Smart textile sensor measures body movement to detect onset of fatigue

The researchers “integrated it into a pair of athletic leggings”, according to the university. “Simply by glancing at their smartphone, testers were able to see when they were reaching their limit and if they ought to take a break.”

This is not that first time a capacitive sensing yarn has been constructed, but this “yarn is considerably more sensitive to minimal movements. Stretching it even a little produces distinctly measurable fluctuations in the sensor’s charge”, said researcher Tyler Cuthbert.

A great deal of experimental and theoretical work went into finding a structure that achieves this characteristic, which produced a yarn where one electrode is a relatively plump conductive elastomer (left-est), around which thin insulated copper wire – the other electrode – is wound in a helix. On pulling, the copper wire straightens, biting into the elastomer and bulging it into a fatter helix (close left).

Fattening when stretched is so unusual that the effect has its own name: ‘auxetic’ behaviour.

“Capacitance change comes from a combination of the distance between the electrodes and then the change in the area between the electrodes,” Cuthbert told Electronics Weekly. “The auxetic character results in an interaction between the two electrodes in the form of engulfment, which in turn results in greater area between the electrodes. The proper combination of parameters – diameters, helical pitch and ratios – results in obtaining the auxetic character. This in turns allows it to reach a superior sensitivity.”

Theoretical grasp was such, that the team also designed helical combinations with almost zero capacitance change as the double helix yarn was stretched.

The final sensing assembly was passive, with the terminals of the sensing capacitor connected directly to the ends of a loop antenna (right), which would eventually couple to a phone.

For realistic testing, sensor was sewn onto close-fitting sports leggings (left), with the antenna stitched with conductive thread directly into the wast band.

“Since the sensor is located so close to the body, we can capture body movements very precisely without the wearer even noticing,” said researcher Professor Carlo Menon.

Yarn-based capacitive sensor team, from the left: Carlo Menon, Valeria Galli, Chakaveh Ahmadizadeh and Tyler Cuthbert

Less regular strides and shortened gait are indicators of the onset of tiredness in runners, said ETH, and the sensing shorts were able to detect this situation.

The work is covered in the thorough and clearly presented paper ‘HACS: Helical auxetic yarn capacitive strain sensors with sensitivity beyond the theoretical limit‘, published in Advanced Materials, and available in full without payment.

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