# Advances in Valveless Piezoelectric Pumps
## 3 Comparison of Various Valveless Piezoelectric Pumps  
In the past 30 years, the valveless piezoelectric pump has developed rapidly. In particular, the valveless piezoelectric pump with an external flow tube of great concern to many scholars. In regard to the structure features, both the valveless piezoelectric pump with external flow tube and valveless piezoelectric pump with built-in structures can realize the unidirectional flow of liquid through the volume change of the pump chamber and the flow resistance structure. The output performance of the valveless piezoelectric pump can be controlled and regulated by optimizing the flow tube structure. In the development process of valveless piezoelectric pumps, many types of valveless piezoelectric pumps with external flow tubes have appeared, such as, valveless piezoelectric pumps with diffuser/nozzle tubes, valveless piezoelectric pumps with Y-shape tubes, valveless piezoelectric pumps with spiral tubes, and valveless piezoelectric pumps with special flow tubes. As shown in Figures 7 and 8, the flow rate of valveless piezoelectric pumps with external flow tubes usually exhibit a relative value, i.e., 35 mL/min, and sometimes can be realized at a \(\upmu\)L/min level.  
The flow resistance structure of valveless piezoelectric pumps with built-in structures is installed in the pump chamber, which can reduce the overall size of the valveless piezoelectric pump. The internal structure of the pump chamber was built to improve the output behavior of these pumps. It consists of valveless piezoelectric pumps with hemisphere-segment bluff-bodies, and valveless piezoelectric pumps with a crescent-shaped structure. As compared with the valveless piezoelectric pump with an external flow tube, the pump flow of a valveless piezoelectric pump with built-in structures is dramatically enhanced, and the maximum pump flow reaches 286 mL/min.  
In addition, the bionic valveless piezoelectric pump is also considered to be one of the main research hotspots. The valveless piezoelectric pump imitating caudal-fin and semi-flexible valve piezoelectric pumps are the two most common bionic valveless piezoelectric  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**First Author** & **Type of** & **Structure** & \(\mathbf{U_{p}}\) & \(\mathbf{f_{p}}\) & \(\mathbf{P_{max}}\) & \(\mathbf{Q_{max}}\) \\
**and Year** & **Valveless** & **Material** & **(V\({}_{p\text{-}p}\))** & **(Hz)** & **(kPa)** & **(mL/min)** \\ \hline
2016 [69] Ma & Diffuser/nozzle & PMMA & N/A & 70 & N/A & 9.1 \\
2018 [70] Munas & \(\dashv\)-shape & PMMA & N/A & 100 & 0.24 & 31.15 \\
2019 [71] Huang & Vortex diodes & PA & 200 & 49 & 0.34 & 9.86 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Summary of properties for other valveless piezoelectric pumps.  
Figure 7: Maximum flow rates for valveless piezoelectric pump: flow rates versus voltages.  
pumps. The valveless piezoelectric pump imitating a caudal-fin is used to simulate a fish swimming, and make the fish static to obtain the one-way flow of liquid from front to back. The semi-flexible valve piezoelectric pump is a new breakthrough in the field of piezoelectric pumps. We can adjust the driving voltage to realize the integration of \"valve-based\" and \"valveless\" in one body.