(Nanowerk News) Most robotic grippers are built using soft plastics – to grab objects without damaging them – which melt at high temperatures, or expensive, rigid metals. A team of researchers from the National University of Singapore (NUS), together with their collaborators from Northeast Forest University, have created a wooden robotic gripper that can be used in very hot environments but still has a soft touch.
This innovative wooden robotic gripper also has another advantage – it is driven by changes in humidity, temperature and lighting in the environment, thereby lowering energy consumption.
“Wood has excellent mechanical properties, natural deformation, is available in large reserves, and is relatively inexpensive. In our recent work, we have shown that a wood-based robotic gripper can overcome the limitations of traditional actuators and manipulators,” explained Assistant Professor Tan Swee Ching, who led the research team from the Department of Materials Science and Engineering under the NUS College of Design and Engineering.
He adds, “Our wooden robotic gripper can spontaneously stretch and bend itself in response to moisture, heat and light stimulation. It also has good mechanical properties, is capable of complex deformations, a wide working temperature range, low manufacturing costs, and is biocompatible. This unique feature sets it apart from conventional alternatives.”
The wooden gripper opens when exposed to high humidity (above 95% RH), and closes tightly when the ambient temperature rises above 70 °C or when exposed to solar radiation.
The research team published their findings in Advanced Materials (“Hygrothermic Wood Actuated Robotic Hand”).
Wood gripper sensitive to moisture, heat and light
The researchers created a wooden robotic gripper using 0.5 mm thin strips of Canadian maple. The wood chips are first treated with sodium chloride to remove lignin – a component found in plant cell walls. The large pores in wood are filled with a polymer called polypyrrole to allow 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 piece is coated with a moisture absorbing gel. The hydrophobic film is fixed on the other side. This wet-dry difference allows the wood to quickly absorb moisture on one side, and it accelerates the deformation of the wooden handle when exposed to high humidity.
The pieces of wood are then formed into grippers at 70 °C using a special mould. When placed in an environment with 95% RH (i.e. moisture stimulation), the hygroscopic gel absorbs moisture and the wooden handle stretches and gradually opens outwards.
“When exposed to high ambient temperatures above 70°C (heat stimulation), the wooden gripper begins to bend inward and reaches its maximum bend at 200°C.” explains Ms Bai Lulu, doctoral student in the NUS Department of Materials Science and Engineering and first author of the paper.
The team also tested the wooden gripper under different light intensities, with the hydrophobic film coating facing the light source. As light illumination increases the surface temperature of the gripper to approx. 42 °C, the moisture-absorbing gel begins to lose water and the gripper begins to bend inward before reaching significant bending at about 57 °C.
The wooden handle remains intact after 100 actuation cycles, demonstrating its stability and sturdiness for long-term use.
Lifting objects at high temperatures
“We also verified the performance of the wooden handle by subjecting it to high temperatures. In our experiment, a wooden handle lifted a 200 gram weight (the equivalent of a can of soda) at about 170 °C. This is not possible for most actuators made using soft polymers. Depending on the design, wooden grippers can carry loads as high as 10,000 times the gripper’s weight,” explained Professor Chen Wenshuai from Northeast Forest University and co-author of the research paper.
The next step
Asst Prof Tan and his team are now working on improving the performance of the wooden gripper, such as shortening the bending time from about 2 minutes currently, increasing the weight it can carry, and gripping objects of various shapes and sizes. They also found ways to reduce costs and improve the manufacture of wooden grippers.
With further structural design and performance improvements, the team hopes to develop an improved version of the wooden gripper to help firefighters carry out rescue operations.