Wong Y S, Xiong Y, Venkatraman S S, Boey F Y C
School of Materials Science & Engineering, Nanyang Technological University, N4.1-1-30 Nanyang Avenue, Singapore 639798, Singapore.
J Biomater Sci Polym Ed. 2008;19(2):175-91. doi: 10.1163/156856208783432516.
Shape memory is extremely useful in minimally-invasive deployment of medical devices, particularly stents used in the cardiovascular system. Current stents employ Nitinol, which exhibits shape memory on the basis of a phase transformation. Cross-linked polymers may also be made to exhibit shape-memory effects, but are sometimes precluded for use in stents for other reasons. Un-cross-linked poly(lactide-co-glycolide) (PLGA) and poly(L-lactic acid) (PLLA) have been used in biodegradable stent prototypes. Their shape memory polymers (SMPs) were investigated in this paper. The three important parameters in SMPs are strain fixity, strain recovery and permanent strain. The effects of deformation temperature, deformation strain level and creeping time on those parameters were evaluated in depth. High deformation temperatures will give high fixity, high recovery, but also high permanent deformation for both PLGA and PLLA. Generally, a lower stress applied for a longer duration (the 'creeping' method) is preferred to a higher stress applied 'instantly' for achieving shape memory. These effects are explained on the basis of molecular orientation and slippage effects.
形状记忆在医疗设备的微创部署中极其有用，尤其是在心血管系统中使用的支架。当前的支架采用镍钛诺，其基于相变表现出形状记忆。交联聚合物也可表现出形状记忆效应，但有时因其他原因而不能用于支架。未交联的聚（丙交酯-共-乙交酯）（PLGA）和聚（L-乳酸）（PLLA）已用于可生物降解支架原型。本文对它们的形状记忆聚合物（SMP）进行了研究。SMP中的三个重要参数是应变固定率、应变恢复率和永久应变。深入评估了变形温度、变形应变水平和蠕变时间对这些参数的影响。对于PLGA和PLLA而言，高变形温度会产生高固定率、高恢复率，但也会产生高永久变形。一般来说，为实现形状记忆，采用较低应力并施加较长时间（“蠕变”方法）比“立即”施加较高应力更可取。这些效应是基于分子取向和滑移效应来解释的。
