Metal-free conductive inks based on biodegradable hydrogels for direct ink writing of flexible circuits

Abstract

Additive manufacturing via conductive ink printing enables rapid, on-demand prototyping of electronic circuits. In particular, direct ink writing has emerged as a pivotal technique due to its broad material versatility and design freedom. To mitigate the growing concern of electronic waste, the aim is to develop conductive inks that meet performance requirements while addressing environmental considerations through the integration of green fabrication processes, such as printing techniques, and the use of materials that naturally degrade after their use. Here, we report a novel environmentally-friendly conductive ink for direct ink writing based on agarose and sodium alginate -biodegradable algae-derived polymers- hydrogels, and functionalized with carbon-based fillers. The ink formulation was optimized to ensure that its rheological properties guarantee good printability. The optimal formulation (0.5% agarose/1% sodium alginate) exhibits shear-thinning characteristics, and a sol-gel transition that ensures viscous behavior during extrusion for ink flow and elastic behavior after deposition for high-fidelity printing. Conductive inks are obtained by incorporating multi-walled carbon nanotubes (MW-CNTs) and graphite particles into the hydrogel, resulting in a low-cost, scalable, and energy-efficient manufacturing approach. MW-CNT inks outperform their graphite counterparts, achieving a conductivity of 303  ± 24 S m −1 for 1.5 wt% nanotubes concentration, thus obtaining a functional ink composed of over 98.5 wt% of biodegradable components, with a low percolation threshold of 0.15±0.04 wt% and electrical stability under cyclic bending. This demonstrates the potential of agarose/alginate hydrogel-based inks as a sustainable alternative to conventional manufacturing for flexible electronics and Internet of Things applications.