Stretchable Substrate Research Articles

Textile-based wearable solid-contact flexible fluoride sensor: Toward biodetection of G-type nerve agents

Rising global concerns posed by chemical and biological threat agents highlight the critical need to develop reliable strategies for the real-time detection of such threats. While wearable sensing technology is well suited to fulfill this task, the use of on-body devices for rapid and selective field identification of chemical agents is relatively a new area. This work describes a flexible printed textile-based solid-contact potentiometric sensor for the selective detection of fluoride anions liberated by the biocatalytic hydrolysis of fluorine-containing G-type nerve agents (such as sarin or soman). The newly developed solid-contact textile fluoride sensor relies on a fluoride-selective bis(fluorodioctylstannyl)methane ionophore to provide attractive analytical performance with near-Nernstian sensitivity and effective discrimination against common anions, along with excellent reversibility and repeatability for dynamically changing fluoride concentrations. By using stress-enduring printed inks and serpentine structures along with stretchable textile substrates, the resulting textile-based fluoride sensor exhibits robust mechanical resiliency under severe mechanical strains. Such realization of an effective textile-based fluoride-selective electrode allowed biosensing of the nerve-agent simulant diisopropyl fluorophosphate (DFP), in connection to immobilized organophosphorus acid anhydrolylase (OPAA) or organophosphorus hydrolase (OPH) enzymes. A user-friendly portable electronic module transmits data from the new textile-based potentiometric biosensor wirelessly to a nearby smartphone for alerting the wearer instantaneously about potential chemical threats. While expanding the scope of wearable solid-contact anion sensors, such a textile-based potentiometric fluoride electrode transducer offers particular promise for effective discrimination of G-type neurotoxins from organophosphate (OP) pesticides, toward specific field detection of these agents in diverse defense settings.

Highly Sensitive and Durable Sea-Urchin-Shaped Silver Nanoparticles Strain Sensors for Human-Activity Monitoring.

High-performance strain sensors, composed of various artificial sensing materials on/in stretchable substrates, show great promise for applications in flexible electronic devices. Here, we demonstrated a highly sensitive and durable strain sensor consisting of a ribbon of close-packed sea-urchin-shaped silver nanoparticles (SUSNs) sandwiched between two layers of poly(dimethylsiloxane) (PDMS). Each of SUSNs possesses high-density and spherically distributed sharp spines over the body, which promotes electron transduction and further improves signal detection. This SUSN-based sensor possesses a desirable integration of high sensitivity (a gauge factor of 60) and large stretchability (up to 25%) at tensile sensing, broadening its application in wearable devices. Moreover, it also shows fast response (48 ms), good reproducibility, and long-term stability (>2500 cycles at 20% strain). It can also be used to detect compressing (sensitivity up to 31.5) and folding-type bending deformations. The sensing mechanism, the resistance of the sensors varying as the deformation load, results from the inter-spine contacts change and the microcracks evolution caused by variation in the gap between SUSNs. The sensor's sensitivity at different degrees of strain was also achieved by controlling the width of the close-packed SUSNs ribbon. For practical demonstration, the SUSN-based sensors could be used as wearable devices for monitoring human activities ranging from subtle deformations to substantial movements.

Green Natural Rubber Composites Reinforced with Black/White Rice Husk Ashes: Effects of Reinforcing Agent on Film's Mechanical and Dielectric Properties.

Green natural rubber (NR) composites reinforced with black rice husk ash (BRHA)/white rice husk ash (WRHA), using alginate as a thickening and dispersing agent and crosslinking by CaCl2, was developed to improve mechanical, chemical and dielectric properties of NR-based films by using a latex aqueous microdispersion process. A maximum of 100 per hundred rubbers (phr) of rice husk ashes (RHAs) could be integrated in NR matrix without phase separation. Mechanical properties of the composite films were considerably enhanced, compared to the neat NR film. The composite films reinforced with WRHA demonstrated relatively better mechanical properties than those reinforced with BRHA, whereas the composites filled with BRHA demonstrated higher elongation at break. The crosslinking by CaCl2 improved the film tensile strength but lowered the film elasticity. The reinforcement strongly improved chemical resistance of the composite films in toluene. The films are biodegradable in soil, with weight loss of 7.6–18.3% of the initial dry weight after 3 months. Dielectric constant and dielectric loss factors of the composite films were enhanced with RHAs loading. According to the obtained properties, the composites offer potential for further development as stretchable conductive substrate or semiconducting polymer films for electronic applications.

The Enhancement of Visible Absorption of Cu Doped ZnO Nanorods on Patterned Substrates

: We report the antireflection and light absorption in visible region by new stretchable substrates with patterned structure. Mogul substrates with 3-Dimentional structures were fabricated by using polydimethyl – siloxane that imitate the nanostructures surface. Then, Copper doped ZnO NRs on mogul-patterned surface by hydrothermal method at low temperature. The optical properties, morphology and structures of ZnO:Cu NRs were investigated through out of measurement the scanning electron microscopy, X-Ray diffraction and ultraviolet-visible spectroscopy, respectively. The results show the Cu doped ZnO NRs were uniformly and dense grown on mogul substrates, well oriented in the (002) plane. Additionally, the light absorption can be significantly enhanced to more 10% in a wide spectral range (400-800 nm) due to the reduce reflection. Growing ZnO NRs doping on new stretchable substrates with a mogul-patterned surface were successfully fabricated and applicable in the flexible and stretchable optoelectronic devices.

Effect of Viscosity on the Formation of Porous Polydimethylsiloxane for Wearable Device Applications.

Medical devices, which enhance the quality of life, have experienced a gradual increase in demand. Various research groups have attempted to incorporate soft materials such as skin into wearable devices. We developed a stretchable substrate with high elasticity by forming a porous structure on polydimethylsiloxane (PDMS). To optimize the porous structure, we propose a manufacturing process that utilizes a high-pressure steam with different viscosities (400, 800, 2100, and 3000 cP) of an uncured PDMS solution. The proposed method simplifies the manufacturing of porous structures and is cost-effective compared to other technologies. Porous structures of various viscosities were formed, and their electrical and mechanical properties evaluated. Porous PDMS (3000 cP) was formed in a sponge-like three-dimensional porous structure, compared to PDMS formed by other viscosities. The elongation of porous PDMS (3000 cP) was increased by up to 30%, and the relative resistance changed to less than 1000 times with the maximum strain test. The relative resistance increased the initial resistance (R0) by approximately 10 times during the 1500-times repeated cycling tests with 30% strain. As a result, patch-type wearable devices based on soft materials can provide an innovative platform that can connect with the human skin for robotics applications and for continuous health monitoring.

Rolling reliability of polyurethane and polyurethane-acrylic ICAs interconnections on printed stretchable electronics

In this study, bending reliability of the surface mounted devices (SMD) attached using isotropic conductive adhesive (ICAs) on screen printed stretchable devices polyurethane substrate are investigated. The performance of polyurethane and polyurethane-acrylic ICAs and the impact of rolling speed are studied. The rolling test was performed using the customized automatic rolling test device. It was found that the polyurethane-acrylic blend gives poorer adhesion with the silver conductor and very high cohesive force. The polyurethane resin brings adhesive high adhesive force with the silver conductive layer while lower adhesion with the Tin surface of the SMD, and it exhibited poorer cohesive force. The resistance of all samples experienced a steady and slow increase period before it accelerated to failure. During the slow and steady increase phase, the increased resistance was mainly from the conductive layer, whereas during the dramatic increase phase, it was mainly from the ICA connected area. The increased rolling speed had accelerated the resistance evolution process and lowered the samples' reliability severely. It also lowered the adhesion of the polyurethane-acrylic blend with the ink layer and lowered cohesive force of polyurethane ICA.

Effect of interfacial stiffness on the stretchability of metal/elastomer bilayers under in-plane biaxial tension

Flexible electronic devices are often subjected to large and repeated deformation, so that their functional components such as metal interconnects need to sustain strains up to tens of percent, which is far beyond the intrinsic deformability of metal materials (~1%). To meet the stringent requirements of flexible electronics, metal/elastomer bilayers, a stretchable structure that consists of a metal film adhered to a stretchable elastomer substrate, have been developed to improve the stretch capability of metal interconnects. Previous studies have predicted that the metal/elastomer bilayers are much more stretchable than freestanding metal films. However, these investigations usually assume perfect bonding between the metal and elastomer layers. In this work, the effect of the metal/elastomer interface with a finite interfacial stiffness on the stretchability of bilayer structures is analyzed. The results show that the assumption of perfect interface (with infinite interfacial stiffness) may lead to an overestimation of the stretchability of bilayer structures. It is also demonstrated that increased adhesion between the metal and elastomer layers can enhance the stretchability of the metal layer.

Stretchable ITO‐Free Organic Solar Cells with Intrinsic Anti‐Reflection Substrate for High‐Efficiency Outdoor and Indoor Energy Harvesting

AbstractFlexible photovoltaic devices are promising candidates for triggering the Internet of Things (IoT). However, the power conversion efficiencies (PCEs) of flexible organic photovoltaic (OPV) devices with high conductivity poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes on plastic are lagging behind the rigid devices due to the low transmittance of polyethylene terephthalate (PET)/PEDOT:PSS. Moreover, the poor stretchability of the commonly used plastic substrates largely hinders the practical application of wearable devices. Herein, a novel stretchable indium tin oxide (ITO)‐free OPV device with a surface‐texturing polydimethylsiloxane (PDMS) substrate for outdoor strong‐ and indoor dim‐light energy harvesting is reported. The high diffuse transmittance and haze effect of the substrate enable stretchable ITO‐free devices, yielding a high PCE of 15.3% under 1 sun illumination. More excitingly, the stretchable device based on textured PDMS/PEDOT:PSS maintains a comparable PCE of 20.5% (20.8% for the rigid device) under indoor light illumination. Notably, the stretchable device is much more insensitive to the light direction, maintaining 38.5% of the initial PCE at an extremely small incident angle of 10° (16.3% for glass/ITO‐based counterpart). The texturing stretchable substrate provides a new direction for achieving high performance and enhanced light utilization for the stretchable light‐harvesting device, suitable for indoor and outdoor applications.

Fully‐Printed Piezoelectric Devices for Flexible Electronics Applications

AbstractRecent advances in materials and manufacturing processes pave the way for the establishment of piezoelectric materials via printing techniques as flexible sensors, actuators, and generators. Such flexible devices are key building blocks for future advanced robotic skin and conformable medical devices. Herein special focus is given to printed devices for its lightweight, flexibility, and manufacturing by high throughput techniques, offering a disruptive advantage in integration technologies and a wide range of opportunities for industrialization routes, where cost‐effective applications are required. In this Progress Report, the different system parameters are discussed, pinpointing the ones that affect the production of reliable flexible printed piezoelectric devices and limit this technology to achieve higher technological maturity. Focus is made on screen and inkjet printing as fabrication techniques and the well‐established piezoelectric polymer poly(vinylidene fluoride‐co‐trifluoroethylene). Key limiting factors found for the manufacturing of robust scalable all‐printed piezoelectric devices stems mainly from the piezoelectric ink production and processing. Finally, the integration of these materials, via printing technologies, into soft, flexible, and even stretchable substrates is analyzed and insights are gathered on the manufacturing trends to achieve low‐cost production of flexible piezoelectric devices embedded in electronic skin and smart wearables.

Selective Laser Pyrolytic Micropatterning of Stretched Elastomeric Polymer Surfaces

The patterning of surface structures is a critical step for interfacial engineering applications that are based on soft lithography, casting, or imprinting techniques in general. However, photolithography and mold/die fabrication processes are expensive and time-consuming. To solve this problem, we developed a novel laser-based process that exploits the high stretchability of a polymer. In this study, continuous-wave laser-induced pyrolysis was conducted on a pre-stretched Ecoflex polymer substrate to obtain a controllable hydrophobic surface with functional structures at the micro/nanoscale, which were constructed via the facile removal of pyrolytic byproducts. Under the optimum strain rate and laser parameters, periodic wrinkle generation and a proportional channel pitch reduction were observed, which provided hierarchical micro/nanostructures and enlarged the air gaps between the target droplet and the hydrophobic structures. The processed surface exhibited improved hydrophobic properties (WCA ~ 135°) compared with a flat Ecoflex polymer surface (WCA ~ 105°). Thus, a facile manufacturing method for stretchable polymer substrates with hydrophobic patterned surfaces is proposed.

Elastomer coils for wearable MR detection

To explore the use of conductive elastomer for MR signal detection and the utility of this approach for wearable detector arrays. An elastomer filled with silver microparticles was used to form stretchable radiofrequency coils for MR detection. Their electrical performance in terms of the Qunloaded and Q ratio was assessed in the relaxed state and under repeated strain up to 40%. In a phantom imaging study, the signal-to-noise ratio yield of conductive elastomer coils was compared with that of a reference copper coil. Four elastomer coils were integrated with a stretchable textile substrate to form a wearable array for knee imaging. The array was employed for multiple-angle and kinematic knee imaging in vivo. The elastomer coils proved highly stretchable and mechanically robust. Upon repeated stretching by 20%, a medium-sized coil element settled at Qunloaded of 42 in the relaxed state and 32 at full strain, reflecting sample-noise dominance. The signal-to-noise ratio of elastomer coils was found to be 8% to 16% lower than that achieved with a conventional copper coil. Multiple-angle and kinematic knee imaging with the wearable array yielded high-quality results indicating robustness of detection performance against stretching and warping of the array. Conductive elastomer is a viable material for MR detection. Coils made from this material reconcile high stretchability and adequate electrical performance with ease of manufacturing. Conductive elastomer also offers inherent restoring forces and is readily washable and sanitizable, making it an excellent basis of wearable detector front ends.

Hydrogen-doped viscoplastic liquid metal microparticles for stretchable printed metal lines.

Conductive and stretchable electrodes that can be printed directly on a stretchable substrate have drawn extensive attention for wearable electronics and electronic skins. Printable inks that contain liquid metal are strong candidates for these applications, but the insulating oxide skin that forms around the liquid metal particles limits their conductivity. This study reveals that hydrogen doping introduced by ultrasonication in the presence of aliphatic polymers makes the oxide skin highly conductive and deformable. X-ray photoelectron spectroscopy and atom probe tomography confirmed the hydrogen doping, and first-principles calculations were used to rationalize the obtained conductivity. The printed circuit lines show a metallic conductivity (25,000 S cm-1), excellent electromechanical decoupling at a 500% uniaxial stretching, mechanical resistance to scratches and long-term stability in wide ranges of temperature and humidity. The self-passivation of the printed lines allows the direct printing of three-dimensional circuit lines and double-layer planar coils that are used as stretchable inductive strain sensors.

Carbon nanofiber-reinforced strain sensors with high breathability and anisotropic sensitivity

In this work, we present a new method of creating fibre-metal composites to effectively modulate the in-plane fracture behaviour of brittle conductive thin metal films on stretchable PDMS substrates via insertion of a toughening interlayer of CNFs.

Inkjet Printing of PEDOT:PSS Based Conductive Patterns for 3D Forming Applications.

This paper presents the formulation, inkjet printing, and vacuum forming of a conductive and stretchable polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), ink on a stretchable and transparent thermoplastic polyurethane (TPU) substrate. The formulation of the conductive and stretchable ink is achieved by combining PEDOT:PSS with additional solvents, to achieve the right inkjet properties for drop-on-demand (DoD) inkjet printing. A conductive pattern can be printed from the 21 µm orifice on a flexible and stretchable TPU substrate, with a linewidth down to 44 µm. The properties of the printed pattern, in terms of sheet resistance, morphology, transparency, impact of weather conditions, and stretching are investigated and show sheet resistances up to 45 Ohm/sq and transparencies as high as 95%, which is comparable to indium tin oxide (ITO). Moreover, in contrast to ITO, one-time stretching up to 40% can be achieved, increasing the sheet resistance up to 214 Ohm/sq only, showing the great potential of this ink for one-time stretching. Finally, as a proof of this one-time stretching, the printed samples are vacuum formed around a 3D object, still showing sufficient conductivity to be applied as a capacitive touch sensor.

Performance and Durability of Thread Antennas as Stretchable Epidermal UHF RFID Tags

Epidermal sensors based on battery-less Radiofrequency Identification (RFID) aim at collecting biophysical parameters with a high level of comfort for the user. This paper investigates the performance and durability of epidermal RFID tags, equipped with a self-tuning RFID IC, that are based either on copper wires or conductive yarns. The tags are deployed onto an ultra-thin stretchable and transparent substrate to achieve comformability to body discontinuities. A statistical analysis on volunteers showed that, in the whole UHF band (860-960 MHz), reliable read ranges of 1 m are easily achieved while up to 2 m can be reached in some favorable configurations. Both tags withstand wear, mechanical stress due to the movements of the body, sweating, and water. In particular, the tag made of conductive yarn lasts for more than 20 days. This new family of epidermal tags is moreover suitable for low-cost and large-scale manufacturing through the widely available machines used for wire-laying, bending, and shaping.

Highly Strained Silicon on Polymer Obtained By Temporary Polymer Wafer Bonding

Strain engineering can be called upon to tune the band structures of semiconductors. It is widely used in different types of microelectronics devices and it is expected to enable unprecedented responses in optoelectronics [1]. In the case of single crystal Silicon (sc-Si), several approaches have been proposed, relying either on the lattice mismatch inherent to hetero-epitaxial growth (Si on relaxed SiGe) or on substrate expansion (porous Si) [2, 3]. However, these methods are limited to bi-axial strain and by the maximum strain achievable in sc-Si thin film. We recently evaluated a method enabling to transfer a 200nm thick sc-Si film on a stretchable polymer substrate. Tensile tests proved that applying an external mechanical load on the substrate injected significant amounts of strain into sc-Si [4]. This work is focused on studying different parameters to improve the maximum strain in a Silicon On Polymer (SOP) stack obtained from a 200mm Silicon On Insulator (SOI) wafer using temporary polymer bonding.The overall fabrication process is described in figure 1(a). 2 × 24mm2 rectangles are defined using photolithography in a SOI thin film. The wafer is temporary bonded to a handling silicon substrate with a 40µm thick glue layer. Then, the backside of the SOI is removed with mechanical grinding and selective chemical etching. Finally, the sc-Si thin film is transferred to a 230µm thick flexible substrate thanks to a release layer between the glue and the sc-Si layer. The substrate is cut into tensile test samples as described in figure 1(a).In a previous study we have shown that a 200nm thick sc-Si film could be uniaxialy strained up to 1.5% along the [110] direction [4]. In the present work, we quantify the impact of strain orientation, sc-Si film thickness and pattern edge roughness on maximum uniaxial strain. In order to change the sc-Si film thickness a dry etching step was used after removing the backside of the SOI ubstrate. A soft lithography mask and a standard chrome 1X lithography mask were compared for the fabrication of rectangles. To achieve even smoother pattern edges, TetraMethylAmmonium Hydroxide (TMAH) etching was also used (for 90 minutes in a 25 %wt solution at 80◦C) on a patterned SOI with a silicon oxide layer as a hard mask. Resulting samples were tested on a uniaxial tensile stage coupled to a Raman spectrometer, giving us access simultaneously to the sample macroscopic strain and the silicon local strain.Tensile test results are summarized in figure 1(b). Reducing the thickness of the sc-Si did not improve the strain limit while better edges definitions did. Indeed, using TMAH, uniaxial strains up to 3% along [100] were reached in a 200nm thick sc-Si film on a flexible polymer substrate. These results are highly encouraging for the development of films with a wide range of strain. Furthermore, this method is easily transferable to other materials such as AlN or Ge.

A Stretchable Lithium-Ion Battery Using Liquid Metal Nano Particle

Introduction Stretchable devices have been highly demanded because of wearable devices in the IoT society. Stretchable lithium ion batteries are crucial to growth of IoT society, since the current developed devices possesses only interface part. The batteries and devices require higher stretchability and robustness to deformation. Liquid metal (LM) is one of the prominent materials for stretchable sensors(1) and wiring circuit(2). In addition, LMs have self-healing characteristics(3) and high capacity (Ga:769 mAh/g, Sn:990 mAh/g) in lithium ion batteries. Here, we investigated the electrochemical characteristics and improvement of robustness using liquid metal nano particles (LMNP) anode in stretchable lithium ion batteries. In this study, stretchable 3D porous LMNP electrodes are fabricated and assembled into stretchable batteries. The stretchable batteries consist of stretchable LMNP and LiFePO4 (LFP) electrodes based on 3D porous polydimethylsiloxane (PDMS)/Ecoflex/Super P carbon composites, stretchable separator, [Li(G4)][TFSI] electrolyte and stretchable package. The batteries have high stretchability and robustness to deformation. Experiment 3D porous PDMS/Ecoflex/Super P carbon composites were used for stretchable electrode substrates. The porous structure was prepared by sacrifice-template method using sugar(4). 3D porous PDMS/Ecoflex/Super P carbon composites was composed of PDMS, Ecoflex with the weight ratio of 6:4 and coating Super P carbon on the surface. LMNP was prepared by ultra-sonication GaSn (Ga:Sn=88:12) alloy. The LMNP alloy was used as active materials of anode. The anode slurries were prepared by mixing the LMNP, Super P carbon, poly(vinylidene fluoride) (PVDF) in N-methyl-2-pyrrolidione (NMP) solvent. LFP was used as active materials of cathode. The cathode slurries consisted of LFP, Super P carbon, PVDF. Stretchable electrodes were fabricated by drop cast method with the electrode slurries on the 3D porous substrates. [Li(G4)][TFSI] was composed of LiTFTI and Tetraethylene glycol dimethyl ether (G4). Stretchable batteries were assembled using the PDMS/Ecoflex/Super P carbon/LMNP anode, PDMS/Ecoflex/Super P carbon/LFP cathode, stretchable separator and stretchable packaging. The nickel/aluminum tabs are attached to PDMS/Ecoflex/Super P carbon/LMNP and PDMS/Ecoflex/Super P carbon/LFP electrode by using small amount of epoxy resin adhesive(Fig. 1). Results and Discussion To investigate the electrochemical characteristics of the stretchable electrode on lithium ion batteries, we respectively conducted galvanostatic tests of the stretchable LMNP anode and the stretchable LFP cathode in half-cell configurations paired with lithium metal as counter and reference electrode. The results show the electrodes can charge/discharge on lithium ion batteries at 1 C. In addition, the electrochemical performance of the stretchable LMNP electrode was investigated. The charge/discharge curves were measured after 100% strain-release cycle 50 and 100 times. The results of the stretchable LMNP electrode showed the capacity retention was higher than the stretchable Si NP (silicone nano particle) electrode. The LMNP used as active materials improve robustness to stretching were confirmed. The full cells were tested using PDMS/Ecoflex/Super P carbon/LMNP and PDMS/Ecoflex/Super P carbon/LFP electrode with charge/discharge measurement. As a result, the full battery could work. Acknowledgement This work was supported by JST CREST Grant Number JPMJCR1905, Japan, by the Japan Science and Technology Agency, PRESTO Grant Number JPMJPR18J2, and by JSPS Grant-in-Aid for Challenging Exploratory Research. References (1) Yuji Gao et al, Advanced. Materials , 2017, 1701985(2) Guangyong Li et al, Sensors and Actuators B , 2015, 221, 1114-1119(3) Yingpeng Wui et al, Energy & Environmental Science , 2017, 10, 1854-1861(4) Hongsen Li et al, Advanced. Materials , 2017, 1700898 Image Figure 1 Self-healing behavior of LMNP (left). Schematic diagram of stretchable battery (right). Figure 1

Vertically Aligned 2D MoS2 Layers with Strain-Engineered Serpentine Patterns for High-Performance Stretchable Gas Sensors: Experimental and Theoretical Demonstration.

Two-dimensional (2D) molybdenum disulfide (MoS2) with vertically aligned (VA) layers exhibits significantly enriched surface-exposed edge sites with an abundance of dangling bonds owing to its intrinsic crystallographic anisotropy. Such structural variation renders the material with exceptionally high chemical reactivity and chemisorption ability, making it particularly attractive for high-performance electrochemical sensing. This superior property can be further promoted as far as it is integrated on mechanically stretchable substrates well retaining its surface-exposed defective edges, projecting opportunities for a wide range of applications utilizing its structural uniqueness and mechanical flexibility. In this work, we explored VA-2D MoS2 layers configured in laterally stretchable forms for multifunctional nitrogen dioxide (NO2) gas sensors. Large-area (>cm2) VA-2D MoS2 layers grown by a chemical vapor deposition (CVD) method were directly integrated onto a variety of flexible substrates with serpentine patterns judiciously designed to accommodate a large degree of tensile strain. These uniquely structured VA-2D MoS2 layers were demonstrated to be highly sensitive to NO2 gas of controlled concentration preserving their intrinsic structural and chemical integrity, e.g., significant current response ratios of ∼160-380% upon the introduction of NO2 at a level of 5-30 ppm. Remarkably, they exhibited such a high sensitivity even under lateral stretching up to 40% strain, significantly outperforming previously reported 2D MoS2 layer-based NO2 gas sensors of any structural forms. Underlying principles for the experimentally observed superiority were theoretically unveiled by density functional theory (DFT) calculation and finite element method (FEM) analysis. The intrinsic high sensitivity and large stretchability of VA-2D MoS2 layers confirmed in this study are believed to be applicable in sensing diverse gas species, greatly broadening their versatility in stretchable and wearable technologies.

Silver nanowire-network-film-coated soft substrates with wrinkled surfaces for use as stretchable surface enhanced Raman scattering sensors

We demonstrate silver nanowire -network-film (AgNWNF)-coated soft polydimethyl siloxane (PDMS) substrates with wrinkled surfaces for use as stretchable SERS sensors. The AgNWNF-coated stretchable substrates showed strong SERS effects compared to those of Ag film-deposited substrates, which were likely attributed to the electromagnetic field enhancement induced by the relatively high density of hot spots in the AgNWNF. To investigate the stability of the stretchable SERS sensors, we examined the adhesion strength of AgNWNFs on the ultraviolet-ozone (UVO)-treated PDMS substrates, which was much stronger than the adhesion strength of AgNWNFs on the PDMS without prior UVO treatment. In addition, the strain-dependent SERS activity of the AgNWNF-coated stretchable sensor showed the detection of 10−7 M Rhodamine 6G (R6G) molecules despite the decreased Raman intensity under strain increased to 100%, resulting from the surface plasmon resonance tuning by the modulation of the active gap distance between neighboring AgNWs through the application of strain. This was further supported by finite-difference time domain (FDTD) numerical evaluation.

Development of a full roll-to-roll manufacturing process of through-substrate vias with stretchable substrates enabling double-sided wearable electronics

In the recent years, there has been a growing interest towards printed stretchable electronics used in diagnostics, health-monitoring, and wearable applications. Double-sided electronic circuits with through-substrate vias offer a solution where the amount of printed circuitry and assembled SMDs (surface-mount devices) in direct contact with the human skin can be minimized. This improves not only the wearability and cost-effectiveness of the printed electronic devices but also the product safety and comfort to wear. Another factor decreasing the unit costs in printed electronics is the use of high volume, high speed, and continuous roll-to-roll (R2R) manufacturing processes. In this current paper, a full R2R process for the manufacturing of through-substrate vias on stretchable thermoplastic polyurethane (TPU) substrate was developed and verified. The through-substrate via-holes were manufactured in R2R using either laser-cutting or die-cutting. Rotary screen printing was used to print conductive tracks onto both sides of the stretchable substrate and to fill the via-holes. Eventually, conductive and stretchable through-substrate vias with low sheet resistance and low resistance deviation were reliably achieved with the developed process.