A Fluidically Tunable Metasurface Absorber for Flexible Large-Scale Wireless Ethanol Sensor Applications

A dual functional tunable terahertz metamaterial absorber based on vanadium dioxide

A dual-functional switchable metamaterial absorber (MMA) based on vanadium dioxide (VO2), which achieves flexible switching between broadband absorption and four-band absorption by adjusting the VO2 conductivity, was proposed. The device has a broadband absorption function when VO2 is in the metal phase, and the conductivity is 3 × 105 S m-1. Numerical simulation shows that the absorption rate of the device reaches over 90% in the frequency range of 3.36-6.98 THz. The absorber exhibits polarization insensitivity and wide-angle absorption to transverse electric (TE) and transverse magnetic (TM) waves. When VO2 is in the insulator phase, and the conductivity is 3 × 102 S m-1, the device switches to a narrowband absorber with a band-efficient absorption function. Numerical simulation shows that the device has an absorption rate of 99.7% at 2.39 THz, 98.3% at 2.83 THz, 95.6% at 3.84 THz, and 96.1% at 4.61 THz. It can be used as a sensor with high sensitivity. In addition, to verify the absorption mechanism of the absorber, we introduced impedance matching theory to analyze the device. Finally, the influence of structural parameters on the performance of resonators was investigated. Through the joint action of multi-layer structures, the proposed MMA concentrates broadband and narrowband absorption functions on one device, achieving flexible switching between tasks without changing the structure. The switchable metamaterial absorber designed through simple tuning methods has broad application prospects in stealth technology and thermal emitters. It provides a wide range of ideas for the design of terahertz functional devices.

Electric Split-Ring Metamaterial Based Microfluidic Chip with Multi-Resonances for Microparticle Trapping and Chemical Sensing Applications

In this work, an integration of terahertz (THz) electrical split-ring metamaterial (eSRM) with microfluidic chip is presented. This eSRM-based microfluidic chip exhibits multiple resonances in the THz spectrum and trapping selectively microparticle size characteristics. The arrangement of eSRM array is dislocation. It generates the fundamental inductive-capacitive (LC) resonant mode, quadrupole, and octupolar plasmon resonant modes and then exhibits high sensitivity to the environmental refraction index. The trapping structures of microparticles are elliptical barricades on eSRM surface. Thus, the electric field energy is strongly confined within the gap of eSRM in transverse electric (TE) mode and then the elliptical trapping structures are anchored on both sides of the split gap to ensure the microparticles can be trapped and located on the gap. To imitate the microparticle sensing ambient environment qualitatively and quantitatively in the THz spectrum, the microparticles are designed different feature sizes with different refraction index from 1.0 to 2.0 in ethanol medium. The results show the proposed eSRM-based microfluidic chip possesses the trapping and sensing abilities in single microparticle and high sensitivity for fungus, microorganism, chemical and environmental applications.

Recent Development in Metasurfaces: A Focus on Sensing Applications

One of the fastest-expanding study areas in optics over the past decade has been metasurfaces (MSs). These subwavelength meta-atom-based ultrathin arrays have been developed for a broad range of functions, including lenses, polarization control, holography, coloring, spectroscopy, sensors, and many more. They allow exact control of the many properties of electromagnetic waves. The performance of MSs has dramatically improved because of recent developments in nanofabrication methods, and this concept has developed to the point that it may be used in commercial applications. In this review, a vital topic of sensing has been considered and an up-to-date study has been carried out. Three different kinds of MS absorber sensor formations, all-dielectric, all-metallic, and hybrid configurations, are presented for biochemical sensing applications. We believe that this review paper will provide current knowledge on state-of-the-art sensing devices based on MSs.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Microwave Polarization Sensing for Dielectric Materials Based on a Twisted Dual-Layer Meta-Surface

A microwave sensor is proposed based on a chiral twisted dual-layer meta-surface. Elliptical angle and polarization rotation angle are used to characterize the different dielectric constants of materials. The dielectric films consisting of polydimethylsiloxane and barium titanate with volume fractions 0%, 10%, 15%, 20% are prepared and tested for a proof of concept. The measured results show that the Q factors of polarization rotation angle and elliptical angle peak are 11.85 when the volume fraction of barium titanate is 20%, which is 75.5% higher than 6.75 of the transmission resonance peak, and the figures of merit of the polarization rotation angle and elliptical angle peak are 0.99 and 0.86, which are 73.7% and 50.9% higher than the 0.57 of transmission resonance, respectively. Compared to the resonance sensing method, polarization sensing not only has a better Q factor and figure of merit while maintaining similar sensitivity, but also obtains more characterization information due to the double-parameter sensing, which provide a new idea for the development of high-sensitivity microwave sensors.

Metasurfaces for Sensing Applications: Gas, Bio and Chemical

Performance of photonic devices critically depends upon their efficiency on controlling the flow of light therein. In the recent past, the implementation of plasmonics, two-dimensional (2D) materials and metamaterials for enhanced light-matter interaction (through concepts such as sub-wavelength light confinement and dynamic wavefront shape manipulation) led to diverse applications belonging to spectroscopy, imaging and optical sensing etc. While 2D materials such as graphene, MoS₂ etc., are still being explored in optical sensing in last few years, the application of plasmonics and metamaterials is limited owing to the involvement of noble metals having a constant electron density. The capability of competently controlling the electron density of noble metals is very limited. Further, due to absorption characteristics of metals, the plasmonic and metamaterial devices suffer from large optical loss. Hence, the photonic devices (sensors, in particular) require that an efficient dynamic control of light at nanoscale through field (electric or optical) variation using substitute low-loss materials. One such option may be plasmonic metasurfaces. Metasurfaces are arrays of optical antenna-like anisotropic structures (sub-wavelength size), which are designated to control the amplitude and phase of reflected, scattered and transmitted components of incident light radiation. The present review put forth recent development on metamaterial and metastructure-based various sensors.

MS, Almalki SHA, Islam MS. Polarization Independent Metamaterial Absorber with Anti-Reflection Coating Nanoarchitectonics for Visible and Infrared Window Applications

The visible and infrared wavelengths are the most frequently used electromagnetic (EM) waves in the frequency spectrum; able to penetrate the atmosphere and reach Earth's surface. These wavelengths have attracted much attention in solar energy harvesting; thermography; and infrared imaging applications for the detection of electrical failures; faults; or thermal leakage hot spots and inspection of tapped live energized components. This paper presents a numerical analysis of a compact cubic cross-shaped four-layer metamaterial absorber (MA) structure by using a simple metal-dielectric-metal-dielectric configuration for wideband visible and infrared applications. The proposed MA achieved above 80% absorption in both visible and near-infrared regions of the spectrum from 350 to 1250 nm wavelength with an overall unit cell size of 0.57λ × 0.57λ × 0.59λ. The SiO2 based anti-reflection coating of sandwiched tungsten facilitates to achieve the wide high absorption bandwidth. The perceptible novelty of the proposed metamaterial is to achieve an average absorptivity of 95.3% for both visible and infrared wavelengths with a maximum absorptivity of 98% from 400 nm to 900 nm. Furthermore, the proposed structure provides polarization insensitivity with a higher oblique incidence angle tolerance up to 45°.

A Recent Approach towards Fluidic Microstrip Devices and Gas Sensors: A Review

This paper aims to review some of the available tunable devices with emphasis on the techniques employed, fabrications, merits, and demerits of each technique. In the era of fluidic microstrip communication devices, versatility and stability have become key features of microfluidic devices. These fluidic devices allow advanced fabrication techniques such as 3D printing, spraying, or injecting the conductive fluid on the flexible/rigid substrate. Fluidic techniques are used either in the form of loading components, switching, or as the radiating/conducting path of a microwave component such as liquid metals. The major benefits and drawbacks of each technology are also emphasized. In this review, there is a brief discussion of the most widely used microfluidic materials, their novel fabrication/patterning methods.

A THz graphene metasurface for polarization selective virus sensing

We propose a novel method to exploit chirality of highly sensitive graphene plasmonic metasurfaces to characterize complex refractive indexes (RI) of viruses by detecting the polarization state of the reflected electric fields in the THz spectrum. A dispersive graphene metasurface is designed to produce chiral surface currents to couple linearly polarized incident fields to circularly polarized reflected fields. The metasurface sensing sensitivity is the result of surface plasmon currents that flow in a chiral fashion with strong intensity due to the underlying geometrical resonance. Consequently, unique polarization states are observed in the far-field with the ellipticity values that change rapidly with the analyte's RI. The determination of bimolecular RI is treated as an inverse problem in which the polarization states of the virus is compared with a pre-calculated calibration model that is obtained by full-wave electromagnetic simulations. We demonstrate the polarization selective sensing method by RI discrimination of three different types of Avian Influenza (AI) viruses including H1N1, H5N2, and H9N2 is possible. Since the proposed virus characterization method only requires determination of the polarization ellipses including its orientation at monochromatic frequency, the required instrumentation is simpler compared to traditional spectroscopic methods which need a broadband frequency scan.

Soil moisture remote sensing using SIW cavity based metamaterial perfect absorber

Continuous and accurate sensing of water content in soil is an essential and useful measure in the agriculture industry. Traditional sensors developed to perform this task suffer from limited lifetime and also need to be calibrated regularly. Further, maintenance, support, and deployment of these sensors in remote environments provide additional challenges to the use of conventional soil moisture sensors. In this paper, a metamaterial perfect absorber (MPA) based soil moisture sensor is introduced. The ability of MPAs to absorb electromagnetic signals with near 100% efficiency facilitates the design of highly accurate and low-profile radio frequency passive sensors. MPA based sensor can be fabricated from highly durable materials and can therefore be made more resilient than traditional sensors. High resolution sensing is achieved through the creation of physical channels in the substrate integrated waveguide (SIW) cavity. The proposed sensor does not require connection for both electromagnetic signals or for adding a testing sample. Importantly, an external power supply is not needed, making the MPA based sensor the perfect solution for remote and passive sensing in modern agriculture. The proposed MPA based sensor has three absorption bands due to the various resonance modes of the SIW cavity. By changing the soil moisture level, the absorption peak shifts by 10 MHz, 23.3 MHz, and 60 MHz, which is correlated with the water content percentage at the first, second and third absorption bands, respectively. Finally, a [Formula: see text] cell array with a total size of [Formula: see text] has been fabricated and tested. A strong correlation between measurement and simulation results validates the design procedure.

Rapid prototyping of flexible terahertz metasurfaces using a microplotter

Additive manufacturing is a promising tool for the rapid prototyping of terahertz metamaterials at low-cost. In this letter, a terahertz metamaterial is fabricated using a microplotter system on a flexible polyimide film. The limits of the rapid prototyping technique is investigated both experimentally and numerically in order to determine the spectral range accessible by the fabricated metamaterials. Here, the metamaterial is composed of four arrays of metal-insulator-metal (MIM) antennas exhibiting a Fabry Perot resonance at frequencies from 0.25 to 0.8 THz. From a structural analysis of the printed antennas, we determined that the printing resolution is limited to about 5 μm. The arrays are analyzed by terahertz time-domain spectroscopy (THz-TDS). The good agreement between THz-TDS measurements and numerical simulations showed that the microplotter system can be used for rapid prototyping by adjusting a limited number of fabrication parameters.

VO2 based dynamic tunable absorber and its application in switchable control and real-time color display in the visible region

Metasurface-based near perfect absorbers exhibit a wide range of potential applications in the fields of solar energy harvesting, thermal images and sensors due to their unique absorption regulation function. However, absorption characteristics of devices are locked by the device structure, leading to the limitation in real-time dynamic applications. In this work, we integrate the phase change material VO₂ thin film into the metal-insulator-metal structured metasurface based absorber, and design a fully visible band switchable dynamically tunable absorber (DTA). By controlling the phase transition of VO₂, the DTA can realize a novel switch function in the full band of visible light (400 ∼ 780 nm), with absorption contrast ranges from 42% to 60%. Furthermore, via accurate structural parameter control, the vivid cyan, magenta, and yellow pixels based on the VO₂ DTA are designed and proposed in the real-time optical anti-counterfeiting, exhibiting outstanding characteristics of anti-glare interference and real-time encryption ability. The absorption spectrum and local electric field are simulated and analyzed to study the internal operation mechanism of DTA. The dynamic absorption adjustable function is attributed to the synergistic effect of insulator-metal transition of VO₂ and Fabry-Pérot resonance of absorber.

Achieving broadband absorption and polarization conversion with a vanadium dioxide metasurface in the same terahertz frequencies

We present the bifunctional design of a broadband absorber and a broadband polarization converter based on a switchable metasurface through the insulator-to-metal phase transition of vanadium dioxide. When vanadium dioxide is metal, the designed switchable metasurface behaves as a broadband absorber. This absorber is composed of a vanadium dioxide square, silica spacer, and vanadium dioxide film. Calculated results show that in the frequency range of 0.52-1.2 THz, the designed system can absorb more than 90% of the energy, and the bandwidth ratio is 79%. It is insensitive to polarization due to the symmetry, and can still work well even at large incident angles. When vanadium dioxide is an insulator, a terahertz polarizer is realized by a simple anisotropic metasurface. Numerical calculation shows that efficient conversion between two orthogonal linear polarizations can be achieved. Reflectance of a cross-polarized wave can reach 90% from 0.42 THz to 1.04 THz, and the corresponding bandwidth ratio is 85%. This cross-polarized converter has the advantages of wide angle, broad bandwidth, and high efficiency. So our design can realize bifunctionality of broadband absorption and polarization conversion between 0.52 THz and 1.04 THz. This architecture could provide one new way to develop switchable photonic devices and functional components in phase change materials.

Optically Transparent Metamaterial Absorber Using Inkjet Printing Technology

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 26.8–28.2 GHz. Therefore, it is successfully demonstrated by electromagnetic simulation and measurement results.

Thermal Frequency Reconfigurable Electromagnetic Absorber Using Phase Change Material

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.

Quarter-mode spoof plasmonic resonator for a microfluidic chemical sensor

In this paper, we propose a microfluidic chemical sensor based on quarter-mode spoof plasmonic resonators with more compact overall size and higher sensitivity. First, a microfluidic channel engraved on polydimethylsiloxane is aligned to the upper part of the spoof plasmonic metal-insulator-metal (MIM) ring resonator where the strongest electric fields are observed at resonance. Although a resonant frequency shift of 270 MHz has been observed when the microfluidic channel is filled with pure ethanol, there is no resonant frequency shift when the ethanol concentration is changed from 40% to 60%. Then the spoof localized surface plasmons modes on the quarter corrugated MIM ring are analyzed, and a microfluidic sensor based on the quarter-mode spoof plasmonic resonator has been proposed. The proposed microfluidic sensor requires a very small amount (3.9 μL) of liquid for testing. After infilling the microfluidic channel with pure ethanol, the resonant frequency shift of 940 MHz has been observed on account of the dielectric changes. It is observed that the resonant frequency of the proposed sensor shifts from 5.07 to 6.62 GHz when the ethanol concentration is varied from 10% to 90%. It has been demonstrated that such quarter-mode spoof plasmonic resonator is well suited to a highly sensitive and compact microfluidic chemical sensor.

Comparative Study of Square and Circular Loop Frequency Selective Surfaces for Millimeter-Wave Imaging Diagnostics Systems

A design method of large-sized square-loop and circular-loop frequency selective surface (FSS) filters for protection of mm-wave imagining receivers is presented. Due to fine cell structure requirements, the performance of the FSS structures at mm-wave frequencies can be significantly affected by fabrication tolerances, especially involved with large-size panel fabrication. Through a comprehensive parametric variation study on the performance of square-loop and circular-loop FSS structures, it is found that the circular-loop FSS structure performs much less sensitively to the fabrication tolerances, thereby producing better and consistent performances with given design values. As a design example, square-loop and circular-loop notch filters resonating at 105 GHz were designed and the performances were evaluated with multiple prototypes. The resonant frequencies of the implemented circular-loop FSS filters deviated by only about 0.5 GHz from the accurate designed value, which can be easily adjusted in the design process. The implemented square-loop and circular loop FSS filters provided low-loss in the pass-band and high rejection of 23 dB at the resonant frequency with good oblique angle performance.

Hybrid Metasurface Based Tunable Near-Perfect Absorber and Plasmonic Sensor

We propose a hybrid metasurface-based perfect absorber which shows the near-unity absorbance and facilities to work as a refractive index sensor. We have used the gold mirror to prevent the transmission and used the amorphous silicon (a-Si) nanodisk arrays on top of the gold mirror which helps to excite the surface plasmon by scattering light through it at the normal incident. We numerically investigated the guiding performance. The proposed absorber is polarization independent and shows a maximum absorption of 99.8% at a 932 nm wavelength in the air medium. Considering the real applications, by varying the environments refractive indices from 1.33 to 1.41, the proposed absorber can maintain absorption at more than 99.7%, with a red shift of the resonant wavelength. Due to impedance matching of the electric and magnetic dipoles, the proposed absorber shows near-unity absorbance over the refractive indices range of 1.33 to 1.41, with a zero-reflectance property at a certain wavelength. This feature could be utilized as a plasmonic sensor in detecting the refractive index of the surrounding medium. The proposed plasmonic sensor shows an average sensitivity of 325 nm/RIU and a maximum sensitivity of 350 nm/RIU over the sensing range of 1.33 to 1.41. The proposed metadevice possesses potential applications in solar photovoltaic and photodetectors, as well as in organic and bio-chemical detection.

Microwave Metamaterial Absorber for Non-Destructive Sensing Applications of Grain

In this work, we propose a metamaterial absorber at microwave frequencies with significant sensitivity and non-destructive sensing capability for grain samples. This absorber is composed of cross-resonators periodically arranged on an ultrathin substrate, a sensing layer filled with grain samples, and a metal ground. The cross-resonator array is fabricated using the printed circuit board process on an FR-4 board. The performance of the proposed metamaterial is demonstrated with both full-wave simulation and measurement results, and the working mechanism is revealed through multi-reflection interference theory. It can serve as a non-contact sensor for food quality control such as adulteration, variety, etc. by detecting shifts in the resonant frequencies. As a direct application, it is shown that the resonant frequency displays a significant blue shift from 7.11 GHz to 7.52 GHz when the mass fraction of stale rice in the mixture of fresh and stale rice is changed from 0% to 100%. In addition, the absorber shows a distinct difference in the resonant absorption frequency for different varieties of grain, which also makes it a candidate for a grain classification sensor. The presented scheme could open up opportunities for microwave metamaterial absorbers to be applied as efficient sensors in the non-destructive evaluation of agricultural and food product quality.

Simultaneous Detection of Two Chemicals Using a TE20-Mode Substrate-Integrated Waveguide Resonator

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/ε_r) of 7.5%, 216%, and 1170% compared with three previously existing multichannel microwave chemical sensors.

Review of Recent Metamaterial Microfluidic Sensors

Metamaterial elements/arrays exhibit a sensitive response to fluids yet with a small footprint, therefore, they have been an attractive choice to realize various sensing devices when integrated with microfluidic technology. Micro-channels made from inexpensive biocompatible materials avoid any contamination from environment and require only microliter-nanoliter sample for sensing. Simple design, easy fabrication process, light weight prototype, and instant measurements are advantages as compared to conventional (optical, electrochemical and biological) sensing systems. Inkjet-printed flexible sensors find their utilization in rapidly growing wearable electronics and health-monitoring flexible devices. Adequate sensitivity and repeatability of these low profile microfluidic sensors make them a potential candidate for point-of-care testing which novice patients can use reliably. Aside from degraded sensitivity and lack of selectivity in all practical microwave chemical sensors, they require an instrument, such as vector network analyzer for measurements and not readily available as a self-sustained portable sensor. This review article presents state-of-the-art metamaterial inspired microfluidic bio/chemical sensors (passive devices ranging from gigahertz to terahertz range) with an emphasis on metamaterial sensing circuit and microfluidic detection. We also highlight challenges and strategies to cope these issues which set future directions.

A Microwave Microfluidic Sensor Based on a Dual-Mode Resonator for Dual-Sensing Applications

In this paper, we propose a novel microwave microfluidic sensor with dual-sensing capability. The sensor is based on a dual-mode resonator that consists of a folded microstrip line loaded with interdigital lines and a stub at the plane of symmetry. Due to the specific configuration, the resonator exhibits two entirely independent resonant modes, which allows simultaneous sensing of two fluids using a resonance shift method. The sensor is designed in a multilayer configuration with the proposed resonator and two separated microfluidic channels-one intertwined with the interdigital lines and the other positioned below the stub. The circuit has been fabricated using low-temperature co-fired ceramics technology and its performance was verified through the measurement of its responses for different fluids in the microfluidic channels. The results confirm the dual-sensing capability with zero mutual influence as well as good overall performance. Besides an excellent potential for dual-sensing applications, the proposed sensor is a good candidate for application in mixing fluids and cell counting.

Review of Recent Inkjet-Printed Capacitive Tactile Sensors

Inkjet printing is an advanced printing technology that has been used to develop conducting layers, interconnects and other features on a variety of substrates. It is an additive manufacturing process that offers cost-effective, lightweight designs and simplifies the fabrication process with little effort. There is hardly sufficient research on tactile sensors and inkjet printing. Advancements in materials science and inkjet printing greatly facilitate the realization of sophisticated tactile sensors. Starting from the concept of capacitive sensing, a brief comparison of printing techniques, the essential requirements of inkjet-printing and the attractive features of state-of-the art inkjet-printed tactile sensors developed on diverse substrates (paper, polymer, glass and textile) are presented in this comprehensive review. Recent trends in inkjet-printed wearable/flexible and foldable tactile sensors are evaluated, paving the way for future research.

Recent Advancements towards Full-System Microfluidics

Microfluidics is quickly becoming a key technology in an expanding range of fields, such as medical sciences, biosensing, bioactuation, chemical synthesis, and more. This is helping its transformation from a promising R&D tool to commercially viable technology. Fuelling this expansion is the intensified focus on automation and enhanced functionality through integration of complex electrical control, mechanical properties, in situ sensing and flow control. Here we highlight recent contributions to the Sensors Special Issue series called "Microfluidics-Based Microsystem Integration Research" under the following categories: (i) Device fabrication to support complex functionality; (ii) New methods for flow control and mixing; (iii) Towards routine analysis and point of care applications; (iv) In situ characterization; and (v) Plug and play microfluidics.

A Stretchable Electromagnetic Absorber Fabricated Using Screen Printing Technology

A stretchable electromagnetic absorber fabricated using screen printing technology is proposed in this paper. We used a polydimethylsiloxane (PDMS) substrate to fabricate the stretchable absorber since PDMS exhibits good dielectric properties, flexibility, and restoring capabilities. DuPont PE872 (DuPont, Wilmington, CT, USA), a stretchable silver conductive ink, was used for the screen printing technique. The reflection coefficient of the absorber was measured using a vector network analyzer and a waveguide. The proposed absorber was designed as a rectangular patch unit cell, wherein the top of the unit cell acted as the patch and the bottom formed the ground. The size of the patch was 8 mm × 7 mm. The prototype of the absorber consisted of two unit cells such that it fits into the WR-90 waveguide (dimensions: 22.86 mm × 10.16 mm) for experimental measurement. Before stretching the absorber, the resonant frequency was 11 GHz. When stretched along the x-direction, the resonant frequency shifted by 0.1 GHz, from 11 to 10.9 GHz, demonstrating 99% absorption. Furthermore, when stretched along the y-direction, the resonant frequency shifted by 0.6 GHz, from 11 to 10.4 GHz, demonstrating 99% absorption.