Bonding of PDMS to a wide range of materials

1. Microfluidic chips

As a carrier of microfluidic technology, microfluidic chip, also known as lab-on-a-chip or micrototal analytical system (μTAS), can integrate the sample input, reaction and analytical detection units on a chip only a few square centimeters in size, which can meet the diversified analytical detection needs.

2. SAW microfluidic devices

The basic principle of surface acoustic wave (SAW) microfluidics is to use a piezoelectric material, lithium niobate (LiNbO3), as a substrate, deposit a pair of fork-finger electrodes on its surface, and apply a radio frequency signal to the electrodes to generate acoustic waves that propagate along the surface of the piezoelectric substrate.

Utilizing the acoustic radiation force generated by surface acoustic waves, it can be used to control micro-liquids or micro-particles to separate, extract or enrich specific samples for efficient detection.

3. Microfluidic chip materials and bonding methods

Bonding is defined as a technology in which two identical or different materials are brought into contact with each other by some specific process, and at the interface of the contact, the materials are interconnected at the molecular level through van der Waals forces, chemical bonds or even atomic forces, so that the two materials become one.

Early microfluidic chip materials are mainly silicon and glass, the reason is that the material is chemically stable, the optical properties of the glass is excellent, and the processing of silicon-based materials has been in the semiconductor field to obtain a more mature development.

The main bonding methods for microfluidic chips for this material are thermal bonding, anodic bonding and surface-modified bonding.

The basic principle of thermal bonding is to subject the device to high temperature and high pressure, which is generally applicable to the bonding of homogeneous materials. At high temperatures, the molecular motion within the material becomes active, and molecular or atomic diffusion occurs at the bonding interface, so that the two materials can become one when cooled to room temperature.

Other bonding technologies, such as ultrasonic bonding, laser bonding, etc. also apply high temperatures to the bonding interface in their own unique ways, causing the local material to transform into a molten or glassy state to realize bonding, and the core principle is similar to that of thermal bonding.

Anodic bonding is currently the most commonly used bonding technique for silicon and glass materials, in which a silicon wafer is kept in close proximity to the glass, in which the glass is connected to the negative terminal of the power supply, and the silicon is connected to the positive terminal, and a voltage of 500~1000V is applied. Under the action of the electric field, Na+ inside the glass moves toward the cathode and O2- moves toward the bonding interface, combining with Si atoms to form Si-O-Si bonds to realize bonding.

Surface-modified bonding is a process that promotes diffusion or chemical bonding at bonded interfaces through various surface modification methods, but bonds cannot be formed by surface modification alone, and high-temperature and high-pressure conditions, such as pressurized annealing, are also required.

4. Theory of plasma surface modification

Plasma is produced by ionizing a specific gas under the action of an electric field from a radio frequency electrode, transforming it into a complex substance containing positive and negative ions, free electrons, neutral particles, and a variety of reactive chemical groups. By using plasma for surface modification, both physical and chemical actions occur on the surface of the material.

1) Physical effect of plasma on material surface

In the process of plasma surface modification, high-energy active particles bombard and flush the surface of the material, so that the adsorption of impurities and pollutants on the surface of the decomposition of surface impurities can be removed, can play a role in cleaning the surface.

2) Chemical effect of plasma on material surface

Under the effect of plasma bombardment, the chemical bonds within the surface molecules will be broken, and the free radicals and reactive groups in the plasma will be accessed to the surface of the material to activate the surface of the material, and the generation of polar groups will make the surface of the material become extremely hydrophilic.

Therefore, plasma surface modification is a process in which physical and chemical effects assist and promote each other, with chemical effects being the main influence.

Depending on the effect on the surface of the material, plasma modification has three main application scenarios, namely plasma cleaning, plasma activation and plasma etching.

5. Plasma-modified bonding

The basic principle of plasma-modified bonding is that when plasma-activated material surfaces come into contact with each other, the reactive groups introduced on the surface of the material undergo a dehydration reaction and form chemical bonds with each other, thus realizing bonding between the two materials.

The gases used to excite plasma are divided into two main categories, non-reactive and reactive gases.

The non-reactive gases are mainly inert gases such as Ar, He and so on. This kind of gas generally will not occur with the material surface chemical reaction, so the use of this kind of gas plasma treatment of the main purpose of cleaning the material surface impurities, for the subsequent process to provide good surface conditions, generally as a whole set of processes in the pre-treatment process.

The reactive gases are mainly high-purity oxygen, nitrogen and carbon tetrafluoride. The use of plasma generated by this gas to modify the surface of the material, in addition to physical effects, more often than not, a chemical reaction with the surface components of the material, the introduction of chemical groups.

If the introduced chemical groups react with the surface components of the material so that the surface material is removed by the reaction, it is a plasma etching process. If the introduced chemical group reacts with the surface structure to generate a layer of reactive groups grafted on the surface, it is a plasma modification activation process.

For PDMS materials, the main gases for plasma surface modification are oxygen and nitrogen, of which the most applied gas is oxygen.

6. PDMS bonding with lithium niobate plasma surface modification

1) Chemical Mechanism of Plasma Modified Bonding

Plasma modification causes a change in the surface properties of the material, a process that dramatically improves wettability, making the surface extremely hydrophilic and promoting the adsorption and wetting of water molecules on the surface.

Thanks to the reactive chemical groups introduced by plasma excitation, the PDMS and lithium niobate surfaces are activated but not etched by particle washout or group reactions.

When the modification is complete and the two materials are in contact with each other, the reactive groups at the bonding interface will react with each other to form covalent bonds and realize the connection at the molecular level.

2) Bonding step of PDMS with lithium niobate plasma surface modification

Plasma surface modification: First, the surface of PDMS with lithium niobate is plasma treated to introduce reactive chemical groups. The plasma modification can be carried out at room temperature, avoiding the problems of thermal stress and deformation that may be caused by heating treatment.

Selection of modifying gases: According to different needs, different gases can be selected for plasma modification, such as air, oxygen and nitrogen. Among them, the plasma modification excited by high purity gases (e.g. oxygen and nitrogen) can significantly improve the bond strength.

Modification parameter setting: Modification parameters include RF power, gas flow rate and modification time.

Bonding process: After the modification is complete, keeping the PDMS in contact with the lithium niobate surface, bonds can be formed within 5 minutes.

Secondary plasma modification: If there is a need to further improve bond strength, secondary plasma modification can be used. For example, oxygen plasma modification is used first, and then nitrogen plasma modification is used to form oxygen-nitrogen secondary plasma modification.