Microfluidic Fabrication Techniques

Different microfluidic control manufacturing technologies will lead to different types of chips made of different materials. This article will list four types of microfluidic chip manufacturing technologies: one is based on etching, wet or dry, and is usually used to manufacture silicon or glass microfluidic chips; the second uses thermoforming technology, such as heat/rolling patterns and injection molding; the third is direct writing because of micromachines or lasers; the fourth is about polymer casting using different soft lithography processes.

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Etching involves protecting certain parts of the substrate and attacking another part to remove material at a certain depth. Etching is usually used for silicon or glass substrates. Therefore, the microfluidic chip can be cast or directly molded for the polymer.

Wet Etching

Liquid chemicals or etchants are usually used to remove materials in wet etching. The etching can be isotropic. The material will etch in all 3D directions and cause the extension of the channel. Etching can also follow some crystal plans more preferentially and result in more or less complete anisotropic etching. In fact, the liquid etchant etches the crystalline material at different rates, depending on which crystal face is exposed to the etchant.

The wet etching process can be divided into three steps: the first is the diffusion of the liquid etchant to the surface; the second is the reaction between the liquid etchant and the material, usually a reduction-oxidation reaction; the third is the diffusion of by-products from the reaction surface to the surface.

Dry Etching

In dry etching, plasma or etchant gas is used to remove material.

Physical Dry Etching	It requires high kinetic energy provided by beams of ions, electrons or photons; there is no chemical reaction; only particle energy can knock atoms off the surface.
Chemical Dry Etching	It uses liquid instead of gas. The gas molecules react with the surface and remove atoms from the surface.
Reactive Ion Etching (RIE)	This method combines physical and chemical etching. The high-energy collisions from ionization help dissociate the etchant molecules into more reactive species. Therefore, this technique is the most diverse and widely used, and it is faster than other dry etching techniques.

Thermoforming

Thermoforming involves heating a material to soften it to make a specific shape. It is usually used for many plastic products and is made by injection molding or hot embossing.

• Injection molding introduces thermoplastic particles into a heated mold to manufacture chips. Once the parameters are optimized, this technology provides high-frequency manufacturing. However, the high cost of equipment and molds makes this solution rarely used in laboratories.
• Hot embossing is a method of pressing heated silicon or metal molds on a thermoplastic sheet. Materials such as PMMA or COC are commonly used in the embossing process.

Polymer Ablation

You can use traditional mechanical drilling, sawing, laser and powder spraying, and other direct writing processes to achieve microstructures, which can produce micro-devices very quickly, and sometimes lasers can be used for 3D production. But the resolution may be limited. Most importantly, the shape of the channel is not uniform because of the mechanical ablation of the material.

Polymer Casting

The method is to create a mold and use polymers to replicate it. There are different ways to make molds, but the principles are always the same. Hard materials can be used to make the designed negative, and then pour liquid polymer on it. The polymer can be cured by heating or ultraviolet rays, and then peeled from the mold to obtain a microfluidic device.

References

• Wang T, et al. (2018). "Fabricating Microstructures on Glass for Microfluidic Chips by Glass Molding Process." Micromachines. 9: 269.
• Scott S. M, et al. (2021). "Fabrication Methods for Microfluidic Devices: An Overview." Micromachines. 12: 319.

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