Microelectronic fluid detector

A resistive microelectronic fluid sensor implemented as an integrated voltage divider circuit can sense the presence of a fluid within a fluid reservoir, identify the fluid, and monitor fluid temperature or volume. Such a sensor has biomedical, industrial, and consumer product applications. After fluid detection, the fluid can be expelled from the reservoir and replenished with a fresh supply of fluid. A depression at the bottom of the sample reservoir allows a residual fluid to remain undetected so as not to skew the measurements. Electrodes can sense variations in the resistivity of the fluid, indicating a change in the fluid chemical composition, volume, or temperature. Such fluctuations that can be electrically sensed by the voltage divider circuit can be used as a thermal actuator to trigger ejection of all or part of the fluid sample.

BACKGROUND

Technical Field

The present disclosure relates to microelectronic fluid sensors on board integrated circuit chips.

Description of the Related Art

Fluids can be integrated with microelectronic circuitry in many applications including, for example, biomedical devices configured to perform physiological tests on samples of bodily fluids, and thermal actuators that eject fluid in response to changes in temperature.

Approximately 25-30 million people in the United States are diagnosed with type II diabetes requiring monitoring of blood glucose at least twice daily. A common example of a biosensor device that relies on a microelectronic fluid sensor is a disposable test strip used for measurement of blood glucose levels in diabetic patients as shown inFIG. 1. A typical blood glucose monitoring apparatus80includes a conventional disposable strip biosensor82and a portable electronic monitor83. The biosensor82is made of a semi-rigid backing material84approximately an inch long, impregnated with an electrolytic chemical reagent86at one end and printed with electrodes88at the other end. The patient pricks a fingertip, applies a blood sample89to the electrolytic chemical reagent86, and inserts the electrodes88into the portable electronic monitor83. The electrolytic chemical reagent86conducts a current that is proportional to an amount of glucose in the blood sample89. Current flow conducted via the electrodes88in the biosensor82closes a circuit when the biosensor82is inserted into the portable electronic monitor83. The current in the circuit can then be measured by the portable electronic monitor83. The portable electronic monitor83is configured with software that converts the current measurement into a numerical value that represents the blood glucose level. The portable electronic monitor83then provides a digital readout of the numerical value and stores the numerical value as blood glucose data in an electronic memory. By either recording or downloading the blood glucose data, the patient can track blood glucose values over time to adjust insulin dosage.

Use of an impregnated biosensor strip is problematic for several reasons. The chemical reagent86may degrade over time such that the biosensor strip has a finite shelf life and must be stamped with an expiration date. In addition, this type of biosensor strip is expensive, and available only on a prescription basis, as opposed to being an item that is sold over-the-counter. When the liquid sample is applied, sometimes the strip fails to take up enough of the liquid volume to make an accurate reading, and the test must be repeated, which incurs even more expense. Finally, the strips are disposable and cannot be re-used.

Another type of fluid detector that can be used to detect electrical properties of a fluid sample such as the blood sample89, uses an open fluid reservoir instead of an impregnated reagent. An example of such a detector is a capacitive fluid detector as shown and described below with respect toFIGS. 2A, 3A, and 3B. Such a capacitive fluid detector transmits electrical signals through the fluid sample in the reservoir. The electrical signals can be compared against previous signals or an independent standard to detect changes. Changes in the electrical signals can indicate the presence or absence of fluid in a region between two electrodes, for example. Once presence of the fluid sample is detected, further changes in such signals can indicate fluctuating levels of fluid components that are charge-dependent such as glucose, electrolytes, or ions such as calcium, magnesium, potassium, and the like. The electrical signal data can then be sent to a microprocessor to calculate corresponding electrical properties of the fluid sample.

Depending on the design of the sensor, such a capacitive detection system may provide information regarding the presence of fluid, or the presence of certain components within the fluid, but not necessarily information regarding an amount of fluid present. For example, if the fluid participates in the circuit as part of a capacitive electrode rather than part of a capacitive dielectric, the capacitor geometry may not allow distinguishing between a small volume of fluid and a large volume. In the parallel plate capacitor sensor described above, the fluid is typically incorporated as a portion of one of the electrodes. However, where the fluid is incorporated as the dielectric, or a portion of the dielectric, it becomes possible to identify the fluid based on the dielectric constant. Such an arrangement is not feasible in the case of a capacitive sensor, however, because the dielectric, being sandwiched between two metal plates, is not easily accessible for introduction of a fluid sample by a user. Furthermore, capacitive sensor measurements may be affected by parasitic capacitances elsewhere in the circuit that are not actually related to the fluid sample and can therefore skew the test results. For at least these reasons, it may be desirable to have other types of fluid sensors available on an integrated circuit chip in addition to, or in place of, capacitive fluid sensors.

BRIEF SUMMARY

A resistive microelectronic fluid detection system can determine more information about a fluid than a typical capacitive fluid detector can, and with greater accuracy, in part because the resistive fluid detector is not subject to parasitic effects. In addition to sensing the presence of a fluid in a reservoir, a resistive microelectronic fluid sensor can identify the fluid and determine its volume.

Embodiments of a resistive microelectronic fluid detector can be built so as to include a fluid reservoir that is at least partially exposed. An electrical signal can then be applied laterally across the fluid reservoir. Presence or absence of a fluid can then be determined directly and with less influence from external factors, for example, by applying a voltage and measuring whether or not a current flows through the sample reservoir. If a current flows and closes the circuit, it can be deduced that a sample is present. In one embodiment of a resistive microelectronic fluid detector, the bottom of the sample reservoir is modified with a depression so that a certain volume of fluid can be present without conducting the electrical signal. Using such a modified reservoir, when a signal is detected, it is known that the volume of the sample is above a certain threshold value.

In another embodiment, if a second resistor is constructed adjacent to the fluid reservoir, simple equations for a voltage divider circuit allow determination of the fluid sample volume if the fluid resistivity is known. Conversely, identification of the fluid is possible if the volume of the sample is known.

In another embodiment, the sample reservoir can be extended vertically and capped to form a partially or substantially enclosed chamber within a microfluidic ejection system. In such a system, detection of a threshold volume of fluid within the reservoir can trigger ejection of the fluid sample out of the chamber, through a nozzle in the cap. Electrodes contacting the fluid within the reservoir can further be used to sense differences or variation in the resistivity of the fluid, indicating a different type of fluid is present, or a change in the fluid chemical composition or the fluid volume. In some applications, for example, changes in resistivity of the fluid can indicate temperature fluctuations. Such fluid characteristics that can be electrically sensed by a voltage divider circuit can in turn be used as a thermal actuator to trigger ejection of all or part of the fluid sample.

The fluid detector described herein may be used in conjunction with a universal flexible micro-sensor as described in U.S. Patent Publication No. 2015/0253276.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods of semiconductor processing comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure.

Reference throughout the specification to integrated circuits is generally intended to include integrated circuit components built on semiconducting substrates, whether or not the components are coupled together into a circuit or able to be interconnected. Throughout the specification, the term “layer” is used in its broadest sense to include a thin film, a cap, or the like and one layer may be composed of multiple sub-layers.

Reference throughout the specification to conventional thin film deposition techniques for depositing silicon nitride, silicon dioxide, metals, or similar materials include such processes as chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), metal organic chemical vapor deposition (MOCVD), plasma-enhanced chemical vapor deposition (PECVD), plasma vapor deposition (PVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), electroplating, electro-less plating, and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. For example, in some circumstances, a description that references CVD may alternatively be done using PVD, or a description that specifies electroplating may alternatively be accomplished using electro-less plating. Furthermore, reference to conventional techniques of thin film formation may include growing a film in situ. For example, in some embodiments, controlled growth of an oxide to a desired thickness can be achieved by exposing a silicon surface to oxygen gas or to moisture in a heated chamber.

Reference throughout the specification to conventional photolithography techniques, known in the art of semiconductor fabrication for patterning various thin films, includes a spin-expose-develop process sequence typically followed by an etch process. Alternatively or additionally, photoresist can also be used to pattern a hard mask (e.g., a silicon nitride hard mask), which, in turn, can be used to pattern an underlying film.

Reference throughout the specification to conventional etching techniques known in the art of semiconductor fabrication for selective removal of polysilicon, silicon nitride, silicon dioxide, metals, photoresist, polyimide, or similar materials includes such processes as wet chemical etching, reactive ion (plasma) etching (RIE), washing, wet cleaning, pre-cleaning, spray cleaning, chemical-mechanical planarization (CMP) and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. In some instances, two such techniques may be interchangeable. For example, stripping photoresist may entail immersing a sample in a wet chemical bath or, alternatively, spraying wet chemicals directly onto the sample.

Specific embodiments are described herein with reference to microelectronic fluid sensors and fluid ejection devices that have been produced; however, the present disclosure and the reference to certain materials, dimensions, and the details and ordering of processing steps are exemplary and should not be limited to those shown.

FIG. 2Ashows a fluid detection circuit100that provides context for a conventional capacitive fluid sensor102, according to the prior art. When nodes1and2are energized, a transistor103turns on and applies a signal to the conventional capacitive fluid sensor102. The capacitance value is sensed and this provides an indication of the fluid being tested.

FIG. 2Bshows a resistive microelectronic fluid detector104, as described herein, that is proposed to replace the conventional capacitive fluid sensor102, for example, adjacent to nodes1and2in the circuit100. In one embodiment, the resistive microelectronic fluid detector104is in the form of a voltage divider that includes a device under test (DUT)106coupled in parallel to a known reference device108having a resistance R that is much larger than the resistance r of the DUT106. For example, R can be on the order of 10-100 kΩ, while r can vary over a range of a very high value (several GΩ or higher) when the fluid under test is not present, to a lower value, such as approximately 1 kΩ when the fluid under test is present.

FIG. 3shows an integrated circuit embodiment of the conventional capacitive fluid sensor102. The integrated capacitive fluid sensor102is in the form of a parallel plate capacitor that includes a bottom electrode112, a dielectric114, and a top electrode116. The bottom and top electrodes112and116are generally metallic while the dielectric114can be made of a passivation material or another dielectric available in a semiconductor fabrication process such as an inter-layer dielectric (ILD) material. The top electrode116includes a fluid reservoir118intended to contain a volume of electrolytic fluid under test, for example, a blood sample, a DNA sample, an industrial chemical sample, or the like. The electrical conductivity of the fluid sample in the fluid reservoir118is thus exploited to engage the fluid sample as an integral part of the top electrode116of the parallel plate capacitor. If the fluid reservoir118is empty, the capacitance of the parallel plate capacitor will be reduced proportionally, according to the relationship C=KA/d, wherein A is the surface area of the top electrode116, K is the dielectric constant of the dielectric114, and d is the thickness of the dielectric114. When the fluid reservoir118is empty, the electrode surface area A is reduced. Therefore, the presence or absence of fluid in the fluid reservoir118is detectable by the conventional integrated capacitive fluid sensor102. However, information regarding an amount of fluid present in the fluid reservoir118is not provided by such a conventional capacitive sensor.

A microelectronic fluid detector104, as described herein, is realized as a miniature integrated voltage divider circuit that is proposed as a substitute sensor to use in place of the conventional integrated capacitive fluid sensor102. The integrated circuit voltage divider includes a known reference element (R) in the form of a resistive thin film layer, and an unknown resistance in the form of a fluid reservoir126(r), positioned between two electrodes. The resistance r varies with changes in the fluid present in the reservoir126.

Two embodiments of such an integrated circuit voltage divider,104aand104b, are presented herein inFIGS. 4B and 4C, andFIGS. 6B and 6C, respectively.FIG. 4Areproduces the schematic representation of the voltage divider.FIG. 4Ais shown for comparison against the top plan view shown inFIG. 4Band the cross-sectional view shown inFIG. 4C. During operation of the resistive microelectronic fluid detector104, an electrical signal is applied at electrodes120and122to test a fluid sample, which is the device under test106.

FIGS. 4B and 4Cshow different views of the first embodiment104aof the microelectronic fluid detector104. With reference toFIGS. 4B and 4C, the first embodiment104ais one integrated circuit implementation of the resistive microelectronic fluid detector104. The first embodiment104aof the voltage divider circuit104includes a high resistance material131as the reference device (R) and a fluid reservoir126as the resistor r. The two resistors are coupled by electrodes123aand123b, and by vias120and122through a first insulator133. A second insulator124separates the electrodes123aand123bso that they are laterally spaced apart from one another, and can be coupled through the fluid in the reservoir126. The first embodiment104acan be built on a semiconductor substrate119, according to steps in an exemplary method130, as shown in FIG.5. Alternatively, the substrate119can be an insulator, such as a polymer layer, a polyimide layer, a glass layer, or the like.

At132, a high resistance material131that serves as the reference device108is formed overlying the substrate119. In one embodiment, the high resistance material131is a high resistance conductor such as silicon carbide (SiC). The high resistance material131could also be a lightly doped polysilicon or other high resistance material. In other embodiments, the material serving as the reference device108can be a passivation material such as a doped or conductive polyimide that can be spun onto the substrate in liquid form. Alternatively, another insulator with some conductive properties can be deposited using a conventional thin film deposition process.

At134, the first insulator133is formed on top of the high resistance material131by depositing an inter-layer dielectric (ILD) material such as, for example, silicon dioxide (SiO2) using a conventional thin film deposition process.

At136, vias are patterned in the first insulator133using a conventional photolithography and etching process sequence.

At138, the vias are filled with metal to form coupling wires120and122.

At140, a metal layer is deposited on top of the first insulator133. The metal layer can be made of copper, silver, platinum, gold, titanium, tungsten, and the like, or alloys thereof.

At142, a first portion of the metal layer is removed and filled with a second insulator124as shown inFIG. 4B, thus creating electrodes123aand123b. The second insulator124can be made from an oxide or nitride material, for example. If the height of the second insulator124, as deposited, exceeds that of the electrodes, the top surface of the device can be planarized by polishing the insulator material and stopping on the surface of the electrodes. Otherwise, the electrodes can be polished to stop on the second insulator124. The second insulator124ensures that the electrodes123a,123bremain spaced apart from one another so that an applied signal will be conducted through the fluid sample as the device under test106, and not be short-circuited between the electrodes123a,123b.

At144, a second portion of the metal layer is removed, for example by etching, to create a fluid reservoir126. Thus, the electrodes123a,123band the second insulator124together form walls that bound the fluid reservoir126on four sides, while the first insulator133forms a floor that bounds the fluid reservoir126from below. The reservoir126can extend below the dielectric-metal interface, into the first insulator133in one embodiment, but this is not required.

The second embodiment104bof the voltage divider circuit104also includes a high resistance material131as the reference device (R) and a fluid reservoir126as the resistor r. The two resistors are coupled by electrodes120and122which are formed with lower portions120aand122aon either side of the reference device R and upper portions120band122bon either side of the fluid reservoir126(r). The second insulator separates the upper portions of the electrodes120band122bso that they are laterally spaced apart from one another, and can be coupled through the fluid in the reservoir126.

FIG. 6Areproduces the schematic representation of the resistive microelectronic fluid detector104in the form of a voltage divider that is proposed as a substitute sensor to use in place of the conventional integrated capacitive fluid sensor102.FIG. 6Ais provided for comparison against the top plan view shown inFIG. 6Band a cross-sectional view shown inFIG. 6Cof a second embodiment104bof the resistive microelectronic fluid detector104. The second embodiment104bcan be built on the substrate119, according to a method150, as shown inFIG. 7.

At152, a high resistance material131that serves as the reference device108is formed on top of the substrate119, using a conventional thin film deposition process. In one embodiment, the reference device108has such a high resistance R, it conducts little to no current and, thus, could be considered an open circuit. In other embodiments, the reference device108is a high resistance conductor, such as SiC or lightly doped polysilicon.

At154, a first insulator133is formed in contact with the high resistance material131.

At156, trenches are patterned in the first insulator133using a conventional photolithography and etching process sequence, to create an insulator block that provides vertical separation between the device under test106from the reference device108.

At158, the trenches are filled with metal to form lower portions of the electrodes,120aand122a, shown inFIG. 6C.

At160, an additional metal layer is deposited on top of the filled trenches which will form upper portions of the electrodes120band122b, shown inFIG. 6C.

At162, a first portion of the additional metal layer is removed and filled with a second insulator124. The second insulator124can be made from the same material as the first insulator133, or it can be made of a different material. If the height of the second insulator124, as deposited, exceeds that of the electrodes, the top surface of the device can be planarized by polishing the second insulator124and stopping on the upper portions of the electrodes120b,122b. Otherwise, the electrodes can be polished to stop on the second insulator124. The second insulator124ensures that the electrodes120b,122bremain spaced apart from one another so that an applied signal will be conducted through the fluid sample as the device under test106, and not be short-circuited between the electrodes.

At164, a second portion of the metal layer is removed to create the fluid reservoir126. Thus, the electrodes120b,122band the second insulator124together form walls that bound the fluid reservoir126on four sides, while the high resistance material131forms a floor that bounds the fluid reservoir126from below.

FIG. 8shows a microelectronic fluid sensor system166including a microprocessor168and an electronic memory169, both of which can be located on the same integrated circuit chip as the microelectronic fluid detector104that includes the device under test106and the reference device108. The electronic memory169stores instructions for execution by the microprocessor168to test fluid samples. In addition, the electronic memory169stores other data such as, for example, material information including resistivity values, material constants, and the like to support calculations such as those described herein.

There are two alternative embodiments to use for the substrate119when it takes the form of a semiconductor substrate. In a first embodiment, the semiconductor substrate119has, in addition to a foundation of a semiconductor substrate layer, electronic circuits formed therein. Such electronic circuits include transistors having source and drain regions and electrical interconnections coupling them to form logic gates. In this embodiment, the microprocessor, memory and other electronic circuits are formed in the semiconductor substrate119, as well as numerous other layers overlying the semiconductor substrate119itself which are not specifically shown inFIGS. 4B and 4C. For example, there will be numerous interconnect metal layers and insulating layers which overlay the transistors formed in the substrate itself. Accordingly, reference to the semiconductor substrate119includes such integrated circuits as a whole, that is to say, the transistors and the interconnect layers connecting them. The numerous layers making up the transistors and the interconnect structure are omitted herein for ease of reference, and it is well understood by those of skill in art how to build an integrated circuit overlying a semiconductor substrate119.

According to one embodiment, the high resistance material131will be one of the top most metal layers of the integrated circuit which overlays the semiconductor substrate119. The various insulating and metal interconnect layers which form the microprocessor are formed on a different part of the die than that section shown inFIG. 4Cand, thus, are not shown for ease in illustration. In most embodiments, there will be an insulating layer positioned between the conductive high resistance material131and the semiconductor substrate119itself and, as just explained herein, there may be numerous alternating conductive and insulating layers which electrically isolate the high resistance material131from the semiconductor substrate itself.

In an alternative embodiment, the substrate119is composed of a fully insulating material instead of being a semiconductor substrate. For example, the substrate119may be a polymer, a polyimide film or other supporting substrate which is capable of having a high resistance material131formed thereon. One technique by which the sensor104can be formed in which the substrate119is a polymer layer is described in U.S. patent application Ser. No. 14/200,828, incorporated herein by reference in its entirety.

FIG. 9shows steps carried out by the microprocessor168in an exemplary method170of operating the microelectronic fluid detector104, according to one embodiment.

At171, the fluid reservoir126is tested by applying an electrical signal to the voltage divider. For example, a voltage can be applied between electrodes120and122by coupling a power source to the electrodes120and122.

At172, a current is measured between the electrodes120and122. If the fluid reservoir126is empty, all the current will flow through the reference device108, according to the relationship
I=Vapplied/R,(1)
and no current will flow through the resistor r, which is the fluid reservoir126. On the other hand, when the fluid reservoir126contains a fluid sample, the total electric current is divided so that some current flows through each of the fluid reservoir126(r) and the insulator block (R). The total current is thus given by
I=Vapplied(1/R+1/r)  (2)

At173, based on the measured value of I, it is determined by the microprocessor whether or not a current is flowing through the fluid reservoir.

At174, if a current is flowing through the reservoir, it is concluded that a fluid is present in the fluid reservoir126.

At175, given that a fluid is present in the fluid reservoir126, the resistance of the fluid sample, r, can be computed from equation (2) above, wherein V is applied, I is measured, and R is a known resistance that can be computed from the known geometry and material parameters of the high resistance material131.

At176, since the geometry of the fluid reservoir126is known, a resistivity, ρ, of the fluid sample can be calculated according to the well-known relationship
r=ρL/A(3)
wherein A is a surface area of the reservoir transverse to the direction of current flow, and L is the length of the reservoir. It is assumed, in the present embodiment, that the fluid sample substantially fills the reservoir126.

At177, once the resistivity is known, a lookup table of various material parameters stored in the electronic memory169can be consulted to identify the type of fluid present in the fluid reservoir126.

If the type of fluid is known,FIG. 10shows steps carried out by the microprocessor168in an exemplary method180of operating the microelectronic fluid detector104, to determine fluid volume of a known fluid sample, according to one embodiment.

At182, an electric current is applied to the microelectronic fluid detector104, wherein the fluid reservoir126contains a sample of an identified fluid.

At184, a voltage is measured across the fluid reservoir126.

At186, the resistance, r, of the fluid sample is computed from equation (2).

At188, the volume of the fluid sample is computed as V=AL wherein A is known and L is determined from equation (3).

FIGS. 11A and 11Bshow a top plan view and a cross-sectional view, respectively, of a microfluidic ejection system190according to one embodiment as described herein. The microfluidic ejection system190includes elements of the microelectronic fluid sensor104adescribed above, including the high resistance material131used as the reference device108, the electrodes120and122, the second insulator124, and the fluid reservoir126.

In the microfluidic ejection system190, the lower boundary of the fluid reservoir126further includes a depression192that allows a small volume of fluid193to be captured on the bottom of the reservoir while remaining isolated from the electrodes120and122. In addition, the microfluidic ejection system190further includes an encapsulant194on top of the electrodes, the encapsulant enclosing a chamber196located directly above the reservoir126. The chamber196is then substantially enclosed by a cap198. In the embodiment shown inFIG. 10B, the encapsulant194and the cap198are both made of the same flexible insulating material, which can be a polymer or a dielectric.

The cap198includes a nozzle200substantially vertically aligned with the depression192, through which fluid in the reservoir126can be ejected in response to a signal from a controller, for example, the microprocessor168. Following ejection of fluid, the reservoir126can be refilled via microfluidic channels from a fluid supply external to the microfluidic ejection system190.

FIG. 12illustrates an exemplary method200of operating the microfluidic ejection system190that can be carried out by a controller such as the microprocessor168, for example. The basic operation of the microfluidic ejection system190as shown includes a probe to obtain information about the fluid in the reservoir, to determine whether or not to eject the fluid and refill the reservoir, or to allow the fluid to remain in the reservoir.

At202, a probe of the fluid in the reservoir126is activated to determine information about the fluid sample using the resistive fluid detector104within the microfluidic ejection system190. For example, at204, the type of fluid is verified; at206, the fluid temperature is sensed and evaluated; and at208the volume of fluid in the reservoir126is determined and evaluated. Each of these determinations is described in further detail below.

At204, if the fluid in the reservoir126is not known, a fluid verification method can be executed to identify the fluid and decide whether or not to clear the reservoir126. First, the fluid is identified using the method160described above.

At210, it is determined whether or not the fluid present in the reservoir matches a desired fluid that is expected to be present.

At212, if the desired type of fluid is present, the identification method160can be repeated continuously or periodically to automatically monitor the fluid sample for compositional changes.

At214, if the fluid is not the correct type of fluid, for example, if the expected fluid is a whole blood sample, while measured sample has a different resistivity that matches blood plasma, fluid can be ejected from the reservoir.

At216, the reservoir can be re-filled from an external supply. To do this, the microprocessor168can send a signal to the external supply to release a sample into a microfluidic channel for delivery into the fluid reservoir126.

In parallel with identification of the fluid in the reservoir, a thermal actuation method206can be executed to monitor the fluid temperature and, in response to changes in the temperature, clear the reservoir126.

First, the fluid temperature is sensed using the method160described above to measure resistivity. At step172of the method160, instead of interpreting the resistivity as a particular type of fluid, the resistivity can be correlated to a fluid temperature.

At218, it is determined whether or not the fluid temperature exceeds a maximum or a minimum threshold temperature. If the threshold is not exceeded, the method160can be repeated continuously or periodically to automatically monitor the fluid sample for changes. It is noted that heat dissipated by the reference device108can be a source of thermal excitation of the fluid under test within the reservoir126.

At214, if the temperature of the fluid sample is not within a desired range, some or all of the fluid can be ejected from the reservoir126.

At216, the reservoir126can be re-filled from an external supply.

Additionally or alternatively, a volume actuation method208can be executed to adjust the volume of fluid in the reservoir126.

First, the fluid volume is measured using the method180described above.

At220, it is determined whether or not the volume of the fluid present in the reservoir126exceeds a volume setpoint. If the setpoint is not exceeded, the method180can be repeated continuously or periodically to automatically monitor the fluid volume for changes.

At214, if the volume of the fluid sample exceeds the setpoint, some or all of the fluid can be ejected from the reservoir126. The reference device108can be pulsed so as to effectively trigger a thermally-induced fluid ejection at regular intervals.

At216, the reservoir126can be refilled with fresh fluid from an external supply. Alternatively, a user can be alerted to supply a fresh fluid sample instead of feeding the reservoir from an external supply.

By executing one or more of the methods202,204, and206, it is possible to monitor fluid samples for quality, or to perform an electrolytic verification analysis of the fluid sample prior to performing other biological or chemical testing. Such monitoring can also detect errors in sample preparation so as to reduce the number of false negative results obtained by subsequent biochemical testing using an incorrect sample, or an insufficient volume of the sample. Based on the results of monitoring, if the fluid sample fails to meet standards and is ejected, a message can be displayed or transmitted to a user to communicate the status of the sample. Furthermore, monitor data can be recorded in the memory169for subsequent statistical analysis. In addition, a fluid sensor that contains a reservoir and an ejection mechanism can be re-usable as opposed to disposable.

So, in addition to the improved form factor that a sensor reservoir offers over that of an impregnated sensor strip, it has been demonstrated herein that an integrated resistive sensor can be significantly more useful than an integrated capacitive sensor. In general, more information can be gathered by the resistive sensor and, furthermore, such additional information can be used to control operation of the sensor itself.