Correction of refraction induced distortion in optical coherence tomography corneal reconstructions for volume deformation measurements

In this project, the depth-resolved full-field deformation of the porcine cornea under changing intraocular pressure was investigated by performing digital volume correlation (DVC) on the reconstructed volume images generated through swept source optical coherence tomography (SS-OCT). Posterior inflation test of porcine cornea sample for two load steps were performed and the distribution patterns of displacement and strain fields were produced. The error sources for the measurements were analyzed. The refraction induced OCT image distortion is a main error source for the measurement results. Then, a methodology was developed to correct the OCT distortion based on the Fermat’s principle

Fourier-series-based virtual fields method for the identification of 2-D stiffness distributions

The Virtual Fields Method (VFM) is a powerful technique for the calculation of spatial distributions of material properties from experimentally-determined displacement fields. A Fourier-series-based extension to the VFM (the F-VFM) is presented here, in which the unknown stiffness distribution is parameterised in the spatial frequency domain rather than in the spatial domain as used in the classical VFM. We summarise here the theory of the F-VFM for the case of elastic isotropic thin structures with known boundary conditions. An efficient numerical algorithm based on the 2-D Fast Fourier Transform reduces the computation time by 3-4 orders of magnitude compared to a direct implementation of the F-VFM for typical experimental dataset sizes. Reconstruction of stiffness distributions with the FVFM has been validated on several stiffness distribution scenarios, one of which is presented here, in which a difference of about 0.5% was achieved between the reference and recovered stiffness distributions

Depth-resolved full-field measurement of corneal deformation by optical coherence tomography and digital volume correlation

The study of vertebrate eye cornea is an interdisciplinary subject and the research on its mechanical properties has significant importance in ophthalmology. The measurement of depth-resolved 3D full-field deformation behaviour of cornea under changing intraocular pressure is a useful method to study the local corneal mechanical properties. In this work, optical coherence tomography was adopted to reconstruct the internal structure of a porcine cornea inflated from 15 to 18.75 mmHg (close to the physical porcine intraocular pressure) in the form of 3D image sequences. An effective method has been developed to correct the commonly seen refraction induced distortions in the optical coherence tomography reconstructions, based on Fermat’s principle. The 3D deformation field was then determined by performing digital volume correlation on these corrected 3D reconstructions. A simple finite element model of the inflation test was developed and the predicted values were compared against digital volume correlation results, showing good overall agreement

A Fourier‐series‐based virtual fields method for the identification of three‐dimensional stiffness distributions and its application to incompressible materials

We present an inverse method to identify the spatially-varying stiffness distributions in three-dimensions (3-D). The method is an extension of the classical Virtual Fields Method (VFM) – a numerical technique which exploits information from full-field deformation measurements to deduce unknown material properties – in the spatial frequency domain, which we name the Fourier-series-based Virtual Fields Method (F-VFM). Three-dimensional stiffness distributions, parameterised by a Fourier series expansion, are recovered after a single matrix inversion. A numerically efficient version of the technique is developed, based on the Fast Fourier Transform. The proposed F-VFM is also adapted to deal with the challenging situation of limited or even non-existent knowledge of boundary conditions. The 3-D F-VFM is validated with both numerical and experimental data. The latter came from a phase contrast MRI experiment containing material with Poisson’s ratio close to 0.5; such a case requires a slightly different interpretation of the F-VFM equations, to enable the application of the technique to incompressible materials

Fast Fourier virtual fields method for determination of modulus distributions from full-field optical strain data

Inspection of parts for manufacturing defects or in-service damage is often carried out by full-field optical techniques (e.g., digital speckle pattern interferometry, digital holography) where the high sensitivity allows small anomalies in a load-induced deformation field to be measured. Standard phase shifting and phase unwrapping algorithms provide full-field displacement and hence strain data over the surface of the sample. The problem remains however of how to quantify the spatial variations in modulus due, for example, to porosity or damage-induced micro-cracking. Finite element model updating (FEMU) is one method to solve problems of this type, by adjusting an approximate finite element model until the responses it produces are as close to those acquired from experiments as possible

PrestoBlue cell viability assay in the IBUCS Device compared with a cell culture flask.

PrestoBlue cell viability assay in the IBUCS Device compared with a cell culture flask.</p

Phase derivative maps of the grid images taken without phase contrast.

Phase derivative maps of the grid images taken without phase contrast.</p

DIC parameters used for PMMA calibration, according to Jones, E. M. C. and M. A. Iadicola (2018).

A Good Practices Guide for Digital Image Correlation, International Digital Image Correlation Society.</p

Images of IBUCS test setup adapted to image live cell deformation under microscopic conditions.

(a) Images of IBUCS test setup. (b) Close up of custom device under microscope.</p

Displacement of cells adhered to the PMMA compared with the substrate scratch over time.

Displacement of cells adhered to the PMMA compared with the substrate scratch over time.</p