Bhattacharya Shambo, Dubey Devendra K
Department of Mechanical Engineering, Indian Institute of Technology, Delhi, New Delhi 110016, India.
J Biomech Eng. 2022 Apr 1;144(4). doi: 10.1115/1.4052563.
Radial variation in water concentration from outer to inner lamellae is one of the characteristic features of annulus fibrosus (AF). In addition, water concentration changes are also associated with intervertebral disc (IVD) degeneration. Such changes alter the chemo-mechanical interactions among the biomolecular constituents at molecular level, affecting the load-bearing nature of IVD. This study investigates mechanistic impacts of water concentration on the collagen type I microfibrils in AF using molecular dynamics simulations. Results show, in axial tension, that increase in water concentration (WC) from 0% to 50% increases the elastic modulus from 2.7 GPa to 3.9 GPa. This is attributed to combination of shift in deformation from backbone straightening to combined backbone stretching- intermolecular sliding and subsequent strengthening of tropocollagen-water (TC-water-TC) interfaces through water bridges and intermolecular electrostatic attractions. Further increase in WC to 75% reduces the modulus to 1.8 GPa due to shift in deformation to polypeptide straightening and weakening of TC-water-TC interface due to reduced electrostatic attraction and increase in the number of water molecules in a water bridge. During axial compression, increase in WC to 50% results in increase in modulus from 0.8 GPa to 4.5 GPa. This is attributed to the combination of the development of hydrostatic pressure and strengthening of the TC-water-TC interface. Further increase in WC to 75% shifts load-bearing characteristic from collagen to water, resulting in a decrease in elastic modulus to 2.8 GPa. Such water-mediated alteration in load-bearing properties acts as foundations toward AF mechanics and provides insights toward understanding degeneration-mediated altered spinal stiffness.
从外层到内层薄片，水分浓度呈径向变化是纤维环（AF）的特征之一。此外，水分浓度变化也与椎间盘（IVD）退变有关。这种变化在分子水平上改变了生物分子成分之间的化学-机械相互作用，影响了IVD的承重特性。本研究使用分子动力学模拟研究了水分浓度对AF中I型胶原微纤维的作用机制。结果表明，在轴向拉伸时，水分浓度（WC）从0%增加到50%，弹性模量从2.7 GPa增加到3.9 GPa。这归因于变形从主链伸直转变为主链拉伸与分子间滑动相结合，以及随后通过水桥和分子间静电引力加强了原胶原-水（TC-水-TC）界面。当WC进一步增加到75%时，模量降至1.8 GPa，这是由于变形转变为多肽伸直，以及由于静电引力降低和水桥中水分子数量增加导致TC-水-TC界面减弱。在轴向压缩过程中，WC增加到50%会导致模量从0.8 GPa增加到4.5 GPa。这归因于静水压力的产生和TC-水-TC界面的加强。当WC进一步增加到75%时，承重特性从胶原转移到水，导致弹性模量降至2.8 GPa。这种由水介导的承重特性改变是AF力学的基础，并为理解退变介导的脊柱刚度改变提供了见解。
