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Abstract Recently, intelligent reflecting surfaces (IRSs) have emerged as potential candidates for overcoming the line-of-sight issue in 5 G/6 G wireless communication. These IRSs can manipulate the direction of reflected beams, enabling efficient beam steering to enhance the performance of wireless communication. Each unit cell (or unit structure) of an IRS commonly consists of electrical elements for phase modulation. However, by employing phase modulation alone, an IRS can steer the reflected electromagnetic waves toward only discrete and specific angles, leaving a wide range of out-of-beam areas. In this work, an IRS that uses both phase modulation and space modulation is presented to improve the beam resolution and continuously cover out-of-beam areas that phase modulation alone cannot address. A positive-intrinsic-negative diode is mounted on a unit cell for phase modulation, and a 4D-printed reconfigured structure is fabricated to demonstrate space modulation. The beam-steering function is achieved by alternating the states of the diodes in the same columns, while the beam resolution is improved by controlling the gaps between the columns. The functions are first theoretically and numerically analyzed and then experimentally verified, demonstrating that additional angles of −46°/+50°, −22°/+14°, and −16°/+12° are achieved with space modulation and −60°/+62°, −30°/+22°, and ±16° are achieved by phase modulation alone. The proposed IRS offers the possibility of functional integration in a variety of indoor applications within the wireless communication field.

ABSTRACTA metamaterial can be used for fabricating an electromagnetic absorber by tailoring its effective permittivity and permeability. In general, a metamaterial absorber works at a specific resonant frequency, and its absorption frequency does not change after the structure is fabricated. A three-dimensionally (3D)-printed shape memory polymer (SMP) is a potential candidate for overcoming this limitation and realising reconfigurability with its fabrication versatility. In this work, we propose a frequency – and bandwidth-controllable electromagnetic absorber using 3D-printed shape memory meta-wires (SMMWs). Unlike the conventional usage of SMPs with two states, thermally deformable SMMWs have three different modes that are realised by controlling the states of each column. Multi-material additive manufacturing is used to print the SMMW, with an SMP substrate and a high-temperature filament being the materials that are printed. The high absorptivity and frequency – and bandwidth-tuning capability are numerically and experimentally demonstrated for three different modes.

Transparent metasurface absorbers are highly sought after for their diverse applications, but achieving broad absorption bandwidths without sacrificing optical transparency remains a challenge. Traditionally, enhancing absorption involves increasing substrate thickness, which reduces transmittance. We address this by introducing a resistance-gradient metasurface (RGM) that combines broad absorption with high optical transparency. Unlike conventional methods requiring various inks for different resistances, our approach uses a single silver nanowire ink and screen-printing to create multiple resistances. This technique allows for flexible design and multiple resonances within the metasurface, achieving broad absorption even with a thin substrate. The RGM demonstrates an absorption bandwidth from 8.68 to 11.48 GHz with a substrate thickness of just 0.045 λ0, and optical transmittance of 66.3 % at 550 nm. This innovation promises to enhance both substrate efficiency and bandwidth in transparent metasurface absorbers, broadening their application potential in advanced devices.

Abstract Polarization conversion and beam scanning metasurfaces are commonly used to reduce polarization mismatch and direct electromagnetic waves in a specific direction to improve the strength of a wireless signal. However, identifying suitable active and mechanically reconfigurable metasurfaces for polarization conversion and beam scanning is a considerable challenge, and the reported metasurfaces have narrow scanning ranges, are expensive, and cannot be independently controlled. In this paper, we propose a reconfigurable transmissive metasurface combined with a scissor and rotation actuator for independently controlling beam scanning and polarization conversion functions. The metasurface is constructed with rotatable unit cells (UCs) that can switch the polarization state between right-handed (RHCP) and left-handed circular polarization (LHCP) by flipping the UCs to reverse their phase variation. Moreover, independent beam scanning is achieved using the scissor actuator to linearly change the distance between the UCs. Numerical and experimental results confirm that the proposed metasurface can perform beam scanning in the range of 28° for both the positive and negative regions of a radiation pattern (RHCP and LHCP beams) at an operational frequency of 10.5 GHz.

In stealth applications, there is a growing emphasis on the development of radar-absorbing structures that are efficient, flexible, and optically transparent. This study proposes a screen-printed metamaterial absorber (MMA) on polyethylene terephthalate (PET) substrates using indium tin oxide (ITO) as the grounding layer, which achieves both optical transparency and flexibility. These materials and methods enhance the overall flexibility and transparency of MMA. To address the limited transparency caused by the silver nanoparticle ink for the top pattern, a metal mesh was incorporated to reduce the area ratio of the printed patterns, thereby enhancing transparency. By incrementing the fractal order of the structure, we optimized the operating frequency to target the X-band, which is most commonly used in radar detection. The proposed MMA demonstrates remarkable performance, with a measured absorption of 91.99% at 8.85 GHz and an average optical transmittance of 46.70% across the visible light spectrum (450 to 700 nm), indicating its potential for applications in transparent windows or drone stealth.

Abstract Spiral inductors are required to realise high inductance in radio frequency (RF) circuits. Although their fabrication by using micro-electrical–mechanical systems, thin films, actuators, etc., has received considerable research attention, current approaches are both complex and expensive. In this study, we designed and fabricated a thermal spiral inductor by using a three-dimensional (3D) printed shape-memory polymer (SMP). The proposed inductor was inspired by kirigami geometry whereby a two-dimensional (2D) planar geometric shape could be transformed into a 3D spiral one to change the inductance by heating and manually transform. Mechanical and electromagnetic analyses of the spiral inductor design was conducted. Hence, in contrast with the current processes used to manufacture spiral inductors, ours can be realised via a single facile fabrication step.

In this paper, we propose an optically transparent dual-band metamaterial absorber (MMA) that uses Ag nanowire screen-printed fractal structures. The proposed MMA exhibits near-perfect absorption in the C- and K-bands. This dual-band absorption property is achieved through two inductive–capacitive (L-C) resonances located at 6.45 and 21.14 GHz, which are generated by the second-order fractal structures. We analyzed the microwave absorbing mechanisms through the distributions of the surface current and electromagnetic field on the top and bottom layers. The MMA demonstrates an optical transmittance of 63.1% at a wavelength of 550 nm. This high optical transmittance is attained by screen printing transparent Ag nanowire ink onto a transparent PET substrate. Since screen printing is a simple and low-cost fabrication method, the proposed MMA offers the advantages of being low cost while having the properties of optical transparency and effective dual-band absorption. Consequently, it holds great potential for the radar stealth application of C- and K-bands in that it can be attached to the windows of stealth aircraft due to its optical transparency and dual-band near-perfect absorption property.

Periprosthetic joint infection (PJI) is one of the most devastating complications that can occur after shoulder arthroplasty. Although staged revision arthroplasty is the standard treatment in many cases, surgical intervention with debridement, antibiotics, and implant retention (DAIR) can be an effective option for acute PJI. We report a complex case of infected reverse shoulder arthroplasty (RSA) in a 73-year-old male. The patient had been previously treated for infected nonunion of a proximal humerus fracture caused by methicillin-resistant Staphylococcus epidermidis. He presented with a sinus tract 16 days after the implantation of RSA and was diagnosed with PJI caused by Serratia marcescens. The patient was successfully treated with DAIR and was free of infection at the last follow-up visit at 4 years postoperatively.

Abstract The development of multifunctional and reconfigurable metasurfaces capable of manipulating electromagnetic waves has created new opportunities for various exciting applications. Extensive efforts have been applied to exploiting active metasurfaces with properties that can be controlled by externally controlling active components. However, previous approaches have poor switch isolation, power handling limitations due to nonlinear effects, and complex biasing networks. Therefore, dynamically tunable metasurfaces have become a burgeoning field in many research areas. This paper reports a hydrodynamic metasurface (HMS) that can be programmed to realize electromagnetic beam scanning on the azimuth and elevation planes. The proposed HMS platform incorporates four micropumps, each controlling four metasurface elements via microfluidic channels, built into the HMS base. The proposed platform regulates microfluidic flow through micropumps, causing irregularities in incident wave transmission phase. An HMS was built as a proof of concept, and far-field scanning experiments were performed. Numerical and experimental results verify the feasibility of electromagnetic beam scanning using a hydrodynamic metasurface. This work advances metasurface research, with very high potential for wide-ranging application and a promising route for replacing bulky cascading active components.

A sinuous antenna is a frequency-independent antenna known for its wide bandwidth and consistent gain, which makes it valuable in broadband applications such as ultrawideband (UWB) radar and ground-penetrating radar (GPR). However, sinuous antennas tend to be rather large. Consequently, numerous studies have explored miniaturization methods, with the gap-loading method emerging as a prominent approach. Unfortunately, it is still difficult to achieve broad bandwidths for conventional miniaturized sinuous antennas. In this paper, we use a novel approach incorporating a meander shape into the sinuous curve and employing gap loading with meandering. This innovative technique results in the development of a fractal-structured two-arm sinuous antenna characterized by an ultra-compact size and significantly expanded bandwidth. Adding a meander line in the outermost part maximizes the capacitance, thereby enhancing the gap-loading effect and minimizing the overall size of the sinuous antenna. In addition, the introduction of an inner meander line increases the inductance, contributing to a further expansion of the antenna’s bandwidth. For example, the electrical length of the antenna without the meander line is 0.552 × 0.552 × 0.052 λg3, while the electrical length of the antenna with the meander line is only 0.445 × 0.445 × 0.036 λg3, i.e., 19.4% smaller. The antenna lacking the outermost meander line exhibits a 10 dB impedance bandwidth, spanning from 0.74 to 10.53 GHz. In contrast, the antenna featuring the outermost meander line has a 10 dB impedance bandwidth, extending from 0.51 to 10.72 GHz, which results in a remarkable enhancement in the fractional bandwidth (by 8.1%). Hence, the proposed antenna design is a good candidate for broadband applications that require miniaturization.

In this study, a novel microfluidic frequency reconfigurable and optically transparent water antenna is designed using three-dimensional (3D) printing technology. The proposed antenna consists of three distinct parts, including a circularly shaped distilled water ground, a sea water-based circular segmented radiator, and a circularly shaped distilled water-based load, all ingeniously constructed from transparent resin material. The presented antenna is excited by a disk-loaded probe. The frequency of the antenna can be easily tuned by filling and emptying/evacuating sea water from the multisegmented radiator. The radiator consists of three segments with different radii, and each segment has a different resonant frequency. When the radiator is filled, the antenna resonates at the frequency of the segment that is filled. When all the radiator segments are filled, the antenna operates at the resonant frequency of 2.4 GHz and possesses an impedance bandwidth of 1.05 GHz (40%) in the range of 2.10–3.15 GHz. By filling different radiator segments, the frequency could be tuned from 2.4 to 2.6 GHz. In addition to the frequency-switching characteristics, the proposed antenna exhibits high simulated radiation efficiency (with a peak performance reaching 95%) and attains a maximum realized gain of 3.8 dBi at 2.9 GHz. The proposed antenna integrates water as its predominant constituent, which is easily available, thereby achieving cost-effectiveness, compactness, and transparency characteristics; it also has the potential to be utilized in future applications, involving transparent and flexible electronics.

Reconfigurable intelligent surface‐assisted wireless networks have been proposed to overcome the current technical issues in fifth‐generation (5G) wireless communication systems. Generally, the beam control functions of a reconfigurable intelligent surface require the independent phase control of the unit cells, and each unit cell requires an active device. Although reconfigurable intelligent surfaces are considered a promising solution for 5G and sixth‐generation (6G) wireless communication, their practical implementation faces a bottleneck at high frequencies (e.g., with the millimetre‐wave spectrum) because they require a large number of active devices, leading to high costs, parasitic effects, increased design complexity, and modulated signal distortion. Herein, a mechatronic reconfigurable intelligent surface is reported for indoor millimetre‐wave wireless communication. Our mechatronic device strengthens the innovative reflective beam control functions through rotatable unit structures and rack gear systems. Furthermore, the criteria required for future 5 G wireless communication are experimentally verified by transmitting and receiving quadrature phase‐shift keying 16‐/32‐/64‐quadrature amplitude modulation signals based on 5 G new radio (NR) frequency range two standards for indoor environments.

Digital programmable coding metasurfaces (DPCMs) have recently attracted enormous attention and have been broadly applied, owing to their ability to manipulate electromagnetic (EM) wave behaviours and programmable multi-functionality. Recent DPCM works are divided into reflection and transmission types (R-DPCM and T-DPCM, respectively); however, there are only a few reported T-DPCM works in the millimetre-wave spectrum, owing to the difficulty of realising the large-phase controllable range while maintaining low transmission losses with electronic control components. Consequently, most millimetre-wave T-DPCMs are demonstrated only with limited functions in a single design. Additionally, all these designs use high-cost substrate materials that constrain practical applicability, owing to cost-ineffectiveness. Herein, we propose a 1-bit T-DPCM that simultaneously performs three dynamic beam-shaping functions with a single structure for millimetre-wave applications. The proposed structure is completely constructed using low-cost FR-4 materials, and operation of each meta-cell is manipulated using PIN-diodes, thus driving the achievement of multiple effective dynamic functionalities including dual-beam scanning, multi-beam shaping, and orbital-angular-momentum-mode generation. It should be noted that there are no reported millimetre-wave T-DPCMs demonstrating multi-function design, thus showing a gap in the recent literature of millimetre-wave T-DPCMs. Moreover, cost-effectiveness can be significantly enhanced, owing to the construction of the proposed T-DPCM using only low-cost material.

In recent years, replacing the external control stimuli with internal control using characteristics of radio frequency (RF) signals (such as waveform or power level) has attracted considerable attention to the design of reconfigurable multifunctional RF devices. However, even with the most exciting techniques, the control process always needs a chip-based system to sense the power level or waveform of the incident RF signal, which is realised by additional supporting electronic components of the sensing and microcontroller circuits. Therefore, to achieve a batteryless structure, a majority of conventional works focus on using energy harvesters to convert energy from external environmental sources to DC energy for the electronic circuits. Herein, we propose a novel alternative approach to replace the traditional energy harvesters in batteryless RF devices with all-passive energy smart-sensing circuits. By exploiting the features of a nonlinear semiconductor device under different incident RF power values, the proposed network can passively self-sense the RF power level and dynamically self-control RF signal flow and power ratio. The operation of the proposed network can be considered as purely self-adaptation with control from the RF power level. Moreover, normal RF devices can be transformed to all-passive and batteryless purely self-reconfigurable devices via integration with the proposed structure. As proof of concept, the proposed network is integrated with a two-port antenna to experimentally demonstrate its purely self-reconfigurable polarisation. The proposed strategy is hereby expected to extend the field of batteryless self-reconfigurable multifunctional RF devices and pave promising new paths for the development of future intelligent, smart RF devices.

Metasurfaces are two-dimensional (2D) artificial electromagnetic (EM) materials comprising subwavelength periodic or non-periodic arrays of resonator elements. Multi-functional metasurfaces have shown magnificent potentials for many applications, including wireless communication, radar systems, and security. EM functions of metasurfaces can generally be switched or controlled by electrical or optical components, but these conventional tuning technologies suffer from complexity, high cost, and design limitations for large scales and function integration. Therefore, this paper proposes a novel thermal–mechanical anisotropic metasurface to provide multiple functionalities through mechanical transformation due to thermal stimulus on two shape memory alloy actuators, switching EM functionality between absorption and reflection. The absorptivity and reflectivity at 10.5 GHz are 96.5% and 81.33%, respectively. It consists of the motional metasurface and stational metasurface where mechanical transformation is achieved by the motional metasurface. In addition to the mechanical transformation, its function is switched depending on incident EM wave direction due to anisotropy that could achieved the transmission enhancement in ratio of 11.02 dB at operation frequency 5.9 GHz in opposing excitation. The proposed idea is numerically and experimentally demonstrated and discussed.

This paper presents a W-band planar type via-monopole based quasi Yagi-Uda antenna using metal coated through glass silicon via structures. The antenna was designed and fabricated on 350 μm thick borosilicate glass substrate, which has very low dielectric loss compared with silicon at millimeterwave frequencies. We used a microstrip line to feed the antenna, and the Yagi-Uda configuration using via structured radiator; reflectors; and director were fabricated using tungsten coated silicon via structures embedded in reflowed glass substrate with good high frequency characteristics. The proposed antenna achieved vertical polarization in planar configuration with height 0.09λo. High gain with end-fire radiation was achieved due to the Yagi-Uda configuration. Measured results confirmed the fabricated antenna operated in the W-band with 10 dB fractional bandwidth (FBW) of 12.5% from 76.3 to 86.5 GHz and peak gain of 7.82 dBi at 81 GHz in the end-fire direction. Thus, the proposed antenna with end-fire radiation will be useful for millimeter-wave onboard wireless communication, radar imaging, and tracking applications.

An all-dielectric transparent metamaterial absorber with encapsulated water is demonstrated in this paper. Because the proposed absorber is realized using only water and polydimethylsiloxane (PDMS) without any conductive patterns, optical transparency is achieved. In addition, the high dielectric loss of water renders it a good candidate for an electromagnetic absorber. The absorptivity is increased by encapsulating the water within the PDMS, and the absorptivity of the proposed absorber is numerically compared with that of the PDMS and water with the same size. When the proposed absorber is realized using two layers, 92.5% absorptivity is achieved at 10.8 GHz, and the absorptivity exceeds 90% in the range of 10.45 to 11.20 GHz, which corresponds to 6.9% bandwidth. Therefore, the proposed absorber has advantages of high transparency, low cost, wide absorption bandwidth, and eco-friendly fabrication.

Electrically conformal antenna arrays (ECAA) for 2-dimensional (2-D) beamforming configured by wide beamwidth multipole radiation elements are proposed. Unlike the conventional conformal antenna array with physically curved shape, the proposed arrays are planar structures with wide-angle beam steering. The wide range of beam steering with a low gain variation allows the planar array structure to achieve the electrically conformal phased array patterns. In order to verify the ECAA concept, two different types of ECAA in a 1 × 8 configuration using planar multipole antenna elements were fabricated at 5.8 GHz and compared to the conventional patch array. Each multipole element had half-power-beamwidth (HPBW) wider than 110° at both E- and H-planes with the maximum HPBW of 175°. The first type of ECAA showed a measured peak gain of 10.8 dBi with a measured half power steering angle of 155° while maintaining a side lobe level (SLL) less than -10 dB. Further, the second ECAA showed a measured peak gain of 12.8 dBi with a measured half power steering angle of 144° while keeping the SLL less than -10 dB. These scan angles of the first and second types were extended about 66.7% and 54.8%, respectively from the conventional patch array with the same size. Lastly, two types of 8 × 8 array modules for 2-D beam steering were verified at 5.8 GHz and the first type showed a maximum peak gain of 21.0 dBi. Further, the 2-D half power steering angles in E- and H-planes of the second type were about 145° and 137°, respectively.

Despite the low cost and easy fabrication afforded by 3D printing technology, high dielectric loss of the 3D printing filament degrades the performance of 3D printed radio frequency electronics at high frequencies. In this paper, we propose a 3D-printed substrate-integrated waveguide (SIW) using a lossy polylactic acid or polylactide (PLA) filament. To minimize the insertion loss of the SIW, a non-uniform honeycomb is 3D-printed as the dielectric material of the SIW. The non-uniform honeycomb-shaped substrate is composed of large unit cells, which achieve low insertion loss (0.01 dB/mm) because of the low volume of the dielectric material. The performance characteristics of the proposed SIW is compared with those of SIWs made of solid and uniform honeycomb-shaped structures. The average insertion loss of the microstrip line-fed SIW with the proposed non-uniform honeycomb substrate is 0.92 dB in the frequency range of 1.97 to 3.35 GHz and those of the solid PLA and uniform honeycomb substrates are 2.49 dB and 1.38 dB, respectively. The proposed 3D-printed SIW additionally has the advantages of light weight and low cost.

This article proposes a low-cost, high-gain, and vertically polarized deployable antenna utilizing kirigami pop-up geometry. It primarily utilizes a foldable polyethylene terephthalate sheet to produce kirigami geometry in association with a rectangular radiating monopole, two reflectors, and a parasitic strip director. The reflectors and director increase the antenna gain and provide a frequency-independent tilted radiated beam with a higher beamwidth in the azimuth plane. In addition, electromechanically excited shape memory alloy (SMA) actuators enable folding and unfolding, make the antenna easily transportable and swiftly deployable. We describe the step-by-step fabrication of the kirigami geometry and shape memory spring actuator characterization. The designed, fabricated, and tested antenna achieves a -10 dB reflection bandwidth of 48.8% (1.7-2.8 GHz) providing a peak gain of more than 10 dBi at 2.45 GHz. The tilted radiated beam has a significantly wider beamwidth (90°) in the azimuth plane, compared to 40° in the elevation plane. The measured results agree well with simulations, verifying the proposed design concept. The fabricated prototype offers cost-effectiveness, more rapid fabrication, unprecedented performance, and significant potential for use in a range of microwave applications.

Single-resonance-based radio frequency (RF) resonators cannot detect multiple cracks simultaneously nor localize the position of a crack. To address these drawbacks, we propose a complementary split-ring resonator (CSRR)-loaded array. In this array, there are four channels and each channel consists of three CSRRs, forming a 4 × 3 sensing array that is developed in the ground plane of a microstrip line using a low-cost FR4 substrate. A voltage-controlled oscillator (VCO) generates three discrete frequencies: 1.88 GHz, 2.60 GHz, and 3.61 GHz to each channel, which is sequentially selected using a single-pole four-throw (SP4T) switch. The transmitted RF signals are converted into the DC voltage levels and are interpreted by a microcontroller. Aluminum sheets with cracks embedded in the surface are used to demonstrate the detection of cracks of various shapes, sizes and locations/orientations (horizontal, diagonal, and vertical) with simulations and measurements. The detection of the minimum detectable crack (Wc × Lc × Dc = 1 mm × 10 mm × 0.1 mm) is experimentally verified. Full-length longer cracks (Lc = 100 mm) are also detected using our proposed detection system with the SP4T switch in addition to our proposed algorithm that scans the CSRRs of each selected channel.

Microwave sensors have attracted interest as non-destructive metal crack detection (MCD) devices due to their low cost, simple fabrication, potential miniaturization, noncontact nature, and capability for remote detection. However, the development of multi-crack sensors of a suitable size and quality factor (Q-factor) remains a challenge. In the present study, we propose a multi-MCD sensor that combines a higher-mode substrate-integrated waveguide (SIW) and complementary split-ring resonators (CSRRs). In order to increase the Q-factor, the device is miniaturized; the MCD is facilitated; and two independent CSRRs are loaded onto the SIW, where the electromagnetic field is concentrated. The concentrated electromagnetic field of the SIW improves the Q-factor of the CSRRs, and each CSRR creates its own resonance and produces a miniaturizing effect by activating the sensor below the cut-off frequency of the SIW. The proposed multi-MCD sensor is numerically and experimentally demonstrated for cracks with different widths and depths. The fabricated sensor with a TE20-mode SIW and CSRRs is able to efficiently detect two sub-millimeter metal cracks simultaneously with a high Q-factor of 281.

Abstract This paper presents our investigation and experimental validation of a thermally reconfigurable shape memory alloy (SMA)‐based metamaterial with the offered degree of freedom of SMA mechanical features. The metamaterial unit cell consists of a two‐way helical SMA acting as the resonator element and a cartridge heater for controlling the excitation temperature of the helical SMA. Experimental results of electrical control of the cartridge heater's excitation temperature show that as the temperature increases from room temperature (25°C) to approximately 92°C—corresponding to input DC voltages of 0 and 4 V, respectively—the metamaterial structure acts as a switchable absorber with an absorptivity of 97.2% and a reflector with a reflectivity of 89.4% at 7.13 GHz. Additionally, it deforms to its initial state as the temperature decreases to room temperature.

In this paper, we demonstrate a V-band planar micromachined helical antenna (PHA) with end-fire radiation on the glass substrate. The planar rectangular helical configuration is realized using the novel through-glass silicon via (TGSV) technology. The proposed micromachined antenna is designed and fabricated on a borosilicate glass substrate of thickness 350 μm, which has a very low-dielectric loss compared to silicon at millimeter-wave bands. The proposed PHA is fed by a microstrip line, and the planar helical configuration with 3.25 turns with a truncated ground plane is designed for achieving wideband end-fire radiation with seven tungsten-coated silicon vias and six connected gold arm patterns, which are fabricated using the TGSV technology. The electrical length of the proposed antenna is (3 λo ×1.4 λo). The designed antenna operates at the center frequency of 58 GHz. A prototype of the proposed antenna is fabricated by micromachining technology and tested. The simulated and measured results show that the proposed antenna has a wide operational bandwidth of 50.3 to 65 GHz for |S11|

This paper proposes a millimeter-wave lens antenna using 3-dimensional (3D) printing technology to reduce weight and provide stable gain performance. The antenna consists of a four-layer cylindrical gradient-index (GRIN) lens fed by a wideband Yagi antenna. We designed a fractal cell geometry to achieve the desired effective permittivity for a GRIN lens. Among different candidates, the honeycomb structure is chosen to provide high mechanical strength with light weight, low dielectric loss, and lens dispersion for a lens antenna. Therefore, the measured peak gain was relatively flat at 16.86 ± 0.5 dBi within 25−31.5 GHz, corresponding to 1 dB gain bandwidth = 23%. The proposed 3D-printed GRIN lens is cost-effective, with rapid and easy manufacturing.

In this paper, a MEMS (Micro Electro Mechanical Systems)-based frequency-tunable metamaterial absorber for millimeter-wave application was demonstrated. To achieve the resonant-frequency tunability of the absorber, the unit cell of the proposed metamaterial was designed to be a symmetric split-ring resonator with a stress-induced MEMS cantilever array having initial out-of-plane deflections, and the cantilevers were electrostatically actuated to generate a capacitance change. The dimensional parameters of the absorber were determined via impedance matching using a full electromagnetic simulation. The designed absorber was fabricated on a glass wafer with surface micromachining processes using a photoresist sacrificial layer and the oxygen-plasma-ashing process to release the cantilevers. The performance of the fabricated absorber was experimentally validated using a waveguide measurement setup. The absorption frequency shifted down according to the applied DC (direct current) bias voltage from 28 GHz in the initial off state to 25.5 GHz in the pull-down state with the applied voltage of 15 V. The measured reflection coefficients at those frequencies were −5.68 dB and −33.60 dB, corresponding to the peak absorptivity rates of 72.9 and 99.9%, respectively.

Origami technology is promising for the development of antennas operating in the microwave and terahertz regime. This study summarizes recent advances in origami antennas and their construction regarding frequency, patterns, and polarization switching capabilities for the microwave frequency band. However, there are some challenges of origami antennas related to stability, robustness, and self-deployment, which should be addressed to design more practical deployable origami antennas in the microwave frequency band. In this review, we explained the design principle of various reconfigurable origami antenna. The dielectric and conductive materials of previous origami antennas are discussed and provided the promission materials to satisfy both easy transformation and high robustness for origami antennas. In addition, various actuators are discussed to pave the way for the practical application. Origami technology has also been employed for nanoscale antennas working in the terahertz and optical regime for several applications, including single molecule detection and artificial light harvesting. Although DNA molecules provide a scaffold for the origami antennas at nanoscale size, various issues require attention for development of more advanced nanoscale origami antennas in the terahertz regime. The most pressing issues are the high cost of DNA and the probability of high error rates in self-assembly fabrication.

Background: Active restraint for the elbow joint is provided by the soft tissue component, which consists of a musculoligamentous complex. A lesion of the lateral collateral ligament complex (LCLC) is thought to be the primary cause of posterolateral rotatory instability in the elbow. Its role as a protective reflexogenic structure is supported by the existence of ultrastructural mechanoreceptors. The aim of this study was to describe the existence and distribution of LCLC mechanoreceptors in the human elbow joint and to determine their role in providing joint stability. Methods: Eight LCLCs were harvested from fresh frozen cadaver elbows. Specimens were carefully separated from the lateral epicondyle and ulna. The ligament complex was divided into 7 regions of interest and stained with modified gold chloride. Microscopic evaluation was performed for Golgi, Ruffini, and Pacinian corpuscles. The number, distribution, and density of each structure were recorded. Results: Golgi, Ruffini, and Pacinian corpuscles were observed in LCLCs, with variable distribution in each region of interest. Ruffini corpuscles showed the highest total mechanoreceptor density. Mechanoreceptor density was higher at bony attachment sites. Conclusion: The existence and role of each mechanoreceptor defined the purpose of each region of interest. Mechanoreceptors are beneficial for its proprioceptive feature towards a successful elbow ligament reconstruction. Keywords: Mechanoreceptor, Lateral collateral ligament complex, Elbow, Human

Objective: The aim of this study was to investigate the functional role of each head of the triceps brachii muscle, depending on the angle of shoulder elevation, and to compare each muscle force and activity by using a virtual biomechanical simulator and surface electromyography. Methods: Ten healthy participants (8 males and 2 females) were included in this study. The mean age was 29.2 years (23–45). Each participant performed elbow extension tasks in five different degrees (0, 45, 90, 135, and 180°) of shoulder elevation with three repetitions. Kinematics data and surface electromyography signal of each head of the triceps brachii were recorded. Recorded kinematics data were then applied to an inverse kinematics musculoskeletal modeling software function (OpenSim) to analyze the triceps brachii's muscle force. Correlation between muscle force, muscle activity, elbow extension, and shoulder elevation angle were compared and analyzed for each head of triceps brachii. Results: At 0° shoulder elevation, the long head of the triceps brachii generates a significantly higher muscle force and muscle activation than the lateral and medial heads (p

In this paper, we propose a metamaterial absorber using 3-D printing technology. To miniaturize the footprint size of a unit cell, we introduce a 3-D printed stair-like Jerusalem cross (JC) structure. It is demonstrated that the absorption frequency can be decreased by building up one and two stairs from the planar JC. In this paper, the unit cell footprint size of the three-stair JC is reduced by 41% compared with the unit cell footprint size of the planar JC. The proposed metamaterial absorber with 9×9 unit cells is fabricated using a 3-D printer. Poly lactic acid is used as a dielectric material for 3-D printing, and a conductive pattern is realized by applying the silver paste to the 3-D printed dielectric structure. The simulated and measured absorptivity are 99.9% and 99.8%, respectively, at 5.18 GHz. Due to the symmetric unit cell structure, the absorptivity does not change, although the incident polarization gets rotated.

This paper proposes a switchable metasurface for broadband and polarization-insensitive absorption, whose absorption spectrum can be fluidically switched using liquid metal. The proposed metasurface consists of a top metallic pattern, a flexible printed circuit board substrate, a polydimethylsiloxane substrate, and a bottom metallic ground plane. The proposed unitcell is inspired by a four-circular-sector structure. To maintain the symmetrical geometry and increase the switching range, a symmetrical microfluidic channel is designed with a circular ring capillary and four isolated patches. To measure the absorptivity of the proposed metasurface, 10 × 10 unitcells are fabricated with 20 inlets and 20 outlets. When the microfluidic channels of the metasurface are empty, the fabricated metasurface achieves an absorptivity of higher than 90%, from 6.23 to 12.14 GHz. When eutectic gallium-indium alloy is injected into the microfluidic channels, the fabricated metasurface achieves higher than 90% absorptivity, from 5.44 to 6.12 GHz. Thus, for both cases, the absorptivity is constant at different incident polarization angles.

Glucose-monitoring sensors are necessary and have been extensively studied to prevent and control health problems caused by diabetes. Spoof localized surface plasmon (LSP) resonance sensors have been investigated for chemical sensing and biosensing. A spoof LSP has similar characteristics to an LSP in the microwave or terahertz frequency range but with certain advantages, such as a high-quality factor and improved sensitivity. In general, microwave spoof LSP resonator-based glucose sensors have been studied. In this study, a millimeter-wave-based spoof surface plasmonic resonator sensor is designed to measure glucose concentrations. The millimeter-wave-based sensor has a smaller chip size and higher sensitivity than microwave-frequency sensors. Therefore, the microfluidic channel was designed to be reusable and able to operate with a small sample volume. For alignment, a polydimethylsiloxane channel was simultaneously fabricated using a multilayer bonding film to attach the upper side of the pattern, which is concentrated in the electromagnetic field. This real-time sensor detects the glucose concentration via changes in the S11 parameter and operates at 28 GHz with an average sensitivity of 0.015669 dB/(mg/dL) within the 0–300 mg/dL range. The minimum detectable concentration and the distinguishable signal are 1 mg/dL and 0.015669 dB, respectively, from a 3.4 μL sample. The reusability and reproducibility were assessed through replicates.

Abstract In this study, the novel electronically switchable broadband metamaterial absorber, using a PIN diode, is proposed. The unit cell of the absorber was designed with a Jerusalem-cross resonator and an additive ring structure, based on the FR-4 dielectric substrate. Chip resistors and PIN diodes were used to provide both a broadband characteristic and a switching capability. To satisfy the polarization insensitivity, the unit cell was designed as a symmetrical structure, including the DC bias network, electronic devices, and conductor patterns. The performance of the proposed absorber was verified using full-wave simulation and measurements. When the PIN diode was in the ON state, the proposed absorber had a 90% absorption bandwidth from 8.45–9.3 GHz. Moreover, when the PIN diode was in the OFF state, the 90% absorption bandwidth was 9.2–10.45 GHz. Therefore, the absorption band was successfully switched between the low-frequency band and the high-frequency band as the PIN diode was switched between the ON and OFF states. Furthermore, the unit cell of the proposed absorber was designed as a symmetrical structure, and its performance showed insensitivity with respect to the polarization angle.

Abstract In this paper, a wide incidence angle-insensitive metamaterial absorber is proposed using eight-circular-sector (ECS). Under normal incidence, the proposed absorber shows high absorptivity at different polarizations due to its symmetric geometry. Under oblique incidence, zero-reflection conditions for transverse electric (TE) and transverse magnetic (TM) polarization are different. Nevertheless, the proposed absorber shows high absorptivity under oblique incidence of both TE and TM polarization due to ECS. The performance of the proposed absorber was demonstrated with full-wave simulation and measurements. The simulated absorptivity at the specular angles exceed 90% and the frequency variation is less than 0.7% at approximately 9.26 GHz up to a 70° incidence angle in both TM and TE polarization. We built the proposed absorber on a printed-circuit board with 20 × 20 unit cells, and we demonstrated its performance experimentally in free space. The measured absorptivity at 9.26 GHz for the specular angles is close to 98% for all polarization angles under normal incidence. As the incidence angle is varied from 0° to 70°, the measured absorptivity at 9.26 GHz for the specular angles remain above 92% in both TE and TM polarization.

Electromagnetic responses are generally controlled electrically or optically. However, although electrical and optical control allows fast response, they suffer from switching or tuning range limitations. This paper controls electromagnetic response by mechanical transformation. We introduce a novel kirigami-inspired structure for mechanical transformation with less strength, integrating a shape memory alloy actuator into the kirigami-inspired for mechanical transformation and hence electromagnetic control. The proposed approach was implemented for a reconfigurable antenna designed based on structural and electromagnetic analyses. The mechanical transformation was analyzed with thermal stimulus to predict the antenna geometry and electromagnetic analysis with different geometries predicted antenna performance. We numerically and experimentally verified that resonance response was thermally controlled using the kirigami-inspired antenna integrated with a shape memory alloy actuator.

Liquid materials’ characterization using commercial probes and radio frequency techniques is expensive and complex. This study proposes a compact and cost-effective radio frequency sensor system to measure the dielectric constant using a three-material calibration. The simplified approach measures reflection coefficient magnitudes for all four materials rather than the complex values in conventional permittivity detection systems. We employ a sensor module based on a circular substrate-integrated waveguide with measured unloaded quality factor = 910 to ensure measurement reliability. Miniaturized quarter-mode substrate-integrated waveguide resonators are integrated with four microfluidic channels containing three known materials and one unknown analyte. Step-wise measurement and linearity ensures maximum 4% error for the dielectric constant compared with results obtained using a high-performance commercial product.

In this paper, we propose a frequency-tunable electromagnetic absorber that uses the mechanical control of substrate thickness. The absorption frequency of the proposed absorber can be changed by varying the substrate thickness. In order to mechanically control the substrate thickness, we introduce a 3D-printed molding with air space. The proposed structure consists of two layers and one frame: the FR4 substrate, polylactic acid (PLA) frame, and air substrate. The FR4 and PLA thicknesses are fixed, and the air thickness is varied using the PLA frame. Therefore, the effective dielectric constant of the overall substrate can be changed. The metallic rectangular patch and ground are patterned on the top and bottom FR4 substrates, respectively. The performance of the proposed tunable absorber is demonstrated from full-wave simulation and measurements. When both of the FR4 substrate thicknesses are 0.3 mm and the air thickness is changed from 1 to 3.5 mm, the absorption frequency is changed from 8.9 to 8.0 GHz, respectively. Therefore, the frequency-tuning capability of the proposed absorber is successfully demonstrated.

In this paper, we propose a paper-based pattern switchable antenna system using inkjet-printing technology for bi-direction sensor applications. The proposed antenna system is composed of two directional bow-tie antennas and a switching network. The switching network consists of a single-pole-double-throw (SPDT) switch and a balun element. A double-sided parallel-strip line (DSPSL) is employed to convert the unbalanced microstrip mode to the balanced strip mode. Two directional bow-tie antennas have different radiation patterns because of the different orientation of the reflectors and antennas. It is demonstrated from electromagnetic (EM) simulation and measurement that the radiation patterns of the proposed antenna are successfully switched by the SPDT switch.

The proposed structure in this research is constructed on substrate integrated waveguide (SIW) technology and has a mechanism that produces 16 different and distinct resonant frequencies between 2.45 and 3.05 GHz by perturbing a fundamental TE10 mode. It is a unique method for producing multiple resonances in a radio frequency planar structure without any extra circuitry or passive elements is developed. The proposed SIW structure has four vertical fluidic holes (channels); injecting eutectic gallium indium (EGaIn), also known commonly as liquid metal (LM), into these vertical channels produces different resonant frequencies. Either a channel is empty, or it is filled with LM. In total, the combination of different frequencies produced from four vertical channels is 16.

In this study, we demonstrated a new class of frequency-switchable metamaterial absorber in the X-band. Eutectic gallium-indium (EGaIn), a liquid metal alloy, was injected in a microfluidic channel engraved on polymethyl methacrylate (PMMA) to achieve frequency switching. Numerical simulation and experimental results are presented for two cases: when the microfluidic channels are empty, and when they are filled with liquid metal. To evaluate the performance of the fabricated absorber prototype, it is tested with a rectangular waveguide. The resonant frequency was successfully switched from 10.96 GHz to 10.61 GHz after injecting liquid metal while maintaining absorptivity higher than 98%.

In this paper, we propose a bi-directional loop antenna array using magic cube origami. The proposed antenna array consists of three one-wavelength loop antenna elements with series feeding. Each loop antenna is realized on a single magic cube, and three cubes are connected in series to form the array. The three cubes can be easily folded and unfolded due to being constructed in the form of a magic cube origami. Antenna volume can be minimized for high mobility by folding the array, which radiates a bi-directional pattern with full volume when unfolded. The proposed antenna was designed at 1.39 GHz. When the single antenna is realized on the single cube, the peak gain is 4.03 dBi. The peak gain increased to 5.2 and 5.53 dBi with two and three antennas, respectively. Half-power beam width (HPBW) with three antenna elements decreased to 40° from 360° compared to the HPBW with the single antenna. The proposed antenna performance was assessed numerically and experimentally.

In this paper, we propose a switchable band-pass/band-stop filter using liquid metal alloy as a fluidic switch. The filter is designed based on the Chebyshev response and implemented using a three-stage quarter-wavelength resonant structure. The fluidic switch is realized by injecting eutectic gallium–indium (EGaIn) in the microfluidic stubs, engraved in the polydimethylsiloxane (PDMS) material. When the fluidic switch selects the short stub using a micro-pump and microprocessor for switching, the filter acts as a bandpass filter (BPF) with the short stubs. When the fluidic switch selects the open stub, the filter acts as the bandstop filter (BSF) with the open stubs. At the BPF mode, the center frequency is 2.5 GHz and the 1-dB bandwidth is 1.75–3.07 GHz. The insertion loss is 0.5-dB ± 0.4-dB. At the BSF mode, the 15-dB bandstop bandwidth is 2.4–2.65 GHz with 2.5 GHz center frequency.

In this study, we propose a thermal frequency reconfigurable electromagnetic absorber using germanium telluride (GeTe) phase change material. Thermally-induced phase transition of GeTe from an amorphous high-resistive state to a crystalline low-resistive state by heating is used to change the resonant frequency of the absorber. For full-wave simulation, the electromagnetic properties of GeTe at 25 °C and 250 °C are characterized at 10 GHz under normal incidence for electromagnetic waves. The proposed absorber is designed based on the characterized electromagnetic parameters of GeTe. A circular unit cell is designed and GeTe is placed at a gap in the circle to maximize the switching range. The performance of the proposed electromagnetic absorber is numerically and experimentally demonstrated. Measurement results indicate that the absorption frequency changes from 10.23 GHz to 9.6 GHz when the GeTe film is altered from an amorphous state at room temperature to a crystalline state by heating the sample to 250 °C. The absorptivity in these states is determined to be 91% and 92%, respectively.

In this paper, a frequency reconfigurable quasi-Yagi dipole antenna is proposed by leveraging the properties of microfluidic technology. The proposed antenna comprises a metal-printed driven dipole element and three directors. To tune resonant frequencies, microfluidic channels are integrated into the driven element. To maintain a high gain for all the tuned frequencies, microfluidic channels are also integrated into the directors. Therefore, the length of the driven-element as well as directors can be controlled by injecting liquid metal in the microfluidic channels. The proposed antenna has the capability of tuning the frequency by varying the length of the metal-filled channels, while maintaining a high gain for all the tuned frequencies. The proposed antenna’s performance is experimentally demonstrated after fabrication. The injected amount of liquid metal into the microfluidic channels is controlled using programmable pneumatic micropumps. The prototype exhibits continuous tuning of the resonant frequencies from 1.8 GHz to 2.4 GHz; the measured peak gain of the proposed antenna is varied in the range of 8 dBi to 8.5 dBi. Therefore, continuous tuning with high gain is successfully demonstrated using liquid-metal-filled microfluidic channels.

This paper proposes a microfluidic impedance tuner that is applied to a planar inverted-F antenna (PIFA). The proposed microfluidic impedance tuner is designed while using a simple double-stub and the impedance is changed by tuning the stub length. In this work, the stub length can be tuned by injecting a liquid metal alloy to the microfluidic channels. Initially, the PIFA operates at 900 MHz with impedance matching of 50 Ω. The impedance is mismatched when a hand is placed close to the antenna. The mismatched impedance is matched to 50 Ω by injecting the liquid metal alloy. The antenna is fabricated on the FR-4 substrate, and the impedance tuner is fabricated on polydimethylsiloxane (PDMS). In order to inject the liquid metal alloy, a piezoelectric micropump and microprocessor are used in the measurement. At 900 MHz, the return loss is successfully tuned from 4.69 dB to 18.4 dB when a hand is placed 1 mm above the antenna.

Owing to the rapid growth in wireless data traffic, millimeter-wave (mm-wave) communications have shown tremendous promise and are considered an attractive technique in fifth-generation (5G) wireless communication systems. However, to design robust communication systems, it is important to understand the channel dynamics with respect to space and time at these frequencies. Millimeter-wave signals are highly susceptible to blocking, and they have communication limitations owing to their poor signal attenuation compared with microwave signals. Therefore, by employing highly directional antennas, co-channel interference to or from other systems can be alleviated using line-of-sight (LOS) propagation. Because of the ability to shape, switch, or scan the propagating beam, phased arrays play an important role in advanced wireless communication systems. Beam-switching, beam-scanning, and multibeam arrays can be realized at mm-wave frequencies using analog or digital system architectures. This review article presents state-of-the-art phased arrays for mm-wave mobile terminals (MSs) and base stations (BSs), with an emphasis on beamforming arrays. We also discuss challenges and strategies used to address unfavorable path loss and blockage issues related to mm-wave applications, which sets future directions.

In this paper, a microwave cavity resonator is presented for chemical sensing applications. The proposed resonator is comprised of a three dimensional (3D) split-ring resonator (SRR) residing in an external cavity and capacitively coupled by a pair of coaxial probes. 3D-printing technology with polylactic acid (PLA) filament is used to build the 3D SRR and cavity. Then, the surfaces of the SRR and the inside walls of cavity are silver-coated. The novelty of our proposed structure is its light weight and inexpensive design, owing to the utilization of low density and low-cost PLA. A Teflon tube is passed through the split-gap of the SRR so that it is parallel to the applied electric field. With an empty tube, the resonance frequency of the structure is measured at 2.56 GHz with an insertion loss of 13.6 dB and quality factor (Q) of 75. A frequency shift of 205 MHz with respect to the empty channel was measured when deionized water (DIW) was injected into the tube. Using volume occupied by the structure, the weight of the proposed microwave resonator is estimated as 22.8 g which is significantly lighter than any metallic structure of comparable size.

The detection of multiple fluids using a single chip has been attracting attention recently. A TM02 quarter-mode substrate-integrated waveguide resonator designed at 5.81 GHz on RT/duroid 6010LM with a return loss of 13 dB and an unloaded quality factor of Q ≈ 13 generates two distinct strong electric fields that can be manipulated to simultaneously detect two chemicals. Two asymmetric channels engraved in a polydimethylsiloxane sheet are loaded with analyte to produce a unique resonance frequency in each case, regardless of the dielectric constants of the liquids. Keeping in view the nature of lossy liquids such as ethanol, the initial structure and channels are optimized to ensure a reasonable return loss even in the case of loading lossy liquids. After loading the empty channels, Q is evaluated as 43. Ethanol (E) and deionized water (DI) are simultaneously loaded to demonstrate the detection of all possible combinations: [Air, Air], [E, DI], [DI, E], [E, E], and [DI, DI]. The proposed structure is miniaturized while exhibiting a performance comparable to that of existing multichannel microwave chemical sensors.

Microwave resonators working as sensors can detect only a single analyte at a time. To address this issue, a TE20-mode substrate-integrated waveguide (SIW) resonator is exploited, owing to its two distinct regions of high-intensity electric fields, which can be manipulated by loading two chemicals. Two microfluidic channels with unequal fluid-carrying capacities, engraved in a polydimethylsiloxane (PDMS) sheet, can perturb the symmetric electric fields even if loaded with the two extreme cases of dielectric [ethanol (E), deionized water (DI)] and [deionized water, ethanol]. The four layers of the sandwiched structure considered in this study consisted of a top conductive pattern and a bottom ground, both realized on a Rogers RT/Duroid 5880. PDMS-based channels attached with an adhesive serve as the middle layers. The TE20-mode SIW with empty channels resonates at 8.26 GHz and exhibits a −25 dB return loss with an unloaded quality factor of Q ≈ 28. We simultaneously load E and DI and demonstrate the detection of the four possible combinations: [E, DI], [DI, E], [E, E], and [DI, DI]. The performance of our proposed method showed increases in sensitivity (MHz/εr) of 7.5%, 216%, and 1170% compared with three previously existing multichannel microwave chemical sensors.

In this study, a high-Q circular substrate-integrated waveguide (SIW) cavity resonator is proposed as a non-contact and non-invasive radio frequency (RF) sensor for chemical sensing applications. The design of the structure utilizes SIW technology along with a circular shape to achieve a high unloaded Q factor, which is one of the important requirements for RF sensors. The resonant frequency of the proposed circular SIW cavity sensor changes when a liquid material or a chemical (microliters) is inserted in the sensitive area of the structure. The sensing of liquid materials with different permittivities is accomplished via the perturbation of the electric fields in the SIW configuration. When a microwell that is 4 mm in radius is installed vertically through the center of the bare circular SIW cavity, the operating frequency varies from 5.26 to 5.34 GHz. Similarly, when the microwell contains ethanol, the frequency shifts from 5.26 to 5.18 GHz, and the amplitude of reflection coefficient is shifted from −29 dB to −17 dB; when the microwell contains mixing deionized (DI)-water, the frequency moves from 5.26 to 4.98 GHz (which is also 0% Ethanol in our study), and the amplitude of reflection coefficient is shifted from −29 dB to −8 dB. A high unloaded Q factor is maintained throughout all experimental results. To demonstrate our idea, different concentrations of ethanol are tested and recorded. The experimental validation yields a close agreement between the simulations and the measurements.