Selective stimulation of sacral nerve roots for bladder control: astudy by computer modeling

The aim of this study was to investigate theoretically the conditions for the activation of the detrusor muscle without activation of the urethral sphincter and afferent fibers, when stimulating the related sacral roots, Therefore, the sensitivity of excitation and blocking thresholds of nerve fibers within a sacral root to geometric and electrical parameters in tripolar stimulation using a cuff electrode, have been simulated by a computer model. A 3D rotationally symmetrical model, representing the geometry and electrical conductivity of a nerve root surrounded by cerebrospinal fluid and a cuff was used, in combination with a model representing the electrical properties of a myelinated nerve fiber. The electric behavior of nerve fibers having different diameters and positions in a sacral root was analyzed and the optimal geometric and electrical parameters to be used for sacral root stimulation were determined. The model predicts that an asymmetrical tripolar cuff can generate unidirectional action potentials in small nerve fibers. While blocking the large fibers bidirectionally. This result shows that selective activation of the detrusor may be possible without activation of the urethral sphincter and the afferent fibers     

Selectivity of multiple-contact nerve cuff electrodes: a simulation analysis

Advances in functional neuromuscular stimulation (FNS) have increased the need for nerve cuff designs that can control multiple motor functions through selective stimulation of selected populations of axons. This selectivity has proved to be difficult to achieve. Recent experiments suggest that it is possible to slowly reshape peripheral nerve without affecting its physiological function. Using computer simulations we have tested the hypothesis that changing the cross section of a nerve from a round to a flat configuration can significantly improve the selectivity of a nerve cuff. We introduce a new index to estimate selectivity to evaluate the various designs. This index is based on the ability of a nerve electrode to stimulate a target axon without stimulating any other axons. The calculations involve a three-dimensional finite element model to represent the electrical properties of the nerve and cuff and the determination of the firing properties of individual axons. The selectivity rating was found to be significantly higher for the Flat Cuff than the Round Cuff. The result was valid with uniform or random distribution of axons and with a random distribution of fascicles diameters. Flattening of individual fascicles also improved the selectivity of the Flat Cuff but only when the number of contacts used was increased to maintain uniform contact density. Therefore, cuff designs that can reshape the nerve into flatter configurations should yield better cuff performance than the cylindrical cuffs but will require higher contact density.     

Closed-loop stimulation of hypoglossal nerve in a dog model of upper airway obstruction

Electrical stimulation of upper airway (UAW) muscles has been under investigation as a treatment method for obstructive sleep apnea (OSA). Particular attention has been given to the electrical activation of the genioglossal muscle, either directly or via the stimulation of the hypoglossal nerve (HG), since the genioglossus is the main tongue protrusor muscle. Regardless of the stimulation site or method, an implantable electrical stimulation device for OSA patients will require a reliable method for detection of obstructive breaths to apply the stimulation when needed. In this paper, we test the hypothesis that the activity of the HG nerve can be used as a feedback signal for closed-loop stimulation of the HG nerve in an animal model of UAW obstruction where a force is applied on the submental region to physically narrow the airways. As an advantage, the method uses a single electrode for both recording and stimulation of the HG nerve. Simple linear filtering techniques were found to be adequate for producing the trigger signal for the electrical stimulation from the HG recordings. Esophageal pressure, which was used to estimate the size of the UAW passage, returned to the preloading values during closed-loop stimulation of the HG nerve. The data demonstrate the feasibility of the closed-loop stimulation of the HG nerve using its activity as the feedback signal.     

A Two-Part Model for Determining the Electromagnetic and Physiologic Behavior of Cuff Electrode Nerve Stimulators

A two-part model for determining the electrophysiological behavior of a cuff electrode designed for unidirectional nerve stimulation is presented. In the first part, a model is described which evaluates the potential field of the cuff electrode system in the absence of a nerve. The model recognizes boundary conditions through the specification of secondary sources on the cuff; the unknown source strengths are found through the solution of a Fredholm integral equation. In the second part of the study, the potential field derived for the cuff is taken to be the applied field of a myelinated nerve fiber considered through a linear core-conductor model. The simulation confirms the existence of a "window" of the unidirectional stimulation, a condition that must exist to implement a collision block. The result corresponds to that obtained experimentally by van den Honert and Mortimer [11], [12], and suggests the applicability of such models to determine optimum cuff electrode designs.     

A nerve cuff technique for selective excitation of peripheral nervetrunk regions

Numerical modeling and experimental testing of a nerve cuff technique for selective stimulation of superficial peripheral nerve trunk regions is presented. Two basic electrode configurations ("snug" cuff monopolar and tripolar longitudinally aligned dots) have been considered. In addition, the feasibility of "steering" excitation into superficial nerve trunk regions using subthreshold levels of current flow from an electrode dot located on the opposite side of the nerve has been tested. Modeling objectives were to solve for the electric field that would be generated within a representative nerve trunk by each electrode configuration; and to use a simple nerve cable model to predict the effectiveness of each configuration in producing localized excitation. In three acute experiments on cat sciatic nerve the objective was to characterize the effectiveness of each electrode configuration in selectively activating only the medial gastrocnemius muscle. Modeling and experimentation both suggest that longitudinally aligned tripolar dot electrodes on the surface of a nerve trunk, and bounded by a layer of insulation (such as a nerve cuff), will restrict excitation to superficial nerve trunk regions more successfully than will monopolar dot electrodes. Excitation "steering" will improve the spatial selectivity of both monopolar and tripolar electrode configurations.     

Model of Heat Exchangers for Waste Heat Recovery from Diesel Engine Exhaust for Thermoelectric Power Generation

The performance and operating characteristics of a hypothetical thermoelectric generator system designed to extract waste heat from the exhaust of a medium-duty turbocharged diesel engine were modeled. The finite-difference model consisted of two integrated submodels: a heat exchanger model and a thermoelectric device model. The heat exchanger model specified a rectangular cross-sectional geometry with liquid coolant on the cold side, and accounted for the difference between the heat transfer rate from the exhaust and that to the coolant. With the spatial variation of the thermoelectric properties accounted for, the thermoelectric device model calculated the hot-side and cold-side heat flux for the temperature boundary conditions given for the thermoelectric elements, iterating until temperature and heat flux boundary conditions satisfied the convection conditions for both exhaust and coolant, and heat transfer in the thermoelectric device. A downhill simplex method was used to optimize the parameters that affected the electrical power output, including the thermoelectric leg height, thermoelectric n-type to p-type leg area ratio, thermoelectric leg area to void area ratio, load electrical resistance, exhaust duct height, coolant duct height, fin spacing in the exhaust duct, location in the engine exhaust system, and number of flow paths within the constrained package volume. The calculation results showed that the configuration with 32 straight fins was optimal across the 30-cm-wide duct for the case of a single duct with total height of 5.5?cm. In addition, three counterflow parallel ducts or flow paths were found to be an optimum number for the given size constraint of 5.5?cm total height, and parallel ducts with counterflow were a better configuration than serpentine flow. Based on the reported thermoelectric properties of MnSi_1.75 and Mg₂Si_0.5Sn_0.5, the maximum net electrical power achieved for the three parallel flow paths in a counterflow arrangement was 1.06?kW for package volume of 16.5?L and exhaust flow enthalpy flux of 122?kW.     

On the induced electric field gradients in the human body for magnetic stimulation by gradient coils in MRI

Prior theoretical studies indicate that the negative spatial derivative of the electric field induced by magnetic stimulation may be one of the main factors contributing to depolarization of the nerve fiber. This paper studies this parameter for peripheral nerve stimulation (PNS) induced by time-varying gradient fields during MRI scans. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic human, full-body model. Whole-body cylindrical and planar gradient sets in MRI systems and various input signals have been explored. The spatial distributions of the induced electric field and their gradients are calculated and attempts are made to correlate these areas with reported experimental stimulation data. The induced electrical field pattern is similar for both the planar coils and cylindrical coils. This study provides some insight into the spatial characteristics of the induced field gradients for PNS in MRI, which may be used to further evaluate the sites where magnetic stimulation is likely to occur and to optimize gradient coil design.     

A Possible Mechanism for the Influence of Electromagnetic Radiation on Neuroelectric Potentials

This paper explores the idea that the electrical component of applied microwave and radiowave radiation might induce transmembrane potentials in nerve cells and, thereby, disturb nervous function and behavior. The paper estimates the transmembrane currents and potentials induced in nerve cells by applied electrical fields and currents. Estimates are made for steady and for oscillating stimulation. The primary conclusion is that intracranial electrical fields associated with low-intensity irradiation in the frequency range of 10/sup 6/-10/sup 10/ Hz may induce transmembrane potentials of tenths of millivolts (or more) and that, therefore, such externally applied fields may distrub normal nervous function through this mechanism. The paper also presents a discussion which indicates that the induced transmembrane potential should exhibit a maximum at about 10/sup 8/Hz. Although some researchers suggest that the direct mechanism explored here may not represent the main influence of microwaves and radiowaves on biological tissue, this model together with a recent model by Barnes and Hu suggest that the results so produced may indeed be significant.     

Estimation of fiber diameters in the spinal dorsal columns fromclinical data

Lack of human morphometric data regarding the largest nerve fibers in the dorsal columns (DCs) of the spinal cord has lead to the estimation of the diameters of these fibers from clinical data retrieved from patients with a new spinal cord stimulation (SCS) system. These patients indicated the perception threshold of stimulation induced paresthesia in various body segments, while the stimulation amplitude was increased. The fiber diameters were calculated with a computer model, developed to calculate the effects of SCS on spinal nerve fibers. This computer model consists of two parts: (1) a three-dimensional (3-D) volume conductor model of a spinal cord segment in which the potential distribution due to electrical stimulation is calculated and (2) an electrical equivalent cable model of myelinated nerve fiber, which uses the calculated potential field to determine the threshold stimulus needed for activation. It is shown that the largest fibers in the medial DCs are significantly smaller than the largest fibers in the lateral parts. This finding is in accordance with the fiber distribution in cat, derived from the corresponding propagation velocities. Moreover, it is shown that the mediolateral increase in fiber diameter is mainly confined to the lateral parts of the DCs. Implementation of this mediolateral fiber diameter distribution of the DCs in the computer model enables the prediction of the recruitment order of dermatomal paresthesias following increasing electrical stimulation amplitude     

Simulations for the Development of Thermoelectric Measurements

In thermoelectricity, continuum theoretical equations are usually used for the calculation of the characteristics and performance of thermoelectric elements, modules or devices as a function of external parameters (material, geometry, temperatures, current, flow, load, etc.). An increasing number of commercial software packages aimed at applications, such as COMSOL and ANSYS, contain vkernels using direct thermoelectric coupling. Application of these numerical tools also allows analysis of physical measurement conditions and can lead to specifically adapted methods for developing special test equipment required for the determination of TE material and module properties. System-theoretical and simulation-based considerations of favorable geometries are taken into account to create draft sketches in the development of such measurement systems. Particular consideration is given to the development of transient measurement methods, which have great advantages compared with the conventional static methods in terms of the measurement duration required. In this paper the benefits of using numerical tools in designing measurement facilities are shown using two examples. The first is the determination of geometric correction factors in four-point probe measurement of electrical conductivity, whereas the second example is focused on the so-called combined thermoelectric measurement (CTEM) system, where all thermoelectric material properties (Seebeck coefficient, electrical and thermal conductivity, and Harman measurement of zT) are measured in a combined way. Here, we want to highlight especially the measurement of thermal conductivity in a transient mode. Factors influencing the measurement results such as coupling to the environment due to radiation, heat losses via the mounting of the probe head, as well as contact resistance between the sample and sample holder are illustrated, analyzed, and discussed. By employing the results of the simulations, we have developed an improved sample head that allows for measurements over a larger temperature interval with enhanced accuracy.     

Investigation of the on-state behaviors of the variation of lateral width LDMOS device by simulation

The main content revolves round the on-state characteristics of the variation of a lateral width (VLW) LDMOS device. A three-dimensional numerical analysis is performed to investigate the specific on-resistance of the VLW LDMOS device, the simulation results are in good agreement with the analytical calculation results combined with device dimensions. This provides a theoretical basis for the design of devices in the future. Then the self-heating effect of the VLW structure with a silicon-on-oxide (SOI) substrate is compared with that of a silicon carbide (SiC) substrate by 3D thermoelectric simulation. The electrical characteristic and temperature distribution indicate that taking into account the SiC as the substrate can mitigate the self- heating penalty effectively, alleviating the self heating effect and improving reliability.     

Enhancing polymer thermoelectric performance using radical dopants

Organic thermoelectric materials provide a means to reclaim, in part, potentially lost energy through a solid-state process that converts low-value thermal gradients (in the form of heat) to electricity; however, a new avenue towards improving the performance of these emerging thermoelectric materials must be brought to light before their widespread implementation becomes warranted. Here, we develop a blend of open-shell small molecules and closed-shell, conjugated polymers in order to evaluate how the chemical composition of the distinct open-shell, charge-neutral molecular dopants impacts the thermoelectric performance of a common hole-transporting (p-type) polymer semiconductor, poly(3-hexylthiophene) (P3HT). In doing so, we are able to increase the electrical conductivity of the P3HT composite both with and without affecting the oxidation state of the polymer. Specifically, the electrical conductivity of the conjugated polymer increases without changing the oxidation state of the P3HT when the preferentially-oxidized species 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) radical is incorporated into the hole-conducting thin film. Moreover, the inclusion of the preferentially-reduced galvinoxyl (GAV) radical also increases the electrical conductivity of the thin film; however, this small molecule dopant changes the oxidation state of the P3HT moiety. The ability to achieve the gains in the electrical conductivity of P3HT with the TEMPO radical in a manner that is independent of its oxidation state was confirmed using optical spectroscopy, cyclic voltammetry, and x-ray diffraction techniques. Importantly, this ability to enhance the electrical conductivity of P3HT in a manner that is independent of the oxidation state of the polymer provides a means to circumvent the oft-observed inverse relationship between the electrical conductivity and thermopower of the semiconducting polymer, and this combined effect allows for an ∼170-fold increase for the TEMPO-containing composites compared to a ∼70-fold increase for the GAV-containing composites. Thus, the described phenomena for charge-neutral radical dopants provides a set of critical design parameters for future polymer thermoelectric materials and composites, and it opens a new pathway towards high-performance organic thermoelectric devices.     

Neuro-fuzzy extraction of angular information from muscle afferentsfor ankle control during standing in paraplegic subjects: an animalmodel

This paper is part of a project whose aim is the implementation of closed-loop control of ankle angular position during functional electrical stimulation (FES) assisted standing in paraplegic subjects using natural sensory information. In this paper, a neural fuzzy (NF) model is implemented to extract angular position information from the electroneurographic signals recorded from muscle afferents using cuff electrodes in an animal model. The NF model, named dynamic nonsingleton fuzzy logic system is a Mamdani-like fuzzy system, implemented in the framework of recurrent neural networks. The fuzzification procedure implemented was the nonsingleton technique which has been shown in previous works to be able to take into account the uncertainty in the data. The proposed algorithm was tested in different situations and was able to predict reasonably well the ankle angular trajectories especially for small excursions (as during standing) and when the stimulation sites are far from the registration sites. This suggests it may be possible to use activity from muscle afferents recorded with cuff electrodes for FES closed-loop control of ankle position during quite standing.     

Modeling a Thermoelectric Generator Applied to Diesel Automotive Heat Recovery

Thermoelectric generators (TEGs) are outstanding devices for automotive waste heat recovery. Their packaging, lack of moving parts, and direct heat to electrical conversion are the main benefits. Usually, TEGs are modeled with a constant hot-source temperature. However, energy in exhaust gases is limited, thus leading to a temperature decrease as heat is recovered. Therefore thermoelectric properties change along the TEG, affecting performance. A thermoelectric generator composed of Mg₂Si/Zn₄Sb₃ for high temperatures followed by Bi₂Te₃ for low temperatures has been modeled using engineering equation solver (EES) software. The model uses the finite-difference method with a strip-fins convective heat transfer coefficient. It has been validated on a commercial module with well-known properties. The thermoelectric connection and the number of thermoelements have been addressed as well as the optimum proportion of high-temperature material for a given thermoelectric heat exchanger. TEG output power has been estimated for a typical commercial vehicle at 90°C coolant temperature.     

Effect of oxidation on the thermoelectric properties of PbSe thin films

The effect of oxidation at room temperature on the thermoelectric properties of PbSe/KCl (001) thin films prepared by thermal evaporation was investigated. The dependences of the electrical conductivity, the Hall coefficient, charge carrier mobility, and thermopower on the PbSe layer thickness (d=4–200 nm) were obtained. An inversion of the sign of the dominant carriers from n to p at d∼80 nm was observed under decreasing d. The d dependences of the thermoelectric properties were interpreted, taking into consideration the oxidation processes at the film/air interface within the framework of models considering both n-type and p-type carriers.     

Can Thermotunneling Improve the Currently Realized Thermoelectric Conversion Efficiency?

In a tunnel junction, electrons can overcome a nanoscale vacuum gap after the application of an electrical voltage. A temperature difference rather than an electrical voltage applied at the junction gives rise to an analogous thermoelectric tunnel effect called thermotunneling. This effect opens the possibility of thermoelectric conversion without phononic thermal backflow, which has encouraged optimism regarding the potential of thermotunneling for power generation and refrigeration. However, thermotunneling implies a photonic thermal backflow caused by radiative heat exchange amplified by photon tunneling of evanescent modes. An investigation based on a free electron model and comprising the combined influence of both electronic and photonic heat transfer through a vacuum tunnel gap is presented. An upper limit M = π ²/12 on the dimensionless thermoelectric figure of merit practically attainable by thermotunneling can be stated. This means that thermo- tunneling cannot outperform the maximum M values achieved by thermoelectric materials research to date.     

Thermoelectric Energy Harvesting from Transient Ambient Temperature Gradients

We examine a thermoelectric harvester that converts electrical energy from the naturally occurring temperature difference between ambient air and large thermal storage capacitors such as building walls or the soil. For maximum power output, the harvester design is implemented in two steps: source matching of the thermal and electrical interfaces to the energy source (system level) followed by load matching of the generator to these interfaces (subsystem level). Therefore, we measure thermal source properties such as the temperature difference, the air velocity, and the cutoff frequency in two application scenarios (road tunnel and office building). We extend a stationary model of the harvester into the time domain to account for transient behavior of the source. Based on the model and the source measurements, we perform the source and load matching. The resulting harvester consists of a pin fin heat sink with a thermal resistance of 6.2?K/W and a cutoff frequency 2.5?times greater than that of the source, a thermoelectric generator, and a DC/DC step-up converter starting at a total temperature difference of only ??T?=?1.2?K. In a final road tunnel field test, this optimized harvester converts 70?mJ of electrical energy per day without any direct solar irradiation. The energy provided by the harvester enables 415?data transmissions from a wireless sensor node per day.     

Electrical conductivity and thermoelectric power of liquid tellurium doped with 3<Emphasis Type="Italic">d</Emphasis> transition metals

The electrical conductivity and thermoelectric power of liquid tellurium with transition 3d metal impurities were measured in a wide temperature range (from 1700 K to the crystallization temperature) under argon pressure (up to 25 MPa). It is shown that Ti, V, Cr, and Mn impurities decrease the conductivity and slightly increase the thermoelectric power, whereas Fe, Co, Ni, and Cu impurities increase the conductivity and slightly decrease the thermoelectric power. In the region above 1200 K, the thermoelectric power remains virtually constant, whereas the conductivity decreases. The results obtained are interpreted in the context of the model of s-d hybridization. To describe electron scattering in such systems, the Friedel-Anderson procedure is used, based on the ideas about the existence of virtual bound states.     

The effect of stimulus parameters on the recruitment characteristics of direct nerve stimulation

The effect of stimulus parameters on the recruitment characteristics of motor nerve was studied for regulated current monophasic and balanced charge biphasic stimuli. Results of a nerve model investigation indicated that the threshold difference between different diameter nerve fibers would be dependent on pulse width, the choice between monophasic and biphasic stimuli, and the delay between the primary cathodic and secondary anodic pulses. Threshold difference increased with decreasing pulse width, the greatest effects evident for pulses less than 100 ?s. Biphasic stimulation with no delay between pulses provided greater threshold separation than monophasic stimulation or biphasic stimulation with delay. Animal experiments, in which recruitment in a nerve trunk composed of mixed diameter nerve fibers was examined, showed a decrease in recruitment slope with a decrease in pulse width and with the use of a biphasic, zero delay pulse. These results were examined through muscle force measurements using both a metal loop electrode encircling the nerve trunk and a nerve cuff electrode, i. e., a loop electrode in an insulating tube.