Development of an ultrasound-responsive oxygen sensor for chronic wounds monitoring

The aim of this thesis was to develop an ultrasound-responsive sensor for monitoring oxygen levels in chronic wounds. The proposed device consisted of a hydrogel matrix embedding oxygen-sensitive elements that can modulate their speed of sound (SoS) in response to variations in oxygen concentration in the surrounding environment. Alginate was selected as bulk material and three different oxygen-sensitive elements were explored: hemoglobin, perfluorooctyl bromide, and carbon nanotubes. First, an optimization process was carried out to define the most effective protocol for fabricating the hydrogel samples with embedded oxygen-sensitive elements. Acoustic characterization tests were performed using the through transmission ultrasound technique. To assess if a variation in SoS occurred in the material, the samples were exposed to known concentrations of oxygen, in a range from 20 mmHg to 70 mmHg, and subsequently tested to obtain calibration curves for the sensors. The samples with embedded sensing elements showed a decrease in SoS with increasing oxygen concentration, instead the control sample made of alginate hydrogel exhibited a negligible variation. The sensitivities obtained for the sensors were respectively: −0.1272 m/s·mmHg for alginate, −0.2989 m/s·mmHg for alginate + Hb, −0.1543 m/s·mmHg for alginate + PFOB, and −0.5424 m/s·mmHg for alginate + SWCNTs. For the hydrogel with the highest sensitivity, the resolution was evaluated, obtaining an overall value of 20 mmHg inside the measurement range. In the 20–30 mmHg range, the resolution was found to be 10 mmHg.