The wooden robotic gripper developed by NUS researchers can spontaneously stretch and bend itself in response to moisture, thermal and light stimulation. It also has good mechanical properties, able to perform complex deformation, wide working temperature range, low manufacturing cost, and is bio-compatible.

A research team from the National University of Singapore (NUS) has developed a wooden gripper that is driven by changes in moisture, temperature, and lighting in the environment.

Most robotic grippers are made using either soft plastics – to pick up objects without damaging them – but they melt at high temperatures, or use metals that are stiff and costly.

The researchers from NUS, together with their collaborators from the Northeast Forest University, have created a wooden robotic gripper that could be used in a very hot environment and yet maintain a tender touch.

This innovative wooden robotic gripper also has another advantage – it is driven by changes in moisture, temperature and lighting in the environment, hence lowering energy consumption.

“Wood has excellent mechanical properties, natural deformation, available in large reserves, and is relatively cheap. In our latest work, we have shown that wood-based robotic gripper can overcome the limitations of traditional actuators and manipulators,” said Assistant Professor Tan Swee Chan, who leads a research team from the Department of Materials Science and Engineering under the NUS College of Design and Engineering.

His team members include laboratory technologist Qu Hao and NUS doctoral student Bai Lulu.

“Our wooden robotic gripper can spontaneously stretch and bend itself in response to moisture, thermal and light stimulation. It also has good mechanical properties, able to perform complex deformation, wide working temperature range, low manufacturing cost, and is bio-compatible. These unique features set it apart from conventional alternatives,” Tan Swee added.

The wooden gripper opens up when exposed to high humidity (above 95% RH), and closes tightly when the temperature of the surroundings increases beyond 70 degrees Centigrade, or when they are exposed to solar radiation.

The research team published their invention in the online version of the scientific journal Advanced Materials in February this year.

The researchers created the wooden robotic gripper using pieces of Canadian maple that are 0.5 mm thin. The pieces of wood were first treated with sodium chloride to remove the lignin – a component found in the cell walls of plants.

The large pores in the wood are filled with a polymer called poly-pyrrole to enable the material to absorb heat and light more easily.

The team also formulated a new nickel-based hygroscopic gel for moisture absorption. One side of the modified wood pieces was coated with the moisture-absorbing gel.

A hydrophobic film was attached to the other side. This wet-dry difference allows the wood to quickly absorb water vapour on one side, and this accelerates changes in the shape of the wooden gripper when exposed to high humidity.

The wood pieces were then shaped into a gripper at 70 degrees C using special moulds. When placed in an environment with 95% RH (moisture stimulation), the hygroscopic gel took in moisture and the wooden gripper stretched and gradually opened outwards.

“When exposed to high ambient temperature of beyond 70 degrees C (heat stimulation), the wooden gripper started to bend inwards and achieved maximum bending at 200 degrees C,” explained Bai Lulu.

The team also tested the wooden gripper under different light intensities, with the hydrophobic film layer facing the light source. When the light illumination increased the gripper’s surface temperature to about 42 degrees C, the moisture-absorbing gel started to lose water and the gripper began to bend inwards before achieving significant bending at about 57 degrees C.

The wooden gripper remained intact after 100 actuating cycles, demonstrating its stability and robustness for long-term use.

“We also verified the performance of the wooden gripper by pick up objects at high temperatures. In our experiments, the wooden gripper successfully lifted a weight of 200 gm (equivalent to a can of soda) at around 170 degrees C. This is impossible for most actuators made using soft polymer,” noted Professor Chen Wenshuai from Northeast Forest University and the co-corresponding author of the research paper.

Depending on the design, the wooden gripper could carry loads as high as 10,000 times the weight of the gripper.

Tan Swee and his team are now looking at improving the performance of the wooden gripper, such as shortening the bending time from about 2 minutes currently, increasing the weight load it can carry, as well as gripper objects with different shapes and sizes.

They are also finding ways to reduce cost and to scale up the fabrication of the wooden gripper.

With further structural design and performance improvement, the team hopes to develop improved version of the wooden gripper to help fire fighters carry out rescue operations.

With further structural design and performance improvement, the team hopes to develop improved version of the wooden gripper to help fire fighters carry out rescue operations.

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