A method for treating a surface of a plastic substrate, the method comprising: optionally, a conditioning plasma treatment carried out by treating a surface of a plastic substrate with conditioning plasma of one or more non-polymerizing compounds generated at a remote point from the surface, where the ratio of the radiant energy density at the remote point to the radiant energy density at the brightest point of the conditioning plasma is less than 0.5, optionally less than 0.25, forming a conditioned surface; and a conversion plasma treatment carried out by treating the conditioned surface (if the optional step is performed) or unconditioned surface (if the optional step is omitted) with conversion plasma of water vapor, the conversion plasma being generated at a remote point from the conditioned surface, where the ratio of the radiant energy density at the remote point of conversion plasma treatment to the radiant energy density at the brightest point of the conversion plasma is less than 0.5, optionally less than 0.25, wherein the radiant energy density at the brightest point of the plasma is determined spectrophotometrically by measuring the radiant intensity of the most intense emission line of light in the spectrum from 380 nm to 750 nm wavelength at the brightest point, and the radiant energy density at the remote point is determined spectrophotometrically by measuring the radiant energy density of the same emission line of light at the remote point, to form a converted surface having a biomolecule recovery percentage, for an aqueous protein dispersion having a concentration from 0.01 nM to 1.4 nM, optionally 0.05 nM to 1.4 nM, optionally 0.1 nM to 1.4 nM, in contact with the converted surface, greater than 80%, wherein the biomolecule recovery percentage is determined according to the method "Testing of all embodiments" described in the description. optionally, a conditioning plasma treatment carried out by treating a surface of a plastic substrate with conditioning plasma of one or more non-polymerizing compounds generated at a remote point from the surface, where the ratio of the radiant energy density at the remote point to the radiant energy density at the brightest point of the conditioning plasma is less than 0.5, optionally less than 0.25, forming a conditioned surface; and a conversion plasma treatment carried out by treating the conditioned surface (if the optional step is performed) or unconditioned surface (if the optional step is omitted) with conversion plasma of water vapor, the conversion plasma being generated at a remote point from the conditioned surface, where the ratio of the radiant energy density at the remote point of conversion plasma treatment to the radiant energy density at the brightest point of the conversion plasma is less than 0.5, optionally less than 0.25, wherein the radiant energy density at the brightest point of the plasma is determined spectrophotometrically by measuring the radiant intensity of the most intense emission line of light in the spectrum from 380 nm to 750 nm wavelength at the brightest point, and the radiant energy density at the remote point is determined spectrophotometrically by measuring the radiant energy density of the same emission line of light at the remote point, to form a converted surface having a biomolecule recovery percentage, for an aqueous protein dispersion having a concentration from 0.01 nM to 1.4 nM, optionally 0.05 nM to 1.4 nM, optionally 0.1 nM to 1.4 nM, in contact with the converted surface, greater than 80%, wherein the biomolecule recovery percentage is determined according to the method "Testing of all embodiments" described in the description.