Toggling Axial Stiffness Via Multistability

Soft robotics is transforming biomedical tools, search-and-rescue, and industrial automation with compliant machines that adapt to complex environments. A key element in the designs and applications are variable stiffness devices. It hinges on the capacity to switch from supple flexibility to full rigidity on demand. Most current approaches, however, still depend on bulky multilayer builds, complex fluidics, or labor-intensive fabrication that restrict design freedom and scalability. We address these limitations with a monolithic metastructure device that modulates the compliance of interactions between an actuator and external objects. By reorienting manually its architected mechanical unit cells, the structure offers four distinct stiffness states. The entire component is fabricated in a single 3D-printing pass using just two materials: flexible thermoplastic polyurethane (TPU) as the functional body and water-soluble BVOH as sacrificial support.