Eggshell-like columnar nanocomposites with structural gradients

Structural gradients are prevalent in biological hard tissues such as nacre, bones, tooth enamel, and eggshells, where they play a crucial role in optimizing load-bearing performance. While biomineralization naturally creates elegant gradations using a diverse range of anisotropic minerals, replicating this concept in synthetic materials with 1D building blocks is challenging due to the lack of mechanisms for controlling axial alignment at the micro- and nanoscale. In this work, a layer-by-layer, self-regulated growth of fluorapatite crystallites is used to synthesize columnar nanocomposites with structural gradients that closely resemble the architecture of eggshells. As fluorapatite layers stack, the crystallographic alignment gradually improves due to spatial restrictions imposed by radially growing crystallites. Epoxy infiltration results in columnar nanocomposites with surface modulus and hardness comparable to tooth enamel, decreasing gradually toward the interior. The variation in crystallite orientation also governs the gradient in strain rate sensitivity and resistance to creep deformation, as confirmed by finite element simulations. These gradient mechanical properties can be tailored by adjusting the growth kinetics of fluorapatite crystallites, which control lateral growth cessation and the magnitude of alignment gradients. Eggshell-like columnar nanocomposites provide a promising biomimetic prototype for designing structural materials with intricate features.