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This paper reports the successful fabrication of softening polymer (SP)-based devices with inter-layer coatings for neural interface applications. SPs are flexible biocompatible polymers that soften upon implantation into the body, thus capable of reducing mechanical stress on the surrounding tissue and increasing the conformability of the implant. The softening process relies on water uptake into the polymer that can affect its electrical insulation capabilities. Employing an amorphous silicon carbide (a-SiC) thin film inter-layer between the metal traces and the SP is expected to provide a reliable water-barrier while allowing the SP plasticization once the device is implanted. We report the electrochemical properties and leakage current characteristics of various structures with a-SiC and polyimide inter-layer coatings under simulated physiological conditions. The fabricated structures demonstrated appropriate electrical properties and were found to be reliable up to a 2.5 mm radius of curvature when bending in compression.

We have studied the charge-injection characteristics and electrochemical impedance of sputtered ruthenium oxide (RuO(x)) films as electrode coatings for neural stimulation and recording electrodes. RuO(x) films were deposited by reactive DC magnetron sputtering, using a combination of water vapor and oxygen gas as reactive plasma constituents. The cathodal charge storage capacity of planar RuO(x) electrodes was found to be 54.6 ± 9.5 mC/cm(2) (mean ± SD, n = 12), and the charge-injection capacity in a 0.2-ms cathodal current pulse was found to be 7.1 ± 0.3 mC/cm(2) (mean ± SD, n = 15) at 0.6 V positive bias versus Ag|AgCl, in phosphate buffer saline at room temperature for ~250 nm thick films. In general, the RuO(x) films exhibited high charge-injection capacities, with or without a positive interpulse bias, comparable to sputtered iridium oxide (SIROF) coatings. The charge-injection capacity increased monotonically with film thickness from 120 to 630 nm, and reached 11.30 ± 0.34 mC/cm(2) (mean ± SD, n = 5) at 0.6 V bias versus Ag|AgCl at 630 nm film thickness. In addition, RuO(x) films showed minimal changes in electrochemical characteristics over 1.5 billion cycles of constant current pulsing at a charge density of 408 μC/cm(2) (8 nC/phase, 200 μs pulse width). The findings of low-impedance, high charge-injection capacity, and long-term pulsing stability suggest the suitability of RuO(x) as a comparatively inexpensive and favorable choice of electrode material for neural stimulation and recording.

Thin film titanium nitride (TiN), with a geometric surface area of 2,000 μm

Objective. Iridium oxide films are commonly used as a high charge-injection electrode material in neural devices. Yet, few studies have performed in-depth assessments of material performance versus film thickness, especially for films grown on three-dimensional (instead of planar) metal surfaces in neutral pH electrolyte solutions. Further, few studies have investigated the driving voltage requirements for constant-current stimulation using activated iridium oxide (AIROF) electrodes, which will be a key constraint for future use in wirelessly powered neural devices. Approach. In this study, iridium microwire probes were activated by repeated potential pulsing in room temperature phosphate buffered saline (pH 7.1–7.3). Electrochemical measurements were recorded in three different electrolyte conditions for probes with different geometric surface areas (GSAs) as the AIROF thickness was increased. Main results. Maintaining an anodic potential bias during the inter-pulse interval was required for AIROF electrodes to deliver charge levels considered necessary for neural stimulation. Potential pulsing for 100–200 cycles was sufficient to achieve charge injection levels of 2.5 mC cm−2 (50 nC/phase in a biphasic pulse) in PBS with 2000 µm2 iridium probes. Increasing the electrode surface area to 3000 µm2 and 4000 µm2 significantly increased charge-injection capacity, reduced the driving voltage required to deliver a fixed amount of charge, and reduced polarization of the electrodes during constant-current pulsing. Significance. This study establishes methods for choosing an activation protocol and a desired GSA for three-dimensional iridium electrodes suitable for neural tissue insertion and stimulation, and provides guidelines for evaluating electrochemical performance of AIROF using model saline solutions.

We have investigated the deposition and electrochemical properties of sputtered ruthenium oxide coatings for neural stimulation and recording electrodes. A combination of oxygen and water vapor was used as a reactive gas mixture during DC magnetron sputtering from a ruthenium metal target. The sputtering plasma was monitored by optical emission spectroscopy to determine the reactive species present and confirm the control of plasma chemistry by reactive gas flow rates into the deposition chamber. The effect of the O(2):H(2)O gas ratio on the microstructure and electrochemical properties of the ruthenium oxide were studied in detail. We employed a combination of surface characterization techniques, including scanning electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy, to understand the relationship between plasma chemistry and the microstructure of the films produced under different gas flow conditions. Electrochemical characterization included cyclic voltammetry, electrochemical impedance spectroscopy, and voltage transient measurements, performed on planar ruthenium oxide electrodes with a geometric surface area of 1960 μm(2). At an O(2):H(2)O gas flow rate ratio of 1:3, a cathodal charge-storage capacity per unit film thickness of 228.7 mC cm(−2) μm(−1) (median, Q1 = 134.5, Q3 = 236.6, n = 15) and a charge-injection capacity (0.6 V anodal interpulse bias) of 7.4 mC cm(−2) (median, Q1 = 6.9, Q3 = 8.3, n = 15) were obtained in phosphate buffered saline. The charge-injection capacity of ruthenium oxide sputtered with water vapor in the reactive plasma is comparable with sputtered iridium oxide (SIROF) and higher than reported values for porous TiN, a commonly employed high-surface area stimulation electrode coating.

The change in residual stress in plasma enhanced chemical vapor deposition amorphous silicon carbide (a-SiC:H) films exposed to air and wet ambient environments is investigated. A close relationship between stress change and deposition condition is identified from mechanical and chemical characterization of a-SiC:H films. Evidence of amorphous silicon carbide films reacting with oxygen and water vapor in the ambient environment are presented. The effect of deposition parameters on oxidation and stress variation in a-SiC:H film is studied. It is found that the films deposited at low temperature or power are susceptible to oxidation and undergo a notable increase in compressive stress over time. Furthermore, the films deposited at sufficiently high temperature (≥325 C) and power density (≥0.2 W cm-2 ) do not exhibit pronounced oxidation or temporal stress variation. These results serve as the basis for developing amorphous silicon carbide based dielectric encapsulation for implantable medical devices. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1654-1661, 2019.

Multisite implantable electrode arrays serve as a tool to understand cortical network connectivity and plasticity. Furthermore, they enable electrical stimulation to drive plasticity, study motor/sensory mapping, or provide network input for controlling brain-computer interfaces. Neurobehavioral rodent models are prevalent in studies of motor cortex injury and recovery as well as restoration of auditory/visual cues due to their relatively low cost and ease of training. Therefore, it is important to understand the chronic performance of relevant electrode arrays in rodent models. In this report, we evaluate the chronic recording and electrochemical performance of 16-channel Utah electrode arrays, the current state-of-the-art in pre-/clinical cortical recording and stimulation, in rat motor cortex over a period of 6 mo. The single-unit active electrode yield decreased from 52.8 ± 10.0 ( week 1) to 13.4 ± 5.1% ( week 24). Similarly, the total number of single units recorded on all electrodes across all arrays decreased from 106 to 15 over the same time period. Parallel measurements of electrochemical impedance spectra and cathodic charge storage capacity exhibited significant changes in electrochemical characteristics consistent with development of electrolyte leakage pathways over time. Additionally, measurements of maximum cathodal potential excursion indicated that only a relatively small fraction of electrodes (10–35% at 1 and 24 wk postimplantation) were capable of delivering relevant currents (20 µA at 4 nC/ph) without exceeding negative or positive electrochemical potential limits. In total, our findings suggest mainly abiotic failure modes, including mechanical wire breakage as well as degradation of conducting and insulating substrates. NEW & NOTEWORTHY Multisite implantable electrode arrays serve as a tool to record cortical network activity and enable electrical stimulation to drive plasticity or provide network feedback. The use of rodent models in these fields is prevalent. We evaluated chronic recording and electrochemical performance of 16-channel Utah electrode arrays in rat motor cortex over a period of 6 mo. We primarily observed abiotic failure modes suggestive of mechanical wire breakage and/or degradation of insulation.

Stretchable and flexible electronic applications require mechanically suitable electrical wiring. This article presents, for the first time, the microfabrication of photolithographically patterned microwiring on an electrospun nanofiber mat. The use of a photoresist-based technology allows for better than 10 μm pattern resolution on a good quality nanofiber mat, providing an order of magnitude smaller feature sizes than what has been published before. We demonstrate metallic wiring patterned from a 75 nm thick coating on top of the nanofiber mat. A silicone elastomer was incorporated to serve as a matrix material and form a composite substrate and an encapsulation layer on top of the microwiring. We demonstrate clean and anisotropic dry etching of the elastomer to open electrode sites that can be smaller than 10 μm in size. We speculate that these structures will be mechanically robust while being soft at the same time and provide the properties necessary for potential use in stretchable and flexible electronics.

In this work, we present three compositions of thiol-click polymers engineered to soften at various degrees upon implantation into the living body. Dynamic mechanical analysis has been used to assess the softening process and the storage modulus under simulated physiological conditions. In detail, we present polymers that are engineered to soften under physiological conditions within 10 to 20 minutes from 1.7-2.5 GPa to 1.7 GPa, 800 MPa, or 20 MPa, dependent on their composition. Critical material parameters such as the glass transition temperature and the cross-link density can be tuned by the choice of monomers and their relative proportions within the pre-polymer solution, as well as by the specific curing conditions used in the polymerization process. Because these polymers are used as substrate material for bioelectronic interfaces, we also present their electrical properties under simulated physiological conditions. For this, electrochemical impedance spectroscopy was used to assess dielectric constant and electrical resistivity. Typical relative permittivity of our compositions ranges from 4 to 6, while volume resistivity ranges from 10E10 to 10E13 Ohm*m, both measured at 37 ° C temperature in phosphate buffered saline solution. We discuss the results in the perspective of building functional electrical circuits on the presented polymer substrates.

Thiol-ene/acrylate-based shape memory polymers (SMPs) with tunable mechanical and thermomechanical properties are promising substrate materials for flexible electronics applications. These UV-curable polymer compositions can easily be polymerized onto pre-fabricated electronic components and can be molded into desired geometries to provide a shape-changing behavior or a tunable softness. Alternatively, SMPs may be prepared as a flat substrate, and electronic circuitry may be built directly on top by thin film processing technologies. Whichever way the final structure is produced, the operation of electronic circuits will be influenced by the electrical and mechanical properties of the underlying (and sometimes also encapsulating) SMP substrate. Here, we present electronic properties, such as permittivity and resistivity of a typical SMP composition that has a low glass transition temperature (between 40 and 60 &deg, C dependent on the curing process) in different thermomechanical states of polymer. We fabricated parallel plate capacitors from a previously reported SMP composition (fully softening (FS)-SMP) using two different curing processes, and then we determined the electrical properties of relative permittivity and resistivity below and above the glass transition temperature. Our data shows that the curing process influenced the electrical permittivity, but not the electrical resistivity. Corona-Kelvin metrology evaluated the quality of the surface of FS-SMP spun on the wafer. Overall, FS-SMP demonstrates resistivity appropriate for use as an insulating material.

Shape memory polymers (SMP) exhibit unique physical properties attractive for potential application in medical devices. These materials can assume a rigid solid state but soften after exposure to saline solution and heat, a feature which may aid the implantation process. Exposure to the biological environment leads to fluid absorption by the polymer coating resulting in the desired change in mechanical properties. The fluid uptake, however, also results in a change of electric properties that need to be taken into account when designing electrical components integrated with SMP. We developed procedures based on electrochemical impedance spectroscopy (EIS) to study the electric properties of saline-soaked SMP coatings. With these, we can observe the softening process as well as long-term electrical behavior. Simple circuit models are used to model the coating properties and calculate relative permittivity and electrical resistivity of the SMP material.

Objective Clinical applications of implantable microelectrode arrays are currently limited by device failure due to, in part, mechanical and electrochemical failure modes. To overcome this challenge, there is significant research interest in the exploration of novel array architectures and encapsulation materials. Amorphous silicon carbide (a-SiC) is biocompatible and corrosion resistant, and has recently been employed as a coating on biomedical devices including planar microelectrode arrays. However, to date, the three-dimensional Utah electrode array (UEA) is the only array architecture which has been approved by the food and drug administration (FDA) for long-term human trials. Approach Here, we demonstrate, for the first time, that UEAs can be fabricated with a-SiC encapsulation and sputtered iridium oxide film (SIROF) electrode coatings, and that such arrays are capable of single-unit recordings over a 30 week implantation period in rat motor cortex. Over the same period, we carried out electrochemical measurements, including voltage transients, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS), to evaluate potential failure modes. Furthermore, we evaluated chronic foreign body response via fluorescence immunohistochemistry following device explantation. Main results During the indwelling period, we observed a reduction in active electrode yield percentage from 94.6 ± 5.4 (week 1) to 16.4 ± 11.5% (week 30). While the average active electrode yield showed a steady reduction, it is noteworthy that 3 out of 8 UEAs recorded greater than 60% active electrode yield at all times through 24 weeks and 1 out of 8 UEAs recorded greater than 60% active electrode yield at all times through the whole implantation period. Significance In total, these findings further suggest that a-SiC may serve as a mechanically and electrochemically stable device encapsulation alternative to polymeric coatings such as Parylene-C.

The deposition and properties of sputtered iridium oxide films (SIROFs) using water vapor as a reactive gas constituent are investigated for their potential as high-charge-capacity neural stimulation electrodes. Systematic investigation through a series of optical and electrochemical measurements reveals that the incorporation of water vapor as a reactive gas constituent, along with oxygen, alters the reduction-oxidation (redox) state of the plasma as well as its morphology and the electrochemical characteristics, including the cathodal charge-storage capacity (CSCc ) and charge-injection capacity (CIC), of the SIROF. An apparent optimal O2 :H2 O gas ratio of 1:3 produced SIROF with a CSCc of 182.0 mC cm-2 μm-1 (median, Q1 = 172.5, Q3 = 193.4, n = 15) and a CIC of 3.57 mC cm-2 (median, Q1 = 2.97, Q3 = 4.58, n = 12) for 300-nm-thick films. These values are higher than those obtained with SIROFs deposited using no water vapor by a factor of 2.3 and 1.7 for the CSCc and CIC, respectively. Additionally, the SIROF showed minimal changes in electrochemical characteristics over 109 pulses of constant current stimulation and showed no indication of cytotoxicity toward primary cortical neurons in a cell viability assay. These results warrant investigation of the chronic recording and stimulation capabilities of the SIROF for implantable microelectrode arrays.

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

Silicone nerve cuff electrodes are commonly implanted on relatively large and accessible somatic nerves as peripheral neural interfaces. While these cuff electrodes are soft (1–50 MPa), their self-closing mechanism requires of thick walls (200–600 µm), which in turn contribute to fibrotic tissue growth around and inside the device, compromising the neural interface. We report the use of thiol-ene/acrylate shape memory polymer (SMP) for the fabrication of thin film multi-electrode softening cuffs (MSC). We fabricated multi-size MSC with eight titanium nitride (TiN) electrodes ranging from 1.35 to 13.95 × 10−4 cm2 (1–3 kΩ) and eight smaller gold (Au) electrodes (3.3 × 10−5 cm2; 750 kΩ), that soften at physiological conditions to a modulus of 550 MPa. While the SMP material is not as soft as silicone, the flexural forces of the SMP cuff are about 70–700 times lower in the MSC devices due to the 30 μm thick film compared to the 600 μm thick walls of the silicone cuffs. We demonstrated the efficacy of the MSC to record neural signals from rat sciatic and pelvic nerves (1000 µm and 200 µm diameter, respectively), and the selective fascicular stimulation by current steering. When implanted side-by-side and histologically compared 30 days thereafter, the MSC devices showed significantly less inflammation, indicated by a 70–80% reduction in ED1 positive macrophages, and 54–56% less fibrotic vimentin immunoreactivity. Together, the data supports the use of MSC as compliant and adaptable technology for the interfacing of somatic and autonomic peripheral nerves.

Microelectrode arrays that consistently and reliably record and stimulate neural activity under conditions of chronic implantation have so far eluded the neural interface community due to failures attributed to both biotic and abiotic mechanisms. Arrays with transverse dimensions of 10 &micro, m or below are thought to minimize the inflammatory response, however, the reduction of implant thickness also decreases buckling thresholds for materials with low Young&rsquo, s modulus. While these issues have been overcome using stiffer, thicker materials as transport shuttles during implantation, the acute damage from the use of shuttles may generate many other biotic complications. Amorphous silicon carbide (a-SiC) provides excellent electrical insulation and a large Young&rsquo, s modulus, allowing the fabrication of ultrasmall arrays with increased resistance to buckling. Prototype a-SiC intracortical implants were fabricated containing 8 - 16 single shanks which had critical thicknesses of either 4 &micro, m or 6 &micro, m. The 6 &micro, m thick a-SiC shanks could penetrate rat cortex without an insertion aid. Single unit recordings from SIROF-coated arrays implanted without any structural support are presented. This work demonstrates that a-SiC can provide an excellent mechanical platform for devices that penetrate cortical tissue while maintaining a critical thickness less than 10 &micro, m.

Current intracortical probe technology is limited in clinical implementation due to the short functional lifetime of implanted devices. Devices often fail several months to years post-implantation, likely due to the chronic immune response characterized by glial scarring and neuronal dieback. It has been demonstrated that this neuroinflammatory response is influenced by the mechanical mismatch between stiff devices and the soft brain tissue, spurring interest in the use of softer polymer materials for probe encapsulation. Here, we demonstrate stable recordings and electrochemical properties obtained from fully encapsulated shape memory polymer (SMP) intracortical electrodes implanted in the rat motor cortex for 13 weeks. SMPs are a class of material that exhibit modulus changes when exposed to specific conditions. The formulation used in these devices softens by an order of magnitude after implantation compared to its dry, room-temperature modulus of ~2 GPa.

Objective. We sought to develop a cervical spinal cord stimulator for the rat that is durable, stable, and does not perturb the underlying spinal cord. Approach. We created a softening spinal cord stimulation (SCS) array made from shape memory polymer (SMP)-based flexible electronics. We developed a new photolithographic process to pattern high surface area titanium nitride (TiN) electrodes onto gold (Au) interconnects. The thiol-ene acrylate polymers are stiff at room temperature and soften following implantation into the body. Durability was measured by the duration the devices produced effective stimulation and by accelerated aging in vitro. Stability was measured by the threshold to provoke an electromyogram (EMG) muscle response and by measuring impedance using electrochemical impedance spectroscopy (EIS). In addition, spinal cord modulation of motor cortex potentials was measured. The spinal column and implanted arrays were imaged with MRI ex vivo, and histology for astrogliosis and immune reaction was performed. Main results. For durability, the design of the arrays was modified over three generations to create an array that demonstrated activity up to 29 weeks. SCS arrays showed no significant degradation over a simulated 29 week period of accelerated aging. For stability, the threshold for provoking an EMG rose in the first few weeks and then remained stable out to 16 weeks; the impedance showed a similar rise early with stability thereafter. Spinal cord stimulation strongly enhanced motor cortex potentials throughout. Upon explantation, device performance returned to pre-implant levels, indicating that biotic rather than abiotic processes were the cause of changing metrics. MRI and histology showed that softening SCS produced less tissue deformation than Parylene-C arrays. There was no significant astrogliosis or immune reaction to either type of array. Significance. Softening SCS arrays meet the needs for research-grade devices in rats and could be developed into human devices in the future.

Introduction: Iridium oxide films have been investigated for multiple uses including electrochromic materials, pH sensors, and neural stimulation electrodes. In neural interfaces, activated iridium oxide film (AIROF) electrodes with planar or 3-dimensional geometries have been used to increase charge storage capacity and charge injection capacity for electrical stimulation of neural tissue. General properties of iridium oxide films have been investigated by several publications, leading to proposed film structures, observations on differences between anhydrous and hydrous iridium oxide films, and reported super-Nernstian changes in potential with pH. [1,2,3] In the present work, we investigated the activation of sharpened iridium microwire electrodes to AIROF using the same approach discussed in prior work. [4,5] We first sought to establish the charge-injection properties of AIROF electrodes in phosphate buffered saline and in an inorganic model of interstitial fluid as a function of the number of activation pulses. As the electrochemical properties of AIROF are known to be highly dependent on the prior oxidation state of the film, [1] we also sought to understand the charge injection Qinj capacity of AIROF when held at 0V compared to +0.6V vs. Ag|AgCl reference. Film cracking and delamination was observed in some electrodes during characterization, and so, we sought to understand the origin of the observed film instability and the role of limiting the maximum current allowed during activation. Methods: Iridium wire probes with 2000µm2 surface area were electrochemically cleaned in phosphate buffered saline (PBS). Cyclic voltammetry (CV) was recorded at 0.05V/s and 50V/s scan rates before and after cleaning. Each probe was activated at 25°C in air equilibrated PBS with 0.05Hz square wave potential pulsing between +0.8V and -0.6V vs. Ag|AgCl. Electrochemical impedance spectroscopy, CV, and voltage transient measurements at 0V and +0.6V bias vs. Ag|AgCl were recorded in both PBS and model interstitial fluid (m-ISF) at 37°C. Changes to the electrode surface were evaluated through cyclic voltammetry at multiple scan rates followed by scanning electron microscope (SEM) images of the film after soaking in DI water and drying. Results: Charge storage capacity (CSC) and maximum charge injection (Qinj) capacity increase significantly within the first 20 activation pulses. While CSC values in m-ISF are similar to values in PBS, Qinj capacity measured in m-ISF is significantly lower. Growth of the iridium oxide film was influenced by limiting the maximum current allowed during potential pulsing for activation. CSC and Qinj capacity of the 2000µm2 AIROF electrodes increased with increasing activation up to 1300 pulses. However, SEM images and CV profiles of probes activated without current limiting showed evidence of cracking in the microstructure of the film, and delamination of the film after 1300 pulses. Conclusions: Kinetics that are more informative for evaluating potential performance of neural stimulation devices in vivo can be observed when AIROF electrodes are measured in m-ISF in addition to PBS. Despite the long history of iridium oxide film characterization, parameters influencing activation are not yet fully understood as the dependence of film growth on current limiting found in this study was not expected. The degree to which activation parameters influence film growth currently appear to be specific to the electrode geometry and the electrochemical cell used and must be further understood to accomplish reliable transition to a stable oxide film covering the metal surface. Further study of the iridium oxide film structure and activation process is needed in order to standardize the activation process for the use of AIROF in neural devices. References: Steegstra, P., and Ahlberg, E. (2012). Influence of oxidation state on the pH dependence of hydrous iridium oxide films. Electrochim Acta, 76, 26-33. doi:10.1016/j.electacta.2012.04.143 Steegstra, P., Busch, M., Panas, I., and Ahlberg, E. (2013). Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution. J Phys Chem C, 117(40), 20975-20981. doi:10.1021/jp407030r Cherevko, S., Geiger, S., et al. (2016). Oxygen evolution activity and stability of iridium in acidic media. Part 2. – Electrochemically grown hydrous iridium oxide. J Electroanal Chem, 774, 102–110. doi:10.1016/j.jelechem.2016.05.015 Robblee, L. S., Lefko, J. L., and Brummer, S. B. (1983). Activated Ir: An Electrode Suitable for Reversible Charge Injection in Saline Solution. J Electrochem Society, 130(3), 731-733. doi:10.1149/1.2119793 Cogan, S. F., Guzelian, A. A., Agnew, W. F., Yuen, T. G., and McCreery, D. B. (2004). Over-pulsing degrades activated iridium oxide films used for intracortical neural stimulation. J Neurosci Methods, 137(2), 141-150. doi:10.1016/j.jneumeth.2004.02.019

The use of microelectronic devices for electrical stimulation and recording of neural electrical activity is a rapidly emerging technique that intends to restore lost or impaired physiological function in humans, such as spinal cord injury, stroke, sensory deficits and neurological disorders. The electrode material is one the most important components of such devices as it constituents the chronic electrode-electrolyte interface to the physiological environment. Therefore understanding the electrochemical properties of the electrodes is essential to provide electrical stimulation without adversely affecting the surrounding tissue. In this study, we explore the charge delivery mechanism of two transition metal oxides namely, iridium oxide (IrOx) and ruthenium oxide (RuOx) electrodes, systematically sputtered using water vapor as one of the reactive plasma constituents in addition to oxygen and argon. Additionally, we examine the effect of microstructure controlled by the composition of the reactive plasma constituents, on electrochemistry and charge density characteristics. Electrode sites with a geometric surface area (GSA) of 1960 μm2 were fabricated on silicon wafers using photolithography to create conductive gold metallization to define contact pads, electrode sites, and interconnect traces. The electrode sites were coated with sputtered IrOx and RuOx films using DC magnetron sputtering with a range of reactive gas ratios (O2:H2O) in the plasma. Material characterization techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to study the microstructure and composition of the electrode coatings. Electrochemical analysis included open circuit potential (Eoc) measurements, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and voltage transient (VT) measurements in electrolyte solutions that mimic the inorganic constituents of the physiological environment. Charge storage capacity (CSC) as a function of sweep and charge injection capacity (Qinj) as a function of pulse width and interpulse bias where calculated from the CV and VT measurements. Varying the plasma constituents show a systematic change in the morphology of both the IrOx and RuOx films. The magnitudes of CSC and Qinj also have a systematic dependence on the morphology of the films as controlled by the composition of the sputtering gases. Figure 1(a) shows a representative dependence of morphology and Qinj as a function of plasma composition for IrOx deposition with similar results obtained for RuOx films. The Eoc of the films was measured as a function of pH, as shown in figure 1(b), with IrOx and RuOx showing a near-Nernstian 62 mV/pH-unit and 59 mV/pH-unit, respectively. In this study, we demonstrate the dependence of IrOx and RuOx film morphology on the composition of the reactive plasma constituents. The relationship between film microstructure and electrochemistry at near-neutral pH has been investigated and the charge delivery mechanisms of sputtered IrOx and RuOx films characterized. The addition of water vapor as a reactive gas constituent increased the charge injection capacity of both IrOx and RuOx films using typical neural stimulation waveforms. Figure 1

Introduction: Emerging applications involving electrical stimulation and recording of nerve employ highly dense thin-film multielectrode arrays fabricated on flexible polymer substrates. Several failure mechanisms including the degradation of encapsulation and interlayer delamination have been identified in multilayer thin-film neural devices. A number of materials such as silicon oxide and silicon nitride have been used enhance the insulation hence increasing the density of the circuits; however, the incompatibility to biological environment, low breaking down field strength and frequency dependence behavior impede the use of these materials for bio applications. In this research, we report on the use of amorphous silicon carbide (a-SiC) which is a biocompatible insulator for electrical components and its electrical characterization in the metal insulator metal(MIM) capacitors. Materials and Methods: Different thicknesses of amorphous SiC (100-500 nm) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) was sandwiched between two metal layers (TiAuTi 40,300,40nm thick) to create MIM capacitors. Metal layers were deposited by e-beam evaporation and patterned using thin film photolithography techniques. Studies involved current leakage measurement of insulator between two metal electrodes, current-voltage measurement and dielectric constant measurement using parametric test systems. Tests and Results: In Current-Voltage (I-V) measurement method, voltage was swept from 0-40V with a rate of 50mV/s and current response was accordingly recorded to find the breaking down field potential. Resistivity of material for different thicknesses have been calculated to be around 5e12 ohm.cm ([Fig. A] comparable to silicon nitride), in addition, a low leakage current in the order of picoamp in electric potential of 0.125MV/cm was measured indicating reliability of a-SiC insulation property. Furthermore, dielectric constant of a-SiC was calculated from the capacitance-voltage and capacitance-frequency measurements [Fig. B] showing a linear behavior over the frequency of 0.1 MHz to 10 MHZ which is an indicator of great insulation within biological recording-stimulation frequency range. Conclusion: We have fabricated a-SiC based MIM capacitor and studied its electrical properties and achieved a comparable dielectric constant with a high resistivity to commonly used materials. The data suggests that a-SiC is a promising candidate for encapsulating highly dense electrical circuits for bioapplication and compatible to thin film microfabrication. Future work will include additional chemical characterization of a-SiC using FTIR to assess the long term chemical stability of this insulator. Figure 1

The cost-effective self-assembly of 80 nm Au nanoparticles (NPs) into large-domain, hexagonally close-packed arrays for high-sensitivity and high-fidelity surface-enhanced Raman spectroscopy (SERS) is demonstrated. These arrays exhibit specific optical resonances due to strong interparticle coupling, which are well reproduced by finite-difference time-domain (FDTD) simulations. The gaps between NPs form a regular lattice of hot spots that enable a large amplification of both photoluminescence and Raman signals. At smaller wavelengths the hot spots are extended away from the minimum-gap positions, which allows SERS of larger analytes that do not fit into small gaps. Using CdSe quantum dots (QDs) a 3-5 times larger photoluminescence enhancement than previously reported is experimentally demonstrated and an unambiguous estimate of the electromagnetic SERS enhancement factor of {approx}10{sup 4} is obtained by direct scanning electron microscopy imaging of QDs responsible for the Raman signal. Much stronger enhancement of {approx}10{sup 8} is obtained at larger wavelengths for benzenethiol molecules penetrating the NP gaps.

Objective: Foreign body response to indwelling cortical microelectrodes limits the reliability of neural stimulation and recording, particularly for extended chronic applications in behaving animals. The extent to which this response compromises the chronic stability of neural devices depends on many factors including the materials used in the electrode construction, the size, and geometry of the indwelling structure. Here, we report on the development of microelectrode arrays (MEAs) based on amorphous silicon carbide (a-SiC). Approach: This technology utilizes a-SiC for its chronic stability and employs semiconductor manufacturing processes to create MEAs with small shank dimensions. The a-SiC films were deposited by plasma enhanced chemical vapor deposition and patterned by thin-film photolithographic techniques. To improve stimulation and recording capabilities with small contact areas, we investigated low impedance coatings on the electrode sites. The assembled devices were characterized in phosphate buffered saline for their electrochemical properties. Main results: MEAs utilizing a-SiC as both the primary structural element and encapsulation were fabricated successfully. These a-SiC MEAs had 16 penetrating shanks. Each shank has a cross-sectional area less than 60 µmlsupg2l/supg and electrode sites with a geometric surface area varying from 20-200 μmlsupg2l/supg. Electrode coatings of TiN and SIROF reduced 1 kHz electrode impedance to less than 100 kΩ from ~2.8 MΩ for 100 µmlsupg2l/supg Au electrode sites and increased the charge injection capacities to values greater than 3 mC/cmlsupg2l/supg. Finally, we demonstrated functionality by recording neural activity from basal ganglia nucleus of Zebra Finches and motor cortex of rat. Significance: The a-SiC MEAs provide a significant advancement in the development of microelectrodes that over the years has relied on silicon platforms for device manufacture. These flexible a-SiC MEAs have the potential for decreased tissue damage and reduced foreign body response. The technique is promising and has potential for clinical translation and large scale manufacturing.

Superomniphobic surfaces are extremely repellent to virtually all liquids. By combining superomniphobicity and shape memory effect, metamorphic superomniphobic (MorphS) surfaces that transform their morphology in response to heat are developed. Utilizing the MorphS surfaces, the distinctly different wetting transitions of liquids with different surface tensions are demonstrated and the underlying physics is elucidated. Both ex situ and in situ wetting transitions on the MorphS surfaces are solely due to transformations in morphology of the surface texture. It is envisioned that the robust MorphS surfaces with reversible wetting transition will have a wide range of applications including rewritable liquid patterns, controlled drug release systems, lab-on-a-chip devices, and biosensors.

This study presents a new 3D printing process, the Diels–Alder reversible thermoset (DART) process, and a first generation of printable DART resins, which exhibit thermoset properties at use temperatures, ultralow melt viscosity at print temperatures, smooth part surface finish, and as-printed isotropic mechanical properties. This study utilizes dynamic covalent chemistry based on reversible furan-maleimide Diels–Alder linkages in the polymers, which can be decrosslinked and melt-processed during printing between 90 and 150 °C, and recrosslinked at lower temperatures to their entropically favored state. This study compares the first generation of DART materials to commonly 3D printed high-toughness thermoplastics. Parts printed from typical fused filament fabrication compatible materials exhibit anisotropy of more than 50% and sometimes upward of 98% in toughness when deformed along the build direction, while the first generation of DART materials exhibit less than 4% toughness reduction when deformed along the build direction. At room temperature, the toughest DART materials exhibit baseline toughness of 18.59 ± 0.91 and 18.36 ± 0.57 MJ m−3 perpendicular and parallel to the build direction, respectively. DART printing will enable chemists, polymer engineers, materials scientists, and industrial designers to translate new robust materials possessing targeted thermomechanical properties, multiaxial toughness, smooth surface finish, and low anisotropy.

We report on magnetotransport experiments investigating the effect of a regular array of nanoscale holes on the anisotropic response in the transition temperature of a superconducting niobium thin film. We find that the angle dependence of the critical temperature exhibits two strong anisotropic effects: Little-Parks oscillations whose period varies with field direction and a smooth background arising from one-dimensional confinement by the finite lateral space between neighboring holes. The two components of the anisotropy are intrinsically linked and appear in concert with one superimposed on top of the other.

A fundamental study on the sterilization of thiol-ene/acrylate polymers for biomedical applications is presented. These polymer networks belong to the emerging field of shape memory polymers and have the capability to undergo softening after insertion into the body. The impact of various sterilization methods, such as radiation, steam, and ethylene oxide on the thermomechanical properties of these stimuli responsive materials is investigated. Time and temperature dependent thermomechanical properties of sterilized and nonsterilized samples are determined by means of dynamic mechanical analysis in an aqueous environment to allow testing of polymers in phosphate buffered saline. The findings show that ethylene oxide sterilization is appropriate for thiol-ene and thiol-ene/acrylate based shape memory polymers. This method does not adversely affect thermomechanical and self-softening properties and after sterilization, endotoxin levels remain below the thresholds recommended in the FDA Guidance.

T he unique ability of plasmonic nanostructures to guide, enhance, and manipulate subwavelength light offers multiple novel applications in chemical and biological sensing, imaging, and photonic microcircuitry. Here the reproducible, giant light amplifi cation in multiscale plasmonic structures is demonstrated. These structures combine strongly coupled components of different dimensions and topologies that resonate at the same optical frequency. A light amplifi er is constructed using a silver mirror carrying light-enhancing surface plasmons, dielectric gratings forming distributed Bragg cavities on top of the mirror, and gold nanoparticle arrays selfassembled into the grating grooves. By tuning the resonances of the individual components to the same frequency, multiple enhancement of the light intensity in the nanometer gaps between the particles is achieved. Using a monolayer of benzenethiol molecules on this structure, an average SERS enhancement factor ∼ 10 8 is obtained, and the maximum enhancement in the interparticle hot-spots is ∼ 3 × 10 10 , in good agreement with FDTD calculations. The high enhancement factor, large density of well-ordered hot-spots, and good fi delity of the SERS signal make this design a promising platform for quantitative SERS sensing, optical detection, effi cient solid state lighting, advanced photovoltaics, and other emerging photonic applications.

Electron-beam-induced deposition (EBID) using gas-phase precursor molecules is an extensively studied fabrication technique. Liquid-phase metal deposition has recently been shown to achieve higher purity levels than traditional gas-phase deposition [1]. The goal of this investigation was to characterize liquid-phase silver deposition for further studies in photonics. A Scanning Electron Microscope (SEM) (FEI Nova 600 NanoLab Dual Beam) was used to deposit silver on polyimide membranes from aqueous AgNO3 solution by accelerating electrons into the solution for silver ion reduction. Atomic Force Microscopy (AFM) and SEM were subsequently used to characterize the size dependence to electron dosage. We observed granular silver deposits with sub-75 nm particle size and 200-250 nm total aggregate diameters. The CASINO (monte CArlo SImulation of electroN trajectory in sOlids) program was used to model electron trajectory in the solution to relate the size to the electron spread.

Close-packed monolayers self-assembled from ligated nanoparticles can form 10 nm thin sheets that stretch over micrometer-wide holes. Employing electron and focused ion beams, we show that one can locally tailor the strain in such sheets while they remain clamped around their perimeter, making it possible to imprint strain fields by design. Furthermore, using the nanoparticles themselves to track imposed strain gradients allows for the first direct measurement of Poisson’s ratio in these two-dimensional materials.

The simulation, fabrication and measurement of nonlinear photonic crystals (PhCs) with hexagonal symmetry in epitaxial BaTiO 3 were investigated. The optical transmission properties of a PhC were simulated by a 2-D finite-difference time domain (FDTD) method. A complete bandgap exists for both the TE and TM optical modes. The fabricated PhC has a well-defined stop band over the spectral region of 1525 to 1575 nm. A microcavity structure was also fabricated by incorporation of a line defect in the PhC. Transmission of the microcavity structure over the spectral region from 1456 to 1584nm shows a well-defined 5 nm wide window at 1495nm. Simulations indicate that the phase velocity matched PhC microcavity device of 0.5 mm long can potentially serve as modulator with a 3 dB bandwidth of 4 THz. Keywords : Hexagonal PhC; Bandgap; EO modulator. 1. INTRODUCTION Photonic crystal (PhC) devices, such as tunable filters, 1 optical switches, 2 optical delay lines 3 as well as low V Πelectro-optic modulators (EOM)

Two-dimensional photonic crystal waveguide structures were fabricated from BaTiO3thin film using focused ion beam method. At a wavelength of 1.55 μm, drive voltage has a factor of 6.6 improvement compared to conventional waveguide structure.

The optical properties of BaTiO3 two dimensional photonic crystal (PhC) nanocavities were investigated. Two types of nanocavities consisting of dopants and vacancies with PhC periodicities ranging from 200 to 550 nm were evaluated. The images from laser scanning confocal microscopy show the optical scattering of the PhC cavities is highly wavelength dependent. An optical intensity reversal is observed when the wavelength of probe light shifts by 29 nm. Meanwhile, intensity contrast between the nanocavity and its adjacent PhCs is enhanced as the PhC periodicity becomes shorter than the probe wavelength. To determine the photonic band structures fluorescence from dye covered PhCs were imaged and analyzed. A strong enhancement of fluorescence is observed for the PhC with a period of 200 nm. Upon comparison to the 2D finite difference time domain calculations, the enhancement is attributed to strong light localization within the PhC nanocavity. As a result, the in-plane lightwave propagation is prohibited that results in an increase in the vertical light scattering.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Deposits of aggregated silver particles were grown from aqueous silver-nitrate solution by electron-beam induced deposition in a liquid cell in a scanning electron microscope. Electron energies of 2, 5, and 20 keV were evaluated and found to produce distinguishably different deposits. Optimal energies exist for maximum growth rate and for minimum feature sizes. The physical structure of the deposits was found to evolve with increasing thickness as silver particles grow larger and develop more structure. Optical absorbance measurements and Raman spectroscopy on adsorbed dye suggest appropriate silver purity for using this fabrication method in nano-optics applications.

Self-assembled copper-silicide (Cu-Si) nanowires were prepared by the evaporation of Cu onto Si(001) under high vacuum conditions. The Cu-Si nanowires were studied in situ by scanning electron microscopy. Crystallographic, structural, and chemical properties of the nanowires were investigated by transmission electron microscopy at cross-sections of these nanowires fabricated by focused ion beam. In addition, chemical of nanowires were also studied by scanning transmission electron microscopy. The morphology of Cu-Si nanowires due to oxidation at ambient conditions was investigated as well. Experimental results obtained from the Cu-Si nanowires were compared to bulk data reported previously by other authors.

We report an elasticrelaxation and increase in local strain variation correlated with ferroelectric domains within epitaxial BiFeO3thin filmnanostructuresfabricated by combined electron-beam and focused ion-beam nanolithography. Nano-focused x-ray diffraction microscopy provided new insights into the relationship between film strain and ferroelectric domains in nanostructures, namely: (i) an out-of-plane (C-axis) elasticrelaxation of as much as −1.8% Δc/c in a BFO film-based nanostructure relative to the planar filmlattice constant; (ii) an out-of-plane rotation trending from the center towards all released edges of the nanostructure; and (iii) an increase of inter-domain strain variation within the nanostructure of approximately 10 times the inter-domain variation found within the planar film, correlated with ferroelectric domain boundaries as confirmed by piezoresponse-force microscopy. These results indicate that the release of in-plane BFO/SRO mismatch strain in a planar film is taken up by the local ferroelectric domain structure after patterning, resulting in greatly increased mechanical strain gradients within the structure.