Nacre is a natural material that displays unique mechanical properties due to its complex layered nano- and microarchitectures, allowing for effective dissipation of deformation energy. Many bio-mimicking approaches aim to reproduce nacre’s impressive mechanical properties, especially in terms of toughness. However, these chemical processes are complex and involve toxic chemicals, high temperatures and/or pressures. Eco-friendly production methods of high-performance nacre-like materials in ambient conditions are currently unavailable.
We are using Sporosarcina pasteurii bacteria to produce macroscopic amounts of calcium carbonate-based materials, including organic-inorganic layered composites that resemble nacre and bacterially-induced inorganic samples. The organic phase of the composite materials, polyglutamate, is produced by another species of bacteria: Bacillus licheniformis. Purely inorganic calcium carbonate samples precipitated in a chemical process serve as controls.
Nanoindentation reveals no significant differences in stiffness between bacterially-produced composite and inorganic samples. However, composites show a substantially higher toughness measured in 3-point bending tests, as compared to either bacterially-produced or chemically-precipitated inorganic samples. These results indicate that the organic layers in the composites allow the recovery of higher fractions of the elastic energy of deformation, perhaps by allowing locally for the relief of high stresses in the ductile organic layers. Interestingly, the bacterially-produced composite materials show little of the brittleness that is characteristic of calcium-carbonate-based materials. Moreover, at the nano-scale, the bacterially-produced materials show a characteristic granular structure and nano-asperities, similar to those found in nacre and likely involved in improving mechanical properties, while no such structure is present in the chemically-produced materials.
By bio-mimicking the complex architectures of natural nacre, we can produce high-performance materials, that attain high toughness and stiffness. This approach uses bacteria exclusively, thus allowing the fabrication of materials at ambient conditions in a more sustainable, economical, and simple manner.