11 October 2012
Neural interfaces are of interest for a vast range of applications from controlling games to restoring body functions like hearing, sight or movement via prostheses. In medical prosthetics a neural interface may be used for reading signals from the human neural system in order to control a prosthesis and also for passing signals the other way.
Neural interfaces often consist of an array of electrodes that read or stimulate electrical activity in neural tissues. Implantable neural interfaces, such as the Michigan and Utah electrode arrays, introduce advantages from CMOS and MEMS technologies allowing miniaturisation, high-density arrays, integrated signal processing circuits and other sophisticated functions.
However, arrays using these technologies tend to be more rigid than the tissues they interface with, making the arrays invasive and difficult to apply over larger areas. Alternatives, typically mounting metal electrodes on a flexible substrate, have better affinity to the neural tissues making them less invasive, but the wiring required can become infeasible when the number of electrodes is increased, limiting the resolution and size of the interface.
The multi-chip architecture presented in this issue by the Japanese team is designed to combine flexibility with the advantages of CMOS microchips. The array electrodes are ‘smart’, i.e. capable of sophisticated functions, as each has a dedicated CMOS microchip incorporating a current generator for neural stimulation and operation control logic circuitry. This also reduces the wiring required to connect the array to a prosthesis to 4 connections, so more electrodes doesn’t mean more external connections.
At the same time the individual chips are small enough and distributed on a flexible substrate in such a way as to make the array very flexible. Each electrode site consists of a CMOS microchip on one side of the substrate connected by through-silicon-via (TSV) to an electrode on the other. Team member Professor Toshihiko Noda said: “The key technology here is the microchip design and the fabrication process of the chip, which are newly developed for our retinal prosthesis system. The main challenges are to form TSV and flip-chip bonding in a microchip. Although flip-chip bonding technology is commonly used, flip-chip bonding of through-hole formed microchip with high yield is difficult. We have had to carefully optimise the conditions of the flip-chip bonding.”
The team have shown the effectiveness of the array by implanting it into the eye of a rabbit as a retinal stimulator. Although the rabbit’s eye was healthy the experiment has direct relevance to some common forms of vision loss in humans, as Noda explained: “In age-related macular degeneration (AMD) and retinitis pigmentosa (RP), although photoreceptor cells are degenerated, some retinal cells remain including ganglion cells, which are the main target of retinal stimulation. If ganglion cells are evoked as the result of retinal stimulation, brain response in the visual cortex is observed: this response can be evoked even if the retina is degenerated.”
This work is the fruit of a collaboration between the Nara Institute of Science and Technology, NIDEK, Kyushu University and Osaka University to realise fully-implantable bio-medical devices based on CMOS technology. This is partially supported by the Strategic Research Program for Brain Sciences by the Ministry of Education, Culture, Sports, Science and Technology of Japan. For the team ‘fully-implantable’ means no physical external connections in order to avoid infection problems in long-term use. CMOS technology is suitable for this in respect to its compatibility with wireless systems but there are other issues that the team are investigating.
They view long-term reliability as a major challenge in terms of waterproof and bio-compatible packaging. Options like hermetically sealed packaging are too bulky for applications like retinal implants. Their current solution is to use a parylene-C coating, but they have yet to test its long-term viability.
In the context of visual prosthetics the team also plan to improve the ‘picture’ their array could provide. Their proof-of-concept device has 9 electrodes but they are confident that a redesign of the substrate could allow up to 1000 electrodes with the same device configuration, improving the definition of visual information that could be transferred. They also plan to capitalise on the ability to increase the area of the device by optimising the design for smooth implantation into the eyeball even if the area is large, as covering more of the retina would improve viewing angle for a user.
Beyond, visual prosthetics, although their letter reports the use of the array as a stimulator, it is designed as a neural interface for both stimulating and measuring neural activity. The team plan to use the design to create an array for electrocorticography .
The Letter presenting the results on which this article is based can be found on the IET Digital Library.
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