Large Stretchability Research Articles

Biocompatible and self-healing ionic gel skin as shape-adaptable and skin-adhering sensor of human motions

Artificial electric skin, which is capable of strongly adhering to different parts of human bodies and precisely detect different types of human motions, shows great promise for biomedical prosthetics, human/machine interfaces, wearable devices and soft robotics. In this study, biocompatible ionic gels with shape-adaptability and skin-adhering are produced through in situ polymerizing (3-acrylamidophenyl) boronic acid and acrylamide in the presence of chitosan containing catechol groups. The chemical cross-linkers are capable of modulating their elasticity, toughness and stretching tolerance. The reversible cross-linkers of H-bonding and dynamic covalent bonds endow the gels not only with strong adhering strength on different surfaces (including skin) and rapid self-healing in minutes, but also with large stretchability (e.g., 12–200 times of tensile length) and plasticity for shape-adaptability on irregular surfaces. Thus, by introducing mussel-inspired catechol groups into biomass-based macromolecules, a novel type of artificial ionic skin is designed with high sensitivity in combination with mechanical stretchability and bio-compatibility. They will promise great potential as (but not limited to) the skin-friendly sensor to detect various human motions with high accuracy and repeatability.

Water-Enabled Room-Temperature Self-Healing and Recyclable Polyurea Materials with Super-Strong Strength, Toughness, and Large Stretchability.

The synthesis of polymeric materials that simultaneously possess multiple excellent mechanical properties and high-efficient self-healability at room temperature is always a huge challenge. Here, we report the synthesis of a transparent polyurea material that can self-heal at room temperature with the aid of water and, meanwhile, has multiple remarkable mechanical performances, including super-high strength, excellent toughness, and large stretchability. Thanks to the synergistic enhancement of both dynamic imine bonds and hierarchical hydrogen bonds within the networks, the resulting polyureas have a world-record tensile strength of 41.2 MPa when compared with other polyurethanes that can self-heal at room temperature and, at the same time, a large breaking strain of 823.0% and a superior toughness of 127.2 MJ/m3. Besides the influence of imine bonds, the mechanical properties of the polyureas are also strongly related to the density and strength of the hierarchical hydrogen bonds within the polyurea networks, and these two factors could be finely controlled by adjusting the mass ratio of the soft segments with different chain lengths and the types of diisocyanates used for polyurea synthesis, respectively. More importantly, the highly dynamic characteristic of both imine bonds and hierarchical hydrogen bonds within the polyureas endows the materials with repeated water-enabled room-temperature self-healing capacity with a high healing efficiency of 92.2%. Moreover, the polyureas can also be recycled or remolded under mild conditions by the hot-pressing or dissolution/casting process. The synthesized polyureas also show great potential in damping applications with a loss factor larger than 0.3 over the temperature range from 12 to 75 °C. It is believed that polyureas with super-high and well-tunable mechanical properties and high-efficient room-temperature self-healing ability have great potential to substitute traditional irreparable polymers in diverse practical applications.

Hyaline and stretchable haptic interfaces based on serpentine-shaped silver nanofiber networks

Abstract Skin-like epidermal electronic technologies are making significant marks in the field of human and machine interaction, with critical applications in prosthetics and robotic systems. However, currently existing skin-interface devices can not statisfy both the high transparency and the large stretchability simultaneously, especially for flexible smart screens, curved optical projections, and patient-friendly medical skin-devices. This study presents a hyaline and stretchable haptic interface (HSHI) based on 20 × 20 capacitive sensor array, simultaneously demonstrating both high transparency and large stretchability. The serpentine design of netlike conductive architectures with narrow nanofibers under surface plasmons in the HSHI can maximize light transmittance (>90%), and form the compatibility of structures and functions with the considerable mechanical strain (>50%). Multifunctional HSHI performs the vary great tactile and geometric adaptation from obviously capacitive-effect shifts for interfacing and switching systems on the robotic arm and the screen. The routes for designing and manufacturing the HSHI also provide effective ways for various complex performances and special application outreaching usual methods through integrating serpentine stretchable technologies and the transparent nanowire arrays.

Simultaneously Achieving Ultrahigh Sensitivity and Wide Detection Range for Stretchable Strain Sensors with an Interface‐Locking Strategy

AbstractThe development of stretchable strain sensors is crucial for the implementation of electronic skins and wearable electronics. Resistive‐type strain sensors are typically composed of an electrically conductive sensing layer coupled to a stretchable substrate. When a sensor is stretched, microstructural changes in the sensing layer lead to a strain‐dependent change in resistance. However, strain sensors with high sensitivity and specificity often suffer from a limited detection range along with reliability issues. Here, a novel strain sensor composed of Ag particles and long‐range entangled carbon nanotubes (CNTs) with a special composite design is proposed, in which a strain‐induced self‐locking effect is introduced at the interface between the CNT film and a percolating network consisting of Ag particle filler. As a result, the sensors achieve significantly enhanced performance such as large stretchability (≥120%), high gauge factor (3990.8), fast response time (<33 ms), and high mechanical durability. A range of wearable applications including radial artery, facial expression, and joint motion monitoring are demonstrated, which range from minute deformation to excessive stretching of the sensor.

2D Percolation Design with Conductive Microparticles for Low‐Strain Detection in a Stretchable Sensor

AbstractAlthough a variety of stretchable strain sensors based on electrical percolation have been reported, stretchable sensors detecting low strains have been rarely demonstrated. This is because large stretchability of a strain sensor conflicts with high strain resolution at low strains. Here, the electrical percolation into 2D is confined and a strain sensor that is highly sensitive at low strains and simultaneously highly stretchable is presented. The 2D confinement of the electrical percolation is accomplished by a close‐packed monolayer assembly of conductive microparticles (MPs) on an elastomer substrate. The current profiles of the MP monolayer at low strains are in situ visualized using conductive atomic force microscopy. When the lattice of the MP monolayer is aligned vertically to the strain direction, the resistance is highly sensitive to low‐strain deformations (ε = 0 – 0.05), but the sensor has reasonable stretchability (ε = 0.3). The simultaneous achievement of the high sensitivity at low strains and the reasonable stretchability is explained by the relationship between the strain‐dependent current profile and the relative position changes of the MPs. A high‐precision pulse sensor clearly showing the representative peaks is demonstrated.

Highly Stretchable Bilayer Lattice Structures That Elongate via In‐Plane Deformation

AbstractMany emerging technologies such as wearable batteries and electronics require stretchable functional structures made from intrinsically less deformable materials. The stretch capability of most demonstrated stretchable structures often relies on either initially out‐of‐plane configurations or the out‐of‐plane deflection of planar patterns. Such nonplanar features may dramatically increase the surface roughness, cause poor adhesion and adverse effects on subsequent multilayer processing, thereby posing a great challenge for flexible devices that require smooth surfaces (e.g., transparent electrodes in which flat‐surface‐enabled high optical transmittance is preferred). Inspired by the lamellar layouts of collagenous tissues, this work demonstrates a planar bilayer lattice structure, which can elongate substantially via only in‐plane motion and thus maintain a smooth surfaces. The constructed bilayer lattice exhibits a large stretchability up to 360%, far beyond the inherent deformability of the brittle constituent material and comparable to that of state‐of‐the‐art stretchable structures for flexible electronics. A stretchable conductor employing the bilayer lattice designs can remain electrically conductive at a strain of 300%, demonstrating the functionality and potential applications of the bilayer lattice structure. This design opens a new avenue for the development of stretchable structures that demand smooth surfaces.

Mechanics of Buckled Kirigami Membranes for Stretchable Interconnects in Island-Bridge Structures

Abstract Island-bridge structures incorporated with kirigami membranes emerge as a novel design strategy for flexible/stretchable electronics, taking advantages of large stretchability, high-surface filling ratio and low resistance. However, it is hard to determine the mechanical properties of this design due to its complex geometries and nonlinear deformation configuration, thereby limiting its further applications. In this paper, we present a model for the postbuckling behavior of kirigami membranes through a combination of theoretical modeling, finite element analysis, and experiments. Scaling laws for elastic stretchability are developed, showing good agreement with numerical results and experimental images. Investigations on the critical height of post array are conducted to ensure the boundary condition of the kirigami membranes in the analytical model. These results can serve as design guidelines for kirigami structures and facilitate their applications in flexible/stretchable electronics.

Super soft but strong E-Skin based on carbon fiber/carbon black/silicone composite: Truly mimicking tactile sensing and mechanical behavior of human skin

Besides the prominent tactile sensing ability, human skin is also well known for its unique mechanical behavior – combination of excellent softness with certain stretchability of a strain-limiting effect to prevent damage from excessive strain. Here a stretchable e-skin truly mimicking the tactile sensing and mechanical behavior of human skin is fabricated. Carbon black (CB)/silicone composite with excellent piezo-resistive performance is employed as mechanical stimuli sensing material, which provides super softness, large stretchability, high strain sensitivity and long-term reliability for the e-skin. High performance carbon fiber (CF) with quasi-sinusoidal shape is designed to serve as electrode material and reinforcement filler simultaneously, which renders the e-skin a typical J-shape stress-strain curve similar as that of human skin. The Young's modulus, tensile strength and failure strain of the e-skin can be readily adjusted by controlling the content and shape of the CF electrode to match them well with those of human skin. Finally, the fabricated e-skin exhibits the capability of spatiotemporal recognition of the location and magnitude of contact forces in monitoring dynamic stress distribution. The as-prepared stretchable e-skin based on CF/CB/silicone composite is highly promising for robotics and prosthetics, etc.

Highly stretchable two-dimensional auxetic metamaterial sheets fabricated via direct-laser cutting

The design and production of multifunctional materials possessing tailored mechanical properties and specialized characteristics is a major theme in modern materials science, particularly for implementation in high-end applications in the biomedical and electronics industry. In this work, a number of metamaterials with perforated architectures possessing the ability to exhibit a plethora of 2D auxetic responses with negative Poisson's ratios ranging from quasi-zero to large negative values (lower than −3.5), stiffnesses, stretchability and surface coverage properties were manufactured. These systems were produced through the introduction of microstructural cuts in a rubber sheet using direct laser cutting, and analysed using a dual approach involving experimental tests and Finite Element Analysis. In addition to examining the mechanical properties of the perforated metamaterials, the influence of edge effects and material thickness on the deformation behaviour of these systems were investigated, with re-entrant systems shown to possess anomalous deformation profiles which are heavily dominated by the boundary regions. These findings highlight the effectiveness of this method for the fabrication of auxetic metamaterial sheets as well as the large variety of mechanical properties, deformation mechanisms and load responses which may be obtained through what may be effectively described as simply the introduction of patterned cuts in a thin sheet.

Stretchable and Highly Sensitive Optical Strain Sensors for Human-Activity Monitoring and Healthcare.

Flexible and stretchable strain sensors are essential to developing smart wearable devices for monitoring human activities. Such sensors have been extensively exploited with various conductive materials and structures, which, however, are normally in need of complex manufacturing processes and confronted with the challenge to achieve both large stretchability and high sensitivity. Here, we report a simple and low-cost optical strategy for the design of stretchable strain sensors which are capable of measuring large strains of 100% with a low detection limit (±0.09%), a fast responsivity (<12 ms), and high reproducibility (over 6000 cycles). The optical strain sensor (OS2) is fabricated by assembling plasmonic gold nanoparticles (GNPs) in stretchable elastomer-based optical fibers, where a core/cladding structure with step-index configuration is adopted for light confinement. The stretchable, GNP-incorporated optical fiber shows strong localized surface plasmon resonance effects that enable sensitive and reversible detection of strain deformations with high linearity and negligible hysteresis. The unique mechanical and sensing properties of the OS2 enable its assembling into clothing or mounting on skin surfaces for monitoring various human activities from physiological signals as subtle as wrist pulses to large motions of joint bending and hand gestures. We further apply the OS2 for quantitative analysis of motor disorders such as Parkinson's disease and demonstrate its compatibility in strong electromagnetic interference environments during functional magnetic resonance imaging, showing great promises for diagnostics and assessments of motor neuron diseases in clinics.

One-Step Preparation of a Highly Stretchable, Conductive, and Transparent Poly(vinyl alcohol)-Phytic Acid Hydrogel for Casual Writing Circuits.

Conductive hydrogels have shown great potential applications in a wide variety of fields, including artificial intelligence devices and biomedical engineering. However, it still remains a great challenge to develop a facile and cost-effective approach to achieve a conductive hydrogel with favorable qualities. Herein, we have changed the traditional ingredient of poly(vinyl alcohol) (PVA) hydrogel by the addition of phytic acid (PA), which could yield a conductive hydrogel through one freeze-thaw cycle. The PVA-PA hydrogel holds several virtues including a large stretchability (about 1100% strain), excellent conductivity (1.34 kΩ cm), and high optical transparence (about 95%). By assembling the PVA-PA hydrogel into a wearable strain sensor, the gel-based sensor has shown good performance for the real-time monitoring of human daily activities and health conditions. Moreover, one formula of the PVA-PA sol ink could rapidly convert to the gel state just by being injected on a flexible substrate under an ice-bath, which would satisfy the demand of casual writing circuits. This one-step preparation method of the PVA-PA hydrogel may open an innovative avenue for the fabrication of easy-molding and functional hydrogels with only two components under mild ambient conditions.

Dielectric elastomer actuators based on stretchable and self-healable hydrogel electrodes.

Dielectric elastomer actuator (DEA) based on dielectric elastomer holds promising applications in soft robotics. Compliant electrodes with large stretchability and high electrical conductivity are the vital components for the DEAs. In this study, a type of DEA was developed using carbon nanotube/polyvinyl alcohol (CNT/PVA) hydrogel electrodes. The CNT/PVA hydrogel electrodes demonstrate a stretchability up to 200% with a small relative resistance change of approximately 1.2, and a self-healing capability. The areal strain of the DEA based on the CNT/PVA hydrogel electrodes is more than 40%, much higher than the ones based on pure PVA electrodes.

Transparent Soft Robots for Effective Camouflage

AbstractTransparency is a surprisingly effective method to achieve camouflage and has been widely adapted by natural animals. However, it is challenging to replicate in synthetic systems. Herein, a transparent soft robot is developed, which can achieve effective camouflage. Specifically, this robot is driven by transparent dielectric elastomer actuators (DEAs). Transparent and stretchable conductive polymers, based on blends of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and water‐borne polyurethane (WPU), are employed as compliant solid‐state electrodes in the DEAs. The electrode exhibits large stretchability, low stiffness, excellent conductivity at large strain, and high transmittance. Consequently, the DEA can achieve a large voltage‐induced area strain of 200% and a high transmittance of 85.5%. Driven by these soft actuators, the robot can realize translations using its asymmetric vibration mode, which can be explained by dynamics analysis and is consistent with finite element modeling. This soft robot can achieve effective camouflage, due to its high transparency as well as thin structure. Furthermore, the robot can become completely flat for even better camouflage by converting its 3D structure to 2D. The transparent soft robot is potentially useful in many applications such as robots for battlefield, reconnaissance, and security surveillance, where effective camouflage is required in dynamic and/or unstructured environments.

Multifunctional Two-Dimensional PtSe2-Layer Kirigami Conductors with 2000% Stretchability and Metallic-to-Semiconducting Tunability.

Two-dimensional transition-metal dichalcogenide (2D TMD) layers are highly attractive for emerging stretchable and foldable electronics owing to their extremely small thickness coupled with extraordinary electrical and optical properties. Although intrinsically large strain limits are projected in them (i.e., several times greater than silicon), integrating 2D TMDs in their pristine forms does not realize superior mechanical tolerance greatly demanded in high-end stretchable and foldable devices of unconventional form factors. In this article, we report a versatile and rational strategy to convert 2D TMDs of limited mechanical tolerance to tailored 3D structures with extremely large mechanical stretchability accompanying well-preserved electrical integrity and modulated transport properties. We employed a concept of strain engineering inspired by an ancient paper-cutting art, known as kirigami patterning, and developed 2D TMD-based kirigami electrical conductors. Specifically, we directly integrated 2D platinum diselenide (2D PtSe2) layers of controlled carrier transport characteristics on mechanically flexible polyimide (PI) substrates by taking advantage of their low synthesis temperature. The metallic 2D PtSe2/PI kirigami patterns of optimized dimensions exhibit an extremely large stretchability of ∼2000% without compromising their intrinsic electrical conductance. They also present strain-tunable and reversible photoresponsiveness when interfaced with semiconducting carbon nanotubes (CNTs), benefiting from the formation of 2D PtSe2/CNT Schottky junctions. Moreover, kirigami field-effect transistors (FETs) employing semiconducting 2D PtSe2 layers exhibit tunable gate responses coupled with mechanical stretching upon electrolyte gating. The exclusive role of the kirigami pattern parameters in the resulting mechanoelectrical responses was also verified by a finite-element modeling (FEM) simulation. These multifunctional 2D materials in unconventional yet tailored 3D forms are believed to offer vast opportunities for emerging electronics and optoelectronics.

Kinematics analysis of scissor-type inspired interconnects

Scissor-type inspired structure is used to design stretchable electronics, which aims to reduce local deformation of interconnects during large stretchability of stretchable electronics for better fatigue design. An additional ultra-soft grid method is developed to compute the deformation and shape transformations of the scissor-type inspired structures. It has been proved that the additional ultra-soft grid is only used to supply additional constraint equations and will not affect the deformation and shape transformations of the structures. As case study, the method can be used to compute the deformation and shape transformations of the structures. The simulation results show that the scissor-type inspired interconnects design can be effective to reduce the local maximal strain amplitude during cyclic stretching, which can be expected to improve the fatigue life of stretchable electronics devices dramatically.

Large-area, kirigami topology structure-induced highly stretchable and flexible interconnects: Directly printing preparation and mechanic mechanism

Integrating the topology design and printing method offers a promising methodology to realize large stretchability for interconnects. Herein, eco-friendly and water-based Ag nanowires (NWs) inks were formulated and used for screen-printing highly stretchable and flexible interconnects on a large area (more than 335 mm × 175 mm). The stretchability of the interconnects was realized by introducing kirigami topology structures. The topology designed models were established to simulate the influence of kirigami patterns on wire compliance and to estimate the maximum stretchability via finite element analysis (FEA). The mechanic mechanism results demonstrate that an increase of the wave numbers results in larger stretchability, and the rectangular type of wave shows better stretchability than the zigzag and sine structures. Comparatively, the electrical and mechanical properties of the interconnects were measured and analyzed, and the experimental results were consistent with FEA. The electric conductivity of the interconnects is stable at ~10,427 S cm−1 even after 1000 cycles of 15.83 mm radius bending, 280% stretching and 200% twisting-stretching deformation, demonstrating outstanding mechanical reliability of the interconnects. The topology designed interconnects have been applied in stretchable flexible light-emitting diode, indicating their broad application prospects in next-generation stretchable electronics.

High-sensitive and stretchable resistive strain gauges: Parametric design and DIW fabrication

The main advantages of the direct ink write (DIW) printed strain gauge are capable of achieving both the high sensitivity and the large stretchability. The performance of the DIW-printed strain gauge is mainly determined by the design parameters and fabrication process. However, it is still a big challenge for parametric design and precision manufacturing of the DIW-printed strain gauge due to the lack of universal analytical model and optimized process parameters. Here we propose an analytical model for designing widely used sandwich-structure strain gauges with coating-filament-base layers on substrates. The resistive strain gauges with a commercial carbon paste are fabricated by the DIW technology on polyimide and aluminium alloy substrates, respectively. Both its high sensitivity and large stretchability are designed from the present analytical model and reproduced well by the DIW-based fabrication. Our analytical model and DIW-based fabrication will be of great help for designing and fabricating other sandwich-structure gauges.

Strong and Stretchable Polypyrrole Hydrogels with Biphase Microstructure as Electrodes for Substrate‐Free Stretchable Supercapacitors

AbstractThe development of stretchable supercapacitors (SSCs) is heading to compact and robust devices with higher capacitance and simpler preparation process. Herein, a new strategy is reported to prepare a highly stretchable and conductive polypyrrole hydrogel with a unique biphase microstructure (loose phase and dense phase), which is formed by the supramolecular assembly of polypyrrole (PPy), poly(vinyl alcohol), and anionic micelles. The loose phase enables the PPy hydrogel large stretchability (elongation at a break of 500%) and good electrochemical capacitive behavior, while the dense phase enables the PPy hydrogel high tensile strength (2 MPa) and good conductivity (0.8 S cm−1). Without using any substrate, the SSC made of this polypyrrole hydrogel provides an areal capacitance of 950 mF cm−2 at a current density of 1.6 mA cm−2, exceeding most of reported SSCs. This SSC can withstand repeated deformation and retain 81% capacitance after 500 stretching–releasing cycles. Besides, at subzero temperature down to −20 °C, this SSC can still retain its good stretchability and capacitance. The combination of high areal capacitance, good stretchability, and high retention of capacitance under various circumstances enables the polypyrrole hydrogel–based SSC an economical and robust SSC for stretchable electronics.

Fluorine-free Superhydrophobic and Conductive Rubber Composite with Outstanding Deicing Performance for Highly Sensitive and Stretchable Strain Sensors.

Conductive polymer composite (CPC) based strain sensor due to its lightweight, tunable electrical conductivity, and easy processing has promising application in wearable electronics. However, it is still challenging to develop the CPC strain sensors with excellent stretchability, sensitivity, durability, anticorrosion, and deicing performance. Herein, a facile method is proposed to prepare fluorine-free superhydrophobic and highly conductive rubber composite. Ag nanoparticles (AgNPs) are first decorated on the RB (rubber band) surface, forming a conductive shell. Then, the RB/AgNPs experiences PDMS (polydimethylsiloxane) modification, which could not only endow the composite with superhydrophobicity and hence excellent corrosion resistance but also improve the interfacial adhesion between the AgNPs. The RB composite possesses a good self-cleaning performance and remains superhydrophobic even after experiencing cyclic abrasion or stretching-releasing test. Also, the high conductivity and superhydrophobicity endow the RB composite with excellent Joule heating performance and water repellence, broadening its application in deicing and water removal. Moreover, the obtained RB composite exhibits both large stretchability with a break elongation larger than 900% and high sensitivity with a response intensity as high as 3.6 × 108 at a strain of 60%. In addition, the RB composite strain sensor can be used to detect full-range human motions including large and subtle body movement. The flexible, durable, and anticorrosive RB composite has potential applications in flexible electronic, health monitoring, physical therapy, and so on.

Coaxial Printing of Silicone Elastomer Composite Fibers for Stretchable and Wearable Piezoresistive Sensors.

Despite the tremendous efforts dedicated to developing various wearable piezoresistive sensors with sufficient stretchability and high sensitivity, challenges remain pertaining to fabrication scalability, cost, and efficiency. In this study, a facile, scalable, and low-cost coaxial printing strategy is employed to fabricate stretchable and flexible fibers with a core–sheath structure for wearable strain sensors. The highly viscous silica-modified silicone elastomer solution is used to print the insulating sheath layer, and the silicone elastomer solutions containing multi-walled carbon nanotubes (CNTs) are used as the core inks to print the conductive inner layer. With the addition of silica powders as viscosifiers, silica-filled silicone ink (sheath ink) converts to printable ink. The dimensions of the printed coaxial fibers can be flexibly controlled via adjusting the extrusion pressure of the inks. In addition, the electro-mechanical responses of the fiber-shaped strain sensors are investigated. The printed stretchable and wearable fiber-like CNT-based strain sensor exhibits outstanding sensitivities with gauge factors (GFs) of 1.4 to 2.5 × 106, a large stretchability of 150%, and excellent waterproof performance. Furthermore, the sensor can detect a strain of 0.1% and showed stable responses for over 15,000 cycles (high durability). The printed fiber-shaped sensor demonstrated capabilities of detecting and differentiating human joint movements and monitoring balloon inflation. These results obtained demonstrate that the one-step printed fiber-like strain sensors have potential applications in wearable devices, soft robotics, and electronic skins.