Patent Publication Number: US-2012035541-A1

Title: Single use safety syringe

Description:
TECHNICAL FIELD 
     The present invention relates to a single use, safety syringe. In particular, the present invention relates to a syringe having a manually retractable needle that can be retained safely, after a single use, within the barrel of the syringe by a needle locking assembly. 
     BACKGROUND ART 
     Contaminated syringes pose a very serious danger to the health of persons who handle them, and many devices and techniques have been proposed to protect such persons from direct contact with, or injury by, contaminated syringes. Preventing needle stick injury to persons using syringes to collect, transfer and/or inject liquids for medical and other uses is especially important where there is a risk of transmitting blood borne diseases, such as Hepatitis B and Human Immunodeficiency Virus (HIV). Many of the proposed solutions provide an arrangement for retracting the contaminated needle into the barrel of the syringe so that both can be disposed of at one time without the person coming into contact with the needle. Such retractable needle syringes can be so arranged as to retract their needles either automatically or manually. With automatically retractable syringes, when an injection has been administered, the plunger is further depressed and deactivates a locking mechanism which normally locks the hub of the needle within the front or leading end of the barrel under tension of a spring or other such biasing means. Deactivation of the locking mechanism allows the needle to retract into the barrel under the influence of the spring. Similarly, with manually retractable syringes, when an injection has been administered, the plunger becomes locked to the hub of the needle by a mechanism that allows the needle to retract into the barrel as the plunger is withdrawn or pulled away from the front of the barrel. 
     However, with both automatically and manually retractable needle syringes, measures need to be taken to ensure that the plunger is not able to be, by deliberate or accidental means, completely withdrawn from the rear or trailing end of the barrel or, if possible, reloaded into the barrel to cause the needle to re-emerge from the front of the barrel, and thereby expose a person to the risk of contact with contaminated liquid or needle stick injury. 
     One approach to ensuring that the plunger cannot be reused in these ways after the needle has been manually retracted into the barrel is to snap off the emerging end of the plunger at a pre-weakened point of the plunger adjacent the opening to the rear of the barrel, but this is not entirely satisfactory as access may still be gained to the snap off point of the plunger retained within the barrel and there is no or little resistance to moving the so retained plunger relative to the barrel once such access has been obtained. 
     DISCLOSURE OF INVENTION 
     It is an object of the present invention to provide a single use, safety syringe that is arranged to retract its needle manually, and which has a needle locking assembly that can retain a retracted needle within the barrel of the syringe in so reliable a way as to prevent any subsequent movement of the plunger to which the needle is attached relative to the barrel, and to restrict access to the snap off point of the plunger. 
     According to the present invention, there is provided a single use safety syringe comprising a barrel having a leading end and a trailing end, a needle sealably, but removably, attached to an opening in the leading end, a plunger slidably received through an opening in the trailing end, wherein, upon completion of an injection, the plunger engages the needle and the needle can then be manually retracted into the barrel, a locking tab assembly securely connected to a main body of the barrel and forming the opening in the trailing end, the locking tab assembly including at least one first locking member and at least one first holding member when assuming a locking-ready position, wherein the or each first locking member is adapted to engage by either of a hooked fit or a snapped fit a respective one or more second locking member connected with the plunger when the needle is fully retracted to prevent any subsequent movement of the plunger with the engaged needle towards the leading end, and wherein, upon such engagement, the or each first holding member is adapted to butt up against a respective one or more second holding member connected with the plunger to prevent any subsequent movement of the plunger with the engaged needle towards the trailing end. 
     In a first preferred embodiment, when engagement is by a hooked fit, the or each first locking member is a locking finger, and the respective one or more second locking member is a locking port. 
     Alternatively, the or each first locking member is a locking port, and the respective one or more second locking member is a locking finger. 
     It is preferred that there are two locking fingers and there are two respective locking ports. Preferably, the or each first holding member is a holding ring, and the respective one or more second holding member is a holding plate. 
     In one preferred form of this embodiment, the holding plate is adapted to hold a sealing element that slidably seals against the inside surface of the barrel, the holding plate and sealing element forming a piston head of the plunger. 
     In another preferred form of this embodiment, the two locking ports are formed in a locking plate located between the holding plate and a weakness in a stem of the plunger. 
     The weakness preferably serves as a snap off point of the plunger when the needle is fully retracted, the weakness being so located along the stem of the plunger that access to the snap off point, after the emerging end of the plunger has been snapped off, is restricted by the locking tab assembly. 
     In a particularly preferred form of this embodiment, the locking tab assembly has a pair of opposed finger tabs integrally formed with the main body of the barrel, and a pair of top tabs, each pivotally connected to a respective finger tab, wherein, when the top tabs are pivotally opened from the finger tabs, the opening in the trailing end of the barrel is of a first size to allow the plunger to be inserted through that opening for assembly of the syringe prior to its use, and wherein, when the top tabs are pivotally closed against the finger tabs, the opening in the trailing end of the barrel is of a second size less than the first size to allow only a stem of an already inserted plunger to slide through that reduced opening for use of the syringe. 
     In this embodiment, the top tabs may be pivotally closed against the finger tabs by engagement of an inner guide finger extending from each top tab with a respective guide aperture formed through each finger tab, and by engagement of an outer closing finger extending from each top tab with a respective slot formed at the periphery of opposed collar portions which join together the finger tabs, thereby allowing the locking tab assembly to assume the locking-ready position. 
     In a second preferred embodiment, when engagement is by a snapped fit, the or each first locking member is a one-way locking tongue, and the respective one or more second locking member is a locking plate. 
     Alternatively, the respective one or more second locking member is a locking wedge. 
     It is preferred that there are four locking tongues and there is one locking plate having a circumferential rim that is an inclined plane. 
     Preferably, the or each first holding member is a holding ring, and the respective one or more second holding member is a holding plate. 
     In one preferred form of this embodiment, the holding plate is adapted to hold a sealing element that slidably seals against the inside surface of the barrel, the holding plate and sealing element forming a piston head of the plunger. 
     In another preferred form of this embodiment, the locking plate is located between the holding plate and a weakness in a stem of the plunger. 
     Alternatively, there are four locking wedges, each one of which is located between the holding plate and the weakness in the stem of the plunger. 
     The weakness preferably serves as a snap off point of the plunger when the needle is fully retracted, the weakness being so located along the stem of the plunger that access to the snap off point, after the emerging end of the plunger has been snapped off, is restricted by the locking tab assembly. 
     In a particularly preferred form of this embodiment, the locking tab assembly has a pair of opposed finger tabs integrally formed with the main body of the barrel, and a pair of top tabs, each pivotally connected to a respective finger tab, wherein, when the top tabs are pivotally opened from the finger tabs, the opening in the trailing end of the barrel is of a first size to allow the plunger to be inserted through that opening for assembly of the syringe prior to its use, and wherein, when the top tabs are pivotally closed against the finger tabs, the opening in the trailing end of the barrel is of a second size less than the first size to allow only a stem of an already inserted plunger to slide through that reduced opening for use of the syringe. 
     In this embodiment, the top tabs may be pivotally closed against the finger tabs by engagement of an inner guide finger extending from each top tab with a respective guide aperture formed through each finger tab, and by engagement of an outer closing finger extending from each top tab with a respective slot formed at the periphery of opposed collar portions which join together the finger tabs, thereby allowing the locking tab assembly to assume the locking-ready position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings, in which: 
         FIG. 1  is an upright front view of a syringe according to one form of a first preferred embodiment of the invention, with the plunger located within the barrel in preparation for the drawing of liquid into the barrel, 
         FIG. 2  is an upright front view of the barrel with attached needle of the syringe of  FIG. 1 , with a locking tab assembly of the syringe shown in an artificially separated state, 
         FIG. 3  is an upright front view of the plunger of the syringe of  FIG. 1 , 
         FIG. 4  is a plan view of the syringe of  FIG. 1 , 
         FIG. 5  is an upright side view of the barrel with attached needle, and the artificially separated locking tab assembly, of the syringe (less the plunger) shown in  FIG. 2 , 
         FIG. 6  is a plan view of the opened locking tab assembly of the syringe of  FIG. 1 , showing detail of the locking plate and the stem of the plunger, 
         FIG. 7  is an upright front view of the syringe of  FIG. 1  in a final stage of assembly prior to its use, with the plunger being inserted into the barrel, and the top tabs about to be pivotally closed against the finger tabs to allow the locking tab assembly to assume a locking-ready position, 
         FIG. 8  is an upright side view of the syringe of  FIG. 1  after the needle has been fully retracted into the barrel and the part of the plunger that emerges from the barrel has been snapped off, 
         FIG. 9  is an upright front view of a syringe according to another form of a first preferred embodiment of the invention, with the plunger located within the barrel in preparation for the drawing of liquid into the barrel, 
         FIG. 10  is a plan view of the opened locking tab assembly of the syringe of  FIG. 9 , showing detail of the holding plate and the stem of the plunger, 
         FIG. 11  is an isolated upright front view of the opposed top tabs of the locking tab assembly of the syringe of  FIG. 9 , 
         FIG. 12  is an isolated upright side view of one of the top tabs shown in  FIG. 11 , 
         FIG. 13  is an upright front view of the syringe of  FIG. 9  in a final stage of assembly prior to its use, with the plunger being inserted into the barrel, and the top tabs about to be pivotally closed against the finger tabs to allow the locking tab assembly to assume a locking-ready position, 
         FIG. 14  is an upright front view of the syringe shown in  FIG. 13  after its final stage of assembly and ready for use, 
         FIG. 15  is an upright front view of the syringe shown in  FIG. 14  after a first stage of use in drawing liquid into the barrel of the syringe, 
         FIG. 16  is an upright front view of the syringe shown in  FIG. 15  after a second stage of use in releasing liquid through the needle and allowing the plunger to engage the needle in preparation for the manual retraction of the needle into the barrel, 
         FIG. 17  is an isolated, partly cut away, perspective view of a lower part of the plunger at a first stage of movement towards the locking tab assembly in a locking-ready position during manual retraction of the needle of the syringe shown in  FIG. 16 , 
         FIG. 18  is an isolated, partly cut away, perspective view of the lower part of the plunger shown in  FIG. 17  at a second stage of movement during manual retraction of the needle, where the locking fingers from the holding plate are in the process of engaging the locking ports in the locking tab assembly, 
         FIG. 19  is an isolated, partly cut away, perspective view of the lower part of the plunger shown in  FIG. 18  after the second stage of movement, where the locking fingers have engaged the locking ports to activate the needle locking assembly and thereby fully retract the needle within the barrel, 
         FIG. 20  is an upright front view of the syringe shown in  FIG. 16  after a third stage of use in fully retracting the needle into the barrel, 
         FIG. 21  is an upright side view of the syringe shown in  FIG. 20  after a fourth stage of use in snapping off the part of the plunger that emerges from the barrel, 
         FIG. 22  is an upright front view of a syringe according to one form of a second preferred embodiment of the invention, with the plunger located within the barrel in preparation for the drawing of liquid into the barrel, 
         FIG. 23  is an upright front view of the barrel with attached needle of the syringe of  FIG. 22 , with a locking tab assembly of the syringe shown in an artificially separated state, 
         FIG. 24  is an upright front view of the plunger of the syringe of  FIG. 22 , 
         FIG. 25  is a plan view of the syringe of  FIG. 22 , 
         FIG. 26  is an upright side view of the barrel with attached needle, and the artificially separated locking tab assembly, of the syringe (less the plunger) shown in  FIG. 23 , 
         FIG. 27  is a plan view of the opened locking tab assembly of the syringe of  FIG. 22 , showing detail of the locking plate and the stem of the plunger, 
         FIG. 28  is an upright front view of the syringe of  FIG. 22  in a final stage of assembly prior to its use, with the plunger being inserted into the barrel, and the top tabs about to be pivotally closed against the finger tabs to allow the locking tab assembly to assume a locking-ready position, 
         FIG. 29  is an upright side view of the syringe of  FIG. 22  after the needle has been fully retracted into the barrel and the part of the plunger that emerges from the barrel has been snapped off, 
         FIG. 30  is an enlarged view of part of the syringe shown in  FIG. 29 , 
         FIG. 31  is an upright front view of the syringe of  FIG. 22  in a final stage of assembly prior to its use, with the plunger being inserted into the barrel, and the top tabs about to be pivotally closed against the finger tabs to allow the locking tab assembly to assume a locking-ready position, 
         FIG. 32  is an upright front view of the syringe shown in  FIG. 31  after its final stage of assembly and ready for use, 
         FIG. 33  is an upright front view of the syringe shown in  FIG. 32  after a first stage of use in drawing liquid into the barrel of the syringe, 
         FIG. 34  is an upright front view of the syringe shown in  FIG. 33  after a second stage of use in releasing liquid through the needle and allowing the plunger to engage the needle in preparation for the manual retraction of the needle into the barrel, 
         FIG. 35  is an isolated, partly cut away, perspective view of a lower part of the plunger at a first stage of movement towards the locking tab assembly in a locking-ready position during manual retraction of the needle of the syringe shown in  FIG. 34 , 
         FIG. 36  is an isolated, partly cut away, perspective view of the lower part of the plunger shown in  FIG. 35  at a second stage of movement during manual retraction of the needle, where the locking tongues are in the process of engaging the locking plate, 
         FIG. 37  is an isolated, partly cut away, perspective view of the lower part of the plunger shown in  FIG. 36  after the second stage of movement, where the locking tongues have engaged the locking plate to activate the needle locking assembly and thereby fully retract the needle within the barrel, 
         FIG. 38  is an upright front view of the syringe shown in  FIG. 34  after a third stage of use in fully retracting the needle into the barrel, and 
         FIG. 39  is an upright side view of the syringe shown in  FIG. 38  after a fourth stage of use in snapping off the part of the plunger that emerges from the barrel. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     The syringe  10  shown in  FIGS. 1 to 8  is one form of a first preferred embodiment of the invention, and has a hollow barrel  12  made of a clear plastic material with a leading end  14  and a trailing end  16 , a needle  18  sealably, but removably, attached to an opening in the leading end  14 , and a plunger  20  slidably received through an opening in the trailing end  16 . 
     The needle  18  has a needle hub  22  and a stainless steel needle tip  24  mounted to the hub  22 . The hub  22  has an external part  23  located outside the barrel  12  and an internal part  25  located inside the barrel  12 , the internal part  25  having a hollow bore that provides for liquid communication between the barrel  12  and a passageway through the external part  23  of the hub leading to the needle tip  24 . There is a flexible annular shoulder (not shown) having a constricting inner diameter within the passageway through the external part of the needle hub  22 . 
     The plunger  20  has a piston head  26 , a stem  28 , and a finger pad  30 . The stem  28  is formed of four radially extending, equally spaced apart, webs, except where there is a localised narrowing or weakness  32  of the stem  28  located just above the piston head  26 . The weakness  32 , when required, serves as a snap off point of the plunger  20 . 
     The piston head  26  has a rubber gasket or liquid tight sealing element  34  that slidably seals against the inside surface of the barrel  12 , and a holding plate  36  for the sealing element  34 , and there is a stepped shaft  38  extending forwardly of the sealing element  34 . 
     The shaft  38  has a domed catch member  40  at its free end. The catch member  40  has an outer diameter that, when required, allows it to engage by a forced snap fit behind the annular shoulder within the passageway through the external part  23  of the needle hub  22 . 
     There is also a locking plate  41  securely located on the stem  28  between the weakness  32  and the holding plate  36  of the piston head  26 . The structure and function of the locking plate  41  will be described in detail later in the specification. 
     The opening in the trailing end  16  of the barrel  12  through which the plunger  20  is slidably received is formed by a locking tab assembly that is closed into a locking-ready position prior to use of the syringe  10 . The locking tab assembly has a pair of integrally formed finger tabs  42 ,  44 , each extending outwardly in opposed directions from the main body of the barrel  12 , but also joined together by collar portions  46 ,  48 . The locking tab assembly also has a pair of pivotable top tabs  50 ,  52 , each connected by an integral hinge  54 ,  56  to an outermost edge of a respective finger tab  42 ,  44 . In an alternative embodiment, the top tabs may be separate from the finger tabs and may be clipped into position. When the top tabs  50 ,  52  are pivotally opened from the finger tabs  42 ,  44 , the opening in the trailing end  16  of the barrel  12  is at its optimum size and of a circular shape to allow the piston head  26  of the plunger  20  to be inserted therethrough for assembly of the syringe prior to its use. When the top tabs  50 ,  52  are pivotally closed against the finger tabs  42 ,  44 , the opening in the trailing end  16  of the barrel  12  is reduced in size and of a cruciform shape to allow only the webbed stem  28  of the already inserted plunger  20  to slide therethrough. A semicircular spacer or holding ring  82 ,  84  extending downwardly from each top tab  50 ,  52  engages by interference fit a respective semicircular inner side wall portion of the opening in the trailing end  16  of the barrel  12 . For closure of the top tabs  50 ,  52  against the finger tabs  42 ,  44  to occur, and for the locking tab assembly to assume a locking-ready position, an inner guide finger  57 ,  58  extending from each top tab  50 ,  52  engages by interference fit a respective guide aperture  59 ,  60  formed through each finger tab  42 ,  44 , and a pair of outer closing fingers  62 ,  64  and  66 ,  68  extending from each top tab  50 ,  52  engages by hooked fit a respective slot  70 ,  72  and  74 ,  76  formed at the periphery of the collar portions  46 ,  48  of the finger tabs  42 ,  44 . 
     In the locking-ready position, the locking tab assembly forms part of a needle locking assembly for reliably retaining a fully manually retracted needle, after use of the syringe, say, for an injection, within the barrel of the syringe. The needle locking assembly includes a locking finger  78 ,  80  extending downwardly from each top tab  50 ,  52  and which terminates in a barb located a distance from each top tab  50 ,  52  that is just above the distance each holding ring  82 ,  84  extends downwardly from each top tab  50 ,  52 . The locking fingers  78 ,  80  are located within a respective semicircular holding ring  82 ,  84 . 
     The needle locking assembly also includes locking ports  86 ,  88  formed in the locking plate  41 . A locking finger  78 ,  80  engages by hooked fit a respective locking port  86 ,  88  of the locking plate  41  when the needle is fully manually retracted after use. When so fully retracted, the peripheral top surface of the holding plate  36  butts up against the bottom surface of each holding ring  82 ,  84  because the outer diameter of the locking plate  41  is just less than the inner diameter of each holding ring  82 ,  84 . 
     The part of the plunger  20  that emerges from the opening in the trailing end  16  may then be snapped off. Because of the location of the weakness or snap off point  32  along the stem  28  of the plunger  20 , access to the snap off point, after the emerging end of the plunger has been snapped off, is restricted by the locking tab assembly. 
     The syringe  100  shown in  FIGS. 9 to 21  is another form of a first preferred embodiment of the invention, and is similar in structure and function to the syringe  10  shown in  FIGS. 1 to 8 , and so like features have been accorded like numerals in the Figures. However, syringe  100  does not have a locking plate  41  with locking ports  86 ,  88 , and does not have locking fingers  78 ,  80  extending downwardly from the top tabs  50 ,  52 . Rather, syringe  100  has locking fingers  102 ,  104  extending upwardly from its holding plate  105 , each of which terminate in a barb located a distance from the holding plate  105  that is just above the distance its stem weakness  32  is from the holding plate  105 . The syringe  100  also has locking ports  106 ,  107  formed in each top tab  108 ,  109 . The locking ports  106 ,  107  are located within a respective semicircular holding ring  82 ,  84 . A locking finger  102 ,  104  engages by hooked fit a respective locking port  106 ,  107  of the top tabs  108 ,  109  when the needle is fully manually retracted after use. When so fully retracted, the peripheral top surface of the holding plate  105  butts up against the bottom surface of each holding ring  82 ,  84 . 
     For both syringes  10  and  100 , the plunger  20 , to which the retracted needle  18  is attached, is locked firmly in the fully retracted position by the engagement of the locking fingers with the locking ports (to prevent any subsequent downward movement of the plunger relative to the barrel), and by the holding plate  36  (for syringe  10 ) or the holding plate  105  (for syringe  100 ) butting up against the holding rings of the locking tab assembly in its locking-ready position (to prevent any subsequent upward movement of the plunger relative to the barrel). The secure means by which the locking tab assembly is closed into its locking-ready position prior to use of the syringe  10 ,  100  provides the necessary strength to prevent such downward and upward movement when the needle is fully manually retracted after use. 
     In the stepped sequence of events shown in  FIGS. 13 to 21 ,  FIG. 13  shows a final stage of assembly of the syringe  100  prior to its use, with the plunger  20  inserted into the barrel  12 , and the top tabs  108 ,  109  about to be pivotally closed against the finger tabs  42 ,  44  to allow the locking tab assembly to assume a locking-ready position. 
       FIG. 14  shows the syringe  100  with the plunger  20  pushed into the barrel  12  in preparation for the drawing of liquid into the barrel of the syringe. 
       FIG. 15  shows the syringe  100  after the plunger  20  has been pulled back to draw liquid into the barrel  12  of the syringe. 
       FIG. 16  shows the syringe  100  after the plunger  20  has been pushed back into the barrel  12  to release liquid through the needle  18  for the purpose of injection and the stepped shaft  38  has been engaged by a forced snap fit to the external part  23  of the needle hub  22 . 
       FIG. 20  shows the syringe  100  after the plunger  20 , to which the needle  18  is attached, has been pulled back to fully retract the needle into the barrel  12  and reliably retain the so retracted needle within the barrel by activation of the needle locking assembly. 
       FIG. 21  shows the syringe  100  after the emerging part of the plunger  20  has been snapped off, whereupon both the snapped off plunger and the barrel  12  containing the retracted needle  18  may be disposed of safely. 
     The syringe  110  shown in  FIGS. 22 to 39  is one form of a second preferred embodiment of the invention, and has a hollow barrel  112  made of a clear plastic material with a leading end  114  and a trailing end  116 , a needle  118  sealably, but removably, attached to an opening in the leading end  114 , and a plunger  120  slidably received through an opening in the trailing end  116 . 
     The needle  118  has a needle hub  122  and a stainless steel needle tip  124  mounted to the hub  122 . The hub  122  has an external part  123  located outside the barrel  112  and an internal part  125  located inside the barrel  112 , the internal part  125  having a hollow bore that provides for liquid communication between the barrel  112  and a passageway through the external part  123  of the hub leading to the needle tip  124 . There is a flexible annular shoulder (not shown) having a constricting inner diameter within the passageway through the external part of the needle hub  122 . 
     The plunger  120  has a piston head  126 , a stem  128 , and a finger pad  130 . The stem  128  is formed of four radially extending, equally spaced apart, webs, except where there is a localised narrowing or weakness  132  of the stem  128  located just above the piston head  126 . The weakness  132 , when required, serves as a snap off point of the plunger  120 . 
     The piston head  126  has a rubber gasket or liquid tight sealing element  134  that slidably seals against the inside surface of the barrel  112 , and a holding plate  136  for the sealing element  134 , and there is a stepped shaft  138  extending forwardly of the sealing element  134 . 
     The shaft  138  has a domed catch member  140  at its free end. The catch member  140  has an outer diameter that, when required, allows it to engage by a forced snap fit behind the annular shoulder within the passageway through the external part  123  of the needle hub  122 . 
     There is also a locking plate  141  securely located on the stem  128  between the weakness  132  and the holding plate  136  of the piston head  126 . The structure and function of the locking plate  141  will be described in detail later in the specification. 
     The opening in the trailing end  116  of the barrel  112  through which the plunger  120  is slidably received is formed by a locking tab assembly that is closed into a locking-ready position prior to use of the syringe  110 . The locking tab assembly has a pair of integrally formed finger tabs  142 ,  144 , each extending outwardly in opposed directions from the main body of the barrel  112 , but also joined together by collar portions  146 ,  148 . The locking tab assembly also has a pair of pivotable top tabs  150 ,  152 , each connected by an integral hinge  154 ,  156  to an outermost edge of a respective finger tab  142 ,  144 . In an alternative embodiment, the top tabs may be separate from the finger tabs and may be clipped into position. When the top tabs  150 ,  152  are pivotally opened from the finger tabs  142 ,  144 , the opening in the trailing end  116  of the barrel  112  is at its optimum size and of a circular shape to allow the piston head  126  of the plunger  120  to be inserted therethrough for assembly of the syringe prior to its use. When the top tabs  150 ,  152  are pivotally closed against the finger tabs  142 ,  144 , the opening in the trailing end  116  of the barrel  112  is reduced in size and of a cruciform shape to allow only the webbed stem  128  of the already inserted plunger  120  to slide therethrough. A semicircular spacer or holding ring  182 ,  184  extending downwardly from each top tab  150 ,  152  engages by interference fit a respective semicircular inner side wall portion of the opening in the trailing end  116  of the barrel  112 . For closure of the top tabs  150 ,  152  against the finger tabs  142 ,  144  to occur, and for the locking tab assembly to assume a locking-ready position, an inner guide finger  157 ,  158  extending from each top tab  150 ,  152  engages by interference fit a respective guide aperture  159 ,  160  formed through each finger tab  142 ,  144 , and a pair of outer closing fingers  162 ,  164  and  166 ,  168  extending from each top tab  150 ,  152  engages by hooked fit a respective slot  170 ,  172  and  174 ,  176  formed at the periphery of the collar portions  146 ,  148  of the finger tabs  142 ,  144 . 
     In the locking-ready position, the locking tab assembly forms part of a needle locking assembly for reliably retaining a fully manually retracted needle, after use of the syringe, say, for an injection, within the barrel of the syringe. The needle locking assembly includes a pair of spaced apart, one way, locking tongues  178 ,  179  and  180 ,  181  extending from a starting position inwardly and upwardly from the inner side wall of a respective pair of semicircular holding rings  182 ,  184  of each top tab  150 ,  152  and which are structurally biased to flex upwardly but not downwardly. 
     The needle locking assembly also includes a locking plate  144  having a circumferential rim that is an inclined plane. A locking tongue  178 ,  179 ,  180 ,  181  engages by snapped fit behind a respective trailing end portion of the locking plate  141  when the needle is fully manually retracted after use. When so fully retracted, the peripheral top surface of the holding plate  136  butts up against the bottom surface of each holding ring  182 ,  184  because the outer diameter of the locking plate  141  is just less than the inner diameter of each holding ring  182 ,  184 . 
     In another form (not shown) of this embodiment, the locking plate  141  may be replaced by four locking wedges, each formed between adjacent webs of the stem  128 , and located between the weakness  132  and the holding plate  136 . Each locking wedge has a sharp end nearest the weakness  132  and a blunt end nearest the holding plate  136  so as to define an upwardly inclined plane against which a respective locking tongue  178 ,  179 ,  180 ,  181  flexes progressively more upwardly as the plunger approaches the end of its range of retraction. When each locking tongue simultaneously passes the blunt end of its respective locking wedge, it is snapped back to its inward and upward starting position. 
     The locking tongues engage the inclined plane at the rim of the locking plate  141  in a similar manner (see especially  FIGS. 35 to 37 ). 
     The part of the plunger  120  that emerges from the opening in the trailing end  116  may then be snapped off. Because of the location of the weakness or snap off point  132  along the stem  128  of the plunger  120 , access to the snap off point, after the emerging end of the plunger has been snapped off, is restricted by the locking tab assembly. 
     For syringe  110 , the plunger  120 , to which the retracted needle  118  is attached, is locked firmly in the fully retracted position by the engagement of the locking tongues with the locking plate (or with the locking wedges for the aforementioned, but not shown, other form of this embodiment) to prevent any subsequent downward movement of the plunger relative to the barrel, and by the holding plate  136  butting up against the holding rings of the locking tab assembly in its locking-ready position to prevent any subsequent upward movement of the plunger relative to the barrel. The secure means by which the locking tab assembly is closed into its locking-ready position prior to use of the syringe  110  provides the necessary strength to prevent such downward and upward movement when the needle is fully manually retracted after use. 
     In the stepped sequence of events shown in  FIGS. 31 to 39 ,  FIG. 31  shows a final stage of assembly of the syringe  110  prior to its use, with the plunger  120  inserted into the barrel  112 , and the top tabs  150 ,  152  about to be pivotally closed against the finger tabs  142 ,  144  to allow the locking tab assembly to assume a locking-ready position. 
       FIG. 32  shows the syringe  110  with the plunger  120  pushed into the barrel  112  in preparation for the drawing of liquid into the barrel of the syringe. 
       FIG. 33  shows the syringe  110  after the plunger  120  has been pulled back to draw liquid into the barrel  112  of the syringe. 
       FIG. 34  shows the syringe  110  after the plunger  120  has been pushed back into the barrel  112  to release liquid through the needle  118  for the purpose of injection and the stepped shaft  138  has been engaged by a forced snap fit to the external part  123  of the needle hub  122 . 
       FIG. 38  shows the syringe  110  after the plunger  120 , to which the needle  118  is attached, has been pulled back to fully retract the needle into the barrel  112  and reliably retain the so retracted needle within the barrel by activation of the needle locking assembly. 
       FIG. 39  shows the syringe  110  after the emerging part of the plunger  120  has been snapped off, whereupon both the snapped off plunger and the barrel  112  containing the retracted needle  118  may be disposed of safely. 
     It will be readily apparent to persons skilled in the art that various modifications may be made in details of design, construction and use of the single use safety syringe described herein without departing from the scope or ambit of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The single use safety syringe as described in various forms and embodiments in the specification is particularly useful for preventing needle stick injury to persons using syringes to collect, transfer and/or inject liquids for medical or other uses, and is especially applicable to reducing the risk of transmitting blood borne diseases.