The document summarizes air lift fermenters, which are impeller-free bioreactor systems that use pressurized air circulation to culture shear-sensitive cells. The fermenters have an inner riser region for upward air flow and an outer downcomer region for recirculating degassed media and cells. A density gradient between the two regions drives continuous liquid circulation without mechanical agitation. Key advantages are homogeneous gas/nutrient distribution without focal points of energy dissipation.