Wearable device for remote monitoring of transcutaneous tissue oxygenation

Abstract

Wearable devices have found widespread applications in recent years as both medical devices as well as consumer electronics for sports and health tracking. A metric of health that is often overlooked in currently available technology is the direct measurement of molecular oxygen in living tissue, a key component in cellular energy production. Here, we report on the development of a wireless wearable prototype for transcutaneous oxygenation monitoring based on quantifying the oxygen-dependent phosphorescence of a metalloporphyrin embedded within a highly breathable oxygen sensing film. The device is completely self-contained, weighs under 30 grams, performs on-board signal analysis, and can communicate with computers or smartphones. The wearable measures tissue oxygenation at the skin surface by detecting the lifetime and intensity of phosphorescence, which undergoes quenching in the presence of oxygen. As well as being insensitive to motion artifacts, it offers robust and reliable measurements even in variable atmospheric conditions related to temperature and humidity. Preliminary in vivo testing in a porcine ischemia model shows that the wearable is highly sensitive to changes in tissue oxygenation in the physiological range upon inducing a decrease in limb perfusion.

Fig. 1.

Optical wireless wearable prototype for transcutaneous oxygen monitoring based on the phosphorescence emission of a highly breathable oxygen sensing film. The prototype is composed of an oxygen-sensing film, a sensor head, and control electronics. The block diagram represents the control electronics and sensor head circuits.

Fig. 2.

Response of the oxygen sensing film to $p{O}_2$ and temperature during a calibration run. (a) Variation of the temperature (measured by the thermistor in the sensor head) and $p{O}_2$ (measured by a commercial oxygen sensor) in a sealed chamber during calibration, in which the oxygen partial pressure is varied by mixing nitrogen and air at different ratios. (b) ADC output of the photodiode and reference signal channels at different points in time throughout the calibration. The photodiode signal reveals how the phosphorescence of the oxygen sensing film changes in amplitude and phase with respect to the reference signal during changes in oxygen (and temperature to a lesser degree). The reference signal remains stable during the measurement. (c) Phase (minus the initial value at $t=0$) of the reference and photodiode signal vs time during the calibration period. The relative phase between the photodiode and reference signal, $\theta ={\theta }_p-{\theta }_r$, exhibits high sensitivity to changes in $p{O}_2$ throughout the whole physiological range. (d) Amplitude of the emission $I$ vs time, presenting a similar response to the phase. Phase and amplitude are obtained with a multiple linear regression algorithm from the data in (b).

Fig. 3.

Fits of the temperature dependent Stern-Volmer equation. (a) Stern-Volmer plot of lifetime data and its fit to the temperature dependent Stern-Volmer equation. The model is able to describe the variation of the measured lifetime with changes in partial pressure of oxygen and temperature. (b) Complimentary lifetime data vs temperature. The dependence of lifetime with temperature is accounted for by modelling $K_{\mathit{eff}}$ as a second order polynomial depending on temperature. (c) Comparison of $p{O}_2$ measured by the developed device and a commercial reference sensor along with a 95% confidence interval (CI) of measurements. The $p{O}_2$ estimated from lifetime data reproduces all features observed in the reference $p{O}_2$ data with slight differences attributed to mismatches in sensor speed and temperature compensation. (d), (e) and (f) show equivalent plots for the intensity data revealing similar features.

Fig. 4.

In vivo testing of the oxygen sensing prototype in a porcine model. (a) Photograph of the experimental set-up. Blood flow was occluded in a front limb of a Yorkshire swine by applying a tourniquet over the elbow joint to induce changes in tissue oxygenation. The wearable was placed over a shaved area of skin on the upper limb. (b) Temperature and $p{O}_2$ (estimated from lifetime and intensity) during the experiment. As seen in both the estimates of $p{O}_2$ and temperature, the device is sensitive to physiological changes due to the compromised blood flow to the limb during the full occlusion. (c) Time derivatives of both partial oxygen pressure estimates reveal a faster rate of change of the local oxygenation for the minutes following application and removal of the tourniquet.