Rigidity tunable multifunctional composites for soft robotics

Soft robotics for human machine interactions requires development of new multifunctional materials that can adapt their physical properties, geometries and functionalities according to the environment they encounter. Among these, tunable rigidity is an important feature that is essential in allowing soft structures to become load bearing and perform mechanical work. Previous approaches for rigidity tuning typically demanded rigid and bulky supporting hardware for pneumatics, magnetic activation and heating, which are not suitable for soft robotics intended for human use. We have recently introduced methods for rigidity tuning in soft robotics that is directly powered with electrical current. This is accomplished with multilayered composites in which embedded elements are melted or softened through Joule heating. First we achieved four orders of magnitude change in tensile rigidity with embedded sheets of low melting point alloy. Activation times were on the order of 10-100s. For faster activation, we also designed and fabricated a conductive thermoplastic with low glass transition temperature that can be easily reached through joule heating of the thermoplastic. With a conductive thermoplastic elastomer we achieved rigidity tuning within seconds. The potential applications of these smart composites were demonstrated by incorporating them into a bio-mimetic soft robotic finger with multiple bending directions, which would otherwise be very complicated to fabricate and control.