Two-Way 4D Printing： A Review on the Reversibility of 3D-Printed Shape Memory Materials

The rapid development of additive manufacturing and advances in shape memory materials have fueled the progress of four-dimensional （4D） printing. With the right external stimulus, the need for human interaction, sensors, and batteries will be eliminated, and by using additive manufacturing, more complex devices and parts can be produced. With the current understanding of shape memory mechanisms and with improved design for additive manufacturing, reversibility in 4D printing has recently been proven to be feasible. Conventional one-way 4D printing requires human interaction in the programming （or shapesetting） phase, but reversible 4D printing, or two-way 4D printing, will fully eliminate the need for human interference, as the programming stage is replaced with another stimulus. This allows reversible 4D printed parts to be fully dependent on external stimuli; parts can also be potentially reused after every recovery, or even used in continuous cycles-an aspect that carries industrial appeal. This paper presents a review on the mechanisms of shape memory materials that have led to 4D printing, current findings regarding 4D printing in alloys and polymers, and their respective limitations. The reversibility of shape memory materials and their feasibility to be fabricated using three-dimensional （3D） printing are summarized and critically analyzed. For reversible 4D printing, the methods of 3D printing, mechanisms used for actuation, and strategies to achieve reversibility are also highlighted. Finally, prospective future research directions in reversible 4D printing are suggested.

增材制造技术的快速发展和形状记忆材料的进步推动了四维(4D)打印的发展。由于一定程度上的外部刺激,人机交互作用、传感器和电池的需求将被消除,通过使用增材制造技术,可以生产出更复杂的设备和零部件。随着目前对形状记忆机制的理解和对增材制造技术的改进设计,4D打印的可逆性已经被证明是可行的。传统的单向4D打印需要在编程(或定型)阶段进行人机交互,但是可逆的4D打印或双向4D打印将完全消除对人为干预的需求,因为编程阶段被另一种外界刺激所取代。这使得可逆4D打印部件完全依赖外部刺激。零部件在每次回收后都可能被重复利用,甚至在某个周期中可以持续使用——这是一个具有工业运用吸引力的方面。本文综述了影响4D打印的形状记忆材料的机制,目前在合金和聚合物上的4D打印研究结果,以及它们各自存在的一些局限性。对形状记忆材料的可逆性和利用三维(3D)打印技术制作的可行性进行了总结和分析。在对可逆4D打印技术相关内容的介绍中,本文也强调了3D打印技术的方法、相关驱动的机制以及实现可逆性的策略。最后,提出了可逆4D打印技术未来的研究方向。