Tough metal-ceramic composites with multifunctional nacre-like architecture

Nacre-like ceramic composites: Properties, functions and fabrication in the context of dental restorations

Dental restorations are in increasing demand, yet their success rate strongly decreases after 5-10 years post-implantation, attributed in part to mismatching properties with the surrounding buccal environment that causes failures and wear. Among current research to address this issue, biomimetic approaches are promising. Nacre-like ceramic composites are particularly interesting because they combine multiple antagonistic properties making them more resistant to failure in harsh environment than other materials. With the rapid progress in 3D printing producing nacre-like structures has open up new opportunities not yet realised. In this paper, nacre-like composites of various compositions are reviewed in the context of hypothetical biomimetic dental restorations. Their structural, functional and biological properties are compared with those of dentin, enamel, and bone to determine which composition would be the most suitable for each of the 3 mineralized regions found in teeth. The role of complex microstructures and mineral orientations are discussed as well as 3D printing methods that allow the design and fabrication of such complex architectures. Finally, usage of these processes and anticipated prospects for next generation biomimetic dental replacements are discussed to suggest future research directions in this area. STATEMENT OF SIGNIFICANCE: With the current ageing population, dental health is a major issue and current dental restorations still have shortcomings. For the next generation of dental restorations, more biomimetic approaches would be desirable to increase their durability. Among current materials, nacre-like ceramic composites are interesting because they can approach the various structural properties found in the different parts of our teeth. Furthermore, it is also possible to embed self-sensing functionalities to enable monitoring of oral health. Finally, new recent 3D printing technologies now permit the fabrication of complex shapes with local compositions and local microstructures. With this current status of the research, we anticipate new dental restorations designs and highlight the remaining gaps and issues to address.

Multifunctional nacre-like materials

Nacre, the iridescent inner layer of seashells, displays an exceptional combination of strength and toughness due to its 'brick-wall' architecture. Significant research has been devoted to replicating nacre's architecture and its associated deformation and failure mechanisms. Using the resulting materials in applications necessitates adding functionalities such as self-healing, force sensing, bioactivity, heat conductivity and resistance, transparency, and electromagnetic interference shielding. Herein, progress in the fabrication, mechanics, and multi-functionality of nacre-like materials, particularly over the past three years is systematically and critically reviewed. The fabrication techniques reviewed include 3D printing, freeze-casting, mixing/coating-assembling, and laser engraving. The mechanical properties of the resulting materials are discussed in comparison with their constituents and previously developed nacre mimics. Subsequently, the progress in incorporating multifunctionalities and the resulting physical, chemical, and biological properties are evaluated. We finally provide suggestions based on 3D/4D printing, advanced modelling techniques, and machine elements to make reprogrammable nacre-like components with complex shapes and small building blocks, tackling some of the main challenges in the science and translation of these materials.

On the damage tolerance of 3-D printed Mg-Ti interpenetrating-phase composites with bioinspired architectures

Bioinspired architectures are effective in enhancing the mechanical properties of materials, yet are difficult to construct in metallic systems. The structure-property relationships of bioinspired metallic composites also remain unclear. Here, Mg-Ti composites were fabricated by pressureless infiltrating pure Mg melt into three-dimensional (3-D) printed Ti-6Al-4V scaffolds. The result was composite materials where the constituents are continuous, mutually interpenetrated in 3-D space and exhibit specific spatial arrangements with bioinspired brick-and-mortar, Bouligand, and crossed-lamellar architectures. These architectures promote effective stress transfer, delocalize damage and arrest cracking, thereby bestowing improved strength and ductility than composites with discrete reinforcements. Additionally, they activate a series of extrinsic toughening mechanisms, including crack deflection/twist and uncracked-ligament bridging, which enable crack-tip shielding from the applied stress and lead to “Γ”-shaped rising fracture resistance R-curves. Quantitative relationships were established for the stiffness and strengths of the composites by adapting classical laminate theory to incorporate their architectural characteristics.

Bioinspired architectures are desired to achieve improved mechanical properties, but challenging to achieve in metallic systems. Here the authors fabricate a Mg-Ti interpenetrating phase composite with brick-and-mortar, Bouligand, and crossed-lamellar architectures by pressureless infiltrating method.

Evaluation of remodeling and geometry on the biomechanical properties of nacreous bivalve shells

Mollusks have developed a broad diversity of shelled structures to protect against challenges imposed by biological interactions(e.g., predation) and constraints (e.g., [Formula: see text]-induced ocean acidification and wave-forces). Although the study of shell biomechanical properties with nacreous microstructure has provided understanding about the role of shell integrity and functionality on mollusk performance and survival, there are no studies, to our knowledge, that delve into the variability of these properties during the mollusk ontogeny, between both shells of bivalves or across the shell length. In this study, using as a model the intertidal mussel Perumytilus purpuratus to obtain, for the first time, the mechanical properties of its shells with nacreous microstructure; we perform uniaxial compression tests oriented in three orthogonal axes corresponding to the orthotropic directions of the shell material behavior (thickness, longitudinal, and transversal). Thus, we evaluated whether the shell material's stress and strain strength and elastic modulus showed differences in mechanical behavior in mussels of different sizes, between valves, and across the shell length. Our results showed that the biomechanical properties of the material building the P. purpuratus shells are symmetrical in both valves and homogeneous across the shell length. However, uniaxial compression tests performed across the shell thickness showed that biomechanical performance depends on the shell size (aging); and that mechanical properties such as the elastic modulus, maximum stress, and strain become degraded during ontogeny. SEM observations evidenced that compression induced a tortuous fracture with a delamination effect on the aragonite mineralogical structure of the shell. Findings suggest that P. purpuratus may become vulnerable to durophagous predators and wave forces in older stages, with implications in mussel beds ecology and biodiversity of intertidal habitats.