Permittivity-Customizable Ceramic-Doped Silicone Substrates Shaped with 3D-Printed Molds to Design Flexible and Conformal Antennas

3-D-Printing in antenna design is a recent research branch which is attracting academic and industrial interest. Nevertheless, despite the advantages in terms of antenna customizability, common filaments exhibit limitations in some contexts where platform tolerability, flexibility, and compactness are simultaneously required. Indeed, other than a rather low permittivity of commercial 3-D-printable materials, when flexibility is required, also significant loss tangent values must be accounted. To address this problem, a method to realize conformable flexible low-loss substrates with increased and customizable permittivity is proposed in this communication. It is based on the synthesis of ceramic-doped silicone structures shaped through 3-D-printed molds by using alumina and barium titanate as ceramic filler. Different substrates were firstly realized and characterized in terms of permittivity and loss tangent at different doping percentages. Then, a first validation on 2.4 GHz conformal patch antennas exploiting both third dimension and increased permittivity is presented. Finally, a wearable and compact bracelet-shaped UHF RFID planar inverted-F antenna (PIFA)-inspired flexible antenna is designed, tested when applied on the human body, and compared with a previously realized version 3-D-printed in polylactic acid. In spite of comparable performance, the novel device exhibits considerable size reduction and improved wearability, thus confirming the effectiveness of the proposed approach.