Development of Handheld Impedance Spectroscopy Instrument Suitable for Biological Tissue Characterization

Abstract

Impedance spectroscopy (IS) is a powerful and sophisticated tool to characterize intrinsic electrical properties of any materials and its interface, measuring the electric and transport properties of the materials, investigating the mechanism of electrochemical reaction etc. The fundamental principle of IS is the measurement of the impedance (equivalent to resistance in DC analysis) of the system under investigation over a wide frequency range. The IS study provides quantitative information about the conductance, the dielectric coefficient, the static properties of the interfaces like contact resistance, and its dynamic change due to adsorption or charge transfer phenomenon. Commercial Impedance analyzers that are available today are bulky and cannot be carried or deployed at remote locations for monitoring so easily, also such systems are expensive. Though IS concept is not new, standard Impedance analyzers are interfaced to the computer and require detailed analysis of the data by experts. Advancement in data conversion and signal processing is opening up opportunities for handheld smart devices. Hence it is felt that there is a need to develop a versatile low cost, portable (handheld) impedance analyzer instrument which can be used to characterize non linear materials like semiconductors, biological objects, and electrochemical samples. The instrument should be capable accepting test parameters using a touch screen based keypad and display test results on a Graphical LCD for quick analysis. Hence a standalone handheld instrument is designed to work independently without any PC control. The designed instrument consists of Digital Direct Synthesis (DDS) signal generator, a constant current source, Analog to digital convertor, Microcontroller to control frequency sweep parameters and acquire data, TFT Graphics LCD for displaying various plots, touch screen user interface to input sweep parameters and data storage section for offline analysis of obtained data. The instrument is designed to work on a battery or a regulated power supply. The instrument has options to display Nyquist plot, capacitance versus frequency plot, real and imaginary value of impedance versus frequency plot etc. Experimental investigations are conducted using this instrument on few passive components like resistor, inductor, capacitor and combinations. The instrument is also used to monitor the performance of polyimide based capacitive humidity sensor and its characteristics like sensitivity, linearity, repeatability and hysteresis. The measurement accuracy of the impedance varies from 0.7% to 4.67% depending on the impedance range. Bio impedance measurements are carried out on biological samples like Banana and the measured values are compared with the standard LCR meter. The bio impedance measurements are repeatable and comparable with the standard LCR meter readings and it is found to be within ±1% accuracy at the calibrated frequency. The instrument is also validated using industry standard Gauge R & R procedure to understand the repeatability and reproducibility of the instrument. The instrument exhibited good Repeatability and Reproducibility in current mode excitation than voltage mode excitation and Gauge R & R is 55% with P/T ratio is less than 2.5%.