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Chips in microfluidics

Microfluidic chips are the devices used in microfluidic studies in which micro-channels have been molded or patterned. An application dependent number of inlet and outlet port allow fluids to pass through different channels, going from one place to another. The channels may have different diameters, typically ranging from 5 to 500 µm (see our tubing article here). The micro-channel network must be specifically designed for the application and the analysis one wants to carry out (cell culture, organ-on-a-chip, DNA analysis etc.).

Microfluidic devices such as chips have many advantages as they:

  • Decrease sample and reagent consumption.
  • Increase automation capabilities.
  • Minimize analysis time.

Such devices allow applications in many areas such as medicine, biology, chemistry and physics. Three types of materials are commonly used to create microfluidic chips:

  • Silicon
  • Glass
  • Polymers

For some experiments, a combination of these three materials will be needed to create the desired microfluidic chip. Each material has its specific chemical and physical characteristics. The choice of

Figure 1: Example of a complex lab-on-chip for (From Beatie et al.)

the material depends on

  • the needs and conditions of your applications.
  • the type of solvent, samples, buffers…
  • the design.
  • the budget.

Microfluidic chips in silicon

Advantages of silicon are:

  • superior thermal conductivity.
  • surface stability.
  • solvent compatibility.

However, no optical detection can be done due to its optical opacity. As a result, they are not widely used in microfluidic research.


Microfluidic chips in glass

Figure 2: Glass chip from Micronit for droplet generation

 

Glass shares with silicon the same advantages mentioned above. But one can add:

  • Well-defined surface chemistries
  • Superior optical transparency
  • Excellent high-pressure resistance
  • Biocompatibility
  • Chemically inert
  • Allows efficient coatings

These make it a material of choice for many applications. The main hurdle with this material remains its rather high cost, even though prices have been significantly reduced.


Microfluidic chips in polymers

Polymers offer an attractive alternative to glass and silicon as they are cheaper, robust and require faster fabrication processes. Many polymers can be used to build chips:

  • Polystyrene (PS)
  • Polycarbonate (PC)
  • Polyvinyl chloride (PVC)
  • Cyclic Olefin Copolymer (COC)
  • Polymethylmethacrylate (PMMA)
  • Polydimethylsiloxane (PDMS)

 

PDMS is the material of choice for fast prototyping microfluidic devices. PDMS chips are commonly used in laboratories, especially in the academic community due to their low cost and ease of fabrication. Here are listed the main advantages of such chips:

Figure 3 : PDMS chip with a labyrinth network

  • Oxygen and gas permeability
  • Optical transparency
  • Robustness
  • Non toxicity
  • Biocompatibility

 

 

One of the main drawbacks of PDMS chips is its hydrophobic nature. Consequently, introducing aqueous solutions into the microchannels is difficult and hydrophobic analytes can adsorb onto the PDMS surface, thus interfering with analysis. There are now PDMS surface modification to avoid issues due to hydrophobicity. Another main issue of PDMS chips is that they are non-suitable for high pressure operation as it can alter channel geometry and be prone to leaking at elevated pressure.


Some key information

  • Transparent materials are favored to enable optical observation / analysis.
  • Materials must be biocompatible for biological applications.
  • Most of the chips need surface treatment to adapt their surface properties to the application and to limit non-specific adsorption.
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