Stretchable Polymer Research Articles

Silver Nanowires in Stretchable Resistive Strain Sensors.

Silver nanowires (AgNWs), having excellent electrical conductivity, transparency, and flexibility in polymer composites, are reliable options for developing various sensors. As transparent conductive electrodes (TCEs), AgNWs are applied in optoelectronics, organic electronics, energy devices, and flexible electronics. In recent times, research groups across the globe have been concentrating on developing flexible and stretchable strain sensors with a specific focus on material combinations, fabrication methods, and performance characteristics. Such sensors are gaining attention in human motion monitoring, wearable electronics, advanced healthcare, human-machine interfaces, soft robotics, etc. AgNWs, as a conducting network, enhance the sensing characteristics of stretchable strain-sensing polymer composites. This review article presents the recent developments in resistive stretchable strain sensors with AgNWs as a single or additional filler material in substrates such as polydimethylsiloxane (PDMS), thermoplastic polyurethane (TPU), polyurethane (PU), and other substrates. The focus is on the material combinations, fabrication methods, working principles, specific applications, and performance metrics such as sensitivity, stretchability, durability, transparency, hysteresis, linearity, and additional features, including self-healing multifunctional capabilities.

Highly Stretchable and Transparent Conductive Electrodes for Resistive Strain Sensors

AbstractConductive organic materials are a crucial component of resistive strain sensors because phase separation limits the concentration of inorganic materials in polymer matrices, leading to poor electrical conductivity. However, traditional conductive polymers are not good candidates for use in flexible electric devices due to their low or ultralow sensitivity and stretchability. In this paper, the development of transparent, highly stretchable polymer electrodes by a one‐step solution process is reported. The polymer electrodes tolerate extreme strains exceeding 150% and feature an ionic conductivity of 1.4 × 10−4 S cm−1. These metal‐free electrodes also exhibit a high optical transparency of 60%, suggesting that they have great potential for optoelectronic applications. Strain sensors are fabricated by covering the conductive polymer electrodes with polyimide film in a simple, low‐cost, and scalable process. The as‐assembled strain sensors can be used for both stretching and compressing with high sensitivity (a maximum gauge factor of 1049.9), an ultralow limit of detection (0.5% strain), and excellent reliability and stability (>5000 stretching cycles).

Self-Healable, Highly Stretchable, Ionic Conducting Polymers as Efficient Protecting Layers for Stable Lithium-Metal Electrodes.

Although numerous studies on polymeric protective films to stabilize lithium (Li)-metal electrodes have been reported, the construction of self-healing polymers that enables the long-term operation of Li-metal batteries (LMBs) at relatively low temperatures has rarely been demonstrated. Herein, a highly stretchable, autonomous self-healable, and ionic-conducting polymer network (SHIPN) is synthesized as an efficient protective film for LMBs. The network backbone, synthesized from copolymerization of poly(ethylene glycol)-mono-methacrylate (PEGMMA) and 2-[[(butylamino)carbonyl]oxy]ethyl acrylate (BCOE), is chemically cross-linked via diisocyanate. With SHIPN-modified electrodes, enhanced electrochemical performance can be achieved in Li/Cu, Li/Li, and Li/LiFePO4 (Li/LFP) cells. The SHIPN@Li/LFP cell delivers a capacity retention of 85.6% after 500 cycles at 5 °C, resulting from the low-temperature self-healability of SHIPN. In full cells with a high-mass-loading LFP cathode (∼17 mg cm-2), the capacity retention is at least 300% higher than that with a bare Li electrode. Further physical characterizations of electrodes confirm the effect of SHIPN in enhancing the interfacial stability and suppressing Li dendrite growth. Our results will provide insights into rationally designing soft and hybrid materials toward stable LMBs at different temperatures.

Highly stretchable and robust transparent conductive polymer composites for multifunctional healthcare monitoring

ABSTRACT Soft, stretchable, conductive thin films have propelled to the forefront of applications in stretchable sensors for on-skin health monitoring. Stretchable conductive films require high conformability, stretchability, and mechanical/chemical stability when integrated into the skin. Here, we present a highly stretchable, conductive, and transparent natural rubber/silver nanowire (AgNW)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film. Overcoating the PEDOT:PSS layer results in outstanding mechanical robustness and chemical stability by suppressing the mechanical and chemical degradation of the nanowire networks. Moreover, the introduction of the organic surface modifier enhances the bonding strength between the natural rubber substrate and AgNW at the interface. The highly conformable composite films are integrated into multifunctional on-skin sensors for monitoring various human motions and biological signals with low-power consumption. We believe that the highly stretchable, robust, and conformable natural rubber/AgNW/PEDOT:PSS composite film can offer new opportunities for next-generation wearable sensors for body motion and physiological monitoring.

Design and Synthesys of Highly Stretchable and Tough Adhesive Hydrogels for Dry and Wet Biomedical Applications

The development of less invasive procedures and devices in the biomedical applications of wound closure and healing is an important and essential task. The healing process is facilitated by proper closure of the surgical wound; traditionally a particularly invasive practice. Tissue adhesives design should incorporate simplicity, safety, and painless removal from the skin. Moreover, these materials should have adequate adhesive strength, be tough enough to incorporate the mechanical skin behaviors, and must also show compatibility with body fluids as well as cells and tissues. The design of tough adhesives (TAs) can potentially enable many applications, including the gluing of tissues and attaching devices in vivo, tissue repair, and can be used as a hemostatic dressing because of their compatibility with blood exposure. This work reviews many strategies to design tough hydrogels with the introduction of non-covalent bonds and the construction of stretchable polymer networks and interpenetrated networks, such as a double-network hydrogel. To overcome such difficulties, some synthetic wet adhesives emulating natural adhesive materials of marine organisms have been investigated.

Stretchable Redox-Active Semiconducting Polymers for High-Performance Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) represent an emerging device platform for next-generation bioelectronics owing to the uniquely high amplification and sensitivity to biological signals. For achieving seamless tissue-electronics interfaces for accurate signal acquisition, skin-like softness and stretchability are essential requirements, but they have not yet been imparted onto high-performance OECTs, largely due to the lack of stretchable redox-active semiconducting polymers. Here, a stretchable semiconductor is reported for OECT devices, namely poly(2-(3,3'-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-[2,2'-bithiophen]-5)yl thiophene) (p(g2T-T)), which gives exceptional stretchability over 200% strain and 5000 repeated stretching cycles, together with OECT performance on par with the state-of-the-art. Validated by systematic characterizations and comparisons of different polymers, the key design features of this polymer that enable the combination of high stretchability and high OECT performance are a nonlinear backbone architecture, a moderate side-chain density, and a sufficiently high molecular weight. Using this highly stretchable polymer semiconductor, an intrinsically stretchable OECT is fabricated with high normalized transconductance (≈223 S cm-1 ) and biaxial stretchability up to 100% strain. Furthermore, on-skin electrocardiogram (ECG) recording is demonstrated, which combines built-in amplification and unprecedented skin conformability.

Stretchable polymer composite film based on pseudo-high carbon-filler loadings for electromagnetic interference shielding

Polymer composites with carbon-based filler (C-filler) are regarded as promising alternatives for flexible and stretchable electromagnetic interference (EMI) shielding materials. However, the poor mechanical properties caused by high-loadings largely hinders their practical applications, especially for the film. Herein, a class of pseudo-high C-filler (PHCF) composite films are constructed by introducing lean C-filler polymer phase on both sides of the high C-filler polymer phase. Not only does conductive structure of the high C-filler phase not transform, but the lean C-filler phase serves as both absorbing microwaves and supporting function. Strong interfacial compatibility and multi-level “zigzag” cracks synergistically enhance the mechanical stretchability with elongation at break of 200 ± 8%. Notably, its EMI shielding effectiveness reaches 31.8 dB with enhanced absorptivity (>0.2). Further, EMI shielding performance remains reliable after bending and stretching. This work offers a regulation strategy to upgrade EMI shielding materials for burgeoning stretchable, wearable and mini electronics.

High-brightness all-polymer stretchable LED with charge-trapping dilution.

Next-generation light-emitting displays on skin should be soft, stretchable and bright1-7. Previously reported stretchable light-emitting devices were mostly basedon inorganic nanomaterials, such as light-emitting capacitors, quantum dots or perovskites6-11. They either require high operating voltage or have limited stretchability and brightness, resolution or robustness under strain. On the other hand, intrinsically stretchable polymer materials hold the promise of good strain tolerance12,13. However, realizing high brightness remains a grand challenge for intrinsically stretchable light-emitting diodes. Here we report a material design strategy and fabrication processes to achieve stretchable all-polymer-based light-emitting diodes with high brightness (about 7,450 candela per square metre), current efficiency (about 5.3 candela per ampere) and stretchability (about 100per cent strain). We fabricate stretchable all-polymer light-emitting diodes coloured red, green and blue, achieving both on-skin wireless powering and real-time displaying of pulse signals. This work signifies a considerable advancement towards high-performance stretchable displays.

Highly stretchable multifunctional polymer ionic conductor with high conductivity based on organic-inorganic dual networks

Ionogels based on ionic liquids have attracted great attention in the field of energy storage and sensing due to their intrinsic advantages, including good thermal stability, low flammability and volatility, and favorable electrochemical properties. However, the introduction of ionic liquids usually sacrifices the mechanical properties of ionogels, such as stretchability and strength. Herein, inspired by reinforced concrete structures, an ionogel with rigid inorganic and flexible organic interpenetrating dual network is created, which exhibits high mechanical strength, excellent elongation (nearly 900%), good elasticity and high room temperature ionic conductivity (1.34 mS cm−1) by exploring the synergistic effect between the rigid and flexible components. Such a highly stretchable conductive organic–inorganic dual network is constructed through non-hydrolyzed sol–gel reaction of tetrabutyl titanate and in situ polymerization of butyl acrylate through UV irradiation. Impressively, the assembled energy stroage device (lithium-ion batteries) based on the ionogel exhibits over 1000 stable cycles with average Columbic efficiency of ∼ 100%, and excellent rate property. The resulting stretchable lithium-ion battery can power an LED at 50% strain. The ionogel, as ionic skin, also shows highly repeatable electrical responses in reciprocal deformation cycles. In terms of the above unusual features, the as-perpared ionogel manifests great promise for versatile applications in the field of flexible electronics.

Molecular Design of Stretchable Polymer Semiconductors: Current Progress and Future Directions.

Stretchable polymer semiconductors have advanced rapidly in the past decade as materials required to realize conformable and soft skin-like electronics become available. Through rational molecular-level design, stretchable polymer semiconductor films are now able to retain their electrical functionalities even when subjected to repeated mechanical deformations. Furthermore, their charge-carrier mobilities are on par with the best flexible polymer semiconductors, with some even exceeding that of amorphous silicon. The key advancements are molecular-design concepts that allow multiple strain energy-dissipation mechanisms, while maintaining efficient charge-transport pathways over multiple length scales. In this perspective article, we review recent approaches to confer stretchability to polymer semiconductors while maintaining high charge carrier mobilities, with emphasis on the control of both polymer-chain dynamics and thin-film morphology. Additionally, we present molecular design considerations toward intrinsically elastic semiconductors that are needed for reliable device operation under reversible and repeated deformation. A general approach involving inducing polymer semiconductor nanoconfinement allows for incorporation of several other desired functionalities, such as biodegradability, self-healing, and photopatternability, while enhancing the charge transport. Lastly, we point out future directions, including advancing the fundamental understanding of morphology evolution and its correlation with the change of charge transport under strain, and needs for strain-resilient polymer semiconductors with high mobility retention.

Flexible, Elastic, and Superhydrophobic/Superoleophilic Adhesive for Reusable and Durable Water/Oil Separation Coating.

This study investigates a highly flexible/stretchable and mechanically durable superhydrophobic/superoleophilic coating for efficient oil/water separation and oil absorption. The coating is applied via a simple immersion process using a mixed solution of a biocompatible adhesive (ethyl cyanoacrylate, ECA), a highly stretchable polymer (polycaprolactone, PCL), and superhydrophobic/superoleophilic nanoparticles (fluorine-coated silica nanoparticles, F-SiO2 NPs) in a solvent, followed by solvent evaporation and ECA polymerization. Polymerized ECA (poly-ECA) in the coating material strongly adheres the F-SiO2 NPs to the substrate surface, while PCL bestows the rigid poly-ECA with high flexibility. A coated polyurethane sponge exhibits superhydrophobicity (water contact angle of >150°), while retaining robust mechanical stability and flexibility/elasticity. This provides an efficient means of cleaning oil spills with high selectivity, even after mechanical abrasion (>99% separation efficiency is retained after 120 tape test cycles and 50 rubbing test cycles), with excellent reusability.

Release properties of plasma polymeric coated polymer films and adhesive strength of transferred polyurethane coatings to fiber-reinforced thermosets

An investigation of plasma-polymeric coatings on stretchable polymer films utilizing hexamethyldisiloxane and oxygen has been performed. The influence of the plasma parameters on release properties to polyurethane paints as well as the unpreventable transfer of traces of plasma-polymeric coatings to paints or thermosets were investigated. The elemental composition and physical properties have been analyzed using contact angle measurement and X-ray photoelectron spectroscopy (XPS) with peak fitting. The release properties of the coatings are discussed. The results show that a higher ratio of oxygen to hexamethyldisiloxane during the plasma process leads to an increase of peel forces to polyurethane paints. The XPS shows that the transfer of plasma coating to the paint is on average below 0.4 at% of silicon, which is below the critical fraction of 2 at% silicon reported in the literature. Therefore, the clear-coat paint shows no difference in its adhesion to thermosets between untreated and pretreated surfaces.

A hyperbranched polymer elastomer-based pressure sensitive adhesive

A highly stretchable hyperbranched polymer elastomer pressure sensitive adhesive can instantly adhere to various substrates in air, and its outstanding water dispelling capability allows for direct deployment and long-lasting tough adhesion in water.

Mechanical strain, thermal and pressure effects on the absorption edge of an organic charge-transfer polymer for flexible photovoltaics and sensors

A highly stretchable elastic organic charge-transfer polymer displays improved photovoltaic characteristics under high pressure.

Development of stretchable and bendable polymer wearable antenna for 5G applications

Development of stretchable and bendable polymer wearable antenna for 5G applications

Biaxial Stretching Leads to Abrupt Electrical Failure in Printed Inks on Stretchable Polymer Substrates

Biaxial Stretching Leads to Abrupt Electrical Failure in Printed Inks on Stretchable Polymer Substrates

Hydrogel‐based triboelectric nanogenerators: Properties, performance, and applications

The development of triboelectric nanogenerators (TENGs) in 2012 revolutionized the vision of environmental energy harvesting. Nowadays, TENG assembly, working mode, and material selection are investigated continuously in order to obtain high-performance and long-lasting devices. Hydrogels are flexible and stretchable water-swollen 3D polymer networks, which can be tailored to conduct electricity and render outstanding mechanical properties. Hydrogels have been used to develop novel flexible wearable TENGs. Here, we review the current knowledge concerning hydrogel-based TENGs, including an overview of relevant hydrogel characteristics, hydrogel-based TENG performance, and their practical applications. The single-electrode TENG working mode is the most popular in hydrogel-based TENGs as they can be easily attached and stretched for biomechanical energy harvesting. Hydrogel-based TENGs have demonstrated to be capable of delivering high electrical output (250-400 V, ≥10 μA) and being robust enough for devices that last for several months. Biomechanical energy sensing and harvesting, smart farming, biomedical, and human–machine control interfaces are investigated as potential applications of hydrogel-based TENGs. Interestingly, energy harvesting from human motion is of particular interest for this type of TENG due to its outstanding stretchability, strength, and additional physical properties, such as self-healing ability. In most cases, the devices are capable of powering small electronics entirely from harvested biomechanical energy. Biomedical applications involved wound healing acceleration driven by TENG-powered electrical stimuli and disease monitoring, including implantable devices. Despite showing promising electrical performance, controlling water evaporation is still challenging to maintain the mechanical and conductive properties of hydrogel-based TENGs. On the other hand, fully biodegradable TENGs are largely unexplored, as well as many applications, such as blue energy harvesting, the internet of things, and others. The next steps in this line of research must focus on addressing the main challenges of hydrogel-based TENGs and filling the application knowledge gaps.

Stretchable, self-healable, conductive and adhesive gel polymer electrolytes based on a deep eutectic solvent for all-climate flexible electrical double-layer capacitors

With the development of all-climate flexible/wearable supercapacitors, it is highly desired to prepare multifunctional gel polymer electrolytes (GPEs) with low-temperature tolerance. Herein, a GPE containing a deep eutectic solvent (DES), which is consisted of choline chloride and glycerol, is developed as the electrolyte of all-climate and flexible supercapacitors by in situ polymerization of acrylic acid monomers in the DES. The as-prepared DES-based GPE shows a room-temperature ionic conductivity of 3.33 mS cm−1, intrinsical adhesion, mechanical stretchability of 1000%, and quickly healable performance within 10 s. More significantly, the carbon-based supercapacitors assembled by using such a GPE have an areal specific capacitance of 101.3 mF cm−2 and all-climate stability from -20 to 80 °C. It also demonstrates a remarkable flexibility and an excellent self-healing capability. The presented work provides a facile and eco-friendly way to design GPEs for next-generation energy storage devices with high performance and all-climate tolerance.

High-frequency and intrinsically stretchable polymer diodes.

Skin-like intrinsically stretchable soft electronic devices are essential to realize next-generation remote and preventative medicine for advanced personal healthcare1-4. The recent development of intrinsically stretchable conductors and semiconductors has enabled highly mechanically robust and skin-conformable electronic circuits or optoelectronic devices2,5-10. However, their operating frequencies have been limited to less than 100 hertz, which is much lower than that required for many applications. Here we report intrinsically stretchable diodes-based on stretchable organic and nanomaterials-capable of operating at a frequency as high as 13.56 megahertz. This operating frequency is high enough for the wireless operation of soft sensors and electrochromicdisplaypixels using radiofrequency identification in which the base-carrier frequency is 6.78 megahertz or13.56 megahertz. This was achieved through a combination of rational material design and device engineering. Specifically, we developed a stretchable anode, cathode, semiconductor and current collector that can satisfy the strict requirements for high-frequency operation. Finally, we show the operational feasibility of our diode by integrating it with a stretchable sensor, electrochromicdisplay pixel and antenna to realize a stretchable wireless tag. This work is an important step towards enabling enhanced functionalities and capabilities for skin-like wearable electronics.

Self-healable, stretchable, and nonvolatile solid polymer electrolytes for sustainable energy storage and sensing applications

Self-healing materials that autonomously recover physical and functional failures have been investigated to improve the operational durability of stretchable and wearable devices. Although various self-healable materials have been reported, imparting self-healability to solid-state nonvolatile polymer electrolytes remains challenging. In this work, highly conductive, stretchable, self-supporting, transparent, and nonvolatile polymer electrolytes, or ionogels, capable of repairing physical and functional damages over 50% in 10 min and full recovery in 1 h without external stimuli were successfully developed using dynamic disulfide metathesis. Self-healable ionogels consisting of a cross-linked tetra-arm polyethylene glycol possessing disulfide linkages and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, were successfully employed in stretchable/self-healable strain sensors and solid-state supercapacitors as piezoresistive components and electrolyte membranes, respectively. Both devices completely recovered their electrical/electrochemical performance even after repetitive cutting/healing cycles. Therefore, these results offer an effective strategy for developing both mechanically and electrically sustainable solid electrolytes for high-performance stretchable and wearable electronic/electrochemical devices.