A porous filter is fabricated directly inside a microfluidic circuit using a photoreticulable hydrogel. The filter could be used for separation of cells from blood, removal of particles or solutes, such as proteins, in microdialysis and microfiltering. The filter is realized by in situ polymerization approach: a liquid hydrogel is injected in a microfluidic circuit channel where the filter is formed in a specific location by polymerization of UV light, focused by an optical microscope.