Long-term evaluation of a liquid crystal polymer (LCP)-based retinal prosthesis

Laser-Assisted Structuring of Graphene Films with Biocompatible Liquid Crystal Polymer for Skin/Brain-Interfaced Electrodes

The work presented here introduces a facile strategy for the development of flexible and stretchable electrodes that harness the robust characteristics of carbon nanomaterials through laser processing techniques on a liquid crystal polymer (LCP) film. By utilizing LCP film as a biocompatible electronic substrate, control is demonstrated over the laser irradiation parameters to achieve efficient pattern generation and transfer printing processes, thereby yielding highly conductive laser-induced graphene (LIG) bioelectrodes. To enhance the resolution of the patterned LIG film, shadow masks are employed during laser scanning on the LCP film surface. This approach is compatible with surface-mounted device integration, enabling the circuit writing of LIG/LCP materials in a flexible format. Moreover, kirigami-inspired on-skin bioelectrodes are introduced that exhibit reasonable stretchability, enabling independent connections to healthcare hardware platforms for electrocardiogram (ECG) and electromyography (EMG) measurements. Additionally, a brain-interfaced LIG microelectrode array is proposed that combines mechanically compliant architectures with LCP encapsulation for stimulation and recording purposes, leveraging their advantageous structural features and superior electrochemical properties. This developed approach offers a cost-effective and scalable route for producing patterned arrays of laser-converted graphene as bioelectrodes. These bioelectrodes serve as ideal circuit-enabled flexible substrates with long-term reliability in the ionic environment of the human body.

Liquid Crystal Polymer-Based Miniaturized Fully Implantable Deep Brain Stimulator

A significant challenge in improving the deep brain stimulation (DBS) system is the miniaturization of the device, aiming to integrate both the stimulator and the electrode into a compact unit with a wireless charging capability to reduce invasiveness. We present a miniaturized, fully implantable, and battery-free DBS system designed for rats, using a liquid crystal polymer (LCP), a biocompatible and long-term reliable material. The system integrates the simulator circuit, the receiver coil, and a 20 mm long depth-type microelectrode array in a dome-shaped LCP package that is 13 mm in diameter and 5 mm in height. Wireless powering and control via an inductive link enable device miniaturization, allowing for full implantation and, thus, the free behavior of untethered animals. The eight-channel stimulation electrode array was microfabricated on an LCP substrate to form a multilayered system substrate, which was monolithically encapsulated by a domed LCP lid using a specialized spot-welding process. The device functionality was validated via an in vivo animal experiment using a neuropathic pain model in rats. This experiment demonstrated an increase in the mechanical withdrawal threshold of the rats with microelectrical stimulation delivered using the fully implanted device, highlighting the effectiveness of the system.

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.

Flexural bending to approximate cortical forces exerted by electrocorticography (ECoG) arrays

Objective.The force that an electrocorticography (ECoG) array exerts on the brain manifests when it bends to match the curvature of the skull and cerebral cortex. This force can negatively impact both short-term and long-term patient outcomes. Here we provide a mechanical characterization of a novel liquid crystal polymer (LCP) ECoG array prototype to demonstrate that its thinner geometry reduces the force potentially applied to the cortex of the brain.Approach.We built a low-force flexural testing machine to measure ECoG array bending forces, calculate their effective flexural moduli, and approximate the maximum force they could exerted on the human brain.Main results.The LCP ECoG prototype was found to have a maximal force less than 20% that of any commercially available ECoG arrays that were tested. However, as a material, LCP was measured to be as much as 24× more rigid than silicone, which is traditionally used in ECoG arrays. This suggests that the lower maximal force resulted from the prototype's thinner profile (2.9×-3.25×).Significance.While decreasing material stiffness can lower the force an ECoG array exhibits, our LCP ECoG array prototype demonstrated that flexible circuit manufacturing techniques can also lower these forces by decreasing ECoG array thickness. Flexural tests of ECoG arrays are necessary to accurately assess these forces, as material properties for polymers and laminates are often scale dependent. As the polymers used are anisotropic, elastic modulus cannot be used to predict ECoG flexural behavior. Accounting for these factors, we used our four-point flexure testing procedure to quantify the forces exerted on the brain by ECoG array bending. With this experimental method, ECoG arrays can be designed to minimize force exerted on the brain, potentially improving both acute and chronic clinical utility.

Recent Advances in Encapsulation of Flexible Bioelectronic Implants: Materials, Technologies, and Characterization Methods

Bioelectronic implantable systems (BIS) targeting biomedical and clinical research should combine long-term performance and biointegration in vivo. Here, recent advances in novel encapsulations to protect flexible versions of such systems from the surrounding biological environment are reviewed, focusing on material strategies and synthesis techniques. Considerable effort is put on thin-film encapsulation (TFE), and specifically organic-inorganic multilayer architectures as a flexible and conformal alternative to conventional rigid cans. TFE is in direct contact with the biological medium and thus must exhibit not only biocompatibility, inertness, and hermeticity but also mechanical robustness, conformability, and compatibility with the manufacturing of microfabricated devices. Quantitative characterization methods of the barrier and mechanical performance of the TFE are reviewed with a particular emphasis on water-vapor transmission rate through electrical, optical, or electrochemical principles. The integrability and functionalization of TFE into functional bioelectronic interfaces are also discussed. TFE represents a must-have component for the next-generation bioelectronic implants with diagnostic or therapeutic functions in human healthcare and precision medicine.

Thin Film Encapsulation for LCP-Based Flexible Bioelectronic Implants: Comparison of Different Coating Materials Using Test Methodologies for Life-Time Estimation

Liquid crystal polymer (LCP) has gained wide interest in the electronics industry largely due to its flexibility, stable insulation and dielectric properties and chip integration capabilities. Recently, LCP has also been investigated as a biocompatible substrate for the fabrication of multielectrode arrays. Realizing a fully implantable LCP-based bioelectronic device, however, still necessitates a low form factor packaging solution to protect the electronics in the body. In this work, we investigate two promising encapsulation coatings based on thin-film technology as the main packaging for LCP-based electronics. Specifically, a HfO2–based nanolaminate ceramic (TFE1) deposited via atomic layer deposition (ALD), and a hybrid Parylene C-ALD multilayer stack (TFE2), both with a silicone finish, were investigated and compared to a reference LCP coating. T-peel, water-vapour transmission rate (WVTR) and long-term electrochemical impedance spectrometry (EIS) tests were performed to evaluate adhesion, barrier properties and overall encapsulation performance of the coatings. Both TFE materials showed stable impedance characteristics while submerged in 60 °C saline, with TFE1-silicone lasting more than 16 months under a continuous 14V DC bias (experiment is ongoing). The results presented in this work show that WVTR is not the main factor in determining lifetime, but the adhesion of the coating to the substrate materials plays a key role in maintaining a stable interface and thus longer lifetimes.

A Study on Biocompatible Polymer-Based Packaging of Neural Interface for Chronic Implantation

This paper proposed and verified the use of polymer-based packaging to implement the chronic implantation of neural interfaces using a combination of a commercial thermal epoxy and a thin parylene film. The packaging’s characteristics and the performance of the vulnerable interface between the thermal epoxy layer and polyimide layer, which is mainly used for neural electrodes and an FPCB, were evaluated through in vitro, in vivo, and acceleration experiments. The performance of neural interfaces—composed of the combination of the thermal epoxy and thin parylene film deposition as encapsulation packaging—was evaluated by using signal acquisition experiments based on artificial stimulation signal transmissions through in vitro and in vivo experiments. It has been found that, when commercial thermal epoxy normally cured at room temperature was cured at higher temperatures of 45 °C and 65 °C, not only is its lifetime increased with about twice the room-temperature-based curing conditions but also an interfacial adhesion is higher with more than twice the room-temperature-based curing conditions. In addition, through in vivo experiments using rats, it was confirmed that bodily fluids did not flow into the interface between the thermal epoxy and FPCB for up to 18 months, and it was verified that the rats maintained healthy conditions without occurring an immune response in the body to the thin parylene film deposition on the packaging’s surface.

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.

Achieving long-term stability of thin-film electrodes for neurostimulation

Implantable electrodes that can reliably measure brain activity and deliver an electrical stimulus to a target tissue are increasingly employed to treat various neurological diseases and neuropsychiatric disorders. Flexible thin-film electrodes have gained attention over the past few years to minimise invasiveness and damage upon implantation. Research has previously focused on optimising the electrode's electrical and mechanical properties; however, their chronic stability must be validated to translate electrodes from a research to a clinical application. Neurostimulation electrodes, which actively inject charge, have yet to reliably demonstrate continuous functionality for ten years or more in vivo, the accepted metric for clinical viability. Long-term stability can only be achieved if the focus switches to investigating how and why such devices fail. Unfortunately, there is a field-wide reluctance to investigate device stability and failures, which hinders device optimisation. This review surveys thin-film electrode designs with a focus on adhesion between electrode layers and the interactions with the surrounding environment. A comprehensive summary of the abiotic failure modes faced by such electrodes is presented, and to encourage investigation, systematic methods for analysing their origin are recommended. Finally, approaches to reducing the likelihood of device failure are offered. STATEMENT OF SIGNIFICANCE: Neural electrodes that can deliver an electrical stimulus to a target tissue are widely used to treat various neurological diseases. Essential to the function of these electrodes is the ability to safely stimulate the target tissue for extended periods (> 10 years); however, this has not yet been clinically achieved. The key to achieving long-term stability is an increased understanding of electrode interactions with the surrounding tissue and subsequent systematic analysis of their failure modes. This review highlights the need for a change in the approach to investigating electrode failure, and in doing so summarizes the common ways in which neural electrodes fail, methods for identifying them and approaches to preventing them.

A Fully Implantable Miniaturized Liquid Crystal Polymer (LCP)-Based Spinal Cord Stimulator for Pain Control

Spinal cord stimulation is a therapy to treat the severe neuropathic pain by suppressing the pain signal via electrical stimulation of the spinal cord. The conventional metal packaged and battery-operated implantable pulse generator (IPG) produces electrical pulses to stimulate the spinal cord. Despite its stable operation after implantation, the implantation site is limited due to its bulky size and heavy weight. Wireless communications including wireless power charging is also restricted, which is mainly attributed to the electromagnetic shielding of the metal package. To overcome these limitations, here, we developed a fully implantable miniaturized spinal cord stimulator based on a biocompatible liquid crystal polymer (LCP). The fabrication of electrode arrays in the LCP substrate and monolithically encapsulating the circuitries using LCP packaging reduces the weight (0.4 g) and the size (the width, length, and thickness are 25.3, 9.3, and 1.9 mm, respectively). An inductive link was utilized to wirelessly transfer the power and the data to implanted circuitries to generate the stimulus pulse. Prior to implantation of the device, operation of the pulse generator was evaluated, and characteristics of stimulation electrode such as an electrochemical impedance spectroscopy (EIS) were measured. The LCP-based spinal cord stimulator was implanted into the spared nerve injury rat model. The degree of pain suppression upon spinal cord stimulation was assessed via the Von Frey test where the mechanical stimulation threshold was evaluated by monitoring the paw withdrawal responses. With no spinal cord stimulation, the mechanical stimulation threshold was observed as 1.47 ± 0.623 g, whereas the stimulation threshold was increased to 12.7 ± 4.00 g after spinal cord stimulation, confirming the efficacy of pain suppression via electrical stimulation of the spinal cord. This LCP-based spinal cord stimulator opens new avenues for the development of a miniaturized but still effective spinal cord stimulator.

Flexible, high-resolution thin-film electrodes for human and animal neural research

Objective.Brain functions such as perception, motor control, learning, and memory arise from the coordinated activity of neuronal assemblies distributed across multiple brain regions. While major progress has been made in understanding the function of individual neurons, circuit interactions remain poorly understood. A fundamental obstacle to deciphering circuit interactions is the limited availability of research tools to observe and manipulate the activity of large, distributed neuronal populations in humans. Here we describe the development, validation, and dissemination of flexible, high-resolution, thin-film (TF) electrodes for recording neural activity in animals and humans.Approach.We leveraged standard flexible printed-circuit manufacturing processes to build high-resolution TF electrode arrays. We used biocompatible materials to form the substrate (liquid crystal polymer; LCP), metals (Au, PtIr, and Pd), molding (medical-grade silicone), and 3D-printed housing (nylon). We designed a custom, miniaturized, digitizing headstage to reduce the number of cables required to connect to the acquisition system and reduce the distance between the electrodes and the amplifiers. A custom mechanical system enabled the electrodes and headstages to be pre-assembled prior to sterilization, minimizing the setup time required in the operating room. PtIr electrode coatings lowered impedance and enabled stimulation. High-volume, commercial manufacturing enables cost-effective production of LCP-TF electrodes in large quantities.Main Results. Our LCP-TF arrays achieve 25× higher electrode density, 20× higher channel count, and 11× reduced stiffness than conventional clinical electrodes. We validated our LCP-TF electrodes in multiple human intraoperative recording sessions and have disseminated this technology to >10 research groups. Using these arrays, we have observed high-frequency neural activity with sub-millimeter resolution.Significance.Our LCP-TF electrodes will advance human neuroscience research and improve clinical care by enabling broad access to transformative, high-resolution electrode arrays.

The Future of Neuroscience: Flexible and Wireless Implantable Neural Electronics

Neurological diseases are a prevalent cause of global mortality and are of growing concern when considering an ageing global population. Traditional treatments are accompanied by serious side effects including repeated treatment sessions, invasive surgeries, or infections. For example, in the case of deep brain stimulation, large, stiff, and battery powered neural probes recruit thousands of neurons with each pulse, and can invoke a vigorous immune response. This paper presents challenges in engineering and neuroscience in developing miniaturized and biointegrated alternatives, in the form of microelectrode probes. Progress in design and topology of neural implants has shifted the goal post toward highly specific recording and stimulation, targeting small groups of neurons and reducing the foreign body response with biomimetic design principles. Implantable device design recommendations, fabrication techniques, and clinical evaluation of the impact flexible, integrated probes will have on the treatment of neurological disorders are provided in this report. The choice of biocompatible material dictates fabrication techniques as novel methods reduce the complexity of manufacture. Wireless power, the final hurdle to truly implantable neural interfaces, is discussed. These aspects are the driving force behind continued research: significant breakthroughs in any one of these areas will revolutionize the treatment of neurological disorders.

Evaluation methods for long-term reliability of polymer-based implantable biomedical devices

Long-term reliability of implantable biomedical devices is a critical issue for their practical usefulness and successful translation into clinical application. Reliability is particularly of great concern for recently demonstrated devices based on new materials typically relying on polymeric thin films and microfabrication process. While reliability testing protocol has been well-established for traditional metallic packages, common evaluation methods for polymer-based microdevices has yet to be agreed upon, even though various testing methods have been proposed. This article is aiming to summarize the evaluation methods on long-term reliability of emerging biomedical implants based on polymeric thin-films in terms of their theories and implementation with specific focus on difference from the traditional metallic packages.

Liquid Crystalline Polymers: Opportunities to Shape Neural Interfaces

OBJECTIVES

Polymers have emerged as constituent materials for the creation of microscale neural interfaces; however, limitations regarding water permeability, delamination, and material degradation impact polymeric device robustness. Liquid crystal polymers (LCPs) have molecular order like a solid but with the fluidity of a liquid, resulting in a unique material, with properties including low water permeability, chemical inertness, and mechanical toughness. The objective of this article is to review the state-of-the-art regarding the use of LCPs in neural interface applications and discuss challenges and opportunities where this class of materials can advance the field of neural interfaces.

MATERIALS AND METHODS

This review article focuses on studies that leverage LCP materials to interface with the nervous system in vivo. A comprehensive literature search was performed using PubMed, Web of Science (Clarivate Analytics), and Google Scholar.

RESULTS

There have been recent efforts to create neural interfaces that leverage the material advantages of LCPs. The literature offers examples of LCP as a basis for implantable medical devices and neural interfaces in the form of planar electrode arrays for retinal prosthetic, electrocorticography applications, and cuff-like structures for interfacing the peripheral nerve. In addition, there have been efforts to create penetrating intracortical devices capable of microstimulation and resolution of biopotentials. Recent work with a subclass of LCPs, namely liquid crystal elastomers, demonstrates that it is possible to create devices with features that deploy away from a central implantation site to interface with a volume of tissue while offering the possibility of minimizing tissue damage.

CONCLUSION

We envision the creation of novel microscale neural interfaces that leverage the physical properties of LCPs and have the capability of deploying within neural tissue for enhanced integration and performance.

A Review: Electrode and Packaging Materials for Neurophysiology Recording Implants

To date, a wide variety of neural tissue implants have been developed for neurophysiology recording from living tissues. An ideal neural implant should minimize the damage to the tissue and perform reliably and accurately for long periods of time. Therefore, the materials utilized to fabricate the neural recording implants become a critical factor. The materials of these devices could be classified into two broad categories: electrode materials as well as packaging and substrate materials. In this review, inorganic (metals and semiconductors), organic (conducting polymers), and carbon-based (graphene and carbon nanostructures) electrode materials are reviewed individually in terms of various neural recording devices that are reported in recent years. Properties of these materials, including electrical properties, mechanical properties, stability, biodegradability/bioresorbability, biocompatibility, and optical properties, and their critical importance to neural recording quality and device capabilities, are discussed. For the packaging and substrate materials, different material properties are desired for the chronic implantation of devices in the complex environment of the body, such as biocompatibility and moisture and gas hermeticity. This review summarizes common solid and soft packaging materials used in a variety of neural interface electrode designs, as well as their packaging performances. Besides, several biopolymers typically applied over the electrode package to reinforce the mechanical rigidity of devices during insertion, or to reduce the immune response and inflammation at the device-tissue interfaces are highlighted. Finally, a benchmark analysis of the discussed materials and an outlook of the future research trends are concluded.

Fabrication and Evaluation of Cyclic Olefin Copolymer Based Implantable Neural Electrode

OBJECTIVE

The purpose of this paper is to establish fabrication method of cyclic olefin copolymer(COC)-based neural electrode.

METHODS

The fabrication started with preparing COC pellets into COC films by compression molding. Metal layers were deposited on the COC film and attached to a silicon wafer. Laser ablation was used to cut the outer edges and mark alignment keys. The metal layers were patterned using standard photolithography procedures. Finally, the isolated electrodes were laminated. To ensure that the resulting electrode is safe and suitable for long-term implants, in vitro biocompatibility test, impedance evaluation, accelerated soak test, and repeated bend test were conducted.

RESULTS

Cytotoxicity test and elution test confirmed the biocompatibility in vitro. The basic performance was not hindered compared to other polymer-based electrodes, and the longevity of the electrode was validated by accelerated soak test. However, repeated bend test revealed that the material might not be suitable for applications where constant bending is required.

CONCLUSION

The COC-based neural electrode was successfully fabricated. The material showed several merits such as biocompatibility, thermoplasticity, low water absorption rate, and high transparency, but should be limited to applications where repeated bending is not required.

SIGNIFICANCE

Electrical circuits in implantable prosthetic devices must be hermetically encapsulated for a long period of time. Material such as COC with extremely low water absorption rate could have a significant impact on the longevity of these devices.

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.

Preliminary Study of Palatal Implant for Sleep Apnea Control

A fully-implantable device for treating obstructive sleep apnea (OSA) is conceptually suggested using soft palate stimulation. In this research, two in vivo studies were conducted to demonstrate electrical and physical feasibilities of the suggested device. First, electrical stimulation was delivered to the soft palate of a rabbit using a stimulator ASIC. The stimulation frequencies were swept from 20 Hz to 200 Hz to find out the appropriate parameter. Also, threshold level of the current pulse was evaluated to be 1.10 mA with an observance of a C-arm fluoroscopy. Second, a mock-up was fabricated with liquid crystal polymer (LCP), reflecting dimensions of the suggested device. The mock-up was inserted toward the soft palate of a rabbit by incising the hard palate in a lateral direction. After the mock-up was inserted, protrusion of the device was not detected and the subject stayed alive for at least a month at the time of this writing. Finally, several discussions on the palatal implant fabrication with LCP are presented.

Injection Molded Liquid Crystal Polymer Package for Chronic Active Implantable Devices With Application to an Optogenetic Stimulator

Implanted electronics require protection from the body's fluids to avoid moisture induced failure. This study presents an injection molded liquid crystal polymer (LCP) package to protect active implantable devices for chronic applications, such as in optogenetic research. The technology is applied and assessed through a custom package for a fully implantable optogenetic stimulation system, built on a versatile telemetry system that can incorporate additional stimulating and recording channels. An adapted quasi-steady state model predicts the lifetime of an enclosure, where the definition of the lifetime is the time before the internal relative humidity (RH) reaches a time constant, or 63%RH, a conservative limit to minimize the risk of corrosion. The lifetime of the LCP optogenetic device is 94 days, and can be extended to 326 days with the inclusion of 5% w/v silica gel desiccant. Samples of the LCP optogenetic device containing humidity sensors testing in saline at 38 °C support the RH change predictions. Desiccants inside the implant enclosure can store permeating moisture and prolong the life expectancy of LCP-based implants to years or decades. The results of this study demonstrates the feasibility of providing reliable protection for chronic optogenetic implants, and the technology can be transferred to other applications as an easily-manufactured, cost-effective, radiofrequency compatible alternative to hermetic packaging for chronic studies.

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.

Remote-Controlled Fully Implantable Neural Stimulator for Freely Moving Small Animal

The application of a neural stimulator to small animals is highly desired for the investigation of electrophysiological studies and development of neuroprosthetic devices. For this purpose, it is essential for the device to be implemented with the capabilities of full implantation and wireless control. Here, we present a fully implantable stimulator with remote controllability, compact size, and minimal power consumption. Our stimulator consists of modular units of (1) a surface-type cortical array for inducing directional change of a rat, (2) a depth-type array for providing rewards, and (3) a package for accommodating the stimulating electronics, a battery and ZigBee telemetry, all of which are assembled after independent fabrication and implantation using customized flat cables and connectors. All three modules were packaged using liquid crystal polymer (LCP) to avoid any chemical reaction after implantation. After bench-top evaluation of device functionality, the stimulator was implanted into rats to train the animals to turn to the left (or right) following a directional cue applied to the barrel cortex. Functionality of the device was also demonstrated in a three-dimensional (3D) maze structure, by guiding the rats to better navigate in the maze. The movement of the rat could be wirelessly controlled by a combination of artificial sensation evoked by the surface electrode array and reward stimulation. We could induce rats to turn left or right in free space and help their navigation through the maze. The polymeric packaging and modular design could encapsulate the devices with strict size limitations, which made it possible to fully implant the device into rats. Power consumption was minimized by a dual-mode power-saving scheme with duty cycling. The present study demonstrated feasibility of the proposed neural stimulator to be applied to neuroprosthesis research.

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.

Long-term recording reliability of liquid crystal polymer µECoG arrays

OBJECTIVE

The clinical use of microsignals recorded over broad cortical regions is largely limited by the chronic reliability of the implanted interfaces.

APPROACH

We evaluated the chronic reliability of novel 61-channel micro-electrocorticographic (µECoG) arrays in rats chronically implanted for over one year and using accelerated aging. Devices were encapsulated with polyimide (PI) or liquid crystal polymer (LCP), and fabricated using commercial manufacturing processes. In vitro failure modes and predicted lifetimes were determined from accelerated soak testing. Successful designs were implanted epidurally over the rodent auditory cortex. Trends in baseline signal level, evoked responses and decoding performance were reported for over one year of implantation.

MAIN RESULTS

Devices fabricated with LCP consistently had longer in vitro lifetimes than PI encapsulation. Our accelerated aging results predicted device integrity beyond 3.4 years. Five implanted arrays showed stable performance over the entire implantation period (247-435 d). Our regression analysis showed that impedance predicted signal quality and information content only in the first 31 d of recordings and had little predictive value in the chronic phase (>31 d). In the chronic phase, site impedances slightly decreased yet decoding performance became statistically uncorrelated with impedance. We also employed an improved statistical model of spatial variation to measure sensitivity to locally varying fields, which is typically concealed in standard signal power calculations.

SIGNIFICANCE

These findings show that µECoG arrays can reliably perform in chronic applications in vivo for over one year, which facilitates the development of a high-density, clinically viable interface.

Interface Adhesion in Implantable Chip-in-Foil Systems

Bioelectronic medicine requires miniaturized implants to selectively interface small target structures in the autonomous nervous system. Long-term stable non-hermetic packaging techniques have to be developed for smallest implantable electronics and interfaces. A process for the fabrication of chip-in-foil implants is proposed that combines a flip-chip approach for bare die embedding with a silicone rubber backbone. The conducting tracks are structured on polyimide (PI), enabling the use of microsystems fabrication technologies. The long-term stability of the interface between PI and silicone rubber is investigated by peel tests in phosphate buffered saline after prolonged soaking at $37 ^{circ}\mathrm {C}$. With a peel force of 721 mN after 14 days of soaking, the combination of 10-nm-thick titanium oxide and the adhesion promoter Dow Corning 1200 OS leads to the highest interface stability of the tested methods. This conforms to the results of atomic force microscopy measurements, where this treatment increased the surface roughness from 0.44 nm to 46.45 nm. The devised interface enables the construction of a chip-in-foil system with silicone rubber for height levelling in combination with polyimide-based micro structuring.

Investigation of Surgical Techniques for Optimization of Long-Term Outcomes of LCP-Based Retinal Prosthesis Implantation

Purpose

To investigate reproducible surgical techniques to optimize the long-term safety of liquid crystal polymer (LCP)-based retinal prosthesis implantation.

Methods

An LCP-based retinal prosthesis is fabricated monolithically on a single-body LCP substrate with all components, including the package and electrode array. We implanted the electrode array into the suprachoroidal space and anchored the package and transition part to the sclera in rabbits (n = 11). The safety profile was assessed upon the completion of the surgery and postoperatively.

Results

The surgical procedures for implantation of the entire system were easily performed in nine eyes (81.8%) without any intraoperative complications. In the other two eyes (18.2%), surgical complications related to electrode insertion, including optic nerve damage and retinal tear, arose. In 10 eyes (90.9%), the devices were well tolerated for at least 3 months. However, in most eyes (nine; 81.8%), two complications began to appear after 3 months, postoperatively, including conjunctival erosion or dehiscence over the package or transition part. The electrode arrays were maintained safely in the suprachoroidal space after surgery without any complications, regardless of the status of the extraocular components in all cases except two intraoperative complications.

Conclusions

We established safe and reproducible surgical techniques for implantation of our LCP-based retinal prosthesis into the suprachoroidal space. Although issues related to surgical technique or device configuration were identified, further technical solutions would improve the long-term safety of device implantation.

Translational Relevance

This study presents successful implantation of LCP-based retinal prosthesis. The technical solutions will permit an optimization of surgical techniques.

Liquid Crystal Elastomer-Based Microelectrode Array for In Vitro Neuronal Recordings

Polymer-based biomedical electronics provide a tunable platform to interact with nervous tissue both in vitro and in vivo. Ultimately, the ability to control functional properties of neural interfaces may provide important advantages to study the nervous system or to restore function in patients with neurodegenerative disorders. Liquid crystal elastomers (LCEs) are a class of smart materials that reversibly change shape when exposed to a variety of stimuli. Our interest in LCEs is based on leveraging this shape change to deploy electrode sites beyond the tissue regions exhibiting inflammation associated with chronic implantation. As a first step, we demonstrate that LCEs are cellular compatible materials that can be used as substrates for fabricating microelectrode arrays (MEAs) capable of recording single unit activity in vitro. Extracts from LCEs are non-cytotoxic (>70% normalized percent viability), as determined in accordance to ISO protocol 10993-5 using fibroblasts and primary murine cortical neurons. LCEs are also not functionally neurotoxic as determined by exposing cortical neurons cultured on conventional microelectrode arrays to LCE extract for 48 h. Microelectrode arrays fabricated on LCEs are stable, as determined by electrochemical impedance spectroscopy. Examination of the impedance and phase at 1 kHz, a frequency associated with single unit recording, showed results well within range of electrophysiological recordings over 30 days of monitoring in phosphate-buffered saline (PBS). Moreover, the LCE arrays are shown to support viable cortical neuronal cultures over 27 days in vitro and to enable recording of prominent extracellular biopotentials comparable to those achieved with conventional commercially-available microelectrode arrays.

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.

Fluoropolymer-Based Flexible Neural Prosthetic Electrodes for Reliable Neural Interfacing

We covalently bound fluoropolymer (FP) films by plasma treatment followed by thermal pressing at temperatures below their melting point and fabricated an adhesion-metal-free flexible gold electrode array entirely encapsulated by the FP film, excepting the active electrode openings. The fabricated device was chemically resistant and was modified to have a lower impedance and efficient charge injection capability. The fabricated device was evaluated in vivo in rats and was confirmed to record the epidural epileptiform activity induced by chemical administration. The chemically inert nature of FPs and the gold electrode is expected to facilitate reliable neural interfacing without abiotic issues. Plasma treatment-induced covalent binding of FP films can also be utilized in a variety of applications requiring durability, such as implantable biosensors and sensor platforms operating under chemically harsh environments, including humid conditions.

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.

Advances in Retinal Prosthetic Research: A Systematic Review of Engineering and Clinical Characteristics of Current Prosthetic Initiatives

PURPOSE

To date, reviews of retinal prostheses have focused primarily on devices undergoing human trials in the Western Hemisphere and fail to capture significant advances in materials and engineering research in countries such as Japan and Korea, as well as projects in early stages of development. To address these gaps, this systematic review examines worldwide advances in retinal prosthetic research, evaluates engineering characteristics and clinical progress of contemporary device initiatives, and identifies potential directions for future research in the field of retinal prosthetics.

METHODS

A literature search using PubMed, Google Scholar, and IEEExplore was conducted following the PRISMA Guidelines for Systematic Review. Inclusion criteria were peer-reviewed papers demonstrating progress in human or animal trials and papers discussing the prosthetic engineering design. For each initiative, a description of the device, its engineering considerations, and recent clinical results were provided.

RESULTS

Ten prosthetic initiatives met our inclusion criteria and were organized by stimulation location. Of these initiatives, four have recently completed human trials, three are undergoing multi- or single-center human trials, and three are undergoing preclinical animal testing. Only the Argus II (FDA 2013, CE 2011) has obtained FDA approval for use in the United States; the Alpha-IMS (CE 2013) has achieved the highest visual acuity using a Landolt-C test to date and is the only device presently undergoing a multicenter clinical trial.

CONCLUSION

Several distinct approaches to retinal stimulation have been successful in eliciting visual precepts in animals and/or humans. However, many clinical needs are still not met and engineering challenges must be addressed before a retinal prosthesis with the capability to fully and safely restore functional vision can be realized.

Flexible, Penetrating Brain Probes Enabled by Advances in Polymer Microfabrication

The acquisition of high-fidelity, long-term neural recordings in vivo is critically important to advance neuroscience and brain⁻machine interfaces. For decades, rigid materials such as metal microwires and micromachined silicon shanks were used as invasive electrophysiological interfaces to neurons, providing either single or multiple electrode recording sites. Extensive research has revealed that such rigid interfaces suffer from gradual recording quality degradation, in part stemming from tissue damage and the ensuing immune response arising from mechanical mismatch between the probe and brain. The development of "soft" neural probes constructed from polymer shanks has been enabled by advancements in microfabrication; this alternative has the potential to mitigate mismatch-related side effects and thus improve the quality of recordings. This review examines soft neural probe materials and their associated microfabrication techniques, the resulting soft neural probes, and their implementation including custom implantation and electrical packaging strategies. The use of soft materials necessitates careful consideration of surgical placement, often requiring the use of additional surgical shuttles or biodegradable coatings that impart temporary stiffness. Investigation of surgical implantation mechanics and histological evidence to support the use of soft probes will be presented. The review concludes with a critical discussion of the remaining technical challenges and future outlook.