血管支架类生物医用材料系列4--记忆金属材料

3.
有关记忆金属NITINOL的基本事实
4. 其基本组成成分一般为镍钛原子数各一半(即50% atomic each; 或55-57%Wt的镍和43-45%Wt左右的 钛,镍钛两者比例的略为变化会对材料的性质产生重 大影响;其它微量元素Cu, Nb, Fe, Hf 可加入以调整 镍钛形状记忆合金的性质
Because the material can change from one phase to the other with a simple shearing motion of the atoms within the crystal structure and no diffusion or large movement of atoms is required, the transition can occur virtually instantly; and more over, the “tilted” twin forms of the martensite can be flipped to the opposite “tilt” by the application of stress without damaging the crystal structure).
微创手术与介入医疗工程学导论
有关记忆金属的基本事实:
2. 镍钛合金的记忆效应是美国海军兵工实验室( Naval Ordinance Laboratory)的Wiley 和Buechler 1962年 偶然发现的。 在医学上常用的记忆合金为镍钛（Ni-Ti ）合金或 称Nitinol,取自(Nickel/ Titanium / Naval Ordinance Laboratory).
Shape Memory Effect:
NiTi alloys after an apparent deformation in the martensitic phase have the ability to recover their original shape upon heating through the phase transformation temperature range above the Af temperature.

Superelasticity, also termed “pseudoelasticity”, describes a nonlinear recoverable deformation behavior of NiTi alloys at temperatures above the Af ( Austenite Phase Transformation Finish Temperature), that arises from the stress-induced martensitic transformation on loading and the spontaneous reversion of the transformation upon unloading. A transformation-induced strain up to 6% is recoverable. When deformation exceeds 6% strain, the materials can further extend the deformation via linear elasticity of the stress-induced martensite. A total strain of 8% is recoverable.
有关记忆金属的基本事实（续）: 镍钛两者 比例的略 为变化会 对材料的 性质产生 重大影响
微创手术与介入医疗工程学导论
有关记忆金属的基本事实（续）:
For a given regular metal, the stress –strain relation is initially linear (called elastic deformation. When the yield stress is reached, plastic deformation occurs, which result in permanent deformation, until reaching the fracture point.
Heat treatment and mechanical strains (wire drawing) of 316L during room temperature influences yield stress, ductility, and fracture toughness.
A brittle material has low range of plastic deformation (low ductility) and low fracture toughness. A ductile material shows in contrast a large range of plastic deformation
有关记忆金属的基本事实（续）:
6.
For Nitinol material, the stress-strain curve shows initially elastic deformation followed by a non-linear yet reversible deformation phase, as long as the stress is not increased beyond the yield point.
Nitinol则与人体组织极为类似：即可塑性变形（非线性地） 至应变量的8-10%且两者(Nitinol 与人体组织）的应力/应变 曲线（Stress/ Strain Curve）极为相似，均有一个滞后回线 （Hysteresis Loop）； 微创手术与介入医疗工程学导论
有关记忆金属的基本事实（续）
特点：
1）形变须在马氏体状态（martensite)(较低温度Ms）下进行； 2）马氏体状态：柔软态，易形变； 3）应变不要高于 8% ； 4）加热到奥氏体状态（austenite) （较高温度>Af），可恢复到 原来的形状。
微创手术与介入医疗工程学导论
What is Shape Memory Effect ?
第二节 记忆金属材料 Shape Memory Materials 两大基本特性:
1. 记忆 (Shape Memory)
2. 超弹 (Super Elastic)
微创手术与介入医疗工程学导论
有关记忆金属的基本事实:
1. 形状记忆效应：材料即使在极端变形的情况下也能通过加热至
一定温度（不需受力）而迅速迅速恢复到先前形状的现象；
Because the material can change from one phase to the other with a simple shearing motion of the atoms within the crystal structure and no diffusion or large movement of atoms is required, the transition can occur virtually instantly; and more over, the “tilted” twin forms of the martensite can be flipped to the opposite “tilt” by the application of stress without damaging the crystal structure).

超弹性 (Super-elasticity)之特点

形变须在奥氏体（austenite) 状态较高温度>Af）下进行

超弹性应变范围 8%（无真 正的塑变） 从0.5 %至8%的应变范围内， 应力基本保持恒定
当应力撤除时，材料弹性地 恢复至原来的形状


有关记忆金属的基本事实（续）:
8. 除具有不同与一般金属材料的超弹性外，Nitinol的另 一个重要特性为形状记忆效应（Shape Memory Effect, 或称作热记忆效应）：即一种通过加热提高材料温度 而使该材料从塑变状态恢复至初始形态的现象。
Nitinol 这种在变形量超过应变量8-10%（不锈钢的最 大可恢复变形量为0.80%)的情况下尚能弹性恢复的超 常行为称作超弹（Superelasticity or Pseudoelasticity）， 有很强的柔软性(Flexibility)和抗弯折能力(KinkResistance) ；由该材料制作的医疗器械可以被压缩成 极小的形状（如压缩在一个很小的输送导管内，处于 畸变状态）, 在达到一定温度（如体温）下，该器械 可以自行膨胀（膨胀比率一般为1:2至1:5)并恢复至预 先”训练” (Training) 而成的初始形状。
5. 常规材料如不锈钢等的塑变（Elastic Deformation）特点与人体组成材料相比有显著不同；常规材料的形变一般为应 变量（Strain）的1%左右，且拉伸量（Elongation）随负荷的变化而呈正比线性（linearly and proportionally）变化； Elastic deformation is temporary, reversible, while plastic deformation is permanent and irreversible. Plastic deformation plays an 微创手术与介入医疗工程学导论 important role when a balloon expandable stent is delivered in the lumen. The ability of a material to undergo plastic deformation is called its ductility. Heat treatment reduces the carbon content of steel and can influence the crystallographic structure.