12 April, 2021

Engineers from Duke University have developed an electronics-free, entirely soft robot shaped like a dragonfly that can skim across water and react to environmental conditions. 

Soft robots are a growing trend in the industry due to their versatility. Their soft parts can handle delicate objects and the soft bodies can help robots float or squeeze into tight spaces.

The soft robot is described online on March 25 in the following journal. The inspiration struck from the mind of Shyni Varghese, professor of biomedical engineering, mechanical engineeringmaterials science, and orthopaedic surgery at Duke. Vardhman Kumar, a PhD student in Varghese’s laboratory and first author of the paper commented:

“I got an email from Shyni from the airport saying she had an idea for a soft robot that uses a self-healing hydrogel that her group has invented in the past to react and move autonomously. But that was the extent of the email, and I didn’t hear from her again for days. So the idea sort of sat in limbo for a little while until I had enough free time to pursue it, and Shyni said to go for it.”

In 2012, Varghese created a self-healing hydrogel that reacts to changes in pH in a matter of seconds. Whether it be a crack in the hydrogel or two adjoining pieces “painted” with it, a change in acidity causes the hydrogel to form new bonds, which are completely reversible when the pH returns to its original levels.

Varghese’s idea was to find a way to use this hydrogel on a soft robot that could travel across water and indicate places where the pH changes. With the help of Ung Hyun Ko, a postdoctoral fellow, Kumar began designing a soft robot based on a fly. After several iterations, the shape of a dragonfly engineered with a network of interior microchannels was settled. This shape allows the soft robot to be controlled with air pressure.

The body of the robot was created to be about 2.25 inches long with a 1.4-inch wingspan—by pouring silicone into an aluminium mould and baking it. The team used soft lithography to create interior channels.

That’s how DraBot was born.

DraBot works by controlling the air pressure coming into its wings. Microchannels carry the air into the front wings, where it escapes through a series of holes pointed directly into the back wings. If both back wings are down DraBot goes nowhere but if both wings are up, it goes forward.

The team also designed balloon actuators under each of the back wings close to the robot’s body so that they can control it. When inflated, the balloons cause the wings to curl upward. By changing which wings are up or down, the researchers control the robot movements.

“We were happy when we were able to control DraBot, but it’s based on living things. And living things don’t just move around on their own, they react to their environment.”

Here comes the self-healing hydrogel. By painting one set of wings with the hydrogel, the researchers were able to make DraBot responsive to changes in the surrounding water’s pH. If the water becomes acidic, the front wing fuses with the back wing. Instead of travelling in a straight line, the imbalance causes the robot to spin in a circle. Once the pH returns to a normal level, the hydrogel “un-heals,” the fused wings separate, and DraBot once again becomes fully responsive to commands.

The researchers also created sponges under the wings and doped the wings with temperature-responsive materials. When the robot skims over the water with oil floating, the sponges will soak it up and change colour. And when the water becomes overly warm, DraBot’s wings change from red to yellow.

The team believe these types of measurements could play an important part in an environmental robotic sensor in the future. The team also sees many ways that they could improve their work in the future.

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