Elastomeric Substrate Research Articles

On the Development of a New Flexible Pressure Sensor.

The rapid advancement of the Internet of Things (IoT) serves as a significant driving force behind the development of innovative sensors and actuators. This technological progression has created a substantial demand for new flexible pressure sensors, essential for a variety of applications ranging from wearable devices to smart home systems. In response to this growing need, our laboratory has developed a novel flexible pressure sensor, designed to offer an improved performance and adaptability. This study aims to present our newly developed sensor, detailing the comprehensive investigations we conducted to understand how different parameters affect its behaviour. Specifically, we examined the influence of the resistive layer thickness and the elastomeric substrate on the sensor's performance. The resistive layer, a critical component of the sensor, directly impacts its sensitivity and accuracy. By experimenting with varying thicknesses, we aimed to identify the optimal configuration that maximizes sensor efficiency. Similarly, the elastomeric substrate, which provides the sensor's flexibility, was scrutinized to determine how its properties affect the sensor's overall functionality. Our findings highlight the delicate balance required between the resistive layer and the elastomeric substrate to achieve a sensor that is both highly sensitive and durable. This research contributes valuable insights into the design and optimization of flexible pressure sensors, paving the way for more advanced IoT applications.

Stretchable electronic strips for electronic textiles enabled by 3D helical structure

The development of stretchable electronic devices is a critical area of research for wearable electronics, particularly electronic textiles (e-textiles), where electronic devices embedded in clothing need to stretch and bend with the body. While stretchable electronics technologies exist, none have been widely adopted. This work presents a novel and potentially transformative approach to stretchable electronics using a ubiquitous structure: the helix. A strip of flexible circuitry (‘e-strip’) is twisted to form a helical ribbon, transforming it from flexible to stretchable. A stretchable core—in this case rubber cord—supports the structure, preventing damage from buckling. Existing helical electronics have only extended to stretchable interconnects between circuit modules, and individual components such as printed helical transistors. Fully stretchable circuits have, until now, only been produced in planar form: flat circuits, either using curved geometry to enable them to stretch, or using inherently stretchable elastomer substrates. Helical e-strips can bend along multiple axes, and repeatedly stretch between 30 and 50%, depending on core material and diameter. LED and temperature sensing helical e-strips are demonstrated, along with design rules for helical e-strip fabrication. Widely available materials and standard fabrication processes were prioritized to maximize scalability and accessibility.

Multifunctional Nanocomposite Yield-Stress Fluids for Printable and Stretchable Electronics.

Compliant materials are crucial for stretchable electronics. Stretchable solids and gels have limitations in deformability and durability, whereas active liquids struggle to create complex devices. This study presents multifunctional yield-stress fluids as printable ink materials to construct stretchable electronic devices. Ionic nanocomposites comprise silica nanoparticles and ion liquids, while electrical nanocomposites use the natural oxidation of liquid metals to produce gallium oxide nanoflake additives. These nanocomposite inks can be printed on an elastomer substrate and stay in a solid state for easy encapsulation. However, their transition into a liquid state during stretching allows ultrahigh deformability up to the fracture strain of the elastomer. The ionic inks produce strain sensors with high stretchability and temperature sensors with high sensitivity of 7% °C-1. Smart gloves are further created by integrating these sensors with printed electrical interconnects, demonstrating bimodal detection of temperatures and hand gestures. The nanocomposite yield-stress fluids combine the desirable qualities of solids and liquids for stretchable devices and systems.

Variable density wrinkling in polymer thin film by gradient stress induced in the elastomeric substrate

Variable density wrinkling in polymer thin film by gradient stress induced in the elastomeric substrate

Mitigating the Overheat of Stretchable Electronic Devices Via High-Enthalpy Thermal Dissipation of Hydrogel Encapsulation.

The practical application of flexible and stretchable electronics is significantly influenced by their thermal and chemical stability. Elastomer substrates and encapsulation, due to their soft polymer chains and high surface-area-to-volume ratio, are particularly susceptible to high temperatures and flame. Excessive heat poses a severe threat of damage and decomposition to these elastomers. By leveraging water as a high enthalpy dissipating agent, here, a hydrogel encapsulation strategy is proposed to enhance the flame retardancy and thermal stability of stretchable electronics. The hydrogel-based encapsulation provides thermal protection against flames for more than 10 s through the evaporation of water. Further, the stretchability and functions automatically recover by absorbing air moisture. The incorporation of hydrogel encapsulation enables stretchable electronics to maintain their functions and perform complex tasks, such as fire saving in soft robotics and integrated electronics sensing. With high enthalpy heat dissipation, encapsulated soft electronic devices are effectively shielded and retain their full functionality. This strategy offers a universal method for flame retardant encapsulation of stretchable electronic devices.

Biomimetic wafer-scale alignment of tellurium nanowires for high-mobility flexible and stretchable electronics.

Flexible and stretchable thin-film transistors (TFTs) are crucial in skin-like electronics for wearable and implantable applications. Such electronics are usually constrained in performance owing to a lack of high-mobility and stretchable semiconducting channels. Tellurium, a rising semiconductor with superior charge carrier mobilities, has been limited by its intrinsic brittleness and anisotropy. Here, we achieve highly oriented arrays of tellurium nanowires (TeNWs) on various substrates with wafer-scale scalability by a facile lock-and-shear strategy. Such an assembly approach mimics the alignment process of the trailing tentacles of a swimming jellyfish. We further apply these TeNW arrays in high-mobility TFTs and logic gates with improved flexibility and stretchability. More specifically, mobilities over 100 square centimeters per volt per second and on/off ratios of ~104 are achieved in TeNW-TFTs. The TeNW-TFTs on polyethylene terephthalate can sustain an omnidirectional bending strain of 1.3% for more than 1000 cycles. Furthermore, TeNW-TFTs on an elastomeric substrate can withstand a unidirectional strain of 40% with no performance degradation.

Design strategy of porous elastomer substrate and encapsulation for inorganic stretchable electronics

ABSTRACT The emergence of stretchable electronic technology has led to the development of many industries and facilitated many unprecedented applications, owing to its ability to bear various deformations. However, conventional solid elastomer substrates and encapsulation can severely restrict the free motion and deformation of patterned interconnects, leading to potential mechanical failures and electrical breakdowns. To address this issue, we propose a design strategy of porous elastomer substrate and encapsulation to improve the stretchability of serpentine interconnects in island-bridge structures. The serpentine interconnects are fully bonded to the elastomer substrate, while segments above circular pores remain suspended, allowing for free deformation and a substantial improvement in elastic stretchability compared to the solid substrates. The pores ensure unimpeded interconnect deformations, and moderate porosity provides support while maintaining the initial planar state. Compared to conventional solid configurations, finite element analysis (FEA) demonstrates a substantial enhancement of elastic stretchability (e.g. ≈9 times without encapsulation and ≈ 7 times with encapsulation). Uniaxial cyclic loading fatigue experiments validate the enhanced elastic stretchability, indicating the mechanical stability of the porous design. With its intrinsic advantages in permeability, the proposed strategy has the potential to offer insightful inspiration and novel concepts for advancing the field of stretchable inorganic electronics.

Broadband and strong visible-light driven photocatalytic enhancement utilizing two-dimensional plasmonics of Au/Cu-modified SrTiO3 perovskite nanostructures

Addressing limited access to clean drinking water demands innovative solutions to energy and environmental crises. Photocatalysis offers promise without eliciting secondary pollution, yet traditional metal oxide semiconductor photocatalysts encounter impediments in solar energy utilization and charge carrier recombination rates. To overcome these limitations, we present for the first time, a novel design, fabrication, and characterization of two-dimensional plasmonic noble metal/perovskite nanostructure hybrids for photocatalysis, combining Au as the plasmonic material with Cu-modified SrTiO3 as the semiconductor, fabricated on elastomeric polydimethylsiloxane substrates. Integrating localized surface plasmon resonance and Schottky junction-related band-bending significantly extends optical absorption into the ultraviolet-visible-infrared bands and enhances the effective carrier lifetimes, ensuring state-of-the-art photocatalytic efficiency. Exceptional photocatalytic performance in a 5-ppm aqueous methylene blue solution can be attained with 72.69-wt% Cu incorporation in the perovskite oxide, reaching a visible light-mediated photodegradation efficiency of 26% - an astounding 62.5-% efficiency increase compared to previous experiments without Cu modification. This advancement underscores the synergistic potential of noble metals and photocatalytic semiconductors in managing water purification challenges.

Magnetically responsive manipulation of droplets and bubbles

AbstractDroplets and bubbles have a wide range of applications in industry, agriculture, and daily life, and their controllable manipulation is of significant scientific and technological importance. Versatile magnetically responsive manipulation strategies have been developed to achieve precise control over droplets and bubbles. To manipulate nonmagnetic droplets or bubbles with magnetic fields, the presence of magnetic medium is indispensable. Magnetic additives can be added to the surface or interior of droplets and bubbles, allowing for on‐demand manipulation by direct magnetic actuation. Alternatively, magnetically responsive elastomer substrates can be used to actuate droplets and bubbles by controlling the deformation of microstructures on the substrates through magnetic stimulation. Another strategy is based on untethered magnetic devices, which enables free mobility, facilitating versatile manipulation of droplets and bubbles in a flexible manner. This paper reviews the advances in magnetically responsive manipulation strategies from the perspective of droplets and bubbles. An overview of the different classes of magnetic medium, along with their respective corresponding droplet/bubble manipulation methods and principles, is first introduced. Then, the applications of droplet/bubble manipulation in biomedicine, microchemistry, and other fields are presented. Finally, the remaining challenges and future opportunities related to regulating droplet/bubble behavior using magnetic fields are discussed.

Highly Stretchable, Self-Healing, and Sensitive E-Skins at -78 °C for Polar Exploration.

Ultralow temperature-tolerant electronic skins (e-skins) can endow polar robots with tactile feedback for exploring in extremely cold polar environments. However, it remains a challenge to develop e-skins that enable sensitive touch sensation and self-healing at ultralow temperatures. Herein, we describe the development of a sensitive robotic hand e-skin that can stretch, self-heal, and sense at temperatures as low as -78 °C. The elastomeric substrate of this e-skin is based on poly(dimethylsiloxane) supramolecular polymers and multistrength dynamic H-bonds, in particular with quadruple H-bonding motifs (UPy). The structure-performance relationship of the elastomer at ultralow temperatures is investigated. The results show that elastomers with side-chain UPy units exhibit higher stretchability (∼3257%) and self-healing efficiency compared to those with main-chain UPy units. This is attributed to the lower binding energy variation and lower potential well. Based on the elastomer with side-chain UPy and man-made electric ink, a sensitive robotic hand e-skin for usage at -78 °C is constructed to precisely sense the shape of objects and specific symbols, and its sensation can completely self-recover after being damaged. The findings of this study contribute to the concept of using robotic hands with e-skins in polar environments that make human involvement limited, dangerous, or impossible.

Ultrathin damage-tolerant flexible metal interconnects reinforced by in-situ graphene synthesis

Conductive patterned metal films bonded to compliant elastomeric substrates form meshes which enable flexible electronic interconnects for various applications. However, while bottom-up deposition of thin films by sputtering or growth is well-developed for rigid electronics, maintaining good electrical conductivity in sub-micron thin metal films upon large deformations or cyclic loading remains a significant challenge. Here, we propose a strategy to improve the electromechanical performance of nanometer-thin palladium films by in-situ synthesis of a conformal graphene coating using chemical vapor deposition (CVD). The uniform graphene coverage improves the thin film’s damage tolerance, electro-mechanical fatigue, and fracture toughness owing to the high stiffness of graphene and the conformal CVD-grown graphene-metal interface. Graphene-coated Pd thin film interconnects exhibit stable increase in electrical resistance even when strained beyond 60% and longer fatigue life up to a strain range of 20%. The effect of graphene is more significant for thinner films of < 300 nm, particularly at high strains. The experimental observations are well described by the thin film electro-fragmentation model and the Coffin-Manson relationship. These findings demonstrate the potential of CVD-grown graphene nanocomposite materials in improving the damage tolerance and electromechanical robustness of flexible electronics. The proposed approach offers opportunities for the development of reliable and high-performance ultra-conformable flexible electronic devices.

Manufacturing of soft capacitive strain sensor based on dielectric elastomer material for an elastic element of a jaw coupling

In this work, we present a procedure for manufacturing a soft capacitive strain sensor in the form of a multi-layer electrical capacitor for further integration into the elastic gear rim of a jaw coupling. The dielectric elastomer sensor (DES) is based on an elastomeric substrate with alternating layers of conductive carbon black based ink as electrode layers and elastomeric film as a dielectric and electrical insulation. A variety of thin multi-layer sensors were produced to evaluate the manufacturing process. Additionally, using an LCR meter and a tensile test machine, the equivalent electrical capacitance (C) at the two sensor contacts and the applied force are measured, respectively. It is shown that C varies depending on the amount of sample strain caused by the applied force. By testing two versions of DES, a maximum change in capacitance of ΔC = 1.55 pF was achieved. The obtained characteristics show that the presented manufacturing process for the DES can be used as a soft strain sensor to measure the strain caused by the force applied to the elastic element between the jaw couplings.

Single-step fabrication of liquid gallium nanoparticles via capillary interaction for dynamic structural colours

Incorporating structural coloured materials in flexible and stretchable elastomeric substrates requires numerous steps that compromise their scalability and economic viability for prospective applications in visual sensors and displays. Here we describe a one-step approach for fabricating plasmonic Ga nanostructures embedded in a polydimethylsiloxane substrate exhibiting tunable chromaticity, in response to mechanical stimuli. The process exploits the capillary interactions between uncrosslinked oligomeric chains of the substrate and Ga metal deposited by thermal evaporation, as elucidated by a theoretical model that we developed. By tuning the oligomer content in polydimethylsiloxane, we attain a range of colours covering a substantial gamut in CIE (Commission Internationale de l’Éclairage) coordinates. This mechanochromic flexible substrate shows reversible response to external mechanical stimuli for ~80,000 cycles. We showcase the capabilities of our processing technique by presenting prototypes of reflective displays and sensors for monitoring body parts, smart bandages and the capacity of the nanostructured film to map force in real time.

Flexible antibacterial degradable bioelastomer nanocomposites for ultrasensitive human–machine interaction sensing enabled by machine learning

AbstractFlexible wearables have attracted extensive interests for personal human motion sensing, intelligent disease diagnosis, and multifunctional electronic skins. However, the reported flexible sensors, mostly exhibited narrow detection range, low sensitivity, limited degradability to aggravate environmental pollution from vast electronic wastes, and poor antibacterial performance to hardly improve skin discomfort and skin inflammation from bacterial growth under long‐term wearing. Herein, bioinspired from human skin featuring highly sensitive tactile sensation with spinous microstructures for amplifying sensing sensitivity between epidermis and dermis, a wearable antibacterial degradable electronics is prepared from degradable elastomeric substrate with MXene‐coated spinous microstructures templated from lotus leaf assembled with the interdigitated electrode. The degradable elastomer is facilely obtained with tunable modulus to match the modulus of human skin with improved hydrophilicity for rapid degradation. The as‐obtained sensor displays ultra‐low detection limit (0.2 Pa), higher sensitivity (up to 540.2 kPa−1), outstanding cycling stability (&gt;23,000 cycles), a wide detection range, robust degradability, and excellent antibacterial capability. Facilitated by machine learning, the collected sensing signals from the integrated sensors on volunteer's fingers to the related American Sign Language are effectively recognized with an accuracy up to 99%, showing excellent potential in wireless human movement sensing and smart machine learning‐enabled human–machine interaction.

Recent Advances in Stretchable and Permeable Electrodes for Epidermal Electronics

AbstractEpidermal electronics is an emerging wearable platform that involves attaching deformable forms of devices to the skin. Epidermal electrodes represent a vital component of this technology, as they provide a direct electronic interface with the skin for sensing and stimulation. However, most of the current electrodes are built on non‐permeable elastomer substrates, which can limit their long‐term, continuous operations in a non‐invasive manner. Fortunately, recent advancements in conductive materials and fabrication techniques have enabled high‐performance epidermal electrodes that are comfortable to wear. In order to track the latest progress, this review article first introduces the designs of permeable structures and the preparation of conductive electrodes. The subsequent discussion elaborates on effective strategies to achieve desirable properties, such as high conductivity, stretchability, skin adhesion, and biocompatibility. The emerging applications of permeable epidermal electrodes are also summarized. Finally, this review concludes with the current challenges and future directions of breathable epidermal electrodes.

Transparent PDMS Surfaces with Covalently Attached Lubricants for Enhanced Anti-adhesion Performance.

Liquid-like surfaces featuring slippery, omniphobic, covalently attached liquids (SOCALs) reduce unwanted adhesion by providing a molecularly smooth and slippery surface arising from the high mobility of the liquid chains. Such SOCALs are commonly prepared on hard substrates, such as glass, wafers, or metal oxides, despite the importance of nonpolar elastomeric substrates, such as polydimethylsiloxane (PDMS) in anti-fouling or nonstick applications. Compared to polar elastomers, hydrophobic PDMS elastomer activation and covalent functionalization are significantly more challenging, as PDMS tends to display fast hydrophobic recovery upon activation as well as superficial cracking. Through the extraction of excess PDMS oligomers and fine-tuning of plasma activation parameters, homogeneously functionalized PDMS with fluorinated polysiloxane brushes could be obtained while at the same time reducing crack formation. Polymer brush mobility was increased through the addition of a smaller molecular silane linker to exhibit enhanced dewetting properties and reduced substrate swelling compared to functionalizations featuring hydrocarbon functionalities. Linear polymer brushes were verified by thermogravimetric analysis. The optical properties of PDMS remained unaffected by the activation in high-frequency plasma but were impacted by low-frequency plasma. Drastic decreases in solid adhesion of not just complex contaminants but even ice could be shown in horizontal push tests, demonstrating the potential of SOCAL-functionalized PDMS surfaces for improved nonstick applications.

Topologically Engineered Strain Redistribution in Elastomeric Substrates for Dually Tunable Anisotropic Plasmomechanical Responses.

The prompt visual response is considered to be a highly intuitive tenet among sensors. Therefore, plasmomechanical strain sensors, which exhibit dynamic structural color changes, have recently been developed by using mechanical stimulus-based elastomeric substrates for wearable sensors. However, the reported plasmomechanical strain sensors either lack directional sensitivity or require complex signal processing and device design strategies to ensure anisotropic optical responses. To the best of our knowledge, there have been no reports on utilizing anisotropic mechanical substrates to obtain directional optical responses. Herein, we propose an anisotropic plasmomechanical sensor to distinguish between the applied force direction and the force magnitude. We employ a simple strain-engineered topological elastomer to mechanically transform closely packed metallic nanoparticles (NPs) into anisotropic directional rearrangements depending on the applied force direction. The proposed structure consists of a heterogeneous-modulus elastomer that exhibits a highly direction-dependent Poisson effect owing to the periodically line-patterned local strain redistribution occurring due to the same magnitude of applied external force. Consequently, the reorientation of the self-assembled gold (Au)-NP array manifests dual anisotropy, i.e., force- and polarization-direction-dependent plasmonic coupling. The cost-effectiveness and simple design of our proposed heterogeneous-modulus platform pave the way for numerous optical applications based on dynamic transformation and topological inhomogeneities.

Opposite Mechanical Preference of Bone/Nerve Regeneration in 3D-Printed Bioelastomeric Scaffolds/Conduits Consistently Correlated with YAP-Mediated Stem Cell Osteo/Neuro-Genesis.

To systematically unveil how substrate stiffness, a critical factor in directing cell fate through mechanotransduction, correlates with tissue regeneration, novel biodegradable and photo-curable poly(trimethylene carbonate) fumarates (PTMCFs) for fabricating elastomeric 2D substrates and 3D bone scaffolds/nerve conduits, are presented. These substrates and structures with adjustable stiffness serve as a unique platform to evaluate how this mechanical cue affects the fate of human umbilical cord mesenchymal stem cells (hMSCs) and hard/soft tissue regeneration in rat femur bone defect and sciatic nerve transection models; whilst, decoupling from topographical and chemical cues. In addition to a positive relationship between substrate stiffness (tensile modulus: 90-990kPa) and hMSC adhesion, spreading, and proliferation mediated through Yes-associated protein (YAP), opposite mechanical preference is revealed in the osteogenesis and neurogenesis of hMSCs as they are significantly enhanced on the stiff and compliant substrates, respectively. In vivo tissue regeneration demonstrates the same trend: bone regeneration prefers the stiffer scaffolds; while, nerve regeneration prefers the more compliant conduits. Whole-transcriptome analysis further shows that upregulation of Rho GTPase activity and the downstream genes in the compliant group promote nerve repair, providing critical insight into the design strategies of biomaterials for stem cell regulation and hard/soft tissue regeneration through mechanotransduction.

Stretchable Thin-Film Transistors Based on Wrinkled Graphene and MoS2 Heterostructures.

Two-dimensional (2D) materials are outstanding candidates for stretchable electronics, but a significant challenge is their heterogeneous integration into stretchable geometries on soft substrates. Here, we demonstrate a strategy for stretchable thin film transistors (2D S-TFT) based on wrinkled heterostructures on elastomer substrates where 2D materials formed the gate, source, drain, and channel and characterized them with Raman spectroscopy and transport measurements. The 2D S-TFTs had initial mobility of 4.9 ± 0.7 cm2/(V s). The wrinkling reduced the strain transferred into the 2D materials by a factor of 50, allowing a substrate stretch of up to 23% that could be cycled thousands of times without electrical degradation. The stretch did not alter the mobility but did lead to strain-induced threshold voltage shifts by ΔVT = -1.9 V. These 2D S-TFTs form the foundation for stretchable integrated circuits and enable investigations of the impact of heterogeneous strain on electron transport.

Adhesive-free PDMS/PUA bilayer using selective photopolymerization for transparent, attachable, and wearable triboelectric nanogenerator

Polydimethylsiloxane (PDMS) is a widely used elastomer substrate in various fields, including wearable devices, because of its diverse advantages. However, PDMS has low stretchability and lack of adhesion, thus requires additive adhesives, such as conventional tapes, which have potential to cause skin trauma. Meanwhile, polyurethane acrylate (PUA) is also a broadly used elastomer given its various advantages, such as biocompatibility and mechanical properties. This study introduces, for the first time, a novel laser process method to form an adhesive-free PDMS/PUA bilayer film. By adjusting UV laser scanning parameters, the interfacial bonding between the PDMS and PUA can be improved without additional additives. The fabricated PDMS/PUA bilayer showed high transparency, improved mechanical properties, and adhesiveness properties. Combining with PUA (as a backbone and adhesive layer) and PDMS (as a triboelectric layer), the PDMS/PUA bilayer was further used to fabricate a transparent, attachable, and stretchable PDMS/PUA-based triboelectric nanogenerator (PP-TENG) device. The transparent attachable PP-TENG successfully detected minute human motions and functioned as a virtual piano keyboard and a virtual computer keyboard when attached to an arbitrary paper.