Mechanics of Hierarchical ArchitectureProjects:
Mechanics of Nano-Architected Materials
Personnel: Carlos Portela (Ph.D. student in Mechanical Engineering) and Widi Moestopo (Ph.D. student in Mechanical Engineering)
Micro- and nanolattices represent an emerging class of structural metamaterials that possess unique combinations of properties like ultralight weight, energy absorption, damage tolerance, enhanced deformability, metafluidic behavior, extremely low dielectric constants, and negative thermal expansion coefficients to name a few. Incorporating three-dimensional (3D) architecture into materials design across multiple length scales has led to the creation of advanced materials with novel mechanical properties, such as ultralight weight, negative Poisson’s ratios, enhanced heat capacity, and near infinite bulk-to-shear modulus ratios. Despite many proof-of-concept demonstrations, very few guiding principles exist for designing architectures that efficiently integrate structural and microstructural deformation mechanisms. Understanding the complex interplay between constituent materials and architecture is crucial to creating and optimizing new materials with tunable properties. As the characteristic features of architected materials are reduced to microscale, a question arises whether they are better described as discrete structures or continuum-like materials – especially in the context of flaw sensitivity and failure.
Using novel 3D patterning fabrication and a combination of experiments, finite element analysis, and theory, we are developing new principles that describe and quantify the mechanics of lattice architectures because classical theories are insufficient to capture and predict the full range of their mechanical properties. We are bridging the gap between in-situ mechanical experiments and computationally efficient models, with the objective of developing scaling laws that can better predict the mechanical behavior of the nano-architected materials that are currently being manufactured.
Microstructural Complexity of Biomaterials and Bio-Mimicking: Human Bone and Diatoms
Personnel: Ottman Tertuliano (Ph.D. student in Materials Science) and Dr. Alessandro Maggi (Ph.D. student in Medical Engineering, just defended)
Human bone is a sophisticated, hierarchically arranged composite that has intrigued scientists for a long time. Part of the reason why it has been challenging to fully understand the structure and properties of bone is because of its multi-scale nature: components range in size from some nanometers to microns to millimeters and centimeters. Isolating a particular feature has been very challenging. We use the Focused Ion Beam (FIB), nanoindenters, and transmission electron microscopy (TEM) to excise micron-sized site-specific sections of human bone to characterize its microstructure and to conduct in-situ nanomechanical experiments on these tiny bone samples to determine their strength, failure, and fracture toughness.
In addition to studying human bone, our group is investigating the role of scaffolds for bone cell proliferation and tissue formation. The precise mechanisms that lead to orthopedic implant failure are not well understood; it is believed that the micromechanical environment at the bone-implant interface regulates structural stability of an implant. We are interested in understanding how the 3D mechanical environment of an implant affects bone formation during early osteointegration. We design and fabricate three-dimensional (3D) rigid polymer scaffolds with architectures whose strut dimensions and geometries are on the same order as osteoblasts’ focal adhesions and pore sizes on the order of cell size coated with biocompatible materials and assess the cells’ viability and proliferation as a function of structural stiffness and design parameters.
Our group is also interested in bio-mimicking natural hard biomaterials because they are exceptionally resilient. We have been particularly interested in understanding the mechanical response of natural marine diatoms, as well as in fabricating life-sized synthetic replicas of diatom coscinodiscus sp frustules out of cyclohexyl polyhedral oligomeric silsesquioxanes (POSS). We demonstrate that these synthetic structures have biosilica-like amorphous atomic-level microstructure and mechanical attributes similar to those of a natural diatom. We conduct In-situ beam bending and fracture experiments on micron-sized excised sections of natural and synthetic diatoms and demonstrate that their mechanical properties are indistinguishable. The demonstrated ability to fabricate a synthetic hard biomaterial that is virtually indistinguishable from its natural counterpart while capturing its complex architecture, microstructure, and mechanical properties provides a powerful platform for investigating the specific role of each geometrical feature at every relevant length scale in the often sophisticated, multi-scale hierarchical construct of hard biomaterials and provides a robust pathway for property optimization.