Growing clinical evidence suggests a correlation between diabetes and more frequent and severe intervertebral disc failure, partially attributed to accelerated advanced glycation end-products (AGE) accumulation in the annulus fibrosus (AF) through non-enzymatic glycation. However, in vitro glycation (i.e., crosslinking) reportedly improved AF uniaxial tensile mechanical properties, contradicting clinical observations. Thus, this study used a combined experimental-computational approach to evaluate the effect of AGEs on anisotropic AF tensile mechanics, applying finite element models (FEMs) to complement experimental testing and examine difficult-to-measure subtissue-level mechanics. Methylglyoxal-based treatments were applied to induce three physiologically relevant AGE levels in vitro. Models incorporated crosslinks by adapting our previously validated structure-based FEM framework. Experimental results showed that a threefold increase in AGE content resulted in a ∼55% increase in AF circumferential-radial tensile modulus and failure stress and a 40% increase in radial failure stress. Failure strain was unaffected by non-enzymatic glycation. Adapted FEMs accurately predicted experimental AF mechanics with glycation. Model predictions showed that glycation increased stresses in the extrafibrillar matrix under physiologic deformations, which may increase tissue mechanical failure or trigger catabolic remodeling, providing insight into the relationship between AGE accumulation and increased tissue failure. Our findings also added to the existing literature regarding crosslinking structures, indicating that AGEs had a greater effect along the fiber direction, while interlamellar radial crosslinks were improbable in the AF. In summary, the combined approach presented a powerful tool for examining multiscale structure-function relationships with disease progression in fiber-reinforced soft tissues, which is essential for developing effective therapeutic measures. STATEMENT OF SIGNIFICANCE: Increasing clinical evidence correlates diabetes with premature intervertebral disc failure, likely due to advanced glycation end-products (AGE) accumulation in the annulus fibrosus (AF). However, in vitro glycation reportedly increases AF tensile stiffness and toughness, contradicting clinical observations. Using a combined experimental-computational approach, our work shows that increases in AF bulk tensile mechanical properties with glycation are achieved at the risk of exposing the extrafibrillar matrix to increased stresses under physiologic deformations, which may increase tissue mechanical failure or trigger catabolic remodeling. Computational results indicate that crosslinks along the fiber direction account for 90% of the increased tissue stiffness with glycation, adding to the existing literature. These findings provide insight into the multiscale structure-function relationship between AGE accumulation and tissue failure.

Functionally gradient materials, characterized by a smooth compositional change over space, have gained attention in recent years due to their tunable properties and ability to reduce mechanical stress concentrations. Specifically, attaining high stiffness without compromising toughness is a challenge, as the properties are inversely related. Varying moduli gradients, as seen abundantly in biological materials, do not present this trade-off, facilitating increased rigidity without altering maximum elongation. Herein, the application of stiffness gradients in porous structures is investigated through compressive mechanical testing, computerized tomography imaging, and finite element modeling. Gradients are compared to several alternative designs to investigate the importance of a smooth material transition and increased radial stiffness. All groups are compressed to 50% strain and the pre- and postpore closure modulus, toughness, and dimensional change in the transverse and radial directions are measured. The findings indicate that a mechanical gradient with increasing radial stiffness allows for balance of mechanical integrity and favorable recovery characteristics, crucial for long-lasting performance. Furthermore, the structures with an increasing radial stiffness outperform those with a decreasing radial stiffness in all aspects. The results emphasize the mechanical advantages and optimization potential of a radial stiffness gradient for a wide range of load-bearing applications.