Semiaquatic mammals like beavers and sea otters keep warm and even dry while diving by trapping warm pockets of air in dense layers of fur. Inspired by these furry swimmers, MIT engineers have fabricated fur-like rubbery pelts and used them to identify the mechanism by which air is trapped between individual hairs when the pelts are plunged into liquid. Their findings may serve as a guide for designing bio-inspired materials like warm, furry wetsuits. “We are particularly interested in wetsuits for surfing, where the athlete moves frequently between air and water environments,” says Anette Hosoi, a professor of mechanical engineering. “We can control the length, spacing, and arrangement of hairs, which allows us to design textures to match certain dive speeds and maximize the wetsuit's dry region.” To make hairy surfaces, the team first created several molds by laser-cutting thousands of tiny holes in small acrylic blocks. With each mold, a software program to alter the size and spacing of individual hairs was used. The team then filled the molds with a soft casting rubber called PDMS (polydimethylsiloxane), and pulled the hairy surfaces out of the mold after they had been cured.