As the micro/nanofabrication methods develop, the applications also expand demanding a wider range of materials for microfabrication of microfluidic chips. For many years the main material of choice in microfluidic experiments has been PDMS. Although PDMS is still the main and most popular material for microfabrication, gradually other materials such as PMMA started to be used in cases where PDMS could not meet the requirements. Here, we provide a brief introduction about some commonly used materials in microfabrication. Let’s start with PDMS.
Polydimethylsiloxane (PDMS) is a silicon-based organic polymer that is the most commonly used material for the fabrication of microfluidic chips.
PDMS is hydrophobic meaning that it naturally repels aqueous solutions. However, various methods can be used to coat and functionalize the PDMS surface to make it hydrophilic or more hydrophobic. The coatings are normally stable unless in harsh conditions.
PDMS offers several advantages that make it the main material of choice for microfluidic applications. Some of the advantages are listed in the following.
• Optical Transparency: The superior optical transparency of PDMS is advantageous for imaging and microscopy. For example, it allows for real-time imaging and monitoring of a microfluidic cell culture chamber.
• Low cost and easy fabrication: The methods for fabrication of PDMS microfluidic chips are quite standard and straight forward as mentioned below. Additionally, as opposed to thermoplastics, PDMS can easily be bonded to be sealed and form a closed channel.
• High resolution and fine features: The PDMS chips are usually made by casting the liquid premix over a master mold. This casting process enables even the finest features (down to few microns) to be imprinted on the PDMS with a wide range of aspect ratios.
• Flexibility: As the microfluidics research grows, its applications also grow. Flexible electronics researchers, for example, have started to use PDMS microchip due to its flexibility.
• Bio-inertia: PDMS is a bioinert material that ensures its neutrality in biological applications making it a suitable choice as a cell culture substrate.
• Tunability: The elastic modulus of the PDMS is relatively low and is easily tunable by adjusting the curing agent ratio offering a wide range of material stiffnesses. There are also methods for tuning the electrical and thermal properties of PDMS microfluidic chips.
• Gas permeability: PDMS is a gas permeable material as opposed to PMMA, and PC with a diffusion coefficient of ~2000-4000µm²/s for Oxygen and ~1000µm²/s for CO2, respectively. This gives PDMS an advantage in long term cell cultures. However, this permeation can also cause non-specific absorption of small hydrophobic molecules into the channels.
To make a microfluidic chip, first a master mold should be made. The master mold is often a silicon wafer or a glass substrate that contains the negative replica of the microfluidic chip design.
1- The mixture of PDMS and the curing agent (often in 10:1 ratio) is cast over the master mold.
2- The master mold covered with the PDMS mixture goes to the oven so the PDMS gets cured and solidifies.
3- The cured PDMS over which the microchannels are engraved gets peeled off from the master mold.
4- The part then needs to be bonded to a substrate to form a closed channel. Since PDMS parts have rubber-like mechanical properties, they are not stiff enough for handling especially when the parts are less than 1 mm (0.04”) thick. Therefore, these PDMS parts are often bonded to a glass slide or thin glass cover slide, or even custom size glass substrates. Sufficient bonding strength between PDMS and glass is needed for continuous flow applications, otherwise, the fluid would leak out of the microfluidic channels. uFluidix has special expertise in bonding and sealing PDMS parts to glass and a variety of other substrates. Here, you see the picture of a PDMS chip bonded to the glass. In some cases, the substrate can be a thin layer of PDMS instead of a glass slide.
5- The microfluidic chip is now ready for post-modification processes such as surface functionalization if required.
Although PDMS is still by far the material for microfabrication, there are conditions under which other materials can be beneficial. Thermoplastics such as PMMA, Polycarbonate, and Polystyrene are probably the second most used materials used in microfabrication. Other materials that have been used less frequently include Poly(ethylene glycol) diacrylate (PEGDA), Cyclic Olefin Copolymer (COP), and Cyclic Olefin Polymer (COP).
Thermoplastics are plastic polymers that become soft by heating and harden by cooling. This feature can be used to reshape them into desired forms such as a microfluidic chip. They are more rigid and less gas permeable compared to PDMS. PMMA, PC, and PS are some of the more common thermoplastics in the microfluidic research.
Thermoplastics microfluidic chips require a thermoforming method for fabrication. Thermoforming techniques such as hot embossing and microinjection molding are reasonable for commercialization and mass production. However, for rapid prototyping and small scale microfabrication, they can be costly and not suitable. Briefly, thermoforming involves heating a thermoplastic sheet to a pliable forming temperature and pressing it against a template. The template can be any material that does not deform or react when heated and pressed against the thermoplastic sheet. Silicon, metal, or PDMS templates are among those that have been used for this purpose. Other methods of fabrication include CNC machining and laser ablation. These methods are better options for prototyping microfluidic chips and are not ideal for mass production.
The main challenge of working with is bonding to other substrates to seal the microfluidic chips. The thermoplastics are much more difficult to seal and are typically bonded using a thermal or glue-assisted bonding technique. Additionally, they are not permeable to gases as PDMS, therefore are not suitable for long-term cell culture.
One of the widely used thermoplastic materials is PMMA (Poly(methyl methacrylate)) which is commonly known as acrylic or plexiglass. It is an optically transparent thermoplastic that has been used for the fabrication of microfluidic chips using a variety of methods including milling, hot embossing, micromachining, laser ablation, microinjection molding, etc. PMMA microchips are known to be the least hydrophobic material used in microfabrication. They are more rigid than PDMS and are suitable for mass production. However, bonding of PMMA is more challenging compared to the straight forward bonding of PDMS which makes it rather inconvenient for rapid prototyping. Methods of bonding include thermal bonding, solvent bonding, polymerization bonding, glue-assisted bonding, microwave bonding, etc.
The PMMA chips are less likely to absorb small hydrophobic biomolecules. However, their rigidity limits their applications. For example, PMMA is not suitable for making microfluidic valves. Moreover, due to its fabrication methods, the resolution of PMMA microfluidic chips is not as high as PDMS chips.
Polycarbonate is a tough thermoplastic which is optically transparent and highly durable. It has a higher glass transition temperature (~145oC) compared to PMMA so it is a better choice in applications where higher temperatures are required for example for polymerase chain reaction (PCR) microfluidic chips. The high temperature required for bonding of the PC, however, can damage the microchannels.
Polystyrene (PS) is an optically transparent and rigid material that biocompatible and commonly used for cell culture dishes and microtiter plates. It is hydrophobic but can be rendered hydrophilic through various chemical and physical means. Although it is not as widespread as the abovementioned materials in microfluidic chips, it has been used for a variety of bioapplications such as DNA synthesizers.