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.

We present a design for an energetically autonomous artificial organism, combining two subsystems; a bio-inspired energy source and bio-inspired actuation. The work is the first demonstration of energetically autonomy in a microbial fuel cell (MFC)-powered, swimming robot taking energy from it's surrounding, aqueous environment. In contrast to previous work using stacked MFC power sources, the Row-bot employs a single microbial fuel cell as an artificial stomach and uses commercially available voltage step-up hardware to produce usable voltages. The energy generated exceeds the energy requirement to complete the mechanical actuation needed to refuel. Energy production and actuation are demonstrated separately with the results showing that the combination of these subsystems will produce closed-loop energetic autonomy. The work shows a crucial step in the development of autonomous robots capable of long term self-power. Bio-inspiration for the design of the Row-bot was taken from the water boatman beetle. This proof of concept study opens many avenues for the further development of the subsystems comprising the Row-bot, and the functionality of the robot itself.