Abstract:
A syringe includes a piston ( 16 ) that is slidably guided in a casing ( 10 ), and a needle ( 20 ) adapted to be withdrawn into the casing. The piston ( 16 ) and the needle ( 20 ) are connected by a coupling ( 22; 22′ ) which is adapted to assume three states: a releasable coupling state in which the needle ( 20 ) can be moved towards the front end of the casing ( 10 ) together with the piston ( 16 ), an uncoupled state in which the piston ( 16 ) can be moved back in the casing ( 20 ) separately from the needle ( 20 ), and a permanent coupling state in which the needle ( 20 ) can be withdrawn into the casing ( 10 ) by the piston ( 16 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a syringe comprising a piston that is slidably guided in a casing, and a needle adapted to be withdrawn into the casing. 
   Injection syringes for the administration of medicine or vaccine are preferably configured as one-way syringes that are to be disposed after they have been used once, and wherein a mechanism assures that they are non-usable after the first use, so as to avoid infections caused by a repeated use of the syringe. However, there is a risk that the unprotected needle of the syringe causes injuries and hence infections during or after the disposal of the syringe. In order to reduce the risk of injury, it has been common practice to mount a protective cap on the needle after the syringe has been used, so that the needle is protected in the condition in which the syringe is disposed of. Frequently, however, it is just the process of mounting the protective cap that leads to injuries. 
   In practice, injection syringes have become known, wherein the needle is elastically biased, so that it automatically retreats into the casing after the syringe has been used. This, however, requires a relatively complex triggering mechanism which increases the production costs of the syringes. However, disposable syringes for mass vaccinations or for the combat of endemic diseases, in particular in the third world, should be producible at low costs. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to provide a disposable syringe that can be produced at low costs and offers an improved safety. 
   According to the invention, this object is achieved by the feature that the piston and the needle are connected by a coupling which assumes three states: a releasable coupling state in which the needle can be moved together with the piston towards the front end of the casing, an uncoupled state, in which the piston is separated from the needle and can be withdrawn into the casing, and a permanent coupling state, in which the needle can be withdrawn into the casing by means of the piston. 
   In a delivery state of the syringe, the coupling is in the releasable coupling state, and the piston is withdrawn, so that the needle is protected in the interior of the casing. Thus, the risk of injuries is reduced even in the state prior to the use of the syringe. 
   When the syringe is to be used, the user pushes the piston towards the front end, so that the needle is projected out of the casing. Then, the tip of the needle can be immersed into the vaccine or medicine, as usual, and the liquid can be sucked into the casing of the syringe by withdrawing the piston. During the withdrawal movement of the piston, the coupling transits into the uncoupled state, so that the needle remains in its projected position relative to the casing, and the piston alone is moved rearwards. Then, during the proper injection process, the piston is again moved forward, until the coupling parts of the piston and the needle are finally re-engaged with one another. During this process, the coupling assumes the permanent coupling state in which the needle and the piston are persistently connected to one another. When, now, the user or an unauthorized third person attempts to suck-in liquid once again, the needle moves rearward into the casing together with the piston, so that no subatmospheric pressure can be built up in the casing and, consequently, no liquid can be sucked-in. In this way, the syringe is protected against re-use. At the same time, the permanent coupling state has the advantage that the needle can be withdrawn again into the casing by means of the piston, without any risk that the user is injured by the tip of the needle. Then, in the disposal condition, the needle is again protected in the interior of the casing, so that other persons can no longer be injured or infected by the needle, neither. 
   Useful embodiments and further developments of the invention are indicated in the dependent claims. 
   For the coupling that goes sequentially through the three states described above, several embodiments are conceivable which can easily be implemented by injection molding techniques. Preferably, the permanent coupling state is established by a snap connection with tensile strength between the needle and the piston. In contrast, in the releasable coupling state, an elastically biased element establishes a thrust-resistant connection between the piston and the needle, so that the needle will be pushed forward by the piston without establishing the snap connection that defines the permanent coupling state. When the piston is withdrawn, the elastically biased element is relaxed, so that, now, the snap connection for the permanent coupling state can be established as soon as the coupling members of the needle and the piston are again brought into engagement with one another. 
   Preferably, a coupling member, that is formed at the rear end of the needle, and the casing of the syringe have such a configuration that they can be snap-fastened to one another in a fluid-tight manner when the needle is in its front terminal position. In this way, the needle can be fixed in the casing in the foremost position, and it is assured that the coupling makes a transition from the releasable coupling state into the uncoupled state when the piston is withdrawn in the process of loading the syringe, i.e. when the liquid is sucked-in. At the same time, the fluid-tight connection makes it possible to build up a subatmospheric pressure in the casing and, accordingly, to suck-in the liquid. The snap connection between the needle and the casing is dimensioned such, that the resistance that is experienced when the needle penetrates into the body of the patient is not sufficient for releasing the snap connection, whereas, in the permanent coupling state, the snap connection between the needle and the piston is so strong that the needle can again be freed of its snap connection with the casing. 
   The elastically biased element is preferably formed by at least one elastic tongue that, when in the biased state, acts as a spacer between the coupling member of the needle and the coupling member of the piston. When the elastic tongue is relaxed while the piston is withdrawn, it assumes a position in which it can plunge into a recess of the respective other coupling member, so that it does no longer act as a spacer and permits to establish the snap connection for the permanent coupling state. 
   In a preferred embodiment, the casing is internally formed with a lock which, in a manner known per-se, forms a stop or detent when the piston is withdrawn in the process of loading the syringe, in order to prevent the piston from being drawn back too far. When, after the injection, the syringe is transformed into the disposal state, this detent can however be overcome by applying a certain force, and then the lock prevents the needle from being moved forward again. Thus, it is assured that the needle is permanently accommodated and protected inside of the casing in the disposal state. 
   In a modified embodiment, the coupling has such a configuration that it provokes a slightly inclined position of the needle in the permanent coupling state, so that the tip of the needle will abut a shoulder at the front end of the casing and can no longer be projected out of the casing once it has been withdrawn after the injection process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiment examples of the invention will now be described in detail in conjunction with the drawings, wherein: 
       FIGS. 1 to 5  are axial sectional views of a syringe in different stages prior to, during, and after an injection process; 
       FIG. 6  is an enlarged axial section of a coupling between a needle and a piston of the syringe in a releasable coupling state; 
       FIG. 7  is a sectional view of the coupling in a permanent coupling state, in a sectional plane that is rotated in comparison to  FIG. 6 ; 
       FIG. 8  is a partial sectional view of a syringe according to a modified embodiment; 
       FIG. 9  is a sectional view similar to  FIG. 7  for a modified embodiment of the coupling; and 
       FIG. 10  is a sectional view of a syringe having the coupling shown in  FIG. 9  in a disposal condition. 
   

   DETAILED DESCRIPTION 
   The syringe shown in  FIG. 1  comprises a cylindrical casing  10  of plastics that is constricted at its front end to form a spout  12 . A lock in the form of a peripheral annular groove  14  is formed in the internal wall of the casing in the transition zone between the spout  12  and the main body of the casing  10 . 
   A disk-shaped piston  16  is formed at the front end of a piston rod  18  and is guided to be slidable in the cylindrical part of the casing  10 . 
   In the condition shown in  FIG. 1 , a needle  20  is accommodated in the interior of the casing  10  in such a manner that the tip thereof is accommodated and protected in the spout  12 . The rear end of the needle  20  is connected to the piston  10  by a coupling  22 . The coupling  22  comprises a needle-side coupling member  24  and a piston-side coupling member  26 . In  FIG. 1 , these coupling members are in a releasable coupling state in which a thrust-resistant connection between the piston and the needle is formed, which connection, however, is releasable when the piston  16  is withdrawn. The needle-side coupling member  24  has an annular boss  28  forming the complement of the annular groove  14  of the casing  10 . 
   The condition of the syringe shown in  FIG. 1  corresponds to the delivery state. When the syringe is to be transformed into a condition ready for use, which is shown in  FIG. 2 , the user pushes the piston  16  forward by means of the piston rod  18 . During this process, the needle  20  is projected out of the casing, and the boss  28  is locked in the annular grove  14 , so that the needle  20  is locked in the casing with a certain retention force by means of the coupling member  24 . Then, a cone  30  formed on the coupling member  24  fits in the spout  12 , so that the cavity delimited by the piston  16  in the front part of the casing  10  is sealed-off at the front end, to one part by engagement of the peripheral wall of the cone  30  at the internal surface of the spout  12 , and to the other part by engagement of the boss  28  in the annular grove  14 . At the same time, in this way, the needle  20  is stably held in the casing  10 . 
   In order to load the syringe, the user withdraws the piston rod  18  into the position shown in  FIG. 3 . During this, the coupling members  24 ,  26  may be disengaged from one another, so that the piston  16  travels backward alone, whereas the needle  20  remains snap-fastened to the casing  10  by the coupling member  24 . Since the cavity  32  in the interior of the casing  10  is sealed fluid-tightly, the liquid to be injected is sucked-in through the interior of the needle  20 . 
   Then, the user pricks the tip of the needle  20  into the skin of the patient and pushes the piston rod  18  forward again in order to inject the liquid into the body of the patient. The retaining force of the lock formed by the boss  28  and annular grove  14  is so large that the needle, when penetrating into the body, will not retreat but will retain its position relative to the casing  10 . During the injection process, the user may repeatedly move the piston rod  18  and the piston  16  back and forth, and this will not compromise the function of the syringe. 
   In  FIG. 4 , the injection process is completed, and the piston  16  has reached its front terminal position in the casing  10 . The coupling members  24 ,  26  have again been brought into engagement with one another, but, because of a mechanism that will be described below, they now form a permanent snap connection. The strength of this snap connection is larger than the strength of the snap connection between the needle  20  and the casing  10  formed by the boss  28  and the annular grove  14 . 
   When the needle  20  has been withdrawn from the skin of the patient, and the user wants to dispose of the syringe, he withdraws the piston rod  18  into the position shown in  FIG. 5 . Thanks to the permanent snap connection between the coupling members  24 ,  26 , the needle  20  is now entrained rearward, until the tip thereof is again protected in the spout  12  of the casing. This avoids the risk that anybody is injured by the tip of the needle. When a user or an unauthorized third person now moves the piston rod  18  and the piston  16  forward again and attempts to load the syringe once again, the permanent snap connection persists, and when the piston is withdrawn, the needle  20  is also moved rearward again. In this process, the cone  30  retreats from the spout  12 , and air may enter into the interior of the casing  10  through an annular gap formed between the needle  20  and the internal surface of the spout  12 , so that no subatmospheric pressure can be built-up in the interior of the casing and, consequently, no liquid can be sucked-in. In this way, a forbidden re-use of the syringe is prevented. 
   The coupling  22  has been shown in greater detail in  FIGS. 6 and 7 .  FIG. 6  shows the coupling in the releasable coupling state corresponding to  FIG. 1 . The coupling member  24 , as a whole, is shaped like a bell which opens towards the piston  16  and the peripheral wall of which is penetrated in at least two positions by cut-outs  34 . Each of these cut-outs  34  accommodates an elastic tongue  36  that is formed integrally with the coupling member  24  and projects freely towards the piston  16 . In the condition shown in  FIG. 6 , the tongues  36  are slightly compressed, and they are held with elastic bias in a recess  38 , e.g. an annular groove of the piston  16 . In this condition, the tongues  36  act as spacers between the piston  16  and the coupling member  24 . The piston-side coupling member  26  is formed directly on the front side of the piston and forms a collar that surrounds the bell-shaped needle-side coupling member  24  on a portion of the periphery thereof and has a profile shaped as an inwardly cranked locking lug  40 . Corresponding counter-locks  42  are formed at the outer periphery of the bell-shaped coupling member  24 . In the condition shown in  FIG. 6 , however, the elastic tongues  36  prevent the lugs  40  from gripping behind the counter-locks  42 . The lugs  40  will only slide onto the counter-locks  42  with their ramp surfaces (this occurs in a peripheral portion that is not visible in  FIG. 6 ), whereby the collar is slightly expanded. When the piston  16  is thrust forward (to the left in  FIG. 6 ) by means of the piston rod  18 , the thrust force is transmitted by the elastic tongues  36  onto the coupling member  24  and further onto the needle  20 , so that the piston and the needle move forward together without allowing the lugs  40  to lock at the counter-locks  42 . During this process, a stable guidance for the needle  20  is assured by a pin  44 , which centrally projects from the piston  16 , engaging into a sleeve  46  formed in the interior of the coupling member  24 . At the same time, the pin  44  and the sleeve  46  provide for frictional contact by which the coupling is held in the state illustrated in  FIG. 6 . 
   When now, after the coupling member  24  has reached its foremost position and has been locked with the boss  28  in the annular grove  14  of the casing, the piston  16  is withdrawn again, the free ends of the elastic tongues  36  retreat from the recess  38 , and they spring back into a relaxed position in which the extend essentially in parallel to one another. In this position, the free ends of the tongues  36  are aligned with deeper recesses  48  of the piston. When, then, the piston  16  reaches again the foremost position at the end of the injection process, the free ends of the elastic tongues  36  enter into the recesses  48 , so that they do no longer act as spacers. For this reason, the distance between the coupling members  26  and  24  can be reduced to such an extent that the locking lugs  40  slide over the counter-locks  42  and lock behind the same, as has been shown for a different sectional plane in  FIG. 7 . The lower part of  FIG. 7  also shows the edge of one of the elastic tongues  36  with its free end accommodated in the recess  48 . In this condition, the lugs  40  and counter-locks  42  connect the coupling members  24 ,  26  in a manner to resist tensile strain, so that the piston  16  and the needle  20  are now non-releasably connected to one another. As a consequence, when the piston is withdrawn, the boss  28  of the coupling member  24  is separated from the annular grove  14  of the casing  10 . 
   The collar-shaped coupling member  26  and the counter-locks  42  of the coupling member  24  are all formed only on a part of the periphery. When the coupling members are produced by injection molding, this permits an easy demolding of the undercuts formed by the lugs  40  and the counter logs  42 . At the same time, when the coupling is mounted, there is offered the possibility to compress the elastic tongues  36  and to insert them into the annular groove or recess  38 . Optionally, the lugs  40  and the counter-locks  42  can be angularly offset from one another to such an extent that they do not come into contact with one another. Subsequently, the needle-side coupling member  24  can be rotated about its longitudinal axis until the counter-locks  42  are aligned with the lugs  40 . A lock which has not been shown may be used for securing the coupling members  24 ,  26  in this angular position, so that they may not be rotated relative to one another. Finally, the unit formed by the piston rod, the piston, the coupling and the needle, that have been pre-mounted in this way, are inserted into the casing  10  of the syringe. 
   As is shown in  FIG. 6 , the cylindrical internal wall of the casing  10  forms a flat peripheral collar  50  that serves as a stop for the piston  16 . This defines the position of the piston in the delivery state as shown in  FIGS. 1 and 6 . When the syringe is to be used, i.e. when passing from the condition shown in  FIG. 1  to the condition shown in  FIG. 2 , the resistance formed by the collar  50  can be overcome with a certain effort. Then, when the syringe is loaded ( FIG. 3 ) the collar serves as a detent in the opposite direction, indicating to the user that the maximum fill volume of the syringe has been reached. The injection process can then be performed without having to overcome a detent. Then, before the syringe is disposed of, the piston  16  is again drawn back and moved beyond the collar  50 , so that the piston can only be moved forward again with a certain effort. This reduces the risk that the tip of the needle projects again from the spout  12 . 
     FIG. 8  shows another embodiment, wherein the piston  16  is immobilized in the delivery condition ( FIG. 1 ) between a front collar  50  and a rear collar  52 . In addition, in this embodiment, the piston rod  18  forms an elastic tongue  54  which locks at the rear end of the casing  10  and must be depressed manually in order to convert the syringe from the state shown in  FIG. 1  to the state shown in  FIG. 2 . When, after the injection, the syringe is to be converted into the disposal state ( FIG. 5 ), the tongue  54  will again snap-in behind the end of the casing  10  and will thus prevent the needle from projecting out of the spout  12  of the casing. 
     FIG. 9  shows a coupling  22 ′ which differs from the coupling  22  according to  FIG. 7  in that a certain play exists between the lug  40  and the counter-lock  42  on one side (bottom side in  FIG. 9 ), and in that the lug  40  is supported at the piston  16  by an elastic tongue  56 . This modification has no effect in the releasable coupling state, and the needle  20  is aligned with the spout  12 , so that it may be projected out of the spout. In the condition at the end of an injection process ( FIG. 4 ), the needle is held coaxially in the spout  12  by the cone  30 , and the tongue  56  is biased elastically, as shown in  FIG. 9 . In this embodiment, the needle  20  is so short that it may completely be drawn out of the spout  12  when the piston rod  18  is withdrawn. Then, the elastic tongue  56  and the play of the lower lug  40  assure (in conjunction with a certain resiliency of the pin  44 ) that the coupling member  24  and the needle  20  assume an inclined position, as shown in  FIG. 10 , in which, when the piston rod is pushed forward again, the needle will no longer enter into the spout  12  but will be caught with its tip at a shoulder  58  on the front wall of the casing  10 .