As a new class of smart materials, shape memory polymers and their composites (SMPs and SMPCs) can respond to specific external stimulus and remember the original shape. There are many types of stimulus methods to actuate the deformation of SMPs and SMPCs, of which the thermal-and electro-responsive components and structures are common. In this review, the general mechanism of SMPs and SMPCs are first introduced, the stimulus methods are then discussed to demonstrate the shape recovery effect, and finally, the applications of SMPs and SMPCs that are reinforced with fiber materials in aerospace are reviewed. SMPC hinges and booms are discussed in the part on components; the booms can be divided again into foldable SMPC truss booms, coilable SMPC truss booms and storable tubular extendible member (STEM) booms. In terms of SMPC structures, the solar array and deployable panel, reflector antenna and morphing wing are introduced in detail. Considering the factors of weight, recovery force and shock effect, SMPCs are expected to have great potential applications in aerospace.

Four-dimensional (4D) active shape-changing structures based on shape memory polymers (SMPs) and shape memory nanocomposites (SMNCs) are able to be controlled in both space and time and have attracted increasing attention worldwide. However, conventional processing approaches have restricted the design space of such smart structures. Herein, 4D active shape-changing architectures in custom-defined geometries exhibiting thermally and remotely actuated behaviors are achieved by direct-write printing of ultraviolet (UV) cross-linking poly(lactic acid)-based inks. The results reveal that, by the introduction of a UV cross-linking agent, the printed objects present excellent shape memory behavior, which enables three-dimensional (3D)-one-dimensional (1D)-3D, 3D-two-dimensional (2D)-3D, and 3D-3D-3D configuration transformations. More importantly, the addition of iron oxide successfully integrates 4D shape-changing objects with fast remotely actuated and magnetically guidable properties. This research realizes the printing of both SMPs and SMNCs, which present an effective strategy to design 4D active shape-changing architectures with multifunctional properties. This paves the way for the further development of 4D printing, soft robotics, flexible electronics, minimally invasive medicine, etc.

Implantation of occlusion devices is an effective approach for the treatment of congenital heart diseases in the clinic. However, most commercial clinical occlusion devices are currently made of nondegradable metals, which may lead to complications such as perforation, allergies, and erosion. In this work, 4D-printed novel, biodegradable, remotely controllable, and personalized shape memory occlusion devices are demonstrated and atrial septal defect occluders are exemplified. By incorporating Fe 3 O 4 magnetic particles into the shape memory poly(lactic acid) matrix, the deployment of the occluders can be controlled remotely after implantation. The excellent cytocompatibility and histocompatibility are conducive to cell adhesion and ingrowth of granulation tissues into the occluders, thus facilitating rapid endothelialization. In addition, personalized shape memory occluders ensure an ideal fit and provide sufficient support for defects. Therefore, 4D-printed shape memory occluders can be used as a potential substitute for metal occlusion devices.

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Shape-memory polymers (SMPs) are synthesized with adjustable glass transition temperature (T g ) ranging from 299 to 322 °C, higher than those reported previously. The polyimide containing thermal stable but flexible linkages within the backbone act as reversible phase, and high molecular weight (M n ) is necessary to form physical cross-links as fixed phase of thermoplastic shape-memory polyimide. The critical M n is 21.3 kg/mol, and the relationship between M n and T g is explored. Thermoset polyimides show higher storage modulus and better shape-memory effects than thermoplastic counterparts due to covalent cross-linking, and the effective cross-link density with the influence on their physical properties is studied. The mechanism of high-temperature shape-memory effect of polyimide on the basis of chain flexibility, molecular weight, and cross-link density is proposed, which will benefit further research on high-temperature SMPs.