We present a novel technological approach for the in situ realization of a micron-thin poly-acrylamide membrane in the center of a microfluidic channel. The membrane is formed by interfacial polymerization of an inner stream of monomer solution in between two streams of initiator/catalyst solution in a hydrodynamic focusing chip. 20 μm thick SU-8 structures are used to replicate the chip in poly dimethylsiloxane (PDMS). The chip is fitted within an adaptor allowing easy fluidic connections, temperature control and optical monitoring under a microscope. With this system, we can easily tune the internal stream width from 5 to 100 μm, by varying the internal/external flow ratio.