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In a study published online this week, robotics researchers, engineers and materials scientists from Rice University and Harvard University showed it is possible to make programmable, nonelectronic circuits that control the actions of soft robots by processing information encoded in bursts of compressed air.

“Part of the beauty of this system is that we’re really able to reduce computation down to its base components,” said Rice undergraduate Colter Decker, lead author of the study in the Proceedings of the National Academy of Sciences. He said electronic control systems have been honed and refined for decades, and recreating computer circuitry “with analogs to pressure and flow rate instead of voltage and current” made it easier to incorporate pneumatic computation.

Decker, a senior majoring in mechanical engineering, constructed his soft robotic control system primarily from everyday materials like plastic drinking straws and rubber bands. Despite its simplicity, experiments showed the system’s air-driven logic gates could be configured to perform operations called Boolean functions that are the meat and potatoes of modern computing.

“The goal was never to entirely replace electronic computers,” Decker said. He said there are many cases where soft robots or wearables need only be programmed for a few simple movements, and it’s possible the technology demonstrated in the paper “would be much cheaper and safer for use and much more durable” than traditional electronic controls.

As a freshman, Decker began working in the lab of Daniel Preston, an assistant professor of mechanical engineering at Rice. Decker studied fluidic control systems and became interested in creating one when he won a competitive summer research fellowship that would allow him to spend a few months working in the lab of Harvard chemist and materials scientist George Whitesides.

The project turned into a monthslong collaboration between the two research groups, and Decker had nine co-authors on the study, including co-corresponding authors Preston and Whitesides.

Decker and colleagues created two components, a pistonlike actuator that translates air pressure into mechanical force and a valve that can be switched between two states — off and on. The components were made from parts that included plastic drinking straws, flexible plastic tubing, rubber bands, parchment paper and thermoplastic polyurethane sheets that could be bonded together with a desktop heat press or a hot iron.

The research team showed the two components could be combined in a single device, a bistable valve that operates like a switch and uses air pressure as both input and output. A specific amount of air pressure is needed to flip the switch between off and on states. The valves are held closed by rubber bands, and they are programmed by adding or subtracting rubber bands, which changes the amount of pressure required for activation. In tests, Decker showed the circuits could be used to control a soft, hand-shaped robot, a pneumatic cushion and a shoebox-sized robot that could walk a preprogrammed number of steps, retrieve an object and return to its starting location.

“The biggest achievement in this work is the incorporation of both digital and analog control in the same system architecture,” said Preston. Having both means the pneumatic control circuits can be programmed digitally, with the “ones and zeroes that you think of in a traditional computer. But we can also bring in analog capabilities, things that are continuous,” he said. “That allows us to really simplify the overall system architecture and achieve new capabilities that weren’t accessible in prior work.”

It is rare for an undergraduate to be the lead author of a study in a journal as prestigious as the Proceedings of the National Academy of Sciences, but Preston said Decker’s success was no fluke.

“The undergraduates at Rice are truly top notch, and Colter, in his case, actually has risen to essentially what I would say is the level of a Ph.D. student in terms of some of his output as an undergraduate researcher,” Preston said.

Additional co-authors include Haihui Joy Jiang, Samuel Root, Jonathan Alvarez, Jovanna Tracz and Lukas Wille of Harvard, Anoop Rajappan of Rice and Markus Nemitz of Worcester Polytechnic Institute.

The research was supported by the Department of Energy (DE-SC0000989), the National Science Foundation (2144809, 2011754, 2025158), the Rice University Academy of Fellows and the Harvard University Center for Nanoscale Systems.

Video: https://youtu.be/0D22s2cMlxc

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Materials provided by Rice University. Original written by Jade Boyd. Note: Content may be edited for style and length.

The paper is published in the Proceedings of the National Academy of Sciences (PNAS).

“This is the first time these concepts have been fundamentally demonstrated,” said Chris Ellison, a lead author of the paper and professor in the University of Minnesota Twin Cities Department of Chemical Engineering and Materials Science. “Developing new ways of manufacturing are paramount for the competitiveness of our country and for bringing new products to people. On the robotic side, robots are being used more and more in dangerous, remote environments, and these are the kinds of areas where this work could have an impact.”

Soft robotics is an emerging field where robots are made of soft, pliable materials as opposed to rigid ones. Soft growing robots can create new material and “grow” as they move. These machines could be used for operations in remote areas where humans can’t go, such as inspecting or installing tubes underground or navigating inside the human body for biomedical applications.

Current soft growing robots drag a trail of solid material behind them and can use heat and/or pressure to transform that material into a more permanent structure, much like how a 3D printer is fed solid filament to produce its shaped product. However, the trail of solid material gets more difficult to pull around bends and turns, making it hard for the robots to navigate terrain with obstacles or winding paths.

The University of Minnesota team solved this problem by developing a new means of extrusion, a process where material is pushed through an opening to create a specific shape. Using this new process allows the robot to create its synthetic material from a liquid instead of a solid.

“We were really inspired by how plants and fungi grow,” said Matthew Hausladen, first author of the paper and a Ph.D. student in the University of Minnesota Twin Cities Department of Chemical Engineering and Materials Science. “We took the idea that plants and fungi add material at the end of their bodies, either at their root tips or at their new shoots, and we translated that to an engineering system.”

Plants use water to transport the building blocks that get transformed into solid roots as the plant grows outward. The researchers were able to mimic this process with synthetic material using a technique called photopolymerization, which uses light to transform liquid monomers into a solid material. Using this technology, the soft robot can more easily navigate obstacles and winding paths without having to drag any solid material behind it.

This new process also has applications in manufacturing. Since the researchers’ technique only uses liquid and light, operations that use heat, pressure, and expensive machinery to create and shape materials might not be needed.

“A very important part of this project is that we have material scientists, chemical engineers, and robotic engineers all involved,” Ellison said. “By putting all of our different expertise together, we really brought something unique to this project, and I’m confident that not one of us could have done this alone. This is a great example of how collaboration enables scientists to address really hard fundamental problems while also having a technological impact.”

The research was funded by the National Science Foundation.

In addition to Ellison and Hausladen, the research team included University of Minnesota Department of Chemical Engineering and Materials Science researchers Boran Zhao (postdoctoral researcher) and Lorraine Francis (College of Science and Engineering Distinguished Professor); and University of Minnesota Department of Mechanical Engineering researchers Tim Kowalewski (associate professor) and Matthew Kubala (graduate student).

Video of a soft growing robot navigating a tortuous path: https://youtu.be/-Ez7m9LO4ZY

Video explaining the idea behind the plant-inspired research: https://youtu.be/8zZ8RivrrWc

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Materials provided by University of Minnesota. Note: Content may be edited for style and length.