Although PDMS has been widely studied in the field of microfluidics, the highly hydrophobic nature of its surface makes it difficult for water-soluble media to fill the microchannels, which tends to lead to nonspecific adsorption, thus affecting efficient separation and analysis. To overcome this limitation, various surface modification methods have been developed to improve the wettability and increase the hydrophilicity of the PDMS surface.
For example, through various surface modification techniques, the hydrophilicity and biocompatibility of the PDMS chip were not only improved, but also the interaction force between the analyte and the wall of the tube was significantly reduced, which led to a more effective immobilization of bioactive molecules and improved electroosmotic flow performance, thus enhancing the separation and analytical capabilities of the PDMS chip.
Surface modification methods for microfluidic chips include intrinsic doping, plasma treatment, polymer-induced grafting, dynamic modification, chemical vapor deposition, layer-by-layer assembly, and sol-gel.
1. Ontological doping
Intrinsic doping modification is achieved by doping specific chemicals into PDMS prepolymers by utilizing the property of microporous structures to adsorb and attach small molecules or embed macromolecules on the surface of the chip, thus enabling the modification of the material matrix during the fabrication process. The advantage of this approach is that it avoids the need for additional post-processing steps on the already molded PDMS chip.
For example, by adding melt mold powder as a dopant to a PDMS prepolymer, an enhanced PDMS mold was developed that exhibited excellent performance during hot embossing of micropatterned cyclic olefin polymer sheets (180 °C, 103 kPa, 5 min).
2. High-energy oxidation
High-energy oxidation methods include several methods such as plasma, UV and corona discharge. Under high-energy conditions, the Si-O and Si-C bonds of the PDMS chip are broken to generate surfaces containing Si-OH groups, and these Si-OH groups form a reticular structure similar to -Si-O-Si- through a dehydration reaction.
After plasma treatment, the surface properties of the PDMS chip are changed, the water contact angle is significantly reduced, and the surface exhibits a high degree of hydrophilicity. However, during the treatment process, small molecules inside the PDMS migrate to the surface, while hydrophilic groups on the surface are transferred to the interior.
The silanol groups on the PDMS surface may undergo a dehydration reaction due to the lack of active ozone molecules or other high-energy particles, which can make the surface texture rougher, leading to difficulty in maintaining hydrophilicity for a long period of time and a gradual return to the original hydrophobicity.
3. Polymer-induced grafting
During the grafting process, the photoinitiator and monomer solution along with the PDMS chip are exposed to UV radiation, causing the surface of the PDMS chip to be activated. The team generated a layer of compounds with hydrophilic properties on the PDMS surface by directly irradiating the monomer solution with UV light.
Meanwhile, this method successfully attached fluorescent probes such as benzophenone to the surface of PDMS, which greatly accelerated the polymerization process and completed the polymerization reaction in only a few minutes, while maintaining the stability of the electromobility.
4. Chemical vapor deposition
Chemical vapor deposition techniques generate solid films on material substrates through gaseous chemical reactions. For example, graphene nanosheets were modified using PDMS by a simple chemical vapor deposition-based dry chemistry method that significantly enhanced the dispersion of PDMS in common coating solvents.
The researchers also sealed the PDMS microfluidic channel using a vapor-phase deposited nanobinder layer, which effectively inhibited the nonspecific adsorption of fluorescent molecules in the PDMS chip channel and enabled the capture of yeast cell information.
5. Dynamic modification method
The dynamic modification method utilizes a solution containing surface-active molecules to clean the channels of the PDMS microfluidic chip or to dope the surface-active molecules into the buffer solution in use. In this way, the surface-active molecules achieve effective adsorption on the surface of the PDMS chip, thereby modulating the electroosmotic flow.
It was shown that dynamically modified PDMS channels using 2-morpholinoethanesulfonic acid had lower adsorption rates and higher efficiencies in separating biomolecules compared to unmodified PDMS channels.
6. Layer-by-layer self-assembly
Layer-by-layer self-assembly is an efficient, versatile and simple technique to construct various thin films utilizing non-covalent bonding forces. When the surface of a PDMS microfluidic chip is repeatedly immersed in oppositely charged polyelectrolyte solutions, the surface undergoes regular alternating charge changes.
In this way, positively charged chitosan and negatively charged hyaluronic acid polysaccharides were assembled layer by layer on PDMS silicone-oxygen hydrogel, and the modified silicone hydrogel exhibited hydrophilicity and resistance to protein adsorption, which provided a new material for contact lens fabrication.
7. Sol-gel method
The sol-gel technique uses metal-organic compounds as precursors, such as highly chemically active metal alkoxides, which are hydrolyzed and condensed to produce sols that are ultimately converted to gels by polymerization reactions. Some researchers have prepared acidic silica sols using hydrochloric acid as a catalyst and ethanol as a dispersant by sol-gel method.
The permeation-enhancing film obtained after the addition of acidic silica sol to the substrate and acrylate-co-siloxane emulsion showed significant permeation-enhancing effects. In another experiment, an aerogel layer was constructed on the surface of PDMS by the sol-gel method and chemically grafted with cinnamaldehyde, and the structural improvement enhanced the liquid storage stability of the coating, thereby improving the bioadhesion resistance of PDMS.
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