On-Chip Micro-Supercapacitor with High Areal Energy Density Based on Dielectrophoretic Assembly of Nanoporous Metal Microwire Electrodes.

Advances in the Internet of Things (IoT) technology have driven the demand for miniaturized electronic devices, prompting research on small-scale energy-storage systems. Micro-supercapacitors (MSCs) stand out in this regard because of their compact size, high power density, high charge-discharge rate, and extended cycle life. However, their limited energy density impedes commercialization. To resolve this issue, a simple and innovative approach is reported herein for fabricating highly efficient on-chip MSCs integrated with nanoporous metal microwires formed by dielectrophoresis (DEP)-driven gold nanoparticle (AuNP) assembly. Placing a water-based AuNP suspension onto interdigitated electrodes and applying an alternating voltage induces in-plane porous microwire formation in the electrode gap. The DEP-induced AuNP assembly and the gold microwire (AuMW) growth rate can be adjusted by controlling the applied alternating voltage and frequency. The microwire-integrated MSC (AuMW-MSC) electrically outperforms its unmodified counterpart and exhibits a 30% larger electrode area, along with 72% and 78% higher specific and areal capacitances, respectively, than a microwire-free MSC. Additionally, AuMW-MSC achieves maximum energy and power densities of 3.33 µWh cm-2 and 2629 µW cm-2 , respectively, with a gel electrolyte. These findings can help upgrade MSCs to function as potent energy-storage devices for small electronics.

Foldable RF Energy Harvesting System Based on Vertically Layered Metal Electrodes within a Single Sheet of Paper.

Radio frequency energy harvesting (RFEH) systems have emerged as a critical component for powering devices and replacing traditional batteries, with paper being one of the most promising substrates for use in flexible RFEH systems. However, previous paper-based electronics with optimized porosity, surface roughness, and hygroscopicity still face limitations in terms of the development of integrated foldable RFEH systems within a single sheet of paper. In the present study, a novel wax-printing control and water-based solution process are used to realize an integrated foldable RFEH system within a single sheet of paper. The proposed paper-based device includes vertically layered foldable metal electrodes, a via-hole, and stable conductive patterns with a sheet resistance of less than 1 Ω sq-1 . The proposed RFEH system exhibits an RF/DC conversion efficiency of 60% and an operating voltage of 2.1 V in 100 s at a distance of 50 mm and a transmitted power of 50 mW. The integrated RFEH system also demonstrates stable foldability, with RFEH performance maintained up to a folding angle of 150°. The single-sheet paper-based RFEH system thus has the potential for use in practical applications associated with the remote powering of wearable and Internet-of-Things devices and in paper electronics.

Highly Uniform and Conducting Single-Walled Carbon Nanotube Thin Films via Controlling the Dewetting Phenomenon.

Carbon nanotubes (CNTs) have received substantial attention as alternatives to indium tin oxide for the production of transparent conductors. However, problems associated with the dewetting of liquid thin films have hindered the reliable fabrication of networked conducting CNT films via solution-based processes. In this study, the dewetting of liquid thin films containing single-walled carbon nanotubes (SWCNTs) on substrates is successfully retarded by simply adding ethylene glycol to the SWCNT dispersion, and highly uniform SWCNT thin films are obtained using the meniscus-dragging deposition (MDD) method. The dewetting-free coating conditions for the uniform SWCNT films are determined by calculating the dewetting and drying times of the liquid thin films formed by the MDD method. When the dewetting time was 2.5 times longer than the drying time of the liquid thin layers, uniform SWCNT films are formed over the entire substrates without breakage or rupture of the films. In addition, the transmittance and sheet resistance of the transparent SWCNT films are easily controlled over a wide range by varying the coating parameters.

Highly Transparent, Flexible Conductors and Heaters Based on Metal Nanomesh Structures Manufactured Using an All-Water-Based Solution Process.

Metal mesh is a promising material for flexible transparent conducting electrodes due to its outstanding physical and electrical properties. The excellent control of the sheet resistance and transmittance provided by the metal mesh electrodes is a great advantage for microelectronic applications. Thus, over the past decade, many studies have been performed in order to realize high-performance metal mesh; however, the lack of cost-effective fabrication processes and the weak adhesion between the metal mesh and substrate have hindered its widespread adoption for flexible optoelectronic applications. In this study, a new approach for fabricating robust silver mesh without using hazardous organic solvents is achieved by combining colloidal deposition and silver enhancement steps. The adhesion of the metal mesh was greatly improved by introducing an intermediate adhesion layer. Various patterns relevant to optoelectronic applications were fabricated with a minimum feature size of 700 nm, resulting in high optical transmittance of 97.7% and also high conductivity (71.6 Ω sq-1) of the metal mesh. In addition, we demonstrated an effective transparent heater using the silver mesh with excellent exothermic behavior, which heated up to 245 °C with 7 V applied voltage.

Microencapsulation by pectin for multi-components carriers bearing both hydrophobic and hydrophilic active agents.

Oil/water microencapsulation by microfluidic systems has been a prominent delivery method to prepare functional microcapsules in the food, cosmetic, and pharmaceutical industries because it is an easy way to control the shape and size of structures and functionalities. We prepared biocompatible and multi-component microcapsules using the precipitation and ionic crosslinking of pectin in a poor solubility environment and with multivalent cations, respectively. When the aqueous solution (including calcium ions and ethanol) in a sheath flow met the flow of a pectin aqueous solution containing oil droplets, ethanol-gelation and ionic cross-linking occurred, enclosing the inner oil phase droplets by solidified pectin shells. Furthermore, the resulting microcapsules stabilized by pectin shells exhibited functionalities using a hydrophobic agent and nanoparticles of a hydrophilic species that were dissolved and dispersed, respectively, in the oil phase.

Highly sensitive metal-grid strain sensors via water-based solution processing.

Strain sensor technologies have been spotlighted for their versatility for healthcare, soft robot, and human-robot applications. Expecting large future demands for such technology, extensive studies have investigated flexible and stretchable strain sensors based on various nanomaterials and metal films. However, it is still challenging to simultaneously satisfy parameters such as sensitivity, stretchability, linearity, hysteresis, and mass producibility. In this work, we demonstrate a novel approach for producing highly sensitive metal-grid strain sensors based on an all-solution process, which is suitable for mass production. We investigated the effects of the width of the metal grid and width/spacing ratio on the piezoresistivity of the strain sensors. The metal grid strain sensors exhibited high sensitivity (gauge factor of 4685.9 at 5% strain), rapid response time (∼18.6 ms), and superior strain range (≤5%) compared to other metal-based sensors. We demonstrated that the sensors could successfully convert voice signals and tiny movements of fingers and muscles into electrical signals. In addition, the metal-grid strain sensors were produced using a low-cost procedure without toxic solvent via an all water-based solution process, which is expected to allow the integration of such metal-grid strain sensors into future highly sensitive physical sensing devices.

Large-Area, Highly Sensitive SERS Substrates with Silver Nanowire Thin Films Coated by Microliter-Scale Solution Process.

A microliter-scale solution process was used to fabricate large-area, uniform films of silver nanowires (AgNWs). These thin films with cross-AgNWs were deposited onto Au substrates by dragging the meniscus of a microliter drop of a coating solution trapped between two plates. The hot spot density was tuned by controlling simple experimental parameters, which changed the optical properties of the resulting films. The cross-AgNW films on the Au surface served as excellent substrates for surface-enhanced Raman spectroscopy, with substantial electromagnetic field enhancement and good reproducibility.

Highly Sensitive Piezocapacitive Sensor for Detecting Static and Dynamic Pressure Using Ion-Gel Thin Films and Conductive Elastomeric Composites.

A new class of simple and highly sensitive piezocapacitive sensors that are capable of detecting static and dynamic pressure changes is reported. The pressure sensor structure is formed by vertically sandwiching a sandpaper-molded carbon nanotube/poly(dimethylsiloxane) composite (CPC) dielectric layer between two ion-gel thin film electrodes. Such a capacitive sensor system enables the distinguishable detection of directional movement of applied pressure as well as static pressure variation by modulating ion distribution in the ion-gel thin films. The resulting capacitive pressure sensors exhibit high sensitivity (9.55 kPa-1), high durability, and low operating voltage (0.1 V). Our proposed pressure sensors are successfully applied as potential platforms for monitoring human physiological signals and finger sliding motions in order to demonstrate their capability for practical usage. The outstanding sensor performance of the pressure sensors can permit applications in wearable electronic devices for human-machine connecting platforms, health care monitoring systems, and artificial skin.

Enhancing surface functionality of reduced graphene oxide biosensors by oxygen plasma treatment for Alzheimer's disease diagnosis.

We performed oxygen plasma treatment on reduced graphene oxide (rGO) to improve its surface reactivity with respect to biomolecular interactions. Oxygen-plasma-treated rGO surfaces were employed as reactive interfaces for the detection of amyloid-beta (Aß) peptides, the pathological hallmarks of Alzheimer's disease (AD), as the target analytes. By measuring the changes in electrical characteristics and confirmation through topographic analysis, the oxygen-plasma-treated rGO sensors had enhanced surface functionality for better antibody immobilization and sensing performance, with a 3.33-fold steeper slope for the electrical responses versus analyte concentration curve (logarithmic scale) compared to the untreated. The elicited biomolecular reactivity of the rGO surfaces with the oxygen plasma treatment remained at 46-51% of the initial value even after aging for 6h in ambient conditions. This phenomenon was also confirmed by pretreating the rGO surfaces with a blocking agent and subsequently subjecting them to antibody immobilization. Finally, the feasibility of the oxygen-plasma-treated rGO sensors as a diagnostic tool was evaluated with clinical samples of neural-derived exosomal Aß peptides extracted from apparent AD patients and normal controls (NC). In contrast to the untreated sensors (p=0.0460), the oxygen-plasma-treated rGO sensors showed a significant p-value in the identification of clinical samples of AD and NC subjects (p<0.001). These results suggest that oxygen plasma treatment improves sensor performance without complicated fabrication procedures and should aid in the development of novel diagnostic tools based on carbon nanomaterials.

Synergetic Evolution of Diketopyrrolopyrrole-Based Polymeric Semiconductor for High Reproducibility and Performance: Random Copolymerization of Similarly Shaped Building Blocks.

A new random copolymer consisting of similarly shaped donor-acceptor building blocks of diketopyrrolopyrrole-selenophene-vinylene-selenophene (DPP-SVS) and DPP-thiophene-vinylene-thiophene (DPP-TVT) is designed and synthesized. The resulting P-DPP-SVS(5)-TVT(5) with an equal molecular ratio of the two building blocks produced significantly enhanced solubility when compared to that of the two homopolymers, PDPP-SVS and PDPP-TVT. More importantly, despite the maximum segmental randomness of the PDPP-SVS(5)-TVT(5) copolymer, its crystalline perfectness and preferential orientation are outstanding, even similar to those of the homopolymers thanks to the similarity of the two building blocks. This unique property produces a high charge carrier mobility of 1.23 cm2 V-1 s-1 of PDPP-SVS(5)-TVT(5), as determined from polymer field-effect transistor (PFET) measurements. The high solubility of PDPP-SVS(5)-TVT(5) promotes formulation of high-viscosity solutions which could be successfully processed to fabricate large-areal PFETs onto hydrophobically treated 4 in. wafers. A total of 269 individual PFETs are fabricated. These devices exhibit extremely narrow device-to-device deviations without a single failure and demonstrate an average charge carrier mobility of 0.66 cm2 V-1 s-1 with a standard deviation of 0.064. This is the first study to report on successfully realizing large-areal reproducibility of high-mobility polymeric semiconductors.

Wafer-scale high-resolution patterning of reduced graphene oxide films for detection of low concentration biomarkers in plasma.

Given that reduced graphene oxide (rGO)-based biosensors allow disposable and repeatable biomarker detection at the point of care, we developed a wafer-scale rGO patterning method with mass productivity, uniformity, and high resolution by conventional micro-electro-mechanical systems (MEMS) techniques. Various rGO patterns were demonstrated with dimensions ranging from 5 µm up to several hundred µm. Manufacture of these patterns was accomplished through the optimization of dry etching conditions. The axis-homogeneity and uniformity were also measured to verify the uniform patternability in 4-inch wafer with dry etching. Over 66.2% of uniform rGO patterns, which have deviation of resistance within range of ±10%, formed the entire wafer. We selected amyloid beta (Aß) peptides in the plasma of APP/PS1 transgenic mice as a study model and measured the peptide level by resistance changes of highly uniform rGO biosensor arrays. Aß is a pathological hallmark of Alzheimer's disease and its plasma concentration is in the pg mL(-1) range. The sensor detected the Aß peptides with ultra-high sensitivity; the LOD was at levels as low as 100 fg mL(-1). Our results provide biological evidences that this wafer-scale high-resolution patterning method can be used in rGO-based electrical diagnostic devices for detection of low-level protein biomarkers in biofluids.

Highly Transparent and Stretchable Conductors Based on a Directional Arrangement of Silver Nanowires by a Microliter-Scale Solution Process.

We report an effective method for fabricating highly transparent and stretchable large-area conducting films based on a directional arrangement of silver nanowires (AgNWs) driven by a shear force in a microliter-scale solution process. The thin conducting films with parallel AgNWs or cross-junctions of AgNWs are deposited on the coating substrate by dragging a microliter drop of the coating solution trapped between two plates. The optical and electrical properties of the AgNW thin films are finely tuned by varying the simple systematic parameters in the coating process. The transparent thin films with AgNW cross-junctions exhibit the superior electrical conductivity with a sheet resistance of 10 Ω sq(-1) at a transmittance of 85% (λ = 550 nm), which is well described by the high ratio of DC to optical conductivity of 276 and percolation theory in a two-dimensional matrix model. This simple coating method enables the deposition of AgNW thin films with high optical transparency, flexibility, and stretchability directly on plastic substrates.

Highly Stretchable and Transparent Microfluidic Strain Sensors for Monitoring Human Body Motions.

We report a new class of simple microfluidic strain sensors with high stretchability, transparency, sensitivity, and long-term stability with no considerable hysteresis and a fast response to various deformations by combining the merits of microfluidic techniques and ionic liquids. The high optical transparency of the strain sensors was achieved by introducing refractive-index matched ionic liquids into microfluidic networks or channels embedded in an elastomeric matrix. The microfluidic strain sensors offer the outstanding sensor performance under a variety of deformations induced by stretching, bending, pressing, and twisting of the microfluidic strain sensors. The principle of our microfluidic strain sensor is explained by a theoretical model based on the elastic channel deformation. In order to demonstrate its capability of practical usage, the simple-structured microfluidic strain sensors were performed onto a finger, wrist, and arm. The highly stretchable and transparent microfluidic strain sensors were successfully applied as potential platforms for distinctively monitoring a wide range of human body motions in real time. Our novel microfluidic strain sensors show great promise for making future stretchable electronic devices.

Novel Synthesis, Coating, and Networking of Curved Copper Nanowires for Flexible Transparent Conductive Electrodes.

In this work, a whole manufacturing process of the curved copper nanowires (CCNs) based flexible transparent conductive electrode (FTCE) is reported with all solution processes, including synthesis, coating, and networking. The CCNs with high purity and good quality are designed and synthesized by a binary polyol coreduction method. In this reaction, volume ratio and reaction time are the significant factors for the successful synthesis. These nanowires have an average 50 nm in width and 25-40 µm range in length with curved structure and high softness. Furthermore, a meniscus-dragging deposition (MDD) method is used to uniformly coat the well-dispersed CCNs on the glass or polyethylene terephthalate substrate with a simple process. The optoelectrical property of the CCNs thin films is precisely controlled by applying the MDD method. The FTCE is fabricated by networking of CCNs using solvent-dipped annealing method with vacuum-free, transfer-free, and low-temperature conditions. To remove the natural oxide layer, the CCNs thin films are reduced by glycerol or NaBH4 solution at low temperature. As a highly robust FTCE, the CCNs thin film exhibits excellent optoelectrical performance (T = 86.62%, R(s) = 99.14 Ω â—»(-1)), flexibility, and durability (R/R(0) < 1.05 at 2000 bending, 5 mm of bending radius).

Pectin Micro- and Nano-capsules of Retinyl Palmitate as Cosmeceutical Carriers for Stabilized Skin Transport.

Retinyl palmitate (RP)-loaded pectinate micro- and nano-particles (PMP and PNP) were designed for stabilization of RP that is widely used as an anti-wrinkle agent in anti-aging cosmeceuticals. PMP/PNP were prepared with an ionotropic gelation method, and anti-oxidative activity of the particles was measured with a DPPH assay. The stability of RP in the particles along with pectin gel and ethanolic solution was then evaluated. In vitro release and skin permeation studies were performed using Franz diffusion cells. Distribution of RP in each skin tissue (stratum corneum, epidermis, and dermis) was also determined. PMP and PNP could be prepared with mean particle size diameters of 593~843 µm (PMP) and 530 nm (i.e., 0.53 µm, PNP). Anti-oxidative activity of PNP was greater than PMP due largely to larger surface area available for PNP. The stability of RP in PMP and PNP was similar but much greater than RP in pectin bulk gels and ethanolic solution. PMP and PNP showed the abilities to constantly release RP and it could be permeated across the model artificial membrane and rat whole skin. RP was serially deposited throughout the skin layers. This study implies RP loaded PMP and PNP are expected to be advantageous for improved anti-wrinkle effects.

A novel miniature dynamic microfluidic cell culture platform using electro-osmosis diode pumping.

An electro-osmosis (EOS) diode pumping platform capable of culturing cells in fluidic cellular micro-environments particularly at low volume flow rates has been developed. Diode pumps have been shown to be a viable alternative to mechanically driven pumps. Typically electrokinetic micro-pumps were limited to low-concentration solutions (≤10 mM). In our approach, surface mount diodes were embedded along the sidewalls of a microchannel to rectify externally applied alternating current into pulsed direct current power across the diodes in order to generate EOS flows. This approach has for the first time generated flows at ultra-low flow rates (from 2.0 nl/s to 12.3 nl/s) in aqueous solutions with concentrations greater than 100 mM. The range of flow was generated by changing the electric field strength applied to the diodes from 0.5 Vpp/cm to 10 Vpp/cm. Embedding an additional diode on the upper surface of the enclosed microchannel increased flow rates further. We characterized the diode pump-driven fluidics in terms of intensities and frequencies of electric inputs, pH values of solutions, and solution types. As part of this study, we found that the growth of A549 human lung cancer cells was positively affected in the microfluidic diode pumping system. Though the chemical reaction compromised the fluidic control overtime, the system could be maintained fully functional over a long time if the solution was changed every hour. In conclusion, the advantage of miniature size and ability to accurately control fluids at ultra-low volume flow rates can make this diode pumping system attractive to lab-on-a-chip applications and biomedical engineering in vitro studies.

Micropatterned single-walled carbon nanotube electrodes for use in high-performance transistors and inverters.

We demonstrated the solution-processed single-walled carbon nanotube (SWNT) source-drain electrodes patterned using a plasma-enhanced detachment patterning method for high-performance organic transistors and inverters. The high-resolution SWNT electrode patterning began with the formation of highly uniform SWNT thin films on a hydrophobic silanized substrate. The SWNT source-drain patterns were then formed by modulating the interfacial energies of the prepatterned elastomeric mold and the SWNT thin film using oxygen plasma. The SWNT films were subsequently selectively delaminated using a rubber mold. The patterned SWNTs could be used as the source-drain electrodes for both n-type PTCDI-C8 and p-type pentacene field-effect transistors (FETs). The n- and p-type devices exhibited good and exactly matched electrical performances, with a field-effect mobility of around 0.15 cm(2) V(-1) s(-1) and an ON/OFF current ratio exceeding 10(6). The single electrode material was used for both the n and p channels, permitting the successful fabrication of a high-performance complementary inverter by connecting a p-type pentacene FET to an n-type PTCDI-C8 FET. This patterning technique was simple, inexpensive, and easily scaled for the preparation of large-area electrode micropatterns for flexible microelectronic device fabrication.

High performance organic nonvolatile flash memory transistors with high-resolution reduced graphene oxide patterns as a floating gate.

High-performance organic nonvolatile memory transistors (ONVMTs) are demonstrated, the construction of which is based on novel integration of a highly conductive polymer as a semiconductor layer, hydroxyl-free polymer as a tunneling dielectric layer, and high-resolution reduced graphene oxide (rGO) patterns as a floating gate. Finely patterned rGO, with a line width of 20-120 µm, was embedded between SiO2 and the polymer dielectric layer, which functions as a nearly isolated charge-trapping center. The resulting ONVMTs demonstrated ideal memory behavior, and the transfer characteristics promptly responded to writing and erasing the gate bias. In particular, the retention time of written/erased states tended to increase as the rGO line width was reduced, implying that the line width is a critical factor in suppressing charge release from rGO. Using a 20-µm-wide rGO pattern, a nonvolatile large memory window (>20 V) was retained for more than 5 × 10(5) s, which is 50 times longer than non-patterned rGO films.

Formation and characterization of wrinkle structures of chemically-derived graphene thin films and micropatterns.

In this study, we report a simple and effective process for the fabrication of wrinkle structures of chemically derived graphene thin films and patterns. Reduced graphene oxide (rGO) thin films/patterns formed on glass substrates are transferred to pre-strained elastomeric layers by improving adhesion strength at the rGO/PDMS interface with the assistance of oxygen plasma treatment. The morphology of rGO wrinkle structures is investigated in the various applied strains and film thicknesses. The experimental results were interpreted by theoretical models and well fitted to the estimated values. The techniques for such well-defined rGO wrinkle structures could be used for flexible and stretchable graphene-based electronic devices.

Development of a new contactless dielectrophoresis system for active particle manipulation using movable liquid electrodes.

This study presents a new DEP manipulation technique using a movable liquid electrode, which allows manipulation of particles by actively controlling the locations of electrodes and applying on-off electric input signals. This DEP system consists of mercury as a movable liquid electrode, indium tin oxide (ITO)-coated glass, SU-8-based microchannels for electrode passages, and a PDMS medium chamber. A simple squeezing method was introduced to build a thin PDMS layer at the bottom of the medium chamber to create a contactless DEP system. To determine the operating conditions, the DEP force and the friction force were analytically compared for a single cell. In addition, an appropriate frequency range for effective DEP manipulation was chosen based on an estimation of the Clausius-Mossotti factor and the effective complex permittivity of the yeast cell using the concentric shell model. With this system, we demonstrated the active manipulation of yeast cells, and measured the collection efficiency and the dielectrophoretic velocity of cells for different AC electric field strengths and applied frequencies. The experimental results showed that the maximum collection efficiency reached was approximately 90%, and the dielectrophoretic velocity increased with increasing frequency and attained the maximum value of 10.85 ± 0.95 µm/s at 100 kHz, above which it decreased.