WO1989004185A1

WO1989004185A1 - A syringe of the disposable type

Info

Publication number: WO1989004185A1
Application number: PCT/DK1988/000182
Authority: WO
Inventors: Steen Rupert Christoffersen; Niels SØRENSEN
Original assignee: Schubert Seals A/S
Priority date: 1987-11-04
Filing date: 1988-11-04
Publication date: 1989-05-18
Also published as: DK579687D0; AU2725088A

Abstract

A syringe, such as a hypodermic syringe of the type to be used only once, comprises a syringe cylinder, a piston member (12) arranged within the cylinder, a piston rod (14) extending outwardly from the cylinder, and means (15) for interconnecting the piston member and the piston rod. The interconnecting means are adapted to cooperate during the piston strokes so as to release the piston member (12) from the piston rod (14) after a predetermined number of consecutive piston strokes and comprises track surfaces (53-58) and a track follower (16). The track follower is connected to the piston rod (14), and the track surfaces are formed on the piston member (12) or vice versa. The track follower (16) is biased in a transverse direction or in transverse directions into engagement with the track surface and may, for example, be in the form of an elastically, flexible arm.

Description

A Syringe of the Disposable Type

The present invention relates to a syringe, such a hypodermic syringe, of the type which is marketed in a sterilized condition, and which is intended to be used only once and to be discarded after such first use. Re-use of disposable syringes is especially tempting in undeveloped countries and other locations, where new sterilized syringes are not readily available. Such temptation to re-use syringes may also exist among certain groups of syringe users, such as drug addicts. Obviously, re-use of syringes without any or without sufficient sterilization thereof involves the risk of spreading hepatitis, AIDS, and other serious diseases.

Hypodermic syringes of a structure which allows for a single use of the syringe only, are known from European patent publication No. 0,229,017 and US patent No. 4,699,614. The syringe known from the said European patent publication is of a type where the connection between the piston rod and the piston will break during the first pressure stroke of the piston. This means that the piston cannot be used in a situation where two pressure strokes are needed, for example when the injection medium has to be sucked into the cylinder from an ampoule and remaining air has to be expelled from the syringe cylinder. In the syringe disclosed in the said U.S. patent, the piston and the piston rod are interconnected by interconnecting means releasing the piston from the piston rod after a certain number of strokes. These interconnecting means are in the form of zigzag-shaped grooves formed in the inner wall of the hollow piston rod, and the piston comprises a rotatably mounted collar having radial projections which cooperate with the guiding grooves of the piston rod so that the piston is released from the piston rod after a predetermined number of piston strokes. However, due to the rotatable collar this known structure is rather complicated and expensive co produce and assemble. Consequently, this known syringe structure is less suited for syringes of^' the disposable type. Furthermore, the groove-shaped track must be especially shaped to ensure that the collar projections are moved from one track section into a consecutive track section, when movement of the piston is shifted from a pressure stroke to a suction stroke or vice versa.

The present invention provides a syringe of the disposable type and comprising a syringe cylinder, a piston member arranged within the cylinder so as to be displaceable between advanced and retracted positions through suction and pressure strokes, a piston rod extend¬ ing outwardly from the cylinder, means for interconnecting the piston member and the piston rod, said interconnecting means cooperating during the piston strokes so as to release the piston member from the piston rod after a predetermined number of consecutive piston strokes, the interconnecting means defining track surfaces and a track follower arranged on either one of the piston member and the piston rod, said track surfaces comprising consecutive track surface sections having axial and/or transverse components extending parallel and at right angles, respectively, to the axis of the syringe cylinder, and biasing means for biasing the track follower in said transverse direction or directions, and into engagement with the track surface. The transverse bias of the biasing means ensure that the cam follower follows the track surface.

The sterilized syringe may be delivered with an injection medium in the cylinder. In that case the connection means could be adapted to allow for only a single pressure stroke for expelling the injection medium from the cylinder through a cannula or a hollow needle, such as a hypodermic needle, and the interconnecting means would then release the piston member from the piston rod, if and when the piston rod is moved towards its retracted position. This means that it would not be possible to move the piston member through a succeeding suc¬ tion stroke, but the piston member would remain in its forward posi¬ tion. In most cases, however, an injection medium should be sucked into the cylinder of the sterilized syringe through a cannula or a hollow needle arranged thereon, prior to the injection procedure. If, for example, the injection medium is to be withdrawn from an ampoule, a wall part of this ampoule is pierced by the hollow needle of the syringe, whereafter the piston member of the syringe is moved through a pressure stroke to pump air into the ampoule so as to create a superatmospheric pressure therein. Thereafter the piston member is moved through a suction stroke to suck injection medium into the cylinder of the syringe. Then, the first part of a second pressure stroke is used for expelling possible remaining air from the syringe through the hollow needle, and the remaining part of the second pressure stroke may then be used for injection of the medium into a patient. Thus, in that case, the interconnecting means of the syringe according to the invention should be adapted to release the piston member from the piston rod after three consecutive strokes, namely after a first pressure stroke followed by a suction stroke, and a second pressure stroke. In any case the interconnecting means should be "pre-programmed" to release the piston member from the piston rod after a predetermined number of consecutive piston strokes so as to prevent a second use of the syringe. Such interconnecting means being pre-programmed to release the piston member from the piston rod may be provided in any of a large number of possible embodiments.

The traction and pressure forces are at least partly transmitted from the pistoii rod to the piston member through the track follower and the track surface, and the track follower may be adapted to move along one of the consecutive track surface sections for each piston stroke, so that the track follower may leave the track surface and thereby release the piston rod from the piston member when the piston member has been moved through the predetermined number of piston strokes. Every second of the consecutive track surface sections may comprise an axial component extending parallel to the axis of the syringe cylinder. The other track surface sections could then, for example, extend in planes at right angles to the syringe cylinder axis. In such a case, the axial components of the track surface sections may extend in the same direction whereby the track surface may obtain a stepped form. Preferably, at least every second of the track surface sections has a transverse component extending at right angles to the axis of syringe cylinder.

The transverse components of the various track surface sections may extend in different directions. However, the transverse components of the track surface sections extending between first and second points of the track surface preferably extend in the same general direction or two differen directions defining a right angle therebetween. The track follower may then start at said first point, and by the manipulation of the piston member of the syringe through the various predetermined strokes the track follower may move in the same transverse direction to said second point of the track, where the track follower may be released. Simultaneously with the transverse movement of the track follower in said same direction the track follower is also moved axially in order to follow the track surface sections having an axial component. Preferably, consecutive sections of the track surface have oppositely directed axial components, so that the track surface may have a wave-like form or may follow a zigzag course. The track follower is caused to move in engagement with the consecutive track surface sections by the bias of the track follower in the said same general direction or in said two different directions in which the track surface sections extend transversely from said first to said second point of the track surface. The track surface and the track follower cooperating therewith may form some kind of lost motion connection between the piston member and the piston rod to allow the track follower to move along track surface sections having an axial component. It should be understood that each track surface section could be rectilinear or formed by rectilinear subsections, or it could be curved with single or double curvature.

The track follower may be mounted on the piston member or on the pis¬ ton rod by elastically or resiliently flexible mounting means, and in a preferred embodiment the track follower and the mounting means are formed integrally with the syringe part carrying the track follower. Alternatively, the track follower and its resilient mounting means may form a separate part, which may, for example, be a metal spring. When the track follower is mounted by resilient mounting means, the track surface may further comprise a cam surface for cooperating with the track follower so as to resiliently move the same to said first point of the track during a first piston stroke, whereby the track follower is resiliently biased in said transverse direction or directions.

Alternatively or additionally, the interconnecting means may comprise a force transmitting member defining an abutment surface and a cutting edge axially spaced therefrom, and a breakable member ar¬ ranged between the abutment surface and the cutting edge and extend¬ ing transversely to the latter. If the sterilized syringe is marketed with the piston member in its advanced innermost position, an injec- tion medium may be sucked into the syringe cylinder during a suction stroke provided by pulling the piston rod outwardly. The pulling force applied to the piston rod may be transferred to the piston mem¬ ber via the breakable member abutting the abutment surface. When the piston rod is later pushed inwardly to move the piston member through a pressure stroke so as to expel the injection medium from the syringe, the cutting edge is moved into contact with the breakable member to cut the latter. Therefore, if after termination of the pressure stroke the piston rod is pulled outwardly, the piston rod will be separated from the piston member so that the piston member cannot be moved outwardly through a further suction stroke. The use of a breakable member arranged between a cutting edge and an abutment surface as described above may be combined with, the previously de¬ scribed structure comprising a track surface and a track follower. Thus, according to the invention, the breakable member may form part of the track follower, while the abutment surface and/or the cutting edge may define one of said track surface sections.

When an injection medium, such as a liquid drug or drug solution, which should be injected into a blood vessel, such as a vein, has been sucked into the syringe cylinder during a first suction stroke, and the piston member has been moved through a succeeding short first pressure stroke to expel remaining air from the syringe cylinder, the patient or drug addict normally desires to make sure that the pointed end of the hollow needle of the syringe has been properly placed within a vein or blood vessel. This may be done by moving the piston member outwardly to provide a short second suction stroke prior to moving the piston member inwardly through a second pressure stroke so as to inject the injection medium. During the second short suction stroke blood should be sucked into the cylinder to indicate that a vein or another blood vessel has been pierced by the hollow needle.

In order to permit manipulation of the syringe as just described the connecting means may further comprise an axially extending flexible traction member arranged eccentrically on the piston member for cooperating with a projection formed on the piston rod and intersect¬ ing the path of movement of the traction member, said projection de¬ fining an inclined cam surface part and a transversely extending traction transmitting surface part thereon, whereby the traction mem¬ ber may cooperate with the inclined cam surface so as to be flexed inwardly and back into engagement with the traction transmitting sur¬ face, when the piston rod is moved inwardly in relation to the piston member, while the piston member may tilt and move the traction member out of engagement with the traction transmitting surface, when the piston rod is pulled outwardly in relation to the cylinder. Thus, the described cooperation between the traction member and the projection makes it possible to move the piston member only a small distance outwardly to provide said second suction stroke, before the piston is tilted, whereby the traction member and the pro ection are disengaged.

The invention will now be further described with reference to the drawings, wherein

Fig. la is a side view and partially sectional view of part of a first embodiment of the syringe according to the invention, wherein the piston member and the piston rod are shown in an first mutual position,

Fig. lb is a sectional view substantially along the line B-B in Fig. la, Fig. lc is a sectional view substantially along the line C-G in Fig. la,

Figs. 2a, 2b, and 2c are sectional views corresponding to Figs, la, lb, and lc, respectively, but with the piston member and the piston rod in a second mutual position, Figs. 3a, 3b, and 3c are sectional views corresponding to Figs, la, lb, and lc, but with the piston member and the piston rod in a third mutual position,

Figs. 4, 5, and 6 are perspective views and partially sectional views of a second embodiment of the syringe according to the invention showing the piston member and the piston rod in different mutual positions, Fig. 7 is a side view of a piston member and part of a piston rod interconnected therewith illustrating a third embodiment of the syringe according to the invention, and

Figs. 8-13 are perspective views showing a piston member and a piston rod of a preferred fourth embodiment of the syringe according to the invention in various mutual operational positions.

The various embodiments illustrated in the drawings comprise a syringe cylinder 10 having a spout or cone 11 at one end forming a seat for a cannula or a hollow needle, not shown, and a piston member 12 having a sealing ring 13 and being displaceably mounted within the cylinder 10. The inner end of a piston rod 14 is connected to the piston member 12 by connecting means generally designated 15. These connecting means may be formed integrally with the piston member 12 and with the piston rod 14 as shown in Figs. 1-6, but the connecting means may also comprise separate connecting members as shown in Figs. 7.

In the embodiment shown in Figs. 1-3 the connecting means 15 com¬ prises a loop-like shaped, flat track follower 16 defining an axially extending slot 17 therein and having a thin, breakable portion 18 defining the slot 17 at the free end of the track follower. The track follower 16 extends axially.from and is formed integrally with the piston member 12 and is connected to an adjacent surface part 19 through a thin, hinge-like part 20. The connecting means 15 further comprise a cutting member 21 formed integrally with and extending transversely from the inner end of the piston rod, and an oppositely arranged hook-like arm 22 extending transversely to the flat track follower 16 and through the slot 17 formed therein. The cutting member has a cutting edge 23 defining an acute angle with the axis of the piston cylinder and being directed towards the piston member 12. The connecting means 15 also comprise a traction member 24, which extends axially from the surface part 19 of the piston member 12 and is provided with a head 25 at its free end defining a shoulder 26 facing the surface part 19 of the piston member 12. The traction member 24, which forms part of the piston member 12, is guided in a narrow space defined between a pair of parallelly extending wall parts or ribs 27 and 28, which form part of the piston rod 14 and extend in a radial direction opposite to the hook-like arm 22. A cam 29 is formed on and projects from the inner side of the wall part 27 for cooperating with the shoulder 26 of the head 25. The track follower 16 as well as the traction member 24 are eccentrically positioned on the piston member 12 on either side of the cylinder axis. The connecting means 15 interconnecting the piston member 12 and the piston rod 14 are formed so as to be "pre-programmed" to release the piston rod from the piston member after a predetermined number of piston strokes which are necessary for a single use of the syringe, whereby use of the syringe more than a single time is rendered impossible. The syringe illustrated in Figs. 1-3 operates as follows:

The syringe is marketed in a sterilized, empty condition with the piston member 12 and the piston rod 14 in a retracted position within the cylinder 10, and the mutual position of the piston member 12 and the piston rod 14 is shown in Figs, la, lb, and lc. In this position the cutting member 21 as well as the hook-like arm 22 extends through the slot 17 in the track follower 16, and the shoulder 26 defined on the traction member head 25 is in engagement with the upper surface of the cam 29 as shown in Figs, la and lc. From this retracted position the piston member 12 may be pushed inwardly through a first pressure stroke to the position shown in Figs, la, lb, and lc. During this first pressure stroke the innermost end of the piston rod 14 is in direct contact with the surface part 19 of the piston member 12 so that the pressure exerted on the piston rod is transferred directly to the piston member 12. The movement of the piston member from its retracted position to its advanced position shown in Figs, la, lb, and lc causes air to be expelled from the syringe cylinder 10. If an injection medium is to be sucked from an ampoule or a similar container, this ampoule may be pierced by a hollow needle mounted on the cone 11 so that the air expelled from the syringe cylinder is transferred to the ampoule. Thereafter the piston rod 14 is retracted so as to move the piston member 12 through a first suction stroke, whereby injection medium is sucked into the syringe cylinder. The traction force applied to the piston rod 14 is initially transferred to the piston member 12 through the cam 29 and the head 25 engaging therewith. However, the eccentric arrangement of the traction member 24 on the piston member 12 causes a tilting movement of the piston member 12 sufficient to disengage the traction member head 25 from the cam 29, whereafter the track follower 16 will slide downwardly along the rounded upper side of the cutting member 21 to a position in which the breakable portion 18 of the loop-shaped track follower 16 is in engagement with the upper side of the hook-like arm 22. This mutual position of the piston member 12 and the piston rod 14 is shown in Figs. 2a, 2b, and 2c, and the outward movement of the piston rod 14 during the first suction stroke will now be transferred to the piston member 12 through the arm 22 and the track follower 16. When the first suction stroke has been completed and injection medium has been sucked into the syringe cylinder 10, the piston rod 14 is pushed inwardly and moved axially in relation to the piston member 12, whereby the cutting edge 23 of the cutting member 21 is brought into engagement with the thin breakable portion 18 to cut the same as illustrated in Fig. 3b. The axial movement of the piston rod 14 in relation to the piston member 12 brings these parts into a mutual position illustrated in Figs. 3a, 3b, and 3c, where the lower part of the piston rod 14 is in engagement with the surface part 19 of the piston member 12, and where the shoulder 26 of the head 25 is again in engagement with the upper surface of the cam 29. Further inward movement of the piston rod 14 is now directly transferred to the sur¬ face part 19 of the piston member to move the same through a short second pressure stroke to expel remaining air from the syringe cylinder 10. The hollow needle of the syringe may now be inserted into an arm or another body part of a patient. As explained above, in some cases the injection should be made into a blood vessel, such as a vein, and in order to make sure that the pointed end of the^"hollow needle has pierced the vein, the piston rod 14 may be retracted for a small distance. This outward movement of the piston rod 14 is trans¬ ferred to the piston member 12 through the cam 29 and the traction member head 25, so that the piston member 12 may be moved through a short second suction stroke to suck vein blood into the cylinder so as to make sure that the vein has been pierced. However, the ec- centric arrangement of the traction member 24 causes the head 25 and the cam 29 to become disengaged after a short movement of the piston member 12 due to tilting of the piston member as previously de¬ scribed. The injection medium may now be expelled from the syringe cylinder 10 through the hollow needle by pressing the piston rod 14 inwardly through a third long pressure stroke to move the piston mem¬ ber 12 to its fully advanced position, the pressure being transferred directly from the inner end of the piston rod 14 to the surface part 19 of the piston member 12. It should be understood that it would be impossible to re-use the syringe, because the breakable portion 18 of the track follower (which may follow a track defined by the upper side of the cutting member 21 and the upper edge of the hook-like arm 22) is cut so that a traction force may be transferred from the pis- ton rod 14 to the piston member 12 only via the traction member head 25 and the cam 29. However, as explained above these parts automati¬ cally disengage after a short movement.

Figs. 4-6 illustrate a modified embodiment of the syringe according to the invention which is suitable for hypodermic and intramuscular injections, but not especially suited for intravenous injections. The parts shown in Figs. 4-6 corresponding to parts shown in Figs. 1-3 have been designated the same reference numerals.

In the embodiment shown in Figs. 4-6 the loop-shaped track follower 16 is arranged centrally on the surface part 19 of piston member so that the hinge-like part 20 extends substantially diametrically along this surface part. A channel-shaped member 30 extends axially from the surface part 19 and is formed integrally with the piston member 12. This channel member may be adapted to engage with an axially ex¬ tending flange or rib 31 forming part of the piston rod 14. The en- gagement between the channel member 30 and the flange 31 positions the piston member 12 and the piston rod 14 in a mutual angular posi¬ tion in which the plane of the cutting member 21 and the hook-like arm 22 extends transversely to the plane of the flat track follower 16. The syringe shown in Figs. 4-6 may be operated as follows.

The syringe is normally marketed in an empty, sterilized condition with the piston member 12 in a retracted position. The inner end of the flange 31 is in engagement with and guided by the channel member 30, and the inner edge of the hook-like arm 22 is positioned closely adjacent to or in abutting relationship with the breakable portion 18. Alternatively, the breakable portion 18 may be arranged in abut- ting relationship with an abutment surface 59 formed on the piston rod 14 outside the arm 22, the free end of which may then be in en¬ gagement with the channel member 30. A user may now push the piston rod 14 inwardly so as to move the piston member through a first pres- sure stroke from the retracted starting position to an advanced posi¬ tion. The inner edge of the arm 22 defines an inclined cam surface 32. Therefore, in case the piston member 12 and the piston rod 14 are in the mutual positions shown in Fig. 4, the pressure transmitted from the cam surface 32 to the track follower 16 causes a deflection of the track follower during the first pressure stroke so that the breakable portion 18 will ride along the cam surface to the free end of the arm 22 and thereafter snap back into engagement with the abut¬ ment surface 59. hen air has been expelled from the syringe cylinder 10 through the first pressure stroke, an injection medium may be sucked into the syringe cylinder during a succeeding first suction stroke which is provided by moving the piston rod outwardly to its retracted position, whereby the breakable portion 18 is moved along the passage defined between the cutting edge 23 and the adjacent edge surface of the area 22 into a position as that shown in Fig. 5. The outward movement of the piston rod 14 is transmitted to the piston member 12 through the hook-like arm 22 and the loop-shaped track fol¬ lower 16 engaging therewith. Now, the piston rod 14 may be pushed inwardly to provide a second pressure stroke for expelling remaining air from the syringe cylinder 10 and for successively expelling the pressure medium from the cylinder through a hollow needle for making an injection. At the beginning of the first pressure stroke, the pressure force applied to the piston rod 14 is transmitted from the cutting edge 21 to the breakable portion 18, so that this portion is cut as illustrated in Fig. 6. Thereafter, the piston rod 14 is moved axially in relation to the piston member 12 till the inner end of the piston rod 14 comes into engagement with the surface part 19 of the piston member so that pressure may be transmitted directly from the piston rod 14 to the piston member 12. After completion of the second pressure stroke, the piston member 12 will remain in its fully advanced position even if the piston rod 14 is pulled outwardly, be¬ cause the breakable portion 18 has been cut. This means that re-use of the syringe is impossible. Fig. 7 shows an embodiment wherein the piston rod 14 and the piston member 12 are interconnected by means of a wire spring 40. One end 41 of the wire spring 40 is mounted on the piston member 12, and the other free end 42 of the spring is guided in a zigzag-shaped groove or track 43 formed in the piston rod 14. Fingers 44 extending axially from the surface part 19 of the piston member 12 engage with com- plementarily shaped axial grooves or recesses formed in the piston rod 14 so that the piston rod and the piston member may be moved axially in relation to each other, while being guided by the fingers 44 engaging with the grooves 45. Fig. 7 shows the piston assembly in its starting position in which the wire spring 40 is resiliently dis¬ torted so that the free end 42 is biased in a direction indicated by an arrow.

The mutual position of the piston member 12 and the piston rod 14 shown in Fig. 7 will not change during a first pressure stroke. However, when the piston rod 14 is moved through a first suction stroke, the free end 42 of the spring will move to a point 46 of the track. During a succeeding second pressure stroke the free spring end 42 will move to a point 47 of the track 43. If somebody tries to re-use the syringe by pulling the piston rod 14 outwardly, the free spring end 42 will move out from the free end of the track 43, where¬ by the piston rod 14 will be disconnected from the piston member 12.

Figs. 8-13 illustrate a presently preferred embodiment, wherein an axially extending guide surface 47 and an axially extending, rod-like track follower 16 are formed at the inner end of the piston rod 14.

The track follower 16, which is preferably formed integrally with the piston rod 14, for example from plastics material, has a transversely extending head, or end portion 48 which is resiliently biased towards to the guide surface 47 by inherent elasticity. The piston member 12 has a flat axially extending engaging part 49 which is adapted to be inserted between and to engage with the guide surface 47 and the head 48 of the track follower 16. Thus, one side surface of the engaging part is shaped substantially complementarily to the guide surface 47 so as to guidingly cooperate therewith, and the opposite side of the engaging part 49 is shaped so as to define projecting cams 50, 51, and 52 with cam surfaces defining a tortuous track. When the piston member and the piston rod are moved through a predetermined number of suction and pressure strokes, the head 48 of the track follower 16 will move along its track and finally be released therefrom as will be explained more in detail below.

Fig. 8 shows the piston assembly comprising the piston member 12 and the piston rod 14 in a starting position. This piston assembly is arranged within the syringe cylinder (not shown) , when the syringe is marketed in a sterilized condition. In the starting position the track follower 16 is flexed transversely, and its head 48 is in en- gagement with a step 53 and the cam 51. The piston rod 14 may now be retracted to move the piston member 12 through a first suction stroke to suck injection medium into the cylinder. As shown in Fig. 9 such outward movement of the piston rod causes a short movement of the piston rod 14 in relation to the engaging part 49 of the piston mem- ber 12, whereby the head 48 of the track follower 16 is moved from the step 53 down into a transversely extending channel or groove 54 forming an acute angle with the axis of the syringe cylinder. The traction force may now be transferred from the piston rod 14 to the piston member 12 via the track follower 16 and the cam 50. Possible air may now be expelled from the piston cylinder by pressing the pis¬ ton rod 14 inwardly so as to move the piston member 12 through a first pressure stroke. The pressure exerted on the piston rod 14 may be transferred to the piston member 12 through the track follower 16, but as shown in Fig. 10 the inwardly directed pressure causes the track follower 16 to move along the channel 54 to an open end there¬ of, whereby the track follower is flexed transversely. A succeeding outward movement of the piston rod 14 to move the piston member 12 through a short second suction stroke so as to ascertain that the needle has pierced a blood vessel as described above causes the head 48 of the track follower 16 to move along an axially extending cam surface 55 till the head 48 drops into a transversely extending chan¬ nel 56 located between the cams 50 and 52, vide Fig. 11. A traction or pulling force may now be transmitted from the piston rod 14 to the piston member 12 via the track follower 16 and a cam 52. The track follower 16 will tend to flex back so that the head 48 is moved along the channel 56 into engagement with an adjacent cam surface 57 of the cam 50, vide Fig. 12. In order to expel the injection medium from the syringe cylinder, the piston rod 14 is now pressed inwardly, and because the channel 56 extends obliquely, the head 48 is moved along the channel 56, past a step and down into engagement with a surface part 58. When the second pressure stroke and the injection has been completed, an outward movement of the piston rod 14 results in separation of the piston rod from the piston member as illustrated in Fig. 13.

It should be understood that various amendments and modifications of the embodiments described above could be made within the scope of the present invention. Thus, any connecting means could be used for interconnecting the piston rod 14 and the piston member 12, provided that such connecting means are pre-programmed to release or disconnect the piston rod from the piston member when the piston mem¬ ber 12 has been moved through a predetermined number of pressure and suction strokes.

Claims

1. A syringe of the disposable type and comprising a syringe cylinder, a piston member arranged within the cylinder so as to be displaceable between advanced and retracted positions through suction and pressure strokes, a piston rod extending outwardly from the cylinder, means for interconnecting the piston member and the piston rod, said interconnecting means cooperating with the piston strokes so as to release the piston member from the piston rod after a predetermined number of consecutive piston strokes, the interconnecting means defining a track surface, and a track follower arranged on either one of the piston member and the piston rod, said track surface comprising consecutive track surface sections, having axial and/or transverse components extending parallel and at right angles, respectively, to the axis of the syringe cylinder, and biasing means for biasing the track follower in said transverse direction or directions and into engagement with the track surface.

2. A syringe according to claim 1, wherein the track follower is mounted on the piston member, or on the piston rod by elastically flexible mounting means.

3. A syringe according to claim 2, wherein the track follower and the mounting means are formed integrally with the syringe part carrying the track follower.

4. A syringe according to any of the claims 1-3, wherein the transverse components of track surface sections extend between first and second points of the track surface in the same general direction.

5. A syringe according to any of the claims 1-3, wherein the transverse components of the track surface sections, which extend between first and second points of the track surface, extend in two different directions defining a right angle therebetween.

6. A syringe according to any of the claims 1-5, wherein consecutive sections of said track surface have oppositely directed axial components.

7. A syringe according to any of the claims 4-6, said track surface further comprising a cam surface for cooperating with the track follower so as to resiliently move the same to said first point of the track surface during a first piston stroke, whereby the track follower is resiliently biased in said transverse direction or directions.

8. A syringe according to any of the claims 1-7, wherein said inter- connecting means comprise a force transmitting member defining an abutment surface and a cutting edge axially spaced therefrom, and a breakable member arranged between the abutment surface and the cut¬ ting edge and extending transversely to the latter.

9. A syringe according to any of the claims 1-7 and to claim 8, wherein the breakable member forms part of the track follower, while the abutment surface and/or the cutting edge define one of said track surface sections.

10. A syringe according to claim 8 or 9, wherein the force transmit¬ ting member forms part of the piston rod, while the breakable member is connected to the piston member.

11. A syringe according to any of the claims 1-10, wherein said con¬ necting means further comprise an axially extending flexible traction member arranged eccentrically on the piston member for cooperating with a projection formed on the piston rod and intersecting the path of movement of the traction member, said projection defining an in¬ clined cam surface part and a transversely extending traction trans¬ mitting surface part thereon, whereby the traction member may cooper- ate with the inclined cam surface so as to be flexed inwardly and back into engagement with the traction transmitting surface when the piston rod is moved inwardly in relation to the piston member, while the piston member may tilt and move the traction member out of en- gagement with the traction transmitting surface when the piston rod is pulled outwardly in relation to the cylinder.