Smart yarns hold the potential to transform everyday textiles into functional platforms, yet current methods remain constrained. These include conductive yarns, made from silver or stainless steel, which retain the feel of conventional yarns but offer limited functions, and PCB-based solutions, which add capability at the cost of bulk and rigidity. We present Circuit2Yarn, a fabrication framework that transforms planar printed circuits into flexible yarns by rolling copper-traced TPU films with soldered surface-mount components, preserving the capabilities of rigid electronics while producing yarn-like forms suitable for textile integration. We demonstrate yarns as small as 0.8 mm that integrate LEDs and sensors, including temperature, humidity, light, IMU, and capacitive sensing modules, enabling applications ranging from smart garments and interactive musical instruments to responsive tea bags. Characterization confirms durability under bending/stretching. By rolling planar circuits into yarns, Circuit2Yarn paves the way toward comfortable, multifunctional, and interactive textiles in everyday life.
