模拟生理应力作用下Mg/PLA复合材料的分阶协同降解行为研究

During the healing process of bone fracture, the stress mode, magnitude and dynamic frequency of the physiological stresses acting on the bone tissues and the biodegradable orthopedic fixation materials possess the step-wise evolution characteristics, which could further have a significant impact on the degradation behaviors of the fixation materials. Both magnesium alloys (Mg) and poly-lactic acid (PLA) are the promising biodegradable orthopedic fixation materials. Moreover, the composites composed with these two components have the designable mechanical properties and the highly tailorable degradation behaviors. Based on our previous research work on the biodegradable PLA based - composite reinforced unidirectionally with high-strength magnesium alloy wires, this project will focus on the step-wise synergistic degradation behaviors of the Mg/PLA composite in the simulated physiological stress environments, especially on the evolution characteristics of the mechanical and degradation properties of the composite under different dynamic physiological stresses with various stress modes, magnitudes or frequencies. Furthermore, the degradation behaviors of each component and their synergistic effects will be investigated. The aim of this project is to establish a step-wise synergistic degradation kinetics model to reflect the relationship between the properties of the composite with the physiological stress, temperature and the degradation time. And finally the in vitro test method for the scientific evaluation to the degradation behaviors and lifetime of the orthopedic fixation materials will be designed. This project provides a common reference for the studies of most biodegradable implants. It could offer the scientific basis to ideally choose the applied stress and exercise method, tailor the degradation rate and predict the lifetime for the biodegradable implants during the healing process of bone fracture.

骨折愈合过程中，可降解骨科固定材料与骨组织受到的生理应力在应力形式、大小、动态频率上呈现分阶段演变特点，并显著影响固定材料的降解行为。镁合金（Mg）与聚乳酸（PLA）同属可降解材料，两者以不同形式复合所制得的Mg/PLA类复合材料具有性能可设计、降解可调控等特性，是当前骨科固定材料的研究热点之一。在镁合金微细丝定向增强聚乳酸复合材料前期研究基础上，本项目围绕该复合材料在模拟生理应力作用下的分阶协同降解行为展开，重点研究其在变频微载、变频高载、复合载荷等动态生理应力作用下的力学和降解性能响应机制、两组元的降解行为及其协同效应，建立性能与应力、温度、时间之间的分阶降解动力学模型，制定能科学评价材料降解行为的体外试验方法和寿命周期预测方法。本项目对可降解骨科材料研究具有普遍借鉴意义，可为骨折愈合过程中合理选择外加应力和运动形式、控制材料降解速度以及预测其寿命周期提供科学依据和评价方法。

项目期内，首先搭建了可实现变频微载、变频高载、复合载荷等模拟生理动态应力环境的实验平台，基于该实验平台，系统开展了镁合金微细丝定向增强聚乳酸复合材料分别在不同静态载荷、不同频率和应力幅值动态载荷、分阶段复合动态载荷等不同模拟生理应力环境中的力学和降解行为响应机制及两组元的协同降解效应。.研究发现，复合材料中镁合金和聚乳酸两组元的降解存在协同效应。低含量的镁合金丝对复合材料中聚乳酸的降解有一定缓和作用，这与镁合金降解产物镁离子与聚乳酸的降解产物中端羧基发生交联作用有关；而更高含量的镁合金丝则抑制了这种缓和作用，这是因为两组元降解产物之间形成中和反应，可维持pH值稳定，但也促进了两组元各自的降解反应。在静态应力环境中，提高温度或施加应力，会打破两组元间的降解平衡，加速两组元的降解及复合材料力学性能的衰减。发现了聚乳酸降解存在活化熵和活化焓之间的“补偿效应”，指前因子与降解活化能间服从Meyer-Neldel准则。动态应力会加速复合材料的降解，增大应力大小和频率，其降解速度增大，其影响比静态应力更为显著。在动态压应力环境中，复合材料中聚乳酸的早期降解行为服从一阶降解动力学规律。建立了动态应力环境中复合材料的弯曲强度与频率、应力大小和降解时间之间的数学模型。分阶动态应力作用下，即使后期载荷相同，前期载荷越大，对复合材料降解的促进作用越大；而较大压应力作用时间越长，这种促进作用越明显。与分阶动态压应力条件相比，压应力与弯曲应力结合的分阶动态载荷能更有效促进复合材料的快速降解。建立了适用于单一动态压应力和分阶动态压应力作用下复合材料的弯曲强度变化与施加载荷、降解时间之间的数学模型。.本研究为可降解骨科材料的生理动态应力实验设计、降解行为和性能预测提供了理论指导，为骨折愈合过程中合理选择外加应力和运动形式以控制其降解速度提供科学依据。