Antennas are a vital component of the wireless body sensor networks devices. A wearable antenna in this system can be used as a communication component or energy harvester. This paper presents a detailed review to recent advances fabrication methods for flexible antennas. Such antennas, for any applications in wireless body sensor networks, have specific considerations such as flexibility, conformability, robustness, and ease of integration, as opposed to conventional antennas. In recent years, intriguing approaches have demonstrated antennas embroidered on fabrics, encapsulated in polymer composites, printed using inkjets on flexible laminates and a 3-D printer and, more interestingly, by injecting liquid metal in microchannels. This article presents an operational perspective of such advanced approaches and beyond, while analyzing the strengths and limitations of each in the microwave as well as millimeter-wave regions. Navigating through recent developments in each area, mechanical and electrical constitutive parameters are reviewed, and finally, some open challenges are presented as well for future research directions.

A conformal antenna with electronically tuning capability of its radiation pattern between broadside and monopolelike patterns is proposed. The antenna is based on a proximityfed circular patch, loaded with a ring patch and four rectangular slots. The design is planar without any use of rigid shorting posts or complex feeding network. The reconfigurability is achieved by activating and deactivating the slots using PIN diodes, to switch between TM02 (monopole-like mode) and perturbed TM02 distributions (broadside mode) of the antenna. For conformability, the antenna is fabricated using highly flexible PDMSconductive fabric composite. All the antenna parts, including the RF switches, wires, and DC biasing circuit are fully encapsulated by PDMS to provide resilience against deformation and harsh environment. Investigations on the RF performance and mechanical stability of the antenna were conducted. Under various bendings, it was demonstrated that all the antenna components, including those for electronic switching, remained intact and in working order even under radius bending of 30 mm, thus maintaining good pattern reconfigurability and overall performance. When bent, the measured results at 5.2 GHz show a stable radiation performance relative to those of the flat case (i.e., maximum gain of 2.9 dBi and efficiency of 64% in broadside mode, corresponding to 1.75 dBi and 52% in monopole-like mode). To the best of our knowledge, all these features have never been demonstrated in previously published pattern reconfigurable antennas.

A simple, low profile and pattern reconfigurable antenna with a full-ground plane is presented for use in wireless body area networks at 5.8 GHz ISM band. The antenna consists of a probe-fed circular patch, and a parasitic ring surrounding it. The ring is loaded with a rectangular slot on each side of the patch, and two PIN diodes are added in the center of each slot. With all the diodes in on-state, the antenna presents monopole-like radiation patterns with the gain of 4.2 dBi for maximum coverage on the body, whereas the pattern of the antenna can be directed to AE15 with an increased gain of 2.9 dBi, by turning off each diode for the semi-directional patterns states. Measured results of the fabricated prototype antenna show good agreement with numerical results obtained using Ansys HFSS. K E Y W O R D S circular patch antenna, monopole-like pattern, pattern reconfigurable antenna, wireless body area networks