Design Of Bio-inspired Materials Research Articles

The Role of Structural Polymorphism in Driving the Mechanical Performance of the Alzheimer's Beta Amyloid Fibrils.

Alzheimer's Disease (AD) is related with the abnormal aggregation of amyloid β-peptides Aβ1−40 and Aβ1−42, the latter having a polymorphic character which gives rise to U- or S-shaped fibrils. Elucidating the role played by the nanoscale-material architecture on the amyloid fibril stability is a crucial breakthrough to better understand the pathological nature of amyloid structures and to support the rational design of bio-inspired materials. The computational study here presented highlights the superior mechanical behavior of the S-architecture, characterized by a Young's modulus markedly higher than the U-shaped architecture. The S-architecture showed a higher mechanical resistance to the enforced deformation along the fibril axis, consequence of a better interchain hydrogen bonds' distribution. In conclusion, this study, focusing the attention on the pivotal multiscale relationship between molecular phenomena and material properties, suggests the S-shaped Aβ1−42 species as a target of election in computational screen/design/optimization of effective aggregation modulators.

Impact resistance of nanocellulose films with bioinspired Bouligand microstructures.

The Bouligand structure features a helicoidal (twisted plywood) layup of fibers that are uniaxially arranged in-plane and is a hallmark of biomaterials that exhibit outstanding impact resistance. Despite its performance advantage, the underlying mechanisms for its outstanding impact resistance remain poorly understood, posing challenges for optimizing the design and development of bio-inspired materials with Bouligand microstructures. Interestingly, many bio-sourced nanomaterials, such as cellulose nanocrystals (CNCs), readily self-assemble into helicoidal thin films with inter-layer (pitch) angles tunable via solvent processing. Taking CNC films as a model Bouligand system, we present atomistically-informed coarse-grained molecular dynamics simulations to measure the ballistic performance of thin films with helicoidally assembled nanocrystals by subjecting them to loading similar to laser-induced projectile impact tests. The effect of pitch angle on the impact performance of CNC films was quantified in the context of their specific ballistic limit velocity and energy absorption. Bouligand structures with low pitch angles (18–42°) were found to display the highest ballistic resistance, significantly outperforming other pitch angle and quasi-isotropic baseline structures. Improved energy dissipation through greater interfacial sliding, larger in-plane crack openings, and through-thickness twisting cracks resulted in improved impact performance of optimal pitch angle Bouligand CNC films. Intriguingly, decreasing interfacial interactions enhanced the impact performance by readily admitting dissipative inter-fibril and inter-layer sliding events without severe fibril fragmentation. This work helps reveal structural and chemical factors that govern the optimal mechanical design of Bouligand microstructures made from high aspect ratio nanocrystals, paving the way for sustainable, impact resistant, and multi-functional films.

The melanization road more traveled by: Precursor substrate effects on melanin synthesis in cell-free and fungal cell systems

Natural brown-black eumelanin pigments confer structural coloration in animals and potently block ionizing radiation and antifungal drugs. These functions also make them attractive for bioinspired materials design, including coating materials for drug-delivery vehicles, strengthening agents for adhesive hydrogel materials, and free-radical scavengers for soil remediation. Nonetheless, the molecular determinants of the melanin "developmental road traveled" and the resulting architectural features have remained uncertain because of the insoluble, heterogeneous, and amorphous characteristics of these complex polymeric assemblies. Here, we used 2D solid-state NMR, EPR, and dynamic nuclear polarization spectroscopic techniques, assisted in some instances by the use of isotopically enriched precursors, to address several open questions regarding the molecular structures and associated functions of eumelanin. Our findings uncovered: 1) that the identity of the available catecholamine precursor alters the structure of melanin pigments produced either in Cryptococcus neoformans fungal cells or under cell-free conditions; 2) that the identity of the available precursor alters the scaffold organization and membrane lipid content of melanized fungal cells; 3) that the fungal cells are melanized preferentially by an l-DOPA precursor; and 4) that the macromolecular carbon- and nitrogen-based architecture of cell-free and fungal eumelanins includes indole, pyrrole, indolequinone, and open-chain building blocks that develop depending on reaction time. In conclusion, the availability of catecholamine precursors plays an important role in eumelanin development by affecting the efficacy of pigment formation, the melanin molecular structure, and its underlying scaffold in fungal systems.

10 Years of Soft Nanomaterials Research and Training at the Adolphe Merkle Institute

10 Years of Soft Nanomaterials Research and Training at the Adolphe Merkle Institute

Biomacromolecules within bivalve shells: Is chitin abundant?

Bivalve shells are inorganic-organic nanocomposites whose material properties outperform their purely inorganic mineral counterparts. Most typically the inorganic phase is a polymorph of CaCO3, while the organic phase contains biopolymers which have been presumed to be chitin and/or proteins. Identifying the biopolymer phase is therefore a crucial step in improving our understanding of design principles relevant to biominerals. In this work we study seven shells; four are examples of nacroprismatic shells (Alathyria jacksoni, Pinctada maxima, Hyriopsis cumingii and Cucumerunio novaehollandiae), one homogeneous (Arctica islandica), and two are crossed lamellar (Callista kingii, Tridacna gigas). Both intact shells, their organic extracts as isolated after decalcification in acid, and the periostracum overlay have been studied by solid-state CP-MAS NMR, FTIR, SEM and chemical analysis. In none of the shells examined in this work do we find a significant contribution to the organic fraction from chitin or its derivatives despite popular models of bivalve biomineralization which assume abundant chitin in the organic fraction of mollusk bivalve shells. In each of the nacroprismatic extracts the 13C NMR spectra represent similar proteinaceous material, Ala and Gly-rich and primarily organized as β-sheets. A different, yet highly conserved protein was found in the periostracum covering each of the three nacreous shells studied. The Arctica islandica shells with homogeneous microstructure contained proteins which do not appear to be silk-like, while in the crossed lamellar shells we extracted too little organic matter to characterize. Statement of SignificanceHydrophobic macromolecules are structural components within the calcareous inorganic matrix of bivalve shells and are responsible for enhanced materials properties of the biominerals. Prevalent models suggest that chitin is such major hydrophobic component. Contrary to that we show that chitin is rare within the hydrophobic biopolymers which primarily consist of proteinaceous matter with structural motifs as silk-like β-sheets, or others yet to be determined. Recognizing that diverse proteinaceous motifs, devoid of abundant chitin, can yield the optimized mechanical properties of bivalve shells is critical both to understand the mechanistic pathways by which they regulate biomineralization and for the design of novel bioinspired materials.

Functional Polymeric Materials Inspired by Geckos, Mussels, and Spider Silk

AbstractNature has developed elegant and economical strategies to produce materials that are well‐adapted to their purposes. As biology evolved to remarkable and complex designs, synthetic mimics are evolving toward new levels of complexity achieving larger combinations of properties within one material. The field of bioinspiration encompasses a wide range of advanced materials ranging from biooptics to energy materials, to biomaterials. In this paper an overview is given of selected recent developments in the field of bioinspired material design focusing on gecko‐inspired adhesives, mussel‐inspired coatings, and spider silk‐inspired biomaterials.

Effect of strain rate on mechanical properties of the bamboo material under quasi-static and dynamic loading condition

As a kind of fiber-reinforced bio-composite, bamboo material exhibits superior mechanical performance. In present work, quasi-static and dynamic Split Hopkinson Pressure Bar (SHPB) compression experiments at the nominal strain rate range of 500 s−1–2200 s−1 were carried out to investigate the effect of strain rate on mechanical behaviors of the bamboo material. In consideration of the effect of fiber directions on mechanical behaviors, the specimens were crushed with the loading direction parallel to the fiber longitudinal direction and perpendicular to the fiber direction, respectively. The experiments under both loading directions exhibited remarkable strain rate sensitivity. The yield stress for the specimens crushed along the fiber longitudinal direction increased from 38.59 MPa to 94.25 MPa, as the strain rate varied from 1.85 × 10−3 to 2200 s−1, while it increased from 7.18 MPa to 42.04 MPa for the specimens crushed perpendicularly to the fiber direction. Final deformation photographs and scanning electron micrographs of the bamboo microstructures clearly illustrated their damage morphologies and various failure mechanisms, including fiber buckling, fiber fractures, PCs collapse, delamination and crack propagation at different strain rates and loading directions. These results may provide a novel perspective on the mechanical behaviors of bamboo microstructure and design of new bio-inspired structural materials.

Kaxiras's Porphyrin: DFT Modeling of Redox-Tuned Optical and Electronic Properties in a Theoretically Designed Catechol-Based Bioinspired Platform.

A detailed computational investigation of the 5,6-dihydroxyindole (DHI)-based porphyrin-type tetramer first described by Kaxiras as a theoretical structural model for eumelanin biopolymers is reported herein, with a view to predicting the technological potential of this unique bioinspired tetracatechol system. All possible tautomers/conformers, as well as alternative protonation states, were explored for the species at various degrees of oxidation and all structures were geometry optimized at the density functional theory (DFT) level. Comparison of energy levels for each oxidized species indicated a marked instability of most oxidation states except the six-electron level, and an unexpected resilience to disproportionation of the one-electron oxidation free radical species. Changes in the highest energy occupied molecular orbital (HOMO)–lowest energy unoccupied molecular orbital (LUMO) gaps with oxidation state and tautomerism were determined along with the main electronic transitions: more or less intense absorption in the visible region is predicted for most oxidized species. Data indicated that the peculiar symmetry of the oxygenation pattern pertaining to the four catechol/quinone/quinone methide moieties, in concert with the NH centers, fine-tunes the optical and electronic properties of the porphyrin system. For several oxidation levels, conjugated systems extending over two or more indole units play a major role in determining the preferred tautomeric state: thus, the highest stability of the six-electron oxidation state reflects porphyrin-type aromaticity. These results provide new clues for the design of innovative bioinspired optoelectronic materials.

Numerical Study on Helical Fiber Fragmentation in Chiral Biological Materials

Chiral microstructures exist widely in natural biological materials such as wood, bone, and climbing tendrils. The helical shape of such microstructures plays an important role in stress transfer between fiber and matrix, and in the mechanical properties of biological materials. In this paper, helical fiber fragmentation behavior is studied numerically using the finite-element method (FEM), and then, the effects of helical shape on fiber deformation and fracture, and the corresponding mechanical mechanisms are investigated. The results demonstrate that, to a large degree, the initial microfibril angle (MFA) determines the elastic deformation and fracture behavior of fibers. For fibers with a large MFA, the interfacial area usually has large values, inducing a relatively low fragment density during fiber fragmentation. This work may be helpful in understanding the relationship between microstructure and mechanical property in biological materials, and in the design and fabrication of bio-inspired advanced functional materials.

Lamellae spatial distribution modulates fracture behavior and toughness of african pangolin scales.

Pangolin scales form a durable armor whose hierarchical structure offers an avenue towards high performance bio-inspired materials design. In this study, the fracture resistance of African pangolin scales is examined using single edge crack three-point bend fracture testing in order to understand toughening mechanisms arising from the structures of natural mammalian armors. In these mechanical tests, the influence of material orientation and hydration level are examined. The fracture experiments reveal an exceptional fracture resistance due to crack deflection induced by the internal spatial orientation of lamellae. An order of magnitude increase in the measured fracture resistance due to scale hydration, reaching up to ~ 25kJ/m2 was measured. Post-mortem analysis of the fracture samples was performed using a combination of optical and electron microscopy, and X-ray computerized tomography. Interestingly, the crack profile morphologies are observed to follow paths outlined by the keratinous lamellae structure of the pangolin scale. Most notably, the inherent structure of pangolin scales offers a pathway for crack deflection and fracture toughening. The results of this study are expected to be useful as design principles for high performance biomimetic applications.

Anomalous evolution of broadband optical absorption reveals dynamic solid state reorganization during eumelanin\xa0build-up in thin films

The origin of eumelanin optical properties remains a formidable conundrum preventing a detailed understanding of the complex photo-protective role of these widespread natural pigments and the rational design of innovative bioinspired materials for optoelectronic applications. Here we report the unusual kinetic and thickness-dependent evolution of the optical properties of black eumelanin polymers generated by spontaneous aerial polymerization of 5,6-dihydroxyindole (DHI) thin films (0.1–1 μm), consistent with peculiar solid state reorganization mechanisms governing broadband absorption. The complete reversal of eumelanin UV-visible transmittance spectrum curvature on passing from 0.2 to 0.5 μm thick films, the marked increase in visible extinction coefficients with increasing film thickness and the higher UV extinction coefficients in slowly vs. rapidly generated polymers concur to support distinct dynamic regimes of solid-state molecular reorganization at the nanoscale level and to do affect the development of broadband visible absorption. Solid state control of molecular reorganization disclosed herein may delineate new rational strategies for tuning optical properties in eumelanin thin films for optoelectronic applications.

Mechanical behavior of idealized, stingray-skeleton-inspired tiled composites as a function of geometry and material properties

Tilings are constructs of repeated shapes covering a surface, common in both manmade and natural structures, but in particular are a defining characteristic of shark and ray skeletons. In these fishes, cartilaginous skeletal elements are wrapped in a surface tessellation, comprised of polygonal mineralized tiles linked by flexible joints, an arrangement believed to provide both stiffness and flexibility. The aim of this research is to use two-dimensional analytical models to evaluate the mechanical performance of stingray skeleton-inspired tessellations, as a function of their material and structural parameters. To calculate the effective modulus of modeled composites, we subdivided tiles and their surrounding joint material into simple shapes, for which mechanical properties (i.e. effective modulus) could be estimated using a modification of traditional Rule of Mixtures equations, that either assume uniform strain (Voigt) or uniform stress (Reuss) across a loaded composite material. The properties of joints (thickness, Young’s modulus) and tiles (shape, area and Young’s modulus) were then altered, and the effects of these tessellation parameters on the effective modulus of whole tessellations were observed. We show that for all examined tile shapes (triangle, square and hexagon) composite stiffness increased as the width of the joints was decreased and/or the stiffness of the tiles was increased; this supports hypotheses that the narrow joints and high tile to joint stiffness ratio in shark and ray cartilage optimize composite tissue stiffness. Our models also indicate that, for simple, uniaxial loading, square tessellations are least sensitive and hexagon tessellations most sensitive to changes in model parameters, indicating that hexagon tessellations are the most “tunable” to specific mechanical properties. Our models provide useful estimates for the tensile and compressive properties of 2d tiled composites under uniaxial loading. These results lay groundwork for future studies into more complex (e.g. biological) loading scenarios and three dimensional structural parameters of biological tilings, while also providing insight into the mechanical roles of tessellations in general and improving the design of bioinspired materials.

Damping behavior investigation and optimization of the structural layout of load-bearing biological materials

Load-bearing biological materials, such as bone and nacre, exhibit superior mechanical properties to support and/or protect biological functionalities. To achieve this, an important ability of such materials is to effectively shield external dynamic impacts. The damping behavior of the biological materials mostly stems from their architecture and also the organic portion which intrinsically shows viscoelastic-plastic behavior. The loss modulus, a crucial representative of dynamic behavior, is a key index on how strong the bio-composite is able to damp the dynamic energy within its structure. This work aims to study the damping behavior of mostly seen staggered architectures of biological materials such as regular, offset, stairwise, herringbone and random staggering patterns to highlight the optimal layout, geometry, and feature sizes of the bio-composite in which the energy dissipation within the structure is maximal. Our studies show that not only the stairwise staggering arrangement overall offers the highest loss modulus amongst other existing architectures but also the aspect ratio of the platelet leading to the maximum loss modulus is remarkably in excellent agreement with experimental observations which might explain why this kind of arrangements is extensively observed in nature. Our findings here not only gain valuable insights into the dynamic behavior of load-bearing biological materials, but also provide useful guidelines to the design of bio-inspired engineering materials.

Spider silk: Dynamic mechanics.

The discovery of a hypersonic bandgap in spider silk could drive the design of bio-inspired and biocompatible dynamic materials.

Protein viscosity, mineral fraction and staggered architecture cooperatively enable the fastest stress wave decay in load-bearing biological materials

One of the key functions of load-bearing biological materials, such as bone, dentin and sea shell, is to protect their inside fragile organs by effectively damping dynamic impact. How those materials achieve this remarkable function remains largely unknown. Using systematic finite element analyses, we study the stress wave propagation and attenuation in cortical bone at the nanoscale as a model material to examine the effects of protein viscosity, mineral fraction and staggered architecture on the elastic wave decay. It is found that the staggered arrangement, protein viscosity and mineral fraction work cooperatively to effectively attenuate the stress wave. For a typical mineral volume fraction and protein viscosity, an optimal staggered nanostructure with specific feature sizes and layouts is able to give rise to the fastest stress wave decay, and the optimal aspect ratio and thickness of mineral platelets are in excellent agreement with experimental measurements. In contrary, as the mineral volume fraction or the protein viscosity goes much higher, the structural arrangement is seen having trivial effect on the stress wave decay, suggesting that the damping properties of the composites go into the structure-insensitive regime from the structure-sensitive regime. These findings not only significantly add to our understanding of the structure-function relationship of load-bearing biological materials, and but also provide useful guidelines for the design of bio-inspired materials with superior resistance to impact loading.

Bioinspired toughening mechanism: lesson from dentin

Inspired by the unique microstructure of dentin, in which the hard peritubular dentin surrounding the dentin tubules is embedded in the soft intertubular dentin, we explore the crack propagation in the bioinspired materials with fracture process zone possessing a dentin-like microstructure, i.e. the composite structure consisting of a soft matrix and hard reinforcements with cylindrical voids. A micromechanical model under small-scale yielding conditions is developed, and numerical simulations are performed, showing that the rising resistant curve (R-curve) is observed for crack propagation caused by the plastic collapse of the intervoid ligaments in the fracture process zone. The dentin-like microstructure in the fracture process zone exhibits enhanced fracture toughness, compared with the case of voids embedded in the homogeneous soft matrix. Further computational simulations show that the dentin-like microstructure can retard void growth, thereby promoting fracture toughness. The typical fracture mechanism of the bioinspired materials with fracture process zone possessing the dentin-like structure is void by void growth, while it is the multiple void interaction in the case of voids in the homogeneous matrix. Based on the results, we propose a bioinspired material design principle, which is that the combination of a hard inner material encompassing voids and a soft outer material in the fracture process zone can give rise to exceptional fracture toughness, achieving damage tolerance. It is expected that the proposed design principle could shed new light on the development of novel man-made engineering materials.

Rational design of bio-inspired high-performance ambipolar organic semiconductor materials based on indigo and its derivatives

Indigoids have received much attention as the candidates of sustainable ambipolar organic semiconductor. However, the low charge carrier mobilities extremely limit their practical applications. Therefore, in-depth understanding of their electronic-structure properties and rational molecular modifications are urgently required. Here, we propose a promising strategy to design ambipolar organic semiconductors based on indigo fragments. Moreover, we predicted the organic crystal structures by evolutionary algorithm combined with DFT-D method. Charge transport properties have been significantly improved for the designed molecules, such as narrower energy gaps, higher electron affinity, larger transfer integrals as well as much smaller reorganization energies for hole and electron. Thusly, remarkable ambipolar charge transport behavior has been predicted, for example, the charge carrier mobilities are up to μh/μe=7.71/5.42cm2V−1s−1 for NN-indigo-6,6′-2CN and μh/μe=5.15/2.13cm2V−1s−1 for C9-NN-indigo-6,6′-2CN respectively.

Kinetics of aggregation and crystallization of polyaspartic Acid stabilized calcium phosphate particles at high concentrations.

Bone is an important material to study due to its exceptional mechanical properties and relevance with respect to hard tissue regeneration and repair. A significant effort has been directed toward understanding the bone formation process and the production of synthetic bone mimicking materials. Here, the formation and structural evolution of calcium phosphate (CaP) was investigated in the presence of relatively high concentrations of calcium, phosphate, and polyaspartic acid (pAsp) using dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM). The incipient CaP aggregates were comprised of spherical nanoparticles (diameter ≈ 3-4 nm); they became preferentially aligned over time and eventually transformed into nanorods. The nanorods remained stable in suspension with no signs of further aggregation for at least four months. Detailed cryo-TEM suggested that the CaP nanorods formed through an oriented attachment mechanism. These results show that the reaction concentration greatly influences the mechanism and final properties of CaP. Mechanistic insights gained from this study will facilitate better design and fabrication of bioinspired materials.

Structural and Mechanical Characterization of Thermally Treated Conch Shells

Seashells are natural nanocomposite armors with an exceptional combination of strength and toughness. Conch shells have a crossed-lamellar structure constructed with aragonite and biopolymer. Thermal treatment uncovered a nanoscale hierarchical structure in shell’s third-order lamellae. Individual third-order lamellae were found to consist of aragonite nanoparticles cemented with biopolymer. The biopolymer renders conch shells joint increase in strength, ductility and fracture energy, and especially the fracture energy increase is more remarkable. The shell’s aragonite transformed to calcite at 407°C and lime at 607°C. The shell’s biopolymer was burned out in the thermal treatment at 310°C, leading to 1.7% mass loss. The crossed-lamellar structure remained in the 500°C thermally treated shell. The 900°C heat treatment destroyed the crossed-lamellar architecture completely. Thermal treatment resulted in reduction in mechanical properties due to the joint effects—burning out of biopolymer, phase transformation, and destruction of structural integrity. The findings advance our understanding of conch shell’s architecture and provide new guidelines for the design and manufacturing of bio-inspired materials.

Mechanical Response of Hybrid Cross-Linked Networks to Uniaxial Deformation: A Molecular Dynamics Model

Networks combining physical and covalent chemical cross-links can exhibit a large amount of dissipated inelastic energy along with high stretchability during deformation. We present our analysis of the influence of the extent of covalent cross-linking on the inelasticity of hydrogels. Four model networks, which are similar in structure but strongly differ in elasticity, have been studied. The aim was the identification of a key structural factor responsible for observing a hysteresis or an elastic deformation. In the employed molecular dynamics study this factor is derived from the underlying structure of each particular hydrogel network. Several structural characteristics have been investigated like the extent of damage to the network, chains sliding, and the specific properties of load-bearing chains. By means of such a key factor, one can predict the deformation behavior (hysteresis or elasticity) of some material, provided a precise description of its structure exists and it resembles any of the four types of a network. The results can be applied in the design of bio-inspired materials with tailored properties.