Molecular crystals are attractive as mechanically responsive materials, contrary to their appearance as hard and fragile objects. We have intensively developed various photomechanical crystals such as azobenzene and salicylideneaniline based on photoisomerization over the recent decade. Not only bending but also various motions such as robot-like locomotion are possible by thermal phase transition as well as photo-triggered phase transition. Crystals have the advantage of exerting greater mechanical power than polymers. Mechanically responsive crystals have great potential as ultimate actuation materials for soft robots. We hope this chapter will bring great interest and attention to many readers about mechanical crystal materials.
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