Table 1.

Summary of MMM in terms of deformation mechanism at unit cell level. Their potential engineering applications are highlighted along with the challenges, limitations and scalability issues.

Feature	Deformation mechanism	Potential application	Challenges	Scalability issues
Auxetic behavior	Bending-dominated, stretching-dominated	1) Multi-axial energy absorption [51]	1) Energy absorption investigation restricted to compressive loads	1) Anisotropic behavior for printed structures, due to interlayer defects and pores [55]
	Chirality	2) Improved stiffness and auxetic behavior for larger deformations [55]	2) Energy absorption process relies on plastic deformation, reducing the use to a single application
Negative stiffness	Elastic buckling due to geometry nonlinearity,	1) Multi-axial energy absorption with self-recovery snap-through response [60], [66]	1) Further investigation of energy absorption under distinct impact conditions	1) Study required for NS lattice structure with sufficient number of cells per side [60]
	Snap-through behavior	2) Energy absorbing feature by using cylindrical structures [67], [68]	2) Damping effects of functionality [67]	2) Numerical models cannot predict well asymmetric modes caused by manufacturing imperfections [67]
		3) Development of stretchable devices [68]		3) Boundary constraint alters stiffness of outer layers
Foldable structures	Crease folding and panel bending; scissor mechanism	1) In-situ stiffness manipulation and curved origami-based grippers [74]	1) Manually tunability of stiffness [74]	1) Connection of the panels [74]
		2) Automotive airbags, implants [75], smart textiles, and wearable electronics	2) Property evaluation in linear-elastic regime [75]	2) Planar configurations not suitable for three-dimensional construction
Multi- functional	Snap-through behavior	1) Shape reconfiguration and shock isolation [29]	1) Require investigation under different loading conditions	1) Amount of unit cells not enough for demonstrating scalability
		2) Impact protectors in aerospace, soft robotics [84], [85], [86], [89]	2) Large friction coefficient [86]