The folks at Engadget got a lab tour at Harvard where they saw some interesting technologies that scientists and engineers are working on, including this mechanism called the Cranial Drilling Device with Retracting Drill Bit After Skull Penetration. As the name implies, the bit recedes back into the housing once pressure on its tip drops, preventing it from entering the soft tissue of the brain.
Currently, drilling is typically done with a hand-cranked device that surgeons are trained to safely operate. The new tool should help increase safety while speeding up the drilling process, and perhaps may lend itself to use by non-neurosurgeon clinicians for performing trepanations for intracranial pressure monitoring and other uses.
Check out the following video: