Soft devices employ variable stiffness to ensure safety and improve the robustness in the interaction between robots and objects. Using soft materials is one of the most popular approaches to design a variable-stiffness device, while the use of silicone sponge remains less explored in this field. Here we present a novel silicone-sponge-based variable-stiffness device (SVD). The SVD is easy-to-make and low-cost, and fabricated by an air-tight bellow enclosing a silicone sponge core. This allows easy access to the hyper-elastic response of the porous sponge whilst stiffness tuning of the device via pneumatic pressure difference. A detailed mathematical model of the SVD is proposed, by which the stiffness can be precisely controlled by the pressure difference applied. The stiffness of SVD can be tuned in the range of$[\mathbf{1.55}, \mathbf{2} \mathbf{2.82}]\times \mathbf{10}^{\mathbf{3}}\ \mathbf{N}/\mathbf{m}$, up to 14.7 times increase. The high stiffness is easily triggered by a low pressure difference$(\mathbf{\Delta} \boldsymbol{P} < \mathbf{12}\mathbf{kPa})$. The SVD is a versatile and compact module, with small axial size (10 mm height) and light weight (14.3 g), making it highly suitable for integration in a wide range of robotics and industrial applications. This, in addition to its easy-to-fabricate and low-cost features, may appeal to the robotics community at large. We further detail its working principle, fabrication processes, mathematical model and automated control methods to show its versatility.

Snails’ unique locomotion abilities help them realise stable movement by muscular exploiting travelling waves and friction modulation. Inspired by these characteristics, snaillike robots have recently become the focus of growing research. In this paper, we present a novel friction-driven three-foot snaillike robot which employs a simple mechanism to partially mimic and replicate snail-like motion, but in a novel form. This robot is driven by two servo motors, which makes it easy and low-cost to fabricate. The robot operates by breaking frictional symmetry in the cyclic motion of the three feet, in much the same way as the three-sphere Golestanian swimmer. The symmetry of its structure and properties of friction give the robot distinctive movements. We present a mathematical model of the robot’s locomotion, focusing on its kinetic harmonic-peristaltic movement. We designed and fabricated the robot, then undertook simulations and experiments, which closely match the analytic solutions. This robot provides a new approach to realising simpler and lower cost biomimetic mobile robots.