PDMS Plasma Bonding & Microfluidics

PDMS Bonding & Microfluidics Functional Principles

PDMS is a castable inert silicone that is used to take on the shape or form of the microfluidic channels and structures of the mold it is cast into. These casted structures are formed from raised shapes produced in the bottom of a mold. When the PDMS is cast in the mold and removed, the shapes in the bottom of the mold are transferred into the casted part.

These casted parts are then placed in oxygen plasma with the glass or silicon containing mating parts. When the parts are activated with oxygen plasma and then placed together with precision these parts will bond. The bond characteristics are covalent, meaning the PDMS part and the mating part share electrons producing a very strong bond.

When bonding PDMS devices it is important to determine if you require equipment for a simple proof of ability or tooling to optimize and repeat the process in volume. A wide range of products are available to bond PDMS. Understanding the subtle differences is important to ensure your application of plasma treatment is successful.

PDMS Bonding & Microfluidics Industries & Uses

The application of producing microfluidic devices is relatively new technology with applications in industries performing process evaluation of fluid chemistries. The ability to reduce the size of the fluid testing hardware and the volume of the fluid required to test through the application of microfluidics are the key criteria driving the industry. These key variables determine the need of the medical industry, biotech and others to continually apply these design aspects to their individual needs.

Here is a more exhaustive list of industries that use PDMS, very often in combination with plasma surface treatment or plasma coating technologies:

• Biomedicine: The number of PDMS related applications in this field is enormous. It ranges from microfluidic devices over micro channels to high-tech surfaces, which can show enhanced cytocompatibility and decreased genotoxicity. Depending on the envisioned usage the kind of plasma treatment is either surface modification (coating or activation) or even plasma enhanced ion implantation. The plasma treatment of a PDMS surface with oxygen plasma can also improve the biocompatibility of the substrate.
• Microelectronics: PDMS in combination with plasma surface treatment (plasma cleaning, surface activation or deposition) is frequently used to either deposit PDMS on microchip substrates or to coat PDMS substrates with other materials. Furthermore, also surface cleaning or surface activation is regularly performed. For some processes that are relevant to microelectronics a hydrophilic surface is preferable, since it enhances the surface wettability. This comes into play when the fabrication process involves some form of printing technique.
• Sensors: There is a large variety of sensors that can be produced with PDMS and plasma. One interesting example is a composite of a plasma etched PDMS and carbon nanotube composite sensor for measuring physical activity. Another one is stretchable PDMS sensors, for which the surface is first plasma treated and then coated with silver and carbon nanotubes.
• Glass manufacturing: For some applications it is useful to coat a glass substrate with PDMS but this is not so easy without extensive cleaning of the glass and, possibly, surface activation. For both of these cases plasma treatment is usually chosen as a workhorse. The main advantage being that plasma can be used for each process step without changing the equipment. One has just to adjust the composition of the working gas(es). It is especially important for window glasses that the PDMS surface layer is hydrophobic with a high contact angle so that it can act as a rain repellant.
• Textiles: The reason of using of PDMS in this field is either to make fabrics more durable, hydrophobic or to produces smart materials for different applications. Different fibers have been plasma treated and coated with PDMS. Some of those fibers are used for administering different substances, such as caffeine over a long period of time. Others are expected to become oleophobic or superhydrophobic and self-cleaning.
• Nanofiltration: Nowadays nano membranes can be manufactured from PDMS and it has shown that plasma treatment of such membranes have an influence on the transport of liquids and gasses through them. This influence depends strongly on the type of working gas mixture that is used to generate the plasma. For example, mixtures of argon and hydrogen, argon and oxygen or just pure argon have been studied.
• Self-healing coatings: Certain plasma treated PDMS polymer films have the ability of self-healing, which means that they can cure themselves of smaller mechanical damages such as minor scratches or the impact of small objects. The same holds for polystyrene/SiO2-PDMS composites that were treated with air plasma.

PDMS Material Properties

PDMS stands for polydimethylsiloxane, which is also known as dimethylsiloxane or dimethicone. It is an organic polymer that is based on silicon, which makes it also a member of the so-called organosilicon compounds. The properties of PDMS are quite remarkable, since it is optically clear, chemically inert and, thus, non-flammable, non-toxic and biocompatible. These properties enable a wide range of applications. It is used in the food industry to suppress foaming of liquids. In the medical field it is applied for manufacturing contact lenses or medical devices. PDMS is also used in cosmetics as an important ingredient in hair products or skin lotions. It can also be used as a remedy for head lice or fleas on pets. Depending on the usage PDMS coatings can either be obtained with sol-gel methods or via plasma deposition. For the latter, both atmospheric and low-pressure plasma can be utilized. After a plasma treatment of a PDMS surface the polydimethylsiloxane can be either hydrophobic or hydrophilic, depending on the plasma parameters. The most important aspect is the composition of the feed gas.

PDMS for Microchips

PDMS is an important raw material in microchips for biomedical applications. Such microchips can be used, for example, as monitoring devices for micro dialysis, contactless conductivity detection or genetic analysis. A quite recent development is the printing of PDMS patterns with air plasma assistance. This plasma treatment allows a simple fabrication of such patterns directly on large area gold or PDMS substrates. Other researchers have demonstrated the possibility of surface coating of bonded dimethylsiloxane microchannels with an atmospheric pressure plasma source. On the other hand, there are also PDMS based microchips, which can be plasma treated and coated with other polymers. For this purpose the surface has to be extremely clean and is, thus, prepared with low or atmospheric pressure plasma (mostly with oxygen).

PDMS Bonding on Glass and Polymers

PDMS can stick to glass even permanently when the glass substrate is correctly prepared. This preparation often includes a plasma cleaner or another form of plasma treatment, which cleans and activates the surface before the PDMS can stick to the substrate. The cleanliness of the glass is of utmost importance because the PDMS has to come in direct contact with the glass. Some research suggests that the additional use of oxygen plasma enables the formation of chemical bonds between glass and PDMS. However, there are many applications where a bonding between PDMS and some polymer is needed. A very prominent example is poly(methacrylate) or PMMA. Again, plasma surface treatment can achieve exactly this – either PDMS bonding to PMMA or even using PDMS as an intermediate layer to achieve adhesion between PMMA and glass. Plasma-treated polymers can also exhibit a greatly enhanced adhesion with plasma-treated PDMS. If such a process is also accompanied by heat treatment, one can even renounce adhesives entirely.

PDMS-Metal Composites

Combinations of PDMS and metals, such as copper, gold or silver are of particular interest, since they enable a broad range of useful applications. Either the PDMS is coated with metal or vice versa. Plasma treatment of the PDMS is very important as it has been proven that it can reduce cracking and the formation of wrinkles in thin metal films. Oxygen plasma is also commonly used to deposit stable metal layers on PDMS, as well as obtaining micro and nano patterned metal surfaces with large surface area.