Polydimethylsiloxane (PDMS) has natural gas permeability, physicochemical stability, and biocompatibility, and is a common basic material for micro/nano devices such as microfluidics and micro contact printing [2]. It also plays an important role in medical prosthesis implantation and repair. However, it has natural hydrophobicity, which makes it easy to adhere to non-polar substances and produce non-specific adsorption. Therefore, converting the hydrophobic surface of PDMS into hydrophilic can further expand its application fields.

At present, there are various methods for surface modification of PDMS, mainly including UV/ozone modification, plasma surface treatment, surface activator method, graft copolymerization method, and corona discharge instrument method. Plasma surface treatment is a commonly used PDMS modification method and also a frequently used chip packaging method, suitable for PDMS and glass, PDMS and PDMS, and belongs to irreversible packaging methods. This article will provide a detailed introduction to the principles, advantages, and disadvantages of plasma surface treatment methods.

PDMS thin film plasma surface treatment

Due to the hydrophobicity of PDMS film, the droplet spreading range on its surface is relatively small, and the liquid adsorption effect is not ideal. Therefore, plasma surface treatment equipment can be used to treat its surface, which can improve the surface energy of PDMS film and expand the spreading range of droplets.

Plasma is an ionized gaseous substance composed of atoms that have been partially electron deprived and electrons generated by ionization of atoms. Plasma surface treatment refers to the chemical and physical processes in which non polymeric gases act on the surface of polymer materials in a plasma state.

In the plasma surface treatment device, as the gas becomes thinner, the molecular spacing and free movement distance of molecules or ions also become longer. Under the action of an electric field, they collide to form a plasma. These ions have high activity and their energy is enough to break almost all chemical bonds, causing chemical reactions on any exposed surface. The oxygen radicals generated by the plasma are very active and easily react with hydrocarbons, causing non-polar siloxanes (SiCH3) on the PDMS surface to be converted into polar silanol (SiOH), which is a hydrophilic group (Figure 1.1).

Figure 1.1 PDMS plasma surface treatment

The hydrophilic angle is a measure of the wetting degree of a liquid on the surface of an object, which refers to the angle between the tangent of the gas-liquid interface at the intersection of the gas, liquid, and solid phases and the boundary between the liquid and the solid-liquid interface.

The contact angle of PDMS film before and after plasma treatment was measured using a contact angle measuring instrument, and the change in wettability of PDMS film can be obtained. The measurement results are shown in Figure 1.2. The contact angle of untreated PDMS film is about 116 °, indicating hydrophobicity. Deionized water droplets completely spread on the treated PDMS film, indicating hydrophilicity.

Figure 2.2 Comparison of Wetting Properties of PDMS Thin Films Before and After Plasma Surface Treatment

However, the hydrophilicity of the PDMS film surface treated with plasma surface treatment technology is short-lived. After being left in air for several hours, the surface wettability gradually returns from hydrophilicity to hydrophobicity. This method is environmentally friendly, causes minimal damage to the biological properties of the sample, and has a promising application prospect.