A reversible alteration of kinematic degree of freedom via local hydration of hydrogel

Summary
In this paper, a strategy that manipulates micro-water of hydrogel in reversible, selective, and on-demand manner is reported.
It realizes instant switching between a transparent and homogeneously soft gel and an opaque and heterogeneously rigid solid with a monolithic mechanical gradient.
These two mechanical dual modes mimicking the characteristic of invertebrate and vertebrate demonstrate multi-modal kinematics.

Professor Ko's team in Seoul National University announced that they have developed a technology that can reversibly transform the state of a material from an octopus-like state to a vertebrate articular shape inspired that the mechanical rigidity is related to the amount of water in bio tissue.

The body of an organism has evolved into an optimum form regarding the environment in which it lives. Animals such as octopuses have a soft body to survive in narrow rock crevices, and vertebrates have an articular structure composed of bone and cartilage for efficient power transmission.

In general, the mechanical properties of tissues are determined by the amount of water they contain. The body of an octopus contains evenly distributed water, but the articular structure of vertebrate is composed of a combination of water rich cartilage and water poor bones.

Decisive deformation of the articular structure occurs on relatively weak part (knee, elbow), consequently, it has finite kinematic degree of freedom. These characteristic is suitable for accurate force transmission but not for entering narrow gaps. The soft feature of an octopus has an opposite character to that of an articular structure.

Prof. Ko's research team developed a technology that can control the amount of local hydration of soft hydrogel in on-demand manner by using the phenomenon that the supersaturated liquid hydrated salt takes away water from the hydrogel when it crystallizes. The team reversibly demonstrated this transformation by spatially controlling the thermodynamic interaction of hydrogel molecules and salt hydrate.

This research is expected to bring an innovative leap forward in the field of next-generation soft robots with a technology that can reversibly switch an articular structure of body in on-demand manner.

Primary Keyword
mechanical kinematics, monolithic mechanical gradient, reversible switching between invertebrate and vertebrate

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