Kirigami-based metastructures with programmable multistability.

SignificanceDifferent from most existing multistable structures whose multiple stable states are achieved through the combinational effect of bistable units, we invent a generic tristable kirigami cuboid. The three stable states have fundamentally distinct geometric configurations and chirality, and the transformation among them can be realized by tension/compression or clockwise/counterclockwise twist. Tessellating the units in series, a family of multistable metamaterials can be constructed, the mechanical behaviors of which are programmable by the unit geometry, the material of the elastic joints, the number of units, and the loading conditions. As a demonstration of the potential applications, a frequency reconfigurable antenna for 5G triple-band communication is developed based on a tristable unit, and the frequency tunability is verified by experiments.

Thermal Emission Manipulation Enabled by Nano-Kirigami Structures.

The nano-kirigami metasurfaces have controllable 3D geometric parameters and dynamic transformation functions and therefore provide a strong spectral regulation capability of thermal emission. Here, the authors propose and demonstrate a dynamic and multifunctional thermal emitter based on deformable nano-kirigami structures, which can be actuated by electronic bias or mechanical compression. Selective emittance and the variation of radiation intensity/wavelength are achieved by adjusting the geometric shape and the transformation of the structures. Particularly, a thermal management device based on a composite structure of nano-kirigami and polydimethylsiloxane (PDMS) thin film is developed, which can dynamically switch the state of cooling and heating by simply pressing the device. The proposed thermal emitter designs with strong regulation capability and multiple dynamic adjustment strategies are desirable for energy and sensing applications and inspire further development of infrared emitters.

Superlubric Sliding of Graphene Auto-Kirigami with Interfaces Containing Self-Assembled Stripe-Pattern Adsorbates.

Van der Waals heterostructures formed by stacked 2D materials show exceptional electronic, mechanical, and optical properties. Superlubricity, a condition where atomically flat, incommensurate planes of atoms result in ultra-low friction, is a prime example enabling, for example, self-assembly of optically visible graphene nanostructures in air via a sliding auto-kirigami process. Here, it is demonstrated that a subtle but ubiquitous adsorbate stripe structure found on graphene and graphitic surfaces in ambient conditions remains stable within the interface between twisted graphene layers as they slide over each other. Despite this contamination, the interface retains an exceptional superlubricious state with an estimated upper bound frictional shear strength of 10 kPa, indicating that direct atomic incommensurate contact is not required to achieve ambient superlubricity for 2D materials. The results suggest that any phenomena depending on 2D heterostructure interfaces such as exotic electronic behavior may need to consider the presence of stripe adsorbate structures that remain intercalated.

Kirigami tripod-based electrode for the development of highly stretchable dengue aptasensor.

Kirigami is one of the interesting paper art forms and the modified sub-class of origami. Kirigami paper art is widely employed in a variety of applications, and it is currently being used in biosensors because of its outstanding advantages. This is the first study on the use of a Kirigami-based aptasensor for DENV (Dengue virus)-antigen detection. In this study, the kirigami approach has been utilized to develop a stretchable, movable, and flexible sensor. The constructed stretchable-kirigami electrode helps in adjusting the connection of electrodes without disturbing the electrochemical cell zone during the experiment. To increase the sensitivity of this biosensor we have synthesized Ag-NPs (Silver nanoparticles) via chemical methods and characterized their results with the help of TEM & UV-Vis Spectroscopy. Different electrochemical approaches were used to validate the sensor response i.e., CV (Cyclic voltammetry) and LSV (Linear sweep voltammetry), which exhibited great detection capability towards dengue virus with the range of 0.1 µg/ml to 1000 µg/ml along with a detection limit of 0.1 µg/ml and showing no reactivity to the chikungunya virus antigen, making it more specific to the DENV antigen. Serum (healthy-human) was also successfully applied to validate the results of the constructed aptasensor. Integration of the Kirigami approach form with the electrochemical aptasensor that utilizes a 3-E setup (three-electrode setup) which is referred to as a tripod and collectively called Kirigami-tripod-based aptasensor. Thus, the developed integrated platform improves the sensors capabilities in terms of cost efficiency, high stretchability, and sensitivity.

Elastocaloric Effect in Graphene Kirigami.

Kirigami, a traditional Japanese art of paper cutting, has recently been explored for its elastocaloric effect (ECE) in kirigami-based materials (KMs), where an applied strain induces temperature changes. Importantly, the feasibility of a nanoscale graphene kirigami monolayer was experimentally demonstrated. Here, we investigate the ECE in GK representing the thinnest possible KM to better understand this phenomenon. Through molecular dynamics simulations, we analyze the temperature change and coefficient of performance (COP) of GK. Our findings reveal that while GKs lack the intricate temperature changes observed in macroscopic KMs, they exhibit a substantial temperature change of approximately 9.32 K (23 times higher than that of macroscopic KMs, which is about 0.4 K) for heating and -3.50 K for cooling. Furthermore, they demonstrate reasonable COP values of approximately 1.57 and 0.62, respectively. It is noteworthy that the one-atom-thick graphene configuration prevents the occurrence of the complex temperature distribution observed in macroscopic KMs.

Deterministic and stochastic control of kirigami topology.

Kirigami, the creative art of paper cutting, is a promising paradigm for mechanical metamaterials. However, to make kirigami-inspired structures a reality requires controlling the topology of kirigami to achieve connectivity and rigidity. We address this question by deriving the maximum number of cuts (minimum number of links) that still allow us to preserve global rigidity and connectivity of the kirigami. A deterministic hierarchical construction method yields an efficient topological way to control both the number of connected pieces and the total degrees of freedom. A statistical approach to the control of rigidity and connectivity in kirigami with random cuts complements the deterministic pathway, and shows that both the number of connected pieces and the degrees of freedom show percolation transitions as a function of the density of cuts (links). Together, this provides a general framework for the control of rigidity and connectivity in planar kirigami.

Origami and Kirigami Structure for Impact Energy Absorption: Its Application to Drone Guards.

As the use of drones grows, so too does the demand for physical protection against drone damage resulting from collisions and falls. In addition, as the flight environment becomes more complicated, a shock absorption system is required, in which the protective structure can be deformed based on the circumstances. Here, we present an origami- and kirigami-based structure that provides protection from various directions. This research adds a deformation capacity to existing fixed-shape guards; by using shape memory alloys, the diameter and height of the protective structure are controlled. We present three protective modes (1: large diameter/low height; 2: small diameter/large height; and 3: lotus shaped) that mitigate drone falls and side collisions. From the result of the drop impact test, mode 2 showed a 78.2% reduction in the maximum impact force at side impact. We incorporated kirigami patterns into the origami structures in order to investigate the aerodynamic effects of the hollow patterns. Airflow experiments yielded a macro understanding of flow-through behaviors on each kirigami pattern. In the wind speed experiment, the change in airflow velocity induced by the penetration of the kirigami pattern was measured, and in the force measurement experiment, the air force applied to the structure was determined.

Kirigami Engineering of Suspended Graphene Transducers.

The low mass density and high mechanical strength of graphene make it an attractive candidate for suspended-membrane energy transducers. Typically, the membrane size dictates the operational frequency and bandwidth. However, in many cases it would be desirable to both lower the resonance frequency and increase the bandwidth, while maintaining overall membrane size. We employ focused ion beam milling or laser ablation to create kirigami-like modification of suspended pure-graphene membranes ranging in size from microns to millimeters. Kirigami engineering successfully reduces the resonant frequency, increases the displacement amplitude, and broadens the effective bandwidth of the transducer. Our results present a promising route to miniaturized wide-band energy transducers with enhanced operational parameter range and efficiency.

Kirigami-Inspired Flexible Lithium-Ion Batteries via Transformation of Concentrated Stress into Segmented Strain.

Emerging directions in the growing wearable electronics market have spurred the development of flexible energy storage systems that require deformability while maintaining electrochemical performance. However, the traditional fabrication approaches of lithium-ion batteries (LIBs) are challenging to withstand long-cycle bending alternating loads due to the stress concentration caused by the nonuniformity of the actual deformation. Herein, inspired by kirigami, a segmented deformation design of full-cell scale thin-type flexible lithium-ion batteries (FLIBs) with large-scale manufacturing characteristics via the current collector's mechanical blanking process is reported. This strategy allows the battery's elliptical deformation of the actual state to be transformed into the circular strain of the ideal configuration, thereby dispersing the stress concentration on the top of the battery. According to the results, the designed battery maintains >95% capacity after >20 000 harsh in situ dynamic tests. In addition, finite element analysis further reveals the mechanism that the segmented deformation strategy bears the mechanical stress. This work can enlighten the rational design and customization of electrode patterns for high compatibility with various devices, thereby providing potential opportunities for the application of FLIBs.

Modulating the mass sensitivity of graphene resonators via kirigami.

The unique mechanical properties of graphene make it an excellent candidate for resonators. We have used molecule dynamic to simulate the resonance process of graphene. The kirigami approach was introduced to improve the mass sensitivity of graphene sheets. Three geometric parameters governing the resonant frequency and mass sensitivity of Kirigami graphene NEMS were defined. The simulation results show that the closer the kirigami defect is to the center of the drum graphene, the higher the mass sensitivity of the graphene. The kirigami graphene shows up to about 2.2 times higher mass sensitivity compared to pristine graphene. Simultaneously, the kirigami graphene has a higher out-of-plane amplitude and easy access to nonlinear vibrations, leading to higher mass sensitivity. Besides, the kirigami structure can restrict the diffusion of gold atoms on graphene under high initial velocity or large tension condition. It is evident that a reasonable defect design can improve the sensitivity and stability of graphene for adsorption mass.

Failure mechanism of graphene kirigami under nanoindentation.

Though graphene is the strongest material in nature, its intrinsic brittleness hinders its applications where flexibility is the key figure of merits. In this work, we report the enhanced flexibility of graphene under nanoindentation by using kirigami technique. Based on molecular dynamics simulations, we find that graphene kirigami designed at the optimal cut parameter can sustain more than 45% larger out-of-plane deformation than its pristine counterpart while the maximum impact load is reduced by 20% due to the flexible cut edges. This trade-off between flexibility and strength in a graphene kirigami can be overcome by adding a pristine graphene as a supporting substrate. This double-layer structure consisting of one graphene kirigami and one pristine graphene can stand the maximum impact load three times larger than the single-layer graphene kirigami but its maximum indentation depth is merely 8% smaller. Our simulation results provide useful insights into the failure mechanism of the graphene kirigami under nanoindentation and useful guidelines to enhancing the flexibility of graphene for its applications as protection materials.

Propagation of pop ups in kirigami shells.

Kirigami-inspired metamaterials are attracting increasing interest because of their ability to achieve extremely large strains and shape changes via out-of-plane buckling. While in flat kirigami sheets, the ligaments buckle simultaneously as Euler columns, leading to a continuous phase transition; here, we demonstrate that kirigami shells can also support discontinuous phase transitions. Specifically, we show via a combination of experiments, numerical simulations, and theoretical analysis that, in cylindrical kirigami shells, the snapping-induced curvature inversion of the initially bent ligaments results in a pop-up process that first localizes near an imperfection and then, as the deformation is increased, progressively spreads through the structure. Notably, we find that the width of the transition zone as well as the stress at which propagation of the instability is triggered can be controlled by carefully selecting the geometry of the cuts and the curvature of the shell. Our study significantly expands the ability of existing kirigami metamaterials and opens avenues for the design of the next generation of responsive surfaces as demonstrated by the design of a smart skin that significantly enhances the crawling efficiency of a simple linear actuator.

Buckling and twisting of advanced materials into morphable 3D mesostructures.

Recently developed methods in mechanically guided assembly provide deterministic access to wide-ranging classes of complex, 3D structures in high-performance functional materials, with characteristic length scales that can range from nanometers to centimeters. These processes exploit stress relaxation in prestretched elastomeric platforms to affect transformation of 2D precursors into 3D shapes by in- and out-of-plane translational displacements. This paper introduces a scheme for introducing local twisting deformations into this process, thereby providing access to 3D mesostructures that have strong, local levels of chirality and other previously inaccessible geometrical features. Here, elastomeric assembly platforms segmented into interconnected, rotatable units generate in-plane torques imposed through bonding sites at engineered locations across the 2D precursors during the process of stress relaxation. Nearly 2 dozen examples illustrate the ideas through a diverse variety of 3D structures, including those with designs inspired by the ancient arts of origami/kirigami and with layouts that can morph into different shapes. A mechanically tunable, multilayered chiral 3D metamaterial configured for operation in the terahertz regime serves as an application example guided by finite-element analysis and electromagnetic modeling.

Programmable active kirigami metasheets with more freedom of actuation.

Kirigami (cutting and/or folding) offers a promising strategy to reconfigure metamaterials. Conventionally, kirigami metamaterials are often composed of passive cut unit cells to be reconfigured under mechanical forces. The constituent stimuli-responsive materials in active kirigami metamaterials instead will enable potential mechanical properties and functionality, arising from the active control of cut unit cells. However, the planar features of hinges in conventional kirigami structures significantly constrain the degrees of freedom (DOFs) in both deformation and actuation of the cut units. To release both constraints, here, we demonstrate a universal design of implementing folds to reconstruct sole-cuts-based metamaterials. We show that the supplemented folds not only enrich the structural reconfiguration beyond sole cuts but also enable more DOFs in actuating the kirigami metasheets into 3 dimensions (3D) in response to environmental temperature. Utilizing the multi-DOF in deformation of unit cells, we demonstrate that planar metasheets with the same cut design can self-fold into programmable 3D kirigami metastructures with distinct mechanical properties. Last, we demonstrate potential applications of programmable kirigami machines and easy-turning soft robots.

MetaMembranes for the Sensitivity Enhancement of Wearable Piezoelectric MetaSensors.

The low stretchability of plain membranes restricts the sensitivity of conventional diaphragm-based pressure and inflatable piezoelectric sensors. Using theoretical and computational tools, we characterized current limitations and explored metamaterial-inspired membranes (MetaMems) to resolve these issues. This paper develops two MetaMem pressure sensors (MPSs) to enrich the sensitivity and stretchability of the conventional sensors. Two auxetic hexagonal and kirigami honeycombs are proposed to create a negative Poisson's ratio (NPR) in the MetaMems which enables them to expand the piezo-element of sensors in both longitudinal and transverse directions much better, and consequently provides the MPSs' diaphragm a higher capability for flexural deformation. Polyvinylidene fluoride (PVDF) and polycarbonate (PC) are considered as the preferable materials for the piezo-element and MetaMem, respectively. A finite element analysis was conducted to investigate the stretchability behavior of the MetaMems and study its effect on the PVDF's polarization and sensor sensitivity. The results obtained from theoretical analysis and numerical simulations demonstrate that the proposed MetaMems enhance the sensitivity of pressure sensors up to 3.8 times more than an equivalent conventional sensor with a plain membrane. This paper introduces a new class of flexible MetaMems to advance wearable piezoelectric metasensor technologies.

Soft Robots' Dynamic Posture Perception Using Kirigami-Inspired Flexible Sensors with Porous Structures and Long Short-Term Memory (LSTM) Neural Networks.

Soft robots can create complicated structures and functions for rehabilitation. The posture perception of soft actuators is critical for performing closed-loop control for a precise location. It is essential to have a sensor with both soft and flexible characteristics that does not affect the movement of a soft actuator. This paper presents a novel end-to-end posture perception method that employs flexible sensors with kirigami-inspired structures and long short-term memory (LSTM) neural networks. The sensors were developed with conductive sponge materials. With one-step calibration from the sensor output, the posture of the soft actuator could be calculated by the LSTM network. The method was validated by attaching the developed sensors to a soft fiber-reinforced bending actuator. The results showed the accuracy of posture prediction of sponge sensors with three kirigami-inspired structures ranged from 0.91 to 0.97 in terms of R2. The sponge sensors only generated a resistive torque value of 0.96 mNm at the maximum bending position when attached to a soft actuator, which would minimize the effect on actuator movement. The kirigami-inspired flexible sponge sensor could in future enhance soft robotic development.

Artificial Propeller Chirality and Counterintuitive Reversal of Circular Dichroism in Twisted Meta-molecules.

Here we demonstrate an optical propeller chirality in artificially twisted meta-molecules, which is remarkably different from conventional optical helical chirality. Giant circular dichroism (CD) is realized in a single layer of meta-molecule array by utilizing the surface lattice resonances that are formed by the coupling of chiral electric quadrupole modes to the diffractive lattice mode. Due to the special twist of the propeller blades, the periodic meta-molecule array is hybridized by unit cells with two different chiral centers. As a result, the CD response is readily reversed by tailoring the interference phase through engineering the structural blades without inverting the geometric chirality. Importantly, the enhanced CD and its sign reversal are demonstrated in experiments by using a nano-kirigami fabrication technique.

Engineering Tough Metallosupramolecular Hydrogel Films with Kirigami Structures for Compliant Soft Electronics.

A simple and effective approach is demonstrated to fabricate tough metallosupramolecular hydrogel films of poly(acrylic acid) by one-pot photopolymerization of the precursor solution in the presence of Zr4+ ions that form coordination complexes with the carboxyl groups and serve as the physical crosslinks of the matrix. Both as-prepared and equilibrated hydrogel films are transparent, tough, and stable over a wide range of temperature, ionic strength, and pH. The thickness of the films can be easily tailored with minimum value of ≈7 µm. Owing to the fast polymerization and gelation process, kirigami structures can be facilely encoded to the gel films by photolithographic polymerization, affording versatile functions such as additional stretchability and better compliance of the planar films to encapsulate objects with sophisticated geometries that are important for the design of soft electronics. By stencil printing of liquid metal on the hydrogel film with a kirigami structure, the integrated soft electronics shows good compliance to cover curved surfaces and high sensitivity to monitor human motions. Furthermore, this strategy is applied to diverse natural and synthetic macromolecules containing carboxyl groups to develop tough hydrogel films, which will open opportunities for the applications of hydrogel films in biomedical and engineering fields.

Electromagnetic Control by Actuating Kirigami-Inspired Shape Memory Alloy: Thermally Reconfigurable Antenna application.

Electromagnetic responses are generally controlled electrically or optically. However, although electrical and optical control allows fast response, they suffer from switching or tuning range limitations. This paper controls electromagnetic response by mechanical transformation. We introduce a novel kirigami-inspired structure for mechanical transformation with less strength, integrating a shape memory alloy actuator into the kirigami-inspired for mechanical transformation and hence electromagnetic control. The proposed approach was implemented for a reconfigurable antenna designed based on structural and electromagnetic analyses. The mechanical transformation was analyzed with thermal stimulus to predict the antenna geometry and electromagnetic analysis with different geometries predicted antenna performance. We numerically and experimentally verified that resonance response was thermally controlled using the kirigami-inspired antenna integrated with a shape memory alloy actuator.

Bioinspired Color Switchable Photonic Crystal Silicone Elastomer Kirigami.

Bioinspired dynamic structural color has great potential for use in dynamic displays, sensors, cryptography, and camouflage. However, it is quite rare for artificial structural color devices to withstand thousands of cycles. Male hummingbird's crowns and gorgets are brightly colored, demonstrating frequent color switching that is induced by regulating the orientation of the feathers through movement of skin or joints. Inspired by this unique structural color modulation, we demonstrate a flexible, mechanically triggered color switchable sheet based on a photonic crystal (PhC)-coated polydimethylsiloxane (PDMS) kirigami (PhC-PDMS kirigami) made by laser cutting. Finite element modeling (FEM) simulation reveals that the thickness of PDMS kirigami and the chamfer at the incision induced by laser cutting both dominate the out-of-plane deformation through in-plane stretching. The bioinspired PhC-PDMS kirigami shows precisely programmable structural color and keeps the color very well after recycling over 10 000 times. This bioinspired PhC-PDMS kirigami also shows excellent viewability even in bright sunlight, high readability, robust functionality, technical flexibility, and mechanical durability, which are readily exploitable for applications, such as chromic mechanical monitors for the sports industry or for medical applications, wearable camouflage, and security systems.