Smart textiles powered by soft transmission lines

Professor Fabien Sorin and doctoral assistant Andreas Leber, at the Laboratory of Photonic Materials and Fibre Devices (FIMAP) in EPFL’s School of Engineering, have developed a technology that can be used to detect a body’s movements – and a whole lot more. “Imagine clothing or hospital bed sheets capable of monitoring your breathing and other vital movements, or AI-powered textiles that allow robots to interact more safely and intuitively with humans” says Leber. “The soft transmission lines that we’ve developed open the door to all of this.”

Stretching, pressing and twisting

The researchers invented a sensor that can detect different kinds of fabric deformation such as stretch, pressure and torque – all at the same time. “Finding a method for differentiating all these convoluted movements was our biggest challenge, because it is very difficult for sensors to measure several stimulations simultaneously,” says Leber. “Also, conventional sensors in textiles have several drawbacks. First, they are fragile and break easily. Second, you need a lot of them to cover a large area, which eliminates many of the advantages of fabrics. And third, each type of conventional sensor can detect only one kind of deformation.”

But by incorporating concepts from reflectometry, Sorin and Leber were able to create soft fiber-shaped sensors that open up new doors for smart textiles. “Our technology works similar to a radar, but it sends out electrical pulses instead of electromagnetic waves,” explains Leber. “That means our fiber sensors operate like transmission lines, known from high-frequency communication. The system measures the time between when a signal is sent out and when it’s received, and uses that to determine the exact location, type and intensity of deformation.” This kind of detection technology has never before been used in structures combining extended mechanical flexibility and high electronic performance, which are key for measuring deformations.

Liquid metal and fiber optics processing

Creating the fibers is a complex task involving an optical fiber fabrication process applied to unusual materials such as elastomers or liquid metals that serve as the conductors. “The structure includes micrometer-size features and has to be perfect, otherwise it won’t work,” says Leber. With these fibers, the entire surface of a fabric becomes one large sensor. “The trick was to create transmission lines made entirely of soft materials, using a simple method that can be scaled up,” adds Sorin. The team’s research drew on a variety of disciplines including electrical engineering, mechanical engineering, and materials science. The next step will be to make the technology more portable by reducing the footprint of peripheral electronics.