When Don Hall saw a robot arm made of balloons while visiting Carnegie Mellon University’s Robotics Institute several years ago, he knew instantly that Baymax, a pivotal character in the animated feature he was co-directing for Disney, also would be an inflatable robot.

In the new comedy-adventure, “Big Hero 6,” Baymax, a gentle robot designed to care for humans, is transformed into a warrior and joins a band of high-tech heroes.

Though fictional, the balloon-like robot reflects a growing field of research at Carnegie Mellon University known as soft robotics.

“The movie is a tremendous win for soft robotics,” said Chris Atkeson, professor of robotics, in whose lab the inflatable robotic arm was developed by former student Siddharth Sanan. He said mobile robots made from soft materials — fabrics, balloons, light plastics — offer advantages over metal robots, including lower weight, lower cost and greater safety when operating near people.