Fully 3D printed multi-material soft bio-inspired frog for underwater synchronous swimming

Inspired by the relatively simple morphology of Anura such as Rana Esculenta (a semi aquatic frog), we propose the design, fabrication, characterization of a soft biomimetic robotic frog based on multilayer structural design of Shape Memory Alloy (SMA) and ultra-flexible material. Dual thrust generation approach has been proposed to achieve the synchronous swimming motion of frog, utilizing four SMA (BMF 150) muscle wires. The frog robot is named as Exploratory Frog (EXPOG) and is fabricated using multiheaded 3D printing system. Whereas the body of the EXPOG is printed using polylactic acid (PLA), which is connected with functionally responsive limbs (50 mm length and average 6 mm gradient thickness). Each limb consists of two muscles (SMA-1 and SMA-2). SMA-1 works as the main thrust creating unit, which is powered by 5V, 250 mA whereas SMA-2 creates back thrust force for antagonistic motion. The proposed kinematic model was simulated in COMSOL to verify the surface velocity and vortex generation in water using the frog's strokes. The design motivated from the rigorous modelling and real frog dynamics analysis, enabled the as-developed EXPOG swim synchronously in significant level of consistency with the real frog. Electrical and mechanical characterizations have been performed. Moreover, experimental data was further processed using TRACKER for angle, displacement, and velocity analysis. The EXPOG (weighing 57 g) can swim at different controllable frequencies (0.5 – 2 Hz) which enables it to swim at the speed up to 15 mm/s (0.3 BLS) on the surface of deep water (150 cm) with excellent weight balance.