Abstract:
A retractable syringe is proposed having a cutting head mounted on a plunger of the syringe. When the plunger is advanced to expel fluid from the syringe, the cutting crown cuts a retention mechanism which holds a needle unit of the syringe, allowing a drive mechanism to drive the needle unit into the syringe. The cutting crown is shaped to reduce the force which has to be applied to cut the retention mechanism. Furthermore, a disposable needle assembly for attachment to a syringe unit is proposed. The disposable needle assembly has a needle and connector element for connecting the needle to the syringe unit. The connector element encircles the needle and has a thread on its outer surface for mating with the syringe unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of and claims priority to U.S. application Ser. No. 14/494,602, filed on Sep. 24, 2014, which is a continuation of U.S. patent application Ser. No. 13/696,617, filed Nov. 7, 2012 to Ang and entitled “Retractable Syringe with a Cutting Crown,” which is a filing under 35 U.S.C. 371 as the National Stage of International Application No. PCT/SG2010/000492, filed Dec. 31, 2010 and entitled “A Retractable Syringe with a Cutting Crown,” which claims the benefit of and priority to both U.S. Provisional Patent Application No. 61/332,270, filed May 7, 2010 and entitled “Retractable Syringe with Self-Destruct Design” and U.S. Provisional Patent Application No. 61/373,926, filed Aug. 16, 2010 and entitled “Fittings for Syringe, Needles and Certain Other Medical Equipment, all of which are incorporated herein by reference in their entirety for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to medical syringes. More particularly it relates to a retractable syringe, and to a disposable needle unit for attachment to a syringe unit to form a syringe assembly. 
       BACKGROUND 
       [0003]    Syringe assemblies having a needle (“cannula”) are commonly used for the delivery of fluids, e.g. medicaments, into patients, and/or for aspirating fluids from the patients. Desirably, these syringe assemblies can be operated by a single hand, so that a doctor or nurse is able to operate them while using his or her other hand for another purpose. The force required should not be very great, or the doctor or nurse&#39;s hand will become tired if many injections are performed. Furthermore, it is often undesirable for the same needle of the syringe assembly to be used for multiple patients, and several forms of syringe assemblies are known which prevent or discourage this. 
         [0004]    A first form of syringe assembly comprises a disposable needle assembly consisting of a needle and a connector element. Using the connector element, the needle assembly is temporarily attached to a forward end of an elongate syringe unit, to form the syringe assembly. The syringe unit has a barrel, and a plunger movable forward within the barrel towards the needle assembly. A medicament fluid is located in a volume between a front face of the plunger and the needle assembly. The volume communicates with the inside of the needle, so that as the barrel is advanced the fluid is driven through the needle. When this happens, it is undesirable to have fluid leaking through the junction where the circumference of the front face of the plunger meets the walls of the barrel. In order to be compliant with the ISO 7886-1 standard, the syringe unit has to withstand a compression force resulting in an internal test pressure of 300 kPa (3 bars) without leaking. When the medicament has been dispensed, the needle assembly is removed and disposed of. 
         [0005]    Examples of such syringe assemblies are the Luer Taper and Luer Lock designs standardized under ISO 594. The Luer Taper variant uses a press fit connection between the needle assembly and syringe unit that works using friction. Unfortunately, the needle assemblies are prone to loosening and dropping off the syringe unit. The Luer Lock variant seeks to resolve the problem of loosening associated with the Luer Taper by using a thread locking system. The Luer Lock however has a complicated design involving multiply nested interlocking portions. Specifically, the syringe unit has a conical spout encircled by an internally threaded collar. The connection element of the needle assembly carries an external thread for engaging the collar, and defines a cavity for receiving the spout. The connection element is inserted into the gap between the spout and the collar. 
         [0006]    Both these syringe assemblies contain a large amount of dead space—i.e. volumes in the syringe assembly which contain medicament which cannot be expelled from the syringe assembly. There is typically dead space within the connector unit and/or the interface between the connector unit and syringe unit. This dead space is an issue of concern as the cost of medicament may be high. It is possible to reduce the size of the dead space by reducing the tolerances in the production process, but this increases the production cost of the syringe assembly. 
         [0007]    Another known form of syringe assembly is a “retractable syringe”, which comprises a syringe unit, a needle unit (which in some forms of the retractable syringe is just a needle, but in others is a needle and a “needle hub”, i.e. an encircling body of material, typically molded plastics material) located at one end of the syringe unit, a retention mechanism for maintaining the position of the needle unit with respect to the syringe unit, and a drive mechanism. Operation of the syringe causes the retention mechanism to be disabled after the syringe assembly has been used to deliver a medicament, and the drive mechanism then drives the needle unit into the syringe unit, so that the needle is no longer exposed. Retractable syringes are illustrated in U.S. Pat. No. 6,994,690, WO 2005/053779, U.S. Pat. No. 6,494,863, US 2008/0033355 and U.S. Pat. No. 7,351,224. Another retractable syringe is the BD Integra Syringe marketed by Becton, Dickinson and Company of New Jersey, United States. This retractable syringe has a detachable needle, and uses a form of connection between the syringe unit and the needle reminiscent of the Luer Lock system. Experiments have found that it may take an “activation” force of about 55N in order to break the retention mechanism of a 1 ml version of the Integra syringe. This force is estimated to be over 5 times the compression force which the syringe unit has to withstand without leaking in order to achieve the ISO 7886-1 standard. 
         [0008]    Some of the earlier retractable syringes have thus reduced the amount of “activation” force required by having more dead space within the syringe and/or by staggering the activation process. In cases where staggering is used, dead space may be required to provide room for the cascaded movement of the individual elements involved in the activation process. Therefore, most designs of retractable syringe have dead spaces, resulting in wastage of the medicament. In some designs air may be trapped in the dead space, and is hard to expel this air before the syringe assembly is used. 
         [0009]    Furthermore, since the retention mechanism has to be strong enough to resist the urging force generated by the drive mechanism, the user has to apply an inconveniently large force to break the retention mechanism, and release the needle unit. 
         [0010]    Furthermore, the needle unit, syringe unit, retention mechanism and drive mechanisms of many known retractable syringes have too many parts, leading to high production cost. Since retractable syringes are expensive, some users are tempted to try to re-use them, and there is little to prevent this since almost all components remain intact following operation of the device. 
       SUMMARY 
       [0011]    Embodiments relate to a retractable syringe of the kind including a needle unit, a barrel, a plunger movable within the barrel towards the needle unit, a drive mechanism for driving the needle unit into the barrel, and a retention mechanism for retaining the needle unit with respect to the barrel. The plunger has a cutting crown at its forward end, for cutting the retention mechanism as the plunger is advanced, so that the drive mechanism retracts the needle unit into the barrel. The profile of the cutting crown may include one or more cutting teeth which cut a first portion of the retention mechanism before other portions of the cutting crown cut other portions of the retention mechanism. In other words, a user does not have to apply sufficient force to the plunger simultaneously to break all portions of the retention mechanism. This reduces the required activating force. 
         [0012]    A piston rides on the front of the plunger, such that a volume for holding a medicament is defined between the piston and the needle unit. This piston comprises a weakened portion which is cut by the cutting crown before the cutting crown cuts the retention mechanism. The weakened portion comprises a plurality of notches in a surface of the piston. 
         [0013]    The needle unit may consist of the needle alone. In certain embodiments it comprises both a needle and an encircling body of material (e.g. plastics material) which functions as a needle holder. The needle holder may be formed by molding. 
         [0014]    Other embodiments relate to a disposable needle assembly for attachment to a syringe unit having a plunger movable within a barrel, to form a syringe assembly. The disposable needle assembly has a needle and connector element for connecting the needle to the syringe unit. The connector element encircles the needle and has a thread on its outer surface for mating with the syringe unit. The needle assembly has a central bore of substantially constant bore extending along the whole length of the needle assembly. Thus, there is very little dead space within the needle assembly itself. 
         [0015]    Furthermore, when the needle assembly mates with the syringe unit, the central bore of the needle assembly meets a passage of the syringe unit having substantially the same bore and communicating with the barrel of the syringe unit. This substantially reduces dead space at the interface between the syringe unit and the needle assembly. 
         [0016]    Before attachment to the syringe, the needle of the needle assembly is preferably encased in a protector formed (e.g. by molding) as a one-piece unit together with the hub. The protector may be connected to the hub by a frangible portion. 
         [0017]    Features of the retractable syringe and disposable needle assembly are combinable by forming a syringe unit which mates with the needle assembly using an element which defines the passage, and such that when the needle assembly and element are connected together, they together play the role of the “needle unit”. 
         [0018]    Certain embodiments of the present invention may have the advantages of:
       having a design that may be simple and effective but yet resulting in almost no dead space within the assembly;   having a simple design that may allow for simple tooling and production economy;   having a compact design that may allow for an efficient material usage, thus resulting in a product that may be environmentally sound;   having a design with a minimal part count as components may be fabricated as integrated parts, e.g. the needle protector and the needle hub may be made as a single piece;   having minimized medication wastage as there is almost no dead space;   allowing for the easy elimination of hazardous air bubbles as there is almost no dead space;   preventing the trapping of air bubbles;   allowing for easy inventory management as the needle assembly may be used with both retractable and conventional syringes;   assuring the integrity of a virgin needle as the integrity is easily verified; and   allowing for a tamper proof design that may prevent reuse or recycling.       
 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0029]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0030]      FIG. 1  is an exploded view of a syringe according to an embodiment; 
           [0031]      FIG. 2A  is a cross-sectional view of the syringe of  FIG. 1  when assembled; 
           [0032]      FIG. 2B  is a cross-sectional view of a piston of the syringe of  FIG. 2A  when the piston is proximate a retention mechanism; 
           [0033]      FIG. 2C  is a cross-sectional view of the piston as in  FIG. 2B  when a cutting crown of the syringe is brought to bear against a weak portion of the retention mechanism; 
           [0034]      FIG. 2D  is a cross-sectional view of the piston as in  FIG. 2B  after the cutting crown cuts the piston and the retention mechanism; 
           [0035]      FIG. 3A  is a cross-sectional view of the cutting crown of  FIG. 2B ; 
           [0036]      FIG. 3B  is a side view of the cutting crown as in  FIG. 3A ; 
           [0037]      FIG. 3C  is a perspective view of the cutting crown of  FIG. 2B ; 
           [0038]      FIG. 3D  is a cross-sectional view of a retraction assembly of the syringe of  FIG. 1 ; 
           [0039]      FIG. 3E  is a cross-sectional view of the piston of  FIG. 2B ; 
           [0040]      FIG. 3F  is a cross-sectional view of a part of the retraction assembly of  FIG. 3D  when it is fitted into a front portion of a barrel; 
           [0041]      FIG. 4  is a cross-sectional view of an assembled syringe according to an embodiment; 
           [0042]      FIG. 5A  is a drawing of a needle assembly according to an embodiment; 
           [0043]      FIG. 5B  is a cross-sectional view of the needle assembly of  FIG. 5A  installed onto a syringe unit; 
           [0044]      FIG. 5C  is a cross-sectional view of the needle assembly of  FIG. 5B  but with a needle protector removed; 
           [0045]      FIG. 6  is a cross-sectional view of an embodiment including a portion of the needle assembly of  FIG. 5A ; 
           [0046]      FIG. 7  is a schematic drawing showing two teeth of the cutting crown of  FIG. 2B  with different cutting distances; 
           [0047]      FIG. 8A  is a schematic side view of the cutting crown of  FIG. 2B  with five cutting teeth; 
           [0048]      FIG. 8B  is a schematic side view of the cutting crown as shown in  FIG. 8A  but with two cutting teeth; 
           [0049]      FIG. 9A  is a perspective view of a variation of a plunger of the syringe of  FIG. 1 ; 
           [0050]      FIG. 9B  is a perspective view of a variation of a barrel of the syringe of  FIG. 1 ; 
           [0051]      FIG. 10A  shows a syringe piston of an embodiment; and 
           [0052]      FIG. 10B  is an enlarged view of part of the syringe piston of  FIG. 10A . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0053]    A Retractable Syringe 
         [0054]      FIG. 1  is an exploded view of a syringe  100  according to embodiments of the present invention.  FIG. 2A  is a cross-sectional view of the syringe of  FIG. 1  when assembled. The arts of the syringe  100  are described next with reference to  FIGS. 1 and 2A . 
         [0055]    The syringe  100  comprises a hollow barrel  140  defining a chamber, a plunger  120  insertable within the chamber, a piston  130  which is inserted into the chamber before the plunger  120 , and a retraction assembly  150  to be fitted at a front portion  1420  of the barrel  140 . The barrel  140  functions as a housing providing support to the other parts of the syringe  100 , e.g. the retraction assembly  150 . In different versions of the embodiment, the size of the barrel  140  differs, so as to have varying fluid capacities. In these various versions of the embodiment, the barrel  140  has differing diameters, and the diameters of the piston  130 , plunger  120  and/or retraction assembly  150  also differ to suit the diameter of the barrel  140 . A spring  1600  surrounds the needle holder  1560 . 
         [0056]    The retraction assembly  150  includes a needle  1520  and a needle holder  1560  which encircles the needle  1520 . The needle holder  1560  is integral with a needle hub  1510  and with a seal  1540 . The junction between needle hub  1510  and the seal  1540  is an annular weak portion  1590  (shown in  FIG. 3D ). As described below, the needle  1520 , needle holder  1560  and needle hub  1510  together function as a needle unit which is eventually driven into the barrel  140 , and the seal  1540  and annular weak portion  1590  function as a retention mechanism for retaining the needle unit relative to the syringe unit until the retention mechanism is disabled. Once this happens, the spring  1600  functions as a drive mechanism to drive the needle unit into the plunger  120 . 
         [0057]    The barrel  140  comprises a hollow cylinder portion  1410 , a mouth  1440  and the front portion  1420 . The front portion  1420  may be tapered. The cylinder portion  1410  forms the main body of the syringe, thus permitting a user to hold the syringe steady in place when performing an injection. Optionally, the barrel  140  comprises one or more flanges  1430  for the purpose of providing additional holding surfaces. These flanges  1430  may be as shown in  FIG. 1 , situated at the mouth  1440  of the barrel away from the front portion  1420 . Alternatively, the flanges  1430  may be situated anywhere along the external surface of the cylinder portion  1410  or even the front portion  1420 .  FIG. 9B  is a perspective view of a variation of the barrel  140  where the flanges  1430  are disposed a suitable distance away from the mouth  1440 . The distance is chosen to allow for a separation between the flanges  1430  and the exposed end of the plunger  120  which may allow a user to have a better grip of the syringe. Further, the skilled person will also understand that the flanges  1430  may take on other forms apart from that shown in  FIGS. 1 ,  2 A or  9 B, for example taking the form of finger loops or an annular lip. 
         [0058]    The plunger  120  comprises a stem  1290  with an end cap  1100  fitted over a top end of the plunger  120 , and a cutting crown  1200  at the bottom end of the plunger  120 . The stem  1290  is tubular. As described below, the tubular stem permits the retraction of the needle  1520  into stem  1290  and thus into the main body of the syringe  100 . The purpose of the end cap  1100  is to improve a user&#39;s grip when the user is performing the push and pull pumping action associated with performing an injection. Further, the end cap  1100  may prevent the needle holder  1560  and/or needle  1520  from dropping out of the plunger  120  after the needle  1520  is driven into the plunger. The end cap  1100  may also be suitable for a user to depress the plunger  120  within the barrel  140  with a thumb. Such an end cap  1100  may be made as a separate piece to be fitted onto the top end of the stem  1290  in which case the plunger  120  contains a retention means for holding the end cap  1100  in place. Optionally, as shown in  FIG. 9A , the end cap  1100  may be made integrated with the stem  1290 .  FIG. 9A  is a perspective view of a variation of the plunger  120  where the end cap  1100  and the stem  1290  is formed together as a singular item. By integrating the end cap  1100  together with the stem  1290 , the part count may be lowered, thus allowing for a more efficient use of production material and an optimization of production cost. 
         [0059]    Similarly, the cutting crown  1200  may be made either integrated as part of the plunger  120 , or it may be made as a separate piece to be fitted at the bottom end of the plunger  120 . The cutting crown  1200  will be described to a greater detail below. 
         [0060]    The piston  130  is disposed over the cutting crown  1200 . The piston  130  comprises a cylindrical portion  1340  which has an outer diameter slightly less than that of the inner diameter of the cylinder portion  1410  of the barrel  140 , and further comprises one or more annular sealing lips  1310  running around the outer surface of the cylindrical portion  1340 . The cylindrical portion  1340  forms a wall around the cutting crown  1200  when the piston  130  is disposed over the cutting crown  1200 . The cylindrical portion  1340  provides support for the piston  130 , thus allowing the piston  130  to withstand the compression forces exerted on the piston  130  when the plunger  120  is depressed. The sealing lips  1310  allow the piston  130  to have a tight fit within the body of the barrel  140  and may thus permit the proper expelling or aspiration of fluids out of or into the barrel  140 . Further, the piston  130  may also be made out of a resilient material so achieve a tighter fit between the piston  130  and the barrel  140 . The tight fit may help to ensure the proper expelling or withdrawing of liquid or gas by the syringe  100 . 
         [0061]    The piston  130  will now be further described with the aid of  FIG. 3E .  FIG. 3E  is a cross-sectional view of the piston  130 . The piston  130  comprises a central domed elevated portion  1385 , a weak portion  1380  forming a circumferential perimeter about the elevated portion  1385 , and a guiding means  1330  extending inwardly from the inner surface of the piston  130 . As described below, the piston  130  will be cut by the cutting crown  1200 , and the weak portion  1380  reduces the effort required for cutting the piston  130 . 
         [0062]    The elevated portion  1385  serves to thicken the central portion of the piston  130 . Thickening the central portion reinforces it and thus prevents an inward collapse of the piston  130  as it withstands the resistive forces exerted by the fluids contained within the barrel  140  when the plunger  120  is depressed. Further, having a domed shape for the elevated portion  1385  may serve to better diffuse the resistive forces, thus providing better reinforcement. It is noted that having such a reinforced piston  130  may also be advantageous as it serves to reduce the flexing of the piston  130  when the plunger  120  is depressed or withdrawn. 
         [0063]    Turning now to the guiding means  1330 , the guiding means  1330  may also provide support for the piston  130  so as to enable the piston  130  to withstand the compression forces exerted on the piston  130  when the plunger  120  is depressed. The elevated portion  1385  and the guiding means  1330  guide cutting teeth  1220  of the cutting crown  1200  towards the weak portion  1380 . There may be any number of cutting teeth, but there are preferably in the range 2 to 5 such teeth. The guiding means  1330  may be annular with a chamfered edge  1350  facing away from the inner surface of the piston  130 . A ledge  1230  of the cutting crown  1200  rests against the chamfered edge  1350  when the plunger  120  advances. Thus, the guiding means  1330  also plays the role of a spacer preventing the cutting teeth  1220  from prematurely cutting into the weak portion  1380  when the piston  120  is expelling fluid from the syringe  100 . The chamfered edge  1350  may also aid in the guiding of the cutting teeth  1220  towards the weak portion  1380 . Further, the guiding means  1330  may also play the role of a biasing means providing a biasing force against the cutting crown  1200  when the end of the plunger  120  approaches the seal  1540 . Such a biasing means  1330  provides a tactile feedback to a user depressing the plunger  120  in the form of a “click”. This allows the user to know when the cutting crown  1200  is about to bear against the weak portion  1380  of the piston  130  and the weak portion  1590 . In such a case, after the cutting crown  1200  breaks the weak portion  1590 , the biasing means  1330  may be deformed. 
         [0064]    The outer cylindrical portion  1340  has a catch  1370  for retaining the piston  130  over the cutting crown  1200 . This catch  1370  may take the form of an annular recess running around the inner surface of the cylindrical portion  1340  in which case the catch  1370  may also provide added rigidity to the piston  130 , thus restricting any buckling of the piston  130  when a large compression force acts upon it. With reference to  FIG. 2A , the catch  1370  may be mated with a ridge  1250  running around the cylindrical sides of the cutting crown  1200 . Such a ridge  1250  and catch  1370  may permit the piston  130  to be retained over the plunger  120  when the plunger  120  is slid up or down the length of the barrel  140 . The plunger  120  and piston  130  arrangement thus enables the push and pull pumping function for injecting or aspirating a fluid. 
         [0065]    The weak portion  1380  of the piston  130  is shown in more detail in  FIGS. 10A and 10B . In particular, weak portion  1380  comprises a plurality of notches  1382 , forming a patterned blind perforation. That is, the notches  1382  do not penetrate completely through the surface of the piston  130  which is contacted by the cutting crown  1200 . 
         [0066]    The notches  1382  are formed as a series of blind apertures or blind furrows around the elevated portion  1385  of the piston  130 , at a diameter which is less than the internal diameter of the cutting crown  1200 . By providing a weak portion  1380  thus formed, a series of stress points is provided on the plunger  130  such that the plunger  130  is more easily cut by the cutting crown  1200 , to allow the cutting crown  1200  to more readily proceed to rupture the retention means which retains the needle unit. Further, the cutting edge at the periphery of the elevated portion  1385  is smoother than if an unbroken annular weak portion is used. In addition, the ease of manufacturing the plunger  130  by molding is increased, as weak portion  1380  may be made significantly thicker. 
         [0067]    Returning to  FIGS. 1 and 2A , the inner dimensions of the mouth  1440  and the cylinder portion  1410  of the barrel  140  are made suitable for receiving the piston  130  and the plunger  120 . In normal assembly, the piston  130  may be fitted to the plunger  120  before the plunger  120  is inserted into the barrel  140 . Alternatively, optionally, the piston  130  and plunger  120  may be formed as a one-piece (i.e. integral) item. After the plunger  120  and piston  130  are inserted into the barrel  140 , the plunger  120  and piston  130  are slideable within the barrel  140 . 
         [0068]    A passage  1530  runs through the needle holder  1560 , so that the volume  1470  within the barrel  140  between (i) the piston  130 , and (ii) the needle hub  1510  and seal  1540 , communicates with the inside of the needle  1520 . Otherwise, fluid cannot escape from the volume  1470 . 
         [0069]      FIGS. 3D and 3F  respectively show a cross-sectional view of the retraction assembly  150 , and a cross-sectional view of part of the retraction assembly  150  of  FIG. 3D  when it is fitted into the front portion  1420  of the barrel  140 . The weak portion  1590  around the seal  1540  is frangible so that when it is broken, the needle holder  1560  will be released and be pushed upwardly by the spring  1600 . The weak portion  1590  allows the cutting crown  1200  of the plunger  120  to break the retention mechanism formed by the weak portion  1590  and the seal  1540  with greater ease. Further, the thinness of the weak portion  1590  allows the seal  1540  to be flexed. By flexing the seal  1540 , the retraction assembly  150  may be fitted more easily into the front portion  1420  of the barrel  140 . 
         [0070]    When the syringe  100  is assembled, the retraction assembly  150  is attached to the front portion  1420  of the barrel  140  by the outer surface  1580  of the seal  1540 . The outer circumference  1580  comes to rest against the side wall of the front portion  1420 . An annular recess  1460  may be formed into the inner wall of the front portion  1420  to function as a retaining means for holding the seal  1540 . Such an annular recess  1460  may also have the advantage of creating a more water tight seal between the seal  1540  and the front portion  1420 . An adhesive may also be applied around the outer circumference of the seal  1540  to create a tighter seal. Further, it is also envisaged that in variants of the embodiment, parts or the whole of the retraction assembly  150  may be formed integrated with the front portion  1420 . As an example, the seal  1540  may be integrated with the front portion  1420  to form a single piece. 
         [0071]    The spring  1600  is compressed within the front portion  1420  of the barrel  140 . The spring  1600  may be attached at a first end  1610  to the needle holder  1560  and at the other end  1620  attached to the front portion  1420  of the barrel  140 . The needle holder  1560  comprises one or more anchor points  1570  holding the end  1610  of the spring  1600 . The anchor point  1570  may take the form of a ledge with an outer diameter wider than that for the spring  1600 . In such a case, the end  1610  may rest upon the anchor point  1570 , the anchor point  1570  thus providing a point of resistance which permits the spring  1600  when compressed to push the needle holder  1560  upwardly. 
         [0072]    At the other end  1620  of the spring  1600 , the end  1620  of the spring  1600  bears against an annular lip  1450  of the front portion  1420 . The annular lip  1450  provides a point of support which the compressed spring  1600  pushes against. The compressed spring  1600  thus expands and pushes in opposite directions against the annular lip  1450  and the anchor point  1570 . 
         [0073]    Returning to  FIGS. 1 and 2A , the syringe  100  optionally further comprises a needle protector  1700 . Such a needle protector  1700  may be fitted over the front end  1420  of the barrel  140 . The protector  1700  has a retention means for keeping the protector  1700  in place over the front end  1420  and the needle  1520 . 
         [0074]    The cutting crown  1200  will now be further described with the aid of  FIGS. 3A ,  3 B and  3 C.  FIG. 3A  is a cross-sectional view of the cutting crown  1200 .  FIG. 3B  is a side view of the cutting crown  1200  of  FIG. 3A .  FIG. 3C  is a perspective view of the cutting crown  1200  of  FIG. 3A . 
         [0075]    The cutting crown  1200  comprises a ledge  1230  that rubs against the chamfered edge  1350  of the piston  130  as the cutting teeth  1220  approach the seal  1540 . The cutting crown  1200  further comprises one or more cutting teeth  1220  located at the leading end of the plunger  120 . These cutting teeth  1220  are arranged circumferentially around the longitudinal axis  122  of the plunger  120  and taper inwardly towards the longitudinal axis  122 . The edge of the teeth  1220  may be chamfered to yield a sharpened cutting edge suitable for cutting into the weak portion  1380  of the piston  130  and the weak portion  1590  of the retraction assembly  150 . The teeth  1220  are thus be capable of cutting the piston  130  and the weak portion  1590  with a minimal amount of effort. Interspaced between each consecutive cutting tooth  1220  along the cutting edge are bridges  1210 . Each bridge  1210  may have an arcuate profile spanning from tooth to tooth. When the cutting crown  1200  is brought to bear against the piston  130  and/or the weak portion  1590 , such an arcuate profile may result in a better diffused distribution of resistive forces acting upon each tooth  1220 , thus resulting in teeth  1220  that are stronger and better able to cut through the piston  130  and/or the weak portion  1590 . 
         [0076]    When designing the cutting crown  1200 , a cutting distance d (as indicated in  FIG. 3A ) and a puncturing force F (as indicated in  FIG. 2C ) may be minimized. As is illustrated in  FIGS. 3A and 3B , the cutting distance d is defined as the perpendicular distance from the apex of the teeth  1220  to the nadir of the bridge  1210 . The puncturing force F is defined as the force required for the teeth  1220  of the cutting crown  1200  to cut the piston  130  and/or the retraction assembly  150 . 
         [0077]    A short cutting distance d may be advantageous because it means a user will have to provide a sustained cutting force on the plunger  120  for a shorter sliding distance. A lesser puncturing force F may be advantageous because it means the user will have to provide less instantaneous force to pierce into the piston  130  and/or the retraction assembly  150 . 
         [0078]    There exists a trade-off between the cutting distance d and the puncturing force F.  FIG. 7  shows two possible profiles for bridges  1210  and tooth  1220 , each profile having a different value of d. It can be seen that lower values of d are associated with lower sharpness of the tooth  1220 . The reduced “sharpness” of each tooth  1220  however is undesirable as it requires a greater puncturing force F. Consequently, in order to have a lower puncturing force F, the distance d may be made larger in order to increase the “sharpness” of the tooth  1220 . 
         [0079]    Holding the cutting distance d constant, the “sharpness” of each tooth  1220  may be improved by increasing the number of teeth  1220  arranged around the end of the plunger  120 .  FIG. 8A  shows a side view of a cutting crown  1200  with five cutting teeth  1220  (the solid lines in  FIG. 8A  show the port of the cutting crown which face the viewer, while the dashed portions show the part of the cutting crown facing away).  FIG. 8B  shows an alternative cutting crown  1200  with two cutting teeth  1220 . It can be seen that the teeth  1220  of  FIG. 8A  are “sharper” than the teeth  1220  of  FIG. 8B . 
         [0080]    It is thus noted that there exists a further trade-off between the number of teeth  1220  and the puncturing force F. While having more cutting teeth  1220  may result in “sharper” teeth  1220 , it also results in the puncturing force F having to be greater because it is divided between a greater number of teeth  1220 . Thus in the interest of having a lesser puncturing force F, a lesser number of cutting teeth  1220  may be employed. 
         [0081]    The typical operation of the assembled syringe  100  is next described with reference to  FIGS. 2A to 2D .  FIG. 2A  is a cross-sectional view of the assembled syringe  100  before it is used.  FIG. 2B  is a cross-sectional view of the piston  130  when the piston  130  is proximate the seal  1540 .  FIG. 2C  is a cross-sectional view of the piston  130  when the cutting crown  1200  is cutting the weak portion  1380  of the piston  130 . Finally,  FIG. 2D  is a partial cross-sectional view of the piston  130  after the cutting crown  1200  has cut the piston  130  and the weak portion  1590 . 
         [0082]    A user of a fresh syringe  100  first removes the needle protector  1700 . The tip  1570  of the needle may be placed in a fluid e.g. a medicament that is to be drawn into the syringe  100 . The plunger  120  is then withdrawn from the barrel  140 . This may be done by the user using a first hand to firmly grasp the outer surface of the barrel  140  while using a second hand to hold the end cap  1100  and draw back the plunger  120 . 
         [0083]    The syringe  100  has a space  1470  that is formed within the barrel  140  between an outer surface of the piston  130  facing away from the cutting crown  1200  and the top face  1565  of the needle holder  1560 . When the plunger  120  is withdrawn from the barrel  140 , the increase in the volume of the space  1470  sucks fluid into the space  1470 . The fluid is fed into the space  1470  via the passage  1530  leading through the needle holder  1560  of the retraction assembly  150 . 
         [0084]    The user then depresses plunger  120  in order to expel the fluid from the syringe  100 . This may be done by the user grasping the outer surface of the barrel  140  with the index finger and the middle finger of a hand, while using the thumb of the hand to press against the end cap  1100 . The flanges  1430  aid this action as they provide additional holding surfaces for the index and middle finger. 
         [0085]    When the plunger  120  is depressed, the piston  130  forces the fluid contained within the space  1470  out via the passage  1530 . The outer surface  1320  of the piston  130  that faces away from the cutting crown  1200  has a domed profile which matches the concave receiving surface disposed at the front portion  1420  of the barrel  140 . The receiving surface consists of the top face  1565  of the needle holder  1560 , the seal  1540  and the end wall  1461  of the cylinder portion  1410  of the barrel  140 . By having such a concave receiving surface, the syringe  100  may have the advantage of being easier to “bleed”. What this means is that air bubbles contained within the space  1470  may be removed via the passage  1530  more easily. Also, the receiving surface is smooth and there are no gaps or protruding seams at the points where the top face  1565  of the needle holder  1560  meets the seal  1540 , and where the seal  1540  meets the end wall  1461  of the barrel  140 . Such a smooth surface may further contribute to the advantage of being easier to “bleed” as there will be an absence of irregularities on the surface which may retain air bubbles. 
         [0086]    The process of “activating” the syringe  100  is described next across three “activation” states. Referring now to  FIG. 2B , the plunger  120  is depressed until the cutting crown  1200  is approaches the weak portion  1380  of the piston  130 . The ledge  1230  of the cutting crown  1200  rubs against the chamfered edge  1350  of the piston  130  and the guiding means  1330  provides a tactile feedback to a user depressing the plunger  120  in the form of a “click”. When this happens, the syringe  100  is in the first “activation” state and the guiding means  1330  is also forced outwardly away from the longitudinal axis  1440 . This frees the guiding means  1330  from its role as a spacer preventing the cutting crown  1200  from prematurely cutting the weak portion  1380 . Further, the cylindrical portion  1340  may be deformed as the ridge  1250  runs down the inside of the piston  130 . As the plunger  120  is further depressed, the elevated portion  1385  and the guiding means  1330  guide the one or more cutting teeth  1220  of the cutting crown  1200  towards the weak portion  1380 . It is estimated that the force required to achieve the first “activation” state may be about 1.2 to 2 times the compression force which the syringe has to withstand without leaking in order to achieve the ISO 7886-1 standard. 
         [0087]    During this process, a radially-outer portion of the front surface  1320  of the piston  160  first meets the end wall  1461  of the cylinder portion  1410  of the barrel  140 . Gradually, more radially-inward portions of this surface  1320  contact the end wall  1461  of the portion  1410 , and then the seal  1540  and top face  1565  of the needle holder  1560 . In other words, the contact area of the plunger with the end of the chamber gradually expands radially-inwardly. When the syringe  100  is emptied, the top face  1565  fully meets the outer surface  1320 . This may assist in fully expelling the fluids contained within the space  1470 , thus preventing as fluid wastage. 
         [0088]    Turning now to  FIG. 2C , the plunger  120  is depressed further until the cutting crown  1200  cuts through the weak portion  1380  of the piston  130 . The syringe  100  is now in the second “activation” state. The application of the puncturing force F on the plunger  120  forces the cutting teeth  1220  to pierce into the weak portion  1380  of the piston  130  and thereafter also cuts through the weak portion  1590  of the retraction assembly  150 . 
         [0089]    Referring now to  FIG. 2D , the plunger  120  is depressed even further for a cutting distance d, at which time syringe  100  is in the “third” activation state and the sharpened cutting edge of the teeth  1220  cuts completely through the weak portion  1590 . When this happens, the retention mechanism formed by the weak portion  1590  and the seal  1540  can no longer retain the needle unit (i.e. the needle  1570 , the needle holder  1560  and the hub  1510 ). Furthermore, the elevated portion  1385  has been released from the main body of the piston  130 . This permits the needle unit to be pushed upwards by the spring  1600  and into the stem  1290  of the plunger  120 . When doing so, the spring  1600  also pushes the released elevated portion  1385  into the stem  1290 . The spring  1600  is allowed to expand to its maximum extend thus bringing the needle holder  1560  and any attached needle  1520  entirely within the stem  1290 . The needle holder  1560  and attached needle  1520  are thus retracted into the syringe  100 . The syringe  100  in this state may thus be said to have been “activated”. It is noted that the force needed to complete the cut through the weak portion  1590  in the “third” activation state may be less than the puncturing force F. 
         [0090]    It is noted that once the syringe  100  has been “activated”, the piston  130  and the retraction assembly  150  are destroyed. The syringe  100  thus may no longer be reused and this may prevent the recycling of syringes. 
         [0091]    A second embodiment of the present invention is described next with the aid of  FIG. 4 .  FIG. 4  is a cross-sectional view of an assembled syringe  110  according to a second embodiment of the present invention. The syringe  110  may be especially suitable for small fluid volumes (e.g. 1 ml or 10 ml), but may also be used with any other volume of fluid. The second embodiment has elements in common with the first embodiment and these like elements are designated using like reference numerals. 
         [0092]    The second embodiment differs from the first embodiment by having a wider cutting crown  1201  of a diameter that is substantially the same as that for the stem  1290 . Conspicuously, the guiding means  1330  previously present in the second embodiment is now absent and the end wall  1461  is now reduced to an annular ledge. The roles previously played by the guiding means  1330  may now be fulfilled by the outer cylindrical portion  1340  of the piston  130 . 
         [0093]    The syringe  110  may be operated the same way as the syringe  100 . When “activating” the syringe  110 , in the first “activation” state i.e. when the plunger  120  is depressed until the cutting crown  1201  approaches the weak portion of the piston  130 , the lower of the sealing lips  1310  comes to rest against the end wall  1461 . The piston  130  is thus restricted from travelling further down the barrel  140 . The cylindrical portion  1340  now performs the role of a spacer preventing the cutting crown  1201  from prematurely cutting the weak portion  1380 . 
         [0094]    A further application of downward pressure dislodges the ridge  1250  from the catch  1370 , thus permitting the cutting crown  1201  to travel downwardly free of the piston  130 . The dislodgment of the ridge  1250  from the catch  1370  may result in a tactile feedback to the user in the form of a “click”. As the cutting crown  1201  travels downwardly towards the weak portion  1380  of the piston  130 , the elevated portion  1385  and the cylindrical portion  1340  guide the one or more cutting teeth  1220  of the cutting crown  1201  towards the weak portion  1380 . Meanwhile, the ridge  1250  also deforms the cylindrical portion  1340  outwardly away from the longitudinal axis  1440  as it travels down the cylindrical portion  1340 . 
         [0095]    As in the case of the first embodiment i.e. the syringe  100 , the downward travel of the cutting crown  1201  causes the one or more cutting teeth  1220  of the cutting crown  1201  to eventually bear against the weak portion  1380  of the piston  130 . The second “activation” state is thus arrived at. In the second and third “activation” states, the further functioning of the elements of the syringe  110  are the same as that for the syringe  100 . 
         [0096]    We now turn to a third embodiment of the invention, which is a needle assembly  200 . The needle assembly  200  will be described with reference to  FIGS. 5A to 5C .  FIG. 5A  is a needle assembly  200  in its initial state.  FIG. 5B  is a cross-sectional view of the needle assembly  200  of  FIG. 5A  when installed onto a syringe.  FIG. 5C  is a cross-sectional view of the needle assembly  200  of  FIG. 5B  but with a protector  1700  portion removed. 
         [0097]    The needle assembly  200  comprises a needle hub  1510  with threads  520  on the outer side wall, a needle (cannula)  1520  extending from a first end  522  of the hub  1510  to a distal tip  1570 , and a tubular neck  510  that is suitable for insertion into the needle holder  1560  of a syringe extending from a second end  524  of the hub. A cylindrical bore  515  runs continuously length-wise through the center of the needle  1520  from the tip  1570 , and on through the center of the hub  1510 , and further on through the center of the neck  510  before terminating as an opening  565  at an end face  560  of the neck  510 . The bore  515  functions as a conduit for the delivery of fluids from the syringe out through the tip  1570 . 
         [0098]    The needle  1520  may be made out of a metal e.g. stainless steel, or optionally other materials such as a rigid plastic. The tip  1570  of the needle  1520  may be chamfered and sharpened to assist in piercing surfaces. Optionally, the needle  1520  extends through the first end  522  of the hub  1510  and part of the way through the neck  510 . In other embodiments, the needle  1520  may extend through the entire length of the needle assembly  200  from the tip  1570  to the end face  560 . In either case, the bore  515  has a substantially constant shape (typically a circle) and size along its length, making it possible to much reduce any dead spaces. 
         [0099]    The hub  1510  is cylindrical and has one or more nested helical threads  520  running on at least part of the outer cylindrical side wall of the hub  1510 . 
         [0100]    The threads  520  of the hub  1510  mate with corresponding threads  1585  present on the inner surface of the needle holder  1560 . It is however envisaged that in a variant of the embodiment, the hub  1510  may not necessarily use a twist-locking system in order to mate the hub  1510  with the needle holder  1560 . For example, a stopper based locking system may be used. 
         [0101]    The neck  510  has a smooth outer surface. The neck  510  terminates with the end face  560  which may be beveled circumferentially so as to allow an easier fitting of the neck  510  into the needle holder  1560 . The smooth outer surface of the neck  510  may function as a guiding surface leading the corresponding threads  1585  to the threads  520 . Further, the neck  510  may result in a more secure seal, thus preventing the leaking of fluids originating from a passage  1471  of the syringe. 
         [0102]    As mentioned above, the needle assembly  200  comprises a needle protector  1700  that is disposed over the needle  1520 , thus encasing the needle  1520 . Such a needle protector  1700  may prevent the needle  1520  from becoming a sharps hazard. It is envisaged that the protector  1700  may be formed as a single piece together with the hub  1510  in which case the protector  1700  may be connected to the hub  1510  by a frangible portion  550 . Such a single piece design may confer the advantage of manufacturing ease, and a tamper proof needle assembly  200 . The integrity of a virgin needle assembly  200  with the frangible portion  550  intact may thus be assured. 
         [0103]    The frangible portion  550  is intended to break easily when a user twists the protector  1700  away relative to the hub  1510 . The protector  1700  may further comprise one or more wings  530  extending from the outer wall of the protector  1700 . The wings  530  may permit a user to better deliver rotational force to the protector  1700  about the longitudinal axis  202  of the protector  1700  when the protector  1700  is encasing the needle  1520 . The skilled person however will understand that the wings  530  may take on other forms apart from that shown in  FIGS. 5A and 5B , for example taking the form of finger loops or simply just a roughened surface. 
         [0104]    The needle protector  1700  may further comprise a biasing means for holding onto the needle  1520  when the protector  1700  is encasing the needle  1520 . Such a biasing means may be disposed internally within the protector  1700  and may serve to prevent the protector  1700  falling off the needle  1520  after the frangible portion  550  is broken. 
         [0105]    The needle holder  1560  comprises a tubular recess  1575  that is suitable for receiving the needle assembly  200 . The internal surface of the recess  1575  has a threaded portion containing threads  1585  which correspond to the threads  520  of the needle assembly  200 . 
         [0106]    It is noted that the hub  1510  of the needle assembly  200  has the form of a male body with outer threads  520 , and the needle holder  1560  of the syringe has the recess  1575  which results in a female body with inner threads  1585 . This results in a needle assembly  200  and needle holder  1560  that are easier to manufacture since the interlocking portions of both the needle assembly  200  and the needle holder  1560  i.e. the hub  1510  and the recess  1575  respectively, are not multiply nested when mated. Also, the compact design of the needle assembly  200  may require less material when manufacturing, thus allowing for more efficient material usage. 
         [0107]    The needle assembly  200  may be installed into the syringe by inserting the neck  510  of the needle assembly  200  into the tubular recess  1575  first. The neck  510  is inserted past the inner threads  1585  until where the outer threads  520  of the needle assembly  200  are ready to be screwed onto the inner threads  1585 . 
         [0108]    The user may then screw the outer threads  520  of the needle assembly  200  onto the inner threads  1585  by holding onto the wings  530  of the protector  1700  and twisting. Such a mating of the outer threads  520  with the inner threads  1585  may be described as a “lock” because the interlocking threads may prevent the leakage of any fluid. Once the needle assembly  200  is fully inserted in the needle holder  1560 , the hub  1510  of the needle assembly  200  may twist no further. By further twisting the protector  1700 , the user delivers rotational force to the protector  1700  about the longitudinal axis  202  of the protector  1700 . The rotational force breaks the frangible portion  550  connecting the protector  1700  to the hub  1510 . When this happens, the protector  1700  becomes free from the rest of the needle assembly  200  and may be removed. The further twisting action that is used to break the frangible portion  550  may also assist in lodging the outer threads  520  together with the inner threads  1585  more tightly. 
         [0109]    The recess  1575  is in fluid communication with the cylindrical chamber  1470  of the barrel  140  through the passage  1471 . Thus when the needle assembly  200  is fully inserted into the recess  1575 , the opening  565  of the end face  560  coincides with the passage  1471 . The fact that the opening  565  of the end face  560  faces the passage  1471  and has substantially the same narrow diameter, prevents the existence of a dead space between the end face  560  of the needle and the passage  1471 . A substantially constant bore  515  is formed from the space  1470  in the barrel  140  of the syringe  100 , through the passage  1471 , into the opening  565  of the needle assembly  200  and out through the tip  1570  of the needle  1520 . Such a continuous bore  515  reduces dead spaces within the entire delivery channel since the diameter of each section of the continuous bore  515  may be similar. Also, the uniform diameter avoids trapping of air within the bore  515 . The continuous bore  515  may also be made to be thin so as to reduce the amount of residual fluid present in the delivery channel when the syringe  100  is fully depressed. 
         [0110]    It is noted that once the needle assembly  200  is installed onto the needle and the protector  1700  is broken free of the needle hub  1510 , there may be no practical way for unscrewing the needle assembly  200  from the syringe  100  as the protector  1700  which provides traction for a user to twist the needle assembly  200  is now free of the needle hub  1510 . This may thus also discourage the recycling of the needle assembly  200 . 
         [0111]    A fourth embodiment of the invention is shown in cross-section in  FIG. 6 . The needle assembly of  FIG. 6  is the third embodiment shown in  FIG. 5A , but the syringe is a variant of the syringe  100  of  FIG. 1 , having corresponding portions labeled by the same reference numbers. In this variant, the retraction assembly  150  of  FIG. 1  is replaced by a structure which is formed by inserting the needle assembly of  FIG. 5A  into a needle holder  1560 . The operation of this embodiment is then the same as that of the first embodiment, except that the “needle unit” which is drawn into the syringe unit is composed of the needle  1570 , and the needle hub  1510 , and the needle holder  1560 . Note that since the needle assembly  200  is usable both in the context of a retractable syringe (as in  FIG. 6 ) and a non-retractable syringe (as in  FIG. 5B ) only a single type of needle assembly needs to be stocked to enable both these functions. 
         [0112]    In this specification, the terms “needle” and “cannula” have been used interchangeably to refer to the needle  1520 . Further, the term “fluid” may refer to either a “liquid” or a “gas”. 
         [0113]    Whilst example embodiments of the invention have been described in detail, many variations are possible within the scope of the invention as will be clear to a skilled reader.