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Allbirds, the San Francisco-based startup that makes wool sneakers, just closed on $7.25 million in a new round of funding.

The Series A round was led by Maveron, the venture capital firm co-founded by Starbucks CEO Howard Schultz. Seed investors Lerer Hippeau, Peterson Partners, Great Oaks VC, Red Sea Ventures and Slow Ventures also participated, Allbirds said.

The $7.25 million adds to Allbirds' $2.7 million in seed funding.

Launched last March, Allbirds makes men's and women's shoes whose main appeal is their material. The New Zealand merino wool Allbirds uses is designed specifically to be breathable, flexible and lightweight. The shoes are meant for casual wear, not active use.

Co-founder Tim Brown grew up in New Zealand, which gave him the idea for a shoe that would be designed around his country's trademark material. Co-founder Joey Zwillinger came from a background in renewables and biotech, helping to develop Allbirds shoes as a sustainable product.

Allbirds says the shoes are sustainable because they're made with wool, a natural material.

"We go out and look for a need in the market and try to engineer a material that is both natural and sustainable," Zwillinger told Mashable. "That creates an amazing experience."

Allbirds isn't releasing specific numbers about the shoes it has sold since launching in March, but Zwillinger says the company ran through its first round of inventory planned for three months in three weeks. Its plan for 2016 will include three times the inventory it presented to investors during its seed rounds.

The bulk of Allbirds' customers are between 25 and 35 years old, and the company has sold shoes to customers in all 50 states. It also has a strong presence in New Zealand.

The sneakers retail for $95 and are only sold through Allbirds' website.

With this recent round of funding, Allbirds plans to beef up its inventory and support new product development.

Right now, the company is focused on footwear, but it has plans to look into other products down the line, possibly apparel or travel accessories.