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The outbreak of the coronavirus disease has drawn public attention to the transmission of infectious pathogens, and as major carriers of those pathogens, respiratory droplets play an important role in the process of transmission. This Review describes respiratory droplets from a physical and mechanical perspective, especially their correlation with the transmission of infectious pathogens. It covers the important aspects of (i) the generation and expulsion of droplets during respiratory activities, (ii) the transport and evolution of respiratory droplets in the ambient environment, and (iii) the inhalation and deposition of droplets in the human respiratory tract. State-of-the-art experimental, computational, and theoretical models and results are presented, and the corresponding knowledge gaps are identified. This Review stresses the multidisciplinary nature of its subject and appeals for collaboration among different fields to fight the present pandemic.

The approach to improve the output power of piezoelectric energy harvester is one of the current research hotspots. In the case where some sources have two or more discrete vibration frequencies, this paper proposed three types of magnetically coupled multi-frequency hybrid energy harvesters (MHEHs) to capture vibration energy composed of two discrete frequencies. Electromechanical coupling models were established to analyze the magnetic forces, and to evaluate the power generation characteristics, which were verified by the experimental test. The optimal structure was selected through the comparison. With 2 m/s2 excitation acceleration, the optimal peak output power was 2.96 mW at 23.6 Hz and 4.76 mW at 32.8 Hz, respectively. The superiority of hybrid energy harvesting mechanism was demonstrated. The influences of initial center-to-center distances between two magnets and length of cantilever beam on output power were also studied. At last, the frequency sweep test was conducted. Both theoretical and experimental analyses indicated that the proposed MHEH produced more electric power over a larger operating bandwidth.

Soft robotics revolutionized human-robot interactions, yet there exist persistent challenges for developing high-performance soft actuators that are powerful, rapid, controllable, safe, and portable. Here, we introduce a class of self-contained soft electrofluidic actuators (SEFAs), which can directly convert electrical energy into the mechanical energy of the actuators through electrically responsive fluids that drive the outside elastomer deformation. The use of special dielectric liquid enhances fluid flow capabilities, improving the actuation performance of the SEFAs. SEFAs are easily manufactured by using widely available materials and common fabrication techniques, and display excellent comprehensive performances in portability, controllability, rapid response, versatility, safety, and actuation. An artificial muscle stretching a joint and a soft bionic ray swimming in a tank demonstrate their effective performance. Hence, SEFAs offer a platform for developing soft actuators with potential applications in wearable assistant devices and soft robots.

This paper proposes and investigates a piezoelectric energy harvesting system based on the flow induced vibration of a piezoelectric composite cantilever pipe. Dynamic equations for the proposed energy harvester are derived considering the fluid-structure interaction and piezoelectric coupling vibration. Linear global stability analysis of the fluid-solid-electric coupled system is done using the numerical continuation method to find the neutrally stable vibration mode of the system. A measure of the energy harvesting efficiency of the system is proposed and analyzed. A series of simulations are conducted to throw light upon the influences of mass ratio, dimensionless electromechanical coupling, and dimensionless connected resistance upon the critical reduced velocity and the normalized energy harvesting efficiency. The results provide useful guidelines for the practical design of piezoelectric energy harvester based on fluid structure interaction and indicate some future topics to be investigated to optimize the device performance.

A new type of micro rotary motor with a single piezoelectric stack actuator has been proposed, developed and tested. The conventional principle of elliptical motion is realized by a proposed mechanism which utilizes a Y-shaped stator and a ring rotor, and the biomimetic gait of an inchworm is achieved by integrating one piezoelectric stack actuator into the stator. The performance of the motor is predicted on the basis of computational simulation. An experimental setup was built to validate the working principles and evaluate the performance. Preliminary results have verified the working principle. Differences between the measured and predicted performances of the motor are analyzed.

Inspired by the fluttering of leaves in wind, we propose a novel type of wind energy harvesting device composed of a piezoelectric bimorph beam and flexible extensions. Working principle and advantages of the proposed design are discussed with numerical simulations conducted to investigate the influence of extension shape upon the device performance. Fabricated prototypes are tested in a simple experiment setup to give a description of the actual device performance. Results show that the leaf-inspired sector-shaped flexible extension corresponds to a better device performance. The test for the dual attachment case of the flexible extensions shows unexpected results, showing that the interaction and collision between the two flexible extensions are to be further explored and optimized.

It is well known that three-dimensional (3D) printing is an emerging technology used to produce customized implants and surface characteristics of implants, strongly deciding their osseointegration ability. In this study, Ti alloy microspheres were printed under selected rational printing parameters in order to tailor the surface micro-characteristics of the printed implants during additive manufacturing by an in situ, controlled way. The laser path and hatching space were responsible for the appearance of the stripy structure (S), while the bulbous structure (B) and bulbous–stripy composite surface (BS) were determined by contour scanning. A nano-sized structure could be superposed by hydrothermal treatment. The cytocompatibility was evaluated by culturing Mouse calvaria-derived preosteoblastic cells (MC3T3-E1). The results showed that three typical microstructured surfaces, S, B, and BS, could be achieved by varying the 3D printing parameters. Moreover, the osteogenic differentiation potential of the S, B, and BS surfaces could be significantly enhanced, and the addition of nano-sized structures could be further improved. The BS surface with nano-sized structure demonstrated the optimum osteogenic differentiation potential. The present research demonstrated an in situ, controlled way to tailor and optimize the surface structures in micro-size during the 3D printing process for an implant with higher osseointegration ability.