A Wireless Battery-Free Cerebral-Oxygen-Monitoring Micro-System Featuring 0.028% Sensing Resolution

Cerebral oxygen monitoring (COM) presents the partial pressure of oxygen $\left(\mathrm{PO}_{2}\right)$ in human brain, which helps with the treatment of various brain diseases such as traumatic brain injury (TBI). Longtermly COM can detect the potential cerebral hypoxia due to severe brain injuries, which is necessary in invasive medical treatment for TBI patients. To conduct accurate portable diagnosis in a long term, the COM devices are required to be wireless, high-resolution, long-lifetime, and miniature. Traditional PO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> sensing products such as Licox relied on bulk hardware and cables, which was not suitable for long-term portable monitoring. In recent years, plenty of researches try to change the bulky cable-based COM instruments into portable wireless devices, where low-power techniques are studied to extend the lifetime. As shown in Fig.1, there are two conventional PO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> sensing methods:1) Optical sensing method. A $4.5 \mathrm{~mm}^{3}$ optical PO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> sensing device offered a resolution up to 0.09% [1], but the power-hungry LED driver limited the life time. For example, the LED driver consumed 20 mW power in [3], which presented to be the bottleneck in long-termly monitoring. 2) Electrochemical sensing. A resolution of $\pm 0.13 \%$ was achieved by a $42 \mu \mathrm{~W}$ cable-based electrochemical sensor [2]. Then, the PO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> sensing resolution was improved to 0.06% in [5], while it’s at the penalty of 1.4 mW power consumption due to the power-hungry trans-impedance amplifier (TIA). As a result, it’s difficult for the conventional COM devices to realize both a high resolution and a low power.