Tri-WOB: A Bistability-Driven Variable Stiffness Origami Structure for Soft Robotic Arms.

Soft robotic arms offer high compliance but suffer from limited load-bearing capacity and energy-intensive stiffness programming. Here, we present a bistability-enabled origami structure, Tri-WOB, composed of three Waterbomb-origami-based (WOB) units that achieve rapid, discrete, and energy-free stiffness reconfiguration. When stacked into a modular robotic arm, Tri-WOB enables large deformation while allowing stiffness modulation to define discrete end configurations, thereby improving positioning accuracy and repeatability. In the fully stiff configuration, the arm supports loads an order of magnitude greater than in the compliant state, while hybrid stiffness states preserve load-bearing capacity during controlled directional bending. By leveraging structural bistability rather than powered stiffening mechanisms, this work proposes a novel design for energy-efficient variable stiffness in origami robotics.