Objectives Neural stimulation is well-accepted as an effective therapy for a wide range of neurological disorders. While the scale of clinical devices is relatively large, translational and pilot clinical applications are underway for microelectrode-based systems. Microelectrodes have the advantage of stimulating a relatively small tissue volume which may improve selectivity of therapeutic stimuli. Current microelectrode technology is associated with chronic tissue response which limits utility of these devices for neural recording and stimulation. One approach for addressing the tissue response problem may be to reduce physical dimensions of the device. “Thinking small” is a trend for the electronics industry, and for implantable neural interfaces, the result may be a device that can evade the foreign body response. Materials and Methods This review paper surveys our current understanding pertaining to the relationship between implant size and tissue response and the state-of-the-art in ultra-small microelectrodes. A comprehensive literature search was performed using PubMed, Web of Science (Clarivate Analytics), and Google Scholar. Results The literature review shows recent efforts to create microelectrodes that are extremely thin appear to reduce or even eliminate the chronic tissue response. With high charge capacity coatings, ultra-microelectrodes fabricated from emerging polymers and amorphous silicon carbide appear promising for neurostimulation applications. Conclusion We envision the emergence of robust and manufacturable ultra-microelectrodes that leverage advanced materials where the small cross-sectional geometry enables compliance within tissue. Nevertheless, future testing under in vivo conditions is particularly important for assessing the stability of thin film devices under chronic stimulation.

Objective.-With ever increasing applications of neural recording and stimulation, the necessity for developing neural interfaces with higher selectivity and lower invasiveness is inevitable. Reducing the electrode size is one approach to achieving such goals. In this study, we investigated the effect of electrode geometric surface area (GSA), from 20 μm 2 to 1960 μm 2 , on the electrochemical impedance and charge-injection properties of sputtered iridium oxide (SIROF) coated electrodes in response to current-pulsing typical of neural stimulation. These data were used to assess the electrochemical properties of ultra-small SIROF electrodes (GSA < 200 μm 2 ) for stimulation and recording applications.Approach.-SIROF charge storage capacities (CSC), impedance, and charge-injection characteristics during current-pulsing of planar, circular electrodes were evaluated in an inorganic model of interstitial fluid (model-ISF).Main results.-SIROF electrodes as small as 20 μm 2 could provide 1.3 nC/phase (200 μs pulse width, 0.6 V versus Ag|AgCl interpulse bias) of charge during current pulsing. The 1 kHz impedance of all electrodes used in this study were below 1 MΩ, which is suitable for neural recording.Significance.-Ultra-small SIROF electrodes are capable of charge injection in buffered saline at levels above some reported thresholds for neural stimulation with microelectrodes.

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At high frequencies (>10 kHz) electrode double-layer charging is the principal mechanism of charge-injection and selection of the electrode material has little effect on polarization, with platinum, iridium oxide, and titanium nitride exhibiting similar behavior. High frequency stimulation is dominated by a highly nonuniform primary current distribution.

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Size and material considerations are important in the development of next-generation chronically reliable neural interface devices. In this review, we discuss the use of amorphous silicon carbide (a-SiC) for the fabrication of indwelling electrode arrays with ultrathin penetrating shanks for neural stimulation and recording. The a-SiC film is stable in saline environments and has a high intrinsic stiffness that allows fabrication of tissue-penetrating arrays with extremely small cross-sectional areas (<60 μm2). Present literature on arrays with extremely small shanks and/or ultramicroelectrode (UME) sites are reviewed. Properties of a-SiC films and their current biomedical applications are summarized. Reduced shank dimensions increase the flexibility of high Young's modulus a-SiC arrays. Iridium oxide-coated UMEs had electrochemical properties suitable for neural recording and stimulation, and recorded neural signals with high amplitudes and high signal-to-noise ratios. UMEs and a-SiC may provide a platform for next-generation high-density chronic neural interface devices.

A dispersion relation for the plasma loaded free-electron laser (FEL), with a helical wiggler and an axial magnetic field is derived. The cold fluid formulation is used with self-fields of the electron beam taken into account. By solving the dispersion relation numerically the influence of self-fields on the FEL resonance and the two-stream instability is investigated. It was found that although self-fields have strong effect on the FEL resonance, their effects on the two-stream instability is much weaker.