Fabrication and evaluation of an improved polymer-based cochlear electrode array for atraumatic insertion

Assessing the manufacturable 32-channel cochlear electrode array: evaluation results for clinical trials

Reliability evaluation results of a manufacturable 32-channel cochlear electrode array are reported in this paper. Applying automated laser micro-machining process and a layer-by-layer silicone deposition scheme, authors developed the manufacturing methods of the electrode array for fine patterning and mass production. The developed electrode array has been verified through the requirements specified by the ISO Standard 14708-7. And the insertion trauma of the electrode array has been evaluated based on human temporal bone studies. According to the specified requirements, the electrode array was assessed through elongation & insulation, flexural, and fatigue tests. In addition, Temporal bone study was performed using eight fresh-frozen cadaver temporal bones with the electrode arrays inserted via the round window. Following soaking in saline condition, the impedances between conducting wires of the electrode array were measured over 100 kΩ (the pass/fail criterion). After each required test, it was shown that the electrode array maintained the electrical continuity and insulation condition. The average insertion angle of the electrode array inside the scala tympani was 399.7°. The human temporal bone studies exhibited atraumatic insertion rate of 60.3% (grade 0 or 1). The reliability of the manufacturable electrode array is successfully verified in mechanical, electrical, and histological aspects. Following the completion of a 32-channel cochlear implant system, the performance and stability of the 32-channel electrode array will be evaluated in clinical trials.

Liquid Crystal Materials for Biomedical Applications

Liquid crystal is a state of matter being intermediate between solid and liquid. Liquid crystal materials exhibit both orientational order and fluidity. While liquid crystals have long been highly recognized in the display industry, in recent decades, liquid crystals provide new opportunities into the cross-field of material science and biomedicine due to their biocompatibility, multifunctionality, and responsiveness. In this review, the latest achievements of liquid crystal materials applied in biomedical fields are summarized. The start is made by introducing the basic concepts of liquid crystals, and then shifting to the components of liquid crystals as well as functional materials derived therefrom. After that, the ongoing and foreseeable applications of liquid crystal materials in the biomedical field with emphasis put on several cutting-edge aspects, including drug delivery, bioimaging, tissue engineering, implantable devices, biosensing, and wearable devices are discussed. It is hoped that this review will stimulate ingenious ideas for the future generation of liquid crystal-based drug development, artificial implants, disease diagnosis, health status monitoring, and beyond.

Development of a miniaturized, reconnectable, and implantable multichannel connector

Objective. Connectors for implantable neural prosthetic systems provide several advantages such as simplification of surgery, safe replacement of implanted devices, and modular design of the implant systems. With the rapid advancement of technologies for neural implants, miniaturized multichannel implantable connectors are also required. In this study, we propose a reconnectable and area-efficient multichannel implantable connector.Approach. A female-to-female adapter was fabricated using the thermal-press bonding of micropatterned liquid crystal polymer films. A bump inside the adapter enabled a reliable electrical connection by increasing the contact pressure between the contact pads of the adapter and the inserted cable. After connection, the adapter is enclosed in a metal case sealed with silicone elastomer packing. With different sizes of the packings, leakage current tests were performed under accelerated conditions to determine the optimal design for long-term reliability. Repeated connection tests were performed to verify the durability and reconnectability of the fabricated connector. The connector was implanted in rats, and the leakage currents were monitored to evaluate the stability of the connectorin vivo. Main results. The fabricated four- and eight-channel implantable connectors, assembled with the metal cases, had a diameter and length of 6 and 17 mm, respectively. Further, the contact resistances of the four- and eight-channel connectors were 53.2 and 75.2 mΩ, respectively. The electrical contact remained stable during repeated connection tests (50 times). The fabricated connectors with packings having 125%, 137%, and 150% volume ratios to the internal space of the metal case failed after 14, 88, and 14 d, respectively, in a 75 °C saline environment. In animal tests with rats, the connector maintained low leakage current levels for up to 92 d.Significance. An implantable and reconnectable multichannel connector was developed and evaluated. The feasibility of the proposed connector was evaluated in terms of electrical and mechanical characteristics as well as sealing performance. The proposed connector is expected to have potential applications in implantable neural prosthetic systems.

Manufacturable 32-Channel Cochlear Electrode Array and Preliminary Assessment of Its Feasibility for Clinical Use

(1) Background: In this study, we introduce a manufacturable 32-channel cochlear electrode array. In contrast to conventional cochlear electrode arrays manufactured by manual processes that consist of electrode-wire welding, the placement of each electrode, and silicone molding over wired structures, the proposed cochlear electrode array is manufactured by semi-automated laser micro-structuring and a mass-produced layer-by-layer silicone deposition scheme similar to the semiconductor fabrication process. (2) Methods: The proposed 32-channel electrode array has 32 electrode contacts with a length of 24 mm and 0.75 mm spacing between contacts. The width of the electrode array is 0.45 mm at its apex and 0.8 mm at its base, and it has a three-layered arrangement consisting of a 32-channel electrode layer and two 16-lead wire layers. To assess its feasibility, we conducted an electrochemical evaluation, stiffness measurements, and insertion force measurements. (3) Results: The electrochemical impedance and charge storage capacity are 3.11 ± 0.89 kOhm at 1 kHz and 5.09 mC/cm2, respectively. The V/H ratio, which indicates how large the vertical stiffness is compared to the horizontal stiffness, is 1.26. The insertion force is 17.4 mN at 8 mm from the round window, and the maximum extraction force is 61.4 mN. (4) Conclusions: The results of the preliminary feasibility assessment of the proposed 32-channel cochlear electrode array are presented. After further assessments are performed, a 32-channel cochlear implant system consisting of the proposed 32-channel electrode array, 32-channel neural stimulation and recording IC, titanium-based hermetic package, and sound processor with wireless power and signal transmission coil will be completed.

Current Stimulation of the Midbrain Nucleus in Pigeons for Avian Flight Control

A number of research attempts to understand and modulate sensory and motor skills that are beyond the capability of humans have been underway. They have mainly been expounded in rodent models, where numerous reports of controlling movement to reach target locations by brain stimulation have been achieved. However, in the case of birds, although basic research on movement control has been conducted, the brain nuclei that are triggering these movements have yet to be established. In order to fully control flight navigation in birds, the basic central nervous system involved in flight behavior should be understood comprehensively, and functional maps of the birds’ brains to study the possibility of flight control need to be clarified. Here, we established a stable stereotactic surgery to implant multi-wire electrode arrays and electrically stimulated several nuclei of the pigeon’s brain. A multi-channel electrode array and a wireless stimulation system were implanted in thirteen pigeons. The pigeons’ flight trajectories on electrical stimulation of the cerebral nuclei were monitored and analyzed by a 3D motion tracking program to evaluate the behavioral change, and the exact stimulation site in the brain was confirmed by the postmortem histological examination. Among them, five pigeons were able to induce right and left body turns by stimulating the nuclei of the tractus occipito-mesencephalicus (OM), nucleus taeniae (TN), or nucleus rotundus (RT); the nuclei of tractus septo-mesencephalicus (TSM) or archistriatum ventrale (AV) were stimulated to induce flight aviation for flapping and take-off with five pigeons.

Biomedical invasive pressure sensor coatings: calibration and waveform perspectives

The biocompatibility of invasive biomedical sensors is a fundamental issue in the design of implanted sensors. Therefore, the unique packaging of the sensor is a crucial design procedure that must be performed and evaluated correctly. Under steady-state measurements, the sensor calibration can be done quickly, and the corresponding errors and loss in the sensor's sensitivity, because of the packaging material, can be compensated easily. This paper investigates the effect of the presence and absence of biocompatible silicone paste as packaging material on the catheter sensor's output waveform morphology, and the sensor's response time under dynamic measurements. The procedure to calibrate the sensor during the design is presented to compensate for the effect of packaging material in terms of state-space formulation. In conclusion, errors in peak pressure and waveform shape in the catheter sensor can be significantly reduced by the geometry and the packaging materials of the catheter sensor. At last, we believe that using biocompatible silicone paste as packaging material on the catheter sensor is scalable.

Hemispherical Microelectrode Array for Ex Vivo Retinal Neural Recording

To investigate the neuronal visual encoding process in the retina, researchers have performed in vitro and ex vivo electrophysiological experiments using animal retinal tissues. The microelectrode array (MEA) has become a key component in retinal experiments because it enables simultaneous neural recording from a population of retinal neurons. However, in most retinal experiments, it is inevitable that the retinal tissue is flattened on the planar MEA, becoming deformed from the original hemispherical shape. During the tissue deforming process, the retina is subjected to mechanical stress, which can induce abnormal physiological conditions. To overcome this problem, in this study, we propose a hemispherical MEA with a curvature that allows retinal tissues to adhere closely to electrodes without tissue deformation. The electrode array is fabricated by stretching a thin, flexible polydimethylsiloxane (PDMS) electrode layer onto a hemispherical substrate. To form micro patterns of electrodes, laser processing is employed instead of conventional thin-film microfabrication processes. The feasibility for neural recording from retinal tissues using this array is shown by conducting ex vivo retinal experiments. We anticipate that the proposed techniques for hemispherical MEAs can be utilized not only for ex vivo retinal studies but also for various flexible electronics.

Implantable electrical stimulation bioreactor with liquid crystal polymer-based electrodes for enhanced bone regeneration at mandibular large defects in rabbit

The osseous regeneration of large bone defects is still a major clinical challenge in maxillofacial and orthopedic surgery. Previous studies demonstrated that biphasic electrical stimulation (ES) stimulates bone formation; however, polyimide electrode should be removed after regeneration. This study presents an implantable electrical stimulation bioreactor with electrodes based on liquid crystal polymer (LCP), which can be permanently implanted due to excellent biocompatibility to bone tissue. The bioreactor was implanted into a critical-sized bone defect and subjected to ES for one week, where bone regeneration was evaluated four weeks after surgery using micro-CT. The effect of ES via the bioreactor was compared with a sham control group and a positive control group that received recombinant human bone morphogenetic protein (rhBMP)-2 (20 μg). New bone volume per tissue volume (BV/TV) in the ES and rhBMP-2 groups increased to 132% (p < 0.05) and 174% (p < 0.01), respectively, compared to that in the sham control group. In the histological evaluation, there was no inflammation within the bone defects and adjacent to LCP in all the groups. This study showed that the ES bioreactor with LCP electrodes could enhance bone regeneration at large bone defects, where LCP can act as a mechanically resistant outer box without inflammation. Graphical abstract To enhance bone regeneration, a bioreactor comprising collagen sponge and liquid crystal polymer-based electrode was implanted in the bone defect. Within the defect, electrical current pulses having biphasic waveform were applied from the implanted bioreactor.

A 3D Microscaffold Cochlear Electrode Array for Steroid Elution

In cochlear implants, the electrode insertion trauma during surgery can cause damage residual hearing. Preserving the residual hearing is an important challenge and the localized administration of drugs, such as steroids, is one of the most promising ways, but remains a challenge. Here, a microscaffold cochlear electrode array (MiSCEA) consisting of a microfabricated flexible electrode array and a 3D microscaffold for steroid reservoir is reported. The MiSCEA without loaded drug is tested by measuring the electrically evoked auditory brainstem response of the cochlea in guinea pigs (n = 4). The scaffold is then coated with steroid (dexamethasone) encapsulated in polylactic-co-glycolic acid and the continuous release of the steroid into artificial perilymph during six weeks is monitored. The steroid-containing scaffolds are then implanted into guinea pigs (n = 4) and threshold shifts are analyzed for four weeks by measuring the acoustically evoked auditory brainstem response. The threshold shifts tend to be lower in the group implanted with the steroid-containing MiSCEAs. The feasibility of 3D MiSCEA opens up the development of potential next-generation cochlear electrode with improved steroid release dynamics into cochlea.

Emerging Encapsulation Technologies for Long-Term Reliability of Microfabricated Implantable Devices

The development of reliable long-term encapsulation technologies for implantable biomedical devices is of paramount importance for the safe and stable operation of implants in the body over a period of several decades. Conventional technologies based on titanium or ceramic packaging, however, are not suitable for encapsulating microfabricated devices due to their limited scalability, incompatibility with microfabrication processes, and difficulties with miniaturization. A variety of emerging materials have been proposed for encapsulation of microfabricated implants, including thin-film inorganic coatings of Al2O3, HfO2, SiO2, SiC, and diamond, as well as organic polymers of polyimide, parylene, liquid crystal polymer, silicone elastomer, SU-8, and cyclic olefin copolymer. While none of these materials have yet been proven to be as hermetic as conventional metal packages nor widely used in regulatory approved devices for chronic implantation, a number of studies have demonstrated promising outcomes on their long-term encapsulation performance through a multitude of fabrication and testing methodologies. The present review article aims to provide a comprehensive, up-to-date overview of the long-term encapsulation performance of these emerging materials with a specific focus on publications that have quantitatively estimated the lifetime of encapsulation technologies in aqueous environments.

CNT bundle-based thin intracochlear electrode array

OBJECTIVE

It is known that the insertion of the intracochlear electrode is critical procedure because the damage around cochlear structures can deteriorate hearing restoration. To reduce the trauma during the electrode insertion surgery, we developed a thin and flexible intracochlear electrode array constructed with carbon nanotube (CNT) bundles.

METHODS

Each CNT bundle was used for an individual electrode channel after coated with parylene C for insulation. By encapsulating eight CNT bundles with silicone elastomer, an 8-channel intracochlear electrode array was fabricated. The mechanical and electrochemical characteristics were assessed to evaluate the flexibility and feasibility of the electrode as a stimulation electrode. The functionality of the electrode was confirmed by electrically evoked auditory brainstem responses (eABR) recorded from a rat.

RESULTS

The proposed electrode has a thickness of 135 μm at the apex and 395 μm at the base. It was demonstrated that the CNT bundle-based electrodes require 6-fold the lower insertion force than metal wire-based electrodes. The electrode impedance and the cathodic charge storage capacitance (CSCc) were 2.70 kΩ ∠-20.4° at 1 kHz and - 708 mC/cm², respectively. The eABR waves III and V were observed when stimulation current is greater than 50 μA.

CONCLUSION

A thin and flexible CNT bundle-based intracochlear electrode array was successfully developed. The feasibility of the proposed electrode was shown in terms of mechanical and electrochemical characteristics. A proposed CNT bundle-based intracochlear electrode may reduce the risk of trauma during electrode insertion surgery.

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.

Progress in artificial vision through suprachoroidal retinal implants

Retinal implants have proven their ability to restore visual sensation to people with degenerative retinopathy, characterized by photoreceptor cell death and the retina's inability to sense light. Retinal bionics operate by electrically stimulating the surviving neurons in the retina, thus triggering the transfer of visual sensory information to the brain. Suprachoroidal implants were first investigated in Australia in the 1950s. In this approach, the neuromodulation hardware is positioned between the sclera and the choroid, thus providing significant surgical and safety benefits for patients, with the potential to maintain residual vision combined with the artificial input from the device. Here we review the latest advances and state of the art devices for suprachoroidal prostheses, highlight future technologies and discuss challenges and perspectives towards improved rehabilitation of vision.

Study of the Carrier-Aided Thin Film Electrode Array Design for Cochlear Insertion

The micro-fabricated thin film electrode array (TFEA) has been a promising design for cochlear implants (CIs) because of its cost-effectiveness and fabrication precision. The latest polymer-based cochlear TFEAs have faced difficulties for cochlear insertion due to the lack of structural stiffness. To stiffen the TFEA, dissolvable stiffening materials, TFEAs with different structures, and TFEAs with commercial CIs as carriers have been invested. In this work, the concept of enhancing a Parylene TFEA with Kapton tape as a simpler carrier for cochlear insertion has been proved to be feasible. The bending stiffness of the Kapton-aided TFEA was characterized with an analytical model, a finite element model, and a cantilever bending experiment, respectively. While the Kapton tape increased the bending stiffness of the Parylene TFEA by 10³ times, the 6-μm-thick TFEA with a similar Young's modulus, as a polyimide, in turn significantly increased the bending stiffness of the 170-μm-thick Kapton carrier by 60%. This result indicated that even the TFEA is ultra-flexible and that its bending stiffness should not be neglected in the design or selection of its carrier.

Optimization of cochlear implant stimulation resolution using an intracochlear electric potential model

Designing an electrode array with a high stimulation resolution (SR) is the main challenge in cochlear implant development. In this work, a thin-film electrode array (TFEA) and partial tripolar (pTP) mode were combined in the design stage to optimize the SR. A finite-element model of the intracochlear electric potential V_e incorporating a TFEA and pTP mode was built and validated using previous experimental measurements. Based on this model, the SR was analyzed by using a defined stimulation factor V_s, which takes both the amplitude and bandwidth of V_e into account. A co-simulation method integrating the model and genetic algorithm was employed to maximize V_s with an optimized parameter set including the electrode diameter d, electrode interval g, and compensation coefficient σ. The results indicated that a TFEA combined with pTP mode outperforms their individual utilization to improve the SR and that d has an independent negative correlation with the SR, but it is more effective and feasible to consider all three parameters in the design stage with the proposed model and co-simulation optimization method. In our design, the optimized parameters were d = 150 μm, g = 200.5 μm, and σ = 0.746.

Wireless navigation of pigeons using polymer-based fully implantable stimulator: A pilot study using depth electrodes

A polymer-based implantable stimulator for wirelessly navigating pigeons was conceptually suggested and a pilot study using depth electrodes was conducted. In this study, depth electrodes based on liquid crystal polymer (LCP) with eight channels were designed and fabricated. Electrochemical impedance spectrum (EIS) assessments were performed to measure impedances of the electrodes. The average value of the measured impedances was 16.8∠15.8 ° kμ. The electrodes were then advanced to a target nucleus (formatio reticularis medialis mesencephalic, FRM) of a pigeon to prove their in vivo feasibilities. Biphasic current pulses were generated by a custom-made stimulator and delivered to the electrodes to stimulate the FRM electrically. Pulses with an amplitude level of 0.567 μA, a rate of 58.0 Hz, and a duration of 1.00 ms were applied with inter-stimulus intervals of three minutes. Turning and circling behaviors were consistently shown when the FRM was stimulated. The feasibilities of the electrodes were proved in both in vitro and in vivo tests, as a pilot study for the suggested scheme. Finally, several discussions of the assessments and extensions for a fully implantable stimulator were described.