Hansjoerg Beutel

A flexible, light-weight multichannel sieve electrode with integrated cables for interfacing regenerating peripheral nerves

Abstract It has been shown previously that peripheral nerve axons regenerate through microvias in silicon devices. A major challenge in the design of a biocompatible interface is to establish a reliable electrical and mechanical interconnection to signal-processing and transmission electronics which allows simultaneous multichannel recordings or stimulation of nerves. This paper describes the on-going work of developing a new generation of flexible and extremely light-weight electrode arrays with integrated cables. A process technology has been established to fabricate a multilayer device with micromachining methods, which overcomes the ‘classical’ separation of substrate and insulation layers. The micromachined electrodes exhibit promising mechanical stability and high insulation resistance.

“Microflex”—A New Assembling Technique for Interconnects:

A new interconnection technique has been developed that allows versatile multiple strand connections between microsensors, sensor arrays, and chips designed for wire bonding. The new technique has been termed “Microflex interconnection” technique (MFI). Conventional wire bonding technique is commonly restricted to planar interconnects with a limited degree of freedom for placing microsystem components in hybrid assemblies. The MFI technique has overcome this limitation by interconnecting microsystem components through custom designed flexible substrates with embedded metallized conductors, pad arrays for integrated circuits’ assembly and substrate integrated electrodes. Standard CMOS components without additional pad metallization can be used. The integration density of the MFI technique corresponds to one of the flip-chip technology. Special advantages of the MFI technique are three-dimensional interconnects, the flexibility in design and shape, and easy visual inspection of alignment qualities. The method is especially suitable for small volumes of customer specified devices and biomedical implants because all materials used are biocompatible. Within this paper, the MFI technique is introduced, described in detail, and tested according to international standards. The evaluation of the electrical and mechanical properties of the interconnection sites exhibited promising results regarding stability and reliability. First applications in the biomedical field were presented on the example of a neural implant and a sensorized cardiac catheter.

Fast and precise positioning of single cells on planar electrode substrates

For cell biosensors and for studying neural networks using planar electrode substrates, a suitable technique for positioning single cells on electrodes was needed. We reported a new method for fast and efficient positioning of single cells on ring electrodes by controlled suction through holes. We described the microfabrication of electrode substrates with microholes and the cell positioning procedure. L929 cells and Neuro 2A cells could be positioned in parallel without cell damage.

Flexible, polyimide-based neural interfaces

Micromachining technologies were established to fabricate microelectrode arrays and devices for interfacing parts of the central or peripheral nervous system in case of neuronal disorders. The devices were part of a neural prosthesis that allows simultaneous multichannel recording and multisite stimulation of neurons. Overcoming the brittle mechanics of silicon devices and challenging housing demands close to the nerve, we established a process technology to fabricate light-weighted and highly flexible polyimide based devices with integrated interconnects. A new assembling technique-the microflex interconnection (MFI)-has been applied for the connection of the flexible microsystems to silicon microelectronics. In this paper, we present different shapes and applications of the flexible electrodes. The discussion is focused on electrode properties and the hybrid assembly of a fully implantable neural prosthesis.

Microflex: a new technique for hybrid integration for microsystems

This paper describes a new interconnection wire method which allows versatile multiple strand connections between microsensors: sensor arrays and integrated circuits (IC). The interconnection method is termed Micro Flex Interconnects (MFI). One example for this technology is the connection of implantable, highly flexible neural micro devices to electronics for interfacing to the external world. The interconnection technique is based on a novel multilayer process using polyimide (Du Pont PI 2611). The thickness of the polyimide structure ranges from 5 to 15 /spl mu/m including the insulation layers. Several metallization layers can be embedded in the material. This approach exhibits same advantages. The involved material is non-toxic and the ICs do not need any additional bond pad metallization. The MFI technique has been proven long-term stable. The metallization material can be chosen accordingly for electrodes, conducting lines, and connection pads. An commercial ball wedge bonder is the only equipment needed to perform the MFI method.

Micromachined devices for interfacing neurons

Micromachining technologies were established to fabricate microelectrode arrays and devices for interfacing parts of the central or peripheral nervous system. The devices were part of a neural prosthesis that allows simultaneous multichannel recording and multisite stimulation of neurons. Overcoming the brittle mechanics of silicon devices and challenging housing demands close to the nerve we established a process technology to fabricate light-weighted and highly flexible polyimide based devices. Platinum and iridium thin-film electrodes were embedded in the polyimide. With reactive ion etching we got the possibility to simply integrate interconnections and to form nearly arbitrary outer shapes of the devices. We designed multichannel devices with up to 24 electrodes in the shape of plates, hooks and cuffs for different applications. In vitro tests exhibited stable electrode properties and no cytotoxicity of the materials and the devices. Sieve electrodes were chronically implanted in rats to interface the regenerating sciatic nerve. After six months, recordings and stimulation of the nerve via electrodes on the micro-device proved functional reinnervation of the limb. Concentric circular structures were designed for a retina implant for the blind. In preliminary studies in rabbits, evoked potentials in the visual cortex corresponded to stimulation sites of the implant.