the dry state under the electron microscope the cross sectional structure of an example of the hollow fiber membrane according to this invention can be seen from the electron photomicrograph magnification x 1200 of fig1 and the microstructure of the same membrane can be seen from the electron photomicrograph magnification x 12 000 of fig2 to examine the structure of the membrane the dry membrane is first frozen in liquid nitrogen and then broken to fracture which is then vapor deposited with gold to a thickness of 100 angstrom units and examined with an electron microscope for example hitachi model hfs 2 of hitachi ltd the porosity can be calculated according to the following equation equ1 the hollow fiber membrane according to this invention is of about 40 to 3000 u03bc preferably about 100 to 2000 u03bc in outside diameter and about 10 to 1000 u03bc preferably about 20 to 500 u03bc in membrane thickness the membrane can be used as a filtering membrane in the separation and purification of various solutions or the separation and concentration of blood and other body fluids the following examples are further illustrative of this invention unless otherwise stated all percentages and parts are by weight an eva copolymer with an ethylene.