![]() ![]() In the process of DLW and laser-scribing, GO is reduced to reduced GO (RGO), so RGO/GO bilayer structures form on a specific zone of the GO fiber and GO film, which is vital for the formation of actuators due to the apparent difference in expansion and shrinkage between RGO and GO upon adsorption/desorption of water molecules. A simple laser-scribing procedure also has been used to fabricate GO-based programmable actuators 57. GO fiber-type and GO film-type smart actuators with adjustable structural changes are successfully prepared by DLW on GO fibers and GO films along a predesigned path 18, 51. A solo GO film-based actuator has proven the ultrafast response and large deformation degree due to the anisotropic structures (called Quantum confined superfluidics channels), and distinct advantage in long-term stability owing to the single composition 56. In particular, graphene oxide (GO), as an important graphene derivative, has been chosen as building blocks for the fabrication of GO-based actuators 18, 24, 25, 26, 28, 51, 52, 53, 54, 55. Graphene and its derivatives are outstanding candidates for the fabrication of smart and high-performance bilayer actuators due to their fascinating properties, including extraordinary flexibility, low weight density, high mechanical strength and electrical conductivity, unique thermal and optical properties, and good stability 48, 49, 50. In total, photoinduced reactions are the only strategy for fabricating actuators that can achieve programmable structure changes, particularly actuators with bilayer structures. ![]() Lithography and direct laser writing (DLW) belong to the scope of light-mediated manufacture 46 and are very commonly used for the fabrication of actuators with layered structures, which are amenable to photoinitiation reactions, allow designable patterning and create a wide range of proof-of-concept devices, including various self-folding, origami and other complex deformable structures 47. ![]() Electrospinning has been employed to produce thermoresponsive bilayer hydrogel fibrous membranes and hydrogel fibers with different orientations at every layer showing reversible coiling, rolling, bending, and twisting deformations 19, 20, 21. The other type is actuators with bilayer or multilayer structures, which are widely researched and consist of a single component with different orientations of different layers or several components in different layers, including an active layer that contracts or expands under external stimulation and a passive layer that remains intact 43, 44, 45. Plant-inspired composite hydrogel architectures similar to orchids and calla lily flowers are fabricated by four-dimensional (4D) printing, and these hydrogels are encoded with localized, anisotropic swelling behavior controlled by the alignment of cellulose fibrils along prescribed 4D printing pathways 1. Stiff reinforcing magnetic nanoparticles or platelets (iron oxide nanoparticles and functional ceramic platelets) with aligned orientation that can be controlled by an external magnetic field are introduced into networks of hydrogels and ceramics to produce hydrogel-based and ceramic-based actuators with programmable structural changes 38, 40. Hydrogel stripes have been incorporated into the network of certain hydrogels with different moduli by lithography to fabricate hydrogel actuators, and these hydrogel actuators can change from planar to cylindrical and conical helical structures 41, 42. In one type, which is inspired by fibrous plant organs, aligned and rigid matrices are embedded in a soft and pliant matrix to mimic the life behaviors of plant organs 37, 38, 39, 40, 41. Several methods have been proposed to fabricate actuators with desired structural changes, which can be classified into two main types. Actuators with fast, controllable/programmable structure changes/shape transformations are the key factor for the initial design to achieve different applications. Actuators, which can be defined as machines that can convert external stimuli such as magnetic/electric fields 14, 15, 16, pH 17, temperature 18, 19, 20, 21, solvent 22, 23, humidity 24, 25, 26, 27, and light 28, 29 to mechanical movements, hold great potential in many frontier applications, including soft robots 30, 31, microswimmers 32, tissue engineering 33, artificial muscles 34, 35, electronic skins 36, target capture/release and biomimetic actuations 1, 25. In recent years, smart materials, particular responsive materials such as actuators, have attracted great attention 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. ![]()
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