Parylene photonics: a flexible, broadband optical waveguide platform with integrated micromirrors for biointerfaces

Fully flexible implantable neural probes for electrophysiology recording and controlled neurochemical modulation

Targeted delivery of neurochemicals and biomolecules for neuromodulation of brain activity is a powerful technique that, in addition to electrical recording and stimulation, enables a more thorough investigation of neural circuit dynamics. We have designed a novel, flexible, implantable neural probe capable of controlled, localized chemical stimulation and electrophysiology recording. The neural probe was implemented using planar micromachining processes on Parylene C, a mechanically flexible, biocompatible substrate. The probe shank features two large microelectrodes (chemical sites) for drug loading and sixteen small microelectrodes for electrophysiology recording to monitor neuronal response to drug release. To reduce the impedance while keeping the size of the microelectrodes small, poly(3,4-ethylenedioxythiophene) (PEDOT) was electrochemically coated on recording microelectrodes. In addition, PEDOT doped with mesoporous sulfonated silica nanoparticles (SNPs) was used on chemical sites to achieve controlled, electrically-actuated drug loading and releasing. Different neurotransmitters, including glutamate (Glu) and gamma-aminobutyric acid (GABA), were incorporated into the SNPs and electrically triggered to release repeatedly. An in vitro experiment was conducted to quantify the stimulated release profile by applying a sinusoidal voltage (0.5 V, 2 Hz). The flexible neural probe was implanted in the barrel cortex of the wild-type Sprague Dawley rats. As expected, due to their excitatory and inhibitory effects, Glu and GABA release caused a significant increase and decrease in neural activity, respectively, which was recorded by the recording microelectrodes. This novel flexible neural probe technology, combining on-demand chemical release and high-resolution electrophysiology recording, is an important addition to the neuroscience toolset used to dissect neural circuitry and investigate neural network connectivity.

Fabrication and Characterization of PDMS Waveguides for Flexible Optrodes

With the growth of optogenetic research, the demand for optical probes tailored to specific applications is ever rising. Specifically, for applications like the coiled cochlea of the inner ear, where planar, stiff, and nonconformable probes can hardly be used, transitioning from commonly used stiff glass fibers to flexible probes is required, especially for long-term use. Following this demand, polydimethylsiloxane (PDMS) with its lower Young's modulus compared to glass fibers can serve as material of choice. Hence, the long-term usability of PDMS as a waveguide material with respect to variations in transmission and refractive index over time is investigated. Different manufacturing methods for PDMS-based flexible waveguides are established and compared with the aim to minimize optical losses and thus maximize optical output power. Finally, the waveguides with lowest optical losses (-4.8 dB cm-1 ± 1.3 dB cm-1 at 472 nm) are successfully inserted into the optogenetically modified cochlea of a Mongolian gerbil (Meriones unguiculatus), where optical stimuli delivered by the waveguides evoked robust neuronal responses in the auditory pathway.

A snapshot review on materials enabled multimodal bioelectronics for neurological and cardiac research

Seamless integration of the body and electronics toward the understanding, quantification, and control of disease states remains one of the grand scientific challenges of this era. As such, research efforts have been dedicated to developing bioelectronic devices for chemical, mechanical, and electrical sensing, and cellular and tissue functionality modulation. The technologies developed to achieve these capabilities cross a wide range of materials and scale (and dimensionality), e.g., from micrometer to centimeters (from 2-dimensional (2D) to 3-dimensional (3D) assemblies). The integration into multimodal systems which allow greater insight and control into intrinsically multifaceted biological systems requires careful design and selection. This snapshot review will highlight the state-of-the-art in cellular recording and modulation as well as the material considerations for the design and manufacturing of devices integrating their capabilities.

En route to sound coding strategies for optical cochlear implants

Hearing loss is the most common human sensory deficit. Severe-to-complete sensorineural hearing loss is often treated by electrical cochlear implants (eCIs) bypassing dysfunctional or lost hair cells by direct stimulation of the auditory nerve. The wide current spread from each intracochlear electrode array contact activates large sets of tonotopically organized neurons limiting spectral selectivity of sound coding. Despite many efforts, an increase in the number of independent eCI stimulation channels seems impossible to achieve. Light, which can be better confined in space than electric current may help optical cochlear implants (oCIs) to overcome eCI shortcomings. In this review, we present the current state of the optogenetic sound encoding. We highlight optical sound coding strategy development capitalizing on the optical stimulation that requires fine-grained, fast, and power-efficient real-time sound processing controlling dozens of microscale optical emitters as an emerging research area.

Emerging trends in the development of flexible optrode arrays for electrophysiology

Optical-electrode (optrode) arrays use light to modulate excitable biological tissues and/or transduce bioelectrical signals into the optical domain. Light offers several advantages over electrical wiring, including the ability to encode multiple data channels within a single beam. This approach is at the forefront of innovation aimed at increasing spatial resolution and channel count in multichannel electrophysiology systems. This review presents an overview of devices and material systems that utilize light for electrophysiology recording and stimulation. The work focuses on the current and emerging methods and their applications, and provides a detailed discussion of the design and fabrication of flexible arrayed devices. Optrode arrays feature components non-existent in conventional multi-electrode arrays, such as waveguides, optical circuitry, light-emitting diodes, and optoelectronic and light-sensitive functional materials, packaged in planar, penetrating, or endoscopic forms. Often these are combined with dielectric and conductive structures and, less frequently, with multi-functional sensors. While creating flexible optrode arrays is feasible and necessary to minimize tissue-device mechanical mismatch, key factors must be considered for regulatory approval and clinical use. These include the biocompatibility of optical and photonic components. Additionally, material selection should match the operating wavelength of the specific electrophysiology application, minimizing light scattering and optical losses under physiologically induced stresses and strains. Flexible and soft variants of traditionally rigid photonic circuitry for passive optical multiplexing should be developed to advance the field. We evaluate fabrication techniques against these requirements. We foresee a future whereby established telecommunications techniques are engineered into flexible optrode arrays to enable unprecedented large-scale high-resolution electrophysiology systems.

2D optical confinement in an etchless stratified trench waveguide

We demonstrate novel trapezoidal and rectangular stratified trench optical waveguide designs that feature low-loss two-dimensional confinement of guided optical modes that can be realized in continuous polymer thin film layers formed in a trench mold. The design is based on geometrical bends in a thin film core to enable two-dimensional confinement of light in the transverse plane, without any variation in the core thickness. Incidentally, the waveguide design would completely obviate the need for etching the waveguide core, avoiding the scattering loss due to the etched sidewall roughness. This new design exhibits an intrinsic leakage loss due to coupling of light out of the trench, which can be minimized by choosing an appropriate waveguide geometry. Finite-difference eigenmode simulation demonstrates a low intrinsic leakage loss of less than 0.15 dB/cm. We discuss the principle of operation of these stratified trench waveguides and present the design and numerical simulations of a specific realization of this waveguide geometry. The design considerations and tradeoffs in propagation loss and confinement compared with traditional ridge waveguides are discussed.

On the Fabrication and Characterization of Polymer-Based Waveguide Probes for Use in Future Optical Cochlear Implants

Improved hearing restoration by cochlear implants (CI) is expected by optical cochlear implants (oCI) exciting optogenetically modified spiral ganglion neurons (SGNs) via an optical pulse generated outside the cochlea. The pulse is guided to the SGNs inside the cochlea via flexible polymer-based waveguide probes. The fabrication of these waveguide probes is realized by using 6" wafer-level micromachining processes, including lithography processes such as spin-coating cladding layers and a waveguide layer in between and etch processes for structuring the waveguide layer. Further adhesion layers and metal layers for laser diode (LD) bonding and light-outcoupling structures are also integrated in this waveguide process flow. Optical microscope and SEM images revealed that the majority of the waveguides are sufficiently smooth to guide light with low intensity loss. By coupling light into the waveguides and detecting the outcoupled light from the waveguide, we distinguished intensity losses caused by bending the waveguide and outcoupling. The probes were used in first modules called single-beam guides (SBGs) based on a waveguide probe, a ball lens and an LD. Finally, these SBGs were tested in animal models for proof-of-concept implantation experiments.

Flexible passive integrated photonic devices with superior optical and mechanical performance

Flexible integrated photonics is a rapidly emerging technology with a wide range of possible applications in the fields of flexible optical interconnects, conformal multiplexing sensing, health monitoring, and biotechnology. One major challenge in developing mechanically flexible integrated photonics is the functional component within an integrated photonic circuit with superior performance. In this work, several essential flexible passive devices for such a circuit were designed and fabricated based on a multi-neutral-axis mechanical design and a monolithic integration technique. The propagation loss of the waveguide is calculated to be 4.2 dB/cm. In addition, we demonstrate a microring resonator, waveguide crossing, multimode interferometer (MMI), and Mach-Zehnder interferometer (MZI) for use at 1.55 µm, each exhibiting superior optical and mechanical performance. These results represent a significant step towards further exploring a complete flexible photonic integrated circuit.

Wettability and Surface Roughness of Parylene C on Three-Dimensional-Printed Photopolymers

The use of poly-(para-chloro-xylylene) (Parylene C) in microelectromechanical systems and medical devices has increased rapidly. However, little research has been conducted on the wettability and surface roughness of Parylene C after being soaked in solutions. In this study, the contact angle and surface roughness (arithmetic average of roughness) of Parylene C on three-dimensional (3D)-printed photopolymer in 10% sodium hydroxide, 10% ammonium hydroxide, and 100% phosphate-buffered saline (PBS) solutions were investigated using a commercial contact angle measurement system and laser confocal microscope, respectively. The collected data indicated that 10% ammonium hydroxide had no major effect on the contact angle of Parylene C on a substrate, with a Shore A hardness of 50. However, 10% sodium hydroxide, 10% ammonium hydroxide, and 100% PBS considerably affected the contact angle of Parylene C on a substrate with a Shore A hardness of 85. Substrates with Parylene C coating exhibited lower surface roughness than uncoated substrates. The substrates coated with Parylene C that were soaked in 10% ammonium hydroxide exhibited high surface roughness. The aforementioned results indicate that 3D-printed photopolymers coated with Parylene C can offer potential benefits when used in biocompatible devices.

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.

Prediction of Surface Roughness as a Function of Temperature for SiO2 Thin-Film in PECVD Process

An analytical model to predict the surface roughness for the plasma-enhanced chemical vapor deposition (PECVD) process over a large range of temperature values is still nonexistent. By using an existing prediction model, the surface roughness can directly be calculated instead of repeating the experimental processes, which can largely save time and resources. This research work focuses on the investigation and analytical modeling of surface roughness of SiO₂ deposition using the PECVD process for almost the whole range of operating temperatures, i.e., 80 to 450 °C. The proposed model is based on experimental data of surface roughness against different temperature conditions in the PECVD process measured using atomic force microscopy (AFM). The quality of these SiO₂ layers was studied against an isolation layer in a microelectromechanical system (MEMS) for light steering applications. The analytical model employs different mathematical approaches such as linear and cubic regressions over the measured values to develop a prediction model for the whole operating temperature range of the PECVD process. The proposed prediction model is validated by calculating the percent match of the analytical model with experimental data for different temperature ranges, counting the correlations and error bars.

Flexible optoelectric neural interfaces

Understanding the neural basis of brain function and dysfunction and designing effective therapeutics require high resolution targeted stimulation and recording of neural activity. Optical methods have been recently developed for neural stimulation as well as functional and structural imaging. These methods call for implantable devices to deliver light into the neural tissue at depth with high spatiotemporal resolution. To address this need, rigid and flexible neurophotonic implants have been recently designed. This article reviews the state-of-the-art flexible passive and active penetrating optical neural probes developed for light delivery with minimal damage to the tissue. Passive and active flexible neurophotonic implants are compared and insights about future directions are provided.

Parylene Photonic Microimager for Implantable Imaging. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2021;2021:7511-7514. [PMID: 34892830 DOI: 10.1109/embc46164.2021.9630912] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We have recently introduced a fully flexible, compact photonic platform, Parylene photonics. Here, we demonstrate a Parylene photonic waveguide array microimager with a light source localization accuracy of 17.04 µm along the x-axis and 30.07 µm along the y-axis over a 200 µm×1000 µm region. We show the feasibility of fluorescent imaging from mouse brain tissue using the microimager array.Clinical Relevance- Implantable microimagers can be used for clinical intraoperative monitoring as well as structural and functional imaging with cell-type specificity in research.

A Semi-Automated System for Wafer-Scale Optical Waveguide Characterization

Integrated photonic waveguide systems are used in biomedical sensing and require robust, high-throughput methods of characterization. Here, we demonstrate a semi-automated robotic system to characterize waveguides at the wafer-scale with minimal human intervention based on imaging the outscattered light to measure the propagation loss. We demonstrate automated input coupling efficiency optimization using closed-loop control of the input fiber position. The automated characterization system collects and combines multiple images of the waveguide to measure the propagation loss. This system allows high-throughput characterization of integrated photonic waveguides and lays the foundation for a fully automated and high throughput system to characterize photonic waveguides at the wafer scale.Clinical Relevance- This method enables high precision, high throughput characterization of optoelectrical neural probes to maximize the yield of surgical implantation and electrophysiology recording.

Structural and optical properties of Nd:YAB-nanoparticle-doped PDMS elastomers for random lasers

We report the structural and optical properties of Nd:YAB (NdxY1-x Al3(BO3)4)-nanoparticle-doped PDMS elastomer films for random lasing (RL) applications. Nanoparticles with Nd ratios of x = 0.2, 0.4, 0.6, 0.8, and 1.0 were prepared and then incorporated into the PDMS elastomer to control the optical gain density and scattering center content over a wide range. The morphology and thermal stability of the elastomer composites were studied. A systematic investigation of the lasing wavelength, threshold, and linewidth of the laser was carried out by tailoring the concentration and optical gain of the scattering centers. The minimum threshold and linewidth were found to be 0.13 mJ and 0.8 nm for x = 1 and 0.8. Furthermore, we demonstrated that the RL intensity was easily tuned by controlling the degree of mechanical stretching, with strain reaching up to 300%. A strong, repeatable lasing spectrum over ~ 50 cycles of applied strain was observed, which demonstrates the high reproducibility and robustness of the RL. In consideration for biomedical applications that require long-term RL stability, we studied the intensity fluctuation of the RL emission, and confirmed that it followed Lévy-like statistics. Our work highlights the importance of using rare-earth doped nanoparticles with polymers for RL applications.

Technological Challenges in the Development of Optogenetic Closed-Loop Therapy Approaches in Epilepsy and Related Network Disorders of the Brain

Epilepsy is a chronic, neurological disorder affecting millions of people every year. The current available pharmacological and surgical treatments are lacking in overall efficacy and cause side-effects like cognitive impairment, depression, tremor, abnormal liver and kidney function. In recent years, the application of optogenetic implants have shown promise to target aberrant neuronal circuits in epilepsy with the advantage of both high spatial and temporal resolution and high cell-specificity, a feature that could tackle both the efficacy and side-effect problems in epilepsy treatment. Optrodes consist of electrodes to record local field potentials and an optical component to modulate neurons via activation of opsin expressed by these neurons. The goal of optogenetics in epilepsy is to interrupt seizure activity in its earliest state, providing a so-called closed-loop therapeutic intervention. The chronic implantation in vivo poses specific demands for the engineering of therapeutic optrodes. Enzymatic degradation and glial encapsulation of implants may compromise long-term recording and sufficient illumination of the opsin-expressing neural tissue. Engineering efforts for optimal optrode design have to be directed towards limitation of the foreign body reaction by reducing the implant's elastic modulus and overall size, while still providing stable long-term recording and large-area illumination, and guaranteeing successful intracerebral implantation. This paper presents an overview of the challenges and recent advances in the field of electrode design, neural-tissue illumination, and neural-probe implantation, with the goal of identifying a suitable candidate to be incorporated in a therapeutic approach for long-term treatment of epilepsy patients.