Blast-induced electromagnetic fields in the brain from bone piezoelectricity Ka Yan Karen Lee a, ⁎, Michelle K. Nyein b , David F. Moore c , J.D. Joannopoulos d , Simona Socrate e , Timothy Imholt f , Raul Radovitzky b , Steven G. Johnson g a Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge MA 02139, USA b Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge MA 02139, USA c Defense and Veterans Brain Injury Center, Walter Reed Army Medical Center, Building 1, Room B207, 6900 Georgia Ave. NW, Washington DC 20309, USA d Department of Physics, Massachusetts Institute of Technology, Cambridge MA 02139, USA e Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge MA 02139, USA f Raytheon Co., 870 Winter St., Waltham MA 02451, USA g Department of Mathematics, Massachusetts Institute of Technology, Cambridge MA 02139, USA abstract article info Article history: Received 25 January 2010 Revised 5 May 2010 Accepted 16 May 2010 Available online 12 June 2010 In this paper, we show that bone piezoelectricity—a phenomenon in which bone polarizes electrically in response to an applied mechanical stress and produces a short-range electric field—may be a source of intense blast-induced electric fields in the brain, with magnitudes and timescales comparable to fields with known neurological effects. We compute the induced charge density in the skull from stress data on the skull from a finite-element full-head model simulation of a typical IED-scale blast wave incident on an unhelmeted human head as well as a human head protected by a kevlar helmet, and estimate the resulting electric fields in the brain in both cases to be on the order of 10 V/m in millisecond pulses. These fields are more than 10 times stronger than the IEEE safety guidelines for controlled environments (IEEE Standards Coordinating Committee 28, 2002) and comparable in strength and timescale to fields from repetitive Transcranial Magnetic Stimulation (rTMS) that are designed to induce neurological effects (Wagner et al., 2006a). They can be easily measured by RF antennas, and may provide the means to design a diagnostic tool that records a quantitative measure of the head's exposure to blast insult. © 2010 Elsevier Inc. All rights reserved. Introduction In this paper, we show that a blast pressure wave traversing the skull is a direct source of potentially intense electric fields in the brain, which may have significant neurological effects. This unexpected source does not appear to have been considered in studies of primary blast traumatic brain injury (TBI) (blast-induced neurotrauma (Cernak and Noble- Haeusslein, 2009)). The mechanism is based on the fact that bone is a piezoelectric material: it polarizes electrically in response to an applied mechanical stress and produces a short-range electric field. A shockwave from an explosion generates large stresses in the skull and, consequently, large electric fields. Using computed stresses from full- head-model simulations of typical shockwave exposures from impro- vised explosive device (IED) blasts, we calculate the induced charge density in the skull and estimate the resulting electric fields. We find fields in the brain on the order of 10 V/m in millisecond-scale pulses, more than 10 times stronger than the corresponding IEEE safety guidelines for controlled environments (IEEE Standards Coordinating Committee 28, 2002), and comparable in strength and timescale to fields from repetitive transcranial magnetic stimulation (rTMS) that are known to have neurological effects (Wagner et al., 2006a). Independent of whether these fields play a role in TBI, they can be easily measured by RF antennas and may therefore provide a direct measure of the stresses on the skull: a ‘blast dosimeter’ that could be useful for diagnosis and study of blast-induced TBI. In order to assess the potential neurological impact of any blast- induced electromagnetic fields in the brain, we compare them to published safety standards and also to medical procedures where neurological effects are intentionally induced in the brain via electric fields. At millisecond timescales (ms pulses, ≈1 kHz frequencies) typical of IED-scale blasts, the IEEE safety standard for in-brain electric fields is 0.3 V/m for the general public and 0.9 V/m in controlled environments (IEEE Standards Coordinating Committee 28, 2002). Another point of reference is the medical procedure rTMS, which uses magnetic-field pulses to create electric fields in the brain that, in turn, induce currents which can disrupt brain activity in the short term or have long-term effects by stimulating the release of neurochemicals (Wagner et al., 2006b; Muller et al., 2000). A recent full-head finite- element simulation of a typical commercial rTMS device found maximum in-brain currents densities of around 4.4 A/m 2 (in ms- scale pulses) (Wagner et al., 2006a); the brain has a conductivity of about 0.28 A/Vm at kHz frequencies (Wagner et al., 2006a), and hence NeuroImage 54 (2011) S30–S36 ⁎ Corresponding author. E-mail address: kylkaren@mit.edu (K.Y.K. Lee). 1053-8119/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2010.05.042 Contents lists available at ScienceDirect NeuroImage journal homepage: www.elsevier.com/locate/ynimg