Four-dimensional printing allows for the transformation capabilities of 3D-printed architectures over time, altering their shape, properties, or function when exposed to external stimuli. This interdisciplinary technology endows the 3D architectures with unique functionalities, which has generated excitement in diverse research fields, such as soft robotics, biomimetics, biomedical devices, and sensors. Understanding the selection of the material, architectural designs, and employed stimuli is crucial to unlocking the potential of smart customization with 4D printing. This review summarizes recent significant developments in 4D printing and establishes links between smart materials, 3D printing techniques, programmable structures, diversiform stimulus, and new functionalities for multidisciplinary applications. We start by introducing the advanced features of 4D printing and the key technological roadmap for its implementation. We then place considerable emphasis on printable smart materials and structural designs, as well as general approaches to designing programmable structures. We also review stimulus designs in smart materials and their associated stimulus-responsive mechanisms. Finally, we discuss new functionalities of 4D printing for potential applications and further development directions.

Atomistic mechanisms of frictional energy dissipation have attracted significant attention. However, the dynamics of phonon excitation and dissipation remain elusive for many friction processes. Through systematic fast Fourier transform (FFT) analyses of the frictional signals as a silicon tip sliding over a graphite surface at different angles and velocities, we experimentally demonstrate that friction mainly excites non-equilibrium phonons at the washboard frequency and its harmonics. Using molecular dynamics (MD) simulations, we further disclose the phononic origin of structural lubrication, i.e., the drastic reduction of friction force as the contact angle between two commensurate surfaces changes. In commensurate contacting states, friction excites a large amount of phonons at the washboard frequency and many orders of its harmonics that perfectly match each other in the sliding tip and substrate, while for incommensurate cases, only limited phonons are generated at mismatched washboard frequencies and few low order harmonics in the tip and substrate.