Elastomer Film Research Articles

Enhancing actuation control in monolithic liquid crystal elastomer films with dual heat and light responsiveness

This study presents a dual heat and light-responsive monolithic liquid crystal elastomers (LCEs) film designed to enhance actuation control. The LCE films were capable of various motion types, including curvature reversal, oscillation, and sustainable flipping. By incorporating an azobenzene compound, the films exhibited additional functionalities such as enhanced curvature reversal, accelerated oscillation, and the ability to lock continuous flipping motion upon light stimulation. Furthermore, by adjusting the aspect ratio (AR) of a single film, different thermal motion states were realized, demonstrating the flexibility of this approach. Observations using an NIR camera confirmed temperature fluctuations corresponding to thermal exchange, and the influence of UV light on motion regulation was quantitatively analyzed. The ability to regulate motion through dual stimuli paves the way for improved dynamic motion control and thermal management in advanced robotic applications.

Ultrasensitive and Breathable Hydrogel Fiber‐Based Strain Sensors Enabled by Customized Crack Design for Wireless Sign Language Recognition

AbstractWearable strain sensors, capable of continuously detecting human movements, hold great application prospects in sign language gesture recognition to alleviate the daily communication barriers of the deaf and mute community. However, the unsatisfactory strain sensing performance (such as low sensitivity, narrow detection range, etc.) and wearing discomfort severely hinder their practical application. Here, high‐performance breathable hydrogel strain sensors are proposed by introducing an adjustable localized crack in a closed‐loop connected hydrogel fiber encapsulated by porous elastomer films. Upon loading/unloading of external strain, the dynamic opening/closing of the pre‐cut crack causes a rapid switching in the hydrogel conductive path, resulting in sharp changes in resistance and a high sensitivity. Consequently, the hydrogel‐based crack‐effect strain sensor exhibits a superb sensitivity (GF up to 3930), a broad detection range (from 0.02% to 80%), fast response/recovery time (78/52 ms), repeatability, and structural stability. Based on the capability to accurately detect various strains across the full range of human movements, a wireless sign language recognition system is developed to achieve a high recognition accuracy of 98.1% by encoding and decoding various sign language gestures with the assistance of machine learning, providing a robust platform for efficient sign language intelligibility and barrier‐free communication.

Ultrasensitive, Fast-Response, and Stretchable Temperature Microsensor Based on a Stable Encapsulated Organohydrogel Film for Wearable Applications.

Ionic conductive hydrogel-based temperature sensors have emerged as promising candidates due to their good stretchability and biocompatibility. However, the unsatisfactory sensitivity, sluggish response/recovery speed, and poor environmental stability limit their applications for accurate long-term health monitoring and robot perception, especially in extreme environments. To address these concerns, here, the stretchable temperature sensors based on a double-side elastomer-encapsulated thin-film organohydrogel (DETO) architecture are proposed with impressive performance. It is found that the water-polyol binary solvent, organohydrogel film, and sandwiched device structure play important roles in the temperature sensing performance. By modifying the composition of binary solvent and thicknesses of organohydrogel and elastomer films, the DETO microsensors realize a thickness of only 380 μm, unprecedented temperature sensitivity (37.96%/°C), fast response time (6.01 s) and recovery time (10.53 s), wide detection range (25-95.7 °C), and good stretchability (40% strain), which are superior to those of conventional hydrogel-based sensors. Furthermore, the device displays good environmental stability with negligible dehydration and prolonged operation duration. With these attributes, the wearable sensor is exploited for the real-time monitoring of various physiological signals such as human skin temperature and respiration patterns as well as temperature perception for robots.

Stretchable carbon nanotube/Ecoflex conductive elastomer films toward multifunctional wearable electronics

Conductive elastomer films (CEFs) have found widespread applications toward flexible electronics and wearable devices. Silicones are particularly advantageous for fabricating high-performance CEFs. However, processing silicone/conductive filler composite prepolymers is challenging because of their high viscosity, especially when incorporating nanofillers like carbon nanotubes (CNTs) that can greatly increase the viscosity of silicone prepolymers. Herein, we present a simple, facile and versatile hexane-assisted air-spraying method for fabricating silicone-based CEFs from Ecoflex and CNTs. This method allows for processing CNT/Ecoflex prepolymers with CNT concentrations of 0.5 ∼ 5 wt%, resulting in stretchable CEFs with controllable structural, mechanical and electrical properties. We leveraged the distinct advantages of CNT/Ecoflex CEFs, including the excellent mechanoelectrical sensitivity, workable strain range and elasticity of the CNT1/Ecoflex group, and the high conductivity and low mechanoelectrical sensitivity of the CNT5/Ecoflex group. These properties enable their multifunctional applications as wearable and waterproof resistive strain sensors, multilayered capacitive pressure sensors, and wearable electric heaters. This work provides valuable insights into the development of high-performance and multifunctional silicone-based composite materials for advanced wearable electronics.

Single-layer iron network microstructure magnetorheological elastomer for transparent soft actuator

The motion of a soft magnetic magnetorheological elastomer, a composite material consisting of soft magnetic fillers and an elastomeric matrix, is limited by the gradient of the applied magnetic field, making its application as an out-of-plane soft actuator difficult. In this study, an anisotropic soft magnetic magnetorheological elastomer-based transparent soft actuator with large out-of-plane deformation was fabricated using a very low concentration (1 vol%) of carbonyl iron particles as the soft magnetic fillers. An applied AC electric field treatment method was used to manipulate iron particles in silicone oil, assembling and depositing them onto a glass substrate to form a single-layer iron-network microstructure. A single-layer iron-network microstructure-based magnetorheological elastomer (SL-sMRE) film with initial self-buckling was fabricated by exploiting the swelling properties of the residual silicone oil and polydimethylsiloxane. The SL-sMRE film exhibited out-of-plane deformation with a very short reaction time (228 ms) under a uniform magnetic field. When stimulated by a non-uniform magnetic field, the SL-sMRE film deformed away from the permanent magnet, which was not possible with an isotropic film. An optically transparent SL-sMRE film was applied to a transparent flexible gripping device that could detect the surface of a gripped object in real time.

Optimized design and performance testing of hydraulic electrostatic actuator

ABSTRACT Hydraulic electrostatic actuators have become a research hotspot for their inherent flexibility and safety of human-machine interaction. This paper aims to combine the characteristics of dielectric elastomers and fluid actuators, enhancing the dielectric constant and breakdown field strength by modifying Al2O3 on the surface of nanostructured BaTiO3 and in order to develop a hydraulic electrostatic actuator with silicone rubber as the matrix material. Single-factor experiments were firstly conducted to confirm the range of three factors affecting the actuation strain of the actuator: BaTiO3@Al2O3 content, thickness of the silicone rubber elastomer film, and pre-stretching ratio coefficient. The response surface methodology was used to study the interactive influence of the above three influencing factors on the actuation strain. It was obtained that A has an insignificant influence on the actuation strain, AB has a significant influence on the actuation strain, and B, C, AC, and BC have a highly influence on the actuation strain. Maximizing actuation strain as the optimization objective yielded the combination results of each factor: BaTiO3@Al2O3 content of 2.57%, thickness of 0.60 mm, and pre-stretching ratio coefficient of 2.50. Actuation strain tests were conducted with optimized parameters under no-load. The experimental results of the actuation strain test show that the relative error between the test value and the model prediction value of 16.47% is less than 5%, and the optimization model results are reliable. The optimized hydraulic electrostatic actuator was tested for actuation strain and electrical performance under different loads. The experiment showed that the maximum actuation strain of the hydraulic electrostatic actuator was 17.20% under a load of 100 g. The critical breakdown current of the actuator ranged from 115 μA to 130 μA, and the maximum electromechanical conversion efficiency of the actuator under different loads was 67.93%. Finally, a joint actuating device was developed based on the structure of the actuator, amplifying the output displacement of the actuator to 30 mm, and the linear displacement of the soft electro-fluid actuator can be converted into a 20° rotation angle through the gear steering mechanism, thus validating the effectiveness of the hydraulic electrostatic actuator.

A piezoresistive dielectric elastomer switch consisting of inkjet-printed carbon black

The piezoresistive effect is a key physical property utilized in soft and stretchable electronics, functioning as intrinsic electromechanical sensors. However, most conventional piezoresistive sensors can only passively measure and quantify external stimuli. To address these limitations, this work integrates an inkjet-printed piezoresistive pattern and a dielectric elastomer actuator (DEA) into a single monolithic dielectric elastomer film, creating a dielectric elastomer switch (DES). This DES can realize dramatic resistance changes along with DEA actuation, leveraging DEA-driven piezoresistivity for active and effective circuit control and signal processing. Theoretically, the DES operates by exhibiting resistance change through the periodic compression induced by DEA actuation. Experimentally, the piezoresistive pattern and the DEA electrodes were fabricated on an acrylic dielectric elastomer (VHB) using inkjet-printed piezoresistive carbon black (CB) and manually brushed conductive grease, respectively. The optimized DES demonstrated approximately 3.77 (±0.11) orders of magnitude in resistance change at a DEA frequency of 0.1 Hz. Compared to previously reported methods (smearing and stamping), inkjet-printed CB patterns showed significant improvements. These included negligible Joule heating impact, precise digital fabrication control, larger resistance change (up to 3.77 orders of magnitude), faster response speeds to DEA actuation (∼0.25 s for 3 orders of magnitude in resistance change), and greater durability (operating at 0.5 Hz for 112 min). Scanning electron microscopy (SEM) characterization revealed that these improvements were attributed to a more uniform distribution of conductive “CB islands” and appropriate non-conductive gap size among them. For demonstrating the applications of DES, it was employed for LED control, DEA-based soft gripper control, and as a multiplexer for signal processing.

Optical Micro/Nanofiber Enabled Multiaxial Force Sensor for Tactile Visualization and Human-Machine Interface.

Tactile sensors with capability of multiaxial force perception play a vital role in robotics and human-machine interfaces. Flexible optical waveguide sensors have been an emerging paradigm in tactile sensing due to their high sensitivity, fast response, and antielectromagnetic interference. Herein, a flexible multiaxial force sensor enabled by U-shaped optical micro/nanofibers (MNFs) is reported. The MNF is embedded within an elastomer film topped with a dome-shaped protrusion. When the protrusion is subjected to vector forces, the embedded MNF undergoes anisotropic deformations, yielding time-resolved variations in light transmission. Detection of both normal and shear forces is achieved with sensitivities reaching 50.7dB N-1 (14% kPa-1) and 82.2dB N-1 (21% kPa-1), respectively. Notably, the structural asymmetry of the MNF induces asymmetrical optical modes, granting the sensor directional responses to four-directional shear forces. As proof-of-concept applications, tactile visualizations for texture and relief pattern recognition are realized with a spatial resolution of 160µm. Moreover, a dual U-shaped MNF configuration is demonstrated as a human-machine interface for cursor manipulation. This work represents a step towards advanced multiaxial tactile sensing.

Injectable and photocurable precursors with their improved adhesive elastomeric films by nature-inspired marine mussels chemistry

Photocurable materials, capable of being delivered as liquids and rapidly cured in situ within seconds using UV light, are garnering increased interest in advanced minimally invasive procedures. Examining living organisms to extract novel principles and technologies, and subsequently applying them into synthetic materials to enhance their performances holds a central position in biomimetics (bioinspiration). In this exploration, we delve into the multifaceted world of marine mussel adhesion, emphasizing the pivotal role of 3,4-dihydroxy-L-phenylalanine (L-DOPA) in adhesive proteins. Simultaneously, we navigated the promising realm of elastomers derived from fatty acid dimers. 90° peeling test and fluorescence microscope indicate that the adhesiveness of the catechol-containing samples (5 % and 10 %) to the hydrophilic surface versus control samples were ∼ 4 and 8 times higher, respectively, as compared to within the tested group. Overall, our results suggest that the incorporation of methacrylated L-DOPA in the synthesis of photocured elastomeric networks leads to lower water contact angle and improved adhesiveness, creating new avenues for potential biomedical applications.

The Hydrogen Bonding in the Hard Domains of the Siloxane Polyurea Copolymer Elastomers.

For probing the structure-property relationships of the polyurea elastomers, we synthesize the siloxane polyurea copolymer elastomer by using two aminopropyl-terminated polysiloxane monomers with low and high number-average molecular weight (Mn), i.e., L-30D and H-130D. To study the influence of the copolymer structures on the film properties, these films are analyzed to obtain the tensile performance, UV-vis spectra, cross-sectional topographies, and glass transition temperature (Tg). The two synthetic thermoplastic elastomer films are characterized by transparency, ductility, and the Tg of the hard domains, depending on the reacting compositions. Furthermore, the film elasticity behavior is studied by the strain recovery and cyclic tensile test, and then, the linear fitting of the tensile data is used to describe the film elasticity based on the Mooney-Rivlin model. Moreover, the temperature-dependent infrared (IR) spectra during heating and cooling are conducted to study the strength and recovery rate of the hydrogen bonding, respectively, and their influence on the film performance is further analyzed; the calculated Mn of the hard segment chains is correlated to the macroscopic recovery rate of the hydrogen bonding. These results can add deep insight to the structure-property relationships of the siloxane polyurea copolymer.

The interfacial behavior of a liquid crystal elastomer film bonded to an orthotropic substrate under light illumination

Bilayer soft actuators composed of a liquid crystal elastomer (LCE) film and a soft substrate have been widely used in soft robotics, bioengineering, and other fields. The existence of stress concentration at the interface edge easily leads to delamination damage of layered structures. Therefore, this article focuses on the distribution of interfacial stresses of the light-driven LCE film/orthotropic substrate system. The governing equations of the LCE film/substrate system are derived. The distributions of peeling and shear stresses on the interface are given and validated by using the finite element method (FEM). The effects of light illumination intensity, light attenuation distance and the director direction on the distribution of interfacial stresses are comprehensively analyzed. It is found that the signs of the shear stress and peeling stress depend on the director direction. This indicates that aligning the director in a certain direction may enhance the interface strength. The results may be useful for designing light-responsive LCE bilayer drivers.

Influence of Initial Film Properties in UVO-Mediated Patterning of an Elastomeric Film Using a TEM Grid.

Ultraviolet irradiation of a cross-linked polydimethylsiloxane (PDMS) Sylgard 184 film in the presence of atmospheric oxygen (UVO) through a bare transmission electron microscope (TEM) sample holding grid is a rather simple and widely utilized technique for creating micropatterned surfaces. The surface oxidation of a Sylgard 184 film due to UVO exposure is associated with densification and the formation of a silica-like surface layer, which under a TEM grid happens only over the exposed areas of the film, resulting in a physicochemical pattern. It is known that the depth (hD) of the features depends on the duration of UVO exposure (tE). In this article, we show for the first time that hD also depends on the initial film thickness (hF) and the cross-linker percentage (CL, ratio of part A to part B) in a Sylgard 184 thin film. We show that for a specific tE, hD progressively decreases with the reduction in hF. On the other hand, hD shows a nonmonotonic dependence with CL, resulting in patterns with maximum depth for CL ≈ 10.0%. We attribute this observation to the combined effect of resistance against the penetration of the propagation front by the rigid substrate as well as stress relaxation within the exposed parts of the film below the propagating front in films with higher CL values leading to the variation of hD. The observation reported here would allow the potential fabrication of polymer films with physicochemical patterns with feature height on demand by a one-step, facile technique.

Synthesis, Characterization, and Evaluation of a Hindered Phenol-Linked Benzophenone Hybrid Compound as a Potential Polymer Anti-Aging Agent.

Hindered phenol antioxidants and benzophenone UV absorbers are common polymer additives and often used in combination applications to enhance the anti-aging performance of polymer materials. This study primarily aims to incorporate hindered phenol and benzophenone structures into a single molecule to develop a multifunctional polymer additive with good anti-aging performance. Thus, a novel potential polymer anti-aging agent, namely 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid 3-(4-benzoyl-3-hydroxyphenoxy)propyl ester (3C), was synthesized using 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid, 3-bromo-1-propanol, and 2,4-dihydroxybenzophenone as raw materials by two-step procedure. The structure of compound 3C was characterized by nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS), Fourier-transform infrared (FT-IR) spectroscopy, and X-ray single crystal diffraction. Its thermal stability and UV resistance were assessed using thermogravimetric analysis (TGA) and UV absorption spectroscopy (UV). The compound 3C as an additive was incorporated into the preparation of polyolefin elastomer (POE) films. The anti-aging performance of POE films was evaluated by measuring parameters such as oxidation induction time, melt flow index, transmittance, and infrared spectra of the artificially aged POE films. The results indicate that the compound 3C exhibits a promising anti-aging performance in both thermo-oxidative aging and ultraviolet aging tests of POE films and is a potential polymer anti-aging agent.

A new strategy for fabricating a stacked flexible capacitive sensor

Currently, flexible capacitive sensors have a wide range of application scenarios in the field of wearable electronic devices. In order to detect more subtle joint movements of the human body, a method of fabricating stacked capacitive sensors is demonstrated. An ultrathin dielectric elastomer film of about 110 μm by the “secondary calendering” method was prepared. The shape of the electrode layers was designed, printed the electrode materials on the dielectric elastomer film by screen-printing, realized the stacked-layer technology, and connected each sensor unit in parallel by the electrode columns formed inside. A 12-layer flexible capacitive sensor with an initial capacitance of 10.2nF, good resolution (1% strain), high sensitivity (1.09) and stability under 10,000 cycles is fabricated. The sensor fabricated in this paper can recognize the motion at various joints of the human body, such as elbow and knee joints. This paper provides a new method for fabrication of stacked flexible capacitive sensors, which opens up new applications in flexible sensors, wearable electronic devices and human-computer interaction.

Structural study of an electroactive allyl ester thiophenoazomethine for PDMS blending

A conjugated thiophene azomethine was prepared having pendant allyl esters. X-ray crystallography confirmed its structure and hydrogen bonding. The compound switched its absorption between 495 and 645 nm with electrochemical oxidation/neutralization and between 495 and 710 nm with chemical oxidation. The azomethine could be blended with PDMS for homogeneous elastomeric film preparation while maintaining its intrinsic color.

Control the handedness of CPL using a cholesteric liquid crystal elastomer film

The reversible modulation of the handedness of circularly polarized luminescence (CPL) is highly important for anti-counterfeiting, flexible displays, and encrypted transmission applications. A thermochromic cholesteric liquid crystal (CLC) oligomer was prepared and applied for printing colourful patterns. The structural colours of the patterns were sensitive to the temperature. A CLC elastomer (CLCE) film was prepared using this CLC oligomer, which exhibited mechanochromic property with a wide range of structural colour changes. A CPL-active CLCE film was obtained with the addition of a fluorescent dye. The handedness of the CPL was reversibly switched upon uniaxial stretching of the film. This work not only sheds insight into CPL handedness control based on the CLCE system, but also provides possibilities for flexible decorations and displays.

Compliant frame geometry for DEMES-based gripper and flapping wing actuators: A comprehensive design study

Dielectric elastomer minimum energy structures (DEMES) have attracted significant attention in the recent past because of their ability to switch between multiple equilibrium states. The DEMES is formed when a pre-stretched elastomer film adheres to an inextensible frame and is further allowed to attain an equilibrium configuration as a result of energy minimization. While several researchers have investigated; both theoretically and experimentally, the underlying mechanics of DEMES, a majority of them deploy simplistic boundary frames (square/rectangular/circular) to obtain the requisite minimum energy configuration. In this paper, we demonstrate that this seemingly restrictive choice of using simplistic frame geometries can be given away by designing more complex-shaped compliant frames capable of producing useful modes of deformation. To demonstrate this idea, two prototypes; namely, the four-arm gripper actuator and the flapping-wing actuator, are studied numerically and experimentally. In both cases, a single compliant frame is used to realize the respective configuration, thus circumventing the need to assemble multiple MES on a common platform. In tandem, we investigate the role of reinforcements in (a) controlling the warping in MES and (b) maximizing actuation in the desired mode of deformation. Finite element analysis are carried out using the ABAQUS to determine the equilibrium configuration of the actuators, and subsequently their electromechanical behavior. Experimental investigations involve the utilization of the commercially available VHB 4910 acrylic tape in conjunction with PET frames and 3D-printed reinforcements. Excellent qualitative agreement is achieved between the numerical predictions and the experimental observations. Finally, we allude to a few innovative frame architectures leading the MES of engineering interest.

Magnetoactive, Kirigami-Inspired Hammocks to Probe Lung Epithelial Cell Function

IntroductionMechanical forces provide critical biological signals to cells. Within the distal lung, tensile forces act across the basement membrane and epithelial cells atop. Stretching devices have supported studies of mechanical forces in distal lung epithelium to gain mechanistic insights into pulmonary diseases. However, the integration of curvature into devices applying mechanical forces onto lung epithelial cell monolayers has remained challenging. To address this, we developed a hammock-shaped platform that offers desired curvature and mechanical forces to lung epithelial monolayers.MethodsWe developed hammocks using polyethylene terephthalate (PET)-based membranes and magnetic-particle modified silicone elastomer films within a 48-well plate that mimic the alveolar curvature and tensile forces during breathing. These hammocks were engineered and characterized for mechanical and cell-adhesive properties to facilitate cell culture. Using human small airway epithelial cells (SAECs), we measured monolayer formation and mechanosensing using F-Actin staining and immunofluorescence for cytokeratin to visualize intermediate filaments.ResultsWe demonstrate a multi-functional design that facilitates a range of curvatures along with the incorporation of magnetic elements for dynamic actuation to induce mechanical forces. Using this system, we then showed that SAECs remain viable, proliferate, and form an epithelial cell monolayer across the entire hammock. By further applying mechanical stimulation via magnetic actuation, we observed an increase in proliferation and strengthening of the cytoskeleton, suggesting an increase in mechanosensing.ConclusionThis hammock strategy provides an easily accessible and tunable cell culture platform for mimicking distal lung mechanical forces in vitro. We anticipate the promise of this culture platform for mechanistic studies, multi-modal stimulation, and drug or small molecule testing, extendable to other cell types and organ systems.

Linear Film Strain Sensors with Nanosphere‐on‐Fiber Hetero‐Modulus Microstructure for Human‐Bionic Finger Interfacing

AbstractStretchable film strain sensors show potential applications in healthcare and human‐machine interfacing, while poor signal linearity restricts their practical applications owing to inaccurate and instable strain indication. Signal sensitivity and linearity can be improved via constructing hetero‐modulus microstructure on elastomeric substrate to some extent, though it is challenging to construct hetero‐structure with distinct modulus differences for electrical signal linearity modulation over a wide strain range. Herein, a simple strategy is proposed to tune signal sensitivity, linearity, and detection range of stretchable strain sensors by adjusting the distribution of rigid polystyrene (PS) nanospheres coated on electrospun thermoplastic polyurethane (TPU) fiber mats. Heterostructure characteristics can be controlled by spray coating times of PS nanospheres, which are immobilized onto the fiber mats by weak swelling of TPU and the linking layer of reduced graphene oxide (rGO) between TPU and PS induced by hydrogen bonding and π–π stacking. Relatively, the sensors with moderate spray coating times of PS nanospheres show balanced signal sensitivity and linearity over a strain range of 0–80%, as well as fast response to tensile strain and good signal reproducibility. Such elastomeric film strain sensors can be used for monitoring physiological activities and interfacing human hand and bionic fingers.

Design of interconnected graphene loaded thermoplastic elastomeric blend composite films for minimizing electromagnetic radiation and efficient heat management

AbstractIn this work, electrically conductive thermoplastic elastomeric blend composite films based on polystyrene (PS)/ethylene‐co‐methyl acrylate (EMA) filled with functionalized graphene were developed via the solution mixing technique. Morphological analysis revealed that selective localization of amine‐functionalized reduced graphene oxide (G‐ODA) sheets in the EMA phase of co‐continuous binary blend formed a well‐connected dense conductive pathway by graphene sheets ultimately facilitating the double percolation phenomenon. The electrical percolation threshold was achieved at ~2 wt% of G‐ODA loading which was much lower than that for both single polymer composites. An electrical conductivity of 0.9 S/cm was obtained for blend composite film with 10 wt% of graphene concentration whereas for the same filler loading, PS and EMA composites exhibited electrical conductivity of 1.9 × 10−1 and 2.3 × 10−1 S/cm, respectively. The obtained thermal conductivity of the blend composite with 10 wt% of G‐ODA loading was 0.95 W/m K with 400% enhancement compared to the neat blend system. The same composite exhibited increased real and imaginary permittivity of 92 and 83, respectively. The electrical percolation threshold is well‐correlated with the percolation concentration found from storage modulus and thermal conductivity data. The fabricated PS/EMA blend composite film exhibited absorption‐dominant electromagnetic interference SE of −25 and − 35 dB in X‐band frequency (8.2–12.4 GHz) for 10 wt% of graphene loading with a sample thickness of 0.5 and 1 mm, respectively.