Enhanced microfluidic electromagnetic measurements

Abstract

Techniques for enhanced microfluidic impedance spectroscopy include causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid. Flow in the channel is laminar. A dielectric constant of a fluid constituting either sheath flow is much less than a dielectric constant of the core fluid. Electrical impedance is measured in the channel between at least a first pair of electrodes. In some embodiments, enhanced optical measurements include causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid. An optical index of refraction of a fluid constituting either sheath flow is much less than an optical index of refraction of the core fluid. An optical property is measured in the channel.

What is claimed is: 
     
         1 . A method comprising:
 causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a dielectric constant of a fluid constituting either sheath flow is substantively less than a dielectric constant of the core fluid, and wherein flow in the channel is laminar; and   determining impedance in the channel based on measurements at a first pair of one or more pairs of electrodes.   
     
     
         2 . A method as recited in  claim 1 , further comprising determining impedance of a particle in the core fluid. 
     
     
         3 . A method as recited in  claim 2 , wherein determining impedance of the particle in the core fluid further comprises determining a temporal change in measured impedance in the channel between the first pair of electrodes. 
     
     
         4 . A method as recited in  claim 2 , wherein determining impedance of the particle in the core fluid further comprises determining a difference between a first determined impedance in the channel at the first pair of electrodes and a second determined impedance at a different second pair of electrodes disposed along the channel separately from the first pair of electrodes. 
     
     
         5 . A method as recited in  claim 2 , wherein a largest spatial dimension of the particle is much less than a narrowest spatial dimension of the channel. 
     
     
         6 . A method as recited in  claim 2 , wherein a largest spatial dimension of the particle is not substantively greater than a narrowest spatial dimension of the core fluid. 
     
     
         7 . A method as recited in  claim 2  wherein the particle is a platelet. 
     
     
         8 . A method as recited in  claim 7 , further comprising distinguishing an activated platelet and resting platelet based, at least in part, on impedance determined for the particle. 
     
     
         9 . A method as recited in  claim 1 , wherein determining impedance in the channel at a first pair of electrodes further comprises determining impedance in the channel at the first pair of electrodes at a plurality of alternating current frequencies. 
     
     
         10 . A method as recited in  claim 9 , wherein the plurality of alternating current frequencies span a frequency range greater than about 100 megahertz (MHz). 
     
     
         11 . A method as recited in  claim 1 , wherein causing the core fluid to flow into the channel between two sheath flows further comprises controlling a narrowest spatial dimension of the core by controlling relative pressure or flow rate of a core fluid compared to a corresponding pressure or flow rate of one or more of the sheath fluids. 
     
     
         12 . A method as recited in  claim 11 , wherein controlling relative pressure or flow rate of the core fluid compared to the corresponding pressure or flow rate of the one or more of the sheath fluids further comprises controlling the relative pressure or flow rate to stabilize a measurement of a property of the core. 
     
     
         13 . A method as recited in  claim 12 , wherein the measurement of the property of the core is impedance determined in the channel at the first pair of electrodes. 
     
     
         14 . A method as recited in  claim 12 , wherein the measurement of the property of the core is impedance determined in the channel at the first pair of electrodes when a particle is absent from the channel between the first pair of electrodes. 
     
     
         15 . A method as recited in  claim 12 , wherein the measurement of the property of the core is impedance determined in the channel at the first pair of electrodes at a particular set of one or more alternating current frequencies. 
     
     
         16 . A method as recited in  claim 12 , wherein the measurement of the property of the core is the width of the core between the two sheath flows in the channel in a vicinity of the first pair of electrodes. 
     
     
         17 . A method as recited in  claim 12 , wherein the measurement of the property of the core is based on a measurement of a property of an internal control. 
     
     
         18 . A method as recited in  claim 17 , wherein the internal control is a plurality of known particle of uniform properties. 
     
     
         19 . A method as recited in  claim 1 , wherein the core fluid comprises substantively polar molecules and the sheath fluid comprises substantively non-polar molecules. 
     
     
         20 . A method as recited in  claim 19 , wherein a width of the core between the sheath flows in the channel is controlled, at least in part, by a width of a strip of material with affinity for the polarity of the core fluid in at least one of a top wall or a bottom wall of the channel. 
     
     
         21 . A method as recited in  claim 19 , wherein a width of the core between the sheath flows in the channel is controlled, at least in part, by topographical features on at least one of a top wall or a bottom wall of the channel, which features extend into the channel. 
     
     
         22 . A method as recited in  claim 1 , wherein the sheath fluid is mineral oil and the core fluid is an aqueous mixture. 
     
     
         23 . A method as recited in  claim 1 , wherein the sheath fluid is a fluorocarbon solvent and the core fluid is an aqueous mixture. 
     
     
         24 . A method as recited in  claim 1 , wherein the sheath fluid is a gas. 
     
     
         25 . A method as recited in  claim 24 , wherein the gas is air. 
     
     
         26 . A method as recited in  claim 1 , wherein the method further comprises measuring an optical property in the channel. 
     
     
         27 . A method as recited in  claim 1 , wherein the core fluid has a higher index of refraction for optical waves than do the one or more sheath fluids. 
     
     
         28 . A method as recited in  claim 27 , wherein measuring the optical property further comprises directing incident light to produce substantively total internal reflection within a core flow that encompasses the core fluid. 
     
     
         29 . A method as recited in  claim 1 , further comprising determining impedance in the channel at a plurality of pairs of electrodes disposed separately along the channel and disposed separately from the first pair of electrodes. 
     
     
         30 . A method as recited in  claim 2 , wherein:
 the method further comprises introducing a plurality of uniform particles of known impedance value into the core flow as an internal control; and,   determining impedance of the particle in the core fluid further comprises normalizing features of measured impedance based, at least in part, on measured value of impedance for at least one particle of the internal control.   
     
     
         31 . A method comprising:
 causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein an optical index of refraction of a fluid constituting either sheath flow is much less than an optical index of refraction of the core fluid; and   measuring an optical property in the channel between an optical source and an optical detector.   
     
     
         32 . A method as recited in  claim 31 , further comprising determining impedance in the channel based on measurements at a first pair of one or more pairs of electrodes. 
     
     
         33 . A method as recited in  claim 31 , wherein measuring the optical property further comprises directing incident light to produce substantively total internal reflection within a core flow that encompasses the core fluid. 
     
     
         34 . A method as recited in  claim 31 , further comprising determining a property of a core flow that encompasses the core fluid based on the optical property measured. 
     
     
         35 . A method as recited in  claim 31 , further comprising determining a property of a particle in a core flow that encompasses the core fluid based on the optical property measured. 
     
     
         36 . An apparatus comprising:
 means for causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a dielectric constant of a fluid constituting either sheath flow is substantively less than a dielectric constant of the core fluid, and wherein flow in the channel is laminar; and   means for determining impedance in the channel based on measurements at a first pair of one or more pairs of electrodes.   
     
     
         37 . A non-transient computer-readable medium carrying one or more sequences of instructions, wherein execution of the one or more sequences of instructions by one or more processors causes an apparatus to:
 receive first data indicating measurements of impedance in a channel between a first pair of electrodes, wherein a core fluid flows into the channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a dielectric constant of a fluid constituting either sheath flow is much less than a dielectric constant of the core fluid, and wherein flow in the channel is laminar; and   determine impedance of a particle in the core fluid based at least in part on the first data.   
     
     
         38 . A computer-readable medium as recited in  claim 37 , wherein the apparatus is further caused to determine control data that indicates, at least in part, relative pressure of a source of the core fluid compared to pressure of a source of one or more of the sheath fluids. 
     
     
         39 . A computer-readable medium as recited in  claim 38 , wherein to determine control data further comprises to determine control data to stabilize a value of the first data. 
     
     
         40 . A computer-readable medium as recited in  claim 38 , wherein:
 the apparatus is further caused to receive second data that indicates measurements of a property of the core fluid; and   to determine control data further comprises to determine control data to stabilize a value of the second data.   
     
     
         41 . A computer-readable medium as recited in  claim 40 , wherein the second data indicates measurements of width of the core fluid. 
     
     
         42 . An apparatus comprising:
 means for receiving first data indicating measurements of impedance in a channel between a first pair of electrodes, wherein a core fluid flows into the channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a dielectric constant of a fluid constituting either sheath flow is much less than a dielectric constant of the core fluid, and wherein flow in the channel is laminar; and   means for determining impedance of a particle in the core fluid based at least in part on the first data.   
     
     
         43 . A method comprising:
 causing a core fluid to flow into a channel between two sheath flows of one or more sheath fluids different from the core fluid, wherein a value of a first electromagnetic property of a fluid constituting either sheath flow is substantially different from a value of the first electromagnetic property of the core fluid, and wherein flow in the channel is laminar; and   measuring a second electromagnetic property in the channel using an electromagnetic signal that is concentrated in the core fluid by a difference in the value of the first electromagnetic property of either sheath flow and the value of the first electromagnetic property of the core fluid.