Electrical properties of thermally stable e-textiles. (a) Optical image of cocoons from the Bombyx mori silkworms. (b) SEM image of cocoons composed of silk fibroins and sericins. (c) Schematic of the structure of silk fibroin with hydrogen-bonded b-sheets and amorphous domains (H: white, C: gray, O: red, N: blue). (d) Optical image and (e) SEM image of silk fibroins after removing silk sericins via degumming process. (f) Optical image and (g) SEM image of LOPy obtained by the pyrolysis of silk fibroins under axial tension. (h) A lighted blue LED connected with LO-Py1000 stitched into a CS fabric using a needle. (i) Electrical conductivity of LO-Py as a function of temperature. Variation in the conductance of the LO-Py1000 as a function of (j) bending degrees and (k) bending cycles. The inset images indicate each bending state. Optical images of (l) the lighted blue LED stitched into a piece of fire blanket using the LOPy1000 (m) the blue LED lamp remains lighted even after heating using an alcohol lamp.
Electrical properties of thermally stable e-textiles. (a) Optical image of cocoons from the Bombyx mori silkworms. (b) SEM image of cocoons composed of silk fibroins and sericins. (c) Schematic of the structure of silk fibroin with hydrogen-bonded b-sheets and amorphous domains (H: white, C: gray, O: red, N: blue). (d) Optical image and (e) SEM image of silk fibroins after removing silk sericins via degumming process. (f) Optical image and (g) SEM image of LOPy obtained by the pyrolysis of silk fibroins under axial tension. (h) A lighted blue LED connected with LO-Py1000 stitched into a CS fabric using a needle. (i) Electrical conductivity of LO-Py as a function of temperature. Variation in the conductance of the LO-Py1000 as a function of (j) bending degrees and (k) bending cycles. The inset images indicate each bending state. Optical images of (l) the lighted blue LED stitched into a piece of fire blanket using the LOPy1000 (m) the blue LED lamp remains lighted even after heating using an alcohol lamp.

A new silk-based textile can both conduct electricity and withstand high temperatures, according to the team of Korean researchers that developed it [Jeon et al., Materials Today (2018), https://doi.org/10.1016/j.mattod.2018.03.038]. Electronic or e-textiles could enable a new generation of portable, flexible electronic devices, particularly if embedded into clothing, packaging or other objects. But most current e-textiles, such as graphene oxide-coated nylon, cotton, polyester and silk, are complex to produce and cannot withstand heat or high-temperature treatments.

Natural silk from the silkworm Bombyx mori consists of chains of biopolymer proteins that can survive heating even to extreme temperatures (up to 2800°C). Byung Hoon Kim and his colleagues at Incheon National University, Inha University, Korea Institute of Science and Technology, Korea University of Science and Technology, Sungkyunkwan University, and the University of Seoul used this attribute to create e-textiles from heat-treated, stretched silk proteins. Long-range ordered pyroproteins (LO-Py) are stretched and heat-treated (or annealed) at different temperatures ranging from 800°C to 2800°C and then fabricated into yarn and e-textiles.

When the silk proteins are pyrolysed, the crystalline beta-sheets in the structure, which are interspersed with amorphous chains, are transformed into electrically conducting sheets of carbon atoms arranged in a hexagonal pattern. The e-textiles produced from pyroproteins are both conducting (on the order of 103 S/cm) and thermally durable, while maintaining the flexible properties of silk. The electrical conductivity also increases as temperature increases from 30-400°C, but returns to its original value when cooled.

“This is the first time that e-textiles have been fabricated from pyroprotein,” says Byung Hoon Kim, who led the research. “The fabrication method is very simple compared with previously reported e-textiles, and the fabric is highly conductive and thermally durable.”

The natural strength and flexibility of silk mean that the e-textile can withstand repeated cycles of bending and flexing without losing conductivity. Meanwhile, because silk-based e-textiles can withstand high temperatures, other materials can be deposited onto the fabric surface using standard techniques like sputtering or evaporation. This attribute provides an easy route to tailor the electrical properties. For example, depositing ZnO, niobium nitride (NbN), or molybdenum diselenide (MoSe2), respectively, gives the e-textile semiconducting, superconducting or light emitting properties.

 “We are now investigating our pyroprotein-based e-textiles for energy harvesting devices such as piezoelectric, thermoelectric, or photovoltaic devices,” Kim told Materials Today, “but we are not sure yet when we will be able to apply our e-textile in actual devices.”