High-grade optical polydimethylsiloxane for microfluidic applications

Commercially available polydimethylsiloxane (PDMS) elastomers, such as Sylgard 184A (R) are widely used in soft lithography and for microfluidic applications. These PDMS elastomers contain fillers to enhance their mechanical stability. The reinforcing fillers, often sub-micrometer small SiO(2) particles, tend to aggregate, swell with water, and thereby become cognoscible in a way that can strongly interfere with the visualization of micro-scale events taking place next to PDMS structures. As PDMS microfluidics are often used for studying cells and micro-/nanoparticles and for creating/handling nanodroplets, it has become highly desirable to employ a PDMS having high optical quality and that allows microscopy observation without artifacts. Here, we present a PDMS formulation that is free of fillers and has sufficiently low viscosity to perform a filtration step of the mixed prepolymers before curing. By molding a bi-layer microfluidic network (MFN), composed of a thin filler-free PDMS layer and a thicker Sylgard 184A (R) backing layer, PDMS MFNs featuring both high optical quality and mechanical stability, can be fabricated.