Silk features exceptional mechanical properties such as high tensile strength and great extensibility, making it one of the toughest materials known. The exceptional strength of silkworm and spider silks, exceeding that of steel, arises from beta-sheet nanocrystals that universally consist of highly conserved poly-(Gly-Ala) and poly-Ala domains. This is counterintuitive because the key molecular interactions in beta-sheet nanocrystals are hydrogen bonds, one of the weakest chemical bonds known. Here we report a series of large-scale molecular dynamics simulations, revealing that beta-sheet nanocrystals confined to a few nanometres achieve higher stiffness, strength and mechanical toughness than larger nanocrystals. We illustrate that through nanoconfinement, a combination of uniform shear deformation that makes most efficient use of hydrogen bonds and the emergence of dissipative molecular stick-slip deformation leads to significantly enhanced mechanical properties. Our findings explain how size effects can be exploited to create bioinspired materials with superior mechanical properties in spite of relying on mechanically inferior, weak hydrogen bonds.

Crumpled graphene films are broadly used, for instance in electronics1, energy storage2, 3, composites4, 5, and biomedicine6. Although it is known that the degree of crumpling affects graphene's properties and the performance of graphene-based devices and materials3, 5, 7, the controlled folding and unfolding of crumpled graphene films has not been demonstrated. Here we report an approach to reversibly control the crumpling and unfolding of large-area graphene sheets. We show with experiments, atomistic simulations and theory that, by harnessing the mechanical instabilities of graphene adhered on a biaxially pre-stretched polymer substrate and by controlling the relaxation of the pre-strains in a particular order, graphene films can be crumpled into tailored self-organized hierarchical structures that mimic superhydrophobic leaves. The approach enables us to fabricate large-area conductive coatings and electrodes showing superhydrophobicity, high transparency, and tunable wettability and transmittance. We also demonstrate that crumpled graphene-polymer laminates can be used as artificial-muscle actuators.

Amyloid or amyloid-like fibrils represent a general class of nanomaterials that can be formed from many different peptides and proteins. Although these structures have an important role in neurodegenerative disorders, amyloid materials have also been exploited for functional purposes by organisms ranging from bacteria to mammals. Here we review the functional and pathological roles of amyloid materials and discuss how they can be linked back to their nanoscale origins in the structure and nanomechanics of these materials. We focus on insights both from experiments and simulations, and discuss how comparisons between functional protein filaments and structures that are assembled abnormally can shed light on the fundamental material selection criteria that lead to evolutionary bias in multiscale material design in nature.

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Despite decades of studies of calcium-silicate-hydrate (C-S-H), the structurally complex binder phase of concrete, the interplay between chemical composition and density remains essentially unexplored. Together these characteristics of C-S-H define and modulate the physical and mechanical properties of this ''liquid stone'' gel phase. With the recent determination of the calcium/silicon (C/S ‫؍‬ 1.7) ratio and the density of the C-S-H particle (2.6 g/cm 3 ) by neutron scattering measurements, there is new urgency to the challenge of explaining these essential properties. Here we propose a molecular model of C-S-H based on a bottom-up atomistic simulation approach that considers only the chemical specificity of the system as the overriding constraint. By allowing for short silica chains distributed as monomers, dimers, and pentamers, this C-S-H archetype of a molecular description of interacting CaO, SiO2, and H2O units provides not only realistic values of the C/S ratio and the density computed by grand canonical Monte Carlo simulation of water adsorption at 300 K. The model, with a chemical composition of (CaO)1.65(SiO2)(H2O)1.75, also predicts other essential structural features and fundamental physical properties amenable to experimental validation, which suggest that the C-S-H gel structure includes both glass-like short-range order and crystalline features of the mineral tobermorite. Additionally, we probe the mechanical stiffness, strength, and hydrolytic shear response of our molecular model, as compared to experimentally measured properties of C-S-H. The latter results illustrate the prospect of treating cement on equal footing with metals and ceramics in the current application of mechanism-based models and multiscale simulations to study inelastic deformation and cracking.atomistic simulation ͉ mechanical properties ͉ structural properties B y mixing water and cement, a complex hydrated oxide called calcium-silicate-hydrate (C-S-H) precipitates as nanoscale clusters of particles (1). Much of our knowledge of C-S-H has been obtained from structural comparisons with crystalline calcium silicate hydrates, based on HFW Taylor's postulate that real C-S-H was a structurally imperfect layered hybrid of two natural mineral analogs (2) (4)]. While this suggestion is plausible in morphological terms, this model is incompatible with two basic characteristics of real C-S-H; specifically the calcium-tosilicon ratio (C/S) and the density. Recently, small-angle neutron scattering measurements have fixed the C/S ratio at 1.7 and the density at 2.6 g/cm 3 (1), values that clearly cannot be obtained from either tobermorite (C/S ϭ 0.83, 2.18 g/cm 3 ) or jennite (C/S ϭ 1.5 and 2.27 g/cm 3 ). From the standpoint of constructing a molecular model of C-S-H, this means that these crystalline minerals are not strict structural analogs. Here we adopt the perspective that the chemical composition of C-S-H is the most essential property in formulating a realistic molecular description. We show that once the C/S ratio is described...

Collagen is a protein material with superior mechanical properties. It consists of collagen fibrils composed of a staggered array of ultra-long tropocollagen (TC) molecules. Theoretical and molecular modeling suggests that this natural design of collagen fibrils maximizes the strength and provides large energy dissipation during deformation, thus creating a tough and robust material. We find that the mechanics of collagen fibrils can be understood quantitatively in terms of two critical molecular length scales S and R that characterize when (i) deformation changes from homogeneous intermolecular shear to propagation of slip pulses and when (ii) covalent bonds within TC molecules begin to fracture, leading to brittle-like failure. The ratio S͞R indicates which mechanism dominates deformation. Our modeling rigorously links the chemical properties of individual TC molecules to the macroscopic mechanical response of fibrils. The results help to explain why collagen fibers found in nature consist of TC molecules with lengths in the proximity of 300 nm and advance the understanding how collagen diseases that change intermolecular adhesion properties influence mechanical properties.deformation ͉ fracture ͉ mechanics ͉ tropocollagen ͉ length scale M aterials found in nature often feature hierarchical structures ranging from the atomistic and molecular scales to macroscopic scales (1-5). Many biological materials found in living organisms, often protein-based, feature a complex hierarchical design.Collagen, the most abundant protein on earth, is a fibrous structural protein with superior mechanical properties, and it provides an intriguing example of a hierarchical biological nanomaterial (4, 6-18). Collagen consists of tropocollagen (TC) molecules that have lengths of L Ϸ 280 nm and diameters of Ϸ1.5 nm, leading to an aspect ratio of Ϸ190 (6, 7, 9, 18-20). Staggered arrays of TC molecules form fibrils, which arrange to form collagen fibers (Fig. 1).Collagen plays an important role in many biological tissues, including tendon, bone, teeth, and cartilage (6,7,13,15,19,21). Severe mechanical tensile loading of collagen is significant under many physiological conditions, as in joints and in bone (22, 23). Despite significant research effort over the past couple of decades, the geometry and typical length scales found in collagen fibrils, the deformation mechanisms under mechanical load, and, in particular, the relationship between those mechanisms and collegen's molecular and intermolecular properties, are not well understood. Moreover, the limiting factors of the strength of collagen fibrils and the origins of toughness remain largely unknown.Some experimental efforts focused on the deformation mechanics of collagen fibril at nanoscale, including the characterization of changes of D-spacing and fibril orientation (18,20,24), analyses that featured x-ray diffraction (18) and synchrotron radiation experiments (19). Other experimental studies were focused on the averaged response of arrays of collagen fibrils, considering nan...

CONSPECTUS: Polydopamine (PDA), a black insoluble biopolymer produced by autoxidation of the catecholamine neurotransmitter dopamine (DA), and synthetic eumelanin polymers modeled to the black functional pigments of human skin, hair, and eyes have burst into the scene of materials science as versatile bioinspired functional systems for a very broad range of applications. PDA is characterized by extraordinary adhesion properties providing efficient and universal surface coating for diverse settings that include drug delivery, microfluidic systems, and water-treatment devices. Synthetic eumelanins from dopa or 5,6-dihydroxyindoles are the focus of increasing interest as UV-absorbing agents, antioxidants, free radical scavengers, and water-dependent hybrid electronic-ionic semiconductors. Because of their peculiar physicochemical properties, eumelanins and PDA hold considerable promise in nanomedicine and bioelectronics, as they are biocompatible, biodegradable, and exhibit suitable mechanical properties for integration with biological tissues. Despite considerable similarities, very few attempts have so far been made to provide an integrated unifying perspective of these two fields of technology-oriented chemical research, and progress toward application has been based more on empirical approaches than on a solid conceptual framework of structure-property relationships. The present Account is an attempt to fill this gap. Following a vis-à-vis of PDA and eumelanin chemistries, it provides an overall view of the various levels of chemical disorder in both systems and draws simple correlations with physicochemical properties based on experimental and computational approaches. The potential of large-scale simulations to capture the macroproperties of eumelanin-like materials and their hierarchical structures, to predict the physicochemical properties of new melanin-inspired materials, to understand the structure-property-function relationships of these materials from the bottom up, and to design and optimize materials to achieve desired properties is illustrated. The impact of synthetic conditions on melanin structure and physicochemical properties is systematically discussed for the first time. Rational tailoring strategies directed to critical control points of the synthetic pathways, such as dopaquinone, DAquinone, and dopachrome, are then proposed, with a view to translating basic chemical knowledge into practical guidelines for material manipulation and tailoring. This key concept is exemplified by the recent demonstration that varying DA concentration, or using Tris instead of phosphate as the buffer, results in PDA materials with quite different structural properties. Realizing that PDA and synthetic eumelanins belong to the same family of functional materials may foster unprecedented synergisms between research fields that have so far been apart in the pursuit of tailorable and marketable materials for energy, biomedical, and environmental applications.

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