Experimental Soft Condensed Matter Group Harvard University, Prof. D. A. Weitz Glass coating for PDMS channels by sol-gel methods

Polydimethylsiloxane (PDMS) is widely used for fabrication of microfluidic devices: it is inexpensive and it can be fashioned to have complex channel structures. However, PDMS channels have several drawbacks. Even when cured, PDMS remains permeable to liquids and gases, which can affect reactions that occur in the channels. Organic solvents can swell PDMS significantly, seriously degrading device performance. The limited chemical compatibility of PDMS is, therefore, a major problem that can limit the wider application of PDMS to microfluidic technology.

We coat PDMS microfluidic devices with a glass-like layer using sol-gel chemistry. The coating greatly increases the chemical resistance of the channels, enabling the use of organic solvents. In addition, the coating can be functionalized with a wide range of silanes, for example, to make the channels either hydrophilic or hydrophobic.

As a demonstration of the chemical resistance afforded by the coating, we flow toluene through uncoated and coated PDMS channels. The toluene swells the uncoated channels almost instantly, so that a diffusion wave of toluene is visible moving into the PDMS walls, as shown in this movie [Fig 1]. By contrast, with the sol-gel coating the toluene is contained in the channel and no diffusion wave into the PDMS is visible, as shown in the coated channel movie [Fig 2]. Even after hours of use, the coated channel remains open and functional, as shown in the 30x sped up movie of toluene flowing through a sol-gel coated channel at 250 ml/hr[Fig 3]. We verify that the channel is open over its entire length, as shown in this movie [Fig 4].

In this final movie we show the first channel that we coated. The coating in this channel cracked numerous times as it hardened; a large crack is visible to the left and just out of focus in the movie [Fig 5]. When the toluene is flushed into the channel a diffusion wave and swelling can be seen originating from the crack; however, no diffusion wave is visible through the coated parts of the channel. As the toluene continues to diffuse into the crack, the PDMS beneath the coating begins to swell, eventually constricting the channel, though it remains open. Heating the coating liquid at 65 C for 12 hours speeds preconversion, which reduces cracking and yields more homogenous coatings.

More information

To learn more about this research please contact Adam Abate (aabate@seas.harvard.edu), or read our Lab on a Chip paper.