Abstract:
A piezoelectric actuated device for acquiring and dispensing fluid samples is described. Fluid samples are acquired or drawn up into the device by dipping the tip into a fluid and applying an alternating current electrical signal to the piezoelectric element. Removing the tip from the fluid and applying another electrical signal causes drops of fluid to be ejected for non-contact dispensing. The device optionally incorporates a second piezoelectric element functioning as a sensor to detect if it is empty, plugged or working properly. An optional heater, temperature sensor and feedback circuitry regulate the temperature of the device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a piezoelectric actuated device for manually or automatically transferring very small volumes of fluid. Transferring fluids from one container to another is one of the most common tasks performed in a typical chemical or biological laboratory. For example, various chemicals from different containers may have to be mixed together and then the mixture may have to be divided out equally or ‘aliquoted’ into other containers or onto microscope slides or some other substrates. Currently transferring of fluids is done either by hand using manual pipettors or automatically using an automated robotic pipetting instruments. Motivated by the high costs of chemical reagents there is a trend in chemical and biological laboratories to perform experiments using smaller and smaller fluid samples. 
     Prior to the 1990&#39;s fluid transfer was done in laboratories by hand using glass tubes called pipettes. Samples were aspirated by sucking on the end of the pipette by mouth like a straw and then sealing the end with a finger. Fluid samples were then dispensed or aliquoted by briefly unsealing and then resealing the pipette with the finger allowing a small volume of fluid to flow out. This method required a fair amount of dexterity and had some serious drawbacks in terms of ergonomics, precision and health risks. During the same time period glass capillaries were also used for transferring smaller volumes of fluid with greater precision. These techniques for manually transferring fluid samples were made obsolete in the early 1990&#39;s by the introduction of the manual pipette as described, for example, in U.S. Pat. Nos. 3,613,952, 3,766,785, 3,827,305 and 3,991,617, with ejectable, disposable plastic tips. Currently the manual pipette is a ubiquitous and indispensable tool for transferring fluid samples in the modern laboratory. 
     Although manual pipetting is a big improvement over earlier techniques it is tedious for the human operator and can cause repetitive motion injuries as well as being prone to human error. Currently, a great variety of automated pipetting instruments are commercially available which address these specific drawbacks. These instruments, as advanced by Tecan AG of Hombrechtikon Switzerland, typically use a Cartesian X,Y,Z robot to move a pipetting head among various aspirating, dispensing and washing stations. The various types of instruments currently available differ mainly in the mechanisms used to acquire and then dispense fluid samples. The most common mechanism for fluid transfer is the syringe pump as used in instruments manufactured by Tecan, Hamilton Company, Cavro Scientific Instruments, Robbins Scientific, Qiagen and Tomtec among others. In most cases the syringe pumps and pipette tips are connected via long flexible tubing, however, in the instruments supplied by Tomtec and Robbins Scientific the syringe pumps are mounted directly to the back of the pipette tips. The smallest volume that can be accurately transferred using syringe pumps is approximately 0.1 microliters. U.S. Pat. Nos. 5,743,960 and 5,741,554 describe an instrument which combines a syringe pump with a solenoid valve allowing drops as small as 0.1 microliters to be ejected onto a substrate for non-contact printing or arraying applications. By contrast, the drops that are dispensed using the present invention are a thousand times smaller. 
     Another method for transferring small volumes of fluid for arraying applications is the use of pins as described in U.S. Pat. No. 5,807,522. Instruments using pins for fluid transfer are used by Synteni, among other companies, to generate DNA arrays and are commercially available, for example, from BioRobotics and GeneMachines. Using pins is a simple, robust and practical means for fluid transfer but it suffers from some limitations. First of all it is slow. The pins have to stop at each spot and then wait for over a second for capillary action to transfer the fluid onto the substrate. By contract, the piezoelectric based dispensing of the present invention is almost a thousand times faster. Pin based fluid transfer is sensitive to the wetting properties of the substrate. Also it can damage some substrates like Nylon membranes for example. These are not concerns for the piezoelectric dispensing which is non-contact. Pins generate relatively low density, poor quality arrays with approximately 50% variability in spot size. By contrast, piezoelectric dispensers generate arrays with almost an order of magnitude higher density and better than 3% spot size variability. Finally, pins are limited to acquiring and dispensing a fixed volume of fluid. Piezoelectric dispensers have thousands of times higher dynamic range. Sub nanoliter to tens of microliter volumes can be aspirated and subsequently dispensed in volumes ranging from 100 picoliters to several microliters per second. 
     A number of companies including Microdrop, Packard Instruments and GeSiM, use piezoelectric fluid dispensing devices to dispense drops of fluid with volumes on the order of 100 picoliters. Originally piezoelectric dispensing technology was used for “drop-on-demand” “ink-jet” printing. These devices as described in U.S. Pat. No. 2,512,743 have a fluid filled chamber with an inlet at one end and a nozzle on the other. A piezoelectric element induces an acoustic wave in the fluid causing a drop to be ejected from the nozzle. For printing applications ink is supplied to the back end of the piezoelectric fluid dispenser from a reservoir. For fluid transfer applications fluid is drawn up through the nozzle. The instruments supplied by Microdrop, Packard Instruments and GeSiM all use syringe pumps to aspirate fluids up through the nozzle of the piezoelectric devices. Syringe pumps impose several serious limitations on current piezoelectric based pipetting instruments. It turns out that properly aspirating samples prior to dispensing is one of the most critical considerations for reliable operation of piezoelectric fluid dispensers. Syringe pumps can sometimes aspirate air bubbles and small particles that can adversely affect the dispensing characteristics. Also, variations in back pressure in the flexible tubing leading to the syringe pumps causes the drop-on-demand dispensing properties to change. Packard Instruments uses a closed loop pressure controller to actively regulate this back pressure. This system adds to the cost and complexity of the instrument. Additionally the syringe pumps themselves add significantly to the cost, size and complexity of the instrument and they limit the smallest fluid volumes that can be transferred to around 0.5 microliter. By contrast, with the present invention, volumes over 100 times smaller can be aspirated. Most importantly, aspirating is much more uniform and precise making dispensing more reliable and repeatable. In the present invention, the dispensers are opened to ambient pressure so the back pressure always remains uniform. Additionally the novel sensing technology of the present invention detects almost instantly if there is a problem with the dispensers, e.g. if they are clogged, empty or attempting to aspirate from an empty well. 
     The present invention provides improvements to the cylindrical piezoelectric fluid dispenser described in U.S. Pat. No. 3,840,758. These improvements enable bi-directional operation of the device, sensing of the operational state and thermal control. As a summary of some related devices. Humberstone in U.S. Pat. No. 5,518,179 describes a bi-directional device with a piezoelectric driven, thin, perforated membrane capable of drawing a fluid up through the perforations as well as dispensing fluid out through them. Beckman et al. in U.S. Pat. No. 4,432,699 describes a peristaltic piezoelectric pump with an internal load sensor. Hayes in U.S. Pat. No. 5,622,897 describes a process of manufacturing a drop-on-demand ink-jet print head having n-type and p-type thermoelectric carriers. 
     SUMMARY OF THE INVENTION 
     The invention provides a device enabling the automatic or manual transfer of small volumes of fluid from one container to another or from one container to the surface of a substrate. The device is a self contained, bi-directional pump in the sense that it can acquire fluid samples as well as dispense them. Acquiring samples in accordance with the present invention requires dipping the tip of the device into the sample fluid. Dispensing, however, does not require any contact with either a fluid or solid surface. Fluid can be dispensed one drop at a time in a “drop-on-demand” mode or continuously at frequencies from over several thousand to fifty thousand drops per second. 
     In accordance with further aspects of the present invention, an integral monitoring sensor detects if the piezoelectric pipette is functioning normally, is empty or if the nozzle is plugged. This type of feedback of the operational state of the device is critical for automated pipetting instruments to alert a human operator if the device requires some attention or to initiate an automated failure recovery procedure. 
     The characteristics of drop-on-demand ink-jet dispensers can change with temperature. In accordance with a further aspect of the present invention, a piezoelectric pipette incorporates a built in temperature regulator. This helps maintain uniform dispensing characteristics even if the ambient temperature changes. The temperature regulator can also be used to raise the temperature of the fluid being dispensed. Elevated temperatures can reduce the viscosity of fluids that are too viscous to dispense at ambient temperature. 
     In summary, the invention is a compact, bi-directional pump for acquiring and dispensing small volumes of fluid with a built in monitoring sensor and temperature control. The invention is suitable for automated or manual fluid transfer applications. 
     Accordingly, the invention provides a diffuser pump including a pipette having a capillary having a first end including a nozzle and a piezoelectric actuating element adjacent to the capillary and an alternating current voltage source coupled across the piezoelectric actuating element. With the nozzle immersed in a sample fluid and the capillary at least partially filled with a working fluid, alternating current voltage applied to the actuating element causes sample fluid to be drawn into the capillary. 
     The invention further provides a pipette sensing arrangement for sensing a working condition of a pipette having a capillary with an end including a nozzle, a piezoelectric actuating element adjacent to the capillary, and first electrodes for applying electrical energy across the piezoelectric actuating element. The sensing arrangement includes a piezoelectric sensing element adjacent to the capillary and second electrodes for coupling the piezoelectric sensing element to a voltage sensor. 
     The invention still further provides a temperature control arrangement for a pipette having a capillary having an end including a nozzle, a piezoelectric actuating element adjacent to the capillary, and actuating electrodes for applying electrical energy across the piezoelectric actuating element. The temperature control arrangement includes a current source for passing an electrical current through one of the actuating electrodes, and a temperature sensor closely adjacent the capillary for sensing the temperature of the capillary. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof may best be understood by making reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify identical elements, and wherein: 
     FIG. 1 is a cross-sectional view of piezoelectric pipette embodying the present invention including a sensor, temperature controller, Teflon housing and reservoir; 
     FIG. 2 is a cross-sectional view of piezoelectric pipette in a diffuser pump mode in accordance with the present invention with a reservoir attached to the back end and a mounting bracket arm for automated dipping into the sample fluid; 
     FIG. 3 is a manually operated piezoelectric pipette; 
     FIG. 4 is a cross-sectional view of piezoelectric pipette embodying further aspects of the present invention with a tube attached to the back end and another tube attached to the front end; 
     FIG. 5 is a cross-sectional view of two piezoelectric pipettes back-to-back forming a reversible continuous flow pump in accordance with the present invention; 
     FIG. 6 is a cross-sectional view of piezoelectric pipette with a separate sensor embodying the present invention; 
     FIG. 7 is a graph of the power spectral densities of the sensors for water vs. Fluorinert fluids with the device operating in drop-on-demand mode illustrating a further aspect of the present invention; 
     FIG. 8 is a block diagram of a dispenser actuation and sensor detection system embodying the present invention; 
     FIG. 9 is a flow diagram illustrating an actuation and detection method embodying the present invention; 
     FIG. 10 is a cross-sectional view of a piezoelectric pipette with a thermocouple and heating system including an inner electrode functioning as an integrated heating element embodying the present invention; and 
     FIG. 11 is a block diagram of a temperature control system which may be employed with the pipette of FIG. 10 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, it illustrates a piezoelectric pipette  10  with sensor, temperature control, housing and reservoir embodying the present invention. It includes an piezoelectric actuating element  12  with a glass capillary  14  bonded concentrically on the inside. The glass capillary has a nozzle  16  formed on one end by heating and then polishing the end. Applying a voltage to the piezoelectric element  12  across an inner electrode  18  and an outer electrode  20  causes it to deflect which induces an acoustic wave in the fluid filled glass capillary  14 . A drop is ejected from the nozzle if the acoustic energy is high enough to overcome surface tension at the nozzle opening  22 . In accordance with the present invention, a separate sensing piezoelectric ceramic tube  24  is concentric with the glass capillary  14  and adjacent to the actuating piezoelectric element  12 . These elements may also be seen in FIG. 6 with the corresponding elements having corresponding reference numerals with an “a” subscript. The piezoelectric pipette is housed in Teflon heat shrink tubing  26  and  41  as shown in FIG.  1 . This housing is compact, provides mechanical protection and electrical insulation for the piezoelectric element. 
     Diffuser Pump Mode 
     With the nozzle immersed in fluid  28  as shown in FIG. 1 the device  10  operates as a diffuser pump capable of acquiring or drawing fluid samples up through the nozzle. Due to the shape of the nozzle, fluid has less resistance flowing from the sample fluid up into the glass capillary than out. Periodically deflecting the piezoelectric element by applying an alternating voltage from an alternating current source  19  to the electrodes  18  and  20  causes fluid to flow in and out of the nozzle with a net inward flow due to the lower flow resistance in this direction. Based on the diffuser pump principle it is possible to draw up fluid samples into the piezoelectric pipette when the nozzle is dipped in a fluid. The sample fluid may then be dispensed when the device operates as a drop-on-demand ‘ink-jet’ printer when the nozzle is removed from a fluid. 
     The diffuser pump works over a large range of periodic excitation voltage frequencies, amplitudes and waveforms. Pumping speed increases with frequency and amplitude. This diffuser pump mode has been demonstrated to successfully pump fluid with excitation signal frequencies ranging from 6 KHz to 25 KHz and amplitudes from 20 V to over 90 V. Pumping was demonstrated with a variety of waveform shapes including saw-tooth, square-wave and sine-wave waveforms. 
     The glass capillary of the device must be at least partially filled with fluid to transmit acoustic energy provided by the piezoelectric element  12  to the fluid in the area of the nozzle  16  for both drop-on-demand and diffuser pump modes to function properly. This ‘working’ fluid does not necessarily have to be the same as the sample fluid as illustrated in FIG. 2 which shows another pipette  10   b  wherein corresponding elements are designated by corresponding reference numerals with a “b” subscript. Here the working fluid  30  and the sample fluid  28   b  are different. The working and sample fluids may be immiscible with each other allowing small fluid samples to be drawn up without diffusing the sample into the working fluid. A wide variety of fluids immiscible in, for example, water in practicing the present invention may be selected including Fluorinert manufactured by 3M, 200 Fluid manufactured by Dow Corning and paraffin. Fluorinert has a good combination of properties for many applications including low viscosity, good bio-compatibility and it does not leave a residue when dried out. 
     As also illustrated in FIG. 2, the piezoelectric pipette  10   b  can be mounted on a supporting mounting bracket arm  32 . This arm can be attached to an automated robotic instrument which dips the device into the sample fluid  28   b  for aspirating and then moves it to another station for dispensing. Alternatively, the piezoelectric pipette  10   b  with wires  33  can be attached to a manual pipettor  34  having a disposable tip  36 , as shown in FIG.  3 . In this case the device is dipped into the sample fluid by a human operator and then manually moved to another container or substrate for dispensing. The plunger  38  of the manual pipettor can be used for priming, rinsing and emptying the device. 
     Three different versions of the piezoelectric pipette  10   b  operating as a diffuser pump are shown in FIG. 2, FIG.  4  and FIG.  5 . As seen in FIG. 2, attaching a reservoir  40  to the back end  42  of the piezoelectric pipette  10   b  allows larger volumes of fluid  28   b  to be drawn up. As seen in FIG. 4, by attaching tubing  44  and  46  to respective ends  48  and  42   b  respectively of pipette  10   b , the piezoelectric pipette operates as a pump for a continuous flow system. A reversible continuous flow pump can be made by putting two piezoelectric pipettes  10   b  back to back as shown in FIG.  5 . In each of the embodiments of FIGS. 2 and  4 , the voltage source  19  of FIG. 1 may be used to apply an alternating current voltage across the inner and outer electrodes of the actuating piezoelectric elements. In the embodiment of FIG. 5, separate voltage sources may be used for the actuating electrodes or a single source may be selectively applied to one or the other of the actuating elements. 
     Integrated Sensor 
     The devices  10  and  10   a  shown in FIG.  1  and FIG. 6 respectively each incorporate a separate piezoelectric element  24  and  24   a  respectively which functions as a sensor to monitor the operational state of the device. The sensing elements  24  and  24   a  are proximal to their corresponding actuating elements  12  and  12   a  respectively with respect to the nozzles  22  and  22   a . The sensor  24 ,  24   a  detects vibrations induced in the fluid  28 ,  28   a , the glass capillary  14 ,  14   a  and the actuating piezoelectric element  12 ,  12   a . By analyzing the voltage signal generated by this sensor across electrodes  25  and  27  and  25   a  and  27   a  it is possible to identify the operational state of the device, specifically, if the dispenser is operating properly both in drop-on-demand and diffuser pump modes of operation. Specifically it is possible to detect if the device is working properly, if it is empty or full when aspirating or dispensing, if the nozzle is partially or completely plugged when aspirating or dispensing, what type of fluid is being dispensed or if the device is trying to aspirate fluid from an empty container. The power spectral densities of the time signals are computed to make it easier to distinguish the differences between these various operating conditions. For example, FIG. 7 shows the power spectrial density (PSD) of the output from the sensor while the dispenser is dispensing in drop-on-demand mode. The PSDs in FIG. 7 were computed by averaging the fast Fourier transforms (FFTs) of 16 individual Hanning windowed data records of 128 points each sampled at a frequency of 375 KHz. The figure shows the difference in the sensor output if water  43  vs. Fluorinert  45  is being dispensed. At 90 KHz there is a difference of three orders of magnitude between the PSDs of water  43  and Fluorinert  45 . Detection software will compare the PSDs at this frequency to determine if the sample fluid (water) or the working (Fluorinert) fluid is being dispensed. 
     A block diagram of the dispenser actuation and sensor detection system  50  is depicted in FIG.  8 . It incorporates a digital computer  52 , drive electronics  54  for the actuating piezoelectric element  12 , 12   a , the piezoelectric pipette  10 ,  10   a  itself with an integrated sensor  24 ,  24   a  and an analog to digital converter (ADC)  56  for the sensor signal. The computer  52 , upon implementing operating instructions in accordance with this embodiment of the present invention forms an actuation and detection stage  58 . A flow diagram  60  of the actuation and detection stage operation is depicted in FIG.  9 . First in step  62 , an actuating trigger  51  command to actuate the device is issued to the drive electronics  54 . The drive electronics  54  in turn generates an actuating voltage  53  to drive the actuating piezoelectric element  12 ,  12   a  with a voltage waveform. To obtain the sensor data, for FIG. 7 for example, the piezoelectric element may be driven with a 60 microsecond, 35 volt amplitude pulse. The voltage causes the actuating piezoelectric element to deflect which induces vibrations in the fluid, the glass capillary and the piezoelectric element itself. These vibrations are in turn converted to a sensor voltage signal  55  by the sensing piezoelectric element  24 ,  24   a . The analog to digital converter (ADC)  56  is then, in step  64 , triggered to sample and digitize this signal. The computer then in step  66  stores the digitized sensor signal  57  output values. Steps  64  and  66  are repeated. When the voltage signal is completely sampled, the data is made available for digital signal processing and analysis by the computer. The power spectral densities of FIG. 7 are then computed in step  68  using any accepted method of power spectral estimation. The PSD is then evaluated in step  70  by comparing the PSD value at different frequencies to known corresponding values for a particular operational state of the device. For each evaluation, the operational state of the device  10 ,  10   a  is determined in accordance with step  72 . Steps  70  and  72  are repeated for all of the various operational states of the device. 
     Temperature Control 
     FIG. 10 shows a piezoelectric pipette  70  wherein the temperature of the glass capillary  72  and thus the fluid  74  within the capillary may be controlled in accordance with a further aspect of the present invention. Here, the actuating element  76  is associated with an outer electrode  78  and an inner electrode  80  having two wrap-around electrode portions  82  and  84  on each end of the piezoelectric element  76 . Applying a current between the wrap-around electrode portions  82  and  84  and thus through the inner electrode  80  causes the temperature of the device to increase. A thermocouple  86 , depicted in FIG. 10, is closely adjacent the nozzle  87  and senses the temperature of the device. The temperature may be regulated with a temperature regulator  88  (FIG. 11) such as an Analog Devices AD594 as specified in the  Monolithic Thermocouple Amplifier Data Sheet  from Analog Devices. This integrated circuit controls the current passing through the inner electrode to maintain a set-point temperature using feedback from the thermocouple as depicted in the block diagram in FIG.  11 . 
     As can be seen in FIG. 11, the temperature regulator  88  provides a control output to a heater driver  90  which applies the current to the inner electrode  80 . The amount of current applied is dictated by the temperature regulator  88  and more specifically the difference between the output voltage of the thermocouple  86  and a set point reference voltage applied to a subtractor stage  92 .