Sebastian Kisban

Fabrication technology for silicon-based microprobe arrays used in acute and sub-chronic neural recording

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.

Brain–computer interfaces: an overview of the hardware to record neural signals from the cortex

Brain-computer interfaces (BCIs) record neural signals from cortical origin with the objective to control a user interface for communication purposes, a robotic artifact or artificial limb as actuator. One of the key components of such a neuroprosthetic system is the neuro-technical interface itself, the electrode array. In this chapter, different designs and manufacturing techniques will be compared and assessed with respect to scaling and assembling limitations. The overview includes electroencephalogram (EEG) electrodes and epicortical brain-machine interfaces to record local field potentials (LFPs) from the surface of the cortex as well as intracortical needle electrodes that are intended to record single-unit activity. Two exemplary complementary technologies for micromachining of polyimide-based arrays and laser manufacturing of silicone rubber are presented and discussed with respect to spatial resolution, scaling limitations, and system properties. Advanced silicon micromachining technologies have led to highly sophisticated intracortical electrode arrays for fundamental neuroscientific applications. In this chapter, major approaches from the USA and Europe will be introduced and compared concerning complexity, modularity, and reliability. An assessment of the different technological solutions comparable to a strength weaknesses opportunities, and threats (SWOT) analysis might serve as guidance to select the adequate electrode array configuration for each control paradigm and strategy to realize robust, fast, and reliable BCIs.

Microprobe Array with Low Impedance Electrodes and Highly Flexible Polyimide Cables for Acute Neural Recording

This paper reports on a novel type of silicon- based microprobes with linear, two and three dimensional (3D) distribution of their recording sites. The microprobes comprise either single shafts, combs with multiple shafts or 3D arrays combining two combs with 9, 36 or 72 recording sites, respectively. The electrical interconnection of the probes is achieved through highly flexible polyimide ribbon cables attached using the MicroFlex Technology which allows a connection part of small lateral dimensions. For an improved handling, probes can be secured by a protecting canula. Low-impedance electrodes are achieved by the deposition of platinum black. First in vivo experiments proved the capability to record single action potentials in the motor cortex from electrodes close to the tip as well as body electrodes along the shaft.

Comparative study on the insertion behavior of cerebral microprobes

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.

A Novel Assembly Method for Silicon-Based Neural Devices

This paper reports on a novel interconnection technology for silicon-based neural devices used in extracellular recording. It is based on a flip-chip bonding process establishing the electrical connection of the silicon specimen and a highly flexible polymer cable via electroplated contact structures. This process makes possible the parallel connection of multiple bonding pads with high integration density. By the integration of a structurable fluoropolymer on the cables as an adhesion layer the mechanical connection is improved simultaneously. Further, the adhesion layer inherently encapsulates the bonding sites and allows omitting the application of an additional underfill. Test structures were characterized with respect to the electrical and mechanical reliability of this interconnection approach. The storage in Ringer’s solution for 12 days minimally affected the cable connection and its inherent encapsulation capability. Furthermore, a neural device was assembled with this technology and its capability for intracortical neural recording was demonstrated.

A 3D slim-base probe array for in vivo recorded neuron activity

This paper introduces the first experimental results of a new implantable slim-base three-dimensional (3D) probe array for cerebral applications. The probes are assembled perpendicularly into the slim-base readout platform where electrical and mechanical connections are achieved simultaneously. A new type of micromachined interconnect has been developed to establish electrical connection using extreme planarization techniques. Due to the modular approach of the platform, probe arrays of different dimensions and functionality can be assembled. The platform is only several hundred microns thick which is highly relevant for chronic experiments in which the probe array should be able to float on top of the brain. Preliminary tests were carried out with the implantation of a probe array into the auditory cortex of a rat.

Hybrid microprobes for chronic implantation in the cerebral cortex

This paper reports on a neural device for chronic implantation into the cerebral cortex. Silicon microprobes with 36 electrodes arranged on four shafts are fabricated using MEMS technology. The hybrid integration of a ribbon cable with high flexibility provides the connection of the electrodes to external instrumentation. Crosstalk between the channels is investigated, as well as the electrode stability for a time period of one month in vitro. Due to the geometry and the mechanical stability of these microprobes, insertions are possible without the need for prior opening of the dura mater. A dedicated insertion tool has been fabricated to achieve a precise insertion of the microprobes and their subsequent mechanical decoupling. Additionally, a protection chamber allowing the secure attachment of two connector units on the skull is introduced. The short-time chronic implantation of microprobes showed that neural activity can be recorded, including single unit activity, which was present after four days.

Novel method for the assembly and electrical contacting of out-of-plane microstructures

This paper reports on the assembly of out-of-plane microstructures with multiple electrical contacts established at the same time. The reported structures are intended for neuroscientific applications. Dedicated bays in a platform ensure the orthogonal orientation of the microstructure. In addition, a 10-µm-thick polymer cable is clamped between bays and microstructures and establishes the fixation of the probes. This cable, enhanced by gold bumps, ensures the transfer of conducting lines from the horizontal into the vertical direction. It simultaneously provides the electrical contacts to the microstructures. The contact resistance of individual contacts was found to be smaller than 2 Ω.

Discrete cortical responses from multi-site supra-choroidal electrical stimulation in the feline retina

Exploration into electrical stimulation of the retina has thus far focussed primarily upon the development of prostheses targeted at one of two sites of intervention - the epi- and sub-retinal surfaces. These two approaches have sound, logical merit owing to their proximity to retinal neurons and their potential to deliver stimuli via the surviving retinal neural networks respectively. There is increasing evidence, however, that electric field effects, electrode engineering limitations, and electrode-tissue interactions limit the spatial resolution that once was hoped could be elicited from electrical stimulation at epi- and sub-retinal sites. An alternative approach has been proposed that places a stimulating electrode array within the supra-choroidal space - that is, between the sclera and the choroid. Here we investigate whether discrete, cortical activity patterns can be elicited via electrical stimulation of a feline retina using a custom, 14 channel, silicone rubber and Pt electrode array arranged in two hexagons comprising seven electrodes each. Cortical responses from Areas 17/18 were acquired using a silicon-based, multi-channel, penetrating probe developed at IMTEK, University of Freiburg, within the European research project NeuroProbes. Multi-unit spike activity was recorded in synchrony with the presentation of electrical stimuli. Results show that distinct cortical response patterns could be elicited from each hexagon separated by 1.8 mm (center-to-center) with a center-to-center electrode spacing within each hexagon of 0.55 mm. This lends support that higher spatial resolution may also be discerned.

Building the human-chip interface

This paper summarizes several interconnection issues related to brain-computer interfaces and neuroprosthetics. Silicon-based out-of-plane integration and interconnection to achieve partially-invasive brain-computer interfaces will be described.