Caulfield, Richard David Robert; (2021) High-resolution 3D printing enabled, minimally invasive fibre optic sensing and imaging probes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Minimally invasive surgical procedures have become more favourable to their traditional surgical counterparts due to their reduced risks, faster recovery times and decreased trauma. Despite this, there are still some limitations involved with these procedures, such as the spatial confinement of operating through small incisions and the intrinsic lack of visual or tactile feedback. Specialised tools and imaging equipment are required to overcome these issues. Providing better feedback to surgeons is a key area of research to enhance the outcomes and safety profiles of minimally invasive procedures. This thesis is centred on the development of new microfabrication methods to create novel fibre optic imaging and sensing probes that could ultimately be used for improving the guidance of minimally invasive surgeries. Several themes emerged in this process. The first theme involved the use and optimisation of high-resolution 3D injection of polymers as sacrificial layers onto which parylene-C was deposited. One outcome from this theme was a series of miniaturised parylene-C based membranes to create fibre optic pressure sensors for physiological pressure measurements and for ultrasound reception. The pressure sensor sensitivity was found to vary from 0.02 to 0.14 radians/mmHg, as the thickness of parylene was decreased from 2 to 0.5 μm. The ultrasound receivers were characterised and exhibited a noise equivalent pressure (NEP) value of ~100 Pa (an order of magnitude improvement compared to similarly sized piezoelectric hydrophones). A second theme employed high-resolution 3D printing to create microstructures of polydimethylsiloxane (PDMS) and subsequently formed nanocomposites, to create microscale acoustic hologram structures. This theme included the development of innovative manufacturing processes such as printing directly onto optical fibres, micro moulding and precise deposition which enabled the creation of such devices. These microstructures were investigated for reducing the divergence of photoacoustically-generated ultrasound beams. Taken together, the developments in this thesis pave the way for 3D microfabricated polymer-based fibre optic sensors that could find broad clinical utility in minimally invasive procedures.

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