School of Mechanical and Aerospace Engineering, Nanyang Technological University , 50 Nanyang Avenue, 639798 Singapore, Singapore.
Institute of Soft Matter Mechanics, College of Mechanics and Materials, Hohai University , Nanjing, Jiangsu 210098, PR China.
Acc Chem Res. 2017 Feb 21;50(2):141-150. doi: 10.1021/acs.accounts.6b00539. Epub 2017 Feb 9.
The phenomenon of recovering the permanent shape from a severely deformed temporary shape, but only in the presence of the right stimulus, is known as the shape memory effect (SME). Materials with such an interesting effect are known as shape memory materials (SMMs). Typical stimuli to trigger shape recovery include temperature (heating or cooling), chemical (including water/moisture and pH value), and light. As a SMM is able not only to maintain the temporary shape but also to respond to the right stimulus when it is applied, via shape-shifting, a seamless integration of sensing and actuation functions is achieved within one single piece of material. Hydrogels are defined by their ability to absorb a large amount of water (from 10-20% up to thousands of times their dry weight), which results in significant swelling. On the other hand, dry hydrogels indeed belong to polymers, so they exhibit heat- and chemoresponsive SMEs as most polymers do. While heat-responsive SMEs have been spotted in a handful of wet hydrogels, so far, most dry hydrogels evince the heat and water (moisture)-responsive SMEs. Since water is one of the major components in living biological systems, water-responsive SMMs hold great potential for various implantable applications, including wound healing, intravascular devices, soft tissue reconstruction, and controlled drug delivery. This provides motivation to combine water-activated SMEs and swelling in hydrogels together to enhance the performance. In many applications, such as vascular occlusion via minimally invasive surgery for liver cancer treatment, the operation time (for both start and finish) is required to be well controlled. Due to the gradual and slow manner of water absorption for water-activated SMEs and swelling in hydrogels, even a combination of both effects encounters many difficulties to meet the timerequirements in real procedures of vascular occlusion. Recently, we have reported a bioabsorbable radiopaque water-responsive shape memory embolization plug for temporary vascular occlusion. The plug consists of a composite with a poly(dl-lactide-co-glycolide) (PLGA) core (loaded with radiopaque filler) and cross-linked poly(ethylene glycol) (PEG) hydrogel outer layer. The device can be activated by body fluid (or water) after about 2 min of immersion in water. The whole occlusion process is completed within a few dozens of seconds. The underlying mechanism is water-responsive shape recovery induced buckling, which occurs in an expeditious manner within a short time period and does not require complete hydration of the whole hydrogel. In this paper, we experimentally and analytically investigate the water-activated shape recovery induced buckling in this biodegradable PEG hydrogel to understand the fundamentals in precisely controlling the buckling time. The molecular mechanism responsible for the water-induced SME in PEG hydrogel is also elucidated. The original diameter and amount of prestretching are identified as two influential parameters to tailor the buckling time between 1 and 4 min as confirmed by both experiments and simulation. The phenomenon reported here, chemically induced buckling via a combination of the SME and swelling, is generic, and the study reported here should be applicable to other water- and non-water-responsive gels.
从严重变形的临时形状恢复到永久形状的现象，但仅在存在正确刺激的情况下，称为形状记忆效应（SME）。具有这种有趣效果的材料称为形状记忆材料（SMM）。触发形状恢复的典型刺激包括温度（加热或冷却）、化学物质（包括水/湿度和 pH 值）和光。由于 SMM 不仅能够保持临时形状，而且能够在施加时通过形状转变对正确的刺激做出反应，因此在单个材料中实现了传感和致动功能的无缝集成。水凝胶的定义是其能够吸收大量水（从 10-20%到其干重的数千倍），从而导致显著膨胀。另一方面，干水凝胶确实属于聚合物，因此它们表现出热和化学响应 SME，就像大多数聚合物一样。虽然已经在少数湿水凝胶中发现了热响应 SME，但到目前为止，大多数干水凝胶表现出热和水（湿度）响应 SME。由于水是生命生物系统的主要成分之一，因此水响应 SMM 具有在各种可植入应用中的巨大潜力，包括伤口愈合、血管内装置、软组织重建和控制药物输送。这为将水激活 SME 和水凝胶中的肿胀结合在一起以提高性能提供了动力。在许多应用中，例如通过微创外科手术治疗肝癌的血管闭塞，操作时间（开始和结束）都需要得到很好的控制。由于水激活 SME 和水凝胶中的肿胀吸收缓慢，即使两者结合也很难满足实际血管闭塞过程中的时间要求。最近，我们报道了一种用于临时血管闭塞的可生物吸收的射线可透水性响应形状记忆栓塞塞。该塞子由聚（DL-丙交酯-共-乙交酯）（PLGA）核（负载射线不透射性填充剂）和交联聚乙二醇（PEG）水凝胶外层组成。该装置在浸入水中约 2 分钟后可通过体液（或水）激活。整个闭塞过程在几十秒内完成。其潜在机制是水响应形状恢复诱导的屈曲，该屈曲在短时间内迅速发生，并且不需要整个水凝胶完全水合。在本文中，我们通过实验和分析研究了这种可生物降解的 PEG 水凝胶中的水激活形状恢复诱导的屈曲，以深入了解精确控制屈曲时间的基本原理。还阐明了 PEG 水凝胶中引起水诱导 SME 的分子机制。通过实验和模拟证实，原始直径和预拉伸量被确定为两个有影响力的参数，可以将屈曲时间调整为 1 至 4 分钟之间。这里报道的现象是通过 SME 和肿胀的化学诱导屈曲，是通用的，并且这里报道的研究应该适用于其他水响应和非水响应的凝胶。
