Abstract:
An injection device ( 10; 110, 210 ) is described of the type that receives a syringe, extends it, discharges its contents and then retracts it automatically. The injection device makes use of a fluid-damped delay mechanism.

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. 
     However, problems have arisen in devices such as these that make it difficult to ensure both complete discharge of the syringe contents and reliable release of the syringe from the drive 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 applications no. 0325596 make use of a fluid-damped delay mechanism that is particularly effective in ensuring complete discharge of the syringe contents, but creates problems of its own. Firstly, the use of a fluid-damped delay mechanism requires the creation of a fluid-tight reservoir. Thus, the manufacturing tolerances of those components that define the fluid reservoir must be fine, or seals must be used to prevent the fluid from leaking out before its job is done. Secondly, it is undesirable for the fluid to leak out of its reservoir even when the device has been actuated, because that could give rise to a simulated wet injection, or to the impression that the syringe contents may have leaked within the device. Neither is conducive to the peace-of-mind of self-administered drug users. Again, fine tolerances or seals are called for, which pushes up the price of manufacture. For injection devices that are designed to be disposable, as many will be, every penny counts. 
     SUMMARY OF THE INVENTION 
     The injection devices of the present invention make use of a fluid-damped delay mechanism, but suffer from none of the disadvantages just described, as will now be explained. 
     An injection device according to a first aspect of the present invention comprises:
         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;   an actuator;   a drive acted upon by the actuator and in turn acting on the syringe to advance it from its retracted position to its extended position and discharge its contents through the discharge nozzle;   a decoupling mechanism, activated when the drive has been advanced to a nominal decoupling position, to decouple a first component of the device from a second component, whereupon the first component of the device moves relative to the second component;   a release mechanism, activated when the first component has reached a nominal release position relative to the second, to release the syringe from the action of the actuator, whereupon the biasing means restores the syringe to its retracted position; and   a highly viscous fluid damping the movement of the first component relative to the second, so that the release of the syringe is delayed after the activation of the decoupling mechanism to allow the remaining contents of the syringe to be discharged before the syringe is released.       

     The delay between the activation of the decoupling mechanism and the activation of the release mechanism is used to compensate for any stacking of tolerances. Although triggering of the decoupling mechanism can be designed to occur before the contents of the syringe are fully discharged, the delay is so chosen that, for all variations within the intended tolerances of the components, release of the syringe will not occur until after its contents have been fully discharged. It thus becomes possible to ensure that the syringe contents have been discharged before it is retracted, without having to comply with unrealistically fine tolerances. 
     By “highly viscous fluid” is here meant a fluid that, at 25° C., has a dynamic viscosity of 3000 centiPoise or more. Methods are known in the art for determining the dynamic viscosity of both Newtonian fluids, which are preferred in this invention, and non-Newtonian fluids. A preferred method, which is applicable to both Newtonian and non-Newtonian fluids is described in the Annex to this application. This method derives an average value for dynamic viscosity at shear rates that are determined by the test apparatus and the fluid under test and are reproducible. 
     Greater improvements can be obtained with fluids that, at 25° C., have a dynamic viscosity of 6000 centiPoise or more and, better still, 12000 centiPoise or more. The preferred fluid is DOW CORNING 111 Silicone Compound valve lubricant and sealant which, at 25° C., has a dynamic viscosity of about 12500 centiPoise. 
     Because a highly viscous fluid is, by definition, highly resistant to flow, certain constraints are avoided. Firstly, it is no longer necessary to create a completely fluid-tight reservoir, because imperfections in the reservoir boundaries will not provide an escape route for a that does not flow under the prevailing conditions. Thus, the manufacturing tolerances of those components that define fluid reservoir need not be fine and nor need seals be used. Secondly, simulated wet injections and the impression that the syringe contents may have leaked within the device are problems no longer, since the highly viscous fluid will not flow to a sufficient extent to give rise to these misapprehensions. 
     To reduce the component count and ensure the injection device remains compact, the first and second components of the device may be constituted by first and second elements of the drive, of which the first is acted upon by the actuator and the second acts upon the syringe, the first drive element being capable of movement relative to the second when the former is acted upon by the actuator and the latter is restrained by the syringe. As will be recognised, the relative movement of the first and second drive elements that is damped by the highly viscous fluid, is driven by the actuator. Use of the actuator in this way keeps down the component count. 
     A reservoir for the highly viscous fluid may be defined in part by the first drive element and in part by the second drive element, the volume of the reservoir tending to decrease as the first drive element moves relative to the second when acted upon by the actuator, the reservoir containing the highly viscous fluid and having a vent through which the fluid escapes as the volume of the reservoir decreases. This probably provides the simplest and most compact realisation of the fluid damping mechanism using a highly viscous fluid. 
     An injection device according to a second aspect of the present invention comprises:
         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;   an actuator;   first and second drive elements, of which the first is acted upon by the actuator 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 by the actuator and the latter is restrained by the syringe;   a reservoir defined in part by the first drive element and in part by the second drive element, the volume of the reservoir tending to decrease as the first drive element moves relative to the second when acted upon by the actuator, the reservoir containing a highly viscous fluid and having a vent through which the fluid escapes as the volume of the reservoir decreases; and   a release mechanism, activated when the first drive element has been advanced to a nominal release 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 this aspect of the invention, account is taken of the fact that, where the highly viscous fluid damps relative movement of two elements of the drive, the decoupling of the two drive elements need not be accomplished by a decoupling mechanism. Other possibilities exist, including the use of two components that include a frangible coupling or no coupling other than that provided by static friction between the two components. Nonetheless, a delay between the decoupling of the drive elements and the activation of the release mechanism is present, and is used as described above. 
     Thus, the injection device may further comprise a coupling that prevents the first drive element from moving relative to the second until they have been advanced to a nominal decoupling position that is less advanced than the said nominal release position. The coupling may, and preferably does, comprises a decoupling mechanism, activated when the drive elements have been advanced to the said nominal decoupling position. 
     Two forms of coupling and decoupling mechanisms are specifically proposed, although it is acknowledged that other possibilities exist. In its first form, the coupling is a third drive element acting upon the first and second drive elements. In this case, the decoupling mechanism is adapted to decouple the third drive element from the second so that the third drive element acts only it no longer once the said nominal decoupling position has been reached, thus allowing the first drive element to move relative to the second, and the release mechanism is adapted to decouple the third drive element from the first so that the third drive element acts upon it no longer once the said nominal release position has been reached, thus releasing the syringe from the action of the actuator. 
     In its second form, the coupling comprises cooperating features of the first and second drive elements allowing the first to act upon the second. In this case, the decoupling mechanism is adapted to decouple the first drive element from the second so that the first drive element acts no longer on the second once the said nominal decoupling position has been reached, thus allowing the first drive element to move relative to the second, and the release mechanism is adapted to decouple the first drive element from the actuator so that the actuator acts upon it no longer once the said nominal release position has been reached, thus releasing the syringe from the action of the actuator. 
     In general, for simplicity of manufacture of the component parts by injection moulding, one drive element may include a stem and the other a bore that is open at one end to receive the stem, the bore and the stem thus defining the fluid reservoir. 
     To reduce further the possibility of simulated wet injections or the impression that the syringe contents may have leaked within the device, the vent may be in communication with a collection chamber defined by one drive element, within which the escaped fluid is collected. In this case, for simplicity of manufacture, it is preferred that one drive element include a stem and define the vent and the collection chamber and the other drive element include a blind bore that is open at one end to receive the stem and closed at the other, the bore and the stem thus defining the fluid reservoir. Again, for greater simplicity of manufacture by injection moulding, the collection chamber may be defined by a bore in the said one element, being open at one end and closed at the other but for the vent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of a first embodiment; 
         FIG. 2  is a second; and 
         FIG. 3  is likewise a third. 
         FIG. 4  is a schematic view of a liquid filled damper in accordance with the present invention. 
         FIG. 5  is a schematic view of a test apparatus in accordance with the present invention. 
         FIG. 6  is a schematic view of a test apparatus in accordance with the present invention. 
         FIG. 7  is a schematic view of a test apparatus in accordance with the present invention. 
         FIG. 8  is a schematic view of a test apparatus in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an injection device  10  in which a housing  12  contains a hypodermic syringe  14 . The syringe  14  is of conventional type, including a syringe body  16  terminating at one end in a hypodermic needle  18  and at the other in a flange  20 , and a rubber bung  22  that constrains a drug  24  to be administered within the syringe body  16 . The conventional plunger that would normally be connected to the bung  22  and used to discharge the contents of the syringe  14  manually has been removed and replaced with a drive element as will be described below. 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  18 . As illustrated, the housing includes a return spring  26  that biases the syringe  14  from an extended position in which the needle  18  extends from an aperture  28  in the housing  12  to a retracted position in which the discharge nozzle  18  is contained within the housing  12 . 
     At the other end of the housing is an actuator, which here takes the form of a compression drive spring  30 . Drive from the drive spring  30  is transmitted via a multi-component drive to the syringe  14  to advance it from its retracted position to its extended position and discharge its contents through the needle  18 . The drive accomplishes this task by acting on the bung  22 . Static friction between the bung  22  and the syringe body  16  initially ensures that bung  22  and body  16  advance together, until the return spring  26  bottoms out or the syringe body  16  meets some other obstruction (not shown) that retards its motion. 
     The multi-component drive between the drive spring  30  and the syringe  14  consists of three principal components. A first drive element  32  and a second drive element  34  are each acted upon by a third drive element  36 , in internal shoulder  38  of which is acted upon by the drive spring  30 . Thus, the drive spring  30  causes the third drive element  36  to move, which in turn causes the first and second drive elements  32 ,  34  to move in tandem. The third drive element  36  is coupled to the first and second drive elements  32 ,  34  by means of respective ball latches  52 ,  54 , of which more later. 
     The first drive element  32  includes a hollow stem  40 , the inner cavity of which forms a collection chamber  42  in communication with a vent  44  that extends from the collection chamber through the end of the stem  40 . The second drive element  34  includes a blind bore  46  that is open at one end to receive the stem  40  and closed at the other. As can be seen, the bore  46  and the stem  40  defining a fluid reservoir  48 , within which a highly viscous fluid is contained. 
     A trigger  50  is provided at the end of the housing  12  remote from the exit aperture  28  for the hypodermic needle  18 . The trigger, when operated, serves to decouple the third drive component  36  from the housing  12 , allowing it to move relative to the housing  12  under the influence of the drive spring  30 . The operation of the device is then as follows. 
     Initially, the drive spring  30  moves the third drive element  36  and the third drive element  36  moves the first and second drive elements  32 ,  34  by acting through the ball latches  52 ,  54 . The second drive element  34  moves the rubber bung  22 , which by virtue of static friction and hydrostatic forces acting through the drug  24  to be administered moves the syringe body  16  against the action of the return spring  26 . The return spring  26  compresses and the hypodermic needle  18  emerges from the exit aperture  28  of the housing  12 . This continues until the return spring  26  bottoms out or the syringe body  16  meets some other obstruction (not shown) that retards its motion. Because the static friction between the bung  22  and the syringe body  16  and the hydrostatic forces acting through the drug  24  to be administered are not sufficient to resist the full drive force developed by the drive spring  30 , at this point the bung  22  begins to move within the syringe body  16  and the drug  24  begins to be discharged. Dynamic friction between the bung  22  and the syringe body and hydrostatic and hydrodynamic forces now acting through the drug  24  to be administered are, however, sufficient to retain the return spring  26  in its compressed state, so the hypodermic needle  18  remains extended. 
     Before the bung  22  reaches the end of its travel within the syringe body  16 , so before the contents of the syringe have fully discharged, the ball latch  54  linking the third drive element  36  with the second drive element  34  reaches a region  56  of the housing  12  at which the inner diameter of the housing  12  is enlarged. The balls in the ball latch  54  move laterally outwards from the position shown to a position at which they no longer couple the third drive element  36  to the second drive element  34 , aided by the bevelled surfaces on the second drive element  34 , fast against which they are normally retained by the inner surface of the housing  12 . Once this happens, the third drive element  36  acts no longer on the second drive element  34 , allowing the first and third drive elements  32 ,  36  to move relative to the second drive element  34 . 
     Because the highly viscous fluid is contained within a reservoir  48  defined between the end of the first drive element  32  and the blind bore  46  in the second drive element  34 , the volume of the reservoir  46  will tend to decrease as the first drive element  32  moves relative to the second drive element  34  when the former is acted upon by the drive spring  30 . As the reservoir  48  collapses, highly viscous fluid is forced through the vent  44  into the collection chamber  42 . Thus, once the ball latch  54  has been released, some of the force exerted by the drive spring does work on the highly viscous fluid, causing it to flow though the constriction formed by the vent  44 ; the remainder acts hydrostatically through the fluid and through friction between the first and second drive elements  32 ,  34 , thence via the second drive element  34  and onto the bung  22 . Losses associated with the flow of the highly viscous fluid do not attenuate the force acting on the body of the syringe to a great extent. Thus, the return spring  26  remains compressed and the hypodermic needle remains extended. 
     It has been found that with a highly viscous fluid possessing a dynamic viscosity of 12,000 centistokes or more, the vent  44  may consist of a circular aperture 0.7 mm in diameter. This is a relatively large diameter and is easy to form using conventional injection moulding techniques. Thinner fluids require smaller holes and thicker ones require larger holes. Forcing such a fluid through such a vent  44  is effective to damp the movement of the first and second drive elements  32 ,  34  relative to each other. Moreover, such a fluid resists flow to such an extent that it will not, under its own weight, flow from the open end of the collection chamber  42 . Thus, the collection chamber  42  need not be closed at the end remote from the vent  44 , making the first drive element  32  easy to manufacture by injection moulding. 
     After a time, the bung  22  completes its travel within the syringe body  16  and can go no further. At this point, the contents of the syringe  14  are completely discharged and the force exerted by the drive spring  30  acts to retain the bung  22  in its terminal position and to continue to cause the highly viscous fluid to flow though the vent, allowing the first drive element  32  to continue its movement. 
     Before the reservoir  48  of fluid is exhausted, the ball latch  52  linking the third drive element  36  with the first drive element  32  reaches the region  56  of the housing  12  at which the inner diameter of the housing  12  is enlarged. The balls in the ball latch  52  move laterally outwards from the position shown to a position at which they no longer couple the third drive element  36  to the first drive element  32 , aided by the bevelled surfaces on the first drive element  32 , fast against which they are normally retained by the inner surface of the housing  12 . Once this happens, the third drive element  36  acts no longer on the first drive element  32 , allowing the first and third drive elements  32 ,  36  to move relative each other. At this point, of course, the syringe  14  is released, because the forces developed by the drive spring  30  are no longer being transmitted to the syringe  14 , and the only force acting on the syringe will be the return force from the return spring  26 . Thus, the syringe  14  is now returned to its retracted position and the injection cycle is complete. 
       FIG. 2  shows another injection device  110  in which a housing  112  contains a hypodermic syringe  114 . The syringe  114  is again 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 have been removed and replaced with a drive element  134  as will be described below, which terminates in 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. 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 an actuator, which here takes the form of 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 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 highly viscous fluid is contained. 
     A trigger (not shown) is provided in the middle 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  32  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  moves and, by virtue of static friction and hydrostatic forces acting through the drug  124  to be administered, moves 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 second drive element  134  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 second drive element  134  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  within the housing  112 . 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 highly viscous 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  146  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, highly viscous 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 highly viscous fluid, causing it to flow though the constriction formed by the vent  144 , and acts hydrostatically through the fluid and through friction between the first and second drive elements  132 ,  134 , thence via the second drive element  134 . Losses associated with the flow of the highly viscous 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 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 highly viscous fluid to flow though 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  within the housing  112 . 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 each other. At this point, of course, the syringe  114  is released, because the forces developed by the drive spring  130  are 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  is now returned 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 that fits within the sleeve  119  when the cap  111  is installed on the housing  112 , is hollow at the end and the 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. 3  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 . Hydrostatic forces acting through the drug  224  and, to a lesser extent, static friction between the bung  222  and the syringe body  216  initially ensure 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 highly viscous 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 initially free to move relative to all other components, but that is constrained between the syringe flange  220  and additional flexible arms  247  on the second drive element  234 . These 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. 3  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 highly viscous 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, highly viscous 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 highly viscous fluid, causing it to flow into the collection chamber  242 , and also acts hydrostatically through the fluid and through friction between the first and second drive elements  232 ,  234 , thence via the second drive element  234 . Losses associated with the flow of the highly viscous 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 highly viscous 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 highly viscous 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. 
     In the injection devices described, and in any injection device according to the invention, the highly viscous fluid may be any fluid that has the appropriate properties. Silicone oil and silicone grease are examples of fluids that may be selected to have a kinematic viscosity at 20° C. of 12500 centistokes or more. Moreover, both are excellent lubricators and certainly silicone grease is sufficiently resistant to flow that it will not accidentally discharge from the open end of the collection chamber. Furthermore, the reservoir in the second drive element is simple to fill before stem of the first drive element is pushed into place. The volume of fluid need not be accurately controlled, since excess fluid will be expelled into the collection chamber. The fluid will then fills the vent and prevents the ingress of dirt or other contaminants that could lead to blockage. 
     Although preferred damping mechanisms using a highly viscous fluid have been described, it will of course be understood that other damping mechanisms that use a highly viscous fluid are possible. Thus, the highly viscous fluid may be used to damp the movement of components of the device other than elements that transmit drive from the drive actuator to the syringe. Many of the advantages associated with the use of a highly viscous fluid are independent of the other details of the damping mechanism. 
     A functional upper limit on the dynamic viscosity of the highly viscous fluid is set by the need for it to act as an effective damper. In practical embodiments of this invention, including the embodiments just described, it is unlikely that dynamic viscosities in excess of 150,000 centiPoise would be effective. Even fluids with dynamic viscosities in excess of 60,000 centiPoise would appear to have limited applicability. 
     ANNEX 
     Measurement of Dynamic Viscosity 
     1. Introduction 
     A liquid filled damper has been proposed for use in the device. The damper consists of a small bore filled with fluid and a hollow piston with a small hole in the centre. When a force is applied the fluid is forced through the hole and into the centre of the piston. 
     This document describes a test method by which the dynamic viscosity of the fluid may be determined. 
     2. Description of Damper 
     3. Theoretical Treatment of Flow Through Bleed Hole 
     3.1 Derivation 
     The following analysis is applied to laminar flow of fluid in an axisymmetric pipe, in this case the bleed hole in the piston. 
     Resolving forces on a cylindrical element: 
     
       
         
           
             
               
                 
                   
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       τ 
                     
                     = 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       P 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       A 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     A 
                     = 
                     
                       
                         
                           π 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             r 
                             2 
                           
                         
                         ⁢ 
                         
                           
 
                         
                         ∴ 
                         τ 
                       
                       = 
                       
                         
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             P 
                           
                           L 
                         
                         ⁢ 
                         
                           r 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   i 
                   ) 
                 
               
             
           
         
       
     
     Assuming the fluid is Newtonian and referencing flow from the centreline: 
     
       
         
           
             
               
                 
                   
                     
                       
                         τ 
                         = 
                         
                           
                             - 
                             µ 
                           
                           ⁢ 
                           
                             
                               ⅆ 
                               u 
                             
                             
                               ⅆ 
                               r 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           
                             Equating 
                             ⁢ 
                             
                               
                                   
                               
                               ⁢ 
                               
                                   
                               
                             
                             ( 
                             i 
                             ) 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           and 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ( 
                             ii 
                             ) 
                           
                         
                         : 
                       
                     
                   
                   
                     
                       
                         
                           
                             
                               
                                 - 
                                 Δ 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               P 
                             
                             
                               2 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               µL 
                             
                           
                           ⁢ 
                           r 
                         
                         = 
                         
                           
                             ⅆ 
                             u 
                           
                           
                             ⅆ 
                             r 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   ii 
                   ) 
                 
               
             
           
         
       
     
     Integrating: 
     
       
         
           
             u 
             = 
             
               
                 
                   
                     - 
                     Δ 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     Pr 
                     2 
                   
                 
                 
                   4 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   µL 
                 
               
               + 
               
                 C 
                 1 
               
             
           
         
       
     
     Boundary conditions: 
     u=0 at r=R 
     u=max at r=0 
     
       
         
           
             
               
                 
                   
                     ∴ 
                     
                       C 
                       1 
                     
                   
                   = 
                   
                     
                       
                         
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             P 
                           
                           L 
                         
                         ⁢ 
                         
                           
                             R 
                             2 
                           
                           
                             4 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             µ 
                           
                         
                       
                       ⁢ 
                       
                         
 
                       
                       ∴ 
                       u 
                     
                     = 
                     
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           P 
                         
                         
                           4 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           µL 
                         
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             R 
                             2 
                           
                           - 
                           
                             r 
                             2 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   iii 
                   ) 
                 
               
             
           
         
       
     
     Examining an annular element:
 
δQ=uδA Elemental volumetric flow rate
 
δA≈2πrδr
 
     
       
         
           
             
               ∴ 
               
                 δ 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Q 
               
             
             = 
             
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   P 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   π 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   r 
                 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   µL 
                 
               
               ⁢ 
               
                 ( 
                 
                   
                     R 
                     2 
                   
                   - 
                   
                     r 
                     2 
                   
                 
                 ) 
               
               ⁢ 
               δ 
               ⁢ 
               
                   
               
               ⁢ 
               r 
             
           
         
       
     
     Integrating between r=0 and r=R: 
     
       
         
           
             
               
                 
                   Q 
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         P 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         π 
                       
                       
                         2 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         µL 
                       
                     
                     ⁢ 
                     
                       
                         ∫ 
                         O 
                         R 
                       
                       ⁢ 
                       
                         
                           r 
                           ⁡ 
                           
                             ( 
                             
                               
                                 R 
                                 2 
                               
                               - 
                               
                                 r 
                                 2 
                               
                             
                             ) 
                           
                         
                         ⁢ 
                         δ 
                       
                     
                   
                 
               
               
                 
                   Volumetric 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   flow 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   rate 
                 
               
             
             
               
                 
                   Q 
                   = 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       P 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         R 
                         4 
                       
                     
                     
                       8 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       µL 
                     
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   Q 
                   = 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       P 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         d 
                         4 
                       
                     
                     
                       128 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       µL 
                     
                   
                 
               
               
                 
                     
                 
               
             
           
         
       
     
     For the case of the damper, neglecting friction in the piston, mass of the piston and assuming a perfect seal between piston and bore: 
     
       
         
           
             
               
                 
                   Q 
                   = 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       P 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         d 
                         4 
                       
                     
                     
                       128 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       µL 
                     
                   
                 
               
               
                 
                   ( 
                   iv 
                   ) 
                 
               
             
           
         
       
         
         
           
             Volumetric flow rate through bleed hole 
           
         
       
    
     
       
         
           
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               P 
             
             = 
             
               
                 4 
                 ⁢ 
                 F 
               
               
                 π 
                 ⁡ 
                 
                   ( 
                   
                     
                       d 
                       2 
                       2 
                     
                     - 
                     
                       d 
                       1 
                       2 
                     
                   
                   ) 
                 
               
             
           
         
       
         
         
           
             (v) 
             Pressure from piston above atmospheric pressure
 
Q=A piston v piston  
 
           
         
       
    
     
       
         
           
             
               ∴ 
               
                 v 
                 piston 
               
             
             = 
             
               
                 4 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Q 
               
               
                 π 
                 ⁡ 
                 
                   ( 
                   
                     
                       d 
                       2 
                       2 
                     
                     - 
                     
                       d 
                       1 
                       2 
                     
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             
               v 
               piston 
             
             = 
             
               
                 ΔP 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 d 
               
               
                 32 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   µL 
                   ⁡ 
                   
                     ( 
                     
                       
                         d 
                         2 
                         2 
                       
                       - 
                       
                         d 
                         1 
                         2 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
       
         
           
             
               v 
               piston 
             
             = 
             
               
                 Fd 
                 1 
                 4 
               
               
                 8 
                 ⁢ 
                 π 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   µL 
                   ⁡ 
                   
                     ( 
                     
                       
                         d 
                         2 
                         4 
                       
                       + 
                       
                         d 
                         1 
                         4 
                       
                       - 
                       
                         2 
                         ⁢ 
                         
                           d 
                           1 
                           2 
                         
                         ⁢ 
                         
                           d 
                           2 
                           2 
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
       
         
           
             
               1 
               
                 v 
                 piston 
               
             
             = 
             
               
                 
                   8 
                   ⁢ 
                   π 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     µL 
                     ⁡ 
                     
                       ( 
                       
                         
                           d 
                           2 
                           4 
                         
                         + 
                         
                           d 
                           1 
                           4 
                         
                         - 
                         
                           2 
                           ⁢ 
                           
                             d 
                             1 
                             2 
                           
                           ⁢ 
                           
                             d 
                             2 
                             2 
                           
                         
                       
                       ) 
                     
                   
                 
                 ⁢ 
                 
                     
                 
               
               
                 F 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   d 
                   1 
                   4 
                 
               
             
           
         
       
         
         
           
             Volumetric flow rate equal to rate at which piston displaces fluid (vi) 
             Substituting (iv) for Q 
             Substituting (v) for P 
             Time delay per unit of piston travel
 
4. Apparatus and Method
 
           
         
       
    
     The test apparatus shall consist of two rigid, rotationally symmetrical, coaxial bodies as are illustrated schematically in  FIG. 5 . One contains a cylindrical bore having an internal diameter in the range 4.45 to 4.55 mm. Let this diameter be d 2 . It also includes a coaxial, circular bleed hole having a diameter in the range 0.65 to 0.75 mm. Let this diameter be d 1 . The length of the bleed hole is in the range 1.95 to 2.05 mm. Let this length be L. The bleed hole leads to a collection chamber, also of diameter d 2 . 
     The second coaxial body has a hollow cylindrical piston that forms a sufficiently good seal with the bore in the other body that there is no significant loss of fluid between the cylindrical surfaces of the bodies during the course of the test. Any force necessary to overcome dynamic friction between the cylindrical surfaces of the bodies can be measured in the presence of an amount of test fluid sufficient to lubricate the interface. 
     With the bleed hole temporarily stopped, the first coaxial body is inverted and a sample of the fluid to be tested is introduced into the cylindrical bore to a depth of at least 6 mm. The second coaxial body is then inserted into the first. The apparatus is then righted and the bleed hole unstopped. The second coaxial body is held stationary and the first is lowered until the fluid emerges from the bleed hole, where it is collected. There must be at least 5 mm of travel remaining at this stage. 
     A downward force is applied to the first coaxial body causing it to move. The size of this force is such that the net force acting on the surface of the fluid, which is the applied force, less the force necessary to overcome dynamic friction between the cylindrical surfaces of the bodies, plus the weight of the first coaxial body, is in the range 9.95 to 10.05 N. Let this net force be F. 
     A position transducer is attached to the second coaxial body and to a data logger, by means of which a plot of position vs. time is obtained. Once the second coaxial body has moved by at least 1.5 mm in response to the applied force, the time taken for it to move by a further 2.0 mm is measured from the position vs. time plot. At least 1.5 mm of travel must remain after this 2 mm interval. This time measured is divided by two to yield an average time to travel 1.0 mm. Let this time be t 1 . 
     According to the analysis presented above, if v piston  is measured in SI units, 
               1     v   piston       =       1000     t   1       =       8   ⁢   π   ⁢           ⁢     µL   ⁡     (       d   2   4     +     d   1   4     -     2   ⁢     d   1   2     ⁢     d   2   2         )           Fd   1   4               
Or, in other words,
 
     
       
         
           
             µ 
             = 
             
               
                 125 
                 ⁢ 
                 
                   t 
                   1 
                 
                 ⁢ 
                 
                   Fd 
                   1 
                   4 
                 
               
               
                 π 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   L 
                   ⁡ 
                   
                     ( 
                     
                       
                         d 
                         2 
                         4 
                       
                       + 
                       
                         d 
                         1 
                         4 
                       
                       - 
                       
                         2 
                         ⁢ 
                         
                           d 
                           1 
                           2 
                         
                         ⁢ 
                         
                           d 
                           2 
                           2 
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     Thus is the dynamic viscosity determined. 
     The test procedure is to be repeated another four times with different samples of the fluid and the mean of the five results obtained is taken as the dynamic viscosity of the fluid. 
     This procedure is applicable to both Newtonian and non-Newtonian fluids. Especially in the case of fluids that depart considerably from Newtonian behaviour, the various dimensions of the apparatus and the applied force should be exactly at the mid-point of the ranges given above.