J.-U. Meyer

Comparative study on micromorphology and humidity sensitive properties of thin-film and thick-film humidity sensors based on semiconducting MnWO4

Abstract 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.

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.

A novel thick-film ceramic humidity sensor

Abstract The paper describes the results of studies on the fabrication and characterisation of a thick-film humidity sensor based on the semiconducting metal oxide MnWO 4 . The sensor element possesses a novel ‘sandwich’-configuration with a 40 μm porous MnWO 4 ceramic layer sandwiched by two 10 μm polarity-reversed, interdigitated metal films. Instead of traditional glass frits, LiCl powders are used as adhesion promoters for sintering the sensor paste. With this method, MnWO 4 powders with an average particle size of 3.0 μm are sintered at the standard thick-film firing temperature of 850°C. The sintered ceramic layer exhibits a porous structure. The novel electrode arrangement combines the advantages of humidity sensors in the form of a parallel capacitor with those in the form of an interdigital capacitor, permitting a high sensitivity and a fast response. The influence of temperature on the sensor characteristics has been compensated for by integrating a thick-film NTC resistor. The humidity sensor shows no cross-sensitivity to organic vapour. The organic contamination on the sensor surface can be burned out by heating the sensor element at about 400°C with the refresh heater printed on the back side of the substrate.

Characterization and optimization of microelectrode arrays for in vivo nerve signal recording and stimulation

Revealing the complex signal-processing mechanisms and interconnection patterns of the nervous system has long been an intriguing puzzle. As a contribution to its understanding the optimization of the impedance behavior of implantable electrode arrays with via holes is discussed here. Peripheral axons will regenerate through these holes allowing for simultaneous nerve stimulation and signal recording. This approach is part of the ESPRIT project INTER and may eventually lead to devices driving sensory motor prosthesis with closed loop control. In the first set of experiments, micromachined platinum electrode arrays were prepared, characterized and optimized for nerve signal recording. The results of these studies are based on impedance spectroscopy and microscopic techniques. Equivalent circuits were modeled describing formally the electrical response behavior with ohmic resistances between 500 omega and 10 k omega. To attain low impedances for all electrodes on the INTER device, platinum from H2PtCl6 was electrodeposited, and sputter technology as well as electrochemical deposition from H2IrCl6 solution were used to produce thin iridium films. For the former, a lift-off process was established at one of the institutes to generate electrode structures with a line width of 5 microns. As a result in all three cases the electrodes showed almost constant impedances over the entire frequency range (10 Hz-1 kHz), which is relevant for nerve signal recording. In the second set of experiments, electrodes were optimized to allow for nerve stimulation. For this purpose, the charge delivery capacity (CDC) had to be increased and the impedance had to be decreased. Iridium oxide is the material of choice, because its CDC is much higher than the CDC of platinum at 75 microC/cm2 (Ziaie et al., 1991, IEEE Sensors & Actuators Transducers, 6, 124-127). A significant increase of the electrochemically active surface of the electrode structures could be observed by measuring the surface roughness. In first experiments, an activated iridium oxide film was formed with cyclic voltammetry and was evaluated using scanning force microscopy and impedance spectroscopy. The evaluation of the cyclic voltammograms showed a CDC up to 400 mC/cm2 for sputter deposited and oxidatively treated iridium films. Further investigations are directed towards increasing the stability of the iridium oxide electrodes with regard to long-term implants. Parallel experiments aim at the controlled axon adhesion without changing the impedance behavior of the described electrodes.

A novel thick film conductive type CO2 sensor

Abstract A low-cost CO2 sensor in thick film technology has been developed using BaTiO3 and various semiconducting oxides. The sensing principle of this new CO2 sensor is based on changes in conductance of the metal oxides in the presence of carbon dioxide. In this report, effects of preparation parameters and effects of various additives on the sensitivity of the CO2 sensor are described, and measurements of the sensor resistance as a function of CO2 concentration are presented. The sensor is applicable over a wide range of CO2 concentrations. Humidity changes have only minor effects on the sensor response.

Retina Implant — A BioMEMS Challenge

A key-application for microsystems in life-science are active microimplants for restoring and substituting lost or impaired biological functions in humans. The development of functional micro devices that fit in the human eye and that take over lost biological functions to restore vision is a challenge that several international research groups have engaged in. This paper describes the joint multi-team efforts of German scientists to develop a microelectromechnical implant to stimulate retinal structures for regaining lost visional functions. Different types of active eye implants have been developed for stimulation in the epi-retinal and sub-retinal space. Microelectrode arrays for electrical stimulation have been integrated into flexible and rigid substrates. Innovative microtechniques have been developed for obtaining biocompatible flexible microstructures, interconnects, and hybrid assemblies to fit in the eye ball. Biocompatibility of material and devices was investigated in all design phases utilizing in vitro cell and tissue culture test. Devices at various stages of development have been evaluated in acute and chronic animal models. Passive stimulator electrode arrays (without electronic components) remained in animals for more than six months after implantation Furthermore, animal experiments have shown that active devices equipped with silicon chips could be inserted and housed into the eye. Fully functional chronic devices are presently tested in animals. Experiments on humans are planned in the near future.

A biohybrid system to interface peripheral nerves after traumatic lesions: design of a high channel sieve electrode

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 kOmega at 1 kHz which ought to permit recording of signals from nerves sprouting through the sieve holes.

A new process for fabricating CO2-sensing layers based on BaTiO3 and additives

Abstract We present a new process for fabricating CO2 gas sensors. The process was to introduce heat-treatment in vacuum into fired sensing layers. The powders used for paste materials were BaTiO3 and additives, which were all microcrystalline in the range of 2–5 μm. Thick films were fired in air and subsequently heat-treated in vacuum. Their impedance was measured during vacuum-treatment. When the paste materials contained the additive of LaCl3, changes of impedance were observed. A low sensor resistance was achieved by adding 10 wt.% LaCl3 or higher amount to an equimolar mixture of BaTiO3 and CuO. When added LaCl3 amounted to 10 wt.%, a good sensitivity was obtained. Conventionally fired and vacuum-treated layers showed a different response to a change in partial oxygen pressure. Sensor surfaces were analysed using scanning electron spectroscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and Auger electron spectroscopy (AES). No grain size effects were observed. A change of the chemistry in the sensing material was measured. The change indicated that LaOCl, being thermally transformed from LaCl3 during firing, is further oxidised to LaxOy during vacuum-treatment. The analysis gives a possible explanation of the changes in sensor resistance and sensitivity induced by the vacuum process.

Electrostatically excited and capacitively detected flexural plate waves on thin silicon nitride membranes with chemical sensor applications

Abstract A new method for excitation and detection of flexural plate waves on thin silicon nitride membranes has been developed. The interdigital transducers employ electrostatic and capacitive principles. They operate without a piezoelectric substrate or layer. A micromachined FPW resonator based on the new principles has been developed and tested in practice. Due to the extremely thin membrane, the design allows the development of very sensitive gravimetric chemical and biological sensors working in gases, liquids and gels. A microcontroller signal processing unit has been built for controlling the FPW delay line input and for measuring the output parameters. With this unit, sensor response processing and drift compensation is achieved by means of software.

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.