Answer: a smart prosthetic, and that depends.
Danielle Clode, a graduate student in product design at London's Royal College of Arts, has created a 3-D-printed, sensor-controlled digit called the third thumb. While it may initially sound a bit creepy, the extra gripper might be helpful — especially in our mobile-device-crazed society. The smart device is controlled by Bluetooth-enabled pressure sensors in the wearer’s shoes, which activate the wrist-worn motors that move the thumb. The actual curling of the third thumb itself is controlled by wires attached to the motor.
According to the Hackster’s blog, controlling the additional digit via toe pressure is not at all uncommon for upper-body prosthetics, and is easily managed after only a short familiarization process. The blog's author compares the toe control to skiing, where subtle pressure changes are all that’s needed to control direction and stability. Other devices of a similar nature have been developed, but this appears to be somewhat original in the sense that it is not replacing a lost digit, but supplementing the existing digits.