Schematic of a bone morphogenic protein (BMP)-7-producing bioartificial kidney (BAK). The patient's blood (red) first enters the hemofiltration unit (left), which contains hollow fiber membranes for ultrafiltration. The blood and the ultrafiltrate (yellow) leaving the hemofiltration unit then flow into the bioreactor unit (right), which contains hollow fiber membranes with an epithelium of renal cells (green) on the inner surfaces. The cells secrete BMP-7 (violet), which becomes enriched in the ultrafiltrate during processing and in the blood flowing on the outside of the hollow fiber membranes. The blood enriched in BMP-7 flows back into the patient, and the processed ultrafiltrate (orange) is discarded. An enlarged cross-section of a hollow fiber membrane from the bioreactor unit is shown in the lower right corner. The ultrafiltrate flows in the lumen of the hollow fiber membrane, and the blood flows on the outside. The inner surface of the hollow fiber membrane is covered with BMP-7-secreting renal cells, and BMP-7 becomes enriched in the ultrafiltrate and the blood in the bioreactor unit. BMP-7 in the ultrafiltrate would regulate HPTC performance, whereas BMP-7 in the bloodstream would be delivered to the patient.