Through-holes, cavities and perforations in polydimethylsiloxane (PDMS) chips

Abstract

We present a method to fabricate through-holes between 10 to 180 μm between polydimethylsiloxane (PDMS) layers of microfluidic large-scale integration platforms. Therefore we employed standard PDMS spin-coating processes onto silicon molds with microstructures formed from SU-8 and AZ photoresists. Our approach is based on the modification of the surface polarity of the PDMS prototyping molds by a 250 nm thick layer of octafluorocyclobutane (C₄F₈), which resulted in a contact angle of 125 ± 3° for water. This super hydrophobic surface repelled PDMS from microstructures protruding out of the spin coated PDMS layer. Subsequently, we applied and characterized the C₄F₈ coating for the robust fabrication of interlayer connectors between PDMS membranes of 40 μm thickness. To enable embedding of through-holes, perforations and/or cavities in very thin layers of PDMS (<20 μm) we mixed PDMS with a PDMS based silicone oil to reduce its viscosity. In contrast to previous attempts to lower the viscosity of PDMS using organic solvents, the silicone oil cross-linked to PDMS and was thus, unable to freely diffuse into the polymerized PDMS. This reduces the risk for bleeding of hazardous components in biological applications. Finally, we manufactured a three layer mLSI chip with integrated cavities for catching fluorescently labeled beads and cells. The presented process parameters can easily be adapted to specific needs in the fabrication of multi-layer PDMS arrangements by following the systematic parameter screening.