Biogenic crystals produced by organisms have been known for several decades to exhibit intracrystalline organic macromolecules. Here, using a reductionist approach, we tackle the question of whether the incorporation of single amino acids is driven by kinetics or by thermodynamics. We show that when calcite is grown in the presence of amino acids under nonambient conditions, extremely high loading levels of up to 6.12 mol % of aspartic acid (Asp) are achieved. This incorporation leads to marked changes in the host calcite crystal’s structure and expansion coefficient. The latter is as much as twice as high as that of pure calcite. This is the first example showing that an organic molecule incorporated into an inorganic host can strongly affect the expansion coefficient. Most importantly, we show that the incorporation of amino acids in calcite is controlled by their thermodynamic solubility in calcite rather than kinetically and that hybrid amino acid-calcite crystals can indeed be considered a solid solution.

As organisms can form crystals only under ambient conditions, they demonstrate fascinating strategies to overcome this limitation. Recently, we reported a previously unknown biostrategy for toughening brittle calcite crystals, using coherently incorporated Mg-rich nanoprecipitates arranged in a layered manner in the lenses of a brittle star, Ophiocoma wendtii. Here we propose the mechanisms of formation of this functional hierarchical structure under conditions of ambient temperature and limited solid diffusion. We propose that formation proceeds via a spinodal decomposition of a liquid or gel-like magnesium amorphous calcium carbonate (Mg-ACC) precursor into Mg-rich nanoparticles and a Mg-depleted amorphous matrix. In a second step, crystallization of the decomposed amorphous precursor leads to the formation of high-Mg particle-rich layers. The model is supported by our experimental results in synthetic systems. These insights have significant implications for fundamental understanding of the role of Mg-ACC material transformation during crystallization and its subsequent stability.

The development of residual strains within a material is a valuable engineering technique for increasing the material's strength and toughness. Residual strains occur naturally in some biominerals and are an important feature that is recently highlighted in biomineral studies. Here, manifestations of internal residual strains detected in biominerals are reviewed. The mechanisms by which they develop, as well as their impact on the biominerals' mechanical properties, are described. The question as to whether they can be utilized in multiscale strengthening and toughening strategies for biominerals is discussed.

Calcite has the ability to host large amounts of intracrystalline inclusions, a phenomenon that is known to be the case in biominerals and has been demonstrated in bioinspired synthetic systems. In this study, barium ion is focused on as the inclusion. Highly substituted Ba-calcite possesses disordered carbonate orientations, characteristic of 3 3 R R m m symmetry. It is shown that calcite experiences an order-disorder transition, in which the carbonate groups undergo progressive loss of their rotational order with increasing amounts of incorporated Ba, and reach complete rotational disorder for a critical amount of Ba. This transition is characterized and a theoretical model justifying the influence of Ba is proposed. Moreover, the disordered 3 3 R R m m Ba-substituted calcite has been previously identified as a high-temperature phase or as a highly metastable room-temperature phase. Those descriptions are challenged by successfully synthesizing it under slow-rate conditions and by studying its thermal behavior, and it is concluded that the fully disordered Ba-calcite is stable, whereas the transitional, partially disordered Ba-calcite phase is metastable.

Hybrid perovskites demonstrate high potential in optoelectronic applications. Their main drawback is their low stability under humid conditions. In this paper, one of nature's strategies is implemented-the incorporation of amino acids into the crystal lattice-in order to improve the stability of methylammonium lead bromide (MAPbBr 3) in water, and to tune its structure, as well as its optical and thermal properties. The amino acid lysine, which possesses two NH 3 + groups, is incorporated into the hybrid unit cell, by substituting two methylammonium ions and serves as a "molecular bridge". This incorporation induces a decrease in the lattice parameter of the host, accompanied with an increase in its bandgap and noticeable changes in its morphology. Furthermore, a substantial decrease in the thermal expansion coefficient of MAPbBr 3 and a shift of its cubic-to-tetragonal phase transformation temperature are observed. Two different modes of incorporation are identified, which depend on the conditions of crystallization. These modes dictate the level of lysine incorporation and the magnitude of MAPbBr 3 bandgap changes. Notably, lysine incorporation strongly increases the perovskite stability in water. This study demonstrates a unique and promising approach to tune the properties and improve the stability of hybrid perovskites via this novel bioinspired route.