US 20020008032 A1
A device for sensing fluid movement within a microfluidic channel which uses feedback to control its operation. The device measures electric parameters to interpret fluidic parameters such as flow speed, and the presence or absence of fluid within the channel.
1. A microfluidic device, comprising:
a microfluidic channel having an inlet and an outlet;
means associated with said channel and located between said inlet and said outlet, for sensing fluid flow within said channel, and generating electrical signals corresponding to fluidic properties of the fluid flow;
and means, coupled to said sensing means, for controlling fluid flow through said channel as a result of information obtained from said electrical signals from said sensing means.
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means for analyzing said electrical signals from said sensing means, and
means for pumping a fluid into the inlet of said microfluidic channel in response to commands from said analyzing means.
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 This patent application claims benefit from U.S. Provisional Patent Application Serial No. 60/213,865, filed Jun. 23, 2000, which application is incorporated herein by reference.
 1. Field of the Invention
 This invention relates generally to microscale devices for performing analytical testing and, in particular, to a microfluidic cartridge which uses embedded electrodes for feedback control in the operation of its device.
 2. Description of the Prior Art
 Microfluidic devices have recently become popular for performing analytical testing. Using tools developed by the semiconductor industry to miniaturize electronics, it has become possible to fabricate intricate fluid systems which can be inexpensively mass produced. Systems have been developed to perform a variety of analytical techniques for the acquisition of information for the medical field.
 Microfluidic devices may be constructed in a multi-layer laminated structure where each layer has channels and structures fabricated from a laminate material to form microscale voids or channels where fluids flow. A microscale channel is generally defined as a fluid passage which has at least one internal cross-sectional dimension that is less than 500 μm and typically between about 0.1 μm and about 500 μm. The control and pumping of fluids through these channels is affected by either external pressurized fluid forced into the laminate, or by structures located within the laminate.
 The use of electrodes within microfluidic channels for the manipulation of fluids has been practiced extensively in the prior art. U.S. Pat. No. 5,126,022 teaches a device for moving molecules by the application of a plurality of electrical fields by the use of a plurality of electrodes which were placed at regular intervals along a gel-filled channel which produced traveling electrical waves propelling charged particles through the medium within the channel for separation and resolution purposes.
 U.S. Pat. No. 5,989,402 teaches an electrically controlled microfluidic system having an electrical interfere array with a plurality of electrode pins which are oriented for insertion into a plurality of ports. The electrode pins on each electrically coupled to a separate electrical lead, which leads are connected to an electrical control system which concomitantly delivers a voltage to each of the leads.
 U.S. Pat. No. 6,007,690 is directed to a device for performing microchannel electrophoresis in capillaries, in which the main electrophoretic flow path has associated with it at least one pair of electrodes for applying an electric field to the medium present in the flow path, thus providing for precise movement of entities along the flow path.
 U.S. Pat. No. 6,171,850 is directed to a device for performing temperature controlled reactions and analyzes in microfluidic systems. Heat exchangers are fabricated from a material that is both thermally and electrically conductive, so that they can function as both a heat exchanger and an electrode when placed into a fluid filled reservoir.
 It is therefore an object of the present invention to provide a device which uses feedback from a sensing means to control operation of a microfluidic device.
 It is a further object of the present invention to provide a device which measures electric parameters to determine the presence or absence of fluid in microfluidic channels.
 It is a still further object of the present invention to provide a device which uses electrodes within microfluidic channels to measure flow speed and wetout.
 It is still a further object of the present invention to provide a device which uses optical sensors located in proximity to microfluidic channels to measure wetout.
 These and other objects of the present invention will be more readily apparent in the description and drawings that follow.
FIG. 1 is a representation of a flow sensor according to the present invention used within a microfluidic channel;
FIG. 2 is an enlarged view of the sensor and sensor holder of the flow assembly shown in FIG. 1;
FIG. 3 is an exploded view of the sensor carrier device shown in FIG. 1;
FIG. 4 is an assembled view of the device shown in FIG. 3; and
FIG. 5 is a plan view of an optical sensor assembly according the present invention.
 Referring now to FIG. 1, there is shown a fluid sensor assembly, generally indicated at 10, according to the present invention. Assembly 10 uses a fluid sensor 12, which is located within a microfluidic channel 14 to monitor the flow of fluids within channel 14. Fluid is provided to channel 14 via a fluid driving and control means 16. Means 16, which may contain a separate fluid supply or merely pump fluid from another circuit into channel 14, is coupled to channel 14 at a channel inlet 18. The opposite end of channel 14 comprises a channel outlet 20, which end may be coupled into additional microfluidic circuitry.
 Sensor 12 is electrically coupled to driving and control means 16 by a cable 22. Cable 22 provides electrical signals from sensor 12 to means 16 relative to fluid flow within channel 14 at the location where sensor 12 is positioned. These signals from sensor 12 may be used by means 16 to measure specific electric parameters, such as conductivity or capacity across channel 14. In addition, these electric parameters can be used to interpret fluidic parameters, such as flow speed, or the presence or absence of fluid within channel 14.
 Means 16 is capable of analyzing signals received from sensor 12, and adapting the flow within channel 14 in response to information received from sensor 12 via cable 22. This use of feedback signals to control the operation of assembly 10 may be implemented by a computer, programmable controller, or any other device well known to persons familiar with this art.
 Fluid flow within channel 14 may be in the form of a liquid or a gas, and sensor 12 may be located at any point within channel 14, or may consist of multiple sensors located at different regions of channel 14. Means 16 may detect a gradient between different regions of channel 14 and react to the differences, such as differences in flow speed or conductivity. As means 16 analyzes the information returned from sensor 12, it reacts to adjust the operation of fluid driving within channel 14.
 Sensor 12 may consist of a single electrode, a series of electrodes, an optical sensing device, or even a hot wire anemometer capable of monitoring temperature changes within the fluid flowing within channel 14. Many sensing devices capable of performing these operations are well known in the art.
FIG. 2 illustrates an embodiment of the sensor and sensor holder which may be used in conjunction with the present invention. A sensor 30 is shown having a pair of electrodes 32, 34 embedded within the body of sensor 30. Sensor 30 is adapted to be inserted into an opening 36 within a sensor holder 38. Opening 36 is sized such that sensor 30 slides into said opening from the rear and is prevented from sliding out of the top by extensions 38 a of holder 38.
FIG. 3 is an exploded view of a sensor for use in the present invention. Sensor carrier device, generally indicated at 50, consists of an upper layer 52, a central layer 54, and a bottom layer 56. Upper layer 52 includes a plurality of apertures 60, 62, 64. Aperture 64 allows sensor 30 to monitor conditions within a microfluidic channel in which carrier device 50 is mounted, while apertures 62, 64 provide access for the cabling to operate fluid sensor assembly 10. Layer 54 includes a cutout section 66 for accommodating sensor 30 within carrier device 50, while bottom layer 56 is used to hold sensor 30 in its proper position for operation. The assembled sensor carrier device 50 can be seen in FIG. 4.
FIG. 5 illustrates the use of an optical sensor to determine the wetout of a microfluidic channel. Light source 70 is positioned in optical proximity to channel 71 such that it illuminates a portion of channel 71. As channel 71 is filled, meniscus 72 creates an optically detectable signal (e.g., absorption or light scattering), which is picked up by detector 73. The detector signal is then fed back through leads 74 into fluid driver 75 to control the flow.
 While the present invention has been shown and described in terms of several preferred embodiments thereof, it will be understood that this invention is not limited to these particular embodiments and that many changes and modifications may be made without departing from the true spirit and scope of the invention as defined in the appended claims.