Microfluidic Flow Control System Syringe Pumps

Microfluidics flow control systems Syringe pumps are the most commonly used flow control systems in microfluidics even if in the last 5 years researchers have begun to use more alternative flow control systems.

Let’s say that syringe pumps can be divided in two categories. Classic syringe pumps, which are quite inexpensive but generate flow oscillations when dealing with microfluidics, and pulseless microfluidic syringe pumps, which are quite expensive but clearly offer better performances in terms of flow stability. In this tutorial, we will focus only on pulseless microfluidic syringe pumps. If you decide to use common syringe pumps, the information we provide in this tutorial will apply, but keep in mind the fact that your flows will not be stable at low flow rates.

The main advantage of syringes is that they are quite easy to use. The main weak point of pulseless syringe pumps is their responsiveness, since it depends on the microfluidics setup. Flow changes inside chips can take seconds to hours (see our tutorial on syringe pump responsiveness in microfluidics). This lack of reactivity is one of the main limitations of syringe pumps for numerous applications. However, in 2013 and 2014, new solutions can help to overcome these problems.

Strengths Syringe pumps generally allow fast setup for fluidic experiments. New pulseless syringe pumps may give you a flow stability below 1% The amount of dispensed liquid can be known for long term experiment (not during transient periods because of the flow rate uncertainty). Maximum pressure generated by a syringe pump can be at several hundred bars. (High pressure syringe pumps are not pulseless, but could be very usefull in nanofluidics). The mean flow rate in the device does not vary with eventual changes in the fluidic resistance of the device (except if the syringe pump stalls due to high pressure).

A Brief Thing to Know About Microfluidic Chips

A microfluidic chip is a set of micro-channels etched or molded into a material (glass, silicon or polymer such as PDMS, for PolyDimethylSiloxane). The micro-channels forming the microfluidic chip are connected together in order to achieve the desired features (mix, pump, sort, control bio-chemical environment).

This network of micro-channels trapped into the microfluidic chip is connected to the outside by inputs and outputs pierced through the chip, as an interface between the macro- and micro-world.

It is through these holes that the liquids (or gas) are injected and removed from the microfluidic chip (through tubing, syringe adapters or even simple holes in the chip) with external active systems (pressure controller, push-syringe or peristatic pump) or passive ways (e.g. hydrostatic pressure). If searchers can now choose between a full set of materials to build his microfluidic chips, one must consider that, initially, the fabrication process of a microfluidic chip was based on photolithographic methods, derived from the well-developped semiconductor industry.

The use of diverse materials for microfluidics chips such as polymers (e.g. PDMS), ceramics (e.g. glass), semi-conductors (e.g. silicon) and metal is currently possible because of the development of specific processes: deposition and electrodeposition, etching, bonding, injection molding, embossing and soft lithography (especially with PDMS).

Accessing these materials makes possible to design microfluidic chips with new features like specific optical characteristics, biological or chemical compatibility, faster prototyping or lower production costs, possibility of electrosensing, etc… The final choice depends on the aimed application.

Nowadays, a lot of searchers use PDMS and soft-lithography due to their easiness and fast process. They allow searchers to rapidly build prototypes and test their applications/setups, instead of wasting time in laborious fabrication protocols. Contrary to common beliefs, soft-lithography does not require hundreds of square meters of clean room space. Indeed, a little bench space under a lab fume hood is sufficient to place essential rapid PDMS prototyping instruments to quickly assess microfluidic concepts and obtain publishable results.