The object of the invention is a unit element forming a membrane comprising a semi-permeable material able to separate in two compartments a filtration device for extracorporeal treatment of a fluid such as blood or plasma, said material being negatively charged, said element having a first internal face intended to be in direct contact with blood or plasma and a second external face intended to be in contact with filtrate, wherein said second face of said element is treated on the surface by one selected in the group comprising at least a cationic polymer, a mixture of polymers of which at least one polymer is cationic. The unit element can also have said first face treated on the surface by one selected in the group comprising: at least one cationic polymer, a mixture of polymers of which at least one polymer is cationic.The object of the invention is also a semipermeable membrane consisting of such unit elements; a filtration device for extracorporeal treatment of a fluid such as blood or plasma comprising two compartments separated by such a semipermeable membrane.The object of the invention is also the processes for manufacturing such devices.