Micromotors are extensively applied in various fields, including cell separation, drug delivery and environmental protection. Micromotors with high speed and good biocompatibility are highly desirable. Bubble-driven micromotors, propelled by the recoil effect of bubbles ejection, show good performance of motility. The toxicity of concentrated hydrogen peroxide hampers their practical applications in many fields, especially biomedical ones. In this paper, the latest progress was reviewed in terms of constructing fast, bubble-driven micromotors which use biocompatible fuels, including low-concentration fuels, bioactive fluids, and enzymes. The geometry of spherical and tubular micromotors could be optimized to acquire good motility using a low-concentration fuel. Moreover, magnesium- and aluminum-incorporated micromotors move rapidly in water if the passivation layer is cleared in the reaction process. Metal micromotors demonstrate perfect motility in native acid without any external chemical fuel. Several kinds of enzymes, including catalase, glucose oxidase, and ureases were investigated to serve as an alternative to conventional catalysts. They can propel micromotors in dilute peroxide or in the absence of peroxide.

Micromotors, which can be moved at a micron scale, have special functions and can perform microscopic tasks. They have a wide range of applications in various fields with the advantages of small size and high efficiency. Both high speed and efficiency for micromotors are required in various conditions. However, the dynamical mechanism of bubble-driven micromotors movement is not clear, owing to various factors affecting the movement of micromotors. This paper reviews various factors acting on micromotor movement, and summarizes appropriate methods to improve the velocity and efficiency of bubble-driven micromotors, from a mechanical view. The dynamical factors that have significant influence on the hydrodynamic performance of micromotors could be divided into two categories: environment and geometry. Improving environment temperature and decreasing viscosity of fluid accelerate the velocity of motors. Under certain conditions, raising the concentration of hydrogen peroxide is applied. However, a high concentration of hydrogen peroxide is not applicable. In the environment of low concentration, changing the geometry of micromotors is an effective mean to improve the velocity of micromotors. Increasing semi-cone angle and reducing the ratio of length to radius for tubular and rod micromotors are propitious to increase the speed of micromotors. For Janus micromotors, reducing the mass by changing the shape into capsule and shell, and increasing the surface roughness, is applied. This review could provide references for improving the velocity and efficiency of micromotors.

This study proposed a general framework for adaptive delay compensation in real-time hybrid simulation(RTHS). This framework is composed of two main parts: an adaptive feedforward controller to handle variable time delay and amplitude discrepancies, and a feedback controller to enhance the robustness of compensation. In particular, the adaptive feedforward controller is determined using a discrete physical testing system (PTS) model, whose parameters are estimated using the least-squares method. Moreover, the feedback controller is introduced to reduce dependency on the PTS model of the feedforward controller. Results of virtual and actual RTHS alongside five other compensation strategies revealed the superiority of the proposed compensation method.

Micromotors have shown significant potential for diverse future applications. However, a poor understanding of the propelling mechanism hampers its further applications. In this study, an accurate mechanical model of the micromotor has been proposed by considering the geometric asymmetry and fluid viscosity based on hydrodynamic principles. The results obtained from the proposed model are in a good agreement with the experimental results. The effects of the semi-cone angle on the micromotor are re-analyzed. Furthermore, other geometric parameters, like the length-radius aspect ratio, exert great impact on the velocity. It is also observed that micromotors travel much slower in highly viscous solutions and, hence, viscosity plays an important role.