Towards Half-moon-shaped Soft Pneumatic Cilia

Symmetry breaking mechanisms lead to locomotion with a reduced number of inputs, with soft robotic solutions emerging for both terrestrial and aquatic propulsion [1], [2]. For fluids, natural cilia are a source of inspiration that display an asymmetric actuation cycle where the forward and backwards stroke are following a different path. To instigate a similar path asymmetry, this paper reports on the design, simulation, fabrication and characterization of a soft bending pneumatic actuator with an unconventional half-moon shaped cross-section. The cross-section is constant along the actuator length and a design is proposed where both the outer profile and the inflatable inner cavity are half-moon shaped. The actuator is dimensionalised trough iterative structural simulations performed with Ansys simulation software, optimising the bending angle and the compatibility with a low Reynolds environment when submerged in a fluid with high viscosity. The actuator is fabricated through a single-step molding process out of silicone rubber. In order to obtain the complex geometry, the mold is made out of multiple aluminium components that are machined with wire electrical discharge machining and conventional milling. An experimental characterization is performed, analysing both the pressure-volume and the bending-angle characteristic. Results show that the actuator achieves a bending angle of about 60° at 50 kPa with a constant curvature. Structural simulations reveal that the corners of the half-moon cross-sections are stress-free, which is optimal in the perspective of coupling in hydrodynamic forces to change the cross section while being submerged in a liquid.