Methods and apparatus for transdermal measurement of impedance

1 . A non-invasive transdermal apparatus for measuring complex impedance of body tissue, the apparatus comprising:

at least two electrodes coupled, in use, to the surface of the skin of a subject;

a signal source to deliver an electrical potential to the electrodes;

a monitor to detect changes in a signal passing through body tissue between the electrodes; and

a processor operatively coupled to the signal source and to the monitor and configured to:

use the signal source to apply a potential to the electrodes to reduce an electrical impedance of the stratum corneum; and

use the monitor to measure complex transdermal impedance AC signals corresponding to both:

α dispersion spectra below 1 kHz, and

β dispersion spectra below 10 KHz.

2 . The apparatus of claim 1 , wherein the apparatus also monitors AC signals corresponding to γ dispersion spectra.

3 . The apparatus of claim 1 , wherein the potential applied to the electrodes to reduce the electrical impedance of the stratum corneum is between 1 V and 150 V.

4 . The apparatus of claim 1 , wherein the electrodes have a uniform electrical impedance of a magnitude close to that of the stratum corneum of human skin.

5 . The apparatus of claim 1 , wherein the processor is configured to use the electrodes to input a common signal to provide a potential to reduce electrical impedance of the stratum corneum and to provide a measurement input signal.

6 . The apparatus of claim 1 , wherein the processor is configured to use the electrodes to input separate signals to provide a potential to reduce electrical impedance of the stratum corneum and to provide a measurement input signal.

7 . The apparatus of claim 1 , wherein the processor is configured to use the monitor to detect and/or measure phase shift in the electrical signal.

8 . The apparatus of claim 1 , wherein the processor is configured to use the complex transdermal impedance AC signals to detect biosignals from the subject.

9 . The apparatus of claim 8 , wherein the biosignals include one or more of: Electroencephalogram (EEG), Electrocardiogram (ECG), Electromyogram (EMG), Electrooculogram (EOG), Electroretinogram (ERG), Electrogastrogram (EGG), Galvanic skin response (GSR) or electrodermal activity (EDA).

10 . The apparatus of claim 1 , wherein the processor is configured to use the complex impedance of body tissue to measure impedance variations indicative of at least one target substance in the body.

11 . The apparatus of claim 10 , wherein the at least one target substance includes one or more of blood sugar, blood alcohol, and cholesterol.

12 . The apparatus of claim 1 , wherein the apparatus comprises a wearable device.

13 . A method of non-invasive transdermal measurement of complex impedance of body tissue, the method comprising the steps of:

coupling at least two electrodes to the skin of a subject;

applying a potential to the electrodes to reduce an electrical impedance of the stratum corneum; and

monitoring complex transdermal impedance AC signals corresponding to

α dispersion spectra below 1 kHz, and

β dispersion spectra below 10 KHz.

14 . The method of claim 13 , wherein the step of applying a potential to the electrodes further comprises simultaneously applying a measurement input signal.

15 . The method of claim 13 , wherein the step of monitoring complex transdermal impedance AC signals corresponding to a and β dispersion spectra further comprises monitoring AC signals corresponding to γ dispersion spectra.

16 . The method of claim 13 , wherein the step of monitoring comprises detecting and/or measuring phase shift in the electrical signal.

17 . The method of claim 13 , wherein the method further comprises using the complex impedance of body tissue to detect biosignals from the subject.

18 . The method of claim 13 , wherein the method further comprises using the complex impedance of body tissue to measure impedance variations indicative of at least one target substance in the body.