Soft materials damp energy well, but if autonomous systems such as soft robots are to become a reality, researchers need to develop a way to transmit energy through soft materials.

Now, scientists at Harvard University's John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with colleagues at the California Institute of Technology, have developed a way to send mechanical signals through soft materials.

The system uses the concept of bistable beams to store and release elastic energy along the path of a wave. Image credit: Bertoldi Lab/Harvard SEAS.The system uses the concept of bistable beams to store and release elastic energy along the path of a wave. Image credit: Bertoldi Lab/Harvard SEAS.

“Soft autonomous systems have received a lot of attention because, just like the human body or other biological systems, they can be adaptive and perform delicate movements. However, the highly dissipative nature of soft materials limits or altogether prevents certain functions,” says Jordan Raney, postdoctoral fellow at SEAS. “By storing energy in the architecture itself, we can make up for the energy losses due to dissipation, allowing the propagation of mechanical signals across long distances.”

The system uses the centuries-old concept of bistable beams—structures stable in two distinct states—to store and release elastic energy along the path of a wave. It consists of a chain of bistable elastomeric beams connected by elastomeric linear springs. When those beams are deformed, they snap and store energy in the form of elastic deformation. As the signal moves down the elastomer, it snaps the beams back into place, releasing the stored energy and sending the signal downstream. The bistable system prevents the signal from dissipating downstream.

“This design solves two fundamental problems in transmitting information through materials,” says Katia Bertoldi, professor of natural sciences. “It not only overcomes dissipation, but it also eliminates dispersive effects so that the signal propagates without distortion. As such, we maintain signal strength and clarity from start to end.”

The beam geometry requires precise fabrication techniques. If the angle or thickness of one beam is off only slightly, the whole system fails.The team designed and 3D printed a soft logic gate using this system. The gate, which looks like a tuning fork, can be controlled to act as either an AND or an OR gate.

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