Petreikis, Matas; (2025) 3D-printing enabled, hybrid flexible sensors for multipoint relative and absolute oxygen concentration monitoring for healthcare applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The variety of reasons for oxygen sensing in a hospital environment has not stopped growing since the 1950s and the development of the first polarographic oxygen sensors. Today they range from continuous tracking of pulsatile oxygen saturation as a vital sign in the intensive care units to determining oxygen concentration within tumours to aid in the evaluation of their malignancy. The rise in the development of wearable and flexible electronics has shown how sensor flexibility can enable improvements in oxygen sensing in the body in terms of sensor performance and ergonomics, especially where conformal coupling is beneficial, for example, when continuously tracking oxygen saturation in the skin of healthy or injured tissue, i.e. around chronic wounds or after reconstructive surgeries. The goal of this thesis is to demonstrate the fabrication of flexible electronic circuits using additive manufacturing techniques, such as direct-write 3D printing, that would enable production not only of flexible and conformal, but also low-cost and potentially custom-made oxygen sensors for tissue oxygen saturation. This effort was split into two distinctive parts. The first part involved designing and fabricating a flexible printed circuit board (fPCB) by 3D-printing silver nanoparticle ink that would form the basis for a near-infrared spectroscopy (NIRS)-based oxygen sensor. The potential for simultaneous, multipoint measurements was demonstrated by detecting the photoplethysmography signal from an index finger using 660 nm and 940 nm light. By integrating four different wavelengths, i.e. 660 nm, 740 nm, 880 nm, and 940 nm, instead of the more common case of two in flexible sensor designs, it was shown how changes in deoxygenated and oxygenated haemoglobin could be detected during forearm ischemia caused by arterial occlusion. Consequently, these changes were then used to estimate tissue oxygen saturation variation during arterial occlusion, which is one of the complications of free flap reconstructive surgeries. Multipoint measurements of tissue oxygenation during forearm ischemia were also demonstrated, they revealed significant differences in the measured values between the individual sensors. This suggested that further development of multipoint flexible tissue oximeters should include efforts in sensor packaging to ensure optimum coupling between all of the individual sensors and the probed skin or to counteract the differences in optical between the sensors via advances data processing techniques. The second part involved investigating how 3D-printed fPCBs could be utilised in conjunction with phosphorescent nanoporous films for phosphorescence quenching-based oxygen sensors. Optimisation of parameters, such as light source brightness and light source-detector distance, was first performed through calibra tion of the fPCB and phosphorescent film system in a closed chamber with a controlled oxygen environment. To showcase an application of such a sensor for healthcare, another fPCB, tailored for a bioreactor in which artificial mice liver can be grown, was designed and fabricated; the sensor was calibrated in a closed chamber in a replica bioreactor. This research effort revealed that introducing multiple measurement points using photodiode-based systems necessitates per-sensor calibration, the solution to which could be increasing the complexity of the optical system, e.g. adding a reference phosphorescent dye or an additional light source. Lastly, a technique to integrate the phosphorescent film onto the fPCB itself, involving 3D-printing a water-soluble PVP mould, was demonstrated. This step, involving submerging of the phosphorescent film in an elastomer, worsened the response time of the sensor to ∼10s of minutes due to the inhibited oxygen diffusion through the elastomer. This led to a conclusion that the full optical system of the sensor should be miniaturised in the future to speed up the gas exchange process. Taken together, the advancement in the 3D-printing enabled oxygen sensors depicted in this thesis should provide useful insight for further development of flexible sensors and their arrays using additive manufacturing techniques.

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