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Ball V.,University of Strasbourg | Ball V.,French Institute of Health and Medical Research | Gracio J.,University of Aveiro | Vila M.,Complutense University of Madrid | And 9 more authors.
Langmuir | Year: 2013

Eumelanin is not only a ubiquitous pigment among living organisms with photoprotective and antioxidant functions, but is also the subject of intense interest in materials science due to its photoconductivity and as a possible universal coating platform, known as "polydopamine films". The structure of eumelanin remains largely elusive, relying either on a polymeric model or on a heterogeneous aggregate structure. The structure of eumelanin as well as that of the closely related "polydopamine films" can be modified by playing on the nature of the oxidant used to oxidize dopamine or related compounds. In this investigation, we show that dopamine-eumelanins produced from dopamine in the presence of either air (O2 being the oxidant) or Cu2+ cations display drastically different optical and colloidal properties in relation with a different supramolecular assembly of the oligomers of 5,6 dihydroxyindole, the final oxidation product of dopamine. The possible origin of these differences is discussed on the basis of Cu 2+ incorporation in Cu dopamine-eumelanin. © 2013 American Chemical Society. Source

Giesa T.,Laboratory for Atomistic and Molecular Mechanics | Buehler M.J.,Laboratory for Atomistic and Molecular Mechanics | Buehler M.J.,Massachusetts Institute of Technology
Annual Review of Biophysics | Year: 2013

This review examines size effects observed in the mechanical strength of biopolymers that are organized in microstructures such as fibrils, layered composites, or particle nanocomposites. We review the most important aspects that connect nanoconfinement of basic material constituents at critical length scales to the mechanical performance of the entire material system: elastic modulus, strength, extensibility, and robustness. We outline theoretical and computational analysis as well as experimentation by emphasizing two strategies found in abundant natural materials: confined fibrils as part of fibers and confined mineral platelets that transfer load through a biopolymer interface in nanocomposites. We also discuss the application of confinement as a mechanism to tailor specific material properties in biological systems. Copyright © 2013 by Annual Reviews. Source

Garcia A.P.,Laboratory for Atomistic and Molecular Mechanics | Buehler M.J.,Laboratory for Atomistic and Molecular Mechanics
Computational Materials Science | Year: 2010

The ability of nature to integrate disparate properties in materials is astounding. Diatoms, unicellular algae that originated approximately 200 million years ago, contain frustules, or silicified cell walls with nanoscale size pores, which are surprisingly tough when compared to bulk silica, which is one of the most brittle materials known. For example, the frustule morphology of the Coscinodiscus sp. diatom consists of a hierarchy of three overlapping porous silica layers that lie on a hexagonal grid, much like a honeycomb within a honeycomb. Here we utilize the porous structure found in diatoms to develop a bioinspired nanoporous material implemented in silicon. By varying the size of the trusses in the constituting silicon nanostructure, we examine associated mechanical properties, as well as fracture and toughening mechanisms by carrying out a series of molecular dynamics simulations with the first principles based reactive force field ReaxFF. We find that by controlling the wall width of the silicon nanostructure, it is possible to significantly enhance the mechanical response of the material, creating a highly deformable (up to 80% strain) and extremely tough material. Our findings provide fundamental insight into transforming a brittle material (s.a. silicon or silica) into a ductile material, through alterations of its structural arrangement at the nanoscale. © 2010 Elsevier B.V. All rights reserved. Source

Sen D.,Laboratory for Atomistic and Molecular Mechanics | Sen D.,Massachusetts Institute of Technology | Garcia A.P.,Laboratory for Atomistic and Molecular Mechanics | Buehler M.J.,Laboratory for Atomistic and Molecular Mechanics
Journal of Nanomechanics and Micromechanics | Year: 2011

Porous silica structures with intricate design patterns form the exoskeleton of diatoms, a large class of microscopic mineralized algae, whose structural features have been observed to exist down to nanoscale dimensions. Nanoscale patterned porous silica structures have also been manufactured for the use in optical systems, catalysts, and semiconductor nanolithography. The mechanical properties of these porous structures at the nanoscale are a subject of great interest for potential technological and biomimetic applications in the context of new classes of multifunctional materials. Previous studies have established the emergence of enhanced toughness and ductility in nanoporous crystalline silica structures over bulk silica. The authors undertake molecular dynamics simulations and theoretical size-scaling studies of elasticity and strength of a simple model of generic nanoporous silica structures, used to establish a theoretical model for the detailed mechanisms behind their improved properties, and show through theoretical analysis that below a critical length scale around 60-80 Å, the silica struts in the nanoporous structure undergo plastic shear deformation before fracture, leading to enhanced ductility. Corresponding molecular dynamics simulations directly confirm that at this critical length scale, the fracture mechanism changes from crack propagation starting at regions of stress concentration to plasticity showing shearing and necking. This drastic change in the material behavior arises from the flaw-tolerant size of the constituent silica struts, where below a critical width, the strut fails at the theoretical shear strength of silica. The insight developed from these theoretical-computational studies could be used to engineer silica structures that deform plastically before fracture and are much tougher than bulk silica. Such structures have potential application in carrying load, along with catalytic, optical, and adsorbent applications for novel nanodevices and nanomaterials. © 2011 American Society of Civil Engineers. Source

Nova A.,Laboratory for Atomistic and Molecular Mechanics | Nova A.,Polytechnic of Milan | Keten S.,Laboratory for Atomistic and Molecular Mechanics | Pugno N.M.,Laboratory for Atomistic and Molecular Mechanics | And 8 more authors.
Nano Letters | Year: 2010

Spider dragline silk is one of the strongest, most extensible and toughest biological materials known, exceeding the properties of many engineered materials including steel. Silk features a hierarchical architecture where highly organized, densely H-bonded beta-sheet nanocrystals are arranged within a semiamorphous protein matrix consisting of 31-helices and beta-turn protein structures. By using a bottom-up molecular-based approach, here we develop the first spider silk mesoscale model, bridging the scales from Angstroms to tens to potentially hundreds of nanometers. We demonstrate that the specific nanoscale combination of a crystalline phase and a semiamorphous matrix is crucial to achieve the unique properties of silks. Our results reveal that the superior mechanical properties of spider silk can be explained solely by structural effects, where the geometric confinement of beta-sheet nanocrystals, combined with highly extensible semiamorphous domains, is the key to reach great strength and great toughness, despite the dominance of mechanically inferior chemical interactions such as H-bonding. Our model directly shows that semiamorphous regions govern the silk behavior at small deformation, unraveling first when silk is being stretched and leading to the large extensibility of the material. Conversely, beta-sheet nanocrystals play a significant role in defining the mechanical behavior of silk at large-deformation. In particular, the ultimate tensile strength of silk is controlled by the strength of beta-sheet nanocrystals, which is directly related to their size, where small beta-sheet nanocrystals are crucial to reach outstanding levels of strength and toughness. Our results and mechanistic insight directly explain recent experimental results, where it was shown that a significant change in the strength and toughness of silk can be achieved solely by tuning the size of beta-sheet nanocrystals. Our findings help to unveil the material design strategy that enables silk to achieve superior material performance despite simple and inferior material constituents. This concept could lead to a new materials design paradigm, where enhanced functionality is not achieved using complex building blocks but rather through the utilization of simple repetitive constitutive elements arranged in hierarchical structures from nano to macro. © 2010 American Chemical Society. Source

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