Martin Schuettler

Micromachined, Polyimide-Based Devices for Flexible 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, we established a process technology to fabricate light-weighted and highly flexible polyimide based devices. Concerning the challenging housing demands close to the nerve to prevent mechanical induced nerve traumatization, we integrated interconnects to decouple the nerve interface from plugs and signal processing electronics. Hybrid integration with 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: sieve electrodes for regeneration studies, cuff electrodes for interfacing peripheral nerves, and a retina implant for ganglion cell stimulation. The discussion is focused on electrode and material properties and the hybrid assembly of a fully implantable neural prosthesis.

Fabrication of implantable microelectrode arrays by laser cutting of silicone rubber and platinum foil

A new method for fabrication of microelectrode arrays comprised of traditional implant materials is presented. The main construction principle is the use of spun-on medical grade silicone rubber as insulating substrate material and platinum foil as conductor (tracks, pads and electrodes). The silicone rubber and the platinum foil are patterned by laser cutting using an Nd:YAG laser and a microcontroller-driven, stepper-motor operated x-y table. The method does not require expensive clean room facilities and offers an extremely short design-to-prototype time of below 1 day. First prototypes demonstrate a minimal achievable feature size of about 30 microm.

Very Low-Noise ENG Amplifier System Using CMOS Technology

In this paper, we describe the design and testing of a system for recording electroneurographic signals (ENG) from a multielectrode nerve cuff (MEC). This device, which is an extension of the conventional nerve signal recording cuff, enables ENG to be classified by action potential velocity. In addition to electrical measurements, we provide preliminary in vitro data obtained from frogs that demonstrate the validity of the technique for the first time. Since typical ENG signals are extremely small, on the order of 1 1 muV, very low-noise, high-gain amplifiers are required. The ten-channel system we describe was realized in a 0.8 mum CMOS technology and detailed measured results are presented. The overall gain is 10 000 and the total input-referred root mean square (rms) noise in a bandwidth 1 Hz-5 kHZ is 291 nV. The active area is 12 mm2 and the power consumption is 24 mW from plusmn2.5 V power supplies

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.

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.

Flexible organic field effect transistors for biomedical microimplants using polyimide and parylene C as substrate and insulator layers

Biomedical micro implants are used as neural prostheses to restore body functions after paraplegia by means of functional electrical stimulation (FES). Polymer electronic technology offers the potential to integrate flexible electronic circuits on microelectrodes in order to overcome the limit of traditional FES systems. This paper describes an approach of flexible organic transistors in order to develop a flexible biomedical micro implant for FES use. Polyimide shows excellent biocompatibility and biostability properties for flexible multi-channel microelectrodes in neural prosthetics application (Stieglitz et al 1997 Sensors Actuators A 60 240–3). Therefore, it was used as a flexible substrate on which polymer transistors have to be integrated. Gold or platinum was sputtered as the gate, drain and source. In this paper polyimide has been investigated as a gate isolator because of its high flexibility and biocompatibility. Polyimide was spin coated and imidized at different temperatures and times. Pentacene (C14H22) was evaporated at UHV and 75 °C substrate temperature as an active layer in an organic field effect transistor (OFET). Plasma activation and self-assembled monolayer surface modification were used to advance the electrical properties of organic transistors. The whole transistor was encapsulated in parylene C that was evaporated at room temperature using a standard Gorham system (Gorham 1966 J. Polym. Sci. A-1 4 3027–39). Investigation of the electrical properties of the OFET using polyimide as the isolator led to promising results.

Design, in vitro and in vivo assessment of a multi-channel sieve electrode with integrated multiplexer

This paper reports on the design, in vitro and in vivo investigation of a flexible, lightweight, polyimide based implantable sieve electrode with a hybrid assembly of multiplexers and polymer encapsulation. The integration of multiplexers enables us to connect a large number of electrodes on the sieve using few input connections. The implant assembly of the sieve electrode with the electronic circuitry was verified by impedance measurement. The 27 platinum electrodes of the sieve were coated with platinum black to reduce the electrode impedance. The impedance magnitude of the electrode sites on the sieve (geometric surface area 2,200 microm(2)) was |Z(f=1kHz)| = 5.7 kOmega. The sieve electrodes, encased in silicone, have been implanted in the transected sciatic nerve of rats. Initial experiments showed that axons regenerated through the holes of the sieve and reinnervated distal target organs. Nerve signals were recorded in preliminary tests after 3-7 months post-implantation.

A voltage-controlled current source with regulated electrode bias-voltage for safe neural stimulation

A current source for neural stimulation is presented which converts arbitrary voltage signals to current-controlled signals while regulating the offset-voltage across the stimulation electrodes in order to keep the electrodes in an electrochemical state that allows for injecting a maximum charge. The offset-voltage can either be set to 0V or to a bias-voltage, e.g. of a few 100mV, as it can be advantageous for fully exploiting the charge injection capacity of iridium oxide electrodes.

On the stability of poly-ethylenedioxythiopene as coating material for active neural implants.

This article deals with the stability of poly-ethylenedioxythiopene (PEDOT) coatings under high loads of current pulses. Test parameters were chosen to match many peripheral nervous system applications in regard of charge injection, pulse width, and repetition frequency. PEDOT coatings were characterized with electrochemical impedance spectroscopes and pulse tests. After 60-100 million pulses, impedance increased and a more capacitive behavior was observed. A mean-time-to-failure of 127 million pulses could be calculated, suggesting a stable coating for at least this time frame with superior properties in regard of conventional platinum electrodes.

Fabrication of multi-layer, high-density micro-electrode arrays for neural stimulation and bio-signal recording

The electrode-tissue interface is of principal importance in neuroprosthesis. Indeed the successes of the cochlear implant and other therapeutic devices are directly attributable to the design and fabrication techniques of their interfaces with neural tissue, that is, the electrode or electrode array. Traditional fabrication techniques are often labor-intensive and do not lend themselves to automation thereby increasing the cost of the electrode, and owing to fabrication variability, potentially compromising the reliability of the devices incorporating them. Exacerbating the difficulties in electrode fabrication further is the fact that only a handful of materials have been demonstrated to be biologically inert. These same materials are often among the most difficult to utilize in the fabrication of neural electrodes. In the present paper, a new methodology for automated fabrication of high-density electrode arrays is presented. Using exclusively biologically-inert raw materials, laser machining techniques combined with multiple layer structuring is shown to achieve feature sizes of the order of 25 mum. As an illustrative example, a 98 electrode array for interfacing with surviving retinal tissue through a visual prosthesis for the blind is presented. Overall dimensions of the array are of the order of 8.7 times 9.4 mm, consistent with approximately 25 degrees of visual field.