The porosity offilters is typicallyfixed; thus, complex purification processesrequire application of multiple specializedfilters. In contrast, smartfilters withcontrollable and tunable properties enable dynamic separation in a single setup.Herein, an electroactivefilter with controllable pore size is demonstrated. Theelectroactivefilter is based on a metal mesh coated with a polythiophene polymerwith ethylene glycol sidechains (p(g3T2)) that exhibit unprecedented voltage-driven volume changes. By optimizing the polymer coating on the mesh, con-trollable porosity during electrochemical addressing is achieved. The poresreversibly open and close, with a dynamic range of more than 95%, corre-sponding to over 30μm change of pores’widths. Furthermore, the pores’widthscould be defined by applied potential with a 10μm resolution. From amonghundreds of pores from different samples, about 90% of the pores could beclosed completely, while only less than 1% are inactive. Finally, the electroactivefilter is used to control theflow of a dye, highlighting the potential forflow controland smartfiltration applications.