## 4 Micromanipulation by Vibration-Induced Steaming Flow  
### Vibrated Needles  
Vibrated-needle-driven manipulation is another kind of popular method. A simple example of needle-driven micromanipulation is depicted in Figure 1e. Different from in-plane manipulation systems, needle-driven manipulation is more flexible and versatile. Different needle-driven manipulation methods differ in terms of needle shape, operating environment, oriented object, and principle. In general, the principle of needle-driven manipulation can be divided into two categories: inducing the flow of the surrounding fluid through the vibration of the tip, thereby driving the movement of the object in the fluid; and stimulating the acoustic streaming and radiation force, which we discussed in the previous session, in the vicinity of the tip through its high-frequency vibration. The term needle here refers to a generalized needle shape, which means an operating end with a large aspect ratio. The shape of the needle directly influences the manipulation effect, according to the [PERSON] et al. [63]. Their results showed that the ultrasonic needle's vibration generates the acoustic field, which aggregates materials at the micro or nano scales, and the ARF contributes little to the concentration. The elliptical, rectangular, and rhombic cross-sections of the needle were investigated as well. The needles with elliptical or rhombic cross-sections were found to perform better in concentrating ability than those with rectangular cross-sections. The development of needle-driven micromanipulation started with contact operation. The tip did not vibrate but directly pushed the cell to move. The research [PERSON] et al. [64, 65] is shown in Figure 5a, providing a typical example of contact operation. They presented a driving method for an on-chip robot, for which a piezoelectric ceramic is applied to induce ultrasonic vibration to the microfluidic chip. The tip and the base constitute a controllable miniature robot, which is actuated by permanent magnets in a microfluidic chip. The high-frequency vibration reduces the sufficient friction on the magnetically driven microtool (MMT) significantly, so the operation accuracy, output force, movement velocity, and execution efficiency were significantly improved. However, high-frequency vibration does not directly affect the manipulation of the robot on the cells, which is still a type of contact operation. The rotation of a single cell needs to be coordinated by the needle tips of two robots. This is troublesome, but due to the simple structure, the cost is still acceptable. Figure 5: Several vibrated-needle-driven manipulators: (**a**) [PERSON] et al. [64, 65]. The rear end of the needle is connected to a square base that can be rotated and translated by the magnet below. An ultrasound stream is applied to the operation units consisting of needle and base to suspend these units stably in the liquid, but the needle manipulates the cells in a contact operation. (Republished with permission of the Royal Society of Chemistry from reference [65]. Copyright © The Royal Society of Chemistry 2011; permission conveyed through Copyright Clearance Center, Inc.) (**b**) The methods proposed by [PERSON] and [PERSON] et al. [30, 66] generate high-frequency vibration (~100 kHz) to form acoustic microstreaming, which can trap tiny objects near the tip to realize movement. (Reprinted from [30], with the permission of AIP Publishing.) (**c**) [PERSON] et al. [28, 67] proposed a universal and efficient operating system. The needle tip is fixed to a movable piezoelectric actuator that generates a micro-vortex in the liquid medium through low-frequency vibration (~350 Hz) to drive the particle to move and rotate. (Republished with permission of the John Wiley and Sons from reference [28]. Copyright © 2018, the John Wiley and Sons; permission conveyed through Copyright Clearance Center, Inc.)Although contact operation is low cost and high efficiency, microparticles, especially cells, are considerably influenced.