摘要
镁合金具有良好的生物相容性和可降解性,已被用于作为可降解冠脉支架材料进行研究,可降解镁合金支架有望成为新一代心血管支架。然而,与不锈钢材料相比,镁合金作为支架材料,抗拉强度低、屈服强度低、断后延伸率低、塑性变形能力差,使得镁合金支架存在径向回弹率大,径向支撑力低以及应力应变不均匀(应力集中)等问题。因而本文针对镁合金材料的力学性能特点,对镁合金支架结构进行优化设计,以弥补材料本身性能的不足。本文以有限元模拟为研究方法,采用AZ31镁合金为支架材料,对课题组自行开发的镁合金支架结构进行优化研究。通过改变支架结构单元的长度、丝宽、圆弧半径等结构参数,同时利用系统分析中的敏感性分析方法,对上述不同结构参数组合下支架的径向回弹率、径向支撑力以及应力应变分布进行参数敏感性研究。结果表明,参数是决定结构性能的重要因素,不同支架性能受不同结构参数影响的敏感性不同,通过模拟分析,对于径向回弹率而言,影响其性能的主要因素按影响力大小为长度变量、丝宽变量以及半径变量以及壁厚；对于支撑力而言,影响其性能的主要因素按影响力大小依次为长度变量、丝宽变量以及壁厚变量；对于最大主应变而言影响其性能的主要因素按影响力大小依次为长度变量、丝宽变量以及半径变量。在这些因素中,以长度因素和丝宽因素为甚,分别对支架的三个性能能产生较大影响。而改变半径对径向回弹率以及最大主应变的影响较大,对支撑力的影响较小；改变壁厚能够影响支架的支撑力性能和径向回弹率性能,对最大主应变的影响较小。这些变量对各自性能的影响力大小,对支架结构设计尤其是镁合金支架结构的设计尤为重要,可以在支架结构设计中作为参考。
根据课题组支架的实际实验中发现,支架在变形过程中存在着变形不均匀性,主要是“之”字形支撑环在撑开时“V”形梁张的开角度存在差异。根据这种情况,我们对完整的支架和折叠球囊装配在一起的组合系统进行动态模拟分析,通过改变支架的结构、球囊的折翼的变化以及球囊的厚度的厚薄等变量,分别模拟支架在不同情况下的变形行为从模拟的结果看,球囊折翼的数量、支架与球囊的配合度、连接筋数量以及球囊自身厚度等情况对支架的扩张不均匀性有着重要的影响。具体来讲,支架与球囊的对称性越接近,支架的扩张越均匀,在实际可行的情况下,应采用球囊的折翼数量应与支架轴向最简单元的重复个数相同；球囊的厚度越小,支架的扩张越均匀,因此在选择球囊时,应尽量选择薄壁球囊进行扩张；同时,支架的连接筋分布越对称,连接筋越多,支架扩张越均匀,因此在支架连接筋的分布和数量选择上,应使连接筋的分布应尽量均匀,且不能过渡减少连接筋的数量,这些举措都有利于改善支架的对称不均匀性。
Magnesium alloys has recently been studied as biodegradable coronary stent material for its good biocompatibility and biodegradability. However, compared with stainless steel, magnesium alloy as the coronary stent material has lower tensile strength, yield strength, elongation and poorer plastic deformation ability, which cause magnesium alloy stent to emerge higher radial recoil rate, lower radial force and non-uniform stress and strain (stress concentration) and other poorer mechanical properties. In consideration of the premises, this paper studied about optimization design of magnesium alloy stent structure in order to cover the shortage of material property itself. The group self-developed AZ31 magnesium alloy stent has been studied to optimizing structure using the finite element method. By means of changing length, width, radial and other structure parameters of coronary stent unit, the sensitivity factor of stent radial resilience, radial force and the stress and strain distribution was researched by sensitivity analysis method.. The results show that the structure parameters is an important factor of stent properties. The different stent performance were impacted by different sensitivity to different structural parameters. Through simulation analysis, the radial recoil rate, radial force and stress and strain distribution is concerned. For the radial recoil rate, the main factors affecting its performance is variable length, wire width, radius and wall thickness, according to the order of the influence. For the radial force, the main factors affecting its performance is variable length, wire width and wall thickness. For the stress and strain distribution, the main factors affecting its performance is variable length, wire width and radius. In all of these factors, respectively, the significant impact on three performance factors are the length and wire width variable. Changing the radius of stent units can cause a great impact on both radial recoil and maximum principle strain, but less impact on the radial force; Changing the wall thickness of stent units can affect the radial force and radial recoil, but less impact on the maximum principle strain. The influence of structure parameter is important to designing the coronary stent.
According to the actual experimental on self-developed stent, it was found that stent structure is non-uniform after the deformation process, mainly due to "V"-shaped beam opening angle is different while the support ring-shaped is unfolded. This is the case, the stent assembly with balloon was were dynamically simulated by changing the structure of the stent, balloon wings and balloon thickness under different circumstances. From the simulation results, the number of balloon wings, the compatibility of stent and balloon, the number of connected muscle and the balloon itself thickness were highly impact on non-uniformity of stent expansion.Specifically, the symmetry of Stent and balloon closer, more uniform stent expansion, in practical cases, the number of balloon wings and the number of stent easiest repeat element of the axial should be the same; more thin balloon thickness, more uniform stent expansion, and in the choice of balloon, the thin-wall balloon should be selected to expand; in the meantime, more symmetrical connected muscle structure and more the number of connected muscle, more uniform stent expansion, in the choice of reinforcement bracket, the number of connect tendons can not to reduce in excess, connect tendons should be symmetrically distributed. Therefore, these initiatives are conducive to improved stent symmetry.
引文
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