This work aims at solving these issues, through developing a tribo-induced color tuner which can be integrated into the vastly-distributed commercial solid-state lighting (SSL) system. Through this approach, the sensing is achieved by the tribo-induced time-variant color without the need of pre-amplification, which can be wirelessly transmitted with no additional energy consumption, and the signal can be sent by everywhere-existed lamps and processed by everyone-owned smartphone cameras or closed circuit televisions (CCTVs). The accurate self-powered wireless sensing of the rotation speed was performed, with the accuracy evaluateddemonstrated. The smart lighting and an underwater photographing system were demonstrated by the developed color-tunable SSL system with the best photographing quality achieved.
Internet of things (IoTs) enables a cloud of physical devices to be network-connected, providing real-time information of the physical environment for decision-making [1,2]. The recently advocated 5th-generation (5G) communication with nearly zero time-delay has greatly contributed the development of IoTs. It is estimated that 20.4 billion connected items will be in use by 2020, forecasted by Gartner, Inc . An IoT system usually consists of sensors, actuators, controlling and communication circuits. Each sensor serves as a node in the network, and there are usually more than ten thousand sensor nodes in an IoT system. It is a challenging issue to supply electrical power to a cluster of sensor nodes. The employment of battery in a sensor will greatly affect the device reliability and durability where the battery replacement is usually difficult, and the chemical substances in batteries are normally harmful to environments [4,5]. To solve this issue, self-powered electronic devices have emerged as an alternative approach to operate without external electrical power supply, where the electrical power can be harvested from the ambient environment energy including solar, thermal and mechanical energies
6], , , [9
. Among various paradigms for energy harvesting, the recently proposed triboelectric nanogenerator (TENG) has capacity to harvest the mechanical energy from the ambient environment by coupling the triboelectrification and electrostatic induction effects, where the extremely high voltage can be attained and the thickness of the device can be shrank down to be an extremely small size
10], , [12
. A TENG is intrinsically a self-powered mechanical sensor with a high signal to noise ratio owing to its high-voltage output. In this case, a variety of self-powered TENG stress sensors have been proposed including seawater pressure sensor, human-machine interface, smart keyboard, etc
13], , [15
However, even though TENG can achieve extremely high voltage owing to the nature of the low capacitance, the transferred charge between electrodes is very small where the short-circuit current is usually lower than 100 μA. This causes the design of signal processing circuits to be difficult, where extremely high sensitivity is needed. Furthermore, cable connection is usually needed to transfer the mechanical signal detected by a TENG to the control panel, however, for several specific applications including infrastructure health monitoring in severe environments, turbulence detection in the deep sea, wireless smart keyboard, and so forth, cables for electrical connection are not allowed. Thus, cable-free self-powered TENG based mechanical sensing is desirable. Free-space optical communication (FSOC) is an effective approach for the wireless sensing, where information can be carried through the light [16,17]. Optical switch is a key component for the FSOC, where the signal can be loaded by switching on and off
18], , [20
Optical switching have been achieved by various physical mechanisms including twisted nematic (TN) cells, surface-stabilized ferroelectric liquid crystals (SSFLCs), polymer-dispersed liquid crystals (PDLCs), electrowetting actuation, etc
20], , , , , [25
. Among these approaches, electrowetting optical switch (EOS) presents several advantages such as low manufacturing cost, high reliability as well as long lifetime
25], , [27
. The majority of raw materials including conducting fluids, insulating oils for EOS are all available from daily life, which are inexpensive and non-toxic. The fabrication of the EOS is simple, where chemical reactions and micro-fabrications are not needed. In order to actuate the liquid by varying the interfacial tension between the conducting droplet and the dielectric substrate or oil, high-voltage outputs are required. Conventional high-voltage sources are usually bulky, bringing difficulties in system integrations, and dangerous in several specific situations. Circuit design for connecting the high-voltage source and the mechanical sensor is extremely complicated. TENGs, in contrast, can achieve high voltages and mechanical sensing at the same time, of which the structure can be very compact.
In this study, it is the first time ever that a TENG-driven electrowetting optical switch was developed by a combination of a freestanding sliding mode triboelectric nanogenerator (FS-TENG) and an electrically tunable liquid lens (ETULL). The ETULL was fabricated by a conducting fluid, an insulating oil, an acrylic cylindrical spacer as well as two indium tin oxide (ITO) electrodes. Upon applying a voltage on the ETULL, the interface between two liquids was curved due to the electrowetting effect, forming an insulating oil based concave lens. The light propagation through the ETULL could be switched between the on state, which means no light diverging at the flat interface, and the off state, where the light was diverged by the concave lens. The switch was controlled by the voltage generated by the FS-TENG. To verify the effectiveness of the proposed self-powered EOS, a wireless sensing system was performed and the mechanical motions were remotely detected. Such a mechanical-electrical-optical signal conversion enabled the wireless sensing, which can be applied for various fields such as human-machine interfaces, remote monitoring of the infrastructure health, security detections, wireless smart keyboard, etc.
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To quantitatively evaluate the output performance of triboelectric nanogenerators (TENG), the figure-of-merits (FOM) have been developed. However, the current version of FOM has limitations in applications, especially, it does not consider the breakdown effect that seriously affects the effective maximized energy output. At the same time, the standardized method to evaluate output capability that can be universally applied for all nanogenerators is missing. Here, a standardized assessment method is firstly proposed for output capability assessment of nanogenerators, in which the breakdown limits have been considered. Contact separation (CS) and contact freestanding-triboelectric-layer (CFT) modes TENGs are used to demonstrate this method, and the maximized effective energy output and revised FOM are calculated based on the experimental results. These results are consistent with that theoretically calculated based on Paschen’s law. The standardized evaluation on a film-based piezoelectric nanogenerator is also conducted based on this method, demonstrating its universal applicability for nanogenerators. This study proposes a standardized method for evaluating the effective output capability of nanogenerators considering the breakdown effect, which is crucial for standardized evaluation and applications of nanogenerator technology.