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In this work, we present a wireless ultrasonic neurostimulator, aiming at a truly wearable device for brain stimulation in small behaving animals. A 1D 5-MHz capacitive micromachined ultrasonic transducer (CMUT) array is adopted to implement a head-mounted stimulation device. A companion ASIC with integrated 16-channel high-voltage (60-V) pulsers was designed to drive the 16-element CMUT array. The ASIC can generate excitation signals with element-wise programmable phases and amplitudes: 1) programmable sixteen phase delays enable electrical beam focusing and steering, and 2) four scalable amplitude levels, implemented with a symmetric pulse-width-modulation technique, are sufficient to suppress unwanted sidelobes (apodization). The ASIC was fabricated in the TSMC 0.18- ${\mu \rm {m}}$ HV BCD process within a die size of 2.5 × 2.5 ${\rm {mm}^2}$ . To realize a completely wearable system, the system is partitioned into two parts for weight distribution: 1) a head unit (17 mg) with the CMUT array, 2) a backpack unit (19.7 g) that includes electronics such as the ASIC, a power management unit, a wireless module, and a battery. Hydrophone-based acoustic measurements were performed to demonstrate the focusing and beam steering capability of the proposed system. Also, we achieved a peak-to-peak pressure of 2.1 MPa, which corresponds to a spatial peak pulse average intensity ( $\mathrm{I_{SPPA}}$ ) of 33.5 ${\rm {W}/\rm {cm}^2}$ , with a lateral full width at half maximum (FWHM) of 0.6 mm at a depth of 3.5 mm.

Development of micro electrode arrays for neural recording is an active field that thrives on novel materials and fabrication techniques offered by micro fabrication technology. The material and mechanical properties of microelectrode arrays have a critical role on the quality and longevity of neural signals. In this study, carbon fiber microelectrode (CFME) bundles were developed and implanted in the spinal cord of experimental animals for textbf{\textit{in vivo{recording. Neural data analysis revealed that single spikes could successfully be recorded and sorted. Removal of approximately $75 \mu \mathrm{m}$ of the parylene-C coating at the tips of the fibers increased the signalto-noise ratio. Connecting multiple (three) carbon fiber filaments to the same recording channel did not deteriorate the signal quality compared to that of undesheathed fibers. Immunohistochemistry showed that electrode tips were splayed in tissue after implantation and CF bundles had a small footprint with mild encapsulation around them. These results are very promising for the use of CFME bundles for recordings of spinal cord signals in behaving animals.

As a non-invasive brain stimulation technique, transcranial electrical stimulation (TES) and specifically the transcranial direct current stimulation (tDCS) has gained popularity in recent years for treatment of a wide variety of cognitive and neurological disorders. Recent studies have shown that TES can alter the motor cortex excitability. Animal studies to demonstrate the underlying mechanisms of TES are clearly lacking in literature. Clinical studies have agreed on the critical role of the current intensity and the montage of the electrodes for the treatment to be effective. In this study, we used a rat model for in vivo investigation of the vertical electrical (E) field distribution due to electrodes placed over the skin and through a craniotomy hole. A mono-phasic current pulse was used as a substitute for DC currents by taking advantage of primarily resistive properties of the brain tissue at low frequencies. The electrical potentials induced by the current pulses were recorded with penetrations at 0mm, 2mm, and 4mm away from the stimulation electrode. The results showed that the E-field was maximum immediately under the anodic electrode and decreased both in the vertical and horizontal directions rapidly by distance. The magnitude of the electric field varied from tens of mV/mm to a fraction of mV/mm by distance for a 100 μ A stimulus amplitude. The results also show that the E-field amplitudes and distribution strongly depend on whether the stimulus electrode is placed over the skin or into a craniotomy hole.

The corticospinal tract (CST) is one of the descending tracts that carry the forelimb volitional information. Using flexible multi-electrode arrays (MEAs), the CST signals were recorded in rats during active and resting states. The power spectral density (PSD) of CST signals during the active state were notably higher than those observed during resting or anesthesia. Average inter-channel coherences below 30 Hz were considerably higher for reach-to-pull task compared to face grooming and the quiet state, suggesting the presence of volitional information in the recorded CST signals. A novel feature extraction technique (spectrum of cross-spectrum) was applied to obtain feature vectors and used in a classification algorithm. The extension and flexion phases of the forelimb movements were classified with 95% accuracy. The results warrant further investigations for extraction of more detailed volitional motor control information from the CST signals.

Analytical solutions for voltage fields in a volume conductor are available only for ideal electrodes with radially symmetric contacts and infinitely extending substrates. Practical electrodes for neural stimulation may have asymmetric contacts and finite substrate dimensions and hence deviate from the ideal geometries. For instance, it needs to be determined if the analytical solutions are adequate for simulations of narrow shank electrodes where the substrate width is comparable to the size of the contacts. As an extension to this problem, a ldquofloatingrdquo stimulator can be envisioned where the substrate would be finite in all directions. The question then becomes how small this floating stimulator can be made before its stimulation strength is compromised by the decrease in the medium impedance between the contacts as the contacts are approaching each other. We used finite element modeling to solve the voltage and current profiles generated by these radially asymmetric electrode geometries in a volume conductor. The simulation results suggest that both the substrate size and the bipolar contact separation influence the voltage field when these parameters are as small as a few times the contact size. Both of these effects are larger for increasing elevations from the contact surface, and even stronger for floating electrodes (finite substrate in all directions) than the shank-type electrodes. Location of the contacts on the floating electrode also plays a role in determining the voltage field. The voltage field for any device size and current, and any specific resistance of the volume conductor can be predicted from these results so long as the aspect ratios are preserved.

Objective. Microelectrodes implanted in the central nervous system (CNS) often fail in long term implants due to the immunological tissue response caused by tethering forces of the connecting wires. In addition to the tethering effect, there is a mechanical stress that occurs at the device– tissue interface simply because the microelectrode is a rigid body floating in soft tissue and it cannot reshape itself to comply with changes in the surrounding tissue. In the current study we evaluated the scar tissue formation to tetherless devices with two significantly different geometries in the rat brain and spinal cord in order to investigate the effects of device geometry. Approach. One of the implant geometries resembled the wireless, floating microstimulators that we are currently developing in our laboratory and the other was a (shank only) Michigan probe for comparison. Both electrodes were implanted into either the cervical spinal cord or the motor cortices, one on each side. Main results. The most pronounced astroglial and microglial reactions occurred within 20 μm from the device and decreased sharply at larger distances. Both cell types displayed the morphology of non-activated cells past the 100 μm perimeter. Even though the aspect ratios of the implants were different, the astroglial and microglial responses to both microelectrode types were very mild in the brain, stronger and yet limited in the spinal cord. Significance. These observations confirm previous reports and further suggest that tethering may be responsible for most of the tissue response in chronic implants and that the electrode size has a smaller contribution with floating electrodes. The electrode size may be playing primarily an amplifying role to the tethering forces in the brain whereas the size itself may induce chronic response in the spinal cord where the movement of surrounding tissues is more significant.

Evoked motor signals descending down the corticospinal tract can be recorded selectively with multi-contact electrodes from the spinal cord surface. This method of extracting motor signals from the spinal cord may provide a means of communication for people with spinal cord injury. The information rate obtained with such an interface will improve if the separation of neural channels can be increased. In this study, the feasibility of increasing the channel separation was investigated using the blind source separation (BSS) technique. Neural signals recorded with multi-contact surface electrodes were treated as a linear mixture of independent neural sources located inside the spinal cord. Principal component analysis (PCA) was applied to estimate the dimensionality of the raw signals, and then the fixed-point FastICA algorithm was used to separate the primary neural sources from the secondary (smaller) ones. In all trials but one, the separation between the neural channels has increased by eliminating the secondary sources. These results suggest that the information rate of a spinal cord interface can be improved by separating the neural recordings into their independent components and selecting the ones with the largest distance between them. Comparison of independent component analysis (ICA) and PCA reveals that ICA performs better in this application.

Recording the motor output of the central nervous system from the cervical spinal cord was investigated as a method of generating voluntary command signals, potentially to be used in quadriplegic individuals. Corticospinal volleys evoked by motor cortex stimulation were recorded from the spinal cord surface with multicontact electrodes in anesthetized cats. The multicontact recordings were analyzed for their information-carrying capacity as a neural interface. Neural signals resulting from the stimulation of various points in the motor cortex were considered as symbols of an alphabet that were sent through a discrete information channel. The information capacity of this channel at the thermal noise level of the electrode contacts was calculated. The maximum information rate was 1.57 bits in a trial for a 4-symbol alphabet. The background noise that reduces the information rate to 50% of its maximum theoretical value was defined as the half-bitrate-noise-tolerance (HBR-NoiseTol) and used as a measure of symbol distinguishability. The HBR-NoiseTol for all trials on average was 24 +/- 12%, 18 +/- 10%, and 15 +/- 9% for interfaces with 2-, 3-, and 4-symbol alphabets (n = 11 trials). The average peak-to-peak amplitude of the neural volleys was 13.5 +/- 6.7 microV (n = 11). These results suggest that the corticospinal signals can be recorded with spatial selectivity from the spinal cord surface and thus warrant further investigation of their potential use for a spinal cord-computer interface.

Neural electrodes and associated electronics are powered either through percutaneous wires or transcutaneous powering schemes with energy harvesting devices implanted underneath the skin. For electrodes implanted in the spinal cord and the brain stem that experience large displacements, wireless powering may be an option to eliminate device failure by the breakage of wires and the tethering of forces on the electrodes. We tested the feasibility of using optically clear polydimethylsiloxane (PDMS) as a waveguide to collect the light in a subcutaneous location and deliver to deeper regions inside the body, thereby replacing brittle metal wires tethered to the electrodes with PDMS-based optical waveguides that can transmit energy without being attached to the targeted electrode. We determined the attenuation of light along the PDMS waveguides as 0.36 ± 0.03 ?? dB / cm and the transcutaneous light collection efficiency of cylindrical waveguides as 44 % ± 11 % by transmitting a laser beam through the thenar skin of human hands. We then implanted the waveguides in rats for a month to demonstrate the feasibility of optical transmission. The collection efficiency and longitudinal attenuation values reported here can help others design their own waveguides and make estimations of the waveguide cross-sectional area required to deliver sufficient power to a certain depth in tissue.

Electrical neural stimulation with micro electrodes is a promising technique for restoring lost functions in the central nervous system as a result of injury or disease. One of the problems related to current neural stimulators is the tissue response due to the connecting wires and the presence of a rigid electrode inside soft neural tissue. We have developed a novel, optically activated, microscale photovoltaic neurostimulator based on a custom layered compound semiconductor heterostructure that is both wireless and has a comparatively small volume (

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.

Neural microstimulation is becoming a powerful tool for the restoration of impaired functions in the central nervous system. Microelectrode arrays with fine wire interconnects have traditionally been used in the development of these neural prosthetic devices. However, these interconnects are usually the most vulnerable part of the neuroprosthetic implant that can eventually cause the device to fail. In this paper, we investigate the feasibility of floating-light-activated microelectrical stimulators (FLAMES) for wireless neural stimulation. A computer model was developed to simulate the micro stimulators for typical requirements of neural activation in the human white and gray matters. First, the photon densities due to a circular laser beam were simulated in the neural tissue at near-infrared (NIR) wavelengths. Temperature elevation in the tissue was calculated and the laser power was retrospectively adjusted to 325 and 250 mW/cm(2) in the gray and white matters, respectively, to limit ΔT to 0.5 °C. Total device area of the FLAMES increased with all parameters considered but decreased with the output voltage. We conclude that the number of series photodiodes in the device can be used as a free parameter to minimize the device size. The results suggest that floating, optically activated stimulators are feasible at submillimeter sizes for the activation of the brain cortex or the spinal cord.

One of the roadblocks in the field of neural prosthetics is the lack of microelectronic devices for neural stimulation that can last a lifetime in the central nervous system. Wireless multi-electrode arrays are being developed to improve the longevity of implants by eliminating the wire interconnects as well as the chronic tissue reactions due to the tethering forces generated by these wires. An area of research that has not been sufficiently investigated is a simple single-channel passive microstimulator that can collect the stimulus energy that is transmitted wirelessly through the tissue and immediately convert it into the stimulus pulse. For example, many neural prosthetic approaches to intraspinal microstimulation require only a few channels of stimulation. Wired spinal cord implants are not practical for human subjects because of the extensive flexions and rotations that the spinal cord experiences. Thus, intraspinal microstimulation may be a pioneering application that can benefit from submillimeter-size floating stimulators. Possible means of energizing such a floating microstimulator, such as optical, acoustic, and electromagnetic waves, are discussed.

Chronic tissue response to microelectrode implants stands in the way as a major challenge to development of many neural prosthetic applications. The long term tissue response is mostly due to the movement of interconnects and the resulting mechanical stress between the electrode and the surrounding neural tissue. Wireless microstimulators are a potential solution to the problem. As a method of energy transfer to the microstimulator, we propose to use a laser beam at near infrared (NIR) wavelengths. Microstimulators of various sizes were fabricated with two cascaded GaAs PIN photodiodes. The voltage field of the device was measured in saline solution as a response to an NIR laser source. The voltage in medium had a peak of around 190 mV above the cathodic contact and stayed flat for the duration of 0.2 ms pulse. In rats, microstimulators were inserted into the ventral horn of cervical spinal cord. A train of NIR pulses (0.2 ms, 100 Hz) were applied to wirelessly activate the devices. The forces generated due to stimulation of the motor neurons were measured from the ipsilateral forelimb with a force transducer. The largest force generated was around 0.9 N. The volume conductor and force measurements suggest that the floating light activated micro-electrical stimulator (FLAMES) approach is feasible for intraspinal stimulation.

Recordings were performed on the rat paramedian lobule of the cerebellum with a surface micro-electrode array during hand licking and quiet states. The Fast Fourier Transform (FFT) and average coherence both showed a change in the frequency distributions between active and quiet conditions. The signals were segregated into different frequency bands and the signal compositions were analyzed. In each of the bands an increase in neural activity was seen at the onset of activity. Frequency coherence analysis was performed between electrodes at two different separations for active and quiet conditions. The coherence analysis showed that there was an increase in coherence for shorter distances and during activity.

Optical tissue properties have been widely investigated and studied in human and animal tissue due to its importance in the medical laser applications. However, published data on the optical properties of neural tissue in rodents are rare. The aim of this study was to measure the penetration depth of light into the rat brain gray mater at NIR wavelengths. Sample preparation is an important factor in measurements of tissue optical properties. Although shock and slow freezing are standard procedures, they alter the optical properties of biological tissue. In this study, intact slices of gray matter were cut in the range of 300 to 1500µm without prior freezing to the brain samples. We found the penetration depth in the rat gray matter to be 0.39±0.036mm. Compared to the data reported in literature for the human brain, the rat brain gray matter attenuates the NIR light much more strongly.

Historically the rectangular pulse waveform has been the choice for neural stimulation. The strength–duration curve is thus defined for rectangular pulses. Not much attention has been paid to alternative waveforms to determine if the pulse shape has an effect on the strength–duration relation. Similarly the charge injection capacity of neural electrodes has also been measured with rectangular pulses. In this study we questioned if non-rectangular waveforms can generate a stronger stimulation effect, when applied through practical electrodes, by minimizing the neural activation threshold and maximizing the charge injection capacity of the electrode. First, the activation threshold parameters were studied with seven different pulse shapes using computer simulations of a local membrane model. These waveforms were rectangular, linear increase and decrease, exponential increase and decrease, Gaussian, and sinusoidal. The chronaxie time was found to be longer with all the non-rectangular pulses and some provided more energy efficient stimulation than the rectangular waveform. Second, the charge injection capacity of titanium nitride microelectrodes was measured experimentally for the same waveforms. Linearly decreasing ramp provided the best charge injection for all pulse widths tested from 0.02 to 0.5 ms. Finally, the most efficient waveform that maximized the charge injection capacity of the electrode while providing the lowest threshold charge for neural activation was searched. Linear and exponential decrease, and Gaussian waveforms were found to be the most efficient pulse shapes.

Near infrared (NIR) light energy has been used in medical applications both for diagnostic and treatment purposes. A priory knowledge of optical tissue properties is necessary in these applications; not only of human but also in animals for testing of devices. However, published data on the optical properties of neural tissue in rodents are rare. The aim of this study was to measure the penetration depth of light into the rat peripheral nerve and brain cortex at NIR wavelengths. Penetration depth was calculated from measurements of transmitted light for various thicknesses of the neural tissue. We found the penetration depth in the rat sciatic nerve to be 0.35plusmn0.023 mm and in the white matter 0.35plusmn0.026 mm. The penetration depth of the gray matter was 0.41plusmn0.029 mm. Compared to the data reported in literature for the human brain, the rat peripheral and the brain cortex attenuate the NIR light much more strongly.

Injury at the cervical region of the spinal cord results in the loss of the skeletal muscle control from below the shoulders and hence causes quadriplegia. The brain–computer interface technique is one way of generating a substitute for the lost command signals in these severely paralyzed individuals using the neural signals from the brain. In this study, we are investigating the feasibility of an alternative method where the volitional signals are extracted from the cervical spinal cord above the point of injury. A microelectrode array assembly was implanted chronically at the C5–C6 level of the spinal cord in rats. Neural recordings were made during the face cleaning behavior with forelimbs as this task involves cyclic forelimb movements and does not require any training. The correlation between the volitional motor signals and the elbow movements was studied. Linear regression technique was used to reconstruct the arm movement from the rectified–integrated version of the principal neural components. The results of this study demonstrate the feasibility of extracting the motor signals from the cervical spinal cord and using them for reconstruction of the elbow movements.

A single-stage, low-noise preamplifier is designed using the concept of noise matching for recordings of neural signal with cuff electrodes. The signal-to-noise ratio is approximately 1.6 times higher than that of a low-noise integrated amplifier (AMP-01) for a cuff impedance of 1.5 k/spl Omega/. The bandwidth is 230 Hz-8.25 kHz (R/sub s/=2 k/spl Omega/), and the common-mode-rejection-ratio is 91.2 dB at 1 kHz.

The functional effects of selective hypoglossal nerve (HG) stimulation with a multi-contact peripheral nerve electrode were assessed using images of the upper airways and the tongue in anesthetized beagles. A biphasic pulse train of 50 Hz frequency and 2 s duration was applied through each one of the tripolar contact sets of the nerve electrode while the pharyngeal images were acquired into a computer. The stimulation current was limited to 20% above the activation threshold for maximum selectivity. The images showed that various contact sets could generate several different activation patterns of the tongue muscles resulting in medial and/or lateral dilation and closing of the airways at the tongue root. Some of these patterns translated into an increase in the oropharyngeal size while others did not have any effect. The pharyngeal sizes were not statistically different during stimulation either between the two different positions of the head (30 degrees and 60 degrees), or when the lateral contacts were compared with the medial ones. The contacts that had the least effect generated an average of 53 +/- 15% pharyngeal dilation relative to the best contacts, indicating that the results are marginally sensitive to the contact position around the HG nerve trunk. These results suggest that selective HG nerve stimulation can be a useful technique to produce multiple tongue activation patterns that can dilate the pharynx. This may in turn increase the size of the patient population who can benefit from HG nerve stimulation as a treatment method for obstructive sleep apnea.

Electrical activation of the tongue protrusor muscle has been demonstrated as an effective technique for alleviating upper airway (UAW) obstructions and is considered a potential treatment for obstructive sleep apnea (OSA). Recent studies, however, have shown marked improvements in UAW patency by coactivating the tongue protrudor and retractor muscles. As such, selective stimulation of the hypoglossal nerve (XII) using a single implantable device presents an attractive approach for treating OSA. In order to demonstrate the feasibility of such a device, the maximum achievable stimulation selectivity of the Flat Interface Nerve Electrode (FINE) was investigated. The XII nerve of beagles was stimulated with an acutely implanted FINE, while the corresponding neural and muscular responses were recorded and analyzed. The overall performance of the FINE, as depicted by the average of the maximum target-specific selectivity values, S(i), confirmed that high degrees of selectivity can be achieved at both the fascicular and muscular levels: 0.93 +/- 0.03 (n = 5) and 0.88 +/- 0.03 (n = 4), respectively. The results of this study demonstrate the feasibility of the FINE for selective stimulation of the XII nerve branches and the innervated tongue muscles.

Isolated activation of the tongue protrusor muscles is the current technique for treating obstructive sleep apnea (OSA) via functional electrical stimulation. Recent studies, however, have shown marked improvements in upper airway (UAW) patency by co-activating the tongue protrudor and retractor muscles. As such, selective stimulation of the hypoglossal (XII) nerve with a single implantable device presents an attractive alternative for treating OSA. In order to demonstrate the feasibility of such a device, the maximum achievable stimulation selectivity of the Flat Interface Nerve Electrode (FINE) was investigated. The beagle XII nerve was stimulated with an implanted FINE, while the corresponding neural and muscular responses were recorded. The overall performance of the FINE, as defined by the selectivity index (SI), showed a high degree of selectivity at both the fascicular and muscular levels: 0.91 /spl plusmn/ 0.05 (n = 5) and 0.85 /spl plusmn/ 0.03 (n = 4), respectively. The results clearly demonstrate the feasibility of the FINE for selective stimulation of the XII nerve and UAW muscles.

In anesthetized animals, evoked motor signals descending through the corticospinal tract were recorded from the spinal cord with selectivity using multi-contact surface electrodes. However, the spatial selectivity needs to be improved for this approach to be used as a multi-channel neural interface. In this study, we applied the blind source separation (BBS) technique to improve the separation between the neural channels. The BSS algorithm improved the selectivity from an initial value of less than 1% to 91% although the signal-to-noise ratio of the signals was as low as 0.46 on average.

A multi-contact cylindrical nerve cuff electrode was evaluated for its ability to record neural signals selectively in an in vitro preparation, Three branches of a Beagle hypoglossal nerve are stimulated sequentially while compound action potentials (CAP) are recorded from its trunk with the multi-contact cuff electrode. A selectivity index (SI) is defined and applied to the CAP recorded from the 4 sets of tripolar contacts (12 contacts in total) that are equally spaced around the cuff. The results show that the cuff can record selectively from different fascicles, but these effects are small. Connecting the contacts of the opposite set together while recording from a tripole improved the selectivity. The signal amplitudes from various contacts are consistent with the location of the fascicles relative to the contacts.

A nerve cuff electrode interface capable of both stimulating and recording from a nerve is described. The interface also rejects the EMG contamination in the recordings using reactive components without adding noise to the ENG signal. A transformer is added to the design for noise matching and the signal-to-noise ratio improvement is evaluated for a specific amplifier (AMP-O1).

The thermal effect may be a desired outcome or a concerning side effect in laser-tissue interactions. Research in this area is particularly motivated by recent advances in laser applications in diagnosis and treat- ment of neurological disorders. Temperature as a side effect also limits the maximum power of optical transfer and harvesting of energy in implantable neural prostheses. The main objective was to investigate the thermal effect of a near-infrared laser beam directly aimed at the brain cortex. A small, custom-made thermal probe was inserted into the rat brain to make direct measurements of temperature elevations induced by a free-air circular laser beam. The time dependence and the spatial distribution of the temperature increases were studied and the maximum allowable optical power was determined to be 2.27 W∕cm 2 for a corresponding temperature increase of 0.5°C near the cortical surface. The results can be extrapolated for other temperature elevations, where the margin to reach potentially damaging temperatures is more relaxed, by taking advantage of linearity. It is con- cluded that the thermal effect depends on several factors such as the thermal properties of the neural tissue and of its surrounding structures, the optical properties of the particular neural tissue, and the laser beam size and shape. Because so many parameters play a role, the thermal effect should be investigated for each specific application separately using realistic in vivo models. © The Authors. Published by SPIE under a Creative Commons Attribution

A method has been developed for improving the signal amplitudes of the recordings obtained with nerve cuff electrodes. The amplitude of the electroneurogram (ENG) has been shown to increase with increasing distance between the contacts when cuff electrodes are used to record peripheral nerve activity [9]. The effect is directly related to the propagation speed of the action potentials. Computer simulations have shown that the propagation velocity of action potentials in a length of a nerve axon can be decreased by subthreshold extracellular anodic currents. Slowing the action potentials is analogous to increasing the cuff length in that both result in longer intercontact delays, thus, larger signal outputs. This phenomenon is used to increase the amplitudes of whole nerve recordings obtained with a short cuff electrode. Computer simulations predicting the slowing effect of anodic currents as well as the experimental verification of this effect are presented. The increase in the amplitude of compound action potentials (CAP's) is demonstrated experimentally in an in vitro preparation. This method can be used to improve the signal-to-noise ratios when recording from short nerve segments where the cuff length is limited.

Near-infrared (NIR) lasers find applications in neuro-medicine both for diagnostic and treatment pur- poses. Penetration depth and profile into neural tissue are critical parameters to be considered in these applications. Published data on the optical properties of rodent neural tissue are rare, despite the frequent use of rats as an animal model. The aim of this study was to measure the light intensity profile inside the rat brain using a direct method, while the medium is being illuminated by an NIR laser beam, and compare the results with in vitro measurements of transmittance in the rat brain slices. The intensity profile along the vertical axis had an exponential decline with multiple regions that could be approximated with different coefficients. The Monte Carlo method that was used to simulate light-tissue interactions and predict the scattering coefficient of brain tissue from the measurements sug- gested that more scattering occurred in deeper layers of the cortex. A single scattering coefficient of 125 cm −1 was estimated for cortical layers from 300 to 1500 μm and a gradually increasing value from 125 to 370 cm −1 for depths of 1500 to 3000 μm. The deviations of in vivo results from the in vitro transmittance measurements, as well as the postmortem in vivo results from the alive measurements were significant. © The Authors. Published by SPIE under a Creative

Microelectrodes of neural stimulation utilize fine wires for electrical connections to driving electronics. Breakage of these wires and the neural tissue response due to their tethering forces are major problems encountered with long term implantation of microelectrodes. The lifetime of an implant for neural stimulation can be substantially improved if the wire interconnects are eliminated. Thus, we proposed a floating light-activated micro electrical stimulator (FLAMES) for wireless neural stimulation. In this paradigm, a laser beam at near infrared (NIR) wavelengths will be used as a means of energy transfer to the device. In this study, microstimulators of various sizes were fabricated, with two cascaded GaAs p-i-n photodiodes, and tested in the rat spinal cord. A train of NIR pulses (0.2 ms, 50 Hz) was sent through the tissue to wirelessly activate the devices and generate the stimulus current. The forces elicited by intraspinal stimulation were measured from the ipsilateral forelimb with a force transducer. The largest forces were around 1.08N, a significant level of force for the rat forelimb motor function. These in vivo tests suggest that the FLAMES can be used for intraspinal microstimulation even for the deepest implant locations in the rat spinal cord. The power required to generate a threshold arm movement was investigated as the laser source was moved away from the microstimulator. The results indicate that the photon density does not decrease substantially for horizontal displacements of the source that are in the same order as the beam radius. This gives confidence that the stimulation threshold may not be very sensitive to small displacement of the spinal cord relative to the spine-mounted optical power source.

Computer simulations indicate that the propagation velocity of action potentials in a length of a nerve axon can be decreased by sub-threshold extracellular anodic currents. This phenomenon can be used to increase the amplitude of whole nerve recordings made with a short cuff electrode with circumferencial metal bands, since larger propagation delays between the bands result in larger recorded signals. Computer simulations predicting the slowing effect of anodic currents and experimental data verifying the simulations are presented. The increase in the amplitude of nerve signals (fivefold), recorded experimentally from a short cuff, is demonstrated.