Unconventional application of Image Correlation techniques on Biomaterials and cardiovascular applications

Image Correlation techniques are increasing in popularity and are now suitable to investigate complex structural and fluid-dynamics of cardiovascular systems. These approaches operate through a digital correlation of a pair of images to determine physical quantities of complex systems. In this thesis, their application to the mechanical characterisation of complex biomaterials (Nitinol and soft materials), and the characterisation of complex biofluid-dynamics was investigated, with the aim to analyse their reliability and enhance their accuracy and field of application. Digital Image Correlation, supported by Infrared Thermography, was used to achieve a more in depth understanding of the mechanical behaviour of Nitinol shape memory alloys. The analysis led to the design of a new approach that minimise the error of standard characterisation methods. Application of DIC to soft materials focused on biaxial test protocols commonly adopted in their characterisation. These were investigated, identifying major drawbacks that reduce the accuracy of the results. A new setup arrangement of easy implementation was developed to reduce the source of errors from above 50% to below 20%. For the characterisation of complex biofluid-dynamics, a new Particle Image Velocimetry (PIV) approach was attempted to improve the current estimation of the Effective Orifice Area (the leading parameter in heart valves characterisation) by allowing its direct measurement. Previously unreported limitations in the application of PIV to the analysis of the fluid-dynamics in complex cardiovascular cases were identified and investigated by implementing a new approach based on the use of SPH (Smoothed-particle hydrodynamics) Synthetic Images. Image correlation techniques were also used to address another experimental issue typical of PIV applications in cardiovascular engineering, the refractive index mismatch between blood equivalent test fluid and the anatomical phantoms. In particular, a new blood equivalent was developed, with similar density and viscosity as human blood at body temperature. This has same refractive index as a new optically transparent silicone (FER-7061), for which a prototyping protocol was implemented. In summary, this thesis presents a range of studies focused on applications of digital image correlation to cardiovascular engineering, making a distinct contribution to the state of knowledge in the area by identifying main limitations of the different methods and proposing new original approaches that allow to minimise them, to better support the development and assessment of safer and more effective cardiovascular solutions.