The core of the fibre comprises a liquid metal made from indium and gallium incorporated into a soft elastomer matrix of styrene–isoprene–styrene (SIS). This core is clad with a sheath of styrene-ethylene-butylene-styrene (SEBS).
The fibre is produced using a thermal drawing technique. The process begins with the construction of a macroscopic preform that contains the liquid-metal components carefully arranged in a three-dimensional (3D) pattern and clad with SEBS. The preform is then drawn through a narrow opening to produce a fibre that contains all of the components, in their same relative positions, but greatly reduced in diameter. The researchers say that the process allows them to control which areas of an individual fibre are active (electrically conductive) or inactive (insulating).
A PhD student at EPFL, Stella Laperrousaz, explains: “When the liquid metal is mixed with a soft elastomer matrix, it forms many small droplets. The process of heating and stretching the preform breaks these droplets and activates the liquid metal. This means that we can finely tune the functionality of a single fibre by controlling which areas become active through the shear stress caused by the preform stretching process.”
Experiments conducted by the researchers have shown that capacitive fibre sensors produced using the method remain highly sensitive, even when stretched to over ten times their original length, giving the technique a significant advantage over other methods for producing such fibres, which struggle to guarantee electrical performance, elasticity and ease of processing.
As a proof-of-concept, the researchers integrated their electronic fibres into a soft knee brace and then recorded the device’s performance while a subject walked, ran, squatted and jumped. The brace reliably monitored the bending angle of the wearer’s knee and was even able to reconstruct accurately their gait during running.
The Head of the Laboratory of Photonic Materials and Fiber Devices (FIMAP) in EPFL’s School of Engineering, Fabien Sorin(1), says: “Thanks to its ease of integration, our fibre could easily be used to monitor motion and detect anomalies in other joints, such as the ankle, shoulder or wrist”. He adds that the thermal drawing process is also potentially highly scalable.
Sorin concludes: “Conventional electronic devices can be too fragile or too rigid to be integrated into textiles, but our fibre could be integrated into metres – or even kilometres – of fabric with sufficient scale-up, which is what we are working on next. Such fabric could then be used to produce wearables, soft prostheses, or sensors for robotic limbs."