there is shown in the drawings and will be described in detail herein a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as exemplary of the principles of the invention and is not intended to limit the invention to the embodiment illustrated . referring first to fig1 – 3 , a single use syringe assembly 20 includes a barrel 22 having an inside surface 24 defining a chamber 26 for retaining fluid . the barrel 22 includes an open end 28 and a distal end 30 having a passageway 32 therethrough in communication with the chamber . a needle cannula 34 projects outwardly from the distal barrel end . the needle cannula has a lumen ( not shown ) therethrough in fluid communication with the passageway and a sharpened distal tip . the syringe assembly of the present invention is preferably used with a needle cannula that is attached to the distal end of the barrel by an adhesive or other suitable means . it will be appreciated that the invention could be applied to syringe assemblies having removable needle / hub assemblies , or fixed or removable blunt cannulas . as used in the preceding paragraph and hereafter , the term “ distal end ” refers to the end furthest from the person holding the syringe assembly . the term “ proximal end ” refers to the end closest to the holder of the syringe assembly . in the preferred embodiment , the proximal end of the barrel 22 includes a flange 36 to facilitate handling and positioning of the syringe assembly and to maintain the relative position of the barrel with respect to the plunger rod during filling and medication administration . a plunger assembly 38 includes a plunger rod 40 having an elongate body portion 42 . the distal end of the elongate body portion includes a projection having an enlarged end 44 . a stopper 45 having a recess therein is affixed to this end . a disc - shaped flange 46 is provided at the proximal end of the plunger rod for allowing the user to apply the force necessary to move the plunger rod with respect to the barrel . the elongate body portion 42 includes several sections between the proximal and distal ends thereof . a first section 42 a thereof is substantially cylindrical . the distal end of this section adjoins an enlarged plunger rod surface that functions as a stop surface 48 as described hereafter . a second section 42 b adjoins the proximal end of the first section , and is also substantially cylindrical . the second section is larger in diameter than the first section , and accordingly defines a shoulder 50 at the proximal end of the first section . a third section 42 c extends between the second section 42 b and the disc - shaped flange 46 . a second shoulder 52 adjoins the proximal end of the second section 42 b , and separates the second and third sections . a locking element 54 is positioned within the barrel 22 . in accordance with the preferred embodiment , the locking element is generally trough - shaped . the locking element defines a channel through which the elongate body portion 42 of the plunger rod 40 extends . it will be appreciated that the locking element can be u - shaped in cross section or otherwise similarly configured , all of which should be considered generally trough - shaped as the term is used herein . the locking element includes a base 56 that includes a bottom wall 58 and first and second opposing side walls 60 , 62 . a first leg 64 extends proximally from the first wall and a second leg 66 extends proximally from the second wall . the legs 64 , 66 are also in opposing relation . a spring element 68 in the form of a third leg extends proximally from the bottom wall 58 . the spring element may be pivotally attached to the bottom wall . each of the legs 64 , 66 includes an end portion that is angled generally towards a longitudinal axis extending through the channel defined by the locking element . they further include inner and outer edges . ( the terms “ inner ” and “ outer ” are relative terms as used herein .) the inner edges thereof are substantially adjacent to the spring element . barbs 70 , 72 are integral with the outer edges of the first and second legs . the spring element 68 includes a pair of bends 68 a , 68 b therein . it accordingly extends beneath the plane of the bottom wall 58 of the base 56 , as best shown in fig1 . the locking element is preferably formed from a thin sheet of metal such as stainless steel . it is preformed into the flat configuration shown in fig7 . the broken lines show the folds that are made in the flat substrate 74 to form the locking element 54 shown in fig7 and 9 – 11 . the dimensions of the locking element are selected in accordance with the barrel and plunger rod assembly with which it is to be used . the syringe assembly is easily constructed from the component parts thereof . the locking element is positioned on the section 42 b of the plunger rod 40 such that the angled end portion of the legs 64 , 66 adjoin the shoulder 52 at the proximal end of this section . the legs 64 , 66 and spring member extend proximally , and the barbs 70 , 72 are angled proximally with respect to the plunger rod . the plunger rod / locking element assembly is then inserted into the barrel 22 through the proximal end thereof . as the assembly is moved distally within the barrel , the angular orientation of the barbs allows them to slide along while engaging the inside surface 24 of the barrel . the locking element moves distally with the plunger rod due to the engagement of the ends of the legs 64 , 66 with the shoulder . the plunger rod / locking element assembly is moved distally to the positions shown in fig2 and 4 where the stopper engages the end wall of the barrel . the syringe assembly is then ready for use or storage . in use , the plunger rod assembly 38 is retracted from the position shown in fig4 to the position shown in fig5 in order to draw fluid through the needle cannula 34 and passageway 32 and into the chamber 26 of the barrel 22 . the locking element 54 remains stationary during such retraction , and the plunger rod assembly is moved proximally with respect to both the barrel 22 and the locking element . this is due to the engagement of the barbs 70 , 72 with the inside surface 24 of the barrel . the barbs are preferably made from a harder material than the barrel , which enhances their ability to resist proximal movement . the barbs are resiliently urged by the spring member towards the inside surface 24 of the barrel , further enhancing their effectiveness . retraction of the plunger rod assembly 38 is limited by the locking assembly . as shown in fig5 , the stop surface 48 on the plunger rod 40 engages the distal end of the locking element 54 . as the locking element cannot be moved proximally , further retraction of the plunger rod assembly is not possible . the amount of fluid that can be drawn into the chamber 26 is accordingly limited by the distance between the stop surface 48 and the second shoulder 52 as well as the length of the locking element . it will be appreciated that the distance between the stop surface 48 and second shoulder 52 and the length of the locking element 54 can be chosen to meet the needs of particular applications . the end portions of the legs 64 , 66 of the locking element adjoin the first shoulder 50 when the plunger rod assembly is retracted to the position shown in fig5 . the distance between the first shoulder 50 and the stop surface 48 , being substantially the same as the distance between the distal end of the locking element and the proximal end portions of the legs , causes the locking element to be substantially immovable with respect to the plunger rod assembly . as discussed above , the locking element is substantially immovable in the proximal direction within the barrel due to the engagement of the barbs 70 , 72 with the inside surface of the barrel 22 . once the fluid has been drawn into the barrel from a vial or other fluid source , the needle cannula can be removed from the fluid source and used for injection . during the injection of a patient , the plunger assembly 38 and locking element both move distally from the positions shown in fig5 to the positions shown in fig6 . in fig6 , the stopper 45 again adjoins or engages the end wall of the barrel 22 . the locking element 54 remains positioned between the stop surface 48 and the first shoulder 50 . both the plunger rod assembly 38 and the locking element are substantially immovable from their positions . the syringe assembly 20 accordingly cannot be reused . the syringe barrel of the present invention may be constructed of a wide variety of rigid materials with thermoplastic materials such as polypropylene and polyethylene being preferred . similarly , thermoplastic materials such as polypropylene , polyethylene and polystyrene are preferred for the plunger rod . a wide variety of materials such as natural rubber , synthetic rubber and thermoplastic elastomers are suitable for the stopper . the choice of stopper material will depend on compatability with the medication being used . in the preferred embodiment of this invention , the stopper 45 , made of medical grade rubber , includes a partially hollow interior with an undercut ledge which is snap fit over complementary structure 44 on plunger rod 40 to secure the stopper to the plunger rod . the stopper and plunger rod may also be integrally formed of the same material or different materials , or secured together in other suitable ways . as previously recited , it is preferable that the locking element be fabricated from a material which is harder than the barrel so that the locking barbs may effectively engage the barrel . resilient spring like properties are also desirable along with low cost dimensionally consistent fabrication . with this in mind , sheet metal is the preferred material for the locking element with stainless steel being preferred for medical applications . although the locking element of the preferred embodiment is fabricated from a single sheet , it is within the purview of the instant invention to include locking elements made of other forms and / or containing multiple parts and materials . thus , it can be seen that the present invention provides a simple , reliable , easily fabricated , single use syringe which becomes inoperable or incapable of further use without any additional act on the part of the user .
a single use syringe assembly is provided as well as a locking element for such an assembly . the single use syringe assembly includes a barrel , a plunger rod assembly and a locking element positioned within the barrel . the locking element includes one or more barbs for engaging the inside surface of the barrel . the barbs prevent the locking element from moving proximally within the barrel , but allow its distal movement therein with the plunger rod assembly . a spring member may be provided for urging the barbs towards the inside surface of the barrel . the barbs are preferably positioned on the edge portions of opposing legs that form part of the locking element .