Abstract:
An apparatus and a method are disclosed for providing point of care testing for osmolarity of a bodily fluid. An apparatus is disclosed as having a fluid pathway passing through it for receiving and testing a sample fluid. The invention permits osmolarity testing of a sample fluid wherein the sample fluid has a volume of less than approximately 30 nL, and implements a method and device to measure fluid osmolarity in a clinical setting quickly and accurately, while also reducing evaporation of the fluid.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to the field of devices for measuring the osmolarity of a relatively small volume of fluid, and in particular to a method and an apparatus for measuring, in vivo, the osmolarity of human tears. 
     2. Related Art 
     Dry eye syndrome (DES), a condition that occurs due to loss of water from the tear film, is one of the most common complaints seen by optometrists. Studies have found that DES is common in about 15% of patients over the age of 50, with prevalence increasing with age. Dry eye in general is caused by any condition that increases tear film evaporation, or by any condition that decreases tear production. For some patients, evaporation is increased as a result of having larger eyes. Larger eyes cause greater evaporation due to the larger surface area and the loss of water. Tear production can also decrease from any condition that decreases corneal sensation. Long-term contact lens wear, LASIK eye surgery, trauma to the 5th nerve, and certain viral infections cause decrease in corneal sensation. The treatment of DES depends on the severity of the condition. Some patients find relief from DES through the use of various artificial tears available on the market. Additionally, some patients are prescribed Omega-3 containing supplements. There are cases where “punctual plugs” need to be inserted to stop drainage of tears. 
     Osmolarity is the measure of the concentration of osmotically active particles in a solution, which may be quantitatively expressed in osmoles of solute per liter of solution. It is known that when the tear film loses water, salt and protein concentrations increase relative to the amount of water. When the concentration of salt and protein increases relative to the amount of water, osmolarity increases. Therefore, in order to diagnose and treat DES patients, it is desirable for a treating physician to quantify the osmolarity of a sample tear fluid. Some current osmolarity measurement methods and devices available include: osmotic pressure measurement, freezing point measurement, and vapor pressure measurement. 
     In one approach, an osmometer is used to measure the osmotic pressure exerted by a solution across a semi-permeable membrane. In this approach, a solvent and solution are separated by the semi-permeable membrane, which allows only solvent molecules to pass through. The osmotic pressure of the solution can be determined by measuring the excess pressure that must be applied to the solution to prevent the solvent from passing into the solution. 
     In another approach, the osmolarity of a sample fluid (e.g., a tear) can be determined by an ex vivo technique called “freezing point depression.” In this technique, solutes or ions in a solvent (i.e., water) cause a lowering of the fluid freezing point from what it would be without the ions. In the freezing point depression analysis, the freezing point of the ionized sample fluid is found by detecting the temperature at which a quantity of the sample (typically on the order of about several milliliters) first begins to freeze in a container (e.g., a tube). To measure the freezing point, a volume of the sample fluid is collected into a container, such as a tube. Next, a temperature probe is immersed in the sample fluid, and the container is brought into contact with a freezing bath or Peltier cooling device. The sample is continuously stirred so as to achieve a supercooled liquid state below its freezing point. Upon mechanical induction, the sample solidifies, rising to its freezing point due to the thermodynamic heat of fusion. Deviation of the sample freezing point from 0 degrees C. is proportional to the solute level in the sample fluid (i.e., osmolarity value). 
     Another ex vivo technique for osmolarity testing measures vapor pressure. In this method, a small, circular piece of filter paper is lodged underneath a patient&#39;s eyelid until sufficient fluid is absorbed. The filter paper disc is placed into a sealed chamber, whereupon a cooled temperature sensor measures the condensation of vapor on its surface. Eventually the temperature sensor is raised to the dew point of the sample. The reduction in dew point proportional to water is then converted into osmolarity. However, because of induced reflex tearing, osmolarity readings are not as accurate. Similarly, in vivo techniques, which attempt to measure osmolarity by placing electrodes directly under the eyelid of a patient, are likely to induce reflex tearing. As a result the above-described approaches are neither convenient nor accurate for an eye doctor operating in a clinical environment. 
     There is a need for a clinically feasible, nanoliter-scale osmolarity measurement device, with the capability for reduced evaporation, that does not suffer from the problems of the related art. 
     SUMMARY OF THE INVENTION 
     An apparatus and a method are disclosed for providing point of care testing for osmolarity of a bodily fluid. An apparatus is disclosed as having a fluid pathway passing through it for receiving and testing a sample fluid. The invention permits osmolarity testing of a sample fluid wherein the sample fluid has a volume of less than approximately 1 mL, with a preferred volume of less than 30 nL, and implements a method and device to measure fluid osmolarity in a clinical setting quickly and accurately, while also reducing evaporation of the fluid. 
     A first aspect of the invention is directed to a sample receiving chip comprising: a substrate having a fluid pathway passing through the substrate for receiving a sample fluid, the fluid pathway including a first port, at least one second port, and a recessed channel, the recessed channel enclosed in the substrate; and at least two electrodes positioned in the substrate to contact the sample fluid in the recessed channel to measure properties of the sample fluid. 
     A second aspect of the invention is directed to a device for osmolarity testing, comprising: a base member; a sample receiving chip fixed to the base member for receiving a sample fluid; and a conduit fixed to the base member for depositing the sample fluid on the sample receiving chip, the conduit including a first end and a second end. 
     A third aspect of the invention is directed to a method for determining osmolarity of a sample fluid, comprising the steps of: communicating a sample fluid through a conduit fixed to a base member directly to a sample receiving chip; and determining osmolarity of the sample fluid. 
     The foregoing and other features of the invention will be apparent from the following more particular description of the embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of this invention will be described in detail, with reference to the following figures, wherein the like designations denote like elements, and wherein: 
         FIGS. 1A-B  show a cross sectional view of a sample receiving chip according to one embodiment of the invention. 
         FIGS. 2A-B  show a plan view of two embodiments of a first substrate layer of the sample receiving chip of  FIG. 1 . 
         FIG. 3  shows a plan view of a second substrate layer of the sample receiving chip of  FIG. 1 . 
         FIG. 4  shows a plan view of a third substrate layer of the sample receiving chip of  FIG. 1 . 
         FIG. 5  shows a cross sectional view of the electrode windows which provide access to electrodes for osmolarity testing. 
         FIGS. 6A-B  show a cross sectional view of the electrode contacts positioned on different surfaces the sample receiving chip of  FIG. 1 . 
         FIG. 7  shows a plan view an osmolarity testing device to collect a sample fluid and to test osmolarity of the sample fluid. 
         FIG. 8  shows a plan view of an osmolarity testing device to collect a sample fluid and to test osmolarity of the sample fluid. 
         FIG. 9  shows a plan view of an osmolarity testing device to collect a sample fluid and to test osmolarity of the sample fluid. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments are described below for measuring the osmolarity of a sample fluid. The embodiments are configured to provide quick and accurate testing of a relatively small amount of fluid. 
     Referring to  FIGS. 1-4 , a sample receiving chip for testing osmolarity of a sample fluid according to one embodiment of the invention is shown. It can be appreciated, that even though three substrate layers are shown in the present embodiment, any number of substrate layers can be used. Furthermore, while sample receiving chip  2  is initially discussed in isolation, during operation sample receiving chip  2  may be coupled to a device, as will be described further below, including a base member; sample receiving chip  2  fixed to the base member for receiving a sample fluid; and a conduit fixed to the base member for depositing the sample fluid on sample receiving chip  2 . Coupling receiving chip  2  to a device allows for more convenient and effective point-of-care testing. 
     When the various substrate layers shown in  FIGS. 1-4  are combined, sample receiving chip  2  comprises: substrate  4  having fluid pathway  6  passing through substrate  4  for receiving a sample fluid. Fluid pathway  6  may include a first port  8 , at least one second port  10  (hereinafter simply “second port  10 ”), and a recessed channel  12 . As shown in  FIG. 1 , recessed channel  12  is enclosed in substrate  4 . Sample receiving chip  2  also includes at least two electrodes  14  positioned in substrate  4  to contact the sample fluid in the recessed channel to measure properties of the sample fluid. Electrode windows  18 , which are shown in  FIGS. 2A ,  3 ,  5 ,  7 , and  8 , are not shown in  FIG. 1  for clarity. However, it should be noted that substrate  4  may include electrode windows  8 . 
     Referring to  FIGS. 2A-B , a plan view of first substrate layer  16  is shown. First substrate layer  16  forms an upper layer of chip  2 , as shown in  FIG. 1 . As shown in  FIG. 2A , first port  8 , second port  10 , and electrode windows  18  are openings formed in first substrate layer  16  by, for example, mechanically punching-out portions of first substrate layer  16 . It can be appreciated, however, that any technique for creating openings in a substrate layer can be used. As will be described in further detail below, at least two electrode windows  18  provide access to at least two electrodes  14 . In an alternative embodiment, shown in  FIG. 2B , first substrate layer  16  may include first port  8 , and second port  10 , but no electrode windows. As will be described in further detail below, when substrate  4  does not include electrode windows  18 , substrate  4  includes at least two electrodes (not shown) connected to contacts  20  positioned on an external surface of substrate  4 . Although contacts  20  are shown in  FIG. 2B  as circular in shape, it can be appreciated that contacts  20  can be any suitable geometric shape. 
     Referring to  FIG. 3 , a plan view of second substrate layer  22  is shown. Second substrate layer  22  constitutes a middle layer of chip  2 , as shown in  FIG. 1 . In this embodiment, second substrate layer  22  includes openings for first port  8 , second port  10 , and recessed channel  12 . Additionally, second substrate layer  22  may include openings for electrode windows  18 . First port  8 , second port  10 , and recessed channel  12  are formed, by example, by mechanically punching out the desired portion of second substrate layer  22 . In a preferred embodiment, second substrate layer  22  is positioned below first substrate layer  16 . 
       FIG. 4  shows a plan view of third substrate layer  24 . Third substrate layer  24  constitutes a bottom layer of chip  2 , as shown in  FIG. 1 . Third substrate layer  24  comprises at least two electrodes  14  in the recessed channel to contact the sample fluid and contacts  20  to connect to testing circuit  50  to measure properties of the sample fluid. In a preferred embodiment, third substrate layer  24  is positioned below first substrate layer  16  and second substrate layer  22 , respectively. Electrodes  14  are positioned under recessed channel  12  to make contact with the sample fluid, as shown in  FIG. 3 , and are preferably cosintered with multilayer ceramic. 
     Due to traditional manufacturing methods for ceramic substrates, traditional metal electrodes begin to deteriorate under the higher temperatures necessary to bond and cure the substrate. Ceramic particles and metal particles coalesce at different temperature ranges and rates during sintering. Therefore, reasonably matching metals and ceramics with similar densification rates helps to obtain controlled part dimensions (outer and feature dimensions), and defect free (cracks/breakage, etc) devices. In the present invention, a cordierite based glass ceramic is preferred as the base device material and a copper+nickel+glass ceramic is preferred as the conductor material. The nickel and copper combination helps to avoid corrosion during use and storage of the chip, as chemical reactions, such as corrosion, negatively interfere with measurement. Additionally, the maximum sinter temperature in a preferred embodiment is less than approximately 1000 degrees C. 
     Referring again to  FIGS. 1-4 , operation of a sample receiving chip  2  will now be described in greater detail. During operation, a relatively small amount of sample fluid is deposited into first port  8 . In a preferred embodiment, reliable osmolarity measurement is obtained with a fluid sample volume of less than approximately 30 nL. The sample fluid passes through first port  8  and recessed channel  12  formed in substrate layers  16  and  22 , respectively. First port  8  narrows as the sample liquid passes through first substrate layer  16 , and second substrate layer  22 . The fluid is drawn through first port  8  and recessed channel  12  by venting second port  10 . It can be appreciated that first port  8  and second port  10  of sample receiving chip  2  may be a variety of geometric configurations, so long as first port  8  funnels the sample fluid into recessed channel  12  and second port  10  vents recessed channel  12 . However, the geometries of first port  8 , recessed channel  12 , and second port  10 , can influence fluid flow. Second port  10  can be designed to control the rate at which the sample fluid flows through recessed channel  12 . As shown by  FIG. 1B , additional second port  10  (or any number of additional second ports) can be added to further influence fluid flow through recessed channel  12 . In a preferred embodiment, once the sample liquid is drawn through recessed channel  12  by capillary action, second port  10  becomes partially filled with the sample fluid, the sample fluid being held by surface tension. Furthermore, a hydrophilic substrate surface is preferably used to promote fluid flow through recessed channel  12 . This combination of surface chemistry, channel geometry, and vent geometry is used to control flow uniformity, rate, and residence time. 
     Referring now to  FIG. 5 , a cross sectional view of one embodiment of substrate  4 , including electrode windows  18 , is shown. In this embodiment, recessed channel  12 , containing the sample fluid, flows in a direction perpendicular to electrodes  14 . It can be appreciated however, that different electrode configurations can be used, as long as the sample fluid comes into contact with the electrodes. Also shown in  FIG. 5 , at least two electrode windows  18  provide access to at least two electrodes  14 . An external measurement device (not shown) can be inserted into the openings formed by electrode windows  18  to contact electrodes  14 , via contacts  20 . As a result, the conductivity of the sample fluid may be determined. In alternative embodiments, as shown in  FIGS. 6A-B , at least two electrodes  14  are connected to contacts  20  that extend to and are positioned on an external surface of substrate  4 . As shown by comparing  FIGS. 6A-B , contacts  20  may be positioned on various external surfaces of substrate  4 , so long as electrodes  14  come into contact with the sample fluid flowing through recessed channel  12 . 
     Referring now to  FIG. 7 , a point of care osmolarity testing device  26  is shown. In one embodiment, device  26  for testing osmolarity comprises: base member  28 ; sample receiving chip  2  fixed to base member  28  for receiving a sample fluid; and conduit  30  fixed to base member  28  for depositing the sample fluid on sample receiving chip  2 . Conduit  30  includes first end  31  and second end  33 . It should be noted, that sample receiving chip  2  may be substantially identical to that described above, except for any required mounting structure. In one embodiment, osmolarity testing device  26 , as shown in  FIG. 7 , further includes capillary receptacle  32  including: base unit  34 , including fastener  36  for fixing conduit  30  to base unit  34 , and chamber  38  for receiving first end  31  of conduit  30 . Conduit  30 , containing the sample fluid, may be fastened to capillary receptacle  32 . Chamber  38  includes substantially flexible partition  40 . Device  26  also includes external pressure applying mechanism  42  to apply an external pressure to substantially flexible partition  40  for altering chamber pressure to discharge the sample fluid from second end  33  of conduit  30 . Mechanism  42  may include structure, for example, to pump air, to provide a piezoelectric change that causes flexible partition  40  to expand and contract in a controlled manner, or any other now known or later developed structure to apply a force to substantially flexible partition  40 . 
     Referring again to  FIG. 7 , a preferred method for determining osmolarity of a sample fluid will be described in greater detail. In one embodiment, a method for determining osmolarity of a sample fluid comprises the steps of: communicating a sample fluid through conduit  30  fixed to base member  28 ; and determining osmolarity of the sample fluid. Communicating a sample fluid through conduit  30  may include contacting an in vivo sample of bodily fluid on the human eye, whereby the sample fluid is drawn into conduit  30  by capillary force. Typically, a treating physician opens the lower eyelid of a patient and touches the tear in the tear cavity with conduit  30 . The tear is drawn into conduit  30  by capillary force and held by surface tension. After the sample fluid is collected by conduit  30 , conduit  30  is placed in capillary receptacle  32 . The receptacle contains fastener  36  to isolate first end  31  of conduit  30  extending into chamber  38 . In the present embodiment, the step of communicating also includes applying external pressure  42  to base unit  34 , base unit  34  including chamber  38  for receiving first end  31  of conduit  30 , wherein chamber  38  includes substantially flexible partition  40 . A positive external pressure  42 , such as low-pressure air, is applied to substantially flexible partition  40 . Partition  40  transfers the pressure to chamber  38  and forces the sample fluid out as a drop from second end  33  of conduit  30 . 
     Next, the osmolarity of the sample fluid is determined by a testing circuit  50 . The osmolarity of the sample fluid can be measured by sensing the energy transfer properties of the sample fluid. The energy transfer properties can include, for example, electrical conductivity, such that the impedance of the sample fluid is measured, given a particular current that is transferred into the sample fluid. Testing circuit  50  applies a current source across the electrodes of sample receiving chip  2 . Osmolarity of the sample fluid may be determined by measuring the conductivity of the sample fluid using conductivity measuring device  52  to obtain a conductivity value and converting the conductivity value to a corresponding osmolarity value using conversion system  54  (e.g., by a calibration knowledge base). In this case, testing circuit  50  includes an electrical conductivity measurement circuit  56  to determine osmolarity of the sample fluid. For example, measurement circuitry  56  may provide electrical energy in a specified waveform (such as from a function generator) to the at least two electrodes bridged by the sample fluid. Furthermore, as shown in  FIG. 7 , base member  28  may include a device for communicating results to a user, e.g., a display device  142  for displaying a visual representation of the osmolarity value. Alternatively, the osmolarity results can be communicated and displayed at a remote location in any known fashion. 
     In another embodiment, shown in  FIG. 8 , a treating physician may pre-position a conduit  130  to a base member  128  of an osmolarity testing device  126 . Device  126  is similar to device  26  ( FIG. 7 ) except Conduit  130  is fixed to base member  128  for depositing the sample fluid on sample receiving chip  102 . A tear is then collected from the patient and is drawn into conduit  130  by capillary force. First end  131  of conduit  130  extracts the sample fluid, and second end  133  of conduit  130  deposits the sample fluid on sample receiving chip  102 . Therefore, the method for determining osmolarity of a sample fluid, comprises: communicating a sample fluid through conduit  130  fixed to base member  128  directly to sample receiving chip  102 ; and determining osmolarity of the sample fluid. Furthermore, osmolarity testing device  133  may include hinged-cover  144  to protect conduit  130  and to make handling of device  126  more convenient. In another embodiment, as shown in  FIG. 9 , conduit  130  may be fastened to hinged-cover  44 . It should be noted, that osmolarity testing device  126  can be a hand-held device, allowing for convenient and effective point-of-care treatment. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.