A Multimodal, SU-8 - Platinum - Polyimide Microelectrode Array for Chronic In Vivo Neurophysiology

Flexible multifunctional titania nanotube array platform for biological interfacing

Abstract

The current work presents a novel flexible multifunctional platform for biological interface applications. The use of titania nanotube arrays (TNAs) as a multifunctional material is explored for soft-tissue interface applications. In vitro biocompatibility of TNAs to brain-derived cells was first examined by culturing microglia cells-the resident immune cells of the central nervous system on the surface of TNAs. The release profile of an anti-inflammatory drug, dexamethasone from TNAs-on-polyimide substrates, was then evaluated under different bending modes. Flexible TNAs-on-polyimide sustained a linear release of anti-inflammatory dexamethasone up to ~11 days under different bending conditions. Finally, microfabrication processes for patterning and transferring TNA microsegments were developed to facilitate structural stability during device flexing and to expand the set of compatible polymer substrates. The techniques developed in this study can be applied to integrate TNAs or other similar nanoporous inorganic films onto various polymer substrates.

Impact statement

Titania nanotube arrays (TNAs) are highly tunable and biocompatible structures that lend themselves to multifunctional implementation in implanted devices. A particularly important aspect of titania nanotubes is their ability to serve as nano-reservoirs for drugs or other therapeutic agents that slowly release after implantation. To date, TNAs have been used to promote integration with rigid, dense tissues for dental and orthopedic applications. This work aims to expand the implant applications that can benefit from TNAs by integrating them onto soft polymer substrates, thereby promoting compatibility with soft tissues. The successful direct growth and integration of TNAs on polymer substrates mark a critical step toward developing mechanically compliant implantable systems with drug delivery from nanostructured inorganic functional materials. Diffusion-driven release kinetics and the high drug-loading efficiency of TNAs offer tremendous potential for sustained drug delivery for scientific investigations, to treat injury and disease, and to promote device integration with biological tissues. This work opens new opportunities for developing novel and more effective implanted devices that can significantly improve patient outcomes and quality of life.

Graphical abstract

Supplementary information

The online version contains supplementary material available at 10.1557/s43577-023-00628-y.

Tunable Conductive Hydrogel Scaffolds for Neural Cell Differentiation

Multielectrode arrays would benefit from intimate engagement with neural cells, but typical arrays do not present a physical environment that mimics that of neural tissues. It is hypothesized that a porous, conductive hydrogel scaffold with appropriate mechanical and conductive properties could support neural cells in 3D, while tunable electrical and mechanical properties could modulate the growth and differentiation of the cellular networks. By incorporating carbon nanomaterials into an alginate hydrogel matrix, and then freeze-drying the formulations, scaffolds which mimic neural tissue properties are formed. Neural progenitor cells (NPCs) incorporated in the scaffolds form neurite networks which span the material in 3D and differentiate into astrocytes and myelinating oligodendrocytes. Viscoelastic and more conductive scaffolds produce more dense neurite networks, with an increased percentage of astrocytes and higher myelination. Application of exogenous electrical stimulation to the scaffolds increases the percentage of astrocytes and the supporting cells localize differently with the surrounding neurons. The tunable biomaterial scaffolds can support neural cocultures for over 12 weeks, and enable a physiologically mimicking in vitro platform to study the formation of neuronal networks. As these materials have sufficient electrical properties to be used as electrodes in implantable arrays, they may allow for the creation of biohybrid neural interfaces and living electrodes.

Fully desktop fabricated flexible graphene electrocorticography (ECoG) arrays

Objective:Flexible Electrocorticography (ECoG) electrode arrays that conform to the cortical surface and record surface field potentials from multiple brain regions provide unique insights into how computations occurring in distributed brain regions mediate behavior. Specialized microfabrication methods are required to produce flexible ECoG devices with high-density electrode arrays. However, these fabrication methods are challenging for scientists without access to cleanroom fabrication equipment.Results:Here we present a fully desktop fabricated flexible graphene ECoG array. First, we synthesized a stable, conductive ink via liquid exfoliation of Graphene in Cyrene. Next, we established a stencil-printing process for patterning the graphene ink via laser-cut stencils on flexible polyimide substrates. Benchtop tests indicate that the graphene electrodes have good conductivity of ∼1.1 × 103S cm-1, flexibility to maintain their electrical connection under static bending, and electrochemical stability in a 15 d accelerated corrosion test. Chronically implanted graphene ECoG devices remain fully functional for up to 180 d, with averagein vivoimpedances of 24.72 ± 95.23 kΩ at 1 kHz. The ECoG device can measure spontaneous surface field potentials from mice under awake and anesthetized states and sensory stimulus-evoked responses.Significance:The stencil-printing fabrication process can be used to create Graphene ECoG devices with customized electrode layouts within 24 h using commonly available laboratory equipment.

Materials for Implantable Surface Electrode Arrays: Current Status and Future Directions

Surface electrode arrays are mainly fabricated from rigid or elastic materials, and precisely manipulated ductile metal films, which offer limited stretchability. However, the living tissues to which they are applied are nonlinear viscoelastic materials, which can undergo significant mechanical deformation in dynamic biological environments. Further, the same arrays and compositions are often repurposed for vastly different tissues rather than optimizing the materials and mechanical properties of the implant for the target application. By first characterizing the desired biological environment, and then designing a technology for a particular organ, surface electrode arrays may be more conformable, and offer better interfaces to tissues while causing less damage. Here, the various materials used in each component of a surface electrode array are first reviewed, and then electrically active implants in three specific biological systems, the nervous system, the muscular system, and skin, are described. Finally, the fabrication of next-generation surface arrays that overcome current limitations is discussed.

Minimally-invasive insertion strategy and in vivo evaluation of multi-shank flexible intracortical probes

Chronically implanted neural probes are powerful tools to decode brain activity however, recording population and spiking activity over long periods remains a major challenge. Here, we designed and fabricated flexible intracortical Michigan-style arrays with a shank cross-section per electrode of 250 μm[Formula: see text] utilizing the polymer paryleneC with the goal to improve the immune acceptance. As flexible neural probes are unable to penetrate the brain due to the low buckling force threshold, a tissue-friendly insertion system was developed by reducing the effective shank length. The insertion strategy enabled the implantation of the four, bare, flexible shanks up to 2 mm into the mouse brain without increasing the implantation footprint and therefore, minimizing the acute trauma. In acute recordings from the mouse somatosensory cortex and the olfactory bulb, we demonstrated that the flexible probes were able to simultaneously detect local field potentials as well as single and multi-unit activity. Additionally, the flexible arrays outperformed stiff probes with respect to yield of single unit activity. Following the successful in vivo validation, we further improved the microfabrication towards a double-metal-layer process, and were able to double the number of electrodes per shank by keeping the shank width resulting in a cross-section per electrode of 118 μm[Formula: see text].

Intracortical Microelectrode Array Unit Yield under Chronic Conditions: A Comparative Evaluation

While microelectrode arrays (MEAs) offer the promise of elucidating functional neural circuitry and serve as the basis for a cortical neuroprosthesis, the challenge of designing and demonstrating chronically reliable technology remains. Numerous studies report “chronic” data but the actual time spans and performance measures corresponding to the experimental work vary. In this study, we reviewed the experimental durations that constitute chronic studies across a range of MEA types and animal species to gain an understanding of the widespread variability in reported study duration. For rodents, which are the most commonly used animal model in chronic studies, we examined active electrode yield (AEY) for different array types as a means to contextualize the study duration variance, as well as investigate and interpret the performance of custom devices in comparison to conventional MEAs. We observed wide-spread variance within species for the chronic implantation period and an AEY that decayed linearly in rodent models that implanted commercially-available devices. These observations provide a benchmark for comparing the performance of new technologies and highlight the need for consistency in chronic MEA studies. Additionally, to fully derive performance under chronic conditions, the duration of abiotic failure modes, biological processes induced by indwelling probes, and intended application of the device are key determinants.

Graphene active sensor arrays for long-term and wireless mapping of wide frequency band epicortical brain activity

Graphene active sensors have demonstrated promising capabilities for the detection of electrophysiological signals in the brain. Their functional properties, together with their flexibility as well as their expected stability and biocompatibility have raised them as a promising building block for large-scale sensing neural interfaces. However, in order to provide reliable tools for neuroscience and biomedical engineering applications, the maturity of this technology must be thoroughly studied. Here, we evaluate the performance of 64-channel graphene sensor arrays in terms of homogeneity, sensitivity and stability using a wireless, quasi-commercial headstage and demonstrate the biocompatibility of epicortical graphene chronic implants. Furthermore, to illustrate the potential of the technology to detect cortical signals from infra-slow to high-gamma frequency bands, we perform proof-of-concept long-term wireless recording in a freely behaving rodent. Our work demonstrates the maturity of the graphene-based technology, which represents a promising candidate for chronic, wide frequency band neural sensing interfaces.

Guidelines to Study and Develop Soft Electrode Systems for Neural Stimulation

Electrical stimulation of nervous structures is a widely used experimental and clinical method to probe neural circuits, perform diagnostics, or treat neurological disorders. The recent introduction of soft materials to design electrodes that conform to and mimic neural tissue led to neural interfaces with improved functionality and biointegration. The shift from stiff to soft electrode materials requires adaptation of the models and characterization methods to understand and predict electrode performance. This guideline aims at providing (1) an overview of the most common techniques to test soft electrodes in vitro and in vivo; (2) a step-by-step design of a complete study protocol, from the lab bench to in vivo experiments; (3) a case study illustrating the characterization of soft spinal electrodes in rodents; and (4) examples of how interpreting characterization data can inform experimental decisions. Comprehensive characterization is paramount to advancing soft neurotechnology that meets the requisites for long-term functionality in vivo.

Amorphous silicon carbide ultramicroelectrode arrays for neural stimulation and recording

OBJECTIVE

Foreign body response to indwelling cortical microelectrodes limits the reliability of neural stimulation and recording, particularly for extended chronic applications in behaving animals. The extent to which this response compromises the chronic stability of neural devices depends on many factors including the materials used in the electrode construction, the size, and geometry of the indwelling structure. Here, we report on the development of microelectrode arrays (MEAs) based on amorphous silicon carbide (a-SiC).

APPROACH

This technology utilizes a-SiC for its chronic stability and employs semiconductor manufacturing processes to create MEAs with small shank dimensions. The a-SiC films were deposited by plasma enhanced chemical vapor deposition and patterned by thin-film photolithographic techniques. To improve stimulation and recording capabilities with small contact areas, we investigated low impedance coatings on the electrode sites. The assembled devices were characterized in phosphate buffered saline for their electrochemical properties.

MAIN RESULTS

MEAs utilizing a-SiC as both the primary structural element and encapsulation were fabricated successfully. These a-SiC MEAs had 16 penetrating shanks. Each shank has a cross-sectional area less than 60 µm² and electrode sites with a geometric surface area varying from 20 to 200 µm². Electrode coatings of TiN and SIROF reduced 1 kHz electrode impedance to less than 100 kΩ from ~2.8 MΩ for 100 µm² Au electrode sites and increased the charge injection capacities to values greater than 3 mC cm^-2. Finally, we demonstrated functionality by recording neural activity from basal ganglia nucleus of Zebra Finches and motor cortex of rat.

SIGNIFICANCE

The a-SiC MEAs provide a significant advancement in the development of microelectrodes that over the years has relied on silicon platforms for device manufacture. These flexible a-SiC MEAs have the potential for decreased tissue damage and reduced foreign body response. The technique is promising and has potential for clinical translation and large scale manufacturing.

Method for spike detection from microelectrode array recordings contaminated by artifacts of simultaneous two-photon imaging

The simultaneous utilization of electrophysiological recordings and two-photon imaging allows the observation of neural activity in a high temporal and spatial resolution at the same time. The three dimensional monitoring of morphological features near the microelectrode array makes the observation more precise and complex. In vitro experiments were performed on mice neocortical slices expressing the GCaMP6 genetically encoded calcium indicator for monitoring the neural activity with two-photon microscopy around the implanted microelectrodes. A special filtering algorithm was used for data analysis to eliminate the artefacts caused by the imaging laser. Utilization of a special filtering algorithm allowed us to detect and sort single unit activities from simultaneous two-photon imaging and electrophysiological measurement.

Histological evaluation of flexible neural implants; flexibility limit for reducing the tissue response?

OBJECTIVE

Flexible neural probes are hypothesized to reduce the chronic foreign body response (FBR) mainly by reducing the strain-stress caused by an interplay between the tethered probe and the brain's micromotion. However, a large discrepancy of Young's modulus still exists (3-6 orders of magnitude) between the flexible probes and the brain tissue. This raises the question of whether we need to bridge this gap; would increasing the probe flexibility proportionally reduce the FBR?

APPROACH

Using novel off-stoichiometry thiol-enes-epoxy (OSTE+) polymer probes developed in our previous work, we quantitatively evaluated the FBR to four types of probes with different softness: silicon (~150 GPa), polyimide (1.5 GPa), OSTE+_Hard (300 MPa), and OSTE+_Soft (6 MPa).

MAIN RESULTS

We observed a significant reduction in the fluorescence intensity of biomarkers for activated microglia/macrophages and blood-brain barrier (BBB) leakiness around the three soft polymer probes compared to the silicon probe, both at 4 weeks and 8 weeks post-implantation. However, we did not observe any consistent differences in the biomarkers among the polymer probes.

SIGNIFICANCE

The results suggest that the mechanical compliance of neural probes can mediate the degree of FBR, but its impact diminishes after a hypothetical threshold level. This infers that resolving the mechanical mismatch alone has a limited effect on improving the lifetime of neural implants.

Microfluidic neural probes: in vivo tools for advancing neuroscience

Microfluidic neural probes hold immense potential as in vivo tools for dissecting neural circuit function in complex nervous systems. Miniaturization, integration, and automation of drug delivery tools open up new opportunities for minimally invasive implants. These developments provide unprecedented spatiotemporal resolution in fluid delivery as well as multifunctional interrogation of neural activity using combined electrical and optical modalities. Capitalizing on these unique features, microfluidic technology will greatly advance in vivo pharmacology, electrophysiology, optogenetics, and optopharmacology. In this review, we discuss recent advances in microfluidic neural probe systems. In particular, we will highlight the materials and manufacturing processes of microfluidic probes, device configurations, peripheral devices for fluid handling and packaging, and wireless technologies that can be integrated for the control of these microfluidic probe systems. This article summarizes various microfluidic implants and discusses grand challenges and future directions for further developments.