When you’re a professional soccer player, companies practically pelt you with free shoes. Just ask Tim Brown, who spent his twenties playing for New Zealand’s national team, going all the way to the 2010 World Cup in South Africa. “I got tons of shoes from brands like Nike that were brightly colored, covered in logos, and made of synthetics,” Brown says. “But I was always interested in finding a shoe that was simple, beautiful, and made from natural materials.”

Tim Brown

So in his spare time, Brown set out to invent a sustainable, biodegradable shoe. He would go down to the local fabric store to find alternative materials. Since he was in New Zealand, a country where there are six sheep for every human, wool was abundant and renewable. But wool also has many properties that you might want in a shoe: It regulates temperature, wicks away moisture, and can help to control bad odors.

Brown tinkered with creating a high-performing wool shoe. His main struggle was developing a wool fabric that was strong enough to use in a shoe. “Nothing existed,” he says. “That probably should have been a sign that I should stop, go back to my Playstation, and live a quiet, simple life somewhere in New Zealand.”

Instead, he applied for a grant from the New Zealand wool industry and with his funding began a project with AgResearch, a textile institute, to engineer a new fabric that would be tough but also soft and comfortable enough to wear without socks. The result is a patent-pending woolen material that he has used to create what he describes as “wool runners”–performance woolen shoes that can be used for working out, but also wearing every day.

Two years ago, he launched a Kickstarter campaign to see if there was a demand for his product, which was pitched as a shoe you could wear without socks without your feet getting smelly. And there certainly was demand: In five days, he received over $119,000 from 970 backers. The shoes were so popular that he decided to turn this project into a career. He teamed up with biotech engineer and renewable materials expert Joey Zwillinger to improve the construction and introduce additional sustainable materials such as a vegetable oil-based polyurethane insole. Today, they launch their company, Allbirds, and a line of wool running shoes that cost $95.

Joey Zwillinger

Brown’s not the only one who is thinking about how to integrate wool’s properties into shoes. London-based Mahabis has developed a woolen slipper with a sole that can be worn outside. A Swiss startup called Baabuk has invented a wool sneaker. And in the U.S., Tom’s has developed a line of wool shoes. Allbirds hopes to make inroads into this market among consumers who are interested in sustainability in their footwear, but who also want hardy shoes that can be used for going to the gym or for a run. The shoe fits nicely into the atheleisure trend of products that allow people to go from athletic activity into their everyday lives.

Manufacturing a wool shoe from scratch is a complex process. Shoe makers often rely on large companies that produce leather and other synthetic materials that are used to produce shoes. “We discovered that few shoe companies manage the supply chain and the raw material sourcing closely,” Zwillinger says.