The two Poisson’s ratios in annulus fibrosus: relation with the osmo-inelastic features

Abstract

The annulus fibrosus of the intervertebral disc is a highly complex layered structure in which the inelastic features of the tangled extracellular matrix interact with the surrounding physiological fluid by osmotic effect. In this in vitro study, the time-dependent transversal behavior in the two planes (fibers and lamellae planes) of multi-lamellae annulus tissues is reported by means of an accurate optical strain measuring technique based upon digital image correlation. Fresh annulus specimens of square cross section, extracted from bovine cervical discs, are tested under quasi-static (cyclic uniaxial stretching) and relaxation (interrupted stretching) loading with variation in osmolarity and strain rate conditions. Significant osmotic and strain rate effects are found on the elastic stiffness and the apparent Poisson's ratios (p < 0.05, ANOVA). Under quasi-static loading, the apparent Poisson's ratio is found higher than 0.5 in fibers plane and negative (i.e., auxetic) in lamellae plane. This material property evolves progressively towards classical bounds with relaxation time, i.e., between 0 and 0.5. The strong dependence of the auxetic behavior on time and chemical environment provides valuable insights about internal fluid exchanges. An interpretation of the osmo-inelastic mechanisms is proposed in which mechanical-based and chemical-based fluid flow interact until chemo-mechanical equilibrium. The new information allows a better understanding of the disc functionality and must be considered in accurate modeling of the disc annulus.

Interlamellar matrix governs human annulus fibrosus multiaxial behavior

“…The three sets of experiments are replicated in-silico using the finite element method according to the experimental recommendations available in the literature regarding specimen shapes, sizes and mechanical conditions in terms of strain rate and maximum strain. As observed in the works of Gregory and Callaghan 49 and Derrouiche et al 45 the strain rate has an effect on the stress-strain and transversal responses of the multilamellar annulus specimen. As a consequence, for all the simulated tests, quasi-static conditions were ensured by using strain rates of 0.002 s −1 and 0.0002 s −1 depending on the strain rate level of the reproduced experimental test.…”

“…All previous layered models considered these zones as common sliding zones separating the lamellae but the role of the ILM zones does not stop trivially there. Their essential role as chemical actuator of the transfer fluid through the annulus tissue and the corresponding unusual transversal behavior was only appreciated through very recent contributions 17,18,45 . In the present paper, we highlighted for the first time the determining role of the ILM zones on the multiaxial response of the annulus.…”

Structural Mechanisms in Soft Fibrous Tissues: A Review

“…Engineering these mechanical motifs in simpler material systems will also allow us to isolate the effect of the structural motif on the mechanical behavior and explain how these structural elements interact in an assembly to generate the required function. Although the biological aspect is very important in this area, it was widely discussed in other reviews (Dhandayuthapani et al, 2011;Nikolova and Chavali, 2019). Herein, we will further discuss the latest advances in soft tissue biomimetics from the material and mechanical point of view.…”

“…Soft tissues are also characterized by viscoelasticity, prominent fatigue life, and exceptional tear resistance. Some tissues also demonstrate unique Poisson behaviors, such as Poisson value larger than one and negative Poissons (auxetic behavior) (Derrouiche et al, 2020;Pissarenko and Meyers, 2020). This review will mainly focus on the large deformation behaviors.…”

“…Biomaterials for soft tissue engineering are extensively explored and developed. These materials cover multiple aspects; such as cell adhesion, biocompatibility, biodegradability, bioprinting, structural hierarchy, and mechanical compatibility (Venkatraman et al, 2008;Dhandayuthapani et al, 2011;Mazza and Ehret, 2015;Nikolova and Chavali, 2019;Buwalda, 2020;Guimarães et al, 2020). Herein, we will focus on the biomimetics of structural motifs and their effect on the mechanical compatibility of the materials.…”