Optical Filter Piezo O2-Sensing Film µLED LED Holder Thickness = 1.2 mm Flex Board IC 4.5 mm 3 mm Volume ~4.5 mm3 Encapsulation (a) (b) FIGURE 11: (a) An exploded view of (b) the implantable O2 sensor designed by University of California, Berkeley. Adapted from [21]. Sensor IC Sensor Piezo Vrect φmod Active Rectifier Cstore Vdc-dc Modulation Switch Voltage Doubler Power on Reset RO Clock Clock Division φref φref Bias and Reference LDOs LED Driver OOK Demod. VOOK POR Analog PD φmod FSM CLK 10-bit AFE Vmid φref TDC Data ILED ILED (a) O2 Concentration (mmHg) 1.96 mm 1.96 mm path between the LED driver and the AFE. The power management blocks consist of 5-V CMOS devices to withstand a wide range of battery voltages for regular silicon LEDs and organic LEDs. The AFE consists of two stages: a TIA and a variable-gain amplifier (VGA). The purpose of the TIA is to sense the current generated by the photodiode and convert it into voltage. The VGA provides the additional tunable gain required to measure low-intensity input signals. The film's fluorescence can be measured in terms of either intensity or lifetime, as presented in [39]. The performance of the two techniques was analyzed and compared in [38]. The lifetime (decay time) and intensity (amplitude) were measured for different concentrations of O2, ranging from 0.5 to 500 mm of mercury. Figure 14(a) and (b) conveys the result for both the low and high reflectance, respectively. This simulates the real-world example of the instrument being used on individuals with different skin tones. While both the intensity and the lifetime are inversely proportional to the concentration of O2, which can be observed from Figure 14(c), the intensity of the reemitted light varies strongly with the differences in the optical path. The differences in the lifetime measurement are NEOFOX O2 Probe Wireless Sensor 80 60 40 20 0 0 10 20 30 Time (min) (b) 40 (c) FIGURE 12: (a) A system diagram of an implantable O2 sensor. (b) A micrograph of an SoC in a 65-nm, low-power-CMOS process. (c) The system response to alternating streams of O2 and nitrogen. Adapted from [21]. AFE: analog FE; FSM: finite state machine; RO: ring oscillator; OOK Demod.: on/off keying demodulation; POR: power on reset; Ref.: reference; TDC: time-todigital converter; D-LDO: digital low-dropout regulator; A-LDO: analog low-dropout regulator. IEEE SOLID-STATE CIRCUITS MAGAZINE FA L L 2 0 2 0 43