the non - reusable hypodermic syringe 10 of the present invention includes a cylindrical barrel 12 , a double - landed piston 14 and a plunger rod 16 for use in reciprocating the piston 14 within barrel 12 . rod 16 defines a gripping flange 17 on the rearward end thereof and is preferably secured to the piston 14 at its forward end by means of an enlarged tapered portion 18 held within a correspondingly configured recess 20 in the rearward portion of piston 14 . an upper plug member 22 is disposed below piston 14 within a chamber 23 defined by the lower portion of barrel 12 and a barrel cap 26 . a lower plug member 24 is disposed in chamber 23 axially below plug member 22 . barrel cap 26 is permanently affixed to the lower end of barrel 12 by heat bonding on a suitable adhesive , and a hypodermic needle 28 is permanently secured within a hollow extension 30 extending from barrel cap 26 . the syringe barrel 12 , which is preferably constructed of a transparent durable plastic material , defines an outwardly extending radial flange 32 at the upper end thereof , a constant diameter upper portion 34 within which piston 14 is axially translatable , a lower portion 36 which defines the interior wall of chamber 23 and an intermediary annular horizontal end wall 38 . a stop ring 39 is provided at the rearwardmost end of barrel 12 to prevent withdrawal of the piston from the barrel so as to preclude access to the interior of the barrel . the upper plug member 22 , preferably constructed of a rubber material includes an enlarged diameter upper portion 40 , a body portion 42 , a shoulder 44 disposed therebetween , a resilient annular flange 46 extending outwardly from the lower end thereof , and a fluid channel 48 extending axially therethrough and communicating with a conical recess 50 . shoulder 44 is adapted to abut a first shoulder 52 defined by the lower portion 36 of barrel 12 in chamber 23 to limit the downward movement of the upper plug member 22 within chamber 23 . as seen in fig1 the resilient annular flange 46 extends outwardly and upwardly from the lower end of plug member 22 such that when the upper plug member 22 is in the raised inactive position , flange 46 flexes outwardly and bears against side wall 54 in chamber 23 and abuts a second shoulder 56 defined by the lower portion 36 of barrel 12 in chamber 23 to limit the upward travel of plug member 22 within chamber 23 . the lower portion 36 of syringe barrel 12 also defines a third interior shoulder 58 in chamber 23 located between side wall 54 and a lower side wall 59 disposed about an enlarged diameter portion 60 of chamber 23 adjacent the lower end 62 of barrel 12 . the lower plug member 24 , also preferably constructed of a rubber material , is disposed axially below upper plug member 22 in chamber 23 . lower plug member 24 defines a lower base portion 64 , and a conical extension 66 . lower base portion 64 defines a diameter less than the inner diameter of end cap 26 to allow fluid flow therebetween but greater than the inner diameter of barrel 12 at the end 62 thereof so as to be axially translatable within chamber 23 between the lower end wall 63 of barrel cup 26 and the lower end 62 of barrel 12 . extension 66 projects upwardly from base portion 64 into the recess 50 in the upper plug member 22 , and is configured so as to abut and mate with the conical side wall of recess 50 upon the plug members being forced together so as to seal the lower end of fluid channel 48 in the upper plug member 22 to prevent fluid flow therethrough . as seen in fig5 and 6 , fluid passages 68 are formed in the side and bottom walls of the lower plug member 24 so as to communicate the hypodermic needle 28 with the portion of chamber 23 above the lower plug member 24 so as to allow fluid flow therebetween about lower plug member 24 when the lower plug member is in its lowermost position against end wall 63 of barrel cap 26 . prior to any fluid being drawn into the syringe 10 through needle 28 , the upper plug member is in the raised position in chamber 23 as illustrated in fig1 . in this position , the syringe 10 can be used as an air pump by means of piston 14 , as the downward movement of piston 14 will force air in barrel 12 through chamber 23 via the channel 48 in the upper plug member 22 , about the lower plug member 24 and out needle 28 , but will not create sufficient force against the upper end of plug member 22 to move the plug member downwardly within chamber 23 . accordingly , if one needed an injection of , for example , 20 units of medicine , the user would retract the piston to a point slightly above the 20 unit mark on the barrel ( not shown ), insert the needle into the liquid medicine in the vial through the standard seal thereon , push the piston back down to wall 38 to drive a volume of air slightly in excess of 20 units into the vial and retract the piston to the point slightly above the 20 unit mark to draw slightly over 20 units of medicine into the barrel of the syringe . the syringe is then held in an inverted position with the needle pointing upwardly , and the piston is again pushed back to the 20 unit mark , ejecting the excess medicine from the needle as well as any air remaining in the syringe . the syringe is then ready to inject the measured 20 units of medicine into the user and , due to the air pumping capability of the syringe , the interior of the vial of medicine is left at atmospheric pressure . as the piston 14 is then driven downwardly during the injection , the liquid medicine is forced through the channel 48 in the upper plug member 22 , about the lower plug member 24 through the fluid passages 68 therein and into and out of the hypodermic needle 28 . concurrently , the hydraulic forces created during the injection by the liquid being driven by the piston act on the effective upper surface areas of the upper and lower plug members 22 and 24 to drive the upper plug member 22 downwardly within chamber 23 to the position illustrated in fig2 and the lower plug member 24 downwardly against the lower end wall 63 of cap member 26 . as the resilient flange 46 on the upper plug member 22 passes the shoulder 58 in chamber 23 , the flange flexes outwardly within the enlarged diameter portion 60 of chamber 23 and bears against the side wall 59 of the enlarged diameter portion 60 of chamber 23 . the upper and lower plug members will then remain in this position during the injection . if , either after the completion of the injection or during the injection , one were to attempt to retract the piston 14 within the barrel to aspirate more liquid into the syringe through needle 28 , the resultant vacuum created within the barrel below the piston 14 would draw the upper and lower plug members upwardly to the positions illustrated in fig3 . the upward travel of upper plug member 22 during such an attempted refill would be limited by the abutment of the flange 46 thereon with shoulder 58 in chamber 23 , while the conical extension 66 on the lower plug member 24 would be drawn into recess 50 and against the upper plug member , sealing the lower end of channel 48 , and thereby preventing any fluid flow upwardly through the upper plug member 22 . the sealing engagement of flange 46 with the side wall 59 and shoulder 58 in chamber 23 would prevent any upward fluid flow about the upper plug member 22 . accordingly , prior to drawing any liquid into the syringe 10 , the piston can be moved upwardly and downwardly within the barrel without effecting deactivation of the syringe . however , once a liquid has been drawn into the syringe and the piston moved downwardly a slight distance sufficient to cause a translation of the lower plug member 22 a distance equal to axial length of side wall 54 , flange 46 clears shoulder 58 , flexes outwardly and the syringe is rendered inoperative , other than to eject the fluid contained therein without any affirmative action or other cooperation of the user . while the above mechanism provides a non - reusable syringe which cannot be refilled by the user , to dissuade one from continuing to try to make the device function properly or from even attempting to disassemble the syringe to remove the disabling plug members , the enlarged tapered end 18 of the plunger rod 16 by which the rod is secured to piston 14 is configured to separate from the piston in the event one were to attempt to draw additional fluid into the syringe . once the upper plug member has been driven forwardly to the lower position by injection of liquid from the syringe , any subsequent retraction of the piston within the barrel will create a vacuum within the barrel between the piston and upper plug member as a result of the sealing engagements of the upper and lower plug members 22 and 24 . as this vacuum increases with the retraction of the piston , the opposed forces acting on the upper and lower ends of the piston will cause the enlarged tapered end 18 of the plunger rod 16 to be pulled out of the tapered recessed 20 in the piston , separating the rod 18 from piston 14 and making the destruction of the syringe readily apparent to the user so as to dissuade the user from any further attempts to use the device or to try to disassemble and reconstruct the syringe . in addition , a washer 80 could be provided on barrel wall 38 which extends slightly over the upper perimeter surface of upper plug member 22 to prevent one from being able to somehow pull the plug members from the barrel . alternatively , end wall 38 could be extended radially inwardly to provide the same function . fig4 illustrates a modified form of the above described embodiment of the invention which , while operating in the same manner as the prior embodiment , differs slightly from the prior embodiment in the configuration of the upper plug member 122 and the lower portion 136 of the syringe barrel 112 . in the second embodiment , the upper plug member 122 which , like the upper plug member 22 of the prior embodiment , is preferably constructed of a resilient rubber material , comprises an enlarged diameter upper radial flange portion 140 , a central body portion 142 of reduced diameter , an enlarged lower radial flange portion 146 , a central axial fluid channel 148 , and conical recess 150 . the enlarged lower flange portion 146 is sized so as to be in slight compression against a side wall 154 defined by barrel 112 to retain plug member 122 in place prior to ejection of liquid therethrough and such that upon plug member 122 being forced downwardly by the hydraulic forces acting thereon during liquid ejection , the enlarged lower flange portion 146 will flex outwardly upon clearing a shoulder 158 defined in the lower portion of the barrel to maintain the plug member 122 in a lowered position and , in combination with a lower plug number 24 , disable the syringe from being able to draw additional liquid therein in the same manner as described above with respect to the prior embodiment . in addition , the syringe barrel can be modified as seen in fig4 to provide an inwardly extending radial flange 138 having an upper curvalinear surface 139 . flange 138 prevents withdrawal of the plug members from the barrel , while the upper curvalinear surface 139 thereon will assist the user in aligning any air bubbles in the medicine with the fluid flow channel 148 in the upper plug member 122 for removal through needle 28 . the configuration of flange 138 can be used with either embodiment of the invention . however , when using flange 138 , the lower surface of the piston should be corresponding configured as seen at 114 in fig4 . fig7 - 10 illustrate a second embodiment of the invention which employs only a single plug member to render the syringe non - reusable . the syringe 200 of the second embodiment differs from syringe 10 solely in the lower portion thereof and accordingly only the lower portion of syringe 200 will be described . syringe 200 includes a barrel 202 defining a constant diameter portion 204 within which a piston 206 is disposed . piston 206 is preferably provided with a curvalinear lower surface 208 and is carried by plunger rod 210 . barrel 202 defines a lower syringe chamber 211 , an annular shoulder 212 disposed in chamber 211 and a curvalinear lower surface 214 having a concave position 216 terminating a flat horizontal surface 218 having a central opening 220 therein . a barrel cap 222 is carried by the lower end of barrel 202 and a hypodermic needle 223 is carried by barrel cap 222 such that it communicates with chamber 211 through the opening 220 in barrel cap 222 . an annular stop member 224 , preferably constructed of the same durable plastic material as barrel 202 , is affixed to the interior surface of barrel 202 so as to define the upper end of chamber 211 . stop member 224 defines a curvalinear upper surface 226 adapted to mate with the correspondingly configured lower surface 208 of piston 206 , a central opening 228 therein , a flat horizontal lower surface 230 and an annular depending portion 232 which abuts shoulder 212 in chamber 211 . a plug member , preferably 234 constructed of a rubber material , is disposed within chamber 211 , and defines : an upper portion 236 , a curved downwardly tapered portion 238 which is disposed below portion 236 and inwardly spaced from the perimeter thereof , and a resilient lower annular flange 240 disposed below tapered portion 238 . the outer diameter of the upper portion 236 of plug member 234 is less than the inner diameter defined by the interior side wall 242 of stop member 224 so as to allow fluid flow therebetween . for reasons which will become readily apparent , the diameter of lower flange 240 in the unflexed state , as seen in fig9 is greater than the inner diameter defined by interior side wall 242 . as seen in fig9 fluid passageways 244 and 246 are provided in the upper surface of plug member 234 and in the underside of resilient flange 240 respectively to allow for fluid flow about the plug member . alternatively , the fluid passageways could be formed in the lower surface 230 and depending portion 232 of stop member 224 and in the lower surface 214 of barrel 202 to allow for such flow . in the raised inactive position illustrated in fig7 flange 240 is disposed substantially adjacent the tapered portion 238 of the plug member 234 , flexes outwardly and bears against the side wall 242 so as to retain the plug member 234 in the raised inactive position . prior to any liquid being drawn into the syringe 200 , the syringe can be used as an air pump , as with the prior embodiment , as air is free to pass through chamber 211 about plug member 234 via fluid passageways 244 and 246 without moving the plug member . upon a liquid being drawn into the syringe and the piston 206 being moved downwardly , the liquid is forced through chamber 211 , about plug member 234 through the fluid passageways 244 and 246 therein and out needle 224 . concurrently , the hydraulic forces acting on plug member 234 , force the plug member downwardly within chamber 211 , causing flange 240 to move past the lower end 248 of stop member 224 and against the lower surface 214 of the barrel 202 to the position illustrated in fig8 . as soon as flange 240 clears end 248 , it flexes outwardly in the enlarged diameter portion 241 of chamber 211 and the plug member 234 is in the active position . thereafter , withdrawal of the piston 206 in an effort to draw more liquid into the syringe will cause the plug member to be drawn rearwardly within chamber 211 such that the upper surface 249 of flange 240 will abut the lower end 248 of stop member 224 and prevent fluid passage therebetween as seen in fig9 thereby rendering the syringe non - reusable . the sensitivity of the embodiments of the non - reusable syringe of the present invention is determined by the axial distance the flange 46 on the first embodiment , the enlarged flange portion 146 on the modified form thereof , or the flange 240 on the second embodiment , travels before flexing outwardly to secure the plug member in its lower active position . for example , in the second embodiment this distance depends on the axial length of the depending portion 232 of stop member 224 . by increasing this axial length , or the length of side wall 54 in the first embodiment , the syringe could be designed to allow for ejection of a small amount of liquid prior to the plug member being locked in its lower active position . this would provide the user with some flexibility in the event of inadvertly ejecting slightly too much liquid during air expulsion prior to an injection as the user could then draw a portion of an additional unit of medicine into the syringe to correct the error . various other changes and modifications may be made in carrying out the present invention without departing from the spirit and scope thereof . insofar as these changes and modifications are within the purview of the appended claims , they are to be considered as part of the present invention .
a single use disposable hypodermic syringe having a hollow elongated barrel portion , a piston and sealing engagement with an axially translatable along the barrel portion , a piston rod secured to the piston and projecting from the rearward end of the barrel portion for use in reciprocating the piston , a walled chamber disposed proximate the forward end of the barrel portion and plug means disposed in the chamber and moveable therein from a rearward position to a forward position in response to liquid being forced against the plug member by the piston . the plug member defines fluid passages therein to allow fluid flow about the plug member and an annular flange which cooperates with the interior surface of the chamber for maintaining the plug member in the rearward position upon air being passed through the fluid passages in response to translation of the piston within the barrel portion . the annular flange also cooperates with a second portion of the chamber wall for permanently maintaining the plug member in the forward position upon the plug member being moved to the forward position by the liquid during ejection thereon from the syringe . with the plug member in the forward position , the annular flange thereon seals the fluid passages upon the piston being retracted within the barrel portion toward the rearward end thereof to prevent any additional liquid from being aspirated into the barrel portion of the syringe about the plug member thereby rendering the syringe non - reusable .