Researchers in the United States on Sunday said they had bio-engineered a kidney and transplanted it into rats, marking a step forward in a quest to help patients suffering from kidney failure.

The prototype proves that a “bio-kidney” can work, emulating breakthroughs elsewhere to build replacement structures for livers, hearts and lungs, they said.

Described in the journal Nature Medicine, the work entailed taking a rat kidney and stripping out its living cells using a detergent solution, leaving behind a shell made of collagen.

The next step was to repopulate this empty structure with living cells, comprising human endothelial cells, which line the walls of blood vessels in the kidney, and kidney cells taken from newborn rats.

The trick was then to “seed” these cells in the correct part of the kidney, using a muscle duct called the ureter as a tube.