Microelectrodes that can be implanted for a long time into the central nervous system (CNS) have a wide field of application. In principle, all brain nuclei can be recorded from or stimulated by such electrodes and their functions monitored. Of particular importance is the use of a multichannel design in brain nuclei stimulation. In such a design, groups of electrodes or even individual electrodes can be addressed separately. This allows the user to select those electrodes whose stimulation produces a therapeutic effect that is improved in comparison with unselective stimulation. Stimulation of the brain or spinal cord can be of particular value in situations when brain nuclei are degenerated or injured. In certain situations it would also be useful to be able to combine controlled electrical stimulation and localized gene transfer. A multichannel design may also allow the user to effectively measure the effects on multiple neurons and other cells following systemic or local drug administration or gene transfer. Of particular interest is an ability to simultaneously measure the effects of multiple drug candidates on neuronal function. Monitoring brain activity through implanted electrodes can also be useful if used to control drug delivery either locally or systemically or other therapeutic methods such as electrical stimulation of brain nuclei. Multichannel electrodes may also be used to lesion specific and circumscribed sites in tissue after abnormal impulse activity has been detected by recordings from the electrodes.
To record and stimulate brain structures various forms of implantable electrodes have been developed (U.S. Pat. No. 6,253,110 B1, U.S. Pat. No. 5,957,958, U.S. Pat. No. 4,573,481, U.S. Pat. No. 7,146,221 B2, U.S. Pat. No. 5,741,319, U.S. Pat. No. 4,920,979, U.S. Pat. No. 5,215,008, U.S. Pat. No. 5,031,621, U.S. Pat. No. 6,993,392 B2, U.S. Pat. No. 6,032,062, U.S. Pat. No. 4,852,573, U.S. Pat. No. 3,995,560, U.S. Pat. No. 7,041,492, U.S. Pat. No. 6,421,566 B1, U.S. Pat. No. 4,379,462, U.S. Pat. No. 5,417,719, U.S. Pat. No. 3,822,708, U.S. Pat. No. 5,501,703, U.S. Pat. No. 7,099,718 B1, U.S. Pat. No. 3,724,467; US 2007/0197892 A1).
For the function of an electrode implant it is important to have a fixed spatial relationship between the recording/stimulation sites on the implant and the measured entities. The body and thus the tissue exhibit considerable movements during daily life. Movements are caused by for example respiration, the heart beat, intestinal movement, skeletal movements such as rotating the head in relation to the body. Movements may also be caused by external forces on the body. Relative movements between tissue and electrodes can cause changes in the recorded biological signals such as electrical or chemical signals such as transmitter substances. For example, an action potential corresponds to a voltage change in the order of 100 mV over the neuronal membrane. This potential change fades quickly with distance from the cell. Consequently, movements of the electrode relative to a measured cell can result in a considerable variation in the amplitude of the measured action potential. Likewise, when the electrodes are used for electrical stimulation, a shift in location of the electrode relative to the tissue may result in a shift of the neurons stimulated. It is thus very important that the sites on the medical electrode from where recordings or stimulations are made in the tissue can follow the movements of the tissue in which it is embedded as faithfully as possible. Besides impairing the recorded signal or efficacy of stimulation, movements between implants and tissue may cause injuries to the tissue that in turn can trigger a tissue reaction and loss of function of the implant. Mechanical stability between electrode and tissue is particularly important for intracellular recordings because movements of electrode relative to the cell can easily damage the membrane and cause leakage of extracellular fluid into the cell and vice versa. Today there is no known electrode implants designed or suitable for intracellular recordings simultaneously in many neurons over long time spans such as days, weeks or months in freely moving animals or humans.
Ultra thin electrodes that are flexible and thereby overcome some of the problems related to movements between tissue and electrode are known in the art (WO 2007/040442). By embedding such electrodes in a dissolvable hard matrix it is possible to implant them in soft tissue, without any additional support such as a syringe. Such ultrathin electrodes should be made of a material that is not degraded by the tissue or easily oxidized causing high electrical resistance and thereby decreased signal to noise ratio. Examples of suitable conductors are noble metals such as gold and platinum. Commonly an alloy of platinum and iridium is used as a material for implants used for stimulation.
To achieve a physically stable contact with cells in the nervous system it is also important that the electrode is anchored in the tissue close to the measured or stimulated tissue. Electrodes with electrically conducting barbs and electrode sheets equipped with holes through which the tissue may grow and thereby attach firmly to the electrode are known in the art (WO 2007/040442; WO 2008/091197; WO 2009/075625). However, implants may cause chronic inflammation and even infections and may have to be removed. In the situation when the electrode is withdrawn from the tissue anchoring devices known by the art such as barbs or in particular holes in the electrode body allowing tissue ingrowth may cause extensive damage to the tissue. It is thus desirable to solve the problem of how to anchor a medical electrode in soft tissue such that the medical electrode is physically stabilized in the tissue and yet can be withdrawn from the tissue with reduced tissue damage.