Satoko Hirabayashi

Optimization of Vehicle Deceleration to Reduce Occupant Injury Risks in Frontal Impact

Objective: In vehicle frontal impacts, vehicle acceleration has a large effect on occupant loadings and injury risks. In this research, an optimal vehicle crash pulse was determined systematically to reduce injury measures of rear seat occupants by using mathematical simulations. Method: The vehicle crash pulse was optimized based on a vehicle deceleration-deformation diagram under the conditions that the initial velocity and the maximum vehicle deformation were constant. Initially, a spring–mass model was used to understand the fundamental parameters for optimization. In order to investigate the optimization under a more realistic situation, the vehicle crash pulse was also optimized using a multibody model of a Hybrid III dummy seated in the rear seat for the objective functions of chest acceleration and chest deflection. A sled test using a Hybrid III dummy was carried out to confirm the simulation results. Finally, the optimal crash pulses determined from the multibody simulation were applied to a human finite element (FE) model. Results: The optimized crash pulse to minimize the occupant deceleration had a concave shape: a high deceleration in the initial phase, low in the middle phase, and high again in the final phase. This crash pulse shape depended on the occupant restraint stiffness. The optimized crash pulse determined from the multibody simulation was comparable to that from the spring–mass model. From the sled test, it was demonstrated that the optimized crash pulse was effective for the reduction of chest acceleration. The crash pulse was also optimized for the objective function of chest deflection. The optimized crash pulse in the final phase was lower than that obtained for the minimization of chest acceleration. In the FE analysis of the human FE model, the optimized pulse for the objective function of the Hybrid III chest deflection was effective in reducing rib fracture risks. Conclusions: The optimized crash pulse has a concave shape and is dependent on the occupant restraint stiffness and maximum vehicle deformation. The shapes of the optimized crash pulse in the final phase were different for the objective functions of chest acceleration and chest deflection due to the inertial forces of the head and upper extremities. From the human FE model analysis it was found that the optimized crash pulse for the Hybrid III chest deflection can substantially reduce the risk of rib cage fractures. Supplemental materials are available for this article. Go to the publishers online edition of Traffic Injury Prevention to view the supplemental file.

Effects of Wall Stress on the Dynamics of Ventricular Fibrillation: A Computer Simulation Study of Mechanoelectric Feedback

The prevalence of ventricular fibrillation (VF) is increased in the mechanically compromised heart. Computer simulation is a useful means of investigating the mechanisms underlying this phenomenon. Using our latest research as an example, we show how computer simulations are performed and what they reveal. We have developed a fully coupled electromechanical model of the human ventricular myocardium. The model formulated the biophysics of specific ionic currents, excitation-contraction coupling, anisotropic non-linear deformation of the myocardium, and mechanoelectric feedback through stretch-activated channels. Our model suggested that sustained stretches shortens action potential duration (APD) and flattens the electrical restitution curve, whereas stretches applied at the wavefront prolongs APD. The wavefront around the core was highly stretched, even at lower pressures, resulting in a prolongation of APD and extension of the refractory area in the wavetail. As left ventricular pressures increased, the stretched area became wider and the refractory area was further extended. The extended refractory area in the wavetail facilitated wave break-up and the meandering of tips through the interaction between wavefronts and wavetails. This simulation study indicated that mechanical loading promotes meandering and wave breaks of spiral re-entry through mechanoelectric feedback. Mechanical loading in pathological conditions may contribute to the maintenance of VF through these mechanisms.