Low Stretchability Research Articles

Influence of Rheological and Morphological Characteristics of Polyhydroxybutyrate on Its Meltblown Process Behavior.

Polyhydroxybutyrate (PHB) is a promising biopolymer. However, processing PHB in pure form in thermoplastic processes is limited due to its rapid degradation, very low initial crystallization rate, strong post-crystallization, and its low final stretchability. In this article, we screened commercial PHBs for morphological characteristics, rheological properties, and "performance" in the meltblown process in order to reveal process-relevant properties and overcome the shortcoming of PHB in thermoplastic processes for fiber formation. An evaluation of degradation (extruded (meltblown) material vs. granules) was performed via rheological and SEC analysis. The study revealed large differences in the minimum melt temperature (175 up to 200 °C) and the grade-dependent limitation of accessible throughput on a 500 mm plant. The average fiber diameter could be lowered from around 10 μm to 2.4 μm in median, which are the finest reported values in the literature so far. It was found that the determination of the necessary process temperature can be predicted well from the complex shear viscosity. Different to expectations, it became apparent that a broader initial molar mass distribution (>8) is suitable to overcome the state-of-the art limitations of PHAs in order to stabilize fiber formation, increase the productivity, and obtain better resistance towards thermal degradation in process. Accordingly, longer polymer chain fractions could be more affected by degradation than medium and short polymer chains in the distribution. Further, a low initial narrow distributed molar mass resulted in too brittle fabrics.

Design of a Highly Sensitive Ionic Conductive Hydrogel Sensor based on the Kirigami Structure

Conductive hydrogels are polymers that respond to mechanical stimuli and have been widely used in wearable sensors and soft machines. Myriads applications posed high-performance requirements for hydrogels: compliance, stretchability, and high sensitivity. However, sensors based on flexible polymers often struggle to achieve a highly sensitive response. We propose a highly sensitive hydrogel strain sensor based on the Kirigami structure. The structure solves the problem of low stretchability caused by the high stiffness of double-network hydrogel. It greatly improves the stretchability of the strain sensor by customizing the cutting of a high-toughness PAAM-PAA double-network hydrogel. We further analyze the deformation and sensing characteristics through simulation and experiments and successfully apply them to monitor human motion.

All-in-one sandwich-like PVA/PEDOT:PSS/WPU electrodes with low impedance and high stretchability for ECG monitoring

Conductive hydrogels have attracted considerable attention due to the promising application of flexible bioelectrodes. However, these hydrogel flexible electrodes are prone to damage or aging and readily influenced by external conditions like temperature, pressure and humidity, making them difficult in correcting accuracy real-time signals. Hence, we proposes to prepare all-in-one sandwich-like PVA/PEDOT:PSS/WPU electrodes with low impedance and high stretchability. Inspired by the layered structure of human skin, We fabricated ECG electrodes with high mechanical properties and low contact impedance by succsessively coating PEDOT:PSS conductive layer and PVA adhesive layer on WPU aerogel, in which the WPU aerogels can provide good elasticity on one hand and effectively promote the integration of PEDOT:PSS on the other hand. Furthermore, the outside layer of PVA gives good biocompatibility and comfort feel to the human skin, leading to a significant reduction of skin-electrode impedance. The PVA/PEDOT:PSS/WPU electrodes change resistance by only 45 % at 30 % strain, its conductivity reaches 7.8S/m, the fracture strain is 300 %. The as-prepared electrodes can accurately monitor subtle physical movements when wearing on the human-body, such as vocal cord vibrations, while continuously, consistently and clearly collecting ECG signals. Therefore, multifunctional integrated hydrogels can show promising applications in ECG biosensors.

A high κ, transparent, and stretchable Cu@TiO2 core@shell nanowire/poly(dimethylsiloxane) composite film for sensitive wearable touch sensor

Touch sensors are crucial for the development of advanced wearable devices and enhancement of the user experience through human-machine interaction. The properties of dielectric films, including their high dielectric constants (high-κ), low loss tangents (tan⁡δ), transparency, and stretchability are key factors that impact the accuracy, reliability, and display visibility of wearable touch sensors. Here, we present an approach for fabricating a Poly(dimethylsiloxane) (PDMS)-based dielectric film that can achieve a dielectric constant 6 times larger than that of pure PDMS film by loading a very low wt% (3%) of copper@titanium dioxide (Cu@TiO2) core-shell nanowires (NWs) to PDMS matrix. TiO2 was deposited on surfaces of Cu NWs by a hydrothermal reaction to form the covering shell, and it is experimentally and theoretically proved that the shell can suppress the scattering from metal surfaces, resulting in improving the transparency, and reducing the dielectric loss of the film (tan⁡δ=0.01–0.06). The nanocomposite film also exhibits outstanding mechanical property (standing up to a tensile strain of 200%). The demonstration of a transparent, highly sensitive and low noise multi-touch sensor using high-κ film suggests that this film has potential application in next-generation of wearable electronic devices for human-machine interaction.

Room-Temperature Self-Healing Soft Composite Network with Unprecedented Crack Propagation Resistance Enabled by Supramolecular Assembled Lamellar Structure.

Soft self-healing materials are compelling candidates for stretchable devices because of their excellent compliance, extensibility, and self-restorability. However, most existing soft self-healing polymers suffer from crack propagation and irreversible fatigue failure due to easy breakage of their dynamic amorphous low-energy polymer networks. Herein, inspired by distinct structure-property relationship of biological tissues, we propose a supramolecular interfacial assembly strategy of preparing soft self-healing composites with unprecedented crack propagation resistance by structurally engineering preferentially-aligned lamellar structures within dynamic and super-stretchable poly(urea-ureathane) matrixes (which can be elongated to 24750× its original length). Such a design affords world-record fracture energy (501.6kJ m-2 ), ultra-high fatigue threshold (4064.1 J m-2 ), and outstanding elastic restorability (dimensional recovery from 13 times elongation), and preserving low modulus (1.2MPa), high stretchability (3200%), high room-temperature self-healing efficiency (97%). Thereby, the resultant composite represents the best of its kind and even surpasses most natural biological tissues. The lamellar MXene (2D transition metal carbide/carbonitride) structure also leads to a relatively high in-plane thermal conductivity, enabling composite as stretchable thermo-conductive skins applied in joints of robotics to enable thermal dissipation. The present work illustrates a viable approach how autonomous self-healing, crack tolerance and fatigue resistance can be merged in future material design. This article is protected by copyright. All rights reserved.

Reconfigurable, Solvent-Processable, NIR-Triggerable Shape Memory Cyanate Esters for Smart Molds.

Shape memory polymer (SMP)-based smart molds, which could provide high-resolution mold shape and morph in response to external stimuli for readily demolding the complex structure, attract extensive attention. However, the suitable SMP for smart molds is usually confined with low stretchability that likely causes damage during demolding. Herein, we present a cyanate ester smart composite (CESC) with a reconfigurable, solvent-processable, and near-infrared (NIR)-triggerable shape memory effect (SME), which enables the 2D sheet with a variety of morphed complex shapes through deformation in a mild situation. Notably, the reconfigurable SME and the recyclability of the shape memory cyanate ester (SMCE) were addressed for the first time, attributed to the dynamic covalent bonds of transesterification and the novel cyanurate exchange. In addition, we found that the mechanism of solvent-processable SME is attributed to the varied cross-linking density and the mobility of the polymer chain. Integrating the multiple responsive SME and reconfigurable SME, the CESC demonstrated versatile applications as a smart mold. The results demonstrate a wide scope of application of the integrated SME and provide a new design strategy for thermoset cyanate materials.

Fabrication of Eco-Friendly Wearable Strain Sensor Arrays via Facile Contact Printing for Healthcare Applications.

Wearable flexible strain sensors with spatial resolution enable the acquisition and analysis of complex actions for noninvasive personalized healthcare applications. To provide secure contact with skin and to avoid environmental pollution after usage, sensors with biocompatibility and biodegradability are highly desirable. Herein, wearable flexible strain sensors composed of crosslinked gold nanoparticle (GNP) thin films as the active conductive layer and transparent biodegradable polyurethane (PU) films as the flexible substrate are developed. The patterned GNP films (micrometer- to millimeter-scale square and rectangle geometry, alphabetic characters, and wave and array patterns) are transferred onto the biodegradable PU film via a facile, clean, rapid and high-precision contact printing method, without the need of a sacrificial polymer carrier or organic solvents. The GNP-PU strain sensor with low Young's modulus (≈17.8MPa) and high stretchability showed good stability and durability (10000 cycles) as well as degradability (42% weight loss after 17 days at 74°C in water). The GNP-PU strain sensor arrays with spatiotemporal strain resolution are applied as wearable eco-friendly electronics for monitoring subtle physiological signals (e.g., mapping of arterial lines and sensing pulse waveforms) and large-strain actions (e.g., finger bending).

A Hybrid Strategy‐Based Ultra‐Narrow Stretchable Microelectrodes with Cell‐Level Resolution

AbstractStretchable ultra‐narrow (e.g., 10 µm in width) microelectrodes are crucial for the electrophysiological monitoring of single cells providing the fundamental understanding to the working mechanism of neuro network or other electrically functional cells. Current fabrication strategies either focus on the preparation of normal stretchable electrodes with hundreds of micrometers or millimeters in width by using inorganic conductive materials or develop conductive organic polymer gel for ultra‐narrow electrodes which suffer from low stretchability and instability for long‐term implantation, therefore, it is still highly desirable to explore bio‐interfacial ultra‐narrow stretchable inorganic electrodes. Herein, a hybrid strategy is reported to prepare ultra‐narrow multi‐channel stretchable microelectrodes without using photolithography or laser‐assisting etching. A 10 µm × 10 µm monitoring window is fabricated with enhanced interfacial impedance by the special rough surface. The stretchability achieves to 120% for this 10 µm‐width stretchable electrode. Supported by these superior properties, it is demonstrated that the stretchable microelectrodes can detect electrophysiological signals of single cells in vitro and collect electrophysiological signals more precisely in vivo. The reported strategy will open up the accessible preparation of the fine‐size stretchable microelectrode. It will significantly improve the resolution of monitoring and stimulation of inorganic stretchable electrodes.

Multifunctional Ultraviolet Laser Induced Graphene for Flexible Artificial Sensory Neuron

AbstractNeuromorphic perceptual system (NPS) is inspired by the interaction between organisms and environment, which has boosted many emerging fields like human–machine interfaces and bionic robots. Laser‐induced graphene (LIG) can be a fast, low cost, and accurate patterning technology toward a flexible and biological plausible NPS. While, to unleash the full potential of LIG, thorny issues like low endurance and limited stretchability should be addressed. Herein, high‐performance ultraviolet LIG (UV‐LIG) based sensors and electrodes are introduced for building flexible artificial sensory neuron (ASN). The UV‐LIG can be fabricated with a fine linewidth of ≈75 µm and a high conductivity of 3900 ± 150 S cm−1, which facilitates the demonstration of LIG toolbox, containing bending sensors, flexible heaters, etc. The transferred LIG electrodes retain good conducting property (only ≈12.7% increment in resistance) and enable large stretchability (up to 150% strain), outperforming most of the LIG‐based strain sensors. As a proof‐of‐concept, an artificial sensory neuron that is able to mimic the strain perception of somatosensory system is realized based on the integration of LIG‐based functional components and an InGaZnO (Indium‐Gallium‐Zinc Oxide, IGZO)‐based synaptic transistor. This work can provide an efficient patterning methodology as well as essential components for neuromorphic perceptual systems.

Stretchable, transparent, self-adhesive, anti-freezing and ionic conductive nanocomposite hydrogels for flexible strain sensors

Conductive hydrogels have attracted considerable attentions due to their great potential in the field of flexible strain sensors. However, the low stretchability, poor adhesiveness as well as the lack of freezing-resistant capacity of conventional conductive hydrogels greatly limited their practical applications. Herein, a stretchable, transparent, self-adhesive and anti-freezing conductive nanocomposite hydrogel (PAHS gel) was fabricated via a one-step in situ free-radical polymerization of sulfobetaine methacrylate (SBMA), acrylic acid (AA) and 2-hydroxyethyl methacrylate (HEMA) in the presence of alumina nanoparticles (Al2O3 NPs) and lithium chloride (LiCl) as the inorganic cross-linkers and conductive substance, respectively. The obtained PAHS gels displayed high transparency (higher than 85 % at 550 nm), excellent stretchability (up to 800 %) and good ionic conductivity (2.25 S/m) and could keep flexible and conductive at the temperature at −18 ℃. Furthermore, the various types of functional groups on the polymer chains endow the PAHS gels with strong self-adhesiveness to different substrates such as glass, rubber, skin, etc. In addition, the PAHS gels also revealed superior strain sensitivity (GF = 2.69) in the strain of 0 ∼ 100 %, which can be assembled into wearable strain sensors to monitor various human activity. Based on these combined merits, it is believed that this newly developed conductive nanocomposite hydrogel would have prospective applications in the field of wearable strain sensors and other flexible electronics.

A Review of some Characteristics of a Composite Hybrid Socket for Prosthetics Derived from Plant Fibers

According to the ICF (International Classification of Functioning, Disability, and Health), a prosthetic is a device used to restore or replace the function of a missing limb as a result of a rare genetic defect, dangerous incident, surgical trauma, or disease that limits the amputee's ability to participate in daily life. Prosthetic limb sockets were formerly made out of a wide variety of materials before the development of thermoplastics and composites. Plant fibers are favored due to their low density. Both their strength and rigidity are remarkably high considering their low weight and inexpensiveness. Plant fiber is one of the most popular ways to give polymer composites more strength because of its low production costs and stretchability. Following a brief introduction to the many types of natural fibers (both plant and animal), this article provides a comprehensive assessment of the literature from the past decade that focuses on the mechanical characteristics of plant materials. Reinforced polymers are made with fibers. This literature study will focus on the mechanical properties and behavior of plant fibers used to create sockets for prosthetic limbs. If an article is written about the overall features of composites made with fibers made from the plant, it is expected to be of great use to the research community in the field of composites.

Highly stretchable and robust textile-based capacitive mechanical sensor for human motion detection

Mechanically stretchable capacitive sensors are extremely promising for applications in continuous health-monitoring. Typical capacitive sensors, made from polymer and thin metal film supporters, lack interfacial adhesion or chemical bonding and have intrinsically low stretchability. Moreover, the performance of most sensors is critically affected by the conformity mismatch at the electrode and dielectric layer interface during high mechanical deformation. Many mechanical sensors have limited practical applications due to significant hysteresis, low sensitivity, and lack of reproducibility. Herein, we develop a textile-based capacitive mechanical sensor by utilizing titanium carbide-based MXene (Ti3C2Tx) to fabricate conductive textiles and a composite film of polystyrene (PS) fibers and ecoflex silicone as a dielectric layer. Ti3C2Tx showed strong bonding with the fabric owing to its rich surface chemistry, hydrophilicity, and large surface area. The MXene-coated fabric and integration of the composite film make the sensor mechanically stretchable, thus improving its sensitivity, response time, and stability. The sensor showed negligible hysteresis (∼0.2) and demonstrated a high sensitivity of 1.11 when stretched to 100 % elongation and 13.02 kPa−1 when squeezed at a pressureof 200 kPa and excellent cyclic stability (1000 cycles). The sensor was further utilized for the detection of human motion when specificbody movements were performed.

Cellulose nanocrystals boosted hydrophobic association in dual network polymer hydrogels as advanced flexible strain sensor for human motion detection

Conductive hydrogels attract the attention of researchers worldwide, especially in the field of flexible sensors like strain and pressure. These flexible materials have potential applications in the field of electronic skin, soft robotics, energy storage, and human motion detection. However, its practical application is limited due to low stretchability, high hysteresis energy, low conductivity, long-range strain sensitivity, and high response time. It's still a challenging job to endow all these properties in a single hydrogel network. In the present work, cellulose nano crystals (CNCs) reinforced hydrophobically associated gels were developed using APS as a source of radical polymerization, acrylamide and lauryl methacrylate were used as a monomer. CNCs reinforced the hydrophobically associated hydrogels through hydrogen bonding to retain the hydrogel's network structure. Hydrogels consist of dual crosslinking, which demonstrate exceptional mechanical performance (fracture stress and strain, toughness, and Young's modulus). The low hysteresis energy (10.9 kJm−3) and high conductivity (22.97 mS/cm) make the hydrogels a strong candidate for strain sensors with high sensitivity (GF = 19.25 at 700% strain) and a fast response time of 200 ms. Cyclic performance was also investigated up to 300 continuous cycles. After 300 cycles, the hydrogels were still stable and no considerable change was observed. These hydrogels are capable of sensing different human motions like wrist, finger bending, and neck (up-down and straight and right/left motion of neck). The hydrogels also demonstrate changes in current in response to swallowing, different speaking words, and writing different alphabets. These results suggest that our prepared materials can sense different small and large human motions, and also could be used in any electronic device where strain sensing is required.

스트레처블 일렉트로닉스를 구현하기 위한 기하학적 엔지니어링

Recently, soft and stretchable electronics integrated with various functional devices are attracting attention as they can be used for stretchable display, stretchable battery, and electronic skin (e-skin). It is essential to impart stretchability to the electrical components (e.g., electrodes and devices). However, conventional materials used in electronics have low stretchability, which hinders the development of stretchable electronics. To solve this problem, various strategies for geometrical engineering that enhance stretchability to rigid materials have been reported. In this paper, geometrical engineering such as serpentine, kirigami, and island structures are discussed, focusing on the progress of recent developments and future prospects.

Intrinsic ionic confinement dynamic engineering of ionomers with low dielectric-k, high healing and stretchability for electronic device reconfiguration

Ionomers are gaining much attention in the fields of soft robots, flexible electronics, and artificial intelligence. However, intrinsic dynamics modulation of ionomer molecules to achieve desirable properties is highly required and challenging due to a lack of understanding of the interactions between polymerics and ionic species. In this study, a novel ionic confinement engineering approach was employed, in which the composition of a copolymer, namely, PTAE-Fe, was adjusted to facilitate dynamic ionic crosslinking, which enables high stretchability (>10000 % elongation). The proportion of ionic functional groups in PTAE-Fe significantly enhanced the self-healing efficiency up to 95 %. The ionic confinement strategy also promotes electron momentum locking, resulting in a low dielectric-k property (Dk < 2.5), while a reduction in the heat release rate of 69.1 % makes the ionomer an excellent flame retardant material. In addition, a eutectic gallium-indium (EGaIn)-infused stretchable device and low Dk flexible ink were designed according to the ionic confinement strategy. The as-fabricated ionomer is expected to benefit a wide range of energy and storage technologies.

Stretchable, Skin-Breathable, and Ultrasensitive Respiration Sensor Using Graphite on Paper With Smart Structures

Healthy sleep is an important indicator of good physical and mental health. The current technology employed for monitoring sleep quality is typically expensive and invasive, which demands innovations in respiration monitoring. In addition, recent effort in developing wearable respiration sensors has faced great challenges in concurrently obtaining low cost, high stretchability, and high sensitivity. Herein, we demonstrate a simple fabrication strategy to construct low-cost, stretchable, skin-breathable and ultrasensitive respiration sensors using pencil drawing, electrode pasting, and laser cutting methods. With smart designs, the sensors exhibited high sensitivity, good stability, decent skin-breathability and capability to detect respiration rate and depth in real-time. The sensors fabricated by the simple low-cost method in this study can be utilized to measure sleep quality, providing warning signals for apnea and sleep disorders. The proposed technology holds promise for developing cost-effective wearable ultrasensitive sensors for sleep quality monitoring.

Polydimethylsiloxane-based stretchable conductive elastomer fiber by using oil bath-spinning for wearable devices

Flexible stretchable conductors are widely used in various wearable devices and flexible electronic components due to their unique engineering structure adaptability. Fiber has anisotropic stretchability due to its linear structure and it is used in the study of stretchable conductors. However, traditional fibers have low axial stretchability and poor adhesion to conductive particles, so they are not suitable for high-strain engineering structures. Here, a simple and fast preparation method with oil bath thermal curing and a blade coating method is proposed to obtain polydimethylsiloxane-based stretchable elastomer fibers. The fibers have excellent flexibility, stretchability (greater than 100%), elasticity (recovery rate greater than 96%), conductivity (176.5 S/m), and rapid electromechanical response. In particular, the fibers have good weaving ability for their stable performance under bending and twisting load, which is important to weave flexible high conductivity fabric. The effect of conductive particle types (carbon black, carboxylated multi-arm carbon nanotubes) on the fiber properties is also studied. The results show that the carbon nanotube/polydimethylsiloxane fiber has better stable performance under bending and twisting load due to a uniform and dense conductive layer formed. This work provides a research basis for the preparation of silicone rubber-like fibrous composite materials, and the prepared polydimethylsiloxane-based stretchable conductive elastomer fibers have broad application prospects in flexible wearable electronic devices.

Flexible environment-tolerant electroluminescent devices based on nanocellulose-mediated transparent electrodes

Stretchable electroluminescent (EL) devices show great potential for wearable displays. However, the integration of stretchability, flexibility, temperature-tolerance, waterproofness and biocompatibility into a single EL device remains a challenge. Herein, we report a facile full solution-process method for a novel EL device consisting of a dielectric luminescent layer sandwiched between two silver nanowires-cellulose nanocrystals with II crystalline allomorphs/polydopamine-polydimethylsiloxane (CNC II-AgNWs/PDA-PDMS) electrodes. CNC II is used as a green dispersant, film-forming agent and antioxidant to improve the optical, electrical, mechanical and antioxidant properties of the electrodes. The electrodes exhibit a smooth surface, low sheet resistance (~11 Ω sq−1), high transparency (~79.2 %), ideal stretchability (~100 % strain) and excellent inoxidizability. The assembled EL devices with outstanding tensile stability and fatigue resistance demonstrate excellent luminance, flexibility and stretchability underwater and at extreme temperatures, as well as intrinsic biocompatibility. Our multifunctional EL devices with outstanding integrated properties provide new insights for the next-generation flexible electronics.

Interfacial Coordination Interaction Enables Soft Elastomer Composites High Thermal Conductivity and High Toughness.

Soft elastomers have attracted wide applications, such as soft electronic devices and soft robotics, due to their ability to undergo large deformation with a small external force. Most elastomers suffer from poor toughness and thermal conductivity, which limits their use. The addition of inorganic fillers can enhance the thermal conductivity and toughness, but it deteriorates the softness (low Young's modulus and high stretchability). Integrating thermal conductivity, toughness, and softness into one elastomer is still a challenge. Here, we report a strategy of interfacial coordination interaction to achieve soft elastomer composites with high thermal conductivity and high toughness. We demonstrate the strategy by using poly(lipoic acid) elastomer and silver-coated aluminum filler as model, where silver-sulfur coordination cross-links are formed at the interface. The resultant elastomer composite shows high streachability (450%), high thermal conductivity (2.35 W m-1 K-1), low modulus (321 kPa), and high toughness (3496 J m-2), which cannot be achieve in existing elastomers. The time domain thermoreflectance technique demonstrates that the silver-sulfur coordination interaction lowers the interfacial thermal resistance, resulting in enhanced thermal conductivity of the elastomer composites. The excellent softness stems from lower bonding energy of the silver-sulfur coordination cross-links compared with covalent chemical cross-links. The high toughness also benefits from the interfacial silver-sulfur coordination interaction that can dissipate more energy upon deformation. We further demonstrate the potential application of the thermally conductive, tough, and soft elastomer composites for thermal management of chip and soft electronic devices.

Stretchable, conductive poly(acrylamide‐co‐maleic acid)/triethylene glycol/NaCl double‐crosslinked organohydrogel with excellent antifreezing and sensing properties

AbstractRecently, the wearable electronic devices have attracted extensive attention while the low stretchability and freezing‐induced performance deterioration largely limit the applications in many fields. In this work, the poly(acrylamide‐co‐maleic acid) hydrogels are immersed in the triethylene glycol/sodium chloride/water solution to establish the dual‐crosslinked organohydrogels, which present the extraordinary stretchability (1322 ± 75%) ascribed to the energy dissipation of physical interactions and stability of chemical crosslinked networks. Triethylene glycol forms strong hydrogen bonds with water, which interfere with the formation of intermolecular hydrogen bonds among water molecules at low temperatures and inhibit the formation of ice crystals in the organohydrogel networks. Sodium chloride imparts excellent electrical conductivity to the organohydrogel. As a result, a stretchable electronic sensor based on the organohydrogel is fabricated, which is strain‐sensitive with a wide strain sensing window (0%–440%) and excellent stability. More importantly, the sensor based on the as‐prepared organohydrogel could precisely detect various activities of the human model at −30°C leading to the widely application prospects.