In the early 2000s engineer David Caldwell might have missed the chance of a lifetime. While developing touch-sensitive switches for home appliances, Caldwell was invited to Cupertino, Calif. to demonstrate his patented TouchSensor technology to Apple, which was looking to replace the mechanical scroll wheel in the original iPod. After peppering him with questions about how it worked, an Apple engineer asked, “Can you integrate it into a round wheel?”