With unobtrusive, portable devices for monitoring respiratory rhythm and heart rate during sleep expected to come up in future, scientists have developed physiological-sensing textiles that can be made into sleep garments which they have named 'phyjamas'.

Graduate students Ali Kiaghadi and S Zohreh Homayounfar, with their professors Trisha L Andrew, a materials chemist, and computer scientist Deepak Ganesan, introduced their health-monitoring sleepwear at the Ubicomp 2019 conference in London, UK.

A paper detailing the work has been chosen for publication in the Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies (IMWUT).

"Generally, people assume that smart textiles refer to tightly worn clothing that has various sensors embedded in it for measuring physiological and physical signals, but this is clearly not a solution for everyday clothing and, in particular, sleepwear," explained Professors Trisha L Andrew.

"Our insight was that even though sleepwear is worn loosely, there are several parts of such a textile that are pressed against the body due to our posture and contact with external surfaces. This includes pressure exerted by the torso against a chair or bed, pressure when the armrests on the side of the body while sleeping, and light pressure from a blanket over the sleepwear," Ganesan added.

"Such pressured regions of the textile are potential locations where we can measure ballistic movements caused by heartbeats and breathing," he explained, "and these can be used to extract physiological variables."

The difficulty is that these signals can be individually unreliable, particularly in loose-fitting clothing, but signals from many sensors placed across different parts of the body can be intelligently combined to get a more accurate composite reading.

Researchers explained that their team had to come up with several new ideas to make their vision a reality. They realised that there is no existing fabric-based method to sense continuous and dynamic changes in pressure, particularly given the small signals that they needed to measure.

So they designed a new fabric-based pressure sensor and combined that with a triboelectric sensor -- one activated by a change in physical contact -- to develop a distributed sensor suite that could be integrated into loose-fitting clothing like pyjamas.

The team also developed data analytics to fuse signals from many points that took into account the quality of the signal coming in from each location.

The authors reported that this combination allowed them to detect physiological signals across many different postures.

They performed multiple user studies in both controlled and natural settings and showed that they can extract heartbeat peaks with high accuracy, breathing rate with less than one beat per minute error, and perfectly predict sleep posture.