Machine Metabolism

A garter snake curves and slithers into a pond to escape a circling hawk overhead. A jellyfish moves through the ocean by pulsating its gel-like body. A scientist in her lab reaches for a pipette, and the muscles in her arm expand and contract to pick up the tool.

The capacity for concerted movement is essential for survival in most living things. Yet the slightest movement made by a living creature requires a complex symphony of biochemical effort. Now, a few scientists are exploring a new frontier—whether the capacity for this kind of concerted movement could be created in nonliving objects. Is that even possible?

Two Pitt engineering professors say ‘Yes,’ and their research suggests even more intriguing possibilities—soft robotics.

Anna Balazs, Distinguished Professor of Chemical and Petroleum Engineering and the Robert Von der Luft Professor in Pitt’s Swanson School of Engineering, has been immersed in the field of biomimicry since 2006, investigating whether the basic biological function of autonomous motion can be applied outside the natural world, to synthetic material.

“In biology it’s very common for the act of directed motion to be accompanied by some level of changing of shape, either the muscles or the entire frame,” she says. To survive, a living organism must be able to move around to get food or to escape an enemy. “We don’t have too many synthetic materials that spontaneously pick themselves up and move, so we wanted to see: Can you inscribe that property onto a synthetic material? Can you blur the line between the living and nonliving?”

This was the philosophical conundrum that became the “intellectual quest” for Anna Balazs and Olga Kuksenok, associate professor of chemical engineering in Pitt’s Swanson School. The pair’s research—published earlier this year by Nature Publishing Group’s Scientific Reports—describes a computational model for creating a synthetic material that is able to harness its own internally generated chemical energy to reconfigure its shape and undergo self-sustained autonomous motion.

The two engineers work with polymer gels, which are synthetic materials that have elements of liquid and elasticity, creating a neither-here-nor-there state on the solid-to-liquid spectrum. Their pliability makes them ideal materials for playing around with movement. In their recent work, Balazs and Kuksenok designed a synthetic polymer gel with new and unusual properties

Typically, polymer gels are mono-functionalized, meaning they are designed to do one thing, and one thing only. So, the processes required for directed motion are far more intricate and complex than any single polymer gel can support. But what if the gels were somehow actualized and, possibly, networked?

Balazs and Kuksenok built upon the work of a research colleague, Ryo Yoshida, at the University of Tokyo who explored the notion of taking two gels, each with their own function, and synthesize them into a brand new, composite, dual-functionalized gel, with properties of both its constituents?

The Pitt researchers pushed this further.  Using the dual-functionalized polymer—Spirobenzopyran, (or SP) gel, which flexes and shrinks in the presence of light, and Belousov-Zhabotinsky (or BZ) gel, which swells and deswells in the presence of a chemical catalyst—they experimented with various patterns of light to influence SP’s bending and folding properties in concert with BZ’s pulsation and movement properties.

What resulted was the design of a synthetic polymer gel able to create its own energy and use it to perform functions that result in self-sustained motion and shape-shifting. Balaz says its similar to the process of metabolism in living creatures.

“The next push in materials science is to mimic these internal metabolic processes in synthetic materials, and thereby, create man-made materials that take in energy, transform this energy, and autonomously perform work, just as in biological systems,” said Balaz in the research account published in the April 30 Scientific Reports.

This breakthrough could be the basis for new combinations of polymer gels and networks of these gels in the development of soft robotics.

“If robots are soft and squishy, that gives you more range of functionality than if you have all these hard, noncompliant, nonflexible bits,” Balazs says. “Then they can more readily squeeze into small spaces, they can bend more readily.” Like a garter snake or a jelly fish.

Unlike the stiff, mechanical robot arms used today in manufacturing—or the hard-edged robots of Star Wars fame, C-3PO and R2-D2—the development of soft, responsive, self-energized gels would enable flexible joints and fluid movements, closer to a human arm reaching for a pipette.

So, the two Pitt engineers continue to delve deeper into, as Balaz says, that “wonderful question of blurring the line between living and nonliving.” ■