“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.
“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.”
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.
Penn State already had a strategic partnership with Freiburg, one of Germany’s top universities, and the two institutions had both overlapping and complementary strengths in the area of living materials. Working together, through both research collaborations and educational exchanges, they could become a worldwide leader.
The idea was supported and facilitated by Penn State Global Programs and the International Office in Freiburg, and in July of 2019, Penn State President Eric Barron and the University of Freiburg’s Rector Hans-Jochen Schiewer signed the formal partnership that made LiMC2 a reality. Soon after, the center announced that adaptive architecture would be one of its core research areas.
Adaptive architecture, Ounaies says, is a “no-brainer” for a center built around living materials. “It’s a natural core area for us because of the focus on materials discovery and sustainability, as well as our strengths in smart materials, advanced manufacturing, architecture and design, where the needed expertise spans multiple department and colleges. And it’s also a great platform where all these things can come together to demonstrate what living materials can do.”
Before the center even existed, in fact, Ounaies, whose research focuses on responsive soft materials for sensing, actuation, energy storage and energy harvesting, was collaborating with Penn State architecture professor Jose Duarte and Ph.D. student Elena Vazquez, both in the College of Arts and Architecture.
Duarte’s work focuses on customized buildings that fit into their environmental contexts to create sustainable and affordable housing options. But conventional approaches to customization have their limits. “Once it’s built, it’s static,” he says. So he and his students began to explore the possibilities for buildings that continue to adapt in response to changing conditions.
For her Ph.D., Vazquez designed a shading system that could be added to the façade of an existing building, made up of panels that open or close in response to sunlight, controlling the amount of light and heat entering the building. Because a mechanical system would be expensive and difficult to maintain, Duarte and Vazquez were looking for a materials solution for her design.
“My research interest, broadly defined, is to find new engineered materials and figure out ways of incorporating them into architectural design,” Vazquez says. They turned to Ounaies, aware of her work with electroactive polymers. Vazquez spent time training in Ounaies’s lab, and eventually they settled on bistable materials as a possible solution. With just a little bit of energy, Ounaies explains, a bistable material can be nudged from one stable shape to another—snapping from open to closed, for instance.
Vazquez’s subsequent design incorporates just such a material, actuated by an polymer-based smart materials, to open and close its shade panels. The system works in response to sunlight, without human intervention. A small protoype is mounted in the Stuckeman Building at University Park and a grant awarded by the American Institute of Architects will allow the collaborators to build a full-scale model.
Beyond the façade: Living materials in structural elements
Not surprisingly, the façade or building envelope–what Duarte calls “the membrane that permeates between the interior of the building and the outside”–is a primary focus of adaptive architecture. “Anything you do there has a huge impact on performance, on energy consumption, on environmental conditions inside,” Duarte says.
At Freiburg, Speck and his colleagues are developing their own sun-shading systems, combining the opening and closing movements of a pair of plant models: the Bird-of-paradise and the waterwheel plant. Botanist Linnea Hesse, however, is more interested in a building’s structural elements. During a summer webinar event introducing Penn State and Freiburg researchers to one another, she presented a keynote talk on biomimetic structural design.
Hesse, a group leader in plant biomechanics at Freiburg’s Botanic Garden, noted that the branching structures of trees have already been incorporated into architecture—in the spectacular designs of buildings like Barcelona’s Sagrada Familia basilica and Stuttgart’s international airport, for example. In addition to their beauty, the tree-like pillars in these designs function to spread load-bearing requirements and allow for more interior space.
With the goal of optimizing the load-bearing strength of such components, she is studying the dragon tree, a member of the Monocotyledon family that includes palm trees and bamboo. Monocots have a particularly pronounced fiber-reinforced inner structure, Hesse explains, achieving strength without excess weight and making them a good biological role model.
To better understand how a dragon tree’s inner structure adapts to a load, she turned to medical colleague Jochen Leupold, who trained her in the use of magnetic resonance imaging (MRI). Her time-lapsed scans of living trees, both under load and not, show localized changes in fibrous tissue due to weight-bearing, data that have already been used to improve the design of a fiber-and-concrete pillar being developed for architectural use.
Hesse is also using MRI and CT imaging to study branching mechanisms, how water is transported through monocot tissue, and the growth process itself. Understanding the inner workings of the plant, she says, is only the first step. Her vision is one day to help design houses that can adapt and grow over time, altering their structure to meet changing needs the way plants do. “It’s a bold idea,” she says. “But I think it’s possible. This is where I want to go.”
Juan Pablo Gevaudan, in contrast, wants to know how things break down. Gevaudan, an assistant professor of architectural engineering at Penn State, calls himself a modern cement chemist. He is focused on the problem of concrete degradation.