Scientists have turned rat hearts into miniature human hearts to improve the accuracy of tests on the rodents.
In order to make the rat hearts more human-like, they were removed from the animals and researchers used the 4-Flow cannulation process.
This process means special fluids can be injected into the arteries and veins of the heart, but normal flow and full circulation can continue.
The fluids added by the scientists take away the rat cells, but don't affect the structure of the organ, according to New Atlas.
Human cells are then added to the heart to replace the rat cells and create the miniature human heart.
Scientists now want to use the miniature hearts to test new drugs and treatments more accurately, which should mean testing the safety of drugs and treatments is improved.
Previously researchers have created miniature lungs, livers, kidneys and brains by using stem cells for disease research.
It is hoped that full-size organs will one day be created by using the stem cells for human transplants.