DraBot is a soft robotic dragonfly that senses and monitors its environment to help point out hazardous areas. It uses air force, microarchitectures, and self-healing hydrogels to perceive changes in pH and temperature and find contaminants like oil spills. Though it’s still just a prototype, it could pave the way for future environment-monitoring robotics.
Soft robots are trending in the robotics industry and an emerging research category due to their adaptability. The soft robotics field focuses on creating devices using compliant materials, such as elastomers and hydrogels. These materials allow robots to explore constrained surroundings where mainstream robots based on rigid materials can’t go.
Soft systems can help robots float and squeeze into tight areas where rigid frames would get stuck. They can perform delicate tasks, too, passively interacting with their surroundings. Soft parts can handle fragile things like biological tissues, whereas metal or ceramic components would cause damage.
Shyni Varghese, professor of mechanical engineering, biomedical engineering, materials science, and orthopedic surgery at Duke, had this newly expanding field on her mind when inspiration struck.
As Vardhman Kumar, a biomedical engineering Ph.D. student in Varghese’s laboratory and first author of the paper said:
I got an email from Varghese 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.
They ended up inventing a smart, electronics-free soft robot, dubbed DraBot, that features controlled skimming movement over water surfaces and ecological sensing abilities. It’s shaped like a dragonfly and can respond to environmental conditions like temperature, pH, or the presence of oil as it grazes lakes, rivers, canals, or any other body of water. The proof-of-principle prototype could be the forerunner to more advanced, autonomous, long-range environmental sentinels for monitoring a wide range of telltale signs of hazards.
The hydrogel that Varghese and her colleagues created back in 2012 is self-healing and reacts to modifications in pH in seconds. Any change in acidity causes the hydrogel to create a new bond, reversible once the pH returns to its original levels. So, two adjoining pieces of material “painted” with it would bond in an acidic environment.
Varghese’s spontaneously written idea to Kumar was to find a way to use this hydrogel involving a soft robot that can travel across water and locate places with pH level modifications. She was confident her lab could design such a robot – an autonomous environmental sensor.
Our lab has developed several stimuli-responsive hydrogel systems – one of them being pH-responsive, self-healing hydrogels wherein exposure to low pH results in instantaneous healing of individual hydrogel pieces. This process is reversible, and exposure to a high pH separates the hydrogels. Incorporation of such self-healing materials in soft robotics can be used to enhance their function.
With the help of his colleague Ung Hyun Ko, Kumar began designing a soft robot based on a fly. Eventually, the pair settled on the shape of a dragonfly instead. They engineered it to contain a network of microchannels inside that allow it to be controlled with air pressure.
They created the body (approximately 2.25 inches long with a 1.4-inch wingspan) by pouring silicon into a mold and baking it. The team used soft lithography to make the interior channels and connected them with flexible silicon tubing. Thus, DraBot was born.
We based the structural design of our skimming, soft robot on the body plan of a dragonfly as skimmers are the most common family of dragonflies, hence the name DraBot. This entirely soft robot is fabricated out of silicone elastomers using microfabrication. It is decorated with multiple functional elements such as flexible actuators and stimuli-responsive and self-healing materials at different locations on its body to incorporate sensing capabilities.
Microfluidic and air microchannels inside the robot’s body control its locomotion. Meaning, DraBot works by controlling the air pressure flowing into its wings. The microchannels carry the air into the front wings, where it escapes through a series of holes pointed towards the back wings. If both rear wings are up, DraBot goes forward. But if they’re down, blocking the airflow, DraBot goes nowhere.
The team incorporated balloon actuators under each of the back wings close to DraBot’s body to facilitate the element of control. The balloons make the wings curl upward if inflated. The researchers use this feature to change which wings are down or up, thus telling DraBot where to go.
Co-first author Dr. Ung Hyun Ko, a postdoctoral fellow in Varghese’s research group, said:
The front wings of the robot consist of air channels while the back wings consist of microfluidic channels that enable the flapping movement of the wings via balloon actuators. The air channels in the front wings direct the airflow backward towards the back wings, propelling the robot forward using jet propulsion. Precise control over forwarding movement, taking a turn, and coming to a stop is enabled by flapping the back wings via flexible actuators.
Getting DraBot to respond to air pressure controls over long distances using only self-actuators without any electronics was difficult. That was the most challenging part.
The researchers created the robot to sense and adjust to its environment, thus bringing it nearer to mimicking living systems and exhibiting complex motion.
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.
That’s where the self-healing hydrogel came in. The scientists made DraBot responsive to alterations in the surrounding water’s pH by painting one set of wings, on the same side, with the substance. So, when DraBot encounters an acidic condition, the wings fuse, disabling the flapping and impairing the forward movement of the robot. It causes the robot to fly in circles over-acidity. After the pH returns to an average amount, the hydrogel “un-heals,” the fused wings separate, and the robot once again becomes fully responsive to commands.
The scientists also doped sponges on the underside of the wings with temperature-responsive materials to beef up their ecological awareness. Its wings change from red to yellow when the water is overly warm.
Also, whenever DraBot skims over the water with oil floating on top, the sponges will soak it up and alter the wing’s color. The sponges are microporous hydrophobic structures, part of the soft robot’s design already because they help it stay afloat and assist with locomotion.
Applications for a robot-like DraBot could include:
- A monitor for pH level in freshwater to signal acidification – a severe environmental problem impacting geologically sensitive regions.
- An oil spill early-detection device.
- A red tide and coral bleaching detection device (the color-changing wings in response to temperature could spot indications of such events).
This proof-of-concept study demonstrates the potential of such robots to be multi-functional. Other nature-mimicking soft robots or bio-sensors include a fish that can swim in the Marina Trench, gathering ecological information, and a Venus Flytrap controlled by Smartphone.