Unraveling the Mechanisms Governing Anisotropy in Accordion-Shaped Honeycomb Microlattice Fabricated by Two-Photon Polymerization

High-resolution printing afforded by two-photon polymerization (2PP) has opened up vast design space, spanning over four orders of magnitude (sub-mu m to cm), unleashing the opportunity to fabricate architected materials with intriguing mechanical characteristics. This study seeks to exploit direct laser writing to print accordion-shaped honeycomb lattices constituted of slender micro-membranes to achieve in-plane mechanical anisotropy. The deformation mechanisms governing orientation-sensitive mechanical properties of the lattice are examined by in situ investigations in a scanning electron microscope. The microlattice displays greater flexibility, compliance, and strain hardening during in-plane compression, whereas it is highly stiff and resistant to fatigue in the out-of-plane orientation. The high aspect ratio of accordion-shaped cells imparts prominent in-plane anisotropy, with longitudinal stiffness twice the transverse stiffness. Folding, interlocking, and compaction of micro-walls are the major deformation mechanisms, inducing brilliant energy dissipation ability, with up to 87% mechanical work-absorption. Digital image correlation analysis reveals that strain re-distribution facilitated by hinges dictates the recoverability of the microlattice. This work provides useful mechanistic insights at the micrometer length scale for tuning the orientation-sensitive mechanical response of mesostructures printed by 2PP.