Markus A. Hartmann

Observations of Multiscale, Stress-Induced Changes of Collagen Orientation in Tendon by Polarized Raman Spectroscopy

Collagen is a versatile structural molecule in nature and is used as a building block in many highly organized tissues, such as bone, skin, and cornea. The functionality and performance of these tissues are controlled by their hierarchical organization ranging from the molecular up to macroscopic length scales. In the present study, polarized Raman microspectroscopic and imaging analyses were used to elucidate collagen fibril orientation at various levels of structure in native rat tail tendon under mechanical load. In situ humidity-controlled uniaxial tensile tests have been performed concurrently with Raman confocal microscopy to evaluate strain-induced chemical and structural changes of collagen in tendon. The methodology is based on the sensitivity of specific Raman scattering bands (associated with distinct molecular vibrations, such as the amide I) to the orientation and the polarization direction of the incident laser light. Our results, based on the changing intensity of Raman lines as a function of orientation and polarization, support a model where the crimp and gap regions of collagen hierarchical structure are straightened at the tissue and molecular level, respectively. However, the lack of measurable changes in Raman peak positions throughout the whole range of strains investigated indicates that no significant changes of the collagen backbone occurs with tensing and suggests that deformation is rather redistributed through other levels of the hierarchical structure.

New Suggestions for the Mechanical Control of Bone Remodeling

Bone is constantly renewed over our lifetime through the process of bone (re)modeling. This process is important for bone to allow it to adapt to its mechanical environment and to repair damage from everyday life. Adaptation is thought to occur through the mechanosensitive response controlling the bone-forming and -resorbing cells. This report shows a way to extract quantitative information about the way remodeling is controlled using computer simulations. Bone resorption and deposition are described as two separate stochastic processes, during which a discrete bone packet is removed or deposited from the bone surface. The responses of the bone-forming and -resorbing cells to local mechanical stimuli are described by phenomenological remodeling rules. Our strategy was to test different remodeling rules and to evaluate the time evolution of the trabecular architecture in comparison to what is known from μ-CT measurements of real bone. In particular, we tested the reaction of virtual bone to standard therapeutic strategies for the prevention of bone deterioration, i.e., physical activity and medications to reduce bone resorption. Insensitivity of the bone volume fraction to reductions in bone resorption was observed in the simulations only for a remodeling rule including an activation barrier for the mechanical stimulus above which bone deposition is switched on. This is in disagreement with the commonly used rules having a so-called lazy zone.

Sacrificial Ionic Bonds Need To Be Randomly Distributed To Provide Shear Deformability

Multivalent ions are known to allow for reversible cross-linking in soft biological materials, providing stiffness and extensibility via sacrificial bonds. We present a simple model where stiff nanoscale elements carrying negative charges are coupled in shear by divalent mobile cations in aqueous media. Such a shear coupling through a soft glue has, indeed, been proposed to operate in biological nanocomposites. While the coupling is elastic and brittle when the negative charges are periodically arranged, sufficient randomness in their distribution allows for large irreversible deformation.

Gold Nanoparticles at the Liquid-Liquid Interface: X-ray Study and Monte Carlo Simulation

The behavior of mixed-ligand-coated gold nanoparticles at a liquid-liquid interface during compression has been investigated. The system was characterized by measuring pressure-area isotherms and by simultaneously performing in situ X-ray studies. Additionally, Monte Carlo (MC) simulations were carried out in order to interpret the experimental findings. With this dual approach it was possible to characterize and identify the different stages of compression and understand what happens microscopically: first, a compression purely in-plane, and, second, the formation of a second layer when the in-plane pressure pushes the particles out of the plane. The first stage is accompanied by the emergence of an in-plane correlation peak in the scattering signal and a strong increase of the pressure in the isotherm. The second stage is characterized by the weakening of the correlation peak and a slower increase in pressure.

Imidazole release from the antitumor-active ruthenium complex imidazolium trans-tetrachlorobis(imidazole) ruthenate(III) by biologically occurring nucleophiles

Abstract The antitumor-active complex Hlm[ trans -Ru 111 Cl 4 (im) 2 ], immidazolium trans -tetrachlorobis(imidazole)ruthenate(III), completely changes its ligand configuration within 1 h in water in the presence of l -histidine and l -glutathione. The observed release of the trans -standing imidazole ligands at 37°C that occurs in addition to chloride substitution reactions has to be taken into consideration for further investigations into the mode of action of this new antitumor drug.

Elastic properties of graphene obtained by computational mechanical tests

The basic building block of many carbon nanostructures like fullerenes, carbon onions or nanotubes is the truly two-dimensional material graphene. Commercial finite element codes, widely used to predict the mechanical properties of these structures, rely on the knowledge of the mechanical properties of the basic material. In this paper using an atomistic simulation approach we determine the membrane and bending stiffness of graphene, as well as the corresponding effective parameters: the effective elastic modulus , Poisson ratio and thickness . It is shown that within reasonable accuracy the obtained parameters can be applied to various loading scenarios on carbon nanostructures as long as the characteristic length of these structures is larger than . Thus, for such large and complex structures that withstand an analytical or atomistic description, commercial finite element solvers, in combination with the found effective parameters, can be used to describe these structures.

Structural and mechanical properties of the arthropod cuticle: Comparison between the fang of the spider Cupiennius salei and the carapace of American lobster Homarus americanus

Most biological materials are nanocomposites characterized by a multi-level structural hierarchy. Particularly, the arthropod cuticle is a chitin-based composite material where the mechanical properties strongly depend on both molecular chitin/protein properties, and the structural arrangement of chitin-fibrils within the protein matrix. Here materials properties and structural organization of two types of cuticle from distantly related arthropods, the wandering spider Cupiennius salei and American lobster Homarus americanus were studied using nanoindentation and X-ray diffraction. The structural analysis of the two types of cuticle including the packing and alignment of chitin-fibrils is supported by Monte Carlo simulations of the experimental X-ray data, thereby regions of parallel and rotated fibril arrangement can be clearly distinguished. The tip of the spider fang which is used to inject venom into the prey was found to be considerably harder than the lobster carapace, while its stiffness is slightly lower.

The role of topology and thermal backbone fluctuations on sacrificial bond efficacy in mechanical metalloproteins

Sacrificial bonding is a ubiquitous cross-linking strategy for increasing toughness that is found throughout nature in various biological materials such as bone, wood, silk and mussel byssal threads. However, the molecular mechanism of sacrificial bonding remains only poorly understood. Molecular modeling possesses a strong potential to provide insights into the behavior of these crosslinks. Here we use Monte Carlo simulations to investigate the effect of reversible sacrificial binding sites on the mechanical properties of single linear polymer chains based on load-bearing metalloproteins found in the mussel byssus. It is shown that the topology of the bonds determines the position and spacing of sacrificial force peaks, while the height of these peaks is intimately tied to the magnitude of thermal fluctuations in the chain that are dependent on effective chain length. These results bear important implications for understanding natural systems and for the generation of strong and ductile biomimetic polymers.

Onsager's coefficients and diffusion laws - a Monte Carlo study

For a numerical description of multicomponent diffusion processes, the coefficients of the Onsager diffusion matrix are needed. We compare a number of models relating these parameters to experimentally accessible quantities, such as tracer diffusion coefficients. Since these models present different levels of approximation, we investigate the differences in Onsager parameters when they are determined from tracer diffusion. Moreover, we study an ideal solution alloy by Monte Carlo methods to determine the Onsager coefficients directly and using the model assumptions. Measuring atomic fluxes and site fraction gradients, our simulation method is more closely related to a real physical experiment than the usual simulation method of generalized Einstein equations. Onsagers variation principle for the calculation of the kinetic coefficients is extended and it is shown that a reasonable description, even of a simple alloy system, requires consideration of a non-diagonal dissipation matrix in the derivation of the diffusion equations.

Trabecular bone remodelling simulated by a stochastic exchange of discrete bone packets from the surface

Human bone is constantly renewed through life via the process of bone remodelling, in which individual packets of bone are removed by osteoclasts and replaced by osteoblasts. Remodelling is mechanically controlled, where osteocytes embedded within the bone matrix are thought to act as mechanical sensors. In this computational work, a stochastic model for bone remodelling is used in which the renewal of bone material occurs by exchange of discrete bone packets. We tested different hypotheses of how the mechanical stimulus for bone remodelling is integrated by osteocytes and sent to actor cells on the bones surface. A collective (summed) signal from multiple osteocytes as opposed to an individual (maximal) signal from a single osteocyte was found to lead to lower inner porosity and surface roughness of the simulated bone structure. This observation can be interpreted in that collective osteocyte signalling provides an effective surface tension to the remodelling process. Furthermore, the material heterogeneity due to remodelling was studied on a network of trabeculae. As the model is discrete, the age of individual bone packets can be monitored with time. The simulation results were compared with experimental data coming from quantitative back scattered electron imaging by transforming the information about the age of the bone packet into a mineral content. Discrepancies with experiments indicate that osteoclasts preferentially resorb low mineralized, i.e. young, bone at the bones surface.