A fluidic actuator with an internal stiffening structure inspired by mammalian erectile tissue

One of the biggest problems with soft robots is precisely the fact that they are soft. Indeed the softer they are, the less force they can exert on the environment. Researchers have proposed a number of stiffening methods, but all of them have drawbacks, such as locking the shape of the device in a way that precludes further adjustments. In this paper we propose a stiffening method inspired by the internal structure of the mammalian penis. The soft actuation chamber is divided into small compartments that trap the actuation fluid, leading to locally amplified pressure increase under certain conditions. At the same time, the proposed solution does not affect the actuation mechanism, allowing the actuator to be adjusted in one direction just as if it was in non-stiffened mode, while offering a stiff response in the opposite direction. Our prototype achieves an increase in stiffening of approximately a factor of two. The paper describes the concept, the mathematical justification of the working principle, the prototype design, its implementation and our experimental results.