When woven into clothes, piezoelectric fibres will transform mechanical energy into electric energy every time a person moves. Credit: Katharina Maisenbacher, Max Planck Institute

When woven into clothes, piezoelectric fibres will transform mechanical energy into electric energy every time a person moves. Credit: Katharina Maisenbacher, Max Planck Institute

Researchers have found a way to produce nylon fibres that are smart enough to generate electricity from simple body movement, paving the way for smart clothing that will monitor our health through miniaturised sensors and charge our devices without any external power source.

The discovery – a collaboration between the UK's University of Bath, the Max Planck Institute for Polymer Research (MPI-P) in Germany and the University of Coimbra in Portugal – is based on breakthrough work on solution-processed piezoelectric nylons led by professor Kamal Asadi, former group leader at the MPI-P and now professor in the department of physics at Bath and his former PhD student Saleem Anwar.

Piezoelectricity describes the phenomenon where mechanical energy is transformed into electric energy. When you tap on or distort a piezoelectric material, it generates a charge. Add a circuit and the charge can be taken away, stored in a capacitor for instance and then put to use – for example, to power your mobile phone.

While wearing piezoelectric clothing, such as a shirt, even a simple movement like swinging your arms would cause sufficient distortions in the shirt's fibres to generate electricity.

Asadi said: "There's growing demand for smart, electronic textiles, but finding cheap and readily available fibres of electronic materials that are suitable for modern-day garments is a challenge for the textile industry.

"Piezoelectric materials make good candidates for energy harvesting from mechanical vibrations, such as body motion, but most of these materials are ceramic and contain lead, which is toxic and makes their integration in wearable electronics or clothes challenging."

Scientists have been aware of the piezoelectric properties of nylon since the 1980s, however, the silky, man-made fabric often associated with cheap T-shirts and women's stockings is "a very difficult material to handle", according to Asadi.

In its raw polymer form, nylon is a white powder that can be blended with other materials (natural or man-made) and then moulded into myriad products, when nylon is reduced to a particular crystal form it becomes piezoelectric. The established method for creating these nylon crystals is to melt, rapidly cool and then stretch the nylon. However, this process results in thick slabs (known as 'films') that are piezoelectric but not suited to clothing. The nylon would need to be stretched to a thread to be of woven into garments, or to a thin film to be used in wearable electronics. 

Asadi and Anwar dissolved the nylon powder in an acid solvent rather than by melting it. However, they found that the finished film contained solvent molecules that were locked inside the materials, thereby preventing the formation of the piezoelectric phase.

The pair discovered that by mixing the acid solution with the acetone (a chemical best known as a paint thinner or nail varnish remover), they were able to dissolve the nylon and then extract the acid efficiently, leaving the nylon film in a piezoelectric phase.

"The acetone bonds very strongly to the acid molecules, so when the acetone is evaporated from nylon solution, it takes the acid with it. What you're left with is nylon in its piezoelectric crystalline phase. The next step is to turn nylon into yarns and then integrate it into fabrics."

Developing piezoelectric fibres is a major step towards being able to produce electronic textiles with clear applications in the field of wearable electronics. The goal is to integrate electronic elements, such as sensors, in a fabric, and to generate power while we're on the move. Most likely, the electricity harvested from the fibres of piezoelectric clothing would be stored in a battery nestled in a pocket. This battery would then connect to a device either via a cable or wirelessly.

"In years to come, we could be using our T-shirts to power a device such as our mobile phone as we walk in the woods, or for monitoring our health," said Asadi.