Stretchable Substrate Research Articles

Stretchable and transparent alginate ionic gel film for multifunctional sensors and devices

Flexible and stretchable substrates based on pure natural polymers have attracted widespread attention for next-generation “green” electronics. However, fabrication of stretchable and “green” electronic sensors with integrated high stretchability, optical transmittance and good conductivity still remains tremendous challenges. Herein, alginate ionic gel films (AIGFs) with integrated high stretchability (tensile strength of 4.13 MPa and 191.1 % fracture strain) and excellent transparent properties (transparency of ∼92 %) are achieved by the glycerol inducing physical crosslinking and CaCl2 initiating ionic crosslinking, a simple soaking and drying strategy. The obtained gel films not only exhibit good ionic conductivity, but also high reliability, wide-range sensing, and multiple sensitivity to external stimulus. More importantly, these ionic conductive gel films as green substrates are successfully utilized for construction of flexible and patterned optoelectronic devices. This promising strategy will open up new powerful routes to construct highly stretchable, transparent, and ionic conductive substrates for multifunctional sensors and devices.

Design and Fabrication of Metallic-Conductive Polymer-Based Hybrid Film Interconnections for Stretchable Electronic Devices

Stretchable circuit is a technological innovation that has transformed the microelectronic landscape due to its enormous applications in the field of medicine. The consistency or durability of health monitoring devices can increase the dependability with which non-invasive clinical measures are collected. Metal–conductive polymer (CP) hybrid interconnects and metal–polyimide dual-layered interconnects were all produced as stretchable interconnections. Stretchable substrate for all of the interconnects was selected as soft elastomer polydimethylsiloxane (PDMS). However, the PDMS substrate presents challenges because it is temperature sensitive, limiting the process temperature. The extreme hydrophobic nature of the PDMS surface makes it difficult to deposit components that contain water and results in poor adhesion with different metals. Following the development of processes for fabricating materials on the PDMS substrate, methods for resolving these issues were investigated.

Stretchable multi-mode sensor with single and simultaneous sensing modes for human neck motion tracking

Sensors with stretchability and multimodality are increasingly desired. This paper proposes a multimode sensor with three modes to detect and differentiate clockwise and counterclockwise twisting motions as well as stretching. This sensor consists of piezoelectric and piezoresistive sensors with a yarn-type functional structure and has a stretchable substrate that affords stretchability (∼80%). We use electrohydrodynamic spray to deposit conductive materials, which results in a stable and stretchable conductive network on substrates. The sensor can distinguish forces exerted on it through its different voltage and resistance change responses to external stimuli. We use a mathematical model to expand its ability to a simultaneous sensing mode. As an application, we construct a human neck tracking system with the multimode sensor.

Fatigue Behavior of Polymer Encapsulated Graphene to Mitigate Interfacial Fatigue Damage

In applications such as flexible electronic devices, graphene, and other 2D materials are frequently in contact with stretchable polymeric substrates. The interface between 2D materials and polymers is dominated by weak van der Waals forces and can eventually degrade due to the frequent dynamic mechanical loads that these devices experience. This can lead to significant local delamination and shear fracture of the 2D materials. Using the polydimethylsiloxane (PDMS) encapsulation method, it is shown that the damage in graphene is significantly mitigated when it is capped during dynamic loading. To track the spread of damage in both encapsulated and nonencapsulated graphene, in situ, cyclic loading is performed. The fundamental process driving this substantial reduction in damage propagation in the 2D lattice is explained by the conventional shear lag model. It is also observed that softer PDMS substrate and capping layer completely mitigate the fatigue damage in graphene for 100 cycles at 10% applied fatigue strain.

Wide gamut dynamic color manipulation based on dielectric metasurface on a stretchable substrate

Dynamic color adjustment technology has broad prospects in invisibility, anti-counterfeiting, and display, and is an important research direction in structural color. This study investigates the excitation of magnetic dipole resonances in the visible region by titanium dioxide (TiO2) metasurfaces using elastic dielectric polydimethylsiloxane (PDMS) and applied to dynamic color modulation. The near-field interaction between the resonant dielectric elements can be modulated using the mechanical strain of the PDMS substrate, enabling the advection of the resonance peak of the structure throughout the visible range, thereby achieving a wide color gamut. The color structure of TiO2 metasurface is sensitive to the refractive index of the surrounding medium. Moreover, active switching between the “ON” state (highly reflective at a selected wavelength) and the “OFF” state (transparent) can be achieved through the deformation of the flexible PDMS substrate. The proposed method can be applied to fields such as sensor displays and spectroscopy and for studying stretchable and wearable photonics.

Tensile properties of ferromagnetic nanofilms on stretchable substrates: Links between multi-cracking and magnetic properties

The influence of multi-cracking on the magnetic response of more (Co) or less (Ni78Fe22) magnetostrictive ferromagnetic nanofilms on flexible substrates has been studied by combining tensile tests with in situ magneto-optical Kerr effect magnetometry measurements, up to large strain (20%). The results show that the variations of the magnetic features are more significant in the elastic domain (before cracking), while they are slightly affected in the multi-cracking regime, linked to the fact that the stresses evolve relatively little in this regime. This results in a lesser modification of the magnetization curves of Ni78Fe22, which also means weak magnetostatic (dipolar) effects despite a very high crack density. This is very promising for the applicability of magnetic films in highly curved or stretched systems, especially using weakly magnetostrictive materials.

Deformation behaviour of interconnect encapsulated on functionally graded stretchable substrates

This work proposes an innovative design approach for a stretchable electronics substrate. The primary purpose of the substrate is to establish equilibrium among various elements that may lead to stretch-induced harm, such as plastic strain, out-of-plane deformation, and interfacial delamination. It accomplishes this by enabling the interconnect to flex with minimal resistance from the enclosed interconnect while undergoing stretching. We have utilised the dependency of PDMS with the cross-linking agent (CLA) and curing time during the fabrication process. A fabrication process using a spinning method of the functionally graded PDMS stretchable (FGPS) substrates is demonstrated so that it can be perfect cross-linking diffusion between layers with the desired thickness. Experiments indicated that curing time between poring layers is crucial in fabricating the FGPS. Further, numerical simulation was performed to examine FGPS substrates with the help of encapsulated horseshoe interconnect deformation behaviour, and results were validated with in-situ experimental results. For the numerical simulation to assign the different crosslinking PDMS properties, hyperelastic materials modelling (HMM) was done using different models to predict the behaviour of PDMS X:Y materials accurately under stretching conditions and found that (under 50% stretching) match well with R2>0.99. The computational findings demonstrate that incorporating a FGPS substrate, horseshoe interconnects increase out-of-plane deformation and geometric opening, leading to a decrease in plastic strain in the interconnect by up to 32%. However, the shear strain at the substrate interconnects interfaces increased from 55.38% to 77.7%. The shear strain counter is well-matched with the observed experimental delamination locations. This substrate design approach provides customized material properties at the substrate interconnect interface to meet specific requirements and can delay the interconnect failure.

Defect Engineering of Graphene for Dynamic Reliability.

The interface between two-dimensional (2D) materials and soft, stretchable polymeric substrates is a governing criterion in proposed 2D materials-based flexible devices. This interface is dominated by weak van der Waals forces and there is a large mismatch in elastic constants between the contact materials. Under dynamic loading, slippage, and decoupling of the 2D material is observed, which then leads to extensive damage propagation in the 2D lattice. Herein, graphene is functionalized through mild and controlled defect engineering for a fivefold increase in adhesion at the graphene-polymer interface. Adhesion is characterized experimentally using buckling-based metrology, while molecular dynamics simulations reveal the role of individual defects in the context of adhesion. Under in situ cyclic loading, the increased adhesion inhibits damage initiation and interfacial fatigue propagation within graphene. This work offers insight into achieving dynamically reliable and robust 2D material-polymer contacts, which can facilitate the development of 2D materials-based flexible devices.

Directionality of developing skeletal muscles is set by mechanical forces

Formation of oriented myofibrils is a key event in musculoskeletal development. However, the mechanisms that drive myocyte orientation and fusion to control muscle directionality in adults remain enigmatic. Here, we demonstrate that the developing skeleton instructs the directional outgrowth of skeletal muscle and other soft tissues during limb and facial morphogenesis in zebrafish and mouse. Time-lapse live imaging reveals that during early craniofacial development, myoblasts condense into round clusters corresponding to future muscle groups. These clusters undergo oriented stretch and alignment during embryonic growth. Genetic perturbation of cartilage patterning or size disrupts the directionality and number of myofibrils in vivo. Laser ablation of musculoskeletal attachment points reveals tension imposed by cartilage expansion on the forming myofibers. Application of continuous tension using artificial attachment points, or stretchable membrane substrates, is sufficient to drive polarization of myocyte populations in vitro. Overall, this work outlines a biomechanical guidance mechanism that is potentially useful for engineering functional skeletal muscle.

Soft Electromagnetic Artificial Muscles Using High‐Density Liquid‐Metal Solenoid Coils and Bistable Stretchable Magnetic Housings

AbstractSoft electromagnetic artificial muscles (SEAMs) that use electric currents are reported as their power sources. The proposed actuator consists of fully soft components: microfluidic coils, stretchable magnets, ferromagnetic silicone, and stretchable housings. The soft coils are fabricated by directly printing room‐temperature liquid metal on a stretchable substrate, enabling the generation of high‐density electromagnetic fields. Based on design optimization through modeling and simulation, the proposed actuators have a characteristic of bistability following the relationships of the forces acting on the components. Depending on the design configurations, the proposed actuators generate contraction and expansion motions as well as vibrations in a bidirectional manner, enabled by electromagnetic actuation. The main advantages of the proposed actuators are fully compliant structures, compact form factors, and short response times, which have not been observed in existing polymer‐based artificial muscles. Another advantage is the self‐detection of the actuation states by measuring the inductance change in the coils. Last, the modular design fully packaged with a coil and magnets in a soft housing makes it possible to easily resize and reconfigure the robotic systems with multiple actuator modules for different applications. Examples of applications demonstrated are a modular crawling robot, energy‐efficient grippers, a multi‐degrees of freedom (DOF) soft manipulator, and a high‐frequency swimming robot.

Stretchable and Robust Silver Nanowire Composites on Transparent Butyl Rubber

Silver nanowires (AgNWs) are leading materials for transparent conductive electrodes in optoelectronic devices. The solution processability of AgNWs combined with the optical transparency and electrical conductivity of solution-deposited AgNW networks makes them a promising alternative to conventional indium tin oxide. Furthermore, the ability of AgNW networks to withstand mechanical deformation means that AgNW networks may be the material of choice for next-generation flexible and stretchable optoelectronics. The drawback with the practical use of AgNWs, however, is that corrosion of AgNWs under ambient conditions damages the AgNW network and reduces the workable life span. Environmental degradation is especially problematic in stretchable electronics, which use elastomeric substrate materials that have a high gas permeability like poly(dimethylsiloxane) (PDMS). Covering individual AgNWs with a protective nanoscale coating to prevent corrosion complexifies their preparation and implementation in practical applications. This study presents an alternative approach in which the elastomeric substrate, transparent butyl rubber (TBR), acts as both a stretchable substrate and a protective gas barrier. TBR, an optically transparent formulation of poly(isobutylene-co-isoprene), is a synthetic rubber with an intrinsically low gas permeability. We demonstrate that AgNW networks embedded in a polyurethane adhesive and adhered to a TBR substrate provide high conductivity and optical transparency and are robust to tensile strain as well as extreme environments of humidified air, underwater storage, and corrosive acid vapors. We show that this system outperforms a comparison system that uses PDMS instead of TBR.

Stretching mode deformation analysis for an elastomeric encapsulation-assisted stable flexible electronic substrate

The substrate plays an important role in flexible devices and sensors. In this direction, it is observed that elastomeric encapsulation assists the sensor system to deform successfully under stretching. The encapsulation not only makes it flexible but also protects it from environmental factors and mechanical damage. In this paper, a finite element method analysis is used to study the mechanical effects on the encapsulated system, which provides insight into the design of a stretchable substrate for flexible electronic systems. Here, a serpentine silver electrode is designed on a polyethylene terephthalate substrate, which is then encapsulated by polydimethylsiloxane. With the variation in the ratio of top-to-bottom encapsulation thickness i.e. T en1: T en2, the interfacial stress was studied. The mismatch in T en1 and T en2 may result in compressive bending strain, which can be avoided by making T en1 = T en2. It is observed from the simulation that, there is a spike in von-Mises stress at the interface of the substrate and the encapsulation when stretching mode deformation is applied. Also, this maximum stress varies with the variation in encapsulation thickness. For a range of total encapsulation thickness i.e. T EN = T en1 + T en2 = 30 μm to 100 μm, the optimum thickness is found to be 55 μm, for which the spike in interfacial von-Mises stress is minimum.

Publisher Correction to: Ultrasensitive, Highly Selective, Integrated Multidimensional Sensor Based on a Rigid-Flexible Synergistic Stretchable Substrate

Publisher Correction to: Ultrasensitive, Highly Selective, Integrated Multidimensional Sensor Based on a Rigid-Flexible Synergistic Stretchable Substrate

Dynamic adjustable metalens based on a stretchable substrate with a double-layer metal microstructure.

Based on the impedance-matching theory, a double-layer metal structure dynamical focusing cylindrical metalens with a stretchable substrate was designed at the operation frequency of 0.1THz. The diameter, initial focal length, and NA of the metalens were 80mm, 40mm, and 0.7, respectively. The transmission phase of the unit cell structures could cover 0-2π by changing the size of the metal bars, and then the different unit cells were spatially arranged as the designed phase profile for the metalens. When the stretching range of the substrate was about 100%-140%, the focal length changed from 39.3mm to 85.5mm, the dynamic focusing range was about 117.6% of the minimum focal length, and the focusing efficiency decreases from 49.2% to 27.9%. Then, by rearranging the unit cell structures, a dynamically adjustable bifocal metalens was numerically realized. Using the same stretching ratio, compared to a single focus metalens, the bifocal metalens can provide a larger focal length control range.

Ultrasensitive, Highly Selective, Integrated Multidimensional Sensor Based on a Rigid-Flexible Synergistic Stretchable Substrate

Ultrasensitive, Highly Selective, Integrated Multidimensional Sensor Based on a Rigid-Flexible Synergistic Stretchable Substrate

Interfacial Click Chemistry Enabled Strong Adhesion toward Ultra‐Durable Crack‐Based Flexible Strain Sensors

AbstractFlexible strain sensors, particularly crack‐based ones, have evoked great interests in recent years because of their excellent sensitivity and promising applications in health monitoring and human‐machine interactions. However, the poor durability originated from the weak interfacial bonding between fragile conductive layer and stretchable elastic substrate makes it difficult to meet the needs of practical use. Herein, a strategy enabled by interfacial click chemistry is proposed for effectively enhancing the adhesion strength (τ), thereby improving the durability of the flexible strain sensors. It is found that the strong interfacial adhesion can bear larger cyclic shear force, effectively suppress the crack propagation of the conductive layer, and prevent the conductive layer from peeling off, thus stabilize the conductive path under specific strain during cyclic test, resulting in excellent durability. Moreover, the device durability (number of stable cycles, i.e., Cn) is found to linearly depend on τ, based on which “durability‐adhesion coefficient (CDA)” is defined. So far it is known, this is the first report to quantitatively establish the relationship between interfacial adhesion strength and cycling durability of crack‐based strain sensors, providing a new viewpoint for future design of flexible devices with high reliability.

Experimental investigation on the change of pull-off force between bulk particulate material and an elastic polymeric filter fiber

HypothesisAdhesion between particles and a filter fiber is an important process of the filtration as it dictates the process of separation and in the following the detachment process of particles during filter regeneration. In addition to the shear stress that a new polymeric stretchable filter fiber implements into the particulate structure, the elongation of the substrate (fiber) is also expected to cause a structural change in the surface of the polymer. Thus, the changed contact area and surface energy could affect the adhesion force between particles and fibers. ExperimentsSystematic measurements of adhesion forces between a single particle and the stretchable substrate were performed using Atomic Force Microscope (AFM). The substrate surface characteristics (roughness) was changed directly beneath the modified measurement head using piezo-motors to achieve stepless elongation state. Polystyrene particles and particles made of Spheriglass were applied. FindingsIn the experiments, a reduced adhesion force between the particles and the filter fiber was found for a new high range of substrate roughness and peak-to-peak distance, in which the Rabinovich model has not been used before [1]. Further, the influence of high and low energy surface particulate material was evaluated to understand the detachment process in the new real adaptive filter and in DEM-simulation.

Tunable ultraviolet polarized light switch based on all-dielectric metasurfaces on a stretchable substrate.

Most ultraviolet (UV) passive optics are currently non-tunable and lack external modulation methods because of the poor tunability of wide-bandgap semiconductor materials in UV working media. This study investigates the excitation of magnetic dipole resonances in the solar-blind UV region by hafnium oxide metasurfaces using elastic dielectric polydimethylsiloxane (PDMS). The near-field interactions between the resonant dielectric elements can be modulated by the mechanical strain of the PDMS substrate, which can flatten the structure's resonant peak beyond the solar-blind UV wavelength range, thereby turning on or off the optical switch in the solar-blind UV region. The device has a facile design and can be used in various applications, such as UV polarization modulation, optical communications, and spectroscopy.

Nanotransfer-on-Things: From Rigid to Stretchable Nanophotonic Devices

The growing demand for nanophotonic devices has driven the advancement of nanotransfer printing (nTP) technology. Currently, the scope of nTP is limited to certain materials and substrates owing to the temperature, pressure, and chemical bonding requirements. In this study, we developed a universal nTP technique utilizing covalent bonding-based adhesives to improve the adhesion between the target material and substrate. Additionally, the technique employed plasma-based selective etching to weaken the adhesion between the mold and target material, thereby enabling the reliable modulation of the relative adhesion forces, regardless of the material or substrate. The technique was evaluated by printing four optical materials on nine substrates, including rigid, flexible, and stretchable substrates. Finally, its applicability was demonstrated by fabricating a ring hologram, a flexible plasmonic color filter, and extraordinary optical transmission-based strain sensors. The high accuracy and reliability of the proposed nTP method were verified by the performance of nanophotonic devices that closely matched numerical simulation results.

Low-voltage polymer transistors on hydrophobic dielectrics and surfaces

A set of unique features, including large-area solution processing on flexible and stretchable substrates, make polymer semiconductors a promising material choice for a range of state-of-the-art applications in electronics, optoelectronics and sensing. Yet, an inherent weakness of polymer semiconductors remains their low dielectric constants, increasing their susceptibility toward unscreened dipoles. These dipoles are particularly prevalent at polymer-dielectric interfaces with high-k dielectrics, which are essential for the operation of devices such as low-voltage field-effect transistors. This shortcoming can be addressed by using self-assembled monolayers (SAMs) to passivate surfaces that impact charge transport. However, SAM-treatment also increases the hydrophobicity of surfaces and therefore poses a challenge for subsequent solution processing steps and complex packaging of devices. Here, we report low-voltage polymer transistors processed by spin coating of the polymer semiconductors on highly hydrophobic SAM-treated aluminum and hafnium oxide dielectrics (contact angles >100) through fine-tuning of the interfacial tension at the polymer-dielectric interface. This approach enables the processing and detailed characterization of near-amorphous (indacenodithiophene-cobenzothiadiazole) as well as semicrystalline (poly(2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)diketopyrrolo[3,4-c]pyrrole-1,4-dione-alt-thieno[3,2-b]thiophen)) polymer semiconductors. We demonstrate polymer transistors that exhibit high on-currents and field-independent, charge carrier mobilities of 0.8 cm2 V−1s−1 at low operating voltages (<3 V).