## 5 Discussion and Future Prospects  
Although manipulation actuated by vibration-induced acoustic waves and streaming flow is not a new concept, much remains to be researched. Simple in-plane micro-manipulating devices have been extensively applied in the range of lab-on-a-chip scenarios, which is an important research interest in the related fields of vibration or acoustic micromanipulation. Due to better biocompatibility and lower cost than optical tweezers, it has gradually become universally applied in the field of cell micromanipulation. Magnetic tweezers are relatively common biological manipulation methods, but since most cells are not magnetic, directly manipulating cells with magnetic tweezers is challenging. For easy control, the current VAW devices, especially the in-plane manipulating devices, have restrictions on the shape but not the material of particles, which are different from restrictions on magnetic tweezers. Therefore, in the future, when acoustic tweezers can accurately manipulate irregularly shaped particles, they could replace magnetic tweezers to a certain extent. However, acoustic waves, as a kind of mechanical vibration performance, have strict requirements on the medium (a frequency of 200 MHz in water corresponds to a wavelength of 5 microns). The speed of mechanical wave propagation also affects the flexibility, accuracy, and response speed of acoustic micromanipulation. Performance improvement is the primary trend in the development of acoustic micromanipulation. More sophisticated control strategies may emerge and lead to even more versatile manipulation capabilities. One possible approach is to cast the manipulation challenge as an optimization problem. The operation precision problem could be considered as an optimization project [41; 84]. The theoretical basis of acoustic operation is unstable. The mathematical model we use for analysis is too simplified sometimes, which prevents obtaining ideal results by directly estimating the particle control model and motion model. The optimization problem could reduce the impact of this model's inaccuracy to some extent. In the future, machine learning, a tool based on optimization methods, could be used to predict the motion of particles and control. The working scale of micromanipulators is a challenging issue, especially for in-plane devices, array devices, and beam devices. In the discussion of in-plane devices in this paper, we explained that the same acoustic wave has different effects on particles of different sizes, which is also applicable to other SAW and BAW devices. This phenomenon implies that fixed-frequency devices cannot achieve the same result for particles of various sizes. Chirped IDTs have been used to change the frequency and phase to control particles of different scales [51]. However, the layout of the transducer also affects the results, so this kind of device is only versatile within a small range. The acoustic manipulating devices have a substantial limitation in operating below 1 \(\upmu\)m. The natures of fluids and mechanical waves vary significantly on microscopic scales (especially nanoscale), which is an important factor limiting the minimum scale of acoustic manipulation. Combining acoustic manipulation with the another method is a solution to the problems above. Large amounts of related work have been published per the review by [PERSON] et al. [85]. Because different operating methods have different applicable scales, combining two different methods provides a broader manipulating scale and more flexible performance. Take the work of [PERSON] et al. for example [86]. They combined acoustic transport and photochemical reaction to design an acoustic nanomotor. Compared with a single acoustic field, this method not only moves more flexibly and quickly but also provides a braking function. The fusion between the different acoustic operating modes described in that paper is also an attractive idea. For example, the beam device and the 2 Din-plane device can be easily combined.