Impedance testing with alternating current frequency control

1. An apparatus comprising:

a microfluidic channel;

an electric sensor within the microfluidic channel to form an electric field within the microfluidic channel and to output signals indicating an impedance of fluid within the electric field within the microfluidic channel;

a frequency source; and

a controller to execute machine readable instructions to output control signals controlling the frequency source to selectively apply different nonzero frequencies of alternating current at different times to the electric sensor, wherein the machine readable instructions cause control signals to be output by the controller to apply a first non-zero frequency of alternating current to the electric sensor and a second non-zero frequency of alternating current to the electric sensor, wherein the first nonzero frequency comprises a frequency selected by the machine readable instructions as executed by the controller for a first fluid test based upon signal-to-noise performance of the first frequency for the first fluid test and wherein the second frequency comprises a frequency selected by the machine readable instructions as executed by the controller for a second fluid test, different than the first fluid tests, based upon signal-to-noise performance of the second frequency for the second fluid test.

2. The apparatus of claim 1 , wherein the control signals to be output by the controller apply a first non-zero frequency of alternating current to the electric sensor and a second non-zero frequency of alternating current to the electric sensor, wherein a value of the second non-zero frequency of alternating current to the electric sensor is based upon signals received from the electric sensor during the application of the first non-zero frequency of alternating current.

3. The apparatus of claim 1 , wherein the controller is to output control signals to cause the frequency source to scan across a range of frequencies including the first and second nonzero frequencies.

4. The apparatus of claim 3 , wherein the controller is to output control signals to cause the frequency source to scan across the range of frequencies a plurality of times.

5. The apparatus of claim 3 , wherein the controller is to output control signals to cause the frequency source to continuously scan across the range of frequencies.

6. The apparatus of claim 3 , wherein the controller is to output control signals to cause the frequency source to intermittently scan across the range of frequencies.

7. A method comprising:

applying a first nonzero frequency alternating current to an electric sensor to form a first electric field across a microfluidic channel through which fluid flows;

sensing changes in impedance of the first electric field in response to fluid flow through the first electric field;

selecting a second nonzero frequency alternating current, different than the first nonzero frequency alternating current based upon the changes in impedance sensed during application of the first nonzero frequency alternating current to the electric sensor;

applying the second nonzero frequency alternating current, different than the first nonzero frequency alternating current, to form a second electric field across the microfluidic channel through which fluid flows;

sensing changes in impedance of the second electric field in response to fluid flow through the second electric field.

8. The method of claim 7 , wherein the first nonzero frequency of alternating current and the second nonzero frequency of alternating current are both within a single selected range of frequencies, the single selected range being based upon signal-to-noise performance of frequencies within the range for a single fluid test.

9. The method of claim 7 further comprising continuously adjusting a frequency of alternating current being applied to the electric sensor so as to scan across a range of frequencies.

10. An apparatus comprising:

a non-transitory computer-readable medium containing instructions to direct a processor to:

prompt a user for a selection of a test from amongst a plurality of available tests;

selecting a plurality of nonzero frequencies of alternating current to be applied to an electric sensor within a microfluidic channel based upon the selection of the test by the user;

output control signals applying the plurality of nonzero frequencies of alternating current to the electric sensor within a microfluidic channel; and

receive signals responsive to the application of the plurality of nonzero frequencies to the electric sensor, the signals indicating a characteristic of a fluid within the microfluidic channel;

wherein a second nonzero frequency of the plurality of nonzero frequencies is selected to be different from a first nonzero frequency of the plurality of nonzero frequencies, and wherein the second nonzero frequency is selected based upon a change in impedance sensed during application of the first nonzero frequency to the electric sensor.

11. The apparatus of claim 10 wherein the instructions are to direct the processor to scan across a range of frequencies when applying the plurality of nonzero frequencies to the electric sensor.

12. The apparatus of claim 11 , wherein the instructions are to automatically select a scan profile based upon the selection of the test by the user, from a plurality of available scan profiles and wherein the control signals are to cause the plurality of nonzero frequencies to be applied to the electric sensor according to the selected scan profile.

13. The apparatus of claim 11 , wherein the instructions are to apply continuous scan profile when applying the plurality of nonzero frequencies to the electric sensor.

14. The apparatus of claim 5 , wherein the control signals cause a frequency source to continuously scan across the range of frequencies according to a ramped profile.

15. The apparatus of claim 5 , wherein the control signals cause a frequency source to continuously scan across the range of frequencies according to an arcuate profile.

16. The apparatus of claim 5 , wherein the control signals cause a frequency source to continuously scan across the range of frequencies according to a profile having multiple intermediate linear ramping segments.

17. The method of claim 7 further comprising determining at least one of a size of cells or particles in the fluid and a number of cells are particles in the fluid based upon the sensing of changes in impedance of the second electric field.

18. The apparatus of claim 10 , wherein the characteristic comprises a characteristic selected from a group of characteristics consisting of: a size of cells or particles of the fluid proximate the electric sensor; and a number of cells or particles of the fluid proximate the electric sensor.