# Advances in Valveless Piezoelectric Pumps
## 1 Introduction  
Since [PERSON] [1] designed the first piezoelectric pump in 1987, this started the research and expanded the application field of pumps. The converse piezoelectric effects are utilized to drive the fluid in the piezoelectric pump. With the advantages of such a simple structure [2], low power consumption [3], and no electromagnetic interference [4], the piezoelectric pump has been widely used in the fields of medical drug delivery [5; 6; 7], fuel cells [8; 9; 10], microfluidics [11; 12; 13], liquid cooling [14; 15; 16] and precision machinery [17; 18; 19] etc. According to the structural characteristics, the piezoelectric pump can be divided into two different types, i.e., valve piezoelectric pump and valveless piezoelectric pump. The valve piezoelectric pumps have the weakness of \"pump-lagging of valve\" due to the removable valve body. When the frequency rises to a certain value, the piezoelectric pump will be dynamically balanced and the flow rate will be rapidly dropped to zero in a macro level [20]. In addition, the valve piezoelectric pump also has the disadvantages of large pressure loss, low transmission accuracy, and non-easy miniaturization, which seriously restricts its further promotion and application.  
In 1993, [PERSON] et al. [21] proposed the concept of a valveless piezoelectric pump with a conical flow tube for the first time, and it brings about the research boom of the piezoelectric pump into the valveless era. It should be noted that one-way flow of fluid can be achieved in the valveless piezoelectric pump to achieve using the positive and reverse flow resistance difference to perform the function of the valve. Meanwhile, the valveless piezoelectric pump can effectively solve the problems existing in the valve piezoelectric pump, which is of great concern for researchers. In the past 30 years, some researchers have designed various types of valveless piezoelectric pump. However, up to now, thereis no unified classification of valveless piezoelectric pump, which is not conducive for development in the fierce market competition.  
Figure 1 shows the classifications of valveless piezoelectric pumps. According to different structural characteristics, it can be divided into external flow pipe type, built-in structure type as well as a bionic valveless type. The external tube type of the valveless piezoelectric pump realizes the one-way flow of fluid by using a piezoelectric vibrator and special pipe; for instance, the nozzle/diffuser tubes [22], Y-shape tubes [23], spiral tubes [24] and special flow tubes [25], etc. Built-in structural valveless piezoelectric pump is the flow resistance structure placed inside the pump chamber, common examples include unsymmetrical slope elements [26], hemisphere-segments [27], and conical spiral cavities [28], etc. On the basis of the biological reality of the special structure, the bionic valveless piezoelectric pump is designed to achieve a one-way flow of fluid. Common structures include fishtail Bimorph [29], and built-in compliant structures [30], etc.  
There are already many review articles summarizing the piezoelectric pump [31; 32; 33]. However, there is no research review on valveless piezoelectric pumps, in order to allow people to better grasp the development of valveless piezoelectric pumps. In this paper, based on the application of valveless piezoelectric pumps in various fields, this paper reviews the research progress. According to the description of different types of valveless piezoelectric pumps reported in the literature, the development history is introduced as follows. The influences of material properties and structure on pump performance are introduced, and the valveless piezoelectric pump is systematically summarized. Based on current research status, the future development is analyzed. The future development direction of valveless piezoelectric pump technology is predicted, and the foundation is laid for further promotion.