Ultrasmall implantable composite microelectrodes with bioactive surfaces for chronic neural interfaces.

Implantable neural microelectrodes that can record extracellular biopotentials from small, targeted groups of neurons are critical for neuroscience research and emerging clinical applications including brain-controlled prosthetic devices. The crucial material-dependent problem is developing microelectrodes that record neural activity from the same neurons for years with high fidelity and reliability. Here, we report the development of an integrated composite electrode consisting of a carbon-fibre core, a poly(p-xylylene)-based thin-film coating that acts as a dielectric barrier and that is functionalized to control intrinsic biological processes, and a poly(thiophene)-based recording pad. The resulting implants are an order of magnitude smaller than traditional recording electrodes, and more mechanically compliant with brain tissue. They were found to elicit much reduced chronic reactive tissue responses and enabled single-neuron recording in acute and early chronic experiments in rats. This technology, taking advantage of new composites, makes possible highly selective and stealthy neural interface devices towards realizing long-lasting implants.

Novel multi-sided, microelectrode arrays for implantable neural applications.

A new parylene-based microfabrication process is presented for neural recording and drug delivery applications. We introduce a large design space for electrode placement and structural flexibility with a six mask process. By using chemical mechanical polishing, electrode sites may be created top-side, back-side, or on the edge of the device having three exposed sides. Added surface area was achieved on the exposed edge through electroplating. Poly(3,4-ethylenedioxythiophene) (PEDOT) modified edge electrodes having an 85-µm(2) footprint resulted in an impedance of 200 kΩ at 1 kHz. Edge electrodes were able to successfully record single unit activity in acute animal studies. A finite element model of planar and edge electrodes relative to neuron position reveals that edge electrodes should be beneficial for increasing the volume of tissue being sampled in recording applications.

Conducting-polymer nanotubes improve electrical properties, mechanical adhesion, neural attachment, and neurite outgrowth of neural electrodes.

An in vitro comparison of conducting-polymer nanotubes of poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(pyrrole) (PPy) and to their film counterparts is reported. Impedance, charge-capacity density (CCD), tendency towards delamination, and neurite outgrowth are compared. For the same deposition charge density, PPy films and nanotubes grow relatively faster vertically, while PEDOT films and nanotubes grow more laterally. For the same deposition charge density (1.44 C cm(-2)), PPy nanotubes and PEDOT nanotubes have lower impedance (19.5 +/- 2.1 kOmega for PPy nanotubes and 2.5 +/- 1.4 kOmega for PEDOT nanotubes at 1 kHz) and higher CCD (184 +/- 5.3 mC cm(-2) for PPy nanotubes and 392 +/- 6.2 mC cm(-2) for PEDOT nanotubes) compared to their film counterparts. However, PEDOT nanotubes decrease the impedance of neural-electrode sites by about two orders of magnitude (bare iridium 468.8 +/- 13.3 kOmega at 1 kHz) and increase capacity of charge density by about three orders of magnitude (bare iridium 0.1 +/- 0.5 mC cm(-2)). During cyclic voltammetry measurements, both PPy and PEDOT nanotubes remain adherent on the surface of the silicon dioxide while PPy and PEDOT films delaminate. In experiments of primary neurons with conducting-polymer nanotubes, cultured dorsal root ganglion explants remain more intact and exhibit longer neurites (1400 +/- 95 microm for PPy nanotubes and 2100 +/- 150 microm for PEDOT nanotubes) than their film counterparts. These findings suggest that conducting-polymer nanotubes may improve the long-term function of neural microelectrodes.

In vivo performance of a microelectrode neural probe with integrated drug delivery.

OBJECT: The availability of sophisticated neural probes is a key prerequisite in the development of future brain-machine interfaces (BMIs). In this study, the authors developed and validated a neural probe design capable of simultaneous drug delivery and electrophysiology recordings in vivo. Focal drug delivery promises to extend dramatically the recording lives of neural probes, a limiting factor to clinical adoption of BMI technology. METHODS: To form the multifunctional neural probe, the authors affixed a 16-channel microfabricated silicon electrode array to a fused silica catheter. Three experiments were conducted in rats to characterize the performance of the device. Experiment 1 examined cellular damage from probe insertion and the drug distribution in tissue. Experiment 2 measured the effects of saline infusions delivered through the probe on concurrent electrophysiological measurements. Experiment 3 demonstrated that a physiologically relevant amount of drug can be delivered in a controlled fashion. For these experiments, Hoechst and propidium iodide stains were used to assess insertion trauma and the tissue distribution of the infusate. Artificial CSF (aCSF) and tetrodotoxin (TTX) were injected to determine the efficacy of drug delivery. RESULTS: The newly developed multifunctional neural probes were successfully inserted into rat cortex and were able to deliver fluids and drugs that resulted in the expected electrophysiological and histological responses. The damage from insertion of the device into brain tissue was substantially less than the volume of drug dispersion in tissue. Electrophysiological activity, including both individual spikes as well as local field potentials, was successfully recorded with this device during real-time drug delivery. No significant changes were seen in response to delivery of aCSF as a control experiment, whereas delivery of TTX produced the expected result of suppressing all spiking activity in the vicinity of the catheter outlet. CONCLUSIONS: Multifunctional neural probes such as the ones developed and validated within this study have great potential to help further understand the design space and criteria for the next generation of neural probe technology. By incorporating integrated drug delivery functionality into the probes, new treatment options for neurological disorders and regenerative neural interfaces using localized and feedback-controlled delivery of drugs can be realized in the near future.

Intradural Spinal Cord Stimulation: Performance Modeling of a New Modality.

Introduction: Intradural spinal cord stimulation (SCS) may offer significant therapeutic benefits for those with intractable axial and extremity pain, visceral pain, spasticity, autonomic dysfunction and related disorders. A novel intradural electrical stimulation device, limited by the boundaries of the thecal sac, CSF and spinal cord was developed to test this hypothesis. In order to optimize device function, we have explored finite element modeling (FEM). Methods: COMSOL®Multiphysics Electrical Currents was used to solve for fields and currents over a geometric model of a spinal cord segment. Cathodic and anodic currents are applied to the center and tips of the T-cross component of the electrode array to shape the stimulation field and constrain charge-balanced cathodic pulses to the target area. Results: Currents from the electrode sites can move the effective stimulation zone horizontally across the cord by a linear step method, which can be diversified considerably to gain greater depth of penetration relative to standard epidural SCS. It is also possible to prevent spread of the target area with no off-target action potential. Conclusion: Finite element modeling of a T-shaped intradural spinal cord stimulator predicts significant gains in field depth and current shaping that are beyond the reach of epidural stimulators. Future studies with in vivo models will investigate how this approach should first be tested in humans.

Implantable microelectrode arrays for simultaneous electrophysiological and neurochemical recordings.

Implantable microfabricated microelectrode arrays represent a versatile and powerful tool to record electrophysiological activity across multiple spatial locations in the brain. Spikes and field potentials, however, correspond to only a fraction of the physiological information available at the neural interface. In urethane-anesthetized rats, microfabricated microelectrode arrays were implanted acutely for simultaneous recording of striatal local field potentials, spikes, and electrically evoked dopamine overflow on the same spatiotemporal scale. During these multi-modal recordings we observed (1) that the amperometric method used to detect dopamine did not significantly influence electrophysiological activity, (2) that electrical stimulation in the medial forebrain bundle (MFB) region resulted in electrochemically transduced dopamine transients in the striatum that were spatially heterogeneous within at least 200 microm, and (3) following MFB stimulation, dopamine levels and electrophysiological activity within the striatum exhibited similar temporal profiles. These neural probes are capable of incorporating customized microelectrode geometries and configurations, which may be useful for examining specific spatiotemporal relationships between electrical and chemical signaling in the brain.

Neural probe design for reduced tissue encapsulation in CNS.

This study investigated relationships between a microscale neural probe's size and shape and its chronic reactive tissue response. Parylene-based probes were microfabricated with a thick shank (48 microm by 68 microm) and an integrated thin lateral platform (5 microm by 100 microm, either solid or one of three lattice sizes). Devices were implanted in rat cerebral cortex for 4 weeks before immunostaining for neurons, astrocytes, microglia, fibronectin, laminin, and neurofilament. While nonneuronal density (NND) generally increased and neuronal density decreased within 75 microm of a probe interface compared to unimplanted control regions, there were significant differential tissue responses within 25 microm of the platform's lateral edge compared to the shank. The NND in this region of the lateral edge was less than one-third of the corresponding region of the shank (129% and 425% increase, respectively). Moreover, neuronal density around the platform lateral edge was about one-third higher than at the shank (0.70 and 0.52 relative to control, respectively). Also, microglia reactivity and extracellular protein deposition was reduced at the lateral edge. There were no significant differences among platform designs. These results suggest that neural probe geometry is an important parameter for reducing chronic tissue encapsulation.

Spatiotemporal pH dynamics following insertion of neural microelectrode arrays.

Insertion trauma is a critical issue when assessing intracortical electrophysiological and neurochemical recordings. Previous reports document a wide variety of insertion techniques with speeds ranging from 10 microm/s to 10 m/s. We hypothesize that insertion speed has an effect on tissue trauma induced by implantation of a neural probe. In order to monitor the neural interface during and after probe insertion, we have developed a silicon-substrate array with hydrous iridium oxide microelectrodes for potentiometric recording of extracellular pH (pH(e)), a measure of brain homeostasis. Microelectrode sites were sensitive to pH in the super-Nernstian range (-85.9 mV/pH unit) and selective over other analytes including ascorbic acid, Na(+), K(+), Ca(2+), and Mg(2+). Following insertion, arrays recorded either triphasic or biphasic pH(e) responses, with a greater degree of prolonged acidosis for insertions at 50 microm/s than at 0.5 mm/s or 1.0 mm/s (p<0.05). Spatiotemporal analysis of the recordings also revealed micro-scale variability in the pH(e) response along the array, even when using the same insertion technique. Implants with more intense acidosis were often associated histologically with blood along the probe tract. The potentiometric microsensor array has implications not only as a useful tool to measure extracellular pH, but also as a feedback tool for delivery of pharmacological agents to treat surgical brain trauma.

Fast wave propagation in auditory cortex of an awake cat using a chronic microelectrode array.

We investigated fast wave propagation in auditory cortex of an alert cat using a chronically implanted microelectrode array. A custom, real-time imaging template exhibited wave dynamics within the 33-microwire array (3 mm(2)) during ten recording sessions spanning 1 month post implant. Images were based on the spatial arrangement of peri-stimulus time histograms at each recording site in response to auditory stimuli consisting of tone pips between 1 and 10 kHz at 75 dB SPL. Functional images portray stimulus-locked spiking activity and exhibit waves of excitation and inhibition that evolve during the onset, sustained and offset period of the tones. In response to 5 kHz, for example, peak excitation occurred at 27 ms after onset and again at 15 ms following tone offset. Variability of the position of the centroid of excitation during ten recording sessions reached a minimum at 31 ms post onset (sigma = 125 microm) and 18 ms post offset (sigma = 145 microm), suggesting a fine place/time representation of the stimulus in the cortex. The dynamics of these fast waves also depended on stimulus frequency, likely reflecting the tonotopicity in auditory cortex projected from the cochlea. Peak wave velocities of 0.2 m s(-1) were also consistent with those purported across horizontal layers of cat visual cortex. The fine resolution offered by microimaging may be critical for delivering optimal coding strategies used with an auditory prosthesis. Based on the initial results, future studies seek to determine the relevance of these waves to sensory perception and behavior.

Complex impedance spectroscopy for monitoring tissue responses to inserted neural implants.

A series of animal experiments was conducted to characterize changes in the complex impedance of chronically implanted electrodes in neural tissue. Consistent trends in impedance changes were observed across all animals, characterized as a general increase in the measured impedance magnitude at 1 kHz. Impedance changes reach a peak approximately 7 days post-implant. Reactive responses around individual electrodes were described using immuno- and histo-chemistry and confocal microscopy. These observations were compared to measured impedance changes. Several features of impedance changes were able to differentiate between confined and extensive histological reactions. In general, impedance magnitude at 1 kHz was significantly increased in extensive reactions, starting about 4 days post-implant. Electrodes with extensive reactions also displayed impedance spectra with a characteristic change at high frequencies. This change was manifested in the formation of a semi-circular arc in the Nyquist space, suggestive of increased cellular density in close proximity to the electrode site. These results suggest that changes in impedance spectra are directly influenced by cellular distributions around implanted electrodes over time and that impedance measurements may provide an online assessment of cellular reactions to implanted devices.

In vivo stability and biocompatibility of implanted calcium alginate disks.

Alginate is a commonly used biomedical hydrogel whose in vivo degradation behavior is only beginning to be understood. The use of alginate in the central nervous system is gaining popularity as an electrode coating, cell encapsulation matrix, and for duraplasty. However, it is necessary to understand how the hydrogel will behave in vivo to aid in the development of alginate for use as a neural interface material. The goal of the current study was to compare the rheological behavior of explanted alginate disks and the inflammatory response to subcutaneously implanted alginate hydrogels over a 3-month period. Specifically, the effects due to (1) in situ gelling, (2) diffusion gelling, and (3) use of a poly-l-lysine (PLL) coating were investigated. While all samples' complex moduli decreased 80% in the first day, in situ gelled alginate was more stable for the first week of implantation. The PLL coating offered some stability increases for diffusion gelled alginate, but the stability in both conditions remained significantly lower than that in in situ gelled alginate. There were no differences in biocompatibility that clearly suggested one gelation method over another. These results indicate that in situ gelation is the preferred method in neural interface applications where stability is the primary concern.

Chronic neural recordings using silicon microelectrode arrays electrochemically deposited with a poly(3,4-ethylenedioxythiophene) (PEDOT) film.

Conductive polymer coatings can be used to modify traditional electrode recording sites with the intent of improving the long-term performance of cortical microelectrodes. Conductive polymers can drastically decrease recording site impedance, which in turn is hypothesized to reduce thermal noise and signal loss through shunt pathways. Moreover, conductive polymers can be seeded with agents aimed at promoting neural growth toward the recording sites or minimizing the inherent immune response. The end goal of these efforts is to generate an ideal long-term interface between the recording electrode and surrounding tissue. The goal of this study was to refine a method to electrochemically deposit surfactant-templated ordered poly(3,4-ethylenedioxythiophene) (PEDOT) films on the recording sites of standard 'Michigan' probes and to evaluate the efficacy of these modified sites in recording chronic neural activity. PEDOT-coated site performance was compared to control sites over a six-week evaluation period in terms of impedance spectroscopy, signal-to-noise ratio, number of viable unit potentials recorded and local field potential recordings. PEDOT sites were found to outperform control sites with respect to signal-to-noise ratio and number of viable unit potentials. The benefit of reduced initial impedance, however, was mitigated by the impedance contribution of typical silicon electrode encapsulation. Coating sites with PEDOT also reduced the amount of low-frequency drift evident in local field potential recordings. These findings indicate that electrode sites electrochemically deposited with PEDOT films are suitable for recording neural activity in vivo for extended periods. This study also provided a unique opportunity to monitor how neural recording characteristics develop over the six weeks following implantation.

Suitability of the cingulate cortex for neural control.

Recent neuroprosthetic work has focused on the motor cortex as a source of voluntary control signals. However, the motor cortex can be damaged in upper motor neuron degenerative diseases such as primary lateral sclerosis and amyotrophic lateral sclerosis. The possibility exists that prefrontal areas may also be used in neuroprosthetic devices. Here, we report the use of the cingulate cortex in a neuroprosthetic model. Seven rats were able to significantly modulate spiking activity in the cingulate cortex in order to receive reward. Furthermore, experiments with single neurons provide evidence that the cingulate cortex neuronal modulation is highly flexible and thus useful for a neuroprosthetic device.

Voltage pulses change neural interface properties and improve unit recordings with chronically implanted microelectrodes.

Current neuroprosthetic systems based on electro-physiological recording have an extended, yet finite working lifetime. Some posited lifetime-extension solutions involve improving device biocompatibility or suppressing host immune responses. Our objective was to test an alternative solution comprised of applying a voltage pulse to a microelectrode site, herein termed "rejuvenation." Previously, investigators have reported preliminary electrophysiological results by utilizing a similar voltage pulse. In this study we sought to further explore this phenomenon via two methods: 1) electrophysiology; 2) an equivalent circuit model applied to impedance spectroscopy data. The experiments were conducted via chronically implanted silicon-substrate iridium microelectrode arrays in the rat cortex. Rejuvenation voltages resulted in increased unit recording signal-to-noise ratios (10% +/- 2%), with a maximal increase of 195% from 3.74 to 11.02. Rejuvenation also reduced the electrode site impedances at 1 kHz (67% +/- 2%). Neither the impedance nor recording properties of the electrodes changed on neighboring microelectrode sites that were not rejuvenated. In the equivalent circuit model, we found a transient increase in conductivity, the majority of which corresponded to a decrease in the tissue resistance component (44% +/- 7%). These findings suggest that rejuvenation may be an intervention strategy to prolong the functional lifetime of chronically implanted microelectrodes.

A Novel Lead Design for Modulation and Sensing of Deep Brain Structures.

GOAL: Develop and characterize the functionality of a novel thin-film probe technology with a higher density of electrode contacts than are currently available with commercial deep brain stimulation (DBS) lead technology. Such technology has potential to enhance the spatial precision of DBS and enable a more robust approach to sensing local field potential activity in the context of adaptive DBS strategies. METHODS: Thin-film planar arrays were microfabricated and then assembled on a cylindrical carrier to achieve a lead with 3-D conformation. Using an integrated and removable stylet, the arrays were chronically implanted in the subthalamic nucleus and globus pallidus in two parkinsonian nonhuman primates. RESULTS: This study provides the first in vivo data from chronically implanted DBS arrays for translational nonhuman primate studies. Stimulation through the arrays induced a decrease in parkinsonian rigidity, and directing current around the lead showed an orientation dependence for eliciting motor capsule side effects. The array recordings also showed that oscillatory activity in the basal ganglia is heterogeneous at a smaller scale than detected by the current DBS lead technology. CONCLUSION: These 3-D DBS arrays provide an enabling tool for future studies that seek to monitor and modulate deep brain activity through chronically implanted leads. SIGNIFICANCE: DBS lead technology with a higher density of electrode contacts has potential to enable sculpting DBS current flow and sensing biomarkers of disease and therapy.

Enhanced contrast sensitivity in auditory cortex as cats learn to discriminate sound frequencies.

To better understand the nature and time course for learning-induced cortical reorganization, we examined frequency-specific changes in auditory cortex as cats gradually improved at a difficult sound frequency discrimination task. Three adult cats were trained to discriminate between a tone pip at a fixed target frequency (S-) and a higher deviant frequency (S+). An adaptive training schedule led to an efficient estimate of the frequency discrimination threshold (FDT), which was used to track daily performance. Each cat was also implanted with an array of microwires in auditory cortex. Tone pips with different frequency and amplitude were used to map receptive fields. Onset responses were correlated with training time and the cat's ability to discriminate frequencies. Although lifetime of the neural implants varied among cats, each provided sufficient neural recording to relate at least 3 weeks of learning to response changes in the cortex. An improved FDT was associated with a differential decrease in response strength between the S- frequency and S+ frequencies. Response to the training frequencies gradually located in a local minimum compared to adjacent frequencies (p < 0.001, Cohen's d=0.50). Cortical changes were consistent with a theory of bimodal generalization that enhances stimulus classification by reducing similarity between reinforced and nonreinforced stimuli. Such a strategy may be especially appropriate during an early stage of learning to discriminate similar sounds and differ from later strategies required for fine discrimination.

A finite-element model of the mechanical effects of implantable microelectrodes in the cerebral cortex.

The viability of chronic neural microelectrodes for electrophysiological recording and stimulation depends on several factors, including the encapsulation of the implant by a reactive tissue response. We postulate that mechanical strains induced around the implant site may be one of the leading factors responsible for the sustained tissue response in chronic implants. The objectives of this study were to develop a finite-element model of the probe-brain tissue interface and analyze the effects of tethering forces, probe-tissue adhesion and stiffness of the probe substrate on the interfacial strains induced around the implant site. A 3D finite-element model of the probe-brain tissue microenvironment was developed and used to simulate interfacial strains created by 'micromotion' of chronically implanted microelectrodes. Three candidate substrates were considered: (a) silicon, (b) polyimide and (c) a hypothetical 'soft' material. Simulated tethering forces resulted in elevated strains both at the tip and at the sharp edges of the probe track in the tissue. The strain fields induced by a simulated silicon probe were similar to those induced by a simulated polyimide probe, albeit at higher absolute values for radial tethering forces. Simulations of poor probe-tissue adhesion resulted in elevated strains at the tip and delamination of the tissue from the probe. A tangential tethering force results in 94% reduction in the strain value at the tip of the polyimide probe track in the tissue, whereas the simulated 'soft' probe induced two orders of magnitude smaller values of strain compared to a simulated silicon probe. The model results indicate that softer substrates reduce the strain at the probe-tissue interface and thus may also reduce tissue response in chronic implants.

Cortical microstimulation in auditory cortex of rat elicits best-frequency dependent behaviors.

Electrical activation of the auditory cortex has been shown to elicit an auditory sensation; however, the perceptual effects of auditory cortical microstimulation delivered through penetrating microelectrodes have not been clearly elucidated. This study examines the relationship between electrical microstimulus location within the adult rat auditory cortex and the subsequent behavior induced. Four rats were trained on an auditory frequency discrimination task and their lever-pressing behavior in response to stimuli of intermediate auditory frequencies was quantified. Each trained rat was then implanted with a microwire array in the auditory cortex of the left hemisphere. Best frequencies (BFs) of each electrode in the array were determined by both local field potential and multi-unit spike-rate activity evoked by pure tone stimuli. A cross-dimensional psychophysical generalization paradigm was used to evaluate cortical microstimulation-induced behavior. Using the BFs of each electrode, the microstimulation-induced behavior was evaluated relative to the auditory-induced behavior. Microstimulation resulted in behavior that was dependent on the BFs of the electrodes used for stimulation. These results are consistent with recent reports indicating that electrophysiological recordings of neural responses to sensory stimuli may provide insight into the sensation generated by electrical stimulation of the same sensory neural tissue.

Naive coadaptive cortical control.

The ability to control a prosthetic device directly from the neocortex has been demonstrated in rats, monkeys and humans. Here we investigate whether neural control can be accomplished in situations where (1) subjects have not received prior motor training to control the device (naive user) and (2) the neural encoding of movement parameters in the cortex is unknown to the prosthetic device (naive controller). By adopting a decoding strategy that identifies and focuses on units whose firing rate properties are best suited for control, we show that naive subjects mutually adapt to learn control of a neural prosthetic system. Six untrained Long-Evans rats, implanted with silicon micro-electrodes in the motor cortex, learned cortical control of an auditory device without prior motor characterization of the recorded neural ensemble. Single- and multi-unit activities were decoded using a Kalman filter to represent an audio "cursor" (90 ms tone pips ranging from 250 Hz to 16 kHz) which subjects controlled to match a given target frequency. After each trial, a novel adaptive algorithm trained the decoding filter based on correlations of the firing patterns with expected cursor movement. Each behavioral session consisted of 100 trials and began with randomized decoding weights. Within 7 +/- 1.4 (mean +/- SD) sessions, all subjects were able to significantly score above chance (P < 0.05, randomization method) in a fixed target paradigm. Training lasted 24 sessions in which both the behavioral performance and signal to noise ratio of the peri-event histograms increased significantly (P < 0.01, ANOVA). Two rats continued training on a more complex task using a bilateral, two-target control paradigm. Both subjects were able to significantly discriminate the target tones (P < 0.05, Z-test), while one subject demonstrated control above chance (P < 0.05, Z-test) after 12 sessions and continued improvement with many sessions achieving over 90% correct targets. Dynamic analysis of binary trial responses indicated that early learning for this subject occurred during session 6. This study demonstrates that subjects can learn to generate neural control signals that are well suited for use with external devices without prior experience or training.

Multi-site incorporation of bioactive matrices into MEMS-based neural probes.

Methods are presented to incorporate polymer-based bioactive matrices into micro-fabricated implantable microelectrode arrays. Using simple techniques, hydrogels infused with bioactive molecules are deposited within wells in the substrate of the device. This method allows local drug delivery without increasing the footprint of the device. In addition, each well can be loaded individually, allowing spatial and temporal control over diffusion gradients in the microenvironment of the implanted neural interface probe. In vivo testing verified the following: diffusion of the bioactive molecules, integration of the bioactive molecules with the intended neural target and concurrent extracellular recording using nearby electrodes. These results support the feasibility of using polymer gels to deliver bioactive molecules to the region close to microelectrode shanks. This technique for microdrug delivery may serve as a means to intervene with the initial phases of the neuroinflammatory tissue response to permanently implanted microelectrode arrays.