The Third Thumb is a 3D printed device that wraps around your hand and attaches to a wristband, providing an actual third thumb on the opposite side of your hand from your natural thumb. Its movement is controlled by your feet. Two pressure sensors are placed into your shoes and connected to the thumb through a Bluetooth connection. With an extra thumb, you can extend your musical instrument-playing abilities, more easily grasp objects, or just improve your dexterity overall.
https://3dprint.com/180162/third-thumb-project-3d-printing/