Physiological Sensors

We have developed a suite of sensors integrated into an unobtrusively wearable chestband that can provide a continuous measure across a variety of physiological signals in natural environments. This work is led by the Ohio State University team.

Sensors: These sensors enable capture of physiological indices of stress/emotion. The chestband currently consists of the following sensors.

• Two lead ECG (Lead II configuration) – 0.05 Hz HP, 35 Hz LP Filters, Adaptive Gain
• Respiratory Inductive Plethysmography band for measurement of respiration rate at rib cage – 0.05 Hz HP, 10 Hz LP
• Skin conductance Response (SCR) between two electrodes placed under the chestband – 0.05 Hz HP, 10Hz LP
• Skin temperature with surface probe thermistor – 10 Hz LP
• Three-axis accelerometer for motion sensing – 50 Hz LP

Each sensor, and the associated bio-amplifiers and signal conditioning circuitry is integrated with two wireless motes. The wireless mote platform provides a 250 kpps 802.15.4 wireless radio, 8 channel A/D and an 8 MHz microcontroller for on board digital signal processing. Mote 1 hosts the ECG, Temperature (skin & ambient), GSR, and Accelerometer sensors. Mote 2 hosts the Respiratory Inductive Plethysmograph (RIP) sensor. Features, such as location of r-peaks in ECG signals are computed directly on the mote, while tens of other features are computed once the signals are wirelessly communicated to a smart phone. The form factor for the two motes hosting sensors appear below.

Two Wireless Hops to a Smart Phone: An additional 3rd mote bridges communication between the ultra low power Zigbee radion on the sensor motes and the Bluetooth radio on a modern mobile smartphone (Android G1). The mobile phone assesses signal integrity & artifacts, computes additional features, makes inferences about the user (e.g., user stress level), stores signals and inferences, and sends data to an external database over WiFi or a GSM cellular network. The Bridge mote also supports an SD card, which can be used to locally store data, for scenarios when connection to a smart phone is not available (e.g., phone not in the vicinity, phone out of battery, etc.). It can also act as a replica for data storage providing additional protection to the data.

End-to-End System

Battery Lifetime: The system has an ultra-low power design. It can support one week of continuous measurement across all the sensors and realtime wireless transmission of measurements on a 750 mAh battery, on a single charge.

Signal captured from the AutoSense system on a laptop appear below. Red is respiration and green is the ECG.