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Resonant cavity antennas (RCAs) are suitable candidates to achieve high-directivity with a low-cost and easy fabrication process. The stable functionality of the RCAs over different frequency bands, as well as, their pattern reconfigurability make them an attractive antenna structure for the next generation wireless communication systems, i.e., fifth generation (5G). The variety of designs and analytical techniques regarding the main radiator and partially reflective surface (PRS) configurations allow dramatic progress and advances in the area of RCAs. Adding different functionalities in a single structure by using additional layers is another appealing feature of the RCA structures, which has opened the various fields of studies toward 5G applications. This paper reviews the recent advances on the RCAs along with the analytical methods, and various capabilities that make them suitable to be used in 5G communication systems. To discuss different capabilities of RCA structures, some applicable fields of studies are followed in different sections of this paper. To indicate different techniques in achieving various capabilities, some recent state-of-the-art designs are demonstrated and investigated. Since wideband high-gain antennas with different functionalities are highly required for the next generation of wireless communication, the main focus of this paper is to discuss primarily the antenna gain and bandwidth. Finally, a brief conclusion is drawn to have a quick overview of the content of this paper.

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Stretchable conducting traces are the key component to realize wearable healthcare electronics; a conductor material that can withstand high strain conditions can be crucial. Here, we describe a simple fabrication pathway to achieve stretchable conductive ink for printing. Specifically, silver flakes and fluorine rubber are amalgamated by the aid of triethanolamine (TEA), which enhanced the compatibility of the components in solution state where methylisobutylketone is the co-solvent. Moreover, TEA plasticizes the composites after the printing and drying of the solvent, causing the composite to deform freely without losing conductivity. The composite exhibits a conductivity value of 8.49 × 10 4 S m −1 at rest. The printed composite itself is not mechanically resilient after plastic deformation, but it has remarkable adhesion on elastomeric substrates. Thus, the printed ink on elastomers allows stretchable trace that can accommodate repeated stretching/releasing cycles. We fabricate and characterize stretchable printed antenna with three different designs (loop, patch, and bowtie) for the application of skin-adhesive electronics.

The emergence of the Internet of Things (IoT) necessitates the development of electronic components with various form factors and mechanical properties. 3D printing is an effective tool to realize objects with arbitrary form factors. Various 3D printable materials have recently been commercialized; among them, stretchable materials are particularly useful in the IoT because they enable adaptability in the dimensional change of the electronics. Most of these stretchable materials are, however, not electrically conductive; conductive coating can enable the functionality. Here, we propose a selfreinforcing conductive coating strategy, which reduced graphene oxide (RGO) self-assembles to wrap graphene nanoflakes (GNF) as a conductive binder that can also achieve mechanical integrity. The conductivity of the GNF-RGO coating reaches 4.47 × 10 4 S m −1. To demonstrate the potential applications of the GNF-RGO coating, applying the coating on 3D printed porous elastomers enabled flexible radio frequency (RF) antennas and strain sensors. The RF antenna shows high radiation efficiency and maintains excellent performance under bending conditions. The coating also produces a strain sensor with a gauge factor of ∼13 up to 40% of strain. We foresee that the electrically conductive GNF-RGO composite coating can provide versatile functionalization strategy in flexible electronics and in wearable biomedical devices.