Direct Ink Writing of Hybrid Soft-Rigid Robots using Bio-based Inks

The rise of robots has increased concerns over the use of synthetic, non-recyclable materials, and motivating a shift toward sustainable, biodegradable alternatives that support end-of-life reuse. To address this challenge, we propose a circular material system for fabricating vertebrate-inspired soft robots that incorporate internal skeletal and joint structures made from bio-composite inks. The rigid, bone-mimetic ink was derived from eggshell-alginate bio-composites, while flexibility was introduced by increasing the glycerol content within the same material system. These inks are rheologically tuned for direct ink writing (DIW) and air-dried to enable lowenergy processing with minimal structural shrinkage (${\lt}10 \%$). Mechanical characterization through tensile, compression, and flexural tests quantified the transition from compliant to loadbearing behavior based on composition of bio-ceramics. In flexural test, the rigid ink exhibited a modulus of 1477 MPa, showing intermediate performance between commercial grade thermoplastic PLA and TPU, while tensile tests directly compared the rigid and flexible formulations, revealing their distinct stiffness and strain-at-failure characteristics. The outcome of this research was demonstrated by fabricating an anatomically inspired finger integrated with rigid and flexible regions in a single assembly, a quadruped robot with bio-ceramic limbs, and a skeletal wing structure printed with reprocessed bio-ceramic ink. This work establishes a foundation for sustainable, modular soft-rigid robotic designs in which a significant portion of the structure is derived from biodegradable or recyclable materials, aligning with emerging circular fabrication frameworks in robotics and multifunctional material systems.