Abstract:
A method of controlling fluid flow in a microfluidic process includes the step of providing a piezoelectric pumping apparatus ( 100 ) in fluid communications with the microfluidic process such as an ink jet printer and the like. The piezoelectric pumping apparatus ( 100 ) has a piezoelectric transducer ( 80 ) with a functionally gradient piezoelectric element ( 60 ) arranged in a fluid containment chamber ( 120 ) which fluidically communicates with the microfluidic process. The functionally gradient piezoelectric element ( 60 ) responds to a voltage applied by a power source ( 240 ) by either expanding to expel fluid from the microfluidic process or contracting to permit fluid to enter the fluid containment chamber ( 120 ) and thus the microfluidic process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to commonly owned U.S. application Ser. No. 09/071,485, filed May 1, 1998, entitled CONTROLLED COMPOSITION AND CRYSTALLOGRAPHIC CHANGES IN FORMING FUNCTIONALLY GRADIENT PIEZOELECTRIC TRANSDUCERS, by Dilip K. Chatterjee, Syamal K. Ghosh, and Edward P. Furlani. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of fluid flow control. More particularly, the invention concerns fluid flow in a microfluidic process, such as an ink jet printer and the like, that requires high reliability and accurate fluid flow control. 
     BACKGROUND OF THE INVENTION 
     Piezoelectric pumping mechanisms are used in a wide range of microfluidic applications ranging from the controlled metering and flow of intravenous solutions in biomedical environments to ink jet printing apparatus. Conventional piezoelectric pumps utilize piezoelectric transducers that comprise one or more uniformly polarized piezoelectric elements with attached surface electrodes. The three most common transducer configurations are multilayer ceramic, monomorph or bimorphs, and flextensional composite transducers. To activate a transducer, a voltage is applied across its electrodes thereby creating an electric field throughout the piezoelectric elements. This field induces a change in the geometry of the piezoelectric elements resulting in elongation, contraction, shear or combinations thereof. The induced geometric distortion of the elements can be used to implement motion or perform work. In particular, piezoelectric bimorph transducers, which produces a bending motion, are commonly used in micropumping devices. However, a drawback of the conventional piezoelectric bimorph transducers is that two bonded piezoelectric elements are needed to implement the bending. These bimorph transducers are difficult and costly to manufacture for micropumping applications (in this application, the word micro means that the dimensions of the apparatus range from 100 microns to 10 mm). Also, when multiple bonded elements are used, stress induced in the elements due to their constrained motion can damage or fracture an element due to abrupt changes in material properties and strain at material interfaces. 
     Therefore, a need persists for a piezoelectric pumping apparatus that utilizes a functionally gradient piezoelectric transducer that overcomes the aforementioned problems associated with conventional pumping apparatus. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a method of controlling fluid flow in a microfluidic process which includes a piezoelectric pump that utilizes a functionally gradient transducer in which the pumping action is accomplished with a single functionally gradient piezoelectric element. 
     It is a feature of the method of the invention that a functionally gradient piezoelectric transducer in fluid communications with the microfluidic process expands to expel fluid from the microfluidic process and contracts to cause fluid to enter the microfluidic process. 
     To accomplish these and other objects of the invention, there is provided, in one aspect of the invention, a method of controlling fluid flow in a microfluidic process comprising the step of providing a piezoelectric pump in fluid communications with the microfluidic process. The piezoelectric pump comprises a pump body having a fluid containment chamber and inlet and outlet ports in fluid communication with the fluid containment chamber. The inlet and outlet ports have, respectively, a first valve and a second valve for controlling fluids passing therethrough and through the microfluidic process. A piezoelectric transducer is arranged in the pump body. 
     The piezoelectric transducer includes a functionally gradient piezoelectric element having first and second surfaces and is formed of piezoelectric material having a functionally gradient d-coefficient selected so that the functionally gradient piezoelectric element changes geometry in response to an applied voltage. When the voltage is applied, an electric field is produced in the functionally gradient piezoelectric element. More particularly, first and second electrodes respectively disposed over the first and second surfaces of the functionally gradient piezoelectric element are arranged so that voltage applied to the first and second electrodes induces the electric field in the functionally gradient piezoelectric element. 
     A source of power having first and second terminals connected to the first and second electrodes, respectively, of the piezoelectric transducer enables fluid flow through the fluid containment chamber which is in fluid communications with the microfluidic process. Thus, on the one hand, when the piezoelectric transducer is energized to pump fluid out of the fluid containment chamber and thus into the microfluidic process, the source of power provides a positive voltage to the first terminal and a negative voltage to the second terminal. On the other hand, when the piezoelectric transducer is energized to pump fluid into the fluid containment chamber, and thus out of the microfluidic process, the source of power provides a negative voltage to the first terminal and a positive voltage to the second terminal. 
     Accordingly, the method of piezoelectric pumping apparatus of the invention has numerous advantages over prior art developments, including: it enables the use of a single functionally gradient piezoelectric element to implement a desired geometric distortion thereby eliminating the need for multilayered or composite piezoelectric structures; it eliminates the need for multiple electrodes and associated drive electronics; and it minimizes or eliminates stress induced fracturing that occurs in multilayered or composite piezoelectric structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and objects, features and advantages of the present invention will become apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
     FIG. 1 is a perspective view of the piezoelectric pumping apparatus of the invention, partially torn away to expose the piezoelectric transducer; 
     FIG. 2 is a section view along line  2 — 2  of FIG. 1; 
     FIG. 3 is a perspective view of a functionally gradient piezoelectric element with a functionally gradient d 31  coefficient; 
     FIG. 4 is a plot of the piezoelectric d 31  coefficient across the width (T) of a piezoelectric transducer element of FIG. 3; 
     FIG. 5 is a plot of piezoelectric d 31  coefficient across the width (T) of a conventional piezoelectric bimorph transducer element, respectively; 
     FIG. 6 is a section view along line  6 — 6  of FIG. 3 illustrating the piezoelectric transducer before activation; 
     FIG. 7 is a section view taken along line  7 — 7  of FIG. 3 illustrating the piezoelectric transducer after activation; and 
     FIG. 8 is a section view taken along line  8 — 8  of FIG. 3 illustrating the piezoelectric transducer after activation but under a opposite polarity compared to FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, and particularly to FIGS. 1 and 2, the piezoelectric pumping apparatus  100  of the present invention is illustrated. As depicted in FIGS. 1 and 2, piezoelectric pumping apparatus  100  comprises a pump body  110  having a fluid containment chamber  120  and an inlet port  150  and outlet port  160  in fluid communication with the fluid containment chamber  120 . The inlet and outlet ports  150 ,  160  have, respectively, a first valve  130  and a second value  140  for controlling fluids passing therethrough and through the fluid containment chamber  120 . As seen clearly in FIG. 1, piezoelectric transducer  80  is arranged in the pump body  110  for enabling fluid flow in and out of the fluid containment chamber  120 , as described in detail below. A reservoir  164  has an outflow port  166  which is connected via a fluid conduit  168  to inlet port  150  for supplying fluid to the piezoelectric pump  100 . 
     Referring to FIG. 3, a perspective view is shown of a functionally gradient piezoelectric element  60  with a functionally gradient d 31  coefficient. A functionally gradient piezoelectric element  60  has first and second surfaces  62  and  64 , respectively. The width of the functionally gradient piezoelectric element  60  is denoted by T and runs perpendicular to the first and second surfaces  62  and  64 , respectively, as shown. The length of the functionally gradient piezoelectric element  60  is denoted by L and runs parallel to the first and second surfaces  62  and  64 , respectively, as shown. A functionally gradient piezoelectric element  60  is poled perpendicularly to the first and second surfaces  62  and  64  as indicated by polarization vector  70 . 
     Skilled artisans will appreciate that in conventional piezoelectric transducers the piezoelectric “d”-coefficients are constant throughout the piezoelectric element  60 . Moreover, the magnitude of the induced sheer and strain are related to these “d”-coefficients via the constitutive relation as is well known. However, the functionally gradient piezoelectric element  60  used in the pumping apparatus  100  of the invention is fabricated in a novel manner so that its piezoelectric properties vary in a prescribed fashion across its width as described below. The d 31  coefficient varies along a first direction perpendicular to the first surface  62  and the second surface  62 , and decreases from the first surface  64  to the second surface  64 , as shown in FIG.  4 . This is in contrast to the uniform or constant spatial dependency of the d 31  coefficient in conventional piezoelectric elements, illustrated in FIG.  5 . 
     In order to form the preferred functionally gradient piezoelectric element  60  having a piezoelectric d 31  coefficient that varies in this fashion, the following method may be used. A piezoelectric block is coated with a first layer of piezoelectric material with a different composition than the block onto a surface of the block. Sequential coatings of one or more layers of piezoelectric material are then formed on the first layer and subsequent layers with different compositions of piezoelectric material. In this way, the functionally gradient piezoelectric element  60  is formed having a functionally gradient composition which varies across the width of the functionally gradient piezoelectric element  60 , as shown in FIG.  4 . 
     Preferably, the piezoelectric materials used for forming the functionally gradient piezoelectric element  60  are selected from the group consisting of PZT, PLZT, LiNbO 3 , LiTaO 3 , KNbO 3  or BaTiO 3 . Most preferred in this group is PZT. For a more detailed description of the method, see cross-referenced commonly assigned U.S. patent application Ser. No. 09/071,485, filed May 1, 1998, to Chatterjee et al, hereby incorporated herein by reference. 
     Referring now to FIGS. 6-8, the piezoelectric transducer  80  is illustrated comprising functionally gradient piezoelectric element  60  in the inactivated state, a first bending state, and a second bending state, respectively. The word bending includes elongation, contraction, shear, or combinations thereof. Piezoelectric transducer  80  comprises a functionally gradient piezoelectric element  60 , with polarization vector  70 , and first and second surface electrodes  20  and  22  attached to first and second surfaces  62  and  64 , respectively. First and second surface electrodes  62  and  64  are connected to wires  24  and  26 , respectively. Wire  24  is connected to a switch  30  that, in turn, is connected to a first terminal of voltage source  40 . Wire  26  is connected to the second terminal of voltage source  40  as shown. 
     According to FIG. 6, the transducer  80  is shown with switch  30  open. Thus there is no voltage across the transducer  80  and it remains unactivated. 
     According to FIG. 7, the transducer  80  is shown with switch  30  closed. In this case, the voltage V of voltage source  40  is impressed across the transducer  80  with positive and negative terminals of the voltage source  40  electrically connected to the first and second surface electrodes  20  and  22 , respectively. Thus, the first surface electrode  20  is at a higher potential than the second surface electrode  22 . This potential difference creates an electric field through the functionally gradient piezoelectric element  60  causing it to expand in length parallel to its first and second surfaces  62  and  64 , respectively and perpendicular to polarization vector  70 . Specifically, we define S(z) to be the change in length (in this case expansion) in the x (parallel or lateral) direction noting that this expansion varies as a function of z. The thickness of the functionally gradient piezoelectric element  60  is given by T as shown, and therefore S(z)=(d 31 (z)V/T)×L as is well known. The functional dependence of the piezoelectric coefficient d 31 (z) increases with z as shown in FIG.  4 . Thus, the lateral expansion S(z) of the functionally gradient piezoelectric element  60  decreases in magnitude from the first surface  62  to the second surface  64 . Therefore, when a potential difference is impressed across the transducer  80  with the first surface electrode  20  at a higher potential than the second surface electrode  22 , the transducer  80  distorts into a first bending state as shown. The word bending includes elongation, contraction, shear, or combinations thereof. 
     Referring to FIG. 8, the transducer  80  is also shown with switch  30  closed. In this case, the voltage (V) of voltage source  40  is impressed across the transducer  80  with the negative and positive terminals of the voltage source  40  electrically connected to the first and second surface electrodes  20  and  22 , respectively. Thus, the first surface electrode  20  is at a lower potential than the second surface electrode  22 . As before, this potential difference creates an electric field through the functionally gradient piezoelectric element  60  causing it to contract in length parallel to its first and second surfaces  62  and  64 , respectively and perpendicular to polarization vector  70 . Specifically the change in length (in this case contraction) is given by S(z)=(d 31 (z)V/T)×L as is well known. Since the functional dependence of the piezoelectric coefficient d 31 (z) increases with z as shown in FIG. 4, the lateral contraction S(z) of the functionally gradient piezoelectric element  60  decreases in magnitude from the first surface  62  to the second surface  64 . Therefore, when a potential difference is impressed across the transducer  80  with the first surface electrode  20  at a lower potential than the second surface electrode  22 , the transducer  80  distorts into a second bending state as shown. The word bending includes elongation, contraction, shear, or combinations thereof. It is important to note that the piezoelectric transducer  80  requires only one functionally gradient piezoelectric element  60  as compared to two or more elements for the prior art bimorph transducer (not shown). 
     Referring again to FIGS. 1 and 2, a source of power  240  having first and second terminals  250 ,  260  connected, respectively, to the first and second surface electrodes  20  and  22  of the piezoelectric transducer  80  enables fluid flow through the fluid containment chamber  120 . Thus, on the one hand, when the piezoelectric transducer  80  is energized to pump fluid out of the fluid containment chamber  120 , the source of power  240  provides a positive voltage to the first terminal  250  and a negative voltage to the second terminal  260 . On the other hand, when the piezoelectric transducer  80  is energized to pump fluid into the fluid containment chamber  120 , the source of power  240  provides a negative voltage to the first terminal  250  and a positive voltage to the second terminal  260 . 
     In operation, the piezoelectric pumping apparatus  100  of the invention performs in the manner described below. When the power source  240  connected to the transducer  80  is off, i.e. there is no voltage on either the first or second terminals  250  and  260 , the pump is inactive. To pump fluid out of the fluid containment chamber  120 , the power source  240  provides a positive voltage to first terminal  250  and a negative voltage to second terminal  260 . Thus, the first surface electrode  20  is at a higher potential than the second surface electrode  22 . This creates an electric field through the functionally gradient piezoelectric element  60  causing it to expand in length parallel to the first and second surface electrodes  20  and  22 , as discussed above. Since the functional dependence of the piezoelectric coefficient d 31 (z) increases with (z) as shown in FIG. 4, the lateral expansion of the functionally gradient piezoelectric element  60  decreases in magnitude from the first surface electrode  20  to the second electrode  22 , thereby causing the functionally gradient transducer  80  to deform into a first bending state as shown in FIG.  7 . Thus, the top surface  124  of compliant member  122  takes the shape of dotted line  270  thereby reducing the volume of fluid containment chamber  120 . This, in turn, increases the pressure of the fluid in the fluid containment chamber  120  so that it is greater than that at the exterior part  200  of the outlet port  160 . Under this condition the second valve  140  permits fluid to flow out of the fluid containment chamber  120  through the outlet port  160  as indicated by flow arrow  190 , as is well known. The compliant member  122  is preferably made from plastic, such as nylon, and functions to insulate the transducer  80  from the fluid in the fluid containment chamber  120 . 
     To draw fluid into the fluid containment chamber  120 , the power source  240  provides a negative voltage to terminal  250  and a positive voltage to terminal  260 . Thus, the first surface electrode  20  is at a lower potential than the second surface electrode  22 . Similarly, this potential difference creates an electric field through the functionally gradient piezoelectric element  60  causing it to contract in length parallel to the first and second surface electrodes  20  and  22  as discussed above. Since the functional dependence of the piezoelectric coefficient d 31 (z) increases with (z) as shown in FIG. 4, the lateral contraction of the functionally gradient piezoelectric element  60  decreases in magnitude from the first surface electrode  20  to the second surface electrode  22 , thereby causing the functionally gradient transducer  80  to deform into a second bending state as shown in FIG.  8 . Thus, the bottom surface  126  of compliant member  122  takes the shape of dotted line  280  thereby reducing the volume of fluid containment chamber  120 . This, in turn, decreases the pressure of the fluid in the fluid containment chamber  120  so that it is less than that at the exterior part  180  of the inlet port  150 . Under this condition the first valve  130  permits fluid to flow into the fluid containment chamber  120  through the inlet port  150  as indicated by flow arrow  170 , as is well known. 
     The outflow/inflow operation described above is depicted by the bi-directional arrow  290  which shows the range of motion of the compliant member  122  with enclosed functionally gradient piezoelectric transducer  80 . 
     Therefore, the invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     Parts List 
       20  first surface electrode 
       22  second surface electrode 
       24  wire 
       26  wire 
       30  switch 
       40  voltage source 
       60  functionally gradient piezoelectric element 
       62  first surface 
       64  second surface 
       70  polarization vector 
       80  piezoelectric transducer 
       100  piezoelectric pumping apparatus 
       110  pump body 
       120  fluid containment chamber 
       122  compliant member 
       124  top surface of compliant member 
       126  bottom surface of compliant member 
       130  first valve 
       140  second valve 
       150  inlet port 
       160  outlet port 
       164  reservoir 
       166  outflow port 
       168  fluid conduit 
       170  flow arrow 
       180  exterior part of the inlet port 
       190  flow arrow 
       200  exterior part of the outlet port 
       240  power source 
       250  first terminal 
       260  second terminal 
       270  dotted line 
       280  dotted line 
       290  bi-directional arrow