Technique Creates Hydrophobic and Hydrophilic Regions in Microfluidic Channels

Daniel S. Burgess

Using a photoinitiated chemical process from polymer chemistry, a team of investigators in France has developed a method to create hydrophobic and hydrophilic regions inside microfluidic channels. Potential applications include the design of lab-on-a-chip systems for use in chemical analysis, environmental monitoring, medical diagnostics and pharmaceutical screening.

Using a photoinitiated chemical process from polymer chemistry, researchers have created hydrophobic and hydrophilic regions inside microfluidic channels etched into silicon to modify the flow characteristics, as demonstrated by the introduction of acetone to the channels.

Eric Besson, a researcher at Laboratoire d’Analyse et d’Architecture des Systèmes in Toulouse, explained that the method enables the modification of microfluidic systems, as needed, using commercially available chemicals and near-UV photolithography to control the dynamics of fluid flow within etched silicon structures. Besson worked on the project with colleagues from the Institut de Chimie Moléculaire et des Matériaux d’Orsay at Université Paris Sud in Orsay, and from the Centre de Génie Electrique de Lyon and the Laboratoire d’Electronique, Optoélectronique et Microsystème, both at l’Ecole Centrale de Lyon in Ecully.

After rejecting an alternative process that involves photocleavable organosilane because the reaction was too slow at practical irradiation fluxes, the investigators turned to the UV-induced thiol-ene reaction, using benzophenone as the photoinitiator. They coated a substrate with mercaptopropyltrimethoxysilane, letting it dry into a monolayer at room temperature for 17 hours. Then they coated the surface with a solution of octadecyl acrylate and benzophenone in 1.4-dioxane and exposed it to 365-nm radiation using a Karl Süss mask aligner to define regions of exposure, including those inside pre-etched microchannels.

Where the surface is irradiated, hydrocarbon chains bind to the thiol groups of the mercaptopropyltrimethoxysilane, creating a hydrophobic area. Where the surface is not exposed to the UV radiation, it oxidizes into sulfonic acid, becoming highly hydrophilic.

To demonstrate the potential of the technique, the scientists modified the flow characteristics of 500-μm- to 1-mm-wide channels etched in silicon using the process with irradiation at 20 mW/cm² for 120 s to produce hydrophilic and hydrophobic regions. They further fabricated a series of 100- to 200-μm-diameter hydrophilic spots spaced by 1 mm on a silicon wafer, which they suggest indicates the feasibility of rapidly producing microarrays for various biotechnology research applications.

The investigators plan to use more sophisticated optics and exposure masks with the method to assess its ability to generate more complex patterns in micro- and nanofluidic systems.

Langmuir, Sept. 26, 2006, pp. 8346-8352.