Extracorporeal life support (ECLS) is a well-established technique for the treatment of different cardiac and pulmonary diseases, e.g., congenital heart disease and acute respiratory distress syndrome. Additionally, severely ill patients who cannot be weaned from the heart-lung machine directly after surgery have to be put on ECLS for further therapy. Although both systems include identical components, a seamless transition is not possible yet. The adaption of the circuit to the patients' size and demand is limited owing to the components available. The project I³-Assist aims at a novel concept for extracorporeal circulation. To better match the patient's therapeutic demand of support, an individual number of one-size oxygenators and heat exchangers will be combined. A seamless transition between cardiopulmonary bypass and ECLS will be possible as well as the exchange of components during therapy to enhance circuit maintenance throughout long-term support. Until today, a novel oxygenator and heat exchanger along with a simplified manufacturing protocol have been established. The first layouts of the unit to allow the spill- and bubble-free connection and disconnection of modules as well as improved cannulas and a rotational pump are investigated using computational fluid dynamics. Tests were performed according to current guidelines in vitro and in vivo. The test results show the feasibility and potential of the concept.