Using Microelectrode Arrays for Cerebral Applications

The fundamental understanding of cerebral systems and associated diseases relies on the electrical recording of single neuron activity. This requires in vivo interfacing with neurons using micrometer-scale electrodes. Traditionally, this challenging task is being performed using: single wire electrodes, probes containing small ensembles of electrodes or, more recently, multi-electrode arrays. These electrode arrangements have the potential to gather signals in three-dimensional space and to provide important laminar information. However, the types of arrays that have been proposed so far are either restricted to sampling in a given plane or have difficulty in collecting data in complex regions, such as those found in highly convoluted cortices. Moreover, state-of-the-art micro-electrode systems are not (yet) suitable for obtaining highly stable signals over extended recording periods. In some cases, the probes are just too bulky to follow cortical motion in chronic applications. Also, the damage inflicted to tissue and the way tissue responds to the presence of a foreign body are presently restrictive to chronic neural recordings. Such a chronic use is, however, highly needed, since it allows the study of changes in population activity at single neuron level and at the interaction level with learning, memory and training.

To deal with these shortcomings, European technologists, scientists and industrialists have gathered forces in the NeuroProbes project, funded by the European Commission and coordinated by IMEC. Their goal is ambitious – to develop a new generation of microprobe arrays that addresses the above challenges and incorporates additional functions in one system.

NeuroProbes – An Integrative Concept

At the heart of the NeuroProbes concept is a unique microsystem integration solution that enables a modular integration of diverse features into a common platform. In essence, arrays are assembled in a modular fashion similar to the way Lego® bricks are put together. In this way, customization of arrays for diverse application conditions is possible for the first time with a combination of diverse functions: electrical recording and stimulation, drug delivery, and chemical sensing. Probes of different functionalities, dimensions and configurations can be interchangeably assembled to the array.

The probe arrays are assembled perpendicularly into a backbone using a novel out-of-plane interconnect technique. The individual probe arrays, also known as combs, are made of needle-like structures (or shanks) that are manufactured in silicon using deep reactive ion etching. Their fabrication process is compatible with CMOS fabrication.

Each probe shaft contains a number of electrodes. The resulting probe array has a slim profile which is conducive to a floating operation for chronic use in the brain. The probe arrays are sequentially assembled into a backbone leading to a three-dimensional electrode distribution. The interconnect consists of a gold blade which hangs over the edge of a cavity in which a probe is inserted. Once the probe is inserted into the cavity, electrical contact through mechanical caulking is established between the gold blades on the backbone and matching gold tracks on the probe.

The assembly process is done using a flip-chip bonder. The resulting interconnect density can be currently pushed to a pitch of 35μm. During assembly, both electrical and mechanical connections are established. The electrical connection between probes and external circuitry is performed via highly flexible ribbon cables that are consistent with the floating character of the entire set up (Figure 1).