Could
Explosions Generate Electrical Impulses in the Brain?
Department
of Mathematics
Massachusetts
Institute of Technology
Cambridge
MA 02139
Ka
Yan Karen Lee
Department
of Electrical Engineering
Massachusetts
Institute of Technology
Cambridge
MA 02139
Michelle
K. Nyein
Department
of Aeronautics and Astronautics
Massachusetts
Institute of Technology
Cambridge
MA 02139
David
F. Moore
Defense
and Veterans Brain Injury Center
Walter
Reed Army Medical Center
Washington
DC 20309
J.
D. Joannopoulos
Department
of Physics
Massachusetts
Institute of Technology
Cambridge
MA 02139
Simona Socrate
Department
of Mechanical Engineering
Massachusetts
Institute of Technology
Cambridge
MA 02139
Timothy
Imholt
Raytheon
Company
Waltham,
MA 02451
Raul
Radovitzky
Department
of Aeronautics and Astronautics
Massachusetts
Institute of Technology
Cambridge
MA 02139
Popular
version of paper 2aBB4
Presented
Tuesday morning, April 20, 2010
159th
ASA Meeting, Baltimore, MD
Explosions
can lead to injuries in many ways, from shrapnel impact to lung damage, and
understanding, diagnosing, and mitigating these injuries is a major concern in
order to safeguard soldiers stationed in dangerous areas. Although improvements
in body armor have greatly increased the survivability of improvised explosive
devices (IEDs) and similar explosive blasts, an important area of recent study
has been traumatic brain injuries (TBI). Sometimes, TBI seems to arise from the
blasts pressure wave itself as it impacts the head, even without any obvious
physical injury (such as from shrapnel). The questions then arise: by what
physical mechanism could a blast wave affect the brain, how can such an injury
be diagnosed, and (hopefully) how can it be mitigated?
Our
research has recently uncovered an unexpected possible mechanism by which a
blast wave might affect the brain: electric fields that are created when the
skull is impacted by the blast wave, due to a property of bone called piezoelectricity, which may be large
enough to have a neurological effect. Because our initial work is primarily
theoretical, we cannot yet say for certain whether this mechanism plays a
significant role in TBI, but even if not, it may provide a new pathway for
measurement and diagnosis of blast-induced brain injuries.
Neurons
in the brain communicate with one another via millisecond-scale electrical
signals, and it is well known that this communication can be disrupted by
externally produced electric fields. Most famously, relatively large fields and
currents are used in electroconvulsive therapy -- the modern version of
electroshock therapy -- which is well known to have sometimes-severe side
effects such as temporary amnesia. There are also medical procedures that use
smaller-scale electromagnetic fields, such as transcranial
magnetic stimulation (TMS), which can temporarily disrupt brain functions and
can have longer-term effects by stimulating release of neurochemicals.
When
a blast wave -- a short pulse of rapid pressure variations -- passes through
matter, there are a variety of ways in which electromagnetic fields can be
generated as a side effect. One of the strongest is piezoelectricity, a
property of certain materials by which they electrically polarize in response
to stress, commonly used for pressure sensors and other applications. It turns
out that one piezoelectric material is bone,
a fact that was first demonstrated in 1957 and was thought to play some role in
bone growth, but which had escaped notice in studies of blast injuries. The
impact of a blast wave on bone can thus cause a pulse of electric field to be
generated over a short distance (centimeters)if this happens in the case of
the skull, then these electric fields may have a neurological effect on brain
activity. Theoretical estimates of these fields from an IED-scale blast
indicate that they could exceed IEEE (Institute of Electrical and Electronics
Engineers) safety guidelines by a factor of 10, and may be comparable in
magnitude and timescale to procedures such as TMS that are known to have
neurological effects.
Many
uncertainties remain at this point, the most basic of which is that all
experimental measurements of bone piezoelectricity have been based on femurs
and similar bones, and no published data for cranial bone exists. Even if the
fields are as strong as predicted, it is not certain how their neurological
impact compares with that of the pressure wave itself.
However,
even generated electric pulses turn out to be neurologically irrelevant, they
may open the door to a new class of diagnostic tool for blast-induced brain
injuries. Electric fields are easy to detect with antennas, so a set of small
antennas attached inside a soldiers helmet could record the field produced by
the blast-skull polarization, providing a direct measure of the soldiers head
exposure to the blast -- a blast dosimeter that could be an important and
simple new diagnostic tool.