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, or control the biochemical 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 gases) 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 peristaltic pump) or passive ways (e.g. hydrostatic pressure). A wide variety of flow control solutions can be found on Darwin Microfluidics.
If researchers can now choose between a full set of materials to build their microfluidic chips, one must consider that, originally, the fabrication process of a microfluidic chip was based on photolithographic methods, derived from the well-developed semiconductor industry.
The use of diverse materials for microfluidics chips such as polymers (e.g. PDMS), ceramics (e.g. glass), semiconductors (e.g. silicon) and metal is currently possible because of the development of specific processes: deposition and electro-deposition, etching, bonding, injection molding, embossing and soft lithography (especially with PDMS). To the best of our knowledge, only a few manufacturers of microfluidic chips offer a wide range of chips or materials.
Accessing these materials makes it possible to design microfluidic chips with new features like specific optical characteristics, biological or chemical compatibility, faster prototyping or lower production costs, possibility of electro sensing, etc. The final choice depends on the application.
Nowadays, a lot of researchers use PDMS and soft lithography due to their easiness of use and fast process. They allow researchers 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.