Materials and structures with negative Poisson's ratio exhibit a counter-intuitive behaviour. Under uniaxial compression (tension), these materials and structures contract (expand) transversely. The materials and structures that possess this feature are also termed as 'auxetics'. Many desirable properties resulting from this uncommon behaviour are reported. These superior properties offer auxetics broad potential applications in the fields of smart filters, sensors, medical devices and protective equipment. However, there are still challenging problems which impede a wider application of auxetic materials. This review paper mainly focuses on the relationships among structures, materials, properties and applications of auxetic metamaterials and structures. The previous works of auxetics are extensively reviewed, including different auxetic cellular models, naturally observed auxetic behaviour, different desirable properties of auxetics, and potential applications. In particular, metallic auxetic materials and a methodology for generating 3D metallic auxetic materials are reviewed in details. Although most of the literature mentions that auxetic materials possess superior properties, very few types of auxetic materials have been fabricated and implemented for practical applications. Here, the challenges and future work on the topic of auxetics are also presented to inspire prospective research work. This review article covers the most recent progress of auxetic metamaterials and auxetic structures. More importantly, several drawbacks of auxetics are also presented to caution researchers in the future study.

Auxetic metamaterials and structures: a review

Innovative honeycomb-based structures have been paid substantial attention in recent years due to their superb mechanical performances and specific functions. The presented study made acomprehensive overview on the development of the innovative honeycomb-based structures in the past two decades, including filled-type, embedded, tandem, hierarchical, auxetics with Negative Poisson's Ratio (NPR), etc. Mechanical performances of these structures were commented with advantages and disadvantages, related on their geometric configurations, mechanical performance and dynamic response, respectively. The challenges as well as future directions were also analyzed. The achievements provide significant guidelines in designing of new generation light-weight honeycomb-based structures.

Recent advances in novel metallic honeycomb structure

Different from conventional materials, materials with negative Poisson's ratios expand laterally when stretched longitudinally. Known as ‘auxetic’ materials, the effect means they possess particularly fascinating properties, which have recently attracted considerable attention in the literature. A range of auxetic materials has been discovered, theoretically designed and fabricated. Developments in additive manufacturing (AM) techniques enable fabrication of materials with intricate cellular architectures. This paper outlines recent progress in the development of auxetic materials and structures, and their mechanical properties under quasi-static and dynamic loading are analysed and summarised. Limited experimental studies on 3D printed auxetic materials and structures are given more attention, ahead of extensively finite element (FE) simulations. A special focus is dedicated to their large, plastic deformation behaviour and energy absorption performance, which should be stressed in their engineering applications; no review paper has yet been found regarding this. Finally, this paper provides an overview of current study limitations, and some future research is envisaged in terms of auxetic materials and structures, nano-auxetics and additive manufacturing.

Large deformation and energy absorption of additively manufactured auxetic materials and structures: A review

Following high profile, life changing long term mental illnesses and fatalities in sports such as skiing, cricket and American football—sports injuries feature regularly in national and international news. A mismatch between equipment certification tests, user expectations and infield falls and collisions is thought to affect risk perception, increasing the prevalence and severity of injuries. Auxetic foams, structures and textiles have been suggested for application to sporting goods, particularly protective equipment, due to their unique form-fitting deformation and curvature, high energy absorption and high indentation resistance. The purpose of this critical review is to communicate how auxetics could be useful to sports equipment (with a focus on injury prevention), and clearly lay out the steps required to realise their expected benefits. Initial overviews of auxetic materials and sporting protective equipment are followed by a description of common auxetic materials and structures, and how to produce them in foams, textiles and Additively Manufactured structures. Beneficial characteristics, limitations and commercial prospects are discussed, leading to a consideration of possible further work required to realise potential uses (such as in personal protective equipment and highly conformable garments).

https://www.mdpi.com/2076-3417/8/6/941/pdf

Review of Auxetic Materials for Sports Applications: Expanding Options in Comfort and Protection

A novel re-entrant auxetic honeycomb with enhanced in-plane impact resistance

Impact and close-in blast response of auxetic honeycomb-cored sandwich panels: Experimental tests and numerical simulations

Rational design of sulfur hosts for effectively confining lithium polysulfides (LiPS) and optimizing the sluggish sulfur kinetics is still a major challenge in lithium–sulfur batteries (LSBs). In this work, a simple strategy of introducing single Mo–N4 atoms into N‐doped carbon nano‐flower matrix (Mo‐N‐CNF) as sulfur host cathode materials is developed to realize high‐performance LSBs. These single Mo–N4 atoms have been demonstrated to regulate the hydrophilic nature, Li‐ion diffusion, adsorption capacity, and catalytic conversion of polysulfides via experimental evidences and theoretical calculations. The resulting Mo‐N‐CNF with high loading content of sulfur (>72 wt.%) exhibits a high specific capacity (1248 mAh g–1 at 0.2 C) and excellent rate capability (715 mAh g–1 at 5 C). More importantly, the outstanding cycling performance with a low attenuation rate of only 0.004% per cycle over 400 cycles at 4.27 mA cm–2 is achieved with the area sulfur loading of 5.1 mg cm–2. This work demonstrates a viable strategy for using single atoms‐based carbon materials with high exposed sites as multiple captors for LiPS and an efficient accelerator for sulfur redox kinetics toward next‐generation LSBs with boosted electrochemical performance.

Single Mo–N4 Atomic Sites Anchored on N‐doped Carbon Nanoflowers as Sulfur Host with Multiple Immobilization and Catalytic Effects for High‐Performance Lithium–Sulfur Batteries

In this paper, an ultrathin terahertz metamaterial absorber consisting of Au pattern, polyimide and Au film is proposed. Based on the finite element method, the simulation consequences indicate that when its thickness is 9.4 m, there is a narrow absorption peak for the TE wave at 7.73 THz with near-perfect absorption of 99.8% and a full width at half maximum of 0.0896 THz, resulting from impedance matching and plasma resonance. And the absorption switching from a dual-band to a multi-band or a single-band to a dual-band absorption can be achieved for TM and TE waves by changing the incident angle or polarization angle. Furthermore, the absorption can also be switched from 99% to 0% by changing the polarization angles, and has a high tolerance of polarization angle down to more than 20 degrees. In addition, its application in the sensor is also studied at 7.73 THz, where its refractive index sensitivity and thickness sensitivity can achieve 1.2 THz/RIU and 0.065 THz/m, respectively. This might be utilized for monospectral imaging, biological diagnosis, and environmental monitoring.

Zhi-Hang Yu

Papers

Impact and close-in blast response of auxetic honeycomb-cored sandwich panels: Experimental tests and numerical simulations

Single Mo–N4 Atomic Sites Anchored on N‐doped Carbon Nanoflowers as Sulfur Host with Multiple Immobilization and Catalytic Effects for High‐Performance Lithium–Sulfur Batteries

An Ultra-thin, Switchable Ultra-narrow Band Absorber for High-performance Terahertz Sensing