Printing 3D metallic structures through reduction processes: principle, approaches, and applications

3D printing holds significant promise for the fabrication of functional metal structures in various applications. This is made possible due to the unique properties of metals such as electrical conductivity, electrochemical activity, and catalytic behavior. However, existing methods are hampered by critical limitations. Traditional approaches often require high-temperature sintering and yield conductivities inferior to those of bulk metals. Similarly, existing direct writing techniques face challenges in achieving both fine feature control and high throughput, while mainstream metal 3D printing operates at resolutions too coarse for delicate electronics and metallic micro-/nanostructures. This review addresses these gaps by exploring reduction-based 3D printing strategies, where metal-containing precursors are directly transformed into conductive metals via reduction processes. By systematically examining five cutting-edge approaches, namely reactive ink printing, electroless plating of 3D printed structures, metal precursor printing followed by thermal reduction, in situ photoreduction-based laser fabrication, and electrochemical printing, this work elucidates the underlying reduction mechanisms, energetic considerations, and material behaviours that allows for the fabrication of metallic structures with either enhanced resolution, reduced thermal budget, or both. By unifying insights across these methods, the review outlines a roadmap for overcoming current limitations and advancing integration, resolution, and scalability in future applications of 3D-printed metallic materials.