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

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

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Icebi 07 august 29th september 2nd 2007 in graz austria

ICEBI ’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

Why, what and how

ICEBI ’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


How to measure tissue parameters

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

How to measure tissue parameters?

In July of 1886 Oliver Heaviside introduced term "impedance"

  • Impedance

  • Temperature

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

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.


Bioimpedance

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

rint

C

rext

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.


Design choices

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

Design choices

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

0.01

4.0

0.00

3.0

-0.01

2.0

-0.02

-0.03

1.0

-0.04

0.0

-0.05

-

1.0

-0.06

1

10

100

100

1000

0

0

1

10

100

1000

10000

Sinusoidal signal(s) and square wave

  • (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

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.


Shortened rectangular pulses

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

The 1st harmonic

Relative magnitude of harmonic

Relative magnitude of harmonic

The coinciding harmonics

All the harmonics are coinciding

18º

The 1st harmonic

30º

t/T

Shortened rectangular pulses

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

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


From almost analog

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

Iexc

30

Vref,Re

18

Vref,Im

90

VZ

VRe

VIm

t; 

Texc

From almost analog…

Analog synchronous detection

waveforms for synchronous detection


To mostly digital

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

… to mostly digital -

  • sampling and processing of the signal:

  • uniform sampling with 6°distance between samples

  • 366° undersampling

Digital multichannel solution


Device

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

Device

Block diagram of the measurement unit


Current sources i

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

Current sources I


Current sources ii

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

Current sources II


Electrodes

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

Electrodes

  • Needles from ordinary disposable hypodermic syringe

  • ETHICON Surgical Stainless Steel Suture - Temporary Pacing Wire


Current consumption

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

Current consumption

Design errors (LFO, SWPS) – 10 mA

AD8251 – 4x5mA

Oscillator (12 MHz) – 2 mA

Microcontroller (AVR) – 10 mA

Bluetooth – 15 mA

Power supplies – 10 mA

Device start | communication start | measure

Average (Agilent 34410A) – 100 mA


User interface and some results

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

User interface and some results

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

270 ohms, all 4 channels


Where to go from here

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

Where to go from here

  • 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

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

Acknowledgements

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

  • and by Enterprise Estonia through the Competence Centre ELIKO

THANK YOU !


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