Abstract:
An injection device ( 210; 110 ) is described. A housing ( 212; 112 ) receives a syringe and includes a return spring ( 226; 126 ) for biasing the syringe from an extended position in which its needle ( 218; 118 ) extends from the housing ( 212; 112 ) to a retracted position in which the it does not. A drive spring ( 230; 130 ) acts on a first drive element ( 232; 132 ) and a second drive element ( 234; 134 ) acts upon the syringe to advance it from its retracted position to its extended position and discharge its contents through the needle. The first drive element ( 232; 132 ) is capable of movement relative to the second ( 234; 134 ) once a nominal decoupling position has been reached. A release mechanism is activated when the first drive element ( 234; 134 ) is further advanced to a nominal release position, to release the syringe ( 214; 114 ) from the action of the drive spring ( 230; 130 ), whereupon the return spring ( 226; 126 ) restores the syringe ( 214; 114 ) to its retracted position. The nominal decoupling and release positions are defined relative to the syringe ( 214; 114 ). This may be achieved by interaction between a moving component and a decoupling component ( 162; 262 ) that moves with the syringe as it is advanced.

Description:
BACKGROUND TECHNOLOGY 
     The present invention relates to an injection device of the type that receives a syringe, extends it, discharges its contents and then retracts it automatically. Devices of this general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring. 
     Because of the stack-up of tolerances of the various components of the device, a certain margin of safety must be built into the activation of the release mechanism, to ensure that it is effective. The consequence of underestimating the safety margin is that the release mechanism may fail to operate even once the syringe contents have been discharged, which is unsatisfactory in a device that is supposed to retract automatically, particularly for self-administered drugs. On the other hand, overestimating the safety margin may mean that some of the syringe contents are discharged after the syringe has retracted, which results firstly in a short dose and secondly in what may be termed a “wet” injection. Wet injections are undesirable for the squeamish, particularly in connection with self-administered drugs. 
     UK patent applications nos. 0210123, 0229384 and 0325596 describe a series of injection devices designed to deal with this problem. Each makes use of a neat trick that delays the release of the syringe for a certain period of time after the release mechanism has been activated, in an attempt to ensure that the syringe has been completely discharged. The devices illustrated in UK patent application no. 0325596 make use of a two-part drive incorporating a fluid-damped delay mechanism that is particularly effective in ensuring complete discharge of the syringe contents. In each case, the device relies upon two unlatching mechanisms. The first unlatching mechanism initiates the fluid damping mechanism and the second releases the syringe from the actuator, allowing it to be withdrawn. The unlatching mechanisms are activated by components of the injection device having been advanced to nominal unlatching positions relative to the device casework. 
     A device  10  of this general character is illustrated schematically in  FIG. 1 . The sequence of operation is as follows. Firstly, the device  10  is armed. The user presses a release button and the syringe  14  is advanced a distance d 1  by a drive spring  30 , thereby compressing the retraction spring  26 . This movement inserts the needle  18  into the patient. The plunger  23  is advanced a distance d 2  by the drive spring  30 , injecting most of the dose. Once nearly the entire dose has been injected, the first unlatching mechanism is activated, an operation illustrated schematically by the coincidence of components  1  and  3 . The plunger  23  is then advanced a further distance d 3  by the drive spring  30 , injecting the rest of the dose. Finally, the second unlatching mechanism is activated, an operation illustrated schematically by the coincidence of components  2  and  4 , and the retraction spring  26  then causes the needle  18  to be retracted by the distance d 1 . 
     Since the drive spring acts upon the same component of the device throughout, here referred to as the “actuator”, the distance that component must move between the device being armed and the second unlatching mechanism being activated is, subject to tolerance stack-up, equal to the sum of d 1 , d 2  and d 3 . In the devices described in the applications mentioned above, all of this movement takes place to the rear of the syringe, which means that the overall length of the device must be greater than the sum of the length of the actuator, the distances d 1 , d 2  and d 3  and the length of the syringe body not including the needle. 
     The best design of injection device is one that is compact. This is important both to the ergonomics of the device and to its manufactured cost. The length of the device can be reduced by allowing the actuator to move past the syringe, and by having the unlatching mechanisms activated in front of the syringe. However, this would require the actuator and its unlatching mechanisms to pass around the space occupied by the syringe, involving an increase in diameter of the device that negates the length savings. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide a more compact device. Instead of triggering release of the unlatching mechanisms using a fixed point on the device casework, the present invention does it using one of more features that move forward with the syringe as it is advanced. In other words, the nominal positions at which the unlatching mechanisms are activated are defined relative to the syringe, not relative to the device casework. As illustrated in  FIG. 2 , these nominal positions also move forwards a distance d 1  as the syringe is initially advanced. This in turn means that the initial distance between the actuator and the syringe plunger can be reduced by the distance d 1 . The length of the device can be reduced by d 1  at a stroke. More modest improvements are available when only one of the nominal positions at which the unlatching mechanisms are activated is defined relative to the syringe. 
     Accordingly, a first aspect of the present invention provides an injection device comprising:
         a housing adapted to receive a syringe having a discharge nozzle;   first and second drive elements, of which the first is acted upon and the second acts upon the syringe to advance it from its retracted position to its extended position and discharge its contents through the discharge nozzle, the first drive element being capable of movement relative to the second when the former is acted upon and the latter is restrained by the syringe;   a coupling that prevents the first drive element from moving relative to the second until they have been advanced to a nominal decoupling position relative to the syringe.       

     In this case, the nominal decoupling position, i.e. the first nominal unlatching position, is defined relative to the syringe and not relative to the housing. 
     Preferably, the device includes:
         an actuator that acts upon the first drive element;   means for biasing the syringe from an extended position in which the discharge nozzle extends from the housing to a retracted position in which the discharge nozzle is contained within the housing; and   a release mechanism, activated when the first drive element has been advanced to a nominal release position that is more advanced than the said nominal decoupling position, and adapted to release the syringe from the action of the actuator, whereupon the biasing means restores the syringe to its retracted position.       

     In preferred embodiments of the invention, the nominal decoupling position is defined either by one of the drive elements interacting with a decoupling component that moves with the syringe as it is advanced. 
     For ease of manufacture and assembly, the coupling may comprise flexible arms on one of the drive elements that engage with a drive surface on the other, in which case the decoupling component causes the flexible arms to move when the said nominal decoupling position is reached, thus disengaging them from the drive surface to allow the first drive element to move relative to the second. 
     A second aspect of the present invention provides an injection device comprising:
         a housing adapted to receive a syringe having a discharge nozzle, the housing including means for biasing the syringe from an extended position in which the discharge nozzle extends from the housing to a retracted position in which the discharge nozzle is contained within the housing;   first and second drive elements, of which the first is acted upon and the second acts upon the syringe to advance it from its retracted position to its extended position and discharge its contents through the discharge nozzle, the first drive element being capable of movement relative to the second when the former is acted upon and the latter is restrained by the syringe;   a coupling that prevents the first drive element from moving relative to the second until they have been advanced to a nominal decoupling position; and   a release mechanism, activated when the first drive element has been advanced to a nominal release position relative to the syringe that is more advanced than the said nominal decoupling position, and adapted to release the syringe, whereupon the biasing means restores the syringe to its retracted position.       

     Here, the nominal release position, i.e. the second nominal unlatching position, is defined relative to the syringe and not relative to the housing. 
     Again, in preferred embodiments, the nominal release position is defined by an actuator or the first drive element interacting with a decoupling component that moves with the syringe as it is advanced. It may be defined by the actuator interacting with the first drive element once the nominal decoupling position has been reached, at which position the first drive element is restrained by the syringe against further movement. 
     Once again, for ease of manufacture and assembly, of the actuator and the first drive element, one preferably comprises second flexible arms that engage with a second drive surface on the other, and the release mechanism preferably comprises the said decoupling component, which causes the second flexible arms to move when the said nominal release position is reached, thus disengaging them from the drive surface. 
     Alternatively, of an actuator and the first drive element, one preferably comprises second flexible arms that engage with a second drive surface on the other, allowing the actuator to act upon the first drive element and preventing the former from moving relative to the latter until the nominal release position has been reached, the second flexible arms are preferably biased toward a position at which they engage the second drive surface and the release mechanism preferably causes them to move against their bias, thus disengaging them from the drive surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIGS. 1 and 2  are schematic illustrations to which reference has already been made; 
         FIG. 3  is an illustration of a first embodiment of the invention; and 
         FIG. 4  is likewise a second. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  shows an injection device  110  in which a housing  112  contains a hypodermic syringe  114 . The syringe  114  is of conventional type, including a syringe body  116  terminating at one end in a hypodermic needle  118  and at the other in a flange  120 . The conventional plunger that would normally be used to discharge the contents of the syringe  114  manually has been removed and replaced with a drive element  134  as will be described below, to which is attached a bung  122 . The bung  122  constrains a drug  124  to be administered within the syringe body  116 . Whilst the syringe illustrated is of hypodermic type, this need not necessarily be so. Transcutaneous or ballistic dermal and subcutaneous syringes may also be used with the injection device of the present invention. Generally, the syringe must include a discharge nozzle, which in a hypodermic syringe is the needle  118 . 
     As illustrated, the housing includes a return spring  126  that biases the syringe  114  from an extended position in which the needle  118  extends from an aperture  128  in the housing  112  to a retracted position in which the discharge nozzle  118  is contained within the housing  112 . The return spring  126  acts on the syringe  114  via a sleeve  127 . 
     At the other end of the housing is a compression drive spring  130 . Drive from the drive spring  130  is transmitted via a multi-component drive to the syringe  114  to advance it from its retracted position to its extended position and discharge its contents through the needle  118 . The drive accomplishes this task by acting directly on the drug  124  and the syringe  114 . Hydrostatic forces acting through the drug  124  and, to a lesser extent, static friction between the bung  122  and the syringe body  116  initially ensure that they advance together, until the return spring  126  bottoms out or the syringe body  116  meets some other obstruction that retards its motion. 
     The multi-component drive between the drive spring  130  and the syringe  114  consists of three principal components. A drive sleeve  131  takes drive from the drive spring  130  and transmits it to flexible latch arms  133  on a first drive element  132 . This in turn transmits drive via flexible latch arms  135  to a second drive element, the drive element  134  already mentioned. 
     The first drive element  132  includes a hollow stem  140 , the inner cavity of which forms a collection chamber  142  in communication with a vent  144  that extends from the collection chamber through the end of the stem  140 . The second drive element  134  includes a blind bore  146  that is open at one end to receive the stem  140  and closed at the other. As can be seen, the bore  146  and the stem  140  define a fluid reservoir  148 , within which a damping fluid is contained. 
     A trigger (not shown) is provided on one side of the housing  112 . The trigger, when operated, serves to decouple the drive sleeve  131  from the housing  112 , allowing it to move relative to the housing  112  under the influence of the drive spring  130 . The operation of the device is then as follows. 
     Initially, the drive spring  130  moves the drive sleeve  131 , the drive sleeve  131  moves the first drive element  132  and the first drive element  132  moves the second drive element  134 , in each case by acting through the flexible latch arms  133 ,  135 . The second drive element  134  and the bung  122  move and, by virtue of static friction and hydrostatic forces acting through the drug  124  to be administered, move the syringe body  116  against the action of the return spring  126 . The return spring  126  compresses and the hypodermic needle  118  emerges from the exit aperture  128  of the housing  112 . This continues until the return spring  126  bottoms out or the syringe body  116  meets some other obstruction that retards its motion. Because the static friction between the bung  122  and the syringe body  116  and the hydrostatic forces acting through the drug  124  to be administered are not sufficient to resist the full drive force developed by the drive spring  130 , at this point the second drive element  134  begins to move within the syringe body  116  and the drug  124  begins to be discharged. Dynamic friction between the bung  122  and the syringe body  116  and hydrostatic forces acting through the drug  124  to be administered are, however, sufficient to retain the return spring  126  in its compressed state, so the hypodermic needle  118  remains extended. 
     Before the second drive element  134  reaches the end of its travel within the syringe body  116 , so before the contents of the syringe have fully discharged, the flexible latch arms  135  linking the first and second drive elements  132 ,  134  reach a constriction  137 . The constriction  137  is formed by a component  162  that is attached to the syringe flange  120 , so it will be understood that when the syringe  114  advances from its retracted position to its extended position, the component  162  advances with it. The constriction  137  moves the flexible latch arms  135  inwards from the position shown to a position at which they no longer couple the first drive element  136  to the second drive element  134 , aided by the bevelled surfaces on the constriction  137 . 
     Once this happens, the first drive element  136  acts no longer on the second drive element  134 , allowing the first drive element  132  to move relative to the second drive element  134 . 
     Because the damping fluid is contained within a reservoir  148  defined between the end of the first drive element  132  and the blind bore  146  in the second drive element  134 , the volume of the reservoir  148  will tend to decrease as the first drive element  132  moves relative to the second drive element  134  when the former is acted upon by the drive spring  130 . As the reservoir  148  collapses, damping fluid is forced through the vent  144  into the collection chamber  142 . Thus, once the flexible latch arms  135  have been released, the force exerted by the drive spring  130  does work on the damping fluid, causing it to flow through the constriction formed by the vent  144 , and also acts hydrostatically through the fluid, to drive the second drive element  134 . Losses associated with the flow of the damping fluid do not attenuate the force acting on the body of the syringe to a great extent. Thus, the return spring  126  remains compressed and the hypodermic needle  118  remains extended. 
     After a time, the second drive element  134  completes its travel within the syringe body  116  and can go no further. At this point, the contents of the syringe  114  are completely discharged and the force exerted by the drive spring  130  acts to retain the second drive element  134  in its terminal position and to continue to cause the damping fluid to flow through the vent  144 , allowing the first drive element  132  to continue its movement. 
     Before the reservoir  148  of fluid is exhausted, the flexible latch arms  133  linking the drive sleeve  131  with the first drive element  132  reach another constriction  139 , also provided by the component  162  that is attached to the syringe flange  120 . The constriction  139  moves the flexible latch arms  133  inwards from the position shown to a position at which they no longer couple the drive sleeve  131  to the first drive element  132 , aided by the bevelled surfaces on the constriction  139 . Once this happens, the drive sleeve  131  acts no longer on the first drive element  132 , allowing them to move relative to each other. At this point, of course, the syringe  114  is released, because the force developed by the drive spring  130  is no longer being transmitted to the syringe  114 , and the only force acting on the syringe will be the return force from the return spring  126 . Thus, the syringe  114  now returns to its retracted position and the injection cycle is complete. 
     All this takes place, of course, only once the cap  111  has been removed from the end of the housing  112 . As can be seen from  FIG. 3 , the end of the syringe is sealed with a boot  123 . The central boss  121  of the cap  111  is hollow at the end and a lip  125  of the hollow end is bevelled on its leading edge  157 , but not its trailing edge. Thus, as the cap  111  is installed, the leading edge  157  of the lip  125  rides over a shoulder  159  on the boot  123 . However, as the cap  111  is removed, the trailing edge of the lip  125  will not ride over the shoulder  159 , which means that the boot  123  is pulled off the syringe  114  as the cap  111  is removed. 
       FIG. 4  shows another injection device  210  in which a housing  212  contains a hypodermic syringe  214 . The syringe  214  is again of conventional type, including a syringe body  216  terminating at one end in a hypodermic needle  218  and at the other in a flange  220 , and a rubber bung  222  that constraints a drug  224  to be administered within the syringe body  216 . The conventional plunger that would normally be connected to the bung  222  and used to discharge the contents of the syringe  214  manually, has been removed and replaced with a multi-component drive element as will be described below. Whilst the syringe illustrated is again of hypodermic type, this need not necessarily be so. As illustrated, the housing includes a return spring  226  that biases the syringe  214  from an extended position in which the needle  218  extends from aperture  228  in the housing  212 , to a retracted position in which the hypodermic needle  218  is contained within the housing  212 . The return spring  226  acts on the syringe  214  via a sleeve  227 . 
     At the other end of the housing is a compression drive spring  230 . Drive from the drive spring  230  this transmitted via the multi-component drive to the syringe  214  to advance it from its retracted position to its extended position and discharge its contents through the needle  218 . The drive accomplishes this task by acting directly on the drug  224  and the syringe  214 . Static friction between the bung  222  and the syringe body  216  initially ensures that they advance together, until the return spring  226  bottoms out or the syringe body  216  meets some other obstruction that retards its motion. 
     The multi component drive between the drive spring  230  and the syringe  214  again consists of three principal components. The drive sleeve  231  takes drive from the drive spring  230  and transmits it to flexible latch arms  233  on a first drive element  232 . These elements are shown in detail “A”. The first drive element  232  in turn transmits drive via flexible latch arms  235  to a second drive element  234 . These elements are shown in detail “B”. As before, the first drive element  232  includes a hollow stem  240 , the inner cavity of which forms a collection chamber  242 . The second drive element  234  includes a blind for  246  that is open at one end to receive the stem  240  and closed at the other. As can be seen, the bore  246  and the stem  240  define a fluid reservoir  248 , within which a damping fluid is contained. 
     A trigger (not shown) is provided in the middle of the housing  212 . The trigger, one operated, serves to decouple the drive sleeve  231  from the housing  212  allowing it to move relative to the housing  212  under the influence of the drive spring  230 . The operation of the device is then as follows. 
     Initially, the drive spring  230  moves the drive sleeve  231 , the drive sleeve  231  moves the first drive element  232  and the first drive element  232  moves the second drive element  234 , in each case by acting through the flexible matching arms  233 ,  235 . The second drive element  234  moves and, by virtue of static friction and hydrostatic forces acting through the drug  224  to be administered, moves the syringe body  216  against the action of the return spring  226 . The return spring  226  compresses and the hypodermic needle  218  emerges from the exit aperture  228  of the housing  212 . This continues until the return spring  226  bottoms out or the syringe body  216  meets some other obstruction that retards its motion. Because the static friction between the bung  222  and the syringe body  216  and the hydrostatic forces acting through the drug  224  to be administered are not sufficient to resist the full drive force developed by the drive spring  230 , at this point the second drive element  234  begins to move within the syringe body  216  and the drug  224  begins to be discharged. Dynamic friction between the bung  222  and the syringe body  216  and hydrostatic forces acting through the drug  224  to be administered are, however, sufficient to retain the return spring  226  in its compressed state, so the hypodermic needle  218  remains extended. 
     Before the second drive element  234  reaches the end of its travel within the syringe body  216 , so before the contents of the syringe have fully discharged, the flexible latch arms  235  linking the first and second drive elements  232 ,  234  reach a constriction  237 . The constriction  237  is formed by a component  262  that is attached to the syringe carrier. Additional flexible arms  247  overlie the flexible arms  235  on the first drive element  232 , by means of which drive is transmitted to the second drive element  234 .  FIG. 4  illustrates the injection device  210  at the position where the additional flexible arms  247  are just making contact with the constriction  237  in the component  262 . 
     The constriction  237  moves the additional flexible arms  247  inwards, aided by the bevelled surfaces on both, and the additional flexible arms  247  in turn move the flexible arms  235 , by means of which drive is transmitted from the first drive element  232  to the second drive element  234 , inwards from the position shown to a position at which they no longer couple the first and second drive elements together. Once this happens, the first drive element  232  acts no longer on the second drive element  234 , allowing the first drive element  232  to move relative to the second drive element  234 . 
     Because the damping fluid is contained within a reservoir  248  defined between the end of the first drive element  232  and the blind bore  246  in the second drive element  234 , the volume of the reservoir  248  will tend to decrease as the first drive element  232  moves relative to the second drive element  234  when the former is acted upon by the drive spring  230 . As the reservoir  248  collapses, damping fluid is forced into the collection chamber  242 . Thus, once the flexible latch arms  235  have been released, the force exerted by the drive spring  230  does work on the damping fluid, causing it to flow into the collection chamber  242 , and also acts hydrostatically through the fluid, thence via the second drive element  234 . Losses associated with the flow of the damping fluid do not attenuate the force acting on the body of the syringe to a great extent. Thus, the return spring  226  remains compressed and the hypodermic needle remains extended. 
     After a time, the second drive element  234  completes its travel within the syringe body  216  and can go no further. At this point, the contents of the syringe  214  are completely discharged and the force exerted by the drive spring  230  acts to retain the second drive element  234  in its terminal position and to continue to cause the damping fluid to flow into the collection chamber  142 , allowing the first drive element  232  to continue its movement. 
     A flange  270  on the rear of the second drive element  234  normally retains the flexible arms  233  in engagement with the drive sleeve  231 . However, before the reservoir  248  of fluid is exhausted, the flexible latch arms  233  linking the drive sleeve  231  with the first drive element  232  move sufficiently far forward relative to the second drive element  234  that the flange  270  is brought to register with a rebate  272  in the flexible arms  233 , whereupon it ceases to be effective in retaining the flexible arms  233  in engagement with the drive sleeve  231 . Now, the drive sleeve  231  moves the flexible latch arms  233  inwards from the position shown to a position at which they no longer couple the drive sleeve  231  to the first drive element  232 , aided by the bevelled latching surfaces  274  on the flexible arms  233 . Once this happens, the drive sleeve  231  acts no longer on the first drive element  232 , allowing them to move relative to each other. At this point, of course, the syringe  214  is released, because the forces developed by the drive spring  230  are no longer being transmitted to the syringe  214 , and the only force acting on the syringe will be the return force from the return spring  226 . Thus, the syringe  214  now returns to its retracted position and the injection cycle is complete.