Damien Durville

A multilayer braided scaffold for Anterior Cruciate Ligament : mechanical modeling at the fiber scale

An adapted scaffold for Anterior Cruciate Ligament (ACL) tissue engineering must match biological, morphological and biomechanical requirements. Computer-aided tissue engineering consists of finding the most appropriate scaffold regarding a specific application by using numerical tools. In the present study, the biomechanical behavior of a new multilayer braided scaffold adapted to computer-aided tissue engineering is computed by using a dedicated Finite Element (FE) code. Among different copoly(lactic acid-co-(ε-caprolactone)) (PLCL) fibers tested in the present study, PLCL fibers with a lactic acid/ε-caprolactone ratio of 85/15 were selected as a constitutive material for the scaffold considering its strength and deformability. The mechanical behavior of these fibers was utilized as material input in a Finite Element (FE) code which considers contact/friction interactions between fibers within a large deformation framework. An initial geometry issued from the braiding process was then computed and was found to be representative of the actual scaffold geometry. Comparisons between simulated tensile tests and experimental data show that the method enables to predict the tensile response of the multilayer braided scaffold as a function of different process parameters. As a result, the present approach constitutes a valuable tool in order to determine the configuration which best fits the biomechanical requirements needed to restore the knee function during the rehabilitation period. The developed approach also allows the mechanical stimuli due to external loading to be quantified, and will be used to perform further mechanobiological analyses of the scaffold under dynamic culture.

A Finite Element Approach of the Behaviour of Woven Materials at Microscopic Scale

A finite element simulation of the mechanical behaviour of woven textile materials at the scale of individual fibers is proposed in this paper. The aim of the simulation is to understand and identify phenomena involved at different scales in such materials. The approach considers small patches of woven textile materials as collections of fibers. Fibers are modelled by 3D beam elements, and contact-friction interactions are considered between them. An original method for the detection of contacts, and the use of efficient algorithms to solve the nonlinearities of the problem, allow to handle patches made of few hundreds of fibers. The computation of the unknown initial configuration of the woven structure is carried out by simulating the weaving process. Various loading cases can then be applied to the studied patches to identify their mechanical characteristics. To approach the mesoscopic behaviour of yarns, 3D strains are calculated at the scale of yarns, as post-processing. These strains display strong inhomogeinities, which raises the question of using continuous models at the scale of yarns.

Mechanical-Electrical Modeling of Stretching Experiment on 45

Cable-In-Conduit Conductors made with Nb3Sn strands will be used in ITER magnets. The current carrying capability of these Nb3Sn strands is known to be highly dependant on the strain state resulting from mechanical loading. The intricate cabling pattern of CICC, added to the thermal differential shrinkage between conductor jacket and Nb3Sn filaments induce complex strand trajectories and a highly inhomogeneous strain state. This “cable strain map” also evolves with operating loads (Lorentz force/hoop stress). The SAMAN experiment, conducted in the FBI facility at Karlsruhe Institute of Technology, aimed to stretch subsize, ITER-like conductors, in order to observe the evolution of the critical current associated with these loadings. The application of the Multifil finite element code, developed at Ecole Centrale de Paris, has helped quantifying the local strains along every individual strand, and their evolutions during cooldown (from heat treatment), energizing and stretching phenomena. Using Multifil output mechanical data as input in the CEA electrical code CARMEN has allowed computing the critical current in every strand, thus leading to an understanding of the critical current degradation of such subsize conductors. This paper shows, for two SAMAN samples, what is the impact of bending strain concentration on a CICC current transport capability.

{\rm Nb}_{3}{\rm Sn}

The use of biodegradable scaffolds seeded with cells in order to regenerate functional tissue-engineered substitutes offers interesting alternative to common medical approaches for ligament repair. Particularly, finite element (FE) method enables the ability to predict and optimise both the macroscopic behaviour of these scaffolds and the local mechanic signals that control the cell activity. In this study, we investigate the ability of a dedicated FE code to predict the geometrical evolution of a new braided and biodegradable polymer scaffold for ligament tissue engineering by comparing scaffold geometries issued from FE simulations and from X-ray tomographic imaging during a tensile test. Moreover, we compare two types of FE simulations the initial geometries of which are issued either from X-ray imaging or from a computed idealised configuration. We report that the dedicated FE simulations from an idealised reference configuration can be reasonably used in the future to predict the global and local mechanical behaviour of the braided scaffold. A valuable and original dialog between the fields of experimental and numerical characterisation of such fibrous media is thus achieved. In the future, this approach should enable to improve accurate characterisation of local and global behaviour of tissue-engineering scaffolds.

Strands CICCs

An approach to model contact-friction interactions between beams within assemblies of fibers is presented in this paper in order to simulate the mechanical behaviour of entangled structures at the scale of individual fibers using the finite element method. The determination of contact elements associating pairs of material particles is based on the detection of proximity zones between beams and on the construction of intermediate geometries approximating the actual contact zone, and allowing to consider contact along zones of non-zero lengths. The penalty method for contact is improved by adjusting the penalty parameter for each contact zone, thus stabilizing contact algorithms and allowing to handle high numbers of contact elements. Applications to samples of textile materials involving few hundreds of fibers are presented to demonstrate the abilities of the method. The presented examples are related to the simulation of woven fabrics – computation of the initial configuration and application of test loadings – and the identification of the transverse mechanical behaviour of a twisted textile yarn.

Mechanical behaviour of a fibrous scaffold for ligament tissue engineering: Finite elements analysis vs. X-ray tomography imaging

A simulation of the mechanical behaviour of textile composites at the scale of fibers is presented in this article. The approach, based on a finite element code with an implicit solver, focuses on the taking into account of contact-friction interactions appearing in assemblies of fibers undergoing large transformations. It allows, in a first step, to compute the unknown initial configuration of any woven structure. Then, adding an elastic matrix to the fabric, various loading tests can be simulated in order to identify mechanical properties of composite materials.

Contact Modelling in Entangled Fibrous Materials

Tissue engineering has the potential to overcome the limitations associated with current reconstructions strategies of the Anterior Cruciate Ligament (ACL). However, the design of a scaffold satisfying the key requirements associated with ACL tissue engineering is a challenging task. In order to avoid a costly trial-and-error approach, computer-based methods have been widely used in the case of various applications such as bone or cartilage. These methods can help to define the best scaffold and culture conditions for a given list of criteria, and may also enable to predict the ultimate evolution of the scaffold and to better understand some mechanobiological principles. Some of these methods are reviewed in the current chapter, and are applied for the first time in the case of ACL tissue engineering. The morphological and mechanical properties of a new scaffold based on copoly(lactic acid-co-(\(\upvarepsilon \)-caprolactone)) (PLCL) fibers arranged into a multilayer braided structure will be assessed using dedicated numerical tools. Preliminary biological assessments are also presented, and some conclusions concerning the suitability of the scaffold and the interest of CATE in this case will be drawn.

Finite Element Simulation of the Mechanical Behaviour of Textile Composites at the Mesoscopic Scale of Individual Fibers

Nb3Sn is now commonly used in the design of high-field large-scale magnets. However, it is a brittle material, the superconducting properties of which degrade under mechanical strain. Both ITER TF and CS magnets make use of Nb3Sn strands in cable-in-conduit conductors. Experiments have been carried out in the TARSIS facility at University of Twente aiming at measuring the strand critical current as a function of periodically applied strain/stress. Until recently, these experiments have given good indications of the strand behavior, but they had not been fully understood because of the lack of an accurate description of the local strain along the tested strand. Furthermore, they cannot be extrapolated directly to a real cable-in-conduit conductor because they do not simulate the differential thermal contraction, which puts the strand under longitudinal compression. Using the mechanical code MULTIFIL developed at Ecole Centrale de Paris, associated with the electrical code CARMEN developed at CEA/IRFM, this paper aims at understanding the mechanisms of the critical current reduction during a TARSIS experiment by coupling the local strain map of the strand to the complex current paths between Nb3Sn filaments. Comparison with experimental results and with analytic limiting cases are presented and discussed.

Computer-Aided Tissue Engineering: Application to the Case of Anterior Cruciate Ligament Repair

Abstract.The mechanical behaviour of a wire rope depends highly on the interactions between its elementary wires. We suggest to model this behaviour by the mean of the finite element method, consid...

Coupled Mechanical-Electrical Modeling of the TARSIS Experiment

The purpose of the present work is to understand physical phenomena occurring in roving structures under transverse compression. In order to reach this aim, transverse behavior of prototype rovings featuring a small number of polyamide 6.6 filaments are studied using an experimental device developed in our laboratory. Moreover, the effect of roving characteristics on their transverse compression behavior is examined. The studied characteristics are filament diameter and number, roving twist and tension. It is found that the roving behavior under compression shows plateaus separated by a significant increase of compression force, indicating discontinuous changes in the roving structures. This fact may be attributed to a reorganization of rovings followed by a local slippage between filaments. Transverse properties of rovings are affected by contact-friction inter filaments and the initial filament section fraction. In fact, it is more difficult to compact high-twisted rovings. Rovings with a greater number of filaments require a higher force in order to be compacted. The pre-tension of the rovings has no noticeable effect on their compression behavior.