  The discovery that repetitive mild traumatic brain injury (mTBI) can result
in chronic traumatic encephalopathy (CTE) in high risk contact sports has led
to increased scrutiny of head protective gear. In this work, we asked if it was
physically possible to prevent mTBI in American football with helmets alone.
Here, we show that modern helmets of several types are unlikely to prevent mTBI
from high speed collisions as might be seen in the NFL, but that introducing
liquid as an energy absorbing medium can dramatically reduce the forces of
impact across a spectrum of impact severities. We hypothesized that a helmet
which transmits a nearly constant force during football impacts is sufficient
to reduce biomechanical loading in the brain below the threshold of mTBI. To
test this hypothesis, we first show that the optimal impact force transmitted
to the head, in terms of brain strain, is in fact a constant force profile.
Then, to generate a constant force with a helmet, we implement a computational
model of a fluid-based shock absorber that adapts passively to any given impact
speed. Computer simulation of head impacts with liquid shock absorption
indicate that, at the highest impact speed, the average brain tissue strain is
reduced by 27.6% $\pm$ 9.3 compared to existing helmet padding that is
available on the market. These simulations are based on the NFL's helmet test
protocol and predict that adding liquid shock absorbers could reduce the number
of concussions by at least 75%. Taken together, these results suggest that the
majority of mTBI in football could be prevented with more efficient helmet
technology.
