A dialysis filter can fill in for a kidney, separating body wastes from the blood. It works, but lost are some subtler jobs of kidney cells, such as reclaiming antioxidants and producing vitamin D. At the University of Michigan, tissue engineers have created a “bioartificial” kidney that looks to put cells, and their metabolic function, back in the picture. The team lined hollow polymer fibers with pig kidney cells and packed the fibers together in a cartridge. The device works in series with a dialysis machine: Wastes filtered out by dialysis run through the fibers while blood runs through the spaces between fibers, and the pig cells transport useful molecules across the permeable fiber walls and back into the blood. The researchers say that tests on dogs have been successful and human trials are planned, pending FDA approval. Ultimately, their ambition is to design an implantable bioartificial kidney that could replace dialysis altogether.