Human dental enamel, the hardest tissue in the body, plays a vital role in protecting teeth from wear as a result of daily grinding and chewing as well as from chemical attack. It is well established that the mechanical strength and fatigue resistance of dental enamel are derived from its hierarchical structure, which consists of periodically arranged bundles of hydroxyapatite (HAP) nanowires. However, we do not yet have a full understanding of the in vivo HAP crystallization process that leads to this structure. Mg(2+) ions, which are present in many biological systems, regulate HAP crystallization by stabilizing its precursor, amorphous calcium phosphate (ACP), but their atomic-scale distribution within HAP is unknown. We use atom probe tomography to provide the first direct observations of an intergranular Mg-rich ACP phase between the HAP nanowires in mature human dental enamel. We also observe Mg-rich elongated precipitates and pockets of organic material among the HAP nanowires. These observations support the postclassical theory of amelogenesis (that is, enamel formation) and suggest that decay occurs via dissolution of the intergranular phase. This information is also useful for the development of more accurate models to describe the mechanical behavior of teeth.
In-plane frost growth on chilled hydrophobic surfaces is an inter-droplet phenomenon, where frozen droplets harvest water from neighboring supercooled liquid droplets to grow ice bridges that propagate across the surface in a chain reaction. To date, no surface has been able to passively prevent the in-plane growth of ice bridges across the population of supercooled condensate. Here, we demonstrate that when the separation between adjacent nucleation sites for supercooled condensate is properly controlled with chemical micropatterns prior to freezing, inter-droplet ice bridging can be slowed and even halted entirely. Since the edge-to-edge separation between adjacent supercooled droplets decreases with growth time, deliberately triggering an early freezing event to minimize the size of nascent condensation was also necessary. These findings reveal that inter-droplet frost growth can be passively suppressed by designing surfaces to spatially control nucleation sites and by temporally controlling the onset of freezing events.
Crystalline materials are crucial to the function of living organisms, in the shells of molluscs, the matrix of bone, the teeth of sea urchins, and the exoskeletons of coccoliths. However, pathological biomineralization can be an undesirable crystallization process associated with human diseases. The crystal growth of biogenic, natural and synthetic materials may be regulated by the action of modifiers, most commonly inhibitors, which range from small ions and molecules to large macromolecules. Inhibitors adsorb on crystal surfaces and impede the addition of solute, thereby reducing the rate of growth. Complex inhibitor-crystal interactions in biomineralization are often not well elucidated. Here we show that two molecular inhibitors of calcium oxalate monohydrate crystallization-citrate and hydroxycitrate-exhibit a mechanism that differs from classical theory in that inhibitor adsorption on crystal surfaces induces dissolution of the crystal under specific conditions rather than a reduced rate of crystal growth. This phenomenon occurs even in supersaturated solutions where inhibitor concentration is three orders of magnitude less than that of the solute. The results of bulk crystallization, in situ atomic force microscopy, and density functional theory studies are qualitatively consistent with a hypothesis that inhibitor-crystal interactions impart localized strain to the crystal lattice and that oxalate and calcium ions are released into solution to alleviate this strain. Calcium oxalate monohydrate is the principal component of human kidney stones and citrate is an often-used therapy, but hydroxycitrate is not. For hydroxycitrate to function as a kidney stone treatment, it must be excreted in urine. We report that hydroxycitrate ingested by non-stone-forming humans at an often-recommended dose leads to substantial urinary excretion. In vitro assays using human urine reveal that the molecular modifier hydroxycitrate is as effective an inhibitor of nucleation of calcium oxalate monohydrate nucleation as is citrate. Our findings support exploration of the clinical potential of hydroxycitrate as an alternative treatment to citrate for kidney stones.
Material degradation due to precipitation of implanted helium (He) is a key concern in nuclear energy. Decades of research have mapped out the fate of He precipitates in metals, from nucleation and growth of equiaxed bubbles and voids to formation and bursting of surface blisters. By contrast, we show that He precipitates confined within nanoscale metal layers depart from their classical growth trajectories: They self-organize into elongated channels. These channels form via templated nucleation of He precipitates along layer surfaces followed by their growth and spontaneous coalescence into stable precipitate lines. The total line length and connectivity increases with the amount of implanted He, indicating that these channels ultimately interconnect into percolating “vascular” networks. Vascularized metal composites promise a transformative solution to He-induced damage by enabling in operando outgassing of He and other impurities while maintaining material integrity.
Ice repellent coatings have been studied and keenly sought after for many years, where any advances in the durability of such coatings will result in huge energy savings across many fields. Progress in creating anti-ice and anti-frost surfaces has been particularly rapid since the discovery and development of slippery, liquid infused porous surfaces (SLIPS). Here we use SLIPS-coated differential scanning calorimeter (DSC) pans to investigate the effects of the surface modification on the nucleation of supercooled water. This investigation is inherently different from previous studies which looked at the adhesion of ice to SLIPS surfaces, or the formation of ice under high humidity conditions. Given the stochastic nature of nucleation of ice from supercooled water, multiple runs on the same sample are needed to determine if a given surface coating has a real and statistically significant effect on the nucleation temperature. We have cycled supercooling to freezing and then thawing of deionized water in hydrophilic (untreated aluminum), hydrophobic, superhydrophobic, and SLIPS-treated DSC pans multiple times to determine the effects of surface treatment on the nucleation and subsequent growth of ice. We find that SLIPS coatings lower the nucleation temperature of supercooled water in contact with statistical significance and show no deterioration or change in the coating performance even after 150 freeze-thaw cycles.
Supersaturating Drug Delivery Systems (SDDS) hold the promise of enabling intestinal absorption for difficult-to-formulate, poorly soluble drug candidates based on a design approach that includes (1) converting the drug into a high energy or rapidly dissolving system which presents a supersaturated solution to the gastrointestinal environment and (2) dosage form components that act to stabilize the formed metastable drug solution through nucleation and/or crystal growth inhibition. The appropriate development and study of SDDS require that useful and biorelevant supersaturation and precipitation assays are available. This review summarizes different methodological aspects of currently available in vitro assays, including the generation of supersaturation (solvent shift, pH shift or formulation-induced), the quantification of supersaturation and the detection of precipitation. Also down-scaled approaches, including 96-well plate setups, are described and situated in the pharmaceutical development cycle based on their consumption of API as well as time requirements. Subsequently, the ability to extrapolate in vitro supersaturation assessment to the in vivo situation is discussed as are direct and indirect clinical tools that can shed light on SDDS. By emphasizing multiple variables that affect the predictive power of in vitro assays (e.g. the nature of the test media, hydrodynamics, temperature and sink versus non-sink conditions), this review finally highlights the need for further harmonization and biorelevance improvement of currently available in vitro procedures for supersaturation and precipitation evaluation.
Methane hydrates are ice-like inclusion compounds with importance to the oil and natural gas industry, global climate change, and gas transportation and storage. The molecular mechanism by which these compounds form under conditions relevant to industry and nature remains mysterious. To understand the mechanism of methane hydrate nucleation from supersaturated aqueous solutions, we performed simulations at controlled and realistic supersaturation. We found that critical nuclei are extremely large and that homogeneous nucleation rates are extremely low. Our findings suggest that nucleation of methane hydrates under these realistic conditions cannot occur by a homogeneous mechanism.
Centrosome- and chromatin-based microtubule nucleation pathways have been implicated in spindle assembly. Using total internal reflection fluorescent microscopy and Xenopus egg extracts, Petry et al. demonstrate that new microtubules can also nucleate and branch out from existing ones in animal cells.
Hybrid organic/lead halide perovskites are promising materials for solar cell fabrication, resulting in efficiencies up to 18%. The most commonly-studied perovskites are CH3NH3PbI3 and CH3NH3Pb(I3-xClx) where x is small. Importantly, in the latter system, the presence of chloride ion source in the starting solutions used for the perovskite deposition results in a strong increase in the overall charge diffusion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH3NH3PbI3 and CH3NH3PbBr3. We find that the addition of PbCl2 to the solutions used in the perovskite synthesis has a remarkable effect on the end product, because PbCl2 nanocrystals are present during the fabrication process, acting as heterogeneous nucleation sites for the formation of perovskite crystals in solution. We base this conclusion on SEM studies, synthesis of perovskite single crystals, and on cryo-TEM imaging of the frozen mother liquid. Our studies also included the effect of different substrates and substrate temperatures on the perovskite nucleation efficiency. In view of our findings, we optimized the procedures for solar cells based on lead bromide perovskite, resulting in 5.7% efficiency and Voc of 1.24 V, improving the performance in this class of devices. Insights gained from understanding the hybrid perovskite crystallization process can aid in rational design of the polycrystalline absorber films, leading to their improved performance.
Herbal remedies are increasingly being considered as suitable long-term treatments for renal dysfunction. The objective of the present study was to investigate the effect of some herbal extracts, all previously identified in published studies as influencing kidney stone formation, on the crystallization characteristics of calcium oxalate (CaOx) in synthetic urine (SU). Five herbal extracts were selected for the study: Folium pyrrosiae, Desmodium styracifolium, Phyllanthus niruri, Orthosiphon stamineus and Cystone(®). Concentrated stock solutions of each herbal extract were prepared and were tested at their recommended dosages in in vitro crystallization studies in SU. CaOx crystallization experiments were performed in which the metastable limit (MSL), average particle size, and nucleation and growth rates were determined. The CaOx MSL of SU was unaltered by the five herbal extracts. Three of the herbs (Desmodium styracifolium, Orthosiphon stamineus and Cystone(®)) significantly reduced the average particle size of precipitated crystals relative to undosed SU. All of the extracts increased the rate of nucleation and decreased the rate of growth significantly in SU. Cystone(®) showed the greatest effect on the measured risk factors. It is concluded that all of the herbs have the potential to serve as inhibitors of calcium oxalate stone formation and warrant investigation in clinical trials.