Bending States Research Articles

A variable stiffness method for pneumatic flexible arms

In this study, a three-degree-of-freedom pneumatic flexible arm with a braking function was designed to overcome the shortcomings of low bearing capacity and insufficient rigidity of existing flexible arms. The braking force was adjusted by controlling the air pressure entering the brake, changing the stiffness of the flexible arm, and enhancing the posture maintainability of the flexible arm. A theoretical model of the braking force and stiffness of a flexible arm was established, and applicable experiments were conducted. The theoretical data were consistent with the experimental results. The braking force linearly increased with the increase of braking pressure, at a brake air pressure of 0.40 MPa. The braking force of a single braking unit reached 276 N. The stiffness nonlinearly increased with an increase in the brake air pressure. At a brake air pressure of 0.4 MPa, the axial stiffness of the flexible arm in the initial state had the highest value of 20 kN/m. The stiffness change in the bending direction of the flexible arm in the spatial bending state was the largest and increased by 12.4 times. The proposed flexible arm exhibited high stiffness and flexible movement; thus, it can be used as a flexible arm to support other end effectors. The proposed variable stiffness method can be practically employed to maintain the posture of flexible robots, which has a high practical value.

A frequency tunable resonator using liquid crystals on flexible substrates

ABSTRACT Liquid crystals have already been widely used in reconfigurable microwave devices through external electrically controlled dielectric properties. The traditional method is to sandwich liquid crystals between two metal layers etched on hard substrates. However, this method of adding external electric fields tends to cause short-circuiting between the upper and lower electrodes of liquid crystals in the bending state for the case of electrodes on flexible substrates. In this paper, a novel method is proposed to realise a frequency tunable resonator using liquid crystal fabricated on flexible substrates. The resonator is composed of coplanar waveguide structures. Since the metal layers of the coplanar waveguide placed on the same horizontal plane do not touch each other when bending, it allows the controlled electric field direction to be parallel to the electrode layers. Liquid crystals are placed under the coplanar metals for the electric field control. In order to maximise tunability, the structural parameters of the proposed structure were analysed in detail. The test results agree with the theory and simulation to verify the reliability of this frequency tunable resonator. Furthermore, the test results also illustrate that the applied electric field method can achieve the frequency continuous electrically tunable function when the flexible resonator is bending.

Establishing non-Newtonian flow state K metal electrodes for flexible batteries

Potassium metals are promising anodes owing to their wide distribution and low redox potential. Nevertheless, their application in flexible devices is impeded by their poor mechanical properties. Herein, we fabricate a moldable and processable non-Newtonian flow state K metal anode (NK) by encapsulating molten K into Super P powder in a short time. The NK slurry can be arbitrarily designed in various shapes and precise sizes with excellent viscosity, and stabilize K metal anodes at near room temperature (around 100 °C). Further, with the support of a stainless steel mesh carrier, the mechanical stability and corrosion resistance of the electrodes tend to be more honorable. Remarkably, the NK metal based on stainless steel mesh (NK@SS) exhibits excellent cycling performance at high current density. Paired with a Prussian blue (PPB) cathode, the NK@SS anode shows better electrochemical performance and extended lifetime than bare K. The pouch cell based on the NK@SS anode can function well at various bending states without obvious capacity attenuation. These results may lay the foundation for the design of advanced materials.

Synthesis of Ni Nanosheets by Template-Free Method and Their Application in Conductive and Magnetic Flexible Electrons.

Two-dimensional Ni nanosheets are synthesized by the template-free method using Na3CA as an orientation agent in liquid phase, and then the conductive Ni nanosheet ink is prepared for conductive circuits on flexible electronics. The thickness of the Ni nanosheets is about 800 nm, and the diameter is about 100 μm. Na3CA plays a structural guiding role to form Ni nanocrystals, promoting the self-assembly of Ni nanocrystals into Ni nanosheets effectively. The laminar stackable patterns of the Ni nanosheet circuits increase the contact area of the Ni nanosheets and improve the stability of the conductors under stress. Ni nanosheets can bend with the folding of the structure, while the mutual constraints between their layers promote the circuit to remain stable during the bending state. Therefore, the Ni nanosheet circuits display excellent conductive performance during the tiled and bent stages. In addition, Ni nanosheet/Ag nanowire composites are prepared to enhance conductivity to meet higher demands. Moreover, the experimental results of its application in magnetic guided switch closure circuits show that Ni nanosheet/Ag nanowire composites have the potential to participate in both conductive and magnetic field applications simultaneously.

Cryo-EM structures of full-length integrin αIIbβ3 in native lipids

Platelet integrin αIIbβ3 is maintained in a bent inactive state (low affinity to physiologic ligand), but can rapidly switch to a ligand-competent (high-affinity) state in response to intracellular signals (“inside-out” activation). Once bound, ligands drive proadhesive “outside-in” signaling. Anti-αIIbβ3 drugs like eptifibatide can engage the inactive integrin directly, inhibiting thrombosis but inadvertently impairing αIIbβ3 hemostatic functions. Bidirectional αIIbβ3 signaling is mediated by reorganization of the associated αIIb and β3 transmembrane α-helices, but the underlying changes remain poorly defined absent the structure of the full-length receptor. We now report the cryo-EM structures of full-length αIIbβ3 in its apo and eptifibatide-bound states in native cell-membrane nanoparticles at near-atomic resolution. The apo form adopts the bent inactive state but with separated transmembrane α-helices, and a fully accessible ligand-binding site that challenges the model that this site is occluded by the plasma membrane. Bound eptifibatide triggers dramatic conformational changes that may account for impaired hemostasis. These results advance our understanding of integrin structure and function and may guide development of safer inhibitors.

An elastic luminescent organic single crystal with linear and nonlinear optical waveguide properties

In recent years, organic single crystals combining elastic and photoluminescent (PL) properties have attracted extensive attention due to their promising applications in many fields. However, elastic organic crystals with two-photon luminescence (2 PL) waveguiding properties are still rarely found, which is imperative for various applications, including near-infrared (NIR) bioimaging and nonlinear optical (NLO) photonic devices. This work reports an elastic single crystal with a long needle shape, anthracene-9,10-dicarbaldehyde (An-DC). Its crystal (100) face exhibits highly reversible bending by applying the mechanical force, which is facilitated by the van der Waals (C– H···π), H-bond (C–H⋯O), and π···π interactions among molecules. The crystal emits a strong orange-red fluorescence when excited by blue-violet light. Interestingly, it also exhibits excellent two-photon fluorescence emission when stimulated by near-infrared light. Based on these linear and nonlinear optical characteristics, the waveguide applications of the An-DC crystal were implemented. Notably, in the bending state, the luminescence performance and the crystal structure of the An-DC crystal are maintained well, which can be used as a low-loss material for linear and nonlinear optical waveguide devices, thus expanding the potential use of flexible organic crystals.

Lead free halide perovskite embedded PVDF based efficient mechanical energy harvester: Self-driven respiratory sensor

Lead-free inorganic perovskites have emerged as a promising material for energy harvesting applications, owing to their superior optoelectronic properties. This enables the insertion of lead-free halide perovskites into polyvinylidene difluoride (PVDF) to feasibly enhance its piezoelectric coefficient. Consequently, piezoelectric nanogenerators based on PVDF composite have evolved as a futuristic, sustainable, and renewable energy alternative due to their flexible, durable, and nontoxic behavior. Herein, we realized a novel composite by incorporating different mass-fractions of pre-synthesized CsCuCl3 particles in the polymer matrix. Notably, the composite containing 4 wt% CsCuCl3 perovskite nucleates an optimum electroactive phase fraction of about ∼93 %. A first-principles investigation is also carried out in order to fully comprehend the interfacial interaction of the PVDF dipole with the CsCuCl3 system. Thereafter, the piezo-sensors are fabricated utilizing the composites, where the 4 wt% CsCuCl3/PVDF- based device delivered optimal output performance owing to its highest electroactive phase. This optimized device, viz., PNG 4 achieves an open-circuit voltage and short-circuit current of ∼73.2 V and ∼5.44 μA, respectively. Besides, the optimized device could operate as a piezo-resistive sensor as well as a pressure sensor. An excellent sensing performance was achieved from the PNG 4 device, where the sensing parameter (∆II0) reached values as high as ∼98.7. The optimized device is efficient in the low-pressure region and exhibits a high sensitivity of 13.8 kPa−1 with notable response ≤26ms and recovery time≤75ms. Furthermore, the device is capable of sensing different bending states with a sensitivity of 0.5/degree, which proves its potential for applications in human health care monitoring regarding self-powered real-time respiratory monitoring systems.

Flexible planar Hall effect sensor with sub-200 pT resolution

Flexible sensors are important for applications, such as wearable medical devices, soft robotics, and more, as they can easily conform to soft and irregularly shaped surfaces. This study presents elliptical planar Hall effect magnetic sensors fabricated on a polyamide tape with an equivalent magnetic noise (EMN) better than 200 pT/Hz. The sensor is characterized in flat and bent states with a bent radius of 10 mm. An EMN of 200 and 400 pT/Hz in flat and bent states, respectively, is achieved at a frequency of 100 Hz. The remarkable EMN combined with a simple, low-cost fabrication process makes these sensors a promising candidate for flexible electronics.

Understanding the piezoelectric response of ZnO nanotetrapods: Detailed numerical calculations

The complex tetrapod shape of zinc oxide nanostructure, which is constructed from four one-dimensional arms interconnected together via a central core, is a special 3D geometry with multifunctional applications in advanced technologies. The ZnO hexagonal wurtzite crystal lattice with a non-centrosymmetric structure introduces interesting piezoelectric property in nanorods in the bent state, which has been well reported and utilized in piezo- and tribo-electric nanogenerator applications. Considering the broad technological relevance of tetrapods, it is important to understand the piezoelectric response of zinc oxide tetrapods under different conditions. In this study, we explicate the fundamental mechanical and electrical properties of ZnO nanotetrapods (ZnO NTs) through a detailed finite element method analysis. On this basis, the effects of shape factors (including length, height, and aspect ratio) as well as connection strength and packing density on the deformation and piezoelectric potential of ZnO NTs are examined, offering guidance for the fabrication of ZnO NTs. This theoretical model and numerical simulation provide an avenue for further piezoelectric and piezotronic research of ZnO NTs.

Development of Rapid Dispense-Printed Flexible Interdigitated Electrode Modified with rGO-TiO2 Nanohybrid for Glucose Detection

A rapid and facile method in developing a printed electrochemical glucose sensor constructed using flexible interdigitated electrode (IDE) employed with reduced graphene oxide (rGO)-Titanium dioxide (TiO2) is demonstrated. A 2 × 3 silver (Ag)-based IDE array based on a simulated IDE model, was fabricated via a single-step dispense-printing within less than 5 min, while a rGO-TiO2 nanohybrid and glucose oxidase (GOx) enzyme was coated on the IDE surface simply via drop-casting method. Exceptional reproducibility and repeatability of the printed IDE functionalized with rGO-TiO2/GOx in terms of electrical and mechanical performance were observed. The stability of the sensor was investigated over a week period, in which 6%–7% performance degradation was recorded based on resistance measurement in flat state, whereas no further significant loss was noted over the same period in bending state. The IDE sensor was tested using chronoamperometry with varied glucose concentrations up to 30 mM, resulted in a stabilized current after 5 s. The sensitivity plot attained depicted a best linear fit of 0.988 obtained at response time of 60 s, whilst covering lowest detection at 0.05 mM and selective detection from ascorbic acid and uric acid. The application of this sensor could contribute as an alternative method to develop a reliable and economical glucose sensing wearable for independent monitoring.

Self-Powered Flexible Ultraviolet Photodetectors Based on CuI/a-ZTO Heterojunction Processed at Room Temperature.

For traditional wide-bandgap semiconductor materials, a high-temperature process is unavoidable for improving crystallization quality, so the substrate of the device is greatly limited. In this work, zinc-tin oxide (a-ZTO) amorphous oxide processed by the pulsed laser deposition method was utilized as the n-type layer, which exhibits considerable electron mobility and optical transparency, and can be deposited at room temperature. At the same time, by combining p-type CuI grown by the thermal evaporation method, a vertically structured ultraviolet photodetector based on CuI/ZTO heterojunction was obtained. The detector demonstrates self-powered properties, with an on-off ratio exceeding 104, and rapid response with a rise time of 2.36 ms and a fall time of 1.49 ms. Also, the photodetector shows long-term stability with 92% retention after 5000 s cyclic lighting and maintains reproducible response in frequency dependence measurement. Furthermore, the flexible photodetector on poly(ethylene terephthalate) (PET) substrates was constructed, exhibiting fast response and durability in the bending state. This is the first time that the heterostructure based on CuI has been applied in the flexible photodetector. The excellent results indicate that the combination of amorphous oxide and CuI has the potential for ultraviolet photodetectors, and will broaden the application range of high-performance flexible/transparent optoelectronic devices in the future.

Scalable Quasi-Solid-State Supercapacitor for Wide-Temperature Wearable Devices.

Quasi-solid-state supercapacitors have wide application prospects in flexible and scalable electronics, which require high capacity, simple form factor, and excellent mechanical robustness. However, it is a challenge to have all these benefits in one material. Addressing this, we report a composite hydrogel with excellent mechanical durability and freezing resistance. The designed composite hydrogel acts both as a load-bearing layer to maintain its structure during deformation and as a permeable binder to stimulate the interfacing between the conductive electrode and the electrolyte to reduce the interface resistance. Flexible supercapacitors are assembled with composite hydrogels and high-performance MnO2/carbon cloth, which has excellent performance and can store energy at different temperatures or bending states. These results show that the tough hydrogel facilitates the improvement of electrical and mechanical stability, showing great potential in wide-temperature wearable devices.

Structural analysis of REBCO coated conductors and quasi-isotropic strands under bending using continuum shell element

The electro-mechanical properties of Rare-Earth Barium Copper Oxide (REBCO) coated conductors (CCs) are crucial for practical structures such as power cables, magnet coils and so on. Many tests including tension and bending tests were conducted to estimate the critical current of REBCO CCs under different loadings. However, standard test method for the electro-mechanical properties of REBCO CCs has not been established, especially for bending tests. Therefore, finite element simulations of bending tests of REBCO CCs are still needed to further analyze the electro-mechanical behavior under loading. Structural responses of REBCO CCs and quasi-isotropic (Q-IS) strands under bending are analyzed using continuum shell element in this study. The numerical results indicate that bending strains obtained from finite element simulations match well with the analytical calculations in pure bending state. Furthermore, the influences of former shapes are also discussed to evaluate the bending tolerance of REBCO CCs in different cases. In addition, the mechanical properties and critical current degradation of Q-IS strands under bending are analyzed.

Large‐Area Freestanding Bi2S3 Nanofibrous Membranes for Fast Photoresponse Flexible IR Imaging Photodetector (Adv. Funct. Mater. 24/2023)

Flexible IR Imaging Photodetectors In article number 2300159, Jinzhong Wang, Shiyong Gao, and co-workers, successfully prepared, a large-area freestanding Bi2S3 nanofibrous membrane (NFM) with high flexibility, which is self-assembled from ultralong Bi2S3 nanowires. The flexible photodetector based on the as-prepared Bi2S3 NFM exhibits high responsivity, fast response, excellent stability and splendent imaging capability under both flat and bent state. The as-prepared Bi2S3 NFM has great potential in flexible and wearable optoelectronic devices.

Digital Mechanical Metasurfaces for Reprogrammable Structural Display.

Stimuli-responsive surfaces with reversible surface topography and controllable physical and mechanical properties are highly desirable for various engineering applications: e.g., information encoding, anticounterfeiting, micromanipulations, displays, etc. Here we present a digital type of stimuli-responsive surface composed of discrete silicon scales supported on individual core-shell magnetic micropillars (MMP) and realize precise control over the local topography of the surfaces. The individual MMP can be reversibly modulated between two contrasting bending states (state 0, hard to bend; state 1, easy to bend) by controlling the spatial distribution of the magnetic nanoparticles inside the pillar shells. These two different states of the micropillars induce a completely different topography of the supporting scales that can be utilized as mechanical pixels upon applying actuation magnetic fields. We further build a three-dimensional (3D) microcontrolling platform for digital modulation of the micropillar states. With this platform, a large array of 50 × 50 MMP surfaces can be programmed and reprogrammed into any combination of the local states by simply reading a matrix of binary digits. Such a digital modulation process facilitates the practical application of the MMP surfaces for fast and reprogrammable display of various structural patterns, as demonstrated for microscale letters, millimeter-scale QR code, and complex Chinese characters. The digital modulation and on-demand reprogrammability of the MMP surfaces reported here are expected to advance the development of various other forms of digital mechanical metasurfaces that can easily transform digital information into encoded mechanical responses.

Flexible and frequency reconfigurable CPW-fed monopole antenna with frequency selective surface for IoT applications

This paper proposes a flexible, frequency-reconfigurable monopole antenna design with frequency selective surface (FSS) for Internet of Things (IoT) applications. The proposed antenna operates at three of the IoT frequency bands. This antenna is a coplanar waveguide (CPW)-fed monopole with two balanced arms printed on a thin ROGERS 3003 flexible substrate. The length of the right-hand arm of the antenna is used to achieve frequency reconfiguration by using PIN diodes. Three frequency modes of operation have been obtained; the 2.4 GHz frequency band with the right-hand arm is fully truncated, the 3.5 GHz frequency band with the two arms is completely maintained, and the 4 GHz frequency band with the right-hand arm is partially truncated. To improve the gain of the antenna, a simple FSS surface is designed to be placed under the antenna at a distance of 15 mm. The FSS operates efficiently from 2 to 4.5 GHz and has improved the gain of the antenna. A maximum gain of 6.5 dBi, 7.52 dBi, and 7.91 dBi has been achieved at the three frequency bands respectively. The behavior of the flexible antenna has been evaluated in both the flat and bent states, and stable performance has been observed in both cases.

In-situ etching activation towards flexible carbonized paper-supported electrode with transition-metal oxides embedded into carbonized cellulose fibers

Cellulose paper is considered to be a promising substrate of energy materials for energy storage device due to its flexible, porous, renewable, degradable, and environmental-friendly feature. However, the large weight and low conductivity of cellulose fibers limit the specific capacitance and energy density of the paper electrodes. Converting cellulose fibers into conductive carbon fibers (CCF) by high temperature carbonization is an effective strategy for improving the electrochemical performance of paper electrodes. But the brittleness of carbonized paper limits its application in flexible supercapacitors. In addition, stable loading of energy materials on CCF is another challenge. Here, KNO3 in-situ etching activation method is used to construct the embedded structure of transition-metal oxides (TMOs) on CCF. And the embedding degree of TMOs on CCF can be adjusted by controlling the amount of KNO3. The embedded structure releases the mechanical stress from bending of CCF, endowing good flexibility for carbonized paper. In addition, embedded structure can anchor TMOs into CCF, and increase the porosity and specific surface area of CCF, promoting a rapid ion diffusion and electron transport for high capacitance contributions. The KNO3-etched Co3O4@CCF paper as an electrode exhibits a high volume specific capacitance of 72.3 F cm−3 at current density of 0.5 mA cm−2 (more than three times higher than that of Co3O4/CCF paper) and shows good rate capability (capacitance retention of 83.3% with 10 times current density) and long lifetime (charge-discharge stability of 94.8% after 10000 cycles). Even in bending state, the composite paper still maintains stable electrochemical performances, which can broaden potential application in flexible supercapacitors.

Electronic coupling coordinated vanadium nitride/magnesium oxide hetero-junction for accelerating oxygen reaction and long-life flexible zinc-air batteries

Designing highly active, durable, and cost-effective electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is explored as exciting research trends. However, a deep understanding of electrochemical performance enhancement is still a big challenge. Herein, we develop a well-defined vanadium nitride/magnesium oxide (VN/MgO) heterostructure on nitrogen-doped carbon frameworks (VM-NC) with enhanced active sites and tunable electronic structure. The synergistic effect at VN/MgO interfaces facilitates the charge-transfer rate, increases the active sites, and promotes OER/ORR kinetics in an alkaline medium. Theoretical studies suggest that the tuning of electronic structure in VN-MgO effectively boosts the conductivity and catalytic activity of VM-NC. The catalyst delivers an excellent half-wave potential of ≈ 0.852 V for ORR and a low overpotential of ≈ 253 mV for OER. Furthermore, VM-NC-based zinc-air cells (ZACs) exhibit high peak power-density and ultralong cycle-life. The quasi-solid state flexible ZACs present excellent flexibility with different bending states and outstanding electrochemical performance.

Self-assembled MXene-graphene oxide composite enhanced laser-induced graphene based electrodes towards conformal supercapacitor applications

Since pure laser-induced graphene (LIG)-based supercapacitors suffer poor capacity, additional active materials need to be introduced to promote the capacitive performance of LIG-based electrodes. However, the effective introduction and the removal of redundant active materials remain challenging during the fabrication of planar LIG-based supercapacitors. Here we propose the easy-removed self-assembled MXene-graphene oxide composite (C) for the preparation of LIG-based supercapacitors. Owing to the combination of homogeneous C solution and programmable laser-induced process, the as-prepared supercapacitor based on laser-induced C covered polyimide (PI) films (LIG-C) showed enhanced capacity and improved ion transportation characteristics due to the low sheet resistance (15.0 Ω sq−1) and homogenous microstructures. What’s more, the LIG-C-based supercapacitor prepared under laser power of 3 W exhibited good stability including a specific capacitance retention of 98.9% after six months and 102.4% after 1000 cycles charge–discharge process. Additionally, LIG-C-based supercapacitors also showed good performance uniformity, and series LIG-C-based supercapacitors could power a LED because of a voltage expansion. At the same time, LIG-C-based supercapacitors possessed good deformation-resisting properties under different bending states, which showed promising potential for future conformal electronics device applications. And we hope that this work will pave a new way for LIG-based material in the energy storage field.

A study on the structure of garment-hood-embedded air curtain with fine dust protection function

Fine dust can induce heart and lung challenges. To prevent the entry of fine dust into the human body, this study compared and examined the fine dust blocking efficacies of four wearable air curtain designs in the form of a garment hood with a fine dust protection function. The fine dust blocking efficiency was measured at two measuring areas, “center under the nose” and “center under the eyes,” in the “straight neck” and “30-degree bending forward” states. Notably, the blocking efficiency increased in the order of “basic structure,” “canopy structure,” “standing structure,” and “canopy & standing structure” at both measurement areas in both states. By analyzing the fine dust blocking efficiency of each design according to the angle of the dummy neck, the single structures of the “canopy structure” and “standing structure” in the “30-degree bending forward” state had no significant effect. In contrast, the blocking efficiency of the “canopy & standing structure” increased. Moreover, the efficiency was higher in the “30-degree bending forward” state than in the “straight neck” state. These results suggest that a composite structure in which canopy and standing states are combined is more suitable than a single structure because the actual human neck is more often in a bending state than a standing state during work.