Yves Berthier

Tenascin-X increases the stiffness of collagen gels without affecting fibrillogenesis

Tenascin-X is an extracellular matrix protein whose absence leads to an Ehlers-Danlos Syndrome in humans, mainly characterised by connective tissue defects including the disorganisation of fibrillar networks, a reduced collagen deposition, and modifications in the mechanical properties of dense tissues. Here we tested the effect of tenascin-X on in vitro collagen fibril formation. We observed that the main parameters of fibrillogenesis were unchanged, and that the diameter of fibrils was not significantly different when they were formed in the presence of tenascin-X. Interestingly, mechanical analysis of collagen gels showed an increased compressive resistance of the gels containing tenascin-X, indicating that this protein might be directly involved in determining the mechanical properties of collagen-rich tissues in vivo.

Contact of a Finger on Rigid Surfaces and Textiles: Friction Coefficient and Induced Vibrations

The tactile information about object surfaces is obtained through perceived contact stresses and friction-induced vibrations generated by the relative motion between the fingertip and the touched object. The friction forces affect the skin stress-state distribution during surface scanning, while the sliding contact generates vibrations that propagate in the finger skin activating the receptors (mechanoreceptors) and allowing the brain to identify objects and perceive information about their properties. In this article, the friction coefficient between a real human finger and both rigid surfaces and fabrics is retrieved as a function of the contact parameters (load and scanning speed). Then, the analysis of the vibration spectra is carried out to investigate the features of the induced vibrations, measured on the fingernail, as a function of surface textures and contact parameters. While the friction coefficient measurements on rigid surfaces agree with empirical laws found in literature, the behaviour of the friction coefficient when touching a fabric is more complex, and is mainly the function of the textile constructional properties. Results show that frequency spectrum distribution, when touching a rigid surface, is mainly determined by the relative geometry of the two contact surfaces and by the contact parameters. On the contrary, when scanning a fabric, the structure and the deformation of the textile itself largely affect the spectrum of the induced vibration. Finally, some major features of the measured vibrations (frequency distribution and amplitude) are found to be representative of tactile perception compared to psychophysical and neurophysiologic works in literature.

Stability and tribological performances of fluid phospholipid bilayers: effect of buffer and ions.

We have investigated the mechanical and tribological properties of supported Dioleoyl phosphatidylcholine (DOPC) bilayers in different solutions: ultrapure water (pH 5.5), saline solution (150 mM NaCl, pH 5.8), Tris buffer (pH 7.2) and Tris saline buffer (150 mM NaCl, pH 7.2). Friction forces are measured using a homemade biotribometer. Lipid bilayer degradation is controlled in situ during friction tests using fluorescence microscopy. Mechanical resistance to indentation is measured by force spectroscopy with an atomic force microscope. This study confirms that mechanical stability under shear or normal load is essential to obtain low and constant friction coefficients. In ultrapure water, bilayers are not resistant and have poor lubricant properties. On the other hand, in Tris saline buffer, they fully resist to indentation and exhibit low (micro=0.035) and stable friction coefficient with no visible wear during the 50 min of the friction test. The unbuffered saline solution improves the mechanical resistance to indentation but not the lubrication. These results suggest that the adsorption of ions to the zwiterrionic bilayers has different effects on the mechanical and tribological properties of bilayers: higher resistance to normal indentation due to an increase in bilayer cohesion, higher lubrication due to an increase in bilayer-bilayer repulsion.

Nanomechanical and tribological characterization of the MPC phospholipid polymer photografted onto rough polyethylene implants.

Grafting biomimetic polymers onto biomaterials such as implants is one of the promising approaches to increase their tribological performance and biocompatibility and to reduce wear. In this paper, poly(2-methacryloyloxyethyl phosphorylcholine) (p(MPC)) brushes were obtained by photografting MPC from the rough surface of ultra high molecular weight polyethylene (UHMWPE) joint implants. Such substrates have a high roughness (Ra∼650nm) which often has the same order of magnitude as the brush thickness, so it is very difficult to estimate the vertical density profile of the grafted content. The quality of the p(MPC) grafting was evaluated through a wide range of characterization techniques to reveal the effectiveness of the grafting: atomic force microcopy (AFM) imaging and force spectroscopy, contact angle, SEM/EDX, and confocal microscopy. After testing the methods on smooth glass substrate as reference, AFM nano-indentation proves to be a reliable non destructive method to characterize the thickness and the mechanical properties of the p(MPC) layer in liquid physiological medium. Tribological measurements using a homemade biotribometer confirm that, despite heterogeneity thickness (h=0.5-6μm), the p(MPC) layer covers the roughness of the UHMWPE substrate and acts as an efficient lubricant with low friction coefficient and no wear for 9h of friction.

Experimental analysis of friction-induced vibrations at the finger contact surface:

Abstract When a finger moves to scan the surface of an object, the sliding contact generates vibrations that propagate in the finger skin and transmit the information about the object characteristics to mechanoreceptors. Mechanoreceptors convert vibrations into electric impulses sent to the brain. In this context, by appropriate experiments, a frequency analysis of the signal characterizing the surface scanning can be carried out to investigate the vibration spectrum measured on the finger and to highlight how the tactile sense is connected to the measured frequency spectra. Although the correlation between the surface roughness with respect to the tactile sensation is deeply analysed in the literature, the vibration spectra induced by the finger—surface scanning and the consequent activation of mechanoreceptors on the skin were rarely investigated. In particular, in this paper, interests will be focused on the changes shown in the vibration spectra, caused by variations in the characteristic contact parameters such as scanning velocity and roughness.

Alteration of cartilage mechanical properties in absence of β1 integrins revealed by rheometry and FRAP analyses

CONTEXTnMechanical properties are essential for biological functions of the hyaline cartilage such as energy dissipation and diffusion of solutes. Mechanical properties are primarily dependent on the hierarchical organization of the two major extracellular matrix (ECM) macromolecular components of the cartilage: the fibrillar collagen network and the glycosaminoglycan (GAG)-substituted proteoglycan, mainly aggrecan, aggregates. Interaction of chondrocytes, the only cell type in the tissue, with the ECM through adhesion receptors is involved in establishing mechanical stability via bidirectional transduction of both mechanical forces and chemical signals. In this study, we aimed to determine the role of the transmembrane β1 integrin adhesion receptors in cartilage biomechanical properties by the use of genetic modification in mice.nnnMETHODSnCostal cartilages of wild type and mutant mice lacking β1 integrins in chondrocytes were investigated. Cartilage compressive properties and solute diffusion were characterized by rheometric analysis and Fluorescence Recovery After Photobleaching (FRAP), respectively. Cartilage tissue sections were analyzed by histology, immunohistochemistry and transmission electron microscopy (TEM).nnnRESULTSnAt the histological level, the mutant costal cartilage was characterized by chondrocyte rounding and loss of tissue polarity. Immunohistochemistry and safranin orange staining demonstrated apparently normal aggrecan and GAG levels, respectively. Antibody staining for collagen II and TEM showed comparable expression and organization of the collagen fibrils between mutant and control cartilages. Despite the lack of gross histological and ultrastructural abnormalities, rheological measurements revealed that the peak elastic modulus in compression of mutant cartilage was 1.6-fold higher than the peak elastic modulus of wild-type sample. Interestingly, the diffusion coefficient within the mutant cartilage tissue was found to be 1.2-fold lower in the extracellular space and 14-fold lower in the pericellular (PCM) space compared to control.nnnCONCLUSIONnThe results demonstrate that the absence of β1 integrins on the surface of chondrocytes increases the stiffness and modifies the diffusion properties of costal cartilage. Our data imply that β1 integrins-mediated chondrocyte-matrix interactions directly affect cartilage biomechanics probably by modifying physical properties of individual cells. This study thus highlights the crucial role of β1 integrins in the cartilage function.

Modeling Wear for Heterogeneous Bi-Phasic Materials Using Discrete Elements Approach

A proper understanding of the processes of friction and wear can only be reached through a detailed study of the contact interface. Empirical laws, such as Archards, are often used in numerical models. They give good results over a limited range of conditions when their coefficients are correctly set, but they cannot be predicted: any significant change of conditions requires a new set of experimental coefficients. In this paper, a new method, the use of Discrete Element Models (DEM), is proposed in order to tend to predictable models. As an example, a generic biphasic friction material is modelled, of the type used in aeronautical or automotive brake systems. Micro-scale models are built in order to study material damage and wear under tribological stress. The models show what could be achieved by these numerical methods in tribological studies, and how they can reproduce the behavior and mechanisms seen with real-life friction materials without any empirical law or parameter.

Charged particles interacting with a mixed supported lipid bilayer as a biomimetic pulmonary surfactant

This study shows the interactions of charged particles with mixed supported lipid bilayers (SLB) as biomimetic pulmonary surfactants. We tested two types of charged particles: positively charged and negatively charged particles. Two parameters were measured: adsorption density of particles on the SLB and the diffusion coefficient of lipids by FRAPP techniques as a measure of interaction strength between particles and lipids. We found that positively charged particles do not adsorb on the bilayer, probably due to the electrostatic repulsion between positively charged parts of the lipid head and the positive groups on the particle surface, therefore no variation in diffusion coefficient of lipid molecules was observed. On the contrary, the negatively charged particles, driven by electrostatic interactions are adsorbed onto the supported bilayer. The adsorption of negatively charged particles increases with the zeta-potential of the particle. Consecutively, the diffusion coefficient of lipids is reduced probably due to binding onto the lipid heads which slows down their Brownian motion. The results are directly relevant for understanding the interactions of particulate matter with pulmonary structures which could lead to pulmonary surfactant inhibition or deficiency causing severe respiratory distress or pathologies.Graphical abstract

Self-lubricating composite bearings: Effect of fibre length on its tribological properties by DEM modelling

Self-lubricating polymer-based composites are used in space and in aircraft mechanisms as materials for solid lubricated systems. Such composites mostly consist of a polymeric matrix and fillers of two kinds: hard fillers (fibres made of glass, or of minerals) and solid lubricating particles (made of MoS 2). Their advantages are that they provide their own lubrication, and they can be used in both very high and very low temperatures (from −40 up to ~200 F). Precision ball bearings with these composites are manufactured since the 60s in these bearings the retainer material itself provides the lubrication. From the experimental analyses implemented (X-ray tomography, SEM observations, and experiences in a tribometer); it is possible to observe that the geometry of the fillers has a strong influence on the third body rheology. Nevertheless, the confined nature of the contact does not allow in-situ observation. To overcome this difficulty a combined numerical/experimental approach is carried out. To be able to reproduce the evolution of third-body particles within the contact, Discrete Element Methods (DEM) is used. Such an approach allows to represent wear: by the construction of an equivalent continuous medium resulting from the incorporation of interaction laws between the discrete particles. The motivation to this work is the understanding of the impact of filler geometry o tribological behaviour of these materials. More specifically, the goal is to study the influence of the fibre length in the tribological behaviour of self-lubricating composites by Discrete Element Methods (DEM).

Role of Third Body on Bolted Joints' Self-Loosening

Bolted joints are frequently subjected to self-loosening (gradual loss of clamping force) causing multiple failures, especially leaking and breaking of mechanical systems. Such physical phenomena would occur whatever the considered coating (Ag, MoS2, Zn–Ni and others). To enlighten this phenomenon, which remains rather misunderstood due to the confined nature of bolted joint contacts, a coupled experimental-numerical approach is adopted on a bolted joint with silver coating. Indeed, from tribological expert assessments of disassembled joints without loosening, a local view of nut/screw threads contacts is proposed, using discrete element method. This method becomes essential in tribology since it offers the ability to model the dynamic behavior of a contact interface. The model is based on a Non-Smooth Contacts Dynamics approach. The case of third body formed in contacts during tightening process, which has been ignored so far, is placed at the focus of self-loosening phenomenon.