Describe perfusion cell culture and its advantages over fed-batch.

Perfusion culture continuously removes spent medium and supplies fresh medium while retaining cells using retention devices (ATF - alternating tangential flow, TFF, settlers, spin filters). Cells are maintained at high density (50-100 million cells/mL) for extended periods. Advantages: higher volumetric productivity, consistent product quality (reduced residence time), smaller bioreactor footprint, continuous harvest enabling integrated processing, and gentler conditions (steady metabolite levels). Challenges include: complex operation, cell retention device selection and optimization, bleed rate determination for cell viability, longer validation, and equipment requirements. Perfusion is increasingly used for unstable molecules, high-cost products, and continuous manufacturing platforms.