Embodiments described herein generally relate to passively replacing media in a closed cell expansion system to reduce or prevent the dilution of chemical signaling used to inhibit signaling pathways that keep a cell population in the lag phase of cell growth. To prevent such dilution, active inlet fluid flow to the system may be halted. To replace fluid lost by the system, a bag containing media may be attached to the waste line in replacement of the waste or outlet bag connected thereto. By turning off one or more pumps, media from the replacement bag is added to the system at the rate of evaporation. Chemical signaling dilution may be prevented while conserving system resources. Enhancement of chemical signaling to reduce the lag phase of cell growth may further be accomplished by adding molecules, such as chemical-signaling proteins, from a direct source to the system.