Vibration isolation performance at low-frequency ranges before resonance is a vital characteristic that conventional springs cannot exhibit. This paper introduces a novel zero Poisson’s ratio graded cylindrical metamaterial to fulfill two main goals: (1) vibration isolation performance in low-frequency bands prior to resonance and (2) global buckling control of a long cylindrical tube. For this purpose, “soft and stiff” re-entrant unit cells with varying stiffness were developed. The cylindrical metamaterials were then fabricated using a multi-jet fusion HP three-dimensional (3D) printer. The finite element analyses (FEA) and experimental results demonstrate that the simultaneous existence of multi-stiffness unit cells leads to quasi-zero stiffness (QZS) regions in the force-displacement relationship of a cylindrical metamaterial under compression. They possess significant vibration isolation performance at frequency ranges between 10 and 30 Hz. The proposed multi-stiffness re-entrant unit cells also offer global buckling control of long cylindrical tubes (with a length to diameter ratio of 3.7). The simultaneous existence of multi-stiffness re-entrant unit cells provides a feature for designers to adjust and control the deformation patterns and unit cells’ densification throughout cylindrical tubes.

This review serves as a comprehensive design strategy for designing quasi-zero stiffness (QZS) mechanical metamaterials (MMs). It discusses their underlying deformation mechanisms that enable the attainment of QZS behavior under both compressive and tensile loadings. While the QZS characteristic of metamaterials has garnered considerable attention, further research is essential to unlock their potential fully. Numerous QZS metamaterials have been meticulously reviewed. They comprise various elements and mechanisms, including positive and negative stiffness elements (PS and NS), PS elements with variable stiffness, bending mechanisms employing stiff joints/areas, buckling, buckling-rotating, and bending/buckling deformation mechanisms leading to a QZS feature. Furthermore, the capability of multi-material, adaptive, smart metamaterials, origami (bending around the hinge of the folded joints), and kirigami lattices (out-of-plane buckling via cutting patterns) are weighted. These diverse mechanisms contribute to achieving QZS behavior in metamaterials under both compression and tension loads, which is paramount for various mechanical applications such as passive vibration isolation. This review effectively categorizes QZS metamaterials based on their underlying mechanisms, providing scholars with valuable insights to identify suitable mechanisms for the desired QZS feature.

This study introduces a bio-inspired vibro-drilling system that harnesses bending resonance to improve penetration efficiency in granular soils. Inspired by the lateral undulation of sandfish, the probe combines a compliant polylactic acid (PLA) flexure joint with surface-bonded piezoelectric actuators to generate whirling bending motions independent of axial thrust and torque. Experimental modal analysis was conducted to identify the resonant bending modes of the system. Subsequently, finite-element analysis was performed to visualize the dominant mode shapes and assess the influence of the compliant joint on soil fluidization. Experimental penetration tests indicated that the optimal bending resonance for rapid soil fluidization occurs at approximately 4400 Hz. Complementary FE analysis of the resonance modes revealed that the compliant PLA joint amplifies the bending resonance, leading to a tenfold increase in squared tip velocity. This enhanced motion facilitates greater transfer of vibrational energy to the surrounding soil particles, thereby improving fluidization efficiency. Whirling vibrations at approximately 4400 Hz reduce linear penetration force by 46.4%. When combined with rotary drilling, bending vibrations also reduce penetration force by 31% and the required torque by 12% compared to a non-vibrating probe. These findings demonstrate that bio-inspired bending resonance significantly enhances energy transfer to soil particles, enabling compact, energy-efficient, and waterless drilling systems. This opens new opportunities for planetary subsurface prospecting, miniaturized low-power geotechnical probes for in-situ testing in resource-constrained environments such as the Moon and Mars, trenchless micro-drilling in urban environments, soft robotic systems for underground exploration or inspection, and drone- or rover-mounted penetration-based sensing.