First-of-its-kind wooden robotic gripper can be triggered by light

"Wood has excellent mechanical properties, natural deformation, is available in large reserves, and is relatively cheap. In our latest work, we have shown that wood-based robotic grippers can overcome the limitations of traditional actuators and manipulators," said Ching in a press release.

Creating a wooden gripper

The researchers used 0.5 mm (millimeters) thin maple wood from Canada to make the gripper. The wood pieces were treated with sodium chloride to remove lignin, which is a natural component found in the cell walls of plants and is responsible for providing rigidity and strength to the structure of the plant.

Next, the pores were filled with polypyrrole, a polymer, allowing the wood to absorb heat and light efficiently. Additionally, they formulated a nickel-based hygroscopic gel that can absorb moisture from the air. 

They coated one side of the modified wood pieces with the gel and attached a hydrophobic film to the other side. The difference in wetness and dryness enables the wood to rapidly absorb water vapors on one side, facilitating faster changes in the shape of the gripper when exposed to high humidity.

The wood pieces were then shaped into the structure of a gripper using molds at 70 degrees Celsius (°C). When exposed to a relative humidity of 95 percent, the hygroscopic gel absorbed moisture making the wooden gripper stretch and open outwards. 

In the press release, the first author of the study, Bai Lulu, said, "When exposed to a high ambient temperature of beyond 70 degrees Celsius (heat stimulation), the wooden gripper started to bend inwards and achieved maximum bending at 200 degrees Celsius."

The wooden gripper was tested under varying light intensities, causing the gripper to bend inwards as the moisture-absorbing gel lost water at around 42°C and achieved significant bending at about 57°C. The gripper showed durability and stability after undergoing 100 actuating cycles, indicating its suitability for long-term use.

Lifting objects and next steps

The researchers also used their wooden gripper to lift objects at high temperatures. 

"In our experiments, the wooden gripper successfully lifted a weight of 200 grams (equivalent to a can of soda) at around 170 degrees Celsius. This is impossible for most actuators made using soft polymer. Depending on the design, the wooden gripper could carry loads as high as 10,000 times the weight of the gripper," said Chen Wenshuai, the co-corresponding author of the study from Northeast Forest University.

The wooden robotic hand and grippers showed skilled interactions with objects, making them suitable for tasks like grasping and holding. Their reversible hygrothermic property makes them potentially useful in fire rescue scenarios for rescuing trapped objects. 

This device combines desirable mechanical properties, multi-stimulus-response capabilities, complex deformation, wide temperature range, affordability, and biocompatibility into a single device.

In their future work, the researchers are looking to enhance the wooden gripper's performance by reducing its bending time, increasing its weight-carrying capacity, and enabling it to grip objects of various shapes and sizes. They are also seeking to lower costs and scale up the production process for the wooden gripper.

The findings of their study are published in the journal Advanced Materials.

Study abstract:

Stimulus–responsive actuators play a vital role in the new generation of intelligent systems. However, poor mechanical performance, complicated fabrication processes, and the inability to complex deformation limit their practical applications. These challenges are overcome by designing a strong hygrothermic wood actuator with asymmetric water affinity. The actuator is readily constructed by sandwiching polypyrrole-coated wood with a Ni complex hygroscopic gel top layer for moisture absorption and a polyimide bottom layer as the water barrier. The resulting hygrothermic wood spontaneously stretches and bends itself in response to moisture and thermal/light stimulation. A robotic hand and a series of grippers made of hygrothermic wood demonstrate dexterous object–hand interactions during grasping and holding, while the reversible hygrothermic property allows the actuator to be potentially applied in fire rescue scenarios to rescue trapped objects. A combination of good mechanical properties, multi-stimulus-response, complex deformation, wide working temperature range, low manufacturing cost, and biocompatibility are simultaneously realized by one device. It is thus believed that such a strong wood actuator will open up a new avenue for building intelligent robotic hand systems.