Microfluidics is the science that deals with the flow of liquids in micrometric sized channels. It is enough that a dimension of the channel is of the order of a micrometer or about ten micrometers for us to start talking about microfluidics. Microfluidics can be considered both as a science (studies of the behavior of fluids in microchannels) and a technology (manufacture of microfluidic devices for laboratories on chips).
A microfluidic chip is a set of micro-channels engraved or molded in a material (glass, silicon, or polymer such as PDMS for PolyDiMethylSiloxane). The micro-channels constituting the microfluidic chip are connected together so as to perform the desired function (mixing, pumping, sorting, control of the bio-chemical environment).
This micro-channel network enclosed in the microfluidic chip is connected to the outside by inputs and outputs pierced through the chip, as interfaces between the macro and micro world. It is through these holes that liquids (or gases) are injected and evacuated from the microfluidic chip (through tubes, syringe adapters, or even simple holes in the chip) with external active systems) or passive means.
If researchers can now choose from a wide range of materials to build their microfluidic chip, it must be considered that originally, it was the semiconductor manufacturing processes, and in particular photolithographic processes, which allowed the first developments of microfluidic chips. The use of materials such as polymers (e.g., PDMS), ceramics (e.g., glass), semiconductors (e.g., silicon), and metals is now possible thanks to the development of specific processes. Access to these materials has made it possible to design microfluidic chips with unprecedented properties, such as specific optical characteristics, biological or chemical compatibilities, a possibility of creating prototypes more quickly, reducing production costs, finding precise applications such as electrochemical detection, etc. The final choice depends on the intended application.
The technologies developed to miniaturize the transistors and manufacture the microprocessors have made it possible to manufacture microscopic size channels and to integrate them on chips. Thus the history of microfluidics will take us from the first lunar expedition, passing from our printer heads to our hospitals.
The 1950s saw the invention and development of the first transistors. Manufactured in semiconductor blocks, they have gradually replaced the lamps long used in the manufacture of electronic devices (radio, calculator, etc.)
In the 1960s, space research, via the Apollo program with a budget of 25 billion dollars, made it possible to finance research programs on the miniaturization of computers to allow them to take them into space and especially on the moon.
The development of technologies such as photolithography has made it possible to miniaturize and integrate thousands of transistors on semiconductor wafers, mainly silicon. This research led to the manufacture of the first integrated circuits and, with them, the first microprocessors.
In the 1990s, a lot of research was carried out to apply microsystems in the fields of biology, chemistry, and biomedical. These applications requiring the control of movements of liquids in microchannels have considerably contributed to the development of microfluidics. A major research effort was made for the development of laboratories on chips to allow the integration of almost all the operations necessary for a biological, chemical, or biomedical protocol on a simple microfluidic chip.
At that time, the majority of microfluidic devices was still made of silicon or glass and therefore required the heavy infrastructure of the microelectronics industry.
From the 2000s, technologies based on the molding of micro-channels in polymers such as PDMS have experienced strong development. The reduction in costs and manufacturing time of the devices has enabled a large number of laboratories to conduct research in microfluidics.
Nowadays, thousands of researchers work in microfluidics to extend its field of application, in particular via on-chip laboratories for hospitals.
The simplest current microfluidic devices consist of microchannels molded from a polymer that is bonded to a flat surface (a glass slide, for example). The most widely used polymer for molding microfluidic chips is PDMS. PDMS is a transparent, biocompatible elastomer (very close to the silicone gels used in breast prostheses), deformable, inexpensive, easy to mold, and stick to glass. It is for these reasons that it is appreciated by researchers in the field. The production of a simple microfluidic chip requires several steps.
The manufacturing of a microfluidic device begins with the drawing of the channels on dedicated software (AUTOCAD, LEDIT, and Illustrator). Once this drawing is made, it is transferred to an optical mask, a glass plate covered with chrome, or a plastic film for most masks. This can be done with dedicated manufacturers or in a clean room with glass masks. The microchannels are printed with a UV opaque ink (if the support is a plastic film) or engraved in chrome (if the support is a glass plate).
It is during this stage that the drawings representing the microchannels on the photomask are transformed into real microchannels on a mold. The micro-channels made in relief on the mold will then make it possible to obtain replicas dug in the future material of the microfluidic chip.
(1) A resin is spread on flat support (often a silicon wafer) with the desired thickness (which will determine the height of the channels).
(2) The resin, protected by the mask on which the channels are drawn, is partially exposed to UV. Thus (in the case of a negative resin, type SU8), only the parts representing the channels are exposed to UV and polymerized, the other parts of the mold being protected by the opaque zones of the mask.
(3) The mold is developed in a solvent that destroys all the areas of resin that have not been exposed to UV.
(4) We thus obtain a microfluidic mold with a resin replica of the patterns that were present on the photomask (the future microchannels are “reliefs” on the mold). The height of the channels is determined by the original thickness of the resin spread on the plate. Most of the time, the mold is then treated with silane to facilitate the detachment of the microfluidic components during the molding stages.
Microfluidic technologies have found many applications mainly:
- In biomedical, with laboratories on chips because they allow the integration of numerous medical analyzes on the same chip.
- In cell biology, because the microchannels have the same characteristic size as cells and allow, for example, the manipulation of single cells and the rapid change of drugs.
- In protein crystallization because microfluidic devices make it possible to generate on a single chip a very large number of crystallization conditions (temperature, pH, humidity, etc.)
And also, many other fields: drug screening, blood glucose testers, chemical microreactor, electrochemistry, and microprocessor cooling or micro fuel cells.
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