Three-dimensional (3D) additive manufacturing techniques have been utilized to make 3D electrical components, such as resistors, capacitors, and inductors, as well as circuits and passive wireless sensors. Using the fused deposition modeling technology and a multiple-nozzle system with a printing resolution of 30 μm, 3D structures with both supporting and sacrificial structures are constructed. After removing the sacrificial materials, suspensions with silver particles are injected subsequently solidified to form metallic elements/interconnects. The prototype results show good characteristics of fabricated 3D microelectronics components, including an inductor-capacitor-resonant tank circuitry with a resonance frequency at 0.53 GHz. A 3D "smart cap" with an embedded inductor-capacitor tank as the wireless passive sensor was demonstrated to monitor the quality of liquid food (e.g., milk and juice) wirelessly. The result shows a 4.3% resonance frequency shift from milk stored in the room temperature environment for 36 h. This work establishes an innovative approach to construct arbitrary 3D systems with embedded electrical structures as integrated circuitry for various applications, including the demonstrated passive wireless sensors.

This paper presents a LC strain sensor with a novel encapsulated serpentine helical inductor. The helical coil of the inductor is formed by serpentine wire to reduce the radial rigidity. Also the inductor is encapsulated by material with high Poisson's ratio. When an axial deformation is applied to this encapsulated inductor, the cross-sectional area of the helical coil will have more evident change due to lower radial rigidity and encapsulation. Therefore, the variation of inductance or LC resonant frequency can be enhanced to provide better sensitivity of the LC strain sensor. By using PDMS as encapsulated material, it is shown that the sensitivity of the conventional helical inductor with or without encapsulation are both about 73.0 kHz/0.01ε, which means that encapsulation on the conventional helical inductor does not help to improve the sensitivity due to high radial rigidity of the conventional helical coil. It is also found that the encapsulated serpentine helical inductor has better sensitivity (121.9 kHz/0.01ε) than the serpentine helical inductor without encapsulation (62.7 kHz/0.01ε), which verifies the sensitivity enhancing capability of the proposed encapsulated serpentine helical inductor design. The error between simulation and measurement results on sensitivity of LC strain sensor with the encapsulated serpentine inductor is about 5.57%, which verifies the accuracy of the simulation model. The wireless sensing capability is also successfully demonstrated.

This paper presents novel wireless EWOD/DEP chips that are wirelessly powered and controlled through LC circuits with one-to-many transmitter-receiver coupling. Each receiving LC circuit connected to the EWOD/DEP electrode is designed to have a different resonant frequency. When the input frequency is close to one of the resonant frequencies of receiving LC circuits, the induced voltage on the corresponding EWOD/DEP electrode will increase due to the resonance. Therefore, electrodes can be selectively and sequentially activated to provide sufficient EWOD or DEP force to manipulate the droplet or liquid by modulating the input frequency. Unlike previously reported wireless EWOD or DEP devices powered through one-to-one transmitter-receiver coupling, the transmitting inductor in the one-to-many transmitter-receiver coupling design proposed here is much larger than the total sizes of receiving inductors. Therefore, receiving inductors can be easily covered and coupled by the transmitting inductor. Here, droplet transport, splitting, and merging are successfully demonstrated using 5 receiving LC circuits at different input frequencies (1210-1920 Hz). Liquid pumping with multiple electrodes by wireless DEP is also demonstrated using 5 receiving LC circuits at higher input frequencies (51.2-76.1 kHz). Furthermore, liquid pumping with a continuous meandered electrode by wireless DEP is demonstrated through the resonant frequency shifting effect. It shows that the liquid pumping distance on a continuous electrode also can be tuned by proper frequency modulation.