Polymer-based chips appeared a few years after the advent of silicon/glass chips. A wide variety of polymers provide great flexibility in choosing suitable materials with specific properties and become an attractive alternative to glass and silicon.
Compared with inorganic materials, polymers with good overall physical properties are easy to obtain and inexpensive, and are currently the most commonly used microchip materials. According to physical properties, polymers can be divided into elastomers, thermoplastics and thermosetting plastics. The first two groups are used more as materials for microfluidic chips.
Elastomers are amorphous polymers that remain above their glass transition temperature and are usually composed of intertwined, cross-linked polymer chains. When an external force is applied, it can be stretched or compressed, and when the external force is withdrawn, it can return to its original state. It exhibits weaker intermolecular forces, and most of the time has lower Young's modulus and higher failure strain.
PDMS Microfluidic Chips
The most popular elastomer in microfluidics is polydimethylsiloxane (PDMS), which is the most commonly used microfluidic material in research laboratories. It is easy to manufacture and can be firmly combined with glass and PDMS substrates while having good optical transparency and elasticity. PDMS also has other advantages, such as biocompatibility, gas permeability, and optical transparency. Therefore, it has been widely used in microfluidic rapid prototyping.
Device molds are formed by traditional machining or photolithography methods, and the microstructure of PDMS is cast and cured on these molds. It is also possible to create complex microfluidic designs by stacking multiple layers.
- Low modulus of elasticity, suitable for the manufacture of valves and pumps.
- Its gas permeability may be beneficial for O2 and CO2 transportation in cell research.
- Due to its inherent hydrophobicity, PDMS is susceptible to non-specific adsorption and penetration of hydrophobic molecules. Chemical modification can solve these problems.
Figure 1. A poly(dimethylsiloxane) microfluidic chip. (Ou J. J, et al. 2008)
Although PDMS can be used for rapid prototyping, soft lithography is not suitable for mass production. Repeatability is a challenge. Hydrophobic analytes will adsorb to the PDMS surface and may interfere with the analysis. Surface treatment can be performed to alleviate the problem, but it can also be time-consuming and the treatment can lose efficiency. It is not suitable for high-pressure operation and may easily leak under high pressure.
Thermosetting Polyester Microfluidic Chips
Thermosetting polyester (TPE) is a thermally-initiated material made by polymerization of polyester and styrene through ultraviolet light or heating. It is one of the most commonly used thermosetting materials in microfluidics. TPE is a transparent material in visible light, and its elastic modulus is higher than PDMS but lower than typical thermoset plastics. Such hydrophobic materials require surface modification through buffer additives or chemical reactions.
- Use photopolymerization for 3D micromachining.
- Chlorinated solvents can dissolve TPE, although they can resist many other solvents.
- They do not swell with certain solvents and are gas impermeable, which makes them insufficient for long-term cell culture.
Thermoplastics are materials that can be reshaped multiple times by reaching the glass transition temperature (Tg), which are usually manufactured by thermoforming so that thousands of replicas can be produced at a high rate and low cost. It is a transparent material and can be analyzed microscopically. Thermoplastics are resistant to penetration by small molecules and are harder than elastomers.
Depending on their application, their surface can be modified by dynamic coating or surface grafting. Covalently modified surfaces are generally more stable to thermoplastics than PDMS.
Polystyrene Microfluidic Chip
Polystyrene (PS) is optically transparent, biocompatible, inert, and rigid, and its surface is easy to functionalize. PS is suitable for large-scale manufacturing processes, so it can facilitate the conversion of currently used manufacturing processes into micro-systems.
Polycarbonate Microfluidic Chip
Polycarbonate (PC) is a durable material produced by the polymerization of bisphenol A and phosgene. It has visible light transparency and a very high glass transition temperature, so it is very suitable for DNA thermal cycling applications. Other advantages are low cost, high impact resistance, low moisture absorption and good processing performance.
Polymethyl Methacrylate Microfluidic Chip
PMMA is an inexpensive polymer and a commonly used material in microfluidic systems. Due to its low price, rigid mechanical properties, excellent optical transparency and compatibility with electrophoresis, it is especially suitable for disposable microfluidic chips.
Polyethylene Glycol Diacrylate Microfluidic Chip
Polyethylene glycol diacrylate (PEGDA) is a material that has certain characteristics with PDMS. However, it shows less non-specific adsorption and is more resistant to penetration by small hydrophobic molecules. The polymerization reaction can occur rapidly at room temperature and does not require much energy.
Figure 2. PMMA surface modification for surfactant-free real-time toxicity assay on droplet microfluidic platform. (Ortiz R, et al. 2017)
- Ou J. J, et al. (2008). "Integration of Dialysis Membranes into a Poly(dimethylsiloxane) Microfluidic Chip for Isoelectric Focusing of Proteins Using Whole-Channel Imaging Detection." Anal. Chem. 80(19): 7401-7407.
- Ortiz R, et al. (2017). "Poly(methyl methacrylate) Surface Modification for Surfactant-Free Real-Time Toxicity Assay on Droplet Microfluidic Platform." ACS Applied Materials & Interfaces. 9(15): 13801-13811.
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