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crucial for characterizing the reliability of the actuation behaviour of a reversible two- way shape- memory effect. In contrast to a one- way shape- memory effect, cyclic movement can be generated by a single one- time pro- gramming process and without additional programming after recovery. Therefore, no external stress is applied to the sample during multicyclic testing (Fig. 4b,c), which quantifies the degradation of the material over repeated actuation cycles. The actuation cycle of the thermome- chanical testing of a reversible shape- memory polymer actuator can be visualized on a strain–temperature plot (Fig. 4d), which allows observation of the morphological transition between shape A and shape B. Moreover, the temperatures corresponding to the highest actuation rate during heating Tact,heat and cooling Tact,cool can be determined from this graph. Free- standing reversible memory effects can be tested using stress- free measure- ment techniques. However, the application of an external stress during the actuation cycle (Fig. 4e) leads to a more pronounced shape change than achieved in stress- free testing, because the external stress increases the network anisotropy of the material. Morphology- dependent function The physical manifestation of shape- memory materials has great influence on their performance and function- ality, and by varying device geometry, an additional hierarchical force can be generated. Diverse morpho- logies have been explored to meet the requirements of potential applications. The molecular mechanisms of shape memory underlying these morphologies are similar; however, macroscale shape and microscale structure can be modified to increase the magnitude of actuation and to introduce specific behaviours. The ability of shape- memory materials to dictate mole- cular orientation using macroscopic deformation is essential to enable bespoke movements such as bending, twisting or stretching. Folding. In nature, specific morphological changes are used to enhance actuation; for example, the folding of bird wings and flowers offers an energy- efficient method to amplify motion. However, for shape- memory poly- mers, engineering principles have been typically taken from the mechanical engineering of machines, which have shape- memory metallic alloys limited to a 3–4% reversible change in strain. The incorporation of a fold- ing mechanism in shape- memory polymers has sub- stantially increased their amplitude of actuation. This is especially pertinent for a two- way reversible shape- memory effect, for which the actuation amplitude is typically limited to length changes of up to 20%22. Programming a ribbon of poly[ethylene- co-(vinyl ace- tate)] (PEVA) into a concertina shape enables reversible length changes of approximately 100%39 (Fig. 5a). In nature, folding is also used to increase the diver- sity and complexity of movements. For synthetic mate- rials, coordinated shape change can be used to create 3D structures from polymer films. For example, deter- ministic self- folding processes can be employed to cre- ate robotic actuators, solar cells and biological devices, which respond to external stimuli such as light10,33,53, heat54–57, magnetic fields58,59 and solvent60. Key chal- lenges are the control of molecular chain orientation in layer processing methods and the predictive modelling of multidimensional shape change. The self- folding of shape- memory materials has been achieved using ther- mal actuation, with a variety of triggers, such as light61, Joule heating and thermal radiation62. Origami- inspired design and the incorporation of different stimuli- responsive elements enable the design of multifunctional soft robots63. For example, a tempo- rary 3D crane shape can be created from a polymer sheet by triggering the phase- change of a thermoresponsive structural unit (Fig. 5b). In this case, light is also used as a spatioselective stimulus to programme an active region within the polymer film, enabling flapping movements of the wings. Localizing the linear elongation–contraction movement to a preformed joint facilitates the desired reversible actuation movement of the crane. This ‘4D’ behaviour is enabled by the combination of a traditional shape- memory effect and a two- way shape- memory poly- mer actuation, actuated at a higher temperature range64, or through the formation of additional crosslinks. Similarly, fused deposition modelling or UV curing can be applied to generate 2D structures that are subsequently deformed by a shape- memory effect into 3D shapes. These tech- niques forgo the need for time- intensive layer- by-layer printing by using simple programming to generate complex 3D geometries65–67. Introducing microstructures. Microstructural variation is also an efficient method to amplify the change in shape of shape- memory materials or to generate novel func- tional behaviour. Inspiration can be taken from metal foams, in which the introduction of hierarchical archi- tectures leads to materials with elastomer- like energy absorption and remarkable post- compression recovery68. Applications in minimally invasive surgery69,70 or aero- space70,71 have driven the development of porous, self- deployable structures made of soft polymeric materials. These materials are capable of programmable, stimuli- responsive volume change. Supercritical carbon dioxide foaming or thermally induced phase separation can be used to create materials with a high strength- to-weight ratio and with tailorable pore size, density and pore size distribution72. Variation of these parameters impacts shape- memory function. For example, foam density is important for the constant- strain recovery behaviour of epoxy foams73. The introduction of a hierarchical internal struc- ture amplifies shape recovery and energy storage dur- ing compression of a shape- memory foam (Fig. 5c). By comparing hierarchically structured polyether ure- thane (PEU) foams with PEU foams with a homogene- ous pore- size distribution, it could be shown that the microscale structural response to bending causes perfor- mance improvement74. The large pore walls of the non- hierarchical material buckle; however, the presence of a porous structure within the walls provides structural support for their folding during deformation. The con- trolled response of hierarchically structured foams to compression enables greater energy storage during com- pression and thus better performance. Porous materials Nature reviews | Materials R e v i e w s Reprogrammable recovery and actuation behaviour of shape-memory polymers Morphology-dependent function Folding. Introducing microstructures.
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