A highly stretchable carbon nanotubes/thermoplastic polyurethane fiber-shaped strain sensor with porous structure for human motion monitoring

Feeling strain isn't always bad. Scientist are developing new materials that can detect strain, with likely applications including devices that monitor health and new ways to control computers. In the journal Composites Science and Technology, researchers in China report what they believe is a significant advance in making flexible strain-sensing fabric.

“Although many efforts have been made to improve sensitivity, increase the response range and achieve large-scale production, it is still a great challenge to achieve a nice balance between these three key factors,” explains Kun Dai of the research team at Zhengzhou University. Dai believes his team have achieved a good balance by combining the emerging technology of carbon nanotubes with more conventional polyurethane polymer technology.

Carbon nanotubes are long cylindrical structures composed of many linked hexagonal arrangements of bonded carbon atoms. The versions used by Dai and his colleagues are ‘multi-walled carbon nanotubes’ (MWCNTs), with cylinders contained within others of increasing width. Tubes within tubes, in other words.

To make the materials, commercially available MWCNTs are added to a suitable solvent and dispersed evenly using a 15-minute blast of ultrasound. Molecules of polyurethane polymer are then added, followed by another two hours of mixing stimulated by ultrasound. Extruding the mixture into water through a needle causes fine fibers of the MWCNT-polyurethane composite to form.

Dai explains that this simple “wet-spun” process for making fibers is one of the key advantages, offering a cost-effective procedure that should be suitable for scaling up for eventual commercialization. The fibers can readily stretch and twist and can also be stitched and woven, which is crucial for incorporating them into fabrics. They are also unusually porous, which is an advantage for any material to be worn next to skin.

To demonstrate the potential for making strain-sensors, the researchers mounted copper electrodes on the ends of the fibres. Stretching changes the electrical resistance of the fibres, which is sensed by the electrodes, providing the basis for generating signals that report the level of stretching and strain.

The basic system was tested by taping it to the finger, elbow or knee joints of human subjects. Movements ranging from gentle flexing of a finger to the extreme motions accompanying squat jumping were reliably and repeatedly converted into clear electrical signals.

In tests so far, the material has sustained its signalling performance through 9,700 cycles of 100 percent strain and relaxation. “The results clearly demonstrate that our system is a nice candidate for use in wearable smart materials,” says Dai.

The research team now plan to take some of the first steps required to move the technology toward specific applications. “We plan to send the electrical signals by wireless transmission to a mobile phone to achieve online monitoring everywhere and anytime,” Dai explains.

Eventual applications might range from following a patient's recovery from illness or recording progress in exercise regimes, to controlling external equipment and allowing robots to detect the movements and strains involved in their physical manipulations.

Article details:

Dai, K. et al.: "A highly stretchable carbon nanotubes/thermoplastic polyurethane fiber-shaped strain sensor with porous structure for human motion monitoring," Composites Science and Technology(2018)

Kun Dai on WeChat: DaiKun_ZZU