Five days. That’s how long intracranial pressure and temperature typically need to be monitored in the case of traumatic brain injury. And that’s at least how long flexible, dissolvable sensors created by a research team at the University of Illinois led by professor John Rogers will operate accurately.

http://spectrum.ieee.org/view-from-the-valley/biomedical/devices/siliconbased-sensors-slip-into-the-brain-then-dissolve-when-their-jobs-are-done