Copyright Interesting Engineering
In a major leap for wearable electronics, researchers at EPFL have created a fiber-based electronic sensor that stays fully functional even when stretched to more than ten times its original length. This breakthrough could transform the future of smart clothing, physical rehabilitation gear, and soft robotics. The secret ingredient? Liquid metal. But not the dangerous kind you might imagine. In the Laboratory of Photonic Materials and Fiber Devices (FIMAP) at EPFL’s School of Engineering, “liquid metal” refers to a safe, flexible mix of indium and gallium, a nontoxic alloy that stays liquid at room temperature and offers excellent electrical conductivity. The problem, however, lies in turning this alloy into something that can flex, stretch, and still conduct electricity reliably. “Liquid metals are extremely difficult to process,” explains FIMAP head Fabien Sorin, adding that “it’s especially hard to produce electronic fibers that combine high and stable conductivity with stretchability.” The EPFL team has now solved this problem using thermal drawing, a technique traditionally used to make optical fibers. “We have integrated thermal drawing into a greatly simplified process for producing fiber sensors with finely tailored electronic properties, making them promising candidates for smart textiles for sport and health monitoring applications,” Sorin says. Simple, sensitive, stretchable The process starts with a “preform”, a large-scale version of the electronic fiber that contains liquid metal components carefully arranged in a 3D pattern. This preform is then heated and stretched, much like molten plastic, to produce thin fibers that retain the same intricate internal structure. According to PhD student and first author Stella Laperrousaz, this 3D pattern is key to the breakthrough. It allows researchers to define which areas of a single fiber are electrically active or insulating. “When the liquid metal is mixed with a soft elastomer matrix, it forms many small droplets,” she explains. “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 fiber by controlling which areas become active.” Tests revealed that these fibers remained highly sensitive even when stretched more than tenfold, outperforming conventional stretchable sensors that often lose electrical performance when deformed. This balance between conductivity, elasticity, and simple processing makes EPFL’s method particularly attractive for wearable applications. A smart knee brace To demonstrate its potential, the team embedded the fibers into a soft knee brace capable of tracking a wearer’s movements. The brace successfully monitored joint bending while the subject walked, ran, squatted, and jumped, even reconstructing the person’s running gait with precision. “Thanks to its ease of integration, our fiber could easily be used to monitor motion and detect anomalies in other joints, such as the ankles, shoulder or wrist,” Sorin says. He adds that scalability is next on the team’s agenda. “Conventional electronic devices can be too fragile or too rigid to be integrated into textiles, but our fiber could be integrated into meters – or even kilometers – of fabric with sufficient scale-up. Such fabric could then be used to produce wearables, soft prostheses, or sensors for robotic limbs.”
