A wearable lab-on-a-patch platform with stretchable nanostructured biosensor for non-invasive immunodetection of biomarker in sweat.

Conformable, wearable biosensor-integrated systems are a promising approach to non-invasive and quantitative on-body detection of biomarkers in body fluids. However, realizing such a system has been slowed by the difficulty of fabricating a soft affinity-based biosensor patch capable of precise on-body fluid handling with minimal wearer intervention and a simple measurement protocol. Herein, we demonstrate a conformable, wearable lab-on-a-patch (LOP) platform composed of a stretchable, label-free, impedimetric biosensor and a stretchable microfluidic device for on-body detection of the hormone biomarker, cortisol. The all-in-one, stretchable microfluidic device can precisely collect and deliver sweat for cortisol quantitation and offers one-touch operation of reagent delivery for simultaneous electrochemical signal generation and washing. Three-dimensional nanostructuring of the Au working electrode enables the high sensitivity required to detect the pM-levels of cortisol in sweat. Our integrated LOP detected sweat cortisol quantitatively and accurately during exercise. This LOP will open a new horizon for non-invasive, highly sensitive, and quantitative on-body immunodetection for wearable personal diagnostics.

Fully Stretchable Capillary Microfluidics-Integrated Nanoporous Gold Electrochemical Sensor for Wearable Continuous Glucose Monitoring.

Biosensor systems for wearable continuous monitoring are desired to be developed into conformal patch platforms. However, developing such patches is very challenging owing to the difficulty of imparting materials and components with both high stretchability and high performance. Herein, we report a fully stretchable microfluidics-integrated glucose sensor patch comprised of an omnidirectionally stretchable nanoporous gold (NPG) electrochemical biosensor and a stretchable passive microfluidic device. A highly electrocatalytic NPG electrode was formed on a stress-absorbing 3D micropatterned polydimethylsiloxane (PDMS) substrate to confer mechanical stretchability, high sensitivity, and durability in non-enzymatic glucose detection. A thin, stretchable, and tough microfluidic device was made by embedding stretchable cotton fabric as a capillary into a thin polyurethane nanofiber-reinforced PDMS channel, enabling collection and passive, accurate delivery of sweat from skin to the electrode surface, with excellent replacement capability. The integrated glucose sensor patch demonstrated excellent ability to continuously and accurately monitor the sweat glucose level.

Stretchable, Transparent, and Stretch-Unresponsive Capacitive Touch Sensor Array with Selectively Patterned Silver Nanowires/Reduced Graphene Oxide Electrodes.

Stretchable and transparent touch sensors are essential input devices for future stretchable transparent electronics. Capacitive touch sensors with a simple structure of only two electrodes and one dielectric are an established technology in current rigid electronics. However, the development of stretchable and transparent capacitive touch sensors has been limited due to changes in capacitance resulting from dimensional changes in elastomeric dielectrics and difficulty in obtaining stretchable transparent electrodes that are stable under large strains. Herein, a stretch-unresponsive stretchable and transparent capacitive touch sensor array was demonstrated by employing stretchable and transparent electrodes with a simple selective-patterning process and by carefully selecting dielectric and substrate materials with low strain responsivity. A selective-patterning process was used to embed a stretchable and transparent silver nanowires/reduced graphene oxide (AgNWs/rGO) electrode line into a polyurethane (PU) dielectric layer on a polydimethylsiloxane (PDMS) substrate using oxygen plasma treatment. This method provides the ability to directly fabricate thin film electrode lines on elastomeric substrates and can be used in conventional processes employed in stretchable electronics. We used a dielectric (PU) with a Poisson's ratio smaller than that of the substrate (PDMS), which prevented changes in the capacitance resulting from stretching of the sensor. The stretch-unresponsive touch sensing capability of our transparent and stretchable capacitive touch sensor has great potential in wearable electronics and human-machine interfaces.

Ultrarapid and ultrasensitive electrical detection of proteins in a three-dimensional biosensor with high capture efficiency.

The realization of a high-throughput biosensor platform with ultrarapid detection of biomolecular interactions and an ultralow limit of detection in the femtomolar (fM) range or below has been retarded due to sluggish binding kinetics caused by the scarcity of probe molecules on the nanostructures and/or limited mass transport. Here, as a new method for the highly efficient capture of biomolecules at extremely low concentration, we tested a three-dimensional (3D) platform of a bioelectronic field-effect transistor (bio-FET) with vertically aligned and highly dense one-dimensional (1D) ZnO nanorods (NRs) as a sensing surface capped by an ultrathin TiO2 layer for improved electrolytic stability on a chemical-vapor-deposited graphene (Gr) channel. The ultrarapid detection capability with a very fast response time (∼1 min) at the fM level of proteins in the proposed 3D bio-FET is primarily attributed to the fast binding kinetics of the probe-target proteins due to the small diffusion length of the target molecules to reach the sensor surface and the substantial number of probe molecules available on the largely increased surface area of the vertical ZnO NRs. This new 3D electrical biosensor platform can be easily extended to other electrochemical nanobiosensors and has great potential for practical applications in miniaturized biosensor integrated systems.

High-Performance Flexible Ultraviolet (UV) Phototransistor Using Hybrid Channel of Vertical ZnO Nanorods and Graphene.

A flexible ultraviolet (UV) photodetector based on ZnO nanorods (NRs) as nanostructure sensing materials integrated into a graphene (Gr) field-effect transistor (FET) platform is investigated with high performance. Based on the negative shift of the Dirac point (VDirac) in the transfer characteristics of a phototransistor, high-photovoltage responsivity (RV) is calculated with a maximum value of 3 × 10(8) V W(-1). The peak response at a wavelength of ∼365 nm indicated excellent selectivity to UV light. The phototransistor also allowed investigation of the photocurrent responsivity (RI) and photoconductive gain (G) at various gate voltages, with maximum values of 2.5 × 10(6) A W(-1) and 8.3 × 10(6), respectively, at a gate bias of 5 V. The UV response under bending conditions was virtually unaffected and was unchanged after 10,000 bending cycles at a bending radius of 12 mm, subject to a strain of 0.5%. The attributes of high stability, selectivity, and sensitivity of this flexible UV photodetector based on a ZnO NRs/Gr hybrid FET indicate promising potential for future flexible optoelectronic devices.

Stretchable, Transparent, Ultrasensitive, and Patchable Strain Sensor for Human-Machine Interfaces Comprising a Nanohybrid of Carbon Nanotubes and Conductive Elastomers.

Interactivity between humans and smart systems, including wearable, body-attachable, or implantable platforms, can be enhanced by realization of multifunctional human-machine interfaces, where a variety of sensors collect information about the surrounding environment, intentions, or physiological conditions of the human to which they are attached. Here, we describe a stretchable, transparent, ultrasensitive, and patchable strain sensor that is made of a novel sandwich-like stacked piezoresisitive nanohybrid film of single-wall carbon nanotubes (SWCNTs) and a conductive elastomeric composite of polyurethane (PU)-poly(3,4-ethylenedioxythiophene) polystyrenesulfonate ( PEDOT: PSS). This sensor, which can detect small strains on human skin, was created using environmentally benign water-based solution processing. We attributed the tunability of strain sensitivity (i.e., gauge factor), stability, and optical transparency to enhanced formation of percolating networks between conductive SWCNTs and PEDOT phases at interfaces in the stacked PU-PEDOT:PSS/SWCNT/PU-PEDOT:PSS structure. The mechanical stability, high stretchability of up to 100%, optical transparency of 62%, and gauge factor of 62 suggested that when attached to the skin of the face, this sensor would be able to detect small strains induced by emotional expressions such as laughing and crying, as well as eye movement, and we confirmed this experimentally.

Ultrahigh Responsivity in Graphene-ZnO Nanorod Hybrid UV Photodetector.

Ultraviolet (UV) photodetectors based on ZnO nanostructure/graphene (Gr) hybrid-channel field-effect transistors (FETs) are investigated under illumination at various incident photon intensities and wavelengths. The time-dependent behaviors of hybrid-channel FETs reveal a high sensitivity and selectivity toward the near-UV region at the wavelength of 365 nm. The devices can operate at low voltage and show excellent selectivity, high responsivity (RI ), and high photoconductive gain (G). The change in the transfer characteristics of hybrid-channel FETs under UV light illumination allows to detect both photovoltage and photocurrent. The shift of the Dirac point (V Dirac ) observed during UV exposure leads to a clearer explanation of the response mechanism and carrier transport properties of Gr, and this phenomenon permits the calculation of electron concentration per UV power density transferred from ZnO nanorods and ZnO nanoparticles to Gr, which is 9 × 10(10) and 4 × 10(10) per mW, respectively. The maximum values of RI and G infer from the fitted curves of RI and G versus UV intensity are 3 × 10(5) A W(-1) and 10(6) , respectively. Therefore, the hybrid-channel FETs studied herein can be used as UV sensing devices with high performance and low power consumption, opening up new opportunities for future optoelectronic devices.

Piezoelectric coupling in a field-effect transistor with a nanohybrid channel of ZnO nanorods grown vertically on graphene.

Piezoelectric coupling phenomena in a graphene field-effect transistor (GFET) with a nano-hybrid channel of chemical-vapor-deposited Gr (CVD Gr) and vertically aligned ZnO nanorods (NRs) under mechanical pressurization were investigated. Transfer characteristics of the hybrid channel GFET clearly indicated that the piezoelectric effect of ZnO NRs under static or dynamic pressure modulated the channel conductivity (σ) and caused a positive shift of 0.25% per kPa in the Dirac point. However, the GFET without ZnO NRs showed no change in either σ or the Dirac point. Analysis of the Dirac point shifts indicated transfer of electrons from the CVD Gr to ZnO NRs due to modulation of their interfacial barrier height under pressure. High responsiveness of the hybrid channel device with fast response and recovery times was evident in the time-dependent behavior at a small gate bias. In addition, the hybrid channel FET could be gated by mechanical pressurization only. Therefore, a piezoelectric-coupled hybrid channel GFET can be used as a pressure-sensing device with low power consumption and a fast response time. Hybridization of piezoelectric 1D nanomaterials with a 2D semiconducting channel in FETs enables a new design for future nanodevices.

Improved performance and stability of field-effect transistors with polymeric residue-free graphene channel transferred by gold layer.

One of the most significant issues that occurs when applying chemical-vapor deposited (CVD) graphene (Gr) to various high-performance device applications is the result of polymeric residues. Polymeric residues remain on the Gr surface during Gr polymer support transfer to an arbitrary substrate, and these residues degrade CVD Gr electrical properties. In this paper, we propose that a thin layer of gold be used as a CVD Gr transfer layer, instead of a polymer support layer, to enable a polymer residue-free transfer. Comparative investigation of the surface morphological and qualitative analysis of residues on Gr surfaces and Gr field-effect transistors (GFETs) using two transfer methods demonstrates that gold-transferred Gr, with uniform, smooth, and clean surfaces, enable GFETs to perform better than Gr transferred by the polymer, polymethylmethacrylate (PMMA). In GFETs fabricated by the gold transfer method, field-effect carrier mobility was greatly enhanced and the position of the Dirac point was significantly reduced compared to GFETs fabricated by the PMMA transfer method. In addition, compared to the PMMA-transferred GFETs, the gold-transferred GFETs showed greatly increased stability with smaller hysteresis and higher resistance to gate bias stress effects. These results suggest that the gold transfer method for Gr provides significant improvements in GFET performance and reliability by minimizing the polymeric residues and defects on Gr.

All-solid-state flexible supercapacitors fabricated with bacterial nanocellulose papers, carbon nanotubes, and triblock-copolymer ion gels.

We demonstrate all-solid-state flexible supercapacitors with high physical flexibility, desirable electrochemical properties, and excellent mechanical integrity, which were realized by rationally exploiting unique properties of bacterial nanocellulose, carbon nanotubes, and ionic liquid based polymer gel electrolytes. This deliberate choice and design of main components led to excellent supercapacitor performance such as high tolerance against bending cycles and high capacitance retention over charge/discharge cycles. More specifically, the performance of our supercapacitors was highly retained through 200 bending cycles to a radius of 3 mm. In addition, the supercapacitors showed excellent cyclability with C(sp) (~20 mF/cm(2)) reduction of only <0.5% over 5000 charge/discharge cycles at the current density of 10 A/g. Our demonstration could be an important basis for material design and development of flexible supercapacitors.