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 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.