Electrodes chronic implantation

Spiegel, E.A. and H.T. Wycis, Chronic implantation of intracerebral electrodes in humans, in Electrical Stimulation of the Brain, D.E. Sheer, Ed. 1961, University of Texas Press Austin, TX, pp. 37-44. 

One final but important point that an investigator should realize is that animals which are chronically implanted with electrodes can be tested without interference from anesthetizing drugs or stressful restraint. 

Neural electrodes are often used for chronic implant applications that demand chemical and electrical stability of materials. Yamato et al. found that PPy has limited electrical response under cyclic voltammetry because of structural defects along the CP backbone... 

Although electrical shock has been used widely as a nociceptive stimulus for evaluating opiate activity, problems do arise in controlling the intensity of stimulus thus, electrode currents and vdtages may be readily maintained where as the impedance of the biological tissues can be extremely variable, thereby producing data of poor reproducibility [29 2]. The variability of data can be reduced when the animals can act as their own controls and also by chronically implanting electrodes subcutaneously sometime before the experiment so that impedance is minimised [SI]. 

An encapsulation scheme that combines AI2O3 by atomic layer deposition with parylene-C for implantable electroiuc systems has been presented [85]. The so formed AI2O3/parylene-C bi-layer was used to encapsulate interdigitated electrodes. Accelerated lifetime test showed a very good performance so that the method of encapsulation has been claimed to be suitable for chronic implants. 

The chronic implants for electrochemical measurements are completely conventional. The working electrode(s) are implanted under stereotaxic control the reference and auxiliary electrode, as mentioned, can be located at the experimenter s convenience. Electrode contacts are soldered to some form of multiprong connector affixed to the skull with the usual dental cement. A mating connector, flexible cable, and commutator make connection to the potentiostat for chronic recording in behaving animals. 

A major problem with the chronic implantation of electrodes, especially for control of cellular function, is the likelihood that the electrode will poison or cause trauma to the tissue[16]. When large currents are passed through implanted electrodes, the dissolution of ions and the generation of gases can cause lesions. However, the use of transcutaneous stimulation avoids this problem. For example, an area that has received particular attention is the use of electromagnetically induced currents. 

Polikov, V. S., Tresco, P. A. and Reichert, W. M. (2005) Response of brain tissue to chronically implanted neural electrodes. J. Neurosci. Meth. 148, 1-18. 

In this two-electrode system, the cardiac tissue behaved electrochemically as the electrolyte in a simple electroplating cell. While stainless steel is passive under recording conditions, it loses its passivity in the chronic, active state (Evans, 1960). Rowley also reported that spectrographic analysis of tissue excised from the positive electrode region showed that metal ions from the electrode had migrated into the myocardial tissue. He showed from a theoretical Faradaic calculation that a 10-mg iron electrode placed in an ionic medium would be electrolyzed in 19 days by a 90-per-min, 1.5-msec-duration, 10-mA unidirectional stimulus. This and other problems associated with electrodes for myocardial and endocardial chronic implantation have been discussed by Greatbach and Chardack (1968). Hallen et al (1965) have also examined the problem. 

For emergency use when chronic implantation is contraindicated, a single coaxial bipolar electrode is useful. This scheme requires the use of one needle instead of two. The electrode is shown schematically in Figure 1.1. The unit is a No. 18 standard hypodermic needle to which has been added an external insulating sheath and an insulated internal central electrode. It is used in cases of temporary heart block or until corrective procedures can be instituted in the case of permanent block (Ferris and Cowley, 1968). 

Chronically implanted stimulating electrodes, whether cardiac or other, are all subject to the problems noted above. 

Malmstrom, J.A., TG. McNaughton, and K.W. Horch, Recording properties and biocompatibility of chronically implanted polymer-based intrafascicular electrodes. Ann. Biomed. Eng., 1998, 26 ... 

Babb TL, Kupfer W (1984) Phagocytic and metabolic reactions to chronically implanted metal brain electrodes. Exp Neurol 86(2) 171-182... 

Greatbatch W, Chardack WM (1968) Myoctirdial and endocardiac electrodes for chronic implantation. Ann N.Y Acad Sci 148 234-251... 

One of the most successful commercially available implantable microelectrode arrays is the deep-brain stimulator. Deep-brain stimulation (DBS) uses chronically implanted electrodes to treat neurological conditions such as movement disorders. Electrodes deliver high-frequency electrical stimulation to targeted regions of the brain to treat symptoms of Parkinson s disease. Implantable visual prostheses promise to restore vision by providing stimulation at the retina, optic nerve, or visual cortex. Cochlear implants include an electrode array implanted in the inner ear to stimulate the auditory nerve. Other commercially available implantable neural interfaces include neurostimulation systems for treatment of chronic pain or urinary control. 

Although microfabricated electrodes have demonstrated effective recording and stimulation for acute applications, stability in chronic applications has not been achieved. Even using microwire arrays, which have demonstrated the greatest robustness, Nicolelis et al. reported a steady decline in the number of functional electrodes over 18 months [11]. The failure of chronically implanted microelectrodes is attributed to the CNS response. 

Suner S, Fellows MR, Vargas-Irwin C (2005) Reliability of signals from a chronically implanted, silicon-based electrode array in non-human primate primary motor cortex. IEEE Transactions on Neural Systems and Rehabilitation Engineering 13 524-541. 

Lefurge T, GoodaU E, Horch K (1991) Chronically implanted intrafascicular recording electrodes. Annals of Biomedical Engineering 19 197-207. 

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