Recently, auxetic metamaterials have turned into an area of growing interest, because of their deformation uniqueness, design flexibility, and functional diversity. To achieve their programmable design of densification and energy absorption, the variable stiffness factor method (VSF) emerged. However, the energy absorption capacity of the metamaterials designed by this method has a significant reduction. In this work, to remedy this defect, several different lightweight specimens are proposed and fabricated to extend the stage of energy absorption and retain the tunability. Then quasi-static compression tests and finite element methods are carried out to analyze the mechanical properties of the designed boundary-constrained structure (BCS) and X-shape constrained structure (XCS). The accuracy of the densification points and energy absorption capacity of the two structures at different VSF values are also studied. The results show that the densification points of the designed structures can be adjusted quantitatively, and their deformation modes are more stable than conventional structures during compression. Furthermore, their specific energy absorption is four and five times higher than that of non-lightweight structures with VSF = 40%, respectively. These findings contribute to advancing the implementation of auxetics in applications of multistage protective structures.

Auxetic metamaterials have shown good stability and uniform deformation capabilities against influences (vibration, temperature change, load change), making them significant in maintaining and adjusting the bandgap of phonon crystals. The low-frequency and broadband are the important goals of phonon crystals. For most traditional re-entrant honeycomb structures (T-RHS), the bandgap range is narrow and tunability is poor. Here, an auxetic hybrid structure with tunable acoustic bandgap (AHS-T) consisting of periodic mass inclusions integrated with traditional re-entrant honeycomb and chiral hybrid is proposed. Aiming at investigating the tunability of the bandgap in the low-frequency range. Compared with T-RHS, the bandgap real-time adjustment and wider bandwidth of the AHS-T be realized during compression and tension. The numerical results show that the bandgap of the AHS-T can be flexibly tailored by reasonably adjusting the strain and geometrical configurations of AHS-T. The bandwidth of AHS-T can be increased to 87.1% when the bottom diameter D b and column height H of the scatterer are changed reasonably. Moreover, the deformation behavior of auxetic material has an auxiliary effect on expanding the bandwidth. Compared with the structure which is not subjected to load, the adjustable amplitude of the bandgap is 41%. The findings of this work provide a design idea for manipulating elastic waves in dynamic environment.

In this work, a double tubular structure consisting of auxetic tubular structures and aluminum foam was proposed. The mechanical properties of the aluminum foam-filled auxetic tubular structures and aluminum foam-filled conventional tubes under uniaxial compression were compared numerically and experimentally. Compared with conventional aluminum foam-filled tubular structures (FFT), aluminum foam-filled auxetic double tubular structures (FFADT) have stable compression deformation and superior energy absorption because of stronger interaction between the aluminum foam and the auxetic tube. Although the FFADT exhibits lower bearing capacity but overcomes the shortcomings of larger peak force and larger load fluctuation that usually occurs in conventional tubes.

Hydrogel-based flexible sensors hold significant potential for applications in skin-like electronics, human motion detection, and human-machine interfaces. The development of strain-sensitive hydrogel with high stretchability, self-healing, and high conductivity is highly sought after. A hydrogel sensor with a strain-sensitive semi-interpenetrating network consisting of polyacrylamide (PAM), poly(vinyl alcohol) (PVA), and alginate (SA). With environmentally benign KCl as a conductive filler, SA/PVA/PAM/KCl hydrogel has a high toughness (4.8 MJ m-3), high stretchability (1250%), high tensile strength (510 Kpa), excellent recoverability, and excellent self-adhesiveness toward various substrates. The hydrogel sensors present high sensitivity with good linearity in the majority of the detection range with a gauge factor (GF) over 6.78, a response time of 268 ms, and excellent durability. The ionic conductive hydrogel has shown excellent energy harvesting capability as a triboelectric nanogenerators (TENG). Multifunctional organic hydrogels provide a protocol for the design and preparation of multifunctional hydrogel for wearable electronics.

Auxetic honeycomb sandwich structures (AHS) composed of a single material generally exhibit comparatively lower energy absorption (EA) and platform stress, as compared to traditional non-auxetic sandwich structures (TNS). To address this limitation, the present study examines the use of aluminum foam (AF) as a filling material in the re-entrant honeycomb sandwich structure (RS). Filling the AHS with AF greatly enhances both the EA and platform stress in comparison to filling the TNS with AF, while the auxetic composite honeycomb sandwich structure effectively addresses interface delamination observed in traditional non-auxetic composite sandwich structures. Subsequently, the positive–negative Poisson’s ratio coupling designs are proposed to strengthen the mechanical features of a single honeycomb sandwich structure. The analysis results show that the coupling structure optimizes the mechanical properties by leveraging the high bearing capacity of the hexagonal honeycomb and the great interaction between the re-entrant honeycomb and the filling material. In contrast with traditional non-auxetic sandwich structures, the proposed auxetic composite honeycomb sandwich structures demonstrate superior EA and platform stress performance, suggesting their immense potential for utilization in protective engineering.

A cost-effective and scalable approach for the fabrication of heterostructured microsupercapacitors (MSCs) employing screen-printing followed by sequential electrochemical and microspray deposition techniques has been demonstrated. The microsupercapacitor electrode (MSC) that composed of stacked layers of mesoporous carbon, polyaniline (PANI), and MXene hold significant promise for wearable electronics. By adjusting the deposition and spray cycles, the MSC can be readily coated with PANI and MXene. The sequentially stacked two layers of MXene and PANI on the mesoporous carbon spheres (PMPM-MSC) yielded a specific capacitance of 1003 mF cm-2 at 0.5 mA cm-2, surpassing the performance of PANI/mesoporous carbon electrode by 1.6 times (771 mF cm-2). After 10,000 cycles of charge and discharge, PMPM-MSCs retained more than 86% of their initial capacitance. In-situ Raman spectroscopy confirmed the synergistic effects between MXene and PANI within the heterostructured stacked PMPM-MSC electrodes, including enhanced electronic conductivity and improved electrolyte ion dissociation, which aligned with the electrochemical measurement results, such as fast charge/discharge rates and reduced internal and mass transport resistance. This study demonstrates the potential of screen-printed heterostructured MSC stacks with maximum electrochemical synergy for portable and wearable energy storage devices. Based on screen printing technology, electrodeposition, and micro-spraying technology, micro-supercapacitors composed of mesoporous carbon, polyaniline (PANI), and MXene stack are prepared. Through the combination of mesoporous carbon and nanocellulose fibers, a slurry with a 3D network interconnection structure is obtained, and the printed electrode can easily deposit polyaniline and MXene on the surface of micro-supercapacitors (MSCs) by adjusting the deposition and spray cycle. image

A majority of tubular structures have been widely studied due to their superior mechanical performance. Existing works on star-shaped tubular structures have focused on improving their stability and energy absorption performance. This paper proposes a novel star-shaped tubular lattice structure (STL), which not only possesses excellent mechanical properties but also exhibits exceptional auxetic effect. In addition, such structures exhibited two distinct deformation modes under different loading directions. Numerical and experimental studies of the mechanical behavior of the star-shaped tubular structure demonstrated low peak stresses under lateral loading and superior bearing capacity and stability under axial compression. Most importantly, the structure embodied a significant auxetic effect under the direction of both loads. The mechanical performance of the star-shaped tubular structures was investigated by changing wall thickness and angles, which can realize the optimal design. This study enriches the research on star-shaped tubular structures and provides a new perspective and reference for the design of auxetic tubular metamaterials in the future. Furthermore, the star-shaped tubular structure with auxetic behavior has considerable potential for applications in civil engineering and protective fields.

Aluminum foam-filled circular auxetic metamaterials were designed, fabricated and investigated in this work. Firstly, the mechanical properties and deformation modes of the proposed aluminum foam-filled auxetic tubes (AFFAT) were analyzed using experimental and finite numerical methods. The load capacity of the AFFAT is larger than the sum of the load for the auxetic tube and aluminum foam. Secondly, the effect of aluminum foam’s density on the mechanical properties of AFFAT was studied. The results show that the energy contribution ratio of the interaction between the aluminum foam and auxetic tube improves with the aluminum foam’s density. Then, the effect of unit cell size on the mechanical properties of AFFAT was investigated. It can be concluded that the compression load efficiency of AFFAT with the ratio of the elliptical major axis to the minor axis increases. Finally, the study of tube type’s effect on the mechanical properties of AFFAT was carried out. Among the investigated composite structures, the AFFAT has the highest compression load efficiency. Due to their unique structural design and excellent mechanical properties, the AFFAT has great potential for applications in aerospace, vehicle crashworthiness and protective infrastructure.

Sandwich panels find use as energy-absorbing devices and protective structures in various fields. However, due to their mostly rigid surfaces, they might not meet the protection requirements in complex conditions like non-Euclidean geometries of protected objects. To address this issue, a novel sandwich panel structure inspired by the synclastic curvature behavior of auxetic metamaterial was proposed. The surface of the proposed sandwich panel features rotating squares that allow for horizontal stretching, resulting in an improvement in controllability, auxeticity, flexibility, and synclastic curvature compared to traditional sandwich panels. Experimental and numerical methods were used to compare the similarities and differences between the novel and traditional sandwich panels under uniaxial compression. Additionally, different rotation angles and numbers of unit cells were simulated to examine their mechanical properties. The study also explored the auxeticity, stretchability, synclastic curvature, and potential applications of the novel sandwich panel. It was found that appropriate cuts on the sandwich panel can enhance energy absorption. These findings could pave the way for new opportunities in the application and basic research of sandwich panels while providing ideas for enhancing the survivability of panel structures in complex conditions.

Auxetic metamaterials have many exceptional mechanical properties due to their counter-intuitive deformation characteristics. However, most studies on auxetic metamaterials focus on in-plane properties while auxetic metamaterials are inevitably affected by out-of-plane forces in practical applications. In this study, the out-of-plane mechanical properties and deformation patterns of a recently proposed auxetic structure, the chiral lattice with circular nodes (NCL), were investigated numerically and experimentally under quasi-static uniaxial compression. It also took truss lattice and traditional chiral lattice (TCL) as benchmarks to compare. The results showed that the energy absorption capacity of the NCL structure in the out-of-plane direction was superior to that of the other two benchmarks. The energy absorption (EA) and specific energy absorption (SEA) of the TCL structure and the truss lattice structure were roughly close, compared with the TCL structure, the EA and SEA of the NCL structure were improved by 34.1% and 13.3%, respectively. In addition, parametric studies were carried out to investigate the effects of geometric parameters and volume fraction of the frame on the structural deformation patterns and mechanical properties.

Auxetic structures have theoretically superior bending resistance that was previously inaccessible. Compared with the planar two-dimensional (2D) auxetic structures, the three-dimensional (3D) counterparts may exhibit auxetic behavior in multiple directions. Recently, increasing studies have been devoted to studying the mechanical properties of 3D auxetic structures. In this paper, two 3D honeycomb structures were proposed, namely 3D re-entrant honeycomb and 3D hexagonal honeycomb. Subsequently, the mechanical properties, deformation modes, and deformation mechanisms of these two honeycomb structures under three-point bending were investigated experimentally and numerically. Compared with the 3D non-auxetic hexagonal honeycomb, the 3D auxetic re-entrant honeycomb exhibits higher ductility and fracture resistance. In addition, unlike the conventional honeycomb structure, the mid-span section of the 3D auxetic honeycomb exhibits a trapezoidal deformation mode under bending. Finally, the influence of the number of unit cells and geometric parameters on the mechanical properties of the 3D honeycomb structure were systematically studied by parametric analysis. The excellent bending performance provides a basis for the application of auxetic honeycomb in the fields of biomedicine, soft robots, and buffer devices.

Fabricating auxetic metamaterials via an assembling process is a novel and economic method. In this work, an assembled structure that could realize the conversion of positive and negative Poisson's ratio by changing the length of the connection plates was proposed. Two different honeycomb structures of the assembled re-entrant honeycomb (ARH) and assembled hexagonal honeycomb (AHH) were designed, manufactured and studied numerically and experimentally. Dimensions of the two assembled honeycomb structures are consistent. The nominal stress-strain curves of the two assembled honeycombs have similar initial peak stresses, but the assembled re-entrant honeycomb has a higher bearing capacity and shorter stress plateau. Subsequently, the mechanical properties of ARH were studied by changing the three parameters of corrugated plates. Besides, the energy absorption and specific energy absorption of the assembled re-entrant honeycomb were investigated through the numerical method. Finally, the assembled honeycomb structure with the suitable parameter was determined. Tunable mechanical properties of the assembled auxetic structures can be obtained by varying the length of the connection plates. This strategy of assembly can reduce the costs of manufacturing and improve transportation efficiency. The proposed assembled structures are promising in civil and protective engineering.

In this work, a novel metamaterial with an enhanced programmable CTE range is proposed by combing the bi-material bending-dominated chiral unit cell and stretching-dominated triangle unit cell. In the design process, the three sides of the original triangle unit cell are replaced using bi-material anti-chiral unit cells with positive CTE or negative CTE. The materials with high CTE (Nylon) and low CTE (PVE) are selected to realize thermal deformation. Four models with a chiral block size of 80 x 80 mm(2), namely NTE#1, NTE#2, NTE#3, and NTE#4, are designed, analyzed, and discussed. To examine the tailorable CTE range and zero CTE, the parameter analyses are comprehensively carried out using the verified finite element models. The results demonstrate a more than threefold enhancement in the CTE range of the initial triangle metamaterial, accompanied by substantial improvements in its design smoothness, continuity, and flexibility. The combination design extended the tailorable CTE range to -710 similar to 818 ppm/degrees C when the temperature increases from 30 degrees C to 60 degrees C. In addition, the curve of zero CTE with different parameter combinations is obtained. The aim of this work is to construct a new metamaterial with enhanced thermal-mechanical stability to balance the interface thermal deformation of composites in engineering devices, e.g., aerospace and precision instruments. Based on these findings, the structural stiffness and temperature range are still limited and worth studying further.

In this study, a three-dimensional compression-torsion coupled negative Poisson structure was designed by combining the perforated plate with the inclined rod out of the plane. This structure can realize the selection of the positive and negative Poisson's ratio of the three-dimensional structure by changing the direction of the inclined rod. The absolute value of the Poisson's ratio of the two structures is close to the same but the positive and negative are opposite. Both structures maintain close to the same stress-strain curve. This structural characteristic can realize the selection of an ideal Poisson's ratio without changing the mechanical properties of the structure. Two 3D compression-torsion coupling structures were printed using metal 3D printing technology, and compression tests were carried out in the axial and transverse directions. Then, parametric analysis was carried out under axial compression, including energy absorption and specific energy absorption analyses. The results show that the experimental and finite element results are in good agreement and the finite element simulation is accurate. The Poisson's ratio of this negative Poisson's ratio metamaterial is selectable under both axial and lateral pressure conditions, so the structure has the excellent characteristics of selectable Poisson's ratio in three mutually perpendicular axes. The results show that the absolute value of Poisson's ratio exceeds 1 when the structure is under lateral compression, which indicates that the lateral deformation rate of the structure is greater than the axial deformation rate under this compression form. Finally, two tubular structures with different arrangements were designed by using these two structural cells, and the ball passing test was carried out. The results show that different arrangements will cause the balls to experience different resistances when passing through different positions in the tube. Research shows that the structure has potential applications in energy absorption, biomechanical devices, smart actuators, and devices with special requirements for Poisson's ratio.

The incorporation of aluminum foam in auxetic tubes can significantly enhance the overall stability, stiffness and energy absorption properties. In this work, a comparative study of the mechanical properties between auxetic circular and square tubes was conducted. The foam-filled auxetic square tube (FFAST) has the best specific energy absorption (SEA) up to 1.45 among the four auxetic tubes, and exhibits a pronounced auxetic effect throughout the minor deformation under compression, with a symmetrical and stable deformation mode. The experimental and simulated results are in excellent agreement. Hence, the finite element method is subsequently applied to the parametric analysis of FFAST, including the effects of wall thickness, height and ellipticity. Firstly, it is found that the thinner the wall thickness of FFAST is, the weaker the peak force and the greater the specific energy absorption would be. Secondly, within the same minor strain of different heights, SEA remains almost the same. Finally, with the decrease in ellipticity, the yield load of the bore tube gradually grows and SEA progressively increases. It can be concluded that FFAST with the ellipticity of 1.69 has both a high SEA and non-peak force. These findings are useful for fabricating FFAST for individual scenarios of energy absorption requirements. Such composite tubular structures will have practical applications in, e.g., protective engineering, vehicle engineering and aerospace engineering.

Novel auxetic tubes (NAT) were designed, fabricated and examined. Their mechanical properties were compared with the original auxetic tube (OAT) by the finite element method and experiments. The results show that NAT increases the specific energy absorption (SEA) without sacrificing the auxetic characteristic. Then, the effect of unit cell parameters on the mechanical properties of NAT tubes was explored. The results show that, in a certain range, rib reduction can improve the SEA of NAT without sacrificing the auxetic effect. To further improve the mechanical properties of NAT, three novel ribbed auxetic tubes (NRAT) were proposed and compared with NAT. It is concluded that the SEA of NAT can be improved by adding a straight rib to the inner long axis of the ellipse. In addition, the effect of straight rib width on the mechanical properties of NRAT was investigated. The results show that the larger the straight rib is, the higher the energy absorption is. Finally, the stability of the 3D auxetic tubular metamaterial is greater than that of the 2D auxetic thin-plate metamaterial by investigating. Due to their desirable mechanical properties, the NAT and NRAT have great potential for applications in medical engineering, vehicle crashworthiness and protective infrastructure.

To broaden the family of available matrices for auxetic composites, an auxetic TPU frame with cubic matrix cases is proposed, tested, and optimized. Quasi-static compression was applied to the new structure to investigate its mechanical performance. Verified finite element models were established for demonstrating the programmable strength and stiffness. Except for traditional materials that can be portably filled into auxetic frames, aluminum cubic tubes, as an instance of multiple matrixes, were filled into the frame. The newly fabricated auxetic composite structure was also tested experimentally. Besides, a section on multi-functional perspective and potential applications were given at the end to better illustrate the proposed structure in practice. The optimized auxetic frame can be used alone or with multiple strengthening matrixes in many aspects including energy absorbers and smart sensors.

Auxetic metamaterials as a cutting-edge field have attracted extensive attention from researchers because of their peculiar deformation mode and excellent mechanical properties. However, the unstable deformation and low stiffness of auxetic structure limit their potential development. To address these limitations, a novel auxetic honeycomb (NAH) has been proposed by combining an original re-entrant hexagonal honeycomb (RH) with cross-chiral honeycomb (CH). The proposed structure has higher stiffness than RH, and more stability than CH. Additionally, the stiffness, densification point, and auxetic behavior can be adjusted by the angle of cell ribs (theta). In this study, the static mechanical properties and deformation behavior of NAH are investigated experimentally and numerically. Furthermore, the energy absorption and deformation behavior of NAH under different impact loadings (low-, medium- and high-velocity) are investigated numerically. Finally, a series of functionally graded NAHs with varying thicknesses are studied to improve the impact resistance of NAH. The results show that NAH has better specific energy absorption than RH and CH under static compression. Moreover, NAH with theta< 90 degrees exhibits greater energy absorption capacity compared to NAH with "theta > 90 degrees" at high-velocity impacts.

A costly and inefficient manufacturing process significantly impedes broad applications of auxetics. Herein, a novel fabrication methodology based on assembling plates and tubes is presented. Instead of fabricating auxetic structures by regularly-used 3D printing and laser cutting techniques, inexpensive commonly-used materials were assembled by adhesive in a portable way. Quasi-static compression test was carried out experimentally and numerically. Based on reliable finite element models, parametric study and gradient design were conducted for structural optimization. Numerical results reveal the significance of each geometric parameter and give evidence of the advantages of gradient design. The proposed structures are not only favorable in mechanical performance in terms of multi-stage densification and programmable stiffness and strength, but also promising in low-cost and large-scale fabrication. Such a fabrication methodology has great potential in applications of auxetic structures in protective equipment and smart energy absorbers.

Novel 3D printed square auxetic tubular lattice (SATL) structures were designed, fabricated and investigated. Their mechanical properties were examined by the finite element method and experiments. The height and wall thickness show different effects on the mechanical properties of SATL structures. Compared with the circular auxetic tubular (CATL) structures, the SATL structure has a lower peak force under axial load. Under lateral load, the SATL structure has higher stiffness and specific energy absorption. Moreover, the auxetic effect of the proposed SATL structure is also obvious under lateral load. Then, numerical investigations of several improved SATL structures were carried out, the results show that the improved square auxetic tubular lattice (ISATL) structures have stronger energy absorption capacity under axial and lateral loads. Due to their unique structural design and excellent mechanical properties, the SATL structures and ISATL structures have great potential for applications in civil engineering, vehicle crashworthiness and protective infrastructure.