Fiber-based textiles are the ideal starting point for wearable electronics because they are softer, more flexible, breathable, and biocompatible than plastic- or elastomer-based alternatives. Metals, conducting polymers, and carbon-based materials have all been integrated with textiles to create the building blocks of electronic textiles but do not stand up well to washing.

Now researchers from the Universities of Cambridge and Jiangnan have made flexible capacitors from layers of polyester coated with graphene and hexagonal boron nitride (h-BN) inks that can withstand repeated washing cycles without losing function.

“In this work, we enabled the first energy storage device made entirely of fabric, based on graphene and h-BN, which is breathable, washable, and skin-compatible,” says Felice Torrisi, who led the effort. “All-textile-based electronics are key to enable future wearable electronics.”

Graphene and other two-dimensional materials like h-BN offer a promising alternative to metals and conductive and dielectric polymers for electronic textiles as they can be readily formulated into inks or thin films and are more environmentally stable and biocompatible. Most conductive fabrics to date rely on graphene oxide (GO) because its oxygen functional groups hydrogen bond to textiles like cotton, wool, and silk, but require a chemical or thermal reduction step to improve conductivity.

Torrisi and his colleagues avoid this reduction step by creating conductive inks from exfoliated graphene and h-BN in volatile solvents. The inks can be readily deposited onto polyester using a simple ‘dip and dry’ process, in which the fabric is repeatedly dipped into the ink and dried. A ‘hot press’ step, in which the fabric is heated to 200°C, removes the solvent and ensures good bonding between the graphene nanoplatelets and polyester fibers. Not only are the resulting fabrics resistant to water, they are superhydrophobic, so that water droplets pool on the surface of the material and roll off.

The researchers stacked together layers of the graphene and h-BN fabrics to create a typical parallel plate capacitor structure. The edges of the layers are sealed and then the entire sandwich-like structure squeezed and heated to ensure good adhesion.

“We designed the textile capacitor by stacking the graphene and the h-BN functional fabrics to form a conductive/dielectric/conductive sandwich-like heterostructure,” explains Torrisi. “Our electronic textile is a portable, washable, and breathable energy storage element that can store and release energy when it is needed to power textile devices such as sensors, light-emitting fabric or speakers.”

For now, the textile devices can only store enough energy to power a small sensor, but improved designs could improve storage capacity.

“Turning textiles into functional energy storage elements could open up an entirely new applications, from body-energy harvesting and storage to the Internet of Things,” says Torrisi. “In the future, our clothes could incorporate these textile-based charge storage elements to power wearable devices.”

Qiang et al., Nanoscale (2019), DOI: 10.1039/c9nr00463g