Liquid Metal Antenna Research Articles

Flexible and Stretchable Liquid‐Metal Microfluidic Electronics Using Directly Printed 3D Microchannel Networks

AbstractThis paper describes a method for fabricating microfluidic electronics with 3D interconnected networks by direct ink writing (DIW)‐based 3D printing. Existing 3D printing technologies have yet to simultaneously realize 1) direct printing of interconnected multilayered microchannels without supporting materials or post‐processing and 2) integration of electronic elements with microchannels during the process of printing. This research aims to develop a method to fabricate support‐free microchannels consisting of silicone sealant without the collapse of the extruded structure while integrating electronic elements during the fabrication by DIW. The injection of liquid metal into 3D‐printed microchannels allows the formation of electrical connections between 3D conductive networks and the embedded electronic elements, enabling the fabrication of flexible and stretchable liquid metal antenna coils. To demonstrate a practical application, 1) a skin‐attachable radio‐frequency identification (RFID) tag using a commercial skin‐adhesive plaster as a substrate and 2) free‐standing flexible wireless light‐emitting devices with a small footprint (21.4 mm × 15 mm) for potential implantable applications are produced. This technology will offer a new capability to realize the automated fabrication of stretchable printed circuits with 3D configuration of electrical circuits consisting of liquid metals.

An UHF Reconfigurable Liquid-Metal Monopole Antenna Based on a 2-D Surface

This article demonstrates an ultrahigh frequency (UHF) liquid-metal monopole antenna qualified for reconfiguring its resonant frequency by stretching or shortening its length on a 2-D surface. The proposed antenna consists of liquid eutectic gallium and indium (EGaIn) and NaOH solution. A power supply generates a direct current (DC) potential between the EGaIn and NaOH solution. Depending on the electrically-driven method to push or withdraw the EGaIn, the antenna is capable of forming different lengths to redistribute the current of the radiating element. Compared with the microchannel liquid-metal antenna, the proposed antenna can be tuned on a 2-D surface, rather than relying on the microchannel for shaping. By varying the length of EGaIn on a 2-D surface, the proposed antenna can provide continuously tunable frequency range and repeatability. For EGaIn monopole antenna with lengths from 50 to 10 mm, the measured resonant frequency operates from 0.78 to 2.42 GHz for a tuning ratio 3.1:1. The application of the design, operation of tuning liquid-metal antenna at several frequencies, and measurement process are explained in this article.

Leveraging discrete modulation and liquid metal antennas for interference reduction

Recent progress in the understanding of the behavior of the interference channel has led to valuable insights: first, discrete signaling has been discovered to have tangible benefits in the presence of interference, especially when one does not wish to decode the interfering signal, i.e., the interference is treated as noise, and second, the capacity of the interference channel as a function of the interference link gains is now understood to be highly irregular, i.e., non-monotonic and discontinuous. This work addresses these two issues in an integrated and interdisciplinary manner: it utilizes discrete signaling to approach the capacity of the interference channel by developing lower bounds on the mutual information under discrete modulation and treating interference as noise, subject to an outage set, and addresses the issue of sensitivity to link gains with a liquid metal reconfigurable antenna to avoid the aforementioned outage sets. Simulations illustrate the effectiveness of our approach.

Ultra-Deformable and Tissue-Adhesive Liquid Metal Antennas with High Wireless Powering Efficiency.

Flexible and stretchable antennas are important for wireless communication using wearable and implantable devices to address mechanical mismatch at the tissue-device interface. Emerging technologies of liquid-metal-based stretchable electronics are promising approaches to improve the flexibility and stretchability of conventional metal-based antennas. However, existing methods to encapsulate liquid metals require monolithically thick (at least 100µm) substrates, and the resulting devices are limited in deformability and tissue-adhesiveness. To overcome this limitation, fabrication of microchannels by direct ink writing on a 7 µm-thick elastomeric substrate is demonstrated, to obtain liquid metal microfluidic antennas with unprecedented deformability. The fabricated wireless light-emitting device is powered by a standard near-field-communication system (13.56MHz, 1 W) and retained a consistent operation under deformations including stretching (>200% uniaxial strain), twisting (180° twist), and bending (3.0 mm radius of curvature) while maintaining a high quality factor (q> 20). Suture-free conformal adhesion of the polydopamine-coated device to ex vivo animal tissues under mechanical deformations is also demonstrated. This technology offers a new capability for the design and fabrication of wireless biomedical devices requiring conformable tissue-device integration toward minimally invasive, imperceptible medical treatments.

Directivity-reconfigurable antenna actuated by liquid-gas phase transition

Tunable antennas have become research hotspots due to their applicability and practicability. To get rid of complex peripheral facilities, a novel design featuring an efficient actuating strategy by liquid-gas phase transition is proposed, which consists of liquid metal embedded in soft polymer and alcohol stored above a ceramic heater. By simply applying voltage on the heater to induce evaporation of working medium, there appears a sharp increasement in internal pressure, which pushes the liquid metal antenna continuously from planar to tridimensional state. Both the experimental and numerical results demonstrate the effectiveness of the actuating strategy to adjust the antenna configuration.

The Design and Manufacturing Process of an Electrolyte-Free Liquid Metal Frequency-Reconfigurable Antenna.

This communication provides an integrated process route of smelting gallium-based liquid metal (GBLM) in a high vacuum, and injecting GBLM into the antenna channel in high-pressure protective gas, which avoids the oxidation of GBLM during smelting and filling. Then, a frequency-reconfigurable antenna, utilizing the thermal expansion characteristic of GBLM, is proposed. To drive GBLM into an air-proof space, the thermal expansion characteristics of GBLM are required. The dimensions of the radiating element of the liquid metal antenna can be adjusted at different temperatures, resulting in the reconfigurability of the operating frequency. To validate the proposed concept, an L-band antenna prototype was fabricated and measured. Experimental results demonstrate that the GBLM in the antenna was well filled, and the GBLM was not oxidized. Due to the GBLM being in an air-proof channel, the designed liquid metal antenna without electrolytes could be used in an air environment for a long time. The antenna is able to achieve an effective bandwidth of over 1.25–2.00 GHz between 25 °C and 100 °C. The maximum radiation efficiency and gain in the tunable range are 94% and 2.9 dBi, respectively. The designed antenna also provides a new approach to the fabrication of a temperature sensor that detects temperature in some situations that are challenging for conventional temperature sensing technology.

Liquid metal motor.

SummaryLiquid metal has demonstrated an enormous potential for developing soft functional devices and machines. However, current liquid metal enabled machines suffer from several issues, such as the requirement of a liquid environment, generation of weak actuating forces, and insufficient maneuverability. To overcome these restrictions, here, a motor is developed based on the electrical actuation of liquid metal droplets without the need for conventional electromagnets. The approach is distinguished by (1) the encapsulation of electrolyte and multiple liquid metal droplets within an enclosed system, and (2) the creation of stable and continuous torque outside a liquid environment. In addition, a liquid metal electrical brush is introduced to operate the motor with low friction and low wear. The unique driving mechanism endows the motor with several advantages, including low friction, no sparking, low noise, versatile working environment, and being built from soft materials that could offer new opportunities for developing soft robotics.

Liquid Metal Antennas: Materials, Fabrication and Applications.

This work reviews design aspects of liquid metal antennas and their corresponding applications. In the age of modern wireless communication technologies, adaptability and versatility have become highly attractive features of any communication device. Compared to traditional conductors like copper, the flow property and lack of elasticity limit of conductive fluids, makes them an ideal alternative for applications demanding mechanically flexible antennas. These fluidic properties also allow innovative antenna fabrication techniques like 3D printing, injecting, or spraying the conductive fluid on rigid/flexible substrates. Such fluids can also be easily manipulated to implement reconfigurability in liquid antennas using methods like micro pumping or electrochemically controlled capillary action as compared to traditional approaches like high-frequency switching. In this work, we discuss attributes of widely used conductive fluids, their novel patterning/fabrication techniques, and their corresponding state-of-the-art applications.

A Bright Future for Liquid Functional Materials?

Liquid metals were merely sci-fi fantasy a few decades ago, playing a prominent role in the Terminator film franchise. Current research is leading to an array of emerging functional liquids. Here, we discuss the exciting possibilities of such fluid materials.

Liquid metal antenna-based pressure sensor

In this paper, a centre-fed, finite-length, infinitesimally thin dipole antenna is presented as a pressure sensor to monitor pressure changes in geotechnical engineering. The antenna consists of a liquid metal (eutectic gallium–indium alloy) injected in microfluidic channels, which are enclosed in the elastic material polydimethylsiloxane (PDMS); therefore, the liquid antenna is stretchable and frequency reconfigurable and can thus be used as a pressure sensor when the length of the elastic material PDMS is changed under pressure. The effects of the geometrical parameters of the dipole antenna on its resonant frequency are investigated using the finite-element software HFSS. Based on the numerical results, the liquid-metal antenna is designed, fabricated and measured, and its resonant frequencies are recorded under different pressures. According to the experimental results, the relation between the resonant frequency of the liquid antenna and the applied pressure is defined, and it is consistent with the numerical and theoretical results. Moreover, the experimental results show that the relation between the displacement and the resonant frequency is in good agreement with the numerical and theoretical results. Thus, there is a linear relation between the resonant frequency of the antenna and the applied pressure as well as between the resonant frequency and the displacement. Therefore, this frequency-reconfigurable liquid-metal dipole antenna is suitable for use as a pressure sensor.

Mechanochromic Stretchable Electronics.

Soft and stretchable electronics are promising for a variety of applications such as wearable electronics, human-machine interfaces, and soft robotics. These devices, which are often encased in elastomeric materials, maintain or adjust their functionality during deformation, but can fail catastrophically if extended too far. Here, we report new functional composites in which stretchable electronic properties are coupled to molecular mechanochromic function, enabling at-a-glance visual cues that inform user control. These properties are realized by covalently incorporating a spiropyran mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that a strain threshold has been reached. The resulting colorimetric elastomers can be molded and patterned so that, for example, the word "STOP" appears when a critical strain is reached, indicating to the user that further strain risks device failure. We also show that the strain at color onset can be controlled by layering silicones with different moduli into a composite. As a demonstration, we show how color onset can be tailored to indicate a when a specified frequency of a stretchable liquid metal antenna has been reached. The multiscale combination of mechanochromism and soft electronics offers a new avenue to empower user control of strain-dependent properties for future stretchable devices.

Design of a Reconfigurable Patch Antenna Using the Movement of Liquid Metal

A polarization-agility antenna using liquid metal (eutectic gallium indium) with high radiation efficiency is proposed in this letter. This design approach offers possibilities to use the minimum amount of liquid metal as well as elastomer, so that a higher-radiation-efficiency liquid metal antenna can be realized. The proposed design is based on an aperture-coupled patch antenna composed of both liquid metal alloy and copper tape. The polarization of this antenna is controlled by the position of the pressure-driven liquid metal encased in four triangle cavities constructed by both the elastic dielectric material (i.e., Ecoflex) and polyethylene terephthalate film. A prototype is demonstrated with a center frequency of 2.45 GHz. Three kinds of polarization including left-hand circular polarization, right-hand circular polarization, and linear polarization are successfully demonstrated. A radiation efficiency higher than 90% is observed in the measurement.

Characterization of Intermodulation Distortion in Reconfigurable Liquid Metal Antennas

The intermodulation distortion produced by passive and tunable liquid metal (LM) antennas is investigated. Four kinds of monopoles, including a passive copper monopole, a varactor-tuned copper monopole, a passive LM monopole, and a tunable LM monopole using electrochemically controlled capillarity (ECC), are compared for their linearity and power-handling capabilities. Linearity is assessed using a two-tone distortion test, in which two fundamental tones close in frequency result in third-order intermodulation tones at frequencies above and below the original tones separated by the frequency difference. The passive LM monopole has comparable linearity to the passive copper monopole, while the linearity of the ECC-tuned LM monopole is at least 40 dB better than that of the active varactor monopole. The reconfigurable LM antenna also handles higher power (31 dBm) before failure than does the active varactor-tuned antenna (24 dBm).

An Electrically Actuated Liquid-Metal Gain-Reconfigurable Antenna

A tunable liquid-metal antenna demonstrating gain reconfigurability is presented. This antenna uses a reconfigurable stub made with the liquid metal Galinstan. Using continuous electrowetting, a 60 Hz signal with an amplitude of 1 Vpp and 75% duty cycle can actuate the Galinstan continuously along a channel, tuning the stub length and antenna gain. Zero external power is required to maintain the position of the slug. The 5 GHz antenna offers more than 10 dB of analog gain tuning, from −5.90 to 4.43 dB.

A liquid metal-based structurally embedded vascular antenna: I. Concept and multiphysical modeling

This work proposes a new concept for a reconfigurable structurally embedded vascular antenna (SEVA). The work builds on ongoing research of structurally embedded microvascular systems in laminated structures for thermal transport and self-healing and on studies of non-toxic liquid metals for reconfigurable electronics. In the example design, liquid metal-filled channels in a laminated composite act as radiating elements for a high-power planar zig-zag wire log periodic dipole antenna. Flow of liquid metal through the channels is used to limit the temperature of the composite in which the antenna is embedded. A multiphysics engineering model of the transmitting antenna is formulated that couples the electromagnetic, fluid, thermal, and mechanical responses. In part 1 of this two-part work, it is shown that the liquid metal antenna is highly reconfigurable in terms of its electromagnetic response and that dissipated thermal energy generated during high power operation can be offset by the action of circulating or cyclically replacing the liquid metal such that heat is continuously removed from the system. In fact, the SEVA can potentially outperform traditional copper-based antennas in high-power operational configurations. The coupled engineering model is implemented in an automated framework and a design of experiment study is performed to quantify first-order design trade-offs in this multifunctional structure. More rigorous design optimization is addressed in part 2.

DESIGN AND CHARACTERIZATION OF THE LIQUID METAL ANTENNA OPTIMALLY EMBEDDED IN CONCRETE BEAM PROTOTYPE AS AN ALTERNATIVE STRAIN SENSOR

This paper presents the implementation of the novel dipole liquid metal antenna as an alternative strain sensor when embedded in the optimal location of a concrete beam prototype. The antenna is made up of eutectic Indium Gallium, a fluid metal alloy, encased in a microfluidic channel, namely, polydimethylsiloxane (PDMS) elastomer fabricated using McGyver-esque technique to microfabrication. The fluidic dipole antenna being highly flexible, stretchable, and reversibly deformable mimics the basic characteristics of the strain sensor where its resonant frequency is inversely related to its length. The concrete specimen was subjected to center – point loading tests where the resonant frequency of the liquid antenna embedded in it was measured simultaneously. Statistical analysis of the results show that there is a significant relationship between the displacement of the concrete specimen and the resonant frequency of the embedded antenna.

Frequency tunable liquid metal planar inverted‐F antenna

A frequency tunable liquid metal antenna that can compensate for hand effects is presented for use in mobile phones. The antenna structure is based on a planar inverted-F antenna, which is frequently considered as an internal antenna. The upper arm of the antenna was formed using liquid metal filled in a Teflon tube. The liquid metal used was Galinstan. The length of the filled portion determined the resonant frequency of the antenna. The tuning capability of the antenna was demonstrated through an automatic tuning experiment, in which the resonant frequency of the antenna was automatically recovered from disturbances caused by a human hand being placed in the proximity of the antenna.

In brief

PAGE 126 Researchers in Australia have overcome a common limitation of out-of-phase power dividers. Typically in these dividers, the resistor cannot provide effective isolation as it will absorb the out-of-phase signals. To address this, the team introduced slots placed under the tightly coupled lines at the top layer, which couples the reflected signals from the output ports in an in-phase manner. PAGE 154 A high-speed transimpedance amplifier with an efficient bandwidth and power consumption tuning approach has been proposed by researchers in Germany. Optical interconnects are usually optimised to provide the highest data rate constantly – without concern for the varying use over time – but the presented approach can adapt performance to reduce power consumption by a factor of five while keeping the transimpedance gain constant. Out-of-phase power dividers finally achieve effective isolation PAGE 100 Researchers in Korea have developed a liquid-metal antenna that can help to mitigate the effects of the hand in mobile phones. The use of liquid metal allows the antenna's frequency to be tuned – thus to reduce the hand effect – by adjusting the shape or dimensions of the antenna. Tuning interconnect bandwidth can save power while not affecting overall performance PAGE 122 Researchers in China have presented a coplanar waveguide bandpass filter with controllable transmission zeros. This control is achieved by using separated electric and magnetic coupling paths, such that via hole is not required, and the resulting waveguide is integrable with lumped elements and other microwave components. The length of the antenna is adjusted to deal with the presence of the hand in mobile phones PAGE 152 Through silicon vias (TSVs) are used as interconnects in 3d integrated circuits and as such are becoming vital to modern circuit design. However, researchers in Egypt have demonstrated that the parasitic effects of the TSVs and the inhomogeneous nature of the medium result in unreliable results in the delay performance of digital circuits Separating electric and magnetic coupling paths in coplanar waveguides results in improved tenability Cross-section of a TSV: the design results in parasitic behaviour and this is exacerbated by inhomogeneity

A reconfigurable liquid metal antenna driven by electrochemically controlled capillarity

We describe a new electrochemical method for reversible, pump-free control of liquid eutectic gallium and indium (EGaIn) in a capillary. Electrochemical deposition (or removal) of a surface oxide on the EGaIn significantly lowers (or increases) its interfacial tension as a means to induce the liquid metal in (or out) of the capillary. A fabricated prototype demonstrates this method in a reconfigurable antenna application in which EGaIn forms the radiating element. By inducing a change in the physical length of the EGaIn, the operating frequency of the antenna tunes over a large bandwidth. This purely electrochemical mechanism uses low, DC voltages to tune the antenna continuously and reversibly between 0.66 GHz and 3.4 GHz resulting in a 5:1 tuning range. Gain and radiation pattern measurements agree with electromagnetic simulations of the device, and its measured radiation efficiency varies from 41% to 70% over its tuning range.

Microfluidic Based Ka-Band Beam-Scanning Focal Plane Array

An eight-element 1-D Ka-band focal plane array (FPA) capable of beam scanning based on microfluidic principles is presented. The FPA is placed at the back surface of an 8-cm-diameter extended hemispherical Rexolite ( $\varepsilon_{\rm r}=2.56$ , $\tan\delta=0.0026$ ) lens and consists of interconnected microfluidic reservoirs and channels constructed by bonding polydimethylsiloxane (PDMS) ( $\varepsilon_{\rm r}=2.8$ , $\tan\delta=0.02$ ) and liquid crystal polymer (LCP) ( $\varepsilon_{\rm r}=2.9$ , $\tan\delta=0.0025$ ) substrates. The antenna element of the array is a small volume (2.5 $\mu$ L) of liquid metal residing inside a low-loss Fluorinert FC-77 ( $\varepsilon_{\rm r}=1.9$ , $\tan\delta=0.0005$ ) solution. The array beam is scanned by moving the liquid-metal antenna among the reservoirs using an external pump. The proximity-coupled feed network of the array is passive and designed strategically to accommodate the position variation of the liquid-metal antenna element. The array operates with measured $7^{\circ}$ half-power beamwidth (HPBW), $>$ 21 dB realized gain, and 3.3% $\vert{ S}_{11}\vert dB bandwidth and provides $\pm 30^{\circ}$ beam-scanning range. The presented microfluidic-based beam-scanning technique operates without resorting to RF switches. Consequently, it is promising for high-power handling and low-cost realization of millimeter-wave high-gain beam-scanning antenna arrays.