(Nanowerk News) The coming decades present a host of challenges for our built environments: a rising global population combined with increasing urbanization; crumbling infrastructure and dwindling resources to rebuild it; and the growing pressures of a changing climate, to name a few.
To become more livable for more people, cities themselves will need to become smarter, with buildings, bridges and infrastructure that are no longer static but dynamic, able to adapt and respond to what’s going on around them. If not exactly alive, these structures will need to be life-like, in important ways. And for that, they’ll need to incorporate living materials.
“Engineers and scientists have worked for hundreds of years with so-called smart materials,” says Zoubeida Ounaies. “Piezoelectricity was discovered in the 1880s.” Smart materials can sense and respond to their environment, she explains, “but they always need an external control system or source of power. Living materials that adapt, respond to the environment, self-power, and regenerate—in the way that materials in nature do—are the next logical step.”
Ounaies, a professor of mechanical engineering at Penn State, is director of the Convergence Center for Living Multifunctional Material Systems, a research partnership between Penn State and the University of Freiburg in Germany. Known as LiMC2, the center is one of only a handful in the world focused on this emerging field.
A new paradigm: Engineered materials inspired by nature
Living materials, Ounaies explains, are engineered materials that are inspired by nature. Sometimes they even incorporate biological elements. Their dynamic properties, at any rate, enable them to adapt to changes in their environment, responding to external stimuli. They may change shape, heal themselves, even make simple decisions.
Ounaies’s counterpart at Freiburg is Jurgen Ruhe, director of the Cluster of Excellence in Living, Adaptive and Energy-autonomous Materials Systems (livMatS). At a webinar last summer Ruhe put it this way: “If we look at the materials of today, one of the very key features is that materials have properties which do not vary in time. But if we turn our view to nature, nothing is really constant. For living systems, adaptivity is the key to survival. The goal of our livMatS cluster is to generate materials systems which can adapt to changes in the environment based on sensory input and then improve over their lifetime.”
Importantly, Ounaies says, living materials are multifunctional. They don’t just provide strength or elasticity or hardness, they reduce environmental impacts and promote health; they monitor their own status, and when used up they can be recycled or reabsorbed. They harvest energy from their surroundings, store it, and use it for what they need. They do these things, ideally, while self-powering and without external sensors or motors.
Above all, perhaps, engineered living materials aim to be sustainable. “The concept requires us to look at the whole life cycle,” Ounaies says. “To think about the starting material, the extraction and manufacturing processes, the waste generated, the energy required.” The design must account for all. Thus, unlike many smart materials, living materials don’t put a harmful load on the environment.
The Bird of Paradise plant is an example of a natural system whose mechanisms have inspired engineering solutions. A sun-shading system being developed by Thomas Speck and colleagues at the University of Freiburg incorporates its distinct opening and closing movements. (Image: Sardaka)
“If you think about it,” she says, “adaptive behaviors happen in nature all the time. Maybe not in a material form, but certainly in systems. There are plant systems that do this. There are animals that do this. ” Nature does the original design work. “For example, if one investigates the hierarchical pattern of a mollusk shell or the intricate structure of bird wings, one is inspired to apply them to human made structures in ways that integrate multiple functionalities.”
Thomas Speck has been fascinated by biomimetics for 30 years. Trained as a biophysicist, Speck is now professor of botany at the University of Freiburg. He studies the functional morphology of plants—the relationship between structure and function—and how these “biological role models” might be applied to the world of technology. As director of the University’s Botanic Garden, he has over 6,000 species from which to find his inspiration.
Plants, says Speck, have important lessons to offer. “First, they are mobile, although their movement is often hidden from us,” he explains. “A lot of plant movements are very aesthetic—think of a flower opening. We want to transport this aesthetics into our architectural solutions.”
The livMatS Pavilion at University of Freiburg’s Botanic Garden. A collaboration between Freiburg and the University of Stuttgart, the cottage-sized structure is made of wound flax fiber bundles covered with a waterproof polycarbonate. (Image: IntCDC, University of Stuttgart, Robert Faulkner)
What’s more, Speck says, plants work their magic with a very limited number of structural materials. “Cellulose, hemi-cellulose, lignin, a bit of pectin. Three polysaccharides and one complex polyaromatic polymer. With these materials, which are all relatively easy to recycle, they are able to make fantastic structures, fantastic systems which work incredibly well.”
A simple example is the pine cone, whose paddle-shaped scales open and close in response to changes in environmental humidity. At the Botanic Garden, Speck and his colleagues have analyzed fossilized pinecones 50 million years old and found that they still perform like modern specimens. “And it costs no energy, because humidity changes are brought by sunlight,” he says.
As amazingly robust as the natural mechanism is, the pinecone is merely reactive, Speck notes. “If it’s wet, it’s closed. If it’s dry, it’s open.” In adapting this principle, he says, “We want to design systems that are interactive, that can combine movements, that make decisions. Biomimetics for us means we get inspiration from nature and then reinvent nature. We don’t copy it. We want to combine the best of both worlds: living nature and technics.”
A center for living materials is born
Engineering living materials requires a daunting combination of expertise: in biology, materials, engineering, and design, to name a few. It’s exactly the sort of problem that Penn State’s interdisciplinary institutes were set up to solve. LiMC2 got its start when the directors of two of those institutes, Tom Richard of the Institutes for Energy and the Environment and Clive Randall of the Materials Research Institute, saw this emerging field as one in which the University could excel.
