Inorganic Materials

Inorganic materials are the earliest materials used in microfluidic technology, which have also been used in microchannel applications before, such as glass or quartz capillaries for gas chromatography and capillary electrophoresis. With the development of MEMS technology, the first generation of microfluidic chips are made of silicon or glass. Glass and silicon are highly rigid materials, have high stability at high temperatures, and are highly resistant to organic solvents.

Besides, glass/silicon chips produced by photolithography can reach sub-micron channel sizes and have high reproducibility. Due to high thermal stability and solvent compatibility, on-chip reactions and droplet formation are very suitable for use with silicon/glass.

Silicon Microfluidic Chip

The first material used for microfluidics is silicon because of its resistance to organic solvents, easy metal deposition, high thermal conductivity and stable electroosmotic mobility. Its elastic modulus is very high (130-180 GPa), and it is a silicon microfluidic device that can be manufactured using wet/dry etching methods. Several methods have been developed for manufacturing silicon microfluidic chips.

Bulk Micro-Machining	This is a photolithography method used to pattern the required microfeatures on a silicon substrate. Dry etching: The removal of part of the material from the exposed surface by exposing the material to ion bombardment. Wet etching: The wafer is usually immersed in an etchant bath. After etching, another substrate is bonded to the patterned device to form enclosed channels, chambers and other features.
Surface micromachining	The microstructure is established by depositing and etching structural layers on the substrate.
Buried channel technology	The microstructure is constructed by trench etching, coating on the sidewall of the trench, removal of the coating on the bottom of the trench, and etching the channel isotope into the main body of the silicon substrate.

However, silicon material is difficult to create active microfluidic components, such as valves and pumps, due to its hardness. Silicon is an opaque material, so it cannot be seen through. Chemical modification of the silicon surface may be a way to reduce non-specific adsorption or improve cell growth.

Glass Microfluidic Chip

Glass is an optically transparent and electrically insulating amorphous material, which is usually processed using standard photolithography or wet/dry etching methods. Unless special etching techniques are used, the sidewalls of the etched glass channels will be round. Glass is compatible with biological samples, is airtight, and has relatively low non-specific adsorption. Because gas can pass through glass fragments that usually have closed channels and chambers, this material cannot be used for long-term cell culture.

One of the main applications of glass microfluidic chips is capillary electrophoresis (CE), which is easier to set up for parallel analysis and can also quickly separate analytes. Other typical applications include on-chip reactions, droplet formation, solvent extraction and in-situ preparation. Because glass microchannels have high thermal conductivity and stable electroosmotic mobility on the surface, their performance is better than other materials. Similarly, due to its hardness and high manufacturing cost, the application of glass in microfluidics also has many limitations.

Ceramic Microfluidic Chip

Microfluidic devices made of ceramics usually use low-temperature co-fired ceramics (LTCC), which is an alumina-based material. The material is contained in a laminate, which is patterned, assembled, and then heated at high temperatures. It has been demonstrated that the LTCC device exhibits low non-specific adsorption.

The advantages of the LTCC structure are low price and short development time. It also allows heaters, sensors and electronics to be integrated into a single module, which is the main advantage of this technology over silicon, glass and polymer technologies because it simplifies the measurement system.

References

• Shameli S. M, et al. (2015). "Microfluidic Two-Dimensional Separation of Proteins Combining Temperature Gradient Focusing and Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis." Anal. Chem. 7: 3593-3597.
• Pedro S. G, et al. (2012). "A Ceramic Microreactor for The Synthesis of Water Soluble CdS and CdS/ZnS Nanocrystals with On-line Optical Characterization." Nanoscale. 4: 1328-1335.

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