Sensor Tracks Soldiers' Vital Signs

Brent D. Johnson

Battles may be won or lost on how well troops cope with stress and fatigue. In work sponsored by the US Army Medical Research and Materiel Command at Fort Detrick, Md., for the Warfighter Physiological Status Monitoring System program, investigators are developing wearable optical sensors that will provide electrophysiological data about soldiers so that field commanders can ensure that they are at their best for combat.

Initially developed as a voltage measurement solution for NASA's space power systems, the Photrode optical sensor from Srico Inc. uses radiation to detect the very low amplitude potentials produced by the human body. The output of a superluminescent diode enters the sensor, where it is perturbed by electrical signals sensed in the subject. The intensity-modulated light exiting the sensor is applied to an optical receiver, which detects and digitizes the voltage.

Wearable optical sensors may monitor the vital signs of tomorrow's soldier on the battlefield, detecting stress and fatigue. Courtesy of Srico Inc.

In a study at Walter Reed Army In- stitute of Research, Helen C. Sing subjected the Photrode system to a head-to-head test against standard silver/silver-chloride electrodes. An MP150 physiological monitoring instrument from Biopac Systems Inc. of Santa Barbara, Calif., collected and analyzed the brain activity information from the Photrode, as well as data on 12 other channels that indicated pulse and tracked chin, eye and wrist movements of the test subjects, matching performance to alertness levels.

Sing found that the electroencephalogram signals from the optical system were comparable to those from standard electrodes. She also observed that the system could detect the minute fluctuations in alpha waves when the test subjects opened and closed their eyes. The Photrode, however, displayed minimal sensitivity to electromagnetic interference and allowed the subjects to wear the monitoring system with little discomfort. Unlike systems that use metal electrodes, it does not require the use of conductive gels, which must be renewed every three hours, or of collodion adhesive, which must be removed with acetone.

Drowsiness monitor

In the long term, the Photrode system may be used to detect alertness and drowsiness levels as well as head trauma, possibly triggering an audible alarm for the soldier. With the new information they are collecting about high-frequency electroencephalograms, the Army researchers will develop a drowsiness index that could be used to initiate sleep discipline for soldiers.

Eventually, the system could be extended to the commercial arena, detecting and warning workers of incipient drowsiness in environments that require a continuous high level of alertness.