Innovative 3-D Printing Processing Techniques for Flexible and Wearable Planar Rectennas

Abstract

This work demonstrates the use of a low-cost, lossy, flexible substrate processed by novel 3-D printing techniques which significantly mitigate its intrinsic losses, thus providing performance comparable to those of traditional substrates. These processing techniques are applied to both microstrip and coplanar waveguide structures; they are first derived theoretically, starting from the electromagnetic theory of modes propagation, then numerically validated by full-wave analysis, and finally experimentally verified. The design of a miniaturized 868 MHz rectenna, adopting a coplanar-fed patch antenna based on the proposed fabrication approach, is presented. By means of nonlinear/electromagnetic co-design, the antenna is directly matched to the rectifier. A 30-dB power range starting from -20 dBm is considered. Direct matching allows to get rid of a dedicated matching network and its associated losses, resulting in a slight efficiency increase and a significant reduction of the overall dimensions. Finally, the 3-Dprinted prototype is presented: the overall rectenna performance proves that design freedom enabled by 3-D printing paves the way to the use of low-cost flexible dielectric materials, even with poor electromagnetic properties, to realize wearable battery-free wireless nodes.

Microwave Devices for Wearable Sensors and IoT

“…Within this framework, in [62], it has been demonstrated that a low-cost, lossy, flexible, 3D printable resin material, such as the Flexible-80A, distributed by Formlabs (Somerville, MA, USA), can be internally engineered by means of the electromagnetic computer-aided design (CAD) simulation software CST (Computer Simulation Technology) Microwave Studio in order to modify its effective relative permittivity and loss tangent, to mitigate the intrinsic losses caused by the propagation within such a lossy dielectric, and to improve the radiation efficiency when used as substrate of a miniaturized 868 MHz rectifying antenna (rectenna), achieving performances comparable to those realized on conventional RF substrates.…”

“…In [62], the Flexible-80A, a 3D printable resin material printed using the low force stereolithography (LFS) additive manufacturing process and then post-cured by heating the prototype at 60 • C for ten minutes, gaining a tensile strength of 140%, has been characterized by means of the T-stub resonator method [63] up to 6 GHz, resulting in a relative permittivity ε r = 2.7 and an electrical conductivity σ = 2.34 × 10 −3 •S/m. To minimize propagation losses, Si-micromachining techniques, such as those presented in [64], have led to the idea of selecting CPW technologies as the transmission lines, where the electric field of the quasi-transverse electromagnetic (q-TEM) wave mainly propagates in the air between the microstrip line and the nearby coplanar ground instead of penetrating the lossy dielectric.…”