In the fabrication of microfluidic chips, COC (cyclic olefin co-polymer) is a widely used material. COC however, is a plastic, and therefore in its original form has typically hydrophobic characteristics. Because of this, hydrophilic coating of the material is required to optimise the microfluidic flow properties of the chip. Without coating, the flow of the liquid is very slow or even impossible. Coating facilitates fast and reliable filling of the microfluidic channels.
Recently, a new type of hydrophilic coating has been developed. In this process, the surface is modified using a PEG-like coating. Both exposed glass and polymer surfaces (COC, polystyrene, polycarbonate, PMMA) can be coated with this substance. The coating is applied using the plasma enhanced chemical vapor deposition (PECVD) technique. In this method, the surface is covered with a very thin film that transforms from a gas state (vapor) to a solid state.
Coating provides the surface with more hydrophilic qualities. This can be determined by measuring the contact angle of a water drop and its underlying surface. High contact angles are seen in hydrophobic materials, because a droplet stays very round in contact with this material. On a hydrophilic surface, the droplet ‘spreads’ out on the material and therefore has lower contact angles. Using this specific coating, contact angles can get as low as 50° to 55°.
Another characteristic of a surface modified with this PEG-like coating is a decrease in fouling of the surface when exposed to various solutions. Polymers in their original (uncoated, hydrophobic) state, are subject to a lot of fouling of biological material. For glass materials, fouling through electrostatic interactions is a concern. Fouling could interfere with the function of the device, which in turn might affect test results. Coating decreases the unwanted adsorption of biological material, but can’t completely prevent it. The anti-fouling capabilities of the modification have been tested using BSA (Bovine Serum Albumin) as the fouling agent. Adsorption of BSA was reduced to around 20% when compared to its adsorption on the original polymer surface.
Because of the method in which the coating is applied, it can only be used on surfaces exposed on the outside. This means the modification can’t take place inside an enclosed microfluidic structure, and therefore needs to be performed before bonding. The coating is compatible with the thermal bonding process that is generally used for COC devices. In the tests performed thus far, the hydrophilic and anti-fouling properties of the coating weren’t affected by the bonding procedure.
This technique is still being tested and optimised, for example to find out if it can also be applied to other polymer materials.