An injection pump is an excellent tool for fluid injection in microfluidic systems. Widely used in the medical field for long-term, sustained drug injections, this tool has been naturally transformed into fluid injection research. It is now the most commonly used instrument in microfluidics research.
Working principle of the injection pump
The syringe has been widely used in medical institutions for centuries. However, the syringe involves manual piston movement and is not suitable for the controlled delivery of objects within it. Therefore, an injection pump was invented to correct this problem. It consists of a simple linear motion source that controls the driving speed of the piston. You can find different types of laboratory injection pump suitable for microfluidic operation in the Wenhao microfluidic pressure injection pump.
If the diameter of the syringe is known, the instrument adjusts its linear velocity to the required flow rate according to the following formula:
Q = vS. (Q is the flow rate, V is the speed of the piston, and S is the cross-section.)
Technical characteristics of the injection pump
One of the main advantages of the injection pump is that the user can easily adjust the instrument’s operating range by changing the syringe’s diameter. Small syringe diameters allow for better control at low flow rates. In contrast, larger diameters lead to larger volumes but lower performance at low flow rates. Another significant advantage of the injection pump is the ease of knowing the flow rate.
The injection pump can be characterized primarily by response time and stability. The response time of the injection pump depends not only on its mechanical properties but also on the fluid resistance and fluid smoothness of the experimental device. The channel’s elasticity and high fluid resistance are essential parameters to obtain stable flow velocity in most microfluidic systems.
As the flow rate changes, the piston pushes the syringe, increasing the pressure in the fluid system and deflecting rather than moving the fluid. Depending on the fluid resistance and elasticity of the system, the response time can vary from one-hundredth of a millisecond in a rigid fluid system with a low resistance to a few hours in a flexible fluid system with very high fluid resistance.
To obtain the best response of the injection pump, you must avoid the fluid system’s elasticity and minimize the chip’s fluid resistance. However, you can use additional tools to increase the response speed of the injection pump.
It is essential to deal with microfluidics and injection pumps to respond to the experimental conditions because the uncertainty of the actual flow rate in microfluidic devices is one of the main reasons for the failure of experiments.
The flow stability of an injection pump depends on the minimum movement of its motor. Since the displacement of the piston is related to the amount of injection, this minimum movement results in the minimum amount of injection. Thus, discrete phenomena like oscillations or pulses occur at low flow rates due to motor stepping. Note that the minimum injection volume is proportional to the syringe diameter; this is why smaller syringe diameters improve flow stability at low flow rates.
However, using a smaller syringe limits the range of flow you want to achieve, and the quality of the syringe becomes critical when the expected stability reaches the order of 0.1 μl/min. Also, keep in mind that flexibility in the system can achieve a smoother flow rate and increase its stability, but reduce the response speed. The use of additional tools can also improve the stability of the injection pump.
In a word, the injection pump’s performance depends on the engine’s quality and the mechanical precision of its moving parts. This tool usually provides reliable performance. We have seen that some instability occurs at low flow rates. It can improve by increasing the system’s RC constant, for example, by increasing its elasticity, but this reduces the flow rate responsiveness. In dealing with injection pumps and microfluidics, the user must strike the right balance between stability and responsiveness.