At present, intraoperative electrophysiological monitoring for complicated clinical cases is essential [12, 14–16]. In addition to easy-to-use intraoperative imaging equipment, it is also important that EEG electrodes can be attached easily, safely, promptly, and in a cost-effective manner. A variety of electrodes have been developed, such as staple [6], spiral scalp [17], peg-screw [14], carbon-wire [7], and conductive plastic electrodes [5]. Considering that imaging equipment is used during surgery, we need to develop a new EEG electrode that is: markedly conductive with an impedance of less than 5 kΩ [18], permeable to X-rays and MRI, thin and not an obstacle in the surgical field, easy to attach to the scalp, disposable and inexpensive, and can be easily produced. The thin-film EEG electrode is made from carbon nanotubes, which are resistant to acid and high temperatures and significantly functional with unique electrical characteristics [8]. Carbon nanotubes are applicable in the fields of nanotechnology, electricity, and medicine, particularly for their marked mechanical strength and electrical characteristics [8, 9, 15]. Nanotubes are classified into single- and multi-walled. In theory, the MWCNT is a metal nanotube that can transmit an electrical current with a density of 4 × 109 A/cm2, which is 1,000 times higher than the current density that can be transmitted by a copper tube [9]. The MWCNT fulfilled all the requirements for an electrode used for surgery. The evoked potentials of the MWCNT electrode were measured at 10%- to 40%-MWCNT dispersion concentration levels. The evoked potentials were similar to those of silver electrodes. Electrode impedance was modeled as a parallel circuit of a capacitor and a resistor. The SER was representative of a resister, not a capacitor. The MWCNT satisfied these requirements even when the concentration of the electrode was 10%, presumably because of its low impedance [10]. In fact, the impedance of 20% MWCNT electrodes between the electrode and the scalp was 1.7 KΩ. The capacitance might be large enough. Conductive wires and non-metal cables with a higher level of conductivity than that of copper ones can also be produced, as reported by recent studies [15]. Electrophysiological monitoring has increasingly been adopted and used, along with intraoperative imaging, CT, and MRI equipment [1, 3, 4] as well as cerebral angiography [2]. Medical devices with metals embedded are restricted from the viewpoint of radiology because CT and MRI may generate linear and magnetic susceptibility artifacts. According to a previous study, a silver-epoxy-coated, plastic conductive EEG electrode has been developed, which allows the measurement of brain waves without generating artifacts in CT. However, using this electrode in MRI may cause a magnetic field gradient between the electrode and skin, generating tumor-like artifacts on the image [5]. Metals are not allowed to be used in magnetic fields generated by MRI equipment in actual clinical settings [19]. However, the newly developed non-metal electrode can be used not only for intraoperative CT and MRI, but also in endovascular treatment and balloon occlusion tests.
As heavy electrodes easily detach from the skin during surgery, light electrodes should be used. The thin, non-metal electrode is desirable since it can be used for any type of neurosurgery. Although EGG electrodes are often contaminated with blood, water, and disinfectants, they are generally re-used. Electrodes used for patients with infectious diseases must be disposed of. The MWCNT is not very expensive; the price of one MWCNT electrode is less than five USD. However in Japan, the price of one needle electrode is approximately 20 USD, and one spiral-shaped electrode costs about 40 USD [6].
There has been serious concern over the toxicity of carbon nanotubes. In an experiment, MWCNT electrodes were attached to the lungs of rat and mouse models, and granulomas formed [20]. Furthermore, according to a previous study, a large volume (3 μg/mouse) of intraperitoneally injected MWCNT caused mesothelioma [21]. There have also been health concerns over the absorption of aerosolized MWCNT electrodes. To address these problems, carbon nanotubes used for the newly developed electrode are contained in polymethylmethacrylate (PMMA), a substance widely used for surgery and dental treatment [22], to prevent them from being aerosolized. Researchers and companies have been conducting studies on the safety of carbon nanotubes to promote application to a variety of products [21, 23].