<italic>In Vitro</italic> Evaluation of an Injectable EEG/ECG Sensor for Wireless Monitoring of Hibernation in Endangered Animal Species

Abstract

Hibernation is a unique metabolic adaptation employed by several animal species for survival where its study would further enhance our understanding of metabolic disorders, such as diabetes and obesity. As a primate animal with close genetic ties to humans, the recent discovery of hibernation in dwarf lemurs of Madagascar has attracted the attention of researchers. Traditional recording systems require the physical tethering of the animals to the recording apparatus or the use of implantable devices. Scalp and needle electrodes interfere with the natural hibernation process and limit the continuity of the experiments, while invasive procedures are banned on endangered species. By integrating a full-wave rectifier, low-noise signal conditioning circuit, frequency modulation transmitter, and antenna in a single application specific integrated circuit (ASIC), we have developed an ultra-miniaturized wireless system that measures $34 \times 4 \times 2.6$ mm3 in volume. It only requires three off-chip components (a coil wound around a ferrite rod and two external capacitors) to be powered wirelessly through a 1-MHz inductive link, such that it can be packaged inside a glass or polymer capsule and injected subcutaneously underneath the scalp or chest without requiring a surgery, thereby addressing the shortcomings of the traditional monitoring systems. Our recording device provides an input/output correlation coefficient greater than 80% for input amplitudes ranging from 60 to 260 $\mu V_{\mathrm{ pp}}$ , with a wireless data transmission range of ~2.5 cm while operating near the 902–928 MHz ISM frequency band. This system would enable future studies of electroencephalography and electrocardiography in hibernating dwarf lemurs. The ASIC was fabricated using the ON Semiconductor 0.5- $\mu \text{m}$ CMOS process with an active area of 2.5 $\times $ 1 mm2 and has a power consumption of 7.75 mW from a 3.1 V supply. In this paper, we demonstrate the in vitro functionality of the system using simulated physiological signals directly applied to the ASIC or through standard stainless steel electrodes immersed in saline solution.