Sabine Rolland du Roscoat

Reversible dilatancy in entangled single-wire materials

Designing structures that dilate rapidly in both tension and compression would benefit devices such as smart filters, actuators or fasteners. This property however requires an unusual Poisson ratio, or Poisson function at finite strains, which has to vary with applied strain and exceed the familiar bounds: less than 0 in tension and above 1/2 in compression. Here, by combining mechanical tests and discrete element simulations, we show that a simple three-dimensional architected material, made of a self-entangled single long coiled wire, behaves in between discrete and continuum media, with a large and reversible dilatancy in both tension and compression. This unusual behaviour arises from an interplay between the elongation of the coiled wire and rearrangements due to steric effects, which, unlike in traditional discrete media, are hysteretically reversible when the architecture is made of an elastic fibre.

3D analysis of paper microstructures at the scale of fibres and bonds

The evolution of paper microstructure parameters, such as porosity and fibre orientation, as a function of papermaking conditions is most often studied at a macroscopic scale. However, modelling the physical and mechanical properties of papers using upscaling approaches requires understanding the deformation micro-mechanisms that are induced by papermaking operations within the structure of paper fibrous networks for individual fibres and fibre-to-fibre bonds. We addressed this issue by analysing three-dimensional images of model papers. These images were obtained using X-ray microtomography. The model papers were fabricated by varying forming, pressing, and drying conditions. For each image, this analysis enabled an unprecedented large set of geometrical parameters to be measured for individual fibres (centreline, shape and inclination of the fibre cross sections) and fibre-to-fibre bonds (inter-bond distance, number of bonds per unit length of fibre, bond surface area) within the fibrous networks. The evolution of the as-obtained microstructure parameters was analysed as a function of papermaking conditions. All results were in accordance with the data available in the literature. A key result was obtained for the evolution of the number of fibre-to-fibre contacts per fibre as a function of the network density. A representative number of contacts was obtained using relatively small imaged volumes. These volumes must only contain enough fibre segments the cumulated length of which is of the same order as the mean fibre length. These results were also used to validate microstructure models for the prediction of the number of fibre-to-fibre contacts within fibrous networks.

Patient-specific simulation of endovascular repair surgery with tortuous aneurysms requiring flexible stent-grafts

The rate of post-operative complications is the main drawback of endovascular repair, a technique used to treat abdominal aortic aneurysms. Complex anatomies, featuring short aortic necks and high vessel tortuosity for instance, have been proved likely prone to these complications. In this context, practitioners could benefit, at the preoperative planning stage, from a tool able to predict the post-operative position of the stent-graft, to validate their stent-graft sizing and anticipate potential complications. In consequence, the aim of this work is to prove the ability of a numerical simulation methodology to reproduce accurately the shapes of stent-grafts, with a challenging design, deployed inside tortuous aortic aneurysms. Stent-graft module samples were scanned by X-ray microtomography and subjected to mechanical tests to generate finite-element models. Two EVAR clinical cases were numerically reproduced by simulating stent-graft models deployment inside the tortuous arterial model generated from patient pre-operative scan. In the same manner, an in vitro stent-graft deployment in a rigid polymer phantom, generated by extracting the arterial geometry from the preoperative scan of a patient, was simulated to assess the influence of biomechanical environment unknowns in the in vivo case. Results were validated by comparing stent positions on simulations and post-operative scans. In all cases, simulation predicted stents deployed locations and shapes with an accuracy of a few millimetres. The good results obtained in the in vitro case validated the ability of the methodology to simulate stent-graft deployment in very tortuous arteries and led to think proper modelling of biomechanical environment could reduce the few local discrepancies found in the in vivo case. In conclusion, this study proved that our methodology can achieve accurate simulation of stent-graft deployed shape even in tortuous patient specific aortic aneurysms and may be potentially helpful to help practitioners plan their intervention.

Computational fluid dynamics of cerebral aneurysm coiling using high-resolution and high-energy synchrotron X-ray microtomography: comparison with the homogeneous porous medium approach

Background Computational modeling of intracranial aneurysms provides insights into the influence of hemodynamics on aneurysm growth, rupture, and treatment outcome. Standard modeling of coiled aneurysms simplifies the complex geometry of the coil mass into a homogeneous porous medium that fills the aneurysmal sac. We compare hemodynamics of coiled aneurysms modeled from high-resolution imaging with those from the same aneurysms modeled following the standard technique, in an effort to characterize sources of error from the simplified model. Materials Physical models of two unruptured aneurysms were created using three-dimensional printing. The models were treated with coil embolization using the same coils as those used in actual patient treatment and then scanned by synchrotron X-ray microtomography to obtain high-resolution imaging of the coil mass. Computational modeling of each aneurysm was performed using patient-specific boundary conditions. The coils were modeled using the simplified porous medium or by incorporating the X-ray imaged coil surface, and the differences in hemodynamic variables were assessed. Results X-ray microtomographic imaging of coils and incorporation into computational models were successful for both aneurysms. Porous medium calculations of coiled aneurysm hemodynamics overestimated intra-aneurysmal flow, underestimated oscillatory shear index and viscous dissipation, and over- or underpredicted wall shear stress (WSS) and WSS gradient compared with X-ray-based coiled computational fluid dynamics models. Conclusions Computational modeling of coiled intracranial aneurysms using the porous medium approach may inaccurately estimate key hemodynamic variables compared with models incorporating high-resolution synchrotron X-ray microtomographic imaging of complex aneurysm coil geometry.

Development and evaluation of an experimental protocol for 3-D visualization and characterization of the structure of bacterial biofilms in porous media using laboratory X-ray tomography.

Abstract The development of a reliable model allowing accurate predictions of biofilm growth in porous media relies on a good knowledge of the temporal evolution of biofilm structure within the porous network. Since little is known about the real 3-D structure of biofilms in porous media, this work was aimed at developing a new experimental protocol to visualize the 3-D microstructure of the inside of a porous medium using laboratory X-ray microtomography. A reliable and reproducible methodology is proposed for (1) growing a biofilm inside a porous medium, and (2) X-ray tomography-based characterization of the temporal development of the biofilm at the inlet of the biofilter. The statistical analysis proposed here also validates the results presented in the literature based on a biofilm structure single measurement.

Papers as Functional Green Materials

Paper is constituted of natural fibers and represents a perfect example of structural multifunctional materials. Indeed, its fibrous structure is engineered to fit the different end use properties: both optical and mechanical properties are usually required. These requirements may lead to contradictory needs in terms of structure. The influence of the structure on the physical properties is classically tackled based on standard methods such as the estimation of the porosity. However, this macroscopic property is not sufficient in terms of optimization of the fibrous network. For example, fluid transport has to be controlled either in the bulk of the material or only at its surface in the case of health or printing applications. Consequently, the characterization at the macro-level of the structure has to be complemented with an experimental measurement at the fiber level. The X-ray synchrotron micro-tomography, an imaging technique, is based on X-ray transmission. It allows the structure to be analyzed in 3D. It was carried in a large instrument (ESRF, France). The characterization of samples containing different recycled fibers was carried out. In particular, the influence of the number of cycles of drying-pulping is studied. Both qualitative and quantitative characterizations are obtained. The use of recycled fibers may also be included in the elaboration of materials, taking into account the modification of the fibers in terms of morphology and mechanical properties, essentially flexibility. Mechanical properties (tensile and deformation) constitute the main examples of the analysis showing the effect of the recycling of natural fibers: the decrease in mechanical resistance of the fibrous network is explained in terms of the increase of the global porosity, essentially in the bulk of the materials. The profile of porosity in the thickness direction is found to be essential to understand the evolution of physical properties.

3D multiscale imaging of human vocal folds using synchrotron X-ray microtomography in phase retrieval mode

Human vocal folds possess outstanding abilities to endure large, reversible deformations and to vibrate up to more than thousand cycles per second. This unique performance mainly results from their complex specific 3D and multiscale structure, which is very difficult to investigate experimentally and still presents challenges using either confocal microscopy, MRI or X-ray microtomography in absorption mode. To circumvent these difficulties, we used high-resolution synchrotron X-ray microtomography with phase retrieval and report the first ex vivo 3D images of human vocal-fold tissues at multiple scales. Various relevant descriptors of structure were extracted from the images: geometry of vocal folds at rest or in a stretched phonatory-like position, shape and size of their layered fibrous architectures, orientation, shape and size of the muscle fibres as well as the set of collagen and elastin fibre bundles constituting these layers. The developed methodology opens a promising insight into voice biomechanics, which will allow further assessment of the micromechanics of the vocal folds and their vibratory properties. This will then provide valuable guidelines for the design of new mimetic biomaterials for the next generation of artificial larynges.

Three-dimensional visualization and quantification of the fracture mechanisms in sparse fibre networks using multiscale X-ray microtomography

The structural changes that are induced by the initiation and the propagation of a crack in a low-density paper (LDP) were studied using single edge-notched fracture tests that were imaged under an optical microscope or in laboratory or synchrotron X-ray microtomographs. The two-dimensional optical images were used to analyse the links between the mesoscale structural variations of LDP and the crack path. Medium-resolution X-ray three-dimensional images were used to analyse the variations in the thickness and local porosity of samples as well as their displacement field that were induced by the LDP fracture. High-resolution three-dimensional images showed that these mesostructural variations were accompanied by complex fibre and bond deformation mechanisms that were, for the first time, in situ imaged. These mechanisms occurred in the fracture process zone that developed ahead of the crack tip before the crack path became distinct and visible. They were at the origin of the aforementioned thickness variations that developed more particularly along the crack path. They eventually led to fibre–fibre bond detachment phenomena and crack propagation through the fibrous network. These results can be used to enhance the current structural and mechanical models for the prediction of the fracture behaviour of papers.