Phase-change metal ink with pH-controlled chemical sintering for versatile and scalable fabrication of variable stiffness electronics

Variable stiffness electronics represent the forefront of adaptive technology, integrating rigid and soft electronics in a single system through dynamic mechanical modulation. While gallium's high modulus tuning ratio and rapid phase transitions make it ideal for transformative electronic systems (TES), its liquid-state instability, high surface tension, and unintended phase transitions during processing pose substantial challenges. Here, we introduce STiffness-Adjustable temperature-Responsive ink (STAR ink), a chemically sinterable gallium composite electronic ink designed to overcome these obstacles. STAR ink enables high-resolution (~50 micrometers) circuit patterning, large-scale batch fabrication, and three-dimensional structure coating at room temperature. Through pH-controlled chemical sintering, STAR ink-based TES exhibits exceptional mechanical tunability (tuning ratio: 1465) and electrical conductivity (2.27 × 10⁶ siemens per meter). Demonstrated applications-from multilayered variable stiffness printed circuit boards (PCBs) matching standard PCBs' complexity to body-temperature responsive neural probe-underscore STAR ink's potential for reconfigurable electronics across consumer electronics and biomedical devices.