Understand small-scale living systems

Wang, T., Ren, Z., Hu, W., Li, M. and Sitti, M., 2021. Effect of body stiffness distribution on larval fish–like efficient undulatory swimming. Science Advances, 7(19), p.eabf7364. Young Researcher Award at 11th European Solid Mechanics Conference, Galway, Ireland, 2022.

Energy-efficient propulsion is a critical design target for robotic swimmers. Although previous studies have pointed out the significance of non-uniform body bending stiffness distribution (k) in improving the undulatory swimming energy-efficiency of the adult fish-like robots in the inertial flow regime (Reynolds number Re ≳ 2000), whether such an elastic mechanism is beneficial in the robot swimming of the intermediate flow regime (1 ≲ Re ≲ 2000) on the milliscale remains elusive. To answer this question, we develop a class of untethered soft milliswimmers consisting of a magnetic composite head and a passive elastic body with multiple discrete joints along the body, enabling different k. These robots realize larval zebrafish-like undulatory swimming at the same length scale. Quantitative investigations reveal that a uniform k and high swimming frequency (60 – 100 Hz) are favorable to improve their swimming efficiency. A shape memory polymer-based milliswimmer with the tunable k on-the-fly further confirms the superiority of uniform k and higher frequencies. Such acquired knowledge can guide the design of energy-efficient soft milliswimmers for future environmental and biomedical applications at the same flow regime on the milliscale.

Ren, Z.*, Wang, T.*, Hu, W. and Sitti, M., 2019, June. A Magnetically-Actuated Untethered Jellyfish-Inspired Soft Milliswimmer. In Robotics: Science and Systems. Best Paper Award.

Untethered small-scale soft robots can potentially be used in healthcare and environmental applications. They can access small spaces and reshape their bodies in a programmable manner to adapt to unstructured environments and have diverse dynamic behaviors. However, the functionalities of current miniature soft robots are limited, restricting their applications. Taking the advantage of the shape-programmable ability of magnetic soft composite materials, here we propose an untethered soft millirobot (jellyfishbot) that can swim like a jellyfish by time- and trajectory-asymmetric up and down beating of its lappets. Its swimming speed and direction can be controlled by tuning the magnitude, frequency, and direction of the external oscillating magnetic field. We demonstrate that such jellyfishbot can perform several tasks that could be useful for real-world applications, such as steering and delivering cargo. The actuation mechanism, locomotion, and functionals of the millirobot presented in this paper could inspire the novel interventional tool in hard-to-reach regions in nature.