The living, self-growing materials are in development at the University of Southern California.
“We have been amazed by the sophisticated microstructures of natural materials for centuries, especially after microscopes were invented to observe these tiny structures,” said environmental engineer Qiming Wang. “Now we take an important step forward – we use living bacteria as a tool to directly grow amazing structures that cannot be made on our own."
Wang and his colleagues work with S. pasteurii, known for secreting an enzyme called urease. When urease is exposed to urea and calcium ions, it produces calcium carbonate, a strong mineral compound found in bones and teeth.
“The key innovation in our research,” said Wang, “is that we guide the bacteria to grow calcium carbonate minerals to achieve ordered microstructures which are similar to those in the natural mineralised composites.”
The assistant professor of civil and environmental engineering added: “Bacteria know how to save time and energy to do things. They have their own intelligence, and we can harness their smartness to design hybrid materials that are superior to fully synthetic options.”
Combining living bacteria and synthetic materials, Wang said the new living material demonstrates mechanical properties superior to natural and synthetic materials currently in use. This is largely due to the material's bouligand structure, characterised by multiple layers of minerals laid at varying angles from each other to form a spiral shape, which is difficult to create synthetically.
One of the key properties of a mineralised composite, Wang said, is that it can be manipulated to follow different structures or patterns. Researchers have previously observed the mantis shrimp’s ability to use its ‘hammer’ to break open mussel shells. They found the hammer was arranged in a bouligand structure, offering superior strength to one arranged at more homogeneous angles.
In order to build the material, the researchers 3D printed a lattice structure with empty spaces within it. The bacteria were then introduced to the structure, attaching to surfaces and gravitating to the scaffold, where they secreted urease. Calcium carbonate structures grew from the surface up, eventually filling in the voids in the lattice.
“We did mechanical testing that demonstrated the strength of such structures to be very high. They also were able to resist crack propagation – fractures – and help dampen or dissipate energy within the material,” said doctoral student An Xin.
The material could be used in aerospace or automotive applications. Its lightweight nature and energy dissipation means it could also be suitable for body or vehicle armour.
Bacteria could even be reintroduced to the material after damage to carry out repairs.
“An interesting vision is that these living materials still possess self-growing properties,” said Wang. “When there is damage to these materials, we can introduce bacteria to grow the materials back. For example, if we use them in a bridge, we can repair damages when needed.”
Wang worked with An Xin, Yipin Su, Minliang Yan, Kunhao Yu, Zhangzhengrong Feng and Kyung Hoon Lee. Additional support was provided by Lizhi Sun, professor of civil engineering at the University of California, Irvine, and his student Shengwei Feng.
The research was published in Advanced Materials.
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