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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, 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.

This paper proposes a metamaterial inspired radio frequency touchpad sensor system based on two split ring resonators (SRRs) realized on a Rogers/Duroid 5870 with 2.33 dielectric constant = 2.33, loss tangent = 0.0012, and thickness = 0.8 mm. The touchpad consists of two SRRs resonating at $f_{1} = 2.74$ GHz and $f_{2} = 3.86$ GHz. When a finger is placed on SRR 1, $f_{1}$ changes from 2.74 to 2.57 GHz due to electromagnetic coupling between the SRR and the finger, and similarly, $f_{2}$ changes from 3.86 to 3.76 GHz. The proposed touchpad system can detect a finger wearing a glove, in contrast to capacitive touchpads utilized in contemporary smart phones. We demonstrated 56- and 49-MHz frequency shifts for $f_{1}$ and $f_{2}$ , respectively, with 1.66-mm total thickness (0.69-mm-thick photo paper and 0.97-mm-thick glass slide).

Functional metasurfaces help wireless communication to reach beyond current electromagnetic control device limitations. However, current reconfigurable functional metasurfaces require separate systems for function control. In particular, it is difficult to realize millimeter-wavelength regimes due to the increasing number of active elements with the reduction in unit cell size. This paper proposes a four-dimensional printed memory metasurface to memorize absorption and reflection function in millimeter-wavelength regimes. Thus, metasurfaces with electromagnetic absorption and reflection functions can be realized through mechanical shape memory by memorizing electromagnetic properties using four-dimensional printed structures. The desired electromagnetic performance was experimentally demonstrated and deformation time to memorize the initial structure was measured. The results confirmed that the proposed four-dimensional printed metasurface has potential for considerable contribution to multifunctional wireless devices such as smart electromagnetic wave control systems in reconfigurable intelligent surface, stealth, and wireless sensing systems.

An optically transparent metamaterial absorber that can be obtained using inkjet printing technology is proposed. In order to make the metamaterial absorber optically transparent, an inkjet printer was used to fabricate a thin conductive loop pattern. The loop pattern had a width of 0.2 mm and was located on the top surface of the metamaterial absorber, and polyethylene terephthalate films were used for fabricating the substrate. An optically transparent conductive indium tin oxide film was introduced in the bottom ground plane. Therefore, the proposed metamaterial absorber was optically transparent. The metamaterial absorber was demonstrated by performing a full-wave electromagnetic simulation and measured in free space. In the simulation, the 90% absorption bandwidth ranged from 26.6 to 28.8 GHz, while the measured 90% absorption bandwidth was 26.8&ndash, 28.2 GHz. Therefore, it is successfully demonstrated by electromagnetic simulation and measurement results.

A novel high-efficiency, compact metamaterial-inspired rectifier is proposed for 900-MHz wireless power transmission (WPT) applications. The open-ended composite right/left-handed (CRLH) transmission line (TL) stub, which is introduced to realize metamaterials, replaces the band-pass filter (BPF) of a conventional rectifier (and is smaller than the traditional BPF). The open-ended CRLH TL stub is designed to suppress second- and third-order harmonics and to match the fundamental frequency to a 50-Ω impedance. The device is implemented using lumped elements and a microstrip line. The maximum RF-to-DC conversion efficiency of the proposed rectifier is 69.9% at an input power of 50 mW and a load of 1.8 kΩ. A design procedure that is based on minimizing the phase response of a CRLH TL is presented and validated by simulated and measured results. Because the single CRLH stub is used for harmonic suppression, miniaturization is achieved while retaining efficiency similar to that of previous rectifiers.

Metasurfaces have been studied intensively over the past two decades as they offer the advantages of extraordinary electromagnetic characteristics and highly dispersive responses. Furthermore, electrically or optically reconfigurable metasurfaces can dynamically switch their operation or provide diverse functions via control of their electromagnetic responses. Consequently, mechanical transformations of metasurfaces have paved the way for fabrication of physical sensors. Specifically, in this paper, we propose a stretchable metasurface for wireless strain sensor applications. Because of its dispersive characteristic, the proposed metasurface can reject a specific frequency while transmitting all other frequencies. When the metasurface is stretched, its rejection frequency is shifted. Therefore, the proposed metasurface can wirelessly sense the strain level. The stretchable metasurface is implemented by screen printing a stretchable conductor on a stretchable dielectric material. Further, the frequency response of the fabricated metasurface is measured by loading between the transmitting and receiving antennas. The measured modulus of the fabricated sample is 2.07 MPa for a maximum strain of 15%, and its measured sensitivity is 0.085 × 109 Hz/%.

This paper proposed a hybrid printed electromagnetic pressure sensor using a metamaterial absorber. We used a three-dimensional printed flexible resin for the transformable substrate with inkjet printed conductive patterns stacked on the printed substrate to realize a hybrid printed electromagnetic pressure sensor. A square ring pattern was implemented for the metamaterial absorber unit cell. Since absorption frequency varied with substrate thickness, the device could be used as an electromagnetic pressure sensor with mechanically transformable substrate. Performance was numerically and experimentally measured, and absorption frequency increased from 5.2 to 5.66 GHz by applying 0 and 20 N pressure, respectively; device sensitivity = 7.75 × 108 Hz/mm (0.2 × 108 Hz/N); and repeatability was retained up to 100 cycles.

In this paper, we proposed a broadband Frequency reconfigurable metamaterial absorber using a switchable ground plane (SGP). We proposed the multi-layers and a PIN diode of middle layer for the SGP. Because between the top structure and the bottom ground plane determined the resonant frequency, the switchable ground plane is proposed. When the PIN diode of the SGP turns off, the middle layer becomes reactive and bottom ground plane is working as ground plane of the absorber. On the other hand, when the PIN diode of the SGP turns on, the middle layer becomes ground plane and the bottom ground plane does not affect the absorber. The proposed absorber is demonstrated by full-wave simulation and measurement. The fabricated sample with 27 × 27unit cells is measured for the absorption under normal incidence. When the PIN diode of the SGP turns on, the absorption with more than 90% is achieved from 3.5 to 11 GHz. On the other hand, when the PIN diode of the SGP turns off, the absorption with more than 90% is achieved from 1.7 to 5.2 GHz.

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.

In this paper, a mechanically actuated frequency reconfigurable metamaterial electromagnetic absorber is proposed. The absorber’s metamaterial unit cell is designed to exploit LC resonance from inductive and capacitive coupling. Because this inductance and capacitance determine the absorber’s resonant frequency, we propose a mechanical tuning method that changes the resonant frequency by changing the overall thickness of the metamaterial unit cell. The proposed unit cell consists of an FR4 dielectric substrate with fixed thickness and an air substrate with tunable thickness. When the air substrate thickness is varied over the range of 17 mm to 26 mm, the absorber’s resonant frequency changes from 6.96 GHz to 5.79 GHz in EM simulation. In order to verify the proposed idea, a metamaterial absorber was fabricated as a 17 × 17 array of unit cells and a linear actuator was used to control the thickness of the air substrate. We experimentally demonstrated that the absorption frequency changes from 6.96 GHz to 5.78 GHz with 0.12 (GHz/mm) sensitivity when the air substrate thickness is mechanically changed from 17 mm to 26 mm.