Perfusion bioreactor: continuous mAb culture with cell retention

A continuous mammalian culture run in perfusion — fresh medium is exchanged continuously while an ATF/TFF device retains the cells, and density is set by a deliberate bleed rather than by the medium-exchange rate. That decoupling is the whole point, and it is what an ordinary chemostat cannot represent: at steady state the cell balance pins mu - mu_d to the cell-specific removal rate D_eff = D[phi + (1-phi)(1-R)], not to the dilution rate, so this culture holds roughly ten times the cell density the same feed would support in a chemostat. Lactate accumulates and inhibits growth, cells die at a basal rate (so the reported viability is below 100%), and the secreted antibody follows Luedeking-Piret kinetics — leaving in the cell-free harvest while the bleed carries the cells away. Set retention to 0.0 and this collapses exactly to the plain chemostat the 'bioreactor' example shows. Honesty note: screening fidelity — the kinetic constants are illustrative rather than a regressed cell-line parameter set (no such databank ships with the kernel, deliberately); the model is isothermal, the retention device is a single efficiency number rather than a filter model (no fouling or sieving curve), and dissolved oxygen and pH are folded into mu_max rather than modelled.