This article presents a novel pumpless perfusion cell culture cap, the gravity-driven flow rate of which is kept constant by the height difference of two parallel channel layers. Previous pumpless perfusion cell culture systems create a gravity-driven flow by means of the hydraulic head difference (?h) between the source reservoir and the drain reservoir. As more media passes from the source reservoir to the drain reservoir, the source media level decreases and the drain media level increases. Thus, previous works based on a gravity-driven flow were unable to supply a constant flow rate for the perfusion cell culture. However, the proposed perfusion cell culture cap can supply a constant flow rate, because the media level remains unchanged as the media moves laterally through each channel having same media level. In experiments, using the different fluidic resistances, the perfusion cap generated constant flow rates of 871 +/- 27 mu L h-1 and 446 +/- 11 mu L h-1. The 871 and 446 mu L h-1 flow rates replace the whole 20 mL medium in the petridish with a fresh medium for days 1 and 2, respectively. In the perfusion cell (A549 cell line) culture with the 871 mu L h-1 flow rate, the proposed cap can maintain a lactate concentration of about 2200 nmol mL-1 and an ammonia concentration of about 3200 nmol mL-1. Moreover, although the static cell culture maintains cell viability for 5 days, the perfusion cell culture with the 871 mu L h-1 flow rate can maintain cell viability for 9 days. (C) 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2012