A new 19-channel SQUID magnetometer system has been developed for research use in order to measure the neuromagnetic fields originating from cortices of the human brain.The system could function for 6 days with a one-time supply of about 25 L of liquid helium. The system consists of Nb/Al-oxide/Nb SQUID sensors with 2nd-order gradiometers, tank circuits, readout electronics, a liquid helium dewar, a gantry, and a prefabricated shielded room. The gradiometers cover a circular area of 15 cm radius. We used fine stainless steel leads for electric connection between the sensors and room-temperature electronics with low thermal conduction in a low helium consumption dewar. The system could function for 6 days with a one-time supply of about 25L of liquid helium. The system can be thermally cycled for repeated measurements, with an intervening nonusage period at room temperature. The noise characteristics, for both the time and frequency domains, of all channels were measured. From an analysis of the voltage output at the phase-sensitive detector, the flux-origin noise which is generated by external sources was dominant in the white noise frequency. The power spectra of the noise field were below 10 fT/Hz1/2 at 10-100 Hz and below 18 fT/Hz1/2 at 1-10 Hz. Some other peaks of power line frequencies such as 50 Hz and 150 Hz were observed at several channels. Sound-evoked magnetic fields were measured from the temporal area of the head upon application of tone bursts. The evoked fields were recorded with the amplitude of about 250 fTpp. The isofield contours of the peak response showed that the measurement area is large enough to estimate current dipoles. It is confirmed that the system has the ability to measure magnetic fields from the human brain.

Polyvinyl alcohol gel (PVA gel) has been applied in various fields as a biomedical material in Japan since the mechanical and the electrical characteristics are very similar to the human body. In this paper, the electrical characteristics of the electrode coated with PVA gel containing saline solution are described. The electrode has been applied to a stimulation electrode in an extracochlear prosthesis. It is expected that the coated electrode can stick to the round window membrane without scarring it. From the experimental results, is was found that the electrical impedance of a stimulation electrode coated with PVA gel exhibited good stability and the effective double-layer capacitance of the coated electrode was less dependent on the current than the capacitance of the Pt-Ir electrode. The electrical characteristics of the PVA electrode were proved to be very stable for long term use in-vivo. The coated electrode was ascertained to be able stimulate reliably the auditory nerves of a guinea pig.