Three-dimensional Full-field Measurements of Large Deformations in Soft Materials Using Confocal Microscopy and Digital Volume Correlation
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A three-dimensional (3-D) full-field measurement technique was developed for measuring large deformations in optically transparent soft materials. The technique utilizes a digital volume correlation (DVC) algorithm to track motions of subvolumes within 3-D images obtained using fluorescence confocal microscopy. In order to extend the strain measurement capability to the large deformation regime (>5%), a stretch-correlation algorithm was developed and implemented into the Fast Fourier Transform (FFT)-based DVC algorithm. The stretch-correlation algorithm uses a logarithmic coordinate transformation to convert the stretch-correlation problem into a translational correlation problem under the assumption of small rotation and shear. Estimates of the measurement precision are provided by stationary and translation tests. The proposed measurement technique was used to measure large deformations in a transparent agarose gel sample embedded with fluorescent particles under uniaxial compression. The technique was also employed to measure non-uniform deformation fields near a hard spherical inclusion under far-field uniaxial compression. Introduction of the stretch-correlation algorithm greatly improved the strain measurement accuracy by providing better precision especially under large deformation. Also, the deconvolution of confocal images improved the accuracy of the measurement in the direction of the optical axis. These results shows that the proposed technique is well-suited for investigating cell-matrix mechanical interactions as well as for obtaining local constitutive properties of soft biological materials including tissues in 3-D.
KeywordsStrain measurement Confocal microscopy Image correlation Large deformation Agarose gel
We gratefully acknowledge the support provided by the National Science Foundation (DMR # 0520565) through the Center for Science and Engineering of Materials (CSEM) at the California Institute of Technology. GR acknowledges the support of the Army Research Office for providing the DURIP funds for the acquisition of the confocal microscope used in this study. GR also gratefully acknowledges the Ronald and Maxine Linde Venture Fund for enabling the acquisition of imaging instrumentation used in this investigation. We would like to thank Mr. Petros Arakelian for his valuable help with the experimental setup.
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