Biomimetic linear tunable stiffness robot: Design, structural optimization and stiffness enhancement

Soft robots face challenges in tunable stiffness due to flexibility-force transmission trade-offs. This paper presents a biomimetic linear tunable stiffness robot (LTSR) inspired by biological hydrostats and scales. LTSR integrates a bellows-structured soft driving actuator (SDA) mimicking hydrostatic organisms, enabling efficient axial elongation via fluid-mediated force transmission under constant-volume constraints, and overlapping jamming flaps inspired by biological scales, which enhance load-bearing capacity through adjustable interlayer friction. To maximize elongation performance, the kinematic model was established through equilibrium analysis of elastic and driving force, followed by a multi-objective optimization algorithm to identify optimal structural parameters. Theoretical predictions and finite element analysis (FEA) revealed that the optimized LTSR achieves over 200% greater elongation than the initial design. Experimental validation confirmed that the optimized SDA reaches 130.19 mm elongation at 50 kPa driving pressure, closely aligning with both theoretical and FEA results, thereby confirming model accuracy and superior motion performance. The stiffness model of LTSR was established through axial compression analysis to predict stiffness, with the median friction coefficients (FCs) of conventional sheets and the interlayer resisting force of bio-inspired adhesives incorporated. Comparative results revealed that using bio-inspired adhesives as jamming sheets yields significantly higher stiffness than using conventional jamming materials. The adhesion performance testing showed that the adhesive achieves its minimum macroscopic FC of 0.863 under an applied pressure of 471.81 Pa. Integrating these adhesive flaps into LTSR (forming B-LTSR) enabled load-responsive stiffness adjustment. B-LTSR outperforms other soft robots with the highest stiffness and widest tunable range, validating the exceptional tunable stiffness of bio-inspired adhesive jamming flaps. Kinematic experiments show that B-LTSR retains 95.9% of SDA’s elongation, demonstrating that the optimized bellows structure mitigates the mechanical constraint from the jamming components. The stiffening and softening response experiments indicate that B-LTSR can achieve rapid transitions between rigid and flexible states.