We examined the ability of a novel spinal cord injury (SCI) device to produce graded morphological and behavioral changes in the adult rat following an injury at thoracic level 10 (T10). The injury device uses force applied to the tissue as the control variable rather than tissue displacement. This has the advantage of eliminating errors that may arise from tissue movement prior to injury. Three different injury severities, defined by the amount of force applied to the exposed spinal cord at T10 (100, 150, and 200 kdyn), were evaluated at two different survival times (7 and 42 d). Unbiased stereology was employed to evaluate morphological differences following the injury. Quantitative behavioral assessment employed the Basso, Beattie, and Bresnahan locomotive rating scale. There was a significant force-related decline in locomotive ability following the injury. Animals subjected to a 200-kdyn injury performed significantly worse than animals subjected to a 100- and 150-kdyn injury. The locomotor ability at different days post injury significantly correlated with the amount of force applied to the spinal cord. Statistical analysis revealed several significant force-related morphological differences following the injury. The greatest loss of white and gray matter occurred at the site of injury impact and extended in both a rostral and caudal direction. Animals subjected to the greatest force (200 kdyn) displayed the least amount of spared tissue at both survival times indicative of the most severe injury. The amount of spared tissue significantly correlated with the locomotor ability. This novel rodent model of SCI provides a significant improvement over existing devices for SCI by reducing variability with a constant preset force to define the injury.

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An evaluation is presented of two control strategies for piezoelectric stack actuators: voltage feedback control and charge feedback control. The study consists of two principal parts: an experimental determination of the nonlinearities inherent in piezoelectric actuators using different control strategies and an analysis of how those nonlinearities would affect the stability and performance of feedback control systems that utilize piezoelectric actuators. The voltage control response of the piezoelectric actuator tested was demonstrably more nonlinear than the same actuator with charge feedback circuitry. Nyquist design analysis reveals that these nonlinearities translate into lower stability margins and potential system performance for voltage control of piezoelectric actuators as compared to charge control. The improved system stability that can be achieved with charge control as compared to voltage control is demonstrated to be signi cant in multi-degree-of-freedom systems but almost negligible when the system dynamics are essentially single degree of freedom. NomenclatureA = stack cross-sectionalarea, m 2 C = series capacitance, F c D = modulus at constant electric displacement, Pa c E = modulus at constant electric eld, Pa D = electric displacement, coulomb/m 2 E = total electric eld, V/m e, d, h, g = piezoelectric constants F = force applied to tip mass, N G = voltage ampli er gain K c = controller subsystem K f = displacement feedback subsystem K g = gain margin, dB K m = stack/mechanism subsystem m = target mass, kg N = system describing model discrepancies n = number of stack layers Q t = total charge applied to stack, coulombs S = strain T = stress, Pa t = stack layer thickness, m V = voltage applied to stack, V V in = ampli er input voltage, V x = displacement, m v = phase margin, deg

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