Snap-through instability-based mechanical metamaterials (SIMMs) with bistability, multistability, negative stiffness, or excellent energy absorption and dissipation performance play an important role in various advanced functional applications. They can serve as energy absorbers, energy dampers, or mechanical memory and logic computing devices, while also providing amplified force output and faster response time in flexible robots, or implementing sensing functions combined with piezoelectric or triboelectric electricity. However, thus far, research on SIMMs that have non-fixed boundary constraints, proactive responsiveness, multi-physical field cross-coupling, and deep information processing capabilities is still facing significant challenges, potentially hindering the development and cross-field comprehensive applications of truly intelligent SIMMs. Our objective is to furnish a concise categorization of SIMMs and offer direction for innovative design and functional implementations. We have emphasized that the non-fixed boundary constraint will expand the design possibilities, while the use of stimulus-responsive materials and 4D printing technology will create novel opportunities for the design of SIMMs. These advancements are expected to achieve innovative mechanical properties and functions.

Metamaterials, owing to their exceptional physical characteristics that are absent in natural materials, have emerged as a crucial constituent of intelligent devices and systems. However, there are still significant challenges that necessitate immediate attention, as they have considerably constrained the applicability of metamaterials, including fixed mechanical properties post-fabrication and restricted design freedom. Here, thermo-responsive, photo-responsive, electro-responsive, and magneto-responsive shape memory polymer nano-composites were developed, and shape memory gradient metamaterials were fabricated using multi-material 4D printing technology. The correlation mechanism between the design parameters and the mechanical properties of multi-responsive gradient metamaterials was systematically analyzed, and the highly designable and programmable configuration and mechanical properties of the gradient metamaterials were realized. More importantly, 4D printed multi-responsive shape memory polymer gradient metamaterials can be programmed in situ without additional infrastructure for multi-functional mechanical functions, paving the way for the realization of multiple functions of a single structure. Based on the multi-responsive gradient metamaterials, 4D printed digital pixel metamaterial intelligent information carriers were fabricated, featuring customizable encryption and decryption protocols, exceptional scalability, and reusability. Additionally, 4D printed gradient metamaterial logic gate electronic devices were developed, which were anticipated to contribute to the development of smart, adaptable robotic systems that combine sensing, actuation, and decision-making capabilities.

The wingtip has an important influence on the aerodynamic and control performance of the aircraft. Different flight stages such as take-off, climb, cruise, and descent impose diverse requirements for the geometric parameters of the wingtip. The morphing wingtip technology is a multi-functional, local morphing technology used to improve wing aerodynamic performance and fuel efficiency, reduce gust loads, and enhance control performance. This paper classifies the technical benefits of morphing wingtips from the three aspects of aerodynamic characteristics, structural response, and control characteristics, and discusses the current research status of the morphing wingtip technology from the two perspectives of wingtip deformation form, and material and structure composition. In addition, this article points out that the morphing wingtip technology is developing towards multifunctional integration, combined deformation, and intelligence. Four key technologies that need to be urgently solved for the morphing wingtip technology are presented, including the high-output actuator system, deformation/loading integrative skins, global aerodynamic optimization, and intelligent control technology, and the technical characteristics and research difficulties of each key technology are analyzed. If there is a breakthrough in the key technology of morphing wingtip, the related technology can be transplanted and applied to the global morphing aircraft.

Metamaterials have exotic physical properties that rely on the construction of their underlying architecture. However, the physical properties of conventional mechanical metamaterials are permanently programmed into their periodic interconnect configurations, resulting in their lack of modularity, scalable fabrication, and programmability. Mechanical metamaterials typically exhibit a single extraordinary mechanical property or multiple extraordinary properties coupled together, making it difficult to realize multiple independent extraordinary mechanical properties. Here, the pixel mechanics metamaterials (PMMs) with multifunctional and reprogrammable properties are developed by arraying uncoupled constrained individual modular mechanics pixels (MPs). The MPs enable controlled conversion between two extraordinary mechanical properties (multistability and compression-torsion coupling deformation). Each MP exhibits 32 independent and reversible room temperature programming configurations. In addition, the programmability of metamaterials is further enhanced by shape memory polymer (SMP) and 4D printing, greatly enriching the design freedom. For the PMM consisting of m × n MPs, it has 32^{(m × n)} independent room temperature programming configurations. The application prospects of metamaterials in the vibration isolation device and energy absorption device with programmable performance have been demonstrated. The vibration isolation frequencies of the MP before and after programming were [0 Hz–5.86 Hz], [0 Hz–13.67 Hz and 306.64 Hz–365.23 Hz]. The total energy absorption of the developed PMM can be adjusted controllably in the range of 1.01 J–3.91 J. Six standard digital logic gates that do not require sustained external force are designed by controlling the closure between the modules. This design paradigm will facilitate the further development of multifunctional and reprogrammable metamaterials.