The applicability of parylene C as an encapsulation material for implanted neural prostheses was characterized and optimized. The adhesion of parylene C was tested on different substrate materials, which were commonly used in neural prostheses and the efficiency of different adhesion promotion methods was investigated. On Si(3)N(4), platinum, and on a first film of parylene C, a satisfactory adhesion was achieved with Silane A-174, which even withstood standard steam sterilization. The adhesion to gold and polyimide could not be improved sufficiently with the tested methods. Furthermore, tensile tests and measurements of the degree of crystallinity were performed on untreated, on steam sterilized, and on annealed parylene C layers to investigate the influence of thermal treatment. This led to more brittle and stiffer films due to an increase in the crystalline portion in the parylene layers. Finally, an electrochemical impedance spectroscopy was used to test if a parylene C layer was able to protect a metallic structure against corrosion on a Si(3)N(4) substrate. The results indicated that this could be only possible by treating the substrate with Silane A-174. To receive parylene C layers with a good encapsulation performance, it is important to consider the materials, which are used in the neural prosthesis, to find the best suited process parameters.

Characterization of parylene C as an encapsulation material for implanted neural prostheses

Numerous applications in neuroscience research and neural prosthetics, such as electrocorticogram (ECoG) recording and retinal prosthesis, involve electrical interactions with soft excitable tissues using a surface recording and/or stimulation approach. These applications require an interface that is capable of setting up high-throughput communications between the electrical circuit and the excitable tissue and that can dynamically conform to the shape of the soft tissue. Being a compliant material with mechanical impedance close to that of soft tissues, polydimethylsiloxane (PDMS) offers excellent potential as a substrate material for such neural interfaces. This paper describes an integrated technology for fabrication of PDMS-based stretchable microelectrode arrays (MEAs). Specifically, as an integral part of the fabrication process, a stretchable MEA is directly fabricated with a rigid substrate, such as a thin printed circuit board (PCB), through an innovative bonding technology-via-bonding-for integrated packaging. This integrated strategy overcomes the conventional challenge of high-density packaging for this type of stretchable electronics. Combined with a high-density interconnect technology developed previously, this stretchable MEA technology facilitates a high-resolution, high-density integrated system solution for neural and muscular surface interfacing. In this paper, this PDMS-based integrated stretchable MEA (isMEA) technology is demonstrated by an example design that packages a stretchable MEA with a small PCB. The resulting isMEA is assessed for its biocompatibility, surface conformability, electrode impedance spectrum, and capability to record muscle fiber activity when applied epimysially.

A PDMS-Based Integrated Stretchable Microelectrode Array (isMEA) for Neural and Muscular Surface Interfacing

Multi-channel electrical recordings of neural activity in the brain is an increasingly powerful method revealing new aspects of neural communication, computation, and prosthetics. However, while planar silicon-based CMOS devices in conventional electronics scale rapidly, neural interface devices have not kept pace. Here, we present a new strategy to interface silicon-based chips with three-dimensional microwire arrays, providing the link between rapidly-developing electronics and high density neural interfaces. The system consists of a bundle of microwires mated to large-scale microelectrode arrays, such as camera chips. This system has excellent recording performance, demonstrated via single unit and local-field potential recordings in isolated retina and in the motor cortex or striatum of awake moving mice. The modular design enables a variety of microwire types and sizes to be integrated with different types of pixel arrays, connecting the rapid progress of commercial multiplexing, digitisation and data acquisition hardware together with a three-dimensional neural interface.

Massively parallel microwire arrays integrated with CMOS chips for neural recording

The concept of an electronic visual prosthesis has been investigated since the early 20th century. While the first generation of long-term implantable devices were defined by the turn of the millennium, the greatest progress has been achieved in the past decade. This review describes the current state of the art of visual prosthesis investigated by more than two dozen active groups in this field of research. The focus is on technological solutions in regard to long-term safety of materials, electrode–tissue interfaces and encapsulation technologies. Furthermore, we critically assess the maximum number of stimulating electrodes each technological approach is likely to provide.

Bionic vision: system architectures – a review

Nanotechnology and neurophysiology.

A very large format neural stimulator device, to be used in future retinal prosthesis experiments, has been designed, fabricated, and tested The device was designed to be positioned against a human retina for short periods in an operating room environment Demonstrating a very large format, parallel interface between a 2-D microelectronic stimulator array and neural tissue would be an important step in proving the feasibility of high resolution retinal prosthesis for the blind The architecture of the test device combines several novel components, including microwire glass, a microelectronic multiplexer, and a microcable connector The array format is 80 times 40 array pixels with approximately 20 microwire electrodes per pixel The custom assembly techniques involve indium bump bonding, ribbon bonding, and encapsulation The design, fabrication, and testing of the device has resolved several important issues regarding the feasibility of high-resolution retinal prosthesis, namely, that the combination of conventional CMOS electronics and microwire glass provides a viable approach for a high resolution retinal prosthesis device Temperature change from power dissipation within the device and maximum electrical output current levels suggest that the device is acceptable for acute human tests

N. Fernandez

Papers

A Retinal Prosthesis Technology Based on CMOS Microelectronics and Microwire Glass Electrodes