Novel Double Helix Yarn Piezoresistive Sensor for Pulse Monitoring

Cotton woven fabrics are used in remote medical diagnosis with intelligent electronic sensing technology. However, derived anomalous signals are one of the problems yet to be solved. A research led by a scholar from the City University of Hong Kong (CityU) was inspired by the structure of DNA and found that textiles woven with a double helix structure can help address the said challenge. The team has developed a flexible piezoresistive sensor to detect the human pulse. The sensor responds rapidly, recovers quickly after being pressed, and its signal is stable. The team hopes to introduce the sensor to the growing medical and healthcare market.

The research team is led by Professor Hu Jinlian, Professor in the Department of Biomedical Engineering and Director of the Laboratory of Wearable Materials for Healthcare of CityU. The research results have been published in the academic journal advanced Materials Under the title Ultrafast-Response/Recovery Flexible Piezoresistive Sensor with DNA-Like Double Helix Yarn for Epidermal Pulse Monitoring.

Reducing hysteresis by structural modification of yarn

“Textile-based sensors have many advantages, such as flexibility, stretchability and breathability, so they are used to make flexible piezoresistive sensors,” Professor Hu said. However, textile sensors usually have hysteresis, which means they will be difficult to fix once the sensor is resized. In addition, the signals obtained from ordinary textile sensors are inconsistent, so the resulting measurements are often non-repeatable, which hinders their application.

To solve the problem of hysteresis, a traditional strategy is to integrate elastic polymers into textiles. Instead, Professor Hu’s team started with the structure of the yarn itself. They made yarn with a double helix structure similar to DNA and then twisted two pieces of yarn together to make fabric. Professor Hu explained, “This method efficiently folds two pieces of yarn into one, resulting in a stable binding force between the two pieces of yarn, so that the fabric does not loosen and can recover quickly when deformed.” “

Fabric Based Flexible Piezoresistive Sensor for Epidermal Pulse Detection

Professor Hu pointed out that the main reason for the hysteresis of the general textile sensor is that after the yarn is compressed and deformed, the yarn cannot be cured after mutual friction. However, the yarn made by his team adopts a double helix structure, which ties the yarns tightly together and reduces slip caused by friction, to reduce hysteresis.

On this basis, the team developed a fabric-based flexible piezoresistive sensor with ultrafast response and recovery capability, which was used in experiments with epidermal pulse monitoring.

The textile-based flexible piezoresistive sensor developed by the team consists of five layers. In the middle layer the fabric, which was woven from yarn with double helix structure, was placed on a layer of electrodes and a lower layer of polyimide (PI) substrate. The researchers then sealed the three layers with gel, and covered the top surface with a layer of polyethylene naphthalate (PEN) film.

After the cloth-based flexible piezoresistive sensor is triggered, the cloth will generate resistance at the deformed electrode layer by the epidermal pulse beats and receive different signals according to the change of pulse. The team’s experiments confirmed that the cloth-based flexible piezoresistive sensor can detect the epidermal pulse on the neck and wrist, and that monitoring of wrist pulse signals before and after exercise is different. In addition, the wrist pulse signals collected from normal women and men with sinus bradycardia were also significantly different.

Ultrafast response and recovery time

“Compared to other textile sensors, our device has better sensitivity. It can sense the pressure of grains weighing only nine milligrams. Furthermore, the sensor responds so rapidly that it generates a signal in just two milliseconds (ms). can, and it takes just one ms to recover from a single beat of the pulse after the press,” remarked Professor Hu. The textile-based flexible piezoresistive sensor has been tested and proven to be durable enough to be reused 6,000 times while maintaining 95% of its initial performance. “Most importantly, we have solved the problem of hysteresis of ordinary textile sensors. Experimental results show that the sensor receives signals continuously and exhibits excellent and balanced performance in all aspects,” concluded Professor Hu.

Professor Hu described this set of new and efficient textile-based flexible piezoresistive sensors as an innovation in the technology of epidermal pulse detection. It can be flexibly assembled on wearable devices and helps promote the modernization of pulse palpation in traditional Chinese medicine.