against the tags 7 a of the inner housing . fig1 shows what happens when the device is actuated to deliver an injection . as the inner housing 7 moves forwards with the rear tags 7 a engaged behind the rear flange 302 , the strong driving force of the device &# 39 ; s main energy source causes the viscosity of the shear thickening fluid to change and the fluid thickens . the thick fluid can no longer move freely around the piston 301 and the chamber 300 acts , effectively , as a solid bridge between the rear flange 302 and the plunger 8 , so that the plunger 8 is driven into the barrel 9 of the syringe to deliver the medicament . fig1 shows what happens when the dose of medicament has been delivered . when the plunger 8 reaches the end of its forward travel inside the syringe , the piston 301 continues to move forwards , albeit at a reduced speed because the thickened shear thickening fluid is still able to flow to some extent . therefore the inner housing 7 is able to continue forwards slightly after the full dose of medicament has been delivered . this extra forward movement ensures that the inner housing 7 reaches its desired forward position regardless of its absolute initial axial starting position . this is necessary in order for the rear tags 7 a to move radially - outwardly releasing the rear flange 302 as shown in fig1 . thus , although the rear flange 302 is always initially biased against the rear tags 7 a , the shear thickening fluid chamber provides an axial range of possible initial starting points for the plunger 8 . once the rear tags 7 a are fully disengaged from the rear flange , as shown in fig1 , the rear flange 302 , chamber 300 and plunger 8 are free to move axially rearwardly under the force of a secondary spring at the front of the device ( not illustrated ). as shown in fig1 , the needle 10 , syringe barrel 9 , plunger 8 , chamber 300 and rear flange 302 can all retract into the device so that the needle is fully concealed from the patient &# 39 ; s view . a significant advantage of the chamber of shear thickening fluid is that a precise absolute axial starting point of the plunger is no longer required for proper functioning of the device . the chamber of shear thickening fluid absorbs the effect of minor axial movements of the plunger during storage and transportation . thus , although the rear flange 302 is always initially biased against the rear tags 7 a , the shear thickening fluid chamber provides an axial range of possible initial starting points for the plunger 8 . regardless of the absolute initial starting point for the plunger 8 , the shear thickening fluid enables a defined relative axial movement to occur in order to deliver the required dose . a further improvement is illustrated in fig1 . here , instead of the chamber of shear thickening fluid , a ratchet cap 400 is provided which sites over the rear flange 8 f of the plunger 8 , locked into place by locking tabs 401 . the rear tags 7 a of the inner housing can engage anywhere along the ratchet cap 400 , depending upon the absolute axial position of the plunger ( and consequently the ratchet cap attached thereto ). this also effectively provides a plurality of axial starting positions for the plunger for which the device will work to deliver the required dose of medicament . the ratchet cap embodiment could be used in combination with the modified plunger rod of fig7 . other alternatives , instead of the illustrated ratchet cap , may be used to provide the range of axial starting positions for the plunger , for example a differently shaped component intermediate the inner housing tags and plunger . alternatively , the ratchet cap or equivalent component could be an integral part of a modified plunger , for example a plunger having a plurality of annular ribs . the diameter of the inner housing 7 may need to be increased slightly to accommodate the extra diameter of the ratchet cap . alternatively , the ratchet cap could be shaped as shown in fig1 so that the diameter of the device does not need to be increased . this embodiment has the advantage that the overall length of the device could be reduced with the rear flange positioned further back in the device , inside the main spring ( the device &# 39 ; s main energy source ). the reader &# 39 ; s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification , and the contents of all such papers and documents are incorporated herein by reference . all of the features disclosed in this specification ( including any accompanying claims , abstract and drawings ), and / or all of the steps of any method or process so disclosed , may be combined in any combination , except combinations where at least some of such features and / or steps are mutually exclusive . each feature disclosed in this specification ( including any accompanying claims , abstract and drawings ), may be replaced by alternative features serving the same , equivalent or similar purpose , unless expressly stated otherwise . thus , unless expressly stated otherwise , each feature disclosed is one example only of a generic series of equivalent or similar features . the invention is not restricted to the details of any foregoing embodiments . the invention extends to any novel one , or any novel combination , of the features disclosed in this specification ( including any accompanying claims , abstract and drawings ), or to any novel one , or any novel combination , of the steps of any method or process so disclosed .
an autoinjector comprising an outer housing in which is mounted a syringe , the autoinjector further comprising an inner housing intermediate the outer housing and the syringe and an energy source in communication with said inner housing , wherein the inner housing is moveable by the energy source between three positions , namely a first position in . which the inner housing is in communication with the barrel such that , in use , the plunger and barrel are moveable axially so as to move at least part of said needle out of the outer housing ; a second position in which the inner housing is in communication with the plunger but not the barrel such that , in use , said plunger is moveable axially into said barrel so as to expel medicament through the needle ; and a third position in which the inner housing is in communication with neither the plunger nor the barrel such that , in use , the plunger and barrel are able to retract in order to retract the needle into the outer housing , characterized in that the plunger includes biasing means for axially biasing the barrel , before activation of the energy source , to a position forward of the part of the inner housing which acts on the barrel in said first position .