Knitted ‘muscles’ to get people moving again

A conceptual model of a textile exoskeleton with the textile actuator (black) on an elastic elbow sleeve (white).
Thor Balkhed / Linköping University

Your knitted jumper might one day do more than keep you cosy and warm – textile physicists have shown woven materials could form wearable, artificial muscles to help people with movement problems. 

Researchers in Sweden knitted plant-based fibres into a material that they dipped in a conducting fluid. When they passed a voltage through the material, it deformed and was able to lift small weights. 

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The soft, silent artificial muscle, called an actuator, was unveiled in Science Advances. The researchers hope they will eventually help the elderly stay independent, injured people to assist rehabilitation and perhaps paraplegic people to walk again.

Exoskeleton devices such as prosthetics currently used to help people with limited mobility can work well but are often driven by electric motors. While they are powerful and becoming more natural-looking and sophisticated, they’re generally still bulky, stiff and noisy. 

Wearable electronics are also already on the market, but they’re mostly based on rigid electronic parts. 

Textile actuators, on the other hand, are looking like a promising alternative. Being made from fabric, they could be worn like a suit under your clothes and be barely noticeable, according to materials engineer Jian Fang from Deakin University in Australia, who was not involved in the study.

“They use cellulose, which is a very common material, and a very traditional manufacturing process, to achieve very encouraging results – that’s the most exciting part,” Fang says. 

But to effectively mimic human muscles, the textile actuators must have a certain level of stretchiness and strength to lift objects.

A close-up photograph of a knitted textile actuator (or textile muscle) showing individual loops.
Thor Balkhed / Linköping University

For greater strength, or ‘force’, the scientists used woven crisscrossing yarn. And to make it more stretchy, or ‘stressed’, they knitted and looped the yarn. 

“There is a wide variety of weaving and knitting patterns that we can exploit, some give more force, some more strain,” lead author Ali Maziz from Linköping University says.

“We can mix different materials during the processing, thus integrating different functions into the fabric.” 

So how do you electrify cellulose fibres? 

The scientists coated the material with a fluid conductive polymer called polypyrrole. Interactions between the fluid’s positive and negative charges caused the material to expand or shrink with electrical stimulation.

To test it out, the textile actuator was connected to a small lever made of Lego and dunked into the charged fluid. The material managed to move the lever, but only hold up an extra two grams. 

And for it to work, the material had to remain in the fluid, rather than operating freely in the air. But Maziz says he and his colleagues are now working on how to overcome this obstacle.  

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