The tiny hearts are just 1 mm in diameter and contract at around 30 beats per minute, according to researchers at Abertay University in the UK.
They have been developed specifically to find a cure for heart hypertrophy - a form of heart disease that can lead to sudden death.
Although healthy to begin with, the scientists are using chemicals to simulate the physiological conditions that will make the hearts hypertrophic - enlarged, due to abnormal growth of the cells that make up the heart (cardiomyocytes).
Once diseased, the hearts are then treated with newly developed medications to see if they can prevent the damage from occurring.
"Although human hearts have been grown in labs before, this is the first time it has ever been possible to induce disease in them," said Professor Nikolai Zhelev, who is leading the research.
"Heart hypertrophy can be hereditary, can be caused by diseases such as diabetes, or can be caused by doing too much strenuous exercise.
"The disease causes the heart muscle to thicken and stiffen, and makes it harder for the heart to pump blood around the body.
"In some people, a life-threatening abnormal heart rhythm will develop, and this is the most common cause of sudden death in young people.
"Although there are treatments, these only help to control the symptoms, and there is no known cure at the moment," Zhelev said.
Zhelev believes the miniature hearts could help change that.
Using biosensors, Zhelev has been able to label specific molecules within the hearts to see where they are going - which pathway they follow.
By establishing which molecules cause the hearts to become hypertrophic, he has been able to target drugs at these molecules and prevent them from going down the path they would usually take, and prevent them from becoming hypertrophic.
"We've tested a number of different compounds on these hearts - some of them entirely new ones that haven't been tested in humans yet, which is why we're testing them on these hearts we've grown in the lab," Zhelev said.
"One of these compounds, however, is a drug that we have developed which has just completed phase-two clinical trials in cancer patients and has had very positive results.
"Although heart cells are the only ones in the body that will never get cancer, we noticed that the pathways the molecules in hypertrophic hearts follow are similar to those followed by molecules in cancerous cells, so we thought testing this new drug on these hearts might have the same positive effect. And this has certainly proved to be the case," he said.