Microfluidic Chips

A microfluidic chip is a set of microchannels etched or molded into the material. The microchannels forming the microfluidic chip are connected together to allow fluid to flow from one place to another. The micro-channel network is connected to the outside through the input and output throughout the chip, as an interface between the macro and the micro world.

Liquid or gas is passively or actively (syringe pump or peristaltic pump) injected, managed, and/or removed from the chip. The inner diameter of the microchannel is usually in the range of 5 to 500 µm, but today, its structure can be fabricated with sub-micron precision.

The microchannel network design must be precisely designed to achieve the required functions, such as lab-on-a-chip, pathogen detection, DNA analysis, etc.

How to Build A Microfluidic Chip?

Currently, the simplest microfluidic chip is molded by microchannels. PDMS is a transparent, biocompatible, deformable and inexpensive elastomer, which is often used to mold microfluidic chips. Simple manufacturing requires several steps. Here we describe the fabrication of microfluidic chips by soft lithography.

Design of Microfluidic Channels

Manufacturing begins with the use of special software to design microfluidic channels, such as AUTOCAD, Illustrator, and LEDIT. After the design is completed, it is transferred to the photomask. The most common templates are chrome-plated glass plates or plastic films.

Use Soft Lithography to Make Microfluidic Molds

This step corresponds to transferring the microchannel pattern from the photomask to the real microchannel on the mold. The microchannel is "engraved" on the mold to form a replica, so that the channel can be engraved into the future microfluidic chip material.

• The resin is spread on the surface of the silicon wafer with the required thickness
• The resin part protected by the photomask with the microchannel pattern is exposed to ultraviolet rays.
• The mold is developed in a solvent that etches areas of the resin that are not exposed to ultraviolet light.
• A microfluidic mold with a resin replica pattern is obtained from the photomask. The mold is usually treated with silane to facilitate the release of the microfluidic device during the molding step.

Figure 1. Microfluidics fabrication by soft lithography. (A) Template with microchannel. (B) Pour polydimethylsiloxane (PDMS) prepolymer over the template; cure at 70 ℃ overnight. (C) Remove the patterned PDMS substrate from the template. (D) The flow channel is enclosed using a SiN window. (Zhang F, et al. 2018)

Microfluidic Chip Molding

• The mixture of PDMS and crosslinker is poured into a mold and heated at a high temperature.
• Once the PDMS has hardened, it can be removed from the mold. A copy of the microchannel appears on the PDMS block.
• Perforate the input and output of the microfluidic device to allow future experiments to inject fluid.
• Plasma is used to treat the surface of the PDMS block with microchannels and the glass slide.
• Through plasma treatment, PDMS and glass bonding can seal the microfluidic chip.
• Use the microfluidic tube to connect the chip to the microfluidic reservoir and pump.
• Integration of complex functions: Some microfluidic chips also have other features that require the integration of electrodes, nanostructures or surface functionalization. This type of additional step usually uses standard techniques of micro and nano technology, such as thin film deposition, plasma etching, and self-assembled monolayers.

Applications

• In the field of biomedicine, many medical tests can be integrated on a single chip to form a lab-on-a-chip.
• In cell biology research, microfluidic chips can easily manipulate individual cells and quickly change drugs, because microchannels have the same characteristic dimensions as biological cells.
• In protein crystallization, microfluidic devices allow a large number of crystallization conditions, such as temperature, pH, and humidity, to be generated on a single chip.
• There are many other areas: drug screening, glucose testing, chemical microreactors, etc.

Figure 2. Odontoblast microfluidic chip. (Miu L, et al. 2019)

References

• Miu L, et al. (2019). “Microfluidic Chip for Odontoblasts in Vitro.” ACS Biomaterials Science & Engineering. 5(9): 4844-4851.
• Zhang F, et al. (2018). “Chapter 9-Microfluidics and Interfacial Chemistry in the Atmosphere.” Physical Chemistry of Gas-Liquid Interfaces. Developments in Physical & Theoretical Chemistry: 245-270.

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