Direct Ink Writing of Conductive Hydrogels

Direct ink writing (DIW) is an additive manufacturing technique that has garnered notable interest due to its precise and consistent printing of a wide range of materials, such as viscoelastic hydrogels, pastes, and complex composites, by adjusting the ink's rheology. This material flexibility combined with the ability to print at room temperature makes DIW ideal for diverse applications and is scalable from small to industrial levels. In recent years, DIW of conductive hydrogels has gained significant attention across various fields, ranging from biomedical scaffolds to flexible electronics. Conductive hydrogels are a category of hydrogels which exhibit conductivity in their wet and/or dry state. Precursors like conductive polymers, metallic nanoparticles, and carbon-based materials can be used to induce electronic and/or ionic conductivity in hydrogels. This review presents a comprehensive overview of conductive hydrogels demonstrating printability using the DIW technique. The fundamentals of DIW and conductive precursors are presented. Following, the different pathways for reaching optimal conductive hydrogel properties, including mechanical, conductive, and rheological, with a focus on ink synthesis and printability are introduced. Finally, emerging applications of DIW of conductive hydrogels in flexible electronics and medicine are highlighted, and the anticipated challenges for advancement of printable conductive hydrogels using DIW are discussed.