Investigation and Optimization of Direct-Write Printed Metasurfaces on Fabric

Wearable electronics covers a wide range of devices of varying complexity in a multitude of applications. Fabrication methods include weaving, embroidery, or direct-write printing. Printing has the advantage of coating individual fibers for seamless printed designs. Several printing processes can be used to fabricate wearable devices; however, direct-write printing is cost and time effective, producing consistent line widths using a variety of inks and substrates.<br/><br/>The RF metasurface is a passive device and is an example of a simple, wearable technology that can be produced via additive manufacturing. RF metasurfaces can be tailored by adjusting element shape or dimensions, providing reflection, transmission, or absorption within a frequency regime of choice. Here, a stop-band filter is designed at 5.8 GHz using an array of square loops. Textile substrate-based metasurfaces are versatile and have been used in various applications including anti-jamming and EMF shielding. [1]<br/><br/>The fabric-based printed metasurfaces are fabricated by direct-write printing two silver nanoparticle inks on single-sided polyurethane coated nylon substrates with thread masses of 70D and 200D, using a Nordson PRO4 dispensing system. The quality of the print heavily relies on a reasonable window of textile fiber thread mass and ink viscosity. Low viscosity inks printed on heavy thread mass fibers are more likely to introduce cracks and demonstrate fragility in tests. High viscosity inks are more forgiving and fully coat fibers, providing continuity regardless of thread mass. The textiles are subjected to wash and dry cycles, and mechanical testing of twisting, compression, and abrasion is performed. It is imperative to use a highly flexible ink resistant against mechanical distortion and resistance to wash and dry cycles. Lifetime of the textile metasurface is monitored over several months, demonstrating excellent longevity. Even when subjected to damage, the material may still perform depending on the number of defects and their location. RF performance and conductivity are used to assess the integrity of the wearable metasurface, and analysis of the failure mechanisms are conducted using SEM imaging. Quality and durability of the printed metasurface is dependent on the substrate type, thickness, and presence of a protective polyurethane coating. The protective coating on the reverse, non-printed side provides reinforcement, preventing ink from penetrating the fabric. This opens the possibility of printing on both sides, which can be used for introducing a second resonance, or in antenna applications for improving gain and bandwidth.<br/><br/>As part of this presentation, the performance and stability of the different inks on the different substrate thicknesses will be shown, and details of the ink and substrate interaction along with the mechanisms of failure will be presented. Details of the performance and durability of the metasurface samples will be demonstrated, and optimization of printing and testing processes will be reported.<br/><br/>[1] Printed Metasurfaces for Wearables A. Kajenski, S. Khushrushahi, G. Strack, and A. Akyurtlu; 2020 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting, Denver, CO 2022