Quartz crystals have been at the heart of implantable cardiac pacemakers, defibrillators, and other implantable medical devices since these devices were first introduced.  They are one of the basic building blocks of implantable medical devices in that they provide the basic timing, or rhythm, based on which the device operates.  In many implantable medical devices, quartz crystals have historically been the second largest device on the board after the battery.