Generation of Hydrophilic Poly(dimethylsiloxane) for High-Performance Microchip Electrophoresis

Abstract

Poly(dimethylsiloxane) (PDMS) has become one of the most widely used materials for microchip capillary electrophoresis and microfluidics. The popularity of this material is the result of its low cost, simple fabrication, and rugged elastomeric properties. The hydrophobic nature of PDMS, however, limits its applicability for microchip CE, microfluidic patterning, and other nonelectrophoresis applications. The surface of PDMS can be made hydrophilic using a simple air plasma treatment; however, this property is quickly lost through hydrophobic recovery caused by diffusion of unreacted oligomer to the surface. Here, a simple approach for the generation of hydrophilic PDMS with long-term stability in air is presented. PDMS is rendered hydrophilic through a simple two-step extraction/oxidation process. First, PDMS is extracted in a series of solvents designed to remove unreacted oligomers from the bulk phase. Second, the oligomer-free PDMS is oxidized in a simple air plasma, generating a stable layer of hydrophilic SiO2. The conversion of surface-bound siloxane to SiO2 was followed with X-ray photoelectron spectroscopy. SiO2 on extracted-oxidized PDMS was stable for 7 days in air as compared to less than 3 h for native PDMS. Furthermore, the contact angle for modified PDMS was reduced to <40 degrees and remained low throughout the experiments. As a result of the decreased contact angle, capillary channels self-wet through capillary action, making the microchannels much easier to fill. Finally, the modification significantly improved the performance of the devices for microchip electrophoresis. The electroosmotic flow increased from 4.1 x 10(-4) to 6.8 x 10(-4) cm(2)/V.s for native compared to oxidized PDMS. Separation efficiencies for electrochemical detection also increased from 50 000 to 400 000 N/m for a 1.1-nL injection volume. The result of this modification is a significant improvement in the performance of PDMS for microchip electrophoresis and microfluidic applications.

Hydrophilic surface modification of PDMS for droplet microfluidics using a simple, quick, and robust method via PVA deposition

“…However, this effect is transient, and the hydrophobic nature of PDMS returns several minutes after plasma or UV exposure because of the migration of the uncured hydrophobic polymer chains to the surface. Methods to slow down or prevent this recovery from occurring include keeping the surface in water immediately after treatment and removing uncured polymers using solvent extraction 24,25 . However, the utility of these approaches for droplet microfluidics has not been established, and it is likely that exposure of the surface to oil will negate these effects.…”

Droplet confinement and leakage: Causes, underlying effects, and amelioration strategies

“…Both physical modification and chemical derivatization of PDMS surfaces are complicated by a property of PDMS termed hydrophobic recovery. 24 During hydrophobic recovery, non-crosslinked silicone monomers perfuse from the interior of the material and permeate the elastomer surface. 25 This means that physical modification is ideally performed on a PDMS device that has had this residual monomer removed or at least depleted.…”

Study on surface properties of PDMS microfluidic chips treated with albumin

“…Current monitoring a͒ technique is one of the most exploited; [6][7][8][9][10][11][12][13] however, other techniques such as amperometric detection method, 14 cyclic voltametry, 15 imaging of a caged fluorescent dye, 16,17 and particle tracking technique 18 were also employed.…”