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Nature Communications

Published: 01 February 2017

https://www.nature.com/articles/ncomms14230 

■  Researchers

Hyunwoo Yuk, Shaoting Lin, Chu Ma, Mahdi Takaffoli, Nicolas X. Fang & Xuanhe Zhao

Department of Mechanical Engineering, Massachusetts Institute of Technology

■  Abstract

Sea animals such as leptocephali develop tissues and organs composed of active transparent hydrogels to achieve agile motions and natural camouflage in water. Hydrogel-based actuators that can imitate the capabilities of leptocephali will enable new applications in diverse fields. However, existing hydrogel actuators, mostly osmotic-driven, are intrinsically low-speed and/or low-force; and their camouflage capabilities have not been explored. Here we show that hydraulic actuations of hydrogels with designed structures and properties can give soft actuators and robots that are high-speed, high-force, and optically and sonically camouflaged in water. The hydrogel actuators and robots can maintain their robustness and functionality over multiple cycles of actuations, owing to the anti-fatigue property of the hydrogel under moderate stresses. We further demonstrate that the agile and transparent hydrogel actuators and robots perform extraordinary functions including swimming, kicking rubber-balls and even catching a live fish in water.

■  Results

Osmotic and hydraulic actuation of hydrogels

While the active and transparent hydrogel-based tissues and organs of leptocephali endow them with agile motion and natural camouflage in water, such striking capabilities have not been achieved by synthetic hydrogel actuators and robots (Fig. 1a,b and Supplementary Movie 1)1,2. Despite recent developments of hydrogels responsive to various stimuli11,12,13,14,15,16, existing hydrogel actuators are mostly osmotic-driven (that is, based on swelling/de-swelling) and suffer from the intrinsic coupling between responsive times and the actuation forces. As illustrated in Fig. 1c, the swelling-induced force F from a constrained hydrogel block and the responsive time t scale as

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