Influence of Radial Variations in Biochemical Concentrations in Collagen Type and Water on Mechanical Stability of Annulus Fibrosus' Collagen–Hyaluronan Interfaces at Nanoscale: A Molecular Dynamics Investigation

Multidirectional load transmission ability by annulus fibrosus (AF) requires substantial mechanical stability. Additionally, AF exhibits a unique biochemical concentration gradient with outer AF (OA) dominated by type I collagen (COL-I) and inner AF dominated by type II collagen (COL-II) with higher water and proteoglycan concentration. This indicates an intricate relationship between biochemistry and mechanical stability, which remains unclear. This study uses molecular dynamics (MD) simulations to investigate the impact of water, COL-I and COL-II, concentration gradients on mechanical stability of AF's collagen-hyaluronan (COL-HYL) nano-interfaces during tensile and compressive deformation. For this, COL-HYL atomistic models are created by increasing COL-II concentrations from 0% to 75% and water from 65% to 75%. Additional tensile and compressive deformation simulations are conducted for COL-I-HYL interface (COL-HYL interfaces with 0% COL-II) by increasing water concentration from 65% to 75% to segregate the effects of increasing water concentration alone. Results show that increasing water concentration alone to 75% results in marginal changes in local hydration indicating increase in bulk water. This enhances HYL and COL segment sliding-leading to reduction in mechanical stability in tension, indicated by drop in stress-strain characteristics. Additionally, increase in bulk water shifts load-bearing characteristics toward water-leading to reduction in modulus from 3.7 GPa to 1.9 GPa. Conversely, increasing COL-II and water concentration facilitates stable water bridge formation which impedes sliding in HYL and COL-enhancing mechanical stability. These water bridges further improve compressive load sustenance leading to lower reduction in compressive modulus from 3.7 GPa to 2.8 GPa.