Building electronic devices on ubiquitous paper substrates has recently drawn extensive attention due to its light weight, low cost, environmental friendliness, and ease of fabrication. Recently, a myriad of advancements have been made to improve the performance of paper electronics for various applications, such as basic electronic components, energy storage devices, generators, antennas, and electronic circuits. This review aims to summarize this progress and discuss different perspectives of paper electronics as well as the remaining challenges yet to be overcome in this field. Other aspects included in this review are the fundamental characteristics of paper, modification of paper with functional materials, and various methods for device fabrication.

Heavy metal pollution has shown great threat to the environment and public health worldwide. Current methods for the detection of heavy metals require expensive instrumentation and laborious operation, which can only be accomplished in centralized laboratories. Various microfluidic paper-based analytical devices have been developed recently as simple, cheap and disposable alternatives to conventional ones for on-site detection of heavy metals. In this review, we first summarize current development of paper-based analytical devices and discuss the selection of paper substrates, methods of device fabrication, and relevant theories in these devices. We then compare and categorize recent reports on detection of heavy metals using paper-based microfluidic devices on the basis of various detection mechanisms, such as colorimetric, fluorescent, and electrochemical methods. To finalize, the future development and trend in this field are discussed.

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Oscillating bubbles have proven to be a versatile tool for various microfluidic applications. Despite the existence of the extensive literature on the behavior of acoustically actuated bubbles, a readyto-use approach, capable of predicting the oscillatory motion for the bubbles trapped in the circular microcavities, is still missing. In this study, we propose a theoretical model to quantify the resonant frequencies and viscous dissipation factors for a single trapped bubble and verify it experimentally. We further investigate an interaction of two coupled bubbles of equal and different radii. For the identical bubble pair, coupling results in controllable frequency shift from the modes of a single bubble, whereas the non-identical one can operate as a flow switch.