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Synthetic composite mimics tooth enamel

March 21, 2017
Scanning electron microscope image of four strata of the abiotic enamel composite structure on a silicon substrate. Each stratum comprises a forest of ZnO columns in a polymer matrix. A thin polymer layer separates each stratum. The lightweight structure provides high load-bearing performance with exceptional resistance to vibrations stimulated by repetitive stresses normal to the layers. Credit: Bongjun Yeom, Myongji University, South Korea.

Inspired by the example of tooth enamel, which has evolved to endure a lifetime of mechanical stresses due to chewing, Nicholas Kotov of the University of Michigan and Bongjun Yeom, now at Myongji University in South Korea, together with several collaborators, have gone Nature one better in the form of a lightweight composite that consists of nanosized zinc oxide columns surrounded by a polymer matrix. “Our columnar nanocomposites of hard and soft materials represent a new approach to the design of light, load-bearing structures that are resistant to vibration and ageing,” says Kotov.

The project was aimed at understanding the mechanical effects of vibrations at the nanoscale. Vibrations are a problem in hard materials, such as tooth enamel, where accumulation of strain over time from repetitive stresses can generate defects that lead to fracture. “Vibrations are a killer,” Kotov says, “not only for tooth implants but also for materials used in electronics, buildings, automobiles, airplanes, anyplace where long-term, low-impact wear resistance is essential.”

Long interested in the biomimetic approach for designing new materials with superior performance, Kotov recalled that many hard, natural materials like nacre have a laminar composite structure with hard mineral platelets embedded in a softer biopolymer, while others have a columnar structure. Among the latter, he noticed, tooth enamel with the columns normal to the tooth surface was unique in retaining the same basic microstructure from species to species and from epoch to epoch since teeth first appeared eons ago, suggesting the structure was not an accident and it was worth taking a deeper look.

It turned out that tooth enamel has similar hardness, stiffness, and toughness as several other materials, but has a markedly superior viscoelastic figure of merit (VFOM), the product of the storage modulus (the real part of the complex dynamic modulus for viscoelastic materials) and a damping coefficient that is a measure of energy dissipation. “There has been lots of work on improving the Young’s modulus and the hardness of composites,” says Lennart Bergström of Stockholm University, “but Kotov and his colleagues have shown the importance of viscoelasticity to the durability of structural materials and their resilience when exposed to intermittent shock waves, oscillatory stresses, and environmental vibrations.”

With this insight, the research team hypothesized that the vertical columnar structure was very efficient at damping the shear along the interface between the mineral and the polymer during deformation under a dynamic load. The dampened shear meant that vibrations did not propagate throughout the enamel,  which resulted in resistance to damage from repetitive stresses. The columnar organization also contributed to the material’s stiffness. They then set about devising a structure to test their hypothesis. 

The structure they settled on involved growing vertical columns on a silicon substrate seeded with zinc oxide nanoparticles. A hydrothermal growth process produced a forest of faceted zinc oxide columns, each about 1.4 µm high and 140 nm in diameter. To support the columns and complete the composite, the group resorted to a process called layer-by-layer (LBL) deposition of the positive and negative polyelectrolytes polyallylamine (PAAm) and polyacrylic acid (PAA), initially to coat the columns uniformly and subsequently to laterally fill the gaps between them, in contrast to Nature’s way, where the matrix comes first and serves as a template for biomineralization. To increase the thickness, the group planarized this structure with polymer to create a substrate for repeating the whole growth sequence. In this way, they made multilayered structures with up to five strata. 

“The preparation of multi-composites with three or more components in a controlled way is a hot topic in nanoscience,” says Gero Decher of the University of Strasbourg, “and making anisotropic multilayer structures with columns normal to the film was a major challenge.”

The materials were characterized using scanning electron microcopy, static and dynamic nanoindentation, finite-element analysis, and molecular dynamics simulations. Electron microscopy confirmed the structure, while the nanoindentation measurements yielded results consistent with the properties of tooth enamel, even though the zinc oxide proportion in the composite was less than that of the mineral in tooth enamel. Existing materials engineering models could not reproduce the experimental data, so the group made use of finite-element analysis and molecular dynamics simulations to confirm the role of energy dissipation due to shear at the interface between the stiff inorganic nanowires and soft organic. Bergström sums up: “Kotov’s group has combined a simple design concept combining well-established materials, executed the concept, and provided a detailed analysis to develop a unique material with novel properties.” 

With a new approach in hand, selecting materials, designing structures, and scaling to useful sizes for commercial applications are perennial issues, and the Michigan group’s process is no exception. Despite certain risks, Decher, one of the pioneers of the LBL technique, is confident that scale-up should not be a deal breaker. “The Michigan approach seems scalable,” he says. “In particular, LBL assembly is beginning to enter industry, and even big players like 3M are looking at what this nanofabrication technology can do for them. Millions of square meters per year of complex multilayer architectures can already be produced.” Similarly, wet techniques like hydrothermal growth are better suited to producing large areas of columns than vacuum techniques.

Read the abstract in Nature.