This work presents a new fabrication technology for silicon-based neural probe devices and their assembly into two-dimensional (2D) as well as three-dimensional (3D) microprobe arrays for neural recording. The fabrication is based on robust double-sided deep reactive ion etching of standard silicon wafers and allows full 3D control of the probe geometry. Wafer level electroplating of gold pads was performed to improve the 3D assembly into a platform. Lithography-based probe-tracking features for quality management were introduced. Probes for two different assembly methods, namely direct bonding to a flexible micro-cable and platform-based out-of-plane interconnection, were produced. Systems for acute and sub-chronic recordings were assembled and characterized. Recordings from rats demonstrated the recording capability of these devices.

Extracellular recordings from the brain are the basis for the fundamental understanding of the complex interaction of electrical signals in neural information transfer. Going beyond wire electrodes and bundles of electrode wires such as tetrodes, multielectrode arrays based on silicon technologies are receiving growing attention, since they enable a pronounced increase in the number of recording sites per probe shaft. In this paper, recent innovations contributed by the authors to the development of probe arrays based on microelectromechanical system (MEMS) technologies within the EU-funded research project NeuroProbes are described. The resulting structures include passive electrode arrays based on single-shaft and four-shaft probes comprising nine planar electrodes per shaft with lengths of up to 8 mm. Further, active probe arrays with complementary metal-oxide-semiconductor (CMOS) circuitry integrated on the probe shaft, enabling the arrangement of 188 electrodes in two columns along a 4-mm-long probe shaft with an electrode pitch of only 40 µm, are described. These active probes were developed for an electronic depth control. Further, the paper reports assembly technologies for combining the probe arrays with highly flexible ribbon cables. Applications of the probes in in vivo experiments are summarized. 

A series of experiments has been conducted with probes made from silicon, glass, tungsten and polyimide within a developed brain phantom, and the insertion behavior, forces and dimpling are compared to in vitro and in vivo models. This allows the choice of proper insertion parameters and probe structure to reach a compromise between needle stability and tissue trauma as a result of insertion. According to the performed experiments, the reduced interfacial area between the needle tip and the brain will result in reduced insertion force. High insertion speed (100 mm/min) reduces the dimpling but not the penetration force necessarily. In vivo insertion and retraction of the fragile probes made from silicon is possible without pia and/or dura removal.

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