PHYSIOLOGICAL CONSIDERATIONS IN IMPLANTABLE UNDERWATER TELEMETRY

Abstract

Monitoring the internal environment of marine organisms as well as that of man in the oceanic milieu presents unique problems to the physiologist as signals used for data telemetry may have clear behavioral effects. The question of a tolerable level using different transmission modes is found to be species specific, but general guidelines are drawn on the basis of filtering and detection theory, the properties of the transmission media and the excitability characteristics of biological tissue. Transmission through water is surveyed and attenuation characteristics are presented. Experimental findings indicate that waves propagated through water and tissues are time dispersive and thus at sufficiently high frequencies the received signal process must be carefully evaluated with regard to optimal demodulation. Frequencies up to 1 MHz appear to be within the immediately usable range for distances of 1 meter or less. This bandwidth may be used for frequency-division multiplexed subcarriers or appropriately coded PCM transmission. Successful underwater transmission of physiological signals has been accomplished using frequency modulation of carriers transmitted by return-current-density methods, as part of an external biotelemetry system. Experimental evidence shows that implanted devices operating at high frequencies and at levels far below stimulation threshold can be used for transmission to the external transponder where adequate power is available for transmission to an underwater habitat or through data buoys to remote recording centers.