Shape-Morphing Antenna Array by 4D-Printed Multimaterial Miura Origami.

The rapid development of four-dimensional (4D) printing technology has resulted in its application in various fields, including radiofrequency (RF) electronics. Moreover, because origami-inspired RF electronics provide a physically deformable geometry, they are good candidates for reconfigurable RF applications. However, previous origami-inspired RF electronics have generally been fabricated on paper for easy folding and unfolding. Although this facilitates easy fabrication, the resultant structures suffer from a lack of rigidity and stability. In this paper, we propose a 4D-printed multimaterial Miura origami structure for RF spectrum applications. For thermal actuation and robustness, the proposed structure consists of high-temperature durable cores with shape memory polymer (SMP) hinges. The high-temperature durable cores provide rigidity to the desired part and reduce the level of distortion of the conductive pattern, while the SMP hinges enable shape morphing. To demonstrate the feasibility of the technique for RF electronics, a shape-morphing pattern reconfigurable antenna array is designed at 2.4 GHz using the proposed 4D-printed multimaterial structure. Through numerical and experimental demonstrations, the proposed antenna's maximum beam direction is changed from 0° to 50° by thermally morphing the Miura origami. In addition, the antenna successfully recovers to its memorized original state.

Foldable RF Energy Harvesting System Based on Vertically Layered Metal Electrodes within a Single Sheet of Paper.

Radio frequency energy harvesting (RFEH) systems have emerged as a critical component for powering devices and replacing traditional batteries, with paper being one of the most promising substrates for use in flexible RFEH systems. However, previous paper-based electronics with optimized porosity, surface roughness, and hygroscopicity still face limitations in terms of the development of integrated foldable RFEH systems within a single sheet of paper. In the present study, a novel wax-printing control and water-based solution process are used to realize an integrated foldable RFEH system within a single sheet of paper. The proposed paper-based device includes vertically layered foldable metal electrodes, a via-hole, and stable conductive patterns with a sheet resistance of less than 1 Ω sq-1 . The proposed RFEH system exhibits an RF/DC conversion efficiency of 60% and an operating voltage of 2.1 V in 100 s at a distance of 50 mm and a transmitted power of 50 mW. The integrated RFEH system also demonstrates stable foldability, with RFEH performance maintained up to a folding angle of 150°. The single-sheet paper-based RFEH system thus has the potential for use in practical applications associated with the remote powering of wearable and Internet-of-Things devices and in paper electronics.

Microwave Dual-Crack Sensor with a High Q-Factor Using the TE20 Resonance of a Complementary Split-Ring Resonator on a Substrate-Integrated Waveguide.

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.

Four-Dimensional Printed Shape Memory Metasurface to Memorize Absorption and Reflection Functions.

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.

Dynamic phase control with printing and fluidic materials' interaction by inkjet printing an RF sensor directly on a stereolithographic 3D printed microfluidic structure.

Stereolithographic (SL) three-dimensional (3D) printing of microfluidic channels and inkjet printing of radio frequency (RF) electronics are promising lab-on-a-chip technologies. However, the effective integration of the two techniques has been challenging since the fabricated parts need to be combined via an additional bonding process, such as plasma bonding. This study proposes combining RF electronics with SL printed microfluidic structures by directly inkjet printing onto a 3D printed mould. This allows the inkjet printing of RF electronics with high conductivity (8 × 106 S m-1) and high resolution (50 µm) as a surface modification of the 3D printed mould. This process combines the three-dimensional printing of microfluidic parts and the inkjet printing of RF sensors into a single process. The proposed approach increases the interaction between a printed RF part and a fluid material by adjusting the distance between them, and it can be applied to various resins and 3D printing methods. Furthermore, the proposed fabrication process was applied to a dynamic phase advanced and delayed transmission line (TL) operating at 3.8 GHz as a fluidic sensor. Consequently, using the same pattern, a higher phase shift range per microliter of 10° was obtained than the 1° for conventional phase shift TLs.

Switchable Bandpass/Bandstop Filter Using Liquid Metal Alloy as Fluidic Switch.

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.

Dual-Band Band-Pass Filter with Fixed Low Band and Fluidically-Tunable High Band.

In this work, we present a dual-band band-pass filter with fixed low-band resonant frequency and tunable high-band resonant frequency. The proposed filter consists of two split-ring resonators (SRRs) with a stub and microfluidic channels. The lower resonant frequency is determined by the length of the SRR alone, whereas the higher resonant frequency is determined by the lengths of the SRR and the stub. Using this characteristic, we fix the lower resonant frequency by fixing the SRR length and tune the higher resonant frequency by controlling the stub length by injecting liquid metal in the microfluidic channel. We fabricated the filter on a Duroid substrate. The microfluidic channel was made from polydimethylsiloxane (PDMS), and eutectic gallium-indium (EGaIn) was used as the liquid metal. This filter operates in two states-with, and without, the liquid metal. In the state without the liquid metal, the filter has resonant frequencies at 1.85 GHz and 3.06 GHz, with fractional bandwidths of 4.34% and 2.94%, respectively; and in the state with the liquid metal, it has resonant frequencies at 1.86 GHz and 2.98 GHz, with fractional bandwidths of 4.3% and 2.95%, respectively.