Sophia Chen of MRS Bulletin interviews Barbara Mazzolai and Edoardo Sinibaldi of the Italian Institute of Technology about their robot made from two types of polymers, enabling it to extend and retract like the tendrils of a plant. Read the article in Nature Communications.
SOPHIA CHEN: Barbara Mazzolai is designing robots inspired by plants. Recently, she and her research team at the Italian Institute of Technology have made a robot that looks like a plant tendril, similar to the ones that ivy plants might use to climb a trellis. She says they want to develop a robot that can explore an unknown area just like a plant does.
BARBARA MAZZOLAI: Plants could be the model to develop the robot for a very harsh environment, for colonizing a very difficult environment, to explore a harsh situation. Tendril is one of the ways, one of the strategies that they use to anchor the body.
CHEN: The robot doesn’t just look like a plant tendril. It also moves according to the same mechanism as in nature. Both the robot and the plant move fluid around inside them to extend their tendrils. Both the plant and robot push or extract fluid from inside the tendril by exploiting the process of osmosis. Mazzolai’s colleague, Edoardo Sinibaldi, explains how it works in their robot. If you can imagine the curly tip of the robot tendril, that curl is attached to a longer tube contained within a special sleeve made of active carbon cloth electrodes. This sleeve is placed in a solution containing lots of ions.
EDOARDO SINIBALDI: In our implementation we used sodium sulfate. It’s a common electrolyte, and it’s very stable, and it’s not toxic.
CHEN: They’ve made the tendril using two types of plastic. One type, called polysulfone, is permeable to water. Water can flow across this plastic barrier through pores about 50 nm in size. Imagine this part of the tendril, which is soaking inside the solution within the carbon cloth sleeve. At equilibrium, the ion concentration is the same inside and outside the tube. But Sinibaldi applies a voltage to two electrodes on the cloth sleeve, to make ions collect on the electrodes. This causes the liquid outside the tube to have a lower concentration of ions than inside the tube. This concentration gradient causes water to rush into the tube. This inflow of fluid stiffens the tendril tip, making it extend. The tendril tip is made of another type of plastic, ethylene terephthalate, coated with Al, which has been fabricated to achieve the necessary stiffness.
SINIBALDI: This is the basic fluid transport used by plants to, let’s say, swell cells and tissues in a coordinated manner and consistently stiffen tissue while it’s inflating and decreasing the stiffness of tissue while deflating.
CHEN: It takes about a milliliter of fluid to extend the robot tendril. They were able to coil the tendril 500 degrees in 25 minutes. And they can reverse the coiling or uncoiling as needed. Mazzolai also points out that the whole robot, including the tube and the carbon cloth, are made of flexible, soft material with tunable stiffness. These are properties that could be useful for medical applications.
MAZZOLAI: This is the challenge of the soft robotics community, developing something that can operate in the body without any damage to the human, and at the same time to be able to operate and change the stiffness.
CHEN: Mazzolai has also chosen to make plant-like robots because they have unique adaptations for exploring and functioning in harsh conditions. She thinks that they could be useful for exploring new planets, or in more mundane applications on Earth.
MAZZOLAI: These robots, they can move inside very narrow spaces for exploration for recovering objects in wells, or moving debris after a disaste