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.
