When designing in vivo wireless sensor systems, it is critical that advanced energy scavenging (or harvesting) techniques be incorporated into the system design from the ground-level. The integration of this technology into wireless digital sensors opens up many advanced applications including ingestible sensors, lifetime implantable sensors, and E-textiles. More work is needed in designing these wireless digital systems to ensure maximum efficiency in saving and harvesting energy to bring to reality these types of wireless in vivo sensors.