A team led by researchers from the Indian Institute of Technology (IIT) Bombay has developed an inexpensive, flexible pressure sensor that can be used for a variety of health-care applications. The piezoresistive pressure sensor could efficiently monitor even small-scale movements caused by low-pressure variations.
Pulse rate
The sensor could measure blood pulse rate in real time when placed on the wrist and neck. It had the same sensitivity and accuracy at both sites of the body. The sensor was also tested for its ability to monitor respiration. When placed on the throat, the sensor could detect changes in pressure when different words were pronounced. Interestingly, the fabricated sensor also showed good sensitivity in detecting large-scale motion monitoring, as in the case of bending and extension of fingers joints. The results were published in the journal ACS Applied Materials and Interfaces.
“While researchers have been working to develop sensors that can detect very small change in pressure, our pressure sensor is able to detect both small-scale motion caused by low pressure (less than 2.7 kPa) and large-scale motion at high pressure,” says Amit Tewari at IITB-Monash Research Academy, IIT Bombay and first author of the paper. “The sensor exhibited good flexibility and reproducibility over 5,000 cycles.”
“When you speak, the throat muscles respond differently based on the change in pressure. The vocal muscles undergo different motions when different words are pronounced. The sensor was able to detect the slight change in pressure when different words were said,” says Prof. Dipti Gupta from the Department of Materials Science and Engineering from IIT Bombay, who collaborated with researchers from Cambridge and Monash University, Australia.
Word recognition
The researchers tested its sensitivity in recording the difference in pressure when different words were said, and when the same word was repeated several times. “The sensor can be used as a word-recognition device. This is only preliminary work and more has to be studied before the sensor can be used for speech recognition,” says Prof. Gupta.
The device was also able to detect the pulse rate. “It is a proof-of-concept study and more work needs to be done. We are yet to calibrate the sensor,” she says. Since the sensor was able to detect differences in blood pulses, Prof. Gupta feels it can be used as a wearable sensor for long-term and continuous monitoring of heart rate.
In the case of finger bending and extension, which involves monitoring the large-scale motion, the sensor showed high sensitivity. The sensor generated different current signals when the index finger, to which it was attached, was bent. The current signal was the least when the angle of bending was small (15 degree) and maximum when the angle was high (90 degree).
“We have not tested the change in pressure due to change in direction of movement of the finger,” she clarifies.
The sensor is made using polyurethane foam coated with carbon nanomaterial-based ink. The ink is conductive due to the presence of large number of multiwalled nanotubes which are dispersed in reduced graphene oxide matrix. Conductive sheets were pasted on the top and bottom sides of the foam and electrical wires connected to the sheets for measurements.
“When the PU foam coated with the ink is perturbed, in this case compressed, the air gaps are removed and the foam gets thinner. This provides a conduction path for electrical charges. The resistance drops as the foam is compressed and it becomes more conductive,” says Tewari.
