Ions and vitamins are mostly recycled. Once cell growth has plateaued, cells should be able to survive with minimal medium, so long as oxygen, pH, glucose, and amino acid levels are maintained. Replacing only the consumed nutrients with a concentrated stock cuts medium consumption dramatically. Supplementation with concentrated solutions has been suggested in other systems but is generally impractical because of the metabolic characteristics of the cells. Base is added to maintain pH, and more glucose is added as well.
New glucose produces more lactate, which then requires more base.
Soon ionic strength problems arise. Addition of water to dilute the ionic strength lowers ion concentrations. In most real-world situations, replenishment with whole, fresh medium is the most expedient course. C3A cells present a different situation. A study was conducted to track glucose use and lactate production across the growth curve of a single set of devices.