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The worldwide proliferation of COVID-19 poses the urgent need for sterilizable and transparent air filters to inhibit virus transmission while retaining ease of communication. Here, we introduce copper nanowires to fabricate transparent and self-sterilizable air filters. Copper nanowire air filter (CNAF) allowed visible light penetration, thereby can exhibit facial expressions, helpful for better communication. CNAF effectively captured particulate matter (PM) by mechanical and electrostatic filtration mechanisms. The temperature of CNAF could be controlled by Joule-heating up to 100 °C with thermal stability. CNAF successfully inhibited the growth of E. coli because of the oligodynamic effect of copper. With heat sterilization, the antibacterial efficiency against G. anodireducens was greatly improved up to 99.3% within 10 min. CNAF showed high reusability with stable filtration efficiency and thermal antibacterial efficacy after five repeated uses. Our result suggests an alternative form of active antimicrobial air filter in preparation for the current and future pandemic situations.

Synthetic biomaterials are used to overcome the limited quantity of human-derived biomaterials and to impart additional biofunctionality. Although numerous synthetic processes have been developed using various phases and methods, currently commonly used processes have some issues, such as a long process time and difficulties with extensive size control and high-concentration metal ion substitution to achieve additional functionality. Herein, we introduce a rapid synthesis method using a laser-induced hydrothermal process. Based on the thermal interaction between the laser pulses and titanium, which was used as a thermal reservoir, hydroxyapatite particles ranging from nanometer to micrometer scale could be synthesized in seconds. Further, this method enabled selective metal ion substitution into the apatite matrix with a controllable concentration. We calculated the maximum temperature achieved by laser irradiation at the surface of the thermal reservoir based on the validation of three simplification assumptions. Subsequent linear regression analysis showed that laser-induced hydrothermal synthesis follows an Arrhenius chemical reaction. Hydroxyapatite and Mg

The patterning of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hydrogels with excellent electrical property and spatial resolution is a challenge for bioelectronic applications. However, most PEDOT:PSS hydrogels are fabricated by conventional manufacturing processes such as photolithography, inkjet printing, and screen printing with complex fabrication steps or low spatial resolution. Moreover, the additives used for fabricating PEDOT:PSS hydrogels are mostly cytotoxic, thus requiring days of detoxification. Here, we developed a previously unexplored ultrafast and biocompatible digital patterning process for PEDOT:PSS hydrogel via phase separation induced by a laser. We enhanced the electrical properties and aqueous stability of PEDOT:PSS by selective laser scanning, which allowed the transformation of PEDOT:PSS into water-stable hydrogels. PEDOT:PSS hydrogels showed high electrical conductivity of 670 S/cm with 6-μm resolution in water. Furthermore, electrochemical properties were maintained even after 6 months in a physiological environment. We further demonstrated stable neural signal recording and stimulation with hydrogel electrodes fabricated by laser.

In optical transmission technique using optical polarization, the change of state of polarization (SOP) and the rotation of SOP (RSOP) during optical transmission has a significant impact on the transmission performance. We propose polarized intensity rotational frequency shift keying (PIR-FSK) as a novel modulation technique in change of SOP and RSOP-tolerant optical transmissions. The proposed PIR-FSK signal does not affect the entire intensity of the optical carrier by modulating the sinusoidal signals on each X and Y-polarized optical carrier. Therefore, the proposed PIR-FSK modulation and optical carrier intensity modulation can operate at the same time. Moreover, since the proposed technique does not modulate the signal using the SOP unlike the PolSK, PIR-FSK is not affected by SOP changes that occur during transmission, and the signal does not be degraded even with RSOP. We demonstrated that the signals modulated using the proposed PIR-FSK modulation have higher signal capacity and efficiency compared to the signal modulated using PolSK modulation.

A thermal haptic device (THD) is used to implement temperature information in many virtual environments. The THD enables a user to feel the temperature as well as the thermal conductivity. Moreover, as temperature influences human emotion and preference, the THD enriches senses and experiences in a virtual environment. In this paper, we propose laser-assisted dual-function copper nanowire (CuNW) polyurethane acrylate (PUA) patterns for use as feedback controllable stretchable heaters as a 12-pixels THD, with highly enhanced mechanical and chemical durability. The CuNW–PUA pattern retains the stretchability from its serpentine mesh form, and the CuNW embedded in the PUA structure provides mechanical and chemical stability, facilitating a stable resistance. The CuNW–PUA pattern serves as a simultaneous heater and thermometer with accurate temperature control. Furthermore, the CuNW–PUA pattern is fabricated using a simple, fast, and elaborate laser process under ambient conditions. Finally, the CuNW–PUA pattern was used to realize heat transfer in various virtual environments in the form of 12-pixels on a nylon glove, showing potential for stretchable applications in next generation devices.

Transparency in electronics can provide extra functionality and esthetic impression. Transparency plays an important role in accurate soft robot control because one can directly observe target surface condition that is usually blocked by a robot's body. Nowadays, demand for soft actuators has been rapidly increasing because soft robots have attracted much attention recently. However, conventional soft actuators are usually nontransparent with simple isotropic bending, limited performance, and limited functionality. To overcome such limitations of current soft robots, we developed a novel soft shape morphing thin film actuator with new functionalities such as high transparency and unique directional responses to allow complex behavior by integrating a transparent metal nanowire heater. A figure of merit was developed to evaluate the performance and derive an optimum design configuration for the transparent actuator with enhanced performance. As a proof of concept, various transparent soft robots such as transparent gripper, Venus flytrap, and transparent walking robot were demonstrated. Such transparent directional shape morphing actuator is expected to open new application fields and functionalities overcoming limitations of current soft robots.

Evolution has decided to gift an articular structure to vertebrates, but not to invertebrates, owing to their distinct survival strategies. An articular structure permits kinematic motion in creatures. However, it is inappropriate for creatures whose survival strategy depends on the high deformability of their body. Accordingly, a material in which the presence of the articular structure can be altered, allowing the use of two contradictory strategies, will be advantageous in diverse dynamic applications. Herein, spatial micro‐water molecule manipulation, termed engineering on variable occupation of water (EVO), that is used to realize a material with dual mechanical modes that exhibit extreme differences in stiffness is introduced. A transparent and homogeneous soft material (110 kPa) reversibly converts to an opaque material embodying a mechanical gradient (ranging from 1 GPa to 1 MPa) by on‐demand switching. Intensive theoretical analysis of EVO yields the design of spatial transformation scheme. The EVO gel accomplishes kinematic motion planning and shows great promise for multimodal kinematics. This approach paves the way for the development and application of smart functional materials., In this paper, a strategy that manipulates micro‐water of hydrogel in reversible, selective, and on‐demand manner is reported. It realizes instant switching between a transparent and homogeneously soft gel and an opaque and heterogeneously rigid solid with a monolithic mechanical gradient. These two mechanical dual modes mimicking the characteristic of invertebrate and vertebrate demonstrate multi‐modal kinematics.

To mitigate the nonlinear distortion of a multiband orthogonal frequency division multiplexing (OFDM) radio over fiber (RoF) system, a frequency-separated model-based digital pre-distortion (DPD) technique is proposed and experimentally demonstrated herein. Frequency-separated model DPD, when used as a pre-equalizer, mitigates nonlinearity in a transmitter. The nonlinear characteristic of an RoF system depends on the frequency band. Therefore, the proposed equalizer separates the input by frequency band and mitigates nonlinearities. A least mean square algorithm is used to estimate the variables of the proposed equalizer. Using the proposed DPD technique, performance enhancement was verified experimentally relative to the adjacent carrier leakage ratio (ACLR). The proposed technique obtained ACLR values of 27 to 30 dB. The results demonstrate that signal performance improved in terms of error vector magnitude from 6.3% to 3.5% at an optical modulation index value of 63%.

We propose a spectral power shaping technique for use in intermediate frequency (IF) over fiber systems. The technique redistributes the power of nonlinear components by adjusting the spectral power of the signal. Experiments performed using 10 km distance transmission system show that the proposed technique effectively controls the power of the nonlinear components in a broadband system. The feasibility of the proposed technique was experimentally verified in terms of the error vector magnitude (EVM) and degree of channel linearity.

This paper proposes a semiconductor optical amplifier (SOA)-based multilevel polarization shift on–off keying (MPS-OOK) transmission for free-space optical (FSO) communication. At the transmitter end, the MPS-OOK signal is modulated with a constant linear state of polarization (SOP) at the high-intensity level and various SOPs at the low-intensity level in order to improve the spectral efficiency (SE) with the transmitted power efficiency. At the receiver end, first, a polarization-independent SOA was introduced to optically suppress the turbulence-induced scintillation effect and equalize the intensities of the various SOPs in the deep gain saturation state without polarization distortion. Then, a linear polarizer (LP) with a high extinction ratio (ER) was deployed to convert the equalized SOPs into a known intensity. Finally, the converted MPS-OOK signal was detected using a single photodiode (PD) and distinguished using a multilevel fixed-threshold decision (M-FTD). The proposed technique was evaluated using experiments. A Mach–Zehnder modulator (MZM)-based fading simulator was introduced to emulate the turbulence-induced scintillation effect. The experimental results demonstrated that the scintillation effect was effectively mitigated and the SE was improved by up to 2 bit/s/Hz using the proposed four-level polarization shift on–off keying (4PS-OOK) transmission.

Herein, a new type of smart glass fabricated by using a thermoswtichable salt hydrate with a microheater is reported. The supersaturated sodium hydrate solution is clear and transparent at the liquid, however, it turns solid and opaque state after mechanical impact. The nanomaterial-based transparent micro heater is suitable for flexible applications and it consumes low electrical energy due to its high electrical conductivity and high-efficiency of electrothermal effect. Considering the solubility of the sodium acetate in water and chemical properties such as water of crystallization, a phase transition from the liquid state to solid occurs in few seconds with rapid crystal growth of sodium acetate trihydrate. Therefore, it quickly blocks an intense sunlight and provides privacy protection. In reverse, the phase transition from the solid-state to the liquid is simply controllable within a few minutes by the microheater. Finally, we demonstrate a practical application of mechano-thermo-chromic (MTC) device by constructing a system, which operates by an external environment condition such as intense UV light.

Based on its outstanding mechanical, thermal, and chemical properties, a Polyimide (PI) is useful in a wide range of applications. Its usage in biomedicine is drawing great attention owing to the recent confirmation of the biocompatibility of various PIs. However, the conventional patterning of a PI, based on photolithographic methods, which is expensive and time-consuming, hampers the rapid advancement of research-oriented fields that require frequent design changes. To resolve this problem, we introduce the method of the monolithic digital patterning of PI up to the quasi-three-dimensional (3D) structures at the microscale resolution by laser-induced jetting of highly porous Laser-induced graphene (LIG) from the PI matrix. Pyrolytic jetting of the LIG is dependent not only on the laser-induced temperature but also on its temporal and spatial gradients. However, the surfaces of the remaining PI can be exceptionally smooth at the optimum laser condition, comparable to the pristine surface at the microscopic level, as confirmed by Raman spectroscopy and Atomic force microscopy (AFM) measurements. On-demand microfluidic channels and multilevel imprinting molds are created in a single step using the proposed method as a proof-of-concept, substantiating its potential application in relevant research.

In order to accommodate the demands for the next-generation passive optical network (PON), it is necessary to maximize the transmission capacity and the multiple accesses (MAs) within a single wavelength. However, there is a serious problem that the optical beat interference (OBI) occurs in uplink MA. With optical pulse-division multiplexing (OPDM), the OBI can be reduced simply and effectively; thus, OPDM is a useful optical transmission technique for single-wavelength MA that offers various advantages. To increase the transmission capacity within the limited bandwidth of an optical modulator, we propose a signal-to-noise ratio (SNR) enhancement technique involving receive diversity achieved using the replica inevitably generated in OPDM. The SNR improvement was experimentally verified, via a 20-km optical transmission in OPDM-based single-wavelength MA. The enhanced SNR improves the modulation order and spectral efficiency, and relaxes the requirements of OPDM and the decoding complexity. The proposed replica-based receive diversity can increase the SNR of the uplink signal, reduce the OBI, and guarantees the advantages of OPDM; thus, it is a useful technique for effectively supporting the high data capacity and MA required for the next-generation PON.

Future electronics are expected to develop into wearable forms, and an adequate stretchability is required for the forthcoming wearable electronics considering various motions occurring in human body. Along with stretchability, transparency can increase both the functionality and esthetic features in future wearable electronics. In this study, we demonstrate, for the first time, a highly stretchable and transparent electromagnetic interference shielding layer for wearable electronic applications with silver nanowire percolation network on elastic poly(dimethylsiloxane) substrate. The proposed stretchable and transparent electromagnetic interference shielding layer shows a high electromagnetic wave shielding effectiveness even under a high tensile strain condition. It is expected for the silver nanowire percolation network-based electromagnetic interference shielding layer to be beyond the conventional electromagnetic interference shielding materials and to broaden its application range to various fields that require optical transparency or nonplanar surface environment, such as biological system, human skin, and wearable electronics.

Spatial Micro‐Water Manipulation An octopus with homogeneously soft structure can easily hide in rocks, where an athlete can throw javelin in precisely controlled manner thanks to the articular structure. Nature designed these distinctive structures by adjusting the amount of water inside. For taking both of their kinematics, the spatial micro‐water molecule manipulation maneuvers the amount of water in the hydrogel enabling the alteration between two structures. More details can be found in article number 2102536 by Seung Hwan Ko and co‐workers. [Image: see text]

In this paper, a pulse positioned-differential phase shift keying technique is proposed to enhance the data rate in free space optical communication. Using the schematics of polarization rotation differential phase shift keying, multi-rate functionality can be achieved without using delay-line interferometers. Furthermore, the proposed novel modulation format—differential phase shift keying combined with pulse-position modulation—enables a high data rate owing to the use of an average power limited amplifier. By using the average power limited amplifier, the signal power is increased as the pulse position order increases, which enhances the bit-error-rate performance. The increased signal power can be converted to an enhanced data rate. We demonstrated that the data rate above 625 Mbps can be increased in every step, as the pulse position order increases in the pulse positioned-differential phase shift keying. The performance enhancement of the proposed technique is theoretically and experimentally demonstrated.

A polarization diversity-based PON system can be an attractive network solution for the future PON system that requires high data capacity with both a large number of subscribers and longer transmission-reach. However, the state-of-polarization (SOP) inevitably rotates during the transmission through the single-mode fiber (SMF). Therefore, this system cannot ensure high data capacity owing to the SOP rotation. In this paper, we propose the out-of-band pilot-aided SOP rotation tracking technique for in the polarization diversity-based PON transmission system. The proposed technique tracks the SOP rotation by measuring the variance of the power ratio of the inserted pilot-tone at the reference polarization axis. This technique offers the advantages of a low overhead and dynamic update for SOP variation; further, it is optical control-free and transparent for data format. For the experimental demonstration of the SOP rotation compensation, the pilot insertion was firstly optimized and the correctable range of the SOP rotation was analyzed. The results of the emulation of the uplink transmission indicated that the SNR at both polarization axes could be successfully retained by compensating the SOP rotation. Under the target forward error correction limit, the SE up to 8 bit/s/Hz within single-wavelength was obtained for 60 km PON transmission.

Recent research progress of relieving discomfort between electronics and human body involves serpentine designs, ultrathin films, and extraordinary properties of nanomaterials. However, these strategies addressed thus far each face own limitation for achieving desired form of electronic-skin applications. Evenly matched mechanical properties anywhere on the body and imperceptibility of electronics are two essentially required characteristics for future electronic-skin (E-skin) devices. Yet accomplishing these two main properties simultaneously is still very challenging. Hence, we propose a novel fabrication method to introduce kirigami approach to pattern a highly conductive and transparent electrode into diverse shapes of stretchable electronics with multivariable configurability for E-skin applications. These kirigami engineered patterns impart tunable elasticity to the electrodes, which can be designed to intentionally limit strain or grant ultrastretchability depending on applications over the range of 0 to over 400% tensile strain with strain-invariant electrical property and show excellent strain reversibility even after 10 000 cycles stretching while exhibiting high optical transparency (80%). The versatility of this work is demonstrated by ultrastretchable transparent kirigami heater for personal thermal management and conformal transparent kirigami electrophysiology sensor for continuous health monitoring of human body conditions. Finally, by integrating E-skin sensors with quadrotor drones, we have successfully demonstrated human-machine-interface using our stretchable transparent kirigami electrodes.

We demonstrated a reversible transparency-changing smart glass that can be operated by mechanical impacts and heat., Active control of transparency/color is the key to many functional optoelectric devices. Applying an electric field to an electrochromic or liquid crystal material is the typical approach for optical property control. In contrast to the conventional electrochromic method, we developed a new concept of smart glass using new driving mechanisms (based on mechanical stimulus and thermal energy) to control optical properties. This mechano-thermo-chromic smart glass device with an integrated transparent microheater uses a sodium acetate solution, which shows a unique marked optical property change under mechanical impact (mechanochromic) and heat (thermochromic). Such mechano-thermo-chromic devices may provide a useful approach in future smart window applications that could be operated by external environment conditions.

Pressure-sensitive touch panels provide an intuitive and natural method to sketch and write with new levels of control and interactivity. However, they require a combination of sensors or a stylus-based interface to identify 3D signals, which prevents their implementation in a wide spectrum of applications. Here, we report a transparent and flexible 3D touch which operates in a single device with the assistance of multiscale structures and a nanowire percolation network. The device could assign functionalities to objects without reference to any varying surfaces. Rigorous theoretical analysis allowed us to achieve the target pressure sensitivity, and successful 3D data acquisition was carried out through the 6-wire measuring technique.

The advent of soft robotics has led to great advancements in robots, wearables, and even manufacturing processes by employing entirely soft-bodied systems that interact safely with any random surfaces while providing great mechanical compliance. Moreover, recent developments in soft robotics involve advances in transparent soft actuators and sensors that have made it possible to construct robots that can function in a visually and mechanically unobstructed manner, assisting the operations of robots and creating more applications in various fields. In this aspect, imperceptible soft robotics that mainly consist of optically transparent imperceptible hardware components is expected to constitute a new research focus in the forthcoming era of soft robotics. Here, the recent progress regarding extended imperceptible soft robotics is provided, including imperceptible transparent soft robotics (transparent soft actuators/sensors) and imperceptible nontransparent camouflage skins. Their principles, materials selections, and working mechanisms are discussed so that key challenges and perspectives in imperceptible soft robotic systems can be explored.

We propose and experimentally demonstrate a frequency-separated model based digital pre-distortion (DPD) technique in intermediate frequency over fiber system. Frequency-separated model based DPD could mitigate distortion caused by optical and electrical components in the transmission system. To employ the proposed DPD technique, least-mean-square (LMS) algorithm is used to derive the coefficients of pre-distorting equalizer. By the proposed DPD, error vector magnitude was decreased and adjacent carrier leakage ratio was increased.

A nonlinear mitigation technique with spectral shaping of signal power is proposed in analog multi-IF over Fiber link using OFDM signal. Its feasibility was experimentally verified through EVM performance and channel linearity.

This paper presents the fabrication and characterization of multi-dimensional strain sensor that is capable of quantitative determination of the strain characteristics that can imitate the various surface strain properties. In this study, we demonstrated multi-dimensional strain sensor composed of two interlocked layers of pre-strained silver nanowire network which is capable of detecting transverse and longitudinal strain separately. By acquiring two independent electrical signals, the amplitude and the effective direction of the surface strain are calculated with high stretchability (>35%), good stability, large gauge factor (GF ∼ 20) and excellent agreement to the model predicted with 3D percolation theory. We further applied the strain sensor for controlling artificial 3D model in two dimensions by communicating with a home-made data acquisition system to confirm its high potential as a wearable skin monitoring device.

Controlling the surface morphology of the electrode on the nanoscale has been studied extensively because the surface morphology of a material directly leads to the functionalization in various fields of studies. In this study, we designed a simple and cost-effective method to fine-tune the surface morphology and create controlled nanopores on the silver electrode by utilizing 2-ethoxyethanol and two successive heat treatments. High electrical conductivity and mechanical robustness of nanoporous silver corroborate its prospect to be employed in various applications requiring a certain degree of flexibility. As a proof-of-concept, a high-performance supercapacitor was fabricated by electrodepositing MnO2. This method is expected to be useful in various electronic applications as well as energy storage devices.

Transparent and stretchable energy storage devices have attracted significant interest due to their potential to be applied to biocompatible and wearable electronics. Supercapacitors that use the reversible faradaic redox reaction of conducting polymer have a higher specific capacitance as compared with electrical double-layer capacitors. Typically, the conducting polymer electrode is fabricated through direct electropolymerization on the current collector. However, no research have been conducted on metal nanowires as current collectors for the direct electropolymerization, even though the metal nanowire network structure has proven to be superior as a transparent, flexible, and stretchable electrode platform because the conducting polymer’s redox potential for polymerization is higher than that of widely studied metal nanowires such as silver and copper. In this study, we demonstrated a highly transparent and stretchable supercapacitor by developing Ag/Au/Polypyrrole core-shell nanowire networks as electrode by coating the surface of Ag NWs with a thin layer of gold, which provide higher redox potential than the electropolymerizable monomer. The Ag/Au/Polypyrrole core-shell nanowire networks demonstrated superior mechanical stability under various mechanical bending and stretching. In addition, proposed supercapacitors showed fine optical transmittance together with fivefold improved areal capacitance compared to pristine Ag/Au core-shell nanowire mesh-based supercapacitors.