The insulation performance of reactive parylene films in implantable electronic devices

Recent advances in materials and manufacturing of implantable devices for continuous health monitoring

Implantable devices are vital in healthcare, enabling continuous monitoring, early disease detection, informed decision-making, enhanced outcomes, cost reduction, and chronic condition management. These devices provide real-time data, allowing proactive healthcare interventions, and contribute to overall improvements in patient care and quality of life. The success of implantable devices relies on the careful selection of materials and manufacturing methods. Recent materials research and manufacturing advancements have yielded implantable devices with enhanced biocompatibility, reliability, and functionality, benefiting human healthcare. This paper provides a comprehensive overview of the latest developments in implantable medical devices, emphasizing the importance of material selection and manufacturing methods, including biocompatibility, self-healing capabilities, corrosion resistance, mechanical properties, and conductivity. It explores various manufacturing techniques such as microfabrication, 3D printing, laser micromachining, electrospinning, screen printing, inkjet printing, and nanofabrication. The paper also discusses challenges and limitations in the field, including biocompatibility concerns, privacy and data security issues, and regulatory hurdles for implantable devices.

Parylene C Coating Efficacy Studies: Enhancing Biocompatibility of 3D Printed Polyurethane Parts for Biopharmaceutical and CGT Applications

Additive manufacturing, particularly Vat photopolymerization, presents a promising technique for producing complex, tailor-made structures, making it an attractive option for generating single-use components used in biopharmaceutical manufacturing equipment or cell culture devices. However, the potential leaching of cytotoxic compounds from Vat photopolymer resins poses a significant concern, especially regarding cell growth and viability in cell culture applications. This study explores the potential of parylene C coating to enhance the inertness of a polyurethane-based photopolymer resin, aiming to prevent cytotoxicity and improve biocompatibility. The study includes an analysis of extractables from the resin and photoinitiator to evaluate the resin's composition and to define selected marker compounds for investigating the coating efficiency. The time-dependent accumulation of relevant extractable compounds over a 70-day period are assessed to address the long-term use of the coated components. The impact of irradiation on the material and the coating was evaluated, along with an accelerated aging study to address the long-term performance of the coating. Biocompatibility in terms of in vitro cell growth studies is evaluated using Chinese hamster ovary cells, a standard cell line in biopharmaceutical manufacturing. Results demonstrate that parylene C coating significantly reduces the release of cytotoxic compounds, such as the photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO). Although accelerated aging indicates a reduction in the barrier properties of the coating over time, the parylene C coating still effectively slows the release of extractables and significantly improves cell compatibility of the 3D printed parts. The findings suggest that parylene C-coated components can be safely integrated into biopharmaceutical manufacturing processes, with recommendations to minimize storage times between coating application and use to ensure optimal performance.

Recent advances in implantable sensors and electronics using printable materials for advanced healthcare

This review article focuses on the recent printing technological progress in healthcare, underscoring the significant potential of implantable devices across diverse applications. Printing technologies have widespread use in developing health monitoring devices, diagnostic systems, and surgical devices. Recent years have witnessed remarkable progress in fabricating low-profile implantable devices, driven by advancements in printing technologies and nanomaterials. The importance of implantable biosensors and bioelectronics is highlighted, specifically exploring printing tools using bio-printable inks for practical applications, including a detailed examination of fabrication processes and essential parameters. This review also justifies the need for mechanical and electrical compatibility between bioelectronics and biological tissues. In addition to technological aspects, this article delves into the importance of appropriate packaging methods to enhance implantable devices' performance, compatibility, and longevity, which are made possible by integrating cutting-edge printing technology. Collectively, we aim to shed light on the holistic landscape of implantable biosensors and bioelectronics, showcasing their evolving role in advancing healthcare through innovative printing technologies.

Analysis of the Lifetime of Neural Implants Using In Vitro Test Structures

The aim of this work was to measure the lifetime of neural implant test samples at two different temperatures, using a method that allows the precise measurement of the sample lifetime, further analysis with the use of Weibull statistics, and examination of the applicability of the Van't Hoff rule. The correct estimation of the lifetime of neural implants is important to avoid preliminary failures, when used in humans. The novelty lies in the precise data due to the measurement approach, the application of the Weibull statistics to neural test samples, and the examination of the Van't Hoff rule's applicability to the longevity of polyimide-based neural implant samples. Several samples that consisted of interdigitated gold strands, encapsulated in polyimide were soaked in ringer solution. One batch was soaked at a temperature of 37 °C, and another was soaked at a temperature of 57 °C. Voltage was applied and measured to identify the occurrence of failures. The long-term experiment was stopped after 458 days for the samples at 37 °C and 423 days for the samples at 57 °C, with several samples still being intact at both temperature levels. The time to failure was measured and used to identify the Weibull parameters that would describe the behavior of the samples. The median lifetime of the samples changed from 363 days at 37 °C to 138 days at 57 °C. The scale and shape factor changed from 396 and 3.7 at 37 °C to 138 and 2 at 57 °C, respectively. The measured mean, median times, and Weibull scale factors were lower than expected from the Van't Hoff rule. The use of the Van't hoff rule with 2ΔT/10°C for accelerated lifetime tests would lead to an estimation of longer lifetimes than realistic. A reaction rate constant around 1.47 appears more appropriate. While a fourfold difference in lifetime would be expected, only a 2.65-fold difference in the median lifetime and a roughly 2.2-fold difference in the mean and Weibull scale factor were observed. The shift of the Weibull shape parameter from 3.7 at 37 °C to 2 at 57 °C with rising temperatures was observed, indicating differences in failure reasons and stronger aging at lower temperatures. The used method is simple to apply and interpret and allows for a precise anticipation of sample lifetimes.

Methodology for Safe and Effective Subcutaneous Implantation of Wireless Biotelemetry Sensor Devices in Rodents

In this work, a methodology for assessing the impact of implantation surgery on laboratory mice on behavior was created. The study included the design of several implants fabricated on various printed circuit board (PCB) technologies with overall diameters between 26-28mm and weights between 4.5-6.5g. 11 adult CD1 mice were implanted with the devices and their behavior was analyzed using common behavioral benchmark tests. The results show that implants designed to be 10% of the animal's body weight showed no adverse effects on mobility or social behavior. These results illustrate a method to identify and reduce the adverse behavioral changes inherent to device implantation. Additional considerations for implant surgery are provided. These results are validated with the implantation of a Bluetooth Low Energy (BLE) wireless sensor tag. The implanted wireless tag showed an average Received Signal Strength Indicator (RSSI) of 62.96dBm with a standard deviation of 4.95dBm and a variance of 24.5 dBm2. The high RSSI and variance values show that the implant was working well inside of the mouse's body and that the mouse was fully recovered and readily exploring its surroundings.Clinical Relevance-This work 1) studies the behavioral impact of implantable wireless biopotential devices. This will help clinical researchers conducting behavioral studies using sensor implants. 2) demonstrates a working implanted BLE wireless model inside of a mouse. Various wireless connectivity metrics are studied.

A microwave-assisted and efficient reduction protocol for the synthesis of alkyl-substituted [2.2]-paracyclophanes

[2.2]-Paracyclophane derivatives have been promising platforms for applications in biology and materials science. To access pure derivatives thereof, a safe and non-toxic synthetic method for 4- n-propyl-[2.2]-paracyclophane is developed via a microwave-assisted and NH2NH2/KOH reduction route. We introduce microwave-assisted acylation for a synthesis that successfully improves the yield and reduces the reaction time. Notably, an exploration of the length and number of the alkyl chains on the [2.2]-paracyclophane ring did not significantly affect the outcome of this reaction. We synthesized 4- n-butyl-[2.2]-paracyclophane and 4,12-dipropyl-[2.2]-paracyclophane via our protocol with high yields. The regioselectivity of the second electrophilic substitution on 4- n-propyl-[2.2]-paracyclophane occurs at the para position of the less substituted phenyl ring.

Implantable intracortical microelectrodes: reviewing the present with a focus on the future

Implantable intracortical microelectrodes can record a neuron's rapidly changing action potentials (spikes). In vivo neural activity recording methods often have either high temporal or spatial resolution, but not both. There is an increasing need to record more neurons over a longer duration in vivo. However, there remain many challenges to overcome before achieving long-term, stable, high-quality recordings and realizing comprehensive, accurate brain activity analysis. Based on the vision of an idealized implantable microelectrode device, the performance requirements for microelectrodes are divided into four aspects, including recording quality, recording stability, recording throughput, and multifunctionality, which are presented in order of importance. The challenges and current possible solutions for implantable microelectrodes are given from the perspective of each aspect. The current developments in microelectrode technology are analyzed and summarized.

Chemical Vapor Deposition of Longitudinal Homogeneous Parylene Thin-Films inside Narrow Tubes

The effect of quasi-exponentially decreasing film thicknesses of thin poly-para-xylylene (PPX-N) coatings inside narrow tubes or micro scaled gaps is well known and has been discussed by many authors since the late 1970s. However, for technical applications it is often necessary to provide a longitudinal homogeneous film thickness to ensure the constant properties that are required. In a previous work, it was shown, in principle and for the first time, that a temperature gradient along the tube will effectively counteract the longitudinal decreasing film thickness of the PPX-N coating of the interior wall of a capillary. Therefore, this effect is discussed in theory and the provided model is verified by experiments. Our prediction of a required sticking coefficient curve yields experimentally measured homogeneous film thicknesses and shows a good agreement with the given prognosis. Further, it is shown in theory that there is a maximum achievable homogeneous film thickness in the tube in comparison to a blank surface, which can be understood as a coating efficiency for this type of deposition.

Thermal Analysis of Parylene Thin Films for Barrier Layer Applications

Biocompatible polymer films demonstrating excellent thermal stability are highly desirable for high-temperature (>250 °C) applications, especially in the bioelectronic encapsulation domain. Parylene, as an organic thin film, is a well-established polymer material exhibiting excellent barrier properties and is often the material of choice for biomedical applications. This work investigated the thermal impact on the bulk properties of four types of parylene films: parylene N, C, VT4, and AF4. The films, deposited using the standard Gorham process, were analyzed at varying annealing temperatures from room temperature up to 450 °C. Thermal properties were identified by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) methods, while X-ray diffraction (XRD) analysis showed the effect of high-temperature exposure on the structural properties. In addition to thermal and structural analysis, the barrier properties were measured through the helium transmission rate (HTR) and the water vapor transmission rate (WVTR). Fluorinated parylene films were confirmed to be exceptional materials for high-temperature applications. Parylene AF4 film, 25um thick, demonstrated excellent barrier performance after 300 °C exposure, with an HTR and a WVTR of 12.18 × 103 cm3 (STP) m−2 day−1 atm−1 and 6.6 g m−2 day−1, respectively.

Spinal cord bioelectronic interfaces: opportunities in neural recording and clinical challenges

Bioelectronic stimulation of the spinal cord has demonstrated significant progress in restoration of motor function in spinal cord injury (SCI). The proximal, uninjured spinal cord presents a viable target for the recording and generation of control signals to drive targeted stimulation. Signals have been directly recorded from the spinal cord in behaving animals and correlated with limb kinematics. Advances in flexible materials, electrode impedance and signal analysis will allow SCR to be used in next-generation neuroprosthetics. In this review, we summarize the technological advances enabling progress in SCR and describe systematically the clinical challenges facing spinal cord bioelectronic interfaces and potential solutions, from device manufacture, surgical implantation to chronic effects of foreign body reaction and stress-strain mismatches between electrodes and neural tissue. Finally, we establish our vision of bi-directional closed-loop spinal cord bioelectronic bypass interfaces that enable the communication of disrupted sensory signals and restoration of motor function in SCI.

Recent Progress in Materials Chemistry to Advance Flexible Bioelectronics in Medicine

Designing bioelectronic devices that seamlessly integrate with the human body is a technological pursuit of great importance. Bioelectronic medical devices that reliably and chronically interface with the body can advance neuroscience, health monitoring, diagnostics, and therapeutics. Recent major efforts focus on investigating strategies to fabricate flexible, stretchable, and soft electronic devices, and advances in materials chemistry have emerged as fundamental to the creation of the next generation of bioelectronics. This review summarizes contemporary advances and forthcoming technical challenges related to three principal components of bioelectronic devices: i) substrates and structural materials, ii) barrier and encapsulation materials, and iii) conductive materials. Through notable illustrations from the literature, integration and device fabrication strategies and associated challenges for each material class are highlighted.

A portable multi-sensor module for monitoring external ventricular drains

External ventricular drains (EVDs) are used clinically to relieve excess fluid pressure in the brain. However, EVD outflow rate is highly variable and typical clinical flow tracking methods are manual and low resolution. To address this problem, we present an integrated multi-sensor module (IMSM) containing flow, temperature, and electrode/substrate integrity sensors to monitor the flow dynamics of cerebrospinal fluid (CSF) drainage through an EVD. The impedimetric sensors were microfabricated out of biocompatible polymer thin films, enabling seamless integration with the fluid drainage path due to their low profile. A custom measurement circuit enabled automated and portable sensor operation and data collection in the clinic. System performance was verified using real human CSF in a benchtop EVD model. Impedimetric flow sensors tracked flow rate through ambient temperature variation and biomimetic pulsatile flow, reducing error compared with previous work by a factor of 6.6. Detection of sensor breakdown using novel substrate and electrode integrity sensors was verified through soak testing and immersion in bovine serum albumin (BSA). Finally, the IMSM and measurement circuit were tested for 53 days with an RMS error of 61.4 μL/min.

Fabrication of a Multilayer Implantable Cortical Microelectrode Probe to Improve Recording Potential

Intracortical neural probes are a key enabling technology for acquiring high fidelity neural signals within the cortex. They are viewed as a crucial component of brain-computer interfaces (BCIs) in order to record electrical activities from neurons within the brain. Smaller, more flexible, polymer-based probes have been investigated for their potential to limit the acute and chronic neural tissue response. Conventional methods of patterning electrodes and connecting traces on a single supporting layer can limit the number of recording sites which can be defined, particularly when designing narrower probes. We present a novel strategy of increasing the number of recording sites without proportionally increasing the size of the probe by using a multilayer fabrication process to vertically layer recording traces on multiple Parylene support layers, allowing more recording traces to be defined on a smaller probe width. Using this approach, we are able to define 16 electrodes on 4 supporting layers (4 electrodes per layer), each with a 30 μm diameter recording window and 5 μm wide connecting trace defined by conventional LWUV lithography, on an 80 μm wide by 9 μm thick microprobe. Prior to in vitro and in vivo validation, the multilayer probes are electrically characterized via impedance spectroscopy and evaluating crosstalk between adjacent layers. Demonstration of acute in vitro recordings in a cerebral organoid model and in vivo recordings in a murine model indicate the probe's capability for single unit recordings. This work demonstrates the ability to fabricate smaller, more compliant neural probes without sacrificing electrode density.

Smart Contact Lenses with a Transparent Silver Nanowire Strain Sensor for Continuous Intraocular Pressure Monitoring

Continuous intraocular pressure (IOP) monitoring can provide a paradigm shift in the management of patients with glaucoma as a facile alternative to conventional diagnostic methods. However, the low sensitivity and functional instability of current IOP sensors have limited their clinical utility in the management of glaucoma. Here, we have developed a smart contact lens integrated with a transparent silver nanowire IOP strain sensor and wireless circuits for noninvasive, continuous IOP monitoring. After confirming the robust stability of the IOP sensor within the smart contact lens in the presence of tears and repeated eyelid blink model cycles, we were able to monitor IOP changes on polydimethylsiloxane model eyes in vitro. In vivo tests demonstrated that our fully integrated wireless smart contact lens could successfully monitor the change in IOP in living rabbit eyes, which was clearly validated by the conventional invasive tonometer IOP test. Taken together, we could confirm the feasibility of our smart contact lens as a noninvasive platform for continuous IOP monitoring of glaucoma patients.

Fully Implantable Plantar Cutaneous Augmentation System for Rats Using Closed-loop Electrical Nerve Stimulation

Plantar cutaneous feedback plays an important role in stable and efficient gait, by modulating the activity of ankle dorsi- and plantar-flexor muscles. However, central and peripheral nervous system trauma often decrease plantar cutaneous feedback and/or interneuronal excitability in processing the plantar cutaneous feedback. In this study, we tested a fully implantable neural recording and stimulation system augmenting plantar cutaneous feedback. Electromyograms were recorded from the medial gastrocnemius muscle for stance phase detection, while biphasic stimulation pulses were applied to the distal-tibial nerve during the stance phase to augment plantar cutaneous feedback. A Bluetooth low energy and a Qi-standard inductive link were adopted for wireless communication and wireless charging, respectively. To test the operation of the system, one intact rat walked on a treadmill with the electrical system implanted into its back. Leg kinematics were recorded to identify the stance phase. Stimulation was applied, with a 250-ms onset delay from stance onset and 200-ms duration, resulting in the onset at 47.58 ± 2.82% of stance phase and the offset at 83.49 ± 4.26% of stance phase (Mean ± SEM). The conduction velocity of the compound action potential (31.2 m/s and 41.6 m/s at 1·T and 2·T, respectively) suggests that the evoked action potential was characteristic of an afferent volley for cutaneous feedback. We also demonstrated successful wireless charging and system reset functions. The experimental results suggest that the presented implantable system can be a valuable neural interface tool to investigate the effect of plantar cutaneous augmentation on gait in a rat model.

Inhibition of Na+/H+exchanger modulates microglial activation and scar formation following microelectrode implantation

Objective.Intracortical microelectrodes are an important tool for neuroscience research and have great potential for clinical use. However, the use of microelectrode arrays to treat neurological disorders and control prosthetics is limited by biological challenges such as glial scarring, which can impair chronic recording performance. Microglia activation is an early and prominent contributor to glial scarring. After insertion of an intracortical microelectrode, nearby microglia transition into a state of activation, migrate, and encapsulate the device. Na⁺/H⁺exchanger isoform-1 (NHE-1) is involved in various microglial functions, including their polarity and motility, and has been implicated in pro-inflammatory responses to tissue injury. HOE-642 (cariporide) is an inhibitor of NHE-1 and has been shown to depress microglial activation and inflammatory response in brain injury models.Approach.In this study, the effects of HOE-642 treatment on microglial interactions to intracortical microelectrodes was evaluated using two-photon microscopyin vivo.Main results.The rate at which microglia processes and soma migrate in response to electrode implantation was unaffected by HOE-642 administration. However, HOE-642 administration effectively reduced the radius of microglia activation at 72 h post-implantation from 222.2µm to 177.9µm. Furthermore, treatment with HOE-642 significantly reduced microglial encapsulation of implanted devices at 5 h post-insertion from 50.7 ± 6.0% to 8.9 ± 6.1%, which suggests an NHE-1-specific mechanism mediating microglia reactivity and gliosis during implantation injury.Significance.This study implicates NHE-1 as a potential target of interest in microglial reactivity and HOE-642 as a potential treatment to attenuate the glial response and scar formation around implanted intracortical microelectrodes.

Defect Filling Method of Sensor Encapsulation Based on Micro-Nano Composite Structure with Parylene Coating

The demand for waterproofing of polymer (parylene) coating encapsulation has increased in a wide variety of applications, especially in the waterproof protection of electronic devices. However, parylene coatings often produce pinholes and cracks, which will reduce the waterproof effect as a protective barrier. This characteristic has a more significant influence on sensors and actuators with movable parts. Thus, a defect filling method of micro-nano composite structure is proposed to improve the waterproof ability of parylene coatings. The defect filling method is composed of a nano layer of Al2O3 molecules and a micro layer of parylene polymer. Based on the diffusion mechanism of water molecules in the polymer membrane, defects on the surface of polymer encapsulation will be filled and decomposed into smaller areas by Al2O3 nanoparticles to delay or hinder the penetration of water molecules. Accordingly, the dense Al2O3 nanoparticles are utilized to fill and repair the surface of the organic polymer by low-rate atomic layer deposition. This paper takes the pressure sensor as an example to carry out the corresponding research. Experimental results show that the proposed method is very effective and the encapsulated sensors work properly in a saline solution after a period of time equivalent to 153.9 days in body temperature, maintaining their accuracy and precision of 2 mmHg. Moreover, the sensors could improve accuracy by about 43% after the proposed encapsulation. Therefore, the water molecule anti-permeability encapsulation would have broad application prospects in micro/nano-device protection.

Classifying Intracortical Brain-Machine Interface Signal Disruptions Based on System Performance and Applicable Compensatory Strategies: A Review

Brain-machine interfaces (BMIs) record and translate neural activity into a control signal for assistive or other devices. Intracortical microelectrode arrays (MEAs) enable high degree-of-freedom BMI control for complex tasks by providing fine-resolution neural recording. However, chronically implanted MEAs are subject to a dynamic in vivo environment where transient or systematic disruptions can interfere with neural recording and degrade BMI performance. Typically, neural implant failure modes have been categorized as biological, material, or mechanical. While this categorization provides insight into a disruption's causal etiology, it is less helpful for understanding degree of impact on BMI function or possible strategies for compensation. Therefore, we propose a complementary classification framework for intracortical recording disruptions that is based on duration of impact on BMI performance and requirement for and responsiveness to interventions: (1) Transient disruptions interfere with recordings on the time scale of minutes to hours and can resolve spontaneously; (2) Reversible disruptions cause persistent interference in recordings but the root cause can be remedied by an appropriate intervention; (3) Irreversible compensable disruptions cause persistent or progressive decline in signal quality, but their effects on BMI performance can be mitigated algorithmically; and (4) Irreversible non-compensable disruptions cause permanent signal loss that is not amenable to remediation or compensation. This conceptualization of intracortical BMI disruption types is useful for highlighting specific areas for potential hardware improvements and also identifying opportunities for algorithmic interventions. We review recording disruptions that have been reported for MEAs and demonstrate how biological, material, and mechanical mechanisms of disruption can be further categorized according to their impact on signal characteristics. Then we discuss potential compensatory protocols for each of the proposed disruption classes. Specifically, transient disruptions may be minimized by using robust neural decoder features, data augmentation methods, adaptive machine learning models, and specialized signal referencing techniques. Statistical Process Control methods can identify reparable disruptions for rapid intervention. In-vivo diagnostics such as impedance spectroscopy can inform neural feature selection and decoding models to compensate for irreversible disruptions. Additional compensatory strategies for irreversible disruptions include information salvage techniques, data augmentation during decoder training, and adaptive decoding methods to down-weight damaged channels.

Massively parallel microwire arrays integrated with CMOS chips for neural recording

Multi-channel electrical recordings of neural activity in the brain is an increasingly powerful method revealing new aspects of neural communication, computation, and prosthetics. However, while planar silicon-based CMOS devices in conventional electronics scale rapidly, neural interface devices have not kept pace. Here, we present a new strategy to interface silicon-based chips with three-dimensional microwire arrays, providing the link between rapidly-developing electronics and high density neural interfaces. The system consists of a bundle of microwires mated to large-scale microelectrode arrays, such as camera chips. This system has excellent recording performance, demonstrated via single unit and local-field potential recordings in isolated retina and in the motor cortex or striatum of awake moving mice. The modular design enables a variety of microwire types and sizes to be integrated with different types of pixel arrays, connecting the rapid progress of commercial multiplexing, digitisation and data acquisition hardware together with a three-dimensional neural interface.

A softening laminar electrode for recording single unit activity from the rat hippocampus

Softening neural implants that change their elastic modulus under physiological conditions are promising candidates to mitigate neuroinflammatory response due to the reduced mechanical mismatch between the artificial interface and the brain tissue. Intracortical neural probes have been used to demonstrate the viability of this material engineering approach. In our paper, we present a robust technology of softening neural microelectrode and demonstrate its recording performance in the hippocampus of rat subjects. The 5 mm long, single shank, multi-channel probes are composed of a custom thiol-ene/acrylate thermoset polymer substrate, and were micromachined by standard MEMS processes. A special packaging technique is also developed, which guarantees the stable functionality and longevity of the device, which were tested under in vitro conditions prior to animal studies. The 60 micron thick device was successfully implanted to 4.5 mm deep in the hippocampus without the aid of any insertion shuttle. Spike amplitudes of 84 µV peak-to-peak and signal-to-noise ratio of 6.24 were achieved in acute experiments. Our study demonstrates that softening neural probes may be used to investigate deep layers of the rat brain.

Advances in Penetrating Multichannel Microelectrodes Based on the Utah Array Platform

The Utah electrode array (UEA) and its many derivatives have become a gold standard for high-channel count bi-directional neural interfaces, in particular in human subject applications. The chapter provides a brief overview of leading electrode concepts and the context in which the UEA has to be understood. It goes on to discuss the key advances and developments of the UEA platform in the past 15 years, as well as novel wireless and system integration technologies that will merge into future generations of fully integrated devices. Aspects covered include novel device architectures that allow scaling of channel count and density of electrode contacts, material improvements to substrate, electrode contacts, and encapsulation. Further subjects are adaptations of the UEA platform to support IR and optogenetic simulation as well as an improved understanding of failure modes and methods to test and accelerate degradation in vitro such as to better predict device failure and lifetime in vivo.

Techniques and Considerations in the Microfabrication of Parylene C Microelectromechanical Systems

Parylene C is a promising material for constructing flexible, biocompatible and corrosion-resistant microelectromechanical systems (MEMS) devices. Historically, Parylene C has been employed as an encapsulation material for medical implants, such as stents and pacemakers, due to its strong barrier properties and biocompatibility. In the past few decades, the adaptation of planar microfabrication processes to thin film Parylene C has encouraged its use as an insulator, structural and substrate material for MEMS and other microelectronic devices. However, Parylene C presents unique challenges during microfabrication and during use with liquids, especially for flexible, thin film electronic devices. In particular, the flexibility and low thermal budget of Parylene C require modification of the fabrication techniques inherited from silicon MEMS, and poor adhesion at Parylene-Parylene and Parylene-metal interfaces causes device failure under prolonged use in wet environments. Here, we discuss in detail the promises and challenges inherent to Parylene C and present our experience in developing thin-film Parylene MEMS devices.

The development of neural stimulators: a review of preclinical safety and efficacy studies

OBJECTIVE

Given the rapid expansion of the field of neural stimulation and the rigorous regulatory approval requirements required before these devices can be applied clinically, it is important that there is clarity around conducting preclinical safety and efficacy studies required for the development of this technology.

APPROACH

The present review examines basic design principles associated with the development of a safe neural stimulator and describes the suite of preclinical safety studies that need to be considered when taking a device to clinical trial.

MAIN RESULTS

Neural stimulators are active implantable devices that provide therapeutic intervention, sensory feedback or improved motor control via electrical stimulation of neural or neuro-muscular tissue in response to trauma or disease. Because of their complexity, regulatory bodies classify these devices in the highest risk category (Class III), and they are therefore required to go through a rigorous regulatory approval process before progressing to market. The successful development of these devices is achieved through close collaboration across disciplines including engineers, scientists and a surgical/clinical team, and the adherence to clear design principles. Preclinical studies form one of several key components in the development pathway from concept to product release of neural stimulators. Importantly, these studies provide iterative feedback in order to optimise the final design of the device. Key components of any preclinical evaluation include: in vitro studies that are focussed on device reliability and include accelerated testing under highly controlled environments; in vivo studies using animal models of the disease or injury in order to assess efficacy and, given an appropriate animal model, the safety of the technology under both passive and electrically active conditions; and human cadaver and ex vivo studies designed to ensure the device's form factor conforms to human anatomy, to optimise the surgical approach and to develop any specialist surgical tooling required.

SIGNIFICANCE

The pipeline from concept to commercialisation of these devices is long and expensive; careful attention to both device design and its preclinical evaluation will have significant impact on the duration and cost associated with taking a device through to commercialisation. Carefully controlled in vitro and in vivo studies together with ex vivo and human cadaver trials are key components of a thorough preclinical evaluation of any new neural stimulator.

Insulation of thin-film parylene-C/platinum probes in saline solution through encapsulation in multilayer ALD ceramic films

The long-term electrical leakage performance of parylene-C/platinum/parylene-C (Px/Pt/Px) interconnect in saline is evaluated using electrochemical impedance spectroscopy (EIS). Three kinds of additional ceramic encapsulation layers between the metal and Px are characterized: 50 nm-thick alumina (Al₂O₃), 50 nm-thick titania (TiO₂), and 80 nm-thick Al₂O₃-TiO₂ nanolaminate (NL). The Al₂O₃ and TiO₂ encapsulation layers worsen the overall insulation properties. The NL encapsulation layer improves the insulation when combined with a TiO₂ outer layer to promote adhesion to the Px. Experiments are performed with various insulation promotion treatments: A-174 silane (A174) treatment before Px deposition (to promote adhesion); SF₆ plasma treatment (F) after Px deposition (to increase hydrophobicity); and ion-milling descum (IM) after Px deposition (to prevent parylene oxidation). A174 and F treatments do not have a significant impact, while IM leads to worse insulation performance. A circuit model elucidates the insulation characteristics of Px-ceramic-Pt-ceramic-Px interconnect. These studies provide a foundation for processing ultra-compliant neural probes with long-term chronic utility.

Analysis of Al2O3-parylene C bilayer coatings and impact of microelectrode topography on long term stability of implantable neural arrays

OBJECTIVE

Performance of many dielectric coatings for neural electrodes degrades over time, contributing to loss of neural signals and evoked percepts. Studies using planar test substrates have found that a novel bilayer coating of atomic-layer deposited (ALD) Al₂O₃ and parylene C is a promising candidate for neural electrode applications, exhibiting superior stability to parylene C alone. However, initial results from bilayer encapsulation testing on non-planar devices have been less positive. Our aim was to evaluate ALD Al₂O₃-parylene C coatings using novel test paradigms, to rigorously evaluate dielectric coatings for neural electrode applications by incorporating neural electrode topography into test structure design.

APPROACH

Five test devices incorporated three distinct topographical features common to neural electrodes, derived from the utah electrode array (UEA). Devices with bilayer (52 nm Al₂O₃  +  6 µm parylene C) were evaluated against parylene C controls (N  ⩾  6 per device type). Devices were aged in phosphate buffered saline at 67 °C for up to 311 d, and monitored through: (1) leakage current to evaluate encapsulation lifetimes (>1 nA during 5VDC bias indicated failure), and (2) wideband (1-10⁵ Hz) impedance.

MAIN RESULTS

Mean-times-to-failure (MTTFs) ranged from 12 to 506 d for bilayer-coated devices, versus 10 to  >2310 d for controls. Statistical testing (log-rank test, α  =  0.05) of failure rates gave mixed results but favored the control condition. After failure, impedance loss for bilayer devices continued for months and manifested across the entire spectrum, whereas the effect was self-limiting after several days, and restricted to frequencies  <100 Hz for controls. These results correlated well with observations of UEAs encapsulated with bilayer and control films.

SIGNIFICANCE

We observed encapsulation failure modes and behaviors comparable to neural electrode performance which were undetected in studies with planar test devices. We found the impact of parylene C defects to be exacerbated by ALD Al₂O₃, and conclude that inferior bilayer performance arises from degradation of ALD Al₂O₃ when directly exposed to saline. This is an important consideration, given that neural electrodes with bilayer coatings are expected to have ALD Al₂O₃ exposed at dielectric boundaries that delineate electrode sites. Process improvements and use of different inorganic coatings to decrease dissolution in physiological fluids may improve performance. Testing frameworks which take neural electrode complexities into account will be well suited to reliably evaluate such encapsulation schemes.

A Materials Roadmap to Functional Neural Interface Design

Advancement in neurotechnologies for electrophysiology, neurochemical sensing, neuromodulation, and optogenetics are revolutionizing scientific understanding of the brain while enabling treatments, cures, and preventative measures for a variety of neurological disorders. The grand challenge in neural interface engineering is to seamlessly integrate the interface between neurobiology and engineered technology, to record from and modulate neurons over chronic timescales. However, the biological inflammatory response to implants, neural degeneration, and long-term material stability diminish the quality of interface overtime. Recent advances in functional materials have been aimed at engineering solutions for chronic neural interfaces. Yet, the development and deployment of neural interfaces designed from novel materials have introduced new challenges that have largely avoided being addressed. Many engineering efforts that solely focus on optimizing individual probe design parameters, such as softness or flexibility, downplay critical multi-dimensional interactions between different physical properties of the device that contribute to overall performance and biocompatibility. Moreover, the use of these new materials present substantial new difficulties that must be addressed before regulatory approval for use in human patients will be achievable. In this review, the interdependence of different electrode components are highlighted to demonstrate the current materials-based challenges facing the field of neural interface engineering.

Modulation of collective cell behaviour by geometrical constraints

Intracellular and extracellular mechanical forces play a crucial role during tissue growth, modulating nuclear shape and function and resulting in complex collective cell behaviour. However, the mechanistic understanding of how the orientation, shape, symmetry and homogeneity of cells are affected by environmental geometry is still lacking. Here we investigate cooperative cell behaviour and patterns under geometric constraints created by topographically patterned substrates. We show how cells cooperatively adopt their geometry, shape, positioning of the nucleus and subsequent proliferation activity. Our findings indicate that geometric constraints induce significant squeezing of cells and nuclei, cytoskeleton reorganization, drastic condensation of chromatin resulting in a change in the cell proliferation rate and the anisotropic growth of cultures. Altogether, this work not only demonstrates complex non-trivial collective cellular responses to geometrical constraints but also provides a tentative explanation of the observed cell culture patterns grown on different topographically patterned substrates. These findings provide important fundamental knowledge, which could serve as a basis for better controlled tissue growth and cell-engineering applications.

A novel flexible cuff-like microelectrode for dual purpose, acute and chronic electrical interfacing with the mouse cervical vagus nerve

OBJECTIVE

Neural reflexes regulate immune responses and homeostasis. Advances in bioelectronic medicine indicate that electrical stimulation of the vagus nerve can be used to treat inflammatory disease, yet the understanding of neural signals that regulate inflammation is incomplete. Current interfaces with the vagus nerve do not permit effective chronic stimulation or recording in mouse models, which is vital to studying the molecular and neurophysiological mechanisms that control inflammation homeostasis in health and disease. We developed an implantable, dual purpose, multi-channel, flexible 'microelectrode' array, for recording and stimulation of the mouse vagus nerve.

APPROACH

The array was microfabricated on an 8 µm layer of highly biocompatible parylene configured with 16 sites. The microelectrode was evaluated by studying the recording and stimulation performance. Mice were chronically implanted with devices for up to 12 weeks.

MAIN RESULTS

Using the microelectrode in vivo, high fidelity signals were recorded during physiological challenges (e.g potassium chloride and interleukin-1β), and electrical stimulation of the vagus nerve produced the expected significant reduction of blood levels of tumor necrosis factor (TNF) in endotoxemia. Inflammatory cell infiltration at the microelectrode 12 weeks of implantation was limited according to radial distribution analysis of inflammatory cells.

SIGNIFICANCE

This novel device provides an important step towards a viable chronic interface for cervical vagus nerve stimulation and recording in mice.

Nanofluidic chip for liquid TEM cell fabricated by parylene and silicon nitride direct bonding

Despite the importance of nanofluidic transmission electron microscope (TEM) chips, a simple fabrication method has yet to be developed due to the difficulty of wafer bonding techniques using a nanoscale thick bonding layer. We present a simple and robust wafer scale bonding technique using parylene as a bonding layer. A nanoscale thick parylene layer was deposited on a silicon nitride (SiN) wafer and patterned to construct nanofluidic channels. The patterned parylene layer was directly bonded to another SiN wafer by thermal surface activation and bonding, with a bonding strength of ∼3 MPa. Fourier transform infrared spectroscopy showed that carbon-oxygen bonds were generated by thermal activation. We demonstrated TEM imaging of gold nanoparticles suspended in liquid using the fabricated nanofluidic chip.

Long-term characterization of neural electrodes based on parylene-caulked polydimethylsiloxane substrate

This study investigates the mechanical and long-term electrical properties of parylene-caulked polydimethylsiloxane (PDMS) as a substrate for implantable electrodes. The parylene-caulked PDMS is a structure where particles of parylene fill the porous surface of PDMS. This material is expected to have low water absorption and desirable mechanical properties such as flexibility and elasticity that are beneficial in many biomedical applications. To evaluate the mechanical property and electrical stability of parylene-caulked PDMS for potential in-vivo uses, tensile tests were conducted firstly, which results showed that the mechanical strength of parylene-caulked PDMS was comparable to that of native PDMS. Next, surface electrodes based on parylene-caulked PDMS were fabricated and their impedance was measured in phosphate-buffered saline (PBS) solution at 36.5 °C over seven months. The electrodes based on parylene-caulked PDMS exhibited the improved stability in impedance over time than native PDMS. Thus, with improved electrical stability in wet environment and preserved mechanical properties of PDMS, the electrodes based on parylene-caulked PDMS are expected to be suitable for long-term in-vivo applications.

Neuroadhesive L1 coating attenuates acute microglial attachment to neural electrodes as revealed by live two-photon microscopy

Implantable neural electrode technologies for chronic neural recordings can restore functional control to paralysis and limb loss victims through brain-machine interfaces. These probes, however, have high failure rates partly due to the biological responses to the probe which generate an inflammatory scar and subsequent neuronal cell death. L1 is a neuronal specific cell adhesion molecule and has been shown to minimize glial scar formation and promote electrode-neuron integration when covalently attached to the surface of neural probes. In this work, the acute microglial response to L1-coated neural probes was evaluated in vivo by implanting coated devices into the cortex of mice with fluorescently labeled microglia, and tracking microglial dynamics with multi-photon microscopy for the ensuing 6 h in order to understand L1's cellular mechanisms of action. Microglia became activated immediately after implantation, extending processes towards both L1-coated and uncoated control probes at similar velocities. After the processes made contact with the probes, microglial processes expanded to cover 47.7% of the control probes' surfaces. For L1-coated probes, however, there was a statistically significant 83% reduction in microglial surface coverage. This effect was sustained through the experiment. At 6 h post-implant, the radius of microglia activation was reduced for the L1 probes by 20%, shifting from 130.0 to 103.5 μm with the coating. Microglia as far as 270 μm from the implant site displayed significantly lower morphological characteristics of activation for the L1 group. These results suggest that the L1 surface treatment works in an acute setting by microglial mediated mechanisms.

Histopathological Effects of Parylene C (poly-chloro-p-xylylene) in the Inner Ear

Objective

To assess the histopathological effects of parylene C (PC) (poly-chloro-p-xylylene) in the inner ear.

Methods

Nine adult Dunkin Hartley guinea pigs (500-600 g) were included in the study. PC pieces were inserted into the cochlea in the right ear of the animals (study group). The round windows were punctured in the left ears comprised the control group. After three months, the animals were sacrificed, and the dissected temporal bones were examined under a light microscope.

Results

No significant difference was revealed between the study and control groups regarding histopathological findings such as perineural congestion, perineural inflammation, neural fibrosis, number of ganglion cells, edema, and degeneration of ganglion cells (p>0.05).

Conclusion

PC did not cause any additional histopathologic damage in the cochlea. This finding may be promising regarding the use of PC in cochlear implant electrodes as an alternative to silicon materials in the future.

Scanning electron microscopy of chronically implanted intracortical microelectrode arrays in non-human primates

OBJECTIVE

Signal attenuation is a major problem facing intracortical sensors for chronic neuroprosthetic applications. Many studies suggest that failure is due to gliosis around the electrode tips, however, mechanical and material causes of failure are often overlooked. The purpose of this study was to investigate the factors contributing to progressive signal decline by using scanning electron microscopy (SEM) to visualize structural changes in chronically implanted arrays and histology to examine the tissue response at corresponding implant sites.

APPROACH

We examined eight chronically implanted intracortical microelectrode arrays (MEAs) explanted from non-human primates at times ranging from 37 to 1051 days post-implant. We used SEM, in vivo neural recordings, and histology (GFAP, Iba-1, NeuN). Three MEAs that were never implanted were also imaged as controls.

MAIN RESULTS

SEM revealed progressive corrosion of the platinum electrode tips and changes to the underlying silicon. The parylene insulation was prone to cracking and delamination, and in some instances the silicone elastomer also delaminated from the edges of the MEA. Substantial tissue encapsulation was observed and was often seen growing into defects in the platinum and parylene. These material defects became more common as the time in vivo increased. Histology at 37 and 1051 days post-implant showed gliosis, disruption of normal cortical architecture with minimal neuronal loss, and high Iba-1 reactivity, especially within the arachnoid and dura. Electrode tracts were either absent or barely visible in the cortex at 1051 days, but were seen in the fibrotic encapsulation material suggesting that the MEAs were lifted out of the brain. Neural recordings showed a progressive drop in impedance, signal amplitude, and viable channels over time.

SIGNIFICANCE

These results provide evidence that signal loss in MEAs is truly multifactorial. Gliosis occurs in the first few months after implantation but does not prevent useful recordings for several years. Progressive meningeal fibrosis encapsulates and lifts MEAs out of the cortex while ongoing foreign body reactions lead to progressive degradation of the materials. Long-term impedance drops are due to the corrosion of platinum, cracking and delamination of parylene, and delamination of silicone elastomer. Oxygen radicals released by cells of the immune system likely mediate the degradation of these materials. Future MEA designs must address these problems through more durable insulation materials, more inert electrode alloys, and pharmacologic suppression of fibroblasts and leukocytes.

Effect of bias voltage and temperature on lifetime of wireless neural interfaces with Al ₂O₃ and parylene bilayer encapsulation

The lifetime of neural interfaces is a critical challenge for chronic implantations, as therapeutic devices (e.g., neural prosthetics) will require decades of lifetime. We evaluated the lifetime of wireless Utah electrode array (UEA) based neural interfaces with a bilayer encapsulation scheme utilizing a combination of alumina deposited by Atomic Layer Deposition (ALD) and parylene C. Wireless integrated neural interfaces (INIs), equipped with recording version 9 (INI-R9) ASIC chips, were used to monitor the encapsulation performance through radio-frequency (RF) power and telemetry. The wireless devices were encapsulated with 52 nm of ALD Al2O3 and 6 μm of parylene C, and tested by soaking in phosphate buffered solution (PBS) at 57 °C for 4× accelerated lifetime testing. The INIs were also powered continuously through 2.765 MHz inductive power and forward telemetry link at unregulated 5 V. The bilayer encapsulated INIs were fully functional for ∼35 days (140 days at 37 °C equivalent) with consistent power-up frequencies (∼910 MHz), stable RF signal (∼-75 dBm), and 100 % command reception rate. This is ∼10 times of equivalent lifetime of INIs with parylene-only encapsulation (13 days) under same power condition at 37 °C. The bilayer coated INIs without continuous powering lasted over 1860 equivalent days (still working) at 37 °C. Those results suggest that bias stress is a significant factor to accelerate the failure of the encapsulated devices. The INIs failed completely within 5 days of the initial frequency shift of RF signal at 57 °C, which implied that the RF frequency shift is an early indicator of encapsulation/device failure.

Osteogenic Surface Modification Based on Functionalized Poly-P-Xylylene Coating

The biotechnology to immobilize biomolecules on material surfaces has been developed vigorously due to its high potentials in medical applications. In this study, a simple and effective method was designed to immobilize biomolecules via amine-N-hydroxysuccinimide (NHS) ester conjugation reaction using functionalized poly-p-xylylene coating on material surfaces. The NHS ester functionalized coating is synthesized via chemical vapor deposition, a facile and solvent-less method, creating a surface which is ready to perform a one-step conjugation reaction. Bone morphogenetic protein 2 (BMP-2) is immobilized onto material surfaces by this coating method, forming an osteogenic environment. The immobilization process is controlled at a low temperature which does not damage proteins. This modified surface induces differentiation of preosteoblast into osteoblast, manifested by alkaline phosphatase (ALP) activity assay, Alizarin Red S (ARS) staining and the expression of osteogenic gene markers, Alpl and Bglap3. With this coating technology, immobilization of growth factors onto material surface can be achieved more simply and more effectively.

Parylene-coated ionic liquid-carbon nanotube actuators for user-safe haptic devices

Simple fabrication, high power-to-weight and power-to-volume ratios, and the ability to operate in open air at low voltage make the ionic electroactive polymer actuators highly attractive for haptic applications. Whenever a direct tactile stimulation of the skin is involved, electrical and chemical insulation as well as a long-term stability of the actuator are required. Because of its inherent physicochemical properties such as high dielectric strength, resistance to solvents, and biological inactivity, Parylene C meets the requirements for making biocompatible actuators. We have studied the displacement and the generated force of Parylene-coated carbon nanotube actuators as well as the encapsulation quality. A 2 μm coating creates an effective electrical insulation of the actuators without altering the blocking force at frequencies from 50 mHz to 1 Hz. Moreover, the generated strain is preserved at higher frequencies (from 0.5 to 5 Hz). We employed a simple mechanical model to explain the relation between the key parameters-flexural stiffness, displacement, and force-for uncoated and coated actuators. In addition, we demonstrated that our Parylene-coated actuators are not damaged by rinsing in liquid media such as 2-propanol or water. In conclusion, our results indicate that Parylene C encapsulated actuators are safe to touch and can be used in contact with human skin and in biomedical applications in direct contact with tissues and physiological fluids.

Selective and Reversible Binding of Thiol-Functionalized Biomolecules on Polymers Prepared via Chemical Vapor Deposition Polymerization

We use chemical vapor deposition polymerization to prepare a novel dibromomaleimide-functionalized polymer for selective and reversible binding of thiol-containing biomolecules on a broad range of substrates. We report the synthesis and CVD polymerization of 4-(3,4-dibromomaleimide)[2.2]paracyclophane to yield nanometer thick polymer coatings. Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the chemical composition of the polymer coating. The reactivity of the polymer coating toward thiol-functionalized molecules was confirmed using fluorescent ligands. As a proof of concept, the binding and subsequent release of cysteine-modified peptides from the polymer coating were also demonstrated via sum frequency generation spectroscopy. This reactive polymer coating provides a flexible surface modification approach to selectively and reversibly bind biomolecules on a broad range of materials, which could open up new opportunities in many biomedical sensing and diagnostic applications where specific binding and release of target analytes are desired.

Rapid evaluation of the durability of cortical neural implants using accelerated aging with reactive oxygen species

OBJECTIVE

A challenge for implementing high bandwidth cortical brain-machine interface devices in patients is the limited functional lifespan of implanted recording electrodes. Development of implant technology currently requires extensive non-clinical testing to demonstrate device performance. However, testing the durability of the implants in vivo is time-consuming and expensive. Validated in vitro methodologies may reduce the need for extensive testing in animal models.

APPROACH

Here we describe an in vitro platform for rapid evaluation of implant stability. We designed a reactive accelerated aging (RAA) protocol that employs elevated temperature and reactive oxygen species (ROS) to create a harsh aging environment. Commercially available microelectrode arrays (MEAs) were placed in a solution of hydrogen peroxide at 87 °C for a period of 7 days. We monitored changes to the implants with scanning electron microscopy and broad spectrum electrochemical impedance spectroscopy (1 Hz-1 MHz) and correlated the physical changes with impedance data to identify markers associated with implant failure.

MAIN RESULTS

RAA produced a diverse range of effects on the structural integrity and electrochemical properties of electrodes. Temperature and ROS appeared to have different effects on structural elements, with increased temperature causing insulation loss from the electrode microwires, and ROS concentration correlating with tungsten metal dissolution. All array types experienced impedance declines, consistent with published literature showing chronic (>30 days) declines in array impedance in vivo. Impedance change was greatest at frequencies <10 Hz, and smallest at frequencies 1 kHz and above. Though electrode performance is traditionally characterized by impedance at 1 kHz, our results indicate that an impedance change at 1 kHz is not a reliable predictive marker of implant degradation or failure.

SIGNIFICANCE

ROS, which are known to be present in vivo, can create structural damage and change electrical properties of MEAs. Broad-spectrum electrical impedance spectroscopy demonstrates increased sensitivity to electrode damage compared with single-frequency measurements. RAA can be a useful tool to simulate worst-case in vivo damage resulting from chronic electrode implantation, simplifying the device development lifecycle.

Mechanical failure modes of chronically implanted planar silicon-based neural probes for laminar recording

Penetrating intracortical electrode arrays that record brain activity longitudinally are powerful tools for basic neuroscience research and emerging clinical applications. However, regardless of the technology used, signals recorded by these electrodes degrade over time. The failure mechanisms of these electrodes are understood to be a complex combination of the biological reactive tissue response and material failure of the device over time. While mechanical mismatch between the brain tissue and implanted neural electrodes have been studied as a source of chronic inflammation and performance degradation, the electrode failure caused by mechanical mismatch between different material properties and different structural components within a device have remained poorly characterized. Using Finite Element Model (FEM) we simulate the mechanical strain on a planar silicon electrode. The results presented here demonstrate that mechanical mismatch between iridium and silicon leads to concentrated strain along the border of the two materials. This strain is further focused on small protrusions such as the electrical traces in planar silicon electrodes. These findings are confirmed with chronic in vivo data (133-189 days) in mice by correlating a combination of single-unit electrophysiology, evoked multi-unit recordings, electrochemical impedance spectroscopy, and scanning electron microscopy from traces and electrode sites with our modeling data. Several modes of mechanical failure of chronically implanted planar silicon electrodes are found that result in degradation and/or loss of recording. These findings highlight the importance of strains and material properties of various subcomponents within an electrode array.

Long-term reliability of Al2O3 and Parylene C bilayer encapsulated Utah electrode array based neural interfaces for chronic implantation

OBJECTIVE

We focus on improving the long-term stability and functionality of neural interfaces for chronic implantation by using bilayer encapsulation.

APPROACH

We evaluated the long-term reliability of Utah electrode array (UEA) based neural interfaces encapsulated by 52 nm of atomic layer deposited Al2O3 and 6 µm of Parylene C bilayer, and compared these to devices with the baseline Parylene-only encapsulation. Three variants of arrays including wired, wireless, and active UEAs were used to evaluate this bilayer encapsulation scheme, and were immersed in phosphate buffered saline (PBS) at 57 °C for accelerated lifetime testing.

MAIN RESULTS

The median tip impedance of the bilayer encapsulated wired UEAs increased from 60 to 160 kΩ during the 960 days of equivalent soak testing at 37 °C, the opposite trend to that typically observed for Parylene encapsulated devices. The loss of the iridium oxide tip metallization and etching of the silicon tip in PBS solution contributed to the increase of impedance. The lifetime of fully integrated wireless UEAs was also tested using accelerated lifetime measurement techniques. The bilayer coated devices had stable power-up frequencies at ∼910 MHz and constant radio-frequency signal strength of -50 dBm during up to 1044 days (still under testing) of equivalent soaking time at 37 °C. This is a significant improvement over the lifetime of ∼100 days achieved with Parylene-only encapsulation at 37 °C. The preliminary samples of bilayer coated active UEAs with a flip-chip bonded ASIC chip had a steady current draw of ∼3 mA during 228 days of soak testing at 37 °C. An increase in the current draw has been consistently correlated to device failures, so is a sensitive metric for their lifetime.

SIGNIFICANCE

The trends of increasing electrode impedance of wired devices and performance stability of wireless and active devices support the significantly greater encapsulation performance of this bilayer encapsulation compared with Parylene-only encapsulation. The bilayer encapsulation should significantly improve the in vivo lifetime of neural interfaces for chronic implantation.