Ultrasensitive and Stretchable Conductive Fibers Using Percolated Pd Nanoparticle Networks for Multisensing Wearable Electronics: Crack-Based Strain and H2 Sensors.

The need for wearable electronic devices continues to grow, and the research is under way for stretchable fiber-type sensors that are sensitive to the surrounding atmosphere and will provide proficient measurement capabilities. Currently, one-dimensional fiber sensors have several limitations for their extensive use because of the complex structures of the sensing mechanisms. Thus, it is essential to miniaturize these materials with durability while integrating multiple sensing capabilities. Herein, we present an ultrasensitive and stretchable conductive fiber sensor using PdNP networks embedded in elastomeric polymers for crack-based strain and H2 sensing. The fiber multimodal sensors show a gauge factor of ∼2040 under 70% strain and reliable mechanical deformation tolerance (10,000 stretching cycles) in the strain-sensor mode. For H2 sensing, the fiber multimodal sensors exhibit a wide sensing range of high sensitivity: -0.43% response at 5 ppm (0.0005%) H2 gas and -27.3% response at 10% H2 gas. For the first time, we demonstrate highly stretchable H2 sensors that can detect H2 gas under 110% strain with mechanical durability. As demonstrated, their stable performance allows them to be used in wearable applications that integrate fiber multimodal sensors into industrial safety clothing along with a microinorganic light-emitting diode for visual indication, which exhibits proper activation upon H2 gas exposure.

Nonfluorinated Superomniphobic Surfaces through Shape-Tunable Mushroom-like Polymeric Micropillar Arrays.

Superomniphobic surfaces showing extremely liquid-repellent properties have received a great amount of attention as they can be used in various industrial and biomedical applications. However, so far, the fabrication processes of these materials mostly have involved the coating of perfluorocarbons onto micro- and nanohierarchical structures of these surfaces, which inevitably causes environmental pollution, leading to health concerns. Herein, we developed a facile method to obtain flexible superomniphobic surfaces without perfluorocarbon coatings that have shape-tunable mushroom-like micropillars (MPs). Inspired by the unique structures on the skin of springtails, we fabricated mushroom-like structures with downward facing edges (i.e., a doubly re-entrant structure) on a surface. The flexible MP structures were fabricated using a conventional micromolding technique, and the shapes of the mushroom caps were made highly tunable via the deposition of a thin aluminum (Al) layer. Due to the compressive residual stress of the Al, the mushroom caps were observed to bend toward the polymer upon forming doubly re-entrant-MP structures. The obtained surface was found to repel most low-surface-tension liquids such as oils, alcohols, and even fluorinated solvents. The developed flexible superomniphobic surface showed liquid repellency even upon mechanical stretching and after surface energy modification. We envision that the developed superomniphobic surface with high flexibility and wetting resistance after surface energy modification will be used in a wide range of applications such as self-cleaning clothes and gloves.

Highly Conductive Fiber with Waterproof and Self-Cleaning Properties for Textile Electronics.

Major concerns in the development of wearable textile electronics are exposure to moisture and contamination. The exposure can cause electrical breakdown of the device and its interconnections, and thus continuous efforts have been made to fabricate textile electronics which are free from moisture and pollution. Herein, we developed a highly conductive and waterproof fiber with excellent electrical conductivity (0.11 Ω/cm) and mechanical stability for advanced interconnector components in wearable textile electronics. The fabrication process of the highly conductive fiber involves coating of a commercial Kevlar fiber with Ag nanoparticle-poly(styrene- block-butadiene- block-styrene) polymer composites. The fabricated fiber then gets treated with self-assembled monolayer (SAM)-forming reagents, which yields waterproof and self-cleaning properties. To find optimal SAM-forming reagents, four different kinds of reagents involving 1-decane thiol (DT), 1 H,1 H,2 H,2 H-perfluorohexanethiol, 1 H,1 H,2 H,2 H-perfluorodecyltrichlorosilane, 1 H,1 H,2 H,2 H-perfluodecanethiol (PFDT) were compared in terms of their thiol group and carbon chain lengths. Among the SAM-forming reagents, the PFDT-treated conductive fiber showed superior waterproof and self-cleaning property, as well as great sustainability in the water with varying pH because of nanoscale roughness and low surface energy. In addition, the functionality of the conductive fiber was tested under mechanical compression via repeated washing and folding processes. The developed conductive fiber with waterproof and self-cleaning property has promising applications in the interconnector operated under water and textile electronics.

Stimuli-responsive and on-chip nanomembrane micro-rolls for enhanced macroscopic visual hydrogen detection.

Nanomembrane rolling offers advanced three-dimensional (3D) mesostructures in electronics, optics, and biomedical applications. We demonstrate a high-density and on-chip array of rolled-up nanomembrane actuators with stimuli-responsive function based on the volume expansion of palladium in hydrogen milieu. The uniform stimuli-responsive behavior of high-density nanomembrane rolls leads to huge macroscopic visual detection with more than 50% transmittance change under optimization of micropattern design. The reversible shape changing between rolled and flat (unrolled) statuses can be well explained on the basis of the elastic mechanical model. The strain change in the palladium layer during hydrogen absorption and desorption produces a marked change in the diameter of nanomembrane rolls. We found that a functional palladium layer established an external compressive strain after hydrogen stimuli and thus also reduced the rolls' diameters. The large area of the nanomembrane roll array performs excellent nonelectrical hydrogen detection, with response and recovery speeds within seconds. Our work suggests a new strategy to integrate high-density 3D mesoscale architectures into functional devices and systems.

Highly Sensitive Multifilament Fiber Strain Sensors with Ultrabroad Sensing Range for Textile Electronics.

Highly stretchable fiber strain sensors are one of the most important components for various applications in wearable electronics, electronic textiles, and biomedical electronics. Herein, we present a facile approach for fabricating highly stretchable and sensitive fiber strain sensors by embedding Ag nanoparticles into a stretchable fiber with a multifilament structure. The multifilament structure and Ag-rich shells of the fiber strain sensor enable the sensor to simultaneously achieve both a high sensitivity and largely wide sensing range despite its simple fabrication process and components. The fiber strain sensor simultaneously exhibits ultrahigh gauge factors (∼9.3 × 105 and ∼659 in the first stretching and subsequent stretching, respectively), a very broad strain-sensing range (450 and 200% for the first and subsequent stretching, respectively), and high durability for more than 10 000 stretching cycles. The fiber strain sensors can also be readily integrated into a glove to control a hand robot and effectively applied to monitor the large volume expansion of a balloon and a pig bladder for an artificial bladder system, thereby demonstrating the potential of the fiber strain sensors as candidates for electronic textiles, wearable electronics, and biomedical engineering.

Single-Droplet Multiplex Bioassay on a Robust and Stretchable Extreme Wetting Substrate through Vacuum-Based Droplet Manipulation.

Herein, a droplet manipulation system with a superamphiphobic (SPO)-superamphiphilic (SPI) patterned polydimethylsiloxane (PDMS) substrate is developed for a multiplex bioassay from single-droplet samples. The SPO substrate is fabricated by sequential spraying of adhesive and fluorinated silica nanoparticles onto a PDMS substrate. It is subsequently subjected to oxygen plasma with a patterned mask to form SPI patterns. The SPO layer exhibits extreme liquid repellency with a high contact angle (>150°) toward low surface tension and viscous biofluidic droplets (e.g., ethylene glycol, blood, dimethyl sulfoxide, and alginate hydrogel). In contrast, the SPI exhibits liquid adhesion with a near zero contact angle. Using the droplet manipulation system, various liquid droplets can be precisely manipulated and dispensed onto the predefined SPI patterns on the SPO PDMS substrate. This system enables a multiplex colorimetric bioassay, capable of detecting multiple analytes, including glucose, uric acid, and lactate, from a single sample droplet. In addition, the detection of glucose concentrations in a plasma droplet of diabetic and healthy mice are performed to demonstrate the feasibility of the proposed system for efficient clinical diagnostic applications.

Bio-Inspired Extreme Wetting Surfaces for Biomedical Applications.

Biological creatures with unique surface wettability have long served as a source of inspiration for scientists and engineers. More specifically, materials exhibiting extreme wetting properties, such as superhydrophilic and superhydrophobic surfaces, have attracted considerable attention because of their potential use in various applications, such as self-cleaning fabrics, anti-fog windows, anti-corrosive coatings, drag-reduction systems, and efficient water transportation. In particular, the engineering of surface wettability by manipulating chemical properties and structure opens emerging biomedical applications ranging from high-throughput cell culture platforms to biomedical devices. This review describes design and fabrication methods for artificial extreme wetting surfaces. Next, we introduce some of the newer and emerging biomedical applications using extreme wetting surfaces. Current challenges and future prospects of the surfaces for potential biomedical applications are also addressed.

Graphene as an atomically thin barrier to Cu diffusion into Si.

The evolution of copper-based interconnects requires the realization of an ultrathin diffusion barrier layer between the Cu interconnect and insulating layers. The present work reports the use of atomically thin layer graphene as a diffusion barrier to Cu metallization. The diffusion barrier performance is investigated by varying the grain size and thickness of the graphene layer; single-layer graphene of average grain size 2 ± 1 µm (denoted small-grain SLG), single-layer graphene of average grain size 10 ± 2 µm (denoted large-grain SLG), and multi-layer graphene (MLG) of thickness 5-10 nm. The thermal stability of these barriers is investigated after annealing Cu/small-grain SLG/Si, Cu/large-grain SLG/Si, and Cu/MLG/Si stacks at different temperatures ranging from 500 to 900 °C. X-ray diffraction, transmission electron microscopy, and time-of-flight secondary ion mass spectroscopy analyses confirm that the small-grain SLG barrier is stable after annealing up to 700 °C and that the large-grain SLG and MLG barriers are stable after annealing at 900 °C for 30 min under a mixed Ar and H2 gas atmosphere. The time-dependent dielectric breakdown (TDDB) test is used to evaluate graphene as a Cu diffusion barrier under real device operating conditions, revealing that both large-grain SLG and MLG have excellent barrier performance, while small-grain SLG fails quickly. Notably, the large-grain SLG acts as a better diffusion barrier than the thicker MLG in the TDDB test, indicating that the grain boundary density of a graphene diffusion barrier is more important than its thickness. The near-zero-thickness SLG serves as a promising Cu diffusion barrier for advanced metallization.

Enhancement of X-ray detection by single-walled carbon nanotube enriched flexible polymer composite.

Although organic-based direct conversion X-ray detectors have been developed, their photocurrent generation efficiency has been limited by recombination of excitons due to the intrinsically poor electrical properties of organic materials. In this report, we fabricated a polymer-based flexible X-ray detector and enhanced the X-ray detection sensitivity using a single-walled carbon nanotube (SWNT) enriched polymer composite. When this SWNT enriched polymer composite was used as the active layer of an X-ray detector, it efficiently separated charges at the interface between the SWNTs and polymer, preventing recombination of X-ray-induced excitons. This increased the photocurrent generation efficiency, as measured from current-voltage characteristics. Therefore, X-ray-induced photocurrent and X-ray detection sensitivity were enhanced as the concentration of SWNTs in the composite was increased. However, this benefit was counterbalanced by the slow and unstable time-dependent response at high SWNT concentrations, arising from reduced Schottky barrier heights between the active layer and electrodes. At high SWNT concentration, the dark current also increased due to the reduced Schottky barrier height, leading to decrease the signal-to-noise ratio (SNR) of the device. Experimental results indicated that 0.005 wt.% SWNT in the composite was the optimum composition for practical X-ray detector operation because it showed enhanced performance in both sensitivity and SNR. In mechanical flexibility tests, the device exhibited a stable response up to a bending radius of 0.5 cm, and the device had no noticeable change in diode current after 1,000 bending cycles. PACS CODE: 8.67.Sc.

Gas-driven ultrafast reversible switching of super-hydrophobic adhesion on palladium-coated silicon nanowires.

A gas-driven ultrafast adhesion switching of water droplets on palladium-coated Si nanowire arrays is demonstrated. By regulating the gas-ambient between the atmosphere and H2 , the super-hydrophobic adhesion is repeatedly switched between water-repellent and water-adhesive. The capability of modulating the super-hydrophobic adhesion on a super-hydrophobic surface with a non-contact mode could be applicable to novel functional lab-on-a-chip platforms.