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There is an increasing interest in developing next-generation wearable ultrasound patch systems because of their wide range of applications, such as home healthcare systems and continuous monitoring systems for physiological conditions. A wearable ultrasound patch system requires a stable interface to the skin, an ultrasound coupling medium, a flexible transducer array, and miniaturized operating circuitries. In this study, we proposed a patch composed of calcium (Ca)-modified silk, which serves as both a stable interface and a coupling medium for ultrasound transducer arrays. The Ca-modified silk patch provided not only a stable and conformal interface between the epidermal ultrasound transducer and human skin with high adhesion but also offered acoustic impedance close to that of human skin. The Ca-modified silk patch was flexible and stretchable (∼400% strain) and could be attached to various materials. In addition, because the acoustic impedance of the Ca-modified silk patch was 2.15 MRayl, which was similar to that of human skin (1.99 MRayl), the ultrasound transmission loss of the proposed patch was relatively low (∼0.002 dB). We also verified the use of the Ca-modified silk patch in various ultrasound applications, including ultrasound imaging, ultrasound heating, and transcranial ultrasound stimulation for neuromodulation. The comparable performance of the Ca-modified patch to that of a commercial ultrasound gel and its durability against various environmental conditions confirmed that the Ca-modified silk patch could be a promising candidate as a coupling medium for next-generation ultrasound patch systems.

Citrus fibers are a sidestream of the pectin extraction process from citrus peel. This sidestream can be converted into a functional ingredient through a shear-induced homogenization process. One technical challenge with this material is that dehydration and subsequent rehydration result in reduction of viscosity compared to the original product. In this study, various drying methods were compared with never-dried fibers to investigate the structural changes underlying the viscosity loss. Infrared and X-ray diffraction analyses confirmed no changes in chemical composition and crystalline structure of citrus fibers. The dynamic light scattering and sum frequency generation analyses of citrus fiber suspension showed that the rehydration process could not fully disperse aggregated fibers, which appears to be the main cause for the viscosity loss.

We demonstrated an effective poly(p-chloro-xylylene) (Parylene-C) encapsulation method for MAPbI3 solar cells. By structural and optical analysis, we confirmed that Parylene-C efficiently slowed the decomposition reaction in MAPbI3. From a water permeability test with different encapsulating materials, we found that Parylene-C-coated MAPbI3 perovskite was successfully passivated from reaction with water, owing to the hydrophobic behavior of Parylene-C. As a result, the Parylene-C-coated MAPbI3 solar cells showed better device stability than uncoated cells, virtually maintaining the initial power conversion efficiency value (15.5 ± 0.3%) for 196 h.

Strain engineering of perovskite quantum dots (pQDs) enables widely tunable photonic device applications. However, manipulation at the single-emitter level has never been attempted. Here, we present a tip-induced control approach combined with tip-enhanced photoluminescence (TEPL) spectroscopy to engineer strain, bandgap, and the emission quantum yield of a single pQD. Single CsPbBrxI3-x pQDs are clearly resolved through hyperspectral TEPL imaging with ∼10 nm spatial resolution. The plasmonic tip then directly applies pressure to a single pQD to facilitate a bandgap shift up to ∼62 meV with Purcell-enhanced PL increase as high as ∼105 for the strain-induced pQD. Furthermore, by systematically modulating the tip-induced compressive strain of a single pQD, we achieve dynamical bandgap engineering in a reversible manner. In addition, we facilitate the quantum dot coupling for a pQD ensemble with ∼0.8 GPa tip pressure at the nanoscale estimated theoretically. Our approach presents a strategy to tune the nano-opto-electro-mechanical properties of pQDs at the single-crystal level.

An improved efficiency of non-fullerene acceptor-based organic solar cells based on PBDB-T : ITIC-m composites has been obtained by using post-additive soaking (PAS) treatments on pristine BHJ films. The PAS-treated BHJ films show improved carrier dynamics in charge transfer and transport, leading to increased photocurrent and fill factors in the photovoltaic devices, followed by a high power conversion efficiency of 12.3%, which is an increase of 28% compared to that of the control device. We found that this superior performance predominantly resulted from improved exciton dissociation and enhanced electron transport after photoexcitation of the ITIC-m phase.

Methylammonium lead halide (MAPbI3) perovskites are considered as promising materials owing to their excellent optical and electrical properties. However, perovskite materials suffer from degradation in air, which limits their practical applications. Here, we demonstrate successful passivation of the MAPbI3 photodetectors through monochloro-para-xylylene (Parylene-C) deposition. The time-dependent photocurrent characteristics were systematically investigated, and we achieved significantly improved device performance and stability with Parylene-C passivation. Based on the excitation-power-dependent photoluminescence (PL) data, we confirmed that Parylene-C can reduce the carrier losses in MAPbI3, leading to the enhancement of photocurrent and PL in MAPbI3 photodetectors.

We synthesized the gold nanoparticles (Au NPs)-embedded methylammonium lead iodide (MAPbI3) film for the first time. The effects of metal nanoparticles on MAPbI3 perovskite were systematically studied using UV-Vis absorption and photoluminescence (PL) measurements. As a result, the 20-nm-sized Au NPs-embedded MAPbI3 film exhibited a 4.15% higher absorbance than the bare MAPbI3 film. Moreover, the average PL intensity of the Au NPs-embedded MAPbI3 film increased by about 75.25% over the bare MAPbI3 film. Therefore, we have confirmed that addition of the Au NPs has a positive effect on the optical properties of MAPbI3, and we believe that this study will provide a basic insight into the metal nanoparticles-embedded perovskite thin films for the future optoelectronic applications.

With the anticipation for a more connected world through the Internet of Things, there is still a strong demand for miniaturized chemical sensors. Here, we report on a miniaturized resonant chemical sensor based on a Capacitive Micromachined Ultrasonic Transducer (CMUT) with a low operating voltage suitable for portable gas sensor applications. Previously reported CMUT chemical sensors required a DC operating voltage (16∼50 V) higher than the supply voltages of common circuits (e.g., 1.8–5 V). Thus, additional circuitry such as a charge pump circuit often was required as a part of the sensor interface circuits to supply the DC voltage to CMUT. This resulted in additional power consumption and a larger footprint. In this work, the vacuum gap of the CMUT which determines the operating voltage was reduced to 50 nm through the development of a double oxidation process with a high wafer-level yield. We achieved a significantly smaller pull-in voltage (

Grain boundaries (GBs) in polycrystalline materials act as impediments to dislocation motion and result in strengthening. Understanding slip transmission through GBs, specifically twin boundaries, is essential to understand the plastic deformation behavior of polycrystalline fcc materials. In this study the interaction between a glide dislocation and Σ 3 { 112 } incoherent twin boundary (ITB) in copper is investigated using a combined atomistic and mesoscale approach. The material parameters and structure of the GB in the mesoscale phase field dislocation dynamics (PFDD) model are informed from Molecular Statics (MS) simulations. The structural unit of the ITB consists of an array of three partial dislocations. The interaction between a glide dislocation impinging on each of the GB partial dislocations is investigated using both PFDD and Molecular Dynamics (MD) with two boundary conditions. Transmission planes predicted by both PFDD and MD (NVT) are in agreement, and show that not all transmission events are direct. Critical transmission stresses predicted by PFDD are in the range of 276 MPa to 1380 MPa, while MD predictions are in the range from 100 MPa to 700 MPa. The PFDD and MD predictions of slip transmission are explained using dislocation theory based on isotropic linear elasticity.

Body-Centered Cubic (BCC) metals exhibit anomalous mechanical properties such as twinning/anti-twinning and tension/compression asymmetry, which are attributed to asymmetric behavior of screw dislocations. Unlike Face-Centered Cubic (FCC) metals, the Critical Resolved Shear Stress (CRSS) of BCC metals deviates from the Schmid law. We use a mesoscale modeling approach called Phase Field Dislocation Dynamics (PFDD) to understand the critical factors that control this non-Schmid behavior and reproduce atomistic predictions of the CRSS (Peierls stress). All inputs to the PFDD model are obtained from Molecular Statics (MS) simulations. Multiple pathways for modeling the non-Schmid behavior are investigated by incorporating the representative dislocation properties into different energy terms in the PFDD model. One way to understand non-Schmid behavior is to incorporate stress components projected on inclined planes into the external energy term within PFDD. Alternatively, we propose that non-Schmid behavior can also be accounted for by considering the variation of the { 110 } unstable stacking fault energy and the dislocation core width as a function of the applied tensorial stress field. The CRSS predicted using PFDD modeling is in excellent agreement with MS predictions.

We synthesized multiple-cation Rb(MAFA)PbI3 perovskite single crystals for the first time. The effect of Rb+ substitution was systemically investigated, and the addition of 1.5 M 5% RbI was the optimum condition to obtain high-quality Rb(MAFA)PbI3 single crystals. Lattice shrinkage occurred in the Rb(MAFA)PbI3 single crystal because of the small ionic radius of Rb+, resulting in blue-shifted absorption and photoluminescence (PL) peaks. The 1.5 M 5% RbI-added (MAFA)PbI3 single crystal showed the longest carrier lifetime of 18.35 ns, exhibiting the highest photoresponse than other crystals. We believe that this work will provide a basic insight into the mixed-cation perovskite single crystals for the future optoelectronic applications.

Accurate detection of neuropeptides in cerebrospinal fluid (CSF) plays an important role in both indepth studies and early diagnosis of neurological diseases. Here, we report a biosensor based on Capacitive Micromachined Ultrasonic Transducer (CMUT) which is capable of detecting low concentrations (pg $\sim $ ng/ml) of a neuropeptide involved with the progression of Alzheimer's diseases, somatostatin (SST). A 10-MHz CMUT was fabricated and utilized as a physical resonant sensor which detects the change in the concentration of analyte through the mass-loading mechanism. The resonant plate was sequentially coated with protein G and antibodies to provide specificity to SST; Cysteine-tagged protein G layer enables controlled immobilization of antibodies in a welloriented manner. The change in the resonant frequency of the CMUT sensor was measured after incubating the sensor in various concentrations of SST. The significant shifts in the resonant frequency were observed for SST concentrations in the range of 10 pg/ml $\sim 1$ ng/ml. Compared to the previously reported biosensors developed for SST detection, our sensor shows discernable responses for SST that are $\sim 6$ orders of magnitude lower in concentration. Thus, this work demonstrates the potential of the CMUT resonant sensor as a promising biosensor platform for detection of neuropeptides involved with neurodegenerative diseases that often exist in low concentrations in CSF.

Development of flexible, biocompatible, biodegradable, and small electronic components is of great interest for implantable medical devices. In this work, we present a highly dense silk-fibroin-based memristor array, fabricated using our newly developed fabrication technique based on UV photolithography. This is the first demonstration of silk-fibroin-based memristors implemented in a cross-bar architecture with a high cell density (single cell of 20 × 20 μm2 and a density of 105 cm−2) which allows individual access to cells. In addition, we implement a fully flexible silk-fibroin-based memristor array fabricated on a Parylene-C film and demonstrate its reliable resistive switching and biodegradability in an enzymatic solution.

An accurate bladder volume monitoring system is a critical component in diagnosis and treatment of urological disorders. Here, we report an implantable bladder volume sensor with a multi-level resistor ladder which estimates the bladder volume through discrete resistance values. Discretization allows the sensor output to be resilient to the long-term drift, hysteresis, and degradation of the sensor materials. Our sensor is composed of biocompatible polypyrrole/agarose hydrogel composite. Because Young's modulus of this composite is comparable to that of the bladder wall, the effect of mechanical loading of the sensor on the bladder movement is minimized which allows more accurate volume monitoring. We also demonstrate the patterning and molding capability of this material by fabrication various structures. Lastly, we successfully demonstrate the functionality of the multi-level resistor ladder sensor as a bladder volume sensor by attaching the sensor on the pig's bladder and observing the impedance change of the sensor.

We compute the structural energies, elastic constants, and stacking fault energies, and investigate the phase stability of monoborides with different compositions $\left({X}_{1\ensuremath{-}x}^{1}{X}_{x}^{2}\right)\text{B}$ $(X=\text{Ti/Fe/Mo/Nb/V})$ using density functional theory in order to search for Ti monoborides with improved mechanical properties. Our computed Young's modulus and Pugh's modulus ratio, which correlate with stiffness and toughness, agree well with predictions from Vegard's law with the exceptions of mixed monoborides containing Mo and Fe. Among all the monoborides considered in this paper, TiB has the smallest Pugh's ratio, which suggests that the addition of solutes can improve the toughness of a Ti matrix. When ${X}^{1}\mathrm{B}$ and ${X}^{2}\mathrm{B}$ are respectively most stable in the ${B}_{27}$ and ${B}_{f}$ structures, the mixed monoborides $\left({X}_{1\ensuremath{-}x}^{1}{X}_{x}^{2}\right)\text{B}$ have a lower or similar stacking fault energy than TiB and could therefore improve the ductility of the Ti matrix. Among all $\left({X}_{0.5}^{1}{X}_{0.5}^{2}\right)\text{B}$, mixed $({\mathrm{Ti}}_{0.5}{\mathrm{Mo}}_{0.5})\text{B}$ and mixed $({\mathrm{Ti}}_{0.5}{\mathrm{V}}_{0.5})\text{B}$ have a higher Young's modulus, a higher Pugh's ratio, and a smaller stacking fault energy than TiB. We also construct phase diagrams and find large solubility limits for solid solutions containing Ti compared to those containing Fe.

H.Y. and K.L. contributed equally to this work. This study was supported by the Center for Advanced Soft Electronics as Global Frontier Project (CASE-2013M3A6A5073173) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning. This study was also supported by Multi-Ministry Collaborative R&D Program (Development of Techniques for Identification and Analysis of Gas Molecules to Protect Against Toxic Substances) through the National Research Foundation of Korea (NRF) funded by KNPA, MSIT, MOTIE, ME, NFA (Grant No. 2017M3D9A1073501).

This paper describes a useful means of noninvasively enhancing transdermal delivery efficiency. For this, chitosan nanocapsules with positive surface charges were fabricated by using the in situ precipitation method. These nanocapsules cannot only have an ability to encapsulate the drug molecule (this study used riboflavin 5′-monophosphate), but also electrotatically interact with the stratum corneum layer. To demonstrate this, fluorescence-labeled polymer nanoparticles with different particle sizes as well as surface charges were topically applied onto the skin and their distribution was directly imaged. This demonstration experiment allowed us to figure out that once the nanocapsules were provided with positive surface charges, they readily deposited into the stratum corneum layer due to the electrostatic interaction. Further quantitative characterization of the penetrating amount of riboflavin 5′-monophosphate by using the Frantz diffusion cell method showed that our chitosan nanocapsule system effectively improved transdermal delivery efficiency.

PEG-functionalized graphene quantum dots (GQDs) are shown to promote fast exciton dissociation in organic solar cells. Short-chain PEG promotes the most favorable interaction with other organic layers, and the overall efficiency is improved by 36% when compared to the reference devices. The mechanism of enhancement is shown to be increased absorption due to fewer charges remain-ing in the bound state.