Study of Mechanical Properties of PHBHV/Miscanthus Green Composites Using Combined Experimental and Micromechanical Approaches.

In recent years the interest in the realization of green wood plastic composites (GWPC) materials has increased due to the necessity of reducing the proliferation of synthetic plastics. In this work, we study a specific class of GWPCs from its synthesis to the characterization of its mechanical properties. These properties are related to the underlying microstructure using both experimental and modeling approaches. Different contents of Miscanthus giganteus fibers, at 5, 10, 20, 30 weight percent's, were thus combined to a microbial matrix, namely poly (3-hydroxybutyrate)-co-poly(3-hydroxyvalerate) (PHBHV). The samples were manufactured by extrusion and injection molding processing. The obtained samples were then characterized by cyclic-tensile tests, pycnometer testing, differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction, and microscopy. The possible effect of the fabrication process on the fibers size is also checked. In parallel, the measured properties of the biocomposite were also estimated using a Mori-Tanaka approach to derive the effective behavior of the composite. As expected, the addition of reinforcement to the polymer matrix results in composites with higher Young moduli on the one hand, and lower failure strains and tensile strengths on the other hand (tensile modulus was increased by 100% and tensile strength decreased by 23% when reinforced with 30 wt % of Miscanthus fibers).

Biocomposites Based on Poly(3-Hydroxybutyrate-co-3-Hydroxyvalerate) (PHBHV) and Miscanthus giganteus Fibers with Improved Fiber/Matrix Interface.

In this paper, green biocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) and Miscanthus giganteus fibers (MIS) were prepared in the presence of dicumyl peroxide (DCP) via reactive extrusion. The objective of this study was to optimize the interfacial adhesion between the reinforcement and the matrix, improving the mechanical properties of the final material. To this aim, two fibers mass fractions (5 and 20 wt %) and two different fiber sizes obtained by two opening mesh sieves (1 mm and 45 µm) were investigated. The impregnation of fibers with DCP before processing was carried out in order to promote the PHBHV grafting onto MIS fibers during the process, favoring, in this way, the interfacial adhesion between fibers and matrix, instead of the crosslinking of the matrix. All composites were realized by extrusion and injection molding processing and then characterized by tensile tests, FTIR-ATR, SEM, DSC and XRD. According to the improved adhesion of fibers to matrix due to DCP, we carried out an implementation of models involving that can predict the effective mechanical properties of the biocomposites. Three phases were taken into account here: fibers, gel (crosslinked matrix), and matrix fractions. Due to the complexity of the system (matrix⁻crosslinked matrix⁻fibers) and to the lack of knowledge about all the phenomena occurring during the reactive extrusion, a mathematical approach was considered in order to obtain information about the modulus of the crosslinked matrix and its fraction in the composites. This study aims to estimate these last values, and to clarify the effect caused by the presence of vegetal fibers in a composite in which different reactions are promoted by DCP.

Melanin-concentrating hormone regulates beat frequency of ependymal cilia and ventricular volume.

Ependymal cell cilia help move cerebrospinal fluid through the cerebral ventricles, but the regulation of their beat frequency remains unclear. Using in vitro, high-speed video microscopy and in vivo magnetic resonance imaging in mice, we found that the metabolic peptide melanin-concentrating hormone (MCH) positively controlled cilia beat frequency, specifically in the ventral third ventricle, whereas a lack of MCH receptor provoked a ventricular size increase.

Periosteum, bone's "smart" bounding membrane, exhibits direction-dependent permeability.

The periosteum serves as bone's bounding membrane, exhibits hallmarks of semipermeable epithelial barrier membranes, and contains mechanically sensitive progenitor cells capable of generating bone. The current paucity of data regarding the periosteum's permeability and bidirectional transport properties provided the impetus for the current study. In ovine femur and tibia samples, the periosteum's hydraulic permeability coefficient, k, was calculated using Darcy's Law and a custom-designed permeability tester to apply controlled, volumetric flow of phosphate-buffered saline through periosteum samples. Based on these data, ovine periosteum demonstrates mechanically responsive and directionally dependent (anisotropic) permeability properties. At baseline flow rates comparable to interstitial fluid flow (0.5 µL/s), permeability is low and does not exhibit anisotropy. In contrast, at high flow rates comparable to those prevailing during traumatic injury, femoral periosteum exhibits an order of magnitude higher permeability compared to baseline flow rates. In addition, at high flow rates permeability exhibits significant directional dependence, with permeability higher in the bone to muscle direction than vice versa. Furthermore, compared to periosteum in which the intrinsic tension (pre-stress) is maintained, free relaxation of the tibial periosteum after resection significantly increases its permeability in both flow directions. Hence, the structure and mechanical stress state of periosteum influences its role as bone's bounding membrane. During periods of homeostasis, periosteum may serve as a barrier membrane on the outer surface of bone, allowing for equal albeit low quiescent molecular communication between tissue compartments including bone and muscle. In contrast, increases in pressure and baseline flow rates within the periosteum resulting from injury, trauma, and/or disease may result in a significant increase in periosteum permeability and consequently in increased molecular communication between tissue compartments. Elucidation of the periosteum's permeability properties is key to understanding periosteal mechanobiology in bone health and healing, as well as to elucidate periosteum structure and function as a smart biomaterial that allows bidirectional and mechanically responsive fluid transport.

Interstitial fluid flow within bone canaliculi and electro-chemo-mechanical features of the canalicular milieu: a multi-parametric sensitivity analysis.

Canalicular fluid flow is acknowledged to play a major role in bone functioning, allowing bone cells' metabolism and activity and providing an efficient way for cell-to-cell communication. Bone canaliculi are small canals running through the bone solid matrix, hosting osteocyte's dendrites, and saturated by an interstitial fluid rich in ions. Because of the small size of these canals (few hundred nanometers in diameter), fluid flow is coupled with electrochemical phenomena. In our previous works, we developed a multi-scale model accounting for coupled hydraulic and chemical transport in the canalicular network. Unfortunately, most of the physical and geometrical information required by the model is hardly accessible by nowadays experimental techniques. The goal of this study was to numerically assess the influence of the physical and material parameters involved in the canalicular fluid flow. The focus was set on the electro-chemo-mechanical features of the canalicular milieu, hopefully covering any in vivo scenario. Two main results were obtained. First, the most relevant parameters affecting the canalicular fluid flow were identified and their effects quantified. Second, these findings were given a larger scope to cover also scenarios not considered in this study. Therefore, this study gives insight into the potential interactions between electrochemistry and mechanics in bone and provides the rational for further theoretical and experimental investigations.

Possible role of calcium permselectivity in bone adaptation.

According to the core activity of calcium in the bone cellular expression, a new hypothesis linking calcium transport with the mechanical loading is proposed to explain the mechano-adaptation of bone tissue. Due to the piezoelectric coupling, the tensile and compressive areas of bone produce different electrical environments for the osteocytic cells that are embedded in the lacuno-canalicular porosity. This electrical asymmetry engenders a calcium enrichment-exclusion effect that strongly changes the calcium concentration in the lacuno-canalicular fluid and thus modifies the remodelling process. A bibliographic body of evidence supporting this idea is given and its experimental validation is suggested.

Influence of interstitial bone microcracks on strain-induced fluid flow.

It is well known that microcracks act as a stimulus for bone remodelling, initiating resorption by osteoclasts and new bone formation by osteoblasts. Moreover, microcracks are likely to alter the fluid flow and convective transport through the bone tissue. This paper proposes a quantitative evaluation of the strain-induced interstitial fluid velocities developing in osteons in presence of a microcrack in the interstitial bone tissue. Based on Biot theory in the low-frequency range, a poroelastic model is carried out to study the hydro-mechanical behaviour of cracked osteonal tissue. The finite element results show that the presence of a microcrack in the interstitial osteonal tissue may drastically reduce the fluid velocity inside the neighbouring osteons. This fluid inactive zone inside osteons can cover up to 10% of their surface. Consequently, the fluid environment of bone mechano-sensitive cells is locally modified.

Poroelastic behaviour of cortical bone under harmonic axial loading: a finite element study at the osteonal scale.

Bone fluid flow and its induced effects on the bone cells are important players in triggering and signalling bone formation and bone remodelling. This study aims to numerically investigate the behaviour of interstitial fluid flows in cortical bone under axial cyclic harmonic loads that mimics in vivo bone behaviour during daily activities like walking. Here, bone tissue is modelled as a fluid-saturated anisotropic poroelastic medium which consists of a periodic group of osteons. By using a frequency-domain finite element analysis, the fluid velocity field is quantified for various loading conditions and bone matrix parameters.

Multiphysical modelling of fluid transport through osteo-articular media.

In this study, a multiphysical description of fluid transport through osteo-articular porous media is presented. Adapted from the model of Moyne and Murad, which is intended to describe clayey materials behaviour, this multiscale modelling allows for the derivation of the macroscopic response of the tissue from microscopical information. First the model is described. At the pore scale, electrohydrodynamics equations governing the electrolyte movement are coupled with local electrostatics (Gauss-Poisson equation), and ionic transport equations. Using a change of variables and an asymptotic expansion method, the macroscopic description is carried out. Results of this model are used to show the importance of couplings effects on the mechanotransduction of compact bone remodelling.

Multiphysical modelling of fluid transport through osteo-articular media

In this study, a multiphysical description of fluid transport through osteo-articular porous media is presented. Adapted from the model of Moyne and Murad, which is intended to describe clayey materials behaviour, this multiscale modelling allows for the derivation of the macroscopic response of the tissue from microscopical information. First the model is described. At the pore scale, electrohydrodynamics equations governing the electrolyte movement are coupled with local electrostatics (Gauss-Poisson equation), and ionic transport equations. Using a change of variables and an asymptotic expansion method, the macroscopic description is carried out. Results of this model are used to show the importance of couplings effects on the mechanotransduction of compact bone remodelling.

Neste estudo uma descrição multifísica do transporte de fluidos em meios porosos osteo articulares é apresentada. Adaptado a partir do modelo de Moyne e Murad proposto para descrever o comportamento de materiais argilosos a modelagem multiescala permite a derivação da resposta macroscópica do tecido a partir da informação microscópica. Na primeira parte o modelo é apresentado. Na escala do poro as equações da eletro-hidrodinâmica governantes do movimento dos eletrolitos são acopladas com a eletrostática local (equação de Gauss-Poisson) e as equações de transporte iônico. Usando uma mudança de variáveis e o método de expansão assintótica a derivação macroscópica é conduzida. Resultados do modelo proposto são usados para salientar a importância dos efeitos de acoplamento sobre a transdução mecânica da remodelagem de ossos compactados.

Study of the influence of fibrous pericellular matrix in the cortical interstitial fluid movement with hydroelectrochemical effects.

Fluid flow within cortical bone tissue is modeled through an upscaling approach of a local description of the fluid movement. At the pore scale, the coupled phenomena (Poiseuille effect, osmosis, and electro-osmosis) governing the interstitial fluid movement are considered. Thus, actions of electro-osmotic and osmotic motions, in addition to the classical Poiseuille flow, are studied at the canaliculus scale by deriving a coupled Darcy law. The addition of a Brinkman-like term in this macroscopic result helps us to take into account the influence of the pericellular matrix on the coupled transport phenomena. At the canaliculus scale, the general trends that can be drawn from this study are as follows: (i) The presence of the fibrous matrix tends to reduce the fluid flow considerably; (ii) the role of osmotic and electro-osmotic effects is no longer negligible for dense fibrous media.

Interstitial fluid flow in the osteon with spatial gradients of mechanical properties: a finite element study.

Bone remodelling is the process that maintains bone structure and strength through adaptation of bone tissue mechanical properties to applied loads. Bone can be modelled as a porous deformable material whose pores are filled with cells, organic material and interstitial fluid. Fluid flow is believed to play a role in the mechanotransduction of signals for bone remodelling. In this work, an osteon, the elementary unit of cortical bone, is idealized as a hollow cylinder made of a deformable porous matrix saturated with an interstitial fluid. We use Biot's poroelasticity theory to model the mechanical behaviour of bone tissue taking into account transverse isotropic mechanical properties. A finite element poroelastic model is developed in the COMSOL Multiphysics software. Elasticity equations and Darcy's law are implemented in this software; they are coupled through the introduction of an interaction term to obtain poroelasticity equations. Using numerical simulations, the investigation of the effect of spatial gradients of permeability or Poisson's ratio is performed. Results are discussed for their implication on fluid flow in osteons: (i) a permeability gradient affects more the fluid pressure than the velocity profile; (ii) focusing on the fluid flow, the key element of loading is the strain rate; (iii) a Poisson's ratio gradient affects both fluid pressure and fluid velocity. The influence of textural and mechanical properties of bone on mechanotransduction signals for bone remodelling is also discussed.

Multiscale analysis of the coupled effects governing the movement of interstitial fluid in cortical bone.

A multiscale approach (periodic homogenization) is carried out to model osteon's behaviour, and especially the coupled phenomena that govern its interstitial fluid movement. Actions of electro-osmotic and osmotic motions in addition to the classical Poiseuille flow are studied at the mesoscale of the canaliculus and within the micropores of the collagen-apatite matrix. Use of this fully coupled modelling leads to a comparison of these different effects. Limitation of a classical Darcian description of the fluid flow at the two scales is so studied. For each of these studies a special attention is given to the pore's geometry influence and to their electrical and hydraulic properties.