Abstract
The design of novel bidirectional interfaces for in vivo and in vitro nervous systems is an important step towards future functional neuroprosthetics. Small electrodes, structures and devices are necessary to achieve high-resolution and target-selectivity during stimulation and recording of neuronal networks, while significant charge transfer and large signal-to-noise ratio are required for accurate time resolution. In addition, the physical properties of the interface should remain stable across time, especially when chronic in vivo applications or in vitro long-term studies are considered, unless a procedure to actively compensate for degradation is provided. In this short report, we describe the use and fabrication of arrays of 120 planar microelectrodes (MEAs) of sputtered Iridium Oxide (IrOx). The effective surface area of individual microelectrodes is significantly increased using electrochemical activation, a procedure that may also be employed to restore the properties of the electrodes as required. The electrode activation results in a very low interface impedance, especially in the lower frequency domain, which was characterized by impedance spectroscopy. The increase in the roughness of the microelectrodes surface was imaged using digital holographic microscopy and electron microscopy. Aging of the activated electrodes was also investigated, comparing storage in saline with storage in air. Demonstration of concept was achieved by recording multiple single-unit spike activity in acute brain slice preparations of rat neocortex. Data suggests that extracellular recording of action potentials can be achieved with planar IrOx MEAs with good signal-to-noise ratios.
Highlights
A number of reports, documenting the use of Iridium Oxide (IrOx) microelectrodes for neuroengineering and biomedical applications, have been recently presented in the literature
We report preliminary results employing neocortical acute tissue slices of the rat brain, selected as a target example to characterise IrOx electrode ageing and the recording of weak spontaneous electrical activity (Egert et al, 2002).In this context, in vitro simultaneous multisite recording of the spontaneous and evoked electrical activity of the nervous system represent an important step towards the understanding of the network-level and single-cell correlates of manyphysiological mechanisms (Rinaldi et al, 2007; Silberberg and Markram, 2007), as well as a crucial requirement for the validation of large-scale mathematical models and theories of the brain (Markram, 2006; Traub et al, 2005)
IRIDIUM ACTIVATION PROCESS IrOx microelectrodes were activated and restored to the activated state using a simple electrical protocol: each Microelectrode arrays (MEAs) electrode was connected to a potentiostat (AUTOLAB PGSTAT302, Echochemie, The Netherlands), with the counter and the reference terminals connected to a small platinum plate electrode, dipped into the phosphate buffer solution (PBS 10 mM phosphate buffer, 2.7 mM KCl and 137 mM NaCl, pH 7.4 at 25°C, Sigma-Aldrich, Germany) with a contact surface of a few square millimeters
Summary
A number of reports, documenting the use of IrOx microelectrodes for neuroengineering and biomedical applications, have been recently presented in the literature The interest in this material is driven by its excellent properties as a functional coating for implantable stimulation electrodes, with applications in stimulating/recording heart, neuronal or retinal tissues (Blau et al, 1997; Cogan, 2006; Marzouk et al, 1998; Mokwa, 2007). Low impedance and long-term stability of the interface is an important requirement and IrOx electrodes provide both of these requirements This short paper will examine and discuss the use of arrays of activated IrOx electrodes, and their use for short-term in vitro extracellular recording of neuronal electrical activity. We report preliminary results employing neocortical acute tissue slices of the rat brain, selected as a target example to characterise IrOx electrode ageing and the recording of weak spontaneous electrical activity (Egert et al, 2002).In this context, in vitro simultaneous multisite recording of the spontaneous and evoked electrical activity of the nervous system represent an important step towards the understanding of the network-level and single-cell correlates of many (patho)physiological mechanisms (Rinaldi et al, 2007; Silberberg and Markram, 2007), as well as a crucial requirement for the validation of large-scale mathematical models and theories of the brain (Markram, 2006; Traub et al, 2005)
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