Implantable neurotechnologies: electrical stimulation and applications

A biodegradable and flexible neural interface for transdermal optoelectronic modulation and regeneration of peripheral nerves

Optoelectronic neural interfaces can leverage the photovoltaic effect to convert light into electrical current, inducing charge redistribution and enabling nerve stimulation. This method offers a non-genetic and remote approach for neuromodulation. Developing biodegradable and efficient optoelectronic neural interfaces is important for achieving transdermal stimulation while minimizing infection risks associated with device retrieval, thereby maximizing therapeutic outcomes. We propose a biodegradable, flexible, and miniaturized silicon-based neural interface capable of transdermal optoelectronic stimulation for neural modulation and nerve regeneration. Enhancing the device interface with thin-film molybdenum significantly improves the efficacy of neural stimulation. Our study demonstrates successful activation of the sciatic nerve in rodents and the facial nerve in rabbits. Moreover, transdermal optoelectronic stimulation accelerates the functional recovery of injured facial nerves.

Noninvasive Electrical Stimulation Neuromodulation and Digital Brain Technology: A Review

We review the research progress on noninvasive neural regulatory systems through system design and theoretical guidance. We provide an overview of the development history of noninvasive neuromodulation technology, focusing on system design. We also discuss typical cases of neuromodulation that use modern noninvasive electrical stimulation and the main limitations associated with this technology. In addition, we propose a closed-loop system design solution of the "time domain", "space domain", and "multi-electrode combination". For theoretical guidance, this paper provides an overview of the "digital brain" development process used for noninvasive electrical-stimulation-targeted modeling and the development of "digital human" programs in various countries. We also summarize the core problems of the existing "digital brain" used for noninvasive electrical-stimulation-targeted modeling according to the existing achievements and propose segmenting the tissue. For this, the tissue parameters of a multimodal image obtained from a fresh cadaver were considered as an index. The digital projection of the multimodal image of the brain of a living individual was implemented, following which the segmented tissues could be reconstructed to obtain a "digital twin brain" model with personalized tissue structure differences. The "closed-loop system" and "personalized digital twin brain" not only enable the noninvasive electrical stimulation of neuromodulation to achieve the visualization of the results and adaptive regulation of the stimulation parameters but also enable the system to have individual differences and more accurate stimulation.

Effect of Exogenous Electric Stimulation on the Cardiac Tissue Function In Situ Monitored by Scanning Electrochemical Microscopy

Cardiac tissue is sensitive to and can be easily damaged by exogenous electric stimulation. However, due to the thermal-electric coeffect and the limitation of in situ and quantitative information on the cardiac tissue function under electric stimulation, the detailed effect and the underlying mechanism of exogenous electric stimulation on the cardiac tissue remain elusive. To address this, in this work, we first constructed an in vitro cardiac tissue model and established a thermal-electric coupled theoretical model for simulating the electric field and temperature distributions around the cardiac tissue, from which we selected the electric field strengths (1.19, 2.37, and 3.39 kV cm^-1) and electrical energies (0.001, 0.005, and 0.011 J) for electric stimulations without inducing a thermal effect. Then, we applied electric field stimulations on the cardiac tissue using these parameters and scanning electrochemical microscopy (SECM) to in situ and quantitatively monitor the dynamic changes in the key parameters of the cardiac tissue function, including respiratory activity, membrane permeability, and contraction frequency, after electric field stimulations. The SECM results showed that the oxygen consumption, cell membrane permeability coefficient, and contraction frequency of the cardiac tissue were strongly dependent on electrical energy, especially when the electrical energy was higher than 0.001 J. Our work, for the first time, achieves the in situ and quantitative monitoring of the cardiac tissue function under electric stimulation using SECM, which would provide important references for designing an electric stimulation regime for cardiac tissue engineering and clinical application of electrotherapy.

Active therapy based on the byproducts of micro/nanomotors

Different from traditional colloidal particles based on Brownian motion, micro/nanomotors are micro/nanoscale devices capable of performing complex tasks in liquid media via transforming various energy sources into mechanical motion or actuation. Such unique self-propulsion endows motors with fantastic capabilities to access and enter the deep layer of targeted diseased tissue, which in turn breaks through the limitation of the poor permeability of traditional pharmaceutical preparations, thus providing giant prospects for active therapy. It is noteworthy that recently several studies, which utilized the byproducts generated in situ by micro/nanomotors to achieve active therapy, in a truly green zero-waste manner, have been carried out. In this minireview, we highlight the recent efforts with respect to active therapy based on the byproducts of micro/nanomotors, expecting to motivate readers to expand the practical biomedical application scope of micro/nanomotors in a broader horizon. Accompanied by ever booming enthusiasm and persevering exploration, micro/nanomotors are on their way to revolutionize conventional fields.

Totally Organic Hydrogel-Based Self-Closing Cuff Electrode for Vagus Nerve Stimulation

An intrinsically soft organic electrode consisting of poly(3,4-ethylenedioxythiophene)-modified polyurethane (PEDOT-PU) is embedded into a bilayer film of polyvinyl alcohol (PVA) hydrogels for developing a self-closing cuff electrode for neuromodulation. The curled form of the PVA hydrogel is prepared by releasing internal stress in the bilayer structure. The inner diameter of the cuff electrode is set to less than 2 mm for immobilization to the vagus nerve (VN) of humans and pigs. The stability of the immobilization is examined, while the pressure applied to a nerve bundle is at a harmless level (≈200 Pa). Since the electrode is totally organic, MRI measurements can be conducted without image artifacts. The large electric capacitance of the PEDOT-PU (≈27 mF cm^-2 ) ensures a safe stimulation of living tissues without Faradaic reactions. The practical performance of the cuff electrode for VN stimulation is demonstrated by observation of bradycardia induction in a pig.

Ameliorative and Renoprotective Effect of Electrical Stimulation on Blood Sugar, Blood Urea Nitrogen (BUN), Creatinine Levels, and the Islets of Langerhans Weight in Diabetic Mice

Diabetes mellitus (DM) is a chronic metabolic disease or disorder characterized by high blood sugar levels as well as impaired carbohydrate, lipid, and protein metabolism due to insulin function insufficiency. Insulin deficiency can be caused by impaired or deficient insulin production by Langerhans beta cells in the pancreas or by a lack of responsiveness of the body's cells to insulin. This study aims to the effects of electrostimulation on the ameliorative (improves disease manifestations) or renoprotective (protects the kidneys) in a diabetic rat model using noninvasive (electrical stimulation with the magnetic and nonmagnetic electrode) and invasive (using needles) methods. This study used 25 female rats, with a normal control group (KN), a diabetes control group (KD), a needle treatment group (A), an electro-stimulator treatment group with a magnetic electrode (M), and an ES group with a nonmagnetic electrode (ES) (L). The electro-stimulator used AES-05 with a magnetic field strength of 90 mT at two acupoints, Pishu (BL20) and Shenshu (BL23). The treatment was administered 12 times in one month with a therapy time of 6.6 minutes per session. Body weight and blood sugar levels were compared before and after the treatment. After treatment, the diameter of the islets of Langerhans, as well as levels of creatinine and blood urea nitrogen (BUN), was measured. Furthermore, statistical analysis was performed (α = 0.05). The results of this study showed that electrical stimulation treatments with needle-invasive, noninvasive magnetic electrodes, and nonmagnetic electrodes significantly reduced diabetic rats’ blood glucose levels before and after the treatment. The analysis of the diameter of the islets of Langerhans revealed a significant difference between the treatment groups. The analysis of creatinine levels revealed a significant difference between groups, but creatinine levels in the group with the magnetic electrode (0.58 ± 0.17 mg/dL) were not significantly different from the control group (0.58 ± 0.07 mg/dL). The BUN test results revealed a significant difference compared with the diabetic control group, but no significant difference with the magnetic electrode treatment group. Conclusion. Based on the results, the most effective therapy for diabetes is a noninvasive method with magnetic (M) electrodes.

Bioresorbable thin-film silicon diodes for the optoelectronic excitation and inhibition of neural activities

Neural activities can be modulated by leveraging light-responsive nanomaterials as interfaces for exerting photothermal, photoelectrochemical or photocapacitive effects on neurons or neural tissues. Here we show that bioresorbable thin-film monocrystalline silicon pn diodes can be used to optoelectronically excite or inhibit neural activities by establishing polarity-dependent positive or negative photovoltages at the semiconductor/solution interface. Under laser illumination, the silicon-diode optoelectronic interfaces allowed for the deterministic depolarization or hyperpolarization of cultured neurons as well as the upregulated or downregulated intracellular calcium dynamics. The optoelectronic interfaces can also be mounted on nerve tissue to activate or silence neural activities in peripheral and central nervous tissues, as we show in mice with exposed sciatic nerves and somatosensory cortices. Bioresorbable silicon-based optoelectronic thin films that selectively excite or inhibit neural tissue may find advantageous biomedical applicability.

Ionic transistor using ion exchange membranes

Ionic transistors can be used to modulate ionic current in a way that is analogous to their electronic counterparts. An ionic transistor can reversibly change its ionic conduction to control ionic current by injecting electrical charges. To facilitate its applications in biomedical devices (e.g., controlled drug delivery, rectification of ionic current, and signal processing), an ionic transistor should maintain high performance of ionic current control within physiological solutions (e.g., 0.9% NaCl) for long durations. Here, we introduce an ionic transistor using cation and anion exchange membranes (CEM and AEM). It could impose a 10× impedance change in a channel filled with 0.9% NaCl solution and we observed a stable modulation of ionic current throughout a test of 1000 cycles of on/off switching of the ionic transistor.

Predicting Corrosion Delamination Failure in Active Implantable Medical Devices: Analytical Model and Validation Strategy

The ingress of body fluids or their constituents is one of the main causes of failure of active implantable medical devices (AIMDs). Progressive delamination takes its origin at the junctions where exposed electrodes and conductive pathways enter the implant interior. The description of this interface is considered challenging because electrochemically-diffusively coupled processes are involved. Furthermore, standard tests and specimens, with clearly defined 3-phase boundaries (body fluid-metal-polymer), are lacking. We focus on polymers as substrate and encapsulation and present a simple method to fabricate reliable test specimens with defined boundaries. By using silicone rubber as standard material in active implant encapsulation in combination with a metal surface, a corrosion-triggered delamination process was observed that can be universalised towards typical AIMD electrode materials. Copper was used instead of medical grade platinum since surface energies are comparable but corrosion occurs faster. The finding is that two processes are superimposed there: First, diffusion-limited chemical reactions at interfaces that undermine the layer adhesion. The second process is the influx of ions and body fluid components that leave the aqueous phase and migrate through the rubber to internal interfaces. The latter observation is new for active implants. Our mathematical description with a Stefan-model coupled to volume diffusion reproduces the experimental data in good agreement and lends itself to further generalisation.

Physiologically Self-Regulated, Fully Implantable, Battery-Free System for Peripheral Nerve Restoration

The long-segment peripheral nerve injury (PNI) represents a global medical challenge, leading to incomplete nerve tissue recovery and unsatisfactory functional reconstruction. However, the current electrical stimulation (ES) apparatuses fail perfect nerve repair due to their inability of the variable synchronous self-regulated function with physiological states. It is urgent to develop an implantable ES platform with physiologically adaptive function to provide instantaneous and nerve-preferred ES. Here, a physiologically self-regulated electrical signal is generated by integrating a novel tribo/piezoelectric hybrid nanogenerator with a nanoporous nerve guide conduit to construct a fully implantable neural electrical stimulation (FI-NES) system. The optimal neural ES parameters completely originate from the body itself and are highly self-responsive to different physiological states. The morphological evaluation, representative protein expression level, and functional reconstruction of the regenerated nerves are conducted to assess the PNI recovery process. Evidence shows that the recovery effect of 15 mm length nerve defects under the guidance of the FI-NES system is significantly close to the autograft. The designed FI-NES system provides an effective method for long-term accelerating the recovery of PNI in vivo and is also appropriate for other tissue injury or neurodegenerative diseases.

Nanostructured gold electrodes promote neural maturation and network connectivity

Progress in the clinical application of recording and stimulation devices for neural diseases is still limited, mainly because of suboptimal material engineering and unfavorable interactions with biological entities. Nanotechnology is providing upgraded designs of materials to better mimic the native extracellular environment and attain more intimate contacts with individual neurons, besides allowing for the miniaturization of the electrodes. However, little progress has been done to date on the understanding of the biological impact that such neural interfaces have on neural network maturation and functionality. In this work, we elucidate the effect of a gold (Au) highly ordered nanostructure on the morphological and functional interactions with neural cells and tissues. Alumina-templated Au nanostructured electrodes composed of parallel nanowires of 160 nm in diameter and 1.2 μm in length (Au-NWs), with 320 nm of pitch, are designed and characterized. Equivalent non-structured Au electrodes (Au-Flat) are used for comparison. By using diverse techniques in in vitro cell cultures including live calcium imaging, we found that Au-NWs interfaced with primary neural cortical cells for up to 14 days allow neural networks growth and increase spontaneous activity and ability of neuronal synchronization, thus indicating that nanostructured features favor neuronal network. The enhancement in the number of glial cells found is hypothesized to be behind these beneficial functional effects. The in vivo effect of the implantation of these nanostructured electrodes and its potential relevance for future clinical applicability has been explored in an experimental model of rat spinal cord injury. Subacute responses to implanted Au-NWs show no overt reactive or toxic biological reactions besides those triggered by the injury itself. These results highlight the translational potential of Au-NWs electrodes for in vivo applications as neural interfaces in contact with central nervous tissues including the injured spinal cord.

A Reversible Low Frequency Alternating Current Nerve Conduction Block Applied to Mammalian Autonomic Nerves

Electrical stimulation can be used to modulate activity within the nervous system in one of two modes: (1) Activation, where activity is added to the neural signalling pathways, or (2) Block, where activity in the nerve is reduced or eliminated. In principle, electrical nerve conduction block has many attractive properties compared to pharmaceutical or surgical interventions. These include reversibility, localization, and tunability for nerve caliber and type. However, methods to effect electrical nerve block are relatively new. Some methods can have associated drawbacks, such as the need for large currents, the production of irreversible chemical byproducts, and onset responses. These can lead to irreversible nerve damage or undesirable neural responses. In the present study we describe a novel low frequency alternating current blocking waveform (LFACb) and measure its efficacy to reversibly block the bradycardic effect elicited by vagal stimulation in anaesthetised rat model. The waveform is a sinusoidal, zero mean(charge balanced), current waveform presented at 1 Hz to bipolar electrodes. Standard pulse stimulation was delivered through Pt-Black coated PtIr bipolar hook electrodes to evoke bradycardia. The conditioning LFAC waveform was presented either through a set of CorTec^® bipolar cuff electrodes with Amplicoat^® coated Pt contacts, or a second set of Pt Black coated PtIr hook electrodes. The conditioning electrodes were placed caudal to the pulse stimulation hook electrodes. Block of bradycardic effect was assessed by quantifying changes in heart rate during the stimulation stages of LFAC alone, LFAC-and-vagal, and vagal alone. The LFAC achieved 86.2±11.1% and 84.3±4.6% block using hook (N = 7) and cuff (N = 5) electrodes, respectively, at current levels less than 110 µA_p (current to peak). The potential across the LFAC delivering electrodes were continuously monitored to verify that the blocking effect was immediately reversed upon discontinuing the LFAC. Thus, LFACb produced a high degree of nerve block at current levels comparable to pulse stimulation amplitudes to activate nerves, resulting in a measurable functional change of a biomarker in the mammalian nervous system.

Fully Implantable Plantar Cutaneous Augmentation System for Rats Using Closed-loop Electrical Nerve Stimulation

Plantar cutaneous feedback plays an important role in stable and efficient gait, by modulating the activity of ankle dorsi- and plantar-flexor muscles. However, central and peripheral nervous system trauma often decrease plantar cutaneous feedback and/or interneuronal excitability in processing the plantar cutaneous feedback. In this study, we tested a fully implantable neural recording and stimulation system augmenting plantar cutaneous feedback. Electromyograms were recorded from the medial gastrocnemius muscle for stance phase detection, while biphasic stimulation pulses were applied to the distal-tibial nerve during the stance phase to augment plantar cutaneous feedback. A Bluetooth low energy and a Qi-standard inductive link were adopted for wireless communication and wireless charging, respectively. To test the operation of the system, one intact rat walked on a treadmill with the electrical system implanted into its back. Leg kinematics were recorded to identify the stance phase. Stimulation was applied, with a 250-ms onset delay from stance onset and 200-ms duration, resulting in the onset at 47.58 ± 2.82% of stance phase and the offset at 83.49 ± 4.26% of stance phase (Mean ± SEM). The conduction velocity of the compound action potential (31.2 m/s and 41.6 m/s at 1·T and 2·T, respectively) suggests that the evoked action potential was characteristic of an afferent volley for cutaneous feedback. We also demonstrated successful wireless charging and system reset functions. The experimental results suggest that the presented implantable system can be a valuable neural interface tool to investigate the effect of plantar cutaneous augmentation on gait in a rat model.

Manipulating Neural Circuits in Anesthesia Research

The neural circuits underlying the distinct endpoints that define general anesthesia remain incompletely understood. It is becoming increasingly evident, however, that distinct pathways in the brain that mediate arousal and pain are involved in various endpoints of general anesthesia. To critically evaluate this growing body of literature, familiarity with modern tools and techniques used to study neural circuits is essential. This Readers' Toolbox article describes four such techniques: (1) electrical stimulation, (2) local pharmacology, (3) optogenetics, and (4) chemogenetics. Each technique is explained, including the advantages, disadvantages, and other issues that must be considered when interpreting experimental results. Examples are provided of studies that probe mechanisms of anesthesia using each technique. This information will aid researchers and clinicians alike in interpreting the literature and in evaluating the utility of these techniques in their own research programs.

Electrical stimulation for neuroregeneration in urology: a new therapeutic paradigm

PURPOSE OF REVIEW

The present review highlights regenerative electrical stimulation (RES) as potential future treatment options for patients with nerve injuries leading to urological dysfunction, such as urinary incontinence, voiding dysfunction or erectile dysfunction. Additionally, it will highlight the mechanism of nerve injury and regeneration as well as similarities and differences between RES and current electrical stimulation treatments in urology, functional electrical stimulation (FES) and neuromodulation.

RECENT FINDINGS

It has been demonstrated that RES upregulates brain-derived neurotrophic factor (BDNF) and its receptor to facilitate neuroregeneration, facilitating accurate reinnervation of muscles by motoneurons. Further, RES upregulates growth factors in glial cells. Within the past 2 years, RES of the pudendal nerve upregulated BDNF in Onuf's nucleus, the cell bodies of motoneurons that course through the pudendal nerve and accelerated functional recovery in an animal model of stress urinary incontinence. Additionally, electrical stimulation of the vaginal tissue in an animal model of stress urinary incontinence accelerated functional recovery.

SUMMARY

RES has great potential but future research is needed to expand the potential beneficial effects of RES in the field of urology.

The Microbead: A Highly Miniaturized Wirelessly Powered Implantable Neural Stimulating System

An implant that can electrically stimulate neurons across different depths and regions of the brain currently does not exist as it poses a number of obstacles that need to be solved. In order to address the challenges, this paper presents the concept of "microbead," a fully integrated wirelessly powered neural device that allows for spatially selective activation of neural tissue. The prototype chip is fabricated in 130-nm CMOS technology and currently measures 200 μm × 200 μm, which represents the smallest remotely powered stimulator to date. The system is validated experimentally in a rat by stimulating the sciatic nerve with 195-μs current pulses. To power the ultrasmall on-silicon coil, 36-dBm source power is provided to a highly optimized transmitter (Tx) coil at a coupling distance of 5 mm. In order to satisfy the strict power limit for safe use in human subjects, a pulsed powering scheme is implemented that enables a significant decrease in the average power emitted from the Tx.

Bipedal gait model for precise gait recognition and optimal triggering in foot drop stimulator: a proof of concept

Electrical stimulators are often prescribed to correct foot drop walking. However, commercial foot drop stimulators trigger inappropriately under certain non-gait scenarios. Past researches addressed this limitation by defining stimulation control based on automaton of a gait cycle executed by foot drop of affected limb/foot only. Since gait is a collaborative activity of both feet, this research highlights the role of normal foot for robust gait detection and stimulation triggering. A novel bipedal gait model is proposed where gait cycle is realized as an automaton based on concurrent gait sub-phases (states) from each foot. The input for state transition is fused information from feet-worn pressure and inertial sensors. Thereafter, a bipedal gait model-based stimulation control algorithm is developed. As a feasibility study, bipedal gait model and stimulation control are evaluated in real-time simulation manner on normal and simulated foot drop gait measurements from 16 able-bodied participants with three speed variations, under inappropriate triggering scenarios and with foot drop rehabilitation exercises. Also, the stimulation control employed in commercial foot drop stimulators and single foot gait-based foot drop stimulators are compared alongside. Gait detection accuracy (98.9%) and precise triggering under all investigations prove bipedal gait model reliability. This infers that gait detection leveraging bipedal periodicity is a promising strategy to rectify prevalent stimulation triggering deficiencies in commercial foot drop stimulators. Graphical abstract Bipedal information-based gait recognition and stimulation triggering.

An Implantable Wireless Neural Interface System for Simultaneous Recording and Stimulation of Peripheral Nerve with a Single Cuff Electrode

Recently, implantable devices have become widely used in neural prostheses because they eliminate endemic drawbacks of conventional percutaneous neural interface systems. However, there are still several issues to be considered: low-efficiency wireless power transmission; wireless data communication over restricted operating distance with high power consumption; and limited functionality, working either as a neural signal recorder or as a stimulator. To overcome these issues, we suggest a novel implantable wireless neural interface system for simultaneous neural signal recording and stimulation using a single cuff electrode. By using widely available commercial off-the-shelf (COTS) components, an easily reconfigurable implantable wireless neural interface system was implemented into one compact module. The implantable device includes a wireless power consortium (WPC)-compliant power transmission circuit, a medical implant communication service (MICS)-band-based radio link and a cuff-electrode path controller for simultaneous neural signal recording and stimulation. During in vivo experiments with rabbit models, the implantable device successfully recorded and stimulated the tibial and peroneal nerves while communicating with the external device. The proposed system can be modified for various implantable medical devices, especially such as closed-loop control based implantable neural prostheses requiring neural signal recording and stimulation at the same time.

A CMOS Current Steering Neurostimulation Array With Integrated DAC Calibration and Charge Balancing

An 8-channel current steerable, multi-phasic neural stimulator with on-chip current DAC calibration and residue nulling for precise charge balancing is presented. Each channel consists of two sub-binary radix DACs followed by wide-swing, high output impedance current buffers providing time-multiplexed source and sink outputs for anodic and cathodic stimulation. A single integrator is shared among channels and serves to calibrate DAC coefficients and to closely match the anodic and cathodic stimulation phases. Following calibration, the differential non-linearity is within ±0.3 LSB at 8-bit resolution, and the two stimulation phases are matched within 0.3%. Individual control in digital programming of stimulation coefficients across the array allows altering the spatial profile of current stimulation for selection of stimulation targets by current steering. Combined with the self-calibration and current matching functions, the current steering capabilities integrated on-chip support use in fully implanted neural interfaces with autonomous operation for and adaptive stimulation under variations in electrode and tissue conditions. As a proof-of-concept we applied current steering stimulation through a multi-channel cuff electrode on the sciatic nerve of a rat.

Bidirectional peripheral nerve interface and applications

Peripheral nerves, due to their small size and complex innervation to organs and complex physiology, pose particularly significant challenges towards interfacing electrodes and electronics to enable neuromodulation. Here, we present a review of the technology for building such interface, including recording and stimulating electrodes and low power electronics, as well as powering. Of particular advantage to building a miniature implanted device is a "bidirectional" system that both senses from the nerves or surrogate organs and stimulates the nerves to affect the organ function. This review and presentation will cover a range of electrodes, electronics, wireless power and data schemes and system integration, and will end with some examples and applications.

Implantable neurotechnologies: a review of integrated circuit neural amplifiers

Neural signal recording is critical in modern day neuroscience research and emerging neural prosthesis programs. Neural recording requires the use of precise, low-noise amplifier systems to acquire and condition the weak neural signals that are transduced through electrode interfaces. Neural amplifiers and amplifier-based systems are available commercially or can be designed in-house and fabricated using integrated circuit (IC) technologies, resulting in very large-scale integration or application-specific integrated circuit solutions. IC-based neural amplifiers are now used to acquire untethered/portable neural recordings, as they meet the requirements of a miniaturized form factor, light weight and low power consumption. Furthermore, such miniaturized and low-power IC neural amplifiers are now being used in emerging implantable neural prosthesis technologies. This review focuses on neural amplifier-based devices and is presented in two interrelated parts. First, neural signal recording is reviewed, and practical challenges are highlighted. Current amplifier designs with increased functionality and performance and without penalties in chip size and power are featured. Second, applications of IC-based neural amplifiers in basic science experiments (e.g., cortical studies using animal models), neural prostheses (e.g., brain/nerve machine interfaces) and treatment of neuronal diseases (e.g., DBS for treatment of epilepsy) are highlighted. The review concludes with future outlooks of this technology and important challenges with regard to neural signal amplification.

Implantable neurotechnologies: a review of micro- and nanoelectrodes for neural recording

Electrodes serve as the first critical interface to the biological organ system. In neuroprosthetic applications, for example, electrodes interface to the tissue for either signal recording or tissue stimulation. In this review, we consider electrodes for recording neural activity. Recording electrodes serve as wiretaps into the neural tissues, providing readouts of electrical activity. These signals give us valuable insights into the organization and functioning of the nervous system. The recording interfaces have also shown promise in aiding treatment of motor and sensory disabilities caused by neurological disorders. Recent advances in fabrication technology have generated wide interest in creating tiny, high-density electrode interfaces for neural tissues. An ideal electrode should be small enough and be able to achieve reliable and conformal integration with the structures of the nervous system. As a result, the existing electrode designs are being shrunk and packed to form small form factor interfaces to tissue. Here, an overview of the historic and state-of-the-art electrode technologies for recording neural activity is presented first with a focus on their development road map. The fact that the dimensions of recording electrode sites are being scaled down from micron to submicron scale to enable dense interfaces is appreciated. The current trends in recording electrode technologies are then reviewed. Current and future considerations in electrode design, including the use of inorganic nanostructures and biologically inspired or biocomapatible materials are discussed, along with an overview of the applications of flexible materials and transistor transduction schemes. Finally, we detail the major technical challenges facing chronic use of reliable recording electrode technology.

Implantable neurotechnologies: bidirectional neural interfaces--applications and VLSI circuit implementations

A bidirectional neural interface is a device that transfers information into and out of the nervous system. This class of devices has potential to improve treatment and therapy in several patient populations. Progress in very large-scale integration has advanced the design of complex integrated circuits. System-on-chip devices are capable of recording neural electrical activity and altering natural activity with electrical stimulation. Often, these devices include wireless powering and telemetry functions. This review presents the state of the art of bidirectional circuits as applied to neuroprosthetic, neurorepair, and neurotherapeutic systems.