Peripheral nerve lesions lead to nerve degeneration and flaccid paralysis. The first objective in functional rehabilitation of these diseases should be the preservation of the neuro-muscular junction by biological means and following functional electrical stimulation (FES) may restore some function of the paralyzed limb. The combination of biological cells and technical microdevices to biohybrid systems might become a new approach in neural prosthetics research to preserve skeletal muscle function. In this paper, a microdevice for a biohybrid system to interface peripheral nerves after traumatic lesions is presented. The development of the microprobe design and the fabrication technology is described and first experimental results are given and afterwards discussed. The technical microprobe is designed in a way that meets the most important technical requirements: adaptation to the distal nerve stump, suitability to combine the microstructure with a containment for cells, and integrated microelectrodes as information transducers for cell stimulation and monitoring. Micromachining technologies were applied to fabricate a polyimide-based sieve-like microprobe with 19 substrate-integrated ring electrodes and a distributed counter electrode. Monolithic integration of fixation flaps and a three-dimensional shaping technology led to a device that might be adapted to nerve stumps with neurosurgical sutures in the epineurium. First experimental results of the durability of the shaping technology and electrochemical electrode properties were investigated. The three-dimensional shape remained quite stable after sterilization in an autoclave and chronic implantation. Electrode impedance was below 200 kΩ at 1 kHz which ought to permit recording of signals from nerves sprouting through the sieve holes.

Nerves has been previously interfaced with micromachined sieve electrodes for recording and stimulation purposes. In most cases, so-called guidance channels have to be assembled on the sieve to adjust and fix the nerve in the vicinity of the microstructure. In this paper, a new approach of a flexible, polyimide-based sieve electrode is presented. Monolithically integrated fixation flaps offer the opportunity to directly fix the sieve implant on a transected nerve without the need for a guidance channel. The sieve has been fabricated with a thickness of 10 mum and chronically implanted between the transected ends of the sciatic nerve of rats. First electrophysiological experiments proved a functional reinnervation and a selective and graded stimulation over ring electrodes around single holes of 40 mum diameter in the sieve area.

Peripheral nerves have been previously interfaced with multi-electrode structures including sieve-shaped designs for regeneration of peripheral nerve axons through the sieve and cuff-shaped microelectrodes for interfacing intact peripheral nerve bundles. Both approaches assume intact motor neurons and neuromuscular junctions for excitation of skeletal muscle. An innovative biohybrid microdevice has been developed for housing gene manipulated nerve cell complexes on the microprobe. The microprobe is designed for implantation and for subsequent out-sprouting of probe based neural cells, generating a biological junction with the targeted skeletal muscle. Polymer micromachining technology has been applied to microfabricate 3-D flexible polymer structures with embedded microelectrodes. Adherence and growth of gene manipulated OLN-93 nerve cells on the probes have been investigated. Microprobes without cells have been implanted into the proximal stump of the sciatic nerve in the rat. Results demonstrate that the new design of 3-D biohybrid microprobes is well suited for implantation into the peripheral nervous system. OLN-93 nerve cells could adhere and grow on the microprobes under in-vitro conditions. The microprobes remained biostable and fixed at the original sites of the sciatic nerve 11 month after implantation

A method for manufacturing flexible neural implants with low-ohmic wire traces and improved stimulation electrodes is presented together with the results of electrical characterization. The main objective is to reduce the resistance of the wire traces compared to currently used interconnects which are made from sputtered gold. This is achieved by electroplating to obtain a gold layer with a thickness of 2–3μm. The resistance of the electroplated wire traces shows a reduction by a factor 30 and more compared to the sputtered wire traces. The electrode properties also improve (the electrode impedance is reduced to about a half due to the higher roughness of the gold surface) while the tests of the mechanical stability shows no increase in resistance when bending the implant up to 150,000 times in physiologic saline solution (0.9% NaCl) at 37°C.

Brinker, Thomas; Schlosshauer, Burkhard; Meyer, Jörg Uwe; Wewetzer, Konstantin Author Information

Our goal is to develop biohybrid neural microprobe implants with sieve electrodes for external stimulation of co-implanted neurons whose axons penetrate through the holes of electrodes and innervate host targets such as denervated muscle fibers. For evaluation of implants, potential scar formation was imitated in fibroblast–spinal cord co-cultures. In vitro neurite extension through flexible 10-μm thick polyimide sieves was inhibited by co-cultured fibroblasts. In contrast, the neurite penetration of sieves could be greatly stimulated by oriented exposure to Schwann cells. To our knowledge this is the first direct proof that Schwann cells display a guidance effect on spinal cord neurons in vitro. The results pave the way for novel biohybrid neuro-implants and provide means to circumvent the obstacle of inhibitory scar formation.

Advanced microtechnologies offer new opportunities for the development of active implants that go beyond the design of pacemakers and cochlea implants. Examples of future implants include neural and muscular stimulators, implantable drug delivery systems, intracorporal monitoring devices and body fluid control systems. The active microimplants demand a high degree of device miniaturization without compromising on design flexibility and biocompatibility requirements. With the need for integrating various microcomponents for a complex retina stimulator device, we have developed a novel technique for microassembly and high-density interconnects employing flexible, ultra-thin polymer based substrates. Pads for interconnections, conductive lines, and microelectrodes were embedded into the polyimide substrate as thin films. Photolithography and sputtering has been employed to pattern the microstructures. The novel "MicroFlex interconnection (MFI)" technology was developed to achieve chip size package (CSP) dimensions without the requirement of using bumped flip chips (FC). The MFI is based on a rivet like approach that yields an electrical and mechanical contact between the pads on the flexible polyimide substrate and the bare chips or electronic components. Center to center bond pad distances smaller than 100 /spl mu/m were accomplished. The ultra thin substrates and the MFI technology was proven to be biocompatible. Electrical and mechanical tests confirmed that interconnects and assembly of bare chips are reliable and durable. Based on our experience with the retina stimulator implant, we defined design rules regarding the flexible substrate, the bond pads, and the embedded conductive tracks. It is concluded that the MFI opens new venues for a novel generation of active implants with advanced sensing, actuation, and signal processing properties.

During recent years the authors have developed a variety of microelectrode-array based microdevices for interfacing single nerve cells in vitro and for interfacing neural structures inside the living body. Latter neuroprosthetic devices are designed to restore lost or impaired neural functions. This paper provides an overview on the authors' neural microdevices for interfacing and positioning single neural cells, for interfacing peripheral nerve structures, and for stimulating neural structures in the retina. The paper concludes with the authors' vision on biohybrid implants as future neuroprosthetic devices

In order to restore hand function in spinal cord injured people by functional electrical stimulation of arm nerves, the authors developed an 18polar neural cuff type electrode with integrated multiplexer circuit. This circuit reduces the number of necessary interconnection leads to a stimulator from twelve to four. Cable reduction was intended to reduce the risk of cable breakage which is one of the main reasons for implant failure. The multiplexer cuff electrode was fabricated applying a combination of micromachining, hybrid integration and traditional silicone technology, which enabled one to make the system robust, mechanically flexible and small in size

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.

Based on our experience of developing thick-film MnWO4 humidity sensors, a thin-film humidity sensor with nano-sized MnWO4 grains has been fabricated using the sol–gel technique. The thin-film sensor shows smaller humidity sensitivity than that compared to a thick-film sensor. However, it exhibits a fast response to humidity change and also has a very low temperature coefficient within the temperature range of 20–60°C. The absence of capillary pores in the well defined thin-films makes water condensation, and hence electrolytic conduction, impossible. The physically adsorbed multi-layered water molecules on the surface of the thin sensing film play a dominating role for the humidity sensing mechanism. Studies on thin-film sensors based on MnO and WO3 indicate that the W+6 sites in the hubnerite material contribute strongly to the humidity sensing mechanism.

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

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.

We present that low sensor resistance and high CO2 sensitivity were achieved when heat-treatment in vacuum was used to fabricate BaTiO3-based gas sensors. The powders were BaTiO3, CuO, and LaCl3, all of which were microcrystalline. The effect of heat-treatment in vacuum on sensor resistance and sensitivity were examined.;Surfaces of fired and vacuum-treated layers were analysed with SEM, EDX, XRD and AES. The analysis led to the conclusion that lattice vacancies are generated during the vacuum process which will explain increased electrical conductivity and improved sensing properties. Sensitivity enhancements are most likely due to defect structures in the sensing material.

A new generation of solid state sensors based on nanocrystalline BaTiO3 films containing various semiconducting oxides has been developed. The sensors were fabricated from BaTiO3-based nanopowder mixtures using thick film technology and were characterized regarding their CO2-gas sensitivity at different operation temperatures and gas environments. It was found that the sensors operate over a wide range of CO2 concentrations. Interfering gases have only minor effects on the sensor response. By using miniaturized Al2O3-substrates as carriers for the sensitive nanocrystalline layer a significant decrease of the power and powder consumption for each sensor was achieved.

A new stacking technology -MicroStack (MS)-for the assembly of multible integrated circuits (IC) and micro devices has been developed. It is based on the MicroFlex-Interconnection- (MFI) and Flip-Chip technology. Microdevices have been assembled on high density thin film flexible multilayer substrates in arrangements that minimize conductor path lengths, adjust to various housing shapes, and improve the thermal management of microdevices by integration of heat sinks. Flexible, 10 mum high-density thin film polyimide multilayer substrates (HDI-F) (Pyralin, PI 2611, DuPont) have been fabricated using micromachining techniques. The polyimide substrate serves for both, mechanical support for handling and electrical interconnection. The MFI (connection of chips through holes in the polyimide with bond balls) was used to assemble standard Ii's onto the polyimide substrates. In a first step, one chip is assembled on the back side of the flexible ribbon cable by the MFI technology. In the second step, a second chip is adjusted face to face to the bond balls onto the topside of the substrate. The ball served as a conductor studs for the Flip-Chip bonding of the top side chip. The polyimide substrate and the ball studs separate both chips with a distance of 20-60 mum. Several microsystem pairs were connected to one substrate. Such assembled substrates can build stacks by faulting. Heal sinks can be integrated by placing heating conductor devices between each chip pair. The MS technique was applied to place micro-devices with high interconnection densities of <150 <mu>m pitch. The chip pairs were flexible to bending and robust to mechanical handling at the same time. The MFI proved to be highly reliable in respect of tests according to MIL 883 1010 C.

For single-cell-biosensor applications and for studying neural networks using planar electrode substrates often the position of cells on detectors is necessary. Here, the authors present a new method for fast placement of single cells on ring electrodes by suction on suction holes. They describe the fabrication of a micromachined planar electrode substrate with suction holes and the placement of L929 cells and Neuro 2A cells on particular positions. The results are discussed regarding the deciding factors for placement. Finally, applications of the authors' cell placement technique to form patterned neural networks are outlined.

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) [1]. The thickness of the polyimide structure ranges from 5 to 15 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 IC's 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.

A new generation of solid state sensors using nanocrystalline BaTiO3 and various semiconducting oxides has been developed. In this report, fabrication of nanocrystalline materials, sensor fabrication by using thick film technology and sensor characterization is described. The influence of various operation-temperatures is described and measurements of the sensor with respect to various CO2 concentrations are presented. The sensor is applicable over a wide range of CO2 concentrations. Interfering gases have only minor effects on the sensor response.