In antenna design, the available fluidic antenna materials can generally be classified into non-conductive, partially conductive, and conductive fluids. Non-conductive fluids include de-ionized (DI) water [ 8 9 ], ethyl acetate [ 10 11 ], acetone [ 12 ], and various oils such as mineral/transformer oil [ 13 14 ], which are used in dielectric resonator antennas (DRA). Partially conductive fluids like seawater [ 15 16 ] are also available and typically used in fabricating antennas for maritime applications, while solute concentrations in electrolytic solutions [ 17 ] can be used to fine-tune antenna performance. Finally, the third type, which this review focuses primarily on, is conductive fluids. Liquid conductive metals because of their solid-like oxide skin on the surface, impart mechanical stability to the elastomeric antennas and hence, provide flexibility while maintaining high conductivity suitable for antenna applications.

Generally, solid conductors such as copper exhibit excellent properties as an antenna’s conductive element. However, their rigid nature limits their use for applications requiring flexibility. To overcome this, thin metallic films have been used for fabricating flexible electronics. However, such flexibility has its limits, as the excessive stretching/twisting will eventually result in micro cracks in these conductors, thus increasing risks of breakdown in flexible antenna applications. Embedded antennas within elastomers such as PDMS etc., may use thin metallic wires, which are capable of providing elasticity and satisfactory conductivity [ 18 ]. However, unnecessary design constraints (limitation of stencil dimensions used in patterning of PDMS + silver nanowire (AgNW) composite) along with fabrication complexity also make this approach unsuitable for low-cost flexible antennas.

On the other hand, fluidic materials by their very nature have no deformation limits, making them a suitable alternative to rigid or solid conductors for such applications. The lack of a fixed form and the physical properties of conductive fluids have enabled the development of a wide variety of innovative methods and techniques to fabricate LM antennas. Even in cases where the radiative elements are formed on stiff substrates, the liquid nature of used LM materials provide additional degrees of freedom to achieve greater reconfigurability. The most popular approach is to fabricate LM antennas on soft and flexible substrates to realize elastic antenna designs. Such elasticity is not achievable when using solid radiative elements. A summary of the typical conductive fluids and associated substrates are depicted in Figure 1 and discussed below.

2.1. Conductive Fluids

6 S/m as listed in Fluids considered conductive enough for use as radiative elements in antennas generally possess electrical conductivities (σ) of the order of 10S/m as listed in Table 1 . These conductive fluids are mostly either based on liquid metals or exist as composites formed by suspended conductive nanostructures/particles in otherwise less conductive fluids. LMs traditionally include the likes of mercury and gallium-based alloys like Galinstan and eutectic gallium indium (EGaIn) [ 7 19 ]. However, novel LM based nanocomposites were also developed to further enhance LM’s electrical properties [ 20 ].

Compared to solid conductors such as copper, the use of conductive fluid as a radiative element allows for the design of a much more flexible and highly reconfigurable antenna. In particular, Ga alloy-based LMs enjoy rheological properties at room temperature because of the formation of a thin solid-like oxide skin layer which readily forms on the liquid’s surface when exposed to even minute quantities of oxygen in the air [ 21 ]. This property allows LM to behave like an elastic material and retain structure till a critical stress is applied on the surface allowing it to flow rapidly. Consequently, this skin oxide layer also provides mechanical stability to the elastomeric antenna after it is filled in a microchannel or is 3D printed [ 21 22 ]. This allows an LM fluidic antenna to be highly flexible and deformable while maintaining high conductivity, which is suitable for antenna applications.

6 S/m [ Mercury is the only metal available in liquid form at room temperature with melting point of −39 °C and an electrical conductivity of 1 × 10S/m [ 2 ]. It also has good stiction properties and a low oxidation rate, making it a suitable material for developing a liquid antenna [ 23 ]. However, mercury is extremely toxic and must be handled with care, resulting in its limited usage in antenna design.

6 S/m and a melting point of 16 °C, which is significantly close to room temperature. Most importantly, the non-toxic nature of EGaIn makes it a popular choice for flexible and reconfigurable antennas [29, EGaIn is a liquid metal alloy composed of 75% gallium and 25% indium. It features an electrical conductivity of 3.4 × 10S/m and a melting point of 16 °C, which is significantly close to room temperature. Most importantly, the non-toxic nature of EGaIn makes it a popular choice for flexible and reconfigurable antennas [ 4 30 ]. Once exposed to air, this liquid alloy reacts with oxygen to form a thin surface oxide layer, which improves mechanical stability, surface tension, and significantly prevents evaporation. This results in improved performance of EGaIn without impacting the overall conductivity [ 29 ].

31, The commercially available Galinstan is yet another liquid metal alloy widely used as the antenna-radiating element [ 30 32 ]. It comprises of 68.5% gallium, 21.5% indium and 10% tin with an electrical conductivity similar to EGaIn [ 33 34 ]. Galinstan also benefits from the thin surface oxide skin formed when in contact with oxygen, similar to other LM alloys. However, Galinstan has the added advantage of having a melting point much lower than room temperature (−19 °C) and being non-toxic.

10 based LM ink is another conductive fluid composed of 90% gallium, 10% indium and approximately 0.026% oxygen [6 S/m, which is slightly less than EGaIn and Galinstan and is formed by vigorously stirring GaIn 10 alloy in a controlled fashion till enough gallium oxide is formed to make the fluid viscous enough to be used as an LM ink. Similar methods can also be used to prepare conductive inks from other LM materials like Galinstan as reported in [ GaInbased LM ink is another conductive fluid composed of 90% gallium, 10% indium and approximately 0.026% oxygen [ 25 ]. This conductive fluid possesses an electrical conductivity of 2.9 × 10S/m, which is slightly less than EGaIn and Galinstan and is formed by vigorously stirring GaInalloy in a controlled fashion till enough gallium oxide is formed to make the fluid viscous enough to be used as an LM ink. Similar methods can also be used to prepare conductive inks from other LM materials like Galinstan as reported in [ 35 ]. Here, an addition of 0.30% of oxygen into Galinstan by stirring method provides the LM sufficiently enhanced surface tension and adhesion properties for its subsequent use as an LM ink.

Although electrical conductivity of LMs is around an order of magnitude smaller than copper, this disparity can be reduced by using LM-based composites with suspended conductive nanostructures. This has been achieved for EGaIn, where an addition of 0.5% walled carbon nanotubes (SWNT) resulted in a 100% increase in electrical conductivity of the base fluid [ 20 ]. Increased concentration of SWNT resulted in a corresponding increase of electrical conductivity of the nanocomposite, a phenomenon linked to a gradual buildup of interconnected SWNT pathways within the fluid.

36, Despite major flexibility advantage of LM fluids including long-term stability of their electrical properties, certain LM fluids are known to be corrosive in nature. In particular, Ga in LM alloys might come in contact with the surface of traditional metal interconnects like Cu, Ag, Au, and Al when interfacing with external biasing/output circuitry. The corrosive nature of Ga towards other metals might reduce long-term stability of antenna performance [ 24 37 ]. However, these issues can be addressed by Ni-plating the LM/metal pin interface or by treating it with 1-decylphosphonic acid or hydrochloric acid (HCl) vapor [ 36 ]. Additionally, modern composite materials like layers of graphene oxide (GO) together with poly 3,4-ethylenedioxythiophene: polystyrene sulfonate (PEDOT:PSS) can also be utilized [ 37 ]. In the composite material PEDOT:PSS/GO, GO blocks the Ga from coming in contact with metal surface while PEDOT:PSS overcomes the nominally insulative nature of GO and ensures good electrical contact between the metal interconnect pin and the LM alloy.

6 S/m with 72% suspended Ag nanoparticles by weight. However, such composites are relatively more expensive than traditional LMs. Fluids made out of nanoparticles of highly conductive (but otherwise solid) metals like silver are also available as an alternative to LM for fluidic antenna applications. Similar to [ 20 ], the concentration of the nanoparticles modulates the conductivity of such fluids. Silver nanoink [ 28 ] is an example of such composite fluids and exhibits far superior electrical conductivity of ∼20 × 10S/m with 72% suspended Ag nanoparticles by weight. However, such composites are relatively more expensive than traditional LMs.