Fascicle-selectivity of an intraneural stimulation electrode in the rabbit sciatic nerve

The current literature contains extensive research on peripheral nerve interfaces, including both extraneural and intrafascicular electrodes. Interfascicular electrodes, which are in-between these two with respect to nerve fiber proximity have, however, received little interest. In this proof-of-concept study, an interfascicular electrode was designed to be implanted in the sciatic nerve and activate the tibial and peroneal nerves selectively of each other, and it was tested in acute experiments on nine anaesthetized rabbits. The electrode was inserted without difficulty between the fascicles using blunt glass tools, which could easily penetrate the epineurium but not the perineurium. Selective activation of all tibial and peroneal nerves in the nine animals was achieved with high selectivity (? = 0.98 ± 0.02). Interfascicular electrodes could provide an interesting addition to the bulk of peripheral nerve interfaces available for neural prosthetic devices. Since interfascicular electrodes can be inserted without fully freeing the nerve and have the advantage of not confining the nerve to a limited space, they could, e.g., be an alternative to extraneural electrodes in locations where such surgery is complicated due to blood vessels or fatty tissue. Further studies are, however, necessary to develop biocompatible electrodes and test their stability and safety in chronic experiments.     

A modeling study of nerve fascicle stimulation

A nerve stimulation model has been developed, incorporating realistic cross-sectional nerve geometries and conductivities. The potential field in the volume conductor was calculated numerically using the variational method. Nerve fiber excitation was described by the model of McNeal. Cross-sectional geometries of small monofascicular rat common peroneal nerve and multifascicular human deep peroneal nerve were taken as sample geometries. Selective stimulation of a fascicle was theoretically analyzed for several electrode positions: outside the nerve, in the connective tissue of the nerve, and inside a fascicle. The model results predict that the use of intraneural or even intrafascicular electrodes is necessary for selective stimulation of fascicles not lying at the surface of the nerve. Model predictions corresponded with experimental results of Veltink et al. on intrafascicular and extraneural stimulation of rat common peroneal nerve and to results of McNeal and Bowman on muscle selective stimulation in multifascicular dog sciatic nerve using an extraneural multielectrode configuration.     

A model study of extracellular stimulation of cardiac cells

Point source extracellular stimulation of a myocyte model was used to study the efficacy of excitation of cardiac cells, taking into account the shape of the pulse stimulus and its time of application in the cardiac cycle. The myocyte was modeled as a small cylinder of membrane (diameter 10 μm, length 100 μm) capped at both ends and placed in an unbounded volume conductor. A Beeler-Reuter model modified for the Na⁺ dynamics served to simulate the membrane ionic current. The stimulus source was located on the cylinder axis, close to the myocyte (50 μm) in order to generate a nonlinear extracellular field (φ_e). The low membrane impedance associated with the high frequency component of the make and break of the rectangular current pulse leads to a current flow across the membrane and an abrupt change in intracellular potential (φ_i). Because the intracellular space is very small, φ_e is nearly uniform over the length of the myocyte and the membrane potential (V=φ_i -φ_e) is governed by the applied field φ_e . There is then a longitudinal gradient of membrane polarization which is the inverse of the gradient of extracellular potential. With an anodal (positive) pulse, for instance, the proximal portion of the myocyte is hyperpolarized and the distal portion is depolarized. Based on this principle acid considering the voltage-dependent activation/inactivation dynamics of the membrane, it is shown that a cathodal (negative) pulse is the most efficacious stimulus at diastolic potentials, an anodal current is preferable during the plateau phase of the action potential, and a biphasic pulse is optimal during the relative refractory phase. Thus a biphasic pulse would constitute the best choice for maximum efficacy at all phases of the action potential     

FET photodetectors: A combined study using optical and electron-beam stimulation

The photosensitivity of GaAs FET's has been studied using both optical and electron-beam (e-beam) stimulation at various signal frequencies up to 1.3 GHz. The results indicate that, at high frequencies, the photoconductive mechanism which usually gives a small current gain is the dominant process, whereas at low frequencies photovoltaic mechanisms, which lead to "phototransistor" action are responsible for the observed high photosensitivity and current gain in the device.     

A method for calculating the transient time of a multi-stage thermoelectric cooler

The transient time τ of a cascade thermoelectric cooler is calculated. The method applied does not involve the solution of the heat equations with partial derivatives for semiconductor elements. The transient time τ of a two-stage thermopile was obtained in analytical form as a solution of the quadratic equation whose coefficients depend on semiconductor thermoelectric parameters, the specific heat of the thermopile materials, and the sizes of different parts of the cooler. For a thermopile with an arbitrary number of stages N, the calculation was reduced to the standard problem of finding the eigenvalues of two symmetrical matrices of order N. The calculated values of τ for specific two-stage thermopiles are in satisfactory agreement with the experimental data.     

激光熔池三维非稳态对流传热过程的数值模拟

建立了带有移动热源的激光熔池流体流动及传热过程三维非稳态数学模型。采用自适应网格技术离散求解动量方程,计算出了不同时刻激光熔池温度分布和速度分布。结果表明,激光熔池对流传热非稳态过程是一个预热过程,随着时间的推移,熔池最高温度不断升高,熔深和熔池半径不断增大。非稳态过程按时间先后次序分为3个阶段,初始阶段（加热熔化阶段）、准稳态阶段和快速升温阶段。准稳态阶段熔池形貌、温度分布和速度分布增加幅度不大,且持续时间比另两个阶段长,说明三维准稳态模型是三维非稳态模型的较好近似。计算结果与巳有的实验结果相比大体吻合。     

激光熔池三维非稳态对流传热过程的数值模拟

建立了带有移动热源的激光熔池流体流动及传热过程三维非稳态数学模型.采用自适应网格技术离散求解动量方程,计算出了不同时刻激光熔池温度分布和速度分布.结果表明,激光熔池对流传热非稳态过程是一个预热过程,随着时间的推移,熔池最高温度不断升高,熔深和熔池半径不断增大.非稳态过程按时间先后次序分为3个阶段初始阶段(加热熔化阶段)、准稳态阶段和快速升温阶段.准稳态阶段熔池形貌、温度分布和速度分布增加幅度不大,且持续时间比另两个阶段长,说明三维准稳态模型是三维非稳态模型的较好近似.计算结果与已有的实验结果相比大体吻合.     

Influences of stimulation conditions on recruitment of myelinated nerve fibres: a model study

The dependency of excitation of myelinated nerve fibres on nerve-fibre diameter, and the geometrical spread of recruited nerve fibres in the fascicle during artificial nerve simulations were investigated by varying parameters of a simulation model presented by P.H. Veltink et al. (ibid., vol.35, p.69-75, 1988). The model predicts that fascicle conductivities and the connective tissue sheath surrounding the fascicle, i.e., the extraneural tissue, together with the radius of the fascicle and the thickness of the connective tissue sheath, influence both recruitment order and geometrical spread of recruited motoneuron fibres in the fascicle. The model predicts recruitment probability to increase for increasing nerve-fibre diameter in the entire, or almost the entire range of stimulation pulse-amplitudes in all simulated cases. In a probabilistic sense, this implies an inverse order of recruitment compared to the physiological order.<>     

A model of the stimulation of a nerve fiber by electromagneticinduction

A model is presented to explain the physics of nerve stimulation by electromagnetic induction. Maxwell's equations predict the induced electric field distribution that is produced when a capacitor is discharged through a stimulating coil. A nonlinear Hodgkin-Huxley cable model describes the response of the nerve fiber to this induced electric field. Once the coil's position, orientation, and shape are given and the resistance, capacitance, and initial voltage of the stimulating circuit are specified, this model predicts the resulting transmembrane potential of the fiber as a function of distance and time. It is shown that the nerve fiber is stimulated by the gradient of the component of the induced electric field that is parallel to the fiber, which hyperpolarizes or depolarizes the membrane and may stimulate an action potential. Finally, it predicts complicated dynamics such as action potential annihilation and dispersion.     

A transient numerical scheme for the simulation of GaAs MESFETs and circuits

In this paper, an algorithm, Alternating Direction Scheme (ADS), for solving a two-dimensional time-dependent continuity equation is presented. By separating the continuity equation into two parts based on the direction (x or y) of the vectors and solving them alternatively, ADS deals with the two-dimensional equation in a one-dimensional way and therefore has the advantages of better stability, less calculation, faster convergence and wider time steps than the schemes previously used. The application of the algorithm not only greatly improves the efficiency of the intrinsic response simulation, but also makes the two dimensional GaAs DCFL inverter simulation possible and practical.     

A building block approach to the design and simulation of complex current-memory circuits

A building block approach to the design and simulation of complex current-memory circuits is described. Sampled-data techniques are used to simulate the operation of networks containing a great number of cells. The main building block is the current-memory cell itself, which has been carefully modeled including charge injection and noise. The proposed model has been compared to SPICE simulations on several small examples, and excellent agreement is obtained. Simulation of complex circuits is orders of magnitude faster than a full time-domain simulation with SPICE. The usefulness of the approach is demonstrated through the design and simulation of a current-memory based second order — modulator. The complete characterization of the signal-to-noise and distortion ratio can be achieved in about 4 hours of CPU time on an advanced workstation.This work was partly supported by THOMSON-SINTRA ASM under grant 91.C42.284.     

Electrical stimulation of the auditory nerve: direct current measurement in vivo

Neural prostheses use charge recovery mechanisms to ensure the electrical stimulus is charge balanced. Nucleus cochlear implants short all stimulating electrodes between pulses in order to achieve charge balance, resulting in a small residual direct current (DC). In the present study we sought to characterize the variation of this residual DC with different charge recovery mechanisms, stimulation modes, and stimulation parameters, and by modeling, to gain insight into the underlying mechanisms. In an acute study with anaesthetised guinea pigs, DC was measured in four platinum intracochlear electrodes stimulated using a Nucleus C124M cochlear implant at moderate to high pulse rates (1200-14,500 pulses/s) and stimulus intensities (0.2-1.75 mA at 26-200 microseconds/phase). Both monopolar and bipolar stimulation modes were used, and the effects of shorting or combining a capacitor with shorting for charge recovery were investigated. Residual DC increased as a function of stimulus rate, stimulus intensity, and pulse width. DC was lower for monopolar than bipolar stimulation, and lower still with capacitively coupled monopolar stimulation. Our model suggests that residual DC is a consequence of Faradaic reactions which allow charge to leak through the electrode tissue interface. Such reactions and charge leakage are still present when capacitors are used to achieve charge recovery, but anodic and cathodic reactions are balanced in such a way that the net charge leakage is zero.     

A study of NMOS behavior under ESD stress: simulation and characterization

A full-scale simulation-aided ESD design methodology was used to design a group of NMOS ESD protection units. Silicon results match the simulation data quite well. Both simulation and measurement data show good ESD performance uniformity across NMOS poly finger length and finger number in ladder structures in a large range. Optimal layout pattern for ladder structures was obtained with the aid of simulation.     

A study of transient radiation from the Wu-King resistivemonopole-FDTD analysis and experimental measurements

The cylindrical monopole antenna with a continuous resistive loading is considered as a radiator for temporally short, broad-bandwidth pulses. Specifically, the variation of the resistance used along the monopole is one proposed by Wu and King (1965). This antenna is analyzed by the finite-difference-time-domain (FDTD) method utilizing a new, efficient technique for handling the thin-walled, conducting tube that forms the resistance. The electromagnetic field in the space surrounding the antenna is determined as a function of time, and quantities useful for describing the performance of the antenna are then calculated from these results. Graphical displays of the results are used to give new insight into the physical processes for transient radiation form this antenna. An experimental model is constructed using a discretized version of the Wu-King profile formed from a set of precision, high-frequency resistors. Measurements of both the reflected voltage in the feed line and the time-varying radiated field are in excellent agreement with the theoretical calculations     

海上联合探测系统技术研究

针对海上多平台联合系统探测的作战要求,论述海上联合探测资源适应性优化配置技术和海上联合探测综合信号系统处理技术。提出了基于信息获取和系统抗干扰能力、反隐身能力和电磁兼容能力等的联合探测传感器配置方法和技术途径,并探讨对联合探测传感器配置效能评估要求;提出了传感器目标识别、隐身目标综合处理、联合抗干扰探测等的技术基本途径。     

A simulation study on the evolution of hopping motions in animals

This work was designed to investigate biomechanical aspects of the evolution based on the hypothesis of dynamic cooperative interactions between the locomotion pattern and the body shape in the quadrupedal hopping and the bipedal hopping. The musculoskeletal sagittal-plane model used in the computer simulation consisted of several segments; foot, shank, thigh, trunk, forearm, upper arm, and tail. Two adjacent segments were connected by a hinge joint, and each joint angle was controlled by an extensor and a flexor muscle. The nervous system was represented by a rhythm pattern generator which consisted of 12 neuron models. The genetic algorithm was employed based on the natural selection theory to represent the evolutionary mechanism. The simulation results showed that although hopping could not be seen in the early evolution process, repeated manipulations of the selection and multiplication increased the step length and the locomotion speed and that the resulting hopping motion was close to that of living animals. It was suggested that the advantage of the quadrupedal hopping is high energy efficiency and that of the bipedal hopping is high stability due to the simple and easy motion control. The computational evolution method employed in this study can be a new powerful tool for investigation of the evolution process mostly due to its versatility.     

The roles of traps in the energy loss for aged organic solar cells: A transient photovoltage study

By investigating the turn-on and turn-off photovoltage dynamics as a function of aging time, we reported the roles of traps on the energy loss in organic solar cells composing of copper phthalocyanine (CuPc)/fullerene (C60). Illuminating the device with square pulses of light, a peak of transient photovoltage after turn-on was observed after device degradation. After turn-off, the transient photovoltage first goes to the negative before settling back to zero, which is the result of electron trapping in the C60 layer before being neutralized by re-injected holes. Furthermore, by adding a tris (8-hydroxyquinolinato) aluminum buffer to prevent the traps from propagating into C60 layer, the peak after turn-on is greatly suppressed and the negative peak after turn-off vanishes, supporting the trapped electrons in the C60 layer play the critical role in the appearance of peak of the transient photovoltage.     

Unidirectional block in cardiac fibers: effects of discontinuitiesin coupling resistance and spatial changes in resting membrane potentialin a computer simulation study

The mechanisms for conduction and unidirectional block (UDB) in cardiac tissue under spatial changes in cell-to-cell coupling resistivity (Ri) and resting potential (Vrest) were studied. Cable theory was used to simulate the cardiac fiber, and the Beeler and Reuter model, or a modified model based on the Ebihara-Johnson formulation was used to describe the ionic currents. The effects of discontinuities in Ri as would result from collagenous or fibrotic tissue on propagation characteristics were studied. We were especially interested in the effects on propagation characteristics of discontinuities in Ri in the border zone between normal and ischemic tissue. We found that conduction block is more likely to occur when an abrupt decrease in Ri is encountered as compared to an abrupt increase in Ri. Discontinuities in Ri were found to cause changes in propagation characteristics, changing regions of bidirectional block to UDB or bidirectional propagation. Spatial changes in Vrest were also studied. We found that when Vrest alone was altered, block was not likely to occur, while discontinuities in Ri superimposed with Vrest gradients increased the likelihood of block. We also found that Ri discontinuities located in the border zone between normal and ischemic tissue can create exit block or propagation of a parasystolic focus.     

A study of the magnetic characteristics of permalloy nanostructures by micromagnetic simulation

The dependence of the magnetic characteristics of permalloy nanostructures on their sizes has been studied by the micromagnetic simulation method. The critical size of a square-shaped nanoisland for transition into a single-domain state has been estimated. The dependence of the coercive force of the nanoisland on its thickness and area has been studied.