referring first to fig1 illustrated is a needleless hypodermic injector assembly 1 including a disposable , single use , medicine delivery unit 3 in accordance with the present invention . preferably , injector assembly 1 is an apnhi of the same general type described in aforementioned u . s . patent application ser . no . 08 / 483 , 192 . the assembly includes a main injector housing 5 . the main injector housing contains a cylinder 6 forming an air chamber 7 and a driving piston 8 slidably mounted in chamber 7 . as described in detail in application ser . no . 08 / 483 , 192 ( which is incorporated by reference ), a line 12 is provided for connecting the air chamber with a relatively low pressure source of air 16 . an injection actuator 20 is provided for selectively admitting pressurized air from the source into the air chamber to actuate , i . e ., drive , driving piston 8 forward in chamber 7 . a discharge piston rod 9 is connected to driving piston 8 to move therewith . a compression spring 10 is attached to the driving piston and extends coaxially with discharge piston rod 9 into a passageway 11 of main injector housing 5 . passageway 11 is configured to removably receive medicine delivery unit 3 . spring 10 has a disk - shaped platform 13 attached at its distal end . platform 13 has an axially centered bore allowing free travel of discharge piston rod 10 therethrough . a distal side of platform 13 provides a surface which abuts with a proximal end of medicine delivery unit 3 , when unit 3 is fully inserted into passageway 11 . when delivery unit 3 is removed , platform 13 engages an annular ledge 14 . in this manner , spring 10 serves continuously to bias the driving piston and attached rod 9 in the proximal direction , moving these elements to the beginning of their strokes . this ensures that the driving piston and attached rod 9 are returned to an injection - ready position immediately after each injection , on evacuation of air chamber 7 . preferably , the distal side of platform 13 forms part of a bayonet locking device . lock part 15 forms , together with an opposing internal annular wall ( not visible ) of housing 5 , a channel which receives a pair of wings or shoulder flanges 17 ( only one seen in fig1 ) extending radially from medicine delivery unit 3 adjacent a proximal end thereof . a pair of opposed slots are provided in passageway 11 for allowing insertion of delivery unit 3 with flanges 17 aligned with the slots . once fully inserted , flanges 17 pass out of the slots at the proximal end of passageway 11 . delivery unit 3 is then rotated ( e . g ., a quarter turn ) to secure the flanges within the channel . a cam surface 18 may be provided on lock part 15 . under the biasing force of spring 10 , cam surface 18 serves to index and releasably bind the flanges to / in a proper locked position within the channel . other arrangements may be used to lock delivery unit 3 within main injector housing 5 . for example , a push - button mechanism could be employed for effecting locking and unlocking of delivery unit 3 without requiring direct hand manipulation of delivery unit 3 . this would serve to further reduce the possibility of contamination . in the illustrated embodiment , and as disclosed in application ser . no . 08 / 483 , 192 , a medicine chamber filling mechanism 19 , including a medicament - filled vial 22 , is also removably attached to main injector housing 5 . the mechanism includes a helical slot with detent openings at three positions ( insert / remove , fill , fire ). element 24 is a pump button actuated during filling to deliver fluid into delivery unit 3 . in the manner described in the aforementioned patent application , mechanism 19 serves to deliver a dose of medicament from vial 22 and into delivery unit 3 through a side - loading fill port ( to be described ). alternatively , however , medicine delivery unit 3 may be provided ( without a port ) as a sealed cartridge pre - filled with medicament . medicine delivery unit 3 is generally cylindrical in shape . as best seen in fig2 - 3 , unit 3 further comprises generally cylindrical sidewalls 21 defining an elongated medicine chamber 23 . a discharge nozzle 25 is connected ( preferably molded integrally ) with sidewalls 21 at a distal end of chamber 23 . a discharge piston seal 27 is slidably mounted for axial movement within chamber 23 , from a first position adjacent a proximal end of the chamber to a second position adjacent a distal end of the chamber . as illustrated in fig1 an enlarged cup - like end piece 29 of discharge rod 9 engages a truncated conical proximal end 30 of piston seal 27 to advance it through an injection stroke . ( as shown in fig1 seal 27 has been advanced approximately half - way through its injection stroke .) alternatively , the end of discharge rod 9 could be provided with a truncated conical shape to be engaged by a cup - like proximal end of piston seal 27 . in both arrangements , the engagement between discharge rod 9 and piston seal 27 is a push - only engagement , i . e ., rod 9 can push , but cannot pull - back , piston seal 27 . fig2 - 8 illustrate more clearly the structure and functionality of medicine delivery unit 3 . in fig2 - 4 , it is clearly seen how sidewalls 21 define a side - loading medicine fill port 31 . in fig2 piston seal 27 is shown in a pre - injection position adjacent fill port 31 . significantly , by providing a starting position of piston seal 27 adjacent fill port 31 , fill port 31 is removed from fluid communication with chamber 23 almost immediately after piston 27 begins its injection stroke . this avoids the need for the problematic check - valves of the prior art ( see the background section ). preferably , fill port 31 is conical and provided with an end orifice of 0 . 01 &# 34 ; diameter . in order to help with proper initial positioning of piston seal 27 adjacent fill port 31 ( e . g ., during manufacture ), and to reduce compressive loading on seal 27 prior to injection , sidewalls 21 may be formed to provide a chamber diameter ( e . g ., 0 . 265 &# 34 ;) at a proximal side of fill port 31 , which is slightly larger than a chamber diameter ( e . g ., 0 . 25 &# 34 ;) directly adjacent and distally of fill port 31 . along with suitable drafts in each chamber section ( e . g ., 0 . 5 ° and 0 . 25 °, respectively ), the differential diameters will also facilitate removal of a mold core in an injection molding process . sidewalls 21 also preferably define an annular collar 33 which surrounds nozzle 25 and serves to position the nozzle with respect to a skin surface of an injection recipient . as shown , collar 33 lies substantially flush with a distal end surface 34 of nozzle 25 , so as to position surface 34 in low - force contact with the skin surface when collar 33 is pressed against the skin surface . alternatively , as shown in fig9 a collar 33 &# 39 ; could be configured to extend slightly beyond nozzle 25 , in order to pre - position the nozzle in slightly spaced relation to the skin surface of an injection recipient , e . g ., a 4 mm spacing . it has been found desirable to vent the space between the skin surface and nozzle in order to obtain a highly focused jet stream . to this end , collar 33 &# 39 ; is equipped with a plurality of ( e . g ., three ) arcuate spacer tabs 36 spaced equally from each other in a circumferential direction to form vent passageways therebetween . sidewalls 21 may also define a ribbed or otherwise textured finger grip portion 35 at a distal end of delivery unit 3 , in order to facilitate rotation of unit 3 when installing and removing the same in / from main injector housing 5 . in the illustrated embodiment , shoulder flanges 17 protrude radially from cylindrical sidewalls 21 , adjacent a proximal end thereof alternatively , the flanges could be provided at a position between the ends of unit 3 in order to reduce the length of sidewalls 21 subjected to tensile stresses generated during an injection stroke . in accordance with the invention , medicine delivery unit 3 is equipped with at least one ( and preferably three ) features serving to prevent use of the unit for more than a single injection . first , a distal end region of medicine chamber 23 is provided with a reduced cross - sectional area . as seen in fig3 piston seal 27 is configured relative to the distal end region such that it enters the end region and is securely retained therein ( with a greater holding strength than is otherwise present ) at the end of its injection stroke . in the illustrated preferred embodiment , a distal end region 37 of chamber 23 is defined by conically tapered interior walls 39 ( taper angle δ of about 65 °) of nozzle 25 , and piston seal 27 has a mating but slightly larger conical distal end portion 40 which is received within end region 37 with an interference fit . preferably , entry into distal end region 37 permanently deforms , and thereby destroys , piston seal 27 . the second feature for disabling delivery unit 3 is the previously described push - only engagement between discharge rod 9 and piston seal 27 . this prevents use of rod 9 for retracting seal 27 . the third feature for disabling delivery unit 3 is a protruding nub or nipple 41 extending from piston seal distal end portion 40 . nipple 41 is aligned with an injection orifice 43 ( best seen in fig7 and 8 ) of injection nozzle 25 . in a pre - injection condition , orifice 43 preferably has a minimum diameter at its proximal side of 0 . 005 to 0 . 006 , and conical sidewalls angled with respect to each other by an angle α of about 15 °. upon seal 27 reaching the end of its injection stroke , nipple 41 extends into and permanently deforms orifice 43 , thereby destroying its functionality . as best seen in fig5 in order to ensure a good seal that can withstand the high pressures generated during an injection , piston seal 27 comprises a plurality of circumferential fins 45a - c integrally molded with the main seal body . this arrangement eliminates the need for separate dynamic o - ring type seals . in addition , as shown , at least the leading one of the fins 45a is molded with a concave ( cupped ) cross - sectional shape . this permits the fin ( s ) to flair ( i . e ., fold ) outwardly slightly during an injection , in order to maximize seal - to - cylinder wall contact . preferably , flanges 17 , sidewalls 21 and nozzle 25 ( i . e ., all parts of delivery unit 3 except for piston seal 27 ) are injection molded as a single piece from medical grade thermoplastic material . the material may be a medical grade semi - rigid engineering polymer , such as polycarbonate . however , for widespread public health applications where cost is critical , these parts are preferably integrally molded of a clear , medical grade , low cost commodity polymer such as impact modified styrene . an example of a material which has been successfully used is k - resin kr03 from phillips 66 company , a modified butadiene styrene . this material is less brittle than unmodified styrene ( it has a flexural modulus of 205 , 000 psi versus a typical styrene modulus of 250 , 000 psi ). this property allows the cylinder ( sidewalls 21 ) to expand elastically during an injection . a more brittle material will tend to fracture during the initial high pressure loads of injection . a material with an even lower modulus may expand too much , allowing &# 34 ; blow - by &# 34 ; of liquid medicament past piston seal 27 . with the use of commodity rather than engineering polymers as aforesaid , it may be necessary to increase sectional thicknesses of the parts in order to obtain the necessary strength . in addition , as aforementioned , it may be desirable to move shoulder flanges 17 to a position between the ends of unit 3 in order to reduce the length of sidewalls 21 subjected to tensile injection stresses . particularly with nozzle - skin spaced embodiments of the type shown in fig9 it is critical that nozzle 25 be formed with a high degree of precision . despite the low reynolds ( approximately re = 1000 ) flow in the nozzle orifice , turbulence is expected within the free jet . small imperfections ( e . g ., 3 - 5μ ) will produce lateral flows that impinge on the main core of the free jet . these lateral flows are also believed to be caused by a large radius ( over 0 . 005 &# 34 ;) on the inside upstream ( proximal ) edge of orifice 43 . mixing of these small lateral flows with the main core of the jet causes turbulence . as the turbulent eddies flow out of the orifice , rapid growth of the turbulence occurs due to the adverse pressure gradient along the free jet . by minimizing the radius of the proximal edge of orifice 43 ( creating a sharper edge ), and reducing imperfections along the nozzle flow path surfaces , improved jet coherence may be obtained . as best seen in fig8 a flattened nozzle surface 47 extends perpendicularly to the longitudinal axis of chamber 23 adjacent orifice 43 . this , together with small orifice taper angle α , provides a sharp ( acute ) proximal orifice edge . such a sharp edge is believed to increase coherence of the liquid jet by accelerating laminar fluid flow at the outside of the jet . the sharp edge , along with the taper along the length of the orifice , is also believed to improve jet coherence by reducing contact between the orifice walls and the jet . the piston seal may be injection molded of any semi - flexible medical - grade polymer such as delrin ( polyacetal ), pvc , or urethane . to maintain low cost and to provide the best auto - destruct characteristics , piston seal 27 is preferably molded of a polyolefin such as montell pro - fax sr - 857m polypropylene . this material is flexible enough to enable molded sealing fins 45 to conform to the cylinder walls yet rigid enough to withstand injection pressures of 3000 psi . the flexural modulus of sr - 857m is 130 , 000 psi . this characteristic of the preferred material allows piston seal 27 to deform permanently as it enters distal end region 37 at the end of its injection stroke . this material also has a relatively low tensile strength ( under 4000 psi ) so that it will readily permanently deform if tampered with . the present invention has been described in terms of preferred and exemplary embodiments thereof . numerous other embodiments and variations within the scope and spirit of the appended claims will occur to persons of ordinary skill in the art from a review of this disclosure .
a disposable medicine delivery unit for use in a needleless hypodermic injector is configured to allow economical replacement , after each injection , of a patient contact surface and the entire injection flow - path of the medicament , to thereby ensure sterility with less need for repeated equipment cleaning and sterilization procedures . the delivery unit includes a piston seal configured to prevent improper reuse . the piston seal deforms and sticks in a conical end of the medicine delivery chamber at the end of its injection stroke . a push - only connection between the piston seal and a piston drive rod of the injector ensures that the latter cannot be used to retract the former . preferably , the piston seal also includes a protruding nipple which enters and destroys the injection orifice following an injection , to further disable the delivery unit .