A floating microwire technique for multichannel chronic neural recording and stimulation in the awake freely moving rat

Manufacturing Processes of Implantable Microelectrode Array for In Vivo Neural Electrophysiological Recordings and Stimulation: A State-Of-the-Art Review

Electrophysiological recording and stimulation of neuron activities are important for us to understand the function and dysfunction of the nervous system. To record/stimulate neuron activities as voltage fluctuation extracellularly, microelectrode array (MEA) implants are a promising tool to provide high temporal and spatial resolution for neuroscience studies and medical treatments. The design configuration and recording capabilities of the MEAs have evolved dramatically since their invention and manufacturing process development has been a key driving force for such advancement. Over the past decade, since the White House Brain Research Through Advancing Innovative Neurotechnologies (BRAIN) Initiative launched in 2013, advanced manufacturing processes have enabled advanced MEAs with increased channel count and density, access to more brain areas, more reliable chronic performance, as well as minimal invasiveness and tissue reaction. In this state-of-the-art review paper, three major types of electrophysiological recording MEAs widely used nowadays, namely, microwire-based, silicon-based, and flexible MEAs are introduced and discussed. Conventional design and manufacturing processes and materials used for each type are elaborated, followed by a review of further development and recent advances in manufacturing technologies and the enabling new designs and capabilities. The review concludes with a discussion on potential future directions of manufacturing process development to enable the long-term goal of large-scale high-density brain-wide chronic recordings in freely moving animals.

Implantable Neural Interfaces and Wearable Tactile Systems for Bidirectional Neuroprosthetics Systems

Neuroprosthetics and neuromodulation represent a promising field for several related applications in the central and peripheral nervous system, such as the treatment of neurological disorders, the control of external robotic devices, and the restoration of lost tactile functions. These actions are allowed by the neural interface, a miniaturized implantable device that most commonly exploits electrical energy to fulfill these operations. A neural interface must be biocompatible, stable over time, low invasive, and highly selective; the challenge is to develop a safe, compact, and reliable tool for clinical applications. In case of anatomical impairments, neuroprosthetics is bound to the need of exploring the surrounding environment by fast-responsive and highly sensitive artificial tactile sensors that mimic the natural sense of touch. Tactile sensors and neural interfaces are closely interconnected since the readouts from the first are required to convey information to the neural implantable apparatus. The role of these devices is pivotal hence technical improvements are essential to ensure a secure system to be eventually adopted in daily life. This review highlights the fundamental criteria for the design and microfabrication of neural interfaces and artificial tactile sensors, their use in clinical applications, and future enhancements for the release of a second generation of devices.

Intracortical recording interfaces: current challenges to chronic recording function

Brain Computer Interfaces (BCIs) offer significant hope to tetraplegic and paraplegic individuals. This technology relies on extracting and translating motor intent to facilitate control of a computer cursor or to enable fine control of an external assistive device such as a prosthetic limb. Intracortical recording interfaces (IRIs) are critical components of BCIs and consist of arrays of penetrating electrodes that are implanted into the motor cortex of the brain. These multielectrode arrays (MEAs) are responsible for recording and conducting neural signals from local ensembles of neurons in the motor cortex with the high speed and spatiotemporal resolution that is required for exercising control of external assistive prostheses. Recent design and technological innovations in the field have led to significant improvements in BCI function. However, long-term (chronic) BCI function is severely compromised by short-term (acute) IRI recording failure. In this review, we will discuss the design and function of current IRIs. We will also review a host of recent advances that contribute significantly to our overall understanding of the cellular and molecular events that lead to acute recording failure of these invasive implants. We will also present recent improvements to IRI design and provide insights into the futuristic design of more chronically functional IRIs.

Preparation of an awake mouse for recording neural responses and injecting tracers

It is well known that anesthesia alters neural response properties in various regions of the brain. In the auditory system, fundamental response properties of brainstem neurons including threshold, frequency specificity, and inhibitory sidebands are altered in significant ways under anesthesia. These observations prompted physiologists to seek ways to record from single neurons without the contaminating effects of anesthesia. One result was a decerebrate preparation, where the brainstem was completely transected at the level of the midbrain. The drawbacks of this preparation are a formidable surgery, the elimination of descending projections from the forebrain, and an inability to use sensory stimulation to examine structures above the midbrain. A different strategy has been to implant electrode arrays chronically to record from single neurons and multiunit clusters while the animal is awake and/or behaving. These techniques however are not compatible with injecting tracer dyes after first electrophysiologically characterizing a brain structure. To avoid altering neural response properties with anesthetics while recording electrophysiological response properties from single neurons, we have adapted a head restraint technique long used in bats to mouse. Using this method, we are able to conduct electrophysiological recordings over several days in the unanesthetized mouse. At the end of the recording sessions, we can then inject a dye to reconstruct electrode positions and recording sites or inject a tracer so that pathways to and from the recording loci can be determined. This method allows for well isolated single neuron recordings over multiple days without the use anesthetics.

Implant size and fixation mode strongly influence tissue reactions in the CNS

The function of chronic brain machine interfaces depends on stable electrical contact between neurons and electrodes. A key step in the development of interfaces is therefore to identify implant configurations that minimize adverse long-term tissue reactions. To this end, we here characterized the separate and combined effects of implant size and fixation mode at 6 and 12 weeks post implantation in rat (n = 24) cerebral cortex. Neurons and activated microglia and astrocytes were visualized using NeuN, ED1 and GFAP immunofluorescence microscopy, respectively. The contributions of individual experimental variables to the tissue response were quantified. Implants tethered to the skull caused larger tissue reactions than un-tethered implants. Small diameter (50 µm) implants elicited smaller tissue reactions and resulted in the survival of larger numbers of neurons than did large diameter (200 µm) implants. In addition, tethering resulted in an oval-shaped cavity, with a cross-section area larger than that of the implant itself, and in marked changes in morphology and organization of neurons in the region closest to the tissue interface. Most importantly, for implants that were both large diameter and tethered, glia activation was still ongoing 12 weeks after implantation, as indicated by an increase in GFAP staining between week 6 and 12, while this pattern was not observed for un-tethered, small diameter implants. Our findings therefore clearly indicate that the combined small diameter, un-tethered implants cause the smallest tissue reactions.

A novel system for recording from single neurons in unrestrained animals

To observe neural activity in animals engaged in natural behavior, it is often desirable to minimize or eliminate restraint of the animal. We have developed a simple system for recording from single units in unrestrained cats. An implant with multiple guide tubes and a tiny microdrive is placed inside the recording chamber. An indwelling Pt-Ir microelectrode is advanced incrementally during recording sessions that occur over a period of weeks or months. Electrodes can be easily replaced. We obtain excellent recording stability, and also have been able to sample extensively from a region of cortex or brain stem in a single animal. The essential electronics have been miniaturized and sewn into a light-weight walking jacket, so that we can collect data from a cat who is not connected to any fixed equipment.

An economical multi-channel cortical electrode array for extended periods of recording during behavior

We report the development of a low-cost chronic multi-channel microwire electrode array for recording multi-unit cortical responses in behaving rodents. The design was motivated by three issues. First, standard connector systems tended to disconnect from the head-stage during extended periods of behavior. Disconnections resulted in a loss of data and an interruption of the animals' behavior. Second, the use of low insertion force connectors with locking mechanisms was cost prohibitive. Finally, connecting the head-stage to a skull-mounted connector on an unrestrained animal was highly stressful for both the researcher and animal. The design developed uses a high insertion force DIP socket separated from the skullcap that prevents inadvertent disconnects, is inexpensive, and simplifies connecting unrestrained rodents. Electrodes were implanted in layer IV of primary auditory cortex in 11 Sprague-Dawley rats. Performance of the electrodes was monitored for 6 weeks. None of the behaving animals became disconnected from the recording system during recording sessions lasting 6 h. The mean signal-to-noise ratio on all channels (154) following surgery was 3.9+/-0.2. Of the 154 channels implanted, 130 exhibited driven activity following surgery. Forty percent of the arrays continued to exhibit driven neural activity at 6 weeks.

Electrophysiological and morphological properties of identified crossed tecto-reticular neurons in the rat superior colliculus

Previously we classified randomly sampled neurons in the intermediate layer (SI) of the rat superior colliculus (SC) into six subclasses according to their firing responses to depolarizing current pulses and five subclasses based on their morphological properties in slice preparations. In the present study, we investigated properties of a major output cell group of the rat SC (PND 17-24), crossed tecto-reticular neurons (cTRNs), which project to the contralateral medial pontine reticular formation. The cTRNs were identified by retrograde labeling with a fluorescent tracer (n=112). We compared their properties with those of presumed interneurons (n=127). We found that a majority of cTRNs were regular spiking neurons with moderate firing frequency (73%) and were multipolar-shaped (66%). The cTRNs had larger membrane capacitance, larger soma size and lower input impedance than presumed interneurons. Electrical stimulation of the superficial gray layer induced oligosynaptic EPSPs in the cTRNs. When bicuculline was added to the extracellular solution, the EPSPs were markedly enhanced and bursting spike responses were induced. The bursting responses were suppressed by applying D-2-amino-5-phosphonovalerate. These results suggest that the cTRNs exhibit NMDA receptor-dependent bursting responses to visual inputs. These observations give insights into the neuronal mechanism of generating burst activity in cTRNs, which triggers orienting behaviors.

A comparison of chronic multi-channel cortical implantation techniques: manual versus mechanical insertion

High-density multi-channel intra-cortical electrode arrays allow researchers to record simultaneously from populations of neurons for the purpose of understanding neural coding and plasticity. These devices have tens to hundreds of electrodes spaced within a few square millimeters. During insertion, the high-density probes can compress the cortex several millimeters prior to breaking through the pia. Compression of cortical tissue has been demonstrated to result in traumatic brain injury (TBI) which may be a major contributor to low electrode yield and decreased recording longevity. Two insertion techniques for chronically implanting multi-wire electrode arrays in layer IV of primary auditory cortex were compared. A mechanical insertion device, capable of rapidly inserting the electrode array without visible compression of the brain, was constructed. The neural responses to broadband clicks and pure tones recorded from the arrays inserted with the mechanical device were compared to the results from a manual insertion method using a micromanipulator. Both techniques result in a similar number of active channels directly following surgery with a mean signal-to-noise ratio of approximately 4.5. Over 60% of the animals implanted with the mechanical insertion device had driven activity at week 6 whereas none of the animals with manually inserted arrays exhibited functional responses after 3 weeks. This report provides initial evidence that mechanical insertion devices, which prevent cortical compression, increase electrode recording longevity.

Fiber-optic monitoring coupled with confocal microscopy for imaging gene expression in vitro and in vivo

Detection of fluorescent signals in living cells is a common and powerful technique used to monitor gene expression for multiple biomedical applications. A disadvantage of this approach in vivo, is the limited accessibility for long-term monitoring of the fluorescent signals within organs in living animals. Because of the multiple applications of gene expression monitoring through fluorescent signals, innovative methods for readout are required. We developed a strategy combining gene transfer, fiber-optic or endoscope monitoring, and confocal microscopy for the brain interstitial or cavitary endoscopic visualization of the efficacy of gene delivery and expression in vivo. The approach is also effective in vitro and can be applied to multiple organs in vivo. We show an example of the detection of green fluorescent protein (GFP)-emitted fluorescence following the administration of recombinant GFP-expressing adenovirus or implantation of rat C6 glioblastoma cells infected with the recombinant GFP adenovirus into the rat hippocampus of chronically cannulated rats. The results show that fiber-optic monitoring coupled with confocal microscopy in gene transfer studies is a practical approach that results in a direct, efficient, rapid, and sensitive visualization of fluorescent signals in the brain. This allows for the continuous real-time in vitro or in vivo brain monitoring of gene expression, accurate anatomical localization, multiple experimental manipulations in the same subject or preparation, while no sacrifice of the animal is required to monitor the efficacy of gene transfer and/or expression.

Parallel analyses of nociceptive neurones in rat superior colliculus by using c-fos immunohistochemistry and electrophysiology under different conditions of anaesthesia

Multiple sensory inputs to the superior colliculus (SC) play an important role in guiding head and eye movements toward or away from biologically significant stimuli. Much is now known about the visual, auditory, and somatosensory response properties of SC neurones that mediate these behavioural reactions. Rather less is known about the responses of SC neurones to noxious stimuli, and thus far, most of this information has been obtained in anaesthetised animals. Therefore, the purpose of the present study was to use the c-fos immunohistochemical technique and standard extracellular electrophysiology as parallel measures of nociceptive activity in the SC under different conditions of anaesthesia. In unanaesthetised animals, experimental and control treatments induced a qualitatively similar pattern of Fos-like immunoreactivity (FLI) in the SC, which was quantitatively related to the severity or biologic salience of the treatment; thus, baseline control < control injections of saline < a nonpainful stressor (immobilisation) < noxious injections of formalin. Compared with baseline levels, urethane and avertin anaesthesia induced FLI expression in the SC intermediate layers, although the FLI response to both noxious stimulation and control conditions was differentially suppressed in different layers of the SC by anaesthesia. Parallel electrophysiologic recordings found that anaesthesia was associated with high levels of spontaneous activity in the SC intermediate layers, often in neurones which were also nociceptive. High rates of background spike activity were also induced in the SC intermediate layers by noxious stimulation in chronically recorded awake animals. Although these results point to some differences between the nociceptive responses of SC neurones in anaesthetised and unanaesthetised animals, both data sets support the view that there are different populations of nociceptive neurones in the rodent SC that may be related to different adaptive functions of pain.