Abstract:
A Two-part, Syringe-based Electrochemical Dispenser. The dispenser consists of a disposable syringe-like fluid storage reservoir and a reusable fluid driver. The driver is located at the proximal end of a conventional syringe; the fluid exit port at the distal end. The driver generates a gas that inflates an elongated bladder situated within the filled syringe. Expansion of the bladder releases liquid at a rate nearly identical to the gas generation rate. The dispenser is held in a vertical position with the distal end pointed downward. Liquid is released drop-wise onto a porous material from which it can evaporate. A battery-driven electrochemical oxygen generator is the gas source. Means to attach the dispenser in its vertical position are provided. Such dispensing devices can be used to release pheromones, repellants, insecticides, fragrances, etc.

Description:
This application is filed within one year of, and claims priority to Provisional Application Ser. No. 61/392,873, filed Oct. 13, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention herein relates to vapor release devices/dispensers. More particularly it relates to small two-part dispensers with disposable fluid reservoirs and re-usable fluid pumping mechanisms. 
     2. Description of Related Art 
     Commodity fragrance releasers and pheromone dispensers must be economical in order to be deemed commercially feasible by potential consumers. Three such devices are described in U.S. Pat. No. 5,928,194 for “Self-contained liquid microdispenser” (Maget I), U.S. Pat. No. 6,383,165 for “System for achieving a controlled low emission rate for small volumes of liquid solutions” (Maget V) and in U.S. Pat. No. 7,681,809 for “Electrochemical dispenser” (Maget VI). 
     Most prior art dispensers include an integral battery-powered gas generation unit, such as described in U.S. patent application Ser. No. 12/413,546 for “Electrochemical Gas Generator and Cell Assembly,” as well as in Maget VI. The Ser. No. 12/413,546 device does not, however, embody a dispenser having two parts (one being replaceable). 
     One prior device does teach a two-part fluid dispenser—Maget VI, as well as the system taught by U.S. Pat. No. 5,938,640 for “Two-part fluid dispenser” (Maget II). The teachings of Maget V and Maget VI are incorporated herein by reference since devices are described therein that includes an elongated dispenser similar to the device described herein, but with the following important distinctions: (i) fluid release in the prior system is from the top of the elongated reservoir and not the bottom, and (ii) the pumping mechanism of Maget V is an integral part of the disposable releaser. 
     Another important difference and improvement over Maget VI is the re-usable feature of the driver. While the fluid reservoir will require regular replenishment, the gas generator will not. Since the gas generator has a long service life (several years), the driver can be expected to operate up until the exhaustion of the onboard battery capacity. Even then, replacing the onboard batteries will provide another gas generator operating cycle. Therefore the cost per use of the driver is amortized over the multiple uses of the driver (i.e. multiple battery replacements and fluid replenishments.) 
     Since the fluid reservoir of the instant design is based upon a plastic syringe, the cost of replacement of the disposable component is expected to be as low as that of plastic syringes, a commodity product, that are produced in billions of units/year by the health care industry. 
     While it may seem trivial, to place the syringe with the distal end pointing downward, the reader is assured that this is not the case. In fact, by employing this configuration, all of the fluid can be eventually evacuated from the syringe barrel, whereas in the device of Maget VI, some of the liquid would be entrapped at the base of the elongated reservoir. Furthermore, compression of the bladder of the Maget VI device requires ever-increasing forces (pressures) to expel the fluid or ever-decreasing delivery rates, since the bladder eventually needs to collapse completely in order to surrender the contents that can be trapped in the bladder folds. In contrast, in the present invention, the bladder expands outward, with its outer dimensions being confined by the rigid syringe wall. Gravity assists the syringe driver in forcing the liquid contents towards the distal end. 
     Similarly, replacing the fluid receiver of Maget V with a porous plastic cup of the present invention facilitates the attachment of the fluid emanation surface to the syringe. The combination of an airtight bladder and an airtight generator render it possible to hold the syringe “upside-down” without any loss of fluid. Additionally, fluid stream discontinuity between the Luer tip and the porous receiver prevents fluid “streaming” (capillary extraction of fluid from the syringe) as well as preventing the introduction of ambient air into the syringe. If either of these were permitted, it would be impossible to control the fluid delivery rate, and therefore the rate of emanation. Finally, the orientation of the dispenser allows for the solar cells to be mounted to the top of the gas generator and still be exposed to sunlight. 
     SUMMARY OF THE INVENTION 
     In light of the aforementioned problems associated with the prior devices and methods, it is an object of the present invention to provide a Two-part, Syringe-based Electrochemical Dispenser. The dispenser should be practical and low-cost, while being capable of releasing fluids into the environment under controlled conditions. The dispenser should allow the user to operate the device in a position such that all of the fluid is released. It is a further object that the fluid storage reservoir is a disposable (and economical) syringe-based unit. It is yet another object that the gas generator driver be reusable, and only require replacement of the battery power source. Still further, it is an object to provide users with means to set and later readjust the delivery rate of the dispensed fluid. It is still another object that the dispenser utilize energy from solar or artificial light sources to power the driver thereby reduce the periodicity of the battery replacement. Furthermore, if solar-powered, it is an object that alternate embodiments of the invention become inactive in absence of sunlight or on rainy days. It is a final object to provide users with a simple syringe-attachable porous plastic receiver that receives the fluid in droplet form for future evaporation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings, of which: 
         FIG. 1  is a perspective view of a preferred embodiment of the dispenser of the present invention; 
         FIG. 2  is a cutaway side view of the dispenser of  FIG. 1 ; 
         FIG. 3  is an exploded cutaway side view of the dispenser of  FIGS. 1 and 2 ; 
         FIG. 4A  is a partial cutaway side view of an alternate driver head assembly,  FIG. 4B  is a top view of the driver base of the assembly of  FIG. 4A  and  FIG. 4C  is a top view of the head flange of  FIG. 4A ; 
         FIG. 5A  is a perspective view of a single current driver head assembly of the present invention and  FIG. 5B  is a diagram of a preferred circuit for said assembly; 
         FIG. 6A  is a perspective view of a multi-current driver head assembly of the present invention, and  FIG. 6B  is diagram of a preferred circuit for said assembly; 
         FIG. 7A  is a perspective view of a single current driver head assembly of the present invention, including solar power, and  FIG. 7B  is a diagram of a preferred circuit for said assembly; and 
         FIG. 8  is a graph depicting performance examples of the dispenser of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventors of carrying out their invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the generic principles of the present invention have been defined herein specifically to provide a Two-part, Syringe-based Electrochemical Dispenser. 
     The present invention can best be understood by initial consideration of  FIG. 1 .  FIG. 1  is a perspective view of a preferred embodiment of the dispenser  100  of the present invention. The dispenser  100  employs a conventional syringe barrel  1 , having a gas generator driver head assembly (generically  40  here) attached to replace the conventional syringe&#39;s plunger. 
     The driver head assembly  40  is housed within a protective cover  25  that can be removed for maintenance purposes (e.g. battery changeouts). The head assembly  40  has a liquid- and air-tight seal to the syringe head  8 . The tip end of the syringe barrel  1  terminates in a conventional Luer lock  2 , to which a dispense cup  4  is threadedly attached.  FIG. 2  provides additional detail regarding this novel device. 
       FIG. 2  is a cutaway side view of the dispenser  100  of  FIG. 1 , illustrating the components and assembly of the dispenser refill and driver. The refill consists of a conventional plastic syringe barrel  1  terminated at its distal end by a female Luer lock  2 . The syringe can be made from polyethylene, polypropylene, glass, etc. A syringe head  8  is bonded or welded using an air-tight seal  10  to the proximal face of the syringe flange  7 . The plastic head flange  8  is made of polyethylene or other plastic that is generally compatible with the thin-film plastic bladder  6  material. The bladder  6  is heat-staked (thermally bonded) at attachment area  9  on the outer surface of the head flange boss  11 , to form a leak-free connection between the two. 
     The thin-film bladder  6  is designed to fill the syringe barrel  1  internal volume when it is fully expanded. Prior to its use, the bladder  6  is compressed to occupy only a small fraction of the syringe chamber  30 . In actuality, a nominal 50 mL syringe with an internal volume in excess of 60 mL, in absence of its plunger, will hold in excess of 50 mL of fluid, therefore the compressed bladder occupies less than 5-10 mL of the syringe volume. Bladder  6  material is selected for chemical compatibility with fluids  12  to be delivered and low permeation to oxygen  13  delivered to (and stored in) the bladder. Example materials include multi-layered films where one layer is compatible with the syringe head material, such as Saranex, with a film thickness generally between 25 and 125 microns. 
     The male Luer lock  3  at the distal end of the syringe barrel  1  holds a dispense nozzle  28  with an aperture allowing for the formation of small discrete droplets  14 . The nozzle  28  delivers fluid to the evaporation chamber  5  formed within a porous-walled plastic cup  4 . Once dispensed within the chamber  5 , the droplets  14  will be allowed to evaporate and exit through the porous walls of the cup  4 . Cups having the desired properties are known to be produced by POREX, Inc and/or Genpore, (among other sources). The level of porosity of the plastic cup  4  is defined (i.e. created) during manufacturing. The properties of the plastic material, (hydrophobic, hydrophilic, oleophobic, etc.) is selected depending upon the fluid to be delivered. It is expected that the porosity will generally be in the range of 20-200 microns. 
     An important operating requirement of this fluid releaser is that the fluid droplets  14  are not permitted to contact the porous plastic receiver walls (before they are released into the cup  4 ). If contact occurs, it may result in fluid droplets being withdrawn from the syringe through capillary action. This is undesirable, because in such an event the liquid fluid release rates would not be controlled by the gas generator current but by another withdrawing mechanism. 
     The syringe driver base  15  (and associated elements) is also depicted in  FIG. 2 . The diameter of the base  15  is generally of the same order of magnitude as the syringe head  8 . A boss  16  extends from the bottom face of the base  15 , and is cooperatively designed to mate with the well  34  formed in the head flange  8  using a ring seal  17 . The ring seal  17  is provided to ensure a leak-free interface between syringe barrel  1  (at the head flange  8 ) and driver base  15 . The upper part of the boss  16  has a cavity into which an electrochemical module (ECM)  18  will be bonded in an air-tight manner. ECM&#39;s of this type have been described by Maget et al in U.S. Pat. No. 6,010,317 (Maget IV—“Electrochemical cell module having an inner and an outer shell with a nested arrangement”). 
     The ECM&#39;s cathode  23  is exposed to air, and its counter-electrode (the anode  22 ) releases oxygen according to the following reactions, already described in many of the prior Maget patents:
 
Cathode: O 2 +4H + +4 e   −             2H 2 O
 
Anode: 2H 2 O         O 2 +4H + 4H +   +e   − 
 
Overall process: O 2 (air)         O 2 (pure,compressed)

     More specifically, the use of an electrochemical oxygen generator to operate a glass syringe has been described by Maget et al in U.S. Pat. No. 5,971,722 (Maget III) for the delivery of 100 mL of fluid (drugs) over period of 1 to 7 days. However in that instance, since a syringe plunger is used, considerable force was required to prevent rubber plunger or plunger seal to “seize” on the internal syringe wall. This problem is avoided in the current invention since the bladder material is selected for its ability to deform (i.e. inflate) at low internal pressure differentials. 
     Components assembled on the surface of the driver base  15  are the power source  19  (alkaline batteries), an activation switch  20 , and a circuit board  21  on which is a current controller. The components are connected electrically as depicted in  FIGS. 5B, 6B and 7B . Once the switch  20  is activated by pressing button/activation arm  24 , battery power is provided to the current controller, which maintains a constant current through the ECM  18  regardless of cell impedance. Since the current controller requires a minimum input voltage of 1.4 volts, the circuit requires two (2) series-connected batteries. Alternatively, a voltage booster circuit could operate the system from a single battery; however at a cost of system longevity since inefficiencies inherent in such booster circuits would result in increased battery energy demands. For example, AA batteries have an energy storage capacity for continuous operation at low currents, with voltage as far down as 0.9 volts, of about 2500 mAhr. Since the current consumption of the ECM  18  is 4.4 mAhr per cubic centimeter of oxygen generated, the two series-connected batteries can generate about 570 cc of oxygen gas. 
     Factoring in system inefficiencies, then, the rated volume of liquid delivered in such case would be approximately 400-500 mL. Consequently, the battery would yield approximately 8-10 deliveries of 50 mL syringes. In a scenario wherein a flow rate of 1 mL/day is produced, each pair of AA batteries could deliver fluid continuously for over 1 year. Similarly, 2AAA batteries could operate for one-half of one year, and C-size batteries for over 3 years. The elements of the device  100  are depicted again in  FIG. 3 . 
       FIG. 3  is an exploded cutaway side view of the dispenser  100  of  FIGS. 1 and 2 . The syringe barrel  1  is essentially the same as employed by a conventional (disposable) syringe. Either a storage cap  43  or dispense nozzle  28  threadedly engages the luer lock  2  extending from the tip end of the syringe barrel  1 . The storage cap  43  is utilized when the barrel  1  has been pre-filled with the dispense chemical (i.e. the syringe  1  has been filled, and the head flange  8  and bladder  6  have been attached/sealed to the syringe barrel  1 ). This unit may be referred to as a replacement cartridge. The replacement cartridge, then, consists of syringe  1 , syringe head  8  attached to bladder  6 , and (optionally), the storage cap  43 . 
     Specifically, in order to rejuvenate an expended dispenser  100 , a user need only obtain a replacement cartridge, attach the existing driver head assembly (see  FIG. 1 ) to it, and then replace the storage cap  43  with a suitable dispense nozzle  28  and cup  4 . 
     To fill the syringe place the cartridge “upside-down”, i.e. with the syringe&#39;s Luer lock  3  in the upright position, and fill the syringe by using a blunt (to prevent damaging the bladder) needle inserted in Luer entry port. Once filled, the cartridge can be optionally fitted with a Luer cap (e.g. for transportation) or cup  4  (when in operation). 
     As discussed above in connection with  FIG. 2 , the thin film bladder  6  is bonded to the head flange  8  adjacent to the head flange boss  11 , in order to provide a gas-tight seal thereto. The head flange boss  11  is configured to be insertible into the upper throat  41  of the syringe barrel  1 . There is a airtight seal (element  10 , see  FIG. 2 ) between the upper surface of the syringe flange  7  and the bottom surface of the head flange  8 . This seal prevents liquid from escaping, and also is the mechanism that retains the attachment between the head flange  8  and the syringe flange  7 . 
     The driver base  15  is formed with a driver boss  16  extending therefrom. The boss  16  and head flange well  34  are cooperatively designed so that the boss  16  fits into the well  34 . A ring seal (element  17 , see  FIG. 2 ) creates a gas-tight seal between these two elements so that all of the gas being generated by the module  18  is directed through the inflation orifice  32  and into the bladder chamber  36 . The power source  19  (batteries, solar power and/or other power source) and the other elements in the driver head assembly are protected from the environmental elements by the protective cover  25  detachably attached to the driver base  15 .  FIGS. 4A-4C  depict an alternate switching design. 
       FIGS. 4A, 4B and 4C  are a partial cutaway side view of an alternate driver head assembly  40 AA, a top view of the driver base  15 A of the assembly  40 AA, and a top view of the head flange  8 A of the assembly  40 AA, respectively. This alternate design  40 AA comprises a switch aperture  54  formed in the driver base  15 A, through which a snap-action pushbutton switch  24 A protrudes downwardly. An example of a suitable switch  24 A is provided by C&amp;K Corporation (CKN10158-ND). 
     The switch  24 A and button  20 A protrude downwardly from the circuit board so that the button  20 A will engage the extension segment  51  protruding from the head flange  8 A. In order to activate or deactivate the ECM  18 , one need only rotate the driver base  15 A in relation to the head flange  8 A. When the base  15 A is rotated sufficiently (rotating about the driver boss  16 ), the pushbutton  20 A will engage the extension segment  51  and thereby drive the pushbutton  20 A upward to activate the switch  24 A. Activation of the switch  24 A will supply electrical power to the ECM  18 . Continued rotation of the base  15 A relative to the flange  8 A (or reverse rotation) will eventually result in the pushbutton  20 A being released over the edge of the extension segment  51 , thereby deactivating the ECM  18 . It should be understood that the radial protrusion of the extension segment  51  in the depicted design is only one option—other approaches may be used (e.g. an aperture or peg on the flange  8 A, rather than a radially-extending protrusion). 
     Since both the driver base  15 A and head flange  8 A are deeply engaged into cap  25  and therefore not visible, in its preferred form, extension segment  51  is extended downward along syringe barrel  1  to make it visible to the user. A marking inscribed on cap  25  will allow user to identify the location that the extension segment  51  needs to be in (relative to the cap  25 ) for start-up (or deactivation). 
     The protective cap  25  fits tightly past driver head  15 A, and onto and below the head flange  8 A to prevent moisture or dust to reach the electronics circuitry. Since ECM  18  consumes oxygen (from air), to prevent the ECM from becoming oxygen-starved, a small, 1 mm diameter air intake port  53  is provided in driver base  15 A. Cap  25  is provided with hanging wire  52  to facilitate its being hung from an external structure (e.g. a tree branch, etc.). Each version of the head assembly  40  (and  40 A,  40 AA,  40 B and  40 C) could be provided with the intake port  53  and hanging wire  52 , as well. The following figures depict a variety of embodiments of the instant invention. 
       FIG. 5A  is a perspective view of a single current driver head assembly  40 A of the present invention (having its protective cover removed) and  FIG. 5B  is a diagram of a preferred circuit  42 A for said assembly  40 A. Various current controlling options can be implemented. The simplest system (not depicted here) involves a single resistor, which defines the current according the relationship:
 
( V   B   −V   R )/ R=I  
 
     where
         V B  is the battery voltage,   V C  is the ECM voltage and   R is the resistance in the circuit.       

     For small currents (i.e. &lt;1 mA), the battery voltage is stable over long time periods. If the ECM voltage is also quasi-stable, then the current can be pre-set by resistor selection. However, in the single current versions, the current is at the mercy of two voltages, one decreasing in time (battery supply voltage) the other increasing in time (ECM demand voltage). 
     For example, a system having: battery voltage V B =1.50 volts; ECM voltage V C =0.90 volts and R=3 kilo-ohms, results in a current of 0.2 mA equivalent to a gas generation rate of 1.1 cc/day that in turn yields a fluid pumping rate of about 1 mL/day. If V B  decreases by 20 millivolts and V C  increases by 20 mV, then the current decreases by 5% to 0.19 mA, and a pumping rate reduction of about 5 (five) percent. 
     If this variation is unacceptable, current controllers are preferred. The circuit and driver for a single flow rate device are  FIGS. 5A and 5B . The single current driver  40 A consists of 2 alkaline batteries  19 , a current controller  44 A (such as National Semiconductors LM334M or equivalent), a Fairchild Semiconductors diode (1N457A)  46  and 2 resistors  48  to set the current. The controller  44 A output will drop off at a voltage below 1.4 volts, and therefore the need for 2 batteries  19 . For a desired current of 200 micro-amps the resistor values are R 1 =681 and R 2 =6810 ohms. The start-up switch  20 A holds a push button actuator  24 A that is activated by means of an activation arm (not shown) mounted on the syringe head (not shown). 
       FIG. 6A  is a perspective view of a multi-current driver head assembly  40 B of the present invention, and  FIG. 6B  is diagram of a preferred circuit  42 B for said assembly  40 B. The multiple current driver  40 B,  42 B of  FIGS. 6A and 6B  is identical to the single current unit (see  FIGS. 5A and 5B ), except for a 3-position switch [e.g. C&amp;K Components DIP switch (CKN3002-ND)]  25 B and additional resistors. In  FIG. 6A , the DIP switch  25 B and current controller  44 B are mounted on a surfboard circuit board with 3 resistors  48 , namely R 1 =169, R 2 =340 and R 3 =681 ohms. As depicted in the embedded matrix, such an arrangement allows for selection of 8 different currents, varying in increments of 100 microamps and a range of from 0 to 0.7 mA. This equates to an oxygen generation rate range of from 0 to 3.86 cc/day in increments of 0.50 cc/day, or the equivalent of between about 0 and 3.5 mL/day of fluid delivered. 
       FIG. 7A  is a perspective view of a single current driver head assembly including solar power  40 C of the present invention, and  FIG. 7B  is a diagram of a preferred circuit  42 C for said assembly  40 C. The depicted system  40 C,  42 C is configured to generate a single current of 200 microamps. In this configuration, the circuit includes 2 rechargeable AAA nickel-metal hydride batteries  19 , a Solarbotics SCC2422 solar panel  50 , diodes 1N5818  47  and 1N457A  46 , resistors R 1 =681 ohms and R 2 =6810 ohms, and current controller LM334M  44 C. 
     Rechargeable batteries are required for nighttime operation, but are charged while the solar panel  50  is exposed to sunlight. Either solar cells benefiting from direct or indirect solar illumination, or solar cells capable of operation from illumination with artificial light can be used. It is possible to eliminate the batteries  19  from the circuit, but doing so will only allow operation during periods where the panel  50  is illuminated. Such a system would release fluid only during daylight or presence of artificial light. This might be desirable, for example to avoid the release of expensive pheromones at night or in inclement weather (when the insects to be affected by the pheromones are not active). Alternatively, other techniques can be implemented such as the use of optical transducers that are light activated. Fluid release would be terminated as soon as the panel ceases to be illuminated. 
     Clearly, the advantages of having an independent, autonomous, active electrochemical process that can be turned on and off, has an adjustable chemical release rate, and that can be self-powering when exposed to sunlight, represent considerable improvements over the integrated systems of Maget IV. 
     It will also be obvious to others that more complex control functions can be incorporated in circuits that can bear higher costs, such as timers, interrupted delivery, pulsed delivery, etc. 
     Operating the Dispenser (in Reference to  FIGS. 1-7 , as Appropriate) 
     The syringe refill barrel  1 , separated from the driver base  15 , is first filled by way of female Luer lock  2  at the distal and of the syringe  1  (the porous receiver  4  and attached male Luer lock  3  having been previously removed). The bladder  6  is pre-shrunk by applying gas pressure to its exterior via Luer lock  2  or by applying a vacuum via port  32 . 
     After filling the syringe  1  the Luer lock  2  is capped by means of a conventional Luer cap. The syringe and its fluid charge can now be transported (without the receiver cup  4  and nozzle  28  attached). 
     To ready the unit  100  for dispensing of the fluid, the user removes the conventional Luer cap and replaces it by a fluid receiver cup  4  (which is pre-bonded to nozzle  18 ). Next, the driver base  15  is inserted onto head flange  8 , which will release a small fluid bolus. Once the two parts  15  and  8  are mated, the user rotates the driver base  15  into the designated OFF position (i.e. to place it in stand-by). To start the dispenser  100 , the user simply rotates the driver base  15  onto the designated ON position (relative to the flange  8 ). In that position pressure is applied to the contact switch  24  by means of the activation arm  20 , and the unit is started. 
     The single rate unit does not require any further action. Once in the ON position, the multiple-rate unit requires that the desired current be selected via the 3-way switch  25 B located on the driver head assembly  40 B under cover  25 . The codes for the different switch positions determine specific rates. In other versions of the device  40 B, other start-up mechanisms are possible. 
     During operation, the rate can be changed (or the releaser can be stopped), by operation of the switch  24 B, or by rotating the driver into the OFF position. To replace or refill the syringe barrel  1 , the user need only disengage and remove the barrel  1  and repeat the previous start-up procedures. In some instances the empty syringe  1  can be re-used, however, in that case the user is advised to pressurize the syringe chamber  30  in order to collapse the bladder before re-using it. 
     To replace the batteries, the user removes protective cover  25  to access the batteries  19 . The protective cover  25  should be installed during operation, since it protects the driver head assembly  40  from environmental elements (rain, dust, etc.). The cover  25  fits over both driver and syringe heads ( 8  and  7 , respectively) to prevent water from reaching the electronics and the activation arm  20 . In the case of solar cell-powered drivers, solar panel  50  is embedded in a water-proof manner in cover  25 . 
     Performance of the Dispenser 
       FIG. 8  is a graph depicting performance examples of the dispenser of the present invention. The delivery of two fluids (namely water and ethanol) from two different dispensers is illustrated by the two depicted curves. In both instances the syringe barrel has a nominal 50 mL volume, and is constructed from polyethylene. The driver head assembly is configured for a single current of 200 microamps. This current corresponds to a theoretical gas (oxygen) generation rate of 1.10 cc/day. The fluid receiver for water is a Porex 5542 porous high density polyethylene cup that is 1.3 cm in diameter, and 2.4 cm long, and having an average pore size of 30-70 microns. 
     The fluid receiver for ethanol is a Dragon Shing Rotocasting Co. high molecular weight polyethylene cup  4  with a diameter of 1.3 cm and a length of 1.76 cm and an average pore size of 40 microns. 
     Since no fluid accumulation takes place in the receiver, the average fluid pumping rate and evaporation rate for both water and ethanol are identical and about 0.94±0.04 mL/day. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.