3D-Structured Soft Microwave Plasmonic Resonators via Direct Ink Writing of Stretchable Conductors

Beyond previous physical contact-based sensing technologies, emerging wireless sensing has gained significant interest in wearable electronics applications, particularly in monitoring human motion and acquiring data. However, existing wireless sensor systems face reliability and stability concerns when monitoring dynamic human motion due to the prevalent use of rigid integrated circuits (ICs) and wireless modules, along with complex wiring of electronic components. In this regard, developments of skin-attachable wireless sensor systems integrated with soft components are highly desirable. Recently, 2-dimensional (2D) flexible plasmonic resonators have been proposed to skin-attachable wireless sensors, but the coupling of their resonance to the lossy medium prevent from forming the robust resonances. Besides, additional initial calibration process is required due to the different dielectric properties of each individual and attached part. To address these challenges, we propose a novel approach: fully soft 3-dimensional (3D) plasmonic resonator structures enabling decoupled resonance from lossy substrates and the human body, thereby demonstrating reliable wireless sensing onto the human body.<br/>In contrast to conventional 2D plasmonic resonators, our 3D plasmonic resonator structures allow resonance formation in a lossless free space. This achievement can be advanced by introducing an additional ground plane at the bottom of the resonators, enabling a perfect decoupling between the resonance and the lossy medium. Furthermore, we employ omnidirectional 3D printing to realize these 3D soft resonators using intrinsically stretchable conductor composites. Their composition and rheological properties were optimized to ensure that the conductivity and mechanical robustness of the stretchable conductor should be high enough to formulate high-quality factor resonance under mechanical deformation. Moreover, for the structural integrity of 3D free-form printed features, we introduced the emulsion phase into the conductive composite ink that achieves high storage modulus and yield stress, so the printed resonators can maintain their own shapes after printing. In addition, high conductivity (10,000 S/cm) and low resistance variation during stretching (1.5 at 100 % strain) were successfully achieved by optimizing the ink formulation. This innovative material and fabrication method enables us to realize miniaturized free-form 3D resonator structures, which exhibit superior functionality and a high degree of design freedom compared to conventional 2D resonator structures.<br/>We demonstrate highly robust wireless wearable strain sensors based on fully soft 3D microwave plasmonic resonators. The decoupling of resonance ensures that the resonant frequency shifting of these sensors is solely perturbed by geometric parameters, thereby enabling stable wireless strain sensing. A key merit of this approach is eliminating the initial calibration process, and compensating different dielectric properties among individuals or attachment parts. Our works will pave a promising pathway to develop lossless microwave metamaterials and resonators in wearable electronics, and we will present the state-of-the-art results of 3D-printed free-form plasmonic resonators opening up new avenues for highly efficient wireless sensing of human motion.