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Terahertz electromagnetic waves are widely used in various fields, but their practical application often suffers from significant interfacial losses. In this paper, we present a terahertz antireflection film based on an all-dielectric metasurface consisting of a zirconia microsphere array assembled using a template-assisted method. Simulations and experiments show that the film has a broadband, outstanding transmission-enhancing effect. Because the antireflection film has the advantages of ultralow cost, high compatibility, flexibility, and easy industrialization, it may have potential applications in various fields.

In this paper, a characteristic mode (CM)-based design methodology is developed for the reconfigurable microwave metasurface absorbers (MMA) with using commercially available substrates. Theoretical intrinsic connection between modal behaviors and absorption properties is first formulated for the design of the structures. As a proof of the method, a broadband MMA consisting of butterfly elements is step-by-step designed. The positions and values of the impedance loading are determined based on the modal significances and modal currents for broadband operation. The absorption performance is evaluated by the formulated theoretical model and verified by full-wave simulation. After that, a flexible implementation a employed for free-form surfaces conformal. Experimentally, the proposed MMA is fabricated and measured. The absorption fractional bandwidth with reflectivity less than –10 dB is achieved from 3.6 to 11.6 GHz (105%). Good agreements between the simulation and measurement results for both planar and cylindrical cases validate the developed CM-based design methodology. The mimicry of time-varying scattering cross section (SCS) is then further realized through real-time coding absorption states, which is a promising candidate for practical electromagnetic systems.

An optically transparent chessboard metasurface is proposed to achieve wideband radar cross section (RCS) reduction in this paper. The metasurface is a single-layer sandwich structure composed of indium-tin-oxide (ITO) square patches with different sizes and surface resistances, polyethylene terephthalate (PET) substrate, and ITO backplane from the top downwards. Based on optimized multielement phase cancellation (OMEPC) to select the sizes and surface resistances of ITO patches, the proposed metasurface can achieve the 10 dB RCS reduction from 10.1 to 26.4 GHz with a relative bandwidth of 89% under normal incidence. The total thickness of the metasurface is 2 mm, which makes it highly flexible. When there is a slight cylindrical curvature on the metasurface and the curvature angle is less than 30°, an approximate 10 dB RCS reduction can still be achieved in the frequency range of 10–25 GHz under normal incidence. When the oblique incidence angle is less than 45°, the proposed metasurface also exhibits nearly 10 dB RCS reduction at the specular direction in the frequency range of 10–25 GHz. Finally, a sample is fabricated and measured, and the experimental results are in good agreement with the simulation results.

Proliferation of flexible, wearable and portable electronics come with new challenges of system reliability. Particularly, flexible and wearable electronics cause more frequent and are more susceptible to electromagnetic interference (EMI) effects and electrostatic discharge (ESD) failures, and have unique requirements for design-for-reliability (DfR) associated with EMI and ESD immunity to achieve electromagnetic compatibility (EMC) compliance. This paper outlines the key challenges in design for EMI and ESD immunity for wearable microsystems and discusses potential design for EMI/ESD immunity solutions.

Technical method for strain detection using radio frequency identification (RFID) technology is characterized by flexibility, low cost and high intelligence. The traditional planar-based rigid sensing antennas are basically made of rigid substrates, which have the disadvantages of poor surface fit and flexibility, so flexible electronics are gradually applied in the field of structural health monitoring (SHM) to solve the challenge of non-planar deformation monitoring. In this paper, we propose a flexible reconfigurable strain-gauge RFID sensor with an encoding capability of 4 bits. The encoding structure is based on a coupled U-shaped resonator with strain resonance in the UHF RFID frequency band from 0–4 GHz. Reconfigurability is achieved by introducing a diode with modulated bandstop filtering in the resonator. The proposed antenna uses an FPCB board with total dimensions of 100 mm $\times71.4$ mm $\times0.508$ mm. It is subjected to thermal stability experiments, four-point bending strain and strain monitoring inside concrete, and the simulation and measurement results verify the feasibility of the design. This is important for realizing high-performance integrated strain monitoring in structural health monitoring using RFID technology.

This paper introduces a compact antenna with bandwidth reconfigurable and flexible characteristics for cognitive radio applications. The antenna consists of a monopole as a primary radiator and is excited by a coplanar waveguide to have a single-layer design. To adjust the operating bandwidth (BW) by fixing the lower frequency and varying the upper one, an additional stub is connected to the feeding line. This stub causes a disturbance in the current distribution, which significantly degrades the matching performance. By controlling its length, the rejected band can be changed and thus, the upper frequency is varied. To validate the proposed approach, two stubs are used, and they are connected to the feeding line through two PIN diodes. By switching the ON/OFF state of the diodes, three different states with different operating BW are attained. The fractional BWs for these states are 18.9, 33.2, and 64.3% respectively. Besides, it is also worth noting that the antenna achieves good performance when bent over a curved surface.

This article provides a summary of recent microwave antenna sensor technologies published in peer-reviewed journals. This study primarily focuses on the resonator-based microwave sensor, microwave wearable textile antenna sensors, and resonator-based flexible metamaterial devices along with their possible industrial applications. The survey emphasizes on both the possible disadvantages and advantages of employing this technology. Furthermore, it provides an overview of the key issues addressed in the context of antenna sensors, starting with material selection and finishing with the antenna’s frame, as well as the numerous case applications. This article, in particular, offers a description of the various types of textile materials used in the creation of wearable antenna sensors. This article provides various instances of antenna sensor applications in the human body, including the detection of NaCl and sugar solutions, blood, and other biological variables like strain, finger positions, and temperature. In addition, suggestions are made regarding the potential industry and scientific obstructions, as well as future directions for the study.

Herein, we demonstrate a highly-sensitive, flexible, and self-powered UV photodetector based on perovskites Cs3Cu2I5/PEDOT: PSS heterostructure. The device shows a high photoresponsivity (${R}$) of 170.3 mA/W, on/off ratio of $5.7\times 10^{{3}}$ , response/recovery time of 177.0/ $195.2 \mu \text{s}$ , and good specific detectivity (${D}^{\ast }$) of $1.0\times 10^{{13}}$ Jones to 275 nm light at self-powered mode. Benefiting from the solvent vapor annealing process, the Cs3Cu2I5 film is conformally attached to the flexible substrate and maintains relatively high performance at a curvature radius of 3 mm. Our work could pave the way to open up high-performance self-powered perovskites UV photodetector in flexible solution-based optoelectronics. [ABSTRACT FROM AUTHOR]

A compact, dual polarized, multiband four-port flexible Multiple Input Multiple Output (MIMO) antennae with the connected ground and high isolation is designed with computation and experimental measurement studies. All four monopole radiators are embedded decagon-shaped flexible FR-4 substrate with an outer radius of 10 mm in order to accomplish circularly polarized (CP) radiations, bandwidth enhancement, and compact size of only $45\times 38\times 0.2$ mm3 ( $0.375\lambda \times 0.316\lambda \times 0.0016\lambda $ , at lowest resonating frequency 2.5GHz). The interconnected ground structure is loaded with an Interlaced Lozenge Structure (ILS) to suppress the surface wave radiations resulting in low mutual coupling between the radiators. The proposed MIMO antenna demonstrates measured 10-dB impedance bandwidths of 9.63% (2.37–2.61 GHz), 28.79% (3.30–4.41 GHz), and 16.91% (4.98–5.90 GHz) in the LTE 38/40, Sub-6 GHz 5G NR n77/n78, WLAN and Wi-Fi bands, respectively. Furthermore, broad 3-dB Axial Ratio Bandwidth (ARBW) of 28.79% (3.30–4.41 GHz) with gain greater than 4 dBi and efficiency above 80% are achieved. Finally, the bending analysis of the proposed flexible MIMO antenna along the X- and Y- directions shows good performances in terms of scattering parameters, 3 dB ARBW, and MIMO diversity parameters.

Optically transparent electronic devices have attracted enormous interests in recent years. The development of transparent electronics emerges a lot of new industrial applications in variety of fields, such as displays, glasses, solar panels, satellite communications, terrestrial communications, integrated circuits, and sensors where optical transparency is required for unobtrusive placement of electronic devices on the surface. Over the last couple of years, there have been notable advancements in the development of transparent wireless electronics due to the emerge of new materials and fabrication technologies. Among transparent electronic devices, transparent antennas attract tremendous interests due to their widespread applications in healthcare industry, security sector, defence, sports, smart city, Internet of Things (IoTs) and many more. Many of these applications require antennas that are concurrently transparent and flexible. A transparent and flexible antenna can be easily integrated with displays, windows, solar cells and optoelectronic modules, thus, reducing the space in the integrated circuits. However, still the development of flexible-transparent electronics is associated with some challenges which continue to impede the progress of this emerging field. Among them are the contradictory relationship between the electrical conductivity and optical transparency of the transparent conductors, costly and complex processing of the transparent materials and unavailability of the appropriate materials. In this paper, we discuss current advancements in the development of flexible transparent antennas, including potential applications, various enabling materials and manufacturing approaches, technical hurdles, as well as prospects.

Embedded-chip planar silver-elastomer interconnect technology is developed with flexible substrates and demonstrated for on-skin biophotonic sensor applications. This approach has several benefits and is also consistent with chip-thinning where the chip thickness is 100 microns and less. The key benefits from this approach arise because both the bottom and top sides are now available as flat surfaces for 3D integration of other components. It also results in the lowest electrical parasitics compared to flipchip with adhesives or printed-ramp interconnections with surface-assembled devices. Embedding of chips in flexible carriers was accomplished with direct screen-printed interconnects onto the chip pads in substrate cavities. Silver nanoflake-loaded polyurethane is utilized in the embedded-chip packages to provide the desired lower interconnect resistance and also reliability in flexible packages under deformed configurations. Viscoelastic models were utilized to model the interconnection stresses. Planar interconnects in flexible substrates are developed with conductive silver-loaded elastomer interconnects. This approach is compared to direct chip-on-flex assembly technology for reliability under bending and high-temperature storage. The embedded-chip technology is demonstrated through biophotonic sensor applications where light sources (LEDs) and photodetectors are embedded inside the package. Functional validation in bent configuration at low curvatures is shown by measuring pulse rate and muscle activity with human subjects. By extending this technology to nanowires in elastomers, further enhancement in electrical and reliability performance can be achieved.

A coplanar waveguide (CPW) fed flexible interconnected 4-port MIMO antenna is proposed for UWB, X, and Ku band applications having a size $0.67\lambda \times 0.81\lambda \times 0.0028\lambda $ (at 3.58 GHz). The antenna parameters are characterized by using sets of real measurements to validate the antenna design and modelling. A precise milling machine is used for fabricating the single and 4-port antennas over a flexible FR-4 Substrate. Vector network analyser (VNA) is used for measuring scattering and transmission coefficients. As well as, a shielded anechoic chamber is utilised for measuring the far field radiations and the gain of the antenna under test (AUT). The basic antenna that is a part of the 4-port structure consists of a CPW fed octagonal structure surrounded by an octagonal-shaped slot. The MIMO ability is achieved by arranging the 4 identical elements symmetrically in a successive rotational means for achieving good polarization diversity. The connected ground between the elements is achieved by introducing a plus-shaped slotted structure separated by a slotted diamond at the center in the 4-port structure. None of the slots touches each other thus ensuring the common reference while maintaining the inter-element isolation better than 15 dB. The antenna demonstrated maximum gain and minimum efficiency of 6.8 dBi and 89%, respectively besides attaining a 10-dB impedance bandwidth of 125.83% (3.89-17.09 GHz) and envelope correlation coefficient (ECC) < 0.02. Finally, the 4-port flexible antenna is tested in terms of S parameters and ECC under bending conditions along the X and Y axis followed by a time-domain analysis that exhibited very good performance.

In this study, HATCN is coated on flexible PET/indium tin oxide (ITO) substrate as a modified layer and a hole injection layer to improve the hole injection from ITO. Then a blue organic light emitting diode (OLED) can successfully be fabricated without a surface treatment procedure (O2 plasma or UV Ozone treatment). The new blue TADF series fluorescent material is employed as emitting layer with a luminescence wavelength of 456 nm. Also, it has a narrow full width at a half maximum (FWHM) of 26 nm featuring excellent color chromaticity. Quantum dot (QD) photoresist is a perfect color conversion material featuring good color adjustability, narrow emission spectrum, high luminous efficiency, and simple spin-coating processes. In this study, the photoresist mixed with green and red quantum dots is used as a color conversion layer (CCL), and a blue OLED is utilized to excite green and red CCL. To test the material of QD photoresist, it is coated on the substrate of another piece of glass first and then the blue OLED is utilized to remotely excite green and red QD CCL. The fluorescence characteristic of QD photoresist is explored to acquire a spectrum of 528 nm and 620 nm. In the device structure of a blue OLED, the electron-and hole-only current density is compared and the hole transport layer TAPC thickness is adjusted to improve the luminance and efficiency. Finally, green and red quantum dot photoresist is mixed using a proper ratio and then directly coated on the back of the PET/ITO substrate. Furthermore, the thickness of the QD photoresist is adjusted to increase the QD excited fluorescence. The blue OLED and QD CCL was integrated to generate three primary colors, i.e., blue, green, and red. Finally, a flexible white OLED lighting panel is successfully fabricated using simple processes. Moreover, it features high-spectrum stability. The CIE coordinates will not drift with bias, thus, it can resist the voltage variation.

A spoof surface plasmon polariton (SSPP) antenna with dual-band endfire gain and small frequency ratio (FR) is presented. It uses an asymmetric structure to support two independent surface waves with different nonlinear phase constants. Due to the different phases of the superposed $E$ -field at the terminal of the proposed structure, dual-band endfire gains and endfire radiation null are achieved. The odd- and even-mode feedings, providing additional phase differences, are also investigated. Three prototypes, with FRs of 1.06, 1.11, and 1.5, are designed, simulated, and measured, respectively. All results validate the mechanism and demonstrate that the FR of the proposed antenna can be achieved easily in the range of 1.06–1.5 by choosing different unit cells and feeding modes. Moreover, the measured endfire out-of-band gain suppression, over 17 dB, presents a good directional interference inhibition. This small-FR endfire antenna shows a broad prospect for band coverage and directional noise inhibition with very close operating frequencies. [ABSTRACT FROM AUTHOR]

Efficient lattice-based cryptosystems operate with polynomial rings with the Number Theoretic Transform (NTT) to reduce the computational complexity of polynomial multiplication. NTT has therefore become a major arithmetic component (thus computational bottleneck) in various cryptographic constructions like hash functions, key-encapsulation mechanisms, digital signatures, and homomorphic encryption. Although there exist several hardware designs in prior work for NTT, they all are isolated design instances fixed for specific NTT parameters or parallelization level. This article provides an extensive study of flexible design methods for NTT implementation. To that end, we evaluate three cases: (1) parametric hardware design, (2) high-level synthesis (HLS) design approach, and (3) design for software implementation compiled on soft-core processors, where all are targeted on reconfigurable hardware devices. We evaluate the designs that implement multiple NTT parameters and/or processing elements, demonstrate the design details for each case, and provide a fair comparison with each other and prior work. On a Xilinx Virtex-7 FPGA, compared to HLS and processor-based methods, the results show that the parametric hardware design is on average $4.4\times$ 4. 4 × and $73.9\times$ 73. 9 × smaller and $22.5\times$ 22. 5 × and $19.3\times$ 19. 3 × faster, respectively. Surprisingly, HLS tools can yield less efficient solutions than processor-based approaches in some cases. [ABSTRACT FROM AUTHOR]

Neural probe plays an important role in neuroscience research and neurological diseases diagnosis. The flexible neural probes made by polymer emerging in recent years have been manifested more compliant and compatible with bio-tissues. However, the implantation of the flexible probe becomes another problem to be solved. In this letter, we propose a platinum black (Ptb) enhanced ultrathin flexible neural probe for accurate implantation. Benefiting from the anchoring structure on the flexible probe and the probe coupling with the shuttle by polyethylene glycol (PEG), reliable high-precision implantation has been demonstrated in agar model. Compared with traditional probe made with stiff shank, this flexible probe with anchoring structure is biocompatible and shows less immune reaction. In addition, the population spike (PS) was evoked successfully by biphasic stimulation in vivo. [ABSTRACT FROM AUTHOR]

Fundamental neuroscience research and high-performance neuro-prostheses require large-scale brain interfaces with ever-greater spatial resolution across a large cortex coverage, which cannot be achieved with current passive (micro) electrocorticography (ECoG) technologies. In this article, we present an active micro-electrocorticography ($\mu $ ECoG) implant system that circumvents these challenges while achieving significantly lower noise compared to other existing active $\mu $ ECoG arrays. The proposed brain implant system is composed of a flexible, actively multiplexed 256-electrode $\mu $ ECoG array and an incremental- $\Delta \Sigma $ readout integrated circuit (ROIC). The 1 cm $\times $ 1 cm $\mu $ ECoG array was fabricated in a 3- $\mu \text{m}$ IGZO thin-film transistor (TFT) technology on a 15- $\mu \text{m}$ flexible foil and coupled to a 1.25 mm $\times $ 1.25 mm CMOS ROIC fabricated in a 22-nm fully depleted silicon on insulator (FDSOI) process. Due to the 256:16 time-division multiplexing achieved in the electrode array, only 16 multiplexed channels are required in the ROIC to acquire signals from the 256 electrodes simultaneously. By combining TFT multiplexing with newly proposed bulk-DAC (BDAC) feedback in the readout channel, we can integrate and address 4 $\times $ more electrodes than other passive arrays, achieve >10 $\times $ less noise than existing active arrays, and obtain >2 $\times $ effective channel area reduction in the ROIC while maintaining comparable electrical performance over current state-of-the-art ($\mu $)ECoG readouts. [ABSTRACT FROM AUTHOR]

This letter proposes an organic-inorganic heterojunction structure based flexible Ultraviolet (UV) photodetector. The proposed structure consists of Al/PEDOT:PSS/ Yb doped ZnO (YZO)/Al structure fabricated by the low-cost spin coating method over a flexible PET substrate. At −2V voltage bias and 357 nm operating wavelength, the proposed structure gives the maximum responsivity of 0.33 A/W, sensitivity of 33.07, detectivity of 1.01 $\times \,\,{10}^{14}$ Jones, and external quantum efficiency of 113% (at $0.257~\mu \text{W}$ optical excitation). Further, the rise time of 70 ms and the fall time of 30 ms are obtained at 357 nm. The proposed photodetector thus shows an excellent performance in the UV region. [ABSTRACT FROM AUTHOR]

To address the challenge of dense connectors for cryogenic systems, we describe a face-to-face cable connection approach for connecting microwave superconducting stripline cables. The approach provides a space-efficient and reliable interconnect technology for future cryogenic and quantum technology applications at microwave frequencies. A self-aligned assembly was used and was comprised of bottom and top alignment molybdenum pieces and flexible superconducting stripline cables. SU-8, a photodefinable epoxy-based photoresist, was used to form structures defined on the bottom and top Mo pieces to provide alignment and distribute mechanical pressure in the contact regions of the cables. We measured critical transition temperatures in the range of 8.6-9.0 K and an average critical current of 10.2 mA. Time-domain reflectometry measurements at 4.2 K showed a relatively small impedance discontinuity at the connection point and minimal microwave power dependency was observed. This cable-to-cable connection approach for thin, flexible superconducting cables shows promising results as a solution for dense signal integration for microwave and DC interconnects at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

Fabrication of a physically flexible multi-layer metasurface composed of gallium-based liquid metal encapsulated in polydimethylsiloxane (PDMS) is reported. The second-order bandpass frequency selective surface (FSS) operating in the X-band is comprised of two modified Jerusalem cross resonator layers separated by an aperture layer. The multi-layer spatial filter has a total thickness of 4.81 mm ($\lambda _{0}$ /7.5). An inverse image mold of the FSS layer patterns is fabricated in silicon, and FSS patterned PDMS is replicated from the silicon mold. The FSS patterned PDMS is spray coated with liquid metal to fabricate a massive array of discrete conductor patterns in PDMS and subsequent encapsulation by an additional layer of PDMS. Multiple versions of the m $\times $ n metasurfaces have been fabricated and have been repeatedly tested for their bendability, stretchability, twistability, and foldability. The fabricated metasurface in a waveguide environment demonstrated a band-pass center frequency at 8.33 GHz which agreed well with the simulated frequency at 8.31 GHz. [ABSTRACT FROM AUTHOR]

This work reports 2-bits/cell hafnium oxide-based stacked resistive random access memory devices fabricated on flexible polyimide substrates for neuromorphic applications considering the high thermal budget. The ratio of low-resistance state current (${I}_{ \mathrm{\scriptscriptstyle ON}}$) to high-resistance state current (${I}_{ \mathrm{\scriptscriptstyle OFF}}$) or ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ for the fabricated devices was above $1.4\times10$ 3 with a low device-to-device variation at $100 \boldsymbol {\mu }\text{A}$ current compliance. The mechanical stability over 104 bending cycles at a 5 mm bending radius and endurance over 106 WRITE cycles makes these devices suitable for online neural network training. The data retention capability over 104s at 125°C also infuses these devices’ long-term inference capability. Furthermore, the performance of the devices has been verified for neuromorphic applications by system-level simulations with experimentally calibrated data. The system-level simulation reveals only a 2% loss in inference accuracy over ten years from the baseline. [ABSTRACT FROM AUTHOR]

Air-borne ultrasonic transducers based on irradiated cross-linked polypropylene (IXPP) ferro/piezoelectret films, which exhibit a high longitudinal piezoelectric activity and a low acoustic impedance, were analyzed, fabricated, and measured in this work. Equivalent circuits and finite element models for IXPP film transducers were built to predict and confirm the transducer performances. Circular IXPP transducers which are 20 mm in diameter show a maximum radiated pressure of 3.2 Pa (104 dB) driven by 100 Vrms and a receiving sensitivity of 8 mV/Pa when resonance occurs at around 45 kHz, both at a distance of 60 cm, as confirmed by analytical and numerical models. Pulse-echo response of the transducers was obtained from theoretical calculation and simulation as well, indicating a −6 dB bandwidth of around 25% is achieved. Besides, the directivity patterns of IXPP film transducers at various frequencies were measured and analyzed. This work may introduce a novel material for flexible ultrasonic transducers with a demand of high sensitivity. [ABSTRACT FROM AUTHOR]

Real-life graph datasets extracted from the Web are inevitably full of incompleteness, conflicts, and redundancies, so graph data cleaning shows its necessity. Although rules like data dependencies have been widely studied in relational data repairing, very few works exist to repair graph data. In this article, we introduce a repairing semantics for graphs, called Graph-Repairing Rules (${\sf GRR}$ GRR s). This semantics can capture the incompleteness, conflicts, and redundancies in graphs and indicate how to correct these errors. However, this graph repairing semantics can only repair the graphs strictly isomorphic to the rule patterns, which decreases the utility of the rules. To overcome this shortcoming, we further propose a flexible rule-based graph repairing semantics (called $\delta$ δ -GRR). We study three fundamental problems associated with both ${\sf GRR}$ GRR s and $\delta$ δ -GRRs, consistency, implication, and termination, which show whether a given set of rules make sense. Repairing the graph data using ${\sf GRR}$ GRR s or $\delta$ δ -GRRs involves a problem of finding isomorphic subgraphs of the graph data, which is NP-complete. To efficiently circumvent the complex calculation of subgraph isomorphism, we design a decomposition-and-join strategy to solve this problem. Extensive experiments on real datasets show that our two graph repairing semantics and corresponding repairing algorithms can effectively and efficiently repair real-life graph data. [ABSTRACT FROM AUTHOR]

In this article, a flexible, stretchable, and dynamically tunable attenuator based on coplanar waveguide (CPW) and graphene is proposed. Soft silicone elastomer and serpentine metal structure are used to realize the flexible, stretchable, and biocompatible characteristics of the CPW. A graphene sandwich structure (GSS) is placed on the upper surface of the CPW to dissipate the electromagnetic wave, achieving the dynamically adjustable characteristics of the stretchable attenuator. When the surface impedance of the GSS is tuned by applying a proper bias voltage, the simulated and measured results reveal that the attenuation tuning range is dynamically tuned from 4–11 dB with a low reflection below −10 dB. In addition, the fabricated attenuator is simultaneously optimized to work at conditions of the normal state, 180° bending and 130% stretching, with acceptable changes between insertion loss and return loss up to 6 GHz. The attenuator covers the medical frequency band of 2.4–2.483 GHz and the 5th generation (5G) communication frequency band of 3.5–3.8 GHz, which demonstrates that the attenuator has potential applications in stretchable electronic systems. [ABSTRACT FROM AUTHOR]

Flexible energy-storage capacitor has attrac- ted great interest on account of the rapid development of the combination of intelligent systems and flexible electronics. In this work, we fabricated flexible energy-storage capacitors by depositing HfxZr1-xO2 thin films on polyimide (PI) substrates using atomic layer deposition (ALD). The flexible capacitors exhibit high energy storage density (ESD $\approx 35.4$ J/cm3) and efficiency ($\eta \approx 69.3$ %). Furthermore, ESD and $\eta $ remained stable even bending 5000 cycles with a radius of 2 mm. This work provide reference for the development of high-performance flexible HfO2-based energy storage devices. [ABSTRACT FROM AUTHOR]

The importance of wearable sensors has constantly increased in the current era of smart wearable sensors. Among the carbon-based allotropes used to form the sensors, carbon fibers (CFs) have been very popular due to their low weight, excellent electrical, mechanical and thermal characteristics. The paper presents some of the significant work done on using carbon fibers to form composites-based wearable sensors. These composites were developed by mixing carbon fibers and various polymers at optimized ratios via certain fabrication techniques, including printing, layer-by-layer and self-assembling techniques. A classification of the highlighted research has been done based on three categories. The first two types include the polymer types formed using silicone. Polydimethylsiloxane (PDMS) and silicone rubber are the major categories used to form composites with carbon fibers. This is due to their homogenous mixing, hydrophobicity and biocompatibility. The third category consists of other types of polymers used to form these hybrid prototypes. Finally, some of the challenges existing with the current carbon fiber/polymer-based composites have been mentioned and their possible remedies. [ABSTRACT FROM AUTHOR]

In this work, we develop a flexible capacitive pressure sensor with simple structure, low cost, wide operating range, excel repeatability and high sensitivity. The sensor greatly reduces cost while maintaining performance by using a simple method of preparing the dielectric layer and inexpensive chemical materials. The sensor has a excel dynamic response (80ms) and recovery (55ms) and combines high sensitivity (0.3199kPa−1, < 10kPa) with a wide operating range (0.1-65 kPa). It is capable of stable operation for 1400 cycles. In addition, the capacitive pressure sensor has a large signal variation (4.1033nF/kPa), and the capacitance variation can be recognized and processed by a simple measurement circuit. Later experiments and the development of a wireless communication device have also demonstrated the great potential of this flexible capacitive pressure sensor for human detection and biomedical applications. Overall, this sensor is a promising low-cost wearable pressure sensor. [ABSTRACT FROM AUTHOR]

Electrooculogram (EOG) is a well-known physiological metric picked up by placing two or more electrodes around the eyeball. EOG signals are known to be extremely susceptible to motion artifacts. This paper presents a single channel, wireless, wearable flexible EOG monitoring system with motion artifacts sensing and reduction feature. The system uses two non-contact electrode pairs for EOG/motion artifacts detection and motion artifacts reduction. It is implemented on a four-layer flexible polyimide substrate. It is light-weight (only 8.75 gram), battery operated, and uses a microcontroller and a BLE 5.0 transceiver for wireless EOG data transmission, while consuming only 56 mW of power. The system metrics such as gain around 37 dB, bandwidth from 1 Hz to 40 Hz, and noise are evaluated. The system is tested for different electrode configurations and it is demonstrated that horizontally parallel electrode pairs achieve an acceptable motion artifact reduction at the output, while preserving perfect EOG features (such as eye-blinking). The average sensitivity for horizontally parallel non-contact electrodes is found out to be more than 50 times with respect to commercial gold electrodes, whereas the average response time of the sensor is around 380 mS. The flexible EOG system is comfortable to wear and the use of non-contact electrode eliminates the need of skin preparation. Therefore, the system can be easily integrated with eye-masks and headbands, thus making it an excellent prototype for many smart applications. [ABSTRACT FROM AUTHOR]

In this study, HATCN is coated on flexible PET/indium tin oxide (ITO) substrate as a modified layer and a hole injection layer to improve the hole injection from ITO. Then a blue organic light emitting diode (OLED) can successfully be fabricated without a surface treatment procedure (O2 plasma or UV Ozone treatment). The new blue TADF series fluorescent material is employed as emitting layer with a luminescence wavelength of 456 nm. Also, it has a narrow full width at a half maximum (FWHM) of 26 nm featuring excellent color chromaticity. Quantum dot (QD) photoresist is a perfect color conversion material featuring good color adjustability, narrow emission spectrum, high luminous efficiency, and simple spin-coating processes. In this study, the photoresist mixed with green and red quantum dots is used as a color conversion layer (CCL), and a blue OLED is utilized to excite green and red CCL. To test the material of QD photoresist, it is coated on the substrate of another piece of glass first and then the blue OLED is utilized to remotely excite green and red QD CCL. The fluorescence characteristic of QD photoresist is explored to acquire a spectrum of 528 nm and 620 nm. In the device structure of a blue OLED, the electron-and hole-only current density is compared and the hole transport layer TAPC thickness is adjusted to improve the luminance and efficiency. Finally, green and red quantum dot photoresist is mixed using a proper ratio and then directly coated on the back of the PET/ITO substrate. Furthermore, the thickness of the QD photoresist is adjusted to increase the QD excited fluorescence. The blue OLED and QD CCL was integrated to generate three primary colors, i.e., blue, green, and red. Finally, a flexible white OLED lighting panel is successfully fabricated using simple processes. Moreover, it features high-spectrum stability. The CIE coordinates will not drift with bias, thus, it can resist the voltage variation. [ABSTRACT FROM AUTHOR]

One of the few possible mechanisms for self-propulsion at low Reynolds number (henceforth, Re) is undulations of an elastic tail, as proposed in the classical work of Purcell in 1977. This paper studies this effect by investigating a variant of Purcells three-link swimmer model where the lateral links are flexible, specifically fabricated by ionic polymer metal composite (IPMC), by means of simulations in COMSOL Multiphysics. Firstly, a model of the material is developed in COMSOL and simulated in comparison with results that can be found in the literature. The proposed model presents some benefits against other existing models: 1) it allows actuator displacement, so it can be integrated in the model of a mobile robot; 2) it satisfies Euler-Bernoulli theorem; and 3) it incorporates the interaction of the actuator with a fluid in which the robot is immersed. Based on this model, a Purcell's three-link microswimmer is built whose lateral links are of IPMC within a fluid under low Re conditions. The displacement of the robot is analyzed for one of the classical primitives of motion, namely, the circular primitive. The results demonstrate that the swimmer is able to perform a non-reciprocal motion and, consequently, propel itself within the fluid. [ABSTRACT FROM AUTHOR]

Automatic planning, with dynamic binding and adaptive composition recovery, has been used to tackle complex service provisioning in mobile environments, but given frequent network topology changes, and services with time-dependent QoS, finding composites that can functionally and non-functionally satisfy a user's request remains difficult. Many service composition mechanisms either require a centralised perspective of the environment, or use optimisation mechanisms that trade off computational efficiency for optimality. Stigmergy-based approaches have been used to model decentralised service interactions between service providers, using a community of mobile software agents that share the same goal to approximate the set of QoS-optimal service compositions. Inspired by this model, this article addresses computational efficiency concerns using a collaborative approach to engage multiple communities of agents for provisioning QoS-optimal service compositions in mobile environments. New compositions can emerge from local decisions and interactions with agents from diverse communities. We assess whether having multiple communities improves the diversity and optimality of solutions. We also measure the proposed approach’ efficiency in dealing with incomplete information. The results show that the proposed approach trades optimality for a more diverse set of solutions, at a cost of higher overhead. [ABSTRACT FROM AUTHOR]

Convoluted elements on a frequency selective surface (FSS) allow for low frequency elements to be contained in physically smaller unit cells. Smaller unit cells give the FSS greater angular stability, especially where a curved FSS is required, and so unwanted grating effects are avoided. A convoluted element FSS with a frequency rejection band centred at 2 GHz and unit cell area of 15 mm by 15 mm (0.10 λ × 0.10 λ) has been developed. To test its usefulness, the full structure FSS is used as a parabolic reflector in a dual band FSS reflector antenna operating at 1 GHz and 2 GHz. Simulated and measured results are close at both bands. The reflector antenna has high gain at 2 GHz of 12.7 dBi (simulated) and 11.7 dBi (measured). To observe the angular stability of the FSS and therefore its effectiveness as a reflector, it was compared with a copper test reflector at both bands. Simulation of the reflector antenna with test reflector produced a 2 GHz gain of 13.3 dBi which is very close to that with the FSS reflector. The simulated 2 GHz gain plot of the reflector antenna with FSS reflector is very similar to that with the test reflector indicating that the FSS has good angular stability. The gain at 1 GHz is also high with 9.3 dBi (simulated) and 8.7 dBi (measured). Simulation of the reflector antenna with no FSS and only a rear test reflector produced a 1 GHz gain of 10 dBi which is very close to that with the FSS reflector in place indicating that the FSS causes no significant attenuation at that frequency. The convoluted element FSS would be useful as a curved reflector in the creation of high gain, multiband, conformal antennas.

In recent years, the metamaterial has become a popular and powerful tool in the electromagnetic polarization conversion design. In this paper, a flexible and transparent broadband cross-polarization-converter (CPC) was designed, fabricated and tested. ANSYS HFSS was used for the cross-polarization-converter simulation, and the Floquet port and master-slave boundary condition were set in the software. The multi-layer split-ring-resonator (SRR) structure was used to form the metamaterial for the CPC. As a result, >77% polarization conversion ratio (PCR) was achieved in the range of (7.8-18.2) GHz, with little change within 45° incident angle. The experimental results were consistent with the simulation data.

Frequency-selective surfaces (FSSs), have various applications in microwave electromagnetics. This paper reports a solution to the current FSS challenges of flexibly, low profile, simple fabrication and polarization control using a novel structure operating across X and Ku frequency bands where a linearly polarized wave is rotated by 90°. The FSSs were fabricated by laser engraving a thin layer of $5\mu \text{m}$ aluminum on a $65~\mu \text{m}$ Mylar substrate with a relative permittivity of 2.7, and separated by a dielectric spacing layer of 0.9 mm polypropylene substrate, with a relative permittivity of 3. The co and cross-polarized reflection and transmission response of the structure was investigated using numerical modeling and was measured experimentally. A parametric study was also conducted focusing on key performance indicators, and specifically the bandwidth of the structure. The novelty of this polarization rotation structure lies in its ultra-thin profile ( $0.034~\lambda _{0}$ ), flexibility and significant transmission bandwidth. The fabricated prototypes experimental results were in good agreement with the simulated results, with a simulated −6 dB bandwidth of 61% and a measured −6dB bandwidth of 60%. Applications include antenna radomes where polarization is particularly important, as well as other polarization filtering applications which require a conformal low profile structure.

A flexible transparent wideband four-element MIMO antenna with a connected ground plane is proposed with numerical computation and experimental measurement studies. The optical transparency is obtained using flexible conductive oxide material AgHT-4 and Melinex substrate. The radiating elements are in the form of circular stub-loaded C-shaped resonators, which are positioned in a carefully structured flexible Melinex substrate with an interconnected partial ground plane structured in the form of an L-shaped resonator, attaining an overall antenna size of $0.33\lambda \times 0.48\lambda $ at the lowest operating frequency. The proposed antenna spans over a −10 dB impedance bandwidth of 2.21–6 GHz (92.32%) with an isolation level greater than 15dB among all elements. The maximum gain is 0.53dBi with a minimum efficiency of 41%, respectively which is satisfactory considering flexible structure and sheet impedance of $4\Omega $ /sq. MIMO antenna parameters in terms of the envelope correlation coefficient (ECC) and diversity gain (DG) are also extracted where all the values are satisfactory for MIMO applications. The bending analysis of the proposed transparent MIMO antenna along the X and Y axis has revealed good performance in terms of scattering parameters and radiation pattern along with MIMO diversity performance. All of these technical points make the flexible MIMO antenna suitable for smart devices using sub-6 GHz 5G and WLAN band in IoT applications where visual clutter and co-site location issues need to be mitigated with the integration ease of conformal placement on the curved component/device surfaces.

This article presents a flexible miniaturized wideband branch-line coupler printed on a biodegradable paper substrate. The proposed design consists of a combination of series transmission lines and shunt open-stubs along with the meandering technique as a substitute to the conventional quarter-wavelength transmission line (QWTL). The horizontal QWTLs of the traditional three-section branch-line coupler are replaced by a proposed equivalent structure, and the vertical QWTLs have been meandered. The developed prototype occupies an area of $0.04~\lambda _{\mathrm {g}}^{2}$ , and has been tested where the measurements agree well with the simulations, and the obtained fractional bandwidth is greater than 60% at 0.9 GHz operating frequency. In addition, for flexibility analysis, the prototype is bent in different directions, and the tested results are obtained and compared to the flat design. The proposed device can be used in antenna feeding networks, microwave and conformal applications that require both compact size and frequency wideband response simultaneously.

With the fast development of electronic skin, there is a great demand for wearable sensing devices. Herein, a piezoresistive sensor has been fabricated comprising a conductive film of Polydimethylsiloxane (PDMS)-carbon nanotubes (CNTs) sandwiched by two protection films of PDMS. A simple and low-cost manufacturing method has been proposed to wrap CNTs inside the flexible PDMS to obtain a soft sensor with excellent conductivity, high sensitivity, and fast response to various pressures. Additionally, films with different percentages of CNTs were compared. The results show that the film with 7% CNTs has higher conductivity and sensitivity. Moreover, the surface morphologies and microstructures of the superior PDMS-CNTs film were studied by scanning electron microscope, and sensing properties of the pressure sensor were tested. Furthermore, applications of single sensor and sensor array were studied. The results exhibit that the as-prepared pressure sensor successfully detected different human motions and recognized Morse code and words, indicating that the portable sensor can be potentially applied in wide areas such as smart devices and flexible robotics. [ABSTRACT FROM AUTHOR]

Flexible memristor is one of the most promising wearable devices for abundant data storage and processing. In this work, interface engineering by inserting the Al2O3 barrier layer is carried out to construct Pt/TiOx/Al2O3/Pt/indium tin oxide (ITO) flexible artificial synapse device. The memristor performance can be maintained even under 1000 times of bending without degradation, demonstrating its excellent mechanical property. With the Al2O3 diffusion barrier layer, the oxygen vacancies (${V}_{\text {o}}$) movement is slowed down for filaments formation and rupture, thus it boosts up the synaptic plasticity, including long-term potentiation/depression, paired-pulse facilitation (PPF), and spike-timing-dependent plasticity (STDP). Moreover, on the basis of the enhanced symmetry and linearity of conductance for Pt/TiOx/Al2O3/Pt/indium tin oxide (ITO) memristor, the neural network simulation for supervised learning presents an online learning pattern recognition, the accuracy can achieve to 91.15%. Overall, the Pt/TiOx/Al2O3/Pt/ITO memristor with excellent flexibility is a promising emulator for biological synapses, which could be beneficial to future flexible memristor-based neuromorphic computing. [ABSTRACT FROM AUTHOR]

Compact model of flexible radio frequency (RF) electronic devices is crucial for flexible circuit designs. In this article, a compact model, including external strain effects for flexible RF gallium nitride (GaN) high-electron-mobility transistor, is presented. First, the carrier density and threshold voltage with external mechanical uniaxial strain is analytically characterized by introducing the strained piezoelectric charge $\Delta \sigma $ , Schottky barrier height $\Delta \phi _{B}$ , and surface state density $D_{\mathrm {it}}$ into the model. Then, the variations of the conduction band offset at the AlGaN/AlN and AlN/GaN interfaces are considered. Finally, the complete large-signal model is established after embedding strain effect into intrinsic drain current and nonlinear capacitance models. The proposed model is verified by measurements, including direct current $I$ – $V$ characteristics, small-signal $S$ -parameters up to 40 GHz, and large-signal power performance—output power, efficiency, and gain. The proposed model will be useful for the RF application of flexible electronics. [ABSTRACT FROM AUTHOR]

With the rapid development of the flexible electronics field, flexible pressure sensors are widely used in wearable devices, medical monitoring and other fields. However, it is still a huge challenge to design a sensor with high sensitivity, low cost and a simple manufacturing process. In this paper, a resistive pressure sensor based on a composite conductive layer is developed. The polydimethylsiloxane (PDMS) is used as the matrix of the composite. The electrolyte fully absorbed by the super absorbent polymer (SAP+) and used as the conductive filler of the composite. The elastic polymer is used to replace the common rigid conductive fillers (such as carbon nanotubes, graphene, etc.), can be better combined with the matrix, greatly improving the stability and reducing the Young’s modulus of the sensor. After describing the working mechanism of the sensor, the influence of the composite conductive layer of different thicknesses and the content of SAP+ on the electrical output of the sensor is discussed. In addition, the sensor exhibits high sensitivity (0.062 kPa−1), a fast response time (0.136 s) and excellent cyclic loading/unloading stability (>500 cycles). [ABSTRACT FROM AUTHOR]

To achieve fully flexible microwave devices, we investigated flexible polymers in terms of chemical, mechanical, and electrical properties. Moreover, the fabrication techniques for polymer-based microwave devices have been developed to address chemical adhesion and demolding issues. Finally, based on formulated criteria, we have developed recipes for low-loss (0.001), low-Dk (1.73) flexible dielectric materials and have applied them to the microstrip and CPW transmission lines. The transmission losses of the microstrip and CPW lines are as low as 0.065 and 0.034 dB/cm at 2.5 GHz, respectively, which are comparable to those for rigid PCBs. The effects of various materials on microwave performance have been analyzed, from which we show that the acceptable frequency limits for using our fully flexible microwave devices are 7 GHz for microstrip lines and 10 GHz for CPW lines. The proposed molding process allows us to step out from 2-D PCB designs and build 3-D structures or hybrid PCB-3-D components with a certain freedom in material properties. The new material exhibits unique mechanical properties, which extend the material’s application to other fields. In addition, it has been found that the polymer-based devices have significant performance improvements at elevated temperatures, which can be exploited in a high-temperature application. [ABSTRACT FROM AUTHOR]

Flexible pressure sensors play a vital role in wearable devices, human physiological activity monitoring, smart robots and other fields. However, current flexible piezoresistive sensors typically have the disadvantages of complex preparation processes, inability to take into account sensitivity and pressure response range, and poor cycle stability. Herein, a novel flexible asymmetric pressure sensor has been proposed by adjusting sensing material and optimizing the structure of electrode. As a crucial piezoresistive material, MXene/reduced graphene oxide (MXene/rGO) nanocomposite with a layer by layer stacked structure is developed by an efficient and precise one-step laser scribing technology. Benefit from this designed layer by layer crosslinked structure, the proposed flexible MXene/rGO asymmetric pressure sensor exhibits excellent pressure-sensing performances, such as high sensitivity of 2.25 kPa−1 (0 – 200 Pa), low detection limit of about 2.5 Pa, fast responsive recovery, and long cycle durability. So that our flexible MXene/rGO asymmetric pressure sensor can detect subtle body physiological motions in real time, such as wrist pulse, vocal vibration, finger movement, and breath. Additionally, the as-prepared sensor array integrated by $4\times 4$ identical MXene/rGO asymmetric pressure sensors exhibits good multi-touch function. Therefore, such conspicuous sensing performances endow our MXene/rGO asymmetric pressure sensor for widespread practical applications in future flexible wearable electronics. [ABSTRACT FROM AUTHOR]

Millimeter wave antennas have applications in several sensing and communication systems. Such antennas, designed for modern miniaturized devices and systems, must be low profile, flexible, and low cost. Some applications also require beam steering for detection purposes. Combining all these features into an antenna system and delivering decent antenna performance is challenging. In this study, we combined a partially reflective surface with a parasitic patch array to create a simple beam-switching, low-profile, and flexible wearable detection system. To ensure lower costs as well as compatibility with wearable systems, screen printing was utilized on a flexible substrate. The antenna array was optimized for the 77 GHz band and had a high gain of 11.2 dBi. The designed system has three independent beams, which can be oriented from bore-sight to ±32° through a simple switching mechanism. The antenna array maintains its performance in both flat and flexed conditions. Finally, the antenna array was tested in the field to successfully detect objects moving in three different directions.

Radio frequency identification (RFID) security is a relevant matter. The wide spread of RFID applications in the general society and the persistent attempts to safeguard it confirm it, especially since its use involves payments and the store or transmission of sensitive information. In this contribution, we present an innovative solution for improving the security of RFID passive tags through the use of a screen printed button, that allows the reception and transmission only when a certain level of physical pressure normal to its plane is applied. The materials and fabrication technology used demonstrate an easy to implement and cost-effective system, valuable in several scenarios where the user has straight contact with the tags and where its usage is direct and intentional.

With the development of the Internet of Things, FMCW (Frequency Modulated Continuous Wave) radars are widely used in medical treatment, human behavior detection, Internet of Vehicles and UAV (Unmanned Aerial Vehicle) detection. However, the existing systems have some shortcomings, such as high requirements for synchronization signal, lack of flexibility and high design cost. In this paper, to address these problems, a software-synchronization based, flexible, low-cost FMCW radar is proposed. Firstly, the radar uses periodic intermittent slope signal as modulation signal, and analyze echo signal to detect a target with a software synchronization algorithm. Most interestingly, hardware synchronization signal is not needed in the proposed radar as in conventional designs, which greatly facilitate implementation. Secondly, the radar system has a high flexibility due to the use of a software waveform generator and a software data processor. The system is built without expensive integrated chips and equipment, which greatly reduces the design cost. Finally, two groups of experiments under different environments are designed. Through the analysis of the results, it can be seen that the radar system designed in this paper achieves a good accuracy while greatly reduces the design cost and complexity.

A flexible capacitive force sensor was developed for force sensing applications. The force sensor consists of two electrodes and a dielectric layer. The electrodes were fabricated by depositing conductive silver (Ag) ink on a flexible polyimide platform using screen-printing process. The dielectric layer was prepared by mixing the contents of polydimethylsiloxane (PDMS) (pre-polymer and curing agent) in a 16:1 ratio. Then the PDMS dielectric layer was sandwiched between the screen-printed Ag electrodes. The capability of the fabricated force sensor was investigated by recording its capacitive response for varying applied forces of 100 N. It was observed that the capacitance increased from 5.83 pF to 6.36 pF as the force was increased from 0 N (no load) to 100 N. A sensitivity and correlation coefficient of 0.081%N-1 and 0.998 were calculated for the force sensor. In addition, a response time and recovery time of 3.7 seconds and 5.7 seconds was measured for the fabricated force sensor. The relative humidity (% RH) tests performed from 20% RH to 80% RH, in steps of 20% RH, revealed that there was a minimal effect of RH on the base capacitance of the force sensor at room temperature.

A compact grounded asymmetric coplanar strip (GACS)-fed flexible multiband frequency reconfigurable antenna with two PIN diodes is proposed. The investigated antenna is backed by a flexible polyamide substrate with compact dimensions of 24 mm $\times19$ mm and a thickness of 1.53 mm. The investigated antenna structure contains a monopole patch that facilitates operation for wireless LAN applications, whereas inverted L-shaped and F-shaped monopoles facilitate operation for Bluetooth and 5G NR applications. The investigated antenna operates at 2.4, 3.8, and 5.6 GHz with measured impedance bandwidths of 5.8%, 6.3%, and 6.6%, respectively, over the three frequency bands, thus facilitating coverage for Bluetooth, 5G NR, and WLAN standards. The two PIN diodes are employed to tune the investigated antenna among four modes, including a single band mode (WLAN 5.5 GHz), two dual-band modes (5G NR 3.8/5.6 GHz, and Bluetooth 2.48/5.6 GHz), and one multiband mode with Bluetooth (2.4 GHz), 5G New Radio (NR) N77 band (3.8 GHz), and WLAN (5.6 GHz) modes. The investigated antenna radiates unidirectionally with a peak gain of 3.73 dBi at 5.6 GHz. Measurements are carried out on the human body to investigate the behaviour of the wearable antenna. The simulated SAR values are in a safe limit of 1.6 W/kg for 1 g of tissue, according to the FCC. Moreover, the investigated antenna shows extremely low vulnerability to degradation in performance as a result of bending effects concerning impedance matching with acceptable acquiescence between measurements and simulations.

Degradation of flexible low-temperature poly-Si TFTs under dynamic bending stress is studied in controlled ambient of different kinds. It is demonstrated that synergistic effect of H2O vapor and O2 is the origin of the positive threshold voltage (${V}_{\text {th}}$) degradation of poly-Si TFTs under dynamic bending stress in the air ambient, while the dynamic bending stress alone does not cause any TFT degradation. The degradation is found to strongly depend on the ratio of O2 to H2O vapor in the ambient. A degradation model of “H2O and O2 assisted electron transport and capture” is proposed, which is based on the participation of both reactants in reactions with dangling bonds along the micro-/nano- cracks penetrating through the passivation layer into the active region of TFTs, formed under the dynamic bending stress. [ABSTRACT FROM AUTHOR]

A novel flexible LC-type wind speed sensor, as well as its readout circuit, is proposed in this paper. Two inductors are designed on both surface of the flexible substrate, connecting by a parallel plate capacitor. Hence, substrate bending induced by wind can change the total inductance, then shifting the resonant frequency of this LC-type speed sensor. The readout circuit is designed and realized on the PCB board. The LC-type sensor is tested in the wind tunnel and the readout circuit outputs the frequency spectrum. The experiments demonstrate that the resonant frequency increases with the wind speed up to 25m/s. The measured sensitivity is close to 39.8kHz/(m/s), 50.8kHz/(m/s) and 57.7kHz/(m/s) for the sensor with windward height of 30mm, 40mm and 50mm, respectively. Although large windward height can improve the sensitivity, it’s easy to saturate and the maximum detectable speed is reduced. [ABSTRACT FROM AUTHOR]

Electroocugraphy (EOG) is a simple and non-invasive method in which biopotentials developed across the eyes are measured during various eye activities such as eye blinking, winking, and horizontal/vertical eyeball movements. The measured biopotential is called the electroculogram (EOG) signal. This paper presents a single channel EOG measurement system which is implemented on a four layer flexible polymide substrate. The EOG measurement system with its signal conditioning stages is implemented on the top layer of the flexible board whereas, the EOG measurement electrodes are printed on the bottom layer of the flexible board using gold. This eliminates the requirement of external long wires during EOG monitoring. The middle two layers of the flexible substrate are used for implementing the circuit ground plane and active shielding. The entire circuit is powered by a rechargeable Li-ion coin battery. It also uses a Bluetooth 5.0 transceiver module to send the EOG data wirelessly. The system is designed for an effective EOG signal bandwidth of 1.6 Hz to 47 Hz with an effective signal gain above 68.5 dB over the signal bandwidth. The system also has an excellent common-mode rejection ratio (CMRR) response above 70 dB. The system is validated with eight healthy subjects for the detection of different eye activities with an accuracy of 77.08 %. The mass of the entire flexible board along with its battery is only 7.7 g. Such light mass, flexible substrate, and integrated printed electrodes make this EOG monitoring prototype an ideal unit for long term monitoring of biopotentials, without causing any discomfort to the wearer. [ABSTRACT FROM AUTHOR]