Abstract:
An injection device for use with a syringe having a syringe body, a needle coupled thereto, and a plunger moveable through the body to eject medication from the syringe through the needle, includes a housing defining an inner space for holding a syringe and facilitating movement thereof from a first position in which the needle is contained fully within the housing and a second position in which the needle projects from the housing, and a drive member located within the inner space so that in use the drive member can engage the plunger of a syringe held within the housing. The device further includes a force applicator located within the inner space for providing a force to the drive member so as to move the syringe from the first to the second position and thereafter to move the plunger through the syringe body, a trigger mechanism, and a damper.

Description:
TECHNICAL FIELD 
     The present invention relates to injection devices of the type used to inject medication into patients. In particular, the invention relates to the provision of a damping mechanism within such injection devices. 
     BACKGROUND 
     Spring driven injection devices are known and used to assist patients and medical professionals with the injection of medications held in conventional plunger driven syringes. Such injection devices typically comprise a housing for containing the syringe to which a needle is fixed, and a drive member attached to a relatively strong compression spring for driving the plunger into the syringe body to eject medication through the needle and into a patient. 
     By way of example, International patent application publication number WO94/21316 describes an injection device substantially as illustrated in  FIG. 1 . This device is intended to completely eject all medication from a syringe, i.e. it has no mechanism for adjusting the ejected dose. The injection device comprises a distal housing part  1  into which is screwed a proximal housing part  2  (where the terms “distal” and “proximal” are used here and in the following text to identify respectively locations distant from and close to a patient&#39;s skin when the device is in use). An adjustable needle shield  3  is further screwed into a proximal end of the proximal housing part  2 . A housing sleeve  4  is slideably mounted on top of the distal housing part  1 . As is shown in  FIG. 1 , the injection device comprises a trigger  5  which projects through a window  6  provided in the sleeve  4 . 
       FIG. 2  illustrates the prior art device of  FIG. 1  in cross-section. The device is shown with a syringe  7  located within the housing, where the syringe comprises a syringe body  8  having a pair of radially extending “wings”  9  at one end and a needle coupling  10  formed at the other end. A needle  11  is shown attached to the syringe body using the needle coupling  10 . The syringe also comprises a plunger  12  having a rubber bung  13  attached to one end and an enlarged end face or plate  14  formed at the other end to assist in pushing the plunger through the syringe body. The syringe is supported within the proximal housing part  2  by a short support tube  19  that sits coaxially within the proximal housing part  2 . A collar  23  is provided around the distal end of the support tube  19 , and provides an end stop for a relatively light coil spring  24 . The other end of the coil spring  24  is stopped by a locking tube  25  which is screwed to an interior surface of the proximal housing part  1 . The locking tube  25  also fixes the support tube  19  into the proximal housing part  2  such that this support tube is fixed relative to the proximal housing part. 
     As shown in  FIG. 2 , a drive member  15  is slideably located within the distal housing part  1 . The drive member is generally cylindrical having a generally square cross-section. Located within the interior space of the drive member  15  is a relatively strong compression drive spring  16 . The drive spring is supported on a cylindrical support shaft  17  which is in turn fixably attached to the distal housing part  1  via an elongate pin  18 . 
       FIG. 2  illustrates the device in the primed or cocked position, where the drive member  15  has been pushed into the distal housing part  1  in the distal direction, thereby compressing the drive spring  16  along the support shaft  17 .  FIG. 2  shows the trigger  5  pivotally coupled to the distal housing part  1  and partially projecting through the window  6  formed in the sleeve  4 . The trigger comprises at an outer distal end thereof a button part  20  and, at an inner proximal part, a catch  21 . In the position illustrated in  FIG. 2 , the catch  21  is engaged with a corresponding recess  22  formed in the drive member  15  such that the drive member is held by the trigger in the primed state. Although not shown in the figures, the trigger  5  is biased about its pivot axis such that the catch is pressed against the drive member  15  in the absence of any external force. 
     In order to release the drive member  15  from the primed state and thereby push the needle  11  into a patient&#39;s skin and inject medication from the syringe  7 , a user holds the sleeve  4  in a fist-like grip and presses the needle shield  3  against his or her skin. This causes the sleeve  4  to slide over the distal housing part housing  1 , closing the gap identified in  FIG. 2  with the reference A. This relative movement of the sleeve and the distal housing part causes the window  6  to move wholly over the trigger  5  such that, when the user exerts pressure on the trigger button part  20  with his or her thumb, the trigger pivots, disengaging the catch  21  from the recess  22 . Once the drive member  15  is released, the force exerted by the expanding drive spring  16  pushes the drive member  15  forward and into engagement with the end plate  14  of the syringe plunger  12 . 
     At this stage in the operation of the device, the syringe body  8  is relatively free to move through the proximal housing part  2  such that the force exerted by the drive member  15  on the plunger end plate  14  causes the entire syringe  7  to move towards the distal end of the injection device. The wings  9  formed on the end of the syringe body  8  act on the support tube collar  23 , causing the support tube  19  to also move towards the proximal end of the device, compressing the relatively light coil spring  24 . Once this spring is fully compressed, further movement of the support tube  19  and therefore of the syringe body  8  is prevented. Of course, during this preliminary phase, the needle is projected beyond the end of the needle shield and into the patient&#39;s skin. The extent to which the needle projects beyond the end of the needle shield  3  can be adjusted by screwing the shield into and out of the proximal end of the proximal housing part  2 . 
     As the driver member  15  continues to exert a significant force on the plunger  12  after further movement of the support tube  19  and the syringe body  8  has been prevented, the plunger  12  begins to move through the syringe body  8  thereby ejecting medication through the needle  11 . 
     It is noted that the device illustrated in  FIGS. 1 and 2  comprises a further, relatively light compression spring  26  that is connected between the distal housing part  1  and the sleeve  4 . This spring  26  is relatively light, and allows the sleeve  4  to be returned to its original position once an injection has been completed and the user has both removed the needle shield  3  from contact with the skin and has released the pressure on the trigger button  20 . 
     In order to assemble the device of  FIGS. 1 and 2 , the proximal and distal housing parts are separated, and a complete syringe including medication is dropped into the proximal housing part through the larger open end thereof so that it is supported within the support tube  19 . With the housing parts still separated, the drive member  15  is pressed into the distal housing part  1  (using, for example, a cocking tool) until the catch  21  on the trigger  5  engages with the drive member recess  22  and locks the drive member in the cocked position. The device can then be assembled by screwing the proximal and distal housing parts together. 
     In order to take into account different tolerances within the injection device and the syringe, the injection device is configured such that, when the device is fully assembled, a small gap is present between the end plate  14  of the plunger  12  and the opposed end of the drive member  15 . This gap is identified in  FIG. 2  with the reference B. In the absence of such a gap, the drive member could come into contact with the end plate of the plunger during connection of the housing parts, causing some movement of the syringe through the lower housing towards the distal end of the device. It is also possible that some initial sticking of the syringe body could occur, resulting in the ejection of a small volume of medication from the syringe. 
     A potential problem observed in the prior art device such as that illustrated in  FIGS. 1 and 2  arises from the relatively high force exerted by the drive member  15  on the syringe plunger  12 . A high force is of course required in order to eject the medication through the needle and into the patient. However, the initial impact caused by the drive member  15  hitting the end plate  14  of the syringe plunger  12  may damage the plunger and or the drive member. For example, it is possible that the end of the plunger may fracture. It is also possible that the collar  23  formed on the support tube  19  and/or the syringe wings  9  may be damaged when the spring  24  is compressed to its full extent, i.e. immediately prior to injection of medication from the syringe. 
     SUMMARY 
     It is an object of the present invention to overcome or at least mitigate the problems identified with known injection devices including the problem discussed above. This object is achieved by incorporating a damping mechanism into the device such that the kinetic energy imparted by the drive member to the syringe plunger is reduced during an initial operating phase. 
     According to a first aspect of the present invention there is provided an injection device for use with a syringe having a syringe body, a needle coupled to the syringe body, and a plunger which can be moved through the syringe body to eject medication from the syringe through the needle. The injection device comprises a housing defining an inner space for holding a syringe and facilitating movement of the syringe from a first position in which the needle is contained fully within the housing and a second position in which the needle projects from the housing, and a drive member located within said inner space so that in use the drive member can engage the plunger of a syringe held within the housing. The device further comprises a force applicator located within said inner space for providing a force to the drive member so as to move the syringe from said first to said second position and thereafter to move the plunger through the syringe body, a trigger mechanism for retaining the spring and the drive member in a cocked configuration and, upon activation, for releasing the spring and the drive member, and a damper for damping the kinetic energy imparted by the drive member to the syringe plunger during all or a part of the movement of the syringe from said first to said second position. 
     Due to the damping of the drive member, impact forces on the syringe are reduced, thereby reducing the risk of damage to the syringe and the device during use. 
     The housing may comprise a first housing part for holding said syringe and facilitating said movement of the syringe, and a second housing part having a second coupling for engagement with said first coupling to secure the first and second housing parts together, said drive member being located within the second housing part so that in use the drive member can engage the plunger of a syringe held within the first housing part. Alternatively, the housing may comprise a single main body part. The housing may further comprise a needle shield adjustably coupled to other housing parts. 
     The force applicator may be one of:
         a compression spring;   a torsion spring; and   an electrically driven applicator.       

     The damper may be configured to damp kinetic energy imparted by the drive member to the syringe plunger from a cocked position of the plunger, substantially until the syringe has reached said second position. The damper may be one of:
         a pneumatic damper;   a hydrodynamic damper; and   and a friction damper.       

     The damper may be a pneumatic damper comprising a piston having a piston head in engagement with said drive member and a cylinder within which the piston is mounted, said piston and cylinder defining a chamber and one of the piston and cylinder defining a passage for delimiting a first fluid flow path into and out of said chamber. Said piston and cylinder may be configured such that movement of the piston outwardly with respect to the cylinder beyond a point defined by said second position creates a second fluid flow path into said chamber, reducing or eliminating damping of the drive member. The piston and cylinder may be further configured so that the inner surface of an end of the cylinder tapers radially outward with respect to an end of said piston, thereby establishing said second fluid flow path into said chamber following outward movement of the piston within the cylinder beyond a predefined point. 
     The device may comprise an O-ring mounted on an external surface of said piston or on an internal surface of said cylinder and forming a substantially airtight seal between the piston and the cylinder up to said predefined point. 
     Where said force applicator comprises a spring, said cylinder may be provided by a spindle fixedly coupled to said second housing part, with said spring being coaxially mounted around the outside of the spindle. The piston may extend coaxially inside said spring. 
     The piston head being fixed to the drive member. Alternatively, the piston may be formed integrally with said drive member. 
     The force applicator may comprises a spring, with said drive member being generally cylindrical and defining an inner space within which said spring is located. The trigger mechanism may be configured to engage with said drive member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically a prior art injection device for use with a standard syringe; 
         FIG. 2  shows a cross-section through the prior art injection device of  FIG. 1 ; 
         FIG. 3  shows an exploded view of a distal part of an injection device, including a drive member damping mechanism; 
         FIG. 4  shows a cross-sectional view of the distal part of  FIG. 3 , assembled and in an uncocked state; 
         FIG. 5  shows a cross-sectional view of the assembled distal part of  FIG. 4  in a cocked state; and 
         FIG. 6  shows a cross-sectional view of the assembled distal part of  FIG. 4  in an intermediate firing state. 
     
    
    
     DETAILED DESCRIPTION 
     There will now be described, with reference to  FIGS. 3 to 6  of the accompanying drawings, an improved injection device that is suitable for use with a standard syringe  7  such as has already been described with reference to  FIGS. 1 and 2 . Such a syringe includes a generally cylindrical syringe body  8  having provided at a proximal end thereof a coupling for attachment of a needle. The syringe body is provided at its distal end with pair of radially extending wings  9 , intended to aid holding of the syringe during manual use. The syringe  7  has a needle  11  attached to the needle coupling, whilst a plunger  12  is inserted into the distal end of the syringe body  8  and projects therefrom. As is well known in the art, the plunger  12  has a bung  13  attached to the end that is inserted into the syringe body  8  in order to seal a medication contained with the syringe body  8 . 
     The improved injection device is able to re-use certain components of known devices. In particular, in the embodiment that will now be described, the proximal device part including the proximal housing part  1 , needle shield  3 , support tube  19 , locking tube  25 , and coil spring  24 , is substantially identical to that described with reference to  FIGS. 1 and 2 . Modifications to the known device primarily concern distal device part. In the following discussion, it is assumed that the distal device component illustrated in  FIGS. 3 to 6  is merely connected to the proximal device part of the injection device of  FIGS. 1 and 2  using the respective screw threads. 
     Referring first to  FIGS. 3 and 4 , the distal device part  30  comprises a distal housing part  31  that has a generally cylindrical shape with a rectangular cross-section. At the distal end  32  of the distal housing part  31  (that is the leftmost end as illustrated in the Figure), the cross-section changes to circular to provide a coupling having a screw thread formed around its outer surface suitable for engagement with the screw thread formed on an inner surface of a proximal device part. A trigger  33  is formed integrally with the distal housing part  31  and is provided with a trigger button  34  at one end and a catch  35  at the other end. The trigger is able to pivot about a central pivot access. The trigger  33  is formed such that, when a pressure is applied to the trigger button  34  in a radially inward direction, the catch  35  moves in a radially outward direction, such that a subsequent release of this pressure results in the trigger  33  returning to its original position. The operation of the trigger  33  is thus broadly in line with that described with reference to  FIGS. 1 and 2 . 
     The distal housing part  31  is contained within a sleeve  36 , with a light coil spring  37  interposed between an end face of the distal housing part  31  and an inner end face of the sleeve  36 . The sleeve  36  is formed with a window  38  through which a part of the trigger  33 , including the trigger button  34 , projects. A generally cylindrical support shaft  39  is centrally mounted along the axis of the device, within the distal housing part  31 . The support shaft  39  is held in place by an elongate locking pin  40 , with a bump  41  on the locking pin  40  engaging a recess  42  formed on an inner surface of the support shaft  39  (see  FIG. 4 ). A main drive spring  43  is located coaxially over the support shaft  39  and is free to move over the support shaft in the absence of any further restrictions. The support shaft  39  is formed such that a central, cylindrical space within the spindle is separated into a proximal chamber  44  and a distal chamber  45 . The chambers  44 ,  45  are connected via a channel  46  having a relatively small diameter such that the chambers are in fluid communication with one another. The various components of the distal device part  30  are such that air is able to enter the distal chamber  45  from the exterior of the device. 
     A piston  47  having a generally cylindrical cross-section is inserted into one end of the drive spring  43 . The piston  47  has an enlarged, generally circular head  48  at its proximal end, the outer diameter of the head  48  being slightly greater than the outer diameter of the drive spring  43 . When the drive spring  43  is compressed, the body of the piston  47  is centrally located within the proximal chamber  44  of the support shaft such that said proximal chamber  44  provides a cylinder  44  within which the piston can move. An “O” ring  49  is located around the circumference of one end of the piston  47 , and is held in place by a circumferentially extended groove  50  formed around the surface of the piston. 
     The O-ring  49  ensures an essentially air tight fit for the piston  47  within the proximal chamber or cylinder  44  whilst allowing the piston to move axially within the cylinder  44 . Other means for sealing the piston to the cylinder  44  will be readily apparent to the skilled person. 
     A generally cylindrical drive member  51  having a generally circular cross-section encloses the drive spring  43  and the piston  47 . The drive member  51  has a closed distal end face  52  which butts against and is fixed to the piston head  48 . The drive member  51  has a recess  53  formed therein such that, when the drive member is pushed into the distal housing part  31  to thereby compress the drive spring  43 , the trigger catch  35  engages the recess  53  and locks the drive member  51  in a cocked position. As already described, once engaged with the recess  53 , the trigger catch  35  is held in place by locking engagement of the sleeve  36 . 
       FIG. 4  illustrates the distal device part  30  in an initial, uncocked state. In this state, the distal device part  30  would not be attached to the proximal device part (of  FIGS. 1 and 2 ). In order to prime the distal device part, the user presses the drive member  51  into the distal housing part  31 , for example using a suitable cocking tool. The drive member  51  is pushed into the distal housing part until the trigger catch  35  snaps into place, locking the components together. This cocked state of the device is illustrated in  FIG. 5  from which it can be seen is that the drive spring  43  is almost in its fully compressed state. At this stage, and as discussed above, the device is assembled by first dropping a syringe  7  containing medication into the proximal device part. The proximal and distal device parts are then connected together by engaging and closing the respective screw threads. 
     Once the device is fully assembled, as with the device of  FIGS. 1 and 2 , a small gap B is present between the end face  52  of the drive member  51  and the opposed end of the syringe plunger  12  (for simplicity, the syringe is not shown in  FIGS. 3 to 6 ). In this state, the piston  47  is largely contained within the proximal chamber  44  of the support shaft  39 . Once assembled, the device is ready for use by a patient. 
     As has already been discussed, a patient uses the device by holding the sleeve in a fist-like grip and pressing the end of the needle shield against his or her skin. This causes the sleeve  36  to slide over the distal housing part  31  in a direction towards the patient&#39;s skin, in turn causing the window  38  to open over the trigger catch  35 . The user then presses his or her thumb against the trigger button  34  causing the trigger  33  to pivot, raising the trigger catch  35  from the drive member recess  53 . The drive spring  43  begins to expand axially and in the proximal direction, pushing first the piston  47  and in turn the drive member  51  axially towards the distal end of the device. In the initial drive phase, movement of the piston  47  through the proximal chamber  44  causes air to be sucked into that chamber through the small channel  46  connecting the proximal and distal chambers. As the diameter of this channel  46  is relatively small, the flow of air between the chambers is restricted, resulting in a damping of the movement of the piston  47  through the proximal chamber and therefore a damping of the movement of the drive member  51 . In effect, this damps the kinetic energy applied by the piston  47  to the plunger  12 . The impact of the piston head  48  on the plunger end plate  14  is therefore significantly reduced as compared to the impact from an undamped drive member ( FIGS. 1 and 2 ). 
     Movement of the piston  47  continues to be damped after impact on the plunger end plate  14 , i.e. during movement of the syringe  7  through the proximal device part. The result is that the syringe needle  11  is inserted into the patient&#39;s skin at a slower rate than would otherwise be the case. Movement of the piston is damped right up to the point at which the collar  23  of the support tube  19  comes into contact with the locking tube  25 , such that the impact of the collar  23  on the locking tube  25 , and the impact of the syringe wings  9  on the collar  23 , is considerably reduced. Thus, as compared with the known devices, the risk of damage to the device as a result of these impacts is reduced. 
     The dimensions of the piston  47  and of the proximal chamber  44  are such that, at or about this point in the operation phase, the end of the piston within the proximal chamber enters a tapered or fluted end region  54  of the proximal chamber. A gap appears between the piston and the chamber wall such that air is able to flow around the surface of the O-ring  49  and into the proximal chamber  44 . This air flow is significant as compared to the air flow through the interconnecting channel  46 , such that movement of the piston  47  within the proximal chamber  44  is no longer damped to any significant extent (other than by the resistance presented by the plunger  12 ). The full expansion force of the drive spring  43  is therefore exerted on the drive member  51  such that the drive member  51  accelerates movement of the plunger  12  into the syringe body  8 . The increased force is sufficient to inject medication through the needle  11  at the desired rate. 
       FIG. 6  illustrates the configuration of the distal device part  30  at the point where the needle begins to project from the needle shield (not shown in the Figure) and at the point where movement of the syringe body through the proximal device part is stopped. As described, at this point the O-ring  49  affixed to the piston  47  has just entered the fluted end region  54  of the proximal chamber. Advancement of the drive member  51  into the proximal housing part continues up to and beyond the point at which the entire piston  47  lies outside of the proximal chamber  44  of the support shaft  39 . Eventually, and as described above, the drive member  51  reaches the end of its stroke and delivery of medication from the syringe is completed. At this point, the distal device part  30  is in the configuration shown in  FIG. 4 . The user can now remove the needle from his or her skin, disconnect the proximal and distal device parts, and remove the used syringe and needle. If required, a new syringe can be dropped into the proximal device part, the distal device part cocked, and the device reassembled ready for use. 
     It will be appreciated by the person skilled in the art that various modifications may be made to the above described embodiment without departing from the scope of the present invention. In particular, the skilled person will appreciate that other approaches to dampen the movement of the drive member during an initial operational phase may be used. For example, rather than damping the movement of the piston by restricting the flow of air into the proximal chamber or piston cylinder, damping may be achieved by providing appropriate fictional engagement between an outer surface of the piston and an inner surface of the chamber, with that frictional restriction being removed at or about the point where the plunger begins to move through the syringe. Other modifications include: including a one-way valve into the channel  46  such that air can flow easily out of the proximal chamber  44  through the channel, but not in the reverse direction—this causes the piston and cylinder to operate like an air spring; using a fluid other than air in the damping mechanism; using a force applicator other than a compression spring to push the drive member, e.g. a torsion spring or electrically operated applicator; and forming the housing as a single part device (including an adjustable needle shield).