Application of an object-oriented programming paradigm inthree-dimensional computer modeling of mechanically activegastrointestinal tissues

The aim of this study was to develop a novel three-dimensional (3-D) object oriented modeling approach incorporating knowledge of the anatomy, electrophysiology, and mechanics of externally stimulated excitable gastrointestinal (GI) tissues and emphasizing the “stimulus-response” principle of extracting the modeling parameters. The modeling method used clusters of class hierarchies representing GI tissues from three perspectives: 1) anatomical; 2) electrophysiological; and 3) mechanical. We elaborated on the first four phases of the object-oriented system development life-cycle: 1) analysis; 2) design; 3) implementation; and 4) testing. Generalized cylinders were used for the implementation of 3-D tissue objects modeling the cecum, the descending colon, and the colonic circular smooth muscle tissue. The model was tested using external neural electrical tissue excitation of the descending colon with virtual implanted electrodes and the stimulating current density distributions over the modeled surfaces were calculated. Finally, the tissue deformations invoked by electrical stimulation were estimated and represented by a mesh-surface visualization technique     

Muscle recruitment with intrafascicular electrodes

We have studied muscle recruitment with Teflon-insulated, 25 microns diameter, Pt-Ir intrafasicular electrodes implanted in nerves innervating the gastrocnemius and soleus muscles of cats. The purpose of this study was to measure the performance of these bipolar electrodes, which had been designed to optimize their ability to record unit activity from peripheral nerves, as stimulating electrodes. Recruitment curves identified the optimal stimulus configuration as a biphasic rectangular pulse, with an interphase separation of about 500 microseconds and a duration of about 50 microseconds. The current required for a half-maximal twitch contraction was on the order of 50 microA. Current and charge densities needed for stimulation were well below levels believed to be safe for the tissue and electrode materials involved. When the spinal reflex pathway was interrupted by crushing the nerve, the force produced by a given stimulus changed in some cases, but not in others, implying that the spinal reflex contribution was not the same in all the implants. We conclude that intrafascicular recording electrodes are also a potentially valuable technology for functional neuromuscular stimulation, and warrant further development.     

Parametric study of neural gastric electrical stimulation in acute canine models

Manipulation of gastric motility by gastric electrical stimulation (GES) has been suggested as a minimally invasive alternative treatment of gastric motility disorders and obesity. However, only neural GES (NGES) has been successful in invoking gastric contractions. Nevertheless, the relationship between these contractions and the controlling NGES parameters has not been quantified. We aimed at determining the relationship between the electrical energy delivered to the tissue as a function of NGES parameters, and the strength and duration of the resulting invoked gastric contractions. Five healthy mongrel dogs underwent subserosal prepyloric implantation of two NGES electrode pairs. Gastric motility was captured by a force transducer implanted in the vicinity of the distal pair of stimulating electrodes. Custom-designed implantable stimulator delivered NGES with 8-16 V (peak-to-peak) amplitudes, and 60-100% duty cycles. Normalized motility index (MI) was utilized to quantify the contractions recorded from the force transducer. The MI increased with increasing voltage amplitudes. However, it remained remarkably constant across all duty cycles when voltage was held constant. Calculated motility generation efficiency indices (MGEI) indicated that highest energy efficiency for invoked motility was achieved at the lowest duty cycle. The parametric data obtained in the present study can be utilized to optimize the power efficiency of implantable gastric neurostimulators.     

Design and clinical application of a double helix electrode forfunctional electrical stimulation

An electrode, designed to be implanted without a surgical incision, was developed for skeletal muscle stimulation. Stainless steel, Teflon-insulated wire was wound into a helical lead around a polypropylene core and then rewound into a double helix configuration for stress relief during muscle contractions. The electrode tip was augmented with stainless steel barbs to increase anchoring strength. Electrodes were implanted with the help of specially modified hypodermic needles, sheaths, and passing tubes. 775 electrodes were implanted in a five year period in 22 subjects; accumulated implant time was 1,080 electrode years. 453 electrodes (65%) continue to produce strong, stable, muscle contractions. Electrode longevity varied with the location of implant. Electrodes were removed because of (1) inability to locate and properly place the electrode in a suitable site for stimulation during surgery (28, 4%), (2) unwanted changes in muscle response to stimulation (91, 12%) one-third occurring during the first six weeks post implant), (3) increase in electrode impedance (74, 10%) assumed breakage, mostly occurring during the first year after implant), (4) intolerable pain during stimulation (8, 1%), and (5) infection (4, 0.5%). 67 (8%) electrodes were removed by accident or when the subjects left the program. This double helix electrode design has proven practical for achieving chronic stimulation of selected muscles in hemiplegic, paraplegic, stroke and brain-injured subjects with minimally invasive surgery     

Bipolar stimulation of a three-dimensional bidomain incorporatingrotational anisotropy

A bidomain model of cardiac tissue was used to examine the effect of transmural fiber rotation during bipolar stimulation in three-dimensional (3-D) myocardium. A 3-D tissue block with unequal anisotropy and two types of fiber rotation (none and moderate) was stimulated along and across fibers via bipolar electrodes on the epicardial surface, and the resulting steady-state interstitial (Φ _ϵ) and transmembrane (V_m) potentials were computed. Results demonstrate that the presence of rotated fibers does not change the amount of tissue polarized by the point surface stimuli, but does cause changes in the orientation of Φ_ϵ, and V_m in the depth of the tissue, away from the epicardium. Further analysis revealed a relationship between the Laplacian of Φ _ϵ, regions of virtual electrodes, and fiber orientation that was dependent upon adequacy of spatial sampling and the interstitial anisotropy. These findings help to understand the role of fiber architecture during extracellular stimulation of cardiac muscle     

Application of a rat hindlimb model: a prediction of force spaces reachable through stimulation of nerve fascicles

A device to generate standing or locomotion through chronically placed electrodes has not been fully developed due in part to limitations of clinical experimentation and the high number of muscle activation inputs of the leg. We investigated the feasibility of functional electrical stimulation paradigms that minimize the input dimensions for controlling the limbs by stimulating at nerve fascicles, utilizing a model of the rat hindlimb, which combined previously collected morphological data with muscle physiological parameters presented herein. As validation of the model, we investigated the suitability of a lumped-parameter model for the prediction of muscle activation during dynamic tasks. Using the validated model, we found that the space of forces producible through activation of muscle groups sharing common nerve fascicles was nonlinearly dependent on the number of discrete muscle groups that could be individually activated (equivalently, the neuroanatomical level of activation). Seven commonly innervated muscle groups were sufficient to produce 78% of the force space producible through individual activation of the 42 modeled hindlimb muscles. This novel, neuroanatomically derived reduction in input dimension emphasizes the potential to simplify controllers for functional electrical stimulation to improve functional recovery after a neuromuscular injury.     

A System for Neural Recording and Closed-Loop Intracortical Microstimulation in Awake Rodents

There is growing interest in intracortical microstimulation as a means of providing sensory input in neuroprosthetic systems. We believe that precisely controlling the timing and parameters of stimulation in closed loop can significantly improve the efficacy of this technique. Here, we present a system for closed-loop microstimulation in awake rodents chronically implanted with multielectrode arrays. The system interfaces with existing commercial recording and stimulating hardware. Using custom-made hardware, we can stimulate and record from electrodes on the same implanted array and significantly reduce the stimulation artifact. Stimulation sequences can either be preprogrammed or triggered by neural or behavioral events. Specifically, this system can provide feedback stimulation in response to action potentials or features in the local field potential recorded on any of the electrodes within 15 ms. It can also trigger stimulation based on behavioral events, such as real-time tracking of rat whiskers captured with high-speed video. We believe that this system, which can be recreated easily, will help to significantly refine the technique of intracortical microstimulation and advance the field of neuroprostheses.      

Electrode characterization for functional application to upperextremity FNS

A quantitative method has been developed to characterize the isometric force vectors of electrically stimulated paralyzed muscles of the thumb. The vectorial force output as a function of the stimulus level was measured for individual electrode/muscle combinations in a number of intramuscular and epimysial electrodes implanted in paralyzed thenar muscles of cervical level spinal cord injury subejcts. Vectors are used to determine the output characteristics of each electrode/muscle combination. The characteristics studied include: the strength of the contraction, the stimulus level at which fibers from other muscles are stimulated, the recruitment gain of force, dependency of the output on the skeletal position, and the direction of force produced. These characteristics can then be used to select stimulus parameters to produce coordinated hand motion and force generation by functional neuromuscular stimulation (FNS). The range of muscle force and direction for each electrode/muscle combination showed considerable variation between subjects and between electrodes in the same subject. This variation is primarily due to differences in electrode placement within the muscle. Comparison between intramuscular and epimysial electrodes demonstrated similar characteristics in the force vector output. Preliminary results show the potential for using the force vector output to predict the cocontracted output of two muscles.     

Finite-difference modeling of the anisotropic electric fields generated by stimulating needles used for catheter placement

The use of peripheral nerve blocks to control pain is an increasing practice. Many techniques include the use of stimulating needles to locate the nerve of interest. Though success rates are generally high, difficulties still exist. In certain deeper nerve blocks, two needles of different geometries are used in the procedure. A smaller needle first locates a nerve bundle, and then is withdrawn in favor of a second, larger needle used for injection. The distinct geometries of these needles are shown to generate different electric field distributions, and these differences may be responsible for failures of the second needle to elicit nerve stimulation when placed in the same location as the first. A 3-D finite-difference method has been employed to numerically calculate the electric field distributions for a commercial set of stimulating needles.     

Toward functional magnetic stimulation (FMS) theory and experiment

Examines the use of magnetic fields to functionally stimulate peripheral nerves. All electric fields are induced via a changing magnetic field whose flux is entirely confined within a closed magnetic circuit. Induced electric fields are simulated using a nonlinear boundary element solver. The induced fields are solved using duality theory. The accuracy of these predictions is verified by saline bath experiments. Next, the theory is applied to the stimulation of nerves using small, partially occluded ferrite and laminated vanadium permendur cores. Experiments demonstrate the successful stimulation of peripheral nerves in the African bullfrog with 11 mA, 153 mV excitations. These results offer a new vista of possibilities in the area of functional nerve stimulation. Unlike functional electric stimulation (FES), FMS does not involve any half cell reactions, and thus would not have the commensurate FES restrictions regarding balanced biphasic stimulation, strength duration balances, and oxidation issues, always exercising care that the electrodes remain in the reversible operating regime     

A computational model of electrical stimulation of the retinal ganglion cell

Localized retinal electrical stimulation in blind volunteers results in discrete round visual percepts corresponding to the location of the stimulating electrode. The success of such an approach to provide useful vision depends on elucidating the neuronal target of surface electrical stimulation. To determine if electrodes preferentially stimulate ganglion cells directly below them or passing fibers from distant ganglion cells, we developed a compartmental model for electric field stimulation of the retinal ganglion cell (RGC). In this model a RGC is stimulated by extracellular electrical fields with active channels and realistic cell morphology derived directly from a neuronal tracing. Three membrane models were applied: a linear passive model, a Hodgkin-Huxley model with passive dendrites (HH), and a model composed of all active compartments (FCM) with five nonlinear ion channels. Idealized monopolar point and disk stimulating electrodes were positioned above the cell. For the HH and FCM models, the position of lowest cathodal threshold to propagate an action potential was over the soma. Brief (100 microseconds) cathodic stimuli were 20% (HH with disk electrode) to 73% (FCM with point-source) more effective over the soma than over the axon. In the passive model, the axon is preferentially stimulated versus the soma. Although it may be possible to electrically stimulate RGC's near their cell body at lower thresholds than at their axon, these differences are relatively small. Alternative explanations should be sought to explain the focal perceptions observed in previously reported patient trials.     

A method for evaluating the selectivity of electrodes implanted fornerve simulation

The scale of stimulating electrodes possible for use in functional electrical stimulation to restore motor and sensory function is rapidly approaching that of individual neurons. Although the electrodes may approach the dimensions of single nerve cells, it is unclear if the region of excitation elicited by each electrode will be correspondingly small. Previous techniques for evaluating this have either been tedious or have lacked the resolution necessary. This paper describes a method that uses the refractory interaction of the compound action potentials elicited by a stimulus pulse pair, along with high-resolution recording of those potentials, to achieve measurements of the selectivity of stimulation down to the scale of a few axon diameters. The feasibility of this technique is demonstrated in sciatic nerves of frogs (Rana Catesbiana) acutely implanted with a sapphire electrode array.     

Equivalent source estimation based on the calculation of theelectric field from depth EEG data

This paper describes a method for the estimation of equivalent source position and strength based on the estimation of the electric field from depth electroencephalographic (EEG) data. The calculation method for the electric field is based on a tetrahedral geometry. The proposed approach for source parametrization is twofold. Firstly, the distribution of electrical energy by the squared norm of the electric field vector can give an estimate of the source position, without having to assume a dipole source. Secondly, the average electric field can be related to the dipole magnitude and orientation of the equivalent source. Simulation results demonstrate the potential usefulness of the method. The effects of noise and sampling, and the geometry of the measurement system (i.e., implanted electrodes) relative to the source are also investigated through simulations.     

Photoconduction and magnetic field effect on photoconduction in hole-transporting star-burst amine (m-MTDATA) films

Photoconduction and magnetic field effect on photoconduction have been investigated as a function of electric field strength, excitation light intensity and wavelength in vacuum evaporated films of m-MTDATA (4,4′,4″-tris(N-(3-methylphenyl)-N-phenylylamino) triphenylamine), the starburst amine commonly used as hole-transporting material in organic light-emitting diodes. The photocurrent is found to be generated by the singlet exciton dissociation at the illuminated Al anode/m-MTDATA interface in accordance with the 1-D Onsager mechanism and in the bulk of the sample in terms of the 3-D Onsager model of e–h pair separation. The surface component of photocurrent is magnetic field independent whereas the bulk-generated photocurrent is influenced by external magnetic field of the hyperfine coupling (HFC) as well as fine structure modulation (FSM) scales.     

双光子中心对称光折变宽光束调制不稳定性

为了得到稳态条件下1维宽光束在有偏压双光子中心对称光折变晶体中的调制不稳定性的结果,通过空间电荷场的局域性处理和数值模拟的方法,获得了1维调制不稳定性的增长率。结果表明,在入射光束强度一定的情况下,调制不稳定性增长率最大值随外加电场以及空间频率域的增加而增大;在外加电场给定时,调制不稳定性增长率的最大值随入射光强的增大先达到一峰值后又减小。宽光束的调制不稳定性取决于外加电场及入射光束强度与暗辐射的比值。     

Switchable Faraday shielding with application to reducing the pain of internal cardiac defibrillation while permitting external defibrillation

Switchable Faraday shielding is desirable in situations where electric field shielding is required at certain times and undesirable at other times. In this study, electrostatic finite element modeling was used to assess the effect of different shield geometries on the leakage of an internally applied field and penetration of an externally applied field. "Switching OFF" the shield by electrically disconnecting shield faces from each other was shown to significantly increase external field penetration. Applying this model to defibrillation, we looked at the effect of spacing and size of shield panels to maximize the ability to deliver an external defibrillation shock to the heart when shield panels are disconnected while providing acceptably low leakage of internal defibrillation shocks to avoid painful skeletal muscle capture when shield panels are connected. This analysis may be useful for designing internal defibrillator electrodes that preserve the efficacy of internal and external defibrillation while avoiding the significant morbidity associated with painful defibrillator shocks. Similar analysis could also guide optimizing the switchable Faraday shielding concept for other applications.     

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.     

Closed-loop control of ankle position using muscle afferentfeedback with functional neuromuscular stimulation

Describes a closed-loop functional neuromuscular stimulation system that uses afferent neural activity from muscle spindle fibers as feedback for controlling position of the ankle joint. Ankle extension against a load was effected by neural stimulation through a dual channel intrafascicular electrode of a fascicle of the tibial nerve that innervated the gastrocnemius muscle. Ankle joint angle was estimated from recordings of tibialis anterior and lateral gastrocnemius spindle fiber activity made with dual channel intrafascicular electrodes. Experiments were conducted in neurally intact anesthetized cats and in unanesthetized decerebrate cats to demonstrate the feasibility of this system. The system was able to reach and maintain a fixed target ankle position in the presence of a varying external moment ranging in magnitude between 7.3 and 22 N-cm opposing the action of the ankle extensor, as well as track a sinusoidal target ankle position up to a frequency of 1 Hz in the presence of a constant magnitude 22- or 37-N-cm external moment     

Long-term intramuscular electrical activation of the phrenic nerve:safety and reliability

The safety and reliability of a system for long-term intramuscular electrical activation of the phrenic nerve was evaluated in seven dogs. In this system, electrodes are implanted bilaterally into the diaphragm without directly contacting the phrenic nerve using a laparoscope to direct placement. Five dogs underwent chronic bilateral intramuscular diaphragm stimulation (IDS) for 61 to 183 days at stimulus parameters selected to evoke at least 120% of the animal's basal ventilation. Two dogs maintained as controls did not undergo chronic stimulation. The safety and reliability of the system was evaluated in terms of tissue responses to the electrode, alterations in diaphragm muscle, pulmonary function, electrode reliability, and cardiac activation. No adverse responses to the electrode or stimulation were found. The histochemistry of chronically stimulated diaphragm suggested transformation towards type I (oxidative metabolism) muscle fibers. Two IDS electrodes dislodged out of a total of 32 IDS electrodes implanted. Both electrodes dislodged within seven days of implant. All IDS electrodes had stable and repeatable recruitment properties. No IDS electrode mechanical failures were found and no electrode corrosion was observed. It is concluded from these experiments that intramuscular activation of the phrenic nerve will present a minimal risk to human patients who are good candidates for clinical studies using this technique     

Localized DC Field Produced by Diode Implanted in Isotropic Homogeneous Medium and Exposed to Uniform RF Field

Minimally invasive selective stimulation of biological tissue within the body can be achieved by implanting a small rectifying diode and then applying an RF field by means of electrodes or a coil on or near the surface of the body. A theoretical analysis of a simplified model consisting of a cylindrically symmetrical unit implanted in an isotropic homogeneous medium of conductivity ?and permittivity ? and exposed to a uniform RF field relates the dc component of electrode current to the detailed geometry of the electrodes of the implanted unit and to the amplitude of the applied field.