Electrodes cochlear implants

Clark, G.M. et al.. The University of Melbourne — nucleus multi-electrode cochlear implant. Adv. Oto-Rhino-Laryngol, 1987,38 V-IX. 

Two areas of signal processing research in cochlear implants are coding strategies for multi-electrode excitation and the development of noise-suppression systems. One of the problems in cochlear implants is that there is a large spread of the electrical stimulation within the cochlea. Because of this, simultaneous electrical pulses at... 

Computer-brain interfaces can work two ways. Cochlear implants have been developed to detect sound with an external microphone and relay the electrical signal to electrode arrays that directly stimulate inner ear nerve fibers. A visual prosthesis promises to similarly help the blind by applying electrical signals from a camera to an array of microelectrodes implanted into the visual cortex of the brain. Electrical signals from the brain can be used to control prosthetic limbs, computer software, or robots. Electrodes implanted into the pleasure centers of the brains of rats have been used to train rats to respond to investigators commands. 

In 1984, the Veterans Administration (VA) funded the initial animal studies at the Togus VA Medical Center (Augusta, ME). These were aimed at determining what changes would be required to use a modified cochlear implant with a maximum pulse output of 4.3 mA and 0.4-msec pulse width, to be suitable for FES use in humans. An initial decision was taken to utilize epineuraUy placed electrodes (2.5-mm diameter platinum disks) in preference to epimysial or intramuscular electrodes because it was known that the stimulation currents would be lower and that there would be less movement of the electrodes. In order to determine exactly how low the stimulation currents would be and to determine the stimulation sites, initial anesthetized rabbits studies were conducted [9]. The threshold found for each branch of the split sciatic nerves was of 0.1 to 0.2 mA at 0.2 msec with 50 pps. Maximal stimulation was achieved usually between 0.5 and 1.0 mA. Simultaneous dorsiflexion of both paws as well as co-contraction in the anterior and posterior muscle groups could be achieved. 

Artificial ear implants capable of processing speech have been developed with electrodes to stimulate cochlear nerve cells. Cochlear implants also have a speech processor that transforms sound waves into electrical impulses that can be conducted through coupled external and internal coils. The electrical impulses can be transmitted directly by means of a percutaneous device. 

Cochlear implants can be divided into two components the external speech processor and the implanted electrode array and electronics. A diagram of a cochlear implant is shown in Figure 40.1. Sounds recorded by the microphone are sent to the speech processor, which decomposes the incoming waveform and extracts certain cues that allow the speech signal to be represented as a pulse sequence. The information about the pulse sequence is then transmitted transcutaneously to the implanted electronics through a radio-frequency link, where it is decoded and used to specify stimuli that are delivered via the implanted electrode array. 

The electrode array, depending on the specific device, consists of 16-28 contacts spread over a 12-30 mm distance. Cochlear implants take advantage of the tonotopic arrangement of the cochlea ... 

FIGURE 40.1 Diagram of cochlear implant showing external speech processor and transmitter and internal receiver/stimulator and electrode array. (From Medical illustrations by NIH, Medical Arts Photography Branch.)... 

Cochlear Implant Electrode Improvement for Stimulation and Sensing... 

Therefore, an electrode with rougher surface would exhibit a larger surface area, and thus resulting in higher capacitance. This would not be the case for the smoother platinium strips which are used as stimulation sites in the cochlear implant electrode arrays. 

The microfabrication of electrode arrays built with silicon micromachining techniques illustrates an positive approach towards future Cl electrode array development in respect to the traditional manufacturing method used now days. Also lithography and MEMS technology facilitates the addition of enhanced functionality to the microelectrode arrays. There is, however, still a long way to go until these devices can be used in real Cochlear Implants. The fabrication possibilities and characterization of different CMOS compatible metals (Ti, TiN and Al) provides a strong base to go ahead with further research in this direction. In our electrical tests done we conclude that TiN is able to withstand a high current density 2.8, while aluminium failed... 

J.T.J. Roland, T.C. Huang, A.J. Fishman, Cochlear implant electrode history, in Cochlear Implants, ed. by J. Roush, 2nd edn. (Thieme Medical Publishers, New York, 2006), pp. 110-125... 

Dwivedi, A., and R. Roseman. 2003. In-situ development and study of conducting polymer electrodes on PVDF substrates for electro-acoustic application of cochlear implants. Mat Res Soc Symp Proc 771 123. 

In vivo applications of bioelectronic devices, such as artificial limbs, cochlear or retina implants, will not be stressed here in detail. Many of the problems thathave to be solved for in vitro devices, such as a stable neuron/electrode contact, do also matter for in vivo applications. However, for the latter, much more difficult requirements have to be met, such as biocompatibility not only against the neural cells but the whole body, including resistance to body reactions against the foreign device such as inflammation or scar formation, mechanical stabiHty in a moving system (muscle, eye,...), long-term stabitity over years, as well as practical requirements such as easy implantation. Despite all these difficulties, there are systems such as pacemakers and cochlear implants already on the market [37]. Retina implants are under development [38]. And first studies are made with intelligent artificial limbs. Therefore one can hope that in the twenty-first century many of the above-mentioned problems will be solved. 

Again, the application of MPR is variable since any case that requires integration of axial images with additional planes is valuable. One example is the postoperative evaluation of cochlear implants, where the path of the electrode within the cochlea turn is not easy to assess by native images in such cases the creation of MPRs that allow displaying the entire electrode in a unique plane permits the precise assessment of the number of electrodes introduced. 

Fig. 10.1a,b. Spiral CT follow-up in a patient who underwent cochlear implantation of the right ear for hearing loss, a Axial image crossing through the basal turn of the cochlea shows the initial white arrow) and final (black arrow) portions of the electrode. This latter reaches the middle turn of the cochlea, but the precise location is not measurable, b Volume rendering of the bone labyrinth demonstrates the entire path of the electrode within the cochlea. Electrode path from basal turn (1) to the middle turn (2) is clearly visible. The apical turn (3) is not reached by the elecrode tip. v=vestibule... 

Our group (Neri 2001a) has experienced the use of CT VR in the follow-up of patients that underwent cochlear implantation. VR was helpful to display the path ofthe cochlear electrode and toestablishthelength of the portion inserted in the cochlea (Fig. 10.1). 

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