Pressure Control Technologies
To optimize experiments in microfluidics, one needs to understand the different technologies available to use the most suitable way to control microfluidic flows. This presents a short review of the existing pressure control techniques.
Here is a list of the main microfluidic flow control technologies.
Pressure-based microfluidic control
Based on these methods, two different techniques are available to manage microfluidic flows with pressure actuation: using hydrostatic pressure or a pressure pump.
1. Using hydrostatic pressure
This is the simplest way to move fluids: the basic idea is to put the inlet reservoir higher than the outlet reservoir in order to let the gravity force move the fluid from the inlet to the outlet.
Using water, 1cm (or 1 inch) of level difference generates a pressure difference of 0.981 mbar (respectively 2.49mbar). Because of practical constraints, it is hard to get a level difference larger than 100 cm or 3 feet, limiting the maximum pressure to around 100 mbar. The stability is quite good in the short time range but bad in the long-time range: the flow will empty the inlet reservoir and fill in the outlet reservoir, modifying the level difference and so the pressure difference over time. This means that one needs to move the reservoirs while the liquid flows to obtain a stable flow rate.
The responsiveness is also dependent on how quickly it is possible to move the reservoirs during the manipulation. Another limitation is the sensibility to air bubbles: typically, an air bubble in a 50?m microchannel generates a Laplace pressure of 28 mbar for water, equivalent to 27.4cm. So, large modifications of the level difference are necessary to keep the flow-rate constant in presence of air bubbles in your microfluidic chip. Due to these constraints, this technique is not adequate for complex microfluidic network and is very difficult to automate. It is only suitable for pressures lower than 100mbar.
2. Pressure regulator or pressure controller
The working principle of such pumps is to pressurize the sample reservoirs in order to control the pressure drop between the inlets and the outlets of the microfluidic system. The resolution of the generated flow-rate depends on the resolution of pressure sensor in the pressure pump. For instance, all our MFCS™ series (Microfluidic Flow
Control Systems) offer a resolution of 0.03% full scale (pressure sensor resolution) as well as a stability of ±0.1%. In term of responsiveness, the typical response and settling times of a pressure pump are relatively fast and are a function of the technology of the pressure controller and the configuration of the microfluidic setup. For instance, based on the FASTAB™ technology, the MFCS™ series has a response time down to 40ms and a settling time down to 100ms. Pressure generators can be controlled by computer and the different steps of the experiment can be automated. The pressure pumps, also called pressure controllers or pressure generators allow efficient experiments with complex microfluidic networks containing multiple inlets and outlets. This technique can be coupled with flow sensors and an algorithm in order to reach the desired flow-rate by automatically tuning the applied pressure. To learn more about this solution, have a look at the FRCM to control flow-rates while keeping the benefits of the pressure actuation.