Development of a miniaturized position sensing system for measuring brain motion during impact - biomed 2013.

Since 2000, the Department of Defense has documented more than 253,000 cases of Traumatic Brain Injury (TBI). A significant portion of these injuries were attributed to explosive events, yet ninety-eight percent were non-penetrating. Understanding the response of the brain to blast events is critical, yet the mechanisms of brain injury from explosive trauma are poorly understood. This knowledge gap has led to an increased research focus on devices capable of investigating human brain response to non-penetrating, blast-induced loading. Furthermore, traumatic brain injury is a major issue for the civilian population as well with over 1.7 million cases of TBI per year in the US, primarily from falls and motor vehicle accidents. Current head surrogates and instrumentation are incapable of directly measuring critical parameters associated with TBI, such as brain motion, during dynamic loading. To this end, a novel sensor system for measuring brain motion inside of a human head surrogate was conceptualized and developed. The positioning system is comprised of a set of three fixed “generator” coils and a plurality of mobile, miniaturized “receiver” coil triads. Each generator coil transmits a sinusoidal electromagnetic signal at a unique frequency, and groups of three orthogonally arranged “receiver” coils detect these signals. Because of the oscillatory nature of these signals, the magnetic flux through the coil is always changing, allowing the application of Faraday’s Law of Induction and the point dipole model of an electric field to model the strength and direction of the field vector at any given point. Thus, the strength of the signal measured by a particular receiver coil depends on its position and orientation relative to the fixed position of the generators. These predictable changes are used to determine the six degrees of freedom (6-DOF) motion of the receiver. To calibrate and validate the system, a receiver coil was moved about in a controlled manner, and its actual position recorded by optical methods. Comparing the known position to the computed position at each time instance, a set of calibration constants were developed for each receiver triad. These constants were then utilized to convert receiver signal data into actual receiver position and orientation. Comparing this test case and several others like it, mean error was determined to be almost always less than 1.0 mm, and less than 0.5 mm >85% of the time. Additionally, high rate validation was conducted to confirm operation of the system in the impact domain. A coil was accelerated to approximately 15 m/sec along a fixed axis by ballistic impact and tracked by high speed video. The computed position was within 1 mm of the actual position 93% of the time and within 0.5 mm 83% of the time. The successful development and calibration of this sensing system now enables the direct measurements of brain displacement due to mechanical insults applied to a human head surrogate.

Computational and experimental models of the human torso for non-penetrating ballistic impact.

Both computational finite element and experimental models of the human torso have been developed for ballistic impact testing. The human torso finite element model (HTFEM), including the thoracic skeletal structure and organs, was created in the finite element code LS-DYNA. The skeletal structure was assumed to be linear-elastic while all internal organs were modeled as viscoelastic. A physical human surrogate torso model (HSTM) was developed using biosimulant materials and the same anthropometry as the HTFEM. The HSTM response to impact was recorded with piezoresistive pressure sensors molded into the heart, liver and stomach and an accelerometer attached to the sternum. For experimentation, the HSTM was outfitted with National Institute of Justice (NIJ) Level I, IIa, II and IIIa soft armor vests. Twenty-six ballistic tests targeting the HSTM heart and liver were conducted with 22 caliber ammunition at a velocity of 329 m/s and 9 mm ammunition at velocities of 332, 358 and 430 m/s. The HSTM pressure response repeatability was found to vary by less than 10% for similar impact conditions. A comparison of the HSTM and HTFEM response showed similar pressure profiles and less than 35% peak pressure difference for organs near the ballistic impact point. Furthermore, the peak sternum accelerations of the HSTM and HTFEM varied by less than 10% for impacts over the sternum. These models provide comparative tools for determining the thoracic response to ballistic impact and could be used to evaluate soft body armor design and efficacy, determine thoracic injury mechanisms and assist with injury prevention.

Multisite microprobes for neural recordings.

Multisite, passive microprobes have been developed to allow simultaneous recording of action potential activity from multiple neurons at different locations in the brain. The microprobes were fabricated using standard integrated circuit techniques. The probe is a planar structure that consists of gold electrodes sandwiched between two polyimide dielectric layers and bonded to a molybdenum structural support. Windows in the top dielectric layer expose the electrode sites and bonding pads. In two distinct versions of the probe four or six recordings sites, respectively, of approximately 25 microns 2 are arranged on a dagger-shaped structure which can penetrate the pia. The bonding pads and interconnect wires at the probe head are entirely encapsulated in a tubular fixture that is packed with silicone RTV and sealed with epoxy to protect the interconnections from contact with body fluids. The site impedances at 1 kHz are typically between 2 and 4 M omega. Probe lifetimes for continuous immersion in physiological saline solution, as measured by impedance, have exceeded 750 h. The failure mechanism is believed to be due to moisture and ion absorption in the top dielectric layer. In acute neurophysiological experiments using the four site probes, action potential activity was recorded from physiologically identified neurons in the dorsal column nuclei of anesthetized rat.