Patients with painful open sores on their legs have long used a skin substitute as treatment — but we’ve only now figured out how it works. The skin substitute actually encourages the skin to learn to heal again, researchers say, and understanding this process can help us develop even better technologies to heal wounds.

The skin substitute, called bilayered living cellular construct or Apligraf, is like a patch that you put over these leg ulcers. It seems to help the ulcer improve, and now we know how. In a study published today in the journal Science Translational Medicine, scientists ran a clinical trial in which 30 patients with chronic leg ulcers either had compression therapy (think tight bandages) or compression therapy along with Apligraf. They collected tissue from the edge of the ulcer both before treatment and one week after it. After running various molecular analyses on the tissue, they found that Apligraf actually prompts the body to turn on inflammatory response just like it would use for a normal wound. Basically, Apligraf encourages the body to treat this chronic ulcer like a normal wound that can heal.

This is an important insight because chronic ulcers are both painful and common. In the US, patients spend about $15 billion to treat these ulcers, and so understanding how best to treat them could both help people and save a lot of money. The US Food and Drug Administration approved Apligraf about 15 years ago, so we’ve already been reaping its benefits. But the specific data that this study provides — for example, knowing exactly which cells and processes Apligraf triggers — can be used in future trials to see if we can improve Apligraf. And we can use this information to build alternatives that might be even more helpful.