Textile-embedded Sensors for Wearable Physiological Monitoring Systems

Abstract

For long-term physiological monitoring inside or outside a hospital setting, a reliable, wearable monitoring system would be a convenient platform if biomedical sensors are securely placed in appropriate positions. An article of clothing is an attractive platform to implement such a wearable system. It is highly desirable that the sensors be designed and integrated into the garment in an unobtrusive way.

The purpose of the dissertation is to develop textile-embedded biomedical sensors that can be integrated into textile substrates in a seamless manner for long-term ECG and respiration monitoring while normal daily activities including walking, jogging, sleeping, sitting, and other exercise are transpiring. These sensors should provide both a comfortable textile interface and robustness against noise and motion artifacts.

For ECG monitoring, we developed textile-embedded active electrodes that transform high input impedance signals to low impedance versions by employing a voltage follower circuit. The fabric active electrodes include a transducer layer on the top of the nonwoven substrates and a circuit layer on the bottom. The transducer area, signal path and power lines are filled with Ag⁄AgCl ink by screen printing or hand painting. The electrical components and external wires were attached using adhesive conductive inks and protected by another textile covering layer.

For respiration monitoring, we devised a fabric sensor structure based on double nonwoven substrates. Stretchable and non-stretchable segments of nonwoven fabrics are laterally attached by, for example, ultra sonic bonding. The stretchable fabrics are employed in belts around the chest and abdomen and respond to breathing effort by changing the sensor's length in the direction of the strain applied. Rectangular plates for a capacitive sensor or an open-rectangular spiral for an inductive sensor is deposited on the non-stretchable fabric portions of the sensors by printing or painting silver ink. Their relative positions change when the stretchable portion activates. Each plate is initially placed so that the conducting areas overlap minimally. As the stretchable portions of the device are exercised, the two plates slide in opposite directions, changing the effective area and hence the capacitance or inductance values. These capacitance or inductance variations are transformed into voltage outputs by electronic circuits individually designed for each sensor. For single and differential modes of operation in the capacitive sensor, various electrode patterns are suggested. For the inductive sensor, various configurations of spirals are presented to form three different types of planar inductive displacement sensors: a single inductor sensor, a transformer-type differential sensor, and an autotransformer-type differential sensor.

In addition to the design based on double substrates, we demonstrate a respiratory inductive sensor based on a single substrate. To form an inductive sensing area, fine magnet wires are stitched on a stretchable nonwoven substrate. The textile substrates supporting the conducting materials are then laminated to stabilize the geometric structure relationships and mechanically protect the sensor.

Finally, we transform these textile-embedded sensors into a wearable human physiology monitoring system. The various elements of the system are described. Finally, we discuss the possibility of using the system for sleep apnea detection and sleep staging.