Abstract: Biologically active devices are receiving increasing attention, especially in the management of endocrine pancreatic failure (diabetes) and acute liver failure. In both instances, mechanical devices have been unable to replace the function of the original organ, and consequences range from inconvenient (e.g., regular insulin shots, diabetic vasculopathy) to fatal (e.g., fulminant hepatic failure). In developing a cell-based liver assist device, we concluded that currently available extracorporeal blood treatment systems are not suited to the delivery of high molecular weight substances and that they do not adequately address the metabolic needs of the device. We therefore developed a system that provides safe, continuous perfusion of an extracorporeal organ. We detail the design and first clinical use of the system.