Robert S. Cargill

Mechanical stretch to neurons results in a strain rate and magnitude-dependent increase in plasma membrane permeability.

The mechanism by which mechanical impact to brain tissue is transduced to neuronal impairment remains poorly understood. Using an in vitro model of neuronal stretch, we found that mechanical stretch of neurons resulted in a transient plasma membrane permeability increase. Primary cortical neurons, seeded on silicone substrates, were subjected to a defined rate and magnitude strain pulse by stretching the substrates over a fixed cylindrical form. To identify plasma membrane defects, various sized fluorescent molecules were added to the bathing media either immediately before injury or 1, 2, 5, or 10 min after injury and removed one minute later. The percent of cells that took up dye depended on the applied strain rate, strain magnitude and molecular size. Severe stretch (10 sec(-1), 0.30) resulted in significant uptake of all tested molecules (ranging between 0.5 and 8.9 nm radii) with up to 60% of cells positively stained. Furthermore, the neurons remained permeable to the smallest molecule (carboxyfluorescein, 380 Da) up to 5 min after severe stretch but were only permeable to larger molecules (>/=10 kDa) immediately after stretch. These transiently formed membrane defects may be the initiating mechanism that translates mechanical stretch to cellular dysfunction.

Susceptibility of hippocampal neurons to mechanically induced injury

Experimental models of traumatic cortical brain injury in rodents reveal that specific regions of the hippocampus (e.g., CA3 and hilar subfields) are severely injured despite their distance from the initial insult. Hippocampal neurons may be intrinsically more vulnerable to mechanical insult than cortical neurons due to increased NMDA receptor densities and lower energy capacities, as evidenced by increased susceptibility to ischemic insults. The selective vulnerability of hippocampal neurons was evaluated using an in vitro model of TBI in which either primary rat cortical or hippocampal neurons (E17) seeded onto silicone substrates were subjected to graded levels of mechanical stretch. Although cortical neurons exhibited significantly longer increases in stretch-induced membrane permeability, injury of hippocampal neurons resulted in larger increases in intracellular free calcium concentration [Ca(2+)](i) and cell death. [ATP](i) deficits due to stretch were apparent by 60 min after injury in cortical neurons but recovered by 24 h, whereas significant deficits in [ATP](i) were not observed in hippocampal neurons until 24 h after injury. MK801 pretreatment decreased the stretch-induced [Ca(2+)](i) transients in both hippocampal and cortical cultures, thereby negating the regional specificity. However, MK801 pretreatment did not improve hippocampal viability and paradoxically, significantly increased cell death among cortical neurons. As the hippocampus is the primary brain region responsible for the memory deficits and epileptic seizures associated with TBI, understanding why this region is selectively damaged could lead to the development of more accurate mechanical tolerances as well as effective pharmaceutical agents.

An in vitro model of neural trauma: device characterization and calcium response to mechanical stretch.

An in vitro model for neural trauma was characterized and validated. The model is based on a novel device that is capable of applying high strain rate, homogeneous, and equibiaxial deformation to neural cells in culture. The deformation waveform is fully arbitrary and controlled via closed-loop feedback. Intracellular calcium ([Ca2+]i) alterations were recorded in real time throughout the imposed strain with an epifluorescent microscopy system. Peak change in [Ca2+]i recovery of [Ca2+]i and percent responding NG108-15 cells were shown to be dependent on strain rate (1(-1) to 10(-1)) and magnitude (0.1 to 0.3 Greens Strain). These measures were also shown to depend significantly on the interaction between strain rate and magnitude. This model for neural trauma is a robust system that can be used to investigate the cellular tolerance and response to traumatic brain injury.

An assessment of the strength of NG108-15 cell adhesion to chemically modified surfaces.

The strength of adhesion of NG108-15 cells to glass substrates modified with adsorbed proteins (laminin and poly-ornithine) or modified with covalently bound peptides (tri-ornithine and Tyr-Ile-Gly-Ser-Arg) was quantitatively assessed, by determining the shear stresses necessary to denude the cells from substrates using a spinning disk device. The shear stresses required to detach NG108-15 cells from glass modified with either adsorbed poly-ornithine or with both poly-ornithine and laminin were significantly (P < 0.05) higher than the shear stresses required to detach the cells from plain glass substrates. Covalent surface modifications resulted in higher strengths of NG108-15 adhesion than were exhibited on surfaces modified with adsorbed proteins. NG108-15 cell adhesion strength was maximal on surfaces covalently modified with only amine groups (without any peptides or proteins). These results indicate that general (i.e., not necessarily receptor-specific) surface modification strategies, which increase the net surface charge of a substrate, will elicit strong adhesion of NG108-15 cells.

Development of a computational method to predict occupant motions and neck loads during rollovers

The mechanics of on-road, friction-induced rollovers were studied with the aid of a three-dimensional computer code specifically derived for this purpose. Motions of the wheels, vehicle body, occupant torso, and head were computed. Kanes method was utilized to develop the dynamic equations of motion in closed form. On-road rollover kinematics were compared to a dolly-type rollover at lesser initial speed, but generating a similar roll rotation rate. The simulated on-road rollover created a roof impact on the leading (drivers) side, while the dolly rollover simulation created a trailing-side roof impact. No head-to-roof contacts were predicted in either simulation. The first roof contact during the dolly-type roll generated greater neck loads in lateral bending than the on-road rollover. This work is considered to be the first step in developing a combined vehicle and occupant computational model for studying injury potential during rollovers.

Mechanical Properties of the Fetal Bovine Bladder Lamina Propria and Their Correlation With Changes in Extracellular Matrix

The urinary bladder is an organ whose purpose is to store urine at low pressure and periodically expel it. This system normally operates at relatively low pressure to protect the kidneys from the deleterious effects of increased pressure. In certain pathologies, this organ can be subject to a decrease in compliance (“stiffening”) and an increase of the storage pressure which causes higher back pressure on the kidney and ultimately results in kidney damage if untreated. Clinically, these pathologies are exemplified in disorders such as myelomeningocele, posterior urethral valves, dysfunctional voiding, and disorders associated with spinal cord injuries. In these disorders, bladder structure is altered and the bladder becomes stiff and noncompliant.© 2008 ASME

Injury Biomechanics: Evaluating the Evidence to Determine Causation

Biomechanical engineering is a field that encompasses a wide variety of applications including the development and evaluation of medical devices, research regarding sports and sporting equipment, and investigations of how individuals are injured and how those injuries could be prevented. Understanding human tolerance, injury mechanisms, and the facts regarding a given scenario allows the biomechanical engineer to use these data to determine how an individual was injured. As the field of biomechanics is becoming more broadly understood, the biomechanical engineer is being called upon more frequently to contribute to forensic analyses. According to Merriam-Webster, the definition of forensic is as follows: “relating to or dealing with the application of scientific knowledge to legal problems.” For a biomechanical engineer, an increasingly reasonable option is to pursue a career in forensic analysis, where his/her knowledge and skills are employed to help attorneys, judges, juries, and other participants in legal proceedings understand technical concepts key to understanding the case at hand.Copyright

HEAD MOTION IN THE CORONAL PLANE DURING LOW-SPEED LATERAL IMPACT COLLISIONS

INTRODUCTION Although numerous studies have been done on frontal and rear impact collisions and the resulting occupant kinematics, relatively few have considered the effects of lateral impacts on occupant kinematics. Understanding the kinematics of the head during lateral impacts can assist in determining how the passive response of the human body relate to one another when presented with such a stimulus. This information is invaluable in the design of motor vehicle safety systems, amusement park rides, and other situations where an unexpected lateral impact may occur.