Cédric Laurent

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.

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

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.

New method for 3D reconstruction of the human cranial vault from CT-scan data

This study presents a new method for the 3D reconstruction of the human cranial vault from routine Computed Tomography (CT) data. The reconstruction method was based on the conceptualization of the shape of the cranial vault with a parametric description. An initialization was first realized with the identification of anatomical landmarks and contours on Digitally Reconstructed Radiographs (DRR) in order to obtain a pre-personalized reconstruction. Then an optimization of the reconstruction was performed to segment the internal and external surfaces of the cranial vault for thickness computation. The method was validated by comparing final reconstructions issued from our approach and from a manual slice-by-slice segmentation method on ten CT-scans. Errors were comparable to the CT image resolution, and less than 2 min were dedicated to the operator-dependent marking step. The reconstruction of internal and external surfaces of the cranial vault allows quantifying and visualizing of thickness throughout the cranial vault. This thickness mapping is useful for clinical purposes as additional pre-surgical information. Moreover, this study constitutes a first step in the personalized characterization of skull resistance directly from routine exams.

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

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.

Designing a new scaffold for anterior cruciate ligament tissue engineering

Designing a new scaffold for anterior cruciate ligament tissue engineering C. Laurent a , D. Durville b , X. Wang a c , J.F. Ganghoffer a & R. Rahouadj a a LEMTA, Nancy-Universite, CNRS UMR 7563, 2 Avenue Foret de Haye, 54504, Vandœuvreles-Nancy, France b Laboratoire de Mecanique des Sols Structures et Materiaux (LMSSM), CNRS UMR8579 – Ecole Centrale Paris, Grande Voie des Vignes, 92290, Chatenay-Malabry, France c Faculte de Medecine, PPIA, Nancy-Universite, CNRS UMR 7561, 54504, Vandœuvre-lesNancy, France

Suitability of a PLCL fibrous scaffold for soft tissue engineering applications: A combined biological and mechanical characterisation:

Poly(lactide-co-ε-caprolactone) (PLCL) has been reported to be a good candidate for tissue engineering because of its good biocompatibility. Particularly, a braided PLCL scaffold (PLL/PCL ratio = 85/15) has been recently designed and partially validated for ligament tissue engineering. In the present study, we assessed the in vivo biocompatibility of acellular and cellularised scaffolds in a rat model. We then determined its in vitro biocompatibility using stem cells issued from both bone marrow and Wharton Jelly. From a biological point of view, the scaffold was shown to be suitable for tissue engineering in all these cases. Secondly, while the initial mechanical properties of this scaffold have been previously reported to be adapted to load-bearing applications, we studied the evolution in time of the mechanical properties of PLCL fibres due to hydrolytic degradation. Results for isolated PLCL fibres were extrapolated to the fibrous scaffold using a previously developed numerical model. It was shown that no accumulation of plastic strain was to be expected for a load-bearing application such as anterior cruciate ligament tissue engineering. However, PLCL fibres exhibited a non-expected brittle behaviour after two months. This may involve a potential risk of premature failure of the scaffold, unless tissue growth compensates this change in mechanical properties. This combined study emphasises the need to characterise the properties of biomaterials in a pluridisciplinary approach, since biological and mechanical characterisations led in this case to different conclusions concerning the suitability of this scaffold for load-bearing applications.

An Attempt to Predict the Preferential Cellular Orientation in Any Complex Mechanical Environment

Cells respond to their mechanical environment in different ways: while their response in terms of differentiation and proliferation has been widely studied, the question of the direction in which cells align when subject to a complex mechanical loading in a 3D environment is still widely open. In the present paper, we formulate the hypothesis that the cells orientate in the direction of unitary stretch computed from the right Cauchy-Green tensor in a given mechanical environment. The implications of this hypothesis are studied in different simple cases corresponding to either the available in vitro experimental data or physiological conditions, starting from finite element analysis results to computed preferential cellular orientation. The present contribution is a first step to the formulation of a deeper understanding of the orientation of cells within or at the surface of any 3D scaffold subject to any complex load. It is believed that these initial preferential directions have strong implications as far as the anisotropy of biological structures is concerned.

Prediction of the mechanical response of canine humerus to three-point bending using subject-specific finite element modelling

Subject-specific finite element models could improve decision making in canine long-bone fracture repair. However, it preliminary requires that finite element models predicting the mechanical response of canine long bone are proposed and validated. We present here a combined experimental–numerical approach to test the ability of subject-specific finite element models to predict the bending response of seven pairs of canine humeri directly from medical images. Our results show that bending stiffness and yield load are predicted with a mean absolute error of 10.1% (±5.2%) for the 14 samples. This study constitutes a basis for the forthcoming optimization of canine long-bone fracture repair.

Prediction of the morphological and mechanical properties of a novel scaffold for anterior cruciate ligament tissue engineering

Tissue-engineered solutions offer a good alternative to classical surgeries in the case of anterior cruciate ligament (ACL) injuries. Tissue engineering is based on the culture of reparative cells into a scaffold, whose properties must meet several key requirements such as material biocompatibility, biodegradability at a rate matching that of tissue formation, mechanical properties matching those of the native tissue and high porosity to allow cell infiltration (Vunjak-Novakovic et al. 2004). In particular, a threedimensional porous structure flexible in the low range of strain but very stiff in the large range of strain is needed. The ideal properties to match include stiffness up to 200 N/mm, strain to failure of more than 20%, a minimum pore size of 200–250mm for soft tissue ingrowth and a clearly convex force–displacement (or stress–strain) response (Vieira et al. 2009). We propose a tailorable scaffold based on multilayer braided fibres made of copoly(lactic acid-co-(1-caprolactone)) (PLCL). This scaffold is highly adjustable by playing on someparameters of the custom set-up designed to process this scaffold, such as the fibres diameter, the number of layers in the structure or the pitch length of the braid, as well as by changing the lactic acid/1-caprolactone proportions in the copolymer. The objective of the present work was to predict numerically the effect of these parameters on the morphological and mechanical properties of the scaffold, in view of finding the set of parameters which permits tomatch the required properties for ACL reconstruction.

Mesenchymal stem cell interacted with PLCL braided scaffold coated with poly-l-lysine/hyaluronic acid for ligament tissue engineering: MESENCHYMAL STEM CELL INTERACTED WITH PLCL BRAIDED SCAFFOLD

The challenge of finding an adapted scaffold for ligament tissue engineering remains unsolved after years of researches. A technology to fabricate a multilayer braided scaffold with flexible and elastic poly (l-lactide-co-caprolactone) (PLCL 85/15) has been recently pioneered by our team. In this study, polyelectrolyte multilayer films (PEM) with poly-l-lysine (PLL)/ hyaluronic acid (HA) were deposited on this scaffold. After PEM modification, polygonal (PLL) and particle-like (HA) structures were present on the braided scaffold with no significant variation of fibers Youngs modulus. Whartons jelly mesenchymal stem cells (WJ-MSC) and bone marrow mesenchymal stem cells (BM-MSC) showed good metabolic activity on scaffolds. They presented a spindled shape along the fiber longitudinal direction, and crossed the fibers to form cell bridges. Collagen type I, collagen type III, and tenascin-C secreted by MSCs were detected on day 14. Moreover, one-layer modified scaffold presented increased chemotaxis. As a conclusion, our results indicate that this braided PLCL scaffold with one-layer PEM modification shows inspiring potential with satisfying mechanical properties and biocompatibility. It opens new perspectives to incorporate growth factors within PEM-modified braided PLCL scaffold for ligament tissue engineering and to recruit endogenous cells after implantation.