High performance silicon nanoporous membranes have been developed to investigate the feasibility of an implantable bioartificial kidney. The nanoscale pore size is defined by the thickness of a sacrificial layer of silicon dioxide (SiO2), which is grown by thermal oxidation down to 5 nm with 1% variation. Standardized test protocols to examine biocompatibility revealed that silicon and related MEMS materials did not exhibit any evidence of cytotoxicity or hemolysis. Surface modification with polyethylene glycol (PEG) monolayers reduced adsorption of bovine serum albumin (BSA) onto MEMS surfaces to ∼4% of positive control. PEG-modified membranes exhibited size-dependent rejection of Ficoll 70, a spherical probe solute, dissolved in bovine whole blood. Prolonged hemofiltration revealed constant membrane flux and consistent molecular selectivity for >72 hours.