Surface characterization using chemical force microscopy and the flow performance of modified polydimethylsiloxane for microfluidic device applications

Abstract

The widespread interest in micro total analysis systems has resulted in efforts to develop devices in cheaper polymer materials such as polydimethylsiloxane (PDMS) as an alternative to expensive glass and silicon devices. We describe the oxidation of the PDMS surface to form ionizable groups using a discharge from a Tesla coil and subsequent chemical modification to augment electroosmotic flow (EOF) within the microfluidic devices. The flow performance of oxidized, amine-modified and unmodified PDMS materials has been determined and directly compared to conventional glass devices. Exact PDMS replicas of glass substrates were prepared using a novel two step micromolding protocol. Chemical force microscopy has been utilized to monitor and measure the efficacy of surface modification yielding information about the acid/base properties of the modified and unmodified surfaces. Results with different substrate materials correlates well with expected flow modifications as a result of surface modification. Oxidized PDMS devices were found to support faster EOF (twice that of native PDMS) similar to glass while those derivatized with 3-aminopropyl triethoxysilane (APTES) showed slower flow rates compared to native PDMS substrates as a result of masking surface charge. Results demonstrate that the surface of PDMS microdevices can be manipulated to control EOF characteristics using a facile surface derivatization methodology allowing surfaces to be tailored for specific microfluidic applications and characterized with chemical force microscopy.

Electrokinetic characterization of poly(dimethylsiloxane) microchannels

“…Oxygen-plasma oxidation is known to introduce silanol groups at the surfaces [8], but due to its instability, the surface slowly recovers its original hydrophobicity. The formation of ionizable surface groups using a discharge from a Tesla coil and subsequent chemical modification was also studied [12]. Furthermore, silanization of the oxidized PDMS surface [13], the deposition of alternating layers of positively and negatively charged polymers onto PDMS [14], as well as the adsorption of detergents or quaternary amines [11], protein A [15], phospholipid bilayers [16], and other biomacromolecules [17][18][19] onto the PDMS channel wall, to name but a few, have been described.…”

“…The surface of native PDMS is hydrophobic, making it difficult to wet with aqueous solutions [8,9,11,12]. The source of charge at the PDMS surface is unknown and has previously been ascribed to the presence of silica fillers in the polymer [11].…”

Microwave plasma treatment of polymer surface for irreversible sealing of microfluidic devices

“…The slabs are then brought into conformal contact to form the irreversible seal. 4 Plasma surface treatment using corona discharge [7][8][9] and microwave-oven-generated plasma 10 have also been reported. The corona discharge method requires a relatively long treatment time of 10 min.…”

Surface modification in microchip electrophoresis

“…An electrical discharge from a tesla coil to oxidize the PDMS surface and subsequent chemical modification of oxidized PDMS with 3-aminopropyltriethoxysilane was reported by Wang et al [92]. The EOF in such amine-modified microfluidic PDMS devices together with oxidized and native substrates were compared to conventional glass devices at a pH of 8.…”