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Luminescent oxygen sensing is employed for measuring the partial pressure of oxygen diffusing through the skin, named transcutaneous oxygen. Two well-known approaches are intensity- and lifetime-based measurements for assessing transcutaneous oxygen. The lifetime-based technique is preferable as it offers lower susceptibility to optical path changes and reflections compared to the intensity-based method. High-resolution lifetime capturing is critical to accurate transcutaneous oxygen measurements from the human body. This study proposes a miniaturized prototype based on a multimodal analog front end, ADPD4101, and custom firmware. We have demonstrated that the prototype could detect small changes in the lifetime with high resolution, showing its suitability for future human subject tests. We implemented the prototype on a 68 mm × 43 mm printed circuit board (PCB) and consumes the power ×of 39 mW. 

The ability to monitor blood gases, namely oxy-gen and carbon dioxide, in real-time is of critical importance to clinicians in diagnosing and treating respiratory disorders. Transcutaneous monitors measure the partial pressure of carbon dioxide diffused from the skin. These monitors are noninvasive and capable of continuously monitoring carbon dioxide. Conventional transcutaneous carbon dioxide monitors require a heating element and large calibration equipment for reliable measurements. We propose a miniaturized transcutaneous carbon dioxide monitor based on a luminescence sensing film and dual lifetime referencing technique to assess the partial pressure of carbon dioxide within the 0-75 mmHg range, covering the clinically relevant range for healthy humans, 35-45 mmHg. We measured the partial pressure of carbon dioxide with less than ~1.6% error in the given range without any post-processing and heating. 

This paper presents a nonuniform sampling technique for measuring the lifetime of luminescent materials for oxygen sensing. The system features a switched-capacitor circuit to implement fixed-voltage steps for quantization, enabling long integration times without saturating the front-end amplifier. A control circuit automatically tunes the light emitting diode (LED) excitation pulses to avoid overpowering or starving the front end as photodiode current varies with changes in the partial pressure of oxygen. Time gating of the front-end integrator removes the need for optical filtering. The analog front end (AFE) has a gain bandwidth product of 10 MHz and an input-referred noise of 124 μVrms (measured 200 Hz - 100 kHz). The circuit was realized in 180 nm CMOS technology. The AFE and LED driver consume a maximum of 16 μJ per calculation. We have demonstrated the entire system's functionality by measuring oxygen concentrations from 0 to 240 mmHg in a controlled gas vessel. The results indicate satisfactory linearity on a Stern-Volmer plot covering the human-relevant range of 50 to 150 mmHg.