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Surface and bulk modification of PDMS for microfluidic research and commercialization

Among all of the common microfabrication materials, PDMS is receiving ever-increasing attention in microfluidics research and industry. Its features such as gas permeability, optical transparency, biocompatibility, easy fabrication, and scalability make it an ideal choice for both microfluidic research and microfluidic industry. Although PDMS is the material of choice in a wide range of applications, researchers have developed methods to modify its properties to expand its applications even further.

Hydrophobic and hydrophilic coating of PDMS

A hydrophobic surface repels water and aqueous solutions and polar molecules while a hydrophilic surface has high surface energy, attracts water and polar molecules, and allows wetting of the surface. PDMS is a hydrophobic elastomer. Although it gains some level of hydrophilicity during bonding to the glass and exposure to plasma, it reverts to its hydrophobic state shortly after in a process called “hydrophobic recovery”. In hydrophobic recovery, the uncrosslinked polymeric chains of PDMS in the bulk ascend to the surface to reinstate the hydrophobicity. 

Although PDMS is coating in its nature, its hydrophobicity can be enhanced. For example, in droplet microfluidics, the uniformity and stability of the droplets can be enhanced for oil in water droplet generation. Hydrophobic modification of the PDMS is often conducted by coating the surface with a hydrophobic reagent.

PDMS is a biocompatible material and has been widely used in biology, and chemistry for which a hydrophilic surface can be advantageous. A hydrophilic coating is also necessary for droplet microfluidic experiments in which water in oil droplets need to be generated. Hydrophilic rendering of PDMS can be done via various methods which are classified into wet and dry techniques. Some of these approaches are listed here:

– Plasma treatment
– UV radiation
– Layer by layer deposition
– Chemical vapor deposition

uFluidix provides a proprietary coating to modify the surface of the microfluidic chips. Our hydrophobic and hydrophilic coatings are tailored to suit the microfluidic chips and are quite stable.

PDMS Surface treatment for cell culture

Most of the mammalian cells form a monolayer on the surface of the culture substrate and communicate with their surroundings. Communication of the cell with the environment leads to the adhesion of the cell to the substrate and the neighboring cells through specific surface proteins. Since synthetic cell culture plates do not contain the same proteins of the extracellular matrix, they should be tailored to ensure proper interaction of the cells with the substrates. Conventional cell culture substrates such as Polystyrene (PS) often undergo treatments to suit cell and tissue culture requirements.

Although many cells are compatible with PDMS and can adhere to its surface, its surface can be treated to further improve the adhesion. As mentioned above, although plasma and UV treatments change the surface hydrophilicity, they only work in the short term and are not appropriate for long-term use. Additionally, they do not necessarily enhance surface adhesion. Therefore, to create a favorable condition for cell culture, PDMS often undergoes a coating process to promote cellular adhesion and proliferation. This coating is either through charged molecules that interact with the surface or via extracellular matrix proteins. The following surface treatments for PDMS have been shown to be useful in for cell culture:

– Fetal bovine serum (FBS)
– Bovine serum albumin (BSA)
– Collagen
– Laminin
– Fibronectin
– Poly-D-lysine
– Polysaccharides
– Poly(ethylene oxide)

Further Reading