Custom fabrication of PDMS microfluidic chips
1. PDMS Material
PDMS, short for polydimethylsiloxane, is a common organic polymer used in the prototyping of microfluidic chips. It is inherently elastic, transparent, breathable, and chemically inert. Suitable for chip fabrication related to cell culture, drug screening, and cell capture, the material itself is hydrophobic and can be surface-modified chemically or physically.
Advantages of PDMS Material:
• Optical Transparency: PDMS's excellent optical transparency is beneficial for imaging and microscopy. For example, it allows for real-time imaging and monitoring of microfluidic cell culture chambers.
• Low Cost and Ease of Fabrication: The fabrication method for PDMS microfluidic chips is very standard and straightforward, as described below. Furthermore, unlike thermoplastics, PDMS can be easily bonded to seal and form closed channels.
• High Resolution and Fine Features: PDMS chips are typically fabricated by casting a liquid premix onto a master mold. This casting process allows even the finest features (down to a few micrometers) to be imprinted on PDMS across a wide aspect ratio range.
• Flexibility: Its applications are growing with the advancement of microfluidics research. For example, researchers in flexible electronics have begun using PDMS microchips due to their flexibility.
• Bioinertness: PDMS is a bioinert material, ensuring its neutrality in biological applications and making it a suitable choice for cell culture substrates.
• Adjustability: PDMS has a relatively low elastic modulus, which can be easily adjusted by changing the curing agent ratio, providing a wide range of material stiffness. There are also methods to tune the electrical and thermal properties of PDMS microfluidic chips.
• Gas permeability: PDMS is a gas-permeable material. Unlike PMMA and PC, its oxygen diffusion coefficient is approximately 2000-4000 µm²/s, and its CO2 diffusion coefficient is approximately 1000 µm²/s. This gives PDMS an advantage in long-term cell culture. However, this permeability can also lead to the non-specific absorption of hydrophobic small molecules into the microfluidic channels.
Commonly Used PDMS Materials
The two most commonly used PDMS materials are Momentive's PDMS RTV-615 and Dow Corning's PDMS Sylgard 184. The composition of these two materials is currently a trade secret and not publicly known. Based on years of experience, we can offer some suggestions on how to choose PDMS:
Momentive PDMS RTV-615:
1) The first choice for fabricating microvalves;
2) Robust, with strong bonding, suitable for multilayer bonded chips;
3) Not resistant to dirt, easily becomes soiled.
Dow Corning PDMS Sylgard 184
1) Relatively clean
2) Suitable for single-layer bonding, not suitable for multi-layer bonding
3) Primarily used in cell culture and human organ-on-a-chip applications
2. PDMS Chip Fabrication
PDMS chip fabrication utilizes soft lithography to replicate reverse structures from chip molds (silicon molds, SU8 molds, PFCT molds, acrylic molds, metal molds). This method can easily replicate structures with feature sizes smaller than 0.1µm. Simultaneously, through plasma surface treatment, PDMS can bond with itself, glass, and silicon to form closed flow channels.
2.1 Resin Mixing
PDMS adhesive is a two-component mixture, consisting of component A and component B. Using an electronic balance, the weight ratio is 10:(0.9~1). A higher ratio of component A to component B results in a softer, more solidified adhesive.
2.2 Homogenization and Defoaming
Thoroughly mix component A and component B, place in a vacuum aerator, and allow to stand in a vacuum environment for approximately 30 minutes until all air bubbles are removed. Remove and set aside for use.
2.3 Casting Mold
PDMS molding can utilize molds such as silicone molds, SU8 molds, Perfect molds, acrylic molds, and metal molds. After cleaning the mold, pour the PDMS into it and place it in a constant temperature drying oven at 85℃ for 30 to 40 minutes.
2.4 Removal
After the mold has cooled slightly, separate the cured microfluidic PDMS chip from the mold, taking care not to damage the mold or the microfluidic PDMS chip structure.
2.5 Cutting
Use a dedicated PDMS chip cutter or a utility knife to cut along the chip's edge, keeping the edges neat.
2.6 Drilling A PDMS chip drill is used to drill holes in the microfluidic PDMS chip, and the chip is cleaned with adhesive tape.
2.7 Bonding A plasma cleaner is used to treat the substrate (PDMS, glass) and the PDMS chip surface. The bonding power is 80~90W, and the time is 20~30s (the RF power and time may vary depending on the plasma treatment). After removal, bonding is performed within 30 seconds, and the chip is placed in an 80°C oven for about 30 minutes.