RAZVOJ PRENOSNIH SISTEMOV IN METOD ZA MERJENJE BIOIMPEDANCE

BRAJKOVIĆ, ROBERT (author), Križaj, Dejan (mentor), Jankovec, Marko (co-mentor)

Implementacija bioimpedančnih merilnih sistemov na diagnostično in raziskovalno področje poleg osnovnega znanja o sestavi in električnih lastnostih bioloških tkiv, zahteva tudi poglobljeno znanje s področja načrtovanja bioimpedančnega inštrumentarija in metod. Biološka tkiva imajo zelo kompleksno sestavo, katerih struktura praviloma ni homogena in izotropna, ampak je skupek velikega števila struktur z različnimi električnimi lastnostmi, ki v zaključenih bioloških sistemih opravljajo različne funkcije. Električna impedanca bioloških tkiv (bioimpedanca) kot snovna lastnost je odvisna od mnogih fizioloških, strukturnih in geometrijskih lastnosti tkiva ter uporabna na mnogih področjih kot pokazatelj sprememb. V uvodnem delu naloge sem nekaj besed namenil opisu osnovnih lastnosti celice in bioloških tkiv, tako fizioloških kot električnih. Prav tako sem naštel in opisal osnovne parametre, s katerimi običajno opisujemo in vrednotimo električne lastnosti bioloških tkiv. Čim več informacij imamo o lastnostih merjenca (biološkega tkiva) ter vplivnih parametrih, več vhodnih podatkov za izvedbo meritev in razumevanje rezultatov posedujemo. Na raziskovalnem in medicinskem področju bioimpedančnih meritev so potrebne enostavne, natančne in predvsem zanesljive meritve, kar nam omogočajo dragi in zmogljivi impedančni analizatorji ali LCR metri z dodanimi vmesniki. Njihova slabost pa je, da so običajno preveč univerzalni, stacionarni in neprilagodljivi, ter za nekatere medicinske aplikacije in raziskave neuporabni. V disertaciji obravnavam, raziskujem in razvijam metodologijo merjenja bioimpedance s prenosnimi, prilagodljivimi in medicinsko varnimi sistemi, ki so uporabni predvsem v raziskovalne namene. Če povzamem zapisano, je cilj naloge razvoj prenosnih sistemov in metod za merjenje bioimpedance. Naloga je sestavljena iz treh ločenih tematskih sklopov, ki zajamejo merilni inštrumentarij, metode merjenja in uporabo teh na raziskovalnem biomedicinskem področju. Vsak sklop pa je sestavljen iz teoretičnega uvoda, opisa problematike, ciljev in prikaza dobljenih rezultatov. V prvem sklopu naloge sem se podrobneje ukvarjal z inštrumentarijem, signalnim zajemanjem in obdelavo podatkov. Teoretični del sem pričel s splošnim opisom pojma električne impedance, nadaljeval pa z opisom in analizo najpogosteje uporabljenih merilnih tehnik. Posebno pozornost sem posvetil tehniki s sinusnim signalom določene frekvence in rekonstrukciji merilnih rezultatov s fazno sklenjenimi ojačevalniki. Ti temeljijo na fazno občutljivi detekciji, ki ima dve temeljni operaciji: sinhrono demodulacijo in filtriranje. Na kratko sem predstavil tudi prednosti in slabosti tehnike s sestavljenim sinusnim signalom, ki je v uporabi predvsem pri merjenju impedance dinamičnih materialov s hitro spreminjajočimi se lastnostmi. Na koncu teoretičnega uvoda sem predstavil tudi osnovne komponente inštrumentarija za določitev bioimpedance. Uporaba stacionarnih impedančnih analizatorjev z velikimi dimenzijami in visoko ceno ni vedno sprejemljiva in dostopna, zato so za univerzalno uporabo bolj sprejemljivi prenosni in prilagodljivi merilni sistemi. Pri razvoju prenosnih sistemov je smiselna ločitev merilnega dela od procesirnega, saj se na ta način napravi omogoči manjšo porabo energije, hitrejšo izvedbo meritev, lažje shranjevanje ter bolj podrobno obdelavo izmerjenih signalov. V nadaljevanju prvega sklopa naloge pod rubriko rezultatov sem najprej predstavil smernice za razvoj prenosnih, cenenih in baterijsko napajanih bioimpedančnih merilnih sistemov, ki temeljijo na majhnih mikrokrmilnikih. Razvil sem enostaven merilni sistem, ki ima ločen merilni del od procesirnega, kar omogoča manjšo porabo energije, lažje shranjevanje podatkov in bolj podrobne obdelave izmerjenih signalov. Za prenos podatkov na prenosne naprave sem uporabil komunikacijski protokol WiFi. Analiziral sem pomen posameznih sklopov in s pomočjo testiranja ovrednotil negotovosti razvitega sistema. Izkaže se, da tovrstna izvedba doprinese k enostavnosti in cenenosti same naprave, vendar zgublja v pogledu natančnosti in zanesljivosti meritev. Ključna razloga sta zmogljivost mikrokrmilnika in izostanek vzbujalnega vira. Na podlagi analize sem se odločil za razvoj napetostno krmiljenega konstantnega tokovnega vzbujalnega vira. Razlog temu je natančna nastavitev in zagotovilo vzbujanja merilnega objekta z varnim in konstantnim tokom čez celotno frekvenčno območje meritev, neglede na vrednost impedance merilnega objekta. Osredotočil sem se predvsem na izvedbo temelječo na Howlandovem vezju z enostranskim napajanjem. Za oceno kvalitete in natančnosti vira sem izvedel analizo, ki je zajemala predvsem vpliv parazitnih kapacitivnosti na izhodne parametre. Znano je, da se parazitnim kapacitivnostim ne da v celoti izogniti, vendar z optimalno načrtanim tiskanim vezjem lahko njihov vpliv v veliki meri zmanjšamo. Poleg merilnega dela sem raziskoval tudi procesirni del, predvsem rekonstrukcijo merilnih rezultatov. Kot primerna metoda za rekonstrukcijo izmerjenih signalov znotraj procesirnega dela se je izkazala fazno občutljiva detekcija in pripadajoči frekvenčno odzivi analizator, ki učinkovito ločuje signale majhnih amplitud od šuma. Za verodostojno rekonstrukcijo amplitude in faze potrebujejo referenčni signal. Posebno pozornost sem usmeril v ovrednotenje vpliva frekvenčne sinhronizacije in povprečenja znotraj frekvenčno odzivnih analizatorjev. Analiziral sem vpliv različnih parametrov pri rekonstrukciji in izračunu bioimpedance. Pričel sem z nesinhronizacijo referenčnega in merjenih signalov, nadaljeval pa z analizo in oceno posledic pri odstopanjih frekvence referenčnega signala ter povezavo s faznim kotom med merjenima signaloma, številom zajetih period in različnih vrednostih parametra signal-šum. Na koncu sem vse teoretične predpostavke preveril še s pomočjo eksperimentalne analize. Izkaže se, da v primeru idealnega sinusnega vzbujalnega signala in faznega kota nič med merjenima signaloma toka in napetosti (idealen uporovni značaj merjenca) nesinhronizacija na izračun impedance ne vpliva. Z naraščanjem faznega kota in odstopanja frekvence pa napaka linearno narašča. V zadnjem delu prvega sklopa pa sem na podlagi opravljenih raziskav in izdelanih sestavnih delov načrtal koncept bioimpedančnega merilnika na merilni platformi RedPitaya, ki temelji na tehnologiji FPGA. Jedro merilnega sistema je merilna platforma RedPitaya z dodanim prilagojenim vmesnikom za meritev bioimpedančnih parametrov. Merilnik ima prav tako ločen merilni del od procesirnega in njegova posebnost je prilagoditev različnim aplikacijam v smislu frekvenčnega območja meritev, dinamičnega območja meritev, možnosti stimulacije merjenega objekta, poljubne oblike vzbujalnega signala, več-elektrodnega merjenja, ločljivosti, hitrosti itd. Prav tako sem v procesirni del programsko implementiral frekvenčno odzivni analizator. Predstavil in implementiral sem tudi tri-referenčno kalibracijsko metodo in na koncu tudi ovrednotil merilne negotovosti za magnitudo in fazo bioimpedance. V drugem sklopu naloge sem se podrobneje ukvarjal z metodami merjenja in nadgradnjo teh z vključitvijo 'plavajoče prevodne elektrode'. Teoretični uvod sem pričel z opisom razlike med pojmoma vzajemne in trans-impedance, nadaljeval pa z občutljivostjo elektrodnih sistemov na majhne spremembe kompleksne prevodnosti. V splošnem namreč velja, da večja kot je absolutna vrednost občutljivosti majhnega prostorninskega prispevka, večji je prispevek k skupni merjeni impedanci. Negativna vrednost občutljivosti majhnega prostorninskega prispevka pa pomeni, da povečanje upornosti le tega posledično povzroči zmanjšanje skupne merjene impedance. Postavitev ter velikost elektrod, tako vzbujalnih kot merilnih, je pri merjenju bioimpedance zelo pomembna. S tem namreč neposredno vplivamo na občutljivost in globino meritev. Z večanjem razdalje med elektrodama večamo globino merjenja, medtem ko s spreminjanjem dimenzij elektrod spreminjamo prostorninsko občutljivost. Obdelal sem tudi metode merjenja, kjer sem povzel značilnosti merjenja bioimpedance z dvo, tri in štiri-elektrodnim sistemom. Na koncu teoretičnega dela sem predstavil še reševanje geometrijskih modelov s pomočjo numeričnih izračunov, ki sem jih uporabil. Analiza občutljivosti je pokazala, do so področja blizu in okrog vzbujalnih elektrod bolj občutljiva na celotno meritev kot pa področja daleč stran. V določenih primerih so zanimive električne lastnosti merjenega objekta na omejenem področju merjenja med elektrodami in doprinos tega k rezultatu celotne meritve. Še več, v nekaterih primerih se zahteva zaznava sprememb električnih lastnosti na lokalnem območju merjenja. Običajno zadostuje že premik elektrod na želeno območje, ampak takrat se meri le trenutna vrednost impedance, vrednost pred pojavom spremembe pa ni znana. Kot primerno in enostavno rešitev teh problemov sem predstavil nadgradnjo 4-elektrodne meritve z dodatno neaktivno prevodno elektrodo. Cilj te je povečati lokalno zaznavanje sprememb električnih lastnosti. Dodatna elektroda ima nalogo prerazporejanja toka. V okviru rezultatov sem predstavil osnovni princip nadgradnje obstoječih metod s plavajočo prevodno elektrodo. S pomočjo numeričnih simulacij sem prikazal delovanje in potrdil predpostavko, da je s pomočjo dodane prevodne elektrode možno sklepati na globino in električne lastnosti lokalnih sprememb. Nadaljeval sem s simulacijo prikaza uporabe plavajoče elektrode kot skenirne za zaznavanje lokalni sprememb. Na koncu sem s pomočjo 3D modela prikazal še možno uporabo na raziskovalnem področju. V tretjem sklopu naloge sem razvit in izdelan inštrumentarij prilagodil z uvedbo vmesnika ter ga uporabil v raziskovalni aplikaciji na področju vzbujanja živčnih tkiv. V teoretičnem uvodu sem v grobem predstavil področje vzbujanja živčnih tkiv. Povzel sem lastnosti, katere je potrebno upoštevati pri izbiri vzbujalnega signala. Te so: razmerje med amplitudo in trajanjem, med amplitudo in razdaljo elektrod od živčnega tkiva ter med amplitudo in širino signala. Nadalje sem preučil različne vrste elektrod, ki so v uporabi na področju vzbujanja. Posebno pozornost je potrebno nameniti tudi biokompatibilnosti elektrod z tkivom. Podrobno sem analiziral tudi problem polarizacije, ki nastane pri stiku elektrode in biološkega tkiva. Zaradi različne narave tokov na stiku med elektrodo in biološkim tkivom potekajo reakcije, ki omogočajo pretvorbo med elektroni in ioni. Na površini elektrode se takoj po stiku s tkivom in navlažitvi ustvari tako imenovan dvojni sloj in znotraj tega pride do prenosa elektronov. Poleg kapacitivno značilnih pojavov na stiku elektroda-elektrolit poznamo še elektrokemijske ali faradayske, kjer na stiku poteka reakcija oksidacije ali redukcije. Opisal sem tudi značilne načine karakterizacije elektrod in ekvivalentna vezja s katerimi lahko na podlagi rezultatov meritev modeliramo in optimiziramo stik elektroda-tkivo. Obstoječi sistemi za tovrstne karakterizacije imajo običajno ločene enote, ki jih je potrebno prilagajati med seboj in posamezne meritve izvajati ločeno. To je še posebej očitno ob ločenem stimuliranju in merjenju odziva, saj lahko že med meritvami pride do spremembe razmer in jih kasneje ne moremo več zabeležiti. Kot rešitev sem predstavil prilagodljiv sistem z izdelanim vmesnikom, ki omogoča hkrati poljubno vzbujanje, merjenje napetostnega odziva in izračun bioimpedance. Bolj pomemben je vzbujalni del, saj mehanizem in oblika ter moč vzbujalnega signala vplivata na učinkovitost, selektivnost in varnost stimulacije. Znotraj uporabljene komore za vzbujanje izoliranega živčnega tkiva so platinaste elektrode, na katerih pri stiku z biološkim tkivom zaradi pretvorbe med elektroni in ioni posledično prihaja do ustvarjanja dvojnega sloja ter ostalih elektrokemijski reakcij. Izkaže se, da je za spremljanje dogajanja na elektrodah bolj primerna dvo-elektrodna meritev. Na podlagi raziskav sem pripravil stimulacijsko-merilni sistem in predvidel proceduro selektivne stimulacije živčnega tkiva s kvazi-trapeznim signalom. Primerjal sem razliko odziva med bipolarnim in polarnim vzbujalnim signalom. Meritev odziva in bioimpedance sem beležil tako z dvo-elektrodno kot štiri-elektrodno postavitvijo, ter rezultate primerjal med seboj. Ob primernem vzbujanju se lahko reakcije delno omeji in zagotovi njihovo reverzibilnost, ter s tem prepreči morebitno poškodbo vzbujanega tkiva in elektrod. Poleg meritev odziva sem s sistemom zaznaval sestavljen akcijski potencial ter na koncu sestavil bioimpedančni spekter na širšem frekvenčnem območju. Rezultate sem prvotno prikazal v Bodejevem, nato še v Wesselovem diagramu, kjer sem na podlagi značilnih potekov sklepal o vsebnosti elementov nadomestnega vezja. Dobljeni podatki so dobra osnova za nadaljnjo študijo in optimizacijo stimulacijskih mehanizmov.

• Language: Slovenian
• Keywords: bioimpedančni merilni sistem, biološko tkivo, inštrumentarij, fazno občutljiva detekcija, frekvenčna sinhronizacija, merilne metode, plavajoča prevodna elektroda, vzbujanje živčnega tkiva, dvojni sloj
• Work type: Doctoral dissertation
• Organization: FE - Faculty of Electrical Engineering
• Year: 2018
• PID: 20.500.12556/RUL-105798
• Publication date in RUL: 17.12.2018

Secondary language

• Language: English
• Title: DEVELPOMENT OF PORTABLE SYSTEMS AND METHODS FOR BIOIMPEDANCE MEASUREMENTS

Abstract

The implementati edge of the composition and electrical properties of biological tissues also requires in-depth knowledge in the field of bioimpedance instrumentation and methods design. Biological tissues have a very complex composition whose structure is, as a rule, not homogeneous in isotropic, but is an array of a large number of structures with different electrical properties that perform various functions in closed biological systems. The electrical impedance of biological tissues (bioimpedance) as a material property depends on many physiological, structural and geometric properties of tissues and is used in many fields as an difference indicator. In the introductory part of the thesis, I devoted a few words to the description of the basic properties of cells in biological tissues, both physiological and electrical. I also listed and described the basic parameters that we usually use to describe and evaluate electrical properties of biological tissues. More information we have about the properties of the DUT (device under test or biological tissue) and the influential parameters, more input data for the measurement and the understanding of the results we have. In the research and medical field of bioimpedance measurements, simple, precise and above all reliable measurements are required, which are enabled by expensive and powerful impedance analyzers and LCR meters with aditional front-end. Their weakness is that they are usually too universal, stationary and unflexible, and for some medical applications and research often useless. In the thesis, I analyze, research and develop a methodology for measuring bioimpedance with portable, adaptable and medically safe systems, which are useful primarily for research purposes. To summarize, the goal of the thesis is the development of portable systems and methods for measuring bioimpedance. The thesis consists of three separate thematic sections that capture the measurement instrumentation, the measurement methods and their use in the biomedical research field. Each section consists of a theoretical introduction, a description of the problem, objectives and the presentation of the obtained results. In the first part of the thesis, I analyzed instrumentation, signal capturing and data processing. I started the theoretical part with a general description of the concept of electrical impedance, and continued with the description and analysis of the most commonly used measurement techniques. Special attention was paid to single sine technique and the reconstruction of the measurement results with lock-in amplifiers. They are based on phase sensitive detection, which has two basic operations: synchronous demodulation and filtering. I also presented the advantages and disadvantages of the multi sine technique, which is mainly used in measuring the impedance of dynamic materials with rapidly changing properties. At the end of the theoretical introduction, I also presented the basic components of the instrumentation for determining bioimpedance. The use of stationary impedance analyzers with large dimensions and high price is not always acceptable and affordable, therefore, portable and adjustable measuring systems are more acceptable for universal use. In the development of portable systems, it is reasonable to separate the measuring part from the processing one, in that way, the device is constructed for lower energy consumption, faster measurement performance, easier storage and more detailed later processing of the measured signals. In the continuation of the first section of thesis under the results column, I first introduced the guidelines for the development of portable, cheap and battery-powered bioimpedance measurement systems based on small microcontrollers. I have developed a simple measurement system that has a separate measuring part from the processing part, which allows for lower energy consumption, easier storage of data and more detailed processing of the measured signals. I used the WiFi communication protocol to transfer data to portable devices. I analyzed the importance of individual components and evaluated the uncertainties of the developed system through testing. It turns out that this kind of performance contributes to the simplicity and cheapness of the device itself, but it loses in terms of precision and reliability of measurements. The key reasons are the capacity of the microcontroller and the absence of an excitation source. Based on the analysis, I decided to develop a voltage-controlled constant current source. The reason for this is the precise setting and assurance that the measurement object is excited by a safe and constant current over the entire frequency range of measurements, regardless of the value of the impedance of the DUT. I focused on the solution based on Howland's circuit with single power supply. For the evaluation of the quality and accuracy of the source, I carried out an analysis, whichcovered the influence of parasitic capacitance on the output parameters. It is known that the parasitic capacitance can not be completely avoided, but with an optimaly designed circuit board, their influence can be reduced. Besides the measurement part, I also investigated the processing part, especially the reconstruction of the measurement results. It turned out that an appropriate method for reconstructionof the measured signals within the processing part is a phase-sensitive detection and associated frequency response analyzer, which effectively separates the signals of small amplitudes from noise. For the correct reconstruction of the amplitude and phase, they need a reference signal. I paid special mattention to evaluating the effect of frequency synchronization and averaging within the frequency response analyzers. I analyzed the influence of various parameters in the reconstruction and calculation of bioimpedance. I started with the non-synchronization of the reference and measured signals, and continued with the analysis and estimation of the consequences of the deviations of the frequency of the reference signal and the connection with the phase angle between the measured signals, the number of captured periods and different values of the signal-noise parameter. In the end, I checked all the theoretical assumptions with the help of experimental analysis. It turns out that in the case of an ideal sinusoidal excitation signal and a zero phase angle between the measured current and voltage signals (ideal resistivity character of DUT), the non-synchronization to the calculation of the impedance is not affected. With the increase of the phase angle and the deviation of the frequency, the error is linearly increasing. In the last part of the first section, on the basis of the performed research and developeded components, I designed the concept of a bioimpedance measurement system with platform RedPitaya which is based on FPGA technology. The core of the measuring system is the RedPitaya measuring platform with added custom hardware or an interface for measuring bioimpedance parameters. System also has a separate measuring part from the processor part and its specialty is that it is adaptable to different applications in terms of the frequency range of measurements, the dynamic range of measurements, the possibility of stimulating the measured object with any form of excitation signal, multiple electrode measurement, resolution, speed, etc. I also implemented frequency response analyzer with software in the processing part. In addition to the implementation I introduced the three-reference calibration method and ultimately evaluated the measurement uncertainties for the magnitude and phase of bioimpedance. In the second part of the thesis, I dealt more closely with measurement methods and upgrading these with the usage of a 'floating conductive electrode'. I started the theoretical introduction by describing the difference between the concepts of mutual and trans-impedance, and continued with the sensitivity of electrode systems to small changes in complex conductivity of bioimpedance measurements. In general, the greater the absolute value of the sensitivity of a small volumetric contribution, the greater the contribution to the total measured impedance. The negative value of the sensitivity of a small volum contribution, however, means that the increase in resistivity only results in a decrease in the total measured impedance. The placement and size of electrodes, both exciting and measuring, is very important in measuring bioimpedance. This directly influences the sensitivity and depth of measurements. By increasing the distance between the electrodes we increase the depth of measurement while changing the dimensions of the electrodes changes the volume sensitivity. I also analyzed measurement methods, where I summarized the characteristics of measuring bioimpedance with the two, three and four electrode systems. At the end of the theoretical part, I also presented the solution for geometric models calculating using the numerical methods. The sensitivity analysis showed that the area close to and around the excitation electrodes is more sensitive to the overall measurement than the areas far away. In certain cases, the electrical properties of the measured object in the limited field of measurement between the electrodes are interesting, and also the contribution of this to the result of the total measurement. Moreover, in some cases, it is required to detect changes in electrical properties in the local measuring range. It is usually sufficient to move the electrodes to the desired area, but then only the current impedance value is measured, and the value before the occurrence of the change is not known. As an appropriate and easy solution to these problems I presented the upgrade of a 4-electrode measurement with an additional inactive conductive floating electrode. The goal of this is to increase the local perception of changes in electrical properties. An additional electrode has the task of redistributing the current. In the results chapter, I presented the basic principle of upgrading existing methods with floating conductive electrodes. Using numerical simulations, I showed the action and confirmed the assumption that with the help of an added conductive electrode, it is possible to deduce the depth and electrical properties of local changes. I continued with the simulation of the use of floating electrode as a scan electrode for detecting local changes. Finally, using the 3D model, I have demonstrated the possible use in the field of research. In the third part of the thesis, I adjusted developed measurement instrumentation by introducing the interface and used it in a research application in the field of excitation of nerve tissues. In the theoretical introduction, I roughly presented the field of excitation of nerve tissue. I've raised features that must be considered when choosing an excitation pulse. These are: the relationship between the amplitude and duration, between the amplitude and the distance of the electrodes from the nerve tissue and between the amplitude and the width of the pulse. Furthermore, I analyzed the different types of electrodes used in the excitation field. Particular attention should be paid to the biocompatibility of electrodes with tissue. I also analyzed in detail the problem of polarization due to the contact of the electrode and biological tissue. Because of the different nature of the currents at the contact between the electrode and the biological tissue, there are reactions that allow the conversion between electrons and ions. Immediately after contact with the tissue and moisture, a so-called double layer is created on the surface of the electrode, and within this the electron transfer occurs. In addition to the capacitance-specific phenomena at the electrode-electrolyte contact, there are also electrochemical or faradayic processes, where oxidation and reduction reactions take place. I have also described typical ways of characterizing electrodes and equivalent circuits by which we can model and optimize electrode-tissue contact based on the results of measurements. Existing systems for such characterization are usually separate units that need to be adapted to each other and perform individual measurements separately. This is especially evident when the stimulation and response measurement processes are separated, since in the meantime the situation can be changed and can no longer be recorded later. As a solution, I introduced a adjustable system with a built-in interface that allows simultaneous optional excitation and measurement of the voltage response and the calculation of bioimpedance. The exciting part is more important because the mechanism and shape and the power of the excitation signal influence the efficiency, selectivity and safety of stimulation. Inside the chamber used to excite the isolated nervous tissue, there are platinum electrodes, which, when contacted with biological tissue, result in the formation of a double layer and other electrochemical reactions due to the conversion between electrons and ions. It turns out that a two-electrode measurement is more suitable for monitoring electrodes. Based on the research I developed a stimulation and measurement system and prepared the procedure of selective stimulation of the nervous tissue with a quasi-trapezoidal pulse. I compared the difference in the response between the bipolar and the polar excitation pulse. I recorded both measurement of the response and bioimpedance with a two-electrode and a four-electrode placement, and the results were compared with each other. With proper excitation, the reactions can be partially restricted to ensure their reversibility, thereby preventing possible damage to the excited tissue and electrodes. In addition to the response measurements, I sensed the compound action potential with the system and finally composed the bioimpedance spectrum in the wider frequency range. I originally showed the results in Bode diagram, and then in Wessels diagram, where based on the characteristic course of the graph I concluded about the contents of the elements in the equivalent circuit. The data obtained in experiment are a good basis for further study and optimization of stimulation mechanisms.

• Keywords: bioimpedance measurement system, biological tissue, instrumentation, phase sensitivedetection, fryquency synchronization, measurement methods, floating conductiveelectrode, stimulation of nerve tissue, double layer