Strong and Tough MXene Bridging-induced Conductive Nacre.

Nacre is a classic model, providing an inspiration for fabricating high-performance bulk nanocomposites with the two-dimensional platelets. However, the "brick" of nacre, aragonite platelet, is an ideal building block for making high-performance bulk nanocomposites. Herein, we demonstrated a strong and tough conductive nacre through reassembling aragonite platelets with bridged by MXene nanosheets and hydrogen bonding, not only providing high mechanical properties but also excellent electrical conductivity. The flexural strength and fracture toughness of the obtained conductive nacre reach ~ 282 MPa and ~ 6.3 MPa m1/2, which is 1.6 and 1.6 times higher than that of natural nacre, respectively. These properties are attributed to densification and high orientation degree of the conductive nacre, which is effectively induced by the combined interactions of hydrogen bonding and MXene nanosheets bridging. The crack propagations in conductive nacre are effectively inhibited through crack deflection with hydrogen bonding, and MXene nanosheets bridging between aragonite platelets. In addition, our conductive nacre also provides a self-monitoring function for structural damage and offers exceptional electromagnetic interference shielding performance. Our strategy of reassembling the aragonite platelets exfoliated from waste nacre into high-performance artificial nacre, provides an avenue for fabricating high-performance bulk nanocomposites through the sustainable reutilization of shell resources.

Nonclassical crystallization of goethite nanorods in limpet teeth by self-assembly of silica-rich nanoparticles reveals structure-mechanical property relations.

The intricate organization of goethite nanorods within a silica-rich matrix makes limpet teeth the strongest known natural material. However, the mineralization pathway of goethite in organisms under ambient conditions remains elusive. Here, by investigating the multi-level structure of limpet teeth at different growth stages, it is revealed that the growth of goethite crystals proceeds by the attachment of amorphous nanoparticles, a nonclassical crystallization pathway widely observed during the formation of calcium-based biominerals. Importantly, these nanoparticles contain a high amount of silica, which is gradually expelled during the growth of goethite. Moreover, in mature teeth of limpet, the content of silica correlates with the size of goethite crystals, where smaller goethite crystals are densely packed in the leading part with higher content of silica. Correspondingly, the leading part exhibits higher hardness and elastic modulus. Thus, this study not only reveals the nonclassical crystallization pathway of goethite nanorods in limpet teeth, but also highlights the critical roles of silica in controlling the hierarchical structure and the mechanical properties of limpet teeth, thus providing inspirations for fabricating biomimetic materials with excellent properties.

Myriad Mapping of nanoscale minerals reveals calcium carbonate hemihydrate in forming nacre and coral biominerals.

Calcium carbonate (CaCO3) is abundant on Earth, is a major component of marine biominerals and thus of sedimentary and metamorphic rocks and it plays a major role in the global carbon cycle by storing atmospheric CO2 into solid biominerals. Six crystalline polymorphs of CaCO3 are known-3 anhydrous: calcite, aragonite, vaterite, and 3 hydrated: ikaite (CaCO3·6H2O), monohydrocalcite (CaCO3·1H2O, MHC), and calcium carbonate hemihydrate (CaCO3·½H2O, CCHH). CCHH was recently discovered and characterized, but exclusively as a synthetic material, not as a naturally occurring mineral. Here, analyzing 200 million spectra with Myriad Mapping (MM) of nanoscale mineral phases, we find CCHH and MHC, along with amorphous precursors, on freshly deposited coral skeleton and nacre surfaces, but not on sea urchin spines. Thus, biomineralization pathways are more complex and diverse than previously understood, opening new questions on isotopes and climate. Crystalline precursors are more accessible than amorphous ones to other spectroscopies and diffraction, in natural and bio-inspired materials.

3D relationship between hierarchical canal network and gradient mineralization of shark tooth osteodentin.

Osteodentin is a dominant mineralized collagenous tissue in the teeth of many fishes, with structural and histological characteristics resembling those of bone. Osteodentin, like bone, comprises osteons as basic structural building blocks, however, it lacks the osteocytes and the lacuno-canalicular network (LCN), which are known to play critical roles in controlling the mineralization of the collagenous matrix in bone. Although numerous vascular canals exist in osteodentin, their role in tooth maturation and the matrix mineralization process remain poorly understood. Here, high resolution micro-computed tomography (micro-CT) and focused ion beam-scanning electron microscopy (FIB-SEM) were used to obtain 3D structural information of osteodentin in shark teeth at multiple scales. We observed a complex 3D network of primary canals with a diameter ranging from ∼10 µm to ∼120 µm, where the canals are surrounded by osteon-like concentric layers of lamellae, with 'interosteonal' tissue intervening between neighboring osteons. In addition, numerous hierarchically branched secondary canals extended radially from the primary canals into the interosteonal tissue, decreasing in diameter from ∼10 µm to hundreds of nanometers. Interestingly, the mineralization degree increases from the periphery of primary canals into the interosteonal tissue, suggesting that mineralization begins in the interosteonal tissue. Correspondingly, the hardness and elastic modulus of the interosteonal tissue are higher than those of the osteonal tissue. These results demonstrate that the 3D hierarchical canal network is positioned to play a critical role in controlling the gradient mineralization of osteodentin, also providing valuable insight into the formation of mineralized collagenous tissue without osteocytes and LCN. STATEMENT OF SIGNIFICANCE: Bone is a composite material with versatile mechanical properties. Osteocytes and their lacuno-canalicular network (LCN) are known to play critical roles during formation of human bone. However, the bone and osteodentin of many fishes, although lacking osteocytes and LCN, exhibit similar osteon-like structure and mechanical functions. Here, using various high resolution 3D characterization techniques, we reveal that the 3D network of primary canals and numerous hierarchically branched secondary canals correlate with the mineralization gradient and micromechanical properties of osteonal and interosteonal tissues of shark tooth osteodentin. This work significantly improves our understanding of the construction of bone-like mineralized tissue without osteocytes and LCN, and provides inspirations for the fabrication of functional materials with hierarchical structure.

Bioprocess inspired formation of calcite mesocrystals by cation-mediated particle attachment mechanism.

Calcite mesocrystals were proposed, and have been widely reported, to form in the presence of polymer additives via oriented assembly of nanoparticles. However, the formation mechanism and the role of polymer additives remain elusive. Here, inspired by the biomineralization process of sea urchin spine comprising magnesium calcite mesocrystals, we show that calcite mesocrystals could also be obtained via attachment of amorphous calcium carbonate (ACC) nanoparticles in the presence of inorganic zinc ions. Moreover, we demonstrate that zinc ions can induce the formation of temporarily stabilized amorphous nanoparticles of less than 20 nm at a significantly lower calcium carbonate concentration as compared to pure solution, which is energetically beneficial for the attachment and occlusion during calcite growth. The cation-mediated particle attachment crystallization significantly improves our understanding of mesocrystal formation mechanisms in biomineralization and offers new opportunities to bioprocess inspired inorganic ions regulated materials fabrication.

Bioinspired Strategy for Efficient TiO2/Au/CdS Photocatalysts Based On Mesocrystal Superstructures in Biominerals and Charge-Transfer Pathway in Natural Photosynthesis.

Natural photosynthesis involves an efficient charge-transfer pathway through exquisitely arranged photosystems and electron transport intermediates, which separate photogenerated carriers to realize high quantum efficiency. It inspires a rational design construction of artificial photosynthesis systems and the architectures of semiconductors are essential to achieve optimal performance. Of note, biomineralization processes could form various mesocrystals with well-ordered superstructures for unique optical applications. Inspired by both natural photosynthesis and biomineralization, we construct a ternary superstructure-based mesocrystal TiO2 (meso-TiO2)/Au/CdS artificial photosynthesis system by a green photo-assisted method. The well-ordered superstructure of meso-TiO2 and efficient charge-transfer pathway among the three components are crucial for retarding charge recombination. As a result, the meso-TiO2/Au/CdS photocatalyst displays enhanced visible light-driven photocatalytic hydrogen evolution (4.60 mmol h-1 g-1), which is 3.2 times higher than that of commercial TiO2 (P25)/Au/CdS with disordered TiO2 nanocrystal aggregates (1.41 mmol h-1 g-1). This work provides a promising bioinspired design strategy for photocatalysts with an improved solar conversion efficiency.

Nanocage Ferritin Reinforced Polyacrylamide Hydrogel for Wearable Flexible Strain Sensors.

Biocomposite hydrogels are promising for applications in wearable flexible strain sensors. Nevertheless, the existing biocomposite hydrogels are still hard to meet all requirements, which limits the practical application. Here, inspired by the structure and composition of natural ferritin, we design a PAAm-Ferritin hybrid hydrogel through a facile method. Ferritin is uniformly distributed in the cross-linking networks and acts as a nanocage spring model, leading to the enhanced tensile strength of the hydrogel. The fracture stress is 99 kPa at 1400% maximum elongation. As fabricated PAAm-Ferritin hybrid hydrogels exhibit high toughness and low elastic modulus (21 kPa). The PAAm-Ferritin hybrid hydrogels present excellent biocompatibility and increased conductivity compared with PAAm hydrogel. Impressively, as a wearable flexible strain sensor, the PAAm-Ferritin hybrid hydrogels have high sensitivity (gauge factor = 2.06), excellent reliability, and cycling stability. This study indicates the feasibility of utilizing ferritin to synthesize functional materials, which is conducive to expanding the use of protein synthesis of materials technology and application fields.

Room-temperature growth of fluorapatite/CaCO3 heterogeneous structured composites inspired by human tooth.

Organisms can synthesize heterogeneous structures with excellent mechanical properties through mineralization, the most typical of which are teeth. The tooth is an extraordinarily resilient bi-layered material that is composed of external enamel perpendicular to the tooth surface and internal dentin parallel to the tooth surface. The synthesis of enamel-like heterostructures with good mechanical properties remains an elusive challenge. In this study, we applied a biomimetic mineralization method to grow fluorapatite/CaCO3 (FAP/CaCO3) heterogeneous structured thin films that mimic their biogenic counterparts found in teeth through a three-step pathway: coating a polymer substrate, growing a layered calcite film, and mineralization of a fluorapatite columnar array on the calcite layer. The synthetic heterostructure composites combine well and exhibit good mechanical properties comparable to their biogenic counterparts. The FAP/CaCO3 heterogeneous structured composite exhibits excellent mechanical properties, with a hardness and Young's modulus of 1.99 ± 0.02 GPa and 47.5 ± 0.6 GPa, respectively. This study provides a reasonable new idea for unique heterogeneous structured materials designed at room temperature.

Pressure-induced crystallization and densification of amorphized calcium carbonate hexahydrate controlled by interfacial water.

Amorphous calcium carbonate (ACC) is widely known as a metastable precursor in the formation of crystalline calcium carbonate biominerals. However, the exact role of water during the crystallization of ACC remains elusive. Here, a novel ACC with high specific surface area and nanopores is synthesized by solvent-induced dehydration and amorphization of crystalline calcium carbonate hexahydrate (ikaite), denoted as I-ACC. Comparing I-ACC and typical spherical ACC (S-ACC) nanoparticles, it reveals that the crystallization pathways of ACC under heating or pressure are not dictated by the total amount of water in ACC as reported, but rather the interfacial water that is released from ACC bulk and adsorbed on the surface of the particles. We show that the crystallization pathways of I-ACC to calcite single crystal with high specific surface area or vaterite can be easily controlled by tuning the release of water during heating. In addition, densely packed pure vaterite can be obtained via pressured-induced transformation of I-ACC at room temperature, which is otherwise difficult to form using S-ACC. These insights contribute to the understanding of the biological control of mineral formation via amorphous precursors and offer new opportunities to bioprocess inspired fabrication of strong bulk material at room temperature.

Silk fibroin directs the formation of monetite nanocrystals and their assembly into hierarchical composites.

Natural biominerals are usually composite materials produced through mineralization of inorganic crystals within an organic matrix. Silk fibroin is known to be capable of directing the nucleation and growth of hydroxyapatite crystals. Here, we used silk films as the substrate to induce the mineralization of calcium phosphate. We show that the silk fibroin in solution could induce the formation of monetite crystals with a hierarchical structure, which are assembled by well aligned single crystals of monetite. In addition, we show that silk fibroins are incorporated inside the crystals. Therefore, the self-assembly of silk fibroin during the crystallization process is critical for the formation of such hierarchical structures.

Bioprocess-inspired synthesis of multilayered chitosan/CaCO3 composites with nacre-like structures and high mechanical properties.

The formation of natural structures found in biological systems is wonderful and can be completed at ambient temperatures in contrast to artificial technologies wherein harsh conditions are common prerequisites. A new research direction, "bioprocess inspired manufacturing", is proposed for fabricating advanced materials with novel structures and functions. Nacre consists of an ordered multilayer structure of crystalline calcium carbonate lamellae separated by organic layers exhibiting mechanical toughness, which transcends that of its constituent components. Inspired by the nacre formation process, a microscale additive manufacturing mineralization method is proposed for achieving a multilayered organic-inorganic layered structure. In this work, layered calcite was synthesized on the surface of chitosan (CS) films at room temperature under the coordinated control of magnesium ions (Mg2+) and polyacrylic acid (PAA). The CS films and layered calcite are sequentially assembled in a layer-by-layer deposition approach to form an organic-inorganic hybrid structure. The nacre-like chitosan/CaCO3 (CS/CaCO3) composites exhibit high transparency and underwater superoleophobicity. Impressively, the hardness (2.35 ± 0.03 GPa) and Young's modulus (58.1 ± 0.5 GPa) of the as-prepared (CS/CaCO3) composites are comparable to those of their biological counterparts. This study provides a rational bioprocess-inspired room-temperature mineralization method to develop advanced composite materials with good performance.

Hydroxyapatite-reinforced alginate fibers with bioinspired dually aligned architectures.

Biological materials have excellent mechanical properties due to their organized structures from nano- to macro-scale. Artificial manufacture of materials with anisotropic microstructures still remains challenging. We described a stress-induced method to fabricate anisotropic alginate fibers. Organic-inorganic composite fibers were obtained by incorporating aligned hydroxyapatite (HAP) nanowires into the alginate fiber. Detailed structural characterization revealed the bone-like structure of the HAP-reinforced alginate fibers. Tensile test results showed that the maximum Young's modulus and tensile strength were 4.3 GPa and 153.8 MPa, respectively. A multiscale reinforcing mechanism is proposed after the discussion of the structure-property relationship: highly ordered and compacted nanofibrils aligned along the longitudinal direction at the microscale, and two kinds of alginate gels with different mechanical behaviors at the nanoscale coexisted (acidic alginate gel and calcium-alginate gel). This work validates the effectiveness of the bioinspired fabrication strategy, which inspires further manufacturing and optimization of materials for diverse applications.

Bioprocess-Inspired Room-Temperature Synthesis of Enamel-like Fluorapatite/Polymer Nanocomposites Controlled by Magnesium Ions.

Tooth enamel is composed of arrayed fluorapatite (FAP) or hydroxyapatite nanorods modified with Mg-rich amorphous layers. Although it is known that Mg2+ plays an important role in the formation of enamel, there is limited research on the regulatory role of Mg2+ in the synthesis of enamel-like materials. Therefore, we focus on the regulatory behavior of Mg2+ in the fabrication of biomimetic mineralized enamel-like structural materials. In the present study, we adopt a bioprocess-inspired room-temperature mineralization technique to synthesize a multilayered array of enamel-like columnar FAP/polymer nanocomposites controlled by Mg2+ (FPN-M). The results reveal that the presence of Mg2+ induced the compaction of the array and the formation of a unique Mg-rich amorphous-reinforced architecture. Therefore, the FPN-M array exhibits excellent mechanical properties. The hardness (2.42 ± 0.01 GPa) and Young's modulus (81.5 ± 0.6 GPa) of the as-prepared FPN-M array are comparable to those of its biological counterparts; furthermore, the enamel-like FPN-M array is translucent. The hardness and Young's modulus of the synthetic array of FAP/polymer nanocomposites without Mg2+ control (FPN) are 0.51 ± 0.04 and 43.5 ± 1.6 GPa, respectively. The present study demonstrates a reliable bioprocess-inspired room-temperature fabrication technique for the development of advanced high-performance composite materials.

Bioinspired 3D Printable, Self-Healable, and Stretchable Hydrogels with Multiple Conductivities for Skin-like Wearable Strain Sensors.

Bioinspired hydrogels have promising prospects in applications such as wearable devices, human health monitoring equipment, and soft robots due to their multifunctional sensing properties resembling natural skin. However, the preparation of intelligent hydrogels that provide feedback on multiple electronic signals simultaneously, such as human skin receptors, when stimulated by external contact pressure remains a substantial challenge. In this study, we designed a bioinspired hydrogel with multiple conductive capabilities by incorporating carbon nanotubes into a chelate of calcium ions with polyacrylic acid and sodium alginate. The bioinspired hydrogel consolidates self-healing ability, stretchability, 3D printability, and multiple conductivities. It can be fabricated as an integrated strain sensor with simultaneous piezoresistive and piezocapacitive performances, exhibiting sensitive (gauge factor of 6.29 in resistance mode and 1.25 kPa-1 in capacitance mode) responses to subtle pressure changes in the human body, such as finger flexion, knee flexion, and respiration. Furthermore, the bioinspired strain sensor sensitively and discriminatively recognizes the signatures written on it. Hence, we expect our ideas to provide inspiration for studies exploring the use of advanced hydrogels in multifunctional skin-like smart wearable devices.

Mineralization of calcium phosphate induced by a silk fibroin film under different biological conditions.

Silk fibroin is a promising biomaterial that has been used for tissue engineering applications. However, the influence of silk fibroin on the mineralization of calcium phosphate in different biological environments has not been discussed before. In this work, we fabricated organized silk fibroin film as the organic framework and amorphous calcium phosphate (ACP) deposited on the films as precursors. The transformation pathways and morphology of ACP was then studied in both enzyme and PBS (phosphate buffer saline) solutions. While only hydroxyapatite (HA) crystals formed in enzyme solution, a mixture of tricalcium phosphate (TCP) and HA crystals were obtained in PBS solution, which can be related to the variations of the content of silk fibroin and pH of the solution. Therefore, silk fibroin films can have an important effect on the mineralization process of calcium phosphate in different biological environments. In addition, cell cultivation experiments show that the silk films after mineralization promoted osteogenesis and exhibited good biocompatibility.

Particle-attachment crystallization facilitates the occlusion of micrometer-sized Escherichia coli in calcium carbonate crystals with stable fluorescence.

Inspired from the occlusion of macromolecules in mineral crystals during the biomineralization process, the occlusion mechanism of functional guest species into a host matrix is gradually revealed in artificial systems. However, the guest species within calcite crystals are limited to the nanometer scale. Herein, using amorphous calcium carbonate (ACC) as a precursor and taking advantage of the crystallization of vaterite by the attachment of ACC nanoparticles, micrometer-sized modified Escherichia coli (E. coli) was incorporated into vaterite crystals. The occlusion content of bacteria within the vaterite crystal could reach up to 16 wt%. On the contrary, the occlusion of E. coli into calcite crystals, which proceeded via ion-by-ion addition growth, was only confined to the surface layer. Through modifying the surface structure or chemical composition of bacteria, the strong interaction between the surface of the bacteria and calcium carbonate has proved to be the key factor for successful occlusion. Interestingly, the genetically modified green fluorescent protein (GFP)-E. coli/vaterite composites exhibited stable fluorescence for more than six months with little attenuation and the lifetime could be more than 1.2 µs. It was demonstrated that a combination of the amorphous precursor crystallization pathway and a suitable surface structure of the foreign species can significantly enhance the occlusion efficiency of micrometer-sized species in crystals.

Three-dimensional structural interrelations between cells, extracellular matrix, and mineral in normally mineralizing avian leg tendon.

The spatial-temporal relationship between cells, extracellular matrices, and mineral deposits is fundamental for an improved understanding of mineralization mechanisms in vertebrate tissues. By utilizing focused ion beam-scanning electron microscopy with serial surface imaging, normally mineralizing avian tendons have been studied with nanometer resolution in three dimensions with volumes exceeding tens of micrometers in range. These parameters are necessary to yield sufficiently fine ultrastructural details while providing a comprehensive overview of the interrelationships between the tissue structural constituents. Investigation reveals a complex lacuno-canalicular network in highly mineralized tendon regions, where ∼100 nm diameter canaliculi emanating from cell (tenocyte) lacunae surround extracellular collagen fibril bundles. Canaliculi are linked to smaller channels of ∼40 nm diameter, occupying spaces between fibrils. Close to the tendon mineralization front, calcium-rich deposits appear between the fibrils and, with time, mineral propagates along and within them. These close associations between tenocytes, tenocyte lacunae, canaliculi, small channels, collagen, and mineral suggest a concept for the mineralization process, where ions and/or mineral precursors may be transported through spaces between fibrils before they crystallize along the surface of and within the fibrils.

Synthesis of monodisperse rod-shaped silica particles through biotemplating of surface-functionalized bacteria.

Mesoporous silica particles of controlled size and shape are potentially beneficial for many applications, but their usage may be limited by the complex procedure of fabrication. Biotemplating provides a facile approach to synthesize materials with desired shapes. Herein, a bioinspired design principle is adopted through displaying silaffin-derived 5R5 proteins on the surface of Escherichia coli by genetic manipulations. The genetically modified Escherichia coli provides a three-dimensional template to regulate the synthesis of rod-shaped silica. The silicification is initiated on the cell surface under the functionality of 5R5 proteins and subsequentially the inner space is gradually filled. Density functional theory simulation reveals the interfacial interactions between silica precursors and R5 peptides at the atomic scale. There is a large conformation change of this protein during biosilicification. Electrostatic interactions contribute to the high affinity between positively charged residues (Lys4, Arg16, Arg17) and negatively charged tetraethyl orthosilicate. Hydrogen bonds develop between Arg16 (OH), Arg17 (OH and NH), Leu19 (OH) residues and the forming silica agglomerates. In addition, the resulting rod-shaped silica copy of the bacteria can transform into mesoporous SiOx nanorods composed of carbon-coated nanoparticles after carbonization, which is shown to allow superior lithium storage performance.

Unveiling the Origin of the High Catalytic Activity of Ultrathin 1T/2H MoSe2 Nanosheets for the Hydrogen Evolution Reaction: A Combined Experimental and Theoretical Study.

2 D transition metal dichalcogenide materials with layered nanostructures and specific phases usually exhibit excellent catalytic activities for the hydrogen evolution reaction (HER). A facile solvothermal process was used to prepare ultrathin noble-metal-free 2 D biphasic MoSe2 nanosheets composed of a metastable metallic 1T phase and a semiconducting 2H phase. High metallic 1T phase content and few-layer-thick MoSe2 nanosheets were obtained by tuning the amount of NaBH4 used in the reaction. The optimal integration of a metallic 1T phase and an environmentally stable 2H phase in MoSe2 electrocatalysts provides abundant active sites and good conductivity beneficial for the HER. The combination of experimental results and DFT calculations implies that the electrocatalytic activity for the HER on the MoSe2 surface goes through a collaborative Heyrovsky and Volmer reaction process. The theoretical studies suggest that the presence of 1T-MoSe2 could reduce the band energy relative to 2H-MoSe2 and, consequently, accelerate the sluggish HER kinetics of 2H-MoSe2 . This work provides valuable and novel insights into the understanding of the structure-activity relationships in 2 D transition metal dichalcogenide electrocatalysts.

A hydrated crystalline calcium carbonate phase: Calcium carbonate hemihydrate.

As one of the most abundant materials in the world, calcium carbonate, CaCO3, is the main constituent of the skeletons and shells of various marine organisms. It is used in the cement industry and plays a crucial role in the global carbon cycle and formation of sedimentary rocks. For more than a century, only three polymorphs of pure CaCO3-calcite, aragonite, and vaterite-were known to exist at ambient conditions, as well as two hydrated crystal phases, monohydrocalcite (CaCO3·1H2O) and ikaite (CaCO3·6H2O). While investigating the role of magnesium ions in crystallization pathways of amorphous calcium carbonate, we unexpectedly discovered an unknown crystalline phase, hemihydrate CaCO3·½H2O, with monoclinic structure. This discovery may have important implications in biomineralization, geology, and industrial processes based on hydration of CaCO3.