Proceedings of the Conference on Progress in Electrically Active Implants - Tissue and Functional Regeneration (ELAINE 2020)

The conference on Progress in Electrically Active Implants - Tissue and Functional Regeneration (ELAINE 2020) focused on novel methods in the electric stimulation of bio-material compounds of living cells and implantable electric stimulation devices. ELAINE 2020 provided international scientists a virtual platform to discuss the latest achievements in the form of invited presentations, selected talks from abstract submissions, and virtual poster sessions. In addition, we particularly invited critical reviews and contributions with negative results or unsuccessful replications to foster the scientific discussion and explicitly encourage young scientists to contribute and submit their work

Doctor of Philosophy

dissertationThe development of devices to electrically interact with the brain is a challenging task that could potentially restore motion to paralyzed patients and sight to those with profound blindness. Neural engineers have designed many types of microelectrode arrays (MEAs) with this challenge in mind. These MEAs can be implanted into brain tissue to both record neural signals and electrically stimulate neurons with high selectivity and spatial resolution. Implanted MEAs have allowed patients to control of a variety of prosthetic devices in clinical trials, but the longevity of such motor prostheses is limited to a few years. Performance decreases over time as MEAs lose the ability to record neuronal signals, preventing their widespread clinical use. Microstimulation via intracortical MEAs has also not achieved broad clinical implementation. While microstimulation for the restoration of vision is promising, human clinical trials are needed. Chronic in vivo functionality assays in model systems will provide key insight to facilitate such trials. There are three goals that may help address insufficient MEA longevity, as well as provide insight on microstimulation functionality. First, thorough characterizations of how performance decreases over time, both with and without stimulation, will be needed. Next, factors that affect the chronic performance of microstimulating MEAs must be further investigated. Finally, intervention strategies can be designed to mitigate these factors and improve long term MEA performance. This dissertation takes steps towards meeting these goals by means of three studies. First, the chronic performance of intracortically implanted recording and stimulating MEAs is examined. It is found that while performance of implanted MEAs in feline cortex is dynamic, catastrophic device failure does not occur with microstimulation. Next, a variety of factors that affect microstimulation studies are investigated. It is found that many factors, including device iv damage, anesthesia depth, the application of microstimulation, and the use of impedance as a reporter play a role in observations of performance variability. Finally, a promising intervention strategy, a carbon nanotube coating, is chronically tested in vivo, indicating that carbon nanotubes do not cause catastrophic device failure and may impart benefits to future generations of MEAs

Design and development of an implantable biohybrid device for muscle stimulation following lower motor neuron injury

In the absence of innervation caused by complete lower motor neuron injuries, skeletal muscle undergoes an inexorable course of degeneration and atrophy. The most apparent and debilitating clinical outcome of denervation is the immediate loss of voluntary use of muscle. However, these injuries are associated with secondary complications of bones, skin and cardiovascular system that, if untreated, may be fatal. Electrical stimulation has been implemented as a clinical rehabilitation technique in patients with denervated degenerated muscles offering remarkable improvements in muscle function. Nevertheless, this approach has limitations and side effects triggered by the delivery of high intensity electrical pulses. Combining innovative approaches in the fields of cell therapy and implanted electronics offers the opportunity to develop a biohybrid device to stimulate muscles in patients with lower motor neuron injuries. Incorporation of stem cell-derived motor neurons into implantable electrodes, could allow muscles to be stimulated in a physiological manner and circumvent problems associated with direct stimulation of muscle. The hypothesis underpinning this project is that artificially-grown motor neurons can serve as an intermediate between stimulator and muscle, converting the electrical stimulus into a biological action potential and re-innervating muscle via neuromuscular interaction. Here, a suitable stem cell candidate with therapeutic potential was identified and a differentiation protocol developed to generate motor neuron-like cells. Thick-film technology and laser micromachining were implemented to manufacture electrode arrays with features and dimensions suitable for implantation. Manufactured electrodes were electrochemically characterised, and motor neuron-like cells incorporated to create biohybrid devices. In vitro results indicate manufactured electrodes support motor neuron-like cell growth and neurite extension. Moreover, electrochemical characterisation suggests electrodes are suitable for stimulation. Preliminary in vivo testing explored implantation in a rat muscle denervation model. Overall, this thesis demonstrates initial development of a novel approach for fabricating biohybrid devices that may improve stimulation of denervated muscles

Organic electrode coatings for next-generation neural interfaces

Traditional neuronal interfaces utilize metallic electrodes which in recent years have reached a plateau in terms of the ability to provide safe stimulation at high resolution or rather with high densities of microelectrodes with improved spatial selectivity. To achieve higher resolution it has become clear that reducing the size of electrodes is required to enable higher electrode counts from the implant device. The limitations of interfacing electrodes including low charge injection limits, mechanical mismatch and foreign body response can be addressed through the use of organic electrode coatings which typically provide a softer, more roughened surface to enable both improved charge transfer and lower mechanical mismatch with neural tissue. Coating electrodes with conductive polymers or carbon nanotubes offers a substantial increase in charge transfer area compared to conventional platinum electrodes. These organic conductors provide safe electrical stimulation of tissue while avoiding undesirable chemical reactions and cell damage. However, the mechanical properties of conductive polymers are not ideal, as they are quite brittle. Hydrogel polymers present a versatile coating option for electrodes as they can be chemically modified to provide a soft and conductive scaffold. However, the in vivo chronic inflammatory response of these conductive hydrogels remains unknown. A more recent approach proposes tissue engineering the electrode interface through the use of encapsulated neurons within hydrogel coatings. This approach may provide a method for activating tissue at the cellular scale, however, several technological challenges must be addressed to demonstrate feasibility of this innovative idea. The review focuses on the various organic coatings which have been investigated to improve neural interface electrodes

Doctor of Philosophy

dissertationThe Utah Electrode Array (UEA) is a brain-implanted microelectrode recording device that has shown promise to assist patients with motor-control disabilities. Unfortunately, the UEA suffers from a foreign body response (FBR) that results in device movement away from implantation target, encapsulation of devices in meningeal origin tissue, loss of cortical tissue, and persistent neuroinflammation in the brain. These issues affect device functionality, and thus biocompatibility, and hinder widespread implementation of this technology. This dissertation examines whether device anchoring or extracellular matrix (ECM)-based device coating strategies can influence the biocompatibility of chronically implanted UEAs in the rat cortex. Results show that unanchored UEAs have a reduced FBR in comparison to those anchored to the skull, but also suffer from device movement as a result of cortical tissue remodeling, likely attributable to implantation-associated injury. To address implantation-associated injury, ECM was explored as a surface adsorbed device coating and was shown to be both hemostatic and immunomodulatory with in vitro assays. An apparatus was developed to coat Avitene™, an FDA-approved neurosurgical hemostatic ECM, onto the complex surface geometry of the UEA. Compared to uncoated control devices in a chronic rat model, Avitene™ coated devices experienced an enhanced FBR characterized by larger lesion cavities, enhanced meningeal encapsulation, and increased neuroinflammation, attributed to a higher degree of proinflammatory macrophages found surrounding the device coating. These result imply that future ECM-based coatings should include immunomodulatory components that address device-adherent macrophage activation state. Critical improvements in device anchoring and modulation of the FBR are still necessary to improve the biocompatibility of the UEA. Reducing the prevalence of FBR-related device failure is a necessary step that will require further attention before patients can benefit from this technology

In Vivo Imaging to Characterize Dynamic Tissue Responses after Neural Electrode Implantation

Implantable neural electrodes are promising technologies to restore motor, sensory, and cognitive function in many neural pathologies through brain-computer interfacing (BCI). Many BCI applications require electrode implantation within neural tissue to resolve and/or modulate the physiological activity of individual neurons via electrical recording and stimulation. This invasive implantation leads to acute and long-term deterioration of both the electrode device as well as the neurons surrounding the device. Ultimately, damage to the electrode and neural tissue results in electrode recording failure within the first years after implantation. Many strategies to improve BCI longevity focus on mitigating tissue damage through improving neuronal survival or reducing inflammatory activity around implants. Despite incremental improvements, electrode failure persists as an obstacle to wide-spread clinical deployment of BCIs. This can be partly attributed to an incomplete understanding of the biological correlates of recording performance. These correlates have largely been identified through post-mortem histological staining, which cannot capture dynamic changes in cellular physiology and morphology. In the following dissertation, we use longitudinal two-photon in vivo imaging to quantify how neurons, microglia, and meningeal immune cells are affected by an intracortical electrode during and after implantation in mouse cortex. We go beyond conventional histological techniques to show the time-course of neuronal injury and microglial recruitment after implantation. Neuronal injury occurs instantaneously, with prolonged, high calcium levels evident in neurons within 100 µm of implants. Microglial activation occurs within minutes of implantation and subsequent microglial encapsulation of electrodes can be modulated by bioactive surface coatings. Within the first day post-implant, there is high trafficking of peripheral immune cells through venules at the surface of the brain as well as along the electrode’s shank at the surface of the brain. Over the next month, calcium activity in neurons increases while the collagenous meningeal tissues at the surface of the brain thicken. We further show that meningeal thickening can have profound implications for devices implanted into non-human primates as well. In sum, these results define new potential therapeutic targets and windows that could improve the longevity of implantable neural electrodes

COMMUNICATION: Electrochemical polymerization of conducting polymers in living neural tissue

A number of biomedical devices require extended electrical communication with surrounding tissue. Significant improvements in device performance would be achieved if it were possible to maintain communication with target cells despite the reactive, insulating scar tissue that forms at the device–tissue interface. Here, we report that the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) can be polymerized directly within living neural tissue resulting in an electrically conductive network that is integrated within the tissue. Nano and microscale PEDOT filaments extend out from electrode sites, presumably forming within extracellular spaces. The cloud of PEDOT filaments penetrates out into the tissue far enough that it should be possible to bypass fibrous scar tissue and contact surrounding healthy neurons. These electrically functional, diffuse conducting polymer networks grown directly within tissue signify a new paradigm for creating soft, low impedance implantable electrodes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58177/2/jne7_2_l02.pd

Doctor of Philosophy

dissertationNeural recording devices are a therapeutic and diagnostic option for central nervous system (CNS) diseases and a vital component of neuroscience research. However, poor functional longevity is a major hurdle facing this broad class of devices. Decreases in functionality are associated, in part, with the foreign body response (FBR) surrounding chronically implanted recording devices; which includes chronic inflammation, astrogliosis, blood-brain barrier (BBB) leakiness, and neuronal cell death. Two potential areas for intervention were explored including the initial hemorrhage that results from device insertion and the neuroinflammatory sequela. Researchers have shown that cellular interactions with extracellular matrix (ECM) are able to affect both of these aspects of the FBR. The central hypothesis driving this work is that ECM coatings which target the initial hemorrhage, should decrease the FBR. This was investigated by coating silicon microelectrode arrays (MEAs) with ECM and implanting them into motor cortex of rats. Two ECM coatings were investigated, including the xenogeneic clinically-used Avitene Microfibrillar Collagen Hemostat and allogeneic astrocyte-derived ECM. Results show that the allogeneic astrocyte-derived ECM decreased astrogliosis within the recording zone at the 8-week time point. This decrease in astrogliosis may improve device functionality, as indicated by previous studies that correlated recording metrics to histology. Interestingly, the xenogeneic Avitene coating increased IgG within the recording zone at the 8-week time point. Collectively, these results show that ECM coatings with different genetic backgrounds and compositions are able to differentially affect specific aspects of the FBR. To broaden the knowledge on the FBR to neural recording devices, the FBR of headstage components used to anchor CNS devices to the skull was analyzed. Results showed that the FBR to fixation screws and fixation anchoring adhesive illicit a chronic FBR that has all of the hallmarks described for MEAs implanted in brain tissue. Moreover, results show evidence of persistent neuroinflammation below a variety of fixation screws including chronic macrophage activation, demyelination, and neural tissue loss. Understanding the FBR of fixation techniques, which is common to a wide variety of CNS devices, may improve the biocompatibility of existing devices and provide a reference for future biologically-informed device designs

Invasive intraneural interfaces: foreign body reaction issues

Intraneural interfaces are stimulation/registration devices designed to couple the peripheral nervous system (PNS) with the environment. Over the last years, their use has increased in a wide range of applications, such as the control of a new generation of neural-interfaced prostheses. At present, the success of this technology is limited by an electrical impedance increase, due to an inflammatory response called foreign body reaction (FBR), which leads to the formation of a fibrotic tissue around the interface, eventually causing an inefficient transduction of the electrical signal. Based on recent developments in biomaterials and inflammatory/fibrotic pathologies, we explore and select the biological solutions that might be adopted in the neural interfaces FBR context: modifications of the interface surface, such as organic and synthetic coatings; the use of specific drugs or molecular biology tools to target the microenvironment around the interface; the development of bio-engineered-scaffold to reduce immune response and promote interface-tissue integration. By linking what we believe are the major crucial steps of the FBR process with related solutions, we point out the main issues that future research has to focus on: biocompatibility without losing signal conduction properties, good reproducible in vitro/in vivo models, drugs exhaustion and undesired side effects. The underlined pros and cons of proposed solutions show clearly the importance of a better understanding of all the molecular and cellular pathways involved and the need of a multi-target action based on a bio-engineered combination approach