Wooden robotic gripper opens up in high humidity and closes tightly in hot temperatures

Researchers from the National University of Singapore (NUS) and the Northeast Forest University has developed an innovative robotic gripper made of wood. Unlike traditional grippers made of soft plastics or metals, this wooden gripper maintains a gentle touch while being able to withstand high temperatures, making it suitable for use in hot environments.

One of the key advantages of this wooden gripper is its ability to be driven by changes in moisture, temperature, and lighting in the surrounding environment, which helps to reduce energy consumption. Wood was chosen for its excellent mechanical properties, natural flexibility, abundance, and affordability. Assistant Professor Tan Swee Ching, the leader of the research team, highlighted that the wooden gripper can overcome the limitations of conventional actuators and manipulators.

The wooden gripper exhibits spontaneous stretching and bending responses to moisture, thermal changes, and light stimulation. It possesses favorable mechanical properties, allowing it to perform complex deformations and work across a wide temperature range. Additionally, the gripper is cost-effective, biocompatible, and environmentally friendly, distinguishing it from other alternatives.

When exposed to high humidity (above 95% relative humidity), the wooden gripper opens up, while it closes tightly when the temperature exceeds 70°C or under exposure to solar radiation.

The research team’s work was published in the online version of the scientific journal Advanced Materials on February 26, 2023.

A wooden gripper that is sensitive to moisture, thermal and light

The researchers utilized 0.5 mm thin pieces of Canadian maple wood as the primary material for the wooden robotic gripper. To prepare the wood, they treated it with sodium chloride to eliminate lignin, a plant cell wall component. To enhance the wood’s heat and light absorption capabilities, the researchers filled its large pores with a polymer called polypyrrole.

For moisture absorption, the team developed a new hygroscopic gel based on nickel. They coated one side of the modified wood pieces with this gel, while the other side was covered with a hydrophobic film. This wet-dry contrast enabled the wood to rapidly absorb water vapor, leading to accelerated shape changes when exposed to high humidity.

The wood pieces were then molded into the gripper shape using specialized molds at a temperature of 70°C. When placed in an environment with 95% relative humidity, the moisture-absorbing gel absorbed moisture, causing the wooden gripper to stretch and gradually open.

When subjected to high ambient temperatures above 70°C, the wooden gripper began to bend inward, reaching its maximum bending capacity at 200°C, as explained by Ms. Bai Lulu, a doctoral student at the NUS Department of Materials Science and Engineering and the first author of the paper.

The researchers also tested the gripper’s response to different light intensities, with the hydrophobic film side facing the light source. When the light increased the gripper’s surface temperature to around 42°C, the moisture-absorbing gel released water, resulting in inward bending. Significant bending occurred at approximately 57°C.

After undergoing 100 actuating cycles, the wooden gripper remained intact, indicating its stability and durability for long-term use.

Lifting objects at high temperatures

The researchers conducted additional tests to evaluate the wooden gripper’s performance, specifically focusing on its ability to handle objects at high temperatures. Remarkably, the wooden gripper successfully lifted a weight of 200 grams, equivalent to a can of soda, at around 170°C. This achievement is unprecedented for most soft polymer actuators. Furthermore, depending on the gripper’s design, it could potentially carry loads up to 10,000 times its own weight, showcasing its exceptional strength.

Professor Chen Wenshuai, co-corresponding author of the research paper and affiliated with Northeast Forest University, emphasized these significant advancements. The team’s focus now lies in enhancing the wooden gripper’s capabilities. They aim to reduce the bending time, which currently takes around two minutes, improve its weight-carrying capacity, and enable it to grip objects of various shapes and sizes. Additionally, they are exploring methods to reduce costs and scale up the production process for the wooden gripper.

Looking ahead, the researchers envision an improved version of the wooden gripper that could greatly assist firefighters in conducting rescue operations. By further refining the gripper’s structural design and enhancing its performance, they aim to create a tool that can aid in critical emergency scenarios.

Source: National University of Singapore

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