Happy Meal toys like Transformer figurines and Hot Wheels cars have sparked joy with little kids for decades—and now, one of the designers behind the miniatures is changing lives for thousands of people. Dallas-based entrepreneur Nancy Hairston founded MedCAD, a surgical solutions company, in 2007—and in the decades since, its 3D-printing innovations have helped patients recognize themselves in the mirror again. But before Hairston was building skull implants, she spent most of her career in the design wo ...