Engineering strategies towards overcoming bleeding and glial scar formation around neural probes

Neural probes are sophisticated electrophysiological tools used for intra-cortical recording and stimulation. These microelectrode arrays, designed to penetrate and interface the brain from within, contribute at the forefront of basic and clinical neuroscience. However, one of the challenges and currently most significant limitations is their ‘seamless’ long-term integration into the surrounding brain tissue. Following implantation, which is typically accompanied by bleeding, the tissue responds with a scarring process, resulting in a gliotic region closest to the probe. This glial scarring is often associated with neuroinflammation, neurodegeneration, and a leaky blood–brain interface (BBI). The engineering progress on minimizing this reaction in the form of improved materials, microfabrication, and surgical techniques is summarized in this review. As research over the past decade has progressed towards a more detailed understanding of the nature of this biological response, it is time to pose the question: Are penetrating probes completely free from glial scarring at all possible?

Transparent and Conformal Microcoil Arrays for Spatially Selective Neuronal Activation

Magnetic stimulation represents a compelling modality for achieving neuronal activation with high spatial resolution and low toxicity. Stimulation coils can be designed to achieve localized, spatially asymmetric fields that target neurons of a particular orientation. Furthermore, these devices may be encapsulated within biopolymers thereby avoiding direct metal/tissue interfaces that could induce chronic inflammation and glial scarring. Herein, we report a multiplexed microcoil array for localized activation of cortical neurons and retinal ganglion cells. We designed a computational model that related the activation function to the geometry and arrangement of coils, and selected a geometry with a region of activation <50 microns wide. We then fabricated SU8/Cu/SU8 tri-layer devices which were flexible, transparent and conformal and featured four individually-addressable microcoil stimulation elements. Interfaced with ex vivo cortex or retina slices from GCaMP6-transfected mice, we observed that individual neurons located within 40 microns of the element tip could be activated repeatedly and in a dose (power) dependent fashion. Taken together, these results highlight the potential of magnetic stimulation devices for brain-machine interfaces and could open new routes toward bioelectronic therapies including prosthetic vision devices.

Aging impairs the essential contributions of non-glial progenitors to neurorepair in the dorsal telencephalon of the Killifish N. furzeri

SummaryThe aging central nervous system (CNS) of mammals displays progressive limited regenerative abilities. Recovery after loss of neurons is extremely restricted in the aged brain. Many research models fall short in recapitulating mammalian aging hallmarks or have an impractically long lifespan. We established a traumatic brain injury model in the African turquoise killifish (Nothobranchius furzeri), a regeneration-competent vertebrate model that evolved to naturally age extremely fast. Stab-wound injury of the aged killifish dorsal telencephalon unveils an impaired and incomplete regeneration response when compared to young individuals. Remarkably, killifish brain regeneration is mainly supported by atypical non-glial progenitors, yet their proliferation capacity appears declined with age. We identified a high inflammatory response and glial scarring to also underlie the hampered generation of new neurons in aged fish. These primary results will pave the way for further research to unravel the factor age in relation to neurorepair, and to improve therapeutic strategies to restore the injured and/or diseased aged mammalian CNS.HighlightsAging impairs neurorepair in the killifish pallium at multiple stages of the regeneration processAtypical non-glial progenitors support the production of new neurons in the naive and injured dorsal palliumThe impaired regeneration capacity of aged killifish is characterized by a reduced reactive proliferation of these progenitors followed by a decreased generation of newborn neurons that in addition, fail to reach the injury siteExcessive inflammation and glial scarring surface as potential brakes on brain repair in the aged killifish pallium

Glial Associated Impairment of the Glymphatic System in Experimental Neonatal Hydrocephalus

Background Changes in aquaporin-4 (AQP4) and glial fibrillary acid protein (GFAP) expression by astrocytes have been observed in several pathologies. It is hypothesized that prolonged exposure to pathologically elevated intracranial pressure (ICP) may be linked to impaired glymphatic pathways. In this study we explore histological consequences of prolonged pressure-induced injury in a feline model of neonatal hydrocephalus through changes in AQP4 and GFAP expression. We discuss the implications this may have in gaining a better understanding of the underlying mechanisms of hydrocephalus (HCP). Methods Using a neonatal feline model, obstructive HCP was induced through kaolin injection into the cisterna magna. Time between injection and intervention via ventricular reservoir placement was used to divide groups into early and late treatment groups. Early and late animals received reservoirs at 1- and 2-weeks post kaolin injection, respectively. Controls underwent sham operations (saline injection instead of kaolin). Animals were sacrificed at 4 months allowing for a chronic treated hydrocephalic model at time of brain harvest. Immunofluorescent staining for GFAP, AQP4 and DAPI was performed on histological brain sections from each group, and densitometry was used to quantify the relative signal of protein expression. Results Hydrocephalus was seen in all animals receiving kaolin injection as demonstrated by magnetic resonance imaging, clinical examination and neurological sequelae. Hydrocephalic animals demonstrated lower levels of perivascular AQP4 expression, increased diffuse AQP4 expression and increased glial scarring of perivascular, ependymal and subependymal spaces. Cerebral microvasculature of early treatment groups demonstrated increased astrocytic processes in the perivascular spaces, while late treatment groups demonstrated increased glial scar formation. Overall, the glymphatic system was severely disrupted in chronic treated hydrocephalus compared to controls. Conclusions Reactive astrogliosis and AQP4 mislocation are evident in early and late reservoir-treated HCP. Glial scarring in the perivascular, ependymal and subependymal spaces concurrent with AQP4 internalization from the perivascular region are prominent in HCP conditions present within the neonatal period. Delay in treatment by 1 week demonstrates quantifiable increases in perivascular and ependymal glial scarring at 4 months of age. Further investigation is needed to correlate glymphatic disruption with impaired CSF absorption and its role in promoting progressive hydrocephalus.

A new model of spinal cord injury by cryoapplication: Morphodynamics of histological changes of the spinal cord lesion

Up to 500,000 people worldwide suffer from spinal cord injuries (SCI) annually, according to the WHO. Animal models are essential for searching novel methodological guidelines and therapeutic agents for SCI treatment. We developed an original model of posttraumatic spinal cord glial scar in rats using cryoapplication. The method is based on cryodestruction of spinal cord tissue with liquid nitrogen. Thirty six male SD linear rats of SPF category were included in this experimental study. A T13 unilateral hemilaminectomy was performed with an operating microscope, as it was extremely important not to penetrate the dura mater, and liquid nitrogen was applied into the bone defect for one minute. The animals were euthanized at various intervals ranging from 1 to 60 days after inducing cryogenic trauma, their Th12-L1 vertebrae were removed “en bloc” and the segment of the spinal cord exposed to the cryoapplicator was carefully separated for histological examination. The study results demonstrated that cryoapplication of liquid nitrogen, provoking a local temperature of approximately minus 20°C, produced a highly standardized transmural defect which extended throughout the dorsoventral arrangement of the spinal cord and had an “hour-glass” shape. During the entire study period (1-60 post-injury days), the glial scarring process and the spinal cord defect were located within the surgically approached vertebral space (Th13). Unlike other available experimental models of SCI (compression, contusion, chemical, etc.), the present option is characterized by a minimal invasiveness (the hemilaminectomy is less than 1 mm wide), high precision and consistency. Also, there was a low interanimal variability in histological lesions and dimensions of the produced defect. The original design of cryoapplicator used in the study played a major role in achieving these results. The original technique of high-precision cryoapplication for inducing consistent morphodynamic glial scarring could facilitate a better understanding of the self-recovery processes of injured spinal cord and would be helpful for proposing new platforms for the development of therapeutic strategies.

Replicating infant astrocyte behavior in the adult after brain injury improves outcomes

Infants and adults respond differently to brain injuries. Specifically, improved neuronal sparing along with reduced astrogliosis and glial scarring often observed earlier in life, likely contributes to improved long-term outcomes. Understanding the underlying mechanisms could enable the recapitulation of neuroprotective effects, observed in infants, to benefit adult patients after brain injuries. We reveal that in primates, Eph/ ephrin signaling contributes to age-dependent reactive astrocyte behavior. Ephrin-A5 expression on astrocytes was more protracted in adults, whereas ephrin-A1 was associated only with infant astrocytes. Furthermore, ephrin-A5 exacerbated major hallmarks of astrocyte reactivity via EphA2 and EphA4 receptors, which was subsequently alleviated by ephrin-A1. Rather than suppressing reactivity, ephrin-A1 signaling shifted astrocytes towards GAP43+ neuroprotection, accounting for improved neuronal sparing in infants. Reintroducing ephrin-A1 after middle-aged ischemic stroke significantly attenuated glial scarring, improved neuronal sparing and preserved circuitry. Therefore, beneficial infant mechanisms can be recapitulated in adults to improve outcomes after CNS injuries.

Recent Progress on Non-Conventional Microfabricated Probes for the Chronic Recording of Cortical Neural Activity

Microfabrication technology for cortical interfaces has advanced rapidly over the past few decades for electrophysiological studies and neuroprosthetic devices offering the precise recording and stimulation of neural activity in the cortex. While various cortical microelectrode arrays have been extensively and successfully demonstrated in animal and clinical studies, there remains room for further improvement of the probe structure, materials, and fabrication technology, particularly for high-fidelity recording in chronic implantation. A variety of non-conventional probes featuring unique characteristics in their designs, materials and fabrication methods have been proposed to address the limitations of the conventional standard shank-type (“Utah-” or “Michigan-” type) devices. Such non-conventional probes include multi-sided arrays to avoid shielding and increase recording volumes, mesh- or thread-like arrays for minimized glial scarring and immune response, tube-type or cylindrical probes for three-dimensional (3D) recording and multi-modality, folded arrays for high conformability and 3D recording, self-softening or self-deployable probes for minimized tissue damage and extensions of the recording sites beyond gliosis, nanostructured probes to reduce the immune response, and cone-shaped electrodes for promoting tissue ingrowth and long-term recording stability. Herein, the recent progress with reference to the many different types of non-conventional arrays is reviewed while highlighting the challenges to be addressed and the microfabrication techniques necessary to implement such features.

Materials and Devices for Micro-invasive Neural Interfacing

ABSTRACTThere is widespread research and popular interest in developing micro-invasive neural interfacing modalities. An increasing variety of probes have been developed and reported in the literature. Newer, smaller probes show significant benefit over larger ones in reducing tissue damage and scarring. A different set of obstacles arise, however, as probes become smaller. These include reliable insertion and robustness. This review articulates the impact of various design parameters (material, geometry, size) on probe insertion mechanisms, chronic viability, and glial scarring. We highlight various emerging technologies utilizing novel form factors including micron-scale interfaces and bio-inspired designs for probe insertion and steering.

Chronic Intracortical Recording and Electrochemical Stability of Thiol-ene/Acrylate Shape Memory Polymer Electrode Arrays

Current intracortical probe technology is limited in clinical implementation due to the short functional lifetime of implanted devices. Devices often fail several months to years post-implantation, likely due to the chronic immune response characterized by glial scarring and neuronal dieback. It has been demonstrated that this neuroinflammatory response is influenced by the mechanical mismatch between stiff devices and the soft brain tissue, spurring interest in the use of softer polymer materials for probe encapsulation. Here, we demonstrate stable recordings and electrochemical properties obtained from fully encapsulated shape memory polymer (SMP) intracortical electrodes implanted in the rat motor cortex for 13 weeks. SMPs are a class of material that exhibit modulus changes when exposed to specific conditions. The formulation used in these devices softens by an order of magnitude after implantation compared to its dry, room-temperature modulus of ~2 GPa.

An actuated neural probe architecture for reducing gliosis-induced recording degradation

Glial encapsulation of chronically implanted neural probes inhibits recording and stimulation, and this signal loss is a significant factor limiting the clinical viability of most neural implant topologies for decades-long implantation. We demonstrate a mechanical proof of concept for silicon shank-style neural probes intended to minimize gliosis near the recording sites. Compliant whiskers on the edges of the probe fold inward to minimize tissue damage during insertion. Once implanted to the target depth and retracted slightly, these whiskers splay outward. The splayed tips, on which recording sites could be patterned, extend beyond the typical 50-100 micron radius of a glial scar. The whiskers are micron-scale to minimize or avoid glial scarring. Electrically inactive devices with whiskers of varying widths and curvature were designed and monolithically fabricated from a five-micron silicon-on-insulator (SOI) wafer, and their mechanical functionality was demonstrated in a 0.6% agar brain phantom. Deflection was plotted versus deflection speed, and those that were most compliant actuated successfully. This probe requires no preparation for use beyond what is typical for a shank-style silicon probe.