A one-step react-on-demand (RoD) method for fabricating flexible circuits with ultra-low sheet resistance, enhanced safety and durability. With the special functionalized substrate, a real-time three-dimensional synthesize of silver plates in micro scale was triggered on-demand right beneath the tip in the water-swelled PVA coating, forming a three-dimensional metal-polymer (3DMP) hybrid structure of ˜7 μm with one single stroke. The as-fabricated silver traces show an enhanced durability and ultralow sheet resistance down to 4 mΩ/sq which is by far the lowest sheet resistance reported in literatures achieved by direct writing. Meanwhile, PVA seal small particles inside the film, adding additional safety to this technology. Since neither nanomaterials nor a harsh fabrication environment are required, the proposed method remains low-cost, user friendly and accessible to end-users. the RoD approach can be extended to various printing systems, offering a particle-free, sintering-free solution for high resolution, high speed production of flexible electronics.