Additive Manufacturing of Composite Foam Metamaterial Springs for Vibration Isolation

This research introduces a novel approach for reducing the vibrations experienced by passengers in vehicles using metamaterials embedded in polyurethane foam to improve the existing vibration isolation capacity of car seats. An exploration of quasizero and negative stiffness metamaterials is conducted to develop metamaterial springs that exhibit a region of high-static and low-dynamic stiffness to achieve vibration isolation. Metamaterials are developed using low-cost open-source additive manufacturing methods and thermoplastic polyurethane filament. This investigation follows a process of determining the geometric, material, and systemic design requirements, to identify the quasizero and negative-stiffness force–displacement regions. Small-scale models of a car seat are developed by embedding the designed metamaterials into different grades of polyurethane foam and completing static and dynamic testing. The results demonstrate practical applications for implementing metamaterial springs into polyurethane foam to enhance vibration isolation under dynamic loading. The developed material library and the key geometric variables in the metamaterial design allow for application-specific solutions where the selection of the appropriate metamaterial and foam combination can be tailored to suit the system requirements.     

Multi-Scale Design of Ultra-Broadband Microwave Metamaterial Absorber Based on Hollow Carbon/MXene/Mo2C Microtube

Developing various nanocomposite microwave absorbers is a crucial means to address the issue of electromagnetic pollution, but remains a challenge in satisfying broadband absorption at low thickness with dielectric loss materials. Herein, an ultra-broadband microwave metamaterial absorber (MMA) based on hollow carbon/MXene/Mo₂C (HCMM) is fabricated by a multi-scale design strategy. The microscopic 1D hierarchical microtube structure of HCMM contributes to break through the limit of thickness, exhibiting a strong reflection loss of -66.30 dB (99.99997 wave absorption) at the thinnest matching thickness of 1.0 mm. Meanwhile, the strongest reflection loss of -87.28 dB is reached at 1.4 mm, superior to most MXene-based and Mo₂C-based microwave absorbers. Then, the macroscopic 3D structural metasurface based on the HCMM is simulated, optimized, and finally manufactured. The as-prepared flexible HCMM-based MMA realizes an ultra-broadband effective absorption in the range of 3.7-40.0 GHz at a thickness of 5.0 mm, revealing its potential for practical application in the electromagnetic compatibility field.     

Zero-Power Shape Retention in Soft Pneumatic Actuators with Extensional and Bending Multistability

Power-free shape retention enables soft pneumatic robots to reduce energy cost and avoid unexpected collapse due to burst or puncture. Existing strategies for pneumatic actuation cannot attain motion locking for trajectories combining extension and bending, one of the most common modes of operation. Here, a design paradigm is introduced for soft pneumatic actuators to enable zero-power locking for shape retention in both extension and bending. The underpinning mechanism is the integration of a pneumatic transmitter and a multistable guider, which are programmed to interact for balanced load transfer, flexural and extension steering, and progressive snapping leading to state locking. Through theory, simulations, and experiments on proof-of-concept actuators, the existence of four distinct regimes of deformation is unveiled, where the constituents first interact during inflation to attain locking in extension and bending, and then cooperate under vacuum to enable fully reversible functionality. Finally, the design paradigm is demonstrated to realize a soft robotic arm capable to lock at desired curvature states at zero-power, and a gripper that safely operates with puncture resistance to grasp and hold objects of various shapes and consistency. The study promises further development for zero-power soft robots endowed with multiple deformation modes, sequential deployment, and tunable multistability.     

Curved Architected Triboelectric Metamaterials: Auxeticity-Enabled Enhanced Figure-of-Merit

Triboelectric generators are integrated into curved architected materials to realize triboelectric metamaterials that simultaneously harvest electricity from wasted mechanical energy and perform mechanical energy absorption. Novel triboelectric mechanical metamaterials (TMMs) of distance-changing, angle-changing, and mixed modes are designed, fabricated, and tested under a cyclic compressive load. The open-circuit voltage and short-circuit current of lightweight TMMs are found to be as high as 40 V and 10 nA. The introduced TMMs can effectively harvest energy under loadings from two distinctive directions. A theoretical model for predicting the energy harvesting properties of TMMs is developed, and the role of auxeticity on the energy harvesting figure-of-merit (FOM_es) is elicited. The introduced TMMs exhibit enhanced FOM_es enabled by a decrease in their negative Poisson's ratio and an increase in their resilience. The FOM_es of curved architected TMMs surpasses by more than 16 times the FOM_es of triboelectric materials with conventional architectures (i.e., triangular, quadrilateral, and hexagonal cell topologies). An intelligent skateboard with integrated TMMs is fabricated as a proof of concept to demonstrate motion sensing, shock-absorbing, and energy harvesting functionalities of multimodal triboelectric metamaterials. The introduced design strategy for triboelectric metamaterials unlocks their applications in self-powered and self-monitoring sports equipment, smart soft robots, and large-scale energy harvesters.     

From jammed solids to mechanical metamaterials : A brief review

Here we review recent studies of mechanical metamaterials originating from or closely related to marginally jammed solids. Unlike previous approaches mainly focusing on the design of building blocks to form periodic metamaterials, the design and realization of such metamaterials exploit two special aspects of jammed solids, disorder and isostaticity. Due to the disorder, every single bond of jammed solids is unique. Such a bond uniqueness facilitates the flexible adjustment of the global and local elastic responses of unstressed spring networks derived from jammed solids, leading to auxetic materials with negative Poisson’s ratio and bionic metamaterials to realize allostery and flow controls. The disorder also causes plastic instabilities of jammed solids under load. The jammed networks are thus inherently metamaterials exhibiting multi-functions such as auxeticity, negative compressibility, and energy absorption. Taking advantage of isostaticity, topological mechanical metamaterials analogous to electronic materials such as topological insulators have also been realized, while jammed networks inherently occupy such topological features. The presence of disorder greatly challenges our understanding of jammed solids, but it also provides us with more freedoms and opportunities to design mechanical metamaterials.     

Biomimetic and Biophilic Design of Multifunctional Symbiotic Lichen–Schwarz Metamaterial

Noise and environmental problems are significant issues that affect human beings through the triggering of various stressors, including biological, chemical, physical, and psychological stressors. To address these issues, it is essential to develop environmentally friendly strategies, particularly in the field of materials manufacturing. This study presents a novel biomimetic and biophilic design of a lichen–Schwarz metamaterial (SLSM) that achieves multifunctional properties in noise attenuation and humidity control. The SLSM achieves acoustic and air humidifying multi-functions symbiotically through the use of triply periodic minimal surface (TPMS) and naturally occurring organic lichen. To attain partial or complete sound-blocking capabilities at specific frequencies, a Schwarz meta-symbiont in SLSM requires a parametric design of unit cell characteristics prior to 3D printing. The SLSM structure mimics a biomimetic shell and meta-symbiotic exoskeleton that safeguards the inner symbiont lichen by transitioning it from a dry and brittle state to a hydrated and flexible state during humidity control. The symbiotic lichen in SLSM offers superior sound attenuation across a broader frequency range and adds a unique function of humidity control, which is essential for sustainable architecture and multifunctional furniture in the forthcoming era of buildings.     

Modular Soft Robotic Actuators from Flexible Perforated Sheets

Soft robotic actuators can be designed to achieve complex and tailored motions while simultaneously leveraging their compliance to interact with complex and often delicate environments. Mechanical metamaterials reveal a route to customizable deformations, force exertion, and mechanical energy efficiency attainable by careful arrangement of local geometric features. Herein, modular soft robotic actuators are developed from soft elastomers and flexible thermoplastic sheets of various unit cell designs. The efforts are focused on center-symmetric perforated sheets, which are formed into flexible cylindrical skins that surround the soft inflatable actuators. The results demonstrate the influence of perforation geometry on the spatial stiffness of the reinforcement structure and the proposed actuators’ response through several investigations. It is demonstrated that the free-boundary displacement, maximal force exertion, and mechanical energy efficiency of extensile actuators are dependent on a change of deformation mode in the mesostructure. The spatial stiffness concept is extended to develop soft robotic actuators that can bend, twist, and perform hybrid motions, such as simultaneous bending and twisting. Multisegment soft robotic arms are also developed from the aforementioned actuators. Investigations in this study provide a step toward the development of highly customizable and programmable soft robotic actuators for various applications.