Fiber, as a wearable material, offers breathability, flexibility, and resistance to wear, making it an ideal substrate for wearable devices. Using mature textile technology, color-changing fibers can be integrated into clothing to serve as an interface between humans and computers. The use of light-emitting, color-changing fiber as an interface for communications, navigation, healthcare, and Internet of Things is expected to grow. Inspired by photochromic fibers that exhibit fluorescence effects and polymer optical fibers that emit light when coupled with an external source, scientists from Huazhong University of Science and Technology and Nanjing University created a multicolored, uniformly luminescent, photochromic fiber. They prepared the fiber using a mass-producible, thermal-drawing method that allows versatility in the design of the fiber structures. (a): Photograph of the industrial-scale fabrication line of the photochromic fiber. The scale bar corresponds to 0.5 meters. (b): Schematic illustration of the fabrication of the photochromic fiber. The inset shows a photograph of the fabricated and illuminated photochromic fiber. The scale bar corresponds to 10 centimeters. (c): Comparison of the luminescence attenuation in the transmission direction between this work and the commercial product (light diffusing fiber). (d-f): Cross-sectional optical micrograph of three types of photochromic fiber, showing a different number of cores in the fiber: (d) for single-core red color, (e) for dual-core red and green colors, and (f) for tri-core red, green, and blue colors. The scale bar in each case corresponds to 200 μm. (g-i): Photographs of the photochromic fiber under radial observation: (g) for red color at 0°, 90°, 180°, and 270° angles, (h) for dual-core red and green colors at 0°, 90°, 180°, and 270° angles, and (i) for tri-core red, green, and blue colors at 0°, 90°, 180°, and 270° angles. The scale bar in each case corresponds to 500 μm. Courtesy of Guangming Tao. According to the researchers, currently available commercial light-emitting fibers cannot guarantee uniform brightness in light transmission because they are prone to transmission losses and artificial defects. The new design for a photochromic fiber overcomes these issues. The researchers designed independent waveguides inside the fibers to maintain the internal reflection of light as it traversed the fiber. They used polymethyl methacrylate (PMMA) for the inner, light-guiding layer of the fiber, and integrated fluorescent composite material with a lower refractive index in the outer layer. This coaxial structure enabled total internal reflection of light to occur within the fiber. The photochromic fiber uses the wavelength conversion effect of the fluorescent material to achieve uniform, comprehensive light emission. It takes advantage of the multiple waveguide core layers within a multicore fiber, which can be individually controlled in segments to achieve a broad range of colors. It regulates the colors in a single fiber by modulating the brightness of the light source in the coupled core layers. The team exploited the saturable absorption effect of the fluorescent material to mitigate the impact of excessive light leakage on fiber luminosity and ensure uniform luminescence along the transmission direction. The fluorescent composite materials inside the fiber provided control of the spectral radiation of multiple color systems in each fiber. (a): Schematic illustration of the interaction system based on photochromic fiber. (b): Photograph of the wearable wristband. The scale bar corresponds to 2 centimeters. (c): Correspondence between capacitance response and light-emitting colors under different touch positions. (d): Photograph of the photochromic fiber integrated into T-shirts. The scale bar corresponds to 10 centimeters. (e): Wearable interactive display system that reflects the user’s current emotional state based on his facial expression. (f): Photograph of the photochromic fiber in automotive interiors. The scale bar corresponds to 10 centimeters. (g): Photochromic fiber arranged in a fish tank to demonstrate its capability to illuminate underwater. The scale bar corresponds to 5 centimeters. Courtesy of Guangming Tao. To control a wide range of colors within a single fiber, the researchers optimized the fiber structure to mix the red-green-blue (RGB) colors. Following the principles of RGB color mixing, they encapsulated multiple light-guiding core layers and fluorescent materials with different colors inside each fiber to expand the range of colors and allow each single fiber to be used for color regulation and optimization. In the final step, the researchers integrated stable thermoplastic polymer material to the exterior of the fiber to seal and protect the functional materials. “These fibers can be easily incorporated into various daily wear through sewing and knitting techniques, providing a novel approach to achieving flexible, wearable, interactive interfaces,” the researchers said. “Without raising privacy concerns, this breakthrough could open up new areas for future smart cities, smart homes, human-computer interfaces, and health monitoring.” The allows up to 100 m luminescent fibers to be prepared using an industrial thermal drawing process and addresses limitations of traditional luminescent fiber preparation, including extended cycle times and high costs. The method is expected to be able to support the demand for photochromic fibers within the textile industry. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-024-01383-8).