Hybrid Mechanism of Electromagnetic and Piezoresistive Sensing Using a Soft Microfluidic Coil

Soft magnetic sensors for tactile and pressure sensing based on the intensity change of the magnetic field has been drawing attentions recently due to the simplicity in manufacturing using commercial Hall effect sensors with high linearity and sensitivity. However, these sensors usually show relatively narrow sensing ranges due to fast saturation of the output signals from the magnetic field. In addition, rigid components for creating the magnetic field often limit the structural compliance. In this study, we propose a soft pressure sensor not only sensitive in a small pressure range but also has a large dynamic range using a hybrid mechanism of two different sensing modes: electromagnetic and piezoresistive sensing. By directly printing eutectic gallium-indium (EGaIn) to form a double-layered pancake coil pattern on a silicone elastomer, we fabricate a microfluidic soft coil that can be used as either an electromagnet or a variable resistor. Parameters, such as the sensor height, the stiffness of the material, and the resistance of the soft coil are configured for an enhanced sensitivity, a short recovery time from deformation, and reduction in electrical failures during operation. In this letter, we introduce the design and fabrication of the proposed sensor and characterizes a single sensor prototype. An array of the proposed sensors is also demonstrated as an application.