Special Issue to Celebrate JMM's 30th Anniversary

In this article, an overview of the mechanical reliability of silicon microstructures for micro-electro-mechanical systems is given to clarify what we now know and what we still have to know about silicon as a high-performance mechanical material on the microscale. Focusing on the strength and fatigue properties of silicon, attempts to understand the reliability of silicon and to predict the device reliability of silicon-based microstructures are introduced. The effective parameters on the strength and the mechanism of fatigue failure are discussed with examples of measurement data to show the design guidelines for highly reliable silicon microstructures and devices.

Since the end of the last century, energy harvesting technologies have obtained prominent development as the sustainable power supplies for billions of wireless sensor nodes distributed in both the city and remote areas. Microelectromechanical system (MEMS) energy harvesters based on the energy transferring mechanisms of electrostatic effect, electromagnetic induction, and piezoelectric effect were first proposed to scavenge the vibrational energy from the ambient environment. Thereafter, the piezoelectric nanogenerator and triboelectric nanogenerator emerged as promising techniques to harvest diversified mechanical energy for addressing the energy consumption of flourishing wearable devices. Targeting for a more efficient system, multiple strategies for improving the output performance of individual energy harvesters as well as hybridized energy harvesters are extensively investigated. Merging the well-developed energy harvesters with energy storage, wireless data transmission, and other functional units, self-sustainable systems have been realized. Shortly, with the evolving AI technologies, we can foresee that the AI-assisted self-sustainable systems will also be achieved and play a vital role in the future 5 G era. In this review, we systematically introduce the evolution of energy harvesting techniques in the 5 G and IoT era, with detailed operation principles, structural designs, enhancement strategies, self-sustainable and AI-assisted system development, and our perspectives.

Terahertz (THz) part of the electromagnetic spectrum (0.1–10 THz) holds the key for next-generation high-speed wireless communication, non-destructive biosensing, fingerprint chemical detection and imaging for astronomy and security surveillance. The limited THz response of naturally occurring materials had left a technological gap in the THz region of the electromagnetic spectrum. Artificially engineered materials termed as 'metamaterials', have shown great potential in THz wave interaction and its active counterpart termed as 'metadevices' have been widely reported for on-demand manipulation of THz waves. One of the most efficient means of realizing metadevices is to reconfigure the shape of unit cells and hence the corresponding THz response. The 50+ years of development in microelectromechanical systems (MEMS) and the wide array of microactuator designs provide a perfect platform to achieve structural reconfiguration of microscale metamaterial unit cells in both in-plane and out-of-plane directions. In this review, we present a comprehensive overview of various MEMS approaches adopted for the demonstration of THz metadevices, their advantages and limitations. The future research directions of THz MEMS metadevices are also discussed. The seamless integration of matured MEMS technology with incipient THz metamaterials provides significant advantages in terms of enhanced performances, advanced functionalities and large scale manufacturability, that is critical for the development of future THz technologies.

The emergence and evolution of energy micro-generators during the last 2 decades has delivered a wealth of energy harvesting powering solutions, with the capability of exploiting a wide range of motion types, from impulse and low frequency irregular human motion, to broadband vibrations and ultrasonic waves. It has also created a wide background of engineering energy microsytems, including fabrication methods, system concepts and optimal functionality. This overview presents a simple description of the main transduction mechanisms employed, namely the piezoelectric, electrostatic, electromagnetic and triboelectric harvesting concepts. A separate discussion of the mechanical structures used as motion translators is presented, including the employment of a proof mass, cantilever beams, the role of resonance, unimorph structures and linear/rotational motion translators. At the mechanical-to-electrical interface, the concepts of impedance matching, pre-biasing and synchronised switching are summarised. The separate treatment of these three components of energy microgenerators allows the selection and combination of different operating concepts, their co-design towards overall system level optimisation, but also towards the generalisation of specific approaches, and the emergence of new functional concepts. Industrial adoption of energy micro-generators as autonomous power sources requires functionality beyond the narrow environmental conditions typically required by the current state-of-art. In this direction, the evolution of broadband electromechanical oscillators and the combination of environmental harvesting with power transfer operating schemes could unlock a widespread use of micro-generation in microsystems such as micro-sensors and micro-actuators.

Miniature power sources have gained widespread attention and accelerated progress with portable, wearable, and integrated electronic technologies. Micro lithium-ion batteries (μLIBs) featured small size, lightweight, high capacity, and long cycle life, which also offer stability, safety, and compatibility with microfabrication, make them the ideal choice for energy storage. Researchers have engaged in the performance optimization and application expansion of μLIBs, especially in the on-chip μLIBs with compatible integration for microdevices and flexible μLIBs for wearable applications. This paper reviews the working principles, performance metrics, and design methodologies of μLIBs, highlighting the advantages of the architecture design. Typical materials and fabrication methods are introduced, and their effects on the device performance and system integration are analyzed. The developments of μLIBs are summarized from the perspective of 3D architecture design to the on-chip and wearable applications. At last, the challenges and the prospects that inspire further research and development of μLIBs are concluded.

Flow measurement is an essential requirement in medical, industrial, automotive and environmental applications, Thanks to the rapid development of microelectromechanical system (MEMS) micro-fabrication techniques, silicon-based microflow sensor chips used as the key component of microfluidic control systems have attracted particular attentions due to the miniaturized chip-size, high precision, low power consumption, rapid response and low-cost batch-fabrication capability. Up to present, many different types of silicon microflow sensors have been proposed and developed. This paper provides a review on recently completed works on design, fabrication and properties of the MEMS-based silicon monolithic microflow sensors. Some technical challenges and application outlooks are also discussed in the paper.

Flexible/stretchable electronics have been well-developed in epidermal devices, wearable electronic systems and soft robotics for physiological signal detecting, real-time health monitoring and human–machine interfaces. As one of the most widespread-used transducers, strain sensors are playing a promising role in the developments of ongoing flexible electronics, especially equipped with tunable sensitivity (or gauge factor, GF). However, present investigations mainly focus on the enhancement of the sensitivity which required subtle designs or sophisticated fabrication processes. In this work, we report a facile fabrication strategy for configuring strain sensors with tunable sensitivity by adjusting the orientations and duty ratios of the micro-nano hierarchical Ag-coated microgrooves from laser patterning. The heterogeneous micro-nano structures enable localized large deformation to result in crack propagation on electrical layer during stretching, which endows the device with customized GF from 3.4 to 4570.6. The sensitivity-tunable strain sensor shows a great potential in monitoring various health conditions and voice recognition. This technique provides a facile and robust way to fabricate high-performance strain sensors for wearable physiological monitoring systems.

Silicon-based microelectromechanical systems (MEMS) inertial sensors have become ubiquitous, revolutionizing motion sensing, vibration sensing and accurate positioning in several societal fields. Driven by consumer and automotive markets, companies involved in this technological development focused mostly on low cost, miniaturization and low power consumption, somewhat sacrificing measurement accuracy. In several laboratories all over the world, however, the research toward higher-performance sensors has been going on for more than two decades, with the goal of improving two key parameters for future applications: noise density and bias stability. This review article summarizes, for silicon-based MEMS accelerometers and gyroscopes, the most relevant working principles that appeared in the scientific literature. The collection of several data about the above mentioned key figures enables tracing the roadmap for further developments in the upcoming decade.

Recently, flexible electronics have attracted significant attention as they can be integrated on diverse platforms from curved to flexible surfaces. As flexible electronics are used on a curved surface of wearable or manufacturing devices for health and system monitoring, the working environment of such applications forces electronics to be exposed to diverse stimuli such as deformation, temperature, humidity, and gas, resulting in performance changes. Therefore, rather than research on improving the specific performance of electronics, research on maintaining a stable performance in various environmental stimuli has been receiving tremendous interest. Reflecting the latest research trends, this paper introduces efforts in structural designs heading for both improving and maintaining the performance of flexible electronics in diverse environmental stimuli. Firstly, we will sequentially explain the geometric and structural designs introduced for achieving (a) reliable electronics insensitive to undesired mechanical stimuli, (b) reliable electronics in harsh environments, and (c) flexible electrodes. Also, (d) diverse applications of reliable and flexible electronics are introduced. Finally, a perspective on reliable and flexible electronic devices has been presented for suggesting next-generation research.

The booming growth in environmental conditions sensing and monitoring pushes the need of inexpensive environment sensors with small size and low power consumption. The outbreak of COVID-19 further increases the need for fast monitoring of environment conditions. The micro-electrical-mechanical-systems (MEMS) technologies are considered as promising solutions to realize the required environment sensors. The mature complementary metal-oxide-semiconductor (CMOS) process platforms available in many foundries can be extended to fabricate MEMS sensors to offer the advantage of relatively easier commercialization. Moreover, by leveraging the characteristics of CMOS process platforms, the integration of multiple sensors and sensing circuits to form a compact sensing system can also be achieved. This review paper will focus on introducing the miniaturized environmental sensing devices implemented and integrated using the CMOS-MEMS technologies. In general, the CMOS chips for environment sensing are firstly fabricated using the foundry-available CMOS processes, and then the post-CMOS micromachining processes are performed to implement the CMOS-MEMS environment sensors. This paper respectively reviews five different environment sensors (including the infrared, pressure (barometer), humidity/temperature, and gas sensors) using the CMOS-based MEMS technologies. The advantages and design concerns of sensors fabricated by different CMOS and post-CMOS processes are introduced and discussed. Moreover, the CMOS-MEMS environment sensing hub implemented through the monolithic integration of multiple environment sensors is also introduced.