Abstract:
A method and device for performing fluid analysis by separating cells and/or particles from a fluid, such as a biological, vehicular or industrial fluid. The device is a micromachined filtering device comprising a substrate with through-thickness vias having approximately equal diameters that prevent passage through the substrate of a first material while permitting passage through the substrate of other materials having diametrical dimensions less than the diameter of the vias. Electrodes are located on a surface of the substrate between vias so that as the first material collects at the surface of the substrate, the electrodes become electrically connected to produce an output signal in some proportion to the amount of the first material collected. The device can incorporate multiple micromachined substrates, yielding an analysis system that produces an electrical output for each of a number of properties or parameters.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/343,875, filed Jan. 2, 2002. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention generally relates to fluid analysis methods and equipment. 
     More particularly, this invention relates to a fluid analysis device and method that utilize a micromachined filter to separate cells and/or particles from a fluid, such as a biological fluid, and means for sensing the material selectively separated from the fluid with the filter. 
     2. Description of the Related Art 
     Various fluids undergo some type of quantitative analysis to determine their composition and physical properties. Notable examples of such fluids include urine, blood, beverages, pharmaceutical mixtures, water, lubricating oils, fuels, and many industrial chemicals. With regard to urological analysis, a variety of parameters are typically measured during urology, including pH, specific gravity, and the amount of blood, leukocytes (white blood cells), glucose, protein, urobilinogen, bilirubin, ketones, nitrite, sodium, chlorine, potassium, magnesium, urea, uric acid, bicarbonate, sulfate, phosphate and calcium. The specific gravity of urine can be used as a screen to indicate renal and hepatic problems, with additional urinary tests being performed as necessary if a problem is indicated. 
     The specific gravity of urine has been measured by various methods, including ultrasonic and optical techniques as disclosed in U.S. Pat. Nos. 4,664,124 and 4,834,104, respectively. More recently, commonly-assigned U.S. patent application Ser. No. 09/468,628 to Tadigadapa et al. discloses a resonant mass flow and density sensor suitable for quantitative analysis of fluids. The sensor comprises a suspended tube that is vibrated at resonance. As fluid flows through the tube, the tube twists under the influence of the Coriolis effect. The degree to which the tube twists (deflects) when vibrated can be correlated to the mass flow rate of the fluid flowing through the tube, while the density of the fluid is proportional to the frequency of vibration. The tube is fabricated by micromachining, which as used herein denotes a technique for forming very small elements by bulk etching a substrate (e.g., a silicon wafer) or by surface thin-film etching, the latter of which generally involves depositing a thin film (e.g., polysilicon or metal) on a sacrificial layer (e.g., oxide layer) on a substrate surface and then selectively removing portions of the sacrificial layer to free the deposited thin film. 
     Various other parameters of interest in urological analysis have been measured using reagent test strips. However, there are drawbacks to the use of test strips, including the vulnerability to humidity, finger contamination, and erroneous results due to vitamin C intake prior to testing. Test strips also require a manual operation and the constant expense of replacement since they are consumed by the test. 
     Biological fluid filtration has also been utilized in the field of fluid analysis. For example, physical filtration of donated blood has been used for years to separate leukocytes from plasma. For urological applications, the concentration of leukocytes is often of interest, as their presence in urine can indicate a urinary tract infection from chronic catheter use as well as renal and hepatic problems. Leukocytes are larger (about twenty micrometers) than other blood or urine components, and so can be physically filtered. Micromachined filters, including silicon filters, capable of use in urological analysis have been proposed, as disclosed in U.S. Pat. Nos. 5,660,728 and 5,922,210. 
     While fluid analysis techniques and devices of the types described above have been successfully employed, there is a continuing effort to develop improved devices for performing fluid analysis. For example, the capability for continuous monitoring would be desirable, particularly in the form of remote monitoring of disabled catheterized patients. In addition, it would be desirable if components grouped into a single system could perform multiple analysis steps, such that an accurate diagnosis can be made with a single sample. 
     SUMMARY OF INVENTION 
     The present invention provides a method and device for performing fluid analysis utilizing a micromachined filter to separate cells and/or particles from a fluid, such as a biological fluid. The device has the additional capability of sensing the relative quantity of cells and/or particulate material selectively separated from the fluid with the filter. Multiple micromachined filters of this invention can be integrated into a single device that produces an electrical output for each of a number of urological parameters, providing a rapid and simplified interface capable of remote and continuous monitoring of a fluid. 
     According to a first aspect of the invention, the device is a micromachined filtering device comprising a substrate having a first surface, an oppositely-disposed second surface, and a thickness defined by and between the first and second surfaces. A plurality of vias extend through the thickness of the substrate, with the vias being spaced apart and having approximately equal diameters that prevent passage through the substrate of materials (e.g., cells and/or particles) having a diametrical dimension greater than the diameters of the vias, while permitting passage through the substrate of a fluid and any other materials present in the fluid and having a diametrical dimension less than the diameters of the vias. First and second electrodes are, located on the first surface of the substrate so that the materials too large to pass through the vias, and have therefore collected at the first surface of the substrate, will electrically connect the first and second electrodes to produce an output signal in proportion to the amount of the material collected. 
     As a result of the construction of the device, the present invention makes possible a method of quantitatively analyzing a fluid by flowing the fluid through the vias in the substrate, whereby the fluid and any cells and/or particles smaller than the vias are permitted to pass through the substrate, while cells and/or particles larger than the vias are prevented from passing through the substrate, such that the larger cells/particles collect at the first surface of the substrate. The amount of the collected cells/particles at the first surface of the substrate is indicated by the output signal obtained from the electrodes. 
     Fluid filtering in accordance with this invention may be preceded or followed by additional analysis, such as the measurement of specific density, pH, and various constituents detected with chemical sensors. In the case of urological analysis, such constituents include glucose, protein, urobilinogen, bilirubin, ketones, nitrite, pH, sodium, chlorine, potassium, magnesium, urea, uric acid, bicarbonate, sulfate, phosphate, and calcium. With the present invention, such analysis can be preformed on multiple substrates within a single device, yielding a single-system sensing and filtering system. 
     Other objects and advantages of this invention will be better appreciated from the, following detailed description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIGS. 1 and 2 are cross-sectional and plan views, respectively, of a substrate of a micromachined filtering device in accordance with this invention. 
     FIGS. 3 and 4 are cross-sectional views of micromachined filtering devices in accordance with two embodiments of the invention. 
     FIG. 5 represents a urological analysis system utilizing a micromachined filtering device of this invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 represent a substrate  12  for a micromachined filtering device in accordance with the invention, two embodiments of which are represented in FIGS. 3 and 4. The substrate  12  can be formed of silicon, such as silicon doped to be p-type. Alternatively, the substrate  12  can be formed of another semiconductor material, quartz, ceramic, metal, or a composite material. Vias  14  are micromachined in the substrate  12  to have approximately identical diameters, and to extend through the thickness of the substrate  12  between opposing surfaces of the substrate  12 , referred to herein as upstream and downstream surfaces  16  and  18 . The vias  14  are preferably etched through the substrate  12  using known semiconductor processing. For example, if the substrate  12  is formed of silicon, the vias  14  can be formed by masking either of the surfaces  16  or  18  of the substrate  12 , followed by etching with a wet chemical anisotropic etchant, such as ethylenediamine pyrocatechol (EDP), or an alkali-type etchant, such as potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH). 
     As seen in FIG. 2, the vias  14  are arranged in an array (rows and columns), with rows of the vias  14  being separated by interdigitized portions of two electrodes  20  and  22  on the upstream surface  16  of the substrate  12 . The vias  14  are arranged and adapted to serve as passages through which a fluid, such as urine, blood, beverage, pharmaceutical mixture, water, oil, fuel, industrial chemical, etc., flows for the purpose of performing quantitative analysis of the fluid. More particularly, the vias  14  are sized to filter from the fluid any cells and/or particles  24  that exceed the diameter of the vias  14 , while permitting the entraining fluid and smaller cells/particles  25  to pass through the substrate  12 , as represented in FIG.  1 . For example, if the fluid is urine, the cells/particles that may be filtered with the substrate  12  include blood, or selectively leukocytes and erythrocytes (red blood cells). As will be discussed below in reference to urological analysis that can be performed with the teachings of this invention, a variety of parameters can be measured with a device utilizing the substrate  12 , including pH, specific gravity, and the amount of glucose, protein, urobilinogen, bilirubin, ketones, nitrite, sodium, chlorine, potassium, magnesium, urea, uric acid, bicarbonate, sulfate, phosphate, and calcium. Other fluids that can be processed with such a device include any that contain biological cells, spores, or particles from essentially any source. 
     In view of the above, the diameter of the vias  14  is chosen to prevent the passage through the substrate  12  of cells/particles of a particular size and larger, while permitting the entraining fluid and smaller cells/particles  25  to pass through the substrate  12 . For example, leukocytes (diameter of about twenty micrometers) can be filtered with an array of vias  14  on the order of about fifteen to seventeen micrometers in diameters, while allowing water (95% of urine), electrolytes, protein, glucose, and erythrocytes to pass through. The monitoring of the presence of erythrocytes in urine is also desirable as being useful to detect cardiovascular, renal, and hepatic problems. For this purpose, erythrocytes (about eight micrometers in diameter) can be subsequently filtered with a second substrate  12  having appropriately-sized vias  14 , e.g., having a size range of about three to seven micrometers. The quantity of cells filtered from the fluid is then determined by the electrical resistance or current flow that occurs between the electrodes  20  and  22  when a potential is applied across the electrodes  20  and  22 . In particular, as the electrodes  20  and  22  become electrically connected by cells/particles that collect at the upstream surface  16  of the substrate  12 , current flow between the electrodes  20  and  22  will progressively increase, and electrical resistance progressively decrease, to produce an output signal in some proportion to the amount of material collected at the upstream surface  16 . Suitable materials for the electrodes  20  and  22  include platinum or iridium runners of a type known in the art for thick-film hybrid circuits. If the substrate  12  is formed of silicon or another conductive or semiconductive material, the upstream surface  16  on which the electrodes  20  and  22  are formed is preferably oxidized or otherwise provided with an electrically insulating layer prior to the deposition of the electrodes  20  and  22 . In addition to the electrodes  20  and  22 , a chemically-active material can be deposited on the upstream surface  16  in combination with the electrodes  20  and  22  to increase sensitivity. Such a material can be a biological material that attracts leukocytes through an immunological reaction. 
     As shown in FIGS. 3 and 4, multiple substrates  12  of the type shown in FIGS. 1 and 2 can be utilized in a single filtering device  10  or  110 , so that incrementally, smaller cells/particles can be filtered from a fluid. In FIG. 3, three substrates  12  are bonded together and then packaged in a housing  28  as a single filtering device  10 . The device  110  shown in FIG. 4 differs from that of FIG. 3 by individually packaging the substrates  12  in packages  112 , which are then bonded or otherwise secured together. In both embodiments, the downstream surface  18  of each substrate  12  is shown as having been etched to form a recess  30  that defines a membrane  32  surrounded by a frame  34 . In FIG. 3, the frames  34  of the substrates  12  are bonded directly together, e.g., anodically or with a screen-printed adhesive or glass frit, at the die or wafer bonding level. 
     In each of the embodiments of FIGS. 3 and 4, the uppermost substrate  12  is preferably micromachined to have vias  14  sized to filter relatively large cells or particles, e.g., leukocytes, while the middle and lowermost substrates  12  of FIG.  3  and the lowermost substrate  12  of FIG. 4 are micromachined to have vias  14  sized to filter relatively smaller cells or particles, e.g., erythrocytes. Alternatively or in addition, the lowermost substrates  12  of FIGS. 3 and 4 can be adapted to sense other parameters of the fluid which, depending on the fluid, may include pH or the amount of certain constituents in the fluid. For example, if the fluid is urine, the lowermost substrates  12  can be adapted to determine the amount of glucose, protein, urobilinogen, bilirubin, ketones, nitrite, sodium, chlorine, potassium, magnesium, urea, uric acid, bicarbonate, sulfate, phosphate, or calcium in the fluid. For this purpose, chemical sensors  36  are shown in FIG. 3 as being embedded in the walls of the vias  14 . Alternatively or in addition, the sensors  36  could be located on the upstream surface  16  of the substrate  12 , or in the walls of the recess  30  in the downstream surface  18  of the substrate  12 . For urology, the chemical sensors  36  are preferably located downstream of substrates  12  used to filter leukocytes and erythrocytes, as represented in FIG.  3 . As also represented in FIG. 3, the substrate  12  on which the chemical sensors  36  are provided can be placed directly in the fluid flow stream. Alternatively, the substrate  12  could be placed so as to be immersed in a relatively static pool of the fluid for longer exposure times. Any number of substrates  12  equipped with chemical sensors can be employed to increase the number of chemicals monitored. The chemical sensors  36  may be formed by a variety of materials, such as certain metal oxides and organic films known in the art to be sensitive to the parameters of interest, an example of which is pH-sensitive iridium oxide films. Other suitable chemical sensors and methods for forming such sensors in the substrate  12  are known to those skilled in the art, and therefore will not be discussed in any further detail here. 
     According to the invention, the devices  10  and  110  can be modified to sense the specific gravity of the fluid, such as by including the resonant mass flow and density sensor disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al., incorporated herein by reference. For this purpose, another substrate equipped with a suspended micromachined tube would be placed upstream of the uppermost substrate  12  of FIGS. 3 and 4. In accordance with Tadigadapa et al., the tube is fabricated to comprise a fluid inlet, a fluid outlet, and a freestanding portion therebetween, with the freestanding portion being spaced apart from a surface of the substrate. Means is provided for vibrating the freestanding portion of the tube, preferably at resonance, and for sensing movement of the freestanding portion relative to the substrate in a manner that permits the density of the fluid to be determined, from which specific gravity can be calculated by comparing the density of the fluid to the density of water. In urology, the specific gravity of urine obtained in this manner can then be utilized as a screen for renal and hepatic problems. With the present invention, additional quantitative analysis can be advantageously performed immediately downstream of the resonating tube with one or more of the micromachined filtering devices  10 / 110 . 
     In view of the above, the present invention enables a diagnosis to be obtained using a series of tests performed on a single sample with a single device. FIG. 5 represents such an analysis system, in which a catheter  40  attached to a patient delivers a bodily fluid, e.g., urine, to a filtering device  10 / 110  of this invention. In accordance with the above, the device  10 / 110  may include one or more substrates  12  for filtering urine, as well as one or more substrates  12  equipped with chemical sensors  36  (FIG. 3) and optionally an additional substrate for sensing density in accordance with Tadigadapa et al. The several output signals from the individual substrates  12  are relayed to a computer  42  for analysis, with the filtered urine being dispensed to a drain reservoir  44 . While FIG. 5 shows the device  10 / 110  in series with the catheter  40  and reservoir  44 , the device  10 / 110  could be placed in a passage branching off from the catheter  40  and parallel to a drain tube, with both the drain tube and an output tube from the device  10 / 110  dispensing urine to the reservoir  44 . Other configurations are envisioned, including the sampling of fluid from a sample reservoir that accumulates fluid from the catheter  40 , or sampling fluid directly from the reservoir  44 . 
     In view of the above, the present invention provides the capability of measuring a wide variety of fluid properties and parameters, providing a physician with the ability to monitor and diagnose a variety of ailments, such as renal, hepatic, pancreatic, gastrointestinal, and cardiovascular problems via urology. The ability to fabricate the device  10 / 110  as a reusable device, miniaturized through the use of micromachining technology, is beneficial to both specialists (e.g., urologists) and general practitioners. The device  10 / 110  of this invention is particularly well suited to provide continuous monitoring of disabled, catheterized patients. By appropriately controlling fluid flow through the device  10 / 110 , a manual or automatic back-flushing operation can be performed to remove cells/particles that have collected at the upstream surface  16  as the need requires. Otherwise, healthcare workers need only intervene if the output of the device  10 / 110  indicates that a medical concern exists, which can be relayed in the form of an alarm system triggered if abnormal cell or chemical levels exceed a predetermined limit over a given period of time or for the flow rate through the device  10 / 110 . The electrical output signal(s) that can be produced by the device  10 / 110  also enables remote computer monitoring of urinary output to provide early indicators of ailments, which is especially important for diabetic and disabled patients and can greatly reduce the cost of long-term health care 
     While the invention has been described in terms of certain embodiments, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.