Self-healing organisms inspire CEE research team

Sea urchins, mollusk shells and many other organisms in nature share a useful trait: they are able to regrow and regenerate when damaged. Amorphous calcium carbonate (ACC) plays an important role in the self-healing process. Unlike crystalline minerals, which have a very ordered microstructure, ACC benefits from an amorphous – that is, disordered – arrangement of atoms. This confused microstructure creates many advantages for the mineral: it can mold into any desired shape, is tough due to the lack of smooth breakage planes and can transition between a solid and viscous state in response to application of temperature or stress changes. Two CEE at Illinois researchers believe that introducing an amorphous component into biomimetic composites (synthesized materials with properties that mimic natural ones) will lead to superior engineering materials.

Associate Professor Rosa Espinosa-Marzal and Assistant Professor Ahmed Elbanna have received a three-year, $400,000 grant from the National Science Foundation to explore the properties of ACC-based composites and quantify its mechanical properties. The mainstream approach to working with biominerals is to treat the amorphous component as simply a precursor to a crystalline phase, the researchers note. Harnessing the amorphous phase is a novel approach inspired by nature.

“We looked into some organisms and found they produce this amorphous phase, but then keep it in their structure rather than convert it to a crystalline structure,” Elbanna said. “The complex structure is better for the organisms to resist extreme loads.”

“Biominerals are very tough,” Espinosa-Marzal said. “They can absorb a lot of stress before they break. We believe that ACC contributes partially to the toughness of these materials.”

Creating a biomimetic composite in the lab that retains the amorphous component is the first step of the project. Espinosa-Marzal, a materials scientist who teaches courses in environmental engineering and materials, will undertake to synthesize a piece of material that can be used for testing.

“There are various strategies to create these materials,” she said. “The first step is to stabilize this amorphous component in the laboratory, using chemistry, combining inorganic components with selected polyelectrolytes [organic substances of high molecular weight]. In fact, ACC is found in nature but the stabilizing mechanism is not fully clear, and therefore it is one of the challenging parts of the project to create stable ACC.”

Lab tests will help the researchers to obtain some mechanical properties on the resulting material, which can then be modeled. Elbanna, whose background is in applied mechanics and structural engineering, will attempt to replicate the mechanical behaviors that were recorded in the lab. The research team will run various tests on the bonding interfaces between the organic materials and polymers to explore the behavior of the composites and establish optimal configurations.

Ultimately, optimized biomimetic composites can lead to better-performing materials with maximum crack resistance or stiffness, and self-healing capabilities.

“Amorphous materials have this property where if you change the temperature or pressure they change from a solid material to a viscous material. If you have this amorphous phase, you have a natural reservoir for this self-healing condition,” Elbanna said. “For example, if there is a crack in materials, changing the temperature locally can change it from solid nature to fluid nature and seal the crack.”

In the long-term, if the researchers are successful in reaching their objectives, their work will have a wide impact in the form of better and less-costly infrastructure. More immediately, the joint project is notable for the uniqueness of pairing the two specialties.

“I’m particularly excited about this type of collaboration,” Elbanna said. “Rosa is an experienced materials scientist in the field of environmental engineering and science, and I come from a mechanics background and am a member of the structures group. So this interface between materials science and mechanics – structures and environmental science – is very interesting.”