Table 1.

Selected mechanical metamaterials and their fabrication methods.

Function	Material	Method	Fabrication Challenges	References
Ultra-stiffness (high stiffness-to-weight ratio)	Metals, polymers, composites	Conventional	Time-consuming process for material handling, lack of repeatability, product uncertainties, making small structures	[24–31]
	Polymers, metals, ceramics, nanocomposites	Light-assisted AM	Pre-processing to make material compositions, local imperfections for polymers and metals, surface imperfections	[13,32–42]
	Polymers, composites	Extrusion-based AM	Instability of structure, the anisotropy of structure	[43–49],
Negative-Poisson’s ratio	Polymers, metals	Conventional	Lack of complexity, absence of automation for periodic structures	[50–57,166]
	Metals, polymers, composites	Light-assisted AM	Desired material compositions, light scattering for internal spaces/pores,	[58–64]
	Polymers	Extrusion-based AM	Low resolution in structures, Instability among layers in structures	[48,49,65,66]
High-toughness & super elasticity	Polymers	Conventional	Limited selection of materials, lack of a flexible and versatile platform, low mass, negative mass density	[54–67]
	Polymers, metals	Light-assisted AM	Inhomogeneity in structures, high resolution, and large-scale parts	[13,38,68]
	Polymers	Extrusion-based AM	Multi-stability, a separation between layers, multi-scale fabrication,	[65,69]
Self-assembly & programmable	Shape memory polymers	Conventional	Complexity in design, misalignment, lack of control in patterns	[70–74]
	Shape memory metals/polymers, composites	Light-assisted AM	Material composition, opacity requirement for complex materials, post-processing to activate the structure	[59,62,75–78]
	Shape memory composi-tes, Shape memory polymers, composites	Extrusion-based AM	Complexity of structures, multi-material fabrication for products, incorporation of architecture, size effects	[79–84]