A 512-Channel Multi-Layer Polymer-Based Neural Probe Array.

We present for the first time the design, fabrication, and preliminary bench-top characterization of a high-density, polymer-based penetrating microelectrode array, developed for chronic, large-scale recording in the cortices and hippocampi of behaving rats. We present two architectures for these targeted brain regions, both featuring 512 Pt recording electrodes patterned front-and-back on micromachined eight-shank arrays of thin-film Parylene C. These devices represent an order of magnitude improvement in both number and density of recording electrodes compared with prior work on polymer-based microelectrode arrays. We present enabling advances in polymer micro-machining related to lithographic resolution and a new method for back-side patterning of electrodes. In vitro electrochemical data verifies suitable electrode function and surface properties. Finally, we describe next steps toward the implementation of these arrays in chronic, large-scale recording studies in free-moving animal models.

A Parylene Neural Probe Array for Multi-Region Deep Brain Recordings.

A Parylene C polymer neural probe array with 64 electrodes purposefully positioned across 8 individual shanks to anatomically match specific regions of the hippocampus was designed, fabricated, characterized, and implemented in vivo for enabling recording in deep brain regions in freely moving rats. Thin film polymer arrays were fabricated using surface micromachining techniques and mechanically braced to prevent buckling during surgical implantation. Importantly, the mechanical bracing technique developed in this work involves a novel biodegradable polymer brace that temporarily reduces shank length and consequently, increases its stiffness during implantation, therefore enabling access to deeper brain regions while preserving a low original cross-sectional area of the shanks. The resulting mechanical properties of braced shanks were evaluated at the benchtop. Arrays were then implemented in vivo in freely moving rats, achieving both acute and chronic recordings from the pyramidal cells in the cornu ammonis (CA) 1 and CA3 regions of the hippocampus which are responsible for memory encoding. This work demonstrated the potential for minimally invasive polymer-based neural probe arrays for multi-region recording in deep brain structures.

Passive, wireless transduction of electrochemical impedance across thin-film microfabricated coils using reflected impedance.

A new method of wirelessly transducing electrochemical impedance without integrated circuits or discrete electrical components was developed and characterized. The resonant frequency and impedance magnitude at resonance of a planar inductive coil is affected by the load on a secondary coil terminating in sensing electrodes exposed to solution (reflected impedance), allowing the transduction of the high-frequency electrochemical impedance between the two electrodes. Biocompatible, flexible secondary coils with sensing electrodes made from gold and Parylene C were microfabricated and the reflected impedance in response to phosphate-buffered saline solutions of varying concentrations was characterized. Both the resonant frequency and impedance at resonance were highly sensitive to changes in solution conductivity at the secondary electrodes, and the effects of vertical separation, lateral misalignment, and temperature changes were also characterized. Two applications of reflected impedance in biomedical sensors for hydrocephalus shunts and glucose sensing are discussed.

Polymer-coated micro-optofluidic ring resonator detector for a comprehensive two-dimensional gas chromatographic microsystem: µGC ×µGC-µOFRR.

We describe first results from a micro-analytical subsystem that integrates a detector comprising a polymer-coated micro-optofluidic ring resonator (µOFRR) chip with a microfabricated separation module capable of performing thermally modulated comprehensive two-dimensional gas chromatographic separations (µGC ×µGC) of volatile organic compound (VOC) mixtures. The 2 × 2 cm µOFRR chip consists of a hollow, contoured SiO(x) cylinder (250 µm i.d.; 1.2 µm wall thickness) grown from a Si substrate, and integrated optical and fluidic interconnection features. By coupling to a 1550 nm tunable laser and photodetector via an optical fiber taper, whispering gallery mode (WGM) resonances were generated within the µOFRR wall, and shifts in the WGM wavelength caused by transient sorption of eluting vapors into the PDMS film lining the µOFRR cylinder were monitored. Isothermal separations of a simple alkane mixture using a PDMS coated 1st-dimension ((1)D) µcolumn and an OV-215-coated 2nd-dimension ((2)D) µcolumn confirmed that efficient µGC ×µGC-µOFRR analyses could be performed and that responses were dominated by film-swelling. Subsequent tests with more diverse VOC mixtures demonstrated that the modulated peak width and the VOC sensitivity were inversely proportional to the vapor pressure of the analyte. Modulated peaks as narrow as 120 ms and limits of detection in the low-ng range were achieved. Structured contour plots generated with the µOFRR and a reference FID were comparable.

A Library of Polymer-based Microelectrode Array Designs for Recording from the Brain of Different Animal Models.

Large-scale network recording technology is critical in linking neural activity to behavior. Stable, long-term recordings collected from behaving animals are the foundation for understanding neural dynamics and the plasticity of neural circuits. Penetrating microelectrode arrays (MEAs) can obtain high-resolution neural activity from different brain regions. However, ensuring the longevity of implantable devices and the consistency of neural signals over time remains one big challenge. A potential solution is to use flexible, polymer-based MEAs to minimize the foreign body response and prolong the lifetime of neural interfacing devices. Rodents and nonhuman primates (NHP) are commonly used animal models in neuroscience and neuroengineering studies. Specially designed MEAs that capture morphological features of different animal brains and various brain structures are powerful tools to simultaneously obtain neural activities from multiple brain regions. In this work, we develop a set of prototype designs of polymer MEAs that cover cortical, sub-cortical, and multiple brain regions of rodents and NHP.

Polymer Implantable Electrode Foundry: A shared resource for manufacturing polymer-based microelectrodes for neural interfaces.

Large scale monitoring of neural activity at the single unit level can be achieved via electrophysiological recording using implanted microelectrodes. While neuroscience researchers have widely employed chronically implanted electrode-based interfaces for this purpose, a commonly encountered limitation is loss of highly resolved signals arising from immunological response over time. Next generation electrode-based interfaces improve longitudinal signal quality using the strategy of stabilizing the device-tissue interface with microelectrode arrays constructed from soft and flexible polymer materials. The limited availability of such polymer microelectrode arrays has restricted access to a small number of researchers able to build their own custom devices or who have developed specific collaborations with engineering researchers who can produce them. Here, a new technology resource model is introduced that seeks to widely increase access to polymer microelectrode arrays by the neuroscience research community. The Polymer Implantable Electrode (PIE) Foundry provides custom and standardized polymer microelectrode arrays as well as training and guidance on best-practices for implantation and chronic experiments.

Acute in vivo Recording with a Generic Parylene Microelectrode Array Implanted with Dip-coating Method into the Rat Brain.

Flexible polymer-based microelectrode arrays (MEAs) can reduce tissue inflammation and foreign body response and greatly prolong the lifetime of neural implants. However, standard and customized polymer devices are only accessible to limited groups. To better promote the development and application of polymer MEAs, we have launched the Polymer Implantable Electrode (PIE) Foundry and developed a 64-channel Parylene C-based MEA with generic electrodes layout that can be used to record from both cortical and sub-cortical regions in rodents. In addition, a practical dip-coating protocol for the insertion of the flexible standard Parylene MEA is developed.

Microfluidic blood filtration device.

Rapid decentralized biomedical diagnostics have become increasingly necessary in a medical environment of growing costs and mounting demands on healthcare personnel and infrastructure. Such diagnostics require low-cost novel devices that can operate at bedside or in doctor offices using small amounts of sample that can be extracted and processed on the spot. Thus, point-of-care sample preparation is an important component of the necessary diagnostic paradigm shift. We therefore introduce a microfluidic device which produces plasma from whole blood. The device is inexpensive, reliable, easy to fabricate, and requires only 3.5 kPa pressure to operate. The device is fully compatible with microfluidic diagnostic chips. The output 23-gauge microtube of the former can be directly plugged into the input ports of the latter allowing immediate applicability in practice as a sample-prep pre-stage to a variety of emergent microfluidic diagnostic devices. In addition, the shown approach of filter encapsulation in elastomer has principle importance as it is compatible with and applicable to microfluidic sample-prep integration with analytical stages within the same elastomeric chip. This can eventually lead to finger-prick blood tests in point-of-care settings.

A review of implantable biosensors for closed-loop glucose control and other drug delivery applications.

Closed-loop drug delivery promises autonomous control of pharmacotherapy through the continuous monitoring of biomarker levels. For decades, researchers have strived for portable closed-loop systems capable of treating ambulatory patients with chronic conditions such as diabetes mellitus. After years of development, the first of these systems have left the laboratory and entered commercial use. This long-awaited advance reflects recent development of chronically stable implantable biosensors able to accurately measure biomarker levels in vivo. This review discusses the role of implantable biosensors in closed-loop drug delivery applications, with the intent to provide a resource for engineers and researchers studying such systems. We provide an overview of common biosensor designs and review the principle challenges in implementing long indwelling sensors: namely device sensitivity, selectivity, and lifetime. This review examines novel advances in transducer design, biological interface, and material biocompatibility, with a focus on recent academic and commercial work which provide successful strategies to overcome perennial challenges. This review focuses primarily on the topics of closed-loop glucose control and continuous glucose monitoring biosensors, which make up the overwhelming majority of published research in this area. We conclude with an overview of recent advances in closed-loop systems targeting applications outside blood glucose management.

Acute in vivo testing of a conformal polymer microelectrode array for multi-region hippocampal recordings.

OBJECTIVE: The success of a cortical prosthetic device relies upon its ability to attain resolvable spikes from many neurons in particular neural networks over long periods of time. Traditionally, lifetimes of neural recordings are greatly limited by the body's immune response against the foreign implant which causes neuronal death and glial scarring. This immune reaction is posited to be exacerbated by micromotion between the implant, which is often rigid, and the surrounding, soft brain tissue, and attenuates the quality of recordings over time. APPROACH: In an attempt to minimize the foreign body response to a penetrating neural array that records from multiple brain regions, Parylene C, a flexible, biocompatible polymer was used as the substrate material for a functional, proof-of-concept neural array with a reduced elastic modulus. This probe array was designed and fabricated to have 64 electrodes positioned to match the anatomy of the rat hippocampus and allow for simultaneous recordings between two cell-body layers of interest. A dissolvable brace was used for deep-brain penetration of the flexible array. MAIN RESULTS: Arrays were electrochemically characterized at the benchtop, and a novel insertion technique that restricts acute insertion injury enabled accurate target placement of four, bare, flexible arrays to greater than 4 mm deep into the rat brain. Arrays were tested acutely and in vivo recordings taken intra-operatively reveal spikes in both targeted regions of the hippocampus with spike amplitudes and noise levels similar to those recorded with microwires. Histological staining of a sham array implanted for one month reveals limited astrocytic scarring and neuronal death around the implant. SIGNIFICANCE: This work represents one of the first examples of a penetrating polymer probe array that records from individual neurons in structures that lie deep within the brain.

Application of Parylene-Based Flexible Multi-Electrode Array for Recording From Subcortical Brain Regions From Behaving Rats.

Obtaining multiple single-unit recordings in particular neural networks from behaving animals is crucial for the understanding of cognitive functions of the brain. Attaining stable, chronic recordings from the brain is also the foundation to develop effective cortical prosthetic devices. However, severe immune response caused by micromotion between stiff implants and surrounding brain tissue often limits the lifetime of penetrating, neural recording devices. To reduce the stiffness mismatch between recording devices and brain tissue, we developed a flexible, polymer based multi-electrode array for recording single neuron activities from the rat hippocampus, a major subcortical structure of the rat brain. Parylene C, a biocompatible polymer, was used as the structural and insulation material of the multi-electrode array. 64 platinum (Pt) recording electrodes were placed in groups along each shank to conform to the anatomical distribution of hippocampal principle neurons. The multi-electrode array was chronically implanted in three animals. After recovery, neural activity together with movement traces were collected from the behaving animals.

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.

Chronic multi-region recording from the rat hippocampus in vivo with a flexible Parylene-based multi-electrode array.

Neural activities of free-moving animals provide valuable insights into behavior, memory formation and cognitive function of the hippocampus. Unitary activities simultaneously recorded from multiple sub-regions of the hippocampus enable detailed study of hippocampal neural circuits, but require high fidelity recordings with high temporal and spatial resolution. In this work, we explored the possibility of using Parylene-C as the structural material for a penetrating, multi-electrode array designed to record from multiple sub-region of the rat hippocampus. A 64-channel Parylene-based flexible electrode array was designed and fabricated. The layout of the electrode array was arranged to conform to the shape of cell body layers of the rat hippocampus. An insertion technique of temporarily reduce the effective length of the probe with polyethylene glycol (PEG) was developed and tested in vivo. The multi-electrode array was implanted into a rat hippocampus for chronic experimentation and unitary activities were collected both during the implantation and after recovery while the animal ran freely in an open field. Unitary activities with an average signal to noise ratios (SNR) of 3 to 4 were recorded with the Parylene probe over the period of one month after implantation.

Electron-beam lithography for polymer bioMEMS with submicron features.

We present a method for submicron fabrication of flexible, thin-film structures fully encapsulated in biocompatible polymer poly(chloro-p-xylylene) (Parylene C) that improves feature size and resolution by an order of magnitude compared with prior work. We achieved critical dimensions as small as 250 nm by adapting electron beam lithography for use on vapor deposited Parylene-coated substrates and fabricated encapsulated metal structures, including conducting traces, serpentine resistors, and nano-patterned electrodes. Structures were probed electrically and mechanically demonstrating robust performance even under flexion or torsion. The developed fabrication process for electron beam lithography on Parylene-coated substrates and characterization of the resulting structures are presented in addition to a discussion of the challenges of applying electron beam lithography to polymers. As an application of the technique, a Parylene-based neural probe prototype was fabricated with 32 recording sites patterned along a 2 mm long shank, an electrode density surpassing any prior polymer probe.

A flexible parylene probe for in vivo recordings from multiple subregions of the rat hippocampus.

The hippocampus is crucial to the formation of long-term memory and declarative memory. It is divided into three sub-fields the CA1, the CA3 and the DG. To understand the neuronal circuitry within the hippocampus and to study the role of the hippocampus in memory function requires the collection of neural activities from multiple subregions of the hippocampus simultaneously. Micro-wire electrode arrays are commonly used as an interface with neural systems. However, recording from multiple deep brain regions with curved anatomical structures such as the thin cell body layers of the hippocampus requires the micro-wires to be arranged into a highly accurate, complex layout that is difficult to fabricated manually. In this work, we designed and developed a flexible parylene-C based neural probe which can be easily micro-machined to the desired dimensions. Sixty-four electrical recording sites are micromachined on to 8 parylene shanks and spaced according to the distribution of hippocampal principal neurons in different hippocampus subregions. Together with our collaborators, we developed and optimized the implantation procedure of the flexible parylene probe and tested the insertion method both in brain tissue phantom and in vivo with a sham device. Immunohistochemistry (IHC) staining post-implantation of the sham probe was used to verify the location of the probe and to evaluate immune responses to the probe. Fully functional devices were fabricated and, in future studies, functional probes will be chronically implanted into the rat hippocampus, and neural activities will be recorded and compared with signals obtained with micro-wire arrays.

Materials for microfabricated implantable devices: a review.

The application of microfabrication to the development of biomedical implants has produced a new generation of miniaturized technology for assisting treatment and research. Microfabricated implantable devices (µID) are an increasingly important tool, and the development of new µIDs is a rapidly growing field that requires new microtechnologies able to safely and accurately function in vivo. Here, we present a review of µID research that examines the critical role of material choice in design and fabrication. Materials commonly used for µID production are identified and presented along with their relevant physical properties and a survey of the state-of-the-art in µID development. The consequence of material choice as it pertains to microfabrication and biocompatibility is discussed in detail with a particular focus on the divide between hard, rigid materials and soft, pliable polymers.

Design of a flexible parylene-based multi-electrode array for multi-region recording from the rat hippocampus.

The hippocampus is a critical deep brain structure in several aspects. It is directly related to the formation of new long-term declarative memory. The malfunction of the hippocampus closely relates to various disease and pathological conditions. It is also a model structure for the study of cortical function and synaptic plasticity in general because of its special neuro-anatomical structure and intrinsic connections within the hippocampus formation. Both the understanding of roles that the hippocampus plays in recognition memory and the study of neural plasticity require simultaneously recording of neural activities from multiple sub-regions of the hippocampus from behavioral animals. However the distribution of cells in the hippocampus make the recording from multiple sub-regions a big challenge with the traditional uni-length micro-wire arrays. Well-designed electrode arrays are required to reach multiple regions simultaneously because of the distinctive double C shape of the hippocampus cell body layers. In this work, we designed a multi-shanks electrode which uses Parylene C, a highly biocompatible and flexible polymer, as a base and has multiple recording sites specially positioned along the longitudinal axis to fit the curvy shape of the rat hippocampus.

A microfabricated optofluidic ring resonator for sensitive, high-speed detection of volatile organic compounds.

Advances in microanalytical systems for multi-vapor determinations to date have been impeded by limitations associated with the microsensor technologies employed. Here we introduce a microfabricated optofluidic ring resonator (µOFRR) sensor that addresses many of these limitations. The µOFRR combines vapor sensing and fluidic transport functions in a monolithic microstructure comprising a hollow, vertical SiOx cylinder (250 µm i.d., 1.2 µm wall thickness; 85 µm height) with a central quasi-toroidal mode-confinement section, grown and partially released from a Si substrate. The device also integrates on-chip fluidic-interconnection and fiber-optic probe alignment features. High-Q whispering gallery modes generated with a tunable 1550 nm laser exhibit rapid, reversible shifts in resonant wavelength arising from polymer swelling and refractive index changes as vapors partition into the ~300 nm PDMS film lining the cylinder. Steady-state sensor responses varied in proportion to concentration over a 50-fold range for the five organic vapors tested, providing calculated detection limits as low as 0.5 ppm (v/v) (for m-xylene and ethylbenzene). In dynamic exposure tests, responses to 5 µL injected m-xylene vapor pulses were 710 ms wide and were only 18% broader than those from a reference flame-ionization detector and also varied linearly with injected mass; 180 pg was measured and the calculated detection limit was 49 pg without use of preconcentration or split injection, at a flow rate compatible with efficient chromatographic separations. Coupling of this µOFRR with a micromachined gas chromatographic separation column is demonstrated.

Microfabricated optofluidic ring resonator structures.

We describe the fabrication and preliminary optical characterization of rugged, Si-micromachined optofluidic ring resonator (µOFRR) structures consisting of thin-walled SiO(x) cylinders with expanded midsections designed to enhance the three-dimensional confinement of whispering gallery modes (WGMs). These µOFRR structures were grown thermally at wafer scale on the interior of Si molds defined by deep-reactive-ion etching and pre-treated to reduce surface roughness. Devices 85-µm tall with 2-µm thick walls and inner diameters ranging from 50 to 200 µm supported pure-mode WGMs with Q-factors >10(4) near 985 nm. Advantages for eventual vapor detection in gas chromatographic microsystems are highlighted.