Human Hand-Inspired Stiffness-Tunable Soft Gripper Formed by the Split-Brushing Adhesion of Diverse Hydrogels

Humans, animals, and even insects interact with the environment via their grippers having various morphologies and functions. In contrast to other grippers found in nature, the functions of human hands are unique. Even when heavy objects are placed on them, their high stiffness prevents deformation. In addition, they are flexible, allowing the fingers of hands to bend easily to grab various objects. Various attempts have been undertaken in recent years to mimic the morphologies and functions of human hands using hydrogels. However, the majority of present hydrogel grippers are limited since they are readily deformed but their stiffness is insufficient for lifting heavy objects or supporting huge loads. These two features, high stiffness and ease of bending, are challenging to obtain with a single hydrogel system, as finger stiffness must be low for bending to be possible. In this paper, we demonstrate a hydrogel gripper whose stiffness can be easily adjusted by temperature from a few to hundreds of megapascals. This gripper is comprised of various hydrogels with distinct functionalities, including a hydrogel with high stiffness, a hydrogel with temperature-dependent stiffness change, and a hydrogel with temperature-dependent volume change. In order to build this hydrogel gripper by integrating these three hydrogels, we present a unique hydrogel adhesion approach, which we refer to as split-brushing adhesion, which enables strong adherence between various hydrogels. The only trigger that can activate this hydrogel gripper is temperature. We demonstrate that the bending of the fingers of this gripper can be regulated by temperature and that the bent fingers are stiff, allowing them to grasp heavy things.