Robots Flex Their Stuff

By Leto Sapunar

After seeing the creations in Yigit Mengüç’s robotics lab at Oregon State University, you’d be forgiven for thinking you had wandered into a sci-fi film. Mengüç, an assistant professor of mechanical engineering has tentacle-like arms waving, synthetic snakes wiggling, and an underwater hanging machine, that at first glance resembles a living octopus, all lending life to the lab. These aren’t sights most people expect when they think of robots. However, they are typical for researchers in OSU’s soft robotics lab. Here, on the cutting edge — although there isn’t a solid edge to be found on most of these creations — the group is finding revolutionary new ways to fabricate and design machines that look and feel more like flesh than concrete.

Often inspired by biomimicry — the design of practical systems modeled on living organisms — soft robotic researchers look for ways to make robots safer, cheaper and more versatile. Where a conventional rigid robot arm is difficult to program and complex and expensive to fabricate, a soft robot has the potential to perform the same tasks with none of those limitations. This kind of design is particularly useful for delicate tasks like grasping coral samples on the seafloor or handing a glass of water to an elderly person.

Liquid Metal and Hollow Chambers

Soft robots are made from silicone, similar to the kind used in soft phone cases or ice cube trays, and sometimes infused with liquid metal wires made of gallium — a rare metallic element that is liquid near room temperature and found as a trace element in coal and other minerals.

Researchers can control the motion of these soft robotic limbs by pressurizing internal channels. A tentacle with four pneumatic chambers within can flex in any direction depending on which hollow chamber is inflated. This principle also works to make soft robotic snakes, which can locomote in a variety of different snake “gaits” depending on how their air chambers are pressurized and depressurized.

Most current robots are designed to encounter specific environments — a wheeled machine for flat surfaces or a propeller-driven underwater craft. These robots can function spectacularly in the particular circumstances they’re designed for but can lack the versatility to do much else. Soft robots, in their elastic and dexterous forms, could offer significant improvements. For instance, many living snakes can swim as well as traverse flat, rocky or sandy terrain. A soft robot made to mimic that motion, like the kind OSU Ph.D. student Callie Branyan is working on, can already move across flat surfaces as well as through granular materials like millet seeds, sand and river rocks. It can also slither through a pipe and escape after being buried under the seeds. Branyan is tweaking the design proportions as well in an attempt to build thinner, faster locomoting robots. “More like a garter snake than a python, quick and small,” she says.

Currently, the snake’s component pieces are made using a time-consuming molding process, but the lab is actively working on perfecting silicone 3D printing methods, allowing for any device configuration to be made cheaply and quickly. Osman Dogan Yirmibesoglu, another OSU Ph.D. candidate, is using a massive 3D printer that he built to create a meter-long, tentacle-like soft robotic arm. “It’s actually more powerful than an octopus arm,” he says, explaining that an octopus arm doesn’t do well outside of water, whereas this arm will be able to support its weight in air. Although it won’t look much like a tentacle, the arm will be able to flex in any direction and hold its position.

The material used is, as Yirmibesoglu puts it, “radiation transparent,” meaning it doesn’t interact strongly with most types of high-intensity radiation. That could make it preferable for radiological testing or emergency response robots for nuclear incidents. In such situations, high versatility is required to face intense radiation, pass over irregular terrain, investigate underwater reactor pools and operate controls.

To find out how the material responds to radiation, Yirmibesoglu printed samples of the silicone for Tyler Oshiro, an OSU nuclear engineering master’s student, who irradiated them in the university’s research reactor. Oshiro’s goal was to study how the mechanical properties of the silicone changed with high radiation exposure for his master’s thesis. He found the samples stiffened after exposure to vast amounts of radiation but deal well with it overall. Oshiro finds soft robotics interesting because the field is the epitome of finding different ways to approach traditional problems.

Sensors and Circuits

In the lab, Yirmibesoglu demonstrates how one of the sensors deforms when he presses it with his thumb. It stretches like an octopus tentacle but pops back into place as soon as it’s released.

As the material stretches, the small channels inside containing liquid metal become warped, making the wires thinner or thicker as the material flexes in different ways. Because wires of different thicknesses vary in electrical resistance, a computer hooked up to the electrodes can measure this value and determine how distorted the sensor is from its normal shape. In this way, a robot can know the location of each of its limbs. The technology has promise in other avenues like wearable electronics. The sensors are cheap, relatively lightweight and versatile.

Building a silicone structure with internal channels of liquid metal isn’t easy. Previously, it required a complicated, multistage molding and gluing process. However, the lab has recently devised a way to 3D print freestanding liquid metal and hopes to build a dual-headed printer, capable of constructing both the silicone body of a robot limb or sensor and its liquid metal veins simultaneously.

Oregon State Ph.D. student Nick Bira is working on figuring out the design for “soft valves” to precisely control airflow used to activate the limbs of a small octopus robot. The tricky part is building smart pressure controls that don’t rely on conventional electrical valves to achieve the same end. Elegant design here is key. He says the lab space reached “peak awe factor” a few months ago when, in addition to the usual high-ceilinged workspace filled with 3D printers, squishy limbs, actuators and aquatic prototypes were floating in an underwater testing tank.

New and Growing

Back in Mengüç’s Graf Hall lab, the assistant professor of mechanical engineering puts one inspiring animal center stage writing, “Many of my colleagues and I have chosen to take inspiration from one of the most alien mascots: the octopus. As soon as we take as our goal technology that is entirely soft, squishy and stretchy, yet dynamic, agile and intensely intelligent, we are forced to reevaluate what is possible.”

In 2017, Mengüç also wrote an overview piece on the field of soft robotics, published in American Scientist. He describes the many avenues of a new and growing field such as electrorheological fluids — substances that can change between liquid and solid based on an electric field — and the possibility of introducing artificial muscles into future robots. Among already existing soft robot technologies, he lists other applications including maritime robots for inspection and welding; stealthy naval surveillance robots; safer industrial manufacturing robots; surgical tools like endoscopes; and prosthetics or orthotics. The future possibilities all have been inspired by a creature that has existed for millions of years — the octopus.

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Editor’s note: Leto Sapunar was an undergraduate in the College of Science. He graduated from OSU in 2018.