Sofia Martinez
Polydimethylsiloxane (PDMS) is a silicone polymer with a wide variety of uses, from cosmetics to industrial lubrication. It is optically clear and, in general, inert, non-toxic, and non-flammable. PDMS is particularly known for its unusual rheological (or flow) properties. PDMS is viscoelastic, meaning that at long flow times (or high temperatures), it acts like a viscous liquid, similar to honey. PDMS has been found to irreversibly bond with untreated plastics and metals for microfluidics applications. This is achieved by functionalizing the PDMS surface with (3-aminopropyl) triethoxysilane (APTES) and the plastic surface with (3-glycidoxypropyl) triethoxysilane (GPTES), creating a cross-link between the amines on the PDMS-APTES surface and the epoxy groups on the plastic-GPTES surface.
| Characteristics | Values |
|---|---|
| PDMS irreversible bonding to plastics | PDMS surface functionalized with (3-aminopropyl) triethoxysilane (APTES) and the plastic surface with (3-glycidoxypropyl) triethoxysilane (GPTES) |
| PDMS bonding strength | Comparable to plasma oxygen treatment |
| PDMS biocompatibility | Widely used in microfluidics due to desirable characteristics |
| PDMS surface energy | Low |
| PDMS hydrophobicity | Polar solvents, such as water, struggle to wet the PDMS |
| PDMS oxidation | Produces silanol terminations (SiOH) on its surface |
| PDMS with water, glycerol, methanol, or ethanol | Does not allow infiltration and consecutive deformation |
| PDMS deformation | Deforms and swells in the presence of diisopropylamine, chloroform, and ether |
| PDMS-plastic covalent bonding | Exploiting the amine-epoxy cross-link |
| PDMS-PSA tensile strength | ~82 psi for PDMS-acrylic PSA bond and ~73 psi for PDMS-silicon PSA bond |
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What You'll Learn
- PDMS surface functionalization with (3-aminopropyl) triethoxysilane (APTES)
- Plastic surface functionalization with (3-glycidoxypropyl) triethoxysilane (GPTES)
- Cross-linking between amines on PDMS-APTES and epoxy groups on plastic-GPTES
- PDMS/polymer tape composite for bonding strength
- PDMS oxidation to make the surface hydrophilic
PDMS surface functionalization with (3-aminopropyl) triethoxysilane (APTES)
Polydimethylsiloxane (PDMS) is a silicone polymer with a wide range of applications, including in the biomedical field. Its excellent optical, electrical, and mechanical properties, along with its biocompatibility, make it a popular choice for various engineering and biomedical applications.
One of the key properties of PDMS is its ability to form irreversible bonds with untreated plastics and metals, making it useful for microfluidics applications. PDMS has low surface energy, which can make bonding with other substrates challenging. However, functionalizing the PDMS surface with (3-aminopropyl) triethoxysilane (APTES) can address this issue.
APTES is an aminosilane used in the process of silanization, which involves the functionalization of surfaces with alkoxysilane molecules. By functionalizing the PDMS surface with APTES, amine functionalities are introduced, allowing for further chemical derivatization of the PDMS surface with small organic molecules or biomolecules. This process enables the formation of a stable, covalent bond between PDMS and other materials, such as thermoplastics.
To achieve this, oxygen plasma treatment is applied to both the PDMS surface and the material it will be bonded to (e.g., thermoplastics). Subsequently, a 1% APTES solution is applied to the treated surfaces. Within a few minutes, a stable, covalent bond is formed between the PDMS and the functionalized thermoplastic surface. This method has been demonstrated to enhance the bonding strength between PDMS and various materials, including plastics.
Additionally, by functionalizing the PDMS surface with APTES and the plastic surface with (3-glycidoxypropyl) triethoxysilane (GPTES), a cross-link can be established between the amines on the PDMS-APTES surface and the epoxy groups on the plastic-GPTES surface. This cross-linking further strengthens the bond between PDMS and the plastic substrate.
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Plastic surface functionalization with (3-glycidoxypropyl) triethoxysilane (GPTES)
Polydimethylsiloxane (PDMS) is a silicone polymer with a wide range of applications, including in the fabrication of microfluidic components. PDMS is often used for this purpose due to its low cost, ease and speed of fabrication, and optical transparency. However, PDMS has low surface energy, which makes bonding with many substrates challenging.
One method to improve the bonding of PDMS to plastic surfaces involves functionalizing the plastic surface with (3-glycidoxypropyl) triethoxysilane (GPTES). GPTES is an organo-functional silane that can act as a coupling agent between inorganic materials and polymers. In this case, GPTES can facilitate the bonding of PDMS to plastic by establishing a cross-link between the amines on the PDMS surface and the epoxy groups on the GPTES-treated plastic surface. This results in a strong bond that can support pressures up to 5–6 bars in microfluidic devices.
To functionalize a plastic surface with GPTES, the plastic substrate must first be prepared. Common plastic substrates used in microfluidic applications include poly methyl methacrylate (PMMA), which is favoured for its low cost, optical transparency, and ease of patterning. Once the plastic substrate is selected and prepared, GPTES can be applied to the surface.
The GPTES molecule contains two key functional groups: the glycidoxy group and the triethoxy group. The glycidoxy group is an epoxy group that can react with amine groups on the PDMS surface to form a strong covalent bond. The triethoxy group, on the other hand, can hydrolyse to form silanol groups (Si-OH) that can condense with hydroxyl groups (OH) on the plastic surface, forming a durable siloxane bond (Si-O-Si). This dual functionality of GPTES allows it to act as a bridge between the plastic and PDMS surfaces, promoting adhesion and creating a robust seal.
It is important to note that the plastic surface must be properly cleaned and prepared before applying GPTES to ensure effective bonding. Additionally, while GPTES can significantly enhance the adhesion between PDMS and plastic, it may not be suitable for all applications. For example, if the plastic substrate requires functionalization with probe molecules, such as in biosensors, alternative techniques may need to be considered as GPTES may interfere with the desired surface properties.
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Cross-linking between amines on PDMS-APTES and epoxy groups on plastic-GPTES
Polydimethylsiloxane (PDMS) is a silicone polymer with a wide range of applications, including in the fabrication of microfluidic components. PDMS is favoured in this context due to its low cost, ease and speed of fabrication, and optical transparency.
However, PDMS has low surface energy, which makes bonding to many substrates challenging. To address this issue, researchers have developed a technique for PDMS microchannel bonding on untreated plastic and metal surfaces. This technique involves functionalising the PDMS surface with (3-aminopropyl) triethoxysilane (APTES) to enable cross-linking with epoxy groups.
Specifically, by functionalising the PDMS surface with APTES and the plastic surface with (3-glycidoxypropyl) triethoxysilane (GPTES), a cross-link can be established between the amines on the PDMS-APTES and the epoxy groups on the plastic-GPTES. This process results in a strong and stable bond.
The PDMS-APTES surface can then be coated with Norland Optical Adhesive 74 (NOA74), a transparent UV-curable glue with excellent adhesion to metals, plastics, and glass. The PDMS-APTES-NOA74 can then be put in contact with the target material, and the glue is cured under UV light. This process results in a strong and irreversible bond between the PDMS and the substrate.
Overall, the cross-linking between the amines on PDMS-APTES and the epoxy groups on plastic-GPTES enables the formation of a robust and stable bond, making it a useful technique for PDMS microchannel bonding on untreated plastic surfaces.
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PDMS/polymer tape composite for bonding strength
Polydimethylsiloxane (PDMS) is a silicone polymer with a wide variety of uses, from cosmetics to industrial lubrication. It is known for its unusual rheological (or flow) properties, optical clarity, inertness, non-toxicity, and non-flammability. PDMS is typically difficult to bond using adhesive tapes due to its hydrophobic nature and low surface energy.
To address this issue, a PDMS/polymer tape composite was developed by Kim et al. in 2009. This composite is created by spinning and then baking PDMS on a double-sided adhesive tape. The resulting bonding strength is comparable to that obtained with plasma oxygen treatment. However, the authors had to use a knife plotter to fabricate the microfluidic network as soft-lithography was incompatible with their process.
The PDMS/tape composite was then patterned to create channels using xurography and bonded to a PDMS slab. After removing the backing paper from the tape, a complete microfluidic system could be created by placing the construct onto various substrates, including glass, plastic, or metal-coated glass/silicon. The bond strength was sufficient for typical microfluidic channels used for chemical or biological analysis.
This PDMS/tape composite rapid prototyping technique provides a fast, cost-effective, and simple fabrication method. It enables the easy integration of microfluidic channels with sensors and other components without requiring a cleanroom facility. The method is also biocompatible, as demonstrated by using a tape-bonded device to generate droplets for emulsion PCR.
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PDMS oxidation to make the surface hydrophilic
Polydimethylsiloxane (PDMS) is a hydrophobic material commonly used in the fabrication of microfluidic devices. However, certain applications, such as the generation of oil-in-water droplets, require hydrophilic surfaces. Several methods have been developed to modify PDMS surfaces and make them hydrophilic, including oxidation techniques. Here is a detailed overview of PDMS oxidation techniques to achieve hydrophilic surfaces:
Plasma Oxidation
One well-established approach to make PDMS surfaces hydrophilic is through plasma oxidation. This process involves exposing the PDMS surface to oxygen plasma, which results in the oxidation of the polymer chains. Specifically, oxygen plasma attacks the siloxane backbone of PDMS, forming a silica-like layer rich in oxygen and Si-OH groups. This oxidation process increases the hydrophilicity of the surface. However, it is important to note that the hydrophobic nature of PDMS tends to recover over time due to the migration of uncured hydrophobic polymer chains to the surface. This recovery can be slowed down by keeping the surface in water immediately after treatment.
Ultraviolet (UV) Irradiation
UV irradiation is another method used to oxidize PDMS surfaces and make them hydrophilic. When PDMS is exposed to UV radiation, the top layers of the polymer undergo oxidation, leading to the creation of hydrophilic -OH groups. Longer or more intense UV exposure can result in further oxidation and the formation of a contiguous silica thin 'skin'. However, similar to plasma oxidation, the hydrophobic nature of PDMS may recover due to the migration of uncured polymer chains.
UV-Ozone Treatment
UV-ozone treatment is a variation of UV irradiation, where the combination of UV radiation and ozone gas is used to oxidize the PDMS surface. This treatment can lead to the complete oxidation of thick PDMS membranes. However, the hydrophobicity of PDMS tends to recover over time, and the rate of recovery depends on factors such as curing time and treatment duration.
Sol-Gel Method
The sol-gel method is an alternative approach to oxidize and hydrophilize PDMS surfaces. This method involves soaking a cured PDMS piece in a sol-gel precursor solution, such as tetraethyl orthosilicate (TEOS). This process creates an oxide layer on the PDMS surface, making it hydrophilic. The sol-gel method has been shown to effectively prevent hydrophobic recovery for at least 7 days of storage in air.
Polyvinyl Alcohol (PVA) Deposition
While not a direct oxidation method, polyvinyl alcohol (PVA) deposition is often used in combination with plasma oxidation to create hydrophilic PDMS surfaces. After plasma treatment, the PVA solution is applied to the PDMS surface, resulting in improved hydrophilicity. This combination of plasma oxidation and PVA deposition has been demonstrated to be a simple, quick, and effective method for modifying PDMS surfaces for droplet microfluidics applications.
In summary, PDMS oxidation techniques, such as plasma oxidation, UV irradiation, UV-ozone treatment, the sol-gel method, and PVA deposition, can effectively modify PDMS surfaces to make them hydrophilic. These methods have various advantages and considerations, and the choice of technique depends on the specific application and requirements.
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Frequently asked questions
PDMS adheres to plastic due to the establishment of a cross-link between the amines on the PDMS-APTES surface and the epoxy groups on the plastic-GPTES surface.
PDMS is Polydimethylsiloxane, a silicone polymer with a wide variety of uses, from cosmetics to industrial lubrication.
PDMS is used for fabricating microfluidic components due to its low cost, easy and fast fabrication, and optical transparency.
The PDMS surface is functionalized with (3-aminopropyl) triethoxysilane (APTES) for further cross-linking with epoxy groups. The plastic surface is functionalized with (3-glycidoxypropyl) triethoxysilane (GPTES).
PDMS has low surface energy, making its bonding to many substrates non-trivial. Additionally, PDMS is not ideal for many organic chemistry applications as it deforms and swells in the presence of certain chemicals.
