Bio-Impedance Spectroscopy (BIS) Measurement System for Wearable Devices Bassem Ibrahim *, Drew A. Hall † , Roozbeh Jafari* * Embedded Signal Processing (ESP) Lab, Texas A&M University, TX, USA † BioSensors and BioElectronics Group, University of California, San Diego, CA, USA
Outline • Motivation • Objective • Background • Specific Aims and Novelty • System Description • Performance Evaluation • Experimental Results • Conclusions 2
Motivation • Examples on Bio-Impedance Applications (ICG) – Cardiovascular diseases diagnosis • Using Impedance Cardiogram (ICG) – Body Cell Mass (BCM) Composition • Dehydration detection • Calories consumption • Advantages – Low power – Low cost – Small size – Suitable for wearable devices 3
Objective Proposed Method Conventional Method Whole body - Bulky Devices Body Segment - Wearable Device Bio-Impedance Analysis (BIA) Bio-Impedance Spectroscopy (BIS) Multiple frequencies More Accurate Single fixed frequency Single Time Measurement Continuous Monitoring 4
Background • Bio-impedance = resistance of tissue to an applied external current • Measure fluids inside the body • Measured by – Injecting AC current from the current electrodes – Voltage sensed between voltage electrodes is proportional to bio-impedance 5
Bio-Impedance High Frequency Low Frequency Current Current Cell Membrane C m C m R E Extra-Cellular Fluid (ECF) R I R E Intra-Cellular Fluid (ICF) R I • Bio-Impedance is modeled as R I , R E and C m • Low frequency ECF current (R E ) • High frequency ECF and ICF current (R I R E ) 6
Bio-Impedance Spectroscopy (BIS) 𝟐 𝒈 𝒅 = 𝟑𝝆𝑫 𝒏 𝑺 𝑭 + 𝑺 𝑱 Reactance |I(Z)| Frequency (f) C m R E R I Resistance |R(Z)| 𝑺 𝑭 . 𝑺 𝑱 𝑺 𝟏 = 𝑺 𝑭 𝑺 ∞ = 𝑺 𝑭 + 𝑺 𝑱 • Bio-Impedance Spectroscopy (BIS) = Bio- Impedance response with frequency Accurate estimation of (R I , R E and C m ) 7
Specific Aims • Develop Bio-Impedance Spectroscopy (BIS) device with – continuous-time update (every 125 ms) – wide frequency (4-120 kHz) to enable extraction of bio-impedance equivalent circuit – from small body segment wearable applications Continuous Spectroscopy Monitorig Wide frequency coverage Fast time Bio-Impedance with small frequency update to track Wearable steps to extract the cardiac activity equivalent circuit. 150 ms Small body segment with bio-impedance 4-120 kHz of few ohms (32 points, 4.7ms each) 1-120 Ω compared to 600 Ω for the whole body. Time 8 Frequency
Novelty • A BIS system for measurements from small body segments is presented for the first time, which can be integrated into a wearable device. – The detailed design and implementation of the circuits and signal processing are discussed – Measuring very small variations of bio-impedance across wide frequency range in a short time. • Experimental measurements of upper arm BIS with 4 cm distance between electrodes to accurately capture physiological signals such as: – Heart rate, respiration rate, and muscle contractions. 9
System Description Z bio DAC+ V-I +5V R(Z) SPI AC IA MCU AFE4300 ADC R G Current Digital Signal I(Z) -5V Processing ADC DAC Output Simultaneous Sampling ADC 16-bit 2MSPS • Frequency sweeping from 4 to 120 kHz controlled by MCU • Based on discrete components • Sensed voltage and DAC output sampled simultaneously using 16-bit ADC @ 2MSPS . • AC current amplitude = 375μA RMS – Compliant with safety limits 10
Digital Signal Processing IA Output f c f c Phase & Gain Correction DAC I Output f c LPF Phase & Gain 2 nd Order Correction f c =500 Hz 90 o BPF Q f c 3 nd Order • Band pass filters to remove DC, 60 Hz interference and high frequency noise. • Digital quadrature demodulation to get the real and imaginary parts of impedance • Phase and gain correction were done to compensate for errors. • Low pass filtering with fc=500Hz to allow for fast output every 4.7 ms 11
Phase and Gain Correction • Impedance measurement sensitive to phase and gain error • Error is measured by a reference resistor (R ref ) for all frequency points • Phase error: 𝐽 𝑎 ∅ err 𝑔 = 𝑢𝑏𝑜 −1 𝑆 𝑎 1 • Gain error: 𝑆 𝑎 2 + 𝐽 𝑎 2 𝐻 err 𝑔 = 𝑆 ref • Phase was corrected before demodulation by fractional time delay using an all-pass digital filter with a variable phase shift. 12
System Evaluation • Linearity was tested using reference resistors from 1 to 120 Ω • Resistance RMSE = 0.07 Ω 13
System Evaluation • Impedance was measured for reference resistors and capacitors similar to bio-impedance (R=30.8 Ω, 100.2 nF & 82.1 Ω , 34.2 nF) from 4-120 kHz 14
Performance Summary This work [1] [2] AFE4300 Frequency 4 – 120 kHz 1 – 125 kHz <150kHz Current 375μ A RMSE 1μ – 100μ A pp 375μ A RMSE Impedance 1 Ω – 120 Ω 1 Ω – 10k Ω 0 – 2.8k Ω Range Resolution 70m Ω 100m Ω 100m Ω Experimental Yes No NA Results [1] J. Xu, P. Harpe, J. Pettine, C. Van Hoof and R. F. Yazicioglu, "A low power configurable bio- impedance spectroscopy (BIS) ASIC with simultaneous ECG and respiration recording functionality," ESSCIRC Conference 2015 - 41st European Solid-State Circuits Conference (ESSCIRC) , Graz, 2015. [2] AFE4300, Integrated Analog Front-End for Weight-Scale and Body Composition Measurement , Texas Instruments 15
Experimental Results • Actual Bio-Impedance measurements on the upper arm with variable distance between sensing electrodes (1,2 and 3 cm) 16
Experimental Results • Respiration Rate – Real part of Bio- Respiration Cycle Ω Impedance (R(Z)) Ω Ω across frequency Ω for 30 seconds – Verified by (a) Real Bio-impedance |R(Z)| accelerometer placed on the chest (b) Acceleration 17
Experimental Results • Heart Rate – Real part of Bio- Impedance (R(Z)) shows (a) Real Bio-impedance |R(Z)| the heart rate and the respiration rate at fixed frequency (8 kHz) (b) Bio-impedance highpass filtered – Heart rate extracted by a high pass filter – Verified by ECG (c) ECG Signal 18
Experimental Results • Muscles Contraction – The real and Intervals of muscles contraction Ω imaginary bio- Ω Ω impedance for 30 Ω seconds Ω (a) Real Bio-impedance |R(Z)| Ω Ω Ω Ω Ω Ω Ω (b) Imaginary Bio-impedance |I(Z)| 19
Conclusions • Bio-Impedance Spectroscopy was measured from upper arm for the first time • A measurement system was presented with high accuracy of 0.07 Ω for the frequency range from 4-120 kHz and update every 150 ms • The System was evaluated using reference resistors and capacitors • Experimental results of BIS from upper arm were presented 20
Thanks & Questions Bassem Ibrahim, bassem@tamu.edu 21
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