Liquid Metal Antennas: Materials, Fabrication and Applications

Abstract

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.Most of the conventional antennas are fabricated by etching the copper cladding on the rigid substrates to form static conductor shapes. Such antennas are highly efficient but suffer from irreversible structure deformation and even damage when being bent or stretched beyond certain limits [3]. As alternatives, several types of copper-coated polymer substrates such as Kapton, polyethylene terephthalate (PET), and polyethylene naphthalate (PEN) films have been investigated to accommodate the need in flexible applications such as antennas. Similar to conventional substrates, the electrical properties of these substrates also degrade after severe bending/stretching, which in turn reduces antenna efficiency. Another option is polymer-based materials such as polydimethylsiloxane (PDMS), which has been typically used in combination with copper foils to realize highly flexible antennas. However, the semi-rigid copper foils may potentially suffer from irreversible deformation after certain bending cycles. Alternatively, liquid metals can be injected into microfluidic channels to fabricate highly flexible and mechanically stable antennas without compromising their electrical properties [4].In [5], a broader review of fluidics-based tuning methods for the development of microwave components was presented. Although, this included a review of antennas based on dielectric/conductive fluids, it primarily covered flexible and frequency-tunable antenna/arrays and lacked depth on polarization/pattern reconfigurability techniques. Additionally, discussion of recent advancements in state-of-the-art liquid metal applications was also missing in [5]. In [6], a mini review of flexible and stretchable antennas based on textile materials was discussed. Here, the focus was on the benefits of conductive filler-based elastomers used in antennas for bio-integrated electronics applications and the trade-off between antenna stretchability and its performance. However, a detailed review of materials, novel fabrication te...

Performance Limits of Fluid Antenna Systems

“…Note that FAS may be realized by different ways. One way is to use conductive fluids or liquid metals which can be mobilized in a tube-like structure using a software-controlled microfluidic system, e.g., [2]- [8]. Alternatively, a "fluid" antenna can also be realized by an array of pixel-like electronic switches [9].…”

Recent Advances of Wearable Antennas in Materials, Fabrication Methods, Designs, and Their Applications: State-of-the-Art

“…Conductive materials can be divided into two groups: rigid and flexible conductive materials. The rigid conductors can be further divided into various groups, such as copper, silver, and aluminum [ 116 ], whereas the flexible conductive materials are smart textile, ink, liquid, graphene (FG), graphite (FGF), carbon nanotube (CNT), and polymers [ 117 ]. In addition [ 118 , 119 ], polymer composites are widely used for the wearable applications due to their low loss, flexibility, and stretchability like PDMS-coated silica nanoparticles [ 120 , 121 ], and polymer yarns [ 122 ], etc.…”

“…PCBs can be further classified into various groups, such as Roger, FR4, and Teflon, etc., whereas flexible substrates are textile, paper, polymers, rubber, foam, etc. [ 116 ]. In wearable antennas, a precise characterization of a material is necessary to determine an effective design.…”

“… ( a ) Flexography printing process [ 207 ], ( b ) inkjet printing process [ 177 ], and ( c ) 3D direct ink writing method [ 116 ]. …”

“… ( a ) Development in direct-writing electronics [ 260 ], ( b ) design of a spray painting method (Drexel’s spray paint antenna) [ 116 , 256 ]. …”