Currently, raw materials for microfluidic chips include monocrystalline silicon, glass, quartz, metal, polymethylmethacrylate (PMMA), polycarbonate (PC), polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), and polytetrafluoroethylene (PTFE).
Different materials have different impacts on the functionality of microfluidic chips, while the processing process limits the size of the chip channels and structures. Therefore, it is particularly important to select the appropriate materials and processing techniques according to the specific applications of microfluidic chips.
In terms of process fabrication, photolithography, molding, laser ablation, LIGA, thermocompression, soft lithography, and 3D printing are widely used in the processing and manufacturing of microfluidic chips.
1. Lithography
Photolithography is a key microfabrication process widely used in the manufacturing of microelectronic devices. Its basic principle is to make the material corrosion-resistant through the photoresist's sensitization reaction, so as to copy the pattern on the mask plate to the substrate, and finally form the microfluidic chip with engraved channels on the negative. This technology is currently one of the mainstream technologies and is often used in combination with other technologies in the actual microfluidic chip processing and fabrication process.
2. Molding method
The molding method is mainly applicable to microfluidic chips made of polymers such as PDMS. In the pre-processing stage, the required microstructures are first fabricated using photolithography and used directly as molds after development and drying.
The polymer material is then poured into the mold and heat-treated to cure it. Finally, the cured material is separated from the positive mold to obtain a microfluidic chip having a microstructure.
The selection of polymer and mold is crucial in this method, in which the sealing state of the polymer substrate and cover sheet is related to the surface roughness of the mold, and the flatness of the chip surface is directly affected by it.
3. Hot pressing
Thermocompression is a flexible and cost-effective microfluidic chip fabrication technique with high replication accuracy for feature sizes as small as 50 nm. Thermocompression typically consists of three process stages: heating, holding and cooling, and demolding.
Different processing parameters have an important effect on the replication quality and surface roughness of thermo-pressurized microfluidic chips, with processing temperature, pressure, time and demolding temperature being the main evaluation factors.
Relevant experimental results show that enhanced PDMS molds exhibit excellent performance in the hot pressing of micropatterned cyclic olefin polymer sheets, enabling highly reproducible features as small as 10 μm (drainage channel width).
4. LIGA technology
LIGA is a complex electroforming technology that takes its name from the acronyms of the German words lithographie, galanoformung and abformung.
The implementation process of LIGA technology includes the use of X-rays for deep lithography, electroforming to produce a delicate and tight mold, and then replicating to produce a large number of microstructures, and ultimately making microfluidic chips with different sizes such as high aspect ratios.
To date, LIGA technology has been applied to the fabrication of microdevices in a variety of polymers, metals, metal alloys and ceramic materials.
Researchers designed and fabricated a novel microneedle column discharge chip using LIGA technology and performed corona and glow discharges with applied voltages of -2.1 kV and -3.8 kV to study the sterilization effect of different discharge times
The results showed that the inactivation rate of E. coli and S. aureus reached 100% at the sterilization time of 120 and 180 seconds. This indicates that the microneedle column discharge chip manufactured by LIGA technology meets the sterilization requirements and can be used as a portable sterilizer.
5. Laser ablation technology
As an emerging non-contact microfabrication technology, laser ablation utilizes a mask or directly through computer software to design data and graphics, precisely controlling the position of the laser in the X-Y direction, to create micro-channels and micro-holes of various shapes and sizes in materials such as plastics, metals and ceramics.
To fabricate the microchannels, a copper foil is first used to create the required photolithography mask, and then a UV laser beam is passed through the micro objective and the photolithography mask. Using the polymer material as a substrate, the focused laser energy photodissociates the material, and the photolithography mask immobilizes the laser sputtering on a specific area of the substrate.
The depth of laser ablation is determined by the laser intensity and the number of laser pulses, and by adjusting these two parameters, microchannels of different depths can be formed on the substrate.
Finally, the laser ablation method is combined with a thermocompression bonding process to complete the preparation of microfluidic chips. Advantages of the laser ablation method include minimal damage to the microchannel structure of the chip, a channel wall with a large aspect ratio and perpendicularity, low dependence on the mask, and high flexibility.
6. Soft lithography
In the late 1990s, an innovative micrographics replication process began to emerge. This process uses elastic materials instead of the rigid stencils used in traditional lithography to create tiny shapes and structures, and is known as elastic lithography.
Compared with conventional lithography processes, elastography offers greater flexibility and adaptability. It is capable of creating fine, three-dimensional patterns on the surface of polymer materials and is applicable to a wide range of curved surfaces.
In addition, elastic lithography can change the surface chemistry of materials, which is important for surface modification of microfluidic chips.
The instrumentation required for this processing technology is relatively simple and can be implemented under routine experimental conditions, making soft lithography a relatively economical and easy-to-operate technology suitable for adoption by all types of laboratories.
7. 3D printing
3D printing, also known as additive manufacturing, is a process of building three-dimensional objects from computer-aided design (CAD) models. Under the control of a computer, raw materials are added layer by layer.
Using 3D printing technology, the manufacturing process of microfluidic chips has been significantly simplified, and there is a high degree of diversity in the choice of printing materials. Not only can a variety of polymer materials be used, but also biomaterials, which can be directly processed by 3D printing technology.
Researchers experimented with an integrated 3D molding method for microfluidic chips by dissolving channel molds of polyvinyl alcohol and high-impact polystyrene to fabricate the channels of the microfluidic chips.
The experimental results show that the stability of the process can be improved without the need for operations such as rapid alignment and material modification of the microchannels, in which the quality of the microchannels made of high-impact polystyrene is significantly better than that of the microchannels made of poly(vinyl alcohol).
Compared with other micro- and nanofabrication techniques, this method significantly reduces the technical requirements and production cost of microfluidic chip fabrication, which is of great significance for promoting the popularization and application development of microfluidic chip technology.
ZH_CN 
EN