Stretchable Elastomer Research Articles

Flexible Magnetic Skin Sensor Array for Torsion Perception.

Prosthetic hands help upper limb amputees and people who were born without hands. Currently, these prostheses are rather rudimentary and do not provide adequate sensing capabilities compared to a human hand. People use their natural hands to perceive complex tactile phenomena such as shear and torsion using thousands of mechanoreceptors in their fingertips. The capability to detect torsional loads at the fingertips is a notable gap in prosthetic hand sensation. Flexible tactile sensors are a promising new technology that would be ideal for prosthetic hands since they allow for stretching and movement like human skin without damage to the sensor. Therefore, the purpose of this study is to determine whether a flexible magnetic sensor array combined with an artificial neural network (ANN) can detect and classify torsion. The flexible magnetic sensor is designed as a 3×3 array of magnets embedded in a stretchable elastomer which are situated atop a corresponding array of Hall effect sensors. Torques applied to the soft magnetic skin caused displacement of the magnetic fields that were perceived by the nine Hall effect sensors. In this study, ten different values of torque were applied to the flexible magnetic sensor array using a robotic arm to ensure consistency. Data were used to train an ANN to classify the applied torques. The ANN was trained ten times and could predict the applied torque with an average training classification accuracy of 97.48% ± 0.33%. Given the results of this study, this novel sensor design could enable more refined sensations of touch for people who use prosthetic hands.

Elastic integrated electronics based on a stretchable n-type elastomer–semiconductor–elastomer stack

Elastic integrated electronics based on a stretchable n-type elastomer–semiconductor–elastomer stack

Wearable cardiorespiratory monitoring with stretchable elastomer optical fiber.

This work presents a stretchable elastomer optical fiber sensor incorporated into a belt for respiratory rate (RR) and heart rate (HR) monitoring. Different materials and shapes of prototypes designed were tested in terms of performance and the best choice was identified. The optimal sensor was tested by 10 volunteers to evaluate the performance. The proposed elastomer optical fiber sensor can achieve simultaneous measurement of RR and HR in different body positions, and also ballistocardiography (BCG) signal measurement in the lying position. The sensor has good accuracy and stability, with maximum errors of 1 bpm and 3 bpm for RR and HR, respectively, and average weighted mean absolute percentage error (MAPE) of 5.25% and root mean square error (RMSE) of 1.28 bpm. Moreover, the results of the Bland-Altman method showed good agreement of the sensor with manual counting of RR and with electrocardiogram (ECG) measurements of HR.

Module-Assembled Elastomer Showing Large Strain Stiffening Capability and High Stretchability.

Elastomers are indispensable materials due to their flexible, stretchable, and elastic nature. However, the polymer network structure constituting an elastomer is generally inhomogeneous, limiting the performance of the material. Here, a highly stretchable elastomer with unprecedented strain-stiffening capability is developed based on a highly homogeneous network structure enabled by a module assembly strategy. The elastomer is synthesized by efficient end-linking of a star-shaped aliphatic polyester precursor with a narrow molecular-weight distribution. The resulting product shows high strength (≈26MPa) and remarkable stretchability (stretch ratio at break ≈1900%), as well as good fatigue resistance and notch insensitivity. Moreover, it shows extraordinary strain-stiffening capability (>2000-fold increase in the apparent stiffness) that exceeds the performance of any existing soft material. These unique properties are due to strain-induced ordering of the polymer chains in a uniformly stretched network, as revealed by in situ X-ray scattering analyses. The utility of this great strain-stiffening capability is demonstrated by realizing a simple variable stiffness actuator for soft robotics.

Stretchable and Self-Healing Elastomers with Aggregation-Induced Emission Based on Hydrogen Bonding Cross-Linked Networks

With the fast advancement of flexible and stretchable electronics, the development of organic semiconductors with intrinsic stretchability and self-healing properties is desirable and significant yet remains a substantial challenge. Herein, we propose a simple and effective synthetic strategy to construct intrinsically stretchable and self-healing organic light-emitting elastomers via incorporating aggregation-induced emission (AIE) luminogens into hydrogen-bonding cross-linked networks. Owing to the hydrogen-bonding interactions within the networks, the resulting light-emitting elastomers achieved a stress–strain of about 700% and a self-healing efficiency of over 70% under ambient conditions. More importantly, the resulting elastomers have been examined to be highly adhesive and compatible with the silver nanowire networks, exhibiting smart luminescence behaviors and excellent conductivity. This study sheds light on the design and development of stretchable and self-healing organic light-emitting elastomers with promising optoelectronic characteristics, which would inspire a wide range of fancy applications for flexible and stretchable electronics.

Stretchable Ultraviolet Curable Ionic Conductive Elastomers for Digital Light Processing Based 3D Printing

AbstractHighly stretchable and transparent ionic conducting materials enable electronic devices to have a distinguishable working mechanism and performances from the conventional electronics. Compared with hydrogels which contain large amounts of water to dissolve ions, ionic conductive elastomers (ICEs) are solvent‐free, and intrinsically immune from dehydration. However, the current manufacturing technologies constrain the ICE‐based ionotronics to simple and bulk geometries with single material, which greatly limits functionality and performance of ICEs‐based ionotronics. Herein, a stretchable ultraviolet curable ICEs (SUV‐ICEs) for 3D printing approach to fabricate complex 3D flexible electronics is reported. The developed SUV‐ICEs are highly stretchable and thermally stable. Using a self‐built DLP‐based 3D printer, that is able to print 3D flexible electronics consisting of SUV‐ICE covalently bonded with other dielectric elastomer, such as SUV‐ICE strain sensors, and a flexible mini‐keyboard.

Ultrasmall barium titanate nanoparticles modulated stretchable dielectric elastomer sensors with large deformability and high sensitivity

AbstractLarge deformability and high sensitivity is difficult to be realized simultaneously in flexible sensors. Herein, taking advantage of the high permittivity and highly active surfaces of the ultrasmall barium titanate nanoparticles (BT NPs) and the high stretchability of the p(BA‐GMA) elastomer matrix, we propose a high‐performance soft stretchable sensor. The addition of the ultrasmall BT NPs can not only increase the permittivity and capacitance of polyacrylate‐matrix composite dielectric material to obtain a high sensitivity, but also basically maintains the excellent mechanical properties of the polymer matrix. The dielectric constants of the composite films increase from 5.68 to 13.13 at 10 kHz with the increase of BT NPs content from 0 to 15 vol.%, which results in a high capacitance of 236.16 pF for 15 vol.% BT/p(BA‐GMA) sensor. Combining the high permittivity and the large deformability (a maximal deformation of 87.2%), the 15 vol.% BT/p(BA‐GMA) sensor has high sensitivity and shows high linearity and stable output even if under dynamic measurement. The dual‐mode sensor that utilizes the orthogonality of capacitance‐resistance is designed, which shows excellent performance in monitoring human body movements and noncontact measurement. The results present that the BT/p(BA‐GMA)‐based sensor has high stability and reliability not exceed 65°C, which can meet the application requirements in dynamic monitoring.image

Self-Healable, Stretchable, and Super-Soft Bottlebrush Polyester Elastomers for Highly Sensitive Flexible Sensors

While much effort has been devoted to the creation of super-soft elastomers, there is a dearth of research aimed at self-healable and super-soft renewable elastomers. Herein, self-healing and super-soft renewable elastomers are developed based on bottlebrush architecture design and dynamic associative transesterification. The bottlebrush polymer precursors are generated by ring-opening metathesis polymerization (ROMP) of the norbornene-terminated macromonomer poly(ε-caprolactone-co-l-lactide) (PCLLA) (NB-PCLLA). After introducing alkyne groups onto the end of PCLLA side chains, the bottlebrush polymers are cross-linked to achieve elastomer networks via a UV-induced “click” thiol-yne reaction between the terminal alkyne groups and the dithiothreitol (DTT) additive. The resulting materials exhibit a shear modulus as low as 17 kPa, high stretchability with breaking strains as large as 315%, and a self-healing efficiency of 95.6% at an elevated temperature of 80 °C. Taking the super-softness advantage, flexible sensor devices are assembled based on the prepared elastomer, which are highly sensitive to deformation with a gauge factor of 0.347. This work demonstrates that high-performance biodegradable materials with unique properties that conventional linear polymers impossibly possess can be easily constructed by molecular architecture design using bottlebrush polymers as building blocks.

Vat photopolymerization 3D printing of transparent, mechanically robust, and self-healing polyurethane elastomers for tailored wearable sensors

As flexible sensors are soft and prone to damage in practical applications, stretchable conductors with self-healing capacity can extend their service life. However, existing vat photopolymerization (VP) 3D-printed self-healing elastomers generally have a tensile strength of less than 1 MPa, which limits the load-bearing capacity and application range of the flexible sensors. Herein, a photocurable oligomer comprising dynamic hindered urea bonds (HUBs) was synthesized to prepare self-healing elastomers. By using a surface-functionalized spherical nano-silica (Nano-SiO2) for reinforcement modification, the tensile strength and elongation at break of the 3D-printed elastomer were enhanced by 73.7% and 111.3%, respectively, without compromising the transparency and photopolymerization rate. The fractured elastomer exhibited a high tensile strength of 5.44 MPa and a stretchability of 372% after self-healing. Subsequently, a stretchable ionic conductive elastomer (SICE) with high transparency and self-healing properties was developed by incorporating ionic liquids into the 3D-printed elastomer. The high resolution of VP-based 3D printing allowed the construction of pyramid-Kelvin lattice microstructures in the pressure sensor, which enhanced the pressure sensitivity of the sensor several times compared to the pyramid structure. In addition, combining Voronoi tessellation and kirigami art to fabricate tailored wearable sensors with smart structures, broadens the application range and improves the wearing comfort of the sensors.

Frontal Polymerization and Three-Dimensional Printing of Thermoset Polymers with Tunable Thermomechanical Properties

On-demand curing of thermoset polymers with tunable mechanical properties is of great interest for a variety of applications. In this work, we demonstrate rapid, solvent-free synthesis of a cross-linked copolymer using thermal frontal polymerization of dicyclopentadiene (DCPD) and linseed oil at ambient temperature. A series of thermoset copolymers with various concentrations of comonomers are readily prepared to produce polymers with a wide range of ultimate elongation (11–360%), elastic modulus (1.1 GPa to 5.6 MPa), and glass transition temperature (120–31 °C). The addition of linseed oil comonomers to DCPD resin to increase the molecular weight between crosslinks reduces the available energy density for frontal polymerization and improves the elastomeric properties of the produced polymers. Increasing the concentration of linseed oil comonomers results in highly stretchable and flexible elastomers with self-recovery capability after large mechanical deformations. The DCPD resin and the resin containing 50 wt % of linseed oil comonomers are used to demonstrate three-dimensional (3D) printing of stiff and flexible cellular structures, respectively. This study is the first demonstration of low-cost, facile, and energy-efficient 3D printing of stretchable elastomers via frontal polymerization.

Hard magnetics in ultra-soft magnetorheological elastomers enhance fracture toughness and delay crack propagation

Pre-existing flaws in highly stretchable elastomers trigger fracture under large deformations. For multifunctional materials, fracture mechanics may be influenced by additional physical phenomena. This work studies the implications of hard magnetics on the fracture behaviour of ultra-soft magnetorheological elastomers (MREs). We experimentally demonstrate that MREs with remanent magnetisation have up to a 50% higher fracture toughness than non pre-magnetised samples. Moreover, we report crack closure due to the magnetic field as a mechanism that delays the opening of cracks in pre-magnetised MREs. To overcome experimental limitations and provide further understanding, a phase-field model for the fracture of MREs is conceptualised. The numerical model incorporates magneto-mechanical coupling to demonstrate that the stress concentration at the crack tip is smaller when the MRE is pre-magnetised. Overall, this work unveils intriguing applications for functional actuators, with better fracture behaviour and potential better performance under cyclic loading.

Flexible and Stretchable Elastomer Composites Based on Lightweight Liquid Metal Foam Spheres with Pod-like Contacts

Liquid metal (LM) is increasingly employed as a conductive filler in soft and flexible elastomer composites owing to its favorable conductivity and liquid fluidity. However, the high density of LM inevitably increases the weight of composites, which brings limitations in large-area and weight-sensitive applications. This work reports a flexible and stretchable elastomer composite composed of pod-like contacting lightweight LM foam spheres and polydimethylsiloxane matrix (LMS/PDMS). The lightweight LMS reduces the amount of LM used in the preparation process while imparting good electrical conductivity and deformability to the composite. The different contact modes of LMS endow the final composites with diverse strain sensitivity. The mechanism of interfacial contact conduction between the LMS with different melting points has been systematically studied, and the result shows that the liquid-solid contact mode of LMS further improves the strain sensitivity of the composite. Moreover, the composite also has satisfactory electrothermal properties and the temperature can quickly reach 70 °C within 30 s, showing good applicability in electric heating. Finally, the composites containing LMS with different contact modes can be developed as multifunctional sensors to detect human activities, temperature variation, and even underwater vibration, demonstrating the great potential in next-generation sensors and electronics.

Stretchable elastomer based on viscous sensing fluid with a positive piezoconductive effect

Conductive elastomers based on a fully filled structure have been widely used in soft electronics, such as wearable strain sensor for human motions detection. These elastomers are typically conductors because of the conductive fillers within elastic substrate, which can directly reflect the deformation via a contact-separation effect between the fillers. Nevertheless, most reported elastomers so far display a negative piezoconductivity, i.e., the conductivity decreases under a tensile strain, which is undesirable since catching a weak conductivity under large deformation is inconvenient. Here, we report a sensing fluid-filled elastomer comprising cellulose nanofibers (CNF–C) and sodium chloride crystals (NaCl), which exhibits an unconventional positive piezoconductive effect (gauge factor is −0.504). Our elastomer uniquely possesses the worst conductivity (σ: 0.28 S/m) in a relaxed state, and increases rapidly when stretches due to the reversion of the CNF–C flakes, namely ‘shutter effect’. Such a novel strain response mechanism has been achieved in a fully filled conductive elastomer, making it prospective in advanced applications such as strain sensors, stretchable conductors, and smart robotics.

Frontispiece: Intrinsically Stretchable Electroluminescent Elastomers with Self‐Confinement Effect for Highly Efficient Non‐Blended Stretchable OLEDs

Organic Electronics. Intrinsically stretchable electroluminescent elastomers for highly efficient non-blended stretchable organic light-emitting diodes are reported by Wen-Yong Lai et al. in their Research Article (e202213749).

Study on fabricating transparent, stretchable, and self-healing ionic conductive elastomers from biomass molecules through solvent-free synthesis

We propose a simple and effective strategy for increasing the conductivity of polyTA by copolymerization reactions, and a series of autonomously self-healing, transparent, liquid-free ionic conductors was prepared.

Highly stretchable, tough, healable and mechanoresponsive polyurethane elastomers for flexible capacitor applications

Mechanochromic, tough and healable polyurethane films are synthesized and their properties are thoroughly characterized. The optimum films are utilized to explore the potential applications of shape memory and flexible capacitor devices.

Stretchable superhydrophobic elastomers with on-demand tunable wettability for droplet manipulation and multi-stage reaction

We report a facile approach to fabricating stretchable superhydrophobic surfaces with different microstructures (arc-shaped or V-shaped air pockets) for multi-stage liquid droplet micro-reactors.

Design, Fabrication and Characterisation of Multi-Parameter Optical Sensors Dedicated to E-Skin Applications

For many years there has been a strong research interest in soft electronics for artificial skin applications. However, one challenge with stretchable devices is the limited availability of high performance, stretchable, electrical conductors and semiconductors that remain stable under strain. Examples of such electronic skin require excessive amounts of wires to address each sensing element-compression force and strain-in a conventional matrix structure. Here, we present a new process for fabricating artificial skin consisting of an optical waveguide architecture, enabling wide ranging sensitivity to external mechanical compression and strain. The manufacturing process allows design of a fully stretchable polydimethylsiloxane elastomer waveguide with embedded gratings, replicated from low cost DVD-Rs. This optical artificial skin allows the detection of compression forces from 0 to 3.8 N with controllable sensitivity. It also permits monitoring of elongation deformations up to 135%. This type of stretchable optical sensor is highly robust, transparent, and presents a large sensing area while limiting the amount of wires connecting to the sensor. Thus, this optical artificial skin presents far superior mechanical properties compared to current electronic skin.

Polymer-based wearable nano-composite sensors: a review

The demand for stretchable, skin-mountable, and wearable strain sensors is burgeoning because they are required for several budding applications in various emerging fields, such as health monitoring, human-machine interfaces, wearable electronics, soft robotics, human motion detection, virtual reality (VR), and so forth. Recently, strain sensors based on composite materials comprising stretchable elastomer and conductive nano-filler have gathered attention on account of their ample flexibility, record stretchability, excellent durability, customizable characteristics for strain-sensing, and simpler fabrication techniques. This quality write-up discusses current developments in the field of flexible strain sensors (FSSs). We effectively summarized how advanced mechanisms, such as crack propagation, disconnection, and tunneling effect, which are quite different from traditional strain sensing techniques, are used to develop FSS. Various factors and their effects that need to be kept in mind while developing high-performance, high-quality FSSs are also debated. The performance of recently reported FSSs is comprehensively discussed, and the huge potential that FSSs hold has been comprehensively reviewed in this survey.

Intrinsically Stretchable Electroluminescent Elastomers with Self-Confinement Effect for Highly Efficient Non-Blended Stretchable OLEDs.

Ultra-flexible stretchable organic light-emitting diodes (OLEDs) are emerging as a basic component of flexible electronics and human-machine interfaces. However, the brightness and efficiency of stretchable OLEDs remain still far inferior to their rigid counterparts, owing to the scarcity of satisfactory stretchable electroluminescent materials. Herein, we explore a general concept based on the self-confinement effect to dramatically improve the stretchability of elastomers, without affecting electroluminescent properties. The balanced rigid/flexible chain dynamics under self-confinement significantly reduces the modulus of the elastomers, resulting in the maximum strain reaching 806 %. Ultra-flexible stretchable OLEDs have been constructed based on the resulting ISEEs, achieving unprecedented high-performance non-blended stretchable OLEDs. The results suggest an effective molecular design strategy for highly deformable stretchable displays and flexible electronics.