Recent advances of two-way shape memory polymers and four-dimensional printing under stress-free conditions

In comparison with one-way shape memory polymers (SMPs) under specific stimulation, two-way SMPs can realize reversible shape changes without external manual reprogramming after a shape memory cycle. Thus, two-way SMPs show high prospects and potentials in various applications and attract more attentions recently, especially in addition with the additive manufacturing, namely, two-way four-dimensional (4D) printed SMPs. However, the two-way 4D printed SMPs are still in the preliminary research stage that requires categorizing for further deeply excavation of physical mechanism and applications. This paper provides a comprehensive review of the present research on such junior SMPs, including the preparation of SMPs, methodology of printing, shape memory effect (SME) of printed stress-free composites, two-way SME programs and related mechanism, respectively. Future applications and research trends are examined for the two-way 4D printed SMPs finally.