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

1. 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

2. 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

3. 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.

4. 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.

5. 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

6. 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.

7. 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

8. 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

9. 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

10. ICEBI ’07 August 29th – September 2nd 2007 in GRAZ, AUSTRIA Device Block diagram of the measurement unit

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

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

13. 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

14. 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

15. 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

16. 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

17. 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 !