ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

An energy efficient wearable tissue monitor

ANNUS, Paul; MIN, Mart; PARVE, Toomas; LAND, Raul; HALDRE, Eero; KUUSIK, Alar; POOLA, Gustav

Why, what and how

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Need for monitoring of the tissue parameters after transplantation

Two primary characteristics of tissue are considered for measurement:

Bioimpedance Temperature

Healing process is relatively slow Multiple simultaneous measurement points are needed Shortened pulse wave measurement of impedance and

temperature is used Primary circuitry is preferably close to tissue sample RF link to data collecting and processing equipment - PC

In July of 1886 Oliver Heaviside introduced term

"impedance"

In 1893, Arthur Edwin Kennelly presented a paper

on 'Impedance" to the American Institute of

Electrical Engineers in which he discussed the first use of complex numbers as applied to Ohm's Law in

alternating current circuit theory.

Electrical impedance is a measure of opposition to a sinusoidal electric current.

How to measure tissue parameters? Impedance

Temperature

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Bioimpedance Essentials of the Electrical

Bioimpedance EBI

3-element equivalent of the static EBI

phasor diagram of the static EBI for two frequencies, low ωl and high ωh.

rint C

rext

–Im Ż X Imaginary part

R Real part Re Ż

Ż(ωl) = R1 (ωl) + j X1 (ωl)

Re Ż + j Im Ż R* C*

true complex frequency response

two-element approximation

Ż(ωh)

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Design choices

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Excitation waveform Analog versus Digital Current sources Multisensor or small single device Electrodes Current consumption Frequency range and Impedance range Speed User interface and representation of results

Sinusoidal signal(s) and square wave

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

4.0

-1.0

0.0

1.0

2.0

3.0

100001 10 100 1000

0.01

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

100001 10 100 1000

Frequency responses (kHz) of relative magnitude errors (a) and phase errors (b) caused by odd harmonics of the order of 3, 5, 7, 9, 11, and 13 in the case of using regular rectangular waveforms for both, excitation and

reference signals when measuring 3-element bioimpedance equivalent with a pole at f1 = 160 kHz, and a zero at f2 = 2.5•f1 in its frequency response.

(Multi) Sinusoidal signal: accurate measurements according to definition not very easy to generate

Square wave signal: extremely easy to generate with almost any digital circuitry large measurement errors due to higher harmonics

Shortened rectangular pulses

...5sin

5

5cos3sin

3

3cossin

1

cos4)( tttatf

1.0

-1.0

-0.5

0.0

0.5

1.00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

The 1st harmonic

1.0

0.0

0.2

0.4

0.6

0.8

250 1 3 5 7 9 11 13 15 17 19 21 23

All the harmonics are

coinciding

Relative magnitude of harmonic

1.0

-1.0

-0.5

0.0

0.5

1.00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 t/T

18º 30º The 1st harmonic

1.0

0.0

0.2

0.4

0.6

0.8

250 1 3 5 7 9 11 13 15 17 19 21 23

Relative magnitude of harmonic

The coinciding harmonics

1

sincos4

i

tii

ia

a is the constant amplitude value of the pulse signal, and

β characterizes the shortening of pulses and is equal to the signal’s zero value interval within one half period β = 0 … pi / 2

a) square wave excitation and demodulation b)  same with shortened pulses

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

From almost analog…

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

VZ

Vref,Re

Iexc

Vref,Im

VRe

VImt;

Texc

18

30

90

Analog synchronous detection waveforms for synchronous detection

… to mostly digital -

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

VZ

t;

Texc 2+S

sampling(b)

(a)

Vref,ReIexc

VZ

DRe

t

Sampling instants

sampling and processing of the signal:

(a) uniform sampling with 6°distance between samples

(b) 366° undersamplingDigital multichannel solution

Device

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Block diagram of the measurement unit

Current sources I

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Current sources II

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Electrodes

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Needles from ordinary disposable hypodermic syringe

ETHICON Surgical Stainless Steel Suture - Temporary Pacing Wire

Current consumption

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Design errors (LFO, SWPS) – 10 mA

AD8251 – 4x5mA

Oscillator (12 MHz) – 2 mA

Microcontroller (AVR) – 10 mA

Bluetooth – 15 mA

Power supplies – 10 mA

Average (Agilent 34410A) – 100 mA

Device start | communication start | measure

User interface and some results

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

270 ohms, all 4 channels

270 ohms parallel with 0,25 uF (2358 Hz)

Where to go from here

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

Final sensor unit to build, with: Current source Preamplifier Integrated electrode temperature probe

Search for new waveforms for measurement

Compact and meaningful representation of measured parameters

Search for even lower power solutions

Acknowledgements

THANK YOU !

ICEBI ’07ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA

work was supported by grants no. 7212 and 7243 of Estonian Science Foundation,

and by Enterprise Estonia through the Competence Centre ELIKO