Impedance-based polymer microneedle patch sensor for continuous interstitial fluid glucose monitoring.

Early detection and effective blood glucose control are critical for preventing and managing diabetes-related complications. Conventional glucometers provide point-in-time measurements but are painful and cannot facilitate continuous monitoring. Continuous glucose monitoring systems are comfortable but face challenges in terms of accuracy, cost, and sensor lifespan. This study aimed to develop a microneedle-based sensor patch for minimally invasive, painless, and continuous glucose monitoring in the interstitial fluid to address these limitations. Experimental results confirm painless and minimally invasive penetration of the skin tissue with cylindrical microneedles (3 × 3 array) to a depth of approximately 520 µm with minimal loading. The microneedle sensors fabricated with precision using the complementary metal-oxide semiconductor process were immobilized with glucose oxidase, as confirmed through phase angle analysis. Long-term tests confirmed the effective operation of the sensor for up to seven days. Glucose concentrations determined from the fitted concentration-impedance curves correlated well with those measured using commercial glucometers, indicating the reliability and precision of the microneedle sensor. The flexible and minimally invasive sensor developed in this study facilitates painless and continuous glucose monitoring.

Ultrathin Crystalline Silicon Nano and Micro Membranes with High Areal Density for Low-Cost Flexible Electronics.

Ultrathin crystalline silicon is widely used as an active material for high-performance, flexible, and stretchable electronics, from simple passive and active components to complex integrated circuits, due to its excellent electrical and mechanical properties. However, in contrast to conventional silicon wafer-based devices, ultrathin crystalline silicon-based electronics require an expensive and rather complicated fabrication process. Although silicon-on-insulator (SOI) wafers are commonly used to obtain a single layer of crystalline silicon, they are costly and difficult to process. Therefore, as an alternative to SOI wafers-based thin layers, here, a simple transfer method is proposed for printing ultrathin multiple crystalline silicon sheets with thicknesses between 300 nm to 13 µm and high areal density (>90%) from a single mother wafer. Theoretically, the silicon nano/micro membrane can be generated until the mother wafer is completely consumed. In addition, the electronic applications of silicon membranes are successfully demonstrated through the fabrication of a flexible solar cell and flexible NMOS transistor arrays.

Transfer-Printed Cuprous Iodide (CuI) Hole Transporting Layer for Low Temperature Processed Perovskite Solar Cells.

Perovskite solar cells (PSCs) have achieved significantly high power-conversion efficiency within a short time. Most of the devices, including those with the highest efficiency, are based on a n-i-p structure utilizing a (doped) spiro-OMeTAD hole transport layer (HTL), which is an expensive material. Furthermore, doping has its own challenges affecting the processing and performance of the devices. Therefore, the need for low-cost, dopant-free hole transport materials is an urgent and critical issue for the commercialization of PSCs. In this study, n-i-p structure PSCs were fabricated in an ambient environment with cuprous iodide (CuI) HTL, employing a novel transfer-printing technique, in order to avoid the harmful interaction between the perovskite surface and the solvents of CuI. Moreover, in fabricated PSCs, the SnO2 electron transport layer (ETL) has been incorporated to reduce the processing temperature, as previously reported (n-i-p) devices with CuI HTL are based on TiO2, which is a high-temperature processed ETL. PSCs fabricated at 80 °C transfer-printing temperature with 20 nm iodized copper, under 1 sun illumination showed a promising efficiency of 8.3%, (JSC and FF; 19.3 A/cm2 and 53.8%), which is comparable with undoped spiro-OMeTAD PSCs and is the highest among the ambient-environment-fabricated PSCs utilizing CuI HTL.

Recyclable, Antibacterial, Isoporous Through-Hole Membrane Air Filters with Hydrothermally Grown ZnO Nanorods.

Reusable, antibacterial, and photocatalytic isoporous through-hole air filtration membranes have been demonstrated based on hydrothermally grown ZnO nanorods (NRs). High-temperature (300~375 °C) stability of thermoset-based isoporous through-hole membranes has enabled concurrent control of porosity and seed formation via high-temperature annealing of the membranes. The following hydrothermal growth has led to densely populated ZnO NRs on both the membrane surface and pore sidewall. Thanks to the nanofibrous shape of the grown ZnO NRs on the pore sidewall, the membrane filters have shown a high (>97%) filtration efficiency for PM2.5 with a rather low-pressure (~80 Pa) drop. The membrane filters could easily be cleaned and reused many times by simple spray cleaning with a water/ethanol mixture solution. Further, the grown ZnO NRs have also endowed excellent bactericidal performance for both Gram-positive S. aureus and Gram-negative S. enteritidis bacteria. Owing to the wide bandgap semiconductor nature of ZnO NRs, organic decomposition by photocatalytic activity under UV illumination has been successfully demonstrated. The reusable, multifunctional membrane filters can find wide applications in air filtration and purification.

Structuring of Si into Multiple Scales by Metal-Assisted Chemical Etching.

Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

Selective growth of ZnO nanorods by thickness contrast in In-doped ZnO quantum dots seed layer.

Selective growth of ZnO nanorods (NRs) have been demonstrated using thickness contrast in In-doped ZnO (IZO) quantum dot (QD) seed layer. The use of IZO QD as a seed layer has enabled the direct growth of ZnO NRs on soft substrates such as polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS). Depending on the annealing temperature, the seed layers show different grain sizes: as the annealing temperature increases, the seed grain size also increases accordingly. Interestingly, the hydrothermal growth of ZnO NRs has been found to depend on the seed grain size: the larger grain seed sample shows earlier start of growth compared to smaller seed grain counterpart. The same growth behavior has been found in the growth of ZnO NRs on seed layers having different thickness, due again to the difference in seed grain size. To advantageously exploit the observed growth behavior, the IZO QDs seed layers have been patterned by soft lithographic technique, which led to the formation of alternating thin/thick region periodically. On this patterned seed surface, the thin regions showed earlier start of NRs growth compared to thick regions, enabling the spatially selective growth of ZnO NRs. When applied for acetone gas sensors, the selectively grown sample showed better performance than the non-selectively grown counterpart. The low resistance in air, due to increased amount of chemisorbed oxygen, has been found to be responsible for the inferior sensor performance with non-selectively grown sample.

Droplet-Mediated Deterministic Microtransfer Printing: Water as a Temporary Adhesive.

Deterministic transfer printing of solid objects has been introduced and demonstrated, where capillary force based on water droplet has been utilized. Droplets on pickup head form capillary bridges with chips to be picked, where the capillarity provides enough force to grip or lift the chip. Release or printing of chips can be achieved by simply drying droplet while the chip is in contact with receiver substrate. That is, water droplet acts as a temporary adhesive, adheres onto solid chip by capillary force, and releases it upon drying. The technique has unique features such as self-alignment during pickup, self-correction on pickup head by short exposure to water mist, and pickup capability of solids having a nonflat, corrugated surface, all of which originated from the fluidic nature of water droplet. The technique has been successfully applied for the fabrication of stretchable micro-light-emitting diode chip arrays. The proposed technique can find wide applications in many fields such as displays, sensors, printed electronics, photovoltaics, etc.

Correlation between electrical properties and cation distribution in [(Ni x Mn1- x )0.84Cu0.16]3O4 thin films prepared by metal-organic decomposition for microbolometer applications.

[(Nix Mn 1- x )0.84Cu 0.16]3O4 (0.20 ≤ x ≤ 0.40) thin films have been prepared using the metal-organic decomposition method for microbolometer applications. Spinel thin films with a thickness of approximately 100 nm were obtained from the [(Ni x Mn1- x )0.84Cu0.16]3O4 films annealed at the low temperature of 380 °C for 5 h, which enables their direct integration onto substrates having complementary metal-oxide-semiconductor (CMOS) read-out circuitry. To obtain negative-temperature-coefficient films with reasonable performance through low enough temperature anneal process, Ni content has been systematically varied, and the film microstructure has been found to depend on the relative amount of Ni and Mn. A single phase of cubic spinel structure has been confirmed in the prepared films. The resistivity (ρ) of the annealed films decreases with increasing Mn4+/Mn3+ value due to the hopping mechanism between Mn3+ and Mn4+ cations in octahedral sites of spinel structure. Although the temperature coefficient of resistance (TCR) of the annealed films has been decreased slightly with the increase of Ni content, good enough properties of the film (ρ = 61.3 Ω•cm, TCR = -2.950%/K in x = 0.30 film) has been obtained even with the annealing at rather low temperature of 380 °C, thus enabling the direct integration onto substrates having read-out circuitry. The results obtained in this work are promising for applications to CMOS integrated microbolometer devices.

Stretchable, Bifacial Si-Organic Hybrid Solar Cells by Vertical Array of Si Micropillars Embedded into Elastomeric Substrates.

Stretchable electronics has enabled many unforeseen applications in a variety of fields. Mechanical design concepts to achieve the stretchability without affecting the device functionality, however, are limited to few known practices, such as mechanical buckling, serpentine shape, or simple elastomeric composites. In this paper, we propose another mechanics design principle for high stretchability (>100%) based on the composite of vertical array of Si micropillars embedded into elastomer poly(dimethylsiloxane). The orthogonalization of active functional elements to applied strain direction enables highly stretchable electronic devices, where the applied strain is mostly absorbed into elastomer on interpillar space. On the other hand, the vertical pillars do not experience any noticeable strain at all. As a proof-of-concept demonstration, we fabricate stretchable Si-organic hybrid solar cells using such a design and the cell shows reasonable level of cell efficiency compared with planar counterparts. The cell can be stretched reversibly without any noticeable performance degradation. Furthermore, the cell can be operated in a bifacial mode by employing stretchable, transparent Ag nanowire-based electrodes. The mechanical design for stretchability demonstrated here would provide new opportunities for stretchable electronics.

Facile Patterning of Ag Nanowires Network by Micro-Contact Printing of Siloxane.

A simple, low-cost, scalable patterning method has been demonstrated for chemically welded Ag nanowires (AgNWs) network. The chemically welded network of AgNWs on substrates has been patterned by modified microcontact printing (µCP). As an ink for the µCP, uncured high-viscosity siloxane polymer has been applied. Using elastomeric polydimethylsiloxane (PDMS) stamp that has been replicated from micromachined Si master mold by metal-assisted chemical etching, the printed siloxane ink materials have been cured by simple UV/ozone exposure for 3 min, which acts as an etch barrier in ensuing wet-removal of exposed AgNWs network. The proposed patterning technique has no limitation in the choice of substrates and pattern shape, in addition to high resolution. The patterned AgNWs network electrodes have shown excellent optical, electrical, and mechanical performances, such as high flexibility (up to ∼10%) and stretchability (up to 40%). Finally, the patterned AgNWs network electrodes have been applied as a transparent heater, which can be used for rapid raindrop removal or deicing of car windows and outside mirrors. This can be a valuable help for driving safety under harsh weather conditions.

Room-Temperature Chemical Welding and Sintering of Metallic Nanostructures by Capillary Condensation.

Room-temperature welding and sintering of metal nanostructures, nanoparticles and nanowires, by capillary condensation of chemical vapors have successfully been demonstrated. Nanoscale gaps or capillaries that are abundant in layers of metal nanostructures have been found to be the preferred sites for the condensation of chemically oxidizing vapor, H2O2 in this work. The partial dissolution and resolidification at such nanogaps completes the welding/sintering of metal nanostructures within ∼10 min at room-temperature, while other parts of nanostructures remain almost intact due to negligible amount of condensation on there. The welded networks of Ag nanowires have shown much improved performances, such as high electrical conductivity, mechanical flexibility, optical transparency, and chemical stability. Chemically sintered layers of metal nanoparticles, such as Ag, Cu, Fe, Ni, and Co, have also shown orders of magnitude increase in electrical conductivity and improved environmental stability, compared to nontreated ones. Pertinent mechanisms involved in the chemical welding/sintering process have been discussed. Room-temperature welding and sintering of metal nanostructures demonstrated here may find widespread application in diverse fields, such as displays, deformable electronics, wearable heaters, and so forth.

Direct Visualization of Etching Trajectories in Metal-Assisted Chemical Etching of Si by the Chemical Oxidation of Porous Sidewalls.

We demonstrate a simple method for the visualization of trajectories traced by noble metal nanoparticles during metal-assisted chemical etching (MaCE) of Si. The nanoporous Si layer formed around drilled pores is converted into SiO2 by simple chemical oxidation. Etch removal of the remaining Si using alkali hydroxide leaves SiO2 nanostructures that are the exact replica of those drilled pores or etching trajectories. The differences in etching characteristics between Ag and Au have been investigated using the proposed visualization method. The shape and chemical stability of metal nanoparticles used for MaCE have been found to be critical in determining etching paths. The proposed method would be very helpful in studying the fundamental mechanism of MaCE as well as in micro/nanostructuring of the Si surface for various applications. This approach can also be used for the generation of straight or helical SiO2 nanotubes.

Glass and Plastics Platforms for Foldable Electronics and Displays.

Reversibly, repeatedly foldable electronics and displays are enabled by employing engineered glass or plastics substrates, where folding deformation is localized in thinned parts only. This design concept can further be extended to dual folding, leading to size reduction down to 1/4. Notably, the foldable electronics and displays can be implemented with no need to introduce any novel materials.

Bulk micromachining of Si by metal-assisted chemical etching.

Bulk micromachining of Si is demonstrated by the well-known metal-assisted chemical etching (MaCE). Si microstructures, having lateral dimension from 5 µm up to millimeters, are successfully sculpted deeply into Si substrate, as deep as >100 µm. The key ingredient of this success is found to be the optimizations of catalyst metal type and its morphology. Combining the respective advantages of Ag and Au in the MaCE as a Ag/Au bilayer configuration leads to quite stable etch reaction upon a prolonged etch duration up to >5 h. Further, the permeable nature of the optimized Ag/Au bilayer metal catalyst enables the etching of pattern features having very large lateral dimension. Problems such as the generation of micro/nanostructures and chemical attacks on the top of pattern surface are successfully overcome by process optimizations such as post-partum sonication treatment and etchant formulation control. The method can also be successful to vertical micromachining of Si substrate having other crystal orientations than Si(100), such as Si(110) and Si(111). The simple, easy, and low-cost nature of present approach may be a great help in bulk micromachining of Si for various applications such as microelectromechanical system (MEMS), micro total analysis system (µTAS), and so forth.

Vertical arrays of SiO2 micro/nanotubes templated from Si pillars by chemical oxidation for high loading capacity buoyant aquatic devices.

A simple and facile method to fabricate SiO2 micro- or nanotubes has been demonstrated based on room temperature wet chemical oxidation of a porous layer of Si pillar templates that have been prepared by metal-assisted chemical etching (MaCE). Under typical conditions, Si pillars produced by the MaCE have been found to be covered with a thin nanoporous Si layer. The porous Si skin layer has been chemically oxidized by simple dipping in AgNO3 solution at room temperature, which has led to seamless SiO2 shell layer thanks to the accompanying volume expansion during the wet oxidation. Following wet removal of core Si by KOH yields the SiO2 micro- or nanotubes, either in test tube shape or in open shape at both ends, depending on processing method. The vertical arrays of the SiO2 tube on the Si substrate, after hydrophobic siloxane oligomer printing, has been found to have very large loading capacity on water, due to extremely high porosity (>90%) and good enough mechanical stability. The novel method to fabricate SiO2 tubes can shed new light in design of novel aquatic devices, other than simple mimicking the leg of a water strider. Also, the method may be very helpful in various applications of SiO2 nanotubes.

Facile and clean release of vertical Si nanowires by wet chemical etching based on alkali hydroxides.

A simple method to release Si nanowires (SiNWs) from a substrate, with their original length almost intact, is demonstrated. By exploiting the unique chemistry involved for the fabrication of vertical arrays of SiNWs in metal-assisted chemical etching (MaCE) based either on HF/AgNO3 or HF/H2O2 chemistries, wet etching with alkali hydroxides such as NaOH or KOH preferentially attacks the bottom part of the vertical SiNWs. A protective layer of Si oxide is found to exist on the outer wall of the SiNWs and to play the key role of etch mask during the release-etching by alkali hydroxides. The clean release of SiNWs also enables the repeated use of the Si substrate for the fabrication of vertical SiNW arrays by MaCE. The released SiNWs are further used for the fabrication of field-effect transistors on a flexible plastic substrate. The method developed here, when combined with a suitable assembling technique, can be very useful in implementing flexible electronics, or in the fabrication of SiNW composites with other functional materials.

Hydrophobically recovered and contact printed siloxane oligomers for general-purpose surface patterning.

Hydrophobic recovery of elastomeric polydimethylsiloxane (PDMS) has been well-known in various fields, such as microcontact printing (microCP), microfluidics, and electric insulation, etc., which has been believed to be due to the transfer of out-diffused siloxane oligomers in PDMS. The recovery phenomenon has been used to control surface energy of a substrate, due partly to its nanoscale thickness. In this work, we extend the use of recovered oligomers to a general-purpose surface patterning process, in combination with both dry and wet pattern transfer processes. The out-diffused and transfer-printed oligomers play exactly the same role of "ink" in the conventional microCP; thus, the present method can be termed as "inkless" microcontact printing (ImicroCP). Also, the detailed nature of recovered oligomers has been investigated, and they are found to have a molecular weight approximately 10 times larger than that of pristine, uncured PDMS oligomers. And the molecular weight distribution is very broad with a polydispersity index of approximately 15. Then, we present and discuss various aspects of the ImicroCP process, such as pattern transfer onto substrate via wet or dry etching, effect of process variables on printing results, minimum feature size achieved by the technique, repeated printing with the same stamp, and the generation of more complex patterns from simpler ones by applying multiple ImicroCP.

Synthesis of linked carbon monolayers: films, balloons, tubes, and pleated sheets.

Because of their potential for use in advanced electronic, nanomechanical, and other applications, large two-dimensional, carbon-rich networks have become an important target to the scientific community. Current methods for the synthesis of these materials have many limitations including lack of molecular-level control and poor diversity. Here, we present a method for the synthesis of two-dimensional carbon nanomaterials synthesized by Mo- and Cu-catalyzed cross-linking of alkyne-containing self-assembled monolayers on SiO(2) and Si(3)N(4). When deposited and cross-linked on flat surfaces, spheres, cylinders, or textured substrates, monolayers take the form of these templates and retain their structure on template removal. These nanomaterials can also be transferred from surface to surface and suspended over cavities without tearing. This approach to the synthesis of monolayer carbon networks greatly expands the chemistry, morphology, and size of carbon films accessible for analysis and device applications.

On the mechanism of low-pressure imprint lithography: capillarity vs viscous flow.

Dominant mechanisms in low-pressure imprint lithography processes have been identified for the regimes that are definable in terms of applied pressure, temperature, and mold material characteristics. Capillarity is found to be the dominant mechanism at high temperature and low pressure when stiff, hard molds are used. In the case of flexible thin-film ( approximately 20 microm) molds, both the capillarity and the viscous flow are involved. Both mechanisms are operative in the initial stage of the imprinting, but the capillarity takes over as time progresses.

Molecular scale buckling mechanics in individual aligned single-wall carbon nanotubes on elastomeric substrates.

We have studied the scaling of controlled nonlinear buckling processes in materials with dimensions in the molecular range (i.e., approximately 1 nm) through experimental and theoretical studies of buckling in individual single-wall carbon nanotubes on substrates of poly(dimethylsiloxane). The results show not only the ability to create and manipulate patterns of buckling at these molecular scales, but also, that analytical continuum mechanics theory can explain, quantitatively, all measurable aspects of this system. Inverse calculation applied to measurements of diameter-dependent buckling wavelengths yields accurate values of the Young's moduli of individual SWNTs. As an example of the value of this system beyond its use in this type of molecular scale metrology, we implement parallel arrays of buckled SWNTs as a class of mechanically stretchable conductor.