Medical devices that have been implanted into human bodies (as well as other devices that have been implanted into other objects) and are controllable via RF commands are subject to intentional tampering and unintentional interference. This drawback can be avoided by designing the implanted system so that either (a) the implanted system can only be controlled via ultrasound signals, or (b) the implanted system can only be controlled via non-ultrasound signals (e.g., RF signals) after the system has been unlocked in response to receipt of a particular ultrasound signal. Because ultrasound waves at the relevant frequencies (e.g., 1-5 MHz) can only enter a body when an ultrasound transmitter is positioned in direct contact with the body, these configurations provide a significant amount of additional security.
