Chemically controlled micro-pores and nano-filters for separation tasks in 2D and 3D microfluidic systems

Molecule filtering or particle separation is a complex task in microfluidics. Passive structures or polymer based systems like hydrogels are normally used elements. PNIPAAm (poly-N-isopropylacrylamide) hydrogels are known for their capability to work as nano-filters to separate small molecules from their environment and protect them from degrading enzymes by the polymer network. PNIPAAm based active micro-pores were also demonstrated for particle separation. Normally, changes in temperature are used to alter the permeation properties of such hydrogel elements. To handle limiting factors such as separate temperature control for each element, chemical signals can be used to alter the permeation. Here, we present a new way to adapt the size exclusion functionality of PNIPAAm-based nano-filters or micro-pores. PNIPAAm hydrogels respond to organic solvent concentrations by shrinking or swelling. We use this responsiveness to alter the hydrogel mesh size or to adjust the size of a micro-pore. We show the filter functionality for two model molecules: the enzyme horse radish peroxidase and the fluorescence molecule fluorescein. Furthermore, the adjustment of the size of a micro-pore in a continuous way is demonstrated, so that pore sizes between the closed and open state can be addressed and kept permanently.