Characterization of 3-D Printed Flexible Heterogeneous Substrate Designs for Wearable Antennas

This paper presents a novel and scalable approach to realizing 3-D printed flexible substrates for wearable antennas with customizable gain and bandwidth. The heterogeneous substrate designs presented are printed with a dual-extrusion fused deposition modeling printer consisting of a combination of Acrylonitrile Butadiene Styrene (ABS) tiles surrounded with flexible NinjaFlex (NF). The impact of combining these two materials with similar electric permittivty is a distribution of loss tangent in the substrate, which controls its RF performance while maintaining the flexibility. To demonstrate the versatility of this approach, three flexible 3-D printed microstrip patch antenna prototypes are designed, realized, and measured at 2.4 GHz, where the RF performance is characterized with different curvatures. The predicted and experimental results were observed to be in good agreement. All three designs show antenna performance stability with curvature, and a resonant frequency shifts less than 1.4%. Increases of up to 250% in gain or 400% in bandwidth were observed with varying ABS to NF percentage area. Furthermore, these flexible antenna designs are tested with human body contact for wearable applications, and the measured RF performance remains stable. This heterogeneous substrate design approach using an array of arbitrarily shaped tiles is scalable, simple, and can be implemented in a variety of RF design applications for flexibility.