Volumetric Control Technologies
To perform effective experiments in microfluidics, one needs to be aware of the different volumetric control technologies available to use the most suitable way to control microfluidic flows. The present article presents review of the existing techniques.
Here is a list of the main microfluidic flow control technologies.
- Applying a pressure difference
- Syringe pumps or peristaltic pumps
- Other techniques
Volume-based microfluidic control
Based on these methods, two different techniques are available to manage microfluidic flows: syringe pumps and peristaltic pumps.
1. Syringe pumps
Syringe pumps are widely used in standard laboratories as they are the historical solution. They are based on a mechanical system usually actuated by a stepper motor that will push a syringe at a precise rate. The flow rate is equal to the syringe cross section multiplied by the linear speed applied by the mechanical system. A large range of flow rates can be achieved by changing the syringe volume or linear speed of the piston.
Despite recent improvements, mechanical actuation generates flow pulsations that can be very disturbing, especially at low flow rates with rigid tubing or with viscous liquids. The response and settling times can be quite long. Waiting for more than 15 minutes to reach a steady flow is common. This settling time can be even longer in the presence of air bubbles, viscous liquids, and compliant tubing and PDMS microsystems.
Besides, the actual flow rate within the system is not monitored and the user has only access to the flow rate order. This can lead to result bias if the flow rate orders were not reached due to a leakage, a plug or a wrong set-up of the syringe, or because the experiment was made before reaching a steady flow.
The pressure is not controlled: if the microsystem gets plugged, the pressure will rise. Beyond a certain limit, either the syringe or the microsystem may be damaged. When using syringe pumps, it is important to keep this in mind and to regularly verify that the microsystem is not blocked, especially when using microparticules. In this case, it becomes difficult to automate an experiment since one needs to constantly monitor the experiment.
2. Peristatic pump
The liquid is contained in a flexible tube and alternating compressions and relaxations will draw in the liquid and move it forward. The flow pulsation is very high, leading to stability around 20% CV, and is mainly dependent on the number of rollers compressing the flexible tube. It is also necessary to daily calibrate a peristaltic pump to obtain flow rate precision around 5 %. Compared to syringe pumps, the flow pulsations are much more pronounced. To achieve different flow rates, it is necessary to change the inner diameter of the flexible tube.
The principle is to create an electro-osmotic flow in a porous medium made of glass: an electric potential is applied between the porous media (up to several kV) and a flow rate is generated depending on the ionic force of the liquid. The main advantage is that there are no moving parts, and it can be controlled directly with an electric signal. The main drawback is that it works only with water or alcohol with ionic species. The relation between the flow rate and the applied voltage depends on the ionic force: in practice, it can only be used for liquids with constant chemical properties and a calibration is needed.
4. Integrated micropump
Several kinds of integrated micropumps exist, mostly based on a peristaltic principle with flexible membranes made of PDMS. The flow rate range is typically low but it can achieve 10 to 100uL/min. The main advantage is the opportunity to control fluids down to the pL (10-12 L) range. It is particularly useful for applications where a very low quantity of fluid is to be tested. The integration in the microsystem needs complex steps of micromachining and needs a specific design dedicated to the application.