Chemical claws bend and stretch when exposed to steam

Scientists at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia have developed a tiny “claw machine” capable of picking up and dropping a marble-sized ball in response to exposure to chemical vapors.

The findings, published July 12 in the journal Chem, highlight a technique that allows soft actuators (the parts of a machine that make it move) to perform multiple tasks without the need for expensive additional materials. While existing soft actuators can be “one-trick ponies” limited to a single type of motion, this new composite film twists in different ways depending on the vapor it’s exposed to.

“It can bend and stretch based on molecular interactions, which is very sophisticated in this size range,” says author Niveen M. Khashab, a professor of chemistry at KAUST. “We hope our findings will be used to develop advanced soft robotic systems that are capable of precise and adaptable movement in diverse environments,” she says, suggesting that such systems could be used in medical devices, industrial automation, and tools used to measure temperature, air quality, and humidity.

To test the claw machine’s ability to multitask, the researchers first exposed it to acetone. In the presence of the vapor, the device grabbed a red cotton ball and stretched so it could drop it into a box. When the team exposed the machine to ethanol vapor, it grabbed the cotton ball and pulled it out of the box.

Unlike rigid actuators in “hard robots,” which can be made of metal or tough plastic, soft actuators are flexible, allowing them to perform a range of tasks that their rigid counterparts cannot. As a result, soft actuators are the technology of choice for cutting-edge applications such as precision agriculture, deep-sea exploration, and wearable devices.

But soft actuators still have limitations: They can bend, twist, or stretch, but none of them can move in multiple ways, preventing them from performing more complex tasks that would allow them to be used over an even wider range. While researchers have recently experimented with actuator designs to give devices a greater range of motion, many of these strategies involve combining different materials, making them expensive and difficult to manufacture while increasing their risk of mechanical failure.

To overcome this challenge, Khashab and his colleagues developed a claw machine made of a polymer matrix containing molecular cages containing an organic compound, urea. The researchers chose urea for the cages because the compound can form multiple hydrogen bonds, allowing urea molecules to quickly reconfigure when exposed to different molecules in vapors. As a result, the material’s properties can be precisely controlled, making it easy to customize.

The results suggest that the material the machine is made of can be “efficiently programmed to perform complex movements by judiciously controlling the type and concentration of the vapor stimulus,” the authors write.

“The most remarkable finding was the unique actuation behavior where the soft actuator performed a complex motion involving ‘bending, stretching and returning’, which had not been reported before,” Khashab says.

Next, Khashab and her colleagues plan to study the claw machine’s energy density and how efficiently it converts energy to improve its performance, she says. They will also test its ability to produce electrical signals when the soft actuator is combined with materials that generate an electrical charge, with the ultimate goal of developing flexible wearable electronics, Khashab says.