Bioimpedance is uniquely suited to measure several “vital signs” including heart rate, respiratory rate, and intravascular volume status. (There is also some evidence that the arterial waveform present in the signal correlates to blood pressure.) Most clinically available measurement apparatuses require bedside monitoring and a stationary subject. To test that viability of bioimpedance as a signal in situ, a wearable bioimpedance monitor was developed using a reconfigurable frontend (to assess interface effects between bipolar and tetrapolar configurations) and low-power electronics (to enable long-term measurements).
A block diagram representation of the wearable bioimpedance monitor devel- oped. Electrodes are placed on an objected and connected via a 3.5 mm audio jack to the device. The device itself then measures the impedance, sends that data to a central pro- cessor, then transmits that data either wirelessly (Bluetooth) or through a USB connection. The USB connection also serves to program the device and charge the battery.
Operation of the AD5933. A sinusoidal voltage signal at a designated frequency is generated via the combination of the master clock (MCLK), an oscillator, and a direct digital synthesizer (DDS). This discrete signal is then converted to analog and amplified, reaching the material as VOUT. Impedance in the material, Z(ω), is found by passing the current at VIN through an amplifier, filtering, converting to a digital signal, then taking the discrete Fourier transform at the digital signal processing (DSP) engine with information supplied by the DDS core. The real and imaginary components of the impedance are then passed to an inter-integrated circuit (I2C) communication interface and transmitted out through serial data (SDA) and serial clock (SCL) lines.
The reconfigurable frontend designed to work with the AD5933 shown in Fig- ure 4.3. Such a front end allows a bipolar or tetrapolar electrode arrangement to be specified on the fly, enabling researchers to choose that which best fits their needs.Early breadboard prototype of the AD5933 circuit with reconfigurable frontend being used to test known impedance values.
A simplified representation of the printed circuit board (left) and an actual printed circuit board (right). In the top left corner is a button flanked, in the top right is another button and LED combination, the battery connector, and the sliding switch to turn the device on and off. Towards the middle of the board is a reset button, microcontroller, and AD5933 and its analog front end. At the bottom from left to right is a 3.5 mm audio jack connection, a micro-USB connector, and the Bluetooth antenna.
TJ holding up a printed circuit prototype of the wearable bioimpedance sensor showing that it is about the size of a typical credit card.
Data showing the accuracy of the wearable sensor in bipolar and tetrapolar modes as compared to expectations for various biological relevant impedance values.
