Abstract:
Fluid element device including fluid elements arranged in a matrix configuration having rows and columns with the level of fluid in each fluid element being controllable by electric force. The fluid elements include capillary tubes having electrode segments electrically insulated from one another such that each segments is receivable of a different voltage. A first voltage delivery circuit applies voltage to selected fluid elements in one or more rows of fluid elements and a second voltage delivery circuit applies voltage to selected fluid elements in one or more columns of fluid elements. The fluid level rises in the selected fluid elements to which voltage is applied by both the first and second voltage delivery circuits. The capillary tubes are structured and arranged such that the level of fluid in the selected elements can be retained independent of the application of voltage by the first and second voltage delivery circuits.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device comprising a plurality of fluid elements arranged in a matrix configuration, the level of fluid in each element being controllable by means of an electric force, which device is further provided with: 
     first means for applying an electric force to a number of selected elements in one or more of the rows of fluid elements; 
     second means for applying an electric force to a number of selected elements in one or more of the columns of fluid elements; 
     such that the fluid level rises in the selected elements to which an electric force is applied by both the first means and the second means; and 
     memory means for retaining the risen level in the selected elements. 
     DESCRIPTION OF RELATED ART 
     Such a device is known from the international patent application WO 98/51509, which particularly refers to the delivery of fluids to a receptor, e.g. delivery of pigments to a printing media. 
     The known device uses matrix addressing to control the level of fluid in the fluid elements thereby advantageously reducing the number of drivers necessary to set the fluid level in the different elements. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The invention aims at providing a device of the type described above, which is particularly suited for manipulating the level of fluid in the fluid elements in a precise way without the need for extra components. 
     The device according to the invention is characterized in that the fluid elements comprise capillary tubes which are arranged such that the level of fluid is controllable while exercising a threshold like behaviour. 
     By designing the capillary tubes such that the physical characteristics thereof induce the threshold like behavior extra components, such as transistors or diodes, become redundant. This greatly reduces the size of the device according to the invention, which is an important advantage in the relevant technical fields. 
     According to a first preferred embodiment of the device according to the invention the capillary tubes comprise electrodes which are arranged inside the capillary tubes. Preferably the electrodes comprise a coating layer of an electrically conducting material. 
     It is noted that in U.S. Pat. No. 5,666,396 by the same applicant an X-ray examination device is described which is provided with an X-ray filter comprising a matrix of capillary tubes provided with electrodes comprising a coating layer of an electrically conducting material. The level of X-ray absorbing fluid inside the capillary tubes is controllable by means of application of an electric force to a number of selected capillary tubes. Thin film transistors are used to induce a threshold like behaviour for the fluid rise inside the capillary tubes, which is what the invention aims to overcome. 
     According to a second preferred embodiment of the device according to the invention the electrodes are divided into segments in longitudinal direction of the capillary tubes. In addition or as an alternative thereto the electrodes are divided into segments in transverse direction of the capillary tubes. Dividing the electrodes into segments thereby introducing a gap is an elegant construction which inherently induces the desired threshold like behaviour for the fluid rise. Preferably one or more of the electrode segments are coupled to the first means, leading to the so called ‘liquid/in-capillary electrode embodiment. According to an alternative embodiment one or more of the remaining electrode segments are coupled to the second means, leading to the so called ‘in-capillary/in-capillary’ electrode embodiment. 
     According to a still further preferred embodiment one or more of the electrode segments are coupled to the memory means. A memory effect can be obtained by adding an extra electrode to a capillary tube. In conformance with the embodiments described above an electrode segment can serve as the extra electrode. 
     The invention also refers to an X-ray filter comprising a device according to the invention, wherein the capillary tubes are filled with an X-ray absorbing fluid. 
     Furthermore the invention refers to an X-ray examination device comprising an X-ray filter according to the invention. 
     The invention also refers to a microfluidic chip comprising a device according to the invention. Microfluidic chips as such are known from a publication in SCIENCE Vol 282, p. 399 and comprise a system of capillary channels, wherein, for example, a reactive is being transported. These microfluidic chips are applied in the field of biochemistry, for example to perform a DNA sequencing. Microfluidic chips can be also applied in the field of chemistry, for example for the purpose of chemical analysis. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further explained by means of the attached drawings, in which: 
     FIG. 1 shows a diagrammatic representation of an X-ray examination device provided with an X-ray filter comprising a device according to the invention 
     FIG. 2 shows a side elevation of the X-ray filter of the X-ray examination device shown in FIG. 1; 
     FIG. 3 shows a cross sectional view of a first embodiment of a capillary tube of a device according to the invention; 
     FIG. 4 shows a cross sectional view of a second embodiment of a capillary tube of a device according to the invention; 
     FIG. 5 shows a schematic view of a third embodiment of a capillary tube of a device according to the invention; and 
     FIG. 6 shows a schematic view of a fourth embodiment of a capillary tube of a device according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows diagrammatically an example of an X-ray examination apparatus  1  in which the present invention can be implemented. The X-ray source  2  emits an X-ray beam  15  for irradiating an object  16 . Due to differences in X-ray absorption within the object  16 , for example a patient to be radiologically examined, an X-ray image is formed on an X-ray sensitive surface  17  of the X-ray detector  3 , which is arranged opposite the X-ray source. The X-ray detector  3  of the present embodiment is formed by an image intensifier pick-up chain which includes an X-ray image intensifier  18  for converting the X-ray image into an optical image on an exit window  19  and a video camera  23  for picking up the optical image. The entrance screen  20  acts as the X-ray sensitive surface of the X-ray image intensifier which converts X-rays into an electron beam which is imaged on the exit window by means of an electron optical system  21 . The incident electrons generate the optical image on a phosphor layer  22  of the exit window  19 . The video camera  23  is coupled to the X-ray image intensifier  18  by way of an optical coupling  24 , for example a lens system or a fiber-optical coupling. The video camera  23  extracts an electronic image signal from the optical image, which signal is applied to a monitor  25  for the display of the image information in the X-ray image. The electronic image signal may also be applied to an image processing unit  26  for further processing. 
     Between the X-ray source  2  and the object  16  there is arranged the X-ray filter  4  for local attenuation of the X-ray beam. The X-ray filter  4  comprises a large number of filter elements  5  in the form of capillary tubes whose X-ray absorptivity can be adjusted by application of an electric voltage, referred to hereinafter as adjusting voltage, to the inner side of the capillary tubes by means of the adjusting unit  7 . 
     FIG. 2 is a side elevation of an X-ray filter  4  of the X-ray examination apparatus of FIG.  1 . The Figure shows seven capillary tubes by way of example, but a practical embodiment of an X-ray filter  4  of an X-ray examination apparatus in accordance with the invention may comprise a large number of capillary tubes, for example 16384 tubes in a  128 - 128  matrix arrangement. Each of the capillary tubes  5  communicates with the X-ray absorbing liquid  6  via an end  31 . The inner side of the capillary tubes is covered by an electrically conductive layer  37 , for example of gold or platinum, which layer  37  is coupled to a voltage line  35 . 
     The adhesion of the X-ray absorbing liquid to the inner side of the capillary tubes can be adjusted by means of an electric voltage applied to an electrically conductive layer  37  on the inner side of the capillary tubes  5 . One end of the capillary tubes communicates with a reservoir  30  for an X-ray absorbing liquid. The capillary tubes are filled with a given quantity of X-ray absorbing liquid as a function of the electric voltage applied to the individual tubes. Because the capillary tubes extend approximately parallel to the X-ray beam, the X-ray absorptivity of the individual capillary tubes is dependent on the relative quantity of X-ray absorbing liquid in such a capillary tube. The electric adjusting voltage applied to the individual filter elements is adjusted by means of the adjusting unit  7 , for example on the basis of brightness values in the X-ray image and/or the setting of the X-ray source  2 . To this end, the adjusting unit is coupled to the output terminal  40  of the video camera and to the power supply  11  of the X-ray source  2 . The construction of an X-ray filter  4  of this kind and the composition of the X-ray absorbing liquid are described in detail in the International Patent Application No. IB 95/00874 and in U.S. Pat. No. 5,666,396, which are both incorporated herein by reference. 
     The height of the fluid level inside the capillary tubes is influenced by the electrocapillary pressure, also called electrowetting. The electrocapillary pressure p behaves as p=const·V 2 , with V the electrical potential applied between an in-capillary electrode ( 37  in FIG. 2) and the conducting liquid ( 6  in FIG.  2 ). The height of the fluid level inside the capillary tubes is further determined by the repelling force of the capillary tube walls and the externally applied hydrostatic pressure. In this respect it is noted that use is made of watery solutions and hydrophobic materials. The fluid level in the capillary tubes is a result of the balance between said three forces of which the electrocapillary pressure p is actively used to set the fluid level at a desired height. 
     Dynamic measurements show that the switching takes place in 0.1-1 second (speed 1-10 cm/s, electrode length 1 cm). 
     In an X-ray filter the liquid level in every capillary has to be individually controllable. If every capillary is connected to an individual wire, the number of required electronic control elements scales with N 2 . A well-know method to reduce the number of control elements to a number of the order N, is by matrix-addressing. Matrix addressing means that rows (indexed i, iε{1 . . . N}, voltage V i ) are activated one-by-one while the programming signals are placed on column wires (indexed j, jε{1 . . . N}, voltage V j ). 
     In order to apply matrix addressing in an X-ray filter an electrical matrix structure is needed in every capillary tube, i.e. every capillary tube (i, j) needs to be connected to voltages V i  and V j . Hereinafter three different examples of capillary tubes according to the invention are shown. 
     FIG. 3 shows a cross sectional view of a first embodiment of capillary tube  45  of a device according to the invention. Capillary tube  45  comprises an electrode formed by a conducting layer, an insulator  46  and a hydrophobic coating  47 . The electrode is divided into three segments  42 ,  43  and  44  in longitudinal direction of the tube. The segments are mutually electrically insulated and different voltages are applied thereto. A voltage of V mem  is applied to the lowest segment  42  by means of the row connection. A voltage of V j  is applied to the middle segment  43  by means of the column connection. A voltage of V mem  is applied to the highest segment  44 . A voltage V i  is applied to the liquid which acts as an electrode. This embodiment is therefore also referred to as the liquid/in-capillary electrode embodiment. 
     The segmentation of the data-electrode introduces a gap between the segments  42  and  43 . In order for the liquid to rise over the above mentioned gap a threshold voltage should be applied to the data-electrode. Only for a large enough magnitude of V i −V j  the liquid will jump across the gap and rise above it. 
     The gap size (i.e. the distance between the segments of the data-electrode) as well as the gap geometry determine the threshold-behavior. An example of a gap geometry that reduces the required threshold voltage is shown schematically in FIG. 6, where two electrode segments are indented in the direction of the liquid rise. 
     In a similar fashion a local discontinuity of geometry (such as a local shape change, local opening-up and/or constriction of the capillary), of insulator properties (such as thickness or dielectric constant) or of hydrophobicity introduces a nonlinear behavior of liquid level versus applied voltage. The structuring of the metallic electrodes, insulator thickness or hydrophobic coating can be achieved in a filter that is composed of semi-planar plates or of foils. 
     FIG. 4 shows a cross sectional view of a second embodiment of a capillary tube  55  of a device according to the invention. This embodiment is referred to as the in-capillary/in-capillary electrode embodiment. 
     Capillary tube  55  comprises an electrode formed by a conducting layer, an insulator  56  and a hydrophobic coating  57 . The electrode is divided into three segments  52 ,  53  and  54  in longitudinal direction of the tube. The segments are mutually electrically insulated and different voltages are applied thereto. A voltage of V i  is applied to the lowest segment  52  by means of the row connection. A voltage of V j  is applied to the middle segment  53  by means of the column connection. A voltage of V mem  is applied to the highest segment  54 . A voltage V liq  with approximately zero value is applied to the liquid. 
     At the mouth  51  of the capillary the electrodes  52  and  53  act as a valve between the supply channel and the bulk of the capillary thus introducing a threshold-like behavior. Only when a voltage with a predetermined value is applied to electrode  52  as well as to electrode  53  the liquid fills the capillary. The liquid does not rise if only one of the electrodes is activated. 
     Due to the limited velocity of liquid movement (speed 1-10 cm/s), the meniscus senses a time-averaged electrocapillary pressure. To avoid that programmed rows loose their pattern in the time that other rows are being programmed, a memory function has to be added to every capillary tube. Such a memory function can be achieved with an extra electrode in the capillaries. This electrode can be shared by more than one capillary as is the case with V mem  in FIG.  3 . 
     The value of V mem  is chosen such that the capillary tube  55  remains filled once the liquid has risen, independent of the value of voltages V i  and V j . This is referred to as the ‘memory effect’. The memory effect is advantageous because it allows for the sequential addressing of several rows. In order to empty the filled capillaries, a zero voltage should be applied to electrodes  52 ,  53  and  54  resulting in a reset of the capillaries. After resetting the capillaries can be reprogrammed. Preferably every row of capillaries has a separate V mem  connection allowing the resetting to occur in a row-wise fashion. 
     FIG. 5 shows a cross sectional view of a third embodiment of a capillary tube of a device according to the invention. This embodiment is also referred to as the improved in-capillary/in-capillary electrode embodiment. 
     FIG. 5 shows a preferred embodiment of an electrode architecture that allows filling and emptying of a single capillary tube. Every capillary tube is provided with an electrode sequence  62 ,  63 ,  64 ,  68 ,  69 , whereby a voltage V i  is applied to electrode segments  62  and  69 , a voltage V j  is applied to electrode segments  63  and  68  and a voltage V mem  is applied to electrode segment  64  so that the voltage sequence is V i /V j /V mem /V i /V j . V mem  indicates the voltage applied to the memory electrode, electrode segment  64  in this case. The conducting fluid is supplied from below. In the examples given below, V=0 means that no potential is applied, so that the electrode is being de-wetted. V=1 means that a high potential is applied (e.g. V&gt;200 V), so that the electrode is being wetted. 
     EXAMPLE 1 
     Suppose that all capillary tubes are empty and we want to fill only one capillary tube (i=n, j=m). 
     We can do this by applying for example: 
     V i =1 for all i, V j =0 for all j, and then 
     V mem =1, V i =1 for all i, V j =0 for all j, and then 
     V mem =1, V i =0 for all i, V j =0 for all j, and then 
     V mem =1, V i=n =1, V i≠n =0, V j=m =1, V j≠m =0, (the capillary fills) and then 
     V mem =1, V i =0 for all i, V j =0 for all j. 
     EXAMPLE 2 
     Suppose that all capillary tubes are full and we want to empty only one capillary tube (i=n, j=m). 
     We can do this by applying for example: 
     V i =1 for all i, V j =0 for all j, and then 
     V mem =0, V i =1 for all i, V j =0 for all j, and then 
     V mem =0, V i =1 for all i, V j =1 for all j, and then 
     V mem =0, V i=n =0, V i≠n =1, V j =1 for all j, and then 
     V mem =0, V i=n =0, V i≠n =1, V j=m =0, V j≠m =1, (the capillary empties) and then 
     V mem =0, V i =1 for all i, V j =0 for all j, and then 
     V mem =1, V i =1 for all i, V j =0 for all j, and then 
     V mem =1, V i =0 for all i, V j =0 for all j. 
     It is noted that grey-scale programming is possible when the architecture of FIG. 5 is repeated several times in a capillary tube. 
     FIG. 6 schematically shows an embodiment of a capillary tube  71  according to the invention comprising two electrode segments  72  and  73 , which are indented in the direction of the liquid rise. This gap geometry reduces the required threshold voltage. It will be understood that the indented parts can have a variety of shapes apart from the rectangular shape that is shown. 
     It will be clear for a person skilled in the art that in the matrix structures described above the functions of rows and columns are interchangeable. 
     Summarizing the invention provides the insight that in a device as described above the construction of the fluid elements can be designed such that it induces the desired threshold like behavior of the fluid rise without the necessity of extra components. 
     The invention is of course not limited to the described or shown embodiment(s), but generally extends to any embodiment, which falls within the scope of the appended claims as seen in light of the foregoing description and drawings.