US 20080053206 A1
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.
1. 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.
2. The method of
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 cosintered with multilayer ceramic positioned in the substrate to contact the sample fluid in the recessed channel to measure properties of the sample fluid.
3. The method of
4. The method of
5. The method of
The current application is a divisional application of co-pending U.S. patent application Ser. No. 11/163,327, filed on Oct. 14, 2005, which is hereby incorporated by reference.
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.
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'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.
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.
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
FIGS. 6A-B show a cross sectional view of the electrode contacts positioned on different surfaces the sample receiving chip of
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.
When the various substrate layers shown in
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
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
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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
In another embodiment, shown in
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.