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The annulus fibrosus of the intervertebral disc exhibits an unusual transversal behavior for which a constitutive representation that considers as well regional effect, chemical sensitivity and time-dependency has not yet been developed, and it is hence the aim of the present contribution. A physically-based model is proposed by introducing a free energy function that takes into account the actual disc annulus structure in relation with the surrounding biochemical environment. The response is assumed to be dominated by the viscoelastic contribution of the extracellular matrix, the elastic contribution of the oriented collagen fibers and the osmo-induced volumetric contribution of the internal fluid content variation. The regional dependence of the disc annulus response due to variation in fibers content/orientation allows a micromechanical treatment of the soft tissue. A finite element model of the annulus specimen is designed while taking into consideration the 'interlamellar' ground substance zone between lamellae of the layered soft tissue. The kinetics is designed using full-field strain measurements performed on specimens extracted from two disc annulus regions and tested under different osmotic conditions. The time-dependency of the tissue response is reported on stress-free volumetric changes, on hysteretic stress and transversal strains during quasi-static stretching at different strain-rates and on their temporal changes during an interrupted stretching. Considering the effective contributions of the internal fluid transfer and the extracellular matrix viscosity, the microstructure-based chemo-mechanical model is found able to successfully reproduce the significant features of the macro-response and the unusual transversal behavior including the strong regional dependency from inner to outer parts of the disc: Poisson's ratio lesser than 0 (auxetic) in lamellae plane, higher than 0.5 in fibers plane, and their temporal changes towards usual values (between 0 and 0.5) at chemo-mechanical equilibrium. The underlying time-dependent mechanisms occurring in the tissue are analyzed via the local numerical fields and important insights about the effective role of the interlamellar zone are revealed for the different disc localizations. STATEMENT OF SIGNIFICANCE: The structural complexity of the annulus fibrosus has only been appreciated through recent experimental contributions and a constitutive representation that considers as well regional effect, chemical sensitivity and time-dependency of the unusual transversal behavior has not yet been developed. Here, a microstructure-based chemo-viscoelastic model is developed to highlight the interlamellar-induced time-dependent response by means of a two-scale strategy. The model provides important insights about the origin of the time-dependent phenomena in disc annulus along with regional dependency, essential for understanding disc functionality., (Copyright © 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

With their gradual and continuous properties, functionally graded polymers (FGP) have high potentials to reproduce the regional variation in microstructure/property of the natural intervertebral disc and, therefore, the functional anatomy and biomechanics of the soft tissue. This paper evaluates by finite element analysis the biomechanical response and stress distribution of a novel disc prosthesis using FGP. The kinetics of the FGP parameters is designed using experimental data issued from linear ethylene copolymers over a wide crystallinity range. The radial variation in crystallinity index within the disc prosthesis varies gradually and continuously following a special function in the aim to tailor and optimize the FGP parameters. The experimental data of a healthy human cervical spine segment are used to predict the optimal model of the FGP disc prosthesis loaded under different physiological loading conditions, i.e. rotation, lateral bending and flexion/extension. The results suggest that the FGP parameters can be tailored to control the stiffening, the non-linear behavior, the inelastic effects and the stress distribution in the aim to propose the optimal prosthesis model giving the great opportunity of patient-specific FGP prostheses via 3D printing technologies., (Copyright © 2019 Elsevier Ltd. All rights reserved.)