Ultrahigh Stretchability Research Articles

Piezoresistive fibers with record high sensitivity via the synergic optimization of porous microstructure and elastic modulus

Although different kinds of microengineering have been developed in piezoresistive fibers recently, it is still challenging to achieve a high-pressure sensitivity in these fibers. This is because these microstructures can only geometrically increase the conducting paths under pressure. In this work, via the theoretical calculation and practical experiments, we firstly demonstrated that not only the conducting paths but also the fibers’ elastic moduli of compression exhibit key roles in improving the sensitivities in fibers. Typically, by adjusting the kinetic diffusion coefficient between the spinning dope and the coagulation solution in wet-spinning, various structured (solid, hollow, porous, and hollow-porous) thermoplastic polyurethane (TPU) fibers with variable elastic moduli of compression were developed in this work. As a result, the piezoresistive fiber with a uniform and through porous structure (porous in both the core and the surface) exhibited the highest sensitivity of 0.67 kPa−1 due to its synergic improvement in conducting paths and contact area (with a low elastic modulus of compression of 6.98 kPa) under pressure. To the best of our knowledge, it is the recorded highest sensitivity in the piezoresistive fibers. Beyond the pressure sensing, we also proved the as-prepared porous fibers with an ultra-high stretchability of 2110%, a high performance in strain sensing, and a good candidate in humidity and gas sensing. At last, we successfully applied the piezoresistive fibers in our daily life for wearable sensing, demonstrating its high potential in practical application.

Highly Permeable and Ultrastretchable Liquid Metal Micromesh for Skin-Attachable Electronics

Stretchable electronics represents an emerging technology for next-generation smart wearables toward intimate integration with the human body. In contrast with functional devices constructed over elastomer films with limited moisture permeability, a soft electronic textile may represent the ideal skin-attachable platform to achieve long-term wearing comfort. The advancements in this active area largely hinge on a new generation of permeable conductor. Despite its intrinsic mechanical deformability, gallium-based liquid metal typically represents an impenetrable barrier for gases and liquids. In this study, we introduce a liquid metal micromesh on electrospun microfiber textile as a highly permeable and ultrastretchable conductor. The fabrication process involves dropcasting liquid metal onto an elastomeric microfiber textile followed by high-speed rotation to remove the excessive coating. The liquid metal micromesh exhibits low sheet resistance (0.38 Ω/sq), ultrahigh stretchability (>1000% strain), and mechanical durability. The porous morphology enables a high steam permeability and perception of comfort comparable to those of standard textiles. The conformal interface with the skin gives rise to low contact impedance better than that of state-of-the-art Ag/AgCl gel electrodes. The successful implementation of the liquid micromesh conductor in a multifunctional electronic system demonstrates its practical suitability for a broad range of applications in stretchable and wearable electronics.

Stretchable and transparent multimodal electronic-skin sensors in detecting strain, temperature, and humidity

Smart electronic skins (e-skins) have garnered extensive attentions owing to their potentials in health monitoring, human-machine interactions, and soft robotics. Herein, stretchable and transparent multimodal e-skin sensors composed of poly(vinyl alcohol) (PVA), citric acid (CA) and Ag nanoparticles (AgNPs) are developed by a simple in-situ reduction and solvent casting technology, which possess multiple sensing capabilities in strain, temperature and humidity. Although utilizing the nontransparent AgNPs as sensing components, the as-prepared PVA/CA/AgNPs sensor exhibits not only ultrahigh stretchability (up to 596% strain) but also outstanding transparency (the transmittance > 89%) attributed to the homogeneous dispersion of CA and AgNPs in PVA matrix. As flexible strain sensor, this sensor exhibits wide strain sensing range (1–200%), rapid response (90 ms), and excellent linearity of outputted signals (correlation coefficient: 0.998). Moreover, high-precision thermo-sensation (0.1 ℃) and outstanding temperature sensitivity (−0.076/℃ in the range of 30–40 ℃) make the sensor can detect subtle temperature variation. Furthermore, the good hygroscopicity of PVA and CA endows the fabricated sensor with outstanding humidity sensing performance, which can give human skin moisture information in real-time. More importantly, the designed sensor possesses excellent recyclability and can be reused for many times via the simple dissolution-reshaping treatment, which could reduce the electronical waste.

A facile and scalable patterning approach for ultrastretchable liquid metal features.

Liquid metals represent an attractive class of compliant conductors featuring metallic conductivity and inherent deformability. The widespread implementation of liquid metal conductors in stretchable electronics is currently hindered by the lack of a facile patterning approach. In this study, we introduce a facile and scalable patterning approach to create liquid metal features on an elastomer substrate. A screen-printed Ag nanoflake pattern is employed as a template for the subsequent selective coating of a liquid metal layer. The as-prepared liquid metal conductors show a bulk-level conductivity of ∼2.7 × 104 S cm-1, an ultrahigh stretchability of up to 700% tensile strain, and excellent electromechanical durability. The practical suitability is demonstrated by the successful fabrication of an ultradeformable ribbon cable and a smart sensing glove. The efficient and economical access to ultrastretchable liquid metal features may open up a broad range of emerging applications in soft electronic devices and systems.

Ultrastretchable and Self-Healing Conductors with Double Dynamic Network for Omni-Healable Capacitive Strain Sensors.

Skin-mountable capacitive-type strain sensors with great linearity and low hysteresis provide inspiration for the interactions between human and machine. For practicality, high sensing performance, large stretchability, and self-healing are demanded but limited by stretchable electrode and dielectric and interfacial compatibility. Here, we demonstrate an extremely stretchable and self-healing conductor via both hard and soft tactics that combine conductive nanowire assemblies with double dynamic network based on π-π attractions and Ag-S coordination bonds. The obtained conductor outperforms the reported stretchable conductors by delivering an elongation of 3250%, resistance change of 223% at 2000% strain, high durability, and multiresponsive self-healability. Especially, this conductor accommodates large strain of 1500% at extremely knotted and twisted deformations. By sandwiching hydrogel conductors with a newly developed dielectric, ultrahigh stretchability and omni-healability are simultaneously achieved for the first time for a capacitive strain sensor inspired by metal-thiolate coordination chemistry, showing great potentials in wearable electronics and soft robotics.

3D printing-directed flexible strain sensors of accordion-like architecture to achieve ultrastretchability with the assist of ultrasonic cavitation treatment

A new class of accordion-like cellular architecture with sinusoidal struts is designed to enhance the planar stretchability of cellular solids, aiming to fabricate flexible strain sensors with ultrastretchability. The combination manufacturing process of fused deposition modeling (FDM) 3D printing technique and ultrasonic cavitation-enabled treatment was introduced into the fabrication of flexible strain sensors made of thermoplastic polyurethane (TPU) substrate and carbon nanotubes (CNTs). A negative Poisson’s ratio (NPR) architecture made of TPU was firstly 3D-printed by FDM. The ultrasonic cavitation treatment was then conducted on the soft auxetic structure immersing in CNTs liquid, aiming to embed the CNTs into the surface layer of the flexible TPU substrate with NPR configurations. Instead of 3D printing the TPU matrix composite after hybridization inside the matrix material, the hybrid manufacturing procedure can ensure that the intrinsic excellent mechanical properties of TPU are not embrittled. Besides, the sinusoidal struts in accordion-like cellular architectures offer a design route to extend the material property chart to achieve ultrahigh stretchability in lightweight 3D printable flexible polymers for the applications that require combined stretchability, lightweight, and energy absorption such as soft robotics, stretchable electronics, and wearable protection shields.

Tensible and flexible high-sensitive spandex fiber strain sensor enhanced by carbon nanotubes/Ag nanoparticles

Although the wearable strain sensors have received extensive research interest in recent years, it remains a huge challenge conforming the requirements in both of ultrahigh stretchability and high strain coefficient (gauge factor). Herein, a stretchable and flexible spandex fiber strain sensor coupled with carbon nanotubes (CNTs)/Ag nanoparticles (Ag NPs) that assembled through an efficient and large-scale layer-by layer self-assembly is presented. To ensure CNTs and Ag NPs can attach well to the spandex fiber without falling off, achieving high sensitivity under large tensile, sodium dodecyl benzene sulfonate, polyvinyl alcohol, and polystyrene sulfonic acid are introduced to improve the adhesion via the molecular entanglement and other interactions between them. Consequently, the strain sensor exhibits remarkable performance, such as an ultrahigh gauge factor of 58.5 in the low-strain range from 0% to 20%, a wide strain range (0%–200%), a fast response time of 42 ms and good working stability (>5000 stretching-releasing cycles). Subsequently, detailed mechanism of the sensor and its use in full range of human motion monitoring are further studied. It is worth noting that with the distinctive mechanism and structure, the special spandex fiber sensor is able to monitor minimum strain as low as 0.053%, showing tremendous prospect for the field of smart fabrics and wearable health care devices.

Fully solution processed liquid metal features as highly conductive and ultrastretchable conductors

Liquid metal represents a highly conductive and inherently deformable conductor for the development of stretchable electronics. The widespread implementations of liquid metal towards functional sensors and circuits are currently hindered by the lack of a facile and scalable patterning approach. In this study, we report a fully solution-based process to generate patterned features of the liquid metal conductor. The entire process is carried out under ambient conditions and is generally compatible with various elastomeric substrates. The as-prepared liquid metal feature exhibits high resolution (100 μm), excellent electrical conductivity (4.15 × 104S cm−1), ultrahigh stretchability (1000% tensile strain), and mechanical durability. The practical suitability is demonstrated by the heterogeneous integration of light-emitting diode (LED) chips with liquid metal interconnects for a stretchable and wearable LED array. The solution-based technique reported here is the enabler for the facile patterning of liquid metal features at low cost, which may find a broad range of applications in emerging fields of epidermal sensors, wearable heaters, advanced prosthetics, and soft robotics.

Crumpled MXene Electrodes for Ultrastretchable and High-Area-Capacitance Supercapacitors.

In spite of the excellent electrical and electrochemical properties, two-dimensional transition metal carbide (MXene) is often limited by the high stiffness for the direct implementation in next-generation stretchable and wearable energy storage devices. The improved deformability has been achieved in ultrathin composite electrodes utilizing additives that substantially reduce the specific capacitance. Here, we demonstrate an ultrastretchable and high-performing supercapacitor based on MXene electrodes with crumpled textures. After screening on the thickness, the crumpled MXene film of ∼3 μm in thickness is identified as the optimal choice to mitigate the crack formations under large and repetitive mechanical strains. The as-prepared symmetric supercapacitor, therefore, demonstrates a high specific capacitance of ∼470 mF cm-2, ultrahigh stretchability up to 800% area strain, and >90% retention of the initial capacitance after 1000 stretch-relaxation cycles. The developments offer an attractive avenue to design stretchable electrodes based on various two-dimensional nanomaterials and their composites.

A Surface-Confined Gradient Conductive Network Strategy for Transparent Strain Sensors toward Full-Range Monitoring

The development of transparent and flexible sensors suitable for the full-range monitoring of human activities is highly desirable, yet presents a daunting challenge due to the need for a combination of properties such as high stretchability, high sensitivity, and good linearity. Gradient structures are commonly found in many biological systems and exhibit excellent mechanical properties. Here, we report a novel surface-confined gradient conductive network (SGN) strategy to construct conductive polymer hydrogel-based stain sensors (CHSS). This CHSS showed an ultrahigh stretchability of 4000% strain, transparency above 90% at a wavelength of 600 nm, as well as skin-like Young's modulus of 40 kPa. Impressively, the sensitivity was improved to 3.0 and outstanding linear sensing performance was achieved simultaneously in the ultrawide range of 0% to 4000% strain with a high R-square value of 0.994. With the help of SGN strategy, this CHSS was able to monitor both large-scale and small-scale human motions and activities. This SGN strategy can open a new avenue for the development of novel flexible strain sensors with excellent mechanical, transparent, and sensing performance for full-range monitoring of human activities.

Recent progress in multifunctional hydrogel-based supercapacitors

Hydrogel-based supercapacitors with excellent mechanical properties can overcome several unsettled challenges that current wearable power supply devices confront as a result of the distinctive dense intriguing nanostructures of hydrogels, which serve as an ideal candidate with durability and reliability for supplying power to truly wearable electronics. More importantly, benefiting from the broadly tunable physicochemical characterizations, the hydrogel electrolyte components with high ionic conductivity could possess several additional functions including ultrahigh stretchability, strong self-healability, and low-temperature tolerance, which qualifies these multifunctional supercapacitors for application in a variety of extreme working environments, especially in significant deformations caused by human-motion, conditions of applied destructive external force, and extremely frigid circumstances. Herein, the state-of-the-art advanced multifunctional supercapacitors are discussed, focusing on the introductions and explanations of particularly functionalized hydrogel electrolytes, ultra-flexible textile-based electrodes or hydrogel-based electrodes, and the combination strategies of electrolytes and electrodes.

Abrasion and Fracture Self‐Healable Triboelectric Nanogenerator with Ultrahigh Stretchability and Long‐Term Durability

AbstractSince triboelectrification results from materials in physical contact, the durability of triboelectric nanogenerators (TENGs) must be improved. However, it still poses challenges to continuously repair surface abrasion even when self‐healable materials are introduced. In this study, an ultrastretchable TENG (US‐TENG) that simultaneously heal the fracture and abrasion at room temperature is fabricated. By incorporating hydrogen bonds and dynamic metal‐ligand coordination into polydimethylsiloxane chains, the synthesized elastomers possess ultrahigh stretchability (10 000%) and remarkable self‐healing property (100% efficiency) at room temperature. Working in contact‐separation mode, the electrical outputs with 2 × 2 cm2 area can reach 140 V, 40 nC, 1.5 µA, respectively. Besides, the US‐TENG shows a stretchability of 1800% with high electrical outputs. Even if it is stretched to break or scratched to wear out, it can recover its electrical outputs in 20 min and 2 h at room temperature, respectively. Finally, the application of the US‐TENG as flexible power sources and self‐powered pressure sensors is demonstrated. This designed strategy with ultrahigh stretchability and excellent self‐healing properties can meet the wide applications of flexible and wearable electronics for long‐term use.

A flexible composite phase change material with ultrahigh stretchability for thermal management in wearable electronics

A flexible composite phase change material with ultrahigh stretchability for thermal management in wearable electronics

Highly stretchable self-sensing actuator based on conductive photothermally-responsive hydrogel

Soft robots built with active soft materials have been increasingly attractive. Despite tremendous efforts in soft sensors and actuators, it remains extremely challenging to construct intelligent soft materials that simultaneously actuate and sense their own motions, resembling living organisms’ neuromuscular behaviors. This work presents a soft robotic strategy that couples actuation and strain-sensing into a single homogeneous material, composed of an interpenetrating double-network of a nanostructured thermo-responsive hydrogel poly(N-isopropylacrylamide) (PNIPAAm) and a light-absorbing, electrically conductive polymer polypyrrole (PPy). This design grants the material both photo/thermal-responsiveness and piezoresistive-responsiveness, enabling remotely-triggered actuation and local strain-sensing. This self-sensing actuating soft material demonstrated ultra-high stretchability (210%) and large volume shrinkage (70%) rapidly upon irradiation or heating (13%/°C, 6-time faster than conventional PNIPAAm). The significant deswelling of the hydrogel network induces densification of percolation in the PPy network, leading to a drastic conductivity change upon locomotion with a gauge factor of 1.0. The material demonstrated a variety of precise and remotely-driven photo-responsive locomotion such as signal-tracking, bending, weightlifting, object grasping and transporting, while simultaneously monitoring these motions itself via real-time resistance change. The multifunctional sensory actuatable materials may lead to the next-generation soft robots of higher levels of autonomy and complexity with self-diagnostic feedback control.

DNA-inspired, highly packed supercoil battery for ultra-high stretchability and capacity

Stretchable yarn/fiber energy storage devices with a high energy density are highly beneficial for use in wearable applications. Although some studies on stretchable fiber batteries have been conducted, the development of fiber batteries that combine outstanding stretchability with a high energy storage capacity has been restricted by the tradeoff between stretchability and the active material content. In the present study, a DNA-inspired supercoil battery is proposed and fabricated via the secondary twisting (i.e. supercoiling) of a fiber electrode consisting of an elastomeric core fiber and a conductive sheath containing nano-sized active materials. The structural changes induced by supercoiling facilitate the effective packing of the electrode into coil and supercoil loops that represent only 29% of the initial length. The resulting highly packed supercoil battery exhibits an outstanding stretchability (800%) and linear capacity (0.029 mAh/cm) compared to non-coil, coil, and previously reported stretchable yarn/fiber-based batteries.

Melanin-Inspired Conductive Hydrogel Sensors with Ultrahigh Stretchable, Self-Healing, and Photothermal Capacities

Flexible hydrogel strain sensors have received increased attention due to their potential applications in human–machine interfaces, soft robotics, and electronic skin. However, it still remains a challenge to prepare a multifunctional conductive hydrogel integrating high stretchability, strength, and self-healing property. Here, melanin-inspired polydopamine–Fe (PDA–Fe) nanoparticles were successfully incorporated into the hydrogel, with hydrophobic association serving as dynamic physical cross-linking. The hydrogel exhibited ultrahigh stretchability (1900%) and toughness (1.24 MPa), rapid recovery, self-healing property, photothermal effect, and thermal stability. PDA–Fe enhanced the electrical conductivity of the gel, making it potentially useful for application in flexible strain sensors. The PDA–Fe hydrogel strain sensor showed a broad sensing range (350%), high sensitivity (GF = 3.7), a fast response time, and reliable durability, which could enable detection of large (bending of the finger, elbow, and knee) and subtle (pronunciation) human movements. Therefore, the strategy will shed light on preparation of a multifunctional conductive hydrogel for broad applications.

Thermally Drawn Stretchable Electrical and Optical Fiber Sensors for Multimodal Extreme Deformation Sensing

AbstractHighly stretchable fiber sensors have attracted significant interest recently due to their applications in wearable electronics, human–machine interfaces, and biomedical implantable devices. Here, a scalable approach for fabricating stretchable multifunctional electrical and optical fiber sensors using a thermal drawing process is reported. The fiber sensors can sustain at least 580% strain and up to 750% strain with a helix structure. The electrical fiber sensor simultaneously exhibits ultrahigh stretchability (400%), high gauge factors (≈1960), and excellent durability during 1000 stretching and bending cycles. It is also shown that the stretchable step‐index optical fibers facilitate detection of bending and stretching deformation through changes in the light transmission. By combining both electrical and optical detection schemes, multifunctional fibers can be used for quantifying and distinguishing multimodal deformations such as bending and stretching. The fibers’ utility and functionality in sensing and control applications are demonstrated in a smart glove for controlling a virtual hand model, a wrist brace for wrist motion tracking, fiber meshes for strain mapping, and real‐time monitoring of multiaxial expansion and shrinkage of porcine bladders. These results demonstrate that the fiber sensors can be promising candidates for smart textiles, robotics, prosthetics, and biomedical implantable devices.

Electrically conductive NBR/CB flexible composite film for ultrastretchable strain sensors: fabrication and modeling

In this study, a flexible strain sensor with ultrahigh stretchability, outstanding sensitivity, and excellent repeatability based on conductive carbon black (CB) and nitrile butadiene rubber (NBR) was presented. Two organic solvents with three different coating time during the dissolve-coating process were investigated. The morphology and the degradation temperature of all the NBR/CB composite films were studied, and the tensile strength, elongation at break, as well as the young’s modulus and the dissipated energy for all the specimen were revealed. The strain-sensing tests indicated that the NBR/CB film possess a good sensitivity (max. gauge factor = 24.9) and an ultrahigh sensing range (max. 681% strain). The mathematic model from Simmons for predicting the sensing behavior based on the tunneling theory was further simplified, and a novel equation was proposed for predicting the relative change of resistance as a function of time during the cyclic test, which showed great agreement with the cyclic strain-sensing data in experiments. Overall, this study introduces a simple low-cost fabrication method for preparing the polymeric flexible strain sensors with great strain-sensing performance. The development of the two mathematical modellings makes several noteworthy contributions to the existing knowledge of strain-sensing behavior.

A Printable and Conductive Yield-Stress Fluid as an Ultrastretchable Transparent Conductor.

In contrast to ionically conductive liquids and gels, a new type of yield-stress fluid featuring reversible transitions between solid and liquid states is introduced in this study as a printable, ultrastretchable, and transparent conductor. The fluid is formulated by dispersing silica nanoparticles into the concentrated aqueous electrolyte. The as-printed features show solid-state appearances to allow facile encapsulation with elastomers. The transition into liquid-like behavior upon tensile deformations is the enabler for ultrahigh stretchability up to the fracture strain of the elastomer. Successful integrations of yield-stress fluid electrodes in highly stretchable strain sensors and light-emitting devices illustrate the practical suitability. The yield-stress fluid represents an attractive building block for stretchable electronic devices and systems in terms of giant deformability, high ionic conductivity, excellent optical transmittance, and compatibility with various elastomers.

Buckling of ultrastretchable kirigami metastructures for mechanical programmability and energy harvesting

Metastructures based on kirigami (the Japanese art of paper folding and cutting) have great potential for applications in stretchable electronics, bioprobes, and controllable optical and thermal devices. However, a theoretical framework for understanding the physics of their buckling behavior and to facilitate the search for ultrahigh stretchability has yet to be developed. Here, we present such a framework based on an energy approach. Closed-form analytical solutions and scaling laws are obtained for some key mechanical quantities, including flexibility, normalized flexibility, critical buckling strain, maximum tensile strain, elastic stretchability, normalized stretchability, and out-of-plane stiffness. Both experiments and numerical calculations are performed to validate the accuracy and scalability of the theoretical model. Systematic analyses of key mechanical quantities reveal how it is possible to bridge the gap between kirigami design by experience and by mechanically guided design, as well as how various dimensionless geometric parameters can be used to regulate buckling responses. Illustrative applications show that the ability to predict and tune the mechanical programmability of the proposed theoretical framework enables stable electromechanical conversion and programmable electromechanical kirigami with domino-like buckling. This work provides a foundation for further research and can also aid in the design of kirigami for use in programmable metastructures and metamaterials and energy harvesting.