The bioartificial kidney aims to combine the glomerular membrane-sieving function with the tubular metabolic and endocrine function of the nephron. One component of the bioartificial kidney, the kidney tubular cells containing the kidney assisst device, has been applied in two exploratory pilot trials in patients with acute kidney injury and multi organ failure. The kidney assisst device is based on a standard hemofiltration cartridge containing kidney tubular cells grown along the lumen side of the hollow fibers. The kidney asssisst device is connected in series to a conventional hemofilter in an extracorporeal blood circuit. One pilot trial, designed to demonstrate safety and clinical efficacy of the device, was suspended after an interim analysis because of an unanticipated high survival rate of the sham device arm of the study, which may have been related to the use of citrate as an anticoagulant in the placebo arm, compared with heparin in the active arm. Proof-of-concept, demonstration of durability and efficacy, long-term feasibility, cost and clinical benefits, as well as exclusion of potential immunogenicity will be required before these devices can be applied on humans.