reference should now be made to the figures which depict a preferred embodiment of the injection apparatus , indicated generally with the numeral 2 . referring first to fig1 - 5 , it can be seen that apparatus 2 includes a convenient molded plastic case , made up of a base portion 4 , a pivotable cartridge access door 6 , a slidable dose adjustment door 8 , a syringe collar 9 , a skin sensor 10 , an indicator panel 11 , an initiator switch 13 , and a carrying strap 15 . fig5 depicts how these parts fit together to form an integral unit . referring now to fig8 apparatus 2 can be seen to include several basic components . first , a replaceable co 2 cartridge 12 is disposed at one side of the apparatus , toward the front . a cartridge pressure control system is shown behind cartridge 12 at 14 . as shown best in fig8 b , disposed on the other side of the apparatus , at the front thereof , is a syringe 126 which is adapted to hold and then inject a predetermined amount of medication . positioned rearwardly of the syringe is a syringe control system 18 which controls activation of the syringe . the syringe control system 18 is in turn controlled from pressure which is provided by the cartridge pressure control system 14 . indicator panel 11 is disposed at the rear end of the apparatus , and it includes a power button 171 to activate the apparatus and a series of indicator lamps to keep the operator advised of the condition of the apparatus . skin sensor 10 is disposed at the front end of the apparatus , and is used to prevent initiation of the injection process unless the skin sensor is depressed an appropriate amount as the apparatus is pressed against the skin of the patient . finally , a pair of 1 . 5 volt aaa batteries 26 are mounted in a battery casing 146 disposed between co 2 cartridge 12 and syringe 126 to provide power for the apparatus logic circuit , warning lights , etc . each of these basic components will now be described in more detail . the co 2 cartridge 12 is typically a 33 gram steel cartridge of conventional design , holding 8 grams of co 2 . this is usually enough for approximately 6 - 8 injections , although if the apparatus is used infrequently , passive gas leaks may result in fewer injections per cartridge . co 2 cartridge 12 is positioned within a cartridge receptacle 28 between a forward seat 30 which is curved to complement the curvature of the forward , rounded portion of the cartridge , and a rear area having a resilient cartridge sealing gasket 34 . this gasket is sized and positioned such that a piercing pin 36 is adapted to extend through an annulus at the axial center of the gasket in order to pierce the rear end of the cartridge 12 to release co 2 pressure upon closure of hinged cartridge access door 6 . as seen in fig6 and 7 , hinged cartridge access door 6 is mounted to the ends of a pair of roughly z - shaped cartridge door closure arms 44 by a pair of small bolts 39 which slide in slots 35 as the door is opened and closed . cartridge access door 6 is mounted to a so - called pierce block frame 41 and a pierce block 42 by closure arms 44 which straddle the pierce block and are pivotally connected to the pierce block frame at pivot points 43 . pivot points 43 actually are in the form of rivets , and to ensure that the pierce block travels parallel to the pierce block frame , a slot ( not shown ) extends along each side of the pierce block , and the inner portion of the rivet thereby guides the travel of the pierce block . closure arms 44 are also pivotally mounted to a pair of pivotal legs 50 disposed to each side of pierce block frame 41 at pivot points 48 . the opposite ends of legs 50 pivotally connect to a pierce block pin 46 which extends through and is mounted to pierce block 42 . pivotable legs 50 each include a bend at their mid - portions as shown in fig7 to accommodate the length of closure arms 44 . pierce block pin 46 is mounted to reciprocate in a pair of forked ends 45 in pierce block frame 41 as cartridge access door 6 is opened and closed and pierce block 42 is shifted forwardly and rearwardly . thus , when cartridge access door 6 is closed , legs 50 convey the motion of closure arms 44 to pierce block pin 46 and to pierce block 42 which shifts within pierce block frame 41 . this causes forward seat 30 to exert a rearward force ( to the left in fig6 and 8 ) on co 2 cartridge 12 . as noted above , this causes piercing pin 36 to pierce the rear end of cartridge 12 . as best shown in fig8 a series of 17 so - called belleville spring washers 52 are disposed in series between forward seat 30 and pierce block 42 to provide a predetermined piercing pressure of slightly over 100 pounds , which is maintained the entire time cartridge access door 6 is closed . once cartridge 12 has been breached , pressurized co 2 gas passes from the cartridge through piercing pin 36 , and as best shown in fig8 a , to a solenoid valve 54 through a ( 0 . 25 × 118 &# 34 ; 2 micron ) gas filter 56 and through a conduit 58 extending through the axial center thereof . a space 60 extending entirely across solenoid valve 54 thus is filled with pressurized gas , as is an axially centered spring chamber 62 in which a solenoid spring 64 is disposed . solenoid spring 64 holds a resilient solenoid seal 66 against a solenoid seat 68 to prevent the flow of pressure into an axially extending rear conduit 59 . a pair of o - rings 70 are mounted in the solenoid valve to prevent flow of pressurized gas along the interior wall 72 of the pressure control system 14 . a circumferential ring 74 extends entirely around solenoid valve 54 to ensure that the solenoid valve remains stationary in the pressure control system 14 . a generally cylindrical piston 76 is disposed between space 60 and solenoid seal 66 . as will be described below , piston 76 , in combination with solenoid seal 66 , acts as to control the flow of gas pressure through solenoid valve 54 . a sleeve 61 fits around the piston , and well past space 60 , and an o - ring 63 prevents co 2 pressure from passing forwardly along the sleeve . pressure is , however , able to pass rearwardly along the interface between the sleeve and the piston because another o - ring 65 , disposed rearwardly of space 60 , is positioned outwardly of the sleeve . the cartridge pressure control system 14 also includes a poppet valve 80 ( see fig8 ) having a resilient poppet valve seal 82 which bumps up against rear conduit 59 to create a shuttling phenomenon when the poppet valve shifts forwardly or to the right , as will be described later in this discussion . poppet valve 80 includes a radially extending port 86 which interconnects the inner portion of the poppet valve with a gas reservoir 84 . prior to the point that the reservoir is subjected to co 2 pressure , poppet valve 80 will be in the position depicted in fig8 . a poppet valve spring 88 holds the poppet valve in the depicted position , with a poppet valve seat 90 disposed against the poppet valve to close the poppet valve . upon closure of cartridge access door 6 , and with the solenoid valve in the depicted position , pressurized co 2 flows through piercing pin 36 and filter 56 ( see fig8 a ). it is directed through conduit 58 and into space 60 and spring chamber 62 , and along the interface between sleeve 61 and piston 76 to the rear of the piston . while the pressure is therefore equalized at the two ends of the piston , because the surface area is greater on the front side of the piston if the surface area of solenoid seal 66 is included , the piston will remain in the position shown in fig8 and 8a , with the solenoid seal seated firmly against solenoid seat 68 , thereby preventing pressure from entering reservoir 84 . once the apparatus is initiated to inject medication , solenoid valve 54 is shifted slightly ( approximately 0 . 012 inch ) forward or to the right in fig8 and 8a , but not so far as to close off space 60 . this enables pressurized gas to flow through rear conduit 59 into gas reservoir 84 . from the gas reservoir , pressurized co 2 flows through port 86 in poppet valve 80 ( see fig8 ). when the increased pressure within the poppet valve causes the upward force on the poppet valve to exceed the rearward or leftward force of poppet valve spring 88 , the poppet valve lifts off its seat 90 , permitting pressure to rush into the next section of the apparatus . the poppet valve is normally set to lift off of its seat at a pressure of 480 psi . when poppet valve 80 is in this raised position , poppet valve seal 82 bumps up against rear conduit 59 . when the poppet valve opens , the pressure in gas reservoir drops , so that the force of poppet valve spring 88 again exceeds the force of pressurized gas , thereby causing the poppet valve to close . this in turn permits the pressure in gas reservoir 84 to immediately increase , lifting the poppet valve again . this phenomenon , called shuttling , continues for a short period of time until the medication is fully injected . normally the controller closes the solenoid valve 0 . 8 seconds after it is opened , so that the time of termination of the shuttling is determined by the controller . the initial rush of pressure followed by shuttling produces a pressure profile which is ideal for a needleless injection system . as shown in fig1 , the initial rush of co 2 pressure provides a syringe pressure of approximately 3920 psi to penetrate the patient &# 39 ; s skin , followed by a sustained , substantially constant pressure of approximately 1700 psi for about 0 . 5 seconds during the shuttling phase . the term &# 34 ; substantially constant &# 34 ; as used herein is intended to encompass a variation of from about 2000 psi to 1600 psi as shown in fig1 between the 0 . 1 and 0 . 56 second points of the injection cycle . this pressure profile has been found to be superior to some prior art pressure profiles which peak quickly but then drop off sharply . assuming 1 . 0 cc is injected , it can be seen that approximately 0 . 25 cc is injected at the higher pressures , but much more than half of the medication is injected during the shuttling , lower pressure phase . a threaded poppet valve pressure adjustment face 91 may be threaded inwardly to increase or outwardly to decrease the pressure at which poppet valve 80 opens and closes . a special tool ( not shown ) is used to facilitate this adjustment . referring to fig8 and 8a , the syringe control system 18 , which receives co 2 pressure from poppet valve 80 , will now be described . this system includes a dose compensator cylinder 94 , a dose variation assembly 96 having a pressure piston 98 mounted thereto , an inner cylinder 100 , a rearward outer cylinder 102 , and a forward outer cylinder 104 . so - called u - cup seals 99 and 101 will prevent pressure leakage between the stages of the syringe control system . the co 2 pressure entering the syringe control system 18 causes the entire rearward outer cylinder 102 to shift forwardly or to the right in fig8 against the compressive action of a light helical spring 97 . rearward outer cylinder 102 continues to shift until its forward end contacts the rear end of forward outer cylinder 104 , which is about 1 / 8 - 3 / 16 inch into its travel . at this point , inner cylinder 100 continues to move in a forward direction for approximately another 1 - 1 / 2 inch , for a total travel of approximately 11 / 4 inches . this independent movement of the inner cylinder generally corresponds with the point that the shuttling begins in the cartridge pressure control system 14 . thus , the independent movement of inner cylinder 100 cooperates with the shuttling action to provide a reduced , substantially constant but lower second pressure phase to the injector . at this point , spring 97 will have bottomed out and immediately thereafter the controller will cause the solenoid valve to shut off co 2 pressure . dose compensator cylinder 94 travels with inner cylinder 100 and rearward outer cylinder 102 in their above - described forward motion . dose compensator cylinder 94 is a generally cylindrical member having a soft rubber bumper at the rear end thereof ( not shown due to its small dimensions ), and a centrally disposed axially extending channel 108 with an entry segment 110 at the rear end thereof , as shown best in fig8 . this entry segment 110 selectively interconnects channel 108 with fluid pressure from poppet valve 80 . an o - ring 112 is provided on dose compensator cylinder 94 to prevent the flow of fluid pressure along the outer surface of the cylinder . a seal 114 is provided at the forward end of the dose compensator cylinder to minimize any leakage between the inner cylinder wall defining channel 108 and pressure piston 98 . the purpose of the dose compensator cylinder system is to account for the fact that pressure will tend to act somewhat differently on the syringe control system 18 when there is a greater or lesser amount of medication in the syringe . because pressure piston 98 will move forwardly and rearwardly within channel 108 as the dosage is decreased and increased , respectively , thereby increasing and decreasing , respectively , the size of a chamber defined within channel 108 behind piston 98 , this accommodation is made . a helical spring 115 is positioned between dose compensator cylinder 94 and dose variation assembly 96 as shown in fig8 . spring 115 provides a suitable amount of pressure which is passed onto syringe 126 and the medication provided therein to make sure that no air is in the system . with such a forward biasing of the syringe , the amount of medication in the syringe can be measured . as will be described below , if the dose variation assembly 96 is too far forward or to the right in fig8 which indicates that there is an insufficient amount of medication in the syringe , then an interlock will prevent the apparatus from firing . this condition is sensed by a dose indicator flag 106 disposed in a dose indicator optical interrupter space 107 . dose indicator flag 106 is mounted to a cylindrical dose variation compensator 120 so that the position of the flag generally corresponds to the amount of medication in the syringe . when there is sufficient medication in the syringe , flag 106 will block infrared light from passing across space 107 from an illuminator ( not shown ) to a receptor ( not shown ). when there is an insufficient amount of medication in the syringe , spring 115 will cause flag 106 to be shifted to the right , withdrawing the flag from space 107 and permitting ir light to pass from the illuminator to the receptor , which will send a signal to the controller , thereby lighting a warning lamp and preventing the apparatus from entering its initiation phase . it is possible that micro switches , magnetic switches , or other conventional position sensors ( not shown ) may be utilized in place of the optical interrupter described above . dose variation assembly 96 permits the dosage to easily be adjusted in 1 / 4 cc increments ( see fig9 ). this is done through the use of a thumb - nail manipulator 118 which extends radially outwardly from the unit and which is mounted by a lock nut 111 to an axially extending rod 113 which is threaded into dose variation compensator 120 . the dose variation compensator has a generally semi - spherical protrusion 122 mounted on it , and it is surrounded by a cylindrical jacket 123 shown best in fig9 . this jacket 123 has four circumferentially extending slots 125 interconnected by a single axially extending slot 127 , the four slots being adapted to selectively receive semi - circular protrusion 122 . partitions 124 are disposed between and define the four slots . fig9 shows that the partitions are relatively narrow in their circumferential dimensions , so that with only a 40 - 45 ° twist of compensator 120 with thumbnail manipulator 118 , semi - spherical protrusion 122 can clear the adjacent partition , and under the pressure from springs 115 , will be biased forwardly through axially extending slot 127 into the next adjacent slot 125 , thereby adjusting the dosage by 1 / 4 cc . if the thumbnail manipulator 118 has not been released , then the protrusion can selectively be guided over to another one of the four slots , depending upon the desired dosage . once positioned , releasing the thumbnail manipulator permits a series of rotational biasing springs 119 to cause dose variation compensator 120 to rotate , which in turn moves the protrusion into one of the four slots 125 . the syringe 126 is shown best in fig1 . it includes an ampule 128 and a plunger 130 . the end of the plunger includes a radially extending notch 132 which is interconnected with an axial slot 127 which is sized to fit onto rod 113 extending from dose variation compensator 120 . a flared end 134 on the plunger is designed to abut the forward end of dose variation compensator 120 . thus , the axial drive force imparted to the compensator by rearward outer cylinder 102 and inner cylinder 100 will cause the plunger to drive forwardly , forcing medication out of the syringe . the syringe also includes a pair of opposed flanges 129 disposed adjacent the forward end thereof . the syringe ampule 128 includes a small injection aperture 20 at the forward end . aperture 20 is typically 0 . 0045 inch in diameter , although it may be as large as 0 . 014 inch , depending upon the subcutaneous injection depth which is desired . syringe 126 fits into the apparatus by merely inserting the syringe through collar 9 in the front end of the apparatus , and pushing it in . when it is most of the way in , pressure from spring 115 will be felt . when it bottoms out against a wave spring 131 , the syringe is rotated approximately 90 degrees so that flanges 129 are engaged within the syringe collar 9 as shown in fig8 b . as the syringe is rotated that 90 degrees , it engages a pin 133 which rotates with it . once this pin 133 is rotated , it depresses a syringe lock micro switch 140 , which sends a signal to the controller that the syringe has been properly installed . if this syringe lock micro switch is not depressed , the controller will light a warning lamp and prevent the apparatus from entering its initiation phase . a pressure switch 148 is disposed midway between and to the side of the portions of the apparatus which house the cartridge pressure control system 14 and the syringe control system 18 , as shown best in fig5 . referring now to fig1 , pressure switch 148 includes a bellows 150 , a spring 152 and a central rod 154 which terminates in a flag 156 . flag 156 is disposed within a stationary optical interrupter 158 which transmits infrared light across a space 160 much like the previously - described dose measurement optical interrupter . when the flag is disposed within the space , the light is interrupted and a collector ( not shown ), which otherwise receives light from an emitter ( not shown ), sends a signal to a controller . bellows 150 is subjected to co 2 cartridge pressure because a port 151 interconnects an otherwise - sealed chamber 162 surrounding the bellows with the co 2 pressure present within solenoid valve 54 . the variations in pressure cause the bellows to expand and contract , causing rod 154 and flag 156 to move slightly forwardly and rearwardly in relation to optical interrupter 158 . a pin 155 travels within a short slot 157 such that some contraction or expansion of the bellows is permitted without causing any displacement of flag 156 . if the pressure is relatively high , the flag blocks the transmission of ir light across space 160 , but if the pressure is not as high as it should be , spring 152 causes bellows 150 to extend slightly into chamber 162 , thereby causing rod 154 to withdraw flag 156 from optical interrupter 158 , permitting ir light to be conveyed to a collector . this sends a signal to the controller , which lights an appropriate warning lamp and terminates the initiation cycle . it is possible that a pressure switch other than the above - described bellows / optical interrupter could be used . for example , it may be possible to use a helical or a spiral bourdon tube could be used in place of the bellows , and another type of switch other than the described optical interrupter . to ensure that the apparatus is pressed up against the skin of the patient prior to activation of the apparatus , skin sensor 10 is provided . the skin sensor includes an extension rod 142 which is forwardly biased under the pressure of a skin sensor spring 144 to the extended position shown in fig8 . a soft plastic jacket 138 fits over the extension rod in the depicted embodiment . as the apparatus is sufficiently pressed against the skin of the patient , the extension rod is depressed against the pressure of the skin sensor spring , and a spring sensor micro switch 144 is contacted , sending an electronic signal to the controller to prevent termination of the initiation cycle . if the skin sensor is not sufficiently depressed , the controller lights a warning lamp and the initiation cycle is terminated . skin sensor 10 thereby functions to prevent inadvertent or other discharge of the apparatus when the apparatus is not properly positioned against the skin , which may happen if the patient is reluctant or , again , physical dexterity problems make it difficult for the patient to properly position the apparatus . indicator panel 11 , shown best in fig1 , includes the following red warning lamps : co 2 pressure warning lamp 184 ; dose volume warning lamp 194 ; syringe lock warning lamp 19 1 ; and battery warning lamp 176 . a green &# 34 ; ready &# 34 ; lamp 200 is also included , as is a power button 171 . reference will be made to the control circuit schematic , fig1 , as well as to the indicator panel 11 provided at the rear of the apparatus , and depicted in fig1 . the logic circuit , indicated generally with the numeral 164 , selectively provides power to light the lamps of the indicator panel . central to the circuit is controller 166 which in the preferred embodiment is an atmel programmable logic device , designated as model atf 1500l . this is a low power unit which can effectively control the operation of the apparatus while using a minimal amount of power so that the batteries do not have to be replaced very often . as mention ed earlier , the apparatus includes a number of interlocks which prevent the unit from operating , and warn the operator in the event any one of a number of conditions is not satisfied . the logic circuit provides this capability , but before describing those features , reference will first be made to the general layout of the circuit . the batteries , shown at 26 , are mounted in series to provide 3 volts of dc power to the circuit . a power switch 70 , which corresponds with power button 171 ( see fig1 ), controls the flow of power to a dc - dc converter 172 , which converts the 3 volt charge to a 5 volt charge as needed elsewhere in the circuit . in the event there is low battery power , a signal is sent to the controller via line 174 , and a red &# 34 ; battery &# 34 ; light 176 is activated in indicator panel 11 depicted in fig1 . this light is energized by an led 178 which is connected to an active low pin in controller 166 which sends the 3 volt charge to ground upon a low battery signal from line 174 , thereby energizing the led and warning the operator that the batteries need to be replaced . this event prevents the initiation of the apparatus so that even if the operator ignores the light , the apparatus cannot be initiated . in the event there is sufficient battery power , voltage is provided to the controller to activate the apparatus . another one of the interlocks provides protection for insufficient co 2 pressure . as described above , pressure switch 148 determines whether sufficient co 2 pressure is sufficient . if it is , flag 156 will block light from passing through optical interrupter 158 , and a transistor in a co 2 detect subcircuit 180 will remain open . in this condition , the controller will sense the 5 volt charge coming in from line 181 . if co 2 pressure is insufficient , the flag will withdraw , permitting light to pass through the optical interrupter , which will then close the co 2 detect subcircuit 180 , thus grounding the 5 volt charge , which will be sensed by the controller . simultaneously , an active low pin to which a co 2 cartridge led 182 is connected will ground that led circuit , energizing that led and activating a red co 2 cartridge light 184 in indicator panel 11 , as shown in fig1 . this event also causes the controller to prevent initiation of the apparatus even if the user ignores the indicator panel warning light 184 . yet another interlock is provided to ensure that there is sufficient medication in ampule 128 . a dose detect subcircuit 190 , very similar to co 2 detect subcircuit 180 , is provided . dose detect subcircuit 190 is provided with a 5 volt charge from line 181 , and if a sufficient dosage level is sensed by the dose indicator optical interrupter , a transistor in the dose detector subcircuit will remain open and the controller will sense 5 volts . if the dose is insufficient , the transistor will close and the controller will sense the absence of the 5 volt charge . in that event , a red volume warning lamp 194 is lit in indicator panel 11 by a dose detector led 192 . this light is connected to an active low pin in the controller which sends the 3 volt charge to ground , thereby activating the led . unless the dose is sufficient , this dose interlock will warn the user at the indicator panel and will prevent the apparatus from initiating . yet another interlock is provided with a syringe switch 186 which ensures that the syringe is properly locked into positioned in the apparatus before the unit is initiated . as noted previously , this condition is sensed by syringe lock micro switch 140 . syringe switch 186 receives a 5 volt charge from line 188 . if the syringe is properly locked in place , the syringe switch will be closed . in this condition , the controller senses the 5 volt charge , and the apparatus is ready for initiation . if the syringe is not properly locked in place , a syringe lock led 193 will activate a red syringe lock warning lamp 191 and the controller will prevent the apparatus from entering its initiation cycle . if all of the conditions have been met ( other than the next - to - be - described skin sensor ), the controller operates to flash a green &# 34 ; ready &# 34 ; light 200 in indicator panel 11 . the skin sensor interlock will now be described . a skin sensor switch 196 is provided off 5 volt line 181 . in order to initiate the apparatus , skin sensor 10 must be depressed , thereby closing skin sensor switch 196 and sending a 5 volt charge to the controller . unless this charge is sensed by the controller , the apparatus will be prevented from entering its initiation cycle . when the charge is received , showing that everything is ready for initiation , skin sensor led 198 will provide a steady activation of the green &# 34 ; ready &# 34 ; light 200 in indicator panel 11 , and an audible indicator 202 will emit a beep . an initiator switch 204 is also provided off 5 volt line 181 , which is closed by depressing indicator panel initiator button 171 . if all of the foregoing conditions have been satisfied , closing of the initiator switch will send a 5 volt charge to the controller , which in turn sends power to solenoid valve 54 to cause co 2 pressure to inject medication into the patient . if any of the foregoing conditions have not been satisfied , the appropriate warning lights will be lit , and the controller will prevent the apparatus from entering its initiation cycle . in order to use the apparatus depicted in the figures , the user first must install batteries 26 into battery case 146 . this is done by opening cartridge access door 6 and removing the co 2 cartridge if there is one in place . next , co 2 cartridge 12 is installed into cartridge receptacle 28 . once the cartridge is in place , cartridge access door 6 is pivoted closed . the action of the cartridge door mechanism causes pierce block 42 to shift rearwardly and under the action of springs 52 , forward seat 30 pushes rearwardly on the cartridge , which causes the rear of the cartridge to be pushed down so that piercing pin 36 pierces the rear end of the cartridge . breach of the co 2 cartridge causes pressurized co 2 gas to pass through piercing pin 36 , filter 56 , and axial conduit 58 to space 60 and spring chamber 62 . pressurized gas also flows around piston 76 . because there is greater surface area on the forward side of the piston than on the rear side , solenoid seal 66 is held against solenoid seat 68 to prevent the flow of pressurized gas into rear conduit 59 . of course , it is not always necessary to replace the batteries and the co 2 cartridge . the batteries should last for an extended period , and the co 2 cartridge will last for approximately 6 to 8 injections . in any event , either before or after they are replaced , the dose adjust mechanism is set , and the syringe is filled with medication and placed into the apparatus . before insertion of the syringe into the apparatus , thumbnail manipulator 118 is adjusted to the appropriate dosage by first sliding open the dose adjustment door 8 , and then turning the manipulator slightly and moving it axially to the right to decrease and to the left to increase the dosage , in 1 / 4 cc increments . the dose adjustment door is then closed and the apparatus is ready to receive the syringe . the syringe 126 is first filled with the desired amount of medication , is purged of air and is then mounted in the apparatus . the syringe is normally over - filled so that there is somewhat more medication in the ampule than the apparatus dosage setting , just to make sure the ampule is not under - filled . once the syringe has been filled to an appropriate level , it is inserted through collar 9 in the front of the apparatus . as the syringe is slid into place , the pressure from spring 115 will be felt , and as the syringe is pushed beyond that point , wave spring 131 will also be felt . at that point the syringe is rotated 90 degrees and engaged . the apparatus is now ready to have the power turned on , so this is done by depressing power button 171 on indicator panel 148 . once the power is on , the operator can check to see if co 2 pressure warning lamp 184 is lit , and if it is , then the pressure switch 148 has determined that there is insufficient pressure , and the co 2 cartridge needs to be replaced . similarly , if the syringe has not been properly installed , the red syringe lock lamp 191 on the indicator panel will be lit ; if there is insufficient medication in the syringe as determined by the dose indicator , the red dose volume lamp 194 will be lit ; and if there is insufficient battery power , the red battery warning lamp 176 will be lit . if all conditions are satisfied , the apparatus is ready for use , and the green &# 34 ; ready &# 34 ; light 200 will flash intermittently . to initiate the injection , the user presses the front end of the apparatus against the skin , which depresses skin sensor 10 . this causes the green &# 34 ; ready &# 34 ; lamp 200 on the indicator panel to light , and the audible alarm to sound , informing the user that the apparatus is ready to initiate . depression of initiator button 171 causes the initiator switch to close if all four of the aforementioned conditions are met . this in turn activates solenoid valve 54 which permits pressurized co 2 to rush into gas reservoir 84 , causing poppet valve 80 to lift , sending pressure into the syringe control side of the apparatus . this pressure drives rearward outer cylinder 102 and inner cylinder 100 forward , which causes rod 123 to drive syringe plunger 130 , forcing medication out of syringe aperture 20 and through the skin of the patient at a highly elevated pressure . poppet valve 80 then enters its shuttling mode , during which most of the medication is injected at a somewhat lower pressure . once inner cylinder 100 bottoms out against spring 97 , injection is complete and solenoid valve 54 is closed . after each injection , however , co 2 pressure is maintained in the solenoid , upstream of solenoid piston 76 , so that the unit is immediately ready for another injection once the syringe is replaced .
the present invention typically is in the form of a needleless injector which includes the following components : a needleless syringe installed in the injector for holding medication prior to injection , the syringe including an injection aperture at the forward end thereof ; a syringe plunger slidably mounted to the rear end of the syringe for forcing medication out the syringe aperture ; a syringe plunger drive mechanism providing power to drive the syringe plunger and thereby force medication out of the syringe ; and a syringe drive control mechanism for controlling the operation of the drive mechanism , the drive control mechanism including a warning system which warns the user if a pre - injection condition is not met , an interlock system which prevents injection from taking place if the pre - injection condition is not met , and a sensing system which senses whether the pre - injection condition is not met and conveys a signal to the warning and interlock systems informing as to whether the pre - injection condition is met .