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Polydimethylsiloxane, often known as dimethicone or PDMS, is a polymer commonly used to manufacture and develop microfluidic chips and other microelectronics.
It is a siloxane-family mineral-organic polymer. Aside from microfluidics, engineers utilize it as food additives (E900), shampoo, and anti-foaming ingredients in drinks or lubricating lubricants.
Polydimethylsiloxane (liquid) combined with a cross-linking chemical is poured into a microstructured cast and heated to create an elastomeric duplicate of the mold to manufacture microfluidic devices.
Microelectronics and Microelectromechanical Systems rely heavily on bonding (MEMS). Bonding a pair of polydimethylsiloxane (PDMS) to construct tiny devices was particularly popular in microactuators and BioMEMS.
Microfluidics with PDMS Plasma Bonding
The bonding of microfluidic electronics from polydimethylsiloxane is vital for utilizing low-pressure plasma. When triggered with oxygen plasma, PDMS helps produce microfluidic components. When a PDMS component and a silicon-containing structure are plasma-activated, they can connect without needing glue.
When compared to employing an adhesive, this technique of bonding without a bond allows for the development of more minor features. An intriguing element is that, depending on the application, microfluidic pathways can be either hydrophilic with a reduced contact orientation or hydrophobic with a high contact angle.
Microchips with PDMS
PDMS is a critical raw material in microchips used in biomedical applications. Microchips of this type can be employed as monitoring devices for microdialysis, contactless conductivity detection, or genetic analysis, among other things. Printing PDMS patterns using air plasma help is a relatively new invention. This plasma technique directly produces such patterns on enormous areas of gold or PDMS surfaces.
Other researchers have proven using an atmospheric pressure plasma source to cover the surfaces of bonded dimethylsiloxane microchannels. On the other hand, PDMS-based microchips can be plasma-treated and coated with different polymers. The surface must be spotless for this purpose. Hence it is prepped using low or air-pressure plasma.
The use of PDMS in microchips and microfluidics immensely helped the microelectronic industry. Here are some of the benefits of PDMS in other sectors:
Certain plasma-treated PDMS polymer films may self-heal, which implies they can repair slight mechanical damage, such as tiny scratches or the impact of small objects. The same is valid for air plasma-treated polystyrene/SiO2-PDMS composites.
PDMS is widely used in conjunction with plasma surface modification (plasma cleaning, surface activation, or deposition) to deposit PDMS on microchip substrates or to encapsulate PDMS substrates with other elements. They do surface cleaning or interface activation regularly.
A hydrophilic surface is advantageous for several microelectronics operations because it improves surface wettability. It takes place when they use a printing technique in the fabrication process.
Coating a glass substrate with PDMS is advantageous for some applications, although this is difficult without thoroughly cleaning the glass and, perhaps, surface activation. They typically choose plasma therapy as a workhorse in both circumstances.
The key benefit is that plasma may be utilized for each production stage without changing the equipment. All that is required is changing the working gas composition. For window glasses, it is especially necessary for the PDMS surface layer to be hydrophobic with such a high contact angle to operate as a rain-repellent.
Engineers may develop a wide range of sensors using PDMS and plasma. One fascinating example is a composite sensor for monitoring physical activity consisting of plasma-etched carbon nanotubes and PDMS. Another possibility is stretchable PDMS sensors, which have a plasma-treated surface before being coated with silver and carbon nanotubes.
It reveals how plasma treatment of such membranes influences the flow of liquids and gases across them. Nanomembranes may now be produced from PDMS. The working gas combination has a considerable impact. For example, researchers have studied pure argon, argon, oxygen, and hydrogen mixtures.
Combinations of PDMS with metals such as copper, gold, or silver are especially appealing because they offer a wide range of beneficial uses. The PDMS is either coated with metal or vice versa. Plasma therapy for the PDMS is critical since researchers have shown it to minimize cracking and wrinkle development in thin metal films.
Oxygen plasma is also frequently used to deposit stable metal layers on PDMS and to create micro and nano-patterned metal surfaces with a large surface area.
Glass and Polymer PDMS Bonding
When the glass substrate is prepared correctly, PDMS can adhere to it permanently. This preparation frequently includes using a plasma cleaner or another type of plasma treatment to clean and activate the surface before the PDMS can conform to the substrate.
Because the PDMS must come into close touch with the glass, the cleanliness of the glass is critical.
According to several studies, including oxygen plasma allows the creation of chemical connections between glass and PDMS. This understanding of glass and other polymer bonds aided in creating various microelectronics and microfluidics technologies.
However, there are numerous situations where PDMS-polymer bonding is required. Poly(methacrylate), sometimes known as PMMA, is a well-known example. Similarly, plasma surface treatment can accomplish this – either PDMS attaching to PMMA or employing PDMS as an intermediary layer to produce PMMA-glass adhesion.
Plasma-treated polymers can also have significantly improved adhesion to plasma-treated PDMS. You may avoid adhesives altogether if such a technique is supplemented by heat treatment.
Why is PDMS used in the manufacture of microfluidic devices?
Many people chose PDMS to produce microfluidic chips for the following reasons:
- It is transparent at visible wavelengths (240 nM – 1100 nM), allowing for visual or microscopic examination of contents in microchannels.
- The PDMS adheres securely to glass or another PDMS layer with a simple plasma treatment. It enables the manufacture of multilayered PDMS devices to use the technical possibilities provided by glass substrates, such as metal deposition, oxide deposition, and surface functionalization.
- It exhibits a modest level of autofluorescence. It is biocompatible, but only under certain conditions.
- It is less costly than previously utilized materials.
- It is permeable to gas. It permits cell culture by regulating the quantity of gas passing through the material or filling dead-end channels. To balance atmospheric pressure, residual air bubbles under hydrostatic pressure may leak through PDMS.
- It is also simple to mold since it remains liquid at ambient temperature for several hours after being combined with the cross-linking agent. High-resolution structures may be set with PDMS. Creating designs as small as a few nanometers is feasible with some tweaking.
- It is malleable, allowing the incorporation of microfluidic valves utilizing PDMS micro-channel deformation, the simple coupling of leak-proof fluidic connections, and the detection of meager forces such as biomechanical interactions from cells.