Fluid-flow-templated self-assembly of calcium carbonate tubes in the laboratory and in biomineralization: The tubules of the watering-pot shells, Clavagelloidea

Downward fingering accompanies upward tube growth in a chemical garden grown in a vertical confined geometry

Chemical gardens are self-assembled structures of mineral precipitates enabled by semi-permeable membranes. To explore the effects of gravity on the formation of chemical gardens, we have studied chemical gardens grown from cobalt chloride pellets and aqueous sodium silicate solution in a vertical Hele-Shaw cell. Through photography, we have observed and quantitatively analysed upward growing tubes and downward growing fingers. The latter were not seen in previous experimental studies involving similar physicochemical systems in 3-dimensional or horizontal confined geometry. To better understand the results, further studies of flow patterns, buoyancy forces, and growth dynamics under schlieren optics have been carried out, together with characterisation of the precipitates with scanning electron microscopy and X-ray diffractometry. In addition to an ascending flow and the resulting precipitation of tubular filaments, a previously not reported descending flow has been observed which, under some conditions, is accompanied by precipitation of solid fingering structures. We conclude that the physics of both the ascending and descending flows are shaped by buoyancy, together with osmosis and chemical reaction. The existence of the descending flow might highlight a limitation in current experimental methods for growing chemical gardens under gravity, where seeds are typically not suspended in the middle of the solution and are confined by the bottom of the vessel.

Self-Assembled Structures Formed in CO2-Enriched Atmospheres: A Case-Study for Martian Biomimetic Forms

The aim of this study was to investigate the biomimetic precipitation processes that follow the chemical-garden reaction of brines of CaCl₂ and sulfate salts with silicate in alkaline conditions under a Mars-type CO₂-rich atmosphere. We characterize the precipitates with environmental scanning electron microscope micrography, micro-Raman spectroscopy, and X-ray diffractometry. Our analysis results indicate that self-assembled carbonate structures formed with calcium chloride can have vesicular and filamentary features. With magnesium sulfate as a reactant a tentative assignment with Raman spectroscopy indicates the presence of natroxalate in the precipitate. These morphologies and compounds appear through rapid sequestration of atmospheric CO₂ by alkaline solutions of silica and salts.

Perovskite chemical gardens: highly fluorescent microtubes from self-assembly and ion exchange

We report the shape-preserving conversion of self-assembled CaCO3 microtubes to PbCO3 and MAPbBr3 perovskite.

Controlled self-assembly of chemical gardens enables fabrication of heterogeneous chemobrionic materials

Chemical gardens are an example of a chemobrionic system that typically result in abiotic macro-, micro- and nano- material architectures, with formation driven by complex out-of-equilibrium reaction mechanisms. From a technological perspective, controlling chemobrionic processes may hold great promise for the creation of novel, compositionally diverse and ultimately, useful materials and devices. In this work, we engineer an innovative custom-built liquid exchange unit that enables us to control the formation of tubular chemical garden structures grown from the interface between calcium loaded hydrogel and phosphate solution. We show that systematic displacement of phosphate solution with water (H₂O) can halt self-assembly, precisely control tube height and purify structures in situ. Furthermore, we demonstrate the fabrication of a heterogeneous chemobrionic composite material composed of aligned, high-aspect ratio calcium phosphate channels running through an otherwise dense matrix of poly(2-hydroxyethyl methacrylate) (pHEMA). Given that the principles we derive can be broadly applied to potentially control various chemobrionic systems, this work paves the way for fabricating multifunctional materials that may hold great potential in a variety of application areas, such as regenerative medicine, catalysis and microfluidics.

Nacre morphology and chemical composition in Atlantic winged oyster Pteria colymbus (Röding, 1798)

The microstructure and nanostructure of nacre in Pteria colymbus were studied with high-resolution field emission scanning electron microscopy (FESEM). The tablets were found to be flat and polyhedral with four to eight sides, and lengths ranging from 0.6 to 3.0 µm. They consisted of nanocrystals 41 nm wide, growing in the same direction. X-ray diffraction showed the crystals to be mineral phase aragonite, which was confirmed by Raman spectroscopy. Fourier transform infrared spectroscopy identified a band at 1,786.95 cm⁻¹ attributed to carboxylate (carbonyl) groups of the proteins present in the organic matrix as well as bands characteristic of calcium carbonate. X-ray fluorescence showed the nacre to contain 98% calcium carbonate, as well as minor elements (Si, Na, S and Sr) and trace elements (Mg, P, Cu, Al, Fe, Cl, K and Zn).

The origins, relationships, evolution and conservation of the weirdest marine bivalves: The watering pot shells. A review

The fossil record shows that the two clavagelloid or watering pot families evolved at different times, the Clavagellidae first in the late Mesozoic (100-66mya), the Penicillidae later in the Cenozoic (33-23mya)-the former originally with, thus, a near-global Tethyan distribution, the latter restricted to the Indo-West Pacific. Representatives of the two clavagelloid families, moreover, have wholly different adventitious tube/crypt structures and, thus, methods of formation suggesting that evolutionary experiments have been undertaken to achieve such radical architectural novelties. This has resulted in one of the most surprising examples of convergent evolution in the Bivalvia. But, what were the ancestors of the Clavagelloidea? The shell and internal morphology of representatives of the three recognized genera of the Lyonsiidae, that is, Lyonsia, Entodesma and Mytilimeria, are described. Species of the latter two genera are highly specialized epibenthic, byssate, nestlers and embedded symbionts of ascidian colonies and sponges, respectively. Species of Lyonsia, however, are mostly shallow endobenthic burrowers. On the basis of these studies, it is concluded that species of Lyonsia can be regarded as representative of the ancestral watering pot (Clavagelloidea) condition. Evidence for this conclusion include the mineralogy, characteristics and ligament structure of the shell and features of the anatomy, importantly the modification of the vestigial pedal retractor muscles to form simple (Clavagellidae) and more complex (Penicillidae) proprioreceptors. Such an anatomy-based conclusion is supported to some extent by DNA analyses of representatives of the Lyonsiidae and the two constituent families of the Clavagelloidea. To some extent because all clavagelloids are exceedingly rare hindering such analyses. Such rarity, however, also argues for the strict conservation of all the species of the Clavagelloidea.

Interfacial Mineral Fusion and Tubule Entanglement as a Means to Harden a Bone Augmentation Material

A new bone augmenting material is reported, which is formed from calcium-loaded hydrogel-based spheres. On immersion of these spheres in a physiological medium, they become surrounded with a sheath of precipitate, which ruptures due to a build-up in osmotic pressure. This results in the formation of mineral tubes that protrude from the sphere surface. When brought into close contact with one another, these spheres become fused through the entanglement and subsequent interstitial mineralization of the mineral tubules. The tubular calcium phosphate induces the expression of osteogenic genes (runt-related transcription factor 2 (RUNX2), transcription factor SP7 (SP7), collagen type 1 alpha 1 (COL1A1), and bone gamma-carboxyglutamic acid-containing protein (BGLAP)) and promotes the formation of mineral nodules in preosteoblast cultures comparable to an apatitic calcium phosphate phase. Furthermore, alkaline phosphatase (ALP) is significantly upregulated in the presence of tubular materials after 10 d in culture compared with control groups (p < 0.001) and sintered apatite (p < 0.05). This is the first report of a bioceramic material that is formed in its entirety in situ and is therefore likely to provide a better proxy for biological mineral than other existing synthetic alternatives to bone grafts.

Realtime Observation of Diffusing Elements in a Chemical Garden

The chemical garden, which has been known as the plant-growth-like diffusion of chemicals since the 17th century, has regained much attention in recent years. Significant progress in research not only promoted the understanding of the phenomenon itself but also suggested a prospective method of synthesizing new materials via the chemical garden route. It is extremely important to introduce new characterization techniques to provide more insights into chemical diffusion and element redistribution during the reaction process. The present article describes some successful applications of the realtime X-ray fluorescence (XRF) movie technique to observe each diffusing element. The protagonist of the movie is a chemical garden reaction growing from a seed of calcium salt and ferrous salt mixtures. Through observation by an XRF movie, it has been found that the growth rate and diffusion behavior of calcium and iron are very different. This results in a macroscopic diversity of the element composition in the finally precipitated chemical garden structures. The present research not only reconfirms the potential of fabricating gradient composites through the self-organized chemical garden approach but also demonstrates the attractive achievements of XRF movies. It has been demonstrated that the XRF movie is an indispensable realtime characterization technique for the study of chemical garden reactions or even other related diffusions.

Biologically Analogous Calcium Phosphate Tubes from a Chemical Garden

Calcium phosphate (CaPO₄) tubes with features comparable to mineralized biological microstructures, such as Haversian canals, were grown from a calcium gel/phosphate solution chemical garden system. A significant difference in gel mass in response to high and low solute phosphate equivalent environments existed within 30 min of solution layering upon gel (p = 0.0067), suggesting that the nature of advective movement between gel and solution is dependent on the solution concentration. The transport of calcium cations (Ca²⁺) and phosphate anions (PO₄^3-) was quantified and changes in pH were monitored to explain the preferential formation of tubes within a PO₄^3- concentration range of 0.5-1.25 M. Ingress from the anionic solution phase into the gel followed by the liberation of Ca²⁺ ions from the gel was found to be essential for acquiring self-assembled tubular CaPO₄ structures. Tube analysis by scanning electron microscopy (SEM), X-ray diffraction (XRD), and micro X-ray florescence (μ-XRF) revealed hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) and dicalcium phosphate dihydrate (DCPD, CaHPO₄·2H₂O) phases organized in a hierarchical manner. Notably, the tubule diameters ranged from 100 to 150 μm, an ideal size for the permeation of vasculature in biological hard tissue.